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Article 2

Real-Time Observation of the Defect Dynamics in Working Au/CeO Catalysts by Combined Operando Raman/UV-Vis Spectroscopy Christian Schilling, and Christian Hess

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00027 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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The Journal of Physical Chemistry

Real-Time Observation of the Defect Dynamics in Working

Au/CeO2

Catalysts

by

Combined

Operando Raman/UV-Vis Spectroscopy

Christian Schilling1, Christian Hess1*

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Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität

Darmstadt, Alarich-Weiss-Str. 8, 64287 Darmstadt, Germany *[email protected]

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Abstract Supported gold catalysts are highly active for a variety of reactions including low-temperature CO oxidation. It has been shown that reducible support materials, e.g. ceria or titania, may significantly alter the catalytic performance. In this contribution, we provide the first direct evidence for ceria (CeO2) support dynamics and its relevance in Au/CeO2 catalysts during room temperature CO oxidation. In particular, combined operando Raman and UV-Vis spectroscopy are employed to monitor the surface and subsurface defect dynamics of ceria quantitatively and in real time. The results clearly show a dependence of catalytic activity on the reduction state of the ceria support. In fact, the pre-reduced CeO2 catalyst support increases the activity during CO oxidation initially by 100%. The reduction is not limited to the CeO2 surface but also affects the CeO2 subsurface due to oxygen mobility and charge transfer in CeO2-x. Our results highlight the enormous importance of the support properties for a mechanistic understanding of oxidation reactions over metal/ceria catalyst materials.


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1. Introduction Ceria (CeO2) is one of the most important materials in catalysis, solid oxide fuel cells, and oxygen membranes.1 A very prominent application is the use of ceria-based solutions as oxygen buffers in three-way catalytic converters for gasoline-powered vehicles.2 Ceria is most often employed as a support for active noble metal particles and clusters such as Pt,3 4 5 Rh,6 and Au5

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for oxidation reactions such as CO oxidation,4 water-gas shift,5

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ethanol

reforming6 or preferential CO oxidation.8 Despite their relevance, there is still no detailed mechanistic understanding of the mode of operation of these catalysts. While on one hand it is generally accepted in the literature that for Au/CeO2 catalysts (i) the gold-support interface perimeter acts as the active site 5 9 (ii) O2 activation is mediated by vacancies of the support, 10 11

on the other hand (iii) the dynamical behavior of the reduction state of the ceria support

as well as its dependence on pretreatment conditions have not been addressed previously in a direct (time-resolved) fashion. It is well-known from the literature that the activity of Au/CeO2 catalysts strongly depends on the pretreatment procedure.12 To this end, temporal analysis of products (TAP) experiments at Au/CeO2 powder catalysts at 120°C showed a slow activation of the catalyst after outgassing of the sample at 400°C, but a fast activation and adjustment of steady-state activity after prior CO reduction and re-oxidation with O2 at 120°C. For prior CO treatment without re-oxidation an increase in CO2 formation accompanied by O2 consumption but decreased CO uptake was observed.13 Zhang et al. reported that a 0.34% Au/CeO2 catalyst exposed consecutively to O2 and H2 at 250°C exhibited the highest activity in CO oxidation as compared to catalysts pretreated by reduction in H2 followed by re-oxidation with O2 or catalysts not pretreated at all.12 Abd-El Moemen et al. studied a 4.5 wt% Au/CeO2 catalyst for CO oxidation at 80°C.14 Pretreatment in H2 at 400°C led to the lowest activity, whereas pretreatment in CO at 400°C resulted in an activity decay to steady-state conversion within 1 h. The observed steady-state conversion was slightly lower than the conversion of samples pretreated in N2 or O2 at 400°C. This brief ACS Paragon Plus Environment

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survey shows that despite its enormous potential to significantly increase the catalytic activity, the role of the pretreatment on the catalyst properties has not been elucidated. A support effect has recently been suggested for Au15 but also for Pt containing ceria catalysts by in situ X-ray techniques.3 4 The oxygen vacancy creation and Ce4+ reduction to Ce3+

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has been successfully modeled by DFT+U methods.18 These studies suggest that Au in direct interaction with the stoichiometric CeO2(111) surface transfers electrons to the ceria support and is charged to yield a Au+ or Auδ+ state, while excess charge is located at the cerium ions.9 11

Recently we have shown by a combined DFT+U and experimental study that Raman

spectroscopy is not only sensitive towards subsurface reduction10 and peroxides19 but also to the CeO2(111) surface and its oxidation state20. In this contribution, we demonstrate the dependence of the catalytic activity on the reduction state of the ceria support by directly monitoring the defect dynamics using operando Raman/UV-Vis spectroscopy. In fact, by catalyst pre-reduction the CO oxidation activity can initially be increased by 100% highlighting the relevance of the ceria support properties for understanding the catalytic activity of metal/ceria catalysts as well as the potential of targeted ceria reduction for improving catalyst performance.

