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 HSO_A⁻/SO_A²⁻/H_3−BPO₄^B− species, which are notorious poisons of LA sites during the selective catalytic reduction of NO_x with NH₃ (SCR). The HSO_A⁻/SO_A²⁻ (S) and H_3−BPO₄^B− (P) species served as Brönsted 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 °C, while exhibiting ‘M’-shaped periodicity in a plot of SCR performance versus the type of RM. This periodicity was maintained at ≥300–340 °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 °C, whereas the opposite trend was observed at ≥300–340 °C. Furthermore, Gd-S consumed NO_x and NH₃via 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 °C. This study demonstrates the importance of the RM in HSO_A⁻/SO_A²⁻/H_3−BPO₄^B−-modified RMVO₄ frameworks, whose properties on the BA and redox site and SCR performance varied markedly with the choice of RM.