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Identification of the active complex for CO oxidation over single-atom Ir-on-MgAl2O4 catalysts


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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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

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.

Correspondence to Hongliang Xin or Ayman M. Karim.

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    Supplementary Discussion; Supplementary Figs 1-15; Supplementary Tables 1-10; Supplementary References

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Fig. 1: HAADF-STEM images of different Ir SAC and NP samples.
Fig. 2: CO oxidation kinetic measurements on Ir SAC, Ir NP and SAC+NP mixture.
Fig. 3: In situ infrared and operando HERFD-XANES spectra (experimental and DFT) of Ir single atoms.
Fig. 4: DFT-calculated free-energy diagram of CO oxidation on Ir single atoms supported on MgAl2O4.