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. 2019 Jul 25;4(7):12687–12695. doi: 10.1021/acsomega.9b01581

Molecular and Dissociative Adsorption of Oxygen on Au–Pd Bimetallic Clusters: Role of Composition and Spin State of the Cluster

Manzoor Ahmad Dar †,*, Sailaja Krishnamurty ‡
PMCID: PMC6682065  PMID: 31460390

Abstract

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Utilization of molecular oxygen as an oxidizing agent in industrially important reactions is the ultimate goal to design environmentally benign processes under ambient conditions. However, the high thermal stability and a large O–O dissociation barrier in O2 molecule pose a great challenge toward its successful application in the oxidative chemistry. To achieve this goal, different catalysts based on monometallic and bimetallic clusters have been developed over the years to promote binding and dissociation of molecular oxygen. The successful design of efficient metal cluster catalysis needs an in-depth knowledge of synergistic effects between different metal atoms and intrinsic catalytic mechanisms for O2 adsorption and dissociation. Here, we present a systematic theoretical investigation of reaction pathways for O2 adsorption and dissociation on Au8, Pd8, and Au8–nPdn (n = 1–7) nanoclusters in different spin states. The density functional calculations point out that the O2 dissociation barriers can be significantly reduced with the help of certain bimetallic clusters along specific spin channels. Our results particularly indicate that Au5Pd3 and Au1Pd7 show very large O2 binding energies of 1.76 and 1.69 eV, respectively. The enhanced O2 binding subsequently leads to low activation barriers of 0.98 and 1.19 eV along the doublet and quartet spin channels, respectively, without the involvement of any spin flip-over for O2 dissociation. Furthermore, the computed O2 dissociation barriers are significantly low as compared to the already reported barriers (1.95–3.65 eV) on monometallic and bimetallic Au–Ag clusters. The results provide key mechanistic insights into the interaction and dissociation of molecular oxygen with Au–Pd clusters, which can prove informative for the design of efficient catalysts for oxidative chemistry involving molecular oxygen as a reactant.

1. Introduction

Activation and dissociation of molecular oxygen is one of the primary and key steps for driving environmentally and industrially important reactions such as CO oxidation, ethylene epoxidation, hydrocarbon oxidation, and so on.1−14 Because of its triplet ground-state and strong oxygen–oxygen bond, molecular oxygen is kinetically very stable under ambient conditions. Owing to these challenges, a number of catalysts based on different metal nanoclusters15−28 have been explored toward activation and dissociation of molecular oxygen for its successful utilization in oxidative chemistry. Among the various nanoclusters, gold-based nanoclusters have been at the forefront of both experimental and theoretical research for understanding the intrinsic mechanistic pathways of O2 activation and dissociation on metal nanoclusters.

Early experimental investigations were carried out by the research groups of Kaldor29,30 and Whettan31 on O2 adsorption on a series of anionic gold nanoclusters. The authors found that O2 exhibits profound adsorption with even sized anionic clusters and O2 reactivity strongly depends on the cluster size and charge. Subsequently, Wallace et al.32,33 performed detailed experimental studies to discern the role of CO and OH coadsorption on the reactivity of molecular oxygen with small gold clusters. Huang et al.34 using combined experimental and theoretical methods further demonstrated molecular chemisorption and physisorption of O2 on small sized anionic gold clusters with even and odd number of atoms, respectively. Woodham et al.35 using infrared multiple photon dissociation spectra of anionic O2–gold cluster complexes experimentally provided direct evidence for the formation of a superoxo moiety upon O2 complexation. Recent investigations by Zeng and co-workers36 unraveled that a superoxo to peroxo binding transition of O2 molecule on anionic gold clusters occurs at Au8–.

The above experimental findings of O2 interaction with gold clusters were explained by numerous theoretical studies. For example, Mills et al.37 using density functional theory (DFT) found that O2 binds more strongly with neutral and anionic gold clusters having odd number of electrons. Landman and co-workers38 suggested that small sized anionic gold clusters (n = 1–3) prefer molecular adsorption and large sized clusters (n = 4–8) prefer dissociative adsorption. The authors also revealed that O2 dissociation entails very high barriers. Similar results were found by Wang and Gong39 while investigating O2 chemisorption on Au32 as a model system. Jena et al.40 investigated the promotional effect of hydrogen atom doping on the reactivity of Au8 cluster toward molecular oxygen. Zeng and Gao41 and Lyalin and Taketsugu42 in two independent studies discerned the effect of water and ethylene adsorption on the O2 activation and dissociation on small sized gold clusters. Several other theoretical works were carried out to examine the effect of factors such as charge state,43−46 doping,47−52 coadsorption,53,54 and support55−58 on O2 interactions with gold clusters.

