Tuning the Adsorption of Elemental Mercury by Small Gas-Phase

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Tuning the Adsorption of Elemental Mercury by Small Gas Phase Palladium Clusters: First Principle Study Bulumoni Kalita J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b06910 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 22, 2016

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The Journal of Physical Chemistry

Tuning the Adsorption of Elemental Mercury by Small Gas Phase Palladium Clusters: First Principle Study Bulumoni Kalita* Department of Physics, Dibrugarh University, Dibrugarh, Assam, India-786004

Abstract: Density functional theory (DFT) calculations have been performed to study the nature of interaction of elemental mercury (Hg) with small palladium clusters (Pdn, n=1-6) using generalized gradient approximation (GGA) method. Results of these calculations have shown stronger binding of Hg with Pd2 cluster, which, therefore, has been chosen for further investigation as presented in the later part of the results and discussion section of this report. This extended study explains the binding mechanism of Hg with alloys of Pd dimers, PdM (M= Pd, Pt, Cu, Ag, Au) in neutral, cationic and anionic states. Interaction energy of Hg with palladium dimer follows the trend: Pd2+ > Pd2 > Pd2-. For all of the above PdM complexes, the strength of Hg binding is found to be highest in their cationic states. Mixing of Pt and Au, enhances the reactivity of the cationic Pd2 dimers; decreases it for their neutral counterparts and does not affect much in the anionic states. Natural bond orbital (NBO) analysis indicates that Hg binding takes place because of the charge transfer from its s- orbitals primarily to the d- orbitals of M atoms followed by back donation of charges from their s- orbitals to the p- orbitals of Hg atom. Moreover, the amount of charge transfer from Hg(s)→M(d) correlates with the Hg binding energy in Hg-PdM0,± complexes. Binding of Hg in cationic Hg-PdM complexes conjointly depends on energies of the lowest unoccupied molecular orbitals (LUMO) of the PdM+ dimers as well as NBO partial charges on adsorbed Hg.

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1. INTRODUCTION Coal is the most abundant fossil fuel on earth and it will continue to be a major energy source for many years to come. Release of mercury in coal combustion process is of serious concern due to its toxic effects and its accumulation in the food chain resulting in human health issues1. High levels of methyl mercury in the bloodstream of unborn babies and young children have been demonstrated to be harmful in the development of cognitive nervous system2. In view of the importance of this issue, the U. S. Congress included mercury and its compounds as hazardous elements in the 1990 Clean Air Act. Under the Mercury and Air Toxics Standards (MATS) ruling of the U.S. Environmental Protection Agency (EPA) in December 2011, final standards were issued for limiting mercury, acid gases, and other toxic species from coal-fired power plants. Mercury emitted from coal-fired power plants exists in three forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate-bound mercury (Hgp). Among these three species, Hg0 is the most difficult species to eliminate because of its water insolubility and chemical inertness 3 . To increase the efficiency of the mercury removal processes, various sorbents or catalysts may be used to capture and/or oxidize Hg0 including activated carbon, metal oxides, metal sulfides, and pure metals4. Transition and noble metals such as copper, silver, gold and palladium have been outlined as suitable sorbents for Hg removal because of their thermodynamic and regeneration stabilities 5 - 9 .

A pilot-scale study of Hg oxidation in flue gas

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conducted by Blythe et al suggested that the higher cost of precious metals (Au or Pd) can be offset by requiring less metal (44% less in volume) and a potential longer life compared to carbon-based sorbent catalysts. Recent studies have also predicted Pd to be the most promising candidate for Hg removal7,11. The catalytic reactivity of bulk Pd surfaces may be enhanced significantly in the form of nanoparticles (NPs) through deposition onto various support materials, including oxide supports 12 - 17 . Some recent studies18-20 have shown that the presence of catalytic noble metal NPs such as Pd, Pt, Ag on the surface of some metal oxides, viz. ZnO, TiO2 and SnO2 can enhance the gas sensing performances. Moreover, metal clusters supported on oxide surfaces are well known for their important applications in different fields, such as heterogeneous catalysis, optoelectronic and magnetic devices, chemical sensors, etc.21. These reports clearly indicate the importance of studying Pd atomic clusters on


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suitable oxide support to handle the problem of Hg removal. However, the issue of sulfur (S) poisoning should be considered carefully in using pure Pd sorbents in flue and fuel gas environment while removing Hg22. This problem can be overcome by doping Pd catalyst with noble metals such as Au, Cu and Ag, increasing the S tolerance of Pd23,24. Therefore, tuning the reactivity of Pd alloy clusters on suitable support for Hg adsorption/oxidation is of importance for practical applications. To achieve this, a detailed study of adsorption of elemental Hg on pure and alloyed Pd clusters in gas phase is very important for better understanding of their chemical, physical and electronic properties. Numerous experimental6,25-29 and theoretical8,30-32 studies have been devoted for Hg adsorption on metal surfaces. The study of Hg adsorption on noble metals has been explored experimentally since the work of Anderson et al.33 proposing the use of Au as a collector for Hg. From the study of Hg adsorption on a series of metals viz., Ir, Pt, Ag, Pd, Rh, Ti, Ru by Granite et al.7 at different temperatures, Pd showed the highest capacity of Hg capture at a range of temperatures. This property was unaffected with increment of temperature even for Pd/Al2O3 system.

Density

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functional theory (DFT) method was used by Jain et al. to calculate the enthalpy of amalgamation and oxidation for metals to predict the capability of Hg sorption and oxidation in the gas stream. The results showed the highest amalgamation enthalpy for Pd, clarifying its higher tendency to capture Hg. Another DFT study by Steckel8 showed relatively strong binding of Hg with Cu, Ni, Pd, Pt, Ag and Au surfaces with the maximum binding energy value up to 1 eV for Pt and Pd. The reactivity order was found to be Ag < Au < Cu < Ni < Pt < Pd. Aboud et al.30 from their DFT study revealed increased binding energy of Hg with Pd surface alloying by small amounts of Au, Ag and Cu. As a continuation of this work, Sasmaz et al.35 showed that Pd is the only responsible atom for improving the interaction of mercury with the surface atoms in both Pd binary alloys and overlays due to the interaction of s- and p- states of Pd and the d- states of Hg. There are also some reports available in the literature on the experimental study of Hg adsorption on supported metal systems. Few such investigations showed some evidence of Hg amalgamation on Pd/Al2O3 with maximum occurrence at 2040C and at low loadings of Pd (< 8.5 wt. % Pd) 36,37 . Another observation6 reported alumina supported Pd to be superior to Pt for Hg removal. This study also showed that the Hg removal capacities of Pd and Pt were enhanced on various metal doping. 3 Environment ACS Paragon Plus


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Despite the thrust in designing efficient Hg removal catalysts/sorbents and the availability of evidences in support of Pd in this respect, research attention has not been paid till date to study the Hg adsorption behavior on pure/alloy clusters of Pd on suitable support. In fact, to the best of my knowledge, there is only one study reported by Siddiqui and Bouarissa38 so far, on the interaction of elemental Hg with some small gas phase Pdn (n=1-6) clusters in neutral, cationic and anionic states. In the present study, density functional theory (DFT) has been used to study the nature of binding of Hg atom on gas phase small neutral Pdn (n=1-6) clusters. Based on the results from this calculation, some of the clusters have been chosen to model PdM (M = Pd, Pt, Cu, Ag and Au) alloy catalysts to examine the Hg adsorption behaviour. Further, the affect of charge states of PdM clusters on Hg binding has also been studied.

2. COMPUTATIONAL DETAILS All the DFT caculations in this study have been carried out via Gaussian 09 package39. The calculations have been performed by using both hybrid exchangecorrelation functional, B3LYP40-43 and meta-generalized gradient apporixation (GGA) functional, M06-L of Truhlar et al. 44 , which does not include the Hartree-Fock exchange term. The choice of B3LYP functional in the present work is motivated from many previous studies38,45-47, which have already confirmed the efficiency of the hybrid B3LYP functional in rather accurate description of the structure and energetics of small palladium clusters. However, recent studies48,49 have shown the importance of M06-L functional in studying structural and electronic properties of small palladium clusters. Moreover, Zhao and Truhlar50 reported that it is the most accurate functional for transition metals. Therefore, the results of M06-L functional have been compared with those obtained from popular B3LYP functional. For heavy atoms like Pd and Hg, all electron calculation is rather time consuming. Therefore, relativistic effective core potential (RECP) has been introduced to describe the inner core electrons of these atoms, whereas their outermost valence electrons have been described throug the corresponding Los Alamos LANL2DZ basis sets51. Symmetry unrestricted full geometry optimizations have been carried out for Pd, Hg-Pdn, PdM (M = Pt, Cu, Ag, Au), Hg-PdM0,± (M= Pd, Pt, Cu, Ag, Au) complexes. The bare Pdn(n=2-6) clusters have been optimized in their known lowest energy geometries52-57


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with spin multiplicities, M=1 and 3. All possible structural isomers of Hg-Pdn (n=16), PdM (M= Pt, Cu, Ag, Au), Hg-PdM0,± (M= Pd, Pt, Cu, Ag, Au) complexes have been optimized in the mentioned levels of theory, until the minima was obtained on the potential energy surface. Each of these complexes containing even and odd number of total electrons have been investigated considering spin multiplicities, M=1, 3 and M=2, 4, respectively. Vibrational frequency calculations have been performed in both B3LYP/LANL2DZ and M06-L/LANL2DZ levels for the optimized geometries of all the isomers of neutral and single charged palladium complexes and none of them reported here is found to exhibit any imaginary frequency. Absence of imaginary frequency verifies the structures to be stable on the potential energy surfaces.

