Johnson Matthey Technology Review - Volume 66, Issue 4, 2022

Ammonia is a strong candidate as a hydrogen vector and has the flexibility to be used directly as a fuel or decomposed to form pure hydrogen. The format of an ammonia decomposition plant is only starting to emerge, with two types becoming significant: centralised locations feeding into the national gas network and decentralised units to supply fuelling stations, the chemical industry or remote applications. In this paper, we review the aspects critical to decompose ammonia in both cases. While the centralised cracking flowsheet can use equipment standard to current hydrogen production methods, the localised cracking unit requires a more innovative design. Energy and safety considerations may favour low temperature operation for decentralised applications, requiring high activity catalysts, while centralised industrial sites may operate at higher temperatures and use a base metal catalyst. Purification to deliver hydrogen suitable for fuel cells is one of the biggest challenges in developing the flowsheet.

Bismuth vanadate (BiVO4) is proven to be a promising photocatalyst for water splitting. However, the effect of materials syntheses, electrode preparation and size of photoelectrode on the photocurrent output of BiVO4 photoanodes needs further investigations. In this study, three different BiVO4 nanoparticle synthesis were employed, namely hydrothermal (HT), HT in the presence of ethylene glycol (EG) and HT with the addition of hydrazine hydrate (HH). In addition, two molecular inks (Triton-X and ethyl‐methyl‐imidazole, EMI), were compared for the preparation of BiVO4 photoanodes using a simple doctor-blade technique followed by calcination at 450°C. The photoanodes (9 cm² active surface) were then compared for their photocurrent density at AM1.5G illumination and 1.2 V (vs. standard hydrogen electrode (SHE)) bias in a specifically designed, three-dimensional (3D)-printed electrochemical cell. The highest photocurrent 0.13 ± 0.1 mA cm^–2 was obtained with the EMI ink, whereas tenfold lower photocurrent was obtained with Triton-X due to the higher charge transfer resistance, measured by electric impedance spectroscopy (EIS). The photoresponse was reproducible and relatively stable, with only 8% decrease in five consecutive illumination periods of 1 min.

We review recent research into oxides of platinum group metals (pgms), in particular those of ruthenium and iridium, for use as electrocatalysts for the oxygen evolution reaction (OER). These are used in membrane electrode assemblies (MEAs) in devices such as electrolysers, for water splitting to generate hydrogen as fuel, and in fuel cells where they provide a buffer against carbon corrosion. In these situations, proton exchange membrane (PEM) layers are used, and highly acid-resilient electrocatalyst materials are required. The range of structure types investigated includes perovskites, pyrochlores and hexagonal perovskite-like phases, where the pgm is partnered by base metals in complex chemical compositions. The role of chemical synthesis in the discovery of new oxide compositions is emphasised, particularly to yield powders for processing into MEAs. Part I introduces the electrocatalytic splitting of water to oxygen and hydrogen and provides a survey of ruthenium and iridium oxide structures for oxygen evolution reaction catalysis.

We continue our review of recent research into oxides of platinum group metals (pgms), in particular those of ruthenium and iridium, for use as electrocatalysts for the oxygen evolution reaction (OER). In Part I (1), the electrocatalytic splitting of water to oxygen and hydrogen was introduced as a key process in developing future devices for various energy-related applications. A survey of ruthenium and iridium oxide structures for oxygen evolution reaction catalysis was presented. Part II discusses mechanistic details and acid stability of pgm oxides and presents the conclusions and outlook. We highlight emerging work that shows how leaching of the base metals from the multinary compositions occurs during operation to yield active pgm-oxide phases, and how attempts to correlate stability with crystal structure have been made. Implications of these discoveries for the balance of activity and stability needed for effective electrocatalysis in real devices are discussed.

Platinum has only been known to Europe since the 16th century. This was impure platinum, found as grains of native metal in alluvial deposits and comprising mainly platinum alloyed with the other five platinum group metals. They were exploited by pre-Colombian native populations of Ecuador and Colombia. In more recent times, the use of platinum in jewellery dates from the late 19th or early 20th centuries, often as a basis for diamond (and other precious gemstone) jewellery. Early jewellery alloys tended to be based on the existing industrial alloys and comparatively little development of specific jewellery alloys was carried out. Its acceptance as a hallmarkable jewellery metal came in 1975 when, with wider availability of the metal, platinum was promoted as a high-value jewellery metal. Platinum jewellery started to grow in popularity, mainly at 950 and 900 fineness qualities. Since that time there has been alloy development specifically for jewellery application and tailored to the requirements of different manufacturing technologies. This review examines the evolution of platinum jewellery alloys over the past century against the challenges presented in developing improved alloys for jewellery application. There has been a substantial increase in alloy development over the past 30 years, particularly focused on improved investment (lost wax) casting alloys as well as better mechanical properties.

