Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Atomic dynamics of gas-dependent oxide reducibility

Nature volume 644, pages 927–932 (2025)Cite this article

Subjects

Abstract

Understanding oxide reduction is critical for advancing metal production1,2, catalysis3,4 and energy technologies5. Although carbon monoxide (CO) and hydrogen (H2) are widely used reductants, the mechanisms by which they work are often presumed to be similar, both involving lattice oxygen removal6,7,8,9. However, because of growing interest in replacing CO with H2 to lower CO2 emissions, distinguishing gas-specific reduction pathways is critical. Yet, capturing these atomic-scale processes under reactive gas and high-temperature conditions remains challenging. Here we use environmental transmission electron microscopy, which is capable of real-time, atomic-resolution imaging of gas–solid redox reactions10,11,12,13,14,15,16, to directly visualize the gas-dependent oxide reduction dynamics in NiO. We show that CO drives surface nucleation and the growth of metallic Ni islands, leading to self-limiting surface metallization. Conversely, H2 activates a coupled surface-to-bulk transformation, where protons from dissociated H2 infiltrate the oxide lattice to promote the inward migration of surface-generated oxygen vacancies and enabling bulk metallization. By contrast, oxygen vacancies formed by CO remain confined near the surface, where they rapidly form a metallic Ni layer that inhibits further reduction. These results reveal distinct atomistic pathways for CO and H2 and provide insights that may guide metallurgical processes and catalyst design.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on SpringerLink
  • Instant access to the full article PDF.

USD 39.95

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: In situ TEM observation of NiO reduction dynamics in CO.
Fig. 2: In situ TEM imaging of NiO reduction in H2.
Fig. 3: In situ XRD, NAP-XPS and RGA measurements of NiO reduction at 400 °C.
Fig. 4: DFT modelling for diffusion energy barriers, VO clustering and Bader charge analysis.

Similar content being viewed by others

Data availability

All data generated or analysed during this study are included in the article (and its Supplementary Information). Source data are provided with this paper.

References

  1. Manzoor, U., Mujica Roncery, L., Raabe, D. & Souza Filho, I. R. Sustainable nickel enabled by hydrogen-based reduction. Nature 641, 365–373 (2025).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  2. Spreitzer, D. & Schenk, J. Reduction of iron oxides with hydrogen—a review. Steel Res. Int. 90, 1900108 (2019).

    Article  Google Scholar 

  3. Chee, S. W., Lunkenbein, T., Schlögl, R. & Roldán Cuenya, B. Operando electron microscopy of catalysts: the missing cornerstone in heterogeneous catalysis research? Chem. Rev. 123, 13374–13418 (2023).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  4. Chenna, S., Banerjee, R. & Crozier, P. A. Atomic-scale observation of the Ni activation process for partial oxidation of methane using in situ environmental TEM. ChemCatChem 3, 1051–1059 (2011).

    Article  CAS  Google Scholar 

  5. Zeng, L., Cheng, Z., Fan, J. A., Fan, L. S. & Gong, J. Metal oxide redox chemistry for chemical looping processes. Nat. Rev. Chem. 2, 349–364 (2018).

    Article  CAS  Google Scholar 

  6. Wei, S., Ma, Y. & Raabe, D. One step from oxides to sustainable bulk alloys. Nature 633, 816–822 (2024).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  7. Kim, J. Y., Rodriguez, J. A., Hanson, J. C., Frenkel, A. I. & Lee, P. L. Reduction of CuO and Cu2O with H2: H embedding and kinetic effects in the formation of suboxides. J. Am. Chem. Soc. 125, 10684–10692 (2003).

    Article  CAS  PubMed  Google Scholar 

  8. Wang, X., Hanson, J. C., Frenkel, A. I., Kim, J.-Y. & Rodriguez, J. A. Time-resolved studies for the mechanism of reduction of copper oxides with carbon monoxide: complex behavior of lattice oxygen and the formation of suboxides. J. Phys. Chem. B 108, 13667–13673 (2004).

    Article  CAS  Google Scholar 

  9. Rodriguez, J. A., Hanson, J. C., Frenkel, A. I., Kim, J. Y. & Pérez, M. Experimental and theoretical studies on the reaction of H2 with NiO: Role of O vacancies and mechanism for oxide reduction. J. Am. Chem. Soc. 124, 346–354 (2002).

    Article  CAS  PubMed  Google Scholar 

  10. Luo, L. et al. Atomic origins of water-vapour-promoted alloy oxidation. Nat. Mater. 17, 514–518 (2018).

    Article  CAS  PubMed  Google Scholar 

  11. Sun, X. et al. Dislocation-induced stop-and-go kinetics of interfacial transformations. Nature 607, 708–713 (2022).

    Article  CAS  PubMed  Google Scholar 

  12. Zou, L., Li, J., Zakharov, D. N., Stach, E. A. & Zhou, G. In situ atomic-scale imaging of the metal/oxide interfacial transformation. Nat. Commun. 8, 307 (2017).

