Jeong Young Park

Thermally stable Pt/mesoporous silica core-shell nanocatalysts for high-temperature reactions

Recent advances in colloidal synthesis enabled the precise control of the size, shape and composition of catalytic metal nanoparticles, enabling their use as model catalysts for systematic investigations of the atomic-scale properties affecting catalytic activity and selectivity. The organic capping agents stabilizing colloidal nanoparticles, however, often limit their application in high-temperature catalytic reactions. Here, we report the design of a high-temperature-stable model catalytic system that consists of a Pt metal core coated with a mesoporous silica shell (Pt@mSiO(2)). Inorganic silica shells encaged the Pt cores up to 750 degrees C in air and the mesopores providing direct access to the Pt core made the Pt@mSiO(2) nanoparticles as catalytically active as bare Pt metal for ethylene hydrogenation and CO oxidation. The high thermal stability of Pt@mSiO(2) nanoparticles enabled high-temperature CO oxidation studies, including ignition behaviour, which was not possible for bare Pt nanoparticles because of their deformation or aggregation. The results suggest that the Pt@mSiO(2) nanoparticles are excellent nanocatalytic systems for high-temperature catalytic reactions or surface chemical processes, and the design concept used in the Pt@mSiO(2) core-shell catalyst can be extended to other metal/metal oxide compositions.

Molecular Factors of Catalytic Selectivity

Selectivity--the production of one molecule out of many other thermodynamically feasible product molecules--is the key concept in developing clean processes that do not produce by-products (green chemistry). Small differences in the potential-energy barriers of single reaction steps control which reaction channel is more likely to yield the desired product molecule (selectivity), while the overall activation energy of the reaction controls the turnover rates (activity). Recent studies have demonstrated that tailoring parameters at the atomic or molecular level--such as the surface structures of active sites--gives turnover rates and reaction selectivities that depend on the nanoparticle size and shape. Here, we highlight seven molecular components that influence the selectivity of heterogeneous catalyst reactions on single-crystal model surfaces and colloid nanoparticles: surface structure, adsorbate-induced restructuring, adsorbate mobility, reaction intermediates, surface composition, charge transport, and oxidation states. We show the importance of the single factors by means of examples and describe in situ analyses that permit their roles in surface reactions to be investigated.

Size Effect of Ruthenium Nanoparticles in Catalytic Carbon Monoxide Oxidation

Carbon monoxide oxidation over ruthenium catalysts has shown an unusual catalytic behavior. Here we report a particle size effect on CO oxidation over Ru nanoparticle (NP) catalysts. Uniform Ru NPs with a tunable particle size from 2 to 6 nm were synthesized by a polyol reduction of Ru(acac)(3) precursor in the presence of poly(vinylpyrrolidone) stabilizer. The measurement of catalytic activity of CO oxidation over two-dimensional Ru NPs arrays under oxidizing reaction conditions (40 Torr CO and 100 Torr O(2)) showed an activity dependence on the Ru NP size. The CO oxidation activity increases with NP size, and the 6 nm Ru NP catalyst shows 8-fold higher activity than the 2 nm catalysts. The results gained from this study will provide the scientific basis for future design of Ru-based oxidation catalysts.

A Reactive Oxide Overlayer on Rhodium Nanoparticles during CO Oxidation and Its Size Dependence Studied by In Situ Ambient‐Pressure X‐ray Photoelectron Spectroscopy

