Synthesis, catalysis, surface chemistry and structure of bimetallic nanocatalysts.

Studies of bimetallic catalysts can be tracked back to the early 1960s in the Exxon Research and Engineering Company. The term ‘‘bimetallic clusters’’ was introduced by John H. Sinfelt in the early 1980s to refer to highly dispersed bimetallic entities present on the surface of a support such as silica or alumina. The main bimetallic catalysts at that time included Ni–Cu, Ru–Cu, Os–Cu, Pt–Ir, and Pt–Ru. The library of bimetallic catalysts has been significantly enriched in the past decades. Bimetallic catalysts have been one of the major categories of catalysts in heterogeneous catalysis. Fundamental studies of the synthesis, catalysis, surface chemistry and structure of bimetallic catalysts have been a fast-growing and exciting field of heterogeneous catalysis for energy conversion and chemical transformations. Adding a second metal (here called the guest metal) to the first metal (called the host metal) can tune catalytic performances (activity, selectivity, durability, etc.) through modification of electronic and/or structural factors. A bimetallic catalyst is widely defined as a catalyst crystallite which consists of two metal components. The constituent metals can form an alloy, intermetallic, or nanocomposited structure. Alloy catalysts include bulk alloy, surface alloy, and near surface alloy. Nanocomposited structures include core–shell structured bimetallic nanoparticles, nanodendrites, and others. Due to the diversity of bimetallic catalysts, tuning catalytic performance of a host metal could be performed through (a) an ensemble or geometric effect, in which the coordination of atoms of a guest metal to an atom of the host metal on the surface provides new geometries of active sites, (b) the electronic or ligand effect, wherein the addition of a guest metal alters the electron properties of the active sites of the host metal by electron transfer between guest and host metals. A concept to evaluate the influence of a guest metal on the host metal is the d-band center of the host metal. In most cases, the difference in catalytic performance between a bimetallic catalyst and a monometallic catalyst of the host metal can be rationalized through an electronic and/or geometric effect. Unfortunately, it is quite challenging to distinguish the two effects if both of them have a role. From the catalytic point of view, there could be another effect of a formed bimetallic catalyst which can be understood as a synergetic effect or bi-functional effect. In this case, atoms of the two metals are necessary parts of a catalytic site and thus play a unique role such as adsorption for different reactants or different intermediates. Formation of a bimetallic catalyst exhibits the feature of continual tuning through changes in composition of the host and guest metals and flexible modification of the electronic and/or geometric structure of bimetallic catalysts through synthesis. This largely enhances the capability in tuning the catalytic performance. Thus, bimetallic catalysts have the capability of improving catalytic activity, enhancing catalytic selectivity, increasing catalytic stability, and cutting the cost of catalysts. The spectacular advances in the synthesis of bimetallic nanocatalysts with wet chemistry in the past decade have offered numerous methods and protocols Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN, USA 46556. E-mail: ftao@nd.edu w Part of the bimetallic nanocatalysts themed issue.