Catalytic oxidation of CO on metals involving an ionic process in the presence of H2O: the role of promoting materials

Abstract

A new catalytic oxidation of CO involving an ionic process in the presence of H₂O is proposed on a Pt-catalyst with specific promoting materials (co-catalysts). Oxidation of CO is very slow at room temperature on ordinary Pt-catalysts such as Pt/SiO₂, Pt/Al₂O₃, Pt/TiO₂, Pt/Graphite, and Pt/carbon nano-tube (CNT), and H₂ or H₂O have no effect on the reaction. However, in the presence of specific co-catalysts, the oxidation of CO is markedly enhanced by H₂ or H₂O, so that highly selective preferential oxidation (PROX) of CO in H₂ is attained. The role that co-catalysts play in the oxidation of CO enhanced by H₂ or H₂O was clarified by the experiments with Pt supported on CNT and carbon nano-fiber (CNF) that had Ni–MgO and FeO_x at their one terminal end, respectively. Oxidation of CO was markedly enhanced by H₂ on the Pt/CNT and Pt/CNF, but no enhancement was observed on the Pt/CNT-p and Pt/CNF-p, where the CNT-p and CNF-p were purified by removing Ni–MgO and FeO_x. Similar enhancement of the oxidation of CO by H₂ or H₂O was observed on FeO_x/Pt/TiO₂ and FeO_x/Au/TiO₂, although no enhancement was observed on the Pt/TiO₂ and Au/TiO₂ catalysts. The in situ DRIFT spectra of the FeO_x/Pt/TiO₂ catalyst (Fe : TiO₂ : Pt = ca. 100 : 100 : 1) during reaction in a flow of (CO + O₂ + H₂) suggested the rate-determining slow step was HCOO(a) + OH(a) → CO₂ + H₂O. The oxidation of CO enhanced by H₂O/D₂O and H₂/D₂ showed a common hydrogen isotope effect of r_H/r_D = 1.4–1.5. The highly selective oxidation of CO in H₂ on the Pt/CNT and Pt/CNF catalysts strongly suggests efficient transport of ionic intermediates from Ni–MgO or FeO_x to Pt over the hydrophobic CNT and CNF surface according to the local potential gradient. According to this mechanism, selectivity in the preferential oxidation of CO in H₂ is defined by the turnover number of a H₂O molecule forming CO₂ during its residence time on the catalyst, which is essentially different from the selectivity based on competitive adsorption and/or reaction. The role of the H₂O molecule is as a kind of messenger molecule or a molecular catalyst to promote the reaction on the surface expressed by the equation n(CO + 1/2O₂) + H₂O → nCO₂ + H₂O. In this mechanism, the selectivity is given by n/(n + 1). Curious phenomena previously observed in the PROX reaction of CO in H₂ on various catalysts are well explained by the mechanism including an ionic process proposed in this paper.