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AMORPHOUS ICE. A MICROPOROUS SOLID : ASTROPHYSICAL IMPLICATIONS
E. Mayer, R. Pletzer
To cite this version:
E. Mayer, R. Pletzer. AMORPHOUS ICE. A MICROPOROUS SOLID : ASTROPHYS- ICAL IMPLICATIONS. Journal de Physique Colloques, 1987, 48 (C1), pp.C1-581-C1-586.
�10.1051/jphyscol:1987179�. �jpa-00226330�
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Colloque C1, suppl6ment au no 3, Tome 48, mars 1987
AMORPHOUS ICE. A MICROPOROUS SOLID : ASTROPHYSICAL IMPLICATIONS E. MAYER and R. PLETZER
Institut fiir Anorganische und Analytische Chemie, Universitat Innsbruck, A-6020 Innsbruck, Austria
Rdsumc5.- L1&tude de la glace amorphe dgposde h partir de laphase vapeur par la msthode d'adsorption d'azote h 77 K a montrg que ce solide est microporeux. Le volume des micropores entre 0,21 et 0,12 a gt& dsterming par comparaison avec les diagram- mes de Dubinin-Radushkevich. Un rechauffage de 1'Bchantillon jusqu'2 113 K produit un frittage et une diminution de la surface apparente d'environ un ordre de grandeur ; en prBsence de gaz ad- sorb&, une grande quantitg de gaz est emprisonnge dans le solide.
L'influence des micropores sur la vitesse de recombinaison de Hz sur la glace amorphe dans les poussieres interstellaires et gur l'adsorption des gaz volatils dans les comgtes est brigvement discut6e.
Abstract.- Vapour deposited amorphous ice, investigated by N -adsorption at 77 K, was found to be a mfcroporous solid.
~ ? c r o ~ o r e volumes between 0.21 and 0.12 cm /g were determined by comparison plots and Dubinin-Radushkevich plots. Warming of the adsorbent to 113 K caused sintering and reduction of apparent surface area by about an order of magnitude; in the presence of adsorbed gas, large amounts of gas were enclosed in the solid. The infuence of micropores on the H2 recombination rate on amorphous ice in interstellar dust and on adsorption of volatile gases in comets is discussed briefly.
Introduction.
Vapour deposited amorphous ice, H20(as), is being discussed as a major component of comets (1-61, of satellites of the outer planets (7,8) and of interstellar dust (8-10). Some of the physical properties of H O(as) important for these discussions are contra- dictory: N -ads8rption2isotherms at 77 K evaluated by BET gave surfacq argas of 241 m /g according to Ghormley ( 1 1.12) and of 4 12 m /g according to -Others (1 3,141
.
We have reinvestigated adsorption of N2 on H20(as) and found, first, that it is amicroporous solld if prepared and investigated carefully at 77 K, and second, that reduction of surface area occurs at higher temperatures. W believe that the latter process can be described as sintering, following thereby a recent definition (16), and we will use this term henceforth. For N -adsorption at 77 K compar- able E T surface areas are reported 8y Schmitt et al. (17) in this volume. Independent evidence for small voids ) 17 A diameter in H20 (as) comes from positronium trapping (1 8)
.
Micropores are defined as pores of
<
20 A (1 9) or<
18 A (20)diameter. Because of that small width, there are essential differ-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987179
C1-582 JOURNAL DE PHYSIQUE
ences between adsorption in micropores and on large-porous or nonporous solids (20). In micropores the influence of opposite pore walls overlaps. This leads to an enhancement of the energy of interaction of the solid with a gas molecule and thus to an enhanced adsorption energy (21). The micropores are therefore filled in general at much lower relative pressures than large- porous or nonporous solids. According to Dubinin (21), the basic process is volume filling of the micropores rather than layer-by- layer adsorption on the pore walls. The evidence for large numbers of micropores in H20(as) should be of general importance for physical adsorption on H O(as) in space, on the premises that it really exists, because b8th the equilibrium and the kinetics of adsorption are influenced strongly by the presence of micropores.
In particular, as discussed recently (15), it should be of influence on the H recombination rate on H O(as) in interstellar dust and on adsor6tion of volatile gases i4 cometary nuclei.
Experimental.
The experimental details were described before (15) and are repeated briefly. H O(as) was prepared in a sort of sublimation apparatus made of gzass, H20 vapour entering through a fine metering valve and nozzle with 4 mm diameter at the bottom of the apparatus. For H20(as) formation only the cool finger was cooleg with liquid N2,,the flat bottom part of the finger with 10 cm area acting as cryoplate. For isotherm measurements, the whole apparatus was immersed in liquid N and adsorption and desorption measured in situ. It is imporgant to add gaseous N
(99.9995 %) very slowly at each adsorption step to avoid locaz heating and annealing of the sample. Equilibration for H20(as) samples deposited at 77 K needed 5
-
10 min, for samples sintered at 1133
was nearly instantaneous. A large volume of the apparatus(457 cm ) was necessary for a good vacuum and unobstructed gas flow during H 0 (as) formation, allowing only surface areas
>
10 mto be measure3. Apparatus and measurement techniques were tested with a sample of active carbon of known surface area.
