SUPPORTING INFORMATION

Transcription

1 SUPPORTING INFORMATION

2 Nano-engineered Ir core /Pt shell Nanoparticles with Controlled Pt Shell Coverages for Direct Methanol Electro-oxidation Ehab N. El Sawy a,b and Viola I. Birss a,* a Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 b Chemistry Department, School of Sciences and Engineering, American University in Cairo, New Cairo 11835, Egypt * To whom correspondence should be addressed; birss@ucalgary.ca (a) Calculation of Pt shell layer thickness In the case of metallic core/shell nanoparticles (NPs), prepared in the presence of polyvinylpyrolydone (PVP) as a capping agent, the shell is known to deposit uniformly around the core 1 3. Thus, the addition of one monolayer (ML) coverage of Pt to a single Ir NP should result in an increase in the NP diameter by ~ 0.54 nm (2 Pt atom diameter, which is 0.272). The number of moles (n) in a NP depends on its volume (4/3)πr 3, assuming spherical NPs and using the bulk density (ρ) and molar mass (M). Therefore, the corresponding increase in the number of moles, due to the volume increase resulting from the addition of a Pt shell, can then be calculated, as shown in equation (S1) and demonstrated in Scheme S1 4. As Pt and Ir have a similar density and atomic weight, we can assume that the number of moles in the Ir core /Pt shell NPs is the same as what would be found in an Ir NP with a similar diameter. Assuming an Ir core size of 3 nm, an Ir:Pt atomic ratio of 1:0.6 is required for the formation of a single Pt shell monolayer, while a ratio of 1:1.6 is needed for the formation of two monolayers. In order to obtain the desired Pt shell coverage, based on the previously mentioned assumptions and calculations, the Ir:Pt molar ratio in the synthesis solution was the same as the desired atomic ratio of Ir:Pt in the Ir core /Pt shell NPs. S-1

3 n mole ρ g cm 4 3 πr cm M g mole S1 Scheme S1: Schematic diagram showing the calculated number of moles, obtained using equation (S1), in Ir core /Pt shell NPs before and after the addition of one and two monolayers of the Pt shell, assuming that the Ir core NPs are 3 nm in diameter. (b) Confirmation of metal loading and Ir core /Pt shell NP size Figure S1: TGA analysis (10 C/min heating rate in air) of Pt, Ir, and Ir shell NPs, all loaded at 10 wt% on VC. The final metal mass is ca. 10% of the original total mass, as was anticipated. S-2

4 Figure S2: TEM bright field image of 10 wt % Ir 59 NPs loaded on VC powder. (c) Electrochemical determination of Ir core /Pt shell NP composition and surface area To determine the surface composition of the NPs, the following approach, based on the hydrogen atom adsorption/desorption charges, was used. At a Pt shell thickness < 1 ML, the H w :H s peak ratio should be between that of Ir (3.1) at zero Pt coverage and that of Pt (1.3) at full monolayer Pt shell coverage, according to equation (S2). The Pt shell coverage in equation (S2) was calculated as explained in the Experimental Section, with (1-Pt shell coverage) representing the uncovered fraction of the Ir core surface. At Pt shell 1 ML, the H w :H s peak ratio should be 1.3, similar to that of Pt. Therefore, any mismatch between the theoretically calculated and measured values of the H w :H s ratios should be related to the electronic effect of Ir core on the Pt shell or to the presence of uncovered regions on the Ir core. H :H Pt coverage 1.3 Pt coverage 100 S2 S-3

5 In terms of calculating the electrochemically active surface area (ECSA), the following approach was used in this work. If the ECSA of supported metallic NPs is close to their expected total surface area, based on the TEM-estimated average particle size and shape, this would indicate that very good dispersion of the NPs had been achieved. Also, knowledge of the real catalyst surface area is essential to making a reliable comparison between the activities of the various catalysts under study here. Therefore, several approaches were used to attempt to determine the real surface area of the Ir, Pt, and Ir core /Pt shell nanoparticles, supported on Vulcan carbon (VC) powder. First, the Hupd charges were calculated to determine the Ir core /Pt shell NP surface area, as described below. Figure 7a (main paper) shows the CVs ( V vs. RHE, 20 mv/s) for a range of Ir core /Pt shell NPs/VC catalysts in comparison with those of the Ir core NPs/VC and Pt NPs/VC powders, all in RT 0.5 M H 2 SO 4. Figure 7a (main paper) shows that, as the Ir core coverage with the Pt shell increases, the Hupd charge decreases, at first suggesting that the overall surface area is decreasing. However, this observation needs to be analyzed further. In order to measure the surface area (m 2 ) of the Pt and Ir NPs using the Hupd charge, the measured Hupd charge (C) has to be divided by 2.1 C/m 2 and 1.37 C/m 2, respectively. These constants are the charge assumed for the deposition of one and 0.65 monolayers of H on 1 real m 2 of Pt and Ir, respectively 5,6. In the case of Ir core /Pt shell NPs, both Pt and Ir contribute to the measured Hupd charge, and hence the constant should be altered to accommodate the surface composition change. Therefore, equation (S3) was used for the determination of the specific surface area of the Ir core /Pts hell NPs. The Pt shell coverage and (1-Pt shell coverage) represents the Pt and Ir fractions of the surface, respectively. The SSA, based on the Hupd charges, is shown in Figure 8 (main paper). S-4

