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J. Phys. Chem. C 2010, 114, 22624–22629
Oxidation of HCl over TiO2-Supported RuO2: A Density Functional Theory Study A. P. Seitsonen*,†,‡ and H. Over*,§ IMPMC, CNRS, and UniVersite´ Pierre et Marie Curie, 4 place Jussieu, Case 115, F-75252 Paris, France, Physikalisch-Chemisches Institut der UniVersita¨t Zu¨rich, Winterthurerstr. 190, CH-8057 Zu¨rich, Switzerland, and Department of Physical Chemistry, Justus-Liebig-UniVersity, Heinrich-Buff-Ring 58, D-35392 Gieβen, Germany ReceiVed: September 9, 2010; ReVised Manuscript ReceiVed: October 15, 2010
With density functional theory (DFT) calculations, we studied the oxidation of HCl with oxygen producing Cl2 and water on the TiO2(110)-supported RuO2(110). This so-called Sumitomosa novel Deaconsprocess proceeds via a one-dimensional Langmuir-Hinshelwood mechanism, in which the recombination of two adjacent chlorine atoms on the surface of the catalyst constitutes the rate-determining step. Very important for industrial application is that substantial Ru resources can be saved in the production of the Sumitomo catalyst. According to our DFT calculations already 1 ML of RuO2(110) supported on TiO2(110) suffices to maintain practically the full activity of bulk-RuO2 in the HCl oxidation reaction. The calculated electron density differences of the TiO2(110)-supported 1 ML RuO2 system in comparison with bulk RuO2(110) are localized at the internal interface, leaving the electronic structure of the topmost undercoordinated Ru sites (active sites) unaffected by the support. This explains naturally the invariant activity of supported 1 ML RuO2(110) in comparison with bulk RuO2(110). The stoichiometric TiO2(110) is not active at all in the HCl oxidation reaction. However, if the undercoordinated Ti surface atoms are substituted by Ru then the resulting 1 /2 ML RuO2-TiO2(110) catalyst is active with an activation barrier that is 58 kJ/mol higher than for bulkRuO2(110). 1. Introduction Sumitomo Chemical1 discovered an efficient and stable Deacon-like process for the gas-phase oxidation of HCl to recover Cl2 that is catalyzed by rutile-TiO2 coated with ultrathin RuO2 layers. The Sumitomo process is considered as a true breakthrough in current catalysis research since chlorine can now be recycled from the undesired byproduct HCl with low energy cost and high conversion yields of 95%.2 The unit energy consumption of the Sumitomo process is only 15% of that required by the recently developed Bayer and UhdeNora electrolysis method.3 The activity of RuO2 has recently been elucidated on the atomic scale by a combined experiment/theory approach both on single crystalline RuO2(110) and RuO2(100) model catalysts4,5 and on polycrystalline RuO2 powder catalysts.6 The extraordinary stability of RuO2 in the HCl oxidation has been traced to the selective and self-limiting replacement of bridging O atoms (Obr, cf. Figure 1) at the surface by chlorine,7,8 transforming the catalytically active basic surface centers (Obr) into virtually inactive sites (Clbr). The actual HCl oxidation process has been shown to proceed via a Langmuir-Hinshelwood type reaction with the recombination of adsorbed Cl atoms being rate-determining.4,6 One serious challenge left with the Sumitomo process is the limited abundance of ruthenium. With a present worldwide production of about 8 t/a availability of ruthenium may quickly impose a severe bottleneck for a broader commercialization of the Sumitomo process. Detailed studies performed by Sumitomo * To whom correspondence should be addressed. E-mail: Ari.P.Seitsonen@ iki.fi (A.P.S.);
[email protected] (H.O.). Fax: ++41-446356838 (A.P.S.); ++49-641-9934559 (H.O.). † IMPMC, CNRS, and Universite´ Pierre et Marie Curie. ‡ Physikalisch-Chemisches Institut der Universita¨t Zu¨rich. § Justus-Liebig-University.
Figure 1. Ball and stick model of the clean rutile MeO2(110) surface, when part of the undercoordinated O surface are replaced by Cl; Me ) Ru and/or Ti. Large green balls represent oxygen atoms; small balls represent metal atoms (red, blue), and the gray large ball are Cl atoms. The bridge-bonded oxygen and chlorine atoms Obr, Clbr, and 1f-cus metal sites as well as br-metal species are indicated. The Obr and the 1f-cus Me atoms (red) are one-fold undercoordinated with respect to bulk-coordination. The br-Me sites are those metal atoms which are attached to the bridging O atoms.
