Supported single atoms provide an opportunity to design new heterogeneous catalysts while optimizing the utilization of noble metals. However, identification of the active single-atom structure is required for understanding the reaction mechanism and guiding catalyst design. Here, we use in situ infrared spectroscopy, operando X-ray absorption spectroscopy and quantum chemical calculations to identify the active single-atom complex as well as the resting state of the Ir/MgAl2O4 catalysts during the low-temperature CO oxidation. In contrast to poisoning of iridium nanoparticles by CO, here we show that the formation of Ir(CO) on single atoms results in a different reaction mechanism and high activity for low-temperature CO oxidation. This is due to the ability of single atoms to coordinate with multiple ligands, where Ir(CO) provides an interfacial site for facile O2 activation between Ir and Al and lowers the reaction barrier between gas-phase CO(g) and *O in Ir(CO)(O) through an Eley–Rideal mechanism.

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  1. 1.

    Oh, S. H. & Sinkevitch, R. M. Carbon monoxide removal from hydrogen-rich fuel cell feedstreams by selective catalytic oxidation. J. Catal. 142, 254–262 (1993).

  2. 2.

    Alayoglu, S., Nilekar, A. U., Mavrikakis, M. & Eichhorn, B. Ru–Pt core–shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen. Nat. Mater. 7, 333–338 (2008).

  3. 3.

    Allian, A. D. et al. Chemisorption of CO and mechanism of CO oxidation on supported platinum nanoclusters. J. Am. Chem. Soc. 133, 4498–4517 (2011).

  4. 4.

    Twigg, M. V. Progress and future challenges in controlling automotive exhaust gas emissions. Appl. Catal. B 70, 2–15 (2007).

  5. 5.

    Lin, J. et al. Remarkable performance of Ir1/FeOx single-atom catalyst in water gas shift reaction. J. Am. Chem. Soc. 135, 15314–15317 (2013).

  6. 6.

    Yang, X. F. et al. Single-atom catalysts: a new frontier in heterogeneous catalysis. Acc. Chem. Res. 46, 1740–1748 (2013).

  7. 7.

    Lu, J., Serna, P., Aydin, C., Browning, N. D. & Gates, B. C. Supported molecular iridium catalysts: resolving effects of metal nuclearity and supports as ligands. J. Am. Chem. Soc. 133, 16186–16195 (2011).

  8. 8.

    Kyriakou, G. et al. Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations. Science 335, 1209–1212 (2012).

  9. 9.

    Liu, J. Catalysis by supported single metal atoms. ACS Catal. 7, 34–59 (2017).

  10. 10.

    Shan, J., Li, M., Allard, L. F., Lee, S. & Flytzani-Stephanopoulos, M. Mild oxidation of methane to methanol or acetic acid on supported isolated rhodium catalysts. Nature 551, 605–608 (2017).

  11. 11.

    Cui, X., Li, W., Ryabchuk, P., Junge, K. & Beller, M. Bridging homogeneous and heterogeneous catalysis by heterogeneous single-metal-site catalysts. Nat. Catal. 1, 385–397 (2018).

  12. 12.

    Qiao, B. T. et al. Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat. Chem. 3, 634–641 (2011).

  13. 13.

    Moses-DeBusk, M. et al. CO oxidation on supported single Pt atoms: experimental and ab initio density functional studies of CO interaction with Pt atom on theta-Al2O3(010) surface. J. Am. Chem. Soc. 135, 12634–12645 (2013).

  14. 14.

    Kistler, J. D. et al. A single-site platinum CO oxidation catalyst in zeolite KLTL: microscopic and spectroscopic determination of the locations of the platinum atoms. Angew. Chem. Int. Ed. 53, 8904–8907 (2014).

  15. 15.

    Yang, M. et al. Catalytically active Au-O(OH)x-species stabilized by alkali ions on zeolites and mesoporous oxides. Science 346, 1498–1501 (2014).

  16. 16.

    Yang, M. et al. A common single-site Pt(II)-O(OH)x- species stabilized by sodium on “active” and “inert” supports catalyzes the water-gas shift reaction. J. Am. Chem. Soc. 137, 3470–3473 (2015).

  17. 17.

    Ding, K. et al. Identification of active sites in CO oxidation and water-gas shift over supported Pt catalysts. Science 350, 189–192 (2015).

  18. 18.

    Jones, J. et al. Thermally stable single-atom platinum-on-ceria catalysts via atom trapping. Science 353, 150–154 (2016).

  19. 19.

    Stephens, I. E. L., Elias, J. S. & Shao-Horn, Y. The importance of being together. Science 350, 164–165 (2015).

  20. 20.

    Nie, L. et al. Activation of surface lattice oxygen in single-atom Pt/CeO2 for low-temperature CO oxidation. Science 358, 1419–1423 (2017).

  21. 21.

