To tailor the catalytic activities of metal catalysts at will to achieve efficient conversion in chemical processes remains a challenge, particularly for noble metals, such as Pt. We demonstrate herein that encapsulation within the carbon nanotube (CNT) channels with a diameter of 1.0−1.5 nm not only allows restriction of the size of Pt nanoclusters around 1.0 nm but also enables modulating of the Pt species at the active reduced states through host−guest interaction. The encapsulated Pt is protected from oxygen under reaction conditions in toluene oxidation up to 200 °C, as unveiled by in situ X-ray absorption spectroscopy and density functional theory calculations. As a result, the encapsulated Pt clusters deliver a remarkably higher activity and stability than the clusters located on the open surfaces of the CNT exterior walls and carbon black support, although the latter are much more accessible to reactants. This characteristic of the CNT channels can be explored to tune the properties of other metal catalysts for oxidation reactions.

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