Abstract
The different activity of a 1% Pd/carbon catalyst towards aromatic and aliphatic aldehydes hydrogenation has been explored by 13C NMR relaxation. The ratio between T1 relaxation times of adsorbed (ads) and free diffusing (bulk) molecules (T1ads/T1bulk) can be used as an indicator of the relative strength of interaction between the reactant and the catalytic surface, where the lower the T1ads/T1bulk, the higher the adsorption strength. It can be seen that 1% Pd/carbon showed a reverse catalytic behaviour towards benzaldehyde and octanal hydrogenation, which can be explained by analysing the T1 relaxation times related to each substrate in the presence of the catalyst. Comparing and correlating the different T1ads/T1bulk values, we were able to prove that the different catalytic results mainly depend on the contrasting adsorption behaviour of substrates on the catalyst. Moreover, the role of the solvent has been disclosed, as NMR results revealed that the adsorption of the reactants was strongly affected by the choice of solvent, which is revealed to be critical in modulating catalytic activity. As a consequence, T1ads/T1bulk measurements can provide a guide to the selection of appropriate reaction conditions for improving catalytic activity.
Highlights
Predicting catalytic activities is of vital importance in catalyst design, considering the large efforts often required for synthesis and testing
Some techniques allow studying the interaction between a molecule and the catalyst surface, as for example FT-IR spectroscopy of probe molecules [2] that are able to adsorb on specific sites which can subsequently be detected, identified and quantified, based on the intensity and position of the absorption band
Conclusionshas been explored by 13 C NMR relaxation
Summary
Predicting catalytic activities is of vital importance in catalyst design, considering the large efforts often required for synthesis and testing. The underlying phenomena responsible for catalytic performance is sometimes poorly understood due to limited catalyst characterisation. Microscopic and spectroscopic techniques are very often applied in the characterisation of heterogeneous catalysts, of which a significant number is based on supported metal nanoparticles. High-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR), among other techniques, help untangling the role of these metal nanoparticles and the impact of their dimensions, surface exposure, oxidation state and interaction with the support on the overall catalytic performance. Some techniques allow studying the interaction between a molecule and the catalyst surface, as for example FT-IR spectroscopy of probe molecules [2] that are able to adsorb on specific sites which can subsequently be detected, identified and quantified, based on the intensity and position of the absorption band.
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