Catalyzed Ethanol Chemical Looping Gasiﬁcation Mechanism on the Perfect and Reduced Fe 2 O 3 Surfaces

: Biomass chemical looping gasiﬁcation (CLG) is a novel gasiﬁcation technology for hydrogen production, where the oxygen carrier (OC) transfers lattice oxygen to catalytically oxidize fuel into syngas. However, the OC is gradually reduced, showing different reaction activities in the CLG process. Fully understanding the CLG reaction mechanism of fuel molecules on perfect and reduced OC surfaces is necessary, for which the CLG of ethanol using Fe 2 O 3 as the OC was introduced as the probe reaction to perform density functional theory calculations to reveal the decomposition mechanism of ethanol into the synthesis gas (including H 2 , CH 4 , ethylene, formaldehyde, acetaldehyde, and CO) on perfect and reduced Fe 2 O 3 (001) surfaces. When Fe 2 O 3 (001) is reduced to FeO 0.375 (001), the calculated barrier energy decreases and then increases again, suggesting that the reduction state around FeO(001) favors the catalytic decomposition of ethanol to produce hydrogen, which proves that the degree of reduction has an important effect on the CLG reaction.


Introduction
Realizing the aggregate fossil fuel demand peak and achieving 1.5 • C Pathway of Paris Agreement goals require the acquisition of more new and renewable energy in the global energy mix. As a clean, renewable, zero-emission energy, hydrogen is a promising energy carrier to diversify the energy supply and promote the energy consumption transition. Although hydrogen is the most common element in the universe, natural hydrogen occurs not in an elemental form but in hydrogen-containing compounds. The chemical looping gasification (CLG) approach to hydrogen production has received widespread attention due to its high efficiency and low carbon emissions. Unlike traditional gasification technology with molecular oxygen as the gasification agent, CLG uses lattice oxygen in the oxygen carrier (OC) to produce syngas [1,2]. A schematic diagram of the CLG is shown in Figure 1. CLG technology consists of two interconnected reactors, and the OC transfers lattice oxygen and heat in the reactor. The OC is reduced by fuel feedstock in the fuel reactor (FR) and is oxidized by the high temperature in the air reactor (AR) alternately. The fuel and syngas are inherently separated from air and nitrogen. The fuel gas is subjected to subsequent condensation to remove moisture. The resulting higher concentration of CO 2 could be obtained with low energy consumption [3,4]. The OC plays a key role in the CLG process, which is not only an oxygen carrier but also a heat carrier. A competent OC should have the following characteristics [4][5][6]: high reactivity and oxygen transfer performance, stable chemical and thermodynamic properties, a low price, good environmental friendliness, good sintering resistance, good mechanical strength, and wear resistance. Common OCs used in CLG include nickel-based OCs [7], copper-based OCs, manganese-based OCs, cobalt-based OCs, and iron-based OCs [8,9]. In recent years, iron-based OCs have been thoroughly investigated due to their environmental friendliness, nontoxicity, harmlessness, and low cost [10][11][12]. Huang et al. [13] used iron ore as an OC to study the effects of the oxygen excess number, steam concentration, oxygen source, and cycle times on CLG. Zeng et al. [5,14] used a doublefluidized bed gasifier to conduct CLG experiments of biomass such as sawdust based on iron-based OC. Wei et al. [15] assessed the behavior of an iron-nickel composite OC in a 10 kW CLG unit. In addition, the research results show that iron-based OCs not only effectively transfer oxygen and heat but also have an effect on the catalytic gasification of tar and coke macromolecules, which improves the fuel conversion efficiency. It is found that the multiphase OC complex can improve the specific surface area and promote the delivery of lattice oxygen from the theoretical calculations based on the density functional theory [16][17][18]. However, the solid CLG process still has an insufficient gasification agent, an insufficient solid-solid interaction, and a low carbon conversion rate. In previous work, our team employed simultaneous chemical looping combustion with solid fuel and organic waste liquid. During the degradation of the organic waste liquid, hightemperature water vapor was produced to provide the gasification agent and promote solid fuel gasification [16][17][18][19][20][21]. Dou et al. [22] used nickel-based OC to produce hydrogen in ethanol-steam CLG and explained the decomposition mechanism of ethanol on the nickel-based OC. However, the decomposition mechanism of ethanol on iron-based OCs of different reduction degrees is still rarely reported, and the underlying mechanism of the CLG process is still unclear. The catalysis behavior on the surface of the OC needs to be clarified in detail.
In this paper, the density functional theory calculation method is used to study the adsorption characteristics and decomposition reaction path of ethanol on fully reduced Fe 2 O 3 (001), which reveals the formation mechanism of the synthesis gas products such as H 2 , CH 4 , ethylene, formaldehyde, acetaldehyde, and CO in the ethanol chemical looping reforming experiment. Studying the gradual reduction process of the OC and the interaction mechanism between liquid or solid fuel and the surface of the OC can accumulate basic kinetic and thermodynamic parameters for the chemical looping reaction system of substances. This is conducive to the development and engineering application of chemical looping technology.

