Palladium is an interesting element in surface chemistry. It is a noble metal and an active catalyst in many reactions. Palladium or palladium alloys are used in the fields of catalysts, electronics or hydrogen seperation or purification. One special application is its use as a catalyst for metallising non-conductors like platic materials. In this paper the literature concerning the deposition of catalytic palladium for the surface activation prior to the deposition of electroless nickel and other metals is reviewed.
1. Palladium, the element
Palladium is of particular interest in surface chemistry; it is both a noble metal and an active catalyst in many reactions. Being one of the “light” triad of platinum group metals (PGMs) it has a density of 12.0, as compared with e.g. platinum, 21.45; or iridium, 22.65 . Thus a palladium deposit weighs about 55 % of the same thickness of platinum, per unit area; in addition, metal prices of approximately US$ 22 per gram for palladium, US$ 52 per gram for platinum , would weigh heavily with the customer, if cost was the sole consideration. (As a benchmark, gold was US$ 54 per gram at this same period.) Palladium occurs mainly alongside Pt and other noble metals in e.g. South Africa, Russia and the Americas. The price is greatly affected by world politics and varying demand for Pd to use in automotive exhaust catalysts.
Palladium is described variously as “a lustrous white metal which forms the exception in the platinum group of metals by dissolving in hot nitric acid”  or a grey-white metal, similar to Pt in colour, both being less intensely white than Rh. The writer considers Pd electrodeposits to appear a little yellower than Pt electrodeposits, but this is a matter of opinion.
The wrought metals Pd and Pt when annealed are very ductile; various claims that Pd can be beaten out into very thin foils are made, in the same way as gold leaf is made. In the case of Pd foil a thickness of 100 nm is claimed , which is similar to that sometimes cited for translucent gold leaf.
Palladium world consumption is over 9 million troy ounces per year (one troy ounce is 31.1 grams).
Of this, automotive catalysts demand approximately 5.5 million troy ounces and jewellery demand has fallen to about 0.6 million troy ounces recently. Electronics, petrochemical and other industrial purposes consumed most of the balance of 2.9 million troy ounces. Indeed, plating uses very little palladium in world terms, it seems .
The Periodic Table suggests that Pd and Pt will have more properties in common, than those of Rh, Ir or Ag. For example, Pd and Pt exhibit valencies of 2 or 4, in particular forming a large number of analogous tetra-valent complexes, e.g. diammine dinitrite or diammine dichloride.
An example of a physical property in common is that molten Pd adsorbs oxygen, which is evolved upon solidification, as is also the case with Ag.
Palladium uniquely occludes a remarkably high quantity of hydrogen, e.g. as much as 900 volumes of hydrogen in 1 volume Pd; initially Pd hydride forms as a solid solution, which transforms to the b-phase below 300 °C with a consequent expansion of the metal and damage to deposits or solid Pd components. Use of a Pd-Ag25% alloy, which is dimensionally stable, avoids this disruption; thus ultra-purification of hydrogen gas may be done by passing impure H2 over a heated tube of Pd-Ag alloy  without physically damaging the tube.
This property of Pd needs be borne in mind if unexplained embrittlement or exfoliation of Pd deposits or of those adjacent to Pd are encountered; obviously electrolysis at low cathode efficiency is a source of hydrogen; but an electroless process makes use of a powerful reducing agent, such as hydrazine, and this might lead to unexpected problems.
The writer has encountered a case where parts with a barrel-plated Pd electro-deposit were then barrel-plated with acid gold, in conditions leading to extremely low cathode efficiency in the gold electrolyte. Hydrogen was undoubtedly being released in substantial amounts and was very likely to have been adsorbed in the Pd deposit. The voids observed in the subsequent solder layer after a hot dip soldering operation, were ascribed with some confidence to expulsion of adsorbed H2 from the Pd deposit into the liquid gold-solder alloy formed during hot dip soldering.
As indicated briefly above, the principal uses of Pd (pure or alloyed) are in the fields of catalysis, electronics, hydrogen separation or purification. In the first of these are included both heterogeneous or homogeneous catalysis of organic reactions and as a catalyst for metallising of non-conductors by electroless nickel. Fuel cell development is a growing and important field of research and development; the use of palladium hydride in this field has great potential.
Palladium is a noble metal, admittedly less noble than the other PGMs, in being dissolved not only by aqua regia, but by nitric acid, and also sulphuric acid. It is tarnished by iodine vapour, but since Pd is not attacked by oxygen it does not tarnish in air, except at elevated temperatures; so is very suitable for jewellery, especially rings, mostly plain wedding rings or with precious stones. To the writer’s eye a polished Pd ring is indistinguishable from a similarly polished Pt item, but costs a good deal less.
One wonders at the consumption of 0.6 million oz Pd annually for jewellery – which seems to equate to a very large number indeed of rings, both plain and fancy. This quantity of palladium, as given in Platinum Metals Review , will presumably include Pd applied as a barrier layer in plating of “costume jewellery”.
