Rational Design And Catalytic Performance Of Novel Two-dimensional Catalysts For Formic Acid Dehydrogenation:A First-principles Study

Hydrogen is a renewable and clean energy,which would play an important role in the energy landscape of future.However,the storage and transportation of H₂ remain the bottleneck which limits the large-scale application of hydrogen energy.Formic acid is considered as one of the most promising materials for chemical hydrogen storage.The development of efficient and economic catalysts for hydrogen production from formic acid decomposition reaction is of great significance to promote the practical application of formic acid in fuel cells and other novel clean energy devices.Heterogeneous catalysts are widely adopted in hydrogen production from formic acid.But some disadvantages,such as the large consumption of precious metals as well as the poisoning effect,limit their industrial and commercial applications.With the development of nano-catalysis and the progress of characterization technology,it is found that when the size of nanoparticles is reduced to single atom,their electronic structure and catalytic mechanism will change fundamentally.Based on the unique structural feature,single-atom catalysts generally show significantly different activity and selectivity from the traditional nano-catalysts.Although single-atom catalysts have been widely studied in various types of catalytic reactions,their reports in the field of hydrogen production from formic acid dehydrogenation remain limited.In order to achieve the goal of efficient,sustainable and economic utilization of formic acid for hydrogen generation,in this thesis,using the first-principles method based on the density functional theory,we systematically designed a large category of catalysts with the support of novel two-dimensional materials and studied their catalytic properties and reaction mechanism via the first-principles calculation.We also proposed a new screening method based on the energetic span model for efficient selection of potential catalysts.The research contents and main results of this thesis are as follows:1.Ni single-atom,double-atom and triple-atom anchored two-dimensional C₂N（Ni_x@C₂N,x=1-3）were constructed as the catalysts for hydrogen production from formic acid decomposition.The adsorption and decomposition mechanism of formic acid on Ni₁@C₂N,Ni₂@C₂N and Ni₃@C₂N surfaces were calculated.The results suggested that,formic acid on Ni₁@C₂N and Ni₂@C₂N would undergo dissociation adsorption under the synergistic effect of Ni and N sites,which was not happen on Ni₃@C₂N.By comparing different decomposition reaction pathways,it is found that formic acid prefers to decompose and generate hydrogen through the HCOO dehydrogenation pathway on all these three catalytic surfaces.The dehydration reaction,which leads to the undesired CO product,is difficult to occur in Ni_x@C₂N catalysts.These catalysts also have good selectivity to H₂ product.The energetic span and TOF values of Ni₁@C₂N,Ni₂@C₂N and Ni₃@C₂N were calculated by energetic span model and compared with that of the Pd（111）surface.In the results,Ni₂@C₂N exhibits the most excellent catalytic performance.Meanwhile,Ni₁@C₂N and Ni₃@C₂N also have the higher catalytic activity than Pd（111）.Analysis on the electronic properties of Ni_x@C₂N indicate that the accumulated polarization charge on the Ni_x component and the d-band center of catalyst can be used as descriptors to evaluate the catalytic activity.2.With the consideration of implicit solvent model in calculation method,twelve single-atom catalysts containing transition metals（TM@C₃N,TM=Mn,Fe,Ru,Co,Rh,Ir,Ni,Pd,Pt,Cu,Ag and Au）were constructed with a novel two-dimensional material:C₃N,as the support.By comparing the binding energy of TM single atom,the adsorption energy of formic acid,the hydrogen evolution performance,and the structural stability of the catalyst during the ab initio molecular dynamics simulation,three single-atom catalysts:Ni@C₃N,Pd@C₃N and Pt@C₃N,were finally selected.The adsorption and reaction mechanisms of formic acid decomposition in these three catalysts were calculated.By comparing the dehydrogenation and dehydration reaction through the HCOO and COOH pathways,it was proved that all of these catalysts own good selectivity to H₂ product.The analysis results of catalytic activity showed that,the energy span values of Ni@C₃N,Pd@C₃N and Pt@C₃N are 1.02,0.60 and 1.12 e V,respectively,which are all less than that of the Pd（111）surface（1.23 e V）,indicating that these three single-atom catalysts own excellent catalytic performance.In particular,Pd@C₃N is expected to become a potential alternative to traditional Pd-based nano-catalysts due to its excellent catalytic performance.3.By adsorbing 24 kinds of transition metal atoms（including the 3d,4d and 5d subgroup elements except the Cd,Hg,Os,Re,Tc and lanthanide elements）on two-dimensional BN surface,which contains B-defect,N-defect or B,N-defect（denoted as B_vN,BN_v and B_vN_v）,72 kinds of single-atom catalysts were obtained（TM@B_vN、TM@BN_v and TM@B_vN_v）.By applying the energy span approximating method which was proposed in this research,10 of 72 kinds of single-atom catalysts with potential good catalytic activity in hydrogen production from formic acid were screened out,which are:Pt@B_vN,Pd@B_vN,Zn@B_vN,Co@BN_v,Rh@BN_v,Ir@BN_v,Pd@BN_v,Pt@BN_v,Mn@B_vN_v and Ru@B_vN_v.The mechanism of formic acid decomposition on these 10 catalytic surfaces was studied,and the order of their catalytic activity was obtained by comparing their energetic span values.Comparison against the catalytic property of Pd（111）surface suggests that the selected 10 single-atom catalysts all have the better catalytic activity and selectivity.The results of this study also proved the effectiveness and feasibility of our proposed energy span approximating method in catalysts screening.