Identification of key reaction intermediates during toluene combustion on a Pd/CeO2 catalyst using operando modulated DRIFT spectroscopy

Abstract

Operando DRIFT spectroscopy is used to elucidate the role of key reaction intermediates during toluene oxidation on a Pd/CeO2 catalyst and bare CeO2 support. Selective identification of active surface species and their discrimination from spectators were carried out by combining concentration-modulation excitation spectroscopy (c-MES) experiments with phase-sensitive detection (PSD) spectral analysis. The resolution of highly overlapped infrared bands in MES-PSD spectra was performed by a chemometric multivariate curve resolution-alternating least squares (MCR-ALS) method. Pd/CeO2 catalyst completely oxidizes toluene to CO2 (T50 = 235 ºC), while pure CeO2 activates the toluene molecule but incomplete combustion to CO and formaldehyde is observed. Our results revealed that the methyl group of adsorbed toluene on the ceria surface is activated by subtraction of an H atom by a lattice O−2, forming benzyl (C6H5CH2-) species intermediate. Then, the benzyl species is stepwise oxidized to benzyl alcohol, benzaldehyde, and benzoate. Subsequent decarboxylation of benzoate and oxidation of the aromatic ring produces formate and aldehyde-like species, which are finally oxidized to CO2 and water. Alternatively, the benzoate intermediate can be oxidized to anhydride species (maleic anhydride or succinic anhydride) that accumulates on the surface and are slowly oxidized to CO2. The main role of the palladium metal nanoparticles is to facilitate the replenishing of the lattice oxygen via a metal-assisted Mars-van Krevelen mechanism. These findings provide molecular insights into a key environmental catalysis process, which will improve the rational design of optimized catalytic systems.