Search

The state-of-the-art approach to stabilize nanoparticles (NPs) for applications in heterogeneous catalysis is to support them on inert inorganic material, which is limited to low loadings of the catalytic component. However, applications such as catalytic gas sensing require a high density of catalytically active sites at a low total heat capacity. To offer an alternative to the supporting of NPs, the stabilization of catalytic NPs with organic ligands in solid state is presented. Therefore, the preparation strategy, consisting of NP synthesis and subsequent functionalization with mono- and bifunctional ligands, is introduced. The molecular linkage of Pt NPs with bifunctional amine ligands (ligand-linking) results in three-dimensional porous networks with ligand-free surface sites. The catalytic properties of ligand-stabilized NPs were investigated in a thermoelectric hydrogen sensor. In addition to an enhanced activity, the stability of the NPs can be significantly improved by ligand-linking. One reason may be that the bifunctional ligand is anchored on the NPs by two head groups. The criteria for ligand structures to enable a successful NP stabilization were identified. para-Phenylenediamine (PDA) combines the criteria and, consequently, by linking of Pt NPs with PDA a constant catalytic activity over more than 20 h on stream was achieved. Thus, ligand-stabilized NPs are presented as a novel catalyst for catalytic gas sensing.

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.

Miniaturized micro-machined catalytic gas sensors offer great advantages regarding power consumption and response time. In combination with a porous catalyst layer high sensitivity can be achieved. However, often this approach does not provide sufficient long-term stability. In this article a miniaturized catalytic gas sensor for hydrogen detection is presented based on colloidally prepared and stabilized platinum nanoparticles as a catalytic layer. Due to its miniaturized design and the highly porous catalyst, a very short response time of

Micro-technologically based catalytic gas sensors offer great advantages regarding sensitivity, power consumption and response time, especially in connection with a porous catalyst layer, which however often provides no sufficient long-term stability. This paper presents a miniaturized catalytic gas sensor for hydrogen detection based on colloidally prepared platinum nanoparticles as a catalytic layer. Compared to conventional sensors, the sensitivity of the catalytic layer is increased while the amount of catalytic material is decreased, due to the high surface of nanoparticles and the direct contact to the sensor. Because of the susceptibility of the nanoparticles to thermally induced sintering, an approach is adopted where the nanoparticles are embedded in an organic network, consisting of stabilizing surfactants adsorbed on the nanoparticle's surface. Agglomeration is reduced and the adhesion to the substrate is improved so that a highly improved stability compared to sensors fabricated with nanoparticles without surfactants is observed. Additionally, the sensor design allows a precise control of the temperature of the catalyst, which also contributes to the stability by keeping the temperature constant. Thermal loading effects are avoided. A sensitivity of up to 2.5 mV/100 ppm was observed.

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.

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.

This paper presents a miniaturized catalytic gas sensor for hydrogen detection. Due to its micro technological design and high porous nanoparticle based platinum catalyst, a very short response time

This paper presents a miniaturized catalytic gas sensor for hydrogen detection. As catalyst, functionalized platinum nanoparticles are used whose increased surface allows a high sensitivity while using a minimal amount of material. The particles are stabilized by a ligand shell consisting of a diamine compound. These ligands lead to a cross-linking between the nanoparticles and thereby they prevent agglomeration. Sensors signal has been stable only with a low decrement for several hours. For comparison, a non-stabilized nanoparticle based catalyst is stable for only several minutes. A new sensor design provides a better controlling of the catalytic temperature, which also improves the stability of the sensor.