Conversion of NO into N2 over γ-Mo2N

Cubic molybdenum nitride (gamma-Mo2N) exhibits Pt-like catalytic behavior in many chemical applications, most notably as a potent catalyst for conversion of harmful NOx gases into N-2. Guided by experimental profiles from adsorption of (NO)-N-15 on gamma-(Mo2N)-N-14, we map out plausible mechanisms for the formation of the three isotopologues of dinitrogen (N-14(2), N-15(2), and (NN)-N-14-N-15) in addition to (NNO)-N-14-N-15. By deploying cluster models for the gamma-Mo2N(100) and gamma-Mo2N(111) surfaces, we demonstrate facile dissociative adsorption of NO on gamma-Mo2N surfaces. Surfaces of gamma-Mo2N clearly activate adsorbed (NO)-N-15 molecules, as evidenced by high binding energies and the noticeable elongation of the N-O bonds. (NO)-N-15 molecule dissociates through modest reaction barriers of 24.1 and 28.1 kcal/mol over gamma-Mo2N(100) and gamma-Mo2N(111) clusters; respectively. Dissociative adsorption of a second (NO)-N-15 molecule produces the experimentally observed Mo2OxNy phase. Over the 100 surface, subsequent uptake of (NO)-N-15 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 N-15(2) 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 N-15(2) 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 (NO)-N-15 molecules at surface Mo-N bonds weakens the Mo-N bonds and leads to formation of (NN)-N-14-N-15 molecules (where N-14 denotes a nitrogen atom originated from surfaces of gamma-Mo2N crystals). Liberation of N-14(2) molecules occurs via surface diffusion of two surface N atoms on the (111) N-terminated surface. Formation of (NNO)-N-14-N-15 proceeds via direct abstraction of a surface N-14 atom by a gaseous (NO)-N-15 adduct.