Hidden Oxygen Vacancies at Chromium–Zinc Oxide Interfaces Drive Syngas-to-Olefins Catalysis

Oxygen vacancies—tiny voids where an oxygen atom is missing from a metal oxide lattice—have long been suspected as the true engines of many industrial catalytic reactions. Yet proving exactly where these defects sit on a catalyst surface, and which of them actually do the chemical work, has remained one of heterogeneous catalysis’ most stubborn puzzles. The defects are rare, they move, and they hide among billions of ordinary lattice sites. Now a team of researchers led by Yifeng Zhu of Fudan University, working with colleagues at several Chinese institutions, has cracked the problem for a technologically important catalyst: the chromium–zinc oxide interface that converts syngas, a mixture of carbon monoxide and hydrogen, into valuable olefins. Their findings, published in Nature Catalysis, show that the catalytic action is not spread randomly across the surface but concentrated at specific, predictable defect motifs.

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