2. Experimental Section 2.1 Sample Preparation The ceria support was synthesized according to previous studies21

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by decomposition of

Ce(NO3)3 (Alfa Aesar, 99.5%) at 600°C for 12 h using a heating rate of 6°C/min. After the sample had cooled to room temperature, a second calcination step was applied employing the same protocol. The specific surface area of these ceria samples was determined as 65 m2/g by N2 adsorption and use of the Brunauer–Emmett–Teller (BET) model. XRD (STOE STADI P) revealed a cubic, fluorite-type structure.


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Gold was deposited onto the support via deposition precipitation as originally published by Haruta et al.23 but with parameters taken from Ref24: 2 g of CeO2 was first suspended in 300 ml of deionized water and the pH value was adjusted to 9 using a 0.1 M NaOH solution. Then a 10-3 M solution of HAuCl4·3H2O (Sigma Aldrich, 99.999%) was adjusted to pH 9 and an appropriate amount of the solution was added to the ceria suspension to obtain a nominal fraction of 0.5 wt% gold on ceria. Subsequently, the suspension was heated to 65°C for 2 h, cooled down to room temperature, and treated for 30 min in a sonicator. The product was centrifuged, washed with 0.25% ammonia solution and water three times and finally dried at 85°C for 48 h. This sample will be referred to as as prepared. Please note that no further calcination was done.

2.2 X-Ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) was performed on a modified LHS/SPECS EA200 system using a Mg Kα source (1253.6 eV, 168 W). XP spectra were recorded under UHV conditions. Calibration was done based on the Au4f signal of gold foil at 84.0 eV and the Cu2p signal of a copper plate at 932.7 eV.25 Sample charging was accounted for by setting the Ce3d u’’’ signal to 916.7 eV.26 This corresponds to a C1s position of 284.7 eV, in agreement with literature values for ubiquitous carbon.26

2.3 Transmission Electron Microscopy Transmission electron microscopy (TEM) characterization was performed with a JEOL JEM2100F (Tokyo, Japan) microscope equipped with a Schottky field emitter operating at a nominal acceleration voltage of 200 kV. Energy dispersive X-ray (EDX) spectra were recorded on an Oxford X-MAX 80 silicon drift detector (Oxford Instruments Nanoanalysis, High Wycombe, UK) attached to the JEM-2100F. Samples were prepared by dispersing a small amount of powder in ethanol using an ultrasound bath (Bandelin) for approximately ACS Paragon Plus Environment

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30 s. The dispersion was allowed to settle for a short time to reduce the amount of large particles in the dispersion. A droplet of the dispersion was applied to a holey carbon grid (Plano) and allowed to dry. The grid was coated with carbon (Baltec MED010) to avoid charging under the incident electron beam.

2.4 Operando Raman/UV-Vis Spectroscopy Raman and UV-Vis spectroscopy was performed in one setup using fiber optics as described previously.21

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Raman spectra were recorded on a HL5R transmission spectrometer (Kaiser

Optical) using 532 nm laser excitation from a frequency-doubled Nd:YAG laser (Cobolt). The spectral resolution is specified as 5 cm-1, however, the stability of the band positions is better than 0.3 cm-1. A Super Notch filter allows detection of Raman shifts starting at about 150 cm1

. The laser power at the sample was adjusted to 1 mW to avoid damage caused by the laser

beam. The sampling time was 200 s in total, resulting from 5 accumulations with 40 s exposure time each. Raman intensities were normalized to the most intense band, the F2g band. UV-Vis spectra were recorded in diffuse reflection mode on an AvaSpec-ULS2048 (Avantes) using D2 and halogen light sources. As white standard, MgO powder was employed in the same geometry as the sample. The sampling time was 60 s, resulting from a 300 ms exposure time and averaging over 200 spectra. As a result, the typical temporal resolution for alternating dynamically between Raman und UV-Vis measurements was 260 s. A Fourier transform IR (FTIR) spectrometer (Tensor 20, Bruker, resolution: 4 cm-1) was attached to the exit of the reaction cell to allow quantitative analysis of the gas-phase composition. Gas-phase spectra are averages of 125 scans measured within 1 min. For operando measurements, the Au/CeO2 catalyst (25–35 mg) was placed in a stainless steel sample holder (Ø: 8 mm, depth: 0.5 mm). The gas feed for reaction was composed of 2 vol% CO (99.997%, Air Liquide) and 10 vol% O2 (99.999% Westfalen) balanced with argon ACS Paragon Plus Environment

