David C Bell

Mapping reactive flow patterns in monolithic nanoporous catalysts

Abstract The development of high-efficiency porous catalyst membranes critically depends on our understanding of where the majority of the chemical conversions occur within the porous structure. This requires mapping of chemical reactions and mass transport inside the complex nanoscale architecture of porous catalyst membranes which is a multiscale problem in both the temporal and spatial domains. To address this problem, we developed a multiscale mass transport computational framework based on the lattice Boltzmann method that allows us to account for catalytic reactions at the gas–solid interface by introducing a new boundary condition. In good agreement with experiments, the simulations reveal that most catalytic reactions occur near the gas-flow facing side of the catalyst membrane if chemical reactions are fast compared to mass transport within the porous catalyst membrane.

Effect of nanoscale flows on the surface structure of nanoporous catalysts

The surface structure and composition of a multi-component catalyst are critical factors in determining its catalytic performance. The surface composition can depend on the local pressure of the reacting species, leading to the possibility that the flow through a nanoporous catalyst can affect its structure and reactivity. Here, we explore this possibility for oxidation reactions on nanoporous gold, an AgAu bimetallic catalyst. We use microscopy and digital reconstruction to obtain the morphology of a two-dimensional slice of a nanoporous gold sample. Using lattice Boltzmann fluid dynamics simulations along with thermodynamic models based on first-principles total-energy calculations, we show that some sections of this sample have low local O2 partial pressures when exposed to reaction conditions, which leads to a pure Au surface in these regions, instead of the active bimetallic AgAu phase. We also explore the effect of temperature on the surface structure and find that moderate temperatures (≈300-450 K) should result in the highest intrinsic catalytic performance, in apparent agreement with experimental results.

Multiscale Morphology of Nanoporous Copper Made from Intermetallic Phases

Many application-relevant properties of nanoporous metals critically depend on their multiscale architecture. For example, the intrinsically high step-edge density of curved surfaces at the nanoscale provides highly reactive sites for catalysis, whereas the macroscale pore and grain morphology determines the macroscopic properties, such as mass transport, electrical conductivity, or mechanical properties. In this work, we systematically study the effects of alloy composition and dealloying conditions on the multiscale morphology of nanoporous copper (np-Cu) made from various commercial Zn-Cu precursor alloys. Using a combination of X-ray diffraction, electron backscatter diffraction, and focused ion beam cross-sectional analysis, our results reveal that the macroscopic grain structure of the starting alloy surprisingly survives the dealloying process, despite a change in crystal structure from body-centered cubic (Zn-Cu starting alloy) to face-centered cubic (Cu). The nanoscale structure can be controlled by the acid used for dealloying with HCl leading to a larger and more faceted ligament morphology compared to that of H3PO4. Anhydrous ethanol dehydrogenation was used as a probe reaction to test the effect of the nanoscale ligament morphology on the apparent activation energy of the reaction.

Surface Modifications during a Catalytic Reaction: a Combined APT and FIB/SEM Analysis of Surface Segregation

To improve the understanding of catalytic processes, the surface structure and composition of the active materials need to be determined before and after reaction. Morphological changes may occur under reaction conditions and can dramatically influence the reactivity and/or selectivity of a catalyst. Goldbased catalysts with different architectures are currently being developed for selective oxidation reactions at low temperatures [1]. Specifically, nanoporous Au (npAu) with a composition of Au97-Ag3 is obtained by dealloying a Ag70-Au30 bulk alloy. Recent studies highlight the efficiency of npAu catalysts for methanol oxidation using ozone to activate the catalysts before methanol oxidation. In this work, we studied the morphological and compositional changes occurring at the surface of Au-based catalysts in certain conditions.

Nanoscale Investigation of Belgian Chocolate by Atom Probe Tomography.

The recent trend in food science is to study the structure and its influence on functionality. One of the main challenges for this is the complexity of food systems. As an example, the relevant length scales in food systems extend from the macroscale (typically cm) to the nanoscale. Drastically different structures are observable at different scales, but are all important in the food processing and preparation. In the case of our sample, it is indeed known that cocoa butter can present six different crystallographic structures with their own melting points and densities. These influence the chocolate properties, such as the fat bloom formation, and induce different sensory perception [1]. In the case of chocolate, experiments have been performed to determine the microstructure and crystallinity of dark, mild and white chocolates so as to study phase separation, phase transition and polymorphism, as well as emulsifying and rheological properties. For this, a wide range of microscopy and microanalysis techniques have been used such as light microscopy, X-ray microtomography, X-ray scattering, X-ray diffraction, scanning electron microscopy and atomic force microscopy [2-5]. In this work, we used Atom Probe Tomography (APT) to analyse dark chocolate. APT is a powerful technique for the characterization of both the composition and the 3D structure of materials at the atomic-scale. The versatility of the most recent APT instruments allows the study of samples that were not possible only a few years ago: minerals, biominerals and geological samples, or even biological samples such as bacteria [6].

Sample Preparation and Analysis of Aggregated ‘Single Atom Alloy’ Nanoparticles by Atom Probe Tomography

1. Chemical Physics of Materials and Catalysis, Université libre de Bruxelles, 1050 Brussels, Belgium 2. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge MA, USA 3. Department of Chemistry and Chemical Biology, Harvard University, Cambridge MA, USA 4. Center for Nanoscale Systems, Harvard University, Cambridge MA, USA 5. Department of Chemical and Biological Engineering, Tufts University, Medford MA, USA

Microstructure and Crystallographic Determination of Nanoporous Catalysts

1. Chemical Physics of Materials and Catalysis, Université libre de Bruxelles, 1050 Brussels, Belgium 2. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge MA USA 3. Department of Chemistry and Chemical Biology, Harvard University, Cambridge MA USA 4. Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National Laboratory, Livermore CA USA 5. Center for Nanoscale Systems, Harvard University, Cambridge MA USA