CeO2 Catalyst in the Selective

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Pretreatment Effect on Pt/CeO2 Catalyst in the Selective Hydrodechlorination of Trichloroethylene Noelia Barrabe´s,*,†,‡ Karin Fo¨ttinger,*,‡ Jordi Llorca,§ Anton Dafinov,† Francesc Medina,† Jacinto Sa´,| Christopher Hardacre,| and Gu¨nther Rupprechter‡ Dep. d’Enginyeria Quı´mica, UniVersitat RoVira i Virgili, Tarragona, Spain, Institute of Materials Chemistry, Vienna UniVersity of Technology, Vienna, Austria, Institut de Te`cniques Energe`tiques, UniVersitat Polite`cnica de Catalunya, Barcelona, Spain, and CenTACat, School of Chemistry and Chemical Engineering, Queen’s UniVersity, Belfast, BT9 5AG, Northern Ireland, United Kingdom ReceiVed: May 27, 2010; ReVised Manuscript ReceiVed: August 27, 2010

Pt-ceria catalysts present different surface chemistries depending on the preparation method and the pretreatment. The catalytic behavior of Pt/CeO2 catalysts in the hydrodechlorination of trichloroethylene (TCE) to ethylene was examined as a function of the pretreatment conditions and the noble metal precursor salts. Using FTIR and X-ray photoelectron spectroscopy, significant differences were observed in the surface properties of Pt/CeO2 prepared from the H2PtCl6 precursor after different pretreatment procedures (i.e., reduction or oxidation-reduction). These surface changes are related to chloride residues from the synthesis. Strong changes were observed in the selectivity of the catalysts to ethylene depending on the pretreatment conditions. The 0.5%Pt/CeO2 catalyst showed a 13% selectivity toward ethylene after reduction, whereas after oxidation, followed by reduction, the selectivity increased up to 85% at the same conversion level. This effect was only observed when a chloride-containing precursor was used in the preparation. In this way, it is demonstrated that the use of a Cl-containing Pt precursor and an air treatment prior to reduction strongly improves the ethylene selectivity of Pt-CeO2 dechlorination catalysts. This can be explained by formation of a CeOCl phase during the synthesis that decomposes upon air tempering, producing oxygen vacancies on the ceria support. We propose that these oxygen vacancies are active for cleaving off Cl from the TCE. Pt then supplies H to clean-off Cl as HCl. Reaction of TCE on Pt produces rather ethane, so Pt may be partly Cl-poisoned for the hydrodechlorination reaction but not for H2 dissociation or CO adsorption. 1. Introduction Trichloroethylene (TCE) is a clear, colorless, nonflammable liquid that has a sweet, fruity odor characteristic of chloroform. TCE has become a widely distributed common environmental contaminant resulting from its extensive use in the degreasing of metal parts fabricated in the automotive and metal industries. The emission of chlorocompounds into the environment is now stringently regulated due to the associated adverse health effects and ecological damage. Therefore, the conversion of byproducts of industrial processes, such as chlorocarbons, into environmentally benign or even useful substances is of great interest. In this respect, particularly, hydrodechlorination (HDCl) of chlorinated organics is an attractive alternative to incineration, from both an economic and an environmental point of view.1-4 Noble metals are the most frequently used catalytic phase for hydrodechlorination due to their high activity for transforming chlorinated organic compounds into fully hydrogenated products.5,6 Despite their high activity, noble metals are sensitive to poisoning by HCl formed during the HDCl reaction and by coke deposition.7-10 * To whom correspondence should be addressed. E-mail: nbarrabe@ mail.zserv.tuwien.ac.at (N.B.), [email protected] (K.F.). Tel: 0034 977558535 (N.B.), +43-1-25077-3815 (K.F.). Fax: 0034 977559667 (N.B.), +43-1-25077-3890 (K.F.). † Universitat Rovira i Virgili. ‡ Vienna University of Technology. § Universitat Polite`cnica de Catalunya. | Queen’s University.

