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Novel mechanism for oxidation of CO by Fe2O3 clusters
Fuel 83 (2004) 1537–1541 www.fuelfirst.com
Novel mechanism for oxidation of CO by Fe2O3 clusters B.V. Reddya,*, F. Rasoulia, M.R. Hajaligola, S.N. Khannab,1 b
a Research Center, Philip Morris USA, P.O. Box 26583, Richmond, VA 23261, USA Department of Physics, Virginia Commonwealth University, Richmond, VA 23284-2000, USA
Received 14 May 2003; revised 9 December 2003; accepted 16 December 2003; available online 5 March 2004
Abstract The oxidation of CO by Fe2O3 clusters has been studied using a first principles gradient corrected density functional approach. It is shown that the direct oxidation of CO via surface oxygen atoms is feasible with a low barrier of 0.4 eV. The present studies also suggest a new indirect mechanism for CO oxidation. Here, the first CO molecule adsorbed on a surface Fe site breaks one of the FeO bonds and a subsequent CO molecule is oxidized by the less coordinated O atom in an almost barrierless reaction pathway. Subsequent attachment of CO and O2 to Fe sites results in the formation of a CO3 complex where the heat of formation is enough to allow CO2 to leave the surface reverting the system back to Fe2O3. The role of charge transfer and the nature of the transition states are highlighted. It is shown that small clusters are ferromagnetic indicating that super-exchange interactions of the antiferromagnetic bulk are modified at the reduced sizes. q 2004 Elsevier Ltd. All rights reserved. Keywords: Iron oxide; Catalysis; CO; O2; Clusters
The conversion of CO and NO are probably the most important industrial chemical reactions [1 – 4]. In particular, the oxidation of CO to CO2 has attracted the most attention. The binding energy of CO, O2, and CO2 are 11.23, 5.23, and 17.08 eV, respectively. Thus, from a purely energetics point of view, the oxidation of CO in the presence of O2 is an energetically favorable reaction and should proceed automatically. However, the presence of ionic character in CO bond and the covalent nature in CO and O2 requires charge redistribution and breaking of localized bonds. Both of these actions lead to the formation of a reaction barrier and, under ordinary conditions, the reaction requires an external catalyst to facilitate these tasks. The key steps in the reaction are then the weakening of the covalent O –O bond in O2 and charge rearrangement to facilitate the formation of CO2. In the past, transition metal surfaces and powders, bulk oxides, and more recently, free and supported transition and noble metal clusters have been employed to accomplish the desired task [1 – 4]. Two molecular mechanisms have been invoked to understand the oxidation process. In most cases [5], the reactants are first adsorbed to form an intermediate CO3 complex and the CO2 is desorbed leaving behind the oxygen atoms. Here the catalyst merely helps in the formation of * Corresponding author. Tel.: þ 1-804-274-2200; fax: þ1-804-274-4778. E-mail address: [email protected] (B.V. Reddy). 1 PMUSA Research Center Visiting Scientist. 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2003.12.015
the intermediate complex via charge transfer without first breaking the O –O bonds. Very recently, another mechanism [6] has been proposed for the oxidation on Ru surfaces. Here the surface first adsorbs O2 dissociatively and the oncoming molecules pick up the dissociated O atoms forming CO2 without being first adsorbed on the surface. The fact that the oxidation requires charge rearrangement and breaking of the covalent bonds does raise an interesting possibility. If one could identify systems that contain dissociated oxygen atoms and further, if the energy required to detach oxygen atoms is less than the gain in binding to CO, the oxidation should be feasible. The occurrence of the charged sites would also ensure the continuity of the conversion as the adsorbed O2 atoms at the charged sites would lead to a charged molecule which is known to weaken the O –O bond. This could be accomplished in a variety of ways. Recent experiments on charged clusters and the subsequent theoretical studies have established that several anionic and cationic clusters could provide such systems. In particular, numerous experimental and theoretical studies over the past 10 years have suggested that Aun clusters could provide such systems [3,4]. Detailed theoretical studies [3,4] on Aun clusters supported on a variety of substrates have demonstrated that the charge transfer from the support plays a key role in breaking the O –O bond. The reaction is, however, size specific and the difficulties associated with
