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Catalytic hydrogenation of CO2 over Pt- and Ni-doped graphene: A comparative DFT study
Journal of Molecular Graphics and Modelling 77 (2017) 143–152
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Topical Perspectives
Catalytic hydrogenation of CO2 over Pt- and Ni-doped graphene: A comparative DFT study Mehdi D. Esrafili a,∗ , Fahimeh Sharifi a , Leila Dinparast b a b
Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh, Iran Biotechnology Research Center, Tabriz University of Medical Sciences, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 21 June 2017 Received in revised form 16 August 2017 Accepted 17 August 2017 Keywords: Greenhouse gas CO2 hydrogenation Doped-graphene DFT Catalysis
a b s t r a c t Today, the global greenhouse effect of carbon dioxide (CO2 ) is a serious environmental problem. Therefore, developing efficient methods for CO2 capturing and conversion to valuable chemicals is a great challenge. The aim of the present study is to investigate the catalytic activity of Pt- or Ni-doped graphene for the hydrogenation of CO2 by a hydrogen molecule. To gain a deeper insight into the catalytic mechanism of this reaction, the reliable density functional theory calculations are performed. The adsorption energies, geometric parameters, reaction barriers, and thermodynamic properties are calculated using the M06-2X density functional. Two reaction mechanisms are proposed for the hydrogenation of CO2 . In the bimolecular mechanism, the reaction proceeds in two steps, initiating by the co-adsorption of CO2 and H2 molecules over the surface, followed by the formation of a OCOH intermediate by the transfer of H atom of H2 toward O atom of CO2 . In the next step, formic acid is produced as a favorable product with the formation of C H bond. In our proposed termolecular mechanism, however, H2 molecule is directly activated by the two pre-adsorbed CO2 molecules. The predicted activation energy for the formation of the OCOH intermediate in the bimolecular mechanism is 20.8 and 47.9 kcal mol−1 over Pt- and Ni-doped graphene, respectively. On the contrary, the formation of the first formic acid in the termolecular mechanism is found as the rate-determining step over these surfaces, with an activation energy of 28.8 and 45.5 kcal/mol. Our findings demonstrate that compared to the Ni-doped graphene, the Pt-doped surface has a relatively higher catalytic activity towards the CO2 reduction. These theoretical results could be useful in practical applications for removal and transformation of CO2 to value-added chemical products. © 2017 Elsevier Inc. All rights reserved.
1. Introduction In the recent decades, rapid development of cities and industrial activities have resulted in raising the level of greenhouse gases in the Earth’s atmosphere [1]. Carbon dioxide (CO2 ) is a main greenhouse gas produced in industrial cities [2]. An excessive CO2 emission leads to harmful climate changes such as the global warming and sea level rising [2,3]. Thus, the design and development of inexpensive catalysts with high stability and efficiency for the conversion of CO2 to useful materials is highly desirable. The removal of carbon dioxide from the environment can reduce its greenhouse effects [4]. On the other hand, CO2 is a low cost and abundant carbon source [5]. So, the hydrogenation of CO2 and conversion to value-added chemical products, such as formic acid, methane, or methanol, is the great interest of scientific community [6,7]. The
∗ Corresponding author. E-mail address: esrafi[email protected] (M.D. Esrafili). http://dx.doi.org/10.1016/j.jmgm.2017.08.016 1093-3263/© 2017 Elsevier Inc. All rights reserved.
transformation of CO2 into formic acid through the hydrogenation process, has achieved considerable attention because the product has various industrial applications [8–11]. Formic acid is widely used as a feedstock in perfume and dyeing industry and as a reagent for production of organic derivatives such as aldehydes, ketones, carboxylic acids, and amides [12]. To date, numerous experimental studies have investigated the chemical and electrochemical conversion of CO2 to formic acid [13–15], but many aspects remain unknown, especially the reaction mechanism and the role of the used catalyst. In this context, quantum chemical calculations could be useful in understanding the mechanism of transformations, adsorption modes, activation energies, and the nature of transition states during the catalytic process, as well as the role of catalyst. Graphene is a novel form of carbon which could act as a suitable support for various metal and non-metal atoms to form new carbon–metal nanocomposite [16–18]. These composites could be used in several applications, including catalysis [17,19], drug delivery [20], sensors, memory devices [21,22], photocatalysis [23], conversion and storage of energy [24], solar cells [25,26], molecular
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imaging [27,28], and molecular electronics [29]. For the first time, graphene was discovered by Geim and Novoselov in 2004 [30]. This magic material of the century is a nanosheet with a single-carbon atom thick and has a honeycomb planar sp2 hybridized structure [31,32]. Graphene nanosheets are known for their high surfaceto-volume ratio and the high catalytic activity of graphene is due to its specific surface area [33]. Additionally, graphene exhibits excellent electronic and thermal conductivity [34,35]. Today, due to these unique chemical and physical characteristics, and many other extraordinary properties such as high flexibility [36], simplicity of functionalization, low cost, large 2-dimentional - conjugated structure [37,38], and the mechanical and chemical stability [36], graphene has attracted a great deal of attention in the scientific fields. For example, several reports have been published to show the interaction of toxic gases with the graphene nanosheets [23,39,40]. Moreover, recent theoretical and experimental studies have demonstrated that chemical doping of graphene with foreign atoms such as metal atoms [41–46], Al [47], N [48], P [49], and Si [50] could significantly improve its catalytic activity in oxidation/reduction reaction of CO2 , NO, CO, and N2 O molecules [51]. Very recently, novel Ni-N-modified graphene was successfully synthesized by Su et al. [52]. Their results showed that the resultant carbon-based catalyst could efficiently catalyze the electrochemical reduction of CO2 to CO. Using theoretical calculations, Sirijaraensre and Limtrakul [53] found that Cu-doped graphene can be viewed as a promising catalyst for the hydrogenation of CO2 to formic acid. In this contribution, the adsorption of CO2 and its subsequent hydrogenation reduction to formic acid are studied over Pt- and Nidoped graphene nanosheets (Pt-/Ni-G) using DFT calculations. The geometry, electronic structure and the catalytic activity of Pt-/Ni-G are studied and compared in details. To the best of our knowledge, this is the first report on the hydrogenation of CO2 molecule to formic acid over Pt-/Ni-G. The results of this study could be helpful in understanding the chemical and catalytic properties of metaldoped graphene sheets and in designing highly efficient carbonbased catalysts for the conversion of CO2 and other toxic gases to useful chemicals. In facts, this type of study could be helpful for cleaning of the human society. 2. Computational details In this study, all DFT calculations were performed using the Gaussian 09 suite of programs [54]. The geometry optimization and the subsequent frequency calculation of the complexes were performed using the M06-2X density functional. M06-2X is a global hybrid meta-GGA functional which known as a reliable method to study non-covalent interactions, main group thermochemistry, and reaction barriers [55,56]. In the DFT calculations, all atoms were described by the all electron 6-31G* basis set, except Ni and Pt atoms where the LANL2DZ basis set with the corresponding effective core potential was used. A hexagonal graphene supercell (4 × 4 graphene unit cell), containing 48 carbon atoms, was considered as the pristine graphene for the calculations. A carbon atom of the surface was substituted with a single Pt or Ni atom for the preparation of Pt- or Ni-G. The adsorption energy Eads (A) of the adsorbed molecules over Pt-/Ni-G was calculated as the following equation: Eads (A) = EA-S − ES − EA
(1)
where EA-S , ES , and EA are the total energies of the complex of adsorbate-substrate (A-S), the substrate (S), and the absorbate (A), respectively. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and density of state (DOS) plot of Pt-/Ni-G were obtained at the M06-2X/6-31G*(LANL2DZ) level.