Although O2 adsorption and dissociation are studied comprehensively on monometallic gold clusters both experimentally and theoretically, very few investigations are available wherein the role of composition and spin state vis-a-vis O2 adsorption and dissociation on bimetallic clusters, particularly on Au–Pd clusters, has been elucidated thus far. Recently, García-Cruz et al.59 studied the O2 adsorption and dissociation on small bimetallic AumAgn clusters (m + n < 6). The authors concluded that the O2 binding energies on the pure and bimetallic systems show a strong dependence on the spin state of the cluster. Similarly, Joshi et al.60 revealed that Pt6 and AunPtm (n + m = 6) with a double-planar shape are catalytically more efficient toward O–O bond activation.

Bimetallic clusters hold a special position in the field of catalysis as they possess strikingly distinct properties compared to the parent metals, thereby acting as suitable candidates for design of superior catalysts with enhanced stability, activity, and selectivity.61,62 Among the various bimetallic systems, the Au–Pd cluster catalysts are known to display a promising efficiency for catalyzing a wide range of oxidation reactions, namely, CO oxidation, direct synthesis of hydrogen peroxide from hydrogen and oxygen, and selective oxidation of alcohols to aldehydes.63−67 Despite their superior and promising catalytic behavior, a comprehensive knowledge of the synergic effects between Au and Pd and the intrinsic factors controlling the adsorption and dissociation of molecular oxygen on these bimetallic systems are still elusive. Thus, an in-depth molecular-level understanding of the factors controlling the O2 adsorption and dissociation to produce atomic oxygen species on the bimetallic Au–Pd clusters is highly imperative for their optimization for the aforementioned oxidation reactions involving oxygen.

In this context, in the current work, DFT calculations were carried out to systematically investigate the influence of spin state and composition of Au–Pd bimetallic clusters on the adsorption and dissociation of molecular oxygen. The calculations were performed on selected ground-state conformations of monometallic Au8,/Pd8, and bimetallic Au88–nPdn (n = 1–7) clusters according to the data available in the literature.

2. Computational Details

All the calculations were performed by DFT using the Perdew–Burke–Ernzerhof (PBE)68 functional as implemented in the Gaussian 09 package.69 The coordinates of the starting geometries for Au8, Pd8, and Au–Pd clusters were taken from the available literature70 and fully optimized in the singlet/triplet and doublet/quartet spin states depending on whether the cluster has even or odd number of electrons. Geometry optimizations of the different structures were carried out using the default convergence cutoff of 10–4 with the help of Berny algorithm. The LANL2DZ basis set and the corresponding Los Alamos effective core potential were used for gold and palladium atoms, respectively. For oxygen, the TZVP basis set was used. The energetically most stable configurations of O2 adsorbed complexes of Au8, Pd8, and Au–Pd clusters were found by adsorbing the O2 molecule both via atop/superoxo (with one metal–oxygen bond) and bridged/peroxo (with two metal–oxygen bonds) modes on various distinct sites in these clusters. The oxygen adsorption was studied in two different spin states (singlet/triplet or doublet/quartet) depending on the cluster. It is important to highlight here that O2 interaction and dissociation on closed shell metal clusters may involve spin crossovers, and thus, computation of reaction pathways for O2 dissociation quantitatively requires advanced wave-function-based methods. However, because of computational restrictions, DFT in the framework of PBE has been widely used to study the spin and structural details of O2 adsorption and dissociation44,59,71,72 on monometallic and bimetallic clusters. Moreover, in a recent study, Dononelli and Klüner72 showed that the DFT/PBE and high-levelcoupled cluster singles, doubles, and triples method results for the O2 and CO adsorption energies on group 11 nanoclusters were in close agreement with each other. Vibrational frequency calculations were carried out to guarantee that the optimized structures are local minima. The O2 binding energies (Eb(O2)) were computed with the help of the given equation

2.

where EO2 is the energy of the O2 molecule in the triplet sate, EC is the energy of the cluster in the lowest-energy spin state, and Ecomplex is the energy of the O2–cluster complexes in the lowest-energy spin state. The transition states were located by the linear synchronous transit method and were characterized by the presence of single imaginary frequency corresponding to the O–O scission.

