Structure-dependent activity and stability of Al_2O_3 supported Rh catalysts for the steam reforming of n-dodecane

摘要

Steam reforming of liquid hydrocarbon fuels is an appealing way for the production of hydrogen. In this work, the Rh/Al_2O_3 catalysts with nanorod (NR), nanofiber (NF) and sponge-shaped (SP) alumina supports were successfully designed for the steam reforming of n-dodecane as a surrogate compound for diesel/jet fuels. The catalysts before and after reaction were well characterized by using ICP, XRD, N_2 adsorption, TEM, HAADF-STEM, H_2-TPR, CO chemisorption, NH_3-TPD, CO_2-TPD, XPS, Al~(27) NMR and TG. The results confirmed that the dispersion and surface structure of Rh species is quite dependent on the enclosed various morphologies. Rh/Al_2O_3-NR possesses highly dispersed, uniform and accessible Rh particles with the highest percentage of surface electron deficient Rh~0 active species, which due to the unique properties of AI_2O_3 nanorod including high crystallinity, relatively large alumina particle size, thermal stability, and large pore volume and size. As a consequent, Rh/Al_2O_3-NR catalyst exhibited superior catalytic activity towards steam reforming reactions and hydrogen production rate over other two catalysts. Especially, Rh/Al_2O_3-NR catalyst showed the highest hydrogen production rate of 87,600 mmol g~(-1)_(fuel) g~(-1)_(Rh)min~(-1) among any Rh-based catalysts and other noble metal-based catalysts to date. After long-term reaction, a significant deactivation occurred on Rh/Al_2O_3-NF and Rh/Al_2O_3-SP catalysts, due to aggregation and sintering of Rh metal particles, coke deposition and poor hydrothermal stability of nanofibrous structure. In contrast, the Rh/Al_2O_3-NR catalyst shows excellent reforming stability with negligible coke formation. No significantly sintering and aggregation of the Rh particles is observed after long-term reaction. Such great catalyst stability can be explained by the role of hydrothermal stable nanorod alumina support, which not only provides a unique environment for the stabilization of uniform and small-size Rh particles but also affords strong surface basic sites.