Characterizations and catalytic properties of fine particles of Ni-Sn intermetallic compounds supported on SiO2

Fine particles of Ni-Sn intermetallic compounds (IMCs) of Ni3Sn, Ni3Sn2, and Ni3Sn4 with specific crystal structures were selectively prepared on SiO2 by chemical vapor deposition (CVD) of Sn(CH3)(4) onto Ni/SiO2. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) measurements indicated that the near surface of these fine particles with atomic Ni/Sn ratios of 3/1, 3/2, and 3/4 are similar to those of corresponding unsupported Ni-Sn IMCs. X-ray diffraction (XRD) patterns of the fine particles showed peaks too broad to determine these crystal structures. Each unsupported Ni-Sn IMC was clarified to exhibit a specific Ni K-edge and Sn K-edge X-ray absorption near-edge structure (XANES) spectrum. The crystal structures of fine particles were by the identified by the fingerprinting method by Ni K-edge and Sn K-edge XANES spectra. Both these XANES spectra of Ni-Sn IMC/SiO2 catalysts with Ni/Sn atomic ratios of 3/1, 3/2, and 3/4 were similar to those of unsupported Ni3Sn, Ni3Sn2, and Ni3Sn4, respectively. The particles would have an average diameter of 3-4 nm obtained by TEM. Each Ni-Sn IMC particle showed intrinsic properties for the adsorption of carbon monoxide and hydrogen. The fine-particle catalysts exhibited high benzene selectivity (> 99% C) at high conversion for cyclohexane dehydrogenation. The selectivity was almost the same as that of the Ni-Sn IMC/SiO2 catalysts with large particle sizes (about 15 nm), whose crystal structures were clearly determined by XRD, but was completely different from that of the Ni/SiO2 catalyst. The activities of fine-particle catalysts were about 15 times higher than those of the large-particle catalysts. (C) 2003 Elsevier Inc. All rights reserved.