Bifunctional and Spillover Catalysis

Bifunctional and Spillover Catalysis. - It has been suggested above that the active involvement of the support in a catalytic reaction could appear as a metal-support interaction, and several recent papers demonstrate in some detail how this can come about. Catalysis of the hydrogenation of CO2 by Rh on various supports shows a strong dependence of turnover number on the kind of support used (see Table 2). Dispersions, D, are similar, as are activation energies, so the considerable differences in turnover number reside principally in the pre-exponential factor. A further study of this system by i.r. spectroscopy has identified CO on the metal and the formate ion on the support (although not in the case of Si02), and the favoured mechanism for CH4 formation involves a reaction of spillover H with CO2 chemisorbed on the support by insertion into surface OH groups. 

As a further example of support involvement, the reaction of NO with CO may be cited. A recent study, also by i.r. spectroscopy, has demonstrated clearly that the intermediate isocyanate (-NCO) species resides on the support. This species, which is not observed when the catalytic metal 

Guczi and B. S. Gudkov, React. Kinet. Catal. Lett.., 1977, 7, 105. 

We may also recall that striking differences in catalytic activity and in reaction orders were observed in the oxidation of CO with Pd/Al203 and with Pd/Sn02- It appeared that CO could react more easily with lattice oxygen in the Sn02 if it first chemisorbed on the Pd and then spilled over to the support. The positive participation of the support in the total reaction may be more common than is often supposed. 

The same kind of support effect is shown in the steam dealkylation of toluene catalysed by Rh (see Table 4), although significantly not in its hydrogenolysis. These results confirm those reported earlier to Kochloefl, whose rates and activation energies were however somewhat higher. Activation of H2O by the support is again proposed, with reaction of metal-bonded CH  

---

Bifunctional and spillover catalysis and reaction at interfacial sites may occur to an extent depending upon the components of the catalyst. 

The definition of promotion suggested above excludes cases where the support itself provides an essential service (e.g., in bifunctional and spillover catalysis) or where the promotion occurs unintentionally. Carbon deposition can however sometimes result in a selectivity improvement and of course a species may act at the same time as both an electronic and a selectivity promoter. For these reasons it is not always easy to classify what a particular promoter does, or how it works. 

---