Single Molecule Method for the Study of Catalytic Reaction Kinetics and Thermodynamics
Author:Liu Xiao Dong
Supervisor:xu wei lin
Because of their broad applications in industrial catalysts,nanocatalysts have attracted considerable attention.However,nanocatalysts exhibit structural and functional heterogeneity among individual nanoparticles,thus requiring a capability to study single nanoparticle.Moreover,because of nanometer dimension and constant adsorbate-surface interactions under reaction conditions,they are structurally dynamic.While traditional meansure methods often provide static images of their chemical composition,morphology and structure.Real-time imaging the chemical activity of single nanoparticle is highly desirable for exploring the structure-activity relationship,further to understand reaction mechanisms and kinetics,and to identify a minority subpopulation with extraordinary activity.Here,the single molecule method is used to reveal the structure-activity relationship in temporal resolution.(1)Due to the inhomogeneous structure of nanoparticles,many underlying catalytic details of these catalysts are hidden in the ensemble-averaged measurements.The single-molecule approach enables studying the catalytic behavior of nanoparticles at single-particle level in single-turnover resolution.Here,based on such method,we studied the catalytic behaviors of individual Pt nanoparticles to reveal the catalytic properties of nanoparticles of the product formation and desorption process.It was found that the catalytic reaction on Pt nanoparticles followed competitive mechanism in product formation process,while the product desorption process showed no selectivity between the indirect and direct desorption pathways.Moreover,the dynamic heterogeneity of Pt nanoparticles in product formation and desorption process was revealed to be due to the catalysis-induced surface restructuring.Surprisingly,it was found both experimentally and theoretically that the tiny difference of substrate molecules could lead to huge difference of surface restructuring even on the same type of nanoparticles.(2)Due to the intrinsic heterogeneity of nanocatalysis,many underlying catalytic details on the nanocatalysts are hidden in ensemble-averaged measurements.Here,the single-molecule approach was adopted to study the temperature-dependent catalytic kinetics and dynamics of individual Pt nanoparticles and then reveal the thermodynamics of individual catalytic steps on Pt nanoparticles.In this way,the temperature-dependent catalytic kinetics(the effective rate constant of the product formation process,the rate constants of the direct/indirect production desorption process and the substrate adsorption equilibrium constants)and thermodynamics(free energy,entropy and enthalpy of substrate adsorption)were obtained systematically at single particle level.Based on such results,we further obtained the activation energies of the catalytic product formation step and the direct/indirect product desorption steps.Moreover,by analyzing the temperature-dependent surface restructuring rates of individual Pt nanocatalysts,the activation energies of both the catalysis-induced surface restructuring and the spontaneous surface restructuring were obtained for the first time.All these results obtained here deepen our understanding to the catalytic thermodynamics of nanocatalysts.(3)Understanding of the thermodynamic properties of elementary chemical steps of a reaction is important for the development of fundamental reaction theories and for effective industrial practice.In this work,temperature-variable single molecule fluorescence microscopy was employed to study a reversible redox chemical process and reveal the thermodynamics of chemical elementary reactions at single molecule level.Activation energies of pure elementary steps were measured on the level of single molecules and found to be heterogeneously distributed across the population of individual molecules.The activation parameters measured across the population of individual molecules also exhibited a compensation effect and an isokinetic relationship.These results constitute a new single-molecule-level perspective into a chemical reaction.(4)For the atomical dispersed Platinum,it exhibits high catalytic performances,such as high reactivity and selectivity,due to the high dispersion and interaction with the supports.For the single-atom Pt catalyst,the catalytic performance has been studied by various catalytic reactions.However,these catalytic reactions merely reflect the ensemble-averaged performance from macroscopical view,which cannot show the details in catalytic reaction.Here,the catalytic behaviors and the differences in activity for the atomical dispersed Pt were studied at a micro level by the single-molecule approach.Further,we found that the catalytic reaction on atomical dispersed Pt@CeO2 still followed the model of climb up and then decline with the increasing of the substrate concentration in product formation process,which was the same as Pt nanoparticle;while the product desorption process showed fluctuation with the increase of substrate concentration.Meanwhile,the dynamic activity fluctuation of atomical dispersed Pt@CeO2 in product formation and desorption processes revealed that they didnot change with the rate of turnover,which indicated that the activity fluactuation was largely spontaneous effect while the catalysis-induced effect was minimal here.It relates to the supporter due to the special geometric and electronic properties of CeO2 which can maintain the stability of the Pt atom.