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1 DOI: /NCHEM.1095 Single-atom catalysis of CO oxidation using Pt 1 /FeO x Botao Qiao, 1 Aiqin Wang, 1 Xiaofeng Yang, 2 Lawrence F. Allard, 3 Zheng Jiang, 4 Yitao Cui, 5 Jingyue Liu, 6, 1* Jun Li 2* and Tao Zhang 1* 1 State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , China. 2 Department of Chemistry, Tsinghua University, Beijing , China. 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. 4 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai , China. 5 State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , China. 6 Center for Nanoscience, Department of Physics & Astronomy, and Department of Chemistry & Biochemistry, University of Missouri-St. Louis, Missouri 63121, USA. * To whom correspondence should be addressed. taozhang@dicp.ac.cn (T. Zhang); liuj@umsl.edu (J. Liu); junli@tsinghua.edu.cn (J. Li) NATURE CHEMISTRY 1 1

2 Table of Contents Supplementary Fig. S and 4 Supplementary Fig. S Supplementary Fig. S Supplementary Fig. S Supplementary Fig. S Supplementary Fig. S Supplementary Fig. S Effect of impurity on the catalyst Supplementary Fig. S Supplementary Fig. S Supplementary Fig. S Catalytic stability of Pt 1 /FeO x Supplementary Fig. S Supplementary Fig. S Supplementary Fig. S Computational Details Supplementary Figure S Supplementary Fig. S Supplementary Fig. S Understanding the Remarkable Catalytic Activity of Pt 1 /FeO x Supplementary Fig. S Supplementary Table S Supplementary Table S Supplementary Table S Supplementary Table S Supplementary Table S References NATURE CHEMISTRY 2 2

3 a 2 nm b 2 nm NATURE CHEMISTRY 3 3

4 c 10 nm Supplementary Fig. S1 Representative aberration-corrected HAADF-STEM images of sample A with high magnification (a, b) and with relatively low magnification (c). Only Pt single atoms were observed. NATURE CHEMISTRY 4 4

5 Supplementary Fig. S2 Aberration-corrected HAADF-STEM images of sample B with a relatively low magnification. It showed the presence of sub-nanometer Pt clusters and absence of larger Pt clusters or particles. NATURE CHEMISTRY 5 5

6 Supplementary Fig. S3 The observed frequencies of single Pt atoms and Pt clusters in sample B. By assuming that a cluster of 0.2~0.5 nm contains about10 atoms, a 0.5~1 nm cluster contains about 20 atoms, and a 1-2 nm cluster contains about 60 atoms (a rough estimation based on HAADF images), one can estimate that the single Pt atoms in sample B represent only about 1.8 atom% of the total amount of Pt. NATURE CHEMISTRY 6 6

7 Fe 3 O 4 Fe 2 O Pt (111) Sample B Sample A Supplementary Fig. S4 XRD patterns of samples A and B after reduction at 200 o C for 30 min with 10% H 2 /He. NATURE CHEMISTRY 7 7

8 PtO 2 Pt foil Sample A Sample B Supplementary Fig. S5 k 1 -weighted raw EXAFS spectra at the Pt L 3 -edge for sample A, sample B, Pt foil, and PtO 2. Both sample A and sample B are characterized by the absence of oscillations at high k region of k > 8 Å -1, indicating the dominance of low-z backscatters which should be oxygen in our system. NATURE CHEMISTRY 8 8

9 a 0.1 b 0.5 Supplementary Fig. S6 FT-IR spectra of CO adsorption on sample B. (a) Introduction of 8.1 torr CO followed by evacuation for 30 min, and then introduction of O 2 up to 10.0 torr. A new peak evolved at 2070 cm -1 after introduction of O 2, indicating the Pt 0 clusters were oxidized. (b) Introduction of 5.1 torr CO followed by introduction of H 2 up to 16.2 torr. The peak position and intensity were not changed with introduction of H 2. NATURE CHEMISTRY 9 9

10 a b Supplementary Fig. S7 FT-IR spectra of CO adsorption on sample A. The two figures indicate that the introduction of O 2 (a, pre-adsorption of 8.0 torr CO followed by evacuation for 30 min, and then introduction of O 2 to the corresponding pressure) or H 2 (b, pre-adsorption of 11.5 torr CO followed by evacuation for 30 min, and then introduction of H 2 to the corresponding pressure) did not result in any changes in the vibration frequency of CO. NATURE CHEMISTRY

