Design Of Several Highly-efficient Catalysts For Formic Acid Oxidation

The rapid development of the economy and society has brought about energy crisis and environmental pollution,forcing people to develop renewable and clean energy.Formic acid,due to its molecular structural properties and stable chemical properties,can be used for liquid-phase hydrogen storage and also serve as the fuel for anodic reaction of formic acid fuel cells.In addition,formic acid oxidation（FAO）is also an important prototype reaction for the other small molecule oxidation.Therefore,FAO is of great importance in understanding the reaction mechanisms and practical application.Pd is a known high-performance FAO catalyst,but its performance improvement and the development of new efficient FAO catalysts are still one of the central issues to be solved in this field.At present,the design of catalysts mainly focuses on the three aspects:firstly,regulating the morphology of Pd-based catalysts to improve the mass transfer performance and catalytic activity;secondly,introducing the second phase materials with dual-functional effects or changing the electronic structure by alloying to improve the performance of Pd-based catalysts;thirdly,adjusting the coordination environments to design the graphene-based atomic catalysts without Pd.However,the selection and recombination methods of the second phase material,the choose principles of the alloying elements of the Pd-based alloys,and the modulation methods of complex coordination-environment effects of the atomic catalysts are still significant challenges and limit the development of highly-efficient FAO catalysts.Therefore,we focus on the design of highly-efficient FAO catalysts and carried out the following studies:1.Improving the performance of Pd-based catalysts based on the mass transfer effects and dual-functional effectsTo solve the problem of the liquid-phase mass transfer and the CO toxicity of Pd catalysts,we introduced the second phase Ni when constructing the specific three-dimensional flower-like substrate materials to disperse Pd nanoclusters.Firstly,the carbon-coated Ni（OH）₂ precursor was obtained by a hydrothermal carbonization method using glucose as carbon source.Subsequently,Pd and Ni nanoparticle composites were obtained by pyrolysis under the protection of argon（Pd-Ni/C）.Carbon-based support materials with three-dimensional flower structures can fully contact electrolytes and expose more catalytic active sites,which are conducive to the transfer of substances and electrons,thus improving the catalytic activity.After 500^thcyclic voltammetry test,the peak current density of Pd-Ni/C catalysts still has a high retention rate.Density functional theory calculations proved that Ni was beneficial to the formation of OH*,which could further react with CO*,significantly eliminated the surface toxicity of Pd,and improved the catalytic stability of the composite.Pd-Ni/C catalysts exhibit the highly efficient activity for FAO due to their hybrid structure and the dual-functional effects of Ni and can be used as anode electrocatalysts for direct formic acid fuel cells.2.Designing efficient Pd-based alloy catalysts for FAO based on a single descriptorTo solve the problem of the CO toxicity and the formate transformation on Pd-based alloys,we used a descriptor based on valence-electron properties to analyze the mechanisms and improve the catalytic performance.Pd-based alloys are a common type of FAO catalysts,and their activity is generally related to the composition and proportion of the alloy elements.Here,we adopted the descriptorψ,which is determined by the valence-electron number and electronegativity of the catalytic active center,to study the adsorption energies of FAO reaction intermediates（CO*,COOH*,OH*and HCOO*）on TMs and the trend of the catalytic activity.Furthermore,we discussed the variation of the adsorption energies of the reaction intermediates on the alloy surfaces and constructed the FAO activity volcanic diagram by extending the descriptorψto Pd₃TM and single-atom alloy systems TM@Pd（111）（TMs corresponds to the transition metals）.With the descriptorψ,we obtained the optimal reaction pathway of FAO on Pd-based alloys and understood the effects of doping metals on the reactivity and selectivity of FAO.In addition,the parameters involved in the descriptorψare predictable,which allow the rapid and reasonable design of materials with the best adsorption properties and provide a convenient way to design highly-efficient alloy catalysts.3.Designing efficient FAO catalysts based on the coordination-environment effects of atomic catalystsTo understand the coordination-environment effects of atomic catalysts,we extended the valence-electron-based descriptor to analyze the mechanism and design advanced catalysts.We extend the descriptorψto atomic catalysts and study the variation trend of the adsorption energies of C-based,O-based and N-based small molecules on atomic catalysts with different coordination environments,including the tetrad-coordination,triad-coordination,dangling-bond and doped TMs.These results show that the descriptorψeffectively capture the coordination-environment effects（including the type and number of coordination atoms,dangling-bond and central TMs）on the adsorption performance of atomic catalysts,and could be used to rapidly screen the highly-efficient FAO catalysts.We predict that Ir-N4-SAC,Fe2-N6-BAC,as well as Co-SACs containing P and S coordination atoms,are potentially highly active FAO catalysts.These findings point out the direction for the development of highly-efficient FAO atomic catalysts.