CeO2 Catalysts during CO Oxidation - The

C , 2007, 111 (39), pp 14469–14475. DOI: 10.1021/jp073541k. Publication Date (Web): September 1, 2007. Copyright © 2007 .... The Journal of Physica...
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J. Phys. Chem. C 2007, 111, 14469-14475

14469

Surface Dynamics of Au/CeO2 Catalysts during CO Oxidation Francisca Romero-Sarria,* Leidy M. Martı´nez T., Miguel A. Centeno, and Jose´ A. Odriozola Departamento de Quı´mica Inorga´ nica e Instituto de Ciencia de Materiales de SeVilla, Centro Mixto UniVersidad de SeVilla-CSIC, AVenida Ame´ rico Vespucio 49, 41092 SeVilla, Spain ReceiVed: May 9, 2007; In Final Form: July 16, 2007

The catalytic activity of gold-based catalysts for CO oxidation is influenced by gold particle size, dispersion, and redox properties of the support. The nature of the active site (gold oxidation state) and its modification along the course of the reaction are under discussion. In this work, we studied the modifications of a Au/ CeO2 catalyst in the presence of gas-phase CO by simultaneous in situ diffuse reflectance infrared Fourier transform mass spectrometry (DRIFT-MS) in isothermal conditions. Redox processes involving surface hydroxyl groups, gold atoms, and gas-phase CO molecules play a determinant role in surface gold dynamics. The interaction of CO with the catalyst surface results in the evolution of CO2 and H2 through both the decomposition of the formate species and the formation of the [Au(CO)2]+ species, which accounts for the redispersion of gold atoms. The identification of the involved surface species by DRIFT lets us state that a deep reduction of the surface (oxygen vacancy creation) changes the gold dispersion and migration of oxygen atoms to the surface, generating an oxidized gold species. This result is also confirmed by XRD, where a decrease in the intensity of all metallic gold diffraction peaks and the relative intensity among them is evidenced in the used catalyst. Therefore, this work provides evidence for the surface dynamics of gold in this Au/CeO2 catalyst and hence provides clues for understanding the modification of the catalytic activity of gold catalysts under cyclic operations.

1. Introduction Catalytic oxidation reactions have been extensively studied on gold-based catalysts. Among them are the following: epoxidation of propene,1 direct oxidation of hydrogen to hydrogen peroxide,2 oxidation of alcohols,3 water gas shift reaction (WGS),4-7 preferential (PrOx) and total (TOx) oxidation of CO,8-17 and total oxidation of volatile organic compounds (VOCs).18,19 CO abatement reactions (WGS, PrOx, and TOx) show a growing interest since CO is an atmospheric pollutant and a poison for fuel cells (FC). Voltage losses in FC of 3% have been reported for H2 streams containing 50 ppm CO; this figure increases to 85% for 70 ppm CO levels in the stream.20 The economical importance of these reactions coupled to the high activity shown by supported gold catalysts at low temperatures have driven an increasingly large number of studies in this field. Two main conclusions are extracted from the whole set of works devoted to the catalytic properties of supported gold catalysts: the importance of the gold particle size and the key role of the support. The particle size determines that the catalyst activity is higher when the particle size is smaller. Typically, gold particle diameters between 2 and 3 nm result in the highest activities.21 Surface properties of the support influence the final gold dispersion and the metal-support interaction. Metallic oxides used as supports can be classified as inert or active according to their redox properties. Thus, Al2O3, SiO2, and MgO fit within the inert supports, while reducible transition metal oxides such as Fe2O3, MnO2, or CeO2 have to be considered as active supports. The catalytic activity of this latter group is * Corresponding author. Tel.: +34 954489543; fax: +34 954460665; e-mail: [email protected].

