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Coexistence of hydrogen and carbon solutes in the palladium lattice
Journal of the fess-common
Metals, 131 (1987)
COEXISTENCE OF HYDROGEN IN THE PALLADIUM LATTICE*
157 - 162
AND CARBON
157
SOLUTES
S. B. ZIEMECKI, G. A. JONES and D. G. SWARTZFAGER Central Research and Development Department, Experimental de Nemours and Company, Wilmington, DE 19898 (U.S.A.)
Station, E. I. du Pont
(Received May 5,1986)
Summary Carbon atoms can penetrate into the palladium lattice, forming interstitial solid solutions up to PdCO. i5. No P-PdH is detected on exposure of this phase to H,. However, a continuous series of ternary PdC,H, (0 < x < 0.15, 0 < y < 0.65) can be obtained for samples containing lesser amounts of carbon. The quantity of hydrogen accommodated in such systems depends only on the carbon concentration, and not on the phase homogeneity.
1. Introduction An interstitial solid solution of carbon in palladium can be formed when metallic palladium is exposed to a flow of carbon-containing gases at moderate temperatures. The occurrence of the Pd-C phase in the atmosphere of ethylene, acetylene, or carbon monoxide has been described in an earlier paper from this laboratory [l]. Almost simultaneously Stachurski and Frackiewicz [23 reported formation of the Pd-C phase during hydrogenation of acetylene over palladium catalysts. Formation of the PdC0_15 phase results in a total suppression of &PdH on a subsequent exposure to hydrogen [l, 33. The effect of addition of alloying components to palladium on the hydrogen solubility is well established [4, 51. However, most of the published work involves substitutional solid solutions, and boron is the only interstitial solute studied over a wide range of concentrations [6 - 91. The published absorption isotherms in PdB0.0617 show a decreased capacity of the lattice for hydrogen [7], and a total suppression of the P-PdH has been found for 16 at.% of boron in palladium [ 61. An analogous relation between the hydrogen solubility and the amount of carbon accommodated in the palladium lattice has been reported [lo], and recently Stachurski [ll] presented absorption isotherms in carburized palladium catalysts, showing a decreased capacity for *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuis~n, France, May 25 - 30,1986. 0022-50&3/87/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
hydrogen absorption. The purpose of this paper is to present additional evidence for the coexistence of hydrogen and carbon as interstitial solutes in the palladium lattice, and to address the problems of the mechanism of carbon migration, and phase homogeneity in the ternary system Pd-C-H.
2. ~x~rimen~l
details
Palladium black (Aesar9 Cat. 12066, 100 - 300 nm), and palladium foil (Aesar, Cat. 11517, 0.1 mm thick) were used in the experiments. The PdCO, I5 phase was produced by heating the palladium black samples in a flow of ethylene at 573 K. Lower concentrations of carbon in p~ladium were obtained by a controlled decomposition of the PdCIJ.15 phase by programmed heating in the Hz atmosphere (5% Hz-N2 and 8.5% Hz-He). Similarly, PdC,,.rs was produced by exposing palladium foil to ethylene for 100 h at 575 K, and PdC,,09 by exposure for 50 h, followed by annealing in an inert atmosphere at the same temperature. In situ X-ray diffraction studies were carried out in the microreactor incorporated in the Rigaku 8-8 diffractometer, as described previously [12]. An independent evaluation of hydrogen solubility was provided by dynamic me~urement.s of the hydrogen uptake during the cooling cycle, and its evolution during the pro~~med heating cycle (temperatureprogammed deso~tion (TPD)) [13]. ~nte~ation of the peak area corresponding to the uptake or evolution of hydrogen, recorded in a calibrated system, yields the [H]/[Pd] ratio. The depth profiles of carbon in palladium foils were obtained by ion scattering (ISS) with a mixed He-Xe beam at an incident ion energy of 2.0 keV. Electrical resistivity of Pd-C foils was measured by the four-points method in the range 5 - 300 K.
