Dynamic Phenomena during Reduction of α-NiMoO4 in Different

Mar 21, 2007 - Huaiyuan Wang , Xiaoshuang Cheng , Bo Xiao , Chijia Wang , Li Zhao , Yanji Zhu. Catalysis Surveys from Asia 2015 19 (2), 78-87 ...
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MATERIALS AND INTERFACES Dynamic Phenomena during Reduction of r-NiMoO4 in Different Atmospheres: In-Situ Thermo-Raman Spectroscopy Study Hany M. Abdel-Dayem* Chemistry Department, Faculty of Science, Ain Shams UniVersity, 11566-Abassia, Cairo, Egypt

A study by in-situ thermo-Raman spectroscopy was carried out to understand the structural changes occurring during the reduction of R-NiMoO4 in different atmospheres (viz., H2, CO, and C3H6) at different temperatures and time. In addition, the dynamics of R-NiMoO4 during reduction/reoxidation were also investigated. The products from reoxidation of reduced R-NiMoO4 in different atmospheres were analyzed by XRD. During the reaction of H2 with R-NiMoO4 at 400 °C a new band was detected at 455 cm-1, which is characteristic of reduced surface molybdenum oxide species. Thus, the formation NiMoO3 and/or Ni2Mo3O8 suboxide intermediates was suggested. On the other hand, reversible H2 reduction/O2 reoxidation dynamics was observed, where the main Raman features of H2-reduced R-NiMoO4 were restored completely after reoxidation by oxygen. On the contrary, during the reaction of C3H6 with R-NiMoO4 the formation of a stable suboxide was difficult to detect, and irreversible C3H6 reduction/O2 reoxidation dynamics was suggested as the extensive heating of the C3H6-reduced sample in 10% O2/He for 2 h at 400 °C did not restore the Raman features of the R-NiMoO4 observed prior to reduction. No significant change was observed in the Raman spectra of R-NiMoO4 during heating in a flow of 10% CO/He, except for the steady decrease in the intensities of all bands with increasing temperature. This may be due to the fact that at high temperatures CO decomposed and the produced carbon deposited on the sample surface. 1. Introduction The importance of nickel molybdate in heterogeneous catalysis has been reviewed recently by Maderia et al.1 As R-NiMoO4 is an effective catalyst in hydrodesulfurization of thiophene after activation by H2,2 the phase composition of the products from the reduction with hydrogen have been investigated extensively.3 On the other hand, oxidation of propene over nickel molybdate has been the subject of many studies; the catalyst was characterized extensively before and after reaction to clarify the nature of the active site.4 At the same time, CO is the deep oxidation product of many hydrocarbons over R-NiMoO4 (e.g., propane, propene, etc.), yet the influence of CO in the catalytic performance of nickel molybdate during reaction is still not clear.5 Study of the change in the molecular structure of R-NiMoO4 during exposure to these reducing gases (i.e., H2, C3H6, and CO) could help in enhancement of the catalyst selectivity and control its deactivation during the abovementioned catalytic processes. It is well-known in the literature that R-NiMoO4 is able to adopt different crystal structure or different phases (R- or β-configuration) depending on the temperature.6 A large number of characterization studies on the active phase of this catalyst have been reported since the catalytic performance depends heavily on the structure of the active phase.7,8 Time-resolved XRD was used to study the R-to-β transformation in nickel molybdate at different temperatures.9 The variations in the diffraction pattern of R-NiMoO4 were analyzed as a function of temperature using Rietveld refinement.8 During the time* Tel: +202-483-1836-108. [email protected].

