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The Palladium-Hydrogen System a SURVEY of HYDRIDE FORMATION and the EFFECTS of HYDROGEN CONTAINED WITHIN the METAL LATTICES by F

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The Palladium-Hydrogen System a SURVEY of HYDRIDE FORMATION and the EFFECTS of HYDROGEN CONTAINED WITHIN the METAL LATTICES by F 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,
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