3. Results The process of formation and decomposition of the PdCe,Is phase is illustrated in Fig. 1. It shows changes in the X-ray diffraction pattern of palladium black, heated in a flow of ethylene to achieve the transition palladium to PdC, (curves a - d), and then in air (curves e and f) to restore palladium. The distribution of the carbon concentration in palladium is observed during charging of carbon in the hydrocarbon atmosphere (curve c), and discharging on heating in air (curve e). A similar behavior was found during heating of the PdC o,15 phase in hydrogen (see Fig. 2). Curve b in Fig. 2 was recorded during temperature-programmed heating in 8.5% Hz--He. Pd-C was still the major component of the two-phase system when the (111) region was being scanned, but palladium seems to dominate by the time the (200) region is reached. Subsequent heating to 700 K in &-He leaves only p~ladium (curve c).
159
/
30
40
35
-i..
45
SCATTERING
.-
50
ANGLE
Fig. 1. X-ray diffraction patterns of palladium black, recorded in a flow of ethylene: curve a, at 298 K; curve b, at 370 K; curve c, at 423 K; curve d, at 475 K; and in air: curve e, at 373 K; curve f, at 475 K. C* represents the diamond standard.
1
1
35
I
38
I
j
I
41 SCATTERING
1
44 ANGLE
/
I
47
-I_
4 5(
Fig. 2. Changes in the X-ray diffraction pattern of PdCe.,s recorded during its decomposition in a flow of 8.5% Hz-He: curve a, 300 K in Hz-He; curve b, on a ramp 323 - 548 K in Hz-He; curve c, 300 K in hefium, after preheating to 700 K in Hz-He.
In a separate series of experiments, a sample of PdCO, 15was exposed to temperature-programmed excursions in a fIow of 8.5% Hz-He. After reaching each of the predetermined temperatures the sample was cooled, and the X-ray diffraction pattern recorded, still in Hz-He, at 298 K. The results are shown in Fig. 3. The @-PdH formation was not observed for the PdC0.15 (curve a) nor after preheating to 473 K (curve b). The shift of peak positions to lower 28 values, and peak asymmetry, was found after further preheating to 548 K (curve c), and a P-PdH formation typical for palladium was observed after the 623 K run (curve d). A similar cycle of experiments was of performed on the PdC,.Os foil [lo], using TPD for the measurement hydrogen uptake. The values of [H] /[ Pd] , determined after successive ramps to increasingly higher temperatures, were as follows: 0.025 (no heating), 0.21 (493 K), 0.38 (573 K), 0.53 (683 K) and 0.55 (873 K).
160
I
35
I
38
I
I
41 SCATTERING
I
I
44
I
I
47
I
50
ANGLE
Fig. 3. X-ray diffraction patterns, recorded in a flow of 8.5% HrHe at 298 K, for a sample of PdC0,,5 (curve a), exposed in sequence to programmed heating in HrHe to: curve b, 473 K; curve c, 548 K; curve d, 623 K.
The formation of PdCo.is is more time consuming in palladium foils. A homogeneous carbon distribution was probably achieved after 100 h exposure of the 0.125 mm foil to a flow of ethylene at 575 K (no &PdH was detected by TPD, a single-phase was detected by X-ray diffraction, and a constant [C]/[Pd] ratio was detected by ISS, probed over about 400 nm sputter depth). Shorter exposures at lower temperatures permitted us to probe the depth profile of carbon in palladium in two-phase samples. After sputtering through a carbonaceous layer on the surface, a region of constant [C]/ [Pd] ratio (evaluated as 0.1 to 0.15) was observed for the depth proportions to the time, or temperature, of exposure to hydrocarbon. This was followed by a decay of carbon concentration (see Fig. 4(a)). Similarly, the PdCO.is samples, heated in Hz-He to remove part of the interstitial carbon, show a negative carbon gradient near the interface followed by a region of constant [C] /[ Pd] , and finally a decreasing carbon concentration (see Fig. 4(c)). The non-homogeneous foil prep~a~ons cont~ning PdC and palladium can be equilibrated by annealing. The X-ray diffraction pattern of such a sample before annealing (curve a), and after annealing (curve b) in a flow of helium for 65 h at 575 K is shown in Fig. 5. Heating in an inert gas does not destroy the Pd-C phase and little change was found in the capacity for hydrogen absorption, as measured by TPD: the value of [H]/[Pd J = 0.32 was found before the annealing and 0.35 after the annealing. The consequences of the annealing process are represented schematically in Fig. 4(b). This interpretation is further supported by measurements of electrical resistivity. In two-phase samples annealing changes the p us. T characteristic in the direction of partial restoration of the metallic palladium behavior. For example, the resistivities of 1.7 X lo-’ and 3.6 X 10U6 (at 300 and 5 K respectively) are shifted to 1.3 X 10m5 and 0.7 X 10B6 52 cm after annealing. It is of interest to note that the ratio ppdc/pw, measured at 300 K for nonannealed samples having various amounts of carbon, varies only in a narrow range between 1.55 and 1.77.
