In Situ Spectroscopy and Mechanistic Insights into CO Oxidation on

Aug 29, 2017 - Herein we investigate the reaction intermediates formed during CO oxidation on copper-substituted ceria nanoparticles (Cu0.1Ce0.9O2–x...
3 downloads 23 Views 16MB Size
Download PDF
Subscriber access provided by University of Rochester | River Campus & Miner Libraries

Article

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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01600 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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

∆

O

Department of Chemistry, Research 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 Blvd., West Bethesda, Maryland 20817-5700 United States

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 rate-determining 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. The 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-cell,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 7 (CuMn2O4) 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. 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 storagecapacity 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

1 ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 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 Martínez-Arias and coworkers used EPR, XPS, 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 Martínez-Arias 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 Martínez-Arias (Figures 1B and C) better than that by Flytzani-Stephanopoulos (Figure 1A). Recently we have shed new light on the nature of the active sites for these catalysts,15,16 showing that atomicallydispersed 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 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 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 functional theory, we demonstrated that copper preferen-

Page 2 of 25

tially 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 (AP-XPS)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 metalsupport interactions with catalytic activity.43 While such studies have been important for developing structureactivity relationships, a complete understanding of the origin of the catalytic activity (and hence its optimization) for ceria-based materials requires a rigorous evalua-

2 ACS Paragon Plus Environment

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

tion of the evolution of both copper sites and surfaceintermediates. 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 oxida-

tion, while in situ XAS suggests that dispersed Cu3+ sites are responsible for O2- transfer. Importantly, we propose that the oxygen-ion vacancy formation energy, as calculated by 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 transition-metal substituted ceria nanoparticles (M0.1Ce0.9O2-x, M = Mn, Fe, Co, Ni and Cu).

9

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

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. 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 home-made 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 ovendried sand. The compositions of the feed and down-

3 ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

stream gases were obtained by on-line gas chromatography (Agilent 490 with 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 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 x 12 mm) were pressed and introduced into the

Page 4 of 25

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 massflow controllers at flow-rates of 50 mL min-1. Pellets were slowly heated to 300 °C under O2-rich 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 knotspline polynomial was used to fit the post-edge 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, 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 en-

4 ACS Paragon Plus Environment

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

vironmental 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 Cu(4-x)+2Ce34O72-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 Cu3+2Ce34O71 model described previously, in which the two copper sites are bridged at the ceria surface via a µ-O2ligand.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 (∆GF) for slab models of M(4-x)+2Ce34O72-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 M3+2Ce34O71 models (M = Mn, Co, Ni, and Cu) were determined by the difference in energy of the slabs: 1 ‫ܧ‬௩௔௖ = E(M2+2Ce34O70) + E(Oଶ ) − E(M3+2Ce34O71) 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 = 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 degrees 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/CeO2 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’ve 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-1,10 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 doesn’t require a crystallographic interface between CuO and CeO2. Rather, dispersed Cu3+ sites on the surface of the single-phase 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 pre-adsorbed 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 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

5 ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with respect to CO and fractional order with respect to O2 (PO20.7). This could indicate that the rate-determining 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.

A

B

M0.1Ce0.9O2-x

PCO = 7.6 Torr PO = 19 Torr

M0.1Ce0.9O2-x

PCO = 7.6 Torr PO = 19 Torr

2

2

Cu

Cu Ni Mn

Ni Co Co

Fe

C

Fe

Mn

D Cu0.1Ce0.9O2-x

75 °C

Cu0.1 Ce0.9O2-x

300 °C

O2 lean nCO = 0.2 nO2 = 0.3

O2 rich nCO = 1.1 nO2 = -0.1

nCO = 1.2 PO = 19 Torr 2

n O = 0.0 2

PCO = 7.6 Torr

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 oxygen-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 -1 -1 min g . TOFs were estimated assuming homogeneous transition-metal substitution in M0.1Ce0.9O2-x at conversions less than 12.5%.

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 Supporting Information for derivation): k2 KCO ads PCO r= 1 + KCO ads PCO 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