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76 CHAPTER4. RESULTSANDDISCUSSION and structural relaxations (local and global) are found in the paper[19]. Ru 3d, O 1s and C 1s XPS binding energies were calculated for all the structures considered in the study. The analysis was based on chemical shifts calculated ver- sus either the 3d BE of Ru in bulk RuO2, in the case of Ru 3d and C 1s shifts, and versus the O 1s BE of O in bulk RuO2, in the case of O 1s shifts. Of the structures considered, only the Ru 3d XPS shift of RuO4 is consistent with the 4 eV shift of the 289 eV peak, interpreted as corresponding to RuO(OH)2[51]. However, it is very unlikely that RuO4, a more oxidized, meta-stable, form of RuO2 with a boil- ing point of 40 ◦C[261] would form during cathodic polarization at 90 ◦C. While the peak shifted by ca −0.8 eV could be due to formation of Ru metal, the presence of other structures and structural motifs, including the bare or H-covered CUS on RuO2 and Ru sites in the Ru Gibbsite (001) surface would also result in an XPS peak with that shift. Experimental C 1s BEs for e.g. carboxyl groups[245] can explain the peak shifted by 9 eV from the RuO2 3d5/2 peak. Furthermore, the peak inversion observed in Figure 4.19 might be explained by surface carbons such as CO and CO3, shifted by 4.1 eV and 5.1 eV from the RuO2 3d5/2 peak, respectively. From these results, we are forced to conclude that the appearance of what might look like a new spin-orbit split doublet peak at 285 eV and 289 eV might instead be due to carbon contamination in the near-surface region of the electrode. Con- sidering that the electrodes were prepared by thermal decomposition of precursor chlorides dissolved in isopropanol, this might not be implausible. Furthermore, it was found that the evolution of the O 1s XPS spectra, indicating a peak shift of 2 eV, can be explained by the hydrogenation of oxygens in the rutile RuO2 structure. The O 1s XPS BEs of OH groups in the rutile structures were found to be shifted by around 2 eV from the O 1s XPS BE in RuO2. The much smaller shift associated with introduction of molecular H2 into RuO2, 0.8 eV, indicates that the XPS intensity changes occurring during HER on RuO2 are due to introduction of H rather than H2 into the coating. Furthermore, upon relaxing bulk RuO2 structures with H inside, which resulted in conversion of O groups into OH groups, the structures exhibited similar volume increases as those observed experimentally during HER on RuO2[51–53, 242, 243]. These results can be summarized as follows. During HER, H enters the RuO2 structure, and converts the structure into a Ru(OH)3 structure. This explains the volume increase of the lattice, as well as the observed XPS shifts. The large pos- itive XPS shifts observed by Näslund et al. [51] during HER are likely due to carbon contamination, either from the environment during the electrolysis, or due to residual carbon inside the coating from the solvent used in the preparation of the electrode. The additional peak appearing at a slightly lower binding energy than the Ru 3d5/2 peak could be due to formation of metallic Ru, but it is more likely that this peak is due to either Ru sites in hydroxidic regions of the outer surface of the coating, or indeed to bare or H-covered CUS sites on the electrode surface. This latter result suggests that a reason for the activation of RuO2 that hasPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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