Modulation Of Support Microstructure For Ru-Based Catalysts And Their Catalytic Performance Toward CO₂ Methanation

In recent decades, global warming problem from green house gases (mainly CO2) produced by the burning of fossil has attracted great attention, and how to achieve the carbon cycling has become perhaps the most difficult issue. Currently, CO2 methanation is considered to be the most practical and effective path in CO2 recycling technology. Particularly, reducible metal oxides (TiO2 and CeO2, for example) as catalyst carrier are widely used in CO2 conversion reaction. Although the active components are well studied, the influence of the metal-support interaction, catalytic mechanism, and the modulation of support microstructure (exposed facets and defects) on the catalytic performance is very limited and controversy. By modulating the preferably exposed support facets and the concentrations of oxygen vacancy defect, this thesis uncovers the internal relationships between them and the catalytic activities of methanation catalysts. Using multi-scale research methods (e.g., series of in situ techniques and DFT calculation method), the reaction kinetics models were established to determine the methanation reaction path, the important intermediate species and reaction rate-determining step. The catalytic mechanism of CO2 methanation on oxygen vacancy was quantitatively described at the molecular scale level. The identification of intrinsic active site and corresponding reaction mechanism will be an essential pathway for the design and preparation of appropriate heterogeneous catalysts for CO2 conversion.The main research contents and results are as follows:1. Influence of exposed support facet in Ru/TiO2 on its catalytic performance toward CO2 methanationIn order to explore the metal-support interaction and its influence on the catalytic performance, we prepared two kinds of anatase TiO2 with preferably exposed (001) and (101) facet, respectively. By loading Ru metal clusters on these two TiO2 supports with precipitation-deposition method followed by a reduction process, the influence of preferably exposed facet on the metal-support interaction and the resulting catalytic performance toward CO2 methanation were studied. Compared to Ru/TiO2(001) catalyst, the Ru/TiO2(101) catalyst shows much higher stability and catalytic activity (255 ℃, CO2 conversion 98.0% vs.350 ℃, CO2 conversion 91.1%). Based on XPS, TPR results and DFT calculations, the Ru/TiO2(101) catalyst is proved to have much stronger metal-support electronic interaction, which decreases the energy barrier in the rate-determining step and improves the catalytic performance. This thesis provides a detailed cognition of metal-support interaction originating from preferably exposed crystal facets of substrates, which can be applied in fabrication of heterogeneous catalysts with high performance.2. Modulation of the oxygen vacancies in Ru/CeO2 catalyst and its catalytic performance toward CO2 methanation.In order to explore the catalytic roles of support microstructures in CO2 conversion reaction, Three kinds of CeO2 supports which preferably expose (100), (110) and (111) facet were prepared, respectively, followed by loading of Ru nanoparticles by the precipitation-deposition method. The TPR, Raman spectra and OSC prove that the Ru(3%)/CeO2-NCs catalyst have the highest oxygen vacancies concentration which is the result of the Ru-promoted formation of oxygen vacancies on CeO2. Moreover, in situ infrared spectroscopy shows that the oxygen vacancies facilitate the CO2 activation, accounting for its enhanced low-temperature activity. Therefore, this thesis provides an method for the preparation of heterogeneous catalysts by modulating the support facets and related defects of CeO2 support, which can be potentially used in carbon recycle.3. Intrinsic activity of oxygen vacancy in Ru/CeO2 catalyst and its catalytic mechanismWe report an active site-dependent catalytic mechanism by using Ru/CeO2 and Ru/a-Al2O3 catalyst toward CO2 methanation. Operando IR, NEXAFS and Raman prove the generation process of surface hydroxyl, Ce3+, and oxygen vacancy in Ru/CeO2 catalyst, and their structural evolvements under real reaction conditions are well revealed. The steady-state isotope transient kinetic analysis (SSITKA)-type in situ DRIFT infrared spectroscopy confirms that all these three catalytic structures participate in the catalytic process in the formate route on Ru/CeO2 catalyst; and the formate dissociation to methanol (rate-determining step) is well catalyzed by oxygen vacancy. On the contrary, metal Ru acts as the active site for the CO route. Notebly, the catalytic activity evaluation and the oscillating reaction further shows that the oxygen vacancy catalyzes the rate-determining step at a lower activation temperature compared with metal Ru in Ru/a-Al2O3 system (125 ℃ vs.250 ℃). This work provides an effective strategy to reveal the intrinsic structure-activity correlation for the heterogeneous catalysts.