Methanol Adsorption and Oxidation on Gold, Platinum and Gold-Platinum Bimetallic Clusters: A Case Study on Au6 , Pt6 and Au3Pt3 Clusters Using Density Functional Theory

  1. Dar Manzoor1 and
  2. Zubida Habib2

1 Department of Chemistry, Indian Institute of Science Education and Research, Bhopal-462066, India
2Department of Physics, National Institute of Technology, Srinagar-190006, J&K, India

  1. Corresponding author email

Associate Editor: Dr. Noor Danish Ahrar Mundari
Science and Engineering Applications 2017, 2, 142–147. doi:10.26705/SAEA.2017.2.8.142-147
Received 04 Nov 2017, Accepted 15 Nov 2017, Published 18 Nov 2017

Abstract

Geometric orientation of the metal atoms in a bimetallic nanoparticle can play a pivotal role in its sensing and activation of different types of molecules. In the present paper, we demonstrate this complex structure-property correlation on model Au-Pt clusters by studying methanol adsorption and oxidation on various conformations of the Au3Pt3 cluster with different relative orientations of the Au and Pt atoms. We have further compared the results with the monometallic Au and Pt clusters. Our results reveal that the adsorption of methanol is least favourable on the monometallic Au 6 cluster as reflected from a very low free energy of complexation (-0.92 Kcal/mol), whereas, the free energy of methanol complexation for the Pt6 is -6.69 Kcal/mol. The bimetallic Au3Pt3 conformers show higher free energies of methanol complexation with notable dependence on the relative orientation of the Au and Pt atoms in the conformer. Likewise, the activation barriers for methanol oxidation are found to be large for the monometallic Au6 cluster as compared to the monometallic Pt6 and bimetallic Au3Pt3 clusters. Further, the activation barriers with respect to O-H and C-H bond activation of methanol are found to be very low and highly sensitive to the relative orientation of the Au and Pt atoms in the bimetallic Au-Pt cluster. Thus, the current study demonstrates that it is possible to tune reactivity and catalytic activity of the cluster by varying its structural features.

Keywords: Bimetalic clusters; geometric orientation, methanol oxidation; density functional theory; O-H and C-H dissociation.

Introduction

Bimetallic clusters have an extra degree of freedom, enabling their physical and chemical properties to be tuned by varying composition and degree of atomic segregation resulting in improved performance [1-7]. Experiments resorting to doping so as to have binary or mixed clusters have resulted in a number of surprises such as aromaticity [8-10], enhanced thermal stability as compared to the pure clusters [11-12], better optical [13], magnetic [14] and catalytic properties etc. [12-17]. The properties of these bimetallic systems arise due to the synergistic electronic interactions between the different metal atoms. Further, the bimetallic clusters provide more than one reactive sites and thus act as superior catalysts compared to the monometallic clusters. Until now, a number of bimetallic clustersofd-blockelementssuchas Au-Pt, Rh-Pt, Pd-Pt, Au-Ag, Au-Pd, etc with varying compositions have been reported and currently the catalytic properties of these bimetallic systems is a subject of great interest [18-22].

Among the bimetallic clusters, Au-Pt binary clusters are found to have excellent catalytic propertiesfor a wide range of applications such as alkane conversion [23], NO reduction [24], CO oxidation [25-26], CH3OH oxidation [27-28], isotope exchange [29] , C-N coupling [30] , O2 reduction etc [31]. However, the strength of these properties is seen to be dependent upon the preparation route of the bimetallic catalysts, size, and the ratio of the two metals [32-35]. It is also reported that the preparation of these bimetallic nanoparticles is complicated due to different reduction kinetics of Au and Pt ions. For example, the alloying of AuPt clusters has been a subject of many controversies especially in the size of nanoscale and appearsto be complicated. Due to this, Au-Pt nanostructures inevitably, form core-shell or shellcore like structures due to their thermodynamic immiscibility [36] Most of the studies report a PtcoreAushell type of conformations within Au-Pt bimetallic clusters. In a remarkable experiment [37], the Au core Pt shell configuration was maintained when the bimetallic cluster was thermally annealed at 300 C for 24h but changed to layered segregation after annealing at 600 C.

