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Ga2O3 model catalysts of selective liquid-phase hydrogenation of acetylene to ethylene
Journal of Molecular Catalysis A: Chemical 358 (2012) 152–158
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EXAFS study of Pd/Ga2 O3 model catalysts of selective liquid-phase hydrogenation of acetylene to ethylene N.S. Smirnova a,∗ , D.A. Shlyapin a , O.O. Mironenko a , E.A. Anoshkina a , V.L. Temerev a , N.B. Shitova a , D.I. Kochubey b , P.G. Tsyrul’nikov a a b
Institute of Hydrocarbons Processing of Siberian Branch of Russian Academy of Sciences, Neftezavodskaya Str. 54, Omsk 644040, Russia Boreskov Institute of Catalysis, Prosp. Akad. Lavrentieva 5, Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 15 November 2011 Received in revised form 12 March 2012 Accepted 12 March 2012 Available online 21 March 2012 Keywords: EXAFS spectroscopy Pd/Ga2 O3 catalyst Pd–Ga alloy Selective hydrogenation of acetylene
a b s t r a c t Model catalysts Pd/Ga2 O3 of selective liquid-phase hydrogenation of acetylene to ethylene were prepared by impregnation of -Ga2 O3 with a palladium nitrate solution with following drying and reduction in a hydrogen flow at 200 ◦ C. A part of the samples was then calcined in an argon flow at 200–500 ◦ C. The asprepared samples were tested in the reaction of selective liquid-phase hydrogenation of acetylene. EXAFS was used to study structure of the catalyst components. Based on the EXAFS data it was found that Pd–Ga alloys present in the subsurface region of Pd particles. The presence of these alloys increases activity and selectivity of catalysts. The alloys are decomposed by calcination in inert atmosphere following decrease in catalytic activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Processing of natural and oil-dissolving gases into more valuable products is one of the key problems at the moment. In addition to the traditional technologies (using a stage of the intermediate synthesis gas production), a number of natural gas processing technologies have been developed in the last decade. Thus, the known experimental technology of Synfuels Int. Inc. [1] involves the production of acetylene by oxidative pyrolysis of natural gas with following catalytic liquid-phase hydrogenation of acetylene into ethylene and oligomerization of the latter to yield motor fuel components. The key process stage is the selective hydrogenation of acetylene to ethylene. Palladium supported on Al2 O3 is traditionally used as a catalyst of selective acetylene hydrogenation [2]. In patents [3,4], the mention is made of Ga and In modifiers used to improve the process selectivity to ethylene. Active components Pd–Ga and Pd–In supported on Al2 O3 exhibit high activity and selectivity in the liquid-phase hydrogenation of acetylene to ethylene and outperform catalyst (Pd–Ag)/Al2 O3 [4], however, the modifying effect of IIIA group oxides on palladium in the above systems has not been adequately studied.
∗ Corresponding author. Tel.: +7 3812 672 275; fax: +7 3812 646 156. E-mail addresses: ns [email protected] (N.S. Smirnova), [email protected] (D.I. Kochubey). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2012.03.010
In [5–8], palladium and gallium intermetallic compounds (Pd3 Ga7 , PdGa) were studied as highly selective catalysts of acetylene hydrogenation. An increase in the catalyst stability and selectivity to ethylene was attributed to “isolation” of palladium centers of acetylene adsorption by gallium atoms and this changes the nature of binding between acetylene and the active center [7,9]. In contrast to metallic palladium, Pd–Ga intermetallic compounds do not form hydrides, which results in the additional increase in selectivity to ethylene due to suppression of hydrogenation to ethane [5,9]. On the other hand, Pd/Ga2 O3 catalysts are known to exhibit high selectivity in several reactions involving hydrogen, such as the ethanol dehydrogenation [10] and hydrogenation of acetonitrile to ethylamine [11]. Pd–Ga alloys supported on the carbon nanotubes are highly active in the hydrogenation of CO2 to methanol [12]. Adding of Ga to catalyst Pd/SiO2 [13] increases the catalysts activity in this reaction. In [14], Pd/Ga2 O3 catalysts were studied in the reaction of steam methanol reforming. It was shown that the palladium particles in the reduced catalysts consist of a monometallic palladium core with an outer layer formed by intermetallide or an alloy. Unfortunately, the conclusions on the structure of the catalysts reduced at temperatures below 250 ◦ C were drawn from the electron-microscopy data, which could not evidence the formation of disordered alloys. On the contrary, EXAFS can elucidate the nature of interaction between palladium and gallium. In this work, we used sample Pd/Ga2 O3 with a concentration of palladium approximating that in the industrial catalysts to study
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the probability of formation of bimetallic interaction phases in the palladium catalysts modified by gallium. The aim of the work was to use EXAFS for studying catalytic properties of the palladium–gallium catalysts in the selective liquid-phase hydrogenation of acetylene to ethylene as well as structure and composition of the active centers in catalysts Pd/Ga2 O3 in dependence of the preparation conditions. Literature data [15] suggest that the stoichiometric Pd–Ga intermetallides are generated by hydrogen reduction of PdO/Ga2 O3 at temperatures higher than 250 ◦ C. In this work, the catalysts were reduced at temperatures ≤200 ◦ C. The diffraction methods do not show intermetallide phases for the above samples, whereas activity and selectivity of the palladium catalysts supported on Ga2 O3 during hydrogenation of acetylene are higher than for the catalysts on other supports, -Al2 O3 , as an example [16].
2. Experimental 2.1. Preparation of the samples Samples 1% Pd/Ga2 O3 were synthesized by impregnation of -Ga2 O3 (chemically pure) with a solution of Pd(NO3 )2 following by drying at 120 ◦ C for 2 h. After drying, the samples were reduced in a hydrogen flow at 200 ◦ C during 3 h. Then the samples were additionally calcined in an argon flow (initial purity 99.997%, the residue oxygen content was 5 × 10−4 vol.%, additional cleaning was performed by passing argon through copper shavings at 300 ◦ C). After additional calcination at 200, 300, 400 and 500 ◦ C in argon during 3 h (Table 1), the samples were kept in argon.
