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70 CHAPTER4. RESULTSANDDISCUSSION 4.4.1 The active site for HER on MoS2 The study used an ambient pressure XPS setup, as shown in Figure 4.17. The setup consisted of a Nafion membrane, which separated the cathode side, on which amorphous nanoparticulate MoS3 catalysts were dispersed on a carbon support material, from the anode side, on which a platinum-carbon catalyst was deposited. The Mo 2p XPS measurements were performed at room temperature, with the an- ode side being exposed to saturated water vapor and the cathode side to a vacuum with a leakage stream of water gas achieving a partial pressure of water of 10 torr. The evolution of the X-ray photoelectron spectra is seen in Figure 4.18. Two main peaks are visible, with the main peak before HER decreasing in intensity, and the peak shifted by ca −1 eV from the main peak increasing in intensity during HER. Comparison with the spectra for nanocrystalline MoS2 indicated that the change during electrolysis corresponds to a conversion of MoS3 to MoS2. The inelastic mean-free path of the electrons being ionized from the Mo sites was estimated to be 2.2 nm, indicating that the spectral changes correspond to changes in the near- surface region. Previous studies have shown that MoS2 nanoparticles are present on the support as flat polygons made up of trilayer S-Mo-S structures, where the S edges are active for HER[49]. The thickness of one S-Mo-S trilayer is about 3 Å, a tenth of the penetration depth of the excited electrons, so it is not possible to con- nect the active site structure to the observed XPS shifts directly without modeling the active site and calculating the shifts for representative structures. To further the understanding of the active site for HER, we carried out DFT cal- culations to model flat trilayer structures corresponding to both MoS3 and MoS2. Previous theoretical studies have indicated that the experimental conditions de- cide the S coverage on the Mo edges[239, 240]. To attempt to find the most stable S edge coverages at HER conditions, free energy calculations were car- ried out to examine the stability of different Mo edge structures at varying partial pressures of H2S. In these calculations, the electrode potential was accounted for using the CHE model (further details of this method are found in the work of Bollinger et al. [240]). It was found that, under HER conditions (U = 0 V vs RHE, pH2S = 1 × 10−6 bar, the pressure chosen based on the arbitrary standard for cor- rosion resistance of a material[241]), the most stable Mo-S edge coverage was θS = 0.5 ML and θH = 0.25 ML. Thus, under HER, the active edge of the catalyst should correspond to coordinatively unsaturated Mo sites, mirroring the activity of CUS for OER and ClER on RuO2. XPS calculations were then carried out for Mo edges with increasing coverage of S, from θS = 0.5 ML (the CUS “MoS2” edge structure) to θS = 1 ML (the coordinatively saturated “MoS3” edge structure). The calculated XPS shifts for these structures are seen in Table 4.4. In cases where multiple unique S anions are present at the edge, the XPS shifts for each unique anion is indicated. It is seen that the XPS binding energies for the MoS3 structure is shifted by ca 1.3 eV from the binding energy of the MoS2 structure. S in SPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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