First principles studies of CO2 activation and reduction over indium oxide and copper surfaces

Doktorsavhandling, 2021

Catalytic recycling of carbon dioxide (CO2) to added-value chemicals, such as methanol (CH3OH), have been proposed as a possible path for sustainable production of fuel and chemicals, in addition to providing a route to mitigate anthropogenic carbon emissions. Several catalytic systems are known to be active for conversion of CO2 to methanol, Cu/ZnO/Al2O3 being the main industrial catalyst for the process. This catalyst is, however, known to deactivate over time due to copper sintering. In recent years an alternative In2O3/ZrO2 catalyst has attracted attention, thanks to its reported high selectivity, activity and durability.

In this thesis, the activation and reduction of CO2 over Cu(100) and In2O3(110) are investigated from first principles-based calculations and simulations. Reaction intermediates and thermodynamic calculation of surface energy, coupled with theoretical X-ray photoelectron spectroscopy and mean-field microkinetic modeling, are utilized to describe and rationalize surface conditions and reaction mechanisms for the dissociative adsorption of CO2 on Cu(100) and for its reduction to CH3OH on In2O3(110).

The oxidation process of Cu(100) by dissociative CO2 adsorption is found to be controlled by step sites. The role of the step is found to be two-fold, lowering the dissociation energy and simultaneously providing physical separation of the products. Upon reaction, the surface is found to oxidize from the pristine to a disordered p(2×2) oxygen overlayer to a reconstructed (2√2×√2)R45-missing row structure.

Dissociative adsorption of H2 is investigated on In2O3(110) and In2O3(111). The adsorption is found to be facile, and both surfaces are predicted to be hydroxylated at typical methanol synthesis reaction conditions. CO2 reduction to CH3OH on the hydrogen covered In2O3(110) is investigated along a formate (HCOO) mediated mechanism, where the rate controlling step is found to be formation of H2CO+OH. The role of the competing Reverse Water Gas Shift reaction is also evaluated.

The presented findings exemplify the significance of describing catalytic systems under thermodynamically relevant reaction conditions. Additionally, the results provide some understanding and insight on the mechanistic aspects of CO2 activation and reduction to added-value chemicals.