Construction Of Metal-based Nano/single-atom Catalysts For The Hydrogen Evolution From Formic Acid And The Conversion Of By-product Carbon Dioxide

Hydrogen has emerged as an environmentally attractive fuel and a promising energy carrier for future applications to meet the ever-increasing energy challeges.The safe and efficient storage and release of hydrogen is crucial for realizing the upcoming hydrogen economy.Formic acid(HCOOH)processes a high hydrogen content(4.4 wt%),which is considered as one of promising liquid carriers for the protable hydrogen storage application.However,there are still some problems in the catalytic hydrogen evolution from HCOOH(HCOOH?H₂+CO₂).For instance,the hydrogen evolution catalyst is mainly the precious metal Pd,the Pd-based catalysts containing non-noble metals exhibit the low activity at room temperature,the non-noble metal catalyst shows no activity,and CO₂ as a by-product is generated during the HCOOH decomposition.To solve these problems,in this thesis,we have designed and synthesized a series of supported Pd-based photocatalysts and coordinated non-noble metal single-atom catalysts on the basis of the key factors that restrict the efficient hydrogen evolution from HCOOH and the mitigation of carbon emission.The performance of these catalysts in catalyzing the hydrogen evolution from HCOOH and the CO₂ conversion as well as the relationship between the structures and catalytic performance of as-synthesized catalysts has been systematically studied.The main results are as follows:(1)From the perspective of adjusting the adsorption of substrate molecules and the electron density of the active metal center in the supported metal photocatalyst,a series of carbon nitrides(C₃N₄,named DCN)with rich nitrogen defects and adjustable band structures have been obtained through the plasma modification.Then a series of supported photocatalysts Au Pd M/DCN-APTS(M=Co,Ni)have been synthesized,where Au Pd M nanoparticles(NPs)are immobilized on plasma-induced carbon nitrides grafted by aminosilanes.The characterization results show that the as-synthesized carbon nitrides with nitrogen defects not only exhibit a reduced band gap,improved light absorption capacity and carrier separation efficiency,but also transfer more photoelectrons to the trimetallic NPs through the Schottky junction.This remarkably leads to the enriched electron density of metal NPs.In addition,the adsorption of HCOOH has been promoted with introduction of aminosilane in the catalyst.The optimal catalyst exhibits the record room-temperature hydrogen evolution activity with the total TOF of 1075 h^-1 and possesses 66.9%of apparent quantum yield at 420 nm.Meanwhile,the catalyst has the high resistance to CO.(2)From the perspective of constructing new active sites of catalyst,a series of metal-organic framework(MOF)-based precursors are synthesized by the metal ion spatial separation strategy,the metal ion spatial confinement strategy and the metal ion spatial separation combined with the hard template strategy.The single-atom catalysts,non-noble metals(V,Cr,Mn,Fe,Co,Ni and Cu)coordinated with nitrogen in porous nitrogen-doped carbon matrix,have been constructed through the high-temperature pyrolysis process.These single-atom catalysts can enable the hydrogen evolution from HCOOH efficiently proceed under the photothermal condition.The coordination structures of metal with nitrogen in the single-atom catalysts,such as two-,three-,and four-coordination,have been regulated through controlling the calcination temperature and the type of metals.The catalytic results show that the single-atom-type catalyst breaks the limitation of activity free for the hydrogen evolution from HCOOH over the non-noble metal catalysts.Among the as-synthesized catalysts,the Co single atom catalyst obtained through the space separation strategy exhibits the highest catalytic activity with the total TOF of 501 h^-1.In addition,the coordination structures of the metal with nitrogen influence the activity of catalysts.(3)From the perspective of mitigating the carbon emission and converting CO₂ under mild condition,the by-product CO₂ produced in the HCOOH decomposition process is converted into CO by the synergism of plasma and catalyst.The noble metal nanocatalysts and the non-noble metal single-atom catalysts used in the HCOOH dehydrogenation are applied as catalysts for the CO₂ conversion.The results show that this strategy can efficiently convert CO₂ into value-added CO.The activity of noble metal nanocatalyst is low,while the non-noble metal single-atom catalysts exhibit the high catalytic performance.The optimal Cu-based single atom catalyst possesses 499 umol of CO output with the help of plasma.In addition,the optical emission spectra(OES)results show that the inert CO₂ can be activated by plasma.The XPS results show that the coordination structures of metal with nitrogen are the active sites for the CO₂ conversion by the synergism of plasma and catalyst.