Noble Metal Loaded TiO₂ Catalysts Study On Photocatalytic Oxidation Of Ethanol Or Methanol/Ethanol Mixture

Both ethanol and methanol can be produced from syngas,and are important downstream products of coal chemical industry.It has become a hot research topic to convert ethanol and methanol to more important chemicals through green chemical routes.It has a scientific significance of developing the theory of photocatalytic organic synthesis and a practical significance of developing new green routes to produce aldehydes and esters from methanol and ethanol under light irradiation at ambient temperatures.In this study,two series of Ag/TiO₂ and AgAu/TiO₂ catalysts were prepared and characterized by the UV-Vis diffuse reflectance?UV-Vis?,X-ray diffraction?XRD?,transmission electron microscopy?TEM?,specific surface area?BET?and X-ray photoelectron spectroscopy?XPS?etc.The photocatalytic performances of the catalysts for oxidation of ethanol and ethanol/methanol mixtures under ultraviolet light in the temperature range of 25-45°C were investigated.The preliminary reaction mechanism was studied according to the results of catalyst characterization and activity test.The experimental results are as follows:1.Silver and gold-silver alloy nanoparticles were evenly distributed on the surfaces of the catalysts,with an average size of 5 nm.2.The Ag/TiO₂ catalyst with 5%Ag content exhibited the best catalytic performance in photocatalytic oxidation of ethanol.The ethanol conversion was close to 100%and the acetaldehyde selectivity could reach 70%.The conversions of ethanol and methanol were 85%and 65%,respectively,in the photocatalytic oxidation of ethanol/methanol mixture.The AgAu/TiO₂ catalyst with gold to silver ratio of 3:2 in the catalyst showed the best catalytic activity.The ethanol conversion was about 100%and the acetaldehyde selectivity was more than 80%in the photocatalytic oxidation of ethanol at 45°C.The conversions of ethanol and methanol were 90%and 65%,respectively,in the photocatalytic oxidation of ethanol/methanol mixture.The main product of the both reactions was acetaldehyde,and the silver catalysts were more beneficial to the production of acetone than the alloy catalysts.3.In the photocatalytic oxidation of ethanol,the ethanol conversion increased with oxygen partial pressure.The conversion was almost 100%at the alcohol-oxygen ratio of 1:2.With the increase of the flow rate of feed gas,the conversion of ethanol increased first then decreased.The selectivity of acetaldehyde increased but that of acetone decreased,and the optimal gas flow rate was 40 ml/min.In the photocatalytic oxidation of methanol and ethanol,the conversion of ethanol and methanol increased with the increase of oxygen partial pressure.The content of acetaldehyde decreased but the acetone content increased.No acetaldehyde could be observed in the product,but the selectivity of acetone was up to 60%when the alcohol-oxygen ratio was 1:3.When the ethanol partial pressure increased,the selectivity of acetaldehyde increased but the acetone selectivity decreased.The conversion decreased,the acetaldehyde increased and the acetone decreased with the increase of the mixture flow rate.The light intensity is proportional to the conversion rate of ethanol and methanol.4.Under ultraviolet light irridiation,Ag₂O was reduced to metallic Ag,which formed an Ohmic contact at the interface between TiO₂ and Ag.In the alloy catalyst,the photoexcited electrons transferred to the metal through the interface charge area.Ethanol and methanol molecules were adsorbed on the surface of the catalyst and dissociated to ethoxy and methoxy at room temperatures.Under ultraviolet light irridiation,ethoxy and methoxy were oxidized to acetaldehyde and formaldehyde by the photoexcited holes.Acetaldehyde gave rise to 3-hydroxybutyraldehyde and isopropyl alcohol through condensation reaction on the surface of the catalyst,and finally converted to acetone.Methoxy and ethoxyl reacted to form ethyl formate.Methoxy and formaldehyde formed methyl formate.Oxygen on the negatively charged metal surfaces dissociated and spilled to the surface of the catalyst to fill the oxygen vacancies resulted from the involvement of the reaction of the surface hydroxyls,facilitating the continuous reaction.