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 C-C 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 C-C 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.

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 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.

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 activity under mild reaction conditions. However, for the Mn/Co catalyst prepared via impregnation, Co2C nanospheres are formed, which exhibit high methane selectivity, low C2–4 = 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 x composite oxide, no Co2C phase can be formed for the catalyst promoted by Al.

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Person search aims at jointly solving Person Detection and Person Re-identification (re-ID). Existing works have designed end-to-end networks based on Faster R-CNN. However, due to the parallel structure of Faster R-CNN, the extracted features come from the low-quality proposals generated by the Region Proposal Network, rather than the detected high-quality bounding boxes. Person search is a fine-grained task and such inferior features will significantly reduce re-ID performance. To address this issue, we propose a Sequential End-to-end Network (SeqNet) to extract superior features. In SeqNet, detection and re-ID are considered as a progressive process and tackled with two sub-networks sequentially. In addition, we design a robust Context Bipartite Graph Matching (CBGM) algorithm to effectively employ context information as an important complementary cue for person matching. Extensive experiments on two widely used person search benchmarks, CUHK-SYSU and PRW, have shown that our method achieves state-of-the-art results. Also, our model runs at 11.5 fps on a single GPU and can be integrated into the existing end-to-end framework easily.

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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.