CuO Catalysts

Article pubs.acs.org/JPCC

Improving the CO-PROX Performance of Inverse CeO2/CuO Catalysts: Doping of the CuO Component with Zn A. López Cámara,*,† V. Cortés Corberán,† L. Barrio,†,‡ G. Zhou,‡ R. Si,‡ J. C. Hanson,‡ M. Monte,† J. C. Conesa,† J. A. Rodriguez,*,‡ and A. Martínez-Arias*,† †

Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, Campus de Cantoblanco, 28049 Madrid, Spain Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States

‡

S Supporting Information *

ABSTRACT: An inverse CeO2/CuO catalyst in which the CuO component has been doped with Zn is examined in comparison with the analogous Zn-free catalyst with the aim of enhancing the performance of this type of systems for preferential oxidation of CO in H2-rich stream (CO-PROX). The catalysts are characterized by XRD, Raman spectroscopy, XANES-EXAFS, XPS, and HRTEM. Their catalytic properties are explored in conjunction with redox properties studied by XANES, XRD, and DRIFTS under reaction conditions with MS detection. It is shown that the presence of zinc enhances the CO-PROX performance of the inverse system through decreasing its H2 oxidation activity, while the CO oxidation activity does not practically become affected. This is related to an increase in the CO2 selectivity of the catalyst that has been attributed to a hindering of the reduction of the CuO component, thus preventing the formation of H2 oxidation active sites under reaction conditions. A model of the structural and catalytic effects induced by the presence of zinc in the catalyst is proposed on the basis of the multitechnique approach employed.

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INTRODUCTION Production of H2 for polymer fuel cells (PEMFCs) is usually accomplished by a multistep procedure that includes catalytic reforming of hydrocarbons followed by water gas-shift (WGS) reactions.1,2 However, because of the limited activity of current WGS catalysts, approximately 0.5 to 1.0 vol % of unconverted CO still remains in the effluent and is required to be decreased to a trace level to avoid poisoning of the Pt-based PEMFC anode. Preferential oxidation of CO (CO-PROX) upon external introduction of a small amount of O2 in the H2-rich stream resulting from reforming-WGS processes has been recognized as one of the most straightforward and cost-effective methods to achieve acceptable CO concentrations (below ca. 100 ppm).3 Supported noble metal catalysts, in particular, those containing Pt or Au,3−5 have shown their ability for the process and commercial systems based on supported platinum are available.6 Alternatively, catalysts based on combinations between copper and cerium oxides constitute a cheaper choice while also showing promising properties for the process and being particularly superior in terms of CO2 selectivity.5,7,8 Optimum catalytic properties for CO oxidation over copper-ceria catalysts were shown to be achieved, for classical (direct configuration) systems in the presence of well-dispersed copper oxide patches over ceria nanoparticles.7,9 This was rationalized by means of spectroscopic analysis of catalysts of this type under reaction conditions that indicated that active sites for such reaction are related to partially reduced interfacial copper oxide entities formed during the course of interaction with the reactant © 2014 American Chemical Society

mixture, a process for which ceria apparently plays an important promoter role.8,10,11 H2 oxidation is the main reaction competing with the CO oxidation for the available oxygen over this type of catalysts. Their respective relative activity therefore determines the width of practical conversion window, that is, temperature range at which values close to 100% CO conversion with the lowest possible H2 oxidation activity becomes achieved. H2 oxidation was apparently promoted when the reduction of the copper oxide particles becomes extended to noninterfacial sites.8 Such results suggested that the properties of the copper oxide component could play a key role on modulating the overall CO-PROX behavior of this kind of systems and led to the development of the inverse configuration of the catalyst (i.e., CuO acting as support of CeO2) as optimized form.12 The catalytic enhancement observed for such inverse configuration was mainly based on the fact that interfacial sites responsible for CO oxidation activity can be formed in a similar way as for the direct configuration; in turn, simultaneously, an important decrease in H2 oxidation activity, with consequent increase in the selectivity to CO2, is produced in the presence of less reducible larger copper oxide particles present in the system.12 Thus, the results indicate that acting on the characteristics of the CuO component can apparently produce important modifications of the CO-PROX properties of this type of catalysts. On such Received: January 27, 2014 Revised: April 1, 2014 Published: April 3, 2014 9030

dx.doi.org/10.1021/jp5009384 | J. Phys. Chem. C 2014, 118, 9030−9041

The Journal of Physical Chemistry C

Article

g−1 (roughly corresponding to 80 000 h−1 GHSV) and using a heating ramp of 10 °C min−1. Analysis of the feed and outlet gas streams was done by means of gas chromatography (GC) (Shimazdu GC-2014 chromatograph equipped with a 2 m length and 2.1 mm ID capillary column and a TCD detector) and mass spectrometry (MS) (Pfeiffer Omnistar spectrometer connected on line) under steady-state conditions at 30 °C and every 20 °C between 80 and 220 °C. No products other than those resulting from CO or H2 combustion (i.e., CO2 and H2O) were detected in the course of the runs, in agreement with previous results on catalysts of this type.17 Only a residual contribution of possible WGS or reverse WGS reactions, taking place in any case at temperatures higher than ca. 200 °C, was estimated in independent tests. On this basis, values of percentage conversion and selectivity in the CO-PROX process are defined as

