Poster

Activation of water and CO₂ on Fe₃O₄ (111) thin films studied by molecular beam and IRAS

F. Ivars, F. Mirabella, P. Dementyev, K.-H. Dostert, S. Schauerman, and H.-J. Freund

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 [1]. 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 [2], 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 [3]. 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.

[1] K. M. K. Yu et al., ChemSusChem 1, 893 (2008)

[2] C. Graves et al., Ren. Sus. E. Rev. 15, 1 (2015)

[3] Y. Joseph et al., Chem. Phys. Lett. 314, 195 (1999)