Regulating Catalysis Of Pd Nanoparticles In UiO-66 Structures By Pore Wall Engineering

Metal nanoparticles(MNPs)which are the classic heterogeneous catalytic active center,have broad applications in the catalytic field.Although the chemical environment of metal nanoparticles possesses significant influence on their catalytic performance.Due to the limitation of precise tailoring of the structure of traditional heterogeneous catalysis,there's still a big challenge in the related research.In order to maintain MNPs' sustainable and lasting good performance in the catalytic reaction,we can introduce traditional supports,for example,metal oxides,mesoporous silica and so on or surfactants to protect MNPs from aggregation.However,the aforementioned supports and surfactants could also bring new problems to the system.The introduced surfactants will closely cover the surface of MNPs,which will fail the active sites of MNPs to be exploited.While by incorporating MNPs on the traditional supports such as metal oxide,it has negative effects on the MNPs' stability and the regulation of their selectivity due to the limited interface between the MNPs and the supports.To address these problems,the metal-organic framework(MOFs)was adopted as the host of MNPs as the solution.In this work,tiny Pd NPs have been encapsulated into the pore space of metal-organic frameworks(MOFs),UiO-66-X(X=H,OMe,NH2,2OH,2OH(Hf)),affording Pd@UiO-66-X composites.The surface microenvironment of Pd NPs is readily modulated by pore wall engineering,via the functional group and metal substitution in the MOFs.Consequently,the catalytic activity of Pd@UiO-66-X follows the order of Pd@UiO-66-OH>Pd@UiO-66-2OH(Hf)>Pd@UiO-66-NH2>Pd@UiO-66-OMe>Pd@UiO-66-H toward the hydrogenation of benzoic acid.It is found that the activity difference is not only ascribed to the distinct charge transfer between Pd and the MOF,but also explained by the discriminated substrate adsorption energy of Pd@UiO-66-X(-OH<-2OH(Hf)<-NH2<-OMe<-H),based on the CO-DRIFT spectra and DFT calculations.The Pd@UiO-66-OH possessing a high Pd electronic state and moderate adsorption energy displays the highest activity.