Interplay of adsorption and surface mobility in tracer diffusion in porous media

Olivares, Carlos; Reis, F. D. A. Aarão

doi:10.1103/physreve.100.022120

Cited by 5 publications

(5 citation statements)

References 37 publications

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“…First, surface diffusion [240][241][242][243][244][245][246][247][248][249][250][251][252] of adsorbed species may play an important role in the overall transport of fluids through mesoporous materials, because of their high surface area, especially when there are strong interactions of adsorbed species with the wall. Surface diffusion bears similarities with diffusion in zeolites.…”

Section: General Features Of Diffusion In Mesoporous Materialsmentioning

confidence: 99%

“…Increased or reduced diffusivity was noted in the presence of next-neighbor repulsions or attractions, respectively [445]. Olivares and Reis [245] studied a model of random walks in the interstices of a simple cubic packing of solid spheres, to represent diffusion of a tracer interacting with the internal surface of that medium. A scaling approach showed three different regimes for diffusion in this medium: (1) dominant bulk residence, in which the tracer moves in the bulk most of the time and the diffusion coefficient is of the same order of the coefficient in free solution; (2) dominant surface residence with dominant bulk displacement, in which the tracer is adsorbed on the sphere walls most of the time, but with a very small mobility in that region, so that the average displacement is dominated by hops in the bulk; and (3) dominant surface residence with dominant surface displacement, in which the tracer is adsorbed on the sphere's walls most of the time and executes most hops along those walls.…”

Section: Surface Diffusionmentioning

confidence: 99%

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Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

et al. 2021

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Nanoporous solids are ubiquitous in chemical, energy, and environmental processes, where controlled transport of molecules through the pores plays a crucial role. They are used as sorbents, chromatographic or membrane materials for separations, and as catalysts and catalyst supports. Defined as materials where confinement effects lead to substantial deviations from bulk diffusion, nanoporous materials include crystalline microporous zeotypes and metal–organic frameworks (MOFs), and a number of semi-crystalline and amorphous mesoporous solids, as well as hierarchically structured materials, containing both nanopores and wider meso- or macropores to facilitate transport over macroscopic distances. The ranges of pore sizes, shapes, and topologies spanned by these materials represent a considerable challenge for predicting molecular diffusivities, but fundamental understanding also provides an opportunity to guide the design of new nanoporous materials to increase the performance of transport limited processes. Remarkable progress in synthesis increasingly allows these designs to be put into practice. Molecular simulation techniques have been used in conjunction with experimental measurements to examine in detail the fundamental diffusion processes within nanoporous solids, to provide insight into the free energy landscape navigated by adsorbates, and to better understand nano-confinement effects. Pore network models, discrete particle models and synthesis-mimicking atomistic models allow to tackle diffusion in mesoporous and hierarchically structured porous materials, where multiscale approaches benefit from ever cheaper parallel computing and higher resolution imaging. Here, we discuss synergistic combinations of simulation and experiment to showcase theoretical progress and computational techniques that have been successful in predicting guest diffusion and providing insights. We also outline where new fundamental developments and experimental techniques are needed to enable more accurate predictions for complex systems.

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Section: General Features Of Diffusion In Mesoporous Materialsmentioning

confidence: 99%

Section: Surface Diffusionmentioning

confidence: 99%

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

et al. 2021

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“…A scaling approach is used to calculate the effective diffusion coefficient. The method is similar to that of a recent work on a random walk model in the pores of a packing of spheres . This type of phenomenological approach facilitates the interpretation of different diffusion regimes without solving diffusion equations (although those equations might be written from the model rules).…”

Section: Model and Methodsmentioning

confidence: 99%

“…The MSD is usually written in terms of bulk and surface diffusion coefficients, ,− while the adsorption–desorption transitions are assumed to be responsible only for setting the equilibrium concentrations. However, we will show that this is a reasonable approximation only for wide pores, so here the MSD is written with contributions of bulk diffusion and of adsorption and desorption transitions: ⟨ ( Δ x ) 2 ⟩ = false⟨ false( normalΔ x false) 2 false⟩ B U L K + false⟨ false( normalΔ x false) 2 false⟩ A − D The summation of these contributions is possible because the displacements of length ≳ a in the bulk, in the adsorption, and in the desorption processes are uncorrelated.…”

Section: Model and Methodsmentioning

confidence: 99%

“…The method is similar to that of a recent work on a random walk model in the pores of a packing of spheres. 37 This type of phenomenological approach facilitates the interpretation of different diffusion regimes 38 without solving diffusion equations (although those equations might be written from the model rules). We focus on the diffusion across the sample in a given x direction, where the MSD ⟨(Δ x ) 2 ⟩ is calculated.…”

Section: Model and Methodsmentioning

confidence: 99%

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Adsorption of Diffusing Tracers, Apparent Tortuosity, and Application to Mesoporous Silica

Rodrigues,

Alves Aarão Reis

2024

Langmuir

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The apparent tortuosity due to adsorption of diffusing tracers in a porous material is determined by a scaling approach and is used to analyze recent data on LiCl and alkane diffusion in mesoporous silica. The slope of the adsorption isotherm at small loadings is written as β = q A / q G , where q A is the adsorption−desorption ratio and q G = ϵ/(as) − 1 is a geometrical factor depending on the range a of the tracer-wall interaction, the porosity ϵ, and the specific surface area s. The adsorption leads to a decrease of effective diffusion coefficient, which is quantified by multiplying the geometrical tortuosity factor τ geom by an apparent tortuosity factor τ app . In wide pores or when the adsorption barrier is high, τ app = β + 1, as obtained in previous works, but in narrow pores there is an additional contribution from frequent adsorption−desorption transitions. These results are obtained in media with parallel pores of constant cross sections, where the ratio between the effective pore width ϵ/s and the actual width is ≈0.25. Applications to mesoporous silica samples are justified by the small deviations from this ideal ratio. In the analysis of alkane self-diffusion data, the fractions of adsorbed molecules predicted in a recent theoretical work are used to estimate τ geom of the silica samples, which is ≫1 only in the sample with the narrowest pores (nominal 3 nm). The application of the model to Li + ion diffusion leads to similar values of τ geom and to a difference of energy barriers of desorption and adsorption for those ions of ∼0.06 eV. Comparatively, alkane self-diffusion provides the correct order of magnitude of τ geom , with adsorption playing a less important role, whereas adsorption effects on Li + diffusion are much more important.

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Extended adsorbing surface reach and memory effects on the diffusive behavior of particles in confined systems

Recanello

Lenzi

Martins

et al. 2020

International Journal of Heat and Mass Transfer

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Interplay of adsorption and surface mobility in tracer diffusion in porous media

Cited by 5 publications

References 37 publications

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

Adsorption of Diffusing Tracers, Apparent Tortuosity, and Application to Mesoporous Silica

Extended adsorbing surface reach and memory effects on the diffusive behavior of particles in confined systems

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