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generating size selected clusters in large quantities and finding ways to store them are the obstacles that need to be resolved before such finding could be translated in to any applications. In this article, we show that clusters of metallic oxides may offer a viable alternative. In particular, we show that a Fe2O3 cluster could provide an ideal system as it can oxidize CO via a totally new mechanism [7] where the initial CO adsorption leads to the formation of active catalytic sites that get continuously regenerated to sustain the catalytic conversion. Iron forms stable bulk oxides at several compositions namely FeO, Fe3O4 and Fe2O3. The key distinction is the oxygen to metal ratio that increases from 1 to 1.5 and is reflected in the decrease of the number of Fe sites bonded to an O site. In order to examine the effect of this composition on the catalytic activity, we carried out model studies on FeO, Fe2O2, Fe3O4 and Fe2O3 representative clusters. The actual theoretical electronic structure studies were carried out using the first principles gradient corrected density functional theory [8]. Apart from the choice of exchange correlation functional, the only input parameters in these studies are the atomic number of atoms and the number of electrons. In the implementation employed by us, the molecular orbitals are expressed as a linear combination of atomic orbitals centered at the atomic sites. The particular implementation used by us is called the DMOL code [9] where the atomic orbitals are taken in a numerical form over a radial mesh of points. All calculations were carried out at the all-electron level using the double numeric basis sets with 4p polarization functions for Fe and 3d polarization functions for O and C. The exchange correlation effects were included via a gradient corrected functional proposed recently by Perdew et al. [10] We start by briefly describing the Fe2O3 cluster and its features vis a vis the bulk. Bulk a-Fe2O3 has a corundum structure. It can be described as a hexagonal close-packed array of oxygen atoms in which two-thirds of the octahedral holes are occupied by the Fe atoms. The structure can also be viewed as the FeO6 octahedron linked together by sharing edges, faces or vertices. A small cluster that can depict the bulk structure and composition is a Fe2O3 cluster consisting of an approximate triangular bipyramid structure with two Fe atoms occupying the apex sites. Here, the oxygen sites are bonded to two Fe atoms, which is similar to the situation in its bulk surfaces. In addition to studies on pure clusters, we investigated the bonding of one or several CO and O2 molecules. In all cases, the position of atoms was optimized by moving them in the direction of forces. Since bulk Fe is an itinerant ferromagnet [11] with a magnetic moment of 2.2 mB per atom and some of the iron oxides are magnetic, the clusters could carry net spin magnetic moments. Consequently, different spin states were studied to find the lowest energy configurations. In any catalytic activity, the reaction barriers determine the reaction rates. We therefore carried out an in-depth study of all barriers via a judicious choice of the reaction coordinate and calculation of the energies by optimizing the remaining degrees of freedom.
Fig. 1. The ground state geometries of FenOm clusters. (a) FeO; (b) Fe2O2; (c) Fe2O3; and (d) Fe3O4. The unshaded spheres represent the O atoms ˚ . The while the shaded spheres are the Fe sites. The bond lengths are in A Fe –Fe distance in Fe3O4 is greater than in other clusters. We have, however, shown the Fe–Fe bond in this case for the sake of clarity.
While the focus of the current work is a Fe2O3 cluster, we first present results on small FenOm clusters to show that the energy required to remove an O atom decreases as one increases the ratio of O to Fe atoms. In Fig. 1 we show the lowest energy configurations of the pure FenOm clusters along with the calculated bond lengths. The ground state of ˚ and a FeO is a linear quintet with a bond length of 1.62 A binding energy of 6.66 eV. For Fe2O2, the ground state is a rhombus with a binding energy of 16.49 eV and a spin multiplicity of 7. Here, each Fe site has a local magnetic moment of 2.93 mB. The ground state of Fe3O4 is a Fe3 triangle decorated by two O atoms at the bridge site of an edge. The other two O atoms occupy the two bridge sites of the remainder of the edges of the Fe triangle. Note that Fe ions in bulk Fe3O4 exhibit two kinds of oxidation states as they are bonded to different number of oxygen atoms. It is interesting that small clusters at this size range already begin to show some signatures of their bulk counterparts. Wang and co-workers [12] in a recent study obtained Fe triad with three bridging and one hollow site oxygen atoms for the structure of Fe3O4. Note that this is inconsistent with our results as well as the observed bonding order in the bulk phase. For Fe2O3, the ground state is a distorted triangular bipyramid as shown. The spin configuration is ferrimagnetic with a net moment of 4.0 mB. Note that in all cases, the O –O separations are significantly larger than the bond length in ˚ ) indicating that the oxygen atoms are only bound O2 (1.22 A to Fe sites. A Mulliken population analysis indicates that in all the clusters, there is a charge transfer from Fe to O. The existence of O atoms at the sites with negative charged state
B.V. Reddy et al. / Fuel 83 (2004) 1537–1541
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Fig. 2. The energy required to remove an O atom from various clusters.
