AVS2009 Session SS2-MoA: Formation & Reactivity of Nanoclusters

a) Hydrogen absorption/desorption (hydride formation) studies of Pd nanoparticles in the size range 2-10nm. The main result from these studies includes the size dependent thermodynamics (p-C-T diagram) and kinetics of the H/Pd nanoparticle (1-5 nm) system. The kinetics exhibit a power and power-exponential size dependence of the time scales for the hydrogen uptake and release processes. The uptake is in agreement with Monte Carlo simulations of diffusion controlled hydriding kinetics, while the release (desorption) is explained by the effect of surface tension on the activation energy for hydrogen desorption from the nanoparticle.

b) Kinetic studies of CO and H₂ oxidation on Pt nanoparticles with particular emphasis on kinetic phase transition phenomena in these reactions. The latter occur as one varies reactant concentrations but keeps the total amount of reactants constant and are recognized by a sudden transition from oxygen covered nanoparticle surfaces at low CO (H₂) concentration, to high CO or H coverage for low O₂ concentrations,

c) NO₂ storage and conversion to N₂ on Pt/BaO. In this case NO₂ storage and conversion kinetics is followed via changes in the LSPR resonance of the Au sensing particles, coated with a thin layer of BaO onto which Pt nanoparticles were deposited. Exposure of the sensor to the gas mixture of NO₂+O₂ leads to a conversion of BaO to Ba(NO₃)₂ (NO_x storage) leading to a change of refractive index of the material surrounding the sensing nanoparticle. This leads to a LSPR peak shift that is monitored. Exposure to H₂ converts the stored NO₂ to N₂. Reversible changes were monitored for such NO₂ oxidation/reduction cycles.

I will present our recent work where we explored potential utilization of highly uniform metallic nano-structured materials as selective heterogeneous catalysts. The advantage of these materials compared to conventional catalytic materials is that their structure can be controlled with almost atomic precision, and that it is possible to synthesize highly homogeneous structures. We demonstrated some of these advantages recently when we showed that well-defined, tailored Ag nano-structures are much more selective in heterogeneous epoxidation of ethylene to form ethylene oxide (EO) (ethylene + ½ O₂ → EO) than conventional industrial catalysts.

We showed using quantum chemical Density Functional Theory (DFT) calculations, where we studied critical elementary chemical steps that govern the selectivity to EO in the process, that the Ag(100) surface should be inherently more selective than the Ag(111) surface. We note that catalytic particles, synthesized using conventional synthesis procedure and currently used in commercial ethylene epoxidation process, are dominated by the (111) surface. To synthesize Ag nano-structures which are dominated with the Ag(100) faces, we employed a synthesis procedure which uses organic stabilizer molecules to direct the growth of the nano-structure in a particular direction and to control the surface facets that terminate the nano-structure. This synthetics strategy allowed us to synthesize well-defined and highly uniform Ag nano-wires and nano-cubes which are dominated by the (100) facet. Subsequent experiments showed that Ag nano-wires and nano-cube catalysts can achieve selectivity to EO, which is, at differential conversion, by ~ 40 % higher than for conventional Ag catalysts.

We have also recently started exploring these metallic nano-structures as possible platforms for chemical characterization. The features of these nano-structures that are particularly appealing are: (i) the nanostructures are well defined on atomic level, and their surface to volume ratio is fairly high, which makes these structures inherently better suited for the studies of surface chemical processes compared to traditional single crystal model systems, which are while very well defined, characterized by low surface to volume ratio, (ii) we can synthesize the nanostructures with high degree of uniformity in size and shape, which rules out possible effects due to diversity in size and shape, i.e. these, (iii) the nanostructures are effective scatterers of electromagnetic radiation which make them suitable as platforms for a number of chemical characterization techniques including surface enhanced Raman (SERS) or IR spectroscopies. We will demonstrate the utility of the nano-structures for chemical characterization by a way of an example, where we monitored in-situ ethylene epoxidation.

