AVS 72 Session SS-TuA: Chemical Reactions

Oxide surfaces exhibit a strong interplay between local electronic structures, chemical reactivity, and collective excitations, governing processes relevant for catalysis, energy conversion, and infrared nanophotonics. In this work, we investigate how oxidation state, structural disorder, and local coordination affect both surface reactivity and light–matter interactions in model oxide systems.

The reactivity of transition-metal oxide surfaces was investigated considering CO₂ hydrogenation and hydroxylation conditions. On α-Fe₂O₃(012), near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and low-energy electron diffraction (LEED) were used to correlate surface structure and Fe charge state with CO₂ activation pathways. Exposure of the reconstructed (2×1) surface to CO₂ + H₂ induces partial surface reduction and the formation of electronically enriched Fe²⁺ sites, which stabilize activated CO₂^- intermediates. Complementary studies on MnO(001) and Mn₃O₄(001) surfaces show that hydroxylation by water strongly depends on the oxidation state of surface Mn cations, with Mn²⁺ sites acting as active centers for hydroxyl formation. Together, these results demonstrate that oxide surface reactivity is directly governed by local electronic structure and cation valence.

Beyond surface reactivity, oxide interfaces also support collective excitations that are highly sensitive to local chemical environments. Using synchrotron infrared nanospectroscopy (SINS), we investigated surface phonon polaritons (SPhPs) in amorphous SiO₂ films after fluorine-ion irradiation. The dominant SPhP associated with the asymmetric stretching mode exhibits a pronounced reduction in intensity, revealing modifications in the local dielectric response induced by ion-generated disorder. Finite dipole model calculations indicate that the measured response originates from an effective medium composed of mixed SiO₂ and SiO_x regions, demonstrating how nanoscale chemical heterogeneity modifies polaritonic excitations at oxide surfaces.

Together, these results show how local electronic structure and chemical heterogeneity govern both catalytic activation pathways and confined light–matter interactions at oxide interfaces.

The catalytic conversion of CO₂ into value-added products, such as methanol, presents a promising strategy towards carbon neutrality. Palladium-Indium (Pd-In) intermetallic alloys have emerged as a highly selective and effective catalyst for CO₂ hydrogenation. However, their surface structures and interactions with CO₂ at the solid-gas interface require further understandings. In this study, the CO₂ activation pathways on the oxide-tuned PdIn(111) single crystal alloy surfaces were investigated in situ, using ambient pressure scanning tunneling microscopy (AP-STM) and synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS).

Under ultra-high vacuum (UHV), the metastable PdIn(111) surface adopts unique surface structure consisting of clusters, atomic protrusions and ordered superstructure domains. Exposure to ambient pressure O₂ or CO₂ drives dramatic morphological transformations, as dissociated atomic oxygen triggers indium-selective oxidation, and upward segregation of InO_x. Such surface restructuring generates interfacial Pd-In-O_x nanostructures with defect-rich oxygen vacancies, serving as highly active sites for CO₂ activation and the formation of carbonate intermediates. In contrast, the elevated O₂ pressure and temperatures accelerate the dealloying of the Pd-In lattice and the growth of passivating indium oxide overlayer with significantly lower CO₂ activation capabilities.

These in situ observations provide fundamental insights into the active role of the metastable PdIn(111) intermetallic alloys surface in CO₂ activation. Furthermore, they demonstrate that precisely tuning the PdIn(111) surface oxide structures to maintain active interfacial sites is essential for the rational design of robust Pd-In intermetallic catalyst for CO₂ conversion.

Growth, structure and surface chemistry of Pd/In surface alloys and InO_x thin films on Pd(111)

Thermo-catalytic conversion of carbon dioxide (CO₂) to methanol (CH₃OH) is important for mitigating CO₂ emissions and producing fuels and value-added products. Indium oxide (In₂O₃) has emerged as a promising catalyst for activating CO₂ and selectively converting it to methanol while suppressing competing reactions. Adding transition metals (TMs), such as Pd, improves catalytic performance, presumably by promoting efficient H₂ dissociation. Under reducing conditions and during CO₂ hydrogenation, TMs supported on In₂O₃ can transform into TM-In intermetallic compounds, giving rise to complex couplings between surface chemistry and coexisting metal and metal-oxide structures. These effects motivate fundamental studies using model systems.