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2.4. Specific surface area Specific surface areas of the initial support and the catalyst containing 1 wt.% of Pd was determined by the BET method from the one-point adsorption of nitrogen at 77 K. 2.5. EXAFS EXAFS spectra of the PdK edge absorption were taken at the EXAFS spectroscopy Station of the Siberian Synchrotron Radiation Center (Novosibirsk). The spectra were registered by synchrotron radiation, the electron energy was 2 GeV in a VEPP-3 storage, the average current was 70 mA. A cut-off crystal of Si (1 1 1) was used as a monochromator. All spectra were taken in the fluorescence mode with the steps of 2.5 eV. To register X-ray radiation, we used the ionization chambers filled with argon and an X-ray fluorescent detector. The spectra were analyzed by program Viper using the standard method [18]. The spectra were analyzed as k2 (k) in the wave numbers range 2.50–12.00 A˚ −1 . The background was removed by extrapolating of the pre-edge absorption region to the EXAFS region using the Victoreen polynomials. To calculate a smooth part of the absorption coefficient, we used the approximation based on the three cubic spline functions. The inflection point at the absorption edge was taken as the initial point of EXAFS spectrum E0 . Quantum-chemical data required for calculations of structural parameters were obtained by program FEFF-7. Data on the structure of complexes were taken from the Inorganic Crystal Structure Database (ICSD). We also registered a reference spectrum of the K-edge of palladium absorption using the palladium foil. 3. Results and discussion
2.2. Catalytic tests
3.1. Features of the support
The samples were tested in a gradientless, shaken, flow and temperature-controlled reactor at the following conditions: reaction temperature 40 ◦ C, total flow of the gas mixture was 100 ml/min, sweep frequency of the reactor ≥7–8 s−1 , catalyst weight 10 mg, solvent volume 8 ml. N-methylpyrrolidone was used as a solvent. The reaction gas mixture involved: C2 H2 4 vol.%, H2 90 vol.% and He (balance) 6 vol.%. The experiment lasted for ∼220 min. The initial reaction mixture and the resulting product mixture were analyzed by a «Chromos GH-1000» analyzer using a capillary column with SiO2 (25 m × 0.32 mm, the operation temperature 60 ◦ C) and a flame-ionizing detector. Nitrogen was used as carrier gas, the frequency of gas sampling was 33 min. During the experiment, five-six chromatograms of the initial mixture (IM) and resulting mixture (RM) were taken. From the corresponding peak areas we calculated conversion of acetylene (X, %), selectivity (S, %) to ethylene and analyzed the dynamics of changes of the above values vs. testing time [17]. To compare the characteristics of different samples, we used the values obtained in the range of stationary activity of the catalysts. At the above conditions, the main products of the acetylene hydrogenation are ethylene and oligomers Cn≥4 .
As follows from XRD data, gallium oxide exists as -Ga2 O3 ˚ The average coherent scat(C2/m, a = 12.224, b = 3.038, c = 5.806 A). tering domains (CSD) calculated by the Selyakov–Scherrer equation was 31.7 nm. The calculation of CSD with consideration for different directions, represented by peaks on the XRD pattern, gives the following results for planes: (4 0 0) – 36.1 nm, (0 2 0) – 25.7 nm and (0 0 2) – 39.1 nm. The specific surface area of the initial gallium oxide (SBET ) was 14 m2 /g. This value held as 1 wt.% palladium was supported.
2.3. X-ray analysis Phase composition of gallium oxide was determined by powder diffractometry using a D8 Advance (Bruker) diffractometer with K␣ emission of copper. Phase composition of the active component was not determined because of low palladium concentration (1 wt.%).
3.2. Catalytic tests of the prepared samples Table 1 shows the results of catalytic tests of the initial sample after drying, the initial reduced sample and the samples after additional calcination at different temperatures in argon. In [14], the catalyst calcination in oxygen resulted in the palladium oxidation. In our case, calcination in argon was performed to verify the suggestion that thermal treatments can increase the interaction between metallic palladium and the reduced gallium oxide. In the liquid-phase selective hydrogenation of acetylene, the samples were tested in the kinetic region. In this case, the acetylene conversion was not higher than 30% for the most active sample. The chromatographic data suggest that for all catalysts the selectivity to ethane is close to 0. Pure -Ga2 O3 did not show catalytic activity at our reaction conditions. As follows from the table data, the amount of oligomers (hydrocarbons Cx Hy , where x ≥ 4) is high in all cases. Several reasons are responsible for this observation. When the reaction is performed at 40 ◦ C, solubility of acetylene in N-methylpyrrolidone is high (about 23 m3 /m3 normal conditions) [19], whereas the catalyst activity is low (Table 1). In combination, this provides high concentration
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Table 1 Catalytic properties of 1% Pd/Ga2 O3 samples. Sample number
Preparation conditions
XC2 H2 (%)
SC2 H4 (%)
SC4 (%)
1 2 3 4 5 6
After drying (not reduced) 200 ◦ C, H2 200 ◦ C, H2 + 200 ◦ C, Ar 200 ◦ C, H2 + 300 ◦ C, Ar 200 ◦ C, H2 + 400 ◦ C, Ar 200 ◦ C, H2 + 500 ◦ C, Ar
17 27 24 17 10 9
38 57 53 44 38 26
62 43 47 56 62 74
of acetylene near the catalyst surface during the reaction. Excess acetylene competitively inhibits the hydrogen adsorption on the palladium surface and subsequent hydrogenation of ethylene to ethane, but it provides oligomerization [17]. Besides, acetylene is adsorbed about 2000-times stronger than ethylene and displaces it from the surface, preventing further hydrogenation to ethane [17]. As follows from IR spectroscopy, unreduced sample 1% Pd/Ga2 O3 , containing palladium oxide and several nitro groups, is active in the liquid-phase hydrogenation of acetylene at the abovedescribed conditions (acetylene conversion is 17%). This is probably caused by partial reduction of the catalyst at the reaction conditions, since it is well known that palladium oxide can be easily reduced by hydrogen to metallic palladium even at room temperature [20]. It should be mentioned that the reduction is accompanied by catalyst deactivation by oligomers during the reaction. The highest activity and selectivity to ethylene (X = 27–24% and S = 57–53%, see Table 1) were shown by sample 2 (reduced in hydrogen at 200 ◦ C) and sample 3 (additionally calcined at 200 ◦ C in argon). According to [6], Pd–Ga intermetallic compounds are active and selective in the gas phase hydrogenation of acetylene. High catalytic properties are associated with peculiar features of intermetallide structure provided in “isolation” of palladium active centers. These centers bind acetylene molecules more weakly and acetylene hydrogenates to ethylene easier. Gallium modifies the palladium centers resulting in changes of adsorption and desorption energies of both acetylene and ethylene. It should be noted that in contrast to metal palladium, Pd–Ga intermetallides do not form palladium hydrides which are unselective in hydrogenation of acetylene to ethylene. As the calcination temperature increases from 300 ◦ C to 500 ◦ C, the ethylene conversion decreases from 17 to 9%. Note that selectivity to ethylene decreases from 44 to 26% and the number of oligomers in the reaction mixture increases. The sample calcined at 500 ◦ C in argon flow exhibits lower activity and selectivity to ethylene than the initial unreduced catalyst. The sample calcined at 400 ◦ C and unreduced Pd/Ga2 O3 are comparable in their catalytic properties. The obtained catalytic data permit a suggestion that the number of active centers decreases (XC2 H2 decreases) with an increase of calcination temperature in argon. It is likely that the nature of active centers changes, which manifests itself as a decrease in selectivity to ethylene and an increase in selectivity to oligomers. A decrease in the number of active centers can be caused by changes in the surface composition of palladium particles (see below) and a decrease in dispersion of the reduced palladium sample (sintering). According to [5,6,9,21], many active centers are closely set on the large palladium particles, which provides strong sorption of acetylene molecules on the neighboring centers with following formation of hydrooligomers which block the catalyst active centers. Several publications are devoted to systems Ir/Ga2 O3 , Pt/Ga2 O3 and Ru/Ga2 O3 [22,23]. For system Ir/Ga2 O3 heated at 260 ◦ C, the support partial reduction results in “decoration” (overgrowth) of the active component particles with a thin gallium oxide layer. The formation of bulk alloy phase IrGa proceeds at higher temperature, 580 ◦ C, in the reducing atmosphere [22]. Transmission electron microscopy (TEM) and secondary ion mass spectroscopy
(SIMS) data of the Pt/Ga2 O3 sample reduced at 300 ◦ C suggest a shift of partially reduced GaOx to the surface of a platinum particle by the SMSI mechanism («strong metal-support interaction») [23]. At the same time, no significant sintering of metallic palladium particles was observed. For thorough analysis of the observed regularities, EXAFS data were used. 3.3. EXAFS data on catalysts 3.3.1. Sample Pd/Ga2 O3 treated by drying ˚ Fig. 1 shows a curve of radial atom distribution (scale R–ı (A)) of the PdK edge of unreduced sample 1% Pd/Ga2 O3 . The model spectrum shows the presence of distances Pd–O ˚ Pd–Pd (3.04 A), ˚ Pd–Pd (3.46 A), ˚ belonging to palladium (2.03 A), oxide (ICSD No. 29281), in the palladium surrounding. Metallic palladium is not observed in the sample. The RDF curve also exhibits ˚ a peak modeled by distance Pd–Ga (4.51 A). The structure of massive PdO is formed by linear chains of ions (PdO4 )2− in the planar coordination with a Pd–O–Pd distance of ˚ which are interconnected in packages. A distance of 3.46 A˚ 3.03 A, is the distance between such chains. In PdO the coordination numbers for distances 3.03 and 3.46 A˚ are 4 and 8, respectively (ICSD No. 29281). As follows from modeling, for distances Pd–Pd 3.03 A˚ ˚ the coordination number is 1.7 and 2.1, respectively. and 3.46 A, The structure of -gallium oxide exhibits distances Ga–Ga at 3.03 A˚ ˚ which are similar to distances Pd–Pd (3.03 A˚ and 3.42 A) ˚ and 3.44 A, in the palladium oxide. It can be assumed that on the Ga2 O3 surface palladium mainly exists as oxide chains situated along axis b of Ga2 O3 unit cell. The distances between gallium atoms in ˚ This Ga2 O3 structure (ICSD No. 83645) along axis b are 3.03 A. facilitates the growth of palladium oxide chains along this direction
Fig. 1. Model spectrum of the PdK edge in unreduced sample 1% Pd/Ga2 O3 . Dotted line shows a simulated RDF curve, a firm line shows the experimental spectrum.
