Platinum ethylene adsorption

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

C-Propene adsorption on platinum—alumina and platinum—silica [66] differs from ethylene adsorption insofar as a fraction of the initially retained 14C-propene is relatively easily exchanged or removed by hydrogen treatment. This suggests less extensive dissociation of the adsorbed propene and a 7T-allyl species (structure F) has been proposed in this case, viz. 

Fig. 15. Potential dependence of acetylene and ethylene adsorption on smooth platinum electrodes in 1 N HCIO4 at 30°C (184). The charge required for species removal from the surface is proportional to the surface coverage.

In Section II1.2 we discussed ethylene adsorption to a dispersed silver supported on alumina. This section is separated from the earlier discussion for the principal reason that the spectroscopic methods utilized in the silver investigations are generally not applicable to transition metals with partially filled d-orbitals. The most relevant metals (primarily platinum, rhodium, ruthenium, and palladium)... 

Faraday was the first to carry out experiments to explore why platinum facilitates the oxidation reactions of different molecules. He found that ethylene adsorption deactivates the platinum surface temporarily while the adsorption of sulfur deactivates platinum permanently. He measured the rate of hydrogen oxidation, suggested a mechanism, and observed its deactivation and regeneration. Thus, Faraday was the first scientist who studied catalytic reactions. In 1836 Berzelius (1, 2] defined the phenomenon and called it catalysis and suggested the existence of a catalytic force" associated with the action of catalysts. 

The form of the Langmuir isotherm may depend on the number of sites involved for ethylene adsorption. From steric considerations, it is probable that four platinum atoms are required for the... 

The addition of various Kolbe radicals generated from acetic acid, monochloro-acetic acid, trichloroacetic acid, oxalic acid, methyl adipate and methyl glutarate to acceptors such as ethylene, propylene, fluoroolefins and dimethyl maleate is reported in ref. [213]. Also the influence of reaction conditions (current density, olefin-type, olefin concentration) on the product yield and product ratios is individually discussed therein. The mechanism of the addition to ethylene is deduced from the results of adsorption and rotating ring disc studies. The findings demonstrate that the Kolbe radicals react in the surface layer with adsorbed ethylene [229]. In the oxidation of acetate in the presence of 1-octene at platinum and graphite anodes, products that originate from intermediate radicals and cations are observed [230]. 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

The electrochemical oxidation of polyhydric alcohols, viz. ethylene glycol, glycerol, meso-erythritol, xilitol, on a platinum electrode show high reactivity in alkaline solutions of KOH and K2C03 [53]. This electro-oxidation shows structural effects, Pt(lll) being the most active orientation. This results from different adsorption interactions of glycerol with the crystal planes [59]. 

Fig. II. (a) Schematic representation of hydrocarbons adsorbed on the [111] plane of platinum. Intersections of the lines of triangular net denote positions of the centers of platinum atoms. (1) Cyclohexane (2) all-cis conformation of cij-l,3,5-hexatriene (3) transoid conformations of cis- and trans-1,3,5-hexatriene (S4). (b) Adsorption configurations of acetylene and ethylene found most probable according to LEED studies 141).

C-Tracer studies of acetylene adsorption on alumina- and silica-sup-ported palladium [53,65], platinum [66] and rhodium [53] show the coexistence of at least two adsorbed states, one of which is retained on the surface, the other being reactive undergoing molecular exchange and reaction with hydrogen. Acetylene adsorption exhibits the same general characteristics as those observed with ethylene (see Sect. 3.2). However, there are important differences. The extent of adsorption and retention is substantially greater with acetylene than with ethylene. Furthermore, the amounts of acetylene retained by clean and ethylene-precovered sur-... 

The greater amount of retention observed with acetylene than with ethylene has been ascribed to the ability of the former to polymerise extensively. The existence of surface polymers following acetylene adsorption on alumina- and silica-supported platinum [60], evaporated palladium films [154] and silica-supported rhodium [67] has been demonstrated by thermal desorption studies. 

A systematic attempt to correlate the catalytic effect of different surfaces with their adsorptive capacity was made by Taylor and his collaborators. Taylor and Burns, for example, investigated the adsorption of hydrogen, carbon dioxide, and ethylene by the six metals nickel, cobalt, palladium, platinum, iron, and copper. All these metals are able to catalyse the hydrogenation of ethylene to ethane, while nickel, cobalt, and palladium also catalyse the reduction of carbon monoxide and of carbon dioxide to methane. 

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. 

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