Atomic-Scale Understanding of the HCl Oxidation Over RuO2, A Novel

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Atomic-Scale Understanding of the HCl Oxidation Over RuO2, A Novel Deacon Process Herbert Over* Physikalisch-Chemisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 58, D-35392 Gießen, Germany ABSTRACT: Heterogeneous catalysis of the HCl oxidation by oxygen (Deacon process) over RuO2 is a green chemistry route to recover high purity Cl2 from HCl waste in an almost energy neutral way on a large industrial scale. The outstanding properties of RuO2-based catalysts are long-term stability under such harsh reaction conditions and high catalytic activity, allowing for lower reaction temperatures and hence for higher Cl2 conversions at equilibrium. In this Feature Article, I will be reviewing the atomic-scale insights into this Deacon process gained on single-crystalline RuO2(110) model catalysts. The extraordinary stability of RuO2(110) is traced to the selective and self-limited replacement of bridging surface oxygen by chlorine, thereby transforming active surface sites with basic Brønsted character into inactive sites and thereby suppressing the bulk chlorination of RuO2. The reaction mechanism has been clarified by utilizing experimental surface science techniques together with density functional theory (DFT) calculations. Oxygen adsorption proceeds dissociatively across two neighboring undercoordinated Ru sites, thereby forming two undercoordinated surface on-top O (Oot) atoms (homolytic cleavage). These Oot species are able to accept H from dissociative adsorption of HCl to form on-top Cl (Clot) and on-top hydroxyl groups (OotH). The heterolytic cleavage of HCl requires both acidic (undercoordinated Ru) and basic surface centers (undercoordinated O). Another H-transfer to the hydroxyl groups produces the byproduct water which desorbs at 420 K. The recombination of adjacent adsorbed Clot produces finally the desired product Cl2, an elementary reaction step with the highest activation barrier of 228 kJ/mol. Yet, oxygen adsorption constitutes the rate-determining step in the Deacon process over RuO2(110) under typical reaction conditions since strongly adsorbed Clot blocks dissociative oxygen adsorption. The overall reaction mechanism is governed by a delicate interplay of surface kinetics and thermodynamics, i.e., the adsorption energies of reactants, reaction intermediates and products.

1. INTRODUCTION Industrial chemistry intensively employs chlorine as oxidizing agent in a variety of organic processes, rendering chlorine the energy carrier in large-scale synthesis. The worldwide production of Cl2 is 68 million t/a.1,2 Chlorine is required for the manufacture of about two-thirds of all chemical products including polymers (e.g., polycarbonates), crop protection, pharmaceutical products, products for drinking water purification, and ultrapure silicon for photovoltaics and electronics applications. In the course of these processes, hydrogen chloride is incurred as an inevitable byproduct either directly by the substitution reaction or by the subsequent production steps to attain chlorine-free final products.3 As indicated in the chlorine tree,1 about 30% of the final products are chlorine-free, thus leading to some 10 million tons per year of HCl byproduct. Industrial uses do exist for HCl, for instance, as a chlorine source in the polyvinylchloride (PVC) production, as acid catalysts, or for the neutralization of alkaline streams. However, chlorine-related processes produce much more of the byproduct HCl than the market can absorb, resulting in a severe toxic waste disposal problem. The primary method of HCl disposal is by neutralization, which is costly and far from being sustainable since “energetic” HCl gas is transformed into less useful chloride salts. © 2012 American Chemical Society

Consequently, there has been growing interest in the chemical industry to find cost-effective methods for recycling chlorine from hydrogen chloride.4,5 The heterogeneously catalyzed HCl oxidation (so-called Deacon process), i.e. 2HCl(g) +

1 O2 (g) ⇌ Cl2(g) + H2O(g); 2

Δr H = −59kJ/mol at 300 K

is one such process which allows us to design closed-process cycles in which chlorine is recycled almost energy-neutral from the toxic byproduct hydrogen chloride so that this process therefore be subsumed under green, respectively, sustainable chemistry. Although the Deacon process has been known for some 140 years,6 it has not found its way into broad industrial applications. The reasons are many-fold, but the original Deacon process catalyzed by CuO/CuCl2 has suffered most notably from the missing stability of the deployed catalyst and from too high reaction temperatures above 700 K. Both shortcomings are Received: December 15, 2011 Revised: February 2, 2012 Published: February 7, 2012 6779

dx.doi.org/10.1021/jp212108b | J. Phys. Chem. C 2012, 116, 6779−6792

The Journal of Physical Chemistry C

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coupled since the low activity (due to high activation barriers) requires high reaction temperatures which in turn lead to stability loss of CuO due to the formation of volatile CuCl2. Note that the oxidation of HCl is mildly exothermic by −59 kJ/mol (Cl2) (300 K), so that the equilibrium conversion at 700 K is only 70−80% (cf. Figure 1). In addition to these problems, CuCl2 is volatile above 700 K (melting point CuCl2 is 770 K) and extremely corrosive, thus posing severe problems to the choice of materials for reactors and tubing. Over the past 50 years, numerous alternative strategies have been pursued to overcome the problems with the original Deacon process, albeit only with limited success.7−12 A nice review about the history of Cl2 production can be found in recent review articles.13,14 For these reasons, the original Deacon process had largely been displaced by electrolysis, a highly energy-consuming process.2,15 In the electrolysis of aqueous HCl, both Cl2 and H2 are produced. Advanced electrolyzers return the produced H2 to a kind of integrated fuel cell (called gas diffusion electrodes) to form water, thereby reducing the electric power consumption by 30%.15 Still, about 1600−1700 kW h2 of electric energy is required to produce one ton of Cl2. This makes up a substantial and continuously growing part of the total cost for HCl recovery by electrolysis.

Figure 2. Relative activities of the RuO2-based catalyst for the HCl oxidation dependent on the chosen support. The inset shows 1 ML of RuO2(110) coated on TiO2(110).5

lower investment costs. The only other Deacon-type process currently in industrial operation is the Mitsui MT chlorine process based on the Cr2O3/SiO2 catalyst with an annual production of 60 kt Cl2.17−19 In Figure 1, the HCl conversion at equilibrium is given as a function of the temperature whose slope is determined by the standard reaction enthalpy (about −59 kJ/mol). The pressure and stoichiometry dependence of the conversion yield is a nice exercise for chemical equilibrium applying the law of mass action. The pressure dependence of equilibrium conversion is related to the negative molar volume change of the HCl oxidation reaction: the higher the pressure, the greater the equilibrium constant and thus the conversion at equilibrium.20 For determining the stoichiometry dependence of the HCl conversion at equilibrium, the equilibrium constant K has to be rephrased in terms of the conversion y = p(Cl2)/p(HCl) K=

y2 p(O2 )

⎛ −Δr G° ⎞ ⎟ p° = exp⎜ ⎝ RT ⎠

where p°, T, R, and ΔrG° are the standard pressure, absolute temperature, gas constant, and the standard Gibbs energy, respectively. From this expression (K is constant), it is clear that the higher the partial pressure (p(O2)) of oxygen (keeping the partial pressure of HCl, p(HCl), constant), the higher the conversion y at equilibrium. The true technological innovation in the Sumitomo process constitutes the chosen support rutile-TiO2.21 The catalytic activity of RuO2−rutile-TiO2 is substantially higher than that of RuO2−anatase-TiO2 (cf. Figure 1). This observation may suggest that the rutile structure of the support is important for achieving highly active RuO2-based catalysts. Already in the 60s, Shell had introduced Ru-based Deacon catalysts on silica supports, but these catalysts were surprisingly inactive (cf. Figure 2) and therefore have not been commercialized.22 Pure RuO2 powder is too expensive to be used in the Deacon process and not stable enough under HCl reaction conditions.23 It is not only the activity that counts but also the stability of RuO2−TiO2 which renders RuO2 such a versatile catalyst material under harsh reaction conditions of chlorine evolution (electrochemistry) or the Deacon process (heterogeneous catalysis). In retrospect, RuO2−TiO2 catalysts seem to be the obvious choice for the Deacon process since such catalysts have already been in industrial use as dimensionally stable anodes (DSA) in the chlor-alkali electrolysis for more than 40 years.24

