100 101 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Spectroscopy Group Beamlines I20-EDE, I20-Scanning (and beamline B07 from the Structures and Surfaces Group) Using synchrotron radiation to understandmechanochemically-prepared catalysts Related publication: Blackmore R. H., Rivas M. E.,Tierney G. F., Mohammed K. M. H., Decarolis D., Hayama S.,Venturini F., Held G., Arrigo R., AmboageM., Hellier P., Lynch E., Amri M., CasavolaM., Eralp ErdenT., Collier P. &Wells P. P.The electronic structure, surface properties, and: In situ N 2 O decomposition of mechanochemically synthesised LaMnO 3 . Phys. Chem. Chem. Phys. 22 , 18774–18787 (2020). DOI: 10.1039/d0cp00793e Publication keywords: Mechanochemistry; N 2 O decomposition; Perovskite; Catalysis; XAS; in situ NAP-XPS T he commercial catalysts currently used to remove polluting gases from vehicle exhausts rely on expensive precious metals, with demand continually growing. Preparing these catalysts often requires solvents, waste treatment and elevated temperatures, all with an environmental cost. One solution is to investigate the use of an alternative, more abundant material. LaMnO 3 has shown promising catalytic behaviour and is made by physically mixing two solid reactants. The catalytic activity of materials is highly dependent on how they are produced. In this work, researchers synthesised LaMnO 3 by a novel method, ball milling, to improve its catalytic properties. To replicate or optimise the final material structure, it is vital to investigate the chemical steps occurring within the ball mill. However, the ball mill setup makes it difficult to perform real-time analysis. Therefore, the research team replicated the conditions experienced within the ball mill by applying extreme pressures to the startingmaterials. Using Diamond Light Source’s Energy Dispersive EXAFS beamline (I20-EDE) meant they could monitor how the structure changes with increasing pressure, using X-ray Absorption Fine Structure (XAFS) measurements in real-time. This beamline setup also allowed them to use a specialisedhigh-pressure cell.They used complementarymeasurements onDiamond’sVersatile Soft X-ray (VerSoX) beamline (B07) to study the surface properties of the materials during catalysis. Beamline I20-Scanning was used to look at electronic structure. For industrial companies researching ball milling as an alternative production route, i.e. for autocatalysis or battery materials, this research highlights that though the preparation route produces beneficial properties at a lower environmental cost, understanding its underlying chemistry is hugely challenging. As the UK moves towards a net zero carbon future, as enshrined in law, current technologies will need to adapt to the changing legislative and environmental requirements. Catalysis will be a major component of the solution to reducing our carbon emissions; it will be an essential tool in sustainable energy production, allow us to upgrade waste carbon streams, and by its very nature make processes more atom efficient and reduce the associated energy burden. However, the need to reduce carbon emissions will touch all areas of industrialised processes, including the production of catalysts themselves. This means that catalysts will need to be comprised of ‘Earth abundant’ elements that reduce the need for energy intensive and wasteful extraction processes of scarce resources, alongside routes to preparing materials that produce less waste, have reduced number of process steps, and are less energy intensive. An example of this can be seen in the remediation of environmental pollutants (for example, N 2 O). N 2 O is a by-product of major industrialised processes (e.g. production of nitric acid for manufacture of fertilisers) and has a global warming potential (GWP) that is roughly 310 times higher than that of CO 2 and an atmospheric half-life of over 115 years.The decomposition of N 2 O is traditionally achieved using catalysts based on precious metals. One promising alternative is to use perovskite structures of mixed-metal oxides, for example in this work we have focussed on LaMnO 3 . LaMnO 3 is traditionally prepared in a multi-step process, where chemical precursors of La and Mn are mixed with other chemical reagents to make a ‘sol-gel’. This step allows for intimate mixing of La and Mn components before high thermal treatments allow for the crystallographic perovskite LaMnO 3 structures to be produced. Clearly, it is desirable to move towards catalysts that rely on more plentiful (and secure) resources. However, is there more that can be done to prepare these materials in amore sustainable process? In recent years, mechanochemistry has emerged as a viable alternative to traditional synthetic methods for the production of advanced functional materials; this is to say that chemical reactions can be achieved through the physical action of mechanically mixing materials, without the need for additional solvents and thermal annealing steps. For LaMnO 3 , it is now well documented that this can be prepared through the use of planetary ball-milling applied to single oxide precursors (e.g La 