On Metal Segregation of Bimetallic Nanocatalysts Prepared by a One-Pot Method in Microemulsions
A comparative study on different bimetallic nanocatalysts prepared from microemulsions using a one-pot method has been carried out. The analysis of experimental observations, complemented by simulation studies, provides detailed insight into the factors affecting nanoparticle architecture: (1) The metal segregation in a bimetallic nanocatalysts is the result of the combination of three main kinetic parameters: the reduction rate of metal precursors (related to reduction standard potentials),
... ard potentials), the material intermicellar exchange rate (determined by microemulsion composition), and the metal precursors concentration; (2) A minimum difference between the reduction standard potentials of the two metals of 0.20 V is needed to obtain a core-shell structure. For values ∆ε 0 smaller than 0.20 V the obtaining of alloys cannot be avoided, neither by changing the microemulsion nor by increasing metal concentration; (3) As a rule, the higher the film flexibility around the micelles, the higher the degree of mixture in the nanocatalyst; (4) A minimum concentration of metal precursors is required to get a core-shell structure. This minimum concentration depends on the microemulsion flexibility and on the difference in reduction rates. Catalysts 2017, 7, 68 2 of 17 temperatures [22, 23]. This can be attributed to the presence of a second metal, which changes both geometrical and electronic properties due to the electronic coupling between the individual metals. The overall efficiency observed in bimetallic nanocatalysts accounts for synergistic effects between the two metals [20, 24, 25] . The synergetic catalytic effect was defined by Shi  as "a certain kind of cooperation between different components and/or active sites in one catalyst, which results in significantly, or even strikingly, enhanced catalytic performances than the arithmetic summation of those by corresponding individual components". As an example, the catalysis of CO oxidation is improved by Au-Ag nanocatalysts because of the synergistic interaction between the two metals. In this case study, Ag adsorbs reactive oxygen species and Au adsorbs CO  . The progress in this field has been extremely important, and currently a number of bimetallic catalysts are widely utilized in different kinds of reactions , such as hydrogenation [27-30], reforming reactions [31,32], oxidation [25,33-37], oxygen reduction reactions [1,14,22,38-41], hydrogenolysis [10,42,43] and coupling reactions [20,    . Moreover, it is important to point out that bimetallic nanocatalystst were proposed as candidates in green chemistry and future biomass-based refineries [42, 47] . Another advantage of the bimetallic nanoparticles as catalysts is a superior longevity of the bimetallic catalyst compared with a monometallic one. For example, in the dehydrogenation reaction of formic acid catalyzed by Au-Pd bimetallic nanoparticles, the presence of Au inhibits the poisoning of the active sites of Pd by CO (produced in a secondary reaction)  . In general, the catalyst stability can be improved by the presence of the second metal, which allows its recovery and recycling  . Finally, bimetallic catalysts show well-defined active sites within the nanoparticle, contrary to the mixed and unspecified active sites in bulk heterogeneous catalysts  . Excellent review articles including the role of bimetallic nanoparticles as catalysts are already available [2, 3, 20,     . Metal Arrangement in Bimetallic Nanoparticles Controlling the arrangement of the two metals within the first atomic layers from the surface of the bimetallic nanoparticle is essential to improve the catalytic activity [2,     . It is important to note that the synergistic effect exhibited by bimetallic nanocatalysts is strongly dependent on the surface composition  . It is well known that heterogeneous catalytic reactions take place on the surface of catalysts, where the adsorption and desorption of the reactants, intermediates and products occur. The synergic cooperation between both metals modifies the surface electronic properties, so the improvement of catalysis is conditioned by the ability to control the surface composition. Hence, different Au-Pt nanostructures are used as catalysts depending on the metal distribution: for electro-oxidation of methanol, the preferred structure is a Pt-Au alloyed shell  . Conversely, an Au-core/Pt-shell nanoparticle is better to catalyze oxygen reduction reaction [38, 56] , or formic acid electro-oxidation  . For this reason, the surface composition of bimetallic nanocatalysts is a critical aspect to take into account. According to the mixing pattern, bimetallic nanoparticles can be classified as core-shell, subcluster segregated or nanoalloy [3, 58] . The first one consists of a core composed by one of the metals, which is covered by a shell of another, including some mixing between both of them. In fact, using noble metals as shell materials in a core-shell nanoparticle offers economic advantages because it maximizes the noble element surface to volume ratio. Furthermore, when magnetic elements (such as Fe, Co, and Ni) are used as a core metal, the noble shells' nanoparticles may acquire magnetic properties  . The second type of bimetallic arrangement consists of two clearly separated metals, not surrounding one another, but adjacent to each other. It is a segregated subcluster. Finally, nanoalloys, either crystalline ordered or random, are composed of mixed architectures of the two metals. Magno et al.  pay attention so as to distinguish between intermetallic structures and alloys. In an intermetallic arrangement, the mixtures of metals shows a specific lattice structure, that is, different from those of its constituent metals. On the contrary, in an alloy the lattice structure is the same as that of its main compound.