LOW-TEMPERATURE HYDROGEN ADSORPTION ON COPPER-NICKEL ALLOYS
2775
Low-Temperature Hydrogen Adsorption on Copper-Nickel Alloys by D. A. Cadenhead and N. J. Wagner Department of Chemistrzl, State University of New York at Buffalo, Buffalo, New Yorlc 1.&914 (Received December 8, 1987)
Hydrogen adsorption studies have been carried out on a series of granular copper-nickel alloys ranging from pure copper to pure nickel over a temperature range from -197 to 0" and a pressure range from 10-8 to 20 torr. Studies were carried out in a mercury- and grease-free gravimetric apparatus using a sensitive commercial microbalance. Special care was taken during sample preparation to avoid incomplete reduction or water encapsulation. The results indicate that hydrogen is dissociatively chemisorbed on nickel, with part of the hydrogen undergoing activated adsorption, while nonactivated adsorption takes place on copper, Amounts adsorbed indicate that reduction procedures have produced a significant amount of micropore and defect structure. The variation of total amount adsorbed with the over-all composition indicates that the bulk and surface compositions differ over a wide compositional range and that within this range the surface composition remains essentially constant.
Introduction The series of solid solutions of continually changing composition formed by alloying copper with nickel has frequently been used in an attempt to evaluate the theories of Dowden concerning the role of d-band vacancies in transition metal cata1ysis.l For this particular series of alloys, however, the .results are frequently contradictory. Thus the findings of a benzene hydrogenation study by Emmett and Hall2 are in disagreement with Dowden's theories, in that neither the specific activity nor the apparent activation energy of this reaction shows the predicted behavior as a function of composition. A repeat study of the same system by one of us using a microcatalytic technique8 showed that the variations in these parameters with changing composition were dependent upon the reaction mechanism and/or upon the preparative technique for the various samples, while the absolute values were dependent upon the type of kinetic study carried out. Further insight into this complex situation was provided by S a ~ h t l e r , ~who - ~ showed that for such alloys a miscibility gap exists in the plot of the excess free energy of mixing of copper and nickel, indicating that the observed solid solutions are metastable with respect t o two fixed alloy compositions. Sachtler further predicted this equilibrium state would only be attained where high atomic mobility is retained, This would be the case for materials such as high-defect-structure thin films and at the surfaces of other bulk forms. Finally, he suggested that under certain conditions the outermost surface layer would consist of the copper-rich alloy throughout the miscibility-gap compositional range due to the greater diffusivity of copper, Clearly, a need exists to establish the surface composition of typical, granular alloy-catalyst samples before attempting to interpret the mechanism of socalled simple hydrogenation reactions. We elected to carry out a hydrogen adsorption study in the expectation that selective chemisorption would enable us to
establish the relative copper and nickel composition of the alloy surfaces. Since other partial studies718have been carried out at room temperature or above, we decided to carry out a more extensive investigation a t lower temperatures (-197 to 0").
Experimental Section Alloy Preparation. Copper, nickel, and coppernickel alloys were prepared from the oxides and the basic carbonates according to the method of Long, Fraser, and Ott8 as modified by Best and Russell.10 Starting materials were ACS grade Cu(N0a)z. 3H20 and Ni(NOa)2.6N20. Certified ACS grade NHdHCOa was used to coprecipitate the metals in the form of carbonates from solution which were then converted to the respective oxides by roasting at 400" for 4 hr. The oxides were introduced into the vacuum system and were reduced in situ. The over-all alloy composition was determined using an electrodeposition technique.11 Alloy structure was ascertained using a 14.0-cm DebyeScherrer X-ray diffraction powder camera with Cu K a radiation. The characteristics of these alloys are listed in Table I.
