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CO Oxidation Light-Off Temperature: T50 and T90 Explained

Part of the CO Oxidation: The Complete Guide — this article is one of the detailed pages in the guide.

Direct answer: The light-off temperature is the temperature at which a CO oxidation catalyst becomes effective — conventionally quoted as T50 and T90, the temperatures where 50% and 90% CO conversion are reached under defined test conditions. In real applications the operating temperature must stay above the relevant light-off point, with margin for gas composition effects.

What T50 and T90 mean

  • T50 — the temperature at which conversion reaches 50%. Below T50 the catalyst is essentially asleep; conversion rises steeply through this region.
  • T90 — the temperature at which conversion reaches 90%. This is the practical "working" point for most compliance duties.

The gap between T50 and T90 is typically narrow (tens of degrees) for a healthy precious-metal catalyst because CO oxidation follows a self-accelerating behaviour — the reaction exotherm warms the catalyst, which raises activity further.

Why test conditions matter

Light-off temperatures are not intrinsic properties. They shift with:

  • Space velocity — higher flow per catalyst volume pushes light-off up.
  • CO concentration — higher CO releases more reaction heat and can pull the effective light-off down, but only once the reaction ignites.
  • Moisture — water occupies active sites and raises light-off, especially below 150°C.
  • SO₂ — poisons sites and raises light-off progressively over time.
  • O₂ level — CO oxidation needs O₂; lean streams are fine, sub-stoichiometric streams are not.

A T90 of 170°C in dry clean gas may become 220°C or worse in a humid, sulfur-bearing stream. Never compare light-off numbers without their test gas composition.

Ignition and the cold-start problem

Light-off describes steady-state behaviour. Cold-start is a dynamic problem: at low temperature the reaction cannot self-sustain, so the catalyst needs external heat to reach light-off. Solutions include:

  • Electric pre-heating (heating tube systems)
  • Heat exchange with hot process gas
  • Bypass operation until the stream warms

Light-off as a monitoring tool

In service, a rising light-off (same conversion needs higher temperature) is the earliest sign of deactivation, often visible before conversion falls at the normal operating point. Periodic light-off measurement is a cheap, sensitive health check.

Manufacturer perspective

We quote light-off with the test gas specification and recommend margin: choose and size so that the normal operating temperature is at least 20–40°C above the expected T90 under the real gas — more if the stream is intermittent or SO₂ is present.

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