Contents...Of more general interest are irreversible hydrogenations, of which nitrobenzene...

UK ISSN 0032-1400

PLATINUM METALS REVIEW

A quarterly survey of research on the platinum metals and of developments in their application in industry

VOL. 28 JULY 1984

Contents Some Novel Electronic Effects in Hydrogenation Catalysis

Glassy Alloys Containing Platinum Group Metals

Monitoring Thermocouple Usage

The Published Platinum Metal Alloy Systems

The Catalytic Etching of Platinum and Rhodium-Platinum Gauzes

Palladium-Nickel Plating

Combustion in Wood-Burning Stoves

Osmium Doping Improves Recording Media

Electrodeposition of Palladium-Silver Alloys from Ammoniacal Electrolytes

A History of Thermal Analysis

Nineteenth Century Platinum Coins

Abstracts

New Patents

NO. 3

Communications should be addressed to The Editor, Platinum Metals Review

Johnson Matthey Public Limited Company, Hatton Garden, London E C l N 8EE

Some Novel Electronic Effects in Hydrogenation Catalysis T H E SIGNIFICANCE OF ACIDITY AND REDOX POTENTIAL

By J. W. Jenkins Johnson Matthey Group Research Centre

Recent work at the Johnson Matthey Research Centre has thrown fresh light upon the electronic effects in hydrogenation catalysis that underlie the whole nature 0.f the catalytic act and in particular many o,f the effects that may manifest themselves as support metal interactions. The results show that currently accepted mod& o,f the catalytic act are incomplete, that ionic intermediates are sometimes important and that both the relative acidity 0.f the reaction system and thp redox potential of products and intermediates must be considered.

In the two most commonly accepted models for a hydrogenation reaction, one or both of the reactants are adsorbed on the surface, then reaction takes place and the product desorbs. These models are referred to as Eley-Rideal and Langmuir -Hinshe lwood mechanisms, respectively. Such models frequently give an excellent representation of experimental results, but the parameter values so obtained cannot be confidently extrapolated in a quantitative manner from one reaction system to another.

In the Eley-Rideal and Langmuir- Hinshelwood models two types of parameter are important: adsorption coefficients representing the stabilities of the surface intermediates, and reaction rate constants representing the reactivity of the adsorbed intermediates. Naturally, one expects that these parameters are not uniquely determinable, but are in fact related, which is to say that the reactivity is in part influenced by the strength of adsorption. Thus, for example, in a recent international symposium on support-metal interactions ( I ) it was shown (2) that the ratio of adsorption coefficients for toluene and benzene derived from competitive hydrogenation via a Langmuir-Hinshelwood mechanism varied markedly with the apparent electronic state of the catalytic metal species when this was varied by changing the catalyst support or by using

ammonia or hydrogen sulphide as catalyst modifiers. The changes in relative overall reactivity of these two reactants were, however, much smaller than expected since the increases in relative adsorptivity were offset by reduced relative reactivity.

Similar problems arise in the understanding of the all-important effects of alloying in bimetallic catalyst systems and the effects of metal dispersion on reactivity and selectivity.

In studying catalytic hydrogenation reactions we are accustomed to using the terms, “hydrogenation” and “reduction” syn- onymously. However, in electro-organic chemistry and other branches of chemistry “reduction”, is given a more limited definition as meaning the addition of electrons to a substrate. In such a context hydrogenation involv- ing the addition of a molecule of hydrogen becomes instead the addition of two electrons and two protons. Such additions can in principle be either sequential or in combination.

As a result of the considerations outlined above, we have studied a number of hydrogenation reactions in which the sequential addition of electrons and protons could be observed. In electro-organic chemistry, reactions of this type are customarily investigated by cyclic voltammetry in which the potential of the working electrode is controlled and changed

98 Platinum Metals Rev., 1984, 28, (3), 98-106

systematically. We chose instead to use a powdered carbon supported hydrogenation catalyst as our working electrode and to lower the reduction potential by the addition of hydrogen. These changes in the reduction potential of the reacting system are monitored by an inert gold sensor electrode operated in conjunction with a silver/silver chloride reference electrode. The experimental technique is very similar to that developed by Russian workers (3, 4) although the way we have used this technique differs in many other aspects, as will become apparent later.

Experimental Conditions Hydrogenations were performed in an

aqueous liquid phase system at ambient temperature and atmospheric pressure. The three necked 500ml reaction flask was fitted with a standard glass electrode, a silver/silver chloride reference electrode and a gold wire sensor electrode to follow the potential of the catalyst. The hydrogen was sparged in at a rate of too mumin and vented through a bubbler acting as a gas seal. The reaction flask contents were stirred by a magnetic stirrer at 600 rpm. Typically, 0.1 g 1 0 per cent palladium on carbon catalyst was used. Under these conditions the reactions studied are under gas mass transfer control where the rate is independent of the weight of catalyst used. Such conditions would normally apply in commercial applications of these catalysts. The reaction flask contents were buffered with appropriate combina- tions of sulphuric acid, acetic acid, sodium bicarbonate or sodium hydroxide. Standard phosphate (pH 7), citrate (pH 5.7) and borax (pH 9) buffers were also used.

In operation we customarily first reduced the slurried catalyst and equilibrated the reaction system. The measured redox potential at equilibrium in general was in good agreement with the reversible hydrogen potential for that particular reaction pH. To this equilibrated system we then added a known amount of the substance to be hydrogenated and followed the change in redox potential with time. Typically, upon adding the reactant the potential

immediately rose and then began to fall at a decreasing rate. Eventually, however, the potential falls at an increasing rate and then very rapidly back to the original hydrogen equilibrium potential. The net result is a reduction wave whose half wave potential we can measure and the total time taken to complete a reduction can also be measured.

Results and Discussion Quinone Hydrogenat inn

Our interest in the quinone hydrogenation system was two-fold. First, different quinones have known and well-defined half-wave reduction potentials that are reversible and rapidly established. Figure I shows the variation in redox potential with time when a mixture of various quinones was added to the palladium catalysed reaction system at pH 7. Several reduction waves are observed which correspond to the known half wave reduction potentials of

Platinum Metals Rev., 1984, 28, (3) 99

the different quinones injected. The figure also shows the reversible nature of these reductions in that a mirror image is obtained if, at the completion of the hydrogenation, the oxidation potential is increased by adding air. It is important to note that the reduction of a particular quinone does not commence until all those with a higher potential have been completely hydrogenated. That is to say, the reduction is under complete control of the reduction potential of the system. Similar results have been reported recently in the commercially important hydrogenation of ethyl anthraquinone which is an intermediate in the manufacture of hydrogen peroxide ( 5 ) . Here it was found, in a competitive hydrogenation experiment using both tetrahydro-2-ethyl- anthraquinone and 2-ethyl-anthraquinone, that the former was completely hydrogenated before the latter began to be hydrogenated in line with their relative redox potentials. Perhaps an even more dramatic illustration of redox potential control is shown in Figure 2. In this series of experiments we first hydrogenated a known amount of anthraquinone and to this reaction mixture then added varying amounts of benzoquinone. If less than an equivalent stoichiometric amount of benzoquinone was added, the only result seen was a rehydrogena- tion of the anthraquinone. If greater than a stoichiometric amount of benzoquinone was added, then reduction waves of both the excess benzoquinone and the stoichiometric quantity of anthraquinone produced by the dehydrogen- ation of anthrahydroquinone are seen.

Our second interest in the quinone system was in finding out what would happen at a high pH when hydroquinone, which is a weak acid (pK, 10.35), dissociates to give an anion and at even a higher pH a di-anion. Thus:

ti 0 0 0-

0 0 H

Platinum Metals Rev., 1984, 28, (3)

0 H

100

If, therefore, we “hydrogenate” quinone at a high pH we might form the di-anion by simple electron transfer without incur- ring the necessary kinetic penalty of having to add molecular hydrogen.

That this is indeed the case is 0 -

Q

1 Scheme I

Electrochemical Possibilities for Quinone Hydrogenation

Electron addition - Proton Q Q- Q'

addition I QH+ (QH)' QH- QH2'+ (QH2)' QH2

indicated by the results shown in Figure 3. Here we see the variation in reduction time at various pH values. It will be recalled that since we are under hydrogen mass transfer control, we should have expected that changing the pH would not have changed the reduction time. The marked changes observed and in particular the very high rates at the higher pH show that the reaction is apparently under electron transfer control. Adding acid upon the completion of reaction at the high pH gave no further reaction, showing that indeed reduction was completed much faster at the high pH under this electron transfer control. We are therefore proposing that the hydrogenation of quinone is better represented in the nomenclature of electro-organic chemistry, as shown in Scheme I , in which the possible intermediates include radical anions and radical cations in addition to the normally assumed half hydrogenated neutral radical (QH)'.

One aspect of such a scheme is that we might expect the reaction mechanism and inter-

mediates involved to be pH dependent, a low pH favouring a possible cationic, and a high pH an anionic intermediate. Furthermore, in the presence of an active palladium hydrogenation catalyst we have the following equilibration (6)

H + + e- = HA,)

suggesting that the electronic potential is inversely related to the acidity or proton concentration.

Thus variations in the palladium catalysed hydrogenation rate shown for benzoquinone in Figure 3 are interpreted as shown in Scheme 2 .

