Design Of Transition Metal Hydroxide Single-atom Catalyst And Theoretical Study On Its Efficient Catalytic CO Oxidation Mechanism

With the depletion of global energy and the deterioration of the ecological environment,it is particularly important to find alternative clean energy.As an ideal clean energy,hydrogen has the characteristics of high energy density and zero emissions.Hydrogen fuel cell vehicles are fueled by hydrogen,which has the characteristics of high energy conversion efficiency,clean and zero emissions,and is one of the main development directions of new energy power vehicles in the future.However,a key problem in the promotion of hydrogen fuel cell vehicles is catalyst poisoning by impurity gases such as carbon monoxide,resulting in reduced battery performance and shorter life.Preferential oxidation of CO in hydrogen(PROX)is a possible solution to remove trace CO from hydrogen in vehicles.However,the operating temperature of the existing PROX catalysts is relatively high(above room temperature)and the catalytic interval is narrow,which is difficult to provide effective protection for the frequent cold start of hydrogen fuel cells under cold conditions.The single-atom catalysts has excellent catalytic activity for the oxidation reaction of CO,and it has the characteristics of high selectivity,high atomic utilization rate and tunable high activity.Recently,Cao et al.demonstrated the successful fabrication of single-atom catalysts Fe1(OH)x-Pt on silica supported Pt nanoparticles with high activity and stability by taking advantage of the self-limiting growth and steric hindrance processes inherent to atomic layer deposition.The resulting Fe1(OH)x-Pt catalyst exhibits exceptional activity and selectivity for preferential oxidation of CO in hydrogen ambient over an ultra-wide temperature range of-75℃ to 110℃.However,the specific catalytic mechanism is not well understood at the atomic scale,and the relationship between the structure and catalytic activity of single-atom catalysts has important guiding significance for the design of new catalysts in the future.In this thesis,based on density functional theory,the structure-activity relationship of Fe1(OH)x-Pt catalyzing CO oxidation with high activity was explored,and its catalytic mechanism was further analyzed and extended to Fe1(OH)x-Au and Fe1(OH)x-Ag systems.At the same time,the catalytic mechanism and catalytic activity of other transition metal hydroxide single-atom catalysts for CO oxidation reaction were studied.The research content includes the following five parts:1.Via first-principles calculations,we have discovered two key factors that can explain the extraordinary catalytic performance of Fe1(OH)x-Pt in the experiment:(ⅰ)formation of hydrogen bonds in the rate-determining step of catalysis and(ⅱ)moderate adsorption strength of CO on the surface.The former factor facilitates the reaction by stabilizing the intermediates,while the latter makes the adsorbed reactant more reactive to the catalyst.Interestingly,the coverage of single-atom catalysts on the Pt nanoparticles achieved using atomic layer deposition techniques is the optimal one that favors both factors.Guided by these findings,we have systematically explored the Fe1(OH)3 on different metal supports for CO oxidation.Density functional theory suggests that compared with the experimental catalyst on Pt(100),Fei(OH)3 on Au(100)or Ag(100)shows an equal or even higher activity.These results not only predict alternative highly active catalysts for CO oxidation but also provide design principles for novel and efficient single-atom catalysts for other reactions.2.According to the conclusion of the first part,the catalytic performance of Fei(OH)3 supported on Au(100)and Ag(100)surfaces shows an equal or even higher activity than that of Pt(100).In this chapter,we delve into the complete reaction mechanism of CO oxidation catalyzed by Fe1(OH)3/Au(100)and Fe1(OH)3/Ag(100).The results show that it follows a partially different mechanism compared with Fe1(OH)3/Pt(100)catalyzed CO oxidation.In addition,compared with Pt(100),the catalyst formed by Fe1(OH)3 on Au(100)has double the CO adsorption active site.The results of microkinetic modeling showed that the catalytic activity of Fe1(OH)3 loading on Au(100)was better than that of Pt(100)and Ag(100).We wish that our findings would inspire future experimental and theoretical work in related fields.3.We have found that there exists an energetically more favorable mechanism for CO oxidation over Fe1(OH)3/Pt(100)by considering different possible pathways involving both the Langmuir-Hinshelwood and Eley-Rideal mechanisms,as well as the influence of preadsorption of gaseous reactants.The remarkable catalytic performance of Fe1(OH)3-Pt was further revealed.The rate-determining free energy barrier in the new mechanism is only 0.42 eV,about 40%smaller than that(0.68 eV)in the previous mechanism.According to the results of microkinetic modeling,the apparent activation energy derived from the new mechanism is 0.18 eV,much lower than that(0.63 eV)from the previous mechanism.Our findings may provide a more comprehensive understanding of the experimental observations and valuable insights into the rational design of highly efficient single-atom catalysts.4.The Fe1(OH)3-Pt(111)interface site is also an important reactive site in the CO preferential oxidation reaction.In this chapter,we explore the mechanism by which the singleatom catalyst Fe1(OH)3 catalyzes CO oxidation on Pt(111),taking into account the coverage of CO on the surface of the support.The results of competitive adsorption of reactants CO and O2 on the support and catalyst Fe1(OH)3 showed that the adsorption strength of CO was significantly stronger than O2 on the support,while O2 was more competitive than CO on catalyst Fe1(OH)3.By using the optimized catalyst model and taking into account the competitive adsorption of CO and O2 on the catalyst,we explored all possible reaction pathways and proposed a new reaction mechanism,which has a Gibbs free energy barrier of the rate-determining step of only 0.28 eV,which is significantly lower than what we have previously obtained without model optimization(0.43 eV).The analysis of the key factors affecting catalytic activity shows that CO coverage in the Pt(111)surface can effectively reduce the energy barrier of COOH*formation process,while this effect does not exist in the Pt(100)surface.5.We consider other single-atom catalysts formed by transition metal on Pt nanoparticles,Ti1(OH)3/Pt(100)、V1(OH)3/Pt(100)、Cr1(OH)3/Pt(100)、Zr1(OH)3/Pt(100)、Co1(OH)3/Pt(100)、Ni1(OH)3/Pt(100)、Mn1(OH)3/Pt(100)和 Ru1(OH)3/Pt(100),the catalyst structure is thermodynamically and kinetically stable.Then,we screened the catalysts based on two key factors that can explain the extraordinary catalytic performance of single-atom catalysts proposed in Chapter 3 three.In the reaction of CO oxidation,the best catalytic performance is Ru1(OH)3/Pt(100),Mn1(OH)3/Pt(100),and Cr1(OH)3/Pt(100),and the corresponding energy barriers of the rate-determining step were 0.38 eV,0.49 eV,and 0.62 eV,respectively.Moreover,the adsorption strength of CO in such catalysts can be used as a simple descriptor of catalytic activity,that is,the weaker the adsorption strength of CO on the surface of the catalyst,the higher the catalytic activity,and the hydrogen bond has no significant role.The results of microkinetic model simulations show that the apparent activation energy of Mn1(OH)3/Pt(100)and Cr1(OH)3/Pt(100)is 0.21 eV and 0.47 eV,respectively,which is significantly smaller than that of Ru1(OH)3/Pt(100)(0.64 eV),and the CO turnover frequency of the former at room temperature is approximately 1-2 orders of magnitude higher than the latter.In summary,among the designed catalyst structures,the catalytic performance of Mn1(OH)3/Pt(100)and Cr1(OH)3/Pt(100)for CO selective oxidation reaction is the best.It is believed that the designed catalyst will provide a theoretical reference for the development of highly active metal catalysts in the field of heterogeneous catalysis.