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Density functional theoretical study of water molecular adsorption on surface of MoO3 with the cluster model
Journal of Molecular Structure (Theochem) 684 (2004) 81–85 www.elsevier.com/locate/theochem
Density functional theoretical study of water molecular adsorption on surface of MoO3 with the cluster model Xingfu Songa, Gousheng Liua, Jianguo Yua, A.E. Rodriguesb,* a
Laboratory of Resource Utilization and Engineering-LRUE, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China b Laboratory of Separation and Reaction Engineering-LSRE, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr Roberto Frias, s/n 4200-465 Porto, Portugal Received 11 February 2004; accepted 2 June 2004 Available online 28 August 2004
Abstract Density functional theory on LANL2DZ level was used to optimize the geometry and electronic structures of MoO3 cluster model before and after water molecular adsorption on its surface. The electron correlation energies of the optimized structures were calculated by using the second order Møller–Plesset perturbation theory. Results show that when the water molecule was adsorbed on the model surface, it occupied the vacant site of M5C O ; during this process, its conformation transformed from MoO5H4 model to octahedral complex of model MoO5H4$H2O. Water molecule was adsorbed by the mechanism of p electrons of atom O (in H2O) contributing to d orbital of central Mo atom. During this process, there was a minimum point on the potential energy surface. The process was an exothermal reaction, with adsorption heat of K91.39 kJ/mol. The results are significant for studies on use, deactivation and reactivation of this deoxidizing catalyst. q 2004 Elsevier B.V. All rights reserved. Keywords: Water molecular adsorption; MoO3; Cluster model; Density functional theory (DFT)
1. Introduction The catalysts whose main active component is molybdenum oxide have found extensive applications in petroleum refining and processing in recent years, and studies on their catalytic activities are all significant for research and development [1–4]. For example, a fixed reactor loaded with deoxidizing catalyst Co–Mo/g-Al2O3 can remove oxygen from concentration levels of 0.1–0.5% to 0.3!10K6 (v/v). Furthermore, this catalyst has a good performance in atmosphere of hydrogen sulfide (H 2S concentration 0.11%) [5–7]. But during the industrial application, this catalyst encountered some difficulties, mainly due to deactivation after adsorption of water vapor on its surface. Because the adsorption of reactants on the surface of catalyst active centers is the basis for molecular bonding and electronic structure changes between reactant and catalyst active center, studies on water molecular adsorption on * Corresponding author. Tel.: C351-22508-1671; fax: C351-225081674. E-mail address: [email protected] (A.E. Rodrigues). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.07.011
metal surface, especially on transition metal surface, are significant for studies on use, deactivation and regeneration of catalysts. For molybdenum catalysts, the reduction process of hexad molybdenum is the precondition for their catalytic actions; therefore, researchers recognize active centers by their optimized oxidation states [8]. Studies show that for active component MoO3, supported in g-Al2O3, Mo5C is the active center [9,10]. Several models have also been proposed to study the reaction mechanism [3,11], such as concerted conversion mechanism, non-concerted conversion mechanism, mechanism of carbene complex formation on metal surface, etc. Unfortunately, all these mechanisms had not yet been proved by experiments or by strict theoretical calculations. Nevertheless, in view of the fact that the first step for any catalytic reaction process is reactant’s adsorption on active centers, the adsorption of water vapor on active centers means there are less active centers available for reactant’s occupation, consequently affecting the catalytic activity. In order to explore the nature of this type of adsorption, we recently studied the behavior of water molecular
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adsorption on surface of MoO3 with the cluster model using the methods of quantum chemistry calculations. In this study, the cluster model structures before and after water molecular adsorption were fully optimized at LANL2DZ level by the method of B3LYP in density functional theory (DFT); information about bonding and electronic structure changes, atomic net charges and adsorption energy along this reaction path were obtained, which were helpful to study in detail the adsorption process and mechanism of catalyst deactivation and reactivation.
