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Capability of defective graphene-supported Pd13 and Ag13 particles for mercury adsorption
Applied Surface Science 364 (2016) 166–175
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Capability of defective graphene-supported Pd13 and Ag13 particles for mercury adsorption Jittima Meeprasert a , Anchalee Junkaew a , Chompoonut Rungnim a , Manaschai Kunaseth a , Nawee Kungwan b , Vinich Promarak c , Supawadee Namuangruk a,∗ a
National Nanotechnology Center, NSTDA, 111 Thailand Science Park, Klong Luang, Pathum Thani 12120 Thailand Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand c School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Wangchan, Rayong 21210, Thailand b
a r t i c l e
i n f o
Article history: Received 7 October 2015 Received in revised form 15 November 2015 Accepted 10 December 2015 Available online 12 December 2015 Keywords: Mercury DFT Palladium Silver Nanoparticle Graphene
a b s t r a c t Reactivity of single-vacancy defective graphene (DG) and DG-supported Pdn and Agn (n = 1, 13) for mercury (Hg0 ) adsorption has been studied using density functional theory calculation. The results show that Pdn binds defective site of DG much stronger than the Agn , while metal nanocluster binds DG stronger than single metal atom. Metal clustering affects the adsorption ability of Pd composite while that of Ag is comparatively less. The binding strength of −8.49 eV was found for Pd13 binding on DG surface, indicating its high stability. Analyses of structure, energy, partial density of states, and d-band center (d ) revealed that the adsorbed metal atom or cluster enhances the reactivity of DG toward Hg adsorption. In addition, the Hg adsorption ability of Mn -DG composite is found to be related to the d of the deposited Mn , in which the closer d of Mn to the Fermi level correspond to the higher adsorption strength of Hg on Mn -DG composite. The order of Hg adsorption strength on Mn -DG composite are as follows: Pd13 (−1.68 eV) » Ag13 (−0.67 eV) ∼ Ag1 (−0.69 eV) > Pd1 (−0.62 eV). Pd13 -DG composite is therefore more efficient sorbent for Hg0 removal in terms of high stability and high adsorption reactivity compared to the Ag13 . Further design of highly efficient carbon based sorbents should be focused on tailoring the d of deposited metals. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, most electricity is generated by burning fossil fuels such as natural gas, oil, and coal. Air pollution created from the combustion processes of these fuels, particularly coal, causes serious environmental concern. The flue gas from coal-fired power plants contains toxic compounds such as carbon oxides (COx ), sulfur oxides (SOx ), nitrogen oxides (NOx ), and heavy metals including mercury (Hg). Among them, mercury causes significant environmental concern due to its extreme toxicity and high volatility. Generally, there are three forms of mercury that exist in the flue gas: elementary (Hg0 ), oxidized (Hg+ and Hg2+ ) and particulate mercury (Hgp ) [1,2]. Hg+ and Hg2+ are soluble in water and thus can be captured by wet scrubber equipment. Hgp can be collected by using electrostatic precipitation (ESP) and fabric filtration (FF). The most challenging task is Hg0 removal. Due to its low water solubility, Hg0 is difficult to capture using standard equipment, and thus
∗ Corresponding author. E-mail address: [email protected] (S. Namuangruk). http://dx.doi.org/10.1016/j.apsusc.2015.12.078 0169-4332/© 2015 Elsevier B.V. All rights reserved.
is released into environment. Hence, the abatement of Hg0 produced from coal-fired power plants is a long-term global pollution reduction goal. Most coal-fired power plants reduce mercury emissions from the flue gas using the activated carbon injection (ACI) control system. ACI injects powdered activated carbon (PAC) into the flue gas stream. PAC adsorbs the Hg0 from the flue gas and is then collected with fly-ash in the plant’s particulate collection device [3]. ACI has been recognized as a cost-effective means to reduce Hg0 from flue gas but its adsorption efficiency is still low. Thus, the improvement Hg0 removal efficiency of ACI is essential. From experimental studies, it has been reported that noble metals such as gold (Au), silver (Ag), palladium (Pd), and platinum (Pt) are efficient Hg sorbents [4–6]. Moreover, they are reusable and stable for long-term operation. In theoretical studies, density functional theory (DFT) calculations have shown that the binding strength of Hg0 on the surface of metals increases in the following order: Ag < Au < Cu < Ni < Pt < Pd [7,8]. It was noted that Ag was the least reactive to Hg, and Pd was the most reactive to Hg. Sun et al. [9] studied Hg0 adsorption on Agn clusters (n = 1–6) by using DFT calculations. They reported that the binding energy increases as the
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size of Ag cluster increases from Ag1 to Ag4 , but it decreases when the size of Ag cluster is larger than Ag4 . A similar finding was also observed in Hg0 adsorption on Aun (n = 1–6) clusters, as investigated by Siddiqui et al. [10]. However, they showed a contradictory result for Pdn clusters in which there was no correlation between the binding energy and the size of Pd cluster [11]. Thus, the nature of metals may have a different influence on Hg adsorption. Although, noble metals have high reactivity to Hg0 , it is known that the disadvantage of using noble metals as sorbents or catalysts is their expense. Thus, they are not applicable in large-scale use. Many research groups have attempted to develop novel materials, which possess both high reactivity and cost-efficiency for Hg0 removal. When compromising the advantages and disadvantages of activated carbon and noble metals, the combination of both materials is thought to provide superior characteristics. Depositing a small amount of metal on AC not only makes it cost effective, but also increases reactivity of metal nanoparticles and activated carbon. AC is usually modeled by defective graphene due to its structural imperfection. In synthesis aspect, defective graphene (DG) can be produced by plasma treatments at a power of 10 W and a pressure of 0.1 Torr [12]. The amount of defects can be tuned by changing the treatment time. In addition, doping metal nanoparticle on graphene also can be performed by heating method at one pot synthesis [13,14]. In recent experimental studies, Au [15,16] and Pd [17] nanoparticles doped AC have been reported as highly efficient Hg0 sorbents, since AC prevents the aggregation of metal nanoparticles and the sorbents can be regenerated. Recently, by using combined experimental and theoretical studies, we found that loading small amount of Ag onto TiO2 powder could dramatically enhance the efficiency of sorbent for Hg adsorption [18,19]. The deposited Ag promoted electron transfer from Hg to the TiO2 substrate via the Ag-O interface bonds. We found a correlation between the amount of the electron transfer and Hg0 sorbent binding strength. For metal on carbon-based materials, however, function of deposited metal for Hg adsorption and the criteria for selection of metal for AC, has not been reported. Moreover, both Ag and Pd metals in composited sorbents were not comparatively studied; neither experimentally nor theoretically and thus, their comparative efficiencies for Hg0 removal have not been identified. In theoretical studies, Granatier et al. [20] studied the adsorption of Pd, Au and Ag atoms on pristine graphene (PG) and found that each metal binds with PG in different ways. For example, Ag is predominantly bound through a dispersion interaction, while the Pd binds with a covalent bond and the binding of Au involves both charge transfer and dispersion interactions. However, metals tend to aggregate on PG due to the weak interaction between metal and PG [21,22]. On the other hand, the vacancy defect in graphene was suggested as a strong anchoring site for metal nanoparticles, and has been used as a model to represent AC [23]. Many transition metals such as Pt, Pd, Au, and Fe can be strongly deposited onto DG [24–34]. Recently, the structural and electronic properties of DG-supported Pd nanoparticles have been reported. Jia et al. [27] performed DFT calculations to study the Pdn clusters (n = 1–5) deposition on single-vacancy graphene and their adsorptions to O2 . They found that point defect in graphene is a deposited site for Pd clusters, which affects the charge transfer from Pd cluster to adsorbed O2 . Sen et al. [33] studied the adsorption of small Pdn clusters (n = 1–5) on double-vacancy graphene by using DFT calculations. They suggested that uniform and immobile distribution of Pd on graphene might be obtained by the adsorption of a Pd4 cluster on DG. For larger clusters, the icosahedral (Ih) Pd13 structure is particularly used to deposit on this support structure [24,31,32]. Compared to other Pd13 isomers, the Ih symmetry exhibits unusual clusterorbitals as the d-type cluster orbitals are lower in energy than the p-type cluster orbitals [35]. Liu et al. [31,32] studied the Pd13 and