mainly based on the ability of the support to provide reactive oxygen to the catalytic system.22 However, the nature of the active site is a matter of discussion particularly regarding the gold oxidation state. A second disagreement point is concerned with the observed changes in the activity after successive reaction cycles that have been related to the presence of chlorine, gold sintering, or even gold redispersion. Pillai and Deevi12 assigned the high activity of Au/CeO2 catalysts for CO oxidation to Au+-OH- and highly dispersed Au0 species strongly interacting with defects in the ceria surface. Arena et al.13 concluded that the active species are Au0/Au+ sites in the metal-support interface, and therefore, a catalyst able to generate these species under reaction conditions is the best choice for CO oxidation. According to Minico` et al.,8 who studied a Au/Fe2O3 catalyst, Au+ is the most active species, but it is not stable and tends to form Au0 irreversibly in the studied conditions. On the other hand, Corma et al.23 detected higher activities when the Au3+/Au0 ratio increased. In a previous paper, we have stated the influence of the electronic structure of Au/CeO2/Al2O3 catalysts in the CO oxidation in such a way that overlaps of the gold band with the conduction band of the support result in higher activities of the studied catalysts.16 Changes in the catalyst activity following successive reaction cycles have been often reported. Arena et al.14 associated these changes with chlorine elimination once the first reaction run was finished. The modifications thus depend upon the catalyst preparation method. A second explanation dealing with the hydroxylation degree of the support surface is provided by Carriazo et al.15 Gold catalysts supported on Fe, Ce, and Al pillared bentonites show significant differences in the profiles of three consecutive CO light-off curves. Both the activity of

10.1021/jp073541k CCC: $37.00 © 2007 American Chemical Society Published on Web 09/01/2007

14470 J. Phys. Chem. C, Vol. 111, No. 39, 2007 the catalysts and the amount and temperature of water evolution are modified for each light-off cycle. However, these authors did not discard the effect of deactivation of the catalysts due to, for instance, coke deposition or agglomeration of the gold particles. This later phenomenon is observed by Akita et al.24 in Au/CeO2 catalysts during TEM studies. In their study, they observed the encapsulation of gold nanoparticles by ceria in the reductive conditions of TEM analysis. However, they argued that electron beam irradiation under high vacuum may induce structural changes in gold particles and CeO2 grains. The change in the catalytic activity observed in consecutive CO oxidation cycles over the same catalyst might be assigned to any one of the arguments described previously: deactivation by coke, modifications of the gold particle size and/or oxidation states, presence of chlorine atoms, etc. Therefore, the catalyst preparation procedure, the catalyst pretreatment, and the reaction conditions (temperature, flow composition, and time on stream) should modify the catalyst activity. CO adsorption on gold sites has been proposed as the first step for the mechanism of CO oxidation.25 The bond order of CO adsorbed on the catalyst surface is a function of the oxidation state and the nature of the surface site, and hence, the IR frequencies of the CO stretching modes, ν(CO), are relevant parameters to ascertain the gold species involved in the catalytic oxidation process. Besides this, mass spectrometry (MS) is an ideal technique to identify the gaseous compounds present in the reaction stream upon reaction. The combination of both techniques may allow us to identify mechanistic aspects of the reaction. In this paper, we report a simultaneous in situ diffuse reflectance infrared Fourier transform mass spectrometry (DRIFTMS) study of the CO adsorption on Au/CeO2 catalysts working in isothermal conditions. The temporal evolution of the gas phase and the infrared spectra of the adsorbed phase let us propose a model for the surface dynamics of gold particles and ceria surfaces and hence to extrapolate some conclusions for the observed modification of the catalytic activity of successive reaction cycles. 2. Experimental Procedures 2.1. Catalyst Synthesis. The catalyst was prepared using a non-conventional synthesis procedure for supported gold catalysts. A commercial colloidal solution of cerium acetate (Nyacol) was used as the ceria precursor. An adequate amount of gold acetate (Alfa Aesar 99.99%) to obtain a gold concentration of about 1% weight in the final solid was added to the colloidal solution. After mixing, the resulting suspension was oven dried at 80 °C. The obtained solid was calcined at 300 °C for 4 h. This preparation method allows preparing Au/CeO2 catalysts similar to those deposited on stainless steel metallic monoliths26,27 and potentially on microreactors. The use of chloroauric acid as a source of gold is avoided since chloride ions attack the stainless steel surface, producing pitting corrosion. 2.2. Catalyst Characterization. X-ray diffraction (XRD) spectra were recorded using an X’Pert Pro Philips diffractometer working with Cu- KR radiation (λ ) 15404 Å) in continuous scan mode from 20 to 80° of 2θ using a 0.05° sampling interval and a 1.0 s time interval. The chemical composition of the catalyst was determined by X-ray fluorescence (XRF) in a Siemens SRS 3000 sequential spectrophotometer equipped with a rhodium tube. XRF measurements were performed onto pressed pellets (sample included 10 wt % wax).

Romero-Sarria et al.

Figure 1. XRD for fresh (a) and used (after CO adsorption) (b) Au/ CeO2 catalyst.

