Highly Active Ceria-Supported Ru Catalyst for the Dry Reforming of

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Highly Active Ceria Supported Ru Catalyst for the Dry Reforming of Methane: In-situ Identification of Ru#+-Ce3+ Interactions for Enhanced Conversion Zongyuan Liu, Feng Zhang, Ning Rui, Xing Li, Lili Lin, Luis E. Betancourt, Dong Su, Wenqian Xu, Jiajie Cen, Klaus Attenkofer, Hicham Idriss, Jose A. Rodriguez, and Sanjaya D. Senanayake ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b05162 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Highly Active Ceria Supported Ru Catalyst for the Dry Reforming of Methane: In-situ Identification of Ruδ+-Ce3+ Interactions for Enhanced Conversion Zongyuan Liu,1 Feng Zhang,2 Ning Rui,1 Xing Li,3,7 Lili Lin,1 Luis E. Betancourt,1 Dong Su,3 Wenqian Xu,4 Jiajie Cen,2 Klaus Attenkofer,5 Hicham Idriss,6 José A. Rodriguez,1,2* and Sanjaya D. Senanayake1*

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Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973, USA Materials Science and Chemical Engineering Department, Stony Brook University, Stony Brook, NY 11794, USA 3 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973 USA 4 X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA 5 National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973, USA 6 Fundamental Catalysis, Centre for Research and Development (CRD), SABIC, KAUST, Saudi Arabia 7 Department of Physics and Engineering, Key Laboratory of Material Physics, Zhengzhou University, Zhengzhou, Henan, 450054, China 2

Corresponding Author: *Bldg. 555A, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973-5000, 631-344-4343 [email protected] , [email protected] *

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ABSTRACT The metal-oxide interaction changes the surface electronic states of catalysts deployed for chemical conversion, yet details of its influence on the catalytic performance under reaction conditions remain obscure. In this work, we report the high activity/stability of a ceria supported Ru-nanocluster ( 300 oC). The possible reaction pathways and stable surface intermediates were identified using DRIFTS including ruthenium carbonyls, carboxylate species, and surface -OH groups while polydentate carbonates may be plain spectators at the measured reaction conditions. KEYWORDS: ruthenium, ceria, XRD, EXAFS, AP-XPS, DRIFTS, dry reforming of methane

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1. INTRODUCTION Dry reforming of methane (DRM, CH4 + CO2 ⇋ 2H2 + 2CO; ΔH298 = 246 kJ/mol) has received great attention in the catalysis community as an important process for producing syngas at low ratio (≤1) of H2/CO, that allows the tuning of syngas produced in steam reforming (H2/CO, 3:1). This is especially important for selective long-chain hydrocarbons synthesis in the Fischer-Tropsch reaction.1 In addition, the ability to re-utilize CO2, a known byproduct of numerous chemical processes simultaneously with methane, a cheap and abundant fuel stock, is economically attractive.2 The non-polar, chemically stable nature of methane (C-H bond energy: 104 kcal/mol) along with the highly endothermic character of the reaction enthalpy (ΔH298 = 246 kJ/mol) make the dry reforming reaction a challenging task. The partial oxidation redox balance of the reaction requires catalysts that can take advantage of the activation of both difficult reactants. The penalty for this is the need to undertake the reaction at high temperature, requiring thermally stable catalysts with good selectivity. Another obstacle encountered during the dry reforming process is the deactivation of the catalyst by carbon deposition (coke), metal sintering, and metal oxidation. These phenomena associated with deactivation are linked to the intrinsic nature of the metal catalyst.3-4 Thus, designing catalysts with enhanced activity, high thermal stability and resistance to carbon deposition is the main topic in numerous studies of methane dry reforming and requires a detailed knowledge of the active centers and reaction mechanism for the DRM process. One of the key strategies is to configure the dry reforming catalysts with active metal components anchored on high-surface-area oxides, where the metal-support interfaces play pivotal roles in governing the electronic and chemical properties of the metal catalyst. Such metal-oxide interplay seems to facilitate the activation of methane by lowering the methane activation energy barrier.5 Yet the details of this interaction and its influence on the catalytic process under reaction conditions remain obscure. In this study, we use a series of in-situ techniques to understand the behavior of a Ru-CeO2 catalyst under DRM conditions. In 3


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previous studies, Ru based catalysts have exhibited high activity and stability against the deactivation by carbon deposition.6-9 The choice of oxide support also plays a significant role in enhancing or influencing the catalytic activity and stability. Ru supported on Al2O3,10-13 TiO2,14 ZrO2,15 La2O3,15-16 MgO,7, 17 SiO212-13 and carbon black18 have been shown to catalyze the DRM reaction with different degree of reactivity. Recently, ceria-based catalysts have been considered as promising support for C1 chemistry due to the ability of ceria to anchor metals and donate oxygen readily,19-24 The combination of ceria with Ru seems attractive as a catalyst for the dry reforming of methane.25-26 A recent report has shown that atomic Ru species could be stabilized on ceria by mild oxidative pretreatment.27 It have been postulated by kinetic studies that the coordinately unsaturated Ru surface atoms are more active than those in low index crystal planes predominately exposed on large crystallites.9 However, no evidence has been shown regarding the electronic character of such configuration in an atomic level. Specifically, when Ru downsizes into small nanoclusters in close contact with ceria, the catalytic active sites under reaction conditions, the nature of the metal-support interaction, as well as the mechanistic elementary steps of methane dry reforming remain largely unknown. In this work, we present a stable and highly active Ru-nanocluster/CeO2 catalyst for methane dry reforming. To gain deep insight into the catalytic structure-reactivity relationship, a set of in-situ experiments were conducted to probe the dynamic state of active sites, chemical speciation and transient chemical intermediates under reaction conditions. A combination of in-situ X-ray diffraction (XRD) and in-situ X-ray absorption fine structure (XAFS) spectroscopy was used to elucidate the active structure of catalysts in both long and short range. In-situ Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) was also employed to provide surface sensitive information regarding the surface chemical states and oxygen vacancies of ceria. Finally, the reaction mechanism and active intermediates were identified by using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Our studies show a dynamic evolution in the catalytic chemistry and highlight the 4


