Conversion of NO into N2over γ-Mo2N

Cubic molybdenum nitride (γ-Mo2N) exhibits Pt-like catalytic behavior in many chemical applications, most notably as a potent catalyst for conversion of harmful NOx gases into N2. Guided by experimental profiles from adsorption of 15NO on γ-Mo214N, we map out plausible mechanisms for the formation of the three isotopologues of dinitrogen (14N2, 15N2, and 14N15N) in addition to 14N15NO. By deploying cluster models for the γ-Mo2N(100) and γ-Mo2N(111) surfaces, we demonstrate facile dissociative adsorption of NO on γ-Mo2N surfaces. Surfaces of γ-Mo2N clearly activate adsorbed 15NO molecules, as evidenced by high binding energies and the noticeable elongation of the N–O bonds. 15NO molecule dissociates through modest reaction barriers of 24.1 and 28.1 kcal/mol over γ-Mo2N(100) and γ-Mo2N(111) clusters; respectively. Dissociative adsorption of a second 15NO molecule produces the experimentally observed Mo2OxNy phase. Over the 100 surface, subsequent uptake of 15NO continues to occur until the dissociated O and N atoms occupy all 4-fold hollow and top sites. We find that, the direct desorption of 15N2 from the Mo2OxNy-like phases phase requires a sizable energy barrier to precede. Considering a preoxygen surface covered cluster reduces this energy barrier only marginally. Desorption of 15N2 molecules takes place upon combination of two adjacent N atoms from top sites via a low-energy multistep Langmuir–Hinshelwood mechanism. Dissociative adsorption of gaseous 15NO molecules at surface Mo–N bonds weakens the Mo–N bonds and leads to formation of 14N15N molecules (where 14N denotes a nitrogen atom originated from surfaces of γ-Mo2N crystals). Liberation of 14N2 molecules occurs via surface diffusion of two surface N atoms on the (111) N-terminated surface. Formation of 14N15NO proceeds via direct abstraction of a surface 14N atom by a gaseous 15NO adduct.