July, 1959
SURFACE PROPERTIES OF GERMANIUM
1095
The work was supported by a grant to Professor Health G-4912. The author wishes to express his J. W. Williams from the National Institutes of gratitude both to individual and to institution.
SURFACE PROPERTIES OF GERMANIUM BYY. L. SANDLER AND M. GAZITH Westinghouse Research Laboratories, Pittsburgh 35, Pa. Received November #O, 1968
Some physical and chemical properties of germanium films sputtered by a discharge in an inert atmosphere have been studied. A detailed investigation of the ortho- ara conversion of hydrogen catalyzed by the germanium between 77°K. and room temperature reveals a twofold origin orthe conversion, magnetic and chemical; the activity due to either mechanism is remarkably high. Both mechanisms are believed to be due to a high concentration of single or very weakly coupled electrons in the defect surface. The oxygen adsorption on the films a t 77°K. was studied by measuring adsorption isotherms and by investigating the influence of different amounts of oxygen and of absorbed light on the activity of the films. . A weakly adsorbed layer of oxygen was found at low relative pressures a t 77°K. Failure of this oxygen layer to induce II. magnetic conversion in hydrogen indicates that this reversible adsorption is a weak chemisorption. It is shown to be the precursor of a stronger chemisorption at room temperature. The reversible nature of this adsorption and its interaction with unsaturated surface electrons and with the space charge layer of the germanium is discussed.
I n this paper we report a n investigation of some magnetic, catalytic and electrical properties of germanium and of their changes by adsorption of oxygen. Germanium was chosen because it is available in a state of very high chemical purity and because its semi-conducting properties are relatively well understood. Adsorption properties of clean germanium surfaces have been investigated in recent years by various research groups,1-6 and some work on the catalytic properties of germanium a t elevated temperature also has been reported.'^^ The scarcity of work until recently on the catalytic properties may be due to the high sensitivity of germanium to oxygen poisoning and the resulting technical difficulties8 in studying its properties. I n the present work a new approach was chosen: the parahydrogen conversion a t low temperatures was used to probe the bare surface and the adsorbed oxygen layers. As is known, the parahydrogen conversion on surfaces can have two distinct mecha n i s m ~both , ~ of which were found to be operative in the present case. One of these mechanisms, mainly found a t low temperatures, is a purely physical process consisting of the inversionlo of the nuclear spins of a hydrogen molecule in a strong inhomogeneous magnetic field. This type of conversion requires 110 activation energy and is generally caused by the presence of unpaired electrons in the surface on which the hydrogen is adsorbed. As previously shown in the case of TiO2,” a study of this type of conversion reveals the existence of free
valences in the bare surface and also enables us to distinguish between paramagnetic and diamagnetic forms of adsorbed oxygen. The second mechanism in the parahydrogen conversion has a “chemical” origin; it involves the activation of the H-H bond and it is analogous to the hydrogen-deuterium exchange reaction which we also have studied. A thorough study of the kinetics of the ortho-para conversion was carried out in order to differentiate between the two mechanisms. The adsorption and desorption of oxygen a t low temperatures and its influence on the reactivity of the solid also was investigated. In this way different types of adsorption could be distinguished. Electrical conductivity changes and the influence of absorbed light were studied to elucidate the nature of the interaction between semiconductor and adsorbed oxygen. A convenient technique of sputtering a germanium film by an electric gas discharge was adopted in the present experiments. It gave films of relatively high surface area, and new films could be sputtered on top of old ones to give excellent reproducibility. Experimental
Vacuum System.-Some initial experiments were carried out with an ultra-high vacuum system.12 However, because of the catalytic activity of the metal valves, this system was abandoned in favor of a good conventional vacuum system. Mercury pumps and liquid nitrogen tra s were used giva vacuum ing (in the later experimeuts with “thick i!lms”) of mm. or better, as measured by an ion gauge at the (1) J. A. Dillon and H. E. Farnsworth, J . A p p l . Phys., 28, 174 entrance to the reaction vessel. For isolating the reaction (1957). vessel, a greased stopcock was used with liquid nitrogen (2) P. Handler in “Semiconductor Surface Physics,” University o f traps on either side of it; i t was well ground and polished and Pennsylvania Press, Philadelphia, Pa., 1950,p. 23. required only a very thin film of grease (Apiezon N ) for 13) M.Green, J. A. Kafalas and P. H. Robinson, ref. 2, p . 349. lubrication. After extended degassing, a vacuum of better (4) J. T. Law, THISJOVRNAL, 69,543 (1955). than 10-8 mm. could be maintained for many hours in the (5) R. M.Dell, ibid., 61, 1584 (1957). closed reaction vessel (without the presence of a gettering (6) S. P.Wolsky, J . A p p l . P h y s . , 29, 1132 (1988). film). (7) K. Tamaru and M. Boudart in “Advances in Catalysis,” Vol. Reaction Vessel.-The reaction vessel, made of Nonex IX, Academic Press, New York, N. Y., 1957,p. 099. glass, had a volume of about 90 cc. It contained two ( 8 ) S. Z. Roginskii and V. M. Frolov, Doklady A k a d . N a u k SSSR, pieces of intrinsic germanium having an impurity concen111, 623 (1956). tration of less than 1013/cc. The pieces were perforated and (9)A. Frtrkas, “Orthohydrogen, Parahydrogen and Heavy Hydro- slipped over tungsten leads. Before the leads were sealed gen,” Cambridge University Press, Cambridge, 1935,p . 89. in the tube, the germanium was etched in an H20rHF 110) E. Wigner, Z.physik. Chem., B23, 28 (1933). (11) Y. L. Bandler, THIEJOURNAL, 68, 54 (1954). (12) D. Alpert, J . A p p l . Phye., 2 4 , 860 (1052).
