Active and Selective Conversion of CO 2 to CO on Ultrathin Au Nanowires

Transcription

1 Supporting Information Active and Selective Conversion of CO 2 to CO on Ultrathin Au Nanowires Wenlei Zhu, Yin-Jia Zhang, Hongyi Zhang, Haifeng Lv, Qing Li, Ronald Michalsky, Andrew A. Peterson, *, Shouheng Sun, * Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA School of Engineering, Brown University, Providence, Rhode Island 02912, USA *To whom correspondence should be addressed. andrew_peterson@brown.edu; ssun@brown.edu 1. Materials and Experimental Methods 1.1. Reagents Oleic acid (OA, 99%), Oleylamine (OAm, >70%), 1,2,3,4-tetrahydronaphthalene (Tetralin, 99%), borane t-butylamine (BBA, 97%), triisopropylsilane (TIPS, 99%), 1-methyl-2- pyrrolidinone (MP, 99.5%), and potassium bicarbonate ( 99.99% trace metals basis, % dry basis) were purchased from Sigma Aldrich. Hydrogen tetrachloroaurate (III) hydrate (HAuCl 4 3H 2O, 99.9%) was purchased from Strem Chemicals. Platinum wire (0.5 mm diameter, Premion, %), Toray Carbon Paper (TGP-H-60) was from Alfa Aesar. Polyvinylidene fluoride (PVDF) was from MTI corperation. Deionized water was from a Millipore Autopure system. All reagents were of analytical grade and used without further purification. Argon (99.99%), carbon dioxide ( %) was purchased from Corp Brother, Inc. S1

2 1.2. Synthesis 2 nm Au NPs. 0.2 g HAuCl 4 was dissolved in 10 ml of tetralin and 10 ml of OAm at 20 C under Ar flow and vigorous magnetic stirring. 0.5 mmol BBA was dissolved in 1 ml of tetralin and 1 ml of OAm via sonication. The solution was then injected into the HAuCl 4 solution. The mixed solution was then stirred for 1 h at 60 C before 40 ml of acetone was added to collect Au NPs via centrifugation (8500 rpm, 8 min). The product was dispersed in 10 ml hexane, precipitated out by adding 40 ml of ethanol and centrifugation, and re-dispersed in hexane. 4 nm, 8 nm Au NPs were synthesized as reported 1, x 2 nm Au NWs g HAuCl 4 and g 2 nm Au seeds was dissolved in 20 ml of hexane and 1.5 ml of oleylamine at room temperature under magnetic stirring. Triisopropylsilane was then added to this solution. Two minutes later, magnetic stirring was stopped (note: the reduction did not happen till 2 h later when the solution color turned darker) and the solution was kept still at room temperature for 36 h before ethanol was added to collect NW product via centrifugation (6000 rpm, 5 min). The product was redispersed in 20 ml hexane, precipitated out again by adding 40 ml of ethanol, centrifugated (8500 rpm), and re-dispersed in hexane. This seed-mediated method was unique for preparing 2 nm wide Au NWs when 5 nm Au NPs were used as seeds, no NWs could be obtained, instead, larger NPs were separated. 100 nm Au NWs. As in the synthesis of 500 nm Au NWs, when 0.2 ml of OA was added in the reaction solution, ~100 nm long Au NWs were prepared. C-Au-15 NWs. The 15 nm NWs deposited on the carbon support were obtained by 100% irradiation of the C-Au-500 NWs under 150 W infrared bulb for 30 min. S2

