Many technological as well as biological scenarios are controlled by chemical reactions occurring with nanopores and channels. In particular, this is very true for fuel cells and electrolyzers for which hydrogen production/consumption occurs with a hierarchical porous material. With “Confined Chemical Reactors,” we address this problem from a theoretical perspective aiming at identifying the key physical mechanism controlling these dynamics and exploiting them to improve the catalytic yield of these devices.
Our Publications in this field:
The interplay of shape and catalyst distribution in the yield of compressible flow microreactors (The Journal of Chemical Physics, 2024)
We develop a semi-analytical model for transport in structured catalytic microreactors, where both reactant and product are compressible fluids. Using lubrication and Fick–Jacobs approximations, we reduce the three-dimensional governing equations to an effective one-dimensional set of equations. Our model captures the effect of compressibility, corrugations in the shape of the reactor, and an inhomogeneous catalytic coating of the reactor walls. We show that in the weakly compressible limit (e.g., liquid-phase reactors), the distribution of catalyst does not influence the reactor yield, which we verify experimentally. Beyond this limit, we show that introducing inhomogeneities in the catalytic coating and corrugations to the reactor walls can improve the yield.
Enhancement of bubble transport in porous electrodes and catalysts (The Journal of Chemical Physics, 2024)
We investigate the formation and transport of gas bubbles across a model porous electrode/catalyst using lattice Boltzmann simulations. This approach enables us to systematically examine the influence of a wide range of morphologies, flow velocities, and reaction rates on the efficiency of gas production. By exploring these parameters, we identify critical parameter combinations that significantly contribute to an enhanced yield of gas output. Our simulations reveal the existence of an optimal pore geometry for which the product output is maximized. Intriguingly, we also observe that lower flow velocities improve gas production by leveraging coalescence-induced bubble detachment from the electrode/catalyst.
Precision of radiation chemistry networks: Playing Jenga with kinetic models for liquid-phase electron microscopy (Precision Chemistry, 2023)
Liquid-phase transmission electron microscopy (LP-TEM) is a powerful tool to gain unique insights into dynamics at the nanoscale. The electron probe, however, can induce significant beam effects that often alter observed phenomena such as radiolysis of the aqueous phase. The magnitude of beam-induced radiolysis can be assessed by means of radiation chemistry simulations potentially enabling quantitative application of LP-TEM. Unfortunately, the computational cost of these simulations scales with the amount of reactants regarded. To minimize the computational cost, while maintaining accurate predictions, we optimize the parameter space for the solution chemistry of aqueous systems in general and for diluted HAuCl4 solutions in particular. Our results indicate that sparsened kinetic models can accurately describe steady-state formation during LP-TEM and provide a handy prerequisite for efficient multidimensional modeling. We emphasize that the demonstrated workflow can be easily generalized to any kinetic model involving multiple reaction pathways.
Turning catalytically active pores into active pumps (The Journal of Chemical Physics, 2023)
We develop a semi-analytical model of self-diffusioosmotic transport in active pores, which includes advective transport and the inverse chemical reaction that consumes solute. In previous work [Antunes et al., Phys. Rev. Lett. 129, 188003 (2022)], we have demonstrated the existence of a spontaneous symmetry breaking in fore-aft symmetric pores that enables them to function as a micropump. We now show that this pumping transition is controlled by three timescales. Two timescales characterize advective and diffusive transport. The third timescale corresponds to how long a solute molecule resides in the pore before being consumed. Introducing asymmetry to the pore (either via the shape or the catalytic coating) reveals a second type of advection-enabled transition. In asymmetric pores, the flow rate exhibits discontinuous jumps and hysteresis loops upon tuning the parameters that control the asymmetry. This work demonstrates the interconnected roles of shape and catalytic patterning in the dynamics of active pores and shows how to design a pump for optimum performance.
Pumping and Mixing in Active Pores (Physical Review Letters, 2022)
We show both numerically and analytically that a chemically patterned active pore can act as a micro- or nanopump for fluids, even if it is fore-aft symmetric. This is possible due to a spontaneous symmetry breaking which occurs when advection rather than diffusion is the dominant mechanism of solute transport. We further demonstrate that, for pumping and tuning the flow rate, a combination of geometrical and chemical inhomogeneities is required. For certain parameter values, the flow is unsteady, and persistent oscillations with a tunable frequency appear. Finally, we find that the flow exhibits convection rolls and hence promotes mixing in the low Reynolds number regime.
