Supporting Information

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1 Supporting Information Exceptional Size-Dependent Activity Enhancement in the Electroreduction of CO 2 over Au Nanoparticles Hemma Mistry 1, Rulle Reske 2, Zhenhua Zeng 3, Zhi-Jian Zhao 3, Jeffrey Greeley 3, Peter Strasser 2*, Beatriz Roldan Cuenya 4* 1 Department of Physics, University of Central Florida, Orlando, FL 32816, US 2 Department of Chemistry, Chemical Engineering Division, Technical University Berlin, 1623 Berlin, Germany 3 School of Chemical Engineering, Purdue University, West Lafayette, IN 4797, USA 4 Department of Physics, Ruhr-University Bochum, 4478 Bochum, Germany * Beatriz.Roldan@rub.de; pstrasser@tu-berlin.de Nanoparticle Synthesis Au nanoparticles (NPs) were synthesized via inverse micelle encapsulation as follows: poly(styrene)-block-poly(2-vinylpyridine) (PS-P2VP) diblock copolymers (Polymer Source, Inc.) were dissolved in toluene to form micelles, which were loaded with HAuCl 4 and stirred for 2 days. To control particle size, polymers with different PVP molecular weight were used in each solution, and the metal loading, or metal salt to PVP molar ratio, was varied. Synthesis parameters for the nine samples used in this study can be found in Table S1. The solutions were then filtered, and glassy carbon substrates were dip-coated into each solution. In order to remove the encapsulating polymers, the glassy carbon supported Au NPs were cleaned using an O 2 plasma etcher for 2 minutes at 1 W. To increase the NP density on the support, the glassy carbon substrates were dip-coated and O 2 -plasma treated a total of three times. Nine identical samples were simultaneously prepared on SiO 2 /Si(111). These were measured using XPS to ensure that the O 2 plasma treatment was sufficient for complete polymer removal by the absence of a C-1s signal. Due to the relative roughness of the support, the nanometer-sized Au NPs deposited on glassy carbon could not be accurately resolved with AFM. Therefore, the SiO 2 /Si(111)-supported NPs prepared with the same colloidal solution were additionally used to characterize the NP size and coverage resulting from the multiple dip-coating via AFM. S1

2 Electrochemical Measurement Details Electrochemical measurements were performed in an air-tight electrochemical cell with a three-compartment, three-electrode design. A platinum mesh 1 (Sigma Aldrich 99.9 %) formed to a cylinder was used as counter electrode (CE) and a leak-free Ag/AgCl electrode as reference electrode. The glassy carbon samples with Au NPs were contacted with a gold clamp and used as working electrode (WE). The active geometric surface area of the particle-covered support was 3 cm². 25 ml.1 M KHCO 3 was used as electrolyte and purged with CO 2 (3 ml/min) from the bottom of the cell under the WE until a final stable ph of 6.8 was reached. The CO 2 saturation of the gas atmosphere was controlled with an in situ mass spectrometer. H 2 and CO were analyzed using a gas chromatography (GC) system. The working electrodes were immersed into the electrolyte under potential control at E = +.22 V/RHE. A linear voltammetric sweep was performed with a scan rate of -5 mv/s between E = +.22 V/RHE and E = V/RHE followed by a chronoamperometric measurement at E = V/RHE for 1 min. At the end of the amperometric step, product gas samples were analyzed. CO 2 gas was bubbling through the electrochemical cell at a constant excess flow rate of 3 sccm throughout the electrochemical measurements. All potentials are reported with respect to the reversible hydrogen electrode (RHE) and were corrected by the experimental voltage loss (IR-drop) caused by the uncompensated resistances of the electrolyte and external electrical contacts and connections. ICP-OS measurements of Pt ion concentration before and after the electrochemical experiments showed no detectable evidence of dissolved Pt ions, confirming the lack of any significant dissolution of the Pt anode (counter electrode) during the experiments. Surface-area normalized catalytic activity and selectivity values were calculated from raw current data by subtracting data measured on a bare glassy carbon support and subsequently dividing by the Au surface area for each sample, A Au, according to Table S1. AFM Nanoparticle Characterization AFM images were acquired using the peak force mode of a Multimode 8 AFM (Bruker). For each sample, the average NP height, h p, was measured. Assuming spherical particles, the average height was used to calculate the average NP surface area as A Au = 4π(h p /2) 2, which was S2

