Cobalt particle size effects in Fischer–Tropsch synthesis: structural and in situ spectroscopic characterisation on reverse micelle-synthesised Co/ITQ-2 model catalysts

A series of 10%Co/ITQ-2 model catalysts have been prepared by combining a reverse micellar synthesis with a surface silylated ITQ-2 delaminated zeolite. The catalysts display rather uniform Co0 particle size distributions in the 5–11 nm range as ascertained by XRD, H2-chemisorption and (HR)TEM. Additionally, a low dispersed 30%Co/SiO2 reference sample (d(Co0) = 141 nm) has been prepared by supporting a Co3O4 nanopowder on spherical SiO2. H2-TPR and DR UV–vis spectroscopy reveal that the preparative approach leads to highly reducible catalysts in the d(Co0) range of 5.6–141 nm, while the activation energies for the stepwise Co3O4 → CoO → Co0 reduction are found to be particle size dependent. Formation of barely reducible surface and bulk Co silicate species is observed for samples with d(Co3O4) ⩽ 5.9 nm. Under realistic Fischer–Tropsch synthesis conditions (493 K, 2.0 MPa) the TOF increases from 1.2 × 10−3 to 8.6 × 10−3 s−1 when d(Co0) is increased from 5.6 to 10.4 nm, and then it remains constant up to a particle size of 141 nm. In situ and at work FTIR of adsorbed CO reveal a severe cobalt surface reconstruction towards more open crystal planes and/or defect sites (Co–carbonyl bands in the region of 2000–2025 cm−1) and suggest adsorbed C adatoms (surface carbidic species), derived from CO dissociation, as the true restructuring agent. Under FTS conditions, this Co surface reconstruction occurs similarly irrespective of the metal particle size. Moreover, an enhancement in the proportion of Co–SiO2 interfacial Coδ+ sites (Co–CO band at 2060 cm−1) takes place particularly in small cobalt nanoparticles (5.6 nm) likely as a consequence of nanoparticle flattening, as suggested by TEM after catalysis. These Co–SiO2 interfacial sites are tentatively proposed as responsible for the decreased TOF observed for d(Co0) < 10 nm.