Carbon dioxide methanation over hydrotalcite-based nickel catalysts

Global warming and energy shortage have driven modern society to search for alternative and sustainable energy sources to replace with the fossil fuels. Innovative solutions are required for mitigating CO2 emissions to the atmosphere and storing H2 due to the intermittency of the renewable energy sources. With this thesis, it is possible to cover both of these concerns at the same time while utilizing CO2 to produce CH4 as a synthetic fuel that could also be transported into already existing infrastructures or as a chemical compound that could store H2. Hydrotalcite-based nickel catalysts were synthesized via various techniques such as wet impregnation, co-precipitation and sol-gel with different nickel contents utilising a one-pot synthesis approach. The main approach of utilizing catalysts was with the washcoating of the newly synthesized catalysts inside the reactor tubes with a diameter of 4 mm and a length of 17.5 cm. Catalytic performance of the catalysts was investigated for the hydrogenation of carbon dioxide under atmospheric pressure in a temperature range of 250-500oC. These synthesized catalysts were compared with a Ni/Al2O3 catalyst, which was prepared as a benchmark, in terms of CO2 conversion-to-CH4. Additionally, throughout this thesis, the effect of many parameters to the performance of the catalysts had been studied, including but not limited to, nickel content, GHSV, catalyst amount, reduction temperature and the addition of Ce as well as Zr. Furthermore, the stability of the catalysts was investigated through long-term tests. The CO2 methanation was already initiated below 220oC (XCO2=10.2 %, SCO2=98.4 %) which was ~100oC lower than that over Ni/Al2O3 and reached 80% conversion of CO2 at 300oC with 99.9 % selectivity of CH4 on 15 w% Ni/Mg/Al HT by co-precipitation method. When further increment in the temperature, at 350oC a complete conversion (99.5 %) of CO2 was achieved with ~100 % of CH4 selectivity. The catalysts were also utilised as packed-beds, among those 5 w% Ni/Mg/Al HT by sol-gel showed the highest activity at 300oC, 95.0% conversion of CO2 with 99.8% CH4 selectivity. The performance of the catalysts reduced with an increase in GHSV, while enhanced catalytic activity results obtained in the presence of excess amount of H2 in the feeding gas mixture. Furthermore, additional catalyst amount which was assured by repeating washcoating cycles significantly improved the activity results. Finally, long-term tests proved a good stability of the catalysts. This superior performance of hydrotalcite-based nickel catalysts could decrease the reaction temperature of CO2 methanation to lower temperatures and replace the usage of expensive Ru-based catalysts and conventional Ni catalysts supported on various oxides