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Title: Surface-Mediated Synthesis and Spectroscopic Characterization of Tantalum Clusters on Silica

Abstract

Tantalum clusters were synthesized on the surface of porous silica by treatment of adsorbed Ta(CH{sub 2}Ph){sub 5} in H{sub 2} at temperatures in the range of 523-723 K. The surface species were characterized by UV-vis, far-infrared, and extended X-ray absorption fine-structure (EXAFS) spectroscopies, each of which provided evidence of Ta-Ta bonds similar to those in well-characterized molecular tantalum clusters. The Ta-Ta distance determined by EXAFS spectroscopy was 2.93 Angstroms. The chemistry of the cluster synthesis is similar to that of syntheses of similar tantalum clusters in solution. The supported clusters formed at 523 K are characterized by an EXAFS first-shell Ta-Ta coordination number of nearly 2, indicative of tri-tantalum clusters, although it is expected that a mixture of clusters was present, and reduction in H2 at higher temperatures led to larger tantalum clusters. This is the first example of the surface-mediated synthesis of an early transition metal cluster, and the supported clusters reported here are the first to have been characterized by all three of the spectroscopic methods mentioned above. The similarity of the surface synthesis to that in solution points to opportunities to extend this new class of material to other early transition metal clusters on various supports.

Authors:
;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
914198
Report Number(s):
BNL-78766-2007-JA
Journal ID: ISSN 0743-7463; LANGD5; TRN: US200804%%329
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Langmuir; Journal Volume: 22
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; TANTALUM; MOLECULAR CLUSTERS; SYNTHESIS; SILICON OXIDES; SUBSTRATES; COORDINATION NUMBER; national synchrotron light source

