1 Nano-engineered Ircore@Ptshell Nanoparticles with Controlled Pt

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Nano-engineered Ircore@Ptshell Nanoparticles with Controlled Pt Shell Coverages for Direct Methanol Electro-oxidation Ehab El Sawy, and Viola Birss ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13080 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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ACS Applied Materials & Interfaces

Nano-engineered Ircore@Ptshell Nanoparticles with Controlled Pt Shell Coverages for Direct Methanol Electro-oxidation Ehab N. El Sawya,b and Viola I. Birssa,* a Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 b

Current address: Chemistry Department, School of Sciences and Engineering, American University in Cairo, New Cairo 11835, Egypt * To whom correspondence should be addressed; E-mail: [email protected]

Abstract The design and application of bimetallic alloy nanoparticles (NPs) for electrocatalytic applications are challenged by the need to clearly identify and understand the individual effect of each of component. In the present work, the focus has been on PtIr NPs, with alloyed NPs having been previously shown to be active towards the methanol oxidation reaction (MOR), but for which the mode of action of the Ir component remains uncertain. We have therefore nano-engineered a family of Ircore@Ptshell NPs, using a modified polyol method, to control the Pt shell coverage (up to 2 monolayers) and thus to allow the separation of the bifunctional and electronic effects of Ir on the Pt activity. It is shown that the Ir core size and crystallinity do not change with deposition of the Pt shell, as confirmed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). CO stripping and hydrogen underpotential deposition/removal were used for the first time to determine the surface composition of the Ircore@Ptshell NPs. It is shown that the Ircore enhances the MOR activity of the Ptshell primarily through the bi-functional effect, with an optimum Pt coverage of 0.4 of a monolayer. At 60 °C, an additional electronic effect of Ir on Pt can be discerned, causing an inhibition in the MOR rate by weakening the adsorption of methanol on the Ptshell, thus helping to remove the adsorbed CO intermediate from the shell surface.

Keywords: Methanol oxidation reaction (MOR), Pt-Ir, electronic effect, bi-functional effect, core-shell nanoparticles, CO adsorption/stripping, polyol synthesis, underpotential hydrogen adsorption/desorption. 1

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1. Introduction Improving the activity and durability of the anode catalyst layer is a key challenge hindering the commercial production of direct methanol fuel cells (DMFCs)1. Pt is the most active catalyst for the methanol oxidation reaction (MOR). However, due to the strong adsorption of CO (one of the reaction intermediates), the Pt active sites become blocked, leading to a significant drop in the MOR reaction kinetics with operating time1– 3

. Therefore, in order to make DMFC anodes more resistant to poisoning by CO, binary

Pt-M (M = Ru, Ni, Sn, Pd, etc.) alloys have been investigated1,4–12. However, these materials are often unstable under DMFC operating conditions

13

and thus efforts are

being directed at alloying Pt with metals that are less reactive in nature. In the case of PtRu alloys, as a well-known example of a DMFC electrocatalyst, Ru has been found to dissolve under DMFC conditions, crossing over to the cathode and inhibiting the oxygen reduction kinetics, while also degrading the membrane14–16. As a possible alternative to Pt-Ru, Pt-Ir alloy catalysts, prepared by methods such as electrodeposition17–20, polyol reduction21–24, sol-gel25, hydrothermal26, and NaBH4 reduction

27

methods, have also been investigated. Pt-Ir alloys have exhibited high

catalytic activity towards the MOR, explained as arising from a bi-functional and/or electronic effect of Ir on Pt18,24,26,27, although which of these mechanisms is dominant is not yet fully understood

18,24

. In one of our earlier attempts to investigate Pt-Ir catalysts

for the MOR, thin PtxIry alloy films were formed by electrodeposition, giving good control of the bulk Pt and Ir content17,18. However, the surface composition of these films, which is critical to understanding the MOR mechanism, remained unknown.

2


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In very recent work, we used the polyol method to form a series of PtxIry alloy nanoparticles (NPs) with a wide and controllable bulk composition range (where x and y represent the at% of Pt and Ir, respectively), also shown to have a similar bulk and surface composition 24. Although the results suggested that the bi-functional effect of Ir on Pt is dominant during the MOR

24

, the results obtained with the PtxIry alloy NPs did

not allow the full separation of the bifunctional and electronic effects, as both increased commensurately with increasing Ir content. One approach that has been used in the elucidation of the role of Ru in PtRu electrocatalysts has involved the fabrication of Rucore@Ptshell NPs

28

. Hence, in the

present work, Ircore@Ptshell nanoparticles (NPs), having a controlled Ptshell coverage, were synthesized, with the main aim being to better separate the bi-functional and electronic effects of Ir on the Pt activity. Only a few papers have focussed on the synthesis of Ircore@Ptshell NPs, typically using relatively complex procedures including a very wide range of Pt shell coverages

31

29,30

. In one study

and generally not 29

, Ircore NPs were

prepared by dissolving IrCl3 and polyvinylpyrrolidone (PVP, the capping agent) in ethylene glycol (EG) and then reducing with ethylene glycol (EG)-NaBH4, followed by the addition of NaBr/EG and refluxing25. These Ircore@Ptshell NPs showed

25

a higher

activity towards the preferential oxidation of carbon monoxide in hydrogen than did Pt, Aucore@Ptshell NPs, or Pdcore@Ptshell NPs. In other work involving the deposition of Ircore@Ptshell NPs on Vulcan carbon (VC) powder, the Ir NPs were formed first and the Ptshell was added in a subsequent step30. A correlation between the Ptshell thickness and the catalytic activity towards the MOR was seen, but the individual contributions of the bifunctional and/or electronic effect of Ir on the Pt activity were not elucidated 30. 3



