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

摘要

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 H-2 or H2O have no effect on the reaction. However, in the presence of specific co-catalysts, the oxidation of CO is markedly enhanced by H-2 or H2O, so that highly selective preferential oxidation (PROX) of CO in H-2 is attained. The role that co-catalysts play in the oxidation of CO enhanced by H-2 or H2O was clarified by the experiments with Pt supported on CNT and carbon nano-fiber (CNF) that had Ni-MgO and FeOx at their one terminal end, respectively. Oxidation of CO was markedly enhanced by H-2 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 FeOx. Similar enhancement of the oxidation of CO by H-2 or H2O was observed on FeOx/Pt/TiO2 and FeOx/Au/TiO2, although no enhancement was observed on the Pt/TiO2 and Au/TiO2 catalysts. The in situ DRIFT spectra of the FeOx/Pt/TiO2 catalyst (Fe : TiO2 : Pt - ca. 100 : 100 : 1) during reaction in a flow of (CO + O-2 + H-2) suggested the rate-determining slow step was HCOO(a) + OH(a) - CO2 + H2O. The oxidation of CO enhanced by H2O/D2O and H-2/D-2 showed a common hydrogen isotope effect of r(H)/r(D) = 1.4-1.5. The highly selective oxidation of CO in H-2 on the Pt/CNT and Pt/CNF catalysts strongly suggests efficient transport of ionic intermediates from Ni-MgO or FeOx 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 H-2 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 + 1/2O(2)) + H2O - nCO(2) + H2O. In this mechanism, the selectivity is given by n/(n + 1). Curious phenomena previously observed in the PROX reaction of CO in H-2 on various catalysts are well explained by the mechanism including an ionic process proposed in this paper.