Surface Phase Composition of Iron Molybdate Catalysts Studied by

Jun 3, 2008 - ... Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China ... In this study, the surface enrichment of MoO3 that the ...
0 downloads 0 Views 629KB Size
Download PDF
J. Phys. Chem. C 2008, 112, 9387–9393

9387

Surface Phase Composition of Iron Molybdate Catalysts Studied by UV Raman Spectroscopy Qian Xu, Guoqing Jia, Jing Zhang, Zhaochi Feng, and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China ReceiVed: January 15, 2008; ReVised Manuscript ReceiVed: April 11, 2008

Catalytic performance depends largely on the surface properties. Here, the surface phase composition of iron molybdate catalysts with different Mo/Fe atomic ratios calcined at different temperatures was studied by UV Raman spectroscopy. In this study, the surface enrichment of MoO3 that the Mo-rich catalysts undergo during calcination treatment is revealed by the increase in the Raman intensity of MoO3. X-ray photoelectron spectroscopy (XPS) analysis provides further evidence that the concentration of Mo increases evidently with increasing calcination temperature. The surface enrichment of MoO3 also results in a significant variation of the morphologic appearance as evidenced by the presence of a needle-like material assigned to MoO3 on the surface, while the surface phase composition remains almost unchangeable for the sample with a stoichiometric atomic ratio. These results are attributable to the formation of a substitution structure of Fe3+ ions in an octahedral coordination by Mo6+ ions in Mo-rich catalysts, and MoO3 is easier to segregate from such a structure as a second phase concentrated on the surface. 1. Introduction The iron molybdate catalyst system has been widely used industrially for partial oxidation of methanol to formaldehyde. Commercial iron molybdate catalysts always contain an excess of molybdenum with respect to the necessary stoichiometry for Fe2(MoO4)3. They are believed to be composed of Fe2(MoO4)3, MoO3, and a molybdenum-rich phase.1 There has been a great amount of studies on the catalytic activity, deactivation, and mechanism of iron molybdate catalysts.2 In particular, the Mo/Fe ratio is considered to be important.1,3–8 Among them, there is no agreement with respect to the active phase assignment. Some researchers attributed the catalytic activity only to the Fe2(MoO4)3 phase and the excess of Mo had no influence on the specific activity of the investigated catalysts,3–6,9,10 while others considered Mo-rich iron molybdate as the active phase,7,8,11–14 which originates from the fact that the maximum activity is found for catalysts with a Mo/Fe ratio greater than the stoichiometric value. In fact, for metal molybdates, the active sites are widely associated with surface Mo atoms in an octahedral coordination. Deactivation of iron molybdate catalysts is normally attributed to the loss of Mo from the catalyst surface.14–18 Much work1,3–33 has been done on the study of phase composition of iron molybdate catalysts. Based on X-ray photoelectron spectroscopy (XPS) studies, Okamoto et al.26 concluded that for atomic ratios Mo/Fe e 1.5 the catalyst surface is Ferich, forming a core structure in which the iron oxide covers the iron molybdate; for atomic ratios Mo/Fe g 1.63, a considerable segregation of Mo occurs over the iron molybdate. Hill et al.19,20 studied the fundamental chemical, physical, and catalytic properties of iron molybdate catalysts by visible Raman spectroscopy. They came to the conclusion that Raman spectroscopy is an ideal tool to investigate the system and that the relative intensity of Fe2(MoO4)3 and MoO3 peaks in the Raman * To whom correspondence should be addressed. Telephone: +86-41184379070. Fax: +86-411-84694447. E-mail: [email protected].

spectra is a rough estimation of the Mo/Fe atomic ratio. They also assumed that perhaps the wet calcination resulted in the surface enrichment of MoO3 without altering the overall Mo/ Fe ratio of the bulk solid. Nevertheless, no direct evidence has been obtained. Although the past investigations provided information mainly on the bulk phase composition or the surface element analysis, the surface phase composition has not been well investigated. Actually, catalytic performance depends largely on the surface properties. Therefore, the surface phase composition should more directly contribute to the catalytic reaction, and the research on surface phase composition may well reveal the general essence of the active phase. Our previous studies on the phase transformation of zirconia (ZrO2) and titania (TiO2)34–36 demonstrated that UV Raman spectroscopy is more sensitive to the surface phase of a solid sample when the sample has strong absorption in the UV region. The focus of this work is to investigate the surface phase composition of iron molybdate catalysts with different Mo/Fe atomic ratios by UV Raman spectroscopy, as iron molybdate catalysts also strongly absorb UV light. Consequently, the study of surface phase composition can provide fundamental information about the surface structure-catalytic performance relationship. Furthermore, the investigation on the surface phase composition of the iron molybdate catalyst may pioneer identifying the active phase of multicomponent complex oxide catalysts, which is actually a long-standing issue of heterogeneous catalysis. 2. Experimental Section Preparation of Iron Molybdate Catalysts (MoFe-n). The iron molybdate catalysts with initial Mo/Fe atomic ratios of 1.2, 1.5, and 1.9 were prepared by a coprecipitation method from aqueous solutions of Fe (NO3)3 · 9H2O and (NH4)6Mo7O24 · 4H2O. A total of 27.32 g of iron nitrate in 200 mL of distilled water was added dropwise to 150 mL of ammonium heptamolybdate solution preacidified (pH ) 2) with nitric acid under vigorous stirring. After total addition of the iron containing

