Wetting Induced Oxidation of Pt-based Nano Catalysts Revealed by In Situ High Energy Resolution X-ray Absorption Spectroscopy

In situ high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) was used to systematically evaluate interactions of H2O and O2 adsorbed on Pt and Pt3Co nanoparticle catalysts in different particle sizes. The systematic increase in oxidation due to adsorption of different species (H2O adsorption <O2 adsorption <O2 + H2O coadsorption) suggests that cooperative behavior between O2 and H2O adsorptions is responsible for the overpotential induced by hydrated species in fuel cells. From the alloying and particle size effects, it is found that both strength of O2/H2O adsorption and their cooperative effect upon coadsorption are responsible for the specific activity of Pt catalysts.


Estimation of the particle size by using the Pt XRD pattern
The mean particle size of each sample (as shown in Table I) was estimated by the Scherrer equation 1 : where the error bar of the particle size can be described as: where D is the mean size of the ordered (crystalline) domains, maybe smaller or equal to the particle size, λ is the X-ray wave length, K is a dimensionless shape factor called Scherrer constant, θ is the Bragg angle. B 2θ is the line broadening estimated by the full width at half maximum of the Bragg peak after subtracting the instrumental line broadening. The (220) XRD peak (inset of Fig. S1) fine scanned with Rigaku SmartLab XRD spectrometer was used for evaluation of the particle sizes based on Eq. (1).

The relationship of HEFRD-XAS and transmission mode XANES.
Convolution of the HERFD-XAS spectrum of Pt(A) with a differential lifetime broadening between Pt 2p 3/2 (~5.2 eV) and Pt 3d 5/2 (~2.4 eV) 2 , namely 4.6 eV (√(5.2 2 -2.4 2 )), exactly reproduces the XAS spectrum measured by a transmission method as shown in Fig. S2. The basic knowledge about the linear relationship of Pt L 3 white line intensity and Pt d electron vacancies was reported by Lytle et al. 3 and Horsley 4 . Our peak area fitting of the Pt L 3 XANESs spectra for standard Pt foil and PtO 2 samples is also based on the solution described by Horsley 4 . Considering the relationship between HERFD-XAS and transmission mode XANES shown in Fig. S2, one can use the same fitting to analyse the HERFD-XAS spectra and obtain strength of hybridization between adsorbates and Pt atoms more accurately.
When the white line area of Pt and PtO 2 are denoted as µ Pt and µ PtO2 , the change in the white line area can be written as The white line area (µ) for a certain adsorption to the Pt atom with the ratio x and the charge transfer δ can be written as: Then, The physical meaning of x' is the effective ratio of PtO 2 in the total number of Pt atoms and can be measured experimentally.
Given Θ the number of surface atom adsorbed by adsorbates, S the surface area, the surface coverage (mol/m 2 ) of adsorbates σ can be written as: Here, N T is the number of Pt atoms with mass of m and can be rewritten using the Pt Molar mass M as: Therefore, Here, the surface area per gram of Pt (m 2 /g) is defined as S m = S m , then, • S m can be evaluated not only by CO-stripping voltammetry and hydrogen under potential deposition method but also by the diameter of nanoparticles obtained by XRD or STEM 5 . In this work, S m was evaluated by CO-stripping voltammetry (TKK Co., Ltd.) where, Q CO is the charge correlated with CO-stripping, and a constant of C = 4.20 C/m 2 is commonly used for the unit charge per surface area with CO-stripping on Pt or Pt 3 Co surface as described in Ref. 5.
The error bar of ′ x MS m can be described as: Usually, the system error of / m m S S D was varied from 11% to 17% as reported in Ref.

5.
We applied 15% in this work.
By using the above method, the data shown in Fig. 6 were generated.

Samples evaluated by hard X-ray photoelectron spectroscopy (HAXPES)
For the HAXPES measurements the samples were fixed to Cu holders by carbon conducting tapes. The energy position of the Au 4f 7/2 peak was used for energy calibration. The take-off-angle (TOA) was fixed to 80 ○ . A total energy resolution of 230 meV was evaluated by the energy position of the Au Fermi edge at room temperature.  6. Setup of in situ XAFS experiment and the in situ cells.