A DFT STUDY OF HYDROGEN DISSOCIATION

Доклади на Българската академия на науките Comptes rendus de l’Acad´ emie bulgare des Sciences Tome 67, No 6, 2014

CHIMIE Cin´etique et catalyse

A DFT STUDY OF HYDROGEN DISSOCIATION OVER MoP (001) PLANE Sharif F. Zaman, Mohammad Daous, Lachezar Petrov∗ (Submitted on January 8, 2014)

Abstract Density functional theory was employed to investigate the H2 adsorption and dissociation energy over MoP (001) surface. Hydrogen atom preferred the fcc adsorption site with a binding energy of −72.41 kcal mol−1 whereas hydrogen molecule adsorbed over an Mo atom on top arrangement, showed lower binding energy, −16.8 kcal mol−1 , compared to atomic adsorption. H2 dissociation over MoP (001) plane was found not to be catalytically activated. The dissociation activation barrier is 113.4 kcal/mol, which is close to the gas phase molecular H2 dissociation. Key words: H2 dissociation, activation energy, MoP, DFT

Introduction. The important areas of petrochemical industry are catalytic hydrogenation- dehydrogenation processes for production of fine chemicals, intermediates for pharmaceutical industry, monomers for various polymers, etc. Selective, catalytic hydrogenation of functional groups contained in organic molecules is one of most useful, versatile and environmentally acceptable reaction routes available for organic synthesis. Hydrogenation catalytic processes are used to produce about 20 % of the fine chemicals and pharmaceutical products. A wide variety of catalysts are used in these reactions like supported noble metals Pt, Pd, Rh, supported transition metals Ni, Co, Fe, Cu, Mo, oxide catalysts Cr2 O3 , Fe2 O3 , Al2 O3 -Cr2 O3 , sulfides catalysts MoS2 /Al2 O3 , WS2 /Al2 O3 , NiS/Al2 O3 , CoS/Al2 O3 , and others. The reaction mechanisms of those processes are multistep and complex. Most of the noble metals, i.e. Pt, Ru, Rh, Pd show This work was supported by the Deanship of Scientific Research of King Abdulaziz University, Jeddah, Saudi Arabia under grant No (D-005/431).

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extraordinary activity and selectivity. In search for less expensive and more selective catalysts, molybdenum based catalysts, especially MoS2 , Mo2 C, MoN2 , and MoP are of recent interest. The hydrogen is a key reagent in hydrogenation-dehydrogenation processes and the participation of hydrogen in different elementary steps of reaction mechanism might have a strong impact on the proceeding of the given reaction. Thus, to reveal the reaction mechanism and to find way of improving the performance of the particular catalyst in a given reaction, it is necessary to study adsorption of hydrogen over the catalyst surface. MoP has demonstrated promising properties as catalyst in reactions of H2 and CO. For these reasons, we decided to study the adsorption of H2 over this catalyst. Density functional theory (DFT) can give an insight of the microscopic factors determining the reactivity of hydrogen on the solid surface. Adsorption and dissociation energies of hydrogen have been successfully estimated over various transition metals, oxide and sulfide catalyst surfaces. Facile dissociative adsorption of hydrogen molecule and strong hydrogen atom binding energy was reported for most of the transition metals except for Au, Ag and Cu, where binding is weaker and H2 dissociation is highly activated [1 ]. Kitchin at al. [2 ] reported the adsorption of H atom on both the Mo and C terminated surface of Mo2 C. In this case, the H atom was more strongly attached to the Mo terminated surface. For MoS2 catalyst [3 ], much smaller energy barrier was calculated for dissociative adsorption preferentially taking place on the metal sites. This energy barrier is also smaller compared to the energy barrier for the transition metals [4 ]. In this paper, we are reporting the dissociative adsorption energies of hydrogen molecule and hydrogen atom over MoP (001) plane obtained by using Density functional theory. Calculation procedure. The DMol3 module of Material Studio (version 6.0) from Accelrys Inc. (San Diego, CA, USA) was used to perform the DFT calculations. Accordingly, the wave functions of the electrons are expanded in numerical atomic basis sets defined on an atomic-centered spherical polar mesh. The double-numerical plus P-function (DNP) of all electron basis sets was used for all the calculations. The DND basis set includes one numerical function for each occupied atomic orbital and a second set of functions for valence atomic orbitals, plus a polarisation p-function on all atoms. Each basis function was restricted to a cutoff radius of 5.5 ˚ A, allowing for efficient calculations without loss of accuracy. The Kohn–Sham equations [5 ] were solved by a self-consistent field procedure using PW91 functional with GGA for exchange correlation [6, 7 ]. The techniques of direct inversion in an iterative subspace with a size value of six and thermal smearing of 0.005 Ha were applied to accelerate convergence. The optimisation convergence thresholds for energy change, maximum force and maximum displacement between the optimisation cycles were 0.00001 Ha, 0.002 778