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Y. L. SANDLER AND M. GAZITH
solution and outgassed in vacuo in a uarta vessel, heated to 800” for 2 days. The tube containel 2 strips of evaporated gold serving as electrodes for conductivity measurements on part of the film. Tungsten leads and molybdenum springs were used for making electrical contact with the strips. The vessel was degassed for 3 days at 300’. It was similarly treated overnight each time before a fresh film was deposited over an old film. The molybdenum springs ceased to make contact after repeated heating and cooling of the vessel. Conductivity measurements thus could be made only in the initial experiments on thin films. Conductivity Measurements.-These were carried out in a screened Wheatstone bridge circuit with 1.5 volt d.c. The imbalance due to conductivity changes was measured with a Keithley VTVM, type 200B. Pressure Measurements.-The vacuum system contained what we call a “Toepler Gauge,” as previously described.11 This was used for calibrating volumes, mixing gases, and for calibrating the thermistors. One of the thermistor gages was connected directly to the reaction vessel and one to the gas inlet before the vessel. They were used for ad- and desorption measurements with oxygen, hydrogen and krypton. The electric setup was roughly the same as used by Rosenberg.13 The calibration curves (log pressure versus log voltage) for oxygen gave an almost straight 45 degree line between 3 X 10-4 mm. (1.5 mv.) and 3 X IO-’ mm. (1.5 v.). For calculating the pressure in the reaction vessel when below room temperature, the thermal transpiration corrections of Bennett and Tompkins14were used. Hydrogen Analyses.-Ortho-para analyses were carried out by the Farkas micro-method.16 In hydrogen-deuterium exchange experiments, samples were taken off the system and analyzed by mass spectrometer.16 Gases.-02, Kr (for surface area determinations) and He (for sputtering the film) were of spectroscopic purity. HZand Dn were purified by passage through hot palladium tubes. Parahydrogen and orthodeuterium were prepared by adsorption on charcoal at solid nitrogen and liquid hydrogen temperatures, respectively. Equilibrium hydrogen was obtained by keeping hydrogen in a vessel containing charcoal impregnated with a paramagnetic salt; it was kept in the same temperature bath as the reaction vessel. Because the films were found to be extremely sensitive to traces of oxygen, the gases in the initial experiments were passed over a freshly evaporated magnesium film betore entering the reaction vessel. No ortho-para conversion took place in parahydrogen during the short contact time. In later adsorption experiments the film was discarded. When using normal hydrogen directly from the palladium tube, it was found that no poisoning of the film occurred. The other gases used generally caused a certain loss in activity due to impurities. Therefore the change in activity was frequently checked by remeasuring the rate of conversion of normal hydrogen to equilibrium hydrogen under standard conditions, viz., at 77.2’K. and 4.5 mm. pressure. All curves given in the paper are corrected for changing activity. Sputtering of Films.-The transparent “thin films” used in the first experiments were obtained by a discharge in 0.3 mm. helium, applying 1600 volts, 2 ma. d.c. for 10 minutes with the reaction vessel in a li uid nitrogen bath. “Thick films” of about 1 p thickness qestiniated from interference colors) were produced in the following manner: A strong discharge was passed in krypton between the two germanium elect,rodes, the gas being replaced a number of times. The electrodes became red hot in this process and thus were further freed from active gases. After degassing the coating produced by this discharge, a t 300’ in vacuo,new germanium was sputtered under milder conditions onto this coating in 4 mm. helium (300 volts, 35 ma., 30 minutes, 77’K.). The films were of rather non-uniform thickness; they had a geometric surface area of about 20 (13) A. J. Rosenberg, J . Am. Chem. S o c . , 78, 2929 (1950). (14) M. J. Bennett and F. C. Tompkins, Trans. Faraday ~ o c . , 65, 185 (1957). (15) Ref. 9,p . 25. (16) The authors are indebted t o W. M. Hickam and his group for
carrying out the anslyaelr.
