Surface Composition Tuning of Au-Pt Bimetallic Nanoparticles for Enhanced Carbon Monoxide and Methanol Electro-oxidation

Transcription

1 Supporting Information Surface Composition Tuning of Au-Pt Bimetallic Nanoparticles for Enhanced Carbon Monoxide and Methanol Electro-oxidation Jin Suntivich 1,, Zhichuan Xu 2,, Christopher E. Carlton 2, Junhyung Kim 2, Binghong Han 1, Seung Woo Lee 2, Nicéphore Bonnet 1, Nicola Marzari 1, Lawrence F. Allard 4, Hubert A. Gasteiger 2,, *, Kimberly Hamad-Schifferli 2,3, *, Yang Shao-Horn 1,2, * 1 Department of Materials Science and Engineering 2 Department of Mechanical Engineering 3 Department of Biological Engineering Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA 4 Microscopy Group, Oak Ridge National Laboratory 1 Bethel Valley Rd., Building 4515, Oak Ridge, TN 37831, USA Present Address: Department of Chemistry, Technische Universität München, Lichtenbergstrasse 4, D Garching, Germany The authors have contributed equally to this article * Corresponding authors. Contact: hubert.gasteiger@tum.de, schiffer@mit.edu, shaohorn@mit.edu S1

2 Supporting methods Carbon loading. A solution of Vulcan XC-72 (Premetek, USA) was prepared by sonicating ~320 mg of Vulcan in 600 ml hexane (Sigma-Alrich) in an ice bath for 5 hours. Afterward, a solution of as-prepared AuPt NPs in hexane was added drop-wise into the Vulcan solution and the mixture was further sonicated for 2 additional hours and stirred overnight. The catalyst powders were collected by purging Ar (evaporating hexane) at room temperature and dried in vacuum for 24 hours. The metal loading of Au 0.5 Pt 0.5 /C catalyst was determined from thermogravimetric analysis (TGA) and Inductively Coupled Plasma Spectrometry (ICP). TGA analysis was performed on a (Q50, TA instruments) thermogravimetric analyzer within a temperature range from 25 to 800 C under Air (Airgas Co.) with a heating rate of 5 K/min. The metal loading given by TGA and ICP are 24.0% and 21.3%, respectively, agreeing with each other that the metal loading of Au 0.5 Pt 0.5 /C was reached as expected. Thermal treatment for the surface composition of 68% Pt surf Au 0.5 Pt 0.5 /C. ~30 mg Au 0.5 Pt 0.5 /C catalyst was placed in a tube furnace, and purged with dry air (~20% O 2, Airgas Co.) at a flow rate of 100 ml/min. The Au 0.5 Pt 0.5 /C catalyst was heated up to 250 C and maintained at this temperature for 30 minutes. The heating rate was set at 5 K/min. Afterward, the tube furnace was allowed to cool down naturally to room temperature. Thermal treatment for the surface composition of 30% Pt surf Au 0.5 Pt 0.5 /C. ~30 mg Au 0.5 Pt 0.5 /C catalyst was placed in a tube furnace, and purged with dry air (~20% O 2, Airgas Co.) at a flow rate of 100 ml/min. The AuPt/C catalyst was heated up to 250 C under Air and maintained at this temperature for 30 minutes. Then, the Air atmosphere S2

3 was replaced by high purity Ar (99.99%, Airgas Co.) and the gas flow rate was kept at 100 ml/min. The temperature was further increased up to 350 C with a heating rate of 5 K/min and the temperature was maintained for another 30 min. Afterward, the tube furnace was allowed to cool down to room temperature. Thermal treatment for the surface composition of 13% Pt surf Au 0.5 Pt 0.5 /C. ~30 mg Au 0.5 Pt 0.5 /C catalyst was placed in a tube furnace, and purged with high purity Ar (99.99%, Airgas Co.) at a flow rate of 100 ml/min. The Au 0.5 Pt 0.5 /C catalyst was heated up to 500 C with a heating rate of 5 K/min and maintained at this temperature for 30 min. After the thermal treatment, the tube furnace was allowed to cool down naturally to room temperature. TEM characterization and size analysis. ~2 µl of catalyst dispersed in ethanol was dropped onto ultrathin amorphous carbon coated TEM Cu grid and allowed to dry at room temperature. The TEM images are recorded on a JEOL 2010 TEM at 200 kv. It s found that the Au 0.5 Pt 0.5 /C catalysts gradually showed some slight and reasonable agglomerations as more thermal treatment steps were involved. The number-average diameter d n, and the volume-surface-area averaged diameter d v/a were calculated as follows: d n = n i=1 d i n (1) d v / a = n d i 3 i=1 n 2 d i i=1 (2) S3