S. Zaman, Mo. Daous, L. Petrov

˚ and 0.005 ˚ Ha/A A, respectively. The k-point set of (3 × 3 × 3) was used for all calculations. The activation energy of interaction between two surface species was identified by complete linear synchronous transit and quadratic synchronous transit search methods [8 ] followed by TS confirmation through the nudge elastic band method [9–11 ]. Spin polarisation was imposed in all the calculations. The adsorption energy of an element (i.e. molecule or atom) was found according to the following formula: Ead = Eslab+element − {Eempty

slab

+ Eelement }

MoP has a hexagonal crystal structure belonging to P6m2 space group with A. The unit cell of MoP crystal A and c = 3.165 ˚ lattice parameter a = b = 3.235 ˚ was build according to the following atomic coordinate position (x, y, z); Mo at (0, 0, 0) and P at (0.667, 0.333, 0.5) [12 ]. The unit cell of MoP was built by the cleavage of the (001) surface plane from the unit cell. The size of the plane was increased to 4 × 4 by supercell addition. The final cell unit had four atomic layers in depth. The possible adsorption location for H atom over a MoP (001) surface can be “on top” of an Mo atom, “bridge” between two Mo atoms, a fcc site bonding with three different Mo atoms with no underneath phosphorous atom and finally, a hcp site bonding with three different Mo atoms with an underneath phosphorous atom. Adsorption energy of atomic hydrogen. Adsorption energy of hydrogen atom was initially calculated for H atom adsorbed over the MoP (001) plane at different preferred locations, fcc, hcp, “bridge” and “on top” sites. The obtained energies of adsorption are presented in Table 1. The formation of surface complex consisted of three-fold binding of H atom with three Mo atoms at the fcc site which needed the highest adsorption energy of activation. The adsorption activation energy for forming this surface complex was −3.14 eV or −72.40 kcal mol−1 . Table

1

Atomic hydrogen adsorption at different sites over MoP(001) plane Adsorption arrangement Adsorption energy Charge on H atom (e) dM o−H (˚ A)

