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2008, 112, 9966–9969 Published on Web 06/13/2008
Reaction Mechanism of the Oxidation of HCl over RuO2(110) S. Zweidinger,† D. Crihan,† M. Knapp,† J. P. Hofmann,† A. P. Seitsonen,‡ C. J. Weststrate,§ E. Lundgren,§ J. N. Andersen,§ and H. Over*,† Department of Physical Chemistry, Justus-Liebig-UniVersity, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany, IMPMC, CNRS, and UniVersite´ Pierre et Marie Curie, 4 place Jussieu, case 115, F-75252 Paris, France, and Department of Synchrotron Radiation Research, Lund UniVersity, So¨lVegatan 14, S-22362 Lund, Sweden ReceiVed: April 17, 2008; ReVised Manuscript ReceiVed: May 20, 2008
High-resolution core-level shift spectroscopy and temperature-programmed reaction experiments together with density functional theory calculations reveal that the oxidation of HCl with oxygen producing Cl2 and water proceeds on the chlorine-stabilized RuO2(110) surface via a one-dimensional Langmuir-Hinshelwood mechanism. The recombination of two adjacent chlorine atoms on the catalyst’s surface constitutes the ratedetermining step in this novel Deacon-like process. Chlorine is omnipresent in industrial chemistry and is used to produce chlorine-containing intermediates. Hydrogen chloride can be a direct byproduct of chlorine substitution reactions, but mostly, it is generated in subsequent production steps when chlorine is removed from the intermediates to attain chlorinefree final products. The byproduct HCl is environmentally undesirable and has only a very restricted market. Consequently, there has been growing interest in finding efficient methods for converting HCl back into Cl2. The heterogeneously catalyzed HCl oxidation (so-called Deacon process), that is, 4HCl + O2 h 2H2O + 2Cl2 is one such process which allows design of closed process cycles in which chlorine is recycled energyneutral from the byproduct hydrogen chloride (green chemistry route). Although the Deacon process has been known for some 130 years,1 it has not found its way into commercial applications. The Deacon process suffers most notably from a too-high reaction temperature and from rapid loss of catalyst activity at reaction temperatures typically above 700 K. Note that the oxidation of HCl is only mildly exothermic (-57 kJ/mol (Cl2)) so that the final yield is determined by the equilibrium conversion, which is 70% at 700 K. Therefore, the Deacon process has eventually been abandoned and displaced mainly by electrolysis, a prohibitively energy-consuming process.2 Only very recently, Sumitomo Chemical3 has developed an efficient and stable Deacon-like process over RuO2 (Sumitomo process). The Sumitomo process is a true breakthrough in recent catalysis research since chlorine is recycled from HCl with low energy cost and high conversion yields of 95%. The unit energy consumption of the Sumitomo process is only 15% of that required by the recently developed Bayer&UhdeNora electrolysis method.2 * To whom correspondence should be addressed. E-mail: Herbert.Over@ phys.chemie.uni-giessen.de. Fax: ++49-641-9934559. URL: http://www.uni-giessen.de/cms/fbz/fb08/chemie/physchem/ag-prof-dr-herbert-over. † Justus-Liebig-University. ‡ IMPMC, CNRS, and Universite ´ Pierre et Marie Curie. § Lund University.
