Proof of Hydrogen Spillover by Product-Distribution Analysis in Solid Phase Hydrogenations

Hydrogenation in the solid-phase is important in green-chemistry for the limited use of solvents and use of solid, recyclable catalysts and, in the field of hydrogen-scavenging, preventing risks of explosion, or hydrogen-induced metal corrosion or embrittlement.

In solid-phase hydrogenations the reaction progress is believed to involve hydrogen-spillover, where the activated hydrogen migrates across the catalyst’s carbon-support. The study of solid-phase hydrogenation of 1,4-bis(phenylethynyl)benzene (PEB) was used to demonstrate the effect of different parameters on the solid-phase hydrogenation mechanism.

The samples of PEB containing a Pd/C catalyst were hydrogenated to various degrees at different initial pressures and with the addition of carbon-nanotubes (CNTs) or C₆₀ fullerenes; the partially hydrogenated samples were then analyzed by gas-chromatography, for product quantification.

Comparison of the products distribution at different reaction-conditions gives insight into the nature of the spillover-mechanism of hydrogenation in the solid-phase.

The results of this study show that the product-distribution is greatly affected by the hydrogen pressure.

Addition of CNTs, known to accelerate solid-phase hydrogenations, gives the same effect on product-distribution trend as a hydrogen pressure increase. We suppose that CNTs facilitate and increase the distance of the hydrogen’s migration from the metal catalyst to the substrate.

C₆₀ addition has the opposite effect. The reaction-rate substantially diminishes; and the product-distribution trend resembles that of decreased hydrogen pressure. We suppose that the mechanism of C₆₀ inhibition is by recombination of hydrogen-atoms into molecular hydrogen and subsequent H₂ release to the gas-phase. Thus, we can presume that the nature of the spilled-over hydrogen is radical.

In summary, by studying the product-distribution of the solid-phase hydrogenation at different pressures with the addition of additives, we gain insight into the reaction mechanism.