Zero-point vibrational energy corrections have been included in all

calculations. In addition, partial charges of Hg adsorbed PdM (M= Pd, Pt, Cu, Ag, Au) complexes have been examined by using Natural Population (NPA) scheme of the Natural Bond Orbital (NBO) methods58. In the results to follow, the binding energy (BE) of optimized neutral clusters are computed as

BE(PdM q ) = EPdM q − EPd − EM q where EPdM, EPd and EM are the energies of the lowest energy PdM cluster, neutral Pd atom, doped atom M (M= Pd, Pt, Cu, Ag, Au), respectively. q represents the charges considered in the present calculation, i.e., q = 0, +1, -1. Similarly, binding energies of Hg with PdM (M= Pd, Pt, Cu, Ag, Au) dimers have been calculated by using the following formulae.

BE(Hg − PdM q ) = EHg−PdM q − EHg − EPdM q where EHg-PdM, EHg and EPdM, are the total energies of Hg adsorbed PdM complexes, Hg atom and PdM dimers, respectively with q = 0, +1, -1.

The above definitions of binding energy signify stronger interactions for more negative values of BE.

3. RESULTS AND DISCUSSION 3.1. Structure and energetic The lowest energy structures of small gas phase neutral Pd clusters have been



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investigated several times52-57.

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Therefore, the already established lowest energy

geometrical isomers of these clusters have been chosen for geometry optimization in the present study. The results for these optimized clusters obtained from B3LYP and M06-L functionals have been compared in Table 1. It is observed that higher spin multiplicity is preferred by the small palladium clusters in gas phase. This observation is similar to that of Rösch et al.49. However, their prediction of dominant influence of long Pd-Pd bonds calculated by M06-L functional on the stability of high spin cluster configurations is not strictly followed in the present study. Both functionals yield the lowest energy structure of Pd2 in triplet multiplicity, which agrees with the previous study56.

The Pd-Pd bond length

computed with B3LYP and M06-L are 2.54 Å and 2.50 Å, respectively. Elongation of bond lengths are observed (2.76 Å and 2.72 Å, respectively) in their singlet spin multiplicities.

The bond length value of 2.50 Å (M06-L) matches with the

experimental value of 2.48 Å 59 .

The best performance of M06-L is noticed in

determining binding energy of Pd2. The BE of palladium dimer is underestimated (0.31 eV) with the use of B3LYP functional while it is consistent with the trustworthy value of -0.51 eV/atom 60 ,61 as obtained from M06-L level of calculation. In the present work, values Pd-Pd bond length (2.72 Å) and BE (-0.48 eV/atom) in singlet spin multiplicity of Pd2 calculated with M06-L agrees with the correponding values of Das et al.48. All the remaining clusters exhibit triplet spin multiplicities in B3LYP as well as in M06-L level of calculations, which agrees well with earlier study by Landman et al.53. Prediction of identical magnetic moments for very small size Pd clusters by both of the functionals is in contrast with that for medium size clusters49. Further, similar trends are found to be followed by the BE values obtained from B3LYP and M06-L functionals. The M06-L results in terms of average Pd-Pd bond length and BE of small Pd clusters agree pretty well with previous findings52,56. Therefore, in the following sections, results obtained with both B3LYP and M06-L funcationals have been widely discussed and compared when required.

3.1.1. Interaction of Hg with bare Pdn(n=1-6) clusters Geometry optimizations have been carried out for a large number of possible structural isomers of Hg adsorbed Pdn (n=1-6) clusters corresponding to top, bridge and hollow adsorption sites for the adsorbed Hg atom in the complexes.


Two

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different spin multiplicities (M=1, 3) have been examined for all the isomers of each cluster size. The resulting lowest energy structures are shown in Figures 1(a) and 1(b) and their calculated properties are presented in Table 2. The stable structures of various other isomers that have been computed for HgPdn complexes are shown in Figures S1 and S2 in the supplementary material. Apart form this, prior to the calculations of HgPdn, energy of Hg atom has been checked with possible M=1, 3 states. It has been found that Hg atom is most stable in singlet state, which is lower in energy than the triplet states by -4.70 eV and -5.00 eV as calculated in B3LYP and M06L level of calculation, respectively. These values are in close agreement with the experimental value of -4.9 eV62. The B3LYP optimized lowest energy HgPd1 complex is a singlet with the linear top adsorption geometry. The Pd-Hg bond length and Hg binding energy in this complex are 2.73 Å and -0.61 eV, respectively, which match with the available literature38. This isomer is 1.26 eV lower in energy than the corresponding triplet state. The corresponding values with M06-L functional are 2.68 Å and -0.81 eV with singlet-triplet energy difference of 1.60 eV.

B3LYP calculation shows that Hg

binding in Pd2 cluster takes place exothermically (BE is -1.13 eV) in the bridge geometry with the Hg atom bound to both the Pd atoms in the singlet state. The HgPd2 isomer with top adsorption geometry in triplet state lies 0.51 eV higher in energy than the lowest energy state. Hg binding with Pd2 in M06-L calculation exhibits similar structure as that of B3LYP with BE value of -1.45 eV. The higher value of Hg binding energy in the bridge adsorption geometry with neutral Pd2 cluster as observed by Siddiqui and Bouarissa38 also signifies that this geometry is more stable over the other possible top adsorption geometry of HgPd2 complex. B3LYP calculation results in hollow geometry for HgPd3 complex (M=1) as the most stable structure (BE is -0.74 eV) along with the existence of other higher energy isomers viz., atop and bridge structures. In this case, the corresponding geometry in triplet state lies only 0.002 eV higher in energy. M06-L calculation also reveals that Hg adsorption in Pd3 cluster takes place only in the hollow adsorption geometry, whose lowest energy corresponds to triplet spin mulitiplicity with binding energy value of 1.30 eV. However, there is no information about the three-fold adsorption geometry of HgPd3 complex in earlier report38.

B3LYP results predict the lowest energy

structure of HgPd4 to be in top adsorption (M=3) with slightly higher energy of 0.009 eV for the hollow adsorption geometry (M=3). M06-L level calculation shows that 7 Environment ACS Paragon Plus


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bridge adsorption of Hg in triplet state is favoured in Pd4 cluster, which is followed by the hollow adsorption structure (M=3) with only 0.03 eV higher in energy. The BE values of the most stable HgPd4 structures with B3LYP and M06-L functionals are 0.32 eV and -0.61 eV, respectively. Earlier study38 does not speak about the hollow adsorption geometry of this complex. In case of HgPd5 complex, we have checked different top and bridge adsorption geometries (represented by numbering 1, 2 and 3) corresponding to similar adsorption behaviour of Hg in distinguished sites of Pd5 along with hollow site adsorption. The initial and final geometries of these isomers in both level of calculations are shown in Figure S2 in supplementary material. The lowest energy isomers with their spin multiplicities are presented in Figure 1(b). B3LYP computes HgPd5 to be energetically most stable with bridge adsorption geometry in triplet state (Bridge2 isomer in Figure 1(b)) (BE is -0.49 eV) among several isomers. The same level of calculation with Top2 and Hollow isomers as initial geometries change the trigonal bipyramidal structure of bare Pd5 cluster into tetragonal pyramid with bridge (M=3) (Bridge1 isomer) and hollow (M=1) adsorption geometries, which are shown in Figure S2. These two new isomers have 0.04 eV and 0.67 eV energies higher than the most stable geometry. Hg BE in Bridge1 isomer (0.46 eV) agrees well with the ealier study (-0.463 eV)38, which however reported this isomer to be the lowest energy structure of HgPd5. The same study did not reveal any information about an isomer with hollow geometry for HgPd5.

Another bridge

structure (Bridge3 isomer in Figures S2) has energy difference of 0.09 eV from the lowest energy structure. Hg is found to bind preferably in the hollow adsorption site of Pd5 cluster in triplet state (M06-L) retaining the trigonal bipyramidal structure of Pd5 in gas phase. The binding energy of Hg in this complex is -0.78 eV. The bridge adsorption geometry (M=3) (Figure 1(b)) lies only 0.03 eV higher in energy. Hollow adsorption geometry of this complex in singlet multiplicity (Figure S2) state has energy 0.09 eV higher than the observed ground state. Hg binding in Pd6 cluster has been found to occur only in top and hollow geometries in both level of calculations considered in the present study, whereas bridge geometry has been reported earlier38. The strongest Hg binding takes place in hollow adsorption site in triplet states with binding energy values of -0.59 eV (B3LYP) and -0.97 eV (M06-L). The variation of binding energies of Hg with Pdn(n=1-6) clusters as a function of the cluster size is plotted in Figure 2, which shows similar pattern for the two functionals, B3LYP and M06-L used in the present calculations. It is clear from the 8 Environment ACS Paragon Plus

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results that Hg binds relatively strongly to Pd2 cluster, stronger than that with Pd surface (BE ~ -1.00 eV)8,11. Further, both level of calculations shows smallest value of Hg binding energy in the most stable HgPd4 complex in the series of HgPdn (n=16) complexes. This signifies higher stability of Pd4 cluster in agreement with the previous report56. Table 2 shows the average Pd-Pd and Pd-Hg bond lengths of the lowest energy HgPdn complexes.

