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Novel pathway for CO oxidation on a Fe2O3 cluster
Chemical Physics Letters 384 (2004) 242–245 www.elsevier.com/locate/cplett
Novel pathway for CO oxidation on a Fe2O3 cluster B.V. Reddy a
a,*
, F. Rasouli a, M.R. Hajaligol a, S.N. Khanna
b
Chrysalis Technologies Inc., Research Center, RD&E, Philip Morris, 615 Maury Street, Richmond, VA 23224, USA b Physics Department, Virginia Commonwealth University, Richmond, VA 23284-2000, USA Received 8 September 2003; in final form 23 November 2003 Published online: 29 December 2003
Abstract Density functional studies of CO oxidation by Fen Om (n ¼ 1, 2 and m ¼ 1–3) clusters are carried out. A new reaction path where a CO molecule adsorbed on to a surface Fe site breaks one FeO bond and a subsequent CO molecule is oxidized by the less coordinated O atom, proceeds in a barrier less manner. An attachment of CO and O2 to this active Fe site results in the formation of CO2 via a CO3 intermediate. The role of charge transfer and the nature of intermediate states in reduced sizes are highlighted. Ó 2003 Elsevier B.V. All rights reserved.
The oxidation of CO is probably the most studied heterogeneous chemical reaction. The notable catalysts include transition metal surfaces, free transition metal clusters, and more recently, transition and noble metal clusters/nanoparticles supported on oxide surfaces [1–6]. The oxidation on bulk surfaces usually proceeds via two mechanisms namely the Langmuir–Hinshelwood (L–H) or Eley–Rideal (E–R) mechanism. In most cases, it proceeds via L–H mechanism where the reacting species are first adsorbed before undergoing the reaction. This involves formation of the intermediate complex followed by the desorption of the CO2 molecule. It has recently been suggested that the oxidation of CO in some cases proceeds via E–R mechanism where the O2 is first dissociatively adsorbed on to the surface and the gas phase CO molecules undergo an oxidation by directly detaching the adsorbed O atoms. The key steps in both processes are the weakening of the O–O bond, for example, via partial charge transfers to the host molecule. This accounts for the observation that the charged [7] and some supported [6] clusters are better catalysts than the corresponding neutral clusters. An important class of the materials that are rich in oxygen and yet contain O atoms with no O–O bonds are
*
Corresponding author. Fax: +1-804-274-4778. E-mail address: [email protected] (B.V. Reddy).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.023
the metal oxides [8,9]. It then raises the question whether one can use metal-oxides to accomplish the required task. Recent experiments in our laboratory[10] indicate that Fe2 O3 nanoparticles do oxidize CO. While this is attractive, one may ask as to how the catalytic properties evolve if one could further reduce the size, namely, go to small atomic clusters. In fact, Wang et al. [8] have recently generated Fen Om clusters, in beams, containing up to four Fe and six O atoms. Employing negative ion photoelectron spectra they propose structural change with size and composition of the above clusters. The observation that the composition can be varied over a wider range than permissible in solids and the advantage of flexible atomic rearrangements makes clusters as the most attractive systems to investigate the catalytic properties. The purpose of this Letter is to investigate the CO oxidation on Fen Om (n ¼ 1, 2 and m ¼ 1–3) clusters. The key issue we wish to address is how the catalytic behavior is modified in reduced sizes and whether there are new catalytic mechanisms. Our detailed studies based on FeO, Fe2 O2 , and Fe2 O3 clusters carried out using gradient corrected density functional theory [11] demonstrate that the clusters of specific composition can oxidize CO via conventional as well as novel microscopic mechanisms. In particular, Fe2 O3 is shown to be a potential catalyst with novel mechanisms where many of the reactions proceed in an almost barrier less manner.