Vol. 63
Results The first films (“thin films;” see Experimental Section) were produced under poorer vacuum conditions than later films, and also before the additional cleaning of the germanium electrodes by a strong krypton discharge. They produced only a very slow conversion in hydrogen adsorbed a t 77°K. (in a static system). The first film gave a half-life T for the parahydrogen conversion of 25 hours. The activity of the films increased each time when a fresh film was sputtered on top of the old film. When the conversion was remeasured after certain time intervals, the rate was found to fall off gradually. This loss of activity was probably caused by adsorption of oxygen due to the residual oxygen pressure in the system. The hydrogen reaction was found to have the characteristics of a magnetic spin inversion. No chemical activity was found: after the exposure of a 20-hour film, (Le., a film giving a half-life with light hydrogen of 20 hours at 77°K. and 4.5 mm. pressure) to a hydrogen-deuterium mixture, no measurable amount of HD was found after 16 hours; the exchange reaction must have been a t least 100 times slower than the parahydrogen conversion. With light hydrogen, the conversion a t 893°K. proceeded 2.27 times more slowly than a t 77.1”K. The negative temperature coefficiknt of the reaction again shows that the mechanism cannot be a chemical one; moreover it shows that the magnetic reaction itself is the rate determining step (and not desorption of the hydrogen). The order of the reaction n k = 1 ( b log r / b log p ) was ~ found to be 0.8. We can evaluate the heat of adsorption Q of the hydrogen from the experimental activation energy E, assuming that for a magnetic conversion the “true activation energy” E, is negligibly small. From the relation1’ E = Em - (nk& - RT) (1) we find Q = 1400 cal./mole, a reasonable value for the molecular heat of adsorption of hydrogen. Later experiments made it clear that the surface of the thin films was not atomically clean. The results are characteristic of a surface containing a complete monolayer of strongly chemisorbed oxygen. Reaction Mechanism with “Thick Films.”-The following experiments were carried out with “thick films” (see Experimental Section). They were found to be much more active than the first thin films. Each fresh coating sputtered onto an old film gave a reproducible high activity. The parahydrogen conversion a t 77°K. and 4.5 mm. pressure (“standard conditions”) gave a half-life 7 = 13.5 f 0.5 minute. This value increased in some cases by up to 20% after the film was allowed to warm up to room temperature in vacuo for a short time to release the trapped helium. A measurable hydrogen-deuterium exchange was now found to take place a t 77°K.; it was 12 times slower than the parahydrogen conversion. This shows that also the conversion, at least in part, must involve a chemical exchange reaction of the
-
(17) Y. L. Sandler, J . Cham. Phys., 41, 2243 (1953).
July, 1959
SURFACE PROPERTIES OF GERMANIUM
same type. The H2-Dz exchange generally proceeds at a lower rate because of the smaller zero-point energy of adsorbed deuterium. However, the following investigation showed that in the hydrogen conversion an additional mechanism-the magnetic type of conversion-is operative which predominates a t 77°K. The ortho-para conversion in deuterium was measured under the same conditions; it proceeded 4 times more rapidly than the H2-D2 exchange. This shows that zero-point energy differences alone cannot explain the discrepancy between the rates of the hydrogen conversion and the H2-D2 exchange.. The relative ratesls for light and heavy hydrogen for a magnetic surface conversion cannot be predicted with certainty; their ratio usually lies between 1 and 4 at liquid air temperature. The ratio 3 found in the present case thus is within the expected range for the almost
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100
ac*
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-
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150 125 I
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100 I
750 I
TOK.
