| Summary: | In this work, Ho<sub>2</sub>O<sub>3</sub> nanosheets were synthesized by a hydrothermal method. A series of Sr-modified Ho<sub>2</sub>O<sub>3</sub> nanosheets (Sr-Ho<sub>2</sub>O<sub>3</sub>-NS) with a Sr/Ho molar ratio between 0.02 and 0.06 were prepared via an impregnation method. These catalysts were characterized by several techniques such as XRD, N<sub>2</sub> adsorption, SEM, TEM, XPS, O<sub>2</sub>-TPD (temperature-programmed desorption), and CO<sub>2</sub>-TPD, and they were studied with respect to their performances in the oxidative coupling of methane (OCM). In contrast to Ho<sub>2</sub>O<sub>3</sub> nanoparticles, Ho<sub>2</sub>O<sub>3</sub> nanosheets display greater CH<sub>4</sub> conversion and C<sub>2</sub>-C<sub>3</sub> selectivity, which could be related to the preferentially exposed (222) facet on the surface of the latter catalyst. The incorporation of small amounts of Sr into Ho<sub>2</sub>O<sub>3</sub> nanosheets leads to a higher ratio of (O<sup>−</sup> + O<sub>2</sub><sup>−</sup>)/O<sup>2−</sup> as well as an enhanced amount of chemisorbed oxygen species and moderate basic sites, which in turn improves the OCM performance. The optimal catalytic behavior is achievable on the 0.04Sr-Ho<sub>2</sub>O<sub>3</sub>-NS catalyst with a Sr/Ho molar ratio of 0.04, which gives a 24.0% conversion of CH<sub>4</sub> with 56.7% selectivity to C<sub>2</sub>-C<sub>3</sub> at 650 °C. The C<sub>2</sub>-C<sub>3</sub> yield is well correlated with the amount of moderate basic sites present on the catalysts.
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