161
0
depth
(orb )
depth
(arb)
Fig. 4. Schematic distribution of carbon in palladium (c) discharging. In each case tz > tl.
SCATTERING
ANGLE
during: (a) charging; (b) annealing;
Fig. 5. X-ray diffraction pattern of a foil containing Pd-C and palladium before annealing in helium and curve b, after annealing in helium.
phases: curve a,
4. Discussion The Pd-C phase, distinct by its properties, is formed when palladium is heated in an atmosphere of ethylene, acetylene or carbon monoxide. The process of phase transformation involves deposition of a carbonaceous layer on the palladium surface, followed by an activated diffusion of carbon atoms through the palladium lattice. The position and occupancy of the carbon sites were de~rmined by Rietveld re~nement of the neutron diffraction data [ 11, yielding carbon atoms in the octahedral holes and the formula PdCe_ is. The lattice parameter converged at 0.39956 nm. The lattice constant of the Pd-C phase is, however, somehow dependent on sample preparation, and probably reflects a variability in carbon concentration and distribution. This is illustrated, for example, in Fig. 1: the shape and position of peaks observed under the non-equilibrium conditions should be noted. Thus, carbon migration seems to be the rate-limiting step in the formation and decomposition of the PdCa. i5 phase. A mechanism of carbon migration, based on observations described in Section 3, is proposed in Fig. 4. In the presence of a carbon source (see Fig. 4(a)) a diffused front of the carbon-rich phase moves into the bulk of the sample. The carbonaceous overlayer assures the constant supply of carbon, and provides the driving force for migration. Similarly, in the presence of a carbon sink such as Hz
162
or O2 (see Fig. 4(c)), a negative gradient of carbon at the interface was found by ISS, and decomposition of the PdC a_is proceeds without removal of the carbon-rich subsurface layer, leaving a palladium core surrounded by the PdC layer. If this picture is correct, it implies an enhanced mobility of carbon in Pd-C, compared with its mobility in palladium. A total suppression of &PdH formation in the PdCO.is phase is well documented [I - 31, and a systematic increase of the hydrogen absorption with a decreasing carbon content has been observed [lo, 111. Thus, there is evidence that ternary solid solutions PdC,H, (where y increases from 0 to about 0.65 when x decreases from 0.15 to 0) can exist in the whole range of concentrations. It is tempting to write, as it has been done for other solutes, that [H]/[Pd] + rr [C]/[Pd] = constant. However, the exact values of [Cl/ [Pd] are known only at the extremes of the range (on that basis the n value must be close to 4). Clearly, more indexing of the x: and y parameters in the ternary system is needed. However, the relation between the lattice parameter and carbon concentration has been derived for [C]/[Pd] < 0.03 [14], and can only be assumed to continue linearly. In addition, we have shown that the X-ray measurements on non-equilibrated samples (especially foils) do not give the true carbon content owing to the presence of a PdCo_r5 layer near the surface. The method of determination of [C]/[Pd] by measurement of methane produced during decarburization [Z ] neglects a substantial quantity of carbon deposited on the palladium surface. The total amount of carbon present in the palladium lattice, and not its spatial distribution, is the dominant factor in the formation of ternary PdC-H systems. The effect of a given carbon concentration on the hydrogen absorption seems to be the same in homogeneous PdC, samples after annealing, and in non-homogeneous ones containing PdCa.,, and palladium phases. This argues for the electronic, and against the steric, mechanism of the described effect. References 1 S. B. Ziemecki, G. A. Jones, D. G. Swartzfager, R. L. Harlow and J. Faber, J. Am. Chem. Sot., 107 (1985) 4547. 