CeO2-Interface by In Situ Variation of the Pt Particle Size

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Tuning the Pt/CeO2-Interface by In Situ Variation of the Pt Particle Size Andreas Gänzler, Maria Casapu, Florian Maurer, Heike Störmer, Dagmar Gerthsen, Géraldine Ferré, Philippe Vernoux, Benjamin Bornmann, Ronald Frahm, Vadim Murzin, Maarten Nachtegaal, Martin Votsmeier, and Jan-Dierk Grunwaldt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00330 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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Tuning the Pt/CeO2-Interface by In Situ Variation of the Pt Particle Size

Andreas M. Gänzler1, Maria Casapu1, Florian Maurer1, Heike Störmer2, Dagmar Gerthsen2, Géraldine Ferré3, Philippe Vernoux3, Benjamin Bornmann4, Ronald Frahm4, Vadim Murzin5, Maarten Nachtegaal6, Martin Votsmeier7, Jan-Dierk Grunwaldt*1

1

Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of

Technology (KIT), Engesserstr. 20, 76131 Karlsruhe, Germany. 2

Laboratory for Electron Microscopy, Karlsruhe Institute of Technology (KIT), Engesserstr. 7,

76131 Karlsruhe, Germany. 3

Université de Lyon, Institut de Recherches sur la Catalyse et l’Environnement de Lyon

(IRCELYON), UMR 5256, CNRS, Université Claude Bernard Lyon 1, 2 avenue A. Einstein, 69626 Villeurbanne, France. 4

Department of Physics, University of Wuppertal, Gaußstr. 20, 42119 Wuppertal, Germany.

5

Deutsches Elektronen-Synchrotron (DESY), Notkestr. 85, 22607 Hamburg, Germany.

6

Paul Scherrer Institute (PSI), PSI Aarebrücke, 5232 Villigen, Switzerland.

7

Umicore AG & Co. KG, Rodenbacher Chaussee 4, 63457 Hanau, Germany.

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Abstract Pt-CeO2-Al2O3 catalysts play an important role in diesel oxidation and three-way catalysis. In this study, the fast structural dynamics of both platinum and ceria in a 1wt.%Pt/5wt.%CeO2-Al2O3 catalyst prepared by flame spray pyrolysis have been systematically investigated under reducing and oxidizing conditions to elucidate the role of the Pt-CeO2 interface for CO oxidation and fast oxygen storage/release of ceria. The catalyst showed enhanced catalytic activity particularly after application of a reducing/oxidizing conditioning step at 250 °C, with a pronounced dependence on the reducing agent (C3H6 < H2 < CO). In situ time resolved X-ray absorption spectroscopy (XAS) at the Ce L3-edge unraveled a dependency of the reduction extent of ceria during temperature programmed reduction on the noble metal constituent and the applied reducing agent. Dynamic reducing/oxidizing cycling (2 % H2 ↔ 10 % O2 or 2 % CO ↔ 10 % O2) at various temperatures (150 °C, 250 °C and 350 °C) showed that the reducibility of ceria increased at higher temperature and by using a stronger reducing reaction mixture. This coincides with the trend in catalytic activity. Time resolved XAS data recorded at the Pt L3- and Ce L3-edges during redox cycling revealed a close relationship between the Pt oxidation state and the ceria redox response. The formation of reduced Pt particles was found to induce variations in ceria reducibility under transient conditions and was identified as decisive prerequisite for ceria reduction at low temperatures. Variations in the extent of ceria reduction during the reducing/oxidizing cycles indicate an evolution of the Pt-ceria interface from an inactive state towards an optimal activated one due to reduction and slight sintering of the noble metal particles. Further growth of Pt particles leads to a decrease in ceria reduction rate due to the smaller PtCeO2 interface perimeter. A schematic model illustrating the role of Pt for ceria reducibility is developed and the optimal Pt particle size derived. The results are relevant for various applications, particularly for catalysts operated at low temperature under highly dynamic reaction conditions like exhaust gas catalysts. Keywords: metal support interaction, Ceria, Pt, interface, X-ray absorption spectroscopy, CO oxidation


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1. Introduction Supported noble metals represent one of the most important and widely applied nanoparticles (NPs) in oxidation catalysis, especially exhaust gas aftertreatment systems.1-3 The catalytic performance often depends on the particle size, morphology and the surface, especially if the reaction is structure-sensitive, as in the case of CO or NO oxidation on Pt.4-6 Equally important is the support material. In particular, reducible oxides have gained a lot of attention: Ceria has evolved as the support of choice to enhance the performance of three-way catalysts (TWCs).7-8 In exhaust gas applications ceria usually serves as an oxygen buffer,3 as e.g. demonstrated by in situ monitoring of a catalyst bed using the microwave-based cavity perturbation method.9-10 Furthermore, ceria offers a pathway to maintain high metal dispersion11-14 and it even allows regenerating catalysts which suffer from sintering of the noble metal component by exploiting the strong metal support interaction, as shown recently.12-13, 15 Finally, it can directly contribute to the catalytic performance, as a superior activity of ceria supported noble metal nanoparticles was found for the oxidation of carbon monoxide (CO),16-17 in particular after reductive pretreatment procedures.13, 1819

Close contact between Pt and CeO2 has a prominent impact on the electronic structure of the NPs20 and

time resolved resonant X-ray emission spectroscopic studies attributed a critical role to the Pt-CeO2 interface during CO oxidation.21-22 This enables an enhanced catalytic activity compared to e.g. Pt/Al2O3, which suffers from CO self-poisoning at low temperatures.16 Although there is now a large consensus on the beneficial role of the Pt-CeO2 interface in low-temperature CO oxidation, it remains unclear how the noble metal–ceria tandem acts, particularly under the dynamic reaction conditions present in exhaust gas aftertreatment-systems. Even small Pt particle size variations have a dramatic effect on the catalyst performance.13 Low CO oxidation activity was found for highly oxidized and dispersed Pt on ceria in contrast to very high CO oxidation activity over reduced Pt particles with a diameter larger than ~1.5 nm.13 There may also be a close relationship between the Pt oxidation state and the activation of the Pt-CeO2 interface, which is known to play a prominent role for achieving high catalytic activity and which may be reflected in high ceria redox activity. Therefore, understanding 3 ACS Paragon Plus Environment

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the working-principles of an active Pt-ceria interface is decisive, as the system is not only considered to improve the performance of exhaust gas catalysts, but also a very promising candidate, for example, for future water-gas-shift applications,23-24 small scale catalytic partial oxidation of methane25 and electro catalysts, e.g. in fuel cells.26 To further elucidate the role of the Pt-CeO2 interface under transient conditions, in situ and, preferentially, operando studies are required. In particular, synchrotron X-ray based methods are well suited. X-ray absorption spectroscopy (XAS) allows monitoring the variations in oxidation state and in local structure of Ce ions and Pt sites during operation, conditioning or regeneration. However, such experiments are challenging, as cerium is a heavy element and strongly absorbs X-rays at lower energies, e.g. at the Ce L3 absorption edge (5723 eV). Especially, time resolved XAS experiments like quick-scanning X-ray absorption fine structure (QEXAFS) spectroscopy (in the millisecond range) are therefore difficult.27 Only a few experiments have been reported, which investigate the dynamic structural changes in ceria or ceriabased materials, e.g. using dispersive XAS to monitor the oxygen storage and release in a Pt/CeO2-ZrO2 catalyst28 or Ce K-edge (40433 eV) XAS during temperature cycles of CeO0.5Zr0.5O2-X under realistic solar thermochemical reaction conditions for two step CO2/H2O splitting29 as well as during periodic lean/rich cycles of a Cu/CeO2 catalyst.30 In the present study, we applied QEXAFS to acquire time resolved data at the Pt L3-and Ce L3-edges under transient conditions (reducing/oxidizing cycles, which are denoted redox cycles in the following) to understand the interplay between the structure and oxidation state of both components. The materials used in this study were prepared by two-nozzle flame spray pyrolysis (FSP),31 to achieve an intimate noble metal-support contact/interaction. In addition, the low weight density of the resulting catalyst powders is ideal for QEXAFS measurements at the Ce L3-and Pt L3-edge. First, the impact of reducing pretreatment steps on the CO oxidation rate of Pt-CeO2/Al2O3 was investigated (light-off performance). In this regard the role of the applied reducing agent was of interest. Next, time resolved operando QEXAFS data were acquired during conventional temperature programmed reduction experiments (TPR) and during redox 4 ACS Paragon Plus Environment