The BET area was measured by nitrogen adsorption at a liquid nitrogen temperature in a Micromeritics ASAP 2000 apparatus. Before analysis, the sample was degassed for 2 h at 150 °C in vacuum. Transmission electron microscopy (TEM) observations were carried out in a Philips CM200 microscope operating at 200 kV. The samples were dispersed in ethanol by sonication and dropped on a copper grid with a carbon film. 2.3. DRIFT-MS Experiments. DRIFT images were collected for samples placed in a controlled DRIFT chamber (SpectraTech 101) with SeZn windows that was coupled to a Thermo Nicolet Nexus infrared spectrometer using KBr optics and a MCT/B detector working at liquid nitrogen temperature. Prior to adsorption, the sample was pretreated in 30 mL/min flow of 21% O2 in He for 1 h at 300 °C. At this moment, the oxygen flow was stopped, and a flow of 30 mL/min helium was passed through the catalyst for 1 h at this temperature, with the aim of eliminating the remaining oxygen. After activation, a 30 mL/ min flow of 4% CO in helium was established and stabilized for 15 min before passing through the catalyst. Then, the adsorption was followed by IR spectroscopy and mass spectrometry. The mass spectrometer (Balzers Thermostar) provides gas evolution with time, and simultaneously, one DRIFT spectrum (64 scans, 4 cm-1) was recorded every 2 min, following the surface changes produced in the catalyst by CO adsorption. All the feed mixtures were prepared using mass flow controllers (Bronkhorst). The reaction temperature was chosen with the aim of maximizing CO conversion. Studies of CO oxidation using this catalyst in a classical reactor have shown a total conversion at ca. 200 °C. Since both the mass of catalyst and the hydrodynamic conditions in a DRIFT cell are different than those used in a classical reactor, the conversion values in DRIFT experiments are modified with respect to the classical reactor. Increasing the temperature to 300 °C improves the conversion values, and the obtained results in these conditions can be applied at least from a mechanistic point of view. Results and Discussion The gold content of the catalyst measured by XRF is 0.84%, and the specific surface area is 120 m2/g. The XRD patterns of the fresh catalyst, as well as the pattern for the catalyst after the high temperature adsorption experiment, are shown in Figure 1. In both patterns, diffraction peaks corresponding to the cubic Ia3d anti-fluorite structure of cerium

Surface Dynamics of Au/CeO2 Catalysts during CO Oxidation

Figure 2. Evolution of gas-phase composition during CO adsorption at 300 °C (MS results).

oxide at 2θ ) 28.5, 33.0, 47.4, 56.2, 69.3, 76.5, and 79.0° are observed. Besides this, diffraction peaks at 2θ ) 38.1, 44.3, 64.5, and 77.4° appear, being assigned to the (111), (200), (220), and (311) planes of metallic gold, respectively. An increase in the intensity of all these peaks is evidenced in the used catalyst, indicating that the amount of exposed metallic gold increases as CO reduces the catalyst surface. On the other hand, the intensity ratio of the diffraction peaks corresponding to the Au (111) and Au (200) planes increases upon reduction by CO. The intensity ratio I(111)/I(200) increases from 2.11 for the fresh catalysts to 2.70 for the used sample. According to Wang,28 the preferential plane exposed by a particle depends on both the size and the shape of the particle. Therefore, this observation confirms that a change in the size and/or the shape of the gold particles is produced during the adsorption, pointing out the surface mobility of gold particles under reaction conditions. As mentioned previously, the catalyst reactivity strongly depends on the gold particle size; therefore, surface dynamics of gold must influence the catalyst behavior. The gas-phase composition of the reaction stream during the isothermal adsorption experiment was monitored using mass spectrometry. Simultaneously, DRIFT spectra were recorded. The gas-phase evolutions of CO, CO2, and H2 are shown in Figure 2, using the signals at m/z ) 28, 44, and 2 as representative of their concentrations. Changes were absent in the intensity of the m/z )18 signal, indicating that water does not affect the observed phenomena. The initially stable CO mass signal drops suddenly when the reactant flow begins to pass through the catalyst (time 0), recovering the initial value after ca. 10 min. Once the CO concentration begins to decrease, CO2 evolution begins. Once the CO signal reaches its initial level, the CO2 signal drops to zero (Figure 2). The evolution of H2 shows a maximum at ca. 15 min and stops 20 min after CO adsorption. This indicates that both products, CO2 and H2, are produced either sequentially or through independent processes. The initial decrease of the CO signal is due to both the filling of the cell and CO adsorption on the catalyst surface. The adsorption of CO is confirmed by the observed CO2 production, which is easily explained through the reaction between adsorbed CO on gold particles and oxygen atoms of the ceria support. The hydrogen production is not so evident, and a complementary study is needed to explain the formation of this compound. In Figure 3, DRIFT spectra of the temporal evolution of surface species produced upon CO adsorption are shown. The