10.1021/jp800359p CCC: $40.75  2008 American Chemical Society Published on Web 06/03/2008

9388 J. Phys. Chem. C, Vol. 112, No. 25, 2008 solution, the greenish precipitate was kept in the parent solution at 353 K for 3 h. The slurry containing the precipitate was filtered without washing, dried, and calcined under flowing air at different temperatures for 4 h. The catalysts are denoted as MoFe-n, where n (n ) 1.2, 1.5, 1.9) is the Mo/Fe atomic ratio in the parent solution. Characterization. The UV Raman spectra were collected on a Jobin-Yvon T64000 triple-stage spectrograph with a spectral resolution of 2 cm-1 with the laser excitation at 325 nm generated from a He-Cd laser. The visible Raman spectra were recorded with the excitation line at 532 nm from a semiconductor laser. The scattered photons were collected in a backscattering geometry and focused onto a single-stage monochromator. The notch filter for the visible spectra had a 100 cm-1 cutoff. The crystalline phases were determined by X-ray diffraction (XRD) with a Rigaku D/Max-2500/PC powder diffraction system using Cu KR radiation (40 kV and 150 mA). The morphology, chemical analysis, and homogeneity of the prepared catalysts were examined with a SEM QUANTA 200F instrument with an energy dispersive X-ray (EDX) analyzer. UV-vis diffuse reflectance spectra were acquired on a JASCO V-550 UV-vis spectrophotometer. The X-ray photoelectron spectroscopy (XPS) study was carried out on a SHIMADZKARATOS analytical AMICUS spectrometer; Mg KR radiation was used for excitation (hν ) 1253.6 eV). A final pressure of 2.2 × 10-5 Pa was attained before the XPS recording. The C 1s signal (284.6 eV) was used as the internal reference in all experiments. Inductively coupled plasma (ICP) atomic emission spectrometry analysis was done on a Plasma-spec-II instrument (Leeman Laboratories) to check the Mo/Fe atomic ratios in the catalysts. The measured concentrations of iron and molybdenum in a given solution were accurate to within 5% for a single measurement. 3. Results Spectral Characteristics of Iron Molybdate Catalysts. MoO3 displays the typic Raman bands37 at 996 (Ag, νas MdO stretch), 823 (Ag, νs MdO stretch), 667 (B2g, B3g, νas O-M-O stretch), 473 (Ag, νas O-M-O stretch and bend), 380 (B1g, δ O-M-O scissor), 376 (B1g), 366 (A1g, δ O-M-O scissor), 334 (A1g, B1g, δ O-M-O bend), 293 (B3g, δ OdMdO wagging), 285 (B2g, δ OdModO wagging), 247 (B3g, τ OdModO twist), and 216 (Ag, rotation rigid MoO4 chain mode, Rc) cm-1. The Raman spectrum of Fe2(MoO4)3 gives the major bands at 965 (νs terminal MdO stretch), 930 (νs terminal MdO stretch), 780 (νas O-M-O stretch), and 350 (terminal MdO bend) cm-1.19,20 Obviously, Raman spectroscopy is an ideal technique for identifying the Fe2(MoO4)3 phase and MoO3 phase. The Raman spectra of the iron molybdate catalyst and MoO3 under different laser excitations are presented in Figure 1. Both the visible Raman spectrum and UV Raman spectrum show that the main features of the iron molybdate catalyst are the bands at 817 and 781 cm-1 ascribable to MoO3 and Fe2(MoO4)3, respectively. The characteristic bands of MoO3, usually present in the iron molybdate spectrum, are attributed to the presence of trace amounts of Mo octahedrally coordinated with oxygen. Moreover, it is found that the intensity of 781 cm-1 is significantly stronger than that of 817 cm-1, indicating that the iron molybdate catalyst is mainly in the Fe2(MoO4)3 phase. On the other hand, comparison of the Raman spectra excited by the 325 and 532 nm lines shows that the UV Raman spectrum is free of the bands in the 900-1000 and 200-500 cm-1 regions, mainly arising from the vibration of MoO3. These results indicate that visible Raman spectroscopy is more sensitive to the MoO3 vibration modes than UV Raman spectroscopy.

