Direct Visualization of Catalytically Active Sites at the FeO-Pt(111) Interface

Within the area of surface science, one of the "holy grails" is to directly visualize a chemical reaction at the atomic scale. Whereas this goal has been reached by high-resolution scanning tunneling microscopy (STM) in a number of cases for reactions occurring at flat surfaces, such a direct view is often inhibited for reaction occurring at steps and interfaces. Here we have studied the CO oxidation reaction at the interface between ultrathin FeO islands and a Pt(111) support by in situ STM and density functional theory (DFT) calculations. Time-lapsed STM imaging on this inverse model catalyst in O2 and CO environments revealed catalytic activity occurring at the FeO-Pt(111) interface and directly showed that the Fe-edges host the catalytically most active sites for the CO oxidation reaction. This is an important result since previous evidence for the catalytic activity of the FeO-Pt(111) interface is essentially based on averaging techniques in conjunction with DFT calculations. The presented STM results are in accord with DFT+U calculations, in which we compare possible CO oxidation pathways on oxidized Fe-edges and O-edges. We found that the CO oxidation reaction is more favorable on the oxidized Fe-edges, both thermodynamically and kinetically.

NANOCATALYSTS. Platinum-based nanocages with subnanometer-thick walls and well-defined, controllable facets

A cost-effective catalyst should have a high dispersion of the active atoms, together with a controllable surface structure for the optimization of activity, selectivity, or both. We fabricated nanocages by depositing a few atomic layers of platinum (Pt) as conformal shells on palladium (Pd) nanocrystals with well-defined facets and then etching away the Pd templates. Density functional theory calculations suggest that the etching is initiated via a mechanism that involves the formation of vacancies through the removal of Pd atoms incorporated into the outermost layer during the deposition of Pt. With the use of Pd nanoscale cubes and octahedra as templates, we obtained Pt cubic and octahedral nanocages enclosed by {100} and {111} facets, respectively, which exhibited distinctive catalytic activities toward oxygen reduction.

Palladium-platinum core-shell icosahedra with substantially enhanced activity and durability towards oxygen reduction

Conformal deposition of platinum as ultrathin shells on facet-controlled palladium nanocrystals offers a great opportunity to enhance the catalytic performance while reducing its loading. Here we report such a system based on palladium icosahedra. Owing to lateral confinement imposed by twin boundaries and thus vertical relaxation only, the platinum overlayers evolve into a corrugated structure under compressive strain. For the core-shell nanocrystals with an average of 2.7 platinum overlayers, their specific and platinum mass activities towards oxygen reduction are enhanced by eight- and sevenfold, respectively, relative to a commercial catalyst. Density functional theory calculations indicate that the enhancement can be attributed to the weakened binding of hydroxyl to the compressed platinum surface supported on palladium. After 10,000 testing cycles, the mass activity of the core-shell nanocrystals is still four times higher than the commercial catalyst. These results demonstrate an effective approach to the development of electrocatalysts with greatly enhanced activity and durability.

Core-shell catalysts can enhance activity while reducing the loading of expensive catalyst materials. Here, the authors report a palladium@platinum system in which the platinum shells evolve into a corrugated structure with compressive strains, with subsequent enhancement of oxygen reduction activity.

Atomic layer-by-layer deposition of platinum on palladium octahedra for enhanced catalysts toward the oxygen reduction reaction

We systematically evaluated two different approaches to the syntheses of Pd@PtnL (n = 2-5) core-shell octahedra. We initially prepared the core-shell octahedra using a polyol-based route by titrating a Pt(IV) precursor into the growth solution containing Pd octahedral seeds at 200 °C through the use of a syringe pump. The number of Pt atomic layers could be precisely controlled from two to five by increasing the volume of the precursor solution while fixing the amount of seeds. We then demonstrated the synthesis of Pd@PtnL octahedra using a water-based route at 95 °C through the one-shot injection of a Pt(II) precursor. Due to the large difference in reaction temperature, the Pd@PtnL octahedra obtained via the water-based route showed sharper corners than their counterparts obtained through the polyol-based route. When compared to a commercial Pt/C catalyst based upon 3.2 nm Pt particles, the Pd@PtnL octahedra prepared using both methods showed similar remarkable enhancement in terms of activity (both specific and mass) and durability toward the oxygen reduction reaction. Calculations based upon periodic, self-consistent density functional theory suggested that the enhancement in specific activity for the Pd@PtnL octahedra could be attributed to the destabilization of OH on their PtnL*/Pd(111) surface relative to the {111} and {100} facets exposed on the surface of Pt/C. The destabilization of OH facilitates its hydrogenation, which was found to be the rate-limiting step of the oxygen reduction reaction on all these surfaces.

