WOx promotion on Pd/Al2O3 for toluene combustion: Adopting monolayer dispersion effect to fabricate low Pd content catalysts

Abstract

This study investigates the influence of WO_x loading on the WO_x-modified Pd/Al₂O₃ catalyst with low Pd content for the catalytic combustion of toluene. Pd/W_x-Al₂O₃ catalysts with varying WO_x loadings were synthesized using the wet impregnation method, based on the monolayer dispersion theory. The monolayer dispersion threshold of WO_x on Al₂O₃ is 0.717 mmol/100 m², which corresponds to a WO_x loading of 20 wt%. When the WO_x loading is below this threshold, the WO_x species on the Al₂O₃ surface primarily exist as tungstate. Conversely, if the WO_x loading exceeds the monolayer dispersion threshold, crystalline WO₃ forms. At the monolayer dispersion threshold, the Pd/W₂₀-Al₂O₃ catalyst exhibits the highest concentration of acidic sites and reactive oxygen species (O₂²⁻ and O₂⁻), demonstrating optimal catalytic activity for toluene combustion. Furthermore, Pd/W₂₀-Al₂O₃ exhibits excellent long-term stability and resistance to water and sulfur. WO_x loadings above or below the monolayer dispersion threshold also influence the reaction pathway of toluene catalytic combustion.