Figure 1. HCl conversion at equilibrium as a function of the temperature T depends parametrically on the stoichiometry of reaction feed and the total pressure. The original Deacon process (based on CuO/CuCl2) suffers from a low conversion yield at equilibrium of about 70%−80% due to the high reaction temperature of 430−470 °C which is required to overcome the imposed activation barriers. Quite in contrast, the novel Sumitomo process (based on RuO2/rutile-TiO2 catalyst) needs only a reaction temperature of about 300 °C, thus achieving HCl conversion at equilibrium of 90−95%.4

Only recently, Sumitomo Chemical16 has developed an efficient, stable, and cost-effective Deacon-type process on the basis of RuO2-coated rutile-TiO2 catalysts. This Sumitomo process is a highlight in recent catalysis research since chlorine can now be recycled from HCl with low energy cost and high conversion yields of 95%. The high equilibrium conversion of HCl is the result of an optimum reaction temperature of 573 K, which is by 150 K lower than in the original Deacon process (cf. Figure 1). The low reaction temperature improves concomitantly the stability of the catalyst against Ru-chloride formations. Nevertheless, for small-scale recycling of HCl, electrolysis is still currently more cost-effective than the Deacon/Sumitomo process due to 6780



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Cl2, but the rate-determining step (rds) is identified with oxygen adsorption under typical reaction conditions. The present feature article is organized as follows. In section 2, I start with a brief discussion of the atomic-scale properties of RuO2(110), introducing the concept of undercoordinated surface atoms being the catalytically active sites. Section 3 is devoted to the extraordinary stability of RuO2(110) under harsh HCl oxidation conditions. In section 4, the reaction mechanism of the Deacon process on RuO2(110) and RuO2−TiO2(110) is elucidated. This feature article concludes with a summary and some general remarks about the original Deacon process, a comparison with the Chlor-Alkali electrolysis and with the CO oxidation, as well as the scientific potential of kinetic Monte Carlo simulations for gaining even deeper insights into this complex reaction system.

2. ATOMIC-SCALE PROPERTIES OF RUO2(110) IMPORTANT FOR THE SUMITOMO PROCESS RuO2 crystallizes in the rutile structure where the O atoms adopt the sp2 hybridization, while the Ru atoms are coordinated to six O atoms forming a slightly distorted octahedron (d2sp3 hybridization of Ru). The bulk-truncated RuO2(110) surface exposes two kinds of undercoordinated surface atoms. These are the bridging oxygen atoms (Obr), which are coordinated to two (instead to three) Ru atoms underneath, and the 1f-cus Ru atoms, i.e., 1-fold undercoordinated Ru atoms, which are coordinated to five (instead to six) O atoms (cf. Figure 3a). We may recall that RuO2 is a metallic oxide with electronic conductivity half of that of ruthenium itself,32 thereby facilitating both the experimental and computational work.33 Counting the formal charges of the Ru and O with +4 and −2, respectively, the bulk-truncated RuO2(110) surface turns out to be auto compensated in that the number of electrons missing at the surface O atoms is compensated by the surplus electrons at the 1f-cus Ru sites (electron counting rule34). The actual charge on the Ru and O atoms in RuO2 is however much smaller as estimated by a detailed Bader analysis based on DFT calculations,35 namely, +1.74 for bulk-Ru and −0.87 for bulk-O, while 1f-cus Ru carries a charge of +1.60 and Obr −0.80. For comparison: Assuming that each Ru−O bond polarizes 2/3 of a unit charge, the formal charge of Obr is −4/3 and that of the 1f-cus Ru is +3 1/3. The electronic structure of RuO2(110) can be visualized by pseudo valence charge density difference maps (cf. Figure 4a) which are defined as the difference between the total valence electron density (as determined by DFT calculations) and a linear superposition of radially symmetric atomic charge densities of the valence electrons.38−40 These difference plots visualize the polarization of Ru and O atoms upon bond formation in the solid phase. For bulk-coordinated sites, one can clearly recognize the sp2 hybridization of O and the d2sp3 hybridization of Ru (cf Figure 4a). The lopes of sp2 hybridization are enriched by electron density (red contours), and the d2sp3 hybrides are electron depleted (blue contours), forming strong σ-bonds with coordinated O. Along the Ru−O bonds, a substantial charge transfer from Ru to O occurs, which is in line with the expected ionic bonding. The observed electron accumulation along the Ru−Ru bonds originates from the residual d-orbitals not used in the d2sp3 hybridization, signifying metallic bonding. Surprisingly, the shape of pseudo valence charge density difference contours of a particular chemical species at the RuO2(110) surface is virtually identical to that of bulk RuO2. This observation

Figure 3. Ball and stick model of the stoichiometric RuO2(110) surface (a) and the oxygen exposed RuO2(110) surface (b), where most of the undercoordinated 1f-cus Ru site are occupied by on-top O (Oot). The big green balls are the oxygen atoms, and the small blue and red balls are the Ru atoms. At the stoichiometric RuO2(110) surface, there are two types of undercoordinated atoms, the bridging O atoms (Obr) and the 1f-cus Ru site (red ball). 1f-cus stands for 1-fold coordinatively unsaturated site.

The late invention of the Sumitomo process illustrates emphatically the apparent shortcoming of a highly specialized scientific community in which the expertise and knowledge of even closely related chemical disciplines such as electrocatalysis and heterogeneous catalysis are barely exchanged. Modern catalysis research does need to overcome this “community gap” to keep track with challenges ahead.25−27 In the following, I discuss the atomic-scale processes of the HCl oxidation reaction over RuO2 in detail. Of course, molecular-level understanding calls for idealization of the experimental conditions including the use of proper model catalysts with low structural complexity such as single-crystalline surfaces and investigating them under well-controlled and ultrapure conditions such as ultrahigh vacuum (UHV).28 The trade-off for this so-called surface science approach is the inevitable emergence of a pressure gap (10−13 bar versus 10 bar) and a materials gap (single crystal versus supported nanometersized particles). As a consequence, elementary reaction steps, reaction intermediates, the chemical state of the catalyst, etc. identified under well-defined conditions may not be transferable to realistic reaction conditions. In this review, I elaborate on the HCl oxidation over single-crystalline RuO2(110) (and to a smaller extent also RuO2(100)) where both apparent gaps have been successfully bridged. The extraordinary stability of RuO2(110) is shown to be traced to the selective and selflimited replacement of bridging surface oxygen by chlorine, transforming active basic Brønsted sites into inactive sites.29 The reaction mechanism of HCl oxidation over RuO2(110) has been explored by utilizing experimental surface science techniques such as high-resolution core level shift spectroscopy30 together with density functional theory (DFT) calculations.30,31 The reaction step with the highest activation energy turned out to be the association of adsorbed Cl atoms to form 6781