Metallic nanostructures are of great interest in many scientific fields due to their novel size-dependent physical and chemical properties. The origin of the enhanced catalytic reactivity and selectivity displayed by small metal nanoparticles is yet to be explained. However, it is generally agreed that the design of the next generation of nanocatalysts requires detailed knowledge of the correlation between their reactivity and their physical properties such as morphology, electronic structure, chemical composition, and interactions with their support. Intriguing effects including phonon confinement and phonon localization at interfaces have also been observed in low dimensional systems . Such modifications of the vibrational density of states of nanoscale materials are of scientific and technological relevance, because they profoundly affect their thermodynamic properties. In addition, their understanding could help further our insight in the field of catalysis, since phonons might play a decisive role in certain chemical processes. To systematically study these effects, homogeneous and size-selected nanostructures are needed. Diblock-copolymers can aid the synthesis of such well-defined nanoscale systems.

The first part of my talk will illustrate the formation, stability, as well as the electronic and catalytic properties of size- and shape-selected Au, AuFe, Pt, and Pt-M (M = Au, Fe, Ru and Pd) nanoparticles synthesized by micelle encapsulation. CO oxidation and alcohol decomposition and oxidation have been used as model reactions. Emphasis will be given to the role of the nanoparticle support and the oxidation state of the active catalytic species in their reactivity.

In addition to changes in reactivity, nanostructuring materials also affects their vibrational properties. The second part of my talk will discuss the size- and composition-dependent vibrational dynamics of ⁵⁷Fe, ⁵⁷FePt, and ⁵⁷FeAu clusters as well as of nanostructured metal multilayers prepared by molecular beam epitaxy in UHV. The phonon density of states of those systems will be extracted from nuclear resonant inelastic X-ray scattering measurements. An enhancement of the density of low- and high-energy phonon modes as well as non-Debye-like behavior was observed on ⁵⁷Fe clusters. The latter effects were found to depend on the chemical nature of the surface shell on the nanoclusters. Thickness-dependent phonon confinement and interface localization effects were detected on nanoscale ⁵⁷Fe/M multilayers (M = Cu, Pd or Ag). All these effects show the unique physical properties of metal nanoparticles, and their promise in technological applications.

Website: http://www.physics.ucf.edu/~roldan

Scanning tunneling microscopy (STM) was used to characterize RuSn nanoparticles deposited from a gas phase organometallic precursor (Ru₃(CO)₉(μ-SnPh₂)₃) onto an Al₂O₃ film grown on a Ni₃Al(111) substrate. This novel method allows for the specific preparation of small metal clusters that range between single atoms and larger, bulk-like particles. It also assists in bridging the gap between previous studies based on metal evaporation and industrial catalysis, where the use of precursor compounds for the active species is common. The STM images of the Al₂O₃/Ni₃Al(111) substrate prior to the exposure of the precursor show two distinct areas, with the first one exhibiting the dot structure of the alumina film, as reported in the literature. The second area does not exhibit any regular structure. After exposing the sample to the precursor at room temperature, the precursor was found to adsorb preferentially onto the second area, but also on the first area in significant amounts. In a subsequent series of images acquired for increasingly higher annealing temperatures, the discrepancy between the two areas in the interaction with the adsorbates is strongly increased. While the surface with the dot structure becomes almost fully depleted of the adsorbates, except for a few small (up to 3 nm in diameter) and roundish protrucions, the other area is covered by larger polygonal protrusions. Similarly shaped depressions in the surface are observed along with the protrusions. A control experiment in which the bare Ni₃Al(111) surface was exposed to the precursor indicates that the second area of the Al₂O₃/Ni₃Al(111) surface consists of patches of bare metal. A striking difference between the protrusions on the oxide and the metal is that the size of the latter increases dramatically with increasing annealing temperature, while the former do not grow at all. Nevertheless, both features are observed even up to 925 K, from which it is concluded that the annealing removes the ligands from the metal centers of the precursor molecules, after which the metal nanoparticles can form. These results show that by using an organometallic precursor, small uniform metal nanoparticles with high thermal stability can be successfully deposited onto an oxide thin film. This paves the way for subsequent studies of the surface chemistry associated with model oxide-supported metal catalysts consisting of metal nanoparticles of a uniform size.

