Minglin Xiang

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

Abstract The X-ray photoelectron spectroscopy (XPS) was used to investigate the surface characteristic of potassium-promoted or un-promoted both β-Mo2Cand α-MoC1−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-MoC1−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-MoC1−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.

Catalytic performance of iron carbide for carbon monoxide hydrogenation

Abstract Novel iron carbide and potassium-promoted iron carbide catalysts were prepared and investigated for CO hydrogenation. The iron carbide showed high activity for CO hydrogenation under high pressures; with the addition of potassium, activity and selectivity to C5+ hydrocarbons were greatly enhanced, and the selectivity to methane was suppressed under high pressure.

Single-step thermal carburization synthesis of supported molybdenum carbides from molybdenum-containing methyl-silica

Abstract A novel synthesis route to obtain highly dispersed molybdenum carbides in porous silica is described. The synthesis was carried out by a single-step heat treatment of molybdenum-containing and methyl-modified silica (Mo-M-SiO 2 ) in argon atmosphere at 973 K. Mo-M-SiO 2 precursor was facilely obtained via a one-pot synthesis route, using (NH 4 ) 6 Mo 7 O 24 ·4H 2 O (AHM) as molybdenum sources and polymethylhydrosiloxane (PMHS) as silica sources at the initial synthetic step. The optimal C/Mo molar ratio in reaction system for complete carburization of molybdenum species was 7. The carburization process of molybdenum species followed a nontopotactic route involving a MoO 2 intermediate phase, which was evidenced by XRD, N 2 adsorption-desorption and in situ XPS. Formation mechanism of Mo-M-SiO 2 precursor was also proposed by observation of the reaction between AHM and PMHS with TEM. Furthermore, by adding TEOS into silica sources and adjusting TEOS/PMHS mass ratio, crystal phase of molybdenum carbides transferred from β-Mo 2 C to α-MoC 1- x , and SiO 2 structure changed from microporous to micro/mesoporous. Catalytic performances of samples were tested using CO hydrogenation as a probe reaction. The supported molybdenum carbides exhibited high selectivity for higher alcohol synthesis compared with bulk β-Mo 2 C and αMoC 1- x .

Catalytic performance of Fe modified K/MO2C catalyst for CO hydrogenation

Abstract Fe modified and un-modified K/M02C were prepared and investigated as catalysts for CO hydrogenation reaction. Compared with K/MO2C catalyst, the addition of Fe increased the production of alcohols, especially the C2+OH. Meanwhile, considerable amounts of C5+ hydrocarbons and C=2-=4 were formed, whereas methane selectivity greatly decreased. Also, the activity and selectivity of the catalyst were readily affected by the reaction pressure and temperature employed. According to the XPS results, Mo4+ might be responsible for the production of alcohols, whereas the low valence state of Mo species such as Mo° and/or Mo2+ might be account for the high activity and selectivity toward hydrocarbons.

Nickel and potassium co-modified β-Mo2C catalyst for CO conversion

Abstract Nickel and potassium co-modified β-Mo2C catalysts were prepared and used for CO hydrogenation reaction. The major products over β-Mo2C were C1–C4 hydrocarbons, only few alcohols were obtained. Addition of potassium resulted in remarkable selectivity shift from hydrocarbons to alcohols at the expense of CO conversion over β-Mo2C. Moreover, it was found that potassium enhanced the ability of chain propagation with a higher C2+OH production. Modified by nickel, β-Mo2C showed a relatively high CO conversion, however, the products were similar to those of pure β-Mo2C. When co-modified by nickel and potassium, β-Mo2C exhibited high activity and selectivity towards mixed alcohols synthesis, and also the whole chain propagation to produce alcohols especially for the stage of C1OH to C2OH was remarkably enhanced. It was concluded that the Ni and K had, to some extent, synergistic effect on CO conversion.