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45 Heats of Chemisorption of Oxygen on Palladium and Palladium-Silver Alloys
45
Heats of Chemisorption of Oxygen on Palladium and Palladium-Silver Alloys M. H. BORTNER
AND
G. PARRAVANO*
Franklin Institute Laboratories f o r Research and Development, Philadelphia, Pennsylvania Heats of chemisorption of oxygen have been measured on palladium black and on palladium-silver alloys, in the temperature range 500700" K. and fraction of surface coverages, 8, 1 X lo-' to 0.7. Heat of chemisorption data were obtained by means of measurements of resction equilibrium of water decomposition on palladium and palladiumsilver alloys. For palladium black, a value of the adsorption heat of -24 kcal./mole oxide was obtained. This value is not much different from the value corresponding to the formation of bulk palladium oxide (PdO). A large increase in the value of the heat is obtained by alloying palladium with silver. This effect sets in a t relatively low silver concentration (~17~). This result is taken as an indication that bonding orbitals, used in the alloying process, do not contribute greatly to the adsorptive bond. Furthermore, from the behavior of the adsorption equilibrium function in the range of 8 values studied, it can be concluded that oxygen chemisorption on palladium black is localized and noninteracting and occurs on an apparently thermodynamically uniform surface. On the other hand, surface nonuniformity is produced by alloying palladium with even 1% of silver.
I. INTRODUCTION It has been shown recently that the adsorption equilibrium function ( 1 ) can be used to obtain information on the thermodynamics of chemisorption processes as they occur during a catalytic reaction (2). Thus, the free energy, enthalpy, and entropy of oxygen chemisorption on nickel, platinum, and silver surfaces were determined while these surfaces were being used for the catalytic decomposition of water. Silver is well known to be an active oxidation catalyst while palladium is not. Furthermore, there is a considerable difference in the value of the heats of chemisorption of oxygen on silver and on palladium. It then becomes interesting to investigate the effect of the composition of the silver-
* Address : Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana. 424
45. HEATS
425
OF CHEMISORPTION OF OXYGEN
4.08
-& 5a
4.00
bi z
0 . 0
8
3.92
F
c a
-1
3.84
0'
20
40 60 Pd (% by weight)
80
100
FIG.1. Effect of composition on the lattice constant of the Pd-Ag alloy system.
palladium alloy on the energy of the adsorptive bond between oxygen and the metal surface. The results of this study are presented in this communication, together with data on pure palladium metal.
11. EXPERIMENTAL 1. Materials Palladium black was prepared from palladium nitrate and formaldehyde solution by dropwise addition of potassium hydroxide solution (50 wt. %) a t about 10". The solution and precipitate were warmed a t about 60" and the precipitate washed several times by decant,ation. It was then placed in a Soxhlet extractor and washed for 48 hr. (about 100 times). The precipitate was then dryed at 110". The palladium-silver system is known to be one of complete miscibility (3). Alloys of silver-palladium were prepared following a procedure discussed elsewhere (4). Their preparation involved a lowtemperature coprecipitation of both metals from a solution containing proper amounts of their nitrates. Alloy formation was checked by means of x-ray diffraction patterns which were obtained with Cu-Ka! radiation. The computed lattice constants are shown in Fig. 1 to be a linear function of the alloy composition. Hydrogen, used for pretreatment of all samples, was obtained from a commercial tank and purified by passage through a Deoxo unit, magnesium perchlorate, and a charcoal trap immersed in liquid nitrogen. 2. Apparatus The experimental method and apparatus used in this investigation have already been discussed (2).Essentially, the a,pparatus consisted of a race-
426
M. H. BORTNER AND G. PARRAVANO
track-type closed system. Alloy samples were placed in one side of the system, and heated by means of a surrounding electrical furnace (&l.O0), the other side being kept at room temperature. The ensuing thermosiphon effect was found to provide sufficient circulation and mixing of the gaseous reacting mixture of water vapor and hydrogen to overcome any thermomolecular separation. Samples of the gaseous mixture were periodically withdrawn and analyzed for hydrogen by freezing the water vapor and adsorbing the hydrogen on copper oxide. Hydrogen pressure was determined by means of a McLeod gage. During a run, water-vapor pressure was known and kept constant by means of a constant-temperature liquid water supply directly connected with the equilibrium apparatus. Thus, it was possible to compute the ratio p H , / p H , oin equilibrium with any alloy sample. If K1 is the equilibrium constant of the reaction: Me
