Reaction between Thiophene and Ni Nanoparticles Supported on

Sep 8, 2009 - Xuan Meng , Huan Huang , and Li Shi ... Zhangfeng Qin , Guofu Wang , Mingxian Du , Hui Ge , Xuekuan Li , Zhiwei Wu and Jianguo Wang...
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J. Phys. Chem. C 2009, 113, 17064–17069

Reaction between Thiophene and Ni Nanoparticles Supported on SiO2 or ZnO: In Situ Synchrotron X-ray Diffraction Study I. Bezverkhyy,*,† O. V. Safonova,‡ P. Afanasiev,§ and J.-P. Bellat† Institut Carnot de Bourgogne, UMR5209 CNRS-UniVersite´ de Bourgogne, 9, aV. A. SaVary, BP 47870, 21078 Dijon Cedex, France, Swiss-Norwegian Beamline, European Synchrotron Radiation Facility, 6 rue Jules Horowitz, 38043 Grenoble, France, and Institut de Recherches sur la Catalyse et l’EnVironnement, 2, aV A. Einstein, 69626 Villeurbanne Cedex, France ReceiVed: April 16, 2009; ReVised Manuscript ReceiVed: July 31, 2009

Synchrotron X-ray powder diffraction (XRD) and mass spectrometry were applied in situ to study the interaction of thiophene with Ni0 particles (5-6 nm) deposited on inert (SiO2) or sulfidable (ZnO) supports. The reaction was performed in a hydrogen flow containing 20-40 mbar of thiophene at 280-360 °C for Ni/SiO2 and at 360 °C for Ni/ZnO. Surprisingly, in the first XRD pattern of the solid recorded after introduction of thiophene in the reactor, we observed a considerable increase of the Ni cell parameter: ∆a ) 0.06 Å for Ni/SiO2 and ∆a ) 0.15 Å for Ni/ZnO at 360 °C. The simultaneous presence of CH4 in the gas phase revealed that in this stage thiophene molecules undergo complete hydrogenolysis. During this reaction, carbon adatoms can also be produced and their diffusion into the bulk of Ni nanoparticles was supposed to provoke the observed lattice expansion. After this initial stage, the SiO2-supported Ni nanoparticles transformed progressively into a complex mixture of phases with the composition dependent on the reaction temperature: at 280 °C the main product was identified as Ni3S2 (cubic modification and heazelwoodite), while at 360 °C it was Ni3C. The behavior of ZnO-supported Ni is different. The carbon-modified dilated Ni nanoparticles, formed in the initial stage, remained intact until the end of ZnO sulfidation to be later transformed into Ni3C. On the basis of these results, we concluded that carbon-modified Ni particles play the role of an active phase during the reactive adsorption of thiophene on Ni/ZnO. 1. Introduction Interaction between supported metals and sulfur-containing molecules is an eminently important subject of high industrial relevance. In many catalytic processes sulfur interaction with metallic particles is responsible for the detrimental effect of catalyst poisoning. Researchers strive to diminish these effects by developing sulfur-tolerant catalysts in which the degree of metal particles sulfidation by the reaction medium could be minimized. The examples include hydrogenation,1,2 steam reforming,3,4 hydrodenitrogenation,5 and hydrodesulfurization6,7 processes. In most cases relevant to poisoning, the interaction with sulfur concerns only the topmost surface layer of metal particles because the concentration of sulfur-containing species is insufficient to induce the bulk metal to sulfide transformation. In contrast, in the reactive adsorption of sulfur-containing molecules the sulfidation of metallic particles is a desired effect sought to be accelerated. In this case, the high affinity between metals and sulfur is used to selectively remove sulfur-containing molecules from the hydrocarbon streams.8-10 In both catalysis and reactive adsorption, the information about the exact state of metal in the presence of sulfur-containing molecules under reaction conditions is of great utility. Such information can however be obtained only by in situ techniques because of the high reactivity of finely dispersed metals and metal sulfides in air. X-ray absorption spectroscopy and * Corresponding author. E-mail: [email protected]. Telephone: +33 380 39 60 38. Fax: +33 380 3L9 61 32. † UMR5209 CNRS-Universite´ de Bourgogne. ‡ European Synchrotron Radiation Facility. § Institut de Recherches sur la Catalyse et l’Environnement.

