Low temperature water–gas shift: Differences in oxidation states observed with partially reduced Pt/MnOX and Pt/CeOX catalysts yield differences in OH group reactivity

Abstract

The Pt–ceria synergy may be described as the dehydrogenation of formate formed on the surface of the partially reducible oxide (PRO), ceria, by Pt across the interface, with H2O participating in the transition state. However, due to the rising costs of rare earth oxides like ceria, replacement by a less expensive partially reducible oxide, like manganese oxide, is desirable. In this contribution, a comparison between Pt/ceria and Pt/manganese oxide catalysts possessing comparable Pt dispersions reveals that there are significant differences and certain similarities in the nature of the two Pt/PRO catalysts. With ceria, partial reduction involves reduction of the oxide surface shell, with Ce3+ at the surface and Ce4+ in the bulk. In the case of manganese oxide, partial reduction results in a mixture of Mn3+ and Mn2+, with Mn2+ located at the surface. With Pt/CeOX, a high density of defect-associated bridging OH groups react with CO to yield a high density of the formate intermediate. With Pt/MnOX, the fraction of reactive OH groups is low and much lower formate band intensities result upon CO adsorption; moreover, there is a greater fraction of OH groups that are essentially unreactive. Thus, much lower CO conversion rates are observed with Pt/MnOX during low temperature water–gas shift. As with ceria, increasing the Pt loading facilitates partial reduction of MnOX to lower temperature, indicating metal–oxide interactions should be taken into account.