J. Phys. Chem. 1987, 91, 4298-4301
4298
Accommodation Coefficients of Helium, Neon, Argon, Hydrogen, and Deuterium on Graphitlred Carbon Isao Yasumoto Department of Chemistry, Yonago Technical College, Yonago, 683, Japan (Received: July 15, 1986)
The thermal accommodation coefficients of helium, neon, argon, hydrogen, and deuterium have been measured in the temperature range of 77-340 K on the surface of a graphitized carbon filament. The dependence of the accommodation coefficient upon the molecular weight as well as the temperature of measurement, and the conversion from orthohydrogen to parahydrogen, have been discussed.
Introduction
The thermal accommodation coefficient CY represents the average efficiency of the energy exchange between a gas and a solid surface in contact with each other. It is definedl by a = (T3 - T1)/(T2 - TI)
where TI, T,, and T3 are the temperatures of the gas molecules before they strike the surface, of the surface, and of the molecules which leave the surface, respectively. A number of measurements2-13 of the thermal accommodation coefficient have been made on various gas-solid systems by a variety of experimental methodsl2.I4to investigate the energy exchange between a gas and a solid. However, the measurements are almost all made on gas-metal systems. In the present investigation, thermal accommodation coefficients of He, Ne, Ar, H2, and D, on the surface of carbon have been measured by the method using a Knudsen cell in the temperature range of 77-340 K. The sample, a graphitized carbon filament, is well-known to be nonmetallic, electrically conductive, and one of the light elements. The experimental results obtained have been discussed in connection with the molecular weight of gases used as well as with the temperature of measurement. Experimental Section
Materials. The graphitized carbon fiber used was a high modulus type fiber HM-40, which was made from PAN (poly(acrylonitrile)) at Toho Beslon Co. The cross section of the fiber is approximately circular in shape. According to S h i n d ~ , ’the ~ , basic ~ ~ structural unit in the fiber resembles a continuous ribbon or sheet of graphite. X-ray and electron diffraction photographs obtained by ShindoIs show that the c axis of graphite crystallite in the fiber is orientated in the direction perpendicular to the fiber axis. The electron diffraction photographs also show that the graphite basal planes in the outer part of the fiber are arranged nearly parallel with the fiber surface ( 1 ) Knudsen, M. Ann. Phys. 1911, 34, 593. (2) Roberts, J. K. Proc. R. Soc. London, A 1930, A129, 146. (3) Sasaki, N.; Taku, K.; Mitani, K. Mem. Coll. Sci., Unit;. Kyoto, Ser. A 1949, A25, 75. (4) Eggleton, A. E. J.; Tompkins, F. C. Trans. Faraday SOC.1952,48, 738. (5) Thomas, L. B.: Golike, R. C. J . Chem. Phys. 1954, 22, 300. (6) Thomas, L. B.: Schofield, E. B. J . Chem. Phys. 1955, 23, 861. (7) Goodman, F. 0.;Wachman, H. Y . J . Chem. Phys. 1967, 46, 2376. (8) Trilling, L. Surf. Sci. 1970, 21, 337. (9) Roach, D. V.; Thomas, L. B. J . Chem. Phys. 1973, 59, 3395. (IO) Shields, F. D. J . Chem. Phys. 1975, 62, 1248. ( I I ) Lemons, R. S.; Rosenblatt, G. M. Surf. Sci. 1975, 48, 432. (12) Goodman, F. 0. J. Phys. Chem. 1980, 84, 1431. (13) Asscher, M.; Guthrie, W. L.; Lin, T. H.; Somorjai, G. A. J . Chem. Phys. 1983, 78, 6992. (14) Goodman, F. 0. Prog. Surf. Sci. 1974, 5, 261. (15) Shindo, A. Rep. Gou. Ind. Res. Inst., Osaka 1961. 317, 28. (16) Diefendorf, R. J.: Tokarsky, E. Po1ym. Eng. Sci. 1975. 15. 150.