3. Results and discussion This section is divided in three parts. First, the geometry, electronic structure, and stabilization of Pt-/Ni-G are studied. Next, the adsorption of CO2 and H2 molecules as well as their co-adsorbed configurations is analyzed in detail. Finally, the reaction mechanisms and energy barriers for the hydrogenation of CO2 by a H2 molecule will be discussed. 3.1. Geometry, electronic structure and stabilization of Pt-/Ni-G As mentioned before, the primary goal of this theoretical study is to understand the catalytic pathway for the hydrogenation of CO2 over Pt-/Ni-G surface. To this aim, the interaction modes and adsorption energies of CO2 and H2 molecules with these surfaces are obtained and discussed in details. Fig. 1 shows the optimized structures of Pt-/Ni-G. It is clear that after doping with the Pt or Ni atom, the geometric structure of the graphene is change significantly. The covalent bond distantances between Pt and neighboring C atoms in Pt-G are 1.94 and 1.98 Å, which are much larger than the C C bonds in the pure graphene (1.42 Å). The obtained Pt C bond lengths are shorter than those mentioned by Liu et al. (2.10 Å) [57], but in good agreement with the results of Lee et al. (1.94 Å) [58]. The formation of these Pt C chemical bonds is related to the interaction of the C-p states and the in-plane component Pt-d states. Therefore, the surrounding C atoms will move out of the surface of graphene to could interact with the Pt atom [57]. The Mulliken population analysis also shows that in the optimized Pt-G, there is about 0.8 e charge transfer from the Pt atom to the neighboring C atoms on the surface. Due to such a large charge transfer, there should be strong covalent interaction between the positively charged Pt and the neighboring carbon atoms, which helps to stabilize Pt atom. Similar to Pt-G, when the C atom of graphene is replaced with the Ni atom, Ni atom is move out the basal plane of doped system (Fig. 1). The average bond length of Ni C is 1.85 Å, which is smaller than that of reported by Tang and co-workers (2.12 Å) [59]. The observed discrepancies between the calculated bond lengths in this study and previous studies could be attributed to the differences between the used clusters or to applied DFT methods. In the optimized Ni-G, a charge of about 0.4 e is transferred from the Ni atom to the graphene surface, which is smaller than that of Pt-G surface. To consider the effect of Pt or Ni atom impurity on the electronic structure of the graphene, the total DOS (TDOS) plots of Pt-G and Ni-G are obtained. Fig. 1 depicts the obtained results. One can see that in contrast to the pristine graphene which has almost zero HOMO-LUMO energy gap, the TDOS of both Pt-G and Ni-G is characterized by a quite small energy gap. In particular, as the Ni atom is adsorbed over the defective graphene, a peak is appeared below the Fermi level, which is mainly attributed to the states of carbon atoms around the dopant atom. Moreover, the DOS plot of Pt-G clearly indicates that the Pt 5d peaks below the Fermi level overlap well with those of dangling C atoms around the dopant atom, which may explain the strong interaction between the Pt and neighboring C atoms. The HOMO-LUMO energy gap of Pt-G and Ni-G is calculated to be 3.12 and 3.59 eV, respectively, which indicates that the incorporation of the Pt or Ni atom into the network of graphene can significantly change its electronic structure. To consider the stability of Pt-/Ni-G surfaces, the diffusion of the Pt or Ni atom to its neighboring position on the graphene surface is studied. Fig. S1 of Supporting information indicates the optimized structures and activation energies for the diffusion of these atoms over the surface. As evident, the predicted diffusion barrier is 141.2 and 96.5 kcal mol−1 for the Pt and Ni atoms, respectively. These large energy barriers are also supported by a large adsorption energy of these dopant atoms. More especially, the adsorption
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Fig. 1. The optimized structures and DOS plots of Pt- and Ni-doped graphene sheets. All distances are in Å. In the DOS plots, the partial DOS plots of the dopant atom and dangling C atoms around the dopant atom are also depicted. The Fermi level is indicated by a dotted-dashed line.
Table 1 Calculated adsorption energy (Eads ), net charge transfer (qCT ), change of enthalpy (H298 ), and change of Gibbs free energy (G298 ) for the adsorption/co-adsorption of CO2 and H2 molecules over Pt-/Ni-G surfaces. Complex
Eads (kcal mol−1 )
qCT (e)
H298 (kcal mol−1 )
G298 (kcal mol−1 )
Pt-G A C E G
−7.7 −1.2 −3.8 −13.4
0.1 0.2 0.2 0.2
−7.6 −2.4 −4.9 −13.1
−1.8 5.2 8.3 −3.5
Ni-G B D F H
−8.3 −0.9 −8.0 −14.9
0.1 0.2 0.1 0.2
−8.2 −0.1 −8.6 −14.7
−2.5 7.1 3.6 −2.1
energy of the Pt and Ni atoms is calculated to be about −158 and −121 kcal mol−1 over the defective site of the graphene, respectively, suggesting a strong interaction between these dopant atoms and the surface. These findings reveal that both Pt-G and Ni-G are stable enough to be used in the catalytic hydrogenation of CO2 to formic acid. 3.2. Adsorption of CO2 and H2 over Pt-/Ni-G To study the hydrogenation of CO2 , we first consider the adsorption of individual CO2 and H2 molecules on Pt-/Ni-G surface. In order to find the more stable configurations for each absorbate (CO2 and H2 ), different adsorption orientations are considered. The resulted most energetically favorable configuration of absorbed CO2 over the Pt-G is depicted in Fig. 2 (complex A). As seen, CO2 molecule is absorbed on the Pt atom in a O C O· · ·Pt-G orientation with the calculated adsorption energy (Eads ) of −7.7 kcal mol−1 (Table 1). Note that the absolute value of the obtained Eads is larger than the calculated value for the adsorbed CO2 on Cu-graphene [53], but close to those of over Al- [60] and Pd-doped [61] graphene. The Pt-O intermolecular bond distance is 2.53 Å. The bond length of C O is 1.17 Å, which is 0.01 Å longer than those of free CO2
molecule. According to the Mulliken analysis, there is about 0.1 e charge transfer from CO2 to Pt-G, which leads to a decrease in the charge of the Pt atom from 0.8 e in Pt-G to 0.7 e in the complex A. The electron density difference (EDD) map is also provided to understand the electron density rearrangement due to the CO2 adsorption over the surface (Fig. 3). The blue and red colors of this map show electron gain and electron loss regions of the complex, respectively. As it is clear, there is a significant electron density accumulation between the CO2 and surface, which demonstrates the polarization of the O atom of CO2 by the positively charged Pt atom of the surface. On the other hand, an electron loss region is seen on the O atom not interacting with the Pt atom, which is related to the electron charge flow from CO2 to the surface. The negative G298 (−1.8 kcal mol−1 ) and H298 (−7.6 kcal mol−1 ) values in Table 1 clearly demonstrate that the adsorption of CO2 over Pt-G is exothermic and thermodynamically favorable process at room temperature. The adsorption of CO2 over Ni-G sheet is also investigated and the most stable configuration is shown in Fig. 2 (complex B). One can see that the orientation of adsorbed CO2 molecule over the Ni-G is almost similar to that of over Pt-G. The calculated Ni O intermolecular bond distance is 2.23 Å, and the C O bond length of CO2 (1.17 Å) is slightly elongated with respect to its isolated state (1.16 Å). The absolute value of Eads (−8.3 kcal mol−1 ) for the CO2 adsorption on Ni-G shows that the interaction of CO2 with Ni-G is stronger than with Pt-G. Besides, this Eads value is consistent with that of reported by Zhao and co-workers [62]. The EDD map indicates a great charge accumulation between the Ni and O atoms that confirm the strong polarization of CO2 molecule by the Ni atom (Fig. 3). Moreover, the Mulliken population analysis shows that there is a charge transfer of about 0.1 e from CO2 to the surface. The thermochemistry calculations indicate that the adsorption of CO2 over Ni-G sheet is exothermic and a thermodynamically feasible reaction at room temperature (Table 1). As seen in Table 1, the absolute values of Eads and the thermodynamic parameters (H298 and G298 ) of the adsorbed CO2 over Ni-G are larger than those of Pt-G.
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Fig. 2. The optimized geometries of absorbed/coadsorbed CO2 and H2 molecules over Pt-/Ni-G surface. All distances are in Å.