basis, adequate chemical modification of such component could, in principle, favor further improvement in the COPROX performance of this type of systems. This can be accomplished by doping the CuO component of the inverse catalyst with other transition metals. In this sense, recent work has suggested that zinc can be a promising dopant for this type of catalyst/process.13,14 In turn, catalysts combining copper and zinc oxides are known to exhibit outstanding CO oxidation properties.15,16 In this context, the present work shows that doping with zinc the CuO component of the inverse CeO2/ CuO catalyst indeed enhances the CO-PROX performance of the system, and the physicochemical reasons for such behavior are examined in detail.

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EXPERIMENTAL SECTION Inverse CeO2/CuO and CeO2/(Cu−Zn)O (with Cu:Zn atomic ratio of 9:1) catalysts (with Ce:(Cu+Zn) atomic ratios of 4:6) were prepared by employing reverse microemulsions containing n-heptane, Triton-X-100, and n-hexanol as organic solvent, surfactant, and cosurfactant, respectively, in amounts similar to those previously reported.17 The samples are denoted as Cu6Ce4 and (Cu9Zn1)6Ce4, respectively. The required amount of Cu(NO3)2 (or Cu(NO3)2 + Zn(NO3)2) was dissolved in distilled water and added to the former to form the reverse microemulsion. Simultaneously, another microemulsion of similar characteristics was prepared containing the required amount of tetramethylammonium hydroxide (TMAH) dissolved in its aqueous phase. After 1 h stirring of the two microemulsions, the TMAH-containing one was added to the Cu-containing one, and it was left during 18−24 h to complete the precipitation reaction. Then, the resulting microemulsion was heated gently to 60 °C (to the point of visible copper precipitation) using a water bath. An aqueous solution containing the required amount of Ce(NO3)3 was then added to this microemulsion containing the precipitated copper (or copper and zinc) and, after 1 h of stirring, a TMAH aqueous solution was finally added. This final microemulsion containing both precipitated Cu (or Cu+Zn) and Ce components was kept under agitation for the period of 18− 24 h. The resulting solid was then separated by centrifugation and decantation, rinsed with methanol, and dried overnight at 100 °C. Chemical analysis by X-ray fluorescence (TXRF) confirmed quantitative precipitation of all components for both catalysts (Cu/Ce atomic ratios of 1.547 and 1.316 for Cu6Ce4 and (Cu9Zn1)6Ce4 catalysts, respectively, and Zn/Cu atomic ratio of 0.105, quite close to nominal values, 1.500, 1.350, and 0.111, respectively). The final preparation step consisted of calcination of the dry specimen under air during 2 h at 500 °C following a ramp of 5 °C min−1 up to the calcination temperature. Specific surface area (SBET) determined from the analysis of nitrogen adsorption isotherms was of 98 and 99 m2 g−1 for Cu6Ce4 and (Cu9Zn1)6Ce4, respectively. Two other Zndoped catalysts having Cu/Zn ratios of 8/2 and 9.5/0.5, respectively, were synthesized by the same method. Results presented in main text for Zn-doped samples will, however, be those of the catalyst with Cu/Zn = 9/1, selected on the basis of optimum CO-PROX performance achieved over this catalyst (Supporting Information). The catalysts calcined in situ (under oxygen diluted in He at 500 °C) were tested in a glass tubular catalytic reactor for their activity under an atmospheric pressure flow (using mass flow controllers to prepare the reactant mixture) of 1% CO, 1.25% O2, and 50% H2 (He balance) at a rate of 1 × 103 cm3 min−1