Xu et al.

Figure 1. Raman spectra of MoO3 (commercial) with excitation lines at (A) 325 nm and (B) 532 nm, and of iron molybdate catalyst (denoted as MoFe) with excitation lines at (C) 325 nm and (D) 532 nm.

Figure 2. UV-vis diffuse reflectance spectra of iron molybdate catalysts (the broad absorption band is attributed to the O2- to Mo6+ charge transfer) with different Mo/Fe atomic ratios, Fe2O3 (commercial), and MoO3 (commercial).

Figure 2 shows the UV-vis diffuse reflectance spectra of iron molybdate catalysts with different atomic ratios calcined at 400 °C. The spectra exhibit a characteristic absorption band of Fe2(MoO4)3 at about 460 nm.25,38 The broad absorption in the UV region is ascribed to the presence of both tetrahedral and octahedral oxomolybdenum groups.39 Each Mo atom in the structure of Fe2(MoO4)3 is surrounded by oxygen tetrahedrally.40 The absorption at 360 nm could be ascribed to Mo accommodated in the octahedral voids available in the structure of Fe2(MoO4)3 and/or to trace amounts of MoO3. Moreover, iron molybdate catalysts show much stronger adsorption in the UV region than in the visible region. Accordingly, the UV Raman spectra contain more signals from the surface region than from the bulk region for iron molybdate catalysts because the signal from the bulk is attenuated sharply due to the strong absorption.34–36 ICP Atomic Emission Spectrometry Analysis and XRD Patterns of Iron Molybdate Catalysts with Different Initial Mo/Fe Atomic Ratios. ICP atomic emission spectrometry measurements show that MoFe-1.2 gives the Mo/Fe atomic ratio of 1.54, MoFe-1.5 gives 1.86, and MoFe-1.9 gives 2.36, indicating that the values of the Mo/Fe atomic ratio in the catalysts are greater than those in the parent solutions. Hill et al.19 reported that washing the precursor tends to remove a greater percentage of the iron in the precursor than it does of the molybdenum, because iron is more soluble than molybdenum. The enhancement of the Mo/Fe atomic ratio in the catalyst observed may also be related to the difference in solubility.

Surface Phase Composition of Mo-Rich Catalysts

Figure 3. XRD patterns of iron molybdate catalysts with different Mo/Fe atomic ratios, MoO3 (commercial), and Fe2O3 (commercial). The vertical lines indicate the position and intensity of Fe2(MoO4)3 reflections (Powder Diffraction File No. 33-0661).

Generally, the XRD patterns give information mainly from the bulk of a solid sample. Figure 3 shows the XRD patterns of iron molybdate catalysts with different Mo/Fe atomic ratios. Iron molybdate catalysts with high crystallinity are displayed. Some diffraction peaks of Fe2(MoO4)3 do appear in the vicinities of the peaks due to MoO3, while Fe2(MoO4)3 is still predominant. XPS of Iron Molybdate Catalysts with Different Mo/Fe Atomic Ratios Calcined at 500 and 700 °C. The surface chemical compositions of iron molybdate catalysts calcined at 500 and 700 °C have been studied by XPS. The results are summarized in Table 1. The constant binding energies of electrons Eb (Mo 3d5/2 and Fe 2p3/2) indicate Mo6+ as the only detected species and Fe3+ as the only iron species. No significant variation has been observed upon increasing the calcination temperature or changing the Mo/Fe atomic ratio. According to Table 1, the sample MoFe-1.2 calcined at 500 °C has a surface composition with Mo/Fe ) 1.27, indicating that the surface of the sample is Fe-rich with respect to stoichiometric Fe2(MoO4)3. The samples MoFe-1.5 and MoFe-1.9 calcined at 500 °C have the surface composition with Mo/Fe ) 2.24 and 2.75, respectively. From the above data, it follows that the sample with an excess of Mo displays a Mo-enriched surface. As the calcination temperature is increased to 700 °C, the Mo surface enrichment becomes more prominent as evidenced by the great enhanced values 3.01 and 8.24 for MoFe-1.5 and MoFe-1.9, respectively. A slight decrease in the surface Mo/Fe atomic ratio with increasing calcination temperature for MoFe1.2 may be attributed to the slight loss of MoO3 by volatilization during calcination. SEM of Iron Molybdate Catalysts with Different Mo/Fe Atomic Ratios Calcined at 500 and 700 °C. Scanning electron microscopy (SEM) images of MoFe-1.2, MoFe-1.5, and MoFe1.9 calcined at 500 and 700 °C (shown in Figure 4) exhibit ordered agglomeration of compact particles. Interestingly, needle-like images appear in Figure 4D and F, assigned to the MoO3 phase,1,3,14 present on the surface of MoFe-1.5 and MoFe1.9 calcined at 700 °C, which is a good indication that the surface segregation of the MoO3 phase occurs during calcination. EDX analysis further confirms that the needle-like region is dominated by Mo. Both SEM images and XPS results provide evidence that the MoO3 phase is enriched on the surface of iron molybdate catalysts with an excess of Mo. However, the morphologic appearance of MoFe-1.2 calcined at 700 °C is the same as that calcined at 500 °C except for the increase in particle size, suggesting that no surface phase segregation occurs.

