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Among the orders our plant takes, there is one class of customer unlike any other: they don't ask about activity, they don't ask about selectivity — their first question is usually "how long will this gauze last, and how is the spent gauze settled?"
That is because what they are buying is a woven metal gauze over a metre in diameter and weighing tens of kilograms, installed inside an ammonia oxidation furnace where it runs continuously for more than half a year. This is platinum–rhodium alloy gauze — the core equipment of the nitric acid industry, and one of the most mature and irreplaceable applications of platinum group metals in heavy chemical engineering.
1. What It Looks Like
Start with its form, because it is nothing like the powders and pellets that make up ordinary catalysts.
Platinum–rhodium alloy gauze is a metal mesh woven from platinum–rhodium alloy wire. Wire diameters are typically only a few hundredths of a millimetre, and mesh densities often run to thousands of openings per square centimetre. The whole sheet is as thin as window screening, yet it is a real woven metal fabric whose mechanical strength comes from the wire itself.
The alloy is predominantly platinum with rhodium as the secondary element; common ratios are Pt/Rh 90/10 and 95/5. There are also combined designs in which a palladium-based catchment layer is stacked beneath the main gauze — more on that later.
2. The Reaction Step It Catalyzes
The nitric acid industry follows the ammonia oxidation route, i.e. the Ostwald process. The key step is:
4NH₃ + 5O₂ → 4NO + 6H₂O
A mixture of ammonia and air passes through the incandescent platinum–rhodium gauze and is oxidized to nitric oxide in an extremely short time; the NO is then further oxidized and absorbed to yield nitric acid.
The reaction conditions at this step are severe: temperature around 800–900 °C, and gas residence time on the gauze surface on the order of 10⁻⁴ seconds. In other words, the reaction must be complete in the instant the gas stream sweeps across the gauze.
And there is a fatal fork in the road at this step: ammonia can just as easily be oxidized straight to nitrogen (4NH₃ + 3O₂ → 2N₂ + 6H₂O). Forming N₂ means burning feedstock for nothing — not one gram of nitrogen ends up in the product. The value of the platinum–rhodium gauze lies in its highly selective steering of the reaction down the NO pathway; modern plants sustain very high NO yields steadily.
No other metal can hold up on both selectivity and high-temperature stability. That is the fundamental reason why no alternative has emerged in a century.
3. Why It Has to Be a Gauze, Not a Powder or Pellets
This is the question asked most often. There are three answers:
First, pressure drop. Ammonia oxidation is a continuous process handling very large gas volumes, and the gas must pass through the catalyst layer within 10⁻⁴ seconds. A packed bed of pellets or powder would impose too much resistance; fan energy consumption would be unacceptable and flow distribution would become uneven. A woven gauze has a high open area, so the gas passes through almost unimpeded.
Second, contact time must be extremely short. If residence time is prolonged, the NO formed will be further oxidized or even decomposed, and selectivity falls. The thin-layer structure of a gauze naturally provides the "thin and fast" mode of contact — which is itself one source of its selectivity.
Third, mechanical stability at high temperature. Running continuously around 900 °C while enduring gas impingement and thermal cycling rules out powders (which would be blown away) and pellet beds (which would attrite and collapse). A woven gauze maintains its shape by the strength of the wire itself, and can be lifted and replaced as a single unit.
So the gauze is not a compromise; it is the form tailor-made for this reaction.
4. Why Rhodium Is Added to Platinum
Pure platinum gauze was used in the early days, and the problems were practical. Adding rhodium solves two things:
Higher mechanical strength at temperature. Platinum softens noticeably approaching 900 °C; under its own weight and the force of the gas stream, pure platinum gauze readily deforms, sags and even develops holes. Rhodium has a far higher melting point than platinum, and its addition markedly improves the alloy's high-temperature strength and creep resistance, keeping the gauze flat under prolonged thermal load.
Lower volatilization loss of platinum. This is the more critical point. In a high-temperature oxygen-bearing atmosphere, platinum forms volatile oxides that enter the gas phase; the presence of rhodium suppresses this process and reduces the absolute loss of platinum.
The trade-off on rhodium content is clear:
High-pressure units, with their higher gas velocities, greater thermal loads and larger mechanical stresses, normally choose a higher-rhodium ratio; lower-pressure units can use a specification with less rhodium. The choice must be matched to the operating parameters of the specific unit — more expensive is not automatically better.
