Designing highly active and stable Ni-based catalysts for methanation of carbon dioxide

Ren, Jie; Palkovits, Regina (Thesis advisor); Liauw, Marcel (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Power-to-Gas (PtG) concept is under discussion as a technology for storing energy on a large scale as a result of the fluctuating and locally concentrated availability of renewable energy sources. Therefore, methanation of CO2 with renewable H2 (i.e., via electrolysis) is considered promising due to the fact that it can be integrated in the existing infrastructure of natural gas and electricity grids. CO2 methanation is an exothermic and thermodynamically favorable reaction requiring an effective catalyst. Ni-based catalysts are widely investigated for CO2 methanation due to their low cost, easy availability, and comparable activity during the reaction. Nevertheless, conventional Ni-based catalysts (i.e., Ni/Al2O3) are easily deactivated due to sintering and coke deposition during the exothermic methanation reaction. Hence, Ni-based catalysts with enhanced properties (e.g., special structure, Ni dispersion, oxygen vacancy, reduction degree) need to be deeply investigated for providing crucial knowledge for related researchers. In chapter 2, hydrotalcite-derived Mg-Al oxides with different morphologies were synthesized through co-precipitation and used for Ni-based catalyst preparation. The effect of support morphology on the Ni dispersion and catalytic activity in CO2 methanation was investigated. Obtained supports and catalysts were rigorously characterized by various techniques, determining crystallite size, Ni dispersion, morphologies, and basic sites of the materials. The activity, selectivity, and long-term stability of Ni-based hydrotalcite-derived catalysts were evaluated for CO2 methanation under different conditions (i.e., gas hourly space velocities, reaction temperatures, and reduction temperatures). Based on the results, Mg-Al hydrotalcites prepared under solution pH of 10 and aging temperature of 20 oC (MAH-10) supported 20 wt% Ni, with a "rosette-like" structure, exhibited remarkable CO2 conversion (83.5%), CH4 selectivity (99.4%), and turnover frequency (TOF) of 13.5 min−1 at 400 °C. This superior activity of Ni/MAH-10 was attributed to its high basicity, optimized pore size, and defined support structure, which resulted in a high Ni dispersion and metallic surface area after reduction. In chapter 3, novel La2-xCexNiO4 perovskite-derived catalysts were prepared by a sol-gel method, and various characterization techniques were employed to understand structure-performance relationships in CO2 methanation. Based on the characterization results of La2-xCexNiO4 catalysts, La0.5Ce1.5NiO4 with a La/Ce ratio of 0.5/1.5 and 11 wt.% Ni was found to have tailored basicity, reducibility, oxygen vacancies, better Ni dispersion, and larger Ni (111) crystal plane, which therefore exhibited the highest CO2 conversion rate of 57.4 mmolCO2/molNi/s and 99.8% CH4 selectivity at 350 °C. In agreement with the properties obtained from characterization, in-situ DRIFTS experiments confirmed CO2 methanation over La0.5Ce1.5NiO4 to proceed via CO hydrogenation. At last, the results obtained in this chapter suggest that both basicity and oxygen vacancy content contribute to the outstanding catalytic performance and stability during CO2 methanation. To know the influence of the preparation method, the activity and structure of Ni/(La, Ce)Ox and La0.5Ce1.5NiO4 were compared (in chapter 4). The results demonstrate that Ni/(La, Ce)Ox prepared by impregnation possess a smaller particle size but less Ni (111) crystal plane than La0.5Ce1.5NiO4, which therefore show low activity and stability in CO2 methanation. In chapter 5, the effects of impurities (i.e., N2, steam, and O2) on the activity of CO2 methanation over Ni/ZrO2 were carefully studied. The reducible ZrO2 supporting Ni favors the formation of oxygen vacancy, which enhance CO2 adsorption and subsequent hydrogenation into methane. Interestingly, it was found that trace O2 enhanced CO2 methanation over Ni/ZrO2 due to the generation of more *OH groups, which facilitate the conversion of the intermediates to methane. In summary, the various Ni-based catalysts were fabricated through different methods, and the activity was carefully investigated. Through the advanced characterizations, the effects of metal-support interaction, reaction conditions, preparation method, and the impurities on the CO2 methanation were elucidated. This thesis gathers important findings on Ni-based catalysts design, which are crucial to advance the CO2 methanation in PtG technology.