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Catalytic reduction of SO2 by CO using carbon intercalated gold clusters
Chemical Physics Letters 726 (2019) 111–116
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Research paper
Catalytic reduction of SO2 by CO using carbon intercalated gold clusters Mohan Tiwari, Vinit, C.N. Ramachandran
⁎
T
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India
HIGHLIGHTS
corrected density functional studies of reduction of SO by CO using C Au clusters. • Dispersion pathways of the reaction due to multiple reaction sites are analyzed. • Competing • Less desorption energy of products suggest devoid of sulphur poisoning of the catalyst. 2
n
n
ABSTRACT
The reduction of SO2 by CO at various sites of CnAun (n = 4, 8 and 16) clusters is studied using the dispersion corrected density functional theory. The energy required for the desorption of the final product COS from the respective clusters is lower than the maximum activation energy barrier of the reaction indicating that the clusters considered are free from sulphur poisoning. The rate determining step of the reaction in presence of the above clusters is also determined. The competing pathways which arise due to the presence of other available possible reaction sites are discussed.
1. Introduction Sulphur dioxide (SO2) is an environmentally hazardous gas produced during the combustion of fuels as well as during other industrial processes and is emitted directly to the atmosphere leading to air, water and soil pollution. Removal of SO2 from atmosphere is a challenge [1–3]. The reduction of SO2 to elemental sulphur by various reducing species such as C, H2, CH4 and CO in presence of various catalysts has taken wide attention [4–7]. Among these reducing agents, being a harmful gas the use of CO has a particular interest. Moreover, the byproduct of the reaction COS is useful in agriculture for the synthesis of fertilizers [8–9]. Some experimental studies have been reported on the reduction of SO2 by CO using different catalysts. However, most of them require either high temperature or lead to sulphur poisoning of the catalyst. For example; Ma et al. used mixture of La2O2S and CoS2 as a catalyst for the reduction of SO2 in presence of CO at a temperature of 610 οC and reported a synergistic effect between La2O2S and CoS2 that enhances the activity of the catalyst [10]. It has also been proposed that the activity of a mixture of the above two catalysts can be increased if the sulphur content of cobalt sulphide is reduced from 2S to 1.097S. The catalyst formed by mixing CoS2 with Al2O3 or TiO2 showed bi-functional properties for the reduction of SO2 by CO or H2 [11]. Kim et al. investigated the reaction mechanism of the reduction of SO2 by CO using the catalyst Co3O4-TiO2 which showed significant catalytic ⁎
activity at ∼450 οC [12]. In another study, the catalytic reduction of SO2 by CO in water has been examined using the catalyst Fe2O3/γAl2O3 [13]. Zhao et al. used a series of sulphur containing CoMo/γAl2O3 catalyst for the reduction of SO2 using CO and reported that Co/ (Co + Mo) = 0.31 on γ-Al2O3 is the best catalyst at 400 οC [14]. Similarly, the mixed oxides of Ce and Ti have been also used for the above reaction and it is found that Ce0.7Ti0.3O2 at 500 οC is a good catalyst without sulphur poisoning [15]. There are few theoretical studies reported about the reduction of SO2 by CO over various catalysts using density functional methods [16–24]. For example, Wei et al. investigated the mechanism of the reduction of SO2 by CO over Au4Pt2 and Au6Pt [16]. Their studies showed that the activity of the catalyst can be improved by the preadsorption of CO on that. In another study, the clusters AumPtl (where, m + l = 2) have been also used as a catalyst with and without the preadsorption of CO molecule on Au [17]. The role of preadsorbed CO in platinum based catalyst has also been examined in the reduction of SO2 by CO. In this case, the electrons are transferred from preadsorbed CO to AumPtl (where, m + l = 2) thereby weakening the interaction between the catalyst and the sulphur compounds (SO2 and COS) preventing the sulphur poisoning [18]. The same group also investigated about the catalytic activity of AuPt(CO)n (n = 0–3) and its anionic analogue for the reduction of SO2 by CO at the gold site. Their results indicated more catalytic activity for neutral AuPt(CO)n compared to its anionic analogues [19].
Corresponding author. E-mail address: [email protected] (C.N. Ramachandran).
https://doi.org/10.1016/j.cplett.2019.04.035 Received 24 January 2019; Received in revised form 11 April 2019; Accepted 12 April 2019 Available online 13 April 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 726 (2019) 111–116
M. Tiwari, et al.
The high cost of the catalysts, high temperature conditions for the reaction as well as the sulphur poisoning of the catalysts necessitate the need for an alternate catalyst for the above reaction. A detailed understanding of the mechanism of the reaction on relatively less expensive catalysts with multiple reaction sites can trigger the synthesis of such catalysts. In this regard, we carried out density functional theoretical studies on the reduction of SO2 using CO in presence of the recently proposed carbon intercalated gold clusters C4Au4, C8Au8 and C16Au16 [25]. The adsorption energy of SO2 on gold atoms of C4Au4, C8Au8 and C16Au16 is calculated. The change in free energy for the reaction as a function of reaction coordinates is calculated. To get insight on the poisoning of the catalysts, desorption energy of COS is determined. The scaled rate constant (k) of each of the elementary step of the reaction is also calculated.
below.
