hydrogen

Rhodium/Pailadium/I.~ydrogenSystem

35

group IV hydrides the reactions in germane are more similar to those an silane as expected. The main differences are the formation of a small amount of Gez+ and the precursors to M2H+ and M2H3+. While CZ type ions are certainly formed in methane,l3 the preference for condensation in the silane and germane systems is replaced by a preference for atomic ion transfer. Furthermore, there is no evidence for product ions in methane larger than the C3 type,13 while in silane and germane very large polymers have been observed. This difference can be accounted for partly on the basis of relative size' and by the difference in the primary ion spectra, i.e., the distribution of potential reactants.21 From the collisional stabilization model, (31) and (32), it is easily shown that the measured second- and thirdorder rate conlitants zre respectively

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1973 | doi: 10.1021/j100620a007

k

1'

= kakd/(kL-a

+ kd)

Thus for the case a: = 3, the ratio

ks/kd

is equivalent to

kze,/k7 = 1.4 X The analogous ratios are the same, within experimental error, for all x in both the germane and silane systems, with the exception of that for the reactions involving SizH7+ in silane for which ks/kd = 3 X 10-ls. In terms of the model, this order of magnitude difference results from a difference in the probability of the two paths open to dissociation of the intermediate. For the exceptional case in silane, dissociation to original reactants is favored over dissociation to products ( k - a >> k d ) while for the others both paths are equally likely, and by the same considerations the lifetime of the intermediates for the latter cases is an order of magnitude longer.

Acknowledgment. This work was supported by the United States Atomic Energy Commission under Contract No. At(l1-1)-3416. We also wish to thank the National Science Foundation for providing funds to assist in the original purchases of the mass spectrometers. (21) D. P. Beggs and F. W. Lampe, J. Chem. Phys.. 49,4230 (1968)

ssure Investigation of the Rhodium/Palladium/Hydrogen System

. Baranowski, S. Majchrzak, and Ted B. Flanagan"' Institute of Physical Chemistry, Poksh Academy of Sciences, Warsaw, Poiand (Received May 3, 1972)

An iinvestigation of the absorption of hydrogen a t high pressures by a series of Rh/Pd alloys up to 80% (atoim) rhodium is reported. These alloys absorb large quantities of hydrogen, e.g., the Rh(30%)/Pd alloy attaiiris an H-to-Pd atomic ratio of 1.44 (H-to-metal atomic ratio of 1.00 f 0.02 a t 5100 atm). This demonstrates that rhodium can act as an absorber of hydrogen when situated in the palladium matrix. Data are extrapolated to the experimental conditions necessary to form the @ phase in the Rh/& system.

Introduction The absorption of hydrogen by palladium has been the subject of many investigations2 and the extension to absorption studies of alloys of palladium has been a natural one. The absorption of hydrogen by silver/palladium alloys bas evoked great interest3-6 because of both practical and theoretical reasons. Silver is adjacent to palladium in the periodic table and evidence is available that it donates electrons to the partially empty d band of palladium.? Although the rigid band model is no longer believed to be fully valid,s silver atoms are still believed to act as electron donors to the empty palladium d bands.9 Models of absorption for the silver/palladium/hydrogen system have been proposed i n which both the hydrogen and silver atoms act as electron donators to the d band of palladium.5 Other alloying elements which have filled d bands have been tested si111ilarly.10~~~ By way of contrast, there is no simple picture available for the interpretation of hydrogen absorption data where the alloying elements also have holes in the d band, e.g., platinum and rhodium. The rhodium /pallaclium/hydrogen system is of particular interest because of rhodium's adjacent position to palladi-