Chemical1 suggest, however, that the actual catalyst consists of an ultrathin RuO2 film coating the rutile-TiO2 nanoparticles, which are preferentially oriented along the (110) direction. The epitaxial growth of RuO2 on TiO2 is facilitated by the fact that both materials are of rutile structure. Unfortunately the thickness of the RuO2 layer could not be determined by transmission electron microscopy.1,2 Experimental data of well-defined model RuO2(110)/ TiO2(110) catalysts, which would allow for an atomic level understanding, are not available at the moment. Therefore, density functional theory (DFT) calculations may be beneficially utilized to capture molecular details on the reactivity of such layered catalyst systems as a function of the RuO2 film thickness. These calculations are discussed in the present paper.
10.1021/jp108603a 2010 American Chemical Society Published on Web 12/06/2010
Oxidation of HCl over TiO2-Supported RuO2 2. Computational Details The rutile (110) surfaces of TiO2 and RuO2 expose two kinds of undercoordinated surface atoms which govern the interaction with the surrounding reaction gas mixture (cf. Figure 1). These are the bridging oxygen atoms (Obr) and the one-fold under coordinated metal site (1f-cus-Me). Attached to the bridging O atoms (Obr) are bridging (br-Me) metal atoms. For the ab initio calculations9 we employed density functional theory in the Kohn-Sham formulation with the generalized gradient approximation of Perdew, Burke, and Ernzerhof for the exchange-correlation functional.10 The action of the core electrons on the valence electrons was replaced by the projector augmented method11 that introduces a pseudo potential and a second, radial grid around the ionic cores. A plane wave cutoff of 37 Ry was used. The surface was modeled by seven trilayers of MeO2(110) (supercell approach; Me ) Ru, Ti) starting from the 7 ML thick TiO2(110) and replacing the Ti atoms in the topmost trilayers by Ru ranging from n ) 1/2 monolayer (ML) to n ) 7 ML; the bridging Obr sites are replaced by bridging chlorine Clbr. These coating layers are referred to as nML c-RuO2(110), which are pseudomorpically grown on TiO2 with lattice constants of the TiO2. Adsorbates are placed only at one side of the slab. Consecutive nML c-RuO2/TiO2(110) slabs are separated by a vacuum region of about 16 Å. Calculations of the adsorption energy and the geometry of adsorbed HCl, Cl, O, hydroxyl, and water groups on nML c-RuO2/TiO2(110) were performed using a (1 × 2) surface unit cell with a uniform k point mesh of 4 × 4 k-points, respectively. In the structure optimization only the atomic positions in the top three trilayers are optimized; the other atomic positions are kept fixed. Calculated adsorption energies of Cl, O, and H2O are given with respect to energies of half a molecule of Cl2 and O2 or a molecular H2O in the gas phase, respectively. Positive energy values indicate endothermic adsorption processes. The optimized lattice parameters of pure TiO2(110) turned out to be a ) 4.662 Å, c/a ) 0.63685 and those of pure RuO2(110) a ) 4.521 Å, c/a ) 0.6899. Both sets of calculated lattice parameters agree reasonably well with corresponding experimental data (TiO2: a ) 4.593 Å, c/a ) 0.644;12 RuO2: 4.492 Å, c/a ) 0.69213). The reaction coordinate for the recombination reaction of two adsorbed species on the surface is defined by the separation between the reacting species on the surface, while all other structural parameters are optimized to achieve minimum energy. The transition state (TS) of each reaction pathway and the corresponding activation barrier Eact is searched using a constrained minimization technique. The TS is identified with the configuration at a specific distance of the reacting species, where the forces on the atoms vanish and the energy reaches a maximum along the reaction coordinate. First we revisited the chlorine adsorption behavior of stoichiometric TiO2(110)-(1 × 1) and compare our results in Table 1 with values from the recent literature. Chlorine adsorption on the stoichiometric TiO2(110) surface is endothermic by +44 kJ/mol with reference to 1/2 Cl2 in the gas phase. A similar result was reported by Vogtenhuber et al.14 Quite in contrast, chlorine adsorption in the bridge position is strongly exothermic by -256 kJ/mol. The surface, where all bridging O are replaced by Clbr, is in the following termed chlorinated TiO2 (c-TiO2 and analog for the chlorinated RuO2 (c-RuO2(110)). The averaged chlorine binding energy of on-top Cl on c-TiO2(110) surface is exothermic by -171 kJ/mol. As can be seen from Table 1 the adsorption energies determined by the present DFT study agree well with those of a recent DFT calculation performed by Inderwildi and Kraft.15
J. Phys. Chem. C, Vol. 114, No. 51, 2010 22625 TABLE 1: Adsorption Energies of 1 ML Cl (with Respect to 1/2 Cl2 in the Gas Phase) on TiO2(110) with the Bridge Position (br-Me site) Either Occupied by O (s-TiO2) or by Cl (c-TiO2) Determined Using a (1 × 1) Surface Unit cella system TiO2(110)
br
ot
binding energy of Cl
literature15
s-TiO2-Clot c-TiO2 c-TiO2-Clot
O Cl Cl
Cl
44 kJ/mol (endothermic) -256 kJ/mol (exothermic) -171 kJ/mol (averaged)
41 kJ/mol -279 kJ/mol -151 kJ/mol (averaged)
Cl
a
Negative energy values indicate exothermic adsorption processes.
