In Situ Spectroscopy and Mechanistic Insights into CO Oxidation on

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In Situ Spectroscopy and Mechanistic Insights into CO Oxidation on Transition-Metal-Substituted Ceria Nanoparticles Joseph S. Elias,*,† Kelsey A. Stoerzinger,○ Wesley T. Hong,○ Marcel Risch,§,∥ Livia Giordano,⊥,§ Azzam N. Mansour,‡ and Yang Shao-Horn*,Δ,○,§ †

Department of Chemistry, ΔResearch Laboratory of Electronics, ○Department of Materials Science and Engineering, and Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ⊥ Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via R. Cozzi 53, 20125 Milan, Italy ‡ Naval Surface Warfare Center, Carderock Division, 9500 MacArthur Boulevard, West Bethesda, Maryland 20817-5700, United States §

S Supporting Information *

ABSTRACT: Herein we investigate the reaction intermediates formed during CO oxidation on copper-substituted ceria nanoparticles (Cu0.1Ce0.9O2−x) by means of in situ spectroscopic techniques and identify an activity descriptor that rationalizes a trend with other metal substitutes (M0.1Ce0.9O2−x, M = Mn, Fe, Co, Ni). In situ X-ray absorption spectroscopy (XAS) performed under catalytic conditions demonstrates that O2− transfer occurs at dispersed copper centers, which are redox active during catalysis. In situ XAS reveals a dramatic reduction at the copper centers that is fully reversible under catalytic conditions, which rationalizes the high catalytic activity of Cu0.1Ce0.9O2−x. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) show that CO can be oxidized to CO32− in the absence of O2. We find that CO32− desorbs as CO2 only under oxygen-rich conditions when the oxygen vacancy is filled by the dissociative adsorption of O2. These data, along with kinetic analyses, lend support to a mechanism in which the breaking of copper−oxygen bonds is rate-determining under oxygen-rich conditions, while refilling the resulting oxygen vacancy is ratedetermining under oxygen-lean conditions. On the basis of these observations and density functional calculations, we introduce the computed oxygen vacancy formation energy (Evac) as an activity descriptor for substituted ceria materials and demonstrate that Evac successfully rationalizes the trend in the activities of M0.1Ce0.9O2−x catalysts that spans three orders of magnitude. The applicability of Evac as a useful design descriptor is demonstrated by the catalytic performance of the ternary oxide Cu0.1La0.1Ce0.8O2−x, which has an apparent activation energy rivaling those of state-of-the-art Au/TiO2 materials. Thus, we suggest that cost-effective catalysts for CO oxidation can be rationally designed by judicious choice of substituting metal through the computational screening of Evac. KEYWORDS: catalysis, mechanisms of reactions, in situ spectroscopy, ambient pressure XPS, nanotechnology, DFT, ceria

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Three competing hypotheses dominate the literature with regards to the origin of the catalytic activity of CuO/CeO2 materials for CO oxidation. The hypotheses vary in the roles the copper and cerium sites play in CO adsorption and redox, but all follow a Mars−van Krevelen type mechanism in which lattice O2− is directly transferred to bound CO during the oxidation step (Figure 1). Consistent with previous literature on the oxygen storage-capacity of CeO2, Flytzani-Stephanopoulos proposed that Cu+ sites at the interface between CuO and CeO2 are responsible for adsorbing CO while nearby Ce4+ sites supply the oxidative equivalents required to oxidize CO (Figure 1A).9,10 In this mechanism, all redox was confined to the cerium sites adjacent to Cu+ centers. The observation that only minute amounts of copper are required for the improved

he effective oxidation of carbon monoxide (CO) under ambient conditions remains a technologically relevant challenge and a fundamentally interesting question in heterogeneous catalysis.1 The demand to remove CO from gas streams for polymer electrolyte membrane fuel-cells,2,3 respirator, and automotive exhaust4 applications requires the development of novel materials that catalyze the oxidation of CO at low temperatures. Historically, several heterogeneous catalysts, including nanoparticulate gold on titanium(IV) oxide (Au/TiO2),5 copper(II) oxide on cerium(IV) oxide (CuO/ CeO2),6 and hopcalite (CuMn2O4),7 have been found to have high catalytic activities for this reaction. While CuO/CeO2, first reported by the group of Flytzani-Stephanopoulos,6 is a promising low-cost catalyst for the oxidation of CO, the activity is several orders of magnitude lower than that of Au/ TiO2,5,8 and the physical origin of the activity for such catalysts is still under debate. © 2017 American Chemical Society

