DISPERSION AND ELECTRONIC INTERACTION OF PALLADIUM PARTICLES SUPPORTED ON TIN OXIDE

For a basic understanding of the promoting effect of Pd on SnO2-based gas sensor, the dispersion of Pd (or PdO) particles as well as their electronic interaction with SnO2 were investigated for various Pd loading up to 5 wt% by means of TEM and XPS. The dispersed Pd particles were observed as clear TEM images, when supported on the SnO2 particles calcined at 900-degrees-C. They were rather spherical in shape at reduced state (Pd) but changed to hemispherical at oxidized state (PdO) (Fig. 1), suggesting a better contacting interface between PdO and SnO2 than that between Pd and SnO2. Epitaxy to SnO2 was observed for both Pd (Fig. 2) and PdO. With increasing Pd loading, the mean particle size of Pd remained rather small (less than 5 nm) up to 3 wt%, but it increased to 10 nm rather abruptly at 5 wt% (Figs. 3 and 4). This behavior was well reflected on the total surface area of Pd particles per gram sample (S) and the density of Pd particles on the SnO2 surface (m) both of which had a mximum at 3 wt% (Fig. 5). The XPS binding energies (BEs) of Sn 3 d and O 1s for PdO-SnO2 showed Pd-loading-dependent shifts to the lower energy side from those of pure SnO2, becoming minimum at 3 wt% loading, whereas such shifts disappeared totally when PdO was reduced to Pd (Table 1 and Fig. 7). It is understood that the BE shifts are brought about by the electronic interaction between PdO and SnO2 (Fig. 6), their dependency on Pd loading being determined by the density of such PdO-SnO2 contacts on the SnO2 surface (m) (Fig. 8).