The Palladium-Hydrogen System A SURVEY OF HYDRIDE FORMATION AND THE EFFECTS OF HYDROGEN CONTAINED WITHIN THE METAL LATTICES By F. A. Lewis

’l’he QUCCII’S Ilnibersitv of Belfast

Although it can by no means be wholly 4 wry substantial amount of separated from the fields of adsorption and additional information has hen catalysis (I), and although the term “transition puhlishrd conrerning hydridm of the metal hydrides” has been applied to include platinum group metals owr thc tuo molecular complexes of transition metals which decades since the hydrides of include hydrogen in their structures (z jthe palladium and palladium alloys itere subject area of metal-hydrogen systems seems tiit. subjvct of an earlier review article most generally accepted to be concerned with in this .journal. In addition to the the effects of the subsurface incorporation of rnarw articlcs in the general literature, hydrogen within metal lattices, either in solid the subject matter has formed a solution, or in the formation of hydride phases. major part of thc. programmt’.r v,f With regard to hydrogen systems of the six swcral scientific conferencm and of a platinum metals and their companion elements, number of books and monographs iron, cobalt and nickel, in terms of such a rough appearing over this period. Further- demarcation of area of interest, studies of the more, .silvw-palladiumdiffusion tubes hydrides of palladium and its alloys have con- arc’ incorporated into hvdrogen tinued to dominate the field of investigation generators built hr Johnson 8Matthey, over the interval since an initial review article and utiiised for such diverse applica- on these systems in Platinum Meials Review (3). tions as thr hydrogvnntion of f>dible Publication of an expansion (4) of this review oils, innnufarturf.of serniconductors, coincided quite closely with the appearance of a anitealing o/ stainless stet1 and the collection of articles (5) commemorating the cooling o*f power station altrrnators. centenary of the initial observations of the In viru o~f the considerable intwest absorption of hydrogen by palladium; and with being shown in both thrur&d and the appearance of other books concerning tcichnical aspwts of thrsp systems this metal-hydrogen systems, which generally have uncisirallv long rwipir is presented. included information about various aspects of and will br. publishd in parts during the palladium-hydrogen system (6-1 I). the war. Subsequently the palladium-hydrogen area of research has formed the major part of the Over recent years there has been a substan- programmes of relatively small and informal tial expansion of research activity in the field of conferences in Europe (12), which later study of metal-hydrogen systems. In addition to developed into a series of more extensive and intrinsic academic interests, this expansion has comprehensive meetings (I 3-1 6). An important also stemmed from aspects of technological series of review articles concerning metal- importance such as the effects of hydrogen on hydrogen systems has been published quite the mechanical properties of metals, and of the recently (I 7), including one dealing specifically purification and storage of hydrogen to be used with aspects of palladium- and palladium alloy- as a general chemical reagent or as a fuel. hydrogen systems to which further reference

Platinum Metals Rev., 1982, 26, (l), 20-27 20 will be made below. Significant numbers of Determining the Hydrogen papers concerning the palladium-hydrogen and Contents of Palladium and allied systems also have been included in programmes of other conferences centred on Palladium Alloys metal-hydrogen systems in which the major In all aspects of measurements, the problems emphases have been those of low temperature of being able to obtain as exact as possible a phenomena (18) embrittlement and fracture, or knowledge of the hydrogen content of the speci- of hydrogen utilisation and storage (I 9). mens of palladium and palladium alloys have lhe various studies of the palladium- continued to be most important considerations. hydrogen system have been concerned both For measurements involving establishments with improvements and extensions of of equilibria with gaseous hydrogen, contents knowledge concerning experimental parameters have continued to be calculated from accurately which had already been extensively studied, as determined changes of volume and pressure well as with new types of experimental (20-24) and have quite recently been extended measurements. Significant increases also have up to very high pressure ranges (25) which been made in the quantity and variety of infor- present particular experimental difficulties. mation concerning various palladium alloy- Direct measurements of weight changes also hydrogen systems; and this has seemed to be a have been applied relatively recently in such valuable source of guidance with regard to pressure-composition studies (26,27). Deter- possibilities of, and/or explanations of hydride minations of hydrogen content from direct formation by the remainder of the platinum weighings of specimens also recently have been group and by other transition metals. applied conveniently in measurements of

