The catalyst in question belongs to the zinc–chromium oxide family, a workhorse system for high-temperature methanol synthesis and, more recently, for the direct conversion of syngas to light olefins such as ethylene and propylene—the building blocks of plastics. In 2016, the landmark OX/ZEO process demonstrated that metal oxides could steer syngas chemistry away from conventional Fischer–Tropsch distributions toward selectively formed olefins, igniting intense interest in which surface features govern that selectivity. Computational work by Zhi-Pan Liu and Sicong Ma had earlier suggested that dynamic coordination of zinc and chromium cations plays a decisive role, but the identity and location of the active oxygen defects remained unresolved. The new study closes that gap by making the invisible visible.

The central methodological innovation is a site-resolved analytical platform that fuses operando, time-resolved energy-dispersive X-ray absorption spectroscopy with mass spectrometry, transient kinetic analysis, and microcalorimetry. Energy-dispersive X-ray absorption spectroscopy, or EDXAS, acquires full absorption spectra in milliseconds, allowing the researchers to track changes in the chromium and zinc local environments while the catalyst was actually working under realistic syngas feed. By pairing these rapid spectral snapshots with simultaneous product monitoring and then applying transient kinetic analysis—the mathematical dissection of how product signals respond to deliberate perturbations of the feed—the team could correlate specific spectral signatures with specific catalytic functions. Calorimetry added the thermodynamic dimension, measuring the heat released or absorbed as oxygen vacancies formed and were consumed.

Why is such sophistication necessary? Conventional tools for identifying oxygen vacancies, such as X-ray photoelectron spectroscopy, suffer from well-documented ambiguities: the oxygen 1s signal often attributed to vacancies in the 531–532 electron-volt range can arise from hydroxyl groups or other surface species instead, a pitfall highlighted in recent critical analyses of ZnO spectra. Moreover, vacancies exist at vanishingly low concentrations on a heterogeneous surface that may contain terraces, steps, dopants, and metal–oxide junctions all at once. Electron microscopy can image vacancies in exceptional cases, but typically under conditions far removed from working reactors. The new approach sidesteps these limitations by measuring the catalyst under genuine reaction conditions and resolving the vacancy population into distinct site types rather than a single averaged count.

What the team found is strikingly specific. Isolated chromium(III) dopants sitting at the Cr/ZnO interface selectively stabilize a particular class of oxygen vacancy—stoichiometric vacancies located at Zn–O–Cr bridge sites—both thermodynamically and kinetically. In other words, these interfacial positions are the easiest places to remove an oxygen atom, and once removed, the vacancy lingers there preferentially rather than migrating elsewhere. Density functional theory calculations, performed by Liu’s group with global structure searches, rationalize this preference: the chromium cation’s electronic structure compensates the charge and strain associated with vacancy formation, lowering the energetic penalty precisely at the interface. The result is a self-assembled ensemble of Cr–O(v) motifs—chromium cations adjacent to oxygen vacancies—that behave as a distinct chemical species with its own reactivity.

And that reactivity is remarkable. The site-specific Cr–O(v) motifs exhibit exceptionally high activity and selectivity for converting syngas into olefins, outperforming the rest of the surface by a wide margin. Meanwhile, the study assigned a second, complementary role to the interfacial oxygen atoms themselves: they serve as the active sites for propane dehydrogenation, the reverse-type reaction in which hydrogen is stripped from propane to form propylene. This dual assignment is conceptually elegant. At the very same interface, the presence of an oxygen atom enables dehydrogenation chemistry, while its absence enables hydrogenation and C–C coupling chemistry leading to olefins. The catalyst’s selectivity is thus dictated by which of these two interfacial states dominates under a given feed, a balance the researchers could quantify through their transient kinetic framework.