Several studies have demonstrated the ability of bimetallic catalysts, composed of Group VIII and IV metals, to selectively convert chlorinated alkanes into the more valuable alkenes.1,9 Unlike monometallic catalysts, bimetallic catalysts deactivate less with time-on-stream. The modification of supported Pt and Pd by addition of a second metal, such as Cu, dramatically changes the catalytic performance for vicinal chlorocarbon dechlorination by increasing their selectivity toward olefin formation.11 Nevertheless, the activity of monometallic metal catalysts can also be influenced by several factors, including the method of preparation and pretreatment, both affecting metal particle size.12 Furthermore, metal-support interaction could also change the nature of the active sites, especially when noble metals are supported on reducible metal oxides, such as TiO2, CeO2, Nb2O5, and La2O3.13,14 Several studies have reported that CeO2 exhibits different kinds of interaction with the noble metals, depending on, for example, the synthesis method and catalyst pretreatment, with pronounced effects on catalytic performance.15-18 For example, for the selective hydrogenation of the carbonyl bond in R,βunsaturated aldehydes, a promotion/modification of the noble metal is essential to increasing the selectivity toward the unsaturated alcohol. Among other routes, this promotion can be achieved by using a support that strongly interacts with the metal upon a suitably chosen reduction treatment.15-18 Electronic interface effects and/or the formation of alloy phases, inducing strong metal-support interaction (SMSI), have been proposed as being responsible for the improvement in catalytic properties.

10.1021/jp1048748  2010 American Chemical Society Published on Web 09/22/2010

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The SMSI effect has been repeatedly observed for Pt/CeO2 catalysts; however, the exact origin of a specific SMSI state is often a matter of debate. The mechanisms include formation of a Pt-Ce alloy,19-21 electronic effects on platinum particles due to the partial reduction of ceria,22,23 and decoration of platinum particles by patches of partially reduced ceria.2425 Along these lines, Barrabe´s et al. recently demonstrated that Pt-CeO2 catalysts exhibit high activity and selectivity in the selective hydrodechlorination of trichloroethylene to ethylene.11 However, the catalytic behavior strongly depended on the pretreatment procedure applied. According to Shekhtman et al.,17 different pretreatments of Pt/CeO2 at normal pressure or under evacuation modify the catalyst surface composition. Kepinski et al.18 showed that the effect of the treatment applied correlated with the nature of the noble metal salt precursor. It was shown that, upon using PtCl4 as a precursor, a CeOCl phase was formed during the reduction process and that the CeOCl phase decomposed slowly under an oxidizing atmosphere. The aim of this work was thus to correlate the surface changes induced by different pretreatments (RED or OXRED) and different noble metal precursors (Cl-containing or Cl-free) with the corresponding catalytic behavior. To examine the nanostructure and chemical properties of the catalysts, several characterization techniques have been applied: high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), temperature-programmed reduction (TPR), hydrogen chemisorption, Fourier transform infrared spectroscopy with CO as a probe molecule (FTIR-CO), and X-ray photoelectron spectroscopy (XPS). 2. Experimental Section 2.1. Catalyst Synthesis. The cocombustion method, described in detail elsewhere,26 was used for the preparation of Pt/CeO2 catalysts. Corresponding amounts of platinum salt were added to a mixture containing cerium ammonium nitrate, pluronic, and ethano and stirred to obtain a homogeneous mixture. The mixture was then introduced into a muffle furnace preheated to 350 °C, yielding a voluminous cerium oxide powder containing the platinum. Two different platinum precursors were utilized to evaluate the effect of chloride species: hexachloroplatinic acid (H2PtCl6; samples denoted as “Clcontaining”) and platinum acetylacetonate (Pt(acac)4; samples without chloride denoted as “Cl-free”). Furthermore, the catalysts were pretreated by two different routines. In the first, the sample was pretreated in atmospheric air up to 300 °C for 30 min (OX) and then reduced in a hydrogen flow at 300 °C for 30 min (denoted as “preoxidized” throughout the text (OXRED)), and in the second routine, the sample was reduced in flowing hydrogen at 300 °C for 30 min (denoted as “reduced” throughout the text (RED)). 2.2. Sample Characterization. Fourier transform infrared (FTIR) spectra were recorded on a Bruker IFS 28 instrument with a resolution of 4 cm-1. The spectrometer cell was equipped with a heated sample holder and was connected to a vacuum system working in the 10-6 mbar pressure range. The IR cell was used for in situ pretreatment and gas adsorption studies. Samples were pressed into self-supporting wafers that were placed inside a ring-shaped furnace in the IR cell. The catalysts were heated to 300 °C in a hydrogen atmosphere (500 mbar) using a temperature ramp of 10 °C min-1 and kept at 300 °C for 30 min. After reduction, the cell was evacuated for 30 min at the reduction temperature. CO adsorption measurements by FTIR were carried out in 5 mbar (pure) CO at room temperature. Temperature-programmed reduction (TPR) was performed using a TPR apparatus (1100 ThermoFinnigan) that was

