Whether it’s assisting reactions in energy transformations, providing chemicals with increased efficiency or getting rid of, or preventing, waste, catalysis will help in the move towards a greener future. Progress in catalytic science and its applications requires an understanding of what is taking place with the catalyst at the molecular level, which is where techniques such as neutron scattering are invaluable.
The Royal Society of Chemistry journal Physical Chemistry Chemical Physics recently published a themed collection titled Neutron Scattering in Catalysis and Energy Materials. The collection was guest-edited by ISIS scientists Stewart Parker and Ian Silverwood as well as the chair of the UK Catalysis Hub, Prof. Richard Catlow. The Hub is a collaboration between over 30 UK universities with a permanent team based in the Research Complex at Harwell. Three of the four covers show neutron experiments, with two showing experiments run at ISIS. Many papers in the issue include research from ISIS and some are highlighted below.
Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process
Selective catalytic reduction (SCR) is a method of converting NOx gasses to N2, often using ammonia with metal exchanged zeolite catalysts. Copper-chabazite (Cu-CHA) zeolites are currently commercialised for NH3-SCR catalysis for vehicle emissions. Researchers measured the diffusion of ammonia in the catalysts Cu-CHA and H-CHA using molecular dynamics simulations and quasielastic neutron scattering (QENS) to look at the effect of counterion presence on the movement of ammonia.
The QENS results showed similar activation energies of diffusion for both systems, suggesting that the counterion presence has little impact on the diffusion of the ammonia. These results were explained by the molecular dynamics simulations which showed strong coordination of NH3 with Cu2+ counterions in the centre of the chabazite cage structure, allowing diffusion through the windows of the chabazite structure to continue freely. The paper was a collaboration between University College London, UK Catalysis Hub, Johnson Matthey, and ISIS and can be found on the front cover of this collection.
The application of inelastic neutron scattering to investigate the interaction of methyl propanoate with silica
Methyl methacrylate (MMA) is an essential ingredient for materials used in a wide range of products including furniture, automobile components, and mobile phone screens. New, more efficient methods of production of MMA are being developed across the world including one which involves the reaction of methyl propanoate and formaldehyde over a modified silica catalyst. This modern industrial route has been successfully commercialised, however, the surface interactions responsible are still poorly understood.
Researchers aimed to change this by studying methyl propanoate in the solid, liquid, and gas state as well as characterising the adsorption of methyl propanoate onto a representative silica through a variety of techniques including inelastic neutron scattering. The studies showed methyl propanoate bound to silica hydroxyls with hydrogen bonds and adsorption occurring via the carbonyl group of methyl propanoate.
Different routes to methanol: inelastic neutron scattering spectroscopy of adsorbates on supported copper catalysts
For over 50 years methanol has been produced from H2,CO, and CO2 using Cu/ZnO/Al2O3 catalysts although the exact mechanism is still not fully understood. Researchers looked at methanol synthesis with two different catalysts, Cu/ZnO and Cu/MgO, using CO/H2 and CO2/H2 as feedstocks to test the working hypothesis that methanol formation occurs differently depending on the presence of zinc.
Both catalysts behaved similarly with the CO/H2 however under CO2/H2 the catalysts behaved differently, with formate (an intermediate in the methanol formation reaction) found on the catalyst containing zinc. The results support a recently published model for methanol synthesis, highlighting the crucial role ZnO has in the process.
Heads or tails: how do chemically substituted fullerenes melt?
As the search for renewable energy continues, organic solar cells hope to be a more versatile alternative to silicon solar cells. Fullerene based compounds with additional functional groups, such as organic tails, are more commonly being used as an ingredient in these devices that harvest solar energy. The structure and dynamics of fullerenes have been studied extensively, however, little is understood of the thermodynamic behaviour of these chemically substituted fullerenes.
Using OSIRIS at ISIS, researchers looked at how chemically substituted fullerenes melt, whether it was driven by the dynamics of the fullerene head or the substituted tail. The researchers found that it is the tail driving the melting of the material through temperature-activated motion. Further studies of these materials could mean understanding and controlling their behaviour at the nanoscale which could benefit the design and development of future energy materials.
Characterisation of the surface of freshly prepared precious metal catalysts
The majority of heterogeneous catalysts undergo an activation procedure after synthesis. For precious metal catalysts the synthesis is often carried out in aqueous solution by reduction of a precious metal salt, after which the catalyst is dried and reduced. This activation procedure is carried out under the assumption that an oxide layer has formed on the catalyst.
A combination of structural, spectroscopic (including inelastic neutron scattering), and computational techniques were used to characterise the bulk and surface of freshly prepared metal catalysts. The researchers found that at least half of the catalyst surface was metallic, or nearly so, and half was covered by oxygen, mostly as hydroxide.
Water was also present, bound to the hydroxyls through hydrogen bonds rather than the bare metal surface, and is strongly held, as weeks of pumping under high vacuum could not completely remove it. The hydroxyls on the catalyst surface were shown to be reactive by their reaction with or displacement by CO and could be removed by hydrogenation.
The complete collection can be found at: http://pubs.rsc.org/en/journals/articlecollectionlanding?sercode=cp&themeid=cbd37219-87ce-4634-9940-5cbba62349c5
Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process - DOI: 10.1039/C6CP01160H
The application of inelastic neutron scattering to investigate the interaction of methyl propanoate with silica - DOI: 10.1039/C6CP01276K
Different routes to methanol: inelastic neutron scattering spectroscopy of adsorbates on supported copper catalysts - DOI: 10.1039/C6CP00967K
Heads or tails: how do chemically substituted fullerenes melt? - DOI: 10.1039/C6CP01333C
Characterisation of the surface of freshly prepared precious metal catalysts - DOI: 10.1039/C6CP01027J