Quick X-ray Absorption Fine Structure Studies on the Activation

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Quick X-ray Absorption Fine Structure Studies on the Activation Process of Ni2P Supported on K-USY Kyoko K. Bando,*,† Yuichiro Koike,‡ Toshihide Kawai,§ Gosuke Tateno,§ S. Ted Oyama,*,||,^ Yasuhiro Inada,# Masaharu Nomura,‡ and Kiyotaka Asakura*,§ ‡

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Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organisation, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan † National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan § Catalysis Research Center, Hokkaido University, Kita-ku N21W10, Sapporo, Hokkaido 001-0021, Japan Environmental Catalysis and Materials Laboratory, Department of Chemical Engineering and Chemistry, Virginia Polytechnic Institute and State Unversity, Blacksburg, Virginia 24061-0211, United States ^ Department of Chemical Systems Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan # College of Life Science, Ritsumeikan University, 1-1-1 Noji Higashi, Kusatsu, Shiga 525-8577, Japan ABSTRACT: Ni2P/K-USY (K-promoted ultrastable Y-type zeolite) is an efficient catalyst for hydrotreatment of petroleum feedstocks. The structural changes of the passivated catalyst during activation were followed by quick XAFS (X-ray absorption fine structure) with online gas analysis. Activation was conducted by temperature-programmed reduction (TPR) from ambient temperature to 773 K under a flow of H2. The Ni species was present as Ni(OH)2 in the passivated sample. Two peaks were detected by gas analysis during TPR. The quick XAFS analysis revealed that the first peak at 390 K was due to desorption of water from Ni(OH)2 accompanied by formation of atomically dispersed Ni2þ species. The second peak, observed at 773 K, was due to the regeneration of Ni2P by the simultaneous reductive reaction of the atomically dispersed Ni oxide and the phosphate species, followed by growth of Ni2P nanocrystals. No formation of Ni metallic phase was observed.

1. INTRODUCTION Removal of sulfur from transportation fuels is important to reduce emissions of SO2, reduce deactivation of deNOx catalysts, and prevent corrosion of metallic components inside engines. For decades, great efforts have been expended in the development of new and more efficient hydrotreating catalysts. The performance of commercially available hydrodesulfurization (HDS) catalysts such as Mo and W sulfides promoted by Co and Ni has made remarkable progress.1 On the other hand, transition-metal carbides, nitrides, and phosphides are recognized to have unique catalytic properties and are being investigated as alternative catalysts to the Mo and W sulfides as reported in recent reviews2,3 and papers4
8 In particular, metal phosphide catalysts have received much attention due to their high activity for HDS and hydrodenitrogenation (HDN) of petroleum feedstocks.9
15 The hydrogenation properties of transitionmetal phosphides have been studied in the past,16
19 whereas research focusing on hydrotreating is a newly emerging field. In the course of the study of phosphides, it has been found that the group 6 and iron group compounds show good catalytic performance in HDS, and among these Ni2P showed the highest activity in the HDS of dibenzothiophene (DBT, 3000 ppm S) in the presence of a nitrogen compound (quinoline, 2000 ppm N).15,20,21 Along with the development of better catalysts, investigations into reaction mechanisms and the structure of the r 2011 American Chemical Society

active phase have been conducted.22
24 It is found that the bulk Ni2P structure possesses high tolerance for sulfur. The main species found under working conditions is Ni2P.37 No specific morphology is required for Ni2P to show HDS activity, unlike conventional Co
Mo or Ni
Mo sulfide catalysts which have a layered MoS2 structure with Co and Ni at the edges for high HDS activity. Therefore, it was expected that increasing the surface area of Ni2P particles would improve the catalytic performance. For that purpose, research to produce fine Ni2P particles has been conducted using supports with high surface area, such as K-USY,32 high surface area SiO2, and MCM-41.26 When a support with a larger surface area like SiO2 (208 m2 g
1) was employed, the size of Ni2P particles became smaller and the number of exposed active sites increased. As a result, the overall HDS activity of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was greatly improved, and a shift in the HDS pathway from the direct desulfurization (DDS) to the hydrogenation (HYD) pathway was observed.25,26 In general, zeolites are also good materials as substrates for producing fine particles. However, the acidity of USY was too strong under HDS conditions and cracking of the products occurred, which led to serious Received: November 22, 2010 Revised: February 28, 2011 Published: March 29, 2011 7466

