Structure and Reducibility of NiO-MoO - American Chemical

J. Phys. Chem. C 2008, 112, 17265–17271

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Structure and Reducibility of NiO-MoO3/γ-Al2O3 Catalysts: Effects of Loading and Molar Ratio Yuguang Wang,† Guang Xiong,*,† Xin Liu,† Xiaochuan Yu,† Liping Liu,† Junying Wang,‡ Zhaochi Feng,‡ and Can Li‡ State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, Dalian UniVersity of Technology, P.O. Box 39, No. 158 Zhongshan Road, Dalian 116012, China, and State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Science, P.O. Box 110, Dalian 116023, China ReceiVed: January 9, 2008; ReVised Manuscript ReceiVed: July 7, 2008

The effects of loading and molar ratio on the structure and reducibility of NiO-MoO3/γ-Al2O3 catalysts have been investigated. Particular attention is given to the catalysts with excess of Mo. The catalysts were prepared by impregnation method and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and temperature-programmed reduction techniques. Results from Raman, XPS, and XRD show that different surface species can be formed depending on the loading and molar ratio. At low loading spinel-like NiAl2O4 and dispersed MoO3 are the major species on γ-Al2O3. With increasing the loading the aggregation of the Mo species occurs. Only at high loading are crystalline β-NiMoO4 and MoO3 formed on the surface. For the first time, excess Mo was found to promote the formation of β-NiMoO4 on γ-Al2O3. The change in the structure of the catalysts has a large influence on their reducibility, suggesting that both the structure and oxidation-reduction properties of the NiO-MoO3/γ-Al2O3 catalysts can be controlled by varying the loading and molar ratio. 1. Introduction Nickel molybdate has been found to be a promising catalyst for many reactions, such as hydrotreatment process, oxidative dehydrogenation (ODH), and selective oxidation of light alkanes. 1-16 It is well-known that NiMoO has different crystal structures 4 depending on the temperature. The R-phase is stable at room temperature, whereas the β-phase is stable at high temperature. The Rfβ transition can be achieved above 948 K while the reverse transformation occurs when the sample is cooled to 473 K. 17-19 Studies revealed that the β-phase is easier to be reduced than the R-phase and generally shows better catalytic properties. 10,16,17,20 Therefore, stabilization of the β-phase at room temperature is very important. It has been found that excess of nickel can stabilize the metastable β-phase (bulk) at room temperature. However, these catalysts do not show good catalytic performance for propane ODH. 21,22 The other studies showed that, even without excess NiO, the β-phase can be stabilized on the supports (such as alumina, silica, and titania) at room temperature. 9,11,12,20,23-27 The reducibility and catalytic properties are improved due to the presence of the β-phase on the supports, in comparison with the bulk R-NiMoO4 catalyst. Furthermore, many studies proved that the catalysts containing excess MoO3 significantly improved the behavior of the catalysts.6,8,28,29 A good influence of MoO3 on the catalytic performance is ascribed to either a synergetic effect between NiMoO4 and MoO3 phase or the formation of the fine dispersed MoO3 and NiMoO4 phase. 5,8,13-15,29 Therefore, it is of importance to precisely control the surface Ni and Mo species, especially with excess molybdenum. To the best of our knowledge, the effect of loading and molar * Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. † State Key Laboratory of Fine Chemicals. ‡ State Key Laboratory of Catalysis.