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MePd12 (Me = Fe, Co, Ni, Cu, Zn) nanoparticles supported on DG for an application in electrocatalysis. They reported that Pd13 binds strongly with support due to the strong hybridization between dorbitals of Pd and p-orbitals of graphene carbon atoms. A similar result was observed in a separated study by Xia et al. [24]. Nevertheless, understandings of electronic, physical and chemical properties of DG supported metal clusters are still limited. To the best of our knowledge, the use of a DG-supported metal nanoparticle as a sorbent for Hg0 removal has not been investigated by theoretical calculations. In this work, the adsorption of Hg0 on the DG-supported Ag and Pd adatom and nanocluster (Ag13 and Pd13 ) have been comparatively studied using the DFT calculations. A 13 atoms-size is selected because it is a number of closed-shell high symmetry structures exist. The icosahedron is of particular interest of stable high-symmetry structures because they can be considered building blocks that can be compounded into bigger structures. Ag was reported as the least reactive metal among studied noble metals on PG [7,8] and has never been studied on DG before. Both Ag and Pd metals have never been comparatively studied in terms of binding with DG, and Hg0 adsorption efficiency. Thus, the aim of this work is to study the structural and electronic properties of metal and metal-defect graphene composites toward Hg0 adsorption. The nature of different noble metals when acting as sorbents in isolated- and composite-forms were characterized. The key descriptor, which is responsible for Hg adsorption efficiency, was identified. The obtained information would be useful for experimentalists in the selection of metals, the design, and the development of efficient carbon-based materials for removal of Hg0 , as well as other heavy metals, from flue gas. 2. Computational details The spin unrestricted DFT calculations were performed using the DMol3 program in the Materials Studio 5.5TM package [36,37]. The electron exchange-correlations were treated using the generalized gradient approximation (GGA) with the Perdew and Wang formulation (PW91) functional [38]. The double numerical plus polarization (DNP) basis sets were selected with the effective core potential (ECP) scheme [39,40]. An orthorhombic supercell ˚ with periodic boundary condition, was of 14.74 × 14.76 × 15.09 A, used for the DG and DG-supported metal nanocluster systems. These parameters were obtained from the cell optimization of ˚ The inte6 × 6 graphene supercell with a vacuum space of 15 A. gration of the Brillouin zone was calculated using 5 × 5 × 1 k-point sampling for geometry optimizations and electronic charge calculations. Under unconstrained symmetry, all atoms were fully relaxed until the total energy and the SCF density were converged within 1 × 10−5 Hartree. The maximum force and the maximum ˚ displacement tolerances were set at 0.002 Hartree/Å and 0.005 A, respectively. Reference calculations for isolated Pd13 and Ag13 nanoclusters were performed using the same conditions as for the DG and DG-supported metal nanocluster systems. Charge is evaluated from the Hirshfeld population analysis. For the DG-supported Pdn or Agn nanoclusters (n = 1 and 13), the binding energy (Eb ) between metal and DG support is calculated as: Eb = ENC-DG − (EDG + ENC ),
(1)
where ENC-DG is the total energy of the metal nanocluster deposited on DG, EDG is the total energy of DG, and ENC is the total energy of the isolated metal nanocluster. For the adsorption of Hg atom on the DG-supported metal nanoclusters, the adsorption energy (Eads ) is calculated as: Eads = EHg-(NC-DG) − (EHg + ENC-DG ),
(2)
where EHg-(NC-DG ) is the total energy of Hg adsorbed on the metal supported-DG composite, EHg is the total energy of isolated Hg
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atom, and ENC-DG is the total energy of DG-supported metal nanocluster. Negative value indicates stronger adsorption. 3. Results and discussion Section 1, we constructed the sorbent models comprised of (i) bare DG, (ii) freestanding M1 and M13 , and (iii) composite models of M1 -DG and M13 -DG; M = Ag and Pd. In order to study the reactivity of sorbents toward Hg adsorption, we adsorbed Hg on all sorbent models, see Section 2. 3.1. Sorbent models 3.1.1. Defective graphene The DG supercell consists of 71 carbon atoms with a single carbon atom vacancy at the center as shown in Fig. S1. The vacancy site is the most reactive region since the lowest unoccupied molecular orbital (LUMO) is localized mainly at the vacancy site. This is in agreement with a previous study [23]. Therefore, the vacancy site of DG is used as the deposition site for metal atoms and metal nanoclusters in this study. 3.1.2. Metal nanoclusters The electronic configurations of Pd and Ag are [Kr] 4d10 and [Kr] 4d10 5s1 , respectively. The initial configuration of freestanding metal nanoclusters (Pd13 , Ag13 ) were based on icosahedron (Ih) symmetry (see Fig. S2) before they were fully relaxed using spin unrestricted calculation. The calculated total spin of Pd13 and Ag13 are 8.0 and 5.0 B , respectively, which is the comparable to the previous reported values of 8.0 B [35] and 7.47 B [32]. After optimization, we found that the size of Ag13 is slightly larger than that of the Pd13 cluster, where the distance from the center metal atom to the outer atoms of the metal cluster are 2.66 and 2.78 A˚ for Pd13 and Ag13 , respectively. The vertical electron affinity (EA) and ionization energy (IE) for the clusters are provided in eV unit. The calculated (EA:IE) is (3.84:5.06) for Pd13 and (2.14:3.32) for Ag13 , respectively. The Pd13 results are comparable to the reports by Sun et al. (2.55:6.45) [41] using BPW91/LANL2DZ and by Benítez et al. (N/A:6.41) [42] using B3LYP/LANL2DZ. In addition, the EA of Pd13 and Ag13 were reported as 2.25 ± 0.30 and 2.10 ± 0.15 eV by using the photoelectron spectroscopy technique [43]. The EA related to the ability of electron acceptor of metal clusters predicts that the Pd13 has higher Hg adsorption capability than the Ag13 . 3.1.3. Composite models 3.1.3.1. DG supported metal atom (M1 -DG). The optimized structures of Pd and Ag deposited on DG (Pd1 -DG and Ag1 -DG) are shown in Fig. 1(a) and the structural parameters, charges, and binding energies of Pd1 -DG and Ag1 -DG are listed in Table 1. The Pd and Ag adatoms located at 1.81 and 2.18 A˚ on top of the DG plane, respectively. A distortion on the DG surface was observed after the metal adsorption. The Pd-C bond lengths (1.94, 1.96 and ˚ are shorter than those of the Ag-C bonds (2.14, 2.40 and 1.96 A) ˚ The calculated Pd-C bonds are in agreement with previous 2.41 A). ˚ [27,34]. The binding energy of the Pd adatom studies (1.92–1.95 A) on DG is −5.45 eV, which is similar to −5.63 eV from previous work [27]. Such a large binding energy indicates high stability of Pd1 -DG composite. In the case of Ag1 -DG, a binding energy of −2.00 eV indicates its lower stability. When comparing the binding energies of metal atoms on DG in this work to metal atoms on PG in previous reports, it was found that Pd1 binds significantly stronger with DG than with PG (−0.87 to −1.08 eV) [20,44]. In addition, the binding energy of Ag1 and DG is also significantly larger than that of PG (−0.05 to −0.19 eV) [20,21]. Hence, metal atom and cluster bound to DG stronger than PG.
Fig. 1. (a) Optimized structures for defective graphene-supported adatoms; Pd1 (Pd1 -DG) and Ag1 (Ag1 -DG). (b) The charge density difference plots. The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.02 au). The insets represent top view of the structure.
Next, we analyzed the electronic properties of Pd1 -DG and Ag1 DG. Fig. 1(b) shows the charge density-difference plots of Pd1 -DG and Ag1 -DG composite models. Here, we found that the charge near the vacancy site and the charge near the metal atom are transferred to the bonding region between the metal and carbon atoms of DG. This indicates a bonding characteristic between the deposited metal and the dangling C atoms near the vacancy site. Hirshfeld charge analysis shows that the charge of Pd and Ag adatoms are reduced by 0.546 and 0.331, respectively. We found a correlation between the amount of reduced charge and its binding strength, which for Pd and Ag deposition, were −5.45 and −2.00 eV, respectively. The partial densities of states (PDOS) of deposited metal atoms and the C1, C2 and C3 of DG are shown in Fig. 2(a) and Fig. S3. The large overlap between the p-orbital of C atoms and the d-orbital of metal is observed below the Fermi level, and indicates the strong hybridization of these orbitals. 3.1.3.2. DG supported Pd13 and Ag13 nanoclusters. The optimized Pd13 and Ag13 clusters in Ih symmetry are deposited onto the defective site of the DG. We considered the atop-bridge and tripleatop deposition modes as initial structures for optimizations since they were reported as the two-most stable structures in previous studies [31]. The results indicate that the Pd13 cluster uses the atopbridge mode, while the Ag13 cluster uses the triple-atop mode (see Fig. 3(a)). Following metal clusters deposition, the clusters are slightly distorted from their freestanding forms. The Pd13 cluster binds perpendicular to the DG plane, where the bottom-most atom of the Pd13 bonded with the three carbon atoms near vacancy. Table 1 The binding energy, charges, d-band center (d ) and selected structural parameters of Pd and Ag supported on defective graphene.
