Investigation of the reaction network and catalytic sites in selective catalytic reduction of nitric oxide with ammonia over vanadia catalysts

Selective Catalytic Reduction (SCR) of nitric oxide with ammonia and the role of direct oxidation ammonia in SCR reactions were investigated over both TiO{dollar}sb2{dollar}-supported vanadia catalysts and V{dollar}sb2{dollar}O{dollar}sb5{dollar} catalysts with preferential exposure of different crystal planes. All catalysts were characterized using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and 3-D imaging techniques. TPR and TPD studies were also performed. A steady-state fixed-bed reactor was used for activity and selectivity measurements. The product analysis was carried out by a combination of techniques including on-line chromatography, on-line mass spectrometry, chemiluminescence NO{dollar}sb{lcub}rm x{rcub}{dollar} analyzer, and wet chemistry method.; The catalysts supported over the rutile phase appeared to be more active than those over the anatase phase. The polymeric vanadate species are likely to be the active sites for the SCR reaction. For catalysts supported over the rutile phase, the presence of crystalline V{dollar}sb2{dollar}O{dollar}sb5{dollar} did not seem to affect the catalyst activity significantly.; V{dollar}sb2{dollar}O{dollar}sb5{dollar} catalysts were found to exhibit structure sensitivity in both SCR reaction and ammonia oxidation reaction. Direct ammonia oxidation reaction plays a significant role in the SCR reaction network at high temperatures. Isotopic labeling studies were also performed over the V{dollar}sb2{dollar}O{dollar}sb5{dollar} catalysts by labeling N and/or O. Oxygen exchange experiments revealed no formation of cross-labeled oxygen. NO was found to interact actively with the catalyst surface. The experimental evidence obtained through different phases of this study leads us to suggest a model where NH{dollar}sb3{dollar} adsorption takes place on at least three types of sites. Two of them, the V=O sites and the V=O neighboring V-OH sites, are located on the (010) basal plane. Ammonia adsorbed on the V=O sites promotes the formation of NO. Ammonia adsorbed on the V=O neighboring V-OH sites selectively produces N{dollar}sb2{dollar} and N{dollar}sb2{dollar}O from ammonia oxidation reaction and N{dollar}sb2{dollar}O from SCR reaction. Ammonia adsorbed on the third type of sites, i.e. the V-OH sites located on the side planes, readily forms N{dollar}sb2{dollar} from SCR reaction. Two NO molecules do not react to give one N{dollar}sb2{dollar} or one N{dollar}sb2{dollar}O molecule.