Direct Oxidation of L-Sorbose to 2-Keto-L-gulonic Acid with Molecular Oxygen on Platinum- and Palladium-Based Catalysts

Graphite-Supported Platinum Catalysts: Effects of Gas and Aqueous Phase Treatments

“…Four deactivation mechanisms can be distinguished: (i) Deactivation by over-oxidation or oxygen poisoning has received the most attention. It has been reported for the oxidation of primary alcohols (5-10), secondary alcohols (8,(10)(11)(12), and carbohydrates (10,(13)(14)(15)(16)(17)(18)(19). It is generally accepted that over-oxidation is caused by the strong adsorption of oxygen or oxygen-containing species at the platinum surface (16-18, 20, 21).…”

On-line characterization of platinum/graphite catalysts during liquid phase oxidations using cyclic voltammetry

“…Possible causes of deactivation during liquid phase oxidations are: oxidation of the active metal [4], loss of active surface area due to site coverage by either reaction components or poisons [5], particle growth [6] and corrosion of the metal [5,6]. The actual cause of activity loss depends on the type of reaction and the conditions applied.…”

“…It seems that when the oxidation rate is controlled by the gas/liquid transfer of oxygen, the catalysts are more likely to be the subject of coverage by carbonaceous products than when the catalysts are applied in the kinetic regime [7][8][9][10]. This phenomenon is reported to be more pronounced when the catalyst is prereduced by the organic reactant [5,7]. When the oxidation is performed in the kinetic regime, strong chemisorption of oxygen can take place on the active metal leading to what is referred to as over-oxidation [6,8,11].…”

“…Good results have been achieved with bismuth [7,9,10,16], lead [10,17] and tin [10]. Promoter metals can, however, also be the subject of corrosion [5], and optimal performance depends on the degree of coverage by the promoter metal [7,18]. Beyond this optimal degree of coverage, the promoter metal acts as a poison, decreasing the availability of active sites necessary for reactant adsorption.…”

“…It has also been applied as an ex-situ technique for the characterization of graphite supported catalysts before and after the oxidation of alcohols and carbohydrates [11,28]. The prereduction of platinized platinum by the organic reactant as followed in-situ by cyclic voltammetry showed that a dramatic decrease of the available platinum surface area was caused by the formation of by-products [5,7,29].…”