Liquid Organic Hydrogen Carriers represent a key actor in the play of hydrogen technology. In fact LOHS allows to store a large amount of H2 without the need of high pressure containers hence improving the safety and easing the usage of the H2 stored therein. With “LOHC technology” we aim at a theoretical analysis of the charging/discharging dynamics of H2 and hence to an improvement of these processes.
Our publications in this field:
Heat transfer to a catalytic multiphase dehydrogenation reactor (International Journal of Hydrogen Energy, 2024)
The release of hydrogen from liquid organic hydrogen carriers (LOHC) takes place in an endothermal dehydrogenation reaction that is accompanied by a strong volume expansion. This leads to complex hydrodynamic properties that change drastically along the reactor axis due to product gas evolution. Consequently, heat transfer into the catalytic fixed-bed exhibits a pronounced local dependency. For a better understanding of such multiphase dehydrogenation systems, we have performed heat transport measurements in the presence of the chemical reaction, namely during the dehydrogenation of perhydro benzyltoluene (H12-BT) and perhydro dibenzyltoluene (H18-DBT). The results reveal that overall heat transfer coefficients show a clear local dependency on the axial coordinate. Moreover, the two carriers were found to differ significantly in their thermal behavior.
Based on a global analysis, two main regimes can be distinguished in the dehydrogenation reactor:
1.) With the LOHC mixture being primarily in the liquid phase, heat transport is dominated and intensified by the hydrogen release;
2.) With an increasing proportion of LOHC vapor in the reactor, the heat transport is dominated by the gas phase, resulting in significantly lower thermal parameters.
Nucleation as a rate-determining step in catalytic gas generation reactions from liquid phase systems (Science Advances, 2022)
The observable reaction rate of heterogeneously catalyzed reactions is known to be limited either by the intrinsic kinetics of the catalytic transformation or by the rate of pore and/or film diffusion. Here, we show that in gas generation reactions from liquid reactants, the nucleation of gas bubbles in the catalyst pore structure represents an additional important rate-limiting step. This is highlighted for the example of catalytic hydrogen release from the liquid organic hydrogen carrier compound perhydro-dibenzyltoluene. A nucleation-inhibited catalytic system produces only dissolved hydrogen with fast saturation of the fluid phase around the active site, while bubble formation enhances mass transfer by more than a factor of 50 in an oscillating reaction regime. Nucleation can be efficiently triggered not only by temperature changes and catalyst surface modification but also by a mechanical stimulus. Our work sheds new light on performance-limiting factors in reactions that are of highest relevance for the future green hydrogen economy.
Supported Ionic Liquid Phase catalysis is a novel and promising approach to heterogeneous catalysis which allows to store the catalytic compound within a thin ionic liquid film that combines a strong stability (low volatility) to a quick adsorption/desorption of reactants and products from the liquid phase. Within “SILP” many physical processes occur at the same time. Accordingly, we aim at a theoretical analysis of the dynamics of such active thin film which will allow on the one hand to understand the interplay between the different processe and on the other to improve the yeld of SILP devices.
Our publications in this field:
Resolving the microscopic hydrodynamics at the moving contact line (Physical Review Fluids, 2022)
The molecular structure of moving contact lines (MCLs) and the emergence of a corresponding macroscopic dissipation have made the MCL a paradigm of fluid dynamics. Through novel averaging techniques that remove capillary waves smearing we achieve an unprecedented resolution in molecular dynamics simulations and find that they match with the continuum description obtained by finite element method down to molecular scales. This allows us to distinguish dissipation at the liquid-solid interface (Navier-slip) and at the contact line, the latter being negligible for the rather smooth substrate considered.
Order and information in the patterns of spinning magnetic micro-disks at the air-water interface (Science Advances, 2022)
The application of the Shannon entropy to study the relationship between information and structures has yielded insights into molecular and material systems. However, the difficulty in directly observing and manipulating atoms and molecules hampers the ability of these systems to serve as model systems for further exploring the links between information and structures. Here, we use, as a model experimental system, hundreds of spinning magnetic micro-disks self-organizing at the air-water interface to generate various spatiotemporal patterns with varying degrees of order. Using the neighbor distance as the information-bearing variable, we demonstrate the links among information, structure, and interactions. We establish a direct link between information and structure without using explicit knowledge of interactions. Last, we show that the Shannon entropy by neighbor distances is a powerful observable in characterizing structural changes. Our findings are relevant for analyzing natural self-organizing systems and for designing collective robots.
