Microwave-assisted synthesis of Pd nanoparticles

J Nanopart Res (2014) 16:2477 DOI 10.1007/s11051-014-2477-0

RESEARCH PAPER

Microwave-assisted synthesis of Pd nanoparticles supported on Fe3O4, Co3O4, and Ni(OH)2 nanoplates and catalysis application for CO oxidation Hany A. Elazab • Sherif Moussa • B. Frank Gupton • M. Samy El-Shall

Received: 27 November 2013 / Accepted: 21 May 2014 Ó Springer Science+Business Media Dordrecht 2014

Abstract In this paper, we report a simple, versatile, and rapid method for the synthesis of Pd nanoparticle catalysts supported on Fe3O4, Co3O4, and Ni(OH)2 nanoplates via microwave irradiation. The important advantage of microwave dielectric heating over convective heating is that the reactants can be added at room temperature (or slightly higher temperatures) without the need for high-temperature injection. Furthermore, the method can be used to synthesize metal nanoparticle catalysts supported on metal oxide nanoparticles in one step. We also demonstrate that the catalyst-support interaction plays an important role in the low temperature oxidation of CO. The current results reveal that the Pd/Co3O4 catalyst has particularly high activity for CO oxidation as a result of the strong interaction between the Pd nanoparticles and the Co3O4

Electronic supplementary material The online version of this article (doi:10.1007/s11051-014-2477-0) contains supplementary material, which is available to authorized users. H. A. Elazab B. F. Gupton M. S. El-Shall Department of Chemical Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA S. Moussa B. F. Gupton M. S. El-Shall (&) Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23284, USA e-mail: [email protected] M. S. El-Shall Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

nanoplates. Optimizations of the size, composition, and shape of these catalysts could provide a new family of efficient nanocatalysts for the low temperature oxidation of CO. Keywords CO oxidation Microwave synthesis Pd nanoparticles Magnetite Fe3O4 nanoparticles Hexagonal Co3O4 nanoplates Hexagonal Ni(OH)2 nanoplates

Introduction Heterogeneous catalysis processes have a wide range of applications ranging from hydrocarbon refining, fuelcell technologies, production of chemicals and pharmaceuticals, and removal of harmful gases and volatile organics for the control of pollution in the environment (Ertl 2009; Somorjai and Li 2010; Gao and Goodman 2012). For example, the catalyzed oxidation of CO by O2 is an important process in pollution control since small exposure (ppm) to this odorless invisible gas can be lethal (World Health Organization 1999). Significant advances in air quality and environmental protection have been achieved through the introduction of the three-way catalytic converters in automobiles where Pd, Pt, and Rd nanoparticles are the main active catalysts (Haag et al. 1999; Shelef and McCabe 2000; Somorjai and Li 2010; Freund et al. 2011). Palladium is among the most investigated catalysts as it plays an important role in many different heterogeneous

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catalysis processes such as carbon–carbon cross-coupling reactions, energy conversion, and storage in addition to the catalytic oxidation of CO, a prototype reaction important in many applications such as protonexchange fuel cells, WGS reaction, and catalytic convertor applications (Choudhary and Goodman 2002; Bianchini and Shen 2009; Molnar 2011; Jin et al. 2012). The performance of a heterogeneous catalyst for CO oxidation including activity, selectivity, and stability depends on various factors such as the type of the support, the metal precursor, the preparation conditions, the pre-treatment conditions, and the catalytic reaction conditions (Somorjai and Li 2010; Freund et al. 2011; Gao and Goodman 2012). Among these factors, the nature of the support is expected to play an important role given the strong tendency of CO molecules to adsorb on the surface of the metal nanoparticle catalyst, and therefore the turnover relies on having surface sites open for O2 adsorption and dissociation (Somorjai and Li 2010; Freund et al. 2011; Gao and Goodman 2012). Metal oxides have been used as catalyst supports for Pd and other CO oxidation catalysts since they provide a convenient way to disperse the catalyst nanoparticles and prevent their agglomeration and sintering during the reactions (Campbell et al. 2000; Santra and Goodman 2003; Farmer and Campbell 2010; Somorjai and Li 2010; Freund et al. 2011; Gao and Goodman 2012; Jin et al. 2012). In addition, metal oxides can influence the mechanism of the catalytic oxidation through the metal–metal oxide interface where electron transfer processes can take place. In fact, recent studies have shown that the metal oxide can directly participate along with the metal nanoparticles in the CO oxidation process through concerted or sequential mechanisms (Freund et al. 2011; Green et al. 2011; Rodriguez 2011; Widmann and Behm 2011). Fe3O4 nanoparticles present an interesting application as a catalyst support for CO oxidation since the presence of the Fe2? ion in Fe3O4 provides some reducible oxide characters. Recently, a Pd/Fe3O4 hybrid nanocatalyst has been shown to significantly improve the oxidation of CO at temperatures \100 °C (Chen et al. 2012). Similarly, several reports have shown that cobalt oxide-based catalysts are generally active toward CO oxidation; however, their activity depends on the cobalt oxidation state and the phase formation of cobalt under the reaction