4 where d i is the diameter of individual particles, n is the number of counted particles 1. The dispersion of Au 0.5 Pt 0.5 /C is determined by dividing d v/a surface area by d v/a mass. The density of Au 0.5 Pt 0.5 is assumed as the average of Au and Pt. X-ray diffraction (XRD). The XRD (PANalytical X pert Pro) was collected with a Cu Kα radiation (λ= Å). Within the resolution of the XRD, the results revealed single phase Au-Pt alloy regardless of the thermal treatment. Using the full-width half max values of (111) diffractions, we calculate using the Scherrer s formula crystallite sizes of 3.3 nm (2.5 ) for as-synthesized Au 0.5 Pt 0.5, 3.6 nm (2.3 ) for 250 C heat-treated in Air, 4.1 nm (2 ) for heat-treated in Air at 250 C, followed by heat-treated in Ar at 350 C, and 4.6 nm (1.8 ) for heat-treated in Ar at 500 C. The calculated grain sizes are smaller than the particle sizes observed in TEM, which is consisted with the fact that the grain sizes of multiple-twined noble nanoparticles are usually smaller than their overall particle. Electrode preparation. Au 0.5 Pt 0.5 NPs thin-film was prepared from a catalyst ink consisting of Au 0.5 Pt 0.5 NPs supported on carbon blacks with an appropriate amount of Nafion as binder. The catalyst inks were prepared by horn-sonication of the NPs and an appropriate amount of Nafion in a mixture of 20 wt% 2-propanol (supplier) in Milli-Q water (18 MΩ) using ultrasonicator (Sonics & Materials, Inc). Next, 15 µl of the suspension was dropped on the glassy carbon electrode (5 mm in diameter). The total loading for each electrode was 18 µg Au+Pt cm -2 disk, 68 µg Vulcan cm -2 disk and 38 µg Nafion cm - 2 disk. Commercial Pt NP and Au Pt catalysts supported on high surface area carbon (Tanaka Kikinzoku, TKK ) with a Pt loading of 46 wt% and Au loading of 40 wt% were used for references. For TKK samples, loading of 10 µg Pt cm -2 disk with same S4

5 Nafion:C ratio was used. To avoid the size effect for MOR activity comparison, we also employed Pt NPs with bigger size (~7 nm average diameter, 40 wt% Pt, Johnson Matthey.) For Johnson Matthey sample, we used a loading of 18 µg Pt cm -2 disk. Electrochemical Analysis. For surface composition characterization, the electrodes were cycled in 0.5 M H 2 SO 4 with the potential range between 0.05 and 1.7 V (RHE) at a scan rate of 50 mv/s. For the electrocatalytic characterizations of the MOR and COR activities, the electrodes were cycled in the solution of 1 M CH 3 OH (Sigma-Aldrich) in 0.1 M HClO 4 with a potential range between 0.05 and 1.1 V (RHE) at a scan rate of 50 mv/s or with a CO (Airgas)-saturated 0.1 M HClO 4 with a potential range between 0.05 and 1.1 V (RHE) at a scan rate of 5 mv/s respectively. The capacitance current was collected from the cyclic voltammograms of the catalysts in 0.1 M HClO 4 prior to CH 3 OH addition or CO-saturation. Error bars represent standard deviations from at least three independent repeat measurements, whereby the change in the current over subsequent potential cycles is less than 1%. Calculation of Au 0.5 Pt 0.5 NPs surface composition via CV. The relative concentration of Pt on the surface of Au 0.5 Pt 0.5 /C at different heat-treatment is calculated by comparing the electrochemical surface areas (ESA) of Pt and Au in 0.5 M H 2 SO 4 electrolyte. The Pt ESA is obtained from integrating and averaging the capacity-corrected H 2 underpotential potential deposition in the range of 0.05 to 0.3 V (Figure S4, blue arrows) in both positive and negative scan directions [(Q UPD(+) + Q UPD(-) )/2], which is converted to cm 2 Pt by dividing with 230 µc/cm 2 Pt conversion 2. The relative concentration of Au is calculated by integrating the capacity-corrected Au oxide formation in the range of 0.9 to 1.2 V (Q Au-Ox, orange arrow in Figure S4), which is converted to cm 2 Au by dividing with 340 S5

