Preparation and Catalysis of Supported NiO Nanocluster for Oxidative Coupling of Thiophenol

Abstract

Supported NiO nanocluster was prepared by using Ni colloid. Alkoxide stabilized Ni nanocluster supported on SiO 2 was oxidized by exposing to air at room temperature to produce NiO nanocluster. The cluster size of NiO was estimated by XAFS analysis. Supported NiO nanocluster thus prepared showed the catalytic activity for oxidative coupling of thiophenol, whereas the conventional impregnation NiO catalyst did not. Remarkable NiO cluster size dependency on activity for oxidative coupling of thiophenol was observed. Key words: NiO nanocluster, XAFS, catalyst, oxdative thiophenol coupling reaction, cluster size dependecy 1. INTRODUCTION Supported Ni catalysts are one of the useful catalysts used for various reactions including steam reforming of ethanol and hydrocarbons [1-3], synthesis of carbon nanotube [4, 5], and coupling reactions [6, 7]. It is also reported that NiO is also catalytically active for oxidative dehydrogenation of ethane [8, 9], partial oxidation of methane [10, 11] and so on. Currently, it is a social requirement to develop active catalysts using ubiquitous element, such as Ni. Recently, metal nanoclusters have much attention for novel chemical properties arising from quantum size effect as well as high population of active sites. Thus we can expect that the catalytic performance of Ni-based catalysts will be further improved by reducing their size. However, it is difficult to synthesize monodispersed metal nanoclusters on supports using conventional mehods since the particles aggregate into larger particles during the calcination process at high temperature. To overcome this problem, the use of colloidal nanocluster as precursor is promising. Typically, colloidal particles are prepared by reducing metal salts in the presence of anti-aggregation additives [12, 13]. The practical benefit of this synthesis is controllability of the size. Synthesis of monodispersed nanoclusters of Pd [14-16], Pt [17, 18], Rh [19, 20], Au [21, 22] and Ni [23, 24] has been reported. In this study, we prepared NiO nanocluster on SiO 2 support by using the Ni colloid as precursor. Monodispersed Ni colloidal nanocluster adsorbed on SiO 2 was successfully transformed into NiO nanocluster by spontaneous oxidation in air at room temperature. Characterization of catalyst was carried out by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS), and the catalytic activity for oxidative coupling of thiophenol was also investigated.