Details of H20 (as) pg2paration : for isotherm A, figure 1 : p (H20) before nozzle = 9.10 mbar, 0.133 g sample deposited in 137 h.
Approximate deposition rate 3 A/s.
For isotherm D: similar to A.
For isotherm B: 113 k cryoplate temperature during deposition, p(H20) = 0.04 mbar, 1.70 g H O(as) deposited in 6 h. X-ray diffrac- tion showed that deposition at 113 K gave largely H20(as), with a small amount of ice I only.
Evidence for micropores.
To imitate conditions in space more closely we chose the slowest H20 deposition rate for preparation of H O(as) which was experi- mentally feasible. This gave after a wee2 of continuous deposition a layer about 0.1 to 0.2 mm thick, corresponding to an approximate rate of 3 A/sec. Figure 1, curves A and D, shows N2 adsorption isotherms at 77 K on two deposits prepared this way.
-
A curve like B with a BET surface area of about 40 mL/g was obtained if the adsor- bent was either prepared by deposition at 113 K, or warmed shortly to 113 K. That means there is a drastic decrease of surface area by about an order of magnitude. Warming of H O(as) to 143 K for 5 min caused even further reduction of surfage area such that it could not be measured any more at 77 K with our experimental setup.The isotherms A and D are composite isotherms of type 11, con- taining contributions from micropores and external surface. Composite
isotherms generally are separated into the microporous and the non- porous component by comparison with a nonporous reference isotherm.
There is some debate what is the best reference isotherm for that procedure. IUPAC recommends to choose as a reference an isotherm obtained on a chemically similar solid (22). For that reason we chose B, the isotherm on sintered H20(as), as reference.
Figure 1: N -adsorption iso- therms (stazic volumetric at 77 K) on (A) and (D)
,
H20(as)prepared by vapour deposition at 77 K, and at 113 K (B).
Open symbols, adsorption; solid symbols, desorption.
(from ref. 15).
The presence of micropores in the composite isotherms A and D becomes more apparent by subtracting the referenee isotherm: in curve C, B is subtracted from A, and a type I isotherm is obtained which is characteristic for adsorption in micropores.
Figure 2 shows the evaluation of micropore volumes by comparison plots. These are simple plots of uptake per unit mass of the sample
Kigure 2: Evaluation of micro- pore volumes in H20 (as) used for curves A and D in Fig. 1 by
comparison plqts: for A 0.20 cm /g, for D 0.17 cm /g.
(from ref. 15 with changes)
.
material, in mmol/g, against that of the reference material at the same relative pressures. The micropore volumes are determined from the extrapolated ordinate intercepts. From the slopes external surface areas can be calculated. Micropore volumes can in addition be evaluated by an independent method, by Dubinin-Radushkevich (DR) plots (23). The data for isotherms A and D are summarized in Table I.
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Table I.
Evaluation of N2-adsorption isotherms on H30(as) Sample A
" ~ ~ ~ a r e n t ! BET surface area (m /g) 2
I
421Micropore volume (cm /g) 3 a) by comparison plot
b) by Dubinin-Radushkevich plot
'External" surface area (m /g) 2
1
4 3Sample. D
We have in addition investigated N2-adsorption on seven H O(as) deposits prepared in the same apparatus with deposition rgtes more than two orders of magnitude higher, varying various parameters such as nozzle to cryoplate distance and water vapour pressure. The isotherms were either composite isotherms of type I1 as in figure 1, or pure type I isotherms. The "apparent" BET surface areas varied between 220 and 350 m /g, and the micrq- pore volumes from comparison plots between 0.11 and 0.20 cm /g, In all the adsorption isotherms on high-surface-area H O(as)
we have investigated so far the plateau region was app$oached only gradually. This and small values of the BET constant (c between 17 and 109) are an indication of many larger micropores (7 to 20 A width) which are filled by a mainly cooperative process (20).
With high deposition rates between 1 and 2 g of H O(as) were prepared within 6 h. Measurements on these samples werz of higher accuracy than those shown in Fig. 1 because of the larger samples.
Despite this improved accuracy we have found no evidence for hysteresis loops during desorption. This indicates that in these samples the fraction of mesopores defined as being between 20 and 500 A width (20, 22) must have been only small.
Astrophysical implications.