Platinum Metals Rev., 1982, 26, (1) 21 physical properties-particularly in studies a palladium-coated quartz crystal which can be with relatively large specimens which can be related to changes of weight. conveniently handled and removed from their For certain studies however it has continued connections as, for example, in certain studies to be impractical continuously to monitor related to structural, elastic and anelastic hydrogen contents in the course of experi- properties (28-34). mental measurements. Examples include Changes of hydrogen content and measurements at very low temperatures and particularly continuous changes during experi- with particularly high contents of hydrogen ments also have continued to be determined which, in addition to electrolytic techniques from conjoint measurements of a suitable discussed below, have been introduced under parameter which is an already known function high pressures or else by ion implantation of the content. Such parameters have included techniques (46). In these cases the contents of equilibrium pressure (35), dimensional changes hydrogen during the measurements have been (36), integrals of time dependent functions of estimated by reduction of standard solutions of electrode potential (37-39) and electrical oxidising agents (37,47) or by vacuum outgass- resistivity (37-40). ing at the conclusion of experiments Electrical resistivity measurements have con- (4,3 I ,33,45,46,48,49). For this to be possible, tinued to be found useful for following the the hydrogen contents of such specimens have kinetics of absorption and desorption of to be temporarily maintained by an inhibition hydrogen (41-43) but in gas phase studies (44) a of the surface kinetics, and this may be piezoelectric technique (45) has been recently deliberately aided by surface platings with applied in which changes of content have been metals such as copper or gold (50),or in some followed by variations in vibration frequency of instances by adsorption of iodine

Platinum Metals Rev., 1982, 26, (1) 22 (33,40,5 1-53), which has the advantage of surface activity can be obtained by the being quite readily removed again. temporary formation of an easily reducible oxide layer, produced either by anodisation or Electrolytic Studies Involving relatively low temperature heating in oxygen Aqueous or Alcoholic Solutions (4,39,53). Often a highly active surface can be Electrolytic methods have continued to be attained more reliably and conveniently by the used in a number of instances as convenient electrodeposition of a thin layer of palladium means both of hydrogen introduction and also black (4,37-39,47,62-65). Even thin layers for effecting gradual reductions of hydrogen have been found to retain a sufficiently high contents (34,55). Surface conditions of very activity for a whole series of experiments different levels of catalytic activity for the for- (47,63) over which hydrogen can be absorbed mation and dissociation of hydrogen molecules several times to P-phase concentrations followed have been employed in the various studies. by its removal in oxygen-saturated solutions. For specimens into which it may have been Examples of results with such specimens of desired to introduce and retain high contents of studies of relationships between current hydrogen, either no surface activation proce- densities of electrolysis, i, and the hydride com- dure at all has been employed (34), or indeed ponent of hydrogen overpotential are shown in inhibitors (56) of surface hydrogen combination Figure ~(c).Values of this component (qJ of reactions have been added to the electrolyte overpotential have been calculated (4,39) from (3,40,45,50,57-60). Electrolysis in alcoholic extrapolations of plots against time of open solutions (49,61) at quite low temperatures has circuit electrode potentials and electrical effected the introduction of particularly high resistivities following interruption of hydrogen contents into palladium. In the case electrolysis, such as those shown in Figure I(b). of such catalytically inhibited surfaces, it does The linear plots in Figure I(C) against the func- not seem to be possible (4,39,45) accurately to tion log i + i, are diagnostic of a control of v2 relate components of electrode potential with by the rate of transport of evolving hydrogen hydrogen chemical potential, although estima- molecules from the cathode-and limiting tions of hydrogen chemical potentials within upper values of vz are found to correspond with surface-inhibited electrodes of palladium alloys the onset of general hydrogen bubble evolution have been derived from measurements of (4,39) which for specimens coated thickly with electrical resistivity (39,62). A series of studies palladium black, have shown a strong of the open circuit potential of palladium dependence on the acid strength of the electrodes following cathodisation in solutions catholyte (39,62). These findings have led to the containing surface inhibitors however, has development of explanations based on initial been published recently (60). Nevertheless it has discharge of hydrogen at preferred sites and on been a continued finding in electrochemical an importance of the role of local cell hydrogen studies (4,39) that electrode potentials, which transfer processes in equalising surface can he reliably equated with hydrogen chemical hydrogen chemical potentials (39,62). Con- potentials, are only measureable under experi- sideration of these concepts also has been mental conditions where surface kinetics are found to provide explanations of the extents of sufficiently rapid for the transport of dissolved electrolytic separation of hydrogen and molecular hydrogen to and from the electrode deuterium isotopes by palladium electrodes and surface to be kinetically governing the values of of the activity of palladium surfaces for isotope electrode potential. This conclusion appears to exchange reactions with the surrounding solu- be endorsed by recently reported results of tion (39,53). For example, somewhat higher studies in a series of acidified alcohols (63) in efficiencies in these regards have been related to which particularly high hydrogen solubilities apparent decreases of the electrical resistivities can be attainable. Such a sufficiently high of palladium wires in acidic catholytes, of which