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importance of metal-oxide interactions on tuning the structural and electronic properties of the catalyst, and thus influence the catalytic performance. 2. EXPERIMENTAL SECTION 2.1. Catalysts preparation Cerium oxide (57 m2/g) was prepared by precipitating a solution of cerium nitrate at pH 9 with ammonia. The resulting precipitate was filtrated and washed. Upon drying at 100 oC for overnight, it was calcined in air at 450 oC for 5 hours.28 Ru was then added to different oxide supports by incipient wetness impregnation method. An appropriate amount of Ruthenium chloride (RuCl3, Sigma-Aldrich) to achieve 0.5 wt% of metal loading was first dissolved in de-ionized water at room temperature, and the solution was dropwise added to the as-prepared cerium oxide, commercial cerium oxide (30 m2/g, Sigma-Aldrich), titanium oxide (anatase, Sigma-Aldrich) and aluminum oxide (γ-Al2O3, Sigma-Aldrich) for impregnation, respectively. The mixed slurries were then dried overnight at 100 °C. The resulting products were finally calcined in air at 400 °C (5 °C/min ramping rate) for 6 hours. XRD (Figure S1) of the as-prepared samples indicate the crystallinity of the loaded Ru. Here, following previous articles,29-30 the 0.5 wt% Ru supported on synthesized ceria is designated as Ru nanocluster (NC)-CeO2 while the one prepared using commercial ceria is denoted as Ru nanoparticle (NP)-CeO2. 2.2. Catalytic activity tests A series activity tests for the DRM reaction were carried out for the as-prepared samples. The powder catalysts (10 mg, 60-80 mesh) were diluted by ~ 20mg of pre-calcined quartz (900 oC, 60-80 mesh), and then loaded into a quartz tube and mounted on a flow system. The ratio of CH4 and CO2 was set at 1:1 (10 mL/min CH4 with 10 mL/min CO2) in the catalytic performance test and diluted by N2 (10 mL/min). The catalysts were reduced under a 10 mL/min flow of H2 at 400 °C for 1 h prior to reaction. Their DRM activity was measured at 5


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400, 450, 500, 550, 600, 650 and 700 °C, with 2 hours of isothermal at each step. The concentrations of gas products were analyzed with a gas chromatography instrument (Agilent 7890A) equipped with both flame ionization and thermal conductivity detectors. 2.3. Catalyst Characterization Transmission electron microscopy (TEM) Transmission electron microscopy (TEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and element mapping were conducted on a high-resolution analytical scanning/transmission electron microscope (S/TEM, FEI Talos F200X) operating at 200 keV at Center for Functional Nanomaterials, Brookhaven National Laboratory (BNL). The elemental mappings (Ru L-edge) were acquired with a four-quadrant 0.9-sr energy dispersive X-ray spectrometer (Super EDS). The sample was prepared by drop-casting methanol diluted catalyst solutions onto copper grids coated with amorphous carbon membranes and dried for TEM measurements. In-situ time-resolved X-ray diffraction (XRD) The in-situ time-resolved XRD measurements for the Ru-CeO2 catalyst were performed at beamline 17BM (λ = 0.45260 Å) at Advanced Photon Source (APS) with a Clausen cell flow reactor.31 A 10 cc/min flow rate of pure H2 was first used to pretreat the catalyst at 400 °C for one hour. The gas line was subsequently purged by helium at room temperature before introducing a 10 ml/min flow of a gas mixture containing 20% CO2, 20% CH4, and 60% He for a 1:1 CO2/CH4 molar ratio. The samples were stepwise heated to 700 °C with a 10 °C/min ramping rate. An in-line residual gas analyzer was used to track the evolution of the gaseous species right after the flow cell. Two-dimensional XRD images were collected continuously with a PerkinElmer, a Si flat panel detector through the reaction processes. The XRD data were subsequently processed with GSAS-II to obtain diagrams of intensity versus 2θ, and Rietveld analyses were also performed through GSAS-II.32 In-situ X-ray absorption fine structure (XAFS) 6


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In-situ XAFS measurements of Ru-CeO2 catalyst was performed at 8-ID (ISS) beamline at National Synchrotron Light Source II (NSLS-II), BNL. For the methane TPR experiment, the catalyst was ramped to 700 oC with 10 oC/min under a 5 ml/min flow of CH4 diluted in a 15 ml/min of He. For the methane dry reforming experiment, similar conditions to the in-situ XRD measurement was used. The Ru K edge data were collected in fluorescence yield mode using passivated implanted planar silicon (PIPS) detector. Data processing was performed using the IFEFFIT package. Ru foil and bulk RuO2 was used as standard references for EXAFS fitting. Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) A commercial SPECS AP-XPS chamber equipped with a PHOIBOS 150 EP MCD-9 analyzer at the Chemistry Division of Brookhaven National Laboratory (BNL) was used for XPS analysis.33 The Ce 3d photoemission line with the strongest Ce4+ feature (916.9 eV) was used for the energy calibration. The powder catalyst was pressed on an aluminum plate and then loaded into the AP-XPS chamber. 20 mTorr of H2 was used to pretreat the sample at 400 °C for one hour, before a reaction mixture of 50 mTorr of CH4 and 50 mTorr of CO2 was introduced into the reaction chamber through a high precision leak valve. O 1s, Ce 3d, and C 1s + Ru 3d XPS regions were collected from 25 to 500 °C under the reaction gas environment. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

In-situ DRIFTS spectra were collected in Kubelka−Monk (K−M) mode using an FTIR spectrometer (Bruker Vertex 70) equipped with a Harrick cell, MCT detector and mass spectroscopy at the Chemistry Department of BNL. The catalyst was reduced in H2 (10 ml/min) at 400 °C for one hour and then purged with 40 ml/min of He. The background was collected at 500 oC under He before the introduction of gas reactants (CH4/CO2/He, 5/5/30 ml/min). For the ‘gas-on, gas-off” experiments, the gas flow was always balanced by Helium to a total amount of 40 ml/min. 3. RESULTS AND DISCUSSION 7


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3.1. Structural and Electronic Properties Figure 1 exhibits TEM images of a ceria supported 0.5 wt% Ru nanocluster catalyst (denoted as Ru(NC)-CeO2). Highly crystalline ceria nanoparticles (~20 nm) were the sole phase seen in all the HRTEM images. Because of the similar atomic masses of Ru and Ce, no clear contrast related to Ru nanostructures was visualized in the HRTEM image (Figure 1a). EDX elemental mapping (L-edge) was thus used to reveal the presence of Ru nanoclusters on ceria. From Figure 1b to d, one can see that Ru was well dispersed over ceria with nanoclusters predominantly below 1 nm, and no segregated large ruthenium particles (> 3 nm) were seen in all the examined images. Similarly, XRD of the Ru(NC)-CeO2 sample (Figure S1) shows no diffraction pattern of Ru related phases, and the average ceria particle size was estimated to be 20~23 nm in diameter. The oxidation state and coordination environment of the as-prepared Ru(NC)-CeO2 catalyst were identified from ex-situ Ru K-edge XANES/EXAFS results (Figure 2). The Ru K-edge XANES shows that the oxidation state of Ru is 4+ according to the position of the absorption edge. Compared to reference RuO2, the slightly different white line feature of the catalyst indicates that the Ru clusters may adopt a distorted local symmetry. The Fourier transformed R-space EXAFS spectra (Figure 2b) together with the fitting results (Table 1) show the presence of only Ru-O first shell (bond distance: ~2.0Å, coordination number (C.N.): ~ 4.5) in the Ru(NC)-CeO2 catalyst. The absence of Ru-Ru second shell confirms our TEM observation that the ceria supported Ru is in the form of small RuO2 clusters without long-range order.

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Figure 1. a) HRTEM image, b-d) EDX elemental mapping of the as-prepared 0.5 wt% Ru(NC)-CeO2 catalyst.

