Research On Catalysts For Hydrogen Production By Liquid-phase Reforming Of Polyols

Concerns about the depletion of fossil fuel reserves and the pollution caused by continuously increasing energy demands make hydrogen an attractive alternative energy source. Hydrogen is currently derived from nonrenewable natural gas and petroleum, but could in principle be generated from renewable resources such as biomass or water. However, efficient hydrogen production from water remains difficult and technologies for generating hydrogen from biomass, such as enzymatic decomposition of sugars, steam-reforming of bio-oils and gasification, suffer from low hydrogen production rates and/or complex processing requirements.Aqueous phase reforming (APR) is a promising novel route for the catalytic production of high-purity H2 for fuel cells and light alkanes. The process is simple and H2 could be obtained by low-temperature process in a single step, which is more convenient than multi-reactor steam reforming. The good catalyst for APR process has to be active in the cleavage of C-C bonds and water-gas shift reaction, but has to inhibit the cleavage of C-0 bond and methanation reaction. The effective catalysts for reforming of oxygenated hydrocarbons would be based on group VIIIA metals. We employed deposition-precipitation, coprecipitation and evaporation induced self-assembly (EISA) methods to prepare the catalysts which were applied to the APR of ethylene glycol or glycerol to investigate the relation between their catalytic and structural properties.1. Aqueous phase reforming of ethylene glycol to H2 over NiCo/CeO2 catalystsThe rod CeO2 was prepared by the hydrothermal method. Aqueous phase reforming of ethylene glycol for hydrogen production was investigated over Ni-Co/CeO2 catalysts prepared by the deposition-precipitation method. The result showed that the conversion of ethylene glycol to gas products on the Ni/CeO2 was superior to that of Ni/γ-Al2O3 and Raney Ni reported by literature under the same reaction condition. Different the molar ratio of Ni/Co could have great influence on the conversion of ethylene glycol to gas products. With the molar ratio of Ni/Co changed from 1:0,2:1,1:1 to 1:2, the conversion of ethylene glycol to gas products were increased in the beginning, and then became lower. The Ni2Co1/CeO2 at Ni:Co molar ratio of 2:1 had the highest the conversion of ethylene glycol to gas products of any catalyst tested, and the H2 selectivity of-74% for Ni2Co1/CeO2 was obtained, which were higher than that of N1/γ-Al2O3 and Raney Ni catalyst. The addition of Sn to Raney Ni increased the H2 selectivity but drastically suppressed the conversion. The excellent conversion to gas was attributed to possessing the higher active surface area on the Ni2Co1/CeO2 catalyst. As for selectivity to H2, because both Co and CeO2 could promote WGS, the Ni2Co1/CeO2 catalyst showed the excellent selectivity to H2. Reaction kinetic studies indicated that the Ni2Co1/CeO2 catalyst displayed the superior catalytic activity to any Ni-based catalysts. The value of TOFH2 (3.1 min-1) was 1.15 and 2.2 times higher than Ni/CeO2 and Raney Ni-Sn catalysts, respectively. Bimetallic Ni2Co1/CeO2 catalyst had lower heats of H2 and CO adsorption than pure Ni, causing a decrease in the surface coverage of adsorbed hydrogen and CO, allowing more sites to be accessible to ethylene glycol under APR reaction conditions, which resulted in the higher activity of Ni2Co1/CeO2 catalyst. In addition, the Ni2Co1/CeO2 catalyst had reached stability after 60 h on stream and remained～70% of initial activity along with 63% of selectivity to H2, The stability of Ni2Co1/CeO2 catalyst was superior to Raney Ni catalyst and Ni supported on Al2O3, SiO2, and ZrO2. Both CeO2 and Ni-Co alloy in the Ni2Co1/CeO2 could retard the oxidation and growth of active metal, thus leading to the better stability of the Ni2Co1/CeO2 catalyst in APR of ethylene glycol.2. Aqueous phase reforming of ethylene glycol to H2 on Pd/Fe3O4 catalyst prepared by co-precipitationA high-performance Pd/Fe3O4 catalyst for aqueous-phase reforming (APR) of ethylene glycol (EG) to H2 was prepared facilely by the co-precipitation method. After proper activation, the Pd was present as highly dispersed metallic nanoparticles with dimension of< 3 nm, and the Fe was present as magnetite. As compared to Pd catalyst supported on Fe2O3, NiO, Cr2O3, Al2O3 