6 µc/cm 2, the reference value of Au oxide formation at 1.7 V vs. RHE 3. The activities for CO and methanol oxidation are normalized to the Pt surface area determined via hydrogen underpotential potential deposition in 0.1 M HClO 4 electrolyte, using 210 µc/cm 2 Pt conversion 4. Error bars in surface composition represent standard deviations from at least three independent repeat measurements. CO electro - oxidation activity of commercial Au/C NP. The COR activity of Au/C (Tanaka Kikinzoku, TKK, 40 wt%) was studied using the identical procedure to the Au 0.5 Pt 0.5 /C described above. We found that Au/C catalyst exhibited negligible COR activity (much smaller than Pt/C, Figure S5-6). We note that our activity is lower than what has been reported by the Leiden group 5 using single-crystal Au electrode (approximately 2 orders of magnitude difference). We found this difference to be a result of the potential scanning window; our restriction in potential window places an upper limit of 1.1 V vs. RHE. If the potential window was pushed positively to 1.7 V vs. RHE, the COR activity comparable to the result from the Leiden group can be observed (Figure S14). We believe this could be a result of an irreversible oxidation or oxide formation on the Au surface. CO stripping experiment. The CO stripping activity of commercial Pt/C (Tanaka Kikinzoku, TKK, 40 wt%) was contrasted against Au 0.5 Pt 0.5 nanoparticles with 68 at.% Pt on the surface for stripping activity and CO coverage comparisons. Experimentally, the catalysts were subjected to CO adsorption at 0.05 V vs. RHE for 30 minutes and the solution was subsequently purged with Ar to remove unbound CO. The stripping data were collected using both potential sweeping (~10 mv/s) and potentiostatic holding S6

7 procedures (0.78 V vs. RHE). Both currents were corrected with the background current collected the same sample under identical condition without CO pre-adsorption. Figure S1. The representative TEM images with size distribution histograms of Au 0.5 Pt 0.5 /C catalysts with different surface compositions: ~90% Pt (a), ~13% Pt (b), ~68% Pt (c), and ~30% Pt (d). The average diameters, distributions, and dispersions of AuPt nanoparticles on carbon support were measured based on HRTEM imagery studies at 200K magnification with a point-to-point resolution of 0.19 nm. At least 200 randomly selected nanoparticles from HRTEM images were used to count particle size distributions of each sample. S7

8 Figure S2. The XRD patterns of Au 0.5 Pt 0.5 /C catalysts with different surface compositions: ~90% Pt (as-synthesized colloid NPs), ~68% Pt (heat-treated at 250 C in Air for 30 min), ~30% Pt (heat-treated at 250 C in Air for 30 min and then 350 C in Ar for another 30 min), and ~13% Pt (heat-treated at 500 C in Ar for 30 min). The orange and the blue vertical lines represent the crystallographic diffraction of standard Au (PDF# ) and Pt (PDF# ), respectively. S8

