Heterogeneous Catalysis by Gold-based Bimetallic ...

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Institute of Industrial Catalysis, College of Chemical Engineering and ... chemical industries. ...... Landon P, Collier PJ, Papworth AJ, Kiely CJ, Hutchings GJ.

Send Orders of Reprints at [email protected] Recent Patents on Catalysis, 2013, 2, 2-46

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Heterogeneous Catalysis by Gold-based Bimetallic Catalysts Like Ouyanga, Guo-Jin Daa, Jun Nib, Jing Xua and Yi-Fan Hana* a

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, b People’s Republic of China; Institute of Industrial Catalysis, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, 310032, People’s Republic of China Received: October 30, 2012; Revised: November 14, 2012; Accepted: November 27, 2012

Abstract: This critical review summarizes recent advances on the preparation, characterization and application of Aubased bimetallic Au-M catalysts. Particularly bimetallic Au-Pd, Au-Pt and Au-Ag catalysts have been emphasized. The diversity of the structure and combination of these bimetallic Au-M catalysts leads to various catalytic performances in, i.e., oxidation, hydrogenation, electrocatalytic, and photocatalytic reactions. The promotional effects of Au have been discussed by its electronic and geometric modifications of active metals. Efforts on controlling the structure (morphology, particle size and growth of crystalline) of supported bimetallic Au-M catalysts via advanced synthetic approaches have been elaborated. An overview on the challenges and opportunities for future research toward the understanding of catalytic chemistry of gold-based bimetallic systems has also been presented along with the descrption of few of the recent patents.

Keywords: Bimetallic catalyst, nanoalloy, gold-based catalysts, Au-Pd, Au-Cu, Au-Ag. 1. INTRODUCTION Bimetallic catalysts have been extensively applied in chemical industries. Generally, alloying with a secondary metal is considered as an effective approach to improve the performances (e.g. activity, selectivity and durability) of monometallic catalysts [1-3]. Among bimetallic catalysts, gold-based bimetallic catalysts (Au-M) have gained much attention [4-6]. It is worth noting that Au catalysis has been a hot topic since the discovery of low-temperature catalytic oxidation reaction using nano-Au catalysts by Hurata in 1980’s [7]. Au nanoparticles less than 5 nm possess excellent catalytic performance for low-temperature oxidation reaction, hydrogenation and so on [8-15]. Special gold nanostructures have other applications such as optic materials and sensors [1619]. The structures and applications of monometallic Au catalysts have been summarized in several excellent reviews [20-23]. For instance, Gong [22] described the structure and surface chemistry of Au-based model catalysts in detail, and correlated electronic and structural properties of supported gold clusters with reactivity. The chemical transformation of Au catalysis for both homogeneous and heterogeneous reaction has also been reviewed [20, 24]. Furthermore, Ishida and Haruta [23] highlighted the impact of Au catalysis from a viewpoint of sustainable chemistry. Deng and coworkers [21] summarized the use of nano-Au catalysts in fine chemical synthesis.

*Address correspondence to this author at the State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, People’s Republic of China; Tel: +86-21-64251928; E-mail: [email protected] 2211-5498/13 $100.00+.00

However, historically, Au has rarely been considered as a primary active metal because it usually plays a role as assistant or dilutant in bimetallic catalysts. Studies from both model and practical catalysts have demonstrated that the ligand and/or ensemble effects of Au could be responsible for the improvement of performances. Catalytic properties affected by the ligand effect are attributed to changes occurring in the electronic structure of the active metal, due to electron-transfer between Au and another metal, for instance, between Au4f and Pd3d [25]. The ensemble effect arising from the spatial redistribution of the surface atoms, can lead to the formation of active sites. The overlap of these two effects induces a synergistic effect usually assumed to be responsible for the elementary reactions occurring on different sites in one ensemble. Consequently, active sites created through alloying with Au usually result in a number of catalytic functions for utilizations in clean energy and environmental protections [1, 26-28]. Till now, Au-Pd catalysts for selective oxidation/hydrogenation (i.e., redox reactions) have been reviewed from different aspects [4, 5, 29]. Those Au-Pd catalysts have showed superior performance compared to the single metal Pd for oxidation reactions, such as synthesis of hydrogen peroxide (H2O2) directly from H2 and O2, and alcohol oxidation by molecular oxygen. Recently, the modification of the structure of bimetallic Au-Cu alloy catalysts by changing synthetic methods was also summarized by Hutchings and coworkers [30]. However, advances in Au-Pt and Au-Ag catalysts considered as other two important bimetallic Au-M catalytic systems, have not been reviewed yet. The current contribution overview recent advances in preparation, characterization and applications of bimetallic © 2013 Bentham Science Publishers

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

Au-M catalysts, particularly, focusing on the systems of supported Au-Pd, Au-Pt and Au-Ag. The emphasis is given to the origin of active sites upon the addition of Au and potential applications of those practical catalysts. We will also provide perspectives on the exploring of the role of Au in bimetallic catalysts; the electronic and geometric structure of active metals tuned by alloying with Au will be summarized as well. In order to provide deep insight into those supported catalysts, recent theoretic and model studies about bimetallic Au-M catalysts will also be included in this review. 2. AU-PD ALLOY CATALYSTS Pd is one of the most studied precious metal catalysts for various reactions. By addition of a secondary metal, the surface property of Pd can remarkably be changed, which is responsible for creation of more active sites with unique structures. Among all Pd-M alloy catalysts, Au-Pd catalysts have long been used in industries, for instance, vinyl acetate synthesis [31]. The unique catalytic properties of Au-Pd alloys have stimulated numerous theoretical and practical studies. Au-Pd alloy catalysts can be classified into three types: [32] (i) bulk alloys with specific Pd/Au stoichiometries [33], for example, PdAu and Pd3Au, (ii) surface alloys prepared by the deposition of Pd onto a Au single crystal or vice versa, and (iii) Pd and Au thin films synthesized by depositing metals onto a refractory metal substrate (e.g. Mo (110)) [34, 35]. The latter two types offer considerable flexibilities in varying surface compositions, thus being viable for studying the structure of bare alloy surfaces. Besides, Au-Pd alloys have validated their superiority over to monometallic catalysts for reactions such as selective oxidation, hydrogenation and hydrodechlorization. 2.1. CO Oxidation CO oxidation is one of the most widely studied reactions applied for a number of processes such as the removal of exhaust gas from automobile and purifying H2 for fuel cells [25]. Due to the simplicity of the reaction in nature, CO oxidation is also considered as an ideal model reaction for fundamental studies of catalysis. Furthermore, CO is a probe molecule used for understanding surface structures of alloys since the desorption temperatures and, in particular, CO stretching frequencies are sensitive to the structure of adsorption sites [36-38]. The development of highly active and selective catalysts for CO oxidation under mild conditions is paramount for practical applications [39, 40]. However, compared to monometallic catalysts, the mechanism for CO oxidation over bimetallic catalysts became more complicated because of the coexistence of various active ensembles which change structures (e.g., surface reconstruction) remarkably under different reaction conditions [22]. Au-Pd nanoalloys have been validated as active catalysts for low-temperature CO oxidation, and this property of Au-Pd bimetallic catalysts could be modulated by the ensemble style, the atomic ratio of Au/Pd in surface layers, preparation methods, and the nature of supports [25, 41-44]. Goodman and coworkers [45] have reported that the lowtemperature CO oxidation could occur readily on supported Au-Pd nanoparticles, and the particle size and support prop-

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erties become much less critical compared to monometallic Au catalysts because of different reaction mechanisms. As well known, for monometallic Au, it is crucial to control the Au particle dimension in a range 1-5 nm to maintain high CO oxidation reactivity [8]. The CO oxidation over precious metal catalysts involves several elementary steps, such as adsorption of CO and O2, dissociative adsorption of O2, surface reaction between COad and Oad, and desorption of CO2. Behm and coworkers [46] demonstrated that Pd monomers were the smallest ensemble (“critical ensemble”) for carbon monoxide adsorption and oxidation. But, based on the results obtained from polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), it has been proved that neither Au nor isolated Pd sites were capable of dissociating O2 [45]. Dissociation of molecular oxygen and subsequent CO oxidation only occurred when contiguous Pd sites were available. After the dissociative adsorption of O2 on Pd sites, Oad species was assumed to spill over to Au sites [45]. Hence, although CO2 was detected above 400 K over pure Pd, the temperature could be lowered to room temperature upon alloying with Au. Adsorption of CO, an elementary step for CO oxidation generally leads to the reconstruction of Au-Pd surfaces, which reversely changes the binding energy of CO-Pd. CO adsorption on a AuPd(100) surface [32] has been studied using PM-IRAS over a wide range of CO pressures on wellannealed and freshly ion-sputtered surfaces. The interaction between CO and Pd atoms on the surface showed its dependence on CO pressures. When CO pressures were lowered than ~110-3 Torr; Pd segregation was evidenced by the enhancement of CO band intensity with increasing CO pressure. However, the extent of segregation was not sufficient to form contiguous Pd sites on a well-annealed sample surface. Pd segregation was increased to form contiguous Pd sites when CO pressures were higher than ~0.1 Torr. The bond strength of CO-M on various surface sites follows an order of pure Pd > isolated Pd sites on Au-Pd surface > contiguous Pd sites on Au-Pd surface > Au sites [47, 48]. Due to a reduction in binding energy of CO-Pd on AuPd surfaces, the inhibition of CO2 formation, imposed by the adsorption of CO, could be greatly weakened during CO oxidation. Thus, under stoichiometric conditions and relatively low temperatures, Au-Pd alloys showed superior reactivity compared to pure Pd [49]. On the contrary, under net oxidizing conditions, pure Pd displayed higher reactivity than Au-Pd because of its better capability of dissociating O2. However, pure Pd could be more easily oxidized and deactivated compared to Au-Pd, due to the low electronegativity of Pd (2.20 eV) compared to Au (2.54 eV). Therefore, CO oxidation on Pd-Au alloy catalysts is almost structural insensitive at near-atmospheric pressures. Somorjai and coworkers [50] synthesized unsupported AuxPd1-x nanoparticles (x=0, 0.25, 0.5, 0.75, 1) by a one-step sol-gel method. The turnover rates for the CO/O2 reaction have been elucidated in Fig. (1) for AuxPd1-x (x = 0–1) at 473K. Both XPS (X-ray photoelectron spectroscopy) depthprofiles and STEM (scanning transmission electron microscopy)/EDS (energy dispersive spectroscopy) phase maps show that the as-synthesized AuxPd1-x (x = 0.25, 0.5, 0.75) nanoparticles exhibit gradient alloy (i.e., core-shell) struc-

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reaction and found the activities of these catalysts increased with rise in the Au concentration in bulk. Among Pd-Au metallic catalysts, upon reaching a maximum in reactivity at a ratio of Au/Pd = 0.25, the activity decreased significantly with continuously increasing the Au concentration. High resolution TEM (transmission electron microscopy) images revealed that Pd atoms could be substituted by Au on the surface layer, thus changing the electronic property of Pd around Au atoms. XPS has also demonstrated that electron may transfer from the orbits of Au4f to Pd3d as indicated by the shift of binding energies for both Au4f and Pd3d. Venezia et al. [41] prepared bimetallic Au-Pd catalysts supported on silica with different Au/Pd atomic ratios by simultaneous reduction of Pd and Au precursors by ethanol in the presence of protector. Similar phenomenon was observed as the monometallic palladium and the palladium-rich catalysts behaved quite similarly and were the most active catalysts. A drastic reduction of the CO oxidation activity was observed for the 50/50 Au/Pd catalyst and for samples with increasing amount of Au.