    Article  PubMed  PubMed Central  Google Scholar 

  13. Yuan, W. et al. Visualizing H2O molecules reacting at TiO2 active sites with transmission electron microscopy. Science 367, 428–430 (2020).

    Article  CAS  PubMed  Google Scholar 

  14. Lagrow, A. P., Ward, M. R., Lloyd, D. C., Gai, P. L. & Boyes, E. D. Visualizing the Cu/Cu2O interface transition in nanoparticles with environmental scanning transmission electron microscopy. J. Am. Chem. Soc. 139, 179–185 (2017).

    Article  CAS  PubMed  Google Scholar 

  15. Sun, X. et al. Atomic origin of the autocatalytic reduction of monoclinic CuO in a hydrogen atmosphere. J. Phys. Chem. Lett. 12, 9547–9556 (2021).

    Article  CAS  PubMed  Google Scholar 

  16. Frey, H., Beck, A., Huang, X., van Bokhoven, J. A. & Willinger, M. G. Dynamic interplay between metal nanoparticles and oxide support under redox conditions. Science 376, 4–8 (2022).

    Article  Google Scholar 

  17. Rukini, A., Rhamdhani, M. A., Brooks, G. A. & Van den Bulck, A. Metals production and metal oxides reduction using hydrogen: a review. J. Sustain. Metall. 8, 1–24 (2022).

    Article  Google Scholar 

  18. Chen, J. & Hayes, P. C. Mechanisms and kinetics of reduction of solid NiO in CO/CO2 and CO/Ar gas mixtures. Metall. Mater. Trans. B 50, 2623–2635 (2019).

    Article  CAS  Google Scholar 

  19. Krasuk, J. H. & Smith, J. M. Kinetics of reduction of nickel oxide with CO. AIChE J. 18, 506–512 (1972).

    Article  CAS  Google Scholar 

  20. Antola, O., Holappa, L. & Paschen, P. Nickel ore reduction by hydrogen and carbon monoxide containing gases. Miner. Process. Extr. Metall. Rev. 15, 169–179 (1995).

    Article  CAS  Google Scholar 

  21. Scholz, J. J. & Langell, M. A. Kinetic analysis of surface reduction in transition metal oxide single crystals. Surf. Sci. 164, 543–557 (1985).

    Article  CAS  Google Scholar 

  22. Wang, J. et al. Effect of the chemical states of copper on methanol decomposition and oxidation. J. Phys. Chem. C 128, 4559–4572 (2024).

    Article  CAS  Google Scholar 

  23. Swallow, J. E. N. et al. Revealing the role of CO during CO2 hydrogenation on Cu surfaces with in situ soft X-ray spectroscopy. J. Am. Chem. Soc. 145, 6730–6740 (2023).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  24. Peck, M. A. & Langell, M. A. Comparison of nanoscaled and bulk NiO structural and environmental characteristics by XRD, XAFS, and XPS. Chem. Mater. 24, 4483–4490 (2012).

    Article  CAS  Google Scholar 

  25. Furstenau, R. P., McDougall, G. & Langell, M. A. Initial stages of hydrogen reduction of NiO(100). Surf. Sci. 150, 55–79 (1985).

    Article  CAS  Google Scholar 

  26. Norby, T. Protonic defects in oxides and their possible role in high temperature oxidation. J. Phys. IV 3, C9-99–C9-106 (1993).

    Google Scholar 

  27. Li, S., Ding, W., Meitzner, G. D. & Iglesia, E. Spectroscopic and transient kinetic studies of site requirements in iron-catalyzed Fischer–Tropsch synthesis. J. Phys. Chem. B 106, 85–91 (2002).

    Article  CAS  Google Scholar 

  28. Janbroers, S., Crozier, P. A., Zandbergen, H. W. & Kooyman, P. J. A model study on the carburization process of iron-based Fischer–Tropsch catalysts using in situ TEM–EELS. Appl. Catal. B 102, 521–527 (2011).

    Article  CAS  Google Scholar 

  29. Andersson, D. A., Simak, S. I., Skorodumova, N. V., Abrikosov, I. A. & Johansson, B. Optimization of ionic conductivity in doped ceria. Proc. Natl Acad. Sci. USA 103, 3518–3521 (2006).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  30. Matsubu, J. C. et al. Adsorbate-mediated strong metal-support interactions in oxide-supported Rh catalysts. Nat. Chem. 9, 120–127 (2017).