SO2 + CO + Cn Aun
Cn Aun SO + CO Cn Aun S + CO
ESO2
G# RT
The desorption energy (Edes) of COS was calculated as follows:
Edes = Ecomplex
ECn Aun
as in complex
ECOS
as in complex
The overall reaction for the reduction of SO2 by CO in presence of the catalyst CnAun (n = 4, 8 and 16) can be represented as:
SO2 + 3CO + Cn Aun
Cn Aun + COS
The reaction can be initiated either by the adsorption of SO2 or CO on the catalyst as per the adsorption energy listed in Table S2 of the supplementary information. Most of the earlier studies on the reduction of SO2 by CO were initiated by the pre-adsorption of CO on the catalyst probably due to the high affinity of CO towards the metal atoms of the catalysts [16,18,19]. However, the above reaction in presence of preadsorbed SO2 is not much studied [17]. This motivated us to study the latter. For this purpose, we selected the mixed clusters of carbon and gold mentioned above. To know about the donor and acceptor properties of SO2, CO and the catalysts CnAun, the frontier molecular orbitals of these species are analyzed and are shown in Fig. 1. It is clear from the figure that the energy gap between the highest occupied molecular orbital (HOMO) of CnAun species and the lowest unoccupied molecular orbital of SO2 is less (compared to that of CO), leading to the most favourable interaction between these units. For the pairs C4Au4 and SO2, the above energy gap is 2.09 eV whereas that for the pairs C8Au8 and SO2 is 2.21 eV. Similarly, the energy gap between HOMO of C16Au16 and LUMO of SO2 is 1.84 eV. Due to the less energy difference between the orbitals involved for SO2 and CnAun, the reaction is initiated by the adsorption of SO2 over CnAun clusters. The reduction of SO2 by CO follows a multistep reaction as depicted in Fig. 2a and b. The free energy pathway for the above reaction for different catalysts is shown in Fig. 3. When the reaction is initiated by the adsorption of SO2, a molecule of SO2 is adsorbed on one of the gold atoms of C4Au4 followed by the adsorption of CO resulting in the formation of the intermediate (IM1). The activation barrier (ΔG1#) for the first step of the reaction is 41.85 kcal/mol for C4Au4 in which IM1 is converted to IM2 via the transition state (TS1). In this step, SO2 attached to C4Au4 is activated with the elongation of the S-O bond from 1.46 Å to 1.71 Å. In the next step of the reaction, one molecule of CO2 is produced along with the formation of the complex C4Au4-SO. On the other hand, when reaction is initiated by the adsorption of CO, a molecule of CO is adsorbed over C4Au4 followed by the adsorption of SO2 forming the intermediate (IM1). The activation barrier (ΔG1#) for the conversion of IM1 to IM2 via the transition state (TS1) is 46.52 kcal/ mol which is ∼5 kcal/mol higher than that for the reaction which was initiated by SO2 adsorption. In the next step, the product C4Au4-SO is formed along with the formation of one CO2 molecule in both cases and the reaction proceeds in a similar way. A second molecule of CO is adsorbed over the complex C4Au4-SO, resulting in the formation of the intermediate IM3. The above intermediate is converted to another intermediate via the transition state TS2 with an activation barrier (ΔG2#) of 25.62 kcal/mol. In this step, the S-O bond of the complex C4Au4-SO is elongated from 1.51 Å to 1.64 Å. The removal of CO2 from IM4 leads to the formation of C4Au4-S to which a third molecule of CO is adsorbed forming the transition state TS3. The activation energy for the above step is of 5.84 kcal/mol. Finally, the catalysts C4Au4 is retained by the removal of COS with the
where, Ecomplex , ECn Aun and ESO2 are the energy of optimized geometries of complex, carbon-gold cluster and SO2, respectively. The free energy (ΔG) of the reaction was calculated as the difference in free energy of the final product and initial reactants. The free energy barrier (ΔG#) for each step of the reaction was obtained from the free energies of respective intermediates and transitions states. The vibrational frequencies were computed to identify the nature of stationary points. The transition states (TS) in each case were confirmed with the presence of one imaginary frequency. Furthermore, these transitions states were analysed via intrinsic reaction coordinate (IRC) methodology corresponding to the respective reaction coordinates. The scaled rate constant (k) for each step of the reaction was calculated for 298 K using the expression:
k=e
Cn Aun S + CO2
3. Results and discussions
All calculations were carried out using the quantum chemistry package Gaussian 09 [26]. The geometries of the reactants, catalysts, intermediates, transition states and products were optimized using the hybrid density functional B3LYP (Becke, three-parameter, Lee-YangParr) with Grimme’s empirical dispersion correction (i.e. B3LYP-GD2) using the LANL2DZ basis set for gold atoms and 6-311G* basis sets for the non-metals [27–28]. The calculated values of bond length (2.58 Å), ionization potential (9.49 eV) and electron affinity (1.89 eV) for the small cluster Au2 were found to be in reasonable agreement with the experimental values 2.47 Å, 9.5 ± 0.3 eV and 1.94 ± 0.0006 eV, respectively [29–31]. In addition, the bond length obtained for CAu (1.88 Å) was found to be in good agreement with the value (1.83 Å) obtained at CCSD(T) level justifying the level of theory used in the present work [32]. In order to know the relativistic effects on the present results, we performed the relativistic calculations for the small clusters Au2 and CAu. The calculated values of structural and electronic properties of these clusters with and without the relativistic effects are listed in Table S1 of the supporting information. The excellent agreement between the results justifies the level of calculations used in the present study. The adsorption energy (Eads) for the adsorption of SO2 was calculated as follows:
ECn Aun
C n Aun SO + CO2
To make the study more versatile, we have also studied the reaction in presence of SO2 on all gold atom sites of C4Au4. This also justifies the possibility of the adsorption of SO2 at the available sites of the catalyst and the subsequent reactions.