 Fig. (1). A plot of the turnover rates in CO molecules/metal site/sec at 473 K versus % atom by Pd measured by AP-XPS using 300 eV photoelectrons (a) and EDS (b). Reprinted with permission from ref. 50, Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

tures with Pd-rich shells. As shown in Fig. (2), the Au0.25Pd0.75 and Au0.5Pd0.5 NPs with relatively thick Pd shells are stable at 473 K in vacuum and under various gas atmospheres (H2, O2, CO, etc.) and the CO/O2 reaction conditions. However, the Au0.75Pd0.25 NPs restructure irreversibly to Aurich surfaces dominantly at 473K and under the CO/O2 reaction in the Torr pressure regime. Xu et al. [25] prepared a series of Au-Pd/SiO2 catalysts by an incipient wetness method Fig. (3) for CO oxidation

Fig. (3). Temperature-dependent CO oxidation over the SiO2 supported Au-Pd alloy catalysts with a feed gas of 5.0 kPa CO, 5.0 kPa O2, and Ar balance in a flow rate of 50 mL·min-1 (GHSH: 32000 h-1). Reprinted with permission from ref. 25, Copyright 2010 American Chemical Society.

Fig. (2). Normalized Pd (blue) and Au (gold) fractions are plotted versus various temperatures and pressure (i.e., CO oxidation reaction) conditions from ambient pressure XPS analysis using 380 eV X-rays for Au0.25Pd0.75 (a), Au0.5Pd0.5 (b) and Au0.75Pd0.25 (c). Reprinted with permission from ref. 50, Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

Table 1.

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Catalytic systems for the synthesis of H2O2 over bimetallic Au-Pd catalysts. Catalyst

Preparation methoda

Solventb

Pressure (Mpa)

T(K)

Selectivity(%)

conversion

Productivity (mol kg(catalyst)-1h-1)

Ref.

2.5%Au:2.5%Pd /Al2O3

CP

CH3OH

3.7

275

n/a

n/a

4.46

[64]

1.25%Au:3.75%Pd/ZnO

CP

SCCO2

9.2

307

n/a

n/a

0.007

[64]

2.5%Au:2.5%Pd/ZnO

CP

SCCO2

9.2

307

n/a

n/a

0.012

[64]

3.75%Au:1.25%Pd/ZnO

CP

SCCO2

9.2

307

n/a

n/a

0.008

[64]

1%Au:0.1%Pd/SiO2

IWI

H2 O

0.1

283

30

n/a

3.2 mol h-1

[65]

PVP stabilized AuPd4 (5mg)

Sol-gel

Ethanol

0.1

307

65

6%

n/a

[66]

Pd-Au/ZrO2 (Au/Pd atom ratio range = 0–0.11)

IM

H2 O

0.095

273

n/a

35%-45%

n/a

[67]

3.3%Pd1-Au1.6/SiO2

IM

Ethanol

0.1

283

59

n/a

1.89 mol gPd-1h-1

[68]

-1 -1

3.3%Pd1-Au3.4/SiO2

IM

Ethanol

0.1

283

62

n/a

0.87 mol gPd h

[68]

2.5%Pd-2.5%Au/SiO2(Johnson Matthey)

IWI

Methanol+H2O

3.7

275

n/a

n/a

48

[69]

2.5%Pd-2.5%Au/SiO2(Grace)

IWI

Methanol+H2O

3.7

275

80

n/a

108

[70]

2.5%Au:2.5%Pd/TiO2

IM

H2 O

3.7

275

70

n/a

64

[70, 72]

2.5%Au:2.5%Pd/TiO2(2%HNO3)

IM

H2 O

3.7

275

95

n/a

110

[81

4.2%Au:0.8%Pd /Al2O3

IM

Methanol+H2O

3.7

275

n/a

n/a

21

[74]

2.5%Au:2.5%Pd /Al2O3

IM

Methanol+H2O

3.7

275

14

n/a

9.3

[74]

2.5%Au–2.5%Pd/-Fe2O3

Precipitation

Methanol+H2O

3.7

275

22

n/a

16

[76]

2.5%Au–2.5%Pd/CeO2

IM

Methanol+H2O

3.7

275

30

31

68

[78]

2.5%Au–2.5%Pd/CeO2(2%HNO3)

IM

Methanol+H2O

3.7

275

38

31

85

[78]

2.5% Au-2.5% Pd/Y

IM

Methanol+H2O

3.7

275

n/a

n/a

88.9

[79]

2.5% Au-2.5% Pd/Y

IM

Methanol+H2O

3.7

293

n/a

n/a

35.0

[79]

2.5% Au-2.5% Pd/Y

IM

Methanol+H2O

3.7

313

n/a

n/a

4.2

[79]

2.5% Au-2.5% Pd/Y

IM

Acetone+H2O

3.7

293

n/a

n/a

50.8

[79]

2.5% Au-2.5% Pd/Y

IM

H2 O

3.7

293

n/a

n/a

11.9

[79]

2.5% Au-2.5% Pd/HZSM-5

IM

Methanol+H2O

3.7

275

n/a

n/a

52.3

[79]

2.5% Au-2.5% Pd/HZSM-5

IM

Methanol+H2O

3.7

293

n/a

n/a

71.7

[79]

2.5% Au-2.5% Pd/HZSM-5

IM

Methanol+H2O

3.7

313

n/a

n/a

23.7

[79]

2.5%Pd-2.5%Au/C(Aldrich G60)

IM

Methanol+H2O

3.7

275

80

n/a

110

[69, 70]

2.5%Pd-2.5%Au/C(2%HNO3 )

IM

Methanol+H2O

3.7

275

>98

n/a

160

[71]

2.5%Au-2.5%Pd/C(2% CH3COOH)

IM

Methanol+H2O

3.7

275

>98

n/a

175

[71]

2.5%Au-2.5%Pd/C(2% H3PO4 )

IM

Methanol+H2O

3.7

275

30

n/a

120

[71]

2.5%Au-2.5%Pd/C(2% H3PO4 )

IM

Methanol+H2O

3.7

275

15

n/a

130

[71]

2.5%Au-2.5%Pd/C(2% HCl)

IM

Methanol+H2O

3.7

275

24

n/a

70

[71]

2.5%Au-2.5%Pd/C(2% NH4OH)

IM

Methanol+H2O

3.7

275

80

n/a

100

[71]

2.5%Au-2.5%Pd/C(2% NH4NO3 )

IM

Methanol+H2O

3.7

275

80

n/a

120

[71]

2.5%Au-2.5%Pd/C(2% KNO3 )

IM

Methanol+H2O

3.7

275

80

n/a

122

[71]

2.5%Pd-2.5%Au/C(Waterlink Sutcliffe Carbons Activated Carbon Grade:207A)

IM

Methanol+H2O

3.7

275

n/a

n/a

34

[69]

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Table (1). Contd…

Catalyst

Preparation methoda

Solventb

Pressure (Mpa)

T(K)

Selectivity(%)

conversion

Productivity (mol kg(catalyst)-1h-1)

Ref.

0.5%Pd-0.5%Au/C(H2SO4)

SIM

Methanol+H2O

3.3

275

n/a

n/a

156

[82]

0.34%Pd-0.66%Au/C(H2SO4 )

SIM

Methanol+H2O

3.3

275

n/a

n/a

188

Pd-Au/Ti-MCM-41 Pd-Au/Ti-MCM-41 2.5%Pd-Au/ZrO2(H2 SO4) 2.5%Pd-Au/CeO2(H2SO4) 1.21%Pd-0.95%Au/ZrO2

a b

IM PAD IM IM IWI

H2 O H2 O Methanol Methanol Methanol

0.1 0.1 0.1 0.1 0.1

293 293 293 293 293

n/a n/a 60 45 43

n/a n/a n/a n/a n/a

[82] -1 -1

[83]

-1 -1

[83]

-1 -1

[85]

-1 -1

[85]

1.2 mol gPd h 4.5 mol gPd h

1.27mol gPd h 0.72mol gPd h

-1 -1

[86]

-1 -1

1.027mol gPd h

0.89%Pd-0.74%Au/ZrO2

Pd-IWI +Au-DP

Methanol

0.1

293

60

n/a

1.040mol gPd h

[86]

1.17%Pd-0.82%Au/ZrO2

Au-DP+ Pd-IWI

Methanol

0.1

293

54

n/a

1.429mol gPd-1h-1

[86]

CP:Co-precipitation, IWI: Incipient-wetness impregnation, IM: Impregnation, SIM: Sol-Immobilization, PAD: Photo-assisted deposition, DP: Decomposition-precipitation SCCO 2: Supercritical CO2

However, Guczi and coworkers [51, 52] found that AuPd catalysts with different Au/Pd ratio prepared by sol technique and then deposited on TiO2 support were found to exhibit the same activity as that of the mixture of the monometallic samples. No significant synergism was suggested in the presence of bimetallic samples. For TiO2 supported system, it was believed that the activated oxygen can be produced on the support as well, so the alloying did not cause the degradation of activities. In contrast, Crooks and coworkers [53, 54] prepared TiO2-supported monodispersed Au-Pd nanoparticles using dendrimer-encapsulated nanoparticles (DENs). The compositional fidelity of the original bimetallic DENs was retained in the supported catalysts upon removal of the dendrimer template by calcination at 773 K. Additionally, those catalysts showed higher activity towards CO oxidation than single Au or Pd catalysts. However, Au-Pd nanoparticles could aggregate remarkably during the process for burning-off the ligands. Freund and coworkers [55] examined the surface structure of Au-Pd nanoparticles formed on reducible (i.e., CeO2 and Fe3O4) and irreducible (i.e., MgO) metal-oxide thin films and found that the reducibility of the support does not affect the surface structure. Suo et al. [44] prepared Au-Pd/Al2O3 by modified impregnation method. In their experiments, both metallic Au and PdO phase were found to exist in the 473 Kcalcined Au-Pd/Al2O3 catalyst which presented the highest CO oxidation activity in comparison with the catalysts treated at other temperatures. XRD and XPS demonstrated that the formation of AuxPdy alloy was unfavorable for the catalytic reaction but PdO phase coupled with metallic Au was the active phase for CO oxidation. Ye et al. [56] reported M-doped (M = Pt, Pd) Au/SnO2 as well as Au/SnO 2 for CO oxidation at low temperatures. Oxidized Au species were more active than the metallic Au to CO oxidation. The doping of 1wt% Pd or Pt could significantly enhance the catalytic activity of 3 wt% Au/SnO2 due to the presence of strong interaction of Pd or Pt with Au. In a lean burn internal combustion system, a catalyst comprising an alloy of palladium and gold on a metal oxide support has also been described as useful for removing carbon monoxide [57]. Recently, Deng and coworkers [58] fabricated a novel Au&Pd/

Fe(OH)x catalyst. This catalyst has a structure of two separate active sites that provides a more cost-efficient candidate for CO + H2 co-oxidation at room temperature. Moreover, vapor phase method applied to synthesize Au-Pd catalyst for CO oxidation was also reported [59]. 2.2. Direct Synthesis of H2O2 H2O2, as a green oxidant, has widely been applied in industries, particularly in chemical processes and environmental protection [60]. H2O2 is currently produced mainly by the anthroquinone oxidation (AO) process, which involves sequential hydrogenation and oxidation of alkyanthraquinone. This process has several intrinsic drawbacks including gradual loss of costly anthraquinone, deactivation of catalyst, waste treatment and high capital costs. In addition, this current indirect process is only economically viable with a large scale production (4-6104 tpa), whereas H2O2 is required practically on a much smaller scale [29, 61]. Compared to the indirect process, the direct process used in small scale facilities with water or alcohol as solvent would offer several advantages, such as no production of nondisposable waste. The direct process using Pd and Pd-Au catalysts has been recently reviewed by Samanta [62] and Hutchings [63] groups. Although the direct process has been the subject of significant interest for almost 100 years, no commercial application has been available yet [5]. Currently, the most promising catalyst for this reaction is Pd-based catalysts. For practical applications, several drawbacks of the Pd-only catalyst need to be overcome: (i) the selectivity to H2O2 is usually lower than 50%, which means that more than half of H2 in feedstock is consumed by overoxidation; (ii) the metallic Pd may easily cause a blast by igniting the mixture of H2 and O2; (iii) the leaching of Pd in acidic solutions results in the catalyst deactivation. Therefore, the exploration of highly active catalysts with better safety is strongly desired from an industrial point of view. Representative catalytic systems for the synthesis of H2O2 over bimetallic Pd-Au catalysts are listed in Table 1. Landon et al. [64] firstly reported on the promotional effect of Au for Au-Pd alloy catalysts, which might be related to the depression of H2O2 decomposition or the enhancement in the rate of

