New insights into the catalytic mechanism of VOCs abatement over Pt/Beta with active sites regulated by zeolite acidity

Abstract

Zeolite-supported noble metal catalysts have good low-temperature activity and efficacy in eliminating VOCs contaminants. Regulating the nature of active sites is beneficial for improving the catalytic oxidation performance and decreasing coke deposition for further increase in the durability. In this work, Pt/Beta catalysts for toluene oxidation with different valence states, location, and dispersion of Pt were achieved through regulation of active sites using the acidity of zeolites with different Si/Al ratios. It was revealed that Ptδ+ were highly dispersed on the low Si/Al ratio of 6 including isolated Z[Pt2+(OH)]+ originated from Pt2+ ions anchored on ion-exchange sites (strong Brønsted acid sites) of Beta zeolites, while Pt0 nanoparticles were the main active sites of Beta zeolites with the high Si/Al ratio of 260 without H2 post-treatment. The Pt/Beta catalysts with high silica had better catalytic oxidation performance than that with low silica because Pt0 had stronger activation ability towards O2 as well as more facile oxygen replenishment than Ptδ+. It was noted that the key role of OH groups led to the enhanced oxidation of toluene driven by water vapor. Both high-silica and low-silica Pt/Beta catalysts exhibited excellent durability for 60 h of operation with no detectable decrease in catalytic performance and followed similar toluene oxidation pathways, forming intermediates including alkoxides, carboxylates, and anhydrides. The difference was that the quantity of gaseous benzene generated on high-silica Pt/Beta was much lower than that on low-silica Pt/Beta, indicating that Pt0 rather than Ptδ+ was conducive to the benzene ring opening reaction. The strong ability of Pt0 to activate O2 and the reduction of Ptδ+ into Pt0 during the reaction process could be responsible for the excellent durability and similar reaction pathways towards both low-silica and high-silica Pt/Beta. This study provides guidance for the regulation of active sites and design of efficient zeolite-based catalysts for VOCs abatement.