Diffusion on Surfaces. II. Correlation of Diffusivities of Physically and Chemically Adsorbed Species

Sladek, K. J.; Gilliland, E. R.; Baddour, R. F.

doi:10.1021/i160050a002

Cited by 206 publications

(115 citation statements)

References 9 publications

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“…A mean value of 25 kJ/mol is calculated for Est , which is a characteristic value for physisorption or weak chemisorption . It is generally accepted that the activation energy for surface flow and the heat of adsorption are coupled [10][11][12], the ratio being 0 .3 to 0.5 . This implies an activation energy for surface flow of 8-12 kJ/mol, and it can be expected that part of the adsorbed molecules will be mobile on the surface .…”

Section: Interaction : Adsorptionmentioning

confidence: 99%

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Gas and surface diffusion in modified γ-alumina systems

Uhlhorn

Keizer

Burggraaf

1989

Journal of Membrane Science

171

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SummaryThe transport of pure gases through a microporous membrane is described. The alumina-based membrane (pores 2 .5-4 nm) is suitable for Knudsen diffusion separation . To improve the separation factor, interaction with and mobility on the pore wall of one of the gases of a mixture is necessary . To introduce surface diffusion of oxygen and hydrogen, a y-alumina membrane was impregnated with silver . If temperature and atmosphere are controlled carefully, finely dispersed silver up to 17% by weight can be introduced . At higher loads and under oxidizing conditions, particle growth occurs. In adsorption experiments, little oxygen adsorption on the silver-modified y-alumina could be detected . This is due to a decrease in accessible surface area of the silver because of particle growth of silver under oxygen . The mobility of hydrogen on the surface was tested by counterdiffusion experiments, of which the theory is given. Hydrogen shows a considerable mobility on the surface at 293 K . At low pressures the flux ratio of hydrogen to nitrogen improved from 3 .8 to 8.8. Magnesia was introduced into the y-alumina membrane to enhance the adsorption and mobility of CO2. It is known that 30% of the CO 2 transport on non-modified yalumina is surface diffusion . The highest achievable magnesia load was 2 .2% by weight . Introduction of magnesia into the y-alumina surface gives more strong base sites and fewer weak base sites . This results in stronger bonding of C0 2 on the surface, but the amount adsorbed is comparable with the amount of C02 adsorbed on non-modified y-alumina . The contribution of surface diffusion to the total transport decreases with the introduction of magnesia, as is shown by counterdiffusion. The more strongly bonded CO 2 is less mobile, resulting in a smaller surface flux .

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Section: Interaction : Adsorptionmentioning

confidence: 99%

“…If the adsorbed species is mobile on the surface of the pore, it will diffuse along the surface concentration gradient . The occurrence of this type of transport, called surface diffusion, is frequently reported in porous materials [10][11][12] .…”

Section: Introductionmentioning

confidence: 99%

Gas and surface diffusion in modified γ-alumina systems

Uhlhorn

Keizer

Burggraaf

1989

Journal of Membrane Science

171

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“…This transfer could, as suggested elsewhere [105,106], occur via the gas phase (possibly by evolution of O· radicals or derived O 3 from the catalyst surface), or via surface diffusion to the soot/catalyst interface and possibly onto the soot. In the latter case the heat of oxygen chemisorption on the catalyst would also be important, as the rate of surface diffusion of an adsorbate depends on its heat of chemisorption [107].…”

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confidence: 99%

Importance of the oxygen bond strength for catalytic activity in soot oxidation

Christensen

Grunwaldt

Jensen

2016

Applied Catalysis B: Environmental

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The oxygen bond strength on a catalyst, as measured by the heat of oxygen chemisorption, is observed to be a very important parameter for the activity of the catalyst in soot oxidation. With both intimate contact between soot and catalyst (tight contact) and with the solids stirred loosely together (loose contact) the rate constants for a number of catalytic materials outline a volcano curve when plotted against their heats of oxygen chemisorption. However, the optima of the volcanoes correspond to different heats of chemisorption for the two contact situations. In both cases the activation energies for soot oxidation follow linear Brønsted-Evans-Polanyi relationships with the heat of oxygen chemisorption. Among the tested metal or metal oxide catalysts Co 3 O 4 and CeO 2 were nearest to the optimal bond strength in tight contact oxidation, while Cr 2 O 3 was nearest to the optimum in loose contact oxidation. The optimum of the volcano curve in loose contact is estimated to occur between the bond strengths of α-Fe 2 O 3 and α-Cr 2 O 3 . Guided by an interpolation principle Fe a Cr b O x binary oxides were tested, and the activity of these oxides was observed to pass through an optimum for an FeCr 2 O x binary oxide catalyst, which exhibited a rate constant at 550 °C that was 2.3 times higher than the one for pure α-Cr 2 O 3 and 29 times higher than the one for pure α-Fe 2 O 3 .

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“…This is described in detail by Hwang and Kammermeyer (1966a,b), Gilliland et al (1974), Tamon, et al (1981, Chen and Yang (1991) and Xiao and Wei (1992). Surface diffusion is most pertinent for strongly adsorbing molecules and at relatively low temperatures (< 200 IC).…”

Section: Surface Diffusionmentioning

confidence: 96%

“…Interaction energy in surface diffusion is first discussed by Hwang (1966). Gilliland et al (1974) present the effect of concentration on the diffusivity of physically adsorbed gases on surfaces. That is, the differential heat of adsorption (which is used in the correlation'of the surface diffusivity) varies with coverage.…”

Section: Figure 4 Concentration Dependence Of Surface Diffusionmentioning

confidence: 99%

Molecular dynamics of gases and vapors in nanoporous solids. Final LDRD project report

Pohl¹

1996

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Diffusion on Surfaces. II. Correlation of Diffusivities of Physically and Chemically Adsorbed Species

Cited by 206 publications

References 9 publications

Gas and surface diffusion in modified γ-alumina systems

Gas and surface diffusion in modified γ-alumina systems

Importance of the oxygen bond strength for catalytic activity in soot oxidation

Molecular dynamics of gases and vapors in nanoporous solids. Final LDRD project report

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