Catalytic converters have quietly cleaned the exhaust of the world’s vehicles since the 1970s, yet the ceramic honeycomb blocks at their heart have barely changed in shape. Now a team of German researchers has shown that mathematical surfaces borrowed from geometry textbooks, 3D-printed in stainless steel and coated with platinum, can match and in some respects beat conventional designs when tested on a real running engine. The work, published in Results in Engineering, marks one of the first times additively manufactured triply periodic minimal surface (TPMS) catalyst carriers have been evaluated under genuine compressed natural gas exhaust rather than synthetic laboratory gas mixtures.
The research, led by Fatemeh Mehdipour of the Karlsruhe Institute of Technology together with colleagues including Thomas Koch and Roland Dittmeyer, set out to answer a deceptively simple question: can the design freedom of metal 3D printing produce a catalyst support that converts more pollution without choking the engine? Conventional ceramic monoliths are thermally robust but brittle, and the extrusion process that makes them restricts their interiors to simple square or hexagonal channels. That geometry produces largely laminar flow, modest heat and mass transfer, and susceptibility to maldistribution, all of which limit how efficiently harmful molecules reach the catalytic surface. Cold-start conditions, before the catalyst reaches its light-off temperature, compound the problem.
The team selected three TPMS architectures as the building blocks of their experimental carriers: the Schwarz-primitive, the gyroid, and the Schwarz-diamond surfaces. These are mathematical minimal surfaces that repeat periodically in three dimensions, producing smooth, continuous, fully interconnected channel networks with no sharp junctions. Such geometries can pack a large surface area into a small volume while promoting uniform flow distribution, offering a way to soften the classic trade-off between catalytic surface area and pressure drop. Each monolith was designed as a cylinder 50 millimetres tall and 23.88 millimetres across, sized to slot into a sample holder on an engine test bench, with unit cell size, orientation, and even axial size gradients varied systematically to isolate each design parameter’s influence.
Twenty-seven monoliths were fabricated from gas-atomised 316L stainless steel powder using laser powder bed fusion, with 0.2 millimetre walls guided by prior work showing that gas-tight thin walls in the 200 to 300 micrometre range are achievable with the process. Post-processing was deliberately gentle: ultrasonic cleaning flushed loose powder from the internal channels, and wire electrical discharge machining, rather than force-based cutting, separated the parts from the build plate to avoid distorting the thin-walled circular cross-sections. The finished samples measured within a few hundredths of a millimetre of their nominal dimensions, confirming that the intricate internal lattices could be manufactured reproducibly.
Coating the printed structures was arguably the riskiest step. Washcoating, the deposition of a catalytic ceramic layer onto a substrate, has historically been tricky on additively manufactured surfaces because of roughness, wettability, and adhesion concerns. The researchers prepared a stable suspension of platinum supported on gamma-alumina, with a median particle size near 200 nanometres, and dipped each monolith twelve times, spin-drying between immersions before calcining at 500 degrees Celsius. Drop tests and ultrasonic exposure showed relative washcoat losses below 2.5 percent for nearly all geometries, evidence that the rough printed surface actually helps by mechanically interlocking the ceramic layer. Scanning electron microscopy revealed porous, interconnected washcoat morphologies that should aid gas diffusion, along with an axial thickness profile that was thickest near the monolith ends, likely a consequence of slurry depletion and redistribution during drying.
Pressure-drop screening on longer, uncoated 150 millimetre samples showed that specific surface area alone does not dictate flow resistance. The Schwarz-diamond lattice with 5 millimetre cells, which packs the highest surface area of the set, also produced the highest pressure drop, while the open Schwarz-primitive structure with 10 millimetre cells flowed more freely than even the straight-channel hexagonal reference. In between, the gyroid geometries occupied a promising middle ground, their smooth continuous passages balancing gas-surface contact against hydrodynamic resistance.