particularly extended X-ray absorption fine structure spectroscopy are well-adapted for this purpose and have been successfully employed to characterize the state of mono- or bimetallic noble-metal particles in the presence of sulfur-containing molecules.6,11,12 In contrast, it is difficult to apply X-ray diffraction (XRD) to such systems because the catalytically active metal particles are usually small (∼10 Å) and the metal content in the material is merely ∼1%. To obtain structural information on such materials, the radial electron distribution function should be extracted from the XRD data. For instance, Gallezot and Bergeret13,14 have successfully applied this technique to studying the structural modifications of Pt particles in Y zeolite after the adsorption of benzene and sulfur-containing molecules. In the case of Ni-based materials used in reactive adsorption of sulfur-containing molecules, the in situ XRD can provide very detailed information on the material structure. This is mainly because metal particles in these materials are relatively large (∼50 Å) and their loading is quite high (10-20%). For a relatively fast reactive adsorption process with characteristic times varying from several minutes to a few hours, an intense synchrotron X-ray beam should be used to observe the intermediate states. Continuing our previous studies of the reactive adsorption of thiophene on Ni/SiO2 and Ni/ZnO,15-17 we applied the synchrotron radiation XRD to follow in situ the evolution of the adsorbent phase composition under reaction conditions with the minute-scale time resolution. 2. Experimental Section All chemicals used in the synthesis were high-purity grade and purchased from Aldrich. The Ni/SiO2 (18 wt % Ni) samples

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Thiophene and Ni Nanoparticles on SiO2 or ZnO were prepared by incipient wetness impregnation as described in ref 17. Ni/ZnO (12 wt % Ni) samples were obtained by coprecipitation of Ni(NO3)2 · 6H2O and Zn(NO3)2 · 6H2O from Na2CO3 solutions as described elsewhere.16 As shown previously, the pretreatment of solids, particularly, their prereduction in H2, strongly affects their reactivity toward sulfur-containing molecules. This is why, in the present study, the pretreatment of samples was performed in situ immediately before their reaction with thiophene. Original Ni/ZnO samples were in the oxidized form (i.e., NiO/ZnO). Before the reaction with thiophene, the latter samples were either treated in a N2 flow (1 h, 360 °C) or reduced in an H2 flow (3 h, 360 °C). Ni/SiO2 samples were reduced in an H2 flow (1 h, 360 °C). The in situ XRD experiment was carried out at BM01B beamline (Swiss-Norwegian Beamlines) of the European Synchrotron Radiation Facility (Grenoble, France) using a highresolution powder diffractometer. The measurements were performed at the wavelength of 0.5 Å. The sample powder (ca. 2 mg) was placed in the middle of a 60 mm quartz capillary plug flow reactor (1 mm diameter with 20 µm walls) and fixed from the sides with quartz fibers. The capillary was connected with stainless steel metal tubes using Swagelok fittings with graphite ferrules and attached to the gas flows and the exhaust. The experiments were performed at atmospheric pressure. A gas blower oven was used for heating the sample in the reactor. The samples were preheated to the reaction temperature (280-360 °C) in a flow of H2 or N2 (5 mL/min), after which an H2 flow (2 mL/min) saturated with thiophene (40 mbar for Ni/SiO2 and 20 mbar for Ni/ZnO) was introduced in the reactor. The time necessary for recording an XRD pattern varied from 5 to 20 min depending on the angular range and the data quality requirements. The FullProf Suite software package18 was used to fit selected peaks of the obtained XRD patterns to determine the peak positions and widths. The gaseous products evolved during reaction were analyzed online using a mass spectrometer (Gas Trace A, Fison Instruments) equipped with a quadrupole analyzer operating in the Multiplier mode. The ionization was carried out by an electron impact with the electron energy of 65 eV. A silica capillary tube heated to 180 °C was used for the permanent sampling of gaseous products for the analysis. A full histogram of mass spectra from m/z ) 1 to m/z ) 100 was continuously scanned and stored every 1 min. To simulate the evolution of the Ni-Ni interatomic distance in Ni nanoparticles upon adsorption of sulfur or carbonaceous species, Ni13 stable dodecahedral cluster was used as the smallest representative model of Ni nanoparticle. The geometry was first optimized using molecular dynamics (calculations were carried out using the Open Force Field (OFF) program in the Cerius2 package (version 4.2)19) and then refined using the PM6 parametrization level of a semiempirical method implemented in the MOPAC 2007 software.20 3. Results and Discussion 3.1. Mass Spectrometry. Simultaneously with in situ XRD experiments, the reaction products at the reactor outlet were analyzed online by a mass spectrometer (MS). Figure 1 shows the MS signals of thiophene interaction with Ni/SiO2 at 330 °C and Ni/ZnO at 360 °C. The most surprising result was the appearance of a strong peak of methane (m/z ) 16) in the very beginning of the Ni/SiO2 and Ni/ZnO interaction with thiophene (Figure 1). This signal can be reliably ascribed to CH4 molecules because no variations of in-step signals observed for O2 (m/z ) 32) or H2O (m/z ) 18). The presence of methane in the gas