11 Effect of impurity on the catalyst The stability and catalytic activity of single Pt atoms on the FeO x support are exceptional. To exclude the possible effect of the impurities on the support surface, we made further compositional analyses of the FeO x support. The result shows that the main impurity is sodium (0.49 wt%) originating from the sodium carbonate that we used as a precipitation agent. Other impurities, including Si, Mn, and Ca, are less than 0.2 wt% in total; these impurities were probably introduced by the iron nitrate reagent. The blank test shows that the impurities have little influence on the catalytic performance of the single Pt atom catalyst. Very recently, it has been reported that Na + may play a crucial role in stabilizing the OH - associated with Pt single atoms, and then largely boosts the catalytic activity for water-gas shift reaction 1. To investigate if the Na + in our Pt 1 /FeO x catalyst has a similar promotional effect on the catalytic activity, we conducted a control experiment by replacing sodium carbonate with ammonium carbonate as the precipitating agent. The resulting catalyst was evaluated for the PROX reaction. As shown in Supplementary Fig. S9, the catalytic performance of the Na-free sample is comparable to that of sample A. Therefore, we can unambiguously conclude that the presence of Na + in our catalyst A does not affect the exceptional activity of the Pt 1 /FeO x catalyst. NATURE CHEMISTRY

12 Supplementary Fig. S8 O 1s XPS spectrum of sample A. The shoulder centered at ev is mainly due to the hydroxyl groups on the FeO x support. NATURE CHEMISTRY

13 Supplementary Fig. S9 The catalytic performance of the Na-free sample for PROX reaction. Reaction conditions: flow rate: 25 ml/min; catalyst: 83 mg; reaction temperature: 80 o C. NATURE CHEMISTRY

14 a Sample A Sample B Au/Fe 2 O 3 Fe 2 O 3 b conversion selectivity Purged with He for 30 min at 200 o C Supplementary Fig. S10 CO conversion (solid symbols) and CO 2 selectivity (open symbols) with the time-on-stream for PROX reaction at 80 o C. The feed gas composition was 40 vol% H 2, 1 vol% CO, 1 vol% O 2 and balance He. Space velocities were (a), ml g Pt -1 h -1 for sample A, ml g Pt -1 h -1 for sample B, and ml g Au -1 h -1 for Au/Fe 2 O 3 and (b), ml g Pt -1 h -1 for sample A. NATURE CHEMISTRY

15 Catalytic stability of Pt 1 /FeO x To further investigate the stability of sample A against reduction or oxidation treatments, we first tested the catalytic performance of freshly reduced sample A under an increased space velocity to control the CO conversion at about 30% (Supplementary Fig. S11). Then, after the reaction, the sample was treated with 5 vol% O 2 /He at 200 o C for 30 min followed by re-evaluation under PROX conditions. The result showed that the CO conversion decreased by 5% after such an oxidation treatment. The activity decay was most probably caused by the partial oxidation of low-valence Fe during the oxidation treatment 3,4. However, upon re-reduction at 200 o C with H 2 for 30 min, the CO conversion was restored again to the same level as that of the fresh catalyst. Such oxidation-reduction treatments were conducted several times and no irreversible deactivation was observed. This result suggests that the Pt atoms in sample A did not agglomerate under mild oxidation-reduction conditions. Supplementary Fig. S11 The catalytic performance of sample A after sequential reduction and oxidation treatments at 200 o C for two cycles. NATURE CHEMISTRY

16 Supplementary Fig. S12 Representative HAADF images of sample A after a steady reaction period of 1000 min under typical PROX conditions. The STEM images show that even after 1000 min run the Pt 1 /FeO x catalyst still primarily consists of isolated individual Pt atoms dispersed onto the FeO x nanocrystallites. NATURE CHEMISTRY

17 0.004 Supplementary Fig. S13 In-situ DIRFT spectra of CO adsorbed on sample A with the time-on-stream for PROX reaction at 80 o C. The feed gas composition was 40 vol% H 2, 1 vol% CO, 1 vol% O 2 and balance He. Space velocity was ml g Pt -1 h -1. All the spectra were obtained by purging with He for 5 min to remove the gas peak of CO. NATURE CHEMISTRY