B.V. Reddy et al. / Chemical Physics Letters 384 (2004) 242–245
The electronic structure studies were carried out within a first principles gradient corrected density functional approach. In the particular implementation used by us, the molecular orbitals are expressed as a linear combination of atomic orbitals centered at the atomic sites. Most of the calculations are based on the DMOL code [12] where the atomic orbitals are taken in a numerical form over a radial mesh of points. While such an approach is numerically attractive and does lead to quantitatively meaningful energy differences, the absolute binding energies are not as accurate. We also carried out supplementary studies using another code (NRLMOL) [13,14] where the atomic orbitals are expressed as a combination of Gaussian functions. This synergism enabled us to carry out extensive studies without enormous computational effort. The gradient corrected functional proposed recently by Perdew et al. [15] were used and the double numerical basis sets with 4p polarization functions for Fe and 3d polarization functions for O and C were used for the DMOL studies. We have studied CO oxidation on FeO, Fe2 O2 , and Fe2 O3 clusters. While FeO and Fe2 O2 correspond to equiatomic composition, the choice of Fe2 O3 was inspired by the bulk a-Fe2 O3 that has a corundum structure composed of FeO6 octahedron connected in a complex manner [16]. Interestingly, the ground state of a free Fe2 O3 cluster is a triangular bipyramid structure with two Fe atoms occupying the apex sites. Here, each oxygen is bound to two Fe atoms as in bulk surfaces.To study CO oxidation, we studied the energetics, reactions and the reaction barriers for the CO and O2 molecules approaching the various clusters and combining to form CO2 . Table 1 gives the ground state bond lengths, binding energy and the spin multiplicity of the CO, O2 and CO2 molecules using the DMOL and the NRLMOL approaches and compare them with the experimental quantities wherever available. Note that in all cases, the DMOL results are close to those based on NRLMOL and experiment indicating that the approach is capable of providing quantitatively meaningful results. To investigate CO oxidation, consider how an approaching CO is adsorbed by the clusters. Fig. 1 shows the lowest energy configurations of the pure Fen Om clusters and the effect of adding the CO molecules. For a single FeO, the CO can attach to the Fe or the O site and
243
Fig. 1. The ground state geometries of bare and CO adsorbed Fen Om (n ¼ 1, 2 and m ¼ 13) clusters. The bond lengths are in A.
with the C or O pointing towards the atoms. In every case, we found that the CO molecule prefers to bind with the C end towards the molecule. Further, the ground state corresponds to the CO molecule bound to the Fe site with a binding energy of 1.54 eV. The CO brought towards the O site formed a CO2 molecule but the energy was 0.49 eV higher than the other configuration. For Fe2 O2 , the ground state is a rhombus configuration with a spin multiplicity of seven. Note that although the Fe to O composition is the same as in FeO, there are four FeO bonds and thus the FeO bonds are expected to be weaker. Successive CO molecules were added for a possible oxidation. The first CO attaches to
Table 1 binding energy (eV) and the spin multiplicity (M) of various molecules based on DMOL, NRLMOL, and experiment Bond length (A), Cluster
O2 CO CO2
DMOL
NRLMOL
Experiment
B. L.
B. E.
M
B. L.
B. E.
M
B. L.
B. E.
M
1.22 1.14 1.17
6.75 11.97 18.64
3 1 1
1.20 1.13 1.16
6.30 11.65 18.16
3 1 1
1.21 1.13 1.16
5.23 11.23 17.08
3 1 1
244
B.V. Reddy et al. / Chemical Physics Letters 384 (2004) 242–245
the Fe site and a configuration where the CO attaches to an oxygen atom was 1.87 eV higher in energy. We explored the possibility of a second CO being oxidized. In the lowest energy configuration, the CO binds to the second Fe site. In addition to kinetics, one has to examine the energetics of the oxidation process. Our calculated binding energies indicate that the change in binding energy as an O is added to Fe2 and Fe2 O to form Fe2 O and Fe2 O2 is 6.50 and 6.25 eV, respectively. The decreasing binding of additional O raises the hope that the oxidation may be favorable in Fe2 O3 . The ground state of Fe2 O3 shown in Fig. 1 is a distorted structure where each Fe is attached to three O showing atoms. The distance between the O sites is 2.6 A that the O–O bonds are broken and a Mulliken population analysis showed that each Fe loses 0.82 e while each O gains 0.54 e . To investigate whether the cluster would oxidize the CO molecule, we studied the energetics of the system for an approaching CO molecule. The CO brought towards an O atom formed the CO2 molecule and this state was 0.38 eV less stable than the ground state where the CO was attached to the Fe site. This energy difference