In this talk, I will first discuss the structural evolution of Pd-In alloys grown on Pd(111) as a function of In coverage and temperature. Temperature-programmed X-ray photoelectron spectroscopy (TP-XPS) demonstrates that Pd-In intermixing produces In-rich surface alloy phases at room temperature and that the surface In concentration decreases with increasing temperature as In dissolves into the bulk Pd. XPS fitting identifies distinct Pd 3d and In 3d components associated with In-poor and In-rich Pd-In phases, whose compositional regimes correlate with the extent of Pd-Pd connectivity within the intermetallic structures. We further show that an ordered (√3 × √3)R30° surface alloy (Pd₂In) forms upon heating 2 ML In/Pd(111) to 600 K, while InPd(110) domains develop on an initial 6 ML In/Pd(111) surface between ~400 and 525 K.

I will also discuss the formation and structure of InO_x thin films on Pd(111), characterized using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM shows that InO_x film growth follows the Stranski–Krastanov mechanism, in which InO_x initially forms a well-connected wetting layer up to an oxide coverage of 1 ML (monolayer), after which multilayer InO_x islands develop. Both the wetting layer and thicker oxide islands exhibit (5 × 5) periodicity relative to Pd(111), characteristic of bixbyite In₂O₃(111). Lastly, I will discuss cooperative interactions between InO_x islands and the exposed Pd surface regions. XPS and TPD measurements show that H₂ dissociates readily on Pd domains, and that a substantial fraction of the resulting H-atoms migrates to neighboring InO_x domains, driving InO_x hydrogenation rather than recombinative H₂ desorption. These findings provide molecular-level insights into cooperative metal/oxide interactions to CO₂ hydrogenation chemistry.

Layered transition metal oxides underpin a wide range of functional materials, including intercalation electrodes and oxide catalysts, where oxygen activity strongly influences stability, reactivity, and electronic behavior. Understanding oxygen vacancy formation across compositionally complex chemistries remains challenging due to the interplay between local coordination, electronic structure response, and redox behavior. Here, we investigate oxygen vacancy energetics across layered oxide chemistries using density functional theory (DFT), random forest tree (RFT) models, and electronic structure analysis.

RFT models successfully capture composition-dependent trends in pristine layered oxide formation enthalpies using physically motivated descriptors, demonstrating that bulk thermodynamic properties remain transferable across chemistries. In contrast, prediction accuracy decreases for oxygen vacancy formation energetics, where systematic residual patterns emerge as a function of both local coordination environment (LCE) and host composition. These structured residuals suggest that defect-driven redox behavior introduces mechanistic complexity not fully represented within generalized descriptor-based models.

To probe the origin of these deviations, we analyze charge redistribution following oxygen vacancy formation. Vacancy accommodation mechanisms vary significantly with composition and extend beyond purely local bonding effects. Ni-rich environments preferentially stabilize vacancies through metal-centered reduction and localized electron accommodation, whereas Mn-rich systems exhibit more heterogeneous redistribution pathways. Co-rich and electronically inactive cation environments suppress conventional transition-metal redox, promoting increased oxygen-centered or delocalized compensation behavior.

Together, these results demonstrate that oxygen vacancy energetics in layered oxides arise from a continuum of composition-dependent electronic response mechanisms that are fundamentally more complex than the pristine-state properties used to describe the host lattice. More broadly, this work highlights an important limitation of transferable ML models for complex oxides: while generalized descriptors can successfully predict bulk thermodynamic properties, defect-mediated redox processes depend sensitively on emergent electronic structure effects that require more mechanistically informed representations. These findings demonstrate how structured ML residuals can serve as physically meaningful indicators of unresolved chemical complexity, providing a pathway for integrating interpretable machine learning with physics-based electronic structure analysis.

Understanding CO adsorption on well-defined metal oxide surfaces provides critical benchmark data for interpreting catalytic processes and infrared spectroscopy on complex powder materials. We present a multi-technique study of CO on the Fe₂O₃(012)-(1×1) r-cut surface, which has a structurally homogeneous array of equivalent Fe³⁺ and O²⁻ sites. Nevertheless, temperature-programmed desorption (TPD) reveals three distinct desorption peaks (58, 100, 125 K) despite the homogeneous surface. We attribute this finding to lateral repulsive interactions between CO molecules. Infrared reflection absorption spectroscopy (IRAS) shows blue-shifted CO frequencies relative to the gas phase (>2143 cm⁻¹) at low coverage that red-shift with increasing coverage. This is consistent with electrostatic interaction between CO and cationic Fe³⁺ sites. Light polarization-dependent measurements provide direct evidence for tilted CO orientation. Non-contact atomic force microscopy (nc-AFM) reveals protrusions consistent with adsorption atop surface Fe³⁺ sites, with distinguishable static and mobile CO populations. Our multi-technique approach establishes adsorption energetics, vibrational signatures, spatial ordering, and bonding geometry, providing a molecular-level foundation for interpreting complex powder catalyst systems.