XO2 = XCO = SCO2 =

FOin2 − FOout2 FOin2 in out FCO − FCO in FCO

× 100

× 100

XCO × 100 2.5XO2

where X and S are percentage conversion and selectivity, respectively, and F is the (inlet or outlet) molar flow of the indicated gas. Time-resolved X-ray diffraction (TR-XRD) experiments were carried out on beamline X7B of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory with an incident wavelength of 0.3184 Å. The sample was loaded into a glass cell of 1 mm diameter, which was attached to a flow system. A small resistance heater was placed just below the capillary, and the temperature was monitored with a 0.1 mm chromel−alumel thermocouple that was placed in the capillary near the sample. Two-dimensional powder patterns were collected with a Perkin Elmer amorphous silicon area detector (409.6 mm2, 2048 × 2048 pixels and a 200 mm pixel size), and the powder diffraction rings were integrated using the Fit2D code.18 The instrument parameters (Thompson−Cox−Hastings profile coefficients) were derived from the fit of a LaB6 reference pattern. Lattice constants were determined by Rietveld analysis. Rietveld profile refinements were performed with the aid of GSAS software,19,20 which allowed for the sequential analysis of phase transitions during the CO-PROX reaction, which was carried out under similar flow conditions, as employed for the previously described activity tests. Cu K-edge or Zn K-edge time-resolved XAFS spectra (TRXAFS) were collected at beamline X18A of the NSLS under similar operational conditions, as those employed for the TRXRD experiments. The same cell was used for the XAFS experiments as that for TR-XRD, except that the sample was loaded into a Kapton capillary and heated with a hot air blower. The X-ray absorption spectra were taken repeatedly in the ‘“fluorescence-yield mode”’ using a passivated implanted planar silicon (PIPS) detector cooled with circulating water. The XAFS data were then analyzed using Athena software.21 Additional XAFS spectra at the Zn K-edge were obtained in beamline CLAESS of the synchrotron facility ALBA using a Si(111) double-crystal monochromator and dual-toroid focusing mirror provided with harmonic rejection; fluorescence 9031



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Raman spectra were obtained at room temperature (RT) with a Bruker RFS-100 FT-Raman spectrometer provided with a diode-pumped germanium solid-state detector, which operates at liquid-nitrogen temperature. A NdYAG laser (1064 nm) was used as excitation source at a power of 100 mW. Powdered samples were pressed in a holder and analyzed (100 scans, 4 cm−1 resolution) without further treatment. It must be noted that we have selected infrared excitation for these experiments because the use of visible excitation (results not shown) requires a careful control of exciting power (while keeping reasonable signal/noise ratio) to avoid a blue shift in the main triply degenerate F2g mode of fluorite CeO2 (only occurring for Cu-containing samples, results not shown),29 which is likely a consequence of sample heating (and consequent lattice expansion) after electronic relaxation (considering that light absorption by Cu2+ entities extends to the visible region according to UV−vis analysis);30 similar sample heating problems as a consequence of interaction with the visible Raman laser beam have been pointed out elsewhere.31,32 High-resolution transmission electron microscopy-X-ray energy dispersive spectroscopy (HRTEM-XEDS) examination of the samples was done with an electron microscope JEOL JEM 2100 F UHR. Specimens were prepared by depositing particles of the samples to be investigated from ethanol dispersion onto a nickel grid supporting a perforated carbon film.

detection with a Si drift unit was used except when recording, in transmission mode, calibration spectra from metal foils. Data were handled with the Demeter software.22 For these latter spectra, the main inflection point measured for the XANES spectrum of Zn foil was found at 9658.9 eV; this is close to the value reported in the literature (9658.6 eV).23 Simulation of XANES spectra was carried out using the FEFF 9 software,24,25 including in all cases self-consistency of charge and multiple scattering. The structural models used for these simulations were obtained by relaxing crystal structures containing Zn as absorber atom using the DFT code VASP26 at the GGA theory level, complemented, when Cu or Zn are present, with a Hubbard U term (UCu = 7 eV, UZn = 4 eV), and representing the atom cores with the PAW approach.27,28 The planewave cutoff was 445 eV, and Monkhorst−Pack grids used to sample the Brillouin zone were chosen dense enough that the resulting relaxed structures were convergent in terms of lattice spacings and interatomic distances at the 10−4 Å level. The reaction products from both TR-XRD and TR-XAFS experiments were measured with a 0−100 amu quadruple mass spectrometer (QMS, Stanford Research Systems). A portion of the exit gas flow passed through a leak valve and into the QMS vacuum chamber. QMS signals at mass-to-charge ratios of 2(H2), 4(He), 17(OH), 18(H2O), 28(CO), 32(O2), and 44(CO2) were monitored and recorded during the experiments. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra under reaction conditions were recorded using a Bruker Equinox 55 FTIR spectrometer equipped with a liquid N2-cooled high-sensitivity MCT detector and a Harrick Praying Mantis DRIFTS cell. All spectra were recorded with an accumulation of 25 scans at a resolution of 4 cm−1. Aliquots of ca. 100 mg catalyst were placed in a sample cup inside the DRIFTS cell with CaF2 windows and a heating cartridge that allowed samples to be heated to 500 °C. The DRIFTS cell was connected to a gas handling system to measure operando spectra under controlled gas environments at atmospheric pressure. The samples were activated by in situ calcination for 2 h in a diluted O2 (in He) stream at 500 °C. Subsequently, the system was cooled to room temperature, and the background spectra were recorded. The reaction mixture (1% CO + 1.25% O2 + 50% H2 in He) prepared using mass flow controllers was then passed through the catalyst inside the DRIFTS cell at a total flow rate of 100 cm3 min−1 at atmospheric pressure. Similarly to the activity measurements, DRIFTS spectra were collected under steady-state conditions at 30 °C and every 20 °C between 80 and 220 °C using a heating rate of 10 °C min−1. Circulating water was used to cool the body of the reaction chamber. As outlet of the DRIFTS cell, a Pfeiffer Omnistar mass spectrometer was connected on line, and all relevant masses were monitored during DRIFTS measurement. For comparison with the spectra of gaseous CO molecules whose shape changes with the temperature as a consequence of differences in populations of rotational levels, blank runs were done under the same conditions with an inert KBr sample. All DRIFTS data presented here have accordingly been subtracted from such CO(g) data at each temperature to get rid of contribution from CO gas molecules. All spectra obtained were transformed from reflectance units by the use of the Kubelka− Munk function, which is linearly related to absorber concentration in DRIFTS spectra and referenced to those taken just before CO admission. The CO, O2, H2, and He used here were supplied by Gas Natural (purity higher than 99.95%).