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Figure 4. (a) Pseudovalence charge density contour plots of the RuO2(110) surface cut through the cus-Ru atom along the [11̅ 0] and the [001] directions (left and right). On the right side, the surface is cut through the bridging O atom. These plots are defined as the difference between the total valence electron density, as provided by DFT calculations, and a linear superposition of radially symmetric atomic charge densities. Contours of constant charge density are separated by 0.15 e−/Å3. Areas of electron depletion and accumulation are colored by blue and red, respectively. (b) Pseudovalence charge density contour plots of the bridge-chlorinated RuO2(110) surface cut through the Clbr atom along the [11̅ 0] and the [001] directions.36,37

HCl oxidation is corrosive even without water. Exposing a sample by backfilling the chamber with HCl is problematic since HCl strongly adsorbs on the chamber walls, executing replacement reactions. For instance, with STM it was observed that initial HCl exposure leads actually to adsorption of oxygen on the sample surface rather than to HCl adsorption63since HCl displaces oxygen from the chamber walls which in turn adsorbs on the RuO2(110) surface. HCl and Cl2 attack hot filaments in the chamber. This is particularly troublesome when using a mass spectrometer for detecting HCl and Cl2 since upon exposure the electron yield of the filament and with it the sensitivity of the mass spectrometer change significantly. The pumping system of the UHV system must be HCl resistant, i.e., either by using oil-free rough and turbo pumps or using perfluorinated oils in the rotary pumps. HCl reacts easily with oil, in particular if traces of water are dissolved in oil. The best way to dose HCl (or Cl2) is using a long dosing tube which is directed toward and in proximity to the sample surface. As soon as one increases the pressures of HCl beyond 10−1 mbar, special care has to be taken with the gas outlet in that the HCl stream must be neutralized for instance by a NaOH-impregnated zeolite.64 With in situ surface X-ray diffraction (SXRD), one has to pay special attention to the beryllium window which becomes brittle by Cl2 exposure. Hence, for the HCl oxidation experiments in the millibar range, the reaction cell with Be windows was replaced by a special chamber having a thin aluminum window. The Al window is resistant against HCl and Cl2; however, the X-ray beam is more attenuated by aluminum than by beryllium.64 Therefore, the photon energy has to be increased. Although much care had been exercised with the in situ SXRD experiments, the turbo pump was damaged after the second experiment in that the bearings were severely corroded.

implies that the bulk hybridizations of Ru and O atoms are preserved at the surface, corroborating nicely the concept of dangling bonds.41 The 1f-cus Ru sites provide both electronaccepting and -donating orbitals, explaining naturally the high propensity of the 1f-cus Ru sites for the chemisorption of molecules from the gas phase.42 The dangling bond of Obr reveals a large and more contracted charge accumulation than bulk O which is characteristic of a Brønsted base, i.e., a hydrogen accepting site. Hydrogen adsorption43,44 and dehydrogenation experiments30,45−47 have shown that bridging O atoms serve indeed as Brønsted bases, i.e., accepting H atoms and forming bridging O−H groups. Most of the molecules studied so far on the RuO2(110) surface (CO,48 H2O,49−51 O2,40,52 N2,48 methanol,46,47,63 CO2,54 NO,55 ethylene,53,56,57 ethane, methane,58 and NH343,59) adsorb from the gas phase directly above the 1f-cus Ru atoms. Therefore, the 1f-cus Ru atoms are considered to be the dominating active sites of RuO2(110) governing the interaction with the surrounding gas atmosphere.60 Dissociative adsorption of molecular oxygen from the gas phase leads to the formation of atomic Oot species in terminal position above the 1f-cus Ru atoms (cf. Figure 3b).40 Oot has shown to be prone to pick up readily hydrogen from bridging ObrH groups.50 Obr forms two σ bonds with Ru, whereas Oot forms only one single σ bond. This difference in bond order is also reflected by total energy calculations: The Obr species is by 130−150 kJ/mol stronger bound than Oot.31,40,61,62

3. TECHNICAL NOTES TO HCL OXIDATION I should emphasize that HCl oxidation is a corrosive reaction which is not compatible with typical UHV experiments as performed in surface chemistry. HCl itself is less corrosive, but in the presence of water, HCl is a strongly corrosive agent. Therefore, gas lines have to be “water-free”. The product Cl2 of 6782



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4. STABILITY OF RUO2 UNDER HCL OXIDATION REACTION CONDITIONS On the basis of high-resolution core level shift spectroscopy (HRCLS) experiments, the extraordinary stability of the model catalyst RuO2(110) in the Sumitomo process has been attributed to the selective replacement of the bridging oxygen (Obr, cf. Figure 3a) by chlorine, a process which is confined to the topmost layer of RuO2(110).29 Sumitomo Chemical improved further the stability of RuO2 supported on rutile-TiO2 by codepositing SiO2 to suppress sintering of the RuO2 nanodomains under the reaction conditions.5 A deeper reduction/chlorination of RuO2(110) has not been observed under ultrahigh vacuum (UHV) typical conditions29,65 nor at higher pressures in the millibar range.65,66 The maximum surface chlorination of RuO2 attained has been estimated to be 70−80%, both on single crystals67 and on RuO2 powder, respectively, RuO2/TiO2-supported catalysts.66 From DFT calculations, the (2 × 2) structure with only every second Obr replaced by chlorine is more stable by 35 kJ/mol than a surface where all Obr is replaced by Clbr.31,67,68 Actually, in STM63 one can clearly see that HCl exposure of RuO2(110) leads to the formation of a (2 × 1) modulation along the bridging O rows. A density functional theory (DFT) simulated STM image is overlaid to the experimental STM image of the chlorinated RuO2(110) surface to illustrate this point (cf. Figure 5).63

molecules cleave homolytically. The dissociative adsorption of HCl takes place on RuO2(110) without an activation barrier and is exothermic by 130 and 135 kJ/mol if the eliminated H atom is transferred to bridging O or to on-top O, respectively. There are now several ways conceivable how the bridging O atoms of stoichiometric RuO2(110) are replaced by chlorine. First of all, simple exposure of molecular chlorine Cl2 is not able to chlorinate the RuO2(110) surface since the exchange process Clot + Obr → Clbr + Oot is activated by 230 kJ/mol according to DFT calculations.68 Dissociative HCl adsorption leads to on-top Cl (Clot) and a bridging hydroxyl group (ObrH). The replacement of this bridging hydroxyl group by on-top Cl (Clot + ObrH → Clbr + OotH) is, however, strongly activated by more than 300 kJ/mol as found by DFT-based nudged elastic band (NEB) calculations.68 Therefore, the chlorination process needs the formation of bridging water as a reaction intermediate to replace Obr by chlorine. This can be accomplished by either two consecutive H-transferring to the same Obr, forming bridging water (Mechanism A in Figure 6), or two HCl molecules dissociating above neighboring 1f-cus Ru sites and transferring their H to neighboring Obr, thus forming two adjacent ObrH (Mechanism B in Figure 6). An additional H-transfer between ObrH neighbors leads finally to bridging water. In both cases, the bridging water species is strongly stabilized by about 100 kJ/mol by adsorbed Clot.68 The final replacement (either concerted or sequentially) of bridging water by chlorine is activated by 140 kJ/mol as determined by DFT calculations. For stoichiometry reasons, the formation of bridging water should be promoted by preadsorbed hydrogen: The formation of one water molecule from Obr requires the adsorption of two HCl molecules, resulting in two Clot atoms from which only one Clot can be transferred into a bridging position when Obr has been previously reduced to water and shifted to 1f-cus Ru. The other Clot atom blocks a 1f-cus Ru site for further HCl uptake. Above 600 K, neighboring Clot can recombine to form Cl2 which is instantaneously released into the gas phase. If the surface is preexposed to H2 at 300 K, bridging hydroxyl groups (ObrH) are generated.44 In this way the deficiency of hydrogen on the surface is lifted, and subsequent exposure of HCl leads to a balanced and more efficient chlorination of the bridging oxygen atoms, as summarized by the following reaction equation Obr H + HCl gas → H2Ogas + Clbr

