Studies of Hydrocarbon Reactions on Low-Index Iridium and Platinum Surfaces

Citation

Szuromi, Phillip David (1985) Studies of Hydrocarbon Reactions on Low-Index Iridium and Platinum Surfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ndxy-rb82. https://resolver.caltech.edu/CaltechTHESIS:01072019-124055656

Abstract

The interaction of hydrocarbons with the (110)-(1x2) and (111) surfaces of iridium and the (110)-(1x2) surface of platinum has been studied under ultrahigh vacuum conditions. The principle experimental techniques employed were thermal desorption mass spectrometry and low energy electron diffraction.

Chapter 2 describes the extension of previous studies of the adsorption and reaction of ethane and propane on the Ir(110)-(1x2) surface to the normal isomers of butane, pentane, hexane and heptane. At low coverages, each of these alkanes undergoes dissociative chemisorption at 130 K. At higher coverages, molecular adsorption occurs as well. Thermal desorption spectra of hydrogen are similar in many respects for the dissociatively adsorbed overlayers of all six of these paraffins. Both desorption-limited and reaction-limited adstates of hydrogen are observed, the latter being associated with the dehydrogenation of hydrocarbon fragments on the surface. Ethane, butane and hexane form high temperature adstates, the associated fragments of which are low in hydrogen content, while those for propane, pentane and heptane contain relatively more hydrogen. This difference may be explained by extending a model for dehydrogenation which has been proposed previously [T. S. Wittrig, P. D. Szuromi and W. H. Weinberg, J. Chem. Phys. (76), 3305 (1982)] for understanding the dissociative adsorption of other saturated hydrocarbons on this surface.

Chapter 3 discusses cyclopropane, propylene, propyne and allene on the reconstructed Ir(110)-(1x2) surface. Annealing adlayers of these hydrocarbons (at low coverages) leads to the formation of surface hydrogen and hydrocarbon fragments of approximate stoichiometry C₃H₂. The importance of the β₂ adsite of hydrogen on this surface of iridium has been demonstrated further by inhibition studies with hydrogen, CO and surface carbon. The close-packed Ir(111) surface dehydrogenates propylene, but neither propane nor cyclopropane adsorb dissociately under the same reaction conditions, indicating a strong effect for the activation of carbon-hydrogen bonds of alkanes.

Chapter 4 describes the investigation of this strong effect of surface geometry on the dissociative adsorption of alkanes on surfaces of platinum. Previous work (L. E. Firment, Ph.D. Thesis, Univ. of California, Berkeley, 1976) shows that the close-packed Pt(111) surface does not dehydrogenate the normal alkanes through octane under ultrahigh vacuum conditions. On the reconstructed Pt(110)-(1x2) surface, low coverages of n-butane and n-pentane adsorb dissociatively at approximately 200 K to form surface hydrogen and hydrocarbon fragments, whereas only molecular adsorption is observed for ethane and propane. Inhibition of this reaction by precoverages of hydrogen suggests strongly that carbon-hydrogen bond activation is the initial reaction step, and occurs at the same adsite as for the adsorption of hydrogen at lower coverages. Thus for both iridium and platinum the availability of high coordination adsites on the (110)-(1x2) surface appears to lower the kinetic barriers that must be overcome to activate carbon-hydrogen bonds in alkanes. Differences in the electronic structure of the two metals manifest themselves in such details as the magnitude of that kinetic barrier.

Thesis Files