    Article  CAS  PubMed  Google Scholar 

  31. Sun, X. et al. Atomic‐scale mechanism of unidirectional oxide growth. Adv. Funct. Mater. 30, 1906504 (2020).

    Article  CAS  Google Scholar 

  32. Boyes, E. D. & Gai, P. L. Environmental high resolution electron microscopy and applications to chemical science. Ultramicroscopy 67, 219–232 (1997).

    Article  CAS  Google Scholar 

  33. Gai, P. L. et al. Atomic-resolution environmental transmission electron microscopy for probing gas-solid reactions in heterogeneous catalysis. MRS Bull. 32, 1044–1050 (2007).

    Article  CAS  Google Scholar 

  34. Gai, P. L., Lari, L., Ward, M. R. & Boyes, E. D. Visualisation of single atom dynamics and their role in nanocatalysts under controlled reaction environments. Chem. Phys. Lett. 592, 355–359 (2014).

    Article  CAS  Google Scholar 

  35. LaGrow, A. P., Lloyd, D. C., Gai, P. L. & Boyes, E. D. In situ scanning transmission electron microscopy of Ni nanoparticle redispersion via the reduction of hollow NiO. Chem. Mater. 30, 197–203 (2018).

    Article  CAS  Google Scholar 

  36. Helveg, S. et al. Atomic-scale imaging of carbon nanofibre growth. Nature 427, 426–429 (2004).

    Article  CAS  PubMed  Google Scholar 

  37. Yoshida, H. et al. Visualizing gas molecules interacting with supported nanoparticulate catalysts at reaction conditions. Science 335, 317–319 (2012).

    Article  CAS  PubMed  Google Scholar 

  38. Xie, D. G. et al. In situ study of the initiation of hydrogen bubbles at the aluminium metal/oxide interface. Nat. Mater. 14, 899–903 (2015).

    Article  CAS  PubMed  Google Scholar 

  39. Leapman, R. D., Grunes, L. A. & Fejes, P. L. Study of the L23 edges in the 3d transition metals and their oxides by electron-energy-loss spectroscopy with comparisons with theory. Phys. Rev. B 26, 614–635 (1982).

    Article  CAS  Google Scholar 

  40. Sparrow, T. G., Williams, B. G., Rao, C. N. R. & Thomas, J. M. L3/L2 white-line intensity ratios in the electron energy-loss spectra of 3d transition-metal oxides. Chem. Phys. Lett. 108, 547–550 (1984).

    Article  CAS  Google Scholar 

  41. Grosvenor, A. P., Biesinger, M. C., Smart, R. S. C. & McIntyre, N. S. New interpretations of XPS spectra of nickel metal and oxides. Surf. Sci. 600, 1771–1779 (2006).

    Article  CAS  Google Scholar 

  42. Carley, A. F., Jackson, S. D., O’Shea, J. N. & Roberts, M. W. The formation and characterisation of Ni3+—an X-ray photoelectron spectroscopic investigation of potassium-doped Ni (110)–O. Surf. Sci. 440, L868–L874 (1999).

    Article  CAS  Google Scholar 

  43. McIntyre, N. S. & Zetaruk, D. G. X-ray photoelectron spectroscopic studies of iron oxides. Anal. Chem. 49, 1521–1529 (1977).

    Article  CAS  Google Scholar 

  44. Zhao, X. et al. Multiple metal-nitrogen bonds synergistically boosting the activity and durability of high-entropy alloy electrocatalysts. J. Am. Chem. Soc. 146, 3010–3022 (2024).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  45. Anisimov, V. I., Zaanen, J. & Andersen, O. K. Band theory and Mott insulators: Hubbard U instead of Stoner I. Phys. Rev. B 44, 943 (1991).

    Article  CAS  Google Scholar 

  46. Kresse, G. & Furthmüler, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169–11186 (1996).

    Article  CAS  Google Scholar 

  47. Kresse, G. & Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 15–50 (1996).

    Article  CAS  Google Scholar 

  48. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).

    Article  CAS  PubMed  Google Scholar 

  49. Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59, 1758 (1999).

    Article  CAS  Google Scholar 

  50. Xu, Q., Cheah, S. & Zhao, Y. Initial reduction of the NiO(100) surface in hydrogen. J. Chem. Phys. 139, 024704 (2013).

    Article  PubMed  Google Scholar 

  51. Ferrari, A. M., Pisani, C., Cinquini, F., Giordano, L. & Pacchioni, G. Cationic and anionic vacancies on the NiO(100) surface: DFT + U and hybrid functional density functional theory calculations. J. Chem. Phys. 127, 174711 (2007).