10.1021/jp803346q CCC: $40.75
In this letter, we report on high-resolution core-level spectroscopy (HRCLS) experiments4 exploring the catalytic cycle (cf. Figure 1) of the oxidation of HCl on the chlorinated RuO2(110) surface, including microscopic details of the elementary reaction steps on the surface (cf. Figure 2b). The HCl oxidation turns out to proceed via a Langmuir-Hinshelwoodtype reaction mechanism, with the rate-determining step to be the recombination of adsorbed on-top Cl to form the desired Cl2 product. These sophisticated experiments are corroborated by state-of-the-art density functional theory (DFT) calculations.5 The photon energies for the HRCLS measurements of the Cl 2p and O 1s core levels were chosen to be 250 and 625 eV, with total energy resolutions of 140 and 350 meV, respectively. All HRCL spectra were recorded at a sample temperature of 100 K. For the ab initio calculations,5 we employed the projector augmented wave method6 with a plane wave cutoff of 37 Ry and generalized gradient approximation.7 The surface was modeled by five double layers of RuO2(110) (supercell approach) with adsorbates only at one side of the slab. Consecutive RuO2(110) slabs were 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 chlorinated RuO2(110) were performed using a (2 × 1) or a (2 × 2) surface unit cell with a uniform k point mesh of 4 × 4 and 4 × 2 k points, respectively. The core-level energy levels were calculated by removing half of an electron from the core orbital and performing a self-consistent calculation of the electronic structure.8 Thus, the screening effects (“final state”) of the hole are included. The energy is obtained as the energy difference between the Kohn-Sham eigenvalue of the core state and the Fermi energy. On the basis of these DFT calculations, the experimental Cl 2p and O 1s core level shifts were assigned to specific O species and Cl species on the surface. The RuO2(110) surface is produced by exposure of a wellprepared Ru(0001) surface to 2 × 106 L of O2 (1L ) 1 Langmuir ) 1.33 × 10-6 mbar · s) at a sample temperature of 650 K. In the bulk structure of RuO2, the Ru atoms bind to six oxygen atoms, forming a slightly distorted RuO6 octahedron, while the 2008 American Chemical Society
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J. Phys. Chem. C, Vol. 112, No. 27, 2008 9967
Figure 1. The catalytic cycle of the HCl oxidation over RuO2(110), starting with a selective and self-limiting replacement of the bridging O atoms by bridging chlorine forming RuO2-xClx(110). The reactant molecules O2 and HCl both adsorb first on the 1f-cus-Ru sites. O2 dissociates to form adsorbed O, and HCl dehydrogenates via a hydrogen transfer to form Cl and OH species in on-top positions. H-transfer among the OH species leads to water formation, which is released from the surface at around 420 K. Neighboring on-top Cl atoms recombine to form Cl2, which is immediately released into the gas phase. The activation energies ∆Eact are determined by DFT calculations. The rate-determining step constitutes the association of two neighboring Clot atoms to form Cl2.
O atoms are coordinated to three Ru atoms in a planar sp2 configuration. On the stoichiometric RuO2(110) surface (cf. the ball and stick model of the bulk-truncated RuO2(110) surface; Figure 2a), two kinds of undercoordinated surface atoms are present, which are arranged in rows along the [001] direction: (i) the bridging oxygen atoms Obr, which are coordinated only to two Ru atoms underneath (instead of three) and (ii) the socalled 1f-cus-Ru atoms (1f-cus stands for one-fold coordinatively unsaturated sites).9 RuO2(110) is an appropriate model catalyst for the Sumitomo process since the (110) orientation is by far the most stable surface termination and therefore the most prevailing orientation found on RuO2 powders or supported catalysts.10 The major benefit of studying the HCl oxidation over the model catalyst RuO2(110) is the direct comparability of theory with experiment, thus allowing for firm conclusions. Under HCl oxidation conditions, the RuO2(110) surface transforms into a chlorinated surface, in which most of the bridging O atoms (Obr) are selectively replaced by bridging chlorine atoms (Clbr). In this way, the RuO2(110) catalyst, which otherwise can easily be reduced,11 is self-stabilized against further chemical reduction,12 explaining the observed stability of RuO2 in the Sumitomo process. The actual catalytic cycle of the Sumitomo process was followed by the evolution of O 1s and Cl 2p core-level shift spectra as summarized in Figure 3. We started from the chlorinated RuO2-xClx(110) surface, where the bridging O atoms are replaced by chlorine. The Cl 2p spectrum shows only a single spin-orbit duplet with the major 2p3/2 binding energy of 197.63 eV, which is uniquely ascribed to bridging Cl atoms.12 The O 1s spectrum indicates a RuO2(110) film with no residual