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Figure 1. Evolution of MS signals of the reaction products during reaction with thiophene of Ni/SiO2 at 330 °C (a) and Ni/ZnO at 360 °C (b).

phase means that the hydrocarbon moieties, initially formed from thiophene on Ni particles, undergo complete hydrogenolysis. However, this process rapidly decays as follows from the drop of the m/z ) 16 signal. Active carbon species (precursors of CH4) can also be transformed into other products, concentrate on the surface, or even migrate into the bulk of Ni nanoparticles, giving rise to nickel carbide phase detected by XRD (see below). The MS signals recorded in the in situ experiments are similar to those measured in our previous studies carried out in a fixed-bed microreactor.16,17 For instance, the concentration of butenes (m/z ) 41) is high at the beginning of interaction and then gradually decreases, reflecting the Ni sulfidation and related drop of the hydrodesulfurization (HDS) activity of the solid. The rise of the H2S peak coincides with the stabilization of butenes concentration (Figure 1) and corresponds to the end of solid-phase transformation. The presence of H2S and butenes in the gas phase after this point indicates that transformed Ni particles still have noticeable catalytic activity in thiophene decomposition. 3.2. XRD Study of the Structural Changes in Ni/SiO2 during Its Reaction with Thiophene. The diffraction pattern of a Ni/SiO2 sample recorded in air contains the peaks characteristic of both metallic Ni and NiO (Figure 2). Heating the sample to 360 °C under H2 flow prior to its reaction with thiophene results in a complete reduction of nickel oxide. The mean size of the reduced Ni nanoparticles calculated from the broadening of the (111) peak using the Sherrer equation is 5.5 nm. The cell parameter of Ni (3.55 Å) is slightly larger than the tabulated value for standard metallic Ni (3.5238 Å, JCPDF No. 00-004-0850), which is explained by the thermal expansion of the lattice at high temperature.

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Figure 2. XRD patterns of Ni/SiO2 sample before reduction (A), after heating to 360 °C in H2 flow (B), and after the contact with thiophene at 360 °C (C).

Figure 3. Increase of Ni lattice parameter in Ni/SiO2 after the contact with thiophene as function of interaction temperature.