fcc site −3.14 eV −72.41 kcal/mol −0.15 2.03

hcp site −2.88 eV −66.41 kcal/mol −0.134 2.07

On top −2.33 eV 53.73 kacl/mol −0.173 1.78

Pictorial description

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The adsorption energy for the other two forms of adsorption complexes of H atom with the surface of MoP (001) were lower, at hcp and on top sites the adsorption energy was −66.41 kcal mol−1 and −53.73 kcal mol−1 , respectively. Bridge bonded adsorption arrangement was not favourable over MoP (001) plane. Adsorbed hydrogen atom had negative charge in all arrangements, while the free hydrogen atom had a zero charge. The charge on Mo atoms to which the bond with H was build was also decreased from +0.1095e to +0.04e, and neutral atomic hydrogen gained negative charge of −0.155 e. Similar result was also obtained from modelling the CO adsorption on MoP (001) plane. The reason for these results could be the electrons attraction from the underneath P atoms, which are bonded to the Mo atoms. There was no significant change in density of state upon adsorption of H atom over the MoP plane. In [2 ] authors reported that the atomic hydrogen adsorption at the threefold sites over Mo (100) surface was of −70.01 kcal mol−1 . For atomic hydrogen adsorption energy over the Mo terminated surface of Mo2 C (0001) energy of adsorption is −76.55 kcal mol−1 . These values are very close to the value of atomic hydrogen adsorption energy on the fcc site MoP (001) plane. This suggests that the mechanism of the strongest adsorption of H atom for all of Mo containing solids might be the same. The adsorption proceeds via formation of adsorption complex containing H atom adsorbed at a “three-fold ” site on the Mo surface atoms. The precious transition metals, i.e. Pd, Pt, Rh, Ru [1 ] showed lower adsorption energy for hydrogen atom −2.74 eV < E < −3.0 eV. Adsorption energy of molecular hydrogen and H2 bond dissociation. Molecular hydrogen was absorbed in a planar arrangement horizontal to the MoP surface and bonded to one Mo atom as shown in Fig. 1a. This configuration was accepted as the initial configuration for the H2 dissociation calculation using NEB method. The Mo–H bond length was 1.909 ˚ A. Molecular hydrogen showed low adsorption energy, of −0.73 eV (−16.88 kcal mol−1 ). The distance between the two H atoms in adsorbed molecule was 0.843 ˚ A, whereas the distance between atoms in hydrogen molecule was 0.75 ˚ A. We found that both hydrogen atoms had small negative charge of −0.0328 e. Hence, hydrogen molecule was activated upon adsorption over the MoP surface. Hydrogen molecular adsorption energy on MoS2 (–1010) surface [12 ] is −1.4 eV (−32.28 kcal mol−1 ), which is about 2 times larger than adsorption energy on MoP. In the literature, there are no other available data about the activation energy of the adsorption of molecular hydrogen, as well as about hydrogen dissociation over Mo based catalysts. The DOS structure, Fig. 2, showed an interaction peak at low energy value, −0.3 Ha, (s orbital interaction) for the molecular adsorption of H2 . Vibrational frequency for hydrogen molecule was 3674.80 cm−1 , whereas free H2 molecule has a vibrational frequency of 4358.43 cm−1 . For comparison, we studied two hydrogen atoms adsorbed on their preferred fcc adsorption sites of MoP surface, which were adjacent to each other as displayed 780


Fig. 1. Transition state search for H2 dissociation over MoP (001) plane

in Fig 1b. The adsorption energy for each adsorbed H atom was −3.13 eV (−72.35 kcal mol−1 ) close to the value of the single H atom adsorption on fcc site. The adsorbed hydrogen atoms on the MoP (001) surface as a result of dissociative adsorption are energetically stabilised. This configuration was used as initial approximation for the search of possible transition state of the molecular hydrogen adsorption process. The initial distance between H atoms was 3.238 ˚ A and the H atoms after adsorption obtained charges of −0.1487 e. After performing complete transition state search, we found that the activation barrier, ∆Ea , for dissociation of adsorbed H2 was 113.4 kcal mol−1 . This value is slightly higher than the activation barrier for gas phase dissociation of

Fig. 2. DOS analysis of H2 adsorption over MoP (001) plane 4

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molecular hydrogen, which is equal to 104.3 kcal mol−1 [13 ], though the hydrogen dissociation proceeds with a high heat of the reaction of ∆Er = −22.29 kcal/mol. The most probable transition structure is shown in Fig. 1c. Therefore, the results obtained after the performed calculations indicate that MoP (001) plane has no potential for catalysing processes in which formation of atomic hydrogen is part of reaction mechanism. Probably it might be catalyst for reactions in which H2 reacts in molecular form. Conclusions. A DFT calculation of the activation energy of hydrogen atom adsorption over MoP (001) plane was performed. The fcc sites on MoP (001) plane are the preferred locations for atomic H adsorption with Ea = 72.41 kcal mol−1 . Hydrogen dissociative adsorption on MoP (001) plane needs very high activation energy Ea = 113.4 kcal mol−1 , which means that this process does not take place on this surface.

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SABIC Chair of Catalysis, Chemical and Materials Engineering Department Faculty of Engineering King Abdulaziz University P.O. Box 80204 Jeddah 21589, Saudi Arabia ∗