The experimental campaign drew on major synchrotron infrastructure. X-ray absorption measurements were conducted at the BL05U, BL20U, and BL14W beamlines of the Shanghai Synchrotron Radiation Facility, with microscopic infrared measurements performed at the BL06B beamline. The combination of rapid energy-dispersive acquisition with mass spectrometric product detection meant that every fluctuation in catalyst structure could be time-stamped against fluctuations in catalytic output. Microcalorimetry experiments, carried out by Rui Ma and Xiao Kong, anchored the energetic analysis, while Bader charge analysis of the computed models clarified how electron density redistributes when an interfacial oxygen is removed. The optimized atomic coordinates for all computational models were deposited publicly on Figshare, reflecting a commitment to reproducibility that accompanies the paper’s extensive source data.

Beyond the specific zinc–chromium system, the significance of this work lies in its transferable methodology. The pairing of operando transient kinetic analysis with energy-dispersive X-ray absorption spectroscopy constitutes, in the authors’ framing, a platform for tuning surface defect chemistry in oxide catalysts generally. Oxygen vacancies are implicated in an enormous range of transformations: carbon monoxide and carbon dioxide hydrogenation, oxidative dehydrogenation of light alkanes, water–gas shift chemistry, and photocatalytic water splitting, to name a few. In ceria, titania, and countless mixed oxides, the vacancy has been invoked as the active site, the charge reservoir, or the oxygen shuttle—but usually without site-level proof. A method that delivers quantitative, site-resolved vacancy counts under working conditions offers the catalysis community a way to move from correlational arguments to causal ones: to state not merely that vacancies exist, but which vacancies, where, doing what.

The practical implications extend to industrial chemistry. Syngas, produced from coal, natural gas, or biomass gasification, is a foundational feedstock, and routes that bypass energy-intensive intermediate steps to deliver olefins directly carry substantial economic and environmental weight. Knowing that isolated Cr(III) dopants at oxide interfaces nucleate the catalytically potent vacancies gives catalyst designers an actionable blueprint: control the dopant distribution, control the interface density, and thereby control the population of the most active sites. The same logic applies in reverse for propane dehydrogenation, where preserving interfacial oxygen rather than removing it defines the design target. As the chemical industry seeks to electrify and decarbonize, rational defect engineering of mixed-oxide catalysts—guided by operando, site-resolved spectroscopy—may prove one of the most powerful levers available, and this study provides a compelling demonstration of what that lever looks like when it is finally pulled with precision.

Subject of Research: Site-resolved identification of catalytically active oxygen vacancies at Cr/ZnO interfaces during syngas conversion

Article Title: Preferential locations of catalytically active oxygen vacancies at Cr/ZnO interfaces during syngas conversion

Article References: Yang, C., Ma, R., Zhang, C., Kong, X., Ding, B., Li, R., Ma, S., Liu, Z.-P., Cao, Y., Fu, Q., Yang, F., Bao, X., & Zhu, Y. (2026). Preferential locations of catalytically active oxygen vacancies at Cr/ZnO interfaces during syngas conversion. Nature Catalysis. https://doi.org/10.1038/s41929-026-01615-7

Image Credits: AI Generated

DOI: 10.1038/s41929-026-01615-7

Keywords: oxygen vacancies, Cr/ZnO interface, syngas conversion, olefins, heterogeneous catalysis, operando spectroscopy, X-ray absorption spectroscopy, transient kinetic analysis, propane dehydrogenation, zinc–chromium oxide, defect chemistry, microcalorimetry

Oxygen vacancies are missing oxygen atoms in an oxide lattice, and they are routinely introduced to tune the electronic structure of catalysts for fuel cells, metal–air batteries and green chemical synthesis. Conventionally, scientists count vacancies and assume more is better, or at least that all vacancies behave alike. The new work challenges that assumption at a fundamental level. When an oxygen atom leaves the lattice, it can leave behind electrons that either remain trapped at the vacancy site or are transferred to neighbouring metal cations. These two configurations correspond to distinct colour centres, known as F centres: an electropositive F1 centre, in which the electrons are not localized at the vacancy, and an electroneutral F2 centre, in which two electrons are trapped within the vacancy itself.

