Supported Noble Metal Catalysts For Glucose Oxidation: Regulation Of The Ceria And Carbon Support

Converting renewable biomass resources into value-added chemicals and fuels is one of the sustainable ways to solve the depletion of fossil energy and environmental degradation.It is also the key to achieving the national strategic goal of emission peak and carbon neutrality.Glucose,as the most widespread monosaccharide in nature,is a highly functionalized molecule,which can be transformed into a variety of high value-added compounds.Among them,gluconic acid,as one of the oxidation products of glucose,has been widely used in the food,detergent,and pharmaceutical industries,etc.Gluconic acid can be obtained under green conditions through the methods of thermocatalytic and electrocatalytic oxidation of glucose using heterogeneous catalysts.However,the problems of low catalytic activity and easily poisoned easy poisoning of catalysts have not been effectively solved.Therefore,this thesis aimed to design catalysts with high catalytic activity and high selectivity for glucose oxidation,and the regulation of catalyst supports,the selection of catalytic oxidation methods,the interaction between supports and metals,and the structure-activity relationship of catalysts were explored.The main research contents and conclusions are as follows:（1）Oxygen-vacancy-riched ceria supported Pd catalyst（Pd/CeO₂）was employed,which effectively alleviated the catalyst deactivation,realized high catalytic activity and stability with 100%gluconic acid selectivity,and the glucose conversion reached69.9%at room temperature and atmospheric pressure.The superior catalytic performance was attributed to the modulation of the electronic structure of Pd nanoparticles by the strong metal-support interaction between Pd and CeO₂.The valence band photoemission spectra revealed that the downshift of the d-band center of Pd resulted in the weak adsorption of gluconic acid on Pd/CeO₂,thereby suppressing the deactivation of the catalyst.Kinetic isotope effect experiments showed that oxygen vacancies in CeO₂ can dissociate water to generate active OH^*species,which in turn accelerates the oxidation of glucose.Furthermore,the heat treatment of CeO₂ support under hydrogen atmosphere can significantly increase the content of oxygen vacancies on the surface of CeO₂,and increase the fraction of Pd⁰ in the catalyst with the interaction between the support and metal,so that the Pd/CeO₂ catalyst exhibited higher TOF value(186 h^-1).（2）Au-based catalysts are considered one of the promising catalysts for glucose oxidation under base-free conditions.We prepared CeO₂ supports with three different morphologies（nanorods,nanocubes,nanooctahedrons）by hydrothermal method.When applied to base-free selective oxidation of glucose to gluconic acid,the Au/CeO₂nanorods（R-CeO₂）catalyst delivered the highest catalytic activity with glucose conversion of 79.6%and gluconic acid selectivity of 100%,due to the strong metal-support interaction and abundant oxygen vacancy.Specifically,the kinetic experiments showed the Au/R-CeO₂ remarkably lowered the apparent activation energy for glucose oxidation.Moreover,kinetic isotope effect experiments revealed that R-CeO₂accelerated the activation and dissociation of water into a large amount of surface-active hydroxyls,which promoted the glucose oxidation reaction even the Au particles were poisoned by gluconic acid.（3）N-doped glucose-derived carbon sphere（N-CS）can not only realize the value-added of biomass glucose,but also serve as a support for Pd catalysts,which can enhance the electron density of Pd and improve its catalytic performance for glucose oxidation.The Pd/N-CS catalyst prepared with the amount of melamine was 10 times that of glucose carbon spheres（CS）and the carbonization temperature was 900°C,obtained the highest content of graphitic N and Pd⁰,and the small particle size of Pd particles,and displayed the best catalytic performance for glucose oxidation,reaching a glucose conversion of 78.6%with 100%gluconic acid at room temperature and atmospheric pressure for 6 h.Moreover,the Pd/N-CS catalyst showed high stability during a recycled test.Combining the characterization results of the catalyst with its catalytic performance,we concluded that the high content of Graphitic nitrogen dopant obviously minimized the size of Pd nanoparticles,anchored Pd to prevent its agglomeration and deactivation,and promoted the electron transfer between graphitic nitrogen and metallic Pd,thereby effectively improving the catalytic activity and stability of Pd/N-CS for glucose oxidation.（4）Using Pd/N-CS as an electrocatalyst,the electrooxidation of glucose to gluconic acid was achieved in weak base and base-free electrolytes.The electrochemical results indicated that Pd nanoparticles on N-doped carbon spheres can expose more catalytically active sites,exhibit higher electrochemical active area,and also enhance the electrocatalytic performance for glucose oxidation.Under weak base conditions with a reaction temperature of 50°C,an applied voltage of 0.88 V（vs.RHE）,and a glucose concentration of 300 m M,the 10%Pd/N-CS exhibited excellent electro-oxidative glucose performance and cycling stability,and the glucose conversion could reach 85.0%,the gluconic acid selectivity was 80.0%after 6 h of reaction.Furthermore,it is the first time to realize the electrocatalytic oxidation of glucose over Pd/N-CS under alkali-free conditions.At an applied potential of 1.44 V（vs.RHE）,a reaction temperature of 60°C,and a glucose concentration of 50 m M,the Pd/N-CS catalyst exhibited a glucose conversion of 54.2%,and the gluconic acid selectivity reached 73.9%at 12 h.In addition,the electrocatalytic oxidation method can realize the value-added utilization of glucose while producing hydrogen at the cathode,reducing the applied voltage of electrolysis,thereby reducing the production cost of hydrogen production by electrolysis of water process.