    Luo, J. Y. et al. Advantages of MgAlOx over gamma-Al2O3 as a support material for potassium-based high-temperature lean NOx traps. ACS Catal. 5, 4680–4689 (2015).

  22. 22.

    Li, W.-Z. et al. A general mechanism for stabilizing the small sizes of precious metal nanoparticles on oxide supports. Chem. Mater. 26, 5475–5481 (2014).

  23. 23.

    Li, W.-Z. et al. Stable platinum nanoparticles on specific MgAl2O4 spinel facets at high temperatures in oxidizing atmospheres. Nat. Commun. 4, 2481 (2013).

  24. 24.

    Lin, J. et al. Design of a highly active Ir/Fe(OH)x catalyst: versatile application of Pt-group metals for the preferential oxidation of carbon monoxide. Angew. Chem. Int. Ed. 51, 2920–2924 (2012).

  25. 25.

    Lu, J., Aydin, C., Browning, N. D. & Gates, B. C. Oxide- and zeolite-supported isostructural ir(C2H4)2 complexes: molecular-level observations of electronic effects of supports as ligands. Langmuir 28, 12806–12815 (2012).

  26. 26.

    Hoffman, A. S. et al. High-energy-resolution X-ray absorption spectroscopy for identification of reactive surface species on supported single-site iridium catalysts. Chem. Eur. J. 23, 14760–14768 (2017).

  27. 27.

    Aydin, C., Lu, J., Browning, N. D. & Gates, B. C. A “smart” catalyst: sinter-resistant supported iridium clusters visualized with electron microscopy. Angew. Chem. Int. Ed. 51, 5929–5934 (2012).

  28. 28.

    Berlowitz, P. J., Peden, C. H. F. & Goodman, D. W. Kinetics of CO oxidation on single-crystal Pd, Pt, and Ir. J. Phys. Chem. 92, 5213–5221 (1988).

  29. 29.

    Cargnello, M. et al. Control of metal nanocrystal size reveals metal-support interface role for ceria catalysts. Science 341, 771–773 (2013).

  30. 30.

    Mihaylov, M. et al. New types of nonclassical iridium carbonyls formed in Ir-ZSM-5: a Fourier transform infrared spectroscopy investigation. J. Phys. Chem. B 110, 10383–10389 (2006).

  31. 31.

    Wovchko, E. A. & Yates, J. T. Activation of O2 on a photochemically generated RhI site on an Al2O3 surface: low-temperature O2 dissociation and CO oxidation. J. Am. Chem. Soc. 120, 10523–10527 (1998).

  32. 32.

    Abbet, S., Heiz, U., Hakkinen, H. & Landman, U. CO oxidation on a single Pd atom supported on magnesia. Phys. Rev. Lett. 86, 5950–5953 (2001).

  33. 33.

    Atkins, A. J., Bauer, M. & Jacob, C. R. High-resolution X-ray absorption spectroscopy of iron carbonyl complexes. Phys. Chem. Chem. Phys. 17, 13937–13948 (2015).

  34. 34.

    Boubnov, A. et al. Selective catalytic reduction of NO over Fe-ZSM-5: mechanistic insights by operando HERFD-XANES and valence-to-core X-ray emission spectroscopy. J. Am. Chem. Soc. 136, 13006–13015 (2014).

  35. 35.

    van Bokhoven, J. A. et al. Activation of oxygen on gold/alumina catalysts: in situ high-energy-resolution fluorescence and time-resolved X-ray spectroscopy. Angew. Chem. Int. Ed. 45, 4651–4654 (2006).

  36. 36.

    Safonova, O. V. et al. Identification of CO adsorption sites in supported Pt catalysts using high-energy-resolution fluorescence detection X-ray spectroscopy. J. Phys. Chem. B 110, 16162–16164 (2006).

  37. 37.

    Cai, Q. X., Wang, J. G., Wang, Y. & Mei, D. H. First-principles thermodynamics study of spinel MgAl2O4 surface stability. J. Phys. Chem. C 120, 19087–19096 (2016).

  38. 38.

    Ljungberg, M. P., Mortensen, J. J. & Pettersson, L. G. M. An implementation of core level spectroscopies in a real space projector augmented wave density functional theory code. J. Electron Spectrosc. 184, 427–439 (2011).

  39. 39.

    Lu, J., Serna, P. & Gates, B. C. Zeolite- and MgO-supported molecular iridium complexes: support and ligand effects in catalysis of ethene hydrogenation and H–D exchange in the conversion of H2 + D2. ACS Catal. 1, 1549–1561 (2011).

  40. 40.

    Yardimci, D., Serna, P. & Gates, B. C. Tuning catalytic selectivity: zeolite- and magnesium oxide-supported molecular rhodium catalysts for hydrogenation of 1,3-butadiene. ACS Catal. 2, 2100–2113 (2012).

  41. 41.

    Sokaras, D. et al. A seven-crystal Johann-type hard X-ray spectrometer at the Stanford Synchrotron Radiation Lightsource. Rev. Sci. Instrum. 84, 053102 (2013).