Pd1 -DG Ag1 -DG Pd13 -DG Ag13 -DG
Eb (eV)
q (M)
d a (eV)
d (M-C)b (Å)
−5.45 −2.00 −8.49 −3.28
0.546 0.331 0.729 0.771
−4.84 −4.42 −1.80 −4.11
1.94, 1.96, 1.96 2.14, 2.40, 2.41 1.88, 1.92, 1.94 2.11, 2.12, 2.17, 2.48, 2.51
a d of bare Pd1 , Ag1 , Pd13 and Ag13 clusters are −0.14, −2.59, −1.57, −3.92 eV, respectively. b M = Pd or Ag atom.
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Fig. 2. Partial density of states (PDOS) of (a) M1 -DG and (b) M13 -DG. C stands for the combination of the states for C1-C3.
On the other hand, the Ag13 cluster is deviated from the z-axis due to its triple-atop interaction, where three-bottom Ag atoms bonded to four carbon atoms of DG. The average Pd13 -DG and ˚ respectively. In term Ag13 -DG bond lengths are 1.92 and 2.28 A, of binding strength, DG binds with Pd13 (Eb-Pd13 = −8.49 eV) three times stronger than with Ag13 (Eb-Ag13 = −3.28 eV). This signifies the excellent ability of the DG substrate to deposit a Pd nanocluster and indicates the high stability of the Pd13 -DG composite. It is interesting to compare the cluster results with the single-atom deposition from Section 3.1.3.1. The binding energy of Pd13 -DG is much larger than that of the Pd1 -DG (3.03 eV difference). On the other hand, the binding energy of Ag13 -DG is stronger than that of the Ag1 -DG (1.27 eV difference). Thus, the cluster size significantly affects the
Fig. 3. (a) Optimized structures for defective graphene-supported metal clusters; Pd13 (Pd13 -DG) and Ag13 (Ag13 -DG). (b) The charge density difference plots. The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.02 au). The insets represent top view of the structure.
interaction of the Pd cluster on DG, while that of the Ag cluster is comparatively less. Our Pd13 -DG results are in agreement with previous studies, where the average bond length and the binding energy were reported to be within 1.95 to 2.14 A˚ and −6.18 to −4.71 eV, respectively. In the case of the Ag13 system, only the binding on PG has been reported in literature [21]. The reported binding energy of Ag13 on PG is –1.10 eV, is considerably smaller than that of our Ag13 -DG result (–3.28 eV). Fig. 3(b) shows the charge density difference plots of Pd13 -DG and Ag13 -DG. It is noted that large amount of charge density is accumulated at the metal-DG interface bonds, while the charge depletion region is observed at the metals’ contact point. This observation is confirmed by the Hirshfeld’s net-charge analysis of Pd13 and Ag13 clusters, which decreased by 0.729 and 0.771, respectively, after deposition. Next, we performed the PDOS analysis before and after metal cluster deposition. Fig. 3(b) and Fig. S4 shows PDOS plots of Pd13 -DG and Ag13 -DG. Note that only bonding atoms are included in the PDOS result. The average energy of electronic d-states, the d-band center (d ), is one of electronic properties used as a reactivity descriptor for metal catalysts [45,46]. The closer of dband center to the Fermi level indicates the higher reactivity of the metal. We found that the d-bands of all metal clusters are broadened and shifted away from the Fermi level after deposition. The broadened d-bands of metal overlapped with the p-orbitals of carbon atoms of the DG, indicating the hybridization between d-bands of metal clusters and p-orbitals of carbon atoms. This hybridization of M13 -DG is similar to the M1 -DG cases in Section 3.1.3.1. In order to quantify the reactivity of deposited substrates, we performed d-band center (d ) analysis. Base on the d-band theory, reactivity trends of the most transition metals can be predicted from the d-band center calculation. The closer the transition metal’s energy of the d-band center to the Fermi energy, the higher its adsorption ability [47–49]. In our study, the d-band center of the freestanding Pd1 , Ag1 , Pd13 and Ag13 are −0.14, −2.59, −1.57, −3.92 eV, respectively. After deposition, their d-band centers become −4.84, −4.42, −1.80 and −4.11 eV for Pd1 , Ag1 , Pd13 and Ag13 , respectively (see Fig. 4). The decrease in energy of d-band centers after their depositions suggests that their reactivity is marginally reduced. It is noted that the d-band centers of M1 are significantly decreased while those of M13 are slightly decreased after depositions on DG. In the case of
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Fig. 4. (a) The d-band center energy (d ) of freestanding metal M1 and M13 (solid bars) compared with the deposited M1 and M13 on DG (solid bars with checked pattern), (b) show the plot of d-band center of deposited metal and the adsorption energy of Hg.
Table 2 The Hg adsorption energy and M-Hg distance of Pd and Ag supported on defective graphene.
Hg-Pd1 Hg-Pd13 (atop) Hg-Pd13 (bridge) Hg-Pd13 (3-fold) Hg-Ag1 Hg-Ag13 (atop) Hg-Ag13 (bridge) Hg-Ag13 (3-fold) Hg-Ag13 (4-fold)a a
Eads (eV)
M-Hg (Å)
−0.71 −0.89 −1.30 −1.52 −0.33 −0.43 – −0.72 −0.96
2.660 2.638 2.717, 2.732 2.684, 2.732, 2.813 2.923 2.800 become 4-fold 2.894, 2.904, 2.926 2.954, 2.964, 2.969, 2.975
Symmetry of Ag13 in Hg-Ag13 (4-fold) becomes Oh.
the M1 adatom, the decrease in energy of the d-band center of Pd1 is much more than that of Ag1 . This is due to the outermost shell of Pd being a full-filled 4d orbital while that of Ag is a half-filled 5s orbital. When Ag is adsorbed on DG, the electron in s-orbital of Ag1 transfers to DG while electrons in the d-orbital remain the same. However, for Pd1 , electron from its d-orbital transfers to DG resulting in
Fig. 5. (a) Optimized structures for Hg adsorption on bare defective graphene (HgDG). (b) The charge density difference plots. The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.002 au). The insets represent top view of the structure.
decreasing electrons in its d-orbital. These cause the d-band center energy of Ag not to change much compared with that of Pd. Considering the cluster size effect, the d-band center of deposited Pd13 is significantly increased from that of Pd1 (d-Pd(n = 1,13) = 3.03 eV),
Fig. 6. Hg adsorption on free Pd13 and Ag13 at atop, bridge and 3-fold sites, respectively. (a) Initial adsorption sites and (b) optimized adsorption complexes.
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Fig. 7. Partial density of states (PDOS) and the charge density difference plots of (a) Hg-Pd13 and (b) Hg-Ag13 . The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.01 au).
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while the d-band center of deposited Agn is relatively the same (d-Ag(n = 1,13) = 0.32 eV). The d-band center analysis strongly indicated that Pdn is more reactive than Agn in both freestanding and deposited forms. Remarkably, among the studied models, Pd13 -DG composite has the highest outstanding reactivity. 3.2. Adsorption of Hg0 on sorbent models To determine the efficiency of sorbent toward Hg0 removal, the adsorption of a Hg atom on various sorbents have been reported in literature, such as activated carbon [50], Fe2 O3 [51,52], ZnO [53], noble metals [54], and Ag doped TiO2 [19]. Here in this work, the reactivity of the studied sorbents toward Hg0 adsorption is characterized. The influences of sizes and different metals for Hg0 adsorption are discussed.
