Oxide chemistry and catalysis

Metal oxides are among the most earth abundant resources on the planet. For example, by mass, Fe is the most earth abundant element, Ni is the sixth most abundant, and Al is the eighth most abundant. Like Fe, Ni, and Al, most metals with only a very few exceptions exist as oxides under ambient conditions. Even for the simplest binary metal oxides, a large number of phases and oxidation states can exist depending on the oxygen chemical potential, and this phase space rapidly expands when considering ternary and higher order oxides, doped materials, and metal/metal oxide interfaces. Questions of electronic and crystal structures become even more complicated at a surface or interface compared to the bulk material. This is, in part, because defects and impurities often segregate to surfaces. Surfaces are also accessible for molecular adsorption and interfacial bonding, which require challenging interface-specific spectroscopies to accurately characterize. Additionally, surfaces lack the periodicity of bulk crystals, making them challenging to treat theoretically.

Metal oxides are also inherently reactive and can serve as catalysts for numerous reactions. Additionally, high surface area mesoporous oxides often act as supports for metal nanoparticles or other co-catalysts. In such cases, the oxide framework can modulate the activity of the supported catalyst through strong metal support interactions. In many cases, metal oxides are semiconducting and exhibit strong absorption coefficients for visible light, making these materials attractive for applications in photocatalysis, solar energy conversion, and storage. The highly polar bonds in many metal oxides result in strong electron–phonon coupling, making it difficult to decouple the electronic and nuclear contributions to the wavefunction. This strong coupling gives rise to unique electrical and optical properties, which often dominate electron transport and significantly complicate excited state modeling. All these effects point to the need for chemical physics to provide a fundamental framework required to support the many promising applications of oxide chemistry and catalysis.

To call attention to this need and to highlight the rapid advances taking place in this field, we believe that a special issue focused on oxide chemistry and catalysis is especially timely. This resulting special issue includes 18 theoretical papers, 31 experimental papers, and 11 papers combining both experiment and theory. This array of works demonstrates the range of complex questions requiring collaborative approaches involving both theory and experiment to be adequately addressed. These questions are also driving new methodological developments. For example, the findings in this special issue rely on a multitude of cutting-edge experimental techniques, including ambient pressure x-ray photoelectron spectroscopy (XPS),^1–5 ambient pressure scanning tunneling microscopy,⁶ in situ x-ray diffraction (XRD),¹ in situ vibrational spectroscopy,^7–9 photoemission electron microscopy,¹⁰ solid state nuclear magnetic resonance (NMR),^11,12 Mössbauer spectroscopy,¹³ and cluster anion photoelectron spectroscopy.¹⁴ From the theoretical side, these works employ a range of semilocal, semilocal with Hubbard correction (DFT+U), and hybrid density functionals,^15–25 in addition to multi-configurational methods,²⁶ first-principles and multiscale molecular dynamics,^27,28 machine learning, and neural networks.²⁹ 

Applications of these methods cover such important topics as surface chemical kinetics, photo and electrochemical energy conversion, support effects in heterogeneous catalysis and electrochemistry, electronic and structural effects of complex defect chemistry, and advance functional materials such as multiferroics and interfacial pH buffers. These timely applications showcase the importance of chemical physics to address many of the pressing challenges facing modern science. Below we briefly summarize the topics included in this special issue.

A number of papers in this special issue highlight work focused on solar and electrochemical energy conversion by metal oxides. Wu et al. investigate the role of O vacancy and Ti interstitial defect states on the adsorption, desorption, and photochemical reactivity of small molecules on a TiO₂ surface.³⁰ Pelli Cresi et al. demonstrate the ability to tune the optical absorption properties of CeO₂ by embedding Ag nanoparticles.³¹ In the original work, Bertram et al. use in situ vibrational spectroscopy to investigate energy storage using norbornadiene photoswitches covalently grafted onto oxide surfaces.⁸ 

Electrochemical studies include investigation of NiCo₂O₄ particles as supercapacitors for energy storage.³² Considering the catalytic potential of metal oxides for electrochemical O₂ evolution, Gono and Pasquarello demonstrate that bifunctional catalysis by mixed oxides can overcome kinetic limitations imposed by linear scaling relations,¹⁸ and Hajiyani and Pentcheva utilize first-principles calculations to understand the effects of doping on O₂ evolution activity on a hematite anode.²² Mason et al. investigate H₂ evolution on Mo–Mn ternary oxide clusters using anion photoelectron spectroscopy.¹⁴ Vonrüti and Aschauer use DFT+U to investigate the effects of ferroelectric switching on the water splitting activity of LaTiO₂N and BaTiO₃ catalysts.²⁴ Considering applications of metal oxides as ion conductors, Lackner et al. investigate the effect of Yt doping to stabilize ion conducting phases of ZrO₂.³³ Exploring an impressive array of phase space, Guan et al. employ machine learning via neural networks to identify structure–function relationships responsible for the trade-off between ion conductivity and thermal stability in Yt-doped ZrO₂.²⁹ 

