Facile dissociation of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resolution scanning tunneling microscopy, we show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissociation at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.

We report a novel synthesis of nanoparticle Pd-Cu catalysts, containing only trace amounts of Pd, for selective hydrogenation reactions. Pd-Cu nanoparticles were designed based on model single atom alloy (SAA) surfaces, in which individual, isolated Pd atoms act as sites for hydrogen uptake, dissociation, and spillover onto the surrounding Cu surface. Pd-Cu nanoparticles were prepared by addition of trace amounts of Pd (0.18 atomic (at)%) to Cu nanoparticles supported on Al2O3 by galvanic replacement (GR). The catalytic performance of the resulting materials for the partial hydrogenation of phenylacetylene was investigated at ambient temperature in a batch reactor under a head pressure of hydrogen (6.9 bar). The bimetallic Pd-Cu nanoparticles have over an order of magnitude higher activity for phenylacetylene hydrogenation when compared to their monometallic Cu counterpart, while maintaining a high selectivity to styrene over many hours at high conversion. Greater than 94% selectivity to styrene is observed at all times, which is a marked improvement when compared to monometallic Pd catalysts with the same Pd loading, at the same total conversion. X-ray photoelectron spectroscopy and UV-visible spectroscopy measurements confirm the complete uptake and alloying of Pd with Cu by GR. Scanning tunneling microscopy and thermal desorption spectroscopy of model SAA surfaces confirmed the feasibility of hydrogen spillover onto an otherwise inert Cu surface. These model studies addressed a wide range of Pd concentrations related to the bimetallic nanoparticles.

Platinum is a key component in many heterogeneous hydrogenation catalysts. Because of its high price, fairly strong interaction with intermediates, and susceptibility to CO poisoning, it is often mixed with other elements. These bimetallic alloys have complex surface structures, and the atomic structure of their active sites is not well understood. In this study, we examine the effect of the geometric arrangement of dilute Pt−Cu alloys on H 2 activation, spillover, and release. Using scanning tunneling microscopy, we directly visualize the atomic arrangement of Pt−Cu alloys and show that small amounts of Pt (∼1%) exists as isolated atoms in the Cu surface. These Pt monomers are capable of facile H 2 dissociation and spillover to Cu at temperatures as low as 85 K. Additionally, the low-temperature desorption of H 2 (230 K) suggests a reduced desorption barrier compared to monometallic Pt or Cu. We find these single atom alloy surfaces are robust to multiple adsorption/desorption and heating cycles to 450 K. Larger Pt ensembles in Cu exhibit higher temperature desorption profiles due to the stronger binding of H to extended Pt ensembles, demonstrating how the geometric arrangement of Pt atoms in Cu impacts the binding of H to catalytic surface sites. Overall, dilute Pt−Cu alloys containing only isolated Pt atoms are most favorable for H 2 activation, spillover, and release and hence should be capable of catalyzing hydrogenation reactions with a greatly reduced concentration of the precious metal.

Pt–Cu bimetallic alloys are a key component in many heterogeneous catalysts that have the potential to be used in a range of industrially important reactions. Given the catalytic differences between Pt and Cu, the surface composition and geometry of Pt–Cu alloys can have a large influence on their chemistry. Extensive characterization of bulk Pt–Cu alloys has been performed; however, only a few studies have addressed surface and subsurface alloying of Pt with Cu, and none have examined the atomic scale surface structure of Pt–Cu. In this study, scanning tunneling microscopy was used to determine the local structure of surface alloys formed by physical vapor deposition of Pt onto Cu(111) over a range of alloying temperatures (315–550 K). Our results indicated that Pt and Cu were capable of intermixing at 315 K and forming multiple metastable states. Increasing the temperature of the Cu surface during the deposition of Pt altered the surface geometry and further enhanced the dispersion of Pt. The results are compared to the well-characterized Pd/Cu(111) surface alloy. A distinguishing feature of the Pt/Cu(111) surface alloy is the ability of Pt atoms to alloy directly into the Cu surface. Pt alloys as individual isolated atoms, well separated from each other, rather than more localized in regions at step edges, as is the case with Pd. This work indicates that the highly dispersed nature of Pt–Cu surface alloys should render them useful for understanding the surface chemistry of Pt at the single atom level.

Autocatalytic reaction mechanisms are observed in a range of important chemical processes including catalysis, radical-mediated explosions, and biosynthesis. Because of their complexity, the microscopic details of autocatalytic reaction mechanisms have been difficult to study on surfaces and heterogeneous catalysts. Autocatalytic decomposition reactions of S,S-and R,R-tartaric acid (TA) adsorbed on Cu(110) offer molecular-level insight into aspects of these processes, which until now, were largely a matter of speculation. The decomposition of TA/Cu( 110) is initiated by a slow, irreversible process that forms vacancies in the adsorbed TA layer, followed by a vacancy-mediated, explosive decomposition process that yields CO 2 and small hydrocarbon products. Initiation of the explosive decomposition of TA/Cu(110) has been studied by measurement of the reaction kinetics, time-resolved low energy electron diffraction (LEED), and time-resolved scanning tunneling microscopy (STM). Initiation results in a decrease in the local coverage of TA and a concomitant increase in the areal vacancy concentration. Observations of explosive TA decomposition on the Cu(651) S surface suggest that initiation does not occur at structural defects in the surface, as has been suggested in the past. Once the vacancy concentration reaches a critical value, the explosive, autocatalytic decomposition step dominates the TA decomposition rate. The onset of the explosive decomposition of TA on Cu(110) is accompanied by the extraction of Cu atoms from the surface to form a (±6,7; ∓2,1) overlayer that is readily observable using LEED and STM. The explosive decomposition step is second-order in vacancy concentration and accelerates with increasing extent of reaction.