Theoretical Study On The Dehydrogenation Of Formic Acid Over Single Atom/Cluster Supported By Two-Dimensional Carbon Materials

As the overuse of fossil fuels leads to escalating energy crises and environmental pollution,the challenges associated with energy and environmental issues are becoming increasingly pronounced.Hydrogen energy has emerged as a significant alternative to fossil fuels due to its high energy density and environmentally friendly properties.Nevertheless,achieving secure and efficient storage as well as transportation of hydrogen gas remains a formidable challenge.Formic acid,possessing a high hydrogen content（4.4 wt%）and the ability to catalytically decompose into hydrogen gas at room temperature,holds vast application prospects as a hydrogen storage carrier.To realize efficient formic acid dehydrogenation under mild reaction conditions,the design of suitable catalysts is of paramount importance.Presently,catalysts for formic acid dehydrogenation primarily involve supported metal nanoparticles,wherein only the surface metal exerts catalytic effects while the interior metal remains underutilized,thereby necessitating enhancement in catalyst activity and selectivity.The emergence of single-atom/cluster catalysts has significantly enhanced the utilization efficiency of metal atoms,thereby reducing the usage of precious metals and improving catalytic performance.However,the surface free energy of metals increases as their size decreases.Therefore,the selection of suitable carriers is essential to prevent metal agglomeration.Two-dimensional carbon materials,owing to their large surface area,controllable structure,ease of surface functionalization,and high chemical stability,are considered as ideal carriers for single-atom/cluster catalysts.In this study,graphene and graphyne in two-dimensional carbon materials were chosen as carriers.Theoretical design approaches involving N-doping,curvature adjustment,and tuning of cluster size/composition were employed to modulate the metal-carrier interactions,thereby enhancing the activity and selectivity of Pd single atoms or clusters in the catalytic formic acid dehydrogenation reaction.Density Functional Theory（DFT）calculations were used to elucidate the mechanism of formic acid decomposition on the catalyst surface.The study analyzed the modulation mechanisms of the aforementioned strategies on the activation and conversion of formic acid and its decomposition intermediates,investigated the catalyst’s geometric and electronic structures,and explored the impact of metal-carrier interactions on the catalytic performance of formic acid dehydrogenation.The main achievements of this research are as follows:1.N-Doped Graphene Supported Pd Single Atom and Cluster catalyze Formic Acid DehydrogenationWe conducted a comprehensive investigation into the catalytic performance of Pd monatomic and Pd₄ cluster catalysts loaded on N-doped graphene for the formic acid dehydrogenation.We explored the effects of pyridine-N（pyri N3）,pyrrole-N（pyrro N3）,and graphite-N（graph N3）dopants on N-doped graphene-supported Pd monatomic and Pd4 cluster catalysts.The findings reveal that the decomposition of formic acid on all catalysts proceeds through the formate（HCOO）intermediate,yielding CO₂ and H₂ as final products,demonstrating a 100%H₂ selectivity across all catalysts.The activity sequence（δE values in parentheses）among the six catalysts is as follows:Pd₄@graph N3（0.75 e V）>Pd₁@graph N3（0.93 e V）>Pd₁@pyri N3（0.99 e V）>Pd₄@pyri N3（1.00 e V）>Pd₁@pyrro N3（1.26 e V）>Pd₄@pyrro N3（1.31 e V）.It is evident that the activity sequence for formic acid dehydrogenation on the three supports is graph N3>pyri N3>pyrro N3,indicating that the impact of N-doping type on catalyst activity surpasses the influence of active metal size.Furthermore,through electronic structure calculations,we further dissected the fundamental role of N-doped graphene carrier’s coordination environment and active center size in shaping the catalytic performance for formic acid dehydrogenation.This study furnishes a theoretical basis for enhancing the performance of N-doped graphene-based formic acid dehydrogenation catalysts by modulating N-doping types.2.N-doped egg tray graphene supported Pd single-atom catalyze formic acid dehydrogenationThis study employs a novel two-dimensional carbon allotrope with surface curvature,referred to as Egg-tary graphene（ETG）,as a support material.Three variations of N-doped ETG loaded with Pd single-atom catalysts（SACs）are designed,denoted as Pd@ETG-N_x（x=0～3）.The application of these catalysts in the formic acid dehydrogenation reaction is investigated through Density Functional Theory（DFT）.The computational results indicate that,in comparison to graphene,ETG facilitates N-doping more readily,and surfaces of ETG with varying N-doping concentrations can stably adsorb Pd single atoms.The formic acid dehydrogenation reaction predominantly proceeds via a COOH intermediate-mediated pathway on all catalysts,with dehydrogenation being more favorable than dehydration.Furthermore,as the N-content in the support material increases,the catalytic activity and H₂ selectivity of Pd SACs significantly enhance.Among the four SACs studied,Pd@ETG-N3 exhibits the optimal catalytic performance,surpassing traditional Pd（111）catalysts.Notably,in contrast to the best-performing Pd₄@graph N3 from the initial study,the introduction of curvature in the ETG support material notably enhances H₂ selectivity of the catalyst.Electron structure analysis further confirms the viability of modulating the interaction between active centers and support material through N-doping to enhance formic acid dehydrogenation performance.This research highlights the potential of carbon allotropes in catalyst design,providing novel insights into the rational design of efficient catalysts through modulation of the support material and coordination environment.3.Graphdiyne-supported Pd_xCu_y atomic catalyst for formic acid dehydrogenationBased on the findings of the previous two studies,two-dimensional carbon materials loaded with single atoms and clusters have demonstrated remarkable catalytic performance.We subsequently introduced various substrates to construct a broader range of single-atom catalysts and cluster catalysts.The C-C triple bonds and uniform porous structure within the graphiteyne material are conducive to the adsorption of clusters composed of single atoms or a few atoms.By modulating the composition and quantity of loaded clusters,we devised a series of catalysts for formic acid dehydrogenation.The research reveals that in monometallic catalysts,the catalytic activity sequence is as follows:M₃@GDY>M₂@GDY>M₁@GDY（M=Pd/Cu）,underscoring the enhanced catalytic activity stemming from an increased number of active metal sites.Bimetallic trimetallic catalysts exhibit higher catalytic activity compared to monometallic trimetallic catalysts,such as Pd₁Cu₂@GDY>Pd₂Cu₁@GDY>Pd₃@GDY>Cu₃@GDY.This unveils the synergistic effect of Pd and Cu atoms within the graphdiyne-supported Pd_xCu_3-x@GDY.Electronic structure analysis indicates that the heightened activity of bimetallic trimetallic catalysts is attributed to cluster component adjustments that enhance the interactions between metal and bi-HCOO intermediate.On all catalysts,formic acid dehydrogenation surpasses dehydration.Pd₃@GDY,Pd₁Cu₂@GDY and Pd₂Cu₁@GDY exhibit superior activity to Pd（111）,with energetic spans of 1.31,1.01 and 1.28 e V for the dehydrogenation pathway,and energetic span differences of 0.48,0.84 and 1.37 e V between the dehydration and dehydrogenation pathways,respectively.This signifies that the activity and hydrogen selectivity of the two Pd Cu bimetallic catalysts exceed those of Pd（111）.This study not only guides the synthesis of cost-effective and efficient catalysts for formic acid dehydrogenation but also underscores the immense potential of graphdiyne-supported single-cluster catalysts in the field of formic acid dehydrogenation.