ratio on the structure of NiO-MoO3/γ-Al2O3 catalysts has not been well understood. Raman spectroscopy is an important tool to study the structures of bulk and supported metal oxides. 30-36 In particular, the different NiMoO4 phase can be well distinguished by their characteristic Raman bands.11 In the present study, NiO-MoO3/ γ-Al2O3 catalysts have been prepared by impregnation method. The structure and reducibility of the catalysts were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-Ray diffraction (XRD), and temperature-programmed reduction (TPR). Special attention was paid to the NiO-MoO3/ γ-Al2O3 catalysts with excess molybdenum. The results showed that the structure and oxidation-reduction properties of the NiOMoO3/γ-Al2O3 catalysts can be controlled by varying loading and molar ratio. Furthermore, it was found that sufficiently high loading and excess molybdenum favor the formation of β-NiMoO4 phase, which was considered as an important active phase in many reactions. 2. Experimental Section 2.1. Catalyst Preparation. The NiO-MoO3/γ-Al2O3 catalysts were prepared by impregnation of γ-Al2O3 support (300 m2/g) with aqueous solutions of ammonium heptamolybdate and nickel nitrate. After the impregnation at room temperature for an hour, the samples were dried at 80 °C on a water bath. The obtained samples were placed in an oven and dried at 110 °C for 12 h. Then the catalysts were calcined at 320 °C for 1.5 h, cooled to room temperature, and finally calcined in air at 520 °C for 2 h. The surface coverages were expressed as wt % loading in terms of MoO3 and NiO. Mo:Ni refers to molar ratio. 2.2. Catalyst Characterization. Raman Spectroscopy. Raman spectra were measured on a Jobin-Yvon U1000 scanning double monochromator with the spectral resolution of 4 cm-1. A 532-nm line of DPSS 532 Model 200 532-nm single-

10.1021/jp800182j CCC: $40.75  2008 American Chemical Society Published on Web 10/09/2008

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Figure 2. Raman spectra of the 40 wt % NiO-MoO3/γ-Al2O3 catalysts with different molar ratios: (a) Mo:Ni ) 1:2, (b) Mo:Ni ) 1:1.5, (c) Mo:Ni ) 1:1, (d) Mo:Ni ) 2:1. Figure 1. Raman spectra of the NiO-MoO3/γ-Al2O3 catalysts (Mo:Ni ) 1.27) with different loading: (a) 8.8, (b) 18, (c) 27, (d) 37, (e) 39, (f) 50, (g) 60 wt %.

frequency laser was used as the excitation source. The power of the laser line at the sample was below 10 mw. The Raman spectra were measured under ambient conditions XPS. XPS data were obtained using a Thermon escalab 250 spectrometer (USA). The samples were measured with Al KR X-ray source in an area of 500 µm in diameter. All spectra are referenced to the carbon 1s binding energy of 284.6 eV. XRD. XRD patterns were collected at room temperature with an equipment using Cu KR radiation at 40 kV and 30 mA. The measurement was carried out in 2θ range of 5-90° at a scanning rate of 10° min-1. TPR. TPR of the catalysts was performed using a conventional apparatus using thermal conductivity (TC) detection. An amount (0.1 g) of each sample was placed in a U-shaped quartz cell. The sample was calcined in 10% O2 at 500 °C for 2 h and then cooled to room temperature. TPR profiles were taken from room temperature to 850-950 °C at a rate of 10 °C min-1, and then the temperature was kept isothermal for 30 min. 8% H2 in N2 (30 mL/min) was used as reduction gas. 3. Results Figure 1 shows the Raman spectra of NiO-MoO3/γ-Al2O3 (Mo:Ni ) 1.27) with different loading. The samples at low loading show two broad bands at 870 and 960 cm-1, which are assigned to asymmetric stretching mode of MosOsMo bridge and stretching mode of ModO. 30,31 The weak bands at 200-800 cm-1 are associated with γ-Al2O3, which may interfere with the bending and stretching modes of MoO3 in the region. This suggests that the dispersed polymerized molybdenum oxide is formed on γ-Al2O3, but the existence of the tetrahedral molybdenum species cannot be excluded at this loading. With increasing the loading from 8.8 wt % to 27 wt %, the ModO band shows a red shift from 960 to 968 cm-1, indicating an aggregation of the polymerized molybdenum species. It was