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cycling with different reducing gas mixtures and at various temperatures. The ceria redox properties were correlated to the oxidation state and structure of the Pt component in order to finally elucidate the encountered variations in catalyst activity and the underlying working-principles. 2. Experimental Note that all values shown as % for catalyst loading or gas flow volume are in the whole study calculated as wt.% and vol.%, respectively. Sample Preparation Pt/CeO2-Al2O3 and CeO2-Al2O3 were prepared by two nozzle flame spray pyrolysis using a setup reported in detail in ref. 31. The preparation targeted at 1 wt.% Pt and 5 wt.% CeO2 loading on alumina or only 5 wt.% CeO2 on Al2O3 (denoted in the following as Pt/CeO2-Al2O3 and CeO2-Al2O3). The support precursor solution, 6.49 g (2.0·10-2 mol) aluminum acetylacetonate (anhydrous) in 100 mL xylene, was sprayed separately from the platinum and ceria precursor solution, which consisted of 20.6 mg (5.24·10-5 mol) Pt(II) acetylacetonate (anhydrous, Chempur) and 344 mg (2.96·10-4 mol) Ce(III) ethylhexanoate (49 % in 2-ethylhexanoic acid, Alfa Aesar) in 100 mL xylene. For the Pt-free sample the platinum precursor was not added to the ceria precursor solution. This CeO2-Al2O3 material served as a reference material throughout this study. The solutions were dosed by capillary tubes at 5 mL/min using a syringe pump (World Precision Instruments) and injected into the nozzles with a constant O2 flow of 5 L/min with a 3 bar pressure drop. Each spray was ignited by an annular premixed CH4 flame (1.6 L/min O2 and 750 mL/min CH4). The gas flows were controlled by mass flow controllers (Bronkhorst). The two nozzles were positioned at an angle of 120 ° and a distance of 6 cm. The resulting particles were collected on glass fiber filters (75 cm diameter, Whatman GF6) in a water cooled round holder connected to a vacuum pump (Busch R5). The pressure across the filter was 180 mbar. The obtained catalyst powders were calcined at 500 °C for 5 h in air to remove possible organic residues.


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Characterization The Pt and CeO2 loadings were evaluated by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) using an OPTIMA 4300 DV spectrometer (Perkin Elmer) at the Institute for Applied Materials at KIT (Karlsruhe, Germany). N2-physisorption (BELSORP-mini II, Rubotherm) was conducted at -196 °C to estimate the specific surface area according to the Brunauer-Emmet-Teller (BET) method.32 XRD-patterns were recorded using a Bruker D8 Advance diffractometer with Cu K-alpha (λ =0.154 nm) radiation and a Ni filter in the 2Θ range between 20 ° to 90 ° (step size 0.015 °, 2 s dwell time). High angle annular dark field scanning transmission electron microscopy (HAADF)-STEM images and energy dispersive X-ray spectroscopy (EDXS) mappings were acquired using an FEI OSIRIS microscope operated at 200 kV at the Laboratory for Electron Microscopy at KIT (Karlsruhe, Germany). Catalytic Activity For catalytic tests 50 mg of Pt/CeO2-Al2O3 diluted with 500 mg quartz (both sieved to 125-250 µm) was used resulting in a catalyst bed length of 10 mm in a quartz glass reactor (8 mm inner diameter). The total gas flow was set to 500 mL/min to obtain a WHSV of 600 L g-1catalyst h-1 or 60.000 L g-1noble metal h-1. Gases were mixed by mass-flow controllers (Bronkhorst). An electrically actuated valve (Vici) was used for fast switches between premixed gas flows. The gas mixture behind the reactor was analyzed with an infrared spectrometer (MKS Multigas 2030). The light-off activity was evaluated during heating of the catalyst to 500 °C at a 10 °C/min ramp rate in a model lean diesel exhaust gas mixture after two different pretreatment procedures. The lean mixture was composed of 8 % O2, 1000 ppm CO, 500 ppm C3H6 and N2 as balance. During the first pretreatment, denoted as lean conditioning, the catalyst was exposed to the lean mixture for one hour at 500 °C. For the second pretreatment, denoted as lean/rich conditioning, the reaction mixture was switched between a model rich exhaust gas atmosphere (2 % H2/N2, 2 % CO/N2 or 0.22 % C3H6/N2) and the lean gas mixture described above. The catalyst was exposed to 10 cycles of 30 s rich mixture followed by 90 s of lean mixture (10 times (30 s + 90 s) cycles) at 250 °C. The above mentioned procedures were selected based on 6 ACS Paragon Plus Environment

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a previous study,13 which indicated the redispersion of Pt on ceria during the lean conditioning and formation of Pt nanoparticles with slightly different particle size during the lean/rich conditioning, depending on the reducing agent in the rich mixture. Operando QEXAFS XAS data at the Ce L3-edge were acquired at the SuperXAS beamline at the Swiss Light Source (PSI, Villigen, Switzerland). The beamline offers a fast data acquisition infrastructure based on a quick scanning EXAFS (QEXAFS) monochromator and fast-responding gridded ionization chambers.33-34 The polychromatic X-ray beam from the 2.9 T superbend magnet was collimated before the QEXAFS monochromator using a Si coated collimating mirror which also served to reduce higher harmonic contributions. A Rh coated toroidal mirror located behind the QEXAFS monochromator focused the beam to a spot size of 200 µm x 200 µm at the sample position. A Si(111) channel-cut monochromator was used for acquiring data around the Ce L3-edge (5723 eV) in transmission mode; the X-ray intensity was measured using 15 cm long N2 filled gridded ionization chambers.33 The monochromator oscillated at 2 Hz (5640 eV to 6410 eV), which resulted in an acquisition speed of 4 spectra per second. The X-ray absorption spectrum of a Cr foil (5989 eV) was recorded simultaneously between the second and a third ionization chamber throughout the entire experiments for energy calibration. Pt L3-edge (11564 eV) XAS data were acquired at the new beamline P64 at PETRA III (DESY, Hamburg, Germany). The beamline also offers a fast data acquisition infrastructure, which was used for the first time in this study to obtain data on catalyst samples under in situ and operando conditions. A liquid nitrogen cooled Si(111) channel-cut QEXAFS monochromator was used for fast data acquisition together with fast responding gridded ion chambers.33 The monochromator oscillated at 1 Hz (11410 eV to 12300 eV), resulting in 2 spectra per second. The X-ray absorption spectrum of a Pt foil was recorded simultaneously throughout the entire experiments for energy calibration. During the XAS measurements reported in this study no detectable changes in the sample due to exposure to the X-ray beam were observed.


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For all experiments the samples were measured as powder in a quartz glass capillary (Ø = 1.5 mm, 10 µm wall thickness) heated by a hot air blower (Gasblower, FMB Oxford).35 Gases were dosed by mass flow controllers (Bronkhorst), while rapid changes between different atmospheres were controlled by an electrically actuated valve (Vici, Valco Instruments). The total gas flow was set to 50 mL/min. To compare time resolved data obtained at different synchrotrons the gas analysis was monitored and the exact time when a switching between atmospheres occurred was automatically logged. In addition, the position of the switching valve was continuously written into the XAS data file. This ensured comparability of the experiments conducted at different experimental stations utilizing the same reactor and reaction conditions. QEXAFS during temperature programmed reduction (TPR) was performed for CeO2-Al2O3 and Pt/CeO2Al2O3 using various reductants (2 % C3H6, 2 % H2, and 2 % CO in He). For each experiment a fresh sample was used, which was first heated in 10 % O2 in He to 500 °C at a ramp rate of 10 °C/min as a pretreatment. After change to the reductant at room temperature, the sample was heated to 600 °C with a ramp rate of 5 °C/min. Furthermore, Pt/CeO2-Al2O3 was subjected to redox (reducing/oxidizing) cycles at various temperatures. After pretreatment of the sample in 10 % O2 in He at 500 °C, XAS data were acquired at 150 °C, 250 °C and 350 °C, while the gas feed was switched every 90 seconds between 10 % O2 in He and a reducing mixture (either 2 % H2 in He or 2 % CO in He). At the end of the cycling period the reducing mixture was fed for several minutes to the sample before switching back to 10 % O2 in He. In the case of XAS measurements at the Pt L3-edge, cycling experiments with H2 at 150 °C and 250 °C were also conducted after the oxidative pretreatment at 500 °C, allowing for a direct comparison with Ce L3-edge data. At the corresponding temperature, first cycling with H2 was performed followed by cycling with CO, considering that the effect of CO cycling on the noble metal size is more pronounced.13 In addition, the impact of a redox treatment on the ceria reduction behavior at lower temperature was elucidated by comparing changes in the oxidation state of Ce and Pt during cycling (2 % H2 in He ↔ 8 ACS Paragon Plus Environment