Nitrobenzene Hydrogenation The quinone hydrogenation results discussed

above represent a somewhat special case because these hydrogenations are reversible and can yield ionic products. Of more general interest are irreversible hydrogenations, of which nitrobenzene hydrogenation is an example having commercial significance. Since nitrobenzene is only slightly soluble in water, these experiments were performed in a 50 per cent CHJOH/water solvent.

Very pronounced and well defined reduction waves were obtained upon injection of the nitrobenzene substrate, as shown in Figure 4. However, there was little variation in this case in hydrogenation time, as the pH was changed; but there were marked changes in the potential shift of the half wave reduction potential. These results are best summarised in a potential/pH plot, as shown in Figure 5.

The first point of interest in Figure 5 is that the observed half wave potentials are much less

Scheme 2

Effect of Acidity on Quinone Hydrogenation Rate

Hydrogenation is increasingly facilitated by electron transfer

Hydrogenation is increasingly restricted by proton transfer

Hydrogenation is no longer limited by proton transfer but is increasingly facilitated by electron transfer

o < pH < 3

3 < pH < 9

pH > 9

intermediate pH range 3 to 7 the slope is -59mV and this in turn corresponds to an intermediate to which equal numbers of protons and electrons have been added. Again, in the formalism of Scheme I , this corresponds to a neutral free radical or to a species commonly assumed to represent the half hydrogenated state. At yet higher pHs above pH 7, the slope again changes to only -29 mV which corresponds to the addition of two electrons for every proton and the anion of Scheme I .

We thus see that in the case of nitrobenzene hydrogenation, even though the rate of reaction did not markedly change with pH as in the case of the benzoquinone, we would seem to have

than the reversible hydrogenation potential. This can be calculated thermodynamically from the known change in thermodynamic free energy as being +826 mV relative to the reversible potential for I atmosphere hydrogen using the Nernst expression:

AG = -nF AE

the inference being that these well defined half wave potentials represent hydrogenation intermediates formed reversibly on the surface of the catalyst. Of particular interest, however, are the relative changes in these half wave potentials as the pH changes as given by the slopes of the curves shown in Figure 5. At a low pH, below about three, this slope approximates to - I I 8 mV, which is what we should expect from a species formed by the addition of two protons for each electron. In the Scheme I presented earlier, such an intermediate corresponds to a radical cation. In the


clear evidence of the existence of both anionic and cationic charged reaction intermediates. This could affect reaction selectivity, stereo- chemistry or poison sensitivity of the catalyst.

Hydrogenat ion of 2-butyne-1,4-diol The palladium catalysed hydrogenation of

butyne-diol is of interest because it proceeds in two quite clearly defined sequential stages. The butyne-diol is first selectively hydrogenated to butenediol and this hydrogenation is virtually complete before the butene-diol is then hydrogenated to the butane-diol. This selectivity

is generally ascribed to the much stronger absorption of the butynediol relative to the butene-diol with the consequence that the butenediol intermediate hydrogenation product is desorbed and virtually excluded from the catalyst surface. We were curious to see whether a case might be made for this selectivity being due to redox potential control, much in the way as we saw earlier is the case in the selective hydrogenation of the different quinones. The reversible redox potential for the first step of this reaction (est. 625 mV) is considerably higher than for the second (430 mV).

Our results are shown in Figures 6, 7 and 8. In all cases two sequential reduction waves are clearly in evidence and we can determine both


the reduction half wave potentials (Figure 7) and the reduction times (Figure 8) for each sequential stage. In all cases, however, the reductions are irreversible and the half wave potentials considerably less than those corresponding to the reversible reaction.

Somewhat surprisingly, we note that the reduction half wave potential is higher at a low

Concentration x 1 O5 moles

0

1 .o

2.0

2.7

3.0

pH for the second stage of reaction than for the first. We are not able, therefore, to invoke a simple explanation of relative redox potentials to explain the reaction selectivity as we did for the quinone hydrogenation. The difference between the two systems lies in the reversibility of the quinone hydrogenation in contrast to the irreversibility of the butyne and butene-diol hydrogenation.

As we discussed earlier in the case of nitrobenzene hydrogenation, we can obtain some information on the nature of the surface intermediates from a consideration of the slope of the curves of redox potential versus pH. When we do this (Figure 7) we find that the half wave reduction potentials for butyne-diol parallel the hydrogen curve over the whole pH range, indicating that the active intermediate is an orthodox half hydrogenated neutral radical species. For the triple bond hydrogenation we might speculate that this could be some form of ally1 radical. Neutral alkyl radicals are also indicated for butene-diol hydrogenation at a high pH. Under acidic conditions (pH < 7), however, the slope increases and again a radical cation is suggested as becoming the surface intermediate. We shall return to this point later. At a high pH the half wave potential for formation of the allylic surface radical is higher than

Pb : Pd mole ratio

0

0.06

0.12

0.1 6

0.18

Selective Lead Poisoning o f a Palladium Catalys~

C,' + c,=

0.4 g 5% Pd/C paste

Lead added

c,- + c,o

Normal acetic acid (DH 2.3) 0.2 g 2-butyne-1 ,4-diol

Reduction time minutes

14

17

20

25

3 4

11

16

24

7 0

>180

Half wave potential mV R.H.E.

+145

+140

+125

+90

+60

C,' + c,o

+195

+165

+110

+30

+20

that for the alkyl surface radical, that is to say the former is more stable in line with what we might expect from a consideration of the relative free radical stabilities.

The reduction times for the different reduction stages are correlated in Figure 8. Although for a given pH the alkene hydrogenation is in all cases more rapid than that for the alkyne, the relative difference changes with pH. In particular we note that the rates in both cases go through a maximum (that is reduction times are a minimum) as the pH is varied, but that the optimum pH for alkyne hydrogenation is lower than that for alkene hydrogenation. Such maxima in the rate are implicit in our Scheme I , given the relative promoting effects of proton and electron concentration and the inverse relationship between them.

Effect of Lead Poisoning on Palladium Catalyst Selectivity

In catalysts of the Lindlar type, lead acetate is used to selectively poison a palladium catalyst. Such a poisoned catalyst may still be effective for hydrogenating triple bonds and yet is ineffective for hydrogenating double bonds. We were therefore interested in studying the effect of lead addition to our palladium catalysed butyne-diol hydrogenation system. The lead was added as a very dilute solution of lead nitrate to the reduced catalyst slurry. We found no apparent effect of these modest lead additions either at a low or a high pH. In the pH range 2 to 5, however, marked effects were observed, as shown in the Table. Although the addition of lead decreased the half wave reduction potentials and increased the time taken for reduction for both the first and second stages of the butyne-diol reduction, the effect on the butenediol hydrogenation was much greater. So much so, in fact, that with the addition of 0. I 8 moles lead per mole palladium reduction was not completed within the three hour duration of the experiment.

The lead poisoning of a palladium catalyst is generally assumed to be due to an alloy containing metallic lead on the palladium surface. However, in our conditions

(pH 2.3 [Pb’’] - I O - ~ M ) the redox potential for metallic lead formation at - I 50 mV relative to the reversible hydrogenation potential is considerably more cathodic than we can achieve in our reaction system. It only becomes zero at or above pH 5 (7). Indeed we ascribe the absence of a poisoning effect above pH 5 to deposition of metallic lead and conversely the selective poisoning of the palladium surface to the adsorption of a divalent lead cation. Remembering that, as we showed earlier, the surface intermediate in butenediol hydrogenation is a cationic species under these low pH conditions we conclude that it is the more favourable adsorption of the dicationic lead ion that poisons the sites normally active in forming the monocationic butene-diol hydrogenation intermediate. The relative reductions in the stabilities of these intermediates are reflected in the relative reductions in their half wave redox potentials.

Conclusions It has been our purpose in this article to

indicate the effectiveness of the electrochemical technique when applied to a real working catalytic system. We think it is important to recognise the distinction that must be made between electronic reduction and hydrogenation even in the case of simple double bond saturation reactions. We have stressed compara- tive studies of compounds having different redox potentials and studies made under different acidity conditions. Although these conditions are relatively easy to specify in liquid phase hydrogenation systems, we believe that the same parameters apply in gas phase hydrogenation reactions and that if we are to understand properly support-metal interactions both the acidity and redox potential of the whole catalytic system must be taken into account. Our results indicate that in some conditions ionically charged reaction intermediates occur on the catalyst surface and that at least in the case of a lead poisoned palladium catalyst this has been invoked to explain the observed selective poisoning of the mono olefin hydrogenation reaction.


The ability to quantify the free energy of formation of the surface reaction intermediates by means of the half wave reduction potential presents us with a powerful tool for comparing catalysts of different metals and the effects of alloying and multimetallic composite catalysts.

Finally, there is much useful background information in this area contained in a recent review (8) and in the results obtained by Russian workers who originated and developed the electrochemical tichnique that has been publicised by D. V. Sokolskii (3, 4). Particularly we would refer the reader to the latter for information on how different metals affect the half wave reduction potential and for some of the linear free energy correlations between half wave reduction potentials and activation energy on the one hand and the effects of electrophilic benzene substituents in hydrogenation substrates on the other.

References I “Studies in Surface Science and Catalysis; I I .

