Dehydrogenation of Liquid Organic Hydrogen Carriers on Supported Pd Model Catalysts: Carbon Incorporation Under Operation Conditions

Liquid organic hydrogen carriers (LOHCs) have great potential as a hydrogen storage medium needed for a future sustainable energy system. Dehydrogenation of LOHCs requires a catalyst, such as supported Pd nanoparticles. Under reaction conditions, hydrogen and carbon may diffuse into the bulk of supported Pd catalyst particles and affect their activity and selectivity. The detailed understanding of this process is critical for the use of LOHCs in future hydrogen storage technologies. In this work, we studied these processes in-situ on a Pd model catalyst using high-energy grazing incidence X-ray diffraction. Pd nanoparticles were evaporated in ultra-high vacuum on a polished $α$-Al$_2$O$_3$(0001) substrate. The particles, with an initial average size of ~ 3.4 nm, were investigated at elevated temperature during their interaction with H$_2$ and methylcyclohexane (MCH) representing a model LOHC. The interaction with H$_2$ was studied in-situ at partial pressures up to 1 bar and temperatures between 300 and 500 K. At 300 K, the Pd nanoparticles (NPs) show a transition from $α$-PdH to $β$-PdH as a function of the H$_2$ pressure. The transition occurs gradually, which is attributed to the heterogeneity of the NP system. The hydrogen uptake in $β$-PdH$_x$ at 300 K and 1 bar is estimated to be XH ~ 0.37 ± 0.03 indicating that the miscibility gap is narrowed for the nanoparticular system. With increasing temperature, X$_H$ decreases until no $β$-PdH phase is formed anymore at 500 K. At the same temperature, we studied the interaction of the Pd/sapphire model catalyst with MCH, both in the presence and in the absence of H$_2$. In the absence of H$_2$, carbon is formed and diffuses into the bulk yielding PdC$_x$ with a C concentration of around x ~ 0.05 ± 0.01. In the presence of H$_2$ in the gas phase, bulk carbon formation in the Pd/sapphire model catalyst is completely suppressed. These results show that Pd nanoparticles act as an adequate catalyst for the dehydrogenation of MCH