Research Of Dimethyl Ether Electrooxidation Mechanism On Platinum Single Crystals And Highly Active Catalysts

Dimethyl ether (DME) is a promising fuel for the direct oxidation fuel cell. It has several advantages such as low loxicity, low cross over effect and high electron transfer number. Furthermore, DME can be obtained from various sources on large scale. Therefore, DME has been regarded as a new substitute for methanol. However, the electro-oxidation activity of DME on Pt-based catalysts is too low for practical applications. The mechanism for DME electro-oxidation is still unclear at present, which retards the progress of anode catalysts for the direct dimethyl ether fuel cell (DDFC). This thesis is aimed to investigate the electro-oxidation mechanism of DME using Pt single crystal electrodes with certain surface structures with the help of in-situ electrochemical infrared spectroscopy, and further direct the design and preparation of anode catalysts for the DDFC.Cyclic voltammograms and chronoamperometry are used to study the dependence of DME electrochemical behaviors on the Pt surface structue. It is found that DME electro-oxidation on Pt electrodes is a structure sensitive reaction. Pt(111) and Pt(110) electrodes have low catalytic activity for DME while Pt(100) has much higher activity. High index planes of Pt with different width of (100) terraces are also prepared for the characterization of DME electro-oxidation. It is found that the reactivity of DME declines with the decrease of (100) terrace width, suggesting that DME electro-oxidation prefers long range ordered (100) domains. The intermediates of DME decomposition on Pt(111) and Pt(100) electrodes are studied by in-situ electrochemical infrared spectroscopy. It is found that DME is dissociatively adsorbed on these surfaces through dehydrogenation reaction. The intermediates are converted to more stable linearly or bridge adsorbed CO (COL or COB) and are finally oxidized to CO2 in higher potential region.The influencing factors of DME electro-oxidation on Pt(111) and Pt(100) electrodes are investigated by changing the electrolyte, the electrode potential sequence and bulk concentrations of DME. The dissociative adsorption of DME are suppressed by hrdrogen and anion adsorption. By potential step and scanning experiments, it is found that in 0.5 mol·L-1 H2SO4 solution, DME is dissociatively adsorbed on Pt(111) electrode surface in the low potential region between 0.2 ~ 0.5 V and the initial decomposition rate reaches its maximum at 0.35 V. One of the stable adsorbed intermediates for DME decomposition on Pt(111) is carbon monoxide (CO) and its coverage is approximately constant (ca. 0.37) between 0.3 ~ 0.5 V. Furthermore, no direct oxidation of DME is observed on Pt(111) electrode. On the other hand, the effect of anion on DME electro-oxidation on Pt(100) electrode is not so heavy because anion adsorption on Pt(100) electrode is weaker. DME electro-oxidation on Pt(100) electrode follows"dual pathway mechanism". By comparative study with methanol and the effect of cyanide modification on Pt(111), it is proved that DME dissociative adsorption in low potential region needs at least three contiguous Pt sites. In high potential region, DME is oxidized through C-O bond cleavage, which selectively occurs on Pt(100) surface.Ruthenium were deposited on Pt(hkl) single crystal surfaces using repeated spontaneous deposition procedures. Pt(hkl)/Ru surfaces with different Ru coverages were first applied for the electrocatalytic oxidation of dimethyl ether (DME) to clarify the structure dependence of Ru modification. Cyclic voltammograms (CVs) show that the catalytic activity after Ru modification is promoted on Pt(100) while inhibited on Pt(110) and Pt(111) surfaces. 30 mass % Pt/C and PtRu/C, PtIr/C catalysts are synthesized by the impregnation method. The activities of these catalysts for DME electro-oxidation are characterized by cyclic voltammograms. The results show that DME reactivities decrease with the Ru or Ir content increase. This should be due to the dissociative adsorption of DME is inhibited by the decrease of three or more contiguous Pt sites after addition of Ru or Ir.Shape controlled Pt nanocrystals are synthesized by colloidal method. The catalytic activities of these Pt nanocrystals toward DME electro-oxidation and their stability are investigated. Cubic Pt nanocrystals are obtained when K2PtCl6 and K2PtCl4 aged for three days are used as precursors, sodium polyacrylate (NaPA) as capping material and the proportion of Pt and NaPA were adjusted to 1 : 5 and 1 : 1 respectively. It is found that the cubic Pt nanocrystals has (100) ordered surface domains. The catalytic activities of Pt nanocrystals with different shapes for DME electro-oxidation are investigated by cyclic voltammograms. The results show that cubic Pt nanocrystals have higher activity than other shapes, especially those obtained by K2PtCl4. The activity of cubic Pt nanocrystals for DME electro-oxidation is about 2.9 times higher than commercial Pt black and the CV has similar features with Pt(100) electrode. It is found that cubic Pt nanocrystals can maintain its surface structure after aging in air for three months. Electrochemical treatment on cubic Pt nanocrystals is performed by potential steps to different values. The results show that they are stable in the potential region negative than 1.1 V. When the potential is increased to 1.2 V, the nanocrystals can preserve its structure in short time scale. Potential steps to 1.2 V for long time will destroy the ordered (100) domains and convert them to step sites. DME reactivity on the disturbed cubic Pt nanocrystals is found to decrease obviously.