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The Journal of Physical Chemistry C deactivation. Therefore, in this work, K-USY was used as a support in which the acidity of the original USY was neutralized by K. Owing to the success of producing highly dispersed Ni2P on K-USY, its catalytic activity was not only higher than that of commercially available Ni
Mo
Al2O3 catalysts but also at the highest level among various supported Ni2P catalysts. In this study we therefore investigated the preparation process of Ni2P/ K-USY as typical high-performance Ni2P catalysts for the HDS reaction. The Ni2P was prepared conventionally by temperatureprogrammed reduction (TPR) followed by passivation with 0.5% O2/H2 to protect the bulk Ni2P from further oxidation with a thin oxide layer on the Ni2P surface. However, the K-USYsupported Ni2P particles were very small and, as will be shown, underwent oxidation in the passivation process. However, probably because of the fine dispersion and proximity of the phosphorus source, Ni2P on K-USY was easily regenerated by a second TPR. In this work, we focused on the structural changes of the passivated Ni2P during the regeneration process and followed the structural transformation by the in situ QXAFS (quick X-ray absorption fine structure) technique together with online gas analysis of water desorption. Since increasing numbers of new catalytic reactions over Ni2P catalysts have been reported other than HDS and HDN, such as hydrodeoxygenation,21 hydrodechlorination,27
29 water
gas-shift reaction,30 and hydrogen evolution,31 a fundamental study of Ni2P catalysts is of great importance for various fields of catalysis.

2. EXPERIMENTAL SECTION 2.1. Initial Synthesis of Ni2P/K-USY. A K-USY support (surface area 784 m2/g) was prepared by ion exchange of HUSY (Si/Al = 40, Zeolyst, CBV 780) with potassium nitrate (KNO3, 99%), following a method described elsewhere.32 The supported Ni2P catalyst was prepared with excess phosphorus (Ni/P = 1/2) with a loading of 1.16 mmol Ni/g support (corresponding to 12.2 wt % Ni2P/K-USY) using a supported nickel phosphate precursor prepared by incipient wetness impregnation of a solution of nickel nitrate and ammonium phosphate, followed by calcination at 673 K. The supported metal phosphate was reduced to a phosphide by temperature-programmed reduction (TPR) up to 773 K in a quartz tube reactor. The hydrogen flow rate was set at 1000 μmol s
1 (1500 cm3 (NTP) min
1) per gram of sample. After TPR, the sample was cooled to room temperature in helium and passivated under 0.5% O2/He for 6 h. 2.2. Quick XAFS. In situ quick XAFS was carried out at beamline NW10A of PF-AR in IMSS KEK.33 The ring energy and current were 6.5 GeV and 50 mA, respectively. An incident X-ray beam was focused on the sample with a bent cylindrical mirror. Higher harmonics included in the incident beam were removed by a total reflection mirror installed in front of the incident beam slit. The intensity of the incident beam (I0) and that of the transmitted X-ray (I) were monitored by ionization chambers filled with 100% N2 and 25% Ar in N2, respectively. A Si(311) double-crystal monochromator was rotated by a stepping motor at high speed which enabled quick XAFS measurements at 15 s per spectrum in the Ni K-edge region from 7820 to 9430 eV. The I0 and I signals were recorded with a Digital Multimeter (AD7461), and data were stored in a memory board (IK220). It took another 15 s to transfer data to the PC hard disk

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and to rotate the monochromator back to the original position. As a result, measurement of each spectrum was done every 30 s. 2.3. In Situ Experiments. About 60 mg of passivated sample was pressed into a pellet which was 10 mm in diameter and set in the center of an in situ XAFS cell equipped with water-cooled acrylic resin windows.34 After loading the sample, the cell was purged with He and then H2 was introduced. All the reaction conditions, like gas flow rates and cell temperature, were monitored and controlled outside the X-ray absorption experiment hutch of the NW10A beamline. The TPR experiment was carried out under a flow of H2 of 50 cm3/min. The temperature was raised from RT to 773 at 5 K/min and kept at 773 K for 2 h. The effluent gas was monitored with an IR gas analyzer (mks Multigas 2030). 2.4. XAFS Analysis. XAFS data were processed with the program REX2000 (Rigaku Co.)35 XAFS oscillations were extracted by a cubic smoothing method and normalized by the edge height. k3χ(k) was Fourier transformed over k = 30
150 nm
1, and the Fourier peaks over 0.10
0.35 nm were inversely Fourier transformed to k space where curve-fitting analysis was carried out. Phase shift and back-scattering amplitude functions were derived from FEFF8.36

