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Nanoscaled-VPO (vanadium phosphate oxide) catalysts have been prepared on an alumina support with the objective of obtaining crystallites of the active phase with a high surface-to-volume ratio. Since the catalysed reaction occurs at the surface, these nanoscaled catalysts allows studying surface species transformations with a minimized signal from the bulk dominion. The chemical environment and oxidation state of these species has been determined by a combination of NMR, ESR and XANES spectroscopies. The results have allowed to optimize V + P coverage and V/P molar ratio to minimize the concentration of isolated vanadium oxide species and maximize V4+ concentration. The distribution of phosphorus islands ordered and significantly distorted of tetra- and penta-coordinated aluminium sites of the catalytic support, thus shaping the catalytic material to an optimal structure for the desired partial oxidation reaction. [ABSTRACT FROM AUTHOR]

We investigated the possibility of synthesizing Co nanoparticles in CoZrnH/AlOx(OH)y/Al ceramic–metal catalysts and controlling the catalytic properties of these nanoparticles in syngas conversion by changing the Co/Zr ratio. The CoZr nanocomposites were obtained from metal powders by mechanochemical activation in a high-energy mill under an argon atmosphere, followed by treatment with hydrogen at high pressure and room temperature. Ceramic–metal catalysts were prepared by mixing the corresponding CoZrnH powder nanocomposite (30 wt%) with powdered aluminum (70 wt%), hydrothermal treatment of the mixture and subsequent calcination. The materials were characterized with a set of physicochemical methods: powder X-ray diffraction, scanning electron microscopy, 59Co internal field nuclear magnetic resonance spectroscopy, and temperature programmed reduction. Catalytic studies were performed in a laboratory fixed-bed flow reactor at 2 MPA and 210–270 °C. It is shown that the activity in syngas conversion to C5+ hydrocarbons and selectivity to methane and C2–C4 hydrocarbons depend on the Co/Zr ratio. Thus, with an increase in the zirconium content in the samples, the interaction of metal cobalt with metal zirconium improves in the process of mechanical activation and subsequent treatment with hydrogen. The destruction of the agglomerates of crystallites of metallic cobalt in the form of β-Co (Cofcc) occurs as well as their transformation to α-Co (Cohcp) particles active in the syngas conversion to C5+ hydrocarbons. This can explain the highest specific yield of C5+ hydrocarbons on a cermet with the lowest Co/Zr ratio. [ABSTRACT FROM AUTHOR]

[Display omitted] • 59Co IF NMR reveals the crystal structure of Cobalt nanoparticles supported on alumina. • hcp structure is favored for cobalt on χ-Al 2 O 3 compared to cobalt on γ-Al 2 O 3. • DFT modeling of χ-Al 2 O 3 surface shows that its hydroxyl distribution favors the formation of large hcp cobalt particles. • Alumina structure affects hcp / fcc Co ratio with potential catalytic implications, for example in Fischer-Tropsch synthesis. Metallic Co nanoparticles, widely used and studied as supported heterogeneous catalysts for Fischer-Tropsch synthesis (FTS), display catalytic properties that can vary significantly depending on their size and crystal structure. In this work, we used 59Co Internal Field NMR (59Co IF NMR) complemented by high-resolution transmission electron microscopy (HRTEM) to demonstrate the influence of strong metal-support interaction on two noticeably different metastable alumina phases - γ -Al 2 O 3 and χ -Al 2 O 3. According to 59Co IF NMR and HRTEM, the metallic particles supported on χ -Al 2 O 3 were larger and displayed a significantly higher content of Co in its hcp phase, which is known to be more active and selective to C 5+ in FTS. The 1H NMR chemical shifts of hydroxyl groups anchored to the (1 1 0) and (1 1 1) spinel crystal planes were calculated by DFT. It revealed that the hydroxyl coverage of γ -Al 2 O 3 facilitates the dispersion of Co precursor over the surface of the support, ultimately leading to the formation of smaller metal Co nanoparticles on γ -Al 2 O 3 than on χ -Al 2 O 3. [ABSTRACT FROM AUTHOR]