New insights into the effect of nitrogen incorporation in Mo: catalytic hydrogenation vs. hydrogenolysis

The catalytic effect of nitrogen incorporation into Mo on hydrogenation (of -NO2 to -NH2 in nitrobenzene to aniline) and hydrogenolysis (of -C?O in benzaldehyde to toluene) processes has been assessed. Bulk Mo was prepared by temperature programmed reduction of MoO3 (in H-2 to 933 K) and -Mo2N (confirmed by powder XRD) subsequently synthesised by Mo nitridation in N-2/H-2. Two intermediate samples (MoN-1 and MoN-2) with different Mo/N ratio were prepared by altering the duration (1 and 2 h) of the nitridation step. XPS analysis revealed a nitrogen surface enrichment (Mo/N = 2.2 0.9 from MoN-1 to -Mo2N) relative to the bulk (Mo/N = 5.1 2.5). Incorporation of N did not affect morphology and each sample exhibited (by SEM analysis) aggregates (<5 m) of crystals (27-36 nm) with unchanged specific surface area (ca. 4 m(2) g(-1)). Hydrogen chemisorption and release (by TPD) increased with decreasing Mo/N (Mo < MoN-1 < MoN-2 < -Mo2N). Gas phase hydrogenation of nitrobenzene to aniline exhibited increasing rate from Mo -Mo2N, attributed to higher availability of surface heterolytic hydrogen (on Mo-N). In contrast, conversion of benzaldehyde to toluene was favoured by increasing Mo/N (from -Mo2N Mo) where hydrogenolytic -C?O scission is favoured by homolytic hydrogen chemisorption (on Mo). Our results provide the first evidence that N incorporation in Mo structure can control catalytic hydrogenation vs. hydrogenolysis performance