Materials and Methods
The Fe 2 O 3 unit cell is a rhombohedral R-3c space group structure containing 6 iron atoms and 4 oxygen atoms. The Fe 2 O 3 (001) surface is one of the most stable crystal planes exposed in natural iron ore [23,24]. The Fe 2 O 3 model was imported from the structure file in the Materials Studio (MS) software package. A 5 atomic layer 2 × 2 Fe 2 O 3 (001) surface was constructed along the direction of the crystal plane of Fe 2 O 3 (001), with a 1.2 nm vacuum gap to avoid interaction between surfaces. Oxygen atoms were gradually removed from the surface of the 2 × 2 Fe 2 O 3 (001) to simulate the reduced Fe 2 O 3 (001) surface, including Fe 3 O 4 (001), FeO(001), and FeO 0.375 (001). Through geometric optimization, the stable surface configuration is shown in Figure 2. On a stable surface, with the adsorption and decomposition reactions of ethanol molecules, all calculations were based on dispersioncorrected density function theory (DFT-D) [25]. The electronic exchange-related energy was described by the Perdew-Burke-Ernzerhof (PBE) exchange-correlation function [26,27] and the generalized gradient approximation [12,28], and the ion core was described by the supersoft pseudopotential [29]. Taking into account the strong spin polarization effect, the antiferromagnetic arrangement of Fe atoms in the Fe 2 O 3 unit cell was set to +-+; +, referring to the upward and downward rotation directions relative to the z-axis to achieve the total energy of the smallest stable state model [30]. In the calculation process, all atoms were allowed to relax, the truncation energy was set to 350 eV, the tail width was 0.1 eV, and the K point density was 4 × 4 × 1. In a self-consistent field, the convergence criteria for energy, maximum force, maximum stress, and displacement were 2.0 × 10 −5 eV/atom, 0.002 Ha/Å, 0.1 GPa, and 0.002 Å, respectively. The transition state search adopted the linear/secondary synchronous transition method (LST/QST) [31]. All calculations were performed using the CASTEP code in the MS software package.

Adsorption of Ethanol on the Perfect and Reduced Fe 2 O 3 (001)
The stable configuration of ethanol adsorption on the ideal and reduced Fe 2 O 3 surface is shown in Figure 3. The adsorption of ethanol molecules on each surface is physical adsorption. The adsorption position and direction of ethanol on each surface are different. With the increase of the OC reduction degree, the bond length of the Fe-O bond and Fe-Fe bond increases, and the pores of the surface model increase as well. The surface interaction between ethanol and Fe 2 O 3 (001), Fe 3 O 4 (001), FeO(001), and FeO 0.375 (001) can be characterized by adsorption energy (E ads ). E ads is calculated by the following formula: Among them, E AB represents the total energy of the model system after adsorption, E A represents the energy of the OC (Fe 2 O 3 (001), Fe 3 O 4 (001), FeO(001), and FeO 0.375 (001)), and E B represents the energy of the adsorbate. E AB , E A , and E B can be obtained from "final energy" in the optimized model file. As shown in Figure 4, the adsorption energy of ethanol molecules on each surface is negative, which indicates the adsorption process can achieve stability by releasing heat. The bond length of the ethanol molecule of each adsorption system is shown in Table 1. With the increase in the degree of OC reduction, the bond length of the C1-H bond on the ethanol molecule does not change much, and the bond length is about 1.1 Å. The C2-H bond length gradually decreases, which may be caused by the continuous increase in the C-OH bond length of the hydroxyl bond connected to the C2 atom. The O-H bond length gradually increases. The increase in the bond length of the C-OH bond and O-H bond may be caused by the interaction of the OC surface with ethanol, which provides the necessary conditions for the dehydroxylation of hydrogen atoms of ethanol molecules. Table 1. Bond length parameters of the adsorbed ethanol molecule on each surface.

