Johnson Matthey Technology Review - Volume 68, Issue 2, 2024

The photocatalytic effect of titania has long been studied with respect to water oxidation and hydrogen evolution. At present, the modification of this semiconducting material by platinum single atoms (Pt-SAs) represents an interesting approach that has been developed in the past decade and has given good results in the photocatalytic hydrogen evolution reaction (HER). Experimental studies have shown that the deposition of Pt-SAs on the titania surface, in aqueous systems, is a spontaneous process and can also be promoted by different reducing processes. Theoretical studies suggest that this deposition is a site-specific reaction, which occurs in oxygen vacancies on the titania surface. Under such conditions, the Pt-SAs are not in a metallic state, due to the interaction with neighbouring atoms of the substrate. This complex system can be probed using different advanced characterisation techniques, which provide a deeper understanding about the modified surface and how this modification improves the photocatalytic performance of titania.

In this review, we report on recent advances in the use of mechanochemistry to synthesise new catalytic materials. We report recent results obtained by our groups where a rational design of the milling parameters led to the synthesis of advanced materials with novel properties such as unconventional arrangements of metals on the surface of oxide support materials, highly dispersed metals or the stabilisation of species in particular oxidation states. These properties resulted in superior catalytic performances of the mechanochemically-synthesised catalysts compared to their counterparts prepared by traditional impregnation methods. To illustrate these advances, we review the progress made in two important fields of catalysis where noble metals are used: (i) emission control catalysis using palladium-based materials; (ii) the development of photocatalysts to produce hydrogen based on gold and palladium materials.

The article places emphasis on the latest advancements in this field, particularly focusing on indoor and outdoor microplastic (MP) pollution, including their emission, behaviour and potential health hazards. Gaining an in-depth understanding of these factors is crucial for devising effective strategies to mitigate the impact of microplastics (MPs) on human health and the environment. Indoor MP abundance is generally higher than outdoor levels, with textiles serving as a primary source of indoor airborne MPs. Traffic-derived MP particles, MP fibres in residential areas, agricultural plastic mulch, marine MPs and landfill sites appear to be contributors to outdoor atmospheric MP pollution. Factors such as wind direction, wind speed, precipitation and snowfall, along with the physical characteristics and secondary suspension of MPs, collectively influence their behaviour, distribution and fate. Inhalation and ingestion constitute the main exposure pathways for airborne MPs, potentially leading to health issues like respiratory inflammation. Therefore, gaining a deeper insight into the behaviour and impact mechanisms of atmospheric MPs aids in formulating effective risk management strategies to safeguard human health and maintain environmental sustainability.

Membrane separation is an energy-efficient separation process. Two-dimensional (2D) materials have shown potential as a new generation of membrane materials due to their unique structures and physicochemical properties. The separation performance of 2D material membranes crucially depends on how 2D nanosheets are assembled in membranes, such as interlayer spacing between stacked nanosheets, chemical properties of nanosheet surfaces, alignment of nanosheets and thickness of membranes, which are closely related to their fabrication methods. This short review concisely overviews commonly used membrane fabrication methods for different types of 2D materials, including graphene-based materials, 2D covalent organic frameworks, 2D metal-organic frameworks, MXenes and other 2D materials. The representative 2D material membranes resulting from their essential fabrication methods are discussed. The advantages and shortcomings of different fabrication methods are compared. The critical challenges to realising large-scale production of 2D material membranes for practical applications are highlighted.

Oxy-combustion is a promising concept to achieve an extremely clean combustion, independently of the fuel type, because, on the one hand, it is a NOx-free combustion and, on the other hand, the CO2 produced during combustion can be easily captured once the water vapour is removed from the exhaust gases stream, consequently allowing also carbon neutral operation. An existing zero-dimensional (0D), mixing-controlled combustion model, developed for a standard diesel combustion scenario, has been adapted to the oxy-fuel combustion scenario. Initially, the model over-predicted the heat release at the end of the combustion process. The main model adaptation was to modify the relationship between the average YO2 and the effective YO2 (i.e. the one of the charge actually entrained by the spray), to be consistent with the significant increase in compression ratio needed in the oxy-fuel context. As a result, a model able to correctly predict the combustion behaviour at any operating condition has been obtained, which finally represents a very suitable tool to assist in the concept development.

Green ammonia, produced through renewable energy-powered electrochemical and thermal processes, is emerging as a promising candidate to replace fossil fuel-based ammonia in the fertiliser, transportation and energy sectors. This paper provides an overview of the production methods, utilisation methods and technological advancements for green ammonia. The electrochemical production and Haber-Bosch with renewable hydrogen and energy are discussed in detail highlighting recent material advances. Green ammonia utilisation methods are discussed with direct use cases such as ammonia combustion and direct ammonia fuel cells examined. Green ammonia’s potential as a carbon-free hydrogen carrier is also discussed in regards to ammonia cracking for effective hydrogen recovery. This paper concludes that green ammonia has the potential to play a significant role in the transition to a sustainable energy system and offers new opportunities for the fertiliser, transportation and energy industries.

CO2 regulations are becoming very stringent due to the goal of reducing greenhouse gases and achieving carbon neutrality. It has already become a common situation that electric vehicles are emerging as eco-friendly power systems rather than vehicles equipped with conventional internal combustion engines and their share in the market is increasing. However, even with an internal combustion engine, CO2 can be drastically reduced if a carbon-free fuel such as hydrogen is used. Raw nitrogen oxides (NOx) emissions can be overcome to a certain level through ultra-lean burn operation, but in order to balance the amount of hydrogen and air in the limited space of the combustion chamber, a drop in the engine’s maximum output should be accepted. Hyundai Motor Company (HMC) also previously developed an engine using hydrogen fuel (1), but was unable to progress to mass production. Since then, hybrid technology has become popular, and with the development of hydrogen injection devices, an era has arrived where mass production becomes a possibility. For this reason, studies on internal combustion engines using hydrogen fuel based on existing spark ignition or compression ignition engines are rapidly increasing. In this study, a hydrogen fuel engine was designed and manufactured based on the mass produced gasoline spark ignition engine. CO2 level was confirmed from initial performance evaluation, and it was found that raw NOx levels and maximum power were in a trade-off relationship with each other under the same air-charging system application. In addition, the method to improve maximum engine torque was verified while maintaining the raw NOx level, and the maximum engine power improvement level was confirmed when raw NOx emissions were allowed to increase. Thereby the potential of the carbon-neutral internal combustion engine has been demonstrated.