HYDROGEN ADSORPTION ON SILVER, GOLD,AND ALUMINUM
gratitude to Dr. K. A. Kraus. Discussions with him gave the first impulse to study this problem and many of the ideas aro’sefrom these discussions in such a way
1061
that they cannot be attributed to either one of us. The author is also grateful to Dr. J. S. Johnson and Dr. L. Dresner for very helpful discussions.
Hydrogen Adsorption on Silver, Gold, and Aluminum. Studies of Para hydrogen Conversion
by S. J. Holden and D. R. Rossington Department of Physical Science, State Universitu of NEWYork, College of Ceramics at Aljred University, Alfred, New York (Received October 86,1966)
The para-ortho hydrogen conversion has been investigated in order to study aspects of hydrogen adsorption on silver, gold, and aluminum films. The pressure dependence of the first-order rate constant for the conversion obeys a Langmuir isotherm on silver and aluminum. From the temperature dependence of the ratio of the Langmuir adsorption-desorption rate constants, heats of adsorption of hydrogen were calculated. Values of -2.5 kcal./mole in the range 363-413OK. on silver and -7.7 and -8.4 kcal./mole in the range 308-358°K. on aluminum were obtained. No pressure dependence for the conversion on gold was observed in the range 2-20 mm. The normal compensation effect between activation energy and frequency factor for the conversion on the metals was obtained. Calculated values for ithe activation energies and frequency factors fell on the experimentally (obtained line. By comparing the experimentally obtained entropy change upon adsorption with the calculated entropy changes for adsorption into mobile and immobile layers, it is concluded that the adsorbed hydrogen film on silver, in the temperature range studied, is mobile.
Introduction The para-ortho hydrogen conversion has been shown’ to be a particularly sensitive test for the chemisorp tion of hydrogen on t’hose metals such as copper, silver, gold, and aluminum which have been reported as showing negligible adsorption of hydrogen. The process of a chemical parahydrogen conversion (as opposed to a physical or low temperature mechanism) requires the chemisorption of hydrogen regardless of the re. action mechanism1, which may be prevailing. In such CStSeS where the adsorption of hydrogen is VeV small, conventional gas-uptake and calorimetric measurements are extremely difficult to interpret,
and a study of the kinetics of the parahydrogen conversion enables the nature of the adsorption process to be studied.
Experimental The apparatus consisted of a conventional high vacuum system with provision for the preparation and storage of pure normal hydrogen and para-enriched hydrogen. Pure normal hydrogen was prepared by (1) D. D. Eley and D. R. Rossington, “Chemisorption,” W. E. Garner, Ed., Butterworth, London, 1957, p. 137. (2) B. M . W. Trapnell, Proc. Roy SOC.(London), A218, 566 (1953). (3) s. J. Holden and D. R. Rossington, Nature, 1 9 9 , 5 8 9 (1963).
Volume 68, Number 6 M a g , 1964
S. J. HOLDEN AND D. R. ROSSINGTON
1062
allowing tank hydrogen to diffuse through an electrically heated palladium thimble, and para-enriched hydrogen was prepared by adsorbing pure normal hydrogen on charcoal a t the triple point of nitrogen (65 OK.). This para-enriched hydrogen contained approximately 60% p-Hz and is referred to subsequently as parahydrogen. The kinetics of the parahydrogen conversion were studied as a function of pressure and temperature in a constant-volume reaction system. The parahydrogen was left in contact with a metal film for known times and the reaction mixture was then analyzed in a micropirani thermal conductivity gage. Reaction temperatures were obtained by immersing the reaction vessel in an oil bath which could be controlled to '*0.2O. Reaction pressures were obtained by reading a mercury manometer to k0.03 mm. with a cathetometer. The cylindrical reaction vessels were approximately 15 cm. long with an outside diameter of 2.5 cm. The catalysts were in the form of evaporated metal films with an apparent area of approximately 96 cm.2. These were prepared by evaporating a 0.1-mm. diameter wire of the metal under investigation wound around a 10-mil tungsten filament which was sealed down the center of the reaction vessel. The tungsten filament was electrolytically cleaned prior to use. The weights of the films ranged from 14 to 150 mg. and the thicknesses from approximately 3000 to 8600 A. In order to prevent contamination, the reaction vessel was connected to the main vacuum system via a liquid nitrogen trap. The reaction vessel and cold trap were outgassed a t 450 to 500' for 24 hr. prior to the deposition of each film. No deposited film was used for more than 24 hr. and the reaction vessel and cold trap were connected to the rest of the system by mercury float valves. The metal wires were supplied by Johnson Matthey and Co., Ltd., with a purity of 99.999+% for the gold and silver and 99.99+% for the aluminum. The slopes of all lines shown in the figures were calculated by the method of least squares.
