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Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
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Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production Yanwei Zhang, Jiayi Lin, Zhihua Wang*, Rui Wang, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
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abstract
Article history:
Quantum chemistry calculations using a reasonably simplified char model were performed
Received 30 March 2016
to clarify the mechanisms of hydrogen iodide (HI) decomposition over carbon materials.
Received in revised form
The density functional theory at the B3LYP/3-21G** level was used to optimize the geom-
24 January 2017
etries of the reactants, products, stable intermediates, and transition states in possible
Accepted 27 January 2017
reaction pathways. The main elementary reactions of the homogeneous decomposition of
Available online xxx
HI were simulated to verify the applicability of the chosen calculation method and basis set. The adsorptions of HI on two prototypical model chars were all irreversible chemi-
Keywords:
sorption reactions. The results revealed that HI chemisorption preferentially occurred in
Sulfur-iodine cycle
the zigzag model compared with the energy barriers. Based on the chemisorption results,
Hydrogen production
the decomposition process of HI in the zigzag model was calculated at the same level. The
HI decomposition
process of HI decomposition on carbon materials took place not directly but by a series of
Carbon materials
chemisorption and desorption reactions, which demonstrated that the desorption of I2 and
Density functional theory
H2 controls overall catalysis reaction. The initial carbon structure disappeared and turned into the iodine absorbed structure, which played a real catalytic role in the overall process. The two structures were compared by analyzing their electrostatic potentials (ESPs), which demonstrated that the iodine absorbed structure presented a higher activity than the initial carbon structure. A detailed mechanism of the catalytic decomposition of HI over the carbon materials was proposed based on the pathways that we obtained from the calculation results. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is considered an alternative source of energy because of its environmentally friendly characteristics. An efficient hydrogen production system is necessary for developing a hydrogen energy system. Of the methods of largescale hydrogen production, water decomposition using
thermal energy is a promising technique. The sulfur-iodine (SI or IS) thermochemical cycle was first designed by the General Atomics Corporation in the 1970s [1]. The SI cycle involves three chemical reactions:
Bunsen reaction: SO2 þ H2O þ I2 / H2SO4 þ 2HI (T ¼ 293 Ke393 K);
* Corresponding author. Fax: þ86 571 87951616. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.ijhydene.2017.01.174 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Sulfuric acid decomposition: H2SO4 / SO2 þ H2O þ 0.5O2 (T ¼ 870 Ke1173 K); Hydrogen iodide decomposition: 2HI / H2 þ I2 (T ¼ 573 Ke823 K); The decomposition of hydrogen iodide (HI) serves as a step toward hydrogen evolution in several thermochemical watersplitting steps. However, the homogeneous gas-phase conversion during the decomposition is rather slow and occurs below 833 K. The use of catalysts is therefore desirable to accelerate the reaction rate. Early studies that sought to identify active catalysts were conducted in the late 1970s and 1980s and were summarized well by O'Keefe et al. [2]. A series of studies of activated carbon-based catalysts indicated that Pt catalysts have the best catalytic performance [3e10]. Activated carbon (AC) has also been studied in many catalysis reactions due to its high catalytic performance and stability. The performance of a catalyst depends on the availability of suitable active sites that are capable of chemisorbing the reactants and forming surface intermediates of adequate strength. In recent decades, numerous papers have focused on the structural characterization and nanostructures of AC [11e14]. Therefore, carbon materials such as AC are economical alternative catalysts for large-scale hydrogen production. The catalysis reaction mechanisms have a significant effect on studies of the catalytic decomposition on carbon materials. However, most conclusions about the mechanisms have been based on experimental results, which always consist of a catalytic characterization and an evaluation of the activities. The numerous characteristics and complex factors of homogeneous reaction (including the surface functional group, element compositions, surface area and porous structure) make it difficult to elucidate the reaction pathways of HI decomposition on carbon materials. Quantum chemistry calculations provide a theoretical method to fill this gap, which have proven to be of great utility for understanding reaction mechanisms. Quantum chemistry methods have been widely used in research of kinetics, electronic structure, and bond valance. In addition, there is increasing interest in modeling the structure and reactivity of carbon materials using first principles. Montoya et al. [15] studied the adsorption properties of CO2 on carbon structures and determined that, adsorption sites have a greater influence on the adsorption energy than the sizes of the carbon structures. Silva et al. [16] studied the adsorption properties of O2 on carbon structure using density functional theory (DFT), and found that O2 preferentially adsorbed on the edges of the armchair model with a lower reaction energy barrier than adsorption on the edges of the zigzag model. Karina et al. [17] used the zigzag model as the graphene model to study the mechanisms of O2 on an edge site of the zigzag model, including adsorption, migration and desorption, the resulting calculations were obtained at the B3LYP/6-31G(d) level. Frankcombe et al. [18] demonstrated that the unsaturated carbon atom at the edge was the most active site in the reaction with coke and O2. Stein et al. [19] compared the activities of the edge structures of the zigzag and armchair models, and demonstrated that the edge
sites of the zigzag model are more active than those of the armchair model, given the weaker electronic recombinant ability in the unstable edge structure of the zigzag model. Yang et al. [20] analyzed the change in activity of different structures that exists during the chemisorption of H2 on different planes of graphite. Espinal et al. [21] also studied the reaction rate of H2 with nitrogen containing carbonaceous materials, and indicated that the armchair model has a higher reaction rate than the zigzag model. However, research of HI decomposition on carbon materials has not yet been conducted. The purpose of this research is to perform molecular modeling using DFT to gain insight into the thermodynamics and mechanisms of HI during its decomposition on carbon materials. A reasonably simplified char model and possible reaction pathways were investigated using DFT. The characteristics of HI chemisorption on the edges of different char models are described, because of the importance of reactant adsorption on the catalysts during the process. Based on the chemisorption properties, the possible reaction pathway for HI decomposition on the zigzag model is discussed in detail. A detailed mechanism of the catalytic decomposition of HI over the carbon material at the molecular level is also established.