Also, the adsorption of H2 molecule over Pt-/Ni-G nanosheets is evaluated to find the most stable configurations (Fig. 2, complexes C and D). The calculated Eads values of H2 over Pt-G and Ni-G are −1.2 and −0.9 kcal mol−1 , respectively (Table 1). In both cases, a significant increase in the bond length of absorbed H2 molecule is observed (from 0.60 to 0.76 Å). The binding distance of formed Pt(Ni)· · ·H bonds is about 2.17 (2.22) Å. It should be noted that the estimated Ni.... H2 binding distances in the present study is consistent with those of obtained by Ramos-Castillo et al. (2.14 Å using the PBE functional) [63]. The EDD map for the absorbed H2 molecule over the both of surfaces could also verify the weak and physical nature of these adsorptions. As seen, there exists a small electron density accumulation around the hydrogen molecule, which is due to small polarizability of this molecule in the presence of the Pt or Ni atom. However, Table 1 indicates that there is a sizable charge transfer from H2 to the surface, due to the interaction between the bond of H2 molecule atom and the Pt atom. This indicates that the stability of these complexes can be mainly attributed to the charge transfer effects. Moreover, this finding reveals that Pt-/Ni-G can strongly activate H2 molecule, which may facilitate the hydrogenation of CO2 by this molecule. The adsorption of H2 over Ni-G and Pt-G is exothermic but these interactions cannot be occurred at ambient conditions due to the positive G298 value (Table 1). 3.3. Co-adsorption of CO2 and H2 molecules over Pt-/Ni-G Next, the co-adsorption of CO2 and H2 molecules over Pt-/Ni-G is investigated (Fig. 2, complexes E and F). The optimized coadsorption complexes over Pt-G and Ni-G surfaces have adsorption energies of −3.8 and −8.0 kcal mol−1 , respectively. Note that the absolute values of these adsorption energies are smaller than the sum of individual adsorption energies of CO2 and H2 molecules over each surface. This indicates that the adsorption of each species tends to weaken the adsorption of another one. That is, they do not act cooperatively with each other. The important geometrical parameters of co-adsorbed complexes are shown in Fig. 2. Based on the resulted thermodynamic parameters, the formation of both of complexes is exothermic, but impossible at room temperature (H298 < 0, G298 > 0). As noted above, the interaction between CO2 and Pt-/Ni-G is stronger than H2 . Therefore, the catalysis process for CO2 hydrogenation over Pt-/Ni-G support would initiate with the adsorption of CO2 on the Pt-/Ni-G. Considering the large adsorption energy of CO2 molecule, the co-adsorption of two CO2 molecules is also probable over both sur-
faces. Fig. 2 shows the optimized structures (complexes G and H). One can see that the co-adsorption of CO molecules on the both surfaces is stronger than that of CO2 + H2 adsorption, due to the large co-adsorption energy of −13.4 (Pt-G) and −14.9 kcal mol−1 (Ni-G). This means that, the Pt or Ni atom of the surface could be dominantly covered by CO2 molecules if a CO2 /H2 mixture is injected as the reaction gas. The calculated large charge-transfer values (qCT ) also verify the chemistrption of CO2 molecules over the surfaces. For both configurations, the negative G298 and H298 values show that the adsorption of CO2 molecules is exothermic and a thermodynamically possible process at ambient condition. 3.4. Proposed reaction mechanisms for the CO2 hydrogenation over Pt-/Ni-G According to the previous study [53], the catalytic reduction of CO2 by H2 molecule generally occurs through a bimolecular mechanism. This involves the co-adsorption of CO2 and H2 molecules, followed by the formation of a OCOH intermediate, and finally desorption of HCOOH from the surface. It should be noted that there is also reported an alternative mechanism for the dehydrogenation of CO2 over Cu-doped graphene [53], which starts with the dissociation of H2 molecule on the surface, followed by the addition of CO2 to form of HCOO moiety. However, the latter pathway seems unlikely to proceed over Pt- or Ni-G surface, since the adsorption energy of H2 molecule is very smaller than that of CO2 . Instead, we suggest here a termolecular mechanism with the H2 molecule being directly activated by the two pre-adsorbed CO2 molecules as another possible mechanism for reduction of CO2 . In the following section, we study both bimolecular and termolecular reaction pathways over Pt- or Ni-G surfaces, separately. 3.4.1. Bimolecular mechanism The possible bimolecular reaction pathway for the hydrogenation of CO2 molecule and conversion to formic acid is as follow: CO2 + Pt-/Ni-G → CO2 · · ·Pt-/Ni-G
(2)
H2 + CO2 · · ·Pt-/Ni-G → CO2 /H2 · · ·Pt-/Ni-G
(3)
CO2 /H2 · · ·Pt-/Ni-G → H-Pt-/Ni-COOH-G
(4)
H-Pt-/Ni-COOH-G → HCOOH· · ·Pt-/Ni-G
(5)
The energy profiles and the optimized geometries of the initial states (IS), transition states (TS), and final state (FS) over Pt-/Ni-G
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Fig. 3. EDD maps of absorbed CO2 (complexes A and B) and H2 (complexes C and D) molecules on Pt-/Ni-G. The isovalue of EDD plots is ±0.001 au. Red and blue colors indicate regions of decreased and increased electron densities, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 Calculated activation energy (Eact ), reaction energy (E), change of enthalpy (H298 ) and Gibbs free energy (G298 ) for the proposed pathways of the reduction of CO2 by H2 over Pt-/Ni-G. Reaction
Eact (kcal mol−1 )
E (kcal mol−1 )
H298 (kcal mol−1 )
G298 (kcal mol−1 )
Pt-G IS1 → IS2 IS2 → FS2 IS5 → FS5 IS6 → IS7 IS7 → FS7
20.8 3.4 18.4 28.8 1.8
16.1 −21.1 −19.5 18.2 −28.7
16.4 −20.9 −18.1 16.0 −26.9
16.9 −21.6 −12.4 13.5 −18.4
Ni-G IS3 → IS4 IS4 → FS4 IS8 → FS8 IS9 → IS10 IS10 → FS10
47.9 0.4 41.3 45.5 1.4
43.8 −45.4 −6.2 33.7 −37.7
42.5 −44.3 −4.4 30.8 −34.7
45.7 −46.8 −2.1 24.9 −21.4
are shown in Figs. 4 and 5, respectively. Also, in Table 2, the calculate activation energy (Eact ), energy of the reaction (E), changes of enthalpy (H298 ), and Gibbs free energy (G298 ) are summarized. The mechanism of CO2 conversion to formic acid is proposed to proceed in a two-step process; first the formation of a OCOH intermediate and next the conversion of this intermediate to formic acid. The reduction of CO2 starts with the bimolecular adsorption configuration of CO2 and H2 gases over the Pt-/Ni-G substrate. As noted previously, based on the resulted adsorption energies of CO2 and H2 molecules, at first, CO2 is absorbed over Pt-G surface and then H2 molecule is co-adsorbed to form the complex C. Hence, the stable complex E is chosen as the initial state (IS1). As Fig. 4 indicates, IS1 can be converted to the OCOH intermediate through the transition state TS1. At TS1, the distance between the C atom of CO2 and the Pt becomes shorter from 3.73 in IS1 to 2.46 Å, while the H H bond distance is lengthened from 0.76 to 1.00 Å. As a result, the
dissociated hydrogen atom (H1) of H2 molecule is moved toward the O1 atom of the absorbed CO2 with the distance of 1.27 Å. Also, the intermolecular distance between the C atom of CO2 and the H2 atom is shortened (from 3.91 to 2.71 Å). Moreover, the sp hybridization of the carbon atom of CO2 is changed to sp2 and the angle of O C O is reduced from 177.7 in IS1 to 138.9◦ in TS1. The vibrational frequency analysis of TS1 indicates that there is one imaginary frequency of −1390 cm−1 , which is in accordance with the cleavage of H1–H2 and formation of O1 H1 bond. The computed activation energy for this step of reaction is 20.8 kcal mol−1 which is smaller than those of other metal-doped graphene catalysts [53]. The result of this step is the hydrogenation of O atom of CO2 molecule and the formation of the OCOH intermediate (IS2) which is strongly absorbed on the Pt atom. The calculated reaction energy and thermodynamic quantities obtained for this step demonstrate that the formation of the OCOH intermediate is thermodynamically unfa-
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Fig. 4. The energy profile, optimized geometries of the initial states (IS), transition states (TS), and final state (FS) for bimolecular mechanism of CO2 reduction by H2 molecule over Pt-G. All distances are in Å.
