CO adsorption and reaction on catalytic surfaces: examples of reaction intermediates, spectator species and alloy segregation triggered by CO dissociation

+ING+FRM:YSC:NOG:LCSInternational audienceThe nature of the surface intermediates involved in CO hydrogenation over cobalt-based catalysts is still elusive. The methanol traces observed were shown to be related to formate species, which could not yet account for hydrocarbon formation. The effect of tin on cobalt-based catalysts used for CO hydrogenation was studied by operando diffuse reflectance FT-IR spectroscopy (DRIFTS) and isotopic transient techniques to unravel the precursors of hydrocarbons. Tin was used as a modifier to obtain a poisoning effect that could help discriminating active sites. Tin was selected because this metal does not adsorb CO, hence, the carbonyl bands observed can be wholly attributed to surface cobalt atoms.These combined operando DRIFTS and transient data indicate that the hollow-CO can be considered as active reaction intermediates leading to hydrocarbons under these conditions. Other experiments have shown that linear CO were mere spectator species.The second part of this communication describes a yet unreported mode of alloy segregation upon exposure to CO. In situ IR studies indicated that the alloy Pt-Sn nanoparticles were not stable under (i) H2-free CO at any temperature or (ii) CO/H2 mixtures at temperatures below 175°C. This was rationalized by the known, albeit slow, ability of Pt to dissociate CO. The oxygen atoms formed readily reacted with metallic Sn to form a SnOx species, leading to Pt-Sn segregation, alongside carbon deposition. In situ XPS and XANES analyses confirmed Sn reoxidation. An activation energy of 36 kJ/mol was estimated for the oxidation step of Sn surface atoms by adsorbed CO. These data highlight an important phenomenon affecting metal and alloy stability under CO, which is of major importance in heterogeneous catalysis and electrochemistry