Effect of Crystallographic Phase (β vs. γ) and Surface Area on Gas Phase Nitroarene Hydrogenation Over Mo2N and Au/Mo2N

The catalytic action of Mo 2 N and Au/Mo 2 N has been assessed in the selective gas phase hydrogenation of p -chloronitrobenzene ( p -CNB) to p -chloroaniline ( p -CAN). The nitrides were synthesised via temperature programmed treatment of MoO 3 in H 2 + N 2 and Au introduced by deposition–precipitation with urea. We have examined the influence of nitride crystallographic phase (tetragonal β-Mo 2 N vs. cubic γ-Mo 2 N) and surface area (7–66 m 2 g −1 ) on the catalytic response. Catalyst activation by temperature programmed reduction has been monitored and the reduced catalysts characterised in terms of BET area/pore volume, H 2 chemisorption/temperature programmed desorption (TPD), powder X-ray diffraction (XRD), elemental analysis, scanning (SEM) and transmission (TEM) electron microscopy and X-ray photoelectron spectroscopy (XPS) measurements. The formation of β- and γ-Mo 2 N was confirmed by XRD and TEM. γ-Mo 2 N exhibits a platelet morphology whereas β-Mo 2 N is characterised by an aggregation of small crystallites. Hydrogen chemisorption and TPD analysis have established a greater hydrogen uptake capacity (per unit area) for β-Mo 2 N relative to γ-Mo 2 N, which is associated with surface nitrogen deficiency, i.e. higher surface Mo/N for β-Mo 2 N. Incorporation of Au on both nitrides resulted in an increase in surface hydrogen. The Au phase takes the form of nano -scale particles with a mean size of 7 and 4 nm on β-Mo 2 N and γ-Mo 2 N, respectively. Both β-Mo 2 N and γ-Mo 2 N promoted the exclusive hydrogenation of p -CNB to p -CAN where the β-form delivered a higher specific (per m 2 ) rate; the specific rate for γ-Mo 2 N was independent of surface area. The inclusion of Au on both nitrides served to enhance p -CAN production.