First Principles Study on the Adsorption and Dehydrogenation of Borohydride on Mn ( 111 )

The mechanism of adsorption and dehydrogenation of borohydride (BH4) on Mn(111) is explored through first principles calculations within Density Functional Theory (DFT). It is found that the preferred sites for adsorption are the bridge site wherein the adsorbate dissociates resulting to BH2,ads+2Hads (“ads” means in the adsorbed state on the surface) fragments characterized by the competing dzz and dxz,yz interactions of the Mn-d states of the surface with the H-s and B-p states of the adsorbate, and the fcc hollow site wherein the adsorption is molecular. Water molecule is formed when a hydroxyl radical bonds with hydrogen atom on top of an initially adsorbed borohydride. It passes through a metastable state, then an intermediate state and finally the most stable state. [DOI: 10.1380/ejssnt.2011.257]


I. INTRODUCTION
There has been an increasing interest in the research and development of fuel cells as alternative means of producing energy.This is necessitated by the rapid depletion of our primary energy sources which are most often nonrenewable.An ideal alternative source must be highly renewable, environmental friendly, relatively inexpensive, easily accessible, and easy to manufacture [1].Hydrogen has been foreseen as a viable source of alternative energy through fuel cells [2][3][4][5][6][7].Improvements were attained through the use of different fuels such as methanol, ethanol, hydrocarbons, and hydrazine [8].
Another kind of fuel cell that is recently gaining more research focus is Direct Borohydride Fuel Cell (DBFC).It has the potential to generate high power densities competitive to Direct Methanol Fuel Cell (DMFC) for portable power applications [9].DBFC is an alkaline fuel cell that operates using a base-stabilized aqueous borohydride solution (such as NaBH 4 ) as the anode fuel and oxygen at the cathode.Over an electrocatalyst selective to direct oxidation, each borohydride molecule is capable of producing eight electrons compared to six electrons per methanol molecule [10] in DMFC via the suggested overall reaction: However, the efficiency and power density of DBFCs are limited in part by the lack of an effective anode electrocatalyst [11][12][13].Gold [14] and silver [15] are capable of attaining near 100% coulombic efficiency but high overpotentials are required to achieve an appreciable rate.Platinum [16][17][18], palladium [18] and nickel [18,19], on the other hand, can generate higher current densities at lower overpotentials, but generates non-selective hydrolysis reaction: This competing reaction limits the overall coulombic efficiency of DBFC while hydrogen gas production leads to undesirable bubble formation that can cause mechanical failure and has negative effects on the reaction active area [20].
To address these problems, an optimal anode catalyst must promote strong molecular adsorption to produce high coverage of reactive surface species, indicative of high catalytic activity [21].A stable molecularly adsorbed state was found for borohydride adsorption on Au(111) surface with three H atoms on atop sites [22].However, low adsorption energy produces low coverage of reactive surface species which leads to low catalytic activity.For the case of Pt(111) surface, the borohydride molecule dissociates resulting to BH ads and 3H ads fragments [23].Strong dissociative adsorption on this surface produces a high surface coverage of hydrogen which results to molecular hydrogen evolution that competes with the oxidation process.
There have been a number of theoretical studies on the possibility of using 3d transition metals [3,8,24] as anode catalysts for different fuel cells because of their low cost.As an initial step of looking at the possibility of using manganese as a non-precious anode material for DBFC, we explored the mechanism of adsorption and dehydrogenation of borohydride molecule on Mn(111) through first principles calculations within Density Functional Theory (DFT).Potential energy curve (PEC) and adsorption energies were calculated to compare the relative stability of the different orientations of BH 4 on the high symmetry sites of the surface.Since it was shown in the literature that the initial adsorption structure of borohydride determines the subsequent reaction mechanisms [21][22][23], this paper focuses on its gas-phase adsorption, dehydrogenation, water formation, and the role the metal d-states before proceeding to the analysis of fuel cell reactions.Further, this would allow a comprehensive ISSN 1348-0391 c ⃝ 2011 The Surface Science Society of Japan (http://www.sssj.org/ejssnt)

