Enhancement of biohydrogen production and low coke formation by applying Ni/Zr_xCe_1-xO₂ catalyst in steam gasification of spent coffee ground in monolithic reactor

To produce H₂-rich gas, steam gasification of spent coffee grounds is performed in this study by applying Zr_xCe_1-xO₂ supports (x = 0.2, 0.6) in a monolithic reactor. Compared with conventional Ni/γ-Al₂O₃, Ni/ZrxCe_1-xO₂ catalysts offer higher gas yields and H₂ generation. In particular, Ni/Zr_0.6Ce_0.4O₂ exhibits the highest gas yield (86.32 wt%), H₂ selectivity (56 vol%), and low coke formation owing to its favorable Ni dispersion, high oxygen mobility, and completely homogeneous nanometer-sized morphology. A significant increase in gas yield was observed in the monolith reactor (M@Ni/Zr_0.6Ce_0.4O₂) compared to the fixed bed reactor (F@Ni/Zr_0.6Ce_0.4O₂), from 74.85 wt% to 86.32 wt%. H₂ selectivity (56.87 vol%) is also increased, which can be associated with the unique properties of the monolithic reactor. This phenomenon is attributed to the increase in residence time and the amount of mass and heat transfer between the catalyst and vapor, which favorably direct the selectivity toward H₂. The use of metal-oxide composite supports, especially Ni/Zr_0.6Ce_0.4O₂ catalyst, effectively alleviated coke formation (0.7 wt%), whereas the amount of coke produced by Ni/γ-Al₂O₃ (2.4 wt%) was more than three times higher. Overall, exploiting the remarkable benefits of Zr_xCe_1-xO₂ (x = 0.2, 0.6) composite oxides as supports for Ni catalyst and monolithic reactors would have an emerging outlook for H₂ generation via gasification technology.