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In an effort to correlate the Ircore@Ptshell NPs surface composition with its corresponding electrocatalytic activity, our group was the first to report the synthesis of Ircore@Ptshell NPs, using the polyol synthesis method, with a controlled Pt shell coverage ranging from sub-monolayer to several monolayers of Pt32. More recently, Jaramillo et al31 suggested the use of Ircore@Ptshell NPs as a catalyst for the oxygen reduction reaction, using a synthetic method similar to what was reported by us32. However, only a narrow range of Pt coverages was investigated, and the work included only an Ir:Pt ratio of 1:2. In the present paper, Ircore@Ptshell NPs, with a 3 nm dia. Ircore, were synthesized using the simple polyol method 33–37, showing for the first time that the Pt coverage of the Ircore could be fully controlled in the range of 0 - 2 monolayers (MLs). This controllable Ptshell coverage has allowed for the separation of the bi-functional and electronic effects of the Ir core on the MOR, showing that the best composition is a 0.4 ML coverage at all potentials and both room temperature (RT) and 60 °C. This indicates that the bifunctional effect of Ir on Pt is dominant, as both Ir and Pt are then exposed to the solution. At 60 °C and low potentials, Ir is shown to enhance the Pt activity independently of the Pt shell coverage, revealing that the electronic effect of Ir on Pt enhances CO removal from the Pt shell under these conditions. At higher potentials and Pt shell coverages (0.8-2.3 ML) at 60 °C, the MOR activity is lower than at pure Pt, indicating the suppression of methanol adsorption, likely due to the electronic effect of the underlying Ir phase on the Pt shell.

4


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2. Experimental Methods 2.1. Preparation and Physical Characterization of Pt, Ir, and Ircore@Ptshell Nanoparticles The Pt, Ir (Ircore), and Ircore@Ptshell nanoparticles (NPs), synthesized in this study, were prepared using 0.3 mmol of each of H2PtCl6.6H2O (Sigma-Aldrich, 99.9%), H2IrCl6.6H2O (Alfa Aesar, 99 %), or H2PtCl6.6H2O+H2IrCl6.6H2O as the precursors, respectively. The Ircore NPs were synthesized by dissolving 0.3 mmol of Ir precursor (H2IrCl6.6H2O) and 0.75 mmol of polyvinylpyrrolidone (PVP- Alfa Aesar, M.W. of 40k) in 50 ml of ethylene glycol (EG) at RT, and then heating to 160 °C. The solution was kept at 160 °C for ~ 2 hours, ensuring that the Ir precursor was completely reduced, and then the solution was left to cool to RT. In our prior work 38, Ir NPs prepared using this method were shown by TEM to be ~ 3 nm in size but composed of still smaller crystallites (1.5 nm), as demonstrated by XRD analysis

38

. The details of how the shell

coverage was calculated are given in the Supporting Information. The Pt shell was formed by adding the desired amount of Pt precursor to the previously cooled Ircore NP solution at RT, followed by temperature ramping to 110 °C and then a rapid ramp to 160 °C. The temperature was then maintained at 160 °C for 2 hours to ensure the complete reduction of the Pt precursor to Pt and the formation of the Ircore@Ptshell NPs 24. To determine the composition and crystal structure of the Pt, Ir, and Ircore@Ptshell NPs, they were collected as fine powders. Several cycles of collection by centrifugation

5



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and washing of the NPs with acetone were performed. The powders were then dried under vacuum at RT and then ground using a mortar and a pestle. Wavelength dispersive X-ray spectroscopy (WDS) was used for the determination of the Ircore@Ptshell NP composition, using a JEOL JXA-8200 electron microprobe. Based on the WDS results, the Ircore@Ptshell NPs were given the following notation: ‘Irx@Pty (n monolayers)’. Here, x and y represent the Ir and Pt (at %) bulk content, respectively, while n is the number of Pt monolayers in the shell, calculated based on equation (S1). The crystal structure was determined using Powder X-ray Diffraction (PXRD) using a Rigaku Multiflex X-ray Diffractometer with a CuKα radiation source (λ = 1.5406 nm), operating at 40 kV and 20 mA, with the XRD patterns recorded at a rate of 1°/min. Both Pt and Ir have face-centered cubic (fcc) crystal structure, and hence the full width at half maximum (FWHM) of their (111) XRD peak was used to estimate the average crystallite size using the Scherrer equation 39,40. To confirm the presence of both metals in each NP and to exclude the formation of individual Pt and Ir NPs, highresolution

Transmission

Electron

Microscopy

(HRTEM), coupled with Energy

Dispersive X-ray spectroscopy (EDS), was employed, using a Tecnai F20 G2 FEG-TEM instrument. The NP size and distribution on the Vulcan Carbon powder supports was determined using a TEM-Hitachi H-7650 instrument. The NP mass loading on VC was determined using thermogravimetric analysis (TGA), employing a Mettler-Toledo StarE instrument, all in the air at 23-1000 oC, with a temperature ramp of 10 oC/min.

6


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2.2. Electrochemical studies of the Ircore@Ptshell nanoparticles Catalyst powders of Pt NPs/VC, Ir NPs/VC, and Ircore@Ptshell NPs/VC, with 10 wt% of the metallic component, were prepared simply by adding the colloidal catalyst solution to a homogeneously mixed VC/ethanol solution with continuous stirring for 2-4 hours. The catalyst powders were then exposed to several cycles of collection by centrifugation and washing with acetone and then dried under vacuum at RT and ground. 10 mg of the final catalyst powder was mixed with, 0.6 ml of isopropanol, 0.3 ml of a 1% w/w Nafion solution37, and 0.1 ml of water, and then sonicated for ≥ 30 min, forming a homogeneous catalyst ink. 20 µl of the catalyst ink solution was then deposited on a glassy carbon (GC) electrode with a 7 mm diameter and allowed to dry at RT. The catalyst film thickness and mass were expected to be ~ 20 µm