Heavy transition metals such as Rh are commonly used for catalysis and have been heavily studied for oxidative reactions. The fundamental understanding of an oxidized Rh surface is widely investigated on low index single crystals however; limited information is available on the behavior of oxygen on high index surfaces. Using a bi-faceted Rh(111)/(322) crystal, we were able to analyze how oxidation of the two facets compared using scanning tunneling microscopy (STM) to characterize the reconstruction oxidation by surface defects and step width. Alongside with the STM, other techniques such as temperature programmed desorption, and low energy electron diffraction are used to identify the various species of oxygen present on the surface and the structures those oxygen species form.

Heterogeneous catalysis plays an important role in energy production, environmental remediation, and chemical manufacturing, where reactions occur on solid surfaces. Understanding how oxygen interacts with catalytic metal surfaces is necessary for improving catalytic performance. In this study, we investigate oxygen adsorption on two Rh single-crystal surfaces: the flatRh(111) surface and a bifaceted surface with a flat (111) surface and a stepped (322) surface, which contains a high density of under-coordinated sites. Using Reflection Absorption Infrared Spectroscopy, different oxygen prepared surfaces were exposed to CO to examine how surface structure influences CO adsorption behavior. Comparisons between the two crystal single crystals revealed differences in CO vibrational peak positions, peak shifts, and relative intensities, suggesting the presence of distinct CO adsorption species. The results demonstrate that oxygen pre-coverage and surface morphology affect CO adsorption on Rh surfaces. This study provides understanding into structure sensitive adsorption behavior relevant to catalytic oxidation processes.

Infrared (IR) spectroscopy is one of the most powerful techniques in heterogeneous catalysis and remains the most widely used optical spectroscopy in the field. Its various experimental modes enable the investigation of a broad range of samples, from powders studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to flat single crystals examined by infrared reflection–absorption spectroscopy (IRAS or IRRAS). Owing to this versatility, IR spectroscopy is widely regarded as a bridging technique that connects fundamental surface science with applied catalysis.

In this work, we use a newly developed IRAS setup optimised for dielectric single crystals to investigate CO adsorption on the model single-atom catalyst Rh/Fe₃O₄(001).¹ Our spectra resolve three distinct Rh–CO species: monocarbonyls on isolated twofold-coordinated Rh adatoms, monocarbonyls on fivefold-coordinated Rh atoms incorporated into the surface, and gem-dicarbonyls on isolated twofold-coordinated Rh adatoms.² These assignments are established unambiguously by combining isotope-substitution experiments (¹²CO, ¹³CO, and mixed ¹²CO/¹³CO) with precise control of Rh coverage and surface preparation. The interpretation is further supported by our previous SPM, XPS, and TPD studies, as well as DFT calculations.³

Under UHV conditions, monocarbonyls at Rh adatom sites are the predominant species. Gem-dicarbonyl formation is kinetically hindered, occurring primarily through CO-induced dissociation of Rh dimers rather than sequential CO adsorption on isolated adatoms. However, NAP-STM shows that the sequential adsorption pathway becomes accessible at millibar CO pressures, linking the behaviour observed under UHV to that expected under realistic reaction conditions. Although DFT provides qualitative insights, it does not quantitatively reproduce the experimental frequencies and is highly sensitive to the computational parameters, underscoring the need for robust experimental benchmarks. The spectroscopic fingerprints established here provide benchmark reference spectra for oxide-supported single-atom catalysts and a reliable basis for assigning Rh coordination environments in practical catalysts and for evaluating theoretical predictions.

1 D. Rath, et al., Rev. Sci. Instrum., 2024, 95, 065106.

2 N. E. H. Barama, et al., https://arxiv.org/abs/2512.15194v2,.

2 C. Wang, et al., Angew. Chem. Int. Ed., 2024, 63, e202317347.

Cu-based catalysts are active for partial and full oxidation reactions. Deciphering the local atomic environment and oxidation state of active centers in supported copper catalysts, as well as the design of materials to control their stability under reaction conditions remains a great challenge. We show here that mixed-oxides of copper delafossites with gallium, aluminum or iron (CuMO₂, Cu¹⁺ and M³⁺; M: Ga, Al, Fe) in the form of porous nanoplates and films are promising materials as model catalysts to explore the activity and stability of Cu¹⁺-activated reactions. In situ experiments allow the observation of dynamic processes and phases under reaction conditions.