0022-3654/87/2091-4298$01.50/0
and the surface is basically onion skin. The fiber was baked out at 920 K for an hour under a pressure N/m2, and then AES analysis of the surface was below 7 X carried out by using JAMP-10. The result is presented in Figure 1. As seen, the Auger signal of carbon is distinct, whereas that of oxygen (around 507 eV) is too small to distinguish from noises. The bulk density of the fiber was measured by Shindo to be 1.81 g/cm3 by the liquid density gradient tube method. The fiber diameter was measured by Shindo by the laser diffraction method to be 20.8 X m2/m. A yarn which consisted of 6000 fibers was weighed with a balance by Shindo, and the weight of a fiber was found to be 0.0625 X g/m. The adsorption of krypton on the fiber was measured at 77 K by a conventional adsorption apparatus,I7and the specific surface area was found to be 0.496 m2/g by the BET method, by assuming that the cross-sectional area of a krypton m o l e c ~ l e is ~ ~0.194 ~’~ nm2. Then the surface roughness factor of the fiber can be calculated to be 1.49, being the ratio of the area measured by low-temperature krypton adsorption to that measured by the laser method. The fiber surface consists of two types of surfaces, Le., a basal plane and an edge plane. On the latter surface, oxygen is considered to be chemisorbed in the form of various functional groups. 19-22 In order to know the proportion of the edge surface to the total surface, the following experiment was made by Shindo: the functional groups were removed by evacuation in vacuo at 1230 K and the sample was then exposed to oxygen at 590 K, which permitted the chemisorption of the gas. The specific surface area for the oxygen chemisorption was calculated to be 0.01 3 m2/g, assuming that the oxygen forms the carbonyl groups on the surface. The proportion of the oxygen area thus obtained to the total area, 0.496 m2/g, is found to be 0.026. The gases used were supplied by the Takachiho Shoji Co., and they are sealed in respective glass vessels. The nominal purities of gases in mole percent are 99.999 for helium, 99.99 for neon, 99.999 for argon, 99.999 for hydrogen, and 99.5 for deuterium. Measurements of Thermal Accommodation Coefficient. The experimental arrangements used are illustrated in Figure 2. The details of preparing the cell F are as follows. Both ends of a copper wire 1 mm in diameter and about 58 mm in length were fixed on the respective gold foils with electroconductive resin (silver paste). Each foil is spot-welded on a nickel sleeve, the latter being also spot-welded on a tungsten lead wire 1.5 mm in diameter. The electric resistance between the terminals XI’ and Y”, Le., X”W-Ni-Au-resin-Cu-resin-Au-Ni-W-Y”,was measured and was found to be less than 0.5 ohm. The copper wire was then (17) Rosenberg, A. J . J . A m . Chem. Soc. 1956, 78, 2929. (18) Beebe, R. A.; Beckwith, J. B.; Honig,J. M. J . A m . Chem. Soc. 1945, 67, 1554. (19) Coltharp, M. T.; Hackerman, N. J . Phys. Chem. 1968, 72, 1171. (20) Bansal, R. C.; Vastola, F. J.; Walker, Jr. P. L. Carbon 1970, 8, 443. (21) Tremblay, G.;Vastola, F. J.; Walker, Jr. P. L. Carbon 1978, 16, 35. (22) Morimoto, T.; Miura, K. Langmuir 1986, 2. 43.
0 1987 American Chemical Society
Accommodation Coefficients of He, Ne, Ar, HZ, and D2
The Journal of Physical Chemistry, VOI.91, NO. 16, 1987 4299 KL2 I
12'
IO0
200
K
300
Temperature of filament
Figure 3. Plot of the resistance vs. the temperature of a filament 56.46 mm in length.
Figure 4. Schematic cross section of the cell F. a is a circle drawn at a distance equal to the mean free path of the gas molecules from the filament surface.
