Pd particles of different nanosizes were loaded on the SnO2 surface by using different Pd precursors for the purpose of investigating the Pd size effect on gas sensing properties in humid atmosphere. One kind of Pd-loaded SnO2 nanoparticle was characterized by smaller Pd particles (2.6 nm) with high dispersion, while another kind was characterized by larger Pd particles (5-10 nm) with low dispersion. It was found that both kinds of Pd on the SnO2 surface let the mainly adsorbed oxygen species change from O(-) to O(2-) in humid atmosphere at 350 °C. In addition, the water vapor poisoning effect on electric resistance and sensor response was greatly reduced by loading Pd. Interestingly, for the CO response at 350 °C, Pd-SnO2 with small Pd size showed almost constant sensor response with varying humidity (0.5-4 vol % H2O). While the CO response of Pd-SnO2 with large Pd size even increased with increasing amount of water vapor. Moreover, the former CO response was increased from 300 to 350 °C, but the later response decreased with increase in operating temperature. These behaviors were analyzed by temperature programed reduction (TPR) in H2 and CO atmospheres, and they were supported by the different catalytic activities of different nanosized Pd particles.
Keywords: CO; H2; Pd particle size; SnO2 nanoparticles; oxygen adsorption; water vapor.