DOC catalytic converter

Keeping DOC Catalyst Coating Consistent – Batch After Batch After Batch

I've cut open cheap DOC catalytic converters where the coating looked like someone smeared it on with their fingers. Thick here, thin there. Bare metal showing through.

Those DOC converters don't last. No surprise.

We do it different. Here's how we keep the washcoat and precious metal coating the same on every diesel oxidation catalyst that leaves the shop.

The Slurry Has to Be Right

The washcoat is a ceramic goop. Alumina, water, some binders. If the goop changes, the coating changes. Too thick, it plugs cells. Too thin, not enough surface area.

We make our own washcoat here. Same recipe every time. Same thickness. We use a Ford cup – just a little cup with a hole. Time how long it takes to drain. Same number every batch.

We filter it too. No lumps. Lumps would clog the dip tank.

Once we got a batch of alumina powder that was too coarse. The washcoat looked fine but didn't flow right. Caught it on the viscosity check. Sent the powder back. Saved a whole run.

Dip and Blow – Don't Guess the Pressure

The DOC substrate goes in the washcoat tank. Comes out dripping. Then we blow the excess out with an air knife – a curtain of air that pushes the goop out of the cells.

Air pressure is key. Too low, too much washcoat stays – high backpressure. Too high, you blow too much out – low surface area.

We set the pressure with a regulator. Same pressure every time. Operators check it at the start of each shift.

After that, we weigh the DOC. The weight gain tells us the washcoat loading. Target is something like 50 grams per liter, plus or minus 2. If it's outside that, the part gets tossed.

We log every weight. That's our first check.

Drying – Take It Slow

The wet washcoat has to dry without cracking. Dry it too fast, it shrinks and cracks open. Cracks mean less surface area and the coating can flake off later.

We dry the DOC in an oven. Ramp the temperature slow – from room temp to 120°C over two hours. Then hold.

Air circulates inside to keep it even. No hot spots.

The oven has a temperature recorder. We check the chart after every batch. If the ramp was wrong, we find out why.

Firing – Hot Enough, But Not Too Hot

After drying, the DOC goes into a hotter furnace. That's where the washcoat bonds to the metal.

Too cold, the washcoat doesn't stick – it flakes off. Too hot, the washcoat over‑sinters – pores close up, surface area drops.

We run the same profile every time. Ramp to 500°C, hold 30 minutes, then cool. Thermocouples inside log every batch.

We also run test coupons – little metal pieces with the same washcoat. After firing, we bend the coupon. If the washcoat flakes off, the firing was bad. If it stays put, good.

Precious Metals – Platinum and Palladium

Now the active stuff. We use a liquid solution with platinum and palladium salts.

Dip the DOC. Dry it. Fire it again.

The precious metal loading is in grams per cubic foot of substrate. For a typical DOC, about 30 grams per cubic foot – a mix.

We control loading by the concentration of the solution and the dip time. We send samples to a lab for assay regularly.

After coating, we weigh the DOC again. The gain tells us the loading. It's a small gain – maybe 10 grams for a big DOC – so our scales are precise.

If the loading is off by more than 5%, the batch gets re‑coated or scrapped.

Final Fire – Lock It In

One more firing. This fixes the precious metals to the washcoat.

Same deal – temperature control is everything. Too low, the metals don't bond. Too high, they clump together.

Same furnace profile every time. Same ramp, same soak, same cool. Logged.

After firing, we test a sample DOC on our lab reactor. Flow synthetic exhaust at 300°C, measure CO and hydrocarbon conversion. Numbers within spec? Batch ships.

What We Check for Every Batch

Honestly, there's a list.

Viscosity of the washcoat slurry. Logged.

Washcoat weight gain. Recorded.

Drying oven chart. Saved.

Firing profile. Saved.

Precious metal solution assay. Lab report.

Precious metal weight gain. Recorded.

Final fire profile. Saved.

Lab reactor test. On file.

And we keep a coated sample from every batch. Labeled with batch number and date. A year later, customer has a problem, we can go back and test that sample.

What Goes Wrong – And How We Catch It

Seen plenty of coating failures.

Washcoat too thick. Cells plug. Weight gain too high.

Washcoat too thin. Low surface area. Lab reactor test shows low conversion.

Drying too fast. Cracks. We see it under a microscope.

Firing too hot. Over‑sintered. Lab reactor test fails.

Precious metal loading low. Lab assay catches it.

Precious metal not bonded. Tape test – put tape on, pull, look for metal flakes.

Every one of these has happened at some point. That's why we check at every step. To stop bad parts before they ship.

Customers Can Check Too

If you buy DOC catalytic converters from us, ask for the coating batch records. We'll show you the logs.

Cut one of our DOC substrates open. The washcoat should be even all through. No thick spots, no bare metal.

Send a sample to an independent lab. They'll tell you the precious metal loading and distribution.

We've had customers do that. Every time, the report came back consistent. That's why they stay with us.

DOC catalyst coating consistency doesn't happen by accident.

Slurry viscosity. Dip and blow pressure. Drying ramp. Firing temp. Precious metal solution. Final fire. Testing.

We do all that because we've seen what happens when you don't. Uneven washcoat. Low precious metal. Poor adhesion. Short DOC life.

A diesel oxidation catalyst is only as good as its coating. And a consistent coating is only as good as the process behind it.

We take that seriously. Batch after batch after batch.