How Do Temperature And Feed Ratios Affect Co Oxidation? Master Pilot Plant Kinetics

CO oxidation in a catalytic pilot plant is not a linear process. Temperature and the oxygen-to-carbon monoxide (O₂:CO) feed ratio jointly dictate both how much CO disappears and how much of it ends up as CO₂. In preferential CO oxidation (PROX) over supported metal catalysts, CO conversion often peaks at an optimal low-temperature window—frequently 100 °C to 120 °C—while CO₂ selectivity can suffer a distinct drop in intermediate ranges (160 °C–180 °C) as competing hydrogen oxidation accelerates. A higher O₂:CO ratio improves CO conversion, but pushing the ratio well above the stoichiometric value eventually erodes CO₂ selectivity without a meaningful gain in conversion.

Core Takeaway: CO oxidation in a pilot plant does not reward a “more is better” approach. Temperature sets the balance between conversion and the loss of selectivity to H₂ oxidation, while the O₂:CO ratio acts as a lever that must be tuned within a narrow window—typically around 1–2—to maximize CO removal while keeping CO₂ selectivity high.

How Temperature Controls the Conversion–Selectivity Balance

Temperature influences every microscopic step: CO adsorption, O₂ dissociation, and the competition from hydrogen oxidation. Understanding these competing effects is the key to stable, efficient pilot-plant operation.

The Kinetic-Thermodynamic Balancing Act

The rate constant (k) rises with temperature according to the Arrhenius equation, so you might expect conversion to increase steadily. However, CO oxidation is an exothermic reaction, and its equilibrium conversion is thermodynamically limited—higher temperatures shift the equilibrium unfavorably. In a pilot reactor, this means there is always an optimum temperature where fast kinetics and favorable equilibrium intersect.

Why a Low-Temperature Window Can Give the Best CO Conversion

Certain supported Pt and Pt-Rh catalysts display a surprising behavior: CO conversion peaks around 100 °C–120 °C and then declines as temperature rises toward 180 °C–200 °C. This happens because at very low temperatures the catalyst surface is CO-saturated, blocking O₂ adsorption; as temperature rises just enough to partially desorb CO, oxygen adsorbs more freely and conversion climbs. But if temperature rises further, the surface coverage of CO drops too much, and the reaction rate may be limited by CO supply or by a shift in the preferred adsorption site, causing conversion to fall.

The Selectivity Drop at Intermediate Temperatures

CO₂ selectivity is not a flat curve. In the 160 °C–180 °C range, selectivity can dip markedly. In PROX mixtures, this is the temperature zone where hydrogen oxidation takes off kinetically, competing for O₂ and producing water instead of CO₂. After that drop, selectivity often stabilizes at a lower level, but the net useful CO oxidation has already been compromised. This is why pilot-plant operators see a clear “selectivity valley” on otherwise smooth conversion graphs.

Thermal Sintering: The Hidden Cost of High Temperature

Temperature not only shifts product distribution—it degrades the catalyst. Copper-based low-temperature shift catalysts, for instance, are highly active but start sintering irreversibly above ∼260 °C. Even robust noble-metal catalysts lose active surface area if hot spots develop. Thus, a temperature that looks attractive in a short test can cripple catalyst longevity in a continuous teaching or research pilot run.

The O₂:CO Feed Ratio as a Precision Lever

Oxygen concentration is the other control knob. Adjusting it changes both the driving force for CO oxidation and the potential for unwanted side reactions.

How Raising the Ratio Boosts CO Conversion

More oxygen means more oxidizing power. Moving the O₂:CO ratio from 0.75 to 1.5 sharply increases CO conversion, as shown by microreactor data. The extra O₂ shifts the surface coverage toward an oxygen-rich state that favors CO₂ formation, overcoming kinetic limitations where CO was previously blocking sites.

The Danger of Over-Oxidation (Why >2 Can Hurt)

The benefit plateaus quickly. For Pt-catalyzed PROX, once the O₂:CO ratio exceeds about 2, CO conversion barely improves while CO₂ selectivity drops. The excess oxygen preferentially reacts with H₂, producing water and wasting O₂. This is a classic case of diminishing returns: you pay a penalty in selectivity for little or no gain in CO removal.

Feed Purity Interacts With the Ratio

If trace sulfur or chlorine are present—even at 0.1 ppm—they poison low-temperature copper catalysts. Pilot plants must incorporate feed purification stages. A contaminated feed makes any ratio optimization moot, because the catalyst deactivates before the intended temperature and concentration effects ever stabilize.

Understanding the Trade-offs

Every parameter you dial in creates a conflict between conversion, selectivity, and catalyst durability. Recognizing these trade-offs is what separates a textbook optimization from a working pilot plant.

Temperature trade-off: Low temperatures (∼100 °C) deliver peak CO conversion but can be dangerously close to the condensation point of water, risking liquid formation in the reactor. Moderate temperatures (150 °C–170 °C) improve thermal stability and avoid cold spots, but you will likely sacrifice some selectivity.
O₂ ratio trade-off: Running lean on O₂ (ratio 0.5–1.0) preserves high CO₂ selectivity but leaves unreacted CO. Pushing to 1.5–2.0 maximizes conversion, yet invites hydrogen oxidation. Any ratio above 2 is usually wasteful.
Space velocity impact: Higher flow rates (lower contact time) can keep catalyst end-temperatures within safe limits through heat removal, but they often reduce conversion. In regenerative reverse-flow configurations, the opposite can occur—higher flow improves heat integration and yields without overheating, but this demanding operation requires precise control that may not be the goal of a teaching pilot.

Making the Right Choice for Your Pilot-Plant Goal

Your operating strategy depends entirely on what you are trying to learn or prove in the pilot plant. Align your set points with the primary educational or research objective.

If your primary focus is maximizing CO conversion: Set the temperature near the catalyst’s reported optimum window (often 110 °C–120 °C) and run at an O₂:CO ratio of 1.5 to 2.0. Accept that CO₂ selectivity will not be at its theoretical maximum.
If your primary focus is achieving the highest CO₂ selectivity: Stay at a lower O₂:CO ratio (0.75–1.2) and operate at a temperature just above the light-off point, then gradually increase to find the best balance before the selectivity drop begins.
If your primary focus is catalyst longevity and stability: Operate in a moderate temperature band (150 °C–180 °C), purify the feed gas to ppb levels of poisons, and avoid O₂:CO ratios above 1.5. This trade-off preserves activity over extended teaching runs, even at the cost of slightly lower initial conversion.

Once you understand that temperature and feed ratio are interdependent levers—not independent dials—you can navigate the conversion–selectivity landscape with confidence, turning your pilot plant into a true learning engine rather than a black box.

Summary Table:

Parameter	Target/Optimal Range	Impact on CO Conversion	Impact on CO₂ Selectivity
Low Temperature	100°C – 120°C	Peaks as CO desorbs & O₂ adsorbs	Remains high
Intermediate Temp	160°C – 180°C	Declines / stabilizes	Drops sharply due to competing H₂ oxidation
Optimal O₂:CO Ratio	1.0 – 2.0	Increases significantly	Balanced performance
Excess O₂:CO Ratio	> 2.0	Plateaus with no added benefit	Decreases as excess O₂ reacts with H₂

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