Light produces a banana-bond split

A banana-bond split may sound delicious, but actually, it’s a bit of surface chemistry. Specifically, the point at which an H₂ molecule desorbs from a catalyst after light-induced water splitting. Although the desorption is typically thought to be a thermally driven event, a study conducted at Pacific Northwest National Laboratory indicates that the desorption, too, may be triggered by light (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b05083). A common approach to dissociate water to produce hydrogen for fuel is to combine a semiconductor that can emit photoelectrons with a cocatalyst such as RuO₂ to facilitate redox chemistry. Previously, researchers determined that after hydrogen is produced, it remains adsorbed to RuO₂ by donating electron density from its σ bond to the Ru 4d_z² orbital. This produces a three-center, two-electron system with bent—or banana-shaped—bonds. The PNNL team has now identified an additional electrostatic interaction—one that arises from Ru^IVpolarizing H₂ and RuO₂ counterpolarizing in response. The team also found that shining visible light on the complex induces an optical transition in RuO₂ that redistributes charge and weakens the electrostatic interaction, releasing H₂. The finding suggests that “there are optical transitions in cocatalysts that people haven’t recognized, and they should be recognized, understood, and exploited,” PNNL scientist Michael A. Henderson says.