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10 % O2 in He) at 150 °C, after the catalyst was oxidatively pretreated at 500 °C or conditioned with reducing/oxidizing cycles (2 % CO in He ↔ 10 % O2 in He) at 250 °C. A custom made software tool (JAQ), which is available at both beamlines, was used to split the continuously acquired ionization chamber signals and to convert them into the respective spectra, which were finally energy calibrated.36 Further data treatment was performed using Matlab. In a first step several spectra were averaged to increase the signal to noise ratio (Ce-TPR: 240 spectra  1 data point every 5 °C; Ce-cycling: 1 spectrum  4 data points per 1 s; Pt-cycling: 4 spectra  30 data points per 1 min). The spectra were normalized before a linear combination fitting (LCF) based on reference spectra (representing Ce3+ and Ce4+ or Pt0 and Pt4+, respectively), to evaluate the cerium and platinum redox properties of the samples under the various conditions. The relative accuracy of the reported values for the LCF of Ce L3-edge and Pt L3-edge data was about ±0.5 % and ±2 %, respectively.

3. Results and Discussion 3.1. Basic Characterization and Catalytic Performance of Pt/CeO2-Al2O3 during CO Oxidation after different Conditioning Steps The two-nozzle flame spray pyrolysis synthesis method resulted in CeO2-Al2O3 and Pt/CeO2-Al2O3 materials with a specific surface area of about 130 m2/g. The CeO2 concentration amounted to 3.8 wt.% and the Pt loading in the Pt/CeO2-Al2O3 to 0.8 wt.%. EDX-maps and HAADF-STEM images of the Pt/CeO2-Al2O3 catalyst and the CeO2-Al2O3 reference material are presented in Figure 1(A-D). In both cases, ceria was found to be well dispersed on the alumina support, with a particle diameter of ~3-5 nm. According to EDX maps of the Pt/CeO2-Al2O3, Pt was predominantly in a highly dispersed state on ceria particles (Figure 1D), with Pt NP sizes below 1 nm. Only few Pt particles (> 2 nm) appeared as possibly attached only to the alumina component. The XRD patterns presented in Figure 1E show exclusively reflections of γ-Al2O3 and CeO2. The absence of Pt reflections confirms the presence of small Pt particles. 9 ACS Paragon Plus Environment

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Figure 1 HAADF-STEM image (A)and EDX map (B) of CeO2-Al2O3 reveal the presence of ceria nanoparticles (3-5 nm) on the alumina support. HAADF-STEM image (C) and EDX map (D) of Pt/CeO2-Al2O3 show again ceria nanoparticles in the range of 3-5 nm. In addition, they identify Pt to be in a highly dispersed state and in intimate contact with ceria. XRD patterns (E) confirm the presence of ceria as small crystallites (broad reflections) and the absence of Pt reflections.

The catalytic activity of the Pt/CeO2-Al2O3 catalyst was evaluated in a plug-flow reactor with respect to CO oxidation under model diesel exhaust conditions (1000 ppm CO, 500 ppm C3H6, 8 % O2 and N2 as balance). In Figure 2, the CO conversion during light-off experiments of the differently pretreated Pt/CeO2-Al2O3 catalysts is compared. The as prepared catalyst (calcined in static air at 500 °C for 5 h) showed low catalytic activity, with a light-off temperature T50 (temperature corresponding to 50 % CO conversion) above 225 °C. The activity could be substantially enhanced by applying reductive treatments, based on a redox cycling procedure.13 Here, the low-temperature activity was enhanced by cycling the reaction mixture between the lean and a reducing atmosphere at 250 °C. The extent of the activation 10 ACS Paragon Plus Environment

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strongly depended on the reducing agent (C3H6 < H2 < CO), in line with our earlier results.13 In this way, the CO oxidation light-off curve was shifted by up to 80 °C to lower temperatures (Figure 2). The activation probably originates from a gradual increase in Pt nanoparticle size when C3H6, H2 or CO are used during the lean/rich conditioning step. A positive influence on the catalytic activity due to reducing treatments at elevated temperatures (> 500 °C) has also been reported in earlier studies.17-19 In contrast, the high or possibly atomic distribution of Pt present in the as prepared sample (Figure 1), which is reobtained upon lean conditioning at > 400 °C,13, 37 leads to low catalytic activity (Figure 2: lean).

Figure 2 Light-off curves of Pt/CeO2-Al2O3 during CO-oxidation after different pretreatment procedures. The catalyst was subjected to an oxidative conditioning step (lean: 1 h in 1000 ppm CO, 500 ppm C3H6, 8 % O2, N2 at 500 °C) and to lean/rich activation protocols. By varying the reductant (0.22 % C3H6, 2 % H2 or 2 % CO in N2) during the lean/rich treatment the catalyst was activated to a different extent as reflected by the change in the light-off behavior.

These pronounced changes in the catalyst activity upon exposure to oxidizing or reducing/oxidizing conditions may originate from a variation of the noble metal-ceria interface sites, which have been claimed to be important for high catalytic activity.16 To investigate the impact of the structural dynamics of Pt NPs on the Pt-CeO2 interface as well as the differences between the various reductants with respect to the CeO2 redox behavior during the different conditioning procedures, operando QEXAFS measurements in the sub-second regime were performed. First, QEXAFS data were recorded during conventional temperature programmed reduction with C3H6, H2 or CO. Afterwards, oxidizing/reducing conditioning was applied and studied using time resolved XAS at the Pt L3- and Ce L3-edge in a similar manner as during the activation procedure of the catalyst. 11 ACS Paragon Plus Environment

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3.2. Reduction of Ceria in CeO2-Al2O3 and Pt/CeO2-Al2O3 during Temperature Programmed Reduction with Different Reducing Agents

Figure 3 Ce L3-edge XANES spectra used for linear combination fitting (LCF) analysis. Spectra of oxidized (acquired under 2 % H2 in He at RT) and reduced Pt/CeO2-Al2O3 (acquired under 2 % H2 in He at 600 °C) were used as reference spectra representing Ce3+ and Ce4+in the LCF analysis, due to their similarities to the spectra of bulk reference compounds. Different spectral features are marked A to E.

As a first step, XANES spectra of fully oxidized and fully reduced Pt/CeO2-Al2O3 were recorded at 25 °C after lean conditioning and after reduction at 600 °C in 2 % H2/He. In the latter case a complete reduction of cerium sites to Ce3+ was confirmed by comparison of the resulting Ce L3 spectra with those of cerium(IV)oxide and cerium(III)nitrate references (Figure 3). Hence, complete Ce4+ to Ce3+ reduction occurred already at relatively low temperature (< 600 °C). We ascribe this to a close interaction between Pt and CeO2 in the FSP sample and also to the high Pt:CeO2 molar ratio (approx. 1:5, i.e. large interface), which highly promotes the reducibility of ceria. Figure 3 shows several features in the Ce L3 XANES region (marked A-E), mainly originating from 2p3/2 to 5d5/2 transitions that are also sensitive to the ceria particle size.38-39 The reduction of Ce4+ to Ce3+ is indicated by the appearance of feature ‘E’ and the disappearance of the characteristic double peak feature marked with ‘C’ and ‘D’, with maxima at 5729 eV and 5737 eV for Pt/CeO2-Al2O3 and at 5730 eV and 5737 eV for the bulk CeO2 reference. The differences between the nanoparticulate ceria in Pt/CeO2-Al2O3 and the bulk cerium oxide were more pronounced at the maxima close to the edge (~5729 eV), which was broader in the spectrum of the catalyst compared to 12 ACS Paragon Plus Environment

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the reference. As previously reported,40 the low energy shoulder ‘B’ originates from crystal field splitting within the cubic fluorite structure and increases with decreasing ceria particle size. The XANES region of the reduced catalyst was similar to the one of cerium(III)nitrate and was dominated by a sharp feature (‘E’) at 5725 eV, marginally shifted (about 1 eV) to lower energies and slightly less pronounced compared to the Ce(NO3)3 bulk reference (Figure 3). Hence, the XANES spectra of the oxidized and reduced Pt/CeO2-Al2O3 sample were regarded more appropriate for representing the Ce3+ and Ce4+ oxidation state in the highly dispersed systems in this study and were thus applied for quantitative linear combination fitting (LCF) analysis to assess the cerium redox behavior.