Thus, challenges in preparing these binary nanomaterials and their wide range of potential applications in diverse areas has motivated many researchers to investigate the structural properties of Au-Pt clusters using theoretical methods. Although there have been many experimental studies of Au-Pt clusters, there have been few theoretical studies of these clusters. The available theoretical studies have resulted in some understanding of electronic structure and properties of Au-Pt clusters with 1-20 atoms [38-45].Au has an electronic configuration of 5d10, 6s1 and that of Pt is 5d9, 6s1. The small s-d energy gap in Au, caused by strong relativistic effects leads to directional bonding in homogeneous Au clusters. As a result neutral Au clusters adopt planar geometries up to 14 atoms [28], whereas, homogeneous Pt clusters, prefer 3-d structures [43]. Interestingly, it is noted that in Au-Pt binary clusters with higher Au content, retain the planar geometries [46]. Similarly, binary clusters with higher Pt content prefer to orient themselves as 3-D structures [47]. Thus, the ratio of the components is seen to influence the geometry of the cluster

Recently, binary metal clusters have found considerable interest in organic hydrogen based fuel cells such as direct methanol fuel cells (DMFCs) [48-51]. Traditionally, Pt clusters have been used as catalysts in the DMFCs. However the performance of Pt based DMFCs is limited by several factorswhich include: a)slowanodic oxidation of methanol b) high methanol permeation from anode to cathode and c) fuel flow obstruction at anode due to CO2 formation. Thus efforts have been made for tackling these problems by using better mediums as well as using binary clusters (such as Pt-Ni, Pt-Co, Pt-Pd, Pt-Au etc.) which are more active for methanol oxidation at anode and simultaneously tolerant to methanol at cathode. For example, Au-Pt bimetallic clusters have shown enhanced methanol oxidation activity compared to the monometallic Pt based catalysts and it is believed that the presence of Au reduces the CO poisoning in the case of Au-Pt clusters. Further, Au-Pt nanocatalysts are reported to have greater methanol tolerance at the cathode in the case of DMFCs compared to the pure Pt nanocatalysts.

Although, a number of experimental and theoretical works have reported the electrocatalytic activity of Au- Pt clusters vis a vis methanol, to the best of our knowledge not a single theoreticalstudy has beencarriedout to investigate the precise role played by the relative orientation ofthe Au and Pt atomsfor mathanol activation and oxidation in the bimetalic Au-Pt clusters. Thus, in the current work we have studied the methanol oxidation on the Au-Pt clusters in order to gain an in depth understanding into the dissociation of methanol on the different conformers of Au3Pt3 cluster and compared it with its monoatomic counterparts. We have chosen various conformers of a six atom bimetallic Au-Pt cluster for this purpose with different orientations of Au and Pt atoms.

Computational details

Density functional theory (DFT) with PBE exchange and correlation potential as implemented in the Gaussian 09 was used to perform all the calculations [52] . For each cluster, a number of conformers with different spin multiplicities were used as a starting guess to find the ground state geometry. Geometry optimizations were carried out by using the Berny algorithm with the default convergence criterion. The LANL2DZ basis set and the corresponding Los Alamos relativistic effective core potential (RECP) was used to take into account scalar relativistic effects for the gold and platinum atoms. For the hydrogen, oxygen and carbon atoms, the TZVP basis set was used. In order to locate the energetically most favorable configurations of methanol adsorption on the Au6, Pt6, and various conformers of Au3Pt3, we considered the various possible adsorption modes (via O and C atoms), at the various possible sites on all the clusters. Vibrational frequency calculations were carried out to guarantee the optimized structures are local minima. The free energy of complexation of the methanol-cluster complexes was calculated as the difference between the free energy of complex and its constituents (i.e methanol molecule and atomic cluster). The absolute value of total free energy (Gtot) considered for each structure is calculated using the Gibbs free energy function at 298K using the following equation.

