Zhongfan Zhang

Solving the Structure of Size-Selected Pt Nanocatalysts Synthesized by Inverse Micelle Encapsulation

The structure, size, and shape of gamma-Al(2)O(3)-supported Pt nanoparticles (NPs) synthesized by inverse micelle encapsulation have been resolved via a synergistic combination of imaging and spectroscopic tools. It is shown that this synthesis method leads to 3D NP shapes even for subnanometer clusters, in contrast to the raft-like structures obtained for the same systems via traditional deposition-precipitation methods. Furthermore, a high degree of atomic ordering is observed for the micellar NPs in H(2) atmosphere at all sizes studied, possibly due to H-induced surface reconstruction in these high surface area clusters. Our findings demonstrate that the influence of NP/support interactions on NP structure can be diminished in favor of NP/adsorbate interactions when NP catalysts are prepared by micelle encapsulation methods.

Evolution of the Structure and Chemical State of Pd Nanoparticles during the in Situ Catalytic Reduction of NO with H2

An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO(2)-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H(2) using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO(2) support was observed, and Pd(δ+) species were detected. This process parallels the high production of N(2)O observed at the onset of the reaction (>120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N(2) (∼80%). Concomitant with the onset of N(2) production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H(2)-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdO(x) was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.

Structure, Chemical Composition, And Reactivity Correlations during the In Situ Oxidation of 2-Propanol

Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (∼1 nm) supported on nanocrystalline γ-Al(2)O(3) during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (<140 °C), while the complete oxidation (>140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O(2) adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions.

Noncrystalline-to-crystalline transformations in pt nanoparticles

We show that the noncrystalline-to-crystalline transition of supported Pt nanoparticles (NPs) in the subnanometer to nanometer size range is statistical in nature, and strongly affected by particle size, support, and adsorbates (here we use H2). Unlike in the bulk, a noncrystalline phase exists and is stable in small NPs, reflecting a general mesoscopic feature. Observations of >3000 particles by high-resolution transmission electron microscopy show a noncrystalline-to-crystalline transition zone that is nonabrupt; there is a size regime where disordered and ordered NPs coexist. The NP size at which this transition occurs is strongly dependent on both the adsorbate and the support, and this effect is general for late 5d transition metals. All results are reconciled via a statistical description of particle-support-adsorbate interactions.

Kinetics aspects of initial stage thin γ-Al2O3 film formation on single crystalline β-NiAl (110)

The growth kinetics and mechanisms of thermally-grown thin γ-Al2O3 film at 650 °C in air on single-crystalline β-NiAl (110) was characterized via transmission electron microscopy, X-ray diffractometry, and thermo-gravimetric analyses. The oxidation kinetics as a function of thickness was gradually changing from an inverse-logarithmic to parabolic behavior across the “intermediate thickness regime” as the oxide thickness increases. To define the boundaries of the three thickness regimes, the high field approximation (x1) and Debye-Huckel length (LD) were determined using the existing theoretical kinetics models combined with experimentally measured data. All the relevant constants for each rate law at the three thickness regimes were also experimentally determined to quantitatively describe the initial stage growth kinetics.

Statistical analysis of support thickness and particle size effects in HRTEM imaging of metal nanoparticles

High-resolution transmission electron microscopy (HRTEM) examination of nanoparticles requires their placement on some manner of support - either TEM grid membranes or part of the material itself, as in many heterogeneous catalyst systems - but a systematic quantification of the practical imaging limits of this approach has been lacking. Here we address this issue through a statistical evaluation of how nanoparticle size and substrate thickness affects the ability to resolve structural features of interest in HRTEM images of metallic nanoparticles on common support membranes. The visibility of lattice fringes from crystalline Au nanoparticles on amorphous carbon and silicon supports of varying thickness was investigated with both conventional and aberration-corrected TEM. Over the 1-4nm nanoparticle size range examined, the probability of successfully resolving lattice fringes differed significantly as a function both of nanoparticle size and support thickness. Statistical analysis was used to formulate guidelines for the selection of supports and to quantify the impact a given support would have on HRTEM imaging of crystalline structure. For nanoparticles ≥1nm, aberration-correction was found to provide limited benefit for the purpose of visualizing lattice fringes; electron dose is more predictive of lattice fringe visibility than aberration correction. These results confirm that the ability to visualize lattice fringes is ultimately dependent on the signal-to-noise ratio of the HRTEM images, rather than the point-to-point resolving power of the microscope. This study provides a benchmark for HRTEM imaging of crystalline supported metal nanoparticles and is extensible to a wide variety of supports and nanostructures.

HREM, EXAFS and MD Studies on Size-dependent Crystallinity of Pt Nanoparticles Supported on Gamma-Al2O3

The catalytic system of nanoscale Pt particles on γ-Al2O3 support is widely applied for oxidation of hydrocarbon and CO, in fuel cells, and as a catalyst microsensor. A novel phenomenon of negative thermal expansion (NTE) was found in this system as Pt particle sizes reduced to ~1 nm[1]. The novel size effects may result from the change of atomic structure. We employed high-resolution transmission electron microscopy (HREM) to observe thousands of individual Pt particles with a size range from subup to 5 nm, to gain the statistics of the crystallinity versus Pt particle sizes; with extended X-ray absorption fine-structure spectroscopy (EXAFS) we measured the general order-disorder trends of Pt-Pt bond length distributions from samples with different average sizes. The first-principle molecular dynamics (MD) simulation was applied to Pt37/γ-Al2O3 system to find its stable structures. The samples were prepared by impregnating the Pt precursor, Pt(NH3)4(OH)2⋅H2O, on γ-Al2O3, reducing in H2 gas at 573 K to remove the ligands[1]. The Pt particle sizes were controlled by the loading amount, where 1 wt% produced an average Pt size of ~1nm, 3 wt% produced an average size of 2.1 nm and heavy loading of 5 wt% produced a average size of ~2.7 nm. The TEM samples were prepared by spreading a drop of Pt/γ-Al2O3 suspension in ethanol onto an ultra-thin C-grid, and dried in vacuum. The HREM observations were carried out with JEM 2100FEG S/TEM, operated at 200 kV. In order to enhance the contrast of ~1 nm Pt particles to the γ-Al2O3 support and C-film, focal series reconstruction technique, a software package (HREM Research Inc.) was employed and the TEM images were filtered at zero loss energy with Gatan GIF Tridiem.

Ultra-small and Monodisperse Pt Nanoparticles Supported on Gamma-Al 2 O 3

Metal nanoparticles (NPs) show unusual size-dependent optical, electronic, chemical, and catalytic properties; for example, Au particles on TiO2 are catalytically active for CO oxidation only when the Au NP is ~3 nm [1]. The novel size effects in heterogeneous catalysis must result from the change of atomic and/or electronic structures of supported metal NPs. Hence, an important goal to understanding and controlling heterogeneous catalysis is the ability to synthesis supported metal nanoparticles of a specific size and shape with a very narrow size distribution. Here we report the results of using forefront chemical synthesis methods to create ultra-small Pt NPs supported on gamma alumina and characterized by high-angle annular dark-field imaging (HAADF, or Z-contrast), high-resolution transmission electron microscopy (HREM) and extended X-ray absorption fine-structure (EXAFS) spectroscopy. The EXAFS measures the ensemble-average structure of the particles within the entire sample, while TEM measures individual Pt NPs. More than 600 individual Pt NPs were analyzed by HAADF to gain statistically meaningful information on the size distribution.

The Role of γ-Al 2 O 3 Single Crystal Support to Pt Nanoparticles Construction

Here we report the preparation of a model Pt/γ-Al2O3 catalyst and its characterization by a cross-sectional high-resolution electron microscopy (XHREM) method. Pt/γ-Al2O3 is the most important technologically-relevant heterogeneous catalyst in the fuel cell, oil refining and chemical industries. The nanoparticle/support interactions, particularly the role of their interface, play a key role in the 3-dimensional (3D) particle shape, surface morphology and sintering behaviors of catalyst nanoparticles (NPs) which determine the heterogeneous catalysts’ chemical properties. Experimental measurements [1] and theoretical simulations [2] were initiated to quantitatively study the interfacial atomic and electronic structure and adhesion energy between the NPs and their support. Previous investigators have used single crystal oxide supports to resolve the nanoparticle/support structure and shape relations that can be directly compared to the theoretical simulations [2]. However, commercial γ-Al2O3 is polycrystalline and irregular in shape; hence, our aim is to create a single crystal γ-Al2O3 thin film in order to build up a model system to solve the structural and electronic relations between the Pt and γ-Al2O3 support, and compare our experimental results with theoretical predictions.

Preparation and Characterization of Pt/γ-Al2O3 Model Catalyst on NiAl Alloy

Numerous studies of heterogeneous catalysis systems clearly demonstrate that the metal nanoparticle (NPs)/support interaction is significant in determining the catalytic chemistry. Theoretical simulations have been performed to understand the metal/support interactions [1,2]. For example, theorists discovered that electronic and oxygen defects of γ-Al2O3 anchor the active particles [1]. Platinum NPs dispersed on γ-alumina is one of the most widely used heterogeneous catalysts and Pt performs extremely well as a catalyst for the oxygen-reduction reaction used in fuel cell industries. Hence, we chose Pt/γ-Al2O3 as a model heterogeneous catalyst system to investigate the metal NPs/support interface by electron microscopy methods with the ultimate goal of bridging the gap with theoretical simulations of the interfacial atomic and electronic structure. However, theoretical simulations assume single crystal, planar supports with no impurities, but commercial γ-Al2O3 is polycrystalline and irregular in shape [3]. Hence, we are producing a model catalyst support via oxidation of single crystal NiAl to create crystalline and planar γ-Al2O3.