Methanol Oxidation At Pt Based Electrocatalysts,An Electrochemical In Situ FTIR Spectroscopic And Differential Mass Spectroscopic Study

In order to get a better understanding on the mechanism and kinetics of complex electrocatalytic reactions, information from various perspectives such as reaction products, adsorbed species and the electrochemical characteristics which are simultaneously acquired in a single experiment is necessary. This goad may be realised by developing and applying of hyphenated research techniques, which allow simultaneous sensing of the electrochemical parameters coupled in parallel to an additional physico-chemical probing method. On-line Differential Electrochemical Mass Spectrometry is very powerful in monitoring gaseous (volatile) reaction products and educts, and in-situ Infrared Reflection-Absorption Spectroscopy are good techniques to detect the adsorbed species (reaction poisons and/or intermediates) at the electrode surface. In Ph. D work, I have built a combined in-situ attenuated total reflection-infrared spectroscopy (ATR-FTIRS) and on-line differential electrochemical mass spectrometry (DEMS) via a dual thin-layer electrochemical flow-cell, and carried out systematic studies on the oxidation of C1molecules at Pt based electrocatalysts using such hyphenated techniques. The major achievements are as following:i) The combined (ATR-FTIRS) with DEMS allows simultaneous detection of stable adsorbed species and adsorbed reactive intermediates by ATR-FTIR spectroscopy, quantitative determination of volatile reaction products and by-products by DEMS, and determination of the overall reaction kinetics by measuring the Faradaic current under controlled mass transport of reactants and reaction products to/from the electrode.ii) The relationship of the amount of CO2in the bulk solution with the detected mass signal of m/e=44as a function of methanol concentrationhas been examined systematically. We found that in solution with methanol concentration above0.5M, the a significant decrease in the mass signal of CO2is detected, which is due to the decrease in the Henry constant of CO2since CO2molecules are trapped in the hydrophobic region of methanol clusters in aqueous electrolyte.iii) The interaction of CO, HCOOH, HCHO and CH3OH molecules with a Pt surface initially covered with a layer of oxide under open circuit potential (OCP) is studied by combined infrared and mass spectroscopy. We found that after switching to the fuel containing solution and concomitantly switching off the electrode potential control at1.2V1) the OCP decays from1.2V down to values of0.58V (CO),0.12V (HCOOH),0.08V(HCHO) and0.24V(CH3OH);2) CO is the only adsorbate formed at the Pt surface from the fuels;3) the rates for the decay of OCP and for the buildup of COad adlayer decrease in the order of CO> HCHO> HCOOH> CH3OH;4) the rate of CO2production and the total amount of CO2produced decreases in the order of CO> HCOOH> HCHO> CH3OH;5) a significant amount of HCOOH is formed for the case with HCHO and the main by product from CH3OH is HCHO. Our results indicate that the change in OCP is determined by the change of net charge at the electrode/electrolyte interface due to the production of electrons from fuel oxidation and the consumption of electrons by Oad/OHad reduction; Even without an externally potential control the reactions occurring at the interface are controlled by the electrochemical potential of the respective reactants and products./) Methanol oxidation reaction (MOR) at Pt and Pt electrode surface deposited with various amount Ru (denoted as PtxRuy, nominal coverage y is0.17,0.27and0.44ML) in0.1M HCIO4+0.5M MeOH has been studied under potentiostatic conditions by in-situ FTIR spectroscopy in attenuated-total-reflection configuration and differential electrochemical mass spectrometry under controlled flow conditions. Our results reveal that i) CO is the only methanol-related adsorbate observed by IR spectroscopy at all the Pt and PtRu electrodes examined at potentials from0.3V to0.6V (vs. RHE); ii) at Pto.56Ruo.44, two IR bands, one from CO adsorbed at Ru islands and the other from COL at Pt substrate are detected, while at other electrodes, only a single band for COL adsorbed at Pt is observed, iii) MOR activity decreases in the order of Pt0.73Ru0.27>Pt0.56Ru0.44>Pt0.83Ru0.17>Pt; iv) at0.5V, MOR at Pt0.73Ru0.27reaches a current efficiency of50%for CO2production, the turn-over frequency from CH3OH to CO2is ca.0.1molecule site-1s-1; Suggestions for further improving of PtRu catalysts for MOR are provided based on the present results and the information from literature.