Metal monometallic catalysts

Low pressure operation became routine with the appHcation of new catalysts that are resistant to deactivation and withstand the low pressures. The catalysts are bimetallic most incorporate rhenium as well as platinum (95). The stmctures of these catalysts are stiU not well understood, but under some conditions the two metals form small alloylike stmctures, which resist deactivation better than the monometallic catalyst. 

From a general point of view, a monometallic catalyst can be considered as surface metal atoms linked together, forming an ensemble on the surface [160]. 

Another example from Liu s team in this field concerns the selective hydrogenation of citronellal to citronellol by using a Ru/PVP colloid obtained by NaBH4 reduction method [112]. This colloid contains relatively small particles with a narrow size distribution (1.3 to 1.8 nm by TEM), whereas the metallic state of Ru was confirmed by XPS investigation. This colloid exhibited a selectivity to citronellol of 95.2% with a yield of 84.2% (total conversion 88.4%), which represented a good result for a monometallic catalyst. 

The generation of monometallic catalysts with suitable characteristics is the first key step for obtaining supported bi- and organobimetallic catalysts via SOMC/M techniques. Ion-exchange techniques are generally used in the preparation of these catalysts, which lead to solids with good metallic phase dispersion and homo-... 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

In bimetallic catalysts, the influence of Cu as a second metal in catalytic activity and selectivity is also closely related to the nature of the supports. However, as a general rule Ni-Cu alloying promotes an improvement in selectivity values. In this respect, the most interesting results were obtained in Ni-CU/AIPO4 catalyst containing 20 wtX of both Ni and Cu because the selectivity increased to 98% and the catalytic activity maintained the same level as in Ni/AlPO4 monometallic catalyst. 

The pH of the solution can also play an important role. For nitrate reduction, reaction was faster in lower pH solutions (Horold et al. 1993 Liidtke et al. 1998) however, acidic solutions can dissolve the catalyst metal and negate the effects of the faster rates. The Cu in the bimetallic nitrate-reducing catalyst dissolved at pH values < 5 and deactivation was observed (Horold et al. 1993) Pd dissolution in a monometallic catalyst became significant below a pH of 4. (Munakata et al. 1998)... 

Coates and Darensbourg have separately reported that salen metal catalysts undergo bimetallic initiation, followed by a monometallic propagation [123, 129, 155,163]. In contrast, a theoretical study by Rieger and co-workers predicted chain growth to take place via the attack of the metal-bound alkyl carbonate on a metal-coordinated epoxide [103]. Related bimolecular processes have also been observed by Jacobsen et al. for the asymmetric ring opening of epoxides [164, 165]. Some of the mechanistic routes reported with metal salen catalysts are depicted in Fig. 24. 

Various transition metal-based catalysts not containing preformed metal-carbon bonds have been developed for the polymerisation of conjugated dienes [27-35, 150-158]. These catalysts include monometallic precursors such as Rh, Co and Ni salts and bimetallic precursors such as C0CI2-AICI3. Some of them are soluble in a polymerisation medium, e.g. Rh(N03)3 in protic solvents (ROH, H2O) [27,150-154] and C0CI2—AICI3 in aprotic solvents [155-157], and some others are insoluble in a polymerisation diluent, e.g. TiCL—Ni(PCl3)4 [158]. 

The classic co-impregnation or successive impregnation of two metallic salts often proves to be unsatisfactory and new techniques are being tried [6-10]. In all cases these techniques call for the preparation of a monometallic catalyst (i.e. parent catalyst) which is then modified by addition of the second metal. This modification occurs through a selective reaction which takes place solely on the monometallic particles of the parent catalyst. 

Direct redox reactions can be of great interest for the preparation of bimetallic catalysts, in cases where a parent monometallic catalyst is prepared and is modified by reaction with the oxidized form of a second metal. A typical reaction can be expressed as... 

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. 

Activity-versus-time curves shown in Fig. 25 for alumina-supported Ni and Ni bimetallic catalysts show two significant facts (1) the exponential decay for each of the curves is characteristic of nonuniform pore-mouth poisoning, and (2) the rate at which activity declines varies considerably with metal loading, surface area, and composition. Because of large differences in metal surface area (i.e., sulfur capacity), catalysts cannot be compared directly unless these differences are taken into account. There are basically two ways to do this (1) for monometallic catalysts normalize time in terms of sulfur coverage or the number of H2S molecules passed over the catalysts per active metal site (161,194), and (2) for mono- or bimetallic catalysts compare values of the deactivation rate constant calculated from a poisoning model (113, 195). 

Applications to monometallic catalysts include the analysis of chlorine (Cl Ka) in alumina (Fig. 4.9), and metals analysis of supported Pt (La) used for reforming (implying the presence of chlorine) and analysis of Pd in an alumina matrix for selective hydrogenation catalysts. 

The most active catalysts for methanol oxidation are presently based on bifunctional systems such as Pt-Ru. Yet the evaluation of a metal-support interaction would require the analysis of a simpler catalytic system (i.e., a monometallic catalyst) this would avoid the interference of all those aspects such as degree of alloying, changes in crystallographic parameters, chemical state of the promoting element, which significantly affect the activity, and thus a comprehensive interpretation of the data actually available. 

The NSD of precursors mainly applies when Ml and M2 are transition metals. The basic principle of NSD of precursors, together or successively, is such that the interaction of both precursors with the solid surface was stronger than between the two precursors. On that account the two deposited precursors are separated on the support, and surface diffusion will be necessary to yield the bimetallic aggregates during the activation process. NSD will use the same deposition methods as for monometallic catalysts (vide supra). 

Modem hydroformylation research is almost exclusively focused on four transition metals cobalt, rhodium, platinum and to considerable extent mthenium [13], The generally accepted order of hydroformylation activity for unmodified monometallic catalysts clarifies this picture [14] ... 

Bimetallic Catalysts. - It is hardly surprising that metallic catalysts containing more than one species of metal are more chemically versatile than monometallic catalysts. The fact that an enormous variety of bimetallic (and more complex) catalysts can be made with relatively simple synthetic techniques poses both a great opportunity and a challenge for the development of... 

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