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Organometallic compound as an efficient catalyst toward oxygen reduction reaction
Inorganic Chemistry Communications 108 (2019) 107520
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Short communication
Organometallic compound as an efficient catalyst toward oxygen reduction reaction
T
Yaghoub Bazvanda, Maziar Noeib, , Fereydoon Khazalia, Zohreh Saadatia ⁎
a b
Department of Chemistry, Omidiyeh Branch, Islamic Azad University, Omidiyeh, Iran Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran
GRAPHICAL ABSTRACT
Catalytic activation of the O2 is an essential step for the oxygen reduction reaction process in the fuel cells.
ARTICLE INFO
ABSTRACT
Keywords: O2 activation Oxygen reduction reaction Organometallic compounds Density functional theory
The interaction between oxygen molecule (O2) and organometallic compounds has always been an interesting research topic because O2 plays a benchmark role for demonstrating activity level of the organometallic-based catalysts. In this work, the effect of various transition metals (TMs) on the interaction between O2 and TMCmHm organometallic compounds is investigated using density functional theory (DFT) calculations. Our results show that the TM decoration enhances the reactivity of CmHm, however, it is more effective in the case of Scdecorated on the CmHm systems. In addition, the O2 adsorption energy and OeO bond length activation are considerably changed with changing the TM atoms in the TM-CmHm organometallics compounds. Consistent with the prediction of chemical hardness descriptors, the best catalytic activity toward O2 activation is related to the Sc-decorated on CmHm systems. The Sc-CmHm organometallic compounds strengthen the interaction between O2 and Sc atom, resulting in increased adsorption energy; the corresponding OeO bond length is also elongated. Moreover, the energy barrier of the O2 activation on the Sc-C6H6 is less than 0.50 eV, representing that the reaction can proceed easily at room temperature. Because the bond length of the O2 correlates with its catalytic activation, the results can provide a new application of organometallic-based catalysts.
1. Introduction Fuel cells are devices with important role in the industrial
⁎
applications to convert the chemical energy into electrical energy without pollution. The performance of fuel cell correlates with the oxygen reduction reactions (ORRs) at the cathode [1]. However, the
Corresponding author. E-mail address: [email protected] (M. Noei).
https://doi.org/10.1016/j.inoche.2019.107520 Received 17 July 2019; Received in revised form 12 August 2019; Accepted 12 August 2019 Available online 13 August 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
activation of oxygen molecule by a high efficient catalyst is a crucial step for evaluating the possibility of ORR [2]. On the other hand, the ORR is a kinetically slow reaction that restricts the performance of fuel cells [3–5]. To overcome the above problem, platinum (Pt) is considered as a suitable catalyst to improve the speed of ORR at the cathode [6]. However, the voltage losses at the cathode and high cost restrict the use of Pt-based catalysts in fuel cells. In this respect, replacing the expensive Pt-based catalyst is one of the most important aims in the catalytic reactions [7–10]. Catalysis by supported transition metals (TMs) on the various materials is a very interesting method of research [11,12] since the discovery of TMs catalytic activity [13,14]. In the present study, by using first-principles calculations, we have investigated the activation of the O2 molecule on the surface of organometallic compounds. The metal decorated hydrocarbons, aromatic materials, fullerenes, nanotubes and graphene are widely studied for various applications [15–22]. Recent studies [23–28] show that the TMs (Sc, Ti, V, Fe, Ni and Pd) considerably enhance the H2 adsorption at the metal site [29]. In order to study the generality of organometallic field, we have performed a systematic theoretical study of the O2 activation on the organometallic compounds including the TMs decorated on three different molecular templates, namely, C4H4, C5H5, and C6H6. Since these structures have diverse π-electron deficiencies [29–32], the TM atoms decorated on these different molecular templates are expected to have different O2 activation capabilities. Accordingly, to explore the catalytic activity of TMs decorated on the CmHm (m = 4, 5, and 6) molecular templates for O2 activation, we have chosen Sc, Ti, V, Cr, and Mn TM atoms decorated on the CmHm templates, and performed the DFT calculations, and hope to find out the driving force for O2 dissociation from a molecular level. Thus, our study here is expected to act as a guide in the selection of suitable organometallic systems as potential catalysts for ORR.
hardness associated with a variation in the electron number.