Figure 5. STM images of the clean RuO2(110) surface (bottom) and the surface chlorinated RuO2(110) surface (top). The DFT simulated STM image is overlaid to the experimental STM of the chlorinated RuO2(110) surface. This comparison reveals that the height modulations along the bridging O rows are consistent with (2 × 1) structure, where every second Obr is replaced by chlorine. In the experimental STM image, also on-top Cl atoms are visible as bright features.63

With high-resolution core level shift spectroscopy, both the chlorination process of stoichiometric RuO2(110)29 as well as the hydrogen-promoted chlorination have been studied in detail,68 supporting the above surface chlorination scheme on RuO2(110). In situ surface X-ray diffraction (SXRD)65 reveals that chlorinated RuO2(110) and RuO2(100) model catalysts are long-term stable under reaction conditions where the gas feed p(HCl)/p(O2) was varied from 1:4 to 4:1 for pressures in millibar range and temperatures as high as 850 K. Even pure HCl exposure in the millibar range is not able to chemically reduce RuO2 below 600 K since the bridging oxygen positions are mainly populated by chlorine and on-top adsorbed chlorine blocks part of the undercoordinated Ru sites. Without the presence of undercoordinated surface oxygen, HCl adsorption is suppressed for thermodynamic reasons. Therefore, the RuO2(110) surface is resistant against pure HCl exposure even in the millibar range as long as the temperature is below 600 K. Above 650 K, chemical reduction of RuO2(110) sets in

The degree of surface chlorination of RuO2(110) is not a static but rather a dynamic process depending sensitively on the actual reaction mixture. For instance, in excess O2 in the gas feed the bridging Cl atoms are partly restored by oxygen.67 Surface chlorination of RuO2(110) proceeds via a multistep process. HCl adsorbs on RuO2(110) as an intact molecule by 40−60 kJ/mol.31,68 Therefore, above T = 200 K HCl can only be stabilized on the catalyst’s surface by dissociative adsorption in that the Cl binds to 1f-cus Ru forming Clot and hydrogen is transferred to undercoordinated O atoms forming either bridging (ObrH) or on-top (OotH) hydroxyl groups. HCl undergoes a heterolytic cleavage during adsorption, whereas O2 6783



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Figure 6. (i) Schematic representation of the chlorination of RuO2(110). Ball-and-stick model of bulk-truncated RuO2(110) (left) exposing bridging O atoms (Obr) and 1-fold coordinatively unsaturated Ru sites (Ru 1f-cus). Upon HCl exposure at higher temperatures, the stoichiometric surface transforms into a chlorinated surface (right) where the bridging Obr atoms are partly replaced by bridging chlorine (Clbr) atoms (shown in gray color).68,29 (ii) Mechanism A: HCl adsorbs aside to an ObrH forming bridging water and on-top Cl. Bridging water moves toward the 1f-cus Ru (activation energy of 140 kJ/mol), and without any further activation barrier Clot slips into the previously generated bridging vacancy. (iii) Mechanism B: Dissociation of HCl proceeds to Obr, forming Clot and bridging ObrH. Hydrogen transfer from a second ObrH form bridging water which is stabilized by on-top Cl. The rest is identical to Mechanism A. Reprinted with permission from ref 68. Copyright 2010 American Chemical Society.

We used RuO2-based nanofibers which were synthesized by electrospinning. Details about the synthesis can be found in the PhD thesis of Ostermann.71 The crystallite size of the RuO2 nanofibers is about 9 nm according to Rietvield analysis with a BET surface area of 30 m2/g. Reaction-induced corrosion processes become quite easily visible in scanning electron microscopy (SEM) when the shape of the fibers is altered during the reaction. In Figure 7a,b, we show how pure RuO2 nanofibers (composed of RuO2 nanocrystals and sintered in the form of nanofibers) are changed after HCl oxidation reaction in a flow reactor under oxidizing feed gas composition (p(HCl) = p(O2) = 200 mbar using a buffer gas p(Ar) = 600 mbar; the total flow rate of the reaction mixture was 50 mL min−1 (STP)) keeping the catalyst bed at 650 K for 2 h on stream. The pure RuO2 fibers have been disintegrated. Quite in contrast, mixed RuO2−TiO2 fibers are stable under the same HCl oxidation reaction conditions (cf. Figure 7c,d). The Ru-doped TiO2 nanofibers were prepared by 15 atom % Ru, resulting in phase-pure rutile mixed RuO2−TiO2 fibers (Rietvield analysis). The catalytic activity of mixed RuO2−TiO2 nanofibers is as high as that of pure RuO2 nanofibers.23

by moving the bridging Cl to the 1f-cus Ru sites. Subsequently, the created bridging O vacancies are filled by lattice oxygen via diffusion from the bulk of RuO2. The so generated Obr species can be reduced by HCl to water which immediately desorbs. In this way, bulk RuO2(110) is gradually reduced. With a reaction feed of p(HCl)/p(O2) = 4:1, the catalyst is even stable up to 850 K (close to the decomposition temperature of the oxide), indicating that the additional oxygen stabilizes the chlorinated RuO2(110) surface against chemical reduction by HCl exposure. The formation of Ru-chloride has not been observed. Similar stability results were reported for polycrystalline RuO2 powder in a recent TAP experiment.69 In contrast to bridging O, bridging chlorine does not serve as a Brønsted basic site on RuO2(110) since Clbr is not able to accept H. The H-transfer process toward bridging Cl is activated by more than 200 kJ/mol and causes direct desorption of bridging Cl via HCl. Substitution of the bridging oxygen atom by a bridging chlorine (Clbr) is associated with a change of the formal oxidation state of the underlying 2f-cus Ru atoms from +IV 2/3 to +III 2/3. This change in the oxidation state upon chlorination (being closer to +IV) suggests that Cl exerts a stabilizing influence on the RuO2(110) surface. From the pseudo valence electron density, one can recognize that bridging Cl is sp3 hybridized (cf. Figure 4b). Recently, a new method was developed70 to study the longterm stability of RuO2-based catalysts in the HCl oxidation reaction. RuO2 powder samples have the disadvantage that morphological changes due to reactions cannot be made visible. To visualize reaction-induced morphological changes of the catalysts, one needs to start with a catalyst of well-defined morphology.

