After its launch in 2018, the ISIS Impact Awards was again opened to facility users, celebrating the scientific, social and economic impact generated by our user community.
The winner of the Economic Award is Peter Albers from Evonik Operations GmbH.
Peter's work combining neutron scattering, electron microscopy and surface science (XPS, SIMS) to investigate materials of commercial relevance spans three major areas: carbons, silica and catalysts.
Peter's work on carbons spans fundamental studies on C60 to tyre reinforcement agents. The use of neutrons enables the development of a better understanding of the hydrogen in the materials and its distribution. His use of inelastic scattering to study the hydroxyls present in silica materials used in products including toothpastes has developed the knowledge around their reactions and interaction with water.
Both carbons and silica materials have applications in catalysis, and this is where they link in with his other area of expertise. Peter has contributed significantly to the knowledge of the surface behaviour of precious metal catalysts, particularly platinum and palladium based materials. As an industrial user, his studies are carried out on commercial carbon supported palladium and platinum catalysts and the metal blacks that are used in industry every day, not model systems. The aim of his catalysis work has been to characterise the nature of hydrogen present on, or in, these catalysts.
Palladium (Pd) is well-known for absorbing hydrogen gas to form PdH, with the structure dependant on the concentration of hydrogen in the material. At low H-contents (<5%) it forms a solid solution with a random distribution of hydrogen atoms (α phase) and above this it forms an ordered structure (β phase). It was this β phase that was of interest: although it had been studied extensively, the details of the surface interactions, behaviour at low hydrogen contents and low palladium particle sizes were yet to be determined.
Using TOSCA and IN1-Lagrange, confirmed by density functional theory, Peter was able to observe an additional subsurface site for the hydrogen atoms, which was previously unreported. They found that the surface sites are occupied first; only when these are fully occupied are subsurface sites occupied. As more hydrogen is added, the H atoms move into the bulk to minimise interactions forming the α phase, until it becomes favourable to form β PdH. At hydrogen levels above PdH0.7, the hydrogen at the surface also occupies the newly identified on-top site, where it is bonded to a single palladium surface atom.
To detect hydrogen on this site for the first time, Peter used direct geometry spectrometry on MAPS and MERLIN. This work, alongside further experiments on SANDALS, enabled Peter and his research group to determine the mechanism of the hydrogen absorption by palladium. As this is the last site to be populated, it is likely they are the most reactive, and provide easy access for reactants.
As with palladium, platinum readily dissociates hydrogen gas into its atoms at room temperature, but the solubility of hydrogen in platinum is extremely low; therefore, all of the adsorbed hydrogen is at the surface. The techniques usually used (infrared and Raman spectroscopy) to observe the bonding of the hydrogen atoms to the surface are restricted by selection rules caused by the behaviour of light, which leave some bonding modes unobserved. Inelastic neutron scattering (INS) is the only technique that enables scientists to study all of the modes.
When hydrogen adsorbs onto platinum, it can bond in a number of ways. These are described as on-top, twofold, threefold and fourfold coordination, depending on how many platinum atoms the hydrogen is connected to. Peter and his collaborators used INS experiments on TOSCA and on Lagrange at the ILL, combined with computational modelling, to investigate platinum nanoparticles loaded with hydrogen.
They found that, for adsorbed hydrogen on platinum nanoparticles in general, most of the hydrogen is in twofold coordination sites whereas, in previous reports, the spectra were generally assigned to mostly threefold hydrogen. These twofold sites are proposed to be the most active sites for the hydrogen oxidation reaction, and therefore their dominance may be one of the reasons as to why platinum is the preferred material in fuel cells.
Peter's well-established work at ISIS has provided the scientific community with the most detailed understanding currently available of the surface of palladium and platinum-based catalysts. By using ISIS, his employer Evonik gains a commercial advantage by being able to show that they have cutting-edge technical support for their products. Their continued use of neutron scattering for over twenty years is an illustration of its importance.