The bonding interaction between the bare Pd

clusters and Hg atom is reflected by the elongation of Pd-Pd bond lengths in the HgPdn complexes with maximum value for Pd2 cluster. Moreover, Pd-Hg bond length in HgPd2 is smaller than those in other HgPd complexes of different sizes with exception in case of HgPd1 as observed in both level of calculations. The larger PdPd bond length and smaller Pd-Hg bond length in HgPd2 indicate the highest binding affinity of Hg with Pd2 cluster among the series of complexes studied here. On the basis of these arguments, Pd2 cluster has been chosen as a model system in the next part of the present study to investigate its interaction with Hg. Calculations have been performed to find suitable alloys of Pd2 cluster to enhance/tune its binding with Hg atom. Further, in view of the importance of affect of charge states of metal clusters on their catalytic activities, the binding mechanism of Hg atom with a series of binary alloy clusters of PdM (M= Pd, Pt, Cu, Ag, Au) in different charge states have also been investigated. 3.1.2. Bare PdM0,± (M= Pd, Pt, Cu, Ag, Au) dimers Full geometry optimizations have been performed for binary dimers of Pd with group IB metals such as Cu, Ag and Au in four different possible spin multiplicities, M=2 and 4. Similarly, the possible M values for PdPt dimer chosen in present calculations are 1 and 3. The most stable structures of these dimers are shown in Figure 3(a) and their calculated properties are presented in Table 3.

Both B3LYP and M06-L

functionals predict doublet states of PdCu, PdAg and PdAu dimers and triplet state of PdPt dimer to be the lowest energy structures. The M06-L level of calculation reveals that quartet states of PdCu, PdAg and PdAu dimers are 2.35 eV, 2.36 eV and 2.49 eV higher in energy than their corresponding ground states, respectively whereas the singlet state of PdPt lies 0.25 eV higher in energy than its ground state. These energy differences are computed to be 2.20 eV, 1.95 eV, 1.91 eV and 0.56 eV in the B3LYP level of calculation. The observed spin multiplicities of the ground states of PdAu



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and PdPt agree with the earlier study of Harada and Dexpert63,64. The BE values for these two dimers calculated with M06-L functional (-2.02 eV and -1.69 eV, respectively) are consistent with those reported earlier (-2.01 eV and -1.63 eV, respectively)63,65, however B3LYP functional yields lower BE values (-1.64 eV and 1.15 eV, respectively). BE of PdCu and PdAg dimers have been recently calculated as -1.58 eV and -1.40 eV, respectively66. These values are in agreement with the calculated values in the present study (-1.83 eV, -1.38 eV from M06-L functional and -1.47 eV, -1.14 eV from B3LYP functional). The computed Pd-M bond lengths in PdM (M= Pt, Cu, Ag, Au) dimers (Table 3) agree well with the previous studies63,6568

. Singly charged alloy dimers of Cu, Ag and Au with Pd have been optimized in

spin multiplicities, M=1 and 3, whereas M=2 and 4 have been calculated for Pd2± and PdPt±. Bare Pd2± dimers have the lowest energies in their doublet states as reported earlier56, with partial agreement in their energy differences from the quartet states (1.13 eV, 0.95 eV obtained from B3LYP and 1.16 eV, 1.01 eV obtained from M06-L, respectively). The binding energy per atom of these dimers (Table 3) are higher than those reported by Efremenko and Sheintuch69. The bond lengths of Pd2+ (2.68 Å, 2.65 Å) and Pd2- (2.50 Å, 2.47 Å) calculated with B3LYP and M06-L functionals have been found to be larger and smaller than that of Pd2 (2.54 Å), respectively. These values agree well with the available results56. The PdM± (M= Cu, Ag, Au) dimers are most stable in singlet spin multiplicity states (B3LYP), whose energies from those in M=3 states are found to be lower by 0.64 eV, 0.58 eV, 0.33 eV (for the cationic dimers) and 0.55 eV, 0.56 eV, 0.79 eV (for the anionic dimers), respectively (M06-L). The corresponding values in M06-L calculation are 0.77 eV, 0.95 eV, 0.54 eV (for the cationic dimers) and 0.74 eV, 0.85 eV, 0.89 eV (for the anionic dimers), respectively. Literature is not available for comparison of results for all the charged alloy dimers mentioned here. For neutral PdAu dimer, ionization potentials (IP) are found to be 8.30 (eV) (B3LYP) and 8.21 eV (M06-L), which agree well with the previous report67. The electron affinity (EA) value of the same complex (1.81 eV) with M06-L functional is also in proximity with the reported value67 but that with B3LYP functional is very large (2.23 eV). Similarly, the IP and EA values of PdAg dimer obtained with B3LYP (7.52 eV and 2.23 eV) and M06-L (7.11 eV and 1.24 eV) agree well with the previous results68. Pd-Au± bond lengths calculated from both the functionals (Table 3) match with those reported earlier67. The most stable structures 10 Environment ACS Paragon Plus

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of PdPt+ and PdPt- lie in quartet and doublet states, respectively. The next higher energy states of these complexes exist in doublet and quartet sates with energy differences of only 0.23 eV, 0.64 eV (B3LYP) and 0.03 eV, 0.71 eV (M06-L), respectively from their ground states. The Pd-Pt± bond lengths have been computed to be 2.47 Å, 2.52 Å (B3LYP) and 2.48 Å, 2.49 Å (M06-L), respectively. The lowest energy cationic and anionic PdAu dimers have been observed with singlet spin multiplicities, which agree with the earlier study by Mora et al.64. Computed Pd-M bond lengths and BE values of these dimers (Table 3) agree with the already reported values64. It has been observed that the variations in both BE and Pd-M bond lengths in netural as well as charged PdM dimers are similar with very slight deviations in one or two cases, as calculated by B3LYP and M06-L functionals. Figure 3(b) and 3(c) present the patterns of BE and Pd-M bond lengths, respectively. Figure 3(b) shows that binding energy of PdPt+ is maximum followed by PdAu+ among all the PdM0,± dimers studied here. 3.1.3. Interaction of Hg with PdM0,± (M= Pd, Pt, Cu, Ag, Au) dimers Similar to the Hg-Pd2 complex (already discussed in section III.A.1), symmetry unrestricted full geometry optimizations have been carried out for Hg adsorbed netural PdM (M=Cu, Ag, Au) dimers in spin multiplicities, M=2, 4 and for Hg-PdPt dimer in M=1, 3, using B3LYP and M06-L functionals. Each cationic and anionic Hg-PdM0,± (M=Pd, Pt, Cu, Ag, Au) complex has been optimized in spin multiplicities, M=1, 3 and M=2, 4 depending on whether it is an even or an odd numbered electronic system. The optimized structures of various geometrical and spin isomers of Hg-PdM0,± (M=Pd, Pt, Cu, Ag, Au) complexes are shown in Figures 4(a)-4(f) and properties of the corresponding lowest energy structures are summarized in Tables 4(a)-4(c).

Some of these isomers have been named as Top_M

corresponding to the adsorption of Hg on the top of M atom of PdM0,± dimers. It is seen that the bridge adsorption geometry is the lowest energy structure for each of the neutral Hg-PdM complexes. All of these structures lie in doublet spin multiplicities for Hg-PdM (M= Cu, Ag, Au) complexes, whereas singlet spin state is favoured by Hg-Pd2 complex. For Hg-PdPt complex, all the structural isomers are found to be in triplet spin multiplicities with an exception of singlet spin state for the M06-L



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caculated bridge isomer, which lies only 0.03 eV lower in energy than its corresponding triplet spin isomer. The top adsorption geometry (M=1) of neutral HgPd2 turns into bridge geometry in both B3LYP and M06-L level of calculations. However, their top geometries are preserved in M=3 states with corresponding BE values of only -0.63 eV and -0.53 eV (Figure S1). On the other hand, top adsorption geometries in higher spin states (M=4) become bridge geometries for Hg-PdAu in B3LYP level and Hg-PdCu, Hg-PdAg in M06-L level, which however are not stable as shown by positive values of binding energies. The lowest energy structures of cationic Hg-PdM complexes prefer bridge like geometry with singlet and doublet spin multiplicities for Hg-PdM+ (M= Cu, Ag, Au) and Hg-PdM+ (M=Pd, Pt) complexes, respectively. B3LYP calculation reveals that all the lowest energy isomers of anionic Hg-PdM complexes retain the spin multiplicities of their cationic counterparts (Table 4(b) and 4(c)). However, their structures turn from bridge to tilted top geometries similar to those of the Top_Pd isomers except for Hg-Pd2- complex (Figures 4(c)-4(f)). Lowest energy structures of Hg-Pd2± complexes along with the Hg binding energy values of Pd2± dimers match with earlier prediction38. B3LYP calculation shows that in triplet spin states, bridge isomers of Hg-PdM- (M=Cu, Ag, Au) are thermodynamically unstable with positive binding energy values of +0.04 eV, +0.1 eV, 0.18 eV, respectively. In M06-L calculation, althought most of the Top_Pd isomers (M=1) of Hg-PdM± complexes are found to be linear, their triplet states are deviated from linearity forming bridge like structures with very few exceptions (Figures 4(c)-4(f)).

Similar to B3LYP results, perfect

bridge structures (M=3) lie higher in energy than the lowest energy tilted top structures (M=1) in anionic Hg-PdAg (BE=-0.08 eV) and Hg-PdAu (BE=+0.09 eV). It has been observed so far that the nature of Hg adsorption on almost all the complexes studied here is maninly chemical with the adsorption energies of some of the B3LYP calculated PdM0,- dimers lying at the threshold between physisorption and chemisorption (Figure 5). Further, both B3LYP and M06-L calculation predict that there is competition between Pd and M atom in binding with Hg atom in all neutral Hg-PdM complexes. In contrast to this, distinctive weaker and stronger binding has been observed for Pd atom of Hg-PdM± complexes in comparison with the M± counterparts, respectively.