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9389 Phase Composition Changes of Iron Molybdate Catalysts Treated at Elevated Temperatures. Due to the high exothermicity of the reaction, hot spots develop in the catalytic bed that can lead to catalyst deactivation. Deactivation of iron molybdate catalysts is usually ascribed to the loss of molybdenum on the surface, and the loss of catalytic activity is counterbalanced by increasing reaction temperatures. By this token, thermal treatment has an obvious effect on the structure and phase composition. Therefore, there is a crucial need for techniques that can sensitively detect the surface phase composition upon increasing calcination temperature and consequently explore the effect of surface phase composition on catalytic performance. Figure 5A displays the visible Raman spectra of MoFe-1.2 calcined at different temperatures, the Mo/Fe atomic ratio of which is close to the stoichiometric ratio. Obviously, the sample calcined at 400 °C exhibits dominating spectra characteristics of the iron molybdate catalyst. No obvious change in the relative intensities is observed for the sample calcined from 500 to 700 °C, except for the increase in the absolute intensity. As mentioned above, the intensity of the 817 cm-1 peak relative to the 781 cm-1 one can be used as a rough estimation of the phase composition in the iron molybdate catalyst, the value of which remains constant when increasing calcination temperature. For iron molybdate, visible Raman spectroscopy gives information mainly from the bulk, as there is no electronic absorption in the visible region. It is manifested that the thermal treatment almost has no influence on the bulk phase composition of MoFe1.2. For iron molybdate, UV Raman spectra contain more signals from the surface region than from the bulk region due to the strong absorption in the UV region. It can be seen from Figure 5B that the Raman spectrum displays the band at 781 cm-1 with a shoulder band at 817 cm-1 for the sample calcined at 400 °C. As the calcination temperature is increased, the intensity ratios hardly change. Both the visible Raman spectra and UV Raman spectra suggest that phase composition of the stoichiometric sample almost remains constant under thermal treatment. The presence of the characteristic Raman bands attributed to MoO3 demonstrates that the sample with a nearly stoichiometric Mo/Fe atomic ratio has an excess of iron, as revealed by XPS analysis. It is thought that the excess iron forms a Fe2O3 phase.26 However, for iron oxide, it is a very weak Raman scatterer and does not add a significant contribution to the Raman spectrum. Therefore, the signal/noise ratio of the UV Raman spectra is not very good due to the formation of Fe2O3 on the surface. Parts A and B of Figure 6 present the Raman spectra of MoFe-1.5 calcined at different temperatures with excitation lines at 532 and 325 nm, respectively. Obviously, both the visible Raman spectra and UV Raman spectra of the sample calcined at 400 °C display dominating bands associated with Fe2(MoO4)3 as well as weak MoO3 bands. It is found that, when the sample was calcined at higher temperatures, there is no significant change in the intensity ratio of the 817 cm-1 peak to the 781 cm-1 one except all bands become sharp and strong (Figure 6A). However, UV Raman spectra (Figure 6B) give distinctly different results. After the sample was calcined at 500 °C, the intensity of the band at 817 cm-1 due to MoO3 increases. The UV Raman spectrum of the sample calcined at 500 °C indicates that the relative intensity of the bands at 817 to 781 cm-1 approximates to 0.5. A dramatic change in the UV Raman spectrum occurs upon increasing the calcination temperature to 600 °C. The intensities of the bands due to MoO3 increase evidently. Moreover, the relative intensity of the band at 817 cm-1 to the band at 781 cm-1 is close to 1.5. When the

9390 J. Phys. Chem. C, Vol. 112, No. 25, 2008

Xu et al.