Citation Formats

Nemana,S., and Gates, B. Surface-Mediated Synthesis and Spectroscopic Characterization of Tantalum Clusters on Silica. United States: N. p., 2006. Web. doi:10.1021/la0609322.
Nemana,S., & Gates, B. Surface-Mediated Synthesis and Spectroscopic Characterization of Tantalum Clusters on Silica. United States. doi:10.1021/la0609322.
Nemana,S., and Gates, B. Sun . "Surface-Mediated Synthesis and Spectroscopic Characterization of Tantalum Clusters on Silica". United States. doi:10.1021/la0609322.
@article{osti_914198,
title = {Surface-Mediated Synthesis and Spectroscopic Characterization of Tantalum Clusters on Silica},
author = {Nemana,S. and Gates, B.},
abstractNote = {Tantalum clusters were synthesized on the surface of porous silica by treatment of adsorbed Ta(CH{sub 2}Ph){sub 5} in H{sub 2} at temperatures in the range of 523-723 K. The surface species were characterized by UV-vis, far-infrared, and extended X-ray absorption fine-structure (EXAFS) spectroscopies, each of which provided evidence of Ta-Ta bonds similar to those in well-characterized molecular tantalum clusters. The Ta-Ta distance determined by EXAFS spectroscopy was 2.93 Angstroms. The chemistry of the cluster synthesis is similar to that of syntheses of similar tantalum clusters in solution. The supported clusters formed at 523 K are characterized by an EXAFS first-shell Ta-Ta coordination number of nearly 2, indicative of tri-tantalum clusters, although it is expected that a mixture of clusters was present, and reduction in H2 at higher temperatures led to larger tantalum clusters. This is the first example of the surface-mediated synthesis of an early transition metal cluster, and the supported clusters reported here are the first to have been characterized by all three of the spectroscopic methods mentioned above. The similarity of the surface synthesis to that in solution points to opportunities to extend this new class of material to other early transition metal clusters on various supports.},
doi = {10.1021/la0609322},
journal = {Langmuir},
number = ,
volume = 22,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • No abstract prepared.
  • Binuclear tantalum clusters were synthesized from Ta(CH{sub 2}Ph){sub 5} (Ph is phenyl) on the surface of nonporous SiO{sub 2} (Aerosil), and their reactions with H{sub 2}, D{sub 2}, and ethylene were characterized by time-resolved infrared (IR), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge spectroscopies. The EXAFS data indicate the formation in H{sub 2} of clusters with a Ta-Ta coordination number of approximately 1 and a bonding distance of 2.74 {angstrom}. Reactions of the supported clusters with D{sub 2} and H{sub 2} facilitate the interconversion of O-H and O-D groups on the SiO{sub 2} surface. Reaction ofmore » these clusters with ethylene led to their rapid fragmentation to give mononuclear tantalum complexes, as the tantalum was oxidized and new ligands formed, suggested by IR spectra to be ethyl. The results demonstrate a rough analogy between the chemistry of tantalum clusters on the SiO{sub 2} surface and their chemistry in solution. Because alkenes are suggested intermediates in the catalytic disproportionation of alkanes on supported tantalum, our results indicate how these intermediates might influence the nature of the catalytically active species.« less
  • The clusters HM[sub 3](CO)[sub 10]([mu]-PPh[sub 2]) (M = Ru (1a), Os (1b)) are shown to be convenient precursors of the phosphinidene-stabilized clusters nido-M[sub 4](CO)[sub 13]([mu][sub 3]-PPh) (M = Ru (2a), Os (2b)) via P-C(Ph) activation, reductive elimination of benzene, and condensation. Heating a toluene solution of 1a under a purge of carbon monoxide forms Ru[sub 4](CO)[sub 13]([mu][sub 3]-PPh) (40%) as the major product. Formation of Os[sub 4](CO)[sub 13]([mu][sub 3]-PPh) requires more forcing conditions. Heating a solid sample of 1b at 215[degrees]C for 8 min affords 2b in 20-25% yield. Both 2a and 2 b have been characterized by spectroscopy andmore » by accurate single-crystal X-ray structure analyses. Crystals of 2a and 2b are isomorphous, crystallizing in the orthorhombic space group P2[sub 1]2[sub 1]2[sub 1]. The molecular structures of 2a and 2b possess M[sub 4]P cluster frameworks containing a butterfly arrangement of metal atoms stabilized by a [mu][sub 3]-phosphinidene fragment capping an open triangular face. Both clusters adhere to the effective atomic number rule (62 electrons) and are associated with nido octahedral M[sub 4]P core geometries when the skeletal bonding electrons are considered (seven skeletal electron pairs, five vertices). In the latter instance the [open quotes]PPh[close quotes] fragment is located in the square basal plane of the pyramidal framework. Variable-temperature [sup 13]C NMR studies of 2a revealed four independent dynamic processes involving localized CO exchange, and a direct comparison is drawn between 2a and ([mu]-H)[sub 2]Ru[sub 4](CO)[sub 12]([mu][sub 3]-PPh). At temperatures in excess of 296 K, the authors have observed the onset of total intermetallic CO scrambling. 47 refs., 5 figs., 4 tabs.« less
  • Ultrasonic irradiation of a slurry of amorphous silica microspheres, zinc acetate, and thioacetamide in an aqueous medium for 3 h under ambient air yields zinc sulfide coated on silica. The powder X-ray diffraction of the initial zinc sulfide-silica (ZSS) powder yields diffraction peaks corresponding to the ZnS phase. The TEM image of ZSS shows that the porous ZnS nanoparticles (diameter 1--5 nm) coated the silica (SiO{sub 2}) surface as thin layers or nanoclusters, depending on the reactant concentration. Infrared spectroscopy illustrates the structural changes that occurred in the siloxane network and surface silanol groups of SiO{sub 2} upon the ultrasonicmore » deposition of ZnS. The optical absorption of porous ZnS shows a broad band at around 610 nm, ascribed to unusual surface state transition. The absorption energy of the surface state transition is lower than the band gap of the ZnS particles and probably stems from the dangling surface bonds or defects. On the other hand, the ZSS does not show the surface state transition of ZnS, probably due to the strong surface interaction with SiO{sub 2}. The classical valence-conduction transition band has been observed in the optical reflectance mode, and it shows an absorption edge at around (290--310 nm), which is markedly blue-shifted compared to that of bulk ZnS (345 nm). The photoluminescence spectrum of the porous ZnS and ZSS shows a band with a maximum centered around 420 nm, which is similar to that of quantum ZnS particles. The authors propose that the coating process takes place via ultrasonic-cavitation-induced initial grafting of zinc acetate onto the silica surface, followed by the displacement of acetate ion by in situ generated S{sup 2{minus}} species.« less