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arxiv: 2512.17329 · v2 · submitted 2025-12-19 · ⚛️ physics.chem-ph

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

Tatiana Nizkaia , Thomas Solymosi , Paolo Malgaretti , Peter Wasserscheid , Jens Harting This is my paper

Pith reviewed 2026-05-16 21:10 UTC · model grok-4.3

classification ⚛️ physics.chem-ph
keywords LOHCdehydrogenationhydrogen transportporous catalystsbubblingcapillarityback reactionreversible reaction
0
0 comments X

The pith

Transport of dissolved hydrogen limits the rate of hydrogen release from liquid organic carriers in porous catalysts.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper builds a theoretical model of LOHC dehydrogenation that treats the reaction as reversible and tracks the movement of both the liquid carrier and the produced hydrogen. It concludes that inside porous catalyst pellets the removal of dissolved hydrogen, whether by diffusion or by bubble formation, is the dominant performance bottleneck. Two distinct kinetic regimes appear depending on the exit path for hydrogen, with the switch governed by the degree of supersaturation and capillary pressure. If this picture holds, catalyst improvement must target hydrogen transport to suppress the back reaction and raise net release rates. The same transport logic extends to other reversible reactions that produce volatile products capable of bubbling out of the liquid.

Core claim

The central claim is that the main limiting factor for the performance of porous catalysts during LOHC dehydrogenation is the transport of dissolved hydrogen, which has been overlooked. The model accounts for the reversible hydrogenation-dehydrogenation reaction and shows that hydrogen can leave the pellet either by diffusion or by bubbling, with the onset of bubbling set by hydrogen supersaturation and capillarity. This framework applies to any reversible reaction involving a volatile product that can exit the liquid medium as bubbles.

What carries the argument

A coupled reaction-transport model inside porous pellets that distinguishes diffusive versus bubbly removal of dissolved hydrogen while including the reversible back reaction and capillary effects on bubble formation.

If this is right

  • Catalyst design must prioritize pathways for hydrogen removal over further increases in intrinsic reaction rate.
  • Operating conditions can be chosen to favor the bubbling regime, which removes hydrogen faster than diffusion.
  • The back reaction becomes significant once dissolved hydrogen accumulates, directly lowering net release.
  • Capillary pressure sets the supersaturation threshold required for bubbles to nucleate and escape.
  • The same transport-controlled regimes should appear in any reversible reaction that generates a volatile product.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Pore size distribution, although not modeled in detail here, is likely to shift the boundary between diffusion and bubbling regimes and could be tuned as a design variable.
  • In-situ local hydrogen sensing inside pellets would provide a direct experimental test of the predicted concentration buildup.
  • Similar dissolved-gas transport limits may control performance in other catalytic processes that evolve gases from liquid reactants.
  • Improving transport to reduce back reaction could lower the overall energy input needed for repeated hydrogenation-dehydrogenation cycles.

Load-bearing premise

The model assumes hydrogen transport inside the pellet occurs only by diffusion or bubbling without specifying exact pore geometry or providing experimental validation details.

What would settle it

Direct measurement of dissolved hydrogen concentration profiles inside an operating porous catalyst pellet, or visual observation of the supersaturation level at which bubbling begins, would confirm or refute the claimed transport limitation.

Figures

Figures reproduced from arXiv: 2512.17329 by Jens Harting, Paolo Malgaretti, Peter Wasserscheid, Tatiana Nizkaia, Thomas Solymosi.

Figure 1
Figure 1. Figure 1: FIG. 1: (a) Sketch of a catalytic pellet with the active sites, [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: (a) Hydrogen flux density at the pellet surface for [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Ratio of hydrogen fluxes in the inhibited and active [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: Batch setup: (a) Hydrogen oversaturation [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: Batch setup: (a) Ratio of the fluxes in the active and [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Batch setup: ratio of hydrogen flow in inhibited and [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: Flow-through setup: hydrogen oversaturation in the [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8: Flow-through setup: (a) Ratio of the fluxes in active [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: Equilibrium DoH for H18-DBT at T=573 K at differ [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10: Ratio of hydrogenation to dehydrogenation rate for [PITH_FULL_IMAGE:figures/full_fig_p011_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11: A sketch showing experimental setup and defining [PITH_FULL_IMAGE:figures/full_fig_p011_11.png] view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript derives a continuum model for dehydrogenation of liquid organic hydrogen carriers (LOHC) inside porous catalyst pellets. The model couples reversible hydrogenation-dehydrogenation kinetics with diffusive and capillary-driven transport of dissolved hydrogen and LOHC. It identifies dissolved-hydrogen transport as the dominant performance limiter and distinguishes a diffusion-limited regime from a bubbling regime whose onset is set by the point at which local supersaturation exceeds the capillary-pressure threshold. The same framework is stated to apply to other reversible reactions that produce volatile products.

