Bessarabov, D.G.Kozhukhova, Alina Evgenievna2022-07-192022-07-192022https://orcid.org/0000-0002-5709-5878http://hdl.handle.net/10394/39369PhD (Chemical Engineering), North-West University, Potchefstroom CampusAnodized aluminium oxide (AAO) has attracted the attention of scientists and engineers as a source material for the preparation of catalyst support nanostructures. AAO represents a hexagonally distributed pore structure with parallel arranged pore channels. The oxide layer formed during the anodization of aluminium (Al) has increased hardness, thermal conductivity, and corrosion resistance compared to Al’s native oxide layer. However, widespread utilization of AAO as a support material is perplexed by the high cost of pure Al (>99%) and difficulties related to the anodization process, such as the requirements for low-temperature (approximately 0 °C) and extensive anodization times. Therefore, numerous studies have focused on the use of an Al substrate of lower purity (<99%) and the development of a suitable anodization method. Despite this, a research gap exists in the preparation of ordered (or at least semi-ordered) AAO structures, and their application as a catalyst support material in hydrogen-based applications. A comprehensive study is therefore proposed, to develop an acceptable anodization method for low-purity Al, and then to evaluate and demonstrate its application as a catalyst support for high-temperature catalytic hydrogen combustion (CHC). The anodization process for a low-purity Al substrate (Al6082) was investigated in Chapters 3–6. The effects of voltage (30–60 V), the type and concentration of the electrolyte (oxalic and phosphoric acid solutions), temperature (10–40 °C), and duration of the anodization process (1–4 h) on the AAO pore arrangement were investigated. The morphological characteristics (pore diameter, interpore distance, pore density, porosity, and thickness of the oxide layer) were determined to assess the prepared AAO layers. Because the Al alloy consisted of alloying additives (classified as impurities here), the effects thereof on the anodization process were considered. Results obtained from the anodization experiments were as follows: (i) the traditional anodization method employed for pure Al could not be applied to the anodization of the Al6082 alloy; (ii) the time of Al alloy anodization was reduced to 1 h, while using milder anodization temperatures (30 °C); and (iii) a relatively ordered AAO arrangement (defined as being near ideally ordered–one pore surrounded by six neighboring pores) was achieved. These results indicated the potential of AAO as a catalyst support on a large scale (beyond laboratory scale) due to cost reductions during its fabrication. The wet impregnation method, using hexachloroplatinic acid (H2PtCl6), was applied to prepare a Pt/AAO catalyst (Chapters 5–6). Characterization of the catalyst (i.e., Pt particle size, surface characterization, and Pt loading) was performed using advanced techniques such as scanning electron microscopy, focused ion beam (FIB) and transmission electron microscopy (TEM), and inductively coupled plasma optical emission spectroscopy. The catalytic activity of the Pt/AAO catalysts was evaluated upon its exposure to hydrogen (Chapters 5–6). Results showed high activity (near-complete hydrogen conversion, spontaneous initiation of the reaction at room temperature) and durability (530 h of the reaction with no catalyst deactivation) of the Pt/AAO catalyst towards the CHC reaction. High activity of the Pt/AAO catalyst was achieved due to the structural features of the AAO layer that allowed high catalyst dispersion on the AAO support. Furthermore, it was the prepared AAO layer closely adhered to the metallic core, which promoted high thermal conductivity of the Al/AAO system. This, in turn, promoted a uniform temperature distribution throughout the catalyst surface, which prevented localized Pt aggregation (Chapters 5–6). High thermal conductivity of catalysts plays a key role when the catalyst is intended to be used for safety (e.g. passive autocatalytic recombiner, PAR) and combustion/heating applications (e.g. cooking, spatial heating) as it prevents the formation of hotspots. These hotspots are defined as the areas where localized surface temperature is higher than the average surface temperature over the catalyst surface. The appearance of hotspots results in non-uniform temperature distribution throughout the catalyst surface and can cause a hydrogen ignition in some cases. The new Pt/AAO catalyst was then tested and evaluated for the passive autocatalytic recombination of hydrogen (Chapter 6). Five Pt/AAO catalysts were installed in an in-house-developed recombiner section testing station and then tested for combustion of 0.5–4 vol% hydrogen fuel. The thermal distribution throughout the catalyst surface was investigated using an infrared (IR) camera. The Pt/AAO catalyst showed high thermal conductivity; a temperature gradient throughout the catalyst surface of 23 °C was observed during the experiment. In addition, the CHC reaction was initiated at room temperature and low hydrogen concentrations (<1 vol%), suggesting high catalytic activity of the Pt/AAO catalyst. The activation energy for CHC on the Pt/AAO catalyst was determined to be 19.2 kJ/mol. To confirm that Pt aggregation/catalyst deactivation was avoided, the Pt-containing AAO layer was cross-sectioned by FIB and characterized using TEM before and after prolonged CHC. A Pt/CeO2, ZrO2, Y2O3 mixed oxide (CZY)/AAO catalyst was prepared and evaluated for CHC applications in Chapter 7. The CZY support was prepared by a co-precipitation process with aqueous ammonia solution, using aqueous solutions of Ce(NO3)3, ZrO(NO3)2, and Y(NO3)3. The Pt/CZY/AAO catalyst was then prepared by spray-deposition Pt/CZY intermediate on an AAO layer adhered to aluminium core. Evaluation was carried out in 1–8 vol% hydrogen/air mixtures. The thermal distribution throughout the catalyst surface was investigated using an IR camera. Thermal imaging revealed a maximum temperature gradient of 36 °C throughout the catalyst surface (3 × 1 cm) for all studied hydrogen concentrations (1–8 vol%). Further, to assess the catalyst durability, the Pt/CZY/AAO catalyst was subjected to prolonged CHC. The catalyst maintained a continuous combustion temperature of 239.0±10.0 °C for 53 days at a hydrogen flow rate of 138 NmL/min. Finally, a Pt/CZY/AAO catalytic plate (14.0 × 4.5 cm) was prepared to investigate the thermal distribution. A maximum temperature gradient of 5.4 °C was obtained throughout the catalyst surface.enAluminium (Al)Anodized aluminium oxide (AAO)Anodization methodAl/AAO catalyst supportPt/AAO catalystCatalytic hydrogen combustion (CHC)High thermal conductivityPassive autocatalytic recombiner (PAR)Thesis