Supporting Information for

Transcription

1 Supporting Information for Integration of Quantum Confinement and Alloy Effect to Modulate Electronic Properties of RhW Nanocrystals for Improved Catalytic Performance towards CO 2 Hydrogenation Wenbo Zhang,, Liangbing Wang,, Haoyu Liu, Yiping Hao, Hongliang Li, Munir Ullah Khan, and Jie Zeng *, Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Hefei Science Center & National Synchrotron Radiation Laboratory, Department of Chemical Physics University of Science and Technology of China, Hefei, Anhui , P. R. China *To whom correspondence should be addressed. zengj@ustc.edu.cn These authors contributed equally. S1

2 Experimental section Chemicals and materials. Oleylamine (>70%), oleic acid (90%), rhodium acetylacetonate (Rh(acac) 3, >99%), tungsten hexacarbonyl (W(CO) 6, 97%), rhodium on carbon (Rh/C, 5% mass loading) and dimethyl sulfoxide-d 6 (DMSO-d 6, 99.9 atom% D) were obtained from Sigma-Aldrich. All other solvents and chemicals used were analytical grade and purchased from Shanghai Chemical Reagent Co. Led. All chemical reagents were used as received without further purification. Syntheses of RhW nanosheets with different ratios, Rh 73 W 27 nanoparticles, and Rh nanosheets. In a typical synthesis of Rh 75 W 25 nanosheets, 20 mg of Rh(acac) 3 was dissolved in 5.0 ml of diphenyl ether, 3.7 ml of oleylamine, and 0.7 ml of oleic acid. The formed solution was dipped into oil bath and preheated to 220 o C for 5 min, followed by the addition of 40 mg of W(CO) 6. Afterwards, the temperature was kept at 220 o C for 30 min. After the solution had been cooled down to room temperature, the product was precipitated by ethanol, washed three times with hexane, and then re-dispersed in hexane. Rh 90 W 10, Rh 82 W 18, Rh 69 W 31, and Rh 64 W 36 nanosheets were synthesized via a similar synthetic procedure of Rh 75 W 25 nanosheets, except for varying the amounts of W(CO) 6 to 10, 25, 55, and 70 mg, respectively. The synthetic protocol of Rh 73 W 27 nanoparticles was similar to that of Rh 75 W 25 nanosheets except for increasing the reaction temperature to 250 o C. The synthetic process of Rh nanosheets was also similar to that of Rh 75 W 25 nanosheets, except for that the reaction was conducted under the CO atmosphere at 170 o C without the addition of W(CO) 6. These nanocrystals were loaded on active carbon with 5% mass loading of total metals, followed by the treatment with ultraviolet (UV)-ozone cleaner at 80 o C for 30 min to clean the surface. Catalytic tests. The hydrogenation of CO 2 was conducted in a 100-mL stainless-steel autoclave (Parr Instrument Company). After the addition of 30 ml of H 2 O and 20 mg of loaded catalysts into a Teflon inlet, the autoclave was pressurized with CO 2 (8 bar) and H 2 (24 bar) at room temperature (25 o C). The reaction proceeded at 150 o C with stirring at 300 rpm for 5 h. After the completion of the reaction, the gas phase was determined by GC-FID, and no gaseous products were detected. The reaction mixture in liquid phase was collected by centrifugation at 10,000 rpm for 2 min. 1 mmol of N,N-dimethylformamide was added to 1.0 ml of the reaction mixture as an internal standard. 50 µl of the mixture was dissolved in 0.5 ml of DMSO-d 6 to determine the product yields by 1 H NHR spectroscopy. For each catalyst, the catalytic tests were conducted S2

3 three times. DFT calculations. DFT calculations and geometric optimizations were performed by using the Vienna ab initio simulation package (VASP) with plane-wave basis set. 1 The projector augmented wave (PAW) method 2 was adopted in conjugation with a generalized gradient approximation (GGA) 3 of exchange-correlation function in the Perdew, Burke and Ernzerhof (PBE) 4 form. We used a plane-wave cutoff of 500 ev for all calculations, while setting the convergence criteria of energy as 10-5 ev. The residual force was within 0.05 ev/ for geometric optimizations. Partial occupations of eigenstates were determined by first-order Methfessel-Paxton smearing 5 of σ=0.1 ev. Only the gamma point was used for Brillouin-zone integration because of the isolated nanocrystal configuration. To study the charge transfer between different atoms, Bader charge analysis 6-9 was used to calculate charge populations. X-ray photoelectron spectroscopy (XPS) measurements. X-ray photoemission spectroscopy experiments were conducted at the Catalysis and Surface Science Endstation connected to the BL10B beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The beamline is connected to an in-vacuum undulator and equipped with two gratings that offer soft X-rays from 20 to 600 ev with a typical photon flux of photons/s. The analysis chamber of the endstation connected to the beamline is equipped with a VG Scienta R4000 analyzer, a monochromatic Al Ka X-ray source, and a UV light source. In-situ diffuse reflectance infrared Fourier transform (DRIFT) tests. Before in-situ DRIFT tests, the samples were treated with the ultraviolet (UV)-ozone cleaner at 80 o C for 0.5 h to clean the surface. In-situ DRIFT experiments were conducted in an elevated-pressure cell (DiffusIR Accessory PN XX) with a Fourier transform infrared spectrometer (TENSORⅡSample Compartment RT-DLaTGS) at a wavenumber resolution of 4 cm -1 at 150 o C. After flowing with 1 atm of N 2 for 0.5 h at 150 o C, the background spectrum of the sample was acquired. Then, 1 atm of CO 2 was allowed to flow into the cell at 150 o C for 0.5 h, followed by 1 bar of N 2 at 150 o C for 0.5 h. In-situ DRIFT spectrum was obtained to detect the adsorbed CO 2 species on the samples. The adsorption of CO 2 and intermediates was also investigated. After flowing with 1 bar of N 2 for 0.5 h at 150 o C, the background spectrum of the sample was acquired. Then, 1 bar of CO 2 and H 2 (CO 2 :H 2 = 1:3) was allowed to flow into the cell at 150 o C for 0.5 h, followed by flowing with 1 bar of N 2 at 150 o C for 1 h. Instrumentation. TEM, HAADF-STEM, and STEM-EDX images were collected on a JEOL S3