3.2.1. Adsorption of Hg on DG The initial configuration of Hg adsorption on the bare DG is prepared by placing an Hg atom on top of the vacancy site. The optimized structure is shown in Fig. 5, where the distances between the Hg and the three carbon atoms near the vacancy site are 3.32 (Hg˚ The adsorption energy of the C1), 4.21 (Hg-C2) and 4.25 (Hg-C3) A. Hg atom is −0.23 eV. This physisorption is relatively weak and tends to be unstable under operating temperatures. The net charge on an Hg atom is unchanged after adsorption. The PDOS of the Hg and related carbon atoms of DG are illustrated in Fig. S5. Upon adsorption, the s-orbital of an Hg atom is shifted slightly to a lower energy level. A small overlapping region between the p-orbital of C atoms and sd-orbitals of Hg is observed, confirming the weak interaction. This model could be used to explain the moderate efficiency of ACI system which used AC as a sorbent for Hg capturing [3].
3.2.2. Adsorption of Hg on free metal nanoclusters and metal atoms The optimized structures in Ih symmetry of Pd13 and Ag13 from Section 3.1.2 were used as models for Hg adsorptions. Three initial adsorption sites (atop, bridge and 3-fold) on the metal clusters were selected for Hg adsorption (see Fig. 6(a)). After structural relaxation (see Fig. 6(b)), the most stable site for Hg adsorption on a Pd13 cluster is the 3-fold site, while on Ag13 the most stable site is obtained from the initial bridge site. We noted that Ag13 was deformed into octahedral (Oh) symmetry after Hg adsorption. The selected structural parameters and adsorption energies are listed in Table 2. The calculated Hg adsorption energy on Pd13 (−1.52 eV) is larger than Ag13 (−0.96 eV), which is in agreement with the calculated EA of the two clusters. In addition, the Hg adsorption on the two clusters are much larger than the bare DG (Eads = -0.23 eV). The average ˚ which is shorter than that of the bond length of Hg-Pd is 2.74 A, ˚ It was found that these bonding distances are Hg-Ag with 2.96 A. correlated to their binding energies. Moreover, the binding energy and d-band center energy of metal clusters are also correlated. The energy of d-band centers of Pd13 (d-Pd13 = −1.57 eV) is closer to the Fermi level compared to that of Ag13 (d-Ag13 = −3.92 eV). The closer the d-band center to the Fermi level, the higher reactivity to Hg adsorption. In terms of electronic property, PDOS analysis of the most stable structures show the overlapping orbitals of Hg and deposited metals (see Fig. 7(a, b)); s-orbital of Hg is overlapped with d-orbital of bound Pd and Ag atoms. Charge analysis shows charge transfer from the Hg atom to the metal clusters. In addition, we also calculated the binding energy of Hg with a single Pd and Ag atom. The results show that the binding energy of Hg on Pd and Ag atoms are −0.71 and −0.33 eV, respectively. In summary, the trend of reactivity of all sorbent models to
Fig. 8. (a) Optimized structures and (b) the charge density difference plots for Hg adsorption on Pd1 -DG (up) and Ag1 -DG (down). The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.02 au). The insets represent top view of the structure.
Hg0 adsorption is Agn « Pdn both for n = 1 and 13, Ag1 ∼ Ag13 and Pd1 < Pd13 . 3.3. Adsorption of Hg atom on composite models (i) Adsorption of Hg atom on Pd1 -DG and Ag1 -DG: Fig. 8 shows the optimized structures of Hg adsorption on Pd1 -DG and Ag1 -DG. The Hg is slightly bent but remains on top of the metal atoms. The ˚ while distances between Hg and metal atoms are 2.84 and 2.74 A, adsorption energies of Hg0 are −0.62 and −0.69 eV, for Pd1 -DG and Ag1 -DG sorbents, respectively. This indicates significant improvement for Hg0 adsorption efficiency by depositing metals on DG, since Eads-HgDG = −0.23 eV is relatively small. In addition, this result agrees with the d-band energy analysis discussed in Section 3.1.3.1, in which the energy of d-band centers of Pd1 -DG and Ag1 -DG are not significantly different, as can be seen in Fig. 4 (i.e. −4.84 and −4.42 eV, respectively). (ii) Adsorption of Hg atom on Pd13 -DG and Ag13 -DG: The optimized structures of Pd13 -DG and Ag13 -DG are obtained from Section 3.1.3.2. There are many possible adsorption sites for Hg on the deposited M13 . We examined eight initial adsorption sites for each systems (see Figs. 9 and 10). After structural optimization, their configurations and the calculated adsorption energies are shown in Figs. 11 and 12, while
Fig. 9. All possible adsorption sites of Hg on an M13 (Ih) cluster adsorbed on defective graphene.
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Fig. 10. Hg adsorption at different sites of Ag1 -DG composite.
Fig. 11. Hg adsorption at different sites of Pd13 -DG composite.
Fig. 12. The adsorption energy of Hg on various sorbents (PG, DG, Ag (111), Pd (111), Pd1 , Ag1 , Pd13 , Ag13 , Pd1 -DG, Ag1 -DG, Pd13 -DG and Ag13 -DG).
the detailed selected parameters are listed in Table S1. Hg adsorptions on freestanding Mn and Mn /DG were compared in Fig. 12. It is seen that in terms of adsorption energy, DG insignificantly affect to adsorption strength of Hg, which is different from TiO2 support [19]. Remarkably, the adsorption energies of Hg on various sites of a Pd13 -DG composite range between −0.94 and −1.68 eV, while those of Ag13 -DG are relatively smaller within −0.47 to −0.67 eV (see Fig. 12 and Table S2). Compared to the Hg0 adsorption on
M1 -DG, Hg binding strength on Pd13 -DG is significantly improved compared with the Pd1 -DG (Eads-Pd1DG = −0.62 eV). On the contrary, Hg binding strength on Ag13 -DG is comparable to the Ag1 -DG result (Eads-Ag1DG = −0.69 eV). This indicates that the clustering of Pd atoms enhances the Hg adsorption while the clustering of Ag atoms does not affect Hg adsorption. The most stable configuration of Hg on Pd13 -DG (G site), is where the Hg atom binds with the two Pd atoms located at the lower Pd-pentagonal of Pd13 cluster with the ˚ In the case of the Hg-(Ag13 -DG) sysdistances of 2.71 and 2.78 A. tem, an Hg atom binds with the three Ag atoms located at the side of Ag13 clusters with the distances of 2.95, 2.96 and 3.00 A˚ (D site). There is a correlation between the average distance between the Hg-M and the binding energy; the shorter distance of Hg-M, the higher binding strength of Hg and composite. Noted that there is a correlation between the d-band center of the deposited metal and the Hg adsorption energy, see Fig. 4(b). The d-band center shifts closer to the Fermi level, the higher reactivity and higher Hg adsorption energy of the composite sorbent. This indicates that the d-band center is the Hg adsorption descriptor for the sorbent models. One may say that the efficiency of a sorbent can be tuned by tailoring the d-band center. This information might be useful for experimentalist in selecting metal type for depositing on activated carbon as sorbent for Hg removal in further development. 4. Conclusions Density functional theory calculations were carried out to systematically study the reactivity of Hg0 adsorption on metal deposited DG. We have demonstrated that DG supported with Pd is more reactive to Hg than that with Ag. The structural and electronic properties of each sorbent before and after reacting with Hg were
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investigated. From the energetic aspect, metal deposited on DG are highly stable, which originated from the hybridization of d-orbital of metal and p-orbital of DG carbon atoms. Overall, we found that (i) vacancy site of DG is a strong binding trap for depositing metal, (ii) DG, which is representative of activated carbon, is a good substrate for depositing a Pd13 nanoparticle with very high adsorption strength, (iii) the Pd13 -DG composite is the most reactive sorbent for Hg adsorption, (iv) the Hg adsorption abilities of Pd composites are relatively higher than those of Ag composites, (v) metal clustering of Pd affects to its reactivity but does not for Ag, indicating a contribution of the nature of the metals, and (vi) d-band center is an adsorption descriptor of composite models; the d-band center shifts closer to the Fermi level, the higher reactivity to Hg of the sorbent. These results provide understanding and useful information for further design of activated carbon as a reactive sorbent for removing Hg0 as well as other heavy metals from flue gases.
Acknowledgements The authors thank the National Research Council of Thailand (NRCT) and National Nanotechnology Center (NANOTEC) through the Flagship “Clean Air” program for their financial support.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.12. 078.