2 J. Stachurski and A. Frakiewicz, J. Less-Common Met., 108 (1985) 249. 3 S. B. Ziemecki and G. A. Jones, J. Catal., 95 (1985) 621. 4 F. A. Lewis, Platinum Met. Rev., 26 (1982) 20,70,121. 5 E. Wicke and H. Brodowsky, in G. Alefeld and J. Volkl (eds.), Hydrogen in Metals II, Top. Appl. Phys., 29 (1978) 73. 6 R. Burch and F. A. Lewis, Trans. Faraday Sot., 66 (1970) 727. 7 H. Husemann and H. Brodowsky, 2. Naturforsch. A, 23 (1968) 1693. 8 H. Brodowsky and H.-J. Schaller, Ber. Bunsenges. Phys. Chem., 80 (1976) 656. 9 R. Mehlmann, H. Husemann and H. Brodowsky, Ber. Bunsenges. Phys. Chem., 77 (1973) 36. 10 S. B. Ziemecki, React. Solids, 1 (1986) 195. 11 J. Stachurski, J. Chem. Sot. Faraday Trans. I, 81 (1985) 2813. 12 R. D. Srivastava, A. B. Stiles and G. A. Jones, J. Catal., 77 (1982) 192. 13 S. B. Ziemecki, G. A. Jones and J. B. Michel, J. Catal., 99 (1986) 201. 14 R. H. Siller, R. B. McLellan and M. I. Rudee, J. Less-Common Met., 18 (1969) 432.
Metals, 131 (1987)
COEXISTENCE OF HYDROGEN IN THE PALLADIUM LATTICE*
157 - 162
AND CARBON
157
SOLUTES
S. B. ZIEMECKI, G. A. JONES and D. G. SWARTZFAGER Central Research and Development Department, Experimental de Nemours and Company, Wilmington, DE 19898 (U.S.A.)
Station, E. I. du Pont
(Received May 5,1986)
Summary Carbon atoms can penetrate into the palladium lattice, forming interstitial solid solutions up to PdCO. i5. No P-PdH is detected on exposure of this phase to H,. However, a continuous series of ternary PdC,H, (0 < x < 0.15, 0 < y < 0.65) can be obtained for samples containing lesser amounts of carbon. The quantity of hydrogen accommodated in such systems depends only on the carbon concentration, and not on the phase homogeneity.
1. Introduction An interstitial solid solution of carbon in palladium can be formed when metallic palladium is exposed to a flow of carbon-containing gases at moderate temperatures. The occurrence of the Pd-C phase in the atmosphere of ethylene, acetylene, or carbon monoxide has been described in an earlier paper from this laboratory [l]. Almost simultaneously Stachurski and Frackiewicz [23 reported formation of the Pd-C phase during hydrogenation of acetylene over palladium catalysts. Formation of the PdC0_15 phase results in a total suppression of &PdH on a subsequent exposure to hydrogen [l, 33. The effect of addition of alloying components to palladium on the hydrogen solubility is well established [4, 51. However, most of the published work involves substitutional solid solutions, and boron is the only interstitial solute studied over a wide range of concentrations [6 - 91. The published absorption isotherms in PdB0.0617 show a decreased capacity of the lattice for hydrogen [7], and a total suppression of the P-PdH has been found for 16 at.% of boron in palladium [ 61. An analogous relation between the hydrogen solubility and the amount of carbon accommodated in the palladium lattice has been reported [lo], and recently Stachurski [ll] presented absorption isotherms in carburized palladium catalysts, showing a decreased capacity for *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuis~n, France, May 25 - 30,1986. 0022-50&3/87/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
hydrogen absorption. The purpose of this paper is to present additional evidence for the coexistence of hydrogen and carbon as interstitial solutes in the palladium lattice, and to address the problems of the mechanism of carbon migration, and phase homogeneity in the ternary system Pd-C-H.
