Materials for the photocatalytic treatment of recalcitrant organic waste

The photocatalytic degradation of cinnamic acid, by TiO2, has been studied extensively in aerobic and anaerobic conditions and in the presence of common salts often found in industrial waste waters. Analysis of the intermediates formed found that molecular oxygen is central to forming the important radicals for the main benzaldehyde mechanism by which cinnamic acid initially degrades, as well as a key component required for the mineralisation to carbon dioxide. In the absence of molecular oxygen an alternate, but slower, pathway becomes the prevalent. The hydroxyl driven mechanism is capable of decarboxylation of the initial carboxyl group but further oxidation does not occur at a reasonable rate. By investigating the effect of salts in solution we found that sulfates and chlorides both interfere with degradation mechanisms and decrease the mineralisation efficiency of titania photocatalysis. Sulfates blocked important surface sites needed for substrate binding that inhibited the benzaldehyde pathway and slowed down the degradation pathway. Whilst chloride formed radical chlorine species (Cl∙) in the presence of TiO2 that resulted in the increase of cinnamic acid removal and the emergence of new reaction pathways. Cl∙ competed with the superoxide radical anion (O2∙-) to attack across the double bond of cinnamic acid, decarboxylate and form several new acetophenone-derived intermediates. A widening of the intermediate map, through the formation of new intermediates, is coupled with a significant slowing of total mineralisation which presents real issues for photocatalytic waste water treatment where chloride ions are present. Additionally, the chlorine radical induced pathways produce intermediates of a greater toxicity; bringing the implication that partial oxidative degradation could result in waste water with an increased toxicity. Anodic nanotubes were explored as an option for alternative materials to be used within photocatalytic reactors. Nanotubes anodised for 8 hours were found to be the most photoactive in the liquid phase, and in the surface degradation of contaminants, due to the wider pores that were structurally sound enough to not slope and reduce light penetration. The surface topography was identified as the key factor for promoting photocatalysis. It was also found that the materials had a cross-phase applicability, in that the most active liquid phase nanotubes were also the most efficient for surface degradation. The incorporation of tungsten into the anodisation process did not improve the photocatalytic activity. Photodeposition of palladium and gold resulted in a decrease in the degradative efficiency of the nanotube arrays. Pd/TiO2 and Au/TiO2 powders were found to reduce the degradation rate of cinnamic acid in oxygenated conditions, although both metals improved the oxidation of surface deposits of carbon. In deoxygenated conditions, Pd/TiO2 catalysts exhibited superior degradation of cinnamic acid in comparison to plain TiO2 and gold doped catalysts. Enhancements in the mineralisation rates, to CO2, were also found. The improvements were attributed to the presence of palladium improving charge separation and introducing new reaction sites capable of decarboxylating the alcohol and aldehyde functionalities, respectively. While the gold nanoparticles were poorly dispersed, they were found to increase the selectivity for phenylacetaldehyde, in deoxygenated conditions, by a factor of 5

Hydrogen generation by photocatalytic reforming of potential biofuels: polyols, cyclic alcohols and saccharides

We have studied hydrogen gas production using photocatalysis from C2-C5 carbon chain polyols, cyclic alcohols and mono and di-saccharides using palladium nanoparticles supported on a TiO2 catalyst. For many of the polyols the hydrogen evolution rate is found to be dictated by the number of hydroxyl groups and available α-hydrogens in the structure. However the rule only applies to polyols and cyclic alcohols, while the sugar activity is limited by the bulky structure of those molecules. There was also evidence of ring opening in photocatalytic reforming of cyclic alcohols that involved dehydrogenation and decarbonylation of α Csingle bondC bond

