Selective catalytic reduction (SCR) of nitrogen oxides (NOx) by ammonia is the state-of-the-art NOx emission control technology for stationary sources, including power plants and industrial boilers. Supported V2O5-WO3/TiO2 catalysts, which show high catalytic activity and selectivity, thermal stability and resistance to sulfur poisoning, have long been accepted for commercial use. These catalysts are typically prepared by impregnation of soluble vanadium and tungsten precursors onto a TiO2 support. To evaluate the performance of SCR catalysts prepared by both co-precipitation and impregnation, the structural, physico-chemical properties and reactivity of supported V2O5-WO3/TiO2 catalysts prepared by co-precipitation of TiO(OH)2 with ammonium tungstate were compared with those of the catalysts prepared by the conventional impregnation method.
Materials and Methods
Metatitanic acid was precipitated from titanium isopropoxide by addition of deionized water (molar ratio water/titanium isopropoxide=110). After 60 min of stirring, the solution was filtered. The powder was washed and dried at 120 oC for 6hrs. To the resulting powder was then added an appropriate amount of deionized water, with the concentration of TiO(OH)2 slurry controlled at 2mol/L. Afterwards aqueous ammonium metatungstate (0.06M) and/or ammonium metavanadate (Aldrich; 0.35M) were poured into TiO(OH)2 suspension, respectively. Into this mixed suspension, aqueous ammonia was added dropwise with stirring to obtain pH8 to produce a co-precipitate gel. Water was removed from the gel by evaporation in a water bath. Each sample was dried at 120oC overnight, then calcined at 550 oC for 4 h in an atmosphere of air. For comparison, supported V2O5-WO3/TiO2 catalysts were prepared by incipient-wetness impregnation method as previously reported. In situ Raman and IR studies were carried out as previously described. BET surface areas were determined from adsorption-desorption isotherms of N2 at -196 oC using a Micromeritics ASAP 2020 apparatus. An Altamira AMI-200 spectroscope with a Dycor Dymaxion DME200MS online quadrupole mass spectrometer was used to chemically probe the catalysts with Temperature-Programmed Surface Reaction (TPSR) spectroscopy.
Results and Discussion
In contrast to the impregnation method that only contains surface mono-oxo O=WO4 sites, in situ Raman spectroscopic characterization of the co-precipitation procedure reveals the formation of two distinct WOx species on the TiO2 support: mono-oxo O=WO4 (~1010-1017 cm-1) on low defect density patches of TiO2 and a second mono-oxo O=WO4 (~983-986 cm-1) on high defect density patches of TiO2. Both WOx species were formed whether or not vanadia was present. In situ Raman showed that the structure of vanadium oxide is unaffected by the synthesis method; the vanadia was present as mono-oxo surface O=VO3 species present as both isolated and polymeric vanadate species. Although the kinetics for both catalysts are identical, the co-precipitated catalyst possessed more active sites, which is thought to be related to the second WOx species in the co-precipitated catalyst.
In situ infrared spectroscopic studies revealed that both surface NH4+* and NH3* species are chemisorbed on Brønsted and Lewis acid sites, respectively, and are active during the NO-NH3 SCR reaction. Temperature programmed in situ IR spectroscopy demonstrated that the surface NH4+* species Brønsted sites are more reactive than the surface NH3* species on Lewis sites, especially in the lower temperature region.
For the first time, a NH3/D3 isotope experiment was conducted for SCR of NO (+ O2) with NH3/ND3. The findings demonstrate that there is a kinetic isotope effect involved in breaking the N-H/N-D bond that is involved in the rate-determining-step of the SCR reaction. The implications of this finding for the mechanism of SCR of NO by supported V2O5-WO3/TiO2 catalysts as well as the design of improved SCR catalysts will be discussed.
This study provides new molecular level insights about the SCR reaction by supported V2O5-WO3/TiO2 catalysts and will be applied to the rational design of SCR catalysts.