C1-H(Å) C2-H(Å) C-C(Å) C-O(Å) O-H(Å)
Bond Length To explore the adsorption mechanism of ethanol molecules on each surface, Mulliken charge population analysis was performed. As shown in Table 2, after the adsorption of ethanol on each surface, the overall charge of ethanol molecules is negative, indicating that electrons are transferred from the surface to the ethanol molecules. When the OC is reduced from Fe 2 O 3 to FeO, the amount of charge transferred decreases, which is consistent with the change of C-C bond length, indicating that the adsorption effect gradually decreases. As shown in Figure 4, with the reduction of Fe 2 O 3 , the absolute value of E ads gradually increases, indicating the reduction of Fe 2 O 3 benefits the adsorption process, which corresponds to the previous report [32].

Ethanol Deep Decomposition on Fe 2 O 3 (001)
After ethanol molecules are stably adsorbed on the surface of Fe 2 O 3 (001), they will decompose under the promotion of the OC and produce a series of intermediate products.
To study the decomposition mechanism and reaction path of ethanol on Fe 2 O 3 (001), the stable adsorption of ethanol molecules on Fe 2 O 3 (001) was set as the initial state (IS). Then, LST/QST was used to search for the transition state of the reaction, and the reaction activation energy (Ea) and reaction energy (Er) were obtained at the same time.
According to the principle of minimum activation energy (energy barrier), the decomposition reaction path of ethanol molecules on the Fe 2 O 3 (001) can be obtained, as shown in Figure 5. Among the first step reaction of the decomposition reaction of ethanol molecules on Fe 2 O 3 (001), the energy barrier of the dehydroxylation reaction of ethanol molecules is the lowest, only 3.302 eV, and the reaction process is exothermic, of which the energy is −0.689 eV. Therefore, the first decomposition reaction step of ethanol molecules is most likely to be a dehydroxylation reaction. This is different from that of the first step of the decomposition reaction of ethanol molecules on the nickel-based OC, the dehydration reaction, which was obtained by Dou et al., using Fourier infrared spectroscopy [31]. The energy barrier of the ethanol dehydration reaction on the Fe 2 O 3 (001) surface is 7.132 eV, which is higher than the energy barrier of the ethanol dehydroxylation reaction. This may be due to the different characteristics of nickel-based OCs and iron-based OCs. The lowest energy barrier for the second step of the decomposition reaction of ethanol is the breaking process of the C-C bond (called Reaction Path A). The reaction energy barrier is 3.383 eV, and the Er is 2.756 eV. However, in the second step of decomposition, the cleavage of the C1-H bond of the ethyl group (called Reaction Path B) has a reaction energy barrier of 3.869 eV, and its Er is 0.809 eV. The process model of the decomposition reaction of ethanol on Fe 2 O 3 (001) is shown in Figure 6. In Path A, the ethyl group undergoes a C-C bond cleavage to produce a methyl group. However, the generated methyl group is not absorbed on the surface of the OC, and it combines with the free hydrogen atoms generated by dehydrogenation in the other reaction pathways to generate methane. In Path B, the Fe-C bond between the ethyl group and the iron atom on the surface of the OC is broken. At the same time, the C1-H bond of the ethyl group is broken, forming a free ethylene molecule and a H atom, and the H atom is adsorbed on the iron atoms on the surface. Paths A and B, respectively, explain the reaction mechanism of ethanol decomposition to produce methane [13,[31][32][33] and ethylene [34,35].

