Liquid Adsorption of Organic Compounds on Hematite Î±-Fe2O3 Using

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Adsorption of Organic Compounds on Hematite (#-FeO) using ReaxFF Chung Lim Chia, Carlos Avendano, Flor R. Siperstein, and Sorin Vasile Filip Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02374 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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Liquid Adsorption of Organic Compounds on Hematite α-Fe2O3 using ReaxFF Chung-Lim Chia,† Carlos Avenda˜no,† Flor R. Siperstein,∗,† and Sorin Filip‡ †School Chemical Engineering and Analytical Science, The University of Manchester, Sackville street, Manchester, M3 9PL, U. K. ‡BP Formulated Products Technology, Research and Innovation, Technology Centre, Whitchurch Hill, Pangbourne, Berkshire, RG8 7QR, U.K. E-mail: [email protected]

Abstract ReaxFF-based molecular dynamics simulations are used in this work to study the effect of polarity of adsorbed molecules in the liquid phase on the structure and polarization of hematite (α-Fe2 O3 ). We compared adsorption of organic molecules with different polarity on a rigid hematite surface and on a flexible and polarizable surface. We show that the displacements of surface atoms and surface polarization in a flexible hematite model is proportional to the adsorbed molecule’s polarity. The increase in electrostatic interactions resulting from charge transfer in the outermost solid atoms in a flexible hematite model results in better defined adsorbed layers, but less ordered than those obtained assuming a rigid solid. These results suggest that care must be taken when parametrising empirical transferable force fields as the calculated charges on a solid slab in vacuum may not be representative of a real system, especially when the solid is in contact with a polar liquid.

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Introduction Studies of liquid adsorption on solid surfaces often consider that the solid substrate is inert and its structure and properties unmodified by the presence of the adsorbate. This approximation, used since the pioneering work of Langmuir, 1 remains a common practice in modern molecular simulation studies for predicting adsorption isotherms and adsorbent screening. 2–12 This has been considered a well-founded assumption when a solid surface or rigid porous material is put in contact with a low to moderate pressured gas, 13–21 but it has been shown to be inappropriate for modelling some complex porous materials such as Metal Organic Frameworks (MOFs) where a flexible framework can adopt open or closed configurations 22–24 or where linkers can rotate, 25 changing the porous network environment. This assumption has also been proved to be unsuitable for the study of polymers of intrinsic microporosity (PIM) designed to have a rigid and contorted backbone which swells upon adsorption of carbon dioxide at high pressures. 26 Nevertheless, non-porous solids are often considered inert, regardless of the density and polarity of the liquid. 27–29 In this work we study the effect that organic molecules of different polarity have on the structure of a crystal surface, as well as the effect on the adsorbed phase properties that results from neglecting changes in the solid structure. We focus our attention to adsorption of small organic molecules such as ethanol (C2 H5 OH), toluene (C6 H5 -CH3 ), and iso-octane (C8 H18 ), which are commonly used as solvents in the chemical industry, on hematite (α-Fe2 O3 ) which is often formed on the surface of iron-containing alloys 30,31 that are extensively used for construction of equipment and mechanical structures. Molecular simulations provides a systematic and simple approach to test how different approximations used to describe the solid affect the properties of the adsorbed liquid as it is possible to switch on and off different interactions to assess their relevance in the observed properties. One of the challenges in using molecular simulations to study the solid-liquid interface, however, is the availability of suitable force fields (FFs) to describe simultaneously the properties of both liquid and solid phases. A large effort has been devoted to the devel2


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opment of classical force fields for metal oxides such as ClayFF 32,33 and CHARMM water contact angle, 34 which have been shown to be suitable for the description of non-reactive surfaces. A different approach involves using ab initio methods, but they are computationally expensive making them unsuitable to study large systems. Therefore, QM related adsorption studies 35,36 focus on single molecule adsorption or gas phase systems. In this context, the reactive force field (ReaxFF) approach has become a powerful methodology to incorporate chemical reactivity in classical molecular simulations. 37,38 ReaxFF offers a less computationally intensive alternative to quantum-based molecular dynamics (MD) simulations, and has some advantages over classical force fields, such as the ability to model reactive systems. 38 It is well known that water can dissociate on the hematite surface, thus ReaxFF is an interesting approach to study the structure and properties of polar fluids on hematite, because even when no dissociation is expected for the selected molecules, distortion in bond lengths may be observed which would not be captured by classical force fields. ReaxFF has been used to study several metal oxide/water interfaces. 39,40 Little attention, however, has been given to the interface between iron oxide and organic solvent molecules, despite the industrial relevance of these systems. The key questions, which are the aim of this present work, are to assess the effect that surface distortions can have on the properties of the adsorbed liquid, as well as the ability of a solvent to affect the solid structure and properties. In this work, we study the adsorption of selected organic molecules with different polarity: ethanol, toluene and iso-octane using ReaxFF molecular dynamics (MD) simulations. We show that the polarity of the liquid has a strong influence on the structure of the iron oxide surface model, modifying the positions of the outermost surface atoms and their partial charges, which can lead to larger electrostatic interactions and stronger adsorption of the liquid, even in the absence of chemical reactions. This increase in the solid-liquid interactions results in better defined adsorbed layers, but more disorganised than when an inert solid is considered.

