1. A DFT study of CO and H2 dissociation over MoP surfaces
Sharif F. Zaman*a, Yahia Alhameda, Mohammad Daousa, Abdulrahim Ahmad Al-Zahrania, Lachezar Petrovb
a Chemical and Materials Engineering Department, Faculty of Engineering, King Abdulaziz University, P.O. Box 80204, Jeddah 21589, Saudi Arabia.
 b SABIC Chair of Catalysis, Chemical and Materials Engineering Department, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia.
Introduction:
Fischer-Tropsch (FTS) and High Alcohols Synthesis based on CO and H2 is one of most important industrial process in petrochemistry. Development of new highly selective and active catalyst for these reactions is problem of primary importance for industrial catalysis. It is well known many Molybdenum compounds are active catalysts for processes with H2 participation. Molybdenum Phosphide (MoP) is one of potentially good catalyst for this reactions. The theoretical investigation of the adsorption and dissociation of CO and H2 on MoP surface will be valuable addition for future experimental work.Theoretical study of CO and H2 adsorption and dissociation barrier energy over MoP (001) and (100) planes had been performed using density functional theory in this study.
FCC
Bridge
HCP
On top
The MoP (001) and (100) planes do have different adsorption sites. As shown in the Figure MoP(001) plane has ontop, bridge, FCC and HCP adsorption sites where as MoP(100) plane Two different bridge adsorption sites bridge site I with underneath P atom and bridge site II with underneath Mo atom and ontop position. Two different planes will definitely show different activation towards CO and H2 dissociation.
Calculation methods (DFT) using DMol3 package (Material Studio 6.1), Functional - GGA-PW91, Basis set : DNP, K point settings 3x3x3, Orbital cutoff 4.9 oA. The activation energy of interaction between two surface species was identified by complete linear synchronous transit and quadratic synchronous transit search methods followed by TS confirmation through the nudge elastic band method. 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 = E slab+element – {E empty slab + E element}
MoP(100)
MoP (001)
CO adsorption and dissociation over MoP(001) and MoP(100) plane
CO dissociation over
MoP (100 plane )
Density functional theory (DFT) was employed to investigate the CO dissociation energy over MoP surfaces. Before calculating the dissociation energies of CO, the preferred adsorption position of oxygen (O) and carbon (C) atom on the MoP (001) surface were investigated separately. The results of simulation of preferred adsorption location of C and O atoms were performed by adsorbing C and O atoms separately and then placed them together on the surface i.e. surface product configuration.
Nondissociative “on top” adsorption of CO is favored over the MoP(001) surface with an adsorption energy of kal mol-1. Both the C and O atoms prefer hcp three fold binding location on the surface. The dissociation activation energy is kcal mol-1 which is much higher than the adsorption energy of CO molecule.
Nondissociative “on top” adsorption of CO is also favored over the MoP(100) surface with an adsorption energy of kcal mol-1. The C atom prefers a three fold binding location where phosphorous atom is involved in binding whereas O atom prefers a bridge position between two Mo atoms on the surface. The dissociation activation energy is kcal mol-1, which is much higher than the adsorption energy of CO molecule.
Hence in both cases CO molecule would prefer to desorb from the surface rather undergoing the C-O dissociative adsorption.
CO dissociation over
MoP (001 plane )
Density of states (DOS) has been generated for the top layer of Mo atoms of empty MoP(100) plane and with the adsorbents, CO, C and O as depicted in DOS profile in the Fig. for s, p and d orbitals separately. When CO is molecularly adsorbed on the surface there is no significant change in the d orbital DOS structure except a little alteration near the Fermi level by comparing Fig. 3.IIIa and b. The s orbital DOS structure (Fig .I a and b) introduces a new peak at Ha and peak enhancement at -0.2Ha indicates s orbital’s involvement in the bond formation. Similarly for p orbital (Fig 3.II a and b) a new pick is observed at -0.34Ha (for CO) and peak enhancement at -0.2Ha is observed (contribution from MoP plane). So adsorption of CO on the MoP(100) plane is mainly b the
It became clearer when we compare the two DOS structures to that of C+O atoms on MoP surface (Fig II.c). Three very small peaks are observed at -0.65, Ha ( for O atom) and at Ha (for C atom) and contribution of MoP plane comes at -0.2 to Fermi level by altering the height and shape of the DOS structure. Three new peaks are also seen in s orbital more intensely (Fig. .I.c) and less intensely observed in d orbital (Fig.III.c)
interaction of s and p orbitals CO adsorption process on several transition metals, the 5σ bonding orbital-orbital of CO interacts with the d orbitals (mainly dz2) of the transition metals and electrons are transferred from CO to metal by donation and back donation mechanism. So the interactions are mainly in s type σ orbital and d orbital. But for MoP we found the pronounced interaction of p orbital in the bonding process. The contribution of p orbital mainly comes from the underneath phosphorous atom layer as also evidenced by the Mullikan charge distribution reported in previous section.
Figure : DOS of s, p and d orbitals. (a) Empty MoP (100); (b) CO adsorbed on MoP (100); (c) C and O dissociative adsorbed over MoP(100) plane .
H2 adsorption and dissociation over MoP(001) and MoP(100) plane
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 location for atomic H adsorption with Ea = kcal mol-1. Hydrogen molecule adsorbs on top an Mo atom with an adsorption energy of kcal mol-1. Hydrogen dissociative adsorption on MoP (001) plane of needs very high activation energy Ea = kcal mol-1, which means that this process is not taking place on this surface.
On the other hand, on MoP(100) plane hydrogen atom adsorption bridge site I (underneath P atom) is the preferred location for atomic H adsorption with Ea = 61.3 kcal mol-1. Where as H2 molecule adsorbs with an energy 15.1 kcal mol-1. Interestingly hydrogen dissociative adsorption on MoP (100) plane of needs very low activation energy Ea = 4.15 kcal mol-1, compare to the plane MoP(001). Which discloses the fact that H2 dissociation prefers the MoP(100) plane and hydrogenation reaction simulations must be performed on this plane.
Conclusions
Both the MoP (001) and (100) planes prefer non dissociative adsorption of CO with very close adsorption and C-O dissociation activation energies. But for hydrogen (H2) the activation barrier over MoP(100) plane is 27 times less than MoP(001) plane. Hence MoP(100) plane is the active plane for hydrogenation reactions.
H2 dissociation over
MoP (001 plane )
H2 dissociation over
MoP (100 plane )
Acknowledgement : This work was supported by the Deanship of Scientific Research of King Abdulaziz University, Jeddah, Saudi Arabia, under grant No. (D-005/431).