is promising for the catalytic activity. The only remaining condition, then, is the energy, DE; required to remove the O atoms DE ¼ EðFen Om21 Þ þ EOÞ 2 EðFen Om Þ:
ð1Þ
In Fig. 2, we show this energy for Fe2O, Fe2O2, Fe3O4 and Fe2O3 clusters. For Fe2O and Fe2O2, the energy is significantly high. It decreases in going to Fe3O4 and is only 4.17 eV for Fe2O3! This clearly shows that the Fe2O3 cluster has all the necessary conditions to be a good catalyst. Does it really happen and what is the mechanism? As previously stated, the ground state of Fe2O3 shown in Fig. 1 is a distorted ferromagnetic structure with each Fe attached to three O atoms. We studied the energetics of the system as successive CO molecules are brought towards the Fe, O or a bridge site. As in previous cases, the most stable configuration corresponds to the CO attaching to a Fe site. For the case where the CO was forced to approach an O site, it did form a CO2 molecule but 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 not very high and the oxidation of CO may be possible. We further 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 remaining degrees of freedom were optimized. Fig. 3a shows the energy as a function of separation. The approaching CO molecule does experience a barrier of 0.39 eV. As a comparison, the corresponding barrier for the CO bonding to O atoms on a Ru(0001) surface [6] is about 1.1 eV. Since the CO oxidation is known to occur on Ru surfaces, it should be feasible here as well. Can the reaction barrier be further lowered? For the case where CO became attached to the Fe site, we noted that one of the host O atoms, initially attached to this Fe site detached and moved away from the site (Fig. 4a). Can this loosely bound O atom oxidize a CO molecule? A second CO molecule was added that could go to a Fe site or
Fig. 3. The reaction barriers for various reactions. Curve A corresponds to a CO molecule approaching the Fe2O3 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 Fe2O3CO. The reaction coordinate is the distance between the Fe site and the C atom.
grab an O. The process where this CO combines with the loose O to form CO2 was the energetically most favorable channel. What was even more interesting was the reaction barrier obtained by calculating the total energy of the system as a function of the separation between the position of Fe and the C while relaxing the position of all other atoms. Fig. 3b shows the energy of the system as a function of separation between the C of the approaching CO and the Fe atom. It is important to note that the reaction is almost barrierless. In Fig. 4b we have also shown the geometry of this process. To sum up, these investigations point towards a new mechanism where the first CO creates an active O that oxidizes the subsequent CO in a barrierless reaction. Can the reaction be sustained? The oxidation of CO now reduces the Fe2O3 to a Fe2O2 cluster with a CO attached to a Fe site. A similar situation would also occur in the case where the CO picked an O atom from Fe2O3 and another CO is subsequently adsorbed. The key issue is how the reaction can be sustained? The two possibilities to be considered are the addition of a CO or addition of an O2 molecule. To begin, we studied the energetics of an O2 molecule added to the Fe site with CO bond and then to the Fe site without CO bond. The case where the O2 molecule was placed close to the site with CO, a bent CO3 intermediate state (see Fig. 4c) was formed. The heat of formation is 3.77 eV which is higher than the energy
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Fig. 4. The ground state geometries of various clusters. (a) Fe2O3CO. (b) Process describing the approach of a second CO molecule to from a CO2 with the loosely bound O atom in CO-Fe2O3. (c) Fe2O2CO3 complex. (d) O2 bound to a Fe2O2CO.
