Bimetallic catalysts deposition

Waszczuk et al., 2001b Tong et al., 2002]. Because Ru is deposited as nanosized Ru islands of monoatomic height, the Ru coverage of Pt could be determined accurately. In that case, the best activity with regard to methanol oxidation was found for a Ru coverage close to 40-50% at 0.3 and 0.5 V vs. RHE. However, the structure of such catalysts and the conditions of smdy are far from those used in DMFCs. Moreover, the surface composition of a bimetallic catalyst likely depends on the method of preparation of the catalyst [Caillard et al., 2006] and on the potential [Blasini et al., 2006]. 

The surface structure and characteristics (density and acidity) of the hydroxyl groups presented in Fig. 13.21 (using CrystalMaker 2.1.1 software) give very useful information to understand the reactivity of the surface of the particles, particularly when adsorption of another complex is desired to synthesize a bimetallic catalyst, or to control the interaction with an oxide carrier (the deposition step). The isoelectric point calculated with the model (5.9) is in fair agreement with the experimental value (4.3). 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

Vickerman and Ertl (1983) have studied H2 and CO chemisorption on model Cu-on-Ru systems, where the Cu is deposited on single-crystal (0001) Ru, monitoring the process using LEED/Auger methods. However, the applicability of these studies carried out on idealized systems to real catalyst systems has not been established. Significant variations in the electronic structure near the Eermi level of Cu are thought to occur when the Cu monolayer is deposited on Ru. This implies electron transfer from Ru to Cu. Chemical thermodynamics can be used to predict the nature of surface segregation in real bimetallic catalyst systems. 

The advantages of this method are that the extent of gold deposition is almost quantitative, and thermal treatment is unnecessary because gold is already reduced by the UV irradiation. The gold particles can be very small, but the method has not been used very much, although it has been applied to preparing bimetallic catalysts (see Section 4.6.2.4). 

Bimetallic catalysts can be prepared by a direct redox method when a cationic complex of a metal of higher electrochemical potential is reduced by another metal of lower electrochemical potential that has been deposited and reduced first187 (see Table 4.10) PdAu/SiO 88 and PdAu/C189 have been made in this way, gold being deposited after the palladium. Small amounts of the metals were found in filtrates, and XRD and temperature-programmed decomposition of palladium hydride indicated a substantial... 

Surface segregation phenomena and the surface composition of bimetallic catalysts are controlled by the surface free energies of the constituents of the bimetallic particles. The deposition of metal on metal in relation to bimetallic catalysts has been discussed by Dodson [63]. 

In practice the preparation of bimetallic catalysts using direct redox reactions can be extensively used for depositing a noble metal with a high standard electrochemical potential onto a non-noble metal with a lower standard electrochemical potential (eq 3). 

As a consequence when the difference between equilibrium potentials of the two half redox reactions is low, the modifying metal, during the preparation of a bimetallic catalyst by direct redox reaction, will be deposited selectively on specific sites of the parent metal (i.e. sites that are highly oxidizable such as comers, edges, etc.). However, the equilibrium potentials are defined by Nemst s law which provides facilities to fit the potential values by changing the concentrations of the oxidized and reduced forms (eqs 2 and 2 ) and so induces selective deposition of the modifier on the parent catalyst. 

In summary, the direct redox reactions can be largely used in the preparation of bimetallic catalysts with a close interaction between the metallic constituents. In that case a metal with a high electrochemical potential is deposited on a metal with a lower potential. The applicability of the technique can be extended significantly by using different ligands which, by chelating metallic ions, modify the standard electrochemical potentials. 

Finally, under well-defined experimental conditions (essentially the concentrations), the modifying metal can be deposited selectively on specific sites of the parent metal. Such deposition can significantly influence the selectivity of the bimetallic catalysts [12, 14],... 

Previously, Cu-Ru was prepared by direct redox reaction, with Ru deposited on Cu. In this case, however, Cu is deposited on Ru. Comparison between the two bimetallic catalysts showed, during D-glucitol transformation that the two bimetallic phases, with same surface compositions, but prepared by two different techniques, had very different catalytic properties [22]. 

Szabo, Nagy, Margitfalvi, and co-workers [9, 23, 24] investigated the preparation of platinum-gold and platinum-palladium bimetallic catalysts using the ionization of hydrogen preadsorbed on platinum. The first step is the bulk deposition of additive according to the equations... 