Received: May 16, 2017 Revised: August 28, 2017 Published: August 29, 2017 6843

DOI: 10.1021/acscatal.7b01600 ACS Catal. 2017, 7, 6843−6857

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Figure 1. Mechanisms for CO oxidation on CuO/CeO2 catalysts as proposed by Flytzani-Stephanopoulos (A),9 Martı ́nez-Arias (B),12 Harrison (C),13 and the mechanism proposed here (D). Highlighted in color are the surface species proposed to be redox-active for each proposed mechanism.

mechanism. Kinetic data for CO oxidation on a CuO/CeO2 catalyst recorded over a wide range of inlet conditions14 support the mechanistic models proposed by Harrison and Martı ́nez-Arias (Figure 1B and C) better than that by FlytzaniStephanopoulos (Figure 1A). Recently we have shed new light on the nature of the active sites for these catalysts,15,16 showing that atomically dispersed Cu3+ clusters residing on the {111} and {100} surfaces of CeO2 can be responsible for the catalytic rate enhancement of CuO/ CeO2 and that bulk CuO in these materials acts as a spectator species, contrary to previous studies.17,9,18−22 The synthesis of phase-pure and monodisperse nanoparticles of the composition Cu0.1Ce0.9O2−x enabled us to fully characterize the chemical state of the copper sites in these catalysts using X-ray absorption spectroscopy.15 Copper K-edge X-ray absorption near-edge spectroscopy (XANES) revealed an edge position 3.2 eV higher in energy than that of CuO, being more consistent with those of the formally Cu3+ compounds KCuO2 and YBa2Cu3O7. Additionally, the Cu−O bond distance, as determined by copper K-edge extended X-ray absorption fine structure (EXAFS) analysis (1.93 Å), was found to be intermediate between those found for KCuO2 (1.85 Å) and CuO (1.95 Å). The copper LII,III-edge XAS also revealed the presence of satellite peaks consistent with formally Cu3+ sites. On the basis of these data, we proposed that the copper sites in Cu0.1Ce0.9O2−x consist of a combination of Cu2+ and Cu3+ species. Using a combination of STEM-EDS and density

catalytic rate enhancement of CeO2 and that the activity of these catalysts decreases with copper contents greater than 5% seems to support this conclusion; catalysis is not limited by CO adsorption but, rather, by the redox chemistry occurring at the interfacial zone localized at Ce4+ centers.10 Martı ́nez-Arias and co-workers used electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), Raman spectroscopy, and other techniques to further investigate interactions between dispersed Cu2+ ions, lattice O2−, and CeO2 under CO and O2 atmospheres.11 In contrast to the Flytzani-Stephanopoulos mechanism, the investigators demonstrated that Cu2+ sites are directly involved in redox. The synergetic interaction between copper and cerium sites, they proposed, increases the reducibility of both Ce4+ and Cu2+, which in turn enhances CO oxidation catalysis.12 In this case, the oxidation of CO is facilitated by the redox synergy between Cu2+ and Ce4+, each of which is reduced by one electron during the oxidation step (step 2, Figure 1B). Finally, Harrison et al. used in situ EPR spectroscopy, among other techniques such as X-ray absorption spectroscopy (XAS), to monitor changes in the oxidation state and chemical structure of Cu2+ ions in CuO/CeO2 under catalytically relevant conditions for CO oxidation.13 These studies found that Cu2+ sites can be reversibly reduced under relatively mild atmospheres of CO. In light of these studies, a new mechanism (Figure 1C) was proposed in which Cu2+ centers are solely responsible for O2− transfer to Cu-bound CO, in contrast to the Martı ́nez-Arias 6844

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that dispersed Cu3+ sites are responsible for O2− transfer. Importantly, we propose that the oxygen-ion vacancy formation energy, as calculated by density functional theory (DFT), serves as a suitable descriptor for CO oxidation on CeO2-based catalysts, providing guidelines for catalyst design and capturing an activity span over three orders of magnitude for transitionmetal substituted ceria nanoparticles (M0.1Ce0.9O2−x, M = Mn, Fe, Co, Ni and Cu).

