Promotional effect of silica shell coated NiO physically mixed with Mo/HZSM-5 catalyst on methane dehydroaromatization

In our previous study, the activity and stability of the Mo/HZSM-5 catalyst were enhanced by mixing physically with NiO in methane dehydroaromatization (MDA) reaction. It has been confirmed that the physically mixed NiO not only promoted the dispersion of MoC_x active sites but also reduced the coke formation on the MoC_x owing to the CNTs growth on Ni. However, the promotional effect of NiO was limited when the particle size was reduced, due to the excessive interaction with MoO_x (forming NiMoO₄) which is detrimental to the MoC_x dispersion. In this study, to overcome the limitation, silica shell on NiO particles with various sizes (5, 15, 110 nm) was introduced. The catalyst with silica shell coated NiO with the size of 15 nm exhibited a significant improvement in both BTX yield and stability, and the catalyst with silica shell coated NiO with the size of 5 nm achieved the highest maximum BTX yield, about 7.2%. This study demonstrates that the catalytic performance improved as the NiO particle size decreased with the introduction of the silica shell. Combined transmission electron microscopy-energy dispersive spectroscopy, X-ray diffraction, temperature-programmed surface reaction of methane, CO chemisorption, visible Raman, and thermogravimetric analysis allowed us to confirm that a thin silica shell further enhances the MoC_x dispersion while preventing the formation of Ni-Mo complexes. However, when the size of NiO decreased to 5 nm, CNT growth on Ni was limited during the reaction, which is crucial for reducing coke formation on Mo active sites, thereby resulting in the decreased catalyst stabilization ability of Ni. Overall, this study indicates that the introduction of a silica shell in a controlled way can significantly enhance the promotional effect of physically mixed NiO on MDA.