Barrabe´s et al. equipped with a thermal conductivity detector (TCD) and coupled to a mass spectrometer (QMS 422 Omnistar). Before TPR, each sample (around 20 mg) was dried under flowing helium (20 cm3 min-1) at 120 °C for 24 h. Thereafter, the reduction process was carried out between room temperature and 800 °C at a heating rate of 20 °C min-1 and under a flow of 5% H2 in Ar (flow rate ) 20 cm3 min-1). High-resolution transmission electron microscopy (HRTEM) was carried out with a field-emission JEOL 2010F instrument. The point-to-point resolution was 0.19 nm, and the resolution between lines was 0.14 nm. Samples were deposited on holey carbon-coated grids from alcohol suspensions. At least 100 particles were analyzed to determine particle size distributions. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was performed in an experimental setup consisting of an in situ high-temperature diffuse reflectance IR cell (SpectraTech) equipped with ZnSe windows. The DRIFTS cell was installed in a Bruker Equinox 55 IR spectrometer, operating at a resolution of 4 cm-1. The cell was connected to the gas feed through low-volume stainless-steel lines. The gas flow was controlled by mass flow controllers (Aera), which were repeatedly calibrated. A four-way valve was used to allow fast switching between two reaction feeds, when required. The reduced and preoxidized catalysts were analyzed by X-ray photoelectron spectroscopy (XPS) in a Kratos AXIS Ultra DLD apparatus, equipped with a monochromated Al KR X-ray source, a charge neutralizer, and a hemispherical electron energy analyzer. During data acquisition, the chamber pressure was kept below 10-8 mbar. All spectra were averaged over 30 sweeps for O 1s and C 1s and 200 sweeps for Pt 4f and Ce 3d, with a step size of 0.1 eV, dwell time of 300 ms, and pass energy of 40 eV for Pt and Ce and 160 eV for the other elements. CasaXPS software was employed to treat the data, and the binding energies were based using the C1s peak (284 eV) as a reference. 2.3. Determination of Catalytic Activity. The catalysts were tested for the hydrodechlorination reaction of trichloroethylene (TCE) using a continuous fixed-bed glass reactor at 300 °C, atmospheric pressure, and stoichiometric amounts of hydrogen. The TCE (92 mbar) gas feed was obtained by flowing helium (36 mL/min) and hydrogen (14 mL/min) through a saturator (at T ) 25 °C) containing liquid trichloroethylene. The gas flows were controlled using mass flow controllers (Brooks Instrument 0154) and introduced into the reactor, which was placed in an oven coupled with a temperature control system (T ) 300 °C). The outlet of the reactor was connected online to a gas chromatograph with a flame ionization detector (HP 5890 series II) via a six-way valve. For all activity measurements, 0.1 g of catalyst was used. 3. Results Figure 1 shows the TCE conversion and the ethylene selectivity obtained on 0.5%Pt/CeO2 (Cl-containing and Cl-free) after reduction and after oxidation followed by reduction. The reaction selectivity in the catalytic TCE hydrodechlorination on the Pt/CeO2 catalysts was found to be strongly dependent on the preparation method and pretreatment conditions, as shown in Figure 1. Interestingly, whereas the conversion of TCE removal remained rather constant, the selectivity toward ethylene was found to increase from 13% (RED) to 85% after the preoxidation step (OXRED) on the catalyst prepared from the chloride precursor. The Cl-containing catalysts after preoxidation showed the highest selectivity of all catalysts.

Pretreatment Effect on Pt/CeO2 Catalyst

Figure 1. Catalytic behavior of 0.5%Pt/CeO2 catalysts depending on synthesis and pretreatment (9, TCE conversion; 0, ethylene selectivity).

