Activation of water and CO2 on Fe3O4 (111) thin films studied by molecular beam and IRAS
Department of Chemical Physics, Fritz-Haber Institut der Max-Planck-Gesellschaft, 14195 Berlin, Germany
Conversion of CO2 into fuels and other value-added chemicals is a currently hot topic addressed by obvious environmental and economic motivations discussed everywhere . However, CO2 displays the most oxidized state of carbon which makes it rather inert to chemical transformations, thermodynamically highly unfavorable. On the other hand, the use of water as hydrogen resource (instead of hydrogen) for CO2 reduction/hydrogenation would be the most ideal process which still remains extremely challenging though. Despite advances in this area , further fundamental research needs to be addressed to understand the interaction and behavior of water with catalyst substrates, especially metal-oxide surfaces, in order to rationally design heterogeneous catalytic processes involving water either as a hydrogen source or reaction promoter. Thus, we will present here a fundamental study of the interaction of CO2 and water with the surface of a model Fe3O4 (111) thin film supported on Pt (111) single crystal, using molecular beams technology, in-situ infrared reflection absorption spectroscopy (IRAS) and temperature-programmed-desorption (TPD). IRAS results display evidence of dissociative adsorption of water on Fe3O4 (111), as previously reported in a photoelectron spectroscopy study . Formation of hydroxyl species from water dissociation on Fe3O4 (111) surface was confirmed in our study by using D2O and isotopically labeled oxygen either in the heavy water molecule (D218O) or in the magnetite film (Fe318O4). Thus, no changes were detected for two typically observed vibrational bands related to O-D species when the corresponding iron oxide substrate was grown using 18O2. In contrast, a shift of both vibrational bands to lower energies was observed when exposing regular Fe3O4 to D218O molecular beam. This observation strongly suggests that these O-D vibrational bands originate not from two individual OD groups, formed from dissociation of a single water molecule, but from a complex between an OD-group and molecular water. Theoretical calculations carried out by J. Sauer et al (HU Berlin) on dissociative adsorption of water on Fe3O4(111) reproduce very well both the positions and the isotopic shifts of the OD vibrational bands and with this support formation of a OD-D2O complex.
Adsorption and chemical transformations of CO2 were spectroscopically investigated as a function of surface temperature and CO2 surface coverage in the temperature range 120-280 K. These changes in the spectroscopic signatures can be clearly related to strong interaction, activation and transformation of CO2 on Fe3O4 (111). In order to assign and better understand the evolving IR vibrational bands related to these newly formed chemical species, experiments with isotopically labelled 13CO2 and C18O2 as well as CO2 adsorption on isotopically labeled (O18) iron oxide were carried out. Evolution and spectroscopical assignment of surface species formed will be discussed.
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