required to release a CO2 molecule suggesting its release. The eliminated CO2 will, however, leave behind an O attached to the Fe site thus annealing the O hole and recreating the Fe2O3 cluster. The case where the approaching O2 was close to the site away from CO, it merely attached to the Fe site as shown in Fig. 4d. We next considered the kinetics of a CO molecule brought towards the CO – Fe2O2 –O2 cluster. When CO approached the site of the O2 molecule, a CO2 was formed that left the cluster leaving behind a O attached to the Fe site and recreating the original cluster. For the case where an additional CO was first added, it bonded with the Fe site away from the previous CO bonded site. A subsequent O2 approached this site to form a CO3 complex. Due to the heat of formation,
a CO2 left the molecule, leaving behind an O atom and recreating the original cluster. Note that the original CO remains attached to the other Fe site during the whole process. As mentioned in the beginning, the breaking of the covalent bond and the charge rearrangement are the principal controlling factors for the aforesaid catalysis. To illustrate the role of charge we carried out a Mulliken population analysis of the total charge. The resulting analysis indicated that the oxygen atoms in bare Fe2O2 and Fe2O3 carry a charge of 2 0.70 and 2 0.54, respectively. The existence of the charged sites, in our view, facilitates the conversion process. A similar phenomenon has been observed in CO oxidation on anionic and supported clusters
B.V. Reddy et al. / Fuel 83 (2004) 1537–1541
[4]. As has been previously suggested, the charge transfer facilitates the cleavage of the O – O bond needed for sustaining the reaction. These details will be presented in a separate publication. While a Fe2O3 cluster exhibits catalytic activity like Fe2O3 nanoparticles or bulk surfaces, the cluster has other properties that are very different. One such feature is the magnetic coupling. As mentioned before, bulk a-Fe2O3 is antiferromagnetic. There is no direct bonding between the Fe sites and the magnetic interactions proceed via the intermediate O atoms. The Fe sites then couple antiferromagnetically because of the superexchange. Our results on Fe2O3, on the other hand, show a ferromagnetic ground state with a net moment of 4.0 mb. The ferromagnetic coupling between the Fe sites indicates that small particles would exhibit net magnetic moments. Interestingly, our findings are consistent with experiments [13] on nanocrystallites of Fe2O3, that show that the magnetic susceptibility first increases and then decreases with increasing temperatures typical for superparamagnetic particles [12]. Note that the transition from antiferromagnetic to ferromagnetic state at reduced sizes has previously been seen on Crn [14] and Mnn [15] clusters. The nature of magnetic interactions in those systems, however, are quite different. We are in the process of investigating if the Morin transition [12] observed in large crystallites could be observed in small sizes. These, however, will form the basis of another study. To summarize, our studies elucidate the microscopic mechanism that can enable Fe2O3 to be a potential catalyst for CO oxidation in the absence or presence of O2. In the absence of O2, it is the lattice O atoms that result in the oxidation and as such, it is an oxidant rather than a catalyst. In the presence of O2, however, the reaction proceeds via
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first CO adsorption and then subsequent oxidation. Finally, we have shown that the magnetic structure is modified at the reduced sizes. Since bulk Fe3O4 is magnetic while Fe2O3 is antiferromagnetic, it is interesting to investigate how the local magnetic coupling changes during the reduction or oxidation at the surface. These will be addressed in a separate publication.