functional theory, we demonstrated that copper preferentially segregates to the surface in CuyCe1−yO2−x compounds in the form of small, μ-O2−-bridged Cu3+ clusters.16 The kinetics for CO oxidation on a library of phase-pure CuyCe1−yO2−x and mixed-phase CuO/CuyCe1−yO2−x were consistent with these dispersed Cu3+ serving as the active sites for catalysis. Since we proposed that dispersed Cu3+ sites can be catalytically active, rather than the Cu+ and Cu2+ sites invoked by the above three mechanisms (Figure 1A−C), we sought to investigate the mechanism of CO oxidation on these catalysts, and whether the Cu3+ centers reversibly participate in redox. We aim to address the specific role of the copper centers during CO oxidation catalysis in order to develop design descriptors for catalytic activity. Such a descriptor-based approach to catalyst design hinges on linear free energy scaling relationships and have been successfully employed with perovskite oxide type catalysts for such diverse transformations as CO oxidation23,24 and oxygen electrocatalysis,25,26 but to the best of our knowledge, such an analysis has not been carried out for state-of-the art ceria-based catalysts for CO oxidation. Here we employ phase-pure, monodisperse transition-metal-substituted CeO2 (M0.1Ce0.9O2−x, M = Mn, Fe, Co, Ni and Cu) nanoparticles developed recently15 to conduct a rigorous analysis of design descriptors for CO oxidation catalysis since the local structure of the dispersed trivalent transition-metal sites are analogous, sharing square-planar motifs.15 In this study, we interrogate the mechanism and the origin of the catalytic activity of Cu0.1Ce0.9O2−x nanoparticles for CO oxidation by means of a suite of in situ spectroscopic techniques. We take advantage of recent developments in in situ synchrotron X-ray spectroscopic techniques to probe changes in the oxidation state and the local atomic structure of surface copper sites under catalytic conditions using X-ray absorption spectroscopy (XAS).27 Employing XAS under catalytically relevant conditions serves to verify if the proposed dispersed Cu3+ active sites participate directly in redox chemistry during CO oxidation. We track the evolution of surface adsorbates on Cu0.1Ce0.9O2−x during CO oxidation using ambient pressure X-ray photoelectron spectroscopy (APXPS)28−30 and in situ diffuse reflectance infrared spectroscopy (DRIFTS).31 Although these in situ spectroscopy techniques have been employed to probe surface dynamics under CO oxidation for noble-metal-based catalysts,32−38 the literature applying these techniques to ceria-based materials for CO oxidation is sparse. Previously, researchers have employed both XAS and XPS ex situ to investigate redox processes centered around copper sites in CuO/CeO2 subjected to reductive and oxidative pretreatments.18,39−42 Also, in situ DRIFTS has been employed to investigate structure sensitivity in copper-doped ceria, where Cu−CO stretches were used to correlate metal− support interactions with catalytic activity.43 While such studies have been important for developing structure−activity relationships, a complete understanding of the origin of the catalytic activity (and hence its optimization) for ceria-based materials requires a rigorous evaluation of the evolution of both copper sites and surface-intermediates. Here we propose that Cu0.1Ce0.9O2−x follows a Mars−van Krevelen mechanism for CO oxidation in which the ratedetermining step includes the formation of an oxygen vacancy, a key energetic penalty associated with the regeneration of active copper sites. On the basis of AP-XPS and in situ DRIFTS results, we demonstrate that lattice O2− from Cu0.1Ce0.9O2−x directly participates in CO oxidation, while in situ XAS suggests