TABLE 1: Binding energies and Mo/Fe atomic ratios determined by XPS for iron molybdate catalysts calcined at 500 and 700 °C calcination temperature 500 °C 700 °C

binding energy (eV)

Mo/Fe atomic ratio in parent solution MoFe-1.2a

b

(Mo/Fe: 1.54 ) MoFe-1.5a (Mo/Fe: 1.86b) MoFe-1.9a (Mo/Fe: 2.36b) MoFe-1.2a MoFe-1.5a MoFe-1.9a

Mo 3d5/2

Fe 2p3/2

(Mo/Fe)surface

232.9 233.2 233.0 232.6 232.9 233.0

711.6 712.2 711.6 711.6 711.5 711.8

1.27 2.24 2.75 1.04 3.01 8.24

a MoFe-n where n (n ) 1.2, 1.5, 1.9) is the Mo/Fe atomic ratio in the parent solution. b Mo/Fe atomic ratio in the catalyst as determined by ICP.

Figure 4. SEM images of MoFe-1.2 calcined at (A) 500 °C and (B) 700 °C, MoFe-1.5 calcined at (C) 500 °C and (D) 700 °C, and MoFe-1.9 calcined at (E) 500 °C and (F) 700 °C.

temperature was increased to 700 °C, the UV Raman spectrum exhibited three new bands at 281, 335, and 667 cm-1 together with the bands at 817 and 996 cm-1, which are assigned to MoO3. Additionally, the intensity of the 817 cm-1 band is much stronger than that of the 781 cm-1 band, and the relative intensity is greater than 6. These dramatic changes demonstrate that the MoO3 phase is gradually enriched on the surface at elevated temperature for the sample with an excess of Mo. Comparing results from the UV Raman spectra with those from the visible Raman spectra, we can conclude that the Mo-rich

catalysts display surface enrichment of MoO3 with an unchangeable bulk phase composition at elevated temperature. Figure 7A shows Raman spectra of MoFe-1.9 calcined at temperatures from 400 to 700 °C with the excitation laser at 532 nm. For the sample calcined at 400 °C, the visible Raman spectrum displays characteristic bands of Fe2(MoO4)3 and MoO3. However, the bands correlated with Fe2(MoO4)3 are predominant, and the bands of MoO3 are relatively weak. It is clearly seen that the Raman spectra recorded for the samples calcined at 500, 600, and 700 °C are essentially the same as that for the

Surface Phase Composition of Mo-Rich Catalysts

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9391

Figure 5. (A) Visible Raman spectra of MoFe-1.2 calcined at different temperatures with the excitation line at 532 nm. (B) UV Raman spectra of MoFe-1.2 calcined at different temperatures with the excitation line at 325 nm.

Figure 6. (A) Visible Raman spectra of MoFe-1.5 calcined at different temperatures with the excitation line at 532 nm. (B) UV Raman spectra of MoFe-1.5 calcined at different temperatures with the excitation line at 325 nm.

Figure 7. (A) Visible Raman spectra of MoFe-1.9 calcined at different temperatures with the excitation line at 532 nm. (B) UV Raman spectra of MoFe-1.9 calcined at different temperatures with the excitation line at 325 nm.

sample calcined at 400 °C and that only the bands assigned to MoO3 develop slightly. Figure 7B presents UV Raman spectra of MoFe-1.9 calcined at temperatures from 400 to 700 °C. The Raman spectrum for the sample calcined at 400 °C gives the characteristic bands of iron molybdate at 781 and 817 cm-1. In addition, the intensity ratio between the two bands suggests that the sample mainly consists of the Fe2(MoO4)3 phase. The Raman intensities of MoO3 increased significantly, and the intensity of the band at 817 cm-1 is equal to the band at 781 cm-1 for the sample calcined at 500 °C. This suggests that the portion of MoO3 on the surface is dramatically increased. The intensities of the Raman bands at 281, 335, 667, 817, and 996 cm-1, ascribed to the MoO3 phase, increased further after the sample was calcined at 600 °C. The intensities of MoO3 in the UV

Raman spectrum become considerably stronger after calcination at 700 °C, demonstrating a surface with considerably high MoO3 content. Figures 6 and 7 display Raman spectra of iron molybdate catalysts featured in industrial catalysts calcined at different temperatures. UV Raman spectra reveal that, for the catalyst with an excess of Mo with respect to the stoichiometric atomic ratio, the surface MoO3 enrichment gradually occurs during the calcination process, which is consistent with the results of XPS and SEM. However, the Mo surface enrichment is hardly observed in MoFe-1.2 with an atomic ratio Mo/Fe ) 1.5, namely the nearly stoichiometric sample is hardly affected by the calcination treatment and retains the phase composition to a certain degree.

