Effect of promoters (CeOx, ZrOx, and SiOx) on activity and selectivity in the presence of H2O for selective catalytic reduction (SCR) of NO with NH3 by supported V2O5/TiO2 catalysts

A series of PO x -promoted (P = Ce, Si, and Zr) TiO 2 -supported vanadia catalysts were synthesized by incipient-wetness impregnation in order to examine the relationships between the anchoring sites, extent of oligomerization of the surface VO x sites, nature and number of surface acid sites, and the NO/NH 3 -SCR activity/selectivity under both water vapor-free and water vapor-containing reaction conditions at elevated temperatures. In situ IR spectroscopy showed that the deposited PO x and VO x species anchored at the support Ti-OH and P-OH/Ti-OH surface hydroxyls, respectively. In situ Raman spectroscopy revealed that all the oxides were essentially dispersed on the TiO 2 support as surface PO x and VO x sites under dehydrated conditions and reaction conditions. In situ solid-state 51 V MAS NMR revealed that the dehydrated surface VO x sites are present as monomers, dimers, oligomers, and clusters, with comparable molecular distributions for the unpromoted and promoted catalysts. IR also revealed that the surface acid properties (surface NH 3 * species on Lewis acid sites and surface NH 4 + * species on Brønsted acid sites) of the catalysts varied with the introduction of surface promoters and reaction conditions at elevated temperatures. Raman showed that the addition of vapor phase moisture to the SCR feed minimally perturbed the molecular structures of the surface VO x sites. The SCR reaction rate, however, decreased because of the competitive adsorption between H 2 O and NH 3 /NO that also decreased the production of undesirable N 2 O involving a bimolecular reaction. A correlation between SCR activity and nature and number of surface acid sites was not found to be present. The surface CeO x promoter significantly increased the SCR activity because of its facile redox characteristic both as surface CeO x sites on TiO 2 and bridging V-O-Ce bonds. In contrast, the surface ZrO x and SiO x sites on TiO 2 did not significantly affect the SCR activity because of their low redox propensity. The current studies demonstrate that the SCR activity is dominated by the redox properties of the promoter ligand V-O-P bonds and that the SCR selectivity to N 2 O depends on the surface NH 3 /NO/H 2 O concentrations. • SCR activity of CeO x , SiO x , and ZrO x promoted supported V 2 O 5 /TiO 2 was investigated. • Impregnated PO x and VO x anchored at support Ti-OH and P-OH/Ti-OH surface hydroxyls. • Types of surface VO x sites and surface acid properties vary with specific promoter. • Surface promoters suppress the formation of undesirable N 2 O global warming gas. • Moisture in the feed suppresses NO conversion activity due to competitive adsorption.