CeO

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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In-situ Characterization of Cu/CeO Nanocatalysts during CO Hydrogenation: Morphological Effects of Nanostructured Ceria on the Catalytic Activity Lili Lin, Siyu Yao, Zongyuan Liu, Feng Zhang, Li Na, Dimitriy Vovchok, Arturo Martinez-Arias, Rafael Castaneda, Jin Ying Lin, Sanjaya D. Senanayake, Dong Su, Ding Ma, and Jose A. Rodriguez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03596 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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The Journal of Physical Chemistry

In-situ Characterization of Cu/CeO2 Nanocatalysts for CO2 Hydrogenation: Morphological Effects of Nanostructured Ceria on the Catalytic Activity Lili Linab, Siyu Yaoa, Zongyuan Liua, Feng Zhangc, Na Li d, Dimitriy Vovchoke, Arturo Martínez-Ariasf, Rafael Castañedaf, Jinying Linc, Sanjaya D. Senanayakea, Dong Sud, Ding Ma*b, José A. Rodriguez* a,c,e a

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States b College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China c Materials Science and Chemical Engineering Department, State University of New York at Stony Brook, New York, 11794, United States d Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States e Department of Chemistry, State University of New York at Stony Brook, New York 11794, United States f Instituto de Catálisis y Petroleoquímica, CSIC, Calle de Marie Curie 2, Cantoblanco 28049 Madrid, Spain ABSTRACT: A combination of time-resolved X-ray diffraction (TR-XRD), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to carry out an in-situ characterization of Cu/CeO2 nanocatalysts during the hydrogenation of CO2. Morphological effects of the ceria supports on the catalytic performances were investigated by examining the behavior of copper/ceria-nanorods (NR) and nanospheres (NS). At atmospheric pressures, the hydrogenation of CO2 on the copper-ceria catalysts produced mainly CO through the reverse-water gas shift reaction (RWGS) and a negligible amount of methanol. The Cu/CeO2-NR catalyst displayed the higher activity, which demonstrates that the RWGS is a structure sensitive reaction. In-situ TR-XRD and AP-XPS characterization showed significant changes in the chemical state of the catalysts under reaction conditions with the copper being fully reduced and a partial Ce4+  Ce3+ transformation occurring. A more effective CO2 dissociative activation at high temperature and a preferential formation of active bidentate carbonate and formate intermediates over CeO2(110) terminations are probably the main reasons for the better performance of the Cu/CeO2-NR catalyst in the RWGS reaction.

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1. Introduction CO2, the main source of greenhouse gas, is known to be the main reason for global warming and ocean acidification 1-2. Catalytic conversion of CO2 into valuable chemicals not only mitigates the negative effects of CO2 emission, but also offers an effective way to complete the carbon cycle and build a sustainable carbon-neutral fuel/chemical production strategy 3-4. CO2 hydrogenation is one of the promising processes for CO2 utilization by converting CO2 to CO at atmospheric pressure through a reverse water shift reaction (RWGS) route, or the synthesis of alcohols and hydrocarbons at high pressure 5-7. The RWGS reaction (CO2+H2=CO+H2O) has been extensively studied, because the product, CO, can be used as a feedstock in the Fischer–Tropsch process or methanol synthesis to produce valuable chemicals 8-10. Cerium oxide (CeO2), a reducible oxide support, has received a lot of attention in studies of CO2 hydrogenation11-13, due to its efficient oxygen vacancies14-15 and highly active Ce4+/Ce3+ redox pair that is able to effectively enhance the dissociative activation of CO2. In addition to the intrinsic properties of cerium oxide, the morphology of the support with different preferentially exposed faces has been found to be a crucial factor in a series of reactions, such as CO oxidation 16-17, NO reduction 18 , methanol synthesis 19, and the water gas shift (WGS) reaction 20-21. The origin of the face effect of ceria on the catalytic performance comes from the difference of the electronic properties and the surface atomic arrangement of ceria, which influence the difficulty of the oxygen vacancies formation15, 20, 22 as well as the configuration of surface intermediates 23-25. To the best of our knowledge, an in-situ investigation of the effects of nanostructured ceria on CO2 hydrogenation reactions has not been made in a systematic way. For academic and practical reasons, this topic needs to be explored and analyzed in detail. Copper is one of the most widely studied nonprecious metals for CO2 hydrogenation26 reactions exhibiting high activity and selectivity. At atmospheric pressure, the RWGS is the primary reaction route 27-29. Herein, two Cu/CeO2 catalysts were prepared using different nanostructured ceria supports: nanorods (CeO2-NR) and nanospheres (CeO2-NS). It has been reported that these nanostructures expose mainly the (110) and (111) terminations of ceria, respectively, but their actual surface morphologies could be modified by defects and reconstructions which have predominant (111) facets 20,30. The behavior of the Cu/CeO2 catalysts under the RWGS reaction was investigated to clarify the relationship between ceria morphology and the catalytic performance and further correlate the catalytic behavior of different CeO2 crystal faces with the mechanism of the RWGS process. With the aid of in-situ time-resolved X-ray diffraction (TR-XRD) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS), we show that metallic Cu and partially reduced ceria are the active phase under reaction conditions with the support CeO2 probably being the active site for CO2 activation. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to examine the evolution of surface intermediates on the two nanostructured Cu/CeO2 catalysts and gain mechanistic insights. A stronger CO2 adsorption ability and a higher tendency to form active bidentate oxo-intermediates on the ceria at high temperature are probably the reasons 2


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for the excellent catalytic performance of the Cu/CeO2-NR system. 2. Experimental 2.1 Catalysts preparation. As described in previous studies21, 31, ceria nanospheres were prepared by a microemulsion method. Distilled water was mixed with n-heptane to form a reverse microemulsion using Triton X-100 as the surfactant. Then cerium (III) nitrate was introduced to this reverse microemulsion. After stirring the microemulsion for 1 hr, a tetramethyl ammonium hydroxide (TMAH) alkali solution was added to the microemulsion as a precipitant. And then the product was rinsed with methanol and dried overnight at 100 