XPS study of potassium-promoted molybdenum carbides for mixed alcohols synthesis via CO hydrogenation

The X-ray photoelectron spectroscopy (XPS) was used to investigate the surface characteristic of potassium-promoted or un-promoted both β-Mo₂Cand α-MoC_1?x pretreated by syngas at different temperatures, and the promotional effect of potassium on the catalytic performance was also studied. XPS results revealed that the content of surface Mo and its valence distribution between β-Mo₂Cand α-MoC_1?x were quite different. Promoted by potassium, the remarkable changes were observed for surface composition and valence of Mo distribution over β-Mo₂C. Potassium had strong electronic effect on β-Mo₂C, which led to a higher Mo⁴⁺ content. On the contrary, potassium had little electronic effect on α-MoC_1?x, and K-Mo interaction was weak. Therefore, Mo⁰ and Mo²⁺ became the dominant species on the catalyst surface, and the Mo⁴⁺ content showed almost no increase as the pretreatment temperature enhanced. In terms of catalytic performance of molybdenum carbides, the increase in Mo⁰ most likely explained the increase in hydrocarbon selectivity, yet Mo⁴⁺ might be responsible for the alcohols synthesis.