Alloy catalysts solid solutions

In order to act as a catalyst, a given substance has to bear its specific affinities on the reacting molecules. Obviously, a modification of the field of force of these specific affinities can be achieved by the presence of additional components in a catalyst, either via a combination of the affinities of both components at phase boundaries, or via a gradual change of the affinities of the main catalyst due to the formation of solid solutions, alloys, or mixed crystals with the accessory component. 

A second breakthrough in catalysis by metals was the discovery of alloyed catalysts. Formally, alloyed particles should only be considered when a solid solution or a definite compound is formed. However, we wish to extend the terminology of alloy , taking the proposition of Ponec and Bond [5], we will adopt this term to describe ... any metallic system containing two or more components, irrespective of their intimacy of mixing or the precise manner in which their atoms are... 

Theoretical and experimental studies of model bimetallic catalysts in recent years have distinguished between thermodynamically stable bulk alloys and so-called near surface alloys. Near surface alloys are materials where the top few surface layers are created in a chemically heterogeneous way, for example, by depositing a monolayer of one metal on top of another metal. These structures are often not the thermodynamic equilibrium states of the material. To give one example, Ni and Pt form an fee bulk solid solution under most (but not all) conditions,73 so if a monolayer of Ni is deposited on Pt and the system comes to equilibrium, all of the deposited Ni will dissolve into the bulk. There is, however, a considerable kinetic barrier to this process, so the near surface alloy of a monolayer on Ni on Pt(lll) is quite stable provided a moderate temperature is used.191 If the deposited monolayer in systems of this type has a tendency to segregate away from the surface, a common near surface alloy structure is the formation of a subsurface layer of the deposited metal.85 The deposition of V on Pd(lll) is one example of this behavior.192... 

The alloy catalysts used in these early studies were low surface area materials, commonly metal powders or films. The surface areas, for example, were two orders of magnitude lower than that of platinum in a commercial reforming catalyst. Hence these alloys were not of interest as practical catalysts. The systems emphasized in these studies were combinations of metallic elements that formed continuous series of solid solutions, such as nickel-copper and palladium-gold. The use of such systems presumably made it possible to vary the electronic structure of a metal crystal in a known and convenient manner, and thereby to determine its influence on catalytic activity. Bimetallic combinations of elements exhibiting limited miscibility in the bulk were not of interest. Aspects of bimetallic catalysts other than questions related to the influence of bulk electronic structure received little attention in these studies. 

Since the ability to form bulk alloys was not a necessary condition for a system to be of interest as a catalyst, it was decided not to use the term alloy in referring to bimetallic catalysts in general. Instead, terms such as bimetallic aggregates or bimetallic clusters have been adopted in preference to alloys. In particular, bimetallic clusters refer to bimetallic entities which are highly dispersed on the surface of a carrier. For systems such as ruthenium-copper, it appears that the two components can interact strongly at an interface, despite the fact that they do not form solid solutions in the bulk. In this system the copper is present at the surface of the ruthenium, much like a chemisorbed species. 

It was discovered that the ability of metals to form solid solutions (alloys) in the bulk is not necessary for a bimetallic system to be of interest as a catalyst. An example is the ruthenium-copper system, in which the two components are virtually completely immiscible in the bulk. This system exhibits an effect of the copper (in particular, selective inhibition of hydrocarbon hydrogenoly-sis) similar to that exhibited by the nickel-copper system, in which the components are completely miscible. Although ruthenium and copper do not form solid solutions in the bulk, they do exhibit a strong interaction at copper-ruthenium interfaces. The copper tends to cover the surface of the ruthenium, analogous to a chemisorbed layer. As a result, the copper has a marked effect on the chemisorption and catalytic properties of the ruthenium. 

The term bimetallic was introduced by Sinfelt to account for the fact that a catalyst may contain a multitude of phases containing the active metallic components.22 Of these many phases, a characteristic one is the binary alloy. The term alloy can describe a broad range of situations from well-defined phases or solid solutions to surface alloys in cases where bulk alloys are not thermodynamically favoured but a clearly defined surface local arrangement is obtained. Note that the novel core-shell bimetallic structures are included in this catch-all term. A historical overview of the properties of alloys in connection with catalysis has been published by Ponec.23 At present, a... 

Still another application of thermomagnetic analysis to nickel catalysts relates to the addition of other components, such as copper, which may be thought to have a favorable influence on catalyst behavior. Nickel has a magnetic moment corresponding to 0.6 unpaired electron per atom in the d-band. Alloys of nickel and copper become progressively less magnetic until, at 60 atom % copper, the magnetic moment becomes zero. It is, therefore, a simple matter to determine to what extent solid solution has taken place if, say, some copper nitrate is added to the nickel solution used in preparation of the catalyst. Similarly, any influence of the copper on particle size distribution is readily observed. 