3. Results and Discussion

The geometries of the monometallic (Au8 and Pd8) and bimetallic clusters (Au8–nPdn) are shown in Figure 1. As reported earlier, Au8 has a planar structure and prefers to be in the singlet spin state, whereas the Pd8 cluster possesses a three-dimensional structure with a triplet spin state. All the bimetallic clusters exhibit three-dimensional structures. The bimetallic clusters Au7Pd1 and Au6Pd2 with high percentage of Au atoms prefer to be in the low spin singlet or doublet states as compared to the triplet and quartet spin states. However, Au–Pd clusters with three or more than three Pd atoms favor higher spin states of triplet or quartet multiplicity depending on whether the cluster has even or odd number of electrons. Thus, a spin transition is involved as the composition of the clusters is varied with Pd, playing a dominant role in deciding the overall spin state of the bimetallic clusters.

Figure 1.

Figure 1

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Lowest-energy structures of the Au8, Pd8, and Aun–1Pdn (n = 1–7) clusters and their corresponding spin states.

We next look at the mode of the O2 adsorption and binding on the monometallic Au8 and Pd8 clusters. Figure 2 represents the equilibrium structures of O2 adsorbed complexes of the Au8, Pd8, and Au8–nPdn clusters in their lowest-energy spin states. The structures corresponding to the higher energy spin states and their relative energies with respect to the lowest energy spin state structures are given in Figure S1 of the Supporting Information. It can be seen from Figure 2 that O2 adsorbs via atop or superoxo mode of bonding on the pristine Au8 cluster with a single Au–O bond in both singlet and triplet spin states. Among the singlet and triplet spin states, the Au8–O2 complex in the triplet state has 1.06 eV lower energy than the singlet state. The calculated O2 binding energy on Au8 in the triplet state was found to be 0.31 eV. Moreover, interestingly, it was found that the Au8–O2 complex in the singlet state has higher energy than the isolated Au8 cluster and O2 molecule. Thus, the binding of O2 molecule on Au8 cluster is not favored from a thermodynamic point of view in the singlet spin state. No bridged or peroxo type of bonding was seen for O2 adsorption on the Au8 cluster in both spin states. This is in line with the earlier reported results of O2 adsorption on Au8 cluster.38,40,51,55

Figure 2.

Figure 2

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Most stable geometries of the O2-adsorbed complexes of Au8, Pd8, and Aun–1Pdn (n = 1–7) clusters. The spin state of the system is given below each O2–cluster complex. All the geometrical parameters marked are in angstroms.

In contrast to Au8, pristine Pd8 cluster prefers a peroxo/bridged type of bonding with two Pd–O bonds with O2 molecule both in the singlet and in the triplet spin state. The triplet state of the Pd8–O2 complex is found to be 0.05 eV lower in energy than the singlet state. The Pd8 cluster shows considerably higher O2 binding energies of 1.43 eV in the triplet state. To further corroborate the enhanced O2 binding energy in the case of the Pd8 cluster, we calculated the O–O bond lengths, O–O stretching frequencies, and net natural bond order charges on the O2 molecule in the lowest energy spin states of Au8/Pd8–O2 complexes. The abovementioned parameters are known to qualitatively describe the interaction of the O2 molecule with metal nanoclusters. It is well known that the O2 molecule acts as an electron acceptor and interacts strongly with clusters which can donate electrons easily into the antibonding orbitals of the O2 molecule. This results in the increase in the O–O bond length and red shift in the O–O stretching frequencies. As can be seen from Table 1, there is a notable increase in the O–O bond length and decrease in the O–O stretching frequency on the Pd8 cluster as compared to the Au8 cluster. We further note a significant charge transfer of −0.41 electrons from the Pd8 cluster to the O2 molecule as compared to the −0.05 electrons in the case of Au8 cluster.

