In-Situ TEM on Epitaxial and Non-Epitaxial Oxidation of Pd and Reduction of PdO at P = 0.2–0.7 bar and T = 20–650 °C

The oxidation of Pd particles and subsequent reduction of PdO were studied in 0.2–0.7 bar O2 and H2 gas at 20–650 °C by in-situ transmission electron microscopy. The 15 nm thick Pd particles were 50–300 nm wide, and (111) oriented on the SiN support. Two oxidation mechanisms were observed: 1) the slow formation of epitaxial PdO at the Pd particle edges with orientations OR(1): PdO[201]//Pd[111] and PdO(020)//Pd{022}, 5.8° from OR(2): PdO[110]//Pd[100] and PdO[001]//Pd[001]. 2) the fast formation of randomly oriented PdO nucleating at the Pd particle edges. During the reduction process we observed epitaxial formation [OR(1) and OR(2)] as well as nonepitaxial formation of Pd (and Pd hydride). The formation of randomly oriented (5–10 nm) Pd particles on a (001) surface of PdO was much slower than of the epitaxial Pd grains on other PdO surfaces. A systematic study of the electron beam irradiation effect on the phase transformations showed that this effect can be avoided.