Bimetallic CoNi single atoms supported on three-dimensionally ordered mesoporous chromia: highly active catalysts for n-hexane combustion

Developing the alternative supported noble metal catalysts with low cost, high catalytic efficiency, and good resistance toward carbon dioxide and water vapor is critically demanded for the oxidative removal of volatile organic compounds (VOCs). In this work, we prepared the mesoporous chromia-supported bimetallic Co and Ni single-atom (Co₁Ni₁/meso-Cr₂O₃) and bimetallic Co and Ni nanoparticle (Co_NPNi_NP/meso-Cr₂O₃) catalysts adopting the one-pot polyvinyl pyrrolidone (PVP)- and polyvinyl alcohol (PVA)-protecting approaches, respectively. The results indicate that the Co₁Ni₁/meso-Cr₂O₃ catalyst exhibited the best catalytic activity for n-hexane (C₆H₁₄) combustion (T_50% and T_90% were 239 and 263 ℃ at a space velocity of 40,000 mL g^-1 h^-1; apparent activation energy and specific reaction rate at 260 ℃ were 54.7 kJ mol^-1 and 4.3×10^-7 mol g_cat^-1 s^-1, respectively), which was associated with its higher (Cr⁵⁺ + Cr⁶⁺) amount, large n-hexane adsorption capacity, and good lattice oxygen mobility that could enhance the deep oxidation of n-hexane, in which Ni₁ was beneficial for the enhancements in surface lattice oxygen mobility and low-temperature reducibility, while Co₁ preferred to generate higher contents of the high-valence states of chromium and surface oxygen species as well as adsorption and activation of n-hexane. n-Hexane combustion takes place via the Mars-van Krevelen (MvK) mechanism, and its reaction pathways are as follows: n-hexane → olefins or 3-hexyl hydroperoxide → 3-hexanone, 2-hexanone or 2,5-dimethyltetrahydrofuran → 2-methyloxirane or 2-ethyl-oxetane → acrylic acid → CO_x → CO₂ and H₂O.