Surface adsorbed atoms suppressing hydrogen permeation of Pd membranes

Yamakawa, Kohji; Ege, Michael; Ludescher, B.; Hirscher, Michael

doi:10.1016/s0925-8388(02)01157-x

Cited by 12 publications

(9 citation statements)

References 3 publications

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“…2,3 Thin palladium membranes have excellent hydrogen permeability and exhibit very high hydrogen selectivity. 4,5 It is known that any real sample has defects of crystal structure, such as vacancies, impurities, dislocations, etc. However concentration of most of these defects in a material is very low at ambient conditions.…”

Section: Introductionmentioning

confidence: 99%

First-principles study of vacancy-hydrogen interaction in Pd

et al. 2009

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Hydrogen absorption in face-centered-cubic palladium is studied from first principles, with particular focus on interaction between hydrogen atoms and vacancies, formation of hydrogen-vacancy complexes, and multiple hydrogen occupancy of a Pd vacancy. Vacancy formation energy in the presence of hydrogen, hydrogen trapping energy, and vacancy formation volume have been calculated and compared to existing experimental data. We show that a vacancy and hydrogen atoms form stable complexes. Further we have studied the process of hydrogen diffusion into the Pd vacancy. We find the energetically preferable position for hydrogen to reside in the palladium unit cell in the presence of a vacancy. The possibility of the multiple hydrogen occupancy ͑up to six hydrogen atoms͒ of a monovacancy is elucidated. This theoretical finding supports experimental indication of the appearance of superabundant vacancy complexes in palladium in the presence of hydrogen.

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Section: Introductionmentioning

confidence: 99%

First-principles study of vacancy-hydrogen interaction in Pd

et al. 2009

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“…The values obtained by qualitative analyses were close enough to the literaturereported values. However, this study did not take into account the Pd surface contaminants [69][70][71] or the membrane activation [72] to physically or chemically block the surface. Nevertheless, the better understanding of the assumptions in Sieverts' Law enabled the authors to explain some of the literature values for similar conditions, such the nature of the feed gas, operating conditions, and nature of the membrane (thickness, dense or composite).…”

Section: Explaining the Literature (N) Values And The Future Work Directionmentioning

confidence: 99%

Assessment of Sieverts Law Assumptions and ‘n’ Values in Palladium Membranes: Experimental and Theoretical Analyses

Alraeesi

Gardner

2021

Membranes

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Palladium and palladium alloy membranes are superior materials for hydrogen purification, removal, or reaction processes. Sieverts’ Law suggests that the flux of hydrogen through such membranes is proportional to the difference between the feed and permeate side partial pressures, each raised to the 0.5 power (n = 0.5). Sieverts’ Law is widely applied in analyzing the steady state hydrogen permeation through Pd-based membranes, even in some cases where the assumptions made in deriving Sieverts’ Law do not apply. Often permeation data are fit to the model allowing the pressure exponent (n) to vary. This study experimentally assessed the validity of Sieverts’ Law as hydrogen was separated from other gases and theoretically modelled the effects of pressure and temperature on the assumptions and hence the accuracy of the 0.5-power law even with pure hydrogen feed. Hydrogen fluxes through Pd and Pd-Ag alloy foils from feed mixtures (5–83% helium in hydrogen; 473–573 K; with and without a sweep gas) were measured to study the effect of concentration polarization (CP) on hydrogen permeance and the applicability of Sieverts’ Law under such conditions. Concentration polarization was found to dominate hydrogen transport under some experimental conditions, particularly when feed concentrations of hydrogen were low. All mixture feed experiments showed deviation from Sieverts’ Law. For example, the hydrogen flux through Pd foil was found to be proportional to the partial pressure difference (n ≈ 1) rather than being proportional to the difference in the square root of the partial pressures (n = 0.5), as suggested by Sieverts’ Law, indicating the high degree of concentration polarization. A theoretical model accounting for Langmuir adsorption with temperature dependent adsorption equilibrium coefficient was made and used to assess the effect of varying feed pressure from 1–136 atm at fixed temperature, and of varying temperature from 298 to 1273 K at fixed pressure. Adsorption effects, which dominate at high pressure and at low temperature, result in pressure exponents (n) values less than 0.5. With better understanding of the transport steps, a qualitative analysis of literature (n) values of 0.5, 0.5 < n < 1, and n > 1, was conducted suggesting the role of each condition or step on the hydrogen transport based on the empirically fit exponent value.

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“…They were heated for 2 h in air at 300°C to remove the carbon containing species (e.g. adsorbed hydrocarbons) [35]. Next, the samples were reduced under hydrogen atmosphere (5%H 2 -N 2 ) during 2 h at the same temperature.…”

Section: Equipment and Chemicalsmentioning

confidence: 99%