Formation of platinum sites on layered double hydroxide type basic ...

ISSN 00231584, Kinetics and Catalysis, 2014, Vol. 55, No. 6, pp. 786–792. © Pleiades Publishing, Ltd., 2014. Original Russian Text © O.B. Belskaya, T.I. Gulyaeva, V.P. Talsi, M.O. Kazakov, A.I. Nizovskii, A.V. Kalinkin, V.I. Bukhtiyarov, V.A. Likholobov, 2014, published in Kinetika i Kataliz, 2014, Vol. 55, No. 6, pp. 792–798.

Formation of Platinum Sites on Layered Double Hydroxide Type Basic Supports: III. Effect of the Mechanism of [PtCl6]2– Complex Binding to Aluminum–Magnesium Layered Double Hydroxides on the Properties of Supported Platinum in Pt/MgAlOx Catalysts O. B. Belskayaa, b, *, T. I. Gulyaevaa, V. P. Talsia, M. O. Kazakovc, A. I. Nizovskiib, c, A. V. Kalinkinc, V. I. Bukhtiyarovc, and V. A. Likholobova, b a

Institute of Hydrocarbons Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia b Omsk State Technical University, Omsk, 644050 Russia c Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Science, Novosibirsk, 630090 Russia *email: [email protected] Received April 28, 2014

Abstract—While synthesizing platinum catalysts supported on aluminum–magnesium oxides (Pt/MgAlOx), we established that, in the binding of the Pt(IV) chloro complex to aluminum–magnesium layered double hydroxides (LDHs), the mechanism of the metal complex–support interaction depends on the nature of the interlayer anion of the LDH. The synthesis may yield chemically identical Pt/MgAlOx samples differing in the particle size and electronic structure of supported platinum. The higher dehydrogenating activity of the catalyst obtained by binding the [PtCl6]2– complex in the interlayer space of LDH via exchange with inter layer OH– anions is possibly due to the larger proportion of metallic platinum (Pt0) in this catalyst. In the cat alyst prepared from hydrolyzed platinum complex species using LDH with CO32− interlayer anions, platinum is mainly in an oxidized state similar to Pt2+. DOI: 10.1134/S0023158414060020

Platinum (palladium) catalysts supported on alu minum–magnesium oxide materials are widely used in basecatalyzed reactions (condensation of alcohols and carbonyl compounds) [1–4] and in the conversion of hydrocarbons [5–10]. The formation of the proper ties of the catalysts may depend considerably on the structure of the precursor of the oxide support, namely, layered double hydroxide (LDH). The general formula of LDHs (or hydrotalcitelike compounds) is [M12−+x M3x+ (OH)2]x+[An–]x/n ⋅ yH2O, and they consist of brucitelike layers in which part of the divalent cations (M2+) is isomorphically substituted by trivalent cations (M3+) that have a similar ionic radius. The excess pos itive charge of the layers is compensated by hydrated Аn– anions located in interlayer spaces. With this type of hydroxide precursor, the M2+ and M3+ cations undergo uniform distribution during the subsequent formation of the mixed oxide phase. In addition, anions with different charges and sizes can be intro duced into the LDH structure and can be substituted by other ones without breaking the layered structure. This provides means both to tune the textural charac teristics of the oxide phase and to introduce the neces

sary amount of active component into the anionic complexes in the preparation of supported metal cata lysts. In our earlier studies [11, 12], the precursors of the aluminum–magnesium oxide support were LDHs with a fixed Mg/Al molar ratio of 3.3, whose interlayer space mainly contained CO32− anions (MgAl–CO3) or ОН– anions (MgAl–OH). It was demonstrated that the difference in the nature of the interlayer anion can be used both to prepare oxide supports that have the same chemical composition but differ in pore space organization [11] and to alter the mechanism of the platinum complex–LDH interaction and supported platinum particles formation conditions. Here, we report how the properties of the Pt/MgAlOx catalyst depend on the complex–support interaction mechanism, which, in turn, depends on the nature of the interlayer anion. The model reaction in catalytic tests was propane dehydrogenation, which is of significance in industrial catalysis and needs non acid catalysts [13–19].

