(%) C ) from 4% to 0.05% in air at room temperature. The sensor temperature was recorded at 1 point per second. Inserts of (A) are photograph of a catalytic hydrogen sensor which is a J-type thermocouple coated with Pt NPs or Pt NP assembly, and an expanded view of the temperature response curves. (B) Sensor response (ΔT= T -Tambient, where Tambient is the ambient temperature) as a function of C was decreased from 4% to 0.05% stepwise as in (A).