Platinum–palladium is the volume seller among bimetallics, but on the production line the problems that genuinely get solved are often solved by combinations whose names sound obscure: platinum–tin, platinum–ruthenium, platinum–iridium. What they have in common is this — they exist not to make platinum more active, but to stop platinum failing under specific operating conditions.

Alloys of this kind generally have no versatility; move them to a different reaction and they may even be worse than pure platinum. But on their own home ground they are frequently the choice with no substitute.

1. Platinum–Tin: The Mainstay for Dehydrogenation and Reforming

Platinum–tin is the most mature class of platinum alloy catalyst in industrial use, and its main battlefield is petroleum refining and commodity chemicals.

Catalytic reforming. The conventional monometallic platinum reforming catalyst has a long-standing weakness: abundant hydrogenolysis side reactions and rapid coking, requiring frequent regeneration. Once tin is added, it dilutes and breaks up the contiguous platinum ensembles — and hydrogenolysis is precisely the reaction that requires a larger platinum ensemble to occur. The result is suppressed hydrogenolysis, less coking, improved liquid yield and stability, and a markedly longer run length. This is the most classic industrial success story of the geometric effect.

Propane dehydrogenation and long-chain paraffin dehydrogenation. These reactions must convert alkanes to olefins at high temperature, where thermodynamics are unfavourable, side reactions are numerous and coking is severe. Platinum–tin supported on alumina is one of the mainstream solutions, and is used industrially at large scale to produce the feedstock for linear alkylbenzene. Tin plays two roles here at once: it moderates platinum's excessive adsorption of carbon (reducing cracking and coking), and it provides Lewis acid sites that participate in the reaction pathway.

Selective hydrogenation. Tin alters the electronic state of platinum in such a way that selectivity between C=O and C=C can be reversed. This is highly valuable in fine chemicals and fragrance synthesis — where both functional groups are present in the molecule and only one of them is to be reduced.

Platinum–tin in one sentence: it gives up part of its absolute activity in exchange for selectivity and life. In commodity continuous processes, that trade pays off handsomely.

2. Platinum–Ruthenium: The Standard Answer to Carbon Monoxide

The reputation of platinum–ruthenium rests mainly on one specific situation: platinum is poisoned in a CO-bearing atmosphere, whereas platinum–ruthenium is not (or is far more tolerant).

The bifunctional mechanism mentioned earlier shows up here more clearly than anywhere: CO binds tightly to platinum sites and suffocates the platinum. Ruthenium, however, can activate water at a lower potential and generate oxygen-containing species (OHad); these migrate to adjacent platinum sites and oxidize the adsorbed CO to CO₂, driving it off, whereupon the platinum immediately regains activity.

Typical applications:

• Direct methanol fuel cell (DMFC) anode: methanol reformate gas, and the electrochemical oxidation of methanol itself, inevitably produce CO; PtRu black and PtRu on carbon have long been the standard configuration in this field.
• Reformate hydrogen feed for proton exchange membrane fuel cells: even trace CO in the hydrogen is enough to collapse the performance of a pure platinum anode, and platinum–ruthenium is one of the routes to solving it.
• Low-temperature CO oxidation and reactions related to the water–gas shift.
• Selective hydrogenation and nitrate reduction, among other environmental catalysis directions, are also under research and in application.

One point must be stated clearly: platinum–ruthenium is not "a stronger catalyst" — in clean hydrogen it is often worse than pure platinum. Its value rests entirely on tolerance to impurities. So before deciding whether to use it, check first whether your feed contains CO.

Also, ruthenium's potential window is narrower than platinum's; it dissolves and is lost at high potentials, so long-term durability requires a sufficient number of cycles in testing.

3. Platinum–Iridium: The Alloy Built for Extreme Environments

Platinum–iridium has a relatively narrow range of catalytic applications, but in several specific fields it is all but irreplaceable — and the other half of its value does not lie in catalysis at all.