0
,
I
200
,
I
400
,
I
600
I
800
11
Electron energy, E (eV)
Figure 1. AES spectrum of the fiber surface.
a dry ice-acetone mixture or liquid nitrogen. The graphitized carbon filament was flashed at white heat to clean the surface by supplying a current of 5.9 mA dc and then allowed to cool. Then an appropriate amount of a gas was admitted into the apparatus till the pressure attained was approximately 20 N/m2, which is sufficiently low so that the mean free path of the gas molecules is about 180 times as large as thL filament diameter. The vertical tube H 1.2 m in length was heated at about 450 K. The admitted gas was then continuously circulated by convection through the traps and the cell, UI, Uz, F, U3, and U4, to remove adsorbable impurities in the gas. The cell F was immersed in a liquid bath to maintain a constant temperature,
T1.
Figure 2. Schematic diagram of the apparatus used for thermal accommodation coefficient measurement: (F) cell 650 mm in length and 25 mm in diameter; (U) trap; (C) tap; (X and Y ) terminals; (P) Pyrex tube 8.8 mm in inside diameter; (A) ammeter; (V) voltmeter. A digital multimeter, SC-7404 made by Iwatsu Co., was used. removed. Both ends of a graphitized carbon filament were fixed on the respective gold foils with the same kind of resin. The filament length was measured by a cathetometer. The apparatus was then equipped with the cell. The whole apparatus was evacuated below 1.3 X lou3N/m2, while the traps UI and U4 which contain molecular sieve 13X were outgassed at 630 K. After outgassing, all traps were cooled in
An appropriate dc voltage, corresponding to 3-50 FA, was supplied to the filament, and the electrical resistance of the filament was calculated. By plotting the resistance against the current, and by extrapolating the resistance to zero current, the estimated limiting value of the resistance was obtained at each constant temperature T I . The resistance plotted against the filament temperature thus obtained is shown in Figure 3. A dc voltage, which corresponds to approximately 150 FA, was supplied to the filament to raise the temperature of the filament (T2)some 10 K above that of the bath (Tl). T2was obtained from the resistance of filament. According to Hayward and T r a ~ n e l lwhen , ~ ~ the temperature difference T2 - T1 is small, the heat loss is ascribed to the conduction to the gas, radiation and end losses being negligible. Knudsen' stated also that the simplest and most direct way to obtain a is to work at low pressures so that the mean free path of the gas molecules is large compared with the relevant dimensions of the apparatus. Such a low-pressure condition was found, however, to lead to the low precision encountered in measuring small heat losses from the filament, so that Knudsen's method has limited application in the present measurement. Let us consider schematically the heat loss from the electrically heated filament at the temperature T2stretched down in the center of the cell F at the temperature TI,as shown in Figure 4. When the diameter of the filament is very small compared with the mean free path of gas molecules, the temperature drop between F and a is supposed to be small compared with that between a and the filament surface, because of the so-called temperature jump24at the surface. As a result, the temperature of molecules striking the surface can be assumed practically to be T , of the cell F. (23) Hayward, D.0.;Trapnell, B. M . W. Chemisorption, 2nd ed.; Butterworths: London, 1964; p 50. (24) Kennard, E. H. Kinetic Theory of Gases; McGraw-Hill: New York, 1938; p 311.
4300 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
Yasumoto
TABLE I: Experimental Results for Helium on a Filament 56.46 mm in Length Pmsasd," Torr 181 x 10-3 180 x 10-3 17.2 x 10-3 109 X IO-)
P,bTorr 0.18268
0.180 0.16829 0.10169
T,, K
voltage, V
current, p A
A(kR)c
333.2 283.1 194.6 90.1
2.2419 2.2495 2.2922 1.6979
165.950 160.021 152.803 106.941
0.137 138 0.131 97 0.107933 0.054 675
T2 - Tle 12.697 12.219 10.902 8.5563
d(kR)/dTd 0.0108 0.0108 0.0099 0.00639
asif
0.51 0.48 0.46 0.34
Pressure measured with McLeod gauge. Pressure corrected against the thermal transpiration effect. CDecrementof the resistance caused by the elevation from T , to T2. dMean decrement of the resistance per unit temperature at respective temperatures. 'Calculated with the values in the columns 6 and 7. TABLE I 1 Values of Accommodation Coefficients Obtained at Room Temperature with Various Systems
carbon Pt
W
W
Ni
cu
0.176"
0.0167b (f0.0015)
0.06c f 0.06 0.12' f 0.06 0.42< f 0.08
0.69d
0.5gd
(a)
ffeff _..