No such influence of the pretreatment procedure was observed when the chloride-free Pt precursor was used for catalyst preparation. Thus, the differences in the catalytic behavior can be related to the chloride species in/on the material originating from the synthesis. To clarify the connection between changes

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17677 in catalytic activity and those of surface structure/composition, the catalysts were characterized by several techniques. 3.1. Characterization Results. 3.1.1. Structure: HRTEM. To characterize the structure of the Pt nanoparticles, the Clcontaining catalysts after different treatments (RED and OXRED) were analyzed by HRTEM (Figure 2). A detailed study of reduced samples (Cl-containing, prepared by the combustion method) was presented previously.26 A low-magnification overview image of the Cl-containing sample after reduction shows Pt nanoparticles of uniform size centered around a 3 nm mean diameter, well dispersed on the CeO2 support.26 The uniformity of the Pt nanoparticles in the sample (Cl-containing) deserves attention: more than 90% of all nanoparticles exhibited diameters between 2 and 4 nm. Although the Pt particles were structurally not resolved, they exhibited Moire´ patterns due to interference with the CeO2 lattice. Figure 2A1 shows an atomic resolution image, including the contact area between a Pt nanoparticle and the CeO2 support. The Fourier transform (FT, inset) shows bright spots at 2.26 and 3.13 Å. Spots at 2.26 Å correspond to (111) planes of metallic platinum. The spots at 3.13 Å are ascribed to (111) planes of fcc CeO2. The occurrence of these spots along with weaker spots at 2.7 Å in the FT pattern at 54.7° indicates that the CeO2 support crystallite is oriented along the [110] crystallographic direction. The Pt(111) and CeO2(111) spots are aligned along the same crystallographic direction, indicating an epitaxial relationship. Figure 2A2 shows other Pt nanoparticles. The particle labeled “a” exhibits lattice

Figure 2. HRTEM images of 0.5Pt/CeO2 Cl-containing RED (A1, A2) and 0.5%Pt/CeO2 OXRED (B1, B2).

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Figure 3. XPS of Pt 4f and Ce 3d core levels for 0.5%Pt/CeO2 Cl-containing RED and OXRED.

TABLE 1: Binding Energies (eV) of Core Levels and Surface Atomic Ratios of the 0.5Pt/CeO2 Cl-Containing Catalyst RED

OXFRED

Ce 3d

O1 s

Pt 4f1/2

Cl 2p3/2

O/Ce

Pt/Ce

O/Ce

Cl/Pt

O885/O898

O529/O531

Pt0/Pt2+

882.2 898.2 884.7 888.9 882.2 989.3 885.0 888.9

529.3 531.7

72.9 71.3

198.9

0.22

0.006

0.22

2.42

1.05

1.53

0.22

529.1 531.3

72.9 71.3

198.7

0.24

0.005

0.24

2.28

0.93

1.30

0.20

fringes at 2.3 Å (2.26 Å in the corresponding FT), again characteristic of (111) planes of metallic Pt. In the case of the Cl-containing OXRED catalyst, welldispersed Pt nanoparticles were again observed, but with a slightly larger average particle size (3.9 nm) and a broader size distribution. Figure 2B1 shows lattice images of particles of different diameters, labeled “a” and “b”. Both particles show lattice fringes at 2.3 Å, which are ascribed to (111) planes of metallic Pt. The CeO2 support crystallite below particle “b” shows fringes at 3.1 Å, which correspond to CeO2(111) planes. The image suggests that epitaxy between the particle and the support is preserved upon pretreatment. Figure 2B2 shows a profile view of another Pt particle showing (111) planes at 2.3 Å. The edges of the Pt particles are well-defined (except from some defocus), indicating that no other phases were deposited onto the Pt crystallites (e.g., by migration of CeOx). 3.1.2. Composition: XPS. To obtain information complementary to the structure, such as the oxidation states of cerium and platinum and the exact surface composition, XPS analysis was performed. Photoelectron spectra and spectral fits of the