By way of explanation, low cost decorative jewellery is generally a tin-based casting with a heavy under-plate of bright levelling copper. This is then plated with perhaps 0.3 µm of Pd followed by a bright gold deposit varying from 0.1 to 0.5 µm (perhaps more) in thickness, depending on the product quality and price.
The intermediate palladium deposit restricts inter-diffusion of Cu and Au – the thicker the Pd, the longer the time before copper tarnish disfigures the surface. This is now a well-established technique to produce relatively low-cost necklaces, brooches, bracelets and other such decorations, thus supplying the “bling” essential to modern lifestyle at moderate cost, and most importantly, free from significant nickel or lead content. Indeed, many such products appear more attractive to the writer’s untutored eye than genuine pieces of very high value; much depends on the designs, of course.
To conclude this brief survey, mention must be made of the diminishing use of pure Pd for electronic contacts. This is partly due to its wear resistance being poorer than hard acid golds; and in part due to any organic vapour present in electronic enclosures being catalysed by Pd, to form “frictional polymer” . The source of the organic vapours is presumably insulating varnishes and adhesives, or resists applied to printed circuit boards, etc. A very thin final gold flash vastly improves performance of Pd on e.g. connectors. The use of Pd in Pd-Ni20% electro-deposits with a very thin over-plate of 0.1 µm gold (hard or soft) on electronics connectors has been shown many times to at least match orthodox nickel plus 0.7 to 1.0 µm hard gold, at reduced cost.
There is now a substantial literature on the use of palladium-nickel and other alloy deposits, especially for electronic connectors, and their performance in test and in service. Interestingly and conveniently, both Pd and PdNi alloy have excellent solderability.
2. Why choose electroless plating methods?
Reasons for choosing electroless deposition include the ability to produce very thin uniform coatings, with minimal plant and materials outlay: often more important is the ability to deposit a coating on discrete, that is electrically isolated areas of e.g. printed circuit boards and the like: and the feasibility of producing metal deposits of a purity not generally attainable. The emphasis in this review is on electroless deposition of significant thickness of Pd in aqueous medium by use of a reducing agent, as distinct from the very thin deposits of this metal employed to initiate true electroless (i.e. auto-catalytic) deposition. The writer suggests there are several processes and applications to be differentiated here:
2.1. Displacement (immersion) plating
This is also known as replacement (or cementation) plating; a suitable acidic solution containing cations or complex cations of a more noble metal, attacks the surface of a less noble metal; the redox reaction (below) illustrates the deposition of Pd metal ad-atoms. This reaction is of course identical to that involved in copper sulphate solution depositing a copper film on tool steel blanks prior to “marking-out” – or surprising schoolchildren by magically coppering iron nails.
For example, a clean, active copper surface is immersed in a solution of palladous chloride (PdCl2) and hydrochloric acid (HCl):
Cuº + PdCl2 → CuCl2 + Pdº (overall)
Pd++ +Cuº → Pdº + Cu++ (ionic)
Thus copper is oxidised to Cu+2 ions and Pd+2 ions reduced to metallic Pd. The “driving force” is of course the potential difference between two metals, as indicated by the standard electrode potentials in Table 2. These values are volts, at 25 °C, for unit ion activity, taking the hydrogen electrode as zero volts; factors such as temperature, presence of other ions, agitation etc. will significantly affect these values.
A metal with a greater negative potential has a greater tendency to form positive ions; thus a metal displaces another from a solution when its electrode potential is more negative (or algebraically smaller) than that of the second metal. Examples are that Zn displaces Fe, Cd, Cu or Ag, from solution, and that Cu displaces Ag, Hg or Au from solution (the writer is reminded of the detection of Hg in qualitative analysis by its visible deposition on a clean copper foil).
Free acid is essential for dissolution of substrate to drive the reaction, and appears to be critical, although metal concentration is far less critical than might be supposed – for palladium displacement plating at least. Johnson  found that the optimum acid concentration for an HCl-PdCl2 solution for displacement plating of Pd on copper was 250 ml/l of concentrated (32 % w/v) HCl , but as little as ± 5 % variation in acid content caused exfoliation of the Pd when seeking to obtain maximum deposit thickness. At optimum conditions, up to 1.75 µm Pd was obtainable in 12 minutes at 25 °C without exfoliation, but varying the metal content from 1 up to 10 g/l Pd had no effect.
In contrast, a platinum chloride analogue cited by Johnson will deposit Pt only when hot, e.g. 65 °C; Johnson readily obtained up to a maximum of 1.5 µm Pt on copper in 8 minutes at this temperature using 5 g/l Pt.