Table 1. Calculated O2 Binding Energy (Eb(O2)), O–O Bond Distances (Ro–o), O–O Stretching Frequencies, and Net Natural Bond Orbital Charges (Qo–o) on the Au8–O2, Pd8–O2, and Aun–1Pdn–O2 Complexes in Their Lowest-Energy Spin States.

system spin state Eb(O2) (eV) Ro–o (Å) νo–o (cm–1) Qo–o
Au8–O2 triplet 0.31 1.23 1420 –0.05
Pd8–O2 triplet 1.43 1.33 1002 –0.41
Au7Pd1–O2 doublet 0.64 1.26 1279 –0.14
Au6Pd2–O2 triplet 0.61 1.26 1292 –0.12
Au5Pd3–O2 doublet 1.76 1.26 1271 –0.16
Au4Pd4–O2 triplet 1.37 1.31 1248 –0.16
Au3Pd5–O2 quartet 1.26 1.31 1073 –0.28
Au2Pd6–O2 triplet 1.34 1.32 1032 –0.34
Au1Pd7–O2 doublet 1.69 1.33 1016 –0.36
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The lowest-energy structures of the O2 adsorbed complexes of Au–Pd bimetallic clusters are also presented in Figure 2. As can be seen, O2 prefers a superoxo/atop mode of binding on Au–Pd clusters such as Au7Pd, Au6Pd2, and Au5Pd3 with a high percentage of Au atoms, whereas peroxo/bridged mode of bonding is preferred by O2 on Au4Pd4, Au3Pd5, Au2Pd6, and Au1Pd7 clusters with more than three Pd atoms. Thus, we note a superoxo to peroxo transition in the bonding of oxygen molecule with respect to the composition of the cluster. A similar type of superoxo to peroxo transition of O2 binding on anionic gold clusters as a function of size was revealed by Zeng and co-workers36 in a recent study using photoelectron spectroscopy and DFT calculations. Coming to the spin state, the O2-adsorbed complexes of Au7Pd1, Au5Pd3, and Au1Pd7 clusters having odd number of electrons prefer a doublet spin state, whereas the O2-adsorbed complexes of Au6Pd2 and Au4Pd4 having even number of electrons prefer to be in the triplet spin state. The lowest-energy doublet and quartet states of O2–Au3Pd5 complexes were found to have an energy difference of 0.02 eV, whereas the singlet and triplet spin states of O2–Au2Pd6 complexes were found to have the same energy. The equilibrium structures of the O2-adsorbed complexes of Au–Pd clusters in their high energy spin states and their relative energies are given in Figure S2 of the Supporting Information. A careful analysis of the energetics of the O2-adsorbed complexes reveals that the energy difference between the triplet and singlet spin states of even electron clusters and doublet and quartet spin states of odd electron clusters decreases as the number of Pd atoms increase in the cluster. This is particularly important as O2 adsorption is known to involve a spin transition from singlet to triplet state in the case of less reactive monometallic closed shell gold clusters. Thus, incorporation of heteroatoms such as Pd in closed shell gold clusters can be used to overcome the spin activation energy and thereby leading to facile channels for O2 dissociation.

We next calculated the O2 binding energies on the Au–Pd clusters in the lowest energy spin states which are given in Table 1. O2 adsorbs in atop mode on Au7Pd, Au6Pd2, and Au5Pd3 clusters, with binding energy values of 0.64, 0.61, and 1.76 eV, respectively. For clusters such as Au4Pd4, Au3Pd5, Au2Pd6, and Au1Pd7 where O2 adsorbs in a bridged manner, the calculated binding energies are found to be 1.37, 1.26, 1.34, and 1.69 eV, respectively. Thus, importantly, the binding energies of O2 with Au–Pd clusters are seen to increase significantly with the increase in the Pd content irrespective of whether the cluster is an odd or even electron one. Thus, changing the composition by incorporating atomic species such as Pd can be an efficient strategy to improve the binding and reactivity of O2 with the known less reactive even electron gold clusters. We further looked at the O–O stretching frequencies and net negative charge on the O2 molecule on the O2–Au–Pd clusters in the atop mode as well as the bridged mode. The calculated O–O stretching frequencies as given in Table 1 show a larger red shift for O2 adsorption in bridged mode than in the atop mode. The large red shift in the O–O stretching frequencies in the case of bridged O2 adsorption is also supported by a considerable charge transfer in to the antibonding orbitals of O2 during O2–cluster interaction as compared to the atop mode of O2 adsorption.