um in the periodic table. Tammann and Rocha12 studied this system by electrolytic charging but their data are of limited usefulness. The first detailed study was performed by Tverdovskii and StetsenkoI3 who investigated this system by removing Chemistry Department, University of Vermont, Burlington, Vt. F. A. Lewis, "The Palladium/Hydrogen System," Academic Press New York, N. Y.. 1967. (a) A. Sieverts and H. Hagen, 2. Phys. Chem., Abf. A, 174, 359 (1935); (b) G. Rosenhall, Ann. Phys. (leipzig), 24, 297 ('19%). A. C. Makrides, J. Phys. Chem., 68, 2160 (1964). H. Brodowsky and E. Poeschel, 1. Phys. Chem. (Frankfurt am Main), 44, 143 (1965). A. Carson and F. A. Lewis, Trans. Faraday Soc., 63, 1453 (1967). N. F. Mott and H. Jones, "The Theory of Metals and Alloys," Clarendon Press, Oxford, 1936. E.g., F. M. Mueller,A. J. Freeman. J. Dimmock, and A. M. Furdyna, Phys. Rev. B, 1, 4617 (1970). J. S. Dugdale and A. M. Guenault, Phil. Mag., 13, 503 (1966). H. Brodowsky and A. Husemann, Ber. Bunsenges. Phys. Chem., 70, 626 (1966). K. Allard, A. Maeland, J. Simons, and T. B. Flanagan, J. Phys. Chem., 72,136 (1968). G. Tammann and H. Rocha, Festchr. Siebertsclr. Piafinschmelze, 3, 213 (1931). I. P. Tverdovskii and A. I. Stetsenko, Dokl. Akad. Nauk SSSR, 84, 997 (1952). The Journalof Physical Chemistry, Vol. 77, No. 1, 1973

B. Baranowski, S. Majchrzak, and T. B. Flanagan

36 TABLE I: Lattice Parametcxs of H-Free Rh/Pd Alloys (-190’) Yo Rh

A

0

3.804

5

3.EEll :3.876 3.871

10

15 20


a,

Yo

Rh

30 4.0 60 80

i

i B

A \

_____-_II___---

a,

A

3.857 3.851 3.833 3.816

3.866

hydrogen anodically from fully charged, dispersed alloys. Absorption isotherm were constructed from electrode potential-current passed relationships. Lewis and coworke r ~ also ~ incestigated ~ , ~ ~ this system electrochemically. Both absorption and dcsorption isotherms were derived up to hydrogen pressures of about 5 atm. The only gas phase study available has been that of Brodowsky and Huseman+ which was limited to the (Rh) 5%/Pd alloy. They found that A I ~ H ’ L, e , the relative partial molar enthalpy at infinite dilution, was less exothermic than that for pure palladium. In keeping with predictions of the “lattice strain -electron donation” model’Q.16 this suggests that the H-H interaction is greater in this alloy than in pure palladium and this is borne out by the datalo and is consistent with the smaller lattice spacings of these alloys compared to piire p a l l a d i ~ m . ~These 7 earlier studies, which were liraitedi tlo pressures 1 5 atm,10%13-1sfound no evidence of any unusual behavior in this system save the fact that the 1 atm hydrogen solubility was somewhat greater in alloys of sabozrt 5% rhodium content than in pure palladium (25”). uite recently the authors noted that several rhodium/ palladium alloys absorb hydrogen rather dramatically at high pressures of gaseous hydrogen.ls For example, at 5100 atm (25”) a valiiie of H-to-Pd atomic ratio = 1.44 f 0.02 (I-1-to-metal ratio of TI =: 1.00 i 0.02) was found for a Rh(30%)/Pd allcy. This proves that rhodium can function as an absorber of hydrogen, a t least, when it is situated within the palladium matrix. The observation of values df n, H-to-metal ratios, p= 1.00 f 0.02 is noteworthy because such a. ratio has riot been directly observed in high-pressure studies of the Pd/& system, i.e., direct analysis has not yielded such a nearly stoichiometric ratio but, of course, a stoichiometric ratio may have been formed within the high-pressure vessel prior to analysis,lS however, the estimated piessure a t which P d H would be formed is greater than that needed for the 20 and 30% rhodium alloys. Oates and FLanagan20 have, by a different technique, recently prepared PdH (n = 0.99 i 0.01) using direct reaction of H atoms, with palladium. The motive of the present research was to extend the previous high pressure studyls to alloys of larger rhodium content in order to be able to predict the pressure at which pure rhodium may absorb hydrogen. Another motive was to attempt to understand why such large hydrogen contents weie found m certain of the Rh/Pd alloys a t relatively low pressuses.

