Subject Number : S-16-MS-0046
Support Type : Collaborative research
Proposal Title (English) : Theoretical Study on Hydrodeoxygenation of Dimethyl Sulfoxide on Pt5/MoO3 (010) Catalyst
Username (English) : Supawadee Namuangruk
Affiliation (English) : National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Thailand

1. Summary
Recent report showed that loading Pt on MoO3/TiO2 dramatically enhanced the catalytic activity of MoO3/TiO3 for hydrodeoxygenation (HDO) of sulfoxide to sulfide with excellent turnover frequency and moderate sulfur tolerance. Cooperative mechanism between loaded Pt and MoO3 was speculated. This work, by using density functional theory calculations the mechanism of HDO of dimethyl sulfoxide (DMS) on the Pt5/MoO3 catalyst has been investigated. The results reveal that both Pt and Mo sites are involved throughout the reaction. The reaction proceeds through three sequential steps; firstly, H2 is readily dissociated on Pt site with negligible energy barrier. Then, the dissociated H atoms at doped Pt spill out to MoO3 surface to generate H2O and Mo Lewis acid site with the activation energy barrier of 0.83 eV. Finally, dimethyl sulfoxide is deoxygenated at the Mo Lewis acid site to form dimethyl sulfide by requiring energy 0.25 eV. The sulfur-tolerance of this catalytic system is explained by the difficulty of the direct deoxygenation of sulfide on the Pt cluster. In summary, this work has unrevealed the synergism of doped Pt and Mo Lewis acid sites in Pt5/MoO3 catalyst for the HDO of sulfoxide in which Pt is the active site for the H2 dissociation while the Mo Lewis acid site is responsible to the deoxygenation of DMS.
2. Computational Details
The periodic Density Functional Theory (DFT) calculations were performed using Vienna ab initio simulation program (VASP). The electron-ion interaction was described by the projector augmented wave (PAW) method with a kinetic cutoff energy of 400 eV. The generalized gradient approximation (GGA) combined with the Perdew-Burke-Ernzerhof (PBE) functional was employed to describe the exchange-correlation functional. Methfessel-Paxton smearing method was used in the width of 0.1 eV. The Brillouin zone was sampled using a 3 x 3 x 1 Monkhorst-pack grid for surface model. The van der Waals interaction was corrected by the DFT-D3 approach. Dipole correction was considered where necessary. Structural optimizations were performed until total energy and force of the system were converged to less than 10-5 eV and 0.03 eV/Å, respectively.
The α-MoO3 crystalline phase is the most common and stable phase with the space group of Pbmn and the (010) surface. To model the MoO3 surface and support, we start by fully optimize bulk MoO3 with a 5 x 3 x 5 Monkhorst-pack grid. The tetrahedron with Blöchl corrections was used for electron smearing. The computed lattice parameters, a = 3.878, b = 13.445 and c = 3.682 Å, are in agreement with experimental results.29 The lattice parameters from bulk optimization were used in calculations for surface. Next, the MoO3 (010) surface was modeled by a bilayers sheet of MoO3 with a (3 x 3) periodic supercell and an 18 Å vacuum space. During optimizations, the adsorbates, doped Pt5 cluster and the top layer of MoO3 surface were relaxed while the bottom layer of MoO3 slab was fixed to their bulk configuration. The transition state (TS) was searched by a combination of the Climbing Image-Nudged Elastic Band (CI-NEB) and Dimer methods. A frequency calculations were performed to ensure that a transition state structure has only one imaginary frequency according to the reaction movements.
3. Results and Discussion
3.1 MoO3 and Pt5/MoO3 models
The optimized MoO3 (010) surface is displayed in Figure 1(a). There are three different types of oxygen atom in MoO3 namely OT, OS and OA. OT is a terminal oxygen site, OA is an asymmetric oxygen site and OS is a symmetric oxygen site. The calculated structural parameters of the MoO3 surface are closed to the experimental results.

Figure 1. The optimized structure of (a) MoO3 and (b) Pt5/MoO3.