Figure 2. The catalytic cycle of the HCl oxidation over chlorinated c-(RuO2)1@(TiO2)6. The activation energies Eact are given in kJ/mol. The rate-determining step is constituted by the association of two neighboring Clot atoms to form Cl2.
3. Results and Discussion 3.1. Reaction Mechanism for the HCl Oxidation over RuO2(110). In recent publications4,5 the kinetics of the HCl oxidation reaction over chlorinated RuO2(110) has been shown to be governed by the adsorption energies of the reaction intermediates (water 109 kJ/mol and on-top Cl 114 kJ/mol); no additional kinetic barriers are found for the desorption of these species. This finding complies with a previous approach16 which was intended to identify promising (alternative) catalyst materials for the Deacon process on the basis of thermodynamic data. Hisham and Benson16 concentrated on the thermodynamics involved in the Deacon process over various oxides, assuming that first chlorination of the oxide takes place and in a second step the chlorinated oxide is reoxidized. If the first step is too exothermic then it follows that the second step becomes quite endothermic (since the overall reaction energy is exothermic by only -59 kJ/mol) and therefore being unfavorable as a Deacon catalyst. In this way the authors identified promising Deacon catalysts. The reaction mechanism of the HCl oxidation over RuO2(110) is summarized in Figure 2. Dissociative adsorption of O2 is nonactivated, forming atomic O in atop position of the 1f-cus Ru sites (Oot). The on-top O species are required to stabilize HCl adsorption on the chlorinated RuO2(110) surface; note that without the presence of Oot HCl adsorption is endothermic and therefore suppressed. HCl adsorbs first on a 1f-cus Ru site close to a terminal O species to which the H atom is directly transferred (oxidehydrogenation) without any noticeable activation barrier. The final production of surface water (H2Oot) via a second H transfer17,18 between two neighboring OotH groups is kinetically activated by 29 kJ/mol, an energy barrier that is easily surmounted at typical reaction temperatures of 500-600 K. The recombination of two on-top Cl to form the desired product Cl2 constitutes the rate-determining step with an activation barrier of 2 × 114 kJ/mol. This value differs slightly from that
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Seitsonen and Over
TABLE 2: Cumulative Adsorption Energies of Various Reaction Intermediates on the Fully Chlorinated c-RO2(110)/ TiO2(110) Layers, Where All Bridging Surface O Atoms Are Replaced by Chlorinea energies in kJ/mol + 1 /2 O 2 reaction state
Oot
c-RuO2 c-(RuO2)7@(TiO2)0 c-(RuO2)2@(TiO2)5 c-(RuO2)1@(TiO2)6 c-(RuO2)0.5@(TiO2)6.5 c-TiO2 s-TiO2
-100 -108 -122 -118 -144 -159 +219
+HCl OotH + Cl -226 -215 -231 -247 -286 -274 +101
+HCl ot
OotH2 + 2Cl -404 -404 -412 -432 -478 -404 -31
-H2O ot
2 Clot
Clot
-295 -300 -310 -313 -352 -317 +55
-114 -117 -122 -124 -143 -126 +60
a
All energies are given in kJ/mol. The energy reference is the fully chlorinated surfaces c-RuO2(110), c-TiO2(110), and the stoichiometric s-TiO2(110). The nomenclature of the systems is as follows: c-(RuO2)n@(TiO2)m ) n layers of RuO2 + m layers of TiO2 with n + m ) 7, lattice constant of TiO2(110), with the bridging O replaced by chlorine. c-RuO2 ) 7 layers of RuO2 with the native lattice parameters of RuO2 and all bridging O site replaced chlorine. Notice that exothermic interactions are indicated by negative energies, while endothermic steps are indicated by positive energy values. The energy evolutions for s-TiO2 and (RuO2)1@(TiO2(110))6 are illustrated in Figure 3.