3. RESULTS 3.1. XAFS Studies on the Structure of the Passivated Ni2P in K-USY. Figure 1 shows the Fourier transform of the Ni K-edge

EXAFS observed for the passivated Ni2P/K-USY catalyst. There are two peaks at 0.16 and 0.29 nm which are assigned to Ni
O and Ni
Ni distances, respectively. No contribution of Ni
P scattering was found, unlike the case of a passivated Ni2P on a SiO2 (surface area 90 m2/g), in which the Ni2P structure was retained with a partially oxidized Ni.37 This difference was due to the size of the Ni2P clusters on the K-USY before passivation, which was about 1.1 nm (see section 4) in diameter, corresponding to about 70% Ni atoms being on the outer surface. Since most of the Ni atoms were exposed, the initially synthesized Ni2P was totally oxidized to Ni2þ in the passivation step. The Fourier transform spectrum was different from that of NiO, which showed a strong peak at 0.26 nm.21 The oxidized species was characterized by curve-fitting analysis as shown in Table 1. The Ni
O and Ni
Ni distances were, respectively, 0.205 and 0.311 nm. Structural parameters corresponded well with Ni(OH)2, which has a layered structure. Therefore, it is concluded that the passivated Ni species in K-USY had a Ni(OH)2 structure. Comparing the Ni
Ni coordination number of the passivated sample with that of bulk Ni(OH)2, the size of Ni species was estimated to be 3 ( 1.2 nm in diameter. 3.2. XAFS Studies During TPR. During the TPR process, effluent gas analysis was conducted together with quick XAFS measurements. Figure 2 depicts the signal from water formation with increasing temperature. There were three phases in the TPR process. In phase I water evolution began at 320 K and reached a maximum at 390 K. In phase II the water formation decreased monotonously up to 700 K. In phase III water production started increasing again and showed a maximum at about 5 min after the temperature reached 773 K. Figure 3 shows the changes in Ni K-edge X-ray absorption near-edge structure (XANES) during TPR. A peak at 8349 eV decreased with temperature, while a pre-edge shoulder emerged at around 8340 eV with increasing temperature. Figure 4 shows changes in absorption intensity at two energy points in the course of TPR. The peak at 8349 eV 7467

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Table 1. Results of Curve-Fitting Analysis of EXAFS Observed for Ni2P/K-USY condition before TPR after TPR a

scattering

Na

ΔE/eVc

R/nmb

Ni
O

5.3 ( 0.8

0.205 ( 0.001

Ni
Ni

5.2 ( 0.9

0.311 ( 0.001

Ni
P

2.5 ( 0.8

0.220 ( 0.002

Ni
Ni

4.8 ( 0.8

0.259 ( 0.001


3 ( 2 5(2
7 ( 2 0(2

σ /nmd

R factore

0.008 ( 0.002

0.5%

0.010 ( 0.002 0.010 ( 0.002

0.1%

0.010 ( 0.001

Coordination number. b Distance. c Shift of edge energy. d Debye
Waller factor. e R = ((∑(k3χobs
k3χcal)2)/(∑(k3χobs)2))  100.

Figure 1. Fourier transform of the Ni K-edge EXAFS spectrum observed for the passivated sample. Figure 3. Change of Ni K-edge XANES during TPR. Isosbestic points are indicated with arrows.

Figure 2. Results of gas analysis during TPR observed by the IR gas monitor. Desorption of water was followed with temperature. The TPR process was composed of three phases from I to III as indicated in the figure.