reported that crystalline NiO shows two peaks at 460 and 500 cm-1, and crystalline NiAl2O4 exhibits the bands at 200, 375, and 600 cm-1. The supported NiO possesses a Raman band at 550 cm-1. 37 In Figure 1 no distinct band associated with Ni species is observed, probably because they are overlapped by the bands of the support or too weak to be observed. Study reported elsewhere showed that the Raman bands of NiO appear only when the loading of MoO3 is low. The Ni peaks are absent as the loading of MoO3 increases. 39 The bands at 828, 899, and 952 cm-1 start to appear in the spectrum of 37 wt % of NiO-MoO3/γ-Al2O3, indicating the formation of a new structure. These peaks become prominent when the loading reaches 39 wt %. The peaks cannot be assigned to R-NiMoO4 phase, which possesses the Raman bands at 708, 913, and 961 cm-1.11,38 It has been reported that crystalline β-NiMoO4 show the Raman bands at 827, 900, 945, and 955 cm-1. 11,38 Therefore, it is evident that these peaks are associated with the formation of crystalline β-NiMoO4 phase on the support. The overlap of the peaks at 945 and 955 cm-1 of the supported sample is probably due to its poor crystalline structure in comparison with bulk β-NiMoO4. With increasing the loading the bands at 899 and 952 cm-1 shift to higher wavenumber, and the intensities of the peaks increase. At 60 wt % NiO-MoO3/γ-Al2O3 shows additional peaks at 243, 290, 340, 379, 669, 820, and 996 cm-1, which are characteristic of crystalline MoO3. 39 This indicates that both β-NiMoO4 phase and MoO3 phase can be formed at high loading. At low loading (10 wt %) the molar ratio does not has an obvious influence on the structure of surface species (see Supporting Information) Figure 2 shows the Raman spectra of 40 wt % NiO-MoO3/γ-Al2O3 with different molar ratio. The sample (Mo:Ni ) 1:2) shows the broad bands at 870 and 965 cm-1, which are assigned to MosOsMo and ModO modes of the dispersed polymerized molybdate. In the spectrum of the sample (Mo:Ni ) 1:1.5) the peaks at 880 and 952 cm-1 are observed. The peak at 952 cm-1 is due to the formation of the NiMoO4 precursor. With increasing Mo/Ni ratio this peak becomes sharper and more intense, and two additional peaks at

Structure and Reducibility of NiO-MoO3/γ-Al2O3 Catalysts

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Figure 4. XRD patterns of NiO-MoO3/γ-Al2O3 catalysts (Mo:Ni ) 1.27) with different loading: (a) 8.8, (b) 18, (c) 27, (d) 39, (e) 50, (f) 60 wt %.

Figure 3. XPS of the NiO-MoO3/γ-Al2O3 catalysts (Mo:Ni ) 1.27) with different loading: (a) 8.8, (b) 18, (c) 27, (d) 39, (e) 50, (f) 60 wt %.

