Controlled Synthesis And Catalytic Study Of Thermally Stable Single-Atom Catalysts

Supported metal catalysts can efficiently accomplish many important industrial applications in catalysis including the production of chemicals,pharmaceuticals and clean fuels,and the remediation of environment.However,most of the supported metal nanoparticles(NPs)suffer from sintering,including Ostwald ripening,particles migration and coalescence.This process significantly increases their size and decreases their dispersion and active surface areas,resulting in the severely deactivation.The regeneration and recovery of deactivated catalyst will greatly increase the time and economic cost and affect the industrial benefits.In addition,with the continuous optimization of industrial production,there is a growing desire to improve the atomic utilization of supported metal catalysts.Thus,it is of great significance to develop a sintering-resistant and thermally stable metal catalysts with atomically dispersion to further cut down the costs for the catalyst production and recovery.In this thesis,we explored a series of controlled synthesis methods for thermally stable single-atom catalysts.We found that metal nanoparticles will gradually lose their surface atoms in a special interface and high temperature environment.Subsequently,we can cleverly anchor these moving atoms by the strong interaction between support defects and metal atoms,finally obtaining a highly stable single-atom metal catalyst.These methods are versatile,and can even convert severely deactivated and agglomerated NPs catalysts into highly active and thermally stable atomic metal catalysts,showing a broad application prospects in industry.Additionally,we have prepared a series of thermally stable and highly accessible single-atom catalysts through precise controlling of metal atom coordination and the structure of support.These works further expand the controllable synthesis method of single-atom catalysts and enhance the catalytic effectiveness of single-atom sites.Herein,the formation mechanism and structure-activity relationships are deeply studied and discussed.The main contents are summarized as follows:1.The surface digging effect of Ni nanoparticles on amorphous nitrogen-doped carbon can effectively suppress the sintering and agglomeration of Ni particles,and gradually convert them into thermally stable Ni single atoms in the self-excavated nanopores at high temperatures.In-situ transmission electron microscopy observations and theoretical calculations reveal that the surface digging effect of Ni particles and the strong coordination of Ni atoms and N defects are critical to the successful conversion of Ni particles to single atoms.Compared with Ni nanoparticles catalysts,Ni single-atom catalysts exhibit higher methane oxidation activity and selectivity to phenol at low temperatures.2.We have developed a nitrogen-doped carbon thermal atomization strategy that can efficiently convert Pd/Pt/Au nanoparticles supported on TiO2 into more active atomic-scale metals.This strategy was used to treat severely deactivated and sintered catalysts,which can reduce the size of the supported nanoparticles and recover the catalytic activity.Spherical aberration electron microscopy,in-situ electron microscopy,X-ray absorption spectroscopy,and theoretical calculations all reveal that the nitrogen-doped carbon is an effective diffusion channel for metal atoms,and the defects on surface of TiO2 created by carbon reduction is the key to the further stabilization of metal atoms.3.Due to the above synthesis methods can only prepare several single-atom catalysts,we further explored a cation-exchange strategy to achieve the large-scale preparation of a series of S and N co-decorated single-metal sites catalysts.By rationally designing the precipitation and dissolution balance of the cation exchange reaction,we can exchange guest metals such as Cu,Pt and Pd at atom-level or nanoscale on the anion framework of CdS.At the same time,we relied on the S2-framework of CdS and the N-rich polymer shell to simultaneously generate S and N defects to anchor the exchanged cationic metals during high temperature annealing.Both benzene oxidation tests and theoretical calculations show the edge-selectively sulfidation of the metal site is beneficial to the dissociation of reaction intermediates and water desorption,thus the catalytic activity of the S and N co-decorated metal site is much higher than that of the single N-modified metal site.4.Considering the enhancement of the effectiveness of single-atom sites,a negative pressure pyrolysis strategy was further developed to realize the controllable preparation of single-atom catalysts,with high stability and high accessibility supported on a three-dimensional graphene framework.In a high-temperature negative pressure environment,the pressure difference between the inside and outside of the metal-organic framework material can promote the gradual expansion and splitting of the derived N-doped carbon.Meanwhile,the metal nodes are reduced by the derived carbon.Finally,these metals were atomically dispersed on a three-dimensional graphene framework with high specific surface area and mesoporous porosity.Oxygen reduction tests and theoretical calculations indicate that the carbon defects caused by the cleavage of N-doped carbon can effectively lower the reaction energy barrier of the oxygen reduction for Co single-atom sites.Thus,the prepared Co single-atom catalyst exhibited a superior catalytic activity to those of commercial Pt/C and Co single-atom catalysts obtained by traditional methods.