Metal Oxides Film Catalyst for VOCs Degradation Derived from Hydrotalcite

Emission of volatile organic compounds (VOCs) from industrial gases is major contributors to air pollution which may pollute the atmosphere directly or indirectly as secondary pollutants, such asozone generation and photochemical smog. VOCs may be also found in non-industrial indoor air environments, which can act as irritants to the human organism and have negative health effects, and cause bad indoor air quality. Catalytic oxidation with high efficiency and less or no byproducts is recognized as an effective method for VOCs degradation. The monolithic film catalyst recently attracted much attention for their advantages of sufficient exposure of active sites with superb heat conduction and recycling. Transition metal oxides (Fe₂O₃, Cr₂O₃,CuO, Co₃O₄, NiO and MnO₂) as a potential alternative of precious metals, have received much attention for their superior properties such as low cost, high resistance to poisoning, good reducibility. Studies reveal that mixed metal oxides (MMO) own higher catalytic activities than single metal oxide, even equivalent or better than precious metal because of the synergistic effect. To design smartly catalysts with large surface area ratio, rich oxygen species and low temperature reducibility will lead outstanding catalytic activity.

Layered double hydroxides (LDH) are a class of naturally occur-ring and synthetic materials generally expressed by the formula[M²⁺_1−xM³⁺_x(OH)₂](A⁻)_x/n· mH2O, in which M^II and M^III cations disperse in an ordered and uniform manner in brucite-like layers, and Aⁿ⁻ is a charge compensating anion such as NO₃⁻,·CO₃²⁻. The cation composition of LDH-related mixed oxide catalysts can be simply adjusted during precursor synthesis. After heating at moderate temperatures, the resulting mixed oxides catalysts from a topotactic transformation of LDH give finely dispersed mixed oxides of M^II and M^IIImetals with a sufficiently large surface area and good thermal stability. LDHs can in situ grow into thin film over varied substrate with hexagonal laminar structure stick to the substrate under proper synthetic condition. Furthermore, the synergistic effect due to the good interspersion of the oxide phases would possess some desirable features, like low temperature reducibility and more lattice oxygen for the improvement of catalytic activity. Besides, the presence of LDH matrix inhibits the agglomeration of active components and thus enhances the long-term stability.

The monolithic transitional metal oxides film catalysts were obtained from the calcination of the in situ growth of the corresponding LDHs precursor on aluminum substrates with nearly 1.32 mg/cm² by weight. CoMnAl-MMO sample displays the temperature of 90% benzene conversion (T₉₀) 240 ºC with reaction rate 1.19 mmol g_cat⁻¹ h⁻¹ and space velocity 300,000 mL g_cat⁻¹ h⁻¹, which is higher than CoNiAl-MMO at 275 ºC and CoCuAl-MMO at 315 ºC.