    Article  PubMed  Google Scholar 

  52. Jeon, J., Yu, B. D. & Hyun, S. Adsorption properties of transition metal atoms on strongly correlated NiO(001) surfaces with surface oxygen vacancies. Curr. Appl. Phys. 15, 679–682 (2015).

    Article  Google Scholar 

  53. Silvi, B. & Savin, A. Classification of chemical bonds based on topological analysis of electron localization functions. Nature 371, 683–686 (1994).

    Article  CAS  Google Scholar 

  54. Jónsson, H., Mills, G. & Jacobsen, K. W. in Classical and Quantum Dynamics in Condensed Phase Simulations (eds Berne, B. J. et al.) 385–404 (World Scientific, 1998).

  55. He, Y., Dulub, O., Cheng, H., Selloni, A. & Diebold, U. Evidence for the predominance of subsurface defects on reduced anatase TiO2(101). Phys. Rev. Lett. 102, 106105 (2009).

    Article  PubMed  Google Scholar 

  56. Yu, J., Rosso, K. M. & Bruemmer, S. M. Charge and ion transport in NiO and aspects of Ni oxidation from first principles. J. Phys. Chem. C 116, 1948–1954 (2012).

    Article  CAS  Google Scholar 

  57. Wagner Jr, J. B. in Defects and Transport in Oxides (eds Seltzer, M. S. & Jaffee, R. I.) 283–301 (Springer, 1974).

  58. Malyshev, O. B. & Middleman, K. J. In situ ultrahigh vacuum residual gas analyzer ‘calibration’. J. Vac. Sci. Technol. A 26, 1474–1479 (2008).

    Article  CAS  Google Scholar 

Download references

Acknowledgements

This work was supported by the US National Science Foundation (Grant No. DMR 2303712). The computational modelling work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (Award No. DE-SC0001135). This research used the Electron Microscopy, Proximal Probes, and Theory and Computation resources of the Center for Functional Nanomaterials and beamline 7-BM (QAS) at the National Synchrotron Light Source II, which are US DOE Office of Science User Facilities, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Work at the beamline was supported in part by the Synchrotron Catalysis Consortium, US Department of Energy (Grant No. DE-SC0012335).

Author information

Author notes

  1. These authors contributed equally: Xiaobo Chen, Jianyu Wang

Authors and Affiliations

  1. Department of Mechanical Engineering & Materials Science and Engineering Program, State University of New York at Binghamton, Binghamton, NY, USA

    Xiaobo Chen, Jianyu Wang, Shyam Bharatkumar Patel, Shuonan Ye, Yupeng Wu, Zhikang Zhou, Linna Qiao & Guangwen Zhou

  2. Materials Science and Chemical Engineering Department, Stony Brook University, Stony Brook, NY, USA

    Yuxi Wang

  3. Department of Chemical Engineering, Columbia University, New York, NY, USA

    Nebojsa Marinkovic

  4. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, USA

    Meng Li, Sooyeon Hwang, Dmitri N. Zakharov, Qin Wu, Jorge Anibal Boscoboinik & Judith C. Yang

  5. National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY, USA

    Lu Ma

Authors
  1. Xiaobo Chen
    View author publications

    Search author on:PubMed Google Scholar

  2. Jianyu Wang
    View author publications

    Search author on:PubMed Google Scholar

  3. Shyam Bharatkumar Patel
    View author publications

    Search author on:PubMed Google Scholar

  4. Shuonan Ye
    View author publications

    Search author on:PubMed Google Scholar

  5. Yupeng Wu
    View author publications

    Search author on:PubMed Google Scholar

  6. Zhikang Zhou
    View author publications

    Search author on:PubMed Google Scholar

  7. Linna Qiao
    View author publications

    Search author on:PubMed Google Scholar

  8. Yuxi Wang
    View author publications

    Search author on:PubMed Google Scholar

  9. Nebojsa Marinkovic
    View author publications

    Search author on:PubMed Google Scholar

  10. Meng Li
    View author publications

    Search author on:PubMed Google Scholar

  11. Sooyeon Hwang
    View author publications

    Search author on:PubMed Google Scholar

  12. Dmitri N. Zakharov
    View author publications

    Search author on:PubMed Google Scholar

  13. Lu Ma
    View author publications

    Search author on:PubMed Google Scholar

  14. Qin Wu
    View author publications

    Search author on:PubMed Google Scholar

  15. Jorge Anibal Boscoboinik
    View author publications

    Search author on:PubMed Google Scholar

  16. Judith C. Yang
    View author publications

    Search author on:PubMed Google Scholar

  17. Guangwen Zhou
    View author publications

    Search author on:PubMed Google Scholar

Contributions

G.Z. and X.C. conceived the idea and designed the in situ TEM experiments. X.C. performed the in situ TEM and ex situ TEM experiments. J.W., Z.Z., X.C., Q.W. and G.Z. designed and performed the DFT calculations. S.B.P. and J.A.B. conducted the NAP-XPS experiments. X.C., Y. Wang, N.M. and L.M. conducted the in situ XRD and RGA experiments. X.C., S.Y., Y. Wu, L.Q., M.L., S.H., D.N.Z., J.C.Y. and G.Z. carried out the data analysis. X.C. and G.Z. wrote the paper, and all authors reviewed and approved the final version. This work was done under the supervision of G.Z.

