Oxide-supported Rh catalysts for H2 generation from low-temperature ethanol steam reforming: effects of support, Rh precursor and Rh loading on catalytic performance

A variety of oxide-supported Rh catalysts prepared from frequently used oxides (γ-Al2O3, MgO, SiO2, CeO2, ZrO2, La2O3, Y2O3, TiO2 and ZnO) and Rh compounds (Rh(NO3)3·xH2O, RhCl3·xH2O, Rh(acac)3 and Rh4(CO)12) for H2 generation from ethanol steam reforming (ESR) at low temperatures (250–400 °C) have been studied. Catalytic screening shows that Rh/CeO2 is advantageous over other frequently used oxide-supported Rh catalysts. Rh/CeO2 behaves as the most effective catalyst for the water gas shift (WGS) pathway during ESR. A combination of CeO2 and Rh4(CO)12 with a 1% Rh loading results in an optimal-performance catalyst that brings about a CO-free H2 yield of 4 mol mol−1 C2H5OH at 350 °C with good catalytic stability. A comparison of the thermodynamic and catalytic data of ESR indicates that low-temperature ESR is strongly kinetically controlled over Rh/CeO2 in favour of H2 production via acetaldehyde steam reforming, acetone steam reforming, methane decomposition, steam reforming of adsorbed CHx (x = 1–3) and WGS pathways. Combined studies by catalytic stability, thermogravimetric analysis, X-ray photoelectron spectroscopy, transmission electron microscopy and X-ray diffraction suggest that the catalytic stability of Rh/CeO2 is markedly affected by coking which is the only cause of catalyst deactivation during low-temperature ESR and that both CeO2-supported Rh0 and Rh+ are the active sites for ESR to produce H2. The effects of support, Rh precursor, Rh loading and calcination of the precatalyst on the catalytic performance are discussed. In addition, catalytic roles played by typical oxides in reaction pathways during ESR are elucidated.