Active site requirements for chemoselective hydrogenation with binary and ternary intermetallics

Robert Rioux, Ph.D. 
Friedrich G. Helfferich Professor of Chemical Engineering and Associate Department Head
The Pennsylvania State University 

Seminar Abstract: A reliable method to design site-isolated catalysts is through the synthesis of intermetallic bulk compounds where a small number of active atoms are isolated in an inert matrix of a second metal. Intermetallics – multi-metal systems with a well-defined arrangement of the metal atom – offer a platform to control active site structure and composition [1]. They represent an ideal model system for studying structure-function relations in metal-catalyzed hydrogenation reactions. The Pd-(M)-Zn γ-brass phase (M = Zn, Pd, Cu, Ag, Au) is uniquely suited for the controlled synthesis of Pd-M-Pd active ensembles with controlled nuclearity and composition isolated in a Zn matrix. The prototypical Pd8Zn44 structure contains only isolated Pd atoms, but with increasing Pd concentration by Zn substitution (Pd8+xZn44-x, x = 0-2) within the bounds of the γ-phase, a fixed number of Pd-PdPd trimers form in the bulk. These multi-atomic heteronuclear active sites are catalytically distinct from Pd single atoms, fully coordinated Pd and within the Pd-M-Pd series. With these catalysts, we quantify the unexpectedly large effect of active site nuclearity for acetylene semihydrogenation in an ethylene and hydrogen rich environment. The trimer-containing Pd9Zn43 and Pd10Zn42 have ~106 times higher activity compared to trimer-free Pd8Zn44, but the latter leads to net ethylene gain by only semi-hydrogenating acetylene, consistent with theoretical predictions. Pd2M trimers demonstrate activity and selectivity intermediate to the monomerand trimer-containing surfaces. Experimental trends show strong agreement with density functional theory (DFT) calculations, evidencing our ability to control the nuclearity and composition of active sites in intermetallics [2].

We further demonstrate the well-defined active site afforded by the g-brass phase allows for the development of kinetic models that accurately describe apparent activation energies and reaction orders for ethylene hydrogenation on g-brass Pd-(M)-Zn. Previous studies of the kinetics of ethylene hydrogenation on late transition metal surfaces 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 differences in adsorbate size between ethylene and hydrogen. The isolation of Pd1 and Pd3 sites allows us to provide a precise definition of all possible ethylene-hydrogen co-adsorption structures and hydrogenation reaction paths, enabling a precise definition of site requirements. We demonstrate the high sensitivity of the H2 reaction order on monomer-and trimer-containing Pd9Zn43 surfaces is due to the significant influence of Pd monomers on the degree of rate control due to the competitive hydrogenation of ethylene and hydrogen on the monomers. This sensitivity disappears on trimer-only containing Pd10Zn42 surfaces. Microkinetic modeling predicts an ethylene reaction order close to zero and hydrogen order of unity, in agreement with experimental results.

[1]A. Dasgupta, R. M. Rioux. Catalysis Today 330 (2019) 2-15. 2
[2]A Dasgupta, H. He, R. Gong, S. -L. Shang, E. K. Zimmerer, R. J. Meyer, Z. -K. Liu, M. J. Janik, R. M. Rioux. Nature Chemistry 14 (2022) 523-529 
[3] H. He, G. Canning, A. Dasgupta, A. Nguyen, R. J. Meyer, R. M. Rioux, M. J. Janik. Nature Catalysis. 6 (2023) 596-605.