Figure 3. Energy diagram for the HCl oxidation over a chlorinated monolayer of RuO2(110) supported on 6 layers of TiO2(110) in comparison with the stoichiometric TiO2(110) surface. All energies are given in kJ/mol. The reaction energy ∆E is -66 kJ/mol.
reported by Lopez et al.6 but is in quantitative agreement with that of Studt et al.19 (2 × 110 kJ/mol). 3.2. Reaction Mechanism for the HCl Oxidation over RuO2(110)-Supported on TiO2(110). Next, we conducted extensive state-of-the-art DFT calculations to explore in detail the catalytic cycle of the HCl oxidation on RuO2(110)-coated TiO2 (110) model catalysts of various RuO2 film thicknesses without relying on a tedious preparation of well-defined c-RuO2-TiO2(110) model catalysts.20 Since the kinetics of the HCl oxidation process over RuO2(110) is primarily determined by the adsorption energies of the reaction intermediates,4 we calculated for the layered RuO2/TiO2(110) systems first the energy evolution along the reaction pathway of the HCl oxidation for various film thicknesses of RuO2(110) (cf. Table 2). The reference energy is chosen to be that of the respective chlorinated surface of the composite model catalyst, where all bridging O atoms are replaced by chlorine and no adsorbate is present on the surface. The values in the second column of Table 2 indicate the adsorption energy when 1/2 O2 is added to the catalyst, forming oxygen (Oot) on top of a 1f-cus position. On this partially Oot-
precovered surface HCl is accommodated, thereby forming OotH and Clot; the cumulative adsorption energies of this process are provided in the third column. On this surface another HCl is adsorbed, resulting in OotH2 + 2Clot with cumulative adsorption energies given in the fourth column. Lastly, the water desorbs from the surface, increasing the energies (due to the endothermic desorption step) as indicated in the fifth column. To be able to determine the reaction energies, the adsorption energies of ontop chlorine are compiled in the last column. The difference of energies in the fifth column and those of twice the adsorption energy of Clot results in the reaction energy of the net reaction
1 2HCl + O2 h H2O + Cl2 2 which turns out to be -66 kJ/mol(Cl2), slightly higher than the experimental value of -59 kJ/mol(Cl2).16 From inspection of the last row of Table 2, we learn that without Ru the stoichiometric s-TiO2 (110) slab is inactive toward the HCl oxidation due to the endothermic adsorption process of molecular oxygen (219 kJ/mol), consistent with the corresponding values from the literature.21 However, as soon as the 1f-cus Ti sites are replaced by Ru [c-(RuO2)0.5@(TiO2)6.5] dissociative adsorption of oxygen is facile, making TiO2 a surprisingly good Deacon catalyst. The adsorption energy of on-top chlorine on c-(RuO2)0.5@(TiO2)6.5 is -143 kJ/mol, i.e., 29 kJ/mol more exothermic than on s-RuO2. Therefore the activity of c-(RuO2)0.5@(TiO2)6.5 is expected to be significantly lower than that of bulk c-RuO2(110). However, we have to emphasize that already 1/2 ML of Ru suffices to switch the catalytically “dead” TiO2(110) surface into a reasonably active catalyst for the HCl oxidation. According to our DFT calculations also a substitution of the anionic sublattice substantially improves the catalytic activity as observed with the chlorinated c-TiO2(110) surface (cf. Table 2). Here the bridging O atoms of TiO2 are all replaced by chlorine. All the reaction steps of chlorinated TiO2(110) with the incoming gas molecules (O2 and HCl) are energetically downhill. Only the desorption of water and the association of chlorine followed by desorption are activated for energy reasons. As discussed in a recent paper by Metiu et al.22 further calculations on the basis of hybrid methods23 are required to exclude that the erroneous DFT-induced delocalization of electron density along the 1f-cus Ti is the true reason for the apparently improved activity; these calculations are currently underway in our group. Anyway, this surprising finding may initiate novel reactivity experiments in which the anionic surface O sublattice is purposely substituted by other elements, such as chlorine, nitrogen, sulfur or other main group elements. When proceeding with a single layer of RuO2(110) that is pseudomorphically grown on TiO2(110) [(RuO2)1@(TiO2)6], the adsorption energies of on-top chlorine is -124 kJ/mol so that the activation barrier for the association of terminally bonded chlorine is only 2 × 10 kJ/mol higher than that of a native c-RuO2(110) bulk-like slab. On the basis of our calculations we thus conclude that the activity of 1 ML c-RuO2(110)/ TiO2(110) is practically identical to that of pure c-RuO2(110). Increasing the RuO2 film thickness to two layers does not change the adsorption energies significantly. In Figure 3 we depict the energy profiles for the HCl oxidation reaction over 1 ML of chlorinated RuO2(110) supported on TiO2(110) in comparison with that of the stoichiometric TiO2(110) surface. From these energy diagrams it is evident that the first step in the HCl oxidation on TiO2(110), namely,
Oxidation of HCl over TiO2-Supported RuO2
Figure 4. The catalytic cycle of the HCl oxidation over 1 ML of RuO2(110) supported on TiO2(110), starting with a selective and selflimiting replacement of the bridging O atoms of s-(RuO2)1@TiO2 by bridging chlorine forming c-(RuO2)1@TiO2. The activation energies Eact are given in kJ/mol. The rate-determining step is identified with the association of two neighboring Clot atoms to form Cl2.