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EXPERIMENTAL SECTION General Experimental Considerations. All reagents were purchased from commercial vendors and were used without further purification. Powders of M0.1Ce0.9O2−x (M = Mn, Fe, Co, Ni, Cu) were obtained from the pyrolysis of heterobimetallic Schiff-base precursors as detailed in our previous work.15 The ternary oxides (Cu0.1Ln0.1Ce0.8O2−x, Ln = La, Pr, Sm, Dy, Er) were prepared in a similar manner by using a stoichiometric amount of the lanthanide nitrate with respect to cerium in the reaction mixture. A portion of 1 wt % Au/TiO2 (AUROlite) was purchased from Strem Chemicals and sieved to 300 μm before catalysis. CO Oxidation Catalysis. Kinetic measurements of CO oxidation on M0.1Ce0.9O2−x and Cu0.1Ln0.1Ce0.8O2−x nanoparticles were performed in a homemade 3.81 mm i.d. quartz plug-flow reactor. For each measurement, the catalyst powder (14−68 mg) was mixed with 1.705 g oven-dried sand (Vbed = 1.09 cm3) and loaded into the center of the quartz tube along with a K-type thermocouple. The remaining volume of the quartz tube was filled with oven-dried sand. The compositions of the feed and downstream gases were obtained by online gas chromatography (Agilent 490 with a COX column and thermal conductivity detector). After cooling the catalyst bed to room temperature, a stream of 7.6 Torr CO and 19 Torr O2 balanced in He (733.4 Torr) was passed through the catalyst with the use of mass-flow controllers. The flow rate was set to around 1300 mL min−1 g−1 for each catalyst measured. Measurements of the partial pressure dependence of Cu0.1Ce0.9O2−x at 300 °C were performed with flow-rates of 190 000 mL min−1 g−1 in order to get percent conversions below 12.5% at 300 °C. The catalyst was gradually heated, recording gas chromatographs and catalyst bed temperatures every 5 °C. The rates of reaction were calculated according to the methods described in the Supporting Information, and only data between 3% and 12.5% conversion were used to construct Arrhenius plots to minimize the effects of perturbing the partial pressures of CO and O2 during catalysis. For partial pressure dependence studies, catalysts were allowed to equilibrate at the desired bed temperature and gas composition for 15 min before recording gas chromatographs. The kinetic orders with respect to CO were measured at a constant partial pressure of O2 (19 Torr for O2-rich conditions and 3.8 Torr for O2-poor conditions) while varying the partial pressure of CO (7.6−19 Torr for both conditions), and those with respect to O2 were measured at a constant partial pressure of CO (7.6 Torr for both conditions) while varying the partial pressure of O2 (15.2−26.6 Torr for O2-rich conditions and 1.14−7.6 Torr for O2-poor conditions) while keeping the total pressure 760 Torr with a He balance. In Situ X-ray Absorption Spectroscopy (XAS). X-ray absorption measurements at the copper K-edge were performed at the bending magnet station X11A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory.44 The electron storage ring operated at 2.8 GeV with a stored current in the range of 200−300 mA. The 6845

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which were fitted using a combined Gaussian/Lorentzian line shape according to the fitting model outlined in Tables S3 and S4. Binding energy calibration was performed by fitting the main feature of Au foil (Au 4f 7/2) to a binding energy of 84.0 eV. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). DRIFTS measurements were performed in the mid-infrared with a Bruker Vertex 70 FT-IR spectrophotometer equipped with a Praying Mantis DRIFTS accessory (Harrick) using a high-temperature environmental chamber with KBr windows (Harrick model HVC-DRP-4). Approximately 110 mg of the Cu0.1Ce0.9O2−x powder (400 mesh) was loaded into the environmental chamber for the in situ studies and was slowly heated to the desired temperature under a flow of 100 mL min−1 He. Background spectra were collected at the desired temperature under He by averaging 32 scans. For each temperature condition, the gas was switched to the desired partial-pressure condition at a constant flow rate of 100 mL min−1, and 32 sample scans were acquired after equilibrating for 15 min. Computational Methods. The energies of {111}-terminated Cu2(4−x)+Ce34O72−x slabs were calculated using DFT employing GGA + U along with PAW pseudopotentials, as described previously.15 All calculations were performed using either the Cray XE6 (“Hopper”) or Cray XC30 (“Edison”) supercomputers at the National Energy Research Scientific Computing Center (NERSC). Models for the intermediates were based off of the most stable Cu23+Ce34O71 model described previously, in which the two copper sites are bridged at the ceria surface via a μ-O2− ligand.16 Atoms of carbon and oxygen were added and removed according to the mechanism. As in our previous studies,16,15 a Hubbard Ueff on-site correction term of 4.0 eV was applied to the 4f orbitals of cerium to allow for an accurate description of the electronic structure of oxidized and reduced ceria. Also, a dipole correction was applied to the local potential to correct for systematic errors arising from the periodic boundary conditions. Electronic and ionic optimization of the slabs was carried out using the conjugate gradient algorithm with a planewave cutoff of 400 eV within the VASP suite.50,51 All atomic layers were allowed to relax until all the forces acting on the atoms reached a value below 0.01 eV Å−1. Owing to the large size of the slabs studied here, all data were reported from the integration of the Brillouin zone at the Γ point only. The Gibbs energy of formation (ΔGF) for slab models of M2(4−x)+Ce34O72−x (M = Mn, Fe, Co, Ni, and Cu) was estimated as a function of oxygen partial pressure employing a strategy previously reported by Reuter et al., taking into account the pressure- and temperature-dependent chemical potential of an oxygen atom.52 Details can be found in the Supporting Information. Values for Evac for M23+Ce34O71 models (M = Mn, Co, Ni, and Cu) were determined by the difference in energy of the slabs:

excitation energies were selected with a double crystal monochromator (Si-(111)), which was detuned by 40% to suppress higher harmonics. The incident and transmitted beams were monitored using ionization chambers equilibrated with appropriate mixtures of nitrogen and argon gas. The energy calibration of the monochromator was set by calibrating the inflection point of the absorption spectrum of copper foil to its literature value.45 Copper K-edge spectra were acquired in fluorescence yield (FY) mode using a resistively heated in situ catalyst furnace equipped with a 5-grid Lytle fluorescence detector46 (both from the EXAFS Company). For FY measurements, the signal passed through a silver Soller slit assembly prior to detection by the ionization chamber, which had a continuous flow of Ar. Pellets were prepared by first sieving Cu0.1Ce0.9O2−x (10.5 wt %), boron nitride (75.5 wt %), and Vulcan XC-72 carbon (14 wt %, Cabot) to 400 mesh followed by thorough mixing and grinding with an agate mortar and pestle. Vulcan XC-72 carbon was included in order to increase the surface area and gas exchange through the dense, pressed pellets. 50 mg pellets (5 mm × 12 mm) were pressed and introduced into the catalyst furnace, the window of which was sealed with Kapton tape. Gas mixtures (CO and O2 balanced in He) were flowed through the catalyst pellet by means of mass-flow controllers at flow-rates of 50 mL min−1. Pellets were slowly heated to 300 °C under O2rich conditions (7.6 Torr CO and 19 Torr O2 at a flow rate of 50 mL min−1), and gases were allowed to equilibrate for 15 min before the acquisition of XAFS spectra for each partial-pressure condition. Absorption spectra were normalized using the Autobk algorithm found in the IFEFFIT program47 of the Horae XAFS analysis suite.48 First, a linear fit of the pre-edge line was subtracted from the spectrum. A fourth-order knot-spline polynomial was used to fit the postedge line and the edge step was normalized to unity. Prior to Fourier transform, the EXAFS was multiplied by a Hanning window covering the first and last ∼10% of the data range. The model used for EXAFS fitting is described in the Supporting Information. The first coordination sphere of all seven spectra were refined simultaneously in R space (1−3 Å) using a model including two scattering paths (Cu−O for an oxide and Cu−Cu for metallic copper). A summary of quantitative results from EXAFS fitting is presented in Table S2 of the Supporting Information. The oxidation state of copper under different conditions was estimated from the measured absorption edge energy E0 using a calibration curve constructed from the E0 of known oxides of copper (Cu2O, CuO, and KCuO2) recorded at the same beamline. E0 was taken as the energy at half the edge jump in the Cu K-edge XANES spectra. Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS). AP-XPS studies were carried out at beamline 9.3.2 at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL).49 M0.1Ce0.9O2−x powders (400 mesh) were sonicated in acetone, dropcast onto gold foil and were gently heated to remove excess solvent. After introduction into the sample chamber, each sample was exposed to an atmosphere of 75 mTorr O2 and was slowly heated to 400 °C. Monitoring the C 1s region, samples were heated until no more surface carbon contamination was detected and was cooled to the desired temperature of 300 °C. C 1s, O 1s and Au 4f spectra were acquired after letting the desired partial pressures of CO and O2 equilibrate in the chamber for 15 min. Shirley background correction was applied to the photoemission lines,