2. Models and computational methods Usually, in the MoO3 cluster model, the Mo atom is coordinated with six O atoms, forming a Mo–O octahedral structure [12,13]. For computation simplicity and as a theoretical approximation, single Mo cluster can be selected as subject for study of MoO3 cluster model if suitable boundary condition is obtained so that charges are in reasonable distribution on its surface. Inserting atomic cluster method [14] and hydrogen atom saturation boundary method [15] are the usually adopted techniques. Kenton [15] showed that both methods were similar when ab initio computational method were applied, but the latter can simplify the calculation process and improve computation astringency; therefore, in this study we adopt the hydrogen atom saturation boundary method, and use MoO5H4 as MoO3 cluster model, and MoO5H4$H2O as water molecular adsorption cluster model. Both cluster models are shown in Fig. 1 with atomic numbering. In Fig. 1 there is a double bond connecting Mo(1) and O(2); the other four are single bonds Mo(1)–O, all saturated with hydrogen. The opposite site of Mo(1)–O(2) double bond is the Mo5C vacant site. Some studies [10] showed that this Mo5C vacant site was exactly the location of catalytic activity. Once occupied by other molecule, it is disadvantageous for reactant’s occupation to form ‘carbene complex on metal surface’. From this point of view we assumed that when the water molecule is absorbed on the surface of MoO5H4 cluster model, it occupies this vacant site to form the MoO5H4$H2O cluster model. The basis set of LANL2DZ is used in GAUSSIAN98W; the updated effective core potential method (ECP) developed by Hay and Wadt [16–18] is used for atom Mo(1), while for other atoms (H, C, O) all electrons are considered, and the basis set developed by Dunning and Huzinaga [19] is adopted. All atomic orbitals use double zeta basis, thus for models of MoO5H4, MoO5H4$H2O and H2O molecules, 75, 88 and 13 basis functions are used, respectively. For all geometrical configurations, B3LYP method in DFT is used, bonds and bond angles are optimized by Berny methods [20]. For consideration of electron correlation energy’s influence on adsorption energy, the second order Møller–Plesset perturbation theory (MP2) are calculated for all model molecules after geometrical optimization.
Fig. 1. Molecular structure and atomic numbering for cluster models MoO5H4 and MoO5H4$H2O.
All the calculations are carried out on Pentium-IV computer by GAUSSIAN98W [21] revision-A7 program.
3. Results and discussion 3.1. Geometries optimization The optimized geometries, including bonds, bond angles and dihedral angles for models MoO5H4 and MoO5H4$H2O are listed in Table 1. Table 1 shows that during the process of water molecular adsorption on the surface of model MoO5H4 leading to the model MoO5H4$H2O, the bond length R[Mo(1)–O(11)] changes from infinite distance to 0.2487 nm, but all other bond lengths between Mo(1) and O atoms vary very little. This means that the adsorption of water molecules on model MoO5H4 affects Mo–O bonding properties very slightly, but during the same process, bond angles and dihedral angles vary strongly. For example, bond angle :[O(5)Mo(1)O(3)] changes from 89.6 to 151.98, and :[O(6)Mo(1)O(3)] from 161.3 to 97.68; dihedral angles D[O(5) Mo(1) O(3) H(7)] change from K67.5 to K3.78, and D[H(10) O(6) Mo(1) O(3)] from 179.9 to 84.28, meaning that water molecular adsorption on model MoO5H4 forming model MoO5H4$H2O deeply affects model’s conformation. It can also be seen from Fig. 1 that during this process, because
X. Song et al. / Journal of Molecular Structure (Theochem) 684 (2004) 81–85 Table 1 Optimized bond length, bond angle and dihedral angle for models MoO5H4 and MoO5H4$H2O MoO5H4
Parameters Bond length (nm) R[O(2)–Mo(1)] R[O(3)–Mo(1)] R[O(4)–Mo(1)] R[O(5)–Mo(1)] R[O(6)–Mo(1)] R[O(11)–Mo(1)] R[H(12)–O(11)] R[H(13)–O(11)] Bond angle (8) :[O(3)Mo(1)O(2)] :[O(4)Mo(1)O(3)] :[O(5)Mo(1)O(3)] :[O(6)Mo(1)O(3)] :[O(11)Mo(1)O(3)] :[H(12)O(11)Mo(1)] :[H(13)O(11)Mo(1)] :[H(13) O(11) H(12)] Dihedral angle (8) D[O(2) Mo(1) O(3) H(7)] D[O(5) Mo(1) O(3) H(7)] D[H(10) O(6) Mo(1) O(3)] D[H(9) O(5) Mo(1) O(2)]
MoO5H4$H2O
0.1719 0.1869 0.1921 0.1921 0.1962
0.1717 0.1898 0.1905 0.1990 0.1986 0.2487 0.0986 0.0977
102.8 95.4 89.6 161.3
