REVIEWS
CATALYSIS
Hitchhikers’ guide to chemical reactions
It is difficult to overstate the importance of catalysis in nature because all organisms exist due to chemical reactions catalysed by enzymes. Similarly, manmade catalysts are at the core of the production of fuels, foods, pharmaceuticals and construction materials. Around 30–40% of the global GDP is based on catalytic reactions. The sustainable development of an economy is reliant on understanding how catalysts work in order to optimise existing processes or discover new ones. Modern development in catalysis,
co-written by leading researchers from the UK Catalysis Hub, gives insightful snapshots of the current challenges and opportunities in this area, covering an impressive range of topics, from fuel production to quantum mechanical modelling. A common theme that runs through this book is the challenge of understanding complex, multifaceted catalytic reaction mechanisms. We can think of reacting
molecules as a group of hikers climbing up a mountain: they want to travel from the ‘plateau of reactants’ to the ‘valley of products’, and to do so they must overcome the mountain peaks. Climbing costs them energy and time, but an experienced guide could lead them along a different route, avoiding tall peaks and deep ravines, making their journey faster and safer. The guide is the catalyst that accelerates reaction rates by helping the molecules negotiate energy barriers. If we had a tiny, powerful video
camera we could film the journey of each molecule step-by-step, following the entire pathway provided by the catalyst-guide. Such intimate understanding of the reaction mechanism could enable us to design rationally new catalysts and to optimise conditions of the reactions to maximise their rates and yields. In cases when multiple products are possible, the knowledge of mechanisms can give us the power to
control the formation of one product over another. Moreover, the atomistic dynamics of catalyst interactions with reactants, products and intermediates can help us protect, and reuse the catalyst, which often contains precious and rare metals. A fundamental challenge for chemists is that, in any given reaction, billions or more molecules exist in different states with different kinetic energies, and these molecules collide with each other in a chaotic motion. Even in an ideal case, a reaction observed in a laboratory experiment by ensemble-averaging analytical techniques such as spectroscopy or diffraction can only support, rather than confirm, a proposed mechanism: the macroscopic measurements are unable to rule out that an alternative atomistic mechanism may exist that also results in the same macroscale observation. In heterogeneous catalysis, this problem is exacerbated because the structure, environment and chemical properties of the active sites, which can be affected by interactions with the substrate, may vary significantly over the macroscopic ensemble. Modern spectroscopy, using
synchrotron radiation or neutron beams, allows us to delve deeper into reaction mechanisms. Measurements preformed on a catalyst in situ can help to determine key points of the reaction pathway, but the averaging of information over a large number of species remains an issue. By contrast, microscopy, such
as atomic force (AFM), scanning tunnelling (STM), or transmission electron microscopy (TEM), possesses sufficient spatial resolution of the order of sub-Å levels, that they can track the journey of individual molecules along the reaction energy landscape ̶ at least in principle. However, when scaling down the experiment to a single-molecule, any measurement we take has the potential to perturb the behaviour of the molecule by virtue of the scanning probe or beam of particles.
Modern developments in catalysis
The ‘observer effect’ means that our hikers are wary and can change their behaviour due to the act of observation. While microscopy methods
Editors Graham Hutchings Matthew Davidson, Richard Catlow, Christopher Hardacre, Nicholas Turner, Paul Collier
Publisher World Scientific
Pages 384 Price £115 ISBN 978-1-78634-121-1
Reviewer Andrei N. Khlobystov is director of the Nanoscale & Microscale Research Centre at the University of Nottingham, UK
are now advancing into the realm of intermolecular reactions, the relevance of microscopy measurements to real-life synthetic chemistry remains questionable, particularly in the context of catalysis. Several strategies are being developed to mitigate impact on molecules, for example, by decreasing the energy or dose rate of the radiation. However, it may be better if we simply acknowledge the observer effect on reactants and catalysts, and embrace it as a force contributing to conversion of the reactants into products. Provided that the mechanisms of
interactions between the molecules and radiation – the electron beam in TEM, for instance – are well- understood, they can shed light on the mechanisms of catalytic reactions at the single-molecule level and in real time. This will help us to discover new reactions and new catalytic systems, and bolster our goal of rational catalyst design.
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