Self-doping effect and successive magnetic transitions in superconducting Sr2 VFeAsO3

We have studied a quinary Fe-based superconductor Sr2VFeAsO3 by the measurements of x-ray diffraction, x-ray absorption, Mössbauer spectrum, resistivity, magnetization, and specific heat. This apparently undoped oxyarsenide is shown to be self-doped via electron transfer from the V3+ ions. We observed successive magnetic transitions within the VO2 layers: an antiferromagnetic transition at 150 K followed by a weak ferromagnetic transition at 55 K. The spin orderings within the VO2 planes are discussed based on mixed valence of V3+ and V4+

Advances in Selectivity Control for Fischer–Tropsch Synthesis to Fuels and Chemicals with High Carbon Efficiency

Fischer–Tropsch synthesis (FTS) is a versatile technology to produce high-quality fuels and key building-block chemicals from syngas derived from nonpetroleum carbon resources such as coal, natural gas, shale gas, biomass, solid waste, and even CO2. However, the product selectivity of FTS is always limited by the Anderson–Schulz–Flory (ASF) distribution, and the key scientific problems including selectivity control, energy saving, and CO2 emission reduction still challenge the current FTS technology. Herein, we review recent significant progress in the field of FTS to obtain specific target products including fuels, olefins, aromatics, and higher alcohols with high selectivity. These achievements are enabled by developing highly efficient catalysts and a controlled reaction pathway based on an integrated process. The structural nature of catalytic active sites and established structure–performance relationships are clarified. Moreover, we specially focus on the carbon utilization efficiency, and the efforts to tune the preferential formation of value-added chemicals and strategies to reduce CO2 selectivity are summarized. The challenges and the perspectives for future FTS technology development with high carbon efficiency are also discussed

Mechanism of the Mn Promoter via CoMn Spinel for Morphology Control: Formation of Co₂C Nanoprisms for Fischer–Tropsch to Olefins Reaction

The Fischer–Tropsch to olefins (FTO) reaction over Co₂C catalysts is structure-sensitive, as the catalytic performance is strongly influenced by the surface structure of the active phase. The exposed facets determine the surface structure, and it remains a great challenge to precisely control the particle morphology of the FTO active phase. In this study, the controlling effect of the Mn promoter on the final morphology of the Co₂C nanoparticles for the FTO reaction was investigated. The unpromoted catalyst and several promoted catalysts with Ce, La, and Al were also studied for comparison. For the Mn-promoted catalysts, the combination method of the Co and Mn components plays a crucial role in the final morphology of Co₂C and thus the catalytic performance. For the CoMn catalyst prepared by coprecipitation, Co₂C nanoprisms with specifically exposed facets of (101) and (020) can be obtained, which exhibit a promising FTO catalytic performance with high C_2–4⁼ selectivity, low methane selectivity, and high activity under mild reaction conditions. However, for the Mn/Co catalyst prepared via impregnation, Co₂C nanospheres are formed, which exhibit high methane selectivity, low C_2–4⁼ selectivity, and low activity. For the unpromoted catalyst and the catalysts promoted by Ce and La, Co₂C nanospheres are also obtained, with catalytic performance similar to that of the Mn/Co catalyst prepared via impregnation. Due to the high stability of the Co₂AlO_<i>x</i> composite oxide, no Co₂C phase can be formed for the catalyst promoted by Al

Effects of Sodium on the Catalytic Performance of CoMn Catalysts for Fischer–Tropsch to Olefin Reactions

The effects of a sodium (Na) promoter on the catalytic performance of cobalt-manganese (CoMn) catalysts for Fischer–Tropsch to olefin (FTO) reactions were investigated. For the sample without Na, Co⁰ was found to be the active phase for the traditional Co-based Fischer–Tropsch reaction with low CO₂ selectivity. The olefin/paraffin (O/P) ratio was found to be low with a C_2–4⁼ selectivity of only 15.4 C%. However, with the addition of Na, cobalt carbide (Co₂C) quadrangular nanoprisms with the (101) and (020) facets exposed were formed. The Co₂C nanoprisms displayed a high C_2–4⁼ selectivity (54.2 C%) as well as a low methane selectivity (5.9 C%) under mild reaction conditions. The O/P ratio for C_2–4 reached 23.9, and the product distribution deviated greatly from the classical Anderson–Schulz–Flory (ASF) distribution. Co₂C nanoprisms were considered to be an effective FTO active phase with strong facet effects. The Na promoter played a key role in the evolution of the FTO catalysts. The addition of Na, which acted as an electronic donor to cobalt, resulted in stronger CO adsorption and enhanced CO dissociation, which also benefited the formation of the Co₂C phase, leading to highly stable and active catalysts. The effects of other alkali promoters were also studied, and only the K promoter had an effect similar to that of Na on the CoMn catalysts for promoting the FTO reaction