J. Phys. Chem. C, Vol. 111, No. 39, 2007 14471 spectra shown in Figure 3 are difference spectra, resulting from the subtraction of the DRIFT spectrum collected at 300 °C in the presence of gas-phase CO (2 min after CO admission into the DRIFT cell) from those recorded during the temporal evolution of the CO adsorption. It should be noticed that after 2 min in the reaction stream, 4.5 × 1019 CO molecules/min pass through the catalyst bed in the DRIFT cell. This bed consists of 20 mg of the Au/CeO2 catalyst containing 5 × 1017 gold atoms. These 2 orders of magnitude between the number of gas-phase CO molecules per unit time and gold atoms in our catalyst let us confirm that the composition of the gas-phase remains constant during our experiment, and therefore, the subtraction of the DRIFT spectrum performed ensures that gasphase CO has been successfully removed from the spectra shown in Figure 3. Therefore, the observed bands correspond only to species involved in the CO adsorption, with the positive bands assigned to species that appear and the negative bands to those that disappear. In the 1800-800 cm-1 region, bands ascribed to carbonate species are observed. It has been reported that CO adsorption on metal-supported ceria results in carbonate formation that is proposed as an intermediate in the oxidation of CO.29,30 According to literature data, free carbonate species are characterized by IR absorption bands at 1415 cm-1 [νasym(CO)], 879 cm-1 [π(CO3)], and 680 cm-1 [δ(CO3)], νsym(CO) being only active in Raman spectroscopy. When carbonates are adsorbed, the former band splits due to the lowering of the symmetry. Meunier et al.,31 working with a Pt/CeO2 catalyst, assigned the bands at 1588 and 1289 cm-1 to bidentate carbonates and those at 1473, 1373, 1352, 1073, and 860 cm-1 to polydentate ones. In our case, the formation of bidentate and polydentate carbonate seems to be evident since broad bands at about 1574, 1405, 1289, 1048, and 858 cm-1 are detected. The stretching mode of the gaseous CO molecule appears at 2143 cm-1. Upon surface adsorption, the IR frequency of the stretching mode shifts to higher or lower wavenumbers depending on the acceptor or donor character of the surface sites. The CO bonding model implies that the σ* orbital of the adsorbed molecule donates charge density to Lewis acid sites on the surface, while, at the same time, basic surface Lewis sites might back-bond the CO molecule through the acidic π* orbitals. Depending on the donor/acceptor balance, the CO stretching mode may shift to higher or lower wavenumbers in such a way that higher wavenumbers will be observed if the net charge density is accepted by the surface while it should shift to lower wavenumbers (smaller C-O bond orders) if the catalyst exposes electron-rich surfaces. Surface sites having a net acceptor character besides sites showing a net donor character are encountered in our catalyst as shown by the shape of the CO stretching modes in Figure 3. Once CO enters the reaction DRIFT cell, the total intensity of the bands above 2143 cm-1 is higher than the total intensity of bands below this wavenumber. In our experiences, we detected bands at wavenumbers both lower and higher than 2143 cm-1, meaning that donor and acceptor species are present on the catalyst surface. At the beginning of CO adsorption, the total intensity of bands at frequencies higher than 2143 cm-1 is higher than the intensity of bands at lower wavenumbers. After 107 min reaction time, this situation is reversed: the total intensity of bands below 2143 cm-1 becomes dominant. In the simplest possible explanation, it can be said that oxidized sites (those able to accept electrons) are transformed in sites actuating as electron donors (reduced sites). At the reaction temperature, carbon monoxide reduces the catalyst surface interacting with

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Romero-Sarria et al.