Metal-Support and Metal-Additive Effects in Catalysis”, ed. B. Imelik et al., Elsevier, Amsterdam, 1982 Tran Mahn Tri, J. Massardier, P. Gallezot and B. Imelik, ibid., pp. 141-148 D. V. Sokolskii, “Hydrogenation in Solutions”, Akademii Nauk Kazahkskoi SSR, Alma-Ata I 962, translated by the Israel Program of Scientific Translations, Jerusalem, I 964 ‘Catalysis. Heterogeneous and Homogeneous”, ed. B. Delmon and G. Jannes, Elsevier, Amsterdam, 1975 T. Berglin and N.-H. Schoon, Ind. Eng. Chem., ProcessDes. Dew., 1983, ZZ, ( I ) , 150-153 F. Beck, Ber. Bunsenges Phys. Chem., 1965,69, (3), I 99-206

7 M. Pourbaix, “Atlas of Electrochemical Equilibria in Aqueous Solutions”, Pergamon, Oxford, I 966

8 M. D. Birkett, A. T. Kuhn and G. C. Bond, “Catalysis, Volume 6”, a Specialist Periodical Report, Royal Society of Chemistry, 1983, pp. 61-89

Glassy Alloys Containing Platinum Group Metals Amorphous Metallic Al loys , EDITED BY F. E. LUBORSKY Butterworth & Co. (Publishers) Limited, London, 1983, 534 pages, E35

Amorphous metallic alloys have great scientific and technological importance in the field of materials science. The above-named book, one of the Butterworth’s Monographs in Materials series, brings together much of our basic knowledge and understanding of the atomic, electronic and structural behaviour of such materials with emphasis on magnetic, superconducting, thermal and chemical properties and techniques of production.

In the section dealing with chemical properties of amorphous metallic systems, a series of palladium-phosphorus alloys are mentioned which have been specifically designed as anode materials for the electrolysis of sodium chloride solutions. These materials have shown high catalytic activity for chlorine evolution with low activity for oxygen evolution while maintaining good corrosion resistance in the hot aqueous environment. Surface-activated amorphous palladium-phosphorus alloys for use as fuel cell electrodes are also described where it has been observed that these systems show higher catalytic activity for the oxidation of methanol and its derivatives than either platinised platinum or surface-activated crystalline palladium.

Refractory metal-metalloid superconducting glasses, particularly those of molybdenum- ruthenium-phosphorus and molybdenum- rhodium-phosphorus, show unusually high transition temperatures compared with their crystalline counterparts which is in contrast to normally expected behaviour. In the readily formed glass systems of the early transition-late transition alloys where the late transition metal is one of the platinum group elements, eutectic temperatures are generally high (> I 5ooOC). However, with devitrification temperatures in excess of 725°C it is surprising that few systems have been investigated; those reported include 5s niobium-45 iridium and 55 tantalum-45 rhodium.

The book contains references to 193 amorphous metallic alloys of which 39 involve one or mort of the platinum group metals. It is evident, however, that glassy alloys which contain a platinum group metal are still at the level of scientific interest with few systems being examined for technological application.

With the knowledge that a great deal of work is continuing in this field, our increasing understanding of amorphous alloy behaviour should lead to novel products. I.R.M.


Monitoring Thermocouple Usage NEW DEVICE INDICATES WHEN RE-CALIBRATION REQIJIRKI)

- 6 .

The precise control of temperature during manufacturing processes has assumed an increased importance in recent years. Particularly within the semiconductor industry, this has arisen as more complex devices and higher product yields have been sought. For example, during the processing of silicon wafers the stages of diffusion and epitaxial growth are carried out under very strictly controlled conditions. Diffusion furnaces used during these stages require not only accurate control to within narrow limits, but also the maintenance of temperature profiles. In the past, the measurement of profiles was carried out using single junction thermocouples, although the use of multi-junction assemblies is increasing and more recently such assemblies are being used for direct furnace control.

To achieve the precise control of temperatures, noble metal thermocouples are generally employed because of the accuracy to which they can be calibrated and their stability in service. While both these factors are important, stability presents users with the greater imponderable. Calibrations can be carried out to high degrees of accuracy in laboratories approved by the British Calibration Service, such as the Calibration Laboratory of Johnson Matthey Metals Limited, where standards are traceable to those of the National Physical Laboratory. The stability of thermocouples, on the other hand, is relative, as all thermocouple types are subject to drift during use as shown in Figure I and therefore require re-calibration from time to time.

In the past obtaining a reliable guide to the need for re-calibration has proved difficult, not only because the rate of drift is dependent upon the environment, although this variable can be much reduced by the use of a suitable sheath, but also because of the problems associated with monitoring temperatures and lengths of time of thermocouple usage. Such problems have led

4+*

+

the operators of processes depending upon precise temperature control to adopt one of two practices. Either thermocouple usage is monitored continuously by sampling the temperature at frequent intervals under com- puter control and processing this information to give an estimate of accumulated drift, or thermocouples are re-calibrated or replaced at pre-set intervals of time. The first approach is expensive and requires an extensive knowledge of drift characteristics under conditions of time and temperature, while the second is somewhat arbitrary.

To help overcome the problems of deciding when to re-calibrate thermocouples, Johnson

:I ++ 750'C

+ + f + + +

-31 t

Platinum Metals Rev., 1984, 28, (3), 107-108 107

Fig. 1 The drift characteristics of platinum versus 10 per cent rhodium- platinum thermocouples in air at temperatures of 750 and 1000°C. The points represent individual calibrations and the blue lines are a least squarcs f i t

usage and indicates when re-calibration is necessary.

The indicator consists of a small dot within a glass tube which moves along an adjacent scale at a rate determined by the temperature of usage. When the dot has reached full scale deflection, thermocouple re-calibration is recommended.

A DeltaLog device fitted to a modified Lemo connector of a 3-junction profiling thermocouple is shown in Figure 2. It is completely self contained, being driven by a battery inserted into the device immediately prior to despatch, giving an active life of approximately one year. Thermocouples returned to the Calibration Labratory of Johnson Matthey Metals Limited are checked and re-calibrated while the DeltaLog indicator is re-set to zero and a new battery inserted in preparation for further use. R. A. B.

Acknowledgement Deltalog is a Trademark of Johnson Matthey,

registration and patents have been applied for.

Fig. 2 The DeltaLog drvicv, shown fitied t o a modified Lemo connector or a :$-junction profiling thermocouple, is battery driven and eompletrly self contained

Matthey Metals have developed a small electronic device named DeltaLog which can form an integral part of their range of quartz sheathed multi-junction thermocouples. Housed within the end connector, the device monitors the time and temperature of thermocouple

The Published Platinum Metal Alloy Systems Phase Diagrams of Precious Metal Alloys, COMPILED BY HE CHUNXIAO, MA GUANGCHEN, WANG WENNA, WANG YONGLI AND ZHAO HUAIZHI, The Metallurgical Industry Press, People’s Republic of China, I 983, 301 pages, U.S. $6.66

Knowledge is of only limited value to society unless it is accessible to all those who can understand and make use of it; indeed it was to make information on the fundamental properties and industrial applications of the platinum group metals more readily available that Platinum Meials Review was founded by Johnson Matthey in 1957. Since that time many studies of these metals have been made but, unfortunately, much of the established data still remains widely, and inconveniently, dispersed throughout the literature.

To overcome this difficulty in a particularly important area of materials science a group of colleagues under the guidance of Professor Tan Qinglin, Director of the Institute of Precious Metals, at Kunming in the People’s Republic of China, has collected together the phase diagrams of all alloy systems containing the so- called precious metals published up to the end of 1975. Over 500 systems are presented in

this book, including I 99 binary, I 15 ternary and five quarternary systems that contain a platinum group metal.

In view of the rate of progress in this aspect of physical metallurgy none of the diagrams has been evaluated or reviewed; despite this the publication is a most useful addition to the literature on the platinum group metals. Although nominally in Chinese, English translations are given wherever this is required.

The contents pages list systems in alphabetical order according to the chemical symbols of the component elements. Interest- ingly, the compilation has enabled gaps in the knowledge to be identified; even among binary systems phase diagrams of rhodium, iridium, osmium and ruthenium are still rather scarce.

This important work may be obtained from the China National Publishing Industrial Trading Corporation, P.O. Box 614, Beijing, People’s Republic of China. I.E.C.


The Catalytic Etching of Platinum and Rhodium-Platinum Gauzes CRYSTALLOGRAPHIC CHANCES DURING AMMONIA OXIDATION

By J. Pielaszek Institute of Physical Chemistry of the Polish Academy of Sciences, Warsaw, Poland

Surface morphological and X-ray examinations have been carried out on catalyst gauzes exposed to the conditions encountered during the oxidation of ammonia. A model, based upon observations of platinum and rhodium-platinum single crystals is proposed to explain the structural changes that occur on rhodium-platinum alloys during industrial use.

A major concern during the process of ammonia oxidation is the loss of platinum from the rhodium-platinum catalyst gauze, which occurs simultaneously with its deactivation. The use of rhodium-platinum or palladium- rhodium-platinum alloys causes a noticeable rise of mechanical strength of the gauze, without a significant loss of catalytic efficiency.

Previous studies have shown that the wires of a new catalyst gauze are essentially smooth (1-8). During use they become roughened and etching occurs along grain boundaries. The process spreads into the interior of the grains and ultimately well developed facets appear, the character of which varies from grain to grain. After a prolonged period of use deep etch-pits, often with very regular shape and developing into channels which penetrate the interior of the grains, are also observed. Finally irregular cauliflower-like growths appear on the surface.

