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A general approach for the linking of Pt nanoparticles (NPs) with bifunctional amine ligands (organic molecules with two amine groups) is presented that allows for the preparation of NP catalysts without inorganic supports and high densities of the catalytically active metal. Advantage was taken of the use of "unprotected" NPs, which enables us to prepare different ligand-functionalized NPs from the same particle batch and thus to relate changes of the resulting material properties exclusively to the influence of the ligand. Three bifunctional ligands with similar functional groups (amines) but different hydrocarbon skeletons were used and compared to monofunctional ligands of similar molecular structures (alkyl and aryl amines) showing significantly different material properties. Monofunctional molecules with minor steric demand cover almost completely the NP surface and lead to two-dimensional assembling of the NPs. In contrast, the use of bifunctional amine ligands leads to the formation of porous, three-dimensional NP networks (ligand-linked NPs) with a high density of ligand free surface atoms, thus enabling for the application as catalytic materials. The stabilizing effect of bifunctional ligands serves as an alternative to the use of inorganic support materials and enables for catalytic applications of ligand-linked NP networks.

Different mono- and bifunctional amine ligands have been used to stabilize Pt NPs for catalytic H2 gas sensing. Depending on the chemical structure and properties of the ligand, the catalysts show different overall sensor performances, activation periods, and long-term stabilities. These sensor characteristics are put into relation with chemical processes like cleaning of the surface, degradation processes of the ligands and nanoparticle (NP) sintering. It has been found that during activation free adsorption sites are formed primarily due to desorption of synthetic residues. Furthermore, partial desorption of the ligands followed by their degradation may occur. For monoamines the latter process results in destabilization of the NPs followed by catalyst deactivation through particle sintering. The use of bifunctional ligands that link individual NPs shows significantly enhanced stabilities which can be related to the reduction of the ligand desorption rates and degradation. Besides the functionality of the ligands it was observed that the chemical nature of their hydrocarbon skeleton affects the catalyst stability: aromatic substructures remain intact upon H2 oxidation, while alkyl fragments undergo oxidation and decomposition. The advantages of bifunctionality and an aromatic hydrocarbon skeleton can be combined by the use of para-phenylenediamine (PDA) as a linking ligand. Networks formed by this ligand were indeed found to be stable under the applied catalytic conditions for more than 24 h.

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Applications such as catalytic gas sensing require a high density of catalytically active sites at low total heat capacity. One way to achieve this goal is the molecular linkage of colloidal nanoparticles with bifunctional ligands resulting in 3D-porous networks. The catalytic properties of such structures were investigated in a thermoelectric hydrogen sensor.

The use of catalytic gas sensors usually offers low selectivity, only based on their different sensitivities for various gases due to their different heats of reaction. Furthermore, the identification of the gas present is not possible, which leads to possible misinterpretation of the sensor signals. The use of micro-machined catalytic gas sensors offers great advantages regarding the response time, which allows advanced analysis of the sensor response. By using temperature modulation, additional information about the gas characteristics can be measured and drift effects caused by material shifting or environmental temperature changes can be avoided. In this work a miniaturized catalytic gas sensor which offers a very short response time (<150 ms) was developed. Operation with modulated temperature allows analysis of the signal spectrum with advanced information content, based on the Arrhenius approach. Therefore, a high-precise electronic device was developed, since theory shows that harmonics induced by the electronics must be avoided to generate a comprehensible signal.

We present the preparation and electrochemical application of Pt nanoparticles (Pt NPs) functionalized with 1-naphthylamine. Under electrochemical conditions, Pt surface bound 1-naphthylamine (NA) can be reversibly switched (oxidized and reduced), while simultaneously electrocatalytic reactions (e.g. CO oxidation) can proceed on the Pt surface. While the redox activity of the ligand is established immediately after functionalization, the functionalized NPs have to be stored as a colloidal dispersion in tetrahydrofuran (THF) prior to deposition onto the support material in order to induce their catalytic activity. We interpret this catalytic activation due to partial desorption of ligands from the particle surface induced by storing the particles in THF. However, the experimental results do not indicate a loss of ligands from the ligand shell, but evidence that the ligands form oligomers when kept as colloids in THF. As a result the catalytic surface becomes partially available while the redox activity of the ligands is maintained.

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