CO hydrogenation on cobalt‐based Fisher‐Tropsch catalysts: chlorine poisoning reveals the nature of the most active sites
Résumé
The effect of chlorine, introduced as trichloroethylene (TCE), on the CO hydrogenation activity of an alumina-supported cobalt was studied by operando diffuse reflectance FT-IR spectroscopy (DRIFTS) at atmospheric pressure. The surface of metallic cobalt was covered with CO(ads) under reaction conditions. The corresponding IR signal could be exactly decomposed into three bands, whether chlorine was present or not (data not shown). Chlorine induced a strong and partly reversible poisoning.
A major effect of chlorine on the electronic structure of the cobalt particles was evidenced here, in addition to the previously reported site blocking. The carbonyl band position remained unchanged, while its intensity decreased significantly in the presence of TCE. This is in contrast to typical surface coverage effects. This difference was related to the electronic density removed by chlorine from the metallic cobalt that limited the weakening of the C-O bonds, which occured through electronic back-donation from cobalt to the CO π* anti-bonding orbital.
The CO(ads) exhibiting a wavenumber at ca. 1911 cm-1 were most affected by chlorine, suggesting that those were associated with the most active sites. The 1911 cm-1 species only led to a minor dipole-dipole coupling and were thus most likely forming a mono-dimensional, or even punctual, network of CO(ads). These species were likely bridged CO adsorbed on step sites, where chlorine could preferentially adsorb. The corresponding C-O bond strength increased by about 7 kJ/mol in the presence of chlorine.
Assuming that the rate-determining step of CO hydrogenation was C-O bond dissociation, these observations are consistent with chlorine main poisoning effects being both site blocking (i.e. competitive adsorption) and an electronic effect through the strengthening of the C-O bond that made CO(ads) less likely to dissociate (i.e. modification of the activation energy barrier).