K179.9 K67.5 179.9 K91.2
101.4 89.7 151.9 97.0 74.4 91.9 113.3 111.5
83
changes from 1.4751 to 1.3289 with variation of 0.1462, the net charge of the atom O(11) changes from K0.7112 to K0.6940 with variation of 0.0172. The value decrease of the net charge for Mo(1) is due to the fact that after the adsorption of water molecule on model MoO5H4 forming model MoO5H4$H2O, there is electron transference from the p bonding orbital of the O(11) atom to the d orbital of central Mo(1) atom, verifying the fact that the atom O(11) is adsorbed by the mechanism of p electron in O(11) contributing to d orbital of Mo(1) atom. Because of this transference of p electron to d orbital of Mo(1) atom, bond strengths between O and H in water molecule will be weakened; this is confirmed by calculation, which shows that the O–H bond length changes from 0.0951 to 0.0986 and 0.0977 nm, respectively]. The increase of O–H bond length means a looser O–H bond strength. 3.3. Water adsorption energy According to the formula of interaction energy between water molecule and solid surface suggested by Siegbahn [22], the energy variation for the process of water molecular adsorption may be expressed as
177.0 K3.7 84.2 K78.0
E ¼ Esystem K ðEcluster þ EH2 O Þ Table 2 Mulliken charges for models MoO5H4 and MoO5H4$H2O
¼ EoptðMoO5 H4 ,H2 OÞ K ENðMoO5 H4KH2 OÞ
Parameters
MoO5H4
MoO5H4$H2O
Mo(1) O(2) O(3) O(4) O(5) O(6) O(11)
1.4751 K0.3718 K0.6798 K0.6651 K0.6651 K0.7197 K0.7112
1.3289 K0.3666 K0.6323 K0.6479 K0.6988 K0.7491 K0.6940
water molecule occupies the vacant site opposite to the R[Mo(1)–O(2)] double bond, its conformation varies from model MoO5H4 to octahedral model MoO5H4$H2O. 3.2. Atomic net charges For the calculation of atomic net charges, all the Mulliken population distributions of Mo atom and connected O atoms for models MoO5H4 and MoO5H4$H2O are listed in Table 2. Table 2 shows that during the process of water molecular adsorption on surface of model MoO5H4, there is electron transference to Mo(1) atom, the net charge of atom Mo(1)
where Eopt(MoO 5H 4 $H 2 O) is the energy of the optimized geometry for cluster model MoO5H4-H2O, and EN(MoO5H4$H2O) is the sum of the energy of optimized geometry for cluster model MoO5H4 and water molecule at infinite distance. The calculated energies before and after adsorption of water molecules are listed in Table 3. Table 3 shows that the system energy decreases after the adsorption of water molecules; the energy variation calculated by method B3LYP/LANL2DZ is K74.92 kJ/mol, while considering the influence of electron correlation energy, the correlated energy by using the method of MP2/LANL2DZ is K91.39 kJ/mol. The energy of model MoO5H4$H2O is lower and thus more stable, which means that water molecule has tendency to be adsorbed on surface of model MoO5H4, occupying the active vacant site opposite to R[Mo(1)–O(11)] double bond. 3.4. Energy variation along adsorption path If the process of water molecular adsorption was considered as water molecule approaching model MoO5H4 gradually, in other words, if the distance between Mo(1)
Table 3 Calculated energies before and after water molecular adsorption Methods
E (MoO5H4) (au)
E (H2O) (au)
E (MoO5H4$H2O) (au)
DE (kJ/mol)
B3LYP/LANL2DZ MP2/LANL2DZ
K446.171993 K444.432801
K76.414316 K76.135404
K522.614872 K520.603049
K74.92 K91.39
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in fact 80–90 8C was experimentally selected as optimum temperature as a trade-off between conflicting requirements. Because the mechanism of deoxidizing catalyst contains several steps, i.e. first, it adsorbs a certain amount of O2 and then, under H2 atmosphere stream, water is formed on catalytic active sites with the absorbed O2. If the water formed is not easily removed from catalytic active sites, this behavior will influence further O2 adsorption, thus weakening the catalytic activity. This can be explained as the deactivation mechanism of water vapor adsorption on deoxidizing catalyst. Therefore, it can be understood that catalyst would be deactivated if the feed gas contains any water vapor. For consideration of optimum reaction conditions and increment of catalyst life-time on operation, it is important to assure the dryness of the reaction system including feed gas. Fig. 2. Energy variation along the adsorption path.
