Preparation Of Palladium Nanocatalysts By One-step Approach And Their Applications In Water Splitting And Carbon Dioxide Reduction Reaction

The growth of global energy demand and overuse of global energy have led to excessive greenhouse gas emissions and severe environmental pollution.Electrocatalytic water splitting and CO₂ reduction are effective methods for converting and storing renewable energy,which are of great significance for replacing fossil fuels and addressing climate change issues.However,hydrogen production through water splitting faces problems such as slow kinetics,high overpotential,and poor stability.The preparation of syngas through CO₂RR has shortcomings such as narrow product distribution range,low activity and susceptibility to poisoning.Therefore,as the core of electrochemical reactions,the development of advanced electrocatalysts is the key to improving the efficiency of renewable energy utilization.Starting from the perspective of electronic structure modulation and optimization,this thesis constructed efficient and stable electrocatalysts for water splitting and CO₂ reduction using simple one-step preparation methods by studying the morphology,composition and loading of Pd nanomaterials.The structure-activity relationships of the electrocatalysts were elucidated by the combination of characterization methods,electrochemical analyses and theoretical calculations,and the reaction mechanisms were deeply explored.The main research results are as follows:（1）In response to the problems of insufficient activity and stability of commercial catalysts in the HER process,a one-step method was used to prepare ultra-thin two dimensional single-atom alloy Co/Pdm-4 with excellent activity,conductivity,and stability.By designing the morphology and composition of Pd based nanomaterials,single Co atoms were immobilized on Pd metallene（Pdm）support.The combination of the unsaturated coordination environment formed by single Co atoms and the unique geometric and electronic structures of two-dimensional materials realize the modulation of the d-band center and the optimization of electronic structure of the catalyst,generating highly active electronic states on the surface of Co/Pdm-4.Benefiting from the synergistic interaction and spillover effect generated by single Co atoms loading on the catalyst,the Co/Pdm-4 electrocatalyst exhibits superior HER performance to commercial Pt/C in both acid and alkaline solutions,achieving a Tafel slope of 8.2 m V dec^-1,an overpotential of 24.7 m V,and excellent stability in 0.5 M H₂SO₄.The successful preparation of two-dimensional single-atom alloys provides a promising strategy for the preparation of efficient,highly active and stable HER electrocatalysts.（2）In response to the problems of insufficient conductivity and high energy barrier of Fe OOH during OER process,a highly active amorphous hetero-structure Fe Co OOH-Pd was prepared by immobilizing atomic dispersed Pd species on Co doped Fe OOH through a one-step method.In the case of Co/Fe ratio of 0.68,the coexisting structure of nanorods and nanospheres with appropriate oxygen vacancies enhances the conductivity of catalyst and provides more active sites.At a Pd/Fe ratio of 0.026,atomic dispersed Pd species form unsaturated active centers,modulate the surface electronic structure,and improve the intrinsic activity of the electrocatalyst.The rationally designed Fe Co_0.68OOH-Pd_0.026 exhibits excellent OER activity and stability,achieving a Tafel slope of 37.5 m V dec^-1 and an overpotential of 265.1 m V.The DFT results uncover that the Co doping,oxygen vacancies,and atomic dispersed Pd species optimize the binding energies of*O and*OOH,reduce the energy required for*OOH formation,modulate the free energy changes of reaction pathway,and accelerate the OER kinetics.The successful construction of Fe Co_0.68OOH-Pd_0.026 provides a promising strategy for preparing OER electrocatalysts with high activity,excellent stability and low cost,which is of great significance for the large-scale implementation of water splitting.（3）In response to the issues of insufficient catalyst activity and susceptibility to poisoning in the production of syngas through CO₂RR,two dimensional Pd₉₃Mo₇ CNSs were constructed through the alloying of Mo element in a one-step method,serving as an electrocatalyst for efficient,selective,and stable electroreduction of CO₂ to adjustable syngas.The H₂/CO ratio is tunable from 0.15:1 to 4:1.The current density can be as high as 65.6m A cm^–2 with a H₂/CO ratio of 1:1.DFT calculations illustrate that alloying Mo can optimize the electronic structure around the host Pd atoms by enhancing the density states at the Fermi level,significantly reducing the kinetic energy barrier,thus improving the migration ability of CO*and enhancing the anti-poisoning performance of the electrode to achieve more stable CO₂ electroreduction.The successful preparation of Pd₉₃Mo₇ CNSs offers a strategy for developing novel metal-based 2D nano-electrocatalysts for efficient catalysis of CO₂ to syngas.（4）The one-step introduction of Cu and Co species was used to construct a trimetallic alloy Pd Cu Co to solve the inherent problems of easy Pd H formation and hard CO desorption of Pd-based nanomaterials with Pd as the active center in the process of syngas production through CO₂RR.The Cu species with low valence state reduce the conversion energy barrier of adsorbate and the Co species with oxidized state accelerate the adsorption of reactants and the desorption of products.The introduction of Cu and Co species optimizes the electronic structure of the catalyst and synergizes with Pd atoms,significantly affecting the reaction pathway.The obtained Pd Cu Co can achieve high Faradaic efficiency of CO（87.5%at-1.16 V vs RHE）and has a wide range of tunable CO/H₂ ratio（from 0.44 to 7）.In different downstream application scenarios,Pd Cu Co can provide high current densities for syngas preparation(44.4 m A cm^-2 for Fischer-Tropsch synthesis,54.1 m A cm^-2 for aldehyde production and75.8 m A cm^-2 for methanol production).The successful preparation of Pd Cu Co,a trimetallic alloy with outstanding CO₂RR performances,provides an ideal working electrode for the production of syngas,and the study of the mechanism of each component in the alloy provides a new idea for the design of the alloys.