Nanostructured metal oxides supported on stainless steel wire meshes: versatile monolithic catalysts for environmental and energy applications

Abstract

[EN] Monolithic catalysts have acquired a great importance in industrial processes, energy applications and environmental protection systems. The systems based on these supported catalysts offer several important advantages such as a low pressure drop, good mechanical stability, easy manipulability and good electrical and thermal conductivity, depending on the monolith material. In this PhD Thesis, novel synthesis strategies are presented for the fabrication of catalysts made up of high‐surface area metal oxides supported on flexible stainless steel wire mesh (SSWM). First, several synthesis methodologies for producing non‐polar ZnO nanorods and polar ZnO nanosheets deposited onto SSWM substrates are examined. The use of these materials as photocatalysts has been investigated for the photodegradation of methylene blue in aqueous solution under ultraviolet irradiation. The catalytic activity of some of the tested catalysts is initially higher than that of a commercial TiO2 photocatalyst (P25 TiO2), but it rapidly declines due to photocorrosion. To protect the catalysts against this phenomenon, a new technique that consists in coating the surface of the photocatalyst with a thin layer of polysiloxane has been successfully developed. Second, highly polar high surface area ZnO nanosheets supported on SSWM substrates are used as template for the preparation via sacrificial template accelerated hydrolysis (STAH) of a large variety of nanostructured metal oxides (i.e. CuO, α‐Fe2O3, TiO2, CeO2, CdxZn1‐xO (x=0.01‐0.20) and Ni0.7Zn0.3O) supported on SSWM substrates. The structure of the metal oxides produced is a faithful replica of that of ZnO. The key point of this synthesis strategy is the use, as template, of ZnO nanostructures that have a large surface, consisting mainly of polar facets. These facets are chemically very active and they are responsible for the generation of high surface area metal oxide structures (up to 275 m2 g‐1). These high surface area metal oxides have been investigated in three applications: a) the Fenton‐like process for the degradation of methylene blue in water (α‐Fe2O3), b) the photodegradation of methylene blue in aqueous solution (CdxZn1‐xO and TiO2) and c) the production of hydrogen from methanol (Ni0.7Zn0.3O). SSWM‐supported α‐Fe2O3 has been found to have an outstanding catalytic activity and a good stability in the Fenton‐like process for degrading methylene blue in the presence of hydrogen peroxide. As for the photodegradation of methylene blue in water under UV irradiation with ZnO‐based catalysts, cadmium has been incorporated into the ZnO structure to produce a SSWM‐supported CdxZn1‐xO catalyst that is stable against photocorrosion, while it maintains a significant level of activity. Under visible irradiation this solid solution shows a slightly better behaviour than pure ZnO. For the photodegradation of methylene blue under UV irradiation with SSWM‐supported TiO2, the best results are obtained with the photocatalyst calcined in the 450‐600°C temperature region, where it exhibits an intrinsic catalytic activity much higher than that of P25 TiO2 particles. Finally a SSWM‐supported Ni0.7Zn0.3O has been used for the production of hydrogen in the methanol decomposition reaction, behaving as one of the most active catalysts ever reported in the literature. With the techniques developed in this thesis we are able to fabricate a large number of high surface area metal oxides supported on a monolithic and flexible material (i.e., a stainless steel wire mesh) that can be easily employed in structured catalytic reactors with a high performance in environmental and energy applications.