3 1.3. Characterization TEM images were obtained from a Philips EM 420 (120 kv). Each sample for TEM analysis was prepared by depositing either a drop of diluted NW (or NP) dispersion in hexane or carbon supported NW (or NP) suspension in ethanol on a carbon-coated copper grid. XRD patterns were collected on a Bruker AXS D8-Advanced diffractometer with Cu Kα radiation (λ = Å). ICP-AES measurements were performed on a JY2000 Ultrace ICP Atomic Emission Spectrometer equipped with a JY AS 421 autosampler and 2400 g/mm holographic grating. EDX measurements were carried out on a Joel JSM-6060 scanning electron microscopy (SEM) where each sample was deposited on a graphitized porous carbon support for analysis. Catalyst Preparation and Catalytic Studies. Au NWs were deposited onto Ketjen Carbon (C) (EC300J) by magnetically stirring (750 rpm) the mixture of the Au NW dispersion in hexane and C for 30 min, giving C-Au-500 or C-Au-100 (the C-Au-15 was obtained by irradiation of the C-Au-500 in a 150 W infrared field for 30 min). The solvent was decanted and the precipitate was washed twice with hexane and dried in air. The supported catalyst was then suspended in acetic acid and stirred magnetically for 24 h at room temperature. The acid was decanted and the catalyst was washed with ethanol twice and water twice, and dried at 50 C in air. 40 mg of the dried catalyst powder was ground with 4 mg polyvinylidene fluoride (PVDF) with a few drops of 1-methyl-2-pyrrolidone (MP) added to produce catalyst paste that was painted directly onto a 0.7 cm x 0.7 cm carbon paper. The catalyst-decorated carbon paper was dried in a vacuum-oven overnight and served as a working electrode. S3

4 EC-Lab VSP Ultimate electrochemical workstation was used to conduct CO 2 reduction experiments in aqueous 0.5 M KHCO 3 (ph = 7.2 when saturated with CO 2, ph = 8.8 when saturated with Ar). A % platinum wire was used as counter electrode. All potentials were measured against an Ag/AgCl reference electrode (4.0 M KCl, Pine instrument) and were converted to those against a reversible hydrogen electrode (RHE). The experiments were performed in a gas-tight cell with two-compartments separated by an anion exchange membrane (Nafion 212). Each compartment contained 20 ml electrolyte with approximately 5 ml headspace Electrocatalytic Reduction of CO 2 Solution ph was measured with the Thermo Scientific Orion VERSA STAR ph Benchtop Meter. Before the experiment, the electrolyte in the cathodic compartment was saturated with CO 2 by bubbling CO 2 gas for at least 30 min. During the CO 2 reduction experiments, the electrolyte in the cathodic compartment was stirred at 900 rpm. CO 2 gas was delivered at an average rate of 40 ml/min (at room temperature and ambient pressure) and routed directly into the gas sampling loop of a gas chromatograph (Agilent 7890A). The gas phase composition was analyzed by GC every 30 min. The GC analysis was set up to split the gas sample into two aliquots whereof one aliquot was routed through a packed MoleSieve 5A column and a packed HP-PLOT Q column before passing a thermal conductivity detector (TCD) for CO quantification. Argon (Corp Brother, %) and Helium (Corp Brother, %) were employed as carrier or make-up gases respectively. The second aliquot was routed through a packed HP-PLOT Q + PT column equipped with a flame ionization detector (FID) for analyzing S4

5 all major C 1 to C 3 hydrocarbons. The GC was calibrated using commercially available calibration standards from JJS Technical Services. 1 H NMR was employed at the end of experiments to test for possible production of liquid products such as formate or methanol. 1 H NMR spectra were recorded on Bruker DRX 400 Avance and 600 Avance MHz spectrometers. 1 H chemical shifts were referenced to residual protic solvent signals. The faradaic efficiency (FE) and mass activity for the formation of CO or H 2 were calculated as described Estimation of the Active Surface Area of the Au Catalysts. To obtain specific activity values, we estimated the active surface area of the Au catalysts by using their reduction peaks. Glassy carbon electrode (GCE) was polished by 0.1 µm and 0.05 µm alumina powder and rinsed with deionized water, followed by sonication in ethanol and deionized water. 20 µl Au catalyst ink (prepared from 2 mg/ml catalysts suspended in a mixture of deionized water and isopropanol (V/V = 4/1)) was deposited on the GCE and dried in ambient conditions. The electrode potential was first scanned from 0.26 V to 1.66 V at 50 mv/s to oxidize the Au catalyst surface 3. Upon the cathodic scan from 1.66 V to 0.26 V, the oxidized Au was reduced (Fig. S9). The reduction peak was used to calculate the electrochemcial surface area, as described C/cm 2 was chosen as the reference charge value for Au. 2. Computational Methods 2.1. Stucture and Electronic Energy Calculations S5