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resolved XRD experiments, only the diffraction lines of R- and β-NiMoO4 were seen. However, marked changes were observed in the cell dimensions of the nickel molybdate crystal, which suggest a possible path or “reaction coordinate” for the R-to-β transition. A powder diffraction pattern for a transition state between R- and β-NiMoO4 was calculated. The authors suggested that the system evolves from R to β through amorphous or non-long-range periodic intermediates generated at defect sites or imperfections in the crystal lattice. On the other hand, intermediate molybdenum suboxides are believed to be the active sites in molybdenum-based catalysts; hence, detection of this intermediate may be of crucial importance.10 Recently, in-situ time-resolved X-ray spectroscopy has also been used to study the reduction of R-NiMoO4 and β-CoMoO4 by H2 under atmospheric pressure.11 During the reduction of β-CoMoO4, well-defined intermediate(s) (Co2Mo3O8 and/or CoMoO3) were detected, but the detection of a stable suboxide intermediate did not occur during the reduction of R-NiMoO4. Raman spectroscopy has contributed significantly to the progress in this area of catalysis because of its in-situ capabilities and its ability to identify metal oxide structures.12 Several authors have reported ex-situ Raman spectra of supported and unsupported R- and β-NiMoO4.13 Confocal Raman images of stoichiometric R-NiMoO4 were also recorded at room temperature;14 the most interesting result is that tetrahedral sites characteristic of the β-phase were detected in addition to the expected octahedral ones characteristic of the R-phase.15 In addition, the Raman band shifts of metal molybdate catalysts exchanged with 18O traces at the oxidation sites by the reduction/ reoxidationmethodwereexaminedextensivelyintheliterature.16-18 On the other hand, in-situ laser Raman spectroscopy has been

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Figure 1. Raman spectra of R-NiMoO4: (a) parent sample collected at room temperature (low-wave number Raman spectrum of parent R-NiMoO4 (curve inset)), (b) collected at 400 °C during heating in flow of pure He, and (c) collected at ambient temperature after heating in flow of pure He for 1 h.

proved to be a very powerful characterization technique for obtaining useful information about the molecular structure of the oxides during reaction.19,20 Recently, in-situ thermo-Raman spectroscopy was used to study the change in molecular structure of molybdenum-based catalysts during the catalytic oxidation of hydrocarbons.21 In fact, in-situ thermo-Raman spectroscopy can be used to study the behavior of R-NiMoO4 catalyst at different stages of reduction/reoxidation processes. Until now, however, no detailed in-situ thermo-Raman spectroscopy study was reported in the literature during the reduction of R-NiMoO4 in the above-mentioned reducing atmospheres followed by reoxidation with oxygen. In the present work, the variations in the molecular structure of R-NiMoO4 during reaction with the above-mentioned reducing gases were investigated as a function of temperature using in-situ thermo-Raman spectroscopy, with the hope of identifying an NiMoOx intermediate. 2. Experimental Pure stoichiometeric R-NiMoO4 was prepared by coprecipitation from aqueous solutions of 750 mL of 0.057 M ammonium heptamolybdate (Merck 99+%) and 750 mL of 0.4 M nickel nitrate (Adrich 99+%) in the thermo-regulated conditions at a temperature of 63 °C and at pH ) 6.0.7 The purity of the prepared NiMoO4 was verified from XRD analysis, where XRD analysis did not give any indication about the presence of MoO3 in the prepared sample, namely, no weight loss (endothermic peak) assigned to the sublimation of MoO3 was observed above 700 °C.22 The recorded XRD d-spacings: 6.19, 5.49, 3.714, 3.509, 3.09, 2.74, 2.32, 2.18, 2.09, 2.06, 1.99, 1.95, 1.91, 1.83, 1.80, and 1.71 Å corresponded to those reported by Sleight et al.23 and also corresponded to the monoclinic R-NiMoO4 phase of JCPDs file 31-0902.24 Laser Raman spectroscopy was performed with a Dilor Labram spectrometer equipped with a computerized X-Y transition stage, an in-situ cell from Lincam Scientific (provided by USAID), and a 632.8-nm He-Ne laser as an excitation source. The laser power at the sample surface was 10 mW. The spectrometer resolution was 5 cm-1. The full spectra, hereafter shown for Raman shifts, between 200 and 1200 cm-1, were built by accumulating 10 scans during a total time of 100 s. To reduce the heat over the sample with the laser beam during spectra acquisition, the X-Y transition stage holding the cell

