Industrial catalysis is typically done by flowing gaseous or liquid reactants over a solid catalyst. Because the catalyst is often packed into a chamber in the form of wire mesh, microscopic particles, or some other structured solid with a high surface-to-volume ratio, such reactions tend to be extremely complex—full of hot and cold spots that can affect conversion efficiency. So engineers strive to probe the thermodynamics of the reactions in situ to better model their kinetics and optimize reactor design.

In large systems, a few well-placed thermocouples might suffice. But in laboratory-scale systems, such as microreactors or lab-on-a-chip devices, it’s often impossible to probe the temperature without disrupting the flow and thereby the reaction itself.

Optical techniques, though noninvasive, don’t work when the reactants or the chamber enclosing them are opaque. Nuclear magnetic resonance thermometry is similarly noninvasive but often requires contrast agents that may interfere with a reaction. Traditional...

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