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Enhanced catalytic behavior of Ni alloys in steam methane reforming
Journal of Power Sources 359 (2017) 450e457
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhanced catalytic behavior of Ni alloys in steam methane reforming Yeongpil Yoon, Hanmi Kim, Jaichan Lee* School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
DFT calculation was used to understand steam methane reforming on Ni alloys. CO gas-evolving reaction suppresses carbon deposition. CO gas-evolving reaction enhances the catalytic efficiency of H2 generation. Ni-Ru alloy is an effective way to facilitate the CO gas-evolving reaction.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 April 2017 Received in revised form 19 May 2017 Accepted 22 May 2017 Available online 3 June 2017
The dissociation process of methane on Ni and Ni alloys are investigated by density functional theory (DFT) in terms of catalytic efficiency and carbon deposition. Examining the dissociation to CH3, CH2, CH, C, and H is not sufficient to properly predict the catalytic efficiency and carbon deposition, and further investigation of the CO gas-evolving reaction is required to completely understand methane dissociation in steam. The location of alloying element in Ni alloy needed be addressed from the results of ab-inito molecular dynamics (MD). The reaction pathway of methane dissociation associated with CO gas evolution is traced by performing first-principles calculations of the adsorption and activation energies of each dissociation step. During the dissociation process, two alternative reaction steps producing adsorbed C and H or adsorbed CO are critically important in determining coking inhibition as well as H2 gas evolution (i.e., the catalytic efficiency). The theoretical calculations presented here suggest that alloying Ni with Ru is an effective way to reduce carbon deposition and enhance the catalytic efficiency of H2 fueling in solid oxide fuel cells (SOFCs). © 2017 Elsevier B.V. All rights reserved.
Keywords: Ni alloy anode Methane reforming Carbon deposition Hydrogen gas evolution Density functional theory
1. Introduction Fuel cells are energy-conversion devices that directly transform the chemical energy of a fuel into electricity with high efficiency; furthermore, they are environmentally friendly and can use a variety of fuels including hydrogen [1,2]. Among the fuel cells available, solid oxide fuel cells (SOFCs) are most commonly used because of their high power-conversion efficiency. SOFCs must
* Corresponding author. E-mail address: [email protected] (J. Lee). http://dx.doi.org/10.1016/j.jpowsour.2017.05.076 0378-7753/© 2017 Elsevier B.V. All rights reserved.
operate at high temperatures ranging from 500 C to 1000 C. However, despite such high operation temperatures, SOFCs' anodes are not seriously affected by CO poisoning during operation [3e9]. Hydrogen gas generation is an important process for achieving highly efficient SOFCs. Hydrogen gas is the main fuel for SOFCs and is generated at the anode during methane reforming reaction [10e16]; Carbon is formed as a byproduct. When the byproduct carbon is deposited on the anode surface, the effective area for the reaction is reduced, which seriously hampers H2 generation [17e22]. Therefore, preventing the carbon deposition has been a critical issue in SOFC catalysts, while Ni-based catalyst has been widely used. Many catalysts, including bimetallic catalysts, have
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been proposed to resolve this issue. In bimetallic catalysts, Ni has been alloyed with the following transition metals: Ni-Cu [23,24], Ni-Co [25], Ni-Au [26e28], Ni-Pt [29], Ni-Rh [30], Ni-Pd [31], and Ni-M surface (M ¼ Cu, Ru, Rh, Pd, Ag, Pt, and Au) [32]. These studies suggested that coking inhibition was controlled in the step 4 of the following reaction process during the methane-reforming reaction [23e32]: Step 1: CH*4 / CH*3 þ H* Step 2: CH*3 / CH*2 þ H* Step 3: CH*2 / CH* þ H* Step 4: CH* / C* þ H* In the case, the reaction from adsorbed carbon to carbon cluster leads to the carbon deposition as a byproduct of the methane decomposition, in which the activation energy of carbon diffusion would be important in the carbon deposition [33]. However, the methane decomposition is commonly performed in steam. Blaylock et al. suggested that additional dissociation reactions occur, i.e., steam reforming on a multi-faceted Ni surface [34,35]. Among many allowed reaction paths in the steamreforming reaction, the additional following steps including the steps 1e4 were suggested as a primary reaction path for methane decomposition that may further influence carbon deposition: Step 5: CH* þ O* / CHO* Step 6: CHO* / CO* þ H* Step 7: CO* / CO(g) In the reactions considered above (steps1e7), the reaction step 4 or 5 could be an alternative one. After step 3, the steps 5e7 can be, in other words, substituted for step 4, thereby preventing the carbon deposition that occurs in the step 4 and instead producing CO gas. CO gas evolution (steps 5 to 7) can also affect the catalytic efficiency of H2 generation. Therefore, understanding the reaction pathway including steps 5e7 is essential to reduce carbon deposition and enhance the catalytic efficiency of the methanereforming reaction. Previous theoretical studies considering steps 1e4 showed that among several bimetallic catalysts, Ni-Rh bimetallic catalysts have great potential to reduce carbon deposition [32]. However, experimental studies revealed that Ni-Ru bimetallic catalysts exhibited more efficient coking inhibition than Ni-Rh bimetallic catalysts [36]. Theoretical studies considering only steps 1e4 could generate results that contradict experimental observations. Therefore, further detailed studies are needed to understand the methanereforming reaction that includes CO gas-evolving steps, which can significantly influence the carbon deposition and catalytic efficiency. Furthermore, the previous theoretical studies [23e32] using bimetallic catalysts assumed the location of the alloying elements only on a host metal surface although their location could influence the reaction. However, it is reported that the location of the alloying elements depends on the kind of alloying element relative to a host metal [37,38]. In fact, in Ni-Rh bimetallic catalysts, Rh atoms tended to be located at the surface of Ni [37], whereas Ru atoms in Pt were located at the surface or underneath [38]. This concern necessitates the investigation of the location of alloying element in an alloy as well. In this study, we performed first-principles calculations to investigate the methane-reforming reaction involving steps 1e7 on
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pure Ni and Ni-Ru and Ni-Rh bimetals in terms of carbon deposition and the efficiency of the Ni catalyst. The nudged elastic band (NEB) method was used to calculate the energy barrier at each step and examine the catalytic efficiency of bimetal alloying. Vibration frequency analysis was performed to verify transition states and zero point energy (ZPE) was calculated to examine its influence on the adsorption and activation energies. In addition, ab-initio molecular dynamics (MD) was used to locate alloying element in the bimetals. Our result shows that the activation energy of step 4 is much higher than that of step 5, and thus, step 4 can be substituted by step 5. Therefore, the CO gas-evolving reaction significantly affects the carbon deposition and catalytic efficiency. Furthermore, alloying Ni with Ru results in a lower activation energy than those observed for pure Ni and Ni-Rh bimetal in step 5. As a result, step 5 and the CO gas-evolving reaction are accelerated, leading to reduced carbon deposition. Moreover, the activation energies of steps 1e3 are progressively reduced by Ni-Ru bimetal, leading to the rapid depletion of the reaction product of each step and facilitating fast forward reactions. This result indicates that Ni-Ru bimetal also has enhanced catalytic efficiency for H2 generation as well as coke inhibition. Furthermore, this result remains unchanged when exposed to an electric field. 2. Computational details 2.1. Method All calculations were performed using the Vienna Ab-initio Simulation Package (VASP) [39e41]. The interactions between valence electrons and ion cores were treated by the Projector Augmented Wave (PAW) method [42,43]. The exchange correlation functional was the Generalized Gradient Approximation with the PerdeweBurkeeErnzerhof functional (GGA-PBE) [44]. The MonkhorstePack scheme was used to generate the k-points set of 4 4 1 in k-space [45]. An energy cutoff of 400 eV was used for the plane wave basis set in all calculations. The geometries were optimized until the energy converged to 1.0 104 eV/atom, and the force converged to 0.03 eV/Å. Because of the presence of magnetic atoms, spin polarization was considered in all calculations. The adsorption energies without zero point energies, DEads , were calculated by the following relation:
DEads ¼ Eadsorbate=sub Eadsorbate Esub
(1)
Eadsorbate/sub, Eadsorbate, and Esub represent the total energies of the optimized substrate with the adsorbate, the isolated adsorbate molecule, and the clean surface, respectively. A negative DEads represents exothermic chemisorption, and a positive DEads indicates endothermic chemisorption. As the magnitude of DEads increases, the binding of the adsorbate to the surface becomes stronger. In order to investigate the effect of the zero point energy on the decomposition reactions [46], the calculation was carried with ZPE , which modout without and with zero point energies, DEads ifies the adsorption energies as follows:
with ZPE DEads ¼ Eadsorbate=sub Eadsorbate Esub þ DEZPE
DEZPE ¼
X hvadsorbate=sub 2
X hv
adsorbate
2
(2)
(3)
where Eadsorbate/sub, Eadsorbate, and Esub are same in Eq. (1) and DEZPE is the zero point energy (ZPE) correction, i.e., the difference in the zero point energy between the adsorbed system and the gas phase. vadsorbate=sub and vadsorbate are vibrational frequencies of the adsorbate and isolated molecule, respectively.