the adsorption of oxygen, is energetically strongly uphill by 219 kJ/mol. All other steps on TiO2(110) (except for water desorption) are downhill so that the rate-determining step (rds) is clearly identified with the dissociative adsorption of oxygen. Quite in contrast, all the reaction steps on c-(RuO2)1@(TiO2)6 with the incoming gas molecules are energetically downhill. Only desorption of water and the association of chlorine followed by desorption are activated for energy reasons. The chlorine recombination constitutes the rate-determining step on 1 ML c-RuO2(110)/TiO2(110) with a barrier of 2 × 124 kJ/mol ) 248 kJ/mol/2. Besides activity the stability of the catalyst plays an important role in the Deacon process. For instance, under reaction conditions Ti and Ru will intermix above 600-700 K20 for thermodynamic reasons so that the actual catalytic activity of 1 ML c-RuO2(110)/TiO2(110) may be lower. We shall perform ab initio thermodynamic calculations to clarify this important point. When comparing the energies of native c-RuO2(110) with those of the strained c-RuO2(110) [(RuO2)7@(TiO2)0] in Table 2 we can estimate the influence of the imposed strain on the adsorption energies. Both on-top chlorine and on-top O are slightly more strongly bound by about 5 kJ/mol when the lattice of RuO2(110) is strained by the pseudomorphic growth on TiO2(110). The strengthening of the adsorbate bonding is reconciled with the corresponding shift of the d-band center24 due to compressive strain along the [11j0] direction. The other adsorption energies (HCl, H2O) are equally affected by the epitaxial strain as O2 and Cl2 adsorption. Since the association of on-top chlorine constitutes the rate-determining step in the HCl oxidation reaction, strained RuO2(110) is virtually as reactive as the native RuO2(110). The microscopic steps of the HCl oxidation on a fully chlorinated 1 ML c-RuO2(110)/ TiO2(110) model catalyst are compiled in Figure 4 including activation energies (kinetics) of the elementary reaction steps. The dissociative adsorption of O2 is nonactivated, forming readily on-top O (Oot) on neighboring vacant 1f-cus Ru sites. According to our DFT calculations the adsorption of HCl proceeds via H-transfer to Oot,17 a reaction step without any noticeable activation barrier. The final production of adsorbed water (H2Oot) via H transfer between two neighboring OotH groups is (kinetically) activated by 15 kJ/mol. Water adsorbs with a binding energy of 119 kJ/ mol. The rate-determining step is still identified with the association of two neighboring Clot atoms to form Cl2. This reaction step is activated by 2 × 124 kJ/mol, the adsorption energy of on-top Cl. No additional barrier due to kinetics has
J. Phys. Chem. C, Vol. 114, No. 51, 2010 22627 been identified with DFT calculations. Most of these calculated energies are very close to those of the fully surface-chlorinated bulk-RuO2(110),5 rendering c-RuO2(110) indeed a suitable model catalyst for the practical Sumitomo process. When hydrogen is shifted from OH on-top of 1f-cus Ru toward the adjacent bridging Cl atoms, the energy increases by more than 200 kJ/mol and finally HCl desorbs from the bridge position. This high activation energy suppresses an efficient communication between adjacent 1f-cus Ru rows as long as the bridging Clbr stays in bridging position (which is not necessarily the case25). Therefore the 1 ML c-RuO2(110)/ TiO2(110) catalyst can be envisioned as a one-dimensional catalyst offering isolated rows of 1f-cus Ru sites where a Langmuir-Hinshelwood (LH) type dehydrogenation reaction between HCl and oxygen takes place. This one-dimensionality is expected to impose further constraints to the kinetic behavior of the catalyst via product inhibition by chlorine and water. Under typical reaction conditions in excess of oxygen and temperatures between 500 and 600 K, the surface of chlorinated 1 ML c-RuO2(110) on TiO2(110) is mainly covered with ontop Cl and small amounts of on-top O. The released hydrogen is removed from the catalyst surface via water formation (cf. Figure 4). The remaining Clot species on the surface have to diffuse along the 1f-cus Ru rows to meet a second Clot and to react with. This diffusion process is activated by 35 kJ/mol (DFT) and therefore can easily be overcome at typical reaction temperatures of 500 to 600 K. 3.3. Electronic Properties of c-RuO2(110) Films Supported on TiO2(110). To understand the preserved catalytic activity of a 1 ML c-RuO2(110) layer supported on TiO2(110) in comparison with bulk c-RuO2(110) we studied the induced changes of the electronic density when 1 or 2 ML of cRuO2(110) is supported on TiO2(110). The electron density difference of the n-ML c-RuO2(110)/TiO2(110) composite (n ) 1 or 2) is given by
