(159b) DFT and Micro-Kinetic Studies of Intermetallic Catalysts for Selective Hydrogenation
- Conference: AIChE Annual Meeting
- Year: 2019
- Proceeding: 2019 AIChE Annual Meeting
- Group: Catalysis and Reaction Engineering Division
- Time: Monday, November 11, 2019 - 12:50pm-1:10pm
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 Î³-brass phase is used to control active site nuclearity, which exposes only Pd1 monomers for Pd8Zn44 and includes Pd3 trimers for Pd9-11Zn43-41. A number of previous studies have investigated the kinetics of ethylene hydrogenation on late transition metal surfaces, and have required the use of a âspecialâ 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.