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.