This paper attempts to extend our previous studies on the chemical design and characterization of metal atoms/ensembles attached at oxide surfaces and on the in-situ observation of adsorbed molecules on oxide model surfaces. This paper presents a new aspect of catalysis at designed Co/TiO2, Co/Al2O3, Co/SiO2 surfaces characterized by FT-IR and EXAFS, namely "Surface catalytic reactions assisted by gas phase molecules", in which the amount and reactivity of reaction intermediate can be promoted and the structure of reaction intermediate can also be controlled by gas phase molecules undetectable at the surface. This paper also reports the application of one-atomic-layer TiO2 on SiO2 as a unique support for an Mo dimer complex to prepare a supported Mo-dimer catalyst. Finally, a new aspect of regulation of a catalytic reaction by monoatomic-height steps on TiO2(110) relevant to oxide-layer catalysis characterized by STM is also reported.