Figure 3. In situ DRIFT spectra collected during CO adsorption at 300 °C.

the available oxygen producing CO2. The presence of a band at 2350 cm-1 corresponding to gas-phase CO2 is observed in the DRIFT spectra of the catalyst for the first 10 min of reaction time, showing a temporal evolution parallel to that observed for CO2 in the gas phase (Figure 2). This indicates that a fraction of the CO2 produced on the catalyst surface desorbs and that the rest remains adsorbed in the form of carbonate species as detected by DRIFT. Surface reduction is also evidenced by modification of the absorption bands in the OH stretching region. Hydroxyl species responsible for the bands at 3705, 3630, and 3650 cm-1, ascribed to hydroxyl groups on ceria (OH types (I), (II), and (III), respectively), decrease their intensity (negative bands), while a new band at 3666 cm-1 assigned to OH groups of reduced ceria32 appears upon CO adsorption. The formation of formate species on reduced CeO2 samples after interaction with gaseous CO molecules has been previously reported.33 Formate groups adsorbed on ceria are characterized by IR bands at 1563 and 1551 cm-1 [νasym(OCO)], 1373 cm-1 [δ(CH)], 1357 cm-1 [νsym(OCO)], and 2840 cm-1 [ν(CH)].34 The presence of adsorbed carbonate species may mask the IR absorption bands of formate species since absorption bands of both species overlap in the same frequency range. In our case, the [νasym(OCO)], [δ(CH)], and [νsym(OCO)] bands of formates are not clearly distinguished; however, the stretching C-H band at 2850 cm-1 is indicative of the temporal evolution of adsorbed formate groups. At the beginning of the adsorption process, the C-H stretching band is absent. Once the reaction proceeds, this band begins to appear, being clearly observed after 4 min on the gaseous stream. This fact clearly states that formate species cannot be produced on a fully oxidized CeO2 surface. Therefore, it can be concluded that CO interacts with hydroxyl groups of partially reduced ceria (band at 3666 cm-1) generating formate species. This reaction must also be considered in the reduction mechanism of the catalyst since it is well-known that the formate species may decompose on a Ce4+ site to form hydrogen and carbonate species, generating a Ce3+ reduced site.34 The carbonate species formed at the beginning of the adsorption show a π(CO3) band at 858 cm-1 that is highly symmetrical. After 8 min in the reaction mixture, the symmetry of this band decreases, a shoulder at 840 cm-1 is observed (Figure 3), and simultaneously, H2 production is observed by mass spectrometry

(Figure 2). The shoulder at 840 cm-1 confirms the formation of a new carbonate species (with different symmetry) on the catalyst surface. As previously reported, hydrogen is produced by decomposition of formate species according to the reaction34 24+ 23+ HCOOads + Osurf + Cesurf f CO3ads + 1/2H2 + Cesurf

(1)

The previous reaction requires the existence of Ce4+ sites near the adsorbed formate species. Then, on increasing the reduction degree of the surface, the amount of adsorbed formate species increases since they cannot decompose on adjacent oxidized cerium sites. Therefore, hydrogen production stops, and formate groups remain on the surface. A different redox mechanism involving surface hydroxyl groups, CO, and supported noble metals was proposed for Rh/ Al2O3 catalysts.35 Upon CO adsorption, the noble metal was oxidized by surface hydroxyl groups in such a way that hydrogen evolved and noble metal dicarbonyl species remained adsorbed. In our case, this mechanism cannot be fully discarded since a band corresponding to the dicarbonyl species has been detected (2202 cm-1). However, the fairly low intensity of this band suggests a relatively low participation of this route in our catalytic system. The interaction of CO with the support surface might account for the production of CO2, through interaction with surface oxygen atoms, and H2, via formate species. However, from a catalytic point of view, the role of gold in this type of catalyst must be evidenced. Evidence on the dispersion, oxidation, and coordination state of gold can be inferred from the detailed study of the gold-CO interaction bands generated on the catalyst surface after CO adsorption.36,37 As stated previously, CO stretching bands at frequencies higher and lower than that of the gas-phase carbon monoxide are observed, depending on their relative intensities and the reaction time. This clearly indicates that the relative proportion of electron donor and acceptor sites on the surface change with time, and hence, the oxidation state of the surface sites is modified with the time on stream. A careful analysis of this region will lead to the understanding of the involved surface species as well as to a deeper insight in the dynamics of the surface during reaction.

Surface Dynamics of Au/CeO2 Catalysts during CO Oxidation

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Figure 4. Deconvoluted DRIFT spectra of carbonyl region at 4, 20, and 108 min of CO adsorption at 300 °C.

