Silver surface reactivity

A variety of reactions have been observed to be produced by oxygen preadsorbed on Ag(llO) 137,144). In general these reactions do not proceed on clean silver under the conditions normally utilized in studies of surface reactivity. These reactions are listed below ... 

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. 

SUMMARY - Different forms of adsorbed molecular and atomic oxygen on an Ag(UO) surface are analyzed from a theoretical point of view. It is proposed that the active intermediate for the epoxidation of ethylene is an Ag-O surface species. A parallel is made to discrete silver complexes as catalysts for alkene epoxidation as it is experimentally shown that these have similar properties at the silver surface. 180 Labelling studies indicate an Ag-O complex as the reactive catalyst. 

Keywords Atomic scale characterization surface structure epoxidation reaction 111 cleaved silver surface oxide STM simulations DFT slab calculations ab initio phase diagram free energy non-stoichiometry oxygen adatoms site specificity epoxidation mechanism catalytic reactivity oxametallacycle intermediate transition state catalytic cycle. 

The decrease of the rate of ethene oxide formation with respect to time observed at temperatures >470 K can be explained by the accumulation of oxygen species embedded in the silver surface that decrease the surface area available for the formation of the reactive species. Nucleophilic and electrophilic oxygen, which are the major surface species at 420 K, are still present on the silver surface at 470 K however, they are rapidly removed in the absence of oxygen in the gas phase (see the difference spectrum in Figure 18). 

Silver is a white, lustrous, soft, and malleable metal (mp 961°C) with the highest known electrical and thermal conductivities. It is chemically less reactive than copper, except toward sulfur and hydrogen sulfide, which rapidly blacken silver surfaces. The metal dissolves in oxidizing acids (concentrated HN03) and in cyanide solutions in the presence of oxygen or hydrogen peroxide. 

Barteau, M.A. Madix, R. J. The Surface Reactivity of Silver Oxidation Reactions, The Chemical Physics of Solid and Heterogeneous Catalysis, ESPC, vol. 4, 1982, pp. 96-142. 

As stated earlier, it should be possible to epoxidize non-allylic olefins other than butadiene, or even olefins with allylic hydrogen atoms, as long as the allylic hydrogens are kinetically non-reactive. The olefins in Table 4 indicate this is indeed the case. In all cases, the catalysts were promoted with CsCI the unpromoted catalysts were either inactive or exhibited very low and transient activities. The data for the epoxidation of styrene and 4-vinylpyridine are discussed in greater detail in an earlier patent (12). The epoxidation of styrene over silver surfaces has also been observed by Blum (13) and Hawker, Lambert eta (14), although the catalysts evaluated in Table 4 are much more active and selective than those described by Blum. The transient temperature programmed reaction spectroscopy (TPRS)... 

High reaction pressures are needed for many other systems as well in order to convert the surface into a uniquely reactive state such as has been found for ethylene epoxidation. The epoxidation reaction of ethylene catalyzed by silver shows a distinct pressure gap. Higher oxygen pressures are needed in order to convert the silver surface into a silver-oxide overlayer where weakly adsorbed oxygen atoms are formed, that selectively epoxidize ethylene ]. 

The epoxidation of ethylene which is catalyzed by Ag and promoted by chlorine compounds, for example, is thought to occur in a surface overlayer that has features similar to a melt of Ag ions. The silver-oxychloride reactive surface layer requires Ag + ions (as in the electrochemical system, see Scheme 2.1) to enhance the overall selectivity. Reduced Ag clusters, however, are required to activate molecular oxygen. Dynamic events between these two states are necessary to close the catalytic cycle. Chlorine in combination with Cs is added to promote the Ag catalyst. Eutectic melting points of this phase are close to the reaction temperature 14]... 

M. A. Barteau and R. J. Madix, The surface reactivity of silver oxidation reactions, Chem. Phys. Solid Surf. Heretog. Catalysis, Vol. 4 (D. A. King and D. R Woodruff, eds.), Elsevier, Amsterdam, 1982, p. 95. 

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