Bimetallic Cu/Rh Catalyst for Preferential Oxidation of CO in H2: a DFT Study

Abstract

Preferential oxidation of CO in excess H2 (PROX) has been extensively explored for selective removal of CO with minimum H2 consumption to prevent CO poisoning of the Pt-based anode in a proton-exchange-membrane fuel cell (PEMFC). Unmodified platinum group metal catalysts are widely used for this reaction, yet they still show unsatisfying activity for CO oxidation at low temperatures, because their stronger adsorption of CO would poison the active site. Cu-based catalysts are alternatives, but they suffer from structural instability. Therefore, designing a more efficient catalyst for PROX is highly required. In this work, a bimetallic Cu/Rh catalyst is designed that facilitates significantly weakening the CO poisoning effect due to its comparable adsorption strengths of O2 and CO. As compared to the dissociative mechanism, CO oxidation via the OCOO-mediated associative mechanism on this catalyst is found to be more favorable, and the barriers of the steps in the catalytic cycle are modest, suggesting a high low-temperature activity for CO oxidation. Moreover, it is found that a Cu/Rh catalyst exhibits lower selectivity for H2 oxidation than that for CO oxidation. Additionally, the systematic studies of the surface segregation of Cu/Rh induced by the adsorption of species in PROX show that a Cu/Rh catalyst exhibits a good structural stability under the typical PROX conditions. These results demonstrate that the designed bimetallic Cu/Rh catalyst is promising for the PROX reaction at low temperatures.