The evidence for micropores in H O(as) and the basically different adsorption behaviour in mic$oporous solids is the essential point for a further discussion of astrophysical impli- cations. The effect of larger pores on adsorption of atoms and molecules on H20(as) and their mobility has been treated already
(8-10)
.
WS will discuss two areas where consideration of micropores might have astrophysical implications. The first is the rate of
formation of Hz molecules from H atoms which are adsorbed on interstellar ice grains. This recombination is a very efficient process on single crystal ice according to an analysis by Hollen- bach and Salpeter (24, 25). It might be orders of magnitude slower on ~ ~ O ( a s ) according to Smoluchowski (8-10). And we ~elieve it could again be a very efficient process on microporous H O(as). Adsorption in micropores generally occurs at much lower relatzve pressures than on large-porous or on non-porous solids, the smallest micro- pores being filled first (20, 21). One might expect therefore that in the micropores there is an increased probability for an encounter between two H atoms and the formation of H
,
whether the adsorbed species are immobilized in the pores or arg still able to migrate.Experimental data on the recombination of atomic hydrogen on icy
surfaces have been reported (26). Because these surfaces were not well defined, an investigation of the recombination on microporous H20(as) prepared as described here would be of interest.
Second, H20(as) has been discussed as a major component of cometary nuclel. The additional presence of micropores might well be of importance, and the combination of its adsorption and sintering properties could possibly help explaining some of the surprising properties of comets. According to the determined micropore volumes, large amounts of gas of different volatilities can be stored in these pores. We have shown that during warming of H20(as) to 113 K the surface area is reduced by about an order of magnitude
(figure 1 , curves A and D versus B). If a sample is warmed with filled pores to 113 K, large amounts of adsorbed gas are enclosed in
the solid during sinteringwhich cannot be pumped off any more at this temperature. It is only given off gradually during warming up to the melting point of ice. For 300 to 400 mbar pressure of N and 02, the H20/enclosed gas ratios were 90 and 20. The amount
03
enclosed gas seems to be determined by two competing processes, desorption of gas during warm-up and reduction of surface area by pore closure and sintering in the same temperature regime.This would explain why the amount of enclosed, O2 as the less vola- tile gas is higher than that of N2, the H20/0 ratio already approaching the ratio in type I1 clathrate hyarates.
The advantage of this model in comparison to the crystalline clathrate hydrate model (27, 28) is that filling of micropores and gas enclosure by sintering can occur already at very low temper- atures, possibly far below 77 K, our lowest experimental temper- ature. With increasing temperatures the enclosed gas will either vaporize directly, or devitrification to cubic ice and/or formation of crystalline clathrate hydrates will occur. These astrophysical aspects are only speculations at the moment, but they can be tested experimentally.
Financial support by the "Forschungsforderungsfonds" of Austria is gratefully acknowledged.
References.
..
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Klinger, J., Science
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(1980) 271Smoluchowski, R., Astrophys. J. Lett. 2 4 4 (1981) L31 Klinger, J., Icarus
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87 (1983) 4214 Smoluchowski, R., Science 2 2 (1-8) 809Smoluchowski, R., Astrophys. Space Sci. 65 ( 1979) 29 Smoluchowski, R., Astrophys. Space Sci. 7 5 (1981) 353 Smoluchowski, R., J. Phys. Chem. 87 ( 1 9 8 n 4229 Ghormley, 3. A., J. Chem. Phys. 46(1967) 1321 Ghormley, J. A., 3. Chem. Phys.
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(1968) 503 Adamson, A. W., Dormant, L. M., and Oram, M. J., J. Colloid Interface Sci. 25 (1967) 206(1 4) Ocampo, J., and Klinger, J, 3. Colloid Interface Sci.
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(15) ~ a y e r , E., and Pletzer, R., Nature 319 (1986) 298
(16) Since the reviewer raised the pointwhether sintering is an appropriate description, we include a recent definition of this somewhat ambiguous term (H. H. Hawsner in: Sintering- new developments. ed. by M. M. Ristic; Elsevier scientific Publ. 1979, p. 6): "Sintering refers to a process of reducing
JOURNAL DE PHYSIQUE
the interior and exterior surface of a body or of bodies of particles in contact by reinforcement of contact bridges and the reduction of the void volume". Hausner in addition stresses the great role of porosity and pore characteristics.
Following this definition we feel that the processes occurring during warming of H O(as) while still in the amorphous statie can be descri6ed by "sintering".
Schmitt, B., Ocampo, J., and Klinger, J,, J. Physique, this volume Eldrup, M., Vehanen, A,, Schultz, P. J., and Lynn, K. G.,
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J.P. DEVLIN
Given the highly porous structure that you describe, you may lose thermal contact with the deposit surface. Then your temperature (effective) may be much higher than measured. Do you agree ?
Answer :
.Yes, we only can measure the temperature of the cryoplate.
