title: Size- and support-dependent evolution of the oxidation state and structure by oxidation of subnanometer cobalt clusters.
abstract: Size-selected subnanometer cobalt clusters with 4, 7, and 27 cobalt atoms supported on amorphous alumina and ultrananocrystalline diamond (UNCD) surfaces were oxidized after exposure to ambient air. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) and near-edge X-ray absorption fine structure (NEXAFS) were used to characterize the clusters revealed a strong dependency of the oxidation state and structure of the clusters on the surface. A dominant Co(2+) phase was identified in all samples. However, XANES analysis of cobalt clusters on UNCD showed that ∼10% fraction of a Co(0) phase was identified for all three cluster sizes and about 30 and 12% fraction of a Co(3+) phase in 4, 7, and 27 atom clusters, respectively. In the alumina-supported clusters, the dominating Co(2+) component was attributed to a cobalt aluminate, indicative of a very strong binding to the support. NEXAFS showed that in addition to strong binding of the clusters to alumina, their structure to a great extent follows the tetrahedral morphology of the support. All supported clusters were found to be resistant to agglomeration when exposed to reactive gases at elevated temperatures and atmospheric pressure.
authors.affiliation: Materials Science Division, ‡X-ray Science Division, and ▽Nanoscience and Technology Center, Argonne National Laboratory , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors.forename: Chunrong
authors.initials: C
authors.lastname: Yin
authors.forename: Fan
authors.initials: F
authors.lastname: Zheng
authors.forename: Sungsik
authors.initials: S
authors.lastname: Lee
authors.forename: Jinghua
authors.initials: J
authors.lastname: Guo
authors.forename: Wei-Cheng
authors.initials: WC
authors.lastname: Wang
authors.forename: Gihan
authors.initials: G
authors.lastname: Kwon
authors.forename: Viktor
authors.initials: V
authors.lastname: Vajda
authors.forename: Hsien-Hau
authors.initials: HH
authors.lastname: Wang
authors.forename: Byeongdu
authors.initials: B
authors.lastname: Lee
authors.forename: Janae
authors.initials: J
authors.lastname: DeBartolo
authors.forename: Sönke
authors.initials: S
authors.lastname: Seifert
authors.forename: Randall E
authors.initials: RE
authors.lastname: Winans
authors.forename: Stefan
authors.initials: S
authors.lastname: Vajda