Insight into hydroxyl groups in anchoring Ir single–atoms on vacancy–deficient rutile TiO2 supports for selective catalytic oxidation of ammonia

Abstract

High–performance catalysts are extremely required for controlling NH₃ emission via selective catalytic oxidation (SCO), and the anchoring structural feature of active sites is a key prerequisite for developing them. This study confirms the importance of hydroxyl groups on vacancy–deficient reducible oxides as active groups. On the one hand, spontaneous atomic dispersion of active metal Ir is promoted by the abundant terminal hydroxyl groups. On the other hand, Ir cations anchor on the TiO₂ surface through exchange with H⁺ in Ti–OH groups, and thus occupy the Brönsted acid sites. The adsorption strength of NH₃ is another key factor affecting the reaction rate–determining step, namely NH₃ dehydrogenation, which occurs at a faster rate in the coordinated L–NH₃ rather than the ionic B–NH₄⁺. Meanwhile, the coordinated L–NH₃ significantly avoids the competitive adsorption of water vapor in the NH₃–SCO reaction by reducing the number of hydrogen bonding. The TOF of preferred 0.8Ir/TiO₂ sample is significantly higher than 0.2Ir/TiO₂ sample, although Ir is almost always atomic dispersed. Finally, NH₃ conversion is 85% in a wet circumstance (5% H₂O) at 240 °C (GHSV = 85 000 h^–1), with a N₂ selectivity of up to 65% on 0.8Ir/TiO₂ sample.