9 Figure S3. Cyclic voltammogram of Au 0.5 Pt 0.5 nanoparticles treated in air at 250 C in Ar-saturated 0.5M H 2 SO 4 at 50 mv/s scan rate from 0.05 to 1.7 V (vs. RHE). The relative concentration of Pt is calculated by integrating and averaging the capacity-corrected H 2 underpotential potential deposition in the range of 0.05 to 0.3 V (blue arrows) in both positive and negative scan directions [(Q UPD(+) + Q UPD(-) )/2], which is converted to cm 2 Pt by dividing with 230 µc/cm 2. The relative concentration of Au is calculated by integrating the capacity-corrected Au oxide formation in the range of 0.9 to 1.2 V (Q Au-Ox, orange arrow), which is converted to cm 2 Au by dividing with 340 µc/cm 2. In this sample, the measured surface composition was 67±4 at.% Pt, where the error bar was determined by three independent measurements. S9

10 Figure S4. a) Cyclic voltammogram of Au 0.5 Pt 0.5 nanoparticles treated in Ar at 250 C for 30 min in Ar-saturated 0.5M H 2 SO 4 at 50 mv/s scan rate from 0.05 to 1.7 V (vs. RHE). The treatment did not result in any measurable change in the surface composition. b) Thermogravimetric analysis on Au 0.5 Pt 0.5 nanoparticles treated in Air at 250 C (black line) and in Ar at 250 C (red line). The experiment was carried in Air with a heating rate of 10 C/min from 25 to 800 C. The inset is a close view at the temperature range from 150 to 300 C, where the Ar treated one shows more weight loss than the Air treated one, indicative of the remaining surfactant. S10

11 Figure S5. The COR activities of Au 0.5 Pt 0.5 core-shell nanoparticles normalized to the Pt + Au surface area. The kinetic activities were extracted from the transport-corrected CO oxidation currents in 0.1M HClO 4 at 5 mv/s scan rate, forward scan, taken at 1600 rpm. Error bars represent standard deviations of at least three independent measurements. S11

12 Figure S6. The COR mass activities of Au 0.5 Pt 0.5 nanoparticles of various surface compositions. The mass activities were extracted from the capacitance-corrected CO oxidation currents in 0.1M HClO 4 at 5 mv/s scan rate, forward scan (Fig. 4a). The COR mass activities at 0 at.% Pt (100 at.% Au) and 100 at.% Pt were taken from TKK s commercial Au and commercial Pt electrocatalysts. S12

13 Figure S7. The COR activities of Au 0.5 Pt 0.5 core-shell nanoparticles with 68 at.% Pt (top) and 30 at.% Pt (bottom) surfaces under 10% CO (0.1 atm) and 100% CO (1 atm) gas condition. Notably, the 68 at.% sample has zero reaction order (activity largely invariant with CO concentration), whereas the 30 at.% Pt sample has negative reaction order (decreasing activity with increasing CO concentration). S13

14 Figure S8. (a) The CO stripping current (background-corrected) measured via potential sweep at 10 mv/s in 0.1 M HClO 4 on Pt/C (TKK, blue) and Au 0.5 Pt 0.5 /C (68 at.% Pt surface, red) catalysts. Notably, the integrated charges underneath the stripping curves for both Pt/C (0.44 mc/cm 2 Pt) and 68 at.% Pt Au 0.5 Pt 0.5 /C (0.38 mc/cm 2 Pt) were close to the expected CO monolayer adsorption on Pt (0.42 mc/cm 2 Pt), indicating the absence of CO adsorption on Au. Activity-wise, 68 at.% Pt Au 0.5 Pt 0.5 /C exhibits more stripping current than Pt/C excluding the pre-peak feature, whose origin is unclear at the moment. (b) The CO stripping current S14

15 measured via potentiostatic holding (background-corrected) at 0.78 V vs. RHE. Notably, 68 at.% Pt Au 0.5 Pt 0.5 /C exhibits more stripping current than Pt/C, as expected. S15

16 Figure S9. Bright field high-resolution TEM (HRTEM) of (a) 13% surface Pt and (b) 68 at.% surface Pt Au 0.5 Pt 0.5. Bright field and dark field HRTEM was used to study the surface morphologies of Au 0.5 Pt 0.5 nanoparticles showing the (a) low (13 at.% surface Pt) and (b) high (68 at.% surface Pt) MOR activity in order to determine if there was a systematic difference in the surface crystallography of the two samples. FFT was used to determine the crystallographic orientation of the nanoparticle s facets and the length of the facets were measured. Ten nanoparticles each from the 13% surface Pt and the 68% surface Pt samples were measured and their descriptive statistics are presented in Table S1. S16