Rhoda discusses  displacement processes for precious metals, but cites Johnson’s data for the platinum metals. The writer has serious doubts as to the usefulness of thicker immersion deposits, such as those obtained by Johnson in excess of 1 µm. Indeed, one generic patent for a variety of immersion plating processes  including platinum, claims only 0.035 µm Pt on copper for example, after 15 minutes at 100 °C.
Consideration of the mechanism for producing coatings by a displacement reaction suggests they will in general tend to be rather thin, because this reaction must occur via porosities once overall coverage is formed. This reaction is obviously driven by dissolution of the substrate; deposition is in fact a convenient by-product of a corrosion cell.
As the deposit becomes thicker, some pores may become bridged over, but the dissolution of substrate necessary to maintain the deposition of a coating metal requires that some porosity must remain. Extending the process time to obtain a thicker deposit risks under-cutting the immersion deposit as a result of continued dissolution of the substrate; this is discussed and illustrated by Jones  (in the context of platinum deposition), and includes further comments on displacement electroless plating which may be of interest.
Alternatively, all pores are rapidly sealed and further reaction stifled. Rhoda (op. cit. ) observed that sometimes a polymolecular layer of the precious metal forms on the substrate, sealing it off so no further reaction can occur. Such deposits are usually quite thin, perhaps from 0.05 to 0.25 µm thick. He also noted that thick porous coatings up to 2.5 µm have been reported, with further reaction resulting in exfoliation; up to this critical point adhesion is usually good, Rhoda claims.
2.2. Activation of discrete surfaces prior to electroless plating
Electroless plating processes are also described as “autocatalytic electroless plating”, because the deposited metal is capable of catalysing further deposition via chemical reduction of the metal ion (or complex) so long as reducing agent continues to be available. The first and most widely known was “electroless nickel” employing hypophosphite as the reducing agent, first reported by Brenner & Riddell  in 1946 and having a still-growing field of applications. Another major application is that of electroless copper deposition on printed circuit boards (PCB or PWB), with particular emphasis on deposition of copper within through-holes; a very substantial field in itself.
There is a substantial literature on electroless plating; see for example Brenner  or Mallory & Hajdu .
In fact, initiation of copper or nickel electroless deposition on to copper circuits on PCBs is easily done by a brief application of cathodic current or contact with an active Fe or Ni wire, so as to produce a small area of active Cu or Ni surface – provided all those circuits have a common “ground” connection. With even a modest number of wholly isolated circuits on a PCB, contacting every such area on just one PCB, let alone production quantities, with a piece of fine nickel wire was the stuff of nightmares. Additionally there was a growing requirement for plating the side-walls of through-holes in the boards, the surface being a mix of epoxy resin and glass fibres.
How to initiate the desired catalytic reduction?
Techniques have existed for many years to initiate deposition of adherent Cu or Ni, on to polythene or ABS plastics especially for “Plating on Plastics”. Briefly this was to roughen dielectric surfaces, ensure other surfaces were clean and oxide-free as needed, followed by either the two-stage sequence (I) or the mixed-reagent sequence (II):
(I) “sensitising” in SnCl2 /HCl followed by “activating” in PdCl2/HCl,
(II) “catalysing” in a mixed SnCl2/PdCl2/HCl reagent, followed by an “accelerator”, usually HCl or NaOH.
In both cases the aim is to ensure there are sufficient active palladium foci on a plastics surface for adequate initiation of electroless chemical reduction of a metal, e.g. Cu or Ni onto plastics generally. And it also works very well on the copper circuits of PCBs without needing any individual electrical contact. This is achieved by exchange reaction between Pd++ ions and metallic Cu in acid media. As for any plating process, a poorly cleaned surface will adversely affect adhesion, coverage and uniformity of the deposit. Obviously, when plating discrete areas of Cu or Ni (separated by non-conductive resist), it is essential that this Pd activation does not also activate the resist surface.
It is now well established that in sequence (I), the sensitiser deposits Sn++ ions on the surfaces to be plated, and in the following activator, Pd++ ions become attached to the substrate by displacing the Sn++ ions which are then largely removed by the weakly acidic solution.
An example of type I sequence, taken from Keuler et al is:
Sensitising: SnCl2.2H2O; 1 g/l plus HCl (32 % w/v) 1 ml/l
Activation: Pd(NH3)4(NO3)2 1.5 mls of 10 % sol’n/l plus HCl (32 % w/v) 1 ml/l
Keuler et al were concerned with electroless Pd deposition on alumina membranes and were persuaded that activation is not required for metals, other than stainless steels and Ti. It’s not necessarily so the writer has considerable experience of copper or brass items not initiating at all in electroless baths – unless briefly in contact with a different metal to provide a triggering potential difference.
Examples of Type II sequence: Feldstein et al  examined, via electron microscopy, the mixed reagent route for activating (or catalysing) dielectrics rather than metals, concluding that Pd3Sn was present in the best type II processes which they investigated. These are described in Table 3.
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