AVS2013 Session SS+NS-WeM: Nanostructures: Reactivity & Catalysis

The surface structures of catalysts have in some instances a large impact on their catalytic properties. On the other hand, historically, mild reactions are not considered to be surface structure sensitive. In this work we report on our observation that both the size and the shape of Pt nanoparticles strongly affect the selectivity of glycerol oxidation, a reaction that can proceed at room temperature and under atmospheric pressures. In a series of experiment using Pt/SiO catalysts with average particle sizes varying from 3.9 to 6.2nm, it was determined that selectivity toward primary carbon oxidation increases as the proportion of larger particles increases. Aside from this size effect, supported tetrahedral Pt and cuboctahedral Pt also exhibit different selectivity and kinetic behavior from the regular Pt catalysts.(See Fig. 1&2) We propose that these selectivity differences are due to the different adsorption state of the surface species. Dihydroxyacetone (the product of dehydrogenation on secondary carbon) is thermodynamically more stable than glyceraldehyde (the product of dehydrogenation on primary carbon) but with larger steric hindrance effect in the dehydrogenation process. While on the edge and corner parts of the Pt surface, the steric hindrance could be largely reduced comparing to smooth Pt surface, therefore dihydroxyacetone is more favored on smaller particles. (See Fig. 3) Further characterization of the catalysts and their detailed kinetic behavior is currently under way.

[1] J.-H. Fischer-Wolfarth, J.A. Farmer, J.M. Flores-Camacho, A. Genest, I.V. Yudanov, N. Rösch, C.T. Campbell, S. Schauermann, H.-J. Freund, Phys. Rev. B, 81, 2010, 241416(R)

[2] M. Peter, J. M. Flores Camacho, S. Adamovski, L.K. Ono, K.-H. Dostert, C.P. O’Brien, B. Roldan Cuenya, S. Schauermann, H.-J. Freund, Angew. Chem. Int. Edit. DOI: 10.1002/anie.201209476

We prepare model catalysts or catalyst-decorated electrodes by depositing size-selected Pd_n and Pt_n clusters on planar supports, including TiO₂(110), alumina/Ta(110), alumina/Rh(0001) as well as glassy carbon and ITO/glass electrodes. The samples are prepared in UHV and characterized in situ by a combination of X-ray and UV photoelectron spectroscopy (XPS, UPS), and low energy He⁺ ion scattering (ISS). The activity of the model catalysts for gas-surface reactions such as CO oxidation is studied using mass spectrometric methods, such as temperature-programmed reaction/desorption (TPR, TPD). Electrochemistry is done in an attached UHV antechamber fitted with a three electrode cell that can be sealed against the sample surface, allowing electrochemistry to done both without, and with prior exposure to laboratory air. Electrocatalytic activity is drastically altered by air exposure. Results will be presented for CO oxidation over various M_n/oxide catalysts, and for electrochemical reactions observed under conditions appropriate for oxygen reduction or ethanol oxidation. One interesting result seen in many of these systems is a size-dependent anti-correlation between activity as oxidation catalysts, and shifts in the catalytic metal core level binding energies. Potential reasons for this correlation, and related studied of valence level shifts with UPS, will be discussed.

Recently¹ we predicted that when a reaction involving a small number of molecules occurs in a nanometric-scale domain entirely segregated from the surrounding media, the nanoconfinement can shift the position of equilibrium toward products via reactant-product reduced mixing. In this talk, we demonstrate how most-recently reported single molecule fluorescence measurements of partial hybridization of ssDNA confined within nanofabricated chambers provide the first experimental confirmation of this entropic nanoconfinement effect. Thus, focusing separately on each occupancy-specific equilibrium constant, quantitatively reveals extra stabilization of the product upon decreasing the chamber occupancy or size.² Namely, the DNA hybridization under nanoconfined conditions is significantly favored over the identical reaction occurring in bulk media with the same reactant concentrations. This effect, now directly verified for DNA, can be relevant to actual biological processes, as well as to reactions occurring on nanoparticle surfaces, and within molecular capsules, nanotubes, or other functional nanospaces.

1. Polak, M.; Rubinovich, L. Nano Lett. 2008, 8 (10), 3543−3547.

2. Rubinovich, L.; Polak, M. Nano Lett. 2013, DOI: 10.1021/nl4008198.

Supported metal clusters on ZnO are important catalysts for industrial synthesis of methanol from syn-gas. The active site of this reaction has long been debated. We have employed model single crystal catalysts and surface characterization tools to elucidate key aspects of the cluster structure and reactivity. Specifically, we have probed the CO oxidation reaction at high pressure (tens of Torr). In the case of Cu/ZnO(10-10), both LEIS and EELS show an encapsulation of the cluster with an ultrathin layer of ZnO which is thermally driven (up to 725 K) and promoted with oxygen exposure. Other spectroscopies, including XPS and UPS, suggest a self-limiting ZnO film thickness of 1 – 2 ML. UPS measurements on the bare (un-annealed) Cu/ZnO clusters show that CO adsorbs molecularly at 85K while it dissociates on an encapsulated cluster at the same temperature, indicating that the ZnO coating is crucial to reactivity. Similarly CO₂ reacts strongly with the encapsulated Cu clusters indicating that the ultra-thin ZnO is participating in the adsorption process. High-pressure studies at 50 Torr at RT show that there is a greatly enhanced reactivity for CO oxidation for the encapsulated clusters compared to either the bare ZnO substrate or the un-encapsulated clusters. Similar results have been seen in the case of Au clusters on ZnO(10-10) and corresponding encapsulation, however differences in reactivity and structure exist. These results offer a new understanding of the “support” role of ZnO on this heterogeneous catalyst system, and may lead to new means to drive the mechanism of methanol synthesis and help to develop new designs of this important industrial catalyst.