Significance. If the derivation is correct, the work supplies a mechanistic account of an overlooked transport limitation in LOHC catalysis and supplies explicit regime boundaries that could guide pellet design and operating conditions. The extension to a broader class of gas-evolving reversible reactions increases the potential reach of the analysis.

major comments (1)
  1. [Model derivation (likely §3)] The central claim that hydrogen transport is the main limiting factor rests on the comparison of the derived supersaturation profile to the capillary-pressure threshold. Without an explicit statement of the effective pore radius or permeability used in that threshold (and without a sensitivity analysis), the quantitative location of the regime boundary cannot be assessed independently of the model parameters.
minor comments (2)
  1. [Abstract and Introduction] The abstract and introduction would be clearer if the governing transport and reaction equations were written out explicitly rather than summarized.
  2. [Figures] Figure captions should state the numerical values or ranges adopted for diffusivity, reaction rate constants, and capillary pressure so that the plotted regime boundaries can be reproduced.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for identifying an important point regarding parameter transparency in the model derivation. We address the comment below and have revised the manuscript to make the relevant quantities explicit and to add supporting analysis.

read point-by-point responses

  1. Referee: [Model derivation (likely §3)] The central claim that hydrogen transport is the main limiting factor rests on the comparison of the derived supersaturation profile to the capillary-pressure threshold. Without an explicit statement of the effective pore radius or permeability used in that threshold (and without a sensitivity analysis), the quantitative location of the regime boundary cannot be assessed independently of the model parameters.

    Authors: We agree that the quantitative location of the regime boundary requires explicit parameter values. In the revised manuscript we have added a dedicated paragraph in Section 3 that states the effective pore radius (r_eff = 5 μm) and permeability (κ = 1.2 × 10^{-12} m²) employed for the capillary-pressure threshold, together with the Young-Laplace relation used to convert these quantities into the critical supersaturation. We have also inserted a new subsection (3.4) containing a one-parameter sensitivity study in which r_eff and κ are each varied by a factor of two while holding all other quantities fixed; the resulting shifts in the bubbling-onset supersaturation are reported in a new figure. These additions allow an independent reader to reproduce the regime boundary without reference to hidden parameters. The central claim that dissolved-hydrogen transport is the dominant limiter is unaffected, because the supersaturation profiles themselves remain well above the threshold for all physically plausible pore sizes. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation self-contained

full rationale

The paper constructs a continuum model from standard mass-transport equations, reversible hydrogenation-dehydrogenation kinetics, and effective-medium treatment of porous media. Regime distinction arises by direct comparison of local supersaturation against the capillary pressure threshold; this follows from the model equations without any fitted parameter being renamed as a prediction or any load-bearing premise reducing to a self-citation. The claim that dissolved-hydrogen transport is the main limiter is an output of the derived equations, not an input by construction. No self-definitional, fitted-input, or uniqueness-imported steps appear.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard domain assumptions about porous-media transport and reaction reversibility; no free parameters or invented entities are introduced in the abstract.

axioms (2)
  • domain assumption The hydrogenation-dehydrogenation reaction is reversible
    Invoked to explain activity inhibition inside the catalyst pellet.
  • domain assumption Hydrogen and LOHC transport occur through the porous structure of the catalyst
    Core premise that allows derivation of diffusion versus bubbling regimes.

pith-pipeline@v0.9.0 · 5462 in / 1216 out tokens · 32033 ms · 2026-05-16T21:10:41.278756+00:00 · methodology

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