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EXPERIMENTAL The synthesis of the LDH with carbonate counte rions, which was detailed in our previous report [11], consisted of the coprecipitation of Mg2+ and Al3+ hydroxides from diluted nitrate solutions by reacting the latter with solutions containing carbonate and hydroxide ions. For obtaining a sample mainly con taining hydroxide counterions, the synthesized MgAl–СО3 was calcined at 600°С and was rehydrated at 120°C for 2 h. Both samples—MgAl–СО3 and MgAl–OH—had the same magnesiumtoaluminum molar ratio, Mg/Al = 3.3, and contained 41.3 wt % Mg and 13.7 wt % Al after their calcination at 600°С. The concentrations of the metals in the solution after the dissolution of the samples were determined by atomic absorption spectroscopy on an AA6300 spec trometer (Shimadzu, Japan). Chloroplatinic acid Н2[PtCl6] was sorbed from an excess of its aqueous solutions at room temperature. (The platinum concentration in the impregnating solutions was 1.5 or 10.5 mmol/L.) The platinum con centration in the solutions before and after the sorp tion of the metal complex was determined spectropho tometrically [20]. Prior to measuring the platinum particle size, recording photoelectron spectra, per forming catalytic tests, and determining the supported metal content, the samples with supported complexes were calcined and reduced with flowing hydrogen at 550°С. Depending on the nature of the interlayer anion, thermally activated samples were designated Pt/MgAlОх(CO32− ) or Pt/MgAlОх(OH–). Magic angle spinning NMR (195Pt MAS NMR) spectra were recorded on an Àvance400 spectrometer (Bruker, Germany) using an SB4 multinuclear probe (Bruker, Germany). The external standard was an aqueous solution of H2[PtCl6] (0.03 mol Pt/L). The platinum particle size in reduced samples was determined by pulse chemisorption of Н2 molecules at room temperature using an AutoChem2920 instru ment (Micromeritics, United States) under the assumption that the chemisorption stoichiometry is [Pt] : [H] = 1 : 1 [21]. Xray photoelectron spectroscopy (XPS) was car ried out on a spectrometer (SPECS, Germany) fitted with several isolated vacuum chambers for quick intro duction of samples and for their heat treatment and characterization. Samples were transferred from one chamber to another without being exposed to the atmosphere. Photoelectron spectra were obtained in the analyzer chamber at a residual pressure of 5 × 10 ⎯ 9 Torr. The spectrum of each sample was recorded using MgKα (hν = 1253.6 eV) and AgLα (hν = 2984.3 eV) radiation. The binding energy scale of the spectrometer was precalibrated against the gold metal line Au4f7/2 (84.0 eV) and copper metal line Cu2p3/2 (932.6 eV). For either radiation, the spectra of non conductive samples were calibrated against the C1s line, for which the binding energy was accepted to be KINETICS AND CATALYSIS

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284.8 eV. The photoelectron spectra of platinum were deconvolved using the standard program XPSPeak [22]. This procedure was begun by distinguishing the component characterizing the metallic state of plati num (BE = 2121.8 eV). The component correspond ing to the oxide state was then found automatically. Propane dehydrogenation was carried out in a flow reactor with a fixed catalyst bed (1 g of catalyst) at 550°C, atmospheric pressure, Н2/С3Н8 = 0.25 mol/mol, −1 and a feed hourly space velocity of 4 g g Cat h–1. Catalyst pretreatment included calcination in air at 550°C and reduction in flowing hydrogen at 550°C. The duration of each dehydrogenation run was 5 h. The product was analyzed online on a Khromos GKh1000 gas chro matograph (Khromos, Russia) fitted with a 50mlong RtAlumina PLOT column and a flameionization detector. The specific activity of catalysts was calcu lated as the number of reacted propane molecules per surface platinum atom per hour, with the number of surface platinum atoms calculated using platinum dis persion data obtained by the pulse chemisorption method [23]. The deactivation parameter (DP) was estimated in a similar way [23] using initial and final propane conversion (X) data: DP = (Xin – Xfin)/Xin × 100.

RESULTS AND DISCUSSION The existing views of how anions bind to hydrotal citetype LDHs include both their exchange for inter layer anions and their interaction with OHgroups of brucitelike hydroxide layers [24]. Whether one mech anism or the other takes place is determined by the nature of the interlayer anion in the LDH. Interlayer carbonate anions have an extremely low mobility: they readily displace other anions from the interlayer space but are hardly ionexchangeable. This specific feature of carbonate anions has not been unambiguously explained, but it is most commonly attributed to the formation of М–ОСO 2− bonds via the interaction of the interlayer carbonate ions with OH groups of hydroxide layers [25]. Earlier [12], we observed a low exchange capacity of the carbonate form of the Mg/Al = 3.3 LDH with respect to doubly charged anionic platinum(IV) chloro complexes. For this rea son, for hexachloroplatinate binding in the interlayer space, we presynthesized MgAl–OH by hydrating the aluminum–magnesium mixed oxide with distilled water (making use of the “memory effect”). The inter layer space of this LDH mainly contained ОН– ions, which could be exchanged for [PtCl6]2–. This enabled us to load the interlayer space with the necessary amount of metal complex, which was close to the sto ichiometric amount corresponding to the LDH for mula. The fact of the intercalation of the doubly charged complex anion [PtCl6]2– via exchange for