1098
Y. L. SANDLER AND M. GAZITH
Vol. 63
amount of oxygen adsorbed and the active surface area for the parahydrogen conversion (see Discussioii) . The experiments described from here on were all carried out at higher oxygen pressures or with films already containing a monolayer of strongly adsorbed oxygen. Adsorption 1sotherms.-The adsorbing surface area of the films (“thick films”) was determined by the BET method, using krypton a t 77°K. The film surface area was found to be 400 cm.2, after allowance had been made for adsorption on the bare glass surface (-150 The oxygen adsorption isotherms for the same films a t 77°K. were determined at pressures beand 5 X 10-2 mm. This was done tween 3 X by two different methods giving essentially the same results, except a t the lowest pressures. In one method oxygen was adsorbed on the film in increasing amounts a t constant temperature. In 4 6 810 20, 40 the other method a measured amount of oxygen Pressure, mm. Fig. 2.-Pressure dependence of half-life for two films of was let into the reaction vessel a t room temperature. The vessel then was closed and cooled to different activity. 77°K. After 20 minutes a pressure reading was taken again. From the pressure drop the amdunt of adsorbed gas was calculated (taking into account the temperature change and the thermal transpiration effect). The vessel then was pumped a t 77°K. and slowly warmed up again, before a fresh amount X of gas was let in. These curves were taken with higher pressures first, going to lower pressures. Both methods gave practically identical results for films which had been in contact with oxygen. In Fig. 4 two such curves are shown plotted on a logarithmic scale. The upper curve was obtained b I I I with a fresh film by the first method; the lower curve was obtained by the second method, starting with higher pressures first. The adsorption is seen to be reversible to a large extent. Adsorption was practically instantaneous at the higher pressures. Below mm. a slow adsorption was observable which was completed after about 10 minutes. The highest point taken (5 X mm.) correOO 20 40 60 80 100 sponds to a coverage of 300 Le., 75% of the Percent Activity Loss. BET area, provided that one oxygen molecuie is Fig. 3.-Number of adsorbed oxygen molecules versus activ- adsorbed per two germanium sites (assuming 8 X ity loss. l O I 4 sites/cm.2). Unfortunately no higher points atomically clean parts of the surface. The larger could be measured to indicate whether the curve part of the activity for the parahydrogen conversion levels off, because the thermistors became insensiis due to this bare part of the surface. The H2-D2 tive a t higher pressures. At any rate, the reversiexchange reaction apparently can occur only on ble adsorption extends to a t least a large fraction of the clean parts of the surface. We have previouely a monolayer. From the upper curve and from a few points seen that it is almost completely quenched on more strongly poisoned films, while some conversion taken at 9O”K., a heat of adsorption of 2.8 kcal./ mole was evaluated for the low-pressure end of the activity persists. A number of adsorption experiments also were curve (at 5 X CC. coverage, 20 cm.2); i t should carried out a t room temperature with fresh films be even lower a t higher pressures. The relative a t pressures of to lo-* mm. Again a remark- pressures p / p o of the region investigated here are and 2.5 X This should ably small oxygen uptake was found corresponding between 1.5 X to about 1 coverage. The smallness of the be a region between the first strongly chemisorbed uptake was quite unexpected in view of the high layer and the physically adsorbed layer (BET activity of the films. The reason for this behavior region). may be found in the micro-porous structure of the Dependence of Conversion on O2 Pretreatment. active surface. The pores may be closed by rela- -When increasing amounts of oxygen a t 77°K. tively small quantities of oxygen and, therefore, were adsorbed on a fresh film, the film was inthere may be no direct relationship between the creasingly poisoned for the parahydrogen conver-
2ou
5
1099
SURFACE PROPERTIES OF GERMANIUM
July, 1959
TABLE I INFLUENCE OF OXYGENON ACTIVITY Adsorpt. T ,'IC.
Pretreatment of film
(1) Fresh film Same film Same, slowly warmed up (2) 3 fresh films
(3) Fresh film Same film (4) 13 hour film
77
296
206 296
Oz press.,
5
5 3
x x x x
- const.
X log (time)
Inin.
Pumping, hr.
Pumping T, OK.
2;.
77 77 t o 296 296 296 206 206
22.3 7.6 1.6 5.5 5.5 5.5 7.2
GO
10-2
10-2 10-3 10-1
22 1 0.7 0.7 0.7 0.8
10 10 10 20
1 1 . 2 x lo-'
10-1
sion. The half-life T varied from 13 minutes for the fresh film to 22 hours for 5 X l o + mm. oxygen pressure. (This pressure which is the upper limit for the adsorption curve, coincides with the upper pressure limit a t which the presence of oxygen in the hydrogen did not influence the ortho-para analysis.) I n Table I some typical experiments are summarized. The standard half-life resulting from different pretreatments of the film is given. From example 1 it is seen that a film poisoned to 22 hours a t 77°K. could be reactivated to a certain extent by pumping a t 77°K. When the film was slowly warmed up to room temperature, the activity increased to a half-life of 1.6 hours. On the other hand, if adsorption was carried out at room temperature, (examples 2, 3), the gas became much more strongly adsorbed. Pumping restores a good vacuum almost immediat)ely. The activity is restored to a lesser extent than when ad- and desorbing at low temperatures. Example 4 shows that, with a film strongly poisoned a t room temperature, even a t 300" only partial regeneration occurs. Influence of Light.-With the initial thin films" changes in the electric conductivity and in the conversion activity were observed when the film was illuminated. From the experiments described a,bove, it seems clear that they were covered from the start with a complete monolayer of oxygen. The initial resistivity of a 20 hour film was 4.6 X lo9ohm/square. When left in a vacuum for one month, the resistivity increased to 1.3 X 1O1O ohm/square, while the conversion half-life increased from 25 to 45 hours. The 1 month old film was subjected to illumination by an incandescent lamp, the light being focused through the liquid nitrogen bath onto the inside surface of the film. The resistivity p was now found to decrease with time, obeying within the experimental error a logarithmic law p = po
Contact,
mm.