Iron, nickel, cobalt, and their alloys are the most studied metals for the catalytic growth of CNFs or CNTs. The readiness of these metals to produce metal-carbon solid solutions and to form metastable carbides in the appropriate reaction temperature range should be an important factor to take into account for the comprehension of their reactivity. The different carbon species formed depending on the temperature range employed in the steam reforming of hydrocarbons on nickel catalysts have been discussed [29] and consist of ... 

Studies of small particles by Sinfelt [29] and his co-workers have shown that when the particles sizes become very small and dispersions tend toward unity (that is, when virtually every atom is at the surface), alloy systems exhibit phase diagrams very different from those that characterize bulk systems. For example, microclusters containing Cu and Ru, Cu and Os, or Au and Ni can be produced in any ratio of the two elements, indicating complete miscibility or solid solution behavior. In the bulk phase these elements are completely immiscible. This very different behavior of the surface phases of bimetallic systems finds important applications in the design of catalysts to carry out selective chemical reactions. Moran-Ldpez and Falicov [30] developed a theory—using pairwise interactions—of alloy surface segregation that explains this effect. Bimetallic systems remain miscible at lower temperatures in two dimensions than in three dimensions. 

The observed lattice parameter for the Pt-Ru/C sample follows from the presence of a solid solution of Pt and Ru. According to the variation of afcc with composition for Pt-Ru bulk alloys, an atomic fraction of 45% Ru should be present in the carbon supported alloy. An average particle size for the metal crystallites of 23 A and 20 A in the Pt/C and Pt/Ru/C catalysts, respectively, was determined from the broadening of the (220) diffraction peak by using the Debye-Sherrer equation. 

Mossbauer spectroscopy showed that on silica supported catalysts, only Pd-Sn solid solutions or alloys are present, whereas, on alumina supported catalysts, tin is present at... 

Skeletal Cu-Pd of different compositions were prepared Catalysts were obtained by etching Cu-Pd-Al alloys (70% Al). The prepared catalysts contained Cu and Pd phases and their solid solutions and up to 40-58% bayerite. Linearly increasing the contents of Pd in the catalysts fi om 5 to 100%, increased the rate of hydrogenation of EAA at 100°C and 100 bar. More complicated dependence on concentrations of Pd was found below 5%. In the interval of 1-5% Pd the ee values went through a maximum and at 2% Pd in Cu-Pd the optical yield reached the maximal value of 4-5%. Vedenyapin et ai i 58,2so electrochemical method and found... 

The crystal structure of pure Pt is face-centered cubic (fee), while that of Ru is hexagonal close packed (hep). For Ru atomic fractions up to about 0.7, Pt and Ru form a solid solution with Ru atoms replacing Pt atoms on the lattice points of the fee structure. The lattice constant decreases from 0.3923 (pure Pt) to 0.383 nm (0.675 atomic fraction of Ru). In contrast to bulk Pt-Ru alloys, it has to be remarked that in carbon supported catalysts the amount of Ru alloyed with Pt is lower than the nominal Ru content in the material the amount of Ru alloyed with Pt depends on the preparation method of the supported catalyst. In Pt-Ru-M catalysts, the third metal is an oxophilic element as W, Mo, Os, Ni, Ir, etc. Some of these elements can be fully alloyed, while several form alloys to a limited extent or not at all with Pl ... 

Many studies carried out with bimetallic materials show that Pt-Ru is today one of the best options to oxidize methanol. Thus, a simple way to prepare active Pt-Ru catalysts involves the deposition of metallic nanoparticles from a suspention onto the carbon microparticles by the method known as formic acid method [24]. Considering that the crystal structures of Pt and Ru are different, Pt being fee and Ru hep, the final crystal structure of the alloy depends on the composition. For Ru atomic fractions up to 0.6-0.7, the two metals form solid solutions in which Ru atoms replace Pt lattice points in the fee structure. The opposite situation, Pt atoms replacing Ru atoms in the hep structure is found for Ru atomic fractions higher than about 0.7. However, the crystal structure seems to depend also on the physical state of the material. When nanoparticles are prepared by reduction of ionic metal species, there is at least one report [25] claiming that the fee stmeture prevails up to 80 at.%Ru. On the other hand in sputtered films the hep structure is predominant even at low Ru fractions [26]. 

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