We finally investigated the O2 dissociation pathways in different spin states (singlet and triplet states for even electron clusters and doublet and quartet spin states for odd electron clusters) on the Au8, Pd8, and Au–Pd clusters. The computed reaction energy profiles of O2 dissociation on the Au8 and Pd8 clusters along with the reactants, transition states, and products are given in Figure 3. The energies of the reactants, transition states, and products in the reaction profiles are given with respect to the energy of the cluster in the lowest energy spin state and oxygen in the triplet spin state. As can be seen from the reaction profiles, the singlet 1R → 1TS → 1P and triplet 3R → 3TS → 3P pathways of O2 dissociation on Au8 have very large activation barriers of 2.59 and 3.83 eV, respectively. On the other hand, in the case of Pd8, the activation barriers for the singlet and triplet O2 dissociation pathways are 2.03 and 1.56 eV, respectively. The product 3P is less stable than the reactant 3R in the case of Au8 and thus O2 dissociation is more unlikely to occur on Au8. Moreover, based on the energy analysis of the O2 dissociation pathway, it can be concluded that the process starts with triplet 3Au8–O2 complex, passes through the singlet 1Au8–O2* transition state, and ends with the triplet O–Au8–O product. Thus, there are two spin crossover steps, one before the transition state and the other after the transition state in the O2 dissociation process on the Au8 cluster. The transition states corresponding to O2 dissociation on Au8 both in the singlet and triplet pathways connect the reactants with the transition states as can be seen in Figure S3 of the Supporting Information. For the Pd8 cluster, the O2 dissociation starts with the triplet 3Pd8–O2 complex, passes through the triplet 3Pd8–O2 transition state, and ends with the singlet O–Pd8–O product. Hence, only a single spin crossover step after the transition state occurs in the case of O2 dissociation on the Pd8 cluster. The reaction profile diagrams for O2 dissociation on the bimetallic clusters are presented in Figures 4 and 5. As can be seen from the figures, the calculated activation barriers for O2 dissociation on the Au–Pd clusters range from 0.98 to 1.94 eV depending on the composition of the cluster and spin state of the dissociation pathway. The activation barriers for O2 dissociation along certain specific spin channels on the Au–Pd clusters are found to be very low as compared to the parent Au8 and Pd8 clusters. Among the bimetallic clusters, Au4Pd4 shows high activation barriers of 1.94 and 1.86 eV for the singlet and triplet pathways, respectively. Importantly, very low activation barriers of 0.98 and 1.19 eV are seen for O2 dissociation along the doublet and quartet spin channels on Au5Pd3 and Au1Pd7 clusters. It is important to mention that Au5Pd3 and Au1Pd7 were found to adsorb O2 strongly with very large binding energies. Moreover, the obtained O2 dissociation barriers are significantly lower than the already reported activation barriers (1.95–3.65 eV) on pure gold and bimetallic Au–Ag clusters.59 The activation barriers along the quartet and doublet O2 dissociation pathways on these clusters were found to be 1.45 and 1.55 eV, respectively. From the reaction profile diagrams of O2 dissociation on Au5Pd3 and Au1Pd7, it can be seen that the process starts with doublet 2Au5Pd3–O2 and quartet 4Au1Pd7–O2 complexes, passes through doublet and quartet transition states, and ends with doublet and quartet products, respectively, on these clusters. Thus, not only do Au5Pd3 and Au1Pd7 clusters reveal significantly low activation barriers for O2 dissociation but also do not involve any spin crossing during the O2 dissociation. Further, the reaction energies for O2 dissociation as presented in Table 2 are thermodynamically more favorable on the abovementioned clusters than the other cases studied. The binding energy, activation barrier, and reaction energy analysis of O2 dissociation on various bimetallic clusters confirm that certain compositions can lead to facile reaction pathways with no spin flipping involved during the reaction coordinate. Thus, the current study highlights that the composition of a binary cluster and spin state of the various species during a reaction coordinate on metal clusters can play a pivotal role in deciding the kinetic and thermodynamic feasibility of the reaction.

