Density Functional Theory Study on the Role of Polyacetylene as a Promoter in Selective Hydrogenation of Styrene on a Pd Catalyst

Understanding mechanisms of catalyst–substrate interactions is of essential importance for the design and development of novel catalysts with superior performances. In the present density functional theory study, selective hydrogenation of styrene on a polyacetylene (PA)-supported Pd<sub>4</sub> catalyst (Pd<sub>4</sub>/PA) was employed as a model system to address how catalyst–substrate interactions affect the charge state of Pd, which subsequently influences catalytic activity. It was found that the Pd cluster can be anchored strongly on the CC bond of the polymer substrate through the π–d interaction, which further leads to charge rearrangement on the Pd<sub>4</sub> cluster with the top two Pd atoms being more negatively charged. By comparing the calculated minimum energy profiles of styrene hydrogenation on surfaces of both pure Pd<sub>4</sub> and Pd<sub>4</sub>/PA, the mechanism that dictates the catalytic process on Pd<sub>4</sub>/PA was identified. Charge analysis reveals that the enhanced catalytic activity of Pd<sub>4</sub>/PA is largely attributed to the negative charges on the two topmost Pd atoms, which facilitates both hydrogenation of styrene and desorption of the product. Nevertheless, PA hydrogenation to produce polyethylene (PE) was also found to be a potentially viable process with a moderate activation barrier of 0.43 eV, which may consequently lead to the formation of a PE-supported Pd<sub>4</sub> catalytic system. As a consequence, the absence of π orbitals of the PE substrate may significantly reduce the electronic interaction between Pd<sub>4</sub> and PE, which ultimately leads to the catalytic performance similar to the activity on the pure Pd<sub>4</sub> cluster.