Corresponding author

Correspondence to Guangwen Zhou.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature thanks Hicham Idriss, Patricia Kooyman and Gianfranco Pacchioni for their contribution to the peer review of this work. Peer reviewer reports are available.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 DFT modelling of CO and H2 adsorption on NiO(100) and H2 dissociation at NiO(100).

a, CO adsorption on perfect NiO(100), NiO(100) with either Ni vacancies (VNi) or O vacancies (VO), and step edges containing either VNi or VO. b, CO adsorption at the top, O and Ni sites of the interfacial periphery of a Ni cluster on NiO(100), as well as on flat Ni(100). c, H2 adsorption on perfect NiO(100), NiO(100) terraces containing either VNi or VO, and step edges containing either VNi or VO. d, Heterolytic dissociation of H2 on perfect NiO(100) and along intact step edges, resulting in the formation of NiH and OH species. e, H2 adsorbed at VO-containing terraces and step edges undergoes heterolytic dissociation, forming NiH and OH species. f, H2 adsorbed at the VNi-containing NiO(100) terrace dissociates heterolytically to produce NiH and OH, while H2 at the VNi site of the step edge undergoes spontaneous homolytic splitting, forming two OH groups.

Extended Data Fig. 2 DFT calculations of diffusion energy barriers, adsorption energies and relaxed structures.

a, Diffusion barrier of a Ni adatom on NiO(100). b, Adsorption energies of a Ni adatom on perfect NiO(100), along monoatomic step edge, double-layer step edge, and triple-layer step edge. c, Energy barrier for Ni vacancy migration in the NiO bulk. c, DFT-calculated barrier (1.79 eV) for the migration of Ni to VNi in the NiO bulk. d, DFT-calculated barrier (1.70 eV) for the migration of an O vacancy in the topmost NiO surface layer. e, DFT-calculated barriers and comparison with literature data10,56,57. f, DFT-relaxed structures as a function of O vacancy concentrations in the NiO lattice. The metallization process involves significant Ni diffusion in the highly O-deficient lattice. The resulting nearest Ni-Ni distance of 2.58 Å closely matches the nearest Ni-Ni interatomic distance of 2.46 Å in face-center cubic (FCC) Ni. In a H2 atmosphere, the NiO lattice is primarily dominated by O vacancies, making the diffusion of Ni cations less likely due to the limited availability of Ni vacancies. Additionally, Ni cations are unlikely to migrate to O vacancy sites because of electrostatic repulsion, as these vacancies are surrounded by positively charged Ni ions, making such migration energetically unfavorable. However, as the concentration of O vacancies increases, the NiO lattice becomes increasingly unstable, eventually breaking down into metallic Ni. This metallization process involves significant Ni diffusion in the highly O-deficient lattice.

Extended Data Fig. 3 In-situ TEM imaging of NiO reduction in CO and H2.