+ K O ( g )* Hdg) + Me - 0
(1)
where Me - 0 is a surface site occupied by chemisorbed oxygen, then
where 8 is the fraction of surface covered with adsorbed oxygen. From a knowledge of the geometry of the system and the amount of hydrogen formed, 8 and consequently K 1 can be computed. If K2 is the equilibrium constant for the well-known reaction: Hz(g)
+ %Oz(g) *
(3)
and K I the equilibrium constant for the oxygen chemisorption process :
%02(g)
+ Me
$ Me
-0
(4)
It follows that
K3 = KIeK2 (5) Thus, by measurements of the equilibrium ratios pH,/pH,o, under different experimental conditions, values of K 3 can be computed and thermodynamic functions derived. 3. Procedure
After a known amount of palladium-silver alloy was admitted, the system was evacuated and the water of the constant temperature supply was degassed by repeatedly freezing and melting under vacuum, until the residual pressure was not greater than 1 x mm. Hg. The water was then kept frozen during the subsequent reducing treatment of the alloy. This treatment consisted of passing a stream of pure hydrogen at atmos-
45. HEATS OF
CHEMISORPTION OF OXYGEN
427
pheric pressure into the system and heating the alloy at 400" for 3 to 4 hrs. The reaction system was then evacuated by applying high vacuum for 24 hrs. After evacuation, repeated checks were made of the static vacuum in order to determine the leak rate of the apparatus and whether hydrogen had been totally withdrawn from the system. Within the precision of the experiment, no gas was evolved even after a period of several days. Subsequently, the alloy was brought to the desired temperature and the water of the constant temperature supply melted and its temperature controlled by surrounding it with a constant-temperature bath. The system was then allowed to come to equilibrium, and the formation and amount. of hydrogen produced were determined as discussed above. Surface areas of palladium and palladium-silver alloys were not measured. They were assumed to be about 2 m.2/g., because it was previously found t
111. RESULTS Since the initial conditions were pH2= 0 and p o z = 0, and pHzo and reaction volume were kept constant during a run, the amount of hydrogen present at equilibrium could be used to compute the corresponding value of 0. This was simply obtained from
where Z is the total number of sites available on the alloy surface and Vsystem and Tsystem have been computed by taking into account the different volumes of the apparatus that are a t different temperatures ( 2 ) . The product pHzVSystem has been expressed in calories. The value of 2 was taken as 1 X Wt(Pd) Wt(Pd)
+ 0.22Wt(Ag)
+ Wt(Ag)
X 1015 x weight of sample X B.E.T. area in cm.'/g.;
this assumes that practically every surface metal atom is a site for oxygen chemisorption. As discussed previously ( 2 ) ,the contribution to 0 from hydrogen chemisorption must be negligible. By means of Equations (2) and (6), values for the equilibrium constant, K , , have been determined for palladium black and palladium-silver alloys of different compositions at various temperatures and are collected in Fig. 2. For every sample, data were obtained a t two different water-vapor pressures. The lines drawn through the experimental points were taken as straight lines, whose slopes were used to com-
428
M. H. BORTNER AND G . PARRAVANO I
10-1
10-2
10-3
10-4
KI 10-5
10-7
10-8
I0-9
10-10 1.5
1.6
1.7
1.9
1.8
IOJT-I
2.1
2.0
(OK-')
FIG. 2. Equilibrium constant of reaction (1) as a function of temperature. % Pd: 100 99 90 70 50 30 10 T H ~= O Oo: 0 0 A 0 0 6 3 0 T q o = 23": 0 . A X
v
+
+
pute the enthalpy change involved in reaction (1). By combining the values so obtained with the heat of formation of HzO(g) from (3), calculated for each temperature from known data (5), the enthalpy change involved in reaction (4) was deduced. Similarly, by means of known values of K2 and the values of K1, determined experimentally, K3was obtained by means of Equation ( 5 ) . The results of these computations are reported in Table I. For comparison, the heat of adsorption for oxygen on silver (2) is also presented in Table I. In Table I, the compositions of the samples (in weight per cent) are reported in the first column. The temperature a t which the surface equilibrium (1) has been measured is shown in the second column. In the third
TABLE I Oxygen Chemisorption on Palladium and Palladium-Silver Alloys Sample composition, wt.%
TBsmple, Twarr, 8 X 10' "K . OK.
