Atomic Layer Deposited Ni/ZrO₂-SiO₂ for Combined Capture and Oxidation of VOCs

This work reports on the development of novel Ni nanoparticle-deposited mixed-metal oxides ZrO₂-SiO₂ through atomic layer deposition (ALD) method and their application in combined capture and oxidation of benzene, as a model compound of aromatic VOCs. Concentrating ppm-level VOCs in situ, before their oxidation, offers a practical approach to reduce the catalyst inventory and capital cost associated with VOC emissions abatement. The benzene vapor adsorption isotherms were measured at 25 °C and in the pressure range of 0 to benzene saturation vapor pressure thereof (0.13 bar). In the combined capture-reaction tests, the materials were first exposed to ca. 86 100 ppm benzene vapor at 25 °C, followed by desorption and catalytic oxidation while raising the bed temperature to 250 °C. The textural properties revealed that ALD of Ni or ZrO₂ on SiO₂ decreased surface area and pore volume, while sequential doping with ZrO₂ and then Ni caused the otherwise. The benzene vapor adsorption isotherms followed the type-IV isotherm classification, revealing a combination of monolayer-multilayer and capillary condensation adsorption mechanisms in sequence. At saturation vapor pressure, an average equilibrium adsorption capacity of 15 mmol/g was obtained across the materials. However, the dynamic adsorption capacities were up to 50% less than the corresponding equilibrium uptake for the materials. Benzene desorption temperature was observed around 90 °C, and conversion of 85-95% and TOF of 1.28-16.42 mmol_C6H6/mol_Ni/s were obtained over the materials, with 3Ni/ZrO₂-SiO₂, prepared with 3 ALD cycles, exhibiting the maximum conversion and TOF indicating synergistic effects of Ni nanoparticles and ZrO₂ support based on the number of ALD cycles. However, the yields of CO₂ and H₂O were about 5% and 40%, respectively. The small value of the CO₂ yield was hypothesized to be due to simultaneous high-temperature adsorption of CO₂ as the catalytic reaction progressed. The high adsorption affinity, low desorption temperature, and high catalytic activity of the materials investigated in this study made these materials as promising candidates for the abatement of BTX.