Effects of particle size, composition, and support on catalytic activity of AuAg nanoparticles prepared in reverse block copolymer micelles as nanoreactors

Abstract

Block copolymer (BCP) micelles formed by polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) were used as nanoreactors for preparing stable bimetallic AuAg nanoparticles (NPs) with finely tunable composition and diameters. Micelles loaded with NPs were then deposited onto TiO₂ and Al₂O₃ in order to investigate the catalytic properties of the NPs with respect to CO oxidation. The polymer matrix could be removed by thermal treatment at 400 °C under H₂ or O₂. Different parameters, such as support, particle size, and metal composition were varied independently, so that their influence on the catalytic activity for CO oxidation could be separated. The atomic Au/Ag ratio was varied from 1:2 to 2:1, and the highest activity was obtained for an Au/Ag ratio of 1, showing synergy effects of both metals for catalyzing CO oxidation. Using nanosized TiO₂ as support and an Au/Ag ratio of 1, the nanoparticle diameter was varied between ∼3 and ∼20 nm which led to a variation of activity by a factor of ∼7 (in a continuous flow reactor at 70 °C) with the smallest particles showing the highest turnover frequencies (TOFs). In comparison, regular TiO₂ as catalyst carrier showed significantly lower performance than nanostructured TiO₂ while no activity was found on non-reducible γ-Al₂O₃. Independently, all three parameters (metal particle size, Au/Ag atomic ratio, and the support) showed significant effects on the catalytic activity.

Introduction

Extensive research work both in fundamental and in applied catalysis has been carried out in order to understand the origin of the surprising catalytic properties of gold nanoparticles (NPs) [1], [2], [3], [4], [5], [6], [7], [8], [9]. It has been well-established that the catalytic performance of supported Au NPs crucially depends on the particle size, the support, and the preparation method. For example, gold supported on so-called reducible oxides, such as TiO₂, CeO₂, Fe₂O₃, Co₃O₄ and MnO_x [4], [10], [11], [12], [13], [14], is generally considered to be more active and stable than gold nanoparticles supported on non-reducible oxides (SiO₂, Al₂O₃, etc.). Among the reactions studied, the catalytic oxidation of CO is probably the most intensively investigated one because gold catalyzes this reaction already at room temperature and below. This low-temperature activity opens the door to a number of industrial applications, such as gas purification in CO₂ lasers, CO gas sensors, air-purification devices for respiratory protection, and – perhaps most importantly – more efficient pollution control devices for reducing industrial, environmental, and automotive emission [15], [16], [17], [18].

The reason for why the catalytic activity of Au for oxidation reactions is unexpected at first sight is its inability to dissociate O₂. Suitable supports have to be chosen for sufficient activity, and intensive research was devoted to the question, how Au and the oxide support interact to provide active oxygen. Yet, another way of delivering oxygen to Au so that it can unleash its oxidation chemistry is the combination with a second metal which takes over the role of adsorbing and dissociating O₂. For instance, it has been observed that bimetallic AuAg NPs show improved catalytic activity for CO oxidation, as compared to the monometallic system [19]. Recently, it was also reported that nanoporous gold, a nanostructured porous gold material, is very active for CO oxidation in spite of the fact that no oxide support is present at all and feature sizes are much larger (∼30–50 nm) [20] than those of catalytically active Au nanoparticles (<5 nm) [1], [21]. Theoretical studies revealed that Ag in an Au matrix increases the O₂ adsorption energy and lowers the activation energy for dissociation [22]. Notably, silver is also a catalyst for CO oxidation, but only at much higher temperatures than gold [23], [24].

In view of these results, it seems very attractive to prepare AuAg bimetallic catalysts which are economically more viable (because of the lower Ag price) but exhibit the same favorable catalytic properties as pure Au catalysts (e.g., low-temperature activity). The question is, however, how such bimetallic catalysts can be prepared with a high degree of structural and compositional control. One of the most applied methods is deposition–precipitation (D–P) pioneered by Haruta’s group for pure Au catalysts [25]. The preparation of bimetallic particles of uniform composition and size is difficult in this way. Recently, Sandoval and coworkers achieved to prepare TiO₂-supported bimetallic AuAg NPs by the D–P method [26]. However, as-prepared particles exhibited a very broad distribution of compositions which improved during catalyst activation by heating only at the cost of a broadened particle size distribution.

A very versatile approach which can overcome such problems is colloidal chemistry, providing a better control over size and structure as well as chemical composition of the particles. Since the NPs are obtained dispersed in solution, the same batch of particles can be deposited on different supports, in contrast to the D–P method. In this work, we have used block copolymer (BCP) reverse micelles as nanoreactors for synthesizing stable AuAg NPs. This method was first reported by Antonietti and coworkers [27] as well as by Möller and coworkers [28], who have well-established this technique as a method for the generation of regular arrays of monodisperse NPs on various metals and oxides. The microphase separation within the BCPs is the key to the control of particle size and interparticle distance: when BCPs are dissolved in a suitable solvent at or above their critical micelle concentration, they form reverse micelles. Adequate metal precursors added to the solution link only to the micelle cores and do not stain the shells. In a subsequent reaction step, the metal within the precursor, typically a metal salt, is reduced and aggregation of the metal into one or several particles within each micelle core is induced. Thus, the absolute diameter of the micelles and the relative dimensions of their core and shell determine the particle size. In a previous study using polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) as a block copolymer, we could show that by choosing the proper sequence of Au and Ag addition, bimetallic AuAg nanoalloy particles can be successfully prepared with variable and controllable composition [29].

The present work pursues routes toward preparation of supported AuAg catalysts with well-defined particle size and composition by depositing, from solution, the BCP micelles loaded with AuAg NPs on various oxide supports and removing the BCP shell afterward. Being able to deposit particles from the same batch on different supports and study them under the same conditions for CO oxidation allowed us to draw clear conclusions regarding (a) the quantitative influence of particle composition, (b) the role of particle size, and (c) the role of the support.