828 and 904-906 cm-1 appear. This suggests that a well crystallized NiMoO4 phase is formed as the Mo/Ni ratio increases. The formation of small amount of MoO3 cannot be excluded since MoO3 exhibits the peak at 821 cm-1. XPS has been used to get the information about the Ni species. Figure 3 shows the XPS spectra of Ni 2p region for the NiO-MoO3/γ-Al2O3 (Mo:Ni ) 1.27) with varying loading. The peaks around 856.2 and 873.6 eV correspond to the spin-orbit split lines of Ni 2p3/2 and Ni 2p1/2. 7,13,44 The peaks around 862.2 and 880 eV are assigned to the shakeup satellite structures of Ni 2p3/2 and Ni 2p1/2. The broadband around 856 eV can be deconvoluted into two peaks at bending energies of 854.2 and 856.2 eV. The band at 854.2 eV is associated with Ni 2P3/2 of NiO, and that at 856.2 eV is assigned to the Ni2+ in NiAl2O4 and NiMoO4. As shown in Figure 3, both the peaks are observed in the spectrum of 8.8 wt % NiO-MoO3/γ-Al2O3 (Mo:Ni ) 1.27), confirming the existence of NiO and NiAl2O4. The spectra of the samples with higher loading exhibit only the band at 856.2 eV, suggesting the presence of NiAl2O4 and/ or NiMoO4. Raman data has proved that β-NiMoO4 is formed only at high loading. Thus, at loadings lower than 37 wt % the binding energy at 856.2 eV should be assigned to Ni2+ in NiAl2O4. At higher loading the band is associated with NiAl2O4 and/or NiMoO4. XRD patterns of the NiO-MoO3/γ-Al2O3 (Mo:Ni ) 1.27) with varying loading are shown in Figure 4. γ-Al2O3 exhibits the diffraction peaks at 45.8 and 67.3°, while the NiAl2O4 spinel possesses the diffraction peaks at 45 and 65.5°.7 Therefore, it is difficult to distinguish the peaks between γ-Al2O3 and NiAl2O4 spinel. The diffraction peaks at 45.9 and 66.5° in Figure 4 could be assigned to the overlap of γ-Al2O3 and NiAl2O4 spinel or γ-Al2O3. It was reported that bulklike and bidimensioanl NiAl2O4 can be formed on the surface.23 If a bidimensional NiAl2O4 structure is formed, it should be XRD amorphous. The peaks at 37.3, 43.4, and 63.0°, which are associated

with NiO, are not observed in the XRD patterns. If NiO is formed, it must be highly dispersed over alumina. It was reported that the diffraction lines of β-NiMoO4 are observed at 2θ ) 14.5, 23.4, 25.4, 26.7, 27.3, 28.8, 32.7, 33.9, 43.8, and 47.5°.10,11 The peak at 26.7° is the strongest line. When the loading reaches 39 wt %, two weak peaks at 23.5 and 26.7° appear, indicating the formation of poorly crystalline β-NiMoO4 phase on the surface. In the XRD pattern of 50 wt % NiO-MoO3/γ-Al2O3 these peaks become prominent, and the new peaks at 25.6, 28.8, 32.7, 33.9, 44.0, and 47.9° are observed. This suggests that a well-crystallized β-NiMoO4 phase is formed. With increasing the loading up to 60 wt %, the intensities of these peaks further increase, and a new peak at 27.4° appears. As reported elsewhere crystalline MoO3 shows the diffraction peaks at 23.3, 25.5, 27.2, and 33.6°.7 Therefore, the peak at 27.4° is due to the overlap of the peaks of β-NiMoO4 and MoO3, which is consistent with the Raman results. This suggests that both β-NiMoO4 and MoO3 are formed on the sample with the loading of 60 wt %. XRD profiles of 40 wt % NiO-MoO3/γ-Al2O3 with different molar ratio are shown in Figure 5. The peaks at 45.9 and 66.4°, which is assigned to γ-Al2O3, are observed for all the samples. The peaks at 23.5 and 26.8° (associated with β-NiMoO4) are absent in the XRD pattern of the sample (Mo:Ni ) 1:2). They start to appear in the XRD profile of the catalyst (Mo:Ni ) 1:1.5) and become sharper and more intense as the Mo/Ni ratio increases. TPR profiles of NiO-MoO3, NiO, and MoO3 supported on γ-Al2O3 are shown in Figure 6. The TPR profile of 5 wt % NiO/γ-Al2O3 exhibits the temperature maxima at 592 °C and a shoulder at 729 °C. These peaks shift slightly toward higher temperatures (604 and 750 °C) in the TPR profile of 10 wt % NiO/γ-Al2O3. An increase of their integral area indicates an increase in the H2 consumption needed for the reduction of this species. The presence of two peaks is typical for supported NiO sample, suggesting the formation of NiO with different aluminum surrounding on the surface.24,40,41 The maxima on the profile of 5 wt % MoO3/γ-Al2O3 are observed at 600 and 878 °C. The low-temperature peak is associated with the partial reduction of Mo6+ to Mo4+ of the dispersed polymerized molybdate. The high-temperature peak is due to further reduction of Mo4+ to Mo metal or the reduction of Mo species strongly interacting with the support.40-43 In profile of 10 wt % MoO3/

