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The effects of various alkali metal promoters on the structure–performance relationship of cobalt–manganese (CoMn) catalysts for the Fischer–Tropsch synthesis (FTS) reaction have been investigated. The electronic promotion effects are correlated with the ionic potential (IP) of the alkali metals, which represents their electronic polarizability. The presence of alkali metal promoters in the CoMn catalysts is beneficial for the formation and stabilization of the Co2C phase. With an increase in the IP of the alkali metal promoters, the active phase was found to transition from Co2C nanospheres to Co2C nanoprisms and then to a mixture of fcc Co0 and Co2C nanospheres. In addition, the catalytic performance also changed with the transition in the active phase. For the Co2C nanospheres, high CH4 selectivity and low activity were observed. A promising catalytic performance for Fischer–Tropsch to olefins (FTO) with low CH4 selectivity and high C2–4= selectivity was achieved for the Co2C nanoprisms. However, low C2–4= selectivity with high activity was found for a mixture of fcc Co0 and Co2C nanospheres. It is suggested that the alkali metal promoters in the CoMn catalysts have two different promotion effects that are enhanced with IP: (1) the electronic effect to enhance the CO dissociation probability and benefit the formation of Co2C and (2) the facet stabilization effect to stabilize the Co2C nanoprisms with specific exposed (101) and (020) facets under the FTO reaction conditions.

Fischer-Tropsch to olefins (FTO) is recognized as a surface-catalyzed structure-sensitive reaction, and the catalytic performance is strongly influenced by the morphology and exposed facets of the active phase. Here we report the effect of the reduction process on the morphology of the active phase and the catalytic performance for FTO over the CoMn catalyst. For the catalysts reduced by 10% CO-300 °C, 10% H2-300 °C and 10% H2-250 °C, Co2C nanoprisms were formed after reaching the steady state. However, for the catalysts reduced by CO-300 °C and 10% H2-400 °C, Co2C nanospheres were found instead. Both Co2C nanoprisms and nanospheres were present for the spent sample reduced by 10% H2-350 °C. Kinetic study found Co2C nanospheres to possess higher activation energy, and are more sensitive to hydrogen than Co2C nanoprisms. Density functional theory (DFT) calculations were also performed to clarify the structure-performance relationship of Co2C nanostructures for syngas conversion.

CoMn model catalysts were prepared by co-precipitation and evaluated for higher alcohol synthesis (HAS) via syngas. The selectivity to oxygenates (mainly alcohols and aldehydes) was found to be higher than 20 C% for the Na-promoted CoMn catalyst. Among the oxygenates, C2+ and C6+ fractions accounted for >90 wt% and ∼50 wt%, respectively. Metallic Co0 particles and MnCO3 were found in both Na-promoted and unpromoted CoMn catalysts after the reaction, whereas Co2C nanoparticles could only be observed in the spent Na-promoted catalyst. The addition of Na benefited the carbonization of cobalt and increased the selectivity to oxygenates and CO2. Our studies thus suggested that Co/Co2C were the bifunctional dual-sites for the oxygenates formation over the Na-promoted CoMn catalyst, where Co catalyzed CO dissociation and chain propagation, while Co2C was responsible for CO non-dissociative activation and subsequent insertion. In addition, the sole Co2C nanoparticles with certain exposed facets may also act as another kind of active dual-sites for oxygenates formation.

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 C2–4= selectivity, low methane selectivity, and high ...

A series of Co/SiO2 catalysts with different sodium (Na) loadings (0, 0.1, 0.2, 0.5 and 1 wt%) were prepared and evaluated for Fischer-Tropsch reaction to study the effect of Na on the catalyst structure and catalytic performance. The addition of Na was found to decrease the catalytic activity and hydrocarbon selectivity, but increase CO2 selectivity due to the enhanced WGS activity. The addition of Na also resulted in higher selectivity to oxygenates (alcohols and aldehydes) and O/P ratio as well as the shift of hydrocarbons to lower carbon numbers. Structure characterization revealed a decrease in the surface area and particles size for the calcined samples with the addition of Na. Co2C was formed during the reaction process for the Na-promoted catalysts. As a result, a new Co/Co2C bifunctional active sites were generated for oxygenates formation leading to increasing oxygenates selectivity. In addition, the Co2C nanoparticles alone may also act as dual active sites for oxygenate formation at high reaction pressure over the promoted catalysts with high Na loading.

