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Functional CeOx nanoglues for robust atomically dispersed catalysts

Nature volume 611, pages 284–288 (2022)Cite this article

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Abstract

Single-atom catalysts1 make exceptionally efficient use of expensive noble metals and can bring out unique properties1,2,3. However, applications are usually compromised by limited catalyst stability, which is due to sintering3,4. Although sintering can be suppressed by anchoring the metal atoms to oxide supports1,5,6, strong metal–oxygen interactions often leave too few metal sites available for reactant binding and catalysis6,7, and when exposed to reducing conditions at sufficiently high temperatures, even oxide-anchored single-atom catalysts eventually sinter4,8,9. Here we show that the beneficial effects of anchoring can be enhanced by confining the atomically dispersed metal atoms on oxide nanoclusters or ‘nanoglues’, which themselves are dispersed and immobilized on a robust, high-surface-area support. We demonstrate the strategy by grafting isolated and defective CeOx nanoglue islands onto high-surface-area SiO2; the nanoglue islands then each host on average one Pt atom. We find that the Pt atoms remain dispersed under both oxidizing and reducing environments at high temperatures, and that the activated catalyst exhibits markedly increased activity for CO oxidation. We attribute the improved stability under reducing conditions to the support structure and the much stronger affinity of Pt atoms for CeOx than for SiO2, which ensures the Pt atoms can move but remain confined to their respective nanoglue islands. The strategy of using functional nanoglues to confine atomically dispersed metals and simultaneously enhance their reactivity is general, and we anticipate that it will take single-atom catalysts a step closer to practical applications.

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Fig. 1: Schematic diagrams illustrating the fabrication processes of functional CeOx nanoglue islands and CeOx/SiO2-supported Pt1 single-atom catalysts.
Fig. 2: Electron microscopy, XRD and XPS characterizations of CeOx nanoglue islands uniformly dispersed on high-surface-area SiO2.
Fig. 3: Identification of isolated Pt atoms on CeOx nanoglue islands.
Fig. 4: Evaluation of low-temperature CO oxidation activity and stability.

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Data availability

All data that led us to understand the results presented here are available with the Article or from corresponding author J.L. upon reasonable request. Source data are provided with this paper.

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Acknowledgements

This work was primarily supported by the National Science Foundation under grant no. 1955474 (CHE-1955474) and 1465057 (CHE-1465057). J. Zeng acknowledges support by National Key Research and Development Program of China (2021YFA1500500), National Science Fund for Distinguished Young Scholars (21925204), and NSFC (U19A2015). Y.W. acknowledges support by the US Department of Energy (DOE), Office of Science (SC), Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, Catalysis Science program (DE-AC05-RL01830, FWP-47319). Y.C., C.-Y.F., and B.C.G. acknowledge the support of DOE SC grant DE-FG02-04ER15513. X.I.P.-H. thanks Fulbright Colombia and Colciencias for financial support provided to pursue a PhD degree and acknowledges the support of DOE SC Grant DE-FG02-05ER15712 and DOE EERE/VTO. X.L. and Y.C. acknowledge funding from the China Scholarship Council (CSC) (201706340130, 201806340062). The authors thank Y. Yu and N. Zhang for help in the revision stage. The authors acknowledge the use of facilities within the Eyring Materials Center and the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University and thank the Stanford Synchrotron Radiation Lightsource (beamlines 4-1, 9-3) for providing beam time.

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Authors and Affiliations

  1. Department of Physics, Arizona State University, Tempe, AZ, USA

    Xu Li, Jia Xu & Jingyue Liu

  2. Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, P. R. China

    Xu Li, Jiankang Zhao & Jie Zeng

  3. The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, USA

    Xavier Isidro Pereira-Hernández & Yong Wang

  4. Department of Chemical Engineering, University of California, Davis, Davis, CA, USA

    Yizhen Chen, Chia-Yu Fang & Bruce C. Gates

  5. National Synchrotron Radiation Research Center, Hsinchu, Taiwan

    Chih-Wen Pao

  6. Department of Materials Science and Engineering, University of California, Davis, CA, USA

    Chia-Yu Fang

  7. Key Laboratory of Surface and Interface Chemistry and Energy Catalysis of Anhui Higher Education Institutes, University of Science and Technology of China, Hefei, P. R. China

    Jie Zeng

  8. Department of Chemical Physics, University of Science and Technology of China, Hefei, P. R. China

    Jie Zeng

  9. Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA, USA

    Yong Wang

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  1. Xu Li
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Contributions

