(159b) DFT and Micro-Kinetic Studies of Intermetallic Catalysts for Selective Hydrogenation

Intermetallic materials, different than randomly distributed alloys, have well defined atomic composition and atomic arrangements. Such well-defined catalysts, with a majority inert metal, can offer isolated active sites of defined active metal atom nuclearity, serving as a perfect platform to explore the connection of active site structure and kinetic behavior. We combined experimental and DFT methods to examine selective hydrogenation of various unsaturated functionalities on Pd-Zn γ-brass phase intermetallic. We used a series of hydrogenation reactions to examine active and selective site requirements: linear alkene and aromatics competitive hydrogenation, ethylene hydrogenation, acetylene semi-hydrogenation, and α,β-unsaturated aldehyde hydrogenation.

In this talk, we will present the use of intermetallics to define the active site requirements for ethylene hydrogenation. DFT, microkinetic modeling, and experimental kinetic studies are combined to define site requirements. The Pd-Zn γ-brass phase is used to control active site nuclearity, which exposes only Pd₁ monomers for Pd₈Zn₄₄ and includes Pd₃ trimers for Pd_9-11Zn_43-41^. A number of previous studies have investigated the kinetics of ethylene hydrogenation on late transition metal surfaces, and have required the use of a “special” site on which H* can adsorb without competition from ethylene within a Langmuir-Hinshelwood framework. The molecular level definition of such sites is elusive, arising from the complex combination of mixed coverages and difference in adsorbate size between ethylene and hydrogen. The isolation of Pd₁ and Pd₃ sites allows us to provide precise definition of all possible ethylene-hydrogen co-adsorption structures and hydrogenation reaction paths, providing a precise definition of the site requirements. The Pd₃ trimer has a turnover frequency 3 orders of magnitude higher than the Pd₁ monomer, with the trimer offering sufficient adjacent Pd atoms to allow ethylene and hydrogen co-adsorption and stable hydrogenation transition states. DFT-based microkinetic modeling predicts an ethylene reaction order close to zero and hydrogen order of unity, in agreement with experimental results.

Additionally, varying nuclearity within confined sites will enable us to define site requirements for hydrogenation of various unsaturated functionalities, such as acetylene semi-hydrogenation and unsaturated aldehyde selective hydrogenation. Acetylene has one functionality can be sequentially hydrogenated and unsaturated aldehyde has two different functionalities in conjugation, which can reveal active site ensemble and catalyst performance (activity and selectivity) relationship.