Pd
523 523 573 573 673 673
273 295 273 294 273 297
99% Pd-l% Ag
523 523 623 623 673 673
273 295 273 295 273 298
90% Pd-10% Ag
523 523 573 573 623 623 673 673
273 296 273 296 273 295 273 296
507 523 563 563 603 611 658 658
296 273 273
523 523 573 573 623 623 673 673
273 293 273 297 273 296 273 297
533 533
273 297 273 297 273 297
600
600 644 653
294
273 296 273 297
0.350 0.436 0.721 1.72 8.88 14.70
Kt
m , a
kcal./mole
1.3 X 4.2 X 1.9 x 2.6 X 1.5 X 8.3 x
1013 1Ol2 10" 10" 10'0 109
-24.6
2.9 X 1.1 x 8.3 X 3.8 X 6.6 X 2.3 X
1020 1030 10l6 10l6 1Ol6 10l6
-46.8
73.4 84.5 108 151 178 228 229 317
2.1 x 6.6 X 2.1 x 9.5 x 1.0 x 4.0 x 5.1 X 4.6 X
1017 10l6 10'6 1014 1014 1013 10l2 10l2
-46.1
17.0 16.9 31.1 32.8 41.2 53.8 119 1%
1.5 X 1.5 X 2.7 X 1.7 x 3.0 x 5.4 x 4.3 x 9.5 x
10l6 10l6 1Ol6 1014 1013 1012 10'2 10"
-45.1
296 403 547 732 767 1170 1320 1420
4.1 X 2.1 x 5.4 x 2.2 x 2.0 x 1.1 x 1.9 x 4.7 x
1Ol8 10'8 1016 10'6 10" 10'6 1014 1013
-45.0
127 153 195 204 327 412
1.2 X 4.0 x 6.6 X 1.3 X 6.4 x 1.1 x
10" 1017 10'6 10l6 1014 1014
-48.4
3100 3950 4950 6150 6580 7440
430
M. H. BORTNER AND G . PARRAVANO
TABLE I-Continued Sample composition, wt.% 10% Pd-90% Ag
Tasmpie, Twster,
0 X lo4
AH,a
K3
"K.
O K .
523 523 573 573 623 623 673 673
273 297 273 297 273 297 273 296
45.5 71.3 71.6 92.3 78.6 107.0 115 281
2.0 1.1 2.1 6.6 3.7 1.7 3.0 2.7
x x x
473 473 573 573
273 295 273 295
4.3 5.7 6.0 8.2
1.2 4.0 6.2 2.3
kcal./mole --49.1
x X X X
10'6 10'6 1014 1013 1012 10l2 10" 1011
x x x x
10'8 1017 1013 1013
-54.0
X
is expressed in kcal./mole of surface oxide, which is assumed to have the composition PdO or Ag,O.
column are reported the temperatures of the water supply, which controlled the value of the water vapor pressure of the system, and, therefore, 0. The values of 0 are reported in the fourth column, as computed by means of Equation (6). The calculated values of K 3and the heat of chemisorption, AH, for reaction (4) are presented in the fifth and sixth columns. AF and A S values for reaction (4) could be easily computed, but they would be of little significance, because they correspond to different values of 0.
IV. DISCUSSION If the thermodynamic activity of occupied (0) and unoccupied (1 - 0) sites is assumed to be the same, K , is the adsorption equilibrium function for oxygen in the surface involved. Therefore, it is interesting to follow the behavior of K 3as a function of 8. Although values of K 3were determined for only two different values of 8, an inspection of Table I in this regard is
quite instructive. The experimental error on the data on palladium black is probably larger than that corresponding to the palladium-silver alloys, but, to a first approximation, it is permissible to conclude that K 3for palladium black is nearly independent of 8. This indicates that, under present experimental conditions, K3 is a true equilibrium constant and oxygen chemisorption on palladium is localized and noninteracting and occurs on an energetically homogeneous surface. This homogeneity may also be the result of the opposite effects of strong interaction among adsorbate species and surface heterogeneity ( 1 ) . However, it seems that palladium presents in this respect a transition case between those of platinum and nickel.
45.