and O(11) atoms was considered as a parameter, along with R[Mo(1)–O(11)] varying from infinite distance to a certain value, different energy variations for different simulation states can be obtained after optimized calculation for other parameters, thus the energy variations along water molecular adsorption path can be obtained. Results are shown in Fig. 2. Results indicate that when the distance R[Mo(1)–O(11)] becomes smaller and smaller, the system’s energy decreases gradually, and when R[Mo(1)–O(11)]Z0.2487 nm, the total energy is at the lowest level. Along the path from infinite distance to the optimized equilibrium point, the adsorption process would not pass through any energy barrier, meaning that the adsorption process is a fast step. From this result, the influence of water vapor on catalytic activity can be understood. Because water vapor can easily occupy active sites on catalyst surface to form a stable complex, the coverage of active sites with reactants will decrease, leading to catalytic deactivation. As we know, a good catalyst can easily adsorb reactants and easily desorb formed products. Because the process of water molecular adsorption is an exothermal reaction, high temperature would be advantageous for product desorption, thus it would be advantageous to sweep water vapor away from active sites, and so, active sites would be exposed again. This assumption was verified by our experiments. In a fixed bed microreactor with deactivated deoxidizing catalyst, for space velocity of 15,000 hK1 and temperature 80 8C, the residual O2 was as high as 8.1!10K6 (v/v); after treatment with high purity H2 sweeping at 125 8C for 12 h, the fixed bed temperature was lowered to 80 8C again to measure its catalytic activity, results show that residual O2 value was below 0.2!10K6(v/v), which indicated that the catalyst was reactivated as a result of water vapor removal from occupied active sites after sweeping with high purity H2 at a higher temperature. On the other hand, higher temperature is disadvantageous for O2 adsorption. We found in our experiments that higher temperature of fixed bed was not always beneficial for lowering the residual O2;
4. Conclusion DFT was used to study the behavior of water molecular adsorption on surface of MoO3 with cluster model. The optimized geometry and electron structures were calculated, and energy variations before and after the process of water molecular adsorption were compared. Results show that the water molecule has tendency to be adsorbed on model MoO5H4 forming al MoO5H4$H2O octahedral complex; water molecules occupy the active vacant site opposite to the R[Mo(1)–O(2)] double bond, thus affecting model’s catalytic activity. Because the process of water molecular adsorption is exothermic, higher temperature is advantageous for removing water vapor from active sites, but disadvantageous for O2 adsorption, therefore, optimization on operation temperature of deoxidization and reactivation is required. In view of the complexity of the catalytic mechanism and computation capacity for large systems, only a qualitative theoretical study was performed here to explore the deactivation phenomenon from water molecular adsorption on catalytic active site point of view. In order to have a more detailed explanation to the mechanism, further calculations on a large system should be performed in the future.
Acknowledgements The authors are grateful to the 4th Sino-Portugal Joint Commission, Ministry of Science and Technology of China and Portugal, for providing financial support.
