A Electrocatalytic Study Using Combined Electrochemical And MS/FTIR Methods

As a clean and efficient energy convertor, fuel cells can solve the current environment and energy crisis. The hydrogen-fueled proton exchange membrane fuel cell is relatively matured, yet there are some technical problems in the storage and transportation of hydrogen. The hydrogen gas can be replaced by several alternative fuels, such as ammonia, methanol and ethanol, which, however, suffer from slow reaction kinetics. Therefore, research efforts have been focused on the catalysts and the reaction mechanism of the oxidation of these fuels. In this thesis, two types of spectroelectrochemical methods were constructed for electrocatalysis study, namely, the differential electrochemical mass spectrometry and the on-line electrochemical infrared spectroscopy. These two methods were applied to the study of the electrooxidation reactions of ammonia, methanol and ethanol. Major results of this work are summerized as follows:1. Construction of differential electrochemical mass spectrometry (DEMS)Two types of DEMS were constructed:DEMS system for membrane electrodes and DEMS system for disk electrodes. In the design of DEMS system for membrane electrodes, a PTFE-containing carbon film functions as the working electrode, which is also the membrane inlet connecting the vacuum sampling pipe and the electrochemical cell. Driven by the pressure between the two sides of the carbon film, volatile species, once formed during electrochemical reactions, can be sampled by MS. In the set-up of DEMS system for disk electrodes, volatile products formed on the surface of disk electrodes were sampled by a hydrophobic capillary probe. Besides, the disk electrode is capped by a polyethylene mini-hood for the detection of gaseous products produced by reactions in nonaqueous media.2. DEMS study of ammonia oxidation reaction (AOR)A comparative DEMS study of AOR in both aqueous and nonaqueous media revealed that the inhibition of ammonia oxidation reaction catalyzed by Pt or Pd could be due to the surface blocking of oxygenated species. It was found that, during AOR in nonaqueous media where no surface oxygenated species exist, the catalytic activity of Pt and Pd increases with the increasing potential, which proves the inhibition role of surface oxygenated species in AOR in aqueous media. In addition, Pd shows an even higher activity toward AOR than Pt in nonaqueous media, and the oxidation of ammonia can proceed on Au which is inactive in aqueous media. 3. The combination of electrochemistry and infrared spectroscopyAs the complementation of DEMS, the combination of electrochemistry and infrared spectroscopy was set up for electrochemical mechanism studies. First of all, a conventional electrochemical FTIR reflection spectroscopy was constructed. However, several issues were caused by the thin layer electrolyte between the disk electrode and prism, such as poor ionic conduction, exhaustion of reactants and inapplicability of quantitative analysis. Consequently, a new system combining electrochemistry and FTIR transmission spectroscopy was set up. By the design of a sampling system attached with an ion-exchange membrane for the working electrode, enriched products can be sampled and then detected in the transmission IR cell. The new system possesses several advantages, such as normal electrochemical behaviors, supplement of reactants, quantitative analysis of concentration, and controllable reacting temperature.4. The study of methanol oxidation reaction (MOR) on Pt and Pd in alkaline mediaIn alkaline media, the MOR activity of Pt and Pd is remarkably higher than that in acidic media, and Pd shows a comparable activity with Pt. MOR on both Pt and Pd is a structure-sensitive reaction which is favored on low-coordinated Pt(110) and Pd(110) sites. Besides, the MOR activity of Pt and Pd can be further enhanced by the decoration of Ru, and there is a maximum of the enhancement of MOR activity in a wide range of Ru coverage. Lastly, the products of MOR on Pt in alkaline media were detected by on-line electrochemical flowing FTIR transmission spectroscopy, showing that the efficiency of carbonate formation increases with temperature.5. The study of ethanol oxidation reaction (EOR) on Pt and Pd in alkaline mediaThe EOR activity of Pt and Pd in alkaline media is remarkably higher than that in acidic media. The effect of surface morphology on the catalytic activities of Pt and Pd indicates that low-coordinated Pt(110) and Pd(110) sites favor the formation of surface oxygenated species and the split of C-C bond. The electrodeposited Pd with a large number of (110) sites shows an even higher activity toward EOR than Pt. After the investigation of products of EOR in alkaline media by on-line electrochemical flowing FTIR transmission spectroscopy, it was found that the efficiency of carbonate formation is higher at elevated temperatures and thick catalyst layers. Additionally, the efficiency of carbonate formation is much higher in acetaldehyde oxidation reaction than that in EOR.