Ethanol Deep Decomposition on Fe 3 O 4 (001)
To explore the influence of the OC on the decomposition path of ethanol molecules after the loss of lattice oxygen, the decomposition mechanism and reaction path of ethanol on the surface of Fe 3 O 4 (001) were studied. As shown in Figure 7, there are four decomposition reaction paths of ethanol molecules on Fe 3 O 4 (001). In the first step of the decomposition reaction, the E a of the C-C bond cleavage reaction and the O-H bond cleavage reaction is similar, respectively, 3.760 and 3.802 eV, which are lower than others, thus these two decomposition reactions may be the first decomposition reaction step of the ethanol molecules on Fe 3 O 4 (001). In the second step, the O-H bond cleavage reaction of the CH 2 OH group produced in the first reaction is the lowest energy barrier, only 2.476 eV, and Er is −0.458 eV (called Reaction Path A). According to the reaction energy barrier diagram, Reaction Path A is the optimal reaction path, and it is an exothermic reaction. Path C is an exothermic reaction as well, and Paths B and D are endothermic reactions that are not easy to react. Decomposition Reaction Paths A-D are shown in Figure 8. In Decomposition Reaction Path A, the product obtained after the cleavage of the C-C bond does not chemically adsorb on Fe 3 O 4 (001) to produce chemical bonds. The product methyl group can combine with free hydrogen atoms generated by other reaction pathways to generate methane, which may be a source of methane generated in the ethanol CLG experiment [34,35]. In another product, the CH 2 OH group, the C-O single bond becomes a C=O double bond, and when the O-H bond is broken, it becomes a formaldehyde molecule. In Decomposition Reaction Path B, the C=O double bond in CH 3 CHO becomes the C≡O triple bond. The C≡O triple bond is a unique bond type of CO molecules, which may be the formation mechanism of free CO molecules [22,36].

Ethanol Deep Decomposition FeO(001)
From the perspective of the first decomposition reaction step, compared with Fe 2 O 3 (001), the Ea of the dehydrogenation reaction on FeO(001) is greatly decreased, indicating that FeO has a good catalytic effect on the ethanol decomposition reaction. Additionally, the energy barrier for the dehydration reaction of ethanol on FeO(001) is 2.923 eV, which is different from the ethanol decomposition reaction path on NiO described in the literature [31]. As shown in Figure 9, the energy barrier of Reaction Path A is low, so ethanol molecules are most likely to undergo dehydroxylation on FeO(001) first, and then the C-C bond cleavage reaction occurs. As shown in Figure 10, in Decomposition Reaction Path A, the ethanol molecules first undergo a dehydroxylation reaction on FeO(001). The detached hydroxyl group is adsorbed on FeO(001), and a free ethyl group is generated. Subsequently, the O-H bond is broken, and the broken H atoms are adsorbed on FeO(001). Finally, the C1-H bond breaks, creating a free ethylene molecule with non-coplanar atoms. In Decomposition Reaction Path B, CH 3 CH 2 O groups and H atoms are generated by breaking the O-H bond of the ethanol molecules. Then CH 3 CH 2 O groups are adsorbed on FeO(001), but H atoms are not adsorbed. After the C-O bond in the CH 3 CH 2 O group is broken, an ethyl group and an O atom are generated immediately. The O atom is adsorbed on the OC surface and becomes part of the OC surface, which reduces the degree of reduction on the OC surface. It reveals the mechanism of ethanol oxidation on the surface of the OC. Finally, the C1-H bond of the ethyl group is broken, producing an ethylene molecule [20].

Ethanol Deep Decomposition on FeO 0.375 (001)
As shown in Figure 11, there are six decomposition reaction paths of ethanol molecules on FeO 0.375 (001). In the first step, the Ea of the dehydroxylation reaction is similar to the Ea of the O-H bond cleavage reaction, which is similar to the decomposition reaction on FeO(001). Therefore, both types of decomposition reactions may be the first decomposition reaction step of ethanol molecules on FeO 0.375 (001). In the second step of Path A, the energy barrier of the C-O bond cleavage reaction of the CH 3 CH 2 O group is the lowest, only 1.488 eV, and its Er is 1.202 eV. The C2-H bond cleavage reaction acts as the third step of Path A, with an energy barrier of 0.138 eV, and its Er is −10.716 eV. Thus, among six decomposition reaction paths, Path A is the most feasible reaction pathway. Decomposition Reaction Paths A-F are shown in Figure 12. In Path A, the first decomposition reaction step of the ethanol molecules on the FeO 0.375 (001) surface is the breaking of the O-H bond. Then the hydroxyl O atom is adsorbed on the OC surface. The second step is the breaking of the C-O bond, creating a free CH 3 CH 2 group, and then the C2-H bond is broken, with the H atom and the CH 3 CH group adsorbed on the same Fe atom on the surface of FeO 0.375 (001). Finally, the C2-H bond broke again, and the Fe-C bond becomes the Fe-C-Fe bond. The formation of this bond may cause carbon deposition [10,37]. The broken H atom is adsorbed on FeO 0.375 (001) to form the Fe-H-Fe bond, and the previously adsorbed H atom also forms the Fe-H-Fe bond, which may indicate that the highly reduced OC FeO 0.375 (001) surface is not conducive to the formation of free H atoms, thereby inhibiting the decomposition of ethanol to produce H 2 .

