Electrically Polarised Electrodes in DMSO and DMF electrolytes of Pseudohalide Salts: In situ IR studies supported by EXAFS/XANES and other techniques
Alwis, K. H. K. L. (2015). Electrically Polarised Electrodes in DMSO and DMF electrolytes of Pseudohalide Salts: In situ IR studies supported by EXAFS/XANES and other techniques (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/9624
Permanent Research Commons link: https://hdl.handle.net/10289/9624
This thesis presents, subtractively normalised interfacial Fourier transform infrared spectroscopic (SNIFTIRS) investigations of anodically polarised nickel, copper and gold electrodes in pseudohalide-containing (i.e. NCO⁻, NCS⁻, NCSe⁻, CN⁻ and TeCN⁻) dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) solutions, with a supporting electrolyte, tetrabutylammonium perchlorate (TBAP), is presented. Cyclic voltammograms, and current-potential data were recorded while the infrared spectral acquisition was in progress for nickel, copper and gold electrodes in a thin-layer cell. A thin layer electrochemical cell with CaF₂ IR windows was used to acquire data. For the anodic dissolution of Ni, Cu and Au electrodes in DMF and DMSO media containing pseudohalide ions, it was found that all electrodes anodically dissolved generating Ni²⁺, Cu⁺/Cu²⁺ and Au⁺ coordination complexes consisting of pseudohalide ions (i.e. NCO⁻, NCS⁻, NCSe⁻) and solvent molecules. These conclusions were confirmed by demonstrating that the same complex ion species were formed in model solutions prepared by mixing Ni(II), Cu(I), Cu(II) and Au(III) salts with the corresponding pseudohalide salts (KOCN, NaSCN, KSeCN, KCN and KTeCN) in either DMSO or DMF solvent by comparison of their IR transmission spectra with the in situ IR spectra. Additionally the geometry of the nickel/copper-pseudohalide complex ions formed in particular during anodic dissolution experiments was probed using other techniques which involved X-ray absorption spectroscopy (XAS). Further electrospray ionisation mass spectrometry (ESI-MS) was also used to confirm the presence of such species and to characterize other by-products formed in the model solutions. In general, the data showed that the nickel electrode undergoes irreversible anodic dissolution in all solutions studied at high applied potentials, greater than +500 mV (AgCl/Ag). Nickel predominantly speciated into Ni²⁺ complexes. Insoluble films and dissolved CO₂ were also detected, though mostly in the Ni/NCO⁻ systems studied. In general, the Ni/NCO⁻ electrochemical system behaved differently relative to those of Ni/NCS⁻ and Ni/NCSe⁻ , as observed via the difference in colours in cell solutions produced after SNIFTIRS experiments which was mirrored in the model solutions. Ni(II)-cyanate species had a different coordination geometry and gave a characteristic bright blue colour due possibly to the species [Ni(NCO)₄]²⁻ , while Ni(II) thiocyanate and selenocyanate complex ion species were proposed to have octahedral coordination geometry containing solvent and one coordinated pseudohalide ion, and formed green-yellow solutions. X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) were used to obtain direct information on the coordination state of electrogenerated products. From EXAFS/XANES data, the Ni(II)/NCS⁻ and NCSe⁻ complexes were confirmed to be octahedral bearing at least one monopseudohalide-ligand with the balance of ligands being the coordinated DMSO solvent while the data for Ni/cyanate system suggested a “five-coordinate” Ni/pseudohalide-ion complex. In reality, this suggested species was regarded as the result of XAS being a sample averaging technique and that in this solution there is perceived to be a mixture of 4 coordinate (tetrahedral) [Ni(NCO)₄]²⁻ and octahedral [Ni(DMSO)₆]²⁺ species. These observations of the octahedral geometry for the Ni(II)/thiocyanate and Ni(II)/selenocyanate systems and 5-coordinate geometry in the Ni(II)/cyanate systems are supported by the differences in colour observed between the two samples. An IR spectroelectrochemical and X-ray absorption spectroscopy (XAS) study of anodically polarized copper electrodes in polar aprotic solvents (DMSO and DMF) in the presence of pseudohalide ions and tetrabutylammonium perchlorate has been presented in Chapter 5. Cu dissolves in all 6 systems studied (i.e. DMF and DMSO, in the presence of ⁻, NCS⁻ and NCSe⁻) to produce stable Cu(I) pseudohalide complex ion species in addition to other species such as electrogenerated CO₂. Insoluble films were also observed to be deposited at higher anodic applied potentials. These films were thought to be CuSCN and K(SeCN)₃ depending on the solvent system used. The predominance of the Cu(I) oxidation state in these complexes was clearly proven from examining the single scan spectra and was supported by model solution studies. SNIFTIRS studies of Au electrodes under similar experimental conditions are presented in chapter 6. This work has demonstrated the significance of the Au(I) oxidation state which occurs after applied voltages of +500 mV(AgCl/Ag) in the little characterised electrochemistry of this metal in polar aprotic solvents, DMSO and DMF. Generally, all studies conducted showed that Au electrodes dissolved to form the corresponding Au(I) pseudohalide complexes (i.e. [Au(NCO)₂]⁻, [Au(SCN)₂]⁻ and [Au(SeCN)₂]⁻). The Au(I) species observed electrochemically by SNIFTIRS were confirmed by independent preparation in DMSO/DMF containing mixtures of KAuBr₄ and the pseudohalide salt (KOCN/NaSCN/KSeCN) and exploiting fortuitous redox chemistry where Au(I) formed spontaneously. The model solutions examined by transmission FTIR and ESI-MS confirmed the existence of the Au(I) species posited in the SNIFTIRS experiments but additionally revealed other interesting side reactions occurring in the model solutions. In situ IR studies are reported of the interaction of the little studied tellurocyanate ion with electrically polarised nickel, copper and gold electrodes in TBAP supported DMSO and DMF-based electrolytes for the first time. SNIFTIRS combined with voltammetric methods (and model solution + DFT calculations) have revealed that the TeCN⁻ ion is decomposed at anodic potentials at the metal electrodes. It was found that the speciation observed in the in situ IR spectra reflected more that of an interaction of a metal electrode with a CN⁻ ion species (a decomposition product of the TeCN⁻ ion) rather than with the TeCN⁻ ion itself. This ion was incapable of forming any discrete metal ion complexes. Fouling of the electrode by deposited elemental Te was also found to have influenced electrochemistry by blocking surface reactions. In general, the studies have confirmed the instability of TeCN⁻ ion when subjected to electrical polarisation with the observed speciation being indicative of the difference in chemical reactivity of the “fouled” Ni, Cu and Au electrodes toward anodic polarization in the presence of CN⁻ ion.
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