Selective catalytic reduction of NO_x: mechanistic perspectives on the role of base metal and noble metal ion substitution

Abstract

Selective catalytic reduction (SCR) of NO by NH₃ in presence of excess oxygen is an industrially and environmentally important reaction. Determining a suitable catalyst substituted with noble metals or base metal ions with a suitable reaction mechanism is important. In this study, ionically substituted Mn and Pd in TiO₂ catalysts were synthesized by the solution combustion technique and examined for SCR activity. A seven-step reaction mechanism was proposed to incorporate the role of NH₃ oxidation in the SCR reaction that shows minima in the NO concentration profile. Both modeling and experimental results show that the reduction of NO in SCR condition follows the order Ti_0.9Mn_0.1O_2-δ > Ti_0.89Mn_0.1Pd_0.01O_2-δ > Ti_0.99Pd_0.01O_2-δ. However, the hydrogen uptake study showed that the noble metal ion (Pd²⁺) substituted TiO₂ has better reducibility than the base metal ion (Mn³⁺) substituted TiO₂. The rate of ammonia oxidation by different catalysts followed the reverse order such as that of NO reduction. This clearly indicates that catalysts with higher reducibility and that which exhibit higher rates of ammonia oxidation have poor SCR activity. Thus, the base metal ion substitution is better than noble metal ion substitution for SCR.