The nature of the Fe-graphene interface at the nanometer level

The emerging fields of graphene-based magnetic and spintronic devices require a deep understanding of the interface between graphene and ferromagnetic metals. This paper reports a detailed investigation at the nanometer level of the Fe-graphene interface carried out by angle-resolved photoemission, high-resolution photoemission from core levels, near edge X-ray absorption fine structure, scanning tunnelling microscopy and spin polarized density functional theory calculations. Quasi-free-standing graphene was grown on Pt(111), and the iron film was either deposited atop or intercalated beneath graphene. Calculations and experimental results show that iron strongly modifies the graphene band structure and lifts its π band spin degeneracy.

Adsorbate diffusion on transition metal nanoparticles

Diffusion of adsorbates on transition metal nanoparticles is a precursor process for heterogeneously catalyzed reactions, and as a result, an atomistic understanding of the diffusion mechanism is very important. We systematically studied adsorption and diffusion of atomic and diatomic species (H, C, N, O, CO, and NO) on nanometer-sized Pt and Cu nanoparticles with different sizes and shapes using density functional theory calculations. We show that nanoparticles bind adsorbates more strongly than the corresponding extended single crystal metal surfaces. We find that there is a Brønsted-Evans-Polanyi-type linear correlation between the transition state energy and the initial state energy for adsorbate diffusion across the edges of Pt and Cu nanoparticles. We further show that the barrier for adsorbate diffusion across the nanoparticles edges can be estimated by the binding energy of the adsorbate on the nanoparticles. These results provide useful insights for understanding diffusion-mediated chemical reactions catalyzed by transition metal nanoparticles, which are widely used in heterogeneous catalysis.

Catalytically active Au-O(OH)x-species stabilized by alkali ions on zeolites and mesoporous oxides

We report that the addition of alkali ions (sodium or potassium) to gold on KLTL-zeolite and mesoporous MCM-41 silica stabilizes mononuclear gold in Au-O(OH)x-(Na or K) ensembles. This single-site gold species is active for the low-temperature (<200°C) water-gas shift (WGS) reaction. Unexpectedly, gold is thus similar to platinum in creating -O linkages with more than eight alkali ions and establishing an active site on various supports. The intrinsic activity of the single-site gold species is the same on irreducible supports as on reducible ceria, iron oxide, and titania supports, apparently all sharing a common, similarly structured gold active site. This finding paves the way for using earth-abundant supports to disperse and stabilize precious metal atoms with alkali additives for the WGS and potentially other fuel-processing reactions.

Water clustering on nanostructured iron oxide films

The adhesion of water to solid surfaces is characterized by the tendency to balance competing molecule-molecule and molecule-surface interactions. Hydroxyl groups form strong hydrogen bonds to water molecules and are known to substantially influence the wetting behaviour of oxide surfaces, but it is not well-understood how these hydroxyl groups and their distribution on a surface affect the molecular-scale structure at the interface. Here we report a study of water clustering on a moiré-structured iron oxide thin film with a controlled density of hydroxyl groups. While large amorphous monolayer islands form on the bare film, the hydroxylated iron oxide film acts as a hydrophilic nanotemplate, causing the formation of a regular array of ice-like hexameric nanoclusters. The formation of this ordered phase is localized at the nanometre scale; with increasing water coverage, ordered and amorphous water are found to coexist at adjacent hydroxylated and hydroxyl-free domains of the moiré structure.