2. ~x~rimen~l
details
Palladium black (Aesar9 Cat. 12066, 100 - 300 nm), and palladium foil (Aesar, Cat. 11517, 0.1 mm thick) were used in the experiments. The PdCO, I5 phase was produced by heating the palladium black samples in a flow of ethylene at 573 K. Lower concentrations of carbon in p~ladium were obtained by a controlled decomposition of the PdCIJ.15 phase by programmed heating in the Hz atmosphere (5% Hz-N2 and 8.5% Hz-He). Similarly, PdC,,.rs was produced by exposing palladium foil to ethylene for 100 h at 575 K, and PdC,,09 by exposure for 50 h, followed by annealing in an inert atmosphere at the same temperature. In situ X-ray diffraction studies were carried out in the microreactor incorporated in the Rigaku 8-8 diffractometer, as described previously [12]. An independent evaluation of hydrogen solubility was provided by dynamic me~urement.s of the hydrogen uptake during the cooling cycle, and its evolution during the pro~~med heating cycle (temperatureprogammed deso~tion (TPD)) [13]. ~nte~ation of the peak area corresponding to the uptake or evolution of hydrogen, recorded in a calibrated system, yields the [H]/[Pd] ratio. The depth profiles of carbon in palladium foils were obtained by ion scattering (ISS) with a mixed He-Xe beam at an incident ion energy of 2.0 keV. Electrical resistivity of Pd-C foils was measured by the four-points method in the range 5 - 300 K.
3. Results The process of formation and decomposition of the PdCe,Is phase is illustrated in Fig. 1. It shows changes in the X-ray diffraction pattern of palladium black, heated in a flow of ethylene to achieve the transition palladium to PdC, (curves a - d), and then in air (curves e and f) to restore palladium. The distribution of the carbon concentration in palladium is observed during charging of carbon in the hydrocarbon atmosphere (curve c), and discharging on heating in air (curve e). A similar behavior was found during heating of the PdC o,15 phase in hydrogen (see Fig. 2). Curve b in Fig. 2 was recorded during temperature-programmed heating in 8.5% Hz--He. Pd-C was still the major component of the two-phase system when the (111) region was being scanned, but palladium seems to dominate by the time the (200) region is reached. Subsequent heating to 700 K in &-He leaves only p~ladium (curve c).
159
/
30
40
35
-i..
45
SCATTERING
.-
50
ANGLE
Fig. 1. X-ray diffraction patterns of palladium black, recorded in a flow of ethylene: curve a, at 298 K; curve b, at 370 K; curve c, at 423 K; curve d, at 475 K; and in air: curve e, at 373 K; curve f, at 475 K. C* represents the diamond standard.
1
1
35
I
38
I
j
I
41 SCATTERING
1
44 ANGLE
/
I
47
-I_
4 5(
Fig. 2. Changes in the X-ray diffraction pattern of PdCe.,s recorded during its decomposition in a flow of 8.5% Hz-He: curve a, 300 K in Hz-He; curve b, on a ramp 323 - 548 K in Hz-He; curve c, 300 K in hefium, after preheating to 700 K in Hz-He.
In a separate series of experiments, a sample of PdCO, 15was exposed to temperature-programmed excursions in a fIow of 8.5% Hz-He. After reaching each of the predetermined temperatures the sample was cooled, and the X-ray diffraction pattern recorded, still in Hz-He, at 298 K. The results are shown in Fig. 3. The @-PdH formation was not observed for the PdC0.15 (curve a) nor after preheating to 473 K (curve b). The shift of peak positions to lower 28 values, and peak asymmetry, was found after further preheating to 548 K (curve c), and a P-PdH formation typical for palladium was observed after the 623 K run (curve d). A similar cycle of experiments was of performed on the PdC,.Os foil [lo], using TPD for the measurement hydrogen uptake. The values of [H] /[ Pd] , determined after successive ramps to increasingly higher temperatures, were as follows: 0.025 (no heating), 0.21 (493 K), 0.38 (573 K), 0.53 (683 K) and 0.55 (873 K).