Pt 4f and Ce 3d core levels of Cl-containing catalysts after reduction (RED) and after preoxidation (OXRED) are shown in Figure 3. Table 1 summarizes the binding energies (BEs) for Pt 4f, Ce 3d, O 1s, and Cl 2p core electrons. Pt 4f7/2 peaks were observed at 71.3 and 72.9 eV, consistent with Pt(0) and Pt(II) oxidation states, respectively. The results suggest that, due to the higher percentage of oxidized platinum, Pt is in close contact or even incorporated in the ceria support. If there was ionic substitution of Pt2+ ions for Ce4+ sites in CeO2, the lattice parameter should decrease and an oxide ion vacancy should be created in order to maintain charge neutrality (due to lower valent ion substitution).27 A more intense Pt signal was observed in the reduced sample as compared to the preoxidized one (Table 1). Although the difference is small, this could be related to the different particle size, smaller particle size leading to a higher Pt signal. However, the change in average particle size due to the treatments was small, and this is unlikely to be the only reason. Another possibility is that the increase in the Pt signal may be associated with migration of Cl from the Pt metal to the support.

Pretreatment Effect on Pt/CeO2 Catalyst

Figure 4. TPR profiles of 0.5%Pt/CeO2 catalysts: (a) Cl-free, (b) Clcontaining OX, (c) Cl-containing.

The Cl 2p3/2 region reveals the presence of surface chlorides that have not been effectively removed after catalyst synthesis. The binding energy is consistent with the formation of CeOCl, as reported by Kepinski et al.18 The higher percentage of Cl in the reduced sample (RED) in comparison with the preoxidized one (OXRED) may be due to the decomposition of CeOCl crystallites to cubic CeO2 under an oxidizing atmosphere, whereas during reduction, the CeOCl phase was stable. The Ce 3d XPS spectra were fitted with eight peaks corresponding to four pairs of spin-orbit doublets. The labeling of the peaks follows the convention. The satellite peak u′′′ associated with the Ce 3d3/2 is characteristic of the presence of tetravalent Ce (Ce4+ ions) in Ce compounds.28 The Ce 3d3/2,5/2 spectra are composed of two multiplets (V and u) corresponding to the spin-orbit split of 3d5/2 and 3d3/2 core holes. Four peaks corresponding to the pairs of spin-orbit doublets can be identified in the Ce 3d spectrum originating from Ce(III) oxides, in agreement with other authors.28-30 It is well known that rare earth oxide surfaces may hydroxylate relatively quickly in the presence of moisture. These surface hydroxyl species decompose to oxides under X-ray irradiation, leading to the formation of Ce2O3 and other substoichiometric oxides between CeO2 and Ce2O3.31 The component of O 1s at 529 eV increases with reduction, in comparison with the air-pretreated sample (Table 1), suggesting a small increase in Ce2O3, as expected. 3.1.3. Reduction BehaWior: TPR. To examine the reductive activation of catalyst precursors in detail, TPR measurements were performed. The temperature-programmed reduction results of various 0.5%Pt/CeO2 catalysts, in the range between 25 and 900 °C, are shown in Figure 4: (profile a) obtained using the chloride precursor (Cl-containing) without air pretreatment (and before any reduction), (profile b) obtained using the chloride precursor (Cl-containing) with a proceeding air treatment (OX), and (profile c) obtained using the chlorine-free catalysts (Cl-free) without air pretreatment. Reduction of the CeO2 support (not shown) is characterized by two broad reduction peaks, the first (smaller one) between 400 and 550 C and the second one above 700 °C.16,32 The reduction of CeO2 occurs via a stepwise mechanism, starting at lower temperature (400-550 °C) with the reduction of the outermost layer of Ce4+ (surface reduction), followed at higher temperature (above 700 °C) by reduction of inner/deeper Ce4+ layers (bulk reduction). Quantitative analysis