3.2.1. Initial Stage of the Interaction between Ni/SiO2 and Thiophene. After in situ reduction of Ni/SiO2 sample in H2 flow (1 h at 360 °C), the flow of the thiophene/H2 mixture was introduced to the reactor. A rather unexpected effect was observed in the first XRD pattern (Figure 2 C). Along with the appearance of Ni sulfide (a shoulder at 2θ ) 15.5°), all peaks of metallic Ni are shifted toward lower 2θ values, indicating an increase of the Ni cell parameter. The magnitude of this change increases with the reaction temperature and reaches 0.06 Å (a ) 3.61 Å) at 360 °C (Figure 3). In other words, the presumably surface interaction with thiophene leads to a noticeable change of the bulk lattice parameter of metal nanoparticles. This phenomenon can be induced by the appearance of thiophene decomposition products on the surface of Ni particles. Indeed, as follows from our mass spectrometry data and from surface science studies,21-23 thiophene is easily decomposed on Ni to afford both sulfur atoms and dehydrogenated carbon species. Strong interaction between these species and Ni surface atoms may affect the bulk electronic properties of Ni nanoparticles, resulting in the increase in the Ni lattice parameter. Yet another phenomenon leading to the lattice expansion can be the migration of surface carbon atoms into the bulk of Ni particles. It was shown that such migration can occur at moderate temperatures and result in the increase of the cell parameter by about 0.01 Å for a Ni-C alloy containing 1.47 at. % C.24 The same phenomenon was invoked to explain the formation of a dilated cubic Ni phase observed after thermolysis of Ni acetylacetonate in oleyamine.25 To estimate whether the presence of carbon or sulfur atoms on the surface can influence the bulk structure of Ni particles, we used semiempirical simulation of small representative nickel clusters.

Figure 4. Examples of the clusters optimized using the PM6 semiempirical method. Atoms colors: Ni, black; S, red; C, green; H, blue. Interatomic distances in Å are indicated near the corresponding bonds. Symmetry of the initial Ni13 clusters is indicated in italics above the graphs.

3.2.2. Simulation of the Effect of Carbon or Sulfur Adatoms on the Ni-Ni Distances. Geometry, spin states, and energy of nickel clusters were studied previously using various approaches including molecular mechanics and embedded atoms,26 semiempirical,27 first-principles Hartree-Fock,28 and DFT methods.29,30 Our goal was to estimate whether the Ni-Ni bulk distances could change significantly after the adsorption of sulfur or carbon species. This was performed using the smallest representative clusters that possess the essential features of metallic nickel coordination. We have chosen Ni13 clusters in three highly symmetric structures with D3h, Oh, and Ih symmetries (Figure 4). As the first step, the optimal geometry of three bare Ni13 clusters was found using a semiempirical method (PM6) and the Ni-Ni distances were determined by energy minimization. To check whether the semiempirical method gives reasonable values, we compared the results of our calculations with DFT optimization for the same type of Ni13 clusters29 (Table 1). In all cases the equilibrium Ni-Ni distance is significantly

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TABLE 1: PM6-Optimized Distances in Selected Ni13 Clusters before and after Addition of Adatoms cluster Ih Oh D3h Ih - S µ2 Ih - CH µ3 Oh - S µ2 D3h - S µ4 a

Dmean(Ni-Ni), Å 2.38 2.39 2.40 2.41 2.41 2.41 2.42

D(Ni-adatom), Å

D(Ni-Ni)adj, Åa

literature values 29

2.25 2.2529 2.2629 2.16 2.13 2.17 2.13

2.46 2.43 2.46 2.46

1.92 Ni-C in the Ni3C crystal32 2.21 Ni-S in the Ni3S4 crystal33

D(Ni-Ni)adj is the Ni-Ni distance of atoms involved in the chemical bonding with adatom.