The challenge for the team, led by Xiaoyuan Zhang, Jingwen Sun and Junwu Zhu of Nanjing University of Science and Technology, was to isolate the effect of charge state from every other variable. To do this, they devised a template-assisted synthesis in which the atmosphere during preparation was carefully regulated, allowing them to produce zirconia samples, ZrO2−x, that contain predominantly F1 or predominantly F2 centres while keeping the crystal structure, particle morphology and vacancy concentration essentially unchanged. This clean experimental design meant that any difference in catalytic behaviour could be attributed directly to the charge state of the defects rather than to confounding structural differences.

Characterization confirmed the distinction. Electron paramagnetic resonance spectroscopy, which is sensitive to unpaired electrons, revealed the paramagnetic signature of the F1-type vacancies, while complementary measurements of the electronic structure showed the different local environments around zirconium cations adjacent to each vacancy type—F1-Zr4+ versus F2-Zr3+ configurations. X-ray absorption, electron energy-loss spectroscopy and photoluminescence measurements all supported the picture of two electronically distinct but structurally equivalent defect species. Crucially, electrochemical scanning transmission electron microscopy showed that the catalysts remained stable under operating conditions, ruling out structural reconstruction as the source of the differing reactivity.

The electrochemical consequences were striking. When the electroneutral F2 centres dominated, the catalyst favoured the two-electron oxygen reduction pathway, selectively converting oxygen into hydrogen peroxide. When the electropositive F1 centres dominated, the reaction instead proceeded toward full four-electron reduction, cleaving the oxygen–oxygen bond and producing water. Hydrogen peroxide electrosynthesis is a rapidly growing field because the compound is a green oxidant used in water treatment, disinfection and chemical manufacturing, and producing it on-site in an electrochemical cell could replace the energy-intensive anthraquinone process. A catalyst whose selectivity can be switched by defect charge state therefore has immediate practical appeal.

To understand the mechanism, the researchers deployed in situ electrochemical electron paramagnetic resonance, tracking the paramagnetism of the F centres while the reaction ran, together with in situ Raman spectroscopy to follow the evolution of reaction intermediates. The results overturned a common intuition. The electroneutral F2 centre, despite being the site where electrons are trapped, is not the primary adsorption site for oxygen. Instead, it acts through dynamic electronic compensation: it continuously donates and withdraws electron density to and from adjacent zirconium sites, stabilizing the adsorbed *OOH intermediate that is the hallmark of the two-electron pathway. It is this dynamism, rather than direct binding, that makes F2 centres the gatekeepers of peroxide selectivity.

The electropositive F1 centre behaves in an entirely different manner. It binds molecular oxygen directly at the vacancy site, and the interaction is strong enough to cleave the O–O bond, committing the reaction to the four-electron pathway. In the process, the F1 centre itself is quenched, its paramagnetic signature disappearing as the reaction proceeds. Density functional theory calculations reproduced both behaviours, showing favourable adsorption energetics for O2 at F1 sites and for *OOH at sites electronically modulated by neighbouring F2 centres, and the calculated free-energy landscapes matched the experimentally observed selectivity patterns.

Beyond zirconia, the study carries a broad message for the field. Oxygen vacancies have been invoked to explain catalytic behaviour in ceria, perovskites, cobalt and iron oxides, and countless other systems, but the charge state of those vacancies is rarely controlled or even measured. The authors argue that F-centre charge state should be regarded as an independent design lever, alongside vacancy concentration and position. Because the two charge states can be interconverted by atmosphere control during synthesis, and because in situ electron paramagnetic resonance can now monitor them under working conditions, the toolkit exists to rationally design vacancy chemistry rather than accept whatever defects a preparation happens to deliver.

The work also highlights the power of operando spectroscopy to catch catalysts in the act. Static characterization before and after a reaction can miss the transient electronic exchanges that actually govern selectivity; here, the decisive role of the F2 centre only became visible because its paramagnetism and the Raman signatures of intermediates were tracked simultaneously under electrochemical bias. As the energy transition drives demand for selective, precious-metal-free electrocatalysts for peroxide production, water treatment and chemical synthesis, the ability to dial in defect charge states could prove one of the more consequential ideas to emerge from defect engineering in recent years. What was once an invisible nuance of the oxide lattice has become a switch that chemists can now flip at will.