17 Figure S10. MOR electrocatalytic activities of Au 0.5 Pt 0.5 nanoparticles of various surface compositions in 0.1M HClO 4 (forward scan, 50 mv/s scan rate) (a) Specific activity normalized to Pt surface area (b) the relationship between specific activity normalized to Pt surface area with the MOR electrocatalytic activity, (c)-(d) Same analysis as (a) and (b) with Pt + Au surface area (total area). Error bars represent standard deviations of at least three independent measurements. S17

18 Figure S11. The MOR mass activities of Au 0.5 Pt 0.5 nanoparticles of various surface compositions in 0.1M HClO 4 at 50 mv/s scan rate, forward scan. The activity of Pt/C was taken from Johnson Matthey s commercial Pt/C catalyst, which has comparable size (~7 nm) to the Au-Pt catalysts. Error bars represent standard deviations of at least three independent measurements. S18

19 Figure S12. An example of the effect of the EDS smoothing that was used on Figure 3a from the main text. A Kernel smoothing algorithm was used on the data by the Bruker Espirit software. The Kernel smoothing algorithm averaged the measured values over a specified range of pixels. The above figure compares the raw EDS data to data that has been Kernel smoothed with ranges of 3 by 3 px, 5 by 5 px, 7 by 7 px and 9 by 9 px. The figures presented in the main text and SI all use a 7 x 7 kernel. S19

20 Figure S13. Aberration corrected scanning transmission electron microscopy - energy dispersive X-ray spectroscopy (STEM-EDS) of different Au 0.5 Pt 0.5 nanoparticles from those shown in Figure 3, which shows comparable surface Ptenrichment and Au-enrichment to those shown in Figure 3. (a) as-synthesized Au 0.5 Pt 0.5 (90% surface Pt), (b) Ar treatment at 500 ºC for 30 mins (13% surface Pt), (c) Air treatment at 250 ºC for 30 mins (68% surface Pt), (d) Air treatment at 250 ºC for 30 mins, followed by Ar treatment at 350 ºC for 30 mins (30% surface Pt). The right panel shows Pt, Au, and their combined L α mappings. S20

21 Figure S14. (a) The bulk CO oxidation reaction (COR) polarization curves at 1600 rpm in 0.1 M HClO 4 on un-activated (black) and activated (red) Au/C catalysts (TKK). The activation is defined as that the application of potential sweep to 1.7 V vs. RHE in Ar, where the un-activated Au/C was swetp to only 1.1 V (vs. RHE) in Ar (Au 0.5 Pt 0.5 /C and Pt/C were subjected to only 1.1 V). The inset is the specific activities of the un-activated and the activated Au/C catalysts. (b) The COR activities of Au 0.5 Pt 0.5 /C normalized to the Pt + Au surface area (black, adopted from Figure 4B) with the activityof the activated Au/C catalyst. The S21

22 kinetic activities were extracted from the transport-corrected CO oxidation currents in 0.1M HClO 4 at 5 mv/s scan rate, forward scan, taken at 1600 rpm. Error bars represent standard deviations of at least three independent measurements S22

23 Table S1. Results from the HRTEM analysis of nanoparticles surface structure shown in Figure S8. The mean fraction of various facets reported for the 13% surface Pt (Figure S8a) and 68% surface Pt (Figure S8b), along with standard deviations. The mean facet fraction is nearly identical between the two samples for all crystallographic planes, indicating that there is no measurable difference between the surface crystallography of the two samples. Sample (111) (200) (220) (311) None 13%Pt Average(%) σ(%) %Pt Average(%) σ(%) S23

