Operando Nuclear Magnetic Resonance Study On Photocatalytic Reaction Mechanisms And Design Of Novel Photocatalysts

In the past half century,the natural ecosystem has suffered a series of irreversible damage due to the sharp increase of fossil consumption in human society,such as the greenhouse effect and global extreme climate.As an inexhaustible renewable energy,solar energy can be converted into chemical and electrical energy through modified semiconductors.After years of research,many potential breakthroughs have been achieved in the screening and controllable preparation for photocatalysts.However,the research on the structure-activity relationship and dynamic mechanism of the photocatalytic reactions has still lacked the theoretical guidences and in-situ characterizations,caused by the complex solid-liquid reaction environments.Nuclear magnetic resonance（NMR）technology is well known for its powerful analytical ability and technological diversity,which is widely used in materials,catalysis,batteries,petroleum,medicine and other fields.In particular,in the field of catalysis,NMR technology can not only study the three-dimensional structure and electronic properties of catalysts,but also explore the reaction kinetics and monitor the chemical reaction path at the molecular level.In this work,the advanced NMR technology is used to systematically study the dynamic diffusion,the thermodynamic reaction path and the structure-activity relationship in the heterogeneous photocatalytic systems,and the thermodynamic unity of the alcohol reforming reactions,as follows:（1）In the 2nd chapter,operando photocatalytic NMR technology was applied to prove how the cooperative movement originated from the hydrogen-bond network dominateed photocatalytic efficiency in the solid-liquid system,and supplemented the key kinetic process of solute molecules breaking through the solvent-solute network and diffusing to the active site of the catalyst surface.Through 10 Rutile-Ti O₂photocatalytic systems with different molar ratios of water and methanol,it is found that the CH₃OH or H₂O molecules in the solution can diffuse to the catalyst surface in the form of CH₃OH-H₂O clusters,and then adsorbed on the active sites when the photoreforming reaction occurs in the solid-liquid system.In this dynamic equilibrium between diffusion and adsorption processes,a stronger hydrogen bond of CH₃OH-H₂O clusters indicated a smaller number of methanol molecules reaching the catalytic surface per unit time,which will affect the adsorption and dissociation rate of methanol molecules at the active sites of the catalyst.The observations in this chapter deepen our understanding for the critical role of hydrogen bonding in the heterogeneous photocatalytic reactions.（2）In the 3rd chapter,the control effect of solvent water in the photocatalytic thermodynamic mechanism of CH₃OH,H₂O and Pt/Ti O₂ system was furthrt evaluated by operando NMR technology and kinetic simulations.The experimental results indicated that solvent water can not only affect the product selectivity on the catalyst surface,but also inhibit the H₂ evolution originating from methanol photoreforming.On the one hand,H₂O molecules can significantly reduce the C-H activation barrier of-CH₃O in the form of-OH configuration,thus accelerating the formation of oxidation products such as HOCH₂OH and CH₃OCH₂OH;On the other hand,H₂O molecules directly compete for surface active sites with CH₃OH molecules,significantly inhibiting the hydrogen production process of methanol,switching the proton source of hydrogen from CH₃OH to H₂O molecules.（3）In the 4th chapter,we continue to use operando NMR technology to develop the photocatalytic reactions from methanol systemes in Chapter 2 and 3 to ethanol system.As an important chemical and biomass material,the photoreforming process of ethanol has similar properties compared to methanol photoreforming.Thus,the classic Pt/C₃N₄ photocatalyst was selected to reveal the structure-activity relationship between the active sites and the selectivities of alcohol reforming by constructing different Pt nanoparticles（NP）sizes and crystal facet.The operando NMR results showed that small Pt nanoparticles without discernible crystal facet tended to produce reforming products with low degree of polymerization（such as aldehydes,acetals and acids）,while relatively large nanoparticles with obvious Pt（111）crystal facet tended to produce reforming products with higher degree of polymerization（such as acetals,ethers,and esters）.The operando observations demonstrated that the sizes and crystal facet of the supported NPs have a thermodynamic unity on the methanol and ethanol reforming paths,which may be generally applicable to other kinds of alcohols（CH₃（CH₂）_nOH（n>1））reforming reactions.（4）Based on the above kinetic and thermodynamic studies of interfacial reactions,a two-component synergetic photocatalyst(Co _SAs/Pt Co@CNN)was successfully designed.The photocatalyst contained two synergetic active sites of Co single atoms and Pt Co alloys,which was supported on the g-C₃N₄ nanosheets.Through the C-H correlation of two-dimensional NMR experiments,it can be proved that the CN₂（NH_x）structure,as the photocatalytic active site,has extensive proton exchange with the ²H atom of D₂O during the deuterization reaction of g-C₃N₄ nanosheets,and its exchange degree can be used as the evaluation of phtocatalytic activity.In the process of overall water splitting（OWS）reaction,Co _SAs acted as the active centers of hydrogen evolution reaction（HER）,and Pt Co alloys acted as the active centers of oxygen evolution reaction（OER）.This two-component synergetic photocatalyst has the maximum utilization of noble metal atoms,which can clearly reveal the structure-activity relationship between the active sites and the redox reactions in OWS process,and provides a promising model for the further development of monoatomic catalysts.