Supporting Information for. Size-Dependent Oxidation State and CO Oxidation Activity of Tin

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1 Supporting Information for Size-Dependent Oxidation State and CO Oxidation Activity of Tin Oxide Clusters Yusuke Inomata, Ken Albrecht,, Kimihisa Yamamoto *,, Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, Yokohama , Japan JST-ERATO, Yamamoto Atom Hybrid Project, Tokyo Institute of Technology, Yokohama , Japan *Corresponding author: Tel.: Fax: S1. Experimental details (i) Reagents All reagents and dehydrated solvents were used without further purification. Dehydrated chloroform was purchased from Wako Pure Chemical Industries, Ltd. Tin (II) chloride anhydrate (SnCl 2 ), sodium borohydride (NaBH 4 ), Tin (IV) oxide and Tin (II) oxide as references, and dehydrated organic solvents except chloroform were purchased from Kanto Chemical Co., Inc. MSU-H type mesoporous silica (MPS) as a support was purchased from Sigma-Aldrich. The pore size and specific surface area are ~7.1 nm and ~750 m 2 g -1, respectively. 1 4th-generation dendritic phenylazomethine with a tetraphenylmethane core (TPMG4) was synthesized according to literatures. 2,3 (ii) Synthesis of tin oxide clusters loaded into mesoporous silica For complexation of a precursor to a dendrimer, 2, 14, 18, 28, 44 and 60 equivalents of SnCl 2 acetonitrile solution (7.8 mmol L -1 ) were added to a solution of TPMG4 in a mixed solvent (chloroform/ acetonitrile = 5/ 7, 11 mol L -1 ). In this procedure, the yellowish TPMG4 solution turned into orange solution. The mixture was stirred for few minutes, then MPS dispersed in a mixed solvent (chloroform/ acetonitrile = 5/ 7) was added to be clusters g -1 against total weight as a theoretical amount (0.60, 0.70, 0.90, 1.4, 2.2, 3.0 wt% for Sn 12, Sn 14, Sn 18, Sn 28, Sn 44 and Sn 60, respectively.) and stirred for 1 h for complete adsorption of dendrimer complex to a MPS. The dispersion was filtered and the orange-colored residue was re-dispersed in 10 ml of methanol. 60eq vs. metal of a freshly dissolved NaBH 4 methanol solution (6 mg in 1 ml) was added to the dispersion and stirred for 30 min for reduction of the precursor. The solution was filtered and the resulting yellowish residue was washed with methanol several times, then heated at 600 o C in air atmosphere for 10 min to remove residual dendrimer and to oxidize the clusters completely, and colorless powder was collected. The amount of supported tin on MPS was corrected by inductively - 1 -

2 coupled plasma atomic emission spectroscopy (ICP-AES) every synthesis due to the variation. (iii) Characterization Scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM) images and energy dispersive X-ray spectroscopy (EDX) spectra were collected using a transmission electron microscope (JEOL, JEM-2100F) with an accelerating voltage of 200 kv. For the measurements, the powder of MPS supported tin oxide clusters was deposited on a carbon-coated copper grids (Nisshin EM Co., Ltd). X-ray photoelectron spectra (XPS) were measured by an instrument (ULvac-phi, ESCA1700R) with Mg K radiation. To correct electrical charging of the synthesized sample surface, Si 2p peak at ev for SiO 2 was used for correction of XPS spectra of Sn 3d region. (iv) Temperature programmed reaction measurements Temperature-programmed reaction (CO-TPR and H 2 -TPR) measurements were performed using a catalyst analyzer (MicrotracBEL, BELCAT II). For CO-TPR, 10 mg of sample was loaded into the sample tube and in situ pre-treated by heating in He flow for 30 min and O 2 flow for 30 min at 45 o C. Then, the measurements were carried out by using CO/ He mixture gas (5 vol.%/ 95 vol.%) with a flow rate of 20 cm 3 min -1 and programming rate of 20 o Cmin -1 in the temperature range of 45 to 600 o C. The production of CO 2 (m/z = 44) was monitored by mass spectrometry. The amount of produced CO 2 was calculated from the peak area of CO-TPR spectra and the supported amount of Sn in MPS calculated from ICP-AES measurements. For H 2 -TPR, 55 mg of sample was loaded into the sample tube, and then, the measurements were carried out by using H 2 / N 2 mixture gas (15 vol.%/ 85 vol.%) with a flow rate of 20 cm 3 min -1 and programming rate of 20 o Cmin -1 in the temperature range of 300 to 1100 o C. The consumption of H 2 was monitored by thermal conductivity detector (TCD) based on the following equation: SnO x + xh 2 xh 2 O + Sn - 2 -

3 S2. Studies on bulk tin oxide (i) Crystal structure of tin oxide (a) (b) Figure S1. (a) Crystal structure of SnO 2 (P4 2 / mnm, a=b=4.73 Å, c=3.19 Å). sixfold coordinated Sn atom and threefold coordinated O atom compose the unit cell. (b) Crystal structure of SnO (P4/ nmn, a=b=3.85 Å, c=4.89 Å). fourfold coordinated Sn atom and twofold coordinated O atom compose the unit cell. Crystallographic information files (CIF) were quoted from crystallography open database (COD ID: and )

4 (ii) TEM images of SnO 2 and SnO particles (a) SnO 2 5 n m (b) SnO 5 n m Figure S2. TEM images of commercial (a) SnO 2 and (b) SnO particles used for the reference experiments. Mean diameter of both particles was ca. 10 nm. Inset: Selected area electron diffraction patterns of SnO 2 and SnO particles, which correspond to tetragonal rutile and tetragonal PbO-type phase