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RESULTS AND DISCUSSION Characterization of the Catalysts. X-ray diffractograms of the samples are displayed in Figure 1. Table 1 summarizes

Figure 1. X-ray diffractograms of the indicated catalysts. Peaks for the identified crystallographic phases are indicated.

main structural parameters extracted from their analysis. Both diffractograms exhibit peaks due to the CeO2 fluorite and CuO tenorite phases, while no peak attributable to Zn-containing phases is detected. Crystal sizes around ca. 4 to 5 nm for CeO2 and between ca. 12 and 20 nm for CuO are estimated with the presence of Zn inducing a crystal size decrease in both components (Table 1), particularly for CuO and as typically observed for catalysts combining copper and zinc oxides.33,34 Lattice parameters determined for the two phases fairly correspond to those expected for the pure compounds and appear only slightly modified by the presence of Zn (Table 1).35 The phase fraction estimated from Rietveld analysis shows 9032



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Table 1. Main Structural Parameters Extracted from Rietveld Analysis of the X-ray Diffractograms Shown in Figure 1 CeO2 crystal size (nm)

CuO crystal size (nm)

fluorite CeO2 lattice parameter (Å)

tenorite CuO lattice parameters (Å)

CuO/CeO2 phase fraction (nominal = 1.5)a

Cu6Ce4

4.9

20.1

5.421

1.25

(Cu9Zn1)6Ce4

3.9

11.7

5.415

a = 4.680 b = 3.441 c = 5.133 a = 4.684 b = 3.450 c = 5.124

catalyst

a

0.98

Based on quantitative estimation of CuO and CeO2 crystals contribution to respective diffractograms.

this band.36 The ratio between the areas of the bands at ca. 460 and 590 cm−1 provides in this sense a measurement of the relative amount of oxygen vacancies present in the fluorite ceria lattice in each case.39,40 This is A590/A460 ≈ 1.06 and 0.80 for Cu6Ce4 and (Cu9Zn1)6Ce4, respectively, thus suggesting the presence of a higher amount of oxygen vacancies in the former. This qualitatively correlates with the larger lattice parameter observed for the former (Table 1); in this sense, because Cu2+ incorporation to the ceria lattice is not expected to increase the cell size,35 it appears that it is mainly the presence of Ce3+ cations in the nanosized ceria that could explain the observed results.41 As already observed by XRD, no specific Zn-containing phase is detected in the Raman spectrum of (Cu9Zn1)6Ce4 (Figure 2). The main effect of the presence of Zn appears related to the decrease in the relative contribution of tenorite CuO bands (A296/A460 ≈ 0.23 and 0.16 for Cu6Ce4 and (Cu9Zn1)6Ce4, respectively). This suggests that the presence of Zn induces an increase in the amorphization degree of CuO, which could be related to its incorporation to such phase; the presence of zinc in solid solution within CuO was proposed on the basis of STEM-EDS examination of CO oxidation catalysts combining copper and zinc oxides by Whittle et al.42 Alternatively, according to studies for CuO nanocrystals in the scientific literature,37,43 the apparent decrease in Raman or X-ray diffraction intensity of the copper oxide crystals in (Cu9Zn1)6Ce4 could be related to their smaller size within size in the range up to tens of nanometers (Table 1). In this sense, some differences are observed in the Zn−K edge XANES spectrum and EXAFS radial distribution of (Cu9Zn1)6Ce4 with respect to a ZnO nanocrystalline reference (Figure 3). The XANES spectrum reveals in any case that zinc appears as Zn2+ in this catalyst, as also confirmed by analysis of the XPS Auger L3M45M45 spectrum of (Cu9Zn1)6Ce4 showing a maximum at ca. 989.0 eV, as expected for such chemical state of zinc (not shown). To get further hints on the location and properties of zinc in this catalyst, simulations of the Zn XANES spectra were carried out by using the FEFF 9 software. Different possible situations were tested for Zn located in the cationic sites of the bulk structures of ZnO, CuO, and CeO2 (in the latter case with an anion vacancy in nearest neighbor position), using in the two latter cases multiples of the unit cells so as to obtain Zn:Cu and Zn:Ce atomic ratios of 1:7 and 1:3, respectively. Figure 4 shows the XANES spectra computed for these three cases, in which multiple scattering within a large radius (7.1 Å) around the excited Zn atom was taken into account. For Zn in ZnO, the shoulder and main peak double feature normally occurring at the edge jump (9660−9675 eV range), typical of well-crystallized ZnO,23,44 is clearly observed, although the proportion between both features appears somewhat different. In addition, the distinct peak at ca. 9680

