Fundamentals of CO2 adsorption on model copper oxide surfaces

Copper oxides can catalyze the conversion of CO$_{2}$ selectively into methanol. We are studying fundamental aspects of the adsorption of CO$_{2}$ on model copper oxide surfaces using low temperature (5 K) scanning tunneling microscopy (STM) and spectroscopy (STS). We prepared an oxygen adlayer by exposing Cu(100) to O$_{2}$ at 575K, which results in a (2$\sqrt{2}$x$\sqrt{2}$)R45$^{\circ}$ missing row reconstruction. STM images of the pristine surface show a ladder-like contrast with distinct domains. STS reveals a series of image potential states consistent with an increase in work function and two additional unoccupied states. We introduce CO$_{2}$ to the surface in situ, which allows us to unambiguously identify individual CO$_{2}$ molecules and their adsorption sites. We find that the CO$_{2}$ molecules sit in between the O atoms in the missing row reconstruction. The CO$_{2}$ molecules are easily perturbed by the STM tip under typical imaging conditions, suggesting that the molecules are weakly bound to the surface. STS is used to probe molecular orbital states of CO$_{2}$ molecules. Density functional theory calculations of adsorption sites, vibrational modes, and diffusion barriers are in qualitative agreement with the experiment.