This is because of the accumulaion and depletion of

±

electronic charge at M sites, respectively, which are more and less preferred by Hg adsorption. Binding of Hg increases the Pd-M bond lengths in neutral Hg-PdM 12 Environment ACS Paragon Plus

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complexes from those in bare Pd-M dimers, signifying stronger interaction of PdM with Hg atom. The BE and Pd-M0,± bond length values of Hg-PdM0,± complexes vary in similar ways in both B3LYP and M06-L level of calculation (Figures 5 and 6). Moreover, there is one-to-one correspondence between highest/smallest BE and maximum/minimum elongation in Pd-M bond length in neutral Hg-PdM as plotted in Figure S3 in supplementary material. However, the same Figure shows that the correlation is not good enough considering all Hg-PdM complexes as computed with B3LYP functional (correlation coefficient is 0.51) but it is better with M06-L functional (correlation coefficient is 0.69).

A general trend of shortening and

±

lengthening of Pd-M bond lenghts have been noticed in cationic and anionic HgPdM complexes, respectively without correlating much with their respective BE trends. Binding of Hg with PdM dimers is found to be strongest in cationic state than in neutral and anionic charge states with the largest value of BE for PdPt+ followed by PdAu+. Similar binding energy trend has been observed for bare PdM0,± dimers (Figur 3(b)). This strong interaction in case of some specific cationic dimers or their complexes may be arising out of the high electronic charge deficient states in their Pd atoms, which will be discussed in the following sections. 38

however does not tally with the previous study

The present finding

reporting higher BE for neutral

HgPd2 than its caionic counterpart. 3.2. Reactivity Fukui’s Frontier Molecular Orbital (FMO) theory [70] is an effective method for determining reactivity of different molecules, where the interaction between HOMO (highest occupied molecular orbital) of one species and LUMO (lowest unoccupied molecular orbital) of the other is studied. Alike the successful application of FMO theory in predicting reactivity of different molecules38,71,72, LUMO theory has been found to be important in reactivity analysis73-76. Further, LUMO theory has already been shown to be efficient for studying interaction of Pd clusters with Hg atom since the later is an electron donor38. Following this theory, the energies of HOMO of Hg and LUMOs of Pd-M dimers have been tabulated in Table 5. It is seen that the use of unrestricted formalism in open shell neutral and ionic Pd-M dimers has led them to possess two HOMOs (α and β) and two LUMOs (α and β) because of the fulfilling of different orbitals of different energies by the up and down spins. In such complexes,



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palladium atoms having larger contribution to the low energy LUMO orbitals (among α and β) will have higher affinity towards Hg binding76. Therefore, the LUMO orbital pictures of PdM0,± dimers as presented in Figures 7(a) and 7(b) will be useful in determining their reactivity with Hg atom. Another reactivity parameter is the partial charge analysis, which will also be discussed below to give insight to the binding mechanism of Hg with Pd alloy dimers.

3.2.1. Frontier Molecular Orbital Analysis It is seen from Table 5 that the LUMO energies (either α or β) are lowest in cationic PdM dimers followed by their neutral and anionic counterparts, comparison of which with Hg binding energy values as shown in Figure 5 clearly indicates that lowest values of LUMO energies in PdM+ correlate very well with their highest binding energies with Hg atom. This strong correlation is reflected in Figure S4 in supplementary material with 95% correlation in B3LYP and 71% correlation in M06L level of calculations. However, such correlations in case of PdM0,- dimers have been excluded from the present discussion due to the very poor correlations. The above data signify that cationic PdM dimers interact with Hg atom mainly via their LUMOs. Previous studies also observed dependence of nature of Hg binding on charge states of various metal clusters38,74-76. Since the listed data (Table 5) clearly indicates β-LUMOs to be lower in energy than α-LUMOs, most preferable sites of Hg adsorption can be found out from the contributions of different atoms in PdM0,± dimers towards their β-LUMOs. Careful observation of the LUMO pictures (Figures 7(a) and 7(b)) shows that these orbitals are distributed symmetrically over each of the bare PdM0,± dimers. This leads to favourable binding of Hg at the bridge positions of these metal alloys in neutral and cationic states, which supports our observation for most stable PdM0,+ as shown in Figures 4(a)-4(d). On the other hand, shape and symmetry of LUMOs are found to be ineffective in determining the lowest energy geometries of anionic PdM dimers as we have noticed that only the Pd atoms of these clusters are mostly attacked by Hg atom in tilted geometry (Figures 4(e) and 4(f)). Another reactivity parameter based on partial charges will be helpful for explaining the nature and strength of binding Hg with PdM alloys. The following section presents the results based on partial charge analysis of the complexes under study.


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3.2.2. Charge Analysis Adsorption of Hg on neutral and charged PdM clusters can be examined by charge analysis using NPA scheme of NBO methods. The partial charges and electronic configurations of the complexes obtained from NBO analysis are presented in Tables 6(a)-6(c). The use of significantly large basis sets for Hg and M atoms studied here has led us to prefer NBO based partial charge analysis due to its more reliability over Mulliken charges77. It is seen from Tables 6(a)-6(c) that NBO partial charges on Hg atom are positive in neutral followed by more positive in cationic Hg-PdM complexes and almost unchanged charge distribution in anionic Hg-PdM complexes. In case of neutral Hg-PdM complexes, increment in electronic charge in Pd atom is found to be more than that in its alloy companions in PdM dimers leading to decrement of Hg charge. This charge transfer process influences the structure of these complexes with shorter Hg-Pd bonds than Hg-M bonds in their lowest energy states with an exception for Hg-Pd2 (Figure 4(a) and 4(b)). Schematically, we can model this charge transfer process as shown in Figures 8(a) and 8(b). Augmentation in electronic charge in both Pd and M+ of PdM+ dimers are almost equal as a result of charge withdrawal from Hg atom and hence Hg prefers perfect bridge adsorption geometry in the lowest energy structures of Hg-PdM+ (Figures 4(c) and 4(d)). Figure 8(c) represents the model charge transfer mechanism in Hg-PdM+ complexes. In Hg-PdM- complexes, charge increment and decrement have been observed in Pd and M-, respectively.

The

opposite nature of these charge alterations is reflected in their lowest enegy structures (Figure 4(e) and 4(f)) with Hg adsorption taking place in a tilted top geometry on Pd atom. Figure 8(d) depicts the model of very negligible and equal charge transfers from both Pd atoms to the Hg atom, which is in agreement with its most stable structure in the bridge adsorption geometry of Hg between the two Pd atoms (Figure 4(e)). On the other hand, the noncompetitive charge withdrawal by Pd atom from Hg and M- in other Hg-PdM- complexes is represented by Figure 8(e). Among the various complexes, NBO partial charges on Hg atom have good correlation of 76% with Hg binding energy in only cationic Hg-PdM, which is shown in Figure S5 in supplementary material. Moreover, it is seen from Table 6(b) that presence of the 5d transition metal cations (Pt+ and Au+) produces highest electron deficient states in Pd atom of PdM+ dimers. This effect is responsible for causing maximum binding



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energies in bare and Hg adsorbed cationic Pd alloy dimers doped with Pt and Au (Figure 3(b) and Figure 5). Analysis of the electronic configurations computed from NBO calculations clearly indicates the changes in electronic occupation of different orbitals of the individual atoms in the complexes under study. It is observed that the quantity of electron donation from s- orbital and back-donation into p- orbital of Hg atom (as shown in Figure 9) exactly matches with the charge dependent variation of Hg binding energies in Hg-PdM0,± complexes (Figure 5). It is clear from Figure 9 that change of electronic charge in both s- and p- orbitals of Hg atom on binding with PdM0,± is maximum for cationic dimers followed by neutral and anionic correspondents. This signifies that the strength of Hg binding with PdM0,± dimers is governed by the donation of electronic charge from s- orbital of Hg atom to M=Pd, Pt, Cu, Ag, Au atoms and consequent back-donation to the Hg p- orbitals. It is further observed that in neutral Hg-PdM complexes, d- and s- orbitals of Pd mainly participate in receiving and donating charges, respectively, with Pt dominating such processes exceptionally in Hg-PdPt. Altogether for the neutral complexes, the change in charge states of d- and s- orbitals of Pd atoms have very good linear relationships with Hg binding energy having 88% and 85% correlation, respectively, as shown in Figure S6 in the supplementary material. However, there is no direct relation between the transformation of charges in the orbitals of M atom and adsorption energy of Hg. The above results imply that interaction of Hg with PdM dimers takes place due to charge donation from s- orbital of Hg to d- orbital of Pd and back-donation from sorbital of Pd to p- orbital of Hg. As a result, there is a net charge transfer from Hg to Pd leading to partial positive charges of Hg atoms in these complexes (Table 6(a)). Pd atom of cationic Hg-Pd2 gains and loses electronic charge from its s- and dorbitals, whereas both these Pd orbitals receive charges in presence of other metal atoms (M) in Hg-PdM+ complexes with d- dominating over s- orbital (Table 6(b)). In these complexes, charge gain of s- orbitals of M+ sites are accompanied by slight loss from their d- orbitals. However, presence of Pt+ not only withdrwas maximum charge in its d- orbital but also enhances the charge increment in the d- orbital of Pd atom. This excess charge gain is compensated by significant charge donation from s- orbial of Pt+ to s- orbital of Hg keeping Hg atoms still in maximum charge deficit states (Table 6(b)). Adsorption of Hg in cationic Hg-PdM complexes can be understood as


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a process via dominating charge transfers from s- orbital of Hg to d- orbital of Pd and s- orbital of M+ without significant back-donation (exception in Hg-PdPt+). The higher charge gain by PdM+ yields more positive partial charges on Hg in these complexes than those in neutral complexes (Tables 6(a) and 6(b)). This result is obvious due to the electron donor and acceptor natures of Hg and M+ in these complexes. Besides Hg-Pd2-, M- sites of all other anionic complexes feebly donate electrons from their s- orbitals with no gain. Similar task is performed by the dorbitals of Pd atoms in almost all such complexes with nominal gain in their sorbitals (exceptions in Hg-Pd2- and Hg-PdPt-) (Table6(c)).