In summary, the preparation of bimetallic catalysts by surface redox reaction using a reductant preadsorbed on the parent monometallic catalyst has been studied in detail. Unfortunately, the method is intricate and time consuming, especially if several successive operations are required. Furthermore, when the modifier has a standard electrochemical potential higher than that of the parent metal (AUCI4 deposited on Pt°), the overall reaction is a complex one involving a reduction by adsorbed reductant but also direct oxidation of the metallic parent catalyst. The relative rate of the two parallel reactions determines the catalytic properties of the resulting bimetallic catalyst. 

In bimetallic catalysts prepared by catalytic reduction of copper by hydrogen, copper is deposited as three-dimensional agglomerates which are located, at low copper loadings, on the edges, corners, and rims of the parent metallic particles. The mechanism of deposition can be transferred from that proposed in corrosion and involving a local electrochemical cell ... 

On such bimetallic catalysts the rhenium loading at saturation of a 0.6% Pt/Al2C>3 catalyst depends on the hydrogen pressure. From a thermodynamic point of view the quantity of additive deposited at saturation is defined by the equilibrium conditions of the system under consideration. By taking into account the reduction of Re04 to Re0 at 303 K (Table 1),... 

In summary, the technique of catalytic reduction for the preparation of bimetallic catalysts can be extensively used with a variety of parent metals and re-ductants. However, some structure sensitivities of the reduction reactions become apparent and the modifying metal can be selectively deposited on specific sites of the parent-supported metal. Furthermore, such structure sensitivity depends on the nature of the re-ductant, and a given modifier can be deposited, according to the reductant used, selectively onto different parts of the metallic surface. In fact, a bimetallic catalyst can be tailored to provide the optimum activity, selectivity and lifetime for a given reaction. 

In heterogeneous catalysis, the first tests on UPD were performed on bulk catalysts which allows, for the preparation of the bimetallic catalyst, easy control of the electrochemical potential by an external device (potentiostat). In the same way all electrochemical techniques, particularly the control of catalyst potential required for submonolayer deposition, can be extrapolated to metallic catalysts supported on conductive materials such as carbon or carbides [8]. 

In conclusion, as deposition of adatoms can be controlled on well-defined fractions of the parent metal, the UPD technique, by a precise adjustment of the potential, allows a real tailoring of bimetallic catalysts. 

Although nickel catalysts have served as examples, articles dealing with other metals show that the same concepts apply. This is the case for Co deposition on silica or alumina [82], Cu/y-AhOj [124], and Ag/ /-AhOj and Ag/Ti02 [125], for example. The preparation of bimetallic catalysts is more complicated because of possible preferential reduction of one metal before the other, a phenomenon well known with the bulk oxides [3]. A few studies suggest that approaches similar to those mentioned above can also be used in these cases [122],... 

The studies reviewed here are part of a continuing effort (4-10) to identify those properties of bimetallic systems which can be related to their superior catalytic properties. A pivotal question to be addressed of bimetallic systems (and of surface impurities in general) is the relative importance of ensemble (steric or local) versus electronic (nonlocal or extended) effects in the modification of catalytic properties. In gathering information to address this question it has been advantageous to simplify the problem by utilizing models of a bimetallic catalyst such as the deposition of metals on single- crystal substrates in the clean environment familiar to surface science. 

At high temperature and in the presence of Mo complexes, THF is polymerized to yield crown ethers (scheme 3) [4], Probably, this reaction is responsible for deactivation of the bimetallic catalysts supported on Slbunit. The monometallic catalysts are not deactivated even at high yields of resin complexes and high temperature. It is likely that the polymers do not deposit on the active component and the support surface. 

The understanding of the interaction of S with bimetallic surfaces is a critical issue in two important areas of heterogeneous catalysis. On one hand, hydrocarbon reforming catalysts that combine noble and late-transition metals are very sensitive to sulphur poisoning [6,7]. For commercial reasons, there is a clear need to increase the lifetime of this type of catalysts. On the other hand. Mo- and W-based bimetallic catalysts are frequently used for hydrodesulphurization (HDS) processes in oil refineries [4,5,7,8]. In order to improve the quality of fuels and oil-derived feedstocks there is a general desire to enhance the activity of HDS catalysts. These facts have motivated many studies investigating the adsorption of S on well-defined bimetallic surfaces prepared by the deposition of a metal (Co, Ni, Cu, Ag, Au, Zn, A1 or Sn) onto a single-crystal face of anodier metal (Mo, Ru, Pt, W or Re) [9-29]. 

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