Figure 3.

Figure 3

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(A) Calculated reaction pathways (in eV) for O–O bond rupture on the Au8 and Pd8 clusters in singlet (red) and triplet (green) spin states. The energies of the reactants, transition states, and products are with respect to the energy of oxygen molecules in the triplet spin state and the isolated cluster in the lowest energy spin state. (B) Structures of the reactants, transition states, and products along the reaction coordinate in different spin states.

Figure 4.

Figure 4

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(A) Calculated reaction pathways (in eV) for O–O bond rupture on Au7Pd1, Au5Pd3, Au3Pd5, and Au1Pd7 clusters in doublet (red) and quartet (green) spin states. (B) Structures of the reactants, transition states, and products along the reaction coordinate in different spin states.

Figure 5.

Figure 5

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(A) Calculated reaction pathways (in eV) for O–O bond rupture on Au6Pd2, Au4Pd4, and Au2Pd6 clusters in singlet (red) and triplet (green) spin states. (B) Structures of the reactants, transition states, and products along the reaction coordinate in different spin states.

Table 2. Reaction Energiesa (ΔE) and Activation Energiesb (Ea) in eV for the O–O Bond Dissociation on the Au8, Pd8, and Aun–1Pdn Clusters in Various Spin States.

system reaction pathway ΔE (eV) Ea (eV)
Au8 1R → 1TS → 1P –0.11 2.59
  3R → 3TS → 3P 0.59 3.83
Pd8 1R → 1TS → 1P –0.42 2.03
  3R → 3TS → 3P –0.20 1.56
Au7Pd1 2R → 2TS → 2P 0.45 1.42
  4R → 4TS → 4P 0.44 1.83
Au6Pd2 1R → 1TS → 1P 0.30 1.49
  3R → 3TS → 3P 0.75 1.78
Au5Pd3 2R → 2TS → 2P –0.16 0.98
  4R → 4TS → 4P 0.29 1.45
Au4Pd4 1R → 1TS → 1P 0.37 1.94
  3R → 3TS → 3P 0.15 1.86
Au3Pd5 2R → 2TS → 2P 0.36 1.09
  4R → 4TS → 4P 0.48 1.59
Au2Pd6 1R → 1TS → 1P 0.32 1.42
  3R → 3TS → 3P 0.43 1.43
Au1Pd7 2R → 2TS → 2P –0.01 1.55
  4R → 4TS → 4P –0.40 1.19
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a

The reaction energy is calculated as ΔE = E(iP) – E(iR).

b

The activation energy is calculated as Ea = E(iTS) – E(iR).

4. Conclusions

In summary, we have carried out DFT-based calculations to investigate the effect of composition and spin state of bimetallic Au–Pd clusters on the O2 adsorption and dissociation. Our results indicate that the O2 adsorption energy for both atop and bridged mode of binding increases significantly with the increase in the number of Pd atoms in the bimetallic clusters. Thus, the prominent odd/even effect of O2 adsorption prevalent in pristine metal clusters is seen to disappear in the bimetallic Au–Pd clusters. Moreover, among the bimetallic Au–Pd clusters Au5Pd3 and Au1Pd7 are seen to show very low activation barriers of 0.98 and 1.19 eV along the doublet and quartet spin channels, respectively, without any spin flipping for O2 dissociation. The considerably low activation barriers for the above clusters are attributed to their significantly large O2 adsorption energies. In addition to low activation barriers, O–O bond rupture is found to be thermodynamically favorable with negative reaction energies on Au5Pd3 and Au1Pd7 clusters along the doublet and quartet reaction pathways. The current theoretical study thus points out the critical role of composition and spin state of a cluster to exploit alternate and efficient ways of nanocatalyst design.

Acknowledgments

M.A.D. acknowledges the Head, Department of Chemistry and Dean School of Sciences, Islamic University of Science and Technology, for their constant support and encouragement. S.K. acknowledges MLP035426 for funding.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b01581.

  • Optimized geometries of Au8, Pd8, Au8–nPdn, and their O2-adsorbed complexes in the higher energy spin states and an internal reaction coordinate plot for O2 dissociation on the Au8 cluster (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b01581_si_001.pdf (376KB, pdf)

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