Recent reports have shown that plasmonic nanostructures can be used to drive direct photocatalysis with visible photons, where nanostructures act as the light absorber and the catalytic active site. These reports have showcased direct plasmon driven photocatalysis as a route to concentrate and channel the energy of low intensity visible light into adsorbed molecules, enhancing the rates of chemical transformations, and offering pathways to control reaction selectivity. In this perspective, we will discuss the fundamental photophysics of localized surface plasmon resonance (LSPR) excitation in the context of driving chemical transformations. The various demonstrated chemical conversions executed using direct plasmonic photocatalysis will be reviewed. Experimental observations, such as the dependence of photocatalytic rate on illumination intensity and photon energy, will be related to microscopic mechanisms of photocatalysis. In addition, theoretical treatments of various mechanisms within the process of direct plasmonic photocatalysis will be discussed and related to experimental studies. Throughout the Perspective, the possibility of activating targeted adsorbate bonds to allow rational manipulation of reaction selectivity in direct plasmonic photocatalysis will be discussed.

Engineering heterogeneous metal catalysts for high selectivity in thermal driven reactions typically involves the synthesis of nanostructures with well-controlled geometries and compositions. However, inherent relationships between the energetics of elementary steps limit the control of catalytic selectivity through these approaches. Photon excitation of metal catalysts can induce chemical reactivity channels that cannot be accessed using thermal energy, although the potential for targeted activation of adsorbate-metal bonds is limited because the processes of photon absorption and adsorbate-metal bond photoexcitation are typically separated spatially. Here, we show that the use of sub-5-nanometer metal particles as photocatalysts enables direct photoexcitation of hybridized adsorbate-metal states as the dominant mechanism driving photochemistry. Activation of targeted adsorbate-metal bonds through direct photoexcitation of hybridized electronic states enabled selectivity control in preferential CO oxidation in H2 rich streams. This mechanism opens new avenues to drive selective catalytic reactions that cannot be achieved using thermal energy.

Relationships between geometric structures of active metallic sites and areal rates of reaction (structure sensitivity) are extensively studied for supported metal catalysts. For CO oxidation on irreducible oxide-supported Pt catalysts, there still exists a discrepancy regarding structure sensitivity. Theoretical and single-crystal analyses suggest the CO oxidation reaction rate should be highly structure sensitive, whereas measurements on supported Pt catalysts show only minimal structure sensitivity. Here, we used quantitative in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) to investigate the influence of CO oxidation reaction conditions on the fraction of well-coordinated (WC) and under-coordinated (UC) Pt active sites on a series of four α-Al2O3-supported Pt catalysts with average Pt sizes ranging from ∼1.4 to 19 nm. Pt nanoparticle surfaces were observed to restructure under CO oxidation reaction conditions, increasing the fraction of UC Pt sites. Reconstruction rendered the fraction of WC and UC sites less dependent on Pt particle size than expected from geometric models. A model, coupling the DRIFTS measurements with previous theoretical calculations, was quantitatively correlated to the measured slight structure sensitivity on the same series of catalysts. Our results bridge the gap between previous studies exploiting theory, single crystals, and supported Pt catalysts by demonstrating that WC Pt atoms are the active site for CO oxidation, but that CO-induced restructuring of Pt nanoparticle surfaces masks the inherent structure sensitivity in particle-size-dependent rate measurements.

Atomic-scale insights into how supported metal nanoparticles catalyze chemical reactions are critical for the optimization of chemical conversion processes. It is well-known that different geometric configurations of surface atoms on supported metal nanoparticles have different catalytic reactivity and that the adsorption of reactive species can cause reconstruction of metal surfaces. Thus, characterizing metallic surface structures under reaction conditions at atomic scale is critical for understanding reactivity. Elucidation of such insights on high surface area oxide supported metal nanoparticles has been limited by less than atomic resolution typically achieved by environmental transmission electron microscopy (TEM) when operated under realistic conditions and a lack of correlated experimental measurements providing quantitative information about the distribution of exposed surface atoms under relevant reaction conditions. We overcome these limitations by correlating density functional theory predictions of adsorbate-induced surface reconstruction visually with atom-resolved imaging by in situ TEM and quantitatively with sample-averaged measurements of surface atom configurations by in situ infrared spectroscopy all at identical saturation adsorbate coverage. This is demonstrated for platinum (Pt) nanoparticle surface reconstruction induced by CO adsorption at saturation coverage and elevated (>400 K) temperature, which is relevant for the CO oxidation reaction under cold-start conditions in the catalytic convertor. Through our correlated approach, it is observed that the truncated octahedron shape adopted by bare Pt nanoparticles undergoes a reversible, facet selective reconstruction due to saturation CO coverage, where {100} facets roughen into vicinal stepped high Miller index facets, while {111} facets remain intact.