CO2 activation for methanol synthesis on copper and indium oxide surfaces
Licentiate thesis, 2019
Catalytic recycling of CO2 to added-value chemicals, such as methanol (CH3OH), has been proposed as a possible way for sustainable production of fuel and chemicals, in addition to providing a route to mitigate climate change. Multiple systems are known to be active for the conversion of CO2 to methanol, and the state of the art catalyst is Cu/ZnO/Al2O3. This catalyst is, however, known to deactivate rapidly. Moreover, there is no scientific consensus on either the active phase or the reaction mechanism. In response to this, the search for a longer-lasting catalysts for methanol-synthesis has been intense. In recent years, an In2O3/ZrO2 catalyst has attracted much attention, thanks to its high selectivity, activity and durability.
In this thesis, we investigate the surface active phase and its effect on CO2 adsorption on Cu(100) and In2O3(110) with the use of density functional theory (DFT) calculations and ab-initio thermodynamics. Our results are compared to ambient pressure X-ray photoelectron emission spectroscopy (XPS) experiments. CO2 adsorption is the initial step in the reduction process. Hence, understanding of the active catalyst phase, and its effect on the adsorption process, is the first step for the rationalization of the catalytic processes on these systems. Simultaneously, understanding the electronic structure that allows for the high activity, might aid the rational design of better catalysts for CO2 activation.
Our results show that Cu(100) oxidizes from the pristine surface to a p(2×2) overlayer at 0.25 ML followed by a reconstruction to a (2√2×√2)R45 (MR) structure at 0.50 ML. Moreover, dissociative adsorption of CO2 on Cu(100) occurs predominantly at surface steps. In2O3(110) is found to heavily hydroxylate in presence of H2 and/or H2O. Hydroxylation with H2 causes the undercoordinated In-sites to change oxidation state (from In 3+ to In 2+), while H2O does not. We suggest that the redox capacity of the undercoordinated In-site are responsible for the adsorption of CO2 on indium oxide, whereas oxygen vacancies act as spectators. Our results are in qualitative agreement with the experimental observation of heavy hydroxylation and the suppression of the reverse water gas shift on indium oxide
Density functional theory