Ruthenium-copper alloy catalysts

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. 

Ruthenium and copper are not miscible hence, homogeneous alloy particles will not be formed in supported Ru-Cu catalysts. As copper has a smaller surface free energy than ruthenium, we expect that if the two metals are present in one particle, copper will be at the surface and ruthenium in the interior (see also Appendix 1). This is indeed what chemisorption experiments and catalytic tests suggest [40], EXAFS, being a probe for local structure, is of particular interest here because it investigates the environment of both Ru and Cu in the catalysts. 

Let us first consider what EXAFS might tell us in the case of bimetallic particles that are not too small - say a few nanometer in diameter. For a truly homogeneous alloy with a 50 50 composition, EXAFS should see a coordination shell of nearest neighbors with 50% Cu and 50% Ru around both ruthenium and copper atoms. If, on the other hand, the particle consists of an Ru core surrounded by a Cu shell of monatomic thickness, we expect that the Ru EXAFS shows Ru as the dominant neighbor, because only Ru atoms in the layer directly below the surface are in contact with Cu. The Cu EXAFS should see both Cu neighbors in the surface and Ru neighbors from the layer underneath, with a total coordination number smaller than that of the Ru atoms. The latter situation is indeed observed in Ru-Cu/Si02 catalysts, as we shall see below. 

Following the development of sponge-metal nickel catalysts by alkali leaching of Ni-Al alloys by Raney, other alloy systems were considered. These include iron [4], cobalt [5], copper [6], platinum [7], ruthenium [8], and palladium [9]. Small amounts of a third metal such as chromium [10], molybdenum [11], or zinc [12] have been added to the binary alloy to promote catalyst activity. The two most common skeletal metal catalysts currently in use are nickel and copper in unpromoted or promoted forms. Skeletal copper is less active and more selective than skeletal nickel in hydrogenation reactions. It also finds use in the selective hydrolysis of nitriles [13]. This chapter is therefore mainly concerned with the preparation, properties and applications of promoted and unpromoted skeletal nickel and skeletal copper catalysts which are produced by the selective leaching of aluminum from binary or ternary alloys. 

Investigations into these topics are presented in this volume. Iron, nickel, copper, cobalt, and rhodium are among the metals studied as Fischer-Tropsch catalysts results are reported over several alloys as well as single-crystal and doped metals. Ruthenium zeolites and even meteo-ritic iron have been used to catalyze carbon monoxide hydrogenation, and these findings are also included. One chapter discusses the prediction of product distribution using a computer to simulate Fischer-Tropsch chain growth. 

Green CL, Kucemak A (2002) Determination of the platinum and ruthenium surface areas in platinum-ruthenium alloy electrocatalysts by underpotential deposition of copper. 1. Unsupported catalysts. J Phys Chem B 106 1036-1047... 

In order to produce methanol the catalyst should only dissociate the hydrogen but leave the carbon monoxide intact. Metals such as copper (in practice promoted with ZnO) and palladium as well as several alloys based on noble group VIII metals fulfill these requirements. Iron, cobalt, nickel, and ruthenium, on the other hand, are active for the production of hydrocarbons, because in contrast to copper, these metals easily dissociate CO. Nickel is a selective catalyst for methane formation. Carbidic carbon formed on the surface of the catalyst is hydrogenated to methane. The oxygen atoms from dissociated CO react with CO to CO2 or with H-atoms to water. The conversion of CO and H2 to higher hydrocarbons (on Fe, Co, and Ru) is called the Fischer-Tropsch reaction. The Fischer-Tropsch process provides a way to produce liquid fuels from coal or natural gas. 

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