(known as the white line) displayed two large declines. The first small decline started at around 360 K, and the second one occurred at over 700 K, which corresponded well with formation of water. As will be discussed, the first decline is due to adsorbed water desorption and the second to reduction and phosphidation of Ni2þ. Figure 5 shows the changes in the Fourier transforms of the Ni K-edge EXAFS oscillations during TPR. The peak at 0.25
0.35 nm (assigned to a Ni
Ni distance) decreased and nearly disappeared at the end of phase I. No peak emerged in the region 0.2
0.3 nm, which corresponded to the intermetallic Ni
Ni distance. The first Fourier transform peak slowly decreased in phase I and continuously decreased in phase II. In phase III, where the second water formation occurred above 700 K, the first peak of the Fourier transform (Figure 5) shifted to a

longer distance at 0.184 nm and a new feature emerged as a shoulder on the right side at 0.209 nm. As will be discussed, these changes correspond to the transformation of Ni(OH)2 to Ni2P through isolated Ni2þ ions. Curve-fitting analysis of the EXAFS oscillations was carried out. In order to reduce the correlation problems among the fitting parameters, the energy shift was fixed at the value measured at room temperature. The R factor for each fitting was less than 5%. The temperature dependence of the coordination numbers was plotted against the reduction time as shown in Figure 7. The coordination numbers for Ni
O and Ni
Ni bonds of Ni(OH)2 decreased with temperature. The Ni
Ni bond of Ni(OH)2 disappeared at t = 50 min or around 500 K. Even though the coordination number of Ni
O kept decreasing, it was still observable at 500 K (the value was about 4). It gradually decreased to 3 and suddenly disappeared at 700 K. On the other hand, the Ni
P peak appeared at 0.223 ( 0.004 nm. The coordination number of Ni
P was saturated at 2.5 ( 0.4. The Ni
Ni of Ni2P was also found around 700 K, but initially the coordination number was small around 0.6 ( 0.4. After the sample was kept at 773 K for 120 or 130 min when the Ni
P coordination number was saturated, Ni
Ni bond growth started. The coordination number of Ni
Ni of Ni2P increased with temperature and saturated at 4.5 ( 0.5.

4. DISCUSSION Passivation is a procedure commonly employed in heterogeneous catalysis to “protect” supported metallic catalysts from bulk oxidation during routine handling. This is needed because the interaction of oxygen with the metallic particles is highly exothermic, and the sudden exposure of these particles to air 7468

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Figure 4. Change in absorption intensity at 8340 and 8349 eV of Ni K-edge XANES during TPR. I, II, and III correspond to the reduction phases as shown in Figure 2.

Figure 5. Change of the Fourier transform of Ni K-edge EXAFS oscillation (k3χ(k)) during TPR. I, II, and III correspond to the reduction phases as shown in Figure 2.

causes localized heating that leads to uncontrolled oxidation, The heating and oxidation mutually reinforce each other and accelerate over the entire sample so that oftentimes the material actually bursts into flames. Such materials are said to be pyrophoric, and this is the case for supported Ni2P. The spontaneous combustion of samples can be arrested using passivation, in which ideally a monolayer of oxygen is placed on the surface of the supported metal particles to prevent their bulk oxidation, This is carried out by exposing the samples to low concentrations of oxygen over prolonged periods so as to prevent rapid oxidation. In practice, oftentimes more than one monolayer of oxide is formed38 but still the oxide amount is low and the original reduced form is generally recovered after a mild activation treatment by reduction. The question that is addressed in this study is what happens during activation when the supported metallic particles are nanoscale so that the majority of the atoms are at the surface and thus oxidized. Ni2P nanoparticles in freshly synthesized Ni2P/K-USY were well dispersed (70%) due to the high surface area of the support. It was found that because of their small particle size, the Ni2P particles on K-USY were totally oxidized by passivation. In our previous work, it was found that Ni2P on SiO2 was only partially oxidized in the passivation process and the Ni2P structure was retained even after exposure to air.20 This was

