Support Modification Of Supported Noble Metal Catalysts For Glycerol Oxidation

Biodiesel is a kind of environmental protection diesel,that has many advantages such as renewable,wide source of raw materials,but the high production cost also restricts the development of biodiesel.In the process of biodiesel production,a large amount of byproduct glycerol is produced.At present,it is difficult to obtain both high conversion and high selectivity over Au and Pd-based catalysts for glycerol selective oxidation under alkaline conditions,and Au nanoparticles are easy to agglomerate and cause deactivation.The Pt-based catalysts display low activity for glycerol oxidation and low glyceric acid selectivity under base-free conditions.Therefore,the purpose of this project was to design an efficient and stable glycerol oxidation catalyst.Focusing on catalyst support and regulating the composition,introducing active modifier and doping light element in Pd lattice interstitially.To investigate the relationship between catalyst structure and catalytic performance,to enhance the metal-support interaction,and to prepare the catalyst for the efficient conversion of glycerol to glyceric acid.The main research contents and conclusions are as follows:（1）CeMnO₃ perovskite was used as the functional support of Au NPs to construct Au/CeMnO₃ catalyst for glycerol oxidation reaction.The Au/CeMnO₃ catalyst achieved the superior glycerol conversion of 90.1%with 78.5%GLYA selectivity,and the TOF value was 2004.4 h^-1,which were much higher than other Ce-based or Mn-based Au catalysts.The excellent catalytic performance of CeMnO₃ perovskite supported catalyst was attributed to the following three aspects:Firstly,the strong interaction between Au and CeMnO₃ perovskite facilitated the electron transfer from B-site metal（Mn）in CeMnO₃ perovskite to Au and stabilized Au nanoparticles,which resulted in the enhanced catalytic activity and stability for glycerol oxidation.Secondly,the uplifted d-band center of Au/CeMnO₃ promoted the adsorption of glyceraldehyde intermediate on the catalyst surface,which benefited the further oxidation of glyceraldehyde into glyceric acid.Thirdly,the oxygen vacancy of CeMnO₃ could activate and decompose the reaction solvent H₂O to produce surface-active OH_adsspecies,thus accelerated the oxidation reaction of glycerol.（2）By regulating the A/B site of CeMnO₃ perovskite,the influence of perovskite carrier structure on glycerol oxidation activity and product selectivity was further explored.Among Ce-based perovskite supported catalysts with different B sites,CeMnO₃ supported catalysts mainly selected glyceric acid with high selectivity of 78%.However,CeCrO₃ and CeNiO₃ supported catalysts were more likely to choose lactic acid,and the selectivity reached more than 80%.The optimum catalyst Au/La_0.2Ce_0.8MnO₃ showed a glycerol conversion rate of 61.4%and a glyceric acid selectivity above 78%.Combined with the catalytic performance and characterization,we concluded that the excellent glycerol oxidation activity of the catalyst was due to the strong interaction between the supporting metal and La-doped perovskite support,which enhanced the ability of charge transfer.The catalyst produced more oxygen vacancies,and accelerated the activation and dissociation of H₂O,produced active hydroxyl groups on the catalyst surface,thus promoted the glycerol oxidation process.It was concluded that the method of controlling the structure of perovskite to affect the oxidation property of glycerol was universal.（3）In order to solve the problem of low activity of Pt-based catalyst for base-free glycerol oxidation,carbon quantum dots（CQDs）decorated Pt/Ce O₂（Pt-CQDs/Ce O₂）catalyst for the selective oxidation of glycerol to glyceric acid was employed,which enhanced the conversion glycerol（99.3%）,about 1.5 times over the Pt/Ce O₂ in base-free medium,and the selectivity to glyceric acid was 51.6%.The Raman and XPS characterization indicated that the CQDs had high carbon defects and strong electron interaction with Pt.The experimental and characterization results showed that the d-band center of the catalyst was upshift by the modification of CQDs,which enhanced the glycerol adsorption.Moreover,the optimal regulation between the adsorption of glycerol and desorption of glyceric acid can be achieved by changing the type of carbon dots,and the catalytic activity can be improved while the anti-poisoning ability of the catalyst can be enhanced.This work not only found that carbon dots enhanced the catalytic activity of Pt-based catalysts for glycerol oxidation in base-free,but also provided an innovative method for preparing simple and efficient catalysts for glycerol oxidation.（4）In order to strengthen the metal-support interaction of the catalyst,the light element B was incorporated into the precious metal Pd lattice,and then the interstitial B atoms was oxidized to produce BO_x clusters,which was encapsulated on the surface of Pd.The Pd@BO_x/CNTs catalyst exhibited a notably enhanced glycerol conversion rate reaching up to 100%at 2 h and glyceric acid selectivity reaching up to 75%.It was suggested that Pd@BO_x/CNTs catalyst showed the hydrogen spillover effect,which BO_x as a bridge promoted the hydrogen transfer of primary hydroxyl groups for glycerol and glyceraldehyde intermediate to adsorbed hydroxyl on catalyst surface,and thus enhanced the glyceric acid selectively.In addition,the strong orbital hybridization in interface site between in situ formed BO_x clusters and Pd metal for Pd@BO_x/CNTs catalyst induced the upshift of d-band center,which enhanced the adsorption of the substrate on its surface and further promoted the glycerol oxidation.These findings provided new thoughts for understanding the catalytic mechanism and designing efficient supported catalysts.