During catalyst use, segregation of the components and enrichment of the surface layers with rhodium occurred on rhodium-platinum gauzes, and X-ray examination revealed the formation of rhodium oxide (I, 4, 7,9, 10). This rhodium oxide layer can form a very compact, thick envelope which can be separated from the gauze wire core ( I I , I 2).

The facets observed on the surface of the grains have a very regular form. This suggests that they are crystallographically oriented and

it is reported that their character depends on local structural features (2,6, 8, I 3).

Experiments with single crystals in the form of small diameter balls have shown that faceting depends upon the local orientation of the surface, its curvature and the flow velocity of the reactant gases (14, I 5) .

In the present paper results are presented of studies with scanning electron microscopy (SEM) of surface morphology, and X-ray examinations of platinum and rhodium- platinum alloys exposed to industrial conditions for ammonia oxidation. To explain the structural changes observed in commercially used rhodium-platinum gauzes a model is proposed, based on the observed changes of surface morphology of single crystals of platinum and rhodium-platinum alloys.

Experimental Conditions All the samples were exposed in an experi-

mental reactor designed to follow industrial practice. The samples were placed between the first and second gauzes in a pack of four with the examined surface facing the reaction gas stream. The experiments were performed at pressures of 3.5 to 4.8 atm, the concentration of the ammonia in the gas being 6.3 to 6.8 wt. per cent. The average gas temperature was 820 to 9oo0C, and the catalyst gauze loading was either 24.5, or more generally 49N cubic metres

109 Platinum Metals Rev., 1984, 28, (3), 109-1 14

Fig. 1 4 platinuni ( 1 I I ) surface. which had hc-c-n shielded by the gauzc, showing an early

x :$400 stagr o f catalytic- etching

of nitrogen per hour while the exposure times were 5 to loo hours (for single crystals 5, 10 and loo hours), except for gauzes obtained directly from industrial plant.

High purity platinum and rhodium-platinum polycrystalline wires of 150 and 60 pm diameter, respectively, were used; the 60 pm wires being in both the as-produced condition and as taken from catalyst gauzes. In addition flat polycrystalline samples, with surface dimension 5 x 5 mm, were cut from rods used in the production of gauzes.

Single crystals of platinum and 1 0 per cent rhodium-platinum with surface orientation ( I oo), ( I I I ) , ( I I 0) were cut by spark-erosion from randomly oriented single crystal rods of 4 to 6 mm diameter. The samples were polished with diamond paste and then annealed in vacuum by an electron beam at a temperature of about I 200OC. Berg-Barrett X-ray topographs of the cut and annealed samples displayed a highly developed mosaic structure, which, in the case of the platinum single crystal, was noticeably reduced only after the third annealing treatment.

The 1 0 per cent rhodium-platinum samples were annealed at much lower temperatures and for shorter periods of time than the platinum. This reduced segregation of rhodium, but did

not allow the formation of large mosaic blocks. The characteristic feature of any wire is the

longitudinal structure (texture) resulting from directional plastic deformation during wire production. This structure is usually non- uniform, and changes with the distance from the longitudinal axis of the wire. Wires of face centred cubic metals generally exhibit an axial structure with direction as the axis or with mixed ( 1 I I > + (roo> axes (16).

Using CuK,, X-ray radiation, the half penetration depth for platinum is about 1.6pm. Thus by dissolution it was possible to study changes in the structure as a function of the distance from the surface.

Observations and Results As produced platinum wires had an axial

 texture in the surface layer but in the interior there was a mixed axial texture of the + type. X-ray photographs of the surface layers of pure platinum wire after treatment in the reactor for 10 hours showed a polycrystalline image. However, hardly any changes occurred at the interior of the wire.

Before treatment in the reactor the rhodium- platinum wires had a mixed axial texture near the surface. However, after the treatment the samples exhibited a

Fig. 2 A platinum (100) surface showing an early stage o f catalytir etching. The relief developed on the surface corresponds to its symmetry x 3500

polycrystalline structure with a simultaneous increase of the type of texture when examined by X-ray diffraction. The degree of crystallographic orientation was greater in the 10 per cent rhodium-platinum than in the 5 per cent rhodium-platinum wire. The observed changes in the interior of the wires were those expected after a purely thermal treatment of the sample. The thick 150 pm diameter wires behaved similarly, except that after an initial I o hours exposure, recrystallisation of the interior was less pronounced.

The platinum and rhodium-platinum wires obtained from catalytic gauzes behaved in the same manner. However, the curvature of the wires made X-ray identification of the texture more tedious.

The single crystal samples were treated under the same conditions as the wires. Before treatment the surfaces were optically smooth, except for a few scratches remaining from the mechanical polishing. After the treatment however, both the oriented platinum and rhodium-platinum single crystal samples exhibited some common features. Platinum crystals with surfaces parallel to ( I I I), (100) and ( I 10) planes and also the 10 per cent rhodium-platinum crystals with ( I I I ) and (100) faces, when treated for 20 hours, exhibited only a few pits; these were mostly along the

Fig. 3 A platinum (100) surface after 100 hours exposure to the direct influence of the reactant gases. Crystallites with well defined faces are visible, as are cauliflower-like growths x3400

Fig. 4 A 10 per cent rhodium-platinum ( 1 0 0 ) surfare after treatment for 1 0 0 hours. The surface is not as deeply etched as platinum after the same treatment time X . 7 4 0 0

mechanical scratches produced during the handling of the samples after preliminary SEM examinations. After 100 hours treatment, the whole surface of the samples exposed directly to the reactant gases was heavily etched. The places shielded by the gauze were attacked less and show the earlier stages of the etching process, see Figures I and 2. The analysis of the relief showed that the edges of the etch-pits are oriented along the directions of intersection of low-index planes with the single crystal surface. In the heavily etched areas, shown in Figures 3 to 6, the surface was covered with crystallites of varying regularity and, in the case of platinum, cauliflower-like growths occurred in some places, see Figure 3. For the same treatment time, the surfaces of the platinum crystals were more deeply etched than those of rhodium- platinum crystals, as can be seen by comparing Figure 3 with Figures 4 and 5 .

The relief developed on each surface corresponded to its symmetry. This is especially evident at the earlier stages of catalytic etching of the platinum crystals, illustrated in Figures I and 2. The relief observed on (100) and ( I I I ) rhodium-platinum crystals was not as regular as that seen on pure platinum crystals. This results from the partial recrystallisation of the rhodium-platinum single crystals, the crystals having a highly developed mosaic structure.

Fig. 5 A 10 per cent rhodium-platinum ( I I I ) surface treated for 100 hours x 3400

Both the 5 and 10 per cent rhodium- platinum alloys exhibited the same surface features. Grains with different orientations were etched in different ways, as can be seen in Figure 6, and some preferential etching at grain boundaries took place. Prolonged exposure resulted in very pronounced etching of the interior of the grains, with a distinct relief on different grains. On the surface of some grains, regular square pits occurred (shown in the left hand photomicrograph in Figure 6) similar to those observed on the platinum ( I 00) face at the beginning of the etching process. On the grains of the 5 per cent rhodium-platinum alloy, some cauliflower-like growths were present. It should be noted that these growths were not apparent on the I o per cent rhodium-platinum alloy after the same loo hours of treatment.

Discussion and Conclusions The morphology of the oriented platinum

and rhodium-platinum single crystal surfaces exposed to reactant gases during the ammonia oxidation process indicates that catalytic etching takes place along crystallographically defined plans. This explains why pits with four- fold symmetry on (100) surfaces and with three- fold symmetry on ( I I I ) surfaces were observed, whereas the pits on the ( I 10) plans exhibited a linear form.

It seems reasonable to assume that the planes exposed during the process of catalytic etching

are the most stable. An analysis of SEM photographs indicates that on platinum (100) crystals the etching takes place with the exposure of ( I I 0) planes and to a lesser degree ( I I I ) planes. On the ( I I I ) oriented surface the pits have edges parallel to the intersection of ( I 00) and ( I I I ) or ( I I 0) planes, and the exposed planes are probably (100) and ( I 10) because they form more acute angles with respect to the ( I I I ) plane than the other ( I I I ) planes. For identical treatment conditions the (100) plane is etched more quickly than the ( I I I ) plane and this results in the better developed relief observed on the former plane. The 10 per cent rhodium-platinum crystals are more resistant to etching than the platinum crystals.

The relief observed on both the platinum and rhodium-platinum crystals is very regular only over short distances, although its character remains the same all over the surface. This probably results from the highly developed mosaic structure in these crystals and the fact that in some cases the misorientation between subgrains can be as large as several degrees. Figure 7(left) shows Berg-Barrett X-ray topographs of a platinum (100) crystal before annealing, and Figure 7(right) after the third I hour anneal. In the case of the rhodium- platinum alloy such an annealing treatment was precluded to avoid segregation of rhodium and recrystallisation. This is why the relief observed on these crystals is more confused and is only roughly similar to that observed on platinum single crystals. As suggested elsewhere the loss of material probably proceeds through the formation of volatile platinum oxide (5, I 0, I 3,17), and results in the enrichment of the surface layer with rhodium. The volatile component probably condenses on other parts of the crystal and, together with smaller crystallites detached from the bulk of the crystal and carried by the reactant gas stream until randomly deposited elsewhere, forms the observed cauliflower-like growths. These do not have as regular a form on a large scale as the etched crystallites, but in some cases their general shape is parallel to the low index directions of substrate crystals.