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Fig. 2. Model incorporation of palladium atoms into the structure of gallium oxide.
in -Ga2 O3 . Some chains coordinate between themselves, which ˚ explains the presence of distance Pd–Pd 3.46 A. Epitaxial stabilization of the palladium oxide particles along axis b in -Ga2 O3 , accompanied by fixation of palladium atoms in the crystallographic positions of gallium atoms, explains distance ˚ on the RDF curve which is close to distance Ga–Ga Pd–Ga (4.51 A) ˚ in Fig. 2. in the gallium oxide structure (4.46 A) 3.3.2. Sample Pd/Ga2 O3 reduced in hydrogen Modeling of a spectrum with an assumption that the sample contains only metallic palladium (ICSD No. 41517) does not provide positive results, since the position of peak on the experimental spectrum (scale R–ı), corresponding to distance ∼4.93 A˚ (Fig. 3), dif˚ This fers from the distance existed in metallic palladium (4.76 A). peak is best modeled as a distance between palladium and gallium atoms. Two palladium structure models, corresponding to the above spectrum, were considered. According to the first model, palladium exists as metallic particles decorated with GaOx in the sample. According to the second model, metallic palladium particles exist together with a palladium–gallium alloy in the sample. The results of spectrum modeling are shown in Table 2 and Fig. 4. As follows from the table, the observed spectrum mostly corresponds to model 2, for which the factor of discrepancy (R-factor) is much lower. For the model 2, the introduction of additional coordination sphere Pd–O gives Pd–O distance ∼1.98 A˚ and decreases the
Fig. 3. RDF curve PdK of Pd/Ga2 O3 , reduced in hydrogen. A dotted line shows RDF curve simulated for metallic palladium.
Fig. 4. Modeling of spectrum of the PdK edge for simultaneous presence of metallic Pd and a Pd–Ga alloy.
discrepancy factor from 17 to 13%. This variant give incorrect negative Debye–Waller factor for this sphere and the coordination number is 0.1, which is about the noise level (Fig. 5). Hence, this coordination sphere was not thereafter discussed.
˚ Fig. 5. Model 2 with an additional coordination sphere Pd–O (1.98 A).
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Table 2 Modeling results for the reduced palladium supported on Ga2 O3 (R – interatomic distance, N – coordination number, – the Debye–Waller factor; R-factor – discrepancy coefficient). No. of sphere Model 1 1 2 3 Model 2 1 2 3
2 × 104 A˚ 2
Scattering element
˚ R (A)
N (CN)
O Pd Ga
1.99 2.73 4.93
0.3 7.8 1.7
10.76 77.67 74.47
24.56
Pd Ga Ga
2.72 2.55 4.93
8.1 3.5 0.4
83.80 212.3 24.07
16.81
Nine intermetallic compounds are known for system Pd–Ga [24]. All of them are characterized by complex structure and a large set of distances for Pd–Ga. Almost in all intermetallides Pdx Gay , the palladium surrounding exhibits the presence of distances Pd–Ga 2.54 A˚ and Pd–Pd 2.82–3.0 A˚ (depending on the Pd/Ga ratio), which should be observed in EXAFS spectra if such intermetallides present in the catalysts. A comparison of the characteristic distances obtained from EXAFS spectra and crystallographic data on intermetallides GaPd2 (ICSD No. 103903), Ga5 Pd13 (ICSD No. 107572), Ga2 Pd5 (ICSD No. 103904), Ga3 Pd5 (ICSD No. 103906), Ga5 Pd (ICSD No. 103908), Ga3 Pd7 (ICSD No. 107292), GaPd and Ga7 Pd3 [5] shows that palladium enriched intermetallide Ga3 Pd7 could meet the obtained data. However, one cannot assert that the above or some other stoichiometric intermetallide exists in ˚ does not the studied samples, because distance Pd–Pd (2.8–3.0 A) observed and the coordination sphere of the metallic palladium ˚ preserves [5,25]. (2.73 A) Since the sample does not exhibit distances Pd–Pd longer than ˚ characteristic of stoichiometric intermetallic compounds, 2.82 A, one can conclude that the bulk phase, studied elsewhere [14,25], ˚ simuis not formed in our sample. However, short distance 2.55 A, lated by distance Pd–Ga, may suggest the presence of a nonuniform Pd–Ga alloy. In this system, the alloy results from diffusion of atomic gallium from the surface into palladium particles, metallic palladium exists only in the particle core and the alloy forms on the particle surface. The alloy formation requires the presence of zero-valence gallium, which can form via the following mechanisms: 1. Reduction with hydrogen via reaction Ga2 O3 + 3H2 = 2Ga + 3H2 O at 700 ◦ C [20]. Note that the reduction temperature specified in [20] is typical for the well-crystallized bulk samples. In our case,
R-factor (%)
in the presence of dispersed palladium (hydrogen activating) and dispersed gallium oxide (note that RDF data suggest that the size of CSD Ga2 O3 is ∼30 nm), the temperature of reduction can be much lower. 2. Disproportionation of gallium suboxide via reaction 3Ga2 O = 4Ga + Ga2 O3 at >700 ◦ C [26]. This temperature is also typical of the bulk sample. According to [26], Ga2 O forms on heating of Ga2 O3 in a hydrogen flow at >500 ◦ C. It was shown [27] that at ∼500 ◦ C, the reducing mixture may involve particles GaHx . On cooling in a hydrogen flow, the reduced gallium oxide particles undergo re-oxidation. The authors suggest [27] that the active gallium oxide forms exist only under reaction conditions in a hydrogen flow and at elevated temperatures, i.e. they are labile and able to disproportionate. In the presence of metallic palladium, reduction temperatures significantly decrease due to the spillover of atomic hydrogen. In [28], the method of thermally programmed reduction (TPR) was used to study supported catalysts (Pd–Ga2 O3 )/Al2 O3 . The additional peaks, appeared at temperatures 316 and 460 ◦ C, were related with the reduction of Ga2 O3 particles situated near palladium and far from it, respectively. As follows from [29], the hydrogen adsorbed on palladium can reduce the neighboring particles of Ga2 O3 even at 150 ◦ C. It is quite possible that because of high dispersion of the supported palladium and spillover of atomic hydrogen to the support, the latter undergoes partial reduction to zero-valence gallium along its boundary contacts with palladium with following diffusion into a metallic palladium particle. Ga2 O3 can be reduced by hydrogen at 200 ◦ C in the presence of metal palladium. Thus, XRD in situ showed the formation of Pd2 Ga in hydrogen at 250 ◦ C [15]. In addition, XPS showed
Table 3 EXAFS data on the local surrounding of palladium in the catalysts calcined in argon. The data was subjected to the Fourier filtration in the range 0.89–3.84 A˚ −1 . No. of sphere Pd/Ga2 O3 1 2 3 Pd/Ga2 O3 1 2 3 Pd/Ga2 O3 1 2 3 Pd/Ga2 O3 1 2 3 Pd/Ga2 O3 1 2 3
Scattering element
˚ R (A)
N (CN)
2 × 104 A˚ 2
R-factor (%)
Ga Pd Ga
2.55 2.73 4.92
5.4 9.4 1.8
225.4 85.58 57.96
16.88
O Ga Pd
1.96 2.57 2.74
0.3 0.5 8.1
3.38 82.74 68.74
13.14
O Pd Ga
2.01 2.73 3.32
1.7 6.7 1.4
44.21 65.63 63.77
12.09
O Pd Ga
2.03 2.74 3.31
1.8 5.1 1.7
14.06 51.22 35.18
7.65
O Pd Ga
2.02 2.71 3.32
1.7 5.7 1.4
6.43 45.65 1.90
8.44
◦
H2 200 C
H2 200 ◦ C, Ar 200 ◦ C
H2 200 ◦ C, Ar 300 ◦ C
◦
◦
H2 200 C, Ar 400 C
H2 200 ◦ C, Ar 500 ◦ C
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˚ indicates that a part of Pd–O–Pd in palladium oxide (Pd–Pd 3.03 A) palladium interacts with gallium to yield a mixed oxide. The coordination numbers of Pd–O and Pd–Ga distances do not depend on the heating temperature. This fact permits a suggestion that the oxidized palladium interacts with the gallium extracted from Pd–Ga alloy. The growth of metal palladium concentration with increasing temperature does not change the coordination number, whereas the Debye–Waller factor decreases in this case. This observation is caused by structural relaxation of Pd particles associated with complete extraction of gallium or cleaning of the Pd particle surface from Pd–Ga oxides.
4. Conclusions
Fig. 6. RDF curves of PdK edge for a series of catalysts 1% Pd/Ga2 O3 subject to different thermal treatments: a – unreduced sample; b – sample reduced with hydrogen; c–f – reduced samples subject to calcination in argon at 200 ◦ C (c); 300 ◦ C (d); 400 ◦ C (e) and 500 ◦ C (f).
partial reduction of Ga2 O3 to Gaı+ and Ga0 at 175 ◦ C at hydrogen pressure of 0.25 bar. The formation of a bulk alloy phase requires higher temperatures, 250–500 ◦ C [10–12,30]. Thus in [25], the intermetallic compound formed after reduction of PdO/Ga2 O3 at 350 ◦ C in a hydrogen flow for 30 min. Hence, the formation of bulk homogeneous alloy structures is unlikely during low-temperature reductions (about 200 ◦ C). However, saturation of the surface of a metal palladium particle with gallium can result in the formation of a nonuniform microalloy in the subsurface region, the core formed by metallic palladium preserves. 3.3.3. Pd/Ga2 O3 samples reduced and then calcined at different temperatures in an argon flow The additional experiments in Ar medium were used for studying the behavior of Pd–Ga alloys during heating without additional reduction of Ga2 O3 . We suggested that during heating in argon at higher temperatures, gallium is dissolved in palladium to yield a homogeneous solution which can be detected by EXAFS. A series of the samples was reduced in hydrogen at 200 ◦ C during 3 h and then calcined in argon during 3 h at 200, 300, 400 and 500 ◦ C. The results are shown in Fig. 6 and Table 3. Palladium is in the metallic state in the reduced catalyst 1% Pd/Ga2 O3 , heated at 200 ◦ C in argon (Fig. 6, curve c), as in the case of the sample not subjected to calcination in argon (curve b). The effect of calcination in argon at 200 ◦ C manifests itself as the escape of gallium from the palladium particle volume, distance Pd–Ga vanishes. As the calcination temperature increases from 300 to 500 ◦ C, ˚ does the coordination number of metallic palladium (Pd–Pd 2.75 A) ˚ and Pd–Ga (3.32 A) ˚ appear. not change and distances Pd–O (2.01 A) ˚ is not observed. It is known that after Distance Pd–Ga (2.55 A) calcination in argon, the microalloy formed during the reduction ˚ presented in EXAFS undergoes decomposition. The peak at 2.5 A, spectra (Fig. 3d–f) results from interference of two real peaks at ˚ The absence of the peaks responsible for distance 2.01 and 2.75 A.
In the work we studied model catalysts Pd/Ga2 O3 for selective liquid-phase hydrogenation of acetylene to ethylene, prepared by impregnation of -Ga2 O3 with a palladium nitrate solution following by drying and reduction in a hydrogen flow at 200 ◦ C. Some samples were then calcined in pure argon at 200–500 ◦ C. The prepared catalysts were tested in the selective liquid-phase hydrogenation of acetylene. EXAFS was used to study the state of catalyst components. The fact that unreduced catalyst Pd/Ga2 O3 is active in the liquidphase hydrogenation of palladium is explained by partial reduction of the surface palladium oxide during the experiment. High activity and selectivity of the samples reduced in hydrogen with and without additional calcination in argon at 200 ◦ C could be, as it is suggested from EXAFS data, attributed to the presence of the Pd–Ga microalloy formed along the boundary of palladium contacts and partially reduced gallium oxide. The Pd–Ga microalloy is decomposed under samples calcination at the inert atmosphere. The sample, calcined at the highest temperature 500 ◦ C in an argon flow, exhibits the lowest conversion of C2 H2 and selectivity to C2 H4 among the tested catalysts. As follows from EXAFS data the microalloy decomposition occurs at the temperature 300 ◦ C. The previously dissolved gallium is extracted from palladium and the oxidized part of palladium with the formation of mixed Pd–Ga oxides. The amorphous palladium particles recrystallize with increase of the calcination temperature. This leads to the decrease in catalyst activity.
Acknowledgements The authors wish to thank G.G. Savelieva for the determination of specific surface areas and N.N. Leontieva for X-ray analysis of the samples. The work was supported by the Russian Foundation for Basic Research, Grant No. 10-03-90727-mob st and the Ministry of Education and Science of the Russian Federation, Contract No. 16.518.11.7019.
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
EXAFS study of Pd/Ga2 O3 model catalysts of selective liquid-phase hydrogenation of acetylene to ethylene N.S. Smirnova a,∗ , D.A. Shlyapin a , O.O. Mironenko a , E.A. Anoshkina a , V.L. Temerev a , N.B. Shitova a , D.I. Kochubey b , P.G. Tsyrul’nikov a a b
Institute of Hydrocarbons Processing of Siberian Branch of Russian Academy of Sciences, Neftezavodskaya Str. 54, Omsk 644040, Russia Boreskov Institute of Catalysis, Prosp. Akad. Lavrentieva 5, Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 15 November 2011 Received in revised form 12 March 2012 Accepted 12 March 2012 Available online 21 March 2012 Keywords: EXAFS spectroscopy Pd/Ga2 O3 catalyst Pd–Ga alloy Selective hydrogenation of acetylene
a b s t r a c t Model catalysts Pd/Ga2 O3 of selective liquid-phase hydrogenation of acetylene to ethylene were prepared by impregnation of -Ga2 O3 with a palladium nitrate solution with following drying and reduction in a hydrogen flow at 200 ◦ C. A part of the samples was then calcined in an argon flow at 200–500 ◦ C. The asprepared samples were tested in the reaction of selective liquid-phase hydrogenation of acetylene. EXAFS was used to study structure of the catalyst components. Based on the EXAFS data it was found that Pd–Ga alloys present in the subsurface region of Pd particles. The presence of these alloys increases activity and selectivity of catalysts. The alloys are decomposed by calcination in inert atmosphere following decrease in catalytic activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Processing of natural and oil-dissolving gases into more valuable products is one of the key problems at the moment. In addition to the traditional technologies (using a stage of the intermediate synthesis gas production), a number of natural gas processing technologies have been developed in the last decade. Thus, the known experimental technology of Synfuels Int. Inc. [1] involves the production of acetylene by oxidative pyrolysis of natural gas with following catalytic liquid-phase hydrogenation of acetylene into ethylene and oligomerization of the latter to yield motor fuel components. The key process stage is the selective hydrogenation of acetylene to ethylene. Palladium supported on Al2 O3 is traditionally used as a catalyst of selective acetylene hydrogenation [2]. In patents [3,4], the mention is made of Ga and In modifiers used to improve the process selectivity to ethylene. Active components Pd–Ga and Pd–In supported on Al2 O3 exhibit high activity and selectivity in the liquid-phase hydrogenation of acetylene to ethylene and outperform catalyst (Pd–Ag)/Al2 O3 [4], however, the modifying effect of IIIA group oxides on palladium in the above systems has not been adequately studied.
∗ Corresponding author. Tel.: +7 3812 672 275; fax: +7 3812 646 156. E-mail addresses: ns [email protected] (N.S. Smirnova), [email protected] (D.I. Kochubey). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2012.03.010
In [5–8], palladium and gallium intermetallic compounds (Pd3 Ga7 , PdGa) were studied as highly selective catalysts of acetylene hydrogenation. An increase in the catalyst stability and selectivity to ethylene was attributed to “isolation” of palladium centers of acetylene adsorption by gallium atoms and this changes the nature of binding between acetylene and the active center [7,9]. In contrast to metallic palladium, Pd–Ga intermetallic compounds do not form hydrides, which results in the additional increase in selectivity to ethylene due to suppression of hydrogenation to ethane [5,9]. On the other hand, Pd/Ga2 O3 catalysts are known to exhibit high selectivity in several reactions involving hydrogen, such as the ethanol dehydrogenation [10] and hydrogenation of acetonitrile to ethylamine [11]. Pd–Ga alloys supported on the carbon nanotubes are highly active in the hydrogenation of CO2 to methanol [12]. Adding of Ga to catalyst Pd/SiO2 [13] increases the catalysts activity in this reaction. In [14], Pd/Ga2 O3 catalysts were studied in the reaction of steam methanol reforming. It was shown that the palladium particles in the reduced catalysts consist of a monometallic palladium core with an outer layer formed by intermetallide or an alloy. Unfortunately, the conclusions on the structure of the catalysts reduced at temperatures below 250 ◦ C were drawn from the electron-microscopy data, which could not evidence the formation of disordered alloys. On the contrary, EXAFS can elucidate the nature of interaction between palladium and gallium. In this work, we used sample Pd/Ga2 O3 with a concentration of palladium approximating that in the industrial catalysts to study
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the probability of formation of bimetallic interaction phases in the palladium catalysts modified by gallium. The aim of the work was to use EXAFS for studying catalytic properties of the palladium–gallium catalysts in the selective liquid-phase hydrogenation of acetylene to ethylene as well as structure and composition of the active centers in catalysts Pd/Ga2 O3 in dependence of the preparation conditions. Literature data [15] suggest that the stoichiometric Pd–Ga intermetallides are generated by hydrogen reduction of PdO/Ga2 O3 at temperatures higher than 250 ◦ C. In this work, the catalysts were reduced at temperatures ≤200 ◦ C. The diffraction methods do not show intermetallide phases for the above samples, whereas activity and selectivity of the palladium catalysts supported on Ga2 O3 during hydrogenation of acetylene are higher than for the catalysts on other supports, -Al2 O3 , as an example [16].
2. Experimental 2.1. Preparation of the samples Samples 1% Pd/Ga2 O3 were synthesized by impregnation of -Ga2 O3 (chemically pure) with a solution of Pd(NO3 )2 following by drying at 120 ◦ C for 2 h. After drying, the samples were reduced in a hydrogen flow at 200 ◦ C during 3 h. Then the samples were additionally calcined in an argon flow (initial purity 99.997%, the residue oxygen content was 5 × 10−4 vol.%, additional cleaning was performed by passing argon through copper shavings at 300 ◦ C). After additional calcination at 200, 300, 400 and 500 ◦ C in argon during 3 h (Table 1), the samples were kept in argon.
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2.4. Specific surface area Specific surface areas of the initial support and the catalyst containing 1 wt.% of Pd was determined by the BET method from the one-point adsorption of nitrogen at 77 K. 2.5. EXAFS EXAFS spectra of the PdK edge absorption were taken at the EXAFS spectroscopy Station of the Siberian Synchrotron Radiation Center (Novosibirsk). The spectra were registered by synchrotron radiation, the electron energy was 2 GeV in a VEPP-3 storage, the average current was 70 mA. A cut-off crystal of Si (1 1 1) was used as a monochromator. All spectra were taken in the fluorescence mode with the steps of 2.5 eV. To register X-ray radiation, we used the ionization chambers filled with argon and an X-ray fluorescent detector. The spectra were analyzed by program Viper using the standard method [18]. The spectra were analyzed as k2 (k) in the wave numbers range 2.50–12.00 A˚ −1 . The background was removed by extrapolating of the pre-edge absorption region to the EXAFS region using the Victoreen polynomials. To calculate a smooth part of the absorption coefficient, we used the approximation based on the three cubic spline functions. The inflection point at the absorption edge was taken as the initial point of EXAFS spectrum E0 . Quantum-chemical data required for calculations of structural parameters were obtained by program FEFF-7. Data on the structure of complexes were taken from the Inorganic Crystal Structure Database (ICSD). We also registered a reference spectrum of the K-edge of palladium absorption using the palladium foil. 3. Results and discussion
2.2. Catalytic tests
3.1. Features of the support
The samples were tested in a gradientless, shaken, flow and temperature-controlled reactor at the following conditions: reaction temperature 40 ◦ C, total flow of the gas mixture was 100 ml/min, sweep frequency of the reactor ≥7–8 s−1 , catalyst weight 10 mg, solvent volume 8 ml. N-methylpyrrolidone was used as a solvent. The reaction gas mixture involved: C2 H2 4 vol.%, H2 90 vol.% and He (balance) 6 vol.%. The experiment lasted for ∼220 min. The initial reaction mixture and the resulting product mixture were analyzed by a «Chromos GH-1000» analyzer using a capillary column with SiO2 (25 m × 0.32 mm, the operation temperature 60 ◦ C) and a flame-ionizing detector. Nitrogen was used as carrier gas, the frequency of gas sampling was 33 min. During the experiment, five-six chromatograms of the initial mixture (IM) and resulting mixture (RM) were taken. From the corresponding peak areas we calculated conversion of acetylene (X, %), selectivity (S, %) to ethylene and analyzed the dynamics of changes of the above values vs. testing time [17]. To compare the characteristics of different samples, we used the values obtained in the range of stationary activity of the catalysts. At the above conditions, the main products of the acetylene hydrogenation are ethylene and oligomers Cn≥4 .