2. Computational methodology
Eads = Ecomplex
k3
k2
k1
2CO2 + COS + Cn Aun
The corresponding elementary steps for the above reaction are given
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M. Tiwari, et al.
Fig. 1. The frontier molecular orbitals of CnAun (n = 4, 8 and 16), CO and SO2, respectively.
desorption energy of 7.46 kcal/mol. The low value of desorption energy compared to the activation energies of various steps assures that the catalyst used is not poisoned during the reduction of SO2 by CO. The overall reaction is exothermic with a change in enthalpy of −116.64 kcal/mol. To know the rate determining step of the above reaction, the rate constants corresponding to each of the elementary steps of the reaction was calculated and is listed in table1. From the values of k1, k2 and k3, it can be seen that the value of k1 is the least. Thus, the first step of the reaction can be considered as the rate determining step in the reduction of SO2 by CO using C4Au4 as catalyst. In addition, we also calculated the energy barrier for various steps of the reaction incorporating the relativistic corrections. However, we could not find a significant difference in the energy barrier on
+ SO2
incorporating the relativistic correction. The maximum difference observed was less than 0.3 kcal/mol for the first step of the reaction. 3.1. Reduction of SO2 by CO over C8Au8 and C16Au16 To study the effect of size and shape of CnAun catalysts on the reduction of SO2 by CO, we also investigated the above reaction on the clusters C8Au8 and C16Au16. For both types of clusters, the ratio of the number of carbon to gold remained the same as that in C4Au4. The studies indicated that the mechanism of the reaction over C8Au8 and C16Au16 follows in a same manner as it occurs over C4Au4 as evident from Figs. S1 and S2 of the supporting information. The parameters corresponding to the energy pathways are given in Fig. S3. It can be seen from Fig. S3 that the activation energy barrier (ΔG1#) for the
+ CO
C4Au4
IM2
TS1
IM1
+ CO
COS
- CO2
IM3 IM6
1.64Å
+ CO - CO2 TS3
IM4
IM5
Fig. 2. The mechanism of the catalytic reduction of SO2 by CO in presence of C4Au4. 113
TS2
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M. Tiwari, et al.
Fig. 3. The schematic diagram showing the change in Gibb’s free energy associated with various processes involved in the catalytic reduction of SO2 by CO in presence of C4Au4 as well as its all site SO2 adsorbed state.
conversion of IM1 to IM2 via the transition state (TS1) for C8Au8 is 53.87 kcal/mol which is slightly higher than that for C4Au4. In this step, one of the S-O bonds of SO2 is elongated from 1.49 Å to 1.68 Å resulting in the formation of C8Au8-SO and CO2. In the subsequent step, CO molecule is adsorbed over C8Au8-SO forming the intermediate IM4 via IM3. The free energy barrier (ΔG2#) associated with this transformation through the transition state TS2 is 5.57 kcal/mol, which is significantly less than that for C4Au4. In this step, S-O bond is elongated by 0.16 Å and leads to the formation of another molecule of CO2 and C8Au8-S. Further adsorption of CO over C8Au8-S leads to the formation of IM5 which is converted to C8Au8 and COS via the transition state TS3. The activation barrier for this step is 5.21 kcal/mol. Finally, the catalyst C8Au8 is regenerated by the desorption of COS with a desorption energy of 11.94 kcal/mol. Similar to C4Au4, the cluster C8Au8 is also devoid of catalytic poisoning due to the low desorption energy. The reduction of SO2 by CO over C8Au8 is overall exothermic with the change in enthalpy of −129.87 kcal/mol, which is slightly higher than that for C4Au4. As in the case of C4Au4, the first step of the reaction is the rate determining step in the reduction of SO2 by CO using C8Au8 as a catalyst. For the cluster C16Au16, two different types of gold sites are available viz. terminal and bridged. The mechanism of the reaction using C16Au16 on its bridge and terminal gold sites are shown in Fig. S2 of the supporting information. The parameters corresponding to the free energy pathway of the reaction are given in Fig. S3. Similar to C4Au4 and C8Au8, the adsorption of SO2 at different sites of the catalyst occurs in the initial stage of reaction with adsorption energy of −30.69 kcal/mol and −18.30 kcal/mol at bridged and terminal sites, respectively. This suggests that the adsorption energy is the highest for the bridge site of C16Au16 among all the catalysts considered in the present study. In the subsequent step, CO molecule is adsorbed over the complex C16Au16SO2 resulting in the formation of first intermediate (IM1). The activation barrier (ΔG1#) for conversion of the intermediate IM1 to IM2 via