References [1] Kolmakov AA, Goodman DW. In: Khanna SN, Castleman AW Jr, editors. Quantum phenomenon in clusters and nanostructures. Berlin: Springer; 2003. Guo BC, Kerns KP, Castleman Jr AW. J Chem Phys 1992;96:8177. [2] Hammer B. Phys Rev Lett 2002;89:016102. [3] Wallace WT, Whetten RL. J Am Chem Soc 2002;124:7499. Hakkinen H, Landmann U. J Am Chem Soc 2001;123:9704. [4] Heiz H, Schneider W-D. J Appl Phys 2000;33:R85. [5] Peden CHF, Valden M, Lai X, Goodman DW. Science 1998;281: 1647. [6] Stampfli C, Scheffler M. Phys Rev Lett 1997;78:1500. [7] Reddy BV, Rasouli F, Hajaligol MR, Khanna SN. Chem. Phys. Lett. 2004;384:242. [8] Kohn W, Sham LJ. Phys Rev 1965;140:A1133. [9] Delley B. J Chem Phys 1990;92:508. [10] Perdew JP, Burke K, Ernzerhof M. Phys Rev Lett 1996;77:3865. [11] Cornell RM, Schwertmann U, editors. The iron oxides: structure, properties, reactions, occurrence, and uses. New York: VCH; 1996. [12] Wang L-S, Wu H, Desai SR. Phys Rev Lett 1996;76:4853. Kung HH, editor. Transition metal oxides. New York: Elsevier; 1989. [13] Yamamoto N. J Phys Soc Jpn 1968;24:23. [14] Bloomfield LA, Deng J, Zhang H, Emmert JM. In: Jena P, Khanna SN, Rao BK, editors. Clusters and nanostructure materials. Singapore: World Scientific; 2000. p. 131. [15] Knickelbein MB. Phys Rev Lett 2001;86:5255.
Novel mechanism for oxidation of CO by Fe2O3 clusters B.V. Reddya,*, F. Rasoulia, M.R. Hajaligola, S.N. Khannab,1 b
a Research Center, Philip Morris USA, P.O. Box 26583, Richmond, VA 23261, USA Department of Physics, Virginia Commonwealth University, Richmond, VA 23284-2000, USA
Received 14 May 2003; revised 9 December 2003; accepted 16 December 2003; available online 5 March 2004
Abstract The oxidation of CO by Fe2O3 clusters has been studied using a first principles gradient corrected density functional approach. It is shown that the direct oxidation of CO via surface oxygen atoms is feasible with a low barrier of 0.4 eV. The present studies also suggest a new indirect mechanism for CO oxidation. Here, the first CO molecule adsorbed on a surface Fe site breaks one of the FeO bonds and a subsequent CO molecule is oxidized by the less coordinated O atom in an almost barrierless reaction pathway. Subsequent attachment of CO and O2 to Fe sites results in the formation of a CO3 complex where the heat of formation is enough to allow CO2 to leave the surface reverting the system back to Fe2O3. The role of charge transfer and the nature of the transition states are highlighted. It is shown that small clusters are ferromagnetic indicating that super-exchange interactions of the antiferromagnetic bulk are modified at the reduced sizes. q 2004 Elsevier Ltd. All rights reserved. Keywords: Iron oxide; Catalysis; CO; O2; Clusters
The conversion of CO and NO are probably the most important industrial chemical reactions [1 – 4]. In particular, the oxidation of CO to CO2 has attracted the most attention. The binding energy of CO, O2, and CO2 are 11.23, 5.23, and 17.08 eV, respectively. Thus, from a purely energetics point of view, the oxidation of CO in the presence of O2 is an energetically favorable reaction and should proceed automatically. However, the presence of ionic character in CO bond and the covalent nature in CO and O2 requires charge redistribution and breaking of localized bonds. Both of these actions lead to the formation of a reaction barrier and, under ordinary conditions, the reaction requires an external catalyst to facilitate these tasks. The key steps in the reaction are then the weakening of the covalent O –O bond in O2 and charge rearrangement to facilitate the formation of CO2. In the past, transition metal surfaces and powders, bulk oxides, and more recently, free and supported transition and noble metal clusters have been employed to accomplish the desired task [1 – 4]. Two molecular mechanisms have been invoked to understand the oxidation process. In most cases [5], the reactants are first adsorbed to form an intermediate CO3 complex and the CO2 is desorbed leaving behind the oxygen atoms. Here the catalyst merely helps in the formation of * Corresponding author. Tel.: þ 1-804-274-2200; fax: þ1-804-274-4778. E-mail address: [email protected] (B.V. Reddy). 1 PMUSA Research Center Visiting Scientist. 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2003.12.015