vorable reaction at room temperature. In the next step, the H2 atom transfers from the Pt atom to the carbon of COOH through the transition state TS2. In this state, the bond distance between H2 and C atom decreases from 2.36 to 1.58 Å and the Pt-H distance is increased by about 0.1 Å. The frequency analysis with one imaginary frequency of −638 cm−1 confirms the formation of TS2. This vibration frequency is related to the dissociation of Pt-H bond and the formation of C H bond. The calculated activation energy for this step is just 3.4 kcal mol−1 . Finally, the formic acid is produced and absorbed on the Pt atom of the Pt-G with an adsorption energy of −13.6 kcal mol−1 . Besides, the IS2 → FS2 is exothermic and could be performed at ambient conditions (H298 < 0 and G298 < 0). Furthermore, the CO2 reduction over Ni-G is studied. Similar to Pt-G, a stepwise reaction pathway is considered for the CO2 hydrogenation over this surface (Fig. 5). As mentioned before, at the first step of the reaction, the co-adsorbed complex F as the initial state (IS3) is converted to the OCOH intermediate (IS4) through TS3. In TS3, the distance between O1 atom of CO2 and the H1 atom of the hydrogen molecule becomes shorter (from 2.71 Å in IS3 to 1.20 Å in TS3) and the intermolecular distance of C H2 is reduced by 1.77 Å. Also, CO2 molecule initiates to bend to form the carboxyl group. The observed imaginary frequency at −1230 cm−1 confirms the formation of TS3. This frequency corresponds to the cleavage of the H1–H2 and the formation of the O1 H1 bond. As seen earlier, the OCOH group and an adsorbed H atom is resulted from
this step of the reaction as an intermediate (IS4). The activation energy of this process is computed to be 47.9 kcal mol−1 , which is larger than those calculated over Pt-G. In the second step, the C atom of the OCOH group is hydrogenated by H2 to form the formic acid as a favorable product. It should be noted that this conversion occurs as an almost barrier-less reaction over Ni-G and the activation energy of this step is negligible. The enthalpy and Gibbs free energy changes of the final step of this reaction are −44.3 and −46.8 kcal mol−1 , respectively, which confirm that this reaction is exothermal and possible at room temperatures (Table 2). Based on the obtained results, it is concluded that the formation of the OCOH group over both Pt-G and Ni-G surfaces is the rate determining step. Although the second step of the reaction (OCOH → HCOOH) could proceed rapidly over Ni-G surface, but the activation energy needed for the dissociation of H H bond and the formation of the OCOH intermediate is much smaller over Pt-G than Ni-G surface. This is most likely due to the large ionic nature of Pt C bonds in the former surface, which can greatly stabilize the formed OCOH group in the corresponding transition state structure (TS1). This result shows that the large positive charge on the dopant atom in Pt-G can greatly regulate its surface reactivity, and therefore affect the CO2 reduction. So, compared to Ni-G, the catalytic activity of Pt-G for the reduction of the CO2 is high. Therefore, according to our findings, the Pt-G surface could be used as an effi-
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Fig. 5. The energy profile, optimized geometries of the initial states (IS), transition states (TS), and final state (FS) for bimolecular mechanism of CO2 reduction by H2 molecule over Ni-G. All distances are in Å.
cient catalyst for the reduction and conversion of CO2 to the formic acid in the practical application. 3.4.2. Termolecular mechanism In our proposed termolecular mechanism, first two CO2 molecules are co-adsorbed over the surface and then H2 molecule is activated by the pre-adsorbed CO2 molecules. Fig. 6 shows the energy profile and optimized stationary points related to the termolecular mechanism on Pt-G surface. As seen, we take a H2 molecule and two CO2 molecules co-adsorbed on Pt atom as the initial state of the reaction (IS5), in which Pt-O and Pt-H distances are 2.50 and 3.23 Å, respectively. Subsequently, the H H bond of H2 molecule is elongated by 0.82 Å and the hydrogen atoms approach the co-adsorbed CO2 molecules. The calculated activation energy for the formation of the corresponding transition state (TS5) is calculated to be 18.4 kcal mol−1 , which is 2.4 kcal mol−1 smaller than that of TS-1. At FS5, two OCOH moieties are formed over the Pt atom with Pt-C distances of about 2.30 Å. Besides, the formation of these intermediates is exothermic and a thermodynamically possible process at ambient temperature (Table 2). In the next step, the second H2 molecule is adsorbed over the Pt atom, resulting in the formation of IS6 configuration. The binding distance between H2 and the surface is calculated to be 3.36 Å. By overcoming a large activation energy of 28.8 kcal mol−1 , the H2 molecule is dissociated and the H3 atom approaches the carbon atom of OCOH intermediate.
Note that the calculated activation energy for the hydrogenation of OCOH fragment is larger than that of obtained in the bimolecular pathway. Finally, the H4 atom is attached to the OCOH moiety and the second formic acid is formed over the surface. The average adsorption energy for HCOOH molecules is about −10 kcal/mol, suggesting that the formed formic acid molecules would desorb spontaneously and the Pt-G surface is refreshed to a new cycle of CO2 reduction. Almost a similar termolecular mechanism is obtained for the hydrogenation of CO2 molecules over Ni-G surface (Fig. 7). Briefly, the H2 molecule is first added to the pre-adsorbed CO2 molecules over the Ni atom to form the initial state IS7. The calculated adsorption energy of the H2 molecule is found to be −0.6 kcal mol−1 , which is −0.3 kcal mol−1 smaller (less negative) than that of complex D. An activation energy of about 41.3 kcal mol−1 is needed to cleavage the H H bond of H2 molecule. The formation of the intermediate state FS7 is found to be exothermic with a small negative G298 value (−6.2 kcal mol−1 ) which indicates the possibility of the formation of OCOH intermediates at normal temperature. The calculated activation energy for the hydrogenation of the first OCOH fragments is 45.5 kcal mol−1 , which is about 17 kcal mol−1 larger than that of over Pt-G (Table 2). Besides, the second hydrogen atom is added to the remaining OCOH moiety by overcoming a negligible activation energy (1.8 kcal mol−1 ). Finally, the formed HCOOH molecules are released from the surface due to their small desorption energies.
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Fig. 6. The energy profile and optimized geometries of the corresponding stationary points for the termolecular mechanism of CO2 reduction by H2 molecule over Pt-G. All distances are in Å.
According to these findings, we can predict the outcomes of the reduction of CO2 by H2 molecule on the surface of Ni- and Pt-G. The calculated activation energies indicate that for the bimolecular pathway, the formation of OCOH intermediate over both surfaces is the rate-determining step. In the case of the termolecular mechanism, however, the dissociation of the second H2 molecule and the formation of first HCOOH molecule is found to need a larger activation energy compared to other steps. Thus, considering the termolecular reaction pathway, the major product of CO2 + H2 reaction on both surfaces is OCOH moiety; which is consistent with the previous studies using other catalysts [64,65]. However, comparing the adsorption energies of the isolated CO2 and H2 molecules with the activation energies clearly suggests that the reduction of CO2 molecule is unlikely to take place under normal conditions, due to the relatively larger energy barriers. Consequently, it is concluded that the performance of Pt-G or Ni-G as a catalyst used for the reduction of CO2 is poor. To further improve the surface reac-
tivity of graphene, making modifications such as structural defects or chemical functionalization may be suggested. 4. Conclusion In the present study, the catalytic performance of Pt-/Ni-G for the hydrogenation of CO2 by H2 molecule was investigated using the DFT calculations. According to our results, both Pt-G and Ni-G surfaces are stable enough to be used in the catalytic hydrogenation of CO2 , due to their large energy barriers for the diffusion of the Pt or Ni atom to its neighboring position on the graphene surface. Two reaction mechanisms were proposed for the reduction of CO2 by a H2 molecule. In the bimolecular mechanism, CO2 and H2 molecules are first co-adsorbed over the surface and then a OCOH intermediate is obtained. Finally a HCOOH molecule is formed over the surface, which can be easily released from the surface due to the small desorption energy. On the other hand, in our proposed termolecular mechanism, H2 molecule is directly activated by the
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Fig. 7. The energy profile and optimized geometries of the corresponding stationary points for the termolecular mechanism of CO2 reduction by H2 molecule over Ni-G. All distances are in Å.
two pre-adsorbed CO2 molecules. For both mechanisms, it is found that Pt-G has a higher catalytic activity than Ni-G one. This can be related to the large positive charge on the Pt atom in Pt-G, which significantly regulate the surface reactivity, and consequently affect the CO2 reduction. For the bimolecular mechanism, the formation of the OCOH intermediate was found to be the rate determining step over both surfaces. Also, the second step (OCOH → formic acid) could proceed as a barrier-less reaction over Ni-G surface. On the contrary, the formation of the first formic acid was found as the rate-determining step over Pt- and Ni-G surfaces. However, considering the calculated adsorption energies and activation energies, it is proposed that the reduction of CO2 molecule is unlikely to take place over both surfaces under normal conditions. Overall, these theoretical findings could be helpful in practical applications for designing the graphene-based catalysts for the removal of the toxic CO2 gas and conversion it to useful products such as formic acid.