Figure 4 Ce L3-edge XANES data (A+B) obtained on CeO2-Al2O3 at the middle of the catalyst bed during TPR experiments (heating with 5 °C/min in A: 2 % H2/He and B: 2 % CO/He) and respective LCF-results (C+D).

Next, the reduction of ceria in the absence of the noble metal was investigated during temperature programmed reduction in H2 and CO. As shown in Figure 4, the selection of the reductant had a strong impact on the Ce4+  Ce3+ reduction temperature. Both the XANES-spectra and the corresponding LCF results show that ceria reduction in CO occurs at lower temperature than in H2 for the noble metal free CeO2-Al2O3 reference material. In addition, the well-known two-step reduction profile was observed,41 which allows to distinguish between a surface reduction at about 300-400 °C and a bulk reduction above 500 °C. Moreover, slight reduction was observed below 300 °C, which could originate from very small 13 ACS Paragon Plus Environment

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CeO2 particles on the alumina support. The differences are also reflected by the corresponding variation in apparent activation energy (Eapp(CeO2-Al2O3, CO) = 62 kJ/mol, Eapp(CeO2-Al2O3, H2) = 159 kJ/mol), which was estimated from the cerium reduction degree observed during the TPR experiment following the procedure by Alayoglu et al. 42 (for further details, see ESI and Figure S3). In Figure 5, the corresponding Ce L3 XANES spectra recorded during reduction of the Pt/CeO2-Al2O3 sample are shown together with the results of the respective linear combination fitting analysis. Again, striking changes during the TPR experiments using C3H6, H2 and CO are observed. The addition of platinum substantially enhanced the low temperature Ce4+Ce3+ reducibility. In the presence of H2 and CO the reduction of the Ce4+ sites followed again the two-step profile, similar to the CeO2-Al2O3 reference. Also in this case CO was the most efficient reductant. The presence of Pt particularly improved the reducibility of CeO2 surface sites in close contact with the noble metal. Due to the high ceria dispersion and small particle size it substantially contributed to the overall ceria reduction (~60 %). Considering that the surface layer thickness involved was in the range of ~1 nm 43, this fits well with ceria particles with a size in the range of 3-4 nm as observed by TEM. Reduction of the ceria surface started already at about 100 °C, if CO was applied (Eapp(Pt/CeO2-Al2O3, CO) = 49 kJ/mol), and only above 150 °C, if H2 was used as reducing agent (Eapp(Pt/CeO2-Al2O3, H2) = 58 kJ/mol). While CO appeared to be a more effective reductant for surface sites at low temperatures, the contrary was found at high temperatures, where H2 more efficiently reduced bulk ceria. A higher Ce3+ contribution was encountered compared to the CO-TPR above 450 °C (Figure S2). A different behavior was observed during C3H6-TPR, probably because of a low C3H6 cracking rate at low temperatures. Consequently, substantial ceria surface reduction was only observed above 300 °C and bulk reduction started slowly above 500 °C.


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Figure 5 Ce L3-edge XANES spectra (A-C) of Pt/CeO2-Al2O3 during TPR experiments (heating at 5 °C/min in A: 2 % C3H6/He, B: 2 % H2/He and C: 2 % CO/He) with corresponding LCF-results (D-F). The reduction from Ce4+ to Ce3+ started already at about 100 °C in CO, at about 200 °C in H2 and at about 300 °C in C3H6.

The trends for surface and bulk reduction for the Pt loaded and the Pt free sample are summarized in Scheme 1. The importance of the noble metal state for the ceria reduction behavior was reported earlier.42, 44-45

For Pt/CeO2-Al2O3, differences observed in ceria reduction during the CO- and H2-TPR experiments

are also related to differences in the required activation sites, as dissociative adsorption is necessary in the case of hydrogen.46 Therefore, the oxidation state and the size of the noble metal particles play a decisive role and the formation of reduced Pt nanoparticles represents a crucial prerequisite (< 250 °C, see e.g. respective Pt-TPR experiments in Figure S6). In the light of the significantly smaller differences in Pt reduction temperature (T50% red (H2) = 180 °C, T50% red (CO) = 150 °C), a facilitated activation of CO on very small Pt species (onset of ceria reduction at XPt(red) ~25 %) was observed, as Pt and ceria reduction accompany each other (Figure S7A). In contrast, in the case of hydrogen ceria reduction is only detected after substantial reduction of the platinum component (> 50 %, Figure S7B). Interestingly, once reduced 15 ACS Paragon Plus Environment

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Pt particles emerged, activated hydrogen apparently became the superior reducing agent for CeO2, as spillover of atomic hydrogen started to play an important role.42, 47 Considering that ceria was present as small nanoparticles supported on alumina and the high Pt:CeO2 ratio in comparison to the conventional catalysts (platinum on pure ceria), the bulk-term at high temperatures (> 300 °C) still involves ceria in the closer vicinity of Pt nanoparticles.

Scheme 1 Influence of the choice of reductant on the surface and bulk ceria reduction in the Pt loaded and Pt free material.

3.3. QEXAFS at the Ce L3-Edge during Reduction/Oxidation Cycling Experiments on Pt/CeO2-Al2O3 with the Different Reducing Agents In the next step, isothermal reduction/oxidation cycling experiments at 150 °C, 250 °C and 350 °C were conducted on Pt/CeO2-Al2O3 using either H2 or CO as reducing agent. In all cases, time-resolved Ce L3 XANES data were acquired and analyzed with respect to their oxidation state by linear combination fitting (Figure 6, the corresponding XANES spectra are given in the support information, Figure S4). As demonstrated by the conventional TPR experiments (Figure 4+5), the reduction of CeO2 surface species at low temperatures was strictly governed by the Pt-CeO2 interaction and the activation of the reducing agent at the Pt nanoparticles surface. Hence, any variation of the reduction extent in the low temperature range during the cycling experiment was directly linked to a variation of the Pt-CeO2 interface. In line with the TPR experiment (Figure 5), at 150 °C no ceria reduction was observed during short exposure to hydrogen for 90 seconds and even when the sample was kept in hydrogen for about four minutes (Figure 6A). When CO was used as reductant, about 5 % Ce4+ was reduced to Ce3+ during ten cycles (Figure 6D). During the subsequent “long” exposure for about five minutes, up to 10 % of the cerium species were further reduced to Ce3+.


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Figure 6 Results of linear combination fitting of the Ce L3-edge XANES spectra of Pt/CeO2-Al2O3 during redox cycling with 2 % H2/He (A-C) or 2 % CO/He (D-F) as reducing and 10 % O2/He as oxidizing mixture at various temperatures (150 °C, 250 °C and 350 °C). In addition, the mass spectrometer signals are given for H2 (m/z=2), O2 (m/z=32) and H2O (m/z=18) during H2-TPR and for CO (m/z=28), O2 (m/z=32) and CO2 (m/z=44) during CO-TPR.

At higher temperatures differences between the respective redox cycles were observed. The Ce4+ to Ce3+ reduction rate as well as the final reduction extent of the respective reducing cycles evolved over the entire experiment. For example, if hydrogen was cycled at 250 °C (Figure 6B; see also zoom in Figure S5), the maximum reduction extent increased during the first three cycles in H2 (Ce3+: 25 %  29 %  31 %), which suggests an activation of the hydrogen dissociation and spillover process. This indicates a gradual evolution of the Pt nanoparticles and of the Pt-CeO2 interface during short exposure to reducing conditions. In particular, during the first exposure to hydrogen the reduction of ceria was found to proceed significantly slower compared to the subsequent switches to reducing atmosphere (see Figure 6B and the more detailed Figure S5), which indicates an increased ceria reducibility after a rather slow formation of 17 ACS Paragon Plus Environment

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Pt particles. This is in line with earlier results that report an increase in Pt nanoparticle size and formation of more reduced Pt sites during such redox conditioning steps.13 Hence, these observations could be rationalized with an increase of the interface of reduced platinum and ceria and with the presence of platinum particles of a certain size as decisive parameter for ceria reducibility. A prolonged exposure to reducing/oxidizing steps (the next six cycles) led to a further increase in platinum particle size (sintering) and consequently to an additional decrease in the platinum-ceria interface, resulting in a decrease in the overall maximum extent of reduction. This could be observed for example during the last three hydrogen cycles at 250 °C (Figure 6B). The maximum Ce3+ concentration reached during the first four redox cycles decreased steadily from 31 % in the fourth cycle down to 29 % during the tenth cycle. When CO was used as a reducing agent at 250 °C (Figure 6E), which was identified to more efficiently produce reduced platinum NPs,13 already during the first reduction event the maximum Ce3+ concentration was reached. During the following switches the Ce3+ concentration was dominated by the decrease of its maximum (1st: 51 % Ce3+  10th: 41 % Ce3+). At 350 °C for both reductants the first 90 seconds were sufficient to produce platinum nanoparticles with the maximum Pt-CeO2 interface, and thus able to activate ceria reduction (Figure 6C+F). Further redox cycles led again to a decreasing extent of reduced cerium species due to an increase in platinum nanoparticle size and, consequently, a smaller interface between the metal and the ceria component. In summary, the results indicate that the improvement of the catalyst activity by the application of lean/rich pretreatment steps (Figure 2) originates from an efficient activation of the redox chemistry at the Pt-CeO2 interface. To investigate the evolution of Pt sites during the catalyst activation step in detail and particularly to substantiate the ascribed role of the dynamics in Pt particle size with respect to the ceria redox response, Pt L3-edge XANES data were collected during a similar redox cycling experiment.


