SCR용 TiO2 촉매의 금속(산화물)담지에 따른 특성변화 연구

Abstract

The selective catalytic reduction (SCR) of NO by ammonia is the most successful method for eliminating NO emitted from stationary sources. Mixed oxides of V2O5 and WO3 supported on TiO2 (V2O5-WO3/TiO2) are the most widely studied and technologically important SCR materials because of their high catalytic activity. Although V2O5 is the active component, in some operation conditions, it is reported that the undesirable reactions (poisoning effect) were induced like the oxidation of ammonia and SO2. Therefore, the V2O5 content of the catalyst is generally kept as low as possible (< 3 wt%). WO3 was reported to reduce the oxidations of ammonia as well as SO2 and usually added to the V2O5/TiO2 in larger amount (~10 wt%). WO3 is also used to provide thermal stability to the catalysts with retarding the transition of the anatase phase of titania (support) to rutile. Most researches mainly focused on chemical activities of the V2O5-WO3/TiO2 system, and how V2O5 and WO3 interact with the TiO2 support has not yet been clearly defined.

In the present study, to combine and to improve the catalytic properties of V2O5-WO3/TiO2 system, thermal, physical and chemical analysis were employed and the properties were compared to those of the corresponding binary V2O5/TiO2 and WO3/TiO2 systems. To investigate the effect of tungstenand vanadium on the morphological, phase formation and catalytic properties, binary V2O5/TiO2 and WO3/TiO2, and ternary V2O5-WO3/TiO2 catalysts with different WO3 (10 wt%) and V2O5 (1, 3, 5 and 10 wt%) loadings were prepared by wet impregnation method. The reactive TiO2 (Nano Co. NT) powder was used as the catalysts support. TEM morphology of the TiO22 powder shows the ultra-fine crystallites sized below 20 nm. Ammonium metavanadate (NH4VO3) and ammonium metatungstate hydrate ((NH4)6W12O39xH2O) were utilized as precursors for V2O5 and WO3, respectively. The precursors were dissolved in hot distilled water. To impregnate the catalysts, the raw materials including the solution of catalysts precursors and TiO2 powders were mixed and wet ball-milled for 12 h. The resulting slurries were dried at 90℃ for 24 h and heated in air from ambient temperature to 1200℃.

The prepared catalysts were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and BET surface area test. Catalytic activity has been measured by reduction of NO with NH3 in a fixed bed reactor containing 1 g catalyst. A feed consisting of 500 ppm NO, 600 ppm NH3, and ~6 vol% O2 with N2being a carrier gas (total space velocity = 100000h-1) was used. Activity data have been collected at different temperatures in the range 200-450℃, each temperature was maintained until steady-state conditions were reached.

Up to the heat-treatment temperature of 500℃, all samples are mono phasic and only anatase polymorph of TiO2 is detected. To investigate the effect of calcinations temperatures on the catalytic properties, the impregnated powders were heat treated above and below the temperature, i. e. 450℃ and 650℃. For pure TiO2 support, the anatase to rutile phase transition was complete at 1200℃. For WO3(10wt%)/TiO2, the tungsten loading leads to the lower transition temperature of ~1000℃, which is a controversial to the result of the research as mentioned. The transition temperature was also considerably lowered to 650 and 600℃ for the 5 and 10 wt% V2O5 added compositions, respectively. Crystallites dimensions increase with increasing calcination temperature and then the surface area decreases with the calcination temperature for all samples. For V2O5 loaded catalyst, the increasing in the calcinations temperature over 400℃ leaded to the drastic decrease of the surface area comparing to that of WO3 loaded one, which suggest that titania interacts more strongly with V than with W.

The WO3(10wt%)/TiO2 SCR powder obtained at 450℃ showed near 100% of NOX conversion efficiency at 350-400℃ and for the powder prepared at 650℃ the same efficiency was achieved in wider temperature range of 300-400℃. The highest NOx conversion efficiency of 100% was obtained in the V2O5(5wt%)/TiO2 SCR composition calcined at 650℃ in the relatively wider temperature range of 250-350℃ and the catalytic efficiency considerably decreased for the V2O5 (10wt%)/TiO2. The lower conversion of NOX observed in the V2O5(10wt%)/TiO2 composition calcined at 650℃ was considered to be correlated with the lowered surface area resulting from the increased grain growth by highly reactive vanadium addition. In the light of the thermal stability, it is proposed that the V2O5 loading for a conventional SCR catalyst is limited below 10 wt%.

For the ternary V2O5-WO3/TiO2 catalysts, to investigate the effect of V loading on morphological and catalytic properties of the catalyst, 3 different loadings of V3O5 (1, 3 and 5wt%) have been prepared on WO3(10wt%)/TiO2. The sample V2O5(3 wt%)-WO3(10wt%)/TiO2 showed the highest catalytic efficiency among them. The catalytic efficiency measured by NO conversion efficiency of the V2O5(3 wt%)-WO3(10wt%)/TiO2 catalysts calcined at 450℃ and 650℃ was measured. Considerably high efficiency is obtained at the wide temperature range. For the catalyst calcined at higher temperature (i. e. 650℃), it is shown that the relatively high efficiency at the lowest temperature, i. e. 200℃, has been found and the highest efficiency value remained above 400℃.