Surface Infrared Spectroscopic Study Of Electrocatalytic Adsorption And Reaction:Alcohol In Alkaline Solution And CO In Ionic Liquids

One major mission for contemporary electrochemists is to tackle the key electrocatalytic reactions related to energy conversion and environmental protection. To that end, the mechanistic studies are essential, as they can help to understand the so-called "structure-activity relationship" of catalysts. Along this line, a reasonable research roadmap may be proposed as follows:first to identify the research objects and the related key scientific problems and then to apply highly sensitive and appropriate research methods to clarify interfacial or surface processes at both phenomenological and molecular levels. Work on direct alkaline alcohol fuel cells (DAFCs) is considered as one of the hottest topics in developing low temperature fuel cells in considering that non-Pt strategy could be implemented in DAFCs. Consequently the main focus of the thesis is to probe the mechanisms of methanol and ethanol electro-oxidations on Pd electrode in alkaline media, which may benefit the ultimate use of non-Pt anode catalysts for DAFCs. Of equal interest of this thesis is to understand the influences of electrochemical local environments such as electrolyte, electrode potential and electrode type on surface and interfacial behaviors of electrocatalytic adsorbates. As a result, we discuss the surface adsorption of CO on Pt electrode in ionic liquids and surface coordination reaction between CO and iron porphyrin attached on thiol-modified Au electrode. Besides, in consideration of the importance of research methods, the thesis work is also concerned with the technical development of electrochemical attenuated total-reflection surface-enhanced infrared spectroscopy (ATR-SEIRAS).We start with the investigation of the feasibility of using Ge as the ATR-IR window for electrochemical ATR-SEIRAS measurement, and come to a conclusion of "NO". Then we demonstrate the possibility of combining ATR-SEIRAS and external reflection infrared absorption spectroscopy (IRAS) to understand electro-catalytic reactions. Based on the above technical development, the dissociation and electrooxidation of methanol and ethanol on Pd electrodes are investigated. Spectral results suggest that weakly adsorbed formate and acetyl species are the key reaction intermediates for methanol and ethanol oxidations, respectively. Meanwhile, the potential-induced adsorption site conversion of CO and electric double-layer ions reorientation are probed over a wide potential range enabled by using a room temperature ionic liquid (RTIL). In addition, we monitor the self-assembly process of iron protoporphyrin (FePP) on thiol-modified Au electrode and further investigate the surface coordination between CO and FePP both by ATR-SEIRAS.The main results and conclusions are summarized as follows:Part I. New Technical Progresses of Electrochemical ATR-SEIRASFirstly, a secondary Au chemical deposition on Si is applied to improve the adhesion of Au film to the Si substrate, enabling electrochemical ATR-SEIRAS measurements in alkaline media. Secondly, we examine the feasibility of using Ge as an ATR infrared window. Various metallic films are successfully deposited on the basal plane of the Ge prism. Unfortunately, except for Ni and Ni-P films, the anodic dissolution of the underlying Ge results in the rupture of metal overlayers. In addition, the SEIRA signals on Ni-P electrode are hardly detected since the film thickness grows too far beyond control. Only very weak CO bands are detected on Ni electrode with linear CO adsorption being predominant due to the possible surface contamination by Ge, in contrast to normal CO adsorption on Ni electrode. Last but not the least, we design a new optical path including spectral cell, enabling the convenient switch of ATR-SEIRAS and IRAS measurements to provide spectral informations of surface and dissolved species.Part II. Infrared Spectroscopic Study of Methanol (Ethanol)Electro-oxidation on Pd in Alkaline Solution2.1 Methanol OxidationThe dissociative adsorption and electrooxidation of CH3OH at a Pd electrode in alkaline solution are investigated by using in situ infrared spectroscopy with both internal and external flection modes. The former (ATR-SEIRAS) has a higher sensitivity of detecting surface species, and the latter (IRAS) can easily detect dissolved species trapped in a thin-layer-structured electrolyte. Real-time ATR-SEIRAS measurement indicates that CH3OH dissociates to COad species at a Pd electrode accompanied by a "dip" at open circuit potential, whereas deuterium-replaced CH3OH doesn’t, suggesting that the breaking of the €H bondis the rate-limiting step for the dissociative adsorption of CH3OH. Potential-dependent ATR-SEIRAS and IRAS measurements indicate that CH3OH is electrooxidized to formate and/or(bi)carbonate, the relative concentrations of which depend on the potential applied. Specfically, at potentials negative of ca.