Mechanism And Kinetics Of Dissociative Adsorption And Oxidation Of Dimethyl Ether On Platinum Electrode Surface

As the simplest ether, dimethyl ether (DME) lacks of the C-C bond and has a high energy density with negligible crossover effect. It is less toxic than methanol and friendly to the environment. Thus, DME is being considered as one of the promising candidates of alcohols for direct oxidation fuel cells. Since DME has an additional methyl group compared to methanol, its electrochemical behavior appears quite different and much more complicated.The surface processes and kinetics of DME dissociative adsorption and oxidation on Pt electrode were studied using cyclic voltammetry (CV), programmed potential scan and step techniques, in situ (time-resolved) FTIR spectroscopy. The main results are summarized below.1. Mechanism of the dissociative adsorption and oxidation of DME on Pt electrode. It was shown that the adsorption and desorption of hydrogen were significantly suppressed by the dissociative adsorption of DME. As potential increases, two oxidation peaks appear in the voltammogram, corresponding to the oxidation of dissociative adsorbates (DA) and direct oxidation of DME, respectively. Quantitative analysis results revealed that the average number of electrons released from DA oxidation at each Pt site is 1.265, indicating CO_L and CO_B are the main DA. It has found also that these results may be affected by other surface processes on the electrode.It has been demonstrated that the electrochemical reactivity of DME depends strongly on the concentration of H⁺ ions. No perceptible reactivity of DME in 0.1 mol·L^-1 NaOH solution could be detected. This phenomenon contrasts sharply against that of the case of methanol. Combined with the FTIR spectroscopic results, it was confirmed that H⁺ ions take part in the elctrooxidation of DME, i.e., the protonation of the oxygen atom in DME molecule is the key for the dissociative adsorption and oxidation of DME. FTIR spectroscopy has detected in situly the CO_L, CO_B, HCOOH, CO₂ and other species involved in DME oxidation. A mechanism for the electrooxidation of DME on Pt electrode was proposed based on the molecule-level information: CO as the 'poisoning' intermediate derived from the dissociative adsorption of DME at low potentials (<0.55 V vs. RHE), which is oxidized at higher potentials; The oxidation of DME takes place via the reactive intermediate path (HCOOH) to yield CO₂.2. Kinetics of the dissociative adsorption and oxidation of DME on Pt electrode.Kinetics parameters, including average reaction rate (?), initial reaction rate k_i andtime constantÏ„for the dissociative adsorption of DME at a series of adsorption potential (E_ad) were obtained by using programmed potential scan and programmedpotential step techniques. It was demonstrated that (?) varied with E_ad, yielding avolcano-type distribution. It would take 10 and 90 s for DA to reach half-saturated and saturated adsorption, respectively, indicating that the dissociative adsorption of DME is a quite slow dynamic process.It has revealed that the electrodeposited nanostructured thin film of Pt electrode (nm-Pt/GC) exibits abnormal infrared effects (AIREs). Especially, the enhancement of IR adsorption facilitates the study on the dissociative adsorption and oxidation of DME. MSFTIR spectra illustrated that DME oxidizes to CO₂ via reactive intermediates and 'poisoning' intermediates simultaneously at high potential region (0.62s<0.90 V), and merely dissociatively adsorb at Pt sites in the potential region below 0.62 V during the negative potential scan. Kinetics parameters for DME dissociative adsorption and oxidation were obtained from in situ LSTR-FTIR spectra. It was found that the integrated band intensity of COl (I_COL) increases exponentially(first order) against time (t) at low potentials, which has been affected by both of the growth and oxidation rates of CO_L. Another adsorbed species HCOO_ad was detected at low potentials, and its IR band intensity (I_HCOOad) increases first and then decreaseswith increasing t, indicating that it is an unstable intermediate in DME dissociative adsorption process. The CO₂ species, which is produced mainly from the oxidation of reactive intermediates, increases linearly with increasing t. But its formation rate slows down above 0.80 V, due to the formation of inert oxide species on Pt surface. 3. Surface structure and temperature effects in electrooxidation of DME. When the upper limit potential was 1.05 V, the activity order is ranked as Pt(100)>poly-Pt >>Pt(110)≈Pt(111). It was also found that the electrochemical reactivity of DME depends on temperature, i.e. a higher temperature gives a better reactivity. But the variation of temperature makes hardly any impact on the dissociative adsorption of DME, implying that the temperature effect is caused largely by the 'direct' oxidation of DME. From the analysis of experimental data, it has evaluated that the activation energy for DME oxidation on Pt electrode is about 62.8 kJ·mol^-1.The results obtained in this thesis have contributed to a deeper understanding of the surface processes and kinetics in the dissociative adsorption and oxidation of DME. The current studies are of significant importance in developing fundamental aspects of electrocatalysis, as well as in the research and development of DDMEFC.