Mechanistic insight into the methanol selective catalytic reduction of NO reaction over Cu-containing perovskites

Abstract

The selective catalytic reduction (SCR) of NO by methanol was studied on LaFe0.8Cu0.2O3, and compared with that on an Ag/Al2O3 reference catalyst. A detailed characterization of the catalyst’s physicochemical properties was carried out by X-ray diffraction, N2 physisorption, H2 temperature-programmed reduction, isotopic exchange experiments, NOads + O2ads temperature-programmed desorption, and X-ray photoelectron spectroscopy. Materials reactivity was investigated in the CH3OH-SCR of NO. The LaFe0.8Cu0.2O3 catalytic system, modified using reactive grinding (Cu-RG), exhibits the best catalytic performance in terms of both NO conversion (95% at 450 °C) and N2 yield (YN2 ≈ 93%). Activity is related to low-temperature Cu2+ redox activation, and a strong improvement of structural Fe3+ reducibility occurs with substitution of the structure. In contrast, Ag/Al2O3 was confirmed to be close to inactive in the studied temperature range. To elucidate the main reaction pathways over the two catalysts, in situ diffuse reflectance infrared Fourier transform spectroscopy was performed to determine the nature of the adsorbed species and surface intermediates involved. Over Cu-containing perovskite, CH3OH is observed to form methoxy species (OCH3). The surface adNOx species observed over Cu-RG are mainly reactive nitrite/nitrate species, which further reacted with OCH3 species to form formohydroxamic acid and carboxylate. In the case of Ag/Al2O3, nitrate species continuously accumulated over the alumina surface, on which dehydration of CH3OH indubitably occurs. Therefore, no enolic intermediate of SCR can be observed over Ag/Al2O3. A reaction mechanism over LaFe0.8Cu0.2O3 was finally proposed, supported by density functional theory calculations, in which CN bond coupling accompanied by the first H transfer step is identified as the crucial step for N2 production in the CH3OH-SCR process over Cu-containing perovskites.