Heterogeneous Adsorption and Local Ordering of Formate on a Magnetite Surface

Abstract

We report a novel heterogeneous adsorption mechanism of formic acid on the magnetite (111) surface. Our experimental results and DFT calculations give evidence for dissociative adsorption of formic acid in quasi-bidentate and chelating geometries. The latter is induced by the presence of iron vacancies at the surface, making oxygen atoms accessible for hydrogen atoms from dissociated formic acid. DFT calculations predict that both adsorption geometries are energetically favorable under our experimental conditions. The calculations prove that the locally observed (sqrt(3)xsqrt(3)) R30° superstructure consists of formate in a triangular arrangement, adsorbed predominantly in chelating geometry. The results show how defects can stabilize alternative adsorption geometries, which is a crucial ingredient for a detailed atomistic understanding of reaction barriers on magnetite and other oxide surfaces, as well as for the stability of carboxylic acid based nanocomposite materials.

Modeling Charge Redistribution at Magnetite Interfaces in Empirical Force Fields

“…A forthcoming publication will present the initial experimental observation and characterization in combination with DFT calculations of such novel triangular-shaped superstructures on magnetite (111). 53 However, our force field is capable of independently generating similar structures as those presented herein. Due to the weak bonding of the formate with the hydroxyl group, formate is able to dynamically rotate in steps of 120°around its tetrahedral coordinated iron at room temperature on the (111) magnetite surface (see the second video in the Supporting Information).…”

“…Figure e). A forthcoming publication will present the initial experimental observation and characterization in combination with DFT calculations of such novel triangular-shaped superstructures on magnetite (111) . However, our force field is capable of independently generating similar structures as those presented herein.…”

“…The PBE exchange–correlation functional was used for all calculations . To correct the significant correlation effects of the Fe d-electrons, an on-site Hubbard parameter U eff of 4 eV was used by employing the method of Dudarev et al, which is known to accurately reproduce the structure, magnetic moments, and band gap of magnetite. , More details on the computational setup and the model system can be found in previous publications. , To calculate energy–displacement curves for organic molecules and Fe 3 O 4 surfaces, energetically favorable configurations from previous studies were used , as initial structures. The distance between the adsorbates and the surfaces was varied, and a static calculation was performed for every resulting configuration.…”

“…51,52 More details on the computational setup and the model system can be found in previous publications. 34,53 To calculate energy−displacement curves for organic molecules and Fe 3 O 4 surfaces, energetically favorable configurations from previous studies were used 34,53 as initial structures. The distance between the adsorbates and the surfaces was varied, and a static calculation was performed for every resulting configuration.…”

Interaction between Organic Compounds and Catalyst Steers the Oxidation Pathway and Mechanism in the Iron Oxide-Based Heterogeneous Fenton System

“…In contrast, controversies exist in the explanation of the oxidation mechanism of the organics with functional groups (e.g., FA and aromatic acids) that might form strong interaction with the catalyst. 18,19 Therefore, we suspect that the abovementioned controversies might be associated with the overlooked interaction between the pollutants and the surface �Fe(III) of the catalyst, and a more scientific elucidation of the mechanism should take such interaction into account.…”