is smaller than for Fe2 O2 suggesting that the oxidation of CO is energetically more favorable compared to the previous case. To further analyze the feasibility of this reaction, we calculated the reaction barrier by bringing a CO molecule towards the oxygen. The reaction coordinate was the center of the Fe–Fe bond and the position of the C atom of the approaching CO molecule. For each separation, the total energy was calculated by relaxing all other degrees of freedom. Fig. 2a shows the energy as a function of separation. Note that the approaching CO molecule experiences a barrier of 0.39 eV. This barrier could be compared to 1.1 eV for the CO bonding to O atoms on a Ru(0 0 0 1) surface [1] and shows that the barrier is smaller and the reaction may be feasible. Can the energetics be altered favorably and the reaction barrier further lowered? We then considered the case where the CO was attached to the Fe site. It was found that one of the O atoms, initially attached to this Fe site, moved away from the Fe site with the CO bond (Fig. 3a). A second CO molecule was added and it could go to the site with a CO or to the site with O. The state where this CO combines with the loose O was the energetically most favorable configuration indicating that the oxidation of CO was now the most preferred channel. We also investigated the potential barrier for the approaching CO. As before, the CO molecule was brought towards the singly bound O site and the total energy of the system was calculated as a function of the separation between the position of Fe and the C. Fig. 2b shows the energy of the system as a function of separation between the C of the approaching CO and the Fe atom. The reaction is almost barrier less! Fig. 3b shows the geometry of this
Fig. 2. The reaction barriers for various reactions. Curve A corresponds to a CO molecule approaching the Fe2 O3 cluster. The reaction coordinate is the distance between the center of Fe–Fe bond and the position of C atom. Curve B corresponds to a CO approaching the Fe site of a Fe2 O3 CO cluster shown in Fig. 3b. The reaction coordinate is the distance between the Fe site and the C atom.
process. That the first CO weakens the O bond to a Fe site for it to be available to another CO for oxidation is truly remarkable as it can be naively looked upon as a CO induced cooperative catalysis of CO. What is even more interesting is the fact that the approaching CO picks up the loose O without being first adsorbed signifying an E–R catalytic mechanism. The oxidation leaves a Fe2 O2 cluster with a CO attached to a Fe site. A similar situation would also occur when a CO picks an O atom from Fe2 O3 and another CO is subsequently adsorbed. The key issue is how the reaction will proceed? We consider the effect of adsorption of an additional O2 or a CO molecule. We begin with O2 . An O2 molecule was placed close to the Fe site with CO bond and close to the Fe site without CO bond. In the case where the O2 molecule is placed close to the site with CO, a bent CO3 intermediate state (see Fig. 3c) molecule formed which was the energetically preferred configuration. Depending on the CO concentration, two processes can occur: (1) A CO2 could be desorbed due to heat of formation, leaving behind an O attached to the Fe site thus recreating the Fe2 O3 cluster; (2) if another CO approaches the CO3 complex, we have found that two CO2 molecules would be formed in a barrier less reaction. To our knowledge, the formation of CO2 molecules via a reaction between CO3 and CO has not been emphasized
B.V. Reddy et al. / Chemical Physics Letters 384 (2004) 242–245
245
charge of )0.70 and )0.54, respectively and it requires an energy of 6.25 and 4.40 eV to remove them from the parent cluster. The increased oxygen content also moves the oxygen levels closer to the highest occupied molecular orbital. We believe that it is the reduced binding of O, its anionic state, and the location of energy levels that make Fe2 O3 a superior oxidant over e.g., Fe2 O2 . To summarize, our studies demonstrate that Fe2 O3 clusters are a potential catalyst that can oxidize CO in the absence or presence of O2 . In the absence of O2 , it is the host O atoms that result in the oxidation via E–R mechanism. In the presence of O2 , the reaction proceeds via a novel process where partial CO adsorption facilitates the reaction and the system goes back and forth between the reduced and the oxidized state. What is most interesting is that CO oxidation in clusters proceeds without any barrier. This is attributed to the flexibility in geometrical rearrangement offered by the reduced size. Nonetheless, since the structure of Fe2 O3 resembles the environment of Fe in bulk a-Fe2 O3 , it is possible that some of the features are applicable to bulk. However, one needs studies on much larger clusters to confirm such a conjecture.