Eads = E(O2 /TM CmHm)
E(O2)
E(CmHm) E(TM CmHm)
1 2
E (N ) =
(r )
A N
(r )
(5)
(r )
aN + bN 2 1 + cN
(6)
in which the coefficients a, b and c are related to A, I1 and I2 through
c=
I2 2I2
2I1 + A 2I1 A
I1
b=
A
(7)
I1 + A c 2
(8)
I1 + A I A + 1 c 2 2
(9)
2
a=
Then the hardness and hyper-hardness can be obtained by
= 2 × (b
(10)
ac );
The static dipole polarizability of the considered TM-CmHm systems is also studied in the present work. The components of polarizability tensor have been obtained in terms of the finite field method as the second order derivatives of the total energy with respect to the homogenous external electric field: ij
=
µi (F) = Fj
2E (F)
Fi Fj
i, j = x , y , z
(11)
The values αxx, αyy, and αzz are defined as main semiaxes of polarizability ellipsoid of the molecule. These are also used for calculation of mean polarizability:
=
1 ( 3
xx
+
yy
+
zz )
(12)
3. Results and discussion 3.1. Pristine TM-CmHm organometallic systems
(1)
We have performed full geometry optimizations for the TM-CmHm organometallic systems with the chemical formula TM-C4H4, TM-C5H5, and TM-C6H6. The optimized structures of the TM-CmHm systems are represented in Fig. 1. Our DFT calculations predict the TM-C bond lengths of the TM-CmHm systems with the values being in the range of 1.94 (Mn-C5H5) to 2.68 Å (Sc-C6H6) (Fig. 1). Interestingly, the TM-C bond lengths in the Cr-CmHm and Mn-CmHm indicate same values and vary in range between ~1.99 Å (in Cr-CmHm) and ~1.97 Å (in MnCmHm). Also, Fig. 1 indicates that the CmHm molecular templates in the considered TM-CmHm systems are completely planar. The NBO analysis showed a net charge transfer from TM to the neighbor atoms in the considered systems, indicating an ionic nature. In addition, as shown in Fig. 2, all TM atoms in the Tm-CmHm systems have positive charges. Clearly, there is a remarkable charge transfer from the TM atom to the neighbor atoms in the CmHm molecules, which is likely responsible for the stabilization of the TM-CmHm systems. The range of
(2)
2E
N2
(4)
(r )
For calculating hyper-hardness, we have employed the way developed by Fuentealba and Parr [37,38]. They have computed hardness as well as hyper-hardness from the electron affinity (A), and the first and second ionization potential (I1 and I2), assuming that the energy varies with the number of electrons according to the relation
where E (TM-CmHm) is the total energy of the TM-CmHm systems, E (TM) and E (CmHm) are the total energies of a TM atoms and the CmHm molecular templates, respectively. Also, E(O2/TM-CmHm) and E (O2) are the total energies of the O2/TM-CmHm complexes and isolated O2 molecule, respectively. We have also investigated the reactivity of the TM-CmHm systems by use of chemical hardness descriptor. Chemical hardness is a useful concept for understanding and predicting the chemical behavior of different compounds. Hardness is defined as the second derivative of the energy with respect to the number of electrons,
=
N
(r )
I N
First-principles all-electron calculations of the total energies and optimized structures were carried out using the DFT, as implemented in the GAUSSIAN 09 software [33]. The DFT calculations were performed using the GGA with the parametrization of Wang and Perdew (PW91) [34] and 6-31+g(d) basis set. It has been represented that the PW91 provides an efficient and robust basis for calculations of properties of organometallics compounds with TM atoms [29–32]. Also, we adopted a DFT + D3 (D stands for dispersion) method with Grimme's van der Waals (vdW) correction in our calculations [35]. Further, the natural bond orbital (NBO) analysis [36] was used to calculate the given charge transfer in this work. The binding energy (Ebin) of TMs on the CmHm and the adsorption energy (Eads) for the O2 adsorbed on the TM-CmHm systems are computed using the following equations:
E(TM)
=
N3
Obviously, hyper-hardness gives a comparison between the variation of the ionization potential and the electron-affinity when the number of electron of the system changes.
2. Computational details
Ebin = E(TM CmHm)
3E
=
(3)
and can be used to characterize the relative stability of a system. The next derivative, called the hyper-hardness, measures the change in 2
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
Fig. 1. The optimized structure of TM-C4H4, TM-C5H5, and TM-C6H6 organometallic systems (TM = Sc, Ti, V, Cr, and Mn).
calculated charge on the TM atoms is from 0.472e to 0.835e for the TMC4H4, 0.453e to 0.879e for the TM-C5H5, and 0.517e to 0.880e for the TM-C6H6, respectively. In order to test the thermal stability of the TMCmHm systems, which is important for their applications at high temperatures, the binding energies (Ebin) of all TM-CmHm systems were calculated. As shown in Fig. 2, the binding strength for the TMs located on CmHm molecular templates decreases in the order Sc-C6H6 > ScC5H5 > Sc-C4H4 > Cr-C6H6 > Ti-C4H4 > Ti-C5H5 > TiC6H6 > V-C6H6 > V-C4H4 > Mn-C6H6 > Mn-C5H5 > CrC5H5 > Cr-C4H4 > Mn-C4H4 > V-C5H5 with the Ebin of −6.96, −6.61, −6.54, −6.21, −5.97, −5.91, −5.53, −5.42, −5.15, −5.06, −4.56, −4.10, −3.77, −3.51, and −1.64 eV, respectively. These