Thermodynamic Analysis
Since Fe only has a catalytic effect on the decomposition of ethanol molecules and does not directly participate in the chain reaction, and only Fe 2 O 3 , Fe 3 O 4 , and FeO can take part in the oxidation of ethanol during the CLG process, the reduction of the OC will follow a decreasing of oxygen potential as Fe 2 O 3 →Fe 3 O 4 →FeO→Fe. We compared two CLG cases following the carbon conversion to CO and CO 2 + CH 4 , respectively. The possible reactions are concluded as follows: FeO + ethanol → Fe + CO 2 + CH 4 + H 2 (7) Figure 13 compares the free energy change (∆G) and reaction enthalpy change (∆H) for Reactions (2)-(7) under different reaction temperatures. All ∆G is negative, and the reaction can occur spontaneously. The reaction that only produces CO and H 2 is more likely to occur with the increase of reaction temperature [38], which is similar to the previous experiment results [20], while ∆G for the reaction leads to CO 2 , and CH 4 generation does not change significantly with temperature. Comparing with the decomposition mechanism discussed above, we observed that the reaction of forming CO and H 2 is easier to proceed than that of forming CO 2 and CH 4 , which corresponds to the previous report that it is more possible to cross the energy barrier to form H 2 [39]. From the analysis of the reaction mechanism, the reaction energy barrier of the ethanol molecule on FeO is the lowest and more active for catalytic decomposition of ethanol to produce hydrogen than other reduced surfaces, which is similar to the previous result where FeO possesses superior catalytic hydrogen production capabilities [40]. For Figure 13a,c, the reaction enthalpy appeared to decrease first and then increase, which suggests that these reactions are controlled by both thermodynamics and kinetic. Since these reactions are all exothermic reactions, low temperature is conducive to the reaction equilibrium, but the reaction requires a certain high temperature to cross the reaction energy barrier. Therefore, based on reaction thermodynamics and kinetics, there is an optimal reaction temperature. In our previous experiment, 900 • C is the optimal reaction temperature of ethanol CLG [20], which corresponds to the previously reported experimental results [38]. In addition, the feasibility of Fe 2 O 3 material used as OC in the CLG is demonstrated [41], good cycle performance of an iron-based OC been proven [42], the energy balance of ethanol CLG has been discussed [20], and the significant long-term efficiency potential of the chemical looping combustion (CLC)-based coal-fired power plant system [43], as well as the economic feasibility of the CLC-steam methane-reforming (SMR) system [44], have been revealed. However, after circulation, the accumulation of char will reduce the H 2 production in CLG [45]. It is necessary to determine the optimal reaction pressure [46] and structure of the OC [47,48] to improve the circulation of OC in the CLG process.

Conclusions
This paper used periodic DFT to study the adsorption and decomposition reaction mechanism of ethanol molecules on Fe 2 O 3 (001) and its reduced surfaces. The results show that in the CLG process, ethanol molecules are physically adsorbed on Fe 2 O 3 (001) and its reduced surfaces, which are all exothermic processes. As Fe 2 O 3 loses the lattice oxygen, the net charge obtained by the ethanol molecules first increases and then decreases. As lattice oxygen continues to decrease, the energy barrier of the decomposition reaction of ethanol on the reduced Fe 2 O 3 increases first, decreases, and then increases. The decomposition reaction energy barrier of the ethanol molecules on FeO(001) is the lowest and more active for catalytic decomposition of ethanol than other reduced surfaces. The results of the thermodynamic analysis show that ethanol molecules are more likely to produce hydrogen on FeO, which is consistent with the results of the kinetic analysis.

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Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.