Electrochemical In Situ FTIR Study On The Reaction Mechanism And Dynamics Of Smll Organic Molecules At Pt Electrode

Direct Methanol Fuel Cell (DMFC) has been seen as one of the solutions to the worldwide energy shortage and environmental protection problems. DMFC has plenty of adventages such as rapid starting speed, facile operating temperature, wide fuel source and little pollution. It is very propitious for DMFC to be largely used in socical life and industrial production. There are two problems which restrict the large-scale commercialization of DMFC. Firstly, the reaction intermediate CO from methanol oxidation reaction displays a strong adsorption at Pt-based anodic catalyst, which leads to the decrease of current density and output voltage; Secondly, at present the most worldwide DMFC catalyst is Pt-based materials. As an important expensive metal with a finite storage in planet, the Pt not only restricts the productivity of DMFC but also raises the price. Demonstrating the reaction mechanism and the influence on the dynamics from all kinds of factors in molecular level is the premise of designing DMFC anodic electrocatalyst with high performance and low Pt load.Electrochemical in situ ATR-FRIT spectroscopy with flow cell technique devoloped in our lab, allows the systematic research of initial reaction mechanism and dynamics of electrochemical reaction at clean Pt electrode without the influence of mass tranfer, and then to probe potential, PH and electrode compostion effects on the reaction. Aiming the current DMFC problems and combining the advantage in our lab, this thesis focuses on the research on the reaction mechanism and dynamic process of methanol, carbon monoxide and cyanide adsorption and oxidation at Pt or PtRu electrode.The main content and conclusion of each part is listed as follow:1. Methanol oxidation reaction at Pt electrodeMethanol (MeOH) oxidation reaction (MOR) at Pt electrodes under potentiostatic conditions has been investigated by electrochemical in situ FTIR spectroscopy (FTIRS) in attenuated-total-reflection configuration under controlled flow conditions in0.1M HClO4with2M MeOH, where the mass transport effects are largely eliminated using a flow cell. Our results reveal that (ⅰ) at constant potentials, the methanol dehydrogenation rate decreases while the COad oxidation rate increases with the accumulation of COad until the maximum COad coverage (ca.0.5ML i.e., the steady state) is reached;(ⅱ) at fixed COad coverage, the rates for MeOH decomposition to COad and COad oxidation increases with potential from0.3to0.7V (vs. RHE), with Tafel slopes for MeOH dehydrogenation of ca.440±30mV/dec, which is independent of COad coverage;(iii) the current efficiency of the CO pathway in MOR at0.6and0.7V is below20%and it decreases toward higher potentials. The mechanisms as well as the potential induced change in the kinetics of different pathways involved in MOR are briefly discussed.2. Cyanide and Carbon monoxide adsorption at Pt electrodeThe adsorption of cyanide at Pt film electrode in0.1M HClO4is examined by electrochemical in situ infrared spectroscopy under attenuated total reflection configuration. The time-resolved IR spectral features reveal that depending on the adsorption potential, cyanide adsorbates display three modes of C-N stretching vibration. At0.05V, a trans-(HN=CH) surface species is identified for the appearance of1639and1493cm-1bands. The band with peak frequency below2100cm-1is attributed to N-bound Pt-NC and another band with peak frequency at ca.2150cm-1is attributed to C-bound Pt-CN; the Stark slope of the former is ca.3times larger than that the latter. The identification of the orientation of CNad is further confirmed by the spectral behavior of co-adsorption of CO onto the saturated cyanide adlayer; it causes a blue shift in C-N stretching frequency for Pt-NC without intensity change and an increase in CN band intensity for Pt-CN. The evolution of C-N vibration band demonstrates that the initial adsorption state of cyanide is in the form of Pt-NCHad. And then it is oxidized to Pt-NC, and Pt-NC will slowly reorient to Pt-CN at longer adsorption time. Dipole-dipole coupling effects, inter-space compression and electronic effect are found to be responsible for potential-dependent spectral behavior with and without co-adsorbed CO based on calculations using density functional theory.3. Methanol oxidation reaction at PtRu electrodes The reaction mechanism and dynamic process of methanol oxidation reaction at Pt and PtRu electrodes under potentiostatic conditions has been investigated by electrochemical in situ ATR-FTIR spectroscopy with flow cell in0.1M HC1O4+0.2M MeOH solution. After the deposition of Ru at Pt electrode, it is clearly found that the adsorption and desorption of under potential deposited H (UPD-H) has been restrained. Therefore, the coverage of Ru at Pt could be calculated by the total charge of the adsorption and desorption of UPD-H before and after introduction of Ru. After the deposition of Ru at Pt, the catalytic activity of Pt is clearly increased. The relationship between current density of methanol oxidation at PtxRuy electrodes and Ru coverages displays a volcano curve. The Pt61Ru39has the best catalytic activity towards MOR. In high Ru coverages, which the coverage exceeds39%, the CO can adsorb at Ru atom which leads to decrease of catalytic activity of PtxRuy electrodes. In low Ru coverages, the interaction between Ru and Pt promotes the methanol oxidation reaction and increase the density of CO at Pt. The methanol oxidation reaction at Pt61Ru39electrode under series of different constant potentials (0.3V,0.4V,0.5V and0.6V) indicates that the contribution of methanol dehydrogenation to CO and CO oxidation in totle reaction current displays clear potential effect. Comparing methanol oxidation reation at Pt and Pt61Ru39electrodes, it can be found that the enhancement of Ru to Pt catalytic activity is clearly stronger in lower potentials than that in higher potentials.