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due to the larger particle size (9 nm) of Ni2P on SiO2, compared to that of Ni2P on K-USY (1.1 nm). Studies using high-resolution transmission electron microscopy confirm the presence of Ni2P on silica,38 while the phosphide particles on K-USY cannot be seen.32 The oxidized Ni2P on K-USY was in the form of Ni(OH)2 (Figure 1, Table 1), so likely adventitious water was adsorbed upon exposure to air during sample handling. Effluent gas analysis during TPR (Figure.2) showed that a first water production peak (Phase I) occurred at 390 K, a relatively quiescent period occurred at intermediate temperatures (Phase II), and a second water peak (Phase III) appeared at 773 K. The area of the first peak was larger than that of the second peak. The first peak was mainly due to adsorbed water on the support because EXAFS and XANES spectra showed that no reduction of Ni2þ occurred during the first water desorption. Instead, decomposition of Ni(OH)2 took place to form atomically dispersed Ni2þ species which were stabilized by the interaction with the support. NiðOHÞ2 f NiO þ H2 O

ð1Þ

A similar process where nano-oxide clusters or nano-oxide films were disassembled to form well-dispersed cations strongly interacting with the support surface has been described.39 It was reported that V species are disassembled and assembled reversibly in the presence of H2O. A similar process was reported for Mo oxide clusters where redispersion occurs with desorption of water where polymolybdates are decomposed to monomeric Mo species.40 The changes in the in situ X-ray absorption data taken during reduction of the passivated Ni2P on K-USY are shown in Figure 3. The most notable change is the decline of the large peak (the white line), labeled as phase I, phase II, and phase III, but other changes in the absorption coefficient are evident in the pre-edge region and the postedge region. The decline in the white line indicates reduction of the Ni centers. There are three isosbestic points that indicate the linear conversion of one species to another. In fact, one of them (after the white line) is diffuse, indicating that there may be three species involved,41 which is consistent, as will be discussed, with the proposed transformation of Ni(OH)2 to Ni2þ to Ni2P. The results at different energies are plotted versus time in Figure 4. The signal at 8349 eV, corresponding to the white line, shows a small decline at 360 K and a larger one at above 700 K, which coincides with formation of water in Figure 2. During the first decline, no shift in the edge position was observed as shown in Figure 3, indicating that the oxidation state of Ni was maintained at 2þ and that the Ni atoms were not reduced in phase I. A shift in the edge position occurred suddenly at around 700 K in phase III, coinciding with the second decline of the main peak at 8349 eV. The intensity of the shoulder band at 8340 eV showed a significant increase in phase III. The changes in XANES indicated there were two major structural changes during TPR, which agreed well with the results of water formation. A composite of the changes in the Fourier transforms of the Ni K-edge EXAFS oscillations during TPR are displayed in Figure 5. In phase I the greatest change occurred with the peak at 0.25
0.35 nm, which decreased substantially in intensity to almost disappear at the end of that phase (at 523 K around 50 min). This peak is assigned to the Ni
Ni distance in Ni(OH)2, and its disappearance at the end of phase I is consistent with the TPR data. Also, in phase I the first Fourier transform peak at 0.16 nm, which is assigned to the Ni
O distance in Ni(OH)2, decreased 7469

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Figure 7. Variation of the coordination numbers for Ni
O, Ni
Ni, and Ni
P during the TPR process: (green triangles) Ni
O, (blue upside down triangles) Ni
Ni in Ni(OH)2, (black squares) Ni
P, and (red circles) Ni
Ni in Ni2P. Figure 6. Ni K-edge EXAFS oscillation (k3χ(k)) (a) and its Fourier transform (b) observed at 323 K under H2 after TPR to 773 K.

