Carbon monoxide adsorption supported nickel

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

J.J. Prinsloo and P.C. Gravelle, Volumetric and calorimetric study of the adsorption of hydrogen, at 296 K, on supported nickel and nickel-copper catalysts containing preadsorbed carbon monoxide, J. Chem. Soc., Faraday Trans. I, 1980, 76, 512. 

Another Important concept introduced by Taylor was that of heterogeneity of surface-active centers.(25-26) This stemmed from observation of R. N. Pease that minute amounts of carbon monoxide, much smaller than the amount necessary to cover the surface, were sufficient to poison the surface of a copper catalyst. Taylor proposed that there were active centers on the surface while others argued that nickel impurities segregated preferentially on the surface and acted as catalyst. The variation of the heats of adsorption with surface coverage as determined by R. Beebe was used as evidence supporting the concept of active centers. In spite of the contradictory interpretation of the same experimental data, the concept of active centers has been a fruitful one. It inspired Imaginative research in the field of metal and oxide catalysis and has its present day expression in sophisticated surface physics studies. Subsequent work by coworkers of Turkevich at Princeton refined the nature of active centers in monodisperse metal particles and crystalline oxide catalysts. 

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