emission from bridging O atoms at a binding energy of 528.7 eV,13 consistent with the complete replacement of Obr by Clbr. The RuO2-xClx(110) surface is then exposed to 1 L of HCl at 420 K. The O 1s spectrum is not affected by the HCl exposure, while a second Cl 2p doublet shifted to lower binding energies (BE) appears in the Cl 2p spectrum. On the basis of DFT calculations, we assign the second Cl 2p doublet to intact HCl molecules adsorbed on top of 1f-cus-Ru (HClot). Subsequently, 5 L of O2 was dosed at 200 K. The O 1s spectrum reveals an additional feature at 532 eV, which is attributed to adsorbed water.14 The Cl 2p spectrum shows a slight shift of the bridging Cl 2p (Clbr) doublet to lower binding energies (BE). After this sequence of exposures, the sample temperature was raised stepwise from 200 to 700 K. The Cl 2p core level spectra do not vary with temperature up to 375 K. The O1s spectrum indicates an increasing intensity of the O 1s water feature at 532 eV when heating the sample to 300 and 375 K followed by a decrease upon annealing at 420 K. At 520 K, the waterrelated emission disappears completely, which is reconciled with a maximum desorption at 420 K (cf. Figure 4). Further stepwise annealing to 700 K does not alter the O1s spectrum. The Cl 2p spectra change considerably with temperature above 375 K. Annealing to 420 K leads to an increase of the on-top Cl (Clot) component with the 2p3/2 peak at 196.15 eV. At 520 K, when surface water and residual HCl have desorbed as indicated in Figure 4, the Clot feature is narrow, and the binding energy of Clbr is shifted back to the value of the original chlorinated RuO2(110) surface. Therefore, the observed energy shift of Clbr is related to hydrogen bonds of water or residual HCl with Clbr. With an increase of the sample temperature to 600 K, the Cl 2p emission of Clot declines. We should note that on-top Cl
9968 J. Phys. Chem. C, Vol. 112, No. 27, 2008
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Figure 3. High-resolution spectra of Cl 2p and O 1s during the catalytic cycle of HCl oxidation over the chlorinated model catalyst RuO2-xClx(110). The O 1s spectra around the on-top water are magnified. The color code of the spectra is adjusted to the color code of the catalytic cycle in Figure 1. The dashed lines overlaid in the top spectra of Cl 2p and O 1s are the starting spectra of the chlorinated surface, indicating clearly that neither Clbr nor O in RuO2-xClx(110) is consumed during the cycle. Figure 2. (a) Chlorination of the RuO2(110) during the HCl oxidation reaction between 500 and 600 K. Most of the bridging O atoms (Obr) are replaced by bridging Cl atoms (Clbr). (b) Microscopic reaction steps in the HCl oxidation over RuO2(110) along the rows of 1f-cus-Ru atoms. The dashed lines indicate hydrogen bonds. This cartoon shows the pivotal role of the hydrogen transfer in the HCl oxidation reaction over the chlorinated RuO2(110) surface. Since the bridging Cl atoms are not able to accept hydrogen from adsorbed HCl, the HCl oxidation takes place in a purely one-dimensional fashion along the rows of 1fcus-Ru sites.
has been directly identified with scanning tunneling microscopy for the structurally isomorphic TiO2(110) surface.15 This reduction in Cl 2p emission is related to the recombination of adjacent Clot to form Cl2, which is immediately released into the gas phase. Upon annealing to 650 and 700 K, the Cl 2p emission of Clot disappears, which is reconciled with corresponding temperature-programmed reaction experiments in Figure 4. At 700 K, the catalytic cycle is completed, and both the O 1s and the Cl 2p spectra recover the original starting spectra of the fully chlorinated surface (cf. Figure 3). The microscopic steps of the HCl oxidation are summarized in Figures 1 and 2b and supplemented with activation energies as determined by DFT calculations as well as with characteristic temperatures determined by temperature-programmed reactions (cf. Figure 4). Adsorption of O2 and HCl is nonactivated, forming on-top O (Oot) and HClot on the 1f-cus-Ru sites. According to our DFT calculations, the dehydrogenation of HClot via Oot proceeds without any noticeable activation barrier. The final production of adsorbed water (H2Oot) via H transfer16 between two neighboring ObrH groups is 0.28 eV, an energy barrier to be easily overcome at typical reaction temperatures.
