Catalytic Reduction of SO2 by CO over PtlAum(CO)n: A First

Nov 6, 2012 - ... are investigated using first-principles density functional theory. ... (COS) molecule's desorption from the catalyst are remarkably ...
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Catalytic Reduction of SO2 by CO over PtlAum(CO)n: A First-Principles Investigation Guo-Ping Gao, Shi-Hao Wei,* and Xiang-Mei Duan* Department of Physics, Faculty of Science, Ningbo University, Ningbo-315211, People's Republic of China ABSTRACT: The catalytic activities, to the reduction of SO2 by CO, of clusters PtlAum (l + m = 2) with or without preadsorbing CO molecules are investigated using firstprinciples density functional theory. We find that the PtAu(CO)n (n = 1−3) clusters show more excellent catalytic properties than either pure metallic catalysts. Preadsorption of CO to the catalysts could effectively avoid platinum-based catalyst sulfur poisoning; as more CO molecules preadsorbed to the catalysts, the energy barriers for the carbonyl sulfide (COS) molecule’s desorption from the catalyst are remarkably decreased. We propose an ideal catalytic cycle to simultaneously get rid of SO2 and CO over the catalysts PtAu(CO)3.

1. INTRODUCTION Sulfur dioxide, as a byproduct of combustion process, is one of the main components generating acid rain, acidifying soil, and causing other forms of air pollution. Desulfurization of these combustion exhaust gases through throw-away processes are still of high cost and problematic disposal. Direct catalytic reduction of SO2 to elemental sulfur by various reducing agents, including CO,1−3 H2,4 CH4,5 and carbon,6 has spurred much attention. From a practical point of view, the use of CO as a reducing agent is more attractive because CO is often contained in combustion exhausts. The overall reduction of SO2 through CO to sulfur element can be written as SO2 + 2CO → 2CO2 + [S]

on either of the parent metallic clusters, and thus offer the opportunity to obtain new catalysts with enhanced selectivity, activity, and stability. It is proposed that the platinum catalysts alloyed or modified by transition metals, such as ruthenium, molybdenum, and palladium, have exhibited higher activity for CO oxidation.16−18 Among the platinum-based bimetallic catalysts, platinum−gold nanoparticles are of particular interest because the different electronic configurations of gold (4f145d106s1) and platinum (4f145d96s1) would remodel the electronic structure. It has been demonstrated that platinum− gold clusters show superior catalytic performance compared with pure platinum or gold clusters.19−21 Auten et al.22 reported that the platinum−gold bimetallic catalysts are more active than the monometallic catalysts and have lower apparent activation energies in CO oxidation by O2. More recently, Wang et al.23 claimed that the CO’s oxidation promoted by platinum−gold bimetallic catalysts is less expensive and more efficient. To our knowledge, no study so far has involved the reduction of SO2 by CO over a platinum−gold bimetallic catalyst, neither in ab initio calculations nor experiment. We also note that the resultants in most of the studies are elemental sulfur, which leads to catalyst poisoning by sulfur. If the sulfur product is in the gas phase, not the solid phase, the sulfur poisoning is easy to avoid. In this paper, using the ab initio calculations, we investigate the catalytic properties of three kinds of diatomic clusters (Pt2, Au2, and PtAu) without and with CO preabsorption in the instance of reduction of SO2 by CO to COS, which can be used as an agricultural fumigant.24,25 We find that, with preabsorption of three CO molecules, bimetallic diatomic cluster PtAu

(1)

where [S] represents the various forms of sulfur products (S2, S6, S8). Under fuel-rich conditions, COS is the major byproduct.7 Several types of catalysts, including aluminasupported transition metals,8 perovskite,9 and mixed oxides,10 have been reported to mediate the reduction of SO2 by CO. However, all these catalysts have their limitations. Aluminasupported transition metals typically need to be activated by reducing gases at high temperature.11 Perovskite, for example, La1−xSrxCoO3,9 essentially loses its perovskite structure, which contributes to the catalyst activity,7 after a short period under reaction conditions and becomes a mixture of metal sulfides and oxysulfides. Mixed oxides require a high reaction temperature to complete the conversion of SO2. Platinum-based material is one of the most efficient catalysts for many important processes, such as CO oxidation,12 C−N coupling,13 and methanol dehydrogenation.14 However, the pure platinum catalysts are expensive, unstable, and sulfur sensitive.15 A catalyst with high activity and high sulfur selectivity is required to avoid catalyst sulfur poisoning. Bimetallic clusters often show novel properties, not presenting © 2012 American Chemical Society