(1) D. A. Dowden, J . Chem. SOC.,242 (1950). (2) P.H.Emmett and W. K. Hall, J . Phye. Chem., 63, 1102 (1959). (3) D.A. Cadenhead and N. G. Masse, ibid., 70, 3558 (1966). (4)W. M.H. Sachtler and G. J. H. Dorgelo, J . Catal., 4, 654 (1965). (5) W. M, H. Sachtler and R. Jongepier, ibid., 4, 665 (1965). (6) W. M. H. Sachtler, G. J. H. Dorgelo, and R. Jongepier, Proc. Int. Sump. Basic Problems Thin Film Phya., 218 (1965). (7) T. Takeuchi, T. Takabatake, M. Sakaguohi, and I. Myoshi, Bull. SOC.Chem. Jap., 35, 1390 (1962). (8) W. K. Hall and N. A. Scholtus, T~ans.Faraday Soc., 59, 969 (1963). (9) J. H. Long, J. C. Fraser, and E. Ott, J. Amer. Chem. SOC.,56, 1101 (1934). (10) J. R. Best and W. W. Russell, ibid., 26, 838 (1954). (11) H.H.Willand, N. H. Furman, and C. E. Bricker, "Elements of Quantitative-Analysis," D. Van Nostrand Co., Inc., New York, N. Y.,1956,p 438. Volume 78, Number 8 August 1968
D. A. CADENHEAD AND N. J. WAGNER
2776
Table I Sample no.
Composition, atom % of c u
N-1 N-2 N-3 N-4 N-6 N-7 N-8 N-9 N-10 N-1 1 N-12
10.35 18.58 28.83 47.71 57 * 79 66.47 77.15 86.88 95.18 100.00
0
Area, Cell oonstant (aa),
0.65 2.01 2.04 2.32 2.02 2.26 2.92 1.57 1.16 3.07 2.47
H
3.5210=kO0.O066 3.5255zt0.0044 3.5347ztO.0021 3.5423=kO0.O016 3.5601&0.0039 3.5677 =k 0.0025 3.5765 zt0.0019 3.5875&0.0045 3.5947 zt 0.0018 3.6073st0.0036 3.6079zt0.0054
"-$1' Pump
Figure 1. Gravimetric gas adsorption apparatus.
Gas PuriJication. The hydrogen used in this investigation was supplied by the Liquid Carbonic Division of General Dynamics and had a nominal purity of 99.95%. It was further purified by passing it through a heated palladium diffusion cell manufactured by E. Bishop and Co. (Model A-1-X) and had a maximum flow rate of 900 cm3/min. The purified hydrogen was then stored in glass reservoirs or, during oxide reduction, passed directly over the sample. Nitrogen was also supplied by the Liquid Carbonic Division of General Dynamics and had a nominal purity of 99.95%. It was purified by passage through glass-bead traps cooled to liquid-nitrogen temperatures and was stored for later use. Krypton (research grade) was supplied by Air Products and Chemicals, Inc. It had a nominal purity of 99.99% and was used as obtained. Deuterium, used in H2-D2 exchange experiments, was CP grade supplied by Air Products and Chemicals, Inc. It had a nominal purity of >99.5% and was also used as obtained. Apparatus. The all-glass vacuum system shown in Figure 1 was constructed and was capable of being torr. Liquid evacuated to a pressure of less than nitrogen traps prevented mercury from the diffusion pump from contaminating the otherwise mercury-free system. Granville-Phillips Type C metal valves were used to prevent contamination of the sample with stopcock grease during adsorption studies. Bayard-Alperttype ionization gauges were used to measure the absolute pressures in the system upon evacuation; however, a fused-quartz Bourdon spiral, supplied by Texas to Instruments, Inc. (Model 140, pressure range 300 torr), was used to measure the pressure during the actual adsorption measurements. Hydrogen was admitted to the sample via a two-tap doser located between the gas-reservoir system and the sample chamber. Sample-weight changes due to adsorbed gas were measured using a Cahn RG electrobalance capable of measuring weight changes as small as lo-' g. The weight of a reduced sample was usually about 500 mg. The Journal of Physical Chemistry
0
:
-6
.E
:: -10
B
6 f
-16
-60.C -70-C
z
-aov
E -20
--110.C 100%
r
-90.C
a
4"
I
ox lo-'
IXIO'3
P
I.6xI0-3
EXi0.8
In torr
Figure 2. Thermomolecular-effect calibration curves for hydrogen over the temperature range -110 to -60'.