∆F ) F(nML c - RuO2 + (7 - n)TiO2) - F(nML c RuO2) - F((7 - n)TiO2) (1) On both sides of the interface between the freestanding 1 ML/2 ML c-RuO2(110) slabs and the TiO2(110) substrate there are undercoordinated bridge-O and 1f-cus metal sites exposed. In the composite slab c-RuO2(110) supported on TiO2(110) the bridging O of TiO2(110) is attached to the bottom 1f-cus Ru of the nML c-RuO2(110) slab and the bridging O atom of the bottom side of c-RuO2(110) is coordinated to the 1f-cus Ti site of the TiO2(110) substrate. The atomic positions of the metal and oxygen atoms are those of TiO2(110) regardless whether the metal site is occupied by Ti or Ru. Figure 5 displays the electron density difference of 2 ML c-RuO2(110) (a) and 1 ML c-RuO2(110) (b) supported on 5 ML TiO2(110) and 6 ML TiO2(110), respectively. These electron density difference plots visualize the electronic polarization when the dangling bonds of the nML RuO2(110) layer are capped by the dangling bonds of the TiO2(110) support. As expected, strong electron rearrangements occur at the internal interface primary centered at the connecting O atoms. The Ti atoms next to the interface are virtually not affected. The electronic density around the Ru atom which is terminally coordinated to the bridging O atom of TiO2(110) reveals electron depletion, while electron accumulation occurs at the connecting Obr atom. The observed polarization is consistent with the formation of an ionic bonding between the dangling bonds of
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Figure 5. Electron density difference plots of 2 ML RuO2(110) (a) and 1 ML RuO2(110) (b) supported on 5 ML TiO2(110) and 6 ML TiO2(110), respectively. The 1f-cus Ru sites are indicated as a small red ball. The value of the isosurfaces is 0.05e-/Å3. Electron density accumulation is shown as light red and depletion as light blue.
Obr and Ru. Also the other Obr atom in the RuO2/TiO2 interface accumulates electron density when forming a bond to the dangling bond of the underlying 1f-cus-Ti atoms. This electron density difference distribution is very much confined to the internal interface between c-RuO2(110) and TiO2(110), indicating efficient screening. Only the oxygen atom in the 2 ML c-RuO2(110) layer which is attached to the affected Ru atom shows some electron accumulation and electron depletion with pz symmetry. The on-top Ru and O atoms of the 2 ML c-RuO2(110)@TiO2(110) system are not affected. In particular the 1f-cus Ru site (red ball in Figure 5) does not reveal any alteration in the electronic density. This finding is compatible with our total energy calculations that indicate that the adsorption energies of on-top O and on-top Cl on the 1f-cus Ru sites are virtually identical with those on the 7 ML c-RuO2(110) slab. In the case of 1 ML c-RuO2(110)-TiO2(110) the electronic structure of the 1f-cus Ru site is massively affected when the 1 ML c-RuO2(110) layer is supported on the substrate 6 ML TiO2(110), while the electron density of the Ru atom below the bridging O atom is almost unaffected by the TiO2(110) support. However, we have to bear in mind that the 1f-cus Ru atom in the 1 ML c-RuO2(110) layer is not only one-fold undercoordinated, but rather two of its terminal oxygens are missing, thus explaining its pronounced electronic polarization when one such dangling bond is saturated. The symmetry of the electronic density depletion at the 1f-cus Ru site exhibits dz2 character. The O atom attached to 1f-cus Ru experiences both electron depletion and accumulation, while the other O atom in the interface only accumulates electron density, however, much more than in the case of the 2 ML c-RuO2(110) layer. The strong modification of the electron density at the 1fcus Ru sites is not directly reconciled with the unaltered activity of 1 ML RuO2(110)-TiO2(110) relative to the RuO2(110)-bulk. However, this information can also not be expected to be retrieved from simple electron difference plots. The electron density difference can be correlated with the activity difference of a free-standing c-RuO2(110) layer with regard to the c-RuO2(110) layer supported on TiO2(110). For the 2 ML c-RuO2(110) case this difference is surprisingly small, while for the 1 ML RuO2(110) case, the electronic structure is strongly modified since the 1f-cus Ru site is directly connected to the support. To compare the catalytic activity of a TiO2(110)-supported c-RuO2(110) layer with bulk c-RuO2(110) (considered as 1
Seitsonen and Over
Figure 6. Difference of electron density difference plots of 2 ML RuO2(110) (a) and 1 ML RuO2(110) (b) supported on 5 ML TiO2(110) and 6 ML TiO2(110), respectively, in comparison with a support of 5 ML RuO2(110) and 6 ML RuO2(110), respectively. Contour surfaces for constant electron densities of 0.05 e/A3. Electron density accumulation is shown as light red and depletion as light blue. The 1f-cus Ru sites are indicated as small red balls.