References
Fig. 3. The ground state geometries of various clusters: (a) Fe2 O3 CO; (b) process representing the oxidation of CO in Fe2 O3 CO; (c) Fe2 O2 CO3 complex; (d) O2 bound to a Fe2 O2 CO.
before. In the case where the approaching O2 was close to the site away from CO, it attached to the Fe site as shown in Fig. 3d. We next considered the effect of bringing a CO molecule towards the O2 –Fe2 O2 –CO cluster. When CO approached the site attached to O2 molecule, a CO2 was formed that desorbed from the cluster leaving behind an O attached to the Fe site thus recreating the original cluster. To further probe the nature of the catalytic reaction, we analyzed the energetics and the nature of electronic states in various clusters and carried out a Mulliken population analysis of the total charge. It was found that the oxygen atoms in bare Fe2 O2 and Fe2 O3 carry a
[1] C.H.F. Peden, M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [2] B.C. Guo, K.P. Kerns, A.W. Castleman Jr., J. Chem. Phys. 96 (1992) 8177. [3] J.D. Aiken III, R.G. Finke, J. Mol. Cat. A 145 (1999) 1. [4] B. Hammer, Phys. Rev. Lett. 89 (2002) 016102. [5] W.T. Wallace, R.L. Whetten, J. Am. Chem. Soc. 124 (2002) 7499. [6] H. Heiz, W.-D. Schneider, J. Appl. Phys. 33 (2000) R85. [7] D.M. Cox, R.O. Brickman, K. Creegan, et al., Z. Phys. D 19 (1991) 353. [8] L.-S. Wang, H. Wu, S.R. Desai, Phys. Rev. Lett. 76 (1996) 4853. [9] H.H. Kung (Ed.), Transition Metal Oxides, Elsevier, New York, 1989. [10] P. Li, D.E. Miser, S. Rabiei, R.T. Yadav, M.R. Hajaligol, Appl. Catal. B 43 (2003) 151. [11] W. Kohn, L.J. Sham, Phys. Rev. 140 (1965) A1133. [12] B. Delley, J. Chem. Phys. 92 (1990) 508. [13] M.R. Pederson, K.A. Jackson, Phys. Rev. B 41 (1990) 7453. [14] K.A. Jackson, M.R. Pederson, Phys. Rev. B 42 (1990) 3276. [15] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [16] R.M. Cornell, U. Schwertmann (Eds.), The Iron Oxides: Structure, Properties, Reactions, Occurrence, and Uses, VCH, New York, 1996.
Novel pathway for CO oxidation on a Fe2O3 cluster B.V. Reddy a
a,*
, F. Rasouli a, M.R. Hajaligol a, S.N. Khanna
b
Chrysalis Technologies Inc., Research Center, RD&E, Philip Morris, 615 Maury Street, Richmond, VA 23224, USA b Physics Department, Virginia Commonwealth University, Richmond, VA 23284-2000, USA Received 8 September 2003; in final form 23 November 2003 Published online: 29 December 2003
Abstract Density functional studies of CO oxidation by Fen Om (n ¼ 1, 2 and m ¼ 1–3) clusters are carried out. A new reaction path where a CO molecule adsorbed on to a surface Fe site breaks one FeO bond and a subsequent CO molecule is oxidized by the less coordinated O atom, proceeds in a barrier less manner. An attachment of CO and O2 to this active Fe site results in the formation of CO2 via a CO3 intermediate. The role of charge transfer and the nature of intermediate states in reduced sizes are highlighted. Ó 2003 Elsevier B.V. All rights reserved.