He Ne Ar H2 D2
0.48 (0.36) 0.79 (0.65) 0.90 (0.78) 0.36 (0.26) 0.52 (0.39)
0.71d
"Reference 5; measured by a Knudsen cell. bReference 6; measured by a Knudsen cell. CReference10; measured by an acoustical method. dReference 1 1 ; measured by vibrating-surface method. The effective accommodation coefficient cyefl can be calculated for a monatomic molecule from the formula derived by Knudsen'
0 He
A Ne
a e f f= Q(MT1)'/2/(1.74 X 10-4P(T2 - T I ) )
X Ar 0 Hz 0 D2
where M is the molecular weight of the gas, P the pressure of the gas in dyn/cm2, and Q the heat loss in (cal/cm2)/s. For a diatomic molecule, cyeff can be calculated from the modified formula4 cyeff
cy
= ~ ( ~ ~ , ) 1 / 2 / { 1x. 7140 - 4 ~ -( ~ T ,~) ( c , / ~ R + y4)1
a
I#'
_____----
_ - _ _He -
- ..- ..- Ne
-...-... _ _ -*-.-
Ar -H2
D2 where C, is the specific heat of the gas a t constant ~ o l u m e . ~ ~ ~ * ~ O 100 200 K 300 was corrected for the pressure The measured pressure, Pmeasd, Temperature of filament to be measured against the pressure difference developed between A and B of the connecting tube, P, shown in Figure 2, which is Figure 5. Dependence of the thermal accommodation coefficients of caused by the thermal transpiration2' of the used gas. helium, neon, argon, hydrogen, and deuterium on TZ.The broken lines represent the respective curves of cy. After dipping the cell F in the liquid bath, the value of cyeff was measured with time. A steady state of cyeff was attained within trend.',* The reason why the different trends appear is not as yet about 20 min. It was maintained for about 30 min. Then the understood. And the heavier the molecular weight of the gas is, value of cyeff became gradually large, probably because of the the larger the obtained accommodation coefficient becomes. These contamination of the surface due to the adsorbable impurities in facts seem to be due to the long duration of the effective contact the gas. between the molecule of a gas and the relevant carbon surface. The thermally vibrating net plane of the graphite surface is Results and Discussion somewhat pushed by the hitting molecule, and the duration seems For the discussion of the experimental results obtained, it is to become longer with increasing molecular weight, though the necessary to consider the effect of the surface roughness on the exact analysis of the interaction between the hitting molecule and measured accommodation coefficient. When a colliding molecule the vibrating surface is difficult. makes on the average n consecutive collisions with the surface In a series of monatomic and diatomic molecules, it seems of before it returns to the body of the gas, the effective accommointerest to consider the effect of molecular weight on CY measured dation coefficient cyeff can be expressed by the relation2 cyeff = 1 on the same carbon surface. - (1 - CYy. Roughly approximate relationships can be considered to hold, The value of n is, however, difficult to determine in the present under room temperature conditions, between the molecular weight case. As an approximation, the surface roughness factor which and CY as follows was measured to be 1.49 may be assumed to be equal to the value of n in the succeeding calculation. (MA/MB)l13 N a A / a B for monatomic molecules All the experimental results performed for helium are tabulated for diatomic molecules ( M A / M B ) ' / 'N aA/cyB in Table I. The cyeff values observed for helium, together with those observed for neon, argon, hydrogen, and deuterium, are where subscripts