431
HEATS OF CHEMISORPTION O F OXYGEK
These three metals are chemically grouped in the same column of the periodic system, but the surface aflinity of nickel for oxygen is sharply different from the corresponding affinity of platinum. Indeed, for the nickel-oxygen system the adsorption was found to occur on a homogeneous, interacting type of surface, K3 increasing with 0, while for the platinum-oxygen system, the adsorption was found to occur on a strongly heterogeneous surface, K1 decreasing with 8 (2). The adsorption thermodynamics of the palladium-oxygen system is apparently intermediate between the previous two cases, K3 being nearly constant with 0. This result shows that the chemistry of the metal atoms in question, as determined by their electronic structure and their position in the periodic classification of the elements, is only indirectly involved in determining oxygen adsorption behavior of surfaces. I n all the alloys tested, K 3is found to decrease with increasing 0. This is the behavior shown by silver. It is noteworthy to point out that this effect sets in by alloying as little as 1% of silver with palladium. This strong tendency for the chemisorption process to retain the thermodynamic characteristics of oxygen chemisorption on silver down to dilute amounts of silver is suggestive of the fact that electrons which partake in metal bonding do not contribute greatly to bond formation in oxygen chemisorption. This effect is even more apparent if one compares the values of AH for reaction (4)for the different alloys (Fig. 3). Clearly, the nature of the oxygen surface bond is different from that of the metal-metal bond. Since this latter is thought to be due to hybrid (dsp) orbitals, with a high participation of d electrons in the case of palladium, oxygen chemisorption will not involve d-electron bonds. Probably, it will involve sp electrons only. An alternate
Ag (% by weight)
FIG.3. Effect of alloying with silver on the heat of adsorption of
0 2
on palladium.
432
M. H. BORTNER AND 0. PARRAVANO TABLE I1 Heats of Formation of Bulk and Surface Oxides for Palladium and Silver (in kcal./mole of oxide) Oxide
aH bulka
PdO
-21.0
Ag2O
(1
b
-7.3
AH surfaceb -24.6
-54.0
At 298"K., from L. Brewer, Chem. Revs. 62, 1 (1952). At 500°K.
explanation of the data presented in Fig. 3 would be to assume that silver metal concentrates a t the surface in all the alloys tested. Since the adsorption free energy of oxygen on silver is higher than on palladium (a), this difference should provide enough driving force for establishing the diffusion of silver atoms from the bulk to the surface of the alloy in the temperature range investigated. It is interesting to compare data on the heat of formation of bulk oxides of palladium and silver with values of heats of chemisorption of oxygen on the same metals. This is done in Table 11. An inspection of Table I1 indicates that, while bulk palladium oxide is thermodynamically more stable than silver oxide, the opposite is true for the surface compound. This clearly suggests that great caution should be exercised in deducing thermodynamic properties of surfaces from bulk values.
V. CONCLUSION The method of adsorption and chemical reaction equilibrium has again proved its usefulness in deriving heats of adsorption a t low surface concentration, where the most active sites generally control the energetic behavior of surfaces. In the present case, the application of this method to oxygen chemisorption on palladium and palladium-silver alloys has shown how the different surface affinities of these two metals for oxygen contribute to the resulting adsorption thermodynamics of the alloy. This behavior has given experimental support to the suggestion that oxygen chemisorption on transition metals does not involve direct participation of d electrons. The use of the adsorption equilibrium function has revealed a sharply different behavior for nickel, palladium, and platinum surfaces, with palladium presenting an intermediate case between nickel and platinum. This behavior can be compared with the relative position of these three metals in the periodic classification of chemical elements. Finally, our inability to formulate surface behavior in terms of bulk properties is strikingly demonstrated by the higher stability of the surface
45. HEATS
OF CHEMISORPTION OF OXYGEN
433
oxide film on silver than on palladium. This is in sharp contrast with the known properties of the corresponding bulk phases.
ACKNOWLEDGMENTS This work has been made possible by a grant from the Atlantic Refining Company, the Esso Research and Engineering Company, and the Gulf Research and Development Company to The Franklin Institute Laboratories for fundamental studies in the field of heterogeneous catalysis. This support is greatefully acknowledged.
Received: March 2 , 1956 .
REFERENCES
1. Graham, D., J . Phys. Chem. 67, 665 (1953). 2. Gonzalez, 0. D., and Parravano, G., J . Am. Chem. SOC.78, 4533 (1956).
3. Masing, G., i n “Handbuch der Metallphysik,” Vol. I1 p. 266. Akademische Verlagsges., Leipeig, 1935. 4 . Parravano, G., in press. 6. Spencer, H. M., and Justice, J. L., J. A m . Chem. SOC.66, 2311 (1934); NatZ. BUT.
