Chemisorption of hydrocarbons

IV. Chemisorption of Hydrocarbons on Low and High Miller Index Surfaces of... 

The adsorption and ordering characteristics of the various hydrocarbon molecules on the low Miller index platinum surfaces are discussed in great detail elsewhere. These two surfaces appear to be excellent substrates for ordered chemisorption of hydrocarbons, which permit one to study the surface crystallography of these important organic molecules. The conspicuous absence of C-H and C-C bond breaking during the chemisorption of hydrocarbons below 500 K and at low adsorbate pressures (10 9-10-6 Torr) clearly indicates that these crystal faces are poor catalysts and lack the active sites that can break the important C-C and C-H chemical bonds with near zero activation energy. 

The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (5), the Pt(S)-[9(l 11) x (100)], Pt(S)-[6(l 11) x (100)], Pt(S)-[7(lll) x (310)], and Pt(S)-[4(l 11 x (100)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10 9-10-6 Torr). Hydrocarbons on the [9(100) x (100)] and [6(111) x (100)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)] surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by... 

D. The Chemisorption of Hydrocarbons on Gold and Iridium Crystal Surfaces... 

The chemisorption of hydrocarbons, ethylene, cyclohexene, n-heptane, benzene and naphthalene at room temperature and above were studied on both the Au(l 11) and Au[6(l 11) x (100)] stepped surfaces (29). The difference in the adsorption characteristics of hydrocarbons on gold surfaces and on platinum surfaces is striking. The various light hydrocarbons studied (ethylene, cyclohexene, n-heptane, and benzene) chemisorb readily on the Pt(lll) surface. These molecules, on the other hand, do not adsorb on the Au(lll) surface under identical experimental conditions as far as can be judged by changes that occur in the Auger spectra. Naphthalene, which forms an ordered surface structure on the Pt(lll) face, forms a disordered layer on adsorption on the Au(l 11)surface. 

The chemisorption of hydrocarbon molecules on surfaces presents another class of important and interesting systems for study. We shall discuss the case of acetylene chemisorption on the Ni (111), Rh (111) and Pt (111) surfaces, as they incorporate many features relevant to all hydrocarbon chemisorption systems. 

Since cation-radical formation in the chemisorption of hydrocarbons has not previously been considered in the catalytic literature, the nature, reactions, and mechanism for formation of such species should be of considerable importance to the elucidation of catalytic reaction mechanisms particularly in view of the fact that Webb (20) has found spectral evidence for the formation of species other than carbonium ions from butene-2 adsorbed on silica-alumina. It is not possible at the present time to define either the role of cation-radicals in acid catalysis or the chemical nature of the electrophilic surface sites involved in their formation. 

Chemisorption of hydrocarbons on various metals, such as nickel, platinum, copper, etc., was investigated in great detail (9, 90, 91, 92). Information on chemisorption of ethylene, acetylene and methane on various metals may be found in Trapnell s review (93). However, direct application of the relations obtained to metal oxide catalysts would scarcely be justifiable. As a rule, oxygen covers the whole surface of the metal, and chemisorption of hydrocarbons occurs either on a thin layer of the given metal oxide formed as an individual phase, or on oxygen that was sorbed on the surface and has filled the adjacent-to-surface layers. Thus data on chemisorption of hydrocarbons on oxides of these metals may be of use in the above cases. 

Earlier publications from this laboratory ( -4) have shown that the surface area of the catalysts increases with their alumina content, that chemisorption of hydrogen is more pronounced on the mixed catalysts than on pure Cr20s, and that chemisorption of hydrocarbons occurs, but that the effects with paraffins are obscured by decomposition reactions which quickly set in. It was also shown that Cr203 is an amphoteric semiconductor (5) but that the catalysts behaved as w-type semiconductors under the conditions required to produce aromatic hydrocarbons. The conductivity varied widely with composition, and the energy of activation for conduction decreased as the alumina content increased. 

N. Sheppard. Vibrational Spectroscopic Studies of the Structure of Species Derived from the Chemisorption of Hydrocarbons on Metal Single-Crystal Surfaces. Ann. Rev. Phys. Chem. 39 589 (1988). 

The oxidation state, i.e. the electron density of Pd particles is of significant interest. Recently de Vries et al. have investigated different lanthanum doped automotive catalysts and detected several different Pd species on the catalysts [17-21]. The hi er electron density of the Pd particles has an influence on the chemisorption of hydrocarbon species. 

Alkyl transfer steps in the catalytic alkylation of benzene, toluene, and cyclohexane have been investigated over supported Pt, Ir, Ru, and Au. The influence of hydrocarbon partial pressure ratios, temperature, catalyst support, catalyst acidity and basicity, and method of catalyst preparation have been examined. The results are discussed in terms of competitive chemisorption of hydrocarbons. H2 transfer between benzene and C6-hydrocarbons, " and O2 transfer between CO and C02, ethylene and ethylene oxide, and propylene and propylene oxide have also been studied in an attempt to correlate catalyst and reaction variables with resultant rates of reaction. 

The purpose of this section is to try to construct a map to guide us through the jungle that is the literature on the chemisorption of hydrocarbons, to evolve -if indeed it is possible - some general principles, to delineate the main features, and to outline the strategy to be adopted in later sections. 

In this Section we shall consider (i) detailed structure determinations of ethene, ethyne and benzene, (ii) measurements of the heats of chemisorption of hydrocarbons on single crystals and small particles, and (iii) their spectroscopic characterisation. 

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