The active sites of supported silver particle catalysts in formaldehyde. oxidation

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1 Electronic Supplementary Material (ESI) for ChemComm. This journal is The Royal Society of Chemistry 2016 Electronic Supplementary Information (ESI) The active sites of supported silver particle catalysts in formaldehyde oxidation Yaxin Chen, Zhiwei Huang, Meijuan Zhou, Pingping Hu, Chengtian Du, Lingdong Kong, Jianmin Chen, and Xingfu Tang* Contents: 1. Experimental Section 1.1. Catalyst syntheses High resolution transmission electron microscopy images Synchrotron X-ray diffraction patterns X-ray absorption spectra Catalytic oxidation of HCHO Calculations of the reaction rates and TOFs Theoretical Calculations 6 2. Tables and Figures Table S1. EXAFS analysis results of the samples at the Ag K-edge. 7 Table S2. The CN of the first shells of the Ag atoms and the estimated diameter (D), and the apparent activation energy (E a ) and the calculated d-orbital centres (ɛ orb ) of three catalysts 8 Figure S1. In situ HRTEM images illustrating the shrinking process of the supported Ag NPs 9 Figure S2. R-space spectra at the Ag K-edge of the samples 10 Figure S3. Inverse FT spectra at the Ag K-edge of the samples 11 Figure S4. Average coordination number as a function of the diameter of Ag atoms 12 1

2 Figure S5. The HCHO conversions (X) over the samples as a function of the reaction temperature of kinetic measurements. 13 Figure S6. The HCHO conversions (X) over the samples as a function of the reaction temperature under the same reaction conditions 14 Figure S7. The TOFs of three supported Ag catalysts at 45 o C and E a of three supported Ag catalysts at low temperatures (35-60 o C) 15 Figure S8. Ag K-edge XANES spectra of the samples References 17 2

3 1. Experimental Section 1.1. Catalyst syntheses The HMO NPs were described elsewhere 1. Briefly, KMnO 4 ( g, 73.6 mmol), Na 2 MoO 4 (1.228 g, 5.2 mmol), Na 3 VO 4 12H 2 O (3.840 g, 9.6 mmol), and TiOSO 4 H 2 SO 4 8H 2 O (1.536 g, 9.6 mmol) were dissolved in de-ionized water (170 ml) to get a solution, to which another aqueous solution (80 ml) containing MnSO 4 H 2 O ( g, mmol) and FeSO 4 7H 2 O (2.432 g, 8.8 mmol) was added drop-wise to get a brown slurry, and subsequently refluxed at 100 o C in a 1000 ml round-bottom flask for 24 hours. The resulting solid was washed with de-ionized water, filtered, dried at 110 o C for 12 hours, and calcined at 400 o C for 4 hours. AgNO 3 (0.315 g) was initially dissolved in 30 ml de-ionized water to form a solution, to which ammonia (25 wt%) was slowly added under stirring until the solution became a transparent solution. Subsequently, both the solution and a H 2 O 2 solution (30 wt%, 90 ml) were simultaneously added to an aqueous suspension (80 ml) containing the HMO NPs (2.000 g) under stirring at 0 o C for half an hour. The final suspension was filtered, washed with de-ionized water, and dried in 80 o C for 12 hours to obtain Ag NP. The Ag NP was annealed at different temperature to obtain three supported Ag catalysts with different Ag sizes. Other materials and chemicals were commercially available and were used as received High resolution transmission electron microscopy images High resolution transmission electron microscopy (HRTEM) images were carried out with a JEOL JEM-2100F field-emission gun transmission electron microscope operating at an accelerating voltage of 200 kv and equipped with an ultra-high resolution pole-piece that provides a point-resolution better than 3

