Electrochemical Conversion Of 5-hydroxymethylfurfural And Underlying Catalytic Mechanisms

Currently,most of the energy sources and chemical products that support the survival and development of humans depend on the conversion of non-renewable fossil fuels.The consumption of fossil fuels not only causes the depletion of fossil energy,but also brings environmental pollution and climate change.5-hydroxymethylfurfural（HMF）,as an abundant,renewable and highly processable carbon neutral resource on the earth,is considered to be one of the most ideal alternatives for the sustainable production of biofuels/bioderived chemicals.HMF can be converted into a variety of high value-added chemicals and intermediates.Electrochemical oxidation driven by sustainable energy is a green way to realize HMF conversion.Development of high-performance electrocatalysts is the key to the electrochemical oxidation of HMF.Revealing the reaction mechanism and clarifying the catalytic activity sites are also the focus of HMF oxidation.To address these issues,various parameters that affect HMF conversion were first studied.Then,a series of electrocatalysts ranging from nanoparticle sizes to atom sizes were synthesized.The activity and product distribution of HMF oxidation over the electrocatalysts were investigated.The adsorption,migration,bonding behavior of HMF/intermediates on the catalyst surface were also studied.The effect of catalyst structures on HMF oxidation was analyzed.This project could provide valuable guidance for the large-scale application of HMF electrooxidation.The main content and results of this thesis are listed as follows:（1）The effects of electrode preparation,stability of HMF,p H values of the electrolytes,impurities in the electrolyte,electrolysis cell structure and product quantitative methods on the oxidation of HMF were systematically investigated,to establish an accurate and reliable method to evaluate catalyst performance.The results show that HMF is unstable when the p H of the electrolyte is higher than 12 or lower than 2.The presence of ppb-levels of impurities such as Cu²⁺and Ni²⁺in the electrolytes accelerates HMF electro-oxidation.Four electrochemical cells including an undivided cell and H-type cells separated by Nafion 117 membranes,anion membranes and salt bridges are used for HMF conversion.When nickel foam is used as a catalyst support,the H-type cell with a salt bridge and the undivided cell are not suitable for HMF oxidation because of the cracking of the nickel foam.（2）Nine types of metallic ions including Cu²⁺,Ni²⁺,Fe³⁺,Fe²⁺,Co²⁺,Mn²⁺,Zn²⁺,Ce³⁺and Al³⁺are intentionally introduced into the electrolytes with a controlled manner and their effects on electro-oxidation of water,HMF and glycerol are investigated in details.The results show that Co²⁺has the most pronounced effects on H₂O electro-oxidation and that Cu²⁺displays superior activity toward HMF and glycerol electro-oxidation,but has negligible effects on H₂O electro-oxidation.In addition,the effects of metallic impurities are independent of the composition of the electrodes,only rely on the p H of the electrolytes.In-situ electrochemical Raman spectroscopy,control electrochemical experiments and X-ray photoelectron spectroscopy analyses reveal that the origin of impurity effects is attributed to the formation of hydroxides during the electrochemical measurements.（3）The mechanism of HMF electro-oxidation was studied by in-situ electrochemical surface enhanced Raman spectroscopy（SERS）coupled with an Fe³⁺probe.A multicomponent platform consisting of titanium plates integrated with a gold film and uniform nickel,cobalt and copper nanoparticles（NPs）was employed as an electrode as well as a SERS substrate,enabling operando SERS study on the electrocatalytic process with simultaneous evaluation on catalytic activity and visualization on the variations of active sites.Electrodes integrated with Ni and Cu NPs exhibited excellent HMF oxidation performance but their activity were suppressed by adding 1 ppm Fe³⁺.Fe³⁺inhibits the oxidation of Ni²⁺to Ni³⁺and promotes the formation of more Cu O and Cu^III.Cu^III is better active sites for water oxidation.The presence of Fe³⁺also inhibits the transformation of Co³⁺to Co⁴⁺,but it has no pronounced effects on HMF oxidation performance.A highest 2,5-furandicarboxylic acid（FDCA）yield（94.0%）and a maximum HMF conversion（98%）are obtained from the electrode integrated with Cu NPs at a potential of 1.47 V.HMF oxidation on Cu NPs follows the aldehyde oxidation pathway,which is not affected by Fe³⁺addition.（4）The effects of single atoms on the activity of HMF oxidation were investigated by using Ni,Co and Cu NPs co-doped with single atoms.Among the Ni _NPs@Ni/NCNSS-1100,Co _NPs@Co/NCNSs and Cu _NPs@Cu/NCNSs co-doped catalysts,the Ni _NPs@Ni/NCNSS-1100has the best catalytic activity with a current density of 26 times that of Ni NPs.The doping of single Co atoms is more beneficial to the oxidation of water and has little effect on the oxidation of HMF.The doping of single Cu atoms can promote the oxidation of HMF.Single Ni atoms promote the oxidation of HMF to produce more FDCA.The interaction between single Ni atoms and Ni nanoparticles can promote the oxidation of HMF to FDCA.（5）Single copper atoms loaded on nitrogen-doped carbon nanosheets（Cu/NCNSs）were synthesized.The electrooxidation of HMF,furfural,glucose,formaldehyde,methanol,ethanol,isopropanol,ethylene glycol,glycerol,benzyl alcohol,formic acid,and oxalic acid by Cu/NCNSs were studied.The reaction pathways and mechanisms of HMF oxidation on Cu/NCNSs and Cu NPs were explored.The results show that HMF oxidation on Cu NPs followed the 5-diformylfuran（DFF）path at lower potentials,while it turned to 5-hydroxymethyl-2-furancarboxylic acid（HFCA）path at applied potentials over 1.67 V.For the Cu/NCNSs,HMF oxidation followed the alcohol hydroxyl oxidation pathway,that is,the direction of generating DFF.（6）Synergistic mechanism analysis of copper nanoparticles and copper atoms on HMF oxidation.Single copper atoms and copper nanoparticles were co-embedded on nitrogen-doped carbon nanosheets(Cu _NPs@Cu/NCNSs)and used for HMF electrooxidation.The results show a synergistic effect between single copper atom and copper nanoparticles.Single copper atoms tend to produce 5-formyl-2-furanocarboxylic acid（FFCA）.At a low potential（1.42 V）,Cu _NPs/C can efficiently convert HMF into FDCA.The Cu _NPs@Cu/NCNSs show high FFCA selectivity of 86.7%.Cu（OH）₂ species generated by electrochemical oxidation were identified as the main catalytic sites for HMF oxidation on the Cu _NPs/C.HMF oxidation on the Cu _NPs/C followed the path to HFCA,while on the Cu _NPs@Cu/NCNSs and Cu/NCNSs,HMF was oxidized along the DFF pathway.