PH Effects On Sulfate Anion Adsorption And Formic Acid Oxidation At Pd(1 1 1)Electrode

Formic acid oxidation(FAO)is one of the simplest reactions in electrocatalytic reactions which involves the transfer of only two protons and two electrons.It has been an important model reaction for studying the anodization of small organic molecules,and also an important model reaction helpful in understanding such reactions involving proton-electrons transfer.For this type of reaction,the pH of the solution has varying degrees of influence on the thermodynamics and kinetics of the reaction,since the change of pH will not only change the concentration of the reactants or products,but also affect the state of charge of the electrode,the solution composition in the electric double layer and the adsorption behavior of surface species on the electrode.Understanding the effect of pH on the activity of FAO will deepen our understanding of the basic principles of electrocatalysis.The Pd electrode stands out from single precious metals for its good activity in FAO and good resistance to CO poisoning.The kinetics of FAO on the Pd electrode has a significant structural effect,so the use of a structure-determined Pd single crystal electrode can help us better understand how the pH of the solution affects FAO kinetics.Therefore,this thesis uses Pd(1 1 1)as the working electrode to systematically study the anion adsorption behavior and the pH effect of formic acid oxidation in sulfuric acid-sulfate system on Pd(1 1 1)electrode.The main results are as follows:1.Effect of solution composition and pH on the adsorption behavior of(bi)sulfate on Pd(1 1 1)electrode:Cyclic voltammetry method and thermodynamic analysis are used to study the electrochemical behaviors of Pd(1 1 1)electrode in x M H2SO4 + y M Na2SO4 + z M NaOH solutions.According to the change of thermodynamic equilibrium potential of the relevant adsorption reactions with pH and solution composition,it is speculated that the S-containing adsorbate on the surface of the Pd(1 1 1)electrode is SO4-*when compared with the peak potential,onset potential,and phase transition potential of(bi)sulfate desorption and hydrogen(Had)adsorption in the experimental results.The corresponding adsorption reaction for pH<2 is HSO4-(?)2SO4-*+H++e,while for pH>2 is SO42-(?)SO4-*+e.When the solution pH?3,SO4-*adsorb strongly on the surface in the H-UPD region,and when the pH increases,the onset potential for SO4-*adsorption shifts positively.For pH?10,the potential region for SO4-**adsorption overlay with that for oxygen-containing species such as OHad.The existence of Na+ in the solution would further stabilize the sulfate adsorption adlayer,which caused the irreversibility of the adsoprion/desorption peak.2.pH effects of FAO on Pd(1 1 1)electrode:FAO in 0.1 M H2SO4+0.1 M HCOOH+x M Na2SO4 solutions on Pd(1 1 1)single crystal electrode was studied by cyclic voltammetry and potential step method,which is compared with the results in 0.1 M HClO4+0.1 M HCOOH solution.It was found that the sulfate adsorbed on the electrode surface significantly inhibited the FAO,The addition of Na2SOa promotes the formation of a dense(bi)sulfate layer,which greatly reduces the active site of the FAO.In addition,the effects of pH on the FAO were investigated by cyclic voltammetry in 0.1 M HCOOH+0.1 M H2SO4+x M NaOH solutions.It was found that the FAO current decreased with the increase of pH in the relative low potential range,while showed a volcano curve relationship with pH in the higher potential range.It is speculated that the main reactant at low pH and in the lower potential range of negative surface charge is HCOOH,and the main reactant at higher pH and higher potential range is HCOO-.The decreased FAO current with the increase of pH in the relative low potential range and at low pH is mainly due to the decrease of reactant(HCOOH)concentration,while the volcano-type curve relationship of FAO current with pH in the higher potential range and at higher pH is mainly due to the opposite change of the reactant(HCOO-)concentration and the overpotential of rate-limiting step with pH.In addition,the peak potential of FAO increases with the increase of pH,which is mainly due to the positive shift of the initial adsorption potential of sulfate,and also the potential at which the saturated coverage is reached.