Revealing the Chemical State of Palladium in Operating In2 O3 Gas Sensors: Metallic Pd Enhances Sensing Response and Intermetallic Inx Pdy Compound Blocks It.

The sensing response of metal oxides activated with noble metal nanoparticles is significantly influenced by changes to the chemical state of corresponding elements under operating conditions. Here, a PdO/rh-In2 O3 consisting of PdO nanoparticles loaded onto rhombohedral In2 O3 was studied as a gas sensor for H2 gas (100-40000 ppm in an oxygen-free atmosphere) in the temperature range of 25-450 °C. The phase composition and chemical state of elements were examined by resistance measurements combined with synchrotron-based in situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy. As found, PdO/rh-In2 O3 undergoes a series of structural and chemical transformations during operation: from PdO to Pd/PdHx and finally to the intermetallic Inx Pdy phase. The maximal sensing response (RN2 /RH2 ) of ∼5 ⋅ 107 towards 40000 ppm (4 vol %) H2 at 70 °C is correlated with the formation of PdH0.706 /Pd. The Inx Pdy intermetallic compounds formed around 250 °C significantly decrease the sensing response.