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DISSOCIATIVE ADSORPTION OF HYDROGEN ON

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Jul 20, 2015 - The hydrogen molecule adsorbed over a Mo atom in “on top” arrangement ... Molybdenum phosphide (MoP) has recently attracted interest ...

12-04 Доклади на Българската академия на науките Comptes rendus de l’Acad´ emie bulgare des Sciences Tome 68, No 12, 2015

CHIMIE Cin´etique et catalyse

DISSOCIATIVE ADSORPTION OF HYDROGEN ON MoP (100) PLANE. A DFT STUDY Sharif F. Zaman∗,∗∗, Mohammad Daous∗, Lachezar Petrov∗∗ (Submitted on July 20, 2015)

Abstract Density functional theory was applied to investigate the H2 adsorption and dissociation energy over MoP (100) surface. It was found that hydrogen atom preferred the “bridge adsorption site” between two molybdenum atoms having an underneath phosphorous atom, with a binding energy of −61.28 kcal mol−1 . The hydrogen molecule adsorbed over a Mo atom in “on top” arrangement shows the lower binding energy of −15.13 kcal mol−1 , comparing to hydrogen atom adsorption. H2 dissociation over MoP (100) plane was found to be highly activated. The dissociation activation barrier is 4.2 kcal mol−1 , which makes MoP (100) plane a more suitable plane for the investigation of the mechanism of hydrogenation reactions. Key words: H2 adsorption, MoP, activation energy, dissociation energy

1. Introduction. Transition metal phosphides are a class of refractory metal-like compounds formed from alloying of the transition metals with phosphorous. They exhibit excellent catalytic activity in hydroprocessing reactions like (hydrodenitrogenation (HDN) and hydrodesulfurization (HDS)) [1–4 ]. Even in HDN reactions they demonstrate higher activity than the sulphides of corresponding metals [5 ]. For chemical reactions involving hydrogen the understanding of the mechanism of hydrogen dissociative adsorption over the different planes of the crystallites of the active catalyst components is very important. However, it is also important how we select the criterion to choose the particular suitable active plane This work was supported by the Deanship of Scientific Research of King Abdulaziz University, Jeddah, Saudi Arabia under grant No (D-005/431). 3

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to carry out the theoretical investigation. Most of the hydrogenation catalysts are able to easily dissociate the hydrogen molecule adsorbed on their surfaces. Therefore, we can accept that the hydrogen dissociation is usually a fast reaction step from the mechanism of hydrogenation processes over these catalysts. Using density functional theory (DFT) scientists have been able to investigate the energies of atomic and molecular hydrogen adsorption and molecular hydrogen dissociation barriers for various transition metals [6, 7 ]. Sun et al. [8 ] reported results of the study of H2 adsorption and dissociation over MoS2 , as well as on Ni and Co promoted MoS2 clusters. Recently TAO [9 ] reported data about H2 adsorption over different planes of Mo2 C catalyst at high surface coverage. Molybdenum phosphide (MoP) has recently attracted interest because some of its catalytic characteristics are close to the catalytic characteristics of the some of the Pt group of precious metals [10 ]. In addition to the experimental studies, there are also theoretical studies on reagents adsorption and reaction mechanisms mostly limited to CO hydrogenation, CO and thiophene adsorption [11 ] and hydrogenation of pyridine adsorption and activation on MoP catalysts [12 ]. In most of the cases researchers have accepted the MoP (001) plane as a place where reaction is proceeding, since this plane is most stable and no changes could be observed during the reactions at high temperatures and pressure. Most probably the hydrogen participating is the rate limiting step in the mechanism of the above-mentioned reactions. MoP is an active catalyst for Fischer-Tropsch reaction. Therefore, it is important to study in details the mechanism of this very important industrial process. This paper presents part of the results of our project on this topic. Recently, we have published results of the study of adsorption of CO and H2 on different MoP planes [13, 14 ]. In our previous publication [13 ], we have shown that MoP (001) plane does not show any catalytic activity towards hydrogen dissociation. MoP has been reported for its capability to activate molecular hydrogen in several reactions. Therefore it intrigues us to investigate the molecular hydrogen dissociation over the next stable MoP (100) plane. In this article we focus on the MoP (100) plane to find the hydrogen adsorption and dissociation energy. 2. 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 electronic wave functions 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 set, was used for all the calculations. The DNP basis set includes one numerical function for each occupied atomic orbital and a second set of functions for valence atomic orbitals, plus a polarization p-function on all atoms. Each basis function was restricted to a cutoff radius of 5 ˚ A, allowing for efficient calculations without loss of accuracy. The Kohn-Sham equations [15 ] were solved by a self-consistent 1504