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The NEB method was used to locate the transition state (TS) of methane dissociation, and the most stable adsorption configurations were employed as the initial and final states [47,48]. The transition states were verified by vibrational frequency analysis, which shows unstable modes at the transition states (Table S1). The activation energies, DEa , were calculated by the following relation:
DEa ¼ ESP EIS
(4)
EIS and ESP represent the energies of the initial (before reaction) state and the transient state (saddle point) obtained by NEB method, respectively. 2.2. Model The Ni(111) surface is reported to be more active than the Ni(100) and Ni(110) surfaces for the methane-reforming reaction [49]. Therefore, we studied the reaction mechanism on the (111) surface. Figure 1 shows a Ni(111) slab with several adsorption sites. The lattice parameter of pure Ni (3.52 Å) obtained theoretically [26,33] and experimentally [50] was used in the slab calculation. The supercell consists of 6 layers with a p(2 2) surface and a vacuum layer. In the supercell, the first bottom layer of the slab was fixed, and the second and third layers from the bottom were fixed only along the z-axis; the top three layers and adsorbates were allowed to relax. The vacuum length was 10 Å, which was sufficiently large to separate the slab [51,52]. For the bimetals (MNi(111) alloy systems, M ¼ Rh or Ru), a Ni atom in the topmost layer of the slab was replaced by either Rh or Ru, corresponding to a nominal composition of 1/24. The adsorption sites are designated with respect to the distance from the alloying element, as shown in Fig. 1c and d. 3. Results 3.1. Structure analysis of bimetals Prior to performing the density functional theory (DFT) calculations, ab-initio molecular dynamics (MD) was performed to place the Rh or Ru atoms at stable positions in the Ni slab. The Rh or Ru atoms were initially placed on the Ni surface and allowed to diffuse into the Ni slab at a melting point until 2 1015 s (Fig. 2), when the
stable positions were reached. Figure 2 shows that the Rh and Ru atoms were placed at the first and second layers from the top, respectively, after stabilization. This is consistent with experimental results of bimetal Ni alloy with Rh [37] while no experimental data are available for Ni alloy with Ru. Based on the ab-initio MD result, the bimetals containing Rh atoms in the first layer (Rh(1)) and Ru atoms in the second layer (Ru(2)), i.e., NiRh(1) and NiRu(2), were mainly considered in the methane-dissociation reaction. 3.2. Reaction mechanism Figure 3 shows the reaction pathways of methane decomposition, leading to carbon deposition through steps 1e4 or with CO gas evolution after step 3, which prevents carbon deposition, through steps 5e7. The carbon deposition is primarily influenced by the relative difference in the activation energy between step 4 and 5. Therefore, the activation energies of steps 4 and 5 were calculated and compared. The activation energies of the subsequent reactions (steps 5e7) were also calculated because the CO gas-evolution reaction also influences carbon deposition. On the other hand, H2 gas evolution occurs in steps 1e4 and 6. The catalytic efficiency is affected by the ease of H2 gas evolution and the availability of the reaction surface during reactions. The former is determined by the binding strength (adsorption energy) of the adsorbed hydrogen molecules, whereas the latter is determined by the activation energy and the adsorption energies of the reaction species. The available reaction surface would increase if the activation energy decreases progressively from steps 1 to 4. Therefore, the catalytic efficiency was investigated in terms of the adsorbates' stable geometries and associated adsorption energies and the relative changes in the activation energies of all steps. 3.2.1. CH*4 / CH*3 þ H* First, each stable configuration of CH*4 and CH*3 þ H* molecules adsorbed on the Ni and M-Ni(111) alloy surfaces was determined by calculating the adsorption energy. The adsorption energy of methane is very small (~0.02 eV), indicating that methane molecules are adsorbed via physisorption [53]. Figure 4 shows that the adsorption site of CH*3 on a pure Ni surface is a threefold hollow site (FCC), regardless of the presence of hydrogen. On the NiRu(2) surface, the stable adsorption site is HCP, whereas it remains FCC on
Fig. 1. Side view of the slab and top view of the structures of Ni(111) and M-Ni(111) alloy systems, including possible adsorption sites on the surface: slab (a), pure Ni surface (b), and doped M surface (c, d), where M ¼ Rh, Ru.
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Fig. 2. The relative positions of Rh and Ru over time according to MD simulations.
Fig. 3. The mechanisms of steps 1e7 on Ni (111) and M-Ni(111) alloy systems (M ¼ Rh, Ru).
the NiRh(1) surface. The presence of hydrogen on the metal surface does not affect the stable adsorption sites of CH*3 on the NiRh(1) and NiRu(2) surfaces. This means that the interaction between CH*3 and hydrogen is not significant to affect the stable adsorption sites, which allows the calculations valid in the supercell size of this work. The stable adsorption site of H2 is FCC on the pure Ni and NiRh(1) surfaces but HCP on the NiRu(2) surface. Similar to CH*3, the stable adsorption site of hydrogen is not influenced by the presence of CH*3. The coadsorption energies and stable adsorption sites of each species at each step are listed in Table 1 and shown in Fig. 4. The adsorption energies of separately adsorbed CH*3 and hydrogen are also shown in Table S2. The coadsorption energies of CH*3 and H* are 6.43 eV, 5.35 eV, and 7.93 eV on the pure Ni, NiRh(1), and NiRu(2) surfaces, respectively. Thus, CH*3 and H* adsorb onto the NiRh(1) surface weaker than on the pure Ni and NiRu(2) surfaces. As a result, H2 gas is expected to evolve more easily from the NiRh(1) surface than from the pure Ni and NiRu(2) surfaces. However, SOFCs typically
operate at high temperatures, and the entropy change of the H2 gas-evolving reaction from adsorbed hydrogens increases significantly. The large thermal entropy contribution (TDS) exceeds the enthalpy change (DH) in the gas-evolving reaction, and thus, H2 gas evolution (i.e., the catalytic efficiency) exhibits little dependence on alloying Ni with Rh or Ru in step 1. This process is also minimally influenced by the presence of CHx molecules. The detailed calculations of the H2 gas-evolving reaction are discussed later. Since the difference in the adsorption energies of CH*3 and H* molecules has little influence on H2 gas evolution, the activation energy is more important in H2 gas evolution. The activation energies of step 1 are 1.125 eV, 1.023 eV, and 1.007 eV for pure Ni, NiRh(1), and NiRu(2), respectively, and are clearly quite similar (Table 2 and Fig. 5). More importantly, the relative difference in the activation energies in successive reactions affects the availability of the reaction surface and thus H2 gas evolution. Therefore, the activation energies for the subsequent steps (2e4) were calculated. 3.2.2. CH*x / CH*x1 þ H* (x ¼ 3, 2) In steps 2 and 3, the stable adsorption sites of CH*2and CH* were threefold hollow sites, i.e., FCC, FCC, and HCP for pure Ni, NiRh(1), and NiRu(2), respectively, as observed for CH*3 adsorption. The stable adsorption site of hydrogen is FCC on the pure Ni and NiRh(1) surfaces but HCP on the NiRu(2) surface. The coadsorption energies of CH*2 and H* molecules are 8.44 eV, 7.33 eV, and 10.73 eV on the pure Ni, NiRh(1), and NiRu(2) surfaces, respectively. The coadsorption energy of CH*2 and H* molecules on NiRh(1) is smaller than those on the other two surfaces. The same trend is obtained for the coadsorption energies of CH* and H* molecules. The enhancement of H2 gas evolution from the NiRh(1) surface is not expected to be significant compared with the pure Ni and NiRu(2) surfaces in steps 2 and 3 because of the entropy contribution at high temperatures, as described in the previous section (step 1). The stable adsorption sites and energies are listed in Table 1. The activation energies of steps 2 and 3 on pure Ni are 0.738 eV
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Fig. 4. The most stable adsorption geometries for the final states of steps 1e6 on Ni(111) and M-Ni(111) alloy systems (M ¼ Rh, Ru).
