The effect of hydrogen on the selective catalytic reduction of NO in excess oxygen over Ag/Al2O3

Abstract

The selective catalytic reduction (SCR) of nitrogen oxides (NOx) by propane in the presence of H2 on sol–gel prepared Ag/Al2O3 catalysts (0.5–5wt.% Ag) was investigated. It was confirmed that hydrocarbon-assisted SCR of NOx is remarkably enhanced by co-feeding hydrogen to a lean exhaust gas mixture (λ>1), attaining considerable activity within a wide temperature window (470–825K). The samples had marginal activity at 575K without co-fed H2, but achieved up to 60% NOx conversion in the presence of H2 at a space velocity of 30,000h−1. NO2 as NOx feed component is not converted to N2 by C3H8 to a substantial extent under lean conditions. This points to an activation route of NO through direct conversion to adsorbed nitrite/nitrate or to a dissociation of NO over Ag0, formed through short-term reduction by H2. The nature of Ag species was characterized by X-ray diffraction, temperature-programmed reduction, pulse thermoanalytical measurements, electron microscopy and FTIR spectroscopy. It could be shown that Ag2O nano-sized clusters are predominantly present on all samples, whereas formation of silver aluminate could not be confirmed. Nano-sized Ag2O clusters can reversibly be reduced/reoxidized by H2. A silver loading higher than 2wt.% leads to a part of Ag2O particles, which are thermally decomposed during calcination at 800K or higher. The catalytic role of this metallic silver is still unclear. Formal kinetic analysis of catalytic data revealed that the activation energy of the overall reaction is significantly lowered in the presence of H2. The presence of water does not change the activation energy. It is concluded that hydrogen reduces the nano-sized Ag2O clusters to Ag0 on a short-term scale. Zero-valent silver promotes a dissociation pathway of NOx conversion. The fact that more oxidized ad-species (nitrite/nitrate) are observed in the presence of H2 is attributed to a dissociative activation of gas-phase oxygen on Ag0.