~ x ~ e ~ ~ m ~~?~~~~~ e~taH The reaction vessel and the arrangement suitable for high pressures of gaseaus hydrogen have been described elsewhere.Z* The C O U F S ~of absorption was followed in situ by changes of electrical resistance of the sample as a function of hydrogep pressure. The pressure was determined by the electrical resistance of a manganin wire within the reaction vessel. The Journal of Physical Chenlistry, VO;. 77, No. 1, 1973

Figure 1. Values of R / R o as a function of pressure (25”) for a series of Rh/Pd alloys. The linear solid lines drawn through the data all have the same slope. The region where stoichiometric hydrides are formed for t h e 20 and 30% alloys are indicated along the p axis by arrows. The straight-line segments all have the same slope

The samples were in the form of thin foil, -10 pT which weighed -10 mg. The hydrogen contents of the samples were determined by cooling the high-pressure vessel ( -50”), reducing the pressure, and analyzing the content mass spectrometrically. X-Ray diffraction patterns of the hydrogen-containing samples were determined generally a t -190” in order to avoid loss of hydrogen. The alloys were all face-centered cubic and showed no evidence of any ordering (Table I).

-

Results Since hydrogen contents cannot be measured directly in the high-pressure vessel, electrical resistance measurements were employed to monitor the course of hydrogen absorption. In the special case of rhodium /palladium alloys, it appears that rather definite conclusions can be drawn about the hydrogen content from the behavior of , the electrical resistance-pressure relationships. Figure 1 shows the values of R/Ro, the electrical resistance a t any hydrogen content to the hydrogen-free resistance, as a function of the hydrogen pressure. The relationships all exhibit an initial region where R/Ro is nearly unchanged with pressure (not shown) and absorption does not occur; this is followed by a rapid increase of R/Ro with pressure and thereafter values of R/Ro decline slowly with pressure. The noteworthy feature of this system is that after cooling the vessel (-50”) and analyzing the hydrogen contents of the sample, stoichiometric H-to-metal ratios of 1 were found for the 20 and 30% Rh/Pd alloys in the pressure range between 2335 and 5060 atm which is indicated on Figure 1(Table 11). Since it is known that nearly stoichiometric ratios were formed in the indicated region for the 20 and 30% alloys, it (14) J. Green and F. A. Lewis, Trans. FaradaySoc., 62, 971 (1966). (15) J. Barton, J. Green, and F. A. Lewis, Trans. Faraday SOC., 62, 960 (1966). (16) H. Brodowsky, 2. Phys. Chem. (Frankfurt am Main), 44, 129 (1965). (17) E. Raub, H. Beeskow, and D. Menzel, 2. Metallk., 50,428 (1959). (18) T. B. Flanagan, 8. Baranowski, and S. Majchrzak. J . Phys. Chem., 74,4299 (1970). (19) E.g., B. Baranowski, Platinum Metals Rev., 16,10 (1972). (20) W.A. Oates and T. B. Flanagan, Nature (London), 231,lY (1971). (21) B. Baranowski and W. Bujnowski, Rocz. Chem., 44,2271 (1970).

Rhodium /Palladium/Hydrogen System

37

TABLE I!: Hydr’ogen Contents and Lattice Parameters Observed in Rh/Pd Alloys after Removal from High-pressure Vessel

Alloy,

YORh

I?-to-Pd atam ratio

H-to-metal atom ratio

ao. A . Maximum after Hz pres- removal sure (atm) from attained pressure before vessel cooling (-190’)

2 8

20

5

0.132

20

1.Cil 1.15

0.86 f 0.02 0.82 0.91 0.89 0.92 0.91 0.91

20

l.ii6

1.01

5,100

20 30 30 30 40

1.2!2

0.97

1.34

0.93 1 .OQ 1.01

23,200 2,300 5,100 23,200 2,300

5 5 10 10 15


0.90 O.E6 0.95 O.E9 1.02

1.44 1.4.6

40

1.52 1.51

40

1.52

jL

0.90 0.96 0.90

2,300

5,100 23,200 2,300 21,300 21,300 2,300

5,100 23,200

4.073 4.065 4.079 4.077

5

I:

