Supporting Information

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1 Supporting Information Remarkable performance of Ir 1 /FeO x single-atom catalyst in water gas shift reaction Jian Lin, Aiqin Wang, Botao Qiao, Xiaoyan Liu, Xiaofeng Yang, Xiaodong Wang, Jinxia Liang, Jun Li, Jingyue Liu*,, and Tao Zhang*, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, , China, Department of Chemistry, Tsinghua University, Beijing , China, Department of Physics, Arizona State University, Tempe, Arizona, 85287, USA 1. Preparation procedure for Ir 1 /FeO x and other catalysts All chemicals used in this experiment were of analytical grade, and used without further purification. All catalysts were prepared by co-precipitation methods. Under stirring at 80 o C, an aqueous mixture of H 2 IrCl 6 and Fe(NO 3 ) 3 with appropriate ratio was added dropwise (3 ml min -1 ) to an NaOH solution and the ph of the resulting solution was controlled to around 8. After stirring and aging for 4 h, the resulting precipitate was filtered and washed with 1 L hot distilled water, then dried at 80 o C overnight without any further heat treatment thereafter. The single-atom catalyst (SAC) was denoted as Ir 1 /FeO x, while other samples were denoted as -n with n as the loading amount of Ir. 2. Measurements of catalytic activities Catalytic activity measurements were carried out in a fixed-bed reactor. The feed gases for water gas shift were 2 vol% CO + 10 vol% H 2 O balanced with He. The gas flow rate was 30 ml min -1 which resulted in a space velocity of 18,000 ml g cat -1 h -1. Before evaluation, the catalyst sample was reduced in a flow of 20 ml min -1 of 10 vol% H 2 /He at 300 o C for 30 min. The concentrations of CO in the effluent gas were analyzed by an on-line gas chromatograph (Agilent 6890, TDX-01 column) using He as carrier gas. The contributions of single atoms to the total activity for the mixed catalyst containing single atoms, subnanometer clusters and nanoparticles were calculated based on the following: Firstly, obtaining the frequencies of different size scope as shown in Fig. 1; Secondly, assuming that a cluster of 0.2~0.5 nm contains about 10 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), as reported previously. S1 Then, we can estimate that ratio of single atoms to all Ir species in these mixed catalysts. For example, on the number is around 11% from the following calculation: 60% 1. Accordingly, the weight percentage 60% 1+ 33% % 20 of single atoms in these mixed catalysts was wt% for -0.22, wt% for -0.32, and 0.05 wt% for -2.40, respectively. Finally, from the catalyst test on Ir 1 /FeO x (the activity contributed by support was excluded), we can calculate that the activities contributed by single atoms of -0.22, -0.32, catalysts were 29%, 42% and 60%, respectively. 3. Characterization techniques Ir loadings in the catalyst samples were determined by inductively coupled plasma spectrometry (ICP-AES) on an IRIS Intrepid II XSP instrument (Thermo Electron Corporation). S1

2 Brunauer-Emmett-Teller (BET) surface areas of the catalysts were measured by nitrogen adsorption at -196 o C using a Micromeritics ASAP 2010 apparatus. The Ir/Fe-UC sample was outgassed to 0.1 Pa at 110 o C to ensure little or no changes in the support structure. X-ray diffraction (XRD) patterns were recorded on a PW3040/60 X Pert PRO (PANalytical) diffractometer equipped with a Cu Kα radiation source (λ= nm), operating at 40 kv and 40 ma. A continuous mode was used for collecting data in the 2θ range from 20 to 90 at a scanning speed of 10 min -1. The X-ray absorption near edge structure (XANES) spectra at Ir L III -edge of the samples were measured at beam line 14W of Shanghai synchrotron radiation facility (SSRF) in China. The output beam was delivered by Si(111) monochromator. The energy was calibrated by the Ir foil. Before measurement, the samples were reduced under flowing hydrogen at 300 o C for 1 h and sealed in Kapton films in the glove box after being cooled to room temperature. The data were collected at room temperature under Fluoresce mode by using solid state detector. H 2 temperature-programmed reduction (H 2 -TPR) was performed on an Auto Chem II 2920 automatic catalyst characterization system. First, 50 mg of a catalyst was loaded into a U-shape quartz reactor and purged with He at 120 o C for 2 h to remove adsorbed carbonates and hydrates. Then, after cooling to room temperature, the flowing gas was switched to a 10 vol% H 2 /Ar, and the catalyst was heated to 900 o C at a ramping rate of 10 o C min -1. The ratio of the H 2 consumption amount for surface FeO x to surface Ir species reduction was calculated to reflect the promotion of the reducibility by Ir atoms. High-angle annual dark-filed scanning transmission electron microscopy (HAADF-STEM) images were obtained on a JEOL JEM-ARM200F equipped with a CEOS probe corrector, with a guaranteed resolution of 0.08 nm. Before microscopy examination, the samples were suspended in ethanol with an ultrasonic dispersion for 5-10 minutes and then a drop of the resulting solution was dropped on a holey carbon film supported by a copper TEM grid. Microcalorimetric measurements of CO adsorption were performed using a BT 2.15 heat-flux calorimeter (Seteram, France). The calorimeter was connected to a gas handling and a volumetric system employing MKS 698A Baratron Capacitance Manometers for precision pressure measurement (±1.33x10-2 Pa). Prior to the adsorption, the sample was pre-reduced by H 2 at 300 o C for 60 min in a special treatment cell, followed by evacuation at 310 o C for 30 min. The adsorption experiment was conducted at 40 o C and the detailed procedures for microcalorimetric adsorption have been described earlier. S2 A TPSR experiment with on-line mass spectroscopy (MS) analysis was carried out. The gate time for MS analysis was 0.1 s for each detected component (CO, CO 2, H 2, H 2 O), equivalent to the acquisition of 2 data point per second. Typically, a 100 mg Ir 1 /FeO x catalyst was reduced in situ with 10% H 2 /He at 300 o C for 0.5 h, and then purged with He for 1 h. Then 2% CO + 10% H 2 O at a flow rate of 30 ml min -1 was introduced on the catalysts. The responses of CO 2 and H 2 were mainly recorded to reflect the catalyst activity. The FeO x support was also detected as reference. 4. TOF calculations and kinetic measurements Specific reaction rates and TOFs of Ir 1 /FeO x and -n at different temperatures were obtained by decreasing the weight of catalyst from 100 mg to about 10 mg to ensure the CO conversion below 15%. For each run at a specified reaction temperature, the CO conversions were averaged at the steady state and used for calculations of the specific rate. The TOF was then calculated based on the specific rate and the dispersion of S2