or ZrO2 prepared by incipient wetness impregnation, the Pd/Fe3O4 catalyst displayed superior catalytic performance in terms of activity, selectivity, and stability. The conversion of EG to gas products on the Pd/Fe3O4 catalyst was 3.2 times of that on the Pd/Fe2O3 catalyst under the same reaction conditions. In addition, the Pd/Fe3O4 catalyst retained～80% of its initial activity after reaching the steady state. Notably, the Pd/Fe3O4 catalyst possessed the highest turnover frequency of H2 (109 min-1) reported so far, showing its promise as a new practical catalyst for APR of biomass-derived oxygenates to H2. The excellent catalytic performance of the Pd/Fe3O4 catalyst was attributed to the enhanced synergistic effect between small Pd nanoparticles and magnetite in promoting the water-gas shift reaction, the rate-determining step in APR of EG over Pd-based catalysts.3. Aqueous phase reforming of ethylene glycol to H2 over Pd/FeMnO4-10 catalyst with wormlike structure and low Pd contentDifferent element M (M=Mn, Cr, Zn, Ni, Ce) at the molar ratio of Fe/M=10 was incorporated into 1 wt% Pd/Fe3O4 catalyst by co-precipitation method. The conversion of EG to gas products and selectivity to H2 on the Pd/FeMnO4-10 catalyst was far beyond that on the other catalysts under the same reaction condition. Based on Mn as the modified agent, the catalysts with different molar ratio of Fe/Mn (40/1, 10/1,1/1) were prepared by the co-precipitation method. The results showed that the different molar ratio of Fe/Mn could have great influence on the conversion of EG to gas products. The conversion of EG to gas products at the molar ratio of Fe/Mn= 10/1 reached maximum value (96.0%), which 2.5 times higher than the 1 wt% Pd/Fe3O4 catalyst. The characterization results showed that the active components in the Pd/FeMnO4-10 catalyst were well distributed on the catalyst support, thus allowing more sites to be accessible to ethylene glycol under APR reaction conditions. According to kinetic results, the highest values of TOFH2 for the Pd/FeMnO4-10 catalyst was 1.8 times higher than the 1 wt% Pd/Fe3O4 catalyst. The higher values of TOFH2 for the Pd/FeMnO-10 was caused by the synergistic combination of Pd and Fe-Mn binary oxide. In addition, the Pd/FeMnO4-10 could remain-80% of initial activity after reaching the steady state over 40 h on stream, whereas the 1 wt% Pd/Fe3O4 lost nearly 45% of its catalytic activity after 30 h. The surface Fe2MnO4in the Pd/FeMnO4-10 could make the structure of Pd/FeMnO4-10 catalyst stable in the process of steam, thus leading to the better stability of the Pd/FeMnO4-10 catalyst in APR of ethylene glycol.4. One-step synthesis of mesoporous M-Pt-Al2O3 catalyst for aqueous phase reforming of glycerol to H2 The highly ordered mesoporous M-Pt-Al2O3 catalyst was successfully synthesized by EISA method based on small-angle XRD, TEM and BET results. Compared with the Pt/y-Al2O3 and Pt/M-Al2O3 catalysts prepared by incipient wetness impregnation, the M-Pt-Al2O3 catalyst showed the excellent catalytic performance in terms of the conversion of glycerol to gas products and selectivity to H2 under the same reaction condition, which was attributed to highly dispersed Pt nanoparticles on the support, thus allowing more sites to be accessible to glycerol. Based on kinetics studies, the value of TOFH2 for M-Pt-Al2O3 catalyst was 1.33 and 1.9 times higher than Pt/y-Al2O3 and reported Pt/y-Al2O3 catalysts, respectively. The reasons we attributed to two factors. Firstly, the strongly cooperative effect between small Pt nanoparticles and support with mesoporous structure in the M-Pt-Al2O3 catalyst was expected; Secondly, the most abundant surface group in the M-Pt-Al2O3 was hydroxy which could active adsorbed H2O, thus promoting WGS reaction of Pt. According to the stability test, the M-Pt-Al2O3 catalyst retained～81.5% of its initial activity and-85% of selectivity to H2 after 106 h on stream. However, the reported Pt/γ-Al2O3 catalyst only remained-54% of its initial activity after 35 h on stream. The M-Pt-Al2O3 catalyst exhibited the better stability than the reported Pt/γ-Al2O3 catalyst. Based on characterizations of M-Pt-Al2O3 after reaction, we tentatively attributed the 18%-deactivation of the M-Pt-Al2O3 catalyst after 106 h on stream to the structural collapse of support and some weakly bound Pt nanoparticles with support during APR of glycerol.