Standards Circ. 600 (1934).
Heats of Chemisorption of Oxygen on Palladium and Palladium-Silver Alloys M. H. BORTNER
AND
G. PARRAVANO*
Franklin Institute Laboratories f o r Research and Development, Philadelphia, Pennsylvania Heats of chemisorption of oxygen have been measured on palladium black and on palladium-silver alloys, in the temperature range 500700" K. and fraction of surface coverages, 8, 1 X lo-' to 0.7. Heat of chemisorption data were obtained by means of measurements of resction equilibrium of water decomposition on palladium and palladiumsilver alloys. For palladium black, a value of the adsorption heat of -24 kcal./mole oxide was obtained. This value is not much different from the value corresponding to the formation of bulk palladium oxide (PdO). A large increase in the value of the heat is obtained by alloying palladium with silver. This effect sets in a t relatively low silver concentration (~17~). This result is taken as an indication that bonding orbitals, used in the alloying process, do not contribute greatly to the adsorptive bond. Furthermore, from the behavior of the adsorption equilibrium function in the range of 8 values studied, it can be concluded that oxygen chemisorption on palladium black is localized and noninteracting and occurs on an apparently thermodynamically uniform surface. On the other hand, surface nonuniformity is produced by alloying palladium with even 1% of silver.
I. INTRODUCTION It has been shown recently that the adsorption equilibrium function ( 1 ) can be used to obtain information on the thermodynamics of chemisorption processes as they occur during a catalytic reaction (2). Thus, the free energy, enthalpy, and entropy of oxygen chemisorption on nickel, platinum, and silver surfaces were determined while these surfaces were being used for the catalytic decomposition of water. Silver is well known to be an active oxidation catalyst while palladium is not. Furthermore, there is a considerable difference in the value of the heats of chemisorption of oxygen on silver and on palladium. It then becomes interesting to investigate the effect of the composition of the silver-
* Address : Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana. 424
45. HEATS
425
OF CHEMISORPTION OF OXYGEN
4.08
-& 5a
4.00
bi z
0 . 0
8
3.92
F
c a
-1
3.84
0'
20
40 60 Pd (% by weight)
80
100
FIG.1. Effect of composition on the lattice constant of the Pd-Ag alloy system.
palladium alloy on the energy of the adsorptive bond between oxygen and the metal surface. The results of this study are presented in this communication, together with data on pure palladium metal.
11. EXPERIMENTAL 1. Materials Palladium black was prepared from palladium nitrate and formaldehyde solution by dropwise addition of potassium hydroxide solution (50 wt. %) a t about 10". The solution and precipitate were warmed a t about 60" and the precipitate washed several times by decant,ation. It was then placed in a Soxhlet extractor and washed for 48 hr. (about 100 times). The precipitate was then dryed at 110". The palladium-silver system is known to be one of complete miscibility (3). Alloys of silver-palladium were prepared following a procedure discussed elsewhere (4). Their preparation involved a lowtemperature coprecipitation of both metals from a solution containing proper amounts of their nitrates. Alloy formation was checked by means of x-ray diffraction patterns which were obtained with Cu-Ka! radiation. The computed lattice constants are shown in Fig. 1 to be a linear function of the alloy composition. Hydrogen, used for pretreatment of all samples, was obtained from a commercial tank and purified by passage through a Deoxo unit, magnesium perchlorate, and a charcoal trap immersed in liquid nitrogen. 2. Apparatus The experimental method and apparatus used in this investigation have already been discussed (2).Essentially, the a,pparatus consisted of a race-
426
M. H. BORTNER AND G. PARRAVANO
track-type closed system. Alloy samples were placed in one side of the system, and heated by means of a surrounding electrical furnace (&l.O0), the other side being kept at room temperature. The ensuing thermosiphon effect was found to provide sufficient circulation and mixing of the gaseous reacting mixture of water vapor and hydrogen to overcome any thermomolecular separation. Samples of the gaseous mixture were periodically withdrawn and analyzed for hydrogen by freezing the water vapor and adsorbing the hydrogen on copper oxide. Hydrogen pressure was determined by means of a McLeod gage. During a run, water-vapor pressure was known and kept constant by means of a constant-temperature liquid water supply directly connected with the equilibrium apparatus. Thus, it was possible to compute the ratio p H , / p H , oin equilibrium with any alloy sample. If K1 is the equilibrium constant of the reaction: Me