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Methodology Hematite (α-Fe2 O3 ) is the most stable iron oxide at ambient conditions, and its (0001) surface is the most stable according to DFT calculations. 41 Nevertheless, this surface is difficult to prepare experimentally, and at least three terminations have been proposed in the literature. 42 In this work we focus exclusively on the iron terminated α-Fe2 O3 (0001), taking the initial crystallographic positions of the atoms from reference 43 and the corresponding ReaxFF parameters from the literature. 40,44 ReaxFF is a bond order-based force field. The bond order is defined as a continuous function of the interatomic distance and takes into account the σ, π and π-π bond type contributions. Unlike classical force fields, ReaxFF includes potential energy terms that are dependent on the bond order parameter (coordination number) and its general form is given by:

Esys = Ebond + Elp + Eover + Eunder

(1)

+Eangle + Etor + EvdW + Ecoul ,

where Esys is the total energy of the system, and the bond order dependents include the Ebond energy contribution from the formation of chemical bonds, the over coordination penalty energy Eover , the under coordination energy Eunder , the lone pair energy Elp , and angle Eangle and torsion Etor energies. Non-bonded van der Waals EvdW and Coulombic Ecoul interactions are also included. Detailed description of ReaxFF method can be found in references [ 37,45]. The point charges on the atoms are allowed to vary during the MD simulation and are calculated based on the geometry of all the atoms using the Electronegativity Equalization method (EEM) developed by Mortier et al., 46 which is a methodology that is less computationally expensive than the commonly used Charge Equilibration (Qeq) method. 47 The cut off radii for both bonded and non-bonded interactions are set to 10.0 ˚ A. 4


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Two different ReaxFF parametrizations were used in the present study: the FeC-FF parameterization by Zou et al. 44 and the FeOH-FF parametrization developed by Aryanpour et al. 40 The FeC-FF was developed to describe the surface energy and equation of state for iron carbides (Fe5 C2 and Fe3 C), and was also used to model the adsorption and desorption of hydrocarbons on iron and cementite surfaces, but unfortunately does not contain parameters for Fe-O interactions. The Fe-O interaction parameters were taken from the FeOH-FF which was developed to describe the properties of iron oxides with different oxidation states, as well as their interaction with water. This force field shows good agreement with ab initio calculations and experimental data for heats of formation and lattice parameters for hematite, goethite, lepidocrocite, akaganeite, wustite and magnetite. A combination of the two force fields was necessary to describe the interactions of organic molecules on iron oxide. Details of the parameters used for the simulations are in presented in the Supporting Information (SI). The structure of bulk hematite obtained using MD-N V T simulations (constant number of atoms, volume and temperature) with the ReaxFF parameters provided in the SI, at 298 K and 1 atm is in good agreement with experimental crystallographic data 43 as shown in Figure 1. The powder diffraction patterns showed that locations of the major peaks are retained, with noticeable differences in intensity for the peaks at 2θ = 24◦ , 35◦ , 39◦ and 41◦ . Some noise is also observed in the diffraction pattern of the relaxed structure. Further validations of ReaxFF force field with the bulk hematite equation of state, heat of formation and lattice parameters, as well as the properties of the fluid molecules considered in this work can be found in the SI. Adsorption studies were carried oun in a simulation box of dimensions 41.1 ˚ A × 35.6 ˚ A× 200.0 ˚ A containing 4×4×3 unit cells of hematite, constructed with the initial configuration of the atoms taken from experimental crystallographic information. 43 This creates a hematite slab with the (0001) crystallographic plane perpendicular to the z direction. The large dimension of the box in the z direction was necessary to model the surface without having self-

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fixed in the coordinates obtained after the relaxation in vacuum, and the flexible solid slab system where all the atoms in the solid were allowed to move during the simulation. Different metrics were used to analyse the structure of the adsorbed liquids. The density profile along z-axis was computed for all molecules using position of the centres of mass of each molecule in the liquid phase. The orientational order parameters profiles P1 (z) and P2 (z) along the z-direction were used to determine the order of the adsorbed molecules in different layers. These parameters are based on first and second order Legendre polynomials given by

and

Nf (z) 1 X ˆ i · zˆ, P1 (z) = u Nf (z) i=1

(2)