was automatically moved backward and forward at a velocity of 2 mm s-1. Calibration was made using silicon at 520 cm-1. The Raman experiments were performed in a dark room to avoid any fluorescence contribution. The Raman spectra data were stored on a computer, and peak-shape analysis was then carried out. Curve-fitting was performed by Lorentzian function; the integral intensity of the band at 961 cm-1 of the R-NiMoO4 spectrum collected at ambient temperature in the flow of each reduced gas mixture was used for quantitative interpretation of Raman spectra, particularly normalization of all band intensities. The reduction/reoxidation method was performed as follows. Before each experiment, the sample was heated in 10% of O2 in He at 400 °C for 1 h. A 10-mm-diameter pellet of R-NiMoO4 sample was heated under a flow of gas mixture (dry and free of oxygen) of either 10% H2 or 10% CO or 10% C3H6 with He as a diluent at a rate of 2 °C/min from ambient temperature to 400 °C. The reaction gases were mixed prior to feed to the Raman cell. Every 10 °C the temperature was held for 10 min before collecting the spectrum. After 1 h at 400 °C the reducing gas was substituted for a flow of pure He (free of oxygen) for 20 min, then He was changed with 10% O2/He and the spectrum was collected during a 1-h period. The total gas flow rate was 50 cm3 min-1 in all experiments. XRD analysis was performed in a Philips Pert-MPD (multipurpose X-ray diffractometer) by using the KR1,2 radiation of Cu (λ ) 1.5406 Å) for 2θ angles varying from 10° to 80° at ambient temperature. The fluorescence contribution was eliminated from the diffracted beam using a curved graphite monochromator. The scan rate was 0.4 degree min-1, corresponding to a step size of 0.04 degree and a step time of 3 s. Rietveld refinements were performed with the program Win cell 1.1. 3. Results and Discussion The in-situ laser Raman spectrum of the parent R-NiMoO4 is shown in Figure 1; the Raman spectrum of the sample collected at ambient temperature consisted of a very strong band at 961 cm-1, two strong bands at 913 and 706 cm-1, and a group of weak bands at 416, 387, 368, and 263 cm-1 (Figure 1, inset). This spectrum is consistent with that reported in the literature for stoichiometric R-NiMoO4.7 The assignment of the Raman bands of the R-NiMoO4 is based on the Raman studies

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Figure 2. Raman spectra of R-NiMoO4 collected during the reduction with 10% H2/He at different temperatures (300-400 °C). The change of the integrated normalized intensities of the bands at 961, 913, and 706 cm-1 with temperature is shown in the inset.

Figure 3. Raman spectra of R-NiMoO4 collected during the reduction with 10% H2/He at different time intervals at 400 °C. Raman spectra of R-NiMoO4: (a) collected at 400 °C after reduction for 1 h in flow of 10% H2/He, and (b) collected at 25 °C after reoxidation with 10% O2/He for 1 h at 400 °C are shown in the inset.

for R-CoMoO4, which is isotypic with R-NiMoO4, and based also on Raman studies of other molybdates species.25-28 The bands at 961 and 913 cm-1 have been assigned to the symmetric and asymmetric stretching modes of the terminal ModO bond, and the band at 706 cm-1 has been attributed to the Ni-OMo symmetric stretch. In addition, the bands at 416, 387, and 368 cm-1 have been attributed to the bending mode of Mo-O, and the band at 263 cm-1 has been assigned to the deformation mode of Mo-O-Mo. As shown in Figure 1, heating the R-NiMoO4 sample in a flow of He at 400 °C for 1 h affects the dynamics of the band intensities but produced a small shift in the positions of all the bands to lower wave number by about 5 cm-1. Cooling the R-NiMoO4 back to ambient temperature restored the positions of all the bands present prior to thermal treatment, which indicates that the observed small shift in band positions is due to temperature treatment. The Raman spectra associated with reduction of R-NiMoO4 with H2 are presented in Figure 2. As shown (Figure 2 inset),

once heating was started, the bands at 961, 913, and 706 cm-1 decreased in intensity; however, the intensities of all bands remained nearly constant as the temperature increased from 50 to 300 °C. At temperatures above 300 °C, the intensity of the band at 961 cm-1 decreased sharply. In addition, broadening of the band at 706 cm-1 occurred, that is, the full width at halfmaximum (fwhm) of the peak at 706 cm-1 increased from 16.8 to 27.3 cm-1 as the temperature was increased from ambient temperature to 400 °C. On the other hand, at 400 °C, a new band appeared at 455 cm-1, which could be assigned to reduced surface molybdenum oxide species.29 At the same temperature, continuous exposure of the R-NiMoO4 pellet to a flow of 10% H2 in He resulted in the gradual disappearance with time of the new band at 455 cm-1 (Figure 3). When a flow of 10% O2 in He was used instead of a 10% H2/He mixture (Figure 3 inset), the original Raman features of R-NiMoO4 spectra were restored completely after 1 h of exposure of the sample to oxygen atmosphere at 400 °C.

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Figure 4. Raman spectra of R-NiMoO4 collected during the reduction with 10% C3H6/He at different temperatures (300-400 °C). The change of the integrated normalized intensities of the bands at 961, 913, and 706 cm-1 with temperature is shown in the inset.