II. COMPUTATIONAL METHOD
The Mn (111) surface was modeled using a four-layer slab in a (2×2) unit cell with the top two layers allowed to relax and the bottom two layers constrained to the bulk face-centered-cubic (fcc) lattice positions during the initial structural optimization, providing a 0.25 ML adsorbates coverage.Calculation of surface energies established the stability of this surface over Mn(001) surface.Each slab is separated by ∼15.0 Å of vacuum, which is large enough to avoid surface atom interaction along z axis with the neighboring unit cells.The electric dipole correction layer in the vacuum area was used to cut the dipole interactions between the repeated image layer systems.The calculated lattice parameter for fcc Mn is 3.50 Å which is in excellent agreement with the results from the tightbinding parametrization method (3.30Å to 3.57 Å) [25].
Spin polarized total energy calculations were performed using density functional theory (DFT) by utilizing the Vienna ab initio simulation program (VASP) [26][27][28][29].The interaction between ions and electrons were described using projector augmented wave (PAW) method [30,31].Plane wave basis sets were employed with energy cut-off of 400 eV.The exchange-correlation term was expressed using generalized gradient approximation (GGA) based on Perdew-Burke-Ernzerhof (PBE) [32,33] functional.The surface Brillouin zone integrations were performed on a grid of (6×6×1) Monkhorst-Pack k-points [34] using Methfessel-Paxton smearing [35] of σ = 0.2 eV.A conjugate-gradient algorithm was used to relax the ions into their ground state.Convergence of numerical results with respect to the slab thickness, the kinetic energy cutoff and the k-point set was established.

III. RESULTS AND DISCUSSIONS
Mirkin et al. [36] have proposed a reaction mechanism for the initial steps of electrooxidation over Au catalyst using cyclic voltammetry.They proposed an electrochemical reaction as an initial step for the oxidation process: where "ads" means in the adsorbed state on the surface.The initial adsorption occurs with electron transfer followed by a chemical step to break the B-H bond with subsequent transfer of electron.By virtue of the proposed initial reaction step, the adsorption of borohydride anion can be accompanied by a simultaneous transfer of electron to the electrode.Thus, the borohydride adsorption on the metal surface can be modeled in an overall neutral unit cell.

A. Adsorption structure
A potential energy curve (PEC) was plotted for the high symmetry sites (top, bridge, fcc hollow site and hcp hollow site) on Mn(111) and two orientations of BH 4 indicated by Up and Down as shown in Fig. 1.We repeatedly calculated the total energy of the neutral adsorbate-substrate system while changing the height (z) of boron from the surface.In these calculations, the slab was kept frozen at its optimized structure and the coordinates of boron were fixed while hydrogen atoms were allowed to relax.
After finding the stable configuration of the adsorbate on the surface based on the PEC, the total energy was then calculated for a system with relaxed boron and hydrogen atoms and relaxed top two layers of the Mn slab to compute the adsorption energy using the equation: Here, E BH4/Mn , E BH4 and E Mn are the total energy of the BH 4 -Mn system, free BH 4 and Mn slab, respectively.Figure 2 shows the PEC with respect to the total energy of the system when the adsorbate is sufficiently far from the surface for the different surface symmetry sites and adsorbate orientations.The adsorbate enters the surface with tetrahedral configuration and H-B bond length of 1.24 Å.Based on the PEC, the Up orientations are generally more stable than the Down orientations with barrierless adsorption for the top and hollow sites from z approximately equal to 4.0 Å.The minimum energy is at the fcc hollow site with adsorption energy of −4.09 eV.This is so much larger than the calculated adsorption energy (−1.73 eV [22]) for molecular adsorption on Au(111) and smaller compared to dissociative adsorption (adsorphttp://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology  tion energy of −4.73 eV) on Pt(111) [23].
Table I shows the adsorption energies calculated using Eq. ( 4).The term "fixed" means that the slab was kept frozen at its optimized structure, the position of boron was held fixed and only the hydrogen atoms were allowed to relax."Relaxed" means that the top two layers of the slab and borohydride were allowed to relax.Based on this table, the adsorption energies for fcc hollow site and bridge site are almost the same with a difference of only 0.03 eV (for relaxed condition).However, analysis of the adsorption structure shows that borohydride is adsorbed molecularly on the fcc hollow site (fcc-adsorption) and dissociatively on the bridge site (bridge-adsorption) with the same minimum distance of 1.56 Å from the surface.
As shown in Fig. 3 tion were mapped on the plane parallel to the surface of the slab with a corresponding contour spacing of 0.01 electrons/ Å3 when borohydride is sufficiently far from the slab (Fig. 4(a)) and absorbed (Fig. 4(b)).A distinct localization of electron charge distribution around H atoms is noted which clarifies the dissociation of these atoms.Bader analysis [37] shows that the charge transferred to BH 4,abs on the bridge site is greater than for the fcc hollow site by approximately 0.5e − .This large difference of charge transferred from the surface to the molecule is indicative of preference to greater structural modification for the bridge site compared to fcc hollow site.
The dissociated surface H atoms may combine to form http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) hydrogen gas and desorb from the surface by the reaction: or combine with OH − ions to form H 2 O molecules and release electrons by the reaction: Both of these reactions become more favorable as the surface coverage of adsorbed hydrogen (H abs ) increases because the repulsive interactions between adsorbates become stronger [21].However, since Eq. ( 5) is second order in H ads and Eq. ( 6) is first order, higher surface coverage of H ads will show greater preference towards hydrogen evolution.Since it was shown that borohydride molecule dissociates into BH ads and 3H ads fragments on platinum [23] and BH 2,ads +2H ads species on manganese, we expect lesser surface coverage of adsorbed hydrogen on latter than on the former.Therefore, hydrogen evolution on manganese is expected to be less favorable than on platinum.For fcc-adsorption, it is noted that the bond lengths of hydrogen atoms on the surface with boron are stretched by 0.06 Å with the three H atoms on the top sites.This stable molecular adsorption is preferred over dissociation since the competing hydrolysis reaction limits the overall oxidation process on the surface.The high adsorption energy for the fcc hollow site is indicative of a high surface coverage of reactive surface species which leads to high catalytic activity and low activity towards hydrolysis of BH 4 .The possible hydrogen gas evolution due to the dissociation of adsorbate on the bridge site may be countered by the stronger molecular adsorption on the fcc hollow site.