The wide band in the 2250-2000 cm-1 region of the DRIFT spectra (CO stretching modes) was deconvoluted on the basis of second derivative calculations using commercial software. Representative data of this procedure are shown in Figure 4. In the figure, the results of the deconvolution of this band after 4, 20, and 108 min of CO adsorption spectra are shown. These spectra have been chosen to illustrate three different situations: the beginning of adsorption (where oxidized species are predominant), the moment in which surface oxygen has been consumed by CO oxidation, and the end of the reaction, where mainly reduced species are on the surface. In the spectral region corresponding to reduced surface species (wavenumbers lower than 2143 cm-1), bands located at 2060, 2084, 2110, and 2125 cm-1 are distinguished. According to literature data,39,40 the 2060 cm-1 band is ascribed to CO adsorbed on negatively charged gold clusters (Auδ--CO) and the band at 2084 cm-1 to CO adsorbed on metallic gold sites (Au0-CO).21,39 This latter band is metal dispersion sensitive and shifts to higher frequencies on increasing the gold dispersion.37 In our case, a shift from 2084 to 2091 cm-1 is observed after 20 min of the admission of CO in the reaction chamber (Figure 3). This points to a strong reduction of the surface that begins after all available surface oxygen is consumed (between 10 and 20 min), resulting in an increased gold dispersion. Thus, a modification of gold dispersion as a function of time on the CO flow is proposed. This may justify some experimental observations for this catalytic system such as the dependence of the catalyst activity with the pretreatment and also the differences in activity observed after two or more consecutive reaction cycles. The band at 2125 cm-1 is ascribed to the forbidden 2F5/2 f 2F 3+ ions.26 On reducing CeO with 7/2 electronic transition of Ce 2 hydrogen at high temperatures, the band corresponding to this electronic transition is observed at 2127 cm-1, while in catalysts containing gold species, the observed frequencies are lower.32,39 The band at 2110 cm-1 has been assigned to CO adsorbed on oxidized Au sites of Au/TiO2 catalysts.41 However, in our experiment, both bands (2125 and 2110 cm-1) are observed when the catalyst is treated in hydrogen flow (results not shown), and hence, the ascription to oxidized gold sites is meaningless in this catalyst. Although strictly forbidden, the 2F5/2 f 2F7/2 transition in Ce3+ ions may be allowed through crystal field effects, and the energy and multiplicity of the band is related

to both the covalence of the cerium bond and the site symmetry.42 When cerium ions are placed in C1h symmetry sites of a YAlO3 matrix, this transition appears at 2127 cm-1, as in our experiment, indicating the existence of reduced cerium ions in the catalyst surface (low symmetry site [CeO6] distorted polyhedra).43 As the transition energy between the initial and the final state responsible for the 2F5/2 f 2F7/2 transition depends on the interelectronic repulsion in cerium, the existence of atoms in the coordination sphere of cerium with an increased covalent character (increased L f Ce3+ orbital overlap) must result in a decrease in the transition energy as a result of the decrease in the Racah B parameter. Wang and Hwang,44 in a DFT study, found that the most favorable site for gold on reduced TiO2 (110) rutile surfaces is located on top of the titanium ions bound covalently to them and ionically to the titanium coordinated oxygens. Thus, the existence of cerium surface sites adjacent to gold atoms must be responsible for the band at 2110 cm-1. The reduction of the catalyst surface in the presence of CO results in the reduction of the ceria surface, the redispersion of the metallic gold atoms, and the covalent bonding of gold and cerium atoms that favors the migration of gold atoms on the ceria surface. In the spectral region corresponding to metallic species in high oxidation states (frequencies higher than 2143 cm-1), bands at 2173, 2180, and 2202 cm-1 are detected (Figure 4). Adsorbed CO molecules on Au+ sites are responsible for the band at 2160 cm-1.36,45 Bands at 2173 and 2180 cm-1 are ascribed to CO adsorbed on Ce4+ and Au+ species close to oxygen atoms, respectively.37 Bands in the 2200-2215 cm-1 region have been ascribed to Au3+(CO)2 species formed in a zeolite Y during CO adsorption at low temperatures. In our case, the high pressure of CO in the IR cell may favor the formation of these species at higher temperatures. Moreover, in the literature, one can find the description of rhodium dicarbonyls, which are characterized by IR bands in roughly the same region. The formation of these species is related to the metal oxidation process, and they are detected at room temperature.47 Therefore, the band at 2202 cm-1 is ascribed to dicarbonyl species on oxidized gold. The DRIFT spectra of the adsorbed species along the reaction time show the presence of different gold species and ceria sites. Figure 5 presents the temporal evolution of the CO adsorbed species and Ce3+ sites in the reaction flow.