contracted compared to the bulk value of 2.49 Å. The Ih structure is the most stable (relative stabilization energy 1155 kcal/mol) compared to the other two clusters representing the subsets of the bulk fcc and hcp lattices with Oh and D3h symmetries (1109 and 1124 kcal/mol, respectively). The equilibrium Ni-Ni distance predicted by the PM6 method is longer than the DFToptimized value; however, the order of stability and variations in the bond length for different symmetries are the same. Addition of bidentante or tetradentate sulfur and tridentate CH groups was performed at the polyhedra edges, fourfold sites, and threefold sites, respectively. Then the geometry of clusters was optimized in the framework of the PM6 semiempirical method. For sulfur, fourfold adsorption sites are energetically more advantageous than the threefold and twofold sites, in agreement with the earlier STM studies showing the sulfur preference on the hollow fourfold sites of the Ni(111) surface.31 The increase of Ni-Ni distance in clusters due to the presence of adatoms seems to concern only Ni atoms immediately bound with adatoms. It can be seen that on the cluster side opposite to an adatom, the Ni-Ni distances remain the same as those in bare Ni13 clusters (Figure 4). At the same time, the XRD data evidence that the expansion of the Ni lattice in nanoparticles due to their interaction with thiophene is very pronounced (by up to 0.15 Å, see below) and obviously has long-range order. This means that the increase of Ni lattice parameter cannot be explained only by the presence of carbon or sulfur adatoms on the nanoparticle surface. It is much more probable that this phenomenon is associated with the carbon dissolution in the Ni lattice. 3.2.3. Final Stage of Interaction between Ni/SiO2 and Thiophene. After the initial stage, the bulk Ni transformation proceeds further and finally (after 1-3 h, depending on conditions) attains a steady state. We checked that the corresponding diffraction pattern remains unchanged for at least 3 h under a thiophene/H2 flow. The phase composition of a sample under steady-state conditions is a complex mixture of solid products with the composition depending on the interaction temperature (Figure 5). Characteristic peaks of cubic Ni3-xS2 (JCPDF 00-014-0358) can be distinguished under all conditions. The amount of low-temperature modification of Ni3S2 (heazelwoodite, JCPDF 00-044-1418) is the maximum at the lowest temperature (280 °C) but the corresponding peaks are also present in the product formed at 330 °C. Surprisingly, some intense peaks are observed at all temperatures, which cannot be attributed to any known Ni sulfide or carbide (Figure 5). Thus, we speculate that they can belong to some unknown crystalline modification(s) of Ni sulfide or carbide stable only at high temperatures. The diffraction maxima at 2θ ) 12.46°, 13.10°, 14.09°, and 18.17° observed at 360 °C can be indexed in a hexagonal lattice with a ) 2.66 Å, c ) 4.36 Å. The hexagonal phase with similar lattice parameters was systematically observed when Ni thin films or Ni nanoparticles were prepared in the presence of

Figure 5. XRD patterns of Ni/SiO2 at the steady state after interaction with thiophene at different temperatures.

carbon-containing precursors. In the early studies, it was attributed to a hexagonal modification of metallic Ni.34 However, currently, it seems established that in the majority of cases the presence of these peaks is due to the formation of metastable Ni carbide, Ni3C.32 This conclusion was further supported by the argument of Rodriguez-Gonzalez et al.35 who noted that in Ni hexagonal structure with a ) 2.66 Å, the Ni atomic radius should be equal to 1.33 Å. This value far exceeds the Ni radius in the cubic phase (1.246 Å) and therefore seems rather doubtful. The formation of Ni carbide allows us to complete our explanation of the unexpected phenomenon of partial Ni sulfidation observed in this study and in our previous thermogravimetric analysis experiments.17 Indeed, the equilibrium constant of a metal sulfidation reaction is independent of the amount of solids, being determined by the partial pressures of gases involved in equilibrium.36 Hence, in an open system (under a gas flow), the metal sulfidation, once started, should progress to completion. In our previous work,17 it was shown that the partial transformation of Ni is due to thermodynamic limitations. Also, we showed that in the course of its reaction with nickel, thiophene is first desulfurized and then H2S reacts with the metal. As the HDS activity of Ni diminishes upon sulfidation, the H2S partial pressure should also decrease. If after a certain transformation degree it decreases to the equilibrium value characteristic of the Ni-Ni3S2 system, the sulfidation should stop. The present XRD data revealed an additional effect, which can also prevent Ni from complete transformation. In fact, we observed that at 360 °C nonsulfided Ni transforms into Ni3C rather than remaining in the metallic form. If nickel carbide is more stable toward sulfidation than metallic Ni, its formation should shift the equilibrium in the same direction as the decrease of H2S pressure does and hence stabilize the system at an intermediate transformation degree. 3.3. Interaction of NiO/ZnO with Thiophene. 3.3.1. Reaction without Preliminary Reduction of NiO/ZnO. The contact between an unreduced NiO/ZnO sample and the thiophene/H2