3 multiplied by NP density to estimate the total surface area of Au on each sample. All details related to the NP synthesis and size characterization are included in Table S1. Au Nanoparticle Shape Modeling To construct spherical Au NP models, a MATLAB code was used to generate an fcc structure with lattice constant (L) = 4.8Å. Model shapes were derived from this structure by excluding atoms outside of a sphere with diameter corresponding to the NP diameter. For the database to be comprehensive, different symmetry centers (SC) were considered with (i) the SC located on one of the Au atoms and (ii) the SC shifted.5l along one of the cartesian axes (e.g. x-axis). In addition, the coordination number of all individual atoms was calculated, with atoms of CN 12 considered as bulk atoms and those with CN below 12 as surface atoms. Figure S4a shows three of the model shapes generated for 1.1, 3.2, and 7.7 nm Au particles, and the population density of atoms with different coordination number (CN) as a function of NP size is shown in Figure S4b. It has been well-established that our micellar NPs are approximately spherical in shape after synthesis (i.e., after ligand removal with atomic oxygen at room temperature), S1-4 as demonstrated in Figure S2. Therefore, it is expected that energetically unfavorable low-coordinated atoms will be present on the NP surface, as opposed to lowerenergy facets. We have shown in our previous work that high-temperature annealing treatments are required before significant faceting of our NPs can occur. S5 Consequently, while faceted structures have been shown to be the most favorable for annealed NPs, such as the models constructed by Artrith and Kolpak, S6 we chose spherically shaped NP structures (see Figure S4) as a reasonable model approximation of our unannealed micellar catalysts, as opposed to faceted shapes. Au Foil Measurements Comparison of data measured on Au foils in the literature needs to be considered with care since the selectivity towards CO has been shown to change drastically with potential, S7 and so attention needs to be paid to the potential scale and electrolyte ph used by different groups. Our results can be directly compared to the Au foil measured by Kauffman et al., S8 since the RHE potential scale is also used and the measurement conditions were identical to ours. At E = V vs. RHE, they saw 2% faradaic efficiency towards CO, which agrees well with our S3

4 measurement. Measurements of a Au foil were performed by Hori in a ph = 6.7.1M KHCO 3 solution and are given at E = V vs. SHE, showing 8-9% faradaic efficiency towards CO. S9 Converting this potential to the RHE scale gives approximately E = -.75 V vs. RHE, therefore these results cannot be directly compared to ours measured at E = V vs. RHE. Noda et al. S7 showed the change in faradaic efficiency towards CO at different potentials vs. Ag/AgCl. Maximum selectivity to CO of 81.5% was seen at E = V vs. Ag/AgCl, which is approximately E = -.66 V vs. RHE. As the potential is increased, CO efficiency drops to 15.8% at E = -1.6 V vs. Ag/AgCl, the highest potential they measured, which converts to approximately E = -1. V vs. RHE. This trend agrees with both our results and those of Hori. Quantum Size Effects In addition to the larger population of low-coordinated atoms at the surface of small NPs, the quantum size effects responsible for the alteration of the electronic structure should also be taken into account. DFT calculations on Au clusters ranging from 13 to 1415 atoms ( nm in diameter) were used to investigate the binding energy of CO and O adsorbates. S1,11 To deconvolute the effect of coordination number, the adsorption of CO and O was considered only on Au surface atoms with similar coordination numbers for all cluster sizes. It was found that for Au NPs with more than 56 atoms (2.7 nm in diameter), the electronic structure converges to that of bulk Au. S1-12 Thus, quantum size effects may influence surface adsorption energies for particle sizes smaller than this critical value, and these effects will be in addition to coordination effects, as discussed in the main text. We note that, with the exception of highly electron withdrawing adsorbates on very specific cluster sizes (e.g., atomic O on Au 55 ), these combined effects lead to much stronger binding, overall. In other words, not only do Au NPs smaller than 2.7 nm have a higher population of low-coordinated atoms, but also each of the surface atoms on these small NPs would bind most adsorbates more strongly than similarly coordinated atoms in larger NPs. Although the modified electronic structure should be taken into account, it is still very likely that the main effect that dominates the enhanced activity of our small Au NPs is the higher population of low-coordinated atoms. S4

