Novel Pretreatment To Enhance The Performance Of Ni-Co Bimetallic Catalyst For Biogas Reforming To Hydrogen

The increasing negative effects of conventional energy sources and the limited stock of renewable energy have forced many countries to look into substitute energy sources. Biogas is mostly generated from anaerobic degradation of biomass and is a renewable and source-extensive fuel. Biogas reforming to hydrogen can decrease the emission of greenhouse gases and provide a clean and renewable source for hydrogen production.In our previous research, a novel catalyst, NiCo/La2O3-γ-Al2O3 powder, was developed, which possessed high catalytic activity of biogas reforming. Then the catalyst was preliminary enlarged by using alumina pellets as its carrier and still shows excellent activity and carbon deposition resistance. Based on this catalyst, a scale-up study was performed on the Ni-Co bimetallic catalyst in this paper. However, amagnification effect emerged as the preparation scale was magnified. To address this issue, a number of pretreatment routes were designed derived from our analysis to the cause of the amplification effect and literature survey results. Successfully, one of the pretreatment routes was proven to be effective to eliminate the amplification effect and the mechanistic study was investigated afterwards. The main research achievements are listed as follows:1. The effect of preparation scale on constructure and catalytic performance for biogas reforming to hydrogen was investigated. Preparation scale of the catalyst was enlarged from 5 g to 200 g by excessive dipping. As the preparation scale becomes larger, the activity of the catalysts apparently decreases at the beginning of the reaction and has obvious inducement time. Although the activity will rise back as the reaction progress, but the final activity is still reduced by 5%. Characterization results show that the active metal Ni, Co loadings on the surface of the catalyst decreased as the preparation scale became larger. This is probably the main reason that leads to the overall decline of activity of the catalyst. On the other hand, the Additives La2O3 loading increased as the preparation scale became larger. The increasing of La2O3 is likely to lead to uneven distribution of active metal on the surface of the catalyst, which will aggravate carbon deposition at the beginning of reaction.The carbon deposition will further cover the catalyst active sites and bring about the abnorm low initial activity of catalyst at the beginning of the reaction.2. Effects of pretreatment routes on the biogas reforming performances of catalysts with larger preparation scale were investigated. HCD (pretreated by hydrogen and carbon dioxide) pretreatment was proved to have excellent ability to not only improve the activity and eliminate the long induction period, but also enhance the resistance to both sintering and coke formation during the reaction compared with the catalyst pretreated only by H2. This new pretreatment route is very promising for enhancing the performance of biogas reforming catalysts.3. Effect of pretreatment operating conditions, such as pretreatment time, temperature, gas feeding ratio and gas flow rate, on the catalytic performances of Ni-Co bimetallic catalyst was investigated. The optimal pretreatment time, temperature, gas feeding ratio (CH4/CO2) and gas flow rate should be 0.5 to 1 h,780-800℃,0:10 and 175-200 mL min-1, respectively.4. In a 511 h stability test, the catalyst pretreated with both H2 and CO2 exhibited excellent stability, the average conversion of CH4 and CO2, selectivity for H2 and CO, and ratio of H2/CO were 96%,97%,98%,99%, and 0.98, respectively.5. The average carbon deposition rate of the samples after 27,120 and 511 h of reaction is 0.56,0.36 and 0.20 mg g-1 h-1, respectively. This indicates that the amount of deposited carbon on the catalyst, which is responsible for catalyst deactivation, gradually decreases during the reaction and will reach a dynamic equilibrium at last, which is beneficial to maintain the remarkable stability of the reaction.6. A mechanistic study was investigated by a series of catalytic activity measurements. Ni was found to be the main metal that interacted with CO2. Meanwhile, H2 reduction is an indispensible step of the pretreatment route and CO2 cannot be replaced by O2. FT-IR spectroscopy investigations were utilized to confirm that carboxyl was created during the CO2 pretreatment of the Ni-Al2O3 catalyst. The result reveals that bicarbonate of Ni was formed during the CO2 pretreatment. During CO2-reforming of methane, the bicarbonate of Ni could decompose into CO and provide oxygen species. The oxygen species will react with accumulated carbon on Ni crystallites to produce CO and in turn protect the active sites. Thus, the performance of the catalyst was significantly improved.