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 (CoNPNiNP/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 (T50% and T90% were 239 and 263 °C at a space velocity of 40,000 mL g−1 h−1; apparent activation energy and specific reaction rate at 260 °C were 54.7 kJ mol−1 and 4.3 × 10−7 mol gcat−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 → COx → CO2 and H2O.