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TEM images of the CeO2–MnOx hollow spheres with various shell numbers obtained at different heating rates: (a) before calcination; (b) single-shell, 2 °C min−1; (c) double-shell, 5 °C min−1; (d) triple-shell, 10 °C min−1. Insets show the corresponding individual hollow sphere; (e) Proposed collision processes of reactive gases against hollow spheres with different shells. Figure reproduced from ref. [122].