Subject of Research: Charge-state-dependent oxygen vacancy (F-centre) control of electrocatalytic oxygen reduction selectivity in zirconia

Article Title: F-centre charge state and dynamism govern oxide electrocatalytic selectivity

Article References: Zhang, X., Bukhvalov, D., Su, T., Fang, C., Dai, L., San, S., Duan, H., Wang, Y., Liu, K., Cui, J., Hua, Y., Xue, L., Hou, Z., Zhang, W., Xiong, P., Fu, Y., Sun, J., & Zhu, J. (2026). F-centre charge state and dynamism govern oxide electrocatalytic selectivity. Nature Chemistry. https://doi.org/10.1038/s41557-026-02256-w

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02256-w

Keywords: oxygen vacancies, F centres, zirconia, electrocatalysis, oxygen reduction reaction, hydrogen peroxide, selectivity, electron paramagnetic resonance, vacancy engineering, in situ Raman spectroscopy, defect chemistry, 2e- ORR pathway

The synthesis itself is deliberately low-tech. Cerium and praseodymium nitrates, either individually or as mixtures, serve as the oxidizer, while glycerin acts as the fuel in a self-sustaining combustion reaction. Once the precursors ignite, the exothermic reaction rapidly converts the salt-glycerin gel into an oxide powder. The authors then split their product into two series by annealing in air: one set heated at 600 degrees Celsius and another at 1000 degrees Celsius, each for one hour. This two-temperature design lets them separate the effects of nanoscale crystallinity from those of coarser, more equilibrated grains, because the low-temperature powders retain small crystallite sizes and abundant surface defects while the high-temperature powders approach thermodynamic order.

X-ray powder diffraction delivered the structural backbone of the study. The samples annealed at 600 degrees Celsius remained single-phase cubic fluorite solid solutions across nearly the entire composition range, from pure ceria up to a praseodymium fraction of 0.75. That is a remarkable tolerance, since substituting the larger trivalent praseodymium ion onto the tetravalent cerium site should strain the lattice and eventually force phase separation. In the 1000-degree-Celsius series, the cubic solid solution held together only up to compositions below a praseodymium fraction of 0.5, beyond which the homogeneity breaks down. The difference illustrates a classic principle in defect chemistry: at higher synthesis temperatures, the system has enough mobility to exsolve secondary phases, while kinetically frozen nanocrystals can accommodate far more disorder.

The substitution mechanism follows textbook expectations. Praseodymium entering the fluorite lattice as Pr3+ is larger than Ce4+, so the lattice parameter expands with composition, consistent with the revised ionic radii tabulated by Shannon. Charge compensation requires that every trivalent praseodymium ion be balanced either by an oxygen vacancy or by praseodymium oxidized to the tetravalent state. The powder samples, prepared in air, contain a mixture of both praseodymium valences, and it is the oxygen vacancy population created by the trivalent fraction that dominates the defects the team probed spectroscopically and electrochemically.

Raman spectroscopy provided the most incisive window into those defects. The team recorded spectra using two different laser wavelengths, 532 and 785 nanometers, a strategy that exploits resonance effects to weight the contribution of surface and near-surface layers differently from the bulk. In pure ceria, the spectrum is dominated by the sharp first-order triply degenerate breathing mode of the fluorite lattice near 465 wavenumbers, together with weaker second-order features. As praseodymium content rises, additional broad bands emerge in the 500 to 600 wavenumber region, signals conventionally assigned to oxygen vacancies, and features associated with the longitudinal and transverse optical modes of the lattice become distorted. Both annealing series showed the same trend: more praseodymium means more lattice imperfection, with vacancies concentrated in the thin surface and near-surface layers of the particles rather than distributed uniformly through the bulk.