All samples studied were placed in an externally silvered bucket suspended from one arm of the microbalance with an identical arrangement for an empty bucket on the reference side of the balance beam. Static forces were eliminated by silvering the sample and reference hangdown tubes of the vacuum system and by grounding the silvered walls, balance beam, and sample container. The sample and reference chambers of the vacuum system were both immersed in a large constant-temperature bath during the isotherm measurements. The combination of identical sample and reference temperatures, silvered containers, and silvered walls minimized therinomolecular forces but did not completely eliminate them. The small remaining thermomolecular forces were taken into account by running blanks on the system and correcting the isotherms. Typical calibration curves are shown in Figure 2. Reduction Procedure. The sample to be studied was weighed as the oxide in a silvered bucket and was placed in the vacuum system. The balance was calibrated and the system was evacuated to a pressure of about torr, after which the system was flushed with hydrogen several times to ensure removal of foreign gases from the system. Finally, the pressure
LOW-TEMPERATURE HYDROGEN ADSORPTION ON COPPER-NICKEL ALLOYS of hydrogen was raised to slightly above 1 atm and a stopcock was opened, enabling fresh hydrogen to flow over the sample in a continuous stream. The hydrogen exits from a tube in the lower portion of the lefthand balance chamber. For the sake of clarity, this tube was not illustrated in Figure 1. The flow rate of hydrogen was adjusted to 75 i 5 cm*/min, and a furnace at approximately 80" was installed around the sample. The furnace temperature was then increased slowly (about 10"/hr) until a weight loss, indicative of reduction initiation, was observed in the sample. Difficulty was sometimes experienced in establishing this temperature, since weight loss through desorption of previously adsorbed materials overlapped with that due to the initiation of reduction. Weight-loss initiation temperatures as a function of alloy composition are illustrated in Figure 3. At this point, the temperature of the furnace was held constant until virtually all weight loss had ceased. The sample temperature was then raised rapidly to 350", the hydrogen flow rate was increased to 450 k 10 cm*/min, and both temperature and flow rate were maintained for 12 hr. This procedure is illustrated for sample N-6 in Figure 4. At the end of this time, the hydrogen was pumped out of the system until the pressure in the system reached 5 X torr or lower, the furnace was removed from the sample, and the hydrogen adsorption isotherm determinations were started. With the system evacuated, a bath of appropriate temperature was placed around the sample and the reference hangdown tubes. The pressure gauge and balance were zeroed and the pumping system was isolated from the vacuum system. Purified hydrogen was then admitted to the vacuum system in small increments through the dosing system, and pressure and weight changes were recorded after equilibration. With the determination of each complete adsorption isotherm, the system was evacuated at 200 =k 5" overnight with the sample under high vacuum in preparation for the next adsorption isotherm. This procedure was repeated until the whole series of adsorption isotherms on an individual sample was complete. The surface area of the sample was then determined gravimetrically with nitrogen and/or krypton at -197" using the B E T equation (or point-B method in the case of krypton) to calculate the surface area. Finally, the absolute weight of the sample was determined and recorded. Surface areas using values of 16.2 A2/adsorbed nitrogen molecule are given in Table I. Using a value of 22 A2/adsorbed krypton molecule, surface areas of samples N-9-N-11 agreed with the nitrogen values within i O . 0 3 mz/g.
