One-dimensional (1-D) nanostructured metal oxides for catalytic oxidation of hydrocarbons [electronic resource]
Keywords: Materials Science and Engineering

Catalytic oxidation of hydrocarbons has been intensively studied, with the purpose of minimizing emissions of pollutants and facilitating the combustion process. Noble metals, such as platinum and palladium, are the most effective catalysts for the oxidation of hydrocarbons. However, the limited supply of these noble metals imposes a need for developing alternative catalysts. Transition metal oxides are attractive alternatives due to their high thermal stability and low cost. Previous studies of metal oxide catalysts have focused on metal oxide nanoparticles (NPs) supported on porous substrates, such as Al2O3, ZrO2 and spinel-type (AB2O4) supports. Although the dispersed metal species over large surface area have shown much higher activity than the bulk metal oxide, there are several limitations. First, interactions between the support and NPs at high temperatures impede the fundamental understanding of the catalytic properties of individual NPs, and limit their application conditions. Moreover, the solid supports limit the loading of NPs because NPs tend to aggregate at large loadings, leading to a decrease in catalytic activity. Herein, one-dimensional (1-D) nanostructured metal oxide were directly grown on metal mesh substrates and used as catalysts for hydrocarbons oxidation. The 1-D nanostructured catalysts benefits from reduced interaction with the substrates, great flexibility in increasing the catalyst loading, and convenience in tuning the surface chemistry for higher catalytic activity, thus exhibit comparable or better catalytic activity and stability compared to the supported NPs. As one of the most active metal oxide catalysts, CuO was used as a model system to demonstrate the effectiveness of the 1-D nanostructured metal oxide catalysts. CuO NWs have been grown on Cu mesh by solid phase diffusion and applied to catalyze methane oxidation reactions. The CuO NWs have shown comparable or even better activity and stability than the supported CuO NPs. Moreover, owing to the fact that the NWs were exposed on the substrate surface and easy to access, two methods were used to tune the NWs for enhanced catalytic activity. The first one was to reduce the CuO NWs to more active Cu2O NWs by H2 plasma, which has shown 20% increase activity for CH4 oxidation reactions and several times higher activity for CO oxidation reactions. The kinetics study have shown that the bulk oxygen diffusion in Cu2O was faster, which could be one of the reasons for higher activity of Cu2O than that of CuO. The second tuning method was to decorate the CuO NWs with more active NP materials, such as Co3O4 and noble metals with a newly developed simple, fast and general sol-flame method. After the Co3O4 decoration, the CuO NWs surface was uniformly and densely covered by Co3O4 NP-chain structures, with large NP loading, high surface area and minimal aggregation, resulting in times higher activity in catalyzing CH4 oxidation. Moreover, this sol-flame method is a general method to decorate NWs with various NPs, and even to dope NWs with dopants for desirable properties. Given the generality and simplicity of the sol-flame methods, it can be applied to not only catalysis, but also other important application areas, such as lithium ion battery, supercapacitor and photoelectrochemical devices. In addition, to incorporate Cu and Co, the most active metal oxide catalyst Co3O4 was grown as 1-D structure on stainless steel mesh with the Cu2+ ion enhanced ammonia-evaporation-induced synthesis method. The synergetic effects of Cu and Co in catalytic process were studied, which have shown that the Cu2+ improved the nucleation and growth process of 1-D Co3O4, however, the catalytic activity is mainly from the Co species.