Monodisperse Au Nanoparticles for Selective Electrocatalytic Reduction of CO 2 to CO

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1 Supporting Information Monodisperse Au Nanoparticles for Selective Electrocatalytic Reduction of CO 2 to CO Wenlei Zhu, Ronald Michalsky, Önder Metin,, Haifeng Lv, Shaojun Guo, Christopher Wright, Xiaolian Sun, 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 Department of Chemistry, Faculty of Science, Atatürk University, Erzurum, Turkey This file includes: Materials and Experimental Methods Calculation Methods Figure S1 to S6 1. Materials and Experimental Methods 1.1. Reagents Oleylamine (>70%), 1,2,3,4-tetrahydronaphthalene (tetralin), borane t-butylamine complex (TBAB) and potassium bicarbonate ( 99.99% trace metals basis, % dry basis) were purchased from Sigma Aldrich. Hydrogen tetrachloroaurate (III) hydrate (HAuCl 4 3H 2 O) was purchased from Strem Chemicals. 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 Characterization TEM images were obtained on a Philips EM 420 (120 kv). Samples for TEM analysis were prepared by depositing a drop of diluted NP dispersion in hexane on carbon-coated copper grids. XRD patterns were collected on a Bruker AXS D8- Advanced diffractometer with Cu Kα radiation (λ = Å). ICP 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 S1

2 carried out on a Joel JSM-6060 scanning electron microscopy (SEM) where samples were deposited on a graphitized porous carbon support for analysis. Electrochemical measurements were carried out with the EC-Lab VSP Ultimate electrochemical workstation with Ag/AgCl (filled with 4.0 M KCl) as a reference and Pt wire/gauze as a counter electrode. 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. Probe temperatures were calibrated using ethylene glycol. Solution ph was measured with the Thermo Scientific Orion VERSA STAR ph Benchtop Meter Experiments Synthesis of 6 nm Au NPs. 0.2 g HAuCl 4 was dissolved in 10 ml of tetralin and 10 ml of OAm at 4 C under Ar flow and vigorous magnetic stirring. 0.5 mmol TBAB 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 4 C before 40 ml of acetone was added to collect Au NPs via centrifugation (8500 rpm, 8 min). The product was dispersed in 20 ml hexane, precipitated out by adding 40 ml of ethanol and centrifugation, and re-dispersed in hexane. 4 nm Au NPs were synthesized similarly but the reaction temperature was controlled at 45 C. Synthesis of 8 nm Au NPs. 30 mg of 6 nm Au NPs was added to a solution made from 10 ml ODE, 10 ml OAm and 0.1 g HAuCl 4 at room temperature under Ar flow and vigorous magnetic stirring. The reaction solution was then heated to 80 C at 5 C/min and kept at this temperature for 2 h. The reaction solution was cooled down to room temperature and 8 nm Au NP product was separated as described in the synthesis of 6 nm Au NPs. 10 nm Au NPs were synthesized similarly by mixing 30 mg of 8 nm Au NPs with 10 ml ODE, 10 ml OAm and 0.12 g HAuCl 4. Catalyst Preparation. Au NPs were deposited onto Ketjen Carbon (C) (EC300J) by sonicating the mixture of Au NP dispersion in hexane and C, giving C-Au. The C-Au was annealed overnight in air at 180 C to remove the surfactant. After cooling down to room temperature, the C-Au was grind with polyvinylidene fluoride (PVDF), then a few drops of N-methyl-2-pyrrolidone (NMP) to produce catalyst paste that could be painted directly onto the carbon paper support. The catalyst was dried overnight in a vacuum-oven. Electrocatalytic Reduction of CO 2. EC-Lab VSP Ultimate electrochemical workstation was used to conduct CO 2 reduction experiments in aqueous 0.5 M KHCO 3 saturated with CO 2 at ph of A 99.9% 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 RHE reference: 1 E (vs. RHE) = E (vs. Ag/AgCl) V. The experiments were performed in a gas-tight two-compartment electrochemical cell separated by an anion exchange membrane (Nafion, 212). Each compartment contained 50 ml electrolyte and approximately 20 ml headspace. 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 S2