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Capability of defective graphene-supported Pd13 and Ag13 particles for mercury adsorption Jittima Meeprasert a , Anchalee Junkaew a , Chompoonut Rungnim a , Manaschai Kunaseth a , Nawee Kungwan b , Vinich Promarak c , Supawadee Namuangruk a,∗ a
National Nanotechnology Center, NSTDA, 111 Thailand Science Park, Klong Luang, Pathum Thani 12120 Thailand Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand c School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Wangchan, Rayong 21210, Thailand b
a r t i c l e
i n f o
Article history: Received 7 October 2015 Received in revised form 15 November 2015 Accepted 10 December 2015 Available online 12 December 2015 Keywords: Mercury DFT Palladium Silver Nanoparticle Graphene
a b s t r a c t Reactivity of single-vacancy defective graphene (DG) and DG-supported Pdn and Agn (n = 1, 13) for mercury (Hg0 ) adsorption has been studied using density functional theory calculation. The results show that Pdn binds defective site of DG much stronger than the Agn , while metal nanocluster binds DG stronger than single metal atom. Metal clustering affects the adsorption ability of Pd composite while that of Ag is comparatively less. The binding strength of −8.49 eV was found for Pd13 binding on DG surface, indicating its high stability. Analyses of structure, energy, partial density of states, and d-band center (d ) revealed that the adsorbed metal atom or cluster enhances the reactivity of DG toward Hg adsorption. In addition, the Hg adsorption ability of Mn -DG composite is found to be related to the d of the deposited Mn , in which the closer d of Mn to the Fermi level correspond to the higher adsorption strength of Hg on Mn -DG composite. The order of Hg adsorption strength on Mn -DG composite are as follows: Pd13 (−1.68 eV) » Ag13 (−0.67 eV) ∼ Ag1 (−0.69 eV) > Pd1 (−0.62 eV). Pd13 -DG composite is therefore more efficient sorbent for Hg0 removal in terms of high stability and high adsorption reactivity compared to the Ag13 . Further design of highly efficient carbon based sorbents should be focused on tailoring the d of deposited metals. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, most electricity is generated by burning fossil fuels such as natural gas, oil, and coal. Air pollution created from the combustion processes of these fuels, particularly coal, causes serious environmental concern. The flue gas from coal-fired power plants contains toxic compounds such as carbon oxides (COx ), sulfur oxides (SOx ), nitrogen oxides (NOx ), and heavy metals including mercury (Hg). Among them, mercury causes significant environmental concern due to its extreme toxicity and high volatility. Generally, there are three forms of mercury that exist in the flue gas: elementary (Hg0 ), oxidized (Hg+ and Hg2+ ) and particulate mercury (Hgp ) [1,2]. Hg+ and Hg2+ are soluble in water and thus can be captured by wet scrubber equipment. Hgp can be collected by using electrostatic precipitation (ESP) and fabric filtration (FF). The most challenging task is Hg0 removal. Due to its low water solubility, Hg0 is difficult to capture using standard equipment, and thus
∗ Corresponding author. E-mail address: [email protected] (S. Namuangruk). http://dx.doi.org/10.1016/j.apsusc.2015.12.078 0169-4332/© 2015 Elsevier B.V. All rights reserved.
is released into environment. Hence, the abatement of Hg0 produced from coal-fired power plants is a long-term global pollution reduction goal. Most coal-fired power plants reduce mercury emissions from the flue gas using the activated carbon injection (ACI) control system. ACI injects powdered activated carbon (PAC) into the flue gas stream. PAC adsorbs the Hg0 from the flue gas and is then collected with fly-ash in the plant’s particulate collection device [3]. ACI has been recognized as a cost-effective means to reduce Hg0 from flue gas but its adsorption efficiency is still low. Thus, the improvement Hg0 removal efficiency of ACI is essential. From experimental studies, it has been reported that noble metals such as gold (Au), silver (Ag), palladium (Pd), and platinum (Pt) are efficient Hg sorbents [4–6]. Moreover, they are reusable and stable for long-term operation. In theoretical studies, density functional theory (DFT) calculations have shown that the binding strength of Hg0 on the surface of metals increases in the following order: Ag < Au < Cu < Ni < Pt < Pd [7,8]. It was noted that Ag was the least reactive to Hg, and Pd was the most reactive to Hg. Sun et al. [9] studied Hg0 adsorption on Agn clusters (n = 1–6) by using DFT calculations. They reported that the binding energy increases as the
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size of Ag cluster increases from Ag1 to Ag4 , but it decreases when the size of Ag cluster is larger than Ag4 . A similar finding was also observed in Hg0 adsorption on Aun (n = 1–6) clusters, as investigated by Siddiqui et al. [10]. However, they showed a contradictory result for Pdn clusters in which there was no correlation between the binding energy and the size of Pd cluster [11]. Thus, the nature of metals may have a different influence on Hg adsorption. Although, noble metals have high reactivity to Hg0 , it is known that the disadvantage of using noble metals as sorbents or catalysts is their expense. Thus, they are not applicable in large-scale use. Many research groups have attempted to develop novel materials, which possess both high reactivity and cost-efficiency for Hg0 removal. When compromising the advantages and disadvantages of activated carbon and noble metals, the combination of both materials is thought to provide superior characteristics. Depositing a small amount of metal on AC not only makes it cost effective, but also increases reactivity of metal nanoparticles and activated carbon. AC is usually modeled by defective graphene due to its structural imperfection. In synthesis aspect, defective graphene (DG) can be produced by plasma treatments at a power of 10 W and a pressure of 0.1 Torr [12]. The amount of defects can be tuned by changing the treatment time. In addition, doping metal nanoparticle on graphene also can be performed by heating method at one pot synthesis [13,14]. In recent experimental studies, Au [15,16] and Pd [17] nanoparticles doped AC have been reported as highly efficient Hg0 sorbents, since AC prevents the aggregation of metal nanoparticles and the sorbents can be regenerated. Recently, by using combined experimental and theoretical studies, we found that loading small amount of Ag onto TiO2 powder could dramatically enhance the efficiency of sorbent for Hg adsorption [18,19]. The deposited Ag promoted electron transfer from Hg to the TiO2 substrate via the Ag-O interface bonds. We found a correlation between the amount of the electron transfer and Hg0 sorbent binding strength. For metal on carbon-based materials, however, function of deposited metal for Hg adsorption and the criteria for selection of metal for AC, has not been reported. Moreover, both Ag and Pd metals in composited sorbents were not comparatively studied; neither experimentally nor theoretically and thus, their comparative efficiencies for Hg0 removal have not been identified. In theoretical studies, Granatier et al. [20] studied the adsorption of Pd, Au and Ag atoms on pristine graphene (PG) and found that each metal binds with PG in different ways. For example, Ag is predominantly bound through a dispersion interaction, while the Pd binds with a covalent bond and the binding of Au involves both charge transfer and dispersion interactions. However, metals tend to aggregate on PG due to the weak interaction between metal and PG [21,22]. On the other hand, the vacancy defect in graphene was suggested as a strong anchoring site for metal nanoparticles, and has been used as a model to represent AC [23]. Many transition metals such as Pt, Pd, Au, and Fe can be strongly deposited onto DG [24–34]. Recently, the structural and electronic properties of DG-supported Pd nanoparticles have been reported. Jia et al. [27] performed DFT calculations to study the Pdn clusters (n = 1–5) deposition on single-vacancy graphene and their adsorptions to O2 . They found that point defect in graphene is a deposited site for Pd clusters, which affects the charge transfer from Pd cluster to adsorbed O2 . Sen et al. [33] studied the adsorption of small Pdn clusters (n = 1–5) on double-vacancy graphene by using DFT calculations. They suggested that uniform and immobile distribution of Pd on graphene might be obtained by the adsorption of a Pd4 cluster on DG. For larger clusters, the icosahedral (Ih) Pd13 structure is particularly used to deposit on this support structure [24,31,32]. Compared to other Pd13 isomers, the Ih symmetry exhibits unusual clusterorbitals as the d-type cluster orbitals are lower in energy than the p-type cluster orbitals [35]. Liu et al. [31,32] studied the Pd13 and