2 O 3 and Mn 2 O 3 ) 1 . The activity of a heterogenous catalyst is heavily dependent on the precise structures of materials, which are in turn reliant on the chosen preparation route. The use of mechanochemistry to produce LaMnO 3 gives rise to materials that contain significant amounts of both crystalline and amorphous materials. This is not necessarily a disadvantage. We recently demonstrated that mechanochemically prepared LaMnO 3 has a superior activity towards N 2 O decomposition, when working under low temperature conditions 1 . As the preparation method is crucial in controlling catalyst activity it is important to understand the chemical steps that take place during mechanochemical synthesis. This is an extremely challenging endeavour; the act of ball-milling produces collision events that generate high pressure and temperature conditions that facilitates the chemical transformation. Ideally, the evolution of chemical changes would be followed in real-time during the process of ball-milling. However, the fast rotation of the milling jars and the presence of milling media means that an alternative approach is required. In our study on the I20-EDE beamline at Diamond we were able to simulate the high-pressure conditions during ball-milling and isolate the effect this has on the chemical transformations (Fig. 1). Previously, we have mechanochemically synthesised perovskite LaMnO 3 from Mn 2 O 3 and La 2 O 3 at room temperature in atmospheric conditions using a high energy planetary ball mill. X-ray absorption spectroscopy (XAS) has proven extremely vital in analysing ball-milled samples due to the amorphous, disordered content when analysing ex situ XAS at ‘time slices’ through the synthesis 1 . Our approach was to isolate and simulate high-pressure conditions using a diamond anvil cell (DAC) and the collect XAS data during this process at the Mn K-edge. Using I20-EDE was crucial as the configuration is better suited to coupling with the DAC that generates the high pressure. In our experiment, we were able to use perforated diamond windows (0.5 mm thick) in order to reduce the diamond absorption at the Mn K-edge (6.54 KeV). Pressure readings were performed using ruby fluorescence as a standard, on an iHR320 Raman spectrometer from Horiba using the 532 nm laser. XAFS measurements were performed with a spot size of 0.05 mm diameter with pressure readings every 2 GPa up to 30 GPa at room temperature. Achieving these pressure conditions and being able to measure data at the relatively low-energy Mn K-edge is a significant technical achievement. In this experiment, we were not able to induce any structural transformation as a result of the application of pressure. However, this provides important information on the likely conditions required to produce perovskite structures from the application of ball-milling on Mn 2 O 3 and La 2 O 3 2 . To further improve our knowledge of these mechanochemically prepared materials, we used B07 at Diamond. These measurements allowed us to follow how the surface structure of the catalyst evolved under operating conditions using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) and the effect of the atmospheric conditions during milling. The NAP-XPS experiments allowed us to look at specific oxygen environments on the surface of the catalyst during the decomposition of N 2 O. This oxygen signature, when the catalyst was milled with argon, showed a higher proportion of adsorbed species on exposure to N 2 O at room temperature. This indicated that the argon milled samples were more adept at activating N 2 O, which subsequently resulted in improved performance of N 2 O decomposition at lower temperatures. The overall study – including other work looking at the electronic structure on I20-Scanning – shows the importance of synchrotron radiation methods in studying the structurally diverse materials produced duringmechanochemistry.This is an important necessity as we seek to develop sustainable methods for producing enhanced catalyst materials. References: 1. Blackmore R. H. et al. Understanding the mechanochemical synthesis of the perovskite LaMnO 3 and its catalytic behaviour. Dalt. Trans. 49 , 232–240 (2019). DOI: 10.1039/c9dt03590g 2. Blackmore R. H. et al. The electronic structure, surface properties, and: In situ N 2 O decomposition of mechanochemically synthesised LaMnO 3 . Phys. Chem. Chem. Phys. 22 , 18774–18787 (2020). DOI: 10.1039/d0cp00793e Funding acknowledgement: The UK Catalysis Hub is kindly thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium (portfolio grants EP//K014706/1, EP/K014668/1, EP/K014854/1, EP/K014714/1 and EP/ I019693/1). The University of Southampton and EPSRC are thanked for the iCASE studentship of RHB. Peter Wells and KM wish to acknowledge the STFC for funding the position of KM (ST/R002754/1). Peter Wells and MC wish to acknowledge the EPSRC for funding the position of MC (EP/R011710/1). Corresponding author: Dr Peter Wells, University of Southampton/Diamond Light Source,
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[email protected] Figure 1: DAC installed on the I20-EDE experimental hutch.