Improvement of a Real Gas-Sensor for the Origin of Methane Selectivity Degradation by µ-XAFS Investigation

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.

the real sensor device, the lifetime was not stable enough, especially in the selectivity for methane, although the sensitivity of the SnO 2 thin layer was maintained. The selectivity instability was found to be related to the illcontrolled heating of the l-gas sensor system. Since Pd/ Al 2 O 3 is responsible for the selectivity and the damaging process can hardly be reproduced exactly in a model large sensor system, we directly investigated the structure of the Pd/Al 2 O 3 layer in a real, small l-gas sensor, the diameter of which was about 250 lm, before and after the damaging process. Therefore, the measurement of this sample requires lm order spatial-resolved XAFS, so called l-XAFS. We used a l-XAFS method to investigate the origin of the decrease in selectivity. We used a polycapillary lens to focus the X-ray into a l-beam [6][7][8][9]. The l-X-ray size was smaller than the sensor's Pd/Al 2 O 3 area. We could monitor the structure change of the l-gas sensor. In this paper, we discuss the possible origin for the decreases in the methane selectivity of the l-gas sensor together with the role of the Pd/Al 2 O 3 layer and the importance of homogeneous heating in the gas sensor.

l-XAFS
Pd K-edge l-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 [10]. The original beam size of this beamline was 1 9 1 mm 2 .

l Gas Sensor
The l-gas sensors were prepared as described elsewhere [2,3,5,28,29]. Figure 1 shows the structure of the l-gas sensor. The Pd/Al 2 O 3 thickness was about 50 lm. We set the X-rays parallel to the sensor base, as shown in Fig. 2, where the maximum fluorescence X-ray intensity was obtained. The methane selectivity of the sensor was decreased when the sensor underwent ill-controlled temperature treatment where the heating temperature varied as 653 ± 50 K. When the temperature was well-controlled at  Fig. 2 a Alignments of l-beam and l-gas sensor. The sample surface was mounted roughly parallel to the X-ray incident direction and the detector surface. The cross section area of the X-ray is 25 lm in FWHM diameter as measured by a knife-edge scan. b Photograph of polycapillary lens 703 K, the decrease in the selectivity was not observed, as shown in Fig. 3. The sample was treated to obtain a damaged sample for the acceleration aging test, where the l-gas sensors were driven in a flow of 500 ppm H 2 (a model reductant gas), and 99 % relative humidity (RH). Selectivity (Sel) was defined as follows: where R hydrogen and R methane are the resistance in flow of 1000 ppm H 2 and 1000 ppm CH 4 at 20°C and 65 % RH.
Four typical examples Sel = 5.2 (Fresh), Sel = 3.7 (Sel3.7), Sel = 1.8 (Sel1.8), and Sel = 0.9 (Sel0.9) were measured. 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.

Results and Discussion
The decrease in the methane selectivity seemed to be strongly correlated with the formation of Pd metal particles. We assumed that the change of XANES was due to the increase in Pd metal concentration in the composition. We carried out a linear combination analysis using XANES spectra of Pd foil and PdO. Figure 6 shows the fitting results for the Sel0.9 sample. The data were well reproduced using spectra of metallic Pd (38 at.%) and PdO (62 at.%). The decrease in the selectivity was negatively proportional to the Pd metal content, as shown in Fig. 7. Errors of Pd metal content were estimated using the Hamilton ratio method [30] at a confidence level of 95 %. Therefore, the selectivity decrease occurred due to the formation of Pd particles.
Previous research has shown that the function of the Pd/ Al 2 O 3 catalyst is to improve the selectivity for methane [4]. Methane is inert compared to the other gases that are present. The PdO in the Pd/Al 2 O 3 layer can burn up the other gases (hydrogen, alcohol, carbon monoxide, etc.) completely, but not the methane that reaches the sensor part (SnO 2 thin layer). The formation of Pd changes the activity and selectivity of the Pd/Al 2 O 3 overlayer. Consequently, the selectivity decreases. Pd and PdO have different activation behaviors for hydrogen and other gases [31]. 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 H 2 and reoxidation of Pd metal occurred at higher temperatures [31]. 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 [4]. 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/Al 2 O 3 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 l-sensor has been realized to attain the enough lifetime more than 5 years with the sensor structure to heat the sensor homogeneously.

Conclusion
In this work, an X-ray l-beam made by a polycapillary was used to measure the l-XAFS of a model l-gas sensor. These results suggest that the ill-controlled heating of the l-gas  Fig. 7 Relation between the methane selectivity and PdO content 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 (H 2 ), indicating that the homogeneous heating of the sensor is essential to keep the PdO structure. Based on this result, the l-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 l-XAFS is a powerful analytical tool that gives important information in understanding the mechanism of real devices at the atomic level.