How back reaction, hydrogen transport, and capillarity control the performance of hydrogen release from liquid organic carriers

TL;DR

This paper develops a theoretical model to understand how back reaction, hydrogen transport, and capillarity influence hydrogen release efficiency from liquid organic carriers, highlighting hydrogen transport as a key limiting factor.

Contribution

It introduces a comprehensive model accounting for reversible reactions, transport mechanisms, and capillarity effects, revealing new kinetic regimes and conditions for bubble formation.

Findings

Hydrogen transport limits catalyst performance more than previously recognized.

Two kinetic regimes depend on whether hydrogen leaves as bubbles or diffuses.

Conditions for bubble formation depend on hydrogen supersaturation and capillarity.

Abstract

We derive a theoretical model to elucidate the inhibition of catalytic activity during the dehydrogenation of Liquid Organic Hydrogen Carriers (LOHC). Within our model, we account for the reversible nature of the hydrogenation-dehydrogenation reaction as well as the transport of both LOHC and produced hydrogen. Our analysis reveals that the main limiting factor for the performance of porous catalysts is the transport of dissolved hydrogen, which has been overlooked so far. In particular, we show that two distinct kinetic regimes can arise depending on whether hydrogen leaves the pellet in form of bubbles or via diffusion. Moreover, we derive the conditions for the onset of bubbling depending on hydrogen supersaturation and capillarity. Beyond LOHC systems, our findings are applicable to a broader class of reversible reactions, particularly those involving volatile products that can leave the liquid reaction medium in the form of bubbles.