J.L. conceived the concept, designed the studies and conducted the STEM experiments. X.L. designed the synthesis methods, conducted synthesis work, catalytic tests, DRIFTS experiments and other characterizations. X.I.P.-H. and Y.W. carried out CO DRIFTS experiments during the early stages of this project. Y.C., C.-W.P., C.-Y.F. and B.C.G. performed XAS experiments and analysed XANES and EXAFS data. J.X. conducted relevant experiments. J. Zhao conducted DFT calculations. J. Zeng provided suggestions on the research project. X.L. and J.L. wrote the manuscript. B.C.G., Y.W. and J.Z. revised the manuscript, and all authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Jie Zeng, Yong Wang, Bruce C. Gates or Jingyue Liu.

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Extended data figures and tables

Extended Data Fig. 1 Synthesis of CeOx nanoglue islands.

a,b, SEM backscattered electron images of the CeOx/SiO2 after solution reaction for 3 min (a) and 1 h (b). Bright features in b represent large agglomerates of CeO2 particles produced by Ce(OH)4 precipitation during the prolonged 1 h adsorption process. There were no CeO2 particles detectable in a, either at low or high image magnifications. The inset to a shows a photograph of the as-synthesized CeOx/SiO2 powders. c,d,e, High-resolution HAADF images of the Ce–SiO2 (prior to calcination) at various magnifications and defocus values. f,g, HAADF images show distribution and crystallite structure of the CeOx nanoglue islands after 600 °C calcination for 12 h. h,i, Size distribution of the as-synthesized CeOx nanoclusters along two perpendicular directions.

Extended Data Fig. 2 Characterizations of CeOx/SiO2, CeO2 NPs/SiO2 and CeO2.

a, XRD pattern of 12 wt% CeO2 NPs/SiO2 synthesized by impregnation method. b,c, HAADF images of 12 wt% CeO2 NPs/SiO2 in separate regions. The inset to b shows a photograph of the as-synthesized CeO2 NPs/SiO2 powders. d, Size distribution of CeO2 NPs in CeO2 NPs/SiO2 from HAADF images. e,f, HAADF images of pure CeO2 powders, which were fabricated by precipitation and calcination. g, Normalized Raman spectra of the as-synthesized CeOx/SiO2 and pure CeO2. h, High-resolution HAADF images of, and intensity linescans across, a crystalline CeOx nanocluster and a CeO2 nanoparticle. i, Ce 3d XPS data obtained for the 12 wt% CeO2 NPs/SiO2. Circles and black lines represent the data and the fit, respectively. j, H2-TPR profiles obtained on the CeOx/SiO2 and pure CeO2. TCD, thermal conductivity detector. k, XRD patterns of CeOx/SiO2 support after various treatments. Treatment conditions: black, calcined in air at 600 °C for 12 h; red, reduced in H2 at 600 °C for 24 h; blue, re-calcined in air at 600 °C for 12 h after reduction; and orange, calcined in air at 800 °C for 4 h. Prolonged H2 reduction at 600 °C modified the crystallinity of the CeOx nanoclusters, whereas re-calcination at 600 °C recovered the original structure of the CeOx nanoclusters.

Source data

Extended Data Fig. 3 Characterization of Pt/CeOx/SiO2.

a, ICP-MS measurements of Pt concentrations in the final catalysts. b, BET surface areas of various supports and the 0.4 wt% Pt/CeOx/SiO2 catalyst. The BET surface area of the SiO2 support did not change significantly after deposition of CeOx and Pt. c, Estimate of Pt loading levels (relative to the CeOx nanoclusters) in the CeOx/SiO2. d, Estimated specific surface area of CeOx in CeOx/SiO2 and CeO2 in CeO2 NPs/SiO2, and number density of Pt atoms on CeOx (CeO2) component. e, Plots of attainable wt% Pt loading (with respect to SiO2) versus the specific surface area of SiO2 for various sizes of CeOx nanoclusters and the distance between them, assuming that each CeOx nanocluster hosts only one Pt atom. D, average size (diameter) of CeOx nanoclusters; L, average distance between the edges of CeOx nanoclusters. Experimental parameters characterizing the 0.4 wt% Pt/CeOx/SiO2 were used for the plot (red). f, Plots of attainable wt% Pt loading (with respect to SiO2) versus specific surface area of SiO2 for CeOx nanoclusters (D = 2 nm, L = 9 nm) reported in this work. N, number of Pt atoms on each CeOx nanocluster. The red, black and blue lines represent 1, 2 and 5 Pt atoms, respectively, on each CeOx nanocluster. g,h,i, XPS data characterizing the as-synthesized 0.4 wt% Pt/CeOx/SiO2 catalyst.