Figure 5. Raman spectra of R-NiMoO4 (a) collected at 400 °C after reduction for 1 h in flow of 10% C3H6/He, (b) collected at 25 °C in flow of 10% C3H6/He after reduction for 1 h at 400 °C, and (c) collected at 25 °C after reoxidation with 10% O2/He for 2 h at 400 °C.

In contrast, reduction of R-NiMoO4 with 10% C3H6 in He resulted in a gradual decrease in the intensities of all the bands as the temperature increased from 50 to 370 °C (Figure 4 inset). At temperatures greater than 300 °C, the weak bands at 491, 387, 368, and 263 cm-1 were undetectable (Figure 4). Furthermore, at 400 °C, the bands at 706, 913, and 961 cm-1 disappeared after 1 h of continuous exposure of the R-NiMoO4 pellet to a stream of 10% C3H6 in He (Figure 5a). Cooling the sample back to room temperature did not restore the sharper Raman features present in the spectra of R-NiMoO4 prior to reduction (Figure 5b), which indicates that these observed Raman changes are not due to temperature treatment. Upon heating the C3H6reduced sample in a stream of 10% O2 in He at 400 °C for 2 h, the reappearance of the bands at 706, 491, 387, 368, 268, and 250 cm-1 did not occur; however, the intensity of the band at 961 cm-1 is slightly increased (Figure 5c). In-situ thermo-Raman spectroscopy measurements of the catalyst during the reduction with CO at different temperatures revealed that the Raman features of R-NiMoO4 remain unchanged (Figure 6). The sole change observed is the steady decrease in the intensities of all bands with increasing reduction temperature from ambient to 400 °C, which is associated with a small shift (ca. 7 cm-1) in positions of all bands to lower wave number.

The X-ray diffraction patterns of the parent R-NiMoO4 and the reduced in-situ samples in hydrogen and propene atmospheres after reoxidation by oxygen are shown in Figure S1 (Supporting Information).30 The X-ray diffraction pattern of the reoxidized/H2-reduced sample clearly presents peaks due to R-NiMoO4. However, the intensities of all the peaks decreased after reduction, accompanied with broadening of peaks and increasing of background. This decrease in intensity is more pronounced for the peaks detected at 2θ ) 19.00, 26.76, 41.19, 53.42, 61.83, and 66.20 degree. In contrast, the XRD diffraction pattern for the reoxidized/C3H6-reduced sample presents peaks due to MoO2 of d-spacings 3.407, 2.815, 2.426, 1.847, 1.722, 1.692, 1.604, 1.549, 1.526, 1.381, and 1.290 Å (JCPDS file 781071) and Ni of d-spacings 2.033, 1.760, 1.246 Å (JCPDS file 04-0850). In addition, the peaks characteristic of R-NiMoO4 were also detected with a significant shift and decrease in the intensity (Supporting Information, Figure S1c), which indicates no full conversion of R-NiMoO4 under these reaction conditions. Furthermore, weak peaks of d-spacing 4.518, 3.970, 2.888, 2.855, 2.605, 2.257, 1.991, 1.788, and 1.654 Å were detected, which is possibly referred to intermetallic compounds between Mo and Ni species (viz., MoNi4 of JCPDS file 03-1036 and/or MoNi of JCPDS file 48-1745). No specific change was observed in the XRD pattern of the reoxidized R-NiMoO4 sample reduced in CO atmosphere. The simulated cell dimensions “a ) 9.5741 Å, b ) 8.7440 Å, c ) 7.6552 Å, β ) 114.136°” of the parent R-NiMoO4 are very close to those reported by previous studies for monoclinic R-NiMoO4 [JCPDS file 31-902].23 The simulated cell dimensions “a ) 9.5374 Å, b ) 8.7678 Å, c ) 7.6569 Å, β ) 114.189°” of the reoxidized/H2-reduced sample show a small contraction in the “a” axis and a small expansion in the “b” axis. However, a pronounced modification was observed in the simulated cell dimensions “a ) 8.8919 Å, b ) 9.5110 Å, c ) 8.8145 Å, β ) 104.047°” of the reoxidized/C3H6-reduced sample, namely, a contraction was observed in the “a” axis and expansions were observed in both “b” and “c” axes. This was accompanied with a decrease in the degree of “β” angle. The in-situ thermo-Raman spectra of R-NiMoO4 recorded during reduction in the different reducing atmospheres showed different features. In the case of the reduction of R-NiMoO4 with H2, the new Raman band that appeared at 455 cm-1 at 400 °C for 30 min might indicate the presence of a well-defined

