Promotion of surface SO _x on the selective catalytic reduction of NO by hydrocarbons over Ag/Al ₂ O ₃

The promotion of sulfur oxides on the selective catalytic reduction (SCR) of NO by hydrocarbons in the presence of a low concentration of sulfur oxides over Ag/Al ₂ O ₃ has been investigated by a flow reaction test and in situ infrared spectroscopy. When the C ₃ H ₆ (or C ₁₀ H ₂₂ ) + NO + O ₂ feed-flow reaction was tested, maximum NO reduction was below 30% over fresh Ag/Al ₂ O ₃ . After the addition of SO ₂ to the feed flow, conversion increased slightly. Conversion increased further after SO ₂ was cut-off from the feed flow. This demonstrated that the increase in NO reduction activity of the catalyst was related to SO _x adsorbed on the catalyst. SO _x adsorbed on the catalytic surface (1375 cm ^-1 ) was detected by IR spectroscopy and was stable within the temperature range. NCO species, as an intermediate in NO reduction, on SO _x -adsorbed Ag/Al ₂ O ₃ in a C ₃ H ₆ + NO + O ₂ feed flow was observed in in situ IR spectra during the elevation of the reaction temperature from 473 to 673 K, while it was only observed at 673 K on fresh Ag/Al ₂ O ₃ under the same experimental conditions. We suggest that SO _x in low concentrations depressed the combustion of reductants by contaminating hydrocarbon combustion active sites on the catalyst, resulting in an increase in NO reduction efficiency of the reductants.