a, b, HRTEM images and corresponding diffractograms of selected regions (marked by dashed blue squares), along with simulated electron diffraction patterns, illustrating the reduction of NiO to Ni islands under CO at 400 °C and pressures of ~0.03 Pa (a) and ~1 Pa (b). Moiré fringes in the HRTEM images arise from the overlapping lattices of Ni (FCC) and NiO (rock-salt). The electron diffraction simulations model the cube-on-cube orientation relationship between Ni and NiO, along the [001] zone axis in (a) and the \([01\bar{1}]\) zone axis (b). Red and black spots in the simulated patterns correspond to diffraction spots from NiO and Ni, respectively. The alignment of experimental diffractograms with simulations confirms the crystallographic overlap of Ni and NiO as the source of the Moiré contrast. Simulations of overlapping NiO grains produce distinct patterns that do not match the experimental diffractograms, ruling out alternative origins for the Moiré fringes. c, Time-sequence HRTEM images (Supplementary Video 5) illustrating the internal metallization process, leading to the transformation of NiO to Ni in the bulk at 400 °C and ~0.04 Pa H2. The white dashed lines outline the NiO/Ni interface, demonstrating the outward propagation of the NiO/Ni interface toward the NiO region, resulting in the complete NiO reduction to Ni from the bulk toward the surface. d, Time-sequence HRTEM images viewed along the \([01\bar{1}]\) zone axis, illustrating NiO growth along the top-left stepped region via the re-oxidation of Ni adatoms by lattice O in NiO, and the formation of vacancy clusters in the bulk during the NiO reduction at 400 °C and ≈ 0.04 Pa H2. The white dashed lines outline the initial surface profile of the NiO surface at 0 s, showing the receding motion of the atomic step on the top and bottom-left surface facets, with net NiO growth along the stepped region at the top-left corner. The dashed circles mark the clustering of O vacancies generated during the NiO reduction. e, Real-time HRTEM images (Supplementary Video 6) using low electron flux (~7 × 104 e/nm2 s), depicting NiO reduction processes at 400 °C in ~1 × 10−3 Pa H2, starting from surface decay, followed by vacancy clustering (dashed white cycles) in the bulk, and ultimately leading to the collapse of the NiO lattice. Dashed red lines are the trace of the position and configuration of the outermost surface of the NiO at 0 s, showing the collapse of the NiO volume shrinkage. f, Real-time TEM imaging using low electron flux (~2 × 103 e/nm2 s), showing the dynamic formation of vacancy clusters (marked by white dashed circles) in the bulk, followed by the collapse of the oxide structure during NiO reduction at 400 °C and ~1.33 Pa H2. All the HRTEM images and corresponding diffractograms are presented with pseudo-color overlays.

Extended Data Fig. 4 DFT calculations of O vacancy formation energies.

a, Comparison of O vacancy formation energies for placing a single O vacancy (VO), first in the topmost surface layer (1st), and subsequently in the 2nd, 3rd, and 4th subsurface layers, respectively, show the more favorable VO formation in the 2nd than the 1st and deeper layers, consistent with the trends in the reported studies50,55. b, DFT relaxed structures of the VO-containing atomic layer shown in (a), with measured intralayer Ni-VO distances (left panel) and interlayer Ni-VO distances (right). The VO in the topmost layer exhibits the largest intralayer Ni-VO distance (2.06 Å), compared to those (2.02 Å and 2.03 Å) in the 2nd and 3rd layers. The VO in the second layer leads to the downward relaxation of the Ni atom in the top layer (directly above the VO), resulting in a shorter interlayer Ni-VO distance (1.96 Å) compared to those for the VO in the 1st and 3rd layers. c, Electron localization function (ELF) maps of the VO-containing atomic layers, showing that the VO in the topmost surface layer exhibits the highest electron localization, while the VO in the second layer shows the most electron delocalization. d, DFT calculated PDOS for Ni 3d and O 2p orbitals in the topmost four atomic layers of NiO, including perfect NiO, NiO with the presence of a single VO formed either in the topmost layer, second layer, or third layer, respectively. The vertical dashed lines indicate the Fermi level. e, f, Electron numbers obtained by integrating the PDOS of Ni 3d and O 2p orbitals over the entire energy range (d), illustrating that the Ni atom in the 1st layer, directly above the VO in the 2nd layer, experiences the largest increase in the electron population of the Ni 3d orbital. VO1, VO2, and VO3 represent the presence of a single VO formed either in the 1st, 2nd, or 3rd layers, respectively. The different colors correspond to the Ni and O atoms in the top four atomic layers. The dashed lines in (e) and (f) represent reference values for the electron numbers in the 1st layer of the perfect NiO structure, providing a baseline for comparison.

Extended Data Fig. 5 In-situ TEM imaging of vacancy cluster formation in NiO under H2 and their self-healing upon switching to O2.

a, Real-time HRTEM images (Supplementary Video 7) showing the formation of vacancy clusters during NiO reduction at 450 °C and ~0.03 Pa H2. b, Time-sequence HRTEM images (Supplementary Video 8) illustrating the self-healing of the vacancy cluster regions after switching to ~0.03 Pa O2 at 450 °C. c, d, Zoom-in HRTEM views of the regions marked by the dashed white boxes in (a, b), highlighting the formation and dynamic motion of vacancy clusters (marked by dashed red circles) during H2 exposure, as well as the healing process of the vacancy clusters upon exposure to O2. t0 denotes the start of the imaging sequence. The white arrow in (a) highlights the newly formed NiO in the highly stepped surface regions. Dashed red lines track the initial surface profiles of NiO at t0 and their corresponding overlays into the following snapshots in (a, b). e, Comparison of experimental and simulated HRTEM images using a structure model of an O vacancy cluster in the NiO lattice, preserving the integrity of the Ni sublattice. The dashed red circles highlight the regions with brighter image contrast of the atom columns. Inset intensity profiles (marked by white dashed rectangles in (e)) show a brighter image contrast for the atom columns in the O vacancy cluster region in the simulated HRTEM image, matching the contrast observed in the experimental HRTEM image. These in-situ images capture the dynamic contrast changes in the same sample regions during reduction in H2, where progressive brightening of atom columns correlates with the accumulation of oxygen vacancies, and during oxidation in O2, where the annihilation of O vacancies leads to the dimming of the atom column contrast to baseline intensity. While static HRTEM images alone are insufficient to resolve this, the time-resolved image sequence provides clear evidence of oxygen vacancy dynamics.