ZnO has been used as photocatalyst to degrade organic compounds, such as phenol, phenol derivatives, naphthalene, anthracene, and dye molecules. In order to overcome the limitation of ZnO nanoslurries, immobilized photocatalyst with high S/V ratio nanostructure is needed. Previously, ZnO-nanowire-decorated pore-array films, polymer fiber mats and ZnO nano-plates have been fabricated and reported with enhanced photocatalytic properties. However, diffusion of dye molecules into nanostructures was found to limit the photocatalytic performance. Since the structures of these photocatalysts were not made periodic or uniform in size, the effects of nanostructure dimension could not be investigated. Periodic 3D nanostructures with tunable sizes are therefore needed.

A series of ZnO hierarchical 3D nanostructures were fabricated for photocatalytic applications. Nano-pillars were patterned on photoresist (PFi88A7) using interference lithography, and were coated with a conformal ZnO seed layer (~40nm) using atomic layer deposition (ALD). ZnO nanowires were grown onto the nano-pillars via hydrothermal synthesis. With varied growth time, we decorated the ALD ZnO coated nano-pillars with ZnO nanowires of different lengths (60~150nm). Scanning electron microscopy confirmed the 3D hierarchical nanostructures. Photodegradation kinetics of methyl orange was measured to characterize the photocatalytic property of the ZnO nanostructures. Apparent first order rate constant (k_app) of dye degradation was improved by 35% with ALD ZnO coated nano-pillars, compared with ZnO flat film catalyst. By decorating the nano-pillars with ZnO nanowires of 120 nm length, k_app was further increased by 88% to 1.75 h^-1. We compared the photocatalytic property of our nanostructures with varied sizes. We will show how the nanostructures controlled the diffusion rate of dye molecules and affected the overall photocatalytic performance.

Nanoporous metals have many promising technological applications, such as catalysts, photo-catalysts, and electrodes in batteries, but their practical use is often limited by their tendency to coarsen at elevated temperatures. Here we demonstrate that atomic layer deposition (ALD) of nanometer-thick TiO₂ films can drastically improve the thermal stability and mechanical properties of nanoporous gold (np-Au) while further adding functionality—such as dramatically improved catalytic activity. Aerobic CO oxidation was used to test the effects of annealing on the catalytic activity of the TiO₂/npAu hybrid materials, and the observed changes in catalytic activity will be discussed in terms of annealing induced morphology and phase changes. We characterized these annealing-induced changes of morphology and phase composition by cross-sectional scanning electron microscopy (xSEM), x-ray photoemission spectroscopy (XPS), and near-edge x-ray absorption fine structure (NEXAFS). The Ti L_2,3-edge in NEXAFS is well known to exhibit exceptional sensitivity to Ti coordination and structure. Amorphous, anatase and rutile polymorphs of TiO₂ can be clearly distinguished and reveal a smooth and strong variation in the predominant TiO₂ phase as a function of annealing. Our results provide new insights into the thermal stability of functionalized electrodes for energy storage and harvesting applications.

This work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344

A new class of nanostructured gold model catalysts has been synthesized by depositing gold on single-layer graphene and boron nitride (BN) thin films formed on refractory metals. These thin films, which are oxygen-free and devoid of strong adsorption sites in themselves, develop moiré superstructures due to lattice mismatch with the metal substrates, which present periodic, identical traps on the potential energy surface for possible cluster nucleation. STM studies have shown that gold atoms forms two-dimensional (2-D) islands on graphene/Ru(0001) but small monodisperse clusters on BN/Rh(111). The electronic structures of these gold nanostructures have been characterized by electron energy loss spectroscopy (EELS) and angle resolved photoemission spectroscopy (ARPES), and the geometric structures have been studied by scanning tunneling microscopy (STM). CO and O₂ are used to probe the activity of these gold model catalysts. On 2-D gold islands, adsorbed CO molecules demonstrate a stretching frequency characteristic of anionic Au and can be titrated by O₂ molecules. On gold clusters on BN/Rh(111) substrates, both CO and O₂ chemisorption is demonstrated in temperature programmed desorption (TPD). Furthermore, density function theory (DFT) calculations have been used to elucidate the adsorption sites and adsorption configurations for CO and O₂ on the two different gold model catalysts. Our findings illustrate the critical importance of size to the reactivity of gold catalysts and the exciting possibility of corrugated thin films such as graphene and BN moirés functioning as templates to form model nano-catalysts.