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(99.996%, Westfalen) at a total flow rate of 100 ml/min. Catalytic activity is defined as vol% CO2 determined by FTIR analysis at the outlet of the reaction cell divided by vol% CO applied. The calculated activity (CO2 formation) can be treated as a CO conversion because no products beside CO2 are observed. The temperature was continuously monitored by a thermocouple in proximity to the sample, showing a maximum deviation of 1°C from room temperature (21.5 ± 1°C). In the following, volume concentrations of gases (vol%) will be referred to as %. The catalyst sample was equilibrated in 25% O2 in argon at a total flow rate of 100 ml/min. Then the catalyst was exposed directly to reaction conditions (2% CO, 10% O2 balanced with Ar to 100 ml/min), regenerated and exposed to reaction conditions again, which will be referred to as oxidizing pretreatment. For reducing pretreatment the sample was equilibrated as before. Then the cell was cleaned with 100 ml/min Ar for 10 min before the catalyst was exposed first to 2% CO balanced with Ar to 100 ml/min and subsequently to reaction conditions. Finally, the catalyst was regenerated in 25% O2. During the continuous exposure to the gas conditions Raman and UV-Vis spectra were measured in an alternating fashion. The indicated time in Figures 4 and 5 refers to the end of the respective measurement. The simultaneously recorded catalyst activity of the catalyst is then correlated with the respective spectroscopic information (operando approach).

2.5 Activity Measurements The effluent gas stream of the operando reaction cell is analyzed by gas-phase FTIR spectroscopy. A >6-point calibration for CO2 (CRYSTAL mixture of 1.020% ± 0.020% CO2 (99.995%) and N2 (99.999%), Air Liquide) and CO (99.997%, Air Liquide) allows for direct calculation of the concentration of CO and CO2.The reaction mixture contains 2% CO as mixed by digital mass flow controllers. This value is reached about 3 min after feeding the reaction mixture into the reaction chamber (see Figure 1A, 110 min). By fitting an ACS Paragon Plus Environment

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exponentially decaying curve to the CO signal at ca. 360 min we find a t1 time of only 1 min. Thus we conclude that the reaction chamber strongly resembles the behavior of an ideal plug flow reactor although the geometry is much more complicated.21 The conversion can also be derived from the concentration decrease of CO in the gas stream. Note that the conversion is quantitatively identical to the CO2 increase (maximum 6 %). Owing to the rather low IR absorption cross section of CO as compared to CO2, we monitor the formation of CO2 (black dots in Figures 1 A) and B)).

3. Results The 0.5 wt% Au/CeO2 catalyst was characterized by X-ray photoelectron spectroscopy (XPS) and electron microscopy. After Shirley background subtraction, the Au4f photoemission (see Figure S1) revealed two components at 84.0 and 84.9 eV, assigned to Au0 and Au+, respectively.5

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The fraction of Au+ was determined as 30%. Employing relative sensitivity

factors27 yields a surface gold mass fraction of 0.7%. Analysis of the Ce3d signal in accordance with a previous study21 revealed a fraction of 15% Ce3+ for Au/CeO2 as compared to 13% Ce3+ for the bare support, suggesting a charge transfer from gold to the ceria support as proposed by DFT+U calculations of gold adsorption on the oxidized CeO2(111) surface.28 9 The ceria support consists of 10-15 nm sized crystals. The TEM image of the bare ceria support reveals a CeO2(111)29 surface termination as well as stepped sites (see Figure S2). Gold particles in 0.5 wt% Au/CeO2 were characterized in scanning TEM (STEM) mode, enabling

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Ce and

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Au to be distinguished, due to contrast related to the atomic number Z

rather than Bragg diffraction (see Figure S3 A). STEM and TEM images show gold particles with a size of 10 nm (see Figure S3 B). Point EDX of gold particles reveals reflection peaks dominated by gold (see Figure S3 C). There is hardly any indication for surface faceting of the gold particles, whereas characteristic Au(111) peaks are observed after fast Fourier ACS Paragon Plus Environment

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transform (FFT) and application of a mask (see Figures S3 D and E). It is proposed that highly dispersed gold particles in direct contact with the ceria support rather than the particles observable with TEM are responsible for the reactivity as discussed previously.5 Figure 1 shows the effect of pretreatment on room temperature CO oxidation activity of 0.5 wt% Au/CeO2 catalysts. The oxidized catalyst (A) was obtained by oxidizing pretreatment in 25% O2/Ar for 1 h, the reduced catalyst (B) by reducing pretreatment in 2% CO/Ar for ca. 1 h. As shown in Figure 1A) the catalyst undergoes activation during the first 20 min in reaction atmosphere, reaching a maximum of 3.5% conversion, followed by deactivation to reach a quasi-steady state of around 3% conversion, in agreement with previous results.21 Such an induction period was reported in the literature for dry reaction gas mixtures.30 As shown by IR spectroscopy, the deactivation process is accompanied by an increase in the amount of carbonate species.21 Figure 1A) further illustrates that during the second exposure to reaction conditions the catalyst deactivates only slowly and that the activity remains at around 3%. Figure 1B) depicts the behavior of the catalyst after equilibration in 25% O2, cleaning of the cell with argon for 10 min and pretreatment of the catalyst under reducing conditions (2% CO in Ar) for 1 h before exposing the catalyst to reaction conditions. It is remarkable that the catalyst is initially twice as active as compared to the steady-state activity after oxidizing pretreatment (i.e., 6% catalytic activity). After 1 h the catalyst pretreated under reducing conditions is still significantly more active than after oxidizing conditions, i.e., 4% compared to 3% activity. Note that the bare CeO2 support is inactive under reaction conditions at room temperature (21.5°C) as well as at elevated temperature (35°C) after either oxidative or reducing pretreatment. Summarizing, the activity results indicate that the Au/CeO2 catalyst does not behave identically after the different pretreatments. In fact, pre-reduction with CO leads to a dramatic increase in catalyst activity.