The results show that all the examined low-


Fig. 6 T w o areas on a flat polycrystalline sample of 5 per cent rhodium-platinum after treatment for 100 hours in the reactor. The relief produced by etching depends upon the orientation of the grains. Regular square pits can be observcd on one of the grains x 1 1 3 0

index planes are stable. However their stability depends on the number of points at which the etching process starts.

It is reasonable to assume, as in the case of chemical etching, that the sites of preferential etching are places of high stress concentration, grain boundaries, impurity segregation and other kinds of defects. Once activated, catalytic etching proceeds to expose the nearest low- index planes. This process spreads until new points of preferential etching develop on the newly exposed planes. New low-index planes are

then exposed at these points. In this way the etching process proceeds not only at right angles to the crystal surface, but also in other directions. Some parts of a crystal may be etched from all sides and the crystallites formed in this way will be moved away from the bulk of the crystal by the stream of reactant gases. This model should also be valid for polycrystalline material, where every crystal grain is behaving in the same manner. For such polycrystalline material the existence of grain boundaries makes the number of sites of preferential

Fig. 7 Berg-Barrett X-ray topographs of a ( 100) oriented platinum crystal showing, on the left, the sample after annealing for 1 hour at 12OO0C, and on the right after the third period of I hour annealing at 1200OC. (Reflex (31 1 ), CuK,, radiation). The same low-angle boundary is marked tl on both topographs. The increase in size of the micromosaic blocks after repeated annealing is clearly shown approx. x 8 5


etchinp much hieher and the random orienta- References

sample. The rhodium-platinum wires have a high

degree of longitudinal texture of the (100) type in their surface layers. This means that a higher than average number of grains have the direction parallel to the wire axis. The planes are stable. Thus, etching on these planes will be much slower than on any other adjacent plane, and etching along the planes perpendicular to the wire axis will be even slower. As a result, grains with a longitudinal texture close to will be more stable. The other grains will be attacked more readily and some of them will be separated from the matrix and removed by the stream of reactant gases. The SEM studies (18) of post-reaction dusts revealed crystallites with the dimension of a few pm and with habit planes looking very similar to the crystallites developed on oriented single crystal planes, as shown in Figure 3. As a result,

3 J. P. Contour, G. Mouvier, H. Hoogewys and C.

4 F. Sperner and W. Hohmann, Platinum Metals

5 J. C . Chaston, Platinum Metals Rev., 1975, 19,

6 R. W. McCabe, T. Pignet and L. D. Schmidt, 9.

7 N. H. Harbord, Platinum Metals Rev., 1974, 18,

8 L. D. Schmidt and D. Luss, 3. Caral., 1971, zz,

9 J. A. Busby and D. L. Trimm, 3. Caral., 1979,

1 0 M. Chen, P. Wang and L. D. Schmidt, 3. Catal.,

I I M. Pszonicka and T. Dymkowski, Pol. 3. Chem.,

1 2 M. Pszonicka, 3. Caral., 1979,56, (3), 472 1 3 R. T. K. Baker, R. B. Thomas and J . H. F.

14 M. Flytzani-Stephanopoulos and L. D. Schmidt,

I 5 M. Flytzani-Stephanopoulos, S. Wong and L. D.

Leclers, 3. Catal., I 977,48, ( I /3), 2 I 7

Rev., 1976,20,(1), 1 2

(41, ‘35

Catal., 1974, 3 4 (11, I 14

(31397

(I), 269

60, (3L 430

1979, 60, (3)> 356

1978, S Z , ( I ) , 121

Notton, Platinum Metals Rev., 1974, 18, (4), I 30

Prog. Surf. Sci., I 979,9, 83

Schmidt, 3. Caral., 1977,49, ( I ) , 5 I the relative number of grains in the surface 16 G. Wasserman and J. Grewen, “Tekstury - layer with an orientation of the (loo) type is increased. This was actually observed.

metakzeskich materiallov”, Moscow, 1969 I 7 G. C. Fryburg and H. M. Petrus, 3. Elecrrochem.

SOC., I 96 I , 108, (6), 496

and rhodium-platinum wires and single crystals, presented above, can explain the crystallographic features observed on catalytic gauzes used in ammonia oxidation. It can also explain why well developed crystallites are found in post-reaction dust (18). However, it does not account for the way the loss of material takes place, although it seems that the most effective way for the described model to operate will be through the formation of volatile products.

Acknowledgements The rhodium-platinum gauzes were kindly

supplied by Dr. M. Pszonicka, Technical University, Warsaw. The platinum-rhodium single crystal was made by Metals Research Limited, England. Exposures of all the samples were done at the experimental reactor in Instytut Nawoz6w Sztucznych, Pulawy, where substantial help of Mr. Kozlowski is greatly appreciated.

This work was carried out with the Research Project 03.10.

Palladium-Nic kel Plating The economic advantages of palladium-

nickel as a replacement for electrodeposited gold in the electronics industry have been demonstrated many times during recent years and an extended study of their relative perfor- mance recently reported by K. J. Whitlaw of LeaRonal U.K. (Trans. Insr. Met. Finish., 1984, 62, ( I ) , 9-1 2) serves to substantiate the potential value of these deposits.

The experimental work shows that a duplex layer of 2.5 to 3.opm 70 palladium-30 nickel followed by 0.1 to 0.25 pm of acid hard gold is to be recommended as a replacement f o n . 5 pm gold deposited on a copper substrate such as a printed circuit board. This combination offers freedom from porosity, stability of contact resistance, excellent resistance to wear and to corrosion, and also resistance to copper diffusion at elevated temperatures.

These properties, while being identical to those secured with a conventional gold deposit, offer savings in cost of as much as 65 per cent.

Platinum h4etals Rev., 1984,28, (3) 114

tion of grains causes more non-stable high- index planes to be exposed at the surface of the

l , l z , . = X C.lLC3

I G.J.K.Acres,Plarinum MetalsRev., I 980,24,( I) , 14 2 A. G. Knapton, Platinum Metals Rev., 1978, 22,



(41, ' 3 '


Rev., 1976,20,(1), 1 2

l , l z , . = X C.lLC3

I G.J.K.Acres,Plarinum MetalsRev., I 980,24,( I) , 14 2 A. G. Knapton, Platinum Metals Rev., 1978, 22,



(41, ' 3 '


Rev., 1976,20,(1), 1 2

-__., .,-., ---, ,-,, ~~- The etching processes observed on platinum

and rhodium-platinum wires and single 18 M. Pszonicka, private information

Combustion in Wood-Burning Stoves PLATINUM CATALYST INCREASES THERMAL EFFICIENCY AND GREATLY REDUCES POLLUTION

The use of wood as a residential heating fuel has always been popular in the Scandinavian countries. More recently there has been a significant increase in the use of wood-burning stoves in many other parts of the world, especially in the United States of America. This trend was initiated by the escalating costs of fossil fuels and electricity and, to a certain extent, public concern about the reliability of future supplies of these fuels.

Heating with wood fuel can be less costly than heating with coal, oil or electricity, especially in rural and some suburban areas where wood is relatively inexpensive and plenti- ful. Furthermore, modern wood-burning stoves are at least as efficient as oil and gas fires. Tests have shown that radiant, damped-draught wood-burning stoves deliver from 50 to 70 per cent of the energy available in the wood to the surrounding living area.

Despite all its advantages the wood-burning stove does present some problems, not the least of which is smoke.

The main difficulty is that there is no economical method of continuously feeding a measured amount of wood into the stove to control the heat output. Large amounts of fuel are loaded into the stove and the combustion rate is controlled by the amount of air available. The unburned wood is heated to the point at which combustible volatile components are distilled off. These components comprise hydrocarbons (including polycyclic aromatic hydracarbons), aldehydes, phenols and carbon monoxide and are often incompletely burned in the stove because there is insufficient air available for combustion. Even when sufficient air is available, temperatures are often too low to bring about any significant degree of burning. The nett result is that these volatile components can be released from the stove to either con- dense in the chimney or be emitted to the

atmosphere. The condensed materials con- tribute to the danger of chimney fires while the smoke emissions can cause significant air pollution. Furthermore, the overall effect is a significant loss of potential heating value.

The application of new technology in the design of wood-burning stoves can now largely overcome the basic problems associated with their operation. The requirement is for a method of operation which enables smoke to be

Fig. I The plalinuni nietal catalyst is located in the secondary Combustion chamber of the stove. The three-tier design provides a large surface area and hence efficient heat exchange

Platinum Metals Rev., 1984, 28, (3), 115-116 115

burned at the temperatures existing in a stove operating under damped conditions. These temperatures are typically in the range 200 to 4ooOC. The use of platinum group metal catalysts supported on ceramic honeycomb substrates is now a well established method of con- trolling emissions of unburned hydrocarbons and carbon monoxide from motor vehicle exhausts. The catalyst reduces the combustion temperature of the hydrocarbons and carbon monoxide so that they start oxidising at temperatures around 250°C. The catalytic reaction increases until the system reaches an equilibrium between the inlet gas temperature, the gas-flow rate and the amount of combustible material in the gas stream. The equilibrium temperature can be as high as about 800°C for an optimum sized catalyst system and at this temperature essentially complete oxidation of the combustibles in the gas stream proceeds very rapidly.

