암모니아를 이용한 NO의 SCR반응용 V2O5/□-Al2O3촉매의 특성과 반응활성에 MoO3첨가가 미치는 영향

고형림, 송민우', 강 민', 김경림'

(주)효성 화학연구소, 연세대학교 화학공학과'

Effect of MoO3 Addition on the Characteristics and Reactivity of V2O5/□-Al2O3 Catalyst for SCR of NO by Ammonia

Hyoung-Lim Koh, Min-Woo Song', Min Kang', Kyung-Lim Kim'

Hyosung R&D Center for Chemical Technology, Dep. of Chem. Eng., Yonsei University'

Introduction

Nitrogen oxides contribute to acid rain along with sulfur dioxide (SO2). In addition to being a primary acid rain precusor, NOx play a major role in the chain of photochemical reaction leading to the formation of photochemical smog. From the viewpoint of environmental protection, the removal of nitrogen oxide (NOx) from stationary sources has been a great concern of many people. Among the proposed post-combustion technologies for NOx removal, selective catalytic reduction (SCR) is the most effective and widely commercialized NOx removal technology [1]. SCR removes NOx in flue gas by reaction nitrogen oxides with ammonia to form nitrogen and water.

During the past few decades, much effort has been focused on the development of active catalysts for the removal of NO. Various types of catalysts including noble metals, transition metal oxides and mixed metal oxides, show high activity when used for the reduction of NO with NH3 in the absence of sulfur compounds in flue gas. By now, V2O5 supported on TiO2 is known to be most effective and widely used commercial SCR catalyst due to its high activity and durability to sulfur compounds.

Alumina has a higher specific surface area, superior mechanical strength and is highly resistant to sintering, compared to titania. Therefore, many efforts have been made to apply alumina to the SCR of NO by ammonia [2-6]. When alumina is used as the support for the SCR reaction, SO2 in the flue gas, reacting with alumina or ammonia, deactivates the catalytic activity . But, results of other research show that activity of catalyst used alumina support could show the resistance to SO2 deactivation, depending on the pore size and specific surface area of alumina.

The objective of this study is to enhance the activity and availability of alumina-supported vanadia catalysts for the SCR of NO by ammonia. For this objective, MoO3 was added to alumina-supported vanadia catalysts. Catalytic activity for SCR of NO by ammonia with different vanadia loadings of alumina-supported vanadia catalysts was measured in a fixed-bed reactor system and various catalysts with different loadings of MoO3,, V2O5 were also tested for SCR of NO by ammonia. The effect of MoO3 addition to V2O5/Al2O3 catalyst for reactivity of selective catalytic reduction (SCR) of NO by ammonia and physicochemical characteristics of these catalysts were investigated to discover the role of MoO3 in the catalytic activity improvement.

Experimental

A seires of V2O5/Al2O3 catalyst with different vanadia loadings was prepared by the impregnation of □-Al2O3(B.E.T. surface area :167.5 m2/g) with aqueous solutions of ammonium vanadate(NH4VO3, Yakuri,extra pure), followed by drying in air at 120℃ for 24 hours and calcination in air at 500℃ for 3 hours. 6 wt % MoO3/Al2O3 catalysts was prepared by the impregnation of □-Al2O3 with aqueous solutions of ammonium molybdate ((NH4)6Mo7O24·4H2O, Yakuri, extra pure) followed by drying in air at 120℃ for 24 hours and calcination in air at 500℃ for 3 hours. MoO3-V2O5/Al2O3 with different loadings of MoO3 and V2O5 were prepared by the impregnation of □-Al2O3 with aqueous solutions of ammonium molybdate, followed by drying in air at 120℃ for 24 hours and impregnation of these samples with aqueous solutions of ammonium vanadate, followed by drying in air at 120℃ for 24 hours and calcination in air at 500℃ for 3 hours. These catalysts will be quoted in this dissertation according to their V2O5, NiO, and MoO3 content: e.g., Mo6Ni6V12 identifies a ternary sample with MoO3 = 6wt%, NiO = 6wt%, V2O5 = 12wt%.

Catalytic activity for selective catalytic reduction (SCR) of NO by ammonia has been measured in a tubular, downflow, fixed-bed reactor. In a typical test of catalytic activity, 1cc of catalyst (20-40 mesh, 0.38-0.52g) was used and the typical reactant gases for activity test consisted of 500 ppm NO (5% NO in He balance), 500 ppm NH3 (5% NH3 in He balance), 10% O2 and balance He. The space velocity which is based on the ratio of the total gas flowrate to the volume of catalyst bed was 100,000 hr-1, otherwise specified. The nitrogen oxide concentration of the gas stream was measured before and after the catalyst bed by means of chemiluminescent NO/NOx analyser (Horiba Co.) and for the measurement of nitrogen and oxygen content in the gas stream after reaction, inlet and outlet gases were also analyzed by gas chromatography (Donam Instrument) with PoraPLOT Q fused silica PLOT  (25m × 0.53mm) and Rt-Msieve 5A PLOT (30m × 0.53mm) column.

Characterization of catalysts were investigated by B.E.T., XRD, XPS, TPR, Raman, XANES techniques.

Results and Discussion

MoO3 addition clearly showed the synergism in the SCR reaction between the surface V and Mo oxide species, which was shown by the results obtained from the reactivity studies. In particular the temperature window of the SCR reaction was widened and shifted towards lower temperature. This synergism was studied in two directions: structural effect and chemical effect. First, the effect of MoO3 addition on the dispersion of vanadia over alumina surface was studied by B.E.T., XRD and Raman spectroscopy. As the result of B.E.T. analysis, surface areas of MoO3-V2O5/Al2O3 catalysts were same or slightly higher as alumina supported vanadia catalysts with the same loading of V2O5 and XRD results showed growth of crystalline V2O5 was inhibited by the addition of MoO3. Raman spectroscopy was able to detect small crystalline V2O5 particle which could not be detected by XRD and also showed that the addition of MoO3 increased the well-dispersed vanadia particle, and this vanadia species were more active for SCR reaction and become one of the reason for enhanced reactivity. Second, the higher reducibility of the ternary catalyst, MoO3-V2O5/Al2O5, compared to V2O5/Al2O3 catalyst with the same vanadia loading of ternary catalyst, has been clearly demonstrated by H2-TPR. As the result of XPS analysis of the catalysts after calcination and after reaction, the state of vanadium in MoO3 added catalysts after reaction test was more reduced than V2O5/Al2O3 catalysts. This reducibility was also appeared in XANES analysis.

Conclusion

Addition of MoO3 to V2O5/Al2O3 catalysts for SCR of NO by ammonia enhanced the catalytic activity and this synergism was resulted from the effect of MoO3 on the dispersion and reducibility of V2O5 over alumina support.

Reference

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Fig. 1. Effect of MoO3 addition to V9 and V12 catalysts on the catalytic activity. (GHSV : 100,000/hr, NO : 500 ppm, NH3 : 500 ppm, O2 : 10%,  He balance)

Fig. 2. XPS spectra of calcined and reduced V12 catalysts.

Fig. 3. XPS spectra of calcined and reduced Mo6V12 catalysts.

Fig. 4. TPR spectra of Mo6, V12, Mo6V12 catalysts

Fig. 5. Normalized XANES of V-K-edge of catalysts.