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 C2–4= 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 C2–4= selectivity (54.2 C%) as well as a low methane selectivity (5.9 C%) under mild reaction conditions. The O/P ratio for C2–4 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...

Lower olefins-generally referring to ethylene, propylene and butylene-are basic carbon-based building blocks that are widely used in the chemical industry, and are traditionally produced through thermal or catalytic cracking of a range of hydrocarbon feedstocks, such as naphtha, gas oil, condensates and light alkanes. With the rapid depletion of the limited petroleum reserves that serve as the source of these hydrocarbons, there is an urgent need for processes that can produce lower olefins from alternative feedstocks. The 'Fischer-Tropsch to olefins' (FTO) process has long offered a way of producing lower olefins directly from syngas-a mixture of hydrogen and carbon monoxide that is readily derived from coal, biomass and natural gas. But the hydrocarbons obtained with the FTO process typically follow the so-called Anderson-Schulz-Flory distribution, which is characterized by a maximum C2-C4 hydrocarbon fraction of about 56.7 per cent and an undesired methane fraction of about 29.2 per cent (refs 1, 10, 11, 12). Here we show that, under mild reaction conditions, cobalt carbide quadrangular nanoprisms catalyse the FTO conversion of syngas with high selectivity for the production of lower olefins (constituting around 60.8 per cent of the carbon products), while generating little methane (about 5.0 per cent), with the ratio of desired unsaturated hydrocarbons to less valuable saturated hydrocarbons amongst the C2-C4 products being as high as 30. Detailed catalyst characterization during the initial reaction stage and theoretical calculations indicate that preferentially exposed {101} and {020} facets play a pivotal role during syngas conversion, in that they favour olefin production and inhibit methane formation, and thereby render cobalt carbide nanoprisms a promising new catalyst system for directly converting syngas into lower olefins.

A novel synthetic route to the layered material PLS-4 has been developed using amorphous silica as the silica source and ethyldimethylethanolammonium fluoride as the organic structure-directing agent. This reaction can proceed under high concentration conditions utilizing quaternary ammonium fluoride salt rather than ammonium fluoride or hydrofluoric acid. By introducing aluminum, alkali metal and alkaline earth metal into the aforementioned system, three new morphologies have been obtained. Products were fully characterized using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption analyses.

Two-dimension and three-dimension topography of pulsed lasers ablation of tissues were scanned by means of optical coherent microscopy,in order to analyze the ablation characteristics of pulsed Erbium:YAG lasers for hard biological tissues.Experimental results indicate that under the same laser parameters,ablation efficiency for gall-stone is higher than that for urinary stone.The higher the intensities of pulsed lasers are,the higher the ablation efficiency will be for both gall-stone and urinary stone.The higher ablation efficiency mainly embodies that wider surface diameter,deeper height,and bigger volume for craters will be obtained.Measurements for stone craters using the optical coherent microscopy are higher by about one order than those using the optical coherent tomography.Optical coherent microscopy system can contribute to quantitatively measure hard tissues irradiated by pulsed lasers much better than optical coherent tomography system.

A method for preparing a SiC coating on carbon fibers is presented. The SiC coating was generated from the reaction of silicon monoxide (SiO) with a pyrolytic carbon (PyC) layer deposited on the fibers. The influence of holding time on the microstructure of the SiC layer was discussed. The oxidation behaviors of the uncoated and SiC coated carbon fibers were compared. The formation mechanism of the SiC coating was evaluated. With increased reaction time, the SiC coating becomes thicker and its surface becomes rough. The oxidation resistance of the carbon fiber was improved by the SiC coating. The initial oxidation temperature of the SiC coated carbon fiber is about 200 °C higher than that of the uncoated carbon fiber. The growth of the SiC coating is mainly attributed to the indirect reactions of SiO with PyC in the SiO/SiC/C system, in which silicon is considered a critical intermediate product.