Operando X-ray absorption spectroscopy studies on Pd-SnO2 based sensors.
Dorota Koziej1, Michael Hübner, Nicolae Barsan
1Department of Materials, ETH Zürich, Wolfgang Pauli-Strasse, 10, CH-8093, Switzerland. dorota.koziej@mat.ethz.ch
Physical Chemistry Chemical Physics : PCCP
|September 24, 2009
Summary
Palladium addition to tin dioxide (SnO2) gas sensors influences palladium dispersion and reducibility. Even under realistic operating conditions, palladium in low-concentration sensors remains in an oxidized state, impacting gas sensing mechanisms.
Area of Science:
- Materials Science
- Nanotechnology
- Chemical Engineering
Background:
- Tin dioxide (SnO2) is a widely used semiconductor material for gas sensing applications.
- Palladium (Pd) is often added to SnO2 to enhance its gas sensing performance.
- Understanding the state and behavior of palladium within the SnO2 matrix under operating conditions is crucial for optimizing sensor design.
Purpose of the Study:
- To investigate the structure-function relationships in SnO2 gas sensors with varying palladium (Pd) additive concentrations (0.2 wt% and 3 wt%).
- To correlate the structural state of palladium with the sensing properties of the SnO2 material under both idealized and real operating conditions.
- To elucidate the role of palladium's oxidation state and dispersion in the gas sensing mechanism.
Main Methods:
- In situ X-ray absorption spectroscopy (XAS), including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), was employed for structural analysis.
- Simultaneous measurements of gas sensing properties were conducted.
- A novel in situ spectroscopic cell was designed to accommodate thin sensing layers and enable synchrotron X-ray analysis in fluorescence mode.
- Experiments were performed under realistic conditions, including exposure to hydrogen (H2) and carbon monoxide (CO) in dry and humid air at elevated temperatures (200-300 °C).
Main Results:
- Palladium dispersion was found to be highly dependent on concentration: finely dispersed in oxidized state at 0.2 wt% Pd, forming clusters at 3 wt% Pd.
- Higher palladium concentrations (3 wt%) exhibited easier reduction compared to lower concentrations.
- Under realistic sensing conditions (30-200 ppm H2, 10-50 ppm CO, 200-300 °C), palladium in the low additive concentration (0.2 wt%) samples remained in its oxidized state.
- No significant change in palladium oxidation state was observed in the low-concentration sensors during gas exposure.
Conclusions:
- The dispersion and reducibility of palladium in SnO2 gas sensors are strongly influenced by its concentration.
- Palladium in low-concentration SnO2 sensors (0.2 wt%) maintains an oxidized state even under typical operating conditions with H2 and CO.
- These findings provide critical insights into the gas sensing mechanism and offer a basis for the rational design of improved SnO2-based gas sensors.