Extended Data Fig. 6 Ex-situ HAADF-STEM characterizations of furnace-reduced NiO samples and in-situ EELS measurements of NiO reduction in H2 and CO.

a-c, Ex-situ HAADF-STEM images (with pseudo-color overlays) of NiO reduced at 400 °C and ~0.67 Pa H2 for 60 min, revealing a porous morphology and weak-contrast regions indicative of O vacancy clusters (highlighted by dotted red circles). d-f, Ex-situ HAADF-STEM images (with pseudo-color overlays) of NiO reduced at 400 °C and ~0.67 Pa CO for 60 min, showing a more compact morphology and the absence of weak-contrast regions typically associated with O-vacancy clusters in the crystal lattice. g-l, In-situ HAADF images and corresponding O-K and Ni-L2,3 edges collected from locations 1–5 within the same region of the NiO sample at 400 °C, first in ≈ 0.67 Pa O2 (g-i), and then after switching to ~0.67 Pa H2 (j-l). m-r, In-situ HAADF images and corresponding O-K and Ni-L2,3 edge obtained from spots 1–5 within the same region of the NiO sample at 400 °C, first in the O2 atmosphere at ≈ 0.67 Pa O2 (m-o), followed by switching to a CO atmosphere at ~0.67 Pa CO (p-r). The L3/L2 ratios of the Ni L edge are included in i, l, o and r.

Extended Data Fig. 7 In-situ XRD and RGA measurements of NiO reduction in CO and H2.

a-c, TEM characterizations of the pristine NiO nanoparticles (Johnson Matthey & Co. Ltd) reveal their morphology, crystal structure, and atomic-scale arrangements. d, XRD analysis of the pristine NiO powder confirms its crystal structure and indicates a crystallite size of approximately 270 nm. e, f, Evolution of the partial pressures of He, CO, CO2 during NiO reduction in CO, and He, H2, H2O during NiO reduction in H2, respectively. These RGA spectra are acquired concurrently with the XRD data in Fig. 3(a, b). g, The Clausen cell setup for XRD and RGA measurements. The RGA measurements of partial pressures for the product gases obtained with this setup may be influenced by several factors, including the small sample loading (2 ~ 3 mg), compaction level of the powder samples, gas pressure build-up in the glass tube, and variations in total gas pressure between different experiments. In particular, the higher sticking probability of H2O molecules to the walls of the long gas tube, compared to CO2 molecules, may lead to an underestimation of actual H2O production. h, i, Evolution of the partial pressures of Ar, CO, CO2 during NiO reduction in CO, and Ar, H2, H2O during NiO reduction in H2 using a tube reactor, respectively. j, Calibrated partial pressures of CO2 and H2O measured during NiO reduction in CO (h) and H2 (i)58. The inset in (h) illustrates the tube reactor used for the RGA measurements in (h, i). This setup allows for larger sample loads (~30 mg), controlled gas pressures, and a short distance between the reactor and the RGA sensor. k-m, Real-time XRD and RGA measurements using the setup in (g), showing NiO reduction in 5% CO/He, initially at 500 °C, followed by an increase in temperature to 600 °C, leading to further NiO reduction and the formation of a thicker metallic Ni overlayer (k). l, Normalized intensity evolution of the NiO(111) reflection during NiO reduction in CO (k). Coordinated RGA measurements (m) show the evolution of the partial pressures of CO and CO2, first at 500 °C and then at 600 °C, with the blue boxes highlighting the self-limiting production of CO2 at each temperature.

Extended Data Fig. 8 DFT predicted H adsorption configurations, H diffusion energy barriers, and Barder charges of Ni ions upon H+ adsorption.

a, Decrease in H adsorption energy by 0.48 eV as the adsorption site transitions from Ni to O. b, Initial and final states (IS and FS) of the adsorption configurations of H adsorbed to lattice O and Ni, respectively, in the NiO bulk, showing stable adsorption at the O site and the spontaneous transfer of H from the initially adsorbed Ni site to the adjacent O site. c, DFT-calculated energy barrier for the diffusion of interstitial H in NiO bulk, indicating the ease with which protons can infiltrate the NiO lattice, characterized by a small barrier of 0.62 eV. d, Bader charges of surface Ni ions surrounding the lattice O with an adsorbed H+ atop. e, Bader charges of Ni ions surrounding the H+ adsorbed at a tetrahedral site in the NiO bulk. The black circle highlights the Ni ion nearest to the adsorbed H+, which shows the largest electron transfer from the proton.