As follows from XRD data, gallium oxide exists as -Ga2 O3 ˚ The average coherent scat(C2/m, a = 12.224, b = 3.038, c = 5.806 A). tering domains (CSD) calculated by the Selyakov–Scherrer equation was 31.7 nm. The calculation of CSD with consideration for different directions, represented by peaks on the XRD pattern, gives the following results for planes: (4 0 0) – 36.1 nm, (0 2 0) – 25.7 nm and (0 0 2) – 39.1 nm. The specific surface area of the initial gallium oxide (SBET ) was 14 m2 /g. This value held as 1 wt.% palladium was supported.
2.3. X-ray analysis Phase composition of gallium oxide was determined by powder diffractometry using a D8 Advance (Bruker) diffractometer with K␣ emission of copper. Phase composition of the active component was not determined because of low palladium concentration (1 wt.%).
3.2. Catalytic tests of the prepared samples Table 1 shows the results of catalytic tests of the initial sample after drying, the initial reduced sample and the samples after additional calcination at different temperatures in argon. In [14], the catalyst calcination in oxygen resulted in the palladium oxidation. In our case, calcination in argon was performed to verify the suggestion that thermal treatments can increase the interaction between metallic palladium and the reduced gallium oxide. In the liquid-phase selective hydrogenation of acetylene, the samples were tested in the kinetic region. In this case, the acetylene conversion was not higher than 30% for the most active sample. The chromatographic data suggest that for all catalysts the selectivity to ethane is close to 0. Pure -Ga2 O3 did not show catalytic activity at our reaction conditions. As follows from the table data, the amount of oligomers (hydrocarbons Cx Hy , where x ≥ 4) is high in all cases. Several reasons are responsible for this observation. When the reaction is performed at 40 ◦ C, solubility of acetylene in N-methylpyrrolidone is high (about 23 m3 /m3 normal conditions) [19], whereas the catalyst activity is low (Table 1). In combination, this provides high concentration
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Table 1 Catalytic properties of 1% Pd/Ga2 O3 samples. Sample number
Preparation conditions
XC2 H2 (%)
SC2 H4 (%)
SC4 (%)
1 2 3 4 5 6
After drying (not reduced) 200 ◦ C, H2 200 ◦ C, H2 + 200 ◦ C, Ar 200 ◦ C, H2 + 300 ◦ C, Ar 200 ◦ C, H2 + 400 ◦ C, Ar 200 ◦ C, H2 + 500 ◦ C, Ar
17 27 24 17 10 9
38 57 53 44 38 26
62 43 47 56 62 74
of acetylene near the catalyst surface during the reaction. Excess acetylene competitively inhibits the hydrogen adsorption on the palladium surface and subsequent hydrogenation of ethylene to ethane, but it provides oligomerization [17]. Besides, acetylene is adsorbed about 2000-times stronger than ethylene and displaces it from the surface, preventing further hydrogenation to ethane [17]. As follows from IR spectroscopy, unreduced sample 1% Pd/Ga2 O3 , containing palladium oxide and several nitro groups, is active in the liquid-phase hydrogenation of acetylene at the abovedescribed conditions (acetylene conversion is 17%). This is probably caused by partial reduction of the catalyst at the reaction conditions, since it is well known that palladium oxide can be easily reduced by hydrogen to metallic palladium even at room temperature [20]. It should be mentioned that the reduction is accompanied by catalyst deactivation by oligomers during the reaction. The highest activity and selectivity to ethylene (X = 27–24% and S = 57–53%, see Table 1) were shown by sample 2 (reduced in hydrogen at 200 ◦ C) and sample 3 (additionally calcined at 200 ◦ C in argon). According to [6], Pd–Ga intermetallic compounds are active and selective in the gas phase hydrogenation of acetylene. High catalytic properties are associated with peculiar features of intermetallide structure provided in “isolation” of palladium active centers. These centers bind acetylene molecules more weakly and acetylene hydrogenates to ethylene easier. Gallium modifies the palladium centers resulting in changes of adsorption and desorption energies of both acetylene and ethylene. It should be noted that in contrast to metal palladium, Pd–Ga intermetallides do not form palladium hydrides which are unselective in hydrogenation of acetylene to ethylene. As the calcination temperature increases from 300 ◦ C to 500 ◦ C, the ethylene conversion decreases from 17 to 9%. Note that selectivity to ethylene decreases from 44 to 26% and the number of oligomers in the reaction mixture increases. The sample calcined at 500 ◦ C in argon flow exhibits lower activity and selectivity to ethylene than the initial unreduced catalyst. The sample calcined at 400 ◦ C and unreduced Pd/Ga2 O3 are comparable in their catalytic properties. The obtained catalytic data permit a suggestion that the number of active centers decreases (XC2 H2 decreases) with an increase of calcination temperature in argon. It is likely that the nature of active centers changes, which manifests itself as a decrease in selectivity to ethylene and an increase in selectivity to oligomers. A decrease in the number of active centers can be caused by changes in the surface composition of palladium particles (see below) and a decrease in dispersion of the reduced palladium sample (sintering). According to [5,6,9,21], many active centers are closely set on the large palladium particles, which provides strong sorption of acetylene molecules on the neighboring centers with following formation of hydrooligomers which block the catalyst active centers. Several publications are devoted to systems Ir/Ga2 O3 , Pt/Ga2 O3 and Ru/Ga2 O3 [22,23]. For system Ir/Ga2 O3 heated at 260 ◦ C, the support partial reduction results in “decoration” (overgrowth) of the active component particles with a thin gallium oxide layer. The formation of bulk alloy phase IrGa proceeds at higher temperature, 580 ◦ C, in the reducing atmosphere [22]. Transmission electron microscopy (TEM) and secondary ion mass spectroscopy
(SIMS) data of the Pt/Ga2 O3 sample reduced at 300 ◦ C suggest a shift of partially reduced GaOx to the surface of a platinum particle by the SMSI mechanism («strong metal-support interaction») [23]. At the same time, no significant sintering of metallic palladium particles was observed. For thorough analysis of the observed regularities, EXAFS data were used. 3.3. EXAFS data on catalysts 3.3.1. Sample Pd/Ga2 O3 treated by drying ˚ Fig. 1 shows a curve of radial atom distribution (scale R–ı (A)) of the PdK edge of unreduced sample 1% Pd/Ga2 O3 . The model spectrum shows the presence of distances Pd–O ˚ Pd–Pd (3.04 A), ˚ Pd–Pd (3.46 A), ˚ belonging to palladium (2.03 A), oxide (ICSD No. 29281), in the palladium surrounding. Metallic palladium is not observed in the sample. The RDF curve also exhibits ˚ a peak modeled by distance Pd–Ga (4.51 A). The structure of massive PdO is formed by linear chains of ions (PdO4 )2− in the planar coordination with a Pd–O–Pd distance of ˚ which are interconnected in packages. A distance of 3.46 A˚ 3.03 A, is the distance between such chains. In PdO the coordination numbers for distances 3.03 and 3.46 A˚ are 4 and 8, respectively (ICSD No. 29281). As follows from modeling, for distances Pd–Pd 3.03 A˚ ˚ the coordination number is 1.7 and 2.1, respectively. and 3.46 A, The structure of -gallium oxide exhibits distances Ga–Ga at 3.03 A˚ ˚ which are similar to distances Pd–Pd (3.03 A˚ and 3.42 A) ˚ and 3.44 A, in the palladium oxide. It can be assumed that on the Ga2 O3 surface palladium mainly exists as oxide chains situated along axis b of Ga2 O3 unit cell. The distances between gallium atoms in ˚ This Ga2 O3 structure (ICSD No. 83645) along axis b are 3.03 A. facilitates the growth of palladium oxide chains along this direction
Fig. 1. Model spectrum of the PdK edge in unreduced sample 1% Pd/Ga2 O3 . Dotted line shows a simulated RDF curve, a firm line shows the experimental spectrum.