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BELSKAYA et al. (а) [PtCl6]

2

2–

[PtCl5(OH)]2–

1

1000

800

600 400 δ, ppm (b)

200

0 [PtCl6]2–

[PtCl5(OH)]2–

2 1 1000

800

600 400 δ, ppm

200

0

Fig. 1. 195Pt MAS NMR spectra of platinum(IV) com plexes adsorbed on MgAl–OH: (a) samples containing (1) 2 and (2) 5 wt % Pt, dried at 25°C for 48 h; (b) samples containing 10 wt % Pt, dried at 25°C for (1) 48 h and (2) 300 days.

interlayer ОН– ions was indicated by an increase in the interplanar spacing in the LDH and by the formation of planar platinum particles in the reduced samples [12]. In this study, we used 195Pt MAS NMR spectros copy to reveal the difference between the metal com plex binding mechanisms in the hydroxide and car bonate forms of the MgAl LDH. This method is known to provide information both on the composi tion of adsorbed complexes and on the character of their interaction with the surface [26, 27]. Studies of the [PtCl6]2– complex adsorbed on the support surface demonstrated that 195Pt NMR signals are observed only when the octahedral symmetry of the complex is retained upon adsorption or is only slightly distorted

(formation of outersphere complexes takes place) [26, 27]. For example, in the coordination binding of the complex to alumina, when one or several chloride ligands of the [PtCl6]2– anion are replaced by hydroxyl groups of the support (formation of innersphere com plexes occurs), the NMR peaks weaken considerably and broaden or no NMR signals can be seen [26–28]. Just this situation was observed in the adsorption of the complex on the LDH with carbonate counterions, MgAl–CO3. Hydrolyzed species of platinum com plexes were present on the surface (according to elec tron spectroscopy data [12]), and no 195Pt NMR sig nals from the adsorbed complexes could be obtained. At the same time, with MgAl–OH as the support, the spectrum shows a strong peak due to the chloro complex [PtCl6]2– and a weak signal whose position, according to earlier studies [26, 27], is typical of the monosubstituted chlorohydroxo complex [PtCl5(OH)]2– (Fig. 1). Therefore, the binding of plat inum(IV) complexes to the surface of this support does not cause significant changes in their chemical com position or a distortion of their geometry and is likely due to the Coulomb interaction of the anionic com plexes with positively charged brucitelike layers. Note that these signals are present in the NMR spectrum of [PtCl6]2–/MgAl–OH samples not only at high bound metal complex concentrations of 10 and 5 wt % Pt (Fig. 1a, spectrum 2; Fig. 1b), but also at a low complex concentration of 2 wt % Pt (Fig. 1a, spectrum 1). In addition, as distinct from the NMR spectra of platinum(IV) chloro complexes adsorbed on Al2O3 [29], the spectra considered here do not show signal broadening or weakening as the sample is stored for a long time. For example, as [PtCl6]2–/Al2O3 was being dried, the NMR signals weakened and finally died away [29]. The first species to be coordinatively bound were hydrolyzed complexes, as was indicated by the disappearance of the corresponding signals. As the [PtCl6]2–/MgAl–OH samples are dried, not only does the intensity of its NMR signals remain invariable, but also an increase in the proportion of the hydrolyzed form of the complex is observed (Fig. 1b, spectrum 2). It is significant that, because the LDHs with ОН– and CO32− interlayer anions differ in precursor binding mechanism and location, the platinum sites forming in these LDHs differ in dispersion and catalytic prop erties. The dispersion of supported platinum was determined by chemisorption of hydrogen molecules for Pt/MgAlOx samples that contained 0.3 and 2.0 wt % Pt, had the same oxide support composition (Mg/Al = 3.3), and differed only in the nature of inter layer anions in the hydroxide precursor—MgAl–СО3 and MgAl–OH (Table 1). Before chemisorption mea surements, some LDH samples containing adsorbed complexes were dried at 120°С and the others were calcined at 550°C, so either bound complexes or plat KINETICS AND CATALYSIS

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Table 1. Dispersion and specific activity data for platinum in the Pt/MgAlOx catalysts obtained from the carbonate and hydroxide forms of MgAl LDH Sample no.

