Mass properties have an unexpected impact on surface reactivity, new research suggests. With applications as varied as catalysis, energy storage and structural engineering, this new mode of reaction control could have important implications in applied science.
Despite their ubiquity in everyday life, surfaces present an interesting chemical problem. “We can define mass as atoms bonded to their neighbors like any other atom away from the surface. But the surface atoms have dangling bonds, which means they have very different properties,” explains Izabela Szlufarska, materials scientist at the University of Wisconsin, USA. Surface reactivity has therefore traditionally been understood as a consequence of how atoms interact at the reactant-material interface, with control of the reaction being limited to structural and electronic changes in the upper layer atoms.
However, new simulations by Szlufarska and his colleague from Wisconsin, Jianqi Xi, revealed that certain bulk properties can also influence surface reactions. Using ab initio molecular dynamics, the duo modeled a hydrogen scission reaction on the surface of amorphous silicon carbide and, by varying the stoichiometry of the system, were able to tune the elastic properties of the bulk material. During a reaction, atoms below the surface must move to accommodate surface intermediates, which means more elastic bulk systems are better able to facilitate these reactions. This elasticity depends directly on the bonding forces within the structure: the carbon-rich system contains many strong carbon-carbon bonds, which makes the resulting bulk material stiff, while the silicon-rich system is much more elastic in due to lower silicon prevalence. – silicon bonds.
Szlufarska then determined the effect of these volume changes by measuring the reaction energy for each different stoichiometric system. “Jianqi came up with a really creative approach to these calculations that allowed us to separate electronic energy contributions from mechanical contributions resulting from stiffness,” she explains. By isolating these mechanical effects, Szlufarska and Xi established that the stiffer carbon-rich system has a higher reaction energy barrier, due to the greater amount of energy required to deform the bulk structure during the formation of surface intermediates.
It’s a great piece of work,” comments Stephen Jenkins, a theoretical surface chemist from the University of Cambridge, UK. “But those mechanical contributions are probably still in the upper few atomic layers, so it’s still largely a surface effect, rather than the whole bulk material.” Szlufarska agrees it’s probably a localized effect. “It’s usually a few nanometers from the surface,” she explains. “What is important is that this region has no surface properties.” Regardless of the range, this could still be a valuable tool for surface chemists because bulk properties are much easier and cheaper to control than surface ones.
‘These computational methods are becoming so powerful that they can really tell the experimenter what is interesting to study,’ says Andrzej Kotarba, experimental surface chemist at Jagiellonian University in Poland. “It’s an inspiring article, but what’s important is that today you can verify simulations by experience, and that’s actually what’s missing here.” The duo are keen to work hand in hand with experimenters and hope that other researchers will take up this idea and test it on their own systems. “We think it’s a broader phenomenon,” says Szlufarska. “Our project is to see how this will apply to other materials or classes of materials, with a view to developing applications in catalysis. We hope this will open the doors to controlling chemical reactions in a new way.