Atomic layer-by-layer deposition of Pt on Pd nanocubes for catalysts with enhanced activity and durability toward oxygen reduction

An effective strategy for reducing the Pt content while retaining the activity of a Pt-based catalyst is to deposit the Pt atoms as ultrathin skins of only a few atomic layers thick on nanoscale substrates made of another metal. During deposition, however, the Pt atoms often take an island growth mode because of a strong bonding between Pt atoms. Here we report a versatile route to the conformal deposition of Pt as uniform, ultrathin shells on Pd nanocubes in a solution phase. The introduction of the Pt precursor at a relatively slow rate and high temperature allowed the deposited Pt atoms to spread across the entire surface of a Pd nanocube to generate a uniform shell. The thickness of the Pt shell could be controlled from one to six atomic layers by varying the amount of Pt precursor added into the system. Compared to a commercial Pt/C catalyst, the Pd@PtnL (n = 1-6) core-shell nanocubes showed enhancements in specific activity and durability toward the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations on model (100) surfaces suggest that the enhancement in specific activity can be attributed to the weakening of OH binding through ligand and strain effects, which, in turn, increases the rate of OH hydrogenation. A volcano-type relationship between the ORR specific activity and the number of Pt atomic layers was derived, in good agreement with the experimental results. Both theoretical and experimental studies indicate that the ORR specific activity was maximized for the catalysts based on Pd@Pt2-3L nanocubes. Because of the reduction in Pt content used and the enhancement in specific activity, the Pd@Pt1L nanocubes showed a Pt mass activity with almost three-fold enhancement relative to the Pt/C catalyst.

Significant quantum effects in hydrogen activation

Dissociation of molecular hydrogen is an important step in a wide variety of chemical, biological, and physical processes. Due to the light mass of hydrogen, it is recognized that quantum effects are often important to its reactivity. However, understanding how quantum effects impact the reactivity of hydrogen is still in its infancy. Here, we examine this issue using a well-defined Pd/Cu(111) alloy that allows the activation of hydrogen and deuterium molecules to be examined at individual Pd atom surface sites over a wide range of temperatures. Experiments comparing the uptake of hydrogen and deuterium as a function of temperature reveal completely different behavior of the two species. The rate of hydrogen activation increases at lower sample temperature, whereas deuterium activation slows as the temperature is lowered. Density functional theory simulations in which quantum nuclear effects are accounted for reveal that tunneling through the dissociation barrier is prevalent for H2 up to ∼190 K and for D2 up to ∼140 K. Kinetic Monte Carlo simulations indicate that the effective barrier to H2 dissociation is so low that hydrogen uptake on the surface is limited merely by thermodynamics, whereas the D2 dissociation process is controlled by kinetics. These data illustrate the complexity and inherent quantum nature of this ubiquitous and seemingly simple chemical process. Examining these effects in other systems with a similar range of approaches may uncover temperature regimes where quantum effects can be harnessed, yielding greater control of bond-breaking processes at surfaces and uncovering useful chemistries such as selective bond activation or isotope separation.

Quantum tunneling enabled self-assembly of hydrogen atoms on Cu(111)

Atomic and molecular self-assembly are key phenomena that underpin many important technologies. Typically, thermally enabled diffusion allows a system to sample many areas of configurational space, and ordered assemblies evolve that optimize interactions between species. Herein we describe a system in which the diffusion is quantum tunneling in nature and report the self-assembly of H atoms on a Cu(111) surface into complex arrays based on local clustering followed by larger scale islanding of these clusters. By scanning tunneling microscope tip-induced scrambling of H atom assemblies, we are able to watch the atomic scale details of H atom self-assembly in real time. The ordered arrangements we observe are complex and very different from those formed by H on other metals that occur in much simpler geometries. We contrast the diffusion and assembly of H with D, which has a much slower tunneling rate and is not able to form the large islands observed with H over equivalent time scales. Using density functional theory, we examine the interaction of H atoms on Cu(111) by calculating the differential binding energy as a function of H coverage. At the temperature of the experiments (5 K), H(D) diffusion by quantum tunneling dominates. The quantum-tunneling-enabled H and D diffusion is studied using a semiclassically corrected transition state theory coupled with density functional theory. This system constitutes the first example of quantum-tunneling-enabled self-assembly, while simultaneously demonstrating the complex ordering of H on Cu(111), a catalytically relevant surface.

