Growth and decomposition of aligned and ordered PdO nanoparticles

The formation, thermal decomposition, and reduction of small PdO particles were studied by high-resolution transmission electron microscopy and selected area electron diffraction. Well-defined Pd particles (mean size of 5–7 nm) were grown epitaxially on NaCl (001) surfaces and subsequently covered by a layer of amorphous SiO2 (25 nm), prepared by reactive deposition of SiO in 10–2 Pa O2. The resulting films were exposed to molecular O2 in the temperature range of 373–673 K, and the growth of PdO was studied. The formation of a PdO phase starts at 623 K and is almost completed at 673 K. The high-resolution experiments suggest a topotactic growth of PdO crystallites on top of the original Pd particles. Subsequent reaction of the PdO in 10 mbar CO for 15 min and thermal decomposition in 1 bar He for 1 h were also investigated in the temperature range from 373 to 573 K. Reductive treatments in CO up to 493 K do not cause a significant change in the PdO structure. The reduction of PdO starts at 503 K and is completed at 523 K. In contrast, PdO decomposes in 1 bar He at around 573 K. The mechanism of PdO growth and decay is discussed and compared to results of previous studies on other metals, e.g., on rhodium