Perovskite catalysts for Advanced Oxidation Processes (AOPs) in wastewater treatment

Abstract

Advanced oxidation processes (AOPs) are desirable to treat industrial wastewaters containing dyes, mainly attributed to degradation by in-situ generated highly oxidizing hydroxyl radicals (HO•). However, current AOPs generally require chemical additives or energy that hinder their economic competitiveness. Metal oxides following a general formula ABO, including perovskites, have been applied as heterogeneous catalyst for dye degradation under dark ambient conditions without additional chemical/energy input. Nevertheless, significant research gaps still exist associated with the sluggish degradation kinetics and structural stability of catalysts in addition to a limited number of metal oxides reported to date as catalysts under dark ambient conditions.There is an array of metal oxides that can be used as catalysts for dye degradation under dark conditions which remain unexplored. The research gaps in this theme include oxides of copper, nickel and cobalt as B-site cation, whilst alkaline earth metals (Sr, Ca and Mg) in addition to lanthanide elements (Ce and La) could be incorporate into the A-site, similar to perovskites. These metal oxides can be formulated into binary (ABO), ternary (AA'BO or ABB'O) or quaternary (AA'BB'O) compounds. This thesis therefore aims to understand the fundamental correlation of physicochemical properties and catalytic performance of these metal oxide compounds. It was hypothesized that the partial substitution of A' or B' in ABO3 confers tuneable catalytic properties to enhance the degradation performance for Orange II (OII), which is a major water pollutant from dye-related industries.The first key contribution of this thesis is the successful preparation of CuO based catalysts (AA'BO) as CaSrCuO (CSC) formed both metal oxide and perovskite phases. The varied Ca content resulted in materials with different phases and CSC with high Ca content was very effective, reaching 80% of OII (20 mg L-1) degradation in 10 min with 60% TOC removal after 120 min. CSC proved to be stable and maintained high performance up to nine cycles (75%). CSC was very active in breaking down azo bonds of OII, thus generating electrons which reacted with dissolved O2 in the solutions to yield reactive species (e.g., HO•) for further degradation and mineralization.The second contribution of this thesis is the replacement of Cu by Ni (again AA'BO) as CaSrNiO (CSN), which formed a very active Ni2+ phase in the CSN metal oxide. It was found that 97% OII was discoloured within 5min though TOC removal was low (~10%). The fast degradation was aided by electron donation from Ni2+ to O2, resulting in the formation of Ni3+ and reactive species (e.g., HO•). The Ni3+ proved to be catalytically inactive for OII degradation under dark ambient conditions, and the catalytic performance of CSN decayed rapidly under cycling testing.The third contribution of this work was born out from a desire to stabilise the very active NiO. Therefore, the research approach was to partially substitute Ni in CSN with Cu as AA'BB'O compound to give CaSrNiCuO (CSNC). Indeed, CSNC was stable over 15 cycles with the added benefit of maintaining a high OII degradation efficiency of 84% and good mineralization (54% in 2h). Apart from electron transfer from the breakdown of azo bonds to O2 to generate radical species, extra second electron transfer pathway was postulated from Ni2+/Cu1+ in pristine CSNC to Ni3+/Cu2+ in the spent catalyst, where Cu2+ also proved to be an active phase for the long-term catalytic stability.The fourth contribution of this thesis focused on CoO as another B-site metal oxide and using the A-site cations (Ba, Ca and Mg) and A' site (Sr) as ASrCoO. These compounds resulted in variable morphology, crystallite size, phases and catalytic performance. Ba and Ca was partially substituted in the A-site and formed Ba0.5Sr0.5CoO3 and (Ca0.2Sr0.8)5Co4O12 perovskites, respectively, whereas Mg could not be incorporated, forming MgO and SrCoO perovskite. These compounds reached high degradation efficiency though the degradation kinetics was low (90% in 8h). A major finding here is that ASrCoO compounds proved to be stable over 7 cycles, particularly for BaSrCoO with ~85% degradation efficiency over 7 cycles. However, the catalytic performance of the non-substituted SrCoO for OII degradation decayed at every cycle, thus confirming that the partial substitution as ASrCoO conferred superior stable catalytic properties.The current project designed, synthesized, characterized and evaluated a series of metal oxide based heterogeneous catalysts. The reported catalysts herein dispensed the use of valuable chemicals and energy input while demonstrating high catalytic activity and superior recyclability for OII degradation under dark ambient conditions, which is of great importance for potential practical applications. It is anticipated that they could be applied as efficient alternative materials for low cost AOPs in the field of water treatment.