The critical roles of hydrophobicity, surface Ru0 and active O2−/O22− sites on toluene combustion on Ru/ZSM-5 with varied Si/Al ratios

Abstract

By targeting more feasible catalysts for VOC combustion, 2%Ru/ZSM-5 catalysts were fabricated by supporting RuO₂, a relatively cheaper noble metal, onto HZSM-5 supports with varied Si/Al ratios for toluene combustion. The valence state distribution of Ru and the Ru/RuO₂-support interaction have been explored and elucidated. It has been revealed that the catalytic activity increases with the increase of the Si/Al ratio in the order 2%Ru/ZSM-5-18 < 2%Ru/ZSM-5-40 < 2%Ru/ZSM-5-72 < 2%Ru/ZSM-5-110 < 2%Ru/ZSM-5-255 < 2%Ru/SiO₂-MFI. Interestingly, the hydrophobicity of the samples improves also with the increase in the Si/Al ratio, which impedes H₂O adsorption effectively and its competition for the surface-active sites with the reactants. Both RuO₂ and Ru⁰ are detected on all the catalysts, and the Ru⁰ amount/ratio increases significantly with increasing the Si/Al ratio, which promotes the adsorption/activation of both toluene and O₂ molecules. Furthermore, the amount of surface-active O₂⁻ and O₂²⁻ is evidently improved. Therefore, the mixed interaction of higher hydrophobicity, more surface Ru⁰ and active oxygen sites is the major reason for the enhancement in the activity of a Ru/ZSM-5 having a higher Si/Al ratio. It is concluded that the optimal catalyst can be designed by loading Ru/RuO₂ onto an MFI framework structure support with the highest Si content.