Conversion of NO into N₂ over 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 NO_x 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 Mo₂O_xN_y 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 Mo₂O_xN_y-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.