Catalytic Pt-on-Au Nanostructures: Why Pt Becomes More Active on Smaller Au Particles

Transcription

1 Supporting Information Catalytic Pt-on-Au Nanostructures: Why Pt Becomes More Active on Smaller Au Particles Gui-Rong Zhang, Dan Zhao, Yuan-Yuan Feng, Bingsen Zhang, Dang Sheng Su, Gang Liu, Bo-Qing Xu *, Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing , China. Fritz Haber Institute of the Max Planck Society, Faradayweg 4 6, d Berlin, Germany. National Center for Nanoscience and Technology, Beijing , China * Corresponding author: Prof. Bo-Qing Xu Department of Chemistry Tsinghua University Beijing, , China Tel.: Fax: bqxu@mail.tsinghua.edu.cn S1

2 1. Syntheses of Au-d and Pt m^au-d NPs. Au-d (d = ) NPs were prepared via a seed mediated growth method. The primary seeds with average diameters of 1.9 nm were prepared according to Tsukuda et al s method. 1 Briefly, 555 mg PVP (M W : 10000, Sigma-Aldrich) was added into 50 ml aqueous solution of mol L -1 HAuCl 4 (Acros). Subsequently, an ice-cold solution of NaBH 4 (Sigma-Aldrich) was quickly injected into this mixed solution to produce the primary Au seeds. The general seed mediated growth procedure for obtaining larger Au NPs was as followings. 2 A certain amount of Au seeds were added into the HAuCl 4 solution containing PVP (stabilizer). While the solution was stirring, an aliquot of freshly prepared ascorbic acid ( mol L -1 ) was added dropwise (3 ml min -1 ) into the above mixed solution. The final gold-atom concentration was fixed at mol L -1. Further details for the preparation were given in Table S1. The Au-1.9-AA sample, Au-1.9 sample treated with ascorbic acid (AA), was prepared as following: g AA was added into 50 ml of the Au-1.9 seeds (gold-atom concentration: mol L -1 ). The whole solution was then kept stirring for 24 h at room temperature to induce possible oxidation of the AA in solution. The molar ratio of AA-to-Au in this treatment was kept at AA/Au = 1.5, the same as in the syntheses of the bigger Au-d NPs (d = 3.2, 4.7, 8.0, 10.3, 14.0). TABLE S1. Detailed conditions for the preparation of Au-d NPs sample d seeds (nm) a V seeds (ml) b V HAuCl4 (ml) c V AA (ml) d d (nm) e Au ±0.4 Au ±0.5 Au ±0.7 Au ±1.3 Au ±1.8 Au ±1.9 a d seeds is the mean diameter of Au seeds. b The concentration of seeds by moles of Au is mol L -1. c The concentration of HAuCl 4 is mol L -1. d The concentration of ascorbic acid (AA) is mol L -1. e d is the average size of Au NPs. Pt m^au-d nanostructures were prepared by reducing PtCl 6 2- onto the as-prepared Au-d NPs using hydrogen as reductant according to the procedure described previously. 3,4 In brief, a solution containing mol L -1 K 2 PtCl 6 (Beijing Chemical Reagent Company) was added to the colloidal solution of gold. The gold-atom concentration was S2

3 kept constant at mol L -1 in the final solutions, while the amount of K 2 PtCl 6 in the solution was varied to change the overall atomic Pt/Au ratio (m). After saturation with high purity argon for 30 min, the solution was bubbled with high purity hydrogen for 2 h and then kept airtight for 48 h at room temperature to obtain Pt m^au-d nanostructures. 2. Syntheses of carbon supported Au-d and Pt m^au-d NPs. The carbon supported Au-d and Pt m^au-d NPs were prepared by mixing a desired amount of Vulcan XC-72 carbon black (BET surface area: 240 m 2 g -1, Cabot Company) with Au-d and Pt m^au-d hydrosols, followed by a careful adjustment of solution acidity to ph = 1.5 with 1.0 mol L -1 HNO 3 (Beijing Chemical Reagent Company), and then the suspension was refluxed for 2 h accompanied by vigorously stirring. The solid was separated by filtration, intensively washed with deionized water and air-dried at 110 o C for 2 h to give Au-d/C and Pt m^au-d/c samples. 3. Electrochemical characterization. Electrochemical measurements were performed on a potentiostat/galvanostat model 263A (Princeton Applied Research, AMETEK) controlled by PowerSuite software. A saturated calomel electrode (SCE) and a Pt wire (diameter = 0.5 mm) were used as reference and counter electrodes, respectively. All potentials reported in this work are given with respect to SCE. The preparation of the working electrode was done using the previously reported procedures. 3,4 A catalyst ink was prepared first by dispersing the catalyst powders with sonication in a desired amount of Nafion solution (5wt%, DuPont Company). The obtained ink was then carefully coated onto a piece of carbon paper (0.5 cm 2.0 cm) to cover an area of 0.5 cm 0.5 cm and dried under an infrared lamp. The overall loading of the catalyst in an electrode was controlled at 1.0 mg. The electrooxidation of formic acid was studied by recording cyclic voltammetry (CV) curves at a scanning rate of 20 mv s -1 from 0.2 to 1.0 V in 0.5 mol L -1 H 2 SO 4 containing 2.0 mol L -1 HCOOH. For CO-stripping measurements, the catalyst surface was first saturated with high purity CO by bubbling CO through the electrolyte at open circuit potential for 15 min. Then the remained CO was purged by flowing high purity nitrogen for another 15 min S3

