| Electrocatalytic reactions occur at the electrode surface always accompany with specific adsorption.These species include water,cations,anions,and reaction intermediate.These adsorbates would affect electrocatalytic reactions in different ways,such as blocking electrode surface sites,changing the electrode surface electron structure,and the metal-solution interface double layer structure.The pH effect of formic acid oxidation displays a maximum activity of FAO when the solution pH=pKa,but this pH effect is still controversial.Precise understanding of the pH effect can help us understand the mechanism of FAO.Traditionally,scientists just consider the electrostatic force and the covalent effect occur at the electrical double layer.However,a recent report by Strmcnik et al has provoked a public attention on the non-covalent effect which also have impact on the electrocatalytic reactions activity.In order to have a better understanding on the metal-solution interface double layer structure and the influence of solution pH,cation,etc on the electrocatalytic reactions activity,we conducted following research:pH effect of acetate adsorption at Pt(111)electrode:pH effect on acetate adsorption on the Pt(111)electrode in solutions with pH from 1 to 13 has been studied by cyclic voltammetry and electrochemical impedance spectroscopy.At low pH below 3,the onset and peak potential for acetate adsorption do not show obvious shift on the RHE scale,while,from pH=4.7 to 10.2 it shifts with a ratio of ca.56 mV/pH.While the solution pH=12.8,the adsorption of acetate is totally prohibited by OHad.This phenomenon is caused by the shift of the equilibrium potential of acetate adsorption with the change of solution pH.Besides,based on the potential step results,we calculated that the adsorption rate constant for acetate adsorption is 0.28?0.l0and 0.08 cm s-1 in solutions of pH=1.1,4.7 and 7.6,respectively.Which means that acetate adsorption kinetics gets slower with increasing solution pH.This is also verified by the EIS result.The charge transfer resistance for acetate adsorption is found to be 2.0?6.3 and 7.8 ? cm-2 in solutions of pH=1.1,4.7 and 7.6,respectively.The slower kinetics of acetate adsorption is due to the energy barrier changes with solution pH.Cation effect of acetate adsorption and formic acid oxidation at Pt(111)electrode:we used cyclic voltammetry,chronoamperometry,and electrochemical impedance spectroscopy studied cation effect of acetate adsorption and formic acid oxidation at Pt(111)electrode.We found that different cation would not influence the OHad adsorption region of the base CV of Pt(111)electrode in acid solution which differs from the results in alkaline solution.Besides,different cation would not influence the CV of acetate adsorption,as well as its' mechanism.While the adsorption rate of acetate on Pt(111)electrode follows the order:Li+<Na+<K+.The cyclic voltammetry of formic acid oxidation in solution of pH=2.8 suggests that different cation would not influence the activity of formic acid oxidation but the peak potential in acid solution,which is also different with Varela's result in alkaline solution.We propose that this is due to the precursor of formic acid oxidation is HCOO-,which would go through the M+(H2O)x network first before contact with the electrode surface under the effect of electrostatic force.Another possible explanation is that the ability of different cations to form M+(H2O)x network is different.Thus the network would occupy different space near the electrode.Which would affect the free path for precursor to contact with the electrode.Besides,we designed experiments by changing electrode scan rate and rotating rate to investigate the influence of interfacial pH on FAO.We find that when solution pH=9.2,due to the massive H+ produced by FAO the interfacial pH of solution without buffer would change to pH=4.Thus,the interficial pH change is the cause of two kinds of pH effect of FAO in sulphuric acid and solution with buffer... |
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