In recent years, the potential for ‘green’ ammonia produced from renewable energy has renewed the pursuit of a low-pressure, low-temperature ammonia synthesis process using novel catalysts capable of operating under these conditions. In past decades, the trend of decreasing the pressure in the existing Haber-Bosch process to the de facto limit of condensation at 80 bar has been achieved through catalysts such as Johnson Matthey’s (formally ICI, UK) iron-based KATALCO^TM 74-1. By replacing the separation of ammonia via condensation by absorption, the process loop can be integrated into a single vessel at constant temperature, and the operating region drastically shifts to lower pressures (<30 bar) and temperatures (<380°C) unknown to commercial catalysts. Herein, the low-temperature and low-pressure activity of KATALCO 74-1 and KATALCO 35-8A catalysts is studied and compared to a ruthenium and caesium on ceria catalyst known to have low-temperature activity through resistance to hydrogen inhibition. Due to its low temperature and high conversion activity, KATALCO 74-1 can be deployed in an integrated reaction and absorptive-separation using MnCl2/SiO2 as absorbent. Although further catalyst development is needed to increase compatibility with the absorbent in a feasible reactor design, this study clearly demonstrates the need to re-evaluate the viability of commercial ammonia synthesis catalysts, especially iron-based ones, for their deployment on novel green ammonia synthesis processes driven exclusively by renewable energy.

Traditional microbial synthesis of chemicals and fuels often rely on energy-rich feedstocks such as glucose, raising ethical concerns as they are directly competing with the food supply. Therefore, it is imperative to develop novel processes that rely on cheap, sustainable and abundant resources whilst providing carbon circularity. Microbial electrochemical technologies (MET) offer unique opportunities to facilitate the conversion of chemicals to electrical energy or vice versa, by harnessing the metabolic processes of bacteria to valorise a range of waste products, including greenhouse gases (GHGs). However, the strict growth and nutrient requirements of industrially relevant bacteria, combined with low efficiencies of native extracellular electron transfer (EET) mechanisms, reduce the potential for industrial scalability. In this two-part work, we review the most significant advancements in techniques aimed at improving and modulating the efficiency of microbial EET, giving an objective and balanced view of current controversies surrounding the physiology of microbial electron transfer, alongside the methods used to wire microbial redox centres with the electrodes of bioelectrochemical systems via conductive nanomaterials.

It is imperative to develop novel processes that rely on cheap, sustainable and abundant resources whilst providing carbon circularity. Microbial electrochemical technologies (MET) offer unique opportunities to facilitate the conversion of chemicals to electrical energy or vice versa by harnessing the metabolic processes of bacteria to valorise a range of waste products including greenhouse gases (GHGs). Part I (1) introduced the EET pathways, their limitations and applications. Here in Part II, we outline the strategies researchers have used to modulate microbial electron transfer, through synthetic biology and biohybrid approaches and present the conclusions and future directions.

As the chemicals industry transitions towards a net zero future, rapid assessment of the sustainability metrics of different process results will be essential to support investment decisions in innovation and deployment. Life cycle analysis (LCA) offers the gold standard for process assessment, but LCA can take weeks or months to complete, with incomplete databases and inflexibility in comparing different chemical pathways. In this study, we demonstrate an alternative and complementary methodology. By simplifying the metrics used to describe chemical processes, each process may be linked to another by its feedstocks and products. This generates a network of the chemical industry, which may be investigated using graph theory principles. A case study of the plastics industry is provided, using publicly available information to quantitatively compare with a more formalised and detailed LCA approach. This methodology proves useful for quickly estimating the carbon intensity and water footprint of thousands of routes. Further development, such as including Scope 3 emissions and additional industrial data, may further improve the methodology.

Ammonia will be utilised as a key energy vector for storage and long-distance transport in the developing hydrogen economy. Direct ammonia fuel cells (DAFCs) have the potential to decrease the process complexity of current fuel cell technology and therefore increase overall efficiency and unit footprint where implemented. In this paper, current DAFC technologies are explored, such as solid oxide, alkaline and ammonia borane fuel cells. From this, it is shown that solid oxide fuel cells with oxygen conducting electrolyte (SOFC-O) have high experimental power outputs of 1100 mW cm^–2 but have disadvantages of high nitrogen oxides (NOx) production, lower fuel utilisation and low efficiency. Alkaline and ammonia borane fuel cells are of lesser interest due to complex ammonia pretreatment, high NOx production and lower power outputs of 450 mW cm^–2 and 110 mW cm^–2, respectively. Solid oxide fuel cells with proton conducting electrolyte (SOFC-H) seem to have the most potential due to high theoretical power outputs, high efficiency, increased fuel utilisation and low NOx production. DAFC technology has yet to reach full commercialisation, but as the hydrogen economy develops the potential benefits of DAFCs in complexity and footprint reduction will drive further investment and development, particularly in the shipping sector.