9392 J. Phys. Chem. C, Vol. 112, No. 25, 2008

Xu et al.

Figure 8. Raman intensity ratio of the bands at 817 to 781 cm-1 for iron molybdate catalysts with different Mo/Fe atomic ratios calcined at different temperatures.

The studies of phase composition indicate that, for the samples with an excess of Mo, the surface phase composition is different from that of the bulk and that the MoO3 phase is gradually enriched on the surface during calcination, which implies that the surface enrichment of MoO3 should also occur due to high exothermicity during the reaction process for commercial iron molybdate catalysts. Therefore, UV Raman spectroscopy can be used to detect the surface phase composition, varying from fresh catalysts to postreaction catalysts, and then explore the active phase and mechanism of deactivation. Moreover, an in situ probe of the surface phase composition can be achieved by UV Raman spectroscopy. 4. Discussion For metal molybdate catalysts, the active sites are widely associated with surface Mo atoms in an octahedral coordination and deactivation of iron molybdate catalysts is normally attributed to the loss of Mo from the catalyst surface. Thus, an intensive study of the surface phase composition-active phase relationship will facilitate optimization of the catalysts and understanding of the active phase thoroughly. Although there are many studies on iron molybdate catalysts, the fundamental studies on the relationship between the surface phase composition and the catalytic performance are almost nonexistent due to a lack of suitable techniques that can sensitively detect the surface signal. We provide direct surface information using UV Raman spectroscopy, which is surface sensitive to the sample having strong absorption in the UV region. The Raman spectra of iron molybdate display characteristic bands at 781 and 817 cm-1 attributed to Fe2(MoO4)3 and MoO3, respectively. The intensity ratio of the MoO3 peak at 817 cm-1 to the Fe2(MoO4)3 peak at 781 cm-1 provides a rough measure of the phase composition of MoO3 and Fe2(MoO4)3 in the catalysts. The phase composition evolution of iron molybdate catalysts on the basis of UV and visible Raman spectra is visually depicted in Figure 8. It can be seen that the results of UV Raman spectra are different from those of visible Raman spectra. As presented, the content of the MoO3 phase on the surface is increased dramatically, while that in the bulk remains unchangeable during the thermal treatment. It is worth noting that, for the sample with Mo in excess of the stoichiometric ratio, the MoO3 phase segregation occurs on the surface at elevated temperatures. Nevertheless, the surface phase composition in

Figure 9. Schematic representation of the crystal structure of (A) Fe2(MoO4)3, where the octahedrons around Fe3+ ions (denoted in dark gray) and the tetrahedrons around Mo6+ ions (denoted in light gray) are indicated and each vertex of octahedrons and tetrahedrons is occupied by an O2- ion; (B) the Mo-rich phase (the substitution structure), where the octahedrons around Fe3+ ions substituted by Mo6+ ions are indicated by a dashed circle; and (C) MoO3.

the sample with an approximately stoichiometric ratio keeps almost invariable. It is reasonable to assume that the thermal treatment results in the surface enrichment of the excess molybdenum as the MoO3 phase and the unvarying bulk phase composition to a certain extent. XPS analysis gives the evidence of Mo enrichment on the surface for Mo-rich iron molybdate catalysts at elevated calcination temperature. This result is in accordance with SEM images which present the needle-like material, usually assigned to the MoO3 phase, on the surface of Mo-rich catalysts. However, for stoichiometric catalysts, the surface enrichment of Mo hardly occurs, and SEM images show no significant change. The observed distinct divergence between Mo-rich catalysts and stoichiometric catalysts during calcination may be due to the difference in the physical and chemical properties of iron molybdate catalysts. Plyasova et al.41 reported that Fe2(MoO4)3 presents a crystalline structure with [FeO6] octahedra and [MoO4] tetrahedra connected by their vertices. Each Mo atom has four Fe atom near neighbors, whereas each Fe atom has in its vicinity six Mo atoms located at the octahedra corners (as shown in Figure 9A).The industrial catalysts always contain a

