Comparative Investigation of Benzene Steam Reforming over Spinel Supported Rh and Ir Catalysts

Abstract

In a combined experimental and first-principles density functional theory (DFT) study, benzene steam reforming (BSR) over MgAl2O4-supported Rh and Ir catalysts was investigated. Experimentally, it has been found that both highly dispersed Rh and Ir clusters (1–2 nm) on the spinel (e.g., MgAl2O4) support are stable during the BSR in the temperature range of 700–850 °C. Compared to the Ir/MgAl2O4 catalyst, the Rh/MgAl2O4 catalyst is more active with higher benzene turnover frequency and conversion. At steam conditions with the molar steam-to-carbon ratio >12, the benzene conversion is only a weak function of the H2O concentration in the feed. This suggests that the initial benzene decomposition step, rather than the benzene adsorption, is most likely the rate-determining step in BSR over supported Rh and Ir catalysts. To understand the differences between the two catalysts, we followed with a comparative DFT study of initial benzene decomposition pathways over two representative model systems for each supported metal (Rh and Ir) catalysts. A periodic terrace (111) surface and an amorphous 50-atom metal cluster with a diameter of 1.0 nm were used to represent the two supported model catalysts under low and high dispersion conditions. Our DFT results show that the decreasing catalyst particle size enhances the benzene decomposition on supported Rh catalysts by lowering both C–C and C–H bond scission. The activation barriers of the C–C and the C–H bond scission decrease from 1.60 and 1.61 eV on the Rh(111) surface to 1.34 and 1.26 eV on the Rh50 cluster. For supported Ir catalysts, the decreasing particle size only affects the C–C scission. The activation barrier of the C–C scission of benzene decreases from 1.60 eV on the Ir(111) surface to 1.35 eV on the Ir50 cluster while the barriers of the C–H scission are practically the same. The experimentally measured higher BSR activity on the supported highly dispersed Rh catalyst can be rationalized by the thermodynamic limitation for the very first C–C bond scission of benzene on the small Ir50 catalyst. The C–C bond scission of benzene on the small Ir50 catalyst is highly endothermic although the barrier is competitive with those of both the C–C and the C–H bond-breaking on the small Rh50 catalyst. The calculations also imply that, for the supported Rh catalysts, the C–C and C–H bond scissions are competitive, independent of the Rh cluster sizes. After the initial dissociation step via either the C–C or the C–H bond scission, the C–H bond breaking seems to be more favorable rather than the C–C bond breaking on the larger Rh terrace surface.