Figure 4. Temperature-programmed reaction experiments of HCl and O2 coadsorbed on the chlorinated RuO2-xClx(110) surface where most of the bridging O atoms have been replaced by chlorine atoms. For the preparation of the coadsorption phase, 1 L of HCl was exposed at 420 K and 5 L of O2 was postexposed at 200 K. A small fraction of the HCl molecules desorbs already around 450 K, while the rest of the adsorbed HCl molecules react with on-top O to form the water and desired product Cl2. Excess oxygen leaves the surface at around 400 K.
At around 420 K, water desorbs (cf. Figure 4), consistent with a chemisorption energy of 1.05 eV.14 The rate-determining step is identified with the association of two neighboring Clot atoms to form Cl2. This reaction step is activated by 1.6 eV, in line with a temperature above 600 K as observed in temperatureprogrammed reactions (cf. Figure 4). The activation energy is solely determined by thermodynamics, that is, the adsorption energy of on-top Cl, which decreases significantly with increasing Clot and Oot coverage. An additional barrier due to kinetics has not been identified with DFT calculations.
Letters The derived reaction mechanism for the Sumitomo process is distinctly different from the originally proposed Deacon process. The original Deacon process is considered to be a solidstate reaction, in which CuO is transformed into CuCl217 and Cl2 is liberated by the decomposition of CuCl2. To close the catalytic cycle, reoxidation to CuO is required. Quite in contrast, the HCl oxidation over RuO2(110) proceeds via a LangmuirHinshelwood (LH)-type reaction mechanism. Since hydrogen cannot efficiently be accepted by the bridging Cl atoms (activation barrier is as high as 2.5 eV), an efficient communication between the 1f-cus-Ru rows is suppressed. Therefore, the RuO2-xClx(110) catalyst can be envisioned as a one-dimensional catalyst offering isolated rows of 1f-cus-Ru sites where a LHtype reaction between HCl and O2 takes place. This onedimensionality together with the stability of the RuO2-xClx(110) catalyst against reducing agents opens new ways for a variety of catalyzed dehydrogenation reactions with superior selectivity and stability. Under typical reaction conditions in an excess of oxygen and temperatures between 500 and 600 K, the chlorinated RuO2(110) surface is mainly covered with on-top Cl and on-top O. The released hydrogen is removed from the catalyst surface via water formation. The residual oxygen on the surface desorbs at around 400 K (cf. Figure 4). The remaining Clot species on the surface have to diffuse along the 1f-cus-Ru rows to meet a second Clot to react with. This diffusion process is activated by 0.8 eV (DFT) and therefore is not rate determining at reaction temperatures between 500 and 600 K. Since the adsorption energy of Clot declines considerably with increasing O coverage, the HCl reaction over chlorinated RuO2(110) is more active with the higher O2/HCl feed ratio. This general behavior is actually observed with the Sumitomo process over powder RuO2 at ambient pressures. Steady-state reaction experiments over RuO2 powder catalysts indicate a maximum activity at around 600 K with an O2 to Cl2 ratio of 4:1.18 Detailed DFT calculations of the HCl oxidation reaction in this study identified a similar reactionmechanismwhichisactuallyalsoaLangmuir-Hinshelwood rather than a Mars-van Krevelen-type mechanism. The ratedetermining step is the recombination of neighboring on-top Cl atoms. The DFT calculations in ref 18 