Gtot = Eelec +Etrans+Erot+Evib +EZPE +PV −TS (1)

where Eelec, Etrans, Erot and Evib represent the electronic, translational, rotational and vibrational contributions respectively to the total energy. EZPE is the zero point correction to the total energy and the other terms have their usual meaning. The transition states were found by using the linear synchronous transit method and were characterized bythe presence ofone imaginary frequency.

Results and Discussions

Methanol adsorption on the Au6, Pt6 , and Au3 Pt3 clusters

Several conformations of Au3Pt3 cluster are generated and optimized with different possible spin multiplicities. The ground state structures in all the cases were found to prefer singlet multiplicity over triplet multiplicity. The lowest conformation is a D3h conformation as in case of Au6 and Pt6 clusters, whereas, the various bimetallic conformers are found to have slightly distorted D3h conformations except for the conformer IV, which has a 3-d structure. The conformations considered in this study along with their relative energies are shown in Figure 1. Also shown are the conformations of Au6 and Pt6 ground state geometries. Noticeably, the ground state isomer reported in one of the earlierstudies [53] isfound to be second lowest energy isomer in the current study. It can be seen from the figure that the Au6 and Pt6 clusters have all the A-B and B-B lengths similar and as a result the Au6 and Pt6 clusters are highly symmetrical. How- ever, the bimetallic Au3Pt3 conformers (except II and VI) are slightly deformed with different A-B and B-B bond lengths. Figure 1 also enlists the calculated natural bond orbital (NBO) charges on the Au and Pt atoms in Au6, Pt6 and different conformations of the Au3Pt3 clusters. We note that there is a little charge separation between the cap (A) and the bridged atoms (B) in the pristine Au6 and Pt6 clusters. In contrast, the bridged Pt atoms in the various conformations of Au3Pt3 are seen to accumulate a higher degree of negative charge. This enhanced charge separation is expected to induce different reactivity and catalytic activity in the Au-Pt bimetallic clusters as compared to the pristine Au and Pt clusters.

[2456-2793-2-8-1]

Figure 1: Optimized geometries of the various conformers of Au3Pt3 cluster, and ground state geometries of Au6 and Pt6 clusters. The numbers in the green colour represent the calculated A-B and B-B bond distances where as the numbers in red colour are the computed NBO charges on each atom.

We next look at the adsorption of methanol on the ground state Au6, Pt6 structures and various conformers of bimetallic Au3Pt3 cluster. The optimized geometries ofthelowestenergy structures ofthe methanol complexes of Au6, Pt6 and Au3Pt3 conformers are demonstrated in Figure 2. It is important to mention here that methanol adsorption was studied through various modes (via O or C atoms) at different possible sites in all the clusters re- ported in the current study. We note that the methanol adsorbs on each cluster via its oxygen atom preferably, which is in accordance with the earlier reported studies of methanol adsorption on various types of metal clusters and surfaces [49-50,54-55]. Also the methanol adsorption takes place at the cap atoms of the various clusters except for II and III isomers of Au3Pt3 cluster. From Figure 2, it can be seen that methanol adsorption has a little effect on the structural parameters of the Au6, Pt6 and the various Au3Pt3 conformers with the exception of II isomer, where the methanol adsorption resultsin a 2D to 3D transition. We have also computed the free energy of co mplexatio n (∆Gcomp) of methanol with the Au6, Pt6 and the various Au3Pt3 clusters, and the results are also highlighted in Figure 2. The ∆Gcomp values for the Au6 and Pt6 clusters are -0.92 and -6.69 kcal/mol respectively. Among the bimetallic Au3Pt3 con- formers, the II conformer shows highest value of ∆Gcomp (-16.83 kcal/mol). It is important to mention here that the conformer which shows the highest value of ∆Gcomp is the one which undergoes 2D-3D structural transition upon methanol adsorption. The ∆Gcomp values for I, III, IV and V conformers are -9.45, -12.22, -12.68 and -10.61 kcal/mol respectively and are higher than the ∆Gcomp values of both the Au6 and Pt6 clusters. However, interestingly the VI conformer shows a slightly lower ∆Gcomp value of -5.53 kcal/mol as compared to the Pt6 cluster. Thus, from the ∆Gcomp values, we conclude that the methanol adsorbs much more strongly on the Au3Pt3 clusters as compared to the Au6 and Pt6 clusters. Further, it is found that the methanol adsorption depends strongly on the relative position of Au and Pt atoms in the bimetallic Au3Pt3 cluster.