Sample composition

T(O2) after the deposition of a complex, °C

Dispersion of platinum D(H2), %

TOF, mol C3H8 (at Pts)–1 min–1

1

2% Pt/MgAlOx ( CO 3 )

2–

120

53

–

2

2% Pt/MgAlOx(OH–)

120

26

–

3


2–

550

56

–

4

2% Pt/MgAlOx(OH–)

550

26

–

5

0.3% Pt/MgAlOx ( CO 3 )

2–

550

73

11.5

6

0.3% Pt/MgAlOx(OH–)

550

23

59.1

inum oxide species were subjected to subsequent reduction with hydrogen at 550°C. It follows from the chemisorption data that, irrespective of the metal con tent and pretreatment conditions, the dispersion of platinum particles obtained from the carbonate pre cursor Pt/MgAlОх(CO 32 − ) is higher than that of plati num particles obtained from Pt/MgAlОх(OH–), in which the chloroplatinate ion is mainly bound in the interlayer space of the support. However, the assumption that part of the platinum in the support bulk is blocked upon the ionexchange binding of the precursor is at variance with the cata lytic testing data for the Pt/MgAlОх samples. Although the degree of dispersion of platinum in Pt/MgAlOx(ОН–) is lower, this sample affords a higher propane conversion (Fig. 2). The specific catalytic activity (in terms of TOF) for this sample, which was calculated with the dispersion of platinum particles taken into account, is higher than the activity of Pt/MgAlOx(CO32− ) by a factor larger than 5 (Table 1). Furthermore, while the two catalysts show similar and rather high propane selectivities (94%), the Pt/MgA lOx(ОН–) catalyst is more stable. DP calculations dem onstrated that, with Pt/MgAlOx(ОН–), the propane conversion decreased by 20.5 rel. % in 5 h, while the decrease in propane conversion for Pt/MgAlOx(CO32− ) was 33.2 rel. %. In order to understand why supported platinum shows different activities in the catalysts that have the same chemical composition but were prepared using LDHs with different ionexchange properties and metal complex–support interaction mechanisms, we examined the catalysts by XPS. This method is very informative in the investigation of the electronic state of the surface of heterogeneous catalysts. A significant KINETICS AND CATALYSIS

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limitation encountered in the XPS study of platinum catalysts supported on aluminumcontaining materi als (including petroleum refining catalysts based on Pt/Al2O3) arises from the overlap between the most intense line of platinum (Pt4f) and the Al2p line of the support in the spectra obtained using an Al or Mg anode Xray tube [30, 31]. However, in the case of a harder radiation, AgLα, with hν = 2984.3 eV, the strongest lines in the photoelectron spectra are Al1s (BE ≈ 1560 eV) and Pt3d5/2 (BE ≈ 2123 eV). This elim inates the problem of the overlap between the active Conversion, % 25

C4H6 formation selectivity, %

100 4

20

3 90

15

80 2 70

10 1

60

5

50 0

1

2 3 Time, h

4

5

Fig. 2. (1, 2) Propane conversion and (3, 4) propylene for 2−

mation selectivity for (1, 4) 0.3% Pt/MgAlOx(CO3 ) and (2, 3) 0.3% Pt/MgAlOx(ОН–).

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Table 2. XPS data for Pt/MgAlOx samples Pt3d5/2 binding energy, eV

Atomic ratios

Sample Pt0

Pt2+

Pt0/Pt2+

Pt/Al

Cl/Al


2–

2121.8

2123.5

0.6

0.018

0.023

2% Pt/MgAlOx(OH–)

2121.9

2123.5

1.3

0.010

0.028

The samples were calcined at 550°C and reduced at 550°C.

