Titania-Zirconia에 담지된 Vanadia 촉메의 분광분석 및 촉매활성

박은희, 손종락, 김해원*

경북대학교 공업화학과, *경일대학교 공업화학과

Spectroscopic Analysis and Catalytic Activity of Vanadia Supported on Titania-Zirconia Catalysts

Eun Hee Park, Jong Rack Sohn, Hae Won Kim*

Dept. of Industrial Chemistry, Kyungpook National University, *Dept. of Industrial chemistry, Kyungil University

Introduction

Vanadium oxides are widely used as catalysts in oxidation reactions, e.g., the oxidation of sulfur dioxide, carbon monoxide, and hydrocarbons.1-4 These systems have also been found to be effective catalysts for the oxidation of methanol to methylformate. Vanadia catalysts supported on titania-alumina mixed oxide and titania modified with alumina were found to exhibit superior activities in selective catalytic reduction of NOx. Much research has been done to understand the nature of active sites, the surface structure of catalysts and 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. Silica, titania, zirconia and alumina5,6 have been commonly employed as vanadium oxide supports, and comparatively few studies have been reported on binary oxide TiO2-ZrO2 as a support for vanadium oxide.   In the present investigation, the techniques of Fourier transform infrared(FTIR), 51V solid state NMR, X-ray diffraction, and DSC have been utilized to characterize a series of V2O5 supported on TiO2-ZrO2 with various vanadia loadings.

Experimental

Catalyst preparation: Coprecipitate of Ti(OH)4-Zr(OH)4 was obtained by adding aqueous ammonia slowly into a mixed aqueous solution of titanium tetrachloride and zirconium oxychloride at room temperature with stirring until the pH of mother liquor reached about 8.  Ti(OH)4-Zr(OH)4 mixed hydroxide was synthesized with a 1:1 molar ratio.  The coprecipitate thus chloride ion was not detected by AgNO3 solution and was dried at 110℃ for 12 h.  The dried coprecipitate was powdered below 100 mesh by grinding.  The catalysis containing various vanadium oxide contents were prepared by adding an aqueous solution of ammonium metavanadate(NH4VO3) to the Ti(OH)4-Zr(OH)4 powder followed by ddring and calcining at high temperatures for 3 h in air.  This series of catalysts are denoted by their weight percentage of V2O5 and calcination temperature.

Characterization: FTIR absorption spectra of V2O5/TiO2-ZrO2 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 ordinary single pulse sequence was used, in which the pulse width was set at 2.8 s  and the acquisition time was 0.04 s.  The spectra width was 500 kHz.  The number of scans was varied from 400 to 4000, depending on the concentration of vanadium.  The sample was static, and its temperature was ambient.

Results and Discussion

Figure 1 shows IR spectra of V2O5/TiO2-ZrO2 catalysts with various content calcined at 773 K for 3 h.  Although with samples below 25 wt % of V2O5 the definite peaks were not observed, the absorption bands at 1022 and 820 cm-1 appeared for 25-V2O5/TiO2-ZrO2, 33-V2O5/TiO2-ZrO2, and pure V2O5 containing high V2O5 content. The band at 1022 cm-1 is assigned to the V=O stretching vibration, whereas the band at 820 cm-1 is attributed 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.   The intensity of the V=O absorption gradually decreased with decreasing V2O5 content, although the band position did not change. As shown in Figure 1, the catalysts at vanadia loadings below 25 wt % gave no absorption bands from crystalline V2O5. This observation suggests that vanadium oxide below 25 wt % is in a highly dispersed state. It is reported that V2O5 loading exceeding the formation of monolayer on the surface of support is well crystallized and observed in the spectra of IR and 51V solid state NMR.