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conditions (Jernigan and Somorjai 1994; Radwan et al. 2007; Jia et al. 2011; An et al. 2013). Co3O4 shows significant activity in the oxidation of CO at low temperatures (Jansson 2000; Jansson et al. 2002; Xie et al. 2009; Jia et al. 2011). Also, the Ni(OH)2based materials could provide catalyst support applications for CO oxidation (Radwan et al. 2007; Zhou et al. 2009). These materials have many important applications such as in batteries where they are considered the primary electrode materials and also in supercapacitor applications (Zhou et al. 2009; Wang et al. 2010). In this paper, we report a simple method to prepare Pd nanoparticle catalysts supported on Fe3O4, Co3O4, and Ni(OH)2 nanoparticles and nanoplates, and compare their catalytic activities for CO oxidation. This work introduces the method and demonstrates that the shape and morphology of the support nanoparticles can have a significant effect on the activity of the Pd catalysts. The approach utilized in the present work is based on microwave synthesis of nanoparticles from metal salts in solutions. Microwave irradiation (MWI) methods provide simple and fast routes to the synthesis of nanomaterials since no high temperature or high pressure is needed. Furthermore, MWI is particularly useful for a controlled large-scale synthesis that minimizes the thermal gradient effects (Kappe 2004; Glaspell et al. 2005; Abdelsayed et al. 2008, 2009; Herring et al. 2011; Qiu et al. 2011; Siamaki et al. 2011). The heating of a substance by MWI depends on the ability of the material (solvent or reagent) to absorb microwave radiation and convert it into heat. Due to the difference in the solvent and reactant dielectric constants, selective dielectric heating can provide significant enhancement in reaction rates. By using metal precursors that have large microwave absorption cross sections relative to the solvent, very high effective reaction temperatures can be achieved. The rapid transfer of energy directly to the reactants (faster than they are able to relax) causes an instantaneous internal temperature rise. Thus, the activation energy is essentially decreased as compared with conductive heating, and the reaction rate increases accordingly. Furthermore, reaction parameters such as temperature, time, and pressure can be controlled easily (Kappe 2004; Glaspell et al. 2005; Abdelsayed et al. 2008, 2009; Herring et al. 2011; Qiu et al. 2011; Siamaki et al. 2011).