-0.15 V (vs Ag/AgCl), formate is the predominant product and (bi)carbonate (or CO2 in the thin-layer structure of IRAS) is more favorable at potentials from15 to 0.10 V. Further oxidation of the COad intermediate species arising from CH3OH dissociation is involved in forming (bi)carbonate at potentials above 0.15 V. Altho ugh the partial transformation from interfacial formate to (bi)carbonate may ficd, justi bridge-bonded formate species can be detected over the potential range under investigation.2.2 Ethanol OxidationIn situ ATR-SEIRAS in conjunction with HD isoto pe replacement is used to investigate the dissociation and oxidation of CH3CH2OH on a Pd electrode in 0.1 M NaOH, with a focus on identifying the chemical nature of the pivotal intermediate in the so-called dual-pathway (Cl and C2) reaction mechanism. Real-time spectroelectrochemical measurements reveal a band at ～1625 cm-1 showing up prior to the multiply bonded COad band. CH3CD2OH and D2O were used to exclude the spectral interference with this band from interfacial acetaldehyde and H2O, respectively, confirming for thdirst time that the 1625 cm-1 band is due to the adsorbed acetyl on the Pd electrode in alkaline media. The spectral results suggest that the as-adsorbed acetyl (CH3COad) is oxidized to acetate from approximately-0.4 V as the potential moves positively to conclude the C2 pathway. Alternatively, in the C1 pathway, the CH3COad is decomposed to α-COad and β-CHX species on the Pd electrode at potentials more negative than approximately-0.1 V; the α-COad species is oxidized to CO2 at potentials more positive than approximately-0.3 V, while the β-CHx species may befirst converted to CO ad at approximately-0.1 V and further oxidized to CO2 at more positive potentials.Part Ⅲ. SEIRAS Study of a CO-covered Pt Electrode in Room Temperature Ionic Liquids (RTILs)ATR-SEIRAS is extended for the first time to study potential-induced surface and interface structure variation of a CO-covered Pt electrode in a room-temperature ionic liquid of N-butyl-N-methyl-piperidinium bis((tfluoromethyl)-sulfonyl)imide (or [Pip14]+[TNf2]-).Owing to a wide effective potential window of [Pip14]+[TNf2]-, a gradual conversion from bridged COad (COB) to terminal COad (COL) is observed in response to positively going potentials, suggesting that [Pip14]+ may be involved in a strong electrostatic interaction with the COad.This site conversion enables the ratio of the apparent absorption coefficient of COL to that of COB to be determined. Also, the spectral results reveal the potential-dependent COad frequency variations as well as the potential-induced interfacial ionic reorientation and movement at the Pt/CO/[Pipi4]+[TNf2]- interface.Part IV. ATR-SEIRAS Study of Self-assembly Process of Chemically Modified Electrode And its Surface Coordination with COReal-time ATR-SEIRAS has been applied to monitor the self-assembly process of FeⅢPP onto an alkanedithiol modfied Au film (HSRS-Au) electrode through the thioether linkage. This chemical grafting assembly is a two-stage process with a rapid increase of surface coverage in the initial 1000 s followed by a gradual saturation for the HSRS-modified Au electrode in a 50μM FeⅢPP-contained phosphate buffer solution (PBS, pH 7.4). The cyclic voltammogram of thus obtained electrode shows clearly a pair of quasi-reversible FeⅢPP/FeⅡPP redox peaks in PBS of pH 7.4 with the formal potential ca.0.36 V (SCE),and the surface coverage of the attached FePP is estimated to be ca.2.55×10-11 mol/cm2from the charge contained in the redox peak. In situ ATR-SEIRAS was also extended to study the potential-dependent coordination of CO to the as-grafted FeⅡPP(sub)monolayer. The mid-point transition potential for de-ligating CO off the (CO)FeⅡPP is centered at ca.0.21 V and 0.01 V (SCE) in PBS solutions of pH 7.4 and pH 3.8, respectively. No spectral feature indicated the formation of the bicarbonyl adduct.