(2)
between 10 seconds and 18 hours after the start of illumination. The resistivity during this period decreased from 1.388 X 1O'O to 1.269 X 1Olo ohm/ square (10.003 X 1O'O). In the dark it increased again, but a t a lower time rate. The logarithmic law (2) observed for the conductivity change is equivalent to a logarithmic adsorption law ("Elovitch equation"). After improving the vacuum by degassing the traps, the rate of change of resistance
-
I
410
w41
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I
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I
1 1
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IO-'
Fig. 4.-Reversible
I
I
1 1
Pressure, rnm
I
10-2
I
1 1 1
IC'
oxygen adsorption at 77°K.
on illuminating the film considerably decreased. From this and the slowness of the conductivity changes, it follows that a reversible gas adsorption must have been involved. The films probably were p-type; films from intrinsic germanium have a tendency to be ptype a t low temperatures owing to electrons trapped in surface states.2 I n the present case the old films must have been covered with a chemisorbed layer of oxygen. This is known to lead to an electron transfer into the surface, thus making the space charge layer more p-type. Reversible adsorption of oxygen seems to have led to the observed decrease in conductivity. This change in a p-type layer means that an electron transfer took place from the surface to the space charge layer. Conversely, illumination led to an electron transfer back into the surface and desorption of oxygen. The effect of light on the conversion was studied with a film having a half-life of 134 minutes. When the film was illuminated in the course of a conversion experiment, no effect of the light could be detected. However, on pre-illumination of the film for a longer period, a small but significant effect was found. TABLE I1 EFFECT OF PRE-ILLUMINATION ON HALF-LIFE Pretreatment
Hz pressure. mm.
Dark 24 hr. light 22 hr. dark 22 hr. light 24 hr. dark
4.4 4.2 4.0 3.8 3.7
T " . ,
77.3 77.2 77.0 76.9 77.1
Half;life, min.
134 138 155 156 190
In Table I1 results are summarized showing the
1)
Y . L. SANDLER AND M. GAZITH
1100
Vol. 63
Discussion It appears from our experiments that we may distinguish between three different types of surfaces having a different effect on the conversion: (1) X 10.4mm the “bare” part of the surface, which causes the ro 23 Min. E5 larger part of the magnetic activity a t 77°K. with a e 4 4.3mrn fresh film and also causes the chemical activity; 3 T, 5 Hours (2) the surface which contains a monolayer of $ 3 a strongly chemisorbed oxygen. This type of surface $ 2 is believed to account for a part of the magnetic activity but it is chemically inactive; and (3) I the surface covered by a layer of loosely bound oxygen; this surface appears to be completely inactive. Minutes. (1) The source of the high magnetic and the Fig. 5.-Hydrogen pressure a t 77’K. us. desorption time after one minute pumping, for films of different standard chemical activity of the bare part of the surface half-life and for different initial pressures. appears to be the same; both are connected with influence of illumination on the parahydrogen con- the existence of free valences. It is not known with certainty whether the surface version a t 77°K. The half-life measurements are accurate to better than 1%. It is seen that after electrons must be unpaired to give a “surface illumination of one day the half-life remained paramagnetism’’ (as defined by the ability to catroughly the same, while after an equal period of alyze the magnetic parahydrogen conversion). darkness the half-life went up. The usual drift to It seems probable that if the spins are far enough longer half-lives in the dark (due to oxygen uptake apart from each other, they may cause a conversion even if they are weakly coupled. When in coin an imperfect vacuum) is counteracted by light. The experiments with controlled amounts of ordination with one of the spins, the relatively oxygen have shown that addition of oxygen always small hydrogen molecule may be subjected to an leads t o a slowing down of the conversion. We inhomogeneous magnetic field that is of similar thus conclude that the slow effect of light on the order of magnitude as in case of independent spins. conversion is caused by desorption of reversibly At any rate, the conversion would be caused by adsorbed oxygen, which uncovers a Paramagnetic highly unsaturated valences. (2) The half-life of the magnetic conversion on a surface. Desorption of Adsorbed Hydrogen.