S. F. Zaman, M. Daous, L. Petrov

field procedure using PW91 functional with GGA for exchange correlation [16–18 ]. 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 optimization convergence thresholds for energy change, maximum force and maximum displacement between the optimization cycles were 0.00001 Ha, 0.002 Ha/˚ A and 0.005 ˚ 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 [20 ] followed by TS confirmation through the nudge elastic band method [21 ]. Spin polarization 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 }.

3. Building unit cell structure of MoP crystal. MoP has a hexagonal crystal structure belonging to P6m2 space group with lattice parameter a = b = 3.235 ˚ A and c = 3.165 ˚ A. The unit cell of MoP crystal was built according to the following atomic coordinate position (x, y, z); Mo at (0, 0, 0) and P at (0.667, 0.333, 0.5) [20 ]. The investigation plane (100) was then cleaved from the unit cell. The cleaved plane then increased to four atomic layers and 4x4 supercell slabs as shown in Fig. 1. The MoP (100) plane has mainly four different types

Fig. 1. Atomic hydrogen adsorption over MoP(100) Compt. rend. Acad. bulg. Sci., 68, No 12, 2015

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adsorption sites, (i) “on top”, (ii) “bridge site between two molybdenum atoms with an underneath Mo atom” (we will name it “bridge site No 1”), (iii) “bridge site between two molybdenum atoms with an underneath P atom” (we will name it “bridge site No 2”), and (iv) “threefold site where the elements will be bounded with four molybdenum atoms with an underneath P atom”. The underneath P atom layer is located very close to the top Mo layer only on 0.71 ˚ A. The distance to MoP (001) plane is 1.48 ˚ A, which suggests that electronic influence/contribution of P atoms on the top Mo layer, will be stronger (Fig. 1. Side view) at MoP (100) plane. 4. Adsorption energy for atomic hydrogen. Our simulation finds that threefold site is not suitable for hydrogen adsorption as hydrogen atom threefold adsorption mode did not converge. So the following discussion will focus only on the other three preferable adsorption locations. Simulation was performed on two different bridge adsorption arrangements of hydrogen over MoP (100) plane. Type I has a phosphorus atom underneath and type II has a molybdenum atom underneath. In addition, “on top” adsorption mode for hydrogen was also calculated. The calculated adsorption energies are given in Table 1 also with the charge on the atoms association with the adsorption process, the height of the hydrogen atom, the adsorption angle between surface molybdenum atoms and hydrogen atom and the distance between the adsorbed species and surface Table

1

Atomic hydrogen adsorption over MoP (100)

Adsorption energy

Bridge position

Bridge position

(underneath P atom)

(underneath Mo atom)

On top

−61.28 kcal/mol

−55.75 kcal/mol

−51.75 kcal/mol

Charge on H

−0.031 e

−0.16 e

−0.145 e

Charge on Mo

Mo(1) = +0.010 e

Mo(2) = −0.029 e

Mo(3) = +0.160 e

Charge on Mo

Mo(2) = +0.011 e ˚ 1.16 A

Mo(3) = −0.026 e ˚ 1.17 A



Height of H atom Angle Mo–H–Mo

107.28 ˚ A

108.25 ˚ A

Mo(2) − Mo(3) − H ˚ = 83.5 A

Distance (Mo–H)