slowly in intensity. In phase II that first peak continuously decreased. As there were no more Ni
Ni contributions, the peak is assigned to Ni
O bonds in isolated Ni2þ ions. These could be due to bonds with the surface or hydroxyl groups. In phase III beginning at 773 K this Ni
O peak slowly shifted to the longer distance of 0.184 nm which corresponds to a Ni
P distance. This is the temperature at which the second H2O peak occurs in the TPR. At the same time a new feature emerged as a shoulder at 0.209 nm, which grew in intensity, and this is assigned to the Ni
Ni distance in Ni2P. A detailed pattern is shown in Figure 6, which reports the EXAFS oscillation (Figure 6a) and its Fourier transform (Figure 6b) measured at 323 K after TPR to 773 K. Measurement of the XRD pattern under the same condition showed no discernible peaks assignable to Ni2P.32 A two-shell curve-fitting analysis of the main peaks in the Fourier transform spectrum was carried out, and the results are shown in Table 1. The distances for Ni
P and Ni
Ni bonds were 0.220 and 0.259 nm, respectively, and the corresponding coordination numbers were 2.5 and 4.8. These values were almost the same as those determined for the catalyst under high-temperature reduction conditions.42 The particle size of Ni2P was estimated from the coordination number of Ni
Ni at 0.259 nm, and it is 1.1 nm based on a spherical shape. Analysis of the Ni
P coordination number gave results consistent with this but are not included here because the low sensitivity of the Ni P coordination to size gives substantial errors. The small size is consistent with the X-ray diffraction results, which showed no diffraction pattern of Ni2P for Ni2P/K-USY.32 All evidence suggests that the Ni2P nanoparticles were effectively dispersed on the high surface area K-USY support. Early work on the preparation of Ni2P showed that PH3 was formed simultaneously with H2O formation.20,43 Subsequent work has shown that partly or completely reduced transition metals catalyzes the reduction of phosphite or phosphate components to form phosphine (PH3) that itself reacts with the nickel ions44,45 to form the transition-metal phosphide The transformations are summarized in Figure 7. In phase I the Ni
Ni bonds substantially disappear, and the Ni
O bonds decrease in number. In phase II the decline in Ni
O bonds continues. In phase III the atomically dispersed Ni species are completely reduced at 773 K, and this is accompanied with a second water production peak. In the early part of phase III the

free Ni atoms react with reduced P to form Ni2P nuclei rather than to form metallic Ni particles. Thus, Ni
P bonds are produced first, forming small Ni2P nuclei. This is followed by crystal growth to form nanoparticles, as confirmed by the emergence of the Ni
Ni bonds of Ni2P in EXAFS. Decomposition of Ni(OH)2 and stabilization of the atomically dispersed Ni species in phases I and II were requisites for the formation of Ni2P, which enabled close contact of Ni with P species. Ni2þ was stabilized by support interactions until phase III, when the simultaneous reduction of Ni and P species occurred to form Ni2P. Otherwise, Ni(OH)2 or NiO nanoparticles would have been directly transformed to metallic particles in phase II. The preferable formation of Ni
P over Ni
Ni prevented either Ni metallic particles evolution or further growth of Ni2P particles. A previous TPR study on calcined Ni2P/K-USY showed only one peak at a high temperature,32 whereas the passivated sample in this work has two peaks due to the two phase transitions as discussed above. The difference arises because the as-calcined catalyst is composed of Ni phosphate species like Ni3(PO4)2 or Ni2P2O7; in other words, Ni species have already formed a stable compound with phosphorus after calcination, and therefore, no dehydration process occurred at a lower temperature as in the case of the passivated catalyst. In addition, transformation from phosphate to phosphide required higher temperature for the calcined catalyst, compared to the simultaneous reduction of the Ni and P species to form Ni2P in the passivated catalyst. Although it was found that the highly dispersed Ni2P/K-USY catalyst passivated with oxygen could be regenerated by rereduction with hydrogen, complete disruption of the structure by passivation is of some concern for other systems and points to the need for development of new passivation methods. One example of an alternative technique is the use of H2S as a passivating gas, which in the case of Ni2P produces a highly active catalyst.46 This study thus shows that quick XAFS is a powerful technique that allows structural changes to be followed which happen in the scale of minutes. Simultaneous analyses of gas-phase products and XAFS spectra give insight on detailed microscopic structural changes during a chemical process.

5. CONCLUSIONS Structural changes occurring in a passivated Ni2P/K-USY catalyst during activation by temperature-programmed reduction (TPR) were studied by in situ quick X-ray absorption fine structure (XAFS) combined with online gas analysis. Extended XAFS analysis showed that the passivated catalyst was composed 7470

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The Journal of Physical Chemistry C of Ni(OH)2. Two water formation peaks were detected during TPR, which corresponded well with the structural changes observed by quick XAFS. The reduction process was composed of three phases. In phase I there was a first water evolution at 390 K, which corresponded to decomposition of Ni(OH)2 to atomically dispersed Ni2þ species. In phase II no notable structural change nor gas desorption was observed with gradual loss of Ni
O bonding. In phase III a second water production peak was observed at 773 K, in which Ni2P was generated by simultaneous reduction of NiO and phosphates. No formation of Ni metallic particles was observed, probably due to a strong interaction between atomically dispersed Ni atoms and the support.