Evac = E(M 2 2 +Ce34O70 ) +

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1 E(O2 ) − E(M 2 3 +Ce34O71) 2

RESULTS AND DISCUSSION CO Oxidation Measurements of M0.1Ce0.9O2−x Catalysts. In order to quantify trends in the catalytic activity of M0.1Ce0.9O2−x nanoparticles, CO oxidation catalysis was performed under oxygen-rich conditions in a fixed-bed tube reactor. The light-off curves of M0.1Ce0.9O2−x nanoparticles (M 6846

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Figure 2. CO oxidation catalysis on M0.1Ce0.9O2−x nanoparticles. Light-off curves (A) and site-normalized Arrhenius plots (B) for catalysis under oxygen-rich conditions (7.6 Torr CO + 19 Torr O2 + 733.4 Torr He) in a fixed-bed tube reactor. The partial pressure dependence (C) of CO and O2 on the CO oxidation rate Cu0.1Ce0.9O2−x at 75 °C under oxygen-rich conditions (PCO = 7.6 Torr or PO2 = 19 Torr while varying PO2 or PCO, respectively) and at 300 °C (D) under oxygen-rich and -poor conditions (PCO = 7.6 Torr or PO2 = 3.8 Torr while varying PO2 or PCO, respectively). Rates were measured with 14−68 mg of catalyst at a constant flow rate of 1300 mL min−1 g−1. TOFs were estimated assuming homogeneous transition-metal substitution in M0.1Ce0.9O2−x at conversions less than 12.5%.

= Mn, Fe, Co, Ni, and Cu, d ≈ 3 nm) for CO oxidation with catalyst loadings of 14−68 mg and a flow-rate of 1300 mL min−1 g−1 are shown in Figure 2A. Notably, Ni0.1Ce0.9O2−x and Cu0.1Ce0.9O2−x nanoparticles completely oxidize carbon monoxide at temperatures nearly 100 °C lower than Mn0.1Ce0.9O2−x, Fe0.1Ce0.9O2−x, and Co0.1Ce0.9O2−x. We further compare the estimated intrinsic catalytic activity per transition-metal site (the turnover frequency, TOF) among these catalysts by taking into account the surface area and percent substitution of the transition-metal at the surface of ceria (Figure 2B). The TOF was estimated for all metals according to a model where the metal sites substitute for Ce4+ sites at the surface of a nanoparticle composed entirely of {111} facets (see the Supporting Information for a more detailed discussion on the estimation of TOF). While this model is supported by previous aberration-corrected STEM-EELS studies on Cu0.1Ce0.9O2−x,16 it does not take into account possible differences in metal ion dispersion on the surface relative to bulk between the different metals. Previously, we have estimated that this heterogeneity could account for up to a factor of 0.6 difference in the estimated TOF. Since the TOFs in Figure 2B differ by more than a factor of 0.6 we believe the comparison to be fair. The intrinsic rate follows the trend of Cu > Ni ≫ Co ≈ Mn > Fe, with a difference of at least three orders of magnitude between Cu0.1Ce0.9O2−x and Fe0.1Ce0.9O2−x. Consistent with the trend in the TOF, the apparent activation energies (EA) increase nearly monotonically from the most active Cu0.1Ce0.9O2−x (40 kJ mol−1) to the least active Fe0.1Ce0.9O2−x (EA = 100 kJ mol−1, Table S1 of the Supporting Information). For comparison, previous studies on CuO/CeO 2 and