Received: July 4, 2012 Revised: October 29, 2012 Published: November 6, 2012 24930

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activate energy (Emax a ) for the three elementary reactions as the overall reaction’s activation energy. The desorption energy (Ed) of COS from the catalyst is another key factor to evaluate the catalyst’s property. If Ed > Emax a , the catalyst is poisoned.

can effectively avoid catalyst sulfur poisoning. Based on our results, an ideal configuration of the bimetallic catalysts is proposed, expecting to provide a useful clue to synthesize the most suitable catalysts for purifying air pollution. The paper is organized as follows. In Section 2, we describe the calculation method. Section 3 reports results and discussions for the catalytic performance of the PtAu-based clusters, and Section 4 contains the conclusions.

3. RESULTS AND DISCUSSION Clusters are well known to be used as model systems to disclose the intrinsic mechanisms of nanocatalysis and the ensemble effect of bimetallic catalysts.32,33 For the reduction of SO2 by CO, we use homonuclear diatomic clusters (Pt2 and Au2) to model a monometallic cluster and a heteronuclear diatomic cluster (PtAu) to model a bimetallic cluster. We search the most favorable geometries of the three diatomic clusters without and with preabsorbing up to three CO molecules. The optimized clusters are taken as catalysts to mediate the reduction through the above three elementary steps, and their catalytic properties are compared. For the Pt2 catalyst, we schematically plot the catalytic cycle with the geometric structures of the catalysts, involving intermediates (IM) and transition states (TS) in Figure 1,

2. CALCULATION METHODS All calculations were performed using density functional theory (DFT) in the generalized gradient approximation (GGA) with the revised Perdew, Burke, and Ernzerhof exchange-correlation (RPBE)26 as implemented in the DMol3 computational software package.27,28 The electronic eigenfunctions were expanded in terms of a localized atomic orbital basis set of double numerical plus d-DNP basis except that no p functions are used on hydrogen (DND) quality and a real-space cutoff of 4.0 Å. Our calculations included scalar-relativistic corrections. In the self-consistent-field (SCF) calculation, the optimization convergence thresholds for energy change, maximum force, and maximum displacement between the optimization cycles were 2.0 × 10−5 Ha, 4.0 × 10−3 Ha/Å, and 5.0 × 10−3 Å, respectively. Spin polarization and without symmetry were imposed in all the calculations. The bond length of Pt2 is 2.36 Å, and its binding energy is 3.78 eV, which are in satisfactory agreement with the experimental results29 of 2.33 Å and 3.14 ± 0.02 eV, respectively. The results of Au2 (2.50 Å, 2.22 eV) are also in accord with the experimental values of 2.47 Å and 2.31 eV.30 The vibrational normal mode of CO is 2121.2 cm−1, in reasonable agreement with the experimental result of 2169.7 cm−1.31 The adsorption energy (Eads) of SO2 to PtlAum(CO)n is defined as Eads = −[E PtlAum(CO)n SO2 − E PtlAum(CO)n − ESO2]

(2)

where l + m = 2 and n = 0−3 unless specified otherwise, EPtlAum(CO)nSO2 is the total energy of the absorption system, EPtlAum(CO)n is the total energy of CO preabsorption cluster, and ESO2 is the total energy of an isolated SO2 molecule. The overall reduction of SO2 by CO in this work is as follows:

Figure 1. (Color online) Catalytic cycle over Pt2 diatomic cluster and the geometric structures of the catalysts, intermediates, and transition states involved along with primary bond lengths (in Å). The purple, gray, red, and brown spheres represent platinum, carbon, oxygen, and sulfur atoms, respectively.

catalyst

SO2 + 3CO ⎯⎯⎯⎯⎯⎯→ 2CO2 + COS

(3)

