Preparation Of Activated Carbon Supported Nickel Catalysts And Its Application In Catalytic Steam Reforming Of Toluene

Tar is recognized as one of the most problematic parameters in biomass gasification system. Catalytic cracking could not only remove tar effectively under moderate conditions, but also increase the product gas calorific value. As a promising selection, nickel-based catalysts have high catalytic activity for tar steam reforming. Although nickel-based catalyst as a primary catalyst is easy to be inactivated due to carbon deposition, sintering and morphological change, the nickel catalyst as a secondary catalyst can decrease deactivation when tar is treated downstream of the gasifier in a secondary reactor. In order to decrease the reaction temperature of catalytic cracking of tar derived from biomass gasification and prolong the lifetime of catalyst, activated carbon is employed for support of nickel catalyst. Also, inexpensive promoters, such as copper and iron, are used to improve the activity and carbon deposition resistance of nickel catalysts.Nickel supported on activated carbon(Ni/AC) catalysts were prepared by impregnation method to decompose toluene, a model tar compound, in a fixed bed reactor. The physical and chemical properties of the catalysts were analyzed by N2 adsorption, X-ray diffraction(XRD),and Transmission electron microscope(TEM), etc.The effects of nickel content(8-15wt%), calcination temperature(600-800 oC) and reaction temperature(500-700 oC) on carbon conversion were investigated, and the catalytic performance of the Ni/AC catalyst was compared with those of Al2O3/olivine-supported nickel catalysts.The results showed that the Ni/AC catalyst with 10 wt% of nickel loading and 600 oC of calcination temperature had the best low-temperature catalytic activity at 600 oC for toluene reforming, and toluene carbon conversion achieved about 99.7%. The high performance of the Ni/AC catalyst might be accounted for the AC support, which had a large BET surface area and unique porous structure, and therefore, the fine nickel particle size distribution also could be known by XRD analysis that nickel loaded on the surface of AC was in the form of metallic states after calcination, decreasing the use of hydrogen for reduction before reaction, so the catalyst preparation process can be simplified and the cost will be saved.Ni-Cu/AC bimetallic catalysts were prepared with the molar ratio of Cu/Ni varied from 0.05 to 0.7, and tested for the steam reforming of toluene at low temperature. It is found that with increase in the Cu/Ni ratio caused a rise and then decrease in the carbon conversion of toluene, the bimetallic catalyst with a low Cu/Ni rate(0.08-0.2) exhibited much higher catalytic performance, and the carbon conversion achieved a maximum of 89.8% at Cu/Ni=0.2. According to the effect of Cu/Ni ratio of the bimetallic catalyst on toluene reforming, low Fe/Ni ratio(0.08-0.2) catalysts were prepared and tested for toluene catalytic cracking. The results showed that the Ni-Fe/AC bimetallic catalyst with the optimum composition of Fe/Ni=0.25 had a better catalytic performance for toluene steam reforming, and the carbon conversion achieved 99.1%. The characterizations with XRD and TEM confirmed the formation of Ni-Cu and Ni-Fe alloy particles and a good dispersion of active component on the activated carbon support.The duration of 600-10%Ni/AC and 0.1-Fe-Ni/AC catalysts was tested for 20 h, it found that the carbon conversion of toluene on the 600-10%Ni/AC was achieved from 82.8.0% to 99.7%, and the average carbon conversion was about 92.9%; however, on the 0.1-Fe-Ni/AC catalyst, the carbon conversion was varied in the range of 83.1% to 99.8%, and the average carbon conversion of toluene could be as high as 93.8%. The TEM analyses results revealed that the metal average size of the spent 600-10%Ni/AC was estimated to be 30.0 nm by the statistical analysis, but it was just 19.0 nm for the 0.1-Fe-Ni/AC catalyst. Therefore, the Fe promoter can be effective to prevent aggregation of nickel particles. Additively, the weight of carbon deposits for the 600-10%Ni/AC after reaction achieved about 0.68 g, but for the 0.1-Fe-Ni/AC catalyst, it was just 0.38 g. Thus, 0.1-Fe-Ni/AC bimetallic catalyst can enhance the resistance to coke formation.