[2456-2793-2-8-2]

Figure 2: Optimized geometries of the methanol adsorbed conformers of Au3Pt3, and Au6 and Pt6.

Methanol oxidation on Au3Pt3 , Au6 and Pt6 clusters

In this section, we will focus our attention on the oxidation of methanol on the Au6, Pt6, and different conformers of Au3Pt3 clusters. As shown in earlier studies [49-50,54-55], the oxidation of methanol on metal clusters and surfaces occurs throughthebreakingofO-Hand C-H bonds. Figure 3 summarizes the activation barriers for the first step of methanol oxidation in Au6, Pt6 and Au3Pt3 clusters. The reactants are given in the center and the pathway for C-H and O-H activation is highlighted in red and blue respectively for each conformation. First we discuss the barriers for cleavage of C-H and O-H bonds in monoatomic metal clusters, viz., Au6 and Pt6. It is clearly seen from the figure that activation barrier for cleaving C-H bond is quite low as com- pared to the activation barrier for cleaving O-H bond on Pt6 cluster. The activation barriers on Au6 are substantially high (33 and 48 kcal/mol respectively, for C-H and O-H activation) as compared to those on Pt6. Coming to the bimetallic clusters, the O-H activation barrier ranges from 12.99 to 19.71 Kcal/mol on the different con- formers of the bimetallic Au3Pt3 cluster.

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Figure 3: Reaction pathway for methanol activation on the various conformers of Au3Pt3 cluster, and Au6 and Pt6 clusters through the cleavage of C-H and O-H bond.

The barrier is lowest for the V conformer (12.99 Kcal/mol) and highest for IV conformer (19.71 Kcal/mol). The free energy change for O-H bond dissociation becomes negative particularly in the case of V conformer (conformer which shows the lowest activation barrier), thereby, enhancing the thermodynamic feasibility of the O-H bond dissociation. Among the bimetallic Au-Pt conformers, the II and the III conformers show very low activation barriers of 2.00 and 4.87 Kcal/mol for methanol dissociation via C-H. The I and VI conformers show activation barriers of and 12.26 Kcal/mol respectively. However, the IV and V conformers of Au3Pt3 show very high activation barriers of 35.32 and 31.71 Kcal/mol respectively for the C-H dissociation. In short, the more feasible C-H activation barrier for methanol molecule on Au-Pt bimetallic cluster can be increased to the values found in pure gold nanoclusters (nearly 30 kcal/mol). This is particularly attractive when we need to have methanol tolerant nanoclusters. At the same time, few conformations such as II and III exhibit lower barriers than those noted on Pt cluster indicating them to be better catalysts for methanol electrooxidation.

Conclusion

Adsorption and activation of methanol molecule on monometallic (Pt and Au) clusters as well as bimetallic clusters as a function of the relative orientation of gold and platinum clusters is studied in the present work. It is clearly seen that activation barriers are lower for C-H bond dissociation as compared to that for O-H bond in most of the cases. Barriers are highest for monoatomic gold cluster among the various clustersstudied. In case of bimetallic clusters, our results highlight the critical dependence of relative orientation of the gold and platinum atoms in the cluster with respect to methanol ad- sorption and oxidation. Thus, by fine tuning the atomic orientations within the bimetallic cluster, it is possible to increase or decrease the activation barriers for methanol oxidation substantially and obtain the desired chemical activity.

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