-For calcu- film with one chemisorbed oxygen layer is of the lating the absolute conversion rate on our surfaces, order of 1 hour. Films contacted with oxygen a t the fraction of hydrogen in the adsorbed phase 77°K. can be restored to have a half-life of about 1.5 must be known. This fraction seemed too small to hours by a slow warm-up in uacuo (Table I), while be determined by a conventional adsorption ex- heating an oxidized film to 300” in vacuo gave a periment. Therefore, the following procedure was film of 3.5 hours. Experiments by various adopted, giving us the minimum amount of hy- authors1~‘3Jgon the regeneration of oxidized germanium surfaces by heating in vacuo show that no drogen adsorbed. Hydrogen was admitted to the reaction vessel regeneration to a clean surface occurs below 400”. a t a known pressure PO. The reaction vessel was The partial restoration of activity in our experithen pumped for 60 seconds. After this time the ments shows that the surface containing a monovacuum indicated by the thermistor (connected to layer of oxygen must be at least partly paramagthe reaction vessel) was less than 3 X mm. netic too. The same result is indicated by the The reaction vessel then was isolated and the illumination experiments with thin films; here we pressure increase was determined as a function of concluded that light causes desorption of reversible time. Figure 5 shows three such curves. The oxygen and uncovers a paramagnetic surface. This two outer curves were taken with the same 5-hour must have been a surface already covered with a film at different initial pressures pa,while the middle monolayer of strongly adsorbed oxygen. The paramagnetism of this surface is believed to curve was obtained with a relatively fresh film be caused by residual unsaturated valences. The ( r = 23 min.). Within the limit of reproducibility oxygen adsorbed in the form of 0 - ions on intrinof the amount desorbed for different films (-30%), no significant difference was found between fresh sically diamagnetic faces could also be a source of paramagnetism. and oxidized films. From the 4.3 mm. curve we find that a t least 60% (3) The second layer is loosely bound a t 77°K. of the hydrogen desorbed in 10 minutes; t’he total Most of the gas can be desorbed a t 77°K. or a amount was found by warming up to room temper- somewhat hicher temperature. When, however, ature. From this amount we calculate that a t a surface partly covered with a second layer of least 1 X of the total amount of hydrogen oxygen is allowed to warm up to room temperature, present in the reaction vessel must have been ad- the oxygen is much more strongly bound. The sorbed in a conversion experiment a t 77°K. and 4.5 reversible oxygen adsorption at 77°K. is different from the stronger activated adsorption known to mm. pressure. The coverage corresponds to monolayers. This seems to be a reasonable value exist a t room temperature.a Formation of an for adsorbed molecular hydrogen and is not likely (19) A. J. Rosenberg, P. H. Robinson and H. C. Gatos, Lincoln to be too low to a considerable extent. Laboratory Quarterly Progress Report, group 35, Nov. 1, 1957,p. 16. 7 1 Q 6
t ’
-
po- 18.5mm
T~
5 Hours
-
-
,
July, 1950
SURF.4CE PROPERTIES O F
adsorption layer of the latter kind requires an activation energy (5.5 kcal. at monolayer coverage) and probably involves displacementz0 of surface atoms. The weak adsorption a t 77°K. is, therefore, the precursor of the activated adsorption a t room temperature. The build-up of the weakly adsorbed layer is accompanied by a continuous decrease in the conversion rate. This, in principle, might be due to a combination of lower hydrogen adsorption and a lower transition probability. A mu$ more likely explanation, however, is that this layer is diamagnetic. It cannot then consist of physically bound oxygen molecules which would be paramagnetic. The adsorption apparently is a weak chemisorption involving the pairing of the n-electrons of the oxygen molecules with the surface. (The tendency of oxygen molecules to form weak chemical bonds is, for instance, apparent in the formation of diamagnetic dimersz1in gaseous and liquid oxygen.) The free valences causing the surface paramagnetism would be used up in this process, possibly by formation of a weak peroxide bond
9-9 0
I‘
Ge
0
0
+ Jr G e Ge .I
0 2
0
I
Ge