Table 1 The coadsorption energies (Eads) excluding zero point energy (ZPE) and stable sites of adsorbed molecules on Ni(111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru). Pure Ni system
CH3 þ H CH2 þ H CH þ H CþH CH þ O CO þ H
Ni-Rh system 1st layer doping
Eads
Position
6.43 8.44 10.67 11.55 12.32 5.74
FCC FCC FCC FCC FCC FCC
þ þ þ þ þ þ
FCC FCC FCC FCC Bridge FCC
Ni-Ru system 2nd layer doping
Eads
Position
5.35 7.33 9.61 10.49 11.02 4.59
FCC FCC FCC FCC FCC FCC
þ þ þ þ þ þ
FCC FCC FCC FCC Bridge FCC
Eads
Position
7.93 10.73 13.00 14.01 14.83 7.82
HCP þ HCP HCP þ HCP HCP þ HCP HCP þ HCP HCP þ HCP FCC þ HCP
Table 2 The activation energies excluding zero point energy (ZPE) (eV) of steam methane reforming reaction on Ni(111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru). Activation energy without ZPE (eV) Step1: Step2: Step3: Step4: Step5: Step6: Step7: Step8:
CH*4 / CH*3 þ H* CH*3 / CH*2 þ H* CH*2 / CH* þ H* CH* / C* þ H* CH* þ O* / CHO* CHO* / CO* þ H* CO* / CO(g) 2H* / H2(g)
Pure Ni system
Ni-Rh system 1st layer doping
Ni-Ru system 2nd layer doping
1.125 0.738 0.568 1.541 0.621 0 1.897 1.163
1.023 0.587 0.431 1.506 0.795 0 1.874 1.145
1.007 0.622 0.110 1.320 0.370 0.279 1.807 1.312
and 0.568 eV, respectively. Starting from step 1 (1.125 eV), the activation energy decreases progressively as the reaction proceeds until the adsorbed state of the CH* and H* molecules. This favors methane decomposition because the reaction surface is readily depleted by the current reaction and becomes more available for the preceding reaction, confirming that Ni is a good catalyst. On the other hand, the activation energy on NiRh(1) decreases more rapidly than that on pure Ni: 1.023 eV, 0.587 eV, and 0.431 eV for step 1, 2, and 3, respectively. Furthermore, in the case of NiRu(2), the activation energy significantly decreases in step 3 (Table 2 and Fig. 5). This will enhance H2 gas evolution significantly on NiRu(2). Because H2 gas is evolved in all steps 1e4, the relative change in the activation energy is more important than the difference in the adsorption energy. The coadsorption and activation energies were lowered by 0.3e0.4 eV and 0.1e0.2 eV, respectively, by the correction of the zero point energy but their relative differences along the reaction step were not seriously influenced (Table S3-4, Fig. S1-3) for all Ni and Ni alloys. Therefore, the calculation results
Fig. 5. The activation energies excluding zero point energy (ZPE) for steps 1e4 on Ni(111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru).
show that alloying Ni with Ru is the most effective way to enhance the catalytic efficiency of H2 gas generation, in good agreement with the experimental results. 3.2.3. CH* / C* þ H* and CH* þ O* / CHO* Steps 4 and 5 represent alternative routes, leading to carbon deposition (step 4) or preventing the carbon deposition via CO gas evolution (steps 5e7). Therefore, the activation energies of these steps were calculated and compared. It is important to note that step 5's activation energy (0.621 eV) is lower than that of step 4
Y. Yoon et al. / Journal of Power Sources 359 (2017) 450e457
(1.541 eV) for the pure Ni surface, implying that methane decomposition will proceed through step 5 rather than step 4. The same results were obtained for the Ni bimetals, NiRh(1) and NiRu(2) (Table 2), suggesting that step 5 should be considered because of the concern regarding carbon deposition; however, previous theoretical calculations included only steps 1e4. Furthermore, the activation energy of step 5 (0.37 eV) on NiRu(2) surface is much lower than those on the pure Ni (0.621 eV), whereas its activation energy on NiRh(1) surface (0.795 eV) is higher than that on Ni surface, as shown in Fig. 6. This indicates that more effective suppression of carbon deposition can be achieved by alloying Ni with Ru as far as the zero point energy does not alter the relative difference in the activation energies. The zero point energy correction reduces the activation energy of step 4 and 5 on Ni surface by 0.16 and 0.006 eV, respectively (Table 2, Table S4). Its influence on NiRh(1) and NiRu(2) is similar to the case of Ni surface, which makes the validity of the above discussion remain. Second, the activation energy increases at step 4, exceeding those of the preceding steps (1e3), regardless of the metal alloying. Since the activation energy decreases progressively in the preceding steps (1e3) but increases significantly at step 4, abundant CH* and H* molecules will accumulate on the metal surface if dissociation does not proceed through step 5. In this case, the relative difference in the activation energies of steps 4 and 5 will become even more important in determining the carbon deposition and catalytic efficiency. In fact, the activation energy of step 5 is lower than that of step 4 regardless of metal alloying, and is lower on the NiRu(2) surface than on the other metal surfaces. All the calculations described above were carried out on Ni bimetals with Rh at the first layer (NiRh(1)) and Ru at an underneath layer (NiRu(2)). Further calculations were performed on Ni bimetals with Rh and Ru at an underneath (NiRh(2)) and the first (NiRu(1)) layers, respectively. The tendency of decreasing activation energy along the step 1 to 3 and relative difference in the activation energy between the alternative steps 4 and 5 remains unchanged for both NiRh and NiRu alloys (Table S5, Figs. S4eS7). In the case of NiRh(2), both activation energies of steps 4 and 5 were reduced by 0.09 and 0.22 eV, respectively, compared with NiRh(1) (Figs. S4 and S5). Both activation energies of steps 4 and 5 in NiRu(2) were also reduced by 0.08 and 0.27 eV, respectively, compared with NiRu(1) (Figs. S6 and S7). If the activation energies are considered for NiRh(1) and
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NiRu(1), similar to the case of the previous theoretical calculations, the differences in the activation energy of steps 4 and 5 are 0.71 and 0.76 eV for NiRh(1) and NiRu(1), respectively, and their differences are similar in value. This means no beneficial effect of Ru alloying in the enhancement of catalytic efficiency and coke inhibition, which is a misleading conclusion. However, our ab initio MD calculations and experimental report suggest that Rh atoms are primarily located at a metal surface and Ru atoms tend to be preferentially located underneath the surface. Then examining the activation energy should be made in the comparison between NiRh(1) and NiRu(2). Thus, it can be concluded that alloying Ni with Ru is the most effective method to prevent carbon deposition. The catalytic efficiency is also examined in terms of the relative change in the activation energies of the successive reactions. H2 gas evolution, which determines the catalytic efficiency, occurs in the steps 1e4 and 6. To achieve high catalytic efficiency, fast forward methane-decomposition reactions are required. In the reaction pathway, the forward reaction rate of a specific step will be determined by the activation energy of the current step and the subsequent availability of the reaction surface. The reaction surface availability is, in turn, determined by the activation energy of the following reaction and the ease of H2 gas evolution from the reaction surface. As mentioned above, H2 gas evolution is not significantly influenced by metal alloying because of the relatively large thermal entropy contributions at high temperatures. Therefore, the relative change in the activation energies of the current and following reactions will be important for achieving a fast forward reaction. In other words, the activation energy of the following reaction must be smaller than that of the current reaction. In fact, the activation energies decrease progressively for steps 1e3 for all Ni alloys, as described above, but in NiRu(2), the decrease is more rapid than for the other two metals (pure Ni and NiRh(1)), as shown in Fig. 5. After step 3, the reaction will proceed through step 5 instead of step 4. In step 5, the NiRu(2) surface exhibits the smallest activation energy (Fig. 6), allowing the fastest forward reaction. Therefore, alloying Ni with Ru results in greater catalytic efficiency than the other two examined metals. Another concern is the methane reforming reaction under an electric field. Since the anode surface is often exposed to an electric potential during SOFC operation, the activation energy was also calculated in the steps 4 and 5 under the electric field. The comparison of the activation energies without and with an electric field is shown in Table S6 and Figs. S8eS10 for the Ni alloys. The change in the activation energy upon the electric field is negligible regardless of the Ni alloys, compared with the relative difference of the activation energy in steps 4 and 5. Therefore, the discussions on the preferential reaction path and alloying effect remain effective.
