AVS 69 Session TH2-TuA: Electronic Structure and Reactivity

The interaction of hydrogen species with active sites in transition aluminas such as γ-Al₂O₃ is responsible for performance of these materials in important applications such as heterogeneous catalysis and hydrogen permeation barriers. Understanding these active sites and their interactions with hydrogen species has been inhibited by a lack of a clear crystal structure, let alone surface terminations, for these aluminas. Theoretical studies have had to make assumptions regarding surface terminations and the structure of active sites, leading to ambiguous spectroscopic interpretation and uncertainty in reaction mechanisms. Recent experimental investigation of hydrothermally synthesized γ-Al₂O₃ rhombus-platelets utilizing state-of-the-art FTIR, high-field solid state ²⁷Al NMR, high resolution TEM, and CO/N₂ activation probes have clarified the nature of the active sites on (100) segments of highly reconstructed (110) faces and irrational surfaces. (Khivantsev et al., Angew. Chem. Int. Ed. 2021, 60, 17522–17530.) High resolution FTIR and ab initio thermodynamics and molecular dynamics based on density-functional theory are utilized to characterize the structure and reactivity of these sites as well as surface and bulk transport as a function of temperature and partial pressures of H₂, D₂, H₂O, and D₂O.

Perovskite materials can be used in electrochemical CO₂ reduction processes due to their higher stability than metal catalysts such as Nickel but are limited by their catalytic activity towards CO₂ reduction. For perovskites to be an alternative catalyst for this process, the surface chemistry of perovskites needs to be augmented by the introduction of active sites on the surface such as oxygen vacancies or by applying electric fields. We propose using La-based perovskites (LaNiO₃, LaCoO₃ and LaFeO₃) which are known to be highly active in CO₂ reduction to reduce CO₂ on the surface and we investigate these surfaces using both experimental and theoretical methods. While this proposal specifically focuses on the theoretical nature of the project, we use experimental methods to ground our theoretical calculations. Our first study explored experimental XPS spectra with theoretical calculations that predict the core level binding energy shifts of various adspecies on the surface (H, O, OH, H₂O and CO₂). Adsorption of species on the surface was favorable with considerable charge transfer occurring between the surface and the adspecies. This study is imperative for understanding the surface chemistry of our system before we augment it with oxygen vacancies and electric fields. We found that the higher energy peak for both LaCoO₃ and LaNiO₃ corresponds to water adsorption while the lower energy peak is due to lattice oxygen. The other species correspond to intermediate satellite peaks. Figure 1 shows the XPS spectra generated for LaNiO₃. Literature results and temperature dependent XPS spectra confirmed our results were accurate. We will also investigate the effects of coverage and oxygen vacancies on the XPS spectra for several of our surfaces (LaNiO₃ and LaCoO₃). Finally, we investigate the effects of oxygen vacancies and electric fields on the surface activity and adspecies adsorption. Our preliminary studies show that oxygen vacancies can increase the adsorption of CO₂ on the surface and electric fields change the plane wave averaged potential of the surface. We expect that the influence of electric fields will strengthen the adsorption of CO₂ due to the increase electrons stabilizing the surface. Further work will be completed to explore the effects of electric fields on adsorption strength of CO₂ as well as the formation of oxygen vacancies.

Dopants in insulating and semiconducting oxides are of fundamental importance for the design of new materials and often lead to the presence of holes or trapped electrons in particular sites. The correct identification of these paramagnetic centers is crucial for understanding the optical, magnetic, photocatalytic and transport properties of oxides. The nature of magnetic impurities can be investigated by comparing DFT calculations using hybrid functionals with electron paramagnetic resonance, EPR, measurements. We will provide a historical perspective on the description of holes in the O 2p valence band of SiO₂ as a paradigmatic example of interaction between theory and experiment. Then we will discuss N-dopants in TiO₂, ZnO, SnO₂, ZrO₂ and MgO. A comparison with the EPR data allows one to evaluate the accuracy of the DFT calculations. At high N-dopant concentrations the presence of magnetic ordering in some of these materials has been suggested, implying the existence of magnetic interactions between the isolated defects. The use of hybrid functionals allows to adequately describe the nature of the isolated magnetic defects in the oxides, and shows that no magnetic ordering is expected for the dopant concentrations used in the experiments. Problems related to the theoretical treatment in DFT of magnetic impurities in insulating and semiconductor oxides are discussed.

The dependence on kinetic energy of the electron inelastic mean free path (IMFP) parameter has already been studied extensively, showing a drastic qualitative difference between the high-energy regime (above e.g. 100 eV) and the low-energy regime (below e.g. 40 eV). In fact, definitely opposite energy dependencies are exhibited by the two ‘arms’ of the corresponding λ(Ek) universal curve. Extension of these studies to very low energies, below e.g. 5 eV, is by far more challenging experimentally. Hence, only few works have addressed this challenge so far.

Here, a new technique is applied, resulting in an interesting observation. XPS-based experiments with nanometrically thin self-assembled monolayers (SAMs) systematically indicate on a second inversion in the derivative of energy dependence, appearing as a common general feature of the sub-eV regime. Using a simple theoretical model, these results are explained and further propose interesting applications for future studies of hot-electron interactions with dielectric layers.