Surface Phase Composition of Mo-Rich Catalysts large excess of molybdenum compared with Fe2(MoO4)3. Fagherazzi and Pernicone8 reported that the Mo6+ ions in excess replace some Fe3+ ions in an octahedral coordination (as shown in Figure 9B). On the other hand, according to Abaulina et al.,7 the substitution of Fe3+ ions by Mo6+ ions in Mo-rich iron molybdates is unlikely. These authors favor the hypothesis of the formation of an interstitial solid solution of two Mo6+ ions and six O2- ions per unit cell of iron molybdate. The previous studies disclosed that products prepared by normal preparation methods often contain the MoO3 phase. Raman spectra of iron molybdate in this work also display the coexistence of MoO3 regardless of the Mo/Fe atomic ratio in the catalyst. Chen40 claimed that the major features of the structure are its openness and flexibility. Since each oxygen atom is shared by only two polyhedra, a small degree of rotation or rocking of these polyhedra is possible. Thus, the Mo6+ ions can be inserted in the open structure and acquire octahedral coordination. Combining the theories of the literature with our studies, we propose that the structure of an interstitial solution of MoO3 in Fe2(MoO4)3 is likely to be ubiquitous for iron molybdate catalysts and that the molybdenum in excess may replace parts of iron to form the substitution structure. Figure 9B presents the substitution structure with [MoO4] tetrahedra and [MoO6] octahedra sharing corners besides [FeO6] octahedra and [MoO4] tetrahedra connected by their vertices. In fact, the crystal lattice of MoO3 is made up of [MoO6] octahedra connected by their edges (as shown in Figure 9C). Based on the crystalline structure analysis, our results can be interpreted that thermal treatment allows the reorganization of the excess Mo under the MoO3 structure from the [MoO6] octahedra included in the Fe2(MoO4)3 structure and consequently concentrated on the surface. For the stoichiometric catalysts, due to the lack of the substitution structure, the surface enrichment of MoO3 hardly occurs and the structure of the interstitial solid solution keeps the phase composition invariable to a certain extent. Taking into account that the practical iron molybdate catalysts always contain excess molybdenum and the highest activity can be achieved when the catalysts have an atomic ratio of Mo/Fe ) 1.6-1.8,12 the active phase may be correlated with the Morich phase on the surface. The study on the surface phase composition by UV Raman spectroscopy provides a strategy to detect the phase composition during the reaction and to explore the corresponding catalytic performance. 5. Conclusions According to the visible Raman spectra of iron molybdate catalysts with different Mo/Fe ratios calcined at temperatures ranging from 400 to 700 °C, the content of the MoO3 phase in the bulk is nearly invariable regardless of the Mo/Fe ratio. However, UV Raman spectra indicate that the surface concentration of the MoO3 phase increases dramatically with the increase in the calcination temperature for Mo-rich catalysts. The XPS results also demonstrate that the concentration of Mo increases dramatically. The presence of needle-like material assigned to MoO3 on the surface further confirms the surface enrichment for Mo-rich catalysts. Nevertheless, the phase composition remains almost unchangeable for the stoichiometric catalysts. It is proposed that the excess Mo is easy to segregate from the substitution structure and is enriched on the surface in the form of MoO3. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (NSFC, Grants

