NH3에 의한 NO 제거 반응에서 Sulfated TiO2에 담지된

V2O5 촉매에 대한 조촉매의 영향

추수태, 남인식, 함성원*, 이정빈**, 엄무헌***, 김기영***

포항공과대학교, 화학공학과/환경공학부,

*경일대학교 화학공학과, **전력연구원 발전연구실

***(주) 세신전자

Effects of Promoters for V2O5 Supported

on Sulfated TiO2 Catalyst to Remove NO by NH3

Soo-Tae Choo, In-Sik Nam, Sung-Won Ham*, Jeong-Bin Lee**,

Moo-Hun Ohm*** and Kee-Young Kim***

Dept. of Chemical Engineering/School of Environmental Engineering,

Pohang University of Science & Technology (POSTECH)/

*Dept. of Chemical Engineering, Kyungil University/

**Korea Electric Power Research Institute (KEPRI)

***Seshin Electronics Co.

Introduction

The V2O5/TiO2 catalyst promoted with WO3 (or MoO3) has been commonly employed for the selective catalytic reduction (SCR) of NOx by NH3 due to its high NO removal activity and sulfur tolerance [1].  V2O5 is known to be active reaction sites not only for the reduction of NOx but for the oxidation of SO2.  The SO3 produced by the oxidation of SO2 can cause the catalyst deactivation and the operational problem of SCR process.  Therefore, the content of V2O5 should be adjusted below 2% of the catalyst to minimize the oxidation reaction of SO2, with the relevant NO removal activity.

However, there is still controversy on the role of WO3 for the oxidation reaction of SO2 to SO3 which is critical for the sulfur tolerance of SCR catalyst.  Sazonova et al. [2] observed that the activity of SO2 oxidation for V2O5/TiO2 catalyst decreased by the addition of WO3 to the catalyst.  But, Morikawa et al. [3] have reported that V2O5/TiO2 catalysts promoted with WO3 or MoO3 revealed higher oxidation activity of SO2 to SO3 than V2O5/TiO2 catalyst without promoters.  Dunn et al. [4] also discussed that WO3-promoted V2O5/TiO2 catalyst showed higher SO2 oxidation activity than the corresponding binary catalysts such as V2O5/TiO2 and WO3/TiO2.	The surface structure or state of vanadia which is important for NO removal activity of the catalyst can be easily altered by the addition of the promoters on the catalyst surface [4,5].  In the present study, the vanadia catalyst promoted with tungsten oxide on sulfated titania was examined to elucidate the role of WO3 for SCR reactions.  Barium oxide was also employed as an alternative promoter.

Experimental

The sulfated TiO2 (ST), which is prepared from metatitanic acid provided from Hankook Titanium Co. Ltd, Korea, and P25, which is a sulfur-free TiO2 supplied from Degussa, were employed as supports in the present study.  The catalytic activity of NO reduction and SO2 oxidation were examined in a fixed-bed flow reactor.  The details of reactor system and reaction conditions were well described elsewhere [5].  The characteristics of V2O5/TiO2 catalysts promoted with WO3 or BaO were examined by in-situ Raman, TPD, and XPS measurements.

Results and discussion

The Raman spectroscopy has been widely employed to investigate the surface structure of vanadium species on TiO2 support.  The Raman spectra of V2O5 supported on sulfated TiO2 containing 1 to 10% of WO3 are shown in Fig. 1.  Two major bands appear at around 1030 cm-1 and 930 ∼ 960 cm-1 of Raman shift.  The sharp band near 1030 cm-1 is assigned to monomeric vanadyl species, while the broad band in the range of 930 ∼ 960 cm-1 is to polymeric vanadate.  The intensity of the band at 1010 cm-1 becomes stronger as WO3 loading increases on the catalyst surface.  This is identical to the band observed for the Raman spectra of W6ST, indicating the characteristic peak of tungsten species.  The 930 ∼ 960 cm-1 Raman bands revealing the polymeric vanadate species shift toward the higher position of the Raman band with the increase of WO3 content.  It indicates that the polymeric vanadate species easily form on the catalyst surface as WO3 loading increases, compared to that on the catalyst without WO3.  The band position of monomeric vanadyl and tungsten species hardly change with increasing WO3 loading on sulfated TiO2.  The tungsten oxide seems to independently present on the surface of sulfated TiO2 without much interaction with vanadium species.  However, the formation of polymeric vanadate species becomes stronger as the content of WO3 increases, probably due to fact that the number of available surface sites are reduced by tungsten oxide present on the surface of ST.