3 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 for analysis of all major C 1 to C 3 hydrocarbons and using a flame ionization detector (FID). The GC was calibrated using calibration standards commercially available from JJS Technical Services. The faradaic efficiency (FE) and mass activity for formation of j (= CO or H 2 ) were calculated as below: where: FE j = 2Fv jgp 0 RT 0 i total 100% Mass activity j = 2Fv jgp 0 RT 0 βm cat v j (vol%) = volume concentration of j = CO or H 2 in the exhaust gas from the electrochemical cell (GC data) G (ml/min at room temperature and ambient pressure) = Gas flow rate measured by a FL-1802 rotor meter (Omega Engineering) at the exit of the electrochemical cell i total (ma) = steady-state cell current β (g/g) = Au weight ratio in total carbon-supported Au NPs m cat (g) = Catalyst weight p 0 =1.01 x 10 5 Pa, T 0 = K, F = C mol -1, R = J mol -1 K -1 HCO 2 H was quantified either with a Bruker Avance DRX 400 or 600 spectrometer. For analysis, a 0.5 ml aliquot of the electrolyte was mixed with 0.1 ml D 2 O and 1.67 ppm (m/m) dimethyl sulfoxide (DMSO, Sigma, 99.99%) as internal standard. The 1 H spectrum was measured with water suppression using a pre-saturation method. We should note that conducting electrochemical experiments under an Ar atmosphere yielded only H 2 at all tested voltages, no hydrocarbons or CO could be detected via GC analysis. 2. Computational Methods The (111), (211), and M13 facets were chosen in order to examine planar, stepped, and corner site reactivity, in consideration of prior literature across a large range of S3

4 particle sizes 2 and transition metals 3 that showed that these surfaces could capture the trends and established the principle that adsorption strengths correlate well with the local coordination number of the binding site. 4-6 Structure Optimizations. All density functional theory (DFT) calculations were performed with the Grid-based projector-augmented wave (GPAW) code. 7-9 Exchangecorrelation interactions were treated by the revised Perdew-Burke-Ernzerhof (RPBE) 10 functional. All calculations were run in spin-paired mode, consistent with previous studies of binding energies on gold surfaces and particles 2. Free energies were calculated on a 13-atom Au (Au 13 ) cluster with non-periodic boundary conditions and a facecentered cubic (fcc) crystal with the (111) facet exposed. The Au 13 cluster was chosen for representing the lower size-limit of an Au NP. The cuboctahedra cluster was constructed by cutting the (111) and (100) surfaces of fcc Au. The Au(111) surfaces was modeled with a 2x2x4 atom slabs periodically repeated in the directions parallel to the surface. To exclude possible adsorbate stabilization via hydrogen bonding, adsorption of COOH* was tested also on a 3x3x3 atom slab (which gave free energies that differed by less than 0.05 ev uncertainty of the DFT computations from the results for the 2x2x4 slab). The cluster calculations used a grid spacing of 0.2 Å and sampled the Brillouin-zone at the Γ-point. 11 A Fermi-Dirac smearing of 0.01 ev was used to achieve convergence and results were extrapolated to 0 K. To model on-top, bridge, three-fold and four-fold sites on Au NPs, structure optimizations on the Au 13 cluster were performed with all Au atoms fixed with a lattice constant corresponding to the bulk value except the atoms yielding a given adsorption site which were allowed to relax. 3,11 The cluster was surrounded by 8 Å of vacuum in all directions. The periodic systems employed a Monkhorst-Pack (4,4,1) k-point sampling of the Brillouin zone. The local energy minimizations on the periodic Au(111) slab were performed with the lower 2 layers constrained to the bulk geometry while the upper 2 layers were allowed to relax (optimize their geometric positions). A Fermi-Dirac smearing of 0.1 ev was used to achieve convergence and results were extrapolated to 0 K. The surface was modeled using 10 Å of vacuum in the direction perpendicular to the surface to decouple the interaction between the dipole moments of surfaces. Additionally, the introduction of a dipole layer in the vacuum between the surfaces was tested with DACAPO code 4-6 (which is not implemented for non-orthorhombic cells in GPAW) using the same computational details as outlined above. The difference of between both calculations for the periodic Au(111) and Au(211) models is in average 60 mev, i.e., not significantly above a general uncertainty of 50 mev for the reported calculations. All DFT calculations employed the line search BFGS algorithm for relaxing atomic geometries until the maximum force was less than 0.05 ev/atom. On the Au 13 cluster, the free energies of all adsorbates relevant to the reaction mechanism that is considered in the manuscript (CO 2, COOH, HCOH, CO, and H) are computed. Computations on Au(111) were constrained to the COOH adsorbate given the cited previous work on Au(111) and Au(211). Gibbs Free Energies. Gibbs free energies at K and MPa total pressure, G i, are calculated as below: G i = H i TS i = E i + U ZPE,i TS i S4