During the course of the past few years the application of catalytic oxidation to promote secondary combustion has led to the appearance on the market of a new generation of wood-

burning stoves. One such product has recently been launched by Trolla Brug, a long established Norwegian company. Appropriately named the Pioneer, it combines the traditional appeal of a cast iron stove with the modern technology of catalytic secondary combustion.

The stove is a three-tier design comprising a primary combustion chamber, a secondary combustion chamber incorporating the catalyst and a third storey which acts solely as a heat exchanger. The catalyst, which was specifically developed by Johnson Matthey Chemicals Limited for wood-burning stove applications comprises platinum metal dispersed on a low cell density ceramic honeycomb support. The multi-storey design provides a large surface area for maximum heat exchange efficiency. The incorporation of the catalytic afterburner is claimed to result in a 30 per cent reduction in wood consumption for the same useful heat output compared to the non-catalytic version of the same stove. In addition smoke emission levels are greatly reduced and the potentially dangerous accumulation of inflammable con- densates in the chimney avoided. A.E.R.B.

Osmium Doping Improves Recording Media 'I't l lN FILMS H A V E HIGH COERCIVI'I'Y A N D COERCIVK S Q U A H K N K S S

In magnetic recording the continuing demand for ever increasing recording density has stimulated research on thin films of con- tinuous magnetic materials suitable for the production of high capacity storage discs. Sputtered y-Fe203 thin films are attractive for this application in view of their high coercivity and high remanent magnetisation, combined with their resistance to corrosion and wear, and a y-Fe203 film containing small amounts of cobalt, copper and titanium has been developed. The function of these additions is to increase the coercivity of the film, to improve coercive squareness, to widen and lower the temperature range of the a-Fe203 to Fe304 reduction-so making it possible to obtain uniform magnetic properties-and also to suppress grain growth during heat-treatment.

Now workers at the Ibaraki Electrical Com- munication Laboratory in Japan report that remarkable improvements have been made to

the magnetic properties and microstructure of sputtered y-Fe203 thin films when osmium is used as an additive element (0. Ishii and I. Hatakeyama, J . Appl. Phys., 1984, 55, (6),

Films 0.1 to o.2pm thick have been prepared by reactive magnetron sputtering, the target being an iron plate to which osmium pellets were attached. Coercivity and coercive squareness increased with osmium content, to maximum values of 2100 Oe and 0.81, respectively. Osmium doping also brought about field-induced anisotropy which greatly increased the coercive squareness parallel to the easy axis, a figure of 0.96 being obtained with 0.88 to 5.2 atomic per cent osmium. Osmium also suppressed grain growth during preparation, giving crystallites about 400 A in diameter which improves the signal to noise ratio, an advantage for increasing recording density and read back amplitude.

2269-2271).


Electrodeposition of Palladium-Silver Alloys from Arnrnoniacal Electrolytes By B. Sturzenegger

and J. C1. Puippe Department of Chemical Engineering, Swiss Federal Institute of Technology, Zurich

Werner Fluhmann A.G., Diibendorf, Switzerland

A viable technique for the electrodeposition of palladium-silver alloys could find application during the manufacture of electrical contacts. Previously such a process has not been available but some alloys with useful physical properties can now be deposited from an ammoniacal system, while a better understanding of the mechanisms involved has been gained during the work reported here.

Applications of palladium and palladium alloys in the electrical contact field have recently been reviewed by Antler (I). Among these materials, palladium-silver alloys, particularly the 60 per cent palladium-40 per cent silver composition, are well established in the wrought form, as claddings, weldments or inlays, with advantages over the pure metal in terms of cost and durability. More recently, interest has extended in the direction of electrodeposited coatings of palladium and alloys as economic substitutes for gold plating, and in this context it is clearly attractive to con- sider the possibilities of producing palladium- silver alloys by electrodeposition, a technique which would permit, for example, the application of coatings to pre-formed contact fingers and other components in cases where the inlay technique is not practicable. While, however, a considerable amount of research work has been carried out to this end, notably from Russian sources, it would appear that no viable process has yet been developed.

Early attempts used a solution based on cyanide complexes of the two metals (2, 3), but the alloys produced, with a palladium content of 20 to 22 per cent, were of poor appearance and the cathode efficiency was very low. Improvements were later obtained in terms of increased cathode efficiency by optimising process parameters and bath formulation (4-6).

Deposits from thiocyanate-based electrolytes showed a broader compositional range, up to 70 per cent palladium (2, 7-12), the best coatings from this type of bath containing 2 to 10 per cent palladium. A bath formulation based on palladium and silver ammino-hydroxy salts gave semi-bright deposits with palladium contents from 15 to 85 per cent and good cathode efficiency of 85 to 95 per cent (13-16), while bright coatings have been reported from an electrolyte based on amino-acid complexes (17, 18). Solutions of palladate and argentate salts have been stated to give semi-bright coatings with palladium content between I o and 60 per cent, with thicknesses up to 15,um being obtainable ( I 9).

Work has also been carried out using con- centrated halide baths with lithium chloride to permit increased solubility of silver (20-22). This type of solution is very aggressive to base metals due to vigorous displacement reactions; however, good deposits were reported, with palladium contents ranging from 30 to 60 per cent. Attempts to deposit alloys from ammoniacal solutions of nitrito-complexes were unsuccessful (2), but when nitrate complexes are employed it has been reported that the full range of alloy compositions can be obtained simply by adjustment of the current density (23). Later reports claim the production of bright, pore-free deposits from this type of

117

PVC jacket--

Cathode __ Anode __

Outlet Teflon sleeve

ENLARGED VIEW ON A-A

DIMENSONS IN rnm

Fig. I The elrctrolytes were studied in the cell shown here. Deposits were made onto a rotating cylindrical copper cathode, the anode being a con- centric platinised titanium cylinder

solution, with thicknesses up to 20 pm (24). The object of the present work was to study

the mechanism in palladium-silver alloy deposition in order to gain a better understanding of the influence of process and compositional parameters on alloy composition and properties. For this purpose the ammoniacal solution with palladium and silver nitrates was selected in view of the possibility of obtaining a wide range of alloy compositions.

Experimental Conditions The electrolyte compositions studied are

given in the Table opposite. Deposition was made on a copper cylinder

( 1 0 mm diameter, 10 mm high) attached to a Teflon shaft, the rotational speed (a) of which could be continuously adjusted from o to 10 revolutions per second. The anode was a con- centric cylinder of platinised titanium, which was itself provided with a further jacket of PVC for temperature stabilisation. The electrolytic cell is shown as Figure I . The cathode surface

was prepared for plating by an initial polish with I pm alumina paste, followed by cathodic degreasing at I o mA/cm’ in an alkaline cyanide solution, and a final activating dip in con- centrated sulphuric acid, with intermediate water rinses. Immersion in the electrolyte was made under an applied voltage.

Deposition was carried out at room temperature (22OC) for times selected to produce coating thicknesses in the order of 2pm. The applied current density (j&I was varied between 5 and 40mA/cm2, and the cathode rotational speed between o and 10 revolutions per second.

Deposition Mechanism Studies Polarisation curves for (a) a solution contain-

ing 20 g/l of palladium only, and*(b) a solution containing 20g/l of palladium and 2gf l of silver are shown in Figure 2. From these it is seen that silver begins to deposit at potentials less negative than for palladium, and that palladium deposition occurs when silver is


being deposited under limiting current conditions. The large plateau of curve (b) is associated with the limiting current of silver deposition, as will be quantitatively evidenced later. The fact that palladium, a more noble metal than silver, is deposited only at more negative potentials than the latter, is explained by the much greater stability of the palladium- ammino complex, the relevant stability constants being as follows (25):

Palladium, as Pd(NH,l,(NO,), Silver, as Ag(NH,),(NO,) pH (adjusted by gaseous NH,)

The influence of current density and rotational speed of the cathode on the composition of the alloys was studied for electrolytes containing I and 2 g / l of silver. The results, shown in Figures 3 and 4, are consistent with the hypothesis of a mass transport-controlled codeposition of silver, in that the silver content increases with agitation rate at constant current density, and decreases with increase in current density at constant agitation rate. As shown by comparison of Figure 3 with Figure 4, the increase in the silver content of the deposit is proportional to the increase in silver concentration in the electrolyte, which is again consistent with the mass transport model for silver incorporation.

Figures 3 and 4 also show the experimental conditions under which bright, semi-bright, and matt deposits are obtained. The thickness of the deposits was 2pm. In general, deposits with a silver content below 25 atomic per cent are bright, while higher silver contents are associated with semi-bright or matt coatings. This may be explained on the basis that at low silver incorporation rates the deposit structure is essentially governed by the crystallisation

20 911 2 0 911 20 911 1 911 2 911 3 gA

11.5 11.5 11.5

- 2 0 0 -400 -600 - 8 0 C ELECTRICAL POTENTIAL, Ev,,*p,A,c,,rnV

Fig. 2 Polarisation curves for palladium and palladium-silver electrolytes. Voltage scanning speed: I mV/s ( a ) Palladium: 20 g/l ( b ) Palladium: 20 g/I; silver: 2 g/l

40 r

2 4 6 8 1 0 1 2 ROTATIONAL SPEED.n,rev/s

Fig. 3 Silver content of deposits a5 a function of rotational speed of cathode and current density for electrolyte 1 . 0 bright deposits; 0 semi-brighl deposits; matt deposits

mode for palladium, while at higher rates of silver incorporation the morphology of the deposit is progressively affected by the crystallisation of silver. Since silver deposition takes place under mass transport controlled

Electrolyte Compositions

1 2 3


conditions, there is a tendency for the formation of dendrites and powder, leading to rougher deposits. The morphology of matt, semi-bright and bright deposits with various silver contents is illustrated in the scanning electron micrographs of Figure 5.

