(483g) Understanding and Tuning the Activation of Unsaturated C-C Bonds over Nickel-Based Intermetallic Compounds

Authors: 
Laursen, S., University of Tennessee
He, Y., University of Tennessee
Song, Y., University of Tennessee
Intermetallic compounds (IMCs) comprised of atomically ordered mixtures of transition and post-transition metal elements exhibit uniquely tunable surface chemistry toward unsaturated C–C bonds, which can be beneficial in many reactions that require gentle or limited activation of the unsaturated C–C bond. Specifically, improved selectivity can be achieved in the semi-hydrogenation of alkynes, the production of olefins, and the selective hydrogenation of multiply functionalized unsaturated molecules. It is, however, unclear at this time how the surface chemistry of these promising materials changes as a function of element selection and bulk stoichiometry. This study aims to build a systematic understanding of the surface and catalytic chemistry of a suite of nickel-based ordered IMCs through a combined quantum chemical modeling and experimental approach.

Computational studies have isolated clear trends in the activation of unsaturated hydrocarbons as a function of both p-block element selection and the bulk stoichiometry of the IMCs. A volcano-like trend was encountered with respect to surface reactivity with the minimum in reactivity existing near the optimum of d- and p-orbital overlap (NiAl and Ga). The reduced reactivity encountered for NiAl and NiGa was universal for alkynes, alkenes, and aromatics studied as probe adsorbates. The surface reactivity was further a finer function of bulk stoichiometry, which may lend additional degrees of control.

Experimentally, the synthesis of high surface area oxide-supported IMCs (Ni + Ga and Ni + In) has been developed to control stoichiometry, phase segregation, and particle size and morphology. Critical aspects of the synthesis were precursor choice and the rate of its reduction, reduced constituent metal reactivity towards the oxide support and their diffusion, and annealing time and temperature for particle morphology control. A wide range of stoichiometries of Ni + Ga and Ni + In IMCs over different supports have been synthesized using various methods and shown to exhibit moderate activity and high selectivity in olefin production and selective hydrogenation. Reaction results closely followed predicted activity and selectivity from quantum chemical surface reaction modeling. These studies help to build a fundamental understanding of how the surface chemistry of IMCs is manipulated as a function of constituent element selection and bulk stoichiometry as well as the ability to modify the activation of unsaturated hydrocarbons to various degrees such that different catalytic transformations may be promoted. This degree of control is crucial for the development of any PGM-free catalysts associated with the production or functionalization of olefins or aromatics.