Computational details Choice of char model A reasonably simplified char model is important for substantially accurate results. Our previous study [22], which performed structural analyses of ACs and other amorphous carbons using various characterization techniques, provides a direct perspective for the quantitative analysis of reactive sites. These amorphous carbons generally form what can be described as a microcrystalline group, which can have different sizes and can be disorderly arranged [23,24]. As shown in Fig. 1, the structure of these groups primarily consists of small graphite crystallites that are stacked in parallel carbon net structures, and disordered carbons with weak inter-layer correlations or that are located at the edge of a carbon plane.
Fig. 1 e Model char of amorphous carbon.
Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Small graphite crystallites, which have a sp2/sp3 hybridization, are irregularly stacked in size and orientation by three to seven aromatic rings. The existence of a large p bond in the inerratic homocycle stabilizes the six-membered carbocyclic rings, whereas the carbon atoms on the edge are more active for reactions without the effect of the large p bond. The disordered carbons, which are aliphatic chain structures with a sp3 hybridization, are mostly saturated and have stable chemical properties. Therefore, the most likely active site of the carbon structure appears to be located at the edge of the microcrystalline graphite. Furthermore, the impurities can be ignored because all of the carbon materials used in the experimental research are processed to remove their functional groups. In the calculations, at the molecular level, we select the planar aromatic ring cluster to be the model char for simplicity. Research has indicated that the active sites located at the edges of the graphite carbon possess two main structures: zigzag and armchair. Based on the analysis that was presented above, the basal plane cluster C19H8 in the zigzag configuration and the cluster C22H10 in the armchair configuration [see structures in Fig. 2] were selected to model the microcrystalline graphite. Some of the carbon atoms are numbered for convenience. The zigzag model contains five aromatic rings with three sites that are vacant and available for chemisorption, whereas the armchair model contains six aromatic rings and two vacant sites. The other edge carbon atoms are terminated by hydrogen atoms.
Choice of calculation method The theoretical level of calculation can greatly influence the accuracy of results. The Becke 3 DFT (B3LYP) was used in this study with the 3-21G** basis set. Wang et al. [25] successfully calculated the fully optimized bonds length, CqV;m , Sqm and dipole moments of halogen and halide using 19 DFT methods, and determined that the results calculated by Becke 3 (B3LYP) parameter exchange functional method were the best. Considering the existence of iodine, only the 3-21G** bipolar basis set may be applied in the reaction simulation. In addition, spin contamination at the B3LYP level of theory has been shown to be reasonably small for carbonaceous materials [26]. All of the calculations included the following steps: geometric
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optimization for molecular structures to a minimum on the potential energy surface, frequency and single-point calculations to confirm the nature of all of the structures and obtain zero-point corrections, and intrinsic reaction coordinate (IRC) calculations at the same level to confirm the correct connection between all of the critical structures that are located on the potential energy surface [27]. The Gaussian 03 package was used in this study.
Results and discussion This study mainly analyzed the reaction paths, where the search for transition states plays an important role. The structure of the transition state was obtained by numerous continual tests based on the optimized reactants and products according to the reaction mechanism. The reasonable transition state was confirmed by the existence of only one imaginary frequency, which indicates that the eigenvector of the transition state corresponds to the indicated reaction. The IRC calculation was followed to confirm the connection between the reactant, intermediate and product. Because the assumption of a reasonable reaction mechanism was important, we began the quantum chemistry calculation from the homogeneous decomposition of HI and then studied the chemisorption of HI on two different model chars. Because our interest was to investigate HI decomposition on the edge of the carbon material, we chose the higher activity material to complete the integrated reaction process. For simplicity, the entire reaction was assumed to take place on one reaction path along with one transition state.
Mechanism of the homogeneous decomposition of HI Although 11 elementary reactions are involved in the homogeneous decomposition of HI, the three most important reactions were selected to conduct the quantum chemistry calculations. The reactions are shown by the following equations:
HI þ HI ¼ H2 þ I2
(1)
Fig. 2 e Model char molecules. Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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HI þ H ¼ H2 þ I
(2)
H þ I2 ¼ HI þ I
(3)
In our previous study of the homogeneous decomposition [5] of HI, we determined that Reaction (1) is the dominant reaction in the generation of H2 when the temperature is lower than 600 C, whereas Reaction (2) is dominant at higher temperatures. All of the atoms in the geometries of the three stationary points in each reaction are in the same plane (Fig. 3). Reaction (1) is an endothermic process that absorbs 23.47 kJ mol1 of heat, which means that higher temperatures promote the reaction. However, the high energy barrier of 184.45 kJ mol1 limits the HI decomposition rate. In Reaction (2), the free hydrogen atom is near the iodine atom of the HI molecule in the intermediate (M), but moves toward the hydrogen atom side to form the transition state (TS). Reactions (2) and (3) are both exothermic processes with energy barriers of 18.82 kJ mol1 and 89.11 kJ mol1 respectively. These two reactants contain the free H that is generated by Reaction (1), which means that both processes react more easily at high temperatures, and that the free H is more likely to interact with an HI rather than I2 because of its lower energy barrier. Therefore, the process still promotes the generation of H2 with a high-concentration of free H at high temperatures.