It can be divided into three elementary reactions given by the following:

and list the relative energy of intermediates, transition states, and products through the reaction pathway of Pt2 in Table 1. The SO2 molecule adsorbed on the optimized catalyst and a physical adsorbed CO molecule form the intermediate IM1, which locates below the entrance by 1.87 eV, where the energy of the entrance is set to be zero. IM1 converts into IM2 via transition state TS12 with a barrier of 1.55 eV. One oxygen atom in the SO2 molecule is being activated, which can be confirmed by the elongated bond length of S−O (1.73 Å) (Figure 1). The transition vector corresponds to the imaginary frequency of 758.3i cm−1. During the conversion, an oxygen atom in the SO2 molecule is combined with the CO molecule to form a CO2 molecule (see IM2). IM2 is the complex of Pt2SO with a CO2 molecule, whose total energy is −3.49 eV. In this state, CO2 is dissociative, and the S−C distance is 4.18 Å.

catalyst

SO2 + CO ⎯⎯⎯⎯⎯⎯→ SO + CO2

(4)

catalyst

SO + CO ⎯⎯⎯⎯⎯⎯→ S + CO2 catalyst

S + CO ⎯⎯⎯⎯⎯⎯→ COS

(5) (6)

The activation energies for the above three elementary reactions are identified by complete linear synchronous transition and quadratic synchronous transit search methods, following by transition-state optimization and confirmation. The optimized transitional points on the potential energy surface are verified by the second-order derivatives of the energy with respect to the atomic coordinates (Hessian) through vibrational frequency calculations. We take the largest 24931

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Table 1. Relative Energy, Erel, in eV, of Intermediates, Transition States, and Products through the Reaction Pathway of Pt2a state

Erel

state

Erel

IM1 TS12 IM2 IM3 TS34

−1.87 −0.32 −3.49 −3.51 −2.18

IM4 IM5 TS56 IM6 products

−5.26 −5.30 −4.96 −6.67 −4.76

a

The energy of the entrance, including the free Pt2 dimer, SO2, and three CO molecules, is set to zero as the reference.

Then, the second physisorbed CO molecule is introduced for the rest of IM2 to form IM3. This intermediate evolves into IM4 through TS34 with an imaginary frequency of 758.4i cm−1. The barrier from IM3 to TS34 is 1.33 eV. The CO molecule binds with the oxygen atom of the SO molecule in IM3 to form the second CO2 molecule. IM4 consists of Pt2S and a dissociative CO2 molecule with a S−C distance of 4.24 Å and an energy of −5.26 eV. Now, the third CO molecule is imported to bind with the sulfur atom (see IM5), forming IM6 through TS56 with an imaginary frequency of 305.1i cm−1 and a barrier of 0.34 eV. IM6 dissociates directly to the COS molecule and catalyst, indicating the accomplishment of the reaction. The Ed is as high as 1.91 eV, being the highest barrier along the reaction pathway of Pt2, which indicates that the Pt2 catalyst is poisoned. The overall reaction is exothermic by 4.76 eV. In Section 3.2, we will find that the sulfur poisoning of Pt2 can be overcome by alloying platinum with gold and preabsorbing CO molecules. 3.1. CO Preabsorption on PtlAum and SO2 Adsorbing to PtlAum(CO)n. To find the most favorable geometries of PtlAum(CO)n, we consider various binding positions of the CO molecule on the PtlAum(CO)n−1 clusters and the different relative orientations between the clusters and CO molecules. The optimized structures of PtAu(CO)n and their frontier molecular orbital (FMO) isosurfaces are shown in the left panel of Figure 2. The FMOs are very helpful to identify the adsorption/binding sites of small molecules on metallic clusters.34 We take the PtAu(CO)2 as an example to illustrate how it works: The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the CO molecule mainly locate at the carbon site, while the HOMO and LUMO of PtAu(CO)2 are mostly prominent at the end of the Au atom. Once the third CO molecule is imported, its carbon atom is predicted to bind with the Au atom in PtAu(CO)2 to constitute the PtAu(CO)3 cluster. Our DFT calculations confirm this prediction. For the sequent reduction of SO2 by CO, we consider the adsorption of the SO2 molecule on the catalysts PtlAum(CO)n. The optimized geometries of PtAu(CO)nSO2 and their FMO isosurfaces are shown in the right panel of Figure 2. Clearly, according to the FMO structure, the SO2 molecule adsorbs to the Au site of PtAu(CO) and PtAu(CO)2, while it adsorbs to the Pt site of PtAu(CO)3. The adsorption energy of SO2 to PtlAum(CO)n is listed in Table 2. With the same number of CO molecule preabsorption on the diatomic clusters (n ≥ 1), the Pt2(CO)n has the highest SO2 adsorption energy, followed by PtAu(CO)n and Au2(CO)n. We note that, for Au2(CO)n, the preadsorption of CO on Au2 makes SO2 physical adsorption on the catalysts, which hints these catalysts have relatively weak effect to the reduction of SO2 by CO.

