Development of bifunctional, carbon-supported nanocatalysts for wastewater treatment

Globally 1 in 3 people do not have access to safe drinking water. [1] Households, industry, and agriculture often introduce persistent organic pollutants into their waste streams, and by extension into surface waters, ground water and eventually drinking water supplies.[2] An increasing number of these pollutants are being linked to potentially serious negative health effects.[3-4] The ideal solution to this problem would be to prevent these compounds from reaching surface waters, but failing that, there is a pressing need for sustainable, highly active tertiary water treatment solutions.
The most common experimental tertiary water treatment methods use coagulation flocculation, adsorption on activated carbon materials, ozonation, advanced oxidation processes (AOPs), and various bioremediation processes.[5] Some of these methods do not actually destroy pollutants, just transfer or concentrate them into another phase, which often ends up being incinerated, while others suffer from high costs and/or slow detoxification.
This thesis discusses the development of carbon-supported bifunctional catalysts that are capable of both generating H2O2 and decomposing it into reactive oxygen species, which can then be used to efficiently degrade aqueous organics.
The first chapter of this thesis explores the emergent synergism between a macroscale carbon, graphitic nanofibres (GNFs), and a Fenton like catalyst (MnO2). Extensive characterisation is used to uncover the structure-performance relationships governing the activity of this composite catalyst. A novel metric is also suggested for the field of organics removal, which separates out the different contributions to overall organics removal. This research has shown that the most active MnO2-GNF composite facilitates more than three times as much truly catalytic dye degradation than unsupported MnO2. The ability of the composite to dissipate heat has been identified as a crucial factor in this increased catalytic activity.
The second chapter explores the chemical modification of two macroscale carbon materials, GNF and carbon beads (CB), via acid oxidation, and the effect of this modification on sorption characteristics. Observations show that GNF was mostly unchanged by acid oxidation, but CB went through significant changes. In this case, overall surface area decreased moderately but the ratio of micropore area to overall surface area increased, while the concentration of acidic surface functional groups doubled. This led to a strong suppression in the mostly irreversible adsorption of the studied organics.
These acid oxidised carbon materials (oGNF and oCB) are then used as catalyst supports in Chapter 3. In this chapter the synthesis and thorough characterisation of Co, Mn spinels is discussed, along with the catalytic performance of composites made of the most active spinel, and GNF, oGNF, CB and oCB. It was found that the formation of composites only led to an enhanced catalytic performance in the case of Co2MnO4-oCB, compared to the unsupported catalyst. Results also show that oCB composites facilitate a much higher truly catalytic activity, partly because of the prevention of adsorption.
Finally, in Chapter 4 the electrocatalytic activity of the previously synthesised composites are investigated in O2 reduction. The electrochemical activity of GNF and oGNF supported Co, Mn spinels has been investigated with multiple electroanalytical methods. Results showed negligible O2 reduction activity in the case of all studied materials.
This research highlights the suitability of macroscale carbon materials as catalyst supports in Fenton-like chemistry, while also emphasises the need for a nuanced approach to catalytic activity in the field, as not all organics removal modes are equally desirable. Additionally, the results discussed here show that the acid oxidation of macroscale carbon supports is a viable way of enhancing the catalytic activity of composites made with such materials.