A and B represent different molecular species. plotted against T2in Figure 5 . The various broken lines in Figure Though the molecular weights of helium and deuterium are 5 represent the curves of a for the respective gases. almost similar, it is seen that the value of cy obtained for deuterium As seen in Figure 5 , the a value becomes large with increasing is larger than that obtained for helium at every temperature. This T2 for all kinds of gases used, as in the cases of He and Ne on means that the energy exchange occurs appreciably for the rotungsten at the temperature range over 150 K, whereas the extational motion of the molecule as well as for the translational periments on Ar, Kr, and Xe on tungsten indicate the opposite one. It is seen in the low-temperature region that the value of a for hydrogen decreases rapidly with decreasing T2. The rapid decrease ( 2 5 ) Hilsenrath, J., et al. Tables of Thermal Properties of Gases: 1955, NBS Circ. 564. seems to be due to the c o n ~ e r s i o n ~from * ~ ~orthohydrogen ~ to
(26) Sheki. S., et al. Kagaku Binran 11; Maruzen: Tokyo, 1966; Chapter
7.
(27) Yasumoto, 1. J . Phys. Chem. 1980, 84. 589
(28) Bonhoeffer, K. F.; Harteck, P. Natunvissenschaften 1929, 17, 182.
J. Phys. Chem. 1987, 91, 4301-4305 parahydrogen on the carbon surface. Since parahydrogen possesses a lower internal energy than the ortho form, the conversion is exothermic. The present author supposes that the conversion would be caused by a longer duration of the effective contact between the molecule and the net plane of graphite surface and by a catalysis (29) Bonhoeffer, K. F.; Harteck, P. Z . Phys. Chem. 1929, 8 4 , 113.
Adsorption of NO on Pt/SiO,:
4301
of the field produced by ?r electrons in the net plane. The accommodation coefficients obtained at room temperature with the gas-carbon system are presented in Table 11, together with those obtained with some other systems.
Acknowledgment. I thank Dr. A. Shindo, Governmental Industrial Research Institute, Osaka, for his interest and encouragement in this work, and Dr. S. Nishigaki, Toyohashi University o? Technology, for his cooperation in the AES measurement.
An Infrared Study
Janos Sarkany, Mihaly Bartok, Department of Organic Chemistry, Jozsef Attila University, Szeged, H-6720, Hungary
and Richard D. Gonzalez* Department of Chemical Engineering, The University of Illinois at Chicago, Chicago, Illinois 60680 (Received: August 22, 1986)
The adsorption of NO on Pt/SiO, has been studied by infrared spectroscopy. Three distinct surface species are proposed. A high-frequency band centered between 1770 and 1800 cm-I is assigned to an NO species linearly adsorbed on the rough open crystal planes of Pt. The absorption frequency of this band was observed to decrease with increasing surface coverage. A low-frequency band centered between 1570 and 1600 cm-' is assigned to bridge-bonded NO. The absorption frequency of this band was observed to increase with increasing NO surface coverage. Prolonged pretreatment in H2 at 673 K leads to the collapse of the infrared band assigned to linearly adsorbed NO to give a broad band centered at 1772 cm-I. This band is assigned to strong vibrationally coupled NO adsorbed on the smooth, more densely packed crystal planes of Pt. Pretreatment in 0, at 673 K leads to the enhancement of the linearly adsorbed NO species, while pretreatment in Hz promotes vibrationally coupled NO. These results are explained by considering surface reconstruction rather than by changes in Pt dispersion.
