Improvement of a Real Gas-Sensor for the Origin of Methane Selectivity Degradation by µ-XAFS Investigation
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We have directly investigated the chemical state of the Pd species in a real μ-gas sensor device by examining the μ-fluorescence X-ray absorption fine structure. The μ-gas sensor device was heavily damaged by a heating process in which the temperature was ill-controlled, resulting in decrease of methane selectivity. We found that the PdO in the fresh μ-gas sensor was reduced to Pd metal particles as the methane selectivity decreased. Based on the investigation results, we modified the device structure so as to heat up homogeneously. The lifetime of the sensor was then successfully increased by more than 5 years.
KeywordsMicro gas sensor Micro-XAFS Pd/Al2O3 Deactivation
Natural gas is now used for household energy resources because of its ease of use and environmental friendliness. The development of safety measures, such as a gas leakage sensor is in strong demand. SnO2 is a kind of sensor materials, its electric conductivity changes with the gas composition at high temperature . However, the requirement of the AC power supply hinders the daily use of the SnO2 gas sensor in the house because of the installation problem and the bad appearance.
Pd K-edge μ-XAFS experiments were performed at the NW-10A beamline at the Photon Factory (Institute for Materials Structure Science, High Energy Accelerator Research Organization; KEK-IMSS-PF) using a Si (311) double crystal monochromator in a fluorescence mode . The original beam size of this beamline was 1 × 1 mm2.
2.2 μ Gas Sensor
3 Results and Discussion
The XANES spectrum in the fresh sample was much more similar to those of PdO and Pd(OH)2 rather than Pd foil. When the methane selectivity was decreased, the XANES changed a little. The first edge peak (24,350 eV) decreased with the increase in the higher energy side (24,360 eV) as the methane selectivity decreased. EXAFS Fourier transforms provided more definite structural information. In the Fourier transform of the fresh sample, Pd–O was found at 0.16 nm together with Pd–Pd at about 0.3 nm, indicating the formation of PdO, not Pd(OH)2. As the methane selectivity decreased, we found a new, emerging peak at 0.25 nm in the Fourier transform. This new peak could be assigned to the Pd–Pd bond in the Pd metal. The height of this peak increased with the decrease in methane selectivity. Curve fitting results indicated the presence of Pd at 0.273 ± 0.004 nm with a coordination number of 2.8 ± 0.6 in Sel0.9. In Sel3.7 and Sel1.8, we found the coordination number of the Pd–Pd bond in Pd metal to be 0.9 ± 0.4 and 2.4 ± 1.0, respectively.
Previous research has shown that the function of the Pd/Al2O3 catalyst is to improve the selectivity for methane . Methane is inert compared to the other gases that are present. The PdO in the Pd/Al2O3 layer can burn up the other gases (hydrogen, alcohol, carbon monoxide, etc.) completely, but not the methane that reaches the sensor part (SnO2 thin layer). The formation of Pd changes the activity and selectivity of the Pd/Al2O3 overlayer. Consequently, the selectivity decreases. Pd and PdO have different activation behaviors for hydrogen and other gases . The PdO is necessary for the selective combustion and increases the selectivity.
Under ill-controlled temperature conditions, the Pd was not always heated up to 703 K. In the model system, we found the PdO was reduced at intermediate temperatures (500–600 K) in the presence of H2 and reoxidation of Pd metal occurred at higher temperatures . Therefore, insufficient heating may sometimes create Pd particles, which may aggregate. Once the Pd is aggregated into large metal particles, high-temperature heat treatment cannot redisperse the Pd particles to the PdO again, even at 703 K. In the sample that was subjected to ill-controlled temperature conditions, the aggregation to form large Pd particles might occur slowly but steadily, causing the sensor to gradually lose its selectivity. Moisture also accelerates the aggregation process . Under the influence of moisture, the surface of PdO might consist of Pd(OH)2, which should show high mobility and thus accelerate the aggregation of large Pd particles [32, 33]. In the dry gas, the acceleration rate of the decreasing selectivity was low. The heating treatment at 703 K also helps the removal of surface Pd(OH)2 species. We conclude the PdO is the key factor in keeping the high selectivity for methane. The Pd/Al2O3 layer of micro gas sensors structure was modified to increase the heat retention capability and the contact area to the heater and to decrease the thermal capacity. As results, it allowed that the temperature was kept around 703 K homogeneously without increasing power consumption and PdO structure can be maintained. Based on the knowledge obtained here, the battery-driven μ-sensor has been realized to attain the enough lifetime more than 5 years with the sensor structure to heat the sensor homogeneously.
In this work, an X-ray μ-beam made by a polycapillary was used to measure the μ-XAFS of a model μ-gas sensor. These results suggest that the ill-controlled heating of the μ-gas sensor system caused the reduction of PdO to Pd metal particles at medium temperature. Since the Pd nanoparticle was active for the oxidation reaction of methane, the methane selectivity decreased. However, at a higher temperature than 703 K, the Pd was kept in PdO structure even in the presence of reductant gas (H2), indicating that the homogeneous heating of the sensor is essential to keep the PdO structure. Based on this result, the μ-gas sensor structure has been modified to keep the sensor device at the high temperature homogeneously, and the sensor lifetime has successfully been increased by more than 5 years. The μ-XAFS is a powerful analytical tool that gives important information in understanding the mechanism of real devices at the atomic level.
All XAFS measurements were conducted at the Photon Factory, High Energy Accelerator Organization under Proposal Number 2012G680. The investigation was performed under the support of the New Energy and Industrial Technology Development Organization (NEDO).
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