Accelerating Nitrite Reduction to Ammonia: The Synergistic Effect of Dual Active Site Pt–Ir Catalysts

Abstract

Ir-based catalysts have been demonstrated as promising catalysts for the reduction of NO₂^– to ammonia (NRA). However, the strong competitive adsorption of NO₂^– over H₂ on Ir sites, leading to H* deficiency at high NO₂^– concentrations, would hinder its real application. Here, we proposed an effective strategy to solve this problem by constructing dual active site Ir–Pt/TiO₂ composite catalysts with physical mixing (Ir+Pt/TiO₂) and co-impregnation (IrPt/TiO₂) methods. On such catalysts, the hydrogen spillover effect on Pt sites could alleviate H* insufficiency on Ir sites, thereby accelerating the NRA reaction. Specifically, the optimal dry-mixed Ir+Pt/TiO₂ catalyst exhibited an NRA rate constant of 12.4 L·g_Ir+Pt^–1·min^–1, approximately twice that of the Ir/TiO₂ sample. Moreover, Ir+Pt/TiO₂ catalysts showed first-order reaction kinetics rather than competitive reaction kinetics, confirming the alleviation of the H* deficiency limitation. Various characterization methods revealed that H* derived from H₂ dissociated adsorption on Pt sites could migrate through the TiO₂ support to Ir sites. DFT calculations also proved the thermodynamic feasibility of H* migration on TiO₂. Moreover, to further improve NRA activity, IrPt/TiO₂ alloy catalysts were employed to enhance Pt–Ir synergy, exhibiting the highest NRA rate constant of 17.0 L·g_Ir+Pt^–1·min^–1.