The variation of current efficiency with silver content is shown in Figure 6. The decrease in efficiency with increasing silver content can be explained by the increasing roughness of the deposits, leading to enhance- ment of hydrogen evolution at asperities and hence to a decrease in current efficiency for metal deposition. The current efficiency was calculated on the basis of Faraday's law from the metal contents of deposits, as determined by atomic adsorption on solutions prepared by dissolution in nitric acid, as follows:

= 100 [ii]

110,

where IAg and I,, are partial currents for silver and palladium, Ilo, is total current, mAg and mPd are weights of silver and palladium deposited, AAg and A,, are the atomic weights of the metals, F is the Faraday constant and t is the electrolysis time.

From the silver content of deposits, and on the assumption that silver is deposited under mass transport control, the limiting current density for silver deposition can be calculated from the results of Figures 3 and 4, and is plotted as a function of rotational speed of the cathode in Figure 7. The limiting current density values so obtained fit the following relationship:

j h g = 85.5 f2n.sr9 . C A ~ [iii] That is to say, the limiting current density is directly proportional to the silver concentration CAg in the electrolyte and is a function of 12°.ss9. An exactly similar relationship between limiting current density and rotational speed was found in separate experiments with a ferri- ferrocyanide system, where it is well known

that the current is under full mass transport control, providing a quantitative demonstration of the fact that similar conditions apply to the incorporation of silver in the palladium-silver alloy. This being so, a model may be derived for the calculation of alloy composition as a function of deposition parameters, as given below.

During the electrodeposition of palladium- silver alloy, the total current density is due to contributions from the deposition of palladium, silver and hydrogen.

i t01 = iPd + j A g + i H

xAg = jAg/bAg + 112 jl'dd)

[ivl The molar-ratio of silver in the deposit is:

[VI

Assuming a I 00 per cent current efficiency and combining Equations [iv] and [v],

xAg = 2 iAg/( iAg + jd [Vil [vii]

where J =& = 'to' [viii]

With Equation [vii] one can thus predict the silver content in the deposit for different total current densities, different silver concentrations in the electrolyte and different hydrodynamical conditions.

For not negligible drops in current efficiency, Equation [vii] has to be rewritten as follows:

XAg = 2/(I + J)

LA^ 85.5 nn'ss9. C A g

X A ~ = 2/( I + J . CE) [vii']

4

2 4 6 8 1 0 1 2 ROTATIONAL SPEED, n, rev/s

F i g . 4 Silver content of deposits as a function of rotational speed of cathode and current density for electrolyte 2. 0 bright deposits; 0 semi-bright deposits; matt deposits


Fig. .5 Scanning electron micrographs of palladium- silver alloy deposits from rlectrnlytc 3 (palladium 20 g/l; silver 3 g/ l ) .

( a ) R = 0 rev/s; ,i = 5 mA/cm2; Ag = 52.5 at.% (matt)

( b ) R = 0 rev/s; ,i = 15 mA/cm2; Ag = 29.3 at.% (srmi-bright )

( c ) 0 = 0 rev/s; .i = 3 0 mA/cm*; Ag = 20.8 at.% (bright)

( d ) 0 = 3 rcv/s; ,i = 30 mA/cm2; Ag = 33.0 at.% ( semi-bright)


100. + W U K 90. W

> U $80-

,“ 70.

- u LL LL

0 . O 0 .. . .

m . . . 10 20 30 40

ATOMIC PER CENT SILVER

Fig. 6 Current efficiency for metal deposition as a function of silver content of deposits. 0 bright deposits; 0 semi-bright deposits; matt deposits

Equation [vii] and the experimental results for R > 2/s are plotted in Figure 8. Agreement between calculated and measured values was very good. Only small deviations occur for experimental values corresponding to matt deposits.

Physical Property Measurements For measurement of properties such as

hardness and specific resistance it was necessary to produce alloy coatings of thickness at least I o pm and of good surface finish, both on flat substrates (for hardness) and as isolated foils (for specific resistance). Since, at thicknesses in excess of about 2pm, coatings tended to develop cracks due to internal stress associated with hydrogen occlusion, and also to deteriorate in surface finish, a multi-layer technique was adopted for this purpose, in which coatings were built up in successive increments of 2 pm, with intermediate polishing of the cathode with I pm alumina. Thus it was possible to produce bright, crack-free coatings up to I 2 pm thick.

Deposition was made on both sides of brass plate cathodes (20 x 70 mm), which were masked to expose a total area of 16cm’ and suspended between platinised titanium anodes in an open vessel, with stirring by natural convection. To produce unsupported foils as required, the coating was removed from one side of the plate by grinding, and the support- ing substrate was then dissolved by anodic


treatment at 280mV in a solution of 200g/l copper sulphate, 50 g/l sulphuric acid, to leave an isolated foil for measurements. A series of foils was produced in this way under the following conditions:

Electrolyte I : j l O l = 5 , to, IS,ZO d c m ’ Electrolyte 2: j,,,= 10, 1 5 , 20 mNcm2

A l l o y Composition Alloy composition as a function of current

density under the above conditions is shown in Figure 9, together with analogous results pertaining to coatings produced on the copper electrodes used in the earlier work, with zero rotational speed. From the results it is clear that the change from the one type of cathode to the other has no significant influence on the composition-current density relationship.

Hardness The Knoop hardness of coatings was deter-

mined as the mean of five measurements at various locations on the surface of 12pm deposits at a load of 1 5 p, with indentation period of 30 seconds. It has been experimentally determined that the alloy thickness of I 2 pm was sufficient to avoid any influence of the brass substrate on the Knoop-hardness values. Figure 1 0 shows the hardness of deposits from the two electrolytes as a function of composition. Since both curves are more or less parallel, it appears that the hardness is independent of deposition conditions and increases with increasing silver content.

Specific Resistance Results of measurements of specific

resistance by the four-point probe method (ASTM F390-78) are shown in Figure I I . As for hardness, this property increased with increasing silver content, being practically independent of electrolyte composition and bath conditions.

Contact Resistance Contact resistance measurements were made

on 2 p m coatings by the method according to the ASTM B667 specification, the test surface

>

I 1 2 3 4 5 6 7 8 9 1 0

ROTATIONAL SPEED,IL. mv/s

Fig. 7 Limiting current density for silver as a function o f the rotational speed of the cathode. ( I ) Electrolyte I ; ( 2 ) Electrolyte 2

10 2 0 30 40 CURRENT DENSITY, 1. mA/cm2

Fig. 9 Silver content o f deposits as a function o f current density for cylindrical ( 0 ) and vertical plate ( x ) electrodes by natural convection. ( I ) Eleclrolyte I ; (2) Electrolyte 2

? 5401 a

/ 10 15 ATOMIC PER

/ 20 25

CENT SILVER

Fig. I I Electrical resistivity of electrodeposited palladium-silver foils as a function of the silver content. ( I ) Elertrolyte I ; ( 2 ) Electrolyte 2

4 8 12 16 20 24 CURRENT DENSITY, J , dimensionless

Fig. 8 Silvrr content in the deposit as a funrtion of the dimensionless current density J. - according to equation [ vii] ; 0 experimental values

4 50

VI

9 x I400 v; W z E 2 350

300 5 10 15 2 0 25

ATOMIC PER CENT SILVER

Fig. I 0 linoop hardness of deposits as a function o f silver content. ( I ) Elertrolylc- I ; ( 2 ) Electrolyte 2

4

C E w Y 3 I- 2 W a 2

u I- 2 81

10 15 20 25 ATOMIC PER CENT SILVER

Fig. 12 Conlact resistance of palladium- silver alloy deposits as a function of silver content, for different loads


being flat, and the contact partner being a gold hemisphere of diameter 3.2 mm. T h e results, for loads between 10 and IOO g, are shown in Figure 12, indicating that, in contrast to hardness and resistivity, contact resistance is relatively little affected by alloy composition, but does depend strongly on the test load.

Concluding Remarks T h e present investigation has resulted in a

better understanding of the mechanism govern- ing electrodeposition of palladium-silver alloys from an ammoniacal system, in particular in the clear demonstration that silver incorporation occurs under mass transport controlled conditions. On this basis it is possible to predict the silver content of deposits as a function of the experimental conditions. T h e palladium- silver system from ammoniacal baths provides, in fact, a very good example of the “regular” type of alloy deposition, according to the classification of Brenner (26).

Results of pulsed current plating studies, to be

I

2

3

4

5

6 7

8

reported elsewhere, are also consistent with

the mass transport model, but this technique permitted no increase in the silver content for bright coatings as compared to d.c. conditions.