4 0.19 nm. Fine powders of the materials were dispersed in ethanol, sonified, and sprayed on a carbon coated copper grid, and then allowed to air-dry for imaging Synchrotron X-ray diffraction patterns Synchrotron X-ray diffraction (XRD) patterns were performed at BL14B of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of Å. A Mar345 image plate detector was employed for the data collection and the data were further integrated using the fit2d code. The beam is mono-chromatised using Si(111) and a Rh/Si mirror was used for the beam focusing to a size of around 0.5 mm 0.5 mm. For the in situ XRD patterns, a typical amount of the sample (~1.5 mg) was loaded in a flow cell (a quartz capillary tube with a diameter of ~1 mm) sandwiched between glass wool, and then conducted by a temperature-programmed procedure at a ramp of 3.5 o C min -1. The in situ XRD data were collected at two-minute intervals and analysed by using a CMPR software X-ray absorption spectra The X-ray absorption spectra (XAS) covers the X-ray absorption near-edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) spectra, which were measured at the Ag K-edge at BL14W of the SSRF with an electron beam energy of 3.5 GeV and a ring current of ma. Data were collected with a fixed exit mono-chromator using two flat Si(311) crystals for the Ag K- edge XAS measurements. Harmonics were rejected by using a grazing incidence mirror. The XANES spectra were acquired at an energy step of 0.5 ev. The EXAFS spectra were collected in a transmission mode using ion chambers filled with N 2. The raw data were analyzed using the IFEFFIT software package Catalytic oxidation of HCHO 4

5 The oxidation of HCHO was performed in a fixed-bed quartz reactor (i.d. = 8 mm) under atmospheric pressure. Gaseous formaldehyde (HCHO) was generated by passing a N 2 gas flow over paraformaldehyde (96%, Acros) in an incubator kept at 45 o C, and then mixed with an O 2 flow, leading to a feed gas containing 400 ppm HCHO and 1 vol.% O 2 balanced by N 2. The total flow rate was 100 ml min -1. The effluents from the reactor were analysed with an on-line Agilent 7890A gas chromatograph equipped with TCD and FID detectors. The data were recorded up to the steady state for each run. For normal catalytic oxidation test, the same mass of catalyst (25 mg, mesh) was loaded for each run (Fig. S6). For kinetic measurements, different mass of catalyst (26-28 mg, mesh) was loaded for each run and conversions were always kept well below 20% (Fig. S5) Calculations of the reaction rates and the turnover frequency The number of converted reactant (HCHO) molecules (N conv ) per second over samples (w g) can be calculated according to the number of the reactant molecules in the fed gases (N A ) per second and the conversion ratio (X) at a set reaction temperature. N conv = N A X = (C 0 V s / V m ) X, where C 0 is the concentration in volume fraction of the reactant in the fed gases, V s is the volume of the fed gases per second, and V m is the standard molar volume of gas (22.4 L mol -1 ). Thus, the reaction rate (R) per total mol of Ag per second of the catalyst is calculated as: R = (N conv / (w 0.1 / 108) / t, where 0.1 is the mass fraction of Ag in the catalyst, 108 is the molar mass of Ag. Therefore, the R at a set reaction temperature was calculated according to X% of HCHO at the 5

6 corresponding temperature in Fig. S8 (ESI ). According to Fig. 3, all the surface Ag atoms of the Ag particles were evidenced to be the catalytically active sites in the HCHO oxidation. Therefore, the turnover frequency (TOF) of each catalyst can be calculated as: TOF = R (n s / n p ), where n s is the number of surface Ag atoms of the Ag particles, n p is the total number of the Ag particles Theoretical Calculations All of the configurations were implemented in the Vienna ab initio Simulation Package (VASP). The generalized-gradient approximation 5 with Perdew Burke Ernzerh (PBE) 6 functional was performed in density functional theory calculations. The energy cut-off for the plane waves was set to 450 ev. In the calculation of Ag bulk, the lattice constants for a conventional face-centred cubic cell were Å Å Å, and monkhorst-pack grid was used in the k-point sampling for the geometry optimization of Ag bulk. In the calculation of Ag 1 and Ag 13, the lattice constants for a conventional tetragonal cell were Å Å Å, and monkhorst-pack grid was used in the k-point sampling for the geometries optimization of Ag 1 and Ag 13. 6

