Overlayers and Active Species

Carbon Overlayers and Active Species.—One of the most interesting features of the paper by Nieuwenhuys and Somorjai on dehydrogenation of cyclohexane and dehydrocyclization of n-heptane over (111) and stepped [6(111) x (100)] single crystals of Ir is their observation of carbon overlayers. AES and LEED were employed in the surface study. The stepped Ir surface was three to five times more active in dehydrogenation than the Ir(lll) surface. The dehydrocyclization reaction rate was not sensitive to the face of Ir on which the reaction was conducted. 

What is significant, however, is that on a (111) surface on which C had been intentionally deposited, no difference was observed for either dehydrogenation or dehydrocyclization. AES revealed that C deposits (up to monolayer amounts) are formed during the course of reactions on the clean surfaces. 

They then make some useful comments on the possible meaning of their results for catalysis. The surfaces at the beginning and the end of the series are not likely to be versatile catalysts in C—H and C—C reactions. Pt(lll) reacts too mildly and Ir[6(l 11) x (100)] too strongly with hydrocarbons. The surfaces in the middle are promising for catalysis. Small differences in the electronic structures of Pt and Ir may account for the differences between them. Ir, with its fewer /-electrons, may permit greater electron donation from the hydrocarbon adsorbate to the metal, thereby weakening C—C and C—H bonds. 

This theme of the importance of overlayers on metal catalysts is taken further in the next paper, which forms a real bridge between surface physics and catalysis. 

This modification of a surface by hydrocarbon adsorption has been proposed by other authors in that M—Cj Hy has been suggested as the active species in hydrogenation. Demuth s paper seems to add further evidence for this approach to catalysis. 

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