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1. Structural trends in the dehydrogenation selectivity of palladium alloys

   https://doi.org/10.1039/d0sc00875c  

   Purdy, Stephen C.; Seemakurthi, Ranga Rohit; Mitchell, Garrett M.; Davidson, Mark; Lauderback, Brooke A.; Deshpande, Siddharth; Wu, Zhenwei; Wegener, Evan C.; Greeley, Jeffrey; Miller, Jeffrey T. (May 2020, Chemical Science)

   null (Ed.)

   Alloying is well-known to improve the dehydrogenation selectivity of pure metals, but there remains considerable debate about the structural and electronic features of alloy surfaces that give rise to this behavior. To provide molecular-level insights into these effects, a series of Pd intermetallic alloy catalysts with Zn, Ga, In, Fe and Mn promoter elements was synthesized, and the structures were determined using in situ X-ray absorption spectroscopy (XAS) and synchrotron X-ray diffraction (XRD). The alloys all showed propane dehydrogenation turnover rates 5–8 times higher than monometallic Pd and selectivity to propylene of over 90%. Moreover, among the synthesized alloys, Pd 3 M alloy structures were less olefin selective than PdM alloys which were, in turn, almost 100% selective to propylene. This selectivity improvement was interpreted by changes in the DFT-calculated binding energies and activation energies for C–C and C–H bond activation, which are ultimately influenced by perturbation of the most stable adsorption site and changes to the d-band density of states. Furthermore, transition state analysis showed that the C–C bond breaking reactions require 4-fold ensemble sites, which are suggested to be required for non-selective, alkane hydrogenolysis reactions. These sites, which are not present on alloys with PdM structures, could be formed in the Pd 3 M alloy through substitution of one M atom with Pd, and this effect is suggested to be partially responsible for their slightly lower selectivity. 

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2. Search for molecular corks beyond carbon monoxide: A quantum mechanical study of N-heterocyclic carbene adsorption on Pd/Cu(111) and Pt/Cu(111) single atom alloys

   Simpson, Scott (December 2021, Pacifichem 2021)

   Periodic Density Functional Theory calculations reveal the potential application of 10 imidazole based N-heterocyclic carbenes to behave as “molecular corks” for hydrogen storage on single atom alloys, comprised of Pd/Cu(111) or Pt/Cu(111). Calculations show that functionalizing the NHC with different electron withdrawing/donating functional groups results in different binding energies of the NHC with the alloy surfaces. The results are compared to DFT calculations of carbon monoxide bound to these alloys. The Huynh electronic parameter (is calculated for several simple imidazole NHCs to gauge σ-donor ability, while Se-NMR of and P-NMR calculations of selenourea derivatives and carbene-phosphinidene adducts, respectively, have been utilized to gauge π-acidity of the NHCs. It is demonstrated that consideration of both σ and π donating/accepting ability must be considered when predicting the surface-adsorbate binding energy. It was found that electron withdrawing groups tend to weaken the NHC-surface interaction while electron withdrawing substituents tend to strengthen the interaction. 

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3. The Search for Molecular Corks Beyond Carbon Monoxide: A Quantum Mechanical Study of N-Heterocyclic Carbene Adsorption on Pd/Cu(111) and Pt/Cu(111)

   Simpson, Scott (March 2022, ACS Spring 2022)

   Periodic Density Functional Theory calculations reveal the potential application of 10 imidazole based N-heterocyclic carbenes (NHCs) to behave as “molecular corks” for hydrogen storage on single atom alloys, comprised of Pd/Cu(111) or Pt/Cu(111). Calculations show that functionalizing the NHC with different electron withdrawing/donating functional groups results in different binding energies of the NHC with the alloy surfaces. The results are compared to DFT calculations of carbon monoxide bound to these alloys. The Huynh electronic parameter (HEP) is calculated for several simple imidazole NHCs to gauge σ-donor ability, while Se-NMR and P-NMR calculations of selenourea derivatives and carbene-phosphinidene adducts, respectively, have been utilized to gauge π-acidity of the NHCs. It is demonstrated that consideration of both σ and π donating/accepting ability must be considered when predicting the surface-adsorbate binding energy. It was found that electron withdrawing groups tend to weaken the NHC-surface interaction while electron donating substituents tend to strengthen the interaction. 

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4. Effects of Pd ensemble size in dilute and single atom alloy PdAu catalysts for one-pot selective hydrogenation and reductive amination

   https://doi.org/10.1039/D5CY00441A  

   Lim, Kang_Rui Garrick; Azizli, Toghrul; Kaiser, Selina K; Aizenberg, Michael; Montemore, Matthew M; Aizenberg, Joanna (July 2025, Catalysis Science & Technology)

   The Pd ensemble size in dilute and single atom alloy PdAu catalysts controls competitive adsorption between reactants and consequently directs selectivity in the one-pot hydrogenation and reductive amination between nitroarenes and aldehydes. 

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5. Molecular Modeling Insights for N-Heterocyclic Carbenes as Molecular Corks: Potential for Hydrogen Storage

   Simpson, S (December 2025, 2025 International Chemical Congress of Pacific Basin Societies)

   Hydrogen is a versatile, energy-dense gas that can be used as an alternative to fossil fuels in many applications, including transportation and power generation. However, widespread adoption of hydrogen fuels is limited, in part, by the inability to safely store and transport hydrogen gas outside of carefully controlled industrial environments. An intriguing chemical phenomenon called the "molecular corking effect," may prove useful as a hydrogen gas storage mechanism. The molecular corking effect has been observed when hydrogen gas interacts with a class of materials called single-atom alloys (SAAs). SAAs consist of a relatively inert noble metal surface interspersed with single atoms of catalytically-active metals such as platinum and palladium. When diatomic hydrogen gas contacts the single-atom alloy, the bond between the two hydrogen atoms is broken by the catalytically-active metal. The individual hydrogen atoms then spill over on the inert metal surface. A "cork" molecule that preferentially binds to the catalytically-active metal can be added to prevent the hydrogen atoms from reforming gaseous hydrogen. Hydrogen can be safely stored in this manner until the temperature is increased to remove the cork molecule and release the hydrogen gas from the surface. Fundamental insights into the entire molecular corking process must be developed to fully realize the potential of single-atom alloy hydrogen storage. To these ends, we utilize non-local density functional theory (DFT) to investigate the potential of N-heterocyclic carbenes (NHCs) to behave as these so-called “molecular corks”, given their modular σ-donating/π-accepting abilities. 

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