5 DFT calculations and thermodynamic analysis Our DFT calculations were carried out with the plane-wave based Vienna ab initio simulation package (VASP). S13,S14 We used the generalized-gradient approximation (GGA) in the form of the exchange-correlation functional Perdew-Burke-Ernzerhof (PBE). S15 The interaction between atomic cores and electrons was described by the projector augmented wave (PAW) method. S16, S17 The valence wave functions were expanded in a plane-wave basis with a cutoff energy of 4 ev. The Brillouin zone was sampled using a ( ) Monkhorst-Pack S18 mesh for the bulk fcc Au unit cell. The lattice constant was determined to be 4.17 Å with the above parameters, which is a typical result with GGA and 2.2% larger than the experimental value of 4.8 Å. K-point grids of comparable density were used for the surface calculations [(4x4x1) for Au(111) and (5x4x1) for Au(211)], and single k-point (gamma point) was used for Au clusters in (2.9 Å 21 Å 21.1 Å) and molecules in (1 Å 1.1 Å 1.2 Å) orthorhombic boxes. The Au(111) and Au(211) surfaces were chosen to represent the terraces and edges on large Au nanoparticles, while Au 55 and Au 38 clusters were chosen to represent the corner sites of Au nanoparticles, and also as providing two typical models for small Au particles with sizes around 1 nm. The Au(111) and Au(211) surfaces were modeled by repeated slabs separated by vacuum layers with at least 1 Å of vacuum after the adsorption of intermediates. The slabs are composed of 5 layers in the (111) direction, in which the three bottom layers were kept fixed at the theoretical bulk-terminated geometry (Au Au = 2.95 Å ) and the remaining Au atoms were allowed to relax during geometry optimizations, together with the adsorbates, until the force on each atom was less than.2 ev/å. We used a (3 3) unit cell for Au(111) and a (3 1) unit cell for Au(211), both of them containing 9 surface Au atoms per supercell. The Au 55 and Au 38 clusters were cut with cuboctahedral and truncated-octahedron shapes, and all Au atoms and adsorbates were allowed to relax during the geometry optimization (see Figure S7). In order to understand the experimentally observed trends, both CO 2 electroreduction and hydrogen evolution reactions were considered. For the CO 2 electroreduction to CO, the following reaction mechanism was studied: *+CO 2 (g)+2h + (aq)+2e - COOH*+H + (aq)+e - (S1) COOH*+H + (aq)+e - CO*+H 2 O(l) (S2) CO*+H 2 O(l) *+CO(g)+H 2 O(l) (S3) S5

6 For hydrogen evolution, we simply considered *+H + (aq)+e - H* (S4) H* *+.5H 2 (g) (S5) Here, the asterisk (*) denotes a site on the surface. For the adsorption of intermediates COOH*, CO* and H*, a series of many possible adsorption sites are considered on Au surfaces and particles. The reaction free energy, e.g. the adsorption free energy of COOH* in reaction S1, was calculated with ΔG S1 = ΔG ad (COOH*)=G(COOH*)-G(*)-G(CO 2 )-G(H + +e - ) (S6-1) and can be rewritten as ΔG S1 = ΔG ad (COOH*)= G(COOH*)-G(*)-G(CO 2 )-.5G(H 2 )+eu (S6-2) based on the computational hydrogen electrode reference state. S19 Namely,.5H 2 (g) H + (aq)+e - is in equilibrium at V vs RHE, which gives a relation G(H + +e - )=.5G(H 2 )-eu. Here, G(COOH*), G(*), G(CO 2 ) and G(H 2 ) are the free energy of the COOH* intermediate, the surfaces or particles before COOH adsorption, and CO 2 and H 2 in gas phase, respectively. The Gibbs free energy, G, is calculated with the standard formula: G=E DFT +ZPE+ δh -TS (S7) Where E DFT, ZPE, δh and TS are the total energy from DFT calculations, the zero point energy, the integrated heat capacity, and the product of the temperature (T) and the entropy (for liquid water, we actually calculated the total energy of gas phase water, along with a correction to the entropy, as discussed below). Equation S6-2 can be rewritten as ΔG S1 =G ad (COOH*)=E ad (COOH*)+ΔZPE+ΔδH - TΔS +eu (S6-3) For gas phase species, ZPE, δh and TS are taken from standard tables (numerical values are given in the tables below). For liquid water, the TS was calculated based on gas phase water but at.35 atm the vapor pressure of liquid water at room temperature, through the equation: TS=TS +k B Tln(p/p ) (S8), where p is partial pressure and p is 1 atm. ZPE, δh 298 and TS were neglected for surface species, due to relatively small values and effective cancellation, except the ZPEs of the adsorbates, which were obtained using the vibrational frequencies calculated. The adsorption energies (E ad ) with respect to stable gas phase species are given in Table S3-Table S6. The most favorable reaction/adsorption free energies (ΔG) under the standard conditions and V are also S6