160
I
35
I
38
I
I
41 SCATTERING
I
I
44
I
I
47
I
50
ANGLE
Fig. 3. X-ray diffraction patterns, recorded in a flow of 8.5% HrHe at 298 K, for a sample of PdC0,,5 (curve a), exposed in sequence to programmed heating in HrHe to: curve b, 473 K; curve c, 548 K; curve d, 623 K.
The formation of PdCo.is is more time consuming in palladium foils. A homogeneous carbon distribution was probably achieved after 100 h exposure of the 0.125 mm foil to a flow of ethylene at 575 K (no &PdH was detected by TPD, a single-phase was detected by X-ray diffraction, and a constant [C]/[Pd] ratio was detected by ISS, probed over about 400 nm sputter depth). Shorter exposures at lower temperatures permitted us to probe the depth profile of carbon in palladium in two-phase samples. After sputtering through a carbonaceous layer on the surface, a region of constant [C]/ [Pd] ratio (evaluated as 0.1 to 0.15) was observed for the depth proportions to the time, or temperature, of exposure to hydrocarbon. This was followed by a decay of carbon concentration (see Fig. 4(a)). Similarly, the PdCO.is samples, heated in Hz-He to remove part of the interstitial carbon, show a negative carbon gradient near the interface followed by a region of constant [C] /[ Pd] , and finally a decreasing carbon concentration (see Fig. 4(c)). The non-homogeneous foil prep~a~ons cont~ning PdC and palladium can be equilibrated by annealing. The X-ray diffraction pattern of such a sample before annealing (curve a), and after annealing (curve b) in a flow of helium for 65 h at 575 K is shown in Fig. 5. Heating in an inert gas does not destroy the Pd-C phase and little change was found in the capacity for hydrogen absorption, as measured by TPD: the value of [H]/[Pd J = 0.32 was found before the annealing and 0.35 after the annealing. The consequences of the annealing process are represented schematically in Fig. 4(b). This interpretation is further supported by measurements of electrical resistivity. In two-phase samples annealing changes the p us. T characteristic in the direction of partial restoration of the metallic palladium behavior. For example, the resistivities of 1.7 X lo-’ and 3.6 X 10U6 (at 300 and 5 K respectively) are shifted to 1.3 X 10m5 and 0.7 X 10B6 52 cm after annealing. It is of interest to note that the ratio ppdc/pw, measured at 300 K for nonannealed samples having various amounts of carbon, varies only in a narrow range between 1.55 and 1.77.
161
0
depth
(orb )
depth
(arb)
Fig. 4. Schematic distribution of carbon in palladium (c) discharging. In each case tz > tl.
SCATTERING
ANGLE
during: (a) charging; (b) annealing;
Fig. 5. X-ray diffraction pattern of a foil containing Pd-C and palladium before annealing in helium and curve b, after annealing in helium.
