Mechanism Study On The Electrocatalytic Hydrogenation For Biomass Platform Molecules And The Design Of The Electrocatalysts

Developing advanced and efficient electrocatalytic energy conversion systems are of great and practical significance for propelling the efficient development of clean energy,the energy-saving and emission-reduction of human society,and the construction of a new low-carbon power system network.Among them,the electrocatalytic hydrogenation（ECH）,which is driven by renewable electricity to transform biomass into biofuels and high-value chemicals,provides an effective way to realize the goal of low-carbon transformation of the energy system.However,in contrast to the electrocatalytic reaction of water and small gas molecules,the ECH is much more complex.The whole process is involved with kinds of issues,including the adsorption of various organic functional groups,multi-step electron transfer,and the adsorption and desorption of various organic intermediates.As a result,it not only leads to complicated synthetic products and slow reaction kinetics,but also make ECH difficult to compete with the side reaction of hydrogen evolution reaction（HER）during electrolysis,thus,becoming key factor,to hinder the large-scale application of ECH.Additionally,the application of ECH is also obstructed by the low solubility of some organic compounds and electrode deactivation caused by solvent effects and radical polymerization.Hence,to advance the development of efficient,high-selectivity and inexpensive electrocatalysts towards ECH,it is highly important to clarify the reaction mechanism and explore the key influencing factors of the electrolysis environment on ECH performance,which also own value for basic theory and practical application.Unfortunately,for most of the currently reported ECH reaction systems,problems,such as complex and diverse reaction conditions,unclear understanding towards the mechanism of reaction and catalyst selectivity regulation,are unresolved.Thus,great efforts are still needed to be done for guiding the design of highly selective ECH electrocatalysts,constructing efficient ECH electrocatalytic systems and realizing the application of advanced biomass energy conversion.Aiming at the aforementioned issues,the experimental design and mechanism research of this dissertation,of which originated from the characteristic difference of two representative metal electrodes having different reactive H sources for ECH(noble metal electrodes with low HER overpotential（Pt,Pd,Rh,Ir,etc.）and base metal electrodes with high HER overpotential（Cu,Ni,Co,etc.）,were carried out.Through a comprehensive kinetic study of the electrochemical reaction and tracking the hydrogenation evolution of adsorbed organic intermediates on the catalyst surface,the reaction mechanisms of ECH,which is towards the transformation of biomass platform molecules on the aforementioned two types of metal electrodes,are proposed.Based on the suitable mechanism for the ECH reaction,the optimal electrolysis conditions and directions of catalyst optimization are determined.Firstly,using phenol as the molecular model of the lignin platform molecules,the utilization law of adsorption hydrogen(H_ad)of Pt-based metals with high coverage of H_ad was studied in the ECH reaction,and a highly dispersed Pt Rh nanoalloy particles was designed to adjust the adsorption energies of phenol and H_ad,thus inhibit the side reaction of HER and achieve high ECH Faradaic efficiency(FE_ECH%).Secondly,using phenol as molecular model of hemicellulose platform molecules,the selectivity regulation mechanism of electrolyte p H on hydrogenolysis and hydrogenation reactions was studied on a Cu electrode with medium H_ad coverage,the electrochemical kinetic model in the p H-universal range was constructed,the variation of the hydrogenolysis intermediates on the electrode was investigated,and the mixed mechanism in ECH,in which the hydrogenolysis pathway was jointly dominated by the hydrogen transfer and proton-coupled electron transfer（PECT）,was proposed,and the p H-induced selective regulation mechanism under such electrolysis environment was revealed.Finally,in light of the characteristics of hydrogenolysis and hydrogenation mechanisms,the Cu electrode was respectively modified with Pd and Ce O_x,to enhance the H_ad coverage on the Cu surface and improve the local acidity of Cu,respectively,thus,highly selective formation of furfuryl alcohol and 2-methylfuran during ECH was achieved.The specific research contents are as follows:（1）By studying the ECH reaction of phenol on Pt-based metals in acidic electrolytes,this chapter firstly clarifies the competition mechanism between ECH and HER.On this basis,the Pt Rh/MCN electrocatalyst,which Pt Rh bimetallic nanoparticles were uniformly dispersed on a highly ordered mesoporous carbon nanosphere（MCN）,was designed.Through studying the behavior