Patrick Sonström

Colloidally Prepared Pt Nanoparticles for Heterogeneous Gas-Phase Catalysis: Influence of Ligand Shell and Catalyst Loading on CO Oxidation Activity

In contrast to conventional methods, colloidally prepared heterogeneous supported metal catalysts are excellent systems to study the catalytic properties as a function of metal loading, monodispersity, particle shape, or the type of support without changing the other parameters, as will be demonstrated herein. Colloidal, ligand‐capped Pt nanoparticles deposited on oxide supports are investigated for CO adsorption and oxidation. Dodecylamine and different alkanethiols are used as ligands. IR spectroscopic experiments reveal that small molecules, such as CO, can pass through the ligand shell and can adsorb on the particle surface, even if the ligand shell is not removed by a special pretreatment. The ability to penetrate the shell was found to depend on the type of ligand used which renders ligand‐capped nanoparticles potentially interesting for reaction and selectivity control. In the case of CO oxidation, high activity is detected only at temperatures at which a partial loss of ligands has already occurred, resulting in a rather similar catalytic behavior independent on the type of ligand. However, there are no indications for poisoning of the catalysts by decomposition of the ligand shell. Simple purification procedures of the Pt nanoparticles are sufficient to avoid further poisoning effects. Depositing nanoparticles with the same size in different amounts on a support enabled a detailed study of the influence of metal loading on the activity. The activity per gram metal increases with the metal loading. Local autothermal heating is responsible for this effect, which is also detected for a reference system consisting of Pt nanoparticles prepared without a ligand shell.

Colloidally prepared Pt nanowires versus impregnated Pt nanoparticles: comparison of adsorption and reaction properties.

Ligand-capped Pt nanowires, prepared by colloidal synthesis and deposited on a high surface area γ-Al(2)O(3) support, were subjected to surface characterization by electron microscopy and FTIR spectroscopy using CO as a probe molecule. The structural, adsorption, and catalytic reaction properties of the colloidal Pt nanowires were compared to those of conventional, impregnated Pt nanoparticles on the same Al(2)O(3) support. In situ FTIR spectroscopy indicated ligand effects on the CO resonance frequency, irreversible CO-induced surface roughening upon CO adsorption, and a higher resistance of colloidal catalysts toward oxidation (both in oxygen and during CO oxidation), suggesting that the organic ligands might protect the Pt surface. Elevated temperature induced a transformation of Pt nanowires to faceted Pt nanoparticles. The colloidal catalyst was active for hydrodechlorination of trichloroethylene (TCE), but no ligand effect on selectivity was obtained.

Nanostructured Praseodymium Oxide: Correlation Between Phase Transitions and Catalytic Activity

Praseodymia gives rise to a rich phase diagram with a large number of phases between the limiting stoichiometries Pr2O3 and PrO2 that differ only slightly in oxygen content (PrnO2n−2). This chemical and crystallographic variability allows the system to release or incorporate lattice oxygen easily at sufficiently high temperatures and thus renders the material interesting as a catalyst for redox reactions according to a Mars–van Krevelen mechanism. Nanostructured praseodymia samples are investigated in this study with respect to their catalytic properties, focusing on methane oxidation and selective NO reduction by CO and CH4. To correlate catalytic activity and crystallographic changes, complementary high‐temperature X‐ray diffraction measurements have been carried out. The determined temperatures of transitions between different oxide phases agree well with peaks in the temperature‐programmed reduction measurements, confirming the direct connection between the availability of lattice oxygen and crystallographic transformations. The catalytic activity for methane oxidation and NO reduction sets in at 450–500 °C, at which temperature the starting material—mainly Pr6O11—transforms into the next oxygen‐depleted phase Pr7O12. With respect to NO reduction, the results show that it is possible to employ both methane and carbon monoxide as reducing agents in the absence of oxygen, in agreement with a Mars–van Krevelen mechanism. Nevertheless, the use of CO instead of CH4 offers considerable advantages, as no deactivation due to carbon residues takes place in this case. Whereas, in an excess of oxygen, NO reduction is inhibited independently of the reducing agent, it is shown that NO reduction can proceed if the O2 concentration remains below a critical concentration.

Foam, fleece and honeycomb: catalytically active coatings from colloidally prepared nanoparticles

Although monolithic catalysts offer distinct advantages such as low pressure drops and easy catalyst separation compared to tube bundle reactors filled with pellets and are for instance widely used for automobile exhaust treatment, the coating of the monolithic structures with the catalytically active phase is often complex and recipes need to be established empirically. In contrast to traditional methods, such as dip-coating or impregnation of the monolith, where the active nanoparticles are formed during the preparation on an oxidic washcoat usually used and deposited simultaneously or in a previous step, we demonstrate in this study that alternatively preformed colloidally synthesized nanoparticles can be employed to obtain homogeneous coatings with or without a washcoat. In this way, one can take advantage of the far-reaching possibilities of colloidal methods to control the structure and size of the nanoparticles and also to tune and optimize their binding to the monolithic surface. For cases where beneficial metal–support interactions between the nanoparticles and a washcoat improve the catalytic properties we demonstrate that colloidally prepared nanoparticles can be directly mixed with a washcoat slurry and successfully deposited on monolithic supports. Turnover frequencies comparable to the corresponding powder catalysts could be reached. In a second approach, we present here a facile method to directly coat three different monolithic supports (cordierite honeycomb, Al2O3 foam and Nickel fleece) with preformed Pt nanoparticles in the presence and absence of organic ligands. In order to realize high metal loadings, the beneficial influence of a ligand “double-layer” (coating of nanoparticles and the support by organic ligands) enhancing the adhesion between the Pt nanoparticles and the underlying monolithic support will be discussed. In the case of the metallic Ni substrate, this approach furthermore allows to circumvent alloy formation and nanoparticle diffusion into the metallic substrate. This can greatly increase long-term stability of systems coated directly onto metallic substrates without an additional oxidic washcoat.