3.3. H2O* / OH* þ H* and OH* / O* þ H*
Fig. 6. The activation energies excluding zero point energy (ZPE) for steps 5 and 6 on Ni(111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru).
In step5, the O*is produced by water decomposition in a steam reforming reaction. The decomposition of water is divided into two steps. First, the H2O dissociation (H2O* / OH* þ H*) occurs, followed by hydroxide dissociation (OH* / O* þ H*). The activation energy of the H2O dissociation in pure Ni, NiRh(1) and NiRu(2) are 1.093 eV, 1.043 eV and 0.827 eV, respectively, and the activation energy on NiRu(2) is lowest (Table 3 and Fig. S11). The activation energies of the hydroxide dissociation for NiRu(2) is also lower than other two metals. Therefore, NiRu(2) exhibits a higher efficiency than other catalysts in providing the methane decomposition reaction (step 5) with oxygen. Furthermore, the activation energies of the H2O dissociation are smaller than those of the methane decomposition reaction (step 1), indicating that the water decomposition occurs once methane decomposition reaction is initiated.
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Table 3 The activation energies (eV) of water dissociation on Ni (111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru). Activation energy without ZPE (eV)
Pure Ni system
Ni-Rh system 1st layer doping
Ni-Ru system 2nd layer doping
H2O* / OH* þ H* OH* / O* þ H*
1.093 1.014
1.043 0.961
0.827 0.905
3.4. Gas generation: CO* / CO(g) and 2H* / H2(g) The completion of step 6 followed by CO gas evolution inhibits carbon deposition. The standard entropy of CO gas is 197.66 J/ mol$K; thus, the thermal entropy contribution (TS) is 2.206 eV at 800 C, and the free energy change of CO gas evolution is 0.96 eV for the pure Ni surface. The enthalpy change was calculated from the desorption of molecular CO into air. The free energy changes of CO gas evolution are 0.469 eV and 0.438 eV for NiRh(1) and NiRu(2), respectively. Therefore, CO gas evolution will be complete, regardless of the metal surface used. The activation energies of CO gas evolution from all metals are also similar, although the activation energy for NiRu(2) is slightly lower than those of the other two metals (Table 2). H2 gas evolution was also examined. The desorption energies of hydrogen atoms adsorbed on a metal surface into a H2 molecule are 1.019 eV, 0.811 eV, and 1.033 eV for pure Ni, NiRh(1), and NiRu(2), respectively. The corresponding free energy changes for pure Ni, NiRh(1), and NiRu(2) are 0.383 eV, 0.591 eV, and 0.369 eV, respectively. The activation energies are quite similar to each other. Therefore, all gas-evolution reactions will be completed, and no substantially differences will be observed for different metals once the reaction proceeds to the adsorption of CO* and H*. Then, the relative difference in the activation energies of steps 4 and 5 and the relative change in the activation energies of steps 1e3 will determine the catalytic efficiency and the inhibition of carbon deposition, as discussed above. Relative to NiRh(1) and NiRu(2), NiRh(2) and NiRu(1) showed similar trends in their activation energies and adsorption/desorption behaviors. All of the calculation results suggest that alloying Ni with Ru is the most effective method investigated here.
4. Conclusions DFT calculations were performed to study the mechanism of steam methane reforming on Ni and M-Ni(111) alloy systems (M ¼ Rh and Ru). We investigated methane dissociation in steam in a comprehensive way by examining not only the location of alloying element in Ni alloys but also additional CO gas-evolution. Carbon deposition is primarily determined by the relative difference in the activation energies of two alternative reaction steps that produce adsorbed C* and H* or CHO* from CH*. The activation energy of the CHO*-producing step is lower than that of the step producing C* and H*, and as a result, methane decomposition proceeds preferentially through CO gas evolution, thereby reducing carbon deposition regardless of Ni or Ni alloys. Among the metals investigated, NiRu bimetals exhibited the lowest activation energy, suggesting that this material is an effective catalyst for coking inhibition. The DFT calculation results also revealed that the activation energy of the methane decomposition decreases progressively as the reaction proceeds up to the adsorbed state of CH and increases significantly in the step producing C* and H*, preventing further decomposition of CH* into C* and H*. Instead, the lower activation energy of the alternative reaction leading to CO gas evolution facilitates the methane decomposition continuous and enhances H2 gas evolution, i.e., catalytic efficiency. Methane
decomposition and carbon deposition on Ni alloy catalysts could only be completely understood by performing an in-depth examination of CO gas evolution. This work shows that among the metals investigated, alloying Ni with Ru is an effective method for greatly enhancing catalytic efficiency and inhibiting carbon deposition. Acknowledgements This work was supported by the National Research Foundation of Korea (2015M3C8A8073604) and the computational resources for DFT simulation were supported by KISTI supercomputing center (KSC-2014-C3-048). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.05.076. References [1] J.T.S. Irvine, J. Power Sources 136 (2004) 203e207. [2] S.C. Singhal, Solid State Ion. 152e153 (2002) 405e410. [3] D.L. King, J.J. Strohm, X. Wang, H.S. Roh, C. Wang, Y.H. Chin, Y. Wang, Y. Lin, R. Rozmiarek, P. Singh, J. Catal. 258 (2008) 356e365. [4] D.J.L. Brett, A. Atkinson, N.P. Brandon, S.J. Skinner, Chem. Soc. Rev. 37 (2008) 1568e1578. [5] K. Joon, J. Power Sources 71 (1998) 12e18. [6] S.M. Haile, materialstoday 6 (2003) 24e29. [7] R.M. Ormerod, Chem. Soc. Rev. 32 (2003) 17e28. [8] A. Atkinson, S. Barnett, R.J. Gorte, J.T.S. Irvine, A.J. Mcevoy, M. Mogensen, S.C. Singhal, J. Vohs, Nat. Mater 3 (2004) 17e27. [9] Y. Shiratori, T. Ijichi, T. Oshima, K. Sasaki, Int. J. Hydrogen Energy 35 (2010) 7905e7912. [10] R.J. Gorte, J.M. Vohs, J. Catal. 216 (2003) 477e486. [11] H. Kishimoto, K. Yamaji, T. Horita, Y.P. Xiong, N. Sakai, M.E. Brito, H. Yokokawa, J. Electrochem. Soc. 153 (2006) A982eA988. [12] X.F. Ye, B. Huang, S.R. Wang, Z.R. Wang, L. Xiong, T.L. Wen, J. Power Sources 164 (2007) 203e209. [13] S.G. Wang, X.Y. Liao, J. Hu, D.B. Cao, Y.W. Li, J. Wang, H. Jiao, Surf. Sci. 601 (2007) 1271e1284. [14] S. Souentie, M. Athanasiou, D.K. Niakolas, A. Katsaounis, S.G. Neophytides, C.G. Vayenas, J. Catal. 306 (2013) 116e128. [15] J.R. Rostrup-Nielsen, J. Sehested, Fuel Chem. Div. Prepr. 48 (2003) 218e219. [16] L. Maier, B. Schadel, K.H. Delgado, S. Tischer, O. Deutschmann, Top. Catal. 54 (2011) 845e858. [17] J.H. Koh, Y.S. Yoo, J.W. Park, H.C. Lim, Solid State Ion. 149 (2002) 157e166. [18] E. Nikolla, J. Schwank, S. Linic, J. Catal. 250 (2007) 85e93. [19] J.H. Lee, E.G. Lee, O.S. Joo, K.D. Jung, Appl. Catal. A 269 (2004) 1e6. [20] E. Nikolla, J. Schwank, S. Linic, J. Catal. 263 (2009) 220e227. [21] E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649e651. [22] N.M. Galea, D. Knapp, T. Ziegler, J. Catal. 247 (2007) 20e33. [23] H. Liu, R. Zhang, R. Yan, J. Li, B. Wang, K. Xie, Appl. Surf. Sci. 258 (2012) 8177e8184. [24] W. An, X.C. Zeng, C.H. Turner, J. Chem. Phys. 131 (2009) 174702. [25] H. Liu, R. Zhang, R. Yan, B. Wang, K. Xie, Appl. Surf. Sci. 257 (2011) 8955e8964. [26] M. Wang, Z. Fu, Z. Yang, Phys. Lett. A 377 (2013) 2189e2194. [27] H. Liu, R. Yan, R. Zhang, B. Wang, K. Xie, J. Nat. Gas. Chem. 20 (2011) 611e617. [28] Y. Huang, J. Du, C. Ling, T. Zhou, S. Wang, Catal. Sci. Technol. 3 (2013) 1343e1354. [29] R. Wang, J. Ran, W. Qi, J. Niu, X. Du, Comput. Theor. Chem. 1073 (2015) 94e101. [30] K. Li, M. Jiao, Y. Wang, Z. Wu, Surf. Sci. 617 (2013) 149e155. [31] K. Li, Z. Zhou, Y. Wang, Z. Wu, Surf. Sci. 612 (2013) 63e68. [32] C. Fan, Y.A. Zhu, Y. Xu, Y. Zhou, X.G. Zhou, D. Chen, J. Chem. Phys. 137 (2012) 014703. [33] P. Zuo, Z. Fu, Z. Yang, J. Power Sources 242 (2013) 762e767. [34] D.W. Blaylock, T. Ogura, W.H. Green, G.J.O. Beran, J. Phys. Chem. C 113 (2009) 4898e4908. [35] D.W. Blaylock, Y.A. Zhu, W.H. Green, Top. Catal. 54 (2011) 828e844. [36] T. Takeguchi, R. Kikuchi, T. Yano, K. Eguchi, K. Murata, Catal. Today 84 (2003) 217e222. [37] C. Li, Y. Dou, J. Liu, Y. Chen, S. He, M. Wei, D.G. Ecans, X. Duan, Chem. Commun. 49 (2013) 9992e9994. [38] J.R. Croy, S. Mostafa, L. Hickman, H. Heinrich, B.R. Cuenya, Appl. Catal. A 350 (2008) 207e216. [39] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15e50. [40] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558e561. [41] G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251e14269. €chl, Phys. Rev. B 50 (1994) 17953e17979. [42] P.E. Blo