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Figure 6. Raman spectra of R-NiMoO4 collected during the reduction with 10% CO/He at different temperatures (300-400 °C). The change of the integrated normalized intensities of the bands at 961, 913, and 706 cm-1 with temperature is shown in the inset.

oxide intermediate of short lifetime. The experiments of timeresolved XRD recorded during reduction of β-CoMoO4 with H2, which is isotypic with R-NiMoO4, showed the presence of a well-ordered suboxide intermediate: Co2Mo3O8 and/or CoMoO3.11 At this point, this new Raman band probably belongs to Ni2Mo3O8 and/or NiMoO3 suboxides. In addition, the observed short lifetime of this suboxide intermediate may be explained by the so-called “contracting surface model” where, during the reduction, this suboxide grows uniformly around the sample and forms a spherical core that shrinks with time.30 In analogy, a similar model was also adopted by Rodriguez et al.11 to explain the in-situ XRD results recorded during reaction of R-NiMoO4 with H2. Furthermore, one cannot exclude that this intermediate has an amorphous characteristic, which could not be detected by standard XRD (Supporting Information, Figure S1).30 However, no extra Raman bands could be seen during the reduction of R-NiMoO4 with C3H6, which indicates that it is difficult to predicate the generation of an intermediate that has a stable periodic structure. In fact, the data in Figures 2 and 4, collected as a function of temperature during reduction with H2 and C3H6, respectively, show different behaviors, where reduction of R-NiMoO4 with C3H6 started at lower temperature than with H2. This trend might suggest the fast reduction of R-NiMoO4 with C3H6 that probably prevents the formation of Ni2Mo3O8 and/or NiMoO3 intermediates. On the other hand, the trend of reoxidation of H2-reduced R-NiMoO4 with oxygen is completely different from that of reoxidation of C3H6-reduced sample. The results in Figure 3 (inset) and the simulated cell-dimension data indicate a reversible H2-reduction/oxygen-reoxidation mechanism, where the main Raman features of the H2-reduced R-NiMoO4 sample were restored completely after reoxidation by O2. Such behavior is consistent with the so-called “corner-sharing/edge-sharing rearrangement” model,32 in which the edge-sharing MoO6-x rearrange to corner-sharing MoO6 octahedra through filling the vacancy by oxygen. In contrast, the data in Figure 5 indicate irreversible transformation “no full conversion” of C3H6-reduced sample to its parent structure upon reoxidation with oxygen. A previous kinetic study of the oxidation of propene over R-NiMoO4 indicated that the main products detected are acrolein and acetaldehyde at temperature < 300 °C,33 the first compound

being the result of nucleophilic attack on R-carbon and the second compound being the result of electrophilic attack on the double bond. Due to the industrial importance of oxidation of propene to acrolein over metal molybdate, the nucleophilic mechanism of the reaction has been studied extensively in the literature.34-36 According to this mechanism, it was proposed that both the terminal oxygen of octahedral MO6 (M ) Co, Ni, Bi, etc.) and the “oxo group” of ModO played crucial roles in the oxidation of propene to acrolein. The obtained in-situ thermo-Raman results of the reduction of R-NiMoO4 with propene are consistent with the above-mentioned mechanism, where a significant decrease in the intensities of all the Raman bands was observed, especially the intensity of the band corresponding to the ModO bond vibration at 961 cm-1 at T < 300 °C. However, the electrophilic mechanism of the oxidation of propene to acetaldehyde is still not clear in the literature; this communication attempts also to clarify this point. In this Raman study during reduction of R-NiMoO4 by propene, a pronounced disappearance of the band at 706 cm-1 corresponding to the Mo-O-Ni bond was observed (Figure 4). According to these Raman results and from XRD results, one cannot exclude the involvement of the bridging oxygen between Mo and Ni, which have electrophilic character,37 in an electophilic attack with a propene double bond which leads to the formation of acetaldehyde and collapse of R-NiMoO4 to MoO2 and Ni immediately subsequent to the abstraction of the bridging oxygen between octahedral Mo and octahedral Ni. In this case, the reversible transformation of these phases to nickel molybdate by reoxidation by oxygen is expected to be difficult. This suggestion is in good agreement with the Raman results recorded during reoxidation of the reduced sample by oxygen where the Raman features of R-NiMoO4 reduced by C3H6 were not completely restored upon heating in a flow of oxygen at 400 °C for 2 h, especially the band at 706 cm-1 characteristic of the Mo-O-Ni bond (Figure 5). Moreover, the observed significant change in crystal-structure “lattice parameters” of the C3H6-reduced R-NiMoO4 after reoxidation by O2 could also confirm this suggestion. In another interesting work, it was reported that excessive reduction of R-NiMoO4 with propane leads to the irreversible transformation into MoO2 and Ni (NiO).38