B. Adsorption mechanism
To understand the mechanism of adsorption of borohydride on the surface and clarify the noted difference in adsorption structures, we analyzed the local density of states of the adsorbate and the surface.Figure 5(a) shows the DOS of BH 4 as an isolated molecule and adsorbed at the bridge and fcc hollow sites.The adsorption of BH 4 on the Mn surface modified its DOS considerably especially on peaks near the Fermi level.All peaks were decreased and shifted down away from the Fermi level.The broadening of the states for the bridge-adsorption is more pronounced than for fcc-adsorption.
For the bridge-adsorption, the LDOS projected on the d-states of two distinct Mn atoms (Mn1 and Mn2 shown in Fig. 3 The LDOS projected on the d-orbital of Mn2 (not shown) shows the same bonding state.This new peak is in resonance with the peak denoted by asterisk (*) in Fig. 5(a).These are the states significant to the dissociated structure of the adsorbate on the surface since Mn2 is the atom directly bonded to the dissociated hydrogen atoms.Detailed analysis of the LDOS shown in Fig. 5(c) indicates the significant contribution of the d xz and d yz states to the BH 4 -Mn bonding.As expected, analysis of the electronic states of the adsorbate shows the strong interaction of the dissociated hydrogen atoms labeled H1 and H3 in Fig. 3(a 5(d).We note a decrease in the intensity of d zz states at the Fermi level for both the spin up and spin down components compared to the clean surface.The resonant peak in the DOS of the adsorbate is attributed mainly to the s orbitals of the hydrogen atoms on the surface and p x and p y orbitals of boron.This shows that the molecular adsorption of the adsorbate on the fcc hollow site is characterized only by the interaction of the d zz state of the surface Mn atoms with the adsorbate.
It is noted that for both the bridge and fcc-adsorption, the bonding is determined mainly by the hydrogen atoms : The total energy of the system was calculated for different distances (z) of FIG.6: The total energy of the system was calculated for different distances (z) of hydroxyl from the surface when it is oriented on top of the hydrogen atom of borohydride farthest from the surface.This was done for borohydride initially adsorbed at the (a) bridge site and (b) fcc hollow site.
near the surface with little contribution from boron.In addition, the hydrogen atom on top of the adsorbate (H4 in Fig. 3) does not play a significant role for both the bridge and fcc-adsorption.From these, the adsorption structure of BH 4,ads on the bridge and fcc hollow sites can be understood.The dissociative adsorption of the adsorbate on the bridge site is characterized by the competing d zz and d xz,yz interactions of the Mn-d states of the surface with the H-s and B-p states of the adsorbate.On the other hand, only the d zz state of manganese play a major role in bonding with the surface hydrogen atoms of the adsorbate for the molecular adsorption on the fcc hollow site.

