Metal Substrate

Metal Substrate vs. Ceramic Honeycomb Oxidation Catalyst – Full Performance Comparison

Two types of catalyst substrates dominate the emission control world. Ceramic honeycomb – the old standard. Metal honeycomb – the newer challenger.

They look similar. Both are honeycomb. Both get coated with precious metals. Both clean exhaust.

But they work differently. One heats up faster. One has more surface area. One survives vibration. One doesn't crack under thermal shock.

Here's the full comparison – what each one does well, where each one falls short, and which one you should use.

Material and Structure

Ceramic honeycomb is made from cordierite – a magnesium aluminum silicate ceramic. It's extruded as a single piece. The cells are square or hexagonal. Wall thickness is typically 0.1-0.15 mm for automotive applications.

Metal honeycomb is made from thin metal foil – Fe-Cr-Al alloys, stainless steel, or other heat-resistant alloys. The foil is corrugated and wound or stacked into a honeycomb shape. Cell walls are much thinner – down to 0.03-0.05 mm.

The metal substrate has a sinusoidal cell shape that can be easily modified during production. Cell shapes and geometries are more flexible. You can make oval, racetrack, or custom shapes that ceramic extrusion can't easily do.

The difference: Ceramic is a single extruded block. Metal is a fabricated structure. One is brittle. One is ductile.

Thermal Properties – Heat-Up and Light-Off

This is where metal wins. By a lot.

Ceramic has low thermal conductivity – about 1-3 W/mK. Metal conducts heat much faster. The metal substrate heats up faster because of its lower heat capacity, better heat conduction, and better heat transfer.

An 800 cpsi metallic catalyst heats up faster than a 400 cpsi ceramic one. The metal substrate gets to operating temperature sooner. Light-off happens faster.

One study showed that due to smaller heat capacity, better heat conduction, and better heat transfer, the 800 cpsi metallic catalyst heats up faster than the 400 cpsi ceramic one. The volume of the metallic catalyst was half that of the ceramic, yet the geometric areas were almost identical. Both catalysts contained the same amount of precious metals.

The metal heats evenly, so reactions start sooner and emissions drop faster. Ceramics just can't keep up – they lag, and you end up with cold-start spikes.

At low temperature, the metallic honeycomb catalyst showed CO conversion of 54.0% and CO selectivity of 42.5% at 120°C – 1.7 and 1.4 times that of the ceramic honeycomb catalyst, respectively.

The difference: Metal lights off faster. Ceramic takes longer to warm up.

Surface Area and Flow

Ceramic honeycomb has a high geometric surface area (GSA) per unit volume. The channel walls are thicker than metal, but the extrusion process gives consistent, uniform channels.

Metal honeycomb has thinner walls, which means lower pressure drop. More open area. Less backpressure on the engine.

At high space velocities, the metallic substrate presents better conversions than the ceramic substrate – mainly because of its larger geometric surface area and lower transverse Peclet number. More surface area per volume, better mass transfer.

At low space velocities, the ceramic substrate presents better conversions, particularly for HC and CO, possibly because of its lower thermal conductivity which facilitates local ignition.

The difference: Metal flows better at high flow rates. Ceramic holds its own at low flow rates.

Durability – Thermal Shock and Vibration

This is where metal shines.

Ceramic is brittle. It cracks under thermal shock. A rapid temperature change – cold start, then full load – can create cracks. Once cracked, the substrate loses structural integrity. Exhaust bypasses the catalyst. Conversion drops.

Metal is ductile. It doesn't crack under thermal shock. It can withstand rapid temperature changes without fracturing. In contrast to ceramic honeycomb, metal honeycomb is far more robust and does not suffer from irreparable fractures or breaks created by impact. It also resists vibration better.

Metal substrates have improved durability and resistance to thermal shock and vibration compared to ceramic materials. They are used extensively for diesel oxidation catalysts – applications where vibration and thermal cycling are severe.

The ceramic catalyst seems to have a higher tendency towards thermal shock than the metallic.

The difference: Metal survives thermal shock and vibration. Ceramic cracks.

Cost

Metal costs more. The foil is more expensive than ceramic extrusion. The fabrication process – corrugating, stacking, brazing – is more labor-intensive.

Ceramic is cheaper to produce. Extrusion is fast and efficient. Raw materials are less expensive.

The major disadvantage of high-quality metallic substrates is their high cost. But you get what you pay for – faster light-off, better durability, lower backpressure.

The difference: Ceramic is cheaper upfront. Metal costs more but lasts longer.

Application Fit

Metal substrates are used extensively for diesel oxidation catalysts. They're also used as close-coupled pre-converters in gasoline cars – positioned right at the exhaust manifold where heat is highest and light-off speed matters most.

Ceramic substrates are the standard for most automotive catalytic converters – three-way catalysts, DOC, SCR. They work well in steady-state, moderate-temperature applications.

For motorcycles, metal substrates have advantages over ceramic – lower light-off temperature, higher purification efficiency, larger processing capacity, lower exhaust backpressure, stronger vibration resistance, and longer catalyst life.

The difference: Metal for cold-start and high-vibration applications. Ceramic for steady-state and cost-sensitive applications.

Metal substrates win at cold start and durability. They heat up faster, survive vibration, and don't crack under thermal shock. They cost more. They're worth it when you need fast light-off and long life.

Ceramic substrates win at steady-state performance and cost. They hold their own at low flow rates. They're cheaper. They're the standard for most automotive applications.

There's no universal winner. It depends on what you're building.

If you need fast cold-start emissions control, go metal. If you need cost-effective steady-state conversion, go ceramic. If you need both, sometimes you use metal for the pre-converter and ceramic for the main converter.

We make metal substrates. We know what they do well – and where ceramic still makes sense.

That's what we do.