’ AUTHOR INFORMATION Corresponding Author

*K.K.B.: phone, þ81-29-861-9340; fax, þ81-29-861-4548; e-mail, [email protected]. K.A.; phone/fax, þ81-11-7069113; e-mail, [email protected]. S.T.O.: phone, 540-2315309; fax, 540-231-5022; e-mail, [email protected], oyama@ chemsys.t.u-tokyo.ac.jp.

’ ACKNOWLEDGMENT This work was supported by the US Department of Energy, Office of Basic Energy Sciences, through Grant DE-FG02963414669 and JST Grant-in-Aid for Scientific Research (Category S, no. 16106010). The XAFS experiments were conducted under approval of PF-PAC (Project no. 2008G129). ’ REFERENCES (1) Okamoto, Y.; Kubota, T. Catal. Today 2003, 86, 31–43. (2) Alexander, A.-M.; Hargreaves, J. S. J. Chem Soc. Rev. 2010, 39, 4388–4401. (3) Oyama, S. T.; Gott, T.; Zhao, H.; Lee, Y.-K. Catal. Today 2009, 143, 94–107. (4) Schlatter, J. C.; Oyama, S. T.; Metcalfe, J. E.; Lambert, J. M. Ind. Eng. Chem. Res. 1988, 27, 1648–1653. (5) Abe, H.; Cheung, T. K.; Bell, A. T. Catal. Lett. 1993, 21, 11–18. (6) In The Chemistry of Transition Metal Carbides and Nitrides; Oyama, S. T., Ed.; Blackie Academic and Professional: London, 1996. (7) Robinson, W. R. A. M.; van Gastel, J. N. M.; Koranyi, T. I.; Eijsbouts, S.; van Veen, J. A. R.; de Beer, V. H. J. J. Catal. 1996, 161, 539–550. (8) Koranyi, T. I. Appl. Catal., A 2003, 239, 253–267. (9) Stinner, C.; Prins, R.; Weber, Th. J. Catal. 2000, 191, 438–444. (10) Stinner, C.; Prins, R.; Weber, Th. J. Catal. 2001, 202, 187194. (11) Clark, P.; Li, W.; Oyama, S. T. J. Catal. 2001, 200, 140–147. (12) Oyama, S. T.; Clark, P.; Teixiera da Silva, V. L. S.; Lede, E. J.; Requejo, F. G. J. Phys. Chem. B 2001, 105, 4961–4966. (13) Oyama, S. T.; Clark, P.; Wang, X.; Shido, T.; Iwasawa, Y.; Hayashi, S.; Ramallo-Lopez, J. M.; Requejo, F. G. J. Phys. Chem. B 2002, 106, 1913–1920. (14) Phillips, D. C.; Sawhill, S. J.; Self, R.; Bussell, M. E. J. Catal. 2002, 207, 266–273. (15) Oyama, S. T. J. Catal. 2003, 216, 343–352. (16) Sweeny, N. P.; Rohrer, C. S.; Brown, O. W. J. Am. Chem. Soc. 1958, 80, 799–800. (17) Muetterties, E. L.; Sauer, J. C. J. Am. Chem. Soc. 1974, 96, 3410–3415. (18) Nozaki, F.; Adachi, R. J. Catal. 1975, 40, 166–172. (19) Nozaki, F.; Tokumi, M. J. Catal. 1983, 79, 207–210. (20) Oyama, S. T.; Wang, X.; Lee, Y. K.; Bando, K.; Requejo, F. G. J. Catal. 2002, 210, 207–217.

