Low-temperature aqueous-phase reforming of ethanol on bimetallic PdZn catalysts

Abstract

Bimetallic PdZn catalysts supported on carbon black (CB) and carbon nanotubes (CNTs) were found to be selective for CO-free H₂ production from ethanol at low temperature (250 °C). On Pd, the H₂ yield was low (~0.3 mol H₂/mol ethanol reacted) and the CH₄/CO₂ ratio was high (~1.7). Addition of Zn to Pd formed the intermetallic PdZn_β phase (atomic ratio of Zn to Pd is 1) with increased H₂ yield (~1.9 mol H₂/mol ethanol reacted) and CH₄/CO₂ ratio of <1. The higher H₂ yield and low CH₄ formation was related to the improved dehydrogenation activity of the L1₀ PdZn_β phase. The TOF increased with particle size and the CNTs provided the most active and selective catalysts, which may be ascribed to pore-confinement effects. Furthermore, no significant changes in either the supports or the PdZn_β particles was found after aqueous-phase reforming (APR) indicating that the metal nanoparticles and the carbon support are hydrothermally stable in the aqueous phase at elevated temperatures and pressures (>200 °C, 65 bar). No CO was detected for all the catalysts performed in aqueous-phase reaction, indicating that both monometallic Pd and bimetallic PdZn catalysts have high water-gas shift activity during APR. However, the yield of H₂ is considerably lower than the theoretical value of 6 H₂ per mole ethanol which is due to the presence of oxygenated products and methane on the PdZn catalysts.