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

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H2O2 formation. Ishihara and coworkers [65] also reported the performance of Au/SiO2 and Au-Pd/SiO2 catalysts and found that Au-Pd/SiO2 catalyst was active for H2O2 synthesis without addition of halogen compounds. Furthermore, Schaak et al. [66] observed the formation of H2O2 over a non-supported AuPd4 catalyst with higher selectivity than monometallic Pd catalyst. Choudhary and coworkers [67] found that the yield of H2O2 passed through a maximum with an increase in Au concentration, but the higher Au concentration in the Pd-based catalyst might be attributed to higher H2O2 decomposition activity. Han et al. [68] has demonstrated the existence of electron density transfer between Pd and Au atoms in alloy surfaces with the establishment of the relationship between the surface composition and catalyst performance. Actually, direct synthesis of H2O2 over those catalysts has found to be structural sensitive and it strongly depends on the catalyst properties such as the support properties, the ratio of Au/Pd and the assembly of the surface atoms. The effects of these factors will be elaborated in detail.

been found that the selectivity of H2O2 was increased to ~95% on an Au-Pd/TiO2 catalyst upon pre-treating TiO2 in an acidic solution [81]. The prepared support could enhance the dispersion of Au and the formation of Au-Pd nanoparticles. On the other hand, supports with different surface properties may strongly affect side reactions such as H2O2 decomposition and H2O2 hydrogenation. Au-Pd/C upon pretreated with an acidic solution can even switch off the sequential hydrogenation and decomposition of H2O2, thereby, yielding a selectivity of 98% [71]. (iii) It was found during preparation that H2O content in solution has certain effects on TiO2 supporting catalysts. For instance, Hutchings and coworkers [73] reported that the H2O content during impregnation has a great effect on the performance of Au-Pd/TiO2. The activity increased significantly on Au-Pd/TiO2 synthesized with more H2O in preparation, due to the improvement of Pd dispersion. The durability of the catalyst in H2O2 synthesis was, however fairly poor, whereas the catalyst with less H2O during preparation showed good durability.

In addition, Hutchings and coworkers have intensively studied the support effects on H2O2 synthesis using CO2 as a gas diluent at elevated pressures [5, 29, 61, 63]. Supports such as carbon [69-71], TiO2 [72, 73], SiO2 [69, 70], Al2O3 [64, 74, 75], Fe2O3 [76], MgO [77], CeO2 [77, 78], and zeolite [79] have been explored, and an order of reactivity follows carbon > TiO2 > SiO2 > Al2O3 > Fe2O3 [69, 70]. MgO, a basic support, is not suitable for this reaction [80]. It is noted that the performance of Au-Pd/CeO2 is even inferior to monometallic Pd catalysts [78].

The catalyst activity and selectivity are highly dependent on the Au/Pd ratio. Normally, the Au/Pd ratio in the surface layers changes with different preparation methods. Hutchings and coworkers found catalysts prepared by an impregnation method had better performance than those by coprecipitation [76] and deposition-precipitation methods [72]. Au-Pd supported on activated carbon prepared by a solimmobilization method was also reported and the optimized performance for the formation of H2O2 was observed over a catalyst with an Au/Pd ratio of 0.5 [82]. Further increase of Pd content favored the hydrogenation of H2O2.

The enhancement in activity over alloy catalysts can be explained with following reasons: (i) the support properties have great effects on the structure of catalysts, including distribution of particle size and the atomic ratio of Pd/Au in the surface layers (core-shell or homogeneous alloy). Bimetallic Au-Pd particles on TiO2 and Al2O3 usually exhibit a core-shell structure, and Pd is enriched on the alloy surface. Contrastively, homogeneous alloys can be observed for the Au-Pd/carbon catalysts. Normally, the catalytic activity is remarkably decreased with an increase in the average particle size and the gradual transformation of homogeneous alloy to Pd-rich shell/Au-rich core morphology [75]. (ii) The surface property of support is important for further improvement of the catalytic performance. Recently, it has Table 2.

Han and coworkers [68] prepared Au-Pd catalysts with sequential Au-Pd ratio by an incipient wetness method followed with a reduction in H2. H2O2 synthesis was operated under mild reaction conditions. The existence of electron density transfer between Pd and Au atoms in alloy surfaces was demonstrated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption, quantitative powder X-ray diffraction (XRD) and XPS. The modification of the Pd electronic structure by Au was confirmed using DRIFT spectra. The results from Table 2 indicate that the activity and selectivity of Au-Pd alloy catalysts can significantly be enhanced through adjusting the surface structures by changing the Au content.

Activity and selectivity of SiO2 supported Pd, Au, and Au-Pd alloy catalysts in the H2O2 synthesis directly from H2 and O2. Reprinted with permission from ref. [68], Copyright 2007 American Chemical Society.

catalystb

particle sizec (nm)

purePd

8.0

Pd16Au1

8.0

Pd8Au1

surface compositiond (moleratio Pd/Au)

reaction rate (mol/h·gpd)

concn ofH2O2 (%)

selectivity (%)

0.99

0.34

12

8.2/1

1.34

0.46

35

8.5

6.8/1

1.57

0.54

53

Pd4Au1

6.0

3.6/1

1.7

0.59

52

Pd2Au1

5.5

2.4/1

1.82

0.62

51

Pd1Au1.6

9.3

1/1.2

1.89

0.66

59

Pd1Au3.4

9.0

1/1.5

0.87

0.30

62

pureAu

5.0

0.001

n/a.

8 Recent Patents on Catalysis, 2013, Vol. 2, No. 1

Photo-assisted deposition (PAD) is considered as a more suitable method for preparing Au-Pd alloy compared to the conventional impregnation method, which usually uses Tizeolite as support. Yamashita et al. [83, 84] prepared AuPd/Ti-MCM-41 and Au-Pd/TS-1 catalysts by PAD and demonstrated that photo-excited state of Ti-oxide moiety under UV-light irradiation was important for the formation of AuPd alloy based on results from extended X-ray absorption fine structure (EXAFS) measurements. Most of Au atoms were preferentially located in the region of core, while Pd atoms were dispersed in the region of shell. Strukul and coworkers [85-87] studied H2O2 synthesis using sulfated and non-sulfated Au-Pd/ZrO2 and CeO2 catalysts prepared by incipient wetness methods. A selectivity of 70% was obtained. The introduction of Au was found likely to increase the productivity by reducing the particle size of Pd. That was attributed to the high proportion of low energy sites, which could effectively absorb O2 without dissociation. One needs to note that the relationship between activity and structure is still not well established. A general consensus for this reaction is that the interaction of H2 and O2 on the catalyst surfaces results in the formation of H2O2, while OOH is formed as an intermediate [88]. Thus, the energies for the adsorptionactivation and dissociation of H2 and O2 are of importance in determining the activity and selectivity. Recently, DFT calculations showed that the presence of surface Au atoms could increase the selectivity by blocking O2 from dissociation [89]; while H2 dissociation was little affected. DFT calculations also demonstrated [90] that less active Au atoms on the Au-Pd surface could weaken the interaction of the metal surface with H2O2 and H atoms. Furthermore, the O-Pd bond is usually found to be stronger than the O-O bond in the OOH intermediate and H2O2, while the O-Au bond is weaker than the O-O bond [91]. As a result, H2O2 is easily produced and released from the Au-Pd(111) surface, and the side reactions involving the dissociation of O-O bond are suppressed. Thomson et al. [92] reported kinetic and thermodynamic parameters for elementary steps including (i) molecular adsorption of O2, (ii) the formation of hydroperoxy (OOH) species, and (iii) H2O2 formation. The formation of the OOH and H2O2 species is both kinetically and thermodynamically favorable on alloy surfaces. Hwang and coworkers [93, 94] investigated the ensemble effect by the first principle calculations. The availability of Pd monomers surrounded by less active Au atoms enhances the selectivity by suppressing O-O cleavage. The ensemble contributions strongly impacted on the geometric structure of catalyst, particularly local strain and atomic coordination number at the surface, which were directly related to surface electronic states. Recently, Schiffrin and coworkers [95] also reported that an increase in Pd concentration (Pd8Au92) led to a rapid increase in H2O2 selectivity up to nearly 95% in an electrocatalytic reaction. It has been proposed that the formation of H2O2 was favored on a single Pd atom surrounded by Au, on which O2 could be adsorbed without dissociation. However, it is still unclear which effect among assembly, electronic and mixed in bimetallic Au-Pd alloy catalysts is prevailing for H2O2 synthesis. These mechanisms are currently debatable and deserve further investigations using advanced probe tools.

Ouyang et al.

2.3. Vinyl Acetate Synthesis Vinyl acetate (VA) is known as an important chemical commodity applied widely in paints, adhesives and surface coatings. VA is currently produced through a gas phase acetoxylation of ethylene over a Au-Pd/SiO2 catalyst at a reaction temperature range of 423-463 K and a total pressure of 0.6-1.0 MPa [96]. We should note that Au-Pd is the sole gold-based bimetallic catalyst commercially used. It has been patented [97-99] that core-shell catalysts with an alloy of Pd/Au as an outer shell can be used for highly selective production of vinyl acetate. Vinyl acetate can also be produced by integrated sequential oxidation of ethane with ethylene and acetic acid as intermediate products [100]. Han et al. [101] reported that Pd upon alloying with Au could effectively suppress the formation of Pd carbide (PdCx) during reaction, which has been considered as the primary factor responsible for the deactivation of pure Pd catalysts. Dissociative adsorption of O2 was believed to be the rate-determining step because of the suppression by adsorbed C2H4. On Au-Pd surfaces, the reduction of adsorbed C2H4 leading to the enhancement of the adsorption amount of oxygen was likely responsible for the high reactivity of Au-Pd alloy catalysts for VA synthesis [102]. In order to attain deep insight into the structure of active sites, Yi et al. [34] deposited Au-Pd mixtures by physical vapor deposition onto a Mo (110) substrate to form a stable alloy between 700 and 1000 K with substantial enrichment in Au. Annealing a 1:1 Au-Pd mixture at 800 K led to the formation of a surface alloy with a composition Au0.8Pd0.2 where Pd was predominantly surrounded by Au. The surface concentration of isolated Pd sites could systematically be controlled by altering the composition of Au-Pd alloys. Subsequently, Goodman and coworkers [31, 103, 104] have demonstrated the isolated Pd monomer pairs on Au surfaces was active sites, and a correlation between the structure of the ensemble and reactivity is presented in Fig. (4). Considering the bond lengths of adsorbed ethylene and acetate species, the optimized distance between two active sites (Pd-Pd) was estimated to be 3.3 Å [31]. The distance between a pair of Pd monomers is 4.08 Å on Au (100) and 4.99 Å on Au(111). The latter has a prohibitively long distance for coupling two reactive intermediates, as a result, the reaction rate for Pd/Au(111) is much lower than that for Pd/Au(100). The bonding and relative distances involved between reacting species are illustrated by Fig. (5) [96]. Experimental [105, 106] and theoretical [107] investigations have demonstrated that Pd substituents preferred to form the second neighborhoods on both Au(111) and Au(001) upon annealing. Consequently, Au-Pd surfaces became inactive toward ethylene dehydrogenation [108], primarily due to their inability to provide sufficient affinity to both vinyl and the eliminated H in the transition state [109]. Scanning tunneling microscopy (STM) has revealed that the sample annealing strongly affected the arrangements of surface Pd. The Pd pairs with short distance (e.g. 0.33 nm) were believed to be optimal for VA synthesis [110]. DFT calculations also indicated [111] that this reaction was determined by three factors: (i) electronic contributions that favor the adsorption of reactants on exposed surfaces; (ii) the most successful ensembles that block the formation

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

Recent Patents on Catalysis, 2013, Vol. 2, No. 1

9

 Fig. (4). Vinyl acetate (VA) formation rates (TOFs) as a function of Pd coverage on Au (100) and Au (111). The VA synthesis was carried out at 453K, with acetic acid, ethylene, and O2 pressures of 4, 8 and 2 torr, respectively. The total reaction time was 3 hours. The error bars are SD, based on background rate data. The two insets show Pd monomers and monomer pairs on the Au (100) and Au (111) surfaces. Reprinted with permission from ref. 96, Copyright 2005 American Association for the Advancement of Science.

