Study of cobalt and ruthenium supported on WC catalysts for FT reaction.

An investigation of bulk and cobalt or ruthenium supported tungsten carbide was carried out for their use in the Fischer-Tropsch reaction. Two types of WC were studied : tungsten carbide protected by free carbon and clean tungsten carbide, respectively noticed WC(A) and WC(B). XPS analysis shows that after air exposure, the WC(A) carbide surface is protected from the excess carbon whereas a surface W6+ phase is induced during the passivation step for WC(B). However WC(A) is inert towards aqueous impregnation, whereas WC(B) starts to corrode. A reduction in hydrogen at 673 K for Ru and 773K for Co supported catalysts allows obtaining Co0 and Ru0 dispersed on layers of free carbon covering the WC core for WC(A) and on a surface free of oxygen for WC(B). All the catalysts were active for the FT reaction. WC(A) produces mainly light alkanes (78%) and alcohols (22%), whereas no alcohol production is observed for WC(B). Co/WC(B) has a better activity than Co/WC(A), due to a better dispersion of cobalt on WC(B). The addition of Ru on WC(A) allow to obtain an high active catalyst with production of heavy hydrocarbons. On the contrary, formation of a Ru-W alloy could be consider to explain the low activity of Ru/WC(B) catalyst

Characterization of new Co and Ru on WC catalysts for Fischer-Tropsch reaction. Influence of the carbide surface state.

International audienceAn investigation of the performances in Fischer-Tropsch reaction of 1 wt% M/WC(X) (M = Co, Ru; X=A, B), where A is a tungsten carbide protected by free carbon and B is a clean tungsten carbide, was carried out. Supported catalysts performances were compared to those of the parent tungsten carbides at 473K and 20 bar. It was found that WC(A) produces mainly hydrocarbons but also 20–40% alcohols, whereas WC(B) activity is only towards linear alkanes. Before catalytic test, a reduction in pure hydrogen allows obtaining Co0 and Ru0 dispersed on layers of free carbon covering the WC core for the WC(A), and on a surface free of oxygen for WC(B). Co as Ru dispersions are improved on WC(B) compared to WC(A). A direct consequence is that Co/WC(B) has a better activity than Co/WC(A). Ru–W alloy formation could be responsible of the inobservance of a better activity for Ru/WC(B). On contrary, addition of Ru on WC(A) highly increases the activity and the production of heavy hydrocarbons. This beneficial effect, not observed with cobalt, could be attributed to a better dispersion of ruthenium on a carbon polymeric surface of WC

Effect of calcination temperature on the size of cobalt nanoparticles and catalytic performance of cobalt alumina-supported Fischer Tropsch catalysts

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Agglomeration at the Micrometer Length Scale of Cobalt Nanoparticles in Alumina-Supported Fischer-Tropsch Catalysts in a Slurry Reactor

International audienceCobalt sintering leading to the formation of large, individually detached microscopic metal agglomerates is detected in alumina‐supported cobalt catalysts after Fischer–Tropsch synthesis in a slurry reactor. The macroscopic sintering depends on the H2/CO ratio, the partial pressure of water, and the conversion of carbon monoxide, all of which lead to dramatic catalyst deactivation

H2 production from CH4 decomposition: Regeneration capability and performance of nickel and rhodium oxide catalysts

International audienceNickel–lanthanum (LaNiO3) and nickel–rhodium–lanthanum (LaNi0.95Rh0.05O3) perovskite-type oxide precursors were synthesized by different methodologies (co-precipitation, sol–gel and impregnation). They were reduced in an H2 atmosphere to produce nickel and rhodium nanoparticles on the La2O3 substrate. All samples were tested in the catalytic decomposition of CH4. Methane decomposed into carbon and H2 at reaction temperatures as low as 450 ◦C—no other reaction products were observed. Conversions were in the range of 14–28%, and LaNi0.95Rh0.05O3 synthesized by co-precipitation was the most active catalyst. All catalysts maintained sustained activity even after massive carbon deposition indicating that these deposits are of the nanotube-type, as confirmed by transmission electron microscopy (TEM). The reaction seems to occur in a way that a nickel or rhodium crystal face is always clean enough to expose sufficient active sites to make the catalytic process continue. The samples were subjected to a reduction–oxidation–reduction cycle and in situ analyses confirmed the stability of the perovskite structure. All diffraction patterns showed a phase change around 400 ◦C, due to reduction of LaNiO3 to an intermediate La2Ni2O5 structure. When the reduction temperatures reach 600 ◦C, this structure collapses through the formation of Ni0 crystallites deposited on the La2O3. Under oxidative conditions, the perovskite systemis recomposed with nickel re-entering the LaNiO3 framework structure accounting for the regenerative capability of these solids

Cobalt dispersion, reducibility, and surface sites in promoted silica-supported Fischer–Tropsch catalysts.

International audienceCobalt particle size, cobalt reducibility, and metal surface sites in a series of ruthenium- and rhenium-promoted cobalt silica-supported Fischer– Tropsch catalysts were studied by X-ray diffraction, UV–vis spectroscopy, in situ X-ray absorption, in situ magnetic method, X-ray photoelectron spectroscopy, DSC-TGA thermal analysis, and propene chemisorption. The catalysts were prepared by co-impregnation; in several catalyst syntheses, sucrose was added to the impregnating solutions. Mononuclear octahedral cobalt complexes were observed in the catalysts after impregnation and drying. Cobalt repartition on silica in the impregnated and dried catalysts depended primarily on the pH of the impregnating solution. Cobalt repartition was uniform on the silica surface if the pH of the impregnating solution was higher than the point of zero charge (PZC) of silica, but was less uniform at pH below that of the PZC of silica. Cobalt dispersion proceeded during catalyst calcination in air. Decomposition of cobalt nitrate and crystallization of cobalt oxide seemed to be the crucial steps in the preparation of highly dispersed cobalt catalysts. Promotion with noble metals resulted in greater cobalt dispersion, probably due to higher concentrations of cobalt oxide crystallization sites. Addition of sucrose modified the structure of supported cobalt complexes and led to higher temperatures of crystallization of cobalt oxide and to catalysts with extremely high cobalt dispersion. In situ magnetization measurements show that promotion with Ru moderated the temperature of reduction of cobalt oxide to metal phases, whereas the effect was less significant for Re-promoted catalysts. The addition of sucrose during impregnation, although significantly enhancing cobalt dispersion, did not diminish cobalt reducibility. Due to a combination of high cobalt dispersion and reducibility, the ruthenium- and rhenium-promoted catalysts prepared using sucrose had the highest number of cobalt metal surface sites. Fischer–Tropsch reaction rates were determined principally by the number of cobalt surface sites, with high cobalt dispersion and easy reducibility resulting in more active Fischer–Tropsch catalysts