From the applicational viewpoint, alloy deposits so far obtained show useful physical properties, despite the limitation of the silver content. From the process viewpoint, the very high ammonia content of the bath may con- stitute a potential disadvantage, certainly necessitating effective ventilation. However, it appears to have no adverse effect, as might have been expected, on the adhesion of deposits to copper and copper alloys, and, in total, provides a system of good stability, with very simple and easily monitored bath chemistry.

Acknowledgements The work described in the present paper was

started under the direction of the late Professor Dr. Norbert Ibl, and was later supervised by Dr. 0. Dossenbach. Thanks are due to Mr. F. H. Reid for helpful advice at various stages of the work, and for assistance in the preparation of this paper, and to the firm of Werner Fliihmann AG, Dubendorf, for the provision of financial support and facilities for analytical work.

References M. Antler, Platinum Metals Rev., 1982, 26, (3), I 06 G. Grube and D. Beischer, Z. Elektrochem., I 933, 39, (31, 1 3 1 J. Fischer and H. Barth, German Parent 688, 398; I940 N. P. Fedotev, P. M. Vyacheslavov, B. Sh. Kramer and V. V. Ivanova, Zh. Prikl. Khim.

Nippon Mining Co. Ltd., Japanese Parent 81,

Z. G. Stepanova, Russian Parent 425,98 I ; I 974 N. P. Fedotev, P. M. Vyacheslavov, B. Sh. Kramer and G. K. Burkat, Russian Patent 22 I ,

B. M. Kramer, Zh. Prikl. Khim. (Leningrad),

(Leningrad), 1967, 40, (7) I474

156,790; 198 I

452; 1968

I 973,469 (61, 1242 9 B. M. Kramer, Zh. Prikl. Khim. (Leningrad),

1 0 B. M. Kramer, Zh. Prikl. Khim. (Leningrad),

I I B. Sh. Kramer, Obmen Opytom Radioprom., 197 I ,

I 2 Kumamoto Prefecture, Japanese Patent 82, 076,

1 3 N. T. Kudryavtsev, I. F. Kushevich and N. A.

1973,469 ( 1 0 h 2326

1 9 7 5 ~ 4 8 , ( I ) , 226

( I 1 )

196; 1982

Zhandarova, Zashch. Met., 1971,7,206

.Platinum Metals Rev., 1984, 28, (3)

14 N. T. Kudryavtsev, 1. F. Kushevich and N. A. Zhandarova, Russian Patent 291, 988; 1971

1 5 I. F. Kushevich and N. T. Kudryavtsev, Elektrokhim. Osazhdenie Primen. Pokrytii Dragorsennymi Redk. Metal., I 972

16 I . F. Kushevich and N. T. Kudryavtsev, Zashch.

1 7 J. Culjkovic and R. Ludwig, German Patenr

18 H. J. Schuster and K. D. Heppner, German Patent 2,657,925; 1976

19 N. T. Kudryavtsev, I. F. Kushevich, L. P. Vladimirova and L. V. Galkina, Russian Parent

20 Extended Abstracts No. 309, Electrochem. SOC. Fall meeting, Denver, Oct. I 98 I , I I

21 A. K. Graham, S. Heiman and H. L. Pinkerton, Plating, I 948,35, ( I 2), I 2 I 7

22 Bell Telephone Laboratories Inc., US. Parent

2 3 V. E. Medina, US. Parent 3,053,741; 1961 24 A. Ya. Pronskaya, S. I. Krichmar and S. D.

Okhrimets, Dokl. Akad. Nauk SSSR, Elekrrokhim., 1971,7 , (6), 778

2 5 R. M. Smith and A. E. Martell, “Critical Stability Constants”, Volume 4, Plenum, New York, I 98 I

26 A. Brenner, “Electrodeposition of Alloys”, Volume I, Academic Press, New York, 1963,76

Met., 1 9 7 4 , m 78

29 445,538; I974

379Y 676; 1973

432699671; 1981

124

A History of Thermal Analysis PLATINUM IN T H E MEASUREMENT OF HIGH TEMPERATIJRES

For thousands of years man has been slowly learning how to regulate fire to yield the degree of heat required for individual purposes. No method of temperature measurement was available to him, however, until early in the seventeenth century and then only up to about 300°C~ and the measurement of higher temperatures had to await the discovery and use of platinum.

In a Special Issue of Thermochimica Acta devoted to the history of thermal analysis Dr. R. C. Mackenzie, of the Macaulay Institute for Soil Research in Aberdeen, has presented two most interesting papers that together form a monumental and scholarly survey of the whole subject from the earliest times to the present

The first of many applications of platinum in the measurement of higher temperatures was due to Guyton de Morveau who designed a pyrometer in 1803 that employed a platinum rod supported in a refractory groove with its free end in contact with the short arm of a bent lever, the longer arm serving as a pointer moving over a graduated scale, all made in platinum. Some years later, in 1 8 2 1 , Professor J. F. Daniell, of King’s College, London, devised an improved form that overcame the deficiencies of de Morveau’s instrument and in which the temperature was determined by the difference in expansion of a platinum rod and an earthenware tube.

Neither of these instruments was capable of measuring really high temperatures, nor were they of appreciable accuracy. A discovery was now made, however, as Dr. Mackenzie clearly brings out, that was to lead to one of the two reliable and accurate methods of temperature measurement that are still in extensive use in both the manufacturing industry and scientific research. It was in Berlin in 1 8 2 1 that Thomas Johann Seebeck described the deflection of a magnetic needle caused by the electric current

day ( 1 984,73, (31,249-3671.

generated when one of the junctions of two dissimilar metals was heated. While it did not occur to Seebeck to make use of his discovery for the measurement of temperature, this invaluable effect was employed five years later by Antoine C h a r Becquerel who decided that the most suitable combination of metals was a circuit consisting of platinum and palladium. With this combination he was able to arrive at the determination of temperatures up to I 350°C by extrapolation.

An iron-platinum thermocouple was then used by Professor C. S. M. Pouillet of Paris, while Henri Regnault, making use of the same couple, found such irregularities that he roundly condemned the whole idea of the thermoelectric method, his troubles arising, of course, from the use of iron as one element. Later, in I 862, Becquerel’s son Edmond, again using platinum and palladium “as these two metals are not altered by the action of heat”, succeeded in rehabilitating the reputation of the thermocouple, but it was not until 1872 that Professor Peter Tait of Edinburgh, using platinum against iridium-platinum, devised a sound relationship between e.m.f. and temperature, so making possible the develop- ment of accurate pyrometry.

But the successful practical use of the thermocouple was mainly due to the work of Henri Le Chatelier, Professor of Metallurgy at the h o l e des Mines in Paris, who in 1885 con- cluded that platinum against rhodium-platinum gave the most consistent results.

Dr. Mackenzie’s fascinating account of the history of these and other developments, including the later work of Roberts-Austen and the concept of the platinum resistance thermo- meter proposed by Sir William Siemens in 1 8 7 1 , will be of immense interest to all physicists and metallurgists concerned in any way with the control and measurement of temperature. L.B.H.

Platinum Metals Rev., 1984, 28, (3), 125 125

Nineteenth Century Platinum Coins AN EARLY INDUSTRIAL USE OF POWDER METALLURGY

By Hans-Gert Bachmann and Hermann Renner Degubsa AG, Frankfurt am Main, West Germanv

Powder metallurgy is the metallurgist’s answer to the production of ductile metals of high melting point by methods differing from conventional melting and casting. The history of platinum, extensively and vividly recorded by McDonald and Hunt (I), gives examples of how platinum was worked into objects from earliest times onwards. However, the first real melting of platinum was achieved only as late as I 782, when Lavoisier successfully reached the temperature of 1769OC necessary to melt this metal on a very small scale with the aid of an oxygen torch (2). Three years earlier, Franz Karl Achard (1753--1821), whose contributions to metallurgy have only recently been fully realised (3), made use of the property of platinum to form low-melting point alloys with elements such as phosphorus, mercury and arsenic. A mixture of 13 weight per cent of arsenic and 87 per cent of platinum (equivalent to 28 atomic per cent of arsenic) gives a eutectic with a melting point of 597OC (4, 5) . Achard melted this mixture of arsenic and platinum with the addition of potash as flux, and after evaporation of the volatile arsenic he obtained

platinum sponge which he was able to shape into objects, such as crucibles. This process remained in use until I 8 10.

In the meantime Wollaston produced the first malleable platinum by a “wet” method. As early as I 80 I he solved the problem of how to get rid of the impurities normally accompanying naturally occurring placer platinum. By careful adjustment of the proportions of hydrochloric and nitric acid in aqua regia, and later by using more dilute mixtures, he separated platinum from its associated palladium and rhodium. The solution, containing only hexachloroplatinate, H2FtCI6, was subsequently treated with sal ammoniac, resulting in a precipitate of ammonium hexachloroplatinate, (NH4)dPtC16]. On heating this decomposed to platinum sponge, and thus an economical method was found to produce the pure metal in sufficiently large quantities for industrial use (6).

On a limited scale platinum was already turned into commemorative coins and medals-ften surface-gilded-in Spain in 1780 and in France in 1799. About 1825 a new source of platinum group metals was discovered

Fig. I Russian 3-rouble coins minted in 1829 and 1843, respectively. Retween 1828 and 1845 many hundreds of thousands of platinum coins were struck in the mint

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