results show that the TMs bind strongly on the CmHm molecules. The values for chemical hardness (η) descriptor of the TM-CmHm systems is also calculated and represented in Fig. 2. Obviously, the values of the η show a decreasing trend presenting the increase in the reactivity of considered systems from Mn-C4H4 to Sc-C6H6. Fig. 2 also includes the mean static dipole polarizability (〈α〉) values of the TMs located on the CmHm systems. It should be noted that the 〈α〉 of TM-CmHm systems increase along the following series: Sc-C6H6 > Sc-C5H5 > TiC5H5 > Sc-C4H4 > Ti-C4H4 > Ti-C6H6 > V-C4H4 > Cr-C6H6 > Cr-C5H5 > Mn-C6H6 > V-C6H6 > Mn-C5H5 > Cr-C4H4 > VC5H5 > Mn-C4H4. It is worth pointing out that the calculated 〈α〉 values are consistent with the reactivity trends predicted by η for the 3
sc-C5H5
-6.96 1213 Sc-C6H6
1058 Ti-C5H5
1190
997 Sc-C4H4
947 Ti-C6H6
997
939 V-C4H4
Ti-C4H4
933
866 Cr-C5H5
Cr-C6H6
840
778 V-C6H6
Mn-C6H6
0.05 Sc-C6H6
638
0.12 Sc-C5H5
Mn-C5H5
0.26 Sc-C4H4
637
0.38 V-C4H4
Cr-C4H4
0.40 Cr-C6H6
591
0.45 Ti-C4H4
V-C5H5
0.48 Ti-C5H5
479 0.62 Ti-C6H6
Mn-C4H4
0.65 Cr-C5H5
0.82
1.11 V-C6H6
Mn-C6H6
1.12 Mn-C5H5
Cr-C4H4
V-C5H5
1.22
2.00
2.09
η (eV)
Sc-C5H5
-4.56
-3.77
-4.10
-3.51
Ebin (eV)
<α> (Å3)
Mn-C4H4
sc-C6H6
sc-C4H4 -6.54
Cr-C6H6
-6.61
Ti-C4H4
Ti-C6H6
-6.21
V-C6H6 -5.42
-5.53
Ti-C5H5
V-C4H4
-5.91
Mn-C6H6
-5.15
Mn-C5H5
-5.06
Cr-C5H5
Cr-C4H4
Mn-C4H4
V-C5H5
0.88 sc-C6H6
-1.64
0.879
0.835 sc-C4H4
sc-C5H5
0.829
0.772 Ti-C4H4
Cr-C6H6
0.527 V-C4H4
0.766
0.517
0.754
0.504
Mn-C6H6
Ti-C5H5
0.485 Cr-C5H5
Mn-C5H5
Ti-C6H6
0.483 Cr-C4H4
0.665
0.472 Mn-C4H4
V-C6H6
0.453 V-C5H5
Charge (e)
-5.97
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
Fig. 2. Comparative analyses for the charge on the metal atoms, binding energies, chemical hardness values, and polarizabilities of the studied organometallic systems.
TM-CmHm systems. In other word, the maximum 〈α〉 values for the TM-CmHm systems are related to the systems with the minimum η which had been predicted as the most reactive systems. All in all, among the studied TM-CmHm systems, the higher charge transfer, higher 〈α〉, and lower η belongs to the Sc-C6H6 system (Fig. 2). Accordingly, the Sc-C6H6 is predicted as the most reactive system.
0.539e from Mn-C4H4, Mn-C5H5, and Cr-C4H4 to the O2 molecule, respectively (Fig. 4). Things are significantly different when we consider the other TM atoms decorated on the CmHm molecular templates. As shown in Fig. 4, the charge transfer to the O2 molecule are in the order of O2/Sc-C6H6 (0.907e) > O2/Sc-C5H5 (0.899e) > O2/ScC4H4 (0.879e) > O2/Ti-C4H4 (0.733e) > O2/Ti-C5H5 (0.697e) > O2/TiC6H6 (0.643e) > O2/V-C4H4 (0.629e) > O2/Cr-C6H6 (0.602e) > O2/Cr-C5H5 (0.584e) > O2/Mn-C6H6 (0.573e) > O2/VC6H6 (0.547e) > O2/V-C5H5 (0.540e) > O2/Cr-C4H4 (0.539e) > O2/Mn-C5H5 (0.499e) > O2/Mn-C4H4 (0.496e). Also, the largest and smallest Eads and r values are for the Mn-C4H4 and Sc-C6H6, respectively. On Sc-C6H6 system, O2 binds more strongly (−7.69 eV) and exhibits a higher degree of activation: OeO distance is 1.496 Å, and the charge on O2 is 0.907 e (Fig. 4). These results clearly show a higher level of activation. The fact that the O2 activation is higher for the Sc located on the CmHm, despite Eads is similar to the Ti-CmHm (Fig. 4), is due to the electrostatic interaction of the O2 molecule with the ionic ScCmHm systems. Our results reveal that the ability of the O2 adsorption enhances with the increase in reactivity of the TM-CmHm systems. Accordingly, it is concluded that the O2 is highly activated by the Sc atoms in the Sc-CmHm systems. In fact, the partially ionized Sc metal atom located on the CmHm molecular templates polarizes the O2 molecule. It is worth pointing out that the obtained results are in agreement with the reactivity trends predicted by η values. In other word, the maximum Eads and the higher O2 activation are related to the O2 adsorption on the Sc-CmHm systems, which had been predicted as the most reactive organometallic systems. We explore now the OeO activation path with formation of two
3.2. O2 activation by the TM-CmHm organometallic systems The O2 activation is a prototypical catalytic procedure [39–41]. Accordingly, it is selected in the present work as a benchmark reaction to evaluate the catalytic performance of the TM-CmHm systems. We systematically examine O2 adsorption on the TM-CmHm systems to reveal their catalytic activity evolution toward O2 molecule. In order to obtain the most stable adsorption geometries, we examined both end-on and side-on geometries of O2 adsorption, and found that the side-on geometries of O2 adsorption are more stable as shown in Fig. 3. Our results show that the most stable configuration is O2 axis parallel to the TM-CmHm with two TM-O bonds formed. The activation happens generally by partial electron transfer from the TM-CmHm organometallic systems to the 2π* antibonding orbital of O2 and results in the elongation of the OeO bond. The calculated Eads, OeO bond length (r), and negative charges on the O2 molecule (Q) for the studied O2/TMCmHm complexes are represented in Fig. 4. On Mn-C4H4, Mn-C5H5, and Cr-C4H4 the O2 molecule is captured by −5.61, −5.65, and −6.05 eV, respectively and the OeO distance is elongated from 1.240 Å (free O2, experimental r is 1.210 Å [42]) to 1.412, 1.414, and 1.430 Å respectively; there are the small charge transfers of 0.496, 0.499e, and 4
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
Fig. 3. The side view of the most stable configurations for the O2 adsorbed on the TM-C4H4, TM-C5H5, and TM-C6H6 organometallic systems (TM = Sc, Ti, V, Cr, and Mn).
