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Rui Ran1 Baohuai Zhao1

1, Tsinghua University, Beijing, , China

The rise of more strict environmental regulations have set new challenges for the development of automotive catalysts around the world. It was reported that 50–80% of the CO and total hydrocarbons (HCs) were produced from automobiles during the cold-start period. MnO2-based oxides would be promising materials, owing to its considerable oxidative catalytic activity and lower price. However, their easily-sintering property or low specific surface area are the main bottleneck for catalytic applications. In this work, MnO2 and its related mixed oxides with various morphologies and phases were synthetized by controlling the process parameters including precursor types and concentration, hydrothermal temperature and time, acid/basic etching, etc.
The results showed that the catalytic activities were significantly affected by the MnO2 phase structures, especially by the different linking modes of [MnO6] octahedral units. The urchin-like γ-MnO2 had a disordered structure, resulting in larger numbers of active oxygen species, leading to a better catalytic performance. It showed the satisfying NO catalytic oxidation activity and stability, even compared with those of noble-metal-based catalysts. Furtherly, Fe precursor was used for modifying the γ-MnO2 by different paths. The FeOx-MnOx synthesized by hydrothermal method showed the better NO catalytic oxidation activity than single phase MnO2. in situ IR of NO adsorption indicated that a proper content of Fe could promote the adsorption of NO and decomposition of nitrate at low temperature, leading the improvement of the catalytic activity at low temperature. Those FeOx-MnOx synthesized by self-template and etching route displayed hollow structures with ultra-large surface area over 428 m2 g−1. They exhibited extremely better oxidative activity, which even could convert 79.1% CO at room temperature.
Considering to improving the stability of MnOx-based catalysts, some defect-induced Mn contained perovskite oxides were prepared by citrate method and selective etching. After etching, the perovskite structures of the catalysts remained, the surface area apparently enlarged up to > 110 m2 g-1. Mn ions tended to exist in higher valence state and abundant activated oxygen species and vacancies were created, owing to the loss of La3+. Similarly, the high-surface-area (> 110 m2 g-1) La-Ce-Mn mixed oxide was successfully synthesized by the same process. The etched catalyst exhibited superior catalytic activities to oxidize CO and C3H8, with 90% conversion of CO and C3H8 at only 73 and 196 °C, respectively, which would meet the requirements for the emission control during cold-start period. All the catalysts show acceptable stability during the long-term reactions.

Reference
Rui Ran, et al, Catalysis Communication, 2017, accepted.
Rui Ran, et al, Applied Catalysis A: General, 2017, 545 (2017) 64–71.
Rui Ran, et al, RSC Advances, 2016, 6, 69855-69860
Rui Ran, et al, Applied Catalysis A: General, 514 (2016) 24–34

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