 Fig. (5). Schematic for VA synthesis from acetic acid and ethylene. The optimized distance between the two active centers for the coupling of surface ethylenic and acetate species to form VA is estimated to be 3.3 Å. With lateral displacement, coupling of an ethylenic and acetate species on a Pd monomer pair is possible on Au(100) but implausible on Au(111). Reprinted with permission from ref. 96, Copyright 2005 American Association for the Advancement of Science.

of the dihaptoacetate species; (iii) the Pd template induces small changes in structures and energies that affect the reaction barrier for the coupling step. Indeed, Au in alloys works as a dilutant, which isolates Pd ensembles. More recently, Pd atoms enriched in catalyst surface were reported to be mainly responsible for the loss of activity [112]. However, all these experimental evidences were obtained from model catalysts operated under ultra high vacuum (UHV) systems, thus the obtained results could be questionable under real conditions

due to the so-called “pressure and materials gaps”. We also note that the electronic effect of Au on Pd and its effect on this reaction have rarely been discussed in open literature. 2.4. Alcohol Oxidation Catalytic oxidation of alcohols is considered as an important reaction intensively studied for synthesis of fine chemicals. Compared to traditional methods using inorganic

10 Recent Patents on Catalysis, 2013, Vol. 2, No. 1

Table 3.

Ouyang et al.

Catalytic systems for the oxidation of benzyl alcohol over bimetallic Au-Pd catalysts.

Catalyst

Condition

Prepara-

Con-

Selectivity(%)

Ref.

version

tion methoda

TOF(h-1)

S/Mb

T(K)c

P(MPa)

Tim(h)

Benzal

Benzoic

Tolu-

Benzyl

Ben-

dehyde

acid

ene

benzoate

zene

3:1 Au:Pd/PVP

Sol-gel

500:1

298

0.1(air)

6

n/a

8.6

86.1

n/a

n/a

13.9

n/a

[116]

1:1 Au:Pd/PVP

Sol-gel

500:1

298

0.1(air)

6

n/a

7.1

96.9

n/a

n/a

3.1

n/a

[116]

1:3 Au:Pd/PVP

Sol-gel

500:1

298

0.1(air)

6

n/a

11.5

97.9

n/a

n/a

2.1

n/a

[116]

1:3 Au:Pd/PVP

Sol-gel

500:1

298

0.1(O2)

6

n/a

26.2

98.1

n/a

n/a

1.9

n/a

[116]

1:3 Au:Pd/PVP

Sol-gel

500:1

358

0.1(O2)

6

n/a

57.3

100

n/a

n/a

n/a

n/a

[116]

2.5%Au-

IM

n/a

373(SF)

0.2(O2)

0.5

2.6

n/a

90.5

n/a

n/a

n/a

n/a

[117]

IM

n/a

373(SF)

0.2(O2)

0.5

3.6

n/a

97.3

n/a

n/a

n/a

n/a

[117]

IM

n/a

373(SF)

0.2(O2)

0.5

3.6

n/a

74.9

n/a

n/a

n/a

n/a

[117]

IM

n/a

373(SF)

0.2(O2)

0.5

2.9

n/a

53.9

n/a

n/a

n/a

n/a

[117]

IM

n/a

373(SF)

0.2(O2)

0.5

3.7

6440

95.2

n/a

n/a

n/a

n/a

[117]

2%Au-2%Pd/Y

IM.

n/a

373(SF)

0.2(O2)

0.5

0.5

280

n/a

n/a

n/a

n/a

n/a

[118]

4%Au-1%Pd/TiO2

IM

n/a

373(SF)

1.0(O2)

0.5

1.0

n/a

94

1.3

2.5

2.2

n/a

[119]

2.5%Au-

IM

n/a

373(SF)

1.0(O2)

0.5

1.9

n/a

82.8

3.1

5.6

8.5

n/a

[119]

1%Au-4%Pd/TiO2

IM

n/a

373(SF)

1.0(O2)

0.5

1.0

n/a

80.6

1.8

7.7

5.0

n/a

[119]

1%Au-Pd/C

SIM

n/a

433

1.0(O2)

4

82

29900(0.5h)

48.0

18.0

31

n/a

n/a

[120]

1%Au-Pd/C

IM

n/a

433

1.0(O2)

4

44.6

17265(0.5h)

65.0

7.5

20.5

n/a

n/a

[120]

1%(Au+ Pd)/TiO2

SIM

40mL:0.

393(SF)

1.0(O2)

2

61.2

15360(0.5h)

69.2

1.7

26.7

1.9

n/a

[121]

393(SF)

1.0(O2)

2

57.8

19250(0.5h)

77.1

1.8

18.1

2.8

n/a

[121]

393(SF)

1.0(O2)

2

48.7

17360(0.5h)

72.3

1.9

23

2.5

n/a

[121]

393(SF)

1.0(O2)

2

81.1

35400(0.5h)

55.0

1.3

40.9

2.1

n/a

[121]

393(SF)

1.0(O2)

2

70.4

41930(0.5h)

62.6

2.0

35.1

0

n/a

[121]

393(SF)

1.0(O2)

2

79.2

24310(0.5h)

65

27

28.8

2.7

n/a

[121]

413(SF)

1.0(O2)

2

10.7

n/a

91

2

3

3

n/a

[122]

353

0.3mL/mi

n/a

8.4

n/a

85.5

n/a

12.1

n/a

n/a

[123]

2.5%Pd/Al2O3 2.5%Au2.5%Pd/SiO2 2.5%Au2.5%Pd/Fe2O3 2.5%Au2.5%Pd/C 2.5%Au2.5%Pd/TiO2

2.5%Pd/TiO2

1g 1%([email protected])/TiO2

SIM

40mL:0. 1g

1%([email protected])/TiO2

SIM

40mL:0. 1g

1%(Au+ Pd)/C

SIM

40mL:0. 1g

1%([email protected])/C

SIM

40mL:0. 1g

1%([email protected])/C

SIM

40mL:0. 1g

2.5%Au-2.5%Pd/

IM

scCeO2 1%(Au+Pd)/TiO2 (Micro-reactor)

40mL:2 5mg

SIM

50(Spac e time s)

n (O2)

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

Recent Patents on Catalysis, 2013, Vol. 2, No. 1

11

Table (3). Contd….

Catalyst

Condition

Prepara-

Conver-

1.2%(Au1Pd5)/Ce

Selectivity(%)

Ref.

sion

tion methoda

TOF(h-1)

S/Mb

T(K)c

P(MPa)

Tim(h)

Benzal

Benzoic

Tolu-

Benzyl

Ben-

dehyde

acid

ene

benzoate

zene

ISL

16000

433(SF)

1.0(O2)

8min

95.0

108360

80.7

n/a

n/a

n/a

n/a

[124]

AD

50mmol

413(SF)

20mL/min

1

22.3

8667

94.4

0.8

4.8

n/a

n/a

[125]

O2 0.24%Au0.55%Pd/APS-

(O2)

:20mg

SBA-16 Au-Pd/SBA-15

IM-GF

0.5mL:0

353

air

2

20.5

990(2h)

98

n/a

n/a

n/a

n/a

[126]

.2g 0.73%Au-

SIM

500:1

333

0.15(O2)

2

96

160(2h)

94

6

n/a

n/a

n/a

[127]

SIM

500:1

333(H2

0.15(O2)

n/a

At 90

1189(1h)

46

32

n/a

22

n/a

[128]

0.15(O2)

n/a

At 90

1021(1h)

>99

n/a

n/a

n/a

n/a

[128]

0.27%Pd/C 1%[email protected]/AC

O) 1%[email protected]/AC

SIM

500:1

333(H2O NaOH)

Au–Pd/PR24-PS

SIM

35000:1

393(SF)

0.15(O2)

n/a

At 50

6076(15min)

74

3

18

1

4

[129]

Au–Pd/Baytubes

SIM

35000:1

393(SF)

0.15(O2)

n/a

At 90

33580(15min)

75

7

13

4

1

[129]

Au–Pd/N-PR24-PS

SIM

35000:1

393(SF)

0.15(O2)

n/a

At 90

52638(15min)

75

4

15

4

1

[129]

Au–Pd/N-Baytubes

SIM

35000:1

393(SF)

0.15(O2)

n/a

At 90

43479(15min)

76

7

11

5

1

[129]

Au–Pd/AC

SIM

35000:1

393(SF)

0.15(O2)

n/a

At 90

35426(15min)

76

6

12

3

3

[129]

n/a

373

n/a

3

>99.9

16

98

n/a

n/a

n/a

n/a

[131]

0.32%Au1.5%Pd/polyaniline a b c

ISL:In-situ loading, IM: Impregnation, SIM: Sol-Immobilization, AD: Adsorption method, IM-GF: Impregnation + grafting methods. SF: Solvent free S:M=benzyl alcohol (subtrate):Metal (mol ratio)

oxidants such as KMnO4, catalytic oxidation using H2O2 or O2 is regarded as a green process that yields less byproducts.29 Bimetallic Au-Pd catalysts can be promising candidates for these reactions [113] and show better performance than monometallic Au or Pd catalysts. High catalytic performance of bimetallic Pd-Au catalyst can be tuned by novel preparation methods, adjusting the ratio of Au/Pd, changing supports and optimizing reaction conditions. Alcohol oxidation is found to be strongly dependent on properties of catalyst. However, the mechanism for aerobic alcohol oxidation on metal surface is still controversial. Most of studies agreed that the mechanism for aerobic alcohol oxidation starts with the formation of a metal-alkoxy and then undergoes a -hydride elimination giving rise to a carbonylic product and a metal-hydride intermediate over Ru, Ag, Cu, Au or Pd catalysts [114]. In contrast to that for Pd, the mechanism for Au primarily involves superoxo species via oxygen activation over Au surfaces [115, 116]. However, there is no doubt about the effect of addition of Au to Pd, which improves both catalytic activity and selectivity of desired products. The synergetic effect is considered as the origin of enhancement of catalytic performance in alcohol oxidation over bimetallic Au-Pd catalysts. But it should be noted that the further understanding of the mechanism for

oxidation of alcohols over bimetallic Au-Pd catalysts are required due to the complexity of surface structure of catalysts under realistic conditions. 2.4.1. Benzyl Alcohol Oxidation Benzaldehyde is generally produced by the oxidation of benzyl alcohol, while over-oxidation and dehydration yield benzoic acid and other byproducts such as toluene and benzene [117]. Bimetallic Au-Pd catalysts show better performance than monometallic Au and Pd catalysts not only on the activity of benzyl alcohol oxidation, but also high selectivity to benzaldehyde [117]. Catalytic systems for the oxidation of benzyl alcohol over bimetallic Au-Pd catalysts are listed in Table 3. Supported bimetallic Au-Pd catalysts have been examined for this reaction with various structures, compositions and supports. The size and structure of Au-Pd nanoalloys are affected by supports. Hutchings and coworkers [117-123] studied the oxidation of primary alcohols using a series of supported bimetallic Au-Pd catalysts Table 4. AuPd/TiO2 showed particularly higher activity and selectivity towards the oxidation of benzyl alcohol compared to AuPd/Al2O3 and Au-Pd/Fe2O3 Table 5 [119]. The Au-rich core and Pd-rich shell structure was characterized for Au-Pd/TiO2 catalysts, indicating that the Au electronically influenced the

12 Recent Patents on Catalysis, 2013, Vol. 2, No. 1

Table 4.