Source data

Extended Data Fig. 4 Nature of Pt species in Pt/CeOx/SiO2 catalyst.

a,b,c, HAADF images of the as-synthesized 0.4 wt% Pt/CeOx/SiO2 catalyst. d, HAADF image of a high-loading Pt/CeOx/SiO2 catalyst, which was fabricated by impregnation and reduced in H2 at 300 °C for 1 h, shows presence of small Pt clusters. e–j, EXAFS results characterizing the as-synthesized 0.4 wt% Pt/CeOx/SiO2. e, k1-weighted experimental EXAFS function (black solid line) and the sum of the calculated Pt–O, Pt–Olong and Pt–O–Ce contributions (red dashed line). f, Imaginary part and the magnitude of Fourier transform (k1-weighted) of the experimental EXAFS results (black solid line) and sum of the calculated Pt–O, Pt–Olong and Pt–O–Ce contributions (red dashed line). g–i, Imaginary part and magnitude of Fourier transform (phase- and amplitude-corrected) of the experimental results (black solid line) and the calculated contributions (red dashed line) of the Pt–O shell (k1-weighted) (g), the Pt–Olong shell (k1-weighted) (h), and the Pt–O–Ce shell (k3-weighted) (i). j, Summary of the EXAFS parameters. CN, coordination number; R, distance between absorber and backscatterer atoms; Δσ2, disorder term; ΔE0, inner potential correction. Error bounds (accuracies) characterizing the structural parameters obtained by EXAFS spectroscopy are estimated to be CN, ±10%; R, ±0.02 Å; Δσ2, ±20%; ΔE0, ±20%.

Source data

Extended Data Fig. 5 Changes of Pt atoms on SiO2 or CeO2 supports under various treatment conditions.

a,b, Single Pt atoms (yellow circles) and small Pt clusters (yellow squares) were present in the as-synthesized Pt/SiO2 (synthesized by the SEA method using Pt(NH3)42+ ions in alkaline solution). c,d, After H2 reduction of the as-synthesized Pt/SiO2 at 300 °C for 1 h, Pt sintered to form various sizes of nanoparticles (c) although some small Pt clusters were observable (d). e,f, After calcination of the as-synthesized Pt/SiO2 at 500 °C for 1 h, larger Pt agglomerates (e) and many Pt nanoparticles (e,f) were formed. g,h, Atomic-resolution HAADF images of a 0.3 wt% Pt/CeO2 catalyst prepared by the SEA method. The Pt single atoms (yellow circles) on flat facets of CeO2 nanoparticles are clearly distinguishable. i,j, HAADF images of a 0.3 wt% Pt/CeO2 after reduction treatment (10 ml min−1 of H2/He at 300 °C for 1 h), clearly reveal formation of small Pt nanoparticles. During the H2-activation process, Pt atoms became mobile on CeO2 surfaces and sintered to form Pt clusters/nanoparticles (yellow squares). k, CO-DRIFTS spectrum of the reduced 0.3 wt% Pt/CeO2 indicates presence of Pt clusters/nanoparticles.

Source data

Extended Data Fig. 6 Design and characterization of model catalysts.

a, CO-DRIFTS spectra of 0.3 wt% Pt/SiO2, 4 wt% Cu/SiO2, and Cu-Pt/SiO2. b, TEM image of the reduced Cu-Pt/SiO2 catalyst. c, HAADF image of Cu nanoparticles in reduced Cu-Pt/SiO2, revealing Pt single atoms/clusters on/within Cu nanoparticles. d, Schematic diagram showing synthesis and formation of Pt-Cu species in Cu[(Pt/SiO2)@SiO2] sample. Platinum atoms migrated through the porous SiO2 shell to interact with Cu nanoparticles. e, TEM image of (CeO2/SiO2)@SiO2 shows the CeO2 nanoparticles as markers between the SiO2 core and the coated porous SiO2 shell. The thickness of the porous SiO2 shell was estimated to be ~10 nm. f, HAADF image of the Cu[(Pt/SiO2)@SiO2] catalyst. g, CO DRIFTS spectrum and h, HAADF image of Cu nanoparticles in the reduced Cu[(Pt/SiO2)@SiO2] catalyst (reduction conditions:10% H2, 400 °C for 3 h), clearly revealing Pt atoms on/within Cu nanoparticles. i, Schematic diagram showing synthesis and reduction processes of Cu[(Pt/CeO2)@SiO2] model catalyst. j, TEM images of Cu[(Pt/CeO2)@SiO2] and (Pt/CeO2)@SiO2 (inset). The Pt and Cu loadings were 0.3 wt% and 2 wt%, respectively. The thickness of the porous SiO2 shell was estimated to be ~6 nm. CO adsorption DRIFTS spectrum (k) and HAADF images (l) of Cu nanoparticles in the reduced Cu[(Pt/SiO2)@SiO2] catalyst (reduction conditions: 10% H2, 400 °C for 3 h) confirming the absence of Pt-Cu species.