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Enhanced catalytic behavior of Ni alloys in steam methane reforming Yeongpil Yoon, Hanmi Kim, Jaichan Lee* School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
DFT calculation was used to understand steam methane reforming on Ni alloys. CO gas-evolving reaction suppresses carbon deposition. CO gas-evolving reaction enhances the catalytic efficiency of H2 generation. Ni-Ru alloy is an effective way to facilitate the CO gas-evolving reaction.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 April 2017 Received in revised form 19 May 2017 Accepted 22 May 2017 Available online 3 June 2017
The dissociation process of methane on Ni and Ni alloys are investigated by density functional theory (DFT) in terms of catalytic efficiency and carbon deposition. Examining the dissociation to CH3, CH2, CH, C, and H is not sufficient to properly predict the catalytic efficiency and carbon deposition, and further investigation of the CO gas-evolving reaction is required to completely understand methane dissociation in steam. The location of alloying element in Ni alloy needed be addressed from the results of ab-inito molecular dynamics (MD). The reaction pathway of methane dissociation associated with CO gas evolution is traced by performing first-principles calculations of the adsorption and activation energies of each dissociation step. During the dissociation process, two alternative reaction steps producing adsorbed C and H or adsorbed CO are critically important in determining coking inhibition as well as H2 gas evolution (i.e., the catalytic efficiency). The theoretical calculations presented here suggest that alloying Ni with Ru is an effective way to reduce carbon deposition and enhance the catalytic efficiency of H2 fueling in solid oxide fuel cells (SOFCs). © 2017 Elsevier B.V. All rights reserved.
Keywords: Ni alloy anode Methane reforming Carbon deposition Hydrogen gas evolution Density functional theory
1. Introduction Fuel cells are energy-conversion devices that directly transform the chemical energy of a fuel into electricity with high efficiency; furthermore, they are environmentally friendly and can use a variety of fuels including hydrogen [1,2]. Among the fuel cells available, solid oxide fuel cells (SOFCs) are most commonly used because of their high power-conversion efficiency. SOFCs must
* Corresponding author. E-mail address: [email protected] (J. Lee). http://dx.doi.org/10.1016/j.jpowsour.2017.05.076 0378-7753/© 2017 Elsevier B.V. All rights reserved.
operate at high temperatures ranging from 500 C to 1000 C. However, despite such high operation temperatures, SOFCs' anodes are not seriously affected by CO poisoning during operation [3e9]. Hydrogen gas generation is an important process for achieving highly efficient SOFCs. Hydrogen gas is the main fuel for SOFCs and is generated at the anode during methane reforming reaction [10e16]; Carbon is formed as a byproduct. When the byproduct carbon is deposited on the anode surface, the effective area for the reaction is reduced, which seriously hampers H2 generation [17e22]. Therefore, preventing the carbon deposition has been a critical issue in SOFC catalysts, while Ni-based catalyst has been widely used. Many catalysts, including bimetallic catalysts, have
Y. Yoon et al. / Journal of Power Sources 359 (2017) 450e457
been proposed to resolve this issue. In bimetallic catalysts, Ni has been alloyed with the following transition metals: Ni-Cu [23,24], Ni-Co [25], Ni-Au [26e28], Ni-Pt [29], Ni-Rh [30], Ni-Pd [31], and Ni-M surface (M ¼ Cu, Ru, Rh, Pd, Ag, Pt, and Au) [32]. These studies suggested that coking inhibition was controlled in the step 4 of the following reaction process during the methane-reforming reaction [23e32]: Step 1: CH*4 / CH*3 þ H* Step 2: CH*3 / CH*2 þ H* Step 3: CH*2 / CH* þ H* Step 4: CH* / C* þ H* In the case, the reaction from adsorbed carbon to carbon cluster leads to the carbon deposition as a byproduct of the methane decomposition, in which the activation energy of carbon diffusion would be important in the carbon deposition [33]. However, the methane decomposition is commonly performed in steam. Blaylock et al. suggested that additional dissociation reactions occur, i.e., steam reforming on a multi-faceted Ni surface [34,35]. Among many allowed reaction paths in the steamreforming reaction, the additional following steps including the steps 1e4 were suggested as a primary reaction path for methane decomposition that may further influence carbon deposition: Step 5: CH* þ O* / CHO* Step 6: CHO* / CO* þ H* Step 7: CO* / CO(g) In the reactions considered above (steps1e7), the reaction step 4 or 5 could be an alternative one. After step 3, the steps 5e7 can be, in other words, substituted for step 4, thereby preventing the carbon deposition that occurs in the step 4 and instead producing CO gas. CO gas evolution (steps 5 to 7) can also affect the catalytic efficiency of H2 generation. Therefore, understanding the reaction pathway including steps 5e7 is essential to reduce carbon deposition and enhance the catalytic efficiency of the methanereforming reaction. Previous theoretical studies considering steps 1e4 showed that among several bimetallic catalysts, Ni-Rh bimetallic catalysts have great potential to reduce carbon deposition [32]. However, experimental studies revealed that Ni-Ru bimetallic catalysts exhibited more efficient coking inhibition than Ni-Rh bimetallic catalysts [36]. Theoretical studies considering only steps 1e4 could generate results that contradict experimental observations. Therefore, further detailed studies are needed to understand the methanereforming reaction that includes CO gas-evolving steps, which can significantly influence the carbon deposition and catalytic efficiency. Furthermore, the previous theoretical studies [23e32] using bimetallic catalysts assumed the location of the alloying elements only on a host metal surface although their location could influence the reaction. However, it is reported that the location of the alloying elements depends on the kind of alloying element relative to a host metal [37,38]. In fact, in Ni-Rh bimetallic catalysts, Rh atoms tended to be located at the surface of Ni [37], whereas Ru atoms in Pt were located at the surface or underneath [38]. This concern necessitates the investigation of the location of alloying element in an alloy as well. In this study, we performed first-principles calculations to investigate the methane-reforming reaction involving steps 1e7 on
451