Experimental All chemicals used in the experiments were purchased and used as received without further purifications. Palladium nitrate (10 wt% in 10 wt% HNO3, 99.999 %), Fe(NO3)39H2O, Co(NO3)26H2O, Ni(NO3)26H2O, and hydrazine hydrate (80 %) were obtained from Sigma Aldrich. Deionized water (D.I. H2O, *18 MX) was used for all experiments. For all the syntheses described here, a conventional microwave oven (2.45 GHz) operating at 1,000 W was used. For the Pd/Fe3O4 system, 5, 20, 40, and 50 wt% Pd catalysts were prepared by adding 687.2, 578.7, 434.1, 361.7 mg of Fe(NO3)39H2O into 97, 388, 776, and 970 ll of the Pd nitrate solutions, respectively, in the presence of 20 ml deionized water, and the solutions were sonicated for 1 h followed by the addition of 600 ll hydrazine hydrate at room temperature. The reaction mixture was microwaved in 60 s cycles for a total reaction time of 7 min. For the Pd/ Co3O4 system, 10, 20, 30, 40, and 50 wt% Pd catalysts were prepared by adding 443.9, 394.6, 345.2, 295.9, and 246.6 mg of Co(NO3)26H2O into 194, 388, 582, 776, and 970 ll of the Pd nitrate solutions, respectively, in the presence of 20 ml deionized water, and the solutions were sonicated for 1 h followed by the addition of 600 ll hydrazine hydrate at room temperature. The reaction mixture was microwaved in 60 s cycles for a total reaction time of 5 min. For the Pd/ Ni(OH)2 system, 30, 50, and 70 wt% Pd catalysts were prepared by adding 346.7, 247.6, and 148.6 mg of Ni(NO3)26H2O into 582, 970, and 1,358 ll of the Pd nitrate solutions, respectively, in the presence of 20 ml deionized water, and the solutions were sonicated for 1 h followed by the addition of 600 ll hydrazine hydrate at room temperature. The reaction mixture was microwaved in 60 s cycles for a total reaction time of 9 min. In all cases, the supported catalyst product was washed using hot deionized water 2–3 times and then ethanol 2–3 times, and finally the product was dried in an oven at 80 °C for 2 h. Characterization TEM images were obtained using a Joel JEM-1230 electron microscope operated at 120 kV equipped with a Gatan UltraScan 4000SP 4 K 9 4 K CCD camera. X-ray diffraction (SA-XRD) patterns were measured at room temperature with an X’Pert Philips

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Materials Research Diffractometer using Cu Ka1 radiation. The X-ray photoelectron spectroscopy (XPS) spectra were measured on a Thermo Fisher Scientific ESCALAB 250 using a monochromatic Al KR. All the TEM, XRD, and XPS measurements were obtained on the as-prepared samples without any treatment. Catalysis measurements For the CO catalytic oxidation, the sample was placed inside a Thermolyne 2100 programmable tube furnace reactor as described in the previous publications (Glaspell et al. 2005; Abdelsayed et al. 2006; Yang et al. 2006; Glaspell et al. 2008; Abdelsayed et al. 2009). The sample temperature was measured by a thermocouple placed near the sample. In a typical experiment, a gas mixture consisting of 3.5 wt% CO and 20 wt% O2 in helium was passed over the sample, while the temperature was ramped. The gas mixture was set to flow over the sample at a rate of 100 cc/min controlled via MKS digital flow meters. The conversion of CO to CO2 was monitored using an infrared gas analyzer (ACS, Automated Custom Systems Inc.). All the catalytic activities were measured (using a 50 mg sample) after a heat treatment of the catalyst at 110 °C in the reactant gas mixture for 15 min in order to remove moisture and adsorbed impurities.

Results and discussion Figure 1a displays a typical TEM image of the unsupported Pd nanoparticles prepared by the MWIassisted chemical reduction of Pd(NO3)2 using hydrazine hydrate (HH). As expected, in the absence of a capping agent or a support, the nanoparticles show a significant degree of aggregation although the primary particles have small sizes in the range of 6–8 nm. Figure 1b (i–iii) displays TEM images of the Fe3O4, Co3O4, and Ni(OH)2 nanoparticles, respectively, prepared using HH reduction under MWI in the absence of Pd ions. Additional TEM images are given in Figure S1 (Supplementary Materials). In the case of Fe3O4, [Fig. 1b(i)], most of the nanoparticles have irregular shapes with diameters between 25 and 30 nm, and a few have a hexagonal plate shape. This is similar to the morphology of the Fe3O4 nanoparticle prepared by the HH reduction of FeCl3 under MWI as

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Fig. 1 a TEM of Pd nanoparticles prepared by HH reduction of Pd(NO3)2 without a support under MWI. b TEM of nanoparticles prepared by HH reduction of i Fe3O4, ii Co3O4, iii Ni(OH)2, iv 50 wt% Pd/Fe3O4, v 30 wt% Pd/Co3O4, and (vi) 50 wt% Pd/Ni(OH)2