Results and Discussion Nickel. The hydrogen adsorption isotherms on pure nickel (sample N-1) are illustrated in Figure 5, for the temperature range from -197.2 t o 0" and the pressure
2777
210,
*Oi
s o - l , , o
IO
eo
I
,
so
,
,
40
(10
,
aa
I
so
80
70
100
Atom Zt Cu
Figure 3. Weight-loss initiation temperature for the oxide reduction as a function of over-all sample composition. Errors indicated are for a single sample. For nickel, the initiation temperatures are shown for three separate samples. -1BO-
350.8'0-d
t0.06°C -100-
P c
'
.-
I
4
-50-
.25*
I
t0.05'C
I
5
0
IO
16
e5
10
Time in Hours
Figure 4. Reduction procedure for sample N-6 (59.29 atom % ' Cu). For details, see the text. Weight of the original oxide (plus the adsorbed material) was 0.6882 g. The total weight loss was 143 mg. 180
I TO 160 150 I40 130
_a 8
120
110
?j100
2
90
.E
80
4
;: 50
40
30 e0 10 0
,001
,001
-
,063 8 0 4 ,005 .OD6
.
-
$007 LW
.
,009
.
,010
P in iorr
Figure 5. Hydrogen isotherms on nickel for the temperature range from 197 to 0'.
-
range from to torr. At each temperature, the isotherm exhibits an initially rapid increase in the amount adsorbed with increasing pressure, attaining an Volume 78, Number 8 August 1968
D. A. CADENHEAD AND N. J. WAGNER
2778 adsorption plateau" somewhere between and 5 X torr. Increasing the pressure to values as high as 20 torr did not change plateau values by an amount greater than the detection limit of the microbalance (lo-' g). The time taken for the system to attain equilibrium after admission of a typical adsorbate dose varied between 15 and 30 min, depending upon the amount admitted and the amount already adsorbed. Equilibrium attainment times of less than 15 min could not be detected, owing to microbalance oscillations which always accompanied any increase in pressure. The reversibility of these isotherms was examined by evacuating the system at the original adsorption temperature when it was found that within 1 hr usually between 70 and 80% of the total amount adsorbed could be removed. The remaining 20-30% was not removed with several hours of pumping and 100% removal was only achieved by outgassing at an elevated temperature. The temperatute fluctuation of the adsorption plateau evident in Figure 5 is best illustrated as an isobar in Figure 6. From the increase in adsorption observed between 184 and - 102" as well as -30 and O", it is clear that there are at least three forms of adsorbed hydrogen present on the nickel, two of which have an activation energy of adsorption. The nature of the adsorbed hydrogen is best demonstrated by carrying out a study of the reversibility of the adsorption process with temperature for a similarly prepared nickel sample (Figure 7)- After adjusting the pressure so that the amount adsorbed was maximized at all temperatures, the temperature was taken through the following sequence using liquid Nz, acetone-solid COZ, and ice-water baths: -197, -80, 0, -80, and -197". The amounts adsorbed clearly show that the hydrogen adsorbed between - 197 and -80" is adsorbed irreversibly and is not removed upon lowering the temperature to -197". All subsequent cycling of temperature between -197 and 0" indicated reversible adsorption was taking place. (The existance of irreversible adsorption between -30 and 0" would only be detected by using afourth temperature bath a t -30" in the above experiment.) It is of interest to note a return to the starting point of the experiment could only be achieved by outgassing at an elevated temperature, lowering the temperature to -197", and readsorbing hydrogen. At -197" the hydrogen is presumably more weakly chemisorbed than that a t higher temperatures. Hayward and Trapnell12 have concluded from a variety of results that hydrogen chemisorption on nickel should be a nonactivated process. Explanations of activated adsorption on nickel usually fall into two classes : adsorption on partially oxidized surfacesla and activated penetration in one form or anotherl4Jb beyond the actual surface layers. We checked the thoroughness of our reduction procedure by reexposing "
1
eo
4 -160
-200
temp
In
depress ctnttpradr
Figure 6. Hydrogen isobar (p = 6.3 X 8 '01
W0
1
100
{
0
-50
-100
torr) on nickel.
