Nanomaterial-based catalyst | EPFL Graph Search

Nanomaterial-based catalysts are usually heterogeneous catalysts broken up into metal nanoparticles in order to enhance the catalytic process. Metal nanoparticles have high surface area, which can increase catalytic activity. Nanoparticle catalysts can be easily separated and recycled. They are typically used under mild conditions to prevent decomposition of the nanoparticles. Functionalized nanoparticles Functionalized metal nanoparticles are more stable toward solvents compared to non-functionalized metal nanoparticles. In liquids, the metal nanoparticles can be affected by van der Waals force. Particle aggregation can sometimes decrease catalytic activity by lowering the surface area. Nanoparticles can also be functionalized with polymers or oligomers to sterically stabilize the nanoparticles by providing a protective layer that prevents the nanoparticles from interacting with each other. Alloys of two metals, called bimetallic nanoparticles, are used

Rational Design of Pd-Based Catalysts for Selective Alkyne Hydrogenations

Rocio Micaela Crespo Quesada

Traditionally, the search for active and selective catalysts involved a rather tedious "hit-and-miss" approach in which hundreds, if not thousands catalysts were tested until the optimal substance was identified. With the advancing theoretical understanding of catalysis and the development of computational power, a new era of rational catalyst design is dawning. This approach, grounded on first principles, is based on new advances in synthesis, characterization and modeling with the ultimate aim of predicting the expected behavior of a catalyst based on chemical composition, molecular structure and morphology. The rational design of a catalyst is a complex process which spans across several levels of scale. In this thesis, the pursuit of a rational design for Pd-based catalysts effective in alkyne hydrogenations is presented. A multi-level integrated approach was thus applied ranging from the nano-scale design of the active sites for a specific reaction, taking into account its structure sensitivity, to the micro-scale design of the supported Pd nanoparticles including metal-additive and metal-support interactions as well as mass and heat transfer phenomena. In order to rationally design a catalyst at a nano-scale, i.e. a catalyst's active site, several methodologies have been hitherto applied, such as the study on single crystals or model catalysts. Here we present the use of metal nanoparticles with tuned sizes (5-30 nm) and shapes (cubes, octahedra and cube-octahedra) prepared via colloidal techniques. These nanoparticles can be tested per se and represent a new generation of model catalysts, complementing single crystal studies, which inherently lack the complexity of industrial catalysis. However, metal nanoparticles, especially those prepared in a controlled manner in order to tailor their shape and size, require the use of stabilizing and/or capping agents capable of directing their growth. These substances can mask the true catalytic behavior of the nanoparticles. Thus, it is of great importance to study the interactions of the active phase with the substances that are in close contact with it, in the so-called meso-scaled level of rational catalyst design. Therefore, the effect of the nature of the stabilizing agent on the catalytic response was studied, as well as the promoting effect of some of these substances. Finally, a methodology was developed capable of eliminating organic stabilizing agents from the surface of nanoparticles without compromising their morphological stability. The knowledge gathered in the first two levels can be applied further to reach the microscale rational catalyst design. In this thesis, a final catalyst consisting on well-defined stabilizer-free supported Pd nanoparticles was used in the hydrogenation of acetylene. This deep study of the catalytic behavior of well-defined Pd catalysts throughout several levels of scale and complexity have given us the tools needed to perform a rational catalyst design for alkyne hydrogenations. Depending on the specific reaction, the active phase can be optimized in terms of the desired activity and selectivity and can be tuned even further with the use of specific additives. Finally, the appropriate support also exerts a promoting effect on the nanoparticles and ensures their anchoring in addition to avoiding heat and mass transfer artifacts.

EPFL2011

Two-Dimensional Metal-Organic Networks as a New Class of Electrocatalysts

Benjamin Wurster

This thesis deals with the electrocatalytic properties of two-dimensional metal organic coordination networks (2D-MOCNs) self-assembled on electrode surfaces and catalytic activity of 2D-MOCNs is demonstrated for the first time. The low-coordinated metal centers within these supramolecular structures resemble the catalytically active sites of metallo-enzymes. The activity of the metal centers is determined by the type of the metal and the coordination environment; both parameters can be carefully adjusted in 2D-MOCNs. In order to study and correlate electrocatalytic properties with the atomic structure of 2D-MOCNs, a scanning tunnelling microscope operating in ultra-high vacuum (UHV) was combined with an electrochemical (EC) setup. Sample preparation and scanning tunneling microscopy (STM) were conducted under UHV conditions. By STM the composition and structure of the networks were controlled and characterized prior to EC experiments. A transfer system between UHV and EC instrumentation was constructed to keep the sample at all times in a clean and controlled environment. It is demonstrated that the mechanism of the oxygen reduction reaction (ORR) can be influenced by the type of the metal center in the network. The networks formed by benzene-tricarboxylic acid (TMA) with either Fe or Mn atoms are structurally identical, but their electrochemical signal differs significantly. Both networks catalyze the reduction of O2 to H2O, but on different pathways. These results emphasize the key role of the unsaturated metal centers in the electrocatalytic reduction. The electrocatalytic response in the ORR can also altered by the distinct coordination environment; this is shown for Fe atoms nitrogen-coordinated by either tetracyanoquinodimethane (TCNQ), bis-pyridyl-bipyrimidine (PBP) or phythalocyanine (Pc). Depending on the specific coordination environment formed by the different ligands the the final product (H2O2 or H2O) and the mechanism of the ORR is changed. In the last part of this thesis the two metal binding sites of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) are used to selectively incorporate different metal centers in a fixed organic environment and create homo- and hetero-bimetallic networks. The coupling of the two metals centers promoted by the organic environment modifies the electrocatalytic response. The correct combination of two metal centers in the right positioning makes an efficient catalyst. In the ORR the peak and onset potential is varied depending on the metal combination. In the case of the oxygen evolution reaction a non-linearly increased catalytic activity by the cooperative bimetallic effect is obtained. The results presented in this thesis demonstrate the high potential of 2D-MOCNs for heterogeneous catalytic chemical conversions. Engineering of the network structure and the distinct coordination environment of the metal centers offers the possibility to tune their catalytic activity. This opens up a new route for the design of a new class of nanocatalyst materials.

EPFL2015

Catalyst stabilization by stoichiometrically limited layer-by-layer overcoating in liquid media

Kristopher Brown, Florent Emmanuel Héroguel, Benjamin Pierre Le Monnier, Jeremy Luterbacher

The use of metal oxide overcoats over supported nanoparticle catalysts has recently led to impressive improvements in catalyst stability and selectivity. The deposition of alumina is especially important for renewable catalysis due to its robustness in liquid-phase conditions. However, there are limited reports of work on alumina deposition and stabilization that goes beyond atomic layer deposition (ALD). Here, we present a layer-by-layer deposition technique for the controlled formation of conformal alumina overcoats in the liquid phase. This technique is easy to perform in common wet chemistry conditions. Alternated exposure of the substrate to stoichiometric amounts of aluminum alkoxide and water in liquid-phase conditions leads to the formation of a porous overcoat that was easily tunable by varying synthesis parameters. The deposition of 60 Al2O3 layers onto Al2O3-supported copper nanoparticles suppressed irreversible deactivation during the liquid-phase hydrogenation of furfural – a key biomass-derived platform molecule. The porous overcoat leads to highly accessible metal sites, which significantly reduces the partial site blocking observed in equivalent overcoats formed by ALD. We suggest that the ease of scalability and the high degree of control over the overcoat’s properties during liquid-phase synthesis could facilitate the development of new catalyst overcoating applications.

Elsevier2017