5 where H i and S i are the enthalpy and entropy of an atomic geometry i respectively, E i is the electronic energy determined via DFT-based structure optimization, U ZPE,i is the zeropoint vibrational energy, T is the temperature ( K). Adsorbates are treated using the harmonic approximation where all degrees of freedom are treated as frustrated harmonic vibrations and PV contributions are neglected. Thermodynamic properties were calculated from vibrational frequencies and standard statistical mechanical equations evaluated through the Atomic Simulation Environment (ASE) 7. The electronic energy for CO 2 contains a 0.45 ev correction, conventionally employed to correct for an overestimation by the DFT formalism. Free energies of a molecular reaction step along the CO 2 reduction coordinate: ΔG = p=products G p,i r=reactants G r,i are computed relative to one vacant catalytic surface site (marked with an asterisk), one CO 2 molecule and two free H + /e - pairs (based on the computational hydrogen electrode model at V vs. RHE). 12 For example, ΔG of carbon monoxide adsorbed at a surface site is given as: ΔG[CO ] = G[CO ] + G[H 2 O] (G[ ] + G[CO 2 ] + 2 G[H + + e ]) where G[CO*] is the free energy of CO adsorbed at a surface site (based on local energy minimization at all non-symmetric surface sites) and G[*] is the free energy of the bare cluster or surface respectively. Throughout the manuscript, all free energies are given with the same sign convention, i.e., negative values meaning exergonic reaction steps. The computed free energies are relative to stable gas phase molecules, i.e., CO 2, CO, H 2 O and H 2. Similarly, all referenced literature values were adjusted to the same reference energies, E ref. That is, literature values for adsorption energies of a chemical species j, E A,lit. [j], given for example, as E A,lit. [COOH*] = E[COOH*] - E[*] - E[COOH(gas)] were adjusted via E A,adj. [COOH *] = E A,lit. [COOH*] + E[COOH(gas)] - (E ref [C] + 2 x E ref [O] + E ref [H]). The reference energies are given by E[H] = 1/2 x E[H 2 ], E[C] = E[CO] + E[H 2 ] - E[H 2 O], or E[O] = E[H 2 O] - E[H 2 ] respectively. 13 Density of Adsorption Sites and Cluster Diameter. To determine the density of specific on-top adsorption sites on Au NPs up to 10 nm in diameter, Au clusters with one central Au atom and 1, 2, 3, closed shells, i.e., a total of 13, 55, 147, Au atoms with 0.832, 1.39, 1.95, nm cluster diameter were modeled. The density, δ k, of a specific adsorption on-top site k is given relative to the total mass of a cluster (l = total) or relative the mass of the surface atoms (atoms with coordination numbers below 12, the value for bulk Au) of a cluster or Au(111) or Au(211) (l = surface) respectively: δ k = k=specific site m Au,k / l=total or surface m Au,l 100% Specific adsorption on-top sites are Au atoms on the (111) plane (coordination number, CN 9), on the (100) plane (CN 8), at a (211) step or at the edge between the (111) and (001) planes on the clusters (CN 7) respectively, or atop a corner atom (CN 5). The particle diameter is given as the maximum distance between two opposite edge sites on a given Au clusters plus two times the covalent radius of Au. S5

6 a b c Figure S1. TEM images of (a) 4-, (b) 6-, and (c) 10-nm Au NPs. Figure S2. XRD patterns of the 4-, 6-, 8-, and 10-nm Au NPs. Figure S3. Total current on carbon paper containing 10 mg Ketjen carbon with and without 6mg 8 nm Au NPs on it. S6

7 a b Figure S4. (a) FE s and (b) mass activities of the C-Au-IL with 8 nm Au NPs and various IL loadings at low overpotentials. Figure S5. Mass activities of different C-Au-IL at all tested potentials in the presence of 10 µl IL. Figure S6. Time-dependent mass current density change on the C-Au (8 nm Au NPs) during electrochemical reduction of CO 2 at various potentials. S7

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