4 before the measurement. The CO-stripping curves were recorded at a scanning rate of 20 mv s -1 from 0.2 to 1.0 V. In order to clearly identify the CO-stripping peak potentials, the as-measured CO-stripping signals were corrected by subtracting the background currents associating with the double-layer charging and surface oxidation, which were measured in the subsequent anodic-scan voltammograms. The ORR measurements were performed in O 2 -saturated 0.5 mol L -1 KOH (for Au-d/C) or 0.5 mol L -1 H 2 SO 4 (for Pt 0.10^Au-d/C) electrolyte using a glassy carbon rotating disk electrode (GC-RDE, cm -2, Pine Research Instrumentation) at a rotation rate of 1600 rpm, and polarization curves were recorded at a scanning rate of 10 mv s -1. Prior to the preparation of the working electrode, the GC-RDE was polished to a mirror finish using 0.5 and 0.05 μm alumina powder (CH Instruments, Inc.). A catalyst ink was prepared by sonicating a suspension of the catalyst (5.0 mg) in isopropanol (1.0 ml), and then 10 μl of the suspension was transferred onto the GC-RDE electrode. After solvent evaporation, 10 μl of Nafion solution (0.05wt%, DuPont Company) was cast onto the disk electrode to attach the catalyst. Prior to each measurement, high purity nitrogen or oxygen was used to saturate the electrolyte. During each measurement, a gentle nitrogen or oxygen flow was kept to avoid disturbance from ambient atmosphere. 4. Determination of electrochemically active surface area (EAS) and utilization of Pt (U Pt ). The EAS was obtained by measuring the charges associated with the hydrogen desorption signals (Q H ) on the CV curves in 0.5 mol L -1 H 2 SO 4 in the hydrogen UPD region, assuming 210 μc cm -2 for calibrating the desorption charge of a hydrogen monolayer on a Pt surface based on the following equation: EAS = Q H /(210 W Pt ) where W Pt is the weight of Pt in the working electrode. Given the well-established hydrogen-adsorption stoichiometry at a Pt surface (H : Pt = 1 : 1) 5,6, we could count the number of the exposed Pt atoms (N s ) as the number of adsorbed hydrogen atoms (N H ) which can be easily attracted from Q H. Thus, the ratio of N s to the total number of Pt atoms (N t, determined by ICP-AES) in the working electrode S4

5 gives the utilization (or dispersion) of Pt (U Pt ) in the catalyst: U Pt = N s /N t = N H /N t Additionally, we could correlate U Pt with EAS as following: 3,4 U Pt = N s /N t = (Q H /Q e )/(W Pt NA/M Pt ) = (Q H /(210 W Pt ))/(237 m 2 g Pt -1 ) = EAS/(237 m 2 g Pt -1 ) where EAS is in the unit of (m 2 g -1 Pt ); NA is the Avogadro constant ( mol -1 ); Q e is the elementary charge or charge of an electron ( C); M Pt is molar weight of Pt (195 g mol -1 ). 5. Estimation of the number density of Pt atoms (N Pt ) on the surfaces of Au-d in Pt m^au-d/c nanostructures. Estimation of the number density of Pt atoms on the surface of Au NPs (N Pt : number of Pt atoms per square nanometer Au surface) was based on an onion-like magic cluster model. 7 The particle size is dependent on the number of atomic layers (n). e.g., an Au-1.9 particle would be composed of 3 atomic Au layers containing a total number of 147 Au atoms (including the core atom). Since Pt and Au are neighbors in the periodic table of elements, and have very close lattice parameters and surface atom densities, the mono-layer coverage of Pt atoms on Au NP surfaces could be regarded as the ratio of n Pt /N s(au), which is given in the last column of Table S2. For Pt, the values for the surface atom density of three main low index planes (100), (110) and (111) are , and cm -2, respectively; the surface atomic density of Pt is then determined to be cm -2, that is 13 Pt atoms per square nanometer Pt-surface. 3,4 Thus, a full monolayer of Pt on Au NP would mean N Pt = 13. For sub-monolayer Pt coverage, the number density of Pt atoms on the surface of any Au NP could be obtained as: N Pt = 13 (n Pt /N s(au) ) S5