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Figure 5. XRD patterns of 40 wt % NiO-MoO3/γ-Al2O3 catalysts with different molar ratio: (a) Mo:Ni ) 1:2, (b) Mo:Ni ) 1:1.5, (c) Mo:Ni ) 1:1, (d) Mo:Ni ) 2:1. Figure 7. TPR profiles of NiO-MoO3/γ-Al2O3 catalysts (Mo:Ni ) 1.27) with different loading: (a) 8.8, (b) 18, (c) 27, (d) 39, (e) 50, (f) 60, (g) 40 wt % MoO3/γ-Al2O3.

Figure 6. TPR profiles of NiO-MoO3/γ-Al2O3, NiO/γ-Al2O3, and MoO3/γ-Al2O3 catalysts. (a) 5 wt % NiO/γ-Al2O3. (b) 10 wt % NiO/ γ-Al2O3. (c) 5 wt % MoO3/γ-Al2O3. (d) 10 wt % MoO3/γ-Al2O3. (e) 10 wt % MoNiO4/γ-Al2O3, Mo:Ni ) 1:1.

γ-Al2O3, the peaks shift to the lower temperature side (490 and 866 °C), indicating that the dispersed molybdenum species is easier to be reduced with increasing the loading. The 10 wt % NiO-MoO3/γ-Al2O3 profile shows two major peaks at 544 and

747 °C and one minor peak at 396 °C. As reported elsewhere the peaks at 396 and 747 °C are assigned to the reduction of Mo species, while the peak at 544 °C is due to the reduction of the Ni species.40 All reduction peaks of the NiMoO4/γ-Al2O3 shift to the lower temperature side compared with those of the NiO/γ-Al2O3 and MoO3/γ-Al2O3 samples. This indicates that the reducibility of the Mo and Ni species on γ-Al2O3 is promoted by each other, which is in agreement with the results reported elsewhere. 24,40 The TPR profiles of NiO-MoO3/γ-Al2O3 (Mo:Ni ) 1.27) with different loadings are presented in Figure 7. As discussed above the peak in the middle is due to the Ni species, and the other two are assigned to the Mo species. The Mo peaks are found to shift to the lower temperature side with increasing the loading up to 18 wt % and then shift to the higher temperature at higher loadings. Similarly, the temperature of Ni peak decreases with loading up to 27 wt % and then increases at the loading of 39 wt %. Finally it is overlapped by the low-temperature reduction peak of the Mo species at higher loadings. The above results are in agreement with the reported studies, which indicated that the temperature of the reduction peak of Mo species decreases with increasing Mo loading until the formation of monolayer and then increases when the crystalline phase appears. 42,43 The TPR profiles in Figure 7 suggest that crystalline MoO3 or NiMoO4 are formed at high loadings. A TPR profile of 40 wt % of MoO3 /γ-Al2O3 presents three peaks at 467, 562, and 904 °C, which are due to the formation of crystalline MoO3 phase. Literature has shown that the TPR profile of bulk β-NiMoO4 possesses the peaks at 510 and 711 °C, and that of supported β-NiMoO4 shows the peaks at 426-457, 470-505, and 636-686 °C. 10 As shown in Figure 7, the very broad bands presented at high loadings may be due to the overlap of the

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Figure 9. Diagram of the effect of loading and Mo mol % on the formation of β-NiMoO4. (0) No β-NiMoO4. (9) β-NiMoO4. The size of the symbol is proportional to the intensity of the Raman peak at 952-959 cm-1.

