Dissociative Adsorption of CO2 on Copper: The Role of Steps

Abstract: CO2 chemistry has received significant interest in recent time, due to the greenhouse effects of CO2 emissions and the resulting climate change. CO2 reduction reactions, such as methanol synthesis and reverse water-gas shift, is providing a route for recycling of CO2 and thus limiting the CO2 emissions. These reactions are commonly performed over Cu-based catalysts, making the interaction of CO2 and Cu on the atomic scale of tremendous importance for a fundamental understanding and, as a consequence, the development of new and more efficient catalysts.This thesis presents results on the adsorption and dissociation of CO2 and the initial oxidation of both the low-index Cu(100) and the vicinal Cu(911) surface. Due to the inertness of CO2, techniques with the possibility to measure at elevated pressures are necessary. Hence, the main methods used in this thesis are Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Surface X-Ray Diffraction (SXRD).It was found that the oxidation of Cu(100) starts by forming a p(2×2) structure that transforms to a p(2√2×√2)R45 missing row structure as the oxygen coverage increases. The core-level shifts of the O 1s core-level is shown to be a fingerprint of the Cu coordination number of the absorbed oxygen. The results on the adsorption of CO2 on Cu(100) showed that the increase of oxygen coverage from CO2 dissociation is constant in the range of 0-0.25 ML (MonoLayers, 1 ML = 1.53×1015 cm−2). After 0.25 ML the dissociation is still constant, but with a lower dissociation rate, until the oxygen coverage saturates at 0.50 ML. Results from DFT calculations show that CO2 dissociation on terraces cannot explain the constant dissociation rate as the adsorbed oxygen drastically affects the stability of adsorbed CO2. However, steps were found to both lower the dissociation barrier and separate the products, lowering the probability for recombination. Furthermore, the active site was kept available by oxygen diffusion away from the steps, leading to a constant number of reaction sites. Thus, the inclusion of atomic steps on Cu(100) is necessary to explain the experimental findings.To validate the findings from Cu(100), the vicinal Cu(911), which has closed-packed (111) steps each 11.5 Å, was investigated. It was found that the initialoxidation of the surface proceeds by faceting into (410), (401), and (100) facets. As a consequence, the steps will reform from being closed packed to the more open (110) steps. As Cu2O starts to grow on the surface the (410) and (401) facets disappear. Instead, the (911), (311), and (100) facets are present. Well-ordered oxide is seen to grow on the (311) facets with the orientation of Cu2O(110) || Cu(311). The results on the dsorption of CO2 on Cu(911) show that although the stepped surface facilitates the adsorption, the rate of the increase of atomic oxygen is not faster compared to on Cu(100). This is most likely due to the short terraces where recombination of CO and O takes place more readily. The (110) steps are showed to be able to adsorb CO2 even with the presence of oxygen, hence showing the importance of the (110) steps for the reaction.