Interfacial reactivity of Pd-VOx supported on ZSM-5 catalyst for low-temperature H2-SCR

Abstract

Low-temperature selective catalytic reduction of NO by H2 (H2-SCR) over Pd/ZSM-5 is constrained under lean conditions by retained NOx adsorbates that hinder H2 uptake on Pd sites. Here, we investigated how V promotion mitigates this limitation by forming Pd-VOx interfacial contact while preserving Pd-site accessibility. A series of Pd-VOx/ZSM-5 catalysts with systematically varied V/Pd ratio, together with composition-matched catalysts prepared by sequential impregnation, was examined by catalytic testing, temperature-programmed adsorption/ reduction analyses, and time-resolved in situ DRIFTS. The H2-SCR activity showed a volcano-shaped dependence on V loading, with PdV10Z reaching approximately 79% NOx conversion at 180 degrees C. This optimum performance did not arise from VOx addition alone, framework acidity alone, or bulk textural changes. Instead, PdV10Z preserved Pd sites accessible for NO adsorption while exhibiting modified reduction behavior and stronger NH3 retention. The key catalytic divergence emerged after H2 introduction: compared with PdZ, PdV10Z depleted an H2-responsive monodentate nitrate fraction more rapidly, removed Pd-bound nitrosyl species faster, and accumulated NH4+ more strongly. Sequential impregnation confirmed that this response depends not simply on V addition, but on preserving accessible, catalytically cooperative Pd-VOx interfacial contact. These results identify accessible Pd-VOx interfacial contact as the key structural feature governing sustained low-temperature H2-SCR over Pd/ZSM-5 and establish a design principle for balancing interfacial cooperation with Pd-site accessibility.