The oxidation of CO is probably the most studied heterogeneous chemical reaction. The notable catalysts include transition metal surfaces, free transition metal clusters, and more recently, transition and noble metal clusters/nanoparticles supported on oxide surfaces [1–6]. The oxidation on bulk surfaces usually proceeds via two mechanisms namely the Langmuir–Hinshelwood (L–H) or Eley–Rideal (E–R) mechanism. In most cases, it proceeds via L–H mechanism where the reacting species are first adsorbed before undergoing the reaction. This involves formation of the intermediate complex followed by the desorption of the CO2 molecule. It has recently been suggested that the oxidation of CO in some cases proceeds via E–R mechanism where the O2 is first dissociatively adsorbed on to the surface and the gas phase CO molecules undergo an oxidation by directly detaching the adsorbed O atoms. The key steps in both processes are the weakening of the O–O bond, for example, via partial charge transfers to the host molecule. This accounts for the observation that the charged [7] and some supported [6] clusters are better catalysts than the corresponding neutral clusters. An important class of the materials that are rich in oxygen and yet contain O atoms with no O–O bonds are
*
Corresponding author. Fax: +1-804-274-4778. E-mail address: [email protected] (B.V. Reddy).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.023
the metal oxides [8,9]. It then raises the question whether one can use metal-oxides to accomplish the required task. Recent experiments in our laboratory[10] indicate that Fe2 O3 nanoparticles do oxidize CO. While this is attractive, one may ask as to how the catalytic properties evolve if one could further reduce the size, namely, go to small atomic clusters. In fact, Wang et al. [8] have recently generated Fen Om clusters, in beams, containing up to four Fe and six O atoms. Employing negative ion photoelectron spectra they propose structural change with size and composition of the above clusters. The observation that the composition can be varied over a wider range than permissible in solids and the advantage of flexible atomic rearrangements makes clusters as the most attractive systems to investigate the catalytic properties. The purpose of this Letter is to investigate the CO oxidation on Fen Om (n ¼ 1, 2 and m ¼ 1–3) clusters. The key issue we wish to address is how the catalytic behavior is modified in reduced sizes and whether there are new catalytic mechanisms. Our detailed studies based on FeO, Fe2 O2 , and Fe2 O3 clusters carried out using gradient corrected density functional theory [11] demonstrate that the clusters of specific composition can oxidize CO via conventional as well as novel microscopic mechanisms. In particular, Fe2 O3 is shown to be a potential catalyst with novel mechanisms where many of the reactions proceed in an almost barrier less manner.
B.V. Reddy et al. / Chemical Physics Letters 384 (2004) 242–245
The electronic structure studies were carried out within a first principles gradient corrected density functional approach. In the particular implementation used by us, the molecular orbitals are expressed as a linear combination of atomic orbitals centered at the atomic sites. Most of the calculations are based on the DMOL code [12] where the atomic orbitals are taken in a numerical form over a radial mesh of points. While such an approach is numerically attractive and does lead to quantitatively meaningful energy differences, the absolute binding energies are not as accurate. We also carried out supplementary studies using another code (NRLMOL) [13,14] where the atomic orbitals are expressed as a combination of Gaussian functions. This synergism enabled us to carry out extensive studies without enormous computational effort. The gradient corrected functional proposed recently by Perdew et al. [15] were used and the double numerical basis sets with 4p polarization functions for Fe and 3d polarization functions for O and C were used for the DMOL studies. We have studied CO oxidation on FeO, Fe2 O2 , and Fe2 O3 clusters. While FeO and Fe2 O2 correspond to equiatomic composition, the choice of Fe2 O3 was inspired by the bulk a-Fe2 O3 that has a corundum structure composed of FeO6 octahedron connected in a complex manner [16]. Interestingly, the ground state of a free Fe2 O3 cluster is a triangular bipyramid structure with two Fe atoms occupying the apex sites. Here, each oxygen is bound to two Fe atoms as in bulk surfaces.To study CO oxidation, we studied the energetics, reactions and the reaction barriers for the CO and O2 molecules approaching the various clusters and combining to form CO2 . Table 1 gives the ground state bond lengths, binding energy and the spin multiplicity of the CO, O2 and CO2 molecules using the DMOL and the NRLMOL approaches and compare them with the experimental quantities wherever available. Note that in all cases, the DMOL results are close to those based on NRLMOL and experiment indicating that the approach is capable of providing quantitatively meaningful results. To investigate CO oxidation, consider how an approaching CO is adsorbed by the clusters. Fig. 1 shows the lowest energy configurations of the pure Fen Om clusters and the effect of adding the CO molecules. For a single FeO, the CO can attach to the Fe or the O site and
243
Fig. 1. The ground state geometries of bare and CO adsorbed Fen Om (n ¼ 1, 2 and m ¼ 13) clusters. The bond lengths are in A.