7 given in Table S3-Table S6 and plotted in Fig. 4 in the main text along the reaction paths. We note that, as it is well known experimentally that CO prefers top site adsorption on Au surfaces, instead of multi-fold sites erroneously predicted from semi-local functionals, S2 we only consider the most favorable top site for CO* adsorption. From Table S3-Table S6 and Fig. 4 in the main text, we see that CO adsorption with respect to gas phase CO is very strong on Au particles, in comparison with H* at V. On the other hand, since H* adsorption from protons and electrons is strongly favored at lower potentials, H* might also be present at relatively high coverages on the particles, steps and even terraces, at least at very negative potentials. These considerations suggest that coverage effects for CO and H might have important consequences for the size-dependent trends in CO 2 electroreduction chemistry. A very simple estimate of the coverages of CO* and H* at different potentials was made using Langmuir adsorption thermodynamics. For hydrogen adsorption, we considered Equation S4. For CO adsorption, we consider the reverse reaction of Equation S3, i.e. CO+* CO* (S9) By assuming [1] the adsorptions are fast process and are in equilibrium, [2] there is no lateral interaction between CO*-CO*, CO*-H* and H*-H* (or similar lateral interactions), and [3] the reactions are under standard conditions, then the coverage of CO* and H* can be expressed as and The plots are given in Table S4 using the data in Table S3-Table S6. The isotherms shown in Table S4 clearly indicate that, though there may be some CO accumulation problem at potential higher than.3 V vs RHE, the surface would be fully covered by H* at the experimental potential -1.2 V, which emphasizes the possible importance of the H* coverage effect on CO 2 electroreduction. Although it is challenging to quantify this effect, particularly for small nanoparticles, a basic estimate of its importance could be obtained on the single crystal surfaces. Here we calculated COOH* and CO* on Au(111) and Au(211) in the S7

8 presence of a full monolayer of adsorbed hydrogen. The results at V vs RHE are given in Fig. 4 of the main text with dashed lines. S8

9 Table S1. Synthesis parameters for Au NPs, including polymer molecular weight, metal salt to P2VP ratio (loading), particle size as measured by AFM, and surface area of Au normalized by the geometric surface area of the support. Sample Name Polymer Loading Particle Size h p (nm) Au Surface Area A Au (cm 2 Au/cm 2 geo) Au1 PS(26)-P2VP(48) ± 1..6 Au2 PS(534)-P2VP(88) ±.4.6 Au3 PS(91)-P2VP(1) ± Au4 PS(91)-P2VP(1) ±.8.26 Au5 PS(17)-P2VP(98) ± Au6 PS(91)-P2VP(1) ± Au7 PS(53)-P2VP(438) ± Au8 PS(534)-P2VP(88) ± Au9 PS(33)-P2VP(46) ± Table S2. Thermodynamic data of gas-phase species. Zero-point energies (ZPE) are calculated with experimental vibrational data, the integrated heat capacity (δh ) and entropy (S) at K are obtained from Ref. S 21. For water, the entropy is calculated at.35 bar through S S kbt ln( p / p ), because at this pressure gas-phase H 2 O is in equilibrium with liquid water at K. Molecules ZPE δh TS H H 2 O CO CO S9

10 Table S3. Adsorption properties on Au(111). Reference states are H 2 (g), CO 2 (g) and H 2 O(l). For CO*, values in bracket are with reference states of CO(g). The exact location of each site can be found in Fig. S6. Adsorbate Site ID Site Name E ad (ev) ZPE (ev) ΔG (ev) H 1 top.36 2 bridge.19 3 fcc hcp.14 CO 1 top.44 (-.29).17.9 (.28) 2 bridge.38 (-.35) 3 fcc.41 (-.32) 4 hcp.42 (-.31) COOH 1 top Table S4. Adsorption properties on Au(211). Reference states are H 2 (g), CO 2 (g) and H 2 O(l). For CO*, values in bracket are with reference states of CO(g). The exact location of each site can be found in Fig. S6. Adsorbate Site ID Site Name E ad (ev) ZPE (ev) ΔG (ev) H 1 top@edge.27 2 bridge@edge bridge@111-edge.2 4 bridge@1-edge.17 5 fcc@111-edge.18 CO 1 top@edge.17 (-.56) (.1) 2 bridge@edge.17 (-.56) 3 bridge@111-edge.46 (-.27) 4 bridge@step-facet.45 (-.28) 5 fcc@111-edge.52 (-.21) COOH 1 top@edge S1

