Penner S, Jenewein B, Wang D, Schlögl R, Hayek K
Institut für Physikalische Chemie, Universität Innsbruck, Innrain 52a, A-6020, Innsbruck, Austria.
Phys Chem Chem Phys. 2006 Mar 14;8(10):1223-9. doi: 10.1039/b516841d. Epub 2006 Jan 31.
Rh nanoparticles (mean size 10 and 15 nm), prepared by epitaxial growth on NaCl surfaces, were covered with layers of crystalline vanadium oxide (mean thickness 1.5 and 25 nm) by reactive deposition in 10(-2) mbar O2. The 1.5 nm film was further stabilized with a coating layer of 25 nm amorphous alumina. The so-obtained Rh/vanadia films, containing vanadium in the V3+ and V2+ state, were treated in 1 bar O2 at 673 K for 1 h and thereafter reduced in 1 bar H2 at increased temperatures, particularly between 723 and 873 K. The structural and morphological changes were followed by (high-resolution) transmission electron microscopy and selected area diffraction. Oxidation at 673 K transforms the purely vanadia-supported samples into Rh/V2O5, while in the alumina-supported films containing only small amounts of VOx, the formation of topotactic V2O3 is observed. The formation of Rh-V alloys during the subsequent reduction is strongly determined by the intimate contact and the structural and orientational relationship between Rh particles and the surrounding VOx phase. Reduction above 473 K transforms the support into substoichiometric vanadium oxides of composition VO and V2O. Analysis of high-resolution images and diffraction patterns reveals the presence of different alloy phases after reduction with increasing T (from 573 up to 823 K). In the alumina-supported film (low V/Rh ratio) the epitaxial alignment between the Rh particles and the surrounding V2O3 phase apparently favours the primary formation of defined alloys of type V3Rh and VRh3, followed by VRh at higher temperature. On the contrary, mainly V3Rh5 is formed in the purely VOx-supported Rh/films, due to different epitaxial relations in the initial state. Possible pathways of alloy formation are discussed.
通过在氯化钠表面外延生长制备的铑纳米颗粒（平均尺寸为10和15纳米），在10⁻²毫巴氧气中通过反应沉积覆盖了结晶氧化钒层（平均厚度为1.5和25纳米）。1.5纳米的薄膜用25纳米的非晶氧化铝涂层进一步稳定化。如此获得的含有V³⁺和V²⁺态钒的铑/氧化钒薄膜，在673 K的1巴氧气中处理1小时，然后在升高的温度下，特别是在723至873 K之间，在1巴氢气中还原。通过（高分辨率）透射电子显微镜和选区衍射跟踪结构和形态变化。673 K下的氧化将纯氧化钒负载的样品转变为Rh/V₂O₅，而在仅含有少量VOₓ的氧化铝负载薄膜中，观察到了拓扑规整的V₂O₃的形成。后续还原过程中铑 - 钒合金的形成强烈取决于铑颗粒与周围VOₓ相之间的紧密接触以及结构和取向关系。473 K以上的还原将载体转变为化学计量比不足的组成VO和V₂O的钒氧化物。对高分辨率图像和衍射图案的分析揭示了随着温度升高（从573 K到823 K）还原后不同合金相的存在。在氧化铝负载薄膜（低V/Rh比）中，铑颗粒与周围V₂O₃相之间的外延排列显然有利于首先形成确定的V₃Rh和VRh₃型合金，随后在较高温度下形成VRh。相反，在纯VOₓ负载的Rh/薄膜中主要形成V₃Rh₅，这是由于初始状态下不同的外延关系。讨论了合金形成的可能途径。
