Probing the effects of plasma-induced surface species in ring-opening process of toluene decomposition via plasma-excited TPD and in situ DRIFTS

Abstract

Aromatic ring-opening process is considered as the rate-determining step for catalytic toluene decomposition. However, the ring-opening pathway has not been fully revealed, especially the effect of the surface species in plasma catalytic hybrid system. Herein, a series of Mn based catalysts (with the low Mn content of 1 wt %), Mn/γ-Al2O3, Au-Mn/γ-Al2O3 and Ag-Mn/γ-Al2O3, were used to clarify the mechanism of plasma catalytic toluene degradation by a closely combined temperature-programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Effects of plasma on surface oxygen species were investigated by a comparison study of O2-TPD and plasma-excited O2-TPD. A strong interaction between plasma and Ag-Mn/γ-Al2O3 could make better use of plasma-induced active oxygen species (i.e. O3, O*, O2* etc.) and considerably increase the amount of chemically adsorbed oxygen. Moreover, modified NH3-TPD experiments proved that O2 excited by plasma would greatly change the surface acidity of Ag-Mn/γ-Al2O3, resulting in the desorption activation energy (Ed) of toluene increasing to 19.5 kJ/mol. Finally, possible ring-opening mechanisms over different catalysts were proposed after the evolution of typical intermediates being systematically analyzed by in situ DRIFTS. It was found that the toluene ring-opening reaction on different catalysts might be selectively started at hydroquinone. The present work could provide both a more detailed toluene ring-opening process and new insights into the understanding of the plasma exciting effect on the surface species in VOCs decomposition.