reported by Wang et al. (2007). However, for the Co3O4 nanoparticles, more well-defined hexagonal shape particles are formed as shown in Fig. 1b(ii) with the angles of adjacent edges very close to 120°. For the Ni(OH)2 nanoparticles, smaller particles (6–8 nm) with irregular triangles and hexagonal shapes are formed as shown in Fig. 1b(iii). Figure 1b(iv–vi) displays the TEM images of the Pd nanoparticles supported on the Fe3O4, Co3O4, and Ni(OH)2 nanoplates, respectively, prepared using the MWI-assisted chemical reduction method. It is clear that in the three catalyst systems, most of the Pd particles appear to be deposited on the support nanoplates. Figure 2a displays the XRD patterns of the assynthesized Pd, Fe3O4, 20 wt% Pd/Fe3O4, and 50 wt% Pd/Fe3O4 nanoparticles prepared by the HH reduction under MWI. The Pd nanoparticles show the typical fcc pattern of Pd of crystalline particles (JCPDS-46-1043). The 100 % Fe3O4 nanoparticles show the characteristic peaks for the spinal Fe3O4 phase (ICCD-00-003-0863) in addition to a small percentage of the a-Fe2O3 phase as indicated by the 2h peaks at 33.3 and 49.6 (JCPDS-33-0664). For the 20 wt% Pd/Fe3O4 and 50 wt% Pd/Fe3O4 nanoparticles, peaks due to Pd and Fe3O4 are present with no indication of the presence of the a-Fe2O3 phase.

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To characterize the surface composition of the supported nanocatalysts, we carried out XPS measurements as shown in Fig. 2b, c for the 50 wt% Pd/ Fe3O4 catalyst. The data reveal the presence of Fe(III) as indicated by the observed peaks at 724.2 and 710.5 eV corresponding to the binding energies of the 2p1/2 and 2p3/2 electrons, respectively. The broad Fe(III) 2p3/2 peak centered at 710.5 eV most likely contains contributions from the Fe(II) 2p3/2 which normally occurs at *708 eV. For Pd, the observed binding energies of 334.8 and 340.1 eV indicate the presence of 82 % Pd0 and 18 % Pd2? (due to the presence of surface layer of PdO). However, these values are slightly lower than the binding energies of Pd 3d electrons in pure Pd nanoparticles where the values of 335 and 341.1 eV have been reported for Pd0 and Pd2?, respectively (Chen et al. 2012). The decrease in the binding energy of the Pd 3d electron in the Pd/Fe3O4 supported catalyst indicates that the Pd in the supported catalyst is more electron rich than in pure Pd nanoparticles. This could be due to electron transfer from Fe2O3 to Pd consistent with similar results obtained for the Pd/Fe2O3 hybrid nanocatalysts prepared by a seed-mediated process (Chen et al. 2012), and also for Pd nanoparticles grown by vapor phase deposition on ordered crystalline Fe3O4 films (Schalow et al. 2005).

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Fig. 2 a XRD patterns of Pd nanoparticles, Fe3O4 nanoplates, 20 wt% Pd/ Fe3O4, and 50 wt% Pd/ Fe3O4 (*due to Fe2O3). b XPS binding energy (Fe 2p region) for the 50 wt% Pd/Fe3O4 catalyst. c XPS binding energy (Pd 3d region) for the 50 wt% Pd/Fe3O4 catalyst

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Figure 3a compares the catalytic oxidation of CO over Fe3O4 and Pd nanoparticles as well as 5, 20, 40, and 50 wt% Pd nanoparticles supported on Fe3O4. The catalytic activities of pure Pd and pure support nanoparticles were measured as a benchmark for comparison with the supported catalysts. It is clear that both the individual Fe3O4 and Pd nanoparticles show poor activities with 100 % conversion temperatures of 273 and 179 °C, respectively. In the case of pure Pd, this is mostly due to the aggregation of the Pd nanoparticles and the complete coverage by CO molecules on the catalyst surface so there are not many open sites available for the adsorption of the O2 molecules. The data also show that the 5 wt% Pd/Fe3O4 catalyst has lower activity than the pure Pd nanoparticle catalyst indicating that not enough Pd nanoparticles are interacting with the Fe3O4 nanoparticles to create a sufficient number of active interfaces for the CO oxidation. Increasing the Pd wt% up to 50 % increases the activity as shown in Fig. 3a. However, catalysts containing more than 50 wt% Pd show similar behavior to the pure Pd

