Catalyst Surface Dispersion: Insights into Hydrogenation Kinetics and Mechanism

Catalyst-driven hydrogenation of an unsaturated organic matrix is commonly used for irreversible scavenging of hydrogen. The effect of catalyst dispersion on the hydrogenation reaction was explored as a means to deciphering the hydrogenation mechanism. The hydrogenation rate was thus measured in a model system consisting of a composite of a graphene nanoplatelet (GNP)-supported catalyst (Pd) in an organic matrix comprising the hydrogen scavenger 1,4-di(phenylethynyl)benzene (DEB). A similar activated carbon (AC)-supported Pd system in a DEB matrix was used as a reference system. We found that the rate of the hydrogenation reaction was limited by the migration of hydrogen atoms to the scavenger from the carbon-based catalyst support and could be manipulated by tuning (1) the specific surface area (SSA) of the catalyst support and (2) the mean catalyst-to-scavenger distance (catalyst dispersion quality) in the overall composite. We integrated these two key parameters into a single design parameter, termed the catalyst surface density (CSD). We found that for catalyst particles smaller than 100 nm the rate-determining step of the hydrogenation reaction was mainly dictated by the quality of the catalyst dispersion, expressed by the CSD parameter. Manipulating the CSD at a fixed composite composition yielded up to 7-fold acceleration in the hydrogenation rate, which is identical to the effect of increasing the concentration of the expensive and heavy catalyst by the same factor.