Design of a Highly Active Ir/Fe(OH)x Catalyst: Versatile Application of Pt‐Group Metals for the Preferential Oxidation of Carbon Monoxide

Abstract

The proton-exchange membrane fuel cell (PEMFC) has been regarded as one of the most promising candidates for the efficient use of hydrogen energy. However, small amounts of CO (0.3–1%) in the H2 stream from reforming processes must be selectively removed because CO is highly poisonous to the Pt anode of a PEMFC. The preferential oxidation of CO in a H2-rich gas (PROX) is presently the most effective approach to address this problem. Oxide-supported Au catalysts are highly active for the PROX reaction even at room temperature, but the lower stability and sensitivity to CO2 constrain their practical applications. Supported Pt catalysts, on the other hand, are less active and only a few have shown reasonable activity for conversion of CO at temperatures lower than 60 8C. Therefore, it is highly desirable to develop improved catalysts with better catalytic performance for the PROX reaction at lower temperatures. Ir has a higher melting point and surface energy than other metals with 5f orbitals, such as Pt and Au, and Ir can be well-dispersed on and strongly interact with the support. However, compared to Ptand Au-based catalysts, Ir-based catalysts have limited applications in heterogeneous catalysis and are rarely investigated for the PROX reaction, most probably because of its inferior activity. Although much effort has been made to improve the activity of Ir-based catalysts and remarkable progress has been achieved, their activities for the PROX reaction are still low at low temperatures. In fact, there is no report so far claiming that Ir-based catalysts can show high activity at temperatures below 80 8C; thus it remains a formidable challenge to utilize Ir-based catalysts for the PROX reaction at ambient temperatures. One basic task of modern catalysis is to rationally design catalysts based on the fundamental understanding of their reaction mechanisms. Especially, the contribution of support materials to the performance of the final catalysts should be taken into account. For the PROX reaction, the strong binding of CO to Ir poisons the surface so that O2 cannot competitively adsorb on the Ir surface and be activated at low temperatures, thereby prohibiting the conversion of CO to CO2. Therefore, weakening the adsorption strength of CO and/or promoting the activation of O2 at lower temperatures have become the crucial steps. Ferric oxide has proven effective for O2 activation and has been used extensively as an additive to Pt-based catalysts. Recently, we have designed a bimetallic catalyst by adding FeOx to a supported Ir catalyst, and the activity for the PROX reaction was improved. Further study of the catalytic reactions showed that the reaction rate of CO oxidation correlated well with the presence and amount of Fe, suggesting that Fe sites were indeed the active sites for O2 activation. [13] The coordinatively unsaturated Fe center was also recently identified as the site to activate O2, which helped the design of a highly active FeOx/Pt/SiO2 catalyst to totally convert CO at room temperature. All of these studies suggest that the presence of low-valent Fe (Fe) played a decisive role in improving the PROX activity, thus providing a clue for obtaining a highly effective Ir-based catalyst by incorporating materials containing, or easily forming, Fe species. Ferric hydroxide (Fe(OH)x) is a novel support material which has recently been adopted to stabilize various types of metal species for CO oxidation. It possesses a large surface area and a large amount of OH groups; these unique properties make Fe(OH)x a good candidate for generating highly dispersed metal clusters or even single-atom catalysts. Furthermore, the longer Fe O bonds in Fe(OH)x (compared to those in Fe2O3) make it easier to form Fe 2+