NOx reduction consequences of lanthanide-substituted vanadates functionalized with S or P poisons under oxidative environments

Abstract

Rare-earth metal vanadates (RMVO4) typically possess an iso-structural tetragonal architecture but vary in terms of their Lewis acidic (LA) properties, which depend on the nature of the RM element. This study pioneers the exploitation of the LA sites inherent to RMVO4 on a TiO2 support as grafting points to immobilize HSOA-/SOA2-/H3-BPO4B- species, which are notorious poisons of LA sites during the selective catalytic reduction of NOx with NH3 (SCR). The HSOA-/SOA2- (S) and H3-BPO4B- (P) species served as Bronsted acidic (BA) sites with distinct distributions and modulated the redox cycling characteristics of the resulting RM-S/RM-P catalysts. The SCR performance of Ce-S/Ce-P and the other catalysts was dictated by the redox sites and amount of BA sites, respectively, at <= 300-340 degrees C, while exhibiting &apos;M&apos;-shaped periodicity in a plot of SCR performance versus the type of RM. This periodicity was maintained at >= 300-340 degrees C, although the catalyst performance was primarily dictated by the redox sites. With the exception of Ce-S/Ce-P, the RM-P catalysts outperformed the corresponding RM-S analogues in accelerating the SCR at <= 300-340 degrees C, whereas the opposite trend was observed at >= 300-340 degrees C. Furthermore, Gd-S consumed NOx and NH(3)via diverse pathways of NH4NO3 formation/transformation other than the SCR and production of ammonium sulfate (AS)/ammonium bisulfate (ABS) poisons, thus tolerating AS/ABS poisons in the most efficient manner at 250 degrees C. This study demonstrates the importance of the RM in HSOA-/SOA2-/H3-BPO4B--modified RMVO4 frameworks, whose properties on the BA and redox site and SCR performance varied markedly with the choice of RM.