32

and ~ 0.23 mg, respectively, when 20 wt% Nafion

is used. All of the electrochemical measurements were performed using an EG&G PARC 173 potentiostat and a PARC 175 function generator. PowerLab/400 and Chart for Windows v5 were used to collect the data and plotting, respectively. A glass electrochemical cell containing two compartments was used. The working (WE) and counter (CE) electrodes were placed in one compartment and the reference electrode (RHE) in the other one, connecting the RHE to the main compartment through a Luggin capillary. For the measurements at 60 °C, the main compartment of the electrochemical cell was replaced with a water-jacketed compartment that allowed the cell temperature control by circulating the water from a HAAKE FS recirculating/heating water bath to the cell jacket 32. 7



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Platinized Pt gauze was used as the CE, while a reversible hydrogen electrode (RHE) was used as the RE, with all potentials referred to the RHE in this work. Analar-Grade chemicals and triple distilled water were used to prepare all of the solutions. Before each experiment, N2 was bubbled through the solution for 15 min to thoroughly deaerate it, followed by continuous bubbling of N2 through the solution during the experiments to prevent the accumulation of CO2 gas bubbles at the WE. In the CO stripping voltammetry experiments, the potential was held at 0.05 V vs. RHE while purging the solution with CO gas (PRAXIR, 99.5 %) for 15 min to achieve surface saturation with CO 41. While holding at 0.05 V vs. RHE, the solution was purged with N2 for another 15 min to remove CO from the solution, followed by a CO stripping scan (20 mV/sec, 0.05-1.0 V vs. RHE). A subsequent cyclic voltammogram (CV) was recorded under the same conditions to monitor the NP response after the complete oxidation/removal of the adsorbed CO.

3. Results and Discussion 3.1. Physical characterization of Ircore@Ptshell nanoparticles 3.1.1. Composition of Ircore@Ptshell nanoparticles Since the synthesis method used in this study is known to result in the uniform growth of the shell around the core rather than island formation

42–44

, the Ptshell coverage

obtained should relate directly to the Pt content in the Ircore@Ptshell NPs. Since the Ir:Pt molar ratio in the synthesis solution is expected to control the ratio in the NPs, wavelength dispersive X-ray spectroscopy (WDS) was used to confirm this here. We did not use energy dispersive X-ray spectroscopy (EDS), as Pt and Ir would be very difficult 8


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to differentiate due to the similarity of the atomic weight of Pt and Ir 45. Figure 1 shows that a linear relationship (with a slope of one) is obtained between the synthesis solution composition (atomic % Ir) and the WDS-determined Ir content in the Ircore@Ptshell NPs. These results confirm that the Ir:Pt ratio in the Ircore@Ptshell NPs can be accurately controlled simply by varying the Ir:Pt molar ratio in the synthesis solution.

Figure 1: Relationship between the theoretical Ir content (based on the synthesis solution composition) and the measured Ir content (based on WDS measurements) of Ircore@Ptshell NPs that were collected in the form of fine powders.

In order to prove that both Ir and Pt are present in each Ircore@Ptshell NP, rather than the catalyst consisting of a mixture of Ir NPs and Pt NPs, EDS, coupled with highresolution transmission electron microscopy (HRTEM), was used, as WDS coupled with TEM was not an available option. Figure 2a shows the EDS spectrum of a single Ir51@Pt49 NP, while the inset shows the dark field TEM image of the area that was analyzed to obtain the EDS spectrum. Pt and Ir atomic weights are similar, and hence it is 9



difficult to differentiate between them using the low (2 keV) or high (9 keV) energy peaks. Also, only very weak Ir and Pt signals are seen, due to the small mass within a single NP. For this reason, a small cluster of Ir51@Pt49 NPs was analyzed (Fig. 2b inset), with the EDS spectrum shown in Fig. 2b. The Ir:Pt peak intensity ratio (1:0.8) is found to be quite close to the Ir:Pt bulk ratio (1:1), which qualitatively demonstrate the homogeneous distribution of Ir and Pt at the NP level. The slight mismatch between the EDS and WDS data is likely related to the overlap between the Pt and Ir EDS peaks, as EDS is known to give more qualitative than quantitative results, especially for atoms that have similar atomic weights. However, due to the small size of the nanoparticles under study here, the amorphous structure of the Ir core, and the very similar atomic weight of Pt and Ir, EDS mapping was not able to easily distinguish these two metals from each other in the core@shell NPs.

Figure 2: Energy dispersive X-ray (EDS) spectra of (a) single and (b) multiple Ir51@Pt49 NPs, zooming in on the high energy region. The insets in (a) and (b) show the dark-field TEM images of the Ircore@Ptshell NPs that were analyzed by EDS. 10


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3.1.2 Size and crystal structure of Ircore@Ptshell NPs Based on TEM analysis, the Ircore particle size is ca 3 nm38. The addition of Pt in the shell (through layer by layer growth) is expected to increase the particle size of the Ircore@Ptshell by 0.54 nm for each Pt ML (2* Pt atomic diameter (0.27 nm)), as shown in Scheme S1. For example, in the case of the Ir41@Pt59 (1.9) NPs, adding 1.9 MLs of Ptshell to the Ircore should result in a NP diameter increase of 1 nm. Figure 3 shows a TEM image of the Ir41@Pt59 (1.9) NPs, giving an average particle size of 3.9 ± 0.7 nm, as well as a higher magnification view of the particles shown on the left. The difference between the size of the Ir41@Pt59 NPs and that of the Ircore is only 0.9 nm, which is very close to the theoretical estimate. This observation supports the proposed formation of high-quality core@shell NP structures and shows the validity of the approach that was used for the calculation of the Ptshell coverage in the present work.

Figure 3: TEM bright field image of Ir41@Pt59 (1.9) NPs, deposited on a carbon coated Cu TEM grid. Inset: Histogram of NP diameters, obtained from the image in Fig 3. The image on the left shows a higher magnification view of the nanoparticles in the red square on the right.

