Układy bimetaliczne Pd-Ag, Pd-Pt, Pd-Au, Pt-Ag i Pt-Au jako katalizatory heterogeniczne reakcji z udziałem wodoru

Heterogeneous catalysts play an important role in the manufacture of various chemical substances in large-scale processes, e.g. crude oil processing and organic synthesis [1]. Heterogeneous catalyst most often consists of a transition metal arranged on an oxide support [2]. The transition metal employed is usually one from Group 10 of the Periodic Table (Ni, Pd or Pt). The Group 10 metals are efficient catalysts of reactions with hydrogen. Such reactions occur in the above-mentioned crude oil processing and organic synthesis. In large-scale applications some modifications of the catalyst properties are often necessary to increase the rate of an appropriate stage of catalyzed reaction and to avoid by-products. The change of the catalytic properties can be obtained by incorporating another metal into the catalyst. Heterogeneous catalysts in which the active part contains two metals are called bimetallic catalysts or, more generally, bimetallic systems [3, 4]. Research on bimetallic catalysts was initiated in the 1960s and since then these catalysts have become an object of increasingly in-depth investigations [5]. The aim of this review is to summarize the available knowledge on heterogeneous bimetallic catalysts. The review has been narrowed only to a few combinations of metals, i.e. Pd-Ag, Pd-Pt, Pd-Au, Pt-Ag, and Pt-Au. In the first part of the review some general information on the forms of the bimetallic systems is presented. The term bimetallic system itself is quite broad and includes, among other, the following representatives (Fig. 1): alloys, surface alloys, monometallic monolayer or pseudomorphic overlayers arranged on the surface of the other metal, monometallic nanoparticles and clusters arranged on the surface of the other metal, alloyed nanoparticles and clusters, core/shell nanoparticles and clusters, and heteroaggregates. Recently, the last three of these representatives have been in the centre of interest [5, 6]. They offer properties very different from those characteristic of bulk materials [15]. Later, the methods of synthesis and structural characterization of the bimetallic systems are described. At present, the preparation of the bimetallic catalysts that exhibit an appropriate structure is difficult and expensive. Hence, further progress in this field is still required. Some new methods of preparation [7, 16-41], as well as many experimental [42-45, 48-67] and theoretical papers [69-77] on structural, energetic and electronic properties of the bimetallic systems are reviewed. In the last part of the review the catalytic behaviour of the Pd-Ag, Pd-Pt, Pd-Au, Pt-Ag and Pt-Au systems is discussed in detail. The discussion concentrates on the catalytic reactions with hydrogen, e.g. hydrogenation, dehydrogenation, hydrogenolysis, etc. [106-137]. In such reactions the bimetallic catalysts exhibit higher selectivity than the monometallic ones. They also have better resistance to deactivation. At the very end of this review the theoretical investigations on H2 dissociation and H adsorption on the bimetallic systems have been mentioned [138-155].