Platinum-based catalysts are widely used across industry, and many of these uses are due to the ability of hydrogen to adsorb onto the catalyst surface and then dissociate easily.
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.
These different types of coordination give different signals in spectroscopy measurements, where peaks are seen which correspond to the movement of atoms that are bonded together within a material. Each of these different movements, such as bending or stretching, are described as modes. The techniques usually used (infrared and Raman spectroscopy) 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.
The scientists from the UK, France and Germany used INS experiments on TOSCA here at ISIS, and on Lagrange at the ILL, combined with computational modelling to investigate platinum nanoparticles loaded with hydrogen. This combination of techniques meant they were able to study the full spectra of vibrational modes and assign them to the different types of coordination between hydrogen and platinum, giving an insight that was previously unavailable.
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.
As well as their own computational calculations, the group compared their INS spectra with those from previous publications. They found that their assignment of the peaks in the INS data to different modes was also applicable in these examples, providing further evidence for their proposed bonding model.
“How hydrogen is adsorbed on platinum has been studied for more than 50 years." Stewart Parker, the ISIS scientist involved with the work explains; “using neutrons with theory has enabled us to explain all the previous work on the subject."
The full publication can be read online at Chemistry: A European Journal.
Read more science highlights from TOSCA here.