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Figure 2. a) XANES and b) EXAFS spectra of the as-prepared 0.5wt% Ru(NC)-CeO2 sample with Ru metal and RuO2 references. 3.2. The reaction of Ru-CeO2 with methane The activation of methane is a vital step in dry reforming.1,

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The catalysts could

undergo drastic chemical and structural transformations when exposed to methane. Studying these changes using in-situ method provides important information regarding the interplay between CeO2 and Ru nanoclusters that is closely related to the catalytic performance. In a set of experiments, the pre-calcined sample of 0.5wt% Ru(NC)-CeO2 was exposed to a He diluted CH4 gas mixture (He/CH4=15/5ml/min). The evolution of the Ru chemical state during reaction with methane was examined by in-situ Ru K-edge XANES, as presented in Figure 3a. With an increase of temperature, the intensity decrease of the RuO2 white line feature (~22140 eV) indicates its reduction towards metallic Ru. Linear combination fitting (LCF) of the XANES profiles as a function of temperature (Figure 3b) shows that the reduction occurred rapidly at temperatures between 150~200 oC, and the two phases slowly reached an equilibrium with ~90% metallic Ru at 500 oC. The reduction of RuO2 was accompanied by the removal of lattice oxygen in the oxide support, which is evident from the Ce 3d in-situ AP-XPS measurements (Figure 3c). Upon exposing the catalyst to 50 mTorr of methane, a similar reduction trend was observed on ceria as the surface was gradually reduced starting from 150 oC, generating Ce3+ and oxygen vacancies. Bare ceria surface interacts weakly with methane, and the reduction only occurs at significantly higher temperatures (>300 oC).5 The much lower surface reduction temperature when ceria is combined with Ru clusters can be attributed to a H-spillover effect facilitated by the metal-support interactions. This phenomenon has also been ascertained by H2 TPR experiments in a similar Ru-nanoclusters/CeO2 system for CO2 hydrogenation.29, 35 Here we provide direct spectroscopic evidence of this effect, showing that, besides H2, the H adatom

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generated from methane activation could also spill over to the ceria sites and react with the readily available surface oxygen on ceria. The corresponding gas evolution while exposing the 0.5wt% Ru(NC)-CeO2 catalyst to a methane gas mixture (He/CH4=15/5ml/min) was monitored by an RGA mass-spectrometer and is shown in Figure 4. CO2, CO, H2, and H2O were produced as a consequence of the interaction of methane with the catalyst. The first broad water yielding region between 150 and 200 oC is primarily tracked by the reduction of RuO2 as well as the weak surface reduction of ceria, which is consistent with the in-situ XANES/AP-XPS results. The appearance of the sharp water peak accompanied by CO, CO2, and H2 between 350 and 400 oC

could be attributed to a combination of several surface processes: the surface/subsurface

reduction of ceria, the re-oxidation of surface carbon and the decomposition of methane. Above 400 oC, the high level of CO and H2 production arose from the bulk reduction of ceria.36-37 The total amount of CO and CO2 yield up to 700 oC are estimated as 1230 µmol/g and 101 µmol/g, respectively, which largely excess the amount of oxygen present in the RuO2 phase of the as-prepared 0.5 wt% Ru(NC)-CeO2 catalyst (RuO2 → Ru: ~ 75 µmol/g, CeO2 → Ce2O3: ~ 2892 µmol/g ). The production of CO requires a source of oxygen and considering that the direct reaction of methane with bare ceria is weak, thus the significant CO yield necessitates the interfacial oxygen or oxygen transferred from ceria onto metal sites,38-40 to react with the C that originates from the complete dissociation of methane on metal centers. Our CH4 reaction results show that CO was only produced at temperatures above 350 oC, which is in agreement with the reported temperature regimes that facilitates the oxygen spillover from ceria to metal sites.38 Furthermore, such phenomenon could only take place in the prevalence of metal-oxide interactions, when the metal and ceria nanostructures are brought into intimate contact.38 The details and influences of such metal-oxide interactions on the catalytic performance will be shown and further discussed in our study of methane dry reforming below. 11


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Figure 3. a) In-situ Ru K-edge XANES data of Ru(NC)-CeO2 during reaction with methane, b) The corresponding linear combination fitting result, c) Ce 3d region of the in-situ AP-XPS data of the Ru(NC)-CeO2 catalyst under 50 mTorr of CH4.

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Figure 4. Mass spectrometer signals for the evolution of CO, CO2, H2O and H2 while exposing a 0.5wt% Ru(NC)-CeO2 catalyst to a gas mixture of methane and helium. Flow rate: 5 ml/min CH4 + 15 ml/min He. 3.3. Catalytic Methane Dry Reforming Performance The catalytic performance of the 0.5wt% Ru(NC)-CeO2 sample for methane dry reforming was evaluated in the temperature range of 400 to 700 oC using a flow reactor, under a high space velocity of 180, 000 mL/gcat/h. Figure 5a compares the activity of 0.5wt% Ru supported on different oxide supports (Al2O3 and TiO2) impregnated by the same method. The Ru(NC)-CeO2 catalyst exhibits the highest catalytic performance at 700 oC, with a conversion rate of 343 µmol/g/s and 394 µmol/g/s for CH4 and CO2, respectively. Pure ceria as control has also been evaluated for DRM and shows negligible activity at 700 oC.

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The detailed catalytic activity of 0.5wt% Ru(NC)-CeO2 as a function of temperature is displayed in Figure 5b. The main products of the reaction are CO, H2, and H2O. No other carbon related products were detected, and the H2/CO ratio is given on the right axis of Figure 5b. The production of water and the sub-stoichiometric ratio of H2/CO is due to the presence of the reverse water gas shift (RWGS: H2 + CO2 → H2O + CO). It can be seen that with the increase of temperature, less water was produced, and the H2/CO is closer to the 1:1 stoichiometric ratio, suggesting the shifting of reaction pathway from RWGS towards dry reforming at higher temperatures. To understand the unique influence of ceria, Ru and their interactions on the high reactivity of the catalyst, a series of in-situ characterizations were performed and discussed below, especially with respect to the bulk phase, surface properties, and the adsorbate species.

Figure 5. a) Catalytic reaction rate of 0.5wt% Ru supported on different oxides for dry reforming of methane at 700 oC. b) Reaction rate (left axis) and H2/CO ratio (right axis) as a function

of

temperatures

over

the

0.5

wt%

Ru(NC)-CeO2

CH4/CO2/N2=10/10/10ml/min, space velocity: 180, 000 mL/gcat/h. 3.4. Active Structure under Reaction Conditions

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catalyst.

1

atm,

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The bulk crystal structure change of the Ru(NC)-CeO2 catalyst under H2 pretreatment and steady-state methane dry reforming conditions were monitored by in-situ time-resolved XRD, as shown in Figure 6. The ceria fluorite-type structure was the sole crystalline phase observed over the course of the DRM experiment. The absence of Ru related phases indicates that the Ru clusters with crystallinity sizes below the diffraction limit remained stable under reaction conditions even up to 700 oC. Rietveld refinement of the diffraction profile shown in Figure 6 depicts the change of the ceria lattice parameter during the reaction. The lattice parameter of the catalyst before the H2 pretreatment was determined as 5.409 Å at 25 oC, while after H2 reduction and DRM reaction, it expanded to 5.420 Å at room temperature. The nonthermal lattice expansion of ceria after DRM reaction reflects the formation of Ce3+ in bulk (increased ionic radius of Ce3+), suggesting that the bulk phase of ceria tended to remain at a reduced state instead of being fully oxidized by CO2.