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nanoparticle catalyst confirming the importance of the support in dispersing the catalyst nanoparticles and decreasing their tendency for aggregation and sintering. Therefore, it appears that the 50 wt% Pd/ Fe3O4 catalyst provides reasonable optimization between the adsorption of the CO and O2 molecules on the Pd-Fe3O4 interfaces to allow efficient oxidation of CO. To confirm the stronger interaction between the Pd and Fe3O4 nanoparticles prepared simultaneously using the MWI method as compared to the physical mixing of individual Pd and Fe3O4 nanoparticles, we measured the catalytic activity of a 50 wt% physical mixture of Pd and Fe3O4 nanoparticles prepared separately under identical conditions to the preparation of the supported 50 wt% Pd/Fe3O4 catalyst. As shown in Fig. 3b, the physical mixture exhibits a significantly lower activity than the supported catalyst prepared simultaneously using the MWI. This provides evidence for stronger interaction between the Pd and Fe3O4 nanoparticles, which is reflected in the enhanced catalytic activity.

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Fig. 4 a XRD patterns of Pd nanoparticles, Co3O4 nanoplates, and Pd supported on Co3O4 nanoplates. b XPS (Fe 2p) and c XPS (Pd 3d) of 50 wt% Pd/Co3O4 catalyst

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Fig. 3 a CO catalytic oxidation by Pd nanoparticles supported on Fe3O4 with different compositions. b Comparison between the activity of the 50 wt% Pd/Fe3O4 catalyst and a 50 wt% mixture of Pd and Fe3O4 nanoparticles

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The catalytic activity of the 50 wt% Pd/Fe3O4 catalyst prepared here using the simple one-step MWIassisted chemical reduction is comparable to that of

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the Pd/Fe2O3 hybrid nanocatalysts prepared by a seedmediated process for the synthesis of Pd core nanoparticles followed by deposition of Fe3O4 surface


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layers in solution and then thermal annealing at 300 °C (Chen et al. 2012). The 100 % conversion of CO into CO2 was measured for the Pd/Fe3O4 hybrid catalyst at 125 °C as compared to 128 °C for the 50 wt% Pd/Fe3O4 catalyst prepared in this work. Figure 4a shows the XRD patterns of the Pd, Co3O4, 20 wt% Pd/Co3O4, 30 wt% Pd/Co3O4, and 50 wt% Pd/Co3O4 nanoparticles prepared by the HH reduction under MWI. The 100 % Co3O4 nanoparticles show the characteristic peaks for the spinal Co3O4 phase (ICCD-00-030-044300). For the 20, 30, and 50 wt% Pd/Co3O4 nanoparticles, peaks due to Pd and Co3O4 are present and with no indication of the presence of other phases. The Co and Pd XPS data for the 50 wt% Pd/Co3O4 catalyst are shown in Fig. 4b, c for the Co-2p and Pd3d electron binding energies, respectively. The data reveal the presence of Co(II) and Co(III) as indicated by the observed peaks at 796.8 and 780.7 eV corresponding to the Co(II) 2p1/2 and 2p3/2 binding energies, respectively, and at 802.6 and 786.2 eV corresponding to the Co(III) 2p1/2 and 2p3/2 binding energies, respectively. For Pd, the observed binding energies of 340.7 and 335.4 eV indicate the presence of 86 % Pd0 (3d5/2), and 342.5 and 337.2 eV indicate the presence of 14 % Pd2? (due to the presence of PdO on the surface). Figure 5a compares the catalytic oxidation of CO over Co3O4 and Pd nanoparticles as well as 10, 20, and 30 wt% Pd nanoparticles supported on Co3O4. Unlike the pure Fe3O4 nanoparticles, the Co3O4 nanoparticles exhibit a significant activity for CO oxidation with T50 (the temperature at which CO conversion reaches 50 %) and T100 occurring at 117 and 140 °C, respectively. The same trend of increasing catalytic activity with increasing the Pd wt% in the catalyst is also observed in the Pd/Co3O4 system as in the Pd/Fe3O4 system. However, significant activity is observed for the 20 wt% Pd/Co3O4 catalyst especially at lower temperatures. For example, Fig. 5a shows a 10 % conversion of CO into CO2 at 70 °C and a T50 of about 110 °C for the 20 wt% Pd/Co3O4 catalyst. It appears that Co3O4 is responsible for the low temperature oxidation of CO since this behavior is also weakly observed in pure Co3O4 nanoparticles as shown in Fig. 5a. It should be noted that in spite of the high oxidation activity of pure Co3O4 nanoparticles (T100 occurring at 140 °C), the 30 wt% Pd/Co3O4 catalyst provides slightly higher activity with T100 occurring at

