Oxygen-Promoted Dehydrogenation on Size-Selected Cu–Pd Pentamer Clusters for Low-Temperature Hydrogen Release from Cyclic Hydrocarbons
Mykhailo Vaidulych, Yonghyuk Lee, Federico Loi, Petr Vitek, Michael Vorochta, Stanislav Valtera, Ivan Khalakhan, Claudia de Melo, Samiran Chakraborty, Karolína Simkovičová, Yves Garreau, Benjamin Voisin, Bilal Bawab, Jan M. Macak, Muntaseer Bunian, Moon-Hyung Jang, Yu Lei, Philippe Sautet, Iva Matolinová, Alessandro Coati, Pascal Andreazza, Anastassia N. Alexandrova, Stefan VajdaAbstract
Despite extensive studies on cycloalkane and cycloalkene dehydrogenation for liquid organic hydrogen carrier applications, the mechanisms governing selective H2 release under mild conditions remain poorly understood. Here, subnanometer CuxPdy pentamer clusters supported on ZrO2 represent a highly active and selective class of low-temperature dehydrogenation catalysts. Pd5 exhibits the highest benzene and molecular H2 formation rates from dehydrogenation of cyclohexene, followed by Cu2Pd3 and Cu3Pd2, whereas Cu5 is inactive. All active catalysts favored H2 over H2O as the hydrogen-containing product and completely suppressed combustion pathways, with no detectable CO2 formation. Oxygen-lean conditions further favored H2 formation across all active catalysts, with H2 selectivity exceeding 99% for Pd5. Kinetic analysis of Cu3Pd2 reveals positive fractional apparent orders in both C6H10 and O2 and an effective apparent activation energy of approximately 66 kJ·mol–1, while reactant composition and temperature systematically control H2/H2O branching. In situ XPS and in situ GIWAXS reveal that catalytic activity and cluster stability arise from the interplay of atomic composition, electronic structure, metal–oxygen interactions, and the chemical environment, with Cu exhibiting a composition-sensitive redox response and Pd-containing clusters accommodating oxygen while retaining electronically active Pd-derived states near the Fermi level. DFT/MLIP free-energy calculations identify kinetically accessible O-assisted first C–H activation pathways involving H transfer to cluster-bound O, with barriers strongly dependent on cluster composition and O configuration. These findings establish an ensemble-based structure–activity framework for controlling dehydrogenation pathways and provide design principles for catalysts that efficiently promote selective hydrogen release suitable for hydrogen carrier applications.