11



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Powder XRD (PXRD) analysis was used to determine the crystal structure of the Ircore@Ptshell NPs, as the Ircore crystal structure may have changed by altering the Ptshell coverage. Figure 4a shows the XRD pattern of the Ir, Pt, and Ircore@Ptshell NPs with varying Ptshell coverages (indicated in brackets). In the case of the pure Ir NPs, the crystallite size is 1.5 nm, determined using the Scherrer equation 39,40, which is consistent with our prior work

24

. This small crystallite size causes peak broadening and overlap

between the Ir (111) and (200) peaks, as well as overlap of the Ir(311) and Ir(222) peaks, as shown in Fig. 4a. However, in the case of the pure Pt NPs, the XRD patterns show the typical peaks for Pt, with no overlap, giving a crystallite size of 3.6 ± 0.3 nm, while a NP size of 4.2 ± 0.7 was reported by our group from earlier TEM images 24. The similarity of these values reveals that the Pt NPs are more crystalline, overall, than is Ir, likely due to the high cohesive energy of Ir (670 kJ/mol) compared to Pt (564 kJ/mol) 24,46. The formation of an alloy phase during the synthesis of the Ircore@Ptshell NPs NPs is not possible, as Pt and Ir were reduced in separate reaction steps. This is supported by the significant differences in the XRD pattern of the Ircore@Ptshell NPs and the pattern reported by us for the PtIr alloy NPs24 and also by others47. In the case of the Ircore@Ptshell NPs, the XRD pattern is dominated by Ir for NPs with a Pt shell less than 2 ML in thickness. However, in the case of the PtIr alloy NPs, the XRD pattern is very similar to that of Pt for Ir contents up to 50%, while at higher Ir contents, the XRD pattern gradually shifts to that of Ir24.

12


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Figure 4: (a) Powder XRD patterns of Ir, Pt, and Ircore@Ptshell NPs of varying composition and Pt coverage in the 2θ range of 30°-90°. The solid and dashed vertical lines represent 2θ values for the Ir (PDF#87-0715) and Pt (PDF#87-640) FCC crystal phases, respectively. The Ptshell coverage of the Ircore@Ptshell NPs is indicated in brackets as the number of Pt monolayers in the shell. (b) The relationship between the full width at half maximum (FWHM) of the Ircore@Ptshell NP (111) diffraction peaks and the Ir content of the Ircore@Ptshell NPs.

For the Ircore@Ptshell NPs, the Ptshell coverage increases as the Pt content increases, resulting in a gradual shift in the XRD pattern from that of the Ircore to that of Pt, as shown in Fig. 4a. In addition, the full width at half maximum (FWHM) of the (111) peak is seen to decrease as the Ptshell thickness increases (Fig. 4b). These XRD results may reflect either a change in the Ircore crystal structure or an overall increase in the NP size. In order to determine which of these interpretations is correct, a comparison between the Ircore@Ptshell NP size, calculated from both TEM images and XRD patterns (FWHM 13



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values and the Scherrer equation 39,40) with the theoretical value, obtained using equation (S1) and Ircore size of 3 nm, is shown in Figure 5. The average particle size calculated from the TEM images is significantly higher than those calculated based on XRD peak widths in the case of the Ircore NPs 24 and also for all of the Ircore@Ptshell NPs, as shown in Figure 5.

Figure 5: Comparison between the average Ircore@Ptshell NP size, determined from the TEM image analysis and the XRD data, and calculated using Eq. (S1), as a function of Ir content.

In our prior work on Rucore@Ptshell NPs 28, increasing the Ptshell coverage caused a gradual increase in the Rucore crystallinity, a behavior also observed by other groups

43

.

However, in the case of the Ircore@Ptshell NPs reported here, the very small change in the crystallite size (1.5 nm to 2 nm) as the Pt shell coverage increases should be related to the Ptshell contribution and not to the change in the Ircore crystallinity. If increasing the Ptshell coverage forces the Ircore to become more crystalline, the crystallite size (based on XRD) 14


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should become more similar to the particle size measured based on TEM analysis as the Pt coverage increase. However, the almost constant difference between the crystallite and particle size determined using XRD and TEM, respectively, seen in Fig. 5, argues that the Ptshell does not influence the Ircore crystallinity, even as the Ptshell coverage changes. The similarity between the theoretically predicted NP size and that measured using TEM argues for the formation of Ircore@Ptshell NPs, rather than the formation of individual Pt and Ir NPs.

3.1.3 Loading and distribution of Ircore@Ptshell NPs on Vulcan Carbon powder In order to perform electrochemical experiments with the Ircore@Ptshell NPs, the particles were deposited on Vulcan Carbon (VC) powder using a NP:VC weight ratio of 10:90. In order to confirm this composition, thermogravimetric analysis of the Ir@VC, Pt/VC, and Ircore@Ptshell/VC catalyst powders was carried out in air. Figure S1 shows the anticipated mass loss due to the combustion of VC, forming CO2, with the final, stable mass representing only the Ircore@Ptshell NPs. Fig. S1 confirms that the mass of NPs in all of the catalysts is very close to 10% (the residual mass at high temperatures), as expected, with only a small error observed. Figure S2 shows a bright field TEM image of the Ir41@Pt59/VC NPs, as an example, deposited on a carbon-coated Cu TEM grid. This image reveals the good dispersion of the Ircore@Ptshell NPs on the VC support, with no evidence seen for agglomerates or clusters.