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In contrast to the reduction of R-NiMoO4 with either H2 or C3H6, the reduction with CO did not reveal any structure change, that is, no significant change was observed in Raman features of the R-NiMoO4 during reaction with CO, except the observed decrease in Raman band peak intensities. With regard to oxidation of CO over metal molybdates, it was found that MnMoO4 is not able to oxidize CO in the absence of oxygen.39 However, the observed decrease in the Raman band intensities in a flow of CO with increasing temperature might be due to decomposition of CO at high temperature and deposition of carbon on the sample surface.40 Recalling the results in Figures 1-6, it is clear that the bands at 961, 913, and 706 cm-1 showed shifts to lower wavelength in their positions of barely 5-7 cm-1 or even less, and these shifts become more pronounced during the heating of samples for 1 h at 400 °C. This phenomenon cannot be explained so simplysthe discussion being reliable where the nominal resolution of the spectrometer used is 5 cm-1. However, these phenomena may reflect a little change in the molecular structure “molybdenum coordination” of R-NiMoO4 with temperature, that occurs during transformation of R-NiMoO4 to β-NiMoO4.9 4. Conclusions The in-situ thermo-Raman spectroscopy technique has been proven to be a valuable tool for investigating the change in molecular structure of R-NiMoO4 during reduction in different atmospheres, as well as providing clues as to the identity of the intermediates in this compound when H2 is used as reducing agent. In addition, the results showed that the molecular structure of R-NiMoO4 exhibited a drastic change with respect to the reducing gas used. H2 and C3H6 reducing reagents have a pronounced effect on the structure of the R-NiMoO4 phase; this effect depends on both temperature and duration of reduction. However, it seems that the catalyst did not react with CO during the reduction experiment. The subsequent reduction/reoxidation dynamic process of R-NiMoO4 can also be investigated by insitu thermo-Raman spectroscopy; the results obtained herein indicated different dynamics of reduction/reoxidation of R-NiMoO4 with respect to the reducing gas used. Combination of the results of the in-situ thermo-Raman spectroscopy in this work with the information from in-situ XRD reported in the literature could give a clear picture of R-NiMoO4 molecular structure under hydrocarbon oxidation reaction conditions. Supporting Information Available: The X-ray diffraction patterns of pure R-NiMoO4 and reoxidized/reduced in-situ samples in H2 and C3H6 atmospheres. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Maderia, L. M.; Portela, F. M. Nickel Molybdate Catalysts and their Use in the Selective Oxidation of Hydrocarbons. Catal. ReV. Sci. Eng. 2002, 44, 247. (2) Brito, J. L.; Barboza, A. L.; Albornoz, A.; Severino, F.; Laine, J. Nickel Molybdate as Precursor of HDS Catalysts: Effect of Phase Composition. Catal. Lett. 1994, 26, 329. (3) Kipnis, M. A.; Agievskii, D. A. Phase-Composition of Products from the Reduction of NiMoO4. Kinet. Catal. 1981, 22 (6), 1252. (4) Maderia, L. M.; Portela, F. M.; Mazzocchia, C.; Kaddouri, A.; Anouchinsky, R. Reducibility of Undoped and Cs-Doped R-NiMoO4 Catalysts. Catal. Today 1998, 40, 229. (5) Mazzocchia, C.; Di Renzo, F.; Ce´ntola, P.; Del Rosso, R. Reactivity of Solids; Barret, P., Dufour, L.-C., Eds.; Elsiever: Amsterdam, 1985; p 1061. (6) Smith, G. W.; Ibers, J. A. The Crystal Structure of Cobalt Molybdate CoMoO4. Acta Crystallogr. 1965, 19, 269.

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ReceiVed for reView October 19, 2006 ReVised manuscript receiVed February 7, 2007 Accepted February 10, 2007 IE0613467