the transition state TS1 is 32.12 kcal/mol at the bridged site (33.32 kcal/mol at terminal site). Further, the reaction proceeds in a similar way as that for C4Au4 and C8Au8 as evident from the figures. 3.2. Effect of the adsorption of SO2 at multiple sites of the catalyst Considering the fact that there are multiple reaction sites available on the catalyst for the adsorption and subsequent reactions, the effect of SO2 occupancy at all such sites of the catalyst on the energetics of the reaction was also investigated. For this propose, SO2 molecules are attached to all the gold atoms of C4Au4 which is hereafter referred to as C4Au4-4SO2. The adsorption energy per SO2 molecule obtained (−10.94 kcal/mol) is found to be very close to the adsorption energy of −11.30 kcal/mol obtained for the adsorption SO2 at one of the gold sites of C4Au4. The interaction of CO with one of the SO2 molecules attached to the catalyst is studied further. From Fig. 4, it can be seen that the reaction proceeds in the same manner as earlier leading to the oxidation of CO to CO2. The Gibbs activation energy barrier for this step is 41.29 kcal/mol and is close to the value of 41.85 kcal/mol obtained for C4Au4 with only one SO2 groups attached. This suggests that the presence of SO2 groups in the vicinity does not affect the activation barrier for the reaction. This holds also true for other steps of the reaction as evident from the energy profile diagram shown in Fig. 3. To know about the competitiveness between the partially reduced SO group and the adjacent SO2 group in the next step of the reaction with the incoming CO molecule, we also studied the reaction of CO with adjacent SO2 that leads to the formation of the cluster with two SO units. The mechanisms of the above two competing pathways of the reactions are shown in Fig. S4 of the supplementary information. The studies revealed that the Gibbs free energy barrier for the oxidation of CO to CO2 is significantly less (24.02 kcal/mol) for the pathway in which the partially reduced SO group is reduced to S than the other in
Table 1 The calculated values of the rate constants for the elementary reactions involved in the reduction of SO2 by CO in presence of C4Au4, C4Au4-(SO2)4, C8Au8 and C16Au16 (at bridge and terminal gold atom). Rate constant
k1 k2 k3
C4Au4
1.5 × 10−31 1.5 × 10−19 5.4 × 10−5
C4Au4-(SO2)4
6.5 × 10−31 2.7 × 10−18 1.6 × 10−2
C8Au8
4.1 × 10−40 4.3 × 10−5 1.6 × 10−4
114
C16Au16 At terminal gold atom
At bridge gold atom
4.4 × 10−25 7.2 × 10−19 1.9 × 10−6
3.3 × 10−24 1.2 × 10−10 4.1 × 10−8
Chemical Physics Letters 726 (2019) 111–116
M. Tiwari, et al.
4 SO2
CO IM 2
C4Au4
C4Au4-4SO2 COS + 3SO2
TS1
IM 1
+ CO
- CO2
IM 3
IM 6
+ CO - CO2
TS3
IM 4
IM 5
TS2
Fig. 4. The diagram showing various processes involved in the catalytic reduction of SO2 by CO for the C4Au4 cluster when all the gold sites are adsorbed by SO2.
which two SO units are formed (45.58 kcal/mol). This suggests that once the reaction is initiated at one site, the reaction will continue at that site till the reduction is complete.
Appendix A. Supplementary material
4. Conclusion
References
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.04.035.
The mechanism involved in the reduction of SO2 by CO in presence of three different types of mixed carbon gold catalysts C4Au4, C8Au8 and C16Au16 are studied using the dispersion corrected density functional methods. The studies showed that SO2 adsorbs strongly over CnAun and is activated further. The activation energy required for the conversion is not high and can be conducted at relatively low temperature. The desorption energy for the final product COS is less than the maximum Gibbs activation energy barrier for the reaction suggesting that the catalysts are free from sulphur poisoning. The reaction is exothermic for all types of CnAun clusters. It was also found that the formation of CnAun-SO and CO2 in the first stage of the reaction is the rate determining step. The investigations also revealed that all four sites of gold of C4Au4 are capable for the reduction of SO2 by CO and once the reaction is initiated at one site, it will proceed till the reduction at that site is complete. In summary, the mixed clusters of carbon and gold can be used as a catalyst for the removal of two poisonous gases at relatively low temperatures without any catalytic poisoning.
