Adsorption Properties of O 2 on Cr 5 Nanostructures : A DFT Study

Based on the density functional theory (DFT) adsorption properties of oxygen molecule on Cr5 nanostructures at various positions (top, bridge and central sites) were investigated. Geometry optimizations and vibrational frequencies of them were carried out at B3LYP level of theory with LANL2DZ and 6-31G basis sets using Gaussian 98 suites of program. Their thermodynamic properties were estimated. We have also investigated the total electronic properties, HOMO, LUMO and energy gap for Cr5 nanostructures with O2.The result showed that the adsorption of O2 on Cr5 nanocluster in horizontal bridge site is the most stable state of adsorption and its binding energy is strong, -145.88 kcal/mol.


Introduction
Oxide transition metal clusters and nanocomponents have attracted significant attention due to their important role as components of model-catalysts [1][2][3][4][5][6] , sensors 7 , solar-cells 8 and magnetic recording devices 9 .The adsorption of oxygen on transition metal surface has been studied using a wide range of techniques and methods because of its interest for both basic and applied problems in surface science in the past several decades 10 .Wen. Kaichen et al. 11 explained the results of O 2 adsorption and dissociation on CuCr 2 O 4 surface and Masahiro Katoh et al. 12 studied adsorption properties of gases on chromium silicate 12 .
In this work, we report the most favorable site of adsorption of oxygen on Cr 5 nanostructures and the vibrational frequencies, thermodynamic changes, HOMO, LUMO and the electronic properties of Cr 5 by O 2 adsorption.

Computational details
All calculations have been carried out by using the Gaussian 98 suite of programs 13 .The density functional theoretical method with the B 3 LYP functional was used for all the calculations.Cr 5 nanostructures is considered as a doublet molecules with partitioning of the basis sets, for O 2 molecule described by the standard 6-31G * basis set, but for chromium the standard LANL2DZ basis set was used.Adsorptions energies, the vibrational frequencies, thermodynamic properties, HOMO, LUMO and density of state (DOS) have been carried out at B 3 LYP level of theory with LANL2DZ and 6-31G * basis sets.

DFT calculations on adsorption of O 2 molecule on Cr 5 nanocluster
Chromium has the body -centered cubic structure with a lattice constant of 2.88Å and every metal layer contains five Cr atoms as shown in Figure 1a.The interaction of the O 2 molecule with the surface of Cr 5 nanocluster was studied by performing single point energy (SPE) calculation for several orientations on the molecule.Three possible configurations, named top, bridge and center site with vertical and horizontal orientations, were selected for the approach of O 2 molecule to the nanocluster: (Top site) directly on top of a Cr atom (Figure 1b and 1c), (Bridge site) on the middle of two nearest neighbor Cr atom (Figure 1d and 1e) and (Center site) in the center of the smallest unit structures of the surface (Figure 1f and 1g).The adsorption energy (E adsorption ) of one O 2 molecule on the Cr 5 nanocluster can be calculated as follows: (1) In equation (1), the first term is the SPE of the O 2 /Cr 5 surface super molecule, E Cr5 is the SPE of separated fragment and E O2 is the energy of an optimized O 2 molecule.The adsorption energies in the three cases are plotted in Figure 2 and the adsorption energy with the equilibrium distance in each case is summarized in Table 1.The calculations showed that the obtained adsorption energies depend on orientations and locations of O 2 molecule.The most stable configuration of O 2 is horizontal bridge site; the current calculation showed that the adsorption energy for horizontal bridge site is -111.85kcal/mol.In the next step, the adsorption energies in the equilibrium distance for the three cases were calculated with the increase of distance between oxygen atoms (Figure 3).In this step, r(O=O) for the most stable state in all of the cases is 1.41 Å (Figure 3), then with r(O=O) =1.41 Å, the adsorption energies for all of stats were calculated by increasing the distance between O 2 and Cr 5 surface (r d ).The calculations showed that the adsorption of O 2 on Cr 5 nanocluster in horizontal bridge site is the most stable state of adsorption and its binding energy is strong, -145.88 kcal/mol in r d = 1.25 (Figure 4 and Table 2).

Vibration frequencies
The vibrational frequencies have been calculated at the B 3 LYP level of theory with LANL2DZ and 6-31G * basis sets.Vibrational frequencies are used to simulate the IR spectra (Figure 5) for all of the states.Frequency, cm -1 Frequency, cm -1 Frequency, cm -1 Frequency, cm -1 Frequency, cm -1 Frequency, cm -1 The simulated IR spectra could be used as evidence to identify the Cr/O 2 nanocluster.Here we discuss several vibrational bands in the IR intensities of the all of the configurations to be compared with future experimental identification; (1)