that the amount of CuO crystals with respect to CeO2 crystals is well below nominal value, an effect that appears most pronounced in the presence of Zn (Table 1). In principle, this can be related to either the introduction of a part of the copper in the fluorite lattice, taking into account the fact that this does not necessarily produce changes in the CeO 2 lattice parameter,17,35 or to the presence of a relatively higher amount of the copper in an amorphous phase. To explore this point as well as to get further hints on structural characteristics of the samples, they were examined by Raman spectroscopy and the spectra are shown in Figure 2.

Figure 2. Raman spectra of the indicated samples. A pure CeO2 reference (with ca. 7.5 nm crystal size, as found elsewhere)17 is included for comparative purpose.

The spectra show bands related to the presence of fluorite CeO2 (triply degenerate F2g band at ca. 463 cm−1, as only one allowed for this cubic crystal)36 and CuO tenorite phase (three active modes expected at ca. 296, 350, and 630 cm−1)37 as well as a wide band centered at ∼590 cm−1, which is related to oxygen vacancies in the fluorite ceria lattice.17,36 A small red shift along with a certain widening is observed in the fluorite F2g band for the Cu-containing catalysts. This could be related to the incorporation of a small amount of the copper (as Cu2+) to the fluorite lattice.17 We cannot, however, discard that the shift is partially due to phonon confinement effects in the very small CeO2 crystals present in these samples.36,38 In any case, observation of the wide band at ca. 590 cm−1 reveals the presence of oxygen vacancies in the fluorite ceria lattice of the Cu-containing samples. This could be related to the mentioned presence of a small amount of the copper in the lattice as a consequence of charge compensation upon incorporation of Cu2+.35 Alternatively, the presence of reduced cerium cations (Ce3+) in the nanosized particles can also explain observation of 9033



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Figure 3. Left: XANES spectrum of (Cu9Zn1)Ce4 in comparison with spectra of ZnO and metallic Zn references. Right: EXAFS radial distribution function for the indicated samples.

Figure 4. Zn K-edge simulated XANES spectra (black lines) of Zn2+ inside a ZnO structure (left), CuO structure (middle), and CeO2 structure (right). Purple line: experimental spectrum of (Cu9Zn1)Ce4 after pretreatment under inert diluted O2 at 400 °C. Gray dotted line: experimental Zn K-edge of well-crystallized ZnO (digitized from ref 23).

eV typical of well-crystallized ZnO also appears fairly well-fitted in the simulation. In contrast, this latter feature appears clearly shifted to higher energy for Zn2+ in a cationic position of CuO, while the double feature at the edge jump of well-crystallized ZnO is lost for Zn2+ in CeO2. It must be noted that absolute positions in energy obtained for the simulations (which cannot typically be reproduced by using FEFF) appeared shifted to higher energies (by ca. 4−8 eV) and were thus artificially corrected in all simulated spectra to fit the experimental spectra. In principle, appreciable differences are observed between simulated spectra and the experimental one in the analysis shown in Figure 4. In particular, the double contribution in the white line of well-crystallized ZnO is not present in the experimental spectrum, while the feature at ca. 9680 eV appears significantly broadened in the experimental spectrum. Yet another possibility can be that zinc appears in the form of tiny ZnO clusters.45 To check this, we also made simulations for small atomic clusters of ZnO, having radii between 1.0 and 4.7 Å around the central excited Zn atom. As can be seen in the simulations (Figure 5), smaller clusters lead to a mitigation of the shoulder-and-peak structure at the main edge jump, with the shoulder ultimately disappearing, accompanied as well by the decrease in the amplitude of the feature at 9668 eV. In turn, the feature at ca. 9680 eV also appears quite broadened for such small ZnO-type clusters. This reproduces, at least qualitatively, the experimentally observed spectrum. It seems thus that Zn may be present in the sample as segregated very small clusters of more or less defined ZnO structure. This also agrees with the

Figure 5. Zn K-edge simulated XANES spectra of Zn2+ in the center (except for the smaller one) of several ZnO clusters (radii and number of atoms are indicated). The experimental Zn K-edge spectrum of (Cu9Zn1)Ce4 after pretreatment under inert diluted O2 at 400 °C is shown as a purple line.

fact that the spectrum does not apparently change under reaction conditions, as will be discussed later. The inverse configuration of catalysts prepared by the method employed here was demonstrated by HRTEM in a previous work.12 An HRTEM image of the Cu6Ce4 sample is 9034



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Figure 6. Top: HRTEM image of the Cu6Ce4 sample. Points A−C correspond to zones in which EDX analyses were performed (see main text). Bottom: Details of the top picture at zones close to points A (left) and C (right).

