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

Abstract

Introduction 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). Method STA (NETZSCH, STA 409 CD-QMS 403/5 SKIMMER) is a calorimetric method to monitor the heat consumption (endothermic) or release (exothermic DTA signal) occurring by different processes. Heat release during exothermic oxidation reaction is measured by comparison the temperature of sample placed in an alumina crucible with that of reference crucible after calibration and signal normalization. In contrast to a pellistor, in which each element of the sensor including electronics, contacts and catalytic layer, etc. influences the formation of the sensing signal, STA shows unaffected thermal signal originating from catalytic reaction. Additionally, the complex preparation steps as in case of pellistor can be avoided. Thermal analysis like DSC [5] and DTA ensure comparable conditions for appropriate testing and investigation of different catalytic materials. QMS allows the analysis of gaseous residues and reaction products by ionization.For catalyst investigation, Co3O4 spinel doped with nickel (synth. NiCo2O4) and palladium (4wt% Pd/synth. Co3O4) were synthesized according to [4]. For comparison, commercial Co3O4 (comm. Co3O4) was used. Results and Conclusions Co3O4 spinel and his derivatives have been chosen for the investigation of catalytic activity. The thermal behavior of investigated oxides in synthetic air (reference) and in 1 vol% CH4 is shown in Fig. 1a. The mass spectrum presented for mass ion m/z of 44 for NiCo2O4 (Fig. 1b) confirms the CO2 evolution by CH4 injection into the chamber as a result of catalytic reaction. Figure 1: (a) DTA signal of four investigated catalysts at four temperatures (red line) measured successively in synthetic air (gray background) and 1vol% CH4 (green background); (b) simultaneously recorded mass spectrum (STA-QMS) of mass ion 44 from the NiCo2O4 sample. Spinel NiCo2O4 and 4wt% Pd/synth. Co3O4 (Fig. 1a) reveal first traces of activity already at 250°C, while undoped commercial Co3O4 shows a minor signal at 300°C first. Certainly, with temperature increasing the behavior changes. Consequently, NiCo2O4 is a preferred catalyst for low operation temperature. It shows higher activity at lower temperatures, whereas at higher temperatures (450°C) Pd doped Co3O4 catalysts prevail in catalytic activity.Especially doped NiCo2O4 spinel exhibited excellent catalytic activity towards methane oxidation. The structure-properties relation will be presented and discussed.