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Figure 1: Temporal evolution of the gas-phase composition of 0.5 wt% Au/CeO2 at room temperature. A) Two exposures to reaction conditions (2% CO, 10% O2 balanced with Ar to 100 ml/min, grey background), each after equilibration and oxidation in 25% O2/Ar (white background). B) Exposure to reaction conditions (2% CO, 10% O2, grey background at 160 min) after equilibration in 25% O2/Ar, cleaning of the cell with pure Ar (grey background at 80 min) and reducing pretreatment in 2% CO/Ar. The catalytic activity as derived from the CO2 concentration is indicated by black dots and the CO concentration in vol% of product gas stream by red dots.


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Figure 2. Operando Raman spectra (λex = 532 nm) of 0.5 wt% Au/CeO2 at 21.5°C normalized to F2g showing the low (top) and high (bottom) wavenumber regions. A): Spectra during equilibration (black, 25% O2 /Ar), reaction conditions (light blue, 2% CO and 10% O2 in Ar), treatment in oxidizing conditions (green, 25% O2/Ar), a second exposure to reaction conditions (blue, 2% CO and 10% O2 in Ar), and oxidizing treatment in 25% O2/Ar


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(magenta). B): Spectra during equilibration (black, 25% O2 in Ar), pretreatment in reducing conditions (red, 2% CO/Ar), reaction conditions (blue, 2% CO and 10% O2 in Ar), and oxidizing treatment in 25% O2/Ar (magenta). The Raman spectrum of bare CeO2 (thin black line) was taken in O2/Ar. The indicated color code (F2g, 246 cm-1, 828 cm-1) is used to represent the time-dependent behavior of these modes in Figures 4 and 5.

Figure 2 depicts operando Raman spectra of 0.5 wt% Au/CeO2 for oxidizing pretreatment (A) and reducing pretreatment (B). For comparison, the spectrum of the bare ceria support is shown. Raman spectra are dominated by the F2g band at around 462 cm-1. Red-shifting of the F2g band (as compared to 464.0 cm-1 for bare CeO2) due to oxygen vacancy creation and twoelectron reduction is known from experiment.10 From our DFT+U calculations a red-shift of the F2g band can be rationalized by an expansion of the ceria lattice due to excess electron localization on Ce4+ ions forming Ce3+ ions. Taking the calculated values for the F2g Raman shift position and the corresponding stoichiometry20 one finds a relation between the stoichiometry CeO2-δ and the Raman shift (∆ω) of δ = 0.024 ± 0.005 ∆ω/cm-1 (see Supporting Information). This relation is employed in Figure 3, where the F2g position is monitored over time. Please note that no absolute relation between Raman shift position of the F2g and the stoichiometry is drawn, we rather take the F2g position of bare CeO2 (464.0 cm-1) as the reference state (CeO2-x), and for a F2g shift the deviation is calculated as CeO2-δ-x, whereas x refers to a fixed value accounting for the deviation of the CeO2 stoichiometry of the bare ceria support, i.e., intrinsic defects. A DFT+U study in conjunction with experimental Raman spectra of ceria with different crystal size revealed the dominant component of the band at 246 cm-1 to be assigned to the CeO2(111) surface termination of ceria nanocrystals20 (compare CeO2(111) termination of the CeO2 support in TEM image, Figure S2). The vibration was identified as the longitudinal ACS Paragon Plus Environment