Ouyang et al.

Comparison of the catalytic activity for alcohol oxidation to the corresponding aldehyde. Catalyst is 2.5% Au-2.5% Pd/TiO2 unless noted otherwise; substrates oxidized without solvent unless specified; catalyst mass varied to give the metal concentrations indicated; and TOF was measured after first 0.5 hour of reaction. T, temperature. Reprinted with permission from ref. 117, Copyright 2006 American Association for the Advancement of Science.

Entry

Alcohol

[Metal] (10-5 mol/liter)

Reaction conditions 5

T (K)

P (10 Pa)

Au

Pd

TOF (/hour)

1

Benzyl alcohol

373

2

63.5

118

607

2

Benzyl alcohol*

373

2

63.5

0

213

3

Benzyl alcohol†

373

2

0

118

2,200

4

Benzyl alcohol

373

1

2.1

3.9

6,190

5

Benzyl alcohol

373

2

2.1

3.9

6,440

6

Benzyl alcohol

373

5

2.1

3.9

6,190

7

Benzyl alcohol

373

10

2.1

3.9

5,950

8

Benzyl alcohol

383

1

2.1

3.9

14,270

9

Benzyl alcohol

393

1

2.1

3.9

26,400

10

Benzyl alcohol

433

1

2.1

3.9

86,500

11

1-Phenylethanol

433

1

1.8

3.2

269,000

12

3-Phenyl-1-propanol

433

1

2.1

3.9

2,356

13

Vanillyl alcohol‡

363

1

21.6

40.6

10

14

Cinnamyl alcohol§

363

1

21.6

40.6

97

15

Octan-1-ol

433

1

2.5

4.7

2,000

16

Octan-2-ol

433

1

2.5

4.7

0

17

Octan-2-ol/octan-1-ol

433

1

2.1

3.9

0

18

Octan-3-ol

433

1

2.1

3.9

10,630

19

1-Octen-3-ol

433

1

2.1

3.9

12,600

20

Crotyl alcohol

433

5

2.1

3.9

12,600

21

Butan-1-ol

433

5

2.1

3.9

5,930

22

1,2-Butanediol

433

1

2.1

3.9

1,520

23

1,4-Butanediol

433

1

2.1

3.9

104,200

24

Benzyl alcohol

433

1

2.1

3.9

12,500

25

Benzyl alcohol¶

433

1

2.1

0

12,400

26

Benzyl alcohol#

433

1

0

3.9

24,800

27

**

Benzyl alcohol

433

1

2.4

4.5

36,500

28

Benzyl alcohol††

433

1

0

3.6

37,600

29

1-Phenylethanol††

433

1

0

3.1

11,600

• * 2.5% Au/TiO2. • † 2.5% Pd/TiO2. • ‡ 0.2 mol/liter in toluene as solvent. • § 0.2 mol/liter in water as solvent. •  2.5% Au-2.5% Pd/HAP prepared by impregnation of HAP with HAuCl4·3H2O and PdCl2. • ¶ 2.5% Au/HAP prepared by impregnation of HAP with HAuCl4·3H2O. • # 2.5% Pd/HAP prepared by impregnation of HAP with PdCl2. • ** 2.5% Au-2.5% Pd/TiO2 prepared with the method of Kaneda (6) using TiO2 as support. • †† 0.2% Pd/HAP prepared using the method of Kaneda (6) using HAP as support.

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

Table 5.

Recent Patents on Catalysis, 2013, Vol. 2, No. 1

13

Comparative data for benzyl alcohol oxidation and hydrogen peroxide synthesis. Results were obtained for the oxidation of benzyl alcohol after 0.5 hour and 8 hours of reaction and for H2O2 synthesis for 0.5 hour. The oxidation of benzyl alcohol was carried out at 373 K temperature, 0.2 MPa P O2, and 1500 rpm stirrer speed. Productivities are quoted in units of moles of product per hour per kilogram of catalyst. Reprinted with permission from ref. 117, Copyright 2006 American Association for the Advancement of Science. Conversion (%)

Benzaldehyde selectivity (%) Benzaldehyde productivity*[mol/(hour/kgcat )] 0.5 hour 8 hours

H2O2 productivity [mol/(hour/kgcat)]

0.5 hour

8 hours

2.5% Au-2.5% Pd/Al2O 3

2.6

83.3

90.5

86.6

174

23

2.5% Au-2.5% Pd/TiO 2

3.7

74.5

95.2

91.6

165

64

2.5% Au-2.5% Pd/SiO2

3.6

35.7

97.3

88.0

76

80

2.5% Au-2.5% Pd/Fe2O 3

3.6

63.4

74.9

66.4

102

16

2.5% Au-2.5% Pd/C

2.9

69.2

53.9

46.4

78

30

2.5% Au/TiO 2

0.6

15.3

96.7

63.9

24

70 times compared to monometallic Pd catalysts. The significant enhancement of activity was probably attributed to electronic and geometric effects between Au and Pd [178, 179]. Wong and coworkers [180] studied the relationship between Pd surface coverage on Au core and its catalytic reaction rates in the case of trichloroethene hydrodechlorination. A volcano-type curve was observed with a maximum at near 70% Pd surface coverage. It was proposed that the interface between Pd and Au clusters provided another type of active sites and the amount of these sites maximizes when Au surface was partly covered by Pd ensembles properly. Furthermore, Au could enhance the poisoning resistance of Pd by reducing the binding

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

Recent Patents on Catalysis, 2013, Vol. 2, No. 1

21

affinity of Pd to chlorine, because of the increased d-band electron density of Pd caused by Au [181]. Bimetallic Au-Pd catalysts were also applied in the hydrodesulfurization of thiophene and dibenzothiophene [182]. In both reactions the maximum activity was observed at Au:Pd molar ratio of 1:1. Synergetic effects between Au and Pd have been evidenced to be responsible for the hydrodesulfurization of dibenzothiophene. Au atoms around Pd sites inhibited the formation of less-active palladium sulfide (Pd4S)thus sulfur-resistant Au-Pd catalysts can be synthesized therefrom [183, 184]. We note that the hydrodesulfurization over bimetallic Au-Pd catalysts is not a popular subject under current investigations, and the structureperformance in this reaction system is still elusive. 2.8. Electrocatalytic Reactions Bimetallic Au-Pd catalysts are one of the most potential non-Pt electrocatalysts applied in low-temperature fuel cells using fuels such as H2 [185-187], borohydride [188, 189], formic acid [190-199], methanol [199], ethanol [200-205], and isopropanol [206, 207]. For H2-feed fuel cells, a trace amount of CO (~10 ppm) in H2 stream can strongly poison Pt electrode. It was found that Au-Pd catalysts showed stronger CO-tolerance than Ptbased catalysts. Schmidt et al. [186] proposed that the Au-Pd electrode was governed with a lower CO steady-state surface coverage and higher CO oxidation rates at low overpotentials that offered more free active Pd sites for H2 oxidation. It was found that the rate of H2 oxidation on Au(111)/Pd surface alloys was one order of magnitude lower than that for Pt(111) [186]. Additionally, Au(111)/Pd surface alloys were active for the continuous oxidation of pure CO below 0.2 V, and a positive reaction order with respect to CO was measured. Therefore, due to the unfavorable partial pressure dependence, the CO oxidation in rich H2 was determined by H 2 oxidation kinetics below 0.2 V. However, the steady-state activity of a high-surface area Au/Pd catalyst could be reached at potentials above 0.2 V. Ruvinsky et al. [187] observed a significant increase in the exchange current density of H2 oxidation and CO tolerance when the thickness of Pd overlayer was below two monolayers. Likely, Pd assemblies are the primary active sites responsible for those reactions. The addition of Au can enhance the catalytic performance of Pd due to electronic effects. Compared to monometallic Pd, the binding energy of Pd-CO was found to be weaker over Au-Pd bimetallic catalysts, which resulted in more Pd sites accessible to H2. For borohydride electrooxidation, the addition of Au to Pd led to a maximum eight-electron oxidation of BH4 while the complete eight-electron exchange was not achieved on other metals (such as Ni, Pt, and Pd) [188]. In fact, few studies on Au-Pd bimetallic catalysts for this reaction have been reported and therefore, more works should be carried out on reaction mechanisms. For formic acid electro-oxidation, Pt and Pd are the most frequently employed catalysts. However, Pd is not a good anodic catalyst because its surface can easily be oxidized. The addition of Au can improve its electrocatalytic perform-

Fig. (7). Geometrical models of the octahedral Au-Pd alloy, [email protected] core-shell, Pd, and Au NCs. Violet and yellow denote Pd and Au, respectively. Reprinted with permission from ref. 197, Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

ance and stability for formic acid electrooxidation [190]. Furthermore, the Au-Pd catalyst with Au-core and Pd-shell has significant improvements on the catalytic activity and stability of Pd. This is ascribed to the interaction between Pd shell and Au core, which is related to the thin Pd shell structure and Pd gaining d-electrons from Au core [190, 192]. This structure is responsible for weakening the adsorptive strengths of reaction intermediates and prevents the accumulation of poisoning-intermediates. Thus, more Pd sites are available for the direct HCOOH decomposition via the active-intermediate pathway to CO2. Actually, the overall electrocatalytic performance over various catalysts follows an order of Au-Pd alloy > [email protected] core-shell > Pd >> Au Figs. (7, 8) [197]. Zhang et al. [196] reported that electro-oxidation of formic acid over Au-Pd/C catalysts was favored with high alloying degree. Iordache [198] disclosed that a combination of Pd and Au catalyst with up to about 90% degree of alloying was more stable than single Pd catalysts in a direct formic acid fuel cell. Enhancement of CO tolerance and suppression of dehydration might be mainly responsible for the improvement of the catalytic performance. Chen et al. [195, 195] prepared Au-Pd catalysts supported on multi-wall carbon nanotubes (MWCNTs). The catalysts upon hydrogen treatment having smaller metal particles and better contact with MWCNTs showed higher activity than the core-shell Au-Pd/MWCNTs catalyst prepared by subsequent deposition of Pd and Au on MWCNTs. Zhang et al. [191] studied the effects of the ratio of Pd/Au on formic acid electrooxidation, wherein the Pd4Au1/MWCNTs catalyst exhibited distinctly higher activity and better stability. Liu et al. [193] measured the kinetics of formic acid electrooxidation over Au-Pd/C catalysts in which Pd3Au1/C showed the highest electrochemical activity. Moreover, the reaction was sensitive to the temperature at relatively high potential because the activation energy increased significantly with an increase in potential. Till now, most of the studies regarding formic acid electrooxidation over carbon

22 Recent Patents on Catalysis, 2013, Vol. 2, No. 1

Ouyang et al.