18 Computational Details The (0001) surfaces of α-fe 2 O 3 were represented by a periodic slab model, constructed using bulk cell dimensions: a = b = 5.04 Å and c = Å. Since α-fe 2 O 3 is antiferromagnetic and has atomic moment on iron atoms, we used the primitive rhombohedral unit cell of Fe 2 O 3 with the magnetic configuration (+ +) to build the surface slab, which was previously proved to be energetically the most favored magnetic configuration for α-fe 2 O 5 3. The repeated slabs were separated from their neighboring images by a 12 Å-width vacuum in the direction perpendicular to the surface. Considering the usually very large relaxations of the Fe 2 O 3 surfaces 6,7, we chose slabs containing 12 layers of Fe atoms, and 5, 6, and 7 atomic layers of O 3 to model the double-fe-terminated, single-fe-terminated, and the O 3 -terminated surfaces, respectively. The 10 top-layer slabs of the surface were allowed to relax while the other layers beneath the surface were frozen during the geometry optimizations. The theoretical calculations were performed at the level of relativistic density functional theory (DFT) using the Vienna ab-initio simulation package (VASP) The core and valence electrons were represented by the projector augmented wave (PAW) method and plane-wave basis functions with a kinetic energy cut-off of 400 ev 12,13. Inasmuch as Pt has significant relativistic effects, the mass-velocity and Darwin relativistic effects were included through the PAW potentials. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was used in the calculations 14. A Monkhorst-Pack grid of size of was used to sample the surface Brillouin zone 15. Ground-state NATURE CHEMISTRY

19 atomic geometries were obtained by minimizing the forces on the atoms to below 0.02 ev/å. Because of the strong d-electron correlation effects for Fe, the calculations were carried out with the DFT+U method, using the formalism suggested by Liechtenstein and Dudarev et al. 16. The parameters were set at U = 4 ev and J = 1 ev according to previous reports 17. We have also investigated how the values of U affect the thermodynamics of CO adsorption and its vibrational frequencies. The results of such calculations are listed in Supplementary Table S5. It can be seen that the different U terms have only little influence on the adsorption energy of CO. Especially, the vibrational frequencies of the CO stretching mode at different U terms are almost constant, indicating that the U and J terms are reliable for our systems. The transition states were obtained by relaxing the force below 0.05 ev/å by using the CI-NEB method 18 NATURE CHEMISTRY

20 Supplementary Figure S14 Differently located sites for Pt atom over three Fe 2 O 3 (001) surfaces with different termination (I, Single-Fe-terminated; II, Double-Fe-terminated; III, O 3 -terminated), and the relaxation of surfaces in the z-direction relative to an unrelaxed surface. NATURE CHEMISTRY

21 Supplementary Fig. S15 Top and side view of the Fe 2 O 3 (0001) O 3 -termination hematite surface. Only the threefold hollow (TH) sites are marked. Supplementary Fig. S15 Top and side view of the Fe 2 O 3 (0001) O 3 -termination hematite surface. Only the threefold hollow (TH) sites are marked. NATURE CHEMISTRY

22 a b c Supplementary Fig. S16 H 2 -TPR profiles of Fe 2 O 3 (a), Sample A (b) and Sample B (c). NATURE CHEMISTRY

23 Understanding the Remarkable Catalytic Activity of Pt1/FeO x In order to understand the remarkable catalytic role of single-pt-atoms, we compared the properties of Pt δ+ in the single-pt-atoms anchored on the FeO x support and Pt(0) in the metallic Pt x clusters. Calculations were carried out to reveal the orbital interactions between CO and Pt-(O-) 3 cluster and the orbital interactions between CO and Pt 10 cluster. Here in the Pt-(O-) 3 cluster the oxygen atoms were saturated by H atoms to approximately preserve the correct coordination environment. The Pt 10 cluster was selected to approximately represent a small Pt cluster. All these cluster calculations and orbital interaction analyses were performed with ADF program (ADF, using PBE exchange-correlation functional and the TZ2P basis sets. The orbital interactions are depicted in Supplementary Fig. S17. Our model calculations indicate that upon adsorption of CO the 5d-orbital occupation of the Pt δ+ single-atom anchored on the three O-atoms from the support surface is 7.996e -, which is significantly lower than that of the Pt(0) atoms in the cluster (8.6e - ~ 8.8e - ). Therefore the d-orbital vacancy is larger for the single-pt atoms than for the metallic Pt(0) in the clusters or bulk Pt. As a result, the back-donation interaction from Pt to CO is much smaller for Pt δ+ than for Pt(0). Indeed, as shown in Figure S17, the CO 2π -orbitals are raised only slightly (to ~ ev), but they are pushed to much higher energies ( ~ ev) due to Pt(5d)-CO(2π*) back-donation interaction. Accordingly, the calculated charge-transfer from Pt to CO is only 1/3 for Pt δ+ than for Pt(0), indicating that the CO-adsorption is weaker on Pt δ+ than on Pt(0). This is not only consistent with the experimental IR frequency shifts, NATURE CHEMISTRY

