Platinum ethylene chemisorption

The chemisorption and thermal decomposition of ethylene over platinum (111) surfaces have been extensively studied by several groups using a range of modern surface science techniques (1-3 ). Chemisorption at low temperatures is molecular, with the... 

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations.10 For example, ethylene (ethene, H2C=CH-2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3—C= (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif-... 

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. 

Preparation of metal oxide thin film by means of stepwise absorption of metal alkoxide has been carried out in the past for the activation of heterogeneous catalysts [13]. For example, Asakura et al. prepared one-atomic layer of niobium oxide by repeating chemisorption of Nb(OEt)5 on silica beads. The catalyst obtained by immobilizing platinum particles on a niobum oxide layer showed improved reactivity for hydrogenation of ethylene in comparison with... 

Our studies of olefin and acetylene chemisorption states on platinum surfaces is presently incomplete. Ethylene and acetylene chemisorption on platinum (111) are complicated by the apparent presence of more than one chemisorption state (indicated by thermal desorption studies). When C2HH and C2D1, are chemisorbed on Pt(lll), the small fraction of ethylene thermally desorbed as ethylene comprises nearly a statistical mixture of all possible molecules. Thus we see here reversible C-H (and C-D) bond breaking on this flat platinum surface. In an analogous experiment with C2H2 and C2D2> only a small extent of H-D exchange was observed for the small fraction of acetylene molecules that reversibly desorb from this surface (11). 

These conditions must be satisfied in order to correctly apply the steady-state approximation to a reaction sequence. Consider the H-D exchange with ethylidyne (CCH3 from the chemisorption of ethylene) on a platinum surface. If the reaction proceeds in an excess of deuterium the backward reactions can be ignored. The concentrations of the adsorbed ethylidyne species have been monitored by a technique called secondary ion mass spectroscopy (SIMS). The concentrations of the various species are determined through mass spectroscopy since each of the species on the surface are different by one mass unit. Creighton et al. [Surf. Sci., 138 (1984) L137] monitored the concentration of the reactive intermediates for the first 300 s, and the data are consistent with what are expected from three consecutive reactions. The results are shown in Figure 4.2.2. 

The presence of electrolyte, its possible adsorption on the electrocatalyst, and the electrode-electrolyte potential can alter the strength of reactant adsorption, the surface coverage, and the reaction rate (5,7,8). Thus, electro-generative hydrogenation of ethylene on platinum and palladium electrodes in acidic electrolytes proceeds more slowly than the corresponding gas phase catalytic reactions (33). However, electrocatalytic reduction of cyclopropane is faster than the catalytic one, probably due to a decrease in hydrogen and reactant competitive chemisorption. Some electrolyte ions and impurities can also poison the electrocatalysts (34). 

Figure i 0-3 shows the bonding from the adsorption of ethylene on a platinum surface to form chemisorbed ethylidyne. Like physical adsorption, chemisorption is an exothermic process, but the heat.s of adsorption are generally of the same magnitude as the heat of a chemical reaction fi.e., 40 to 400 kj/mol). If a catalytic reaction involves chemisorption, it must be carried out within the temperature range where chemisorption of the reactants is appreciable. 

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. 

Molecular chemisorption occurs with molecules having multiple bonds or free electron pairs. For example, on platinum surfaces, ethylene gives up two n electrons of its double bond and forms two a bonds with Pt atoms. The resulting sp hybridization results in a tetrahedral arrangement of bonds (Fig. 5-10). 

Hahn F, Beden B, Kadirgan F, Lamy C (1987) Electrocatalytic oxidation of ethylene-glycol. 3. In-situ infrared reflectance spectroscopic study of the strongly bound species resulting from its chemisorption at a platinum-electrode in aqueous-medium. J Electroanal Chem 216(1-2) 169-180... 

The chemisorption of ethylene glycol on platinum has been investigated [59, 60] and the results obtained by pulse methods were interpreted [60] as due to chemisorption process on smooth platinum at low temperatures in the following equation ... 

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