Structures, activities, and mechanisms under the spectroscopes: the quest for unveiling the nature of active sites for highly selective CO2 hydrogenation to methanol

Since the industrial revolution in the 1760s, the CO2 concentration in the atmosphere has been rising incessantly driving global warming closer to the point of no return. The world requires urgent actions to not only reduce CO2 emissions but also capture the CO2 for utilization to mitigate the future environmental crisis. CO2 hydrogenation to CH3OH offers an alternative to produce a feasible and economic substitute for oil. This technology also resembles the nearly 100 years old CH3OH synthesis processes from syngas containing H2, CO, and CO2. The conventional Cu/ZnO/Al2O3 catalyst has also been applied for more than 50 years, and its high performance stems from synergies between Cu and ZnO. However, the true nature of the interfacial sites is still extensively debated. Moreover, lower temperature and higher pressure are thermodynamically favorable for maximum CO2 conversion and CH3OH selectivity according to Le Châtelier’s principle and beneficial in terms of energy consumption and catalyst stability against sintering. The limitation in the catalytic performance of Cu/ZnO/Al2O3 in such conditions demands the exploration of novel catalysts.

Part I of this dissertation is dedicated to gaining a deeper understanding of Cu-ZnO synergistic structure as well as other Cu-based catalysts. In Chapter 2, we proposed a greener synthesis route for Cu/ZnO catalysts via urea hydrolysis of acetate precursors that can achieve comparable activity to commercial Cu/ZnO/Al2O3 catalysts without producing wastewater. Co-precipitated Cu-Zn hydroxycarbonate mineral-like precursors are crucial for a high inter-dispersion between CuO and ZnO after calcination and providing Cu-ZnO interfacial sites for the reaction. In Chapter 3, the effects of key process conditions, namely temperature and pressure, on CO2 hydrogenation over a commercial Cu/ZnO/Al2O3 catalyst were investigated using a space-resolved study. The gradients of reactants/products concentration and catalyst bed temperature within the catalytic reactor can reveal the significant effect of temperature on the dominant reaction pathways. CH3OH is formed through direct CO2 hydrogenation at low temperatures, while CH3OH formation is mediated via CO which is formed by a reverse water–gas shift reaction at a high temperature. Although pressure did not influence the reaction pathway, higher pressure helped suppress CH3OH decomposition to CO. In Chapter 4, the decisive roles of peripheral promoters to Cu nanoparticles in promoting CH3OH selectivity were elucidated. The model Cu-based catalysts (Cu-M/SiO2, M = Zn, Ga, and In) were prepared via surface organometallic chemistry (SOMC). The M+ sites played important roles in stabilizing formate species spillovered from Cu and determining the reactivity of formate hydrogenation. Improving the spillover and tuning the reactivity of formate help suppress formate decomposition to CO over Cu and ultimately boost CH3OH selectivity.

Part II is dedicated to exploring the novel catalysts for low-temperature CO¬2 hydrogenation, as well as, gaining a deeper understanding of the state-of-the-art Re/TiO2 catalyst. In Chapter 5, the bifunctionality of Re supported on TiO2 was deciphered, where metallic Re functions as the H2 activator and cationic Re as the CO2 activator. Re/TiO2 suffers from additional CH4 formation, and the active intermediates and reaction pathways for CH3OH and CH4 were identified. Understanding the nature of active sites and reaction mechanisms over Re/TiO2 led to approaches for CH4 selectivity mitigation in Chapter 6. Exploring various transition metals under low-temperature conditions provided insights into the formate stabilization of the coinage metals (Cu, Ag, and Au). Since the balance between metallic and cationic Re limited the CH3OH selectivity of Re/TiO2, the addition of Ag complemented the role of cationic Re. A synergistic interplay between Ag and Re did not only improve CH3OH selectivity significantly by suppressing intermediates in the reaction pathways toward CH4 but also exhibited superior stability.

Finally, the dissertation conveys a message that obtaining the definitive synthesis of well-defined active sites, expansive structure-activity relationships, and comprehensive reaction mechanisms are the major prerequisites for the rational design of novel catalysts.