HI chemisorption on the edges of different model chars The chemisorption of the reactant on the catalysts is the most important step in the catalytic decomposition. As shown in
Fig. 4(a), the HI molecule attacked the middle vacant site (C2) of the zigzag model in the initial chemisorption process. The relatively stable intermediate (M) formed when the iodine atom and the hydrogen atom were located on both sides of C2. Next, the iodine atom moved closer to C2, whereas the HeI bond elongated and subsequently split. The CeI bond was formed, and the hydrogen atom became free, which led to the formation of the transition state (TS). The hydrogen atom then gradually moved toward the C3 vacant site. Finally, the stable CeH bond was formed, which led to the formation of P with all of the atoms of the three stationary points in the same plane. Fig. 4(b) shows the chemisorption path of HI in the other char model. The process began with the formation of M. The HI molecule moved vertically toward the carbon structure along the central line between the two vacant sites. Immediately after M had formed, the orientation of HI gradually became parallel to the two vacant sites, because the M structure was unstable. TS was subsequently formed. The HeI bond still existed in the TS, although the iodine atom was very close to the vacant sites. The hydrogen atom then moved toward its adjacent vacant side of C2 with the cleavage of the He I bond and the formation of the CeH bond. In addition, the iodine atom combined with the C1 vacant site to form the stable CeI bond, which represented the completion of the chemisorption of HI on the armchair structure with all of the atoms of the stationary points in the same plane. The potential energy surfaces of both processes were obtained (Fig. 5). Both chemisorption processes were exothermic reactions, and the heat that was released by the armchair
Fig. 3 e Reaction paths of the main elementary reactions including (a) HI þ HI ¼ H2 þ I2 (b) HI þ H ¼ H2 þ I and (c) H þ I2 ¼ HI þ I. The dashed lines indicate the elongation of bonds. Purple and green colors represent iodine and hydrogen atoms respectively. The energies barriers (DE e kJ mol¡1) and reaction enthalpies (DH e kJ mol¡1) are also shown. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Fig. 4 e Chemisorption paths of HI in (a) the zigzag model and (b) the armchair model.
model (309.89 kJ mol1) was slightly greater than that of the zigzag model (270.01 kJ mol1). The reaction energy barrier between M and TS was 115.46 kJ mol1 in the armchair model, which was higher than that in the zigzag model (51.09 kJ mol1). The adsorptions in the model are all irreversible chemisorption reactions, given the cleavage of the HeI bond and the formation of the CeI bond and the CeH bond. Compared with the energy barriers, the chemisorption of HI on the edge of the zigzag model occurs more easily. In other words, the unsaturated carbon atoms on the edge of the zigzag model are more active in the adsorption of HI.
HI catalytic decomposition along the char edge Research of HI decomposition on the model char plays an important role in the selection and preparation of catalysts for the HI decomposition reaction. As described above, the chemisorption activity of HI on the edges of the zigzag model is compared with the activity of the other model. Therefore, the research was performed using the zigzag model, which consists of five aromatic rings and three vacant sites. Fig. 6 shows the pathway for the decomposition of the HI molecule in the zigzag model. The entire process began with the chemisorption of HI and ended with the dissociation of H2. Two additional HI chemisorption processes and one I2 dissociation process occurred due to the existence of three vacant sites. To describe the reaction precisely, the entire process was divided into five steps. 1) Chemisorption of the first HI molecule This process was previously described in detail above. The iodine atom of HI was chemisorbed at the middle vacant site to form TS1. Next, the hydrogen atom moved toward the right vacant site to form the P1 state. 2) Chemisorption of the second HI molecule
Fig. 5 e Potential surfaces of HI chemisorption on the edges of the different model chars.
In P1, a hydrogen atom and an iodine atom were connected to each respective vacant site. The second HI was adsorbed on
Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Fig. 6 e HI decomposition path in the zigzag model.
this stable structure (Fig. 7). The P1 product contained an unsaturated vacant site, and the second HI moved toward this site to form the intermediate (M2). In M2, the HI and P1 were not in the same plane because the hydrogen atom was closer to the vacant site. Immediately after M had formed, the HI continued to move to the vacant site and closer to the iodine atom of P1, which led to the formation of TS2. In TS2, the HI and the C1 vacant site were located along the same line, but HI and P1 were still in different planes. The HeI bond then split, and the hydrogen atom combined with C1 to form a stable Ce H bond. The iodine atom moved toward the iodine atom, which led to the formation of P2. P2 was actually an intermediate (M3) because of the free state of the iodine atom.
atom that was already chemisorbed on the carbon structure. The I2 molecule then formed (Fig. 8). In M3, after some energy was absorbed, the CeI bond gradually elongated and subsequently split. The transition state (TS3) was formed with the combination of the IeI bond. Finally, the I2 molecule moved away from the carbon structure, which led to the formation of the stable product P3. In the elementary reaction of HI decomposition, two free iodine atoms may combine to generate the I2. However, I2 generated in this manner was rare because it would decrease the number of sites that possessed catalytic activities. Most of the I2 that was generated was based on the combination and desorption processes that are shown in Fig. 8.
3) Desorption of the I2 molecule
4) Chemisorption of the third HI molecule
As was described above, the product P2 was an intermediate (M3) in the entire desorption process because of its unstable state. The free iodine atom combined with the iodine
In product P3, the C2 vacant site became unsaturated after the dissociation of the I2. The third HI was absorbed on this site again (Fig. 9). The chemisorption process began with the
Fig. 7 e Chemisorption path of the second HI in the zigzag model. Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Fig. 8 e Desorption path of I2 in the zigzag model.
Fig. 9 e Chemisorption path of the third HI in the zigzag model.
approach of the third HI toward the carbon structure. The intermediate (M4) formed when the iodine atom gradually moved toward the C2 site. In M4, the HI was located along the central line of the carbon structure. The iodine atom continued to move toward the vacant site with the elongation of the HeI bond. The transition state (TS4) formed due to the formation of the CeI bond and the cleavage of the HeI bond. The hydrogen atom had a tendency to move to the C3 side of the carbon structure. The free hydrogen atom and the atom that was connected with the C3 site both moved away from the plane of the carbon structure. Finally, the free hydrogen atom moved to the C3 vacant site and combined with C3 to form the stable CeH bond. The C3 atom connected with two hydrogen atoms simultaneously to form the product P4.
5) Desorption of the H2 molecule This desorption represented the final step of the decomposition. In product P4, the two hydrogen atoms that were located on the edges of the zigzag model were desorbed to form H2 after absorbing the excitation energy (Fig. 10). In M5, the two hydrogen atoms that were connected to the carbon atom on C3, are symmetrical with respect to the plane of the char structure. Both CeH bonds split due to the absorption of excitation energy. The two hydrogen atoms combined to generate the H2 molecule, which led to the formation of the transition state (TS5). Finally, the stable product P5 formed as the H2 moved away from the carbon structure. After the complete desorption of H2, the carbon structure returned to
Fig. 10 e Desorption path of H2 in the zigzag model.
Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Table 1 e Relative changes in the energy of each step in the decomposition pathway. Step 1 2 3 4 5
Species M1 M2 M3 M4 M5
/ TS1 / TS2 / TS3 / TS4 / TS5
/ P1 / P2 / P3 / P4 / P5
Energy barrier (kJ mol1)
Reaction enthalpy (kJ mol1)
51.09 57.55 182.95 61.21 358.03
270.01 212.42 152.67 207.43 336.84
P1. This desorption is the end of a complete catalysis decomposition in which two HIs decomposed into one H2 and one I2, and one HI was adsorbed to generate P1. The energy barriers and the reaction enthalpies of each step in the decomposition of HI were calculated and are shown in Table 1. The product P1 plays an important role in the entire catalysis reaction, and the iodine atom that was absorbed on P1 exhibited a high promotion because of the benefit for the dissociation of I2. Throughout the entire reaction, the dissociation processes of H2 and I2 are both endothermic with high energy barriers. These dissociation
Fig. 11 e Electrostatic potentials (kcal/mol) of (a) the initial zigzag model and (b) the P1 structure.
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processes may be the determining steps in the entire reaction, especially the dissociation of the H2, which has the highest barrier of 358.03 kJ mol1. Related experimental research was reported in our previous work [28], which indicated that a Niimpregnated carbon structure demonstrated high catalytic activity in HI decomposition. This phenomenon may be caused by the decrease in the barrier energy of hydrogen desorption by a noble metal or transition metal, which improves the catalytic activities of the carbon materials in the HI decomposition reaction.
Reaction mechanisms The mechanism of HI decomposition on a carbon material may be described as follows:
HI þ I(*) / H(*) þ I þ I(*)
(4)
I þ I(*) / I2
(5) Fig. 12 e XPS patterns of the MWCNT before and after the reaction.
HI þ H(*) / I(*) þ H(*) þ H(*)
(6)
H(*) þ H(*) / H2
(7)
where I(*) and H(*) refer to the iodine and hydrogen atoms that are located on the model char, respectively. All of the equations can be reduced to one in which P1 plays the real catalytic role. P1
2HI!H2 þ I2
(8)
In the reaction, the initial carbon structure changes to a P1 structure with the I atom supported after the interaction with the HI molecule. This can be explained by the electrostatic potential (ESP) with a quantitative analysis of the molecular surface. The theoretical basis is that molecules always tend to approach each other in a complementary manner of ESP. An analysis of the ESP was performed on the molecular van der Waals (vdW) surface using the Multiwfn [29,30], which is a multifunctional wave function analyzer. Molecular HI is a polar molecules in which all of the electrons are located around the I atom and the endpoint of the H atom has a very high electrostatic potential. Therefore, the most negative ESP point of the catalytic structure will easily attract the HI molecule. As shown in Fig. 11, the unsaturated carbon of the P1 structure was more negative than those of the initial zigzag model. The HI molecule would attack the active carbon with the H atom facing toward the lowest ESP point of the P1 structure. The other positive ESP points at the ends of the H atom would not attract the I atom to form HI because of the high-HI-concentrations of both the initial structure and the P1 structure. This may explain how the next step was the desorption the of I2 molecule although all of the carbon atoms of the structure were saturated. It can be assumed that in the actual reaction process, the absorbing iodine atom will improve the activities of the carbon structure, which make it become the real catalyst.
This mechanism also explains the X-ray photoelectron spectroscopy (XPS) results in the practical experimental work, in which we characterized multiple-walled carbon nanotubes (MWCNT) as an example carbon material: before and after the catalysis reaction. Fig. 12 shows that there are two obvious peaks located at 618.9 eV and 630.7 eV in the post-reaction state, which correspond to I3d and I3d3 respectively. The existence of the I peak may be attributed to the change of the catalyst during the reaction, which corresponds to the P1 structure that was obtained from the quantum chemistry calculation. The decomposition of HI on the carbon material did not occur directly but rather by a series of chemisorption and desorption processes. Although the actual numbers of active sites that are available for the reaction on the carbon structure surface may vary, Reactions (5) and (7) reflect the key steps of this catalytic reaction, which control the reaction rate. The iodine that is absorbed on the surface of the carbon structure promotes HI decomposition, which was speculated by Favuzava et al. [7], while reducing the energy barrier of the dissociation of H2 can serves as an effective method to improve the catalytic activities of carbon materials.
Conclusions A systematic theoretical study using a hybrid B3LYP DFT was performed to provide insight into the mechanisms of HI decomposition on carbon materials. The homogeneous decomposition of HI was simulated to verify the applicability of the chosen calculation method and basis set. Two typical model chars were selected to represent small graphite crystallites, which have been demonstrated to possess the active sites in carbon materials. The adsorptions of HI on the model chars were all irreversible chemisorption reactions, because of the cleavage of the HeI bond. According to the energy
Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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barriers, the unsaturated carbon atoms on the edge of the zigzag model are more active in the chemisorption of HI. The reaction pathways and mechanisms of HI catalytic decomposition over carbon materials indicated that the process took place not directly but by a series of HI chemisorption and I2 and H2 desorption processes. After chemisorbing the first HI molecule, the initial carbon structure changed to become the iodine atom-supported structure. The active sites on the I absorbed structure had more negative electrostatic potentials, and attracted HI molecules more easily. They always existed and served as the real catalyst in the overall decomposition process, which was also verified by the result of the XPS characterization. Both the chemisorption and the desorption processes occurred on the edge sites of the carbon structure. The energy barriers for H2 desorption from the catalytic structure formed the highest step in the mechanisms. The results suggest that the catalytic performance of carbon materials may be improved by increasing the percent of carbon atoms at the edge, and impregnating a noble or transition metal that promotes hydrogen evolution.
Acknowledgment This work has been financially supported by the National Natural Science Foundation of China (51276170). The authors gratefully acknowledge the support.
