Radical intermediates play a key role in metal nanoparticle catalysis, report UK chemists. 

Victor Chechik and colleagues at the University of York used spin trapping and EPR spectroscopy to detect free radicals formed in the presence of gold nanoparticles. Chechik talks more about his work in the short interview below. 

1. Please explain, for a non-specialist, the significance of your article. 

Reactions at the surface of metal nanoparticles are extremely important. Nanoparticles are ideal catalysts, with the exceptionally high surface area and tuneable (e.g. size-dependent) physical/chemical properties. For instance, most heterogeneous catalysts are based on nanoparticles. However, mechanistic details for most reactions at the nanoparticle surface are lacking. Understanding reactions mechanisms is essential for rational design and optimisation of catalysts. In our study, we found that gold nanoparticles can initiate formation of free radical intermediates by activating molecular oxygen or abstracting a halogen atom from a number of organic compounds. These results highlight the previously unexplored role of radical intermediates in reactions involving metal nanoparticles, and open up the possibilities for optimising existing and discovering new useful synthetic transformations. 

2. What has motivated you to conduct this work? 

Applications of EPR spectroscopy in organic chemistry flourished in the 70s and 80s but went into decline in the last decade or so. I strongly believe that EPR has much more to offer to organic chemistry, and there are many catalytic reactions occurring by poorly-understood radical mechanisms which can be unravelled using EPR spectroscopy. 

3. Where do you see this work developing in the future? 

There are many different avenues. Mechanisms of free radical reactions (e.g., reduction and oxidation reactions) on the surface of heterogeneous catalysts have not really been explored. The structure-property relationships for nanoparticle-catalysed reactions are also mostly unknown, and the existing knowledge is largely empirical. We believe that application of EPR to study nanoparticle-catalysed reactions can help tune the selectivity and activity of the catalysts, and achieve understanding of the factors that affect catalytic properties. 

4. Are there any particular challenges facing future research in this area? 

There are always challenges in research! This area is no exception. Free radical detection using spin traps has many pitfalls and is prone to artefacts. Reaction at nanoparticle surfaces are notoriously difficult to study due to heterogeneity of the system, for example different binding sites have different properties, different nanoparticles have different diameters and different morphologies which are very hard to control and so on. Nonetheless, I believe there are many new important mechanistic features which can be unravelled using ERP spectroscopy. 

Joanne Thomson