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Extended Data Fig. 7 Reduction of Pt/CeOx/SiO2 catalysts and investigation of PtxOy clusters.

a, HAADF image of the 0.4 wt% Pt1/CeOx/SiO2 after 300 °C reduction in H2 for 10 h shows the absence of detectable Pt clusters. b,c,d, CO adsorption DRIFTS spectra (recorded at 100 °C) characterize the reduced 0.4 wt% Pt1/CeOx/SiO2 (H2 at 400 °C for 3 h, 500 °C for 1 h or 600 °C for 1 h, respectively). e, DFT-predicted CO adsorption modes on Pt single atoms supported on CeOx clusters. f–h, CO-DRIFTS spectra characterizing the PtxOy/CeOx/SiO2 sample at 100 °C (f), after cessation of O2 flow (g) and reduced PtxOy sample (10% H2/Ar, 300 °C, 1 h) (h), reveal transformation of the PtxOy species into metallic Pt clusters (2075 cm−1). i, Ce 3d XPS data characterizing the 0.4 wt% Pt1/CeOx/SiO2 after reduction in 10% H2 at 500 °C for 1 h. The percentage of Ce3+ was ~49% and no metallic Ce was observed. j, CO adsorption DRIFTS spectrum characterizing the 0.02 wt% Pt/CeO2 powder after reduction in H2 at 400 °C for 1 h. k–p, EXAFS results characterizing PtxOy supported on CeOx/SiO2. k, k1-weighted experimental EXAFS function (black solid line) and the sum of the calculated Pt–O, Pt–O–Pt, and Pt–O–Ce contributions (red dashed line). l, Imaginary part and magnitude of Fourier transform (k1-weighted) of the EXAFS data (black solid line) and sum of the calculated Pt–O, Pt–O–Pt, and Pt–O–Ce contributions (red dashed line). m–o, Imaginary part and magnitude of Fourier transform (phase- and amplitude-corrected) of the experimental results (black solid line) and the calculated contribution (fit; red dashed line) of the Pt–O shell (k1-weighted) (m), the Pt–O–Pt shell (k3-weighted) (n), and the Pt–O–Ce shell (k3-weighted) (o). p, Summary of the EXAFS parameters. CN, coordination number; R, distance between absorber and backscatterer atoms; Δσ2, disorder term; ΔE0, inner potential correction. Error bounds (accuracies) characterizing the structural parameters obtained by EXAFS spectroscopy are as stated in legend of Extended Data Fig. 4.

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Extended Data Fig. 8 Pt clusters on CeO2 NPs/SiO2 and CeOx/SiO2.

a,b,c, HAADF images of the reduced 0.4 wt% Pt/CeO2 NPs/SiO2 (treated in H2 at 300 °C for 1 h) show Pt clusters. Images a and b were obtained from the same region with different electron beam defocus value. d, Comparison of CO-DRIFTS spectra characterizing the as-synthesized 0.4 wt% Pt/CeO2 NPs/SiO2 and CeO2 NPs/SiO2. The CeO2 NPs and SiO2 support did not adsorb CO, and only the two CO gas-phase peaks were observed. e, CO-DRIFTS spectra characterizing the reduced (reduction in H2 at 300 °C for 1 h) 0.4 wt% Pt/CeO2 NPs/SiO2. The peak at 2,086 cm−1 is assigned to CO adsorbed on Pt nanoclusters/nanoparticles. f, Schematic diagrams illustrate sintering of Pt atoms on CeO2 NPs under conditions of H2 activation treatment; Ce, O, Pt atoms and SiO2 support are shown in yellow, blue, red and grey, respectively. g, CO-DRIFTS spectra characterizing the 4 wt% Pt/CeOx/SiO2 catalyst after reduction in H2 at 300 °C for 3 h. h,i, HAADF images of the 4 wt% Pt/CeOx/SiO2 after reduction in H2 at 400 °C for 5 h and 500 °C for 12 h, respectively. j, Schematic diagrams illustrate movement of Pt atoms on each individual CeOx nanoclusters during the H2 activation process; Ce, O, Pt and SiO2 support are shown in yellow, blue, red and grey, respectively.