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3.3. QEXAFS at the Pt L3-Edge during Reduction/Oxidation Cycling Experiments on Pt/CeO2-Al2O3 with Different Reducing Agents The observed structural changes of the platinum constituent (Pt L3-edge XANES spectra and LCF analysis) during redox cycling with CO or H2 as reductant are presented in Figure 7. TEM images of the Pt/CeO2-Al2O3 catalyst showed only a few Pt nanoparticles directly on alumina and the catalytic behavior as well as the ceria redox behavior was prominently dominated by the Pt-CeO2 interaction. Therefore, the integral Pt L3-edge data was attributed to Pt in an intimate contact with the ceria, as the contribution of Pt on alumina was only marginal. The redox properties at a certain temperature are directly correlated to the Pt nanoparticle size and can be exploited to estimate the particle dimension.15 During this experiment the pre-conditioned Pt/CeO2-Al2O3 catalyst (10 % O2/He, 500 °C) was exposed successively to redox treatments with 2 % H2/He and 10 % O2/He as well as 2 % CO/He and 10 % O2/He at 150 °C and 250 °C. At first, the redox cycling was conducted at 150 °C using 2 % H2/He as reducing atmosphere. The results of the LCF analysis of the Pt L3 XANES data (Figure 7A) uncovered slow and only partial reduction of the highly oxidized Pt component at this temperature. An apparently stepwise reduction was observed, as Pt was further reduced during each cycle (up to about 40 % at the end of the 10th cycle). After the very slow Pt reduction during the first cycle only partial Pt reoxidation was observed when the mixture was switched back to 10 % O2/He. The next cycles revealed a fast Pt reduction onset followed by slow reduction for the remaining time of the reducing step. Similar as during the 1st cycle, Pt sites did not recover their initial high oxidation state within the following oxidizing cycle. This was probably caused by the formation of slightly larger particles, which are more difficult to completely reoxidize.15 The sintering of Pt NPs continued even after six reducing cycles and about 50 % of Pt sites were reduced after another ten minutes in 2 % H2/He. The absence of CeO2 reduction by H2 at 150 °C could be linked to the slow kinetics of Pt reduction and consequently of H2 activation, at this temperature even under prolonged exposure to 2 % H2/He (Figure 7A).


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Figure 7 left: Pt L3-edge XANES spectra acquired during successive redox cycling (A: 2 % H2/He ↔ 10 % O2/He at 150 °C, B: 2 % CO ↔ 10 % O2/He at 150 °C, C: 2 % H2 ↔ 10 % O2/He at 250 °C and D: 2 % CO ↔ 10 % O2/He at 250 °C). right: Respective LCF results (blue: contribution of oxidized reference, red: contribution of reduced reference, top: indication of applied reducing/oxidizing atmosphere).

In the next step, the cycling was repeated at 150 °C using 2 % CO/He (Figure 7B): Within the first two cycles Pt was reduced to a much higher extent (about 55 % reduction). However, a similar Pt reduction trend was observed during further cycling as during the hydrogen reduction (Figure 7A). Pt still did not show a high tendency towards further reduction (about 60 % after the final five minutes in 2 % CO/He). This was attributed to the presence of oxidized Pt species in close contact with ceria, which are more difficult to reduce. For those conditions only minor ceria reduction was encountered (less than 10 % 20 ACS Paragon Plus Environment

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reduction of Ce4+ to Ce3+ after exposure to 2 % CO/He for several minutes, Figure 6B). Hence, the response of the Pt component to both H2 and CO redox cycling treatments at 150 °C was more pronounced compared to ceria. This behavior indicates that the presence of reduced Pt sites is a critical prerequisite for promoting CeO2 reducibility at low temperatures and CO is the more suitable agent for forming Pt nanoparticles. At 250 °C platinum underwent a more pronounced reduction during the redox cycles. Throughout the first 90 seconds in 2 % H2/He (Figure 7C), Pt reduction occurred slower compared to the following reducing cycles, which is similar to the redox behavior observed for ceria (Figure 6B, see comparison between Pt and Ce oxidation state in Figure 8). This is associated with the formation of slightly larger Pt particles, which were easier to reduce in the following redox cycles. A similar trend with a period of fast onset of the Pt reduction and a following period with slow further reduction was visible at this temperature during the reduction in hydrogen of the redox cycle and during the 10 min long reduction (85 % reduction). Note that the reduction extent of the CeO2 support passed through a maximum after three reducing/oxidizing cycles (about 31 %, Figure 6B), which suggests that the evolution of the Pt/CeO2 interface went through a maximum, i.e. first reduction of platinum followed by a sintering process. At the end of the experiment more than 85 % of the Pt sites were reduced, but only 30 % reduction of CeO2 was observed in the corresponding experiment at the Ce L3-edge. The closer comparison of the changes of the Pt and Ce oxidation states during redox cycling at 250 °C (Figure 8) with 2 % H2/He (extracted from QEXAFS experiments at Ce L3-edge (Figure 6B) and Pt L3edge (Figure 7C)) revealed that Pt reduction always preceded the reduction of ceria. Obviously, the presence and formation of reduced Pt particles was a pre-condition for the reduction of ceria at low temperatures (Figure 8). Highly dispersed Pt species or very small and oxidized Pt particles were not able to efficiently activate hydrogen molecules at 250 °C. As a consequence, during the first H2 pulse at 250 °C only slow ceria reduction was observed and only after reduced Pt particles were formed (depicted in Figure 8B). However, there are further differences if appropriate Pt particles (reduced and of a certain 21 ACS Paragon Plus Environment

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particle size) were present, as for example in the 2nd hydrogen cycle at 250 °C (Figure 8). While Pt was reduced in one step (e.g. Figure 8: 2nd H2 phase) two prominent steps can be identified for ceria: first, a very fast reduction step associated with the direct Pt-ceria interface sites, and second, a slow reduction step, which is attributed to H2 spillover and reduction of ceria sites that are not in direct contact with Pt.47

Figure 8 A) Comparison of LCF-results obtained from Pt L3 (Figure 7C) and Ce L3 (Figure 6B) XANES data of the Pt/CeO2Al2O3 sample during redox cycling (H2 ↔ O2) at 250 °C during the first six minutes. Ceria reduction was observed only after significant reduction of Pt. Particularly during the first H2 pulse ceria reduction was delayed (marked red) and slow due to formation of Pt particles. B) Schematic representation of the Pt state during 1st and 2nd hydrogen pulse and its consequence for hydrogen activation and ceria reduction based on the LCF results of the redox cycling experiment. Metallic Pt particles are required to initiate the reduction of ceria.