adsorbed O atoms on the Sc-decorated C6H6. To obtain the minimum energy path of the O2 activation, we select the most stable configurations of the O2 on the Sc-C6H6 as the initial state as represented in Fig. 3. The final state consists of two chemisorbed O atoms locate at the shoulders of Sc atom. The minimum energy path profile of the O2 activation reaction and the corresponding initial state (IS), transition state (TS) and final state (FS) are represented in Fig. 5. As shown in this
figure, the O2 activation reaction on the Sc-C6H6 system is thermodynamically favorable, approved by their exothermal procedure (0.25 eV). Moreover, the energy barrier of the activation reaction (0.48 eV), shows that the O2 activation on the Sc-C6H6 system will be kinetically preferable. These results propose that the Sc-C6H6 system is an efficient catalyst for the O2 activation reaction. 5
Inorganic Chemistry Communications 108 (2019) 107520
-5.61 -5.65 -6.05 -6.11 -6.14 -6.25 -6.41 -6.54 -6.71 -6.82 -7.24 -7.34 -7.39 -7.49 -7.69
Mn-C4H4 Mn-C5H5 Cr-C4H4 V-C5H5 V-C6H6 Mn-C6H6 Cr-C5H5 Cr-C6H6 V-C4H4 Ti-C6H6 Ti-C5H5 Ti-C4H4 Sc-C4H4 Sc-C5H5 Sc-C6H6
Y. Bazvand, et al.
0.899
0.907 Sc-C6H6
0.643 Ti-C6H6
0.879
0.629 V-C4H4
Sc-C5H5
0.602 Cr-C6H6
0.733
0.584 Cr-C5H5
0.697
0.573
Ti-C4H4
0.547
1.496 Sc-C6H6
Mn-C6H6
1.494 Sc-C5H5
0.540
1.492 Sc-C4H4
V-C5H5
1.486 Ti-C4H4
V-C6H6
1.486 Ti-C5H5
0.539
1.484 Ti-C6H6
0.499
1.484 V-C4H4
Cr-C4H4
1.479 Cr-C6H6
0.496
1.477 Cr-C5H5
Mn-C5H5
1.477 Mn-C6H6
Mn-C4H4
1.471
1.458
1.430
V-C6H6
V-C5H5
1.414 Mn-C5H5
Cr-C4H4
1.412 Mn-C4H4
Ti-C5H5
Q (e)
r (Å)
Sc-C4H4
Eads (eV)
Fig. 4. Comparative analyses for the adsorption energies, OeO distances (r), and charge transfer values (Q) for the O2/TM-C4H4, O2/TM-C5H5, and O2/TM-C6H6 systems (TM = Sc, Ti, V, Cr, and Mn).
Fig. 5. Potential energy profile for activation of the O2 by the Sc-C6H6 organometallic system.
4. Conclusion
following findings. First, the O2 activation on the TM-CmHm organometallic systems is sensitive to the TM atoms. There is a net charge transfer from the TM-CmHm systems to the adsorbed O2 molecule. The high elongation of the OeO bond in the oxygen molecule confirms the charge transfers. Because the elongation of the adsorbed O2 exhibits its catalytic activation, we suggest that a TM-CmHm organometallic compound could be considered as a useful catalyst for O2 activation.
In summary, the structural geometries and the binding mechanism for the O2 adsorbed on the TM-CmHm organometallic systems are studied using DFT calculations. The C4H4, C5H5, and C6H6 molecular templates with Sc, Ti, V, Cr, and Mn TM atoms were investigated. From the results described in the previous sections, we can draw the 6
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
Among the studied TM-CmHm organometallic systems, the Sc-C6H6 shows the best catalytic performance toward the O2 activation. Since the total barrier is small (0.28 eV) and the exothermicity is enough large (0.25 eV), it is confirmed that the Sc-C6H6 can be considered as ORR catalysts. We are expecting that the results of our study will provide a clue for the future development of the organometallic-based catalysts.
[19] [20] [21]
Acknowledgments
[22]
The authors are grateful to Islamic Azad University-Iran for computational resources.