the intermediate complex via charge transfer without first breaking the O –O bonds. Very recently, another mechanism [6] has been proposed for the oxidation on Ru surfaces. Here the surface first adsorbs O2 dissociatively and the oncoming molecules pick up the dissociated O atoms forming CO2 without being first adsorbed on the surface. The fact that the oxidation requires charge rearrangement and breaking of the covalent bonds does raise an interesting possibility. If one could identify systems that contain dissociated oxygen atoms and further, if the energy required to detach oxygen atoms is less than the gain in binding to CO, the oxidation should be feasible. The occurrence of the charged sites would also ensure the continuity of the conversion as the adsorbed O2 atoms at the charged sites would lead to a charged molecule which is known to weaken the O –O bond. This could be accomplished in a variety of ways. Recent experiments on charged clusters and the subsequent theoretical studies have established that several anionic and cationic clusters could provide such systems. In particular, numerous experimental and theoretical studies over the past 10 years have suggested that Aun clusters could provide such systems [3,4]. Detailed theoretical studies [3,4] on Aun clusters supported on a variety of substrates have demonstrated that the charge transfer from the support plays a key role in breaking the O –O bond. The reaction is, however, size specific and the difficulties associated with
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generating size selected clusters in large quantities and finding ways to store them are the obstacles that need to be resolved before such finding could be translated in to any applications. In this article, we show that clusters of metallic oxides may offer a viable alternative. In particular, we show that a Fe2O3 cluster could provide an ideal system as it can oxidize CO via a totally new mechanism [7] where the initial CO adsorption leads to the formation of active catalytic sites that get continuously regenerated to sustain the catalytic conversion. Iron forms stable bulk oxides at several compositions namely FeO, Fe3O4 and Fe2O3. The key distinction is the oxygen to metal ratio that increases from 1 to 1.5 and is reflected in the decrease of the number of Fe sites bonded to an O site. In order to examine the effect of this composition on the catalytic activity, we carried out model studies on FeO, Fe2O2, Fe3O4 and Fe2O3 representative clusters. The actual theoretical electronic structure studies were carried out using the first principles gradient corrected density functional theory [8]. Apart from the choice of exchange correlation functional, the only input parameters in these studies are the atomic number of atoms and the number of electrons. In the implementation employed by us, the molecular orbitals are expressed as a linear combination of atomic orbitals centered at the atomic sites. The particular implementation used by us is called the DMOL code [9] where the atomic orbitals are taken in a numerical form over a radial mesh of points. All calculations were carried out at the all-electron level using the double numeric basis sets with 4p polarization functions for Fe and 3d polarization functions for O and C. The exchange correlation effects were included via a gradient corrected functional proposed recently by Perdew et al. [10] We start by briefly describing the Fe2O3 cluster and its features vis a vis the bulk. Bulk a-Fe2O3 has a corundum structure. It can be described as a hexagonal close-packed array of oxygen atoms in which two-thirds of the octahedral holes are occupied by the Fe atoms. The structure can also be viewed as the FeO6 octahedron linked together by sharing edges, faces or vertices. A small cluster that can depict the bulk structure and composition is a Fe2O3 cluster consisting of an approximate triangular bipyramid structure with two Fe atoms occupying the apex sites. Here, the oxygen sites are bonded to two Fe atoms, which is similar to the situation in its bulk surfaces. In addition to studies on pure clusters, we investigated the bonding of one or several CO and O2 molecules. In all cases, the position of atoms was optimized by moving them in the direction of forces. Since bulk Fe is an itinerant ferromagnet [11] with a magnetic moment of 2.2 mB per atom and some of the iron oxides are magnetic, the clusters could carry net spin magnetic moments. Consequently, different spin states were studied to find the lowest energy configurations. In any catalytic activity, the reaction barriers determine the reaction rates. We therefore carried out an in-depth study of all barriers via a judicious choice of the reaction coordinate and calculation of the energies by optimizing the remaining degrees of freedom.
Fig. 1. The ground state geometries of FenOm clusters. (a) FeO; (b) Fe2O2; (c) Fe2O3; and (d) Fe3O4. The unshaded spheres represent the O atoms ˚ . The while the shaded spheres are the Fe sites. The bond lengths are in A Fe –Fe distance in Fe3O4 is greater than in other clusters. We have, however, shown the Fe–Fe bond in this case for the sake of clarity.
