Controlling Catalytic Selectivity Mediated by Stabilization of Reactive Intermediates in Small-Pore Environments: A Study of Mn/TiO₂in the NH₃-SCR Reaction

Controlling selectivity in heterogeneous catalysis is essential for designing processes that minimize the production of undesired byproducts. For example, TiO2-supported manganese oxide, a promising material for catalyzing the selective reduction of NO with NH3 at very low temperatures, is currently restricted by poor selectivity because of the production of unwanted N2O, which has a greenhouse gas potential 300 times higher than that of CO2. In this study, we located manganese oxides in microporous TiO2 with small pores of molecular dimensions and found that this catalyst exhibited superior N2 selectivity (more than 98% at 100-200 °C) as compared to that of conventional Mn/TiO2 nanoparticles. The enhancement in N2 selectivity was consistently observed regardless of the amount of Mn active sites, demonstrating that the confining void environment is able to affect the site-specific selectivity of manganese oxides. When the reaction was performed at a low temperature (175 °C), it was found that the same reactive intermediate, possibly ammonium nitrate, was formed and deposited on both catalysts. However, the N2 selectivity in the reaction of this intermediate and NO was greatly improved in small-pore environments. In addition, it was demonstrated that such reactive intermediates are effectively stabilized when confined in small pores, thereby blocking the reaction pathway in which the intermediates directly decompose into N2O. This approach to confining active sites within the pores of support materials provides a rational strategy for designing highly selective catalytic materials.