Carbon Monoxide in Catalytic Hydrogenations: A Poison or a Promoter?
The hydrogenation of a,b-unsaturated aldehydes was studied over a platinum catalyst. It was shown that the catalyst was rapidly deactivated by CO formed by decarbonylation of the aldehyde. This deactivation was suppressed by the addition of small amounts of oxygen to the feed, since CO was oxidized to CO2 in the presence of oxygen.
Selective hydrogenation of the C=O group to yield the unsaturated alcohol, which is a difficult task, was achieved at high rates by promoting a platinum catalyst with iron in combination with addition of small amounts of oxygen.
The hydrogenation over palladium, which is an excellent catalyst for selective hydrogenation of the C=C double bond to yield the saturated aldehyde, was not appreciably affected by the addition of oxygen or iron.
A new radioactive tracer technique for adsorption-desorption studies on a working catalyst was developed. This technique was used to study the role of CO in the selective hydrogenation of acetylene with excess ethylene present. It was shown that acetylene, in contrast to ethylene, was able to displace strongly adsorbed CO from the catalyst. Moreover, the technique was used to study the rapid displacement of preadsorbed radioactively labeled CO from a palladium catalyst by unlabeled CO or NO from the gas phase. Several possible mechanisms for the displacement are discussed.
A new catalytic system which exhibits kinetic self-oscillations is also presented. The simultaneous H2-D2 exchange reaction was found to oscillate during the hydrogenation of acetylene in the presence of CO over both a palladium and a platinum catalyst.