Figure 6. Time-resolved in-situ XRD data of 0.5 wt% Ru(NC)-CeO2 under dry reforming of methane, a) in-situ XRD profile, b) Rietveld refinement of ceria lattice parameter.

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b)

a)

Figure 7. In-situ Ru K-edge a) XANES and b) EXAFS data of 0.5 wt% Ru(NC)-CeO2 under H2 pretreatment (400 oC) and dry reforming of methane (700 oC). Table 1. Ru K-edge EXAFS Fitting Results.

To further determine the active structure of the catalyst under steady-state reaction conditions, in-situ Ru K-edge XANES and EXAFS spectra were collected at 400 oC under H2 pretreatment and 700 oC under DRM conditions, as shown in Figure 7. In the XANES region (Figure 7a), the position of the absorption edge and similar pre-edge feature to the Ru foil reference demonstrate that the Ru oxidation states of the catalyst during the reaction were 16


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closer to metallic Ru. However, the shallow valley for the line-shape in the white line position of both H2-pretreatment and 700 oC-DRM spectra suggest that the Ru species possess a partial positive charge (Ruδ+), which is possibly related to a charge transfer associated with the formation of Ru-O-Ce species. The LCF fitting of the XANES part shows that the 400 oC H2-pretreatment spectrum has a ~18% oxide feature while the 700 oC-DRM spectrum adopts a ~10% oxide feature. The R-space of the EXAFS data (Figure 7b) confirms the existence of both Ru-O and metallic Ru-Ru bonds, and the Ru-Ru bonds of the catalyst under reaction conditions are in much lower intensity than that of the Ru-Ru scattering path in a fully coordinated Ru foil (C.N. = 12). The fitting results (Table 1 and Figure S3) reveal that the C.N. of Ru-Ru was ~6.0 at 700 oC under DRM conditions, which corresponds a ~1.0 nm particle size based on the hemisphere hcp model.41 The presence of the Ru-O bonds with low C.N. numbers might be attributed to the O at a Ru-O-Ce interface or the O from ceria that migrates onto Ru sites, and both situations demonstrate that the strong interaction between Ru clusters and CeO2 prevails under steady-state reaction conditions. The resulting effects on the catalytic surface chemistry is further investigated next, by in-situ AP-XPS and DRIFTS. 3.5. Surface Chemistry and Active Intermediates in DRM In-situ ambient pressure XPS was employed to provide the surface sensitive information regarding the chemical state and possible surface species in the presence of gas reactants. AP-XPS spectra in the Ce 3d and C 1s + Ru 3d regions of the 0.5 wt% Ru(NC)-CeO2 catalyst under DRM conditions are displayed in Figure 8. In the Ce 3d region, the ceria surface underwent reduction first in the presence of CH4 + CO2, similar to the case of methane alone (Figure 3c), at temperatures up to 300 oC, suggesting the effective activation of methane accompanied by the H-spillover at low temperature. The concentration of Ce3+ was derived through the deconvolution of the Ce 3d spectra as an indication of the surface reduction degree of the catalyst in the two cases, CH4 alone (Figure 3c) and CH4 + CO2 (Figure 8a), 17


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respectively (details of the peak fitting are shown in Figure S4). As indicated in Figure 8c, upon increasing temperature above 300 oC, a significant degree of re-oxidation was observed in the case of CH4 + CO2, which implies the substantial dissociation of CO2 on either ceria or Ru sites, generating O adatoms and subsequently replenishing the surface oxygen vacancies.

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Figure 8. a) Ce 3d and b) C 1s + Ru 3d regions of the in-situ AP-XPS data of the 0.5 wt% Ru(NC)-CeO2 catalyst under 50 mTorr of CH4 + 50 mTorr of CO2, c) the quantified Ce3+ concentrations and d) the integrated surface carbon amount as a function of temperature. On the other hand, in the C 1s + Ru 3d region, the Ru 3d5/2 is often used for analyzing the charge state of the Ru species as the 3d3/2 split spin-orbit component (around 2/3 of the peak intensity of the 3d5/2 component) overlaps with the C 1s (~284.0 eV).30 Due to the low loading of Ru and attenuated signal in the lab-source X-ray AP-XPS, the Ru signal was weak but one could still see the reduction of Ru species from Ru4+(~281.1 eV)35 to Ruδ+ (~280.5 eV)30, 42 as the reaction proceeded to 500 oC, the highest temperature can be achieved for the instrument. This again confirms the existence of partially charged Ru species under reaction conditions as demonstrated by the results of in-situ XAFS. Figure S5 shows the details of the deconvoluted spectrum at 500 oC. Meanwhile, the continuous build-up of surface carbon (~284.6 eV) as a result of methane dissociation was seen up to 300 oC, and it started to decrease thereafter. It has been well established that ceria could serve as additional sites for the activation of CO2 via the redox behavior of the Ce4+/Ce3+ pair.19, 43-44 Additionally, at higher temperatures (> 300 oC), oxygen transfer from ceria is energetically allowed through metal-support interaction.38-40 Thus, in addition to the direct activation of CO2 on the metal sites, the dual-site mechanism allows the O adatoms generated on the ceria sites also to assist the oxidation of surface carbon on Ru site. To validate this, the development of surface carbon during the reaction was also estimated based on the surface carbon peak intensity (~284. 6 eV) in the corresponding C1s spectra, as shown in Figure 8d. By correlating it with the evolution of the surface Ce3+ concentration, the trend of surface carbon accumulation can be interpreted as follows: 1. At 300 oC or below, activation of methane resulted in the gradual building up of surface carbon on the catalyst in both reaction conditions. The lower level of carbon accumulation in the case of CH4 + CO2 could be due to the occupation of methane activation 19


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sites by the competing adsorption of CO2. 2. At temperatures above 300 oC, the generation of adatom O from CO2 together with the O transfer from ceria to the metal or metal-oxide interface, thanks to the intimate Ru-O-Ce interaction, to complete the catalytic cycle by re-oxidizing the surface carbon, as evident by a clear drop of surface carbon peak area in the case of CH4 + CO2. In contrast, only a small decrease of surface carbon was observed on the catalyst exposed to pure methane, and this is likely due to the limited O fed by the reduction of bulk ceria where there is no O provided by the dissociation of CO2. It has been reported from both experiments and theoretical calculations that, besides the methane activation step, the carbon oxidation (CO formation) could also be the rate-limiting step with a relatively high Gibbs energy barrier in the dry reforming reaction.7, 45-46 By correlating the surface carbon turnover temperature (400 oC) observed in AP-XPS with the onset reactivity temperature of the catalyst shown in Figure 3b, it suggests that the oxidation of the surface carbon on the Ru-ceria catalyst is kinetically relevant to the reaction rate at the current temperature range (400 ~ 500 oC).

