Differential Electrochemical Mass Spectrometry Study Of Methanol Electrooxidation

Differential electrochemical mass spectrometry(DEMS)has been widely used for qualitative and quantitative detection of volatile products or intermediates in Faraday reactions due to its characteristics of on-line monitoring,low detection limit,fast response time and linear proportional relationship between concentration and ion current.On-line coupling of an electrochemical workstation with mass spectrometry is often used to identify intermediates or products of electrochemical reactions.It is the key to apply DEMS to qualitative and quantitative analysis and mechanism study.The electrooxidation of methanol was studied on the basis of electrochemical workstation and differential electrochemical mass spectrometry.The dissertation is mainly divided into the following parts:(1)DEMS is briefly introduced,including its development history,working principle and device composition.Then the study of alcohols oxidation by electrochemical differential mass spectrometry is summarized.(2)In order to detect CO₂ generated in acidic and alkaline electrolytes,Probe-type in-let and film-type in-let DEMS systems were established to detect and analyze electrocatalytic products of methanol on Pt-based electrodes.For DEMS in acidic systems,sampling of volatile species can be done by moving the probe close to the electrode surface.The closer the probe is,the stronger the signal is.The influence of flow rate,concentration and scanning speed on DEMS signal acquisition was systematically investigated.For DEMS in alkaline systems,products on catalyst surfaces can be analyzed by DEMS directly through hydrophobic membranes due to the pressure of the vacuum chamber.It was found that in strong alkaline solution,the product CO₂ could not be collected because it was quickly neutralized.But in the carbonate electrolytes,CO₂ can be detected by DEMS.(3)The electrocatalytic oxidation of methanol in acidic medium was systematically studied on DEMS.The results showed that methanol consumption was observed on the surface of the Pt-based catalysts at the same time as the positive and negative scavenging oxidation peaks in the cyclic voltammetry curves.Therefore,we believed that the two peaks were mainly from the fresh methanol oxidation,rather than the oxidation of the intermediate species.Results obtained from different experimental designs consistently show that the oxidation peak produced by reverse scanning is related to the reducibility of Pt O formed at high potential.Through comprehensive analysis,the S_f/S_b ratio was proposed as an index to evaluate the high potential oxidation resistance and reverse scan reduction ability of the electrode surface,rather than the standard to evaluate the resistance of the catalyst to CO poisoning.(4)The alkali metal cation effect on the methanol electrooxidation was investigated in alkaline electrolyte and carbonate electrolytes.This is due to the formation of hydroxy-hydrated metal cationic clusters affecting the activity of hydroxyl groups at Pt active sites.In alkali metal carbonate electrolytes,we studied the influence of metal cations on electrooxidation performance of methanol by using film type DEMS,and the results showed that the smaller the ion radius is,the higher the yield of CO₂ is.It seems that anions also have a certain influence on MOR.Finally,we think that the alkali metal cations are formed on the electrode surface by(^H2O₎x-1M⁺…^H2O…^OHad cluster structure of OHad modifies the oxidation activity of OHad adsorbed on Pt,thus changing the activity of MOR.