Figure 2. (Color online) Equilibrium structures and their FMO isosufaces (isosurface value = 0.03) for PtAu(CO)n (left) and PtAu(CO)nSO2 (right). The purple, yellow, gray, red, and brown spheres represent platinum, gold, carbon, oxygen, and sulfur atom, respectively.

Table 2. Adsorption Energy, Eads, in eV, of SO2 on the Catalysts Pt2(CO)n Au2(CO)n PtAu(CO)n

n=0

n=1

n=2

n=3

1.72 1.18 1.85

2.36 0.71 0.86

1.73 0.23 1.06

1.10 0.30 0.77

3.2. Reduction of SO2 by CO over PtlAum(CO)n. The potential energy surface profiles for the reduction of SO2 by CO over Pt2(CO)n, Au2(CO)n, and PtAu(CO)n catalysts are shown in Figure 3a, b, and c, respectively. The energies of intermediates, transition states, and products are related to the energy of the entrance (including the free catalyst, one isolated SO2 molecule, and three isolated CO molecules). The strong interaction between sulfur products and catalyst will drag down the potential energy surface of the catalyst. The corresponding 24932

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the number of preabsorbed CO molecules, mainly due to the weak interaction among the CO molecules and the diatomic cluster. Still, the CO preabsorption on the diatomic cluster helps to decrease the Ed from 0.95 to 0.15 eV. If the adsorption site of SO2 is taken as the active center, for the PtAu cluster, the active center is the platinum site. The interaction between sulfur products and the Pt atom is so strong that the COS molecule is difficult to dissociate from the PtAu cluster (see Figure 3c and Table 3). The highest energy barrier, Ed, of 2.11 eV indicates the catalyst poisoning. With one and two CO molecules preadsorbing on the PtAu clusters, the active center switches from the platinum site to the gold site. The first elementary step becomes the rate-determining step, and the poisoning is overcome. When there is preabsorption of three CO on the PtAu cluster, the active center turns back to the platinum site. However, the interaction between sulfur products and the Pt atom becomes much weaker, and the Emax a (1.31 eV) is greater than the Ed (0.2 eV). From Table 3, we also find that the Ed dramatically decreases as more CO preabsorbs and for clusters of PtAu(CO)n the Emax is the smallest in the a same column. Therefore, it is clear that the PtAu(CO)n (n = 1 ∼ 3) clusters are free from the poisoning in the promotion of the reduction of SO2 by CO and show excellent catalytic activity, not shown by either of their parent metallic clusters. 3.3. Promotion of the Catalytic Properties for PtAu(CO)n. To simultaneously get rid of SO2 and CO over the catalyst PtAu(CO)3, we propose an ideal catalytic cycle schematically shown in Figure 4. The cycle contains three elementary steps and the desorption of the COS molecule from the bimetallic cluster. First, one oxygen of the SO2 molecule is seized by the CO molecule (see IM1, TS12, and IM2), where the CO molecule interacts with the SO2 molecule to form a CO2 molecule and a SO molecule with an energy barrier of 1.31 eV. Second, the oxygen of SO is bonded with the CO molecule (see IM3, TS34, and IM4). When the second CO molecule approaches the SO molecule, the oxygen in SO and CO form a chemical bond, where the barrier is 0.95 eV. Third, CO reacts with the sulfur to form the COS molecule with an energy barrier of 0.12 eV (see IM5, TS56, and IM6). At last, the COS molecule desorbs to the bimetallic cluster with a very small energy barrier of 0.20 eV, which means that the COS molecule can be easily removed from the catalysts and the catalytic cycle completes. The highest energy barrier during the whole process is 1.31 eV, which is lower than the experimental data on gasphase reactions35 of 2.01 eV at 298 K.