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Figure 9. a) In-situ DRIFTS spectra collected over the 0.5 wt% Ru(NC)-CeO2 catalyst under steady-state reaction conditions (CH4/CO2/He=5/5/30 ml/min) at 500 oC with subsequently switching off CO2. Spectra were recorded per minute and plotted every third of them. The inset figure shows the blow-up region between 3700-1600 cm-1. The bottom dash line was obtained by subtracting the ‘CH4 + CO2’ spectrum (black line) from the last ‘CH4-only’ spectrum (orange line). b) The corresponding gas phases evolution was monitored by mass spectroscopy. To further understand the influence of the metal-support interaction on the surface adsorbates and intermediates, in-situ DRIFTS experiments were carried out to probe the 21


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surface species evolution under steady-state reaction conditions. As shown in Figure 9, after H2 pretreatment, the sample was held at 500 oC under the reactant gas environment (5 ml/min CO2 + 5 ml/min CH4 + 30 ml/min He). The spectrum shown as a solid black line in Figure 9a was collected after the reaction reached a steady state. Multiple adsorption peaks related to different surface species were identified in the IR range from 1000 to 2200 and 3500 to 3700 cm-1, including -OH groups (3635 cm-1),29,

47-48

CO gas phase (2143 cm-1), ruthenium

carbonyls ((CO)x-Ru: 2060~1960 cm-1, bridged CO-Ru: 1885 cm-1),29-30,

49

formyl group

(-CHO, 1783 cm-1),50 carbonate like species (Polydentate carbonate: 1458, 1389 cm-1, carboxylate: 1565, 1509, 1310 cm-1)29, 51-52 and CH4 gas phase (sharp peak at 1305 cm-1). The simultaneous gas components evolution was monitored by a mass-spec as displayed in Figure 9b. A steady state production of CO, H2, and H2O were observed when both CO2 and CH4 were switched on, manifesting in the occurrence of methane dry reforming, and likely as well as the RWGS and thus validating that the observed surface species could be responsible for the reaction mechanism at this temperature. To determine the active surface intermediates, the consumption and decomposition behavior of the surface intermediates were evaluated by switching off one of the feed reactants, CO2 (Figure 9) or CH4 (Figure S6), respectively. The subtracted spectrum (dash line) was obtained to gain a clear view of the change before and after the cutoff. For the case of switching off CH4 (Figure S6), the ruthenium carbonyl feature decreased together with the CH4 and CO gas phase. The diminish of Ru carbonyls upon the cutoff of CH4 demonstrates that the observed C-O bond formation was contributed by both methane and CO2 instead of from the solitary dissociation of CO2 on Ru sites (CO2(a) + Ru → Ru-CO(a) + O(a)). The other decreased band at 3635 cm-1 indicates that the -OH groups on Ce3+ site were able to react with CO2 or desorbed as H2O by combining with adatom H. In contrast, switching off CO2 (Figure 9) from the gas feeding led to the rapid decrease of absorption bands at ca. 1565, 1509 and 1310 cm-1, suggesting that the carboxylate species were active and can be fast consumed/transformed by reacting with CH4 (see Figure S7 for the full set of spectra as a 22


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function of time). The corresponding rapid decrease of CO/H2/H2O in the mass-spectrometer supports this observation while the polydentate carbonate species (1458, 1389 cm-1) remained intact on the surface through the whole process, suggesting that it might be a spectator at this temperature regime. The formation of carboxylate species points to the occurrence of the RWGS reaction, which probably involves the H-assisted activation of CO2 (CO2(a) + H(a) → CO(a) + OH(a)) with the subsequent formation of H2O (OH(a) + H(a) → H2O(g)).29 Ru carbonyls species are not stable at high temperatures and will desorb from the surface at 500 oC. The observation of this species for a steady-state reaction of 500 oC could be strongly relevant to the continuous surface carbon oxidation process (Ru-C(a) + O(a) → Ru-CO(a)) assisted by the interfacial oxygen or oxygen transferred from ceria through the metal-oxide interaction,38-40 as also evident by the results of AP-XPS (Figure 8 and Figure S5). Another possible route for the C-O bond formation predicted by the theoretical calculation is the CH(a) oxidation by adatom O (CH(a) + O(a) → CHO(a)).46, 53 However, only a weak peak attributed to formyl species (CHO(a), 1783 cm-1) was detected here, and it almost remained unaffected during the gas on/off experiments, we thus consider this route has a minor influence on the CO formation. Therefore, the main reaction pathway involves a dual mechanism for CH4 activates on Ruδ+ or Ruδ+-O-Ce interfacial sites predominantly while the presence of O vacancies on ceria creates an additional driving force for the direct activation of CO2, generating O adatoms which will then react with surface carbon to close the reaction cycle. The side reaction pathways include the RWGS via the H-assisted CO2 activation route with carboxylate intermediates. 3.6. Effect of Metal Particle Size: Importance of Metal-Support Interactions Our in-situ multitechnique approach of the DRM reaction on a Ru(NC)-CeO2 catalyst shows a very complex process where metal-support interactions and the movement of oxygen 23


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between the oxide and metal components in the catalyst are esential to obtain a highly active/stable system for the dry reforming of methane. To validate that such interactions, as well as the associated Ruδ+-CeO2-x active configuration, are strongly correlated to the metal particle size, a second Ru-ceria catalyst was tested where the Ru ↔ Ru interactions are more important than the Ru ↔ ceria interactions. In this catalyst, 0.5 wt% Ru nanoparticles were supported on ceria (denoted as Ru(NP)-CeO2) and the performance of the system was examined for the dry reforming of methane. Contrary to the absence of a Ru phase in the Ru(NC)-CeO2 catalyst, the XRD results for the as-prepared Ru(NP)-CeO2 sample (Figure S1) exhibited weak diffraction peaks from the rutile-type RuO2 nanocrystalline with an estimated particle size of 3~4 nm, which is close to nanoparticles observed in the TEM images (Figure S8). The large particle size significantly weakens the metal-support interaction of Ru on ceria and thus alters the electronic structure of the catalyst under reaction conditions, as shown by the in-situ XAFS results in Figure 10a-b. In comparison to small Ru nanoclusters, the Ru nanoparticles are more ‘bulk-like,’ and transformed to a completely metallic state under both H2 pretreatment and DRM conditions, as evident by the distinct XANES features of metallic Ru in Figure 10a. In the R space (Figure 10b), the absence of Ru-O bonds under reaction conditions confirms the metallic nature of the metal phase in the Ru(NP)-CeO2 catalyst. Additionally, the pronounced peak intensity of the Ru-Ru shell (~10 C.N., Table 1) points to Ru nanoparticles with an average size of ~3.3 nm (hemisphere model), which is in line with the XRD results of higher Ru crystallinity in the Ru(NP)-CeO2 catalyst. Overall, our in-situ XAFS results reveal that, with the increase of metal particle size, the active metal phase under steady-state DRM conditions shifts from partially charged Ru nanoclusters (Ruδ+) with O decoration to metallic Ru nanoparticles (Ru0) with a lower fraction of Ru-O-Ce interfacial sites. The influence of the two active structures on the catalytic stability was compared for the two catalysts and shown in Figure 10c. Less than 10 % conversion loss was observed for the Ru(NC)-CeO2 catalyst at a high space velocity and high temperature (700 oC) for over 25

