The promoting effect of cesium on structure and morphology of silver catalysts

Abstract

Silver supported catalysts on alpha-alumina were prepared and characterized by Brunauer-Emmet-Teller equations, scanning electronic microscopy, X-ray diffraction and atomic force microscopy techniques. Results show that these are powerful techniques for the determination of texture, morphology and surface properties. It has been shown that the addition of Cs in the Ag/Al 2 O 3 catalyst increased the dispersion of silver with the formation of small silver particles over a thin silver film already formed over the alumina support. It is important to stress that atomic force microscopy measurements are significant to observe the film and the dispersion of Ag and, on the contrary, X-ray photoelectronic spectroscopy did not, however, it allows to the conclusion that undistinguishable silver, either as metallic or oxidation state, are present at the surface. X-ray diffraction results confirm predominantly metallic silver.

Surface features on Sahara soil dust particles made visible by atomic force microscope (AFM) phase images

“…Then, however, friction and other lateral forces may become important. Combining this mixed mode with phase information has been widely applied to compliant materials, i.e., synthetics (Viswanathan et al, 1997;Magonov et al, 1997), with water-and organics-coated ammonium sulfate (Dong and Yu, 2003), and in catalysis research (Dokou et al, 2000;Carvalho et al, 2004). To access material properties, Dauphin et al (2003) specifically used the phase shift signal to investigate aragonitic and calcitic layers in shells.…”

Supported Silver Catalysts for Ethylene Epoxidation

“…55,77,78 Özbek and van Santen showed that caesium also influences the reactivity of the active metal surface, as higher caesium concentration leads to lower overall conversion. 75,84,196,325,326 Although caesium generally improves the selectivity, in high concentrations it can even lead to lower overall selectivies, probably because caesium introduces oxygen vacancies on the silver surface, which are known to be active sites for full combustion. 81,84 It is clear that if one wants to study intrinsic support effects, promotors such as caesium should rather be avoided.…”

“…[74][75][76] The activity of the catalyst is less important as high conversions decrease the ethylene oxide selectivity due to the secondary reaction (r 3 in Figure 1.2, the full oxidation of ethylene oxide).The presence of chlorine on the surface suppresses the full oxidation of ethylene, 74,77,78 as the presence of chlorine on the catalyst surface prevents the formation of certain surface species responsible for the unselective pathway, leading to an increase in the intrinsic selectivity. 75 The competitive adsorption of chlorine on the silver surface indiscriminately suppresses oxygen adsorption on the silver surface and therefore decreases the overall activity by suppressing both the selective (r 1 ) and unselective (r 2 ) pathways. 47,77,79,80 A secondary effect of chlorine promotion leads to a suppression of the secondary, unselective reaction (r 3 ); the decomposition of ethylene oxide to form carbon dioxide and water.…”

“…or to increase the stability of the catalyst. [74][75][76] The activity of the catalyst is less important as high conversions decrease the ethylene oxide selectivity due to the secondary reaction (r 3 in Figure 1.2, the full oxidation of ethylene oxide).…”

“…The presence of chlorine on the surface suppresses the full oxidation of ethylene, 74,77,78 as the presence of chlorine on the catalyst surface prevents the formation of certain surface species responsible for the unselective pathway, leading to an increase in the intrinsic selectivity. 75 The competitive adsorption of chlorine on the silver surface indiscriminately suppresses oxygen adsorption on the silver surface and therefore decreases the overall activity by suppressing both the selective (r 1 ) and unselective (r 2 ) pathways. 47,77,79,80 A secondary effect of chlorine promotion leads to a suppression of the secondary, unselective reaction (r 3 ); the decomposition of ethylene oxide to form carbon dioxide and water.…”