Quantitative Analysis Methodology Of Differential Electrochemical Mass Spectrometry And Its Application In CO Reduction

Differential electrochemical mass spectrometry(DEMS),as the only technology that can realize real-time monitoring of product information(such as yield,yield,etc.)with the change of reaction conditions,has been widely used in many electrochemical energy conversion reactions,such as oxygen reduction,small organic molecule oxidation and carbon dioxide reduction.For this kind of complex reaction system with many products,it is necessary to determine the accuracy of qualitative and quantitative analysis of mass spectrum signals of products by DEMS technology.Previously,our team has systematically explored the effects of solution composition and solution flow rate on the quantitative analysis of reaction products by using DEMS.On this basis,this paper will systematically explore how to use the DEMS technology for the qualitative and quantitative analysis of complex reactions with a variety of reaction products and parallel competitive reactions,such as the electrocatalytic reduction of CO2.It is mainly divided into the following four parts:1.The effect of water on the quantitative analysis of DEMS:For the mass spectrometry system with membrane injection,hydrophobic porous membrane is generally used to separate the vacuum chamber from the electrolyte solution system,which can not only effectively prevent the water solution from entering the vacuum chamber,but also allow the volatile species generated on the electrode interface to enter the vacuum chamber through the porous membrane and be collected by the detector..Although the porous membrane can block the aqueous solution,in the actual process,the main species entering into the vacuum chamber are usually a large number of water molecules.Compared with water,the partial pressure of the product molecules that we need to detect in vacuum is usually 3-6 orders of magnitude lower than that of water molecules.These water molecules not only compete with the product molecules to bombard their electrons,but also undergo thermal catalytic reactions under high temperature,especially the reaction between water molecules and filaments,which can produce H2 and O2.These factors will interfere with the qualitative and quantitative analysis of the product molecules.Therefore,we systematically changed the content of water molecules in the vacuum chamber,the time of filament lighting and other factors to study the variation of the background signal intensity and the measured products under different conditions.On this basis,we propose that the filament needs to be preheated for a long time before the experiment can reduce the error of the quantitative analysis of the products.2.The methodology of qualitative and quantitative analysis of multi-products system by using DEMS:In this chapter,the reduction of CO on CuOx electrode is used as the model reaction system,and the methodology of qualitative identification and quantitative analysis of multi-products in complex reaction system by using DEMS is explored.By measuring the mass spectrum signals of ion fragments of a variety of possible target products,it shows how to obtain the fragment ion distribution of all molecules(including reactants and product molecules)in a complex system.In view of the situation that some product molecules and reaction molecules have the same fragment ion mass spectrum,it is important to combine with other detection methods such as NMR to conduct a comprehensive analysis.On the basis of accurate identification of various possible products,the relationship between mass spectrum signal intensity and concentration of related products was established by external standard method,on this basis,the quantitative analysis of products was carried out,and the error of this quantitative method and subsequent improvement suggestions were discussed.3.The interference of chemical reaction in solution on the exploration of electrocatalytic reaction in DEMS:Co-existed electrochemical reactions or chemical reactions in solution may introduce mass signal of the same m/e to that of the detected species,which leads to misjudgment of mass spectrum signal source and error of quantitative estimation of product current efficiency.In this chapter,we use R4NCl(R=CH3,C2H5,n-C3H7)as the electrolyte solution system as the model,and the DEMS system is used to study the variation of mass signal with reaction conditions in the process of hydrogen evolution reaction.It is found that some R4N+reagents may slowly form R3NH+Cl-molecules during storage,which can combine with OH-generated at the solution interface during hydrogen evolution to form R3N molecules.These molecules will volatilize into the vacuum chamber and produce a series of fragment ions of CxHy,whose mass spectrum signals will seriously interfere with the target reaction of CO2 reduction.Through systematic study,we also found that the formation of R3NH+Cl-is related to the composition of alkyl chain and anion.The conversion process of R4N+to R3NH+is similar to the Hoffman elimination under alkaline conditions.Therefore,when R is CH3 and anion is ClO4-,this molecule will not be produced.These information provide specific guidance for exploring how to choose the appropriate organic cation electrolyte to explore how it can affect the CO2 reduction behavior without the interference of other chemical reactions.4.The cation effects on CO reduction reaction on CuOx electrode:The composition of cation in solution can seriously affect the activity and product distribution of CO/CO2 reduction.In this chapter,the effect of cations on the reduction behavior of CO on CuOx nanoparticles electrocatalyst was investigated by using the system of DEMS and FTIRS.Firstly,the effects of alkali metal cations(Na+,Cs+)on CO reduction performance were systematically compared.It was found that Cs+ could improve the yield and selectivity of CO conversion.to C2+.Then,the effects of R4N+(R=CH3,C2H5,n-C3H7,n-C4H9)organic cations on CO reduction performance were systematically investigated.The results show that the yield of C2+products decreases with the increase of the radius of organic cation;and the yield of C1 products first increases and then decreases with the increase of the radius,which reaches the maximum in(C2H5)4N+ solution.We speculate that this may come from two factors.On the one hand,the electric field intensity will decrease with the increase of cation radius,which is unfavorable to the production of C2+intermediates.On the other hand,the hydrophobicity will also increase,which will block the interaction between water molecules and CO to a certain extent,thus affecting the hydrogenation process of CO.This information provides specific guidance for how to effectively control the product distribution of CO/CO2 reduction by selecting appropriate cationic solution system.