with the C or O pointing towards the atoms. In every case, we found that the CO molecule prefers to bind with the C end towards the molecule. Further, the ground state corresponds to the CO molecule bound to the Fe site with a binding energy of 1.54 eV. The CO brought towards the O site formed a CO2 molecule but the energy was 0.49 eV higher than the other configuration. For Fe2 O2 , the ground state is a rhombus configuration with a spin multiplicity of seven. Note that although the Fe to O composition is the same as in FeO, there are four FeO bonds and thus the FeO bonds are expected to be weaker. Successive CO molecules were added for a possible oxidation. The first CO attaches to
Table 1 binding energy (eV) and the spin multiplicity (M) of various molecules based on DMOL, NRLMOL, and experiment Bond length (A), Cluster
O2 CO CO2
DMOL
NRLMOL
Experiment
B. L.
B. E.
M
B. L.
B. E.
M
B. L.
B. E.
M
1.22 1.14 1.17
6.75 11.97 18.64
3 1 1
1.20 1.13 1.16
6.30 11.65 18.16
3 1 1
1.21 1.13 1.16
5.23 11.23 17.08
3 1 1
244
B.V. Reddy et al. / Chemical Physics Letters 384 (2004) 242–245
the Fe site and a configuration where the CO attaches to an oxygen atom was 1.87 eV higher in energy. We explored the possibility of a second CO being oxidized. In the lowest energy configuration, the CO binds to the second Fe site. In addition to kinetics, one has to examine the energetics of the oxidation process. Our calculated binding energies indicate that the change in binding energy as an O is added to Fe2 and Fe2 O to form Fe2 O and Fe2 O2 is 6.50 and 6.25 eV, respectively. The decreasing binding of additional O raises the hope that the oxidation may be favorable in Fe2 O3 . The ground state of Fe2 O3 shown in Fig. 1 is a distorted structure where each Fe is attached to three O showing atoms. The distance between the O sites is 2.6 A that the O–O bonds are broken and a Mulliken population analysis showed that each Fe loses 0.82 e while each O gains 0.54 e . To investigate whether the cluster would oxidize the CO molecule, we studied the energetics of the system for an approaching CO molecule. The CO brought towards an O atom formed the CO2 molecule and this state was 0.38 eV less stable than the ground state where the CO was attached to the Fe site. This energy difference is smaller than for Fe2 O2 suggesting that the oxidation of CO is energetically more favorable compared to the previous case. To further analyze the feasibility of this reaction, we calculated the reaction barrier by bringing a CO molecule towards the oxygen. The reaction coordinate was the center of the Fe–Fe bond and the position of the C atom of the approaching CO molecule. For each separation, the total energy was calculated by relaxing all other degrees of freedom. Fig. 2a shows the energy as a function of separation. Note that the approaching CO molecule experiences a barrier of 0.39 eV. This barrier could be compared to 1.1 eV for the CO bonding to O atoms on a Ru(0 0 0 1) surface [1] and shows that the barrier is smaller and the reaction may be feasible. Can the energetics be altered favorably and the reaction barrier further lowered? We then considered the case where the CO was attached to the Fe site. It was found that one of the O atoms, initially attached to this Fe site, moved away from the Fe site with the CO bond (Fig. 3a). A second CO molecule was added and it could go to the site with a CO or to the site with O. The state where this CO combines with the loose O was the energetically most favorable configuration indicating that the oxidation of CO was now the most preferred channel. We also investigated the potential barrier for the approaching CO. As before, the CO molecule was brought towards the singly bound O site and the total energy of the system was calculated as a function of the separation between the position of Fe and the C. Fig. 2b shows the energy of the system as a function of separation between the C of the approaching CO and the Fe atom. The reaction is almost barrier less! Fig. 3b shows the geometry of this
Fig. 2. The reaction barriers for various reactions. Curve A corresponds to a CO molecule approaching the Fe2 O3 cluster. The reaction coordinate is the distance between the center of Fe–Fe bond and the position of C atom. Curve B corresponds to a CO approaching the Fe site of a Fe2 O3 CO cluster shown in Fig. 3b. The reaction coordinate is the distance between the Fe site and the C atom.