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Figure 6. XRD patterns of the initial NiO/ZnO sample (A) and after its reaction with thiophene at 360 °C for 28 min (B), 100 min (C), and 172 min (D). All unmarked peaks are those of ZnO and/or ZnS.

mixture at 360 °C leads to a rapid formation of Ni0 (Figure 6). As in the case of Ni/SiO2, the lattice parameter of formed Ni particles (a ) 3.70 Å, calculated from the position of the (111) peak) is much larger than that in pure cubic Ni (a ) 3.55 Å at 360 °C). Moreover, the lattice expansion in this case (∆a ) 0.15 Å) is even more pronounced than that after the interaction of thiophene with Ni/SiO2 (∆a ) 0.06 Å). If the lattice expansion is due to the presence of carbon species, a stronger increase of the lattice parameter should reflect the larger amount of carbon dissolved in Ni particles. This effect can be due to the higher catalytic activity of Ni in hydrogenolysis because of the weaker poisoning of Ni with H2S, which is rapidly absorbed by ZnO. The carbon-dilated cubic Ni phase formed in the beginning of reaction remains unchanged during sulfidation of ZnO, showing that the active phase in thiophene reactive adsorption is carbon-modified Ni particles rather than Ni sulfide. Disappearance of ZnO peaks from XRD patterns coincides with the formation of the same hexagonal Ni3C (a ) 2.66 Å, c ) 4.37 Å) as was observed for partially sulfided Ni/SiO2 after the reaction at 360 °C. The fact that Ni carbide appears only after complete ZnO sulfidation suggests that free H2S is required for its formation. Indeed, as long as ZnO is present in the sample, it absorbs all H2S produced from thiophene so that H2S appears in the gas phase only after the end of ZnO transformation.16 It is not clear so far why carbon-modified cubic Ni transforms into Ni3C only after the appearance of free H2S. The fact that in the case of Ni/SiO2 the Ni3C phase was formed simultaneously with Ni sulfide (Figure 5) seems to suggest that partial Ni sulfidation is needed for this transformation. 3.3.2. Reaction after Preliminary Reduction of NiO/ZnO. We observed in our previous work that the preliminary reduction of NiO/ZnO sample in an H2 flow slows down its reaction with thiophene.16 Presumably, the reason for this effect was the partial reduction of ZnO and the formation of a NiZn alloy. This assumption was nicely confirmed by the present in situ XRD study. We observed that Ni0 forms rapidly from NiO after the sample was heated in H2 to 360 °C (Figure 7). However, on further annealing, the pattern changes: after 1 h at 360 °C the NiO peak at 2θ ) 13.83° shifts to 2θ ) 13.73°, (200) peak of Ni0 (2θ ) 15.9°) disappears, and a new peak rises at 2θ ) 14.6°. All these changes are characteristic of the formation of NiZn alloy (JCPDF 01-072-2666). Reaction between reduced Ni/ZnO (containing NiZn alloy) and thiophene proceeds in a manner similar to that of the unreduced NiO/ZnO sample (Figure 8). In the beginning, the alloy is decomposed, producing metallic Ni particles. Their cell

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Figure 7. XRD patterns of NiO/ZnO sample in the initial state (A) after its heating in H2 flow to 360 °C (B) and after annealing in H2 at 360 °C for 1 h (C). All unmarked peaks are those of ZnO.

