Catalytic Active Sites for H2 Production from Bio-alcohols over Transition Metal Oxide Nanoparticles

摘要

Nowadays, over 90% H2 is still produced from steam reforming process. This process requires high temperature operation. To reduce the hydrogen product cost, lowering the reforming operation temperature is a key issue. Different processes have been used to produce hydrogen from ethanol, including partial oxidation, oxidative steam reforming and steam reforming. Steam reforming of ethanol (SRE) is famous for its high efficiency among these processes. Oxides, nickel-based, cobalt-based and noble-metal-based catalysts have been studied for steam reforming of ethanol. Nickel- based catalysts have been more extensively studied because of their high activity and low cost. It was found that Pt supported catalysts showed high selectivity to H2 since Pt is noble metal. In the aqueous reforming of ethylene glycol, the rate of reforming decreased in the order of Pt, Ni > Ru > Rh, Pd > Ir, while hydrogen selectivity showed the following trend Pd > Pt > Ni > Ru > Rh. Overall, Pt was identified as the most promising catalyst and thus it was selected as the active metal in this study. Transition metal oxide has been widely chosen for various catalytic process because the different oxidation states available in these oxides make them possible to control the selectivity in oxidation and the strong metal-support interaction (SMSI) could change the chemisorption properties of the catalysts. Shabaker et al. investigated aqueous-phase reforming of 10 wt% ethylene glycol at temperatures of 483 and 498K over Pt-black and Pt supported catalysts, and found the rate of H2 production decreased in the order of TiO2 > Al2O3, carbon, Pt-black > SiO2-Al2O3, ZrO2 > CeO2, ZnO, SiO2. CeO2 is a good catalyst support for water gas shift, and it is also a good promoter to improve the dispersion of active metal and promote the interaction between the support and the promoter. When Pt was supported on ceria, the highest ethanol conversions (+90%) and hydrogen selectivity (30%) were achieved. The addition of small amount of alkali has long been known to the significant effect (beneficial of otherwise) on the activity and the selectivity and of heterogeneous catalysts, changes in temperature range of operation or their lifetime improvements [1-2].