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Fig. 5 a CO catalytic oxidation by Pd nanoparticles supported on Co3O4 with different compositions. b Comparison between the activity of the 50 wt% Pd/Co3O4 catalyst and a 50 wt% mixture of Pd and Co3O4 nanoparticles

127 °C. The lowest T100 obtained for the 30 wt% Pd/ Co3O4 catalyst at 127 °C is similar to that obtained for the 50 wt% Pd/Fe3O4 catalyst. This indicates that Co3O4 nanoparticles provide a more active support for the Pd nanocatalyst than Fe3O4 nanoparticles, and hence a lower amount of the Pd nanoparticles can be used to achieve the same T100. Also, similar to the result obtained for the 50 wt% Pd/Fe3O4 catalyst, the physical mixture of Pd and Co3O4 nanoparticles results in lower activity than that of the supported 50 wt% Pd/Co3O4 catalyst, as shown in Fig. 5b, indicating that the simultaneous reduction of Pd and Co nitrates under MWI produces supported Pd nanoparticles on the Co3O4 nanoplates and not simply a mixture of Pd and Co3O4 nanoparticles. Figure 6a displays the XRD patterns of the Pd, Ni(OH)2, 30 wt% Pd/Ni(OH)2, 50 wt% Pd/Ni(OH)2, and 70 wt% Pd/Ni(OH)2 nanoparticles prepared by

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Fig. 6 a XRD patterns of Pd nanoparticles, Ni(OH)2 nanoplates, and Pd supported on Ni(OH)2 nanoplates. b XPS (Ni 2p) and c XPS (Pd 3d) of 50 wt% Pd/Ni(OH)2 catalyst

the HH reduction under MWI. The 100 % Ni(OH)2 nanoparticles show the characteristic peaks for the hexagonal phase (ICCD-00-001-1047). Small diffraction peaks due to Ni(OH)2 can be observed in the XRD pattern of the 30 wt% Pd/Ni(OH)2 sample as shown in Fig. 6a. However, for samples containing more than 30 wt% Pd, the XRD patterns are dominated by weak and broad Pd diffraction peaks. This may suggest the partial formation of a Pd–Ni alloy with the main diffraction peak between the 2h values corresponding to the 111 and 101 planes of Pd and Ni(OH)2, respectively. The formation of bimetallic alloys under the microwave-assisted synthesis of metal nanoparticles has been previously reported (Abdelsayed et al. 2009). Figure 6b shows that the binding energies of the Ni 2p1/2 and 2p3/2 electrons are 860.7 and 878.8 eV, respectively, indicating that Ni is present as Ni(OH)2 but there are peaks at 872.1, 853.3, and 855.2 eV

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indicating the presence of NiO at the surface of the nanoparticles. The Pd XPS spectra of the 50 wt% Pd/Ni(OH)2 catalyst shown in Fig. 6c indicate the presence of only 69 % Pd0 and higher percentage of Pd2? as compared to the other Pd catalysts supported on Fe3O4 and Co3O4 nanoplates. This could indicate inefficient reduction of the Pd ions in the presence of Ni(OH)2 under the microwave conditions. The partial formation of an oxidized Pd–Ni alloy suggested by the XRD results would also decrease the percentage of Pd0 in the Pd/Ni(OH)2 catalyst. Figure 7a compares the catalytic oxidation of CO over Ni(OH)2 and Pd nanoparticles as well as 30, 50, and 70 wt% Pd nanoparticles supported on Ni(OH)2. The pure Ni(OH)2 nanoparticles show low activity for CO oxidation with T50 and T100 occurring at 213 °C and 244 °C, respectively. This is very different from the activity of the Co3O4 nanoplates which exhibit


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Table 1 Temperature at which CO conversion reaches 50 % (T50) and 100 % (T100) for Pd, Fe3O4, Co3O4, and Ni(OH)2 nanoplates as well as the supported catalysts Pd/Fe3O4, Pd/ Co3O4, Pd/Ni(OH)2