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MePd12 (Me = Fe, Co, Ni, Cu, Zn) nanoparticles supported on DG for an application in electrocatalysis. They reported that Pd13 binds strongly with support due to the strong hybridization between dorbitals of Pd and p-orbitals of graphene carbon atoms. A similar result was observed in a separated study by Xia et al. [24]. Nevertheless, understandings of electronic, physical and chemical properties of DG supported metal clusters are still limited. To the best of our knowledge, the use of a DG-supported metal nanoparticle as a sorbent for Hg0 removal has not been investigated by theoretical calculations. In this work, the adsorption of Hg0 on the DG-supported Ag and Pd adatom and nanocluster (Ag13 and Pd13 ) have been comparatively studied using the DFT calculations. A 13 atoms-size is selected because it is a number of closed-shell high symmetry structures exist. The icosahedron is of particular interest of stable high-symmetry structures because they can be considered building blocks that can be compounded into bigger structures. Ag was reported as the least reactive metal among studied noble metals on PG [7,8] and has never been studied on DG before. Both Ag and Pd metals have never been comparatively studied in terms of binding with DG, and Hg0 adsorption efficiency. Thus, the aim of this work is to study the structural and electronic properties of metal and metal-defect graphene composites toward Hg0 adsorption. The nature of different noble metals when acting as sorbents in isolated- and composite-forms were characterized. The key descriptor, which is responsible for Hg adsorption efficiency, was identified. The obtained information would be useful for experimentalists in the selection of metals, the design, and the development of efficient carbon-based materials for removal of Hg0 , as well as other heavy metals, from flue gas. 2. Computational details The spin unrestricted DFT calculations were performed using the DMol3 program in the Materials Studio 5.5TM package [36,37]. The electron exchange-correlations were treated using the generalized gradient approximation (GGA) with the Perdew and Wang formulation (PW91) functional [38]. The double numerical plus polarization (DNP) basis sets were selected with the effective core potential (ECP) scheme [39,40]. An orthorhombic supercell ˚ with periodic boundary condition, was of 14.74 × 14.76 × 15.09 A, used for the DG and DG-supported metal nanocluster systems. These parameters were obtained from the cell optimization of ˚ The inte6 × 6 graphene supercell with a vacuum space of 15 A. gration of the Brillouin zone was calculated using 5 × 5 × 1 k-point sampling for geometry optimizations and electronic charge calculations. Under unconstrained symmetry, all atoms were fully relaxed until the total energy and the SCF density were converged within 1 × 10−5 Hartree. The maximum force and the maximum ˚ displacement tolerances were set at 0.002 Hartree/Å and 0.005 A, respectively. Reference calculations for isolated Pd13 and Ag13 nanoclusters were performed using the same conditions as for the DG and DG-supported metal nanocluster systems. Charge is evaluated from the Hirshfeld population analysis. For the DG-supported Pdn or Agn nanoclusters (n = 1 and 13), the binding energy (Eb ) between metal and DG support is calculated as: Eb = ENC-DG − (EDG + ENC ),
(1)
where ENC-DG is the total energy of the metal nanocluster deposited on DG, EDG is the total energy of DG, and ENC is the total energy of the isolated metal nanocluster. For the adsorption of Hg atom on the DG-supported metal nanoclusters, the adsorption energy (Eads ) is calculated as: Eads = EHg-(NC-DG) − (EHg + ENC-DG ),
(2)
where EHg-(NC-DG ) is the total energy of Hg adsorbed on the metal supported-DG composite, EHg is the total energy of isolated Hg
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atom, and ENC-DG is the total energy of DG-supported metal nanocluster. Negative value indicates stronger adsorption. 3. Results and discussion Section 1, we constructed the sorbent models comprised of (i) bare DG, (ii) freestanding M1 and M13 , and (iii) composite models of M1 -DG and M13 -DG; M = Ag and Pd. In order to study the reactivity of sorbents toward Hg adsorption, we adsorbed Hg on all sorbent models, see Section 2. 3.1. Sorbent models 3.1.1. Defective graphene The DG supercell consists of 71 carbon atoms with a single carbon atom vacancy at the center as shown in Fig. S1. The vacancy site is the most reactive region since the lowest unoccupied molecular orbital (LUMO) is localized mainly at the vacancy site. This is in agreement with a previous study [23]. Therefore, the vacancy site of DG is used as the deposition site for metal atoms and metal nanoclusters in this study. 3.1.2. Metal nanoclusters The electronic configurations of Pd and Ag are [Kr] 4d10 and [Kr] 4d10 5s1 , respectively. The initial configuration of freestanding metal nanoclusters (Pd13 , Ag13 ) were based on icosahedron (Ih) symmetry (see Fig. S2) before they were fully relaxed using spin unrestricted calculation. The calculated total spin of Pd13 and Ag13 are 8.0 and 5.0 B , respectively, which is the comparable to the previous reported values of 8.0 B [35] and 7.47 B [32]. After optimization, we found that the size of Ag13 is slightly larger than that of the Pd13 cluster, where the distance from the center metal atom to the outer atoms of the metal cluster are 2.66 and 2.78 A˚ for Pd13 and Ag13 , respectively. The vertical electron affinity (EA) and ionization energy (IE) for the clusters are provided in eV unit. The calculated (EA:IE) is (3.84:5.06) for Pd13 and (2.14:3.32) for Ag13 , respectively. The Pd13 results are comparable to the reports by Sun et al. (2.55:6.45) [41] using BPW91/LANL2DZ and by Benítez et al. (N/A:6.41) [42] using B3LYP/LANL2DZ. In addition, the EA of Pd13 and Ag13 were reported as 2.25 ± 0.30 and 2.10 ± 0.15 eV by using the photoelectron spectroscopy technique [43]. The EA related to the ability of electron acceptor of metal clusters predicts that the Pd13 has higher Hg adsorption capability than the Ag13 . 3.1.3. Composite models 3.1.3.1. DG supported metal atom (M1 -DG). The optimized structures of Pd and Ag deposited on DG (Pd1 -DG and Ag1 -DG) are shown in Fig. 1(a) and the structural parameters, charges, and binding energies of Pd1 -DG and Ag1 -DG are listed in Table 1. The Pd and Ag adatoms located at 1.81 and 2.18 A˚ on top of the DG plane, respectively. A distortion on the DG surface was observed after the metal adsorption. The Pd-C bond lengths (1.94, 1.96 and ˚ are shorter than those of the Ag-C bonds (2.14, 2.40 and 1.96 A) ˚ The calculated Pd-C bonds are in agreement with previous 2.41 A). ˚ [27,34]. The binding energy of the Pd adatom studies (1.92–1.95 A) on DG is −5.45 eV, which is similar to −5.63 eV from previous work [27]. Such a large binding energy indicates high stability of Pd1 -DG composite. In the case of Ag1 -DG, a binding energy of −2.00 eV indicates its lower stability. When comparing the binding energies of metal atoms on DG in this work to metal atoms on PG in previous reports, it was found that Pd1 binds significantly stronger with DG than with PG (−0.87 to −1.08 eV) [20,44]. In addition, the binding energy of Ag1 and DG is also significantly larger than that of PG (−0.05 to −0.19 eV) [20,21]. Hence, metal atom and cluster bound to DG stronger than PG.
Fig. 1. (a) Optimized structures for defective graphene-supported adatoms; Pd1 (Pd1 -DG) and Ag1 (Ag1 -DG). (b) The charge density difference plots. The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.02 au). The insets represent top view of the structure.
Next, we analyzed the electronic properties of Pd1 -DG and Ag1 DG. Fig. 1(b) shows the charge density-difference plots of Pd1 -DG and Ag1 -DG composite models. Here, we found that the charge near the vacancy site and the charge near the metal atom are transferred to the bonding region between the metal and carbon atoms of DG. This indicates a bonding characteristic between the deposited metal and the dangling C atoms near the vacancy site. Hirshfeld charge analysis shows that the charge of Pd and Ag adatoms are reduced by 0.546 and 0.331, respectively. We found a correlation between the amount of reduced charge and its binding strength, which for Pd and Ag deposition, were −5.45 and −2.00 eV, respectively. The partial densities of states (PDOS) of deposited metal atoms and the C1, C2 and C3 of DG are shown in Fig. 2(a) and Fig. S3. The large overlap between the p-orbital of C atoms and the d-orbital of metal is observed below the Fermi level, and indicates the strong hybridization of these orbitals. 3.1.3.2. DG supported Pd13 and Ag13 nanoclusters. The optimized Pd13 and Ag13 clusters in Ih symmetry are deposited onto the defective site of the DG. We considered the atop-bridge and tripleatop deposition modes as initial structures for optimizations since they were reported as the two-most stable structures in previous studies [31]. The results indicate that the Pd13 cluster uses the atopbridge mode, while the Ag13 cluster uses the triple-atop mode (see Fig. 3(a)). Following metal clusters deposition, the clusters are slightly distorted from their freestanding forms. The Pd13 cluster binds perpendicular to the DG plane, where the bottom-most atom of the Pd13 bonded with the three carbon atoms near vacancy. Table 1 The binding energy, charges, d-band center (d ) and selected structural parameters of Pd and Ag supported on defective graphene.