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Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production Yanwei Zhang, Jiayi Lin, Zhihua Wang*, Rui Wang, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
article info
abstract
Article history:
Quantum chemistry calculations using a reasonably simplified char model were performed
Received 30 March 2016
to clarify the mechanisms of hydrogen iodide (HI) decomposition over carbon materials.
Received in revised form
The density functional theory at the B3LYP/3-21G** level was used to optimize the geom-
24 January 2017
etries of the reactants, products, stable intermediates, and transition states in possible
Accepted 27 January 2017
reaction pathways. The main elementary reactions of the homogeneous decomposition of
Available online xxx
HI were simulated to verify the applicability of the chosen calculation method and basis set. The adsorptions of HI on two prototypical model chars were all irreversible chemi-
Keywords:
sorption reactions. The results revealed that HI chemisorption preferentially occurred in
Sulfur-iodine cycle
the zigzag model compared with the energy barriers. Based on the chemisorption results,
Hydrogen production
the decomposition process of HI in the zigzag model was calculated at the same level. The
HI decomposition
process of HI decomposition on carbon materials took place not directly but by a series of
Carbon materials
chemisorption and desorption reactions, which demonstrated that the desorption of I2 and
Density functional theory
H2 controls overall catalysis reaction. The initial carbon structure disappeared and turned into the iodine absorbed structure, which played a real catalytic role in the overall process. The two structures were compared by analyzing their electrostatic potentials (ESPs), which demonstrated that the iodine absorbed structure presented a higher activity than the initial carbon structure. A detailed mechanism of the catalytic decomposition of HI over the carbon materials was proposed based on the pathways that we obtained from the calculation results. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is considered an alternative source of energy because of its environmentally friendly characteristics. An efficient hydrogen production system is necessary for developing a hydrogen energy system. Of the methods of largescale hydrogen production, water decomposition using
thermal energy is a promising technique. The sulfur-iodine (SI or IS) thermochemical cycle was first designed by the General Atomics Corporation in the 1970s [1]. The SI cycle involves three chemical reactions:
Bunsen reaction: SO2 þ H2O þ I2 / H2SO4 þ 2HI (T ¼ 293 Ke393 K);
* Corresponding author. Fax: þ86 571 87951616. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.ijhydene.2017.01.174 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Sulfuric acid decomposition: H2SO4 / SO2 þ H2O þ 0.5O2 (T ¼ 870 Ke1173 K); Hydrogen iodide decomposition: 2HI / H2 þ I2 (T ¼ 573 Ke823 K); The decomposition of hydrogen iodide (HI) serves as a step toward hydrogen evolution in several thermochemical watersplitting steps. However, the homogeneous gas-phase conversion during the decomposition is rather slow and occurs below 833 K. The use of catalysts is therefore desirable to accelerate the reaction rate. Early studies that sought to identify active catalysts were conducted in the late 1970s and 1980s and were summarized well by O'Keefe et al. [2]. A series of studies of activated carbon-based catalysts indicated that Pt catalysts have the best catalytic performance [3e10]. Activated carbon (AC) has also been studied in many catalysis reactions due to its high catalytic performance and stability. The performance of a catalyst depends on the availability of suitable active sites that are capable of chemisorbing the reactants and forming surface intermediates of adequate strength. In recent decades, numerous papers have focused on the structural characterization and nanostructures of AC [11e14]. Therefore, carbon materials such as AC are economical alternative catalysts for large-scale hydrogen production. The catalysis reaction mechanisms have a significant effect on studies of the catalytic decomposition on carbon materials. However, most conclusions about the mechanisms have been based on experimental results, which always consist of a catalytic characterization and an evaluation of the activities. The numerous characteristics and complex factors of homogeneous reaction (including the surface functional group, element compositions, surface area and porous structure) make it difficult to elucidate the reaction pathways of HI decomposition on carbon materials. Quantum chemistry calculations provide a theoretical method to fill this gap, which have proven to be of great utility for understanding reaction mechanisms. Quantum chemistry methods have been widely used in research of kinetics, electronic structure, and bond valance. In addition, there is increasing interest in modeling the structure and reactivity of carbon materials using first principles. Montoya et al. [15] studied the adsorption properties of CO2 on carbon structures and determined that, adsorption sites have a greater influence on the adsorption energy than the sizes of the carbon structures. Silva et al. [16] studied the adsorption properties of O2 on carbon structure using density functional theory (DFT), and found that O2 preferentially adsorbed on the edges of the armchair model with a lower reaction energy barrier than adsorption on the edges of the zigzag model. Karina et al. [17] used the zigzag model as the graphene model to study the mechanisms of O2 on an edge site of the zigzag model, including adsorption, migration and desorption, the resulting calculations were obtained at the B3LYP/6-31G(d) level. Frankcombe et al. [18] demonstrated that the unsaturated carbon atom at the edge was the most active site in the reaction with coke and O2. Stein et al. [19] compared the activities of the edge structures of the zigzag and armchair models, and demonstrated that the edge
sites of the zigzag model are more active than those of the armchair model, given the weaker electronic recombinant ability in the unstable edge structure of the zigzag model. Yang et al. [20] analyzed the change in activity of different structures that exists during the chemisorption of H2 on different planes of graphite. Espinal et al. [21] also studied the reaction rate of H2 with nitrogen containing carbonaceous materials, and indicated that the armchair model has a higher reaction rate than the zigzag model. However, research of HI decomposition on carbon materials has not yet been conducted. The purpose of this research is to perform molecular modeling using DFT to gain insight into the thermodynamics and mechanisms of HI during its decomposition on carbon materials. A reasonably simplified char model and possible reaction pathways were investigated using DFT. The characteristics of HI chemisorption on the edges of different char models are described, because of the importance of reactant adsorption on the catalysts during the process. Based on the chemisorption properties, the possible reaction pathway for HI decomposition on the zigzag model is discussed in detail. A detailed mechanism of the catalytic decomposition of HI over the carbon material at the molecular level is also established.
