Hydrogenation properties of supported nanosized gold particles.

Abstract

Introduction In the last few years, there has been growing interest in nanosized structures in the range 1 to about 20 nanometers in many different fields of research. This is also the size of metal particles usually used in heterogenous catalysis. In general, such nanoparticles of metals like palladium, ruthenium, nickel or platinum are used for hydrogenations (1), since on these Group VIII metals the dissociatively adsorbed hydrogen is easily accessible. For a long time, only very limited attention has been paid to realizing catalysis on the basis of gold because of its electronic structure, namely the completely filled d band ([Xe] [4f.sup.14][5d.sup.10][6s.sup.1]), which is usually accompanied by very low activities (2). The situation has been changed since Haruta and coworkers reported on CO oxidation at room temperature, that is feasible only on very small gold nanoparticles on suitable supports (3). This observation was followed by enhanced search for other possible applications in catalysis (2). Unfortunately, f ocussing on oxidation reactions masked the capabilities of gold in hydrogenation reactions, even though there are some very promising first examples of possible applications [2]. The speciality of these reactions, at least of the examples discussed below, is the control of intramolecular selectivity rather than maximum activities. Preparation of gold nanoparticles There are many different methods of preparing gold particles in the nanometer size range. The main problem is the creation of small metallic units that are stable under reaction conditions. Hence particles are often deposited on a support, which is usually a metal oxide. Even though methods like chemical vapor deposition (CVD), sol-gel synthesis or colloidal routes are well established in chemical laboratories, the most often used methods in large scale industry as well as in academics are the following: Impregnation For impregnation, the gold precursor, mostly [HAuCl.sub.4] or [AuCl.sub.3], is dissolved in water in quantities which correspond to the desired metal loading, followed by wet impregnation (abbreviated as I) of the support material (e.g. [SiO.sub.2], [TiO.sub.2], [ZrO.sub.2]) present in powder form or as extrudates. After drying, calcination (treatment in air) and a subsequent reduction in hydrogen at elevated temperatures (473-773 K) the final supported gold catalyst is obtained. Application of the more special incipient wetness method is also possible. Here the amount of water containing the gold precursor corresponds to the pore volume of the support material. Precipitation An aqueous solution of the gold precursor is adjusted to a fixed value of pH in the range 6-10, then the preformed support material is added (deposition-precipitation, DP). Alternatively the support can be precipitated simultaneously (co-precipitation, P). After further stirring and aging of the solution, the precipitate is dried, washed several times in an appropriate solvent and than filtered, calcined and reduced in a similar manner as for impregnations (3,4). Definitions The overall activity of a catalytic system under specified conditions is often defined as the reaction rate obtained by kinetic measurements, e.g. in terms of mol converted educt per mass of catalyst and time (5). From this, specific activities can be calculated relating the rate to the amount of the metal or per unit surface area, as in the case of evaporated metal films done for the first time by Beeck (6). If two or more catalysts contain the same metal loading and the catalytic reactions are conducted at identical space times (i.e. catalyst weight, W, per molar feed flow, [F.sub.0] of the educt) comparison of catalyst activity is also possible on the basis of the degree of conversion (converted educt per moles introduced). If possible, the rate should be referred to the number of active sites and is than known as turnover rate or turnover frequency (TOF). …