ML/2 ML c-RuO2(110) layer supported on RuO2(110)), one has to focus on electron density changes which are induced when the 1 or 2 ML of RuO2(110) is supported either on TiO2(110) or on RuO2(110). The corresponding electronic variation is reconciled with the difference of the electron density differences, i.e. ∆∆F ) F(nML c - RuO2 + (7 - n)TiO2) F(n MLc - RuO2) - F((7 - n)TiO2) - [F(nML c - RuO2 + (7 - n)RuO2) F(n MLc - RuO2) - F((7 - n)RuO2)] ) F(nML c - RuO2 + (7 - n)TiO2) - F((7 - n)TiO2) F(nML c - RuO2 + (7 - n)RuO2) + F((7 - n)RuO2)
(2) This quantity is shown in Figure 6 in the cases of supported 1 and 2 ML of c-RuO2(110) layers. These plots visualize the difference in interaction of the 1 or 2 ML c-RuO2(110) layer on TiO2(110) in comparison to RuO2(110) support. The double difference of electron density in Figure 6 varies again mostly in the interface region both in the 1 ML and the 2 ML RuO2(110) case, and it is somewhat smaller than the electron density differences shown in Figure 5. The double difference of electron density in the interface region is mainly confined to the side of the support. Closer inspection of Figure 6 reveals that the electronic density of the 1f-cus Ru site is practically invariant when the support is varied from TiO2(110) to RuO2(110), thus corroborating nicely the finding that the activity of 1 ML RuO2(110) on TiO2(110) is practically identical to that of RuO2(110) bulk (considered as 1 ML RuO2(110) supported of 6 ML RuO2(110)). As expected from the electron difference map in Figure 5, the same behavior is also observed for the 2 ML RuO2 layer on 5 ML MeO2(110) substrate. 4. Conclusions DFT calculations reveal that the oxidation of HCl with molecular oxygen, producing Cl2 and the byproduct water, proceeds on TiO2(110)-supported c-RuO2(110) surfaces via a one-dimensional Langmuir-Hinshelwood mechanism. Most of the activation barriers of the elementary steps are determined
Oxidation of HCl over TiO2-Supported RuO2 by the adsorption energies of the reaction intermediates, such as water and on-top chlorine. The recombination of two adjacent chlorine atoms on the surface of the catalyst is rate-determining in the Deacon-like process. For a Ru coverage of 1 ML, where all Ti atoms in the topmost O-Ti-O trilayer are replaced by Ru, the activation barrier for the rate-determining step of Cl association is within 2 × 10 kJ/mol identical to that of a pure c-RuO2(110) surface. If only the 1f-cus Ti atoms are replaced by Ru then the 1/2 ML c-RuO2(110)-TiO2(110) catalyst is still active, however with an activation barrier for Clot-Cot recombination which is 2 × 29 kJ/mol higher than at RuO2(110). The catalytic performance in the Sumitomo process of 1 and 2 ML c-RuO2(110) layers supported on TiO2(110) is practically identical to that of bulk c-RuO2(110). This invariant catalytic behavior is explained by the unaltered electronic density at the 1f-cus Ru sites when the TiO2(110) support is replaced by RuO2(110). From a structural point of view, i.e., pseudomorphic growth of RuO2(110) on TiO2(110), it is mandatory that both RuO2 and TiO2 are of the same rutile type structure. The observed invariance of the catalytic performance on the thickness of the RuO2(110) layer is far from being trivial as demonstrated with Au-TiO2(110) system. Chen and Goodman26 have shown that a double layer of Au supported on TiO2(110) is much more active in the CO oxidation than bulk Au or three layers of Au, so that the thickness of the active catalyst component is a decisive parameter for the catalytic performance. The oxygen activation is the true rate-determining step in the HCl oxidation reaction over stoichiometric TiO2(110), since dissociative adsorption of oxygen molecules is highly endothermic (by more than 200 kJ/mol). Our DFT calculations, however, indicate that