component and support lines. Monochromation of the radiation causes a slight decrease in the energy halfwidth of the initially broad AgLα line and makes it possible to reliably interpret data concerning the elec tronic state of platinum: for platinum metal, BE = 2121.8 eV; for Pt2+ (K2PtCl4), BE = 2123.5 eV; for Pt4+ (K2PtCl6), BE = 2125.3 eV [30]. The other elements making up the catalysts (Mg, Al, Cl) were character ized via a standard XPS technique using MgKα radia tion. Reference data [32] were used to calculate the atomic ratios of these elements and to determine their electronic states. In this study, use of AgLα radiation made it possible to analyze data in the Pt3d5/2 spectral region for 2% Pt/MgAlОх(CO32− ) and 2% Pt/MgAlОх(OH–) (Table 1, samples 3, 4), whose preparation included oxidative and reductive treatments at 550°С. It was found that a satisfactory description of the Xray photoelectron spectra of these samples is possible only under the assumption that platinum in different electronic states characterized by BE = 2121.8 eV (metallic platinum) and BE = 2123.5 eV (oxidized platinum species with a charge of 2+) is present on the catalyst surface (Table 2). The Pt/MgAlOx(OH–) sample (analogue of the more active, lowpercentage catalyst) obtained from the nonhydrolyzed chloride precursor localized in the interlayer space mainly contained metallic plat inum (Fig. 3b, Table 2). The 2% Pt/MgAlОх(CO 32 − ) catalyst prepared from the hydrolyzed complexes was dominated by oxidized platinum species, and the Pt0/Pt2+ integrated intensity ratio for this catalyst was 0.6 (Fig. 3a). The same state of supported platinum, with a considerable proportion of oxidized species, was earlier obtained by the surface hydrolysis of adsorbed chloroplatinate in the presence of palla dium(II) complexes under hydrothermal conditions (Fig. 3c). It was also demonstrated that the oxidized platinum species, unlike Pt0 are inactive in benzene hydrogenation [33]. The data indicating a higher propane dehydroge nation activity of the catalysts dominated by metallic

platinum are in good agreement with the results of an earlier work [23]. In that work, changing the func tional cover of alumina by hydrothermal treatment of the support increased the contribution from the Cou lomb interaction between the chloroplatinate and the support and led to the formation of outersphere plat inum complexes. As a result, the Pt3d5/2 peak in the photoelectron spectra of the reduced catalyst samples was shifted to lower energies, indicating a larger pro portion of reduced metal species (Pt0) [23]. The surface Pt/Al atomic ratio determined by XPS varies from one catalyst to another (Table 2). At a given metal content, the Pt/Al ratio is noticeably smaller for the 2% Pt/MgAlОх(OH–) sample. This is in agree ment with the smaller proportion of surface platinum atoms accessible to chemisorbing hydrogen molecules (lower dispersion of platinum) in this sample. It is sig nificant that the difference between the states of plati num in the 2% Pt/MgAlOx(CO32− ) and 2% Pt/MgAlОх(OH–) samples cannot be attributed to the difference between their chemical compositions, spe cifically, to the difference between the concentrations of chloride ions, which can affect the activity of the catalysts. According to the data presented in Table 2, the Cl/Al atomic ratios in the reduced samples are comparable to 0.028, the value determined earlier for platinum/alumina catalysts [23]. Thus, this study demonstrated that interlayer anion selection in the synthesis of LDHs provides means to alter the metal complex–support interaction mecha nism and thereby change the localization of the adsorbed active component precursor. As a result, at a given chemical composition of the Pt/MgAlOx cata lyst, it is possible to obtain platinum particles differing in their size and electronic state. This report for the first time presents XPS data concerning the state of supported platinum in the Pt/MgAlOx catalysts. It was demonstrated that the higher dehydrogenating activity of the catalyst prepared from the chloride precursor localized in the interlayer space of the LDH can be due to the larger proportion of metallic platinum (Pt0) in this catalyst. At the same time, the catalyst synthesized KINETICS AND CATALYSIS

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from the hydrolyzed complexes is dominated by oxi dized platinum species similar to Pt2+. A correlation was established between the charac ter of binding of the metal complex precursor of the active component and the state and properties of the supported metal. This correlation can be used to tune the properties of the Pt/MgAlOx catalysts prepared using an LDH.

(а) 2+

Pt0

2110

Pt

2120 2125 Binding energy, eV (b) Pt0 Pt2+

2115

791

2130

2135

ACKNOWLEDGMENTS The authors are grateful to O.V. Maevskaya for her participation in catalyst preparation and to T.V. Kire eva and A.V. Shilova for performing the elemental analysis of the synthesized materials. This work was supported by a grant from the Presi dent of the Russian Federation for young scientists and leading scientific schools of the Russian Federa tion (project no. NSh3631.2014.3). REFERENCES

2110

2115

2120 2125 Binding energy, eV (c) Pt2+ Pt0

2130

2135

2110

2115

2120 2125 Binding energy, eV

2130

2135

Fig. 3. Pt3d5/2 photoelectron spectra of (a) 2% 2−

Pt/MgAlОх(CO3 ) , (b) 2% Pt/MgAlOx(OH–), and (c) 1% PtPd/Al2O3 (hydrothermally treated) [32]. The samples were calcined at 550°С and reduced at 550°С. KINETICS AND CATALYSIS

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Translated by D. Zvukov

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