ร400ะC (8)(9)(10)(11). A number of attempts have been made to The promoting effect of Ag 2 O on the physicochemical properties modify the surface properties and catalytic activity of the and catalytic activity of manganese nodules has been investigated. nodules (10, 12, 13); for example, incorporation of lanthan-The surface area and surface oxygen increased with addition of ides by lanthanide-strontium mixture in the nodule followed Ag 2 O, reaching a maximum at 1 mol% of Ag, and then decreased by calcination led to formation of perovskite-type mixed with further addition of Ag 2 O. The catalytic activity of manganese oxides, which showed higher thermal stability and catalytic nodules substantially increased with addition of Ag 2 O over a wide activity. Silver oxide alone cannot be used as a combustion range (0.25-25 mol% Ag). Addition of only 0.25 mol% of Ag to catalyst because of low thermal stability. However, Mn-Ag the nodule decreased the temperature of complete CO oxidation (as discussed above) and also Mn-Co-Ag composite oxfrom 350ะC to as low as 100ะC. This enhanced activity was explained by the concerted action of Ag and Mn, where CO was ides (14) were found to be more effective than individual preferentially adsorbed on Ag 2 O and oxidized. The reduced Ag components for complete CO conversion at low temperature was then reoxidized rapidly by oxygen transferred from Mn. The due to the concerted action of Ag and Mn. Recently, Watapromoting effect of Ag 2 O on H 2 O 2 decomposition and ketonization nabe et al. (15) also observed that addition of Ag to Mn 2 O 3 of acetic acid, however, was less pronounced. แญง 1998 Academic Press enhanced the activity and durability of Mn 2 O 3 in the decom-Key Words: catalytic activity; silver oxide; manganese nodules; position of acetaldehyde. All these studies point to a common ketonization.
mechanism in which the oxidation of substrate occurred at the Ag 2 O site and the higher oxidation state of silver was maintained by a supply of oxygen from Mn. We previously