Reactivity of Hydrogen on and in Nanostructured Molybdenum Nitride: Crotonaldehyde Hydrogenation

Early-transition-metal nitrides, including γ-Mo₂N, are active and selective for a variety of reactions, including the hydrogenation of organics (e.g., hydrodeoxygenation), CO (e.g., Fischer–Tropsch synthesis), and CO₂. In addition to adsorbing hydrogen onto the surface, some of these materials can incorporate hydrogen into subsurface, interstitial sites. Research described in this paper examined, experimentally and computationally, the nature of hydrogen on and in γ-Mo₂N, with a particular focus on characterizing the interactions of these hydrogens with crotonaldehyde. Hydrogen was added to γ-Mo₂N via exposure to gaseous hydrogen at elevated temperatures, forming γ-Mo₂N‑H_<i>x</i>, where 0.061< <i>x</i> < 0.082. Temperature-programmed desorption (TPD) experiments indicate that γ-Mo₂N‑H_<i>x</i> has at least two distinct hydrogen binding sites and that these sites can be selectively populated. Inelastic neutron scattering and density functional theory calculations indicate the presence of surface nitrogen-bound (κ¹-NH_surf), surface Mo-bound (κ¹-MoH_surf), and interstitial Mo-bound (μ₆-Mo₆H_sub) hydrogens. Selectivities for the hydrogenation of crotonaldehyde, a model of species in biomass-derived liquids, correlated with the populations at these sites. Importantly, materials with high densities of interstitial, hydridic hydrogen were selective for CO hydrogenation (i.e., formation of crotyl alcohol). Collectively the results provide mechanistic insights regarding the desorption and reactivity of hydrogen on and in γ-Mo₂N. Hydrogen adsorption/desorption to γ-Mo₂N is heterolytic; in particular, H₂ adds across a Mo–N bond. Because the surface Mo–H site is energetically unfavorable in comparison to the interstitial site, hydrogen migrates into interstitial sites once the surface NH sites are saturated. Crotonaldehyde adsorption facilitates migration of this interstitial hydrogen back to the surface, forming surface Mo–H that is selective for hydrogenation of the CO bond. These insights will facilitate the design of γ-Mo₂N and other early-transition-metal nitrides for catalytic applications