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hours. While the Ru(NP)-CeO2 catalyst exhibited similar activity initially but underwent continuous deactivation for more than 40 % over the period of time studied. The intact small Ru nanoclusters with close contact with the oxide support under reaction conditions are essential for superior catalytic performance. Previous theoretical studies for methane activation over CeO2(111)-supported transition metals indicate that small clusters or even single metal atoms, which interact strongly with the ceria support and adopt positive charges, exhibit much lower methane activation barrier than seen on the extended metal surface or big metal particles.54-55 Large metal nanoparticles (3-20 nm) are representative of fine crystallinity with more bulk-like metal properties and less perturbed geometric or electronic structures.38, 56-57 Our results provide clear evidence for this trend and demonstrate that the metal-support interactions could stabilize the small nanoclusters (< 1nm) under reaction conditions (700 oC), giving rise to a special Ruδ+-CeO2-x active configuration for a facile pathway of the surface species conversion (e.g. activation of methane, surface carbon oxidation) with enhanced stability.

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Figure 10. a) In-situ Ru K-edge XANES and b) EXAFS data of 0.5 wt% Ru(NC)-CeO2 and Ru(NP)-CeO2 catalysts under reaction conditions, c) Catalytic stability test for over 25 hours on the two samples at 700oC. 1 atm, CH4/CO2/N2=10/10/10ml/min, space velocity: ~180, 000 mL/gcat/h. CONCLUSION Nanoclusters of Ru ( 300 oC). Possible surface intermediates identified under reaction conditions included ruthenium carbonyls, carboxylate species, and surface -OH groups, while polydentate carbonates might be plain spectators. The excellent activity/stability of the 0.5wt% Ru(NC)-CeO2 catalyst originated in thermally stable Ru clusters and an improved oxygen mobility produced by metal-support interactions, which deteriorated with an increase of the Ru particle size. Our work highlights the importance of investigating the active structure-reactivity relationship under in-situ reaction conditions to aid a better design of the DRM catalysts by controlling the metal particle size and tuning the metal-oxide interactions. ASSOCIATED CONTENT Supporting Information Ex-situ XRD, in-situ XANES, EXAFS fittings, Ce 3d deconvolution, C 1s deconvolution, in-situ DRIFTS, TEM images of Ru(NP)-CeO2 AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected] ACKNOWLEDGMENT The work carried out at Brookhaven National Laboratory was supported by the US Department of Energy under contract no. DE-SC0012704. S.D.S. is supported by a US 27


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Department of Energy Early Career Award. This research used resources 8-ID (ISS) beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The XRD experiments of this research used resources of the Advanced Photon Source (Beamlines 17BM (XRD) at Argonne National Laboratory, which is a DOE Office of Science User Facility under contract no. DE-AC02-06CH11357. REFERENCES (1) Pakhare, D.; Spivey, J. A Review of Dry (CO2) Reforming of Methane over Noble Metal Catalysts. Chem. Soc. Rev. 2014, 43, 7813-7837. (2) Lavoie, J.-M. Review on Dry Reforming of Methane, a Potentially More Environmentally-Friendly Approach to the Increasing Natural Gas Exploitation. Front. Chem. 2014, 2, 81. (3) Wang, S.; Lu, G.; Millar, G. J. Carbon Dioxide Reforming of Methane to Produce Synthesis Gas over Metal-Supported Catalysts: State of the Art. Energy Fuels 1996, 10, 896-904. (4) Jóźwiak, W.; Nowosielska, M.; Rynkowski, J. Reforming of Methane with Carbon Dioxide over Supported Bimetallic Catalysts Containing Ni and Noble Metal: I. Characterization and Activity of SiO2 Supported Ni–Rh Catalysts. Appl. Catal. A 2005, 280, 233-244. (5) Liu, Z.; Grinter, D. C.; Lustemberg, P. G.; Nguyen ‐ Phan, T. D.; Zhou, Y.; Luo, S.; Waluyo, I.; Crumlin, E. J.; Stacchiola, D. J.; Zhou, J. Dry Reforming of Methane on a Highly ‐ Active Ni ‐ CeO2 Catalyst: Effects of Metal‐Support Interactions on C− H Bond Breaking. Angew. Chem. Int. Ed. 2016, 55, 7455-7459. (6) Basini, L.; Sanfilippo, D. Molecular Aspects in Syn-Gas Production: The CO2-Reforming Reaction Case. J. Catal. 1995, 157, 162-178. (7) Rostrupnielsen, J. R.; Hansen, J. H. B. Co2-Reforming of Methane over Transition Metals. J. Catal. 1993, 144, 38-49. (8) Vernon, P. D. F.; Green, M. L. H.; Cheetham, A. K.; Ashcroft, A. T. Partial Oxidation of Methane to Synthesis Gas, and Carbon Dioxide as an Oxidising Agent for Methane Conversion. Catal. Today 1992, 13, 417-426. (9) Wei, J.; Iglesia, E. Reaction Pathways and Site Requirements for the Activation and Chemical Conversion of Methane on Ru−Based Catalysts. J. Phys. Chem. B 2004, 108, 7253-7262. (10) Bradford, M. C. J.; Vannice, M. A. CO2reforming of Ch4over Supported Ru Catalysts. J. Catal. 1999, 183, 69-75. (11) Ashcroft, A.; Cheetham, A.; Green, M. Partial Oxidation of Methane to Synthesis Gas Using Carbon Dioxide. Nature 1991, 352, 225-226. 28