Platinum Metals Rev., 1982, 26, (1) 23 an example is illustrated in Figure r(a), and cells, as a means of hydrogen storage (45,661 which have been associated (4) with conditions and of hydrogen isotope separation (67). of low impedances for local cell hydrogen transfer processes. Electrolysis with Platinum and Considerations of various aspects of surface High Platinum Alloys: Blistering heterogeneity have received increasing atten- Plots in Figure i(cj correspond with the tion in regard to studies of the kinetics of finding that for catalytically highly active hydrogen absorption by palladium from the gas cathode surfaces, the upper limiting values of phase (I ,43) and have also produced interest the hydride component of overpotential (39,54,64) as to the extents to which total corresponding to the general onset of hydrogen hydrogen overpotentials may contain contribu- bubble evolution effectively limits hydrogen tions associated with high hydrogen chemical chemical potentials to equivalent hydrogen pres- potentials at the initial discharge sites. Allied sures of about IOO atmospheres. The reduction interest also has continued in the general area of surface activity by inhibitors as mentioned of study of low voltage hydrogen concentration above, can however lead to development of cells, analogous in operation to surface local internal hydrogen chemical potentials

Platinum Metals Rev., 1982, 26, (1) 24 equivalent to some thousands of atmospheres hydrogen diffusion coefficient measurements in (39,5062). In the case of platinum at the substrates (86,89), and determination of temperatures near 25T, studies with Pd-Pt Pressure-composition (p-n) relationships (38). alloys (45,50,68,69) have indicated that even such high pressures are likely to introduce only Equilibrium Hydrogen Pressure very small contents of hydrogen into the versus Composition Relationships platinum bulk lattice. These low contents have Several further studies of equilibrium however been sufficient to permit calculations hydrogen pressure (hydrogen chemical poten- of hydrogen diffusion coefficients either by per- tial) against composition (p-c(n)) relationships meation techniques (57) or from dilation- of the palladium-hydrogen(deuterium)(tritium) induced “diffusion-elastic” bending effects system have been reported in recent years for produced by hydrogen entry at one side of specimens in a wide variety of forms, including: metal strips (58,59). Figure 2 shows, however, powders (23,25,27,35), sponge (90), evaporated that in addition to the small “reversible” and supportdispersed films (91-93), pellets (35), bending associated with bulk lattice hydrogen chips, strips, foils and sheets (2 1,22,24,26,27, solubility, there is an accompanying irreversible 94), wires, rods and bars (20,24,36). distortion of platinum strips. This is associated In continuation of an important earlier with the development of “blisters” (59) result- series, a further extensive sequence of pn ing from equivalent high pressures of hydrogen (hydrogen:palladium atomic ratio) relationships gas developed within voids. Figure 2 also has been reported by Frieske and Wicke (27) at illustrates how the irreversible distortion, and temperatures up to 300T and pressures of up accompanying blistering (59), is reduced by to 140 atmospheres, over the course of increases alloying additions of palladium-which also (absorption) and decreases (desorption) of lead to increases in the reversible component of hydrogen content. Examples of the desorption the bending, associated with complementary series are shown in Figure 3(a), and a further higher lattice hydrogen solubilities (50,68,69). extension up to 350°C has very recently been reported by Wicke and Blaurock (27). Studies in Catalytically Active, Hydrogen this range of temperatures and pressures are of Transmitting Palladium Layers interest in regard to estimation of the critical In addition to providing an activated surface temperature and pressure with respect to for palladium itself, electrodeposited layers of regions of hydrogen content where CY and p palladium black also have been effectively hydride phases coexist and a related set of pn applied to further series of palladium alloys isotherms in good agreement have been derived (4,37,70-83) in order to provide an inter- by de Ribaupierre and Manchester (36). Also in mediate region for transmission of hydrogen at this range of experimental parameters measure- closely similar chemical potentials to the subs- ments have been reported by Picard, Kleppa trate, Direct electroplating of palladium on to and Boureau (22) at pressures up to 34 atm, at other hydride-forming metals is complicated by 245-352‘C. Another study of p-n relationships the presence of oxide layers, although for was made for Pd9,Ag,,H, (26) where an example an efficient contact to zirconium (84) analogous region of seeming criticality for Q seems to have been effected by subsequent high and p phase coexistence occurs over lower temperature annealing. Alternatively layers of temperature and pressure ranges (4,37,95-98). palladium or palladium alloys have been con- tacted to niobium and tantalum by shock In the, next issue of thrs journal Dr. Lewis’ impact (85) or by vacuum evaporation after reciew of the relntionship between eqidrbrium flash cleaning (86-88). These palladium over- pressure nnrl rompmition mill be concluded, layers, on which a further layer of palladium nnd other arperts of the hydrogen-palladium black may then be deposited, have enabled system mill then he introdured and considered.

Platinum Metals Rev., 1982, 26, (1) 25 References

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Platinum Metals Rev., 1982, 26, (1) 27