Yuqiao Fan

In this work, Ho₂O₃ nanosheets were synthesized by a hydrothermal method. A series of Sr-modified Ho₂O₃ nanosheets (Sr-Ho₂O₃-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, N₂ adsorption, SEM, TEM, XPS, O₂-TPD (temperature-programmed desorption), and CO₂-TPD, and they were studied with respect to their performances in the oxidative coupling of methane (OCM). In contrast to Ho₂O₃ nanoparticles, Ho₂O₃ nanosheets display greater CH₄ conversion and C₂-C₃ 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 Ho₂O₃ nanosheets leads to a higher ratio of (O⁻ + O₂⁻)/O²⁻ 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-Ho₂O₃-NS catalyst with a Sr/Ho molar ratio of 0.04, which gives a 24.0% conversion of CH₄ with 56.7% selectivity to C₂-C₃ at 650 °C. The C₂-C₃ yield is well correlated with the amount of moderate basic sites present on the catalysts.