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stretching mode of surface oxygen against cerium ions (Ce-O). Moreover, it could be shown that by reduction of the surface, i.e., introduction of oxygen vacancies and two-electron transfer to Ce4+, the intensity of the band is damped. The band at 402 cm-1 was assigned to the corresponding transversal stretch of surface oxygen against the cerium ion of the CeO2(111) surface. The intensity of both bands (at 246 cm-1 and 402 cm-1) can be evaluated after background subtraction as indicated in the top panel of Figure 2A). However, in the following we will focus on the intensity of the 246 cm-1 band to monitor the oxidation state of the ceria surface owing to the possibility of integration. The bottom panels of Figure 2 depict the region between 700 cm-1 and 2200 cm-1 containing adsorbate-related bands e.g. peroxide bands at 828 cm-1 known as indicators of surface oxygen vacancies besides the ceria-related band at 1170 cm-1.19 The band at 1650 cm-1 can be assigned to a carbonyl C=O stretching mode of carbonates and shows an increase under reaction conditions (i.e., agglomeration of carbonate during reaction). As indicated by the band at 1872 cm-1, a residual amount of water is present in the sample, which remains fairly stable during all experiments. We can consider the green spectrum in the top panel of Figure 2A) for oxidizing conditions and the red spectrum in the top panel of Figure 2B) for reducing conditions as starting points before the catalyst was exposed to reaction conditions. Comparison of these two spectra reveals i) a lowered intensity of the CeO2(111) surface modes at 246 cm-1 and 402 cm-1 for reducing pretreatment, ii) an increase of defect band intensity after reducing pretreatment, iii) a redshift of the F2g band of 2.5 cm-1 (not shown in Figure 2), and iv) the disappearance of the peroxide band at 828 cm-1 after reducing pretreatment. These differences in spectral behavior clearly demonstrate that the surface as well as the subsurface of the CeO2 support is reduced in 2% CO and peroxide species are eliminated from the surface by reaction with CO (see CO2 concentration during reducing pretreatment in Figure 1B)). As stated above, an absolute measure of the stoichiometry based on the F2g Raman position cannot be found. However, the


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difference in the F2g position of 2.5 cm-1 can be related to a stoichiometry difference of δ = 0.064, i.e., CeO1.976-x for oxidizing and CeO1.912-x for reducing pretreatment. The defect bands between 540 cm-1 and 580 cm-1 have been attributed to oxygen vacancy formation in the subsurface of ceria, i.e., the distortion of the CeO2 crystal structure due to oxygen vacancy formation. The prior DFT+U results allow the origins of the bands at 540 cm1

and 560 cm-1 (predicted at 480 cm-1 and 500 cm-1, respectively) to be distinguished.20 The

band in the region 560 cm-1 can be assigned to Ce3+ in the vicinity of an oxygen vacancy, whereas a band at 540 cm-1 points to Ce3+ further away from an oxygen vacancy. Based on the assignments a lower concentration of oxygen vacancies leads to a situation, in which the oxygen vacancy position and Ce3+ ion avoid each other,31 thus giving rise to a band at 540 cm1

. For a higher degree of reduction, Ce3+ ions and oxygen vacancies can no longer avoid each

other, leading to a component at 560 cm-1. The spectra shown in the top panels of Figure 2 reveal an increase of the component at 540 cm-1 upon switching from oxidizing to reactions conditions. The component at 560 cm-1 increases under reducing conditions of 2% CO (red line in Figure 2B). This indicates two degrees of ceria reduction and oxygen vacancy concentration, consistent with the F2g changes as will be discussed below. Complementary information to the Raman spectra is provided by the corresponding operando UV-Vis spectra of 0.5 wt% Au/CeO2 recorded for oxidizing and reducing pretreatment (see Figure 3). UV-Vis spectroscopy allows the electronic properties of the catalyst to be probed. The two UV absorption peaks at 330 nm = 3.8 eV and 255 nm = 4.9 eV originate from O2p(filled) Ce4f(empty) transitions in ceria keeping their position and intensity throughout the experiments. The conduction band is characterized by two peaks of the O2p density of states (DOS), which are located at around 4 and 6 eV below the Ce4f states (determined by ultraviolet photoelectron spectroscopy, UPS). Thus, the two peaks in the UV-Vis spectra result from transitions from these states to the Ce4f states.32


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Upon reduction of bulk ceria (Ce4+ Ce3+), Ce4f states are populated, which are located within the original band gap of CeO2, i.e., between the O2p and the empty Ce4f states.33 Based on X-ray and UPS experiments the energy difference between O2p and Ce4f(filled) has been proposed to be within 1.5–2.5 eV34, and at 2 eV33

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, respectively. So Ce4f(filled)