24 Table S2. Benchmark MOR activities of various Pt catalysts. The MOR activities were taken at 0.6 V vs. RHE (acid electrolyte, room temperature, cyclic voltammetry measurement only) and normalized to the surface area of a catalyst. The listed values represent the best data from each literature report. Compound (electrolyte) Specific i (ma cm -2 catalyst) Specific i Pt (ma cm -2 Pt) Mass i (A g -1 metal) Capacitance Correction Au 0.5 Pt 0.5 (68% Pt) (0.1M HClO 4 ) 0.12± ± ±3 Yes Au 0.4 Pt 0.6 (Au 0.8 /Pt sample) n.a. <0.005 <10 No (0.5M H 2 SO 4 ) 6 *Pt-nanoporous Au (0.5M H 2 SO 4 ) 7 n.a. <0.005 n.a. No Au 0.68 Pt 0.32 /C (0.5M H 2 SO 4 ) 8 n.a. n.a. <3 No *Pt(111) (0.5M H 2 SO 4 ) 9 Note: 25mM MeOH *Pt(554) (0.5M H 2 SO 4 ) 9 Note: 25mM MeOH *Pt(553) (0.5M H 2 SO 4 ) 9 Note: 25mM MeOH *Pt-Ru alloys (0.5M H 2 SO 4 ) 10 Note: 5mM MeOH n.a. ~0.01 n.a. No n.a. ~0.05 n.a. No n.a. ~0.1 n.a. No n.a. ~ n.a. No Pt-Ru/C, Pt-Ru/CNT (1M H 2 SO 4 ) 11 n.a. ~0.3 ~1 No Pt 3 Sn/C (annealed) (0.1M HClO 4 ) 12 Note: 0.1M MeOH n.a. ~0.1 ~13* No * Extended surface (not nanoparticle). For calculation, we assume cm -2 catalyst/cm -2 disk ~ 1. Lower estimate. Increasing MeOH concentration generally leads to higher activity. S24

25 Table S3. Benchmark COR activities of various Pt catalysts. The COR activities were taken at 0.6 V vs. RHE (acid electrolyte, room temperature, cyclic voltammetry measurement only) and normalized to the surface area of a catalyst. The listed values represent the best data from each literature report. Compound (electrolyte) Specific i (ma cm -2 catalyst) Specific i Pt (ma cm -2 Pt) Mass i (A g -1 metal) Capacitance Correction Au 0.5 Pt 0.5 (13% Pt) (0.1M HClO 4 ) *Ru (0.5M H 2 SO 4 ) 13 Note: 20 mv/s *Pt-Ru alloys (50%) (0.5M H 2 SO 4 ) 13 Note: 20 mv/s *Pt 3 Sn (0.5M H 2 SO 4 ) 14 Note: 20 mv/s Pt 3 Sn/C (annealed) (0.1M HClO 4 ) 12 Note: 20 mv/s 0.05± ± ±0.32 Yes 0.17 n.a. n.a. no n.a. no >5 n.a. n.a. no n.a no * Extended surface (not nanoparticle). For calculation, we assume cm -2 catalyst/cm -2 disk ~ 1. Lower estimate. At 0.6 V, the activity already reaches the limiting current. S25

26 Table S4. TEM characterization of the Au 0.5 Pt 0.5 NPs studied in this work. The number averaged diameter, d n, with standard deviation (in parentheses), the volume-area averaged diameter, d v/a, and the specific surface area of the oxide (see equation 1), A s. were obtained from particle size distribution measurements, and the electrochemical surface area, A e, was obtained from the electrochemical characterization (see experimental). Pt surf (%) d n (nm) d v/a (nm) A s (m 2 g -1 ) A e (m 2 g -1 ) 90 ± ± ± 1 68 ± ± ± 2 30 ± ± ± 2 13 ± ± ± 5 S26

27 Scheme S1. Schematic depicting contribution to adsorbed CO on Pt sites in Au-Pt surfaces. Interactions involving lateral interaction (Au-CO interaction due to Au being neighbor to Pt-CO and CO-CO interaction due to clustering of Pt surrounded by Au) have been demonstrated to weaken CO adsorption bond on the surface 15,16. S27

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