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Figure 5. Evolution of area of bands in 2250-2000 cm-1 region during CO adsorption at 300 °C.

The two bands ascribed to electronic transitions in Ce3+ ions show different behaviors as a function of time (Figure 5a). The band at 2125 cm-1, characteristic of reduced sites in ceria, increases continuously with time, indicating that the presence of gaseous CO reduces the ceria surface, resulting in the formation of formate and carbonate species that remain adsorbed on the CeO2 surface (Figure 3). On the other hand, the band at 2110 cm-1 (Ce3+ closed to gold sites) increases during the first 10 min and then decreases and becomes roughly stable. In the presence of CO, in the tested reaction conditions, the ceria surface begins to be reduced and to interact with gold atoms. The band at 2110 cm-1, characteristic of this interaction, increases its intensity during the first 10 min, showing a maximum at this time; it has to be considered that this is the period in which CO2 is produced (Figure 2). This band reduces its intensity while hydrogen is being produced, and once the hydrogen evolution ceases, its intensity begins again to increase continuously. Two mechanisms can be envisaged for the production of hydrogen observed in Figure 2. In the first one, hydroxyl groups are reduced by adsorbed formate species that oxidize to adsorbed carbonate (eq 1). This mechanism cannot account for modifications in the intensity of the electronic transition band that describes the Ce3+-Au0 interaction responsible for the band at 2110 cm-1. In the second mechanism, hydroxyl groups are reduced by metallic gold atoms resulting in the formation of adsorbed [Au(CO)2]+ species (eq 2). This second mechanism necessarily implies that Ce3+-Au0 interactions are broken, reducing the intensity of the 2110 cm-1 band while hydrogen is being produced. Once all of the surface hydroxyls susceptible of being reacted are eliminated, this mechanism cannot continue, and further interaction with gaseous CO molecules reduces gold atoms, restoring the Ce3+-Au0 interactions; hence, the intensity of the 2110 cm-1 band increases as observed in Figure 5a. It is worth noting the shift of the band at 2084 cm-1 described previously, which indicates that the increase in the gold dispersion clearly supports the modification of the structure of gold particles.

Au0-CeIII-OH + 2CO f [CeIII-O]-[Au(CO)2]+ + 1/2H2 (2) Figure 5b shows the evolution of the area of the carbonyl bands on reduced gold species Auδ--CO and Au0-CO, 2060 and 2084 cm-1, respectively. Both bands show the expected

Romero-Sarria et al. evolution: their areas continuously increase with time until reaching a constant level at ca. 20 min, when the surface seems to be stable, as deduced from mass spectrometry results (Figure 2). CO recovers its initial concentration, and changes in the concentration of CO2 or H2 in the gas phase are undetectable. However, the temporal profile for both bands is not the same. The band at 2060 cm-1 shows a very low intensity increase for the first 10 min, and once hydrogen begins to be formed, the slope of the growing curve increases significantly. The evolution of the area of the bands corresponding to CO adsorbed on oxidized species (Au+ and Ce4+ at 2160 and 2175 cm-1, respectively) is shown in Figure 5c. The area of these bands increases on increasing the CO adsorption time (up to ca. 6 min), reaches a maximum, and decreases to raise a stable value after about 20 min. Two phenomena can explain this result: hydroxyl elimination, which leaves behind Lewis acid centers capable of adsorbing CO, and surface reduction that decreases the number of available Ce4+ sites. These two parallel mechanisms will result in profiles such as the ones described in Figure 5c. The elimination of type II and III hydroxyl groups upon CO reaction is clearly seen in Figure 3. This elimination increases the number of under-coordinated Ce4+ sites on the surface, and therefore, the number of adsorbed CO species on these sites increases. At the same time, the number of reduced cerium sites increases at the expense of Ce4+ sites as a consequence of the mechanisms depicted in eqs 1 and 2. As stated previously, the results of Akita et al.24 indicate that, in reductive conditions, oxygen vacancies are created in ceria favoring the diffusion of gold particles on the surface, as a function of oxygen vacancy density. The increase in gold dispersion deduced from the shift of the Au0-CO band from 2084 to 2091 cm-1 and the migration of gold into the ceria support, as stated by the Ce3+-Au0 band evolution, justify the increase in the number of oxidized gold species as a result of the interaction with the support as well as the increase in the number of (O)Au+-CO species responsible for the band at 2180 cm-1. Oxygen vacancies created by reaction with CO induce a change in the interaction strength between gold particles and support, resulting in an increase of the gold dispersion. The high temperature of our experiments and the good redox properties of ceria favor the migration of some oxygen atoms from the bulk to the surface generating (O)Au+-CO species. That is, there is a partial oxidation of species on the surface when the density of oxygen vacancies is enough to provoke migration from the bulk. The reduction of the catalyst produced upon CO interaction produces gas-phase CO2, the original hydroxyl groups of the surface being affected by this process, as demonstrated by the band at 3666 cm-1 corresponding to OH on the reduced ceria surface. The formation of these Ce3+ sites explains the observed hydrogen generation as a result of its interaction with CO, producing both adsorbed formate species that decompose according to eq 1 in the close vicinity of Ce4+ sites and dicarbonyl species that decompose according to eq 2. In all these processes, oxygen vacancies in the ceria support are generated. The presence of these vacancies induces the interaction of gold atoms with cerium ions, favoring the increase in gold dispersion by breaking off the original gold crystals. On the other hand, the oxygen mobility and the creation of partially oxidized gold species on the surface of the ceria support the close vicinity of surface Ce3+ species. All these processes indicate that the surface is restructured under reaction conditions and that hydroxyl groups play a significant role in these redox processes, which may explain

