Carboxylic acid induced near-surface restructuring of a magnetite surface

Abstract

A fundamental knowledge of the interaction of carboxylic acids, such as formic acid, with magnetite surfaces is of prime importance for heterogeneous catalysis and the synthesis of novel materials. Despite this, little is known about the atomic scale adsorption mechanisms. Here we show by in-situ surface X-ray diffraction that the oxygen rich subsurface cation vacancy reconstruction of the clean magnetite (001) surface is lifted by dissociative formic acid adsorption, reestablishing a surface with bulk stoichiometry. Using density functional theory, the bulk terminated, fully formic acid covered surface is calculated to be more stable than the corresponding clean, reconstructed surface. A comparison of calculated and experimental infrared bands supports the bidentate adsorption geometry and a specific adsorption site. Our results pave the way for a fundamental understanding of the bonding mechanism at carboxylic acid/oxide interfaces.

Heterogeneous Adsorption and Local Ordering of Formate on a Magnetite Surface

“…brations, ν as and ν s (OCO), respectively. 10,11 On the (111) surface, the bands at 1338 cm −1 and 1588 cm −1 appear, however, at similar positions as reported for the quasi-bidentate adsorption of formate on ZnO(1010), making this a more likely candidate (ν s (OCO) at 1373 cm −1 and ν as (OCO) at 1589 cm −1 ). 13 The smaller splitting of the ν s,as (OCO) bands at 1380 cm −1 and 1548 cm −1 suggests a smaller O-C-O bond angle, such as a chelating adsorption geometry with both formate oxygen atoms bound to one single Fe tet1 ion.…”

“…27 All vibrational bands present a Fano line shape which results from magnetite being neither a perfect metal nor a perfect insulator. 10,11,28 While both bands at 1338 cm −1 and 1380 cm −1 present a similar line shape originating from a dynamic dipole perpendicular to the surface, the line shape around 1588 cm −1 is inverted, because it is excited by the electric field component parallel to the surface. 10,28 The lineshape thus supports our assignment of the vibrational symmetry.…”

“…Formic acid is reported to dissociatively adsorb at room temperature (r.t.) in a bidentate bridging geometry predominantly in a specific adsorption site with both oxygens bound to two octahedral surface iron ions. 10,11 In contrast, on other metal oxide surfaces heterogeneous adsorption of formic acid at r.t. was observed. Bidentate bridging and monodentate geometries were reported on anatase TiO 2 (101).…”

Observation of iron diffusion in the near-surface region of magnetite at 470 K

“…• surface reconstruction on Fe 3 O 4 (001) [18] and a recently observed reversible order-disorder phase transition upon heating the Fe 3 O 4 (001) surface [19].…”

“…Having one extra interstitial tetrahedral Fe ion and two octahedral subsurface Fe vacancies the so-called subsurface cation vacancy stabilized (SCV) surface is nonstoichiometric with one net iron vacancy per unit cell [12,13]. The reconstruction is reported to get lifted upon water, formic acid, and atomic hydrogen adsorption at room temperature, in line with a reduction of the surface [13,[17][18][19][20]. Observations by low energy electron microscopy (LEEM) revealed a regrowth of many Fe 3 O 4 layers upon exposure of the surface to 1.3 × 10 −6 mbar oxygen at 1000 K by diffusion of Fe cations to the surface [14].…”