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7
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Organometallic compound as an efficient catalyst toward oxygen reduction reaction
T
Yaghoub Bazvanda, Maziar Noeib, , Fereydoon Khazalia, Zohreh Saadatia ⁎
a b
Department of Chemistry, Omidiyeh Branch, Islamic Azad University, Omidiyeh, Iran Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran
GRAPHICAL ABSTRACT
Catalytic activation of the O2 is an essential step for the oxygen reduction reaction process in the fuel cells.
ARTICLE INFO
ABSTRACT
Keywords: O2 activation Oxygen reduction reaction Organometallic compounds Density functional theory
The interaction between oxygen molecule (O2) and organometallic compounds has always been an interesting research topic because O2 plays a benchmark role for demonstrating activity level of the organometallic-based catalysts. In this work, the effect of various transition metals (TMs) on the interaction between O2 and TMCmHm organometallic compounds is investigated using density functional theory (DFT) calculations. Our results show that the TM decoration enhances the reactivity of CmHm, however, it is more effective in the case of Scdecorated on the CmHm systems. In addition, the O2 adsorption energy and OeO bond length activation are considerably changed with changing the TM atoms in the TM-CmHm organometallics compounds. Consistent with the prediction of chemical hardness descriptors, the best catalytic activity toward O2 activation is related to the Sc-decorated on CmHm systems. The Sc-CmHm organometallic compounds strengthen the interaction between O2 and Sc atom, resulting in increased adsorption energy; the corresponding OeO bond length is also elongated. Moreover, the energy barrier of the O2 activation on the Sc-C6H6 is less than 0.50 eV, representing that the reaction can proceed easily at room temperature. Because the bond length of the O2 correlates with its catalytic activation, the results can provide a new application of organometallic-based catalysts.
1. Introduction Fuel cells are devices with important role in the industrial
⁎
applications to convert the chemical energy into electrical energy without pollution. The performance of fuel cell correlates with the oxygen reduction reactions (ORRs) at the cathode [1]. However, the
Corresponding author. E-mail address: [email protected] (M. Noei).
https://doi.org/10.1016/j.inoche.2019.107520 Received 17 July 2019; Received in revised form 12 August 2019; Accepted 12 August 2019 Available online 13 August 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
activation of oxygen molecule by a high efficient catalyst is a crucial step for evaluating the possibility of ORR [2]. On the other hand, the ORR is a kinetically slow reaction that restricts the performance of fuel cells [3–5]. To overcome the above problem, platinum (Pt) is considered as a suitable catalyst to improve the speed of ORR at the cathode [6]. However, the voltage losses at the cathode and high cost restrict the use of Pt-based catalysts in fuel cells. In this respect, replacing the expensive Pt-based catalyst is one of the most important aims in the catalytic reactions [7–10]. Catalysis by supported transition metals (TMs) on the various materials is a very interesting method of research [11,12] since the discovery of TMs catalytic activity [13,14]. In the present study, by using first-principles calculations, we have investigated the activation of the O2 molecule on the surface of organometallic compounds. The metal decorated hydrocarbons, aromatic materials, fullerenes, nanotubes and graphene are widely studied for various applications [15–22]. Recent studies [23–28] show that the TMs (Sc, Ti, V, Fe, Ni and Pd) considerably enhance the H2 adsorption at the metal site [29]. In order to study the generality of organometallic field, we have performed a systematic theoretical study of the O2 activation on the organometallic compounds including the TMs decorated on three different molecular templates, namely, C4H4, C5H5, and C6H6. Since these structures have diverse π-electron deficiencies [29–32], the TM atoms decorated on these different molecular templates are expected to have different O2 activation capabilities. Accordingly, to explore the catalytic activity of TMs decorated on the CmHm (m = 4, 5, and 6) molecular templates for O2 activation, we have chosen Sc, Ti, V, Cr, and Mn TM atoms decorated on the CmHm templates, and performed the DFT calculations, and hope to find out the driving force for O2 dissociation from a molecular level. Thus, our study here is expected to act as a guide in the selection of suitable organometallic systems as potential catalysts for ORR.
hardness associated with a variation in the electron number.