(Mo(1) − H) = 1.965 ˚ A ˚ (Mo(2) − H) = 1.965 A

(Mo(2) − H) = 1.991 ˚ A ˚ Mo(3) − H) = 2.002 A

Mo(3) − H = 1.782 ˚ A

(Ead)

Distance (Mo–H)

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1.77 ˚ A



S. F. Zaman, M. Daous, L. Petrov

molybdenum atoms are reported. The adsorption and dissociation over bridge site of type I needs the lowest adsorption energy, −61.28 kcal/mol, for the most stable atomic adsorption configuration of hydrogen over MoP (100) plane. The charge on hydrogen is −0.031 e on“type I” adsorption site, which is lowest in comparison with the other two adsorption modes. The authors of [14 ] reported the atomic hydrogen adsorption energy at the “threefold sites” over Mo (100) surface of −70.01 kcal/mol. For atomic hydrogen adsorption energy over the “Mo terminated surface” of Mo2 C (0001) energy of adsorption is −76.55 kcal/mol. Authors of [13 ] have reported H adsorption energy on “fcc site” of MoP (001) plane is of −72.41 kcal/mol. On MoP (100) plane the adsorption energy of hydrogen atom is not only lower compared to the abovementioned catalysts but also “fcc site” of MoP (001) plane. The lowest adsorption energy over MoP (100) plane is close to “hcp” and “on top” adsorption mode over MoP (001) plane which is around 60 kcal/mol. The precious transition metals i.e. Pd, Pt, Rh, Ru [15 ] showed closer adsorption energy for hydrogen atom about 62.26 kcal/mol < E < −69.19 eV if we compare the “bridge mode” of “type I” adsorption site (−61.28 kcal/mol) with underneath P atom. Table

2

Molecular and dissociative hydrogen adsorption over MoP(100) plane

Molecular H2

Adsorption energy (Ead) Charge on H Charge on Mo

−15.13 kcal/mol

Type I – Dissociative adsorption of H2 (Underneath Mo atom)

Type II – Dissociative adsorption of H2 (Underneath P atom)

−56.40 kcal/mol

−62.08 kcal/mol

+0.036 e Mo(3) = 0.0 e

−0.018 e Mo(2) = −0.024 e Mo(3) = −0.103 e Charge on Mo – M o(4) = −0.024 e Height of H atom 1.84 ˚ A 1.19 ˚ A H(i) − Mo(3) − Mo(4) (Hi) 107.203 ˚ A Angle Mo () Mo ˚ (Hii) 107.107 ˚ A = 77.01 A ˚ Distance (Mo-H) (Mo(3) −H(i) ) = 1.89 A (Mo(3) −H(i) ) = 2.027 ˚ A ˚ (Mo(3) −H(ii) ) = 2.026 ˚ Distance (Mo-H) (Mo(3) −H(ii) ) = 1.89 A A

−0.029 e Mo(1) = +0.004 e Mo(2) = −0.037 e Mo(5) = +0.007 e 1.18 ˚ A (Hi) 106.56 ˚ A (Hii) 106.33 ˚ A (Mo(2) −H(i) ) = 1.98 ˚ A (Mo(2) −H(ii) ) = 1.96 ˚ A

Charge on hydrogen molecule = 0 e; Charge on hydrogen atom = 0 e; charges on molybdenum atom +0.092 e (empty slab) Compt. rend. Acad. bulg. Sci., 68, No 12, 2015

