High activity of a catalyst and its thermal stability over a lifetime are essential for catalytic applications, including catalytic gas sensors. Highly porous materials are attractive to support metal catalysts because they can carry a large quantity of well-dispersed metal nanoparticles, which are well-accessible for reactants. The present work investigates the long-term stability of mesoporous Co3O4-supported Au–Pd catalyst (Au–Pd@meso-Co3O4), with a metal loading of 7.5 wt% and catalytically active mesoporous Co3O4 (meso-Co3O4) for use in catalytic gas sensors. Both catalysts were characterized concerning their sensor response towards different concentrations of methane and propane (0.05–1%) at operating temperatures ranging from 200 °C to 400 °C for a duration of 400 h. The initially high sensor response of Au–Pd@meso-Co3O4 to methane and propane decreased significantly after a long-term operation, while the sensor response of meso-Co3O4 without metallic catalyst was less affected. Electron microscopy studies revealed that the hollow mesoporous structure of the Co3O4 support is lost in the presence of Au–Pd particles. Additionally, Ostwald ripening of Au–Pd nanoparticles was observed. The morphology of pure meso-Co3O4 was less altered. The low thermodynamical stability of mesoporous structure and low phase transformation temperature of Co3O4, as well as high metal loading, are parameters influencing the accelerated sintering and deactivation of Au–Pd@meso-Co3O4 catalyst. Despite its high catalytic activity, Au–Pd@meso-Co3O4 is not long-term stable at increased operating temperatures and is thus not well-suited for gas sensors.

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Many bowel cancer patients are in need of an artificial stoma as part of their surgical treatment, and associated post-surgical odors caused by leaking stoma pouches may lead to social isolation, which is why inconspicuous monitoring of this situation is important for affected persons. The integration of micro-and nanotechnology may offer low-cost, low-power consuming and small solutions to this challenge. To this end, we present an inkjet-printed, heterostructured gas sensor that has been built by incorporating nanosized p-type semiconducting CuO in a porous n-type ZnO matrix. The functional layer is fabricated using a combination of a colloidal suspension and sol-gel approach optimized for inkjet printing thus offering an industry-ready method for integration of nanomaterials in microelectromechanical systems (MEMS) structures. Using a thermal modulation scheme we enhance the information content and classify different events. We demonstrate that a simple MEMS device using a novel hetero-nanomaterial may be used to reliably identify situations where stoma pouch content escapes.

Abstract:The gas sensing properties of functional metal oxide layers depend on multitude parameters, including vacancy concentration, layer morphology and thickness, size and shape of the nano/microstructure, and porosity. Using colloidal tin oxide inks we demonstrate a layer by layer deposition technique to control the thickness and composition of gas sensitive layers. To do this we combine inkjet-printing with colloidal suspensions of SnO2 particles to provide a scalable method to interface microelectromechanical systems (MEMS) with nano particles. The approach may pave the way towards an industry ready integration technique to incorporate gas sensitive quantum dots or hybrid nanomaterials in arbitrary sensing devices.

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It is shown that the operating temperature of pellistors for the detection of methane can be reduced to 300 °C by using Au–Pd nanoparticles on mesoporous cobalt oxide (Au–Pd@meso-Co3O4). The aim is to reduce possible catalyst poisoning that occurs during the high-temperature operation of conventional Pd-based pellistors, which are usually operated at 450 °C or higher. The individual role of Au–Pd as well as Co3O4 in terms of their catalytic activity has been investigated. Above 300 °C, Au–Pd bimetallic particles are mainly responsible for the catalytic combustion of methane. However, below 300 °C, only the Co3O4 has a catalytic effect. In contrast to methane, the sensor response and the temperature increase of the sensor under propane exposure is much larger than for methane due to the larger heat of combustion of propane. Due to its lower activation energy requirement, propane exhibits a higher propensity for oxidation compared to methane. As a result, the detection of propane can be achieved at even lower temperatures due to its enhanced reactivity.

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ZusammenfassungHerkömmliche Pellistoren auf der Basis von porösen Al2O3 als Trägermaterial für einen Pd-Katalysator benötigen eine Arbeitstemperatur von rund 450 °C für den Nachweis von Methan. Durch diese hohe Temperatur sind sie anfällig für „Vergiftungen“ durch Silizium-haltige Gase. Eine gute Alternative bieten Co3O4-basierte Träger in Kombination mit bimetallischen katalytischen Nanomaterialien, mit denen die Arbeitstemperatur auf rund 300 °C gesenkt werden kann. Wir stellen in diesem Beitrag unsere Ergebnisse für ein Materialsystem aus mesoporösem Co3O4 mit Au-Pd-Funktionalisierung vor. Die erzielten Ergebnisse zeigen, dass mesoporöses Co3O4 vorteilhaft gegenüber kommerziell erhältlichem polykristallinem Co3O4 in Bezug auf die katalytische Umsetzung ist. Zudem beeinflusst der Bedeckungsgrad des Co3O4 mit Au-Pd-Partikeln das katalytische Verhalten des Sensors. Die niedrigere Betriebstemperatur des untersuchten Materialsystems verbessert den Explosionsschutz entscheidend und senkt die Heizleistungsaufnahme. Bei den für die Experimente verwendeten Substraten verringert sich die Heizleistungsaufnahme von 610 mW bei 450 °C auf 355 mW bei 300 °C Arbeitstemperatur.