during the CO-PROX process over this type of catalysts) achieved over the two catalysts. Practically no difference in CO conversion level is observed between the catalysts at relatively low reaction temperature (below ca. 150 °C). However, above ca. 110 °C, an appreciable (higher than 10% on the whole) increase in the selectivity to CO2 is observed for the catalyst with Zn. This is also reflected in CO conversion levels, higher for (Cu9Zn1)6Ce4 above ca. 150 °C. It must be noted when comparing the two plots that because the concentration of oxygen employed in the reactant mixture is higher than that stoichiometrically required for full CO conversion, 100% CO conversion can still be kept when appreciable H2 oxidation is simultaneously taking place. On the whole, the activity results reveal that the presence of Zn in the catalyst enhances the COPROX performance and that this is related to achievement of a higher CO2 selectivity. The CO-PROX catalytic behavior of this type of systems is typically explained on the basis of correlation with its redox properties.7,8,46 In turn, it is known that the presence of zinc affects the reduction of copper oxide, although controversial

displayed in Figure 6. Points A−C marked on the image correspond to zones in which EDX analyses were performed, aiming to demonstrate the inverse configuration of the system. Ce/Cu atomic ratios obtained from such analyses were of 0.00, 0.13, and 2.14 in points A−C, respectively. Unfortunately, unlike in our mentioned previous contribution,12 no profile view of the catalyst was possible in any of the taken pictures to directly demonstrate such configuration. Nevertheless, details of the image at zones close to points A (left) and C (right) evidence the prevailing presence of CuO (2.4 and 2.6 Å interplanar distances corresponding to (111) and (002) planes of tenorite CuO, respectively) and CeO2 (3.2 Å, assigned to (111) fluorite plane), respectively, in agreement with mentioned EDX analysis and demonstrating the inverse configuration of this catalyst in this case too. Catalytic and Redox Properties. Figure 7 displays CO conversion and selectivity to CO2 (i.e., the portion of oxygen selectively reacting with CO with respect to that reacting with H2; as mentioned in the Experimental Section, those two oxidation reactions are the only ones practically taking place 9035



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Figure 7. Catalytic activity under 1% CO, 1.25% O2, and 50% H2 (He balance) for the indicated catalysts. Left: CO conversion. Right: selectivity to CO2. Note CO2 and H2O are only products obtained from competing CO and H2 oxidation reactions, respectively.

Figure 8. Evolution of the gases (top) and the various phases detected by XRD along with the relative change in CeO2 lattice parameter (bottom), useful to monitor the redox state of this component, as a function of temperature under 1% CO + 1.25% O2 + 50% H2 (He balance) CO-PROX mixture. Left: Cu6Ce4; right: (Cu9Zn1)6Ce4.

correlation is also evidenced in this case upon examining the catalysts under CO-PROX conditions by XRD, as shown in Figure 8. Thus, appreciable H2 oxidation degree (revealed by the increase in H2O concentration; note a decrease in CO2 selectivity is also noted from the decrease in CO 2 concentration) is only achieved when copper becomes massively reduced. In this particular set of experiments, this occurs at 175 and 200 °C for Cu6Ce4 and (Cu9Zn1)6Ce4,

results are found because both positive and negative effects have been reported.33,34,47 For the CO-PROX process, the level of activity for H2 oxidation, which basically determines the CO2 selectivity,48 has been related to the facility of the catalyst to generate in the copper oxide component reduced sites (either Cu+ or metallic copper, some controversy still exists with respect to the nature of H2 oxidation active sites),7,48,49 which are proposed as the active ones for such reaction.8 Such 9036



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Figure 9. Top: Evolution of copper states under 1% CO, 1.25% O2, and 50% H2 (He balance), as examined by XANES at Cu K-edge. Full symbols: Cu6Ce4; open symbols: (Cu9Zn1)6Ce4. Corresponding spectra are shown in the middle (Cu6Ce4: left; (Cu9Zn1)6Ce4: right). Bottom: Zn K-edge spectra under the same conditions at the indicated reaction temperature.

of reaction temperatures can be made between this experiment and the catalytic activity tests performed with a more or less ideal reactor (Figure 7) because control of certain parameters

respectively, thus revealing that the presence of Zn limits copper reduction under CO-PROX conditions. It must be noted for comparative purpose that only qualitative comparison 9037



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Figure 10. Evolution of the various phases detected by XRD along with the relative change in CeO2 lattice parameter (right axis), as a function of temperature, under 10% H2 in He (H2-TPR tests); left: Cu6Ce4; center: (Cu9Zn1)6Ce4. Right: Evolution of MS signal corresponding to H2O during the H2-TPR tests for the indicated catalysts.