Cu0.01Ce0.99O2−x catalysts under similar conditions have measured activation barriers of 41 and 70 kJ mol−1 , respectively.10 Additionally, as we have noted before,15,16 both the mass-normalized catalytic activity of Cu0.1Ce0.9O2−x (0.1 μmol CO s−1 gcat−1 at 35 °C) and its TOF (0.07 s−1 at 120 °C) are comparable to literature values found for optimized CuO/CeO2 catalysts (0.12 μmol CO s−1 gcat−110 and 0.09 s−1,21 respectively). On the basis of these observations, and our previous studies on mixed-phase CuO/CuyCe1−yO2−x,16 we propose that the high catalytic activity of Cu0.1Ce0.9O2−x does not require a crystallographic interface between CuO and CeO2. Rather, dispersed Cu3+ sites on the surface of the singlephase Cu0.1Ce0.9O2−x catalyst are responsible for the adsorption and subsequent oxidation of CO by adjacent surface oxygen ions. The CO oxidation rate on M0.1Ce0.9O2−x was found generally to increase with CO partial pressure, having the strongest dependence (PCO1.2) for the most active Cu0.1Ce0.9O2−x, as shown in Figure 2C (Figure S1 and Table S1 of the Supporting Information), and negligible dependence on oxygen partial pressure at oxygen-rich conditions. A similar partial pressure dependence was found for Cu0.1Ce0.9O2−x under oxygen-rich conditions at elevated temperatures such as 300 °C (Figure 2D). These data suggest that the presence of preadsorbed CO on the surface contributes to the rate-determining step of the reaction mechanism, while the adsorption of oxygen does not. These observations are consistent with a Mars−van Krevelen type mechanism for CO oxidation (Figure 1D) with step 2 as the rate-determining step in which the formation of an oxygen vacancy serves as a major enthalpic hindrance to catalysis. An 6847

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Figure 3. Investigating the dynamics of the copper site in Cu0.1Ce0.9O2−x by in situ Cu K-edge XAS: XANES spectra (A), copper oxidation state as a function of partial pressure of O2 (B), k2-weighted magnitude of the Fourier transform of the EXAFS (C), and structural changes of Cu0.1Ce0.9O2−x (D) under oxygen-rich (7.6 Torr CO, 19 Torr O2), stoichiometric (7.6 Torr CO, 3.8 Torr O2), and oxygen-poor conditions (7.6 Torr CO, 0 Torr O2). Spectra were collected in fluorescence yield mode at 300 °C on BN/C-supported pellets after an equilibration period of 15 min in each gas stream.

Previous measurements on CuO/CeO2 under oxygen-rich conditions found the same first-order dependence for CO (PCO1) and nearly zeroth-order dependence on O2 (PO20.08).9 It should be noted that, assuming step 2 is rate-determining in each case, each of the four proposed mechanisms (Figures 1A− D) would afford similar rate laws. The respective roles of the copper and cerium sites cannot be determined from the empirical rate law by itself. In order to investigate the role of dispersed copper sites in supplying oxidative equivalents for catalysis, we turned to in situ X-ray absorption spectroscopy performed under CO oxidation conditions. In Situ XAS of Cu0.1Ce0.9O2−x during CO Oxidation. In order to probe whether the atomically dispersed copper sites are the origin of oxidative equivalents for CO oxidation, we turned our attention to in situ X-ray absorption spectroscopy (XAS). The Cu K-edge near-edge spectra (XANES) of Cu0.1Ce0.9O2‑x were largely unchanged upon introduction of an oxygen-rich reaction mixture (7.6 Torr CO and 19 Torr O2) into the sample cell and subsequent heating compared to roomtemperature air conditions (Figure S2), where the oxidation state of copper (2.8+) under this catalytic condition remained close to 3. Spectral changes began to occur in the XANES at 300 °C as the gas stream became more reducing (Figure 3A, PCO = 7.6 Torr and PO2 = 0−19 Torr). Due to the low catalyst loading required for these measurements and limitations with the mass-flow controllers, the temperature was set to 300 °C in order to achieve 10% conversion. As the partial pressure of oxygen was decreased below the stoichiometric amount (3.8 Torr), the edge shifted to lower energies with the appearance of a feature at 8983 eV and the disappearance of the white line at 8998 eV (Figure 3A). Calibrated to known copper oxide

exception to this is the partial pressure dependence for the Co0.1Ce0.9O2−x catalyst (Figure S1C). We found the reaction to be zeroth-order with respect to CO and fractional order with respect to O2 (PO20.7). This could indicate that the ratedetermining step with Co0.1Ce0.9O2−x catalysts involves filling rather than forming the oxygen vacancy (step 4 in Figure 1D), which is analogous for Cu0.1Ce0.9O2−x under oxygen-poor conditions. The rate law for a Mars−van Krevelen mechanism of CO oxidation under oxygen-rich conditions with the formation of an oxygen vacancy as the rate-determining step (step 2, Figure 1D) would be (see the Supporting Information for derivation): r=