pure Ni and Ni-Ru and Ni-Rh bimetals in terms of carbon deposition and the efficiency of the Ni catalyst. The nudged elastic band (NEB) method was used to calculate the energy barrier at each step and examine the catalytic efficiency of bimetal alloying. Vibration frequency analysis was performed to verify transition states and zero point energy (ZPE) was calculated to examine its influence on the adsorption and activation energies. In addition, ab-initio molecular dynamics (MD) was used to locate alloying element in the bimetals. Our result shows that the activation energy of step 4 is much higher than that of step 5, and thus, step 4 can be substituted by step 5. Therefore, the CO gas-evolving reaction significantly affects the carbon deposition and catalytic efficiency. Furthermore, alloying Ni with Ru results in a lower activation energy than those observed for pure Ni and Ni-Rh bimetal in step 5. As a result, step 5 and the CO gas-evolving reaction are accelerated, leading to reduced carbon deposition. Moreover, the activation energies of steps 1e3 are progressively reduced by Ni-Ru bimetal, leading to the rapid depletion of the reaction product of each step and facilitating fast forward reactions. This result indicates that Ni-Ru bimetal also has enhanced catalytic efficiency for H2 generation as well as coke inhibition. Furthermore, this result remains unchanged when exposed to an electric field. 2. Computational details 2.1. Method All calculations were performed using the Vienna Ab-initio Simulation Package (VASP) [39e41]. The interactions between valence electrons and ion cores were treated by the Projector Augmented Wave (PAW) method [42,43]. The exchange correlation functional was the Generalized Gradient Approximation with the PerdeweBurkeeErnzerhof functional (GGA-PBE) [44]. The MonkhorstePack scheme was used to generate the k-points set of 4 4 1 in k-space [45]. An energy cutoff of 400 eV was used for the plane wave basis set in all calculations. The geometries were optimized until the energy converged to 1.0 104 eV/atom, and the force converged to 0.03 eV/Å. Because of the presence of magnetic atoms, spin polarization was considered in all calculations. The adsorption energies without zero point energies, DEads , were calculated by the following relation:
DEads ¼ Eadsorbate=sub Eadsorbate Esub
(1)
Eadsorbate/sub, Eadsorbate, and Esub represent the total energies of the optimized substrate with the adsorbate, the isolated adsorbate molecule, and the clean surface, respectively. A negative DEads represents exothermic chemisorption, and a positive DEads indicates endothermic chemisorption. As the magnitude of DEads increases, the binding of the adsorbate to the surface becomes stronger. In order to investigate the effect of the zero point energy on the decomposition reactions [46], the calculation was carried with ZPE , which modout without and with zero point energies, DEads ifies the adsorption energies as follows:
with ZPE DEads ¼ Eadsorbate=sub Eadsorbate Esub þ DEZPE
DEZPE ¼
X hvadsorbate=sub 2
X hv
adsorbate
2
(2)
(3)
where Eadsorbate/sub, Eadsorbate, and Esub are same in Eq. (1) and DEZPE is the zero point energy (ZPE) correction, i.e., the difference in the zero point energy between the adsorbed system and the gas phase. vadsorbate=sub and vadsorbate are vibrational frequencies of the adsorbate and isolated molecule, respectively.
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The NEB method was used to locate the transition state (TS) of methane dissociation, and the most stable adsorption configurations were employed as the initial and final states [47,48]. The transition states were verified by vibrational frequency analysis, which shows unstable modes at the transition states (Table S1). The activation energies, DEa , were calculated by the following relation:
DEa ¼ ESP EIS
(4)
EIS and ESP represent the energies of the initial (before reaction) state and the transient state (saddle point) obtained by NEB method, respectively. 2.2. Model The Ni(111) surface is reported to be more active than the Ni(100) and Ni(110) surfaces for the methane-reforming reaction [49]. Therefore, we studied the reaction mechanism on the (111) surface. Figure 1 shows a Ni(111) slab with several adsorption sites. The lattice parameter of pure Ni (3.52 Å) obtained theoretically [26,33] and experimentally [50] was used in the slab calculation. The supercell consists of 6 layers with a p(2 2) surface and a vacuum layer. In the supercell, the first bottom layer of the slab was fixed, and the second and third layers from the bottom were fixed only along the z-axis; the top three layers and adsorbates were allowed to relax. The vacuum length was 10 Å, which was sufficiently large to separate the slab [51,52]. For the bimetals (MNi(111) alloy systems, M ¼ Rh or Ru), a Ni atom in the topmost layer of the slab was replaced by either Rh or Ru, corresponding to a nominal composition of 1/24. The adsorption sites are designated with respect to the distance from the alloying element, as shown in Fig. 1c and d. 3. Results 3.1. Structure analysis of bimetals Prior to performing the density functional theory (DFT) calculations, ab-initio molecular dynamics (MD) was performed to place the Rh or Ru atoms at stable positions in the Ni slab. The Rh or Ru atoms were initially placed on the Ni surface and allowed to diffuse into the Ni slab at a melting point until 2 1015 s (Fig. 2), when the
stable positions were reached. Figure 2 shows that the Rh and Ru atoms were placed at the first and second layers from the top, respectively, after stabilization. This is consistent with experimental results of bimetal Ni alloy with Rh [37] while no experimental data are available for Ni alloy with Ru. Based on the ab-initio MD result, the bimetals containing Rh atoms in the first layer (Rh(1)) and Ru atoms in the second layer (Ru(2)), i.e., NiRh(1) and NiRu(2), were mainly considered in the methane-dissociation reaction. 3.2. Reaction mechanism Figure 3 shows the reaction pathways of methane decomposition, leading to carbon deposition through steps 1e4 or with CO gas evolution after step 3, which prevents carbon deposition, through steps 5e7. The carbon deposition is primarily influenced by the relative difference in the activation energy between step 4 and 5. Therefore, the activation energies of steps 4 and 5 were calculated and compared. The activation energies of the subsequent reactions (steps 5e7) were also calculated because the CO gas-evolution reaction also influences carbon deposition. On the other hand, H2 gas evolution occurs in steps 1e4 and 6. The catalytic efficiency is affected by the ease of H2 gas evolution and the availability of the reaction surface during reactions. The former is determined by the binding strength (adsorption energy) of the adsorbed hydrogen molecules, whereas the latter is determined by the activation energy and the adsorption energies of the reaction species. The available reaction surface would increase if the activation energy decreases progressively from steps 1 to 4. Therefore, the catalytic efficiency was investigated in terms of the adsorbates' stable geometries and associated adsorption energies and the relative changes in the activation energies of all steps. 3.2.1. CH*4 / CH*3 þ H* First, each stable configuration of CH*4 and CH*3 þ H* molecules adsorbed on the Ni and M-Ni(111) alloy surfaces was determined by calculating the adsorption energy. The adsorption energy of methane is very small (~0.02 eV), indicating that methane molecules are adsorbed via physisorption [53]. Figure 4 shows that the adsorption site of CH*3 on a pure Ni surface is a threefold hollow site (FCC), regardless of the presence of hydrogen. On the NiRu(2) surface, the stable adsorption site is HCP, whereas it remains FCC on
Fig. 1. Side view of the slab and top view of the structures of Ni(111) and M-Ni(111) alloy systems, including possible adsorption sites on the surface: slab (a), pure Ni surface (b), and doped M surface (c, d), where M ¼ Rh, Ru.
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453
Fig. 2. The relative positions of Rh and Ru over time according to MD simulations.