On the catalytic side:

• Formic acid / methanol electro-oxidation: adding iridium alters platinum's electronic structure and improves tolerance to intermediates.
• Oxygen-evolution-related reactions: iridium itself (especially iridium oxide) is the most stable material for the oxygen evolution reaction in acidic media; platinum–iridium and platinum–iridium oxide combinations are used to balance activity against stability.
• Ammonia oxidation, and particular hydrogenation/dehydrogenation systems: used as a modifying component.

Its principal use, however, is as a structural material. Iridium has an extremely high melting point, good high-temperature strength, and resists attack by molten glass and molten salts. Platinum–iridium alloys (as well as platinum–rhodium and multi-component platinum–iridium–rhodium alloys) are therefore used extensively in the melting and forming equipment of the glass industry — crucibles, feeder channels, stirrers and bushings. Equipment of this kind serves for long periods in corrosive melts above a thousand degrees; pure platinum creeps and distorts and is thinned by corrosion, whereas adding iridium markedly extends service life.

It is also used for electrode materials (electrophysiology electrodes, industrial electrodes), high-temperature components and precision devices.

So platinum–iridium occupies a special position: it is both a catalyst component and, more than that, a precious-metal structural material able to withstand extreme environments. When procuring products of this class, mechanical properties and corrosion resistance often carry as much weight as catalytic activity.

4. Other Combinations Worth Mentioning

• Platinum–cobalt and platinum–nickel: important alloy systems for electrocatalytic oxygen reduction, raising activity per unit of platinum through lattice compression and the electronic effect — one of the main technical routes for cutting platinum consumption in fuel cells. The drawback is that the transition metal dissolves out under acidic conditions, so surface treatment or a core–shell structure is required.
• Platinum–germanium, platinum–lead, platinum–bismuth (some belonging to Zintl phases / intermetallic compounds): research-stage selective hydrogenation catalysts with an extremely strong site-isolation effect, capable of very high selectivity.
• Platinum–copper and platinum–silver single-atom alloys: a small amount of platinum dispersed as isolated atoms in a base-metal matrix, pursuing the ultimate reduction in platinum usage.

5. Common Cautions: Three Points That Apply to Every Platinum Alloy

First, alloying does not equal improved performance. It is a directional tool. Establish the cause of your deactivation (coking? CO poisoning? hydrogenolysis side reactions? high-temperature corrosion?) before choosing the corresponding second component — otherwise you may well pay more money for lower activity.

Second, verify that it really is alloyed. The ratio figures alone mean nothing. Use XRD to look for a diffraction peak shift (a solid solution shifts systematically; phase separation gives two independent sets of peaks); use electron microscopy with energy-dispersive spectroscopy to see the elemental distribution within individual particles; use XAFS to check whether coordination paths other than Pt–Pt exist. Two physically mixed powders do not constitute an alloy catalyst.

Third, preferential dissolution is a long-term risk. The more active component in the alloy (ruthenium, tin, cobalt, and iridium at certain potentials) dissolves and is lost in service; the surface gradually becomes enriched in platinum and the structure changes. Performance decay is often non-linear — steady at first, then a sudden drop past a certain point. Durability testing must run for a sufficient duration and number of cycles; looking only at initial data leads to serious misjudgement.

6. The One-Sentence Summary

The logic behind these "niche" alloys is highly consistent: what pure platinum cannot solve, a second metal is brought in to cover.

Platinum–tin covers selectivity and coke resistance; platinum–ruthenium covers CO poisoning; platinum–iridium covers high-temperature corrosion resistance and mechanical strength. Each holds its own ground, and none is transferable to another. When selecting, establish your principal mode of deactivation first, then choose the corresponding alloy component — reverse that order and you can easily end up buying a catalyst that is "more expensive but worse."



Diethylenetetramethyldisiloxane platinum complex (platinum catalyst) MY 8116