Results As u ~ u a l ,it~ was , ~ found that the kinetics of the conversion were first order at constant pressure a t all temperatures studied. If Co denotes the concentration of parahydrogen in excess of its equilibrium value a t time zero and C2at time t, then the experimental rate constant is given by 2.303 Co k=-log t Ct The Journal of PhysiCal Chemistry
(1)
Co is directly proportional to fro, the difference in resistance of the micropirani gage for normal and parahydrogen and C, is directly proportional to i?,, the gage resistance difference for normal hydrogen and a reacted sample after t minutes. The effect of temperature on the rate constant IC a t constant pressure obeys the Arrhenius equation
IC
=
B exp(2)
where E is the apparent activation energy and B is a frequency factor. Typical plots are shown in Fig. 1 and 2 for the temperature dependence of the rate constant, over temperature ranges of 363-413, 400-433, and 308-373OK.
2.8
2.7
2.6
2.5
2.4
2.3
2.3
2.5
2.4
I/T
x
2.6
2.7
103.
Figure 1. Arrhenius plot for silver and gold films: 0, Ag; 0 , Au.
for the silver, gold, and aluminum films, respectively. I n the case of the aluminum films, there was a sharp decrease in the surface area due to sintering. From the linear parts of Fig. 1 and 2, values for E and B may be obtained. It is more meaningful to correct B to BO, the frequency factor for unit reaction volume (4) J. L. Bolland and H. W. Melville, Trans. FaradaU SOC.,33, 1316 (1937). ( 5 ) D. R. Ashmead, D. D. Eley, and R. Rudham, ibid., 5 9 , 207 (1963).
1063
HYDROGEN ADSORPTION ON SILVER,GOLD,AND ALUMINUM
-
1.4 \ \
-
,
0
AI tllm
x
AI
I
I
fllrnII
1.3
1.2
1.1
-+
i.0
0
4
2.9
2.8
5.7 2 -
2.6 I
2.6
I
I
I
2.8 3.0 1 / T X 108.
3.2
3.4
0
I
I
I
I.
I
I
Figure 2 . Arrhenius plot for aluminum films: 0, AI film I; X, A1 film 11.
and unit surface area, ie., BQ= B V / A , where V is the reaction volume and A is the surface area of the catalyst. The surface area of the catalyst was taken to be equal to its geometric area.1,6,7 Values of E and Bo for all the metal films studied w e shown in Fig. 3 and 4. It has been shown previously,l and was confirmed here, that the pressure dependenc,y of the first-ord’er rate constant obeys a Langmuir isotherm. The pressure dependency is therefore expressed by the equation
where a and b are constants and b represents the ratio of the Langmuir adsorption/desorption rate constant,s. A plot of the reciprocal of the first-order rate constant us. pressure a t constant temperature should yield a straight line and it is thus possible to obtain values for b a t various temperatures. Figures 5 and 6 show such plots for the silver and aluminum films. Eley and Rossingtonl and Kernballs have shown that the heat of adsorption (- AH,) may be calculated from
the temperature dependency of b in eq. 2, using the equation given by Bonds 1 - k d (27r~2kT)~’~ ex(%) b a
(3)
where k d is the rate constant for desorption, a is the condensation coefficient, R is the gas constant, and Tis temperature, or -log b
=
AHa 2.303RT ~
+ constant
(4)
The “constant” in eq. 4 contains the term l//z log T . However, over the temperature range used for these experiments (a maximum of 50°), the variation in this term is small, being of the order of 3%. A plot of the results for the silver film obtained from (6) J. A. Allen and J. W. Mitchell, Discussions Faraday SOC.,8 , 309 (1950). (7) J. M. Saleh, C. Kemball, and M. W. Roberts, Trans. Faraday Soe., 57, 1771 (1961). (8) C. Kemball, Advan. Catalysis, 2, 233 (1950). (9) G. C. Bond, “Catalysis by Metals,” Academic Press, London, 1962, p. 70.