15



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3.2 Electrochemistry of Ir@Pt nanoparticles 3.2.1 Electrochemical determination NP surface composition The gradual increase in the Ircore@Ptshell NPs size with increasing Pt content and the similarity between the calculated and measured (TEM images, Fig. 5) NP sizes argues for the uniform growth of the Ptshell on the Ircore. In order to confirm this finding and to determine the NP surface composition, CO adsorption/stripping voltammetry and hydrogen underpotential deposition (Hupd)/desorption were carried out. Figures 6 shows the CO stripping voltammetry (20 mV/s) at the Ir/VC, Pt/VC, and Ircore@Ptshell/VC catalysts under study here. It is seen that the CO stripping peak potential for Ir/VC is much more positive than for Pt/VC. In our earlier work 24, this was explained as arising from the higher desorption activation energy of CO from Ir (22 kcal/mol) compared to Pt-CO (13 kcal/mol)

39

. A gradual negative shift in the CO

stripping peak potential is seen as the Pt coverage of the Ircore/Ptshell NPs increases from sub- to full monolayer (Fig. 6a). Also, the Ir/VC CO stripping peak is broader than at Pt/VC, also reported previously 24,48. It is known that CO stripping takes place according to reactions 1 and 2. In the case of Pt, 0.3 V (0.8 to 1.1) is required to form a monolayer of OHads (reaction 1)

49

,

while in the case of Ir this process requires 0.65 V (0.45 to 1.1 V)50. This means that the kinetics of reaction 1 is slower in the case of Ir than Pt and hence a broader CO stripping peak is expected at Ir. Mozota et al.

50

explained the slow kinetics of reaction 1 in the

case of Ir to the stronger repulsion between deposited OH species and the greater polarity of IrOH than PtOH surface dipoles, due to the different electrosorption valency of OH at these two metals. El Sawy et al. reported that when Pt/VC and Ir/VC catalysts are 16


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physically mixed together, the CO stripping voltammograms showed two clear peaks, centered at the potentials seen for the Pt/VC and Ir/VC catalysts alone

24

. This behavior

indicates the absence of any measurable catalytic effect of Ir on the Pt activity towards CO oxidation in these types of mixtures. Therefore, the single CO stripping peak seen in Figs. 6a and 6b for all of the Ircore@Ptshell NPs investigated in this work also indicates that both Pt and Ir are present in each NP and that the underlying Ircore is influencing the adsorption strength of CO on the Ptshell. M-H2Oads

M-OHads + H+ + e-

M-COads + M- OHads

2 M + CO2 + H+ + e-

(1) (2)

Figure 6: CO stripping voltammetry (20 mV/s) at Pt/VC, Ir/VC and various Ircore@Ptshell/VC catalysts for (a) a Ptshell coverage of 0 to 1.3 ML and (b) a Ptshell coverage of 1.3 to 2.3 MLs, all in RT 0.5 M H2SO4. (c) The relationship between the CO stripping peak potential (from Fig. 6a) and the calculated Pt coverage for up to one monolayer Ptshell coverage. The objective was to form one monolayer of adsorbed CO by holding the potential at 0.05 V for 15 min in the presence of CO. The CO stripping currents were normalized so that all of the peaks have the same height to enable better comparison of the CO stripping peak shapes and potentials. 17



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The relationship between the CO stripping peak potential (Epeak) and the Ptshell coverage (at least up to one monolayer) is found to be linear (Fig. 6c). In our previous study on PtxIry NPs, the CO stripping peak potential also showed a linear relationship with the PtxIry NP composition

24

, thus validating the use of the CO stripping peak

potential as an indicator of the surface composition of these NPs. The negative shift in the CO stripping peak potential ceases when the Ptshell coverage of the Ircore is ≥ one monolayer (Fig. 6b), with the peak potential then remaining fixed at the value seen for Pt/VC (0.83 V vs. RHE). However, the onset potential is shown to be more negative than that seen for Pt. At a Ptshell coverage ≥ 1 ML, the Ircore can no longer influence the CO oxidation behavior of the Ptshell via the bi-functional effect, as all of the Ir surface sites are covered with Pt. Therefore, the slight negative shift in the onset potential of CO stripping should indicate the presence of an electronic effect of Ir on Pt. However, this electronic effect seems to be weak compared to that of Ru, as in the case of the Rucore@Ptshell NPs, a negative shift of > 200 mV in the onset potential of CO stripping is observed

28,37,44,51,52

. Also, in the case of Ir, as the Ptshell thickness is

increased, from 1 to 2 MLs, no significant change is seen in the CO stripping behavior (Fig. 6b). In contrast, in the case of Ru, CO stripping is found to be much more sensitive to the Ptshell thickness, showing a peak at 0.6 and 0.78 V vs. RHE for 1 and 2 MLs of the Ptshell, respectively 37. Both Ir and Pt are also known to adsorb/desorb H atoms at potentials significantly more positive than the E° value for the H+/H2 redox reaction

53,54

, known as hydrogen

underpotential deposition (Hupd), and yet each metal gives a distinct signature. Thus the 18


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goal of this part of the work was to again confirm the surface composition of the core@shell NPs, based on the Hupd response obtained. Figure 7a shows the CVs (20 mV/s, 0.05-1 V vs. RHE) of the Ircore@Ptshell/VC catalysts in comparison with those of pure Ir/VC and pure Pt/VC, all in RT, 0.5 M H2SO4. The Hupd process occurs in two peaks at both Pt and Ir, with the less positive peak related to weakly adsorbed H (Hw, 0.095-0.115 V vs. RHE) and the more positive one to strongly adsorbed H (Hs, 0.22-0.23 V vs. RHE) 55–57, as shown in Fig. 7b.

Figure 7: (a) CVs (20 mV/s) of Pt, Ir, and Ircore@Ptshell NPs loaded (10 wt %) on VC powder in RT 0.5 M H2SO4, with the hydrogen desorption region shown at a more sensitive current scale in (b), and (c) the relationship between the Hw:Hs peak current ratios and the Ir bulk at % in the catalysts studied in (a and b). The theoretically ‘expected’ peak current ratios were obtained from equation (2), while the ‘measured’ ratios were obtained from the Hupd peak data in (a and b).