Eads = E(O2 /TM CmHm)
E(O2)
E(CmHm) E(TM CmHm)
1 2
E (N ) =
(r )
A N
(r )
(5)
(r )
aN + bN 2 1 + cN
(6)
in which the coefficients a, b and c are related to A, I1 and I2 through
c=
I2 2I2
2I1 + A 2I1 A
I1
b=
A
(7)
I1 + A c 2
(8)
I1 + A I A + 1 c 2 2
(9)
2
a=
Then the hardness and hyper-hardness can be obtained by
= 2 × (b
(10)
ac );
The static dipole polarizability of the considered TM-CmHm systems is also studied in the present work. The components of polarizability tensor have been obtained in terms of the finite field method as the second order derivatives of the total energy with respect to the homogenous external electric field: ij
=
µi (F) = Fj
2E (F)
Fi Fj
i, j = x , y , z
(11)
The values αxx, αyy, and αzz are defined as main semiaxes of polarizability ellipsoid of the molecule. These are also used for calculation of mean polarizability:
=
1 ( 3
xx
+
yy
+
zz )
(12)
3. Results and discussion 3.1. Pristine TM-CmHm organometallic systems
(1)
We have performed full geometry optimizations for the TM-CmHm organometallic systems with the chemical formula TM-C4H4, TM-C5H5, and TM-C6H6. The optimized structures of the TM-CmHm systems are represented in Fig. 1. Our DFT calculations predict the TM-C bond lengths of the TM-CmHm systems with the values being in the range of 1.94 (Mn-C5H5) to 2.68 Å (Sc-C6H6) (Fig. 1). Interestingly, the TM-C bond lengths in the Cr-CmHm and Mn-CmHm indicate same values and vary in range between ~1.99 Å (in Cr-CmHm) and ~1.97 Å (in MnCmHm). Also, Fig. 1 indicates that the CmHm molecular templates in the considered TM-CmHm systems are completely planar. The NBO analysis showed a net charge transfer from TM to the neighbor atoms in the considered systems, indicating an ionic nature. In addition, as shown in Fig. 2, all TM atoms in the Tm-CmHm systems have positive charges. Clearly, there is a remarkable charge transfer from the TM atom to the neighbor atoms in the CmHm molecules, which is likely responsible for the stabilization of the TM-CmHm systems. The range of
(2)
2E
N2
(4)
(r )
For calculating hyper-hardness, we have employed the way developed by Fuentealba and Parr [37,38]. They have computed hardness as well as hyper-hardness from the electron affinity (A), and the first and second ionization potential (I1 and I2), assuming that the energy varies with the number of electrons according to the relation
where E (TM-CmHm) is the total energy of the TM-CmHm systems, E (TM) and E (CmHm) are the total energies of a TM atoms and the CmHm molecular templates, respectively. Also, E(O2/TM-CmHm) and E (O2) are the total energies of the O2/TM-CmHm complexes and isolated O2 molecule, respectively. We have also investigated the reactivity of the TM-CmHm systems by use of chemical hardness descriptor. Chemical hardness is a useful concept for understanding and predicting the chemical behavior of different compounds. Hardness is defined as the second derivative of the energy with respect to the number of electrons,
=
N
(r )
I N
First-principles all-electron calculations of the total energies and optimized structures were carried out using the DFT, as implemented in the GAUSSIAN 09 software [33]. The DFT calculations were performed using the GGA with the parametrization of Wang and Perdew (PW91) [34] and 6-31+g(d) basis set. It has been represented that the PW91 provides an efficient and robust basis for calculations of properties of organometallics compounds with TM atoms [29–32]. Also, we adopted a DFT + D3 (D stands for dispersion) method with Grimme's van der Waals (vdW) correction in our calculations [35]. Further, the natural bond orbital (NBO) analysis [36] was used to calculate the given charge transfer in this work. The binding energy (Ebin) of TMs on the CmHm and the adsorption energy (Eads) for the O2 adsorbed on the TM-CmHm systems are computed using the following equations:
E(TM)
=
N3
Obviously, hyper-hardness gives a comparison between the variation of the ionization potential and the electron-affinity when the number of electron of the system changes.
2. Computational details
Ebin = E(TM CmHm)
3E
=
(3)
and can be used to characterize the relative stability of a system. The next derivative, called the hyper-hardness, measures the change in 2
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
Fig. 1. The optimized structure of TM-C4H4, TM-C5H5, and TM-C6H6 organometallic systems (TM = Sc, Ti, V, Cr, and Mn).
calculated charge on the TM atoms is from 0.472e to 0.835e for the TMC4H4, 0.453e to 0.879e for the TM-C5H5, and 0.517e to 0.880e for the TM-C6H6, respectively. In order to test the thermal stability of the TMCmHm systems, which is important for their applications at high temperatures, the binding energies (Ebin) of all TM-CmHm systems were calculated. As shown in Fig. 2, the binding strength for the TMs located on CmHm molecular templates decreases in the order Sc-C6H6 > ScC5H5 > Sc-C4H4 > Cr-C6H6 > Ti-C4H4 > Ti-C5H5 > TiC6H6 > V-C6H6 > V-C4H4 > Mn-C6H6 > Mn-C5H5 > CrC5H5 > Cr-C4H4 > Mn-C4H4 > V-C5H5 with the Ebin of −6.96, −6.61, −6.54, −6.21, −5.97, −5.91, −5.53, −5.42, −5.15, −5.06, −4.56, −4.10, −3.77, −3.51, and −1.64 eV, respectively. These