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Density functional theoretical study of water molecular adsorption on surface of MoO3 with the cluster model Xingfu Songa, Gousheng Liua, Jianguo Yua, A.E. Rodriguesb,* a
Laboratory of Resource Utilization and Engineering-LRUE, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China b Laboratory of Separation and Reaction Engineering-LSRE, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr Roberto Frias, s/n 4200-465 Porto, Portugal Received 11 February 2004; accepted 2 June 2004 Available online 28 August 2004
Abstract Density functional theory on LANL2DZ level was used to optimize the geometry and electronic structures of MoO3 cluster model before and after water molecular adsorption on its surface. The electron correlation energies of the optimized structures were calculated by using the second order Møller–Plesset perturbation theory. Results show that when the water molecule was adsorbed on the model surface, it occupied the vacant site of M5C O ; during this process, its conformation transformed from MoO5H4 model to octahedral complex of model MoO5H4$H2O. Water molecule was adsorbed by the mechanism of p electrons of atom O (in H2O) contributing to d orbital of central Mo atom. During this process, there was a minimum point on the potential energy surface. The process was an exothermal reaction, with adsorption heat of K91.39 kJ/mol. The results are significant for studies on use, deactivation and reactivation of this deoxidizing catalyst. q 2004 Elsevier B.V. All rights reserved. Keywords: Water molecular adsorption; MoO3; Cluster model; Density functional theory (DFT)
1. Introduction The catalysts whose main active component is molybdenum oxide have found extensive applications in petroleum refining and processing in recent years, and studies on their catalytic activities are all significant for research and development [1–4]. For example, a fixed reactor loaded with deoxidizing catalyst Co–Mo/g-Al2O3 can remove oxygen from concentration levels of 0.1–0.5% to 0.3!10K6 (v/v). Furthermore, this catalyst has a good performance in atmosphere of hydrogen sulfide (H 2S concentration 0.11%) [5–7]. But during the industrial application, this catalyst encountered some difficulties, mainly due to deactivation after adsorption of water vapor on its surface. Because the adsorption of reactants on the surface of catalyst active centers is the basis for molecular bonding and electronic structure changes between reactant and catalyst active center, studies on water molecular adsorption on * Corresponding author. Tel.: C351-22508-1671; fax: C351-225081674. E-mail address: [email protected] (A.E. Rodrigues). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.07.011
metal surface, especially on transition metal surface, are significant for studies on use, deactivation and regeneration of catalysts. For molybdenum catalysts, the reduction process of hexad molybdenum is the precondition for their catalytic actions; therefore, researchers recognize active centers by their optimized oxidation states [8]. Studies show that for active component MoO3, supported in g-Al2O3, Mo5C is the active center [9,10]. Several models have also been proposed to study the reaction mechanism [3,11], such as concerted conversion mechanism, non-concerted conversion mechanism, mechanism of carbene complex formation on metal surface, etc. Unfortunately, all these mechanisms had not yet been proved by experiments or by strict theoretical calculations. Nevertheless, in view of the fact that the first step for any catalytic reaction process is reactant’s adsorption on active centers, the adsorption of water vapor on active centers means there are less active centers available for reactant’s occupation, consequently affecting the catalytic activity. In order to explore the nature of this type of adsorption, we recently studied the behavior of water molecular
82
X. Song et al. / Journal of Molecular Structure (Theochem) 684 (2004) 81–85
adsorption on surface of MoO3 with the cluster model using the methods of quantum chemistry calculations. In this study, the cluster model structures before and after water molecular adsorption were fully optimized at LANL2DZ level by the method of B3LYP in density functional theory (DFT); information about bonding and electronic structure changes, atomic net charges and adsorption energy along this reaction path were obtained, which were helpful to study in detail the adsorption process and mechanism of catalyst deactivation and reactivation.