The ratio of the Hw:Hs peak currents is characteristic for each of the Ir and Pt NPs, being ~3.1 and ~1.3 respectively 24. This difference in Hupd peak current ratios was then used to calculate the Pt shell coverage on the Ircore NPs, assuming the absence of any effect of the inner Ircore on the H adsorption/desorption properties of the overlying Ptshell. As the Ptshell coverage increases, the Hw peak currents gradually decrease, as seen in Fig. 7a and 7b, indicative of a gradual change from the Hupd characteristics of Ir to those of Pt, as expected. 19



Page 20 of 41

In order to interpret the Hw:Hs peak ratios quantitatively, equation (S2) was used, using the factors of 3.1 and 1.3 described above. Figure 7c shows that the experimentally determined values for the Hw:Hs ratio match closely with the calculated values. A small difference between these values is observed when the Pt coverage is close to one monolayer (specifically for 0.8 and 1.3 monolayers of Pt), which could be due to the presence of some uncovered regions of the Ircore or due to an electronic effect of Ircore on the Ptshell. In general, however, the good match between the predicted and experimental Hw:Hs ratios supports the proposed uniform growth of the Ptshell on the Ircore NPs as the Pt coverage increases.

3.2. Electrochemical determination of Ircore@Ptshell NP surface area Knowledge of the catalyst specific surface area (SSA) is critical to compare the activities of the various catalysts under study here. Several approaches, including hydrogen underpotential deposition (Fig. 7a), CO stripping (Fig. S3), and area prediction from the particle size (based on TEM images), were used to determine the real surface area of the Ir, Pt, and Ircore@Ptshell NPs. The comparison is shown in Figure 8. The same trend is obtained for the SSA values using all three methods, where the SSA decreases as the Pt content (Ptshell coverage) increases, largely due to the NP size increase. The SSACO and SSATEM values are quite similar, while the SSAHupd values are all lower. The presence of traces of the PVP capping agent, even after electrochemical cleaning 38, is expected to interfere with the determination of the Hupd charges, but not as much with the CO oxidation process. In fact, CO adsorption at NP surfaces is known to 20


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occur even in the presence of some PVP 58,59, while H adsorption tends to be blocked by capping agent adsorption 60. However, the presence PVP resulted in large fluctuations in the SSACO values, due to the possibility of trapped CO or the necessity for a longer time of the adsorption step to achieve monolayer CO coverage. Therefore, SSATEM was used here to determine the electrochemical activity of the NPs toward methanol oxidation. At high Ir contents, the SSA values, based on the three methods used to calculate them, are quite similar to each other, being within 2-10 % of the mean. This good agreement is likely due to the fact that the NP surfaces are quite clean, overall. However, it was noted that, as the Pt content increased, the SSA obtained from the Hupd charges began to decrease (relative to the values obtained using CO stripping and TEM analysis), likely due to the stronger adsorption of the PVP stabilizer, through its carbonyl group, on any exposed Pt sires (vs. on Ir). This change was not seen for the SSA values obtained from the CO stripping peak, which continued to match quite closely with the values obtained from TEM analysis. This is hypothesized to be due to the ability of CO to compete with the PVP carbonyl groups for adsorption sites on both Pt and Ir, in contrast to the H atom. As the SSACO and SSATEM values are so similar (Fig. 8), this argues that the Ircore@Ptshell NPs are distributed very well on the VC support. According to the TEMdetermined SSAs (Fig. 8), the Ircore@Ptshell NP SSAs are in the range of 65-85 m2/g, the Pt NP SSA is ca. 60 m2/g, and the SSA of the Ir NPs is closer to 90 m2/g.

21



Page 22 of 41

Figure 8: Specific surface area (SSA) of Ircore@Ptshell NPs, based on the hydrogen underpotential deposition charges (Hupd, squares and solid line), the CO stripping peak charges (circles and dashed line), and TEM-determined particle sizes and assuming spherical NP shapes (triangles and dotted line).

3.3 Catalytic activity of the Ircore@Ptshell NPs towards the methanol oxidation reaction (MOR) The material that comprises the core in the Mcore@Ptshell NPs can have several effects on the Ptshell MOR activity, including inducing strain 61,62, an electronic effect 36,62, and/or a bi-functional effect, with the latter being relevant only when the core surface is also exposed to the solution. In fact, both the bi-functional and electronic effects are expected to enhance the rate of CO oxidation

7,28,63,64

. However, a stronger electronic

effect will inhibit the methanol adsorption/dehydrogenation step, which is normally rate determining at high potentials (E ≥ 0.8 V vs. RHE) 28,65.

22


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Both Pt and Ir have a face-centered cubic structure (FCC) and the lattice constant of Ir (3.84 Å) is very close to that of Pt (3.92 Å)

47

. Therefore, the Ircore should exert

minimal strain on the Ptshell and thus on its catalytic activity. For this reason, the strain effect was ruled out for the Ircore@Ptshell NPs under investigation here. However, this small difference in the lattice constants could still affect the electronic properties of the Ptshell, but probably not significantly, especially compared to the electronic interactions between Pt and Ir. Even though the CO desorption data shown in Fig. 6b suggested that the electronic effect of the Ircore on the CO oxidation activity at the Ptshell should be minor, the role of the electronic and bi-functional effects of the Ircore on the MOR at the Ptshell was still examined further here. This was done by comparing the MOR activity of Ircore@Ptshell NP catalysts having a Ptshell coverage in the range of 0.3 to ~ 2 monolayers (MLs). In order to allow the bi-functional effect to vary while fixing the electronic effect, the activity towards the MOR was first examined at Ircore@Ptshell NPs with a Ptshell coverage ranging from 0 to 1 ML. At a Ptshell coverage of > 1, the bi-functional effect of Ir will no longer be present and hence any change in the catalytic activity as the Ptshell thickness increases to > 1 ML must be related to the electronic effect of the Ircore on the Ptshell properties. The MOR activity of the Pt, Ir and Ircore@Ptshell NPs (with varying Ptshell coverages), all supported on VC, was examined using cyclic voltammetry (0.05-0.9 V vs. RHE, 20 mV/s) in both RT (Fig. 9a) and 60 °C (Fig. 9b) 1 M CH3OH + 0.5 M H2SO4 solutions, with the currents normalized to the specific surface area of the Ircore@Ptshell NPs. The methanol oxidation activity at the Ircore@Ptshell NPs at both RT and 60 °C was 23



