Silver ethylene adsorption

It is clear from the kinetics that both ethylene and oxygen adsorption are important since both compounds appear in the rate equations with non-zero orders. Moreover, it is well known that ethylene is not adsorbed on pure silver, but that it does adsorb on a surface that is partially covered with oxygen. This implies that ethylene is either adsorbed on top of pre-adsorbed oxygen or on silver sites that are activated by the presence of oxygen (i.e. by formation of surface oxides, or another form of electron transfer or polarization). Consequently, two different mechanisms arise for the formation of ethylene oxide. The (direct) combustion of ethylene is another point of discussion. Although many favour the idea that different oxygen species are involved, others assume the same oxygen species, but different forms of ethylene adsorption. 

Oxygen and ethylene adsorb on a supported silver catalyst. A Rideal-Eley mechanism is favored for the adsorption. Chloride is a promoter/modifier for the catalyst and reduces the surface oxygen content. Silver on the surface tends to increase ethylene adsorption, which explains why small levels of chloride increase the rate of reaction. 

CHARACTERIZATION AND ETHYLENE ADSORPTION PROPERTIES OF SILVER-LOADED FER ZEOLITE POTENTIALLY USED AS TRAP MATERIAL OF COLD-START HYDROCARBON EMISSION FROM VEHICLES... 

Zimakov (120) suggested earlier that the impossibility of obtaining propene oxide from propene over silver was due to peculiarities of the propene oxide structure and the readiness of its further oxidation. However, Gorokhovatskii and Rubanik have shown that this is not so. Adsorption of propene on the silver surface seems to be different from ethylene adsorption. De Boer, Eischens, and Pliskin (121) suggest that ethylene sorbs on a silver surface covered with oxygen to form complexes... 

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)... 

Aqueous solutions of silver nitrate or perchlorate also react rapidly with olefins. It has been noted that the addition of fiuoroboric acid to silver nitrate gives a surprisingly large increase in ethylene adsorption and it was suggested that this may be due to a lowering of the degree of hydration of the silver ion in solution [1. Non-aqueous solvents such as alcohols, acetone or acetic acid have been used in these reactions and, in the case of hydrolytically unstable complexes such as C5H6AgC104, benzene or ether solutions are used [20]. 

Harder P, Grunze M, Dahint R, Whitesides G M and Laibinis P E 1998 Molecular conformation in oligo(ethylene glycol)-terminated self-assembled monolayers on gold and silver surfaces determines their ability to resist protein adsorption J. Rhys. Chem. B 102 426-36... 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

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. 

Characterization of the Adsorbed Layer of a Silver Catalyst in the Oxidation of Ethylene from Its Transient Adsorption Behavior... 

Numerous papers and several review articles889,899-907 deal with adsorption studies and discuss the kinetics and mechanism of the silver-catalyzed epoxidation of ethylene. A simple triangular kinetic scheme of first-order reactions satisfies the experimental observations (Scheme 9.23). On the best industrial catalysts fci/ 2 is 6, and k2/ 3 is 2.5. 

Chemisorption measurements have shown that ethylene does not adsorb on pure silver, but only on a silver surface which has been preoxidized [339]. Complete coverage with an oxygen monolayer, however, also seems to destroy the capacity to adsorb ethylene, as was demonstrated by Force and Bell [114,116] (favouring the idea of adsorption on silver). Consequently, partial oxygen coverage seems to be a necessary condition for catalytic activity. 

As an example consider the oxidation of ethylene over metallic silver at 200 to 300°C. Let M represent the site of adsorption on the metal surface it can be a single Ag atom or a group of two, three, or four Ag... 

The coupling theory for ethylene oxidation requires only that each adsorbed 02 molecule form two types of reactive O atoms. Many such pairs are conceivable. It will be of great interest to determine what types are participating in the reactions. The adsorption rate measurements of Czanderna (5,6) were interpreted as indicating charged 02 molecules and charged O atoms on a silver surface at about 200°C. 

Schultes and Teil expressed the idea that oxygen was not really chemically adsorbed in the conventional sense, but might instead be bound in a manner intermediate between true chemical adsorption and physioftl adsorption. In other words, the oxygon on the silver surface might still be partially diatomic. Reaction with ethylene was then pictured as a completing to form a transient peroxide species C2H4O2, which could have one of two structures (Eq. 102), and which would react further very quickly, cither on the surface or after desorption. The intermediacy of this peroxide was unfortunately not established experimentally, however. 

Although these reactions have been researched extensively and are the subjects of numerous patents, the precise reaction mechanism is not fully understood. The controversy has mostly centered on the nature of the oxygen species responsible for ethylene oxide formation (103). The results of various surface characterization studies indicate that there are at least three types of adsorbed oxygen species on silver monoatomic chemisorbed oxygen, diatomic (molecular) oxygen, and subsurface oxygen. The first results from a dissociative adsorption of oxygen on a silver surface ... 

Microcontact printing (pCP, see Fig. 10 for an example) has been used for the spatially resolved modification of gold, silver, or titanium surfaces with SAMs of methyl-terminated alkanethiolates, which favor protein adsorption [99-101], Backfilling around the patterned protein-attractive islands was performed by a subsequent self-assembly of an ethylene-glycol-terminated alkanethiol. In a next step, the hydrophobic methyl-terminated SAMs were covered by adsorbed FN or other cell-adhesion-mediating proteins. 

In early proposals the species responsible for epoxidation was identified as the adsorbed molecular oxygen, Ag 02(ads)> while combustion was attributed to monoatomic Ag O(ads) (Equations 14-16). The oxidation step envisages the transfer of one atom of molecularly adsorbed oxygen to the double bond, while the other remains adsorbed on silver. The consumption of the latter by the total oxidation of ethylene restores the site vacancies necessary for the continuation of catalysis. Up to a maximum of six oxygen atoms are required for the combustion of one ethylene molecule. Thus, the combination of the reactions (Equation 14) and (Equation 15) predicts that the maximum attainable selectivity in the epoxidation of ethylene is 6/7, i.e., 85.7% (Equation 16). A lower selectivity should normally be expected because some monoatomic oxygen independently formed by dissociative adsorption (Equation 13) raises the level of ethylene combustion above that predicted by Equation 16. 

Discussion Point DP2 Silver is unique among metal catalysts for its high selectivity in the epoxidation of ethylene. Rationalize this result on the basis of the different properties of surface metal-oxygen bonds and of the different adsorption behaviour of hydrocarbons on transition and non-transition metals. 

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