Alkali metal promoters and catalysts: a single-crystal investigation of ethylene epoxidation on cesium-doped silver(111)

Abstract

Cs adsorbs on Ag(lll) to form layers of relatively low atomic density; unlike the other alkalis, it does not significantly penetrate the Ag lattice. Multilayer growth proceeds by the Frank-van der Merwe mechanism, but oxygen chemisorption followed by heating drastically perturbs the structure of the alkali film; bare Ag sites are created, and there is evidence for the formation of higher oxides of Cs. The desorption properties of the system suggest that oxygen can induce the formation of a "surface alloy". The behavior of such cesiated surfaces in the selective and total oxidation of ethylene was investigated by two different approaches. The results are in good agreement in that they indicate that, in a certain coverage regime, Cs enhances both the total activity and the selectivity for ethylene oxide formation. This can be rationalized in terms of the effect of the alkali in respectively promoting and moderating certain aspects of the primary and secondary chemistry. Detailed molecular mechanisms are suggested.

An investigation on the role of Re as a promoter in Ag Cs Re/α-Al 2 O 3 high-selectivity, ethylene epoxidation catalysts

“…In this mechanism, Cs is thought to be responsible for blocking adsorption of oxygen onto these inherently less selective sites. This contradicts earlier work by Lambert and coworkers [25][26][27] who argued that Cs promotion was mainly electronic in nature. In their interpretation, Cs affected the secondary chemistry by decreasing the rate of EO isomerization to acetaldehyde [reaction (r 3 )], which at typical reaction conditions combusts immediately in the presence of Ag.…”

“…The O 2 adsorption stoichiometry was assumed to be ½ O 2 /Ag = 1 (as used by Vannice) to give a Ag dispersion of 0.95%, corresponding to an average Ag diameter of 0.12 lm. Titration with O 2 for samples containing Cs, Re, and Mo was not performed because Cs [25], Re [47], and Mo [48] might interact with O 2 , which would skew the results and make accurate calculations of the average Ag particle sizes unreliable.…”

“…A chlorine-containing moderator such as ethyl chloride (CH 3 CH 2 Cl) is also continuously fed to the reactor at ppm levels because it similarly increases the selectivity to EO. The use of Cs as a promoter has been discussed at length, but its specific role in the selective oxidation of ethylene is still the subject of considerable debate [22][23][24][25][26][27][28][29][30][31]. For example, Waugh and coworkers [6,[22][23][24] recently argued that the role of Cs is mainly geometric in nature and claimed that Cs is preferentially bound to Ag on unselective, stepped silver sites.…”

Investigation of Ethylene Oxide on Clean and Oxygen-Covered Ag(110) Surfaces

“…Earlier surface science studies [12][13][14][15][16][17][18][19][20] of Ag-catalyzed ethylene epoxidation primarily addressed the roles of oxygen and promoters. By the late 1980s, a reasonable consensus emerged that addition of atomic, rather than molecular, oxygen to the double bond of ethylene led to epoxide formation.…”

Nano‐sized silver crystals and their dispersion over α‐alumina for ethylene epoxidation

“…In addition to this, it has also been suggested that cesium plays a role in binding and dispersing the silver crystals on the catalyst support, as well as increasing the adsorption probability of oxygen on silver [15][16][17][18][19]. According to Minahan and co-workers [20], Cs acts as a binder between the Al 2 O 3 surface and the Ag crystallites to give a more uniform and higher coverage of the Ag component.…”