+ K O ( g )* Hdg) + Me - 0
(1)
where Me - 0 is a surface site occupied by chemisorbed oxygen, then
where 8 is the fraction of surface covered with adsorbed oxygen. From a knowledge of the geometry of the system and the amount of hydrogen formed, 8 and consequently K 1 can be computed. If K2 is the equilibrium constant for the well-known reaction: Hz(g)
+ %Oz(g) *
(3)
and K I the equilibrium constant for the oxygen chemisorption process :
%02(g)
+ Me
$ Me
-0
(4)
It follows that
K3 = KIeK2 (5) Thus, by measurements of the equilibrium ratios pH,/pH,o, under different experimental conditions, values of K 3 can be computed and thermodynamic functions derived. 3. Procedure
After a known amount of palladium-silver alloy was admitted, the system was evacuated and the water of the constant temperature supply was degassed by repeatedly freezing and melting under vacuum, until the residual pressure was not greater than 1 x mm. Hg. The water was then kept frozen during the subsequent reducing treatment of the alloy. This treatment consisted of passing a stream of pure hydrogen at atmos-
45. HEATS OF
CHEMISORPTION OF OXYGEN
427
pheric pressure into the system and heating the alloy at 400" for 3 to 4 hrs. The reaction system was then evacuated by applying high vacuum for 24 hrs. After evacuation, repeated checks were made of the static vacuum in order to determine the leak rate of the apparatus and whether hydrogen had been totally withdrawn from the system. Within the precision of the experiment, no gas was evolved even after a period of several days. Subsequently, the alloy was brought to the desired temperature and the water of the constant temperature supply melted and its temperature controlled by surrounding it with a constant-temperature bath. The system was then allowed to come to equilibrium, and the formation and amount. of hydrogen produced were determined as discussed above. Surface areas of palladium and palladium-silver alloys were not measured. They were assumed to be about 2 m.2/g., because it was previously found t
111. RESULTS Since the initial conditions were pH2= 0 and p o z = 0, and pHzo and reaction volume were kept constant during a run, the amount of hydrogen present at equilibrium could be used to compute the corresponding value of 0. This was simply obtained from
where Z is the total number of sites available on the alloy surface and Vsystem and Tsystem have been computed by taking into account the different volumes of the apparatus that are a t different temperatures ( 2 ) . The product pHzVSystem has been expressed in calories. The value of 2 was taken as 1 X Wt(Pd) Wt(Pd)
+ 0.22Wt(Ag)
+ Wt(Ag)
X 1015 x weight of sample X B.E.T. area in cm.'/g.;
this assumes that practically every surface metal atom is a site for oxygen chemisorption. As discussed previously ( 2 ) ,the contribution to 0 from hydrogen chemisorption must be negligible. By means of Equations (2) and (6), values for the equilibrium constant, K , , have been determined for palladium black and palladium-silver alloys of different compositions at various temperatures and are collected in Fig. 2. For every sample, data were obtained a t two different water-vapor pressures. The lines drawn through the experimental points were taken as straight lines, whose slopes were used to com-
428
M. H. BORTNER AND G . PARRAVANO I
10-1
10-2
10-3
10-4
KI 10-5
10-7
10-8
I0-9
10-10 1.5
1.6
1.7
1.9
1.8
IOJT-I
2.1
2.0
(OK-')
FIG. 2. Equilibrium constant of reaction (1) as a function of temperature. % Pd: 100 99 90 70 50 30 10 T H ~= O Oo: 0 0 A 0 0 6 3 0 T q o = 23": 0 . A X
v
+
+
pute the enthalpy change involved in reaction (1). By combining the values so obtained with the heat of formation of HzO(g) from (3), calculated for each temperature from known data (5), the enthalpy change involved in reaction (4) was deduced. Similarly, by means of known values of K2 and the values of K1, determined experimentally, K3was obtained by means of Equation ( 5 ) . The results of these computations are reported in Table I. For comparison, the heat of adsorption for oxygen on silver (2) is also presented in Table I. In Table I, the compositions of the samples (in weight per cent) are reported in the first column. The temperature a t which the surface equilibrium (1) has been measured is shown in the second column. In the third
TABLE I Oxygen Chemisorption on Palladium and Palladium-Silver Alloys Sample composition, wt.%
TBsmple, Twarr, 8 X 10' "K . OK.
