Preparation Of Single-atom Catalyst By The Supercritical Fluid Deposition And The Performance Evaluation

Single-atom catalyst has the advantages of a special coordination environment,high atom utilization,and highly uniform catalytic sites,which has become a research hotspot in the field of catalysis at recent years.Among the various methods for preparing the single-atom catalyst that have been reported in the literature,organic solvents were commonly used.Meanwhile,the preparation process was cumbersome and the conditions were relatively harsh.It was a new topic that was worth exploring if it was possible to prepare single-atom catalysts in green solvents and try to simplify the process.To this end,this paper attempted to prepare singleatom catalyst by supercritical fluid deposition(SCFD).And the single-atom catalyst was successfully prepared using supercritical carbon dioxide(sc CO2)as the solvent by selecting suitable supports and precursors and simply adjusting the system pressure,temperature,time,and co-solvent.The main work completed was as follows:(1)In order to measure the solubility of the precursor in sc CO2,a simple and practical method was developed.And on the basis that the solute was fully dissolved,the solubility was calculated by measuring the mass difference of the solute before and after the experiment instead of the complicated sampling and analysis method.Compared with the literature,this method was more economical,convenient,and efficient than the traditional method while the solubility data has good accuracy,stability,and repeatability.Taking the precursors used in this paper as the object,the solubility of Cu(acac)2,Co(acac)2 and Zn(acac)2 in sc CO2,as well as the solubility of Co Cl2 and Cu Cl2 in sc CO2 modified with a small amount of co-solvent DMF were measured under the conditions of 313-333 K and 11-24 MPa.The above solubility data was correlated with density-type empirical equations,and the results of which can be used to guide the preparation of the single-atom catalyst in the later stage.And the solubility also was a contribution to the basic thermodynamic data.(2)The Cu-NC single-atom catalyst was synthesized by SCFD using organometallic salt Cu(acac)2 as the precursor,self-made nitrogen-doped porous carbon(NC)as support,sc CO2 as solvent.The dissolution and diffusion of the precursor in sc CO2 and its adsorption and deposition on the support were observed through the window reactor,and the preparation mechanism was understood.Further,the Cu-NC single-atom catalyst was prepared in the reactor under the conditions of 20 MPa,50 °C,and 12 h.The atomic-level dispersion of Cu on the surface of the support was verified by characterizations such as XRD,SEM,STEM,and HAADF-STEM.And the actual loading of Cu measured by ICP was 0.8 wt%.The selective oxidation of benzyl alcohol to benzaldehyde was employed as the probe reaction to evaluate the catalytic performance of Cu-NC,and the 73.9% benzyl alcohol conversion and 85.9%benzaldehyde selectivity were obtained at 50 °C and 1 h.Based on the multiple possibilities of the coordination structure between Cu and N atoms,DFT calculation was used to assess the activation process of TBHP on the surface of various Cu-N configurations in order to explore the effect of the tert-butyl hydroperoxide(TBHP)activation on the oxidation of benzyl alcohol.It was found that the most suitable catalyst configuration for TBHP activation was Cu-N1C3.And the energy wells of the transition and final states reach 2.810 e V when TBHP was activated,which was beneficial to the stabilization of reactive oxygen species.(3)Single-atom catalyst was prepared by using inexpensive inorganic metal salts instead of organometallic salts as precursors.Inorganic metal salts were almost insoluble in sc CO2 while a small amount of organic solvent need to be added as the co-solvent.The effects of different co-solvents,deposition time,theoretical loading,and other factors on the preparation of Co-NC single-atom catalysts were investigated at 50 °C and 20 MPa using Co Cl2 as the precursor,the self-made NC material as the support,and sc CO2 as the solvent.It is found that nanoparticles were easily grown when Et OH was used as the co-solvent,and Co-NC singleatom catalyst could be successfully prepared by using DMF as the co-solvent.The excessive deposition time was not conducive to the dispersion of single atoms.Under the deposition conditions of 50 °C,20 MPa,and 12 h,the actual loading of Co measured by ICP reached 1.7wt%.The atomic dispersion of Co on the NC support was confirmed by SEM,STEM,XRD,HAADF-STEM,and other characterization methods.And the electronic structure of Co-NC was analyzed using XPS.The selective oxidation of benzyl alcohol to benzaldehyde was used as the probe reaction to evaluate the catalytic performance of Co-NC.And the 90.0% benzyl alcohol conversion and 86.0% benzaldehyde selectivity were obtained at 70 °C and 1 h.(4)Single-atom catalyst was prepared by coupling the SCFD method with the micropore confinement effect using the MOFs material ZIF-8 as the support.The Pd-ZIF-8 single-atom catalyst was successfully prepared using Pd(hfac)2 as the precursor and sc CO2 as the solvent under the conditions of 50 °C,20 MPa,12 h,and 0.5 wt% of theoretical Pd loading.The actual loading of Pd measured by ICP was 0.3 wt%.The characterization by SEM,STEM,XRD and HAADF-STEM proved that Pd was atomically dispersed,which provided a new way for the preparation of single-atom catalyst using ZIF-8 as the support.The catalytic performance of Pd-ZIF-8 single-atom catalyst was evaluated by hydrogenation of phenylacetylene to styrene as the probe reaction.Meanwhile,the conversion of phenylacetylene and the selectivity of styrene were 80.7% and 92.2%,respectively,when the reaction was carried out at 70 °C for 4h,showing high catalytic activity.