Figure 3. (Color online) Potential energy surface for reduction of SO2 by CO promoted by different catalysts: (a) for Pt2(CO)n, (b) for Au2(CO)n, and (c) for PtAu(CO)n. The chemical formulas of the stable intermediates are shown on the bottom. The energy of the entrance is shifted to zero.

Emax and Ed are given in Table 3, where the catalyst poisoning a cases are marked in italics. In the following, we will show that the platinum and gold are complementary and cooperative in the PtAu cluster preadsorbing different numbers of CO molecules. As shown in Table 3, for the Pt2 diatomic cluster, the Ed (1.91 eV) is greater than the Emax (1.55 eV). With one CO a molecule preadsorbing on the Pt2 diatomic cluster, the Ed is 2.16 eV, greater than the Emax of 1.87 eV, which indicates that a the catalysts Pt2 and Pt2CO are poisoned. However, with the increasing of n, the activation of the first oxygen atom in the SO2 molecule becomes the rate-determining step, and the Emax a is decreased from 2.25 eV (n = 2) to 1.87 eV (n = 3). Meanwhile the Ed dramatically falls to 0.87 eV for n = 2 and 0.43 eV for n = 3. So, the catalytic performance of the Pt2 diatomic cluster can be improved by the preadsorbing of two or more CO molecules. For the Au2(CO)n cluster, the rate-determining step of the reaction is to activate the oxygen atom in the SO2 molecule (see Figure 3b and Table 3). Unlike in the reactions over Pt2(CO)n, the Emax of Au2(CO)n catalysts is not dependent on a

4. CONCLUSION We have investigated the reactivity of PtlAum(CO)n (l + m = 2, n = 0−3) clusters in the reduction of SO2 by CO using DFT. The dimer Pt2 needs to preabsorb at least one CO molecule on each platinum atom to avoid catalyst poisoning. The Au2 dimer, though free from the catalyst poisoning, requires a relative higher energy to activate the first oxygen of SO2. Preabsorption

a Table 3. Largest Activation Energy, Emax a , and Desorption Energy, Ed, in eV, for the Reaction Pathway of PtlAum(CO)n

n=0 Pt2(CO)n Au2(CO)n PtAu(CO)n a

Emax a 1.55 2.00 1.48

n=1 Ed

Emax a

1.91 0.95 2.11

1.87 1.77 1.33

n=2 Ed

Emax a

2.16 0.17 0.79

2.25 2.01 1.44

n=3 Ed

Emax a

Ed

0.87 0.17 0.59

1.87 1.74 1.31

0.43 0.15 0.20

The numbers in italics represent the corresponding catalysts are poisoned. 24933

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Figure 4. (Color online) Catalytic cycle over PtAu(CO)3 and the geometric structures of the catalysts, intermediates, and transition states along with primary bond length (in Å). The purple, yellow, gray, red, and brown spheres represent platinum, gold, carbon, oxygen, and sulfur atoms, respectively.

of at least one CO molecule on the platinum atom of the PtAu cluster can effectively overcome catalyst poisoning. Compared to the pure dimers Pt2 and Au2, the bimetallic PtAu cluster with the preabsorption of CO molecules is the best catalyst in the reduction of SO2 by CO. PtAu(CO)n (n = 1−3) catalysts are complementary and can be tailored for various applications. We propose an ideal catalytic cycle for the PtAu(CO)3 to get rid of SO2 and CO simultaneously.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 574 87609988 (S.-H.W.); +86 574 87600783 (X.M.D.). Fax: +86 574 87600744 (S.-H.W.); +86 574 87600744 (X.-M.D.). E-mail: [email protected] (S.-H.W.); [email protected] (X.-M.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the Natural Science Foundation of China (grant nos. NSFC 10804058, NSFC 1174164, and NSFC 11275100), the Science Foundation of Zhejiang (grant no. Y607546), the K. C. Wong Magna Foundation in Ningbo University, and SRF for ROCS, SEM. The computation is performed in the Supercomputer Center of NBU.

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REFERENCES

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