Figure 8. XRD patterns of the NiO/ZnO sample after reduction in H2 flow at 360 °C for 3 h (A) and its further reaction with thiophene at 360 °C for 30 min (B), 150 min (C), and 260 min (D). All unmarked peaks are those of ZnO and/or ZnS.

parameter (a ) 3.68 Å) indicates that the structure is dilated similarly to other cases. The particle size determined from the broadening of the (111) peak is 6.9 nm, which significantly exceeds the size observed in the unreduced sample (5.5 nm). Larger Ni particles should exhibit lower catalytic activity in thiophene desulfurization, which is the rate-determining step of the overall reaction.16 This difference in size explains (at least partially) the lower transformation rate of reduced solids. After decomposition of the NiZn alloy, the dilated Ni particles remain unchanged throughout the ZnO sulfidation and yield the hexagonal Ni3C phase after complete ZnO transformation as in the case of the direct reaction with thiophene. Conclusions In situ synchrotron XRD study provided us with detailed information on the nature of products formed during the interaction of Ni nanoparticles supported on SiO2 or ZnO with thiophene in a hydrogen atmosphere. For the first time, we observed that the exposure of supported Ni nanoparticles to thiophene initially leads to a substantial increase of the nickel lattice parameter: from 3.55 to 3.61 Å in the case of Ni/SiO2 and from 3.55 to 3.70 Å in the case of Ni/ZnO at 360 °C. It is assumed that this dilation of the Ni lattice is due to dissolution of carbon produced through the thiophene hydrogenolysis, the latter reaction being confirmed by the simultaneous presence of CH4 in the gas phase.

Thiophene and Ni Nanoparticles on SiO2 or ZnO Further reaction with thiophene proceeds by different routes for Ni/SiO2 and Ni/ZnO. The SiO2-supported Ni particles are transformed into a complex mixture of phases with the composition dependent on the reaction temperature. At 280 °C, the main product is Ni3S2 (mixture of the cubic modification and heazelwoodite) but at 360 °C a hexagonal phase is formed with the parameters close to those of Ni3C. In the case of Ni/ ZnO, the formation of dilated Ni particles is followed by sulfidation of ZnO. It is only after the complete ZnO sulfidation that Ni is transformed into a hexagonal phase (presumably Ni3C). The fact that the dilated Ni particles remain unchanged during ZnO-ZnS transformation allows one to conclude that these particles play the role of the active phase, decomposing thiophene during its reactive adsorption. In situ XRD made it possible to confirm that prolonged treatment of Ni/ZnO samples in H2 results in the formation of a Ni-Zn alloy. Its decomposition upon exposure to the thiophene/H2 mixture yields Ni particles that are coarser than those in the unreduced sample (6.9 vs 5.5 nm). This effect explains (at least partially) why preliminary reduction of Ni/ ZnO decreases its reactivity toward thiophene. References and Notes (1) Simon, L. J.; Kooyman, P. J.; van Ommen, J. G.; Lercher, J. A. Appl. Catal., A 2003, 252, 283. (2) Hu, L.; Xia, G.; Qu, L.; Li, C.; Xin, Q.; Li, D. J. Mol. Catal. A 2001, 171, 169. (3) Lu, Y.; Chen, J.; Liu, Y.; Xue, Q.; He, M. J. Catal. 2008, 254, 39. (4) Song, C. Catal. Today 2002, 77, 17. (5) Devers, E.; Geantet, C.; Afanasiev, P.; Vrinat, M.; Aouine, M.; Zotin, J. L. Appl. Catal., A 2007, 322, 172. (6) Bando, K. K.; Kawai, T.; Asakura, K.; Matsui, T.; Le Bihan, L.; Yasuda, H.; Yoshimura, Y.; Oyama, S. T. Catal. Today 2006, 111, 199. (7) Gomez-Cazalilla, M.; Infantes-Molina, A.; Me´rida-Robles, J.; Rodriguez-Castellon, E.; Jime´nez-Lopez, A. Energy Fuels 2009, 23, 101. (8) Tawara, K.; Nishimura, T.; Iwanami, H.; Nishimoto, T.; Hasuike, T. Ind. Eng. Chem. Res. 2001, 40, 2367. (9) Landau, M. V.; Herskowitz, M.; Agnihorti, R.; Kegerreis, J. E. Ind. Eng. Chem. Res. 2008, 47, 6904.