Upon the change to CO as reductant during redox cycling at 250 °C, a steady-state relative to the reduction/reoxidation extent was already reached after the second cycle (Figure 7D). A similar behavior was reflected in the reduction of ceria with pronounced reduction (40 %, Figure 6E). Although according to LCF analysis maximum Pt reduction appeared to be lower for the CO-treated sample (about 75 %) as compared to the H2-conditioned catalyst, the reoxidation after the long-term reduction step was less pronounced in the latter case (55 % versus 50 % after reduction by CO). In addition, for small Pt nanoparticles the changes in the electronic structure of platinum due to CO or H2 adsorption have to be considered. Due to the high surface to bulk ratio, the XANES spectra were significantly altered resulting 22 ACS Paragon Plus Environment

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in a higher white-line signal (see e.g. 48-49). Since the Pt redox behavior can be correlated to the Pt particle size,15 a smaller Pt particle size is indicated by a stronger Pt reoxidation tendency. During redox cycles with H2 at 250 °C (Figure 7C) Pt was oxidized less during each step, reflecting an increase in Pt particle size during the cycling. The results furthermore reveal that Pt particles became even larger when CO was used (Figure 7D), in line with previous reports.13 Finally, the enhanced reducibility of the larger Pt particles formed during CO cycling at 250 °C and thus also of the ceria component, became obvious if the catalyst was cycled subsequently at 150 °C with H2. In Figure 9, the redox responses of Pt and of ceria to hydrogen pulses at 150 °C are presented for the catalyst after lean conditioning (i.e. after calcination at 500 °C, deactivated state) and the activated catalyst after redox cycling at 250 °C using CO as reductant. In the deactivated sample, which contains Pt in highly dispersed or atomically distributed state (Figure 1: TEM of the as prepared catalyst), neither Pt nor ceria showed a significant response to the hydrogen exposure. However, if slightly larger and reduced Pt particles were present the catalyst was not only in a more active state (Figure 2), but also both Pt and ceria exhibited a fast and pronounced reduction and reoxidation during the H2/O2 redox cycles. This proves the unique role of the structure and particle size of platinum for the ceria redox chemistry.

Figure 9 Top: Schematic presentation of the oxidation state of Pt and its influence on the ceria reducibility. Bottom: Comparison between the LCF-results obtained from Pt L3 and Ce L3 XANES data acquired during redox cycling (H2 ↔ O2) at 150 °C of the A) oxidized and B) redox cycled (250 °C, 2 % CO) Pt/CeO2-Al2O3 sample.


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The corresponding CO oxidation light-off activity (Figure 2) changed from a T50 of 220 °C for the catalyst in its deactivated state (oxidative treatment in lean mixture at 500 °C) to a T50 of 143 °C of the catalyst activated in CO. Hence, the variations in the noble metal and ceria redox chemistry upon slight sintering of Pt nanoparticles directly correlate with the CO oxidation activity of the catalyst. The evolution and the close relationship between the structure and oxidation state of Pt and the redox properties of ceria at low temperatures are schematically summarized in Figure 10A. For further discussion and further analysis, we chose the dynamic changes at the interface upon hydrogen/oxygen redox cycles at 250 °C (Figure 10B). As especially observed during the first pulse, the formation of reduced Pt particles represents an important prerequisite for a reducible and redox active Pt-ceria interface, as earlier reported in literature.16,

42, 44-45

However, further sintering of the noble metal

nanoparticles occurs and the extent of the interface starts to decrease. To substantiate this hypothesis, the relationship between the state of Pt and ceria reducibility was further investigated by estimating an optimal Pt particle size in the case of H2 redox cycles. For this purpose, the fraction of ceria interface sites (related to the total amount of surface ceria) was extracted by evaluating the extent of Ce3+ formation and considering the reduction of interfacial ceria sites until their formation slows down (maxima in derivate of Ce3+ species in Figure 10B). Based on the metal loading and the total ceria surface area, a relationship between the fraction of the ceria surface, which is in contact with the metallic Pt component, and Pt particle size was derived (further details see ESI). This allows to estimate the Pt particles size which is sufficient to activate the ceria redox chemistry. Two different regimes can be recognized (Figure 10B+C): ionic Pt species or too small Pt particles, which do not enhance the ceria reducibility, despite their large interface (indicated with red); larger Pt particles for which fast ceria reduction is encountered (indicated with blue). Consequently, the small Pt NPs must be slightly sintered, e.g. by reducing pulses, in order to activate the ceria redox chemistry. If the particles are just sufficiently large to efficiently activate the ceria redox chemistry and to provide a large amount of interface sites the particle size becomes optimal, which is the case for about 1.4 nm Pt particles in this example (see Figure 10C and ESI for further information).


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Figure 10: A) Underlying processes during redox cycles on the Pt/CeO2-Al2O3 sample exemplary for the case when hydrogen was used as reductant at 250 °C: The initial highly dispersed Pt entities on ceria (1) are reduced during hydrogen pulses and form metallic Pt nanoparticles (2+3). The metallic Pt particle size thus determines the redox properties of the ceria support. The initially formed very small particles (2) enhance a larger amount of reduced ceria (indicated red) compared to the subsequently formed larger Pt particles with a smaller Pt/CeO2 perimeter (3). B) Extraction of the fraction of interfacial ceria surface sites (PtCeO2) related to the ceria surface sites from the H2/O2 redox cycling experiment at 250 °C. C) Identification of the optimal and appropriate Pt particle size for active and high fraction of ceria interface sites.

This highlights the critical role of the noble metal particle size on ceria based catalysts, on the one hand for the redox properties of the ceria component, on the other hand for the catalytic activity. The activation potential of the reductant (Figure 2) correlates to its capability to effectively reduce Pt at the activation temperature (Figure 7) and to activate the Pt-ceria interface (Figure 9). Conclusions Complementary operando QEXAFS measurements at the Ce L3-and Pt L3-edges on a Pt/CeO2-Al2O3 model diesel oxidation catalyst during TPR with CO, H2 and C3H6 as well as redox cycling underlined the critical role and nature of the Pt-CeO2 interface. The presence of reduced Pt and thus Pt NPs was 25 ACS Paragon Plus Environment

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identified as prerequisite for ceria reduction at low temperature. The Pt particle size represents a decisive factor for several reasons: At first, it defines the sites available for activation of the reductant at low temperatures to enable ceria reduction. Therefore, ceria reduction was observed at significantly higher temperatures in the case of reductants which require molecular activation (H2 and C3H6). Hence, the same trend as for the activation of the catalyst by reducing pulses was observed: CO > H2 > C3H6. Secondly, the particle size determines the number of interfacial sites between metallic Pt particles and ceria, which decreases if Pt particles become larger. Consequently, an optimal Pt particle size is encountered, which also provides an optimal interface (active and large) under the respective conditions. Based on a geometric model, an optimum particle size of about 1.4 nm was derived for hydrogen activation, underlining the beneficial nature of the activation procedure using re-dispersion by oxygen treatment and careful sintering by pulsing a reductant into the reaction mixture. In addition, it demonstrates the high importance of an intimate and optimal interaction between the noble metal and ceria in activating the versatile redox chemistry. This is important for various applications alongside high oxygen storage capacity and enhanced low temperature CO oxidation. Supporting Information Further Ce L3 XANES data obtained during TPR experiments and redox cycling, Pt L3 XANES data obtained during TPR, evaluation of apparent activation energy for ceria reduction, estimation of optimal Pt particle size for ceria reduction. Acknowledgements The authors thank the German Federal Ministry for Economic Affairs and Energy (BMWi: 19U15014B) and the French National Research Agency (ANR-14-CE22-0011-02) for financial support of the ORCA project within the DEUFRAKO program, as well as the Federal Ministry of Education and Research for financial support of the ZeitKatMat (BMBF: 05K13VK13) and MatDynamics (BMBF: 05K2016) projects. Paul Sprenger and Gülperi Cavusoglu are acknowledged for their help during catalyst preparation and Angela Beilmann for N2-physisorption measurements. Dmitry Doronkin, Abhijeet Gaur, 26 ACS Paragon Plus Environment

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Gülperi Cavusoglu and Deniz Zengel are thanked for their help during beamtimes. Thomas Bergfeldt (Institute for Applied Materials, KIT) is acknowledged for elemental analysis. Furthermore, the authors wish to thank Olga Safonova and Urs Vogelsang (SuperXAS beamline, SLS, PSI) as well as Wolfgang Caliebe, Mathias Herrmann and Marcel Görlitz (beamline P64, PETRA III, DESY) for their support during the XAS experiments. SLS and PETRA III are acknowledged for providing beamtime.

