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

Abstract

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 CH_x (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.