results show that the TMs bind strongly on the CmHm molecules. The values for chemical hardness (η) descriptor of the TM-CmHm systems is also calculated and represented in Fig. 2. Obviously, the values of the η show a decreasing trend presenting the increase in the reactivity of considered systems from Mn-C4H4 to Sc-C6H6. Fig. 2 also includes the mean static dipole polarizability (〈α〉) values of the TMs located on the CmHm systems. It should be noted that the 〈α〉 of TM-CmHm systems increase along the following series: Sc-C6H6 > Sc-C5H5 > TiC5H5 > Sc-C4H4 > Ti-C4H4 > Ti-C6H6 > V-C4H4 > Cr-C6H6 > Cr-C5H5 > Mn-C6H6 > V-C6H6 > Mn-C5H5 > Cr-C4H4 > VC5H5 > Mn-C4H4. It is worth pointing out that the calculated 〈α〉 values are consistent with the reactivity trends predicted by η for the 3
sc-C5H5
-6.96 1213 Sc-C6H6
1058 Ti-C5H5
1190
997 Sc-C4H4
947 Ti-C6H6
997
939 V-C4H4
Ti-C4H4
933
866 Cr-C5H5
Cr-C6H6
840
778 V-C6H6
Mn-C6H6
0.05 Sc-C6H6
638
0.12 Sc-C5H5
Mn-C5H5
0.26 Sc-C4H4
637
0.38 V-C4H4
Cr-C4H4
0.40 Cr-C6H6
591
0.45 Ti-C4H4
V-C5H5
0.48 Ti-C5H5
479 0.62 Ti-C6H6
Mn-C4H4
0.65 Cr-C5H5
0.82
1.11 V-C6H6
Mn-C6H6
1.12 Mn-C5H5
Cr-C4H4
V-C5H5
1.22
2.00
2.09
η (eV)
Sc-C5H5
-4.56
-3.77
-4.10
-3.51
Ebin (eV)
<α> (Å3)
Mn-C4H4
sc-C6H6
sc-C4H4 -6.54
Cr-C6H6
-6.61
Ti-C4H4
Ti-C6H6
-6.21
V-C6H6 -5.42
-5.53
Ti-C5H5
V-C4H4
-5.91
Mn-C6H6
-5.15
Mn-C5H5
-5.06
Cr-C5H5
Cr-C4H4
Mn-C4H4
V-C5H5
0.88 sc-C6H6
-1.64
0.879
0.835 sc-C4H4
sc-C5H5
0.829
0.772 Ti-C4H4
Cr-C6H6
0.527 V-C4H4
0.766
0.517
0.754
0.504
Mn-C6H6
Ti-C5H5
0.485 Cr-C5H5
Mn-C5H5
Ti-C6H6
0.483 Cr-C4H4
0.665
0.472 Mn-C4H4
V-C6H6
0.453 V-C5H5
Charge (e)
-5.97
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
Fig. 2. Comparative analyses for the charge on the metal atoms, binding energies, chemical hardness values, and polarizabilities of the studied organometallic systems.
TM-CmHm systems. In other word, the maximum 〈α〉 values for the TM-CmHm systems are related to the systems with the minimum η which had been predicted as the most reactive systems. All in all, among the studied TM-CmHm systems, the higher charge transfer, higher 〈α〉, and lower η belongs to the Sc-C6H6 system (Fig. 2). Accordingly, the Sc-C6H6 is predicted as the most reactive system.
0.539e from Mn-C4H4, Mn-C5H5, and Cr-C4H4 to the O2 molecule, respectively (Fig. 4). Things are significantly different when we consider the other TM atoms decorated on the CmHm molecular templates. As shown in Fig. 4, the charge transfer to the O2 molecule are in the order of O2/Sc-C6H6 (0.907e) > O2/Sc-C5H5 (0.899e) > O2/ScC4H4 (0.879e) > O2/Ti-C4H4 (0.733e) > O2/Ti-C5H5 (0.697e) > O2/TiC6H6 (0.643e) > O2/V-C4H4 (0.629e) > O2/Cr-C6H6 (0.602e) > O2/Cr-C5H5 (0.584e) > O2/Mn-C6H6 (0.573e) > O2/VC6H6 (0.547e) > O2/V-C5H5 (0.540e) > O2/Cr-C4H4 (0.539e) > O2/Mn-C5H5 (0.499e) > O2/Mn-C4H4 (0.496e). Also, the largest and smallest Eads and r values are for the Mn-C4H4 and Sc-C6H6, respectively. On Sc-C6H6 system, O2 binds more strongly (−7.69 eV) and exhibits a higher degree of activation: OeO distance is 1.496 Å, and the charge on O2 is 0.907 e (Fig. 4). These results clearly show a higher level of activation. The fact that the O2 activation is higher for the Sc located on the CmHm, despite Eads is similar to the Ti-CmHm (Fig. 4), is due to the electrostatic interaction of the O2 molecule with the ionic ScCmHm systems. Our results reveal that the ability of the O2 adsorption enhances with the increase in reactivity of the TM-CmHm systems. Accordingly, it is concluded that the O2 is highly activated by the Sc atoms in the Sc-CmHm systems. In fact, the partially ionized Sc metal atom located on the CmHm molecular templates polarizes the O2 molecule. It is worth pointing out that the obtained results are in agreement with the reactivity trends predicted by η values. In other word, the maximum Eads and the higher O2 activation are related to the O2 adsorption on the Sc-CmHm systems, which had been predicted as the most reactive organometallic systems. We explore now the OeO activation path with formation of two
3.2. O2 activation by the TM-CmHm organometallic systems The O2 activation is a prototypical catalytic procedure [39–41]. Accordingly, it is selected in the present work as a benchmark reaction to evaluate the catalytic performance of the TM-CmHm systems. We systematically examine O2 adsorption on the TM-CmHm systems to reveal their catalytic activity evolution toward O2 molecule. In order to obtain the most stable adsorption geometries, we examined both end-on and side-on geometries of O2 adsorption, and found that the side-on geometries of O2 adsorption are more stable as shown in Fig. 3. Our results show that the most stable configuration is O2 axis parallel to the TM-CmHm with two TM-O bonds formed. The activation happens generally by partial electron transfer from the TM-CmHm organometallic systems to the 2π* antibonding orbital of O2 and results in the elongation of the OeO bond. The calculated Eads, OeO bond length (r), and negative charges on the O2 molecule (Q) for the studied O2/TMCmHm complexes are represented in Fig. 4. On Mn-C4H4, Mn-C5H5, and Cr-C4H4 the O2 molecule is captured by −5.61, −5.65, and −6.05 eV, respectively and the OeO distance is elongated from 1.240 Å (free O2, experimental r is 1.210 Å [42]) to 1.412, 1.414, and 1.430 Å respectively; there are the small charge transfers of 0.496, 0.499e, and 4
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
Fig. 3. The side view of the most stable configurations for the O2 adsorbed on the TM-C4H4, TM-C5H5, and TM-C6H6 organometallic systems (TM = Sc, Ti, V, Cr, and Mn).