Figure 11. Left: DRIFTS spectra of (Cu9Zn1)6Ce4 in the carbonyl-stretching region under 1% CO, 1.25% O2, and 50% H2 (He balance) at the indicated temperatures. (The spectrum on the bottom corresponds to the one prior to the introduction of the reactant mixture.) Right: Evolution of the intensity of the interfacial Cu+-carbonyl band (full symbols; left axis) and evolutions of MS signals corresponding to CO2 and H2O (open symbols; solid and dashed lines, respectively; right axis) during the course of the DRIFTS tests under mentioned CO-PROX reactant mixture.

The results of H2-TPR tests with simultaneous XRD monitoring are shown in Figure 10. This is also relevant in terms of comparison with results in the literature because H2TPR is a widely used technique when analyzing catalytic/redox correlations in this type of systems,7,46,50 particularly, as shown more recently, when dealing with explaining selectivity differences.49,51 Similarly to the results obtained under the CO-PROX mixture, the presence of Zn apparently limits the reduction of copper oxide under H2, in line with the lower H2 oxidation activity observed under CO-PROX conditions over the (Cu9Zn1)6Ce4 specimen (Figures 8 and 9). The formation of interfacial Cu+ sites, active for the CO oxidation reaction,8 has been evidenced by DRIFTS, as exhibited in Figure 11. Those are related to a band around 2110 cm−1 attributed to interfacial Cu+ carbonyls.52 As explored in previous studies,8,10,53 the CO oxidation rate under COPROX conditions of catalysts combining copper and cerium oxides is proportional to the intensity of such interfacial Cu+ carbonyls (data taken just prior to onset of CO oxidation); an exception to this general rule has recently been pointed out when dealing with comparisons between catalysts with different morphology in the ceria component at the nanostructural level.49 Corresponding interfacial Cu+ sites have accordingly been proposed as the active ones for that reaction. It must be noted that such Cu+ species, located at the surface of the sample in CeO2−CuO interfacial positions, normally corre-

relevant in this sense (particularly external or internal diffusion effects) is not possible with the XRD cell employed as reactor for the tests shown in Figure 8. The same holds for analogous experiments performed with the XANES cell, which are described next. Because, as previously evidenced, an appreciable part of the copper in these catalysts can apparently form amorphous phases to which the XRD technique is not sensitive, the same set of experiments was performed by X-ray absorption (in this case, without MS detection of gases evolutions). Figure 9 shows the evolution of chemical states of copper under the CO-PROX mixture, as examined by this technique. Both catalysts start from a fully oxidized state, and an intermediate Cu+ state appears as a transient in any case. Partial final evolution to metallic copper is observed for both catalysts at high temperature. Similarly to XRD experiments (Figure 8), the main difference between the two catalysts is the temperature at which copper reduction takes place, higher for the Zncontaining system. Thus, as a conclusion, the presence of zinc in the catalyst allows retarding of the copper reduction and consequent formation of sites active for H2 oxidation, which explains the higher CO2 selectivity of the corresponding catalyst (Figure 7). In addition, most interestingly, the Zn Kedge spectra practically do not change during the whole run under CO-PROX conditions despite the fact that copper becomes significantly reduced at high temperature (Figure 9). 9038



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over a CuO/CeO2 catalyst in which the CeO2 support is in the form of nanocubes (exposing metastable (100) faces).49 Therefore, it appears that, as shown schematically in Figure 12, the main role of Zn is related to an indirect effect favoring a decrease in CeO2 crystal size with consequent modification of the interfacial CeO2−CuO properties.

