고체 51V NMR, XRD 및 IR에 의한 V2O5/ZrO2-WO3 촉매의 특성 연구

손 종 락, 도 임 자※, 배 영 일※

경북대학교 공업화학과, ※울산대학교 화학과

Characterization of V2O5/ZrO2-WO3 by Solid-State 51V NMR, XRD, and IR

Jong Rack Sohn, Im Ja Doh※, Young Il Pae※

Dept. of Industrial Chemistry, Kyungpook National

University, ※Dept. of Chemistry, University of Ulsan

Introduction

Vanadium oxides are widely used as catalysts in oxidation reactions, e.g., the oxidation of sulfur dioxide, carbon monoxide, and hydrocarbons.1,2  Much research has been done to understand the nature of active sites, the surface structure of catalysts as well as the role played by the promoter of the supported catalysts, using infrared (IR), X-ray diffraction (XRD), electron spin resonance (E.S.R) and Raman spectroscopy.  So far, silica, titania, zirconia and alumina3,4 have been commonly employed as the vanadium oxide supports, and comparatively very few works have been reported for binary oxide, ZrO2-WO3 as the support for vanadium oxide.  In the present investigation, the techniques of solid-state 51V NMR, X-ray diffraction(XRD), and Fourier transform infrared (FTIR) have been utilized to characterize a series of V2O5 samples supported on ZrO2-WO3 with various vanadia loadings.

Experimental

Catalyst Preparation

Precipitate of Zr(OH)4 was obtained by adding aqueous ammonia slowly into an aqueous solution of zirconium oxychloride at room temperature with stirring until the pH of mother liquor reached about 8.  The catalysts containing various vanadium oxide content were prepared by adding the Zr(OH)4 powder into a mixed aqueous solution of ammonium metavanadate (NH4VO3) and ammonium metatungstate [(NH4)6(H2W12O40)·nH2O] followed by drying and calcining at high temperatures for 1.5h.

Characterization

FTIR absorption spectra of V2O5/ZrO2-WO3 powders were measured by the KBr disk method over the range 1200-400 cm-1.  FTIR spectra of ammonia adsorbed on the catalyst were obtained in a heatable gas cell at room temperature using a Mattson Model GL 6030E spectrophotometer.  51V NMR spectra were measured by a Varian Unity Inova 300 spectrometer with a static magnetic field strength of 7.05 T. Larmor frequency was 78.89 MHz.  The spectra were expressed with the signal of VOCl3 being 0 ppm, and the higher frequency shift from the standard was positive. Practically, solid NH4VO3 (-571.5 ppm) was used as the second external reference.

Results and Discussion

Fig.1 shows IR spectra of V2O5/ZrO2-WO3 catalysts with various content calcined at 773 K for 1.5 h.  Although with samples below 18 wt % of V2O5 the definite peaks were not observed, the absorption bands at 1022 and 820 cm-1 appeared for 18-V2O5/ZrO2-WO3, 23-V2O5/ZrO2-WO3, 28-V2O5/ZrO2-WO3 and pure V2O5.  The band at 1022 cm-1 is assigned to the V=O stretching vibration, while that at 820 cm-1 is attributable to the coupled vibration between V=O and to V-O-V.  Generally, the IR band of V=O in crystalline V2O5 shows at 1020-1025 cm-1 and the Raman band at 995 cm-1.  As shown in Fig.1, the catalysts at vanadia loadings below 18 wt % gave no absorption bands due to crystalline V2O5. This observation suggests that vanadium oxide below 18 wt % is in a highly dispersed state. It is reported that V2O5 loading exceeding the formation of monolayer on the surface of ZrO2 is well crystallized and observed in the spectra of IR and 51V solid state NMR.

As shown in Fig.1, for 4-V2O5/ZrO2-WO3 and 12-V2O5/ZrO2-WO3 calcined at 773 K the crystalline V2O5 was not observed in their IR spectra, suggesting the monolayer dispersion of V2O5 on the surface ZrO2-WO3 as the amorphous phase. However, it is necessary to examine the formation of crystalline V2O5 as a function of calcination temperature. Variations of IR spectra against calcination temperature for 4-V2O5/ZrO2-WO3 and 12-V2O5/ZrO2-WO3 was studied.  For both catalysts, there are no V=O stretching bands at 1022 cm-1 from 673 to 973 K of the calcination temperature, indicating no formation of crystalline V2O5. However, V=O stretching bands due to crystalline V2O5 at 1073 K and 1173 K appeared at 1022 cm-1.  The formation of crystalline V2O5 above 1073 K can be explained in terms of the decomposition of ZrV2O7 compound which was formed through the reaction of V2O5 and ZrO2 at 873-973 K. In this work, on X-ray diffraction patterns the cubic phase of ZrV2O7 was observed in the samples calcined at 873 K and for sample calcined at 1173 K the ZrV2O7 phase disappeared due to the decomposition of ZrV2O7, leaving the V2O5 phase and the monoclinic phase of ZrO2. These results are in good agreement with those of 51V solid state NMR described later.

