particular reaction, often a simple hydrogenation reaction of an unsaturated hydrocarbon. The possibility that the effect of alloying depends on the type of reaction was considered later.24,29,30 A striking example of specificity with regard to the type of reaction is provided by work on Ni-Cu alloy catalysts in which two different reactions were investigated, the hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene. The latter type of reaction is important in the production of gasoline components in the petroleum industry.23 The effect of Cu on the catalytic activity of Ni for cyclohexane dehydrogenation is very different from that found for ethane hydrogenolysis,24 as shown by the data on a series of Ni-Cu alloys in Figure 9. In the case of ethane hydrogenolysis, adding only 5 atomic percent Cu to Ni decreases the catalytic activity by 3 orders of magnitude. With further addition of Cu, the activity continues to decrease. However, the activity of Ni for the dehydrogenation of cyclohexane is affected little over a wide range of Ni-Cu alloy compositions and actually increases on the addition of the first increments of Cu to Ni. Only as the catalyst composition approaches pure Cu is a marked decline in activity for this reaction observed.
The Ni-Cu alloys just discussed were prepared under conditions of complete miscibility of the two components. At this point, it is pertinent to consider a system such as ruthenium-copper (Ru-Cu), the components of which are essentially immiscible in the bulk. The crystal structures of the two metals are different, Ru having a hexagonal close-packed structure and Cu a face-centered cubic structure. Although the Ru-Cu system can hardly be considered as an alloy-forming system, it is possible to prepare bimetallic Ru-Cu aggregates that are similar to alloys such as Ni-Cu in their catalytic behavior. In such aggregates the Cu tends to cover the surface of the Ru.31,32 Evidence for this structure comes from studies of hydrogen chemisorption capacity and ethane hydrogenolysis activity, both of which are markedly suppressed when even small amounts of Cu are present with the Ru. The interaction between the two components may be considered analogous to that which would exist in the chemisorption of Cu on Ru. The behavior of the Ru-Cu system for ethane hydrogenolysis is similar to that observed for Ni-Cu. In cyclohexane dehydrogenation to benzene, the two systems also behave similarly, in that Cu has only a small effect on dehydrogenation activity. However, pure Ru exhibits extensive hydrogenolysis of cyclohexane to alkanes of lower carbon number (mostly methane) in addition to dehydrogenation to benzene. Addition of Cu to Ru suppresses hydrogenolysis strongly relative to dehydrogenation, so that a marked increase in the selectivity to benzene is observed.
Thus copper can influence the selectivity of a Group VIII metal whether