Bond formation will thus decrease the electron affinity of the surface and result in some transfer of negative charge from the surface back to the bulk. This would explain the observed photoconductivity changes. Law4 has found similar adsorptions of other gases (Hz, Nz, CO, COZ) on a germanium singlecrystal filament a t low relative pressures. In view of the low heats of adsorption found, Law considered these to be “purely physical’’ adsorptions and therefore to have no effect on the conductivity of the solid. Our results with oxygen indicate that adsorption of these other gases is also caused by unsaturated valences on the surface and that the gases may cause a small conductivity change. A similar type of weak adsorption also seems to cause the chemical parahydrogen conversion a t low temperatures on metal surfaces. z z , 2 3 Like the reversible oxygen adsorption, the reversible hydrogen adsorption causes a conductivity changez6 opposite to the initial change produced by the strongly adsorbed gas. The weak hydrogen adsorption is the forerunner of a slow adsorptionz4-26 a t higher temperatures, which in most cases is probably an activated adsorption (similar to the activated adsorption of oxygen on germanium a t room temperature) and not bulk solution, as has often been assumed. 2 4 , 2 6 It will be of interest to compare the rates found with the results of previous experiments with paramagnetic surfaces. We have reported previously M. Green, ref. 2, p . 372. G. N. Lewis, J . Am. Chem. Soc., 46, 2027 (1924). D. D. Eley, Proc. R o y . SOC.(London), 8178, 452 (1941). A. Farkas and L. Farkas, J . Am. Chem. Soc., 64, 1594 (1942). 0. Beeck, “Advances in Catalysis.” Vol. 2, Academic Press, New York, N. Y . , 1950, p. 161. (25) F. C. Tompkins, 2.Elektrochsm., 66, 360 (1952). (261 J. H . Singleton, THIS JOURNAL, 60, 1606 (1950). (20) (21) (22) (23) (24)
GERMANIUM
1101
that magnetic conversions seem to proceed a t fairly consistent rates11fz7when reduced to an equal number of paramagnetic centers and an equal fraction of hydrogen adsorbed on the surface. adsorbed oxygen It was foundll that 1 X molecules/cm.2 cause a “true” half-life of r8 = 30 seconds; by definition, r8 = r X f, where f is the fraction of adsorbed hydrogen. On our surfaces the maximum density of paramagnetic centers would be about 8 X 10’4/cm.2 and f = 1X On 400 surface (BET area), this would lead to an expected half-life of the order of 1 hour; or about 3 hours, if we correct for the ratio of the squares of the magnetic moments of an electron and an oxygen molecule. It is seen that the strongly chemisorbed layer (2) causes a conversion of the expected magnitude, if we assume that a considerable percentage of the surface sites is paramagnetic. On the other hand, the residual coilversion on the surface covered with reversibly adsorbed oxygen (3) is low, owing to the formation of the weak chemisorption bond. The very fast conversion on what we termed the “bare surface” (1) is striking. The oxygen poisoning experiments a t the lowest pressures a t 77”K., as well as the adsorption experiments a t room temperature, indicate that the active surface area is only of the order of 1 Supposing that the density of adsorbed hydrogen on the bare surface was roughly the same as on the oxygen covered parts of the surface, then f = 1/400 X The conversion half-life of 13 minutes found for this surface then is more than 103 shorter than expected. A more extended adsorption of hydrogen4 on the bare part of the surface cannot explain this large discrepancy. We conclude that the area accessible to hydrogen is considerably larger than the area accessible to oxygen. The active surface apparently is located in micro-pores which are plugged up by a relatively small amount of oxygen. The effect probably is a consequence of the mode of preparation of the germanium. It is quite possible that these “pores” are dislocations not located in the film but in the solid germanium piece used as a cathode. I n any case the strong surface paramagnetism of the “bare surface” is shown to be a consequence of a highly disordered structure, and not to be a property of certain perfect surface planes (previously considered a possibilityz8~z9). The same surface caused a high chemical activity. The fact that no low temperature activity for the hydrogen-deuterium exchange was found by Tamaru and Boudart’ with their germanium films (produced by the decomposition of germane), (27) Y. L. Sandler, ref. 18, p . 257. (28) Y. L. Sandler, Phys. R e v . , 108, 1642 (1957). (29) If the active area was really small (which does not appear likely), we could not completely rule out the possibility that smell patches of metallic impurities, produced in the sputtering process, may have caused the activity. Therefore, to confirm our conclusions, we recently carried out a conversion experiment on germanium powder which had not come in direct contact with metal. The powder was produced by crushing a crystal in an ultra-high vacuum. Again a conversion was found at 77OK., but the rate per unit area now was considerably lower because of the more nearly perfect structure of the crushed crystals.
