Heats of adsorption of linearly adsorbed CO species on Co2+ and Co° sites of reduced Co/Al2O3 catalysts in relationship with the CO/H2 reaction

Abstract

IR spectra in transmission mode are used to measure the heats of adsorption at different coverages θ:E(θ), of two linearly adsorbed CO species formed on Co2+ and Co° sites (noted LCo2+ and LCo°) of reduced x% Co/Al2O3 (wt%, x≤10) catalysts according to the adsorption equilibrium infrared spectroscopy procedure developed previously. For LCo2+ species characterized by an IR band at 2151cm−1, ELCo2+(θ) varies linearly with θ from ELCo2+(1)=45 kJ/mol to ELCo2+(0)=52 kJ/mol. These values are modified by the presence of neither Co° particles nor carbon deposition (from CO disproportionation reaction) nor H2 using CO/H2 gas mixtures. During the CO adsorption at high temperatures (i.e. 538K), C deposition on the surface and in the bulk of the cobalt particles modifies the Co° adsorption sites. This leads to a transformation LCo° (IR band at 2020cm−1)→LCo°C (IR band at 2060cm−1) where LCo°C denotes a linear CO species formed on Co° sites modified by the C deposition. The heat of adsorption of the LCo°C species varies linearly with its coverage from ELCo°C(1)=93 kJ/mol to ELCo°C(0)=165 kJ/mol. In the presence of H2 with a ratio H2/CO=10, the C deposition is strongly decreased and the LCo° species dominates the surface of the cobalt particles. However, in these conditions its hydrogenation into CH4 disturbs its adsorption equilibrium in a large coverage range and only ELCo°(1)=108 kJ/mol has been determined. For a ratio H2/CO=3, the carbon deposition cannot be prevented leading to the formation of the LCo°C species. It is shown that the ELCo°C(θ) values are not modified by the presence of adsorbed hydrogen. The heats of adsorption of the different adsorbed CO species on Co° sites are consistent with some literature data on DFT calculations and experimental values obtained on model cobalt surfaces (i.e. single crystals). In particular, they confirm DFT calculations which indicate that the C deposition on reduced cobalt particles decreases slightly the heats of adsorption of the linear CO species adsorbed on Co° sites.