While the focus of the current work is a Fe2O3 cluster, we first present results on small FenOm clusters to show that the energy required to remove an O atom decreases as one increases the ratio of O to Fe atoms. In Fig. 1 we show the lowest energy configurations of the pure FenOm clusters along with the calculated bond lengths. The ground state of ˚ and a FeO is a linear quintet with a bond length of 1.62 A binding energy of 6.66 eV. For Fe2O2, the ground state is a rhombus with a binding energy of 16.49 eV and a spin multiplicity of 7. Here, each Fe site has a local magnetic moment of 2.93 mB. The ground state of Fe3O4 is a Fe3 triangle decorated by two O atoms at the bridge site of an edge. The other two O atoms occupy the two bridge sites of the remainder of the edges of the Fe triangle. Note that Fe ions in bulk Fe3O4 exhibit two kinds of oxidation states as they are bonded to different number of oxygen atoms. It is interesting that small clusters at this size range already begin to show some signatures of their bulk counterparts. Wang and co-workers [12] in a recent study obtained Fe triad with three bridging and one hollow site oxygen atoms for the structure of Fe3O4. Note that this is inconsistent with our results as well as the observed bonding order in the bulk phase. For Fe2O3, the ground state is a distorted triangular bipyramid as shown. The spin configuration is ferrimagnetic with a net moment of 4.0 mB. Note that in all cases, the O –O separations are significantly larger than the bond length in ˚ ) indicating that the oxygen atoms are only bound O2 (1.22 A to Fe sites. A Mulliken population analysis indicates that in all the clusters, there is a charge transfer from Fe to O. The existence of O atoms at the sites with negative charged state
B.V. Reddy et al. / Fuel 83 (2004) 1537–1541
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Fig. 2. The energy required to remove an O atom from various clusters.
is promising for the catalytic activity. The only remaining condition, then, is the energy, DE; required to remove the O atoms DE ¼ EðFen Om21 Þ þ EOÞ 2 EðFen Om Þ:
ð1Þ
In Fig. 2, we show this energy for Fe2O, Fe2O2, Fe3O4 and Fe2O3 clusters. For Fe2O and Fe2O2, the energy is significantly high. It decreases in going to Fe3O4 and is only 4.17 eV for Fe2O3! This clearly shows that the Fe2O3 cluster has all the necessary conditions to be a good catalyst. Does it really happen and what is the mechanism? As previously stated, the ground state of Fe2O3 shown in Fig. 1 is a distorted ferromagnetic structure with each Fe attached to three O atoms. We studied the energetics of the system as successive CO molecules are brought towards the Fe, O or a bridge site. As in previous cases, the most stable configuration corresponds to the CO attaching to a Fe site. For the case where the CO was forced to approach an O site, it did form a CO2 molecule but 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 not very high and the oxidation of CO may be possible. We further 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 remaining degrees of freedom were optimized. Fig. 3a shows the energy as a function of separation. The approaching CO molecule does experience a barrier of 0.39 eV. As a comparison, the corresponding barrier for the CO bonding to O atoms on a Ru(0001) surface [6] is about 1.1 eV. Since the CO oxidation is known to occur on Ru surfaces, it should be feasible here as well. Can the reaction barrier be further lowered? For the case where CO became attached to the Fe site, we noted that one of the host O atoms, initially attached to this Fe site detached and moved away from the site (Fig. 4a). Can this loosely bound O atom oxidize a CO molecule? A second CO molecule was added that could go to a Fe site or
Fig. 3. The reaction barriers for various reactions. Curve A corresponds to a CO molecule approaching the Fe2O3 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 Fe2O3CO. The reaction coordinate is the distance between the Fe site and the C atom.