5. REACTION MECHANISM OF THE DEACON PROCESS 5.1. Reaction Mechanism of Deacon over RuO2. Mechanistic studies of the HCl oxidation over chlorinated RuO2(110) were performed using DFT calculations30,72 and HRCLS experiments;30 similar DFT calculations were carried out for the HCl oxidation over stoichiometric RuO2(110).31 The Langmuir type kinetics of the HCl oxidation reaction over 6784



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atop position of the 1f-cus Ru sites (Oot). Actually, the adsorption process of O2 is a little bit more complex since oxygen adsorbs first molecularly (by 40−120 kJ/mol),40,52 from where the dissociation is then activated by about 25 kJ/mol.52 The apparent activation energy for the dissociative adsorption of oxygen is therefore negative at higher temperatures such as encountered in the HCl oxidation reaction. This means that with increasing temperature dissociative adsorption of oxygen declines. HCl molecules adsorb dissociatively with Cl sitting on-top of a 1f-cus Ru site, and the H atom is transferred to ontop O (or bridging O), forming a hydroxyl group: This process occurs without any noticeable activation barrier and is exothermic by 125 kJ/mol for the case of on-top O.30 The next HCl molecule can form a second Clot species and water; this process is exothermic by even 178 kJ/mol (cf. Figure 9). The final production of surface water (H2Oot) can also occur via a H transfer between neighboring OotH groups,44,50 a process which is kinetically activated by 29 kJ/mol. Water desorption is endothermic by 109 kJ/mol and proceeds at 420 K. The DFT calculated value is actually too small since thermal desorption at 420 K translates into an activation energy for desorption of 170 kJ/mol.29 The remaining Clot species on the surface have to diffuse along the 1f-cus Ru rows to meet a second Clot to with which to react. This diffusion process is activated by 35 kJ/mol and is therefore easily overcome at typical reaction temperatures of 500−600 K. However, we have to bear in mind that the interaction among direct neighboring Clot is repulsive by about 20 kJ/mol so that Clot recombination needs high surface Clot coverage.37 The recombination of two on-top Cl to form the desired product Cl2 constitutes the elementary reaction step with the highest activation barrier of 228 kJ/mol.30 This value is higher than that reported by Lopez et al.31 (150 kJ/mol) and is in almost quantitative agreement with that of Studt et al.72 (220 kJ/mol). The DFT-calculated reaction energy ΔE of −66 kJ/mol at T = 0 K agrees well with the experimental value of −59 kJ/mol at T = 298 K. For the case that most of the bridging O atoms are replaced by chlorine, the adsorption of HCl is tightly coupled to the surface concentration of on-top O since HCl adsorbs molecularly too weakly (30−50 kJ/mol), and heterolytic splitting of HCl requires the presence of basic centers, i.e., undercoordinated O atoms on the surface to accept the H-atoms from HCl splitting. Another species, which becomes apparent from the energy diagram in Figure 9, are OH groups whose role in the reaction is less obvious. OH groups can simply be considered as surface intermediates for water formation, a process which dominates at high concentrations of OH. However, the OH groups can also exist as isolated species which are surrounded by adsorbed Cl surrounds. In this case, surface OH groups are detrimental for the reaction since at higher temperatures these OH species induce the desorption of ontop Cl via HCl formation. Recent kMC simulations73 have disclosed that the concentration of OH groups serves as a predictor for the activity of the surface: If the concentration of OH is high, then the overall activity is low and vice versa. The theoretical values for the adsorption energy of chlorine30,31,72 are too small in comparison with values derived from a temperature of 700 K for the maximum of Cl2 desorption (cf. Figure 10),29,30 namely, about 300 kJ/mol. A much higher value for the adsorption energy of chlorine is also reasonable when the desorption temperature of chlorine is compared to the desorption temperature of on-top O (400 K)

Figure 7. High-resolution SEM images (the white bars correspond to 500 nm): RuO2 and TiO2−RuO2 mixed nanofibers before and after HCl oxidation reaction in a flow reactor under oxidizing feed gas composition (p(HCl) = p(O2) = 200 mbar; buffer gas p(Ar) = 600 mbar; total flow rate 50 mL min−1 (STP)) at 650 K for 2 h. Clearly, the RuO2 nanofibers disintegrate, while the TiO2−RuO2 mixed fibers are stable under such harsh reaction conditions.70

chlorinated RuO2(110) is shown to be governed by the adsorption energies of the reaction intermediates (water: 109 kJ/mol and on-top Cl: 228 kJ/mol against Cl2) (cf. Figure 8). The

Figure 8. Catalytic cycle of the HCl oxidation over surface-chlorinated RuO2(110) model catalyst (c-RuO2(110)). The reactant molecules O2 and HCl both adsorb first on the 1f-cus Ru sites. O2 dissociates to form adsorbed O and HCl adsorption via a hydrogen elimination to form Clot and OotH species in terminal positions. H-transfer among the OotH species leads to water which is released from the surface around 400 K. Neighboring on-top Cl atoms recombine to form Cl2 which is immediately liberated into the gas phase. The activation energies ΔEact are determined by DFT calculations and given in kJ/ mol.30 The elementary reaction step with the highest activation energy is identified with the association of two Clot to form Cl2. Reprinted with permission from ref 30. Copyright 2008 American Chemical Society.

reaction mechanism of the HCl oxidation over RuO2(110) is summarized in Figure 8, while the energy diagram along the reaction coordinate is depicted in Figure 9. All given energies were determined by DFT calculations.30 Dissociative adsorption of O2 forms readily atomic O in the 6785



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Figure 9. Energy diagram along the reaction coordinate for the HCl oxidation over chlorinated RuO2(110), providing the adsorption energies of the reaction intermediates. ΔE = calculated reaction energy of −66 kJ/mol.21

under reaction conditions. Therefore, oxygen adsorption constitutes the rate-determining step in the HCl oxidation reaction, although the elementary reaction step with the highest activation energy is Clot−Clot association. Even more important for explaining the small oxygen coverage under reaction conditions is that the desorption temperature of on-top Cl is by 300 K higher than that of oxygen (cf. Figure 10). Consequently, the RuO2(110) surface is overpopulated by on-top Cl under reaction conditions of around 600 K, thus blocking 1f-cus Ru sites and as a consequence inhibiting the dissociative adsorption of oxygen. Undercoordinated surface oxygen is, however, mandatory to oxidize HCl by H-elimination so that the overall activity is limited by oxygen adsorption. Maximum activity for the HCl oxidation over polycrystalline RuO2 powder has been found for 620 K, while the reaction starts already around 500 K.31 Pump−probe experiments of O2 and HCl in a TAP (temporal analysis of products) reactor verified a Langmuir−Hinshelwood type reaction rather than a Mars van Krevelen mechanism74 and a strong dependence of the net Cl2 production on the Cl and O coverage on the RuO2 surface.66 Studt et al.72 studied the HCl oxidation reaction for various transition metal oxides to establish an activity relation among various rutile transition metal oxide catalysts (volcano relation). This study disclosed that the activity of RuO2(110) is already very close to the optimum value. This study indicates also that the adsorption energy of on-top O is a good descriptor for the Brønsted, Evans, Polanyi (BEP) relation,75,76 meaning that the adsorption energy is linearly correlated with the adsorption energy of other reaction intermediates on the surface such as on-top Cl, water, and OH. The energy profile along the reaction coordinate as shown in Figure 9 describes only one part of the reaction mechanism. In addition to these surface processes, the various gases Cl2, O2, H2O, and HCl are in adsorption/desorption equilibrium with the catalyst’s surface (cf. Figure 11). These equilibria can result in reaction inhibitions. Water, for instance, can adsorb and desorb, thereby blocking active 1f-cus Ru sites, but also lead to enhanced desorption of adsorbed Cl in the form of HCl by H-transfer from the adsorbed water molecule toward on-top Cl. If the partial pressure of Cl2 is increased, the reaction rate also decreases. The reason for this product poisoning is related to blocking of the 1f-cus Ru sites by dissociative adsorption process of Cl2, thus inhibiting dissociative oxygen adsorption. If most of the bridging O atoms are replaced by chlorine, then

