Kinetics and mechanism of butene isomerization/hydrogenation and of 1,3-butadiene hydrogenation on palladium

• Very slow hydrogenation of butadiene by monomolecular layer of hydrogen on Pd black.
• However, fast hydrogenation of butadiene/hydrogen mixture due to subsurface hydrogen.
• Hydrogen loading of subsurface region is regarded as rate determining step.

Combined studies of adsorption and hydrogenation have been carried out using Pd black electrochemically deposited in a loose packed thin layer in order to avoid co-adsorption on a support. Pd black covered with a monomolecular layer of strongly adsorbed hydrogen and exposed to butadiene shows a very low hydrogenation rate, more than two orders of magnitude lower than the rate of hydrogenation of a mixture of butadiene and hydrogen. This result reveals that the second step of hydrogenation according to the reaction scheme of Horiuti and Polanyi via a semi-hydrogenated intermediate needs subsurface hydrogen loading. While the first step towards a surface alkyl requires a negatively charged surface hydride for nucleophilic attack the subsequent electrophilic attack proceeds with a positively charged hydrogen from the subsurface region. Strong support for this view is given by the kinetics of isomerization/hydrogenation of cis-2-butene which shows at low hydrogen pressure fast isomerization and slow hydrogenation rate. The access of hydrogen to the catalyst surface is regarded as rate determining step of butene respectively butadiene hydrogenation. The consequence of increased hydrogen access to the Pd surface is an enhanced diffusion of hydrogen from surface to subsurface positions thus leading to an increased hydrogenation rate.Furthermore in the state of selective hydrogenation of 1,3-butadiene towards butenes the surface is densely covered by butadiene.

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