6 All electronic energies were calculated by density functional theory (DFT) using the Gridbased projector-augmented wave (GPAW) calculator 5,6. The RPBE functional was used to describe the exchange-correlation interactions 7. A 0.1 ev Fermi smearing temperature and a 0.18 Å realspace grid spacing were applied. A k-point sampling of (8 1 1) was used for Au NW calculations with periodic boundary conditions (pbc) applied along the direction of wire growth; a k-point sampling of (8 8 1) was used for Au (211) surface calculations with pbc applied along the two dimensions on the gold surface plane. The Broyden Fletcher Goldfarb Shanno (BFGS) algorithm was used to optimize geometric configurations until the maximum force on any unconstrained atom less than 0.05 ev/å. 2 nm diameter Au NW, Au (211) and Au 13 cluster models were created in the Atomic Simulation Environment (ASE) 8. The construction of a full size 2 nm wide Au NW was based on an icosahedron NP precursor, which was observed in many previous studies as the multiply twinned crystal precursor nuclei shape for a pentagonal wire 1,9-12. An icosahedron particle with 13 Au atoms was firstly optimized via varying the distance between two neighbor Au atoms. The NW model was built by using the parameter (distance between atoms 2.96 Å) shown in the lowest energy icosahedron particle configuration. The wire grows along one dimension with a pentagonal end. Along the direction of wire growth, five facets of (100) are exposed and the neighboring facets form the long edge; the end of wire exposes five small area facets of (111). 20 Å vacuum was added to surround the NW. A total of 93 Au atoms were included in one unit cell and periodic boundary conditions were applied along the direction of wire growth. The optimum unit length of Au NW was obtained by using the fmin method and the result 8.92 Å unit length was used in all the following NW calculations. Calculations with adsorbate on NW S6

7 was carried out with the first layer of Au NW (45 atoms) relaxed and the inner Au atoms (48 atoms) constrained (Fig. S1). Au (211) surface was simulated by cutting from an fcc Au bulk with the optimized lattice constant of Å. The unit cell contains 3 layers and each layer has 3 3 atoms. 16 Å vacuum was added between layers and a dipole correction was applied in the vacuum in the direction orthogonal to the slab surface. In calculations, the first layer was allowed to relax and the two layers at bottom were fixed. Au 13 was constructed by following the method described 2 and all the Au atoms were constrained except the one in contact with adsorbate Binding Energy of COOH and CO The binding energy of an adsorbate (E B [ad]) was defined by the following equation (Equation 1): Eq. 1: E B [ad] E[ad on Au] (E[Au without ad] + E ref [ad]) in which ad is the abreviation of adsorbate and it can be COOH or CO in this study. E[ad on Au] is the electronic energy with an adsorbate on Au (Au NW, Au (211) or Au 13 cluster). Various initial adsorption sites in each case were tried. For Au NW, edge on top, edge bridge, plane on top, plane bridge and plane four fold sites were all included. The lowest electronic energy configuration was selected and the energy was recorded as E[ad on Au]. One adsorbate was included in one unit cell and the distance between two neighbor adsorbate is 8.92 Å, which is far enough for the adsorbate-adsorbate effect to be ignored E[Au without ad] is the electronic energy without any adsorbate on Au (Au NW, Au(211) or Au 13 cluster). E ref [ad] refers to the reference electronic energy of the adsorbate. S7

8 E ref [COOH] = E[HCOOH] 1 2 E[H 2] E ref [CO] = E[CO] Where E[HCOOH], E[H 2 ] and E[CO] are the electronic energies for the corresponding molecules in a Å 3 vacuum box NW Diameter and the Density Edge Sites and Corner Sites As the cross-section of an Au NW is a pentagonal shape, the diameter of the NW is calculated by adding the diameter of the circumcircle of the cross-section pentagonal and two times of Au atom covalent radius. The density of edge sites is defined as ratio of the number of on-top adsorption sites along the NW edge (N edge sites ) to the total number of atoms in the NW (N total atoms in nanowire ). The density of corner sites is defined in a similar way, but for the Au NW, the corner sites only exist at both ends. Edge site density (%) = Corner site density (%) = N edge sites N total atoms in nanowire 100% N corner sites N total atoms in nanowire 100% S8