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W. KEITHHALLAND P. H. EMMETT
again points to the conclusion that the activity in our case is a structural property. We might compare the chemical parahydrogen conversion a t -78" on our films with the conversion a t the same temperature on nickel films, investigated by Singleton.26 Although the activation energies are about the same (1400 cal./mole), the germanium is about 75 times more active, if the active area is assumed to be only 1 cm.2. This remarkably high activity again suggests that the true active area is larger than 1 Even so, this surface area is not likely to have been much larger than the measured BET area (-400 cm.2) because we would have found this from the hydrogen desorption experiments carried out
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with films of different cleanliness. We thus conclude that the activity of our films even when related to the true active area, is remarkably high. In the case of transition metals and their oxides it has been demonstrated c o n ~ i n c i n g l ythat ~ ~ the strong low-temperature activity for the hydrogendeuterium exchange is due to surface unsaturation connected with an intrinsic property of the catalyst, viz., the existence of unfilled d-shells. It is interesting that structural unsaturation, as found in the present experiments, may lead to catalytic activity of a similar order of magnitude. (30) D. A. Dowden, N. Mackenzie and B. M. W. Trapnell, ref. 7. p. 65.
STUDIES OF THE HYDROGENATION OF ETHYLENE OVER COPPER-NICKEL ALLOYS BY W. KEITHHALLAND P. H. EMMETT* Mellon Institute, Pittsburgh, Pa. Received November 16, 1968
The hydrogenation of ethylene has been studied over a series of copper-nickel alloy catalysts. The activation energies obtained were relatively constant with composition over most of the range and with pretreatment, indicating that the observed changes in specific activities came about through changes in the frequency factors. No abrupt drop in specific reaction rate near the critical composition of 60 atom % copper was observed. The microcatalytic technique employed was particularly well adapted for studies of promoting and poisoning effects and allowed the characterization of a large promoting effect of chemisorbed hydrogen on the reaction rate. I n the cop er-rich range, this effect was so pronounced that these promoted alloys were more active than pure nickel. Relationships ietween the present work and current theories of catalysis are discussed.
Several years ago, Reynolds' studied the hydrogenations of styrene and benzene over a series of copper-nickel alloy catalysts. I n both instances, the isothermal catalytic activity fell to a very small fraction of a fairly high initial activity, as the critical composition of 60% copper-40yo nickel was approached from the nickel-rich side. This critical composition corresponds to that for which the band theory of metals predicts that the d band should be just filled. The magnetic moments a t a fixed field strength of 5,000 gauss also were measured for these catalysts and found t o fall as expected from the theory. From arguments presented by Dowden2this behavior could be expected if positive ions were involved in the reaction mechanism, and when copper is alloyed with nickel, the activation energy might be expected to increase and the frequency factor t o decrease near the critical composition. Considering the behavior of styrene as observed by Reynolds,l it was a little surprising from the experimental viewpoint that a totally different behavior in the ethylene hydrogenation was observed by Best and Russell3 over a similar series of alloy catalysts. These workers observed activities several orders of magnitude higher than that of the
*
Department of Chemistry, The Johns Hopkins University, Baltimore, Md. (1) P. W. Reynolds, J . Chem. SOC.,265 (1950). (2) D. A. Dowden, ibid., 242 (1950); Ind. Eng. Ckem., 4 4 , 977 (1952). (3) R. J. Best and W. W. Russell, J . A m . Chem. Soc., 76, 838 (1954).
pure nickel catalyst in the copper-rich range, where the d bands of the metals are filled. From the theoretical standpoint, however, this discrepancy could be explained in a multiplicity of ways. According to the theoretical development of Dowden12the behavior of the activation energies as a function of catalyst composition would be expected t o be a better criterion of the nature of the surface processes involved than the isothermal reaction rates, as the frequency factors may contain factors which cannot be properly evaluated. Since the present authors had previously found interesting relationships between activation energy and composition with these same catalysts in the hydrogenation of b e n ~ e n e such , ~ data were obtained for the hydrogenation of ethylene and are reported herein. Experimental Catalysts.-Copper and nickel form a continuous series of homogeneous-substitutional solid solutions. For the present work, a series of unpromoted, high purity coppernickel alloys, a pure copper and a ure nickel catalyst were made. It will suffice to say here &at the preparation procedure followed quite closely Best and Russell's modification* of the method of Long, Fraser and 0 t t . S This involved coprecipitation of the metals as the basic carbonates followed by roasting to the mixed oxide phases. The latter were reduced to the metal phase with hydrogen at temperatures not exceeding 350'. Further details concerning their nature, preparation and purity are to be found in earlier 14) W. K. Hall and P. H. Emmett, TAIEJOURNAL,62, 816 (1958). (5) J. H. Long, J. C. W. Fraser and E. Ott, J . A m . Chem. Soc., 66,
1101 (1934).