Carbon monoxide oxidation is one of the most studied heterogeneous reactions, being scientifically and industrially important, particularly for removal of CO from exhaust streams and preferential oxidation for hydrogen purification in fuel-cell applications. The precious metals Ru, Rh, Pd, Pt, and Au are most commonly used for this reaction because of their high activity and stability. Despite the wealth of experimental and theoretical data, it remains unclear what is the active surface for CO oxidation under catalytic conditions for these metals. Herein we utilize in situ synchrotron ambient pressure X-ray photoelectron spectroscopy (APXPS) to monitor the oxidation state at the surface of rhodium nanoparticles (Rh NPs) during CO oxidation and demonstrate that the active catalyst is a surface oxide, the formation of which is dependent on particle size. The amount of oxide formed and the reaction rate both increase with decreasing particle size. Many single-crystal CO oxidation studies over rhodium suggest that the reaction is structure-insensitive and that the oxide formation decreases the reaction rate. However, recent advances in synthetic techniques and in-situ experimentation have revealed that the oxidation state and stoichiometry of the surface oxide greatly affects CO oxidation rates. At low temperatures or low O2/CO ratios, CO strongly adsorbs onto the catalyst surface and inhibits O2 adsorption. At high temperatures or high O2/CO ratios, the catalyst surface becomes saturated with oxygen atoms and the reaction proceeds more rapidly. It has been demonstrated that small palladium nanoparticles are more active for CO oxidation than larger particles and single crystals, whereas the opposite is reported for platinum. For Rh NPs, no particle size effect was observed for supported rhodium catalysts, but a strong particle size dependence was observed for CO desorption, dissociation, and transient CO oxidation over electron-beam-prepared Rh NPs that were precovered with oxygen. For this investigation we have prepared small, polymerstabilized Rh NPs with a narrow size distribution and studied CO oxidation; polymer stabilized NP syntheses enable control of NP size, shape, and/or composition for reaction studies. The turnover frequency (TOF) for CO oxidation at 200 8C increases five-fold, and the apparent activation energy decreases from 27.9 kcalmol 1 to 19.0 kcalmol 1 as the particle size decreases from 11 nm to 2 nm. APXPS of 2 nm and 7 nm Rh NP films during CO oxidation at about 1 Torr provides the first in-situ measurement of the oxidation state of Rh NPs during CO oxidation and demonstrates that smaller particles are more oxidized than larger particles during reaction at 150–200 8C. A surface oxygen species is also observed during CO oxidation that is not present when heating in O2 alone, possibly indicating a unique active oxide phase on Rh NPs. This oxide phase may alter the relative bonding geometries of CO and/or oxygen on the rhodium surface, thereby lowering the activation energy for the reaction. The synthesis of monodisperse Rh NPs by polyol reduction using poly(vinylpyrrolidone) (PVP) as a capping agent and [Rh(acac)3] as a rhodium precursor [17] was extended to smaller sizes by the addition of sodium citrate. Using this approach, Rh NPs of 3.5 nm (3.6 0.5 nm), 2.5 nm (2.5 0.4 nm), and 2 nm (1.9 0.3 nm) were formed by increasing the amount of sodium citrate. Monolayer films of these particles were then prepared in a Langmuir–Blodgett (LB) trough and characterized with transmission electron microscopy (TEM) and XPS. Figure 1a–c shows TEM images of the NPs, with insets of size distribution histograms taken from 100 particles. Figure 1 f shows X-ray photoelectron spectra for the Rh 3d peak of the as-synthesized (no pretreatment) particles after LB deposition onto a silicon wafer. The ratio of oxidized rhodium to reduced rhodium clearly increases as the particle size decreases. The three samples of small Rh NP (2, 2.5, and 3.5 nm) LB films and two previously synthesized samples, 7 nm (7.1 [*] M. E. Grass, D. R. Butcher, Dr. J. Y. Park, Dr. Y. Li, Dr. H. Bluhm, Dr. K. M. Bratlie, Dr. T. Zhang, Prof. G. A. Somorjai Department of Chemistry; University of California, Berkeley Chemical and Materials Sciences Divisions Lawrence Berkeley National Laboratory; Berkeley, CA 94720 (USA) Fax: (+1) 510-643-9668 E-mail: Somorjai@berkeley.edu

Surface Plasmon-Driven Hot Electron Flow Probed with Metal-Semiconductor Nanodiodes

A continuous flow of hot electrons that are not at thermal equilibrium with the surrounding metal atoms is generated by the absorption of photons. Here we show that hot electron flow generated on a gold thin film by photon absorption (or internal photoemission) is amplified by localized surface plasmon resonance. This was achieved by direct measurement of photocurrent on a chemically modified gold thin film of metal-semiconductor (TiO(2)) Schottky diodes. The short-circuit photocurrent obtained with low-energy photons is consistent with Fowlers law, confirming the presence of hot electron flows. The morphology of the metal thin film was modified to a connected gold island structure after heating such that it exhibits surface plasmon. Photocurrent and optical measurements on the connected island structures revealed the presence of a localized surface plasmon at 550 ± 20 nm. The results indicate an intrinsic correlation between the hot electron flow generated by internal photoemission and localized surface plasmon resonance.