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17069 (10) Park, J. G.; Ko, C. H.; Yi, K. B.; Park, J.-H.; Han, S.-S.; Cho, S.-H.; Kim, J.-N. Appl. Catal., B 2008, 81, 244. (11) Bensaddik, A.; Caballero, A.; Bazin, D.; Dexpert, H.; Didillon, B.; Lynch, J. Appl. Catal., A 1997, 162, 171. (12) Bernardi, F.; Alves, M. C. M.; Scheeren, C. W.; Dupont, J.; Morais, J J. Electron Spectrosc. Relat. Phenom. 2007, 156-158, 186. (13) Gallezot, P.; Bergeret, G. J. Catal. 1981, 72, 294. (14) Bergeret, G.; Gallezot, P. J. Catal. 1984, 87, 86. (15) Bezverkhyy, I.; Ryzhikov, A.; Gadacz, G.; Bellat, J. P. Catal. Today 2008, 130, 199. (16) Ryzhikov, A.; Bezverkhyy, I.; Bellat, J. P. Appl. Catal., B 2008, 84, 766. (17) Bezverkhyy, I.; Gadacz, G.; Bellat, J. P. Mater. Chem. Phys. 2009, 114, 897. (18) Rodriguez-Carvajal, J. Reference Guide for the Computer Program FullProf; Laboratoire Le´on Brillouin, CEA-CNRS, Saclay, France, 2001. (19) Cerius2. Molecular Simulation Inc.: San Diego, CA, June 2000. (20) Stewart, J. J. P. MOPAC2007, ver. 7.065W. Stewart Computational Chemistry: Colorado Springs, CO, 2007. (21) Schoofs, G. R.; Preston, R. E.; Benziger, J. B. Langmuir 1985, 1, 313. (22) Zaera, F.; Kollin, E. B.; Gland, J. L. Langmuir 1987, 3, 555. (23) Huntley, D. R.; Mullins, D. R.; Wingeier, M. P. J. Phys. Chem. 1996, 100, 19620. (24) Zwell, L.; Fasiska, E. J.; Nakada, Y.; Keh, A. S. Trans. Metall. Soc. AIME 1968, 242, 765. (25) Goto, Y.; Taniguchi, K.; Omata, T.; Otsuka-Yao-Matsuo, S.; Ohashi, N.; Ueda, S.; Yoshikawa, H.; Yamashita, Y.; Oohashi, H.; Kobayashi, K. Chem. Mater. 2008, 20, 4156. (26) Cleveland, C. L.; Landman, U. J. Chem. Phys. 1991, 94, 7376. (27) Zakharian, T. Y.; Coon, S. R. Comput. Chem. 2001, 25, 135. (28) Upton, T. H.; Goddard, W. A. Phys. ReV. Lett. 1979, 42, 472. (29) Raghavan, K.; Stave, M. S.; Depristo, E. Chem. Phys. Lett. 1988, 149, 89. (30) Hu, C.; Hu, H.; Li, M.; Tian, A. J. Mol. Struct. THEOCHEM 1999, 491, 155. (31) Yamada, M.; Hirashima, H.; Kitada, A.; Izumi, K.; Nakamura, J. Surf. Sci. 2008, 602, 1659. (32) Nagakura, S. J. Phys. Soc. Jpn. 1958, 13, 1005. (33) Lundqvist, D. ArkiV. Kemi A 1947, 24, 1. (34) Hemenger, P.; Weik, H. Acta Crystallogr. 1965, 19, 690. (35) Rodriguez-Gonzalez, V.; Marceau, E.; Beaunier, P.; Che, C.; Train, C. J. Solid State Chem. 2007, 180, 22. (36) Bartholomew, C. H.; Agrawal, P. K.; Katzer, J. R. AdV. Catal. 1982, 31, 135.

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