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Declaration of interests There are no interests to declare Acknowledgement MT and Vinit thank Indian Institute of Technology Roorkee and University Grants Commission, India, respectively for research fellowships. CNR acknowledges Council of Scientific & Industrial Research (CSIR), India for the research grant 012891/17/EMR-II. 115
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116
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Research paper
Catalytic reduction of SO2 by CO using carbon intercalated gold clusters Mohan Tiwari, Vinit, C.N. Ramachandran
⁎
T
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India
HIGHLIGHTS
corrected density functional studies of reduction of SO by CO using C Au clusters. • Dispersion pathways of the reaction due to multiple reaction sites are analyzed. • Competing • Less desorption energy of products suggest devoid of sulphur poisoning of the catalyst. 2
n
n
ABSTRACT
The reduction of SO2 by CO at various sites of CnAun (n = 4, 8 and 16) clusters is studied using the dispersion corrected density functional theory. The energy required for the desorption of the final product COS from the respective clusters is lower than the maximum activation energy barrier of the reaction indicating that the clusters considered are free from sulphur poisoning. The rate determining step of the reaction in presence of the above clusters is also determined. The competing pathways which arise due to the presence of other available possible reaction sites are discussed.
1. Introduction Sulphur dioxide (SO2) is an environmentally hazardous gas produced during the combustion of fuels as well as during other industrial processes and is emitted directly to the atmosphere leading to air, water and soil pollution. Removal of SO2 from atmosphere is a challenge [1–3]. The reduction of SO2 to elemental sulphur by various reducing species such as C, H2, CH4 and CO in presence of various catalysts has taken wide attention [4–7]. Among these reducing agents, being a harmful gas the use of CO has a particular interest. Moreover, the byproduct of the reaction COS is useful in agriculture for the synthesis of fertilizers [8–9]. Some experimental studies have been reported on the reduction of SO2 by CO using different catalysts. However, most of them require either high temperature or lead to sulphur poisoning of the catalyst. For example; Ma et al. used mixture of La2O2S and CoS2 as a catalyst for the reduction of SO2 in presence of CO at a temperature of 610 οC and reported a synergistic effect between La2O2S and CoS2 that enhances the activity of the catalyst [10]. It has also been proposed that the activity of a mixture of the above two catalysts can be increased if the sulphur content of cobalt sulphide is reduced from 2S to 1.097S. The catalyst formed by mixing CoS2 with Al2O3 or TiO2 showed bi-functional properties for the reduction of SO2 by CO or H2 [11]. Kim et al. investigated the reaction mechanism of the reduction of SO2 by CO using the catalyst Co3O4-TiO2 which showed significant catalytic ⁎
activity at ∼450 οC [12]. In another study, the catalytic reduction of SO2 by CO in water has been examined using the catalyst Fe2O3/γAl2O3 [13]. Zhao et al. used a series of sulphur containing CoMo/γAl2O3 catalyst for the reduction of SO2 using CO and reported that Co/ (Co + Mo) = 0.31 on γ-Al2O3 is the best catalyst at 400 οC [14]. Similarly, the mixed oxides of Ce and Ti have been also used for the above reaction and it is found that Ce0.7Ti0.3O2 at 500 οC is a good catalyst without sulphur poisoning [15]. There are few theoretical studies reported about the reduction of SO2 by CO over various catalysts using density functional methods [16–24]. For example, Wei et al. investigated the mechanism of the reduction of SO2 by CO over Au4Pt2 and Au6Pt [16]. Their studies showed that the activity of the catalyst can be improved by the preadsorption of CO on that. In another study, the clusters AumPtl (where, m + l = 2) have been also used as a catalyst with and without the preadsorption of CO molecule on Au [17]. The role of preadsorbed CO in platinum based catalyst has also been examined in the reduction of SO2 by CO. In this case, the electrons are transferred from preadsorbed CO to AumPtl (where, m + l = 2) thereby weakening the interaction between the catalyst and the sulphur compounds (SO2 and COS) preventing the sulphur poisoning [18]. The same group also investigated about the catalytic activity of AuPt(CO)n (n = 0–3) and its anionic analogue for the reduction of SO2 by CO at the gold site. Their results indicated more catalytic activity for neutral AuPt(CO)n compared to its anionic analogues [19].
Corresponding author. E-mail address: [email protected] (C.N. Ramachandran).
https://doi.org/10.1016/j.cplett.2019.04.035 Received 24 January 2019; Received in revised form 11 April 2019; Accepted 12 April 2019 Available online 13 April 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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The high cost of the catalysts, high temperature conditions for the reaction as well as the sulphur poisoning of the catalysts necessitate the need for an alternate catalyst for the above reaction. A detailed understanding of the mechanism of the reaction on relatively less expensive catalysts with multiple reaction sites can trigger the synthesis of such catalysts. In this regard, we carried out density functional theoretical studies on the reduction of SO2 using CO in presence of the recently proposed carbon intercalated gold clusters C4Au4, C8Au8 and C16Au16 [25]. The adsorption energy of SO2 on gold atoms of C4Au4, C8Au8 and C16Au16 is calculated. The change in free energy for the reaction as a function of reaction coordinates is calculated. To get insight on the poisoning of the catalysts, desorption energy of COS is determined. The scaled rate constant (k) of each of the elementary step of the reaction is also calculated.
below.