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Topical Perspectives
Catalytic hydrogenation of CO2 over Pt- and Ni-doped graphene: A comparative DFT study Mehdi D. Esrafili a,∗ , Fahimeh Sharifi a , Leila Dinparast b a b
Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh, Iran Biotechnology Research Center, Tabriz University of Medical Sciences, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 21 June 2017 Received in revised form 16 August 2017 Accepted 17 August 2017 Keywords: Greenhouse gas CO2 hydrogenation Doped-graphene DFT Catalysis
a b s t r a c t Today, the global greenhouse effect of carbon dioxide (CO2 ) is a serious environmental problem. Therefore, developing efficient methods for CO2 capturing and conversion to valuable chemicals is a great challenge. The aim of the present study is to investigate the catalytic activity of Pt- or Ni-doped graphene for the hydrogenation of CO2 by a hydrogen molecule. To gain a deeper insight into the catalytic mechanism of this reaction, the reliable density functional theory calculations are performed. The adsorption energies, geometric parameters, reaction barriers, and thermodynamic properties are calculated using the M06-2X density functional. Two reaction mechanisms are proposed for the hydrogenation of CO2 . In the bimolecular mechanism, the reaction proceeds in two steps, initiating by the co-adsorption of CO2 and H2 molecules over the surface, followed by the formation of a OCOH intermediate by the transfer of H atom of H2 toward O atom of CO2 . In the next step, formic acid is produced as a favorable product with the formation of C H bond. In our proposed termolecular mechanism, however, H2 molecule is directly activated by the two pre-adsorbed CO2 molecules. The predicted activation energy for the formation of the OCOH intermediate in the bimolecular mechanism is 20.8 and 47.9 kcal mol−1 over Pt- and Ni-doped graphene, respectively. On the contrary, the formation of the first formic acid in the termolecular mechanism is found as the rate-determining step over these surfaces, with an activation energy of 28.8 and 45.5 kcal/mol. Our findings demonstrate that compared to the Ni-doped graphene, the Pt-doped surface has a relatively higher catalytic activity towards the CO2 reduction. These theoretical results could be useful in practical applications for removal and transformation of CO2 to value-added chemical products. © 2017 Elsevier Inc. All rights reserved.
1. Introduction In the recent decades, rapid development of cities and industrial activities have resulted in raising the level of greenhouse gases in the Earth’s atmosphere [1]. Carbon dioxide (CO2 ) is a main greenhouse gas produced in industrial cities [2]. An excessive CO2 emission leads to harmful climate changes such as the global warming and sea level rising [2,3]. Thus, the design and development of inexpensive catalysts with high stability and efficiency for the conversion of CO2 to useful materials is highly desirable. The removal of carbon dioxide from the environment can reduce its greenhouse effects [4]. On the other hand, CO2 is a low cost and abundant carbon source [5]. So, the hydrogenation of CO2 and conversion to value-added chemical products, such as formic acid, methane, or methanol, is the great interest of scientific community [6,7]. The
∗ Corresponding author. E-mail address: esrafi[email protected] (M.D. Esrafili). http://dx.doi.org/10.1016/j.jmgm.2017.08.016 1093-3263/© 2017 Elsevier Inc. All rights reserved.
transformation of CO2 into formic acid through the hydrogenation process, has achieved considerable attention because the product has various industrial applications [8–11]. Formic acid is widely used as a feedstock in perfume and dyeing industry and as a reagent for production of organic derivatives such as aldehydes, ketones, carboxylic acids, and amides [12]. To date, numerous experimental studies have investigated the chemical and electrochemical conversion of CO2 to formic acid [13–15], but many aspects remain unknown, especially the reaction mechanism and the role of the used catalyst. In this context, quantum chemical calculations could be useful in understanding the mechanism of transformations, adsorption modes, activation energies, and the nature of transition states during the catalytic process, as well as the role of catalyst. Graphene is a novel form of carbon which could act as a suitable support for various metal and non-metal atoms to form new carbon–metal nanocomposite [16–18]. These composites could be used in several applications, including catalysis [17,19], drug delivery [20], sensors, memory devices [21,22], photocatalysis [23], conversion and storage of energy [24], solar cells [25,26], molecular
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imaging [27,28], and molecular electronics [29]. For the first time, graphene was discovered by Geim and Novoselov in 2004 [30]. This magic material of the century is a nanosheet with a single-carbon atom thick and has a honeycomb planar sp2 hybridized structure [31,32]. Graphene nanosheets are known for their high surfaceto-volume ratio and the high catalytic activity of graphene is due to its specific surface area [33]. Additionally, graphene exhibits excellent electronic and thermal conductivity [34,35]. Today, due to these unique chemical and physical characteristics, and many other extraordinary properties such as high flexibility [36], simplicity of functionalization, low cost, large 2-dimentional - conjugated structure [37,38], and the mechanical and chemical stability [36], graphene has attracted a great deal of attention in the scientific fields. For example, several reports have been published to show the interaction of toxic gases with the graphene nanosheets [23,39,40]. Moreover, recent theoretical and experimental studies have demonstrated that chemical doping of graphene with foreign atoms such as metal atoms [41–46], Al [47], N [48], P [49], and Si [50] could significantly improve its catalytic activity in oxidation/reduction reaction of CO2 , NO, CO, and N2 O molecules [51]. Very recently, novel Ni-N-modified graphene was successfully synthesized by Su et al. [52]. Their results showed that the resultant carbon-based catalyst could efficiently catalyze the electrochemical reduction of CO2 to CO. Using theoretical calculations, Sirijaraensre and Limtrakul [53] found that Cu-doped graphene can be viewed as a promising catalyst for the hydrogenation of CO2 to formic acid. In this contribution, the adsorption of CO2 and its subsequent hydrogenation reduction to formic acid are studied over Pt- and Nidoped graphene nanosheets (Pt-/Ni-G) using DFT calculations. The geometry, electronic structure and the catalytic activity of Pt-/Ni-G are studied and compared in details. To the best of our knowledge, this is the first report on the hydrogenation of CO2 molecule to formic acid over Pt-/Ni-G. The results of this study could be helpful in understanding the chemical and catalytic properties of metaldoped graphene sheets and in designing highly efficient carbonbased catalysts for the conversion of CO2 and other toxic gases to useful chemicals. In facts, this type of study could be helpful for cleaning of the human society. 2. Computational details In this study, all DFT calculations were performed using the Gaussian 09 suite of programs [54]. The geometry optimization and the subsequent frequency calculation of the complexes were performed using the M06-2X density functional. M06-2X is a global hybrid meta-GGA functional which known as a reliable method to study non-covalent interactions, main group thermochemistry, and reaction barriers [55,56]. In the DFT calculations, all atoms were described by the all electron 6-31G* basis set, except Ni and Pt atoms where the LANL2DZ basis set with the corresponding effective core potential was used. A hexagonal graphene supercell (4 × 4 graphene unit cell), containing 48 carbon atoms, was considered as the pristine graphene for the calculations. A carbon atom of the surface was substituted with a single Pt or Ni atom for the preparation of Pt- or Ni-G. The adsorption energy Eads (A) of the adsorbed molecules over Pt-/Ni-G was calculated as the following equation: Eads (A) = EA-S − ES − EA
(1)
where EA-S , ES , and EA are the total energies of the complex of adsorbate-substrate (A-S), the substrate (S), and the absorbate (A), respectively. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and density of state (DOS) plot of Pt-/Ni-G were obtained at the M06-2X/6-31G*(LANL2DZ) level.