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Fig. 2. Model incorporation of palladium atoms into the structure of gallium oxide.
in -Ga2 O3 . Some chains coordinate between themselves, which ˚ explains the presence of distance Pd–Pd 3.46 A. Epitaxial stabilization of the palladium oxide particles along axis b in -Ga2 O3 , accompanied by fixation of palladium atoms in the crystallographic positions of gallium atoms, explains distance ˚ on the RDF curve which is close to distance Ga–Ga Pd–Ga (4.51 A) ˚ in Fig. 2. in the gallium oxide structure (4.46 A) 3.3.2. Sample Pd/Ga2 O3 reduced in hydrogen Modeling of a spectrum with an assumption that the sample contains only metallic palladium (ICSD No. 41517) does not provide positive results, since the position of peak on the experimental spectrum (scale R–ı), corresponding to distance ∼4.93 A˚ (Fig. 3), dif˚ This fers from the distance existed in metallic palladium (4.76 A). peak is best modeled as a distance between palladium and gallium atoms. Two palladium structure models, corresponding to the above spectrum, were considered. According to the first model, palladium exists as metallic particles decorated with GaOx in the sample. According to the second model, metallic palladium particles exist together with a palladium–gallium alloy in the sample. The results of spectrum modeling are shown in Table 2 and Fig. 4. As follows from the table, the observed spectrum mostly corresponds to model 2, for which the factor of discrepancy (R-factor) is much lower. For the model 2, the introduction of additional coordination sphere Pd–O gives Pd–O distance ∼1.98 A˚ and decreases the
Fig. 3. RDF curve PdK of Pd/Ga2 O3 , reduced in hydrogen. A dotted line shows RDF curve simulated for metallic palladium.
Fig. 4. Modeling of spectrum of the PdK edge for simultaneous presence of metallic Pd and a Pd–Ga alloy.
discrepancy factor from 17 to 13%. This variant give incorrect negative Debye–Waller factor for this sphere and the coordination number is 0.1, which is about the noise level (Fig. 5). Hence, this coordination sphere was not thereafter discussed.
˚ Fig. 5. Model 2 with an additional coordination sphere Pd–O (1.98 A).
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Table 2 Modeling results for the reduced palladium supported on Ga2 O3 (R – interatomic distance, N – coordination number, – the Debye–Waller factor; R-factor – discrepancy coefficient). No. of sphere Model 1 1 2 3 Model 2 1 2 3
2 × 104 A˚ 2
Scattering element
˚ R (A)
N (CN)
O Pd Ga
1.99 2.73 4.93
0.3 7.8 1.7
10.76 77.67 74.47
24.56
Pd Ga Ga
2.72 2.55 4.93
8.1 3.5 0.4
83.80 212.3 24.07
16.81
Nine intermetallic compounds are known for system Pd–Ga [24]. All of them are characterized by complex structure and a large set of distances for Pd–Ga. Almost in all intermetallides Pdx Gay , the palladium surrounding exhibits the presence of distances Pd–Ga 2.54 A˚ and Pd–Pd 2.82–3.0 A˚ (depending on the Pd/Ga ratio), which should be observed in EXAFS spectra if such intermetallides present in the catalysts. A comparison of the characteristic distances obtained from EXAFS spectra and crystallographic data on intermetallides GaPd2 (ICSD No. 103903), Ga5 Pd13 (ICSD No. 107572), Ga2 Pd5 (ICSD No. 103904), Ga3 Pd5 (ICSD No. 103906), Ga5 Pd (ICSD No. 103908), Ga3 Pd7 (ICSD No. 107292), GaPd and Ga7 Pd3 [5] shows that palladium enriched intermetallide Ga3 Pd7 could meet the obtained data. However, one cannot assert that the above or some other stoichiometric intermetallide exists in ˚ does not the studied samples, because distance Pd–Pd (2.8–3.0 A) observed and the coordination sphere of the metallic palladium ˚ preserves [5,25]. (2.73 A) Since the sample does not exhibit distances Pd–Pd longer than ˚ characteristic of stoichiometric intermetallic compounds, 2.82 A, one can conclude that the bulk phase, studied elsewhere [14,25], ˚ simuis not formed in our sample. However, short distance 2.55 A, lated by distance Pd–Ga, may suggest the presence of a nonuniform Pd–Ga alloy. In this system, the alloy results from diffusion of atomic gallium from the surface into palladium particles, metallic palladium exists only in the particle core and the alloy forms on the particle surface. The alloy formation requires the presence of zero-valence gallium, which can form via the following mechanisms: 1. Reduction with hydrogen via reaction Ga2 O3 + 3H2 = 2Ga + 3H2 O at 700 ◦ C [20]. Note that the reduction temperature specified in [20] is typical for the well-crystallized bulk samples. In our case,
R-factor (%)
in the presence of dispersed palladium (hydrogen activating) and dispersed gallium oxide (note that RDF data suggest that the size of CSD Ga2 O3 is ∼30 nm), the temperature of reduction can be much lower. 2. Disproportionation of gallium suboxide via reaction 3Ga2 O = 4Ga + Ga2 O3 at >700 ◦ C [26]. This temperature is also typical of the bulk sample. According to [26], Ga2 O forms on heating of Ga2 O3 in a hydrogen flow at >500 ◦ C. It was shown [27] that at ∼500 ◦ C, the reducing mixture may involve particles GaHx . On cooling in a hydrogen flow, the reduced gallium oxide particles undergo re-oxidation. The authors suggest [27] that the active gallium oxide forms exist only under reaction conditions in a hydrogen flow and at elevated temperatures, i.e. they are labile and able to disproportionate. In the presence of metallic palladium, reduction temperatures significantly decrease due to the spillover of atomic hydrogen. In [28], the method of thermally programmed reduction (TPR) was used to study supported catalysts (Pd–Ga2 O3 )/Al2 O3 . The additional peaks, appeared at temperatures 316 and 460 ◦ C, were related with the reduction of Ga2 O3 particles situated near palladium and far from it, respectively. As follows from [29], the hydrogen adsorbed on palladium can reduce the neighboring particles of Ga2 O3 even at 150 ◦ C. It is quite possible that because of high dispersion of the supported palladium and spillover of atomic hydrogen to the support, the latter undergoes partial reduction to zero-valence gallium along its boundary contacts with palladium with following diffusion into a metallic palladium particle. Ga2 O3 can be reduced by hydrogen at 200 ◦ C in the presence of metal palladium. Thus, XRD in situ showed the formation of Pd2 Ga in hydrogen at 250 ◦ C [15]. In addition, XPS showed