2. Models and computational methods Usually, in the MoO3 cluster model, the Mo atom is coordinated with six O atoms, forming a Mo–O octahedral structure [12,13]. For computation simplicity and as a theoretical approximation, single Mo cluster can be selected as subject for study of MoO3 cluster model if suitable boundary condition is obtained so that charges are in reasonable distribution on its surface. Inserting atomic cluster method [14] and hydrogen atom saturation boundary method [15] are the usually adopted techniques. Kenton [15] showed that both methods were similar when ab initio computational method were applied, but the latter can simplify the calculation process and improve computation astringency; therefore, in this study we adopt the hydrogen atom saturation boundary method, and use MoO5H4 as MoO3 cluster model, and MoO5H4$H2O as water molecular adsorption cluster model. Both cluster models are shown in Fig. 1 with atomic numbering. In Fig. 1 there is a double bond connecting Mo(1) and O(2); the other four are single bonds Mo(1)–O, all saturated with hydrogen. The opposite site of Mo(1)–O(2) double bond is the Mo5C vacant site. Some studies [10] showed that this Mo5C vacant site was exactly the location of catalytic activity. Once occupied by other molecule, it is disadvantageous for reactant’s occupation to form ‘carbene complex on metal surface’. From this point of view we assumed that when the water molecule is absorbed on the surface of MoO5H4 cluster model, it occupies this vacant site to form the MoO5H4$H2O cluster model. The basis set of LANL2DZ is used in GAUSSIAN98W; the updated effective core potential method (ECP) developed by Hay and Wadt [16–18] is used for atom Mo(1), while for other atoms (H, C, O) all electrons are considered, and the basis set developed by Dunning and Huzinaga [19] is adopted. All atomic orbitals use double zeta basis, thus for models of MoO5H4, MoO5H4$H2O and H2O molecules, 75, 88 and 13 basis functions are used, respectively. For all geometrical configurations, B3LYP method in DFT is used, bonds and bond angles are optimized by Berny methods [20]. For consideration of electron correlation energy’s influence on adsorption energy, the second order Møller–Plesset perturbation theory (MP2) are calculated for all model molecules after geometrical optimization.
Fig. 1. Molecular structure and atomic numbering for cluster models MoO5H4 and MoO5H4$H2O.
All the calculations are carried out on Pentium-IV computer by GAUSSIAN98W [21] revision-A7 program.
3. Results and discussion 3.1. Geometries optimization The optimized geometries, including bonds, bond angles and dihedral angles for models MoO5H4 and MoO5H4$H2O are listed in Table 1. Table 1 shows that during the process of water molecular adsorption on the surface of model MoO5H4 leading to the model MoO5H4$H2O, the bond length R[Mo(1)–O(11)] changes from infinite distance to 0.2487 nm, but all other bond lengths between Mo(1) and O atoms vary very little. This means that the adsorption of water molecules on model MoO5H4 affects Mo–O bonding properties very slightly, but during the same process, bond angles and dihedral angles vary strongly. For example, bond angle :[O(5)Mo(1)O(3)] changes from 89.6 to 151.98, and :[O(6)Mo(1)O(3)] from 161.3 to 97.68; dihedral angles D[O(5) Mo(1) O(3) H(7)] change from K67.5 to K3.78, and D[H(10) O(6) Mo(1) O(3)] from 179.9 to 84.28, meaning that water molecular adsorption on model MoO5H4 forming model MoO5H4$H2O deeply affects model’s conformation. It can also be seen from Fig. 1 that during this process, because
X. Song et al. / Journal of Molecular Structure (Theochem) 684 (2004) 81–85 Table 1 Optimized bond length, bond angle and dihedral angle for models MoO5H4 and MoO5H4$H2O MoO5H4
Parameters Bond length (nm) R[O(2)–Mo(1)] R[O(3)–Mo(1)] R[O(4)–Mo(1)] R[O(5)–Mo(1)] R[O(6)–Mo(1)] R[O(11)–Mo(1)] R[H(12)–O(11)] R[H(13)–O(11)] Bond angle (8) :[O(3)Mo(1)O(2)] :[O(4)Mo(1)O(3)] :[O(5)Mo(1)O(3)] :[O(6)Mo(1)O(3)] :[O(11)Mo(1)O(3)] :[H(12)O(11)Mo(1)] :[H(13)O(11)Mo(1)] :[H(13) O(11) H(12)] Dihedral angle (8) D[O(2) Mo(1) O(3) H(7)] D[O(5) Mo(1) O(3) H(7)] D[H(10) O(6) Mo(1) O(3)] D[H(9) O(5) Mo(1) O(2)]
MoO5H4$H2O
0.1719 0.1869 0.1921 0.1921 0.1962
0.1717 0.1898 0.1905 0.1990 0.1986 0.2487 0.0986 0.0977
102.8 95.4 89.6 161.3
