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Researchers at the Van 't Hoff Institute for Molecular Sciences (HIMS) have demystified one of the fuzziest parameters in heterogeneous catalysis: the Arrhenius pre-exponential factor. Using clever catalyst design and a special measurement device constructed in-house, they isolated the kinetics of molecules traveling across catalyst surfaces. The results, published in an open-access paper in Chemical Science, have profound implications for the design and manufacturing of solid catalysts for sustainable chemistry and energy applications.

For most chemical reactions, the rate of the reaction increases exponentially with temperature. This simple empirical relation, described by Chemistry Nobel Laureate Svante Arrhenius at the end of the 19th century, has two parameters: the energy of activation, and a pre-exponential factor.

The latter is somewhat of a theoretical embarrassment: it is well explained for reactions involving gases and liquids, but not for reactions at surfaces. This means that many catalytic reactions (which together account for ca. 40% of worldwide GDP) rely on empirical trial-and-error measurements. For example, catalysts are often optimal when their active sites are separate from each other, but just how far from each other is anyone's guess.

Now, a team led by Prof. Gadi Rothenberg at the Van 't Hoff Institute for Molecular Sciences has solved this age-old puzzle by using, like Arrhenius, a purely experimental approach, yet with state-of-the-art 21st-century tools.

Working as part of the NWO TOP-PUNT project "Catalysis in Confined Spaces", PhD candidate Thierry Slot and Erasmus MSc student Nathan Riley, together with Dr. Raveendran Shiju, and Prof. Will Medlin (from University of Colorado Boulder, and HRSMC visiting fellow at the UvA) devised an elegant strategy to study the amount of a space a catalytic particle needs for its reaction.

Catalyst particles with molecular fences

The researchers used a novel three-step synthesis strategy, developed with Medlin's group, which creates a well-defined “free space” around each active site (see figure). First, they coated the platinum particles with organic thiols, which act as template placeholders. Then, they coated all of the alumina support surface with phosphonic acids.

Finally, they removed the thiols, creating a well-defined space between the platinum particle and the phosphonic-acid barrier. As Slot explains: 'Imagine building a fence at different distances from the active site. We then measure how this fence influences the reactivity of the particle as we gradually place it closer to the active site.'

Electron micrograph of the catalyst surface showing the fenced-in spaces, and schematic of how molecules approach the fenced-in sites

Surface travel

The team used a special bubble counter constructed in-house to monitor the reaction kinetics, generating precise Arrhenius plots with hundreds of data points. They found that the “fence” has to be very close to the active site before the reactivity of drops, at a distance about the kinetic diameter of one reactant molecule. This means that as long as a catalyst is stable, the active sites can be very close to each other and still behave as separate entities. The analysis also confirmed that the surface travel step is primarily associated with entropy which is reflected in the pre-exponential factor, leaving the activation energy unchanged.

Back to Arrhenius, and just in time

Rothenberg is proud that this puzzle was solved through experiments, similar to the empirical approach used by Arrhenius over a century ago. 'Our experiments separate entropic and enthalpic contributions. We show that confinement is only observed at a very close range, and that it only influences entropy. This tells us that confinement in larger systems, such as enzymes or zeolites, must be mostly enthalpic, which is a profound conclusion.' For some measurements, there was little time to spare: 'We had to prove that we indeed made these beautifully precise structures on the catalyst surface, and finally we managed to see them using electron microscopy in AMOLF, but you have to be quick, because the electron beam literally tears down the fences, so you only have a few seconds before you destroy your own catalyst with a million-dollar electron cannon.'

One-minute animation

To help disseminate the message of their research, the team made a one-minute animated clip showing the experimental approach and main results. Rothenberg is a strong advocate for such bite-sized science clips, and sees them as an integral part of future research and education: 'People today are used to getting information through videos, the shorter the better. If the video triggers their interest, they are more likely to contact you for more information or read your research. The 15-second limit of TikTok is perhaps beyond our reach, but you can explain a hell of a lot of science in one minute.'

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One-minute animated clip showing the experimental approach and main results


Original open-access paper: An experimental approach for controlling confinement effects at catalyst interfaces. T.K. Slot, N. Riley, N.R. Shiju, J.W. Medlin and G. Rothenberg, Chem. Sci., 2020, 11, 11024–11029. DOI: 10.1039/D0SC04118A (Open Access)

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