As a result of these

complementary phenomena, equivalent s- donation and p- back-donation leading to uninterrupted NBO partial charges in Hg atoms in anionic Hg-PdM complexes are observed.

4. CONCLUSIONS We have performed DFT calculations on the interaction of Hg atom with a series of neutral Pd clusters.

Both symmetry unrestricted geometry optimization and

vibrational frequency calculations have been performed at B3LYP/LANL2DZ and M06-L/LANL2DZ levels.

Zero-point vibrational energy corrections have been

included in all calculations. The first part of the calculation have shown higher binding affinity of Pd2 dimer towards Hg. Therefore, we have chosen this dimer for alloying with different transition metal atoms (M), viz., Pt, Cu, Ag, Au and studied their interaction with Hg atom. We have examined Hg adsorbed PdM complexes in neutral, cationic and anionic charge states. Binding of Hg with PdM dimers follows the trend: PdM+ > PdM ≈ PdM- with the largest value for PdPt+ followed by PdAu+. Frontier molecular orbital analysis has shown that LUMO energies have strong correlation with Hg binding energies in cationic complexes.

Shape and

symmetry of these LUMOs are found to be useful in determining the lowest energy geometries of neutral and anionic complexes.

NBO analysis suggests different

mechanisms of Hg binding depending on the charge state of PdM dimers. This study implies that there is a net charge transfer from Hg to Pd in neutral Hg-PdM as a result of charge donation from s- orbital of Hg to d- orbital of Pd and back-donation from sorbital of Pd to p- orbital of Hg. Adsorption of Hg in Hg-PdM+ complexes takes place via dominating charge transfers from s- orbital of Hg to d- orbital of Pd and sorbital of M+ without significant back-donation (exception in Hg-PdPt+). In Hg-PdM17 Environment ACS Paragon Plus


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complexes, insignificant charge transfers take place (except for Hg-Pd2-) among the different sites leading to uninterrupted NBO partial charges in Hg atoms. From the charge analysis, it has also been noticed that the electron transfer processes among the atoms in the absorbate and the adsorbent of Hg-PdM0,± complexes are very complicated and hence it is difficult to find some correlation of these processes with the binding energy of Hg. However, we have established very good correlations for charge gain and loss by the d- and s- orbitals of Pd atom with strength of Hg binding in neutral complexes. Moreover, NBO partial charges of Hg atoms representing its charge depletion is found to have good linear relationship with its binding energy in cationic complexes. In anionic complexes, no such trend was observed due to the hindering of charge transfer processes. Based on these observations, we can accept cationic palladium dimers to be more efficient for Hg adsorption. In addition, this process can be tuned by selecting suitable dopant atoms. These arguments could be utilized in studying clusters with larger size, clusters with varying doping concentration and supported metal clusters.

SUPPLEMENTARY MATERIAL See supplementary material for the correlations of various structural and electronic parameters with Hg binding energy in the studied Hg-PdM complexes.

ACKNOWLEDGEMENTS This work is supported by the Start Up grant of University Grants Commission, New Delhi.


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Table 1. Results of spin multiplicity (M), binding energy (BE) per atom (eV) and average Pd-Pd bond lengths of lowest energy Pdn(n=1-6) clusters obtained from B3LYP and M06-L methods.

Cluster

Spin multiplicity (M)

Average Pd-Pd bond length (Å)

BE/atom (eV)

B3LYP

M06-L

B3LYP

M06-L

B3LYP

M06-L

Pd1

1

1

−

−

−

−

Pd2

3

3

2.54

2.50

-0.31

-0.48

Pd3

3

3

2.52

2.58

-0.82

-1.07

Pd4

3

3

2.66

2.63

-1.24

-1.59

Pd5

3

3

2.69

2.67

-1.31

-1.76

Pd6

3

3

2.71

2.69

-1.40

-1.94



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Table 2. Calculated properties such as spin multiplicity (M), binding energy (BE) and average bond lengths of lowest energy HgPdn (n=1-6) complexes obtained from B3LYP and M06-L methods. The most stable isomers and spin multiplicities are shown in bold fonts.

Complex Hg-Pd1 Hg-Pd2 Hg-Pd3

Hg-Pd4

Hg-Pd5

Hg-Pd6

Isomers Top Bridge Top Bridge Hollow Top

B3LYP Spin BE multiplicity (eV) (M) -0.61 1 -1.13 1 3 -0.33 3 -0.24 -0.74 3 3 -0.32

M06-L Bond lengths (Å) Pd-Pd Hg-Pd − 2.73 2.76 2.75 2.58 2.86 2.70 2.85 2.80 2.78 2.66 2.87

Bridge

3

-0.30

2.66

3.05

Top Bridge1 Bridge2 Hollow Top Hollow

3 3 3 3 3 3

-0.40 -0.46 -0.49 -0.40 -0.39 -0.59

2.71 2.69 2.72 2.75 2.72 2.74

2.81 2.95 2.93 3.10 2.83 3.01

Bond lengths (Å) Pd-Pd Hg-Pd − 2.68 2.72 2.72 2.77 2.76 2.77 2.76 2.77 2.76 2.64 2.82 2.65 2.95 2.65 3.09 2.68 2.81

Top Bridge

Spin multiplicity (M) 1 1

-0.81 -1.45

Hollow

1

-1.30

Top Bridge Hollow Top

3 3 3 3

-0.51 -0.61 -0.61 -0.52

Bridge

3

-0.76

2.69

2.90

Hollow Top Hollow

3 3 3

-0.78 -0.56 -0.97

2.69 2.70 2.70

3.01 2.78 2.96

Isomers


BE (eV)

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Table 3. Calculated properties such as spin multiplicity (M), binding energy (BE) and Pd-M bond lengths of neutral, cationic and anionic PdM (M= Pd, Pt, Cu, Ag, Au) complexes obtained from B3LYP and M06-L methods.

B3LYP Complex Spin multiplicity (M)

M06-L BE

(eV)

Pd-M (Å)

Spin multiplicity (M)

BE (eV)

Pd-M (Å)

Neutral Pd2

3

-0.62

2.54

3

-0.96

2.50

PdPt

3

-1.64

2.41

3

-2.02

2.47

PdCu

2

-1.47

2.39

2

-1.83

2.34

PdAg

2

-1.14

2.61

2

-1.38

2.58

PdAu

2

-1.15

2.55

2

-1.69

2.56

Cationic Pd2

2

-1.79

2.68

2

-2.41

2.65

PdPt

4

-2.34

2.47

4

-2.76

2.48

PdCu

1

-1.66

2.52

1

-1.94

2.47

PdAg

1

-1.38

2.75

1

-1.42

2.73

PdAu

1

-2.28

2.69

1

-2.26

2.68

Anionic Pd2

2

-2.20

2.50

2

-2.64

2.47

PdPt

2

-1.61

2.52

2

-2.00

2.49

PdCu

1

-1.90

2.43

1

-2.20

2.47

PdAg

1

-1.45

2.64

1

-1.69

2.60

PdAu

1

-1.22

2.66

1

-1.45

2.63



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Table 4(a). Calculated properties such as spin multiplicity (M), binding energy (BE) and average bond lengths of lowest energy neutral HgPdM (M= Pd, Pt, Cu, Ag, Au) complexes obtained from B3LYP and M06-L methods.

B3LYP Complex Geometry

M06-L M

BE (eV)

2.75

1

-1.45

2.72

2.72

2.72

2.86

−

3

-0.64

2.51

2.79

−

2.56

−

2.88

3

-0.62

2.51

−

2.85

-0.46

2.56

2.84

2.95

1

-0.85

2.65

2.72

2.68

2

-0.28

2.42

2.90

−

2

-0.48

2.37

2.82

−

Top_Cu

2

-0.27

2.40

−

2.76

2

-0.49

2.36

−

2.65

Bridge

2

-0.36

2.42

2.81

3.68

2

-0.67

2.39

2.79

3.00

Top_Pd

2

-0.32

2.87

2.64

−

2

-0.52

2.61

2.80

−

Top_Ag

2

-0.21

2.61

−

2.98

2

-0.41

2.58

−

2.88

Bridge

2

-0.38

2.64

2.80

3.94

2

-0.68

2.61

2.78

3.26

Top_Pd

2

-0.77

2.57

2.87

−

2

-0.56

2.58

2.79

−

Hg-PdAu Top_Au

2

-0.72

2.58

−

2.91

2

-0.55

2.57

−

2.86

2

-0.78

2.60

2.80

3.75

2

-0.76

2.62

2.77

3.14

Hg-Pd2 Hg-PdPt

Hg-PdCu

Hg-PdAg

M

BE (eV)

Bridge

1

-1.13

2.76

2.75

Top_Pd

3

-0.36

2.57

Top_Pt

3

-0.38

Bridge

3

Top_Pd

Bridge

Pd-M Hg-Pd Hg-M (Å) (Å) (Å)



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Table 4(b). Calculated properties such as spin multiplicity (M), binding energy (BE) and average bond lengths of lowest energy cationic HgPdM (M= Pd, Pt, Cu, Ag, Au) complexes obtained from B3LYP and M06-L methods.


