Directly enhancing the photocatalytic transformation of CO2 and water to renewable fuels.

The increases in Greenhouse Gas (GHG) concentrations and depletion of hydrocarbon based fuels have become global concerns. As a result, a number of methods are being developed which aim to restrict GHG and to develop alternative fuels from sustainable sources. Photocatalysis has shown potential to restrict GHG through CO2 recycling and convert solar energy to alternative fuels through solar H2 production. This thesis describes the development of photo reactors to reduce CO2 emissions and to provide a novel method of producing H2 as a fuel source. A number of photo reactors were developed including three fluidised design concepts and a thin film system. The preliminary evaluation of the reactors was performed using methyl orange (MO) as a model compound and utilising TiO2 and ZnO. The reduction of CO2 and H2 evolution was performed over a range of novel catalysts in the optimised system developed, referred to as the propeller fluidised photo reactor (PFPR). A MO photodegradation efficiency of > 95 % was achieved in all the photo reactors using illumination from a low power 36 W lamp. The reduction of CO2 was performed in the PFPR under varying illumination sources including UV, simulated and natural solar exposure. CO2 reduction was not achieved in the unit under any experimental conditions. However, the evolution of H2 was achieved under both simulated and natural solar irradiation. The results demonstrated that reactor properties such as propeller rotational speed were found to enhance the photo activity of the system through the elimination of mass transport limitations and increasing light penetration. The optimum conditions for H2 evolution were found to be a propeller rotational speed of 1035 rpm and 144 W of simulated solar irradiation, which produced a rate of 109 μmol h-1 g-1 over Pt-C3N4. Under solar irradiation 8 μmol h-1 g-1 was evolved over NaTaO3.La. The rate of H2 evolution over Pt-C3N4 increased from 27 to 109 μmol h-1 g-1, upon increasing the rotational speed of the propeller from 0 to 1035 rpm. Furthermore, the use of the PFPR with solar irradiation displayed the potential for solar photocatalysis applications

Photocatalytic reactors for environmental remediation: a review.

Research in the field of photocatalytic reactors in the past three decades has been an area of extensive and diverse activity with an extensive range of suspended and fixed film photocatalyst configurations being reported. The key considerations for photocatalytic reactors, however, remain the same; effective mass transfer of pollutants to the photocatalyst surface and effective deployments and illumination of the photocatalyst. Photocatalytic reactors have the potential versatility to be applied to the remediation of a range of water and gaseous effluents. Furthermore they have also been applied to the treatment of potable waters. The scale-up of photocatalytic reactors for waste and potable water treatment plants has also been demonstrated. Systems for the reduction of carbon dioxide to fuel products have also been reported. This paper considers the main photocatalytic reactor configurations that have been reported to date

Development of a doped titania immobilised thin film multi tubular photoreactor.

This paper describes a novel doped titania immobilised thin film multi tubular photoreactor which has been developed for use with liquid, vapour or gas phase media. In designing photocatalytic reactors measuring active surface area of photocatalyst within the unit is one of the critical design parameters. This dictate greatly limits the applicability of any semi-conductor photocatalyst in industrial applications, as a large surface area equates to a powder catalyst. This demonstration of a thin film coating, doped with a rare earth element, novel photoreactor design produces a photocatalytic degradation of a model pollutant (methyl orange) which displayed a comparable degradation achieved with P25 TiO2. The use of lanthanide doping is reported here in the titania sol gel as it is thought to increase the electron hole separation therefore widening the potential useful wavelengths within the electromagnetic spectrum. Increasing doping from 0.5% to 1.0% increased photocatalytic degradation by ∼17% under visible irradiation. A linear relationship has been seen between increasing reactor volume and degradation which would not normally be observed in a typical suspended reactor system

Cellulose II as bioethanol feedstock and its advantages over native cellulose.

Alternative renewable energy must emerge to sustainably meet the energy demands of the present and future. Current alternatives to fossil fuels are electricity from solar, wind and tidal energies and biofuels. Biofuels, especially bioethanol could be produced from lignocellulosic feedstock via pre-treatment and fermentation. The cellulose I content of most lignocellulosic feedstock is significant, yet its highly crystalline amphiphilic structure interlinked with the lignin network makes it difficult to process for bioethanol production. Processing lignocellulosic biomass via a range of physico-chemical, mechanical and biological pre-treatment methods have been well established, however a relatively new area on the use of cellulose II (a polymorph of native cellulose obtained via mercerisation or regeneration) for the production of bioethanol is still in its early stages. Hence, this review discusses in detail the advantages of using cellulose II over cellulose I as feedstock for bioethanol production. Furthermore, current green and sustainable methods for cellulose II production and the advantages and disadvantages of each method are discussed. In addition, examples from literature reporting higher fermentable sugar and bioethanol yields using cellulose II as feedstock are reviewed, thereby highlighting its importance in the field of bioethanol production. The conclusion from this review suggests that, in all the cases studied, fermentable sugar and/or bioethanol production was found to be higher when cellulose II was used as feedstock instead of native cellulose/lignocellulosic biomass. This higher yield could be attributed to the modified structural and lattice arrangement of cellulose II, its porous volume and degree of polymerisation

Photocatalytic conversion of cellulose into C5 oligosaccharides.