Metal-loaded diblock-copolymer micelles have been used to synthesize size-selected self-assembled Au, Fe, and Au-Fe nanoparticles (NPs). In-situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the formation and thermal stability of Au_xFe_1-x (x = 1, 0.8, 0.5, 0.2, 0) NPs supported on TiO₂(110) [1]. Upon step-wise annealing from 300°C to 1060°C, a remarkable thermal stability of the Au-Fe NPs was observed, maintaining their original hexagonal spatial arrangement on the TiO₂ surface up to 900°C. A majority phase of a gold-iron alloy (solid solution) was achieved for our Au_0.5Fe_0.5 NPs in the temperature range of 700°C - 800°C, and for Au_0.2Fe_0.8 NPs at 800°C, while a phase mixture of bcc Fe and Au-Fe alloy was observed for the Au_0.8Fe_0.2 system at 800°C - 900°C. For all samples the segregation of Au atoms towards the NP surface was detected upon high temperature annealing (800°C) in vacuum. Nearly complete Au desorption was observed by XPS at 900°C for Au_0.2Fe_0.8 NPs, at 1000°C for Au_0.5Fe_0.5 NPs, and at 1060°C for Au_0.8Fe_0.2 NPs. The enhanced thermal stability of Au in the Au_0.8Fe_0.2 NPs is believed to be related to the formation of core(Fe)/shell(Au) structures. Furthermore, contrary to the case of pure Fe or Fe-rich NPs where nearly complete Fe desorption or Fe diffusion into TiO₂ was observed at 1000°C, an Fe signal was detected at this temperature for the Au-rich samples (Au_0.8Fe_0.2 and Au_0.5Fe_0.5).

The reactivity of our pure Au, pure Fe, bimetallic Au-Fe and a sample with a mixture of pure Au and pure Fe NPs will be compared via in-situ (UHV) temperature programmed desorption measurements. CO oxidation has been used as probe reaction.

[1] A. Naitabdi, L.K. Ono, F. Behafarid, B. Roldan Cuenya, J. Phys. Chem. C 113 (2009) 1433.

The bimetallic catalyst is one important category of heterogeneous catalysts for numerous industrial processes and energy conversion. Surface structure and chemistry of three bimetallic catalysts, Rh-Pd, Rh-Pt, and Pd-Pt nanoparticles during catalysis, oxidation, and reducing reactions have been systematically studied in-situ using XPS in a Torr pressure range and high resolution TEM. The Rh_xPd_1-x and Rh_xPt_1-x nanoparticles undergo reversible changes in both atomic fractions and chemical states corresponding to the switch of reaction environments between oxidizing and reducing conditions. Rh atoms in Rh_xPd_1-x and Rh_xPt_1-x segregate to surface layers and are largely oxidized in oxidizing conditions while in reducing atmospheres the Pd atoms in Rh_xPd_1-x and Pt atoms in Rh_xPt_1-x diffuse to the surface regions and Rh atoms are largely reduced. In contrast to Rh_xPd_1-x and Rh_xPt_1-x, no significant segregation of Pd or Pt atoms was found in Pt_xPd_1-x nanoparticles; Pd atoms are alternatively oxidized and reduced under oxidizing and reducing conditions, whereas Pt atoms do not. The capability of restructuring nanoparticles through chemical reactions illustrates the flexibility of the structure of bimetallic nanoparticle catalysts and suggests a new method for the development of new catalysts.

Reference: F. Tao et al. 322, 932 (2008)