Figure 10. Temperature-programmed reaction experiments of Clot, Oot, and OotH coadsorbed on the chlorinated RuO2(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 dosed at 420 K, and 5 L of O2 was postdosed at 200 K. HCl adsorbs dissociatively on RuO2(110), forming Clot and OotH. A small fraction of the HCl molecules desorbs around 450 K via recombination of OH and Cl, while the rest of the dissociatively adsorbed HCl molecules form water and the desired product Cl2. Excess oxygen leaves the surface around 400 K. The optimum temperature range for HCl oxidation is indicated in transparent blue. Reprinted with permission from ref 30. Copyright 2008 American Chemical Society.

having an adsorption energy of 160−200 kJ/mol. According to the thermal desorption spectra of the O + Cl + OH coadsorbate overlayers on chlorinated RuO2(110) (cf. Figure 10), the optimum reaction temperature is in the range of 550− 650 K. This temperature range is high enough to produce chlorine by surface chlorine recombination and maintaining vacant 1f-cus Ru sites for dissociative O2 adsorption. The temperatures are also low enough to keep the surface concentration of on-top O high enough under reaction conditions. Kinetic studies in a flow reactor by Lopez et al.31 disclosed a pronounced promoting effect of the reaction rate on the oxygen partial pressure which is more pronounced than expected from thermodynamics (cf. Figure 2). The higher the O2/HCl feed ratio (while keeping the partial pressure of HCl constant), the higher the Cl2 production. Since the adsorption energy of ontop O is only 80−100 kJ/mol (against 1/2 O2), oxygen desorption takes place already at temperatures of 400−450 K which is much lower than the reaction temperature of 620 K. This finding may suggest that for thermodynamic reasons the concentration of on-top O on the surface is actually too low 6786



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subsequently the surface was saturated with Oot by exposure of 5 L of O2 at room temperature. Figure 12a shows the

Figure 11. Energy diagram along the reaction coordinate for the HCl oxidation over chlorinated RuO2(110), providing the adsorption energies of the reaction intermediates in kJ/mol. ΔE = calculated reaction energy of −66 kJ/mol.21 In addition, the equilibria between the gas phase and the surface are indicated, and the corresponding activation energies for desorption are indicated in kJ/mol. Copyright 2010 American Chemical Society.

Figure 12. Trapped oxygen on 1f-cus Ru sites of stoichiometric RuO2(110). (a) Temperature-programmed desorption experiments of HCl and O2 coadsorption on stoichiometric RuO2(110): 1.5 L of HCl at 420 K and subsequent saturation of the remaining free 1f-cus Ru sites by 5 L of O2 at room temperature. (b) Schematic representation how the O + Cl + OH coadsorbate phase on RuO2(110) changes when the temperature is increased. Red stripes correspond to 1f-cus Ru rows. The bridging oxygen rows are omitted for clarity reasons. Green balls are O atoms and white H atoms, and gray balls represent chlorine atoms.82

heterolytic dissociation of HCl is strictly coupled to the oxygen adsorption, which in turn renders oxygen adsorption even more important for the reaction mechanism, qualifying oxygen adsorption as the rate-determining step. Only recently,65 the first values for turnover frequencies have been reported for the HCl oxidation reaction over RuO2(110) and RuO2(100) model catalysts. Reactivity experiments in a batch reactor indicate that independent of the used surface orientation 0.6 Cl2 molecules are produced per second and active site at 650 K (i.e., the turnover frequency TOF is 0.6 s−1), when starting with a reaction mixture of p(HCl) = 2 mbar and p(O2) = 0.5 mbar. This result suggests a structure insensitive reaction, which, however, conflicts with recent DFT calculations.77 Similar TOF values (0.002 s−1 at 573 K) can be derived for polycrystalline RuO2 from space time yield and the Brunauer−Emmett−Teller78 (BET) surface area reported66 or for RuO2(110) from theory (10−100 s−1 at 573 K and 1 bar).72 Careful microkinetic modeling evidences even a quantitative agreement with the experimental observed TOF = 0.6 s−1 for the RuO2(110) surface but found a significantly higher value of 1.12 s−1 for RuO2(100).77 Teschner et al.77 suggested that the discrepancy with the experimental TOF value for RuO2(100) may be related to a reconstruction of the (100) surface in (110)-like facets as observed with TiO2(100).79 However, this suggestion is inconsistent with the invariant surface structure of RuO2(100) under reaction conditions as monitored with in situ surface X-ray diffraction (SXRD).65 In this microkinetic modeling,77 which was based on ab initio calculated activation barriers but also on a mean field approach,80,81 the surface has been shown to be covered mostly by on-top Cl. This leads to a situation where oxygen adsorption becomes rate determining on RuO2(110), albeit the association of on-top Cl has the highest activation energy. Due to dimensional confinement on the RuO2(110) surface, a mean field approximation for microkinetic modeling may not be justified. For instance, on the stoichiometric RuO2(110) surface trapped oxygen can be prepared by coadsorption of HCl and O2. The term trapped oxygen describes oxygen atoms which are hindered to recombine to molecular oxygen by an inactive spectator/separator species, in this case Clot atoms. The surface was exposed to 1.5 L of HCl at a temperature of T = 420 K, and

temperature-programmed desorption traces of O2 and Cl2. Both O2 and Cl2 desorb from surface via recombination of two neighboring O and Cl species, respectively. Part of the adsorbed oxygen species in the on-top position leaves the surface via recombination at Tmax = 420 K, and another portion of O2 desorbs at temperatures around 700 K concomitant with a small Cl2 desorption signal. How can we rationalize the two distinct O2 thermal desorption features at 420 and 700 K in the spectra (cf. Figure 12b)? HCl adsorption of the stoichiometric RuO2(110) proceeds, dissociatively forming Clot and ObrH. Subsequent exposure of 5 L of O2 leads to saturation of the remaining 1f-cus Ru sites. Note that the adsorption of oxygen requires two adjacent vacant 1f-cus Ru adsorption sites. Therefore, in between two adsorbed Clot species only an even number of Oot can be located. Upon increasing the temperature during thermal desorption spectroscopy (TDS), hydrogen diffuses toward the Oot species, forming first Oot-H and finally H2Oot which desorbs around 400 K.49 Water desorption may leave an uneven number of Oot atoms located in between two neighboring Clot. Around 420 K40 these remaining Oot atoms can recombine in pairs leaving finally a trapped Oot in between two adjacent Clot along the 1f-cus Ru row. The trapped Oot can only leave the surface by recombination with another oxygen atom since desorption of atomic oxygen from 1f-cus Ru sites is energetically disfavored due to an adsorption energy of 4 eV.35,40 Recombination of Oot can therefore either be accomplished by an Obr species or by another Oot after two neighboring Clot atoms have recombined and left the surface. The trapped oxygen cannot be simulated by microkinetic modeling using the mean field approach but rather requires the application of kinetic Monte Carlo simulations.82 5.2. Reaction Mechanism of Deacon over RuO2−TiO2. Actually, the Sumitomo process uses RuO2 supported on rutileTiO2 and not just bare RuO2. RuO2 supported on rutile-TiO2 is 6787



