Johnson Matthey Technology Review - Volume 67, Issue 3, 2023

Renewable and low-carbon hydrogen will contribute to a future climate-neutral economy as a fuel, clean energy carrier and feedstock. One of the main concerns when considering its production by the present proton exchange membrane water electrolysers (PEMWE) is the use of scarce and expensive noble metals as catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), because they contribute to increase the cost of the technology. Several strategies have been developed to overcome this drawback, such as optimising the catalyst loading in the electrodes and alloying or using alternative catalyst supports, always with the aim to maintain or even increase electrolyser performance and durability. In this review, we examine the latest developments in HER and OER catalysts intended for practical PEMWE systems, which point in the short term to the use of platinum and iridium nanoparticles highly dispersed at low loadings on conductive non-carbon supports.

Bushings made of platinum-rhodium alloys are a key component in glass fibre production. While bushings have grown in size and functionality since their introduction in the early 20th century, manufacturing constraints still limit their full potential. Both in terms of design and quality, traditional manufacturing methods such as milling, drilling and welding limit the potential of precious metal bushings. The technical feasibility of the use of additive manufacturing for the production of bushings is greatly dependent on the material properties. For the purpose of this work, an additively manufactured alloy consisting of 90 wt% platinum and 10 wt% rhodium (PtRh10) is investigated with regard to density, electrical resistivity, creep performance and the contact angle of E-glass on the PtRh10 samples.

What’s the long- and short-term prognosis for palladium in organic synthesis, the key platinum group metal (pgm) for transition metal-based catalysis used today by the fine chemicals industries? Are these processes green and sustainable? Are they environmentally respectful of the metals, especially the pgms, so essential to modern day society? Are non-pgm ‘earth abundant’ metals an attractive alternative? Where does the up-and-coming area of chemoenzymatic catalysis, which combines chemo- and biocatalysis in one-pot processes in water, fit into the future of drug syntheses? And what about agricultural targets also being made that include palladium catalysis? These and related timely topics are discussed in this Perspective.

The palladium price has been rising because emissions legislation necessitates using more palladium in catalytic converters. However, this trend will not continue as the energy transition progresses, and in the future there will be considerably more palladium available to use in other applications, including chemicals, pharmaceuticals and agrochemicals catalysts. This is both opportunity and justification for the organic chemistry research community to develop new and significant uses for palladium that can be of global benefit. Any catalyst research needs to include optimisation of circular economy, offering sustainable process and recovery options to support life cycle assessment (LCA).

Dependence of mechanical properties of binary platinum-rhodium alloys on valence electron ratio (VER), number of valence electrons (ev) and average atomic number of the alloys (Z) are investigated. The alloys have a high number of valence electrons (9 ≤ ev ≤ 10) and a wide range of average atomic numbers (Z = 45–78). Clear correlations between VER of the alloys and their mechanical properties are found. By increasing the VER of the alloy from 0.13 to 0.20 following the increase of rhodium content in the composition, the hardness, elastic modulus and ultimate tensile strength (UTS) of the alloy increases. Creep rates of the selected alloys clearly decrease with increasing VER at high temperatures (1500–1700°C), while stress rupture time at different temperatures consistently increases because of higher rhodium content in the alloy solid solution chemistry. Dependence of mechanical properties on valence electron parameters is discussed with reference to the atomic bonding.

Research in the group led by James Wilton‐Ely focuses on catalysis, imaging and sensing. All of this work involves d- and f-elements and is underpinned by the group’s wide-ranging expertise in synthesis. Collaborations with researchers in engineering, medicine and bioimaging allow these discoveries to be used to address major healthcare and sustainability challenges. Professor Wilton-Ely is the author of more than 100 publications and was the recipient of the 2021 Sir Geoffrey Wilkinson Award from the Royal Society of Chemistry for his work on metal-based carbon monoxide sensing. He has a strong interest in sustainability, having directed the MRes Green Chemistry course at Imperial College for over 12 years (2010–2022). He has published on the use of recovered metals in catalysis (1–5), the catalytic transformation of biomass to platform chemicals (6–11) and magnetically-recoverable nanoscale catalysts (12, 13). Elemental sustainability is a pressing concern for all researchers using precious metals and this has led Wilton-Ely to work with colleagues to explore the potential for using gold (3) and palladium (4, 5) complexes recovered from end-of-life products in catalysis.

