Tailoring Noble Metal-loaded Layered Double Hydroxides For Small Molecule Electrochemical Oxidation Reactions

Supported noble metal catalysts play a crucial role in various industrial catalytic reactions due to their outstanding catalytic performance.The advancement of nanoresearch has revealed the differences in reaction activity of metal atoms at different positions within noble metal crystals.Noble metal atoms at the surface,edge,and corner sites of crystals exhibit varying relative activities in specific reactions.In recent years,sub-nanoscale noble metal single-atoms and clusters have garnered widespread attention.At this scale,in addition to the coordination structure of noble metal atoms,their interaction with the support also significantly influences reactivity.The constituent elements,surface defect types,and interlayer anions of layer double hydroxides（LDHs）are adjustable.When noble metals are loaded onto LDHs,their sizes,loading positions,and coordination environments are diverse,forming complex structures that increase the difficulty of identifying active sites.Understanding the impact of loading positions on catalytic performance and the regulatory role of coordination environments on reaction activity is crucial in studying active sites.Therefore,a profound understanding of the relationship between structure and active sites supports a deeper comprehension of noble metal-loaded LDHs systems and provides new insights and technical support for the development of catalytic applications and energy conversion in various fields.This paper focuses on the study of noble metal-loaded LDHs catalytic systems.Initially,starting from a structural understanding,a systematic structural characterization method was developed by integrating advanced spectroscopic techniques and first-principles calculations to deeply investigate the presence and location of noble metals on LDHs.Furthermore,various methods for loading noble metals on LDHs were developed in the study,successfully achieving a technological breakthrough in effectively loading noble metals of different sizes and in different positions.By precisely controlling the size and relative position of noble metals,the electronic structure of noble metals was accurately tuned,enabling the effective design of catalysts for different catalytic reactions.Through this work,a better understanding of the behavior and interaction mechanisms of noble metals on LDHs was achieved,providing a theoretical basis and practical guidance for designing efficient catalysts.The main research of this paper includes the following four systems:1.By utilizing advanced spectroscopy characterization and theoretical calculations,the study revealed the state and location of noble metals in LDHs,achieving precise localization of noble metal single atoms.Using Rh as an example,Rh/Ni Fe catalysts were prepared via coprecipitation method.Electron microscopy techniques were employed to preliminarily identify single atoms,clusters,and particles,confirming the existence of Rh in the single-atom form.Subsequently,modeling of its potential location was conducted,and the final position was determined through fitting and theoretical calculations.The results indicate that Rh atoms were situated within the Ni Fe LDH layers,positioned coplanarly with Ni and Fe atoms,replacing some Fe atoms.2.By precisely controlling the size of noble metal nanopparticles at the sub-nanometer scale,the structural sensitivity of hydrazine electro-oxidation reaction（Hz OR）was explored.Building on the understanding of catalyst structure and reactivity from the previous study（Work 1）,three scales of Pd catalysts-single atoms,clusters,and particles-were designed and prepared.In the Hz OR,the performance trend of the three catalysts was nanoclusters>nanoparticles>single-atoms.By considering this performance trend and the morphological characteristics of the three Pd catalysts,it was elucidated that Pd clusters with multi-sites,low steric hindrance,and strong adsorption were suitable catalysts for the Hz OR.3.Starting from the reaction mechanism,for the two key steps of catalyst proton deintercalation and substrate adsorption in the 5-hydroxymethylfurfural electrooxidation reaction（HMFOR）.We synthesized Pd/Ni Co catalyst by doping Ni（OH）₂ with Co and loading Pd,resulting in a significant enhancement of performance.The results showed that the introduction of Co induced distortion of neighboring Ni sites due to the Jahn-Teller effect in the Co O₆octahedron.This reduced the energy barrier for proton deintercalation at the Ni sites,enabling them to oxidize HMF at a low voltage.The unique electronic structure of Pd nanoclusters exhibited a strong tendency to adsorb the furan ring structure in HMF,significantly boosting the local substrate concentration on the Pd/Ni Co surface and enhancing the probability of substrate interaction with Ni active sites.4.By building adjacent highly active sites and adsorption sites,enhancing the synergistic effect of oxidation and adsorption,the catalytic activity and selectivity towards HMFOR were improved.Based on the understanding of the collaborative mechanism in Work 3,an embedded Pd/Ni Fe catalyst was constructed using the restricted effect of LDHs.During the interlayer in situ reduction process,Pd nanoparticles with sizes exceeding the interlayer spacing tolerance penetrated through the layers.The damaged edge sites of the layer formed highly active oxidation centers in close proximity to Pd nanoparticles.The adjacent positioning of the two enhanced the utilization of the adsorptive properties of the Pd nanoparticles.Additionally,the study offered evidence of interaction between the aldehyde group in HMF and the surface hydroxyl groups of the catalyst,indicating that the removal capability of surface hydroxyl groups of the catalysts was also a crucial factor influencing HMFOR activity.