1 research outputs found
Reactivity of Hydrogen on and in Nanostructured Molybdenum Nitride: Crotonaldehyde Hydrogenation
Early-transition-metal
nitrides, including Ī³-Mo<sub>2</sub>N, are active and selective
for a variety of reactions, including
the hydrogenation of organics (e.g., hydrodeoxygenation), CO (e.g.,
FischerāTropsch synthesis), and CO<sub>2</sub>. 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 Ī³-Mo<sub>2</sub>N, with a particular
focus on characterizing the interactions of these hydrogens with crotonaldehyde.
Hydrogen was added to Ī³-Mo<sub>2</sub>N via exposure to gaseous
hydrogen at elevated temperatures, forming Ī³-Mo<sub>2</sub>NāH<sub><i>x</i></sub>, where 0.061< <i>x</i> <
0.082. Temperature-programmed desorption (TPD) experiments indicate
that Ī³-Mo<sub>2</sub>NāH<sub><i>x</i></sub> 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 (Īŗ<sup>1</sup>-NH<sub>surf</sub>), surface Mo-bound
(Īŗ<sup>1</sup>-MoH<sub>surf</sub>), and interstitial Mo-bound
(Ī¼<sub>6</sub>-Mo<sub>6</sub>H<sub>sub</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 Cī»O hydrogenation (i.e., formation of crotyl
alcohol). Collectively the results provide mechanistic insights regarding
the desorption and reactivity of hydrogen on and in Ī³-Mo<sub>2</sub>N. Hydrogen adsorption/desorption to Ī³-Mo<sub>2</sub>N is heterolytic; in particular, H<sub>2</sub> adds across a MoāN
bond. Because the surface MoāH 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 MoāH that is selective for hydrogenation of
the Cī»O bond. These insights will facilitate the design of
Ī³-Mo<sub>2</sub>N and other early-transition-metal nitrides
for catalytic applications