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As shown in Figure 7, mixed RuO2−TiO2 nanofibers are much more stable than pure RuO2 fibers. The improved stabilization of RuO2 by titanium has been observed on single crystal surfaces of RuO2 as well, namely, on RuO2(110) and RuO2(100).85 It turned out that Ti deposition substantially improved the thermal stability of both orientations of RuO2 in UHV which is fully reconciled with the found stabilization of RuO2−TiO2 nanofibers. To understand this stabilization effect of Ti, we have to recall the atomic-scale reduction process on RuO2(110). Chemical reduction and also the disintegration of RuO2(110) starts off with the removal of bridging O by water formation.86 To stabilize the RuO2(110) surface, it helps to replace the Ru atoms coordinated to Obr by Ti. DFT calculations87 have shown that Obr and Clbr bind to Ti 20 kJ/mol more strongly than to Ru. Lopez et al. studied theoretically the HCl oxidation reaction on epitaxial RuO2 layers on SnO2(110) by using DFT calculations.77 It turned out that a single layer of RuO2(110) binds reactants and intermediates too strongly so that 1 ML RuO2 on SnO2(110) is expected to be much less reactive than pure RuO2(110). Quite in contrast, 2 ML of RuO2(110) supported on SnO2(110) binds the reactants and intermediates much more weakly than pure RuO2(110) so that a promoting effect on the activity of the RuO2 bilayer is induced by the SnO2(110) substrate. The reason for this behavior is attributed to SnO2(110)-induced modifications of the electronic properties of RuO2(110) layers.

significantly more active in the HCl oxidation reaction than RuO2 supported on anatase-TiO25 (cf. Figure 2). The rutileTiO2−RuO2 phase diagram exhibits a miscibility gap between 10% and 90% of TiO2 below 1000 K.83,84 Density functional theory (DFT) calculations were performed to study the oxidation of HCl with oxygen, producing Cl2 and water on the TiO2(110)-supported RuO2(110) film of various thickness21 (cf. Figure 13).

Figure 13. Energy diagram for the HCl oxidation over one chlorinated monolayer of RuO2(110) supported on six monolayers of TiO2(110) and one-half chlorinated monolayer of RuO2(110) supported on 6.5 monolayers of TiO2(110) in comparison with the stoichiometric TiO2(110) surface.21 All energies are given in kJ/mol.

6. SUMMARY AND CONCLUDING REMARKS The original Deacon process (cf. Figure 14) over the CuO catalyst can be decomposed in two separate steps in which the

Very important for industrial application is that substantial Ru resources can be saved in the synthesis of the Sumitomo catalyst. According to these DFT calculations, already 1 ML of RuO2(110) coated on TiO2(110) suffices to maintain practically the full activity of bulk-RuO2 in the HCl oxidation reaction. It turned out that the association of two neighboring Clot is activated by 248 kJ/mol, therefore being only slightly higher than on bulk-RuO2(110). Concomitantly, the adsorption energy of oxygen is increased by 20 kJ/mol, so that desorption of Oot is activated now by 236 kJ/mol. The stoichiometric TiO2(110) is not active at all in the HCl oxidation reaction21,72 since the dissociation of oxygen is endothermic by 219 kJ/mol and therefore prohibitively activated (cf. Figure 13). However, if the undercoordinated 1f-cus-Ti surface atoms are substituted by Ru, then the resulting 1/2 ML RuO2−TiO2(110) catalyst is active with an activation barrier for Clot association that is 58 kJ/mol higher than for bulk RuO2(110).21 Concomitantly, the adsorption energy of oxygen is increased by 88 kJ/mol, so that the desorption of Oot is activated now by 288 kJ/mol. The RuO2−rutile-TiO2 catalyst reveals higher binding energy of oxygen than RuO2. This may mitigate the bottleneck in oxygen adsorption. However, the bottleneck in oxygen adsorption is not only a problem of the strength of 1f-cus Ru−O bonding but is also connected to the site demand for dissociative oxygen adsorption and the strong bonding of 1f-cus Ru−Cl. To populate higher concentration of oxygen on the surface, chlorine must not form densely packed overlayers or (2 × 1) overlayers in which oxygen cannot adsorb. The problem with Clot blocking can only be overcome if the 1f-cus Ru−Cl bonding is reduced so that chlorine desorbs partially, thereby opening up pairs of free 1f-cus Ru sites for dissociative oxygen adsorption.

Figure 14. Decomposition of the catalyzed HCl oxidation reaction over CuO (original Deacon process) in a chlorination and a dechlorination step.

catalyst undergoes a solid state redox cycle. First, HCl reduces CuO to CuCl2, during which water is formed as byproduct. This chlorination process is exothermic. In the second step, CuCl2 is reoxidized by molecular oxygen to recover CuO and releasing the desired Cl2, thereby closing the catalytic cycle. This dechlorination step is endothermic (cf. Figure 14). Since the HCl oxidation reaction is only mildly exothermic by −59 kJ/mol (300 K), the dechlorination step is more endothermic the more exothermic the chlorination step is. This interrelation of chlorination and dechlorination had been utilized to search for promising catalyst materials.88 However, this simple thermodynamic consideration does not help to spot RuO2 as a promising catalyst for the Deacon process since RuO2 does not undergo a solid state transformation. The high reaction temperature of 700 K in the original Deacon process is determined by the endothermicity of the dechlorination step of CuCl2, imposing a high activation barrier for Cl2 release (kinetics). The high reaction temperature in turn results in a low Cl2 yield at equilibrium since at such high temperatures the back reaction, i.e., chlorination of CuO by Cl2 6788



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that relevant reaction intermediates in CER and the Deacon process bind in a similar fashion as oxygen on RuO2(110). The HCl oxidation is much more complex than the CO oxidation reaction (the prototypical reaction in surface chemistry), although both reactions are efficiently catalyzed over RuO2.91 There are similarities but also distinct differences between these catalyzed reactions. In both reactions selectivity does not count, while activity and stability of the catalyst are the decisive issues. In both reactions, the adsorption/desorption equilibria of the reactants play an important role in the overall reaction mechanism. However, the CO oxidation is much more exothermic with −280 kJ/mol (300 K) than the HCl oxidation reaction (−59 kJ/mol). Therefore, the equilibrium conversion of the CO oxidation is practically temperature independent in the typical temperature range of 600−700 K,92 whereas for the HCl oxidation the conversion yield at equilibrium decreases notably at higher temperature (cf. Figure 1). In the CO oxidation reaction over RuO2(110), CO adsorbs molecularly and O2 dissociatively on the catalyst’s surface, whereas in the HCl oxidation reaction both reactants have to adsorb dissociatively. This sets serious constraints (site demands) on the HCl oxidation reaction. For the CO oxidation, O2 desorption is much more activated than the CO desorption, while the product CO2 is only weakly held at the surface. Therefore, O2 adsorption is not rate determining in the CO oxidation reaction as long as the reaction feed does not contain too much CO. Quite in contrast, oxygen adsorption is rate determining in the HCl oxidation since strongly bound chlorine blocks available 1f-cus Ru sites for oxygen dissociation. Water formation in the HCl oxidation is a multistep process with H-diffusion at the surface being critical. Product poisoning is less severe in the CO oxidation (although present in the form of carbonate or bicarbonate formation)93−95 than in the HCl oxidation. Both products in the HCl oxidation cause severe product poisoning.77 Actually unprecedented information on the CO oxidation over RuO2(110) has been gained by kinetic Monte Carlo (kMC) simulation.96−98 It has been demonstrated that kMC simulations can only draw firm conclusions when a tight feedback to available experimental kinetic data is accomplished.97 Recent kMC simulations97,98 have disclosed that the temperature dependence of the reaction rate is a critical benchmark for kMC simulations of the CO oxidation over RuO2(110). Kinetic MC simulations are equally important for gaining molecular insight into the catalyzed HCl oxidation on RuO2(110), especially because both reactant molecules need to dissociate on the catalyst’s surface with specific site demands (homolytic versus heterolytic dissociation) and because of one-dimensional confinement.82 Preliminary kMC simulations73 reveal that the concentration of OH groups serves as a predictor for the catalyst’s activity. Whenever the OH surface concentration is high, the activity is low and vice versa. Experimentally, the OH concentration can readily be monitored with IR spectroscopy even under reaction conditions. Extensive kMC simulations are required to fully account for the one-dimensionality of RuO2(110) but also to include properly the interaction with the gas phase and the mutual interaction of adsorbed intermediates including H-bridge bonds. Altogether more than 60 reaction steps, including diffusion, adsorption/desorption, and surface reactions, have to be considered in a quite complex reaction network. The activation barrier for the elementary reaction can be taken from ab initio calculations and/or from temperature programmed reaction

becomes feasible (thermodynamics). Here kinetics and thermodynamics go hand in hand to control the catalytic performance of CuO/CuCl2 in the original Deacon process. In addition the high reaction temperature degrades the stability of the catalyst due to volatizing CuCl2. The HCl oxidation reaction over RuO2 proceeds via a Langmuir−Hinshelwood type reaction mechanism.30,31,65,66 Both reactants O2 and HCl adsorb dissociatively on the RuO2 surface: while oxygen dissociation needs two neighboring 1f-cus Ru sites (homolytic cleavage), HCl dissociation needs one 1f-cus Ru and one undercoordinated O site (heterolytic cleavage). Since most of the bridging O atoms are replaced by chlorine, dissociative adsorption of HCl is closely coupled to oxygen adsorption, supplying basic centers for H-elimination. As indicated in Figure 9, the oxygen-assisted dissociation of HCl on RuO2(110) can be regarded as the surface chlorination step, while the Clot association may be considered as the surface dechlorination step. Surface oxygen is reduced to water by H coming from dissociative HCl adsorption. Water desorbs around 420 K, and the remaining adsorbed chlorine atoms can recombine to form the desired product Cl2 at temperatures around 600 K. Although the recombination of on-top Cl exhibits the highest activation barrier with 228 kJ/mol, the adsorption of oxygen is rate determining for typical reaction conditions. To mitigate the problem with insufficient oxygen uptake, the Cl−1f-cus Ru bonding has to be weakened. Only then Cl can desorb substantially at reaction temperatures of about 600 K, thereby opening vacant 1f-cus Ru sites for the dissociate oxygen adsorption. The oxygen adsorption in turn is required to accept the hydrogen from dissociative HCl adsorption. An intimate correlation between kinetics and thermodynamics is again obvious. The bulk of RuO2 does not participate in the HCl oxidation reaction (no solid state reaction), quite in contrast to the original Deacon process. The active catalyst of Sumitomo Chemical for the Deacon process is RuO2 which is coated on rutile TiO2.5 According to Figure 2, the rutile structure of the support is mandatory to achieve high activity and also high stability. It is therefore not too surprising that RuO2 supported on rutile SnO2 has shown to be also an efficient and remarkable stable Deacon catalyst.89 For the case of 1 ML of RuO2(110) pseudomorphically grown on rutile TiO2(110), DFT calculations21 have shown that the activity in the Deacon process is virtually equal to that of pure RuO2(110) which is consistent with recent kinetic measurements in a flow reactor.23 It is remarkable that both HCl oxidation in the gas phase and the electrochemical recovery of HCl, respectively, the chloralkali electrolysis, are catalyzed by RuO2 supported on rutileTiO2. In both types of reactions, the distinct property of RuO2 is its chemical stability besides high activity. For electrolysis of HCl and NaCl in aqueous solution, the chlorine evolution reaction (CER) is catalyzed by RuO2. Due to this similarity, one could have anticipated that also the underlying reaction mechanisms are similar. This is actually not the case.33 However, DFT calculations90 have shown that the O−metal bond strength is a good “descriptor”26 for searching for promising catalyst materials for both types of oxidation reactions. For CER and Deacon,90,72 it turned out that RuO2 reveals the optimum O−metal bonding, being not too strong to poison the electrocatalyst and not too weak to make dissociative O adsorption rate limiting (Sabatier principle). The main reason why the metal−O bond is an appropriate descriptor for these catalyzed reactions is 6789



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Schmid, Prof. Dr. Peter Varga, Dr. Rainer Ostermann, Claas Wessel, and Prof. Dr. Bernd Smarsly. I would like to thank Philipp Krause for preparing the cover art. The real heroes of the HCl oxidation project were Dr. Olivier Balmes and Prof. Dr. Roberto Felici who took the challenge of in situ SXRD measurements during the HCl oxidation in the mbar range by designing a special chamber with Al windows (rather than Be windows). This Deacon project was supported by the German Science council (DFG: Ov21/7) and the federal ministry of science and education (BMBF_Deacon: 033R018C). I would like to thank the Leibniz Rechenzentrum in Munich and the Hochleistungsrechenzentrum in Darmstadt for providing parallel supercomputing time for performing DFT calculations. The author appreciated intense and sometime even virulent (but always enjoyable) discussions with partners of the BMBF project: Dr. T. Schmidt, Dr. N. Lopez, Prof. J. Perez-Ramirez, Prof. R. Schomäcker, Dr. D. Teschner, Prof. W.F. Maier, Prof. K. Stöwe, and Dr. J. Kintrup.

experiments of well-defined Cl + O + OH coadsorbate phases on RuO2(110). Current Deacon-related research concentrates on searching for efficient and stable catalyst materials replacing expensive and less abundant ruthenium dioxide.69,99−101 The limited amount of Ru resources102 leads to concerns about Ru shortage in the case of increasing demand in heterogeneous catalysis. One promising alternative to RuO2 is CeO2, which was shown to be stable and active at 700 K.103 Keeping with the RuO2-type of catalysts, it is necessary to find promoters which can selectively destabilize the on-top chlorine in order to facilitate dissociative oxygen adsorption, the rate-determining step.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: ++49641-9934559. URL: http://www.uni-giessen.de/cms/fbz/fb08/ chemie/physchem/over.

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Notes

The authors declare no competing financial interest. The present feature article is based on an invited talk presented at the ECOSS 28 in Wroclaw (Breslau), Poland, August 30, 2011.

REFERENCES

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Biography

Professor Herbert Over studied physics as well as mathematics at the Technical University in Berlin, Germany. In 1991 he graduated from the Fritz Haber Institute and received his PhD from the chemistry department at the Free University in Berlin. Following a postdoctoral appointment at the physics department in Milwaukee (UWM), he joined the FHI in Berlin (Prof. Dr. G. Ertl) in 1993, where he completed his habilitation in physical chemistry (Free University Berlin) in 1996. In 2001, he was appointed C3 professor at the Justus Liebig University in Gießen (Chemistry Department, Physical Chemistry). His current research interests focus on an atomistic understanding of elementary reaction steps on transition metal (oxide) surfaces and how this knowledge can be applied in model systems of heterogeneous and electrocatalysis.

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ACKNOWLEDGMENTS I would like to thank Prof. Dr. Martin Muhler who brought this inspiring reaction system to my attention. All my co-workers are acknowledged who have participated in this exciting project: Dr. Daniela Crihan, Dr. Marcus Knapp, Stefan Rohrlack, Dr. JanPhilipp Hofmann, Franziska Hess, Dr. Attila Farkas, Christian Kanzler, Iman Eskan, Katarzyna Zalewska, Dr. Ari Seitsonen, Prof. Dr. Edvin Lundgren, Dr. Olivier Balmes, Prof. Dr. Michael 6790



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