Andreas Röckert: Finding the features of water and oxygen on metal oxide surfaces: Structure, stability and spectroscopic signatures

Abstract

Metal oxides are utilized in a broad spectrum of surface-chemical processes due to the ma-terial group’s valuable surface chemistry. Interactions between molecules and metal-oxide surfaces are crucial in natural phenomena, such as cloud formation, as well as in diverse technological applications, ranging from chemical synthesis to pollutant degradation. Cerium oxide (CeO2, “ceria”) shows particular strength in both high- and low-temperature oxidative catalysis, under gas-phase and aqueous-phase conditions. Experimental probes of ceria surfaces, such as infrared spectroscopy and atomic force microscopy, provide insights into adsorbate structure and bonding but lack atomic-scale resolution; thus, they often require complementary simulations for a comprehensive interpretation. In this thesis, density-functional theory (DFT) calculations bridge that gap by connecting spectroscopic signatures of water and dioxygen on ceria to specific atomic configurations.

The water–ceria interaction was explored through a series of studies. Thermodynamic analysis indicates that water preferentially adsorbs as hydrogen-bonded chains on the ceria surface, which are allowed to form due to partial dissociation into surface hydroxides that allow for strong water–hydroxide hydrogen bonds. The computed vibrational spectra are correlated with distinct hydrogen-bond motifs among the surface-adsorbed water, showing that certain motifs produce notably red-shifted frequencies. Compared to bulk water, these surface-mediated hydrogen bonds adopt more tilted geometries while maintaining strong interactions and exhibiting pronounced vibrational shifts. I further analyze how vibrational frequencies depend on structural descriptors, such as bond lengths, local electric fields, and high-dimensional embeddings of the surrounding atomic environment, to quantify the amount of structural detail encoded in each spectral feature, which provides insight into the cause of the vibrational shift.

Next, DFT simulations of O2 adsorption, coupled with infrared reflection–absorption spec-troscopy (IRRAS) simulation, reveal that charged O2X – species lying flat on CeO2 exhibit large transition dipoles, contrary to existing interpretations. I demonstrate that these dipoles arise from surface-to-molecule charge transfer perpendicular to the O–O bond, induced by the molecular vibration of O2X –. This insight suggests new heuristics for interpreting oxide-surface spectra and underscores the necessity of explicit atomistic modeling for accurate spectral assignments.

Overall, this thesis presents a systematic framework that integrates DFT calculations with thermodynamic, vibrational, and statistical analyses to bridge the gap between atomic structure and experimental observables. The simulation strategies, analysis software, and spectroscopic “rules of thumb” developed here advance the understanding of water and oxygen interactions on ceria — and by extension, on other metal-oxide surfaces — offering generally applicable methods for studying molecule–metal oxide interfaces.