Ce4f (empty) transitions, i.e., Ce3+ Ce4+ charge transfer transitions, are expected at 1.5–2.5 eV = 820–496 nm. Experiments36 and DFT+U calculations31 evidence that for a low concentration of oxygen vacancies on the CeO2(111) surface, i.e., less than one O vacancy per (2×2) cell, the Ce3+ ions avoid each other and are located in the topmost Ce-layer of the CeO2(111) surface. In this case, the populated Ce4f states are confined within the DOS. For comparison, for one oxygen vacancy per (2×2) cell, the two Ce4f states are separated by about 0.5 eV, as one Ce3+ is located at the surface and the other Ce3+ in the subsurface region, leading to the above-mentioned degeneracy.17 31 For polycrystalline Au/CeO2 nanoparticles a broad band at 570 nm is observed in UV-Vis spectra. This has been assigned to a surface plasmon of the adsorbed gold nanoparticles,8 whereas it could also be attributed to Ce3+ preferably at the CeO2 support surface due to charge transfer from the gold particles to the ceria surface.11 This is corroborated by the XP spectra (i.e., the detection of Au+ species, see Figure S1) and Raman spectra (i.e., the F2g shift from 464.0 cm-1 for bare CeO2 to 463.5 cm-1 for Au/CeO2). A further increase of the band at 570 nm is observed upon exposing the Au/CeO2 catalyst to reducing conditions, indicating reduction of the ceria support in line with the Raman results. Upon exposure to reaction conditions an additional band at 450 nm is observed. After 1 h in the reaction gas mixture (blue spectra) the UV-Vis spectra are fairly identical for both types of pretreatment (see Figures 3A) and B)) as are the Raman spectra in Figure 2. The band at 450 nm is assigned to peroxide species based on a comparison of the UV-Vis with the corresponding Raman spectra (see Figures 2A) and B)). In particular, for reducing conditions (red, 2% CO/Ar), the


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disappearance of the 450 nm band is correlated with the disappearance of the peroxide feature at around 828 cm-1 (see Figure 2B)).

Figure 3. Operando UV-Vis spectra of 0.5 wt% Au/CeO2 at 21.5°C during A) equilibration (black, 25% O2/Ar), reaction conditions (light blue, 2% CO and 10% O2 in Ar), treatment in oxidizing conditions (green, 25% O2/Ar), a second exposure to reaction conditions (blue, 2% CO and 10% O2 in Ar), and oxidizing treatment in 25% O2/Ar (magenta), and during B) equilibration (black, 25% O2/Ar), pretreatment in reducing conditions (red, 2% CO/Ar), reaction conditions (blue, 2% CO and 10% O2 in Ar), and oxidizing treatment (magenta, 25% O2/Ar). The color code used to distinguish the different experimental conditions corresponds to Figure 2. The UV-Vis spectrum of bare CeO2 (dashed line) was recorded in O2/Ar. The indicated color code (570 nm) is used to represent the time-dependent behavior of this band in Figure 4.


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Figure 4. Time-dependent Raman and UV-Vis operando spectroscopic information for the ceria subsurface reduction state of 0.5 wt% Au/CeO2. A) During two consecutive exposures to reaction conditions. B) During reaction after reducing pretreatment. Black squares indicate the catalytic activity of the catalyst, blue squares the position of the F2g Raman mode, and brown squares the UV-Vis reflectivity at 570 nm. The scales are identical for both graphs.

To understand the dynamic surface and subsurface CeO2 support properties during reaction we performed time-resolved operando Raman and UV-Vis measurements, while simultaneously recording the catalytic activity by IR gas-phase spectroscopy. In Figures 4 and 5 each data point refers to spectroscopic information extracted from a Raman or UV-Vis spectra. First, we will focus on the spectroscopic information obtained for the ceria subsurface for oxidative pretreatment (see Figure 4A)). A redshift of the F2g band from 463.5 cm-1 by 1.5 cm-1 is observed when the catalyst is exposed to reaction conditions. The F2g position shifts gradually in accordance with the increase in the activity of the catalyst. Moreover a gradual increase in 570 nm reflectivity is observed followed by only a small dynamic afterwards. The


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F2g positions shift of 1.5 cm-1 is linked to a reversible stoichiometry change from CeO1.988-x prior to reaction to CeO1.952-x during reaction, i.e., the evolution of the F2g position can be correlated with the activity behavior of the catalyst during both exposures to reaction conditions. Starting from the same as prepared catalyst, a gradual redshift of the F2g band of 3 cm-1 in total (corresponding to CeO1.916-x) is observed during reduction of the catalyst (see Figure 4B)) together with a strong increase in 570 nm reflectivity (see Figure 3 B)). Exposing the reduced catalyst to reaction conditions results in a steep increase in activity as described before, followed by a slower decay. Please note that the F2g position and the UV-Vis absorption at 570 nm show a sudden dip. The F2g position then continuously decreases towards the Raman shift values observed after oxidizing pretreatment, however, without reaching a steady state (see Figure 4B)). Based on our findings the strong activity increase to 6% after reducing pretreatment, which is almost double the maximum activity after oxidizing pretreatment, is linked to the subsurface reduction of the ceria support. Even after 1 h the activity is 30% higher and has not reached a steady state behavior. Thus by the use of operando spectroscopy the increased activity can be directly linked to subsurface reduction of the ceria support. Surface reduction and re-oxidation of the CeO2 catalyst is further monitored by the Raman descriptor at 246 cm-1, which is assigned to the longitudinal stretch vibration of the topmost O-Ce layer of the stoichiometric CeO2(111) surface. Figure 5A) depicts the intensity of this band during oxidative pretreatment as green dots. As a guide to the eye the dashed line indicates the steady-state intensity during the second exposure to reaction conditions. For the reducing pretreatment, a continuous decrease in intensity of the 246 cm-1 band (see Figure 5B)) can be rationalized by a continuous increase of the number of oxygen vacancies at the CeO2 surface. During subsequent exposure to reaction conditions the majority of the intensity is restored, indicating re-oxidation of the surface by the accessible molecular gas phase ACS Paragon Plus Environment