C. Borohydride dehydrogenation and water formation
We then considered the interaction of the adsorbed borohydride molecule with hydroxyl radical OH.It was shown in the literature that the dehydrogenation process for gold may be described by the following electrochemical reaction [36]: We adapted this electrochemical step for the dehydrogenation of borohydride.As before, since this reaction can be accompanied by a simultaneous transfer of electron to the electrode, we simulated this process in an overall neutral unit cell.Thus, the reaction was simulated in the following discussion.
The adsorption energy of hydroxyl radical was calculated and found to be 0.05 eV greater than for borohydride.This small difference in the adsorption energy indicates small possibility for surface blocking of hydroxyl radicals for the adsorption of borohydride.The O-H bond length (0.97 Å) is perpendicular to the surface with O atom closer to the surface at the hcp hollow site.
Different orientations and arrangements of borohydride and hydroxyl molecules were explored for the initial relaxation calculations.It was found that water molecule is formed when the hydroxyl radical bonds with hydrogen   atom on top of the initially adsorbed borohydride.We constructed a PEC for this reaction by calculating the total energy of the system for different distances (z) of hydroxyl from the surface as shown in figure 6.In these calculations, all hydrogen atoms were allowed to relax while B, O, and the slab were all fixed in their positions.This was done for the bridge site (Fig. 6(a)) and fcc hollow site (Fig. 6(b)).
The calculated PEC with inset figures that show the initial and final structures of the system, is shown in Fig. 7.The hydroxyl radical is in the isolated state from the Mn-BH 4,ads complex for z ≥∼4.3 Å for the bridge site and z ≥∼4.6 Å for the fcc hollow site.The sharp decrease in the potential energy at z ≈ 4.6 Å for fcc hollow site and z ≈ 4.3 Å for the bridge site corresponds to the protonation of hydroxyl for the formation of BH 3,ads +H 2 O complex.For both cases, there is an activation barrier of E a1 = 0.07 eV for the transition from BH The total charge contour plot with a corresponding contour spa FIG.8: The total charge contour plot with a corresponding contour spacing of 0.01 electrons/ Å3 for the system at the minimum energy for the fcc hollow site.The outermost line of the charge contour plot can be interpreted as the hydrogen bond which stabilizes the Mn-(BH 3,ads ) complex.
of E a2 = 0.17 eV, and approaches the energy minimum at z = 4.1 Å.The bond distances and inner angles of the water molecule and B-O distance for these states are listed in Table II.H5 and H6 are respectively, the hydrogen atoms above and below oxygen.The water molecule has approximately a linear structure at the intermediate state and bent structure at the energy minimum.
Figure 8 shows the total charge contour plot with a corresponding contour spacing of 0.01 electrons/ Å3 for the system at the minimum energy for the fcc hollow site.Though it is very difficult to discriminate a chemical bond and a hydrogen bond by the total charge contours, it can said that O and B interact with each other through H6 since it is the atom between them.Thus, the outermost line of the charge contour plot can be interpreted as the hydrogen bond which stabilizes the Mn-(BH 3,ads ) complex.
Through these results, we were able to understand the geometry of the reaction complexes of borohydride dehydrogenation and potential energy for hydroxyl radical motion on Mn(111) surface.It is hoped that these findings will stimulate further theoretical and experimental investigations of this interesting and important system.http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Volume 9 (2011) Further studies that involve the calculation of free energies, the influence of the solvent, and the treatment of the electrode potential shall be done in the next steps of the research to address the electro-catalytic reactions on the Mn(111) surface.

IV. CONCLUSION
We have shown a density functional theory study on the adsorption of gas-phase borohydride and its dehydrogenation on Mn(111).It was found that the preferred sites for adsorption are the bridge and fcc hollow sites with adsorption energy difference of only 0.03 eV.On the former, the adsorbate dissociates resulting to BH 2,ads +2H ads fragments characterized by the competing d zz and d xz,yz interactions of the Mn-d states of the surface with the H-s and B-p states of the adsorbate.For the fcc hollow site, the adsorption is molecular and the major role of the d zz state of manganese in the adsorbate-surface bonding was noted.Water molecule is formed when a hydroxyl radical bonds with hydrogen atom on top of an initially adsorbed borohydride.It passes through a metastable state, then an intermediate state and finally the energy minimum.
It is interesting to note that the equally strong molecular and dissociative adsorption structures of the adsorbate on Mn(111) is halfway between the weak molecular adsorption on Au(111) and strong dissociative adsorption on Pt(111) as shown in the literature.Compared to gold, the stronger molecular adsorption of the adsorbate on manganese avoids establishing a high surface coverage of adsorbed hydrogen atoms that promotes H 2 evolution and enables oxidation activity at lower potentials.Also, since the dissociation of borohydride results to only 2H ads fragments on manganese compared to 3H ads fragments on platinum, we expect lesser surface coverage of adsorbed hydrogen on former than on the latter.Therefore, hydrogen evolution on manganese is expected to be less favorable than on platinum.

