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

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 RMVO₄on a TiO₂support 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_xwith 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 performanceversusthe 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_xand NH₃viadiverse pathways of NH₄NO₃formation/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 RMVO₄frameworks, whose properties on the BA and redox site and SCR performance varied markedly with the choice of RM.