Reactant-dependent stability of supported metal catalysts for hydrogen storage in N-heterocyclic carriers

Abstract

N-heterocyclic aromatic molecules (N-LHCs) can store hydrogen (H₂) in their chemical bonds through reversible (de)hydrogenation over supported metal catalysts, but catalyst deactivation mechanisms during H₂ storage reactions remain poorly understood. Here, we investigated the reactivity and stability of supported Pd and Ni catalysts for the liquid-phase hydrogenation of N-LHCs with varying methyl group positions. We combine continuous flow reactor studies to probe time-on-stream stability and regenerability, with post-reaction catalyst characterization (CO chemisorption, microscopy, elemental analysis) to assess catalyst deactivation routes, including coking, sintering, and leaching. While decreases in rates with time-on-stream due to coking are observed with both metals (Pd, Ni) across all studied carriers, irreversible deactivation due to sintering strongly depends on the structure of N-LHC and the catalyst surface. Over Pd/SiO₂ and Pd/Al₂O₃, while only reversible deactivation via coking occurred during the hydrogenation of indole (performing >5900 catalytic turnovers), indoles methylated at the N- and 2-positions instead caused irreversible sintering. In contrast, Ni/SiO₂ catalysts were resistant to sintering for indole and N-methylindole hydrogenations (performing up to 150,000 turnovers) but sintered during the hydrogenation of 2-methylindole. To gain further insights into reactant-dependent sintering, we investigated additional N-LHCs with different basicities and steric properties. The lack of a monotonic correlation between catalyst stability and basicity of N-LHCs suggests that a combination of steric and electronic properties of N-LHC molecules impacts their propensity to induce sintering. This work provides new insights into reactant-dependent catalyst stability that guide the selection of catalysts and carriers for H₂ storage in chemical bonds.