Syntheses,Characterizations And Catalytic Properties Of Atomically Dispersed Supported Metal Catalysts

Heterogeneous catalysis is the central enabling technology of fossil fuel conversion,chemical production,and vehicular and power plant emissions clean-up.Therefore,the development of efficient heterogeneous catalysts is an effective way to solve the energy and environmental problems.Many of the industrial heterogeneous catalysts are metals finely dispersed on porous high-area supports,such as metal oxides,zeolites,and carbon.The advantage of heterogeneous catalyst is that it can be easily separated and recycled by precipitation and other processes.Since the metals are usually present as zerovalent nanoparticles,the utilization of metal active sites is usually below 20%.To improve the utilization of metal active components,there is one of the important strategies that to reduce the size of active metal center.When the metal nanoparticles are made smaller to the atomic limit,they become supported cationic and take on new catalytic properties with 100%utilization in heterogeneous catalysis called atomically dispersed supported metal catalysts.Herein,from the perspectives of synthesis methods,structure characterizations,catalytic performances,DFT calculations,and in situ/quasi-situ techniques including in situ FTIR,in situ HERFD-XANES,quasi-situ XPS,quasi-situ XANES,and etc,we reported three kinds of atomically dispersed metal catalysts supported on different supports.We aim to investigate the catalytic mechanisms and the relationships between structure and catalytic performance of atomically dispersed supported metal catalysts deeply.The main contents are concluded as follows:1.We reported atomically dispersed Pt catalysts supported on MIL-101.The metal-ligand cooperativity in Pt1@MIL-101 was found to induce distinct reaction path and improve selective hydrogenation of CO2 into methanol.Such well-defined Pt-0 active sites on Pt1@MIL-101 could convert the H2 to Pt-OH species,directly.From further mechanisms studies,we found that H atoms in Pt-OH species converted CO2 to formate intermediates,then subsequent hydrogenation of formate intermediates into methanol.For the Pt nanoparticles supported on MIL-101,Pt-H are the main species from dissociation of H2,which could convert CO2 to carboxyl intermediates,followed by subsequent conversion of carboxyl intermediates into CO and CH4.2.We designed an atomically dispersed platinum catalyst stabilized by surface embedding in magnesium oxide and characterized the structure deeply by using multiple techniques including XAS,FTIR,STEM and etc.The Pt/MgO catalyzed CO oxidation at near 280℃ to get about 100%CO conversion.To further understand the stability and structural characteristics of the Pt/MgO catalysts,the in operando HERFDXANES experiments which avoided the distortion and broadening of the spectra due to low energy resolution and short core-hole lifetime were carried out in the process of CO oxidation at 210℃,and the CO2 was detected at the end of the reactor by mass spectrometry to confirm that the catalytic reaction was real.HERFD-XANES has the prospect of becoming one of the essential methods for characterizing atomically dispersed supported metal catalysts3.We reported single-site Ir(C2H4)2 catalysts supported in zeolite HY with different Ir loading amount.For ethene hydrogenation reactions,both the TOF and selectivity to ethane of Ir1/HY catalysts with low loading is higher than higher loading one.From the further studies by using in operando FTIR we found the synergic interaction between supported metal centers and zeolite surface sites for ethene hydrogenation,the activated H has spill overed from Ir species to the surface of bare zeolite HY and therefore the hydrogenation reaction prevailed.On the basis of the results,we infer that In-containing samples included two types of actives sites that performed paralleled reaction paths:(1)the In centers catalyzed both ethene hydrogenation and dimerization,and(2)the Br?nsted acid sites only catalyzed ethene hydrogenation.Thus,when the Ir loading—the Ir/Al atomic ratio—decreased,the contribution from the Br?nsted acid sites became more predominant.