Pd1 -DG Ag1 -DG Pd13 -DG Ag13 -DG
Eb (eV)
q (M)
d a (eV)
d (M-C)b (Å)
−5.45 −2.00 −8.49 −3.28
0.546 0.331 0.729 0.771
−4.84 −4.42 −1.80 −4.11
1.94, 1.96, 1.96 2.14, 2.40, 2.41 1.88, 1.92, 1.94 2.11, 2.12, 2.17, 2.48, 2.51
a d of bare Pd1 , Ag1 , Pd13 and Ag13 clusters are −0.14, −2.59, −1.57, −3.92 eV, respectively. b M = Pd or Ag atom.
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Fig. 2. Partial density of states (PDOS) of (a) M1 -DG and (b) M13 -DG. C stands for the combination of the states for C1-C3.
On the other hand, the Ag13 cluster is deviated from the z-axis due to its triple-atop interaction, where three-bottom Ag atoms bonded to four carbon atoms of DG. The average Pd13 -DG and ˚ respectively. In term Ag13 -DG bond lengths are 1.92 and 2.28 A, of binding strength, DG binds with Pd13 (Eb-Pd13 = −8.49 eV) three times stronger than with Ag13 (Eb-Ag13 = −3.28 eV). This signifies the excellent ability of the DG substrate to deposit a Pd nanocluster and indicates the high stability of the Pd13 -DG composite. It is interesting to compare the cluster results with the single-atom deposition from Section 3.1.3.1. The binding energy of Pd13 -DG is much larger than that of the Pd1 -DG (3.03 eV difference). On the other hand, the binding energy of Ag13 -DG is stronger than that of the Ag1 -DG (1.27 eV difference). Thus, the cluster size significantly affects the
Fig. 3. (a) Optimized structures for defective graphene-supported metal clusters; Pd13 (Pd13 -DG) and Ag13 (Ag13 -DG). (b) The charge density difference plots. The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.02 au). The insets represent top view of the structure.
interaction of the Pd cluster on DG, while that of the Ag cluster is comparatively less. Our Pd13 -DG results are in agreement with previous studies, where the average bond length and the binding energy were reported to be within 1.95 to 2.14 A˚ and −6.18 to −4.71 eV, respectively. In the case of the Ag13 system, only the binding on PG has been reported in literature [21]. The reported binding energy of Ag13 on PG is –1.10 eV, is considerably smaller than that of our Ag13 -DG result (–3.28 eV). Fig. 3(b) shows the charge density difference plots of Pd13 -DG and Ag13 -DG. It is noted that large amount of charge density is accumulated at the metal-DG interface bonds, while the charge depletion region is observed at the metals’ contact point. This observation is confirmed by the Hirshfeld’s net-charge analysis of Pd13 and Ag13 clusters, which decreased by 0.729 and 0.771, respectively, after deposition. Next, we performed the PDOS analysis before and after metal cluster deposition. Fig. 3(b) and Fig. S4 shows PDOS plots of Pd13 -DG and Ag13 -DG. Note that only bonding atoms are included in the PDOS result. The average energy of electronic d-states, the d-band center (d ), is one of electronic properties used as a reactivity descriptor for metal catalysts [45,46]. The closer of dband center to the Fermi level indicates the higher reactivity of the metal. We found that the d-bands of all metal clusters are broadened and shifted away from the Fermi level after deposition. The broadened d-bands of metal overlapped with the p-orbitals of carbon atoms of the DG, indicating the hybridization between d-bands of metal clusters and p-orbitals of carbon atoms. This hybridization of M13 -DG is similar to the M1 -DG cases in Section 3.1.3.1. In order to quantify the reactivity of deposited substrates, we performed d-band center (d ) analysis. Base on the d-band theory, reactivity trends of the most transition metals can be predicted from the d-band center calculation. The closer the transition metal’s energy of the d-band center to the Fermi energy, the higher its adsorption ability [47–49]. In our study, the d-band center of the freestanding Pd1 , Ag1 , Pd13 and Ag13 are −0.14, −2.59, −1.57, −3.92 eV, respectively. After deposition, their d-band centers become −4.84, −4.42, −1.80 and −4.11 eV for Pd1 , Ag1 , Pd13 and Ag13 , respectively (see Fig. 4). The decrease in energy of d-band centers after their depositions suggests that their reactivity is marginally reduced. It is noted that the d-band centers of M1 are significantly decreased while those of M13 are slightly decreased after depositions on DG. In the case of
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Fig. 4. (a) The d-band center energy (d ) of freestanding metal M1 and M13 (solid bars) compared with the deposited M1 and M13 on DG (solid bars with checked pattern), (b) show the plot of d-band center of deposited metal and the adsorption energy of Hg.
Table 2 The Hg adsorption energy and M-Hg distance of Pd and Ag supported on defective graphene.
Hg-Pd1 Hg-Pd13 (atop) Hg-Pd13 (bridge) Hg-Pd13 (3-fold) Hg-Ag1 Hg-Ag13 (atop) Hg-Ag13 (bridge) Hg-Ag13 (3-fold) Hg-Ag13 (4-fold)a a
Eads (eV)
M-Hg (Å)
−0.71 −0.89 −1.30 −1.52 −0.33 −0.43 – −0.72 −0.96
2.660 2.638 2.717, 2.732 2.684, 2.732, 2.813 2.923 2.800 become 4-fold 2.894, 2.904, 2.926 2.954, 2.964, 2.969, 2.975
Symmetry of Ag13 in Hg-Ag13 (4-fold) becomes Oh.
the M1 adatom, the decrease in energy of the d-band center of Pd1 is much more than that of Ag1 . This is due to the outermost shell of Pd being a full-filled 4d orbital while that of Ag is a half-filled 5s orbital. When Ag is adsorbed on DG, the electron in s-orbital of Ag1 transfers to DG while electrons in the d-orbital remain the same. However, for Pd1 , electron from its d-orbital transfers to DG resulting in
Fig. 5. (a) Optimized structures for Hg adsorption on bare defective graphene (HgDG). (b) The charge density difference plots. The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.002 au). The insets represent top view of the structure.
decreasing electrons in its d-orbital. These cause the d-band center energy of Ag not to change much compared with that of Pd. Considering the cluster size effect, the d-band center of deposited Pd13 is significantly increased from that of Pd1 (d-Pd(n = 1,13) = 3.03 eV),
Fig. 6. Hg adsorption on free Pd13 and Ag13 at atop, bridge and 3-fold sites, respectively. (a) Initial adsorption sites and (b) optimized adsorption complexes.
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Fig. 7. Partial density of states (PDOS) and the charge density difference plots of (a) Hg-Pd13 and (b) Hg-Ag13 . The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.01 au).
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while the d-band center of deposited Agn is relatively the same (d-Ag(n = 1,13) = 0.32 eV). The d-band center analysis strongly indicated that Pdn is more reactive than Agn in both freestanding and deposited forms. Remarkably, among the studied models, Pd13 -DG composite has the highest outstanding reactivity. 3.2. Adsorption of Hg0 on sorbent models To determine the efficiency of sorbent toward Hg0 removal, the adsorption of a Hg atom on various sorbents have been reported in literature, such as activated carbon [50], Fe2 O3 [51,52], ZnO [53], noble metals [54], and Ag doped TiO2 [19]. Here in this work, the reactivity of the studied sorbents toward Hg0 adsorption is characterized. The influences of sizes and different metals for Hg0 adsorption are discussed.
