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The increasingly deteriorating environment demands the clean fuel to substitute the crude oil and coal in energy consumption. The uses of natural gas have attracted a lot of interest because of their clean burn and abundant supply. Fischer–Tropsch synthesis (FTS) is a promising way to convert coal, biomass and natural gas to clean fuels and chemicals via syngas. The cobalt-based catalyst is excellent in FTS for the direct conversion of syngas because of the high activity, high selectivity for long-chain paraffins used as synthetic diesel, and low water gas shift reaction activity [1]. The activity of supported cobalt catalyst in the FTS is reported to be proportional to the area of the exposed metallic cobalt atoms [2]. The high dispersion of the metallic cobalt is usually obtained by deposition of a cobalt salt on high surface area supports, such as Al2O3 or SiO2, and subsequent reduction. The chemical and textural properties of the catalyst support influence the catalytic activity and the product selectivity of Co catalyst via tuning modifications on the reducibility and dispersion of cobalt or the formation of well-fined phases. For the two popular supports, Al2O3 and SiO2, the former has significant interaction with the supported cobalt oxides, contributing to the formation of a smaller support-metal
cluster and limited reducibility due to the strong interaction by the diffusion of the cobalt ion into the structure of alumina, whereas the latter always yields a large SiO2-supported-metal cluster size and high reduction degree, exhibiting a reverse trend [3,4]. Some modifications on Co-based catalyst in FTS have been conducted with the purpose of improving the catalytic activity. The noble metal Ru enriched on a cobalt surface promoted the reduction of Co, whereas Pt and Pd dispersed uniformly in the bulky form of Pt–Co or Pd–Co alloy enhanced the dispersion but scarcely affected the reducibility of Co catalyst [5]. Even so, the practical application of the noble metal modified Co/SiO2 catalysts is unrealistic owing to their high cost. Furthermore, the addition of Al2O3 onto the Co/SiO2 catalyst significantly improved the catalytic activity of FTS by increasing the dispersion of supported cobalt on a SiO2 support without remarkably obstructing the reducibility of the supported cobalt [6]. Alumina is one of the most employed supports for cobalt FTS catalyst because of its favorable mechanical properties and adjustable surface properties. However, the strong interaction between the supported cobalt and the alumina supports often results in the relative low reducibility. It is considered that the addition of silica to Co/Al2O3 catalyst may optimize the surface properties of alumina supports since the cobalt oxides on the silica supports often exhibit high reduction degree. Based on our previous results [6], the present work investigated the reversible effect of silica addition onto Co/Al2O3 catalyst and it is expected that the designed modification could adjust the strong interaction between the supported cobalt and the alumina supports, realizing
the favored reduction degree, dispersion and metallic surface area of supported cobalt simultaneously, aiming at improved FTS activity. The modified catalyst was examined in a semibatch slurry-phase FTS reaction and its properties were characterized by in situ DRIFT, XRD, TPR, N2 physisorption and H2 chemisorption.
