Bridging Dehydrogenation and Hydrogenation in Heterogeneous Catalysis: A Demonstration of a Unified Catalytic Approach

In the pursuit of enhancing the applications of hydrogen as an energy carrier, this research delved into the utilization of a singular hybrid catalyst capable of performing both dehydrogenation and hydrogenation processes for Liquid Organic Hydrogen Carriers (LOHCs). This study presents the synthesis and characterization of a hybrid catalyst, combining an organometallic pincer complex with Pd-Ru heterostructures supported on γ-alumina. Unlike conventional transition and noble metal nanoparticles, the use of a pincer complex offers exceptional thermal stability due to its aryl backbone, which is advantageous for various endothermic dehydrogenation reactions of hydrocarbons in LOHCs. This pioneering hybrid catalyst is a novel approach, demonstrating a proof of concept. In this study, we utilized the hybrid catalyst to investigate the dehydrogenation and hydrogenation of a lower enthalpic system, specifically the cyclooctane-cyclooctene system. The dehydrogenation of cyclooctane was conducted at 443 K using tertiary butyl ethylene as a sacrificial hydrogen acceptor, while the hydrogenation of cyclooctene reaction catalyzed by Pd-Ru nanostructures occurred at 298 K and 1 atm H2. The results showed successful tandem dehydrogenation-hydrogenation reactions. However, challenges were noted in terms of catalytic activity and recyclability, providing valuable insights for further refinement and optimization.

Chemical Bonds Containing Hydrogen: Choices for Hydrogen Carriers and Catalysts

Compounds containing B─H, C─H, N─H, or O─H bonds with high hydrogen content have been extensively studied as potential hydrogen carriers. Their hydrogen storage performance is largely determined by the nature of these bonds, decomposition pathways, and the properties of the dehydrogenation products. Among these compounds, methanol, cyclohexane, and ammonia stand out due to their low costs and established infrastructure, making them promising hydrogen carriers for large-scale storage and transport. They offer viable pathways for decarbonizing society by enabling hydrogen to serve as a clean energy source. However, several challenges persist, including the high temperatures required for (de)hydrogenation, slow kinetics, and the reliance on costly catalysts. To address these issues, strategies such as chemical modification and catalyst development are being pursued to improve hydrogen cycling performance. This review highlights recent progress in hydrogen carriers with B─H, C─H, N─H, or O─H bonds. It examines the fundamental characteristics of these bonds and carriers, as well as advances in catalyst development. Our objective is to offer a comprehensive understanding of current state of hydrogen carriers and identify future research directions, such as molecular modification and system optimization. Innovations in these areas are crucial to advance hydrogen storage technologies for a large-scale hydrogen deployment.

Interfacial hydrogen evolution reaction from Ouzo-effect-generated bulk nano/micro droplets of liquid organic hydrogen carriers

HYPOTHESIS

Organosilanes as liquid organic hydrogen carriers (LOHCs) offer a promising solution for the safe storage and transport of hydrogen gas as a clean energy source. However, the dehydrogenation reaction of organosilanes in the presence of water faces the challenge of sluggish kinetics in conventional bulk reactions. Dispersing organosilanes as stable nanodroplets in basic water offers a potential strategy to increase the interfacial area, thereby enhancing H2 production efficiency.

EXPERIMENTS

Organosilane nanodroplets were generated through spontaneous emulsification via the Ouzo effect in a ternary organosilane-water-acetone system. The reaction between the organosilane nano/microdroplets and the alkaline aqueous phase led to H2 generation. This study investigates how the composition and size distribution of these droplets influence H2 production yield. To gain deeper insight into the reaction mechanisms, single reacting microdroplets were analyzed using side-view imaging and confocal microscopy.

FINDINGS

Organosilane nano/microdroplets formed from the Ouzo effect in the presence of a co-solvent. H2 formation yields from interfacial reactions of these droplets reached up to 25%, whereas single reacting microdroplets achieved a maximum yield of 3.5%. This study demonstrates that spontaneous emulsification in ternary mixture using the Ouzo effect can enhance reaction kinetics and product yields. Furthermore, detailed insights into the behavior of H2 bubbles, from their nucleation within a microdroplet to their growth and eventual detachment, were obtained through the analysis of single reacting microdroplets.