Cellulose is made up of linear polymers of glucose monomers that could be a crucial source for valuable chemicals and sustainable liquid fuels. Cellulose is however, very stable and its conversion to a useful fuel or platform chemical products remains a significant challenge (Kimura et al 2015 Sci. Rep. 5 16266; Xia et al 2016 Nat. Commun. 7 11162). Photocatalysis is a versatile technology which has demonstrated potential for solar driven processes such as water splitting or solar fuels production and has also been applied to the degradation of pollutants in air and water and for the production of useful products from biomass. Here, we focus on the products that are produced from cellulose (a glucose (C6) based polymer) photocatalysis that compliment hydrogen production. Probing the initial steps via UV-TiO2 photocatalysis, we remarkably find that an array of oligosaccharides containing only five (C5) carbon units is initially produced. As the process continues, C6 oligo oligosaccharides grow to dominate. The photocatalytic process is generally not viewed as a controllable synthetic process; however, these findings show, on the contrary that photocatalysis at semiconductor surfaces can achieve novel reaction pathways yielding new products

Comparative assessment of visible light and UV active photocatalysts by hydroxyl radical quantification.

A simple method for determining hydroxyl radical yields on semiconductor photocatalysts is highly desirable, especially when comparing different photocatalyst materials. This paper reports the screening of a selection of visible light active photocatalysts such as Pt-C3N4, 5% LaCr doped SrTiO3, Sr0.95Cr0.05TiO3 and Yellow TiO2 and compares them against WO3 and ultra violet (UV) light activated TiO2 P25 (standard commercial catalysts) based on their oxidative strengths (OH radical producing capability) using a well-studied chemical probe–coumarin. 7-hydroxycoumarin, the only fluorescent hydroxylation product of this reaction can then be measured to indirectly quantify the OH radicals produced. P25 under UV light produced the highest concentration of OH radicals (16.9μM), followed by WO3 (0.56μM) and Pt-C3N4 (0.25μM). The maximum OH radical production rate for P25, WO3 and Pt-C3N4 were also determined and found to be 35.6μM/hr, 0.28μM/h and 0.88μM/h respectively. The other visible light activated photocatalysts did not produce any OH radicals primarily as a result of their electronic structure. Furthermore, it was concluded that, if any visible light absorbing photocatalysts are to be fabricated in future for the purpose of photocatalytic oxidation, their OH radical producing rates (and quantities) should be determined and compared to P25

The application of a novel fluidised photo reactor under UV-Visible and natural solar irradiation in the photocatalytic generation of hydrogen.

With advancements in the development of visible light responsive catalysts for H2 production frequently being reported, photocatalytic water splitting has become an attractive method as a potential 'solar fuel generator'. The development of novel photo reactors which can enhance the potential of such catalyst, however, is rarely reported. This is particularly important as many reactor configurations are mass transport limited, which in turn limits the efficiency of more effective photocatalysts in larger scale applications. This paper describes the performance of a novel fluidised photo reactor for the production of H2 over two catalysts under UV-Visible light and natural solar illumination. Catalysts Pt-C3N4 and NaTaO3·La were dispersed in the reactor and the rate of H2 was determined by GC-TCD analysis of the gas headspace. The unit was an annular reactor constructed from stainless steel 316 and quartz glass with a propeller located in the base to control fluidisation of powder catalysts. Reactor properties such as propeller rotational speed were found to enhance the photo activity of the system through the elimination of mass transport limitations and increasing light penetration. The optimum conditions for H2 evolution were found to be a propeller rotational speed of 1035rpm and 144W of UV-Visible irradiation, which produced a rate of 89μmol h-1 g-1 over Pt-C3N4. Solar irradiation was provided by the George Ellery Hale Solar Telescope, located at the California Institute of Technology

2023 roadmap on photocatalytic water splitting

As a consequence of the issues resulting from global climate change many nations are starting to transition to being low or net zero carbon economies. To achieve this objective practical alternative fuels are urgently required and hydrogen gas is deemed one of the most desirable substitute fuels to traditional hydrocarbons. A significant challenge, however, is obtaining hydrogen from sources with low or zero carbon footprint i.e. so called ‘green’ hydrogen. Consequently, there are a number of strands of research into processes that are practical techniques for the production of this ‘green’ hydrogen. Over the past five decades there has been a significant body of research into photocatalytic (PC)/photoelectrocatalytic processes for hydrogen production through water splitting or water reduction. There have, however been significant issues faced in terms of the practical capability of this promising technology to produce hydrogen at scale. This road map article explores a range of issues related to both PC and photoelectrocatalytic hydrogen generation ranging from basic processes, materials science through to reactor engineering and applications for biomass reforming

Suppressing cyanobacterial dominance by UV-LED TiO2-photocatalysis in a drinking water reservoir: a mesocosm study.

Cyanobacteria and their toxic secondary metabolites present challenges for water treatment globally. In this study we have assessed TiO2 immobilized onto recycled foamed glass beads by a facile calcination method, combined in treatment units with 365 nm UV-LEDs. The treatment system was deployed in mesocosms within a eutrophic Brazilian drinking water reservoir. The treatment units were deployed for 7 days and suppressed cyanobacterial abundance by 85%, while at the same time enhancing other water quality parameters; turbidity and transparency improved by 40 and 81% respectively. Genomic analysis of the microbiota in the treated mesocosms revealed that the composition of the cyanobacterial community was affected and the abundance of Bacteroidetes and Proteobacteria increased during cyanobacterial suppression. The effect of the treatment on zooplankton and other eukaryotes was also monitored. The abundance of zooplankton decreased while Chrysophyte and Alveolata loadings increased. The results of this proof-of-concept study demonstrate the potential for full-scale, in-reservoir application of advanced oxidation processes as complementary water treatment processes