Combustion of carbonaceous fuels like coal, oil and natural gas cause atmospheric concentrations of CO₂ to continue to rise. CO₂ could be utilized as starting material in a catalytic process to produce valuable chemicals and fuels such as synthesis gas (CO and H₂ mixture), oxygenates (alcohols, ethers), or hydrocarbons. The development of an appropriate heterogeneous catalyst that will actively and selectively convert mixtures of CO₂ and H₂ to methanol as a liquid fuel additive or surrogate, can significantly contribute to a more widespread large scale utilization of CO₂ and renewable energy. The most active catalyst for methanol production is copper-zinc-based materials which are widely used for syngas transformation to methanol. These catalysts also show promising results in CO₂ hydrogenation, however, their activity value is still far from commercial utilization and further catalyst improvement is still required [1-3].

The main goal of this research project was to investigate mechanism of direct selective hydrogenation of CO₂ into CH₃OH over Cu/ZnO based catalyst. During catalyst design, attention was focused on maintaining a high dispersion and intimate contact between Cu and ZnO phases for highest possible number of active sites and consequently highest activity. For this reason nanosized Cu/ZnO clasters were encapsulated into zeolite framework. CuO and ZnO phases were introduced by ion exchange and precipitation methods. Influence of precipitation agent (Na₂CO₃/Na₂S) during preparation step was carefully investigated. It was found that precipitation with 0.05 M Na₂CO₃ results in superior catalytic properties. Furthermore, several zeolite structures (FAU and LTA) with different Si/Al ratio were investigated. Synthesized catalyst samples were further examined by a variety of relevant characterization techniques in order to determine their morphological and surface properties, as well as mechanism of CO₂ hydrogenation to methanol. Accordingly to results of operando XRD investigation, encapsulation of CuO and ZnO into zeolite framework prevents nano-particles sinthering and aggregation during catalytic test. It was confirmed that reducibility of ZnO (operando XAS) and formation of Cu-Zn alloy (operando XRD) play a crucial role in investigated process. The higher the amount of reduced Zn⁰ species were presented in the catalyst during reaction conditions, the higher the overall activity and selectivity to methanol were.

[1] M. D. Porosoff, B. Yan, J. G. Chen, Energy Environ. Sci.,2016, 9, 62-73. 
[2] T. Lunkenbein, J. Schumann, M. Behrens, R. Schlögl, M. G. Willinger, Angew. Chem. Int. Ed. 2015, 54, 4544-4548. 
[3] S. Kuld, M. Thorhauge, H. Falsig, C. F. Elkjær, S. Helveg, I. Chorkendorff, J. Sehested, Science 2016, 352, 969-974.