K179.9 K67.5 179.9 K91.2
101.4 89.7 151.9 97.0 74.4 91.9 113.3 111.5
83
changes from 1.4751 to 1.3289 with variation of 0.1462, the net charge of the atom O(11) changes from K0.7112 to K0.6940 with variation of 0.0172. The value decrease of the net charge for Mo(1) is due to the fact that after the adsorption of water molecule on model MoO5H4 forming model MoO5H4$H2O, there is electron transference from the p bonding orbital of the O(11) atom to the d orbital of central Mo(1) atom, verifying the fact that the atom O(11) is adsorbed by the mechanism of p electron in O(11) contributing to d orbital of Mo(1) atom. Because of this transference of p electron to d orbital of Mo(1) atom, bond strengths between O and H in water molecule will be weakened; this is confirmed by calculation, which shows that the O–H bond length changes from 0.0951 to 0.0986 and 0.0977 nm, respectively]. The increase of O–H bond length means a looser O–H bond strength. 3.3. Water adsorption energy According to the formula of interaction energy between water molecule and solid surface suggested by Siegbahn [22], the energy variation for the process of water molecular adsorption may be expressed as
177.0 K3.7 84.2 K78.0
E ¼ Esystem K ðEcluster þ EH2 O Þ Table 2 Mulliken charges for models MoO5H4 and MoO5H4$H2O
¼ EoptðMoO5 H4 ,H2 OÞ K ENðMoO5 H4KH2 OÞ
Parameters
MoO5H4
MoO5H4$H2O
Mo(1) O(2) O(3) O(4) O(5) O(6) O(11)
1.4751 K0.3718 K0.6798 K0.6651 K0.6651 K0.7197 K0.7112
1.3289 K0.3666 K0.6323 K0.6479 K0.6988 K0.7491 K0.6940
water molecule occupies the vacant site opposite to the R[Mo(1)–O(2)] double bond, its conformation varies from model MoO5H4 to octahedral model MoO5H4$H2O. 3.2. Atomic net charges For the calculation of atomic net charges, all the Mulliken population distributions of Mo atom and connected O atoms for models MoO5H4 and MoO5H4$H2O are listed in Table 2. Table 2 shows that during the process of water molecular adsorption on surface of model MoO5H4, there is electron transference to Mo(1) atom, the net charge of atom Mo(1)
where Eopt(MoO 5H 4 $H 2 O) is the energy of the optimized geometry for cluster model MoO5H4-H2O, and EN(MoO5H4$H2O) is the sum of the energy of optimized geometry for cluster model MoO5H4 and water molecule at infinite distance. The calculated energies before and after adsorption of water molecules are listed in Table 3. Table 3 shows that the system energy decreases after the adsorption of water molecules; the energy variation calculated by method B3LYP/LANL2DZ is K74.92 kJ/mol, while considering the influence of electron correlation energy, the correlated energy by using the method of MP2/LANL2DZ is K91.39 kJ/mol. The energy of model MoO5H4$H2O is lower and thus more stable, which means that water molecule has tendency to be adsorbed on surface of model MoO5H4, occupying the active vacant site opposite to R[Mo(1)–O(11)] double bond. 3.4. Energy variation along adsorption path If the process of water molecular adsorption was considered as water molecule approaching model MoO5H4 gradually, in other words, if the distance between Mo(1)
Table 3 Calculated energies before and after water molecular adsorption Methods
E (MoO5H4) (au)
E (H2O) (au)
E (MoO5H4$H2O) (au)
DE (kJ/mol)
B3LYP/LANL2DZ MP2/LANL2DZ
K446.171993 K444.432801
K76.414316 K76.135404
K522.614872 K520.603049
K74.92 K91.39
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in fact 80–90 8C was experimentally selected as optimum temperature as a trade-off between conflicting requirements. Because the mechanism of deoxidizing catalyst contains several steps, i.e. first, it adsorbs a certain amount of O2 and then, under H2 atmosphere stream, water is formed on catalytic active sites with the absorbed O2. If the water formed is not easily removed from catalytic active sites, this behavior will influence further O2 adsorption, thus weakening the catalytic activity. This can be explained as the deactivation mechanism of water vapor adsorption on deoxidizing catalyst. Therefore, it can be understood that catalyst would be deactivated if the feed gas contains any water vapor. For consideration of optimum reaction conditions and increment of catalyst life-time on operation, it is important to assure the dryness of the reaction system including feed gas. Fig. 2. Energy variation along the adsorption path.