Page 28 of 33

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

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(12) Ferreira-Aparicio, P.; Marquez-Alvarez, C.; Rodrıguez-Ramos, I.; Schuurman, Y.; Guerrero-Ruiz, A.; Mirodatos, C. A Transient Kinetic Study of the Carbon Dioxide Reforming of Methane over Supported Ru Catalysts. J. Catal. 1999, 184, 202-212. (13) Guerrero-Ruiz, A.; Ferreira-Aparicio, P.; Bachiller-Baeza, M.; Rodrıguez-Ramos, I. Isotopic Tracing Experiments in Syngas Production from Methane on Ru/Al2O3 and Ru/SiO2. Catal. Today 1998, 46, 99-105. (14) Bradford, M. C.; Vannice, M. A. The Role of Metal–Support Interactions in CO2 Reforming of Ch4. Catal. Today 1999, 50, 87-96. (15) Matsui, N.-o.; Anzai, K.; Akamatsu, N.; Nakagawa, K.; Ikenaga, N.-o.; Suzuki, T. Reaction Mechanisms of Carbon Dioxide Reforming of Methane with Ru-Loaded Lanthanum Oxide Catalyst. Appl. Catal. A 1999, 179, 247-256. (16) Carrara, C.; Munera, J.; Lombardo, E.; Cornaglia, L. Kinetic and Stability Studies of Ru/La2O3 Used in the Dry Reforming of Methane. Top. Catal. 2008, 51, 98-106. (17) Qin, D.; Lapszewicz, J. Study of Mixed Steam and CO2 Reforming of CH4 to Syngas on Mgo-Supported Metals. Catal. Today 1994, 21, 551-560. (18) Schuurman, Y.; Mirodatos, C.; Ferreira-Aparicio, P.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Bifunctional Pathways in the Carbon Dioxide Reforming of Methane over Mgo-Promoted Ru/C Catalysts. Catal. Lett. 2000, 66, 33-37. (19) Xie, Z.; Yan, B.; Kattel, S.; Lee, J. H.; Yao, S.; Wu, Q.; Rui, N.; Gomez, E.; Liu, Z.; Xu, W.; Zhang, L.; Chen, J. G. Dry Reforming of Methane over CeO2-Supported Pt-Co Catalysts with Enhanced Activity. Appl. Catal. B 2018, 236, 280-293. (20) Chen, J.; Yao, C.; Zhao, Y.; Jia, P. Synthesis Gas Production from Dry Reforming of Methane over Ce0.75zr0.25o2-Supported Ru Catalysts. Int. J. Hydrogen Energy 2010, 35, 1630-1642. (21) Zhang, F.; Liu, Z.; Zhang, S.; Akter, N.; Palomino, R. M.; Vovchok, D.; Orozco, I.; Salazar, D.; Rodriguez, J. A.; Llorca, J. In Situ Elucidation of the Active State of Co–CeOX Catalysts in the Dry Reforming of Methane: The Important Role of the Reducible Oxide Support and Interactions with Cobalt. ACS catalysis 2018, 8, 3550-3560. (22) Senanayake, S. D.; Ramírez, P. J.; Waluyo, I.; Kundu, S.; Mudiyanselage, K.; Liu, Z.; Liu, Z.; Axnanda, S.; Stacchiola, D. J.; Evans, J.; Rodriguez, J. A. Hydrogenation of CO2 to Methanol on Ceox/Cu(111) and Zno/Cu(111) Catalysts: Role of the Metal–Oxide Interface and Importance of Ce3+ Sites. J. Phys. Chem. C 2016, 120, 1778-1784. (23) Graciani, J.; Mudiyanselage, K.; Xu, F.; Baber, A. E.; Evans, J.; Senanayake, S. D.; Stacchiola, D. J.; Liu, P.; Hrbek, J.; Sanz, J. F.; Rodriguez, J. A. Highly Active Copper-Ceria and Copper-Ceria-Titania Catalysts for Methanol Synthesis from CO2. Science 2014, 345, 546-550. (24) Lustemberg, P. G.; Palomino, R. M.; Gutiérrez, R. A.; Grinter, D. C.; Vorokhta, M.; Liu, Z.; Ramírez, P. J.; Matolín, V.; Ganduglia-Pirovano, M. V.; Senanayake, S. D.; Rodriguez, J. A. Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal–Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking. J. Am. Chem. Soc. 2018, 140, 7681-7687. (25) Derk, A. R.; Moore, G. M.; Sharma, S.; McFarland, E. W.; Metiu, H. Catalytic Dry Reforming of Methane on Ruthenium-Doped Ceria and Ruthenium Supported on Ceria. Top. Catal. 2014, 57, 118-124. 29


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(26) Safariamin, M.; Tidahy, L. H.; Abi-Aad, E.; Siffert, S.; Aboukaïs, A. Dry Reforming of Methane in the Presence of Ruthenium-Based Catalysts. Comptes Rendus Chimie 2009, 12, 748-753. (27) Aitbekova, A.; Wu, L.; Wrasman, C. J.; Boubnov, A.; Hoffman, A. S.; Goodman, E. D.; Bare, S. R.; Cargnello, M. Low-Temperature Restructuring of CeO2-Supported Ru Nanoparticles Determines Selectivity in CO2 Catalytic Reduction. J. Am. Chem. Soc. 2018, 140, 13736-13745. (28) Yee, A.; Morrison, S. J.; Idriss, H. A Study of the Reactions of Ethanol on CeO2 and Pd/CeO2 by Steady State Reactions, Temperature Programmed Desorption, and in Situ Ft-Ir. J. Catal. 1999, 186, 279-295. (29) Guo, Y.; Mei, S.; Yuan, K.; Wang, D.-J.; Liu, H.-C.; Yan, C.-H.; Zhang, Y.-W. Low-Temperature Co2 Methanation over CeO2-Supported Ru Single Atoms, Nanoclusters, and Nanoparticles Competitively Tuned by Strong Metal–Support Interactions and H-Spillover Effect. ACS Catalysis 2018, 8, 6203-6215. (30) An, J.; Wang, Y.; Lu, J.; Zhang, J.; Zhang, Z.; Xu, S.; Liu, X.; Zhang, T.; Gocyla, M.; Heggen, M. Acid-Promoter-Free Ethylene Methoxycarbonylation over Ru-Clusters/Ceria: The Catalysis of Interfacial Lewis Acid–Base Pair. J. Am. Chem. Soc. 2018, 140, 4172-4181. (31) Chupas, P. J.; Chapman, K. W.; Kurtz, C.; Hanson, J. C.; Lee, P. L.; Grey, C. P. A Versatile Sample-Environment Cell for Non-Ambient X-Ray Scattering Experiments. J. Appl. Crystallogr. 2008, 41, 822-824. (32) Toby, B. H.; Von Dreele, R. B. Gsas-Ii: The Genesis of a Modern Open-Source All Purpose Crystallography Software Package. J. Appl. Crystallogr. 2013, 46, 544-549. (33) Palomino, R. M.; Hamlyn, R.; Liu, Z.; Grinter, D. C.; Waluyo, I.; Rodriguez, J. A.; Senanayake, S. D. Interfaces in Heterogeneous Catalytic Reactions: Ambient Pressure Xps as a Tool to Unravel Surface Chemistry. J. Electron. Spectrosc. Relat. Phenom. 2017, 221, 28-43. (34) Liu, Z.; Duchoň, T.; Wang, H.; Peterson, E. W.; Zhou, Y.; Luo, S.; Zhou, J.; Matolín, V.; Stacchiola, D. J.; Rodriguez, J. A.; Senanayake, S. D. Mechanistic Insights of Ethanol Steam Reforming over Ni–Ceox(111): The Importance of Hydroxyl Groups for Suppressing Coke Formation. J. Phys. Chem. C 2015, 119, 18248-18256. (35) Wang, F.; Li, C.; Zhang, X.; Wei, M.; Evans, D. G.; Duan, X. Catalytic Behavior of Supported Ru Nanoparticles on the {100}, {110}, and {111} Facet of CeO2. J. Catal. 2015, 329, 177-186. (36) Otsuka, K.; Wang, Y.; Sunada, E.; Yamanaka, I. Direct Partial Oxidation of Methane to Synthesis Gas by Cerium Oxide. J. Catal. 1998, 175, 152-160. (37) Otsuka, K.; Sunada, E.; Ushiyama, T.; Yamanaka, I., The Production of Synthesis Gas by the Redox of Cerium Oxide. In Stud. Surf. Sci. Catal., de Pontes, M.; Espinoza, R. L.; Nicolaides, C. P.; Scholtz, J. H.; Scurrell, M. S., Eds. Elsevier: Amsterdam, 1997; Vol. 107, pp 531-536. (38) Vayssilov, G. N.; Lykhach, Y.; Migani, A.; Staudt, T.; Petrova, G. P.; Tsud, N.; Skála, T.; Bruix, A.; Illas, F.; Prince, K. C.; Matolı´n, V. r.; Neyman, K. M.; Libuda, J. Support Nanostructure Boosts Oxygen Transfer to Catalytically Active Platinum Nanoparticles. Nature Materials 2011, 10, 310. (39) Ruiz Puigdollers, A.; Schlexer, P.; Tosoni, S.; Pacchioni, G. Increasing Oxide Reducibility: The Role of Metal/Oxide Interfaces in the Formation of Oxygen Vacancies. ACS Catalysis 2017, 7, 6493-6513. 30