process. That the first CO weakens the O bond to a Fe site for it to be available to another CO for oxidation is truly remarkable as it can be naively looked upon as a CO induced cooperative catalysis of CO. What is even more interesting is the fact that the approaching CO picks up the loose O without being first adsorbed signifying an E–R catalytic mechanism. The oxidation leaves a Fe2 O2 cluster with a CO attached to a Fe site. A similar situation would also occur when a CO picks an O atom from Fe2 O3 and another CO is subsequently adsorbed. The key issue is how the reaction will proceed? We consider the effect of adsorption of an additional O2 or a CO molecule. We begin with O2 . An O2 molecule was placed close to the Fe site with CO bond and close to the Fe site without CO bond. In the case where the O2 molecule is placed close to the site with CO, a bent CO3 intermediate state (see Fig. 3c) molecule formed which was the energetically preferred configuration. Depending on the CO concentration, two processes can occur: (1) A CO2 could be desorbed due to heat of formation, leaving behind an O attached to the Fe site thus recreating the Fe2 O3 cluster; (2) if another CO approaches the CO3 complex, we have found that two CO2 molecules would be formed in a barrier less reaction. To our knowledge, the formation of CO2 molecules via a reaction between CO3 and CO has not been emphasized
B.V. Reddy et al. / Chemical Physics Letters 384 (2004) 242–245
245
charge of )0.70 and )0.54, respectively and it requires an energy of 6.25 and 4.40 eV to remove them from the parent cluster. The increased oxygen content also moves the oxygen levels closer to the highest occupied molecular orbital. We believe that it is the reduced binding of O, its anionic state, and the location of energy levels that make Fe2 O3 a superior oxidant over e.g., Fe2 O2 . To summarize, our studies demonstrate that Fe2 O3 clusters are a potential catalyst that can oxidize CO in the absence or presence of O2 . In the absence of O2 , it is the host O atoms that result in the oxidation via E–R mechanism. In the presence of O2 , the reaction proceeds via a novel process where partial CO adsorption facilitates the reaction and the system goes back and forth between the reduced and the oxidized state. What is most interesting is that CO oxidation in clusters proceeds without any barrier. This is attributed to the flexibility in geometrical rearrangement offered by the reduced size. Nonetheless, since the structure of Fe2 O3 resembles the environment of Fe in bulk a-Fe2 O3 , it is possible that some of the features are applicable to bulk. However, one needs studies on much larger clusters to confirm such a conjecture.
References
Fig. 3. The ground state geometries of various clusters: (a) Fe2 O3 CO; (b) process representing the oxidation of CO in Fe2 O3 CO; (c) Fe2 O2 CO3 complex; (d) O2 bound to a Fe2 O2 CO.
before. In the case where the approaching O2 was close to the site away from CO, it attached to the Fe site as shown in Fig. 3d. We next considered the effect of bringing a CO molecule towards the O2 –Fe2 O2 –CO cluster. When CO approached the site attached to O2 molecule, a CO2 was formed that desorbed from the cluster leaving behind an O attached to the Fe site thus recreating the original cluster. To further probe the nature of the catalytic reaction, we analyzed the energetics and the nature of electronic states in various clusters and carried out a Mulliken population analysis of the total charge. It was found that the oxygen atoms in bare Fe2 O2 and Fe2 O3 carry a
[1] C.H.F. Peden, M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [2] B.C. Guo, K.P. Kerns, A.W. Castleman Jr., J. Chem. Phys. 96 (1992) 8177. [3] J.D. Aiken III, R.G. Finke, J. Mol. Cat. A 145 (1999) 1. [4] B. Hammer, Phys. Rev. Lett. 89 (2002) 016102. [5] W.T. Wallace, R.L. Whetten, J. Am. Chem. Soc. 124 (2002) 7499. [6] H. Heiz, W.-D. Schneider, J. Appl. Phys. 33 (2000) R85. [7] D.M. Cox, R.O. Brickman, K. Creegan, et al., Z. Phys. D 19 (1991) 353. [8] L.-S. Wang, H. Wu, S.R. Desai, Phys. Rev. Lett. 76 (1996) 4853. [9] H.H. Kung (Ed.), Transition Metal Oxides, Elsevier, New York, 1989. [10] P. Li, D.E. Miser, S. Rabiei, R.T. Yadav, M.R. Hajaligol, Appl. Catal. B 43 (2003) 151. [11] W. Kohn, L.J. Sham, Phys. Rev. 140 (1965) A1133. [12] B. Delley, J. Chem. Phys. 92 (1990) 508. [13] M.R. Pederson, K.A. Jackson, Phys. Rev. B 41 (1990) 7453. [14] K.A. Jackson, M.R. Pederson, Phys. Rev. B 42 (1990) 3276. [15] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [16] R.M. Cornell, U. Schwertmann (Eds.), The Iron Oxides: Structure, Properties, Reactions, Occurrence, and Uses, VCH, New York, 1996.