Low Temperature Methane Combustion Catalysts for Pellistors Investigated by Simultaneous Thermal Analysis

The early detection of explosive gas atmospheres is highly relevant for industrial process measurement technology and avoids the endangerment of people. In the field of safety technology, catalytic combustion sensors, so-called »pellistors«, are used to detect flammable gases such as hydrocarbons or hydrogen. Pellistor sensors measure the heat produced from catalytic oxidation of the gas by detecting the resistance alteration of a Pt-based heater induced by temperature changes using a Wheatstone bridge circuit. The heat generated is related to the kind and concentration of gas by specific combustion enthalpy [1]. The major advantages of catalytic sensors are simple operation principle, easy installation and calibration. However, state-of-the-art pellistors have some disadvantages, such as high operation temperatures of over 400°C, high-power consumption and high susceptibility to catalyst poisons. High-power consumption limits the usage of pellistors in mobile applications because of the short battery lifetime. Reducing operating temperature will contribute to decrease the power consumption. The primary gas recognition element of the pellistor is the catalytic layer. To reduce the operation temperature high active catalysts are required. Especially for detection of methane, which is one of the most inert combustible gases, catalysts of high activity or particularly high working temperatures of at least 450°C are mandatory. For this reason, a reliable detection of methane is the most important challenge. Catalysts implemented in nowadays pellistors were developed already in 1960s and contain Pd or/and Pt nanoparticles stabilized on a porous metal oxide support in its active chemical and physical state. Alumina and, to a lesser extent, zirconia are commonly used due to its high surface area and thermal stability [2]. However, some reports show that the application of metal oxides like CeO2 as support can lead to an improved sensor performance due to high oxygen storage and release ability [3]. The researches in the field of catalytic combustion provided many further evidences that especially metal oxides with spinel structures as Co3O4, NixCo3-xO4, Co3-xCuxO4, Co3-xZnxO4 etc., can contribute to catalytic combustion of methane and therefore considerably decrease the temperature for catalytic reaction [4]. New catalysts have the potential to considerably improve the performance of pellistors. This research focuses on the development of alternative, highly active catalysts based on spinels for catalytic detection of methane in the low-temperature range. Catalytic activity was investigated by Simultaneous Thermogravimetry-Differential Thermal Analysis (TG-DTA, termed as STA) coupled with Quadrupole Mass Spectrometer (STA-QMS).