M06-L M

BE (eV)

−

2

-1.00

2.65

2.75

−

2.84

2.84

2

-1.70

2.60

2.79

2.60

2.49

−

2.86

4

-1.34

2.46

2.97

3.00

-1.70

2.64

−

2.73

−

−

−

−

−

2

-2.10

2.58

2.81

2.74

2

-2.14

2.59

2.78

2.70

Top_Pd

−

−

−

−

−

1

-0.75

2.47

2.82

−

Top_Cu

1

-1.12

2.51

−

2.61

1

-1.24

2.46

−

2.54

Bridge

1

-1.51

2.49

2.73

2.82

1

-1.81

2.46

2.69

2.71

Top_Pd

−

−

−

−

−

1

-0.73

2.74

2.81

−

Top_Ag

1

-0.97

2.75

−

2.81

1

-0.99

2.71

−

2.77

Bridge

1

-1.41

2.71

2.73

3.00

1

-1.64

2.69

2.70

2.93

Top_Pd

3

-1.01

2.71

2.78

−

1

-0.85

2.70

2.81

−

Hg-PdAu Top_Au

1

-1.59

2.68

−

2.73

1

-1.58

2.66

−

2.71

1

-1.85

2.67

2.74

2.89

1

-2.06

2.65

2.70

2.85

Hg-Pd2

Hg-PdPt

Hg-PdCu

Hg-PdAg

M

BE (eV)

Top

2

-1.20

2.66

2.80

Bridge

2

-1.72

2.60

Top_Pd

4

-1.61

Top_Pt

2

Bridge

Bridge





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Page 24 of 51

Table 4(c). Calculated properties such as spin multiplicity (M), binding energy (BE) and average bond lengths of lowest energy anionic HgPdM (M= Pd, Pt, Cu, Ag, Au) complexes obtained from B3LYP and M06-L methods.


M06-L M

BE (eV)

−

2

-0.68

2.55

2.73

−

2.86

2.66

2

-1.00

2.62

2.83

2.62

2.57

2.79

−

2

-0.75

2.50

2.75

−

-0.47

2.50

−

2.90

2

-0.68

2.48

−

2.86

2

-0.60

2.55

2.81

4.75

2

-0.80

2.53

2.88

3.16

Top_Pd

1

-0.72

2.45

2.81

4.51

1

-0.91

2.41

2.77

−

Top_Cu

1

-0.37

2.42

−

2.88

1

-0.62

2.38

−

2.75

Bridge

−

−

−

−

−

1

-0.91

2.40

2.83

3.42

Top_Pd

1

-0.72

2.67

2.79

4.89

1

-0.91

2.64

2.72

−

Top_Ag

1

-0.27

2.63

−

3.14

1

-0.47

2.60

−

3.00

Bridge

−

−

−

−

−

1

-0.93

2.63

2.75

4.77

Top_Pd

1

-0.70

2.68

2.76

5.07

1

-0.91

2.65

2.71

−

Hg-PdAu Top_Au

1

-0.31

2.65

−

3.07

1

-0.50

2.62

−

2.99

−

−

−

−

−

1

-0.91

2.65

2.71

5.20

Hg-Pd2

Hg-PdPt

Hg-PdCu

Hg-PdAg

M

BE (eV)

Top

2

-0.57

2.50

2.84

Bridge

2

-0.67

2.66

Top_Pd

2

-0.56

Top_Pt

2

Bridge

Bridge




Page 25 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5. LUMO energies (eV) of lowest energy neutral, cationic and anionic PdM (M= Pd, Pt, Cu, Ag, Au) dimers obtained from B3LYP and M06-L methods.

Orbital energies (eV) Dimers

B3LYP α/β HOMO

Hg

α LUMO

M06-L β LUMO

-6.4825104

α/β HOMO

α LUMO

β LUMO

-6.0972168 Neutral

Pd2

-2.2625115

-3.66276531

-1.9694598

-4.1634021

PdPt

-3.0374523

-4.31536995

-2.9484756

-5.1734373

PdCu

-2.2913541

-3.4385277

-2.0197983

-3.8004207

PdAg

-2.4059082

-3.51836184

-2.2037379

-3.8499429

PdAu

-3.2015286

-4.84484934

-2.8717434

-4.7764434

Cationic Pd2

-9.613293

-10.20589959

-9.5477169 -10.6328517

PdPt

-8.4938736 -11.04960006

-8.1804144

PdCu

-9.8440338

-9.84392496

-10.0924611 -10.0924611

PdAg

-9.6410472

-9.64115604

-9.6116604

PdAu

-10.8257706 -10.82590665

-11.523435 -9.6116604

-10.8121656 -10.8121656

Anionic Pd2

2.2712187

2.22645825

2.5890315

1.602669

PdPt

2.0551713

2.16708603

2.2423761

1.6508307

PdCu

2.4573351

2.45725347

2.516925

2.516925

PdAg

2.304687

2.30476863

2.4312135

2.4312135

PdAu

2.2415598

2.24153259

2.3084964

2.3084964



Page 26 of 51

1 2 3 4 5 6 Table 6(a). Partial charges (au) and electronic configurations of lowest energy bare and Hg adsorbed neutral PdM (M= Pd, Pt, Cu, Ag, Au) 7 8 dimers based on NPA from NBO analysis using B3LYP functional. M06-L calculated corresponding values are shown in parentheses next to the 9 10 B3LYP values. 11 Partial Charge Electronic Configuration 12Complexes Hg Pd M Hg Pd1 Pd2 M 13 0.00000 6S( 2.00)5d(10.00) 14 Hg (0.00000) (6S( 2.00)5d(10.00)) 15 4d(10.00) 16 Pd 1 (4d(10.00)) 17 0.00000 0.00000 5S(0.53)4d(9.45)5p(0.02) 5S(0.53)4d(9.45)5p(0.02) 18 Pd2 (0.00000) (0.00000) (5S(0.52)4d(9.45)5p(0.03)) (5S(0.52)4d(9.45)5p(0.03)) 19 20 0.23550 -0.23550 5S(0.46)4d(9.29)5p(0.02) 6S(1.55)5d(8.66)6p(0.03) PdPt 21 (0.25967) (-0.25967) (5S(0.36)4d(9.36)5p(0.02)) (6S(1.60)5d(8.62)6p(0.04)) 22 -0.02813 0.02813 5S(0.53)4d(9.48)5p(0.01) 4S(1.00)3d(9.93)4p(0.04) 23 PdCu (-0.04475) (0.04475) (5S(0.47)4d(9.56)5p(0.02)) (4S(1.03)3d(9.87)4p(0.05)) 24 0.00001 -0.00001 5S(0.49)4d(9.50)5p(0.01) 5S(0.99)4d(9.98)5p(0.03) 25 PdAg (-0.01890) (0.01890) (5S(0.40)4d(9.60)5p(0.01)) (5S(0.97)4d(9.98)5p(0.04)) 26 0.31117 -0.31117 5S(0.56)4d(9.12)5p(0.02) 6S(1.45)5d(9.84)6p(0.03) 27 PdAu (0.24248) (-0.24248) (5S(0.29)4d(9.45)5p(0.01)) (6S(1.30)5d(9.91)6p(0.03)) 28 0.23209 -0.11604 -0.11605 6S(1.67)5d(10.00)6p(0.10) 5S(0.23)4d(9.85)5p(0.04) 5S(0.23)4d(9.85)5p(0.04) 29 Hg-Pd2 (0.21104) (-0.10590) (-0.10514) (6S(1.68)5d(10.00)6p(0.11)) (5S( 0.25)4d(9.81)5p(0.04)) (5S(0.25)4d(9.81)5p(0.04)) 30 0.27464 0.07040 -0.34505 6S(1.62)5d( 9.99)6p(0.10) 5S(0.31)4d(9.54)5p(0.08) 6S(1.12)5d(9.17)6p(0.06) 31 Hg-PdPt (0.38538) (0.00673) (-0.39211) (6S(1.49)5d(9.99)6p(0.13)) (5S(0.29)4d(9.66)5p(0.05)) (6S(0.65)5d(9.70)6p(0.05)) 32 -0.26515 0.08395 6S(1.76)5d(10.00)6p(0.06) 5S(0.51)4d(9.66)5p(0.09) 4S(0.92)3d(9.95)4p(0.04) 0.18120 33Hg-PdCu (0.11872) (6S(1.74)5d( 9.99)6p(0.12)) (5S(0.39)4d(9.69)5p(0.07)) (4S(0.98)3d(9.92)4p(0.08)) (0.12620) (-0.24491) 34 -0.24558 0.05988 6S(1.76)5d(10.00)6p(0.05) 5S(0.48)4d(9.68)5p(0.09) 5S(0.93)4d(9.98)5p(0.03) 0.18570 35Hg-PdAg (-0.15027) (0.00883) (6S(1.76)5d(10.00)6p(0.10)) (5S(0.37)4d(9.71)5p(0.07)) (5S(0.96)4d(9.97)5p(0.05)) (0.14144) 36 0.23850 -0.00182 -0.23668 6S(1.72)5d(10.00)6p(0.04) 5S(0.41)4d(9.50)5p(0.09) 6S(1.29)5d(9.93)6p(0.02) 37Hg-PdAu (0.23011) (0.04704) (-0.27715) (6S(1.69)5d(10.00)6p(0.08)) (5S(0.27)4d(9.62)5p(0.07)) (6S(1.29)5d(9.94)6p(0.05)) 38 39 40 41 42 43 26 44 45 46 ACS Paragon Plus Environment 47 48