of H_ad on the electrode surface,it was found that the introduction of phenol increased the amount of H_ad in the hydrogen underpotential deposition region（H-UPD,>0 V vs.RHE）and a high utilization of H_ad by ECH in Pt Rh/MCN(FE_ECH%≈88%)was achieved.Combining synchrotron radiation Fourier transform infrared spectroscopy（SR-FTIR）,in situ sum-frequency spectroscopy（SFG）,and density functional theory calculations（DFT）,the ECH pathway of phenol was confirmed and the adsorption energy variation of phenol and reaction intermediates on the catalyst were presented,and it was found that the strong overlap between the d-orbital electron densities of Pt and Rh and a suitable M-H_ad not only enhanced the specific adsorption of phenol,but also improved the intrinsic ECH activity of H_ad.This study not only reveals the utilization law of H_ad,but also developed Pt Rh/MCN electrocatalyst with a low overpotential for the phenol ECH reaction,thus the low energy consumption of electrolysis can be realized,which is of great significance for reducing the energy consumption of large-scale industrial electrolysis.（2）In this chapter,with the biomass platform molecule furfural as the target organic compound,the effects of electrode potential and electrolyte p H on selective hydrogenation and hydrogenolysis reactions were comprehensively studied on Cu electrode.Two parallel reactions,hydrogenation and hydrogenolysis,were predicted to have different ECH paths based on the microkinetic analysis of the Tafel slope.Spectral evidence of intermediate Cu-O_ad species was obtained by in situ enhanced Raman spectroscopy（SHINERS）technique,confirming the existence of direct deoxygenation of intermediate C-O bonds during hydrogenolysis.Meanwhile,it is clarified that,under different p H electrolyte,the process of re-hydrogenation of Cu-O_ad species to water may via potential-dependent PCET mechanism or potential-independent direct H_ad transfer mechanism,namely Langmuir-Hinshelwood（LH）mechanism.On this basis,the p H-dependence of furfural hydrogenation on Cu electrode was rationally explained by combining key experimental data and DFT calculation results,that is,the hydrogenolysis pathway under acidic conditions is realized via the LH-PCET hybrid mechanism.This study shed light into the mixing mechanisms of ECH processes under complex environmental influences and broadens our understanding of the reaction mechanisms of oxygenate deoxygenation during biomass-derived oil refining.（3）Based on the aforementioned mechanism understanding of the hydrogenation and hydrogenolysis reaction in the ECH process on the Cu electrode,this chapter regulates the ECH selectivity of further by increasing the H_ad coverage（hydrogenation）and surface acidity（hydrogenolysis）on the electrode surface of Cu.Taking advantage of the characteristics of H_ad-rich and hydrogen overflow on noble metal Pd atom,the preparation of Pd/Cu nanowire electrocatalyst was realized by galvanic displacement reaction.Making use of the Lewis acid property of metal oxide Ce O_x,Ce O_x/Cu nanowire electrocatalyst owning acid center was prepared by in-situ electrochemical deposition.Physical characterization results of XRD,XAS and XPS showed that,～1 wt%of the modifiers（Pd and Ce O_x）did not change the main structure of Cu,and Pd and Ce O_x on the Cu surface respectively existed in a form of atomically dispersion and amorphism.The electrochemical test results demonstrated that the both modified electrodes improved the ECH performance.Compared with the Cu nanowire electrode with a selectivity of～50%for furfuryl alcohol,the Pd/Cu nanowire electrode has a selectivity of～90%under the same electrolysis conditions,while the Ce O_x/Cu has a selectivity of 72%for 2-methylfuran,which is in line with the hydrogenation and hydrogenolysis characteristics observed by SHINERS during ECH.The experimental data and DFT calculations indicated that the addition of Pd increases the coverage of H_ad on the Cu surface,which can effectively reduce the hydrogenation energy barrier of the hydrogenation intermediate F-CH₂O*,thereby improving the selectivity of furfuryl alcohol;According to the NH₃-TPD test results,the introduction of Ce Ox can significantly increase the surface acidity of the catalyst and enrich the local environmental proton concentration of the catalyst active site,besides,the DFT calculation results revealed that the surface Ce Ox/Cu electrocatalyst has a stronger adsorption energy of furfural,hence,the PCET process in the hydrogenolysis reaction is effectively enhanced.In this work,based on the understanding of the reaction mechanism of hydrogenation and hydrogenolysis,the functionalized regulation of surface hydrogen coverage and surface acidity on the catalyst was conducted,which significantly improved the selectivity of ECH and provided a new perspective for designing efficient ECH catalysts towards biomass platform molecules.