As a model reaction for the selective hydrogenation of carboxylic acid
s towards aldehydes, the reaction of acetic acid towards acetaldehyde
has been investigated. Various oxides are used as catalysts, including
platinum-enriched oxides. It appeared that oxides with a moderate met
al-oxygen bond strength are good catalysts, provided that a source of
activated hydrogen is present. On exposure to molecular hydrogen zero-
valent metals can produce activated hydrogen. Addition of precious met
als, such as Pt, or completely reducing part of the oxidic catalyst ca
n provide the zero-valent metal. The hydrogenation probably occurs on
the oxidic surface by a Mars and Van Krevelen type of mechanism.