FIG. 1 :
FIG. 1: (a) The four symmetry sites on the surface are the top, bridge, hcp hollow and fcc hollow sites.The unit cell is indicated by the dashed lines.(b) We examined the two orientations of BH4 indicated by Up (left) and Down (right) orientations.The green ball represents boron while the pink and fuchsia represent hydrogen and manganese atoms respectively.Height (z) is measured as the length between boron and the surface.

FIG. 2 :
FIG. 2: Potential energy curve for the Up orientations with respect to the sum of the energies of free BH4 and Mn slab for the different surface symmetry sites and orientations.The Up orientations are generally more stable than the Down orientations (not shown in the figure) with barrierless adsorption for the top and hollow sites.The minimum energy is at the fcc hollow site with adsorption energy of −4.09 eV.

FIG. 3 :
FIG. 3: The adsorption is dissociative on the bridge site (a and b) and molecular on the fcc hollow site (c).The dissociation of the borohydride on the bridge site results to BH 2,ads + 2H ads species with maximum B-H distance of 1.94 Å from the initial 1.24 Å in the isolated molecule.The dissociated hydrogen atoms (H1 and H3) reside on the fcc and hcp hollow sites.BH 2,ads forms a bent structure on the plane of the top sites with inner angle equal to 102 • and bond lengths of 1.19 Å and 1.30 Å.For fcc-adsorption, the bond lengths of hydrogen atoms on the surface with boron are stretched by 0.06 Å with the three H atoms on the top sites.

FIG. 4 :
FIG. 4: Contour plots for charge density distribution on the plane parallel to the surface of the slab with a corresponding contour spacing of 0.01 electrons/ Å3 when the adsorbate is (a) sufficiently far from the slab and (b) adsorbed on the bridge site.A distinct localization of electron charge distribution around H atoms is noted which clarifies the dissociation of these atoms from the adsorbate.
(a)) on the surface were analyzed.The LDOS projected on the d zz states of Mn1 (Fig. 5(b)) shows that new peaks appear just below the bottom of the d zz -band as compared to the clean surface.These new peaks are in resonance with the peaks denoted by asterisks (**) shown in Fig. 5(a), which form the bonding state.Analysis of the LDOS projected on the orbitals of boron and hydrogen atoms of the adsorbate shows that this peak is due mainly on the s orbitals of hydrogen atoms on the surface and p y orbital of boron.This indicates the significant role of the s-d zz and p y -d zz hybridizations of the surface H-s, B-p and Mn-d states.

FIG. 5 :
FIG. 5: (a) DOS of the adsorbate as a free molecule and adsorbed at the bridge and fcc hollow sites.(b) LDOS projected on the dzz-state of Mn1 for bridge-adsorption.(c) LDOS projected on dxz and dyz states of Mn2 for bridge-adsorption.(d) LDOS projected on dzz, dxz and dxx−yy states of Mn directly bonded to the hydrogen atoms for fcc-adsorption.

d
xz and d xx−yy states as shown in Fig.

FIG. 7 :
FIG. 7: The calculated potential energy curve with inset figures that show the initial and final structures of the system.The sharp decrease in the potential energy at z ≈ 4.6 Å for fcc hollow site and z ≈ 4.3 A for the bridge site corresponds to the protonation of hydroxyl for the formation of BH 3,ads + H2O complex.

TABLE I :
The adsorption energies for different symmetry sites and orientations.

TABLE II :
Bond distances and angles of water molecule in the metastable state, intermediate state, and minimum energy.
4,ads +OH to BH 3,ads +H 2 O state.For the fcc hollow site, the water molecule passes through a metastable state at z = 4.4 Å, then reaches an intermediate state at z = 4.2 Å with activation energy