Gold/nickel surface alloy catalyst

It should be noted that, in the interpretation of activity patterns of alloy catalysts, extreme care is needed to ensure that the surface composition is known. It has been shown [321,322] with copper—nickel alloys, which show two phases in the composition range 2—80% copper, that, within this miscibility gap, the surface composition remains constant at 80% Cu—20% Ni, independent of the nominal bulk composition. Furthermore, the surface composition may vary depending upon the catalyst pretreatment [322], No miscibility gap occurs with palladium—gold or palladium-silver alloys [323]. 

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. 

A third factor controlling surface composition is the atmosphere in which the catalyst is used. The surface will be enriched in that component of the alloy that has the highest heat of adsorption of the gas. In an oxygen atmosphere the surface of a nickel-gold catalyst becomes enriched with nickel rather than gold. 57 In the presence of CO the surface of a palladium-silver alloy becomes enriched with palladium while, normally, silver would be the predominant surface component. 58 This enrichment of the surface by palladium should also be observed in a hydrogen atmosphere. 

Figure 2.33. TGA data showing the effect of alloying of nickel with gold on the amount of carbon deposited on the catalyst surface during steam reforming of n-butane.10...

Ensemble control is also involved in carbon-fiee steam reforming on a sulphur passivated catalyst [390] (Section 5.5). Ensemble control was also reported for the addition of Bi [500] or B [526] to nickel and for bimetallic catalysts such as Ni,Au [50], Pt,Re [367], Pt,Sn [474], and Ni,Sn [217] [247] [431] [456] [545]. Alloying nickel with copper [49] [16] can also decrease the rate of carbon formation, but it is not possible to achieve die same high surface coverage with copper as with sulphur and gold, because copper and nickel forms a bulk alloy with a fixed surface concentration of copper over a wide range as alloy composition. 

In yet another method [42], the reaction for pyrolysis of l,2-dichloro-2,2-difluoroethane in the presence of hydrogen was carried out in the absence of a catalyst in an essentially empty reactor at a temperature >400°C. In the absence of a catalyst refers to the absence of a conventional catalyst. A typical catalyst has a specific surface area and is in the form of particles or extrudates, which may optionally be supported to facilitate the dehydrochlorination reaction by reducing its activation energy. The reactors that are suitable are quartz, ceramic (SiC), or metallic reactors. In this case, the material constituting the reactor was chosen from metals such as nickel, iron, titanium, chromium, molybdenum, cobalt or gold, or alloys thereof. The metal, chosen more particularly to limit corrosion or other catalytic phenomena, may be bulk metal or metal plated onto another metal. 

---