Experimental Microkinetic Approach of the Surface Reconstruction of Cobalt Particles in Relationship with the CO/H2 Reaction on a Reduced 10% Co/Al2O3 Catalyst

The adsorption of x % CO/He ( x = 1 and 2) on a reduced 10 wt % Co/Al 2 O 3 catalyst at an adsorption temperature of 560 K > T a > 420 K leads to the formation of a linear CO species on Co° sites (denoted L Co° species) with an infrared (IR) band at ≈2030 cm –1 at full coverage. This adsorption is associated with the formation of carbon that is implicated in the reconstruction of the surface of the cobalt particles, leading to the progressive transformation of the L Co° species into a new linear CO species on Co°C sites of the reconstructed surface with an IR band at ≈2060 cm –1 . An experimental microkinetic approach of the reconstruction process via the L Co° → L Co°C transformation reveals the impact of different kinetic parameters such as the reaction temperature, the CO partial pressure, and the presence of hydrogen. During the reconstruction the coverages of the L Co° and L Co°C species remain very high (>0.95) due to their high heats of adsorption determined by the Adsorption Equilibrium Infra Red spectroscopy method. It is shown that the rate of the reconstruction process is controlled by that of the disappearance of the L Co° species via its dissociation into adsorbed elemental carbon and oxygen species according to a pseudo first kinetic order elementary step involving a small amount of free cobalt sites (≈4 × 10 12 sites/cm 2 of cobalt particles). The impact of the reaction temperature on the L Co° → L Co°C transformation indicates that (a) the activation energy of the L Co° dissociation which controls the reconstruction process is ≈125 kJ/mol and (b) the formation of elemental carbon via the disproportionation of the L Co° species seems unlikely. These two conclusions are consistent with literature data on density functional theory (DFT) calculations. For adsorption temperature >560 K, the formation of superficial graphitic like species overlaps the reconstruction process. In the presence of hydrogen, the reconstruction is observed for H 2 /CO ratios <3 but not for higher values (i.e., H 2 /CO = 10).