4 ARM-200F field-emission transmission electron microscope operating at 200 kv accelerating voltage. ICP-AES (Atomscan Advantage, Thermo Jarrell Ash, USA) was used to determine the concentration of Rh and W. X-ray diffraction (XRD) patterns were recorded by using a Philips X Pert Pro Super diffractometer with Cu-Kα radiation (λ= Å). NMR spectra were recorded on a Brucker-400 MHz spectrometer. CO 2 -TPD profiles were recorded by micromeritics AutoChemⅡ2920 chemisorption analyzer. References: (1) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, (2) Blöchl, P. E. Phys. Rev. B 1994, 50, (3) Teter, M. P.; Payne, M. C.; Allan, D. C. Phys. Rev. B 1989, 40, (4) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 77, (5) Methfessel, M.; Paxton, A. T. Phys. Rev. B 1989, 40, (6) Henkelman, G.; Arnaldsson, A.; Jónsson, H. Comput. Mater. Sci. 2006, 36, 254. (7) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. J. Comp. Chem. 2007, 28, 899. (8) Tang, W.; Sanville, E.; Henkelman1, G. J. Phys. Condens. Matter 2009, 21, (9) Yu, M.; Trinkle, D. R. J. Chem. Phys. 2011, 134, S4

5 Figure S1. (A) TEM and (B) AFM images of Rh nanosheets, respectively. S5

6 Figure S2. TEM image of Rh 73 W 27 nanoparticles. The molar ratio of Rh:W in the nanoparticles was determined as 73:27 by ICP-AES. S6

7 Figure S3. TEM images of (A) Rh90W10, (B) Rh82W18, (C) Rh69W31, and (D) Rh64W36 nanosheets. S7

8 Figure S4. TEM and AFM images of Rh 75 W 25 nanosheets obtained at different reaction time points in the standard synthetic procedure: (A, E) t = 10 min, (B, F) t = 15 min, (C, G) t = 20 min, and (D, H) t = 60 min. S8

9 Figure S5. FTIR spectra of Rh and Rh 75 W 25 nanosheets (A) before and (B) after the treatment with UV-ozone cleaner. S9

10 Figure S6. TEM images of Rh 75 W 25 nanocrystals obtained at (A) 200 o C and (B) 240 o C in the standard synthetic procedure. The inset in panel A is the AFM image of Rh 75 W 25 nanocrystals obtained at 200 o C. S10

11 Figure S7. (A) TEM image and (B) size distribution diagram of Rh nanoparticles. S11

12 Figure S8. (A) Products for CO 2 hydrogenation over RhW nanosheets with different ratios under 32 bar of H 2 /CO 2 mixed gas (H 2 :CO 2 = 3:1) at 150 o C after 5 h. (B) Comparison of TOF Metals and TOF Rh for these catalysts. S12

13 Figure S9. Time course of CO 2 hydrogenation catalyzed by Rh nanoparticles at 150 o C. S13

14 Figure S10. TEM images of Rh 75 W 25 nanosheets (A) before and (B) after reuse. These nanosheets were loaded on active carbon. S14

15 Table S1. Carbon balance and residual metal of Rh 75 W 25 nanosheets after different rounds of reaction. Catalyst Carbon balance (%) Residual metal (%) Rh 75 W 25 nanosheets after one round Rh 75 W 25 nanosheets after six successive rounds S15

16 Figure S11. (A-D) Atomic structures of Rh nanoparticles, Rh nanosheets, Rh 73 W 27 nanoparticles, and Rh 75 W 25 nanosheets, respectively. A yellow sphere represents a Rh atom, whereas a wine red sphere represents a W atom. S16

17 Figure S12. In-situ DRIFT spectrum of CO 2. S17