spond to a small portion of the copper with respect to the whole copper in the sample, which makes quite difficult their detection by bulk techniques like XRD or XANES. As shown in Figure 11, irrespective of reaction temperature (note the intensity of the carbonyl must basically depend on pCO and temperature as well as on the availability of surface exposed Cu+ centers),52 a higher intensity of such carbonyl band is produced in the Zn-containing sample despite the fact that CO conversion appears similar. This suggests that zinc affects the copper oxide component by facilitating the formation of interfacial Cu+ sites, although corresponding active sites would individually present slightly lower CO oxidation activity. Further exploration would be, however, required to confirm this hypothesis. In any case, the results of Figure 11 evidence that the presence of zinc favors the generation of surface interfacial Cu+ sites, in line with observations by Zou et al. for CO-PROX catalysts of this type in direct configuration and modified with zinc;14 note also that previous results on methanol synthesis catalysis also suggested zinc oxide as a possible stabilizer of active Cu+ state in Cu/ZnO-related catalysts.54 Such Zn-induced stabilization of surface Cu+ entities can then be the key for preventing further reduction to metallic copper and formation of sites active for the H2 oxidation reaction. Nevertheless, the fact that the Zn K-edge spectrum does not change during the course of the CO-PROX process (Figure 9), despite the fact that copper and ceria become appreciably reduced (Figures 8 and 9), strongly points toward an indirect effect of zinc rather than to its presence as a pure dopant of copper oxide or ceria, as also pointed out by the study of XANES simulations (Figures 4 and 5). It must be taken into account in this respect that despite the fact that a small part of the copper can remain oxidized under CO-PROX conditions at high temperature (Figure 9), such oxidized portion would most likely remain at the bulk of the copper oxide particles, as recently observed for this type of inverse catalysts under WGS conditions examined by XPS.55 However, zinc in (Cu9Zn1)6Ce4 must mostly be located at the surface of the sample according to the Zn/Cu atomic ratio of 0.43 derived from XPS analysis (atomic sensitivities corrected in accordance to experimentally derived factors),56 well above the experimental bulk ratio of 0.105 (Experimental Section). As possible indirect effects of zinc, a first one to consider is related to the copper oxide crystal size, which appreciably decreases (Table 1), as typically observed for catalysts combining copper and zinc oxides.33,34 In this sense, it is known that the reducibility of copper oxide can decrease with decreasing crystal size,57 also for particles interacting with ceria.58 However, despite such difference, copper oxide crystals in (Cu9Zn1)6Ce4 still appear relatively big as to affect significantly such redox property.38 In turn, the presence of zinc also affects appreciably to the CeO2 crystal size (Table 1). The decrease in CeO2 crystal size in the presence of zinc can influence the catalyst in two senses: first, by increasing the amount of (partially reduced) CeO2−CuO interfacial sites active for the CO oxidation reaction8 and second, by changing the physicochemical properties of the CeO2 nanocrystals in a similar way as observed upon changing the type of exposed face of ceria in CuO/CeO2 catalysts.49 Indeed, the correlation between redox and catalytic effects observed over (Cu9Zn1)6Ce4, that is, relative stabilization of surface Cu+ states under CO-PROX conditions (Figure 11) and slightly lower CO oxidation activity of the interfacial Cu+ entities (discussion in former paragraph), is similar to that observed

Figure 12. Schematic model of the effect of the presence of Zn on the inverse CeO2/CuO catalyst. Zinc is proposed to appear basically as surface-segregated ZnO-type clusters, and its presence favors a lower size in the supported CeO2 nanocrystals. (See the main text for details.)

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CONCLUSIONS A catalyst combining copper and cerium oxides in inverse configuration (CeO2/CuO) is examined with respect to its catalytic properties for preferential oxidation of CO in a H2-rich reactant stream (CO-PROX) in comparison with a similar one in which the copper oxide component is doped with a small amount of zinc. Detailed characterization by XRD, Raman, XANES-EXAFS, XPS, and HRTEM evidences the presence of small CeO2 nanoparticles (ca. 4 to 5 nm) dispersed on larger (12−20 nm) CuO nanoparticles with the presence of zinc inducing a decrease in the crystal size of both components. Catalytic activity tests show that the presence of zinc enhances the CO-PROX performance of the catalysts, which is related to an appreciable increase in the CO2 selectivity. Such beneficial effect appears related, on the basis of examination of the redox characteristics of the catalysts by XRD, XANES, and DRIFTS under CO-PROX reaction conditions, to hindering of the reduction of the copper oxide to metallic copper, a process that leads to the formation of active sites for the H2 oxidation reaction, under the CO-PROX reactant mixture. Theoretical and experimental examination of Zn−K edge XANES spectra for the initial calcined sample as well as under CO-PROX conditions in conjunction with evolutions observed by XANES and XRD for copper and cerium components and results extracted from the multitechnique characterization approach allow establishing a structural model of the role of zinc. On that basis, it is proposed that zinc appears in the form of surface segregated ZnO-type clusters. This indirectly induces a decrease in the size of the supported CeO2 nanocrystals, which results most favorable to CO-PROX catalytic properties of the inverse CeO2/CuO catalyst. Furthermore, the results suggest the crystal size of CeO2 as a relevant parameter to tune CO-PROX characteristics of this type of inverse CeO2/CuO catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

CO-PROX catalytic activity results of (Cu−Zn)Ce inverse systems as a function of Cu/Zn at. ratio. This material is available free of charge via the Internet at http://pubs.acs.org. 9039



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AUTHOR INFORMATION

Corresponding Authors

*A.L.C.: E-mail: [email protected]. *J.A.R.: E-mail: [email protected]. *A.M.-A.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.L.C. and M.M. thanks the CSIC and MINECO for a JAE and FPI Ph.D. grants/contracts, respectively. Financial support by MICINN or MINECO (Plan Nacional CTQ2009-14527 and CTQ2012-32928 projects) and Comunidad de Madrid (Project DIVERCEL S2009/ENE-1475) is acknowledged. Support from EU COST action CM1104 is also thanked. The research at BNL was supported by the U.S. Department of Energy (Chemical Sciences Division, DE-AC02-98CH10886). L.B. acknowledges funding by FP7 People program under the project Marie Curie IOF-219674. Thanks are due to ICP-CSIC Unidad de Apoyo for specific surface area and HRTEM measurements. We thank Dr. Laura Simonelli and the rest of the staff at CLAESS beamline for the help provided during XAFS measurements at ALBA Synchrotron Light Facility.

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