Selective reduction of NO over Cu/Al2O3: Enhanced catalytic activity by infinitesimal loading of Rh on Cu/Al2O3

Abstract

This study demonstrates that an isolated Cu2+ species formed by interacting with tetrahedral Al species on γ-Al2O3 shows a high catalytic activity for NO reduction with C3H6 and CO under high oxygen concentrations (lean conditions), and that an infinitesimal amount of Rh (0.01wt%, 100ppm) loaded on 0.5wt% Cu/Al2O3 is a more effective catalyst for the reactions, under a wide range of oxygen concentrations, than 0.5wt% Pd/Al2O3 or 1.0wt% Rh/Al2O3. Although 0.5wt% Cu/Al2O3 itself is effective for the reaction at lean conditions, it shows low catalytic activity under stoichiometric and low oxygen concentrations (rich conditions). On the other hand, the loading of 0.01wt% Rh on Cu/Al2O3 dramatically improves catalytic activities under stoichiometric and rich conditions, with maintaining the high NO reduction activity of Cu/Al2O3 itself under lean conditions. Therefore, a Rh species in the Rh/Cu/Al2O3 system must play a role as an active site under stoichiometric and rich conditions. Under lean conditions using Rh/Cu/Al2O3, an isolated Cu2+ species on γ-Al2O3 mainly contributes to the NO reduction properties of the catalyst. No such advantage was observed for Pd/Cu/Al2O3; that is, the catalytic activity of Cu/Al2O3 under lean conditions is diminished by Pd loading. Hence, we reveal that the roles of the Rh and Cu species in the Rh-loaded Cu/Al2O3 catalyst are effectively shared according to oxygen concentration in selective reduction of NO, when loaded with an infinitesimal amount of Rh. These observations provide novel guidelines for the development of catalysts that more beneficially use precious metal resources.