 Fig. (8). CVs of the Au-Pd alloy, [email protected] core-shell, Pd, and Au NCs on GCE in a) 0.1m HClO4 and b) 0.1m HClO4 + 0.5m formic acid. Scan rate: 50 mVs-1. Current values were normalized with respect to the ECSA. c) Current densities and mass activities for formic acid oxidation on the four different types of NCs. d) Chronoamperometric curves of the NCs on GCE in 0.1m HClO4 + 0.5m formic acid at 0.2 V vs. Ag/AgCl. Current values were normalized with respect to the ECSA. Reprinted with permission from ref. 197, Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

 Fig. (9). (a) SEM images of the cubic Au nanocrystals used to serve as the cores. (b and c) SEM and TEM images of the THH Au-Pd coreshell nanocrystals. (d) HAADF-STEM image of the THH Au-Pd core-shell nanocrystals. (e) EDS line scan and elemental mapping image of a THH Au-Pd core-shell nanocrystal. (f) Schematic drawings of a THH nanocrystal viewed from different angles. The axes projecting along the [100], [110], [111], and {730} directions are also shown. Length ratio is also presented. Reprinted with permission from ref. 205, Copyright 2010 American Chemical Society.

materials supported Au-Pd catalysts focus on the evaluation of catalytic performances. Further understanding of correlations between catalytic performances and the variation of structure of carbon materials are necessary to elucidate reaction mechanisms and to further design active catalysts. For alcohol electrooxidation, Au-Pd bimetallic catalysts showed better performance than Pd-alone catalysts. Zhu et al. [201] deposited mono- or sub-monolayer Pd atoms on Au surface by a chemically-epitaxial-seeded-growth method, which improved the specific activity as well as minimized the loading amount of Pd. The as-prepared Au-Pd alloy cata-

lysts [203] were then examined for ethanol oxidation reactions in an alkaline medium by a co-reduction method under an ultrasonic process. The peak current density followed an order of Pd/C > Pd3Au1/C > Pd7Au1/C > Pd1Au1/C, while the stability was on an order of Pd1Au1/C > Pd3Au1/C > Pd7Au1/C > Pd/C. The Pd3Au1/C catalyst exhibited an optimum stability and activity. Pd3Au1/C with alloy (a), coreshell (c) and physically-mixed (m) structures have also been investigated [204]. The catalytic activity followed an order of m-Pd3Au1 < c-Pd3Au1 < Pd < a-Pd3Au1. The mPd3Au1/CNT catalyst exhibited the highest catalytic activity

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

for oxygen reduction reaction (ORR). Huang et al. [205] have successfully developed a facile method for the highyield fabrication of Au-Pd core-shell heterostructures with an unusual tetrahexahedral (THH) morphology and an entirely high-index {730} facet Fig. (9). The electrocatalytic activity for the oxidation of ethanol was found to be higher than that of the other two Au-Pd heterostructures with lowindex facets. Lang et al. [199] reported a novel nanoporous Au-Pd alloy, which showed much higher catalytic activity and stability for the electrooxidation of methanol as a free-standing electrode. Furthermore, highly ordered Au-Pd nanowire arrays supported on an anodized aluminum oxide [204], WC/C [200] and Poly(p-phenylene) (PPP) films [207] have also been reported for alcohol electrooxidations. Both the structure and composition of Au and Pd in AuPd alloys are essential for the heterogeneous oxidation and the electrooxidation of alcohols. However, the size effect of Au-Pd catalysts is less remarkable in the electrochemical oxidation of alcohols than other alcohol oxidation reactions. 2.9. Coupling Reactions Coupling reactions, such as Suzuki and Heck reactions, are reported to be some of the most important chemical processes for constructing aromatic C-C bonds between complex organic molecules with potential chemical or pharmaceutical applications. Homogeneous Pd-complex catalysts are still the most efficient catalysts used in coupling reactions. Considering the intrinsic disadvantages of homogeneous catalysts, heterogeneous bimetallic Au-Pd catalysts attracted considerable attention in the last decade. Pd and Au-Pd alloy nanoparticles encapsulated in SBA15 with a G4-poly (amido-amine) stabilizer were used for the Suzuki coupling reaction [208]. The presence of Au3+ in dendrimer before reduction may reduce the complexation between Pd2+ and internal amine groups or other groups, thus in favor of the formation of active site (Pd0) from Pd2+ in the Suzuki reaction. Therefore, Au-Pd showed higher activity than Pd in catalyzing the coupling between aryl bromide/aryl chloride and arylboronic acid. Huang and coworkers [209] presented a core-shell Au-Pd catalyst with a systematic shape evolution. The concave cubes with high-index surface facets showed an excellent activity for the Suzuki coupling reaction. Fang et al. [210] synthesized core-shell Au-Pd catalysts with one or several palladium monolayer(s) shell. It has been proved that the catalytic activity originates primarily from Pd centers or clusters leaching away from nanoparticles in the presence of reaction components.

Recent Patents on Catalysis, 2013, Vol. 2, No. 1

23

maximize photocatalytic efficiencies by improving the separation of photo-generated charge carriers and decreasing the recombination between photoelectrons and holes. However, most of them focused on single metals since the use of bimetallic Au-Pd to improve photocatalysis is rare. Mizukoshi et al. [218] decorated TiO2 with bimetallic Au-Pd particles through a sonochemical method and evaluated photocatalytic activities on H2 evolution from a ethanol aqueous solution. The core/shell immobilized photocatalyst exhibited the highest performance under visible (VIS) illumination, whereas the annealed sample exhibited the highest performance under ultra violet (UV) illumination. The thicker Pd-shell was supposed to effectively shield photogenerated electrons from the recombination with holes. Yu et al. [219] fabricated a Au-Pd comodified TiO2 nanotube films to degrade malathion. The enhancement of photocatalytic activity might be attributed to both the effective separation of photo-generated charge carriers and the higher synthesis rate of H2O2. Vallejo and coworkers [220] modified TiO2 films with Au/Pd bimetallic particles by sputtering methods and found that the Au/Pd-modified TiO2 films exhibited higher photocatalytic activity than pure TiO2 in methylene blue degradation. It was considered that the decrease in the charge transference potential of carriers, which reduced the electron-hole recombination rate, was responsible for the increase in activity. 3. AU-PT ALLOY CATALYSTS Au-Pt alloy catalysts are highly reactive for a number of reactions including water-gas shift reactions (WGS) [221]. Generally, Au-Pt nanoparticles are composed of four possible bimetallic architectures Fig. (10): [222] mixture, contact aggregate, core-shell particles and alloys. These Au-Pt catalysts are tailored for many applications through different mechanisms. However, the optimum collocation between architecture and reaction is still debatable. Pt is the most important electrocatalyst employed in fuel cells as both cathodic catalyst for ORR [223] and anodic catalyst for oxidation of hydrogen, methanol, ethanol, formic acid and other fuels [224,225]. However, Pt suffers from the inherent high cost and under-utilization [226]. Moreover, it is easily poisoned by CO and other surface species produced in the reactions [227, 228]. Many efforts have been devoted to Au-Pd and Au-Pt intermetallics as alloys or nanoclusters used for electro-catalysis. The performance of Au-Pt electrocatalysts is generally influenced by several factors, such as preparation methods, compositions of nanoparticles and calcination conditions [222]. The bimetallic Au-Pt catalysts consisting of Pt and Au segregated phases also were found to exhibit different electrochemical properties.

It is noteworthy that the effects of Au on Pd on coupling reactions have not been fully understood. The promotional effect of Au is only validated for certain reactions. In some cases, Au may even deteriorate the Pd activity [211]. Further research on mechanistic understanding is highly required. 2.10. Photocatalytic Reaction TiO2 is a well known photocatalyst, which has been studied for decades. In literature, it was reported that modifying with noble such as Au [212-214], Pd [215], Ag [216], and Pt [217] on the surface of TiO2, was a feasible approach to

Fig. (10). Possible architectures in a bimetallic system. Reprinted with permission from ref. 222, Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

24 Recent Patents on Catalysis, 2013, Vol. 2, No. 1

Table 8. Sub-

Catalytic systems for the oxidation of alcohols over bimetallic Au-Pt catalysts. catalyst

1%Pt/AC(X40S) 1%(Au-

Reaction conditions

Preparation method a

strates

Glycerol

Ouyang et al.

SIM SIM AuPt

conver-

S/M b

T(K)

O2(Mpa)

Other c

500

323

0.3

NaOH:S=4

500

323

0.3

NaOH:S=4

At 50 At 50

Pt)/AC(X40S) 1%(Au-

SIM PtAu

500

323

0.3

NaOH:S=4

At 50

1953.4(0.2

Ref.

Productd

Selectivity

GLYA+T

48.3

[135]

75.7

[135]

80.4

[135]

44.8

[135]

75

[243]

84.4

[244]

ARAC

1986.8(0.2 5h)

SIM Au+Pt

500

323

0.3

NaOH:S=4

At 50

Pt)/AC(X40S) 1%([email protected])/AC(X40S)

532(0,25h)

Main Product

5h)

Pt)/AC(X40S) 1%(Au-

TOF(h-1)

sion

1257.7(0.2 5h)

SIM AuPt

1000

323

0.3

NaOH:S=4

88(at

3455

0.25h) Pt/TiO2

SBR

n/a

363

0.1

NaOH:S=4

30

405.2

Au-Pt/TiO2(1:1)

SBR

n/a

363

0.1

NaOH:S=4

30

517.1

85.6

[244]

Au-Pt/TiO2(3:1)

SBR

n/a

363

0.1

NaOH:S=4

30

507.4

84.3

[244]

Au-Pt/TiO2(1:3)

SBR

n/a

363

0.1

NaOH:S=4

30

501.3

85.3

[244]

1%Pt/AC

SIM

500

373

0.3

No base

78(at 2h)

n/a

45

[245]

79

[245]

Lactic acid

Glyceric acid

1%Pt/H-mordenite

SIM

500

373

0.3

No base

20(at 2h)

n/a

1%AuPt(6:4)/AC

SIM

500

373

0.3

No base

58

n/a

79

[245]

1%AuPt(6:4)/H-

SIM

500

373

0.3

No base

70

n/a

83

[245]

SIM

1000

323

0.1

pH=11

At 50

62

40

[146]

71

[146]

58

[146]

63

[241]

92

[241]

94

[241]

mordenite D-sorbitol

1-octanol

1%Pt/AC(X40S) 1%Au-Pt/AC(X40S)

SIM

1000

323

0.1

pH=11

At 50

149

1%Au-Pt/AC(X40S)

SIM

1000

323

0.3

NaOH:S=1

At 50

266

n/a

r.t.

0.1

BTF

n/a

n/a

Au-Pt/PI-CB(1 mol%) Au-Pt/PI-CB(2 mol%)

n/a

r.t.

0.1

BTF

n/a

n/a

Au-Pt/PI-CB(1 mol%)

n/a

r.t.

0.1

BTF(K2CO 3)

n/a

n/a

Gluconic/Gulo nic acids

Octaldehyde Octanoic acid

Au-Pt/PI-CB(1 mol%)

n/a

r.t.

0.1

MeOH/H2O=1/

n/a

n/a

Ester

43

[241]

4(at 4h)

10

Octalde-

90

[243]

72

[243]

2(K2CO3) 1%Pt/AC(X40S)

SIM

1000

333

0.3

NaOH:S=4

hyde

1%[email protected]/AC(X40S)

SIM

1000

333

0.3

NaOH:S=4

62(at 4h)

210

0.4%Pt/AC(X40S)

SIM

500

333

0.15

Toluene

6(at 8h)

4

98

[127]

0.6%Au-

SIM

500

333

0.15

Toluene

2(at 8h)

1

94

[127]

0.4%Pt/AC(X40S)

SIM

500

333

0.15

H2O

29(at 8h)

22

30

[127]

0.6%Au-

SIM

500

333

0.15

H2O

28(at 8h)

17

28

[127]

SIM

500

333

0.15

Toluene

15(at 2h)

38

100

[127]

100

[127]

0.4%Pt/AC(X40S)

0.4%Pt/AC(X40S) Cinnamyl

0.4%Pt/AC(X40S)

Cinnamyl aldhyde

alcohol 0.6%Au0.4%Pt/AC(X40S)

SIM

500

333

0.15

Toluene

5(at 2h)

12

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

Recent Patents on Catalysis, 2013, Vol. 2, No. 1

25

Table (8). Contd…

Sub-

catalyst

strates

Reaction conditions

Preparation method

conver-

a

S/M

b

T(K)

O2(Mpa)

Other

c

TOF(h-1)

Main Product

sion Product

d

Ref.

