630-632 Conference-ICSFS-16-DFT Studies of S-Modified Au-Supported Pd with Hydroxyl Group

The actual active species of the recently developed S-modified Au-supported Au-supported Pd (SAPd), with one of the lowest Pd-releasing levels and high recyclability in the Suzuki-Miyaura coupling, was investigated. Also SAPd was found to work repeatedly as an excellent Pd reservoir for liquid-phase combinatorial synthesis. In order to develop more effective catalysts, it is essential to understand the function of the OH-group around Pd of SAPd. [DOI: 10.1380/ejssnt.2012.630]


I. INTRODUCTION
Liquid-phase combinatorial parallel synthesis is an effective method for the generation of a diverse library of small molecules, and it has been used not only in the field of drug discovery but also in organic materials.Pdcatalyzed reactions allow for the rapid access to a diverse range of compounds, but they also present a problem in that the Pd can often be retained in the product even after the purification steps.For pharmaceutically active ingredients, there are typically strict guidelines to limit the levels of contaminating heavy metals, including Pd, in the drug.
Most recently, a practical Pd material, namely Smodified Au-supported Pd (SAPd) was developed [1,2].Due to the lowest recorded Pd-releasing levels (less than 1 ppm in 3 mL of solvent, TON up to 2,760,000) and high recycbility (more than 10 cycles) in the Suzuki-Miyarura coupling, this is one of the best materials developed thus far.
To understand SAPd well, it is very important to know the effect of OH group since it is possible that OH group exists.Therefore, in this paper, we report the stable position, adsorption energy and charge transfer for OH group on Pd/S/Au(111) by DFT calculations using the program package VASP [3].

II. MODEL
In this work, all our calculations are carried out using a first-principle calculation technique based on density functional theory (DFT) within a local density approximation (LDA).We used VASP (Vienna Ab-initio Simulation Package), which is a first-principle calculation code with high precision using the PAW method.
The pure surfaces were modeled as slabs with periodic boundary conditions in the two directions parallel to the surface (infinite in two dimensions), and separated by a 15 Å thick vacuum region.For all surfaces investigated here, we utilized a slab consisting of five layers with the bottom two layers constrained to their bulk geometry.The lattice constant for each calculation was fixed to be the value obtained from optimizing this constant with DFT for the bulk material.1×1 surface cells were used.We used a Monkhorst-Pack k-point set of 6×6×1, corresponding to 30 k points in the irreducible Brillouin zone.The Au(111) face was exposed, assuming a bulk terminated structure.
The adsorption energy, E ad , for the adsorbed atom on the surface is E ad = E slab + E adsorbed−atom − E slab+adsorbed−atom , where E slab+adsorbed−atom is the total energy of relaxed sulfur-surface system, while E slab and E adsorbed−atom are the total energy of the relaxed bare surface and sulfur atom, respectively.Hence, the binding energy is defined as positive if the total energy decreased when the ad atom is brought from infinity and placed onto the surface.We calculated the adsorption of OH group on Pd on the S-terminated Au(111).The positions where we adsorbed OH group are fcc site, top site and hcp site.OH group is stable on top of fcc site.In this case, S is located on the top site and Pd is stable on the fcc site in Fig. 1.Pd atom becomes stable between S and Au(111) surface when OH group is not around, as shown on Fig. 1.However, the position of S and that of Pd are changed when OH group is around Pd (Fig. 2).It means that OH group might   The atomic distance between Pd and Au substrate are shown in Table I.The atomic distance of Pd and Au(111) surface is 2.76 Å and that of Pd and S-terminated Au(111) is 2.34 Å.Table II shows that the adsorption energy for Pd atom on clean Au(111) surface is 1.70 eV.The case of Pd on S-terminated Au(111), the adsorption energy is 1.85 eV.When the surface is terminated by S, the adsorption energy is stronger than that of clean Au(111) surface.It shows us that S exists as a bonding face, and besides, it confirms an experimental result of recyclability [2].When OH is near the Pd, it is thought that it is adsorbing to Pd to a substrate strongly.For comparison to the number of electrons for Pd of SAPd, we calculated the charge transfer of three models, which are Pd/Au, Pd/S/Au and OH/Pd/S/Au in Table III.For these calculations, we used the bader charge analysis [4].The charge transfers of Pd for Pd/Au and Pd/S/Au are almost 0. It means that we can use these catalysts for Suzuki-Miyaura coupling, Heck reaction and so on.However the charge transfer of Pd for OH/Pd/S/Au is +0.47, it is slightly +2 direction.This result shows us OH group might affect in the catalysis reaction.

IV. CONCLUSIONS
In this research, we calculated the stable structure of SAPd with OH group, the adsorption energy of Pd, atomic distance between Pd and surface and charge transfer for Pd using DFT calculation with PAW method at three adsorption sites on Au(111)-(1×1).The stable position of S and Pd are changed due to OH group around Pd. To compare with the case of OH/Pd/S/Au and Pd/S/Au, the atomic distance between S and Pd for OH/Pd/S/Au is shorter than that of Pd/S/Au.Therefore we expect that adoption energy for Pd of OH/Pd/S/Au may be stronger than that of Pd/S/Au.The charge transfer for Pd of OH/Pd/S/Au is slightly +2 direction; therefore probably OH group affect the catalysis reaction.http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) Volume 10 (2012) Yokoyama, et al.

FIG. 1 :
FIG. 1: The top view of OH/Pd/S/Au(111)-(1×1).The solid lines in the top views are the unit cells with the 1×1 structure.The Orange, yellow, silver, red and pink sphere corresponding to Au, S, Pd,O and H atoms respectively.

B.
The comparisons with Pd on the clean Au(111) and Pd on S-terminated Au(111)

TABLE I :
The interatomic distance of neighboring atoms.

TABLE II :
The adsorption energy for Pd.

TABLE III :
The charge transfer for Pd using Bader method.
affect the structure of SAPd and the catalysis reaction.