-
The Journal of Physical Chemistry
80 J -100
-180
-IO0
-60
0
temp [n d ~ p r r s r cenllgrada
Figure 7. Isobar experiment on nickel (p = 6.3 torr). For details, see the text.
x 10-8
the sample to hydrogen at temperatures up to 450" and then outgassing the sample. The outgassed weight of the sample remained unchanged within the weightchange detection limit of lo-' g. Had the surface been reoxidized by a process such as
+ Ni(surface) = Ni(bu1k) + NiO(surface) NiO(surface) + Hz(g) = Ni(surface) + H20(g)
NiO(bu1k)
a weight change should still have been observed. Thus any activated adsorption due to the presence of a surface oxide must have been extremely small and the alternative explanation of some type of penetration would appear to be the correct one. The absence of slow diffusion (all equilibrium times were less than 30 min) would favor defect diffusion rather than bulk solution. It should also be noted that our sorption temperature range was generally too low to produce (12) D. 0. Hayward and B. M. W. Trapnell, Ed., "Chemisorption," 2nd ed, Butterworth Inc., Washington, D. C., 1964, Chapter
rrr.
(13) G. C. A. Schuit and N. H. de Boer, Rec. Trav. Chim. Paya-Bas, 70, 1067 (1961). (14) 0. Beeck, Advan. Catal., 2 , 161 (1950). (16) M. McD. Baker, G. T. Jenkins, and E. K.Rideal, Trans. Faraday ~ o c . ,51, 1692 (1966).
LOW-TEMPERATURE HYDROGEN ADSORPTION ON COPPER-NICKEL ALLOYS hydrogen absorption associated with bulk oxide, as described by Hall and Scholtus.8 The onset of activated adsorption at higher temperatures (-0") might, however, be explained in this way. Rased on nitrogen surface areas, approximately 6 monolayers of hydrogen were calculated as being sorbed at -loo", with 4 monolayers at .- 197", assuming a 1:1 hydrogen atom:nickel atom ratio and that the 100, 110, and 111 faces are equally represented. These unusual findings, we believe, arise through our unique method of reducing the original oxide. A slow, controlled reduction process appears to result in the formation of micropores inaccessible to nitrogen (sorption at - 197") and significant defect structure (sorption between -197 and - 100") A more conventional reduction procedure16 resulted in only 1 monolayer of hydrogen being sorbed at - 197", in agreement with the original observation. Copper. The hydrogen adsorption isotherms on pure copper (sample N-12) are shown in Figure 8 over the same tcmperature and pressure range. I n contrast to the nickel isotherms, the total amount adsorbed at a corresponding temperature is drastically reduced, the isotherms reached an adsorption plateau only at the lowest temperatures, and the amount adsorbed at any pressure decreased with increasing temperature. Furthermore, the rate at which adsorption took place was significantly slower than on nickel. All of these findings suggested that the adsorption on copper differed from lhat on nickel. This was checked by exposing the copper sample to a 50-50 mol % hydrogen-deuterium mixture for several hours at 0". The residual gas sarnples showed a conversion to HD of 3%, as did two blank samples. Since all of these values were less than that normally achieved by the mass spectrometer filament (473,it may be concluded that the copper sarnples were ineffective in achieving hydrogen-deuterium exchange. This is not surprising, since Taylor, et a!., have shown that, while conversion takes place readily on nickel films even at low temperat~res,~' significant conversion on copper foils does not occur much below 300°.18Careful note should, however, be made of the sorption of approximately 1 monolayer of hydrogen at -197". This could be compared with the findings of Takeuchi, et al.,' who reported approximately 0.1 monolayer at about 100" and lo-ltorr. While our reduction technique may well explain differences between differently reduced materials, it does not explain the large gap between powders and films.19 It may be that, in reduced copper oxides, trace impurities can affect the amount of hydrogen adsorbed. Thus our starting material, Cu(NO3)e* 3Hz0, contains l o w s % nickel as the maximum impurity. If all of this were to find its way into the final product and then to the surface, we could have as much as 1 part of nicke1/100 parts of copper at the surface. Even if the ratio were closer to 0.003, this might still have a significant effect on the hydrogen adsorption. Dissociative adsorption
2779
50
"E
:
20
D
4 c
4
IO
0 0
,001
,002 .001 .004
,005
.DO6
.OD?