6 TABLE S2. Calculations of the ratio of n Pt /N s(au) for Pt m^au-d particles sample d Au (nm) N (Au) a N s(au) b n Pt c n Pt /N s(au) Pt 0.10^Au-1.9/C 1.9 ± Pt 0.20^Au-1.9/C 1.9 ± Pt 0.10^Au-3.2/C 3.2 ± Pt 0.15^Au-3.2/C 3.2 ± Pt 0.10^Au -4.7/C 4.7 ± Pt 0.10^Au-8.0/C 8.0 ± Pt 0.10^Au-10.3/C 10.3 ± Pt 0.10^Au-14.0/C 14.0 ± a Average number of Au atoms in one Au-d NP. b Average number of the surface Au atoms in one Au-d NP. c Average number of Pt atoms in one Pt m^au-d NP. 6. Estimation of the numbers of surface Au atoms at different locations (corners, edges, (111) and (100) facets) for Au-d NPs. Estimation of the fractions of surface Au atoms at (100) facets, and edge/corner positions on Au-d NPs of different sizes was also based on the magic cluster model. The Au NPs were classified into two classes according to their particle structure: single crystal (cubo-octahedron) and 5-fold twinned crystal (treated as icosahedron). The calculations for numbers of surface Au atoms at the different positions are summarized in Table S3. The fraction of surface Au atoms at corner, edge and (100) facets was determined by calculating the contribution from both cubo-octahedron and icosahedrons as depicted in Figure S7, using the shape selectivity data of the Au NPs (Figure 3b). TABLE S3. Calculations for the numbers of surface atoms at different positions a particle structure N (100) b N e,c c N (111) d (N (100) + N e,c )/N st e Cubo-octahedron 6(n-1) 2 12(2n-1) 4(n-1)(n-2) 3(n 2 +2n-1)/(5(n 2 +1)) Icosahedron - 6(5n-3) 10(n-1)(n-2) 3(5n-3)/(5(n 2 +1)) a Where n refers to the atomic layer number for a Au cluster. b Number of atoms in (100) facets. c Number of atoms at edges and corners. d Number of atoms in (111) facets. e Total number of surface atoms. S6

7 8. Determination of the kinetic currents for ORR on Au-d/C and Pt m^au-d/c catalysts. For the ORR measurement using RDE technique, the Koutecky-Levich equation can be described as follows: where i is the experimentally measured current, i k is the kinetic current and i d the diffusion-limiting current. The mass-transport corrected kinetic current was calculated based on the following equation: The kinetic current was normalized to the EAS of Pt for every Pt 0.10^Au-d/C catalyst to obtain the Pt intrinsic activity (IA Pt ) for ORR. These IA Pt data, in terms of kinetic current normalized to the EAS of Pt, are given in the inset of Figure 7a. S7

8 Figure S1. Representative TEM images of Pt 0.10^Au-d NPs and their carbon-supported counterparts. (a) Pt 0.10^Au-1.9, (b) Pt 0.10^Au-1.9/C, (c) Pt 0.10^Au-4.7, (d) Pt 0.10^Au-4.7/C, (e) Pt 0.10^Au-10.3, and (f) Pt 0.10^Au-10.3/C. S8

9 Figure S2. UV-vis spectra of the as-prepared Au-d NPs (dotted lines) and Pt 0.10^Au-d nanostructures (solid lines). (a,a ) d = 1.9 nm, (b,b ) d = 3.2 nm, (c,c ) d = 4.7 nm, (d,d ) d = 8.0 nm, (e,e ) d = 10.3 nm, and (f,f ) d = 14.0 nm. S9

10 (1) (2) (3) Figure S3. Schematic illustration of Pt deposition onto an Au NP. The deposited Pt atoms would prefer to locate at the edges (1), corners (2) and (100) facets (3) of the Au NP. S10

11 Figure S4. Plot of U Pt as a function of average size of Au NPs in Pt 0.10^Au-d/C samples. Downsizing of the Au NPs leads to steady enhancement in U Pt. S11

12 Figure S5. Cyclic voltammograms of formic acid electrooxidation on Pt 0.10^Au-d/C catalysts. The black solid lines are for anodic and red dotted lines for cathodic scans. S12

13 Figure S6. Anodic-scan CV curves for formic acid electrooxidation on Pt 0.10^Au-1.9/C and Pt 0.10^Au-1.9-AA/C catalysts. S13

14 Figure S7. Schematic showing of the calculations for surface atoms at different locations on truncated-octahedron (upper) and icosahedron (lower) particles. Atoms at different locations are differently colored (edge/corner: green; (100) facets: blue; (111) facets: orange). S14

15 Figure S8. Representative TEM images and their corresponding size histograms of Au-d/C samples. (a) Au-1.9/C, (b) Au-3.2/C, (c) Au-4.7/C, (d) Au-8.0/C, (e) Au-10.3/C, and (f) Au-14.0/C. S15

16 Figure S9. Correlations between IA Au toward ORR of the Au-d NPs (black) and fraction of Au surface atoms at corners, edges and (100) facets (red) with the particle size of Au. S16

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