24 but also explains the high catalytic activity. Because CO cannot adsorb strongly on the Pt δ+ center, this single-pt-atom is less poisoned by CO and more accessible for O 2, which lead to lower activation barrier. Supplementary Fig. S17 Orbital interactions between CO and Pt single atoms and Pt cluster. All the molecular levels with major Pt 5d-character are represented by the hatched area. The Pt dπ-type orbitals (d xz,yz ) of Pt atoms will interact with the 2π* orbital of CO, while the Pt dσ-type orbital will mix with the 5σ orbital of CO. NATURE CHEMISTRY

25 Supplementary Table S1 EXAFS parameters of three candidate models of samples A (Δk = 2.8 to 10.0 Å -1 ). Sample Shell N R σ 2 x10 3 E o R-factor (Å) (Å 2 ) (ev) Sample A (Model I) Sample A (Model II) Pt-O Pt-Fe Pt-O Pt-Fe Sample A (Model III) Pt-O Pt-Pt N, coordination number; R, distance between absorber and backscatter atoms; Δσ 2, change in the Debye Waller factor value relative to the Debye Waller factor of the reference compound; ΔE 0, inner potential correction accounting for the difference in the inner potential between the sample and the reference compound. Error bounds (accuracies) characterizing the structural parameters obtained by EXAFS spectroscopy are estimated to be as follows: N, ±20%; R, ±1%; Δσ 2, ±20%; and E o, ±20%. R-factor is always used when you are comparing different models or the quality of the fit is actually in question. Better you fitting the curve, smaller R-factor value you ll get it. Generally, an acceptable R-factor should be smaller than 5%. According to the R-factor of different models, model I is the most reasonable model. It is specially noted that the fitting result based on model III has a R-factor of 0.087, which is too large to be acceptable. Therefore, it is convinced that there is no Pt-Pt bonding in sample A. NATURE CHEMISTRY

26 Supplementary Table S2 Specific reaction rates and turnover frequencies (TOFs) of some typical Pt catalysts reported in literatures. Pt loadings (wt%) Reaction Temperature ( o C) Specific rate 10 2 (mol CO h -1 g -1 /Pt ) TOF 10 2 (s -1 ) Pt/Al 2 O CO oxidation Ref 19 Pt/SiO 2 5 CO oxidation Ref 19 Pt/CeO x /Al 2 O CO oxidation Ref 19 Pt/MnO x /SiO 2 5 CO oxidation Ref 19 Pt/CoO x /SiO 2 5 CO oxidation Ref 19 Pt/SiO 2 2 CO oxidation > <7 Ref 20 Pt/TiO CO oxidation 27 < 6.84 < 0.38 Ref 21 Pt/TiO CO oxidation 27 < 0.86 < 0.92 Ref 21 K-Pt/Al 2 O 3 2 PROX Ref 22 a M-Pt/Al 2 O 3 2 CO oxidation, <4 Ref 23 PROX Pt 3 Sn/C ~16.6 PROX Ref 24 a M=Li, Na, K, Rb, Cs -- Note NATURE CHEMISTRY

27 Supplementary Table S3 Differently located sites for Pt atom over three Fe 2 O 3 (001) surfaces with different termination. Single-Fe-terminated Double-Fe-terminated O 3 -terminated Sites Energy/eV Sites Energy/eV Sites Energy/eV TH_a Top_Fe_a TH_a TH_b Top_Fe_b TH_b Top_Fe TH_a TH_c Top_O Bridge_Fe TH_d Bridge_O Top_O Bridge_O_a Bridge_O_b Bridge_O_c *TH, top, and bridge represent the three-fold hollow, top and bridge sites, respectively (see Figure S11) NATURE CHEMISTRY

28 Supplementary Table S4 Bader charge, CO adsorption energies, and vibrational frequencies for CO adsorption at Pt single atoms on Fe 2 O 3 -O vac and on free Pt 10 cluster. Pt/Fe 2 O 3 -O vac Pt 10 Bader charges ( e ) ν C-O (cm -1 ) E ad -CO (ev) NATURE CHEMISTRY

29 Supplementary Table S5 The effects of U terms on the adsorption properties of CO over Pt 1 /Fe 2 O 3 (001). U / ev (J=1 ev) E ad (CO) / ev ν CO /cm NATURE CHEMISTRY

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