k 2K CO adsPCO 1 + K CO adsPCO

where KCO ads is the equilibrium constant for the adsorption of CO by copper centers and k2 is the forward rate constant for step 2 of the mechanism. At sufficiently low partial pressures of CO KCO adsPCO ≪ 1 and the derived rate law is first-order with respect to carbon monoxide and zeroth-order with respect to O2, consistent with Figures 2C and S1. Under oxygen-poor conditions at 300 °C (Figure 2D), we observed a weaker dependence of the reaction rate on CO pressure (PCO0.2) and a stronger dependence on O2 (PO20.3). This trend can be rationalized by the same mechanism (Figure 1D) while taking the reoxidation of the catalyst surface (step 4) as the ratedetermining step. The rate law (see the Supporting Information) derived in this manner gives a kinetic order of 1/2 with respect to O2 and a nonzero, positive order with respect to CO. 6848

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ACS Catalysis

Figure 4. Surface intermediate dynamics as measured by ambient pressure XPS. Fitted AP-XPS spectra at four partial-pressure conditions at the (A) O 1s and (B) C 1s regions at 300 °C. Only spectra acquired during the first cycle are shown in A and B for clarity. The incident energies used for APXPS were 650 eV (O 1s) and 490 eV (C 1s).

formally Cu3+ (1.85 Å for KCuO2) and Cu2+ compounds (1.95 Å for CuO). The intermediate Cu−O length is consistent with the XANES measurements, and our hypothesis that the copper sites in Cu0.1Ce0.9O2−x consist of a linear combination of Cu2+ and Cu3+ species under oxygen-rich conditions. Simultaneously, a new feature at 2.0 Å reduced distance (2.47 Å real distance) appears under oxygen-poor conditions, which can be attributed to Cu−Cu scattering. The Cu−Cu distance is much shorter than typical Cu−Cu distances found in the oxides of copper (3.01, 2.88, and 2.71 in Cu 2 O, CuO, and KCuO 2 , respectively)53−55 or that estimated for edge-sharing CuO4 clusters in Cu0.1Ce0.9O2−x (3.13 Å) from our previous publication.15 While shorter than the Cu−Cu bond length found in bulk copper (2.56 Å),56 we ascribe the feature at 2.47 Å to Cu−Cu scattering in very small metallic copper clusters that form under reducing atmospheres. Indeed, the Cu−Cu bond length shortens with decreasing copper particle size, with lengths found to be as low as 2.23 Å in Cu2 clusters and 2.33 Å in 5 Å copper clusters.57,58 In agreement with the XANES analysis in Figure 3B, fitting of the EXAFS shows the reversible transformation between square-planar Cu3+O4 on the surface of Cu0.1Ce0.9O2−x under oxygen-rich conditions and metallic copper clusters under oxygen-poor conditions (Figure 3D and Table S2). Under oxygen-rich conditions, copper is fully

standards, these data are consistent with reduction at the copper centers by at least one electron (Figure 3B). Readers should note that the estimated average oxidation state of the fully reduced catalyst may be lower than that reported in Figure 3B given the results from EXAFS fitting (vide infra). The edge energy of Cu foil (not used for oxidation state estimation) and that of Cu2O are nearly identical and therefore the actual oxidation state of the fully reduced catalyst may be closer to zero. Nevertheless, the reduction at the copper appears to be completely reversible since the reintroduction of an oxygen-rich stream reverses the changes observed at the near edge. The existence of two isosbestic points at 8990 and 9010 eV also suggests that two copper species are present under these conditions. Structural changes in the coordination environment of copper in Cu0.1Ce0.9O2−x during CO oxidation conditions were tracked as a function of oxygen partial pressure using extended X-ray absorption fine structure (EXAFS). The magnitude of the main feature of the Fourier transform of the EXAFS at 1.5 Å reduced distance (1.88 Å real distance), which we have ascribed to Cu−O scattering paths in the first coordination shell of square-planar CuO4 motifs,15 decreases when lowering the oxygen partial pressure (Figure 3C). The Cu−O length is intermediate between those expected for 6849

DOI: 10.1021/acscatal.7b01600 ACS Catal. 2017, 7, 6843−6857

Research Article

ACS Catalysis

1s regions, which we do not detect here.60 This is in accordance with the in situ DRIFTS data (vide infra) in which HCO2− is not detected. When the partial pressure of oxygen was reduced below the stoichiometric (