Tuning of catalytic CO oxidation by changing composition of Rh-Pt bimetallic nanoparticles

Recent breakthroughs in synthesis in nanoscience have achieved control of size and composition of nanoparticles that are relevant for catalyst design. Here, we show that the catalytic activity of CO oxidation by Rh/Pt bimetallic nanoparticles can be changed by varying the composition at a constant size (9+/-1 nm). Two-dimensional Rh/Pt bimetallic nanoparticle arrays were formed on a silicon surface via the Langmuir-Blodgett technique. Composition analysis with X-ray photoelectron spectroscopy agrees with the reaction stoichiometry of Rh/(Pt+Rh). CO oxidation rates that exhibit a 20-fold increase from pure Pt to pure Rh show a nonlinear increase with surface composition of the bimetallic nanoparticles that is consistent with the surface segregation of Pt. The results demonstrate the possibility of controlling catalytic activity in metal nanoparticle-oxide systems via tuning the composition of nanoparticles with potential applications for nanoscale design of industrial catalysts.

Friction Anisotropy–Driven Domain Imaging on Exfoliated Monolayer Graphene

Otherwise identical regions of supported graphene can be distinguished by changes in friction with sliding direction. Graphene produced by exfoliation has not been able to provide an ideal graphene with performance comparable to that predicted by theory, and structural and/or electronic defects have been proposed as one cause of reduced performance. We report the observation of domains on exfoliated monolayer graphene that differ by their friction characteristics, as measured by friction force microscopy. Angle-dependent scanning revealed friction anisotropy with a periodicity of 180° on each friction domain. The friction anisotropy decreased as the applied load increased. We propose that the domains arise from ripple distortions that give rise to anisotropic friction in each domain as a result of the anisotropic puckering of the graphene.

Intrinsic Relationship between Enhanced Oxygen Reduction Reaction Activity and Nanoscale Work Function of Doped Carbons

Nanostructured carbon materials doped with a variety of heteroatoms have shown promising electrocatalytic activity in the oxygen reduction reaction (ORR). However, understanding of the working principles that underpin the superior ORR activity observed with doped nanocarbons is still limited to predictions based on theoretical calculations. Herein, we demonstrate, for the first time, that the enhanced ORR activity in doped nanocarbons can be correlated with the variation in their nanoscale work function. A series of doped ordered mesoporous carbons (OMCs) were prepared using N, S, and O as dopants; the triple-doped, N,S,O-OMC displayed superior ORR activity and four-electron selectivity compared to the dual-doped (N,O-OMC and S,O-OMC) and the monodoped (O-OMC) OMCs. Significantly, the work functions of these heteroatom-doped OMCs, measured by Kelvin probe force microscopy, display a strong correlation with the activity and reaction kinetics for the ORR. This unprecedented experimental insight can be used to provide an explanation for the enhanced ORR activity of heteroatom-doped carbon materials.

Enhanced Nanoscale Friction on Fluorinated Graphene

Atomically thin graphene is an ideal model system for studying nanoscale friction due to its intrinsic two-dimensional (2D) anisotropy. Furthermore, modulating its tribological properties could be an important milestone for graphene-based micro- and nanomechanical devices. Here, we report unexpectedly enhanced nanoscale friction on chemically modified graphene and a relevant theoretical analysis associated with flexural phonons. Ultrahigh vacuum friction force microscopy measurements show that nanoscale friction on the graphene surface increases by a factor of 6 after fluorination of the surface, while the adhesion force is slightly reduced. Density functional theory calculations show that the out-of-plane bending stiffness of graphene increases up to 4-fold after fluorination. Thus, the less compliant F-graphene exhibits more friction. This indicates that the mechanics of tip-to-graphene nanoscale friction would be characteristically different from that of conventional solid-on-solid contact and would be dominated by the out-of-plane bending stiffness of the chemically modified graphene. We propose that damping via flexural phonons could be a main source for frictional energy dissipation in 2D systems such as graphene.

Superlubric Sliding of Graphene Nanoflakes on Graphene

The lubricating properties of graphite and graphene have been intensely studied by sliding a frictional force microscope tip against them to understand the origin of the observed low friction. In contrast, the relative motion of free graphene layers remains poorly understood. Here we report a study of the sliding behavior of graphene nanoflakes (GNFs) on a graphene surface. Using scanning tunneling microscopy, we found that the GNFs show facile translational and rotational motions between commensurate initial and final states at temperatures as low as 5 K. The motion is initiated by a tip-induced transition of the flakes from a commensurate to an incommensurate registry with the underlying graphene layer (the superlubric state), followed by rapid sliding until another commensurate position is reached. Counterintuitively, the average sliding distance of the flakes is larger at 5 K than at 77 K, indicating that thermal fluctuations are likely to trigger their transitions from superlubric back to commensurate ground states.