Computational details Choice of char model A reasonably simplified char model is important for substantially accurate results. Our previous study [22], which performed structural analyses of ACs and other amorphous carbons using various characterization techniques, provides a direct perspective for the quantitative analysis of reactive sites. These amorphous carbons generally form what can be described as a microcrystalline group, which can have different sizes and can be disorderly arranged [23,24]. As shown in Fig. 1, the structure of these groups primarily consists of small graphite crystallites that are stacked in parallel carbon net structures, and disordered carbons with weak inter-layer correlations or that are located at the edge of a carbon plane.
Fig. 1 e Model char of amorphous carbon.
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Small graphite crystallites, which have a sp2/sp3 hybridization, are irregularly stacked in size and orientation by three to seven aromatic rings. The existence of a large p bond in the inerratic homocycle stabilizes the six-membered carbocyclic rings, whereas the carbon atoms on the edge are more active for reactions without the effect of the large p bond. The disordered carbons, which are aliphatic chain structures with a sp3 hybridization, are mostly saturated and have stable chemical properties. Therefore, the most likely active site of the carbon structure appears to be located at the edge of the microcrystalline graphite. Furthermore, the impurities can be ignored because all of the carbon materials used in the experimental research are processed to remove their functional groups. In the calculations, at the molecular level, we select the planar aromatic ring cluster to be the model char for simplicity. Research has indicated that the active sites located at the edges of the graphite carbon possess two main structures: zigzag and armchair. Based on the analysis that was presented above, the basal plane cluster C19H8 in the zigzag configuration and the cluster C22H10 in the armchair configuration [see structures in Fig. 2] were selected to model the microcrystalline graphite. Some of the carbon atoms are numbered for convenience. The zigzag model contains five aromatic rings with three sites that are vacant and available for chemisorption, whereas the armchair model contains six aromatic rings and two vacant sites. The other edge carbon atoms are terminated by hydrogen atoms.
Choice of calculation method The theoretical level of calculation can greatly influence the accuracy of results. The Becke 3 DFT (B3LYP) was used in this study with the 3-21G** basis set. Wang et al. [25] successfully calculated the fully optimized bonds length, CqV;m , Sqm and dipole moments of halogen and halide using 19 DFT methods, and determined that the results calculated by Becke 3 (B3LYP) parameter exchange functional method were the best. Considering the existence of iodine, only the 3-21G** bipolar basis set may be applied in the reaction simulation. In addition, spin contamination at the B3LYP level of theory has been shown to be reasonably small for carbonaceous materials [26]. All of the calculations included the following steps: geometric
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optimization for molecular structures to a minimum on the potential energy surface, frequency and single-point calculations to confirm the nature of all of the structures and obtain zero-point corrections, and intrinsic reaction coordinate (IRC) calculations at the same level to confirm the correct connection between all of the critical structures that are located on the potential energy surface [27]. The Gaussian 03 package was used in this study.
Results and discussion This study mainly analyzed the reaction paths, where the search for transition states plays an important role. The structure of the transition state was obtained by numerous continual tests based on the optimized reactants and products according to the reaction mechanism. The reasonable transition state was confirmed by the existence of only one imaginary frequency, which indicates that the eigenvector of the transition state corresponds to the indicated reaction. The IRC calculation was followed to confirm the connection between the reactant, intermediate and product. Because the assumption of a reasonable reaction mechanism was important, we began the quantum chemistry calculation from the homogeneous decomposition of HI and then studied the chemisorption of HI on two different model chars. Because our interest was to investigate HI decomposition on the edge of the carbon material, we chose the higher activity material to complete the integrated reaction process. For simplicity, the entire reaction was assumed to take place on one reaction path along with one transition state.
Mechanism of the homogeneous decomposition of HI Although 11 elementary reactions are involved in the homogeneous decomposition of HI, the three most important reactions were selected to conduct the quantum chemistry calculations. The reactions are shown by the following equations:
HI þ HI ¼ H2 þ I2
(1)
Fig. 2 e Model char molecules. Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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HI þ H ¼ H2 þ I
(2)
H þ I2 ¼ HI þ I
(3)
In our previous study of the homogeneous decomposition [5] of HI, we determined that Reaction (1) is the dominant reaction in the generation of H2 when the temperature is lower than 600 C, whereas Reaction (2) is dominant at higher temperatures. All of the atoms in the geometries of the three stationary points in each reaction are in the same plane (Fig. 3). Reaction (1) is an endothermic process that absorbs 23.47 kJ mol1 of heat, which means that higher temperatures promote the reaction. However, the high energy barrier of 184.45 kJ mol1 limits the HI decomposition rate. In Reaction (2), the free hydrogen atom is near the iodine atom of the HI molecule in the intermediate (M), but moves toward the hydrogen atom side to form the transition state (TS). Reactions (2) and (3) are both exothermic processes with energy barriers of 18.82 kJ mol1 and 89.11 kJ mol1 respectively. These two reactants contain the free H that is generated by Reaction (1), which means that both processes react more easily at high temperatures, and that the free H is more likely to interact with an HI rather than I2 because of its lower energy barrier. Therefore, the process still promotes the generation of H2 with a high-concentration of free H at high temperatures.