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(40) Happel, M.; Mysliveček, J.; Johánek, V.; Dvořák, F.; Stetsovych, O.; Lykhach, Y.; Matolín, V.; Libuda, J. Adsorption Sites, Metal-Support Interactions, and Oxygen Spillover Identified by Vibrational Spectroscopy of Adsorbed Co: A Model Study on Pt/Ceria Catalysts. J. Catal. 2012, 289, 118-126. (41) Beale, A. M.; Weckhuysen, B. M. Exafs as a Tool to Interrogate the Size and Shape of Mono and Bimetallic Catalyst Nanoparticles. Phys. Chem. Chem. Phys. 2010, 12, 5562-5574. (42) Soin, N.; Roy, S. S.; Mitra, S. K.; Thundat, T.; McLaughlin, J. A. Nanocrystalline Ruthenium Oxide Dispersed Few Layered Graphene (Flg) Nanoflakes as Supercapacitor Electrodes. J. Mater. Chem. 2012, 22, 14944-14950. (43) Staudt, T.; Lykhach, Y.; Tsud, N.; Skála, T.; Prince, K.; Matolín, V.; Libuda, J. Ceria Reoxidation by Co2: A Model Study. J. Catal. 2010, 275, 181-185. (44) Leitenburg, C. d.; Trovarelli, A.; Kašpar, J. A Temperature-Programmed and Transient Kinetic Study of CO2 activation and Methanation over CeO2 supported Noble Metals. J. Catal. 1997, 166, 98-107. (45) Jones, G.; Jakobsen, J. G.; Shim, S. S.; Kleis, J.; Andersson, M. P.; Rossmeisl, J.; Abild-Pedersen, F.; Bligaard, T.; Helveg, S.; Hinnemann, B.; Rostrup-Nielsen, J. R.; Chorkendorff, I.; Sehested, J.; Nørskov, J. K. First Principles Calculations and Experimental Insight into Methane Steam Reforming over Transition Metal Catalysts. J. Catal. 2008, 259, 147-160. (46) Zhu, Y.-A.; Chen, D.; Zhou, X.-G.; Yuan, W.-K. Dft Studies of Dry Reforming of Methane on Ni Catalyst. Catal. Today 2009, 148, 260-267. (47) Laachir, A.; Perrichon, V.; Badri, A.; Lamotte, J.; Catherine, E.; Lavalley, J. C.; El Fallah, J.; Hilaire, L.; Le Normand, F.; Quéméré, E.; Sauvion, G. N.; Touret, O. Reduction of Ceo2 by Hydrogen. Magnetic Susceptibility and Fourier-Transform Infrared, Ultraviolet and X-Ray Photoelectron Spectroscopy Measurements. J. Chem. Soc., Faraday Trans. 1991, 87, 1601-1609. (48) Binet, C.; Daturi, M.; Lavalley, J.-C. Ir Study of Polycrystalline Ceria Properties in Oxidised and Reduced States. Catal. Today 1999, 50, 207-225. (49) Chin, S. Y.; Williams, C. T.; Amiridis, M. D. Ftir Studies of Co Adsorption on Al2O3- and SiO2-Supported Ru Catalysts. J. Phys. Chem. B 2006, 110, 871-882. (50) Eckle, S.; Anfang, H.-G.; Behm, R. J. Reaction Intermediates and Side Products in the Methanation of Co and Co2 over Supported Ru Catalysts in H2-Rich Reformate Gases. J. Phys. Chem. C 2011, 115, 1361-1367. (51) Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Kröhnert, J.; Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F. C.; Knop-Gericke, A.; Paál, Z.; Schlögl, R. Preferential Co Oxidation in Hydrogen (Prox) on Ceria-Supported Catalysts, Part I: Oxidation State and Surface Species on Pt/CeO2 under Reaction Conditions. J. Catal. 2006, 237, 1-16. (52) Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.-i.; Onishi, T. Carbon Monoxide and Carbon Dioxide Adsorption on Cerium Oxide Studied by Fourier-Transform Infrared Spectroscopy. Part 1.—Formation of Carbonate Species on Dehydroxylated CeO2, at Room Temperature. J. Chem. Soc., Faraday Trans. 1 1989, 85, 929-943.

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(53) Foppa, L.; Silaghi, M.-C.; Larmier, K.; Comas-Vives, A. Intrinsic Reactivity of Ni, Pd and Pt Surfaces in Dry Reforming and Competitive Reactions: Insights from First Principles Calculations and Microkinetic Modeling Simulations. J. Catal. 2016, 343, 196-207. (54) Liu, Z.; Lustemberg, P.; Gutiérrez, R. A.; Carey, J. J.; Palomino, R. M.; Vorokhta, M.; Grinter, D. C.; Ramírez, P. J.; Matolín, V.; Nolan, M.; Ganduglia-Pirovano, M. V.; Senanayake, S. D.; Rodriguez, J. A. In Situ Investigation of Methane Dry Reforming on Metal/Ceria(111) Surfaces: Metal–Support Interactions and C−H Bond Activation at Low Temperature. Angew. Chem. Int. Ed. 2017, 56, 13041-13046. (55) Lustemberg, P. G.; Ramírez, P. J.; Liu, Z.; Gutiérrez, R. A.; Grinter, D. G.; Carrasco, J.; Senanayake, S. D.; Rodriguez, J. A.; Ganduglia-Pirovano, M. V. Room-Temperature Activation of Methane and Dry Re-Forming with CO2 on Ni-CeO2(111) Surfaces: Effect of Ce3+ Sites and Metal– Support Interactions on C–H Bond Cleavage. ACS Catalysis 2016, 6, 8184-8191. (56) Van Santen, R. A. Complementary Structure Sensitive and Insensitive Catalytic Relationships. Acc. Chem. Res. 2009, 42, 57-66. (57) Tyo, E. C.; Vajda, S. Catalysis by Clusters with Precise Numbers of Atoms. Nature Nanotechnology 2015, 10, 577.

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Highly Active Ceria-Supported Ru Catalyst for the Dry Reforming of

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