Interplay between surface structure, reaction condition and mechanism for ammonia decomposition on Ru catalyst

Ammonia decomposition provides a potential route for the production of COx-free hydrogen. Despite numerous studies, the active sites and reaction mechanism remain in debate. To date, only N-N recombination mechanism, in which N-2* is formed by the N* atoms recombination, is investigated on the archetypical Ru catalyst and Ru B-5 site has been identified as the active site. Combined density functional theory calculation and microkinetic modeling, we show that the N2Hy* (y > 0) dehydrogenation mechanism, which involves the stepwise N2Hy* dehydrogenation to N-2*, is important and even dominant on Ru defect sites at low temperatures, elevated PNH3 or P-H2 where the coverage of vacant sites is low so that the NHx (x = 1-3) dehydrogenation is suppressed. Regardless of reaction conditions, Ru A(4) and kink sites show higher activity than the conventional B-5 site, with the rate-determining steps of NH3 adsorption and H-2 desorption. Inhibition of the ammonia decomposition rate by the hydrogen produced originates from the low dissociation barrier of hydrogen. This work reveals the interplay between surface structure, reaction condition and mechanism for ammonia decomposition on Ru catalyst, and the insights can enrich the design principles of catalysts for the ammonia decomposition and other important reactions of technological interest.