suffer, however, from a the assumption that the actual catalyst is RuO2(110) rather than the chlorinated RuO2-xClx(110) surface. Since the H atoms of adsorbed HCl can be abstracted by both bridging O and ontop O for the case of stoichiometric RuO2(110), there is substantial cross talking between the 1f-cus rows, and consequently, the reaction mechanism is two-dimensional in nature rather than one-dimensional, as identified with the chlorinated RuO2 surface. The lowest possible reaction temperature for the HCl oxidation over chlorinated RuO2(110) is determined by the water
J. Phys. Chem. C, Vol. 112, No. 27, 2008 9969 desorption temperature of 420 K. Below this temperature, the chlorine-stabilized RuO2(110) catalyst is self-poisoned by the byproduct water. In conclusion, the oxidation of HCl with oxygen producing Cl2 and water has shown to proceed on the chlorine-stabilized RuO2(110) surface via a one-dimensional Langmuir-Hinshelwood mechanism along the rows of undercoordinated Ru sites (1fcus-Ru). This conclusion is drawn from high-resolution corelevel shift spectroscopy and temperature-programmed reaction experiments together with density functional theory calculations. The recombination of two adjacent chlorine atoms on the catalyst’s surface constitutes the rate-determining step in this novel Deacon-like process. Acknowledgment. We would like to thank Leibniz Rechenzentrum in Munich for providing us with massive parallel supercomputing time, the Deutsche Forschungsgemeinschaft and the Swedish Research Council for financial support, and the MAX-laboratory staff for technical support. References and Notes (1) Deacon, H. U. S. Pat. 1875, 165, 6802. (2) Gestermann, F.; Ottavini, A. Mod. Alkali Technol. 2001, 8, 49. (3) Iwanaga, K.; Seki, K.; Hibi, T.; Issoh, K.; Suzuta, T.; Nakada, M.; Mori, Y.; Abe, T. Kagaku 2004, I, 1. (4) The HRCLS measurements were conducted at the beam line I311 at MAXII in Lund, Sweden: Nyholm, R.; Andersen, J. N.; Johansson, U.; Jensen, B. N.; Lindau, I. Phys. Res. A 2001, 520, 467. (5) We employed the Vienna ab initio simulation package (VASP): (a) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1995, 6, 15. (b) Kresse, G.; Joubert, D. Phys. ReV. B 1998, 59, 1758. (6) Blo¨chl, P. Phys. ReV. B 1994, 50, 17953. (7) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (8) Pehlke, E.; Scheffler, M. Phys. ReV. Lett. 1993, 71, 2338. (9) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474. (10) Over, H.; Muhler, M. Prog. Surf. Sci. 2003, 72, 3. (11) Blume, R.; Ha¨vecker, M.; Zafeiratos, S.; Teschner, D.; Vass, E.; Schno¨rch, P.; Knop-Gericke, A.; Schlo¨gl, R.; Lizzit, S.; Dudin, P.; Barinov, A.; Kiskinova, M. Phys. Chem. Chem. Phys. 2007, 9, 3648. (12) 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. (13) (a) Over, H.; Seitsonen, A. P.; Lundgren, E.; Wiklund, M.; Andersen, J. N. Chem. Phys. Lett. 2001, 342, 467. (b) Over, H.; Seitsonen, A. P.; Lundgren, E.; Smedh, M.; Andersen, J. N. Surf. Sci. 2001, 504, 196. (14) Knapp, M.; Crihan, D.; Seitsonen, A. P.; Lundgren, E.; Resta, A.; Andersen, J. N.; Over, H. J. Phys. Chem. C 2007, 111, 5363. (15) Diebold, U.; Hebenstreit, W.; Leonardelli, G.; Schmid, M.; Varga, P. Phys. ReV. Lett. 1998, 81, 405. (16) Knapp, M.; Crihan, D.; Seitsonen, A. P.; Over, H. J. Am. Chem. Soc. 2005, 127, 3236. (17) Hisham, N. W. M.; Benson, S. W. J. Phys. Chem. 1995, 99, 6194. (18) Lope´z, N.; Gomez-Segura, J.; Marin, R. P.; Pere´z-Ramire´z, J. J. Catal. 2008, 255, 29.
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