Figure 8. TPR profiles of the 40 wt % NiO-MoO3/γ-Al2O3 catalysts with different molar ratios: (a) Mo:Ni ) 1:2, (b) Mo:Ni ) 1:1.5, (c) Mo:Ni ) 1:1, (d) Mo:Ni ) 2:1.

different surface species, such as MoO3, β-NiMoO4, and dispersed MoO3. The TPR profiles of the 40 wt % NiO-MoO3/γ-Al2O3 with varying Mo/Ni ratios are shown in Figure 8. Three peaks at 373-433, 460-530, and 713-795 °C are observed in the spectra. It is clearly shown that the Mo peaks (373-433 and 713-795 °C) shift to the higher-temperature side with increasing Mo/Ni ratio. The maxima of Ni peak first show a shift to the low-temperature side and then to the high-temperature side with increasing Mo:Ni ratio. As proved by XRD results the β-NiMoO4 phase is progressive formed with increasing the Mo/Ni ratio. The β-NiMoO4 shows the peaks at higher temperature side, indicating that this phase is less reducible than the amorphous NiO-MoO3/γ-Al2O3. It was also shown that the temperature peaks of the β-NiMoO4/γ-Al2O3 are different from those of bulk β-NiMoO4 and β-NiMoO4 /SiO2,10 which indicates an effect of the support. 4. Discussion Since the catalytic properties of NiO-MoO3/γ-Al2O3 catalysts are closely related to its structure, a fundamental understanding of the structure and its control factors are highly required. The results from Raman, XPS, and XRD show that the different species and phases can be formed by varying the loading and molar ratio. At low loading the dispersed molybdenum oxide and NiAl2O4 are the major species on the samples (Mo:Ni ) 1.27, see Figures 1 and 3). NiO is also found on some catalysts. With an increase in the loading, the aggregation of the molybdenum species occurs and results in a red shift of the Raman band (960 cm-1). At the loading of 37 wt % the Raman bands due to β-NiMoO4 phase start to appear. As the loading further increases these peaks become sharper, indicating the formation of a well-crystallized β-NiMoO4 phase. This result

is consistent with XRD data, which shows a poorly crystallized phase at the loading of 39 wt % and well-crystallized phase at the loadings of 50 and 60 wt %. At the loading of 60 wt %, crystalline MoO3 phase is formed together with β-NiMoO4 on the surface. The effect of molar ratio on the structure of the NiO-MoO3/ γ-Al2O3 catalysts is dependent on the loading. At low loading (10 wt %), the Mo/Ni molar ratio does not show an obvious influence on the structure. No β-NiMoO4 phase is formed regardless of Mo/Ni ratio. At high loading (40 wt %), the intensities of some Raman bands and diffraction peaks increase as the Mo/Ni ratio increases, see Figures 2 and 5. These peaks have been assigned to β-NiMoO4. The increase in Mo/Ni ratio leads to the increase in the amount of the β-NiMoO4 and also in the degree of crystallization of the phase. Therefore, at the loading of 40 wt %, the formation of β-NiMoO4 is enhanced for the Mo-rich catalysts. The effects of loading and molar ratio on the formation of β-NiMoO4 is shown in Figure 9. It is clear that high loading and Mo/Ni ratio enhance the formation of β-NiMoO4. To better understand the reason, the monolayer capacities of MoO3 and NiO on γ-Al2O3 have been considered. The monolayer capacity of MoO3/γ-Al2O3 or NiO/γ-Al2O3 is found to be 0.12 g/100 m2. This suggests that each Mo or Ni atom would occupy 20 and 9.67 Å2, respectively.45,46 With the increase of the Mo/Ni ratio the total densities of Mo and Ni atoms show a decrease from 13.68 atoms/nm2 to 11.04 atoms/nm2, but the total surface area occupied by Mo and Ni atoms does not show an obvious change. It seems that the formation of the β-NiMoO4 phase cannot be explained by exceeding the monolayer dispersion capacity as Mo/Ni ratio increases but are probably due to some other effect related to the presence of MoO3 and the support. In the case of bulk catalyst the excess of Ni helps to stabilize a nonstoichiometric β-NiMoO4 phase at room temperature by insertion of excess Ni in the NiMoO4 lattice.47-49 It is interesting to note that the effect of excess Ni is not observed for the supported NiMoO4 catalyst, indicating that the support plays an important role in the process. Moreover, the peaks of crystalline MoO3 are not unambiguously observed even for the samples with excess Mo (up to 50 wt %), although crystalline NiMoO4 is formed at the loading of about 40 wt %. This probably suggests the formation of nonstoichiometric β-NiMoO4 with excess Mo.