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17679 of the hydrogen consumption of the first peak(s) indicates that ∼4% of the surface Ce4+ was reduced to Ce3+.26 TPR profiles in Figure 4 show that the addition of a noble metal, sucha as Pt, to CeO2 facilitates surface reduction of the support, with a shift to lower reduction temperatures. Several peaks between 50 and 400 °C can be assigned to the combined reduction of Pt oxide species, Pt oxide species introduced into the ceria lattice, and CeO2 surface species.27 The reduction process proceeds by the reduction of platinum oxide and spillover of atomic hydrogen from Pt to the oxide surface, resulting in the formation of bridging OH groups, which decreases the reduction temperature of the support.16 The Cl-free catalyst (Figure 4) exhibits a broad peak between 50 and 300 °C consisting of at least three features. The lowtemperature peaks between 100 and 180 °C can be attributed to the reduction of PtOx surface species and the very broad peak around 200 °C to the combined reduction of Pt oxide species, surface ceria, and Pt oxide species interacting with ceria. Chloride species as well as an air treatment before reduction influence strongly the TPR profiles, in agreement with reports that chlorine-Pt interaction strongly changes the nature of the metal-support interaction for Pt/CeO2.19 The Cl-containing catalyst (Figure 4) shows three distinct features, again, a lowtemperature peak at 110 °C due to PtOx reduction and two additional features at 241 and 302 °C representing the reduction of the Pt-CeO2 and CeO2 surface. In addition to the shift in reduction temperature, from 400-500 °C of ceria alone to 200-300 °C, an increase in hydrogen consumption was also observed. This suggests an increase in the number of vacancies due to the noble metal (and chlorine) addition, which has been reported to facilitate bridging OH group formation33 due to hydrogen spillover. The shift of the TPR profile to higher temperature for the chloride-containing catalysts suggests a different interaction between platinum and ceria34 than that found for the chloridefree catalysts. The main peak of the Cl-containing OXRED catalyst was found to shift from 301 to 271 °C, whereas the

Figure 5. FTIR-CO spectra of 0.5%Pt/CeO2 prepared from different precursors and after pretreatments: (a) Cl-free RED, (b) Cl-free OXRED, (c) Cl-containing RED, (d) Cl-containing OXRED.

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TABLE 2: Vibrational Assignment of Adsorbed TCE frequency (cm-1) assignment

gas phase

C-Cl stretch C-Cl stretch C-H bend C-Cl stretch CH-Cl bend CdC stretch CdC stretch C-H stretch

635 781 847 941 1254 1564 1593 3098

CeO2

837 930 1252 1573 3082

peak at 110 °C, previously related to PtOx, disappeared. A shoulder at around 240 °C was formed upon air treatment and can be assigned to more stable PtOx species, which require higher temperatures to be reduced. The shift of the main peak to lower temperature indicates significant surface changes that apparently promote ceria reduction. These differences may be related to the decomposition of the CeOCl phase18 (formed upon combustion synthesis; see below), leading to an increase in the number of oxygen vacancies in the material. Previous studies suggested that the product distribution in HDCl reactions was primarily affected by the oxidation state of platinum on alumina during the steady-state reaction. Therefore, the reducibility of the platinum particles should also affect the catalytic activity and selectivity by influencing not only the desorption of products but also the adsorption of reactants on the platinum sites of various oxidation states.35,36 3.1.4. Surface Adsorption Sites: CO-FTIR. The FTIR spectra obtained after room-temperature CO exposure of 0.5% Pt/CeO2, prepared using chloride-containing (Cl-containing) and chloride-free (Cl-free) precursors, are shown in Figure 5, for both reduced (RED) and preoxidized (OXRED) samples. The intense bands observed between 2060 and 2160 cm-1 are attributed to CO linearly bonded to surface-exposed Pt atoms.37

The Pt-CO IR band at 2070 cm-1 is indicative of the presence of highly dispersed (low-coordinated) Pt metal atoms.22,37-39 The band at 2108 cm-1 is attributed to the presence of (partially) oxidized Pt in Pt/CeO2, that is, Pt bonded to surface lattice oxygen of ceria. This band may also be attributed to CO adsorption on Ce3+ or to the effect of residual Cl- species affecting the Pt. A blue shift of the CO stretching vibration frequency upon interaction with chloride species was also found in previous studies on Pd nanoparticles.40 In the range of carbonates (1800-1000 cm-1), several species were observed, originating from CO reaction with the oxide surface. On the basis of the literature, the bands can be assigned to the presence of a variety of different types of carbonates, including mono-, bi-, and polydentate carbonates as well as hydrogencarbonates. On the Cl-containing Pt/CeO2 (RED) sample, broad bands at around 1500 and 1380 cm-1 were detected, which can most probably be assigned to monodentate species. The spectrum observed on the preoxidized sample (OXRED) was significantly different, showing bands at 1610, 1436, and 1320 cm-1, most likely due to hydrogencarbonates and polydentate and/or bidentate carbonates.41,42 The broad bands around 1470 cm-1 can be indicative of polydentate carbonate species.43 Obviously, a significant modification of the ceria surface occurred depending on the treatment applied, whereas adsorption on the noble metal was hardly affected. A detailed assignment of the bands present in this spectral range is not straightforward and is beyond the scope of this work. However, it can clearly be observed that different species are present at the surface after RED and OXRED treatment. In contrast, the pronounced differences in carbonate bands upon pretreatment were not observed when chloride-free Pt precursors were used for catalyst preparation (Figure 5). For both types of treatment (RED and OXRED), IR detected nearly the same spectra characteristic of mono- and bidentate carbon-

