  Nanoparticle surface structure and geometry generally dictate where chemical
transformations occur, with the low-coordination-number,
high-radius-of-curvature sites being energetically-preferred. Here, we show how
optical excitation of plasmons enables spatially-controlled chemical
transformations, including access to sites which, without illumination, would
be energetically-unfavorable. We design a crossed-bar Au-PdHx antenna-reactor
system that localizes electromagnetic enhancement away from the innately
reactive PdHx nanorod tips. Using optically-coupled in situ environmental
transmission electron microscopy, we track the dehydrogenation of individual
antenna-reactor pairs with varying optical illumination intensity, wavelength,
and hydrogen pressure. Our in situ experiments show that plasmons enable new
catalytic sites, including hydrogenation dissociation at the nanorod faces.
Molecular dynamics simulations confirm that these new nucleation sites are
energetically unfavorable in equilibrium and only accessible via tailored
plasmonic excitation.
