Surface properties of high-surface-area powder and thin film molybdenum nitrides treated in H2 and H2S

The surface properties of the high-surface-area powder and thin film Mo nitrides were investigated using X-ray photoelectron spectroscopy (XPS). After H2 and H2S treatments at 673 K for 3 h, these materials showed a variation in the distribution of Mo oxidation states and the atomic ratios (O/Mo, N/Mo, S/Mo), however, no change in the surface electronic structure was observed. The primary species for reduced and sulfided materials was Moδ+ while that for as-prepared materials was Mo6+. For H2-treated thin film Mo nitrides, the degree of reduction in the O/Mo ratios was 94% for TF-1 with γ-Mo2N and that for TF-2 with δ-MoN was 64%. However, the N/Mo ratio decreased 65% for TF-1 while TF-2 exhibited the increase of N/Mo ratio to 63%. These results suggested that Mo nitrides exhibited a strong bulk structure dependence for the degree of variation of O/Mo and N/Mo ratios in H2 treatment. Sulfur incorporation with Mo came at the expense of oxygen, showing that the S/Mo ratio increased with decreasing the O/Mo ratio. For high-surface-area Mo nitrides the sulfur tolerance was associated with differences in the predominant crystal faces exposed by the crystallites of γ-Mo2N. The estimation of S/Mo ratios suggested that the γ-Mo2N(200) was more sulfur-tolerable than γ-Mo2N(111). For thin film Mo nitrides, it was observed that the δ-MoN which had a hexagonal structure was more sulfur-tolerable than γ-Mo2N with a face-centered cubic structure.