SO2 + CO + Cn Aun
Cn Aun SO + CO Cn Aun S + CO
ESO2
G# RT
The desorption energy (Edes) of COS was calculated as follows:
Edes = Ecomplex
ECn Aun
as in complex
ECOS
as in complex
The overall reaction for the reduction of SO2 by CO in presence of the catalyst CnAun (n = 4, 8 and 16) can be represented as:
SO2 + 3CO + Cn Aun
Cn Aun + COS
The reaction can be initiated either by the adsorption of SO2 or CO on the catalyst as per the adsorption energy listed in Table S2 of the supplementary information. Most of the earlier studies on the reduction of SO2 by CO were initiated by the pre-adsorption of CO on the catalyst probably due to the high affinity of CO towards the metal atoms of the catalysts [16,18,19]. However, the above reaction in presence of preadsorbed SO2 is not much studied [17]. This motivated us to study the latter. For this purpose, we selected the mixed clusters of carbon and gold mentioned above. To know about the donor and acceptor properties of SO2, CO and the catalysts CnAun, the frontier molecular orbitals of these species are analyzed and are shown in Fig. 1. It is clear from the figure that the energy gap between the highest occupied molecular orbital (HOMO) of CnAun species and the lowest unoccupied molecular orbital of SO2 is less (compared to that of CO), leading to the most favourable interaction between these units. For the pairs C4Au4 and SO2, the above energy gap is 2.09 eV whereas that for the pairs C8Au8 and SO2 is 2.21 eV. Similarly, the energy gap between HOMO of C16Au16 and LUMO of SO2 is 1.84 eV. Due to the less energy difference between the orbitals involved for SO2 and CnAun, the reaction is initiated by the adsorption of SO2 over CnAun clusters. The reduction of SO2 by CO follows a multistep reaction as depicted in Fig. 2a and b. The free energy pathway for the above reaction for different catalysts is shown in Fig. 3. When the reaction is initiated by the adsorption of SO2, a molecule of SO2 is adsorbed on one of the gold atoms of C4Au4 followed by the adsorption of CO resulting in the formation of the intermediate (IM1). The activation barrier (ΔG1#) for the first step of the reaction is 41.85 kcal/mol for C4Au4 in which IM1 is converted to IM2 via the transition state (TS1). In this step, SO2 attached to C4Au4 is activated with the elongation of the S-O bond from 1.46 Å to 1.71 Å. In the next step of the reaction, one molecule of CO2 is produced along with the formation of the complex C4Au4-SO. On the other hand, when reaction is initiated by the adsorption of CO, a molecule of CO is adsorbed over C4Au4 followed by the adsorption of SO2 forming the intermediate (IM1). The activation barrier (ΔG1#) for the conversion of IM1 to IM2 via the transition state (TS1) is 46.52 kcal/ mol which is ∼5 kcal/mol higher than that for the reaction which was initiated by SO2 adsorption. In the next step, the product C4Au4-SO is formed along with the formation of one CO2 molecule in both cases and the reaction proceeds in a similar way. A second molecule of CO is adsorbed over the complex C4Au4-SO, resulting in the formation of the intermediate IM3. The above intermediate is converted to another intermediate via the transition state TS2 with an activation barrier (ΔG2#) of 25.62 kcal/mol. In this step, the S-O bond of the complex C4Au4-SO is elongated from 1.51 Å to 1.64 Å. The removal of CO2 from IM4 leads to the formation of C4Au4-S to which a third molecule of CO is adsorbed forming the transition state TS3. The activation energy for the above step is of 5.84 kcal/mol. Finally, the catalysts C4Au4 is retained by the removal of COS with the
where, Ecomplex , ECn Aun and ESO2 are the energy of optimized geometries of complex, carbon-gold cluster and SO2, respectively. The free energy (ΔG) of the reaction was calculated as the difference in free energy of the final product and initial reactants. The free energy barrier (ΔG#) for each step of the reaction was obtained from the free energies of respective intermediates and transitions states. The vibrational frequencies were computed to identify the nature of stationary points. The transition states (TS) in each case were confirmed with the presence of one imaginary frequency. Furthermore, these transitions states were analysed via intrinsic reaction coordinate (IRC) methodology corresponding to the respective reaction coordinates. The scaled rate constant (k) for each step of the reaction was calculated for 298 K using the expression:
k=e
Cn Aun S + CO2
3. Results and discussions
All calculations were carried out using the quantum chemistry package Gaussian 09 [26]. The geometries of the reactants, catalysts, intermediates, transition states and products were optimized using the hybrid density functional B3LYP (Becke, three-parameter, Lee-YangParr) with Grimme’s empirical dispersion correction (i.e. B3LYP-GD2) using the LANL2DZ basis set for gold atoms and 6-311G* basis sets for the non-metals [27–28]. The calculated values of bond length (2.58 Å), ionization potential (9.49 eV) and electron affinity (1.89 eV) for the small cluster Au2 were found to be in reasonable agreement with the experimental values 2.47 Å, 9.5 ± 0.3 eV and 1.94 ± 0.0006 eV, respectively [29–31]. In addition, the bond length obtained for CAu (1.88 Å) was found to be in good agreement with the value (1.83 Å) obtained at CCSD(T) level justifying the level of theory used in the present work [32]. In order to know the relativistic effects on the present results, we performed the relativistic calculations for the small clusters Au2 and CAu. The calculated values of structural and electronic properties of these clusters with and without the relativistic effects are listed in Table S1 of the supporting information. The excellent agreement between the results justifies the level of calculations used in the present study. The adsorption energy (Eads) for the adsorption of SO2 was calculated as follows:
ECn Aun
C n Aun SO + CO2
To make the study more versatile, we have also studied the reaction in presence of SO2 on all gold atom sites of C4Au4. This also justifies the possibility of the adsorption of SO2 at the available sites of the catalyst and the subsequent reactions.