3.2.1. Adsorption of Hg on DG The initial configuration of Hg adsorption on the bare DG is prepared by placing an Hg atom on top of the vacancy site. The optimized structure is shown in Fig. 5, where the distances between the Hg and the three carbon atoms near the vacancy site are 3.32 (Hg˚ The adsorption energy of the C1), 4.21 (Hg-C2) and 4.25 (Hg-C3) A. Hg atom is −0.23 eV. This physisorption is relatively weak and tends to be unstable under operating temperatures. The net charge on an Hg atom is unchanged after adsorption. The PDOS of the Hg and related carbon atoms of DG are illustrated in Fig. S5. Upon adsorption, the s-orbital of an Hg atom is shifted slightly to a lower energy level. A small overlapping region between the p-orbital of C atoms and sd-orbitals of Hg is observed, confirming the weak interaction. This model could be used to explain the moderate efficiency of ACI system which used AC as a sorbent for Hg capturing [3].
3.2.2. Adsorption of Hg on free metal nanoclusters and metal atoms The optimized structures in Ih symmetry of Pd13 and Ag13 from Section 3.1.2 were used as models for Hg adsorptions. Three initial adsorption sites (atop, bridge and 3-fold) on the metal clusters were selected for Hg adsorption (see Fig. 6(a)). After structural relaxation (see Fig. 6(b)), the most stable site for Hg adsorption on a Pd13 cluster is the 3-fold site, while on Ag13 the most stable site is obtained from the initial bridge site. We noted that Ag13 was deformed into octahedral (Oh) symmetry after Hg adsorption. The selected structural parameters and adsorption energies are listed in Table 2. The calculated Hg adsorption energy on Pd13 (−1.52 eV) is larger than Ag13 (−0.96 eV), which is in agreement with the calculated EA of the two clusters. In addition, the Hg adsorption on the two clusters are much larger than the bare DG (Eads = -0.23 eV). The average ˚ which is shorter than that of the bond length of Hg-Pd is 2.74 A, ˚ It was found that these bonding distances are Hg-Ag with 2.96 A. correlated to their binding energies. Moreover, the binding energy and d-band center energy of metal clusters are also correlated. The energy of d-band centers of Pd13 (d-Pd13 = −1.57 eV) is closer to the Fermi level compared to that of Ag13 (d-Ag13 = −3.92 eV). The closer the d-band center to the Fermi level, the higher reactivity to Hg adsorption. In terms of electronic property, PDOS analysis of the most stable structures show the overlapping orbitals of Hg and deposited metals (see Fig. 7(a, b)); s-orbital of Hg is overlapped with d-orbital of bound Pd and Ag atoms. Charge analysis shows charge transfer from the Hg atom to the metal clusters. In addition, we also calculated the binding energy of Hg with a single Pd and Ag atom. The results show that the binding energy of Hg on Pd and Ag atoms are −0.71 and −0.33 eV, respectively. In summary, the trend of reactivity of all sorbent models to
Fig. 8. (a) Optimized structures and (b) the charge density difference plots for Hg adsorption on Pd1 -DG (up) and Ag1 -DG (down). The charge accumulation and depletion are shown in red and green dots, respectively (isosurface value = 0.02 au). The insets represent top view of the structure.
Hg0 adsorption is Agn « Pdn both for n = 1 and 13, Ag1 ∼ Ag13 and Pd1 < Pd13 . 3.3. Adsorption of Hg atom on composite models (i) Adsorption of Hg atom on Pd1 -DG and Ag1 -DG: Fig. 8 shows the optimized structures of Hg adsorption on Pd1 -DG and Ag1 -DG. The Hg is slightly bent but remains on top of the metal atoms. The ˚ while distances between Hg and metal atoms are 2.84 and 2.74 A, adsorption energies of Hg0 are −0.62 and −0.69 eV, for Pd1 -DG and Ag1 -DG sorbents, respectively. This indicates significant improvement for Hg0 adsorption efficiency by depositing metals on DG, since Eads-HgDG = −0.23 eV is relatively small. In addition, this result agrees with the d-band energy analysis discussed in Section 3.1.3.1, in which the energy of d-band centers of Pd1 -DG and Ag1 -DG are not significantly different, as can be seen in Fig. 4 (i.e. −4.84 and −4.42 eV, respectively). (ii) Adsorption of Hg atom on Pd13 -DG and Ag13 -DG: The optimized structures of Pd13 -DG and Ag13 -DG are obtained from Section 3.1.3.2. There are many possible adsorption sites for Hg on the deposited M13 . We examined eight initial adsorption sites for each systems (see Figs. 9 and 10). After structural optimization, their configurations and the calculated adsorption energies are shown in Figs. 11 and 12, while
Fig. 9. All possible adsorption sites of Hg on an M13 (Ih) cluster adsorbed on defective graphene.
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Fig. 10. Hg adsorption at different sites of Ag1 -DG composite.
Fig. 11. Hg adsorption at different sites of Pd13 -DG composite.
Fig. 12. The adsorption energy of Hg on various sorbents (PG, DG, Ag (111), Pd (111), Pd1 , Ag1 , Pd13 , Ag13 , Pd1 -DG, Ag1 -DG, Pd13 -DG and Ag13 -DG).
the detailed selected parameters are listed in Table S1. Hg adsorptions on freestanding Mn and Mn /DG were compared in Fig. 12. It is seen that in terms of adsorption energy, DG insignificantly affect to adsorption strength of Hg, which is different from TiO2 support [19]. Remarkably, the adsorption energies of Hg on various sites of a Pd13 -DG composite range between −0.94 and −1.68 eV, while those of Ag13 -DG are relatively smaller within −0.47 to −0.67 eV (see Fig. 12 and Table S2). Compared to the Hg0 adsorption on
M1 -DG, Hg binding strength on Pd13 -DG is significantly improved compared with the Pd1 -DG (Eads-Pd1DG = −0.62 eV). On the contrary, Hg binding strength on Ag13 -DG is comparable to the Ag1 -DG result (Eads-Ag1DG = −0.69 eV). This indicates that the clustering of Pd atoms enhances the Hg adsorption while the clustering of Ag atoms does not affect Hg adsorption. The most stable configuration of Hg on Pd13 -DG (G site), is where the Hg atom binds with the two Pd atoms located at the lower Pd-pentagonal of Pd13 cluster with the ˚ In the case of the Hg-(Ag13 -DG) sysdistances of 2.71 and 2.78 A. tem, an Hg atom binds with the three Ag atoms located at the side of Ag13 clusters with the distances of 2.95, 2.96 and 3.00 A˚ (D site). There is a correlation between the average distance between the Hg-M and the binding energy; the shorter distance of Hg-M, the higher binding strength of Hg and composite. Noted that there is a correlation between the d-band center of the deposited metal and the Hg adsorption energy, see Fig. 4(b). The d-band center shifts closer to the Fermi level, the higher reactivity and higher Hg adsorption energy of the composite sorbent. This indicates that the d-band center is the Hg adsorption descriptor for the sorbent models. One may say that the efficiency of a sorbent can be tuned by tailoring the d-band center. This information might be useful for experimentalist in selecting metal type for depositing on activated carbon as sorbent for Hg removal in further development. 4. Conclusions Density functional theory calculations were carried out to systematically study the reactivity of Hg0 adsorption on metal deposited DG. We have demonstrated that DG supported with Pd is more reactive to Hg than that with Ag. The structural and electronic properties of each sorbent before and after reacting with Hg were
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investigated. From the energetic aspect, metal deposited on DG are highly stable, which originated from the hybridization of d-orbital of metal and p-orbital of DG carbon atoms. Overall, we found that (i) vacancy site of DG is a strong binding trap for depositing metal, (ii) DG, which is representative of activated carbon, is a good substrate for depositing a Pd13 nanoparticle with very high adsorption strength, (iii) the Pd13 -DG composite is the most reactive sorbent for Hg adsorption, (iv) the Hg adsorption abilities of Pd composites are relatively higher than those of Ag composites, (v) metal clustering of Pd affects to its reactivity but does not for Ag, indicating a contribution of the nature of the metals, and (vi) d-band center is an adsorption descriptor of composite models; the d-band center shifts closer to the Fermi level, the higher reactivity to Hg of the sorbent. These results provide understanding and useful information for further design of activated carbon as a reactive sorbent for removing Hg0 as well as other heavy metals from flue gases.
Acknowledgements The authors thank the National Research Council of Thailand (NRCT) and National Nanotechnology Center (NANOTEC) through the Flagship “Clean Air” program for their financial support.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.12. 078.
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