0 I

a 0,

4.083 4.074

0 -

I O

4.070

4.068 4.065 4.072

can be reasoriabty assumed that comparable behavior in the relationships for other alloys implies that a stoichiometric hydride has also been formed for that alloy. Thus, for the 40% alloy, which gave upon analysis values of n = 0.90 and 0.95 (Table 11) and a further value of 0.90 after reaching 22,900 a h , it can be assumed that a stoichiometric ratio was attained, -4000 atm, but hydrogen was lost prior to analysis. It should be added that for this alloy H-to-Pd atom ratios of 1.52 and 1.61 were found at 2300 and 5100 atin, respectively. The small decrease in the values of Ii/Ro with pressure after attainment of stoichiometry can he attributed to the effect of pressure upon the electrical resistance of the hydride. For a pressure change from 10,000 to 20,000 atm Bridgman22 noted a decrease in R/Ro of 2% (Pd) and 1.6% (Rh). The observed changes are in the right direction to be attributed to a purely hydrostatic pressure effect but are somewhat srrialler, -1%. This is not unreasonable because the hydride % o d d be expected to behave differently from the pure metals or alloys. Thus, the presence of hydrogen in paiiadiuni decreases Young’s modulus23 and increases its liaidness.24 The slopes of the region in the R/Ro against pressure relationships corresponding to purely pressure efFects are comparable for the 20 and 30% alloys which suggests that similar slopes for the other Rh/Pd alloys cam be attributed to purely pressure effects and therefore a stoichiometric hydride has been formed. Ratios of E := 1 were noted by analysis to correspond to ratios of R/Ro slightly before the onset of the linear region in the R/& against p relationship. Of course, the term stoichiometric hydride refers to the analysis and undoubtedly unfilled interstitial sites exist when n = 1 f 0.02 and these unfilled sites, which introduce disorder into the system, would be expected to contribute significantly to the electrical resistance For example, in the pure palladium/ FIz system at n = 0.015, R/Ro = 1.05 (25”).25If a comparable effect o~-ccursi n these alloy systems, the small difference observed in values of R/Ro at the “stoichiometry” ratio and i a t the onset of the linear region would correspond to An = 0006. Et is suggested that stoichiometry may, in fact, only occur where the R/Ro against p relationships become linear. On this basis, the pressures at

0 5

I

0

I I

L

I

1 12

I 3

14

i I S

6

R/Ro Figure 2. Absorption and desorption relationships observed the 20 and 30% Rh/Pd alloys (25”).

for

TABLE I I I: Estimated Pressures where Stoichiometric Hydrides Are Formed (25’)

Yo Rh

Pressure, atm

0

>24,000 22,000 f 2,000 19,000 16,000 10,000

5 10 15 20

YO Rh

Pressure, atm

30

7,000

40 60

4,000 18,000

80

> 30,000

which stoichiometric hydrides are formed can be estimated from Figure 1 and are given in Table 111. It is of interest that the pressure at which a stoichiometric hydride forms exhibits a minimum at about the Rh(4O%)/Pd alloy. Figures 2 and 3 show values of R/Ro recorded against log p during absorption and desorption of hydrogen for several alloys. A marked hysteresis can be noted indicating the presence of a two-phase a-/3 region in analogy with the behavior of the Pd/H2 system.2 The hysteresis extends over many thousands of atmospheres of pressure in the alloys of higher rhodium content. The two-phase nature of these systems was confirmed in some cases by the X-ray diffraction patterns of the hydrogen-containing alloys. The fortunate combination of the relatively low-pressure range needed for the formation of the p phase and the large hysteresis helps to retain the hydrogen in the 20 and 30% Rh/Pd alloys prior to their analysis. By contrast, the Rh(4O%)/Pd alloy appeared to lose some of its hydrogen and the 60 and 80% Rh/Pd alloys lost all of their hydrogen prior to analysis. (22) P W Bridgman, Daedalus, 77,187 (1949) (23) F Kruger and H Jungnitz, 2 Tech Phys , 17,302 (1936) (24) T Sugero and H Kowaka, Mem lnst S o Ind R e s , Osaka U n r v , 11, 119 (1954) (25) T 8 Flanagan and F A Lewis, 2 PhyS Chem (Frankfurt am Marn), 27, 104 (1960) The Journal of Physical Chemistry, Vol. 77, No. 1, 7973

B. Baranowski,S. Majchrzak, and T. B. Flanagan

38

3olc

o

A

10

'0

.--. 20 E X

c

CY u

e

t

J.


