Modeling of Formaldehyde Adsorption on Hematite (100) Surface with Reaxff Molecular Dynamics

Abstract

Introduction Iron oxide (α-Fe2O3, hematite) is a multi-functional n-type material, which has been used in the detection of formaldehyde and other gases [1, 2], but the gas sensing mechanism is not clear yet. In this paper, the adsorption of HCHO molecules on α-Fe2O3 (100) surface are studied by ReaxFF (Reactive Force Field) through MD simulations (molecular dynamics) for the first time. Both pure HCHO gas and the gas mixture of HCHO with the environment gases of O2, H2O and H2 are studied. The adsorption structures and energies are calculated at various temperatures. The results show that the HCHO molecules are completely and directly adsorbed on the hematite (100) surface without desorption for all temperatures and models. Oxygen do not obviously affect the adsorption of formaldehyde on iron oxide, which is differ from ZnO or SnO2. RDF (radical distribution function) data show that the presence of water makes formaldehyde adsorbed more tightly on the hematite (100) surface. The dissociation of the HCHO on the Fe2O3 surface with the H2O is observed at 600 K, and the adsorption energy is the smallest among all temperatures. A possible dissociation process of formaldehyde on the surface of α-Fe2O3 with H2O is presented according to the absorption structures. Method Materials Studio is used to establish the models. The MD simulations are performed using the LAMMPS package and the ReaxFF [3] is employed to describe the atomic interactions among Fe, C, O, H atoms. The systems with pure HCHO, and gas mixtures (HCHO, H2O, O2, and H2) are studied. Figure 1 (a) shows the initial structure of the α-Fe2O3 (100) surface and the gas before adsorption. The initial pressure of the gas mixtures is about 0.76 MPa and the gas molecules are put 0.5 nm above the surface. Periodic boundary conditions, NVT ensemble and an integration timestep of 1 fs are used in the simulations. The systems are thermostatted for 60 ps at temperatures of 300 K, 400 K, 500 K, 600 K and 700 K, respectively. Results and Conclusions The dynamic adsorption processes of gas molecules on α-Fe2O3 can be clearly observed by MD simulations, and Figure 1 (b)-(f) presents the snapshots of the systems after adsorption. The adsorption energy shown in Figure 2 is calculated by Ead = Egas/Fe2O3 - Egas - EFe2O3 where Egas/Fe2O3 is the energy of the system after adsorption, Egas and EFe2O3 represent the energy of gas and Fe2O3 before adsorption, respectively. The negative Ead indicates that formaldehyde molecules can be spontaneously adsorbed on the (100) surface at any temperature. In the gas mixture adsorption system, setting 0.2 nm as the distance threshold from Fe2O3, the atom numbers adsorbed on the surface are shown in Table 1. Among the four gases in this work, HCHO is the easiest to adsorb on (100) surface of hematite, followed by water, then hydrogen, and finally oxygen, as shown in Figure 1 and 2, and Table 1. It is well-known that oxygen plays an important role for the surface adsorption of reducing gases on the n-type SnO2 and ZnO, and Barik [1] tried to explain the reaction of HCHO on hematite surface in the same way, however, the simulation results in this work show that O2 has no observable influence on the adsorption of HCHO and the HCHO is directly adsorbed on hematite (100) surface with the typical structure shown in Figure 3 (a).Comparing the adsorption results of pure HCHO and the gas mixture of HCHO and H2O, the present of water makes the chemical adsorption of HCHO on the hematite (100) surface much stronger, as the O atoms in HCHO bind more tightly to the Fe atoms with the presence of H2O for all temperatures according to the RDF results, while it is not stable for pure HCHO adsorption system when the temperature increases. The adsorption energy is the lowest at 600 K for the system with gas mixture of H2, O2, H2O and HCHO. A new adsorption structure of HCHO is found at this temperature: when a HCHO is close to a H2O molecule, the HCHO can be decomposed into a CH2 group and an O atom, and the O atom takes one H atom from the H2O molecule to form the OHgroup, as shown in Figure 3 (b). Based on this adsorption structure, we haveHCHO + H2O → CH2 + 2 OHThis phenomenon is only observed at 600 K, which leads to the lowest adsorption energy. The results indicate that humidity plays an important role in the chemical reaction between α-Fe2O3 (100) surface and formaldehyde.This work is supported by the Nature Science Foundation of China (61874018 and 61274076), the Fundamental Research Funds for the Central Universities, and the Nature Science Foundation of Liaoning (20180550923).