with small amounts of Ru in the outermost surface layer this deficiency of s-TiO2(110) can be overcome. Therefore Ru may be envisioned to promote the activation of oxygen on TiO2, suggesting that TiO2 is a promising starting point for the synthesis of a stable and efficient Deacon catalyst. A second metal such as Ru tunes then the final activity of the catalyst in a second step. We have some indications that also a modification of the anionic surface sublattice by chlorine alters the catalytic activity of the TiO2. This finding may push the development of future Deacon catalysts toward Ru-free TiO2 catalysts where surface O is replaced by other main group elements such as chlorine, nitrogen, sulfur, and other elements. Mandatory for industrial applications is the fact that substantial Ru resources can be saved in the production of the Sumitomo catalyst. DFT calculations indicate clearly that already 1 ML
J. Phys. Chem. C, Vol. 114, No. 51, 2010 22629 of RuO2(110) supported on TiO2(110) is sufficient to accomplish practically the full activity of bulk-RuO2 in the HCl oxidation reaction. Acknowledgment. We would like to thank the Leibniz Rechenzentrum in Munich for providing us with massive parallel supercomputing time and the German Science Foundation for financial support. References and Notes (1) Iwanaga, K.; Seki, K.; Hibi, T.; Issoh, K.; Suzuta, T.; Nakada, M.; Mori, Y.; Abe, T. Sumitomo Kagaku 2004, I, 1–11. (2) Seki, K. Catal. Surf. Asia 2010, 14, 168. (3) Gestermann, F.; Ottavini, A. Modern Alkali Technol. 2001, 8, 49. (4) Zweidinger, S.; Crihan, D.; Knapp, M.; Hofmann, J. P.; Seitsonen, A. P.; Westrate, C. J.; Lundgren, E.; Andersen, J. N.; Over, H. J. Phys. Chem. C 2008, 112, 9966. (5) Zweidinger, S.; Hofmann, J. P.; Balmes, O.; Lundgren, E.; Over, H. J. Catal. 2010, 272, 169. (6) Lo´pez, N.; Go´mez-Segura, J.; Marı´n, R. P.; Pe´rez-Ramı´rez, J. J. Catal. 2008, 255, 29. (7) Crihan, D.; Knapp, M.; Zweidinger, S.; Lundgren, E.; Weststrate, C. J.; Andersen, J. N.; Seitsonen, A. P.; Over, H. Angew. Chem. Int. Ed. 2008, 47, 2131. (8) Hofmann, J. P.; Zweidinger, S.; Knapp, M.; Seitsonen, A. P.; Schulte, K.; Andersen, J. N.; Lundgren, E.; Over, H. J. Phys. Chem C 2010, 114, 10901. (9) (a) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1995, 6, 15. (b) Kresse, G.; Joubert, D. Phys. ReV. B 1998, 59, 1758. (10) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3365. (11) Blo¨chl, P. Phys. ReV. B 1994, 50, 17953. (12) Diebold, U. Surf. Sci Rep. 2003, 48, 53. (13) Boman, C. E. Acta Chem. Scand. 1970, 24, 116. (14) Vogtenhuber, D.; Podloucky, R.; Redinger, J.; Hebenstreit, E. L. D.; Hebenstreit, W.; Diebold, U. Phys. ReV. B 2002, 65, 125411. (15) Inderwildi, O. R.; Kraft, M. Chem. Phys. Chem. 2007, 8, 444. (16) Hisham, N.W. M.; Benson, S. W. J. Phys. Chem. 1995, 99, 6194. (17) Knapp, M.; Crihan, D.; Seitsonen, A. P.; Over, H. J. Am. Chem. Soc. 2005, 127, 3236. (18) Knapp, M.; Crihan, D.; Seitsonen, A. P.; Lundgren, E.; Resta, A.; Andersen, J. N.; Over, H. J. Phys. Chem. C 2007, 111, 5363. (19) (a) Studt, F.; Abild-Pedersen, F.; Hansen, H. A.; Man, I. C.; Rossmeisl, J.; Bligaard, T. Chem. Cat. Chem. 2010, 2, 98. (b) T. Bligaard, private communication. (20) Chambers, S. A. Surf. Sci. Rep. 2000, 39, 105. (21) Rasmussen, M. D.; Molina, L. M.; Hammer, B. J. Chem. Phys. 2004, 120, 988. (22) Chretien, S.; Metiu, H. J. Chem. Phys. 2008, 128, 044714. (23) Di Valentin, C.; Pacchioni, G.; Selloni, A. Phys. ReV. Lett. 2006, 97, 166803. (24) Hammer, B.; Norskov, J. K. Surf. Sci. 1995, 343, 211. (25) Hofmann, J. P.; Zweidinger, S.; Seitsonen, A. P.; Farkas, A.; Knapp, M.; Balmes, O.; Lundgren E.; Andersen, J. N.; Over, H. Phys. Chem. Chem. Phys. 2010, 12, 15358. (26) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252.
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