7 2. Tables and Figures Table S1. EXAFS analysis results of the samples at the Ag K-edge. a Sample Shell N b R c (Å) σ 2 d (Å 2 ) ΔE e 0 R-factor Ag 82 Ag-O (1) 05(1) Ag-O (6) 05(0) Ag-Ag (9) 40(3) Ag 13 Ag-O (1) 05(1) Ag-O (6) 05(0) Ag-Ag (8) 66(1) Ag 1 Ag-O (1) 02(1) Ag-O (6) 04(5) Ag foil Ag-Ag (1) 33(5) Ag 2 O Ag-O (7) 16(1) Ag-Ag (2) 29(3) a R-space fit, Δk = Å -1, Δr = Å. b N, coordination number. c R, distance between absorber and backscatter atoms. d σ 2, Debye-Waller factor. e ΔE 0, energy shift. 7

8 Table S2. The CN of the first shells of the Ag atoms and the estimated diameter (D) of the Ag NPs, obtained from the corresponding EXAFS spectra in Fig. 2b, and the apparent activation energy (E a ) in the HCHO oxidation and the calculated d-orbital centroids (ɛ orb ) of three catalysts. Sample CN D (nm) E a (kj mol -1 ) ɛ orb (ev) Ag a Ag b Ag a -4.3 ev is the d-band centroid of the Ag bulk. b -3.5 ev is the ɛ orb of the Ag atoms (Ag 8 ) acting as the CACs in the Ag 13 catalyst. 8

9 a 100 b nm Fig. S1 Model (a) and HRTEM image (b) of a typical Ag NP supported on the HMO surface. Blue, gray and purple balls represent Ag, O and Mn atoms, respectively. The dash white trapezoid in b shows the size of the pristine Ag NP. 9

10 Ag K (R, k 2 ) (Å -3 ) 0.1 Ag 82 Ag K (R, k 2 ) (Å -3 ) Ag R (Å) R (Å) Ag K (R, k 2 ) (Å -3 ) Ag 1 Ag K (R, k 2 ) (Å -3 ) Ag foil R (Å) R (Å) Ag K (R, k 2 ) (Å -3 ) Ag 2 O R (Å) Fig. S2 R-space spectra (Δk = Å -1 ) at the Ag K-edge of the samples. 10

11 Ag 82 Ag 13 Re ( k 2 ) (Å -2 ) - Re ( k 2 ) (Å -2 ) - Re ( k 2 ) (Å -2 ) k (Å -1 ) Ag 1 Re ( k 2 ) (Å -2 ) k (Å -1 ) Ag foil k (Å -1 ) k (Å -1 ) Re ( k 2 ) (Å -2 ) Ag 2 O k (Å -1 ) Fig. S3 Inverse FT spectra (Δr = Å) at the Ag K-edge of the samples. 11

12 10 Coordination Number Diameter of Ag particles (nm) Fig. S4 Average coordination number as a function of the diameter of Ag atoms. Inset: the model of the Ag particles. 12

13 15 12 Ag 82 Ag 13 Ag 1 X (%) Temperature ( o C) Fig. S5 The HCHO conversions (X) over the samples as a function of the reaction temperature of kinetic measurements. 13

14 X (%) HMO Ag 82 Ag 13 Ag Temperature ( C) Fig. S6 The HCHO conversions (X) over the samples as a function of the reaction temperature under the same reaction conditions. 14

15 E a (ev) TOF (10-6 s -1 ) Diameter of Ag Particle (Å) Fig. S7 The TOFs of three supported Ag catalysts at 45 o C and E a of three supported Ag catalysts at low temperatures (35-60 o C). 15

16 Normalized (E) Ag 1 Ag 13 Ag foil Ag 2 O Absorption Energy (ev) Fig. S8 Ag K-edge XANES spectra of Ag 1, Ag 13, Ag foil and Ag 2 O. 16

17 3. References 1 C. K. King'ondu, N. Opembe, C. Chen, K. Ngala, H. Huang, A. Iyer, H. F. Garcés and S. L. Suib, Adv. Funct. Mater., 2011, 21, B. Hammer and J. K. Nørskov, Nature, 1995, 376, Y. Xu, J. Greeley and M. Mavrikakis, J. Am. Chem. Soc., 2005, 127, N. Acerbi, S. C. E. Tsang, G. Jones, S. Golunski and P. Collier, Angew. Chem., Int. Ed., 2013, 52, G. Kresse and D. Joubert, Phys. Rev. B, 1999, 59, J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77,