461 / 2018-09-05 11:58:21

Low-temperature Steam Reforming Of Toluene And Biomass Tar Over Biochar-supported Ni Nanoparticles: Effects Of Ni Particle Size On Catalytic Activity And Stability

全体主题 > 生物质燃气

Although utilizing biochar and biochar-supported metal catalysts for the steam reforming of tar has been extensively studied, the consumption of biochar by steam gasification at high reaction temperatures has largely been neglected. As carbon materials, biochar and biochar-supported metal catalysts should be used at relatively low temperatures to alleviate their consumption and reduce operation costs. Thus, a series of biochar-supported Ni catalysts (Ni/BC) were explored for steam reforming of toluene at a low temperature of 600 °C, with an emphasis on the effects of Ni particle size on the catalytic activity and stability. Metallic Ni nanoparticles were formed upon pyrolysis treatment of impregnated biomass via carbothermal reduction. The influence of preparation parameters (Ni loading amount and pyrolysis temperature) on the Ni particle size was examined. Ni/BC pyrolyzed at 600 °C with a Ni loading of 5% (5Ni-600/BC) displayed higher specific surface areas and higher metal dispersion (i.e., lower particle size) than other catalysts. In addition, 5Ni-600/BC showed superior catalytic performance in terms of initial activity (turnover frequency values) and stability over other Ni/BC catalysts, indicating the positive role of small particles with more corner and step sites. To rationally interpret the experimental observations, density functional theory (DFT) calculations were performed to elucidate the toluene decomposition pathways. The activation barriers were lowered on the stepped Ni(111) surface compared to the flat one, which explains the better activity on smaller particles. Furthermore, two conventional metal oxide-supported Ni catalysts (Ni/ZrO2 and Ni/γ-Al2O3) were selected for comparison with 5Ni-600/BC in long-term stability tests. The stability of Ni/biochar was inferior to that of Ni/γ-Al2O3, but better than that of Ni/ZrO2. Ni/BC experienced severe metal sintering and coke deposition due to the weak interaction between the Ni particles and char support. 5Ni-600/BC was further examined for its performance in steam reforming of real biomass tar. Compared with BC alone, Ni/BC not only reduced the overall amount of tar, but also cracked some large molecules into smaller ones. Our study demonstrats that Ni/BC is a cheap and easy-to-prepare catalyst with promising prospects for toluene/tar reforming. However, the catalyst stability still needs to be improved by inhibiting metal sintering and coke deposition in future work.