5 (iii) XPS spectra of SnO 2 and SnO particles Figure S3. Sn 3d 5/2 core level XPS spectra of commercial (a) SnO 2 and (b) SnO particles used for the reference experiments. Both SnO 2 and SnO gave single sharp spectral line on the contrary to the synthesized clusters, which were assigned to tetravalent Sn(IV) in tetragonal rutile structure and divalent Sn(II) in tetragonal PbO-type structure, respectively. C 1p peak at ev was used to correct electrical charging of the sample surface

6 (iv) CO-TPR measurement of SnO 2 particles Figure S4. CO-TPR spectra of commercial SnO 2 used for the reference experiments measured by the same condition as the clusters. The production of CO 2 from tin oxide clusters was monitored by mass spectrometry (m/z = 44). Different CO 2 desorption sites ( 1, 2, 3, 4 and ) are labeled based on a previous report. 4 1 to 4 and correspond to the reaction of CO with surface-adsorbed oxygen species and reduction of inner lattice of SnO 2, respectively

7 S3. Supplementary characterizations of tin oxide clusters loaded into MPS (i) XPS spectra of tin oxide clusters loaded into MPS for Cl2p and N1s regions (a) (b) Figure S5. Cl 2p and N 1s core level XPS spectra of tin oxide clusters Sn The gray dots are the raw data and the solid lines are the fitting curve. There is no significant peak in each spectra. Si 2p peak at ev for SiO 2 was used to correct electrical charging of the sample surface

8 (ii) EDX spectra of tin oxide clusters loaded into MPS Figure S6. EDX spectra of tin oxide clusters loaded into MPS for different size. Inset: enlarged view of Sn L-level emission line

9 Figure S6. Continued

10 (iii) Elemental mapping of tin oxide clusters loaded into MPS Figure S7. Elemental mapping of tin oxide clusters loaded into MPS for different size. The element maps of Sn L-level emission line appear uniformly on the corresponding dark field

11 (iv) Clarification of tin oxide clusters loaded into MPS using FFT and IFFT processing Figure S8. (a-c) FFT images of STEM images shown in Figure 1a-c. (d-f) FFT images masked the spots in Figure S8a-c attributed to ordered pore structure of MPS. More obvious particle images of the tin oxide clusters were obtained by FFT and IFFT processing compared to the corresponding original STEM images (shown in Figure1e-g)

12 (v) H 2 -TPR measurement of tin oxide clusters loaded into MPS and tin oxide particles Figure S9. H 2 -TPR spectra of support MPS, tin oxide clusters Sn 60, reference SnO 2 and SnO particles. The consumption of H 2 caused by the reduction of the tin oxide clusters was monitored by TCD. The symbols in the figure correspond to the reduction of : Sn(IV) and : Sn(II) to Sn(0), respectively. As for Sn 60 cluster (light blue line), the reduction peak of Sn(IV) is broaden, suggesting that the Sn(IV) sites arrange various coordination modes. In contrast, the reduction peaks of Sn(II) for Sn 60 are twin. Taking account the narrowness of the peaks, we assume that these reduction peaks of Sn(II) arose from specific geometries of Sn(II) sites in the cluster. The reduction temperatures of both Sn(II) and Sn(IV) for Sn 60 cluster were much higher than those of reference materials (red line for SnO 2 and green line for SnO). We speculate that this is because the ratio of oxidizable atoms exposed over the clusters is greater than that in the bulk, and the affinity between oxygen atoms and tin atoms increases compared with that in the case of larger particles. The peak area ratio (Sn(II)/ Sn(IV)) for Sn 60 was calculated to be 12/ 88, which was in close agreement with the value measured from the XPS spectra (13/ 87, shown in Table S1). H 2 -TPR spectra for other sizes of the clusters were not provided here due to the distinguishability of the peaks and the base line of MPS

13 (vi) Detailed XPS data of tin oxide clusters loaded into MPS Table S1. Detailed XPS Data of Tin oxide Clusters for Different Size N Peak position (Sn3d 5/2 )/ ev FWHM of Sn3d 5/2 / ev Peak area ratio / % Sn(IV) Sn(II) Sn(IV) Sn(II) Sn(IV) Sn(II) Comm. SnO Comm. SnO

14 (vii) Temperature dependence of composition of Sn 12 clusters (a) (b) Figure S10. (a) XPS spectra of Sn 12 clusters calcined at different temperature in air atmosphere for 1 h. The gray dots are the raw data and the solid lines are the fitting curve. Black dotted line with colorless and yellow-colored area correspond to fitting component for different charge state, Sn(IV) and Sn(II), respectively. (b) Sn(II)/ Sn(IV) ratio calculated from XPS spectra as a function of calcination temperature. There was no significant change in the composition of Sn 12 before and after calcination at various temperatures, indicating that the clusters were oxidized immediately by air and the Sn(II) sites were not caused by incomplete oxidation

15 Reference (1) Italy Sigma-Aldrich (accessed Jul 12, 2017). (2) Enoki, O.; Katoh, H.; Yamamoto, K. Org. Lett. 2006, 8, (3) Takanashi, K.; Chiba, H.; Higuchi, M.; Yamamoto, K. Org. Lett. 2004, 6, (4) Kocemba, I.; Rynkowski, J. M. Catal. Today 2011, 169,