Catalytic synthesis of methanol from CO2 is considered as one pathway for production of added-value chemicals by use of a greenhouse gas . Already today, methanol is industrially synthesized by catalytic hydrogenation of CO2 over Cu/ZnO/Al2O3. However, the copper-based catalyst has shortcomings , where one is rapid deactivation . This has been linked to the oxidation of the copper phase upon CO2 dissociation . However, a detailed understanding of the reaction at the catalysts surface is still under debate . In the present contribution, we use density functional theory (DFT) calculations and micro-kinetic modeling to investigate the dissociative adsorption of CO2 on the pristine and oxygen covered Cu(100). Surface stability with respect to oxygen coverage is calculated together with O 1s core level shifts. The results are compared to APXPS experiments. A reaction mechanism for CO2 dissociation at Cu(100) is proposed. As a structural model for monoatomic steps on Cu(100) we use Cu(611). We find that CO2 adsorption and dissociation on Cu (100) and Cu(611) occurs via CO2 adsorption in a bent CO2δ- mode over a hollow position. On the pristine surfaces, adsorption to the CO2δ- mode is associated with a barrier of 0.55 eV and 0.60 eV for Cu(100) and Cu(611), respectively. The corresponding barriers for dissociation from the adsorbed state are 0.83 eV [Cu(100)] and 0.57 eV [Cu(611)]. Only the first barrier is affected by the presence of co-adsorbed oxygen. However, the stability of adsorbed CO2 on the surface is critically dependent on the position of co-adsorbed oxygen atoms. Oxidation of Cu(100) is found to occur via the formation of a p(2x2) overlayer (0.25 ML) followed by a reconstructed (2√2x√2)R45◦ -O missing (MR) structure (0.50 ML). A c(4x6) structure with a 0.3 ML coverage is, however, close in stability for intermediate oxygen chemical potentials. We find that tensile strain (+2%) stabilizes the c(4x6) surface structure with respect to the p(2x2). The calculated shifts in the O 1s binding energy are in good agreement with the measured evolution of the binding energy. The shift to higher O 1s binding energies with increasing oxygen coverage is found to correlate with the charge on neighboring copper atoms. The work suggest a path for oxidation of Cu(100) from CO2 where CO2 adsorbs dissociatively exclusively on the step sites . Oxygen diffusion and clustering initiates surface reconstruction, which eventually leads to growth of MR-patches. The computational results are in good agreement with experimental observations of the evolution of the O 1s core level shift as a function of oxygen coverage upon CO2 exposure .References:
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