The influence of hydrogen on the stability of nitrates during H2-assisted SCR over Ag/Al2O3 catalysts – A DRIFT study

Silver/alumina is a promising catalyst for the selective catalytic reduction of NOx by hydrocarbons (HC-SCR) in excess of oxygen, especially when adding small amounts of hydrogen. The same material shows excellent low-temperature activity for NH3-SCR in the presence of hydrogen. Since NH3 has been proposed to be an intermediate in HC-SCR, it is possible that the role of hydrogen is the same for both systems. Here, we study the effect of H2 in the presence of excess O2 on the differently adsorbed nitrates and nitrites on the Al2O3 support and on nitrates on silver by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and show that nitrites are an intermediate in the formation of monodentate nitrate. Moreover, hydrogen promotes the conversion of nitrites and some bidentate nitrates to mainly monodentate and some bridge-bound nitrates. Although monodentate nitrates are stable, they do not poison the catalyst surface, since the NOx reduction remained constant during 10 h of experiment at 250 °C. Contrary to the effect on nitrates bound to Al2O3, no effect of hydrogen on nitrates formed on AgO was observed. These nitrates were stable in gas mixtures containing hydrogen but decompose easily in the presence of NH3 at 100 °C. This indicates that the silver particles of an Ag/Al2O3 catalyst will not be poisoned by nitrates in H2-assisted NH3-SCR. Moreover, nitrates on silver migrate to the Al2O3 support already at 100 °C.