Applications of metal oxides in heterogeneous catalysis are abundantly addressed. These studies discuss catalytic reactions including CO oxidation,^7,34–37 CO₂ hydrogenation,³⁸ partial methane oxidation,^39,40 oxidation of volatile organics,³ dehydrogenation,^41,42 selective acetylene hydrogenation,²³ chemical looping,¹⁹ and olefin epoxidation.⁹ 

A number of papers in this special issue focus on resolving the atomic-scale structure of metal oxides and metal nanoparticles dispersed on an oxide surface. This includes growth and structural characterization of ternary Co–Fe oxides¹⁰ and Ca–Mo oxides,⁴³ as well as structural characterization of the stepped surfaces on a curved ZnO single crystal.⁴⁴ Merte et al. report a novel two-dimensional structure of ultrathin Fe₃O₄ grown on Ag.⁴⁵ Si et al. employ the SCAN functional to investigate the surface termination of hematite.¹⁵ Freindl et al. explore the reversible interconversion between hematite and magnetite.¹³ Similarly, Jiang et al. employ ambient pressure XPS to investigate the reversible oxidation and reduction of Fe oxide on Pt.⁶ Lodesani et al. investigate the growth of Fe oxide on a Ni surface and show that Fe oxide dispersion is strongly affected by the presence of a graphene layer on the Ni substrate.⁴⁶ Bagus et al. present a detailed analysis of the Fe 2p XPS spectrum in hematite, providing new insights on the Fe–O bond covalency and the role of multibody effects.²⁶ Madej et al. characterize the deposition of Au nanoparticles on TiO₂ and show that a sub-monolayer of pre-adsorbed Fe can greatly increase Au dispersion.⁴⁷ Similarly, Rani et al. characterize the deposition of Pt nanoparticles on SiO₂.⁴⁸ Using DFT, Zhou et al. investigate the formation of small FeO_x nanoparticles from Fe–Pt bimetallic alloys under oxidizing conditions and consider the resulting effects on catalytic activity.²⁵ Xue et al. use liquid phase atomic force microscopy to determine the atomic-scale topography of TiO₂ surfaces in aqueous solution.⁴⁹ 

Closely related to structural studies, a significant number of papers deal with complex defect chemistry in metal oxides. Arrigoni and Madsen compare the performances of different DFT+U methods and hybrid density functionals in predicting defect properties in TiO₂.¹⁶ Nagatsuka et al. investigate the creation of midgap states in TiO₂ by H ion irradiation.⁵⁰ Kim et al. utilize ambient pressure XPS and in situ XRD to study O vacancy formation and associated phase transitions in SrRuO₃ films.¹ Cao et al. show that formaldehyde serves as an effective probe molecule to discern O vacancies in ZnO.⁵¹ Daelman et al. present a perspective highlighting the need of an accurate theoretical description of defect states in reducible oxides.¹⁷ 

Water adsorption on oxide surfaces is important for understanding a number of chemical and environmental phenomena, including geological mineralization, aerosol chemistry, and electrochemistry. From a fundamental perspective, the question of water adsorption on oxide surfaces is extensively addressed in this special issue. Multiple studies deal with the adsorption of water on CeO₂ surfaces.^21,52 Ambient pressure XPS is used by Goodacre et al. to study water adsorption on VO₂ surfaces² and by Jhang et al. to study water adsorption on SiO₂ and Al₂O₃ surfaces.⁵ Liu et al. use multiscale simulations to understand the water structure at the aqueous Fe₃O₄ interface.²⁷ Wen et al. use first-principles molecular dynamics to predict important differences between water structure on reduced and oxidized titania surfaces.²⁸ 

In addition to water, the adsorption and reactivity of a number of other molecules on oxide surfaces are reported, including N₂, CO, CO₂, methanol, formic acid, and cysteine.^4,20,53–58 Interesting applications of facet-dependent adsorption are demonstrated by Sellschopp et al. who report the shape directing effects of molecular adsorption during TiO₂ nanoparticle growth.⁵⁹ Using solid state NMR, Hubbard et al. differentiate multiple modes for mobility of triphenylphosphine oxide on an Al₂O₃ surface.¹¹ 

Questions of interfacial pH, which differs significantly from bulk solution, are also investigated. Singappuli-Arachchige and Slowing study surface pH in mesoporous SiO₂, and it is found that surface functionalization allows tunable buffering of interfacial pH.⁶⁰ Maleki and Pacchioni show that ¹⁷O NMR can serve as a sensitive probe of surface basicity in the family of alkaline earth oxides.¹² 

Together, the contributions contained in this special issue provide a snapshot of the rapidly evolving field of oxide chemistry and catalysis. They highlight the challenges and complexities associated with a rigorous understanding of oxide chemistry at surfaces and demonstrate the necessary role of chemical physics to provide a fundamental framework for understanding and utilizing this chemically rich class of materials.

We would like to thank the authors, whose creative work and novel ideas form the basis for this special issue. We also express appreciation to the many reviewers, whose insightful comments and helpful suggestions have significantly strengthened the work included here. Finally, we acknowledge the tireless efforts of The Journal of Chemical Physics editors, Patricia Thiel and Angelos Michaelides, journal staff, Erinn Brigham and Judith Thomas, and editor-in-chief, Tim Lian, for their assistance and strong support during this process.