• Matthias Meier (Vortragende*r)

Aktivität: VorträgeVortragScience to Science


Iron oxides are used in a wide range of applications and chemical processes in which their surfaces interact with water [1]. Recent studies have found significant complexity in the composition of the first water monolayer, with mixed-mode adsorption and coverage dependent hydrogen bonding [2]. On the Fe3O4(111) surface, it was recently proposed that a partially dissociated water dimer is the most stable species, with an average adsorption energy of ≈100 kJ/mol [3]. This study focuses on magnetite (001) [4], one of the most studied iron oxide surfaces, and it’s interaction with water. Experimental techniques such as temperature programmed desorption (TPD), x-ray photoelectron spectroscopy (XPS), and scanning probe microscopy (SPM) are combined with density functional theory (DFT) based calculations. Quantitative TPD spectra [5] reveal 4 distinct peaks between 150 K and 250 K corresponding to the desorption of 9 molecules from the first monolayer of water, with 1, 2, 3, and finally 3 molecules per unit cell desorbing successively as the temperature is increased. To understand why, the most stable configurations of n molecules (0 < n < 9) were calculated following a systematic approach based on DFT. Using these configurations, the minimal energy path can be deduced and an energy profile created. A micro-kinetic model allows to obtain the Gibbs free energies, and an attempt to simulate the desorption temperatures is made using transition state theory. These calculations reveal that entropy plays a significant role in the desorption, reducing the initially high adsorption energies by more than 0.5 eV (or 100 K). The unusual behavior ultimately occurs because the surface provides an array of active site for the dissociation of water, and can store the hydrogen on specific surface oxygen atoms. Water molecules hydrogen bond to the OH groups, forming isolated and partially dissociated agglomerates. With increasing coverage, additional water molecules hydrogen-bond to the agglomerates and create complex networks on the surface. The most stable agglomerates and networks maximize the adsorbate/substrate and adsorbate/adsorbate interactions. keywords: Magnetite, Fe3O4(001), temperature programmed desorption (TPD), density functional theory (DFT, VASP), hydrogen bond, micro-kinetic model, transition state theory (TST) References [1] Parkinson, G.S. Iron oxide surfaces. Surf. Sci. Rep. 2016. [2] Mulakaluri, N. et al., Coverage-dependent adsorption mode of water on Fe3O4(001): Insights from first principles calculations. J. Phys. Chem. C 2010, 114, 11148–11156. [3] Dementyev, P. et al., Water Interaction with Iron Oxides. Angew. Chemie 2015, 127, 14148–14152. [4] Bliem, R. et al., Subsurface cation vacancy stabilization of the magnetite (001) surface. Science 2014, 346, 1215–1218. [5] Pavelec, J. et al., A multi-technique study of CO2 adsorption on Fe3O4 magnetite. J. Chem. Phys. 2017, 146, 14701.
Zeitraum23 Jan. 2018
EreignistitelXI International Workshop on Oxide Surfaces
OrtGranada, SpanienAuf Karte anzeigen