Fig. 3. The mechanisms of steps 1e7 on Ni (111) and M-Ni(111) alloy systems (M ¼ Rh, Ru).
the NiRh(1) surface. The presence of hydrogen on the metal surface does not affect the stable adsorption sites of CH*3 on the NiRh(1) and NiRu(2) surfaces. This means that the interaction between CH*3 and hydrogen is not significant to affect the stable adsorption sites, which allows the calculations valid in the supercell size of this work. The stable adsorption site of H2 is FCC on the pure Ni and NiRh(1) surfaces but HCP on the NiRu(2) surface. Similar to CH*3, the stable adsorption site of hydrogen is not influenced by the presence of CH*3. The coadsorption energies and stable adsorption sites of each species at each step are listed in Table 1 and shown in Fig. 4. The adsorption energies of separately adsorbed CH*3 and hydrogen are also shown in Table S2. The coadsorption energies of CH*3 and H* are 6.43 eV, 5.35 eV, and 7.93 eV on the pure Ni, NiRh(1), and NiRu(2) surfaces, respectively. Thus, CH*3 and H* adsorb onto the NiRh(1) surface weaker than on the pure Ni and NiRu(2) surfaces. As a result, H2 gas is expected to evolve more easily from the NiRh(1) surface than from the pure Ni and NiRu(2) surfaces. However, SOFCs typically
operate at high temperatures, and the entropy change of the H2 gas-evolving reaction from adsorbed hydrogens increases significantly. The large thermal entropy contribution (TDS) exceeds the enthalpy change (DH) in the gas-evolving reaction, and thus, H2 gas evolution (i.e., the catalytic efficiency) exhibits little dependence on alloying Ni with Rh or Ru in step 1. This process is also minimally influenced by the presence of CHx molecules. The detailed calculations of the H2 gas-evolving reaction are discussed later. Since the difference in the adsorption energies of CH*3 and H* molecules has little influence on H2 gas evolution, the activation energy is more important in H2 gas evolution. The activation energies of step 1 are 1.125 eV, 1.023 eV, and 1.007 eV for pure Ni, NiRh(1), and NiRu(2), respectively, and are clearly quite similar (Table 2 and Fig. 5). More importantly, the relative difference in the activation energies in successive reactions affects the availability of the reaction surface and thus H2 gas evolution. Therefore, the activation energies for the subsequent steps (2e4) were calculated. 3.2.2. CH*x / CH*x1 þ H* (x ¼ 3, 2) In steps 2 and 3, the stable adsorption sites of CH*2and CH* were threefold hollow sites, i.e., FCC, FCC, and HCP for pure Ni, NiRh(1), and NiRu(2), respectively, as observed for CH*3 adsorption. The stable adsorption site of hydrogen is FCC on the pure Ni and NiRh(1) surfaces but HCP on the NiRu(2) surface. The coadsorption energies of CH*2 and H* molecules are 8.44 eV, 7.33 eV, and 10.73 eV on the pure Ni, NiRh(1), and NiRu(2) surfaces, respectively. The coadsorption energy of CH*2 and H* molecules on NiRh(1) is smaller than those on the other two surfaces. The same trend is obtained for the coadsorption energies of CH* and H* molecules. The enhancement of H2 gas evolution from the NiRh(1) surface is not expected to be significant compared with the pure Ni and NiRu(2) surfaces in steps 2 and 3 because of the entropy contribution at high temperatures, as described in the previous section (step 1). The stable adsorption sites and energies are listed in Table 1. The activation energies of steps 2 and 3 on pure Ni are 0.738 eV
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Fig. 4. The most stable adsorption geometries for the final states of steps 1e6 on Ni(111) and M-Ni(111) alloy systems (M ¼ Rh, Ru).
Table 1 The coadsorption energies (Eads) excluding zero point energy (ZPE) and stable sites of adsorbed molecules on Ni(111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru). Pure Ni system
CH3 þ H CH2 þ H CH þ H CþH CH þ O CO þ H
Ni-Rh system 1st layer doping
Eads
Position
6.43 8.44 10.67 11.55 12.32 5.74
FCC FCC FCC FCC FCC FCC
þ þ þ þ þ þ
FCC FCC FCC FCC Bridge FCC
Ni-Ru system 2nd layer doping
Eads
Position
5.35 7.33 9.61 10.49 11.02 4.59
FCC FCC FCC FCC FCC FCC
þ þ þ þ þ þ
FCC FCC FCC FCC Bridge FCC
Eads
Position
7.93 10.73 13.00 14.01 14.83 7.82
HCP þ HCP HCP þ HCP HCP þ HCP HCP þ HCP HCP þ HCP FCC þ HCP
Table 2 The activation energies excluding zero point energy (ZPE) (eV) of steam methane reforming reaction on Ni(111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru). Activation energy without ZPE (eV) Step1: Step2: Step3: Step4: Step5: Step6: Step7: Step8:
CH*4 / CH*3 þ H* CH*3 / CH*2 þ H* CH*2 / CH* þ H* CH* / C* þ H* CH* þ O* / CHO* CHO* / CO* þ H* CO* / CO(g) 2H* / H2(g)
Pure Ni system
Ni-Rh system 1st layer doping
Ni-Ru system 2nd layer doping
1.125 0.738 0.568 1.541 0.621 0 1.897 1.163
1.023 0.587 0.431 1.506 0.795 0 1.874 1.145
1.007 0.622 0.110 1.320 0.370 0.279 1.807 1.312
and 0.568 eV, respectively. Starting from step 1 (1.125 eV), the activation energy decreases progressively as the reaction proceeds until the adsorbed state of the CH* and H* molecules. This favors methane decomposition because the reaction surface is readily depleted by the current reaction and becomes more available for the preceding reaction, confirming that Ni is a good catalyst. On the other hand, the activation energy on NiRh(1) decreases more rapidly than that on pure Ni: 1.023 eV, 0.587 eV, and 0.431 eV for step 1, 2, and 3, respectively. Furthermore, in the case of NiRu(2), the activation energy significantly decreases in step 3 (Table 2 and Fig. 5). This will enhance H2 gas evolution significantly on NiRu(2). Because H2 gas is evolved in all steps 1e4, the relative change in the activation energy is more important than the difference in the adsorption energy. The coadsorption and activation energies were lowered by 0.3e0.4 eV and 0.1e0.2 eV, respectively, by the correction of the zero point energy but their relative differences along the reaction step were not seriously influenced (Table S3-4, Fig. S1-3) for all Ni and Ni alloys. Therefore, the calculation results
Fig. 5. The activation energies excluding zero point energy (ZPE) for steps 1e4 on Ni(111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru).
