Formation of carboxy species at CO/Al2O3 interfaces. Impacts of surface hydroxylation, potassium alkalization and hydrogenation as assessed by in situ FTIR spectroscopy

Abstract

The transition alumina (γ + δ)-Al₂O₃ was modified by intensive dehydroxylation, potassium-alkalization and hydrogenation, and examined for possible impacts of modification on surface (i.e. accessibility, chemical composition, functional groups and acid sites) and bulk properties by N₂ sorptiometry, X-ray diffractometry, X-ray photoelectron spectroscopy, and infrared spectroscopy of adsorbed pyridine. Impacts on CO adsorption and resulting carboxy species on the surface and in the gas phase at 298–773 K, were subsequently examined by in-situ infrared spectroscopy of adsorbed CO at 298 K. Results communicated spectroscopic evidence for formation of various hydrogenocarbonate, formate, carbonate, carbonyl and end-on coordinated CO₂ surface species, as well as CO₂ gas phase species. Accordingly, it is concluded that: (i) surface hydroxylation of alumina (particularly the availability of basic OH-groups) enhances formation and thermal stability of hydrogenocarbonate and formate species, but hampers formation of carbonate species; (ii) surface alkalization with potassium ion additives disfavors appreciably formation of hydrogenocarbonate species, however enhances markedly formation of carbonate and formate species; and (iii) co-existence of H₂ in the CO adsorptive atmosphere promotes both formation and thermal stability of surface formate species. Surface reaction pathways are proposed for the formation of the various surface and gas phase carboxy species at CO/Al₂O₃.