Selectivity

0.4%Pt/AC(X40S)

SIM

500

333

0.15

H2O

22(at 2h)

60

100

[127]

0.6%Au-

SIM

500

333

0.15

H2O

18(at 2h)

45

100

[127]

SIM

500

333

0.15

Toluene

19

30

95

[127]

0.4%Pt/AC(X40S) Benzyl

0.4%Pt/AC(X40S)

Benzyl aldhyde

alcohol SIM

500

333

0.15

Toluene

9

15

92

[127]

0.4%Pt/AC(X40S)

SIM

500

333

0.15

H2O

50

98

80

[127]

0.6%Au-

SIM

500

333

0.15

H2O

33

55

95

[127]

1%Pt/AC(X40S)

SIM

1000

323

0.3

NaOH:S=4

0

0

0

[243]

1% [email protected]/AC(X40S)

SIM

1000

323

0.3

NaOH:S=4

16(at

800

16

[243]

0.6%Au0.4%Pt/AC(X40S)

0.4%Pt/AC(X40S) 3-Octen-

2-Octenal

1-ol

5min) Glucose

Pt(unsupported)

Sol-gel

S/Au

323

0.3

pH=9.5

=20,000 1%Au-

SIM

Pt(2:1)/C(Vulcan

S/Au

AuPd > Au-Ca  Au-W > Pd > Au-Ni > Au. Au perhaps altered the local band structure of ceria, which facilitated its redox

32 Recent Patents on Catalysis, 2013, Vol. 2, No. 1

rates at low temperatures. Similar results were also observed for Au-Pd catalysts, whereas the Au-Cu and pure Au metal catalysts were inactive [310]. Au-Pt (5.0 wt%, core)/CeO 2 (shell) with Pt/Au=1 showed the highest activity among all catalysts. In addition, the doping of rare earth metal did not show any influence on the catalyst performance. Thus, the low-temperature WGS over metal-promoted CeO2 catalysts was attributed to electronic aspect of metal-ceria interface rather than oxygen mobility and oxygen storage capacity properties of the promoted CeO2. Yu et al. [311] prepared bimetallic Au-M (M = Ni, Cu, Ag, Pt, and Pd) catalysts supported on CeO2. Au-Pt/CeO 2 showed the highest activity with a conversion of 78% at 523 K. Smaller CeO2 particles with abundant of oxygen vacancies and Ce3+ ions were detected by addition of Au or Pt. The charge transfer from Au5d to Pt5d band, the high concentration of surface oxygen as well as the low degree of Ce3+ surface defects were responsible for WGS reactivity at low temperatures. In addition to traditional CeO2-supported Au-Pt catalysts for WGS, mesoporous materials such as MCM-41 and FSM (folded-sheet mesporous material) have also attracted much attention because of unique structural properties. Mohamed and Khairou [312] developed bimetallic Au-Pt nanowires (20 nm in diameter, 120-170 nm long) on a FSM-16 by introducing Au+ into in situ designed Pt carbonyl clusters. The presence of Au imposed a dramatic effect on the activity by changing the surface alloy at low temperatures and the morphology of the wire structure. 3.4. Hydrogenation Chen et al. [313] prepared colloidal Au-Pt bimetallic nanoparticles with varying the ratio of Au/Pt in the presence of poly (N-isopropylacrylamide). The alloy colloids were more active than the Pt monometallic colloids for the hydrogenation of allyl alcohol. The phase of alloy colloids could be separated during reaction companying with a significant decline in reactivity. The immobilized catalysts even showed much higher reactivity than colloid catalysts in the hydrogenation of allyl alcohols in aqueous phase [314]. Pawelec et al. [315] studied the effects of supports (-Al2O3 and SiO2 ) in naphthalene hydrogenation and found little difference over those catalysts. 3.5. Other Reactions Pt catalysts have been proven to be effective for VOC removal [316-318] and C-N coupling of methane and ammonia [319]. However, bimetallic Au-Pt catalysts have been rarely reported for these reactions, which would spur a substantial future work. One should note that only a few studies have been reported on hydrocarbon reforming reactions [320, 321]. 4. AU-AG BIMETALLIC SYSTEM Au-Ag alloy catalysts have found to be active for CO oxidation [322-328], epoxidation of alkenes [329], aerobic glucose oxidation [330-333]. These catalysts have also been applied in the reduction of 4-nitrophenol [334-338], fuel cells [332, 339-341] and biochemistry [342, 343]. Au-Ag

Ouyang et al.

nanoalloy can be easily formed due to the close match in lattice constants (dAu=4.0782 Å, dAg=4.0853 Å) of both metals [344]. The morphology [332, 338, 345] and size [323, 324 ,333, 335-337] of Au-Ag nanoparticles can precisely be controlled with advanced synthetic methods. 4.1. Oxidation Reactions 4.1.1. CO Oxidation CO oxidation on bimetallic Au-Ag alloy catalysts are generally affected by three factors [346]: i) the size of nanoparticles [347-350], ii) the methods for catalyst preparation [351-355], iii) the property of supports [356]. Au catalytic activity could be greatly promoted upon alloying with Ag [357,358]. The reaction rate for AuAg/SiO2 (the mole ratio of Au/Ag=3) was nearly 5 times larger than that of Au/SiO2 Fig. (21) [323]. The catalytic performance could be variable with the composition [322, 324, 359, 360]. Wang et al. [324] found that the catalyst with a ratio of Au/Ag=1 exhibited the highest activity. Chou et al. [326] calculated the energy for CO adsorption and reaction barriers on clean and Au-enriched Ag(110) by DFT calculations. CO oxidation on the Au-rich Ag(110) surface resulted in an exponential depletion of oxygen with time, and the reaction rate could remarkably be enhanced by Au. The property of supports could affect the catalytic performance significantly. The activities followed an order of AuAg(Au/Ag=3)/SiO2 > Au-Ag (Ag/Ag=1)/SiO2 > Au/SiO2 > Au/SiO2 >> Ag/SiO2 [327]. Yen et al. [323] fabricated an Au-Ag catalyst supported on an acidic mesoporous aluminosilicate ([email protected]), which showed high activity toward CO oxidation [322, 361]. Ag atoms were found to be enriched in the surface layer by forming assemblies with Au. The surface structure was rather stable and Ag was assumed to adsorb and activate molecular oxygen, meanwhile CO was adsorbed on the neighboring Au sites [360, 361]. 4.1.2. Alcohol Oxidation Au-Ag nanoalloy catalysts have exhibited excellent activity toward alcohol aerobic oxidation [115, 362]. A set of monodispersed Au-Ag alloy clusters (1.6-2.2 nm) with different Ag content (5-30 wt.%) was prepared by a coreduction method using PVP. The catalytic activity of AuAg-PVP clusters was investigated for the aerobic oxidation of p-hydroxybenzyl alcohol, which was often used as a model reaction to understand the effect of Ag on the catalytic activity of Au clusters. Recent theoretical studies have demonstrated that molecular oxygen was activated into a superoxo- or peroxo-like species by electronic charge-transfer from anionic Au clusters isolated in gas phase or deposited on F centers of metal oxide surfaces [363-366]. XPS spectra Fig. (22) [367] showed a clear relationship between the catalytic activity and the charge state of Au in alloy. Au sites with anionic character were evidenced to play an essential role in the reaction. The electron-donating ability of stabilizing reagents was one of the crucial factors in the fabrication of Au cluster catalysts for aerobic oxidation [367, 368].

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

Recent Patents on Catalysis, 2013, Vol. 2, No. 1

33

 Fig. (21). (A) CO conversion profile with reaction temperature over [email protected] after calcinations and reduction with Au/Ag molar ratio: 1/0 (), 8/1 (), 4/1 ( ), and 0/1 ( ). (B) [email protected] (Au/Ag = 8/1) only with calcinations still owns activity toward CO conversion at 353 (C) CO conversion of [email protected] (Au/Ag = 8/1) stored for 1 year. (D) The CO oxidation conversion of moisture condition. Reprinted with permission from ref. 323, Copyright 2009 American Chemical Society

4.1.3. Hydrocarbon Oxidation Catalytic oxidation of ethylene with air or bioxygen over supported silver catalysts has been studied for decades [369]. However, the main problem is that the yields of ethylene oxide are quite low over these catalysts. The particle size and surface properties of catalysts are the paramount factors for enhancing the catalytic performance. Rojluechai et al. [329] introduced an appropriate amount of Au into a Ag/Al2O3 catalyst, and the catalytic activity was enhanced remarkably. Au was suggested to act as a dilutent on the Ag surface, and the creation of new single Ag sites favored the adsorption of molecular oxygen. Among all catalysts, the Au-Ag/Al2O3 alloy catalyst is regarded as the most promising candidate for both ethylene and propylene epoxidation. Propylene was likely oxidized to acrolein on a Au-rich Au-Ag alloy catalyst [370, 371]. The selectivity toward acrolein increased with increasing the Au content, meanwhile the selectivity to ethylene oxide decreased and ultimately dropped to zero over gold-rich alloys.

 Fig. (22). XPS spectra of Au(4f) core level for the 1073 K annealed silica thin films containing (a)Ag and (b) Au-Ag-alloy nanoparticles. Reprinted with permission from ref. 367, Copyright 2007 Elsevier

34 Recent Patents on Catalysis, 2013, Vol. 2, No. 1

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The improvement was ascribed to the adsorption of oxygen on exposed Ag surfaces, whereas 1,5-hexadiene formation was promoted by oxygen species adsorbed on the Ag surface.

lytic activities. However, pure Au also shows the lower catalytic activities than Ag10/Au90 and Ag20/Au80 alloys. The negatively charged Au atoms resulted from the charge transfer effect may act as active sites for glucose oxidation.

Bimetallic Au-Ag catalysts have also been validated to be active for partial oxidation of propene, which involves three reactions (eqs.1-3):

Tokonami et al. [331] synthesized Au-Ag alloys with a core (Au)-shell (Ag) structure. The effect of Au/Ag molar ratio on the catalyst performance was investigated for glucose oxidation. The Ag/Au ratio was optimized to 0.25, which possessed nearly 20 times higher activity compared to Au nanoparticles under the same conditions Fig. (24).

C3 H6 + O2  C3 H4 O + H2 O

Acrolein formation

(1)

2C3H6 + O2  C6H10 + H2O

1,5-Hexadiene formation

(2)

C3H6 + O2  CO2 + H2O

Total oxidation

(3)

Au-Ag alloy electrodes [370] showed higher selectivity to the formation of acrolein than pure Ag [372]. Under reducing conditions (excessive propene), the selectivity could be altered significantly. Kondarides et al. [373] have studied the oxygen adsorption properties of Ag over Au-Ag/-Al2O3 catalysts with different compositions. Three adsorbed oxygen species were detected on the alloy surfaces, namely, molecular, atomic and subsurface oxygen. The Ag-O bond was undermined due to interactions between Ag and neighboring Au atoms. The enthalpy of adsorption decreased linearly with increasing the surface Au content, and reached essentially zero at the surface containing more than 40 at.% Au. On the other hand, the activation energy for the adsorption of molecular oxygen decreased from 44 to17 kJ/mol with increasing the Au content from zero to 24%. This allows O2 to be easily reacted with propene.

Liu et al. [332] found that the addition of a trace amount of Ag into a nanoporous Au catalyst could greatly enhance the catalytic activity for the electro-oxidation of glucose. In contrast, the complete removal of Ag component could greatly decrease the electrocatalytic activity. A novel Au-Ag alloy nanoparticles capped with decanethiolate (DT) monolayer shells were reported by Tominaga et al. [333] and the particle size of this catalyst was precisely controlled within 5 nm.