VOhmS
68, Number 6
M a y , 1964
1064
S. J. HOLDEN AND D. R. ROSSINGTON
0.28 G
0.24
._I
I
E 7 . 0.20
0 x
*
2
0.10
0.12 0.08
0 06 0.0
0
I 4
2
0
I
I
6
8
-
I
1
2
3
4 5 6 7 Pre sure, mm.
8
Q 1 0 1 1 1 2
Figure 6. Pressure dependency of the parahydrogen conversion on an aluminum film.
log BQ.
Fig. 5, according to eq. 4,gave a straight line of slope 0.56 f 0.07, yielding a value for the heat of adsorption of hydrogen, a t equilibrium coverage, of (- A H , ) silver = 2.5 f 0.3 kcal./mole. Figure 7 shows the results obtained on two aluminum films. The pressure dependency results for film I are given in Fig. 6. The results for film I1 were similar in every respect. The lines in Fig. 7 yield values of the heat of adsorption of hydrogen on aluminum of 7.7 and 8.4 f 0.5 kcal./mole. As has been observed previously,lO the rate constant for the conversion on gold films was independent of pressure, over the range 2-20 mm. and the temperature range 393-433OK.
Figure 4. Compensation effect for aluminum films: 0, this work; 0 , from work of D. R. Rossington, Ph.D. Thesis, University of Bristol, 1956.
o‘8
t
Discussion Activation Energies and Frequency Factors. Figure 3 shows the activation energies and frequency factors obtained in this work (except for aluminum which is shown in Fig. 4), together with previously reported values. It is seen that a normal compensation effect is operative, which has been discussed by other workers.“ Thy various results in Fig. 3 and 4 were obtained over a span of 8 years and in different laboratories. These results give support to the view that instead of a characteristic activation energy, there may exist a characteristic E-log Bo relation for a metal, or series of metals, as was first suggested by Couper and E1ey.l2 In such cases, it is not useful to report 0.0 1 0
I
2
I
I
I
I
I
4
6
8
10
12
Pressure, mm.
Figure 5 . Pressure dependency of the parahydrogen conversion on a silver film.
The Journal of Physical Chemistry
~
~
(10) A. Couper, D. D. Eley, M . J. Hulatt, and D. R. Rossington, Bull. Soe. Chim. Belyes, 67, 343 (1958). (11) E. Cremer, Advan. CataEysis, 7, 75 (1955). (12) A. Couper and D. D. Eley, Proc. Roy. Soc. (London), A211, 536: 544 (1952).
HYDROGEN ADSORPTION ON SILVER, GOLD,AND ALUMINUM
-0.8
If the term in brackets is assumed constant, then a plot of E vs. log BQ should yield a line of slope 2.303RT.
\
0.6
x
AI f i l m
The calculated value of the slope for an average temperature of 43.5'K. for the points in Fig. 3 is 1.90 as compared with an experimental slope of 1.50. Other w ~ r k e r s have ~ ~ suggested ~ ~ ~ ~ that ~ ~ the ~ ' com~ pensation effect could be explained in terms of a relation between energy and entropy. EverettI5 has reported that entropies and heats of adsorption are often related and Kemball16 showed that this could lead to the compensation effect. Calculated Activation Energies and Frequencg Factors. An equation for the first-order rate constant has been derived by Eley .
I
0.4
0.2 B
-2
0.0
I
-1.8
A Ue k = - (ko n
i.6
1.4
1.2 1
2.6
2.8
3.0
3.2
8.4
L
3.6
1/T x 108.