9 3. Supporting Figures and Tables Fig. S1: Periodic unit cell of the 2 nm Au NW. The unit cell boundaries are shown in Side View 1; 20 Å of vacuum surround cells. Certain atoms are shown outside of the unit cell for clarity; these atoms repeat inside the cell. Fig. S2. TEM image of the 2 nm Au seeds used for Au NW growth. S9

10 Fig. S3. XRD of the Au NWs and 2-nm Au NPs. Fig. S4. TEM image of the ~100 nm long Au NWs. Fig. S5 TEM images of the 500 nm Au NWs exposed to the 120 kv electron beam for 0 s, 30 s and 60 s. S10

11 Fig. S6. Representative TEM image of the ~15 nm Au NWs obtained from 30 min irradiation of the 500nm Au NWs by the 150 W infrared beam. The NWs were deposited on the carbon support. a b Fig. S7. TEM images of (a) C-Au-100 NWs and (b) C-Au-15 NWs after treated with acetic acid, washed, and dried for catalytic studies. S11

12 Fig. S8. I-V curves of the C-Au-500 NWs obtained from the LSV scans in the Ar- or CO 2- saturated 0.5 M KHCO 3. Scan rate: 5 mv/s. Fig. S9. CV of the C-Au-500 deposited on GCE in 0.1 M HClO 4. Scan rate: 50 mv/s. S12

13 Table S1: Summary of Au electrocatalysts studied for electrochemical reduction of CO2 to CO. Onset Catalyst Electrolyte & potential FE ma/cm 2 A/g Au Ref ph (vs.rhe) ~ -0.35V ~-0.35V ~-0.35V & FE 0.5 M 500 nm long KHCO 3/CO V 94% This work Au NWs % (-0.35 V) 0.5 M 100 nm long KHCO 3/CO V 45% This work Au NWs 7.2 N/A (-0.35 V) 0.5 M 4 nm Au KHCO 3/CO V 14% N/A NPs 7.2 N/A (-0.37 V) 0.5 M 8 nm Au KHCO 3/CO V 22% N/A NPs 7.2 N/A (-0.37 V) 0.1 M Au 25 KHCO 3/CO V 4.2% N/A N/A 16 clusters 7.0 ~7% V 0.5 M Oxide- NaHCO 3/CO V 96% < 3.84 N/A 17 derived Au 7.2 ~10% (-0.35 V) 0.5 M Au Foil KHCO 3/CO V (~87%) 5.5 (aqua regia 7.2 N/A (-0.52 V) (-0.52 V) N/A 18 etched) References 1. Lee, Y.; Loew, A.; Sun, S. H. Chem. Mater. 2010, 22, Zhu, W.; Michalsky, R.; Metin, Ö.; Lv, H.; Guo, S.; Wright, C. J.; Sun, X.; Peterson, A. A.; Sun, S. J. Am. Chem. Soc. 2013, 135, Tremiliosi-Filho, G.; Dall'Antonia, L. H.; Jerkiewicz, G. J. Electroanal. Chem. 1997, 422, Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1992, 327, Mortensen, J. J.; Hansen, L. B.; Jacobsen, K. W. Phys. Rev. B 2005, 71, Enkovaara, J.; Rostgaard, C.; Mortensen, J. J.; Chen, J.; Dulak, M.; Ferrighi, L.; Gavnholt, J.; Glinsvad, C.; Haikola, V.; Hansen, H. A.; Kristoffersen, H. H.; Kuisma, M.; Larsen, A. H.; Lehtovaara, L.; Ljungberg, M.; Lopez-Acevedo, O.; Moses, P. G.; Ojanen, J.; Olsen, T.; Petzold, S13

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