Gold（Au） nanoclusters have recently emerged as ideal models for understanding Au catalysis, because the nanosized Au particles have precise atomic numbers and uniform size. In this work, we studied for the first tim...Gold（Au） nanoclusters have recently emerged as ideal models for understanding Au catalysis, because the nanosized Au particles have precise atomic numbers and uniform size. In this work, we studied for the first time the support shape effect on the catalysis of Au nanoclusters by using CO oxidation as a model reaction. Au22（L8）6（L = 1,8-bis（diphenylphosphino） octane） nanoclusters were supported on CeO2 rods or cubes, then pretreated at different temperatures（up to 673 K）, allowing the gradual removal of the organic phosphine ligands. CO oxidation test over these differently pretreated samples shows that CeO2 rods are much better supports than cubes for Au22 nanoclusters in enhancing the reaction rate. In situ IR spectroscopy coupled with CO adsorption indicates that the shape of CeO2 support can impact the nature and quantity of exposed Au sites, as well as the efficiency of organic ligand removal. Although CeO2 rods are helpful in exposing a greater percentage of total Au sites upon ligands removal, the percentage of active Au sites（denoted by Au d＋, 0 〈 d 〈 1） is lower than that on CeO2 cubes. The in situ extended X-ray absorption spectroscopy（EXAFS） and high-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM） results show that the Au nanoclusters bound more strongly to the CeO2 rods than to the cubes where the Au nanoclusters show more sintering. Considering the typical redox mechanism for CO oxidation over supported Au nanoclusters and nanoparticles, it is concluded that the reactivity of the lattice oxygen of CeO2 is the determining factor for CO oxidation over Au22/CeO2. CeO2 rods offer more reactive lattice oxygen and abundant oxygen vacancies than the cubes and thus make the rods a superior support for Au nanoclusters in catalyzing low temperature CO oxidation.展开更多
This paper presents a systematic study of the growth mechanism for Pd nanobars synthesized by reducing Na_(2)PdCl_(4) with L-ascorbic acid in an aqueous solution in the presence of bromide ions as a capping agent.Tran...This paper presents a systematic study of the growth mechanism for Pd nanobars synthesized by reducing Na_(2)PdCl_(4) with L-ascorbic acid in an aqueous solution in the presence of bromide ions as a capping agent.Transmission electron microscopy(TEM)and high-resolution TEM analyses revealed that the growth at early stages of the synthesis was dominated by particle coalescence,followed by shape focusing via recrystallization and further growth via atomic addition.We also investigated the detailed surface structure of the nanobars using aberration-corrected scanning TEM and found that the exposed{100}surfaces contained several types of defects such as an adatom island,a vacancy pit,and atomic steps.Upon thermal annealing,the nanobars evolved into a more thermodynamically favored shape with enhanced truncation at the corners.展开更多
基金supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Divisionsupported by the U.S. Department of Energy, Office of Science, Office of Basic Energy, Sciences under Contract No. DE-AC02-76SF00515the facilities support at the beamline BL 2-2 provided by the Synchrotron Catalysis Consortium U.S. DOE (No. De-SC0012335)
文摘Gold（Au） nanoclusters have recently emerged as ideal models for understanding Au catalysis, because the nanosized Au particles have precise atomic numbers and uniform size. In this work, we studied for the first time the support shape effect on the catalysis of Au nanoclusters by using CO oxidation as a model reaction. Au22（L8）6（L = 1,8-bis（diphenylphosphino） octane） nanoclusters were supported on CeO2 rods or cubes, then pretreated at different temperatures（up to 673 K）, allowing the gradual removal of the organic phosphine ligands. CO oxidation test over these differently pretreated samples shows that CeO2 rods are much better supports than cubes for Au22 nanoclusters in enhancing the reaction rate. In situ IR spectroscopy coupled with CO adsorption indicates that the shape of CeO2 support can impact the nature and quantity of exposed Au sites, as well as the efficiency of organic ligand removal. Although CeO2 rods are helpful in exposing a greater percentage of total Au sites upon ligands removal, the percentage of active Au sites（denoted by Au d＋, 0 〈 d 〈 1） is lower than that on CeO2 cubes. The in situ extended X-ray absorption spectroscopy（EXAFS） and high-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM） results show that the Au nanoclusters bound more strongly to the CeO2 rods than to the cubes where the Au nanoclusters show more sintering. Considering the typical redox mechanism for CO oxidation over supported Au nanoclusters and nanoparticles, it is concluded that the reactivity of the lattice oxygen of CeO2 is the determining factor for CO oxidation over Au22/CeO2. CeO2 rods offer more reactive lattice oxygen and abundant oxygen vacancies than the cubes and thus make the rods a superior support for Au nanoclusters in catalyzing low temperature CO oxidation.
基金This work was supported in part by the Natural Science Foundation(No.DMR-0804088)startup funds from Washington University in St.Louis.P.H.C.C.was also partially supported by the Fulbright Program and the Brazilian Ministry of Education(CAPES).Part of the work was performed at the Nano Research Facility(NRF),a member of the National Nanotechnology Infrastructure Network(NNIN),which is supported by the National Science Foundation(No.ECS-0335765).
文摘This paper presents a systematic study of the growth mechanism for Pd nanobars synthesized by reducing Na_(2)PdCl_(4) with L-ascorbic acid in an aqueous solution in the presence of bromide ions as a capping agent.Transmission electron microscopy(TEM)and high-resolution TEM analyses revealed that the growth at early stages of the synthesis was dominated by particle coalescence,followed by shape focusing via recrystallization and further growth via atomic addition.We also investigated the detailed surface structure of the nanobars using aberration-corrected scanning TEM and found that the exposed{100}surfaces contained several types of defects such as an adatom island,a vacancy pit,and atomic steps.Upon thermal annealing,the nanobars evolved into a more thermodynamically favored shape with enhanced truncation at the corners.
