Characterizations and catalytic properties of fine particles of Ni-Sn intermetallic compounds supported on SiO_2

摘要

Fine particles of Ni-Sn intermetallic compounds (IMCs) of Ni_3Sn,Ni_3Sn_2,and Ni_3Sn_4 with specific crystal structures were selectively prepared on SiO_2 by chemical vapor deposition (CVD) of Sn(CH_3)_4 onto Ni/SiO_2.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 structural 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/SiO_2 catalysts with Ni/Sn atomic ratio of 3/1,3/2,and 3/4 were similar to those of unsupported Ni_3Sn,Ni_3Sn_2,and Ni_3Sn_4,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/SiO_2 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/SiO_2 catalyst.The activities of fine-particle catalysts were about 15 times higher than those of the large-particle catalysts.