Structural controls on OH site availability and reactivity at iron oxyhydroxide particle surfaces

Abstract

Iron oxyhydroxides (FeOOH) are highly reactive minerals of widespread occurrence in natural and industrial settings. These minerals chiefly occur as nano- to submicron-sized particles and are covered by hydroxyl functional groups coordinated to one (-OH), two (μ-OH), or three (μ(3)-OH) underlying iron atoms. These groups are reaction centers for gases, solutes as well as solvents and thereby play important roles in the fate and transformation of natural and industrial compounds. In this work we provide tools to identify hydroxyl groups on distinct crystallographic planes of two important FeOOH polymorphs, namely lepidocrocite (γ-FeOOH) and goethite (α-FeOOH). Fourier transform infrared spectroscopy was used to monitor O-H stretching vibrations of groups on particles with well-defined and distinct morphologies. Spectral responses to proton loadings and thermal gradients were used to assign bands to hydroxyl groups. These efforts were facilitated by the extraction of pure spectral components obtained by multivariate curve resolution. Molecular dynamics simulations of dominant crystallographic planes of the particles guided band assignment procedures by identifying feasible hydrogen bond networks between surface groups. Our findings provide new possibilities for molecular-scale resolution of important gas-phase processes on the surfaces of these important minerals.

Importance of interlayer H bonding structure to the stability of layered minerals

“…However, when Fe 3+ is substituted as the central atom in the cluster, the model predicted that it would be harder to remove H from boehmite; (Supplementary Table 1 ). Conversely, experimental IR data on γ-FeOOH 48 suggests a weaker O-H bond than reported for γ-AlOOH which would suggest substituting Fe 3+ for Al would weaken the O-H bond. Given the simplicity of the model but the low concentration of Fe 3+ in the doped boehmite, it is not clear how to weight the present information.…”

Direct identification of reaction sites on ferrihydrite

“…Our previous work showed that −OH groups responsible for the 3667 cm −1 band are disposed along rows on dominant faces of FeOOH minerals ( Supplementary Fig. 1) 43,48 . Such rows are also exposed at the edges of sheets of Fe1 octahedra in our idealised representation of Fh (Fig.…”

“…This comparison shows that Fh generates flagship bands of poorly hydrogen bonded −OH groups (3667 cm −1 ), chemisorbed water molecules (−OH 2 ; 3690 cm −1 ) and doubly-coordinated hydroxyl groups (μ−OH; 3620 cm −1 ). Triply-coordinated μ 3 −OH sites were not detected in the expected spectral range where they occur in other crystalline iron (oxyhydr)oxide 43,45,48 minerals (cf. Fig.…”

“…2). Inspired by the vibrational spectroscopy work of the catalytic alumina literature 39,40 and by earlier efforts on iron oxides [41][42][43][44][45][46] , our approach relies on the detection of an elusive set of O-H stretching bands that are phenomenally sensitive to the coordination environments of surface OH groups 43,44 . Although Russell 42 did publish over 40 years ago the first evidence for Fh surface OH groups, and these findings were later substantiated by Hausner et al 47 , these sites have never been clearly identified.…”

Phosphate alteration of chloride behavior at the boehmite–water interface: New insights from ion-probe flow adsorption microcalorimetry

“…Spectroscopic techniques such as Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) have been used to compile information pertaining to the densities and distribution of surface hydroxyls on metal (hydr)oxides [15][16][17], however with the continued advancement in computing, much of what is understood has been to a large extent gathered by a variety of mathematical modeling and chemical computation techniques [17][18][19][20][21][22][23][24][25]. For example, in a recent study, Motta et al [24] used first principle molecular dynamic simulation based on density functional theory (MD-DFT) to gain insights into the factors that influence hydroxyl reactivity at the boehmite (c-AlOOH)/water interface.…”