2. Computational methodology
Eads = Ecomplex
k3
k2
k1
2CO2 + COS + Cn Aun
The corresponding elementary steps for the above reaction are given
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Fig. 1. The frontier molecular orbitals of CnAun (n = 4, 8 and 16), CO and SO2, respectively.
desorption energy of 7.46 kcal/mol. The low value of desorption energy compared to the activation energies of various steps assures that the catalyst used is not poisoned during the reduction of SO2 by CO. The overall reaction is exothermic with a change in enthalpy of −116.64 kcal/mol. To know the rate determining step of the above reaction, the rate constants corresponding to each of the elementary steps of the reaction was calculated and is listed in table1. From the values of k1, k2 and k3, it can be seen that the value of k1 is the least. Thus, the first step of the reaction can be considered as the rate determining step in the reduction of SO2 by CO using C4Au4 as catalyst. In addition, we also calculated the energy barrier for various steps of the reaction incorporating the relativistic corrections. However, we could not find a significant difference in the energy barrier on
+ SO2
incorporating the relativistic correction. The maximum difference observed was less than 0.3 kcal/mol for the first step of the reaction. 3.1. Reduction of SO2 by CO over C8Au8 and C16Au16 To study the effect of size and shape of CnAun catalysts on the reduction of SO2 by CO, we also investigated the above reaction on the clusters C8Au8 and C16Au16. For both types of clusters, the ratio of the number of carbon to gold remained the same as that in C4Au4. The studies indicated that the mechanism of the reaction over C8Au8 and C16Au16 follows in a same manner as it occurs over C4Au4 as evident from Figs. S1 and S2 of the supporting information. The parameters corresponding to the energy pathways are given in Fig. S3. It can be seen from Fig. S3 that the activation energy barrier (ΔG1#) for the
+ CO
C4Au4
IM2
TS1
IM1
+ CO
COS
- CO2
IM3 IM6
1.64Å
+ CO - CO2 TS3
IM4
IM5
Fig. 2. The mechanism of the catalytic reduction of SO2 by CO in presence of C4Au4. 113
TS2
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Fig. 3. The schematic diagram showing the change in Gibb’s free energy associated with various processes involved in the catalytic reduction of SO2 by CO in presence of C4Au4 as well as its all site SO2 adsorbed state.
conversion of IM1 to IM2 via the transition state (TS1) for C8Au8 is 53.87 kcal/mol which is slightly higher than that for C4Au4. In this step, one of the S-O bonds of SO2 is elongated from 1.49 Å to 1.68 Å resulting in the formation of C8Au8-SO and CO2. In the subsequent step, CO molecule is adsorbed over C8Au8-SO forming the intermediate IM4 via IM3. The free energy barrier (ΔG2#) associated with this transformation through the transition state TS2 is 5.57 kcal/mol, which is significantly less than that for C4Au4. In this step, S-O bond is elongated by 0.16 Å and leads to the formation of another molecule of CO2 and C8Au8-S. Further adsorption of CO over C8Au8-S leads to the formation of IM5 which is converted to C8Au8 and COS via the transition state TS3. The activation barrier for this step is 5.21 kcal/mol. Finally, the catalyst C8Au8 is regenerated by the desorption of COS with a desorption energy of 11.94 kcal/mol. Similar to C4Au4, the cluster C8Au8 is also devoid of catalytic poisoning due to the low desorption energy. The reduction of SO2 by CO over C8Au8 is overall exothermic with the change in enthalpy of −129.87 kcal/mol, which is slightly higher than that for C4Au4. As in the case of C4Au4, the first step of the reaction is the rate determining step in the reduction of SO2 by CO using C8Au8 as a catalyst. For the cluster C16Au16, two different types of gold sites are available viz. terminal and bridged. The mechanism of the reaction using C16Au16 on its bridge and terminal gold sites are shown in Fig. S2 of the supporting information. The parameters corresponding to the free energy pathway of the reaction are given in Fig. S3. Similar to C4Au4 and C8Au8, the adsorption of SO2 at different sites of the catalyst occurs in the initial stage of reaction with adsorption energy of −30.69 kcal/mol and −18.30 kcal/mol at bridged and terminal sites, respectively. This suggests that the adsorption energy is the highest for the bridge site of C16Au16 among all the catalysts considered in the present study. In the subsequent step, CO molecule is adsorbed over the complex C16Au16SO2 resulting in the formation of first intermediate (IM1). The activation barrier (ΔG1#) for conversion of the intermediate IM1 to IM2 via