Page 27 of 51


1 2 3 Table 6(b). Partial charges (au) and the electronic configurations of lowest energy bare and Hg adsorbed cationic PdM (M= Pd, Pt, Cu, Ag, Au) dimers 4 5 based on NPA from NBO analysis. M06-L calculated corresponding values are shown in parentheses next to the B3LYP values. 6 7 8 Partial Charge Electronic Configuration 9 Complexes 10 Hg Pd M Hg Pd1 Pd2 M 11 0.50000 0.50000 5S(0.04)4d(9.44)5p(0.02) 5S(0.04)4d(9.44)5p(0.02) 12 Pd2+ (0.50000) (0.50000) (5S(0.05)4d(9.44)5p(0.02)) (5S(0.05)4d(9.44)5p(0.02)) 13 14 0.59645 0.40355 5S(0.20)4d(9.18)5p(0.03) 6S(0.91)5d(8.65)6p(0.03) PdPt+ 15 (0.60684) (0.39316) (5S(0.21)4d(9.15)5p(0.03)) (6S(0.90)5d(8.67)6p(0.04)) 16 0.32967 0.67033 5S(0.08)4d(9.57)5p( 0.02) 4S(0.31)3d(9.98)4p(0.03) + 17 PdCu (0.34934) (0.65066) (5S(0.07)4d(9.56)5p(0.02)) (4S(0.34)3d(9.97)4p(0.03)) 18 0.31686 0.68314 5S(0.08)4d(9.59)5p( 0.02) 5S(0.29)4d(9.99)5p(0.03) + 19PdAg (0.30831) (0.69169) (5S(0.07)4d(9.61)5p(0.02)) (5S(0.28)4d(9.99)5p(0.03)) 20 0.51211 0.48789 5S(0.06)4d(9.41)5p( 0.02) 6S(0.51)5d(9.98)6p(0.03) + 21PdAu (0.48135) (0.51865) (5S(0.05)4d(9.45)5p(0.02)) (6S(0.47)5d(9.98)6p(0.03)) 22 0.44500 0.27745 0.27755 6S(1.52)5d(10.00)6p(0.04) 5S(0.25)4d(9.42)5p( 0.06) 5S(0.25)4d(9.42)5p(0.06) + 23Hg-Pd2 (0.41798) (0.29101) (0.29101) (6S(1.52)5d(9.99)6p(0.06) (5S(0.24)4d(9.42)5p(0.06)) (5S(0.24)4d(9.42)5p(0.06)) 24 0.67067 0.27355 0.05578 6S(1.28)5d(9.99)6p(0.06) 5S(0.21)4d(9.48)5p(0.04) 6S(0.68)5d(9.20)6p(0.07) Hg-PdPt+ 25 (0.61598) (0.31214) (0.07189) (6S(1.32)5d(9.99)6p(0.07)) (5S(0.21)4d(9.44)5p(0.05)) (6S(0.62)5d(9.23)6p(0.08)) 26 0.47923 0.08171 0.43905 6S(1.47)5d(10.00)6p(0.05) 5S(0.17)4d(9.70)5p(0.04) 4S(0.50)3d(9.97)4p(0.09) 27 Hg-PdCu+ (0.46293) (0.61598) (0.42028) (6S(1.86)5d(10.00)6p(0.26)) (5S(0.53)4d(9.82)5p(0.11)6p(0.01)) (4S(1.43)3d(9.89)4p(0.09)) 28 0.45404 0.06931 0.47664 6S(1.49)5d(10.00)6p(0.05) 5S(0.18)4d(9.71)5p(0.05) 5S(0.47)4d(9.98)5p(0.07) 29 Hg-PdAg+ (0.41212) (0.09347) (0.49441) (6S(1.84)5d(10.00)6p(0.18)) (5S(0.60)4d(9.82)5p(0.10)) (5S(1.44)4d(9.97)5p(0.04)) 30 0.61671 0.20700 0.17629 6S(1.33)5d(10.00)6p(0.05) 5S(0.17)4d(9.58)5p(0.04) 6S(0.80)5d(9.96)6p(0.06) 31 Hg-PdAu+ (0.57172) (0.21749) (0.21080) (6S(1.36)5d(10.00)6p(0.06)) (5S(0.16)4d(9.58)5p(0.04)) (6S(0.76)5d(9.95)6p(0.07)) 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

27

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Page 28 of 51

1 2 3 Table 6(c). Partial charges (au) and the electronic configurations of lowest energy bare and Hg adsorbed anionic PdM (M= Pd, Pt, Cu, Ag, Au) dimers 4 5 based on NPA from NBO analysis. M06-L calculated corresponding values are shown in parentheses next to the B3LYP values. 6 7 8 Partial Charge Electronic Configuration 9 Complexes 10 M Hg Pd M Hg Pd1 Pd2 11 -0.50000 -0.50000 5S(1.02)4d(9.45)5p(0.02) 5S(1.02)4d(9.45)5p(0.02) 12 Pd2(-0.50000) (-0.50000) (5S(1.00)4d(9.45)5p(0.04)) (5S(1.00)4d(9.45)5p(0.04)) 13 -0.25950 -0.74050 5S(0.60)4d(9.64)5p(0.02) 6S(1.53)5d(9.18)6p(0.03) 14 PdPt(-0.20721) (-0.79279) (5S(0.57)4d(9.60)5p(0.04)) (6S(1.48)5d(9.27)6p(0.05)) 15 -0.48998 -0.51002 5S(0.58)4d(9.90)5p(0.01) 4S(1.47)3d(9.95)4p(0.08)5p(0.01) 16 PdCu(-0.39682) (-0.60318) (5S(0.50)4d(9.88)5p(0.02)) (4S(1.57)3d(9.91)4p(0.11)5p(0.01)) 17 -0.40851 -0.59149 5S(0.48)4d(9.91)5p(0.01) 5S(1.55)4d(9.98)5p(0.06) 18 PdAg(-0.36709) (-0.63291) (5S(0.46)4d(9.90)5p(0.01)) (5S(1.58)4d(9.97)5p(0.08)) 19 -0.27169 -0.72831 5S(0.36)4d(9.90)5p(0.02) 6S(1.71)5d(9.97)6p(0.05) 20 PdAu (-0.26560) (-0.73440) (5S(0.39)4d(9.87)5p(0.01)) (6S(1.72)5d(9.96)6p(0.06)) 21 -0.03664 -0.48168 -0.48167 6S(1.73)5d(10.00)6p(0.31) 5S(0.60)4d(9.79)5p(0.09) 5S(0.60)4d(9.79)5p(0.09) 22 Hg-Pd2 (-0.10205) (-0.44898) (-0.44898) (6S(1.74)5d(9.99)6p(0.36)) (5S(0.62)4d(9.75)5p(0.09)) (5S(0.62)4d(9.75)5p(0.09)) 23 0.04972 -0.41479 -0.63493 6S(1.83)5d(10.00)6p(0.12) 5S(0.59)4d(9.69)5p(0.13)6p(0.01) 6S(1.47)5d(9.14)6p(0.03) 24 Hg-PdPt (-0.06243) (-0.19407) (-0.74350) (6S(1.82)5d(10.00)6p(0.25)) (5S(0.47)4d(9.60)5p(0.12)) (6S(1.52)5d(9.16)6p(0.06)) 25 -0.00337 -0.61178 -0.38484 6S(1.84)5d(10.00)6p(0.17) 5S(0.64)4d(9.85)5p(0.12)6p(0.01) 4S(1.38)3d(9.95)4p(0.05) 26 Hg-PdCu (-0.11405) (-0.46292) (-0.42303) (6S(1.86)5d(10.00)6p(0.26)) (5S(0.53)4d(9.82)5p(0.11)6p(0.01)) (4S(1.43)3d(9.89)4p(0.09)) 27 0.01626 -0.55258 -0.46368 6S(1.83)5d(10.00)6p(0.15) 5S(0.58)4d(9.85)5p(0.11) 5S(1.45)4d(9.98)5p(0.03) Hg-PdAg(-0.02088) (-0.52996) (-0.44916) (6S(1.84)5d(10.00)6p(0.18)) (5S(0.60)4d(9.82)5p(0.10)) (5S(1.44)4d(9.97)5p(0.04)) 28 0.05602 -0.41639 -0.63963 6S(1.83)5d(10.00)6p(0.12) 5S(0.47)4d(9.84)5p(0.11) 6S(1.66)5d(9.97)6p(0.02) 29 Hg-PdAu(0.02023) (-0.38699) (-0.63324) (6S(1.84)5d(10.00)6p(0.14)) (5S(0.47)4d(9.81)5p(0.10)) (6S(1.65)5d(9.96)6p(0.03)) 30 31 32 33 34 35 36 37 38 39 40 41 42 43 28 44 45 46 ACS Paragon Plus Environment 47 48

Page 29 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1(a). Lowest energy structures of different geometrical isomers of Hg adsorbed neutral Pdn (n=1-4) clusters as calculated with M06-L and B3LYP functionals.

The corresponding spin

multiplicities are shown in brackets. The most stable isomers and spin multiplicities are shown in bold fonts.