Figure 7. Determination of heat of adsorption on aluminum films, yielding values of AH, = -7.7 and -8.4 kcal./mola: x, A1 film I; 0,A1 film 11.
an activation energy without a frequency factor. A similar situation has been found to exist for copper.'* Although many workers have reported6t' that low melting point metals sinter completely a t room temperature, it may be seen that the activity of the films decreased with heat, treatment. Not only the activity, but also the activation energy and frequency factor were changed after the temperature was raised above a certain value, as shown in Fig. 4. These changes could be attributed to a change in the physical nature of the film, and while such changes in the surface structure may be quite small, the parahydrogen conversion is an extremely sensitive method for detecting them. Pritchard14 has recently indicated that structural variations in evaporated films may also affect surface potential measurements. In discussing the compensation effect, Eley and Rossington' have sihown that a temperature-dependent activation energy can yield the expression
E
=
1065
2.303RT log Bo
+ [ E ( T )- 2.303RT log B o ( T ) ] (5)
where E and B O are temperatureindependent values.
+ k,)
where A is the surface area of catalyst (cm.2); a is the number of sites/cm.2 of surface; 0 is the fraction of surface covered; n is the total number of hydrogen molecules in the system; k, is the first-order rate constant for the conversion ortho- to parahydrogen; and, k, is the first-order rate constant for the conversion para- to orthohydrogen. By substituting for the number of hydrogen molecules and replacing 0 by a Langmuir isotherm, we obtain
k
=
AakT V (k, I _
+
+ k,) (1 +b bp)
(7)
and (k, k,) represents the net conversion of hydrogen molecules. As the pressure approaches zero, eq. 7 reduces to
(k,
+ k,)
k,+OV AakTb
= -
For silver, the rate constant as the pressure approaches zero and the value of b at a particular temperature may be obtained from Fig. 5. The reaction volume and surface area of the metal films may be measured and the value of a was taken to be that for the most densely packed plane (111). If the rate constant ( k , k,) calculated from eq. 8 obeys the Arrhenius equation, a plot of its logarithm us. reciprocal temperature should give a line of slope -E/2.303R and intercept log Bo. The resuJts of these calculations are given in Table I and yield a linear plot of log (k, k,) vs. 1/T.
+
+
(13) S. J. Holden and D. R. Rossington, t o be published. (14) J. Pritchard, Trans. Faraday SOC.,59,437 (1963). (15) D.H.Everett, ibid., 46, 957 (1950). (16) C.Kemball, Proc. Roy. SOC.(London), A217, 376 (1953). (17) D.D.Eley, Trans. Faraday Soc., 44, 216 (1948).
Volume 68,Number 6 M a y , 1964
1066
S. J. HOLDEN AND D. R. ROSSINGTON
Table I : Calculations of log ( k , Temperature for a Silver Film T,OK.
363 383 393 403 413
l/kp+o
36.Dl 22.76 20.00 14.81 12.87
kP+o
0.0278 0.0439 0.0500 0.0675 0.0777
+ k,)
b
0.0940 0.1104 0.1259 0.1347 0.1457
as a Function of
(ko
x k,)
log (ko
9.710 X 1.238 X low2 1.205 X IO-’ 1.482 X IO-’‘ 1.539 X lo-’’
x
kp)
-3.987 -2.093 -2.081 -2.171 -2.187
The activation energy and frequency factor calculated for pressures approaching zero for eq. 8 (E = 2.68 kcal./mole, log Bo = -1.61) were lower than the experimental values, and yet fell on the general compensation effect line, as shown in Fig. 3. It was not possible to calculate an activation energy and frequency factor for aluminum as the calculated points did not lie on a straight line. This may have been due to the fact that the higher activity of the aluminum resulted in the pressure dependency experiments being conducted over a smaller temperature range than for the silver films. Heat of Adsorption. The rate constant for gold does not appear to be pressure-dependent between 379 and 433’K. It is possible that the small amount of conversion on gold, due to a very low surface coverage, requires a lower range of pressure measurements in order to detect a pressure dependency. Such a low equilibrium surface coverage could be reached even a t a reaction pressure of 2 mm. I n the present work, the lower pressure limit of reaction of approximately 2 mm. was determined by the fact that there had to be enough reaction mixture to give a pressure of 50 mm. when compressed into the micropirani gage for analysis. Pritchard’4 has recently shown that hydrogen adsorption on gold at - 1 8 3 O is low (6 = 0.15) and it is quite possible that the coverage would be undetectable in the temperature range of the current experiments. There was close agreement between the heats of adsorption obtained from the two aluminum films. The chemisorption of hydrogen on aluminum must involve a mechanism other than d-s promotion of electrons, which has been proposed for the meehanism of hydrogen adsorption on copper and silver. A mechanism involving hydrogen atoms being bonded by a hybrid sp orbital a t the surface has been suggested.2 The chemisorption of hydrogen by aluminum is sufficiently small as to be immeasurable by gasuptake methods.2 For silver, the heat of adsorption at equilibrium The JournaE of Ph,ysieal Chemistry
