The anode oxidation reaction in fuel cell: A DFT study

. Although fuel cell (FC) has been regarded as promising “green” power generator, the Pt-based catalysts in the FC hampered their further development for its high cost and scarcity. Direct methanol fuel cell (DMFC) as another kind of proton exchange membrane fuel cell (PEMFC) has been manifested that Pd also performs a certain activity for methanol oxidation reaction (MOR). To better know the mechanism of MOR, we present a DFT study on the first step reaction of MOR on the Pd(111). The results show that methanol prefers to physically adsorb on the Pd(111) through oxygen atom, while the dehydrogenated hydrogen atoms can adsorb either on face-cantered cubic (FCC) or hexagonal close packed (HCP) sites. The intermediate products will form a much stronger interaction with the Pd(111) since they contain more unsaturated bonds. The energy barrier of O-H bond scission is most favourable, while the C-O bond is unfavourable in the first step of MOR.


Introduction
With the increasing severity of environmental pollution and global energy crisis, fuel cells as "green" energy conversion systems have attracted more and more attentions. [1,2] FC can convert chemical energy directly into electric energy, and the products contain no pollution products. DMFC can also offer remarkably higher energy utilization efficiency but emits only CO 2 . In addition, methanol can be synthesized by decomposing biomass fuel, which has abundant sources and lower energy consumption. However, the sluggish methanol oxidation kinetics is still an important issue, since the MOR is a complex reaction containing 6 electrons transference.
A better understanding of the reaction mechanism of MOR is of great significance to improve the reaction kinetics. Experimentally, research on the mechanism of MOR on the surface of transition metals was mainly focused on the single crystal surface under ultra-high vacuum conditions. And modern surface instruments have been used to study the exact broken bonds during MOR. Levis [3] observed the presence of methyl group on Pd(111) surface with x-ray photoelectron spectroscopy (XPS). Guo [4] did not observe the broken C-O bond with isotope method. While Kruse [5] found both CH 3 and CH 3 O on the Pd(111) at the same time. Many scientists believe that CH x O (x=1-3) is produced only by O-H bond breaking during methanol decomposition on Pd surface. However, Chen [6] considered that the dissociation of methanol is not only the above two processes, but may be carried out simultaneously or affected by the surface coverage. Theoretically, Hu [7] found that the O-H bond breaking was much easier. Though the study on the mechanism of MOR has attracted much attention, the exact broken bond in the first step of MOR is still controversial. Thus, the adsorption of methanol, co-adsorption of its intermediate products, and the reaction energy barrier of the first step in MOR have been studied in this paper.

Calculation model and method
The PW91 method based on the generalized gradient approximation is used to calculate the exchange correlation energy. And the transition state is searched with linear synchronous transit (LST) method. A 2×2 three-layer Pd(111) is chosen as our slab model, and a vacuum of 11.23 Å which is 5 times of Pd layer spacing is built to create a two-dimensional Pd(111) surface. The lower two layers are fixed at their optimized positions while the top atomic layer is thoroughly relaxed in all calculations. A Fermi surface smearing of 0.01 a.u. is utilized to speed up the convergence of k-point sampling. K point of the first brillouin zone is set to 4×4×1. The adsorption energy is calculated according to the formula 1.
E ads is the adsorption energy, E complex is the total energy of the system after species adsorption, E Pd is the energy of clean Pd(111) surface, E A is the energy of adsorbed species.

Results and discussion
Adsorption is always the beginning step in every catalytic reaction. Herein, the adsorption of methanol and its intermediate products on Pd(111) are studied. And all the adsorptions at high symmetry sites on Pd(111) as shown in figure 1 are considered.   The adsorption energy of methanol on Pd(111) is also calculated. As listed in Table 2

Hydrogen atom adsorption
H atom is the main product of methanol dissociation. The adsorption of H atom at four high symmetry positions on Pd(111) surface is calculated in this section, and the results are listed in table 3.
It can be found that H atom mainly adsorbs at HCP and FCC sites, and the H atom adsorbed at the bridge site will spontaneously move to the adjacent FCC sites during the optimization process. The adsorption energy of top H atom is -227.75 kJ/mol, and the Pd-H distance is 1.529 Å. It is about 40 kJ/mol lower than that at both FCC and HCP sites. The average distance of Pd-H distance at the FCC site is 0.932 Å, which is 0.012 Å than that at HCP site. The covalent radii of hydrogen and Pd atom are 0.37 Å and 1.23 Å, respectively. However, it can be found that the Pd-H distances become even shorter than the covalent radii of Pd atoms. The significant shortened distance between H and Pd indicates that H and Pd have strong covalent interaction and form multiple hydrogen bonds, which should be the main reason for the energy reduction. Thus, the hydrogen atom is put on the FCC site when we perform the calculation of transition state search.

Adsorption of other intermediate products
The Compared with that in methanol, the oxygen in the methoxy has a larger electron density due to the lack of a hydrogen atom, which leads to a stronger interaction between methoxy and the d electrons of Pd. This is also the reason why the interaction energy of CH 3 O with Pd is larger at the triple vacancy. For the adsorption of CH 2 OH through C-H bond scission. The adsorption of CH 2 OH on the Pd(111) with the C-O axis parallel to the Pd(111) surface. The adsorption energy is -177.62 kJ/mol, which is about 20 kJ/mol more stable than that of methoxy.
For the adsorption of CH 3 and OH through C-O bond scission. CH 3 prefers to adsorb on the top of Pd with an adsorption energy of -179.61 kJ/mol, and the distance between C and Pd is 2.07 Å. OH prefers to adsorb on the bridge site of Pd(111), and the distance of O-Pd spacing is 2.15 Å.

Reaction energy barrier
The