3 catalysts, which were measured by CO adsorption microcalorimetry at 40 o C with the assumption of the stoichiometric ratio of adsorbed CO/Ir=1. For the Ir 1 /FeO x SAC, Ir single atoms were considered as complete dispersion on FeO x. S3

4 Table S1 Physicochemical properties of catalysts Metal loadings by ICP (wt%) BET (m 2 g -1 ) FeO x Ir 1 /FeO x S4

5 Fig. S1 HAADF-STEM images of Ir 1 /FeO x single-atom catalyst with different magnification. S5

6 (a) (b) (c) Fig. S2 HAADF-STEM images of with the increasing Ir loadings: (a) 0.22 wt%; (b) 0.32 wt%; (c) 2.40 wt%. S6

7 Intensity / a.u. IrO 2 Ir(111) Fe 3 O Ir 1 /FeO x FeO x θ / o Fig. S3 XRD patterns of the support FeO x, Ir 1 /FeO x and -n catalysts. S7

8 Normalized absorption / a.u Energy / ev IrO Ir foil Fig. S4 Normalized XANES spectra at Ir L III -edge of the samples with different Ir loadings. S8

9 CO conversion / % Ir 1 /FeO x Pt 1 /FeO x FeO x Time / h Fig. S5 Comparison of CO conversions at 300 o C on the Ir 1 /FeO x and Pt 1 /FeO x SAC for WGS. S9

10 s -1 Reaction rate / µmol CO g cat Ir Ea=52 kj mol -1 1 /FeO x FeO x Ea=54 kj mol -1 Ea=53 kj mol -1 Ea=83 kj mol -1 Ea=50 kj mol /T / K -1 Fig. S6 Arrhenius plots of the reaction rate vs. 1/T for water gas shift on FeO x, Ir 1 /FeO x and -n catalysts. S10

11 80 CO conversion / % Time / h Fig. S7 Stability of Ir 1 /FeO x with the reaction time at 300 o C. Reaction conditions: 2% CO+10% H 2 O, and balance He. WHSV= 30,000,000 ml g Ir -1 h -1. S11

12 Fig. S8 Representative HAADF-STEM images of Ir 1 /FeO x single-atom catalyst after 20 h run for WGS reaction at 300 o C. S12

13 TCD signals / a.u Ir 1 /FeO x FeO x Temperature / o C Fig. S9 H 2 -TPR results of FeO x support, Ir 1 /FeO x and -n samples. Table S2 Reducibility of catalysts by H 2 -TPR H 2 consumed (µmol g cat -1 ) Peak (T/ o C) a FeO x /Ir b FeO x 964 (313) - Ir 1 /FeO x 1121 (270) (200) (180) (108) 33 a The central reduction temperatures and H 2 consumption amounts. b The ratio of H 2 consumption amount for surface FeO x to surface Ir species reduction. It can reflect the reducibility of support promoted by Ir atoms. S13

14 4.00E-011 Ir 1 /FeO x MS signals / a.u. 3.00E E E-011 FeO x H 2 H 2 CO 2 CO E Time / s Fig. S10 MS analysis of introducing CO+H 2 O on FeO x (dashed line) and Ir 1 /FeO x (solid line) catalysts. S14

15 References [S1] Qiao, B. T.; Wang, A. Q.; Yang, X. F.; Allard, L. F.; Jiang, Z.; Cui, Y. T.; Liu, J. Y.; Li, J.; Zhang, T. Nat. Chem. 2011, 3, 634. [S2] Li, L.; Wang, X. D.; Shen, J. Y.; Zhou, L. X.; Zhang, T. Chin. J. Catal. 2003, 24, 872. S15