The solid state 51V NMR spectra of V2O5/ZrO2-WO3 catalysts calcined at 773 K are shown in Fig.2.  There are three types of signals in the spectra of catalysts with varying intensities depending on V2O5 content.  At the low loadings up to 12 wt% V2O5 a shoulder at about 260 ppm and the intense peak at □590 ∼ □650 ppm are observed.  The former is assigned to the surface vanadium-oxygen structures surrounded by a distorted octahedron of oxygen atoms, while the latter is attributed to the tetrahedral vanadium-oxygen structures.  However, the surface vanadium oxide structure is remarkably dependent on the metal oxide support material. Vanadium oxide on TiO2 (anatase) displays the highest tendency to be 6-coordinated at low surface coverages, while in the case of □-Al2O3 a tetrahedral surface vanadium species is the favored.  As shown in Fig.2, at low vanadium loading on ZrO2-WO3 a tetrahedral vanadium species is exclusively dominant compared with a octahedral species. In general, it is known that low surface coverages favor a tetrahedral coordination of vanadium oxide, while at higher surface coverages vanadium oxide becomes increasingly octahedral-coordinated.

Increasing the V2O5 content on the ZrO2-WO3 surface changes the shape of the spectrum to a rather intense and sharp peak at about 300 ppm(□□) and a broad low-intensity peak at about 1400 ppm(□∥), which are due to the crystalline V2O5 of square pyramid coordination.  These observations of crystalline V2O5 for samples containing high V2O5 content above 12 wt % are in good agreement with the results of the IR spectra in Fig.1.  Namely, this is because V2O5 loading exceeding the formation of monolayer on the surface of ZrO2-WO3 is well crystallized.  Moreover, the increase in V2O5 content resulted appearance of additional signals with a peak in the range from  810 to  840 ppm. The intensity of the signal increases with increase in V2O5 loading.  Different peak positions normally indicate the differences of the spectral parameters and are observed due to different local environments of vanadium nuclei.  Thus species at -590 ∼ - 650 ppm and - 810 ∼ - 840 ppm can be attributed to two types of tetrahedral vanadium complexes with different oxygen environments.

It is necessary to examine the effect of calcination temperature on the surface vanadium oxide structure. The spectra of 4-V2O5/ZrO2-WO3 and 12-V2O5/ZrO2-WO3 containing lower vanadium oxide content and calcined at various temperatures was studied.  The shape of the spectrum is very different depending on the calcination temperature.  For both samples calcined at lower temperatures (673 ∼ 773 K), two peaks at about 260 ppm and - 590 ∼ - 650 ppm due to the octahedral and tetrahedral vanadium-oxygen structures are shown,  indicating the monolayer dispersion of V2O5 on the ZrO2 surface, in good agreement with the results of IR spectra.  However, for samples calcined at 873 K, in addition to the above two peaks a sharp peak at 800 ppm due to crystalline ZrV2O7 appeared, indicating the formation of a new compound from V2O5 and ZrO2 at high calcination temperature.  For samples calcined at 873 ∼ 1073 K X-ray diffraction patterns of ZrV2O7 were observed.  At 973 K of calcination temperature only a peak at 802 ppm due to the ZrV2O7 phase  appeared, saying that most of V2O5 on the surface of ZrO2-WO3 was consumed to form the ZrV2O7 compound.  However, at 1073-1173 K of calcination temperature we can observe only the peaks of crystalline V2O5 at 290 ppm and about 1200 ppm, indicating the decomposition of ZrV2O7. These results are in good agreement with those of IR spectra.  Unlike 4-V2O5/ZrO2-WO3 and 12-V2O5/ZrO2-WO3 catalysts, for 28-V2O5/ZrO2-WO3 calcined even at lower temperatures (673-773K) a sharp peak due to crystalline V2O5 appeared at 290 ppm together with peaks at 670 and 870 ppm due to the tetrahedral surface species.  However, for sample calcined at 873 K, in addition to a peak at 300 ppm due to crystalline V2O5 a sharp peak at 800 ppm due to ZrV2O7 compound appeared.  The peak intensity of ZrV2O7 increased with increase in calcination temperature, consuming the content of crystalline V2O5.  Consequently, at 973 K of calcination temperature only a peak due to the ZrV2O7 phase appeared at 800 ppm.  At 1073 K of calcination temperature a sharp peak of crystalline V2O5 at 290 ppm due to the decomposition of ZrV2O7 was again observed.

Conclusions

On the basis of results of FTIR, XRD, and solid state 51V NMR, at low calcination temperature of 773 K vanadium oxide up to 12 wt% was well dispersed on the surface of ZrO2-WO3.  However, high V2O5 loading (equal to or above 18 wt%) exceeding the formation of monolayer on the surface of ZrO2-WO3 was well crystallized.

References

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79, 2928 (1983).

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3. Centi, G., Pinelli, D., Trifiro, F., Ghoussoub, D., Guelton, M.. and Gengembre, L.

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Faraday Trans.1.  87, 3299(1991).

Fig. 2.  Solid state 51V NMR spectra

of V2O5/ZrO2-WO3 catalysts

calcined  at 773 K.

Fig. 1.  Infrared spectra of catalysts

calcined at 773 K: (a) V2O5,

(b)28-V2O5/ZrO2-WO3,(c)23-V2O5/ZrO2-WO3,

(d)18-V2O5/ZrO2-WO3,(e)12-V2O5/ZrO2-WO3,

(f) 8-V2O5/ZrO2-WO3,(g) 4-V2O5/ZrO2-WO3.