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15. Nagai, Y.; Dohmae, K.; Ikeda, Y.; Takagi, N.; Tanabe, T.; Hara, N.; Guilera, G.; Pascarelli, S.; Newton, M. A.; Kuno, O.; Jiang, H.; Shinjoh, H.; Matsumoto, S., In Situ Redispersion of Platinum Autoexhaust Catalysts: An On-Line Approach to Increasing Catalyst Lifetimes? Angew. Chem. Int. Ed. 2008, 47, 9303-9306. 16. Cargnello, M.; Doan-Nguyen, V. V. T.; Gordon, T. R.; Diaz, R. E.; Stach, E. A.; Gorte, R. J.; Fornasiero, P.; Murray, C. B., Control of Metal Nanocrystal Size Reveals Metal-Support Interface Role for Ceria Catalysts. Science 2013, 341, 771. 17. Serre, C.; Garin, F.; Belot, G.; Maire, G., Reactivity of Pt/Al2O3 and Pt-CeO2Al2O3 Catalysts for the Oxidation of Carbon Monoxide by Oxygen: II. Influence of the Pretreatment Step on the Oxidation Mechanism. J. Catal. 1993, 141, 9-20. 18. Holmgren, A.; Azarnoush, F.; Fridell, E., Influence of Pre-Treatment on the Low-Temperature Activity of Pt/Ceria. Appl. Catal. B 1999, 22, 49-61. 19. Gatla, S.; Aubert, D.; Agostini, G.; Mathon, O.; Pascarelli, S.; Lunkenbein, T.; Willinger, M. G.; Kaper, H., Room-Temperature CO Oxidation Catalyst: Low-Temperature Metal–Support Interaction between Platinum Nanoparticles and Nanosized Ceria. ACS Catal. 2016, 6, 6151-6155. 20. Bruix, A.; Rodriguez, J. A.; Ramírez, P. J.; Senanayake, S. D.; Evans, J.; Park, J. B.; Stacchiola, D.; Liu, P.; Hrbek, J.; Illas, F., A New Type of Strong Metal–Support Interaction and the Production of H2 through the Transformation of Water on Pt/CeO2(111) and Pt/CeOx/TiO2(110) Catalysts. J. Am. Chem. Soc. 2012, 134, 8968-8974. 21. Kopelent, R.; van Bokhoven, J. A.; Szlachetko, J.; Edebeli, J.; Paun, C.; Nachtegaal, M.; Safonova, O. V., Catalytically Active and Spectator Ce3+ in Ceria-Supported Metal Catalysts. Angew. Chem. Int. Ed. 2015, 54, 8728-8731. 22. Kopelent, R.; van Bokhoven, J. A.; Nachtegaal, M.; Szlachetko, J.; Safonova, O. V., X-ray Emission Spectroscopy: Highly Sensitive Techniques for Time-Resolved Probing of Cerium Reactivity under Catalytic Conditions. Phys. Chem. Chem. Phys. 2016, 18, 32486-32493. 23. Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M., Active Nonmetallic Au and Pt Species on CeriaBased Water-Gas Shift catalysts. Science 2003, 301, 935-938. 24. Cavusoglu, G.; Miao, D.; Lichtenberg, H.; Carvalho, H. W. P.; Xu, H.; Goldbach, A.; Grunwaldt, J. D., Structure and Activity of Flame Made Ceria Supported Rh and Pt Water Gas Shift Catalysts. Appl. Catal. A 2015, 504. 25. Pino, L.; Recupero, V.; Beninati, S.; Shukla, A. K.; Hegde, M. S.; Bera, P., Catalytic Partial-Oxidation of Methane on a Ceria-Supported Platinum Catalyst for Application in Fuel Cell Electric Vehicles. Appl. Catal. A 2002, 225, 63-75. 26. Lykhach, Y.; Bruix, A.; Fabris, S.; Potin, V.; Matolinova, I.; Matolin, V.; Libuda, J.; Neyman, K. M., Oxide-Based Nanomaterials for Fuel Cell Catalysis: The Interplay Between Supported Single Pt Atoms and Particles. Catal. Sci. Technol. 2017, 7, 4315-4345. 27. Marchionni, V.; Szlachetko, J.; Nachtegaal, M.; Kambolis, A.; Kröcher, O.; Ferri, D., An Operando Emission Spectroscopy Study of Pt/Al2O3 and Pt/CeO2/Al2O3. Phys. Chem. Chem. Phys. 2016, 18, 29268-29277. 28. Yamamoto, T.; Suzuki, A.; Nagai, Y.; Tanabe, T.; Dong, F.; Inada, Y.; Nomura, M.; Tada, M.; Iwasawa, Y., Origin and Dynamics of Oxygen Storage/Release in a Pt/Ordered CeO2–ZrO2 Catalyst Studied by Time-Resolved XAFS Analysis. Angew. Chem. Int. Ed. 2007, 46, 9253-9256. 29. Rothensteiner, M.; Bonk, A.; Vogt, U. F.; Emerich, H.; van Bokhoven, J. A., Structural Changes in Ce0.5Zr0.5O2−δ under Temperature-Swing and Isothermal Solar Thermochemical Looping Conditions Determined by In Situ Ce K and Zr K Edge X-ray Absorption Spectroscopy. J. Phys. Chem. C 2016, 120, 13931-13941. 30. Nagai, Y.; Dohmae, K.; Nishimura, Y. F.; Kato, H.; Hirata, H.; Takahashi, N., Operando XAFS Study of Catalytic NO Reduction over Cu/CeO2: The Effect of Copper-Ceria Interaction under Periodic Operation. Phys. Chem. Chem. Phys. 2013, 15, 8461-8465. 28 ACS Paragon Plus Environment

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31. Teoh, W. Y.; Amal, R.; Madler, L., Flame Spray Pyrolysis: An Enabling Technology for Nanoparticles Design and Fabrication. Nanoscale 2010, 2, 1324-1347. 32. Brunauer, S.; Emmett, P. H.; Teller, E., Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309-319. 33. Müller, O.; Stötzel, J.; Lützenkirchen-Hecht, D.; Frahm, R., Gridded Ionization Chambers for Time Resolved X-Ray Absorption Spectroscopy. J. Phys. Conf. Ser. 2013, 425, 092010. 34. Müller, O.; Nachtegaal, M.; Just, J.; Lutzenkirchen-Hecht, D.; Frahm, R., Quick-EXAFS Setup at the SuperXAS Beamline for In Situ X-ray Absorption Spectroscopy with 10 ms Time Resolution. J. Synchrotron Radiat. 2016, 23, 260-266. 35. Grunwaldt, J. D.; Caravati, M.; Hannemann, S.; Baiker, A., X-ray Absorption Spectroscopy under Reaction Conditions: Suitability of Different Reaction Cells for Combined Catalyst Characterization and Time-Resolved Studies. Phys. Chem. Chem. Phys. 2004, 6, 3037-3047. 36. Müller, O. Hard X-ray Synchrotron Beamline Instrumentation for Millisecond Quick Extended X-ray Absorption Spectroscopy. PhD Thesis, Bergische Universität Wuppertal, 2016, urn:nbn:de:hbz:46820160926-113328-5. 37. Hatanaka, M.; Takahashi, N.; Takahashi, N.; Tanabe, T.; Nagai, Y.; Suda, A.; Shinjoh, H., Reversible Changes in the Pt Oxidation State and Nanostructure on a Ceria-Based Supported Pt. J. Catal. 2009, 266, 182-190. 38. Nachimuthu, P.; Shih, W.-C.; Liu, R.-S.; Jang, L.-Y.; Chen, J.-M., The Study of Nanocrystalline Cerium Oxide by X-Ray Absorption Spectroscopy. J. Solid State Chem. 2000, 149, 408-413. 39. Paun, C.; Safonova, O. V.; Szlachetko, J.; Abdala, P. M.; Nachtegaal, M.; Sa, J.; Kleymenov, E.; Cervellino, A.; Krumeich, F.; van Bokhoven, J. A., Polyhedral CeO2 Nanoparticles: Size-Dependent Geometrical and Electronic Structure. J. Phys. Chem. C 2012, 116, 7312-7317. 40. Marchbank, H. R.; Clark, A. H.; Hyde, T. I.; Playford, H. Y.; Tucker, M. G.; Thompsett, D.; Fisher, J. M.; Chapman, K. W.; Beyer, K. A.; Monte, M.; Longo, A.; Sankar, G., Structure of Nano-sized CeO2 Materials: Combined Scattering and Spectroscopic Investigations. ChemPhysChem 2016, 17, 3494-3503. 41. Lee, J.; Ryou, Y.; Chan, X.; Kim, T. J.; Kim, D. H., How Pt Interacts with CeO2 under the Reducing and Oxidizing Environments at Elevated Temperature: The Origin of Improved Thermal Stability of Pt/CeO2 Compared to CeO2. J. Phys. Chem. C 2016, 120, 25870-25879. 42. Alayoglu, S.; An, K.; Melaet, G.; Chen, S.; Bernardi, F.; Wang, L. W.; Lindeman, A. E.; Musselwhite, N.; Guo, J.; Liu, Z.; Marcus, M. A.; Somorjai, G. A., Pt-Mediated Reversible Reduction and Expansion of CeO2 in Pt Nanoparticle/Mesoporous CeO2 Catalyst: In Situ X-ray Spectroscopy and Diffraction Studies under Redox (H2 and O2) Atmospheres. J. Phys. Chem. C 2013, 117, 26608-26616. 43. Artiglia, L.; Orlando, F.; Roy, K.; Kopelent, R.; Safonova, O.; Nachtegaal, M.; Huthwelker, T.; van Bokhoven, J. A., Introducing Time Resolution to Detect Ce3+ Catalytically Active Sites at the Pt/CeO2 Interface through Ambient Pressure X-ray Photoelectron Spectroscopy. J. Phys. Chem. Lett. 2016, 8, 102-108. 44. Stark, W. J.; Grunwaldt, J.-D.; Maciejewski, M.; Pratsinis, S. E.; Baiker, A., Flame-Made Pt/Ceria/Zirconia for Low-Temperature Oxygen Exchange. Chem. Mater. 2005, 17, 3352-3358. 45. Jacobs, G.; Graham, U. M.; Chenu, E.; Patterson, P. M.; Dozier, A.; Davis, B. H., Low-Temperature Water–Gas Shift: Impact of Pt Promoter Loading on the Partial Reduction of Ceria and Consequences for Catalyst Design. J. Catal. 2005, 229, 499-512. 46. Dutta, G.; Waghmare, U. V.; Baidya, T.; Hegde, M. S., Hydrogen Spillover on CeO2/Pt: Enhanced Storage of Active Hydrogen. Chem. Mater. 2007, 19, 6430-6436. 47. Karim, W.; Spreafico, C.; Kleibert, A.; Gobrecht, J.; VandeVondele, J.; Ekinci, Y.; van Bokhoven, J. A., Catalyst Support Effects on Hydrogen Spillover. Nature 2017, 541, 68-71.


