Platinum catalysts are extensively used in the chemical industry and as electrocatalysts in fuel cells. Pt is notorious for its sensitivity to poisoning by strong CO adsorption. Here we demonstrate that the single-atom alloy (SAA) strategy applied to Pt reduces the binding strength of CO while maintaining catalytic performance. By using surface sensitive studies, we determined the binding strength of CO to different Pt ensembles, and this in turn guided the preparation of PtCu alloy nanoparticles (NPs). The atomic ratio Pt:Cu = 1:125 yielded a SAA which exhibited excellent CO tolerance in H2 activation, the key elementary step for hydrogenation and hydrogen electro-oxidation. As a probe reaction, the selective hydrogenation of acetylene to ethene was performed under flow conditions on the SAA NPs supported on alumina without activity loss in the presence of CO. The ability to maintain reactivity in the presence of CO is vital to other industrial reaction systems, such as hydrocarbon oxidation, electrochemical methanol oxidation, and hydrogen fuel cells.

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Key descriptors in hydrogenation catalysis are the nature of the active sites for H2 activation and the adsorption strength of H atoms to the surface. Using atomically resolved model systems of dilute Pd-Au surface alloys and density functional theory calculations, we determine key aspects of H2 activation, diffusion, and desorption. Pd monomers in a Au(111) surface catalyze the dissociative adsorption of H2 at temperatures as low as 85 K, a process previously expected to require contiguous Pd sites. H atoms preside at the Pd sites and desorb at temperatures significantly lower than those from pure Pd (175 versus 310 K). This facile H2 activation and weak adsorption of H atom intermediates are key requirements for active and selective hydrogenations. We also demonstrate weak adsorption of CO, a common catalyst poison, which is sufficient to force H atoms to spill over from Pd to Au sites, as evidenced by low-temperature H2 desorption.

Plasmonic nanostructures have been proposed as useful materials for photon harvesting applications. However, the mechanisms by which energy transfer occurs across interfaces formed between plasmonic materials and their environment are under debate. A commonly invoked mechanism is indirect hot charge carrier transfer, where hot carriers are generated in the plasmonic material by nonradiatve plasmon decay, followed by transfer of these carriers to interfacial species in a sequential process. Alternatively, chemical interface damping has been reported to allow direct interaction between surface plasmons and interfacial species electronic states. Here we provide evidence from experiment and theory that for plasmon-mediated catalytic O 2 dissociation on Ag plasmonic nanoparticles, the direct interaction of O 2 molecules with surface plasmon near-fields was responsible for observed photocatalysis. These results offer important mechanistic insights for the design of plasmonic materials that maximize efficiency for promoting catalytic small molecule activation using photon fluxes.

Copper is a common catalyst for many important chemical reactions including low-temperature water gas shift, selective catalytic reduction of NO x , methanol synthesis, methanol steam reforming, and partial oxidation of methanol. The degree of surface oxidation, or the oxidation state of the active site, during these reactions has been debated and is known to have a large influence on the reaction rates. Therefore, elucidating the atomic-scale structure of copper surface oxides is an important step toward a fuller understanding of reaction mechanisms in heterogeneous catalysis. The so-called “29” monolayer oxide film is a common intermediate in the oxidation of Cu­(111). The large size of its unit cell has thus far prevented the development of a definitive model for its structure. Using high-resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we arrive at a model for the “29” Cu x O film on Cu(111). There is very good agreement between experimental and computational STM images over a range of biases. Through the construction of a phase diagram from first-principles, we further find that the “29” structure derived from the DFT calculations is indeed the most stable structure under the experimental conditions considered. This work yields an accurate picture of the atomic scale structure of the “29” oxide film and therefore a basis for beginning to understand adsorption sites and reaction mechanisms on this catalytically relevant surface.

We characterized the change in photon absorption and scattering properties of plasmonic Au nanoparticles by chemical interface damping.

Ni/Au is an alloy combination that while, immiscible in the bulk, exhibits a rich array of surface geometries that may offer improved catalytic properties. It has been demonstrated that the addition of small amounts of Au to Ni tempers its reactivity and reduces coking during the steam reforming of methane. Herein, we report the first successful preparation of dilute Ni−Au alloys (up to 0.04 ML) in which small amounts of Ni are deposited on, and alloyed into, Au(111) using physical vapor deposition. We find that the surface structure can be tuned during deposition via control of the substrate temperature. By adjusting the surface temperature in the 300−650 K range, we are able to produce first Ni islands, then mixtures of Ni islands and Ni−Au surface alloys, and finally, when above 550 K, predominantly island-free Ni−Au single atom alloys (SAAs). Low-temperature scanning tunneling microscopy (STM) combined with density functional theory calculations confirm that the Ni−Au SAAs formed at high temperature correspond to Ni atoms exchanged with surface Au atoms. Ni−Au SAAs form preferentially at the elbow regions of the Au(111) herringbone reconstruction, but at high coverage also appear over the whole surface. To investigate the adsorption properties of Ni−Au SAAs, we studied the adsorption and desorption of CO using STM which allowed us to determine at which atomic sites the CO adsorbs on these heterogeneous alloys. We find that small amounts of Ni in the form of single atoms increases the reactivity of the substrate by creating single Ni sites in the Au surface to which CO binds significantly more strongly than Au. These results serve as a guide in the design of surface architectures that combine Au's weak binding and selective chemistry with localized, strong binding Ni atom sites that serve to increase reactivity.

Pt based materials are used extensively in heterogeneous catalytic processes, but are notoriously susceptible to poisoning by CO. In contrast, highly dilute binary alloys formed of isolated Pt atoms in a Cu metal host, known as PtCu single-atom alloys (SAAs), are more resilient to CO poisoning during catalytic hydrogenation reactions. In this article, we describe how CO affects the adsorption and desorption of H2 from a model PtCu(111) SAA surface and gain a microscopic understanding of their interaction at the Pt atom active sites. By combining temperature programmed desorption and scanning tunneling microscopy with first principles kinetic Monte Carlo we identify CO as a Pt site blocker that prevents the low temperature adsorption and desorption of H2, the so-called molecular cork effect, first realized when examining PdCu SAAs. Intriguingly, for the case of PtCu, H2 desorption occurs before CO release is detected. Furthermore, desorption experiments show a non-linear relationship between CO coverage of the Pt sites and H2 desorption peak temperature. When all the Pt atoms are saturated by CO a very sharp H2 desorption feature is observed 55 K above the regular desorption temperature of H2. Our simulations reveal that the origin of these effects is the fact that desorption of just one CO molecule from a Pt site facilitates the fast release of many molecules of H2. In fact, just 0.7 % of the CO adsorbed at Pt sites has desorbed when the H2 desorption peak maximum is reached. The release of H2 from CO corked PtCu SAA surfaces analogous to the escape of gas from a pressurized container with a small puncture. Given that small changes in CO surface coverage lead to large changes in H2 evolution energetics the punctured molecular cork effect must be considered when modeling reaction mechanisms on similar alloy systems.