Academic Master Dissertation Mechanistic Investigation Of Highly Selective Synthesis Of Acetaldehyde From Photoelectrochemical Oxidation Of Ethanol On Metal Oxides

At present,the use of semiconductor materials for photocatalytic organic synthesis with high selectivity has become a new type of green synthesis method.Among them,metal oxides are considered to be the most promising photoelectrode materials because of their low manufacturing cost and suitable energy structure.However,there are still many by-products in practical photocatalysis of organic molecules.One of which the factors control the selectivity is of great importance towards the application of this technique.Therefore,research in the mechanism of using metal oxide for photocatalytic organic synthesis has attracted great attention in both academia and industry.In this thesis,the mechanistic investigation of selective oxidation of ethanol to acetaldehyde is reported using hematite and titanium oxide photoanodes.The spin-coating deposition method was used to synthesize uniform and denseα-Fe₂O₃ and TiO₂ photocatalysts on fluorine-doped tin oxide single-sided conductive glass（FTO）,and then electrochemical deposition was used to synthesize uniform Ni/FTO electrode material.The morphology and structure of the synthesized materials were characterized by scanning electron microscope（SEM）,X-ray diffractometer（XRD）,ultraviolet-visible spectrophotometer（UV-vis）,etc.And through Mott-Schottky testα-Fe₂O₃ flat band potential.It is confirmed that three different electrodes ofα-Fe₂O₃/FTO,TiO₂/FTO and Ni/FTO have been successfully synthesized.In order to select the most suitable electrode to catalyze the formation of aldehydes from alcohol,the（photo）electrocatalytic performance of the three electrodes prepared above was preliminarily explored,and the kinetics of subsequent ethanol oxidation and possible over-oxidation ofα-Fe₂O₃/FTO were determined.Using gas chromatography-mass spectrometry（GC-MS）and UV-vis,it was determined from both qualitative and quantitative perspectives that the reaction product ofα-Fe₂O₃ photocatalyst ethanol is only acetaldehyde,and the Faraday efficiency is between 80%and 99%.Photo-induced absorption（PIA）spectroscopy and transient photocurrent（TPC）technology were used to study the photo-generated hole changes inα-Fe₂O₃ and the corresponding photocurrent during the photoelectrocatalytic oxidation of ethanol and acetaldehyde under quasi-steady-state conditions.Use rate law analysis to determine reaction kinetics and reaction pathways.Since acetaldehyde cannot compete with ethanol oxidation in terms of kinetics,there is no over-oxidation of acetaldehyde.Finally,PIA and photoelectrocatalytic（PEC）measurements were used to determine the activation energy required for the oxidation of ethanol and acetaldehyde.The analysis of Arrhenius and the law of velocity further shows that the activation energy（198 me V）of photo-generated holes inα-Fe₂O₃ to oxidize ethanol is significantly lower than that of acetaldehyde（398 me V）,andα-Fe₂O₃ cannot offset the energy caused by the activation energy of acetaldehyde.Therefore,the barrier cannot provide sufficient thermodynamic driving force.Therefore,α-Fe₂O₃ can convert alcohols into aldehydes with high selectivity due to its valence band advantage,avoiding the problem of excessive oxidation of aldehydes.The results of this paper show that the oxidation ability is related to the valence band potential of metal oxides,and the activation energy has a guiding significance in regulating the selectivity of organic reactions.From the perspective of thermodynamics,two materials with different practical requirements can be designed for selective production and organic degradation.The conclusions of this article provide a theoretical basis for the further design and modification of such compounds to achieve the goal of low-cost and low-energy green synthetic chemicals.