As shown in Figure 1, for samples below 25 wt% of V2O5 calcined at 773 K the crystalline V2O5 was not observed in their IR spectra, suggesting the monolayer dispersion of V2O5 on the surface of TiO2-ZrO2 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 10-V2O5/TiO2-ZrO2 are shown another Figure. For the sample, there are no V=O stretching bands at 1022 cm-1 from the calcination temperature of 673 K to 1073 K, indicating no formation of crystalline V2O5. However, as shown in another Figure, V=O stretching bands due to crystalline V2O5 at 1173 K appeared at 1022 cm-1 together with lattice vibration bands of V2O5 and TiO2-ZrO2 below 900 cm-1. The formation of crystalline V2O5 at 1173 K can be explained in terms of the decomposition of V3Ti6O17 compound, which was formed through the reaction of V2O5 and TiO2 at 973∼1073 K. In the present work, the triclinic phase of crystalline V3Ti6O17 was confirmed by X-ray  diffraction. X-ray diffraction patterns showed the triclinic phase of V3Ti6O17 ( 2□ : 27.6, 28.3, 28.4, 36.2, and 41.4¸) in the samples calcined at 973∼1073 K, and for sample calcined at 1173 K the V3Ti6O17 phase disappeared due to the decomposition of V3Ti6O17, leaving the V2O5 phase and the rutile phase of TiO2. These results are in good agreement with those of 51V solid state NMR described later.

Figure 3 shows IR spectra of 25-V2O5/TiO2-ZrO2 catalysts calcined at 673∼1173 K for 3 h.  Unlike 10-V2O5/TiO2-ZrO2 for 25-V2O5/TiO2-ZrO2 crystalline V2O5 appeared at lower calcination temperature from 673 K to 873 K and consequently V=O stretching band was observed at 1022 cm-1.  This is because V2O5 loading exceeding the formation of monolayer on the surface of ZrO2 is well crystallized.  However, at 973∼1073 K all V2O5 reacted with ZrO2 or TiO2 and changed into ZrV2O7 or V3Ti6O17 so that V=O stretching at 1022 cm-1 disappeared completely.  At the calcination temperature of 1173 K some of the V3Ti6O17 decomposed into V2O5 and TiO2, and then the V=O stretching band due to the crystalline V2O5 was again observed at 1022 cm-1. These results are in good agreement with those of 51V solid state NMR.

Infrared spectroscopic studies of ammonia adsorbed on solid surfaces have made it possible to distinguish Br□nsted acid sites from Lewis acid sites.  Another Figure shows the IR spectra of ammonia adsorbed on 10-V2O5/TiO2-ZrO2 calcined at 973 K and evacuated at 673 K for 1 h.  For 10-V2O5/TiO2-ZrO2 the bands at 1454 cm-1 are the characteristic peaks of ammonium ion, which are formed on the Br□nsted acid sites and the bands at 1620 cm-1 are contributed by ammonia coordinately bonded to Lewis acid sites, indicating the presence of both Br□nsted and Lewis acid sites. Other samples having different vanadium content also showed the presence of both Lewis and Br□nsted acids. Therefore, these V2O5/TiO2-ZrO2 samples can be used as catalysts for Lewis or Br□nsted acid catalysis.

Conclusions

This studies shows that a combination of FTIR and 51V solid-state NMR can be used to perform the characterization of V2O5 catalysts supported on TiO2-ZrO2.  On the basis of results of FTIR and solid state 51V NMR, at low calcination temperature of 773 K up to 15 wt% of vanadium oxide was well dispersed on the surface of TiO2-ZrO2.  However, high V2O5 loading (equal to or above 25 wt%) exceeding the formation of monolayer on the surface of TiO2-ZrO2 was well crystallized.  The ZrV2O7 compound was formed through the reaction of V2O5 and ZrO2 at 773 ∼ 973 K, whereas the V3Ti6O17 compound was formed through the reaction of V2O5 and TiO2 at 973 ∼ 1073 K. The V3Ti6O17 decomposed to V2O5 and TiO2 at 1173 K, which were confirmed by FTIR and 51V NMR.  Infrared spectroscopic studies of ammonia adsorbed on V2O5/TiO2-ZrO2 catalysts showed the presence of both Lewis and Br□nsted acids.

References

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2. Reddy, B. M.; Ganesh, I.; Chowdhury, B., Catal. Today 1999, 49, 115

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87, 761

그림

a

b

c

d

e

f

g

1022

820

1500       1000         500

Wavenumber, cm-1

Fig. 1 Infrared spectra of catalysts calcined at 773 K:

(a) TiO2-ZrO2, (b) 5-V2O5/TiO2-ZrO2,

(c) 10-V2O5/TiO2-ZrO2 (d) 15-V2O5/TiO2-ZrO2,

(e) 25-V2O5/TiO2-ZrO2, (f) 33-V2O5/TiO2-ZrO2, (g) V2O5