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Extended Data Fig. 9 Characterization and CO oxidation performance of the as-synthesized and activated catalysts.

a,b, CO oxidation catalysis: light-off curves characterizing 0.3 wt% Pt/CeO2 (5 mg of catalyst mixed with 25 mg of SiO2) (a) and 0.4 wt% Pt/CeO2 NPs/SiO2 (30 mg) (b). Activation conditions: H2 at 300 °C for 1 h. c, Light-off curves of 4 wt% Pt/CeOx/SiO2 (3 mg of catalyst mixed with 27 mg of SiO2; pretreated in H2 at 400 °C for 3 h to form Pt clusters). d–g, CO oxidation light-off data and CO-DRIFTS spectra characterizing 0.4 wt% Pt/CeOx/SiO2 (IMP) (d,e) and Pt/SiO2 (IMP) catalyst (f,g). h, Light-off temperatures T50 (50% CO conversion) of various catalysts: as-synthesized 0.4 wt% Pt/CeOx/SiO2 (A), activated 0.4 wt% Pt/CeOx/SiO2 (B), activated Pt/CeO2 (C), activated Pt/CeO2 NPs/SiO2 (D), activated 4 wt% Pt/CeOx/SiO2 (E), activated Pt/CeOx/SiO2 by impregnation (F) and activated Pt/SiO2 by impregnation (G). i, In situ Pt LIII XANES spectra of 0.4 wt% Pt/CeOx/SiO2 catalyst during activation in 10% H2/He. j, Ex situ Pt LIII XANES spectra collected at room temperature characterizing the 0.4 wt% Pt/CeOx/SiO2 catalyst before/after reduction at 300 °C for 1 h; data characterizing Pt foil are included for comparison. k–o, EXAFS results characterizing the activated 0.4 wt% Pt/CeOx/SiO2 catalyst. k, k1-weighted experimental EXAFS function (black solid line) and the sum of the calculated Pt–O, Pt–Olong and Pt–O–Ce contributions (red dashed line). l, Imaginary part and the magnitude of Fourier transform (k1-weighted) of the experimental EXAFS results (black solid line) and sum of the calculated Pt–O, Pt–Olong and Pt–O–Ce contributions (red dashed line). m–o, Imaginary part and magnitude of Fourier transform (phase- and amplitude-corrected) of the data (black solid line) and the calculated contribution (red dashed line) of the Pt–O shell (k1-weighted) (m), the Pt–Olong shell (k1-weighted) (n), and the Pt–O–Ce shell (k3-weighted) (o). p, Summary of the EXAFS parameters. CN, coordination number; R, distance between absorber and backscatterer atoms; Δσ2, disorder term; ΔE0, inner potential correction. q, Apparent activation energy (Ea) for CO oxidation characterizing the as-synthesized and reduced 0.4 wt% Pt1/CeOx/SiO2. r, Long-term stability test of the activated 0.4 wt% Pt1/CeOx/SiO2 at a reaction temperature of 140 °C. s, HAADF image of the 0.4 wt% Pt1/CeOx/SiO2 after stability test. t, Long-term stability test of the activated 0.3 wt% Pt/CeO2 at 140 °C.

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Extended Data Fig. 10 CO-DRIFTS investigation of Pd and Rh atoms supported on CeOx/SiO2 and on CeO2.

a, As-synthesized 1.4 wt% Pd/CeOx/SiO2 and b, 0.4 wt% Pd/CeO2. c, Reduced Pd/CeOx/SiO2 and d, reduced Pd/CeO2. CO adsorption temperature, 25 °C. e,f, CO-DRIFTS of reduced 0.6 wt% Rh/CeOx/SiO2 (e) and 0.3 wt% Rh/CeO2 (f). Reduction conditions: 20 ml min−1 of 10% H2 at 400 °C for 1 h. The loadings of Pd and of Rh are reported with respect to the CeOx species.

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Li, X., Pereira-Hernández, X.I., Chen, Y. et al. Functional CeOx nanoglues for robust atomically dispersed catalysts. Nature 611, 284–288 (2022). https://doi.org/10.1038/s41586-022-05251-6

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