3. Results and discussion This section is divided in three parts. First, the geometry, electronic structure, and stabilization of Pt-/Ni-G are studied. Next, the adsorption of CO2 and H2 molecules as well as their co-adsorbed configurations is analyzed in detail. Finally, the reaction mechanisms and energy barriers for the hydrogenation of CO2 by a H2 molecule will be discussed. 3.1. Geometry, electronic structure and stabilization of Pt-/Ni-G As mentioned before, the primary goal of this theoretical study is to understand the catalytic pathway for the hydrogenation of CO2 over Pt-/Ni-G surface. To this aim, the interaction modes and adsorption energies of CO2 and H2 molecules with these surfaces are obtained and discussed in details. Fig. 1 shows the optimized structures of Pt-/Ni-G. It is clear that after doping with the Pt or Ni atom, the geometric structure of the graphene is change significantly. The covalent bond distantances between Pt and neighboring C atoms in Pt-G are 1.94 and 1.98 Å, which are much larger than the C C bonds in the pure graphene (1.42 Å). The obtained Pt C bond lengths are shorter than those mentioned by Liu et al. (2.10 Å) [57], but in good agreement with the results of Lee et al. (1.94 Å) [58]. The formation of these Pt C chemical bonds is related to the interaction of the C-p states and the in-plane component Pt-d states. Therefore, the surrounding C atoms will move out of the surface of graphene to could interact with the Pt atom [57]. The Mulliken population analysis also shows that in the optimized Pt-G, there is about 0.8 e charge transfer from the Pt atom to the neighboring C atoms on the surface. Due to such a large charge transfer, there should be strong covalent interaction between the positively charged Pt and the neighboring carbon atoms, which helps to stabilize Pt atom. Similar to Pt-G, when the C atom of graphene is replaced with the Ni atom, Ni atom is move out the basal plane of doped system (Fig. 1). The average bond length of Ni C is 1.85 Å, which is smaller than that of reported by Tang and co-workers (2.12 Å) [59]. The observed discrepancies between the calculated bond lengths in this study and previous studies could be attributed to the differences between the used clusters or to applied DFT methods. In the optimized Ni-G, a charge of about 0.4 e is transferred from the Ni atom to the graphene surface, which is smaller than that of Pt-G surface. To consider the effect of Pt or Ni atom impurity on the electronic structure of the graphene, the total DOS (TDOS) plots of Pt-G and Ni-G are obtained. Fig. 1 depicts the obtained results. One can see that in contrast to the pristine graphene which has almost zero HOMO-LUMO energy gap, the TDOS of both Pt-G and Ni-G is characterized by a quite small energy gap. In particular, as the Ni atom is adsorbed over the defective graphene, a peak is appeared below the Fermi level, which is mainly attributed to the states of carbon atoms around the dopant atom. Moreover, the DOS plot of Pt-G clearly indicates that the Pt 5d peaks below the Fermi level overlap well with those of dangling C atoms around the dopant atom, which may explain the strong interaction between the Pt and neighboring C atoms. The HOMO-LUMO energy gap of Pt-G and Ni-G is calculated to be 3.12 and 3.59 eV, respectively, which indicates that the incorporation of the Pt or Ni atom into the network of graphene can significantly change its electronic structure. To consider the stability of Pt-/Ni-G surfaces, the diffusion of the Pt or Ni atom to its neighboring position on the graphene surface is studied. Fig. S1 of Supporting information indicates the optimized structures and activation energies for the diffusion of these atoms over the surface. As evident, the predicted diffusion barrier is 141.2 and 96.5 kcal mol−1 for the Pt and Ni atoms, respectively. These large energy barriers are also supported by a large adsorption energy of these dopant atoms. More especially, the adsorption
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Fig. 1. The optimized structures and DOS plots of Pt- and Ni-doped graphene sheets. All distances are in Å. In the DOS plots, the partial DOS plots of the dopant atom and dangling C atoms around the dopant atom are also depicted. The Fermi level is indicated by a dotted-dashed line.
Table 1 Calculated adsorption energy (Eads ), net charge transfer (qCT ), change of enthalpy (H298 ), and change of Gibbs free energy (G298 ) for the adsorption/co-adsorption of CO2 and H2 molecules over Pt-/Ni-G surfaces. Complex
Eads (kcal mol−1 )
qCT (e)
H298 (kcal mol−1 )
G298 (kcal mol−1 )
Pt-G A C E G
−7.7 −1.2 −3.8 −13.4
0.1 0.2 0.2 0.2
−7.6 −2.4 −4.9 −13.1
−1.8 5.2 8.3 −3.5
Ni-G B D F H
−8.3 −0.9 −8.0 −14.9
0.1 0.2 0.1 0.2
−8.2 −0.1 −8.6 −14.7
−2.5 7.1 3.6 −2.1
energy of the Pt and Ni atoms is calculated to be about −158 and −121 kcal mol−1 over the defective site of the graphene, respectively, suggesting a strong interaction between these dopant atoms and the surface. These findings reveal that both Pt-G and Ni-G are stable enough to be used in the catalytic hydrogenation of CO2 to formic acid. 3.2. Adsorption of CO2 and H2 over Pt-/Ni-G To study the hydrogenation of CO2 , we first consider the adsorption of individual CO2 and H2 molecules on Pt-/Ni-G surface. In order to find the more stable configurations for each absorbate (CO2 and H2 ), different adsorption orientations are considered. The resulted most energetically favorable configuration of absorbed CO2 over the Pt-G is depicted in Fig. 2 (complex A). As seen, CO2 molecule is absorbed on the Pt atom in a O C O· · ·Pt-G orientation with the calculated adsorption energy (Eads ) of −7.7 kcal mol−1 (Table 1). Note that the absolute value of the obtained Eads is larger than the calculated value for the adsorbed CO2 on Cu-graphene [53], but close to those of over Al- [60] and Pd-doped [61] graphene. The Pt-O intermolecular bond distance is 2.53 Å. The bond length of C O is 1.17 Å, which is 0.01 Å longer than those of free CO2
molecule. According to the Mulliken analysis, there is about 0.1 e charge transfer from CO2 to Pt-G, which leads to a decrease in the charge of the Pt atom from 0.8 e in Pt-G to 0.7 e in the complex A. The electron density difference (EDD) map is also provided to understand the electron density rearrangement due to the CO2 adsorption over the surface (Fig. 3). The blue and red colors of this map show electron gain and electron loss regions of the complex, respectively. As it is clear, there is a significant electron density accumulation between the CO2 and surface, which demonstrates the polarization of the O atom of CO2 by the positively charged Pt atom of the surface. On the other hand, an electron loss region is seen on the O atom not interacting with the Pt atom, which is related to the electron charge flow from CO2 to the surface. The negative G298 (−1.8 kcal mol−1 ) and H298 (−7.6 kcal mol−1 ) values in Table 1 clearly demonstrate that the adsorption of CO2 over Pt-G is exothermic and thermodynamically favorable process at room temperature. The adsorption of CO2 over Ni-G sheet is also investigated and the most stable configuration is shown in Fig. 2 (complex B). One can see that the orientation of adsorbed CO2 molecule over the Ni-G is almost similar to that of over Pt-G. The calculated Ni O intermolecular bond distance is 2.23 Å, and the C O bond length of CO2 (1.17 Å) is slightly elongated with respect to its isolated state (1.16 Å). The absolute value of Eads (−8.3 kcal mol−1 ) for the CO2 adsorption on Ni-G shows that the interaction of CO2 with Ni-G is stronger than with Pt-G. Besides, this Eads value is consistent with that of reported by Zhao and co-workers [62]. The EDD map indicates a great charge accumulation between the Ni and O atoms that confirm the strong polarization of CO2 molecule by the Ni atom (Fig. 3). Moreover, the Mulliken population analysis shows that there is a charge transfer of about 0.1 e from CO2 to the surface. The thermochemistry calculations indicate that the adsorption of CO2 over Ni-G sheet is exothermic and a thermodynamically feasible reaction at room temperature (Table 1). As seen in Table 1, the absolute values of Eads and the thermodynamic parameters (H298 and G298 ) of the adsorbed CO2 over Ni-G are larger than those of Pt-G.
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Fig. 2. The optimized geometries of absorbed/coadsorbed CO2 and H2 molecules over Pt-/Ni-G surface. All distances are in Å.