I

4B /

x,0.9

1.1

R/

R.

1.5

I .3

Figure 3. Absorption and desorption relationships observed for the 60 and 80% Rh/Pd alloys (25'). A ternary phase diagram is shown in Figure 4. Again the marked contrast with previously examined binary palladium alloy/Hz systems can be noted. The hydrogen contents have been estimated from the lattice parameters corresponding to the ct and (3 phases and these estimations should be accurate to n = zk0.05.zs From the behavior shown in Figure 4 there is no reason to suspect the pure rhodium will no:, absorb hydrogen to form a hydrogen-rich p phase, although attempts made in this laboratory to date have failed.

iscussion In marked contrast to the behavior of the other Pd binary alloy/Ha systenis which have been studied,Z the lattice parametw of the /3 phase hydrides of the Rh/Pd alloys increase with Rh content at least for those with Rh contents be1o.w 50%. This is consistent with the large hydrogen contents observed in these alloys. Values of AGo--a0, Le., the Gibbs' free energy change for the reaction Ilz(1atm) (Rh/Pd/Hz), (Rh/Pd/Hz)s can be estimated. from the fugacity at which the relative resistance rises steeply with pressure (Figure 2). Although data are available for the fugacity-pressure relationship only to about 3000 atm,2' the relationship can be extrapolated further28 if the constants determining the relationship are assumed to be independent of pressure. It is of interest .that on this Elasis a pressure of 21,000 atm equals 109 atm in. fugacity.l9 A plot of AG,,a" against percentage of rhodium in the alloy is shown in Figure 5 together with the data of Green and Lewis for the 2.7, 7.1, and 10.0% Rh/Pd alloys. The data of the two studies are in good agreement. It may be anticipated from Figure 5 that pure rhodium will form a second rionstoichiometric (3 phase at log f = 1.2f25") or p = 35,000 atm (25"). Since this pressure exceeds the range of the present high-pressure equipment, it is desirable to find alternative experimental conditions for the synthesis of 9 phase Rh/& A value of AHo,,' can be estimated from the extrapolated value of f, 1012 atm, at 298°K and the observation that the average value of AS,.-bLjn for various face-centered-cubic metals and alloys

+

The .io?"

-

of Pbys/ca/ Chemistry, Voi. 77, No. 1 , 1973

Figure 4. Ternary phase diagram for the Rh/Pd/W system (25') as compared to several others systems: Ag/Pd/H (ref 5 ) ; Pt/ Pd/H (A. Maeland and T. B. Flanagan, J. Phys. Chem., 68, 1419 (1964)); A u / P d / H (A. Maeland and T. 13. Flanagan, ibid., 69, 3575 (1965)). Since the 60 and 80% Rh/Pd alloys form ,5 phases, the immiscibility gap can be extended as shown by the dotted lines. The concentration variables XU, XPd, and X M are atomic fractions of hydrogen, palladium, and added metal, respectively.

'%16

-a -I

"0.

2

Q O

0

20

I 40

I

Rh%

60

)O

80

Figure 5. Relationship between AG, 0' and percentage Rh as determined from the fugacity at which the values of R/Ro increase rapidly during absorption: e , ref 14: 0, present data. is -25 + 5 cal/deg mol H Z . On ~ this basis, a value of AH,,@" of 8800 f 1500 cal/mol HZ can be estimated. A pressure of 30,000 atm ( f = loT1 atm) can be obtained in the present apparatus and at this pressure the temperap ture at which absorption corresponding to the a transformation should occur is 83 f 12". An attempt is being made to prepare (3 phase Rh/Hz under these conditions. One problem connected with the attempted synthesis of this hydride is that the diffusion coefficient of hydrogen in rhodium is probably very small. One of the au-

-

(26) B. Baranowski, S. Majchrzak, and T. B. Flanagan, d. Pbys. F, 1, 258 (1971). (27) W. DeGraaff, Thesis, University of Amsterdam, 1960. (28) B. Baranowski, unpublished results.