Purification of therapeutic & prophylactic mRNA by affinity chromatography

In vitro transcribed mRNA is an emerging therapeutic and prophylactic modality with the potential to transform medicine. The drug platform features exceptionally rapid development and versatility of manufacturing processes. Despite the prompt advancement of mRNA from trials to market, purification challenges remain. The cell-free synthesis of mRNA is responsible for the generation of product and process-related impurities, creating the potential for immunogenic effects and decreased translatability into the clinic. Affinity chromatography presents itself as an effective primary capture step for the isolation of functional transcripts from product and some process related impurities. Developing platform processes for the affinity purification of mRNA is hindered by the varying strand lengths of non-amplifying, self-amplifying, and trans-amplifying constructs, with disparities in capacity being observed. Ligand chemistries may contribute to non-specific binding events which remain challenging to characterise. Improved elution and wash conditions may be pursued through novel ligand chemistries, enhanced density and spacing. Regardless of the size or application of the product, the impurities generated by in vitro transcription represent a significant obstacle to the safe administration and long-term storage of mRNA. Affinity chromatography is a valuable tool in overcoming these challenges, with current commercially available products relying heavily on oligo deoxythymidine ligand chemistries. Whilst affinity chromatography is highly valuable in the purification of mRNA, the inability to separate key secondary structures such as double-stranded RNA means it remains to be seen if this technology will adopt the same position as protein A does in mAb manufacture

The effect of acid treatment on the surface chemistry and topography of graphite

Highly oriented pyrolytic graphite (HOPG) samples were investigated as model catalyst supports. The surfaces were treated with dilute HCl and HNO3 under ambient conditions and examined with atomic force microscopy and scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Raised features were formed on the HOPG surface after acid treatment. These protrusions were typically 4–6 nm in height and between 10 and 100 nm in width, covering 5–20% of the substrate for acid concentrations between 0.01 and 0.2 M. Both width and surface density of the features increases with acid concentration but the heights are not affected. STM images show that the graphite lattice extends over the protrusions indicating that the features are “blisters” on the surface rather than deposited material, a view that is supported by the XPS which shows no other significant adsorbates except for oxygen in the case of the nitric acid. We propose that penetration of the acid at defective sites leads to a decrease in the interplanar van der Waals forces and a local delamination similar to the “bubbles” reported between exfoliated graphene sheets and a substrate. These findings are important in the context of understanding how carbon supports stabilise active components in heterogeneous catalysts

The photocatalytic destruction of cinnamic acid and cinnamyl alcohol: Mechanism and the effect of aqueous ions

Cinnamic acid was chosen as an exemplar molecule to study the effect of potential contaminants on the kinetics and mechanism of the photocatalytic destruction of hydrocarbons in aqueous solutions. We identify the principal intermediates in the photocatalytic reaction of the acid and corresponding alcohol, and propose a mechanism that explains the presence of these species. The impact of two likely contaminants of aqueous systems, sulfate and chloride ions were also studied. Whereas sulfate ions inhibit the degradation reaction at all concentrations, chloride ions, up to a concentration of 0.5 M, accelerate the removal of cinnamic acid from solution by a factor of 1.6. However, although cinnamic acid is removed, the pathway to complete oxidation is blocked by the chloride, with the acid being converted (in the presence of oxygen) into new products including acetophenone, 2-chloroacetophenone, 1-(2-chlorophenyl)ethenone and 1,2-dibenzoylethane. We speculate that the formation of these products involves chlorine radicals formed from the reaction of chloride ions with the photoinduced holes at the catalyst surface. Interestingly, we have shown that the 1-(2-chlorophenyl)ethenone and 1,2-dibenzoylethane products form from 2-chloroacetophenone when irradiated with 365 nm light in the absence of the catalyst. The formation of potentially dangerous side products in this reaction suggest that the practical implementation of the photocatalytic purification of contaminated water needs to considered very carefully if chlorides are likely to be present

The effect of acid treatment on the surface chemistry and topography of graphite

Highly oriented pyrolytic graphite (HOPG) samples were investigated as model catalyst supports. The surfaces were treated with dilute HCl and HNO3 under ambient conditions and examined with atomic force microscopy and scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Raised features were formed on the HOPG surface after acid treatment. These protrusions were typically 4–6 nm in height and between 10 and 100 nm in width, covering 5–20% of the substrate for acid concentrations between 0.01 and 0.2 M. Both width and surface density of the features increases with acid concentration but the heights are not affected. STM images show that the graphite lattice extends over the protrusions indicating that the features are “blisters” on the surface rather than deposited material, a view that is supported by the XPS which shows no other significant adsorbates except for oxygen in the case of the nitric acid. We propose that penetration of the acid at defective sites leads to a decrease in the interplanar van der Waals forces and a local delamination similar to the “bubbles” reported between exfoliated graphene sheets and a substrate. These findings are important in the context of understanding how carbon supports stabilise active components in heterogeneous catalysts