  42. 42.

    Ravel, B. & Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron. Radiat. 12, 537–541 (2005).

  43. 43.

    Newville, M. IFEFFIT: interactive XAFS analysis and FEFF fitting. J. Synchrotron. Radiat. 8, 322–324 (2001).

  44. 44.

    Brooks, C. S. Characterization of iridium catalyst surfaces by gas chemisorption. J. Colloid Interface Sci. 34, 419–427 (1970).

  45. 45.

    Bonet, F. et al. Kinetics of Heterogeneous Catalytic Reactions (Princeton Univ. Press, Princeton, NJ, 984).

  46. 46.

    Koros, R. M. & Nowak, E. J. A diagnostic test of the kinetic regime in a packed bed reactor. Chem. Eng. Sci. 22, 470 (1967).

  47. 47.

    Madon, R. J. & Boudart, M. Experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions. Ind. Eng. Chem. Fund. 21, 438–447 (1982).

  48. 48.

    Mozaffari, S. et al. Colloidal nanoparticle size control: experimental and kinetic modeling investigation of the ligand metal binding role in controlling the nucleation and growth kinetics. Nanoscale 9, 13772–13785 (2017).

  49. 49.

    Hutter, J., Iannuzzi, M., Schiffmann, F. & VandeVondele, J. CP2K: atomistic simulations of condensed matter systems. Wires Comput. Mol. Sci. 4, 15–25 (2014).

  50. 50.

    Paier, J., Hirschl, R., Marsman, M. & Kresse, G. The Perdew–Burke–Ernzerhof exchange-correlation functional applied to the G2-1 test set using a plane-wave basis set. J. Chem. Phys. 122, 234102 (2005).

  51. 51.

    Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 132, 154104 (2010).

  52. 52.

    Henkelman, G., Uberuaga, B. P. & Jónsson, H. A climbing image nudged elastic band method for finding saddle points and minimum energy paths. J. Chem. Phys. 113, 9901 (2000).

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This research was primarily sponsored by the Army Research Office and was accomplished under grant number W911NF-16-1-0400. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office or the US Government. The US Government holds copyright license rights specified under the aforementioned grant. Additional support by SABIC (Saudi Basic Industries Corporation) and by the US Department of Energy (DOE) Office of Basic Energy Sciences to the SUNCAT Center for Interface Science and Catalysis is acknowledged. Use of the Stanford Synchrotron Radiation Light Source (SSRL, beamlines 6-2, user proposal 4645), SLAC National Accelerator Laboratory is supported by the US Department of Energy, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515. STEM imaging was performed at the William R. Wiley Environmental Molecular Science Laboratory (EMSL) sponsored by the US Department of Energy, Office of Biological and Environmental Research located at Pacific Northwest National Laboratory (PNNL) under science theme proposal 49326. Computing time was awarded at EMSL under the same proposal. L.Y. and H.X. acknowledge the partial financial support from the American Chemical Society Petroleum Research Fund (ACS PRF 55581-DNI5) and computational support from the Advanced Research Computing group at Virginia Polytechnic Institute and State University.

Author information


  1. Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA

    • Yubing Lu
    • , Jiamin Wang
    • , Liang Yu
    • , Xiwen Zhang
    • , Hongliang Xin
    •  & Ayman M. Karim
  2. Pacific Northwest National Laboratory, Richland, WA, USA

    • Libor Kovarik
    •  & Vanessa Dagle
  3. Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

    • Adam S. Hoffman
    • , Simon R. Bare
    • , Dimosthenis Sokaras
    •  & Thomas Kroll
  4. Department of Chemical Engineering, Stanford University, Stanford, CA, USA

    • Alessandro Gallo
  5. SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

    • Alessandro Gallo


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Y.L. performed the synthesis, characterizations (including helping with HERFD) and catalytic tests and wrote the first draft of the paper. J.W. performed the DFT calculations for the reaction barriers and infrared frequencies. L.Y. performed the XANES DFT calculations. L.K. conducted the STEM analysis and contributed to the writing of the STEM section. X.Z. performed synthesis and catalytic reproducibility tests. A.S.H., A.G. and S.R.B. designed and performed the HERFD experiments and data analysis and contributed to writing the XAS section. D.S. and T.K. were responsible for optimizing the crystal optics, developing the scripts to allow the HERFD data to be collected, and monitoring the initial data quality. V.D. synthesized and characterized the 1% Ir/MgAl2O4 catalyst. H.X. designed and directed the computational part of the study and the writing of the DFT results. A.M.K. conceived the idea, and planned and directed the project. Y.L., H.X. and A.M.K. co-wrote the paper. All of the authors discussed the results and commented on the paper.

Competing interests

The authors declare no competing interests.

Corresponding authors

Correspondence to Hongliang Xin or Ayman M. Karim.

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