Page 24 of 41

also normalized to the total Pt+Ir mass or to the Pt mass, as shown in Figures S4 and S5, respectively. Figure 9a shows that Pt and Ir behave similarly towards the MOR at < 0.6 V vs. RHE. However, at higher potentials, the Pt activity increases while the Ir activity drops, likely due to some Ir oxide formation, which can start at 0.45 V

50

. In the case of the

Ircore@Ptshell/VC catalysts with Ptshell coverages of 0.3-0.4 MLs, a significantly higher catalytic activity is observed at < 0.7 V vs. RHE than at the Pt NPs alone (Figure 9a). As the Ptshell coverage increases further, the MOR activity drops until it becomes similar to that seen at the Pt NPs. In contrast, at higher potentials (e.g., 0.8 V vs. RHE), the Ircore@Ptshell NP/VC catalysts with a Ptshell coverage of 0.3-0.4 ML exhibit the lowest MOR activity, while the activity levels increase with increasing coverage of the Ptshell. As well, Figure 9a shows that the MOR activity of the Ircore@Ptshell NP/VC catalysts with a Ptshell coverage of roughly 2 MLs is similar to what is seen at the pure Pt NPs at all potentials. The MOR activity of the Ircore@Ptshell NPs was also examined at higher solution temperatures (60 °C), with Figure 9b showing 5-6 times higher MOR currents vs. at RT, as expected. Furthermore, the Ir and Ircore@Ptshell NPs both show a higher catalytic activity than do the Pt NPs at low potentials, but a lower catalytic activity at higher potentials.

24


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Figure 9: MOR currents measured at Pt/VC, Ir/VC, and Ircore@Ptshell/VC catalysts, all supported on Vulcan carbon powder, after 1 min of potential cycling (0.05-0.9 V vs. RHE, 20 mV/s) in 1 M CH3OH + 0.5 H2SO4 at (a) RT and (b) 60 °C. The measured currents were normalized to the SSATEM. In the labels, the Pt coverage is indicated in brackets and the Ir and Pt at% values are indicated by the subscripts. To clearly show the effect of the Ircore on the Ptshell MOR activity, the currents (imeasured) at the Ircore@Ptshell NPs were divided by the theoretical MOR activity that would have been seen if the Pt and Ir components were not influencing each other. This was achieved by multiplying the MOR currents at the individual Pt (iPt) and Ir (iIr) NPs by the surface fraction of each of Pt and Ir at the Ircore@Ptshell NPs [(iPt * Ptshell coverage) + iIr * (1- Ptshell coverage)]. This gives the catalytic enhancement factor (CEF), obtained using equation (1). At Pt coverages > 1 ML, the CEF was obtained by dividing the Ircore@Ptshell MOR activity by that seen at the Pt NPs alone, as the Ircore is totally covered by the Pt shell and could contribute only through the electronic modification of the Pt activity. The CEF values obtained for the MOR at the Ircore@Ptshell NPs (eq. (1)) are given in Table 1. CEF

i i 1! i i Pt coverage! " i# 1 % Pt coverage!

25



Page 26 of 41

Both bi-functional and electronic effects should enhance the rate of CO oxidation at low potentials, resulting in CEF values > 1. At high potentials, the bi-functional effect enhances CO oxidation and hence the rate of the MOR, thus giving CEF values > 1. However, the presence of a strong electronic effect will inhibit the methanol adsorption/dehydrogenation step, which controls the kinetics at high potentials, thus decreasing the MOR rate and resulting in CEF values < 1.

Table 1: The relationship between the Pt coverage (in MLs of Ptshell) of the Ircore/Ptshell NPs and the MOR catalytic enhancement factor (CEF, eq. (1)). Sample

Ir85@Pt15 Ir80@Pt20 Ir65@Pt35 Ir51@Pt49 Ir41@Pt59 Ir36@Pt64

Pt MLs

0.3

0.4

0.8

1.3

1.9

2.3

CEFRT at 0.6 V

2.1

2.4

1.6

1.4

1.2

0.9

CEFRT at 0.8 V

1.7

1.6

1.1

0.9

1.1

0.9

CEF60 °c at 0.6 V

1.4

1.5

1.5

1.3

1.5

1.7

CEF60 °c at 0.8 V

1.1

1.1

0.7

0.7

0.8

0.9

At RT conditions, the highest CEF values are obtained when the Pt coverage is in the range of 0.3-0.4 ML at both low and high potentials. The CEF value is then seen to decrease when the Ircore is fully covered by the Ptshell (Ir51@Pt49 NPs (1.3 MLPt)). This demonstrates that the bi-functional effect of the Ir core on the Pt shell is dominant in the removal and oxidation of adsorbed CO under these < 1 ML Pt shell conditions. Notably, a CEF value > 1 is still seen at low potentials for the Ir51@Pt49 NPs, which have a Ptshell coverage of 1.3 MLs. This likely indicates that there is a weak electronic effect of the 26


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Ircore on the Ptshell, as the Ir core surface should now be fully covered by Pt and thus the bifunctional effect can be ruled out. As the Pt coverage increases from 1.3 to 2.3 ML, the CEF value at 0.6 V decreases, indicating that the electronic effect of Ir on Pt becomes weaker. This is presumably because thickening of the Ptshell decreases the electronic effect of Ircore on the outer layer (exposed to the solution) of the Ptshell. At 60 °C, the CEF values are quite similar for all of the Ircore@Ptshell NPs at low potentials. It is known that CO adsorption on Pt becomes weaker as the temperature increases 34. Therefore, at low potentials, the presence of exposed Ir sites (Ptshell coverage < 1 ML) at 60 °C (Table 1) does not result in any significant advantage as it did in the RT experiments (Table 1). CO removal is achieved even when the Ircore is covered with ~ 2 MLs of Pt. This is likely indicative of an electronic effect of Ir on Pt under these conditions, consistent with the CO stripping results (Fig. 6). However, at higher potentials (Table 1) in the 60 °C solution, only catalysts with some Ir exposed to the solution (Ptshell coverage < 1 ML) show an improvement in the MOR activity, indicative of the presence of the bi-functional effect. Ircore@Ptshell NPs with a > 1 ML Pt coverage showed a MOR activity equal to or lower than what is seen at the Pt NPs alone, indicating a negative effect of the Ir core on the Pt shell MOR activity. At higher potentials and temperatures, the rate of methanol adsorption increases and the presence of even a minor electronic effect of the underlying Ir on the Ptshell should weaken the adsorbed methanol/NP interactions and hence also the methanol oxidation rate. This is accompanied by slow CO removal, caused by the absence of any exposed Ir. 27