5. How Platinum "Runs Away"
Platinum–rhodium gauze loses weight continuously in service. This is not wear; it is primarily high-temperature volatilization.
In an oxidizing atmosphere around 900 °C, the platinum surface forms volatile platinum oxides that leave the gauze with the gas stream. These gaseous platinum species re-decompose and deposit at cooler points downstream, coating vessel walls, waste-heat boilers, piping and subsequent catchment devices.
The visible symptoms are: the gauze surface gradually roughens, wire diameter thins, and pinholes appear locally; downstream equipment surfaces develop a grey-black deposit containing platinum.
The loss rate is governed by several factors: operating temperature (the dominant factor — each step up in temperature sharply accelerates volatilization), system pressure, gas velocity and flow rate, ammonia-to-air ratio, and time on stream.
Controlling furnace temperature is therefore the number one lever for extending gauze life. The short-term conversion gain bought by running above temperature often comes at the cost of scrapping the entire gauze prematurely.
6. Catchment Gauze: Capturing the Platinum That Escapes
Since volatilization cannot be entirely prevented, the industrial response is to install a catchment layer beneath the main gauze.
The common practice is to lay a palladium-based catchment gauze or a precious-metal-bearing catchment pad. The principle is straightforward: pass the downstream gas through a material of high specific surface area capable of adsorbing gaseous platinum species, intercepting the volatilized platinum.
The economic value of this arrangement is high. With a catchment layer in place, the net loss of platinum drops significantly, and the catchment layer itself is sent for recovery after a period of service; the platinum concentrated in it is considerable.
For a nitric acid plant this is not optional — it is standard configuration. An ammonia oxidation unit without a catchment system is, in effect, scattering platinum up the stack.
7. Life and Recovery: This Gauze Is an Asset, Not a Consumable
Platinum–rhodium gauze is positioned very differently from ordinary catalysts.
An ordinary catalyst becomes waste when spent; a platinum–rhodium gauze remains a platinum-bearing asset. The reason is simple: it is itself a platinum–rhodium alloy, with defined composition, regular form and virtually no impurity contamination, so the recovery path is extremely short — spent gauze goes straight into smelting and refining, both platinum and rhodium are recovered efficiently, and new wire is drawn and woven into new gauze.
This closes the loop: new gauze → service → weight-loss spent gauze → recovery and refining → new gauze.
Procurement and settlement therefore work differently from other products. The industry norm is pricing on a processing-fee basis: the customer supplies old gauze or settles on the basis of platinum content, the supplier charges a processing and service fee, and the recovered platinum quantity is assayed and measured according to methods agreed by both parties.
Judging what a batch of spent gauze is worth turns on three numbers: weighing, assay of platinum and rhodium content, and a sampling and testing method accepted by both sides. Sampling representativeness is where disputes most often arise — loss across a gauze is not uniform, and edges differ from the centre, windward face from leeward.
Recommended practice: hand over the gauze intact, weigh jointly, sample according to agreed rules, assay by a reliable quantitative method, and retain samples for future reference. Our plant's principle in this business is traceable data and measurement protocols written into the contract up front.
8. Poisoning and Operating Taboos
Beyond weight loss by volatilization, platinum–rhodium gauze also suffers poisoning. Ammonia oxidation is no less sensitive to impurities than fine chemicals work:
Hence several hard requirements: air must be efficiently filtered, feed ammonia purity must meet specification, compression systems must be oil-free, and abnormal operating conditions must be avoided during shutdown and start-up.
Installation also demands care: use dedicated lifting tools to avoid scratching the wire with hard objects; lay the gauze flat with even tension — local sagging causes flow maldistribution and hot spots; follow the prescribed heat-up curve before start-up, since thermal shock will distort the gauze.
One common mistake on site is pushing furnace temperature up to chase output. Higher temperature does raise conversion in the short term, but volatilization loss rises steeply with temperature, and the overall account usually comes out worse.
9. Directions of Technical Evolution
Mature as this gauze is, development has never stopped:
One point must be stated clearly: the demands placed on the catalyst
at the ammonia oxidation step are uniquely exacting — high temperature, extremely short contact time, high selectivity, long-term
mechanical stability — and to date no
non-precious-metal system can match them across the board. Platinum–rhodium gauze will remain in this position for a long time to come.
Diethylenetetramethyldisiloxane platinum complex (platinum catalyst) MY 8116