.OD8
.OD9
,010
P in iorr,
Figure 8. Hydrogen isotherms on 1 0 0 ~ copper o (N-12) cover the temperature range from - 197 to 0".
could take place on nickel with subsequent movement of the hydrogen atoms onto the copper. Pritchardlg indicates significant hydrogen atom adsorption can take place at low temperatures, the layer being mobile but not desorbed at -78". Such an adsorbed layer might not show significant Hz-Dt exchange (our samples were taken from the gas phase over the sample). Our results could also be explained, in part, by molecular hydrogen adsorption. Clearly, more work needs to be done to establish the nature of the adsorbed hydrogen on this copper. Copper-Nickel Alloys. The hydrogen-adsorption isotherms on the copper-nickel alloys may be grouped into two categories. Samples N-2-N-10 (10-87 atom % copper) showed similar adsorption behavior to that of nickel, though the amounts adsorbed at given temperatures and pressures were significantly reduced. Sample N-11 showed a gradual change to the behavior of copper. I n Figure 9, the isobars for copper, nickel and two copper-nickel alloys (samples N-3 and N-10, 18.58 and 86.88 atom % copper, respectively) illustrate this classification. The isobar for the 95.18 atom % copper sample (not illustrated) shows a reduced but still perceptible amount of activated adsorption. The isobars for all other compositions fall within the limits defined by those for the two alloys illustrated. It is particularly enlightening to examine the total amount adsorbed at a given temperature as a function of composition. This is done in Figure 10 for the two temperatures -197 and -100". Details of the samples involved are given in Table 11. The trend observed is the same for the two temperatures. After a rapid decrease with the first 10 atom % Cu added, the amount adsorbed remains essentially constant (though exhibiting some scatter), up to about 90 atom % Cu, (16) M. W. Roberts and K. W. Sykes, Trans. Faraday SOC.,54, 648 (1958). (17) A. J. Gould, W '. Bleakney, and H. 8. Taylor, J. Chem. Phys., 2, 362 (1934). (18) R. J. Mikovsky, M. Boudart, and H. S. Taylor, J. Amer. Chem. floc., 76, 3814 (1954). (19) J. Pritchard, Trans. Faraday Soc., 59, 437 (1963). Volume 7.9, Number 8 August 1068
D. A. CADBINHEAD AND N. J. WAQNER
2780 Table I1 Sample no. E
IZOj 110
2
100-
N
.-s E.
*
a
1 2 3 4 0
;;70-
60-
6040 80
20
I
-
-
'200
100%cu -160
-50
-100
9 10 11 12
25.0 19.3 20.1 16.8 10.0 9.7 7.5 13.5 14.9 6.2 6.2
26.3 16.8 17.5 14.6 10.5 10.2 7.8 14.2 15.7 6.5 6.5
0.6035 0.6073 0.5725 0.6027 0.5460 0.4555 0,4592 0.4739 0.5740 0.5537 0,6568
7 8
.:
Microbalance correction, pg - 1000 - 197'
Sample wt,g
Temperaturo OC
Figure 9. Hydrogen isobars ( p = 6.3 X 10-8 torr) for samples N-1 (100 atom % nickel), N-3 (18.58 atom % copper), N-10 (86.88 atom % copper), and N-12 (100 atom % ' copper).
