New approaches to preferential CO oxidation over noble metals

Features of preferential CO oxidation (CO-PROX) over alumina-supported Pt-, Rh-, RLI-, Au-catalysts are investigated. As found in experiments (flow reactor of quartz, hydrogen-containing gas with admixtures of CO, O-2 similar to 1 vol.%, high gas flow rate), the nature of CO-PROX is found to be governed by a combination of two main decisive factors: high exothermicity of reactions (CO, H-2 oxidation), and peculiarities of interaction of reaction mixture components with metal. Due to high exothermicity, CO-PROX can be realized in a special macrokinetic mode, catalyst surface ignition (CSI), which is an external mass transfer control regime on O-2 as the key component. The reaction can be transferred into CSI mode either by heating up to the critical temperature of ignition or by feeding the reaction gas to the catalyst at temperatures above the critical value. In heating, the transition into CSI mode starts with an overheat of the catalyst downstream, after which "hot spot" of the reaction drifts upstream. Variations of temperature allow to observe both ignition and extinction of catalyst surface. Experiments performed with gas mixtures of different composition showed that in CO absence from the reaction gas, H-2 is easily oxidized over Pt and Rh at rather low temperatures close to a room one; reaction proceeds in CSI mode with the "hot spot" at the top of the catalyst bed. In CO-PROX over Pt, Rh, there is a deactivating effect of strong CO adsorption, while for Ru it is strong O-2 adsorption. As in the case of Ru-catalysts, hydrogen makes substantial positive effect on CO oxidation over Au-catalysts. Effects of temperature, O-2 content in gas, Rh content in catalysts are investigated. It is found that a decrease in residual CO content in CSI mode is favored by higher O-2 content, lower temperature and lower metal content (for example, down from 1 to 0.2 wt.%). (C) 2010 Elsevier B.V. All rights reserved.