For the Pt5/MoO3 model, a square pyramidal Pt5 cluster is deposited on the surface of MoO3. Four basal Pt atoms are placed between four OT sites in which two of the four Pt atoms are connected to the OA atom (PtA) while others are connected to the OS sites (PtS). This model was used for study hydrogen spillover on Pt decorated MoO3 (010) surface. The optimized Pt5/MoO3 structure is shown in Figure 1(b). We found that the doped Pt5 cluster is slightly distorted after optimization. The distances between PtA and PtS are in the range of 2.89-2.91 Å while the bond distances between Pt basal (PtA, PtS) and the apex Pt (PtT) are increased from 2.56 to 2.66-2.77 Å. Upon the doping of Pt5 cluster, the distance between the two sublayers of MoO3 is suppressed compared with the clean MoO3 surface. The binding energy of the Pt5 cluster on the MoO3 (010) surface is calculated to be -2.88 eV indicating the stability of the complex.

3.2 Adsorption of H2 and Me2SO on MoO3 and Pt5/MoO3 models
3.2.1 On MoO3.
The optimized structures of Me2SO and H2 adsorbed on the MoO3 (010) surface are shown in Figure 2(a-b). For the adsorption of Me2SO, its S atom is interacting with an OT site of MoO3 surface with the distance of 1.50 Å. Upon the adsorption, the OT-Mo bond of MoO3 surface is greatly lengthened from 1.68 to 2.05 Å implying that the weak OT-Mo bond is ready to break to generate a reduced MoO3. The adsorption energy of Me2SO on the MoO3 surface is calculated to be -0.24 eV. In the case of H2 adsorption, the adsorbed H2 is unable to induce the deformation of MoO3 surface and its H-H bond distance is also unaltered. After optimization, H2 molecule is repelled from the surface and located near the OT site with the distance around 2.5-2.9 Å. Such a long distance indicates a weak physisorption of H2 on the perfect MoO3. The calculated adsorption energy is to be -0.06 eV.

Figure 2. Optimized structures of (a) Me2SO on MeO3, (b) H2 on MoO3, (c) Me2SO on Pt5/MoO3 and (d) H2 on MoO3.

3.2.2 On Pt5/MoO3.
The most stable structures for Me2SO and H2 adsorbed on the Pt5/MoO3 are shown in Figure 2(c-d). For the adsorption of Me2SO, it bonds to the PtT and PtA sites of doped Pt5 cluster via its S and O atoms with the distance of 2.14 and 2.07 Å, respectively. Upon adsorption, the S-O bond of Me2SO is elongated from 1.49 to 1.56 Å. The interaction energy is -3.06 eV indicating a strong adsorption of Me2SO on the Pt5/MoO3. In the case of H2 adsorption, H2 approaches to the doped Pt5 cluster and bound with the PtT site with the distance of 1.57 Å. The H-H bond distance expanded from 0.76 to 1.81 Å indicating that it already dissociated. The adsorption energy is calculated to be -1.15 eV which is significantly stronger than that on bare MoO3 surface (-0.06 eV).
3.3 Reaction mechanisms
The conversion of Me2SO to Me2S in the presence of H2 catalyzed by Pt5/MoO3 catalyst comprises two main classes of reaction: (I) H2 dissociation and (II) Me2SO deoxygenation to Me2S. Each reaction is considered at two distinct active sites: (A) bare MoO3 surface and (B) doped Pt5 nanocluster. The details are discussed below.
3.3.1 H2 dissociation on MoO3 surface

Figure 3. Energy profile for the H2 dissociation and the reduction of MoO3.

In this section, the dissociative H2 adsorption followed by the reduction of MoO3 are displayed in Figure 3. Firstly, H2 molecule is dissociated at the two neighboring OT sites of MoO3 surface. At the TS structure (TS-IA1), The H-H bond is lengthened to 1.05 Å while each of H atom moves toward the nearest terminal oxygen with the distance of 1.29-1.35 Å. Then, two surface hydroxyls are formed at the OT site (HOT_HOT). The activation barrier is calculated to be 2.01 eV suggesting that this process is not facile.
After the two surface hydroxyls are formed, MoO3 can be reduced by the combination of two surface hydroxyls to produce H2O and O vacancy. Starting from HOT_HOT structure, the distance between the two surface hydroxyls is 3.22 Å. Then, the H bonding to the OT site can migrate to the OA site which is in between the two OT sites. This migrated H atom at the OA site approaches to another surface hydroxyl and produces water molecule. At the H migration step, it requires energy 0.67 eV for migrating H from OT to OA site. The OT—H and OA—H distances of the transition state structure (TS-IA2) are calculated to be 1.20 and 1.27 Å, respectively. This process is an exothermic since the HOA_HOT is more stable than the HOT_HOT structure by about 0.24 eV. The HOA_HOT was reported as the most stable structure for two surface hydroxyls formed on MoO3 (010) surface.19 Subsequently, the H of H-OA transfers to hydroxyl HOT to form H2OT-MoO3 structure. At the transition state (TS-IA3), the transferring H atom is in the midway between the OA and OT sites in which the distances are 1.21 and 1.23 Å, respectively while the HOT-Mo bond is strengthened from 1.88 to 2.06 Å. This process is an endothermic with the activation barrier of 0.83 eV. Finally, the bonding between H2OT and the Mo atom is broken to produce water molecule adsorbed on Mo Lewis acid site of the defective MoO3 (H2OT-MoO3*). Mo Lewis acid site of defective MoO3 is reported as the active sites for deoxygenation of many organic molecules by C=O scission. Finally, the desorption of water from the reduced MoO3 surface (MoO3*) requires energy 0.10 eV.

3.3.2 H2 dissociation on doped Pt5 nanocluster
In this section, the dissociative H2 adsorption is followed by the hydrogen spillover on doped Pt5 nanocluster of Pt5/MoO3 catalyst are studied. The mechanism and energy profiles are shown in Figure 4. As described in section 2.2, adsorbed H2 is readily dissociated on doped Pt nanocluster with negligible energy barrier. Thus, doped Pt nanocluster of Pt5/MoO3 catalyst plays an important role in the dissociation of H2 molecule since this process on the bare MoO3 surface requires very high activation energy (2.01 eV).
For the H spillover process, the reaction starts with the dissociative H2 adsorption on the apex of doped Pt5 nanocluster forming symmetric platinum hydrides (H2-PtT). Noted that this structure could rearrange to form a more stable structure which is asymmetric (HH-PtT). The energy difference between the symmetric and asymmetric structure is 0.39 eV. Next, a hydride spills over to a PtA site with a tiny activation energy barrier (0.08 eV). The distances between the transferring hydride with the PtT and PtA of transition state structure (TS-IB2) is calculated to be 1.65 and 2.09 Å, respectively. Then, an intermediate HPtA-HPtT is generated in which one hydride binds with PtA and another bind with PtT site. The relative energy of this structure is -1.54 eV. Finally, the hydride bonding with PtT site transfers to an unoccupied PtA site and form HPtA-HPtA structure. The relative energy of this structure is calculated to be -1.86 eV. Overall, the dissociative H2 adsorption and hydrogen spillover on doped Pt5 nanocluster are both thermodynamically and kinetically preferable. This result is in line with the results of Li et al.16 but the energy difference is due to the Van der Waals force is included in our calculations.
After the H spillover are occurred on doped Pt5 nanocluster, there are two possible routes for the further reaction. (1) Adsorbed hydrogen on doped Pt5 cluster might diffuse to bind with oxygen of MoO3 forming surface hydroxyl and follows hydrogen spillover process as described above which finally produces the Mo Lewis acid site on the defective MoO3. The spillover of H atom from Pt cluster to MoO3 surface is reported as facile process (2) Me2SO might come to co-adsorb at platinum hydride followed by the HDO process to produce Me2S and H2O. The second route is investigated in the section 3.3.4.2.

Figure 4. The energy profile for the dissociative H2 adsorption and H spillover on Pt site of Pt5/MoO3 catalyst.

3.4 Deoxygenation of Me2SO to Me2S
3.4.1 DDO on MoO3*
This work, the Mo Lewis acid site of the reduced MoO3 is suggested as an active site for the direct deoxygenation (DDO) of Me2SO. In the DDO process, the S=O bond of Me2SO is cut resulting in the direct conversion of Me2SO to Me2S without the formation of intermediate. The reaction mechanism is depicted in Figure 5. The initial structure is the adsorption of Me2SO on the reduced MoO3 (Me2SO-MoO3*). In this structure, Me2SO interacts with the Mo site through its oxygen with the distance of 2.02 Å. The S=O bond of the adsorbed Me2SO is strengthened to 1.59 Å compared with 1.49 Å in the gas phase. Next, Me2SO is deoxygenated by breaking the S=O bond and leave its O atom at the Mo Lewis acid site of the surface. At the TS structure (TS-IIA), the S-O bond of Me2SO is lengthened to 1.81 Å while the distance between the O atom of Me2SO and Mo site is shortened to 1.87 Å. Then, the Me2S is produced and regenerate the perfect MoO3 (Me2S-MoO3); the produced Me2S is adsorbed on MoO3 surface with the distance about 3.00 Å. This reaction is an exothermic process and the activation energy is small as 0.25 eV. This indicates that the DDO of Me2SO on the defective MoO3 surface is both kinetically and thermodynamically favorable. Finally, the Me2S is desorbed from the surface requiring energy of 0.35 eV.

Figure 5. The energy profile for the direct deoxygenation of Me2SO on the reduced MoO3.

3.4.2 HDO on doped Pt5 nanocluster
In this section, platinum hydrides which is the result of H-spillover process is considered as an active site for the hydrodeoxygenation (HDO) of Me2SO. The reaction mechanism is shown in Figure 6. The HDO reaction comprises two consecutive elementary steps; hydrogenation followed by dehydration. For the hydrogenation step, the reaction starts with the adsorption of Me2SO on HPtA-HPtA structure forming Me2SO-(HPtA)2 complex. At this complex, Me2SO binds with a PtT site via its S atom with the distance of 2.19 Å while its O atom interacts with a hydride with the distance of 2.20 Å. Next, Me2SO is protonated by receiving a hydrogen from platinum hydride forming Me2SOH. At the transition state (TS-IIB1), the PtA-H bond is greatly elongated from 1.56 to 1.84 Å. At the same time, the distance between the transferring H and Me2SO is shortened to 1.19 Å. Then, the Me2SOH is generated as an intermediate of the reaction. This step is an endothermic process and requires activation energy of 0.33 eV.
For the dehydration step, the adsorbate is protonated again resulting in Me2S and H2O formations. The starting structure is Me2SOH-HPtA in which a hydroxyl group of Me2SOH is rotated to form hydrogen bonding with platinum hydride. This step requires energy of 0.63 eV. At the transition state (TS-IIB2), a H atom of platinum hydride transfers to the hydroxyl group of Me2SOH. The transferring H atom sits in between the PtA and -OH at 1.69 and 1.42 Å, respectively. This causes the breaking of the S-O bond (2.01 Å) simultaneously with the forming of H2O. Finally, the Me2S is produced together with the byproduct H2O (Me2S-Pt5-H2O). This process is an exothermic with the activation energy of 0.67 eV indicating that this is the rate-determining step of the HDO of Me2SO on doped Pt5 nanocluster. Noted that this activation energy is higher than that of the DDO on MoO3* (0.25 eV) suggesting that the deoxygenation of Me2SO prefers to occur on Mo Lewis acid site. Moreover, we also found that the produced Me2S strongly binds with the doped Pt5 nanocluster (Eads = -6.98 eV). It requires energy 2.39 eV to desorb Me2S from the Pt site while the desorption energy of Me2S on MoO3 surface is only 0.35 eV. The strong binding of Me2S on the doped Pt nanocluster might lead to the deactivation of Pt5/MoO3 catalyst. However, the feasibility of DDO on Mo site may lead to the moderate S tolerance of the catalyst observed in the HDO of sulfoxides to sulfides.

Figure 6. The energy profile for the hydrodeoxygenation of Me2SO on Pt site of Pt5/MoO3.

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