HgPd complexes

HgPd1

B3LYP Isomers

Structures

Top

M06-L Isomers Top

(M=1) HgPd2

Structures

Bridge

(M=1) Bridge

(M=1)

(M=1)

Top (M=3) HgPd3

Bridge

Hollow (M=3)

(M=1)

Hollow (M=3)

Top

Top (M=3) HgPd4

Bridge (M=3)

Bridge (M=3)

Hollow (M=3)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 51

Figure 1(b). Lowest energy structures of different geometrical isomers of Hg adsorbed neutral Pdn (n=5-6) clusters as calculated with M06-L and B3LYP functionals.

The corresponding spin

multiplicities are shown in brackets. The most stable isomers and spin multiplicities are shown in bold fonts.

Top

Top (M=3)

(M=3) Bridge1 (M=3)

HgPd5

Bridge (M=3)

Bridge2 (M=3) Hollow

Hollow (M=3)

(M=3)

Top

Top

HgPd6

(M=3)

(M=3)

Hollow

Hollow (M=3)

30 ACS Paragon Plus Environment

(M=3)

Page 31 of 51

Figure 2. Variation of BE of different HgPdn (n=1-6) complexes with cluster size.

-0.2 -0.4 -0.6

BE (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.8 -1.0 -1.2

M06-L

-1.4

B3LYP

-1.6

Hg-Pd clusters



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 51

Figure 3(a). Lowest energy structures of bare neutral, cationic and anionic PdM(M= Pd, Pt, Cu, Ag, Au) dimers.

Complex

Pd20,±

PdPt0,±

PdCu0,±

Structure


PdAg0,±

PdAu0,±

Page 33 of 51

Figure 3(b). Variation of BE of bare neutral, cationic and anionic PdM(M= Pd, Pt, Cu, Ag, Au) dimers calculated with B3LYP and M06-L functionals.

0.0 -0.5 B3LYP(neutral) -1.0

BE (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


M06-L(neutral) B3LYP(cationic)

-1.5

M06-L(cationic) B3LYP(anionic)

-2.0

M06-L(anionic) -2.5 -3.0 Pd2

PdPt

PdCu

PdAg

PdAu

PdM0,± dimers



Figure 3(c). Variation of Pd-M bond lengths of bare neutral, cationic and anionic PdM(M= Pd, Pt, Cu, Ag, Au) dimers calculated with B3LYP and M06-L functionals.

2.8 2.7 B3LYP(neutral)

Pd-M bond lengths (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 51

2.6

M06-L(neutral) B3LYP(cation)

2.5

M06-L(cation) B3LYP(anion)

2.4

M06-L(anion) 2.3 2.2 Pd2

PdPt

PdCu

PdAg

PdAu

PdM0,± dimers


Page 35 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4(a). Lowest energy structures of Hg adsorbed neutral PdM(M= Pd, Pt, Cu) dimers. Colour code for atoms: Gray (Hg), Green (Pd), Blue (Pt) and Orange (Cu). Spin multiplicities are shown in parentheses and the lowest energy structures are represented by bold fonts.

Complex

Isomer

B3LYP

M06-L

Top Hg-Pd2

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=2)

(M=4)

Bridge

Top_Pd

Hg-PdPt Top_Pt

Bridge

Top_Pd (M=4) (M=2) Hg-PdCu

Top_Cu (M=2)

(M=4)

(M=2)

(M=4)

Bridge (M=2)

(M=4)

(M=2)


(M=4)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 51

Figure 4(b). Lowest energy structures of Hg adsorbed neutral PdM(M= Ag, Au) dimers. Colour code for atoms: Gray (Hg), Green (Pd), Sky blue (Ag) and Yellow (Au). Spin multiplicities are shown in parentheses and the lowest energy structures are represented by bold fonts.

Top_Pd (M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

Hg-PdAg Top_Ag

Bridge

Top_Pd

Hg-PdAu

Top_Au

Bridge


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Figure 4(c). Lowest energy structures of Hg adsorbed cationic PdM(M= Pd, Pt, Cu) dimers. Colour code for atoms: Gray (Hg), Green (Pd), Blue (Pt) and Orange (Cu). Spin multiplicities are shown in parentheses and the lowest energy structures are represented by bold fonts.

Complex

Isomer

B3LYP

M06-L

Top Hg-Pd2

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=4)

(M=2)

(M=4)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

Bridge

Top_Pd (M=2) Hg-PdPt Top_Pt (M=2) Bridge

Top_Pd

Hg-PdCu

(M=1)

(M=3)

Top_Cu

Bridge



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Figure 4(d). Lowest energy structures of Hg adsorbed cationic PdM(M= Ag, Au) dimers. Colour code for atoms: Gray (Hg), Green (Pd), Sky blue (Ag) and Yellow (Au). Spin multiplicities are shown in parentheses and the lowest energy structures are represented by bold fonts.

Top_Pd (M=1)

(M=3)

(M=3) (M=1)

Hg-PdAg Top_Ag (M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

Bridge

Top_Pd

Hg-PdAu Top_Au

Bridge


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Figure 4(e). Lowest energy structures of Hg adsorbed anionic PdM(M= Pd, Pt, Cu) dimers. Colour code for atoms: Gray (Hg), Green (Pd), Blue (Pt) and Orange (Cu). Spin multiplicities are shown in parentheses and the lowest energy structures are represented by bold fonts.

Complex

Isomer

B3LYP

M06-L

Top Hg-Pd2

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=2)

(M=4)

(M=1)

(M=3)

(M=1)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

Bridge

Top_Pd

Hg-PdPt Top_Pt

Bridge

Top_Pd (M=3)

Hg-PdCu Top_Cu

Bridge



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Figure 4(f). Lowest energy structures of Hg adsorbed anionic PdM(M= Ag, Au) dimers. Colour code for atoms: Gray (Hg), Green (Pd), Sky blue (Ag) and Yellow (Au). Spin multiplicities are shown in parentheses and the lowest energy structures are represented by bold fonts.

Top_Pd (M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

(M=1)

(M=3)

Hg-PdAg Top_Ag

Bridge

Top_Pd

Hg-PdAu

Top_Au

Bridge


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Figure 5. Variation of BE of Hg with neutral, cationic and anionic PdM(M= Pd, Pt, Cu, Ag, Au) dimers calculated with B3LYP and M06-L functionals.

Hg binding energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0 -2.2 -2.4

B3LYP(neutral) M06-L(neutral) B3LYP(cationic) M06-L(cationic) B3LYP(anionic) M06-L(anionic)

Hg-PdM0,± complexes



Figure 6. Variation of Pd-M bond lengths of bare neutral, cationic and anionic Hg-PdM(M= Pd, Pt, Cu, Ag, Au) dimers calculated with B3LYP and M06-L functionals.

2.8

Pd-M0,± bond lengths (Å)

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2.7 B3LYP(neutral) 2.6

M06-L(neutral) B3LYP(cationic ) M06-L(cationic)

2.5 2.4 2.3

Hg-PdM0,± complexes


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Figure 7(a). HOMO and LUMO pictures of bare PdM0,± (M= Pd, Pt, Cu) dimers calculated with B3LYP and M06-L functionals in their orientations as shown in Figure 3(a). Green and red colour represent positive and negative isosurfaces, respectively. Complex

α/β-HOMO

B3LYP α-LUMO

β-LUMO

α/β-HOMO

Hg

Pd2

Pd2+

Pd2-

PdPt

PdPt+

PdPt-

PdCu

PdCu+

PdCu-


M06L α-LUMO

β-LUMO


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7(b). HOMO and LUMO pictures of bare PdM0,± (M= Ag, Au) dimers calculated with B3LYP and M06-L functionals in their orientations as shown in Figure 3(a). Green and red colour represent positive and negative isosurfaces, respectively.

PdAg

PdAg+

PdAg-

PdAu

PdAu+

PdAu-


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Figure 8. Transfer of electronic charge between Hg and M= Pd, Pt, Cu, Ag, Au atoms of (a), (b) neutral Hg-PdM (c) cationic and (d) anionic Hg-PdM complexes as observed from NBO analysis in B3LYP level of calculation. Directions of significant and negligible amount of charge transfer among the atoms are represented by solid and dashed arrows, respectively.

Hg

Hg

Pd

Pd

Pd

M

(b) Hg-PdPt, Hg-PdCu, Hg-PdAg, Hg-PdAu

(a) Hg-Pd2

Hg

Pd

M

+

(c) Hg-PdM+

Hg Hg

-

Pd

Pd

(d) Hg-Pd2

-

Pd

-

M

(e) Hg-PdPt-, Hg-PdCu-, Hg-PdAg-, Hg-PdAu-

ACS Paragon Plus Environment


Figure 9. Variation of Hg binding energy with electronic charge donation from s- and back-donation into p- orbitals of Hg atom in Hg-PdM0,± (M= Pd, Pt, Cu, Ag, Au) complexes calculated with B3LYP functional.

-0.4 -0.6

Hg binding energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-0.8

s- donation (Neutral)

-1.0

s- donation (Cationic)

-1.2 s- donation (Anionic)

-1.4 -1.6

p- back-donation (Neutral)

-1.8

p- back-donation (Cationic)

-2.0

p- back-donation (Anionic)

-2.2 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Change in electronic charge in s- and p- orbitals of Hg (au)


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Figure: TOC graphic for the manuscript “Tuning the Adsorption of Elemental Mercury by Small Gas Phase Palladium Clusters: First Principle Study”


Tuning the Adsorption of Elemental Mercury by Small Gas-Phase

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