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9393 20673112, 20673115, 20590363) and the National Basic Research Program of China (Grants 2003CB615806, 2005CB221407). References and Notes (1) Sunkou, M. R.; Mendioroz, S.; Fierro, J. L. G.; Palacios, J. M.; Guerreroruiz, A. J. Mater. Sci. 1995, 30, 496. (2) Soares, A. P. V.; Portela, M. F. Catal. ReV. 2005, 47, 125. (3) Soares, A. P. V.; Portela, M. F.; Kiennemann, A.; Hilaire, L.; Millet, J. M. M. Appl. Catal., A 2001, 206, 221. (4) Boreskov, G. K.; Kolovertnov, G. D.; Kefeli, L. M.; Plyasova, L. M.; Karakchive, L. G.; Mastikhin, V. N.; Popov, V. I.; Dzisko, V. A.; Tarasova, D. V. Kinet. Catal. 1965, 7, 125. (5) Demidov, A. V.; Danilova, I. G.; Kustova, G. N.; Plyasova, L. M.; Skomorokhova, N. G.; Sedova, L. L.; Nakrokhin, V. B.; Popov, B. I. Kinet. Catal. 1992, 33, 910. (6) Kolovertnov, G. D.; Boreskov, G. K.; Dzisko, V. A.; Popov, B. I.; Tarasova, D. V.; Belugina, G. C. Kinet. Catal. 1965, 6, 950. (7) Abaulina, L. I.; Kustova, G. N.; Klevtsova, R. F.; Popov, B. I.; Bibin, V. N.; Melekhina, V. A.; Kolomiichuk, V. N.; Boreskov, G. K. Kinet. Catal. 1976, 17, 1126. (8) Fagherazzi, G.; Pernicone, N. J. Catal. 1970, 16, 321. (9) Alessandrini, G.; Cairati, L.; Forzatti, P.; Villa, P. L.; Trifirò, F. J. Less-Common Met. 1977, 54, 373. (10) Kim, T. H.; Ramachandra, B.; Choi, J. S.; Saidutta, M. B.; Choo, K. Y.; Song, S. D.; Rhee, Y. W. Catal. Lett. 2004, 98, 161. (11) Popov, B. I.; Pashis, A. V.; Shkuratova, L. N. React. Kinet. Catal. Lett. 1986, 30, 129. (12) Pernicone, N. CATTECH 2003, 7, 196. (13) Trifir”, F.; Notarbartolo, S.; Pasquon, I. J. Catal. 1971, 22, 324. (14) Arruano, J.; Wanke, S. Can. J. Chem. Eng. 1975, 53, 301. (15) Pernicone, N. Catal. Today 1991, 11, 85. (16) Popov, B. I.; Bibin, V. N.; Boreskov, G. K. Kinet. Catal. 1976, 17, 322. (17) Soares, A. P. V.; Portela, M. F.; Kiennemann, A. Catal. Commun. 2001, 2, 159. (18) Soares, A. P. V.; Portela, M. F.; Kiennemann, A.; Hilaire, L. Chem. Eng. Sci. 2003, 58, 1315. (19) Hill, C. G.; Wilson, J. H. J. Mol. Catal. 1990, 63, 65. (20) Wilson, J. H.; Hill, C. G.; Dumesic, J. A. J. Mol. Catal. 1990, 61, 333. (21) Wachs, I. E.; Briand, L. E. U.S. Pat. Appl. 6,037,290, 14 March 2000. (22) Jacques, S. D. M.; Leynaud, O.; Strusevich, D.; Beale, A. M.; Sankar, G.; Martin, C. M.; Barnes, P. Angew. Chem., Int. Ed. 2006, 45, 445. (23) Belhekar, A. A.; Ayyappan, S.; Ramaswamy, A. V. J. Chem. Technol. Biotechnol. 1994, 59, 395. (24) Delarco, M.; Martin, C.; Rives, V.; Estevez, A. M.; Marquez, M. C.; Tena, A. F. J. Mater. Sci. 1989, 24, 3750. (25) Forzatti, P.; Villa, P. L.; Ferlazzo, N.; Jones, D. J. Catal. 1982, 76, 188. (26) Okamoto, Y.; Morikawa, F.; Ohhiraki, K.; Imanaka, T.; Teranishi, S. J. Chem. Soc., Chem. Commun. 1981, 19, 1018. (27) Massarotti, V.; Flor, G.; Marini, A. J. Appl. Crystallogr. 1981, 14, 64. (28) Jeziorowski, H.; Knozinger, H. J. Phys. Chem. 1979, 83, 1166. (29) Villa, P. L.; Szabo, A.; Trifirò, F.; Carbucicchio, M. J. Catal. 1977, 47, 122. (30) Carbucicchio, M.; Trifirò, F. J. Catal. 1976, 45, 77. (31) Popov, B. I.; Osipova, K. D.; Pankratev, Y. D. Kinet. Catal. 1971, 12, 642. (32) Trifirò, F.; De Vecchi, V.; Pasquon, I. J. Catal. 1969, 15, 8. (33) Delarco, M.; Martin, C.; Rives, V.; Estevez, A. M.; Marquez, M. C.; Tena, A. F. Mater. Chem. Phys. 1989, 23, 517. (34) Li, M. J.; Feng, Z. C.; Xiong, G.; Ying, P. L.; Xin, Q.; Li, C. J. Phys. Chem. B 2001, 105, 8107. (35) Li, C.; Li, M. J. J. Raman Spectrosc. 2002, 33, 301. (36) Zhang, J.; Li, M. J.; Feng, Z. C.; Chen, J.; Li, C. J. Phys. Chem. B 2006, 110, 927. (37) Dieterle, M.; Weinberg, G.; Mestl, G. Phys. Chem. Chem. Phys. 2002, 4, 812. (38) Wolf, M. W. J.; Batist, P. A. J. Catal. 1974, 32, 25. (39) Mitchell, P. C. H.; Trifirò, F. J. Chem. Soc. A 1970, 3183. (40) Chen, H. Y. Mater. Res. Bull. 1979, 14, 1583. (41) Plyasova, L. M.; Klevtsova, R. F.; Borisov, S. V.; Kefeli, L. M. SoV. Phys. Dokl. 1966, 11, 189.

JP800359P