Theoretical Insights into Nitrate Reduction to Ammonia over Pt/TiO2: Reaction Mechanism, Activity Regulation, and Catalyst Design

Rational design of improved catalysts is one of the ultimate goals in catalytic research, the basis of which is clarifying the reaction mechanism and regulation trends. Here, we took nitrate reduction to ammonia as an example and revealed the complete reaction mechanism, rate-determining steps, and charge density regulation trends over Pt/TiO2. The dissociation of the three N–O bonds in NO3– favors the H*-assisted pathway via HONO2*, ONOH*, and HNOH* intermediates, producing the preliminary ammonia source in the form of NH*. Subsequent hydrogenation steps of NH* + H* → NH2* + * and NH2* + H* → NH3* + * show the two largest reaction barriers, being the rate-determining steps of the reaction. Further, by regulating the Pt charge density, we showed that all of the dissociation steps are slightly deactivated, whereas the hydrogenation steps, particularly those involving NH* and NH2*, are apparently promoted as positive charges accumulate on Pt particles. Accordingly, doping of Zn or Cu into TiO2 was proposed and furthermore verified as an effective strategy to improve the nitrate reduction activity. Such a promotional effect was attributed to the reduced H* adsorption energy on the metal surface as it became positively charged, manifesting itself as a general principle in boosting the hydrogenation activity.