Also, the adsorption of H2 molecule over Pt-/Ni-G nanosheets is evaluated to find the most stable configurations (Fig. 2, complexes C and D). The calculated Eads values of H2 over Pt-G and Ni-G are −1.2 and −0.9 kcal mol−1 , respectively (Table 1). In both cases, a significant increase in the bond length of absorbed H2 molecule is observed (from 0.60 to 0.76 Å). The binding distance of formed Pt(Ni)· · ·H bonds is about 2.17 (2.22) Å. It should be noted that the estimated Ni.... H2 binding distances in the present study is consistent with those of obtained by Ramos-Castillo et al. (2.14 Å using the PBE functional) [63]. The EDD map for the absorbed H2 molecule over the both of surfaces could also verify the weak and physical nature of these adsorptions. As seen, there exists a small electron density accumulation around the hydrogen molecule, which is due to small polarizability of this molecule in the presence of the Pt or Ni atom. However, Table 1 indicates that there is a sizable charge transfer from H2 to the surface, due to the interaction between the bond of H2 molecule atom and the Pt atom. This indicates that the stability of these complexes can be mainly attributed to the charge transfer effects. Moreover, this finding reveals that Pt-/Ni-G can strongly activate H2 molecule, which may facilitate the hydrogenation of CO2 by this molecule. The adsorption of H2 over Ni-G and Pt-G is exothermic but these interactions cannot be occurred at ambient conditions due to the positive G298 value (Table 1). 3.3. Co-adsorption of CO2 and H2 molecules over Pt-/Ni-G Next, the co-adsorption of CO2 and H2 molecules over Pt-/Ni-G is investigated (Fig. 2, complexes E and F). The optimized coadsorption complexes over Pt-G and Ni-G surfaces have adsorption energies of −3.8 and −8.0 kcal mol−1 , respectively. Note that the absolute values of these adsorption energies are smaller than the sum of individual adsorption energies of CO2 and H2 molecules over each surface. This indicates that the adsorption of each species tends to weaken the adsorption of another one. That is, they do not act cooperatively with each other. The important geometrical parameters of co-adsorbed complexes are shown in Fig. 2. Based on the resulted thermodynamic parameters, the formation of both of complexes is exothermic, but impossible at room temperature (H298 < 0, G298 > 0). As noted above, the interaction between CO2 and Pt-/Ni-G is stronger than H2 . Therefore, the catalysis process for CO2 hydrogenation over Pt-/Ni-G support would initiate with the adsorption of CO2 on the Pt-/Ni-G. Considering the large adsorption energy of CO2 molecule, the co-adsorption of two CO2 molecules is also probable over both sur-
faces. Fig. 2 shows the optimized structures (complexes G and H). One can see that the co-adsorption of CO molecules on the both surfaces is stronger than that of CO2 + H2 adsorption, due to the large co-adsorption energy of −13.4 (Pt-G) and −14.9 kcal mol−1 (Ni-G). This means that, the Pt or Ni atom of the surface could be dominantly covered by CO2 molecules if a CO2 /H2 mixture is injected as the reaction gas. The calculated large charge-transfer values (qCT ) also verify the chemistrption of CO2 molecules over the surfaces. For both configurations, the negative G298 and H298 values show that the adsorption of CO2 molecules is exothermic and a thermodynamically possible process at ambient condition. 3.4. Proposed reaction mechanisms for the CO2 hydrogenation over Pt-/Ni-G According to the previous study [53], the catalytic reduction of CO2 by H2 molecule generally occurs through a bimolecular mechanism. This involves the co-adsorption of CO2 and H2 molecules, followed by the formation of a OCOH intermediate, and finally desorption of HCOOH from the surface. It should be noted that there is also reported an alternative mechanism for the dehydrogenation of CO2 over Cu-doped graphene [53], which starts with the dissociation of H2 molecule on the surface, followed by the addition of CO2 to form of HCOO moiety. However, the latter pathway seems unlikely to proceed over Pt- or Ni-G surface, since the adsorption energy of H2 molecule is very smaller than that of CO2 . Instead, we suggest here a termolecular mechanism with the H2 molecule being directly activated by the two pre-adsorbed CO2 molecules as another possible mechanism for reduction of CO2 . In the following section, we study both bimolecular and termolecular reaction pathways over Pt- or Ni-G surfaces, separately. 3.4.1. Bimolecular mechanism The possible bimolecular reaction pathway for the hydrogenation of CO2 molecule and conversion to formic acid is as follow: CO2 + Pt-/Ni-G → CO2 · · ·Pt-/Ni-G
(2)
H2 + CO2 · · ·Pt-/Ni-G → CO2 /H2 · · ·Pt-/Ni-G
(3)
CO2 /H2 · · ·Pt-/Ni-G → H-Pt-/Ni-COOH-G
(4)
H-Pt-/Ni-COOH-G → HCOOH· · ·Pt-/Ni-G
(5)
The energy profiles and the optimized geometries of the initial states (IS), transition states (TS), and final state (FS) over Pt-/Ni-G
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Fig. 3. EDD maps of absorbed CO2 (complexes A and B) and H2 (complexes C and D) molecules on Pt-/Ni-G. The isovalue of EDD plots is ±0.001 au. Red and blue colors indicate regions of decreased and increased electron densities, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 Calculated activation energy (Eact ), reaction energy (E), change of enthalpy (H298 ) and Gibbs free energy (G298 ) for the proposed pathways of the reduction of CO2 by H2 over Pt-/Ni-G. Reaction
Eact (kcal mol−1 )
E (kcal mol−1 )
H298 (kcal mol−1 )
G298 (kcal mol−1 )
Pt-G IS1 → IS2 IS2 → FS2 IS5 → FS5 IS6 → IS7 IS7 → FS7
20.8 3.4 18.4 28.8 1.8
16.1 −21.1 −19.5 18.2 −28.7
16.4 −20.9 −18.1 16.0 −26.9
16.9 −21.6 −12.4 13.5 −18.4
Ni-G IS3 → IS4 IS4 → FS4 IS8 → FS8 IS9 → IS10 IS10 → FS10
47.9 0.4 41.3 45.5 1.4
43.8 −45.4 −6.2 33.7 −37.7
42.5 −44.3 −4.4 30.8 −34.7
45.7 −46.8 −2.1 24.9 −21.4
are shown in Figs. 4 and 5, respectively. Also, in Table 2, the calculate activation energy (Eact ), energy of the reaction (E), changes of enthalpy (H298 ), and Gibbs free energy (G298 ) are summarized. The mechanism of CO2 conversion to formic acid is proposed to proceed in a two-step process; first the formation of a OCOH intermediate and next the conversion of this intermediate to formic acid. The reduction of CO2 starts with the bimolecular adsorption configuration of CO2 and H2 gases over the Pt-/Ni-G substrate. As noted previously, based on the resulted adsorption energies of CO2 and H2 molecules, at first, CO2 is absorbed over Pt-G surface and then H2 molecule is co-adsorbed to form the complex C. Hence, the stable complex E is chosen as the initial state (IS1). As Fig. 4 indicates, IS1 can be converted to the OCOH intermediate through the transition state TS1. At TS1, the distance between the C atom of CO2 and the Pt becomes shorter from 3.73 in IS1 to 2.46 Å, while the H H bond distance is lengthened from 0.76 to 1.00 Å. As a result, the
dissociated hydrogen atom (H1) of H2 molecule is moved toward the O1 atom of the absorbed CO2 with the distance of 1.27 Å. Also, the intermolecular distance between the C atom of CO2 and the H2 atom is shortened (from 3.91 to 2.71 Å). Moreover, the sp hybridization of the carbon atom of CO2 is changed to sp2 and the angle of O C O is reduced from 177.7 in IS1 to 138.9◦ in TS1. The vibrational frequency analysis of TS1 indicates that there is one imaginary frequency of −1390 cm−1 , which is in accordance with the cleavage of H1–H2 and formation of O1 H1 bond. The computed activation energy for this step of reaction is 20.8 kcal mol−1 which is smaller than those of other metal-doped graphene catalysts [53]. The result of this step is the hydrogenation of O atom of CO2 molecule and the formation of the OCOH intermediate (IS2) which is strongly absorbed on the Pt atom. The calculated reaction energy and thermodynamic quantities obtained for this step demonstrate that the formation of the OCOH intermediate is thermodynamically unfa-
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Fig. 4. The energy profile, optimized geometries of the initial states (IS), transition states (TS), and final state (FS) for bimolecular mechanism of CO2 reduction by H2 molecule over Pt-G. All distances are in Å.