The decisive test came on a three-cylinder 2.19-litre compressed natural gas engine operating at 1500 revolutions per minute and 150 newton-metres of load, with the air-fuel equivalence ratio swept from 0.9 to 1.4. Exhaust gas was routed through a bypass containing the washcoated monoliths, and inlet and outlet concentrations of methane, carbon monoxide, and nitrogen oxides were measured by Fourier-transform infrared spectroscopy and an exhaust gas analyser. Under stoichiometric conditions, the two best TPMS configurations exceeded 90 percent CO conversion, with the compact Schwarz-diamond SD(5,s) sample reaching approximately 99 percent. Normalising conversion by washcoat loading revealed that catalyst utilisation, not merely the amount of platinum deposited, differed substantially between topologies and was strongly influenced by unit cell orientation.
Equally striking was evidence of three-way catalytic behaviour. Methane and nitrogen oxide conversions peaked near stoichiometric operation, and under fuel-rich conditions the researchers observed ammonia forming inside the monolith, a signature of nitrogen oxides being reduced over platinum with CO and hydrogen acting as reductants. Because no external ammonia was introduced, its appearance confirmed that the printed carriers supported not just oxidation but reductive chemistry, extending their functional repertoire beyond simple CO cleanup. The team is careful to note that these results rest on one engine-tested sample per geometry, so they represent comparative trends within a proof of concept rather than statistically validated rankings.
When conversion and pressure drop were mapped together, the gyroid structure with 10 millimetre cells emerged as the standout, delivering higher washcoat-normalised CO conversion than the straight-channel reference while imposing lower pressure drop per unit length. The compact 5 millimetre designs bought their exceptional conversion with a clear backpressure penalty, whereas the most open Schwarz-primitive structures flowed beautifully but underused their catalyst. The authors conclude that performance is governed by the interplay of specific surface area, topology, effective tortuosity, flow distribution, and catalyst accessibility, and they propose TPMS architectures as a tunable design platform for application-specific catalytic supports. Before such printed converters reach production vehicles, they caution, long-term durability, thermal ageing, vibration resistance, benchmarking against commercial substrates, and techno-economic assessment will all need to be demonstrated. But the core message stands: a catalyst carrier generated from pure mathematics, printed layer by layer in steel, survived an engine’s exhaust and cleaned it remarkably well.
Subject of Research: Additively manufactured washcoated TPMS structures as catalytic converter carriers tested under real engine exhaust conditions
Article Title: Washcoated additively manufactured TPMS structures as catalytic converters for engines: A proof of concept
Article References: Mehdipour, F., Heinrich, J. E., Yu, Z., Kutscherauer, M., Tucker, M. R., Rubin, M., Wagner, U., Koch, T., & Dittmeyer, R. (2026). Washcoated additively manufactured TPMS structures as catalytic converters for engines: A proof of concept. Results in Engineering, 32, Article 113039. https://doi.org/10.1016/j.rineng.2026.113039
Image Credits: AI Generated
DOI: 10.1016/j.rineng.2026.113039
Keywords: additive manufacturing, TPMS, catalytic converter, gyroid, laser powder bed fusion, washcoating, CNG engine, exhaust aftertreatment, pressure drop, platinum catalyst, three-way catalysis, 316L stainless steel
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Denise Maddox. (September 22, 2026). 3D-Printed Gyroid Catalysts Convert Real Engine Exhaust With Less Backpressure. Scienmag. https://scienmag.com/3d-printed-gyroid-catalysts-convert-real-engine-exhaust-with-less-backpressure/
Denise Maddox. “3D-Printed Gyroid Catalysts Convert Real Engine Exhaust With Less Backpressure.” Scienmag, 22 September 2026, https://scienmag.com/3d-printed-gyroid-catalysts-convert-real-engine-exhaust-with-less-backpressure/. Accessed 22 September 2026.
Denise Maddox. “3D-Printed Gyroid Catalysts Convert Real Engine Exhaust With Less Backpressure.” Scienmag. September 22, 2026. https://scienmag.com/3d-printed-gyroid-catalysts-convert-real-engine-exhaust-with-less-backpressure/
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Tags: 316L stainless steeladditive manufacturingcatalytic converterCNG engineexhaust aftertreatmentgyroidlaser powder bed fusionplatinum catalystpressure dropthree-way catalysisTPMSwashcoating