17270 J. Phys. Chem. C, Vol. 112, No. 44, 2008 The effects of the loading and molar ratio on the reducibility of the NiO-MoO3/γ-Al2O3 catalysts are shown in Figures 7 and 8. It has been reported that, for MoO3/γ-Al2O3, the temperature maxima first shifts to lower temperature with the increase of loading and then shifts to higher temperature when MoO3 crystallites start to appear.42,43 As shown in Figure 7, the peak temperatures decrease with increasing the loading, suggesting that the reduction of the Mo species is easier when the surface species aggregates. At high loading the peak temperatures increase when the crystallized β-NiMoO4 and MoO3 are formed. At the loading of 40 wt % β-NiMoO4 is formed on the samples (Mo:Ni ) 1:1.5, 1:1, 2:1). Higher Mo/Ni ratio leads to the increase in the amount of the β-NiMoO4 and also in the degree of crystallization of the phase. The peak maxima of the catalysts shift to higher temperature as the Mo/Ni ratio increases, which is due to the formation of the β-NiMoO4. Therefore, high loading and high Mo/Ni ratio enhance the formation of β-NiMoO4, which results in an increase of the reduction temperature. From the above-mentioned results, it is clear that the structure and oxidation-reduction properties of NiO-MoO3/γ-Al2O3 can be controlled by varying the loading and molar ratio. In particular, it is found that sufficiently high loading and excess Mo favor the formation of β-NiMoO4. Many studies proved that excess MoO3 is crucial for NiMoO4 catalyst in selective oxidation reaction.1 The positive effect of excess Mo on the catalytic properties of bulk catalyst has been ascribed to a close contact of R-NiMoO4 and MoO3, which leads to an easier transfer of electron across the interface.8,13-15 For the supported NiMoO4, its enhanced activity is attributed to the formation of NiMoO4 and highly dispersed MoO3, which are both required for the reaction. It the present study, we provide another possibility. Excess Mo may play a role in enhancing the formation of β-NiMoO4, which is a crucial phase for many reactions. 5. Conclusions 1. NiO-MoO3/γ-Al2O3 catalysts have been prepared by the impregnation method and characterized by Raman spectroscopy, XPS, XRD, and TPR techniques. At low loading the spinellike NiAl2O4 and highly dispersed MoO3 are the major species on the surface. With increasing the loading, the aggregation of the dispersed MoO3 occurs. At high loading crystalline β-NiMoO4 and MoO3 appeared. A change in the structure of the catalyst has a large influence on its oxidation-reduction property. 2. β-NiMoO4 has been proved to be an important active phase or precursor in many reactions. In the present study, it is found that the increase in loading and Mo/Ni ratio leads to the increase in the amount of the β-NiMoO4 and also in the degree of crystallization of the phase. The formation of crystalline β-NiMoO4 thereafter results in the increase in the reduction temperatures. This result provides an alternative interpretation of the change in the catalytic properties observed for the Morich NiO-MoO3 catalysts. Acknowledgment. This work was financially supported by the National Science Foundation of China (NSFC, Grant 20603004). Supporting Information Available: Raman spectra of the 10 wt % NiO-MoO3/γ-Al2O3 catalysts with different molar ratios; XPS of the 10 wt % NiO-MoO3/γ-Al2O3 catalysts with different molar ratios; XPS of the 40 wt % NiO-MoO3/γ-Al2O3

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