vorable reaction at room temperature. In the next step, the H2 atom transfers from the Pt atom to the carbon of COOH through the transition state TS2. In this state, the bond distance between H2 and C atom decreases from 2.36 to 1.58 Å and the Pt-H distance is increased by about 0.1 Å. The frequency analysis with one imaginary frequency of −638 cm−1 confirms the formation of TS2. This vibration frequency is related to the dissociation of Pt-H bond and the formation of C H bond. The calculated activation energy for this step is just 3.4 kcal mol−1 . Finally, the formic acid is produced and absorbed on the Pt atom of the Pt-G with an adsorption energy of −13.6 kcal mol−1 . Besides, the IS2 → FS2 is exothermic and could be performed at ambient conditions (H298 < 0 and G298 < 0). Furthermore, the CO2 reduction over Ni-G is studied. Similar to Pt-G, a stepwise reaction pathway is considered for the CO2 hydrogenation over this surface (Fig. 5). As mentioned before, at the first step of the reaction, the co-adsorbed complex F as the initial state (IS3) is converted to the OCOH intermediate (IS4) through TS3. In TS3, the distance between O1 atom of CO2 and the H1 atom of the hydrogen molecule becomes shorter (from 2.71 Å in IS3 to 1.20 Å in TS3) and the intermolecular distance of C H2 is reduced by 1.77 Å. Also, CO2 molecule initiates to bend to form the carboxyl group. The observed imaginary frequency at −1230 cm−1 confirms the formation of TS3. This frequency corresponds to the cleavage of the H1–H2 and the formation of the O1 H1 bond. As seen earlier, the OCOH group and an adsorbed H atom is resulted from
this step of the reaction as an intermediate (IS4). The activation energy of this process is computed to be 47.9 kcal mol−1 , which is larger than those calculated over Pt-G. In the second step, the C atom of the OCOH group is hydrogenated by H2 to form the formic acid as a favorable product. It should be noted that this conversion occurs as an almost barrier-less reaction over Ni-G and the activation energy of this step is negligible. The enthalpy and Gibbs free energy changes of the final step of this reaction are −44.3 and −46.8 kcal mol−1 , respectively, which confirm that this reaction is exothermal and possible at room temperatures (Table 2). Based on the obtained results, it is concluded that the formation of the OCOH group over both Pt-G and Ni-G surfaces is the rate determining step. Although the second step of the reaction (OCOH → HCOOH) could proceed rapidly over Ni-G surface, but the activation energy needed for the dissociation of H H bond and the formation of the OCOH intermediate is much smaller over Pt-G than Ni-G surface. This is most likely due to the large ionic nature of Pt C bonds in the former surface, which can greatly stabilize the formed OCOH group in the corresponding transition state structure (TS1). This result shows that the large positive charge on the dopant atom in Pt-G can greatly regulate its surface reactivity, and therefore affect the CO2 reduction. So, compared to Ni-G, the catalytic activity of Pt-G for the reduction of the CO2 is high. Therefore, according to our findings, the Pt-G surface could be used as an effi-
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Fig. 5. The energy profile, optimized geometries of the initial states (IS), transition states (TS), and final state (FS) for bimolecular mechanism of CO2 reduction by H2 molecule over Ni-G. All distances are in Å.
cient catalyst for the reduction and conversion of CO2 to the formic acid in the practical application. 3.4.2. Termolecular mechanism In our proposed termolecular mechanism, first two CO2 molecules are co-adsorbed over the surface and then H2 molecule is activated by the pre-adsorbed CO2 molecules. Fig. 6 shows the energy profile and optimized stationary points related to the termolecular mechanism on Pt-G surface. As seen, we take a H2 molecule and two CO2 molecules co-adsorbed on Pt atom as the initial state of the reaction (IS5), in which Pt-O and Pt-H distances are 2.50 and 3.23 Å, respectively. Subsequently, the H H bond of H2 molecule is elongated by 0.82 Å and the hydrogen atoms approach the co-adsorbed CO2 molecules. The calculated activation energy for the formation of the corresponding transition state (TS5) is calculated to be 18.4 kcal mol−1 , which is 2.4 kcal mol−1 smaller than that of TS-1. At FS5, two OCOH moieties are formed over the Pt atom with Pt-C distances of about 2.30 Å. Besides, the formation of these intermediates is exothermic and a thermodynamically possible process at ambient temperature (Table 2). In the next step, the second H2 molecule is adsorbed over the Pt atom, resulting in the formation of IS6 configuration. The binding distance between H2 and the surface is calculated to be 3.36 Å. By overcoming a large activation energy of 28.8 kcal mol−1 , the H2 molecule is dissociated and the H3 atom approaches the carbon atom of OCOH intermediate.
Note that the calculated activation energy for the hydrogenation of OCOH fragment is larger than that of obtained in the bimolecular pathway. Finally, the H4 atom is attached to the OCOH moiety and the second formic acid is formed over the surface. The average adsorption energy for HCOOH molecules is about −10 kcal/mol, suggesting that the formed formic acid molecules would desorb spontaneously and the Pt-G surface is refreshed to a new cycle of CO2 reduction. Almost a similar termolecular mechanism is obtained for the hydrogenation of CO2 molecules over Ni-G surface (Fig. 7). Briefly, the H2 molecule is first added to the pre-adsorbed CO2 molecules over the Ni atom to form the initial state IS7. The calculated adsorption energy of the H2 molecule is found to be −0.6 kcal mol−1 , which is −0.3 kcal mol−1 smaller (less negative) than that of complex D. An activation energy of about 41.3 kcal mol−1 is needed to cleavage the H H bond of H2 molecule. The formation of the intermediate state FS7 is found to be exothermic with a small negative G298 value (−6.2 kcal mol−1 ) which indicates the possibility of the formation of OCOH intermediates at normal temperature. The calculated activation energy for the hydrogenation of the first OCOH fragments is 45.5 kcal mol−1 , which is about 17 kcal mol−1 larger than that of over Pt-G (Table 2). Besides, the second hydrogen atom is added to the remaining OCOH moiety by overcoming a negligible activation energy (1.8 kcal mol−1 ). Finally, the formed HCOOH molecules are released from the surface due to their small desorption energies.
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Fig. 6. The energy profile and optimized geometries of the corresponding stationary points for the termolecular mechanism of CO2 reduction by H2 molecule over Pt-G. All distances are in Å.
According to these findings, we can predict the outcomes of the reduction of CO2 by H2 molecule on the surface of Ni- and Pt-G. The calculated activation energies indicate that for the bimolecular pathway, the formation of OCOH intermediate over both surfaces is the rate-determining step. In the case of the termolecular mechanism, however, the dissociation of the second H2 molecule and the formation of first HCOOH molecule is found to need a larger activation energy compared to other steps. Thus, considering the termolecular reaction pathway, the major product of CO2 + H2 reaction on both surfaces is OCOH moiety; which is consistent with the previous studies using other catalysts [64,65]. However, comparing the adsorption energies of the isolated CO2 and H2 molecules with the activation energies clearly suggests that the reduction of CO2 molecule is unlikely to take place under normal conditions, due to the relatively larger energy barriers. Consequently, it is concluded that the performance of Pt-G or Ni-G as a catalyst used for the reduction of CO2 is poor. To further improve the surface reac-
tivity of graphene, making modifications such as structural defects or chemical functionalization may be suggested. 4. Conclusion In the present study, the catalytic performance of Pt-/Ni-G for the hydrogenation of CO2 by H2 molecule was investigated using the DFT calculations. According to our results, both Pt-G and Ni-G surfaces are stable enough to be used in the catalytic hydrogenation of CO2 , due to their large energy barriers for the diffusion of the Pt or Ni atom to its neighboring position on the graphene surface. Two reaction mechanisms were proposed for the reduction of CO2 by a H2 molecule. In the bimolecular mechanism, CO2 and H2 molecules are first co-adsorbed over the surface and then a OCOH intermediate is obtained. Finally a HCOOH molecule is formed over the surface, which can be easily released from the surface due to the small desorption energy. On the other hand, in our proposed termolecular mechanism, H2 molecule is directly activated by the
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Fig. 7. The energy profile and optimized geometries of the corresponding stationary points for the termolecular mechanism of CO2 reduction by H2 molecule over Ni-G. All distances are in Å.
two pre-adsorbed CO2 molecules. For both mechanisms, it is found that Pt-G has a higher catalytic activity than Ni-G one. This can be related to the large positive charge on the Pt atom in Pt-G, which significantly regulate the surface reactivity, and consequently affect the CO2 reduction. For the bimolecular mechanism, the formation of the OCOH intermediate was found to be the rate determining step over both surfaces. Also, the second step (OCOH → formic acid) could proceed as a barrier-less reaction over Ni-G surface. On the contrary, the formation of the first formic acid was found as the rate-determining step over Pt- and Ni-G surfaces. However, considering the calculated adsorption energies and activation energies, it is proposed that the reduction of CO2 molecule is unlikely to take place over both surfaces under normal conditions. Overall, these theoretical findings could be helpful in practical applications for designing the graphene-based catalysts for the removal of the toxic CO2 gas and conversion it to useful products such as formic acid.
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