Selective catalytic reduction of NOx by methane in excess oxygen over Rh based aluminium pillared clays

Pillared clays can be tailored to give high surface areas and porosities with different degrees of acidity. These materials often show better hydrothermal stability than zeolites. Al-PILC based Rh catalysts were tested in the selective catalytic reduction of NO by methane in presence of excess oxygen. The catalysts were prepared by two methods: incipient wetness impregnation (IMP) and wet ion exchange (IEX). The Rh³⁺ ion exchanged PILCs showed better performances than their homologues prepared by IMP. On IEX samples the effect of temperature and O₂ concentration in SCR-CH₄ activity was analyzed. Different activation degrees detected in the catalysts after 25 vol.% O₂ and 450 °C reaction conditions, were related to the various Rh species present in the catalysts as a consequence of the exchange degree of the support in contact with the active phase solutions. From the results, a reaction mechanism is proposed by which exchanged Rh³⁺ in the Al-PILC matrix and oxidized Rh₂O_x in combination with the supports acid sites contribute to form oxygenated species as the process-determining step. The results from XPS and O₂ adsorption isotherms were used to support the proposed mechanism.