coverage was determined as 2.5 kcal./mole, compared to 8.0 kcal./mole for c0pper.l State of the Adsorbed Surface Film. A knowledge of the heat of adsorption a t equilibrium coverage and the ratio of the adsorption-desorption rate constant enables calculations to be made regarding the state of the adsorbed film. Following Kemball18if the standard surface state is defined as a surface coverage of 0.5 and a pressure of 1 atm., the change in free energy upon adsorption, AF,, is given by
AFa = R T In b
(9)
The value of b may be obtained from Fig. 6 or 7 using the average temperature of the experiment as T . Therefore, the entropy change upon adsorption, AS,, may be obtained from ASa
=
-AFa
4- AHa T
This experimentally determined entropy change may be compared to a calculated entropy change, which will be dependent upon whether the adsorbed film is mobile or immobile. The three-dimensional translational entropy of a gas at 1atm. pressure is given by8 BStransl = R
In [M*’’T6/’]- 2.30
(11)
where M is the molecular weight of the gas. The two-dimensional translational entropy is given by8 ZStranal
=
R In [ M T A ]
+ 65.80
(12)
where A is the area per adsorbed molecule. Kemball and Ridea1l8 have defined a standard state corresponding to a surface layer of thickness 6 A. which gives the same volume per molecule at 1 atm. pressure as the three-dimensional state. Thus defined, A = 22.53T For a mobile layer, upon adsorption, a degree of translational freedom is lost and replaced by a vibration perpendicular to the surface. For chemisorption a t ordinary temperatures, the entropy of vibration is small, usually less than 3 e.u. For an immobile adsorbed layer, the translational entropy which is lost is replaced by a configurational entropy. KembalP has shown that for dissociative adsorption, a surface in which each atom has six nearest neighbors has a configurational entropy of (18) C. Kemball and E. K. Rideal, Proc. Roy. SOC.(London), A187, 63 (1946).
HYDROGEN ADSORPTION
Soonfig =
R[o(x-
'/o)
ON SILVER,
In
(2
-
'/6)
GOLD,AND ALUMINUM
+ In u - 4 X
In x - 2(2 - 1) In
(2
Table I1 : Experimental and Theoretical Entropy Changes
- l ) ] (13)
where x = l/O and u is the symmetry number, taken as equal to two if the two ends of a double molecule are indistinguishable. Therefore, if the surface layer of hydrogen on the metal studied is mobile, the entropy change upon adsorption will be approximated by the difference between the translational entropy in three and two dimensions. If the surface film is immobile, the entropy change will be approximated by the difference between the three-dimensional translational entropy and the configurational entropy. Table I1 shows tlhe calculated and experimentally determined entropy changes. The calculated values for the three-dimensional translational entropy have been corrected to 1 atm. pressure. The surface standard states were estimahed, based on the known surfaae coverage for copper8 and the relative activities and areas of copper, silver, and aluminum films. As may be seen from the table, the film on silver is
1067
Metal film
CUI CuII AgI A11 A111 'See
Surface standard
temp.,
state
OK.
ASexpti, e.u.
413 393 391 330 338
-21.0 -23.5 -15.2 -25.0 -23.8
0.17 0.17 0.10 0.25 0.25 ref. 13.
Average
sStPansi aStransi,
e.u.
-17.0 -24.0 -15.4 -15.3 -15.4
sStF,nsi State of Soonfig, adsorbed e.u. layer
-33.2 -30.7 -29.4 -30.8 -30.9
Mobilen Mobilea Mobile ? ?
mobile a t the temperature of the experiment, while a definite conclusion cannot be reached in the case of aluminum. A similar analysis to this has shown that the adsorbed hydrogen film on copper is also mobile under similar conditions.1a
Acknowledgment. Acknowledgment is made to the U. S. Department of Health, Education, and Welfare for financial support to S. J. H.
Volume 68,Number 6
M a y , 1864