Pd
523 523 573 573 673 673
273 295 273 294 273 297
99% Pd-l% Ag
523 523 623 623 673 673
273 295 273 295 273 298
90% Pd-10% Ag
523 523 573 573 623 623 673 673
273 296 273 296 273 295 273 296
507 523 563 563 603 611 658 658
296 273 273
523 523 573 573 623 623 673 673
273 293 273 297 273 296 273 297
533 533
273 297 273 297 273 297
600
600 644 653
294
273 296 273 297
0.350 0.436 0.721 1.72 8.88 14.70
Kt
m , a
kcal./mole
1.3 X 4.2 X 1.9 x 2.6 X 1.5 X 8.3 x
1013 1Ol2 10" 10" 10'0 109
-24.6
2.9 X 1.1 x 8.3 X 3.8 X 6.6 X 2.3 X
1020 1030 10l6 10l6 1Ol6 10l6
-46.8
73.4 84.5 108 151 178 228 229 317
2.1 x 6.6 X 2.1 x 9.5 x 1.0 x 4.0 x 5.1 X 4.6 X
1017 10l6 10'6 1014 1014 1013 10l2 10l2
-46.1
17.0 16.9 31.1 32.8 41.2 53.8 119 1%
1.5 X 1.5 X 2.7 X 1.7 x 3.0 x 5.4 x 4.3 x 9.5 x
10l6 10l6 1Ol6 1014 1013 1012 10'2 10"
-45.1
296 403 547 732 767 1170 1320 1420
4.1 X 2.1 x 5.4 x 2.2 x 2.0 x 1.1 x 1.9 x 4.7 x
1Ol8 10'8 1016 10'6 10" 10'6 1014 1013
-45.0
127 153 195 204 327 412
1.2 X 4.0 x 6.6 X 1.3 X 6.4 x 1.1 x
10" 1017 10'6 10l6 1014 1014
-48.4
3100 3950 4950 6150 6580 7440
430
M. H. BORTNER AND G . PARRAVANO
TABLE I-Continued Sample composition, wt.% 10% Pd-90% Ag
Tasmpie, Twster,
0 X lo4
AH,a
K3
"K.
O K .
523 523 573 573 623 623 673 673
273 297 273 297 273 297 273 296
45.5 71.3 71.6 92.3 78.6 107.0 115 281
2.0 1.1 2.1 6.6 3.7 1.7 3.0 2.7
x x x
473 473 573 573
273 295 273 295
4.3 5.7 6.0 8.2
1.2 4.0 6.2 2.3
kcal./mole --49.1
x X X X
10'6 10'6 1014 1013 1012 10l2 10" 1011
x x x x
10'8 1017 1013 1013
-54.0
X
is expressed in kcal./mole of surface oxide, which is assumed to have the composition PdO or Ag,O.
column are reported the temperatures of the water supply, which controlled the value of the water vapor pressure of the system, and, therefore, 0. The values of 0 are reported in the fourth column, as computed by means of Equation (6). The calculated values of K 3and the heat of chemisorption, AH, for reaction (4) are presented in the fifth and sixth columns. AF and A S values for reaction (4) could be easily computed, but they would be of little significance, because they correspond to different values of 0.
IV. DISCUSSION If the thermodynamic activity of occupied (0) and unoccupied (1 - 0) sites is assumed to be the same, K , is the adsorption equilibrium function for oxygen in the surface involved. Therefore, it is interesting to follow the behavior of K 3as a function of 8. Although values of K 3were determined for only two different values of 8, an inspection of Table I in this regard is
quite instructive. The experimental error on the data on palladium black is probably larger than that corresponding to the palladium-silver alloys, but, to a first approximation, it is permissible to conclude that K 3for palladium black is nearly independent of 8. This indicates that, under present experimental conditions, K3 is a true equilibrium constant and oxygen chemisorption on palladium is localized and noninteracting and occurs on an energetically homogeneous surface. This homogeneity may also be the result of the opposite effects of strong interaction among adsorbate species and surface heterogeneity ( 1 ) . However, it seems that palladium presents in this respect a transition case between those of platinum and nickel.
45.