phases: curve a,
4. Discussion The Pd-C phase, distinct by its properties, is formed when palladium is heated in an atmosphere of ethylene, acetylene or carbon monoxide. The process of phase transformation involves deposition of a carbonaceous layer on the palladium surface, followed by an activated diffusion of carbon atoms through the palladium lattice. The position and occupancy of the carbon sites were de~rmined by Rietveld re~nement of the neutron diffraction data [ 11, yielding carbon atoms in the octahedral holes and the formula PdCe_ is. The lattice parameter converged at 0.39956 nm. The lattice constant of the Pd-C phase is, however, somehow dependent on sample preparation, and probably reflects a variability in carbon concentration and distribution. This is illustrated, for example, in Fig. 1: the shape and position of peaks observed under the non-equilibrium conditions should be noted. Thus, carbon migration seems to be the rate-limiting step in the formation and decomposition of the PdCa. i5 phase. A mechanism of carbon migration, based on observations described in Section 3, is proposed in Fig. 4. In the presence of a carbon source (see Fig. 4(a)) a diffused front of the carbon-rich phase moves into the bulk of the sample. The carbonaceous overlayer assures the constant supply of carbon, and provides the driving force for migration. Similarly, in the presence of a carbon sink such as Hz
162
or O2 (see Fig. 4(c)), a negative gradient of carbon at the interface was found by ISS, and decomposition of the PdC a_is proceeds without removal of the carbon-rich subsurface layer, leaving a palladium core surrounded by the PdC layer. If this picture is correct, it implies an enhanced mobility of carbon in Pd-C, compared with its mobility in palladium. A total suppression of &PdH formation in the PdCO.is phase is well documented [I - 31, and a systematic increase of the hydrogen absorption with a decreasing carbon content has been observed [lo, 111. Thus, there is evidence that ternary solid solutions PdC,H, (where y increases from 0 to about 0.65 when x decreases from 0.15 to 0) can exist in the whole range of concentrations. It is tempting to write, as it has been done for other solutes, that [H]/[Pd] + rr [C]/[Pd] = constant. However, the exact values of [Cl/ [Pd] are known only at the extremes of the range (on that basis the n value must be close to 4). Clearly, more indexing of the x: and y parameters in the ternary system is needed. However, the relation between the lattice parameter and carbon concentration has been derived for [C]/[Pd] < 0.03 [14], and can only be assumed to continue linearly. In addition, we have shown that the X-ray measurements on non-equilibrated samples (especially foils) do not give the true carbon content owing to the presence of a PdCo_r5 layer near the surface. The method of determination of [C]/[Pd] by measurement of methane produced during decarburization [Z ] neglects a substantial quantity of carbon deposited on the palladium surface. The total amount of carbon present in the palladium lattice, and not its spatial distribution, is the dominant factor in the formation of ternary PdC-H systems. The effect of a given carbon concentration on the hydrogen absorption seems to be the same in homogeneous PdC, samples after annealing, and in non-homogeneous ones containing PdCa.,, and palladium phases. This argues for the electronic, and against the steric, mechanism of the described effect. References 1 S. B. Ziemecki, G. A. Jones, D. G. Swartzfager, R. L. Harlow and J. Faber, J. Am. Chem. Sot., 107 (1985) 4547. 2 J. Stachurski and A. Frakiewicz, J. Less-Common Met., 108 (1985) 249. 3 S. B. Ziemecki and G. A. Jones, J. Catal., 95 (1985) 621. 4 F. A. Lewis, Platinum Met. Rev., 26 (1982) 20,70,121. 5 E. Wicke and H. Brodowsky, in G. Alefeld and J. Volkl (eds.), Hydrogen in Metals II, Top. Appl. Phys., 29 (1978) 73. 6 R. Burch and F. A. Lewis, Trans. Faraday Sot., 66 (1970) 727. 7 H. Husemann and H. Brodowsky, 2. Naturforsch. A, 23 (1968) 1693. 8 H. Brodowsky and H.-J. Schaller, Ber. Bunsenges. Phys. Chem., 80 (1976) 656. 9 R. Mehlmann, H. Husemann and H. Brodowsky, Ber. Bunsenges. Phys. Chem., 77 (1973) 36. 10 S. B. Ziemecki, React. Solids, 1 (1986) 195. 11 J. Stachurski, J. Chem. Sot. Faraday Trans. I, 81 (1985) 2813. 12 R. D. Srivastava, A. B. Stiles and G. A. Jones, J. Catal., 77 (1982) 192. 13 S. B. Ziemecki, G. A. Jones and J. B. Michel, J. Catal., 99 (1986) 201. 14 R. H. Siller, R. B. McLellan and M. I. Rudee, J. Less-Common Met., 18 (1969) 432.