Jingping Hong

The nature of cobalt species in carbon nanotubes and their catalytic performance in Fischer–Tropsch reaction

The nature of cobalt species in the catalysts supported by multi-wall carbon nanotubes and their catalytic performance in Fischer–Tropsch synthesis were investigated using nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy, in situ magnetic method, X-ray absorption and temperature programmed reduction. The catalysts were prepared by incipient wetness impregnation using solutions of cobalt nitrate assisted by sonochemical process followed by calcination in nitrogen. The characterization techniques uncovered that acid pretreatment oxidized the carbon nanotube surface and removed impurities. Small cobalt oxide particles of 8–10 nm diameter and irregular shape anchored to the outer surface of carbon nanotubes were detected in the calcined samples by several characterization techniques. The catalysts displayed high cobalt reducibility, which was slightly affected by the pretreatment with nitric acid and nanotube outer diameter. Cobalt catalysts supported on carbon nanotubes exhibited high catalytic activity in Fischer–Tropsch synthesis. Pretreatment with nitric acid leads to a 25% increase in hydrocarbon yield, while carbon nanotube diameter does not seem to significantly affect the Fischer–Tropsch performance of the resulting catalysts.

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

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)]).