The effect of barium oxide on the structure of vanadate on sulfated TiO2 was examined as shown in Fig. 2.  The Raman band position in the range from 1029 cm-1 to 930 cm-1 do not change by the addition of barium oxide.  However, their intensities are slightly reduced, compared to those on V2ST catalyst without BaO.  This indicates that barium oxide on the surface of sulfated TiO2 support weakly interacts with the vanadia and thus slightly reduce the intensity.  No characteristic Raman peak for barium oxide was observed.  In present study, it was observed that NO removal activity of V2O5 supported on sulfated TiO2 catalyst is hardly improved by the addition of WO3 or BaO to the catalyst.  Both WO3 and sulfate species on V2O5/TiO2 catalyst are generally known to promote SCR reaction.  Their advantageous effect on NO removal activity of V2O5/TiO2 catalyst were mainly attributed to the enhancement of reducibility, the increase of surface acidity and the induction of the formation of polymeric vanadate on the catalyst surface.  However, when the two species simultaneously exist on the surface of V2O5/TiO2, the sulfate species seem to be more critical for NO removal activity than WO3 species.  For SO2 oxidation, the addition of WO3 to the catalyst significantly enhances the formation of SO3, since WO3 as well as V2O5 independently acts as an active reaction site for the oxidation reaction.  However, the SO2 oxidation activity of V2O5/sulfated TiO2 catalyst significantly decreases by the addition of barium oxide to the catalyst.  It may be due to the slight alteration of vanadium species, which are also on active reaction site for SO2 oxidation as observed in Fig. 2.

To investigate the role of sulfur for the structure of the surface species of vanadium and tungsten, the Raman spectra of V2O5 supported on the sulfur-free TiO2 (P25) containing 3 to 10 % of WO3 were also examined as shown in Fig. 3.  A sharp Raman band at 1030 cm-1 of Raman shift revealing the monomeric vanadyl species is commonly observed for P25-based catalysts.  No alteration of the structure of vanadium and tungsten species on V2O5/sulfur-free TiO2 was observed, which is similar to that on V2O5/sulfated TiO2.  It indicates that the tungsten oxide species independently exist on V2O5/TiO2 catalyst, regardless of the presence of surface sulfate species on the catalyst surface.  The polymeric vanadates, which are more active reaction sites for NO reduction reaction than the monomeric vanadyl species, can be easily formed on the catalyst surface by the addition of tungsten oxide to V2O5/TiO2 catalyst.  This is one of the reasons for the significant enhancement of NO reduction activity of sulfur-free TiO2-based SCR catalyst by the addition of tungsten oxide.

The barium oxide also interacts with the vanadium species on sulfur-free TiO2 as shown in Fig. 4.  The monomeric vanadyl and polymeric vanadate species corresponding to the Raman band positions in the range of 1030 and 910 ∼ 915 cm-1 nearly disappear by increasing the loading of BaO to 2%.  Moreover, new sharp Raman bands appear at 960, 906, 881 and 831 cm-1.  Although the nature of these new peaks are not clearly identified yet, they are basically attributed to the interaction of barium oxide with vanadium on the catalyst surface.  The formation of vanadium-oxygen-alkali metals including potassium compounds was already discussed on V2O5/TiO2 catalyst, which is an indicative for the strong interaction of alkali metal with vanadium [4].  Based on the results from the present study, V2Ba2P25S catalyst did not lose NO removal activity, while V2Ba2P25 catalyst revealed much lower catalytic activity than that of V2P25 without BaO.  This may be due to the reduction of monomeric vanadyl and polymeric vanadate species on the catalyst surface, which are active reaction sites for NO reduction and SO2 oxidation, by the strong interaction of BaO with the vanadium species on the surface of sulfur-free TiO2.

References

[1] Busca, G., Lietti, L., Ramis, G. and Berti, F.: Appli. Catal. B, 18, 1(1998).

[2] Sazonova, N., Tsykoza, L., Simakov, A., Barannik, G. and Ismagilov, Z.:      React. Kinet. Catal. Lett., 52(1), 101(1994).

[3] Morikawa, S., Yoshida, H., Takahashi, K. and Kurita, S.: Chem. Lett.         251(1981).

[4] Dunn, J. P., Stenger, H. G. Jr and Wachs, I. E.: Catal. Today, 53,	543(1999).

[5] Choo, S. T., Lee, Y. G., Nam, I. S., Ham, S. W. and Lee, J. B.: Appli.        Catal. A, 200, 177(2000).

Fig. 2. In-situ Raman spectra of 2%V2O5

supported on sulfated TiO2 with

1∼2% barium oxide loadings.

Fig. 1. In-situ Raman spectra of 2%V2O5

supported on sulfated TiO2 with

1∼10% tungsten oxide loadings.

Fig. 4 In-situ Raman spectra of 2%V2O5         supported on sulfur-free TiO2

with 1 ∼ 3% barium oxide

loadings.

Fig. 3. In-situ Raman spectra of 2%V2O5          supported on sulfur-free TiO2

with 3 ∼ 10% tungsten oxide

loadings.