Mechanism And Regulation Of The Catalysis Of Small-Molecules Activation On Iron Carbides And Metal Single Atoms

The activation and transformation of small-molecules,such as CO,O2 and H2O,is an important issue in the field of energy and catalysis.The key to realize the rational design of the metal catalyzed-small molecules activation is understanding its reaction mechanism and regulating effect.Metal nanoparticles are one of the most widely studied catalysts for the small molecule activation.Combined with carbon or reducing the size,the metal nanoparticles can form metal carbides or metal single atoms,which will show significantly different physicochemical properties and thus exhibit excellent catalytic performance on certain small-molecule activation.In-depth study of the metal carbides and single atoms-catalyzed small-molecules activation and transformation to reveal its reaction mechanism and regulating effect can guide the catalyst rational design.This dissertation aims to approach the activation of the coal-derived syngas over the iron carbide catalysts and the electrocatalytic activation of O2 and H2O over the single metal atoms by a combination of DFT calculations with experimentations.The reaction mechanism,the active site characteristic as well as the activity descriptor of the iron carbides and metal single atoms-catalyzed small-molecules activation are well elucidated.The main results are summarized as follows:(1)Compared to the traditional monometallic Fischer-Tropsch synthesis(FTS)catalyst,the ?-Fe5C2 catalyst exhibits remarkably different CO activation properties.On this catalyst,the B5 site is not the active site and some terrace-like surfaces show lower overall CO activation barrier compared to the step-like x-Fe5C2 surfaces.The direct CO activation is suggested as the preferred CO activation pathway with high activity.Furthermore,the spatially resolved atomic charge of the involved surface Fe atoms for the CO activation is discriminated as a good descriptor for the facet-dependent CO activation.This simple and efficient descriptor can be extended to quantitatively describe the CO activation on the?-Fe5C2 catalyst with more complex surface properties involving K promoter,non-stoichiometric termination and/or carbon vacancy.(2)The methane formation mechanism is proposed on the basis of DFT calculations and steady-state isotopic transient kinetic analysis(SSITKA).The DFT calculations reveal the role of the carbide and the dissociated carbon.Specifically,the dissociated carbon has lower effective barrier of methane formationis and thus be more favorable for the methane formation compared to the carbide carbon.Meanwhile,the effective barrier of methane formation on the terraced ?-Fe5C2 surfaces is lower than that on the stepped surfaces,indicating the suppression of methane formation on the terraced ?-Fe5C2 surfaces.Furthermore,the SSITKA is performed on both the Co and Fe-based FTS catalysts.The results show that the time for the isotopic replacement of the 12C to 13C in CH4 and C3H6 is similar on the Co-based catalysts while different,i.e.,much time needed for the replacement of the 12C to 13C in C3H6 than that in CE4,on the Fe-based catalysts during the switch between H2/12C0/Ar and H2/13CO/Kr.These results indicate that the carbide carbon is more favorable for the C-C coupling to form C2+ products and the dissociated carbon is more favorable for the methane formation.(3)Considering the involvement of the carbide carbon atoms in the lower olefins formation,we further employed DFT calculations to elucidate the reaction mechanism of lower olefins formation on the ?-Fe5C2 catalyst.The results suggest that for the C2 and C3 species hydrogenation,a good transition state scaling relationship exists between the stability of transition and initial states.The subsequent exploration of the chain termination mechanism shows that the ?-Fe5C2 catalyst is unfavorable for the formation of lower olefins and the K and Mn promoters can enhance the formation of lower olefins.Furthermore,the novel Fe/MnxKy-CNTs nonocomposite catalysts are employed as model catalysts to study the promotion effects of Fe-based FTS catalysts.It is found that the increase in Mn and K loadings shows the enhanced lower olefins selectivity but suppressed activity and the further increasing K promoter content leads to dramatically suppressed CH4 formation and enhanced chain growth.(4)The stability of the carbon-supported Fe,N and P tri-doped catalysts as well as its oxygen reduction reaction(ORR)mechanism and regulating effect are well elucidated.It is found that the proper O coordination can enhance the stability of the models.Specifically,the models with the Fe-O-P bond are more stable than that with only isolated Fe/P-O bonds.Among all these models,the OH*desorption is the rate-determining step of the whole ORR process and the model with both Fe-O-P and Fe-O bonds has the lowerst theoretical overpotential.Interestingly,the Fe-O-P bond would be broken to form the isolated P-O bond during the ORR process and then the Fe-O-P bond would be restored when an ORR catalytic cycle finished.During this dynamic redox process,the removal of OH*is facilitated by Fe-O-P cycle,which leads to better ORR activity.The further electronic structure analysis reveals that the charge of the Fe single atom is discriminated as a good descriptor for the ORR on the Fe,N and P tri-doped catalysts.(5)Based on the above-mentioned carbon-supported Fe,N and P tri-doped catalysts model,we constructed the corresponding models with different transiton metal centres.The ORR activity on these models follow the order of Cu-NPC>Fe-NPC>Co-NPC>Mn-NPC>Ni-NPC>Cr-NPC,which is almost in accordance to the experimental results.In addition,the O2 adsorption energies calculated from the DFT is linealy correlated with the half-wave potential of the M-NPCs catalysts from the electrochemical measurements,which is originated from the 1St ionization energy of different metal centers.(6)The DFT calculations are performed to eluciadate the regulating effect of the noble metal(Pt and Ru)single atoms-catalyzed hydrogen evolution reaction(HER).Compared to the N-doped carbon supported Pt single atoms(Pt SAs)catalysts,the defect-rich carbon supported Pt SAs catalysts are better HER catalysts,in which the Pt SAs is not only strongly anchored on the supports but also exhibits close to zero Gibbs free energy of H*(AGH*)to boost the HER performance.For the less expensive Ru SAs HER catalysts,the phosphorus nitride imide nanotubes(HPN)supported Ru SAs catalyst is suggested as a good HER catalyst among several traditional supports,such as carbon paper and C3N4,supported Ru SAs catalysts.Specifically,the HPN can anchor the Ru SAs strongly and the resultant cataysts show close to zero ?GH*.The different stability of the single atoms and the HER performance between Ru and Pt SAs supported on different supports can be attributed to the electronic structure of the Ru and Pt SAs coordinated with different supports.