grab an O. The process where this CO combines with the loose O to form CO2 was the energetically most favorable channel. What was even more interesting was the reaction barrier obtained by calculating the total energy of the system as a function of the separation between the position of Fe and the C while relaxing the position of all other atoms. Fig. 3b shows the energy of the system as a function of separation between the C of the approaching CO and the Fe atom. It is important to note that the reaction is almost barrierless. In Fig. 4b we have also shown the geometry of this process. To sum up, these investigations point towards a new mechanism where the first CO creates an active O that oxidizes the subsequent CO in a barrierless reaction. Can the reaction be sustained? The oxidation of CO now reduces the Fe2O3 to a Fe2O2 cluster with a CO attached to a Fe site. A similar situation would also occur in the case where the CO picked an O atom from Fe2O3 and another CO is subsequently adsorbed. The key issue is how the reaction can be sustained? The two possibilities to be considered are the addition of a CO or addition of an O2 molecule. To begin, we studied the energetics of an O2 molecule added to the Fe site with CO bond and then to the Fe site without CO bond. The case where the O2 molecule was placed close to the site with CO, a bent CO3 intermediate state (see Fig. 4c) was formed. The heat of formation is 3.77 eV which is higher than the energy
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Fig. 4. The ground state geometries of various clusters. (a) Fe2O3CO. (b) Process describing the approach of a second CO molecule to from a CO2 with the loosely bound O atom in CO-Fe2O3. (c) Fe2O2CO3 complex. (d) O2 bound to a Fe2O2CO.
required to release a CO2 molecule suggesting its release. The eliminated CO2 will, however, leave behind an O attached to the Fe site thus annealing the O hole and recreating the Fe2O3 cluster. The case where the approaching O2 was close to the site away from CO, it merely attached to the Fe site as shown in Fig. 4d. We next considered the kinetics of a CO molecule brought towards the CO – Fe2O2 –O2 cluster. When CO approached the site of the O2 molecule, a CO2 was formed that left the cluster leaving behind a O attached to the Fe site and recreating the original cluster. For the case where an additional CO was first added, it bonded with the Fe site away from the previous CO bonded site. A subsequent O2 approached this site to form a CO3 complex. Due to the heat of formation,
a CO2 left the molecule, leaving behind an O atom and recreating the original cluster. Note that the original CO remains attached to the other Fe site during the whole process. As mentioned in the beginning, the breaking of the covalent bond and the charge rearrangement are the principal controlling factors for the aforesaid catalysis. To illustrate the role of charge we carried out a Mulliken population analysis of the total charge. The resulting analysis indicated that the oxygen atoms in bare Fe2O2 and Fe2O3 carry a charge of 2 0.70 and 2 0.54, respectively. The existence of the charged sites, in our view, facilitates the conversion process. A similar phenomenon has been observed in CO oxidation on anionic and supported clusters
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[4]. As has been previously suggested, the charge transfer facilitates the cleavage of the O – O bond needed for sustaining the reaction. These details will be presented in a separate publication. While a Fe2O3 cluster exhibits catalytic activity like Fe2O3 nanoparticles or bulk surfaces, the cluster has other properties that are very different. One such feature is the magnetic coupling. As mentioned before, bulk a-Fe2O3 is antiferromagnetic. There is no direct bonding between the Fe sites and the magnetic interactions proceed via the intermediate O atoms. The Fe sites then couple antiferromagnetically because of the superexchange. Our results on Fe2O3, on the other hand, show a ferromagnetic ground state with a net moment of 4.0 mb. The ferromagnetic coupling between the Fe sites indicates that small particles would exhibit net magnetic moments. Interestingly, our findings are consistent with experiments [13] on nanocrystallites of Fe2O3, that show that the magnetic susceptibility first increases and then decreases with increasing temperatures typical for superparamagnetic particles [12]. Note that the transition from antiferromagnetic to ferromagnetic state at reduced sizes has previously been seen on Crn [14] and Mnn [15] clusters. The nature of magnetic interactions in those systems, however, are quite different. We are in the process of investigating if the Morin transition [12] observed in large crystallites could be observed in small sizes. These, however, will form the basis of another study. To summarize, our studies elucidate the microscopic mechanism that can enable Fe2O3 to be a potential catalyst for CO oxidation in the absence or presence of O2. In the absence of O2, it is the lattice O atoms that result in the oxidation and as such, it is an oxidant rather than a catalyst. In the presence of O2, however, the reaction proceeds via
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first CO adsorption and then subsequent oxidation. Finally, we have shown that the magnetic structure is modified at the reduced sizes. Since bulk Fe3O4 is magnetic while Fe2O3 is antiferromagnetic, it is interesting to investigate how the local magnetic coupling changes during the reduction or oxidation at the surface. These will be addressed in a separate publication.
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