(89e) Flow Rate Variation in Two Step Multi-Phase Microsynthesis of Nanomaterials Effecting Their Catalytic Activity of Oxidation

Metal nanoparticles of different composition are of interest for a wide spectrum of catalytic applications. Their high effective surface make them suited for catalytic flow-through processes in micro reactors with short residence times. Gold and silver nanoparticles are particular important for selective partial oxidation of organic compounds [1]. The catalytic activity and specifity are influenced by the distribution of sizes and compositions within a population of nanoparticles. The quality, and in particular the uniformity of catalytic nanomaterial can be improved if the nanoparticles are prepared by using a micro flow-through precipitation process [2].

Previous investigations showed that the properties of colloidal solutions of Au/Ag particles obtained by a miniaturized flow-through process are strongly dependend on flow rates.The different qualities are reflected by strong variations in optical spectra [3]. Therefore, the effect of flow rate during preparation of catalyst on the ctalytic activity was studied, here. A multiphase flow system was applied in order to realize a plug-like liquid transport, and therefore a narrow residence time distribution between the used second and third injector positions. At the first injector, fluid segments are formed from aqueous solution of reducing agents (sodium borohydride/ascorbic acid) injected into inert carrier liquid (tetradecane). At the second injector, a tetrachloroaurate solution (5mM) in water was injected into the aqueous liquid segments of reducing agent in order to start reductive nucleation. The length of the following residence multi loop (20 cm) determined the time between starting of metal nucleation and addition of the second metallic component. At the third injector, salt solutions of the second metal component (silver, copper, iron) were added to the aqueous reaction mixtures in the fluid segments.

The fluid actuation was realized by syringe pumps. PTFE ware was used for tubes (0.5 mm diameter) and for injectors. The flow rates were varied between 250 and 5000 μL/min resulting in delay times between the second and the third injector between about 50 ms and one second.

The obtained nanoparticles were precipitated and cleaned by centrifugation and washing. The catalytic activity of the obtained nanomaterials was tested by investigating the oxidative degeneration of dyes (rhodamine B, bromphenolblue) in aqueous solution in the presence of potassium peroxodisulfate. The bleaching rates of reaction suspensions containing about 0.1 weight percent of catalyst increased up to factors of about 15 in comparison with solutions without catalyst. The factor of reaction activity enhancement was dependend on the composition educt solutions as well as on the flow rate during the particle preparation. In some cases optima of catalytic acitivity were observed in dependence on flow rate.

Acknowledgement

We thank F. Möller and S. Schneider for carrying out experiments and for technical support. Financial support by the German Environmental Foundation (DBU) is gratefully acknowledged

References [1] M.D. Hughes, Y-J. Xu, P. Jenkons et al., Nature 437 (2005), 1132 [2] J. Wagner, J.M. Köhler, Nanoletters 5 (2005), 685 [3] J.M. Köhler, U. Hübner, J. Wagner, M. Held, AICHE Spring National Meeting/5th world congress on particle technology (Orlando, USA, April 2006), 98c