Extended Data Fig. 9 In-situ formation of a NiO overlayer on Ni thin film.

a-c, In situ TEM images and corresponding electron diffraction patterns of (a) air-formed native oxide on as-prepared Ni thin film, (b) complete removal of the native oxide by reducing the Ni thin film at 500 °C in ~0.67 Pa H2, and (c) formation of a NiO overlayer by oxidizing the clean Ni thin film at 400 °C in ~0.3 Pa O2, respectively. d, In-situ HRTEM images showing NiO growth during the exposure of Ni to ~0.3 Pa O2 at 400 °C.

Extended Data Fig. 10 “Blank beam” experiments for NiO reduction in CO and H2.

a, HRTEM snapshots of NiO before and after reduction at 400 °C in ~0.67 Pa CO, showing the formation of Ni islands (marked by dashed cyan circles). b, HRTEM snapshots of NiO before and after reduction at 400 °C in ~0.03 Pa H2. Dashed white lines represent the position and configuration of the outermost surface of the NiO at 0 s, showing NiO growth in the top-left corner region, while NiO shrinkage is observed in the left and top surface regions, with no surface metallization. The e-beam was blanked during CO and H2 exposure to minimize e-beam effects in (a, b). c, HRTEM snapshots of NiO at 400 °C in ~0.03 Pa H2, demonstrating the absence of surface metallization and presence of vacancy clusters (marked by dashed red circles) in the bulk, and NiO at 400 °C in ~0.67 Pa CO, revealing the presence of a large number of Ni islands (marked by dashed cyan circles), with the e-beam blanked. The inset images in (a, c) are the corresponding diffractograms of the HRTEM images, illustrating the formation of metallic Ni islands on NiO surfaces. d, HRTEM images of as-prepared NiO from O2 exposure at 400 °C and 450 °C, respectively. The same surface areas after vacuum annealing at 400 °C for 180 s, and at 450 °C for 515 s, respectively. In-situ HRTEM imaging confirms the high stability of NiO under vacuum annealing and continuous electron beam irradiation (e-beam dose rate: ~3.2 × 105 e/nm2 s), and that the NiO reduction observed in CO and H2 is induced by the reducing gases.

Supplementary information

Supplementary Information

Supplementary Figs. 1–5 and Table 1.

Peer Review File

Supplementary Video 1b

In situ TEM imaging of NiO reduction at 400 °C in approximately 0.03 Pa CO, viewed along the NiO [001] zone axis (Fig. 1a).

Supplementary Video 2b

In situ TEM imaging of NiO reduction at 400 °C in approximately 1 Pa CO, viewed along the NiO\([01\bar{1}]\) zone axis (Fig. 1c).

Supplementary Video 3

In situ TEM imaging of NiO reduction at 400 °C in approximately 0.016 Pa H2, viewed along the NiO [001] zone axis (Fig. 2a).

Supplementary Video 4

In situ TEM imaging of NiO reduction at 400 °C in approximately 0.08 Pa H2, viewed along the NiO [001] zone axis (Fig. 2c).

Supplementary Video 5

In situ TEM imaging of internal metallization during NiO reduction at 400 °C and approximately 0.04 Pa H2 (Extended Data Fig. 3a).

Supplementary Video 6

In situ TEM imaging of NiO reduction at 400 °C in approximately 1 × 10−3 Pa H2, viewed along the NiO [001] zone axis under a low electron dose rate (Extended Data Fig. 3e).

Supplementary Video 7

In situ TEM imaging of vacancy cluster formation in NiO under approximately 0.03 Pa H2 at 450 °C (Extended Data Fig. 5a,c).

Supplementary Video 8

In situ TEM imaging of the self-healing of vacancy clusters formed in NiO under approximately 0.03 Pa O2 at 450 °C (Extended Data Fig. 5b,d).

Source data

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Chen, X., Wang, J., Patel, S.B. et al. Atomic dynamics of gas-dependent oxide reducibility. Nature 644, 927–932 (2025). https://doi.org/10.1038/s41586-025-09394-0

Download citation

  • Received:

  • Accepted:

  • Published:

  • Version of record:

  • Issue date:

  • DOI: https://doi.org/10.1038/s41586-025-09394-0

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing