Søren Bredmose Simonsen

Direct Observations of Oxygen-induced Platinum Nanoparticle Ripening Studied by In Situ TEM

This study addresses the sintering mechanism of Pt nanoparticles dispersed on a planar, amorphous Al(2)O(3) support as a model system for a catalyst for automotive exhaust abatement. By means of in situ transmission electron microscopy (TEM), the model catalyst was monitored during the exposure to 10 mbar air at 650 degrees C. Time-resolved image series unequivocally reveal that the sintering of Pt nanoparticles was mediated by an Ostwald ripening process. A statistical analysis of an ensemble of Pt nanoparticles shows that the particle size distributions change shape from an initial Gaussian distribution via a log-normal distribution to a Lifshitz-Slyozov-Wagner (LSW) distribution. Furthermore, the time-dependency of the ensemble-averaged particle size and particle density is determined. A mean field kinetic description captures the main trends in the observed behavior. However, at the individual nanoparticle level, deviations from the model are observed suggesting in part that the local environment influences the atom exchange process.

Eliminating degradation in solid oxide electrochemical cells by reversible operation

One promising energy storage technology is the solid oxide electrochemical cell (SOC), which can both store electricity as chemical fuels (electrolysis mode) and convert fuels to electricity (fuel-cell mode). The widespread use of SOCs has been hindered by insufficient long-term stability, in particular at high current densities. Here we demonstrate that severe electrolysis-induced degradation, which was previously believed to be irreversible, can be completely eliminated by reversibly cycling between electrolysis and fuel-cell modes, similar to a rechargeable battery. Performing steam electrolysis continuously at high current density (1 A cm(-2)), initially at 1.33 V (97% energy efficiency), led to severe microstructure deterioration near the oxygen-electrode/electrolyte interface and a corresponding large increase in ohmic resistance. After 4,000 h of reversible cycling, however, no microstructural damage was observed and the ohmic resistance even slightly improved. The results demonstrate the viability of applying SOCs for renewable electricity storage at previously unattainable reaction rates, and have implications for our fundamental understanding of degradation mechanisms that are usually assumed to be irreversible.

Substrate Size-Selective Catalysis with Zeolite-Encapsulated Gold Nanoparticles†

Over the years, many strategies have been developed to address the problem of sintering of nanoparticle catalysts, including encapsulating metal nanoparticles in protective shells, and trapping nanoparticles in the cavities of certain zeolites in post-synthesis steps. In general, materials that contain metal nanoparticles that are only accessible via zeolite micropores are intriguing, specifically, but not exclusively, for catalytic applications. The encapsulation of carbon nanoparticles during zeolite crystallization is a well-known approach for making carbon–zeolite composites that afford mesoporous zeolites after combustion. Herein, we show that metal nanoparticles can also be encapsulated during zeolite crystallization, as exemplified by silicalite-1 crystals that are embedded with circa 1–2 nm-sized gold nanoparticles that remain stable and catalytically active after calcination in air at 550 8C. Moreover, we show that the encapsulated gold nanoparticles are only are accessible through the micropores of the zeolite, which makes this material a substrate-size selective oxidation catalyst. Currently, more than 175 different zeolite structures have been reported, and these can be tuned according to the desired acidity and/or redox properties. Expanding the scope from pure zeolites to hybrid materials, by combining the properties of zeolites with other components, significantly widens the field of zeolite materials design. Aside from posttreatment methods, two types of approaches have been pursued for preparing hybrid zeolite–nanoparticle materials. The first type of approach involves crystallization of the zeolite from a gel that contains metal ions that are immobilized in the zeolite during crystallization. With this kind of approach, it is very difficult to control the properties of the non-zeolite component in terms of, for example, particle size. The other type of approach is to first synthesize the nonzeolite component and subsequently encapsulate this in the individual zeolite crystals during crystallization. Indeed, this strategy is also well-known and an entire family of materials, known as mesoporous or hierarchical zeolite crystals, are based on the embedding of carbon nanoparticles, nanofibers, nanotubes, or other nanostructures during zeolite crystallization (and subsequent combustion) in a process known as carbon templating. 15, 16] Concerning the embedding of metal nanoparticles in zeolites, Hashimoto et al. reported a top down approach that features downsizing gold flakes to approximately 40 nm particles by laser ablation, and subsequent encapsulation of these particles during crystallization. A reduction in particle size by one order of magnitude is necessary for an efficient use of costly noble metals in catalytic applications. However, a reduction of the particle size enhances the tendency for sintering, owing to the increase in surface free energy. To mitigate this problem, we report herein a bottom-up approach for the preparation of hybrid zeolite-nanoparticle materials that contain small metal nanoparticles, dispersed throughout the zeolite crystals. This synthetic approach comprises three steps (Figure 1): First, a metal nanoparticle colloid is prepared with suitable anchor points for the generation of a silica shell. Second, the particles are encapsulated in an amorphous silica matrix. Third, the silica nanoparticle precursor is subjected to hydrothermal conditions in order for zeolite crystallization to take place. Using this approach, we successfully prepared a material that consisted predominantly of circa 1–2 nm sized gold particles that were embedded in silicalite-1 crystals. X-ray diffraction revealed that the material contained exclusively gold as well as MFI-structured material (generalized silicalite-1 crystal structure type). Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the hybrid material that consists of gold nanoparticles embedded in silicalite-1 crystals. The SEM image reveals that the material is mainly composed of circa 1–2 mm long coffinshaped crystals, with a minor fraction of intergrown coffin[*] A. B. Laursen, K. T. Højholt, L. F. Lundegaard, S. B. Simonsen, S. Helveg, Prof. C. H. Christensen, K. Egeblad Haldor Topsøe A/S Nymøllevej 55, 2800 Kgs. Lyngby (Denmark) E-mail: chc@topsoe.dk kreg@topsoe.dk

Enhancement of the chemical stability in confined [delta]-Bi2O3

Bismuth-oxide-based materials are the building blocks for modern ferroelectrics, multiferroics, gas sensors, light photocatalysts and fuel cells. Although the cubic fluorite δ-phase of bismuth oxide (δ-Bi2O3) exhibits the highest conductivity of known solid-state oxygen ion conductors, its instability prevents use at low temperature. Here we demonstrate the possibility of stabilizing δ-Bi2O3 using highly coherent interfaces of alternating layers of Er2O3-stabilized δ-Bi2O3 and Gd2O3-doped CeO2. Remarkably, an exceptionally high chemical stability in reducing conditions and redox cycles at high temperature, usually unattainable for Bi2O3-based materials, is achieved. Even more interestingly, at low oxygen partial pressure the layered material shows anomalous high conductivity, equal or superior to pure δ-Bi2O3 in air. This suggests a strategy to design and stabilize new materials that are comprised of intrinsically unstable but high-performing component materials.

Structure and crystallinity of water dispersible photoactive nanoparticles for organic solar cells

Water based inks would be a strong advantage for large scale production of organic photovoltaic devices. Formation of water dispersible nanoparticles produced by the Landfester method is a promising route to achieve such inks. We provide new insights into the key ink properties of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoparticles such as the internal structure and crystallinity of the dispersed nanoparticles and the previously unreported drastic changes that occur when the inks are cast into a film. We observe through transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) that the nanoparticles in dispersion are spherical with the nanodomains of P3HT being partly crystalline. When wet processed and dried into films, the nanoparticles lose their spherical shape and become flattened into oblate shapes with a large aspect ratio. Most particles are observed to have a diameter 13 times of the particle height. After casting into a film, the crystal domains adopt a preferred orientation with the majority of the nanocrystals (68%) with face-on orientation to the substrate. We propose that low substrate surface energy is responsible for particle deformation and texturing.

Quantification of tip-broadening in non-contact atomic force microscopy with carbon nanotube tips

Carbon nanotube terminated atomic force microscopy (AFM) probes have been used for the imaging of 5 nm wide surface supported Pt nanoclusters by non-contact (dynamic mode) AFM in an ultra-high vacuum. The results are compared to AFM measurements done with conventional Si-tips, as well as with transmission electron microscopy images, which give accurate measures for cluster widths. Despite their ideal aspect ratio, tip-broadening is concluded to be a severe problem even when imaging with carbon nanotube tips, which overestimates the cluster width by several times the nominal width of the nanotube tip. This broadening is attributed to a bending of the carbon nanotubes, and not to pure geometrical factors, which coincidentally results in a significant improvement for relative height measurements of tightly spaced high aspect ratio structures, as compared to what can be achieved with geometrically limited conventional probes. Superior durability also stands out as a defining feature of carbon nanotube terminated probes, allowing them to give results with a greatly enhanced reproducibility.

Scandium-doped zinc cadmium oxide as a new stable n-type oxide thermoelectric material

Scandium-doped zinc cadmium oxide (Sc-doped ZnCdO) is proposed as a new n-type oxide thermoelectric material. The material is sintered in air to maintain the oxygen stoichiometry and avoid instability issues. The successful alloying of CdO with ZnO at a molar ratio of 1 : 9 significantly reduced the thermal conductivity by up to 7-fold at room temperature. By carefully selecting the Sc-dopant concentrations, a high power factor of 7.1 × 10−4 W m−1 K−2 at 1173 K could be obtained. Therefore, the highest ZT ∼ 0.3 at 1173 K was achieved for the Zn0.9Cd0.1Sc0.01O1.015 sample, and it has so far one of the highest ZT values among those reported for ZnO based thermoelectric materials over the temperature range, e.g., its ZT value at 300 K, which is 0.012, is over 1 order of magnitude higher than that of the state-of-the-art nanostructured Al-doped ZnO, which is 0.0013. It suggests that this material is a good candidate for improving the overall conversion efficiencies in oxide thermoelectric modules. Meanwhile, Sc-doped ZnCdO is robust in air at high temperatures, whereas other n-type materials, such as Al-doped ZnO, will experience rapid degradation of their electrical conductivity and ZT.

Structural evolution during calcination and sintering of a (La 0.6 Sr 0.4 ) 0.99 CoO 3-δ nanofiber prepared by electrospinning

Design of three-dimensional metal oxide nanofibers by electrospinning is being widely explored. However, the impacts of calcination and sintering on the resulting morphology remain unknown. For the first time, (La0.6Sr0.4)0.99CoO3-δ (LSC) nanofiber, which is among the most promising electrode materials for solid oxide fuel cells, was synthesized by sol-gel electrospinning. By elevating the temperature in oxygen using in situ transmission electron microscopy, we discovered the structural transitions from nanofibers to nanotubes and then to nano-pearl strings. This facile and up-scalable method can be widely applied to design metal oxide one-dimensional nanomaterials with precise control in both geometry (nanofiber, nanotube and nano-pearl string) and surface area (by varying grain size).

Size effect studies in catalysis: a simple surfactant-free synthesis of sub 3 nm Pd nanocatalysts supported on carbon

Supported Pd nanoparticles are prepared under ambient conditions via a surfactant-free synthesis. Pd(NO3)2 is reduced in the presence of a carbon support in alkaline methanol to obtain sub 3 nm nanoparticles. The preparation method is relevant to the study of size effects in catalytic reactions like ethanol electro-oxidation.

Coarsening of carbon black supported Pt nanoparticles in hydrogen

This study addresses coarsening mechanisms of Pt nanoparticles supported on carbon black in hydrogen. By means of in situ transmission electron microscopy (TEM), Pt nanoparticle coarsening was monitored in 6 mbar 20% H2/Ar while ramping up the temperature to almost 1000 °C. Time-resolved TEM images directly reveal that separated ca. 3 nm sized Pt nanoparticles in a hydrogen environment are stable up to ca. 800 °C at a heating rate of 10 °C min-1. The coarsening above this temperature is dominated by the particle migration and coalescence mechanism. However, for agglomerated Pt nanoparticles, coalescence events were observed already above 200 °C. The temperature-dependency of particle sizes and the observed migration distances are described and found to be consistent with simple early models for the migration and coalescence.