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5. Adsorption energy for molecular hydrogen. The adsorption energy of molecular hydrogen adsorbed as “on top” configuration on a single molybdenum atom parallel to the MoP surface as shown in Figure (a) in Table 2 is −15.13 kcalmol−1 , which is much less than the atomic adsorption energy for hydrogen. For MoP (001) plane“on top” molecular adsorption energy of molecular hydrogen was reported as −16.88 kcalmol−1 [13 ]. The charges on which each hydrogen atom is obtaining is +0.036 e, which means that the electrons are drawn to the MoP (100) surface from the hydrogen molecule to form the surface bond. Also there is an increase of atomic charge of Mo (3) atom, to which hydrogen molecule is adsorbed from +0.092 to 0 e. When hydrogen molecule is split and two hydrogen atoms are adsorbed on two adjacent bridge positions, “type I” (with underneath Mo atom) and “type II” (with underneath P atom), the adsorption energy is -56.40 kcal/mol and −62.08 kcal/mol, respectively. “Type II”mode is the most stable adsorption mode for hydrogen atom adsorption. For both types of dissociative adsorption of hydrogen the hydrogen atom charge increase is observed from 0 e to −0.018 e and −0.029 e for type I and type II, respectively. 6. Transition state search for hydrogen dissociation. The search for the transition state of hydrogen dissociation was performed between molecular adsorption of hydrogen (Table 2, Fig. a) and “type I” dissociative adsorption mode (Table 2, Fig. b) and type II dissociative adsorption mode (Table 2, Fig. c). Figure 2 depicts the activation barrier and energy of reaction for these two different arrangements. The dissociation activation barrier for “type I” arrangement from molecular adsorption is 15.05 kcal/mol and the reaction energy is −45.0 kcal/mol. The dissociation activation barrier for “type II” arrangement

Fig. 2. Molecular hydrogen dissociation over MoP (100) plane

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S. F. Zaman, M. Daous, L. Petrov

is 4.2 kcal/mol and reaction energy is −47.0 kcal/mol. “Type II” dissociation arrangement is much more favourable for hydrogen over MoP (100) plane and the dissociation activation is much less than the molecular dissociation of hydrogen [13 ] which is −104.3 kcal/mol and also over MoP (001) plane 113.4 kcal/mol [22 ]. Our results suggest that the MoP (100) plane will be more catalytically active for hydrogenation reaction rather than MoP (001) plane. In both cases we have highly exothermic reaction, which is also expected for this kind of reaction. 7. Conclusions. A DFT calculation of the activation energy of hydrogen atom adsorption over MoP (100) plane was performed. The “bridge adsorption sites with underneath ’P’ on MoP (100) plane” are the preferred location for atomic hydrogen adsorption with Ea = −61.28 kcal mol-1. The adsorption energy of atomic hydrogen over MoP (100) plane is compatible with the adsorption energy over the precious metals, i.e. Pt. Molecular hydrogen dissociative adsorption on MoP (100) shows very low activation barrier of Ea = 4.2 kcal mol-1, which means that this process is activated catalytically, which leads to facile dissociation of H2 on MoP (100) plane. This plane should be the preferably selected plane for carrying investigation of hydrogenation reactions as the other (001) plane does not show any catalytic activity towards hydrogen dissociation.

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[15 ] Kohn W., L. J. Sham (1965) Phys. Rev., 140, No 4A, 1133–1138. [16 ] Perdew J. P., J. A. Chevary, S. H. Vosko, K. A. Jackson, M. R. Pederson, D. J. Singh, C. Fiolhais (1992) Phys. Rev. B, 46, No 11, 6671–6687. [17 ] Becke A. D. (1988) J. Chem. Phys., 88, No 4, 2547–2553. [18 ] Perdew J. P., Y. Wang (1992) Phys. Rev. B, 45, No 23, 13244–13249. [19 ] Halgren T. A., W. N. Lipscomb (1977) Chem. Phys. Lett., 49, No 2, 225–232. [20 ] Bell S., J. S. Crighton (1983) J. Chem. Phys., 80, No 6, 2464–2475. [21 ] Zaman S. F., K. J. Smith (2012) Catal. Rev. Sci. Eng., 54, No 1, 41–132. [22 ] Darwent B. deB., Bond Dissociation Energies of Simple Molecules http://www. nist.gov/data/nsrds/NSRDS-NBS31.pdf. Chemical and Materials Engineering Department Faculty of Engineering King Abdulaziz University P.O. Box 80204 Jeddah 21589, Saudi Arabia e-mail: [email protected] [email protected]

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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 ∗∗

S. F. Zaman, M. Daous, L. Petrov

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