Thermodynamic changes
In this study thermodynamic changes for all of the configurations were estimated using total energies obtained by the B 3 LYP level of theory with LANL2DZ and 6-31G * basis sets.The thermodynamic properties were calculated from the standard statistical mechanical equations which include the finite-temperature translational, rotational and vibrational energies 14 .For example, the enthalpy (H) was calculated as follows: H = E elec + E vib (T=0) + E vib (T) + E rot (T) + E tra (T) + PV (1) Where E elec is the total electronic energy at 0 K, E vib (T = 0) is the zero-point vibrational energy which is a linear sum of the basic harmonic frequencies, and E vib (T), E rot (T) and E tra (T) are vibrational, rotational and translational contributions, respectively.Also, the entropy (S) of molecules can also be calculated from a standard formula: S = S elec + S vib + S rot + S tra (2) Where, S elec , S vib , S rot and S tra are the electronic, vibrational, rotational and translational terms.The change in the standard Gibbs free energy is given by ∆G = ∆H -T∆S (3) The thermodynamic properties with the equilibrium distance (Table 2) in each case were calculated.For this cases ∆H, ∆S and ∆G were calculated at the B 3 LYP level of theory with LANL2DZ and 6-31G * basis sets and ∆H of O 2 on Cr 5 nanocluster in horizontal bridge site is more than another cases (Table 3).Also ∆H<0 show that this process is exothermic.The effects of oxygen molecule on adsorption energies in Cr 5 are related to their electronic structure.When oxygen molecule is adsorbed on the Cr 5 , the interaction between them is strong, their electronic properties changes obviously.It is clear that the presence of O 2 molecule increases the energy-gap of Cr 5 nanocluster (Table 4) and reduces its electronic conductance.The TDOS for the (O 2 /Cr) systems are presented in Figure 6.For these systems, significant changes in the DOS are observed near the Fermi level.However, the adsorption of O 2 molecule in horizontal top site further increases the energy gap of Cr 5 nanocluster (-5.63 eV).

Conclusion
We have theoretically studied the adsorptions of oxygen molecule on Cr 5 nanocluster through the DFT calculations.According to our calculations, it seems that the Cr 5 can be used as an oxygen storage medium as long as oxygen is adsorbed on the surface of Cr 5 because of the strong binding energy.The vibrational frequencies have been calculated at the B 3 LYP level of theory with LANL2DZ and 6-31G * basis sets.The thermodynamic properties for all of the configurations are calculated and ∆H of O 2 on Cr 5 nanocluster in horizontal bridge site is more than another the configurations and the process are exothermic.In this study the presence of O 2 molecule increases the energy-gap of Cr 5 nanocluster and reduces its electronic conductance.

Figure 1 .
Figure 1.Atomistic configurations of adsorption of O 2 molecule with vertical and horizontal orientations in top, bridge and center sites

Figure 2 .
Figure 2. Interaction energy curves of an O 2 molecule adsorbed on the Cr 5 nanocluster, obtained by the SPE calculation in which the O=O and all the Cr-Cr bond lengths are kept constant and only the distance between the oxygen molecule and the Cr 5 surface is varied (rd).

Figure 3 .
Figure 3. Interaction energy curves of an O 2 molecule adsorbed on the Cr 5 nanocluster, obtained by the SPE calculation, the O 2 bond length r(O=O) is varied and the equilibrium distance between the oxygen molecule and the Cr 5 surface is kept constant.In this step, r(O=O) for the most stable state in all of the cases is 1.41 Å (Figure3), then with r(O=O) =1.41 Å, the adsorption energies for all of stats were calculated by increasing the distance between O 2 and Cr 5 surface (r d ).The calculations showed that the adsorption of O 2 on Cr 5 nanocluster in horizontal bridge site is the most stable state of adsorption and its binding energy is strong, -145.88 kcal/mol in r d = 1.25 (Figure4and Table2).

Figure 4 .
Figure 4. Interaction energy curves of an O 2 molecule adsorbed on the Cr 5 nanocluster, obtained by the SPE calculation, the O 2 bond length, r(O=O) is 1.41 Å and only the distance between the oxygen molecule and the Cr 5 surface is varied (rd).
Figure 5 simulated IR spectra for the configurations of adsorption of O 2 molecule with vertical and horizontal orientations in top, bridge and center sites.IR spectrum scaled by 1.000000.

Figure 5 .
Figure 5. Simulated IR spectra for the configurations of adsorption of O 2 molecule with vertical and horizontal orientations in top, bridge and center sites IR spectrum scaled by 1.000000

Table 1 .
Adsorption energy (kcal/mol) and equilibrium distance (Å) of an O 2 molecule on the Cr 5 surface (O=O band length is 1.21 Å).
for vertical top site: the stronger IR spectroscopic signals with IR intensities 84.55 and 72.49 are determined at the frequencies 485.65 and 952.57cm -1 respectively, (2) for horizontal top site: two strong modes are observed at the frequencies 1064.08 and 702.99 cm -1 with IR intensities of 63.72 and 139.89 km mol -1 respectively, (3) for vertical bridge: four strong modes are observed at the frequencies 105.88, 326.55, 622.64 and 1023.72 cm -1 with IR intensities of 42.11, 48.33, 60.94 and 129.37 km mol -1 respectively, (4) for horizontal bridge site: five strong modes are observed at the frequencies 242.35, 286.26, 353.29, 489.60 and 896.48 cm -1 with IR intensities of 20.32, 50.52, 33.93, 47.73 and 22.48 km mol -1 respectively, (5) for vertical center site: three strong modes are observed at the frequencies 204.19, 247.67 and 1065.46 cm -1 with IR intensities of 39.17, 31.08 and 92.01 km mol -1 respectively, (6) for horizontal center site: five strong modes are observed at the frequencies 74.10, 246.22,912.56, 982.56 and 1101.59 with IR intensities of 13.01, 37.56, 12.32, 31.64 and 13.48 km mol -1 respectively.

Table 3 .
The ∆H, ∆S and ∆G in kcal/mole for all of the configurations in B 3 LYP level of theory with LANL2DZ and 6-31G * basis sets.

Table 4 .
The HOMO and LUMO energies and their energy gaps for the most favorable states for adsorption of the configurations