and O(11) atoms was considered as a parameter, along with R[Mo(1)–O(11)] varying from infinite distance to a certain value, different energy variations for different simulation states can be obtained after optimized calculation for other parameters, thus the energy variations along water molecular adsorption path can be obtained. Results are shown in Fig. 2. Results indicate that when the distance R[Mo(1)–O(11)] becomes smaller and smaller, the system’s energy decreases gradually, and when R[Mo(1)–O(11)]Z0.2487 nm, the total energy is at the lowest level. Along the path from infinite distance to the optimized equilibrium point, the adsorption process would not pass through any energy barrier, meaning that the adsorption process is a fast step. From this result, the influence of water vapor on catalytic activity can be understood. Because water vapor can easily occupy active sites on catalyst surface to form a stable complex, the coverage of active sites with reactants will decrease, leading to catalytic deactivation. As we know, a good catalyst can easily adsorb reactants and easily desorb formed products. Because the process of water molecular adsorption is an exothermal reaction, high temperature would be advantageous for product desorption, thus it would be advantageous to sweep water vapor away from active sites, and so, active sites would be exposed again. This assumption was verified by our experiments. In a fixed bed microreactor with deactivated deoxidizing catalyst, for space velocity of 15,000 hK1 and temperature 80 8C, the residual O2 was as high as 8.1!10K6 (v/v); after treatment with high purity H2 sweeping at 125 8C for 12 h, the fixed bed temperature was lowered to 80 8C again to measure its catalytic activity, results show that residual O2 value was below 0.2!10K6(v/v), which indicated that the catalyst was reactivated as a result of water vapor removal from occupied active sites after sweeping with high purity H2 at a higher temperature. On the other hand, higher temperature is disadvantageous for O2 adsorption. We found in our experiments that higher temperature of fixed bed was not always beneficial for lowering the residual O2;
4. Conclusion DFT was used to study the behavior of water molecular adsorption on surface of MoO3 with cluster model. The optimized geometry and electron structures were calculated, and energy variations before and after the process of water molecular adsorption were compared. Results show that the water molecule has tendency to be adsorbed on model MoO5H4 forming al MoO5H4$H2O octahedral complex; water molecules occupy the active vacant site opposite to the R[Mo(1)–O(2)] double bond, thus affecting model’s catalytic activity. Because the process of water molecular adsorption is exothermic, higher temperature is advantageous for removing water vapor from active sites, but disadvantageous for O2 adsorption, therefore, optimization on operation temperature of deoxidization and reactivation is required. In view of the complexity of the catalytic mechanism and computation capacity for large systems, only a qualitative theoretical study was performed here to explore the deactivation phenomenon from water molecular adsorption on catalytic active site point of view. In order to have a more detailed explanation to the mechanism, further calculations on a large system should be performed in the future.
Acknowledgements The authors are grateful to the 4th Sino-Portugal Joint Commission, Ministry of Science and Technology of China and Portugal, for providing financial support.
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