431
HEATS OF CHEMISORPTION O F OXYGEK
These three metals are chemically grouped in the same column of the periodic system, but the surface aflinity of nickel for oxygen is sharply different from the corresponding affinity of platinum. Indeed, for the nickel-oxygen system the adsorption was found to occur on a homogeneous, interacting type of surface, K3 increasing with 0, while for the platinum-oxygen system, the adsorption was found to occur on a strongly heterogeneous surface, K1 decreasing with 8 (2). The adsorption thermodynamics of the palladium-oxygen system is apparently intermediate between the previous two cases, K3 being nearly constant with 0. This result shows that the chemistry of the metal atoms in question, as determined by their electronic structure and their position in the periodic classification of the elements, is only indirectly involved in determining oxygen adsorption behavior of surfaces. I n all the alloys tested, K 3is found to decrease with increasing 0. This is the behavior shown by silver. It is noteworthy to point out that this effect sets in by alloying as little as 1% of silver with palladium. This strong tendency for the chemisorption process to retain the thermodynamic characteristics of oxygen chemisorption on silver down to dilute amounts of silver is suggestive of the fact that electrons which partake in metal bonding do not contribute greatly to bond formation in oxygen chemisorption. This effect is even more apparent if one compares the values of AH for reaction (4)for the different alloys (Fig. 3). Clearly, the nature of the oxygen surface bond is different from that of the metal-metal bond. Since this latter is thought to be due to hybrid (dsp) orbitals, with a high participation of d electrons in the case of palladium, oxygen chemisorption will not involve d-electron bonds. Probably, it will involve sp electrons only. An alternate
Ag (% by weight)
FIG.3. Effect of alloying with silver on the heat of adsorption of
0 2
on palladium.
432
M. H. BORTNER AND 0. PARRAVANO TABLE I1 Heats of Formation of Bulk and Surface Oxides for Palladium and Silver (in kcal./mole of oxide) Oxide
aH bulka
PdO
-21.0
Ag2O
(1
b
-7.3
AH surfaceb -24.6
-54.0
At 298"K., from L. Brewer, Chem. Revs. 62, 1 (1952). At 500°K.
explanation of the data presented in Fig. 3 would be to assume that silver metal concentrates a t the surface in all the alloys tested. Since the adsorption free energy of oxygen on silver is higher than on palladium (a), this difference should provide enough driving force for establishing the diffusion of silver atoms from the bulk to the surface of the alloy in the temperature range investigated. It is interesting to compare data on the heat of formation of bulk oxides of palladium and silver with values of heats of chemisorption of oxygen on the same metals. This is done in Table 11. An inspection of Table I1 indicates that, while bulk palladium oxide is thermodynamically more stable than silver oxide, the opposite is true for the surface compound. This clearly suggests that great caution should be exercised in deducing thermodynamic properties of surfaces from bulk values.
V. CONCLUSION The method of adsorption and chemical reaction equilibrium has again proved its usefulness in deriving heats of adsorption a t low surface concentration, where the most active sites generally control the energetic behavior of surfaces. In the present case, the application of this method to oxygen chemisorption on palladium and palladium-silver alloys has shown how the different surface affinities of these two metals for oxygen contribute to the resulting adsorption thermodynamics of the alloy. This behavior has given experimental support to the suggestion that oxygen chemisorption on transition metals does not involve direct participation of d electrons. The use of the adsorption equilibrium function has revealed a sharply different behavior for nickel, palladium, and platinum surfaces, with palladium presenting an intermediate case between nickel and platinum. This behavior can be compared with the relative position of these three metals in the periodic classification of chemical elements. Finally, our inability to formulate surface behavior in terms of bulk properties is strikingly demonstrated by the higher stability of the surface
45. HEATS
OF CHEMISORPTION OF OXYGEN
433
oxide film on silver than on palladium. This is in sharp contrast with the known properties of the corresponding bulk phases.
ACKNOWLEDGMENTS This work has been made possible by a grant from the Atlantic Refining Company, the Esso Research and Engineering Company, and the Gulf Research and Development Company to The Franklin Institute Laboratories for fundamental studies in the field of heterogeneous catalysis. This support is greatefully acknowledged.
Received: March 2 , 1956 .
REFERENCES
1. Graham, D., J . Phys. Chem. 67, 665 (1953). 2. Gonzalez, 0. D., and Parravano, G., J . Am. Chem. SOC.78, 4533 (1956).
3. Masing, G., i n “Handbuch der Metallphysik,” Vol. I1 p. 266. Akademische Verlagsges., Leipeig, 1935. 4 . Parravano, G., in press. 6. Spencer, H. M., and Justice, J. L., J. A m . Chem. SOC.66, 2311 (1934); NatZ. BUT.
Standards Circ. 600 (1934).