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(21) Oyama, S. T.; Wang, X.; Lee, Y.-K.; Chun, W.-J. J. Catal. 2004, 221, 263–273. (22) Oyama, S. T.; Gott, T.; Asakura, K.; Takakusagi, S.; Miyazaki, K; Koike, Y.; Bando, K. K. J. Catal. 2009, 268, 209–222. (23) Bando, K. K.; Wada, T.; Miyamoto, T.; Miyazaki, K.; Takakusagi, S.; Gott, T.; Yamaguchi, A.; Nomura, M.; Oyama, S. T.; Asakura, K. J. Phys.: Conf. Ser. 2009, 190, 012158–65. (24) Kawai, T; Sato, S.; Chun, W.-J.; Asakura, K.; Bando, K. K.; Matsui, T.; Yoshimura, Y.; Kubota, T.; Okamoto, Y.; Lee, Y.-K.; Oyama, S. T. Chem. Lett. 2003, 32, 956–957. (25) Shu, Y.; Lee, Y.
K.; Oyama, S. T. J. Catal. 2005, 236, 112–121. (26) Oyama, S. T.; Lee, Y.-K J. Catal. 2008, 258, 393–400. (27) Chen, J.; Chen, Y.; Yang, Q.; Li, K.; Yao, C. Catal. Commun. 2010, 11, 571. (28) Liu, X. G.; Chen, J. X.; Zhang, J. Y. Ind. Eng. Chem. Res. 2008, 47, 5362. (29) Zhou, S.; Chen, J.; Liu, X.; Zhang, J. Chinese J. Catal. 2007, 28, 498. (30) Liu, P.; Rodriguez, J. A.; Takahashi, Y.; Nakamura, K. J. Catal. 2009, 262, 294. (31) Liu, P.; Rodriguez, J. A. J. Am. Chem. Soc. 2005, 127, 14878. (32) Lee, Y.
K; Shu, Y.; Oyama, S. T. Appl. Catal. A: Gen. 2007, 322, 191–204. (33) Nomura, M.; Koike, Y.; Sato, M.; Koyama, A.; Inada, Y.; Asakura, K. AIP Conf. Proc. 2007, 882, 896–899. (34) Bando, K. K; Saito, T.; Sato, K.; Tanaka, T.; Dumeignil, F.; Imamura, M.; Matsubayashi, N.; Shimada, H. Top.Catal. 2002, 18, 59–65. (35) Asakura, K. In X-ray Absorption Fine Structure for Catalysts and Surfaces; Iwasawa, Y., Ed.; World Scientific: Singapore, 1996; Vol. 2, p 33. (36) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. Rev. B 1995, 52, 2995–3009. (37) Kawai, T.; Sato, S.; Chun, W. J.; Asakura, K.; Bando, K. K.; Matsui, T.; Yoshimura, Y; Kubota, T.; Okamoto, Y.; Lee, Y. K.; Oyama, S. T. Phys. Scr. 2005, T115, 822–824. (38) Sawhill, S. J.; Layman, K. A.; Wyk, D. R.; Engelhard, M. H.; Wang, C.; Bussell, M. E. J. Catal. 2005, 231, 300–13. (39) Yoshida, S.; Tanaka, T.; Nishimura, Y.; Mizutani, H.; Funabiki, T. Proc. Int. Congress Catal. 1988, 1473. (40) de Boer, M.; van Dillen, A. J.; Koningsberger, D. C.; Geus, J. W.; Vuurman, M. A.; Wachs, I. E. Catal. Lett. 1991, 11, 227–239. (41) Moore, J. W.; Pearson, R. G.; Frost, A. A. Kinetics and Mechanism, 3rd ed.; John Wiley and Sons: New York, 1981; p 49. (42) Kawai, T.; Bando, K. K.; Lee, Y. K.; Oyama, S. T.; Chun, W.-J.; Asakura, K. J. Catal. 2006, 24, 20–24. (43) Wang, X.; Clark, P.; Oyama, S. T. J. Catal. 2002, 208, 321–331. (44) Cecilia, J. A.; Infantes-Molina, A.; Rodríguez-Castellon, E.; Jimenez-Lopez, A. J. Phys. Chem. C 2009, 113, 17032–17044. (45) Liua, S.; Liua, X.; Xua, L.; Qiana, Y.; Ma, X. J. Cryst. Growth 2007, 304, 430–434. (46) Duan, X.; Teng, Y.; Wang, A.; Kogan, V. M.; Li, X.; Wang, Y. J. Catal. 2009, 261, 232–240.

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dx.doi.org/10.1021/jp111657z |J. Phys. Chem. C 2011, 115, 7466–7471