the transition state TS1 is 32.12 kcal/mol at the bridged site (33.32 kcal/mol at terminal site). Further, the reaction proceeds in a similar way as that for C4Au4 and C8Au8 as evident from the figures. 3.2. Effect of the adsorption of SO2 at multiple sites of the catalyst Considering the fact that there are multiple reaction sites available on the catalyst for the adsorption and subsequent reactions, the effect of SO2 occupancy at all such sites of the catalyst on the energetics of the reaction was also investigated. For this propose, SO2 molecules are attached to all the gold atoms of C4Au4 which is hereafter referred to as C4Au4-4SO2. The adsorption energy per SO2 molecule obtained (−10.94 kcal/mol) is found to be very close to the adsorption energy of −11.30 kcal/mol obtained for the adsorption SO2 at one of the gold sites of C4Au4. The interaction of CO with one of the SO2 molecules attached to the catalyst is studied further. From Fig. 4, it can be seen that the reaction proceeds in the same manner as earlier leading to the oxidation of CO to CO2. The Gibbs activation energy barrier for this step is 41.29 kcal/mol and is close to the value of 41.85 kcal/mol obtained for C4Au4 with only one SO2 groups attached. This suggests that the presence of SO2 groups in the vicinity does not affect the activation barrier for the reaction. This holds also true for other steps of the reaction as evident from the energy profile diagram shown in Fig. 3. To know about the competitiveness between the partially reduced SO group and the adjacent SO2 group in the next step of the reaction with the incoming CO molecule, we also studied the reaction of CO with adjacent SO2 that leads to the formation of the cluster with two SO units. The mechanisms of the above two competing pathways of the reactions are shown in Fig. S4 of the supplementary information. The studies revealed that the Gibbs free energy barrier for the oxidation of CO to CO2 is significantly less (24.02 kcal/mol) for the pathway in which the partially reduced SO group is reduced to S than the other in
Table 1 The calculated values of the rate constants for the elementary reactions involved in the reduction of SO2 by CO in presence of C4Au4, C4Au4-(SO2)4, C8Au8 and C16Au16 (at bridge and terminal gold atom). Rate constant
k1 k2 k3
C4Au4
1.5 × 10−31 1.5 × 10−19 5.4 × 10−5
C4Au4-(SO2)4
6.5 × 10−31 2.7 × 10−18 1.6 × 10−2
C8Au8
4.1 × 10−40 4.3 × 10−5 1.6 × 10−4
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C16Au16 At terminal gold atom
At bridge gold atom
4.4 × 10−25 7.2 × 10−19 1.9 × 10−6
3.3 × 10−24 1.2 × 10−10 4.1 × 10−8
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4 SO2
CO IM 2
C4Au4
C4Au4-4SO2 COS + 3SO2
TS1
IM 1
+ CO
- CO2
IM 3
IM 6
+ CO - CO2
TS3
IM 4
IM 5
TS2
Fig. 4. The diagram showing various processes involved in the catalytic reduction of SO2 by CO for the C4Au4 cluster when all the gold sites are adsorbed by SO2.
which two SO units are formed (45.58 kcal/mol). This suggests that once the reaction is initiated at one site, the reaction will continue at that site till the reduction is complete.
Appendix A. Supplementary material
4. Conclusion
References
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.04.035.
The mechanism involved in the reduction of SO2 by CO in presence of three different types of mixed carbon gold catalysts C4Au4, C8Au8 and C16Au16 are studied using the dispersion corrected density functional methods. The studies showed that SO2 adsorbs strongly over CnAun and is activated further. The activation energy required for the conversion is not high and can be conducted at relatively low temperature. The desorption energy for the final product COS is less than the maximum Gibbs activation energy barrier for the reaction suggesting that the catalysts are free from sulphur poisoning. The reaction is exothermic for all types of CnAun clusters. It was also found that the formation of CnAun-SO and CO2 in the first stage of the reaction is the rate determining step. The investigations also revealed that all four sites of gold of C4Au4 are capable for the reduction of SO2 by CO and once the reaction is initiated at one site, it will proceed till the reduction at that site is complete. In summary, the mixed clusters of carbon and gold can be used as a catalyst for the removal of two poisonous gases at relatively low temperatures without any catalytic poisoning.
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Declaration of interests There are no interests to declare Acknowledgement MT and Vinit thank Indian Institute of Technology Roorkee and University Grants Commission, India, respectively for research fellowships. CNR acknowledges Council of Scientific & Industrial Research (CSIR), India for the research grant 012891/17/EMR-II. 115
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