6 PtIr m /g Measured q C/g Pt coverage 2.1 Pt coverage C/ m S3 CO stripping is another electrochemical method that can be used for the determination of Pt and Ir-based catalyst surface areas Figure S3 shows the CO stripping voltammogram (20 mv/s) as well as the second CV scan after the total removal of CO for an Ir 41 /Pt 59 NP/VC catalyst in 0.5 M H 2 SO 4 as an example of how the surface area was calculated using the CO stripping method. In the first cathodic scan (Fig. S3), the H deposition peaks are seen at < 0.3 V vs. RHE. The fact that the second CV cycle fully overlaps with the first in the Hupd region confirms the complete removal of CO in the first anodic stripping scan. Figure S3: CO stripping voltammetry (20 mv/s) of Ir 41 /Pt 59 (1.3 MLs of Pt shell ) NPs/VC in RT, 0.5 M H 2 SO 4 followed by a second CV under the same conditions. CO was pre-adsorbed prior to the first CV by saturating the solution with CO while holding the potential at 0.05 V for 15 min. In the CO stripping method, the CO stripping charges should be divided by the NP total mass (C/g) and by 4.2 C/m 2 (the charge density related to the adsorption of one monolayer of CO S-5

7 on Pt or Ir) 11,12 in order to determine the Ir core /Pt shell NP specific surface area (SSA, m 2 /g). However, the CO stripping charge has to be corrected first for the charge related to Pt or Ir oxide formation. For example, in the second CV (Fig. S3), where the surface is free of adsorbed CO, some anodic charge related to surface oxide formation is observed (Fig. S3). This charge must then be subtracted from the CO stripping charge. The similarity of the surface oxide reduction charge in both the first and second CV, which is observed as an envelope of cathodic charge between 1 and 0.4 V vs. RHE, indicates that an equivalent amount of charge must also have been passed in the anodic scan in the presence and absence of CO. This validates the subtraction of the second anodic scan charge from the anodic CO stripping charge. The SSA based on CO stripping is shown in Figure 8 (main paper). As a third method, the SSA of the Ir core /Pt shell NPs was estimated using equation (S4) and the average particle diameter (2r), determined from TEM analysis (SSA TEM ) 13,14 and assuming a spherical particle shape (as suggested in Fig. 3 (main paper)). 4πr 2 is the particle surface area, (4/3)πr 3 is the particle volume, and ρ is the density that was calculated, based on the atomic fractions of Pt and Ir in the NPs. The SSA TEM values are shown in Fig. 8 (main paper) and compared with the SSA Hupd and SSA CO values. SSA m g 4πr m 3 ρ g m 4 3 πr m ρ g m r m S4 S-6

8 (d). Methanol oxidation at Ir core /Pt shel l NPs Figure S4: MOR current measured at Pt/VC, Ir/VC, Ir@Pt NP catalysts, all supported on Vulcan carbon powder, and divided by (a) the total Pt+Ir mass and (b) Pt mass, after 1 min of potential cycling ( V vs. RHE, 20 mv/s) in 1 M CH 3 OH H 2 SO 4 at RT. The Pt coverage is indicated in brackets. S-7

9 Figure S5: MOR current measured at Pt/VC, Ir/VC, NP catalysts, all supported on Vulcan carbon powder, and divided by (a) the total Pt+Ir mass and (b) Pt mass, after 1 min of potential cycling ( V vs. RHE, 20 mv/s) in 1 M CH 3 OH H 2 SO 4 at 60 C. The Pt coverage is indicated in brackets. S-8

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