HI chemisorption on the edges of different model chars The chemisorption of the reactant on the catalysts is the most important step in the catalytic decomposition. As shown in
Fig. 4(a), the HI molecule attacked the middle vacant site (C2) of the zigzag model in the initial chemisorption process. The relatively stable intermediate (M) formed when the iodine atom and the hydrogen atom were located on both sides of C2. Next, the iodine atom moved closer to C2, whereas the HeI bond elongated and subsequently split. The CeI bond was formed, and the hydrogen atom became free, which led to the formation of the transition state (TS). The hydrogen atom then gradually moved toward the C3 vacant site. Finally, the stable CeH bond was formed, which led to the formation of P with all of the atoms of the three stationary points in the same plane. Fig. 4(b) shows the chemisorption path of HI in the other char model. The process began with the formation of M. The HI molecule moved vertically toward the carbon structure along the central line between the two vacant sites. Immediately after M had formed, the orientation of HI gradually became parallel to the two vacant sites, because the M structure was unstable. TS was subsequently formed. The HeI bond still existed in the TS, although the iodine atom was very close to the vacant sites. The hydrogen atom then moved toward its adjacent vacant side of C2 with the cleavage of the He I bond and the formation of the CeH bond. In addition, the iodine atom combined with the C1 vacant site to form the stable CeI bond, which represented the completion of the chemisorption of HI on the armchair structure with all of the atoms of the stationary points in the same plane. The potential energy surfaces of both processes were obtained (Fig. 5). Both chemisorption processes were exothermic reactions, and the heat that was released by the armchair
Fig. 3 e Reaction paths of the main elementary reactions including (a) HI þ HI ¼ H2 þ I2 (b) HI þ H ¼ H2 þ I and (c) H þ I2 ¼ HI þ I. The dashed lines indicate the elongation of bonds. Purple and green colors represent iodine and hydrogen atoms respectively. The energies barriers (DE e kJ mol¡1) and reaction enthalpies (DH e kJ mol¡1) are also shown. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Fig. 4 e Chemisorption paths of HI in (a) the zigzag model and (b) the armchair model.
model (309.89 kJ mol1) was slightly greater than that of the zigzag model (270.01 kJ mol1). The reaction energy barrier between M and TS was 115.46 kJ mol1 in the armchair model, which was higher than that in the zigzag model (51.09 kJ mol1). The adsorptions in the model are all irreversible chemisorption reactions, given the cleavage of the HeI bond and the formation of the CeI bond and the CeH bond. Compared with the energy barriers, the chemisorption of HI on the edge of the zigzag model occurs more easily. In other words, the unsaturated carbon atoms on the edge of the zigzag model are more active in the adsorption of HI.
HI catalytic decomposition along the char edge Research of HI decomposition on the model char plays an important role in the selection and preparation of catalysts for the HI decomposition reaction. As described above, the chemisorption activity of HI on the edges of the zigzag model is compared with the activity of the other model. Therefore, the research was performed using the zigzag model, which consists of five aromatic rings and three vacant sites. Fig. 6 shows the pathway for the decomposition of the HI molecule in the zigzag model. The entire process began with the chemisorption of HI and ended with the dissociation of H2. Two additional HI chemisorption processes and one I2 dissociation process occurred due to the existence of three vacant sites. To describe the reaction precisely, the entire process was divided into five steps. 1) Chemisorption of the first HI molecule This process was previously described in detail above. The iodine atom of HI was chemisorbed at the middle vacant site to form TS1. Next, the hydrogen atom moved toward the right vacant site to form the P1 state. 2) Chemisorption of the second HI molecule
Fig. 5 e Potential surfaces of HI chemisorption on the edges of the different model chars.
In P1, a hydrogen atom and an iodine atom were connected to each respective vacant site. The second HI was adsorbed on
Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Fig. 6 e HI decomposition path in the zigzag model.
this stable structure (Fig. 7). The P1 product contained an unsaturated vacant site, and the second HI moved toward this site to form the intermediate (M2). In M2, the HI and P1 were not in the same plane because the hydrogen atom was closer to the vacant site. Immediately after M had formed, the HI continued to move to the vacant site and closer to the iodine atom of P1, which led to the formation of TS2. In TS2, the HI and the C1 vacant site were located along the same line, but HI and P1 were still in different planes. The HeI bond then split, and the hydrogen atom combined with C1 to form a stable Ce H bond. The iodine atom moved toward the iodine atom, which led to the formation of P2. P2 was actually an intermediate (M3) because of the free state of the iodine atom.
atom that was already chemisorbed on the carbon structure. The I2 molecule then formed (Fig. 8). In M3, after some energy was absorbed, the CeI bond gradually elongated and subsequently split. The transition state (TS3) was formed with the combination of the IeI bond. Finally, the I2 molecule moved away from the carbon structure, which led to the formation of the stable product P3. In the elementary reaction of HI decomposition, two free iodine atoms may combine to generate the I2. However, I2 generated in this manner was rare because it would decrease the number of sites that possessed catalytic activities. Most of the I2 that was generated was based on the combination and desorption processes that are shown in Fig. 8.
3) Desorption of the I2 molecule
4) Chemisorption of the third HI molecule
As was described above, the product P2 was an intermediate (M3) in the entire desorption process because of its unstable state. The free iodine atom combined with the iodine
In product P3, the C2 vacant site became unsaturated after the dissociation of the I2. The third HI was absorbed on this site again (Fig. 9). The chemisorption process began with the
Fig. 7 e Chemisorption path of the second HI in the zigzag model. Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
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Fig. 8 e Desorption path of I2 in the zigzag model.
Fig. 9 e Chemisorption path of the third HI in the zigzag model.
approach of the third HI toward the carbon structure. The intermediate (M4) formed when the iodine atom gradually moved toward the C2 site. In M4, the HI was located along the central line of the carbon structure. The iodine atom continued to move toward the vacant site with the elongation of the HeI bond. The transition state (TS4) formed due to the formation of the CeI bond and the cleavage of the HeI bond. The hydrogen atom had a tendency to move to the C3 side of the carbon structure. The free hydrogen atom and the atom that was connected with the C3 site both moved away from the plane of the carbon structure. Finally, the free hydrogen atom moved to the C3 vacant site and combined with C3 to form the stable CeH bond. The C3 atom connected with two hydrogen atoms simultaneously to form the product P4.
5) Desorption of the H2 molecule This desorption represented the final step of the decomposition. In product P4, the two hydrogen atoms that were located on the edges of the zigzag model were desorbed to form H2 after absorbing the excitation energy (Fig. 10). In M5, the two hydrogen atoms that were connected to the carbon atom on C3, are symmetrical with respect to the plane of the char structure. Both CeH bonds split due to the absorption of excitation energy. The two hydrogen atoms combined to generate the H2 molecule, which led to the formation of the transition state (TS5). Finally, the stable product P5 formed as the H2 moved away from the carbon structure. After the complete desorption of H2, the carbon structure returned to
Fig. 10 e Desorption path of H2 in the zigzag model.