Nf (z) 1 1 X 3 2 , (ˆ ui · zˆ) − P2 (z) = Nf (z) i=1 2 2

(3)

where Nf (z) is the local number of liquid molecules at position z, zˆ is the unit vector normal ˆ i is the unit vector describing the orientation of molecule i as defined to the solid slab, and u in Figure 2. We used the carbon-oxygen (C-O) bond to define ethanol’s orientation, a vector perpendicular to the aromatic ring to define toluene’s orientation, and the vector connecting the two carbons with branching points for iso-octane. Note that P1 provides information with respect to both orientation and direction of the molecule and takes the limiting values ˆ i with of 1, -1, and 0 corresponding to parallel, antiparallel, or perpendicular orientations of u respect to the surface’s normal vector zˆ, respectively.The parameter P2 provides information only of the alignment of the molecular vectors irrespective of the direction of the molecular axis and takes the limiting values of 1, 0, and -0.5 for full alignment, isotropic order, and ˆ i with respect to zˆ, respectively. perpendicular alignment of u

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the origin for the results of the thick slab is the slab’s center minus the length of a unit cell in the z−direction, to enable the comparison with the thin slabs. Integral adsorption properties can be calculated from the density profiles, including the position of the Gibbs equimolecular surface and the integral amount adsorbed. These properties are included in the SI for completeness. The orientational order parameters P1 and P2 for all fluids in the rigid and flexible systems are shown in Figure 5. The first layer of adsorbed ethanol has P1 ∼ 0 and P2 ∼ −0.5, indicating that the C-O bond in ethanol is parallel to the surface as shown in Figure 4(a)’s snapshot. However, P1 ∼ 0.5 and P2 ∼ −0 in the second layer, suggesting that although a clear layer is formed, there is no significant orientational order of the molecules. It is important to note that even though the layering is better defined for the flexible slab, the ordering in the first layer is slightly higher in the rigid slab as shown by the higher values of P2 in Figure 5. The results for the orientational order parameter P1 for toluene are not particularly meaningful due to the molecule’s planar symmetry, but P2 clearly shows that toluene molecules adsorb flat on to the surface, with an average tilted angle of approximately 35◦ with respect to the normal direction of the slab. Again, the layering peaks for toluene are better defined in the case of flexible solid, but the first layer has higher order in the rigid slab system. Iso-octane does not show any particular ordering on the surface, and the density profiles are practically identical in both type of simulations, suggesting that the polarity of the molecules indeed have an effect of the structure of hematite near the surface. The structure of the adsorbed layers is expected to be sensitive to the approximations made when modelling the solid surface. It is known that the structure of the adsorbent can induce order in the structure of the liquid layer: a smooth surface can significantly favor ordering in the adsorbed liquid, 53 while rough surfaces are also known to reduce the order of the adsorbed layers. 54–57 Therefore, intuitively, one can expect that a rigid solid would favor the formation of well defined ordered layers while the vibrational motion of surface

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The polarization of the solid slab in ReaxFF simulations is observed as a result of the charge transfer at the interface, which is possible because the charges in all atoms are calculated at every time step of the simulation. A similar charge transfer phenomenon has been observed in a DFT study where the adsorption of single benzene on hematite causes the reduction in electronic band gap due to shift in conduction band of the Fe atom that can lead to electron transfer 58 . Although there is no mention of structural distortion in the study reported in reference 58, it can be seen from visual inspection that the interacting iron atom protrude slightly from the surface. This induced structural change in hematite has not been quantified using ab initio methods to the best of our knowledge. Despite the observed charge transfer, we did not observed any dissociation of ethanol on hematite at the conditions studied, in contrast to what has been observed of water adsorption on hematite using first principle calculations. 36 The Fe(solid)-O(H2 O) and O(solid)-H(H2 O) distances observed prior to water dissociation using first principle calculations are significantly smaller than the Fe(solid)-O(ethanol) and O(solid)-H(ethanol) distances obtain in this work with ReaxFF, preventing the path for a transition state and molecule dissociation. In this work, we did not considered single molecule adsorption, therefore it is not possible to indicate if the observations are a result of a collective phenomena and fluid-fluid interactions, or that ethanol does not dissociate on hematite.

Conclusions In this work we show that significant differences are observed in the structure of adsorbed molecules at liquid densities when departing from the approximation of an inert solid surface. The changes arise from the polarisation of the outermost surface atoms in the solid slab, and they are proportional to the polarity of the adsorbed molecules, as the difference in the first layer’s density increases with the polarity of the adsorbed molecules, i.e. ethanol > toluene > iso-octane. This suggest that care should be taken when parameterising empirical

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transferable force fields as the calculated charges on a solid slab in vacuum may not be representative of a real system, especially when the solid is in contact with highly polar liquids. The degree of order in the adsorbed layers is larger in the rigid slab model, despite having a lower density than in the flexible model, probably due to the solid vibrations.

Acknowledgements The authors would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (BP-ICAM) which made this research possible. CLC also acknowledges the support from the University of Manchester Alumni Scholarship. The authors would like to acknowledge the assistance given by IT Services and the use of the Computational Shared Facility at The University of Manchester.

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180x140mm (133 x 150 DPI)


Liquid Adsorption of Organic Compounds on Hematite Î±-Fe2O3 Using

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