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oxygen. As indicated by the dashed line, the band under reaction conditions does not reach the value after oxidizing pretreatment. This behavior can be explained by the conditioning of the catalyst during first exposure to reaction conditions, in agreement with the behavior observed for the 570 nm UV-Vis band (see Figure 3B)). Finally, the integral intensity of the peroxide band at 828 cm-1 is described. As can be seen in Figures 5A) and B), the peroxide concentrations are comparable in Ar/O2 prior to both types of pretreatment. Upon first exposure to reaction conditions after oxidizing pretreatment initially a strong increase in intensity is observed before the catalyst becomes active. In the further course of the reaction the high peroxide intensity declines, while the activity increases. This behavior confirms our previous results highlighting the importance of peroxides for the CO oxidation over Au/CeO2 catalysts.19 During reaction after oxidizing pretreatment the peroxides rise to the same initial intensity as after reducing pretreatment. It should be mentioned that peroxides are not observed after exposure to reducing conditions (2% CO), which indicates that peroxides initially present are consumed due to reaction with CO to CO2, consistent with the behavior of the UV-Vis band at 450 nm (see Figure 3B)).


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Figure 5. Time-dependent operando Raman spectroscopic information for the ceria surface reduction state of 0.5 wt% Au/CeO2. A) During two consecutive exposures to reaction conditions. B) During reaction after reducing pretreatment. Black dots indicate the catalytic activity to CO2, red dots the integrated peroxide intensity, and green dots the intensity of the 246 cm-1 band. The scales are identical for both graphs. Dashed lines indicate the steady-state intensity level under reaction conditions (CO/O2/Ar).


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4. Discussion Regarding the characterization of the employed Au/CeO2 catalyst, we show by TEM that the catalyst support terminates with a CeO2(111) surface plane as the most stable ceria surface.31 Besides, stepped sites are observed. With respect to the state of gold after synthesis by deposition precipitation,24 XPS analysis reveals that gold exhibits predominantly a metallic (Au0) state, while 30% of the total amount of Au is present in an oxidized Au+ state. The excess charge is located on the ceria support as shown by analysis of the Ce3d photoemission. This is corroborated by the majority of theoretical studies, which predict a charge transfer from the gold nanoparticle (Au1,37 Au13,11 and Au209) to the stoichiometric CeO2(111) support surface. For a CeO2(111) surface with one surface oxygen vacancy, it was reported that Aun clusters with n ≥ 5 are positively charged, although single gold atoms at vacancy positions are negatively charged.37 As shown by TEM, the ceria support of the used 0.5 wt% Au/CeO2 catalyst terminates with the CeO2(111) surface plane. Thus, it can be directly compared to the existing theoretical studies in terms of CeO2(111) surface termination20 and gold nanoparticle charge, whereas the stepped structure has rarely been studied with DFT methods.11 In this study, gold particles 10 nm in diameter were observed (see Figure S3). However, on a catalyst prepared by the same synthesis route also smaller gold clusters have been identified previously.19 The CO oxidation activity of the 0.5 wt% Au/CeO2 catalyst was analyzed after activation of the as-prepared catalyst in two ways: (1) by treatment in reaction conditions followed by oxidizing pretreatment (1 h in 25% O2 in Ar) and (2) reducing pretreatment (ca. 1 h in 2% CO in Ar). The pre-reduced catalyst exhibited an activity that is initially twice as high (6% conversion as compared to 3% conversion). After 1 h exposure to reaction conditions the difference became smaller (4% conversion as compared to 3% conversion). This observation is consistent with the results of Zhang et al., who reported a 0.34 wt% Au/CeO2 (prepared by a similar synthesis route) with an initially doubled CO oxidation activity after pre-reduction in ACS Paragon Plus Environment

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hydrogen at 250°C.