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Table 1 e Relative changes in the energy of each step in the decomposition pathway. Step 1 2 3 4 5
Species M1 M2 M3 M4 M5
/ TS1 / TS2 / TS3 / TS4 / TS5
/ P1 / P2 / P3 / P4 / P5
Energy barrier (kJ mol1)
Reaction enthalpy (kJ mol1)
51.09 57.55 182.95 61.21 358.03
270.01 212.42 152.67 207.43 336.84
P1. This desorption is the end of a complete catalysis decomposition in which two HIs decomposed into one H2 and one I2, and one HI was adsorbed to generate P1. The energy barriers and the reaction enthalpies of each step in the decomposition of HI were calculated and are shown in Table 1. The product P1 plays an important role in the entire catalysis reaction, and the iodine atom that was absorbed on P1 exhibited a high promotion because of the benefit for the dissociation of I2. Throughout the entire reaction, the dissociation processes of H2 and I2 are both endothermic with high energy barriers. These dissociation
Fig. 11 e Electrostatic potentials (kcal/mol) of (a) the initial zigzag model and (b) the P1 structure.
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processes may be the determining steps in the entire reaction, especially the dissociation of the H2, which has the highest barrier of 358.03 kJ mol1. Related experimental research was reported in our previous work [28], which indicated that a Niimpregnated carbon structure demonstrated high catalytic activity in HI decomposition. This phenomenon may be caused by the decrease in the barrier energy of hydrogen desorption by a noble metal or transition metal, which improves the catalytic activities of the carbon materials in the HI decomposition reaction.
Reaction mechanisms The mechanism of HI decomposition on a carbon material may be described as follows:
HI þ I(*) / H(*) þ I þ I(*)
(4)
I þ I(*) / I2
(5) Fig. 12 e XPS patterns of the MWCNT before and after the reaction.
HI þ H(*) / I(*) þ H(*) þ H(*)
(6)
H(*) þ H(*) / H2
(7)
where I(*) and H(*) refer to the iodine and hydrogen atoms that are located on the model char, respectively. All of the equations can be reduced to one in which P1 plays the real catalytic role. P1
2HI!H2 þ I2
(8)
In the reaction, the initial carbon structure changes to a P1 structure with the I atom supported after the interaction with the HI molecule. This can be explained by the electrostatic potential (ESP) with a quantitative analysis of the molecular surface. The theoretical basis is that molecules always tend to approach each other in a complementary manner of ESP. An analysis of the ESP was performed on the molecular van der Waals (vdW) surface using the Multiwfn [29,30], which is a multifunctional wave function analyzer. Molecular HI is a polar molecules in which all of the electrons are located around the I atom and the endpoint of the H atom has a very high electrostatic potential. Therefore, the most negative ESP point of the catalytic structure will easily attract the HI molecule. As shown in Fig. 11, the unsaturated carbon of the P1 structure was more negative than those of the initial zigzag model. The HI molecule would attack the active carbon with the H atom facing toward the lowest ESP point of the P1 structure. The other positive ESP points at the ends of the H atom would not attract the I atom to form HI because of the high-HI-concentrations of both the initial structure and the P1 structure. This may explain how the next step was the desorption the of I2 molecule although all of the carbon atoms of the structure were saturated. It can be assumed that in the actual reaction process, the absorbing iodine atom will improve the activities of the carbon structure, which make it become the real catalyst.
This mechanism also explains the X-ray photoelectron spectroscopy (XPS) results in the practical experimental work, in which we characterized multiple-walled carbon nanotubes (MWCNT) as an example carbon material: before and after the catalysis reaction. Fig. 12 shows that there are two obvious peaks located at 618.9 eV and 630.7 eV in the post-reaction state, which correspond to I3d and I3d3 respectively. The existence of the I peak may be attributed to the change of the catalyst during the reaction, which corresponds to the P1 structure that was obtained from the quantum chemistry calculation. The decomposition of HI on the carbon material did not occur directly but rather by a series of chemisorption and desorption processes. Although the actual numbers of active sites that are available for the reaction on the carbon structure surface may vary, Reactions (5) and (7) reflect the key steps of this catalytic reaction, which control the reaction rate. The iodine that is absorbed on the surface of the carbon structure promotes HI decomposition, which was speculated by Favuzava et al. [7], while reducing the energy barrier of the dissociation of H2 can serves as an effective method to improve the catalytic activities of carbon materials.
Conclusions A systematic theoretical study using a hybrid B3LYP DFT was performed to provide insight into the mechanisms of HI decomposition on carbon materials. The homogeneous decomposition of HI was simulated to verify the applicability of the chosen calculation method and basis set. Two typical model chars were selected to represent small graphite crystallites, which have been demonstrated to possess the active sites in carbon materials. The adsorptions of HI on the model chars were all irreversible chemisorption reactions, because of the cleavage of the HeI bond. According to the energy
Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
barriers, the unsaturated carbon atoms on the edge of the zigzag model are more active in the chemisorption of HI. The reaction pathways and mechanisms of HI catalytic decomposition over carbon materials indicated that the process took place not directly but by a series of HI chemisorption and I2 and H2 desorption processes. After chemisorbing the first HI molecule, the initial carbon structure changed to become the iodine atom-supported structure. The active sites on the I absorbed structure had more negative electrostatic potentials, and attracted HI molecules more easily. They always existed and served as the real catalyst in the overall decomposition process, which was also verified by the result of the XPS characterization. Both the chemisorption and the desorption processes occurred on the edge sites of the carbon structure. The energy barriers for H2 desorption from the catalytic structure formed the highest step in the mechanisms. The results suggest that the catalytic performance of carbon materials may be improved by increasing the percent of carbon atoms at the edge, and impregnating a noble or transition metal that promotes hydrogen evolution.
Acknowledgment This work has been financially supported by the National Natural Science Foundation of China (51276170). The authors gratefully acknowledge the support.
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Please cite this article in press as: Zhang Y, et al., Study of the mechanism of the catalytic decomposition of hydrogen iodide (HI) over carbon materials for hydrogen production, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.01.174