11 Table S5. Adsorption properties on Au 55. Reference states are H 2 (g), CO 2 (g) and H 2 O(l). For CO*, values in bracket are with reference states of CO(g). The exact location of each site can be found in Fig. S6. Adsorbate Site ID Site Name E ad (ev) ZPE (ev) ΔG (ev) H 1 top@corner.32 2 top@211-edge.11 3 top@1.3 4 bridge@211-edge bridge@1-edge fcc@111-edge.26 8 hcp@111-edge.8 9 hollow@1-edge.17 CO 1 top@corner -.15(-.88) (-.29) 2 top@211-edge.11 (-.62) 3 top@1.3 (-.7) 4 bridge@211-edge.8 (-.65) 5 bridge@1-edge -.11 (-.84) 6 bridge@111-edge.27 (-.46) 7 fcc@111.3 (-.43) 8 hcp@ (-.49) COOH 1 top@corner.39 2 top@211-edge top@1.4 S11

12 Table S6. Adsorption properties on Au 38. Reference states are H 2 (g), CO 2 (g) and H 2 O(l). For CO*, values in bracket are with reference states of CO(g). The exact location of each site can be found in Fig. S6. Adsorbate Site ID Site Name E ad (ev) ZPE (ev) ΔG (ev) H 1 top@corner top@ bridge@211-edge bridge@221-edge bridge@111-edge fcc@111-edge.14 7 hcp@111-edge hollow@1-edge.52 CO 1 top@corner -.11 (-.84) (-.25) 2 top@ (-.78) 3 bridge@211-edge -.21 (-.94) 4 bridge@221-edge -.16 (-.89) 5 bridge@111-edge -.2 (-.93) COOH 1 top@corner top@ S12

13 Frequency Frequency Frequency Frequency Frequency Frequency Frequency Frequency Frequency a) d) g) Au nm Particle Height (nm) Particle Height (nm) Au nm Au nm Particle Height (nm) b) e) h) Au nm Particle Height (nm) Au nm Particle Height (nm) Au nm Particle Height (nm) c) Particle Height (nm) f) Au nm Au nm Particle Height (nm) 1 i) nm Particle Height (nm) Figure S1 a)-i) Histograms of Au NP heights extracted from AFM images of samples Au1 Au9. Au9 S13

14 Au NP SiO 2 Si 5nm Figure S2. Cross sectional TEM image of a spherical Au NP on SiO 2 /Si(111) prepared by inverse micelle encapsulation with polymers removed by an O 2 plasma treatment. S14

15 J / ma/cm² Au Au foil Au8 Au9 Au7-1 Au6 Au1 ( nm) Au5-2 Au2 (1.4.4 nm) Au4 Au3 ( nm) -3 Au4 (3.2.8 nm) Au5 ( nm) -4 Au3 Au6 ( nm) -5 Au7 ( nm) Au2 Au8 ( nm) -6 Au9 ( nm) Au1 Au foil E / V RHE Figure S3. Linear sweep voltammetry of CO 2 electroreduction over all nine Au NP catalyst samples and a gold foil. The data were acquired in.1 M KHCO 3 with -5mV/s scan rate, and normalized by Au surface areas after subtraction of a background signal measured on clean glassy carbon. S15

16 Figure S4. a) Models of spherical Au NPs with a diameter of 1.1, 3.2, and 7.7 nm, corresponding to the experimental samples Au1, Au4, and Au9, respectively. b) Population (relative ratio) of surface atoms with a specific coordination number (CN) as a function of particle diameter. Inset shows a model of a 3.2 nm particle with surface atoms color-coded according to their first neighbor CN: CN<8 (grey), CN=8 (blue), CN=9 (red), CN>9 (green). S16

17 Coverage (ML) H/Au(111) H/Au(211) H/Au55 H/Au38 CO/Au(111) CO/Au(211) CO/Au55 CO/Au Potential (V vs RHE) Figure S5. The coverage of CO* and H* on Au surfaces and particles at different potentials based on using Langmuir adsorption thermodynamics. S17

18 (a) (b) (c) (d) Figure S6. Au models used in current calculations: (a) Au(111); (b) Au(211); (c) Au 55 ; (d) Au 38. Numbers indicates the sites reported in Table S3, Table S6. S18

19 (a) (b) (c) (c) (d) (e) (g) (h) (i) (j) (k) (l) Figure S7. Geometry of CO, COOH and H adsorbed on (a-c) Au(111); (d-e) Au(211); (g-i) Au 55 and (j-l) Au 38 at low coverage. S19

20 (a) (b) (c) (d) Figure S8. Geometry of CO and COOH adsorbed on (a-b) Au(111); (c-d) Au(211) with 1ML coadsorbed H. S2

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