Rhodlicim/Pi~llaciium/t-lydrogenSystem

39

A

A

I

I

60

I

I 80

I


R h% Figure 6. Relationship between t h e value of R/Ro observed when f u r t h e r absorption of hydrogen has apparently ceased (25") and % R h in alloy.

thors and his coworker29 have measured the diffusion coefficients in Rh/Pd alloys and it was observed that in passing from pure palladium to Rh(4O%)/Pd the diffusion coefficient declines by a factor of 1000. Oates and McLell.an30 have recently round a value of 12.8 kcal/mol of Hz for the relative partial molar enthalpy of absorption of hydrogen by rhodium at high temperatures and small hydrogen contents. The difference in this value and that observed, for example, in the Ni/H2 systems0 is only about 4 kcal and therefore it i s not unlikely that rhodium will absorb hydrogen. The difference in the value at infinite dilution p transition is 4.0 rt 1.5 and that estimated for the a kcai/mol of Us.The increase in the exothermicity in passfl ing from a vanishing small concentration to the a transition is due mainly to the H/H attractive interaction in the Pd/H2 system. It can be assumed that a similar effect operates in the Flh/H2 system. Andersen31 has recently calculated a value of 1.27 holes in the d band for rhodium in contrast to the value of 0.37 for If rhodium does indeed absorb hydrogen, it can be assumed. that this large number of holes in the d band is a factor. Switendick and coworkers33 have recently reported that some of the electrons from hydrogen in p Pd/H2 occupy sp states below the d bands of palladium. I t will be of extreme interest to ascertain what such band calculations will give for the @ Rh/H2 system.

-

-

The electrical resistance-pressure relationships (25") observed here require some comment because of the formation of stoichiometric hydrides in several of the alloys. It is surprising that the values of R/Ro (Figure 1) did not decline to very small values when stoichiometry was obtained. In the Pd/H2 system, values of €?/no fall to values below 1 at very high pressures (Figure 1).Such behavior is not unexpected because complete order exists when stoichiometry is attained. The residual resistivity due to scattering by impurities should vanish at this point. The fact that the metallic alloy matrix has itself disorder should not alter this argument because values of R/Ro are measured and both R and Ro contain the disorder inherent in the alloy. The values of H/Ro in the pressure range where the resistance changes are apparently caused only by pressure increases, and not by absorption of hydrogen, increase steadily in passing from pure palladium to Rh(80%)/ Pd (Figure 6). In fact, the relationship is closely linear between alloy content and the limiting value of R/Ro up to Rh(40%)/Pd. The changes of R/Ro with pressure during the desorption of hydrogen (Figure I) is similar to the trends noted by Lewis and coworkers for low content Rh /Pd and Ni/Pd all0ys,14.~5i.e., the values of R/Ro increase to greater values during the desorption cycle than were observed during absorption.

Acknowledgments. The authors wish to thank Dr. H. T. Weaver of Sandia Laboratory for kindly supplying us with the 60 and 80% Rh/Pd alloys. We also wish to thank Professor Dr. E. Wicke for the Rh(l5%)/Pd alloy. We are grateful for experimental assistance by Mr. S. Filipek and Mr. M. Krukowski. One of the authors (T. R. F.) is grateful for his appointment as a participant in the exchange program between the National (U. S.) and Polish Academies of Sciences and for the hospitality of the Polish Academy of Sciences (Warsaw). We are grateful. to Dr. A. 6 . Switendick of Sandia Laboratory for informative discussions of the band structure of rhodium. (29) D. Artman and T. 8.Flanagan, research to be submitted for publication. (30) W. A. Oates and R. McLellan, results to be submitted for publication. (31) 0. K. Andersen, Phys. Rev. 6,2,883 (1970). (32) J. J . Vuillemin and M. G . Priestley, Phys. Rev. Lett., 14, 307 (1965). (33) D. E. Eastman, J. Cashion, and A. C . Switendick, Phys. Rev. Lett., 27,35 (1971).

The Journal of Physical Chemistry, Vol. 77, No. 7, 7973

hydrogen

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