Additive manufacturing of jewellery alloys has been actively investigated for the past 10 years. Limited studies have been conducted on gold and platinum jewellery alloys. Platinum is of increased interest due to the technological challenges in investment casting. In the present paper, typical platinum jewellery alloys have been tested by laser track experiments on sheet materials. The effect of alloy composition on width and depth of the laser tracks was studied by metallography. Optimum parameters of the laser powder bed fusion (PBF-LB) process were determined for a typical 950Pt jewellery alloy by the preparation of dedicated test samples. Densities of >99.8% were reached for a wide range of processing parameters. However, for real jewellery parts the resulting density was found to depend significantly on the part geometry and on the chosen support structure. The supports must take into account the geometrical orientation of the part relative to the laser build direction and the orientation on the build plate. Local overheating gives rise to porosity in these areas. Therefore, the supports play an important role in thermal management and must be optimised for each part. The design of suitable supports was successfully demonstrated for a typical jewellery ring sample.

Jewellery-specific standardised tests as well as bulk metallic glass (BMG)-specific testing methods were performed on a series of platinum-based BMGs with and without phosphorus, to evaluate their suitability as jewellery items. Their mechanical properties (elasticity, Young’s modulus and yield stress) were determined by three-point beam bending measurements. Hardness, wear and corrosion resistance were tested in comparison to state-of-the-art crystalline platinum-based jewellery alloys. The platinum-BMG alloys exhibit elastic elongation of about 2%. Compared to conventional crystalline platinum-alloys, their fracture strength of ca. 2 GPa and their hardness of ca. 450 HV1 is four and two times higher, respectively. However, the BMGs show less abrasion resistance in the pin-on-disc test than the conventional benchmark alloys due to adhesive wear and microcracking. Regarding the corrosion resistance in simulated body fluids, the BMG alloys reveal a slightly higher release of metals, while the tarnishing behaviour is comparable to the benchmark alloys. The phosphorus-free platinum-BMG alloy showed pronounced tarnishing during exposure to air at elevated temperature. The outstanding thermoplastic formability, a special feature of amorphous metals that can be crucial for enabling novel and filigree designs, was determined and quantified for all BMG alloys.

Carbon-hydrogen bond activations and their subsequent functionalisation have long been an important target in chemistry because C–H bonds are ubiquitous throughout nature, making C–H derivatisation reactions highly desirable. The selective and efficient functionalisation of this bond into many more useful carbon-element bonds (for example, C–B, C–Si, C–O and C–S bonds) would have many uses in pharmaceutical and bulk chemical synthesis. Activation of the C–H bond is, however, challenging due to the high strength and low bond-polarity of this bond rendering its cleavage unfavourable. With the correct choice of reagents and systems, especially those utilising directing groups, kinetically and thermodynamically favourable catalytic processes have been developed. However, a key remaining challenge is the development of undirected, intermolecular reactions using catalysts that are both selective and active enough to make useful processes. In this review, the progress towards optimising Group 9 C–H activation catalysts is discussed, particularly focusing on undirected reactions that are kinetically more difficult, starting with a brief history of C–H activation, identifying the importance of auxiliary ligands including the nature of anionic ligand (for example, cyclopentadienyl, indenyl, fluorenyl and trispyrazolylborate) and neutral ligands (such as phosphines, carbonyl, alkenes and N-heterocyclic carbenes (NHCs)) that contribute towards the stability and reactivity of these metal complexes. The tethering of the anionic ligand to strong σ-donating ligands is also briefly discussed. The focus of this review is primarily on the Group 9 metals rhodium and iridium, however, C–H activation using Group 8 and 10 metals are compared where useful. The most recent advances in this field include the development of C–H borylation of many small hydrocarbon substrates such as arenes, heterocycles and n-alkanes as well as the more challenging substrate methane.

The synthesis of platinum-cobalt nanocrystals (NCs) using colloidal solvothermal techniques is well understood. However, for monodisperse NCs to form, high temperatures and environmentally detrimental solvents are needed. We report a room temperature, aqueous method of platinum-cobalt NC synthesis using electrochemical reduction as the driving force for nucleation and growth. It is found that colloidal NCs will form in both the presence and absence of surfactant. Additionally, we report a monodisperse electrochemical deposition of NCs utilising a transparent conducting oxide electrode. The methods developed here will allow for a synthetic method to produce nanocatalysts with minimal environmental impact and should be readily applicable to other NC systems, including single- and multi-component alloys.

Johnson Matthey’s annual review of the platinum group metal (pgm) markets was published on 15th May 2023, providing an outlook for pgm supply and demand for 2023. The report contains a comprehensive review of developments in the automotive market, covering production and powertrain trends and their implications for pgm use as well as an update on future emissions legislation. It also features an in-depth discussion of the ruthenium and iridium markets in view of increasing interest in the use of these metals in the net zero transition.

Join us for our online conference, where we will be asking important questions about the future of platinum group metals (pgms), their contribution to technology development and the net zero transition. The conference brings together industry and academics, professional organisations and policymakers, to discuss the critical role pgms have to play in the technologies of the future.