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Fig. 7 a CO catalytic oxidation by Pd nanoparticles supported on Ni(OH)2 with different compositions. b Comparison between the activity of the 50 wt% Ni(OH)2 catalyst and a 50 wt% mixture of Pd and Ni(OH)2 nanoparticles

higher activities for CO oxidation with T50 and T100 occurring at 117 and 140 °C, respectively. The activity of Pd/Ni(OH)2 nanoparticles does not show much improvement over that of the unsupported Pd nanoparticles. For example, the best Pd/(NiO)2 catalyst (50 wt% Pd) has T50 and T100 occurring at 162 and 168 °C, respectively, which are not significantly different from the T50 and T100 of unsupported Pd nanoparticles (150 and 179 °C, respectively). This indicates a lack of favorable interactions between the Pd nanoparticles and the Ni(OH)2 support. This is consistent with the similar activity shown for the 50 wt% Pd/Ni(OH)2 catalyst and the physical mixture of Pd and Ni(OH)2 nanoparticles as shown in Fig. 7b, indicating that the simultaneous reduction of Pd and Ni nitrates under MWI produces weakly interacting Pd

nanoparticles and Ni(OH)2 nanoplates without strong catalyst-support interactions. This is also consistent with the stronger interactions between the Pd nanoparticles, and the iron or cobalt oxide nanoparticles which can directly enhance the activity of the supported Pd/Fe3O4 and Pd/Co3O4 catalysts for the catalytic oxidation of CO. Table 1 summarizes the catalytic activities of the Pd nanoparticle catalysts supported on Fe3O4, Co3O4, and Ni(OH)2 nanoplates, and Fig. 8 illustrates the effect of the Pd loading on the oxidation activities of the three catalyst systems. Both the Pd/Fe3O4 and Pd/ Co3O4 catalysts show high activity mainly due to stronger interactions between the Pd nanoparticles and the Fe3O4 and Co3O4 nanoplates. The Pd/Co3O4 catalyst shows improved activity over the Pd/Fe3O4 catalyst as indicated by achieving the same T100 of

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Fig. 8 Oxidation activity of the Pd nanoparticle catalysts supported on Fe3O4, Co3O4, and Ni(OH)2 nanoplates as a function of the Pd wt% in the supported catalyst

127 °C for the 50 wt% Pd/Fe3O4 catalyst but with only 30 wt% Pd in the Pd/Co3O4 catalyst. This can be attributed to the favorable oxidation activity of the Co3O4 support where 30 wt% Pd may represent the optimum composition needed to maximize the activities of both Pd and Co3O4 nanoparticles. It appears that when the Pd loading is higher than 40 %, the agglomeration of the Pd nanoparticles decreases the adsorption of CO and O2 on the active sites of the Co3O4 nanoplates, and this leads to lower activity. The lower activity of the Pd/Ni(OH)2 catalyst could be due to a weaker catalyst-support interaction that may result from the partial formation of Pd–Ni alloy with a few active sites for CO oxidation as compared to Pd nanoparticles. It should also be noted that the Pd catalysts supported on Fe3O4, Co3O4, and Ni(OH)2 nanoplates were not pre-reduced prior to the CO oxidation measurements and as indicated by the XPS results, the Pd2? species vary from 18, 14 to 31 % in 50 wt% Pd catalysts supported on Fe3O4, Co3O4, and Ni(OH)2 nanoplates, respectively. This shows that the prereduction step is not necessary in these catalysts which could simplify their practical uses.

Conclusions In conclusion, a simple, versatile, and rapid method has been developed for the synthesis of Pd

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nanoparticle catalysts supported on Fe3O4, Co3O4, and Ni(OH)2 nanoplates via MWI. The important advantage of microwave dielectric heating over convective heating is that the reactants can be added at room temperature (or slightly higher temperatures) without the need for high-temperature injection. Furthermore, the same method can be used to synthesize bimetallic nanoalloys supported on metal oxide nanoparticles as nanocatalysts for CO oxidation. The current results reveal that the Pd/Co3O4 catalyst has particularly high activity for CO oxidation as a result of the strong interaction between the Pd nanoparticles and the Co3O4 nanoplates. Optimizations of the size, composition, and shape of these catalysts could provide a new family of efficient nanocatalysts for the low temperature oxidation of CO. Acknowledgments We thank the National Science Foundation (CHE-0911146 and OISE-1002970) for the support of this work.

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