Ex-nitrate Co/SBA-15 catalysts prepared with calibrated silica grains: Information given by TPR, TEM, SAXS and WAXS

Abstract

The aim of the present work was to check for the presence of an ordered array of Co-based nanoparticles in catalysts prepared by impregnation and by using the two-solvents technique with pentane and cyclohexane on a single SBA-15 batch (prepared by precipitation in strongly acidic conditions (HCl 2 mol L −1)). Cobalt nitrate is used as a precursor for an at. Co/Si ratio of 0.03. Solids are characterized after heat treatments in oxidizing (up to 700 °C, 2 °C/min, under O 2 10% in vol./Ar) and reducing (up to 1000 °C, 7 °C/min, under a H 2 10% in vol./He) conditions. After oxidation up to 700 °C, characteristic spinel diffraction peaks are observed in WAXS. The relative intensities of these peaks suggest that Co 3O 4 dominates when using impregnation whereas mixed Co 3– x Si x O 4 domains are formed when using the two-solvents technique. Broad and symmetrical when using pentane, the X-ray diffraction peaks are more asymmetric with cyclohexane. On a statistical scale, more asymmetric spinel particles are then formed with this solvent. Still with cyclohexane, a correlation diffraction peak at 0.6° in SAXS is observed. Two TPR peaks before 500 °C are associated with the reduction of Co 3O 4 particles into CoO and then metallic Co. Two other TPR peaks (a shoulder at 330–350 °C, a broad peak at 640–680 °C) indicate the presence of particularly small oxidized particles in strong interaction with silica (detected neither by XRD, nor by TEM). By reference to recently published data, additional reductions observed within the range 800–900 °C and 900–1000 °C are associated with cobalt silicate Co 2SiO 4 domains, respectively dispersed on the silica surface and bulk. In reduced samples, two main kinds of metallic particles are identified: (i) large and spherical particles (diameter >20 nm) growing outside silica grains and often covered by a silica layer. These particles are polytypic face-centered cubic with some hexagonal intergrowths, (ii) smaller particles (diameter <10 nm) which remain dispersed inside the silica grains and/or inside silica walls. In SAXS, there is no correlation peak but the diffusion background detected at the lowest angles can be associated with the diffusion due to the largest Co-based particles.