o
10
20
a0
40 KO 60 Alom % Cu.
70
eo
90
composition, consideration should be given to the possibility of hydrogen adsorption at pressures below 10-B torr. Since the total volume of the vacuum torr and chamber is about 14 l., at a pressure of 350" we would have approximately 2 X loL4molecules. If we assume that after reduction in hydrogen and outgassing our samples at 350°,we obtain a clean surface,20 then on cooling the system to low temperatures, we would have a maximum surface coverage of 0 = 7 X 10-8 if all the hydrogen were to absorb on 1 m2 of sample. The isosteric heats illustrated for nickel and copper in Figures 11 and 12, respectively, neither confirm nor deny such a low surface coverage. While the values for nickel might appear somewhat low,2o the increasing error with decreasing amount adsorbed prevents even an approximate estimate at zero coverage
100
Figure 10. Total amount, of hydrogen adsorbed as a function of alloy composition a t 100 and 197".
-
:
IO
50
a0
Wt
-
40
I
60
Hz adrorbtd
Figure 11. Isosteric heats of adsorption of hydrogen on nickel (-80 to -92') as a function of amount adsorbed (100"). Errors indicated are maximum errors.
and finally drops slightly as .the composition approaches 100 atom % ' Cu. Surface Composition. Before attempting to interpret the results illustrated in Figure 10, in terms of surface T h e Journal of Physical Chemistry
'0a75
0
.I
2
.3
A
.s
.6
.?
.E
.s
1.0
Coverage of Hydrogen
Figure 12. Isosteric heats of adsorption of hydrogen on copper aa a function of surface coverage. Heats were calculated using the temperature range -80 to -97". The value of 1.0 is assigned to the maximum amount of hydrogen taken up torr. by the sample a t -197" and (20) See, for example, E. K. Rideal and F. Sweet, Proc. Roy. Soc., A257, 291 (1960).
LOW-TEMPERATURE HYDROGEN ADSORPTION ON COPPER-NICKEL ALLOYS 170
.
0 .IOO'C 0 .197'0
180 *
20.
o
IO
20
ao
40 60 BO Alom % Cu.
ro eo
so
100
Figure 13. Total hydrogen adsorption (for details, see the text): a comparison with the Sachtler prediction.6
(Le., zero amount adsorbed). We, therefore, assume the amounts adsorbed, as shown in Figure 10, to represent the total amounts adsorbed. We have replotted the experimental data from Figure 10 at - 100 and - 197" in Figure 13. I n this graph we have superimposed solid lines indicating the behavior expected, were phase separation t o occur at 350" in accordance with Sachtler's predictiona and with the assumption that both nickel and copper make the same contributions toward hydrogen adsorption in the alloys as they do in the pure state. If phase separation takes place at the surface, we should have a constant surface composition of 23 atom % nickel and 77 atom % copper between the bulk compositional limits of 2 and
278 1
77 atom % copper. Our assumption concerning the adsorption contributions of nickel and copper in the alloys would lead to 55.6 pg of hydrogen adsorbed at -100" on this fixed surface compositional alloy (Le., 0.23 X 155 Fg due to Ni and 0.77 X 27.1 pg due to Cu) and 46.9 pg at - 197". The broken lines shown represent the extension of this ideal behavior, were the bulk and surface compositions identical. The very real deviation of the experimental results strongly suggests that over a wide bulk compositional range, the surface composition remains unchanged a t approximately 80 atom % copper. The scatter of the experimental points presumably, in part, represents the degree of difficulty in controlling physical factors such as porosity and defect structure from sample to sample. Recently, Sachtler published the results of a similar study on copper-nickel alloy thin films.21 Although his work was carried out at higher temperatures (25") and on films prepared at 200", the results are remarkably similar, indicating that, here too, phase separation has taken place.
Acknowledgment. The authors wish to express their thanks to Dr. G. R. Wilson, Gulf Research Fellow, of the Carnegie-Mellon University, Pittsburgh, Pa., for his analyses of the H2, D2, and HD mixtures and to the Atomic Energy Commission for their financial support through Contract AT (30-1)-3217. (21) W. M. H. Sachtler and P. van der Plank, J . Catal., 7, 300 (1967).
Volume 79, Number 8 August 1968
