The selective catalytic reduction of NO_x over Ag/Al₂O₃ with isobutanol as the reductant

This study investigates the potential of isobutanol (iBuOH) as a reductant for the selective catalytic reduction (SCR) of NO_x over 2 wt% Ag/Al₂O₃ between 150 and 550 °C and gas hourly space velocities (GHSV) between 10,000 and 35,000 h^-1. The feed gas consists of 500 ppm NO, 5% H₂O, 10% O₂, and 375-1500 ppm iBuOH (C₁:N ratios of 3-12); additionally, blends of 24 and 48% v/v iBuOH in gasoline are evaluated. Over 90% NO_x conversion is achieved between 300 and 400 °C using pure iBuOH, including a 40% peak selectivity toward NH₃ that could be utilized in a dual HC/NH₃-SCR configuration. The iBuOH/gasoline blends are only able to achieve greater than 90% NO_x conversion when operated at a GHSV of 10,000 h^-1 and employing a C₁:N ratio of 12. Iso-butyraldehyde and NO₂ appear to function as intermediates in the iBuOH-SCR mechanism, which mirrors the mechanism observed for EtOH-SCR. In general, the performance of iBuOH in the SCR of NO_x over a Ag/Al₂O₃ catalyst is comparable with that of EtOH, although EtOH/gasoline blends display higher NO_x reduction than iBuOH/gasoline blends. The key parameter in employing alcohols in SCR appears to be the C-OH:N ratio rather than the C₁:N ratio.