How Does Platinum Make Nitric Acid

The Metal That Won't Burn, But Makes One of Chemistry's Nastiest Acids

Here's the thing about platinum — it's the metal that laughs at fire. On the flip side, toss a platinum wire into a Bunsen burner flame and it just sits there, glowing cherry red, completely unmoved. Practically speaking, meanwhile, everything else either burns or melts. So when I first heard that platinum is used to make nitric acid, my reaction was: "Wait, how? It doesn't even react with oxygen.

That's exactly the puzzle. Platinum doesn't make nitric acid by burning. Practically speaking, it makes it by being so stubbornly unreactive that it can catalyze a reaction between gases that would otherwise just ignore each other. Real talk — this is one of those processes where the catalyst is more interesting than the product.

What Is the Platinum-Catalyzed Ostwald Process?

Nitric acid doesn't just appear in nature. You can't mine it, distill it, or ferment it. It has to be manufactured, and the dominant industrial method has been the Ostwald process for over a century. Here's the basic idea in plain terms: you start with ammonia, burn it in air to make nitrogen monoxide, then oxidize that to nitrogen dioxide, and finally dissolve it in water to get nitric acid.

The catch? Now, left to themselves, these gases react sluggishly at best. The first step — converting ammonia and oxygen into nitrogen monoxide — needs a kick in the pants. But introduce a platinum-rhodium gauze catalyst at around 900°C, and suddenly the reaction roars to life.

This isn't platinum acting as a reactant. It's platinum acting as a matchmaker. The ammonia molecules stick to the platinum surface, break apart, and recombine with oxygen in a completely different way than they would in the gas phase. The platinum provides an alternative pathway with a much lower activation energy.

Why Platinum? Because Nothing Else Works

So why platinum and not, say, iron or nickel? Now, both of those are cheaper and commonly used as catalysts elsewhere. The answer comes down to two brutal realities: temperature and corrosion.

The Ostwald process runs hot — typically between 800°C and 950°C. In real terms, platinum has a melting point of 1,768°C, so it's literally sitting in the reaction zone without breaking a sweat. So most metals either melt, oxidize rapidly, or both at these temperatures. So more importantly, it doesn't react with the oxygen or water vapor present in the reactor. Iron would rust itself into oblivion in minutes.

But here's what most people miss: platinum isn't used in its pure form. Still, industrial catalysts are almost always platinum alloyed with rhodium — typically 10-20% rhodium. Pure platinum works, but it slowly volatilizes at those temperatures, losing mass over time. Also, rhodium stabilizes the structure and extends the catalyst's life. It's expensive, but so is shutting down a nitric acid plant every few weeks to replace catalyst gauze.

How the Reaction Actually Works

Let me walk you through what happens on that platinum surface, step by step.

First, ammonia molecules collide with the hot platinum gauze. The platinum doesn't bond permanently with ammonia — it just holds the molecules briefly enough for the N-H bonds to weaken. The nitrogen-hydrogen bonds start to break apart.

Then oxygen from the air stream reacts with the freed hydrogen atoms, forming water. That's why meanwhile, the nitrogen atoms are now primed to grab oxygen, forming nitrogen monoxide (NO). This is the critical step that wouldn't happen efficiently without the platinum surface.

The nitrogen monoxide gas then flows to a second chamber where it meets more oxygen and gets oxidized to nitrogen dioxide (NO₂). This step happens spontaneously at lower temperatures — no catalyst needed.

Finally, the nitrogen dioxide is absorbed in water, producing nitric acid. On top of that, the classic reaction: 3 NO₂ + H₂O → 2 HNO₃ + NO. That leftover NO gets recycled back through the system.

The whole process is continuous. Ammonia and air go in one end, nitric acid comes out the other, and the platinum gauze sits in the middle, unchanged, doing its job over and over.

The Price of Being Platinum

This is where the story gets real. Platinum is expensive — like, eye-wateringly expensive. A kilogram of platinum metal costs thousands of dollars. But the catalyst gauze in a typical nitric acid plant might weigh several kilograms. And it degrades over time, even with rhodium stabilization.

Plants lose a few percent of their platinum catalyst to evaporation with each batch. Some facilities recover the platinum from the exhaust gases, condensing it back onto the catalyst structure. Over months, that adds up to real money. Others just accept the loss as the cost of doing business.

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There's also the purity problem. In practice, any contamination on the platinum surface — sulfur compounds, heavy metals, even trace amounts of other chemicals — can poison the catalyst and kill the reaction. The feed gases have to be scrubbed clean before they ever touch the platinum.

Common Mistakes People Make Understanding This Process

I've seen this explained wrong more times than I can count. Here's what usually gets mangled:

Thinking platinum is consumed. It's not. The platinum gauze comes out looking identical to when it went in (minus the gradual evaporation loss). If your mental model involves platinum turning into something else, you've missed the whole point of catalysis.

Confusing this with nitric acid production in the lab. In a chemistry lab, nitric acid is often made by distilling saltpeter (potassium nitrate) with concentrated sulfuric acid. That's a totally different process. The platinum-catalyzed Ostwald process is industrial scale, continuous, and feeds most of the world's nitric acid demand.

Assuming all nitric acid plants use platinum. Some older or smaller facilities use other catalysts, like iron oxide-based systems. They're less efficient and produce lower-grade acid, but they avoid the platinum cost. The trade-off is real.

Missing the rhodium detail. Pure platinum works fine in the lab. In industry, you need the rhodium alloy, or the catalyst won't last long enough to be economical. This matters because rhodium is even more expensive than platinum.

What Actually Works in Practice

If you're running an industrial nitric acid plant, here's what you learn quickly:

Keep the ammonia-to-air ratio just right. Because of that, too much ammonia and you get unreacted NH₃ slipping through. Too little and you're wasting oxygen. The sweet spot is usually around a 1:6 to 1:8 ratio of ammonia to air.

Temperature control is everything. Worth adding: drop below 800°C and the reaction slows dramatically. Because of that, go above 1,000°C and you start getting unwanted side products like N₂O and even elemental nitrogen. The platinum gauze also starts evaporating faster.

Catalyst replacement isn't annual — it's measured in years. A well-maintained platinum-rhodium gauze can last three to five years before needing replacement. When you do replace it, recovering the spent catalyst pays for a significant chunk of the new batch.

Pressure matters less than you'd think. The Ostwald process typically runs at slightly above atmospheric pressure. Higher pressure doesn't help much because the reaction is already gas-phase and the equilibrium favors the products.

Frequently Asked Questions

Can you make nitric acid without platinum?

Yes, but not efficiently. Iron oxide catalysts work at higher temperatures but produce lower concentrations of nitric acid. Some labs use non-platinum methods entirely, but for industrial-scale production, platinum's performance is unmatched.

Why doesn't the platinum dissolve in the acid?

Platinum is one of the few metals that resists nitric acid attack. Which means that's actually why it was chosen — it can handle the corrosive environment without degrading. It's attacked by aqua regia, but not by nitric acid alone.

What happens to the platinum over time?

It slowly evaporates at the operating temperature. The rhodium alloy helps reduce this, but some loss is inevitable. Plants often recover platinum from exhaust gases to minimize waste.

Is the platinum catalyst reusable?

Absolutely. After each batch, the gauze is removed, cleaned if necessary, and reused. It's one of the few catalysts that can be practically recycled without significant loss of activity.

Why add rhodium instead of using pure platinum?

Pure platinum works but evaporates too quickly at 900°C.