Table 3 EXAFS data on the local surrounding of palladium in the catalysts calcined in argon. The data was subjected to the Fourier filtration in the range 0.89–3.84 A˚ −1 . No. of sphere Pd/Ga2 O3 1 2 3 Pd/Ga2 O3 1 2 3 Pd/Ga2 O3 1 2 3 Pd/Ga2 O3 1 2 3 Pd/Ga2 O3 1 2 3
Scattering element
˚ R (A)
N (CN)
2 × 104 A˚ 2
R-factor (%)
Ga Pd Ga
2.55 2.73 4.92
5.4 9.4 1.8
225.4 85.58 57.96
16.88
O Ga Pd
1.96 2.57 2.74
0.3 0.5 8.1
3.38 82.74 68.74
13.14
O Pd Ga
2.01 2.73 3.32
1.7 6.7 1.4
44.21 65.63 63.77
12.09
O Pd Ga
2.03 2.74 3.31
1.8 5.1 1.7
14.06 51.22 35.18
7.65
O Pd Ga
2.02 2.71 3.32
1.7 5.7 1.4
6.43 45.65 1.90
8.44
◦
H2 200 C
H2 200 ◦ C, Ar 200 ◦ C
H2 200 ◦ C, Ar 300 ◦ C
◦
◦
H2 200 C, Ar 400 C
H2 200 ◦ C, Ar 500 ◦ C
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˚ indicates that a part of Pd–O–Pd in palladium oxide (Pd–Pd 3.03 A) palladium interacts with gallium to yield a mixed oxide. The coordination numbers of Pd–O and Pd–Ga distances do not depend on the heating temperature. This fact permits a suggestion that the oxidized palladium interacts with the gallium extracted from Pd–Ga alloy. The growth of metal palladium concentration with increasing temperature does not change the coordination number, whereas the Debye–Waller factor decreases in this case. This observation is caused by structural relaxation of Pd particles associated with complete extraction of gallium or cleaning of the Pd particle surface from Pd–Ga oxides.
4. Conclusions
Fig. 6. RDF curves of PdK edge for a series of catalysts 1% Pd/Ga2 O3 subject to different thermal treatments: a – unreduced sample; b – sample reduced with hydrogen; c–f – reduced samples subject to calcination in argon at 200 ◦ C (c); 300 ◦ C (d); 400 ◦ C (e) and 500 ◦ C (f).
partial reduction of Ga2 O3 to Gaı+ and Ga0 at 175 ◦ C at hydrogen pressure of 0.25 bar. The formation of a bulk alloy phase requires higher temperatures, 250–500 ◦ C [10–12,30]. Thus in [25], the intermetallic compound formed after reduction of PdO/Ga2 O3 at 350 ◦ C in a hydrogen flow for 30 min. Hence, the formation of bulk homogeneous alloy structures is unlikely during low-temperature reductions (about 200 ◦ C). However, saturation of the surface of a metal palladium particle with gallium can result in the formation of a nonuniform microalloy in the subsurface region, the core formed by metallic palladium preserves. 3.3.3. Pd/Ga2 O3 samples reduced and then calcined at different temperatures in an argon flow The additional experiments in Ar medium were used for studying the behavior of Pd–Ga alloys during heating without additional reduction of Ga2 O3 . We suggested that during heating in argon at higher temperatures, gallium is dissolved in palladium to yield a homogeneous solution which can be detected by EXAFS. A series of the samples was reduced in hydrogen at 200 ◦ C during 3 h and then calcined in argon during 3 h at 200, 300, 400 and 500 ◦ C. The results are shown in Fig. 6 and Table 3. Palladium is in the metallic state in the reduced catalyst 1% Pd/Ga2 O3 , heated at 200 ◦ C in argon (Fig. 6, curve c), as in the case of the sample not subjected to calcination in argon (curve b). The effect of calcination in argon at 200 ◦ C manifests itself as the escape of gallium from the palladium particle volume, distance Pd–Ga vanishes. As the calcination temperature increases from 300 to 500 ◦ C, ˚ does the coordination number of metallic palladium (Pd–Pd 2.75 A) ˚ and Pd–Ga (3.32 A) ˚ appear. not change and distances Pd–O (2.01 A) ˚ is not observed. It is known that after Distance Pd–Ga (2.55 A) calcination in argon, the microalloy formed during the reduction ˚ presented in EXAFS undergoes decomposition. The peak at 2.5 A, spectra (Fig. 3d–f) results from interference of two real peaks at ˚ The absence of the peaks responsible for distance 2.01 and 2.75 A.
In the work we studied model catalysts Pd/Ga2 O3 for selective liquid-phase hydrogenation of acetylene to ethylene, prepared by impregnation of -Ga2 O3 with a palladium nitrate solution following by drying and reduction in a hydrogen flow at 200 ◦ C. Some samples were then calcined in pure argon at 200–500 ◦ C. The prepared catalysts were tested in the selective liquid-phase hydrogenation of acetylene. EXAFS was used to study the state of catalyst components. The fact that unreduced catalyst Pd/Ga2 O3 is active in the liquidphase hydrogenation of palladium is explained by partial reduction of the surface palladium oxide during the experiment. High activity and selectivity of the samples reduced in hydrogen with and without additional calcination in argon at 200 ◦ C could be, as it is suggested from EXAFS data, attributed to the presence of the Pd–Ga microalloy formed along the boundary of palladium contacts and partially reduced gallium oxide. The Pd–Ga microalloy is decomposed under samples calcination at the inert atmosphere. The sample, calcined at the highest temperature 500 ◦ C in an argon flow, exhibits the lowest conversion of C2 H2 and selectivity to C2 H4 among the tested catalysts. As follows from EXAFS data the microalloy decomposition occurs at the temperature 300 ◦ C. The previously dissolved gallium is extracted from palladium and the oxidized part of palladium with the formation of mixed Pd–Ga oxides. The amorphous palladium particles recrystallize with increase of the calcination temperature. This leads to the decrease in catalyst activity.
Acknowledgements The authors wish to thank G.G. Savelieva for the determination of specific surface areas and N.N. Leontieva for X-ray analysis of the samples. The work was supported by the Russian Foundation for Basic Research, Grant No. 10-03-90727-mob st and the Ministry of Education and Science of the Russian Federation, Contract No. 16.518.11.7019.
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