Introduction
though NO is a relatively simple molecule with single unpaired electron in a 2a antibonding molecular orbital, its mode of adsorption on metals has been the subject of many interpretations. A large number of workers have studied the adsorption of NO on metals using modern spectroscop~ctechniques,i-~s H ~ the interpretations of the vibrational models are quite contradictory. ?he high-frequency band centered in the 17OG1820-cm-' spectral region predominates at high surface coverages of N O and has been assigned to a linear NO specie^.'-'^^^^-^^ The structure of the N O species giving rise to the low-frequency band in the (1) Thomas, G. E.; Weinberg, W. H. Phys. Reu. Lert. 1978, 41, 1181. (2) Thiel, P. A.; Weinberg, W. H.; Yates, J. T. J . Chem. Phys. 1979, 71, 1643. (3) Umbach, E.; Kulkalni, S.; Feuler, P.; M e n d , D.Surf. Sei. 1979.88, 65. (4) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980, 94, 355. (5) Gorte, R. J.; Gland, J. L. Surf. Sei. 1981, 102, 348. (6) de Jong, K. P.;Meima, G. R.; Geus, J. W. Appl. Surf. Sci. 1982-83, 14, 73. (7) Hayden, B. E. Surf. Sei. 1983, 131, 419. (8) Pirug, G.; Bonzel, H. P.; Hopster, H.; Ibah, H. J . Chem. Phys. 1979, 71, 593. (9) Morrow, B. A,; McFarlane, R. A,; Moran, L. E. J . Phys. Chem. 1985, 89, 17. (10) Morrow, B. A,; Chewier, J. P.; Moran, L. E. J . Caral. 1985, 91, 208. (11) Ibach, H.; Lehwald, S. Surf. Sci. 1978, 76, 1. (1 2) Dum, D.S.; Severson, M. W.; Golden, W. G.; Overend, J. J . Cafal. 1980, 65, 271. (13) Golden, W. G.; Dunn, D.S.; Overend, J. J . C a r d 1981, 71, 395. (14) Dunn, D.S.: Severson, M. W.; Hvlden, J. L.; Overend, J. J . Caral. 1982, 78, 225. (15) Severson, M. W.; Overend, J . J . Chem. Phys 1982, 76, 1584
0022-3654/87/209 1-430 1$01.50/0
1400-1600-~m-~spectral region is uncertain. It has been assigned to N O adsorbed in a bridging c~nfiguration,'-~ to bent PtNO,s-lo or to vibrationally coupled NO chain^.'^-^^ In an attempt to sort out these inconsistencies, we have performed an infrared study on the adsorption of N O on Pt/SiO,. Our findings suggest that the adsorption of NO is highly sensitive to surface structure. The surface structure appears to~ be considerably ~ ~ ~ ~ , more important than surface changes induced by changes in Pt dispersion. Experimental Section
Materials. The Pt/Si02 samples used in this study were prepared by the incipient wetness technique. A solution of H2PtC1,.6H,O (Reanal Co., Budapest) sufficient to prepare a Pt/SiO, catalyst having a metal loading of 5 wt % Pt was added to Cab-0-Si1 (grade M-5, BDH Chemicals, Ltd.). The resulting slurry was dried under vacuum and sieved to >225 mesh before use in the infrared cell. The catalyst preparation and the CO, H,, and 0, gas purification procedure have been previously described.16J7 The NO was of commercial purity (99%). It was further purified by the freeze-thaw technique. Apparatus and Procedure. The infrared spectra were recorded at room temperature with a double-beam "Specord 75 IR" infrared grating spectrophotometer (Zeiss, Jena). The scan rate was 4.54 cm-' s-l in the spectral region of interest. The infrared cell was connected to a conventional vacuum and gas handling system and was capable of an ultimate pressure of 1 X Torr ( 1 Torr = 133.3 Pa). The catalysts were pressed into self-supporting disks having an optical density of 10 mg cm-*. They were stored in air before use. (16) Bartok, M.; Sarkany, J.; Sitkei, A. J . Cafal. 1981, 72, 236. (17) Sarkany, J.; Bartok, M. J . Catal. 1985, 92, 388.
0 1987 American Chemical Society