A Conversation with Haldor Topsøe

Haldor Topsøe was born in Copenhagen, Denmark, in 1913. He studied chemical engineering at the Technical University of Denmark (DTU). In 1940, he founded the company Haldor Topsøe, which has become one of the most successful companies in the field of heterogeneous catalysis. The company develops and markets catalysts and processes for petroleum refining, air-pollution control, synthesis gas production, ammonia and methanol production and solid oxide fuel cells. The company Haldor Topsøe is also a leader in the production and state-of-the-art scientific characterization of new catalysts. Dr. Topsøe is currently the sole owner of the company. He has written numerous articles on ammonia synthesis, catalysis, and energy questions. He is the author of several books on economics. Dr. Topsøe has received numerous awards, including honorary doctorates from Aarhus University, DTU, and the Chalmers University in Sweden; the Gold Medal from the Royal Academy of Sciences; the Hoover Medal for his technical abilities and entrepreneurship; and the Grove Memorial Medal for Advances in Fuel Cell Technology. Topsøe is a member of the Danish and the Swedish Academies of Technical Sciences and of the U.S. National Academy of Engineering.

The following interview was conducted on August 16, 2010.

–Manos Mavrikakis

Water-mediated proton hopping on an iron oxide surface

The diffusion of hydrogen atoms across solid oxide surfaces is often assumed to be accelerated by the presence of water molecules. Here we present a high-resolution, high-speed scanning tunneling microscopy (STM) study of the diffusion of H atoms on an FeO thin film. STM movies directly reveal a water-mediated hydrogen diffusion mechanism on the oxide surface at temperatures between 100 and 300 kelvin. Density functional theory calculations and isotope-exchange experiments confirm the STM observations, and a proton-transfer mechanism that proceeds via an H(3)O(+)-like transition state is revealed. This mechanism differs from that observed previously for rutile TiO(2)(110), where water dissociation is a key step in proton diffusion.

Abnormal spatial QRS-T angle, a marker of ventricular repolarisation, predicts serious ventricular arrhythmia in systemic sclerosis

OBJECTIVES

Cardiac involvement may be under-diagnosed in asymptomatic patients with systemic sclerosis (SSc). Standard electrocardiography-derived spatial QRS-T angle (spQRS-Ta) is an established marker of ventricular repolarisation heterogeneity, and a strong independent predictor of cardiac morbidity and mortality, including sudden death, in the general population. We examined whether spQRS-Ta is abnormal in asymptomatic SSc patients and assessed its predictive value for possibly concurrent, serious ventricular arrhythmia.

METHODS

SpQRS-Ta and 24-hour Holter recordings were obtained from 69 SSc patients (aged 51±13 years, 63 women) without clinically evident cardiac involvement and having left ventricular ejection fraction at least 50% by echocardiography. 'Healthy' subjects matched 1:1 with patients for age, gender and body mass index served as controls.

RESULTS

SpQRS-Ta was wider in SSc (median value 15.6°, interquartile range 10.6-24.3°) than controls (10.5°, 7.3-13.5°, p=0.0001) and not associated with skin fibrosis extent or specific clinical manifestations and autoantibodies. Twenty-four-hour Holter recordings revealed couplets of ventricular beats in six (Lown class IVa) and non-sustained ventricular tachycardia in five patients (Lown class IVb); spQRS-Ta was wider in those eleven patients with serious ventricular arrhythmia than the remaining patients (24.9°, 14.9-31.3° vs. 14.4°, 9.6-22.3°; p=0.02). A spQRS-Ta>19.3° demonstrated 80% sensitivity and 68% specificity (area under the curve 0.81, p=0.02) to predict the presence of non-sustained ventricular tachycardia in Holter monitoring.

CONCLUSIONS

Ventricular repolarisation heterogeneity, as reflected by wider spQRS-Ta, is common in SSc. Increased spQRS-Ta could serve as a simple screening test for further investigation to identify patients at risk or prone to develop life-threatening ventricular arrhythmia.

Low-temperature CO oxidation on Ni(111) and on a Au/Ni(111) surface alloy

From an interplay between scanning tunneling microscopy, temperature programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations we have studied low-temperature CO oxidation on Au/Ni(111) surface alloys and on Ni(111). We show that an oxide is formed on both the Ni(111) and the Au/Ni(111) surfaces when oxygen is dosed at 100 K, and that CO can be oxidized at 100 K on both of these surfaces in the presence of weakly bound oxygen. We suggest that low-temperature CO oxidation can be rationalized by CO oxidation on O(2)-saturated NiO(111) surfaces, and show that the main effect of Au in the Au/Ni(111) surface alloy is to block the formation of carbonate and thereby increase the low-temperature CO(2) production.