Surface Dynamics of Au/CeO2 Catalysts during CO Oxidation the previous observations on the modification of the catalytic activity in successive reaction cycles. Conclusion The data reported in this work show that gold dispersion is affected by the redox mechanisms taking place between CO, gold species, ceria ions, and hydroxyl groups on the surface. The interaction of CO with the catalyst surface results in the evolution of CO2 and H2, in which the hydroxyl groups of the surface play a fundamental role both through the decomposition of formate species and through the formation of [Au(CO)2]+ species. A detailed study of IR spectra of the surface permitted us to identify all the involved species in these processes. This study has also concluded that a deep reduction of the surface (oxygen vacancy creation) provokes a change of gold dispersion and the migration of oxygen atoms from the bulk to the surface, generating oxidized species. This result is also confirmed by XRD, where a decrease in the intensity of all metallic gold diffraction peaks and the relative intensity among them is evidenced in the used catalyst. Therefore, this work provides evidence for the existence of surface dynamics in this type of catalyst that may influence the catalytic activity especially when successive cycles are taken into account. Acknowledgment. We thank MEC (MAT2006-12386-CO501) and Junta de Andalucia, Proyectos de Excelencia (P06-TEP01965) for financial support. We are grateful to MEC for a Ramon y Cajal research program and to the Alban program (E04D046878CO). References and Notes (1) Nijhuis, T. A.; Weckhuysen, B. M. Catal. Today 2006, 117, 84. (2) Thompson, D. T. Appl. Catal., A 2003, 243, 201. (3) Abad, A.; Almela, C.; Corma, A.; Garcı´a, H. Tetrahedron 2006, 62, 6666. (4) Jacobs, G.; Ricote, S.; Patterson, P. M.; Graham, U. M.; Dozier, A.; Khalid, S.; Rhodus, E.; Davis, B. H. Appl. Catal., A 2005, 292, 229. (5) Kim, C. H.; Thompson, L. T. J. Catal. 2005, 230, 66. (6) Wang, X.; Rodriguez, J. A.; Hanson, J. C. J. Chem. Phys. 2005, 123, 221101. (7) Aeijelts Averink Silberova, B.; Mul, G.; Makkee, M.; Moulijn, J. A. J. Catal. 2006, 243, 171. (8) Minico`, S.; Scire`, S.; Crisafulli, C.; Visco, A. M.; Galvagno, S. Catal. Lett. 1997, 47, 273. (9) Deng, J.; de Jesus, H.; Saltsburg, M.; Flytzani-Stephanopoulos. Appl. Catal., A 2005, 291, 126. (10) Grunwaldt, J. D.; Maciejewski, M.; Becker, O. S.; Fabrizioli, P.; Baiker, A. J. Catal. 1999, 186, 458. (11) Daniells, S. T.; Overweg, A. R.; Makkee, M.; Moulijn, J. A. J. Catal. 2005, 230, 52. (12) Pillai, U. R.; Deevi, S. Appl. Catal., A 2006, 299, 266.

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