adsorbed O atoms on the Sc-decorated C6H6. To obtain the minimum energy path of the O2 activation, we select the most stable configurations of the O2 on the Sc-C6H6 as the initial state as represented in Fig. 3. The final state consists of two chemisorbed O atoms locate at the shoulders of Sc atom. The minimum energy path profile of the O2 activation reaction and the corresponding initial state (IS), transition state (TS) and final state (FS) are represented in Fig. 5. As shown in this
figure, the O2 activation reaction on the Sc-C6H6 system is thermodynamically favorable, approved by their exothermal procedure (0.25 eV). Moreover, the energy barrier of the activation reaction (0.48 eV), shows that the O2 activation on the Sc-C6H6 system will be kinetically preferable. These results propose that the Sc-C6H6 system is an efficient catalyst for the O2 activation reaction. 5
Inorganic Chemistry Communications 108 (2019) 107520
-5.61 -5.65 -6.05 -6.11 -6.14 -6.25 -6.41 -6.54 -6.71 -6.82 -7.24 -7.34 -7.39 -7.49 -7.69
Mn-C4H4 Mn-C5H5 Cr-C4H4 V-C5H5 V-C6H6 Mn-C6H6 Cr-C5H5 Cr-C6H6 V-C4H4 Ti-C6H6 Ti-C5H5 Ti-C4H4 Sc-C4H4 Sc-C5H5 Sc-C6H6
Y. Bazvand, et al.
0.899
0.907 Sc-C6H6
0.643 Ti-C6H6
0.879
0.629 V-C4H4
Sc-C5H5
0.602 Cr-C6H6
0.733
0.584 Cr-C5H5
0.697
0.573
Ti-C4H4
0.547
1.496 Sc-C6H6
Mn-C6H6
1.494 Sc-C5H5
0.540
1.492 Sc-C4H4
V-C5H5
1.486 Ti-C4H4
V-C6H6
1.486 Ti-C5H5
0.539
1.484 Ti-C6H6
0.499
1.484 V-C4H4
Cr-C4H4
1.479 Cr-C6H6
0.496
1.477 Cr-C5H5
Mn-C5H5
1.477 Mn-C6H6
Mn-C4H4
1.471
1.458
1.430
V-C6H6
V-C5H5
1.414 Mn-C5H5
Cr-C4H4
1.412 Mn-C4H4
Ti-C5H5
Q (e)
r (Å)
Sc-C4H4
Eads (eV)
Fig. 4. Comparative analyses for the adsorption energies, OeO distances (r), and charge transfer values (Q) for the O2/TM-C4H4, O2/TM-C5H5, and O2/TM-C6H6 systems (TM = Sc, Ti, V, Cr, and Mn).
Fig. 5. Potential energy profile for activation of the O2 by the Sc-C6H6 organometallic system.
4. Conclusion
following findings. First, the O2 activation on the TM-CmHm organometallic systems is sensitive to the TM atoms. There is a net charge transfer from the TM-CmHm systems to the adsorbed O2 molecule. The high elongation of the OeO bond in the oxygen molecule confirms the charge transfers. Because the elongation of the adsorbed O2 exhibits its catalytic activation, we suggest that a TM-CmHm organometallic compound could be considered as a useful catalyst for O2 activation.
In summary, the structural geometries and the binding mechanism for the O2 adsorbed on the TM-CmHm organometallic systems are studied using DFT calculations. The C4H4, C5H5, and C6H6 molecular templates with Sc, Ti, V, Cr, and Mn TM atoms were investigated. From the results described in the previous sections, we can draw the 6
Inorganic Chemistry Communications 108 (2019) 107520
Y. Bazvand, et al.
Among the studied TM-CmHm organometallic systems, the Sc-C6H6 shows the best catalytic performance toward the O2 activation. Since the total barrier is small (0.28 eV) and the exothermicity is enough large (0.25 eV), it is confirmed that the Sc-C6H6 can be considered as ORR catalysts. We are expecting that the results of our study will provide a clue for the future development of the organometallic-based catalysts.
[19] [20] [21]
Acknowledgments
[22]
The authors are grateful to Islamic Azad University-Iran for computational resources.
[23] [24]
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