show that alloying Ni with Ru is the most effective way to enhance the catalytic efficiency of H2 gas generation, in good agreement with the experimental results. 3.2.3. CH* / C* þ H* and CH* þ O* / CHO* Steps 4 and 5 represent alternative routes, leading to carbon deposition (step 4) or preventing the carbon deposition via CO gas evolution (steps 5e7). Therefore, the activation energies of these steps were calculated and compared. It is important to note that step 5's activation energy (0.621 eV) is lower than that of step 4
Y. Yoon et al. / Journal of Power Sources 359 (2017) 450e457
(1.541 eV) for the pure Ni surface, implying that methane decomposition will proceed through step 5 rather than step 4. The same results were obtained for the Ni bimetals, NiRh(1) and NiRu(2) (Table 2), suggesting that step 5 should be considered because of the concern regarding carbon deposition; however, previous theoretical calculations included only steps 1e4. Furthermore, the activation energy of step 5 (0.37 eV) on NiRu(2) surface is much lower than those on the pure Ni (0.621 eV), whereas its activation energy on NiRh(1) surface (0.795 eV) is higher than that on Ni surface, as shown in Fig. 6. This indicates that more effective suppression of carbon deposition can be achieved by alloying Ni with Ru as far as the zero point energy does not alter the relative difference in the activation energies. The zero point energy correction reduces the activation energy of step 4 and 5 on Ni surface by 0.16 and 0.006 eV, respectively (Table 2, Table S4). Its influence on NiRh(1) and NiRu(2) is similar to the case of Ni surface, which makes the validity of the above discussion remain. Second, the activation energy increases at step 4, exceeding those of the preceding steps (1e3), regardless of the metal alloying. Since the activation energy decreases progressively in the preceding steps (1e3) but increases significantly at step 4, abundant CH* and H* molecules will accumulate on the metal surface if dissociation does not proceed through step 5. In this case, the relative difference in the activation energies of steps 4 and 5 will become even more important in determining the carbon deposition and catalytic efficiency. In fact, the activation energy of step 5 is lower than that of step 4 regardless of metal alloying, and is lower on the NiRu(2) surface than on the other metal surfaces. All the calculations described above were carried out on Ni bimetals with Rh at the first layer (NiRh(1)) and Ru at an underneath layer (NiRu(2)). Further calculations were performed on Ni bimetals with Rh and Ru at an underneath (NiRh(2)) and the first (NiRu(1)) layers, respectively. The tendency of decreasing activation energy along the step 1 to 3 and relative difference in the activation energy between the alternative steps 4 and 5 remains unchanged for both NiRh and NiRu alloys (Table S5, Figs. S4eS7). In the case of NiRh(2), both activation energies of steps 4 and 5 were reduced by 0.09 and 0.22 eV, respectively, compared with NiRh(1) (Figs. S4 and S5). Both activation energies of steps 4 and 5 in NiRu(2) were also reduced by 0.08 and 0.27 eV, respectively, compared with NiRu(1) (Figs. S6 and S7). If the activation energies are considered for NiRh(1) and
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NiRu(1), similar to the case of the previous theoretical calculations, the differences in the activation energy of steps 4 and 5 are 0.71 and 0.76 eV for NiRh(1) and NiRu(1), respectively, and their differences are similar in value. This means no beneficial effect of Ru alloying in the enhancement of catalytic efficiency and coke inhibition, which is a misleading conclusion. However, our ab initio MD calculations and experimental report suggest that Rh atoms are primarily located at a metal surface and Ru atoms tend to be preferentially located underneath the surface. Then examining the activation energy should be made in the comparison between NiRh(1) and NiRu(2). Thus, it can be concluded that alloying Ni with Ru is the most effective method to prevent carbon deposition. The catalytic efficiency is also examined in terms of the relative change in the activation energies of the successive reactions. H2 gas evolution, which determines the catalytic efficiency, occurs in the steps 1e4 and 6. To achieve high catalytic efficiency, fast forward methane-decomposition reactions are required. In the reaction pathway, the forward reaction rate of a specific step will be determined by the activation energy of the current step and the subsequent availability of the reaction surface. The reaction surface availability is, in turn, determined by the activation energy of the following reaction and the ease of H2 gas evolution from the reaction surface. As mentioned above, H2 gas evolution is not significantly influenced by metal alloying because of the relatively large thermal entropy contributions at high temperatures. Therefore, the relative change in the activation energies of the current and following reactions will be important for achieving a fast forward reaction. In other words, the activation energy of the following reaction must be smaller than that of the current reaction. In fact, the activation energies decrease progressively for steps 1e3 for all Ni alloys, as described above, but in NiRu(2), the decrease is more rapid than for the other two metals (pure Ni and NiRh(1)), as shown in Fig. 5. After step 3, the reaction will proceed through step 5 instead of step 4. In step 5, the NiRu(2) surface exhibits the smallest activation energy (Fig. 6), allowing the fastest forward reaction. Therefore, alloying Ni with Ru results in greater catalytic efficiency than the other two examined metals. Another concern is the methane reforming reaction under an electric field. Since the anode surface is often exposed to an electric potential during SOFC operation, the activation energy was also calculated in the steps 4 and 5 under the electric field. The comparison of the activation energies without and with an electric field is shown in Table S6 and Figs. S8eS10 for the Ni alloys. The change in the activation energy upon the electric field is negligible regardless of the Ni alloys, compared with the relative difference of the activation energy in steps 4 and 5. Therefore, the discussions on the preferential reaction path and alloying effect remain effective.
3.3. H2O* / OH* þ H* and OH* / O* þ H*
Fig. 6. The activation energies excluding zero point energy (ZPE) for steps 5 and 6 on Ni(111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru).
In step5, the O*is produced by water decomposition in a steam reforming reaction. The decomposition of water is divided into two steps. First, the H2O dissociation (H2O* / OH* þ H*) occurs, followed by hydroxide dissociation (OH* / O* þ H*). The activation energy of the H2O dissociation in pure Ni, NiRh(1) and NiRu(2) are 1.093 eV, 1.043 eV and 0.827 eV, respectively, and the activation energy on NiRu(2) is lowest (Table 3 and Fig. S11). The activation energies of the hydroxide dissociation for NiRu(2) is also lower than other two metals. Therefore, NiRu(2) exhibits a higher efficiency than other catalysts in providing the methane decomposition reaction (step 5) with oxygen. Furthermore, the activation energies of the H2O dissociation are smaller than those of the methane decomposition reaction (step 1), indicating that the water decomposition occurs once methane decomposition reaction is initiated.
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Table 3 The activation energies (eV) of water dissociation on Ni (111) and M-Ni(111) alloy systems, (M ¼ Rh, Ru). Activation energy without ZPE (eV)
Pure Ni system
Ni-Rh system 1st layer doping
Ni-Ru system 2nd layer doping
H2O* / OH* þ H* OH* / O* þ H*
1.093 1.014
1.043 0.961
0.827 0.905
3.4. Gas generation: CO* / CO(g) and 2H* / H2(g) The completion of step 6 followed by CO gas evolution inhibits carbon deposition. The standard entropy of CO gas is 197.66 J/ mol$K; thus, the thermal entropy contribution (TS) is 2.206 eV at 800 C, and the free energy change of CO gas evolution is 0.96 eV for the pure Ni surface. The enthalpy change was calculated from the desorption of molecular CO into air. The free energy changes of CO gas evolution are 0.469 eV and 0.438 eV for NiRh(1) and NiRu(2), respectively. Therefore, CO gas evolution will be complete, regardless of the metal surface used. The activation energies of CO gas evolution from all metals are also similar, although the activation energy for NiRu(2) is slightly lower than those of the other two metals (Table 2). H2 gas evolution was also examined. The desorption energies of hydrogen atoms adsorbed on a metal surface into a H2 molecule are 1.019 eV, 0.811 eV, and 1.033 eV for pure Ni, NiRh(1), and NiRu(2), respectively. The corresponding free energy changes for pure Ni, NiRh(1), and NiRu(2) are 0.383 eV, 0.591 eV, and 0.369 eV, respectively. The activation energies are quite similar to each other. Therefore, all gas-evolution reactions will be completed, and no substantially differences will be observed for different metals once the reaction proceeds to the adsorption of CO* and H*. Then, the relative difference in the activation energies of steps 4 and 5 and the relative change in the activation energies of steps 1e3 will determine the catalytic efficiency and the inhibition of carbon deposition, as discussed above. Relative to NiRh(1) and NiRu(2), NiRh(2) and NiRu(1) showed similar trends in their activation energies and adsorption/desorption behaviors. All of the calculation results suggest that alloying Ni with Ru is the most effective method investigated here.
4. Conclusions DFT calculations were performed to study the mechanism of steam methane reforming on Ni and M-Ni(111) alloy systems (M ¼ Rh and Ru). We investigated methane dissociation in steam in a comprehensive way by examining not only the location of alloying element in Ni alloys but also additional CO gas-evolution. Carbon deposition is primarily determined by the relative difference in the activation energies of two alternative reaction steps that produce adsorbed C* and H* or CHO* from CH*. The activation energy of the CHO*-producing step is lower than that of the step producing C* and H*, and as a result, methane decomposition proceeds preferentially through CO gas evolution, thereby reducing carbon deposition regardless of Ni or Ni alloys. Among the metals investigated, NiRu bimetals exhibited the lowest activation energy, suggesting that this material is an effective catalyst for coking inhibition. The DFT calculation results also revealed that the activation energy of the methane decomposition decreases progressively as the reaction proceeds up to the adsorbed state of CH and increases significantly in the step producing C* and H*, preventing further decomposition of CH* into C* and H*. Instead, the lower activation energy of the alternative reaction leading to CO gas evolution facilitates the methane decomposition continuous and enhances H2 gas evolution, i.e., catalytic efficiency. Methane
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