Voltammetric measurements on the 600-degree series independently confirmed that picture. The researchers fabricated carbon paste electroactive electrodes loaded with the oxide powders and recorded their electrochemical responses. The currents associated with surface redox processes grew with praseodymium loading, consistent with an increasing density of accessible oxygen vacancy sites at the electrode surface where electrons and oxide ions can be exchanged. Because voltammetry probes only what the electrolyte can reach, the agreement with the surface-sensitive Raman data is chemically meaningful rather than coincidental: both techniques are, in effect, counting the same population of near-surface defects from different directions.

Optical spectroscopy added a further layer of information. UV-Vis-NIR diffuse reflectance measurements showed a slight red shift of the absorption band edge as praseodymium concentration increased, meaning the materials absorb marginally longer wavelengths as the dopant content climbs. This modest band gap narrowing reflects the introduction of additional electronic states associated with the praseodymium 4f levels and the defect structure. More visually striking was the luminescence behavior. Samples containing low praseodymium fractions, from 0.01 to 0.1, emitted a bright red-orange glow under visible light excitation, a signature of the intra-4f transitions of Pr3+ ions embedded in the oxide host. The emission intensity fell off as the praseodymium content rose, a phenomenon the authors attribute to concentration quenching, in which densely packed luminescent ions transfer energy to one another and to defect sites until the excitation is dissipated non-radiatively.

The photocatalytic tests delivered the study’s most sobering result. The team evaluated the low-temperature powders, the ones richest in surface oxygen vacancies, in the oxidation of hydroquinone to para-benzoquinone, a model reaction widely used to gauge photocatalytic activity. Despite the abundant vacancy population, the materials showed photocatalytic activity only under ultraviolet irradiation, not under visible light. For ceria, whose band gap of roughly 3.2 electron volts already sits at the edge of the ultraviolet, doping with praseodymium did not deliver the visible-light sensitization that many earlier papers hoped for. The slight red shift of the absorption edge proved insufficient to harvest meaningful visible photons, so the practical takeaway is that vacancy engineering alone does not guarantee broad-spectrum photocatalysis in this system.

The broader significance of the work lies in its completeness and its cautionary message. By covering the full compositional range with a single, scalable synthesis method and by combining diffraction, dual-wavelength Raman spectroscopy, luminescence, UV-Vis-NIR spectroscopy and voltammetry on the same sample series, the Ekaterinburg team has produced one of the most internally consistent pictures to date of how praseodymium reshapes ceria. Their data reinforce the view that oxygen vacancies in Pr-doped ceria localize at particle surfaces, where they govern redox and electrochemical behavior, while simultaneously demonstrating that vacancy concentration and photocatalytic performance are not synonymous. For researchers designing ceria-based catalysts for soot combustion, methane oxidation, carbon dioxide conversion or pollutant degradation, the message is that dopant chemistry must be matched to the application: praseodymium-rich cerias excel as redox-active and electroactive materials, but achieving genuine visible-light photocatalysis will demand additional strategies such as co-doping, heterojunction formation or morphological control that go beyond what vacancy creation by itself can deliver.

Subject of Research: Synthesis of praseodymium-doped ceria oxides and their structural, spectral, voltammetric and photocatalytic properties

Article Title: A New Method for the Synthesis of Ce1−xPrxO2−δ Oxides and the Study of Their Structure, Spectral, Voltammetric and Photocatalytic Properties

Article References: Baklanova, I. V., Krasil’nikov, V. N., Tyutyunnik, A. P., Buldakova, L. Y., & Yanchenko, M. Y. (2026). A New Method for the Synthesis of Ce1−xPrxO2−δ Oxides and the Study of Their Structure, Spectral, Voltammetric and Photocatalytic Properties. Catalysis Letters, 156(10), Article 278. https://doi.org/10.1007/s10562-026-05522-1

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05522-1

Keywords: cerium dioxide, praseodymium doping, auto combustion synthesis, oxygen vacancies, Raman spectroscopy, fluorite structure, luminescence, voltammetry, photocatalysis, hydroquinone oxidation, solid solutions, defect chemistry