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48. Gänzler, A. M.; Casapu, M.; Boubnov, A.; Müller, O.; Conrad, S.; Lichtenberg, H.; Frahm, R.; Grunwaldt, J.-D., Operando Spatially and Time-Resolved X-ray Absorption Spectroscopy and Infrared Thermography during Oscillatory CO Oxidation. J. Catal. 2015, 328, 216-224. 49. Safonova, O. V.; Tromp, M.; van Bokhoven, J. A.; de Groot, F. M. F.; Evans, J.; Glatzel, P., Identification of CO Adsorption Sites in Supported Pt Catalysts Using High-Energy-Resolution Fluorescence Detection X-ray Spectroscopy. J. Phys. Chem. B 2006, 110, 16162-16164.

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Figure 1 HAADF-STEM image (A)and EDX map (B) of CeO2-Al2O3 reveal the presence of ceria nanoparticles (3-5 nm) on the alumina support. HAADF-STEM image (C) and EDX map (D) of Pt/CeO2-Al2O3 show again ceria nanoparticles in the range of 3-5 nm. In addition, they identify Pt to be in a highly dispersed state and in intimate contact with ceria. XRD patterns (E) confirm the presence of ceria as small crystallites (broad reflections) and the absence of Pt reflections. 128x206mm (300 x 300 DPI)

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Figure 2 Light-off curves of Pt/CeO2-Al2O3 during CO-oxidation after different pretreatment procedures. The catalyst was subjected to an oxidative conditioning step (lean: 1 h in 1000 ppm CO, 500 ppm C3H6, 8 % O2, N2 at 500 °C) and to lean/rich activation protocols. By varying the reductant (0.22 % C3H6, 2 % H2 or 2 % CO in N2) during the lean/rich treatment the catalyst was activated to a different extent as reflected by the change in the light-off behavior. 55x37mm (300 x 300 DPI)


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Figure 3 Ce L3-edge XANES spectra used for linear combination fitting (LCF) analysis. Spectra of oxidized (acquired under 2 % H2 in He at RT) and reduced Pt/CeO2-Al2O3 (acquired under 2 % H2 in He at 600 °C) were used as reference spectra representing Ce3+ and Ce4+in the LCF analysis, due to their similarities to the spectra of bulk reference compounds. Different spectral features are marked A to E. 61x46mm (300 x 300 DPI)


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Figure 4 Ce L3-edge XANES data (A+B) obtained on CeO2-Al2O3 at the middle of the catalyst bed during TPR experiments (heating with 5 °C/min in A: 2 % H2/He and B: 2 % CO/He) and respective LCF-results (C+D). 80x80mm (300 x 300 DPI)


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Figure 5 Ce L3-edge XANES spectra (A-C) of Pt/CeO2-Al2O3 during TPR experiments (heating at 5 °C/min in A: 2 % C3H6/He, B: 2 % H2/He and C: 2 % CO/He) with corresponding LCF-results (D-F). The reduction from Ce4+ to Ce3+ started already at about 100 °C in CO, at about 200 °C in H2 and at about 300 °C in C3H6. 106x71mm (300 x 300 DPI)


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Scheme 1 Influence of the choice of reductant on the surface and bulk ceria reduction in the Pt loaded and Pt free material. 20x5mm (300 x 300 DPI)


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Figure 6 Results of linear combination fitting of the Ce L3-edge XANES spectra of Pt/CeO2-Al2O3 during redox cycling with 2 % H2/He (A-C) or 2 % CO/He (D-F) as reducing and 10 % O2/He as oxidizing mixture at various temperatures (150 °C, 250 °C and 350 °C). In addition, the mass spectrometer signals are given for H2 (m/z=2), O2 (m/z=32) and H2O (m/z=18) during H2-TPR and for CO (m/z=28), O2 (m/z=32) and CO2 (m/z=44) during CO-TPR. 114x81mm (300 x 300 DPI)


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Figure 7 left: Pt L3-edge XANES spectra acquired during successive redox cycling (A: 2 % H2/He ↔ 10 % O2/He at 150 °C, B: 2 % CO ↔ 10 % O2/He at 150 °C, C: 2 % H2 ↔ 10 % O2/He at 250 °C and D: 2 % CO ↔ 10 % O2/He at 250 °C). right: Respective LCF results (blue: contribution of oxidized reference, red: contribution of reduced reference, top: indication of applied reducing/oxidizing atmosphere). 140x247mm (300 x 300 DPI)


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Figure 8 A) Comparison of LCF-results obtained from Pt L3 (Figure 7C) and Ce L3 (Figure 6B) XANES data of the Pt/CeO2-Al2O3 sample during redox cycling (H2 ↔ O2) at 250 °C during the first six minutes. Ceria reduction was observed only after significant reduction of Pt. Particularly during the first H2 pulse ceria reduction was delayed (marked red) and slow due to formation of Pt particles. B) Schematic representation of the Pt state during 1st and 2nd hydrogen pulse and its consequence for hydrogen activation and ceria reduction based on the LCF results of the redox cycling experiment. Metallic Pt particles are required to initiate the reduction of ceria. 70x31mm (300 x 300 DPI)


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Figure 9 Top: Schematic presentation of the oxidation state of Pt and its influence on the ceria reducibility. Bottom: Comparison between the LCF-results obtained from Pt L3 and Ce L3 XANES data acquired during redox cycling (H2 ↔ O2) at 150 °C of the A) oxidized and B) redox cycled (250 °C, 2 % CO) Pt/CeO2-Al2O3 sample. 59x43mm (300 x 300 DPI)


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Figure 10: A) Underlying processes during redox cycles on the Pt/CeO2-Al2O3 sample exemplary for the case when hydrogen was used as reductant at 250 °C: The initial highly dispersed Pt entities on ceria (1) are reduced during hydrogen pulses and form metallic Pt nanoparticles (2+3). The metallic Pt particle size thus determines the redox properties of the ceria support. The initially formed very small particles (2) enhance a larger amount of reduced ceria (indicated red) compared to the subsequently formed larger Pt particles with a smaller Pt/CeO2 perimeter (3). B) Extraction of the fraction of interfacial ceria surface sites (Pt-CeO2) related to the ceria surface sites from the H2/O2 redox cycling experiment at 250 °C. C) Identification of the optimal and appropriate Pt particle size for active and high fraction of ceria interface sites. 99x62mm (300 x 300 DPI)


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CeO2-Interface by In Situ Variation of the Pt Particle Size

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