4.1.4. Glucose Oxidation The oxidation of glucose has been widely studied for the transformation of biomass materials. PVP-protected Agcore/Aushell bimetallic nanoparticles with 1.4 nm in diameter was synthesized and used for aerobic glucose oxidation [330]. The activity for this catalyst was about three times higher than that of pure Au, and also several times higher than that of Au-Ag catalyst synthesized by other methods [331]. Preliminary results showed that those catalysts can also be applied for glucose-oxygen fuel cells and glucose sensors for medical and food industries [374-377]. The particular properties of Au-Ag alloys were ascribed to the electronic modification of Au upon alloying with Ag [378]. A plausible mechanism of electronic charge-transfer in Au-Ag is illustrated in Fig. (23) [330]. Au-Ag alloys with the high Ag content (more than 30 at.%) usually show low cata-

Fig. (23). Schematic illustration of electronic charge transfer in the Ag/Au BNPs (core/shell structure of Ag10Au90 BNPs (a), alloy structure of Ag10Au90 and Ag20Au80 BNPs (b), Ei: Ionization energy). Reprinted with permission from ref. 330, Copyright 2011 Elsevier

Fig. (24). Glucose oxidation catalytic activity of Ag/Au NPs at various feed composition ratios and Au and Ag monometallic NPs. Each measurement was carried out at least three times, and the average is presented. Reprinted with permission from ref. 331, Copyright 2010 American Chemical Society

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

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35

Fig. (25). (a) Plots of ln(A400) against reaction time and (b) rate constants for the Au-Ag-HSAF nanoparticle catalytic reduction of 4nitrophenol (7.0% standard deviation). Reprinted with permission from ref. 335, Copyright 2010 American Chemical Society

4.2. Reduction Reactions

Table 9.

4.2.1. 4-nitrophenol Reduction Bimetallic Au-Ag nanoalloys were also examined for the reduction of 4-nitrophenol. Albonetti et al. [334] found AuAg nanocrystals functioned as effective catalysts for the reduction of p-nitrophenol in the presence of NaBH4. Shin et al. [335] studied Ag and Au catalysts by gradually increasing the Ag content. The rate constants exponentially grew when the Au content in the Au-Ag-HSAF (horse spleen apoferritin) samples linearly increased. Au appeared to be much more efficient than Ag for the electron transfer from BH4 ion to 4-nitrophenol Fig. (25).

Comparison of values of apparent rate constant for Au, Ag and Au–Ag nanoparticles as catalysts for the reduction of 4-nitrophenol in the presence of excess NaBH4. Reprinted with permission from ref. 336, Copyright 2011 Elsevier. Catalyst

Rate constant, k (103 s 1)

Au

6.133

Au–Ag(96:4)

14.342

Au–Ag(50:50)

27.002

Au–Ag(4:96)

12.168

Ag

19.949

Jiang et al. [337] prepared novel [email protected] core-shell nanoparticles immobilized on a metal-organic framework (MOF) by a sequential deposition-reduction method. The catalytic activity was improved remarkably, probably because of synergistic interactions. Harish et al. [336] prepared Au, Ag and Au-Ag bimetallic nanoparticle sols stabilized with APS (3-aminopropyltrimethoxysilane). The rate constants for reducing 4-nitrophenol were highly improved compared to monomer metals, especially when alloying the equivalent amount of Au and Ag Table 9. Huang et al. [338] synthesized dendrite-based Au-Ag bimetallic nanostructures with excellent activities using a galvanic replacement reaction (GRR) of Ag dendrites in a chlorauric acid (HAuCl4) solution.

Au as an additive element has also been introduced into other transition metals by forming bimetallic catalysts with or without alloying. Generally, the performance improvement can be seen in most of the cases Table 10 [385-400]. The mechanisms for those reaction systems have been rationalized on the basis of ensemble and ligand effects; however, deep insights into each reaction are desirable and worthy of further investigations in the future.

4.2.2. Degradation of 4-chlorophenol

6. CURRENT & FUTURE DEVELOPMENTS

The degradation of 4-chlorophenol is of great importance to the environmental protection because it is a toxic and nonbiodegradable organic compound. Degradation of this compound has attracted much attention, the mostly used approaches are photocatalysis [379-381] and photo-electrocatalytic degradation [382, 383].

Recent advances on preparation, characterization and applications of bimetallic Au-Pd, Au-Pt and Au-Ag and other Au-M catalysts have been summarized. Obviously, Au in bimetallic catalysts has several important characteristics.

The noble metal (Ag, Au, Pt, etc) supported on TiO2 catalysts are also considered for this reaction. Wongwisate et al. [384] have studied the effects of monometallic and bimetallic Au-Ag supported on sol-gel TiO2 for the photocatalytic degradation of 4-chlorophenol and its intermediates. It has been evidenced to be a very effective catalyst for the aim reaction. Clearly, there are more opportunities to develop bimetallic Au-M catalysts for environmental application.

5. OTHER GOLD-BASED BIMETALLICS

(i) By the addition of Au, the surface properties of Pd, Pt, Ag and other active metal can remarkably be altered by forming alloy or surface alloy. In the case of Pd, upon alloying with Au, the electronic structure and geometry of Pd clusters in the surface layers can remarkably be modified. This hypothesis has been demonstrated by the studies on both model and practical catalysts through multi-techniques. The catalytic performance in different reaction systems is dependent on the diversity of

36 Recent Patents on Catalysis, 2013, Vol. 2, No. 1

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Table 10. Catalytic systems over bimetallic Au-based bimetallic (alloy) catalysts. Catalyst

Reaction

T/K

Evaluation indicator

Ref.

[email protected]

Hydrolytic Dehydrogenation of Ammonia Borane

r.t.

Reation time:11min

[386]

AuCo alloy

Hydrolytic Dehydrogenation of Ammonia Borane

r.t.

Reation time:25min

[386]

1.5%Au1.5%Co/TiO 2

Oxidation of -pinene

343

Conversion of -pinene:88%

[387]

Au–[email protected] 2

Hydrolytic Dehydrogenation of Ammonia Borane

295

Reation time:14 min

[388]

2%Au-5%Ni /(MgO)x(Al2O 3)y

Partial oxidation of methan

973-1173

CH4 conversion: 90-100%

[389]

Au-Ni/Al2O 3

gas-phase hydrodechlorination of 2,4-dichloro-phenol

CO selectivity: 100% Initial fractional dechlorination:0.68

[390]

The corresponding values after 8 h onstream:0.35 Au-Ni/SiO 2

gas-phase hydrodechlorination of 2,4-dichloro-phenol

473

*the initial selectivity to 2-CP: 30

[391]

*the initial selectivity to 4-CP: 0 *the initial selectivity to phenol: 70 Au-Ni/TiO 2

gas-phase hydrodechlorination of 2,4-dichloro-phenol

473

the initial selectivity to 2-CP: 14

[391]

the initial selectivity to 4-CP: 6 the initial selectivity to phenol: 80 6% Au20 Cu1/SiO2

Specific rate/molCO g1 Au h 1 :0.21

CO oxidation

[392]

1

TOF /s :0.036 Au–Cu/SBA-15

Specific rate/molCO g1 Au h 1 :0.43

CO oxidation

[393]

1

TOF /s :0.06 Au-Cu/TiO2-Fe2O3

Partial oxidation of methanol (POM) to produce H2

CH3OH conversion: 82–85%

[394]

H2 selectivity: 78–80% Au-Cu/ Fe2O 3

WGS

373

Rate:2.5107mol s-1gcat-1 9

-1

-1

[395]

Au-Cu/ Fe2O 3

WGS

513

Rate:6.310 mol s gcat

1.5%Au1.5%Ru/TiO 2

Oxidation of -pinene

343

Conversion of -pinene:73%

[397]

1.5%Au1.5%Cu/TiO 2

Oxidation of -pinene

343

Conversion of -pinene:97%

[387]

4%Au1Cu3/TiO2

propene epoxidation

573

Selectivity of PO:26.3;

[396]

1

Rate:1.96mmol gcat h 1.2%Au1-Cu1/TiO2

Propene epoxidation

573

[395]

1

Rate: 316 mmol g1M h1

[397]

PO selectivity: 35.7% 1.2%Au1-Cu3/TiO2

Propene epoxidation

573

Rate: 592 mmol g1M h1

[397]

PO selectivity: 41.0% 1% Au–Cu/SiO 2

oxidation of benzyl alcohol

590

Conversion:98;

[398]

selectivity:>99 yield:98` 4%Au/Cu-fiber

Oxidation of benzyl alcohol

493

Conversion: 76.5% Selectivity: 98.8%

[399]

Heterogeneous Catalysis by Gold-based Bimetallic Catalysts

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37

Table (10). Contd…

Catalyst

Reaction

T/K

Evaluation indicator

Ref.

3%Au/Cu-fiber

Oxidation of cyclohexanol

553

Conversion: 76.0%

[399]

Selectivity: 99.0% Au‒Ru/Fe2O3

Partial oxidation of methanol (POM) to produce H2

523

Methanol conversion: 100%

[400]

Hydrogen selectivity:82.0% 8%Au4‒Ir1(Cl)/γAl2O3

Hydrogenolysis of methylcyclopentane

8%Au1‒Ir4(Cl)/γAl2O3

Hydrogenolysis of methylcyclopentane

8% Au4‒Ir1(Si)/γAl2O3

Hydrogenolysis of methylcyclopentane

8% Au1‒Ir8(Si)/γAl2O3

Hydrogenolysis of methylcyclopentane

457

Initial rate:6.281019 MCP molecules s 1 gIr 1

[401]

2-MP/3-MP:2.1 457

Initial rate:3.341019 MCP molecules s 1 gIr 1

[401]

2-MP/3-MP:2.4 457

Initial rate:3.881019 MCP molecules s 1 gIr 1

[401]

2-MP/3-MP:2.1 457

Initial rate:2.921019 MCP molecules s 1 gIr 1

[401]

2-MP/3-MP:2.4

*recycle 1

bimetallic Au-M structures, which can be manipulated by changing active element, surface structure, preparation methods and reaction conditions. However, deep insight into the structure of real bimetallic Au-M catalysts, especially the interaction of Au and M is still unambiguous, mainly due to the limitations of current techniques. Because of the “material and pressure gaps” between real and model catalyst systems, the great difference considering the mechanisms from both systems is not unusual. Moreover, the samples other than Au-Pd and Au-Pt are still sporadically emerging. Therefore, it is still a long way to form a comprehensive theory to explain the promotional effect of Au on bimetallic catalysts. (ii) In order to tune or control the structure of bimetallic AuM catalysts, various preparation methods have been developed to control the morphology of nanoalloys, the particles size and the atomic ratio of Au/M. Actually, most of those preparation methods are dedicated to synthesize the nanoparticles developed by material scientists. They are able to control unsupported Au-M nanoalloys by using stabilizers or reagents. The nanocrystal planes of bimetallic Au-M are usually changed remarkably in the subsequent pretreatment of the supported catalysts (removing organic stabilizer). That is to say, till now, is still very difficult to manipulate the morphology of bimetallic Au-M particles for supported catalysts under realistic conditions. On the other hand, the surface structure of bimetallic Au-M catalysts can also be modified significantly during pretreatment and reaction, such as reconstructing of surface configuration due to thermal and reactant induction. (iii) The diversity of the structure of bimetallic Au-M catalysts results in various performances in catalytic reactions. Generally, by the addition of Au, the adsorption and activation of oxygen over active metals can greatly be changed. It is the primary reason why those Au-M

bimetallics can be excellent catalysts for oxidation reactions such as CO oxidation, H2O2 synthesis, alcohols oxidation and ORR in electrocatalytic reactions. Moreover, the addition of Au may also change the interaction between active metal and supports, thus the structure of catalysts can become more complex than we speculated. By the replacement of different supports, the supported bimetallic catalysts have found to be active for hydrogenation, degradation of organic compounds and photon-induced catalytic reactions. The performance of bimetallic Au-M catalysts can be applied to the cuttingedge techniques, for instance, fuel cells, the production of H2 and environment protections. It is believed that the functionalized support will provide more opportunities to precisely control the structure of bimetallic Au-M catalysts. It is a potentially hot topic in this area, specially in the conversion of biomass. CONFLICT OF INTEREST The authors confirm that this article content has no conflicts of interest. ACKNOWLEDGEMENTS The authors are grateful to the support from the Chinese Education Ministry 111 project (B08021), the National Science Foundation (21176071, 21106041), Shanghai PuJiang Talent Program (2011/11PJ1402400), Science and Technology Commission of Shanghai Municipality (11JC1402700), Innovation Program of Shanghai Municipal Education Commission (11ZZ52, 11ZZ051), Shanghai Natural Science Foundation (11ZR1408400). REFERENCES [1]

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