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A deactivation profile (ca. 1 h) after pretreatment in CO at 400°C was

also observed for a 4.5 wt% Au/CeO2 catalyst at 80°C, where the catalyst was less active compared to a pretreatment in oxygen or nitrogen atmosphere at 400°C.14 The reduction state of the ceria support in 2% CO was monitored by static operando measurements, i.e., by the redshift of the F2g band, by the increase of the defect band at 540 cm-1, and the damping of the CeO2(111) surface modes at 246 cm-1 and 402 cm-1.The dynamic operando spectra reveal that the subsurface reduction state (as monitored by the F2g position) directly correlates with the activity of the catalyst after oxidizing pretreatment (see Figure 4A)), i.e., a reduced catalyst is more active for CO oxidation. This finding is further supported by the strong increase in activity of the catalyst after reducing pretreatment (6% conversion as compared to 3% conversion). Moreover, the dynamic operando Raman and UV-Vis data show that under reaction conditions after reducing pretreatment the subsurface re-oxidized slowly after an initial fast surface re-oxidation step. The level of surface reduction was monitored by the damping of the longitudinal stretch of Ce-O of the stoichiometric CeO2(111) surface at 246 cm-1. This band was assigned on the basis of recent DFT+U calculations of the CeO2(111) surface in comparison with experimental results. In this study, we observed a damping of the intensity of the mode upon oxygen vacancy introduction into ceria (see Figure 5). We therefore introduce this band as a semi-quantitative indicator of the surface reduction state.20 In combination with the results from Figure 4 we are able to demonstrate that the CeO2 surface is only slightly reduced under reaction conditions, but strongly reduced under reducing conditions. Re-oxidation of the surface is fast compared to the re-oxidation of the subsurface. Therefore, based on the observed correlation between activity and spectroscopic data, we conclude that the catalytic activity is strongly influenced by the subsurface reduction state. With respect to the peroxide species, which have been proposed as an intermediate for oxygen activation,11

9

we observe a coverage lower than 0.06 ML based on the presence of only a ACS Paragon Plus Environment

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small shoulder at 860 cm-1 by comparison with detailed DFT+U20 and experimental38 results obtained previously. Integration of the peroxide region reveals that the peroxide coverage shows an immediate increase upon exposure to reaction conditions and a slower decline when switching to oxidative conditions (see Figures 5A) and B)), representing the changes in surface oxygen vacancy concentration as discussed above. Interestingly, after both types of pretreatment, a comparable level is observed upon exposure to reaction conditions. Thus, at the surface a similar concentration of oxygen vacancies is present, in contrast to the subsurface region (compare the F2g band behavior in Figure 4). With respect to recent findings of an electronic metal–support interaction (EMSI) on Au/TiO2, i.e., a transfer of excess charge due to reduction to the gold particle, as observed after CO treatment at 400°C,39 our results for Au/CeO2 do not suggest such a charge transfer from the support to the gold particle upon reduction. On the basis of the Raman und UV-Vis spectroscopic results we rather propose that in the case of ceria the electron remains in the support (Ce3+) and accelerates oxygen activation, while not hindering CO adsorption at the gold particle. Our data clearly show a relation between the state of the ceria support and the activity of the catalyst. As a consequence, a strongly reduced catalyst shows a higher activity for CO oxidation. Spectroscopically a connection between subsurface reduction and activity was found, which is rationalized by the high oxygen mobility in ceria as compared to other oxides.40 This is supported by a recent study, which reported an acceleration of the oxygen mobility in CeO2 upon 1wt% gold deposition.41


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5. Conclusion In summary, the support dynamics of Au/CeO2 catalysts during room temperature CO oxidation is elucidated using combined operando Raman/UV-Vis spectroscopy. This approach allows one to monitor the surface and subsurface defect dynamics quantitatively and in real time demonstrating the dependence of the catalytic activity on the reduction state of the ceria support. As described, the reduction of the support applies not only to the CeO2 surface but extends into the CeO2 subsurface. In fact, pre-reduction of the CeO2 support initially enhances the catalytic activity during CO oxidation by 100%. This highly active state of the catalyst diminishes as a result of oxygen mobility and charge transfer in CeO2-x. The dynamic operando spectra reveal that the sub-surface reduction state directly correlates with the catalyst activity, i.e., after reducing pretreatment a more pronounced dynamic is observed. Moreover, under reaction conditions after reducing pretreatment the subsurface re-oxidizes slowly after an initial fast surface re-oxidation step. Besides demonstrating the enormous influence of the ceria support on oxidation reactions over metal/ceria catalyst materials, our results highlight the potential of operando spectroscopy to elucidate correlations between structure and activity allowing to elucidate the mode of operation of catalysts.


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Acknowledgements C.S. gratefully acknowledges the Merck’sche Gesellschaft für Kunst und Wissenschaft e.V. for providing a fellowship. The authors thank Stefan Lauterbach from the group of Prof. H.-J. Kleebe (TU Darmstadt) for electron microscopy experiments and Karl Kopp for technical support.

Supporting Information The Supporting Information provides characterization data (TEM, XPS) as well as details on the Grüneisen parameter and the relation between Raman shift and CeO2-x stoichiometry based on DFT+U calculations.


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