Page 28 of 41

When the electronic effect of Ir on Pt was lowered by the presence of a second monolayer of Pt on the Ircore (1.3 ML to 1.9 ML), the catalytic activity of the Ircore@Ptshell NPs in the 60 °C solution at 0.8 V increased by only a small amount (1.2 A/m2), likely due to the enhancement of the methanol adsorption rate. In comparison, for Rucore@Ptshell NPs, studied under the same conditions, the current increased by 5.1 A/m2

28

. This

demonstrates the relatively minor electronic effect of Ir, compared to Ru, on the catalytic activity of Pt towards the MOR. Based on the MOR results presented in this work, it can be concluded that, at RT, the Ircore of the Ircore@Ptshell NPs enhances the MOR activity of the Ptshell primarily through a bi-functional mechanism. At higher temperatures, the bi-functional effect is still dominant, although there is some evidence for the presence of a minor electronic effect of Ir on Pt. This is seen by the facile removal of CO at low potentials, even when the Ircore was covered with > 2 MLs of the Ptshell. However, at higher potentials, the presence of a significant amount of Ir on the exposed NP surface is necessary to remove adsorbed CO. In our previous study, focused on the effect of Ru on the methanol oxidation activity observed at Pt, a series of Rucore@Ptshell NPs with a zero to two monolayer Ptshell coverage and two different Rucore sizes (2 and 3 nm) were tested at RT and 60 oC. The main effect of Ru on Pt was found to be both a strain and an electronic effect, with only a minor bifunctional effect observed28. In contrast, in the case of the Itcore@Ptshell NPs, studied in the present work, the bi-functional effect appears to be the most dominant, with a more minor electronic effect of the underlying Ir core on the Pt shell properties seen during the various steps of the MOR. Notably, in our previous study of PtxIry alloy NPs, 28


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it was simply not possible to distinguish the bi-functional and electronic effects of the Ir sites on Pt during the MOR38.

Summary In our recent work, the methanol oxidation activity of electrodeposited Pt-Ir alloy thin films and Pt-Ir alloy nanoparticles (NPs) was shown to be enhanced by the presence of Ir, likely through a bi-functional effect of Ir on Pt. However, it was difficult to be definitive about these conclusions due to the challenges of separating the bi-functional effect of Ir from its electronic effects, as both of these factors would be expected to increase with increasing Ir content. Therefore, in the present work, Ircore@Ptshell NPs, synthesized using the simple polyol method, were produced with a controlled Ptshell coverage and thickness, allowing the bifunctional and electronic effects to be rigorously evaluated. The Ircore@Ptshell NPs consisted of Ircore NPs, covered by a Ptshell that was between 0 and two monolayers in thickness. Transmission electron microscopy (TEM) images and X-ray diffraction (XRD) data showed that the average size of the Ircore NPs remained at ca. 3 nm, independent of the Ptshell coverage (between 0 – 1 monolayers (ML)) and that the Ircore crystallinity also did not change. The coverage of the Ircore by the Ptshell was determined using the hydrogen underpotential deposition/removal (Hupd) peak ratios and the CO stripping peak potentials, both of which are characteristic of Pt and Ir, as well as from the XRD and TEM results. To precisely determine the Ptshell coverage/thickness when it was expected to be in the 1 - 2 ML range, only XRD and TEM methods could be used, as these NPs all then showed similar CO stripping and Hupd behavior. 29



Page 30 of 41

At room temperature, the presence of the Ircore was found to enhance the MOR activity of the Ptshell at all potentials, shown to be due primarily to the bi-functional effect, with an optimum Pt coverage determined to be 0.4 ML. At higher temperatures and high potentials (high CO formation rates), the bi-functional effect of Ir on Pt is still dominant, while the electronic effect of Ir on Pt causes an inhibition in the MOR rate by weakening methanol adsorption on Pt. However, at lower potentials (lower rates of CO formation), the relatively weak electronic effect arising from the Ircore helps to remove adsorbed CO from the Ptshell. These realizations, as well as the method of electrochemical data analysis developed in this work, could be extended to the study of other electrocatalytic reactions at metallic NPs to obtain a more fundamental understanding of their behavior.

Acknowledgements We are very grateful to the Electrometallurgy Consortium, as well as the Natural Sciences and Engineering Research Council of Canada (NSERC) through its Discovery Grant program, for their support of this work. In terms of TEM/EDX analyses, we thank Dr. Tobias Fürstenhaupt from the Microscopy and Imaging Facility (MIF) at the University of Calgary, Alberta, Canada, while for assistance with the WDS analyses, we thank Dr. Robert A. Marr (The University of Calgary Laboratory for Electron Microprobe Analysis (UCLEMA), Calgary, Canada).

30


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Supporting Information: Calculation of Pt shell layer thickness, confirmation of metal loading and Ircore/Ptshell NP size, electrochemical determination of Ircore/Ptshell NP surface composition and area, and methanol oxidation at Ircore/Ptshell NPs normalized to total and Pt mass.

31



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1 Nano-engineered Ircore@Ptshell Nanoparticles with Controlled Pt

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