Figure 6. DRIFT spectra of TCE adsorption on 0.5%Pt/CeO2 (Cl-containing and Cl-free) at 300 °C. Insets: (I) TCE adsorption on ceria reduced at 500 °C and (II) spectral range of C-H stretch vibrations.

Pretreatment Effect on Pt/CeO2 Catalyst ates and hydrogencarbonates44 with bands between 1600 and 1500 cm-1, at 1395, 1380, and 1300 cm-1. According to FTIR-CO results, specifically, the occurrence of different carbonate species, chloride synthesis residues lead to different surface chemistries and reactivities on the CeO2 surface due to the presence of different surface sites. 3.1.5. Reactant Adsorption: DRIFTS. To study the interaction of trichloroethylene with the catalysts, the adsorption of TCE was studied by DRIFT spectroscopy at various temperatures. The gas-phase IR spectrum, included for comparison, shows bands centered at 3095 cm-1 (υCH), 1566 cm-1 υ(CdC), 1251 cm-1 δ(CH), 945 cm-1 (υCCl), 848 cm-1 (γCCl), 781 cm-1 (γCH), and 631 cm-1 (υCCl) (ν, stretching; δ, in-plane bending; γ, out-of-plane bending) (see Table 2). The gas-phase peaks in Figure 6, inset I, spectrum a, are in good agreement with those found in the literature.45,46 Figure 6, inset I, also displays spectra corresponding to TCE adsorption on the ceria support (i.e., in the absence of Pt). Most of the bands show a small shift from the gas-phase value, which indicates weak interaction with the oxide surface. This shift, associated with the adsorption process, varied as a function of catalyst composition.47 TCE adsorbs via the chlorine atoms, keeping the CdC bond either parallel or perpendicular to the surface.46 No significant changes in the CdC and C-Cl bonds have thus been observed, which suggests that both are preserved on the ceria surface at this temperature. The adsorption of the TCE is not purely physical because not all bands disappeared upon evacuation. In this way, a further dechlorination reaction was observed, indicated by the band at 1025 cm-1, related to monoCl species.46 Figure 6 shows DRIFT spectra of TCE adsorption at 300 °C on 0.5%Pt/CeO2 catalysts prepared with different precursors. In comparison with the pure ceria support, addition of Pt induced strong changes. Furthermore, different bands were observed depending on the precursor (Cl-containing vs Cl-free) after reduction (RED). Whereas two strong bands at 1422 and 1023 cm-1, related to ν(CC) and δ(CH)46 of monochloro species, were observed for the Cl-containing RED sample, the Cl-free RED samples exhibited strong bands at 1080-1170 and 1276 cm-1 related to dichloro species. This shows that different TCE adsorption configurations occurred, depending on the presence of surface chloride species (residuals from synthesis). 4. Discussion Depending on the activation treatment applied to Pt/CeO2 catalysts, that is, purely reductive (RED) or oxidation-reduction (OXRED), their surface structure/composition and thus catalytic properties change significantly. However, this only holds when chloride-containing precursor salts are employed, whereas catalysts derived from chloride-free precursors (acac) are hardly affected by different treatments. This indicates that the surface changes, responsible for the different catalytic behaviors, are directly related to the presence of surface chloride-containing species. Characterization of the catalysts after air pretreatment (OXRED) revealed a small increase in particle size, differences in surface carbonate-type species, the presence of chloride species from the synthesis, different Pt-ceria interactions with respect to the reducibility of the catalysts, and different adsorption configurations of TCE in the presence of chloride species. From the point of view of mean particle size, smaller platinum particles possessing a larger electron-deficient character could possibly induce a longer residence time of adsorbed chlorinated olefins.26,35 On the other hand, previous work from Kepinski et

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17681 al.18 showed how, upon using PtCl4 as a precursor, a CeOCl phase was formed during the reduction process, whereas this CeOCl phase decomposed slowly under an oxidizing atmosphere. This could be the effect that is observed in the samples after the OXRED pretreatment. XPS analysis confirmed the presence of chloride species in catalysts prepared from chloride precursor salts and different amounts of the chloride species after RED or OXRED. The decomposition of CeOCl would lead to an increase of oxygen vacancies, changing the catalytic behavior of the catalyst. The vacancies are probably responsible for the cleavage of the C-Cl bond of adsorbed trichloroethylene, producing the dechlorinated product. Consequently, the addition of a noble metal, capable of hydrogen dissociative chemisorption, is required to clean-off the chlorinated surface and to produce HCl, thereby regenerating the clean surface. Furthermore, noble metals are very efficient for the hydrodehalogenation of chlorocompounds to produce deep hydrogenation products.34 In this way, TCE adsorbed on Pt is transformed to ethane, whereas TCE adsorbed on vacancies forms ethylene. Because of the OXRED treatment with the CeOCl decomposition, some Cl could also migrate to the Pt surface, poisoning the noble metal and decreasing the hydrogenation capacity during the reaction. This could be an additional effect leading to a decrease in ethane selectivity. 5. Conclusions In this study, we demonstrate that using Cl-containing Pt precursors and applying a preoxidation treatment leads to a Pt-CeO2 dechlorination catalyst that is highly selective to ethylene formation. This can be explained by formation of a CeOCl phase during the synthesis that decomposes upon oxidation treatment. This produces active oxygen vacancies on the ceria support, which are proposed to perform the dechlorination step of TCE to ethylene. Pt then supplies H to cleanoff Cl as HCl. Reaction on Pt produces rather ethane, so Pt may be partly Cl-poisoned for the TCE reaction but not for H2 dissociation. Acknowledgment. The Ministerio de Educacio´n y Ciencia, Ministerio de Medio Ambiente, and Generalitat de Catalunya (Spain), Projects HU2006-0026, 246/2006/3-7.1, and Q9999/ 2007/309-118, respectively, are acknowledged for financial support. The Austrian Exchange Service, Project ES 05/2009 (Acciones Integradas), is also acknowledged for financial support. J.L. and F.M. are grateful to the ICREA Academia program and to the project CTQ2009-12520 (MICINN). References and Notes (1) Lambert, S.; Ferauche, F.; Brasseur, A.; Pirard, J. P.; Heinrichs, B. Catal. Today 2005, 100, 283. (2) Rupprechter, G.; Somorjai, G. A. Catal. Lett. 1997, 48, 17. (3) Manzer, L. E.; Nappa, M. J. Appl. Catal., A 2001, 221, 267. (4) Karpinski, Z.; Early, K.; dItri, J. L. J. Catal. 1996, 164, 378. (5) Ordonez, S.; Diez, F. V.; Sastre, H. Ind. Eng. Chem. Res. 2002, 41, 505. (6) Schreier, C. G.; Reinhard, M. Chemosphere 1995, 31, 3475. (7) Yuan, G.; Keane, M. A. Catal. Today 2003, 88, 27. (8) Ordonez, S.; Sastre, H.; Diez, F. V. Appl. Catal., B 2001, 29, 263. (9) Kovalchuk, V. I.; d’Itri, J. L. Appl. Catal., A 2004, 271, 13. (10) Ribeiro, F. H.; Gerken, C. A.; Rupprechter, G.; Somorjai, G. A.; Kellner, C. S.; Coulston, G. W.; Manzer, L. E.; Abrams, L. J. Catal. 1998, 176, 352. (11) Barrabes, N.; Cornado, D.; Foettinger, K.; Dafinov, A.; Llorca, J.; Medina, F.; Rupprechter, G. J. Catal. 2009, 263, 239. (12) Gopinath, R.; Lingaiah, N.; Seshu Babu, N.; Suryanarayana, I.; Sai Prasad, P. S.; Obuchi, A. J. Mol. Catal A: Chem. 2004, 223, 289. (13) Srinivas, S. T.; Prasad, P. S. S.; Madhavendra, S. S.; Kanta Rao, P. Stud. Surf. Sci. Catal. 1998, 113, 835.

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