Density functional theory insights into the bonding of CH3OH and CH3Owith Ir(111) surface

Use your smartphone to scan this QR code and download this article ABSTRACT Introduction: Understanding the adsorption characteristics of CH3OH and CH3O on the noble metal surfaces is essential for designing better catalysts for the on-board production of hydrogen from CH3OH. This study aims to provide insights into the adsorption behavior of these molecules on Ir(111) surface. Methodology: The adsorption structure, the adsorption energy, and the bondingmechanism of CH3OH and CH3Owith Ir(111) surfacewere investigated bymeans of the density functional theory (DFT) calculations and the Bader charge analysis. Results: The DFT results show that the adsorption of CH3OH and CH3O is driven by the formation of Ir–O bond at the top site of the surface by the overlap of O-2p and Ir-5d orbitals. The overlap of these orbitals is greater in the absorption of CH3O, resulting in stronger adsorption energy of CH3O (2.23 eV vs. 0.32 eV). In agreement with the adsorption strength, the charge transfer from CH3O to the surface is significantly larger than from CH3OH (0.386 e vs. 0.073 e). Conclusion: Although driven by the same adsorption bond, the difference in themolecular characteristics leads to amarked difference in the absorption strength of CH3OH and CH3O on Ir(111) surface.


INTRODUCTION
The on-board production of hydrogen from methanol (CH 3 OH) is of paramount importance for the operation of polymer electrolyte membrane fuel cells (PEMFCs), which has been attracted as a promising energy source for various portable applications. 1,2 It has been demonstrated that the most active catalysts for hydrogen production by CH 3 OH reforming are the noble metals such as Pt, Pd, Ru, and Ir in the form of nanoparticles dispersed on supporting materials. 3 It is widely known that in the mechanism of CH 3 OH reforming, the dehydrogenation of CH 3 OH is the key step. Therefore, there have been many experimental [4][5][6][7][8][9] and theoretical 10-15 studies on CH 3 OH dehydrogenation over surfaces of the noble metals. The results of experimental studies found that CH 3 OH is molecularly adsorbed on these metal surfaces; then, CH 3 OH dehydrogenation is initiated by the cleavage of the O-H, the C-H, or the C-O bond. The experimental results also found that the O-H bond cleavage is easiest. The CH 3 OH dehydrogenation that occurs through CH 3 O intermediate is the most energetically favourable. 16 The density functional theory (DFT) calculations have provided insights into the adsorption and decomposition mechanism of CH 3 OH on these metal surfaces. However, all the previous DFT calculations only focused on the energetics of the elementary reactions in the mechanism of CH 3 OH decomposition and ignored the bonding of CH 3 OH and the important reaction intermediates such as CH 3

COMPUTATIONAL METHODS
DFT 17 calculations were performed by the Vienna Ab initio Simulation Package (VASP 5.4.1) [18][19][20][21] which treats the interactions between ions and electrons by the projector augmented wave method (PAW) 22,23 . The generalized gradient approximation 24,25 (GGA) with the Perdew-Burke-Ernzerhof (PBE) 26 exchange-correlation functional was used in this study with a kinetic cut-off energy of 450 eV. The Brillouin zone was sampled by a gamma-center k-point 27 with a grid size of 6×6×1. The unit cell of bulk Ir (a = b = c = 3.839 Å) was used to construct Ir(111) surface. A vertical vacuum space of 15 Å was included in the surface model to mitigate the interactions between the repeated slabs. In the geometry relaxation, CH 3 OH or CH 3 O molecule and the top 3 layers were allowed to relax while the bottom 3 layers were kept fixed to mimic their bulk positions. The geometry relaxation was stopped when the electronic energy tolerance (10 −5 eV) was reached, and the residue forces on each atom were less than 0.03 eV/Å. The adsorption energy of CH 3 OH and CH 3 O on Ir(111) surface was calculated as: where E Ir(111) was the total energy of the clean Ir (111) surface, (111) ) was the total energy of the system consisting of adsorbed CH 3 OH (CH 3 O) on Ir(111).

Structure of Ir(111) surface
Firstly, we present the geometrical structure of Ir(111) surface. In this study, Ir(111) surface was modeled as a periodic slab consisting of 6 atomic layers with 9 Ir atoms on each layer. After the surface relaxation, the Ir-Ir bond length between two adjacent Ir atoms on Ir(111) surface was found to be 2.715 Å. It should be noted that Ir crystallizes in a face-centered cubic lattice. Therefore, there are four high-symmetry positions on the Ir(111) surface, namely one-fold top site, two-fold bridge site, and three-fold hcp and fcc sites, as shown in Figure 1. The high-symmetry sites play the binding centers' role for the adsorption of CH 3 OH and CH 3 O on Ir(111) surface.

Adsorption energy and structural properties
The adsorptions of CH 3 OH were calculated at all four high-symmetry sites of Ir(111) surface. CH 3 OH is a polarized molecule with a negative charge on the O atom. Therefore, the adsorption of CH 3 OH on the surface occurs by the bonding between Ir and O atoms. The adsorption energy of CH 3 OH at the bridge, fcc, hcp, and top sites is 0.03 eV, 0.03 eV, 0.04 eV, and 0.32 eV, respectively in Table 1. It means that the top site is the most stable site for the adsorption of CH 3 OH, and our finding agrees with the previous DFT results. 11 Moreover, the adsorption energies of CH 3 OH at the top site are comparable on

DISCUSSION
In this part, the nature of the bondings between CH 3 OH and CH 3 O and Ir(111) surface will be discussed based on the density of states (DOS) calculations and the Bader charge analyses. Figure 3 displays the DOS of the frontier orbitals of adsorbed CH 3 OH and CH 3 O. It is widely known that the DOS of an adsorbed molecule is shifted to the lower energy region with respect to the respective free molecule, and the extent of the down-shift reflects the adsorption strength. 28 Thus, it can be seen that the adsorption of CH 3 O is stronger than CH 3 OH as the down-shift of the DOS is more pronounced in the adsorption of CH 3 O. As has been discussed, the adsorption of  Figure 4 show the overlap between the electronic state of O-2p and Ir-5d orbitals. By comparing Figure 4a and Figure 4b,

AUTHOR'S CONTRIBUTION
Thong Le Minh Pham conceptualized the study and did all the DFT calculations. Anh Thi Le and Nguyen Thi Thai An interpreted the data and did the proof reading. Thong Le Minh Pham, Thanh Khoa Phung and Ho Viet Thang participated in the writing of the manuscript. All the authors read and approved the manuscript.