Structure and chemical stability of Ce-modified Co3O4(111) surfaces

Abstract

The objective of this thesis is to characterize the surface structure and study the chem- ical stability of the CeO2/Co3O4(111) sample, prepared on Ir(100) substrate. The sample was gradually annealed at temperatures ranging from 323 K to 923 K and it was investi- gated using energy dependent (I-V) low energy electron diffraction, scanning tunneling microscopy and X-ray photoelectron spectroscopy. 12-fold symmetric surface structure corresponding to diffraction on spinel Co3O4(111) was observed and it was preserved even after cerium deposition. Substantial changes in I-V curves were discovered after Ce-deposition and minor changes after annealing at 423 K. After annealing at 923 K a breakdown of spinel structure was observed. A gradual oxidation process of cerium from Ce3+ to Ce4+ was observed, leading to full cerium oxidation at 923 K. A study of redox be- havior of cobalt in CeO2/Co3O4(111) revealed a good thermal stability for temperatures up to 773 K, followed by reduction of Co3+ to Co2+ . 1

Cieľom tejto práce je charakterizácia povrchovej štruktúry a štúdium chemickej sta- bility vzorky CeO2/Co3O4(111) pripravenej na substráte Ir(100). Vzorka bola postupne žíhaná na teploty v intervale 323 K až 923 K, pričom bola skúmaná energeticky závislou (I-V) nízkoenergetickou elektrónovou difrakciou, röntgenovou fotoelektrónovou spektro- skopiou a rastrovacou tunelovou mikroskopiou. Pozorovaná bola 12-četne symetrická po- vrchová štruktúra prislúchajúca difrakcii na spineli CeO2/Co3O4(111), zachovávajúca sa aj po depozícii céru. Veľké meny v I-V krivkách boli objavené po Ce-depozícii a menšie zmeny po žíhaní na 423 K. Po žíhaní na 923 K bol pozorovaný kolaps spinelovej štruk- túry. Pozorovaná bola postupná oxidácia Ce3+ na Ce4+ ústiaca v úplné zoxidovanie céru pri 923 K. Štúdium redoxného správania kobaltu v CeO2/Co3O4(111) odhalilo dobrú ter- málnu stabilitu pre teploty až do 773 K nasledovanú redukciou Co3+ na Co2+ . 1