Mechanisms governing selective hydrogenation of acetylene over γ-Mo₂N surfaces

Selective hydrogenation of acetylene (ethyne), present in hydrocarbon feed into ethylene (ethene) plays critical importance in processing operations. Unsupported molybdenum-nitride (Mo₂N) catalysts mediate surface hydrogenation of ethyne with a profound selectivity, similar to that observed over noble metals. However, the underlying mechanisms governing the H₂-C₂H₂-Mo₂N interaction leading to partial, rather than complete, hydrogenation remain largely unclear. In this contribution, we have found that molecular hydrogen adopts several physisorbed states prior to its dissociation, mainly on 3-fold hollow fcc (H1) and 4-fold hollow fcc (H3) sites over the (111) and (100) terminations of γ-Mo₂N, respectively. Our results on the interaction of hydrogen with the γ-Mo₂N surface concur very well with the analogous experimental findings of the dissociation occurring preferentially on the vacant nitrogen sites, the mobility of surface-adsorbed hydrogen atoms from weak to strong adsorption sites (associated with pathways of low and high activation energies for dissociation of adsorbed H₂ on the catalyst surface) and the inhibition effect of pre-adsorbed oxygen on H₂ dissociation. The constructed reaction mechanism, the estimated reaction rate constants, and the micro-kinetic modelling explain the selective hydrogenation of ethyne into ethene, rather than ethane. We demonstrate that the occurrence of selective hydrogenation rests on two aspects: distinctive energy profiles of the hydrogenation steps in partial versus full hydrogenation routes and thermodynamic selectivity entailing higher surface stability of adsorbed C₂H₂ in comparison to C₂H₄. Higher stabilities of the adsorbed open-shell C_xH_y species (C₂H₃∗, C₂H₅∗) on the (100) surface, in comparison to those on the (111) surface, indicate increased tendency for oligomerisation to occur on the (100) surface. Thermo-kinetic parameters reported herein provide molecular-level understanding of the unique and highly selective hydrogenation capacity of the γ-Mo₂N catalysts. Such knowledge is critically important in designing optimum operational conditions for practical processing operations.