Anne Griboval-Constant

Structure and catalytic performance of cobalt Fischer Tropsch catalysts supported by periodic mesoporous silicas

The structure of cobalt catalysts supported by periodic mesoporous silicas at different stages of preparation was characterized by XRD, N 2 adsorption, XPS, in situ X-ray absorption and TGA. It was shown that the size and reducibility of supported cobalt particles were strongly affected by porous structure; larger and more easily reducible particles being detected in wider pore silicas. Cobalt dispersion was found to be controlled by silica pore sizes even at high cobalt contents (up to 30 wt.%) It was shown that catalytic behavior of cobalt supported mesoporous silicas in Fischer Tropsch synthesis strongly depended on cobalt dispersion and catalyst porous structure. Wide pore SBA-15 supported Co catalysts were found to be much (about 5–10 times) more active than narrow pore MCM-41 supported catalysts with the same cobalt content. Product distribution was found to be a function of cobalt particle sizes and cobalt reducibility. Fischer Tropsch reaction rates increased monotonically with increase in cobalt content up to 30 wt %, whereas product distributions for completely reduced wide pore catalysts were nearly the same at high and low cobalt loadings.

Effects of Metal Promotion on the Performance of CuZnAl Catalysts for Alcohol Synthesis

A series of CuZnAl catalysts modified with different promoters (Fe, Co, Ru, Zr, Mo, Mg, Mn, and Cr) have been prepared through co‐precipitation, characterised by applying a combination of techniques, and tested for carbon monoxide hydrogenation. Cu reducibility in CuZnAl catalysts was affected by the addition of promoters. The ease of Cu reduction in the promoted catalysts leads to more active catalysts for the hydrogenation of carbon monoxide and the production of C2+ alcohols, whereas lower catalytic activity was observed over less reducible catalysts. The promotion of CuZnAl catalysts even with small amounts of Cr, Mn, and Fe resulted in a significant modification in the reaction selectivity. The Fe‐containing catalyst demonstrated a dramatic increase in carbon monoxide conversion and C2+ alcohol productivity (30 mg g

Genesis of active sites in silica supported cobalt Fischer-Tropsch catalysts: effect of cobalt precursor and support texture

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A Time-Resolved In Situ Quick-XAS Investigation of Thermal Activation of Fischer–Tropsch Silica-Supported Cobalt Catalysts†

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

Optimization of cobalt deposition and textural properties of catalytic support represent two different but complementary approaches to enhance the performance of cobalt silica-supported Fischer-Tropsch catalysts. It is shown that low temperature decomposition of supported cobalt complexes involves formation of small Co3O4 crystallites. The exothermic effect due to the decomposition of cobalt precursor seems to be a major factor responsible of cobalt silicate formation. Gentle decomposition of cobalt precursor generally leads to Fischer-Tropsch catalysts with higher activity and selectivity to higher hydrocarbons. It was found that the sizes of supported cobalt particles prepared using cobalt nitrate depended on silica pore sizes. Narrow pore size distribution in periodic mesoporous silicas allows relatively high cobalt dispersion to be maintained at high cobalt loading. This leads to higher density of active sites and more active Fischer-Tropsch catalysts.

Pore Size Effects in Fischer Tropsch Synthesis over Cobalt-Supported Mesoporous Silicas

The Fischer–Tropsch (FT) process transforms coal-, natural gasor biomass-derived syngas (a CO/H2 mixture) into liquid hydrocarbons, which can be used as valuable petroleum substitutes because of their high cetane number and low content of sulfur and aromatics. Supported cobalt catalysts are suitable for low temperature FT process. Their catalytic performance is strongly affected by cobalt dispersion: higher metal dispersion supposes higher proportion of reduced metal in the catalyst and lower average nanoparticle size. This can be achieved by optimizing catalyst texture, adding organic compounds, or promoters during catalyst preparation, decomposing cobalt nitrate in a glow discharge or by controlling the catalyst thermal activation. 11] In particular, de Jong et al. prepared smaller Co3O4 and Co 0 particles on SBA-15 silicas by activating the catalyst in a NO-containing atmosphere instead of air prior to the reduction. In the present paper, in situ quick X-ray absorption spectroscopy (QXAS) has been used as a unique tool to accurately monitor the transformations of dispersed phases in supported catalysts under different atmospheres (air, helium and 5 % NO/He), both from the structural, quantitative, and kinetic standpoints. The time-resolved QXAS spectra were continuously collected at the Co K edge in the transmission mode during the catalyst activation. The experimental setup at SAMBA beamline (SOLEIL synchrotron) also allowed simultaneous in situ recording Raman spectra. The details of preparation, activation and characterisation of CoACHTUNGTRENNUNG(10 wt %)/SiO2 catalysts are given in Experimental Section. Figure S1 (see the Supporting Information) displays timeresolved in situ X-ray absorption near-edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) Fourier transform moduli obtained during the activation of SiO2-supported hexahydrated cobalt(II) nitrate [Co ACHTUNGTRENNUNG(H2O)6] ACHTUNGTRENNUNG(NO3)2 in air or helium. The shift of the edge position towards higher energies and changes in the white line intensity and shape above 160 8C are consistent with the transformation of the cobalt salt into Co3O4, characterised by a typical triangular XANES white line. 13] Actually, the presence of two successive series of isobestic points in the XANES spectra, below and above 140 8C, respectively, shows that decomposition of hydrated cobalt(II) nitrate takes place in two distinct steps, each involving two phases: first, dehydration to anhydrous Co ACHTUNGTRENNUNG(NO3)2 (with progressive replacement of aqua ligands by NO3 ions in Co ions coordination sphere), followed by decomposition of CoACHTUNGTRENNUNG(NO3)2 into cobalt oxide ([Eq. (1) and (2)]).

Fischer-Tropsch synthesis over silica supported cobalt catalysts: mesoporous structure versus cobalt surface density

Fischer–Tropsch (FT) synthesis is a promising approach to produce ultraclean hydrocarbon fuels by using syngas obtained from natural gas, coal, or biomass. Alumina-supported cobalt catalysts are generally preferred for FT synthesis because of their high activity, high selectivity to linear paraffins, and low water gas shift activity. [1–4] Nevertheless, structural changes to the cobalt catalysts during the FT reaction may result in a decrease in catalytic activity. These changes may include catalyst contamination; transformations of metallic cobalt into cobalt carbides, cobalt oxides, and/or cobalt aluminates; cobalt restructuring; agglomeration of the metallic cobalt particles; carbon deposition; and catalyst attrition. It has been shown [3] that, because of thermodynamic reasons, cobalt bulk oxidation does not occur and cobalt particles with crystallite sizes larger than 2–3 nm remain in the metallic state under typical FT synthesis conditions. Long-term deactivation may involve carbon deposition and catalyst attrition. The loss of the active catalyst by attrition represents one of the major problems in slurry bubble column reactors in industry. [4] Cobalt sintering at the nanoscale level has been observed by both in situ and ex situ techniques. Cobalt sintering results in an increase in cobalt particle size to several nanometers. [3, 5–7] The decrease in the active surface resulting from nanoscale sintering of metallic cobalt particles is an irreversible process driven by thermodynamic forces. [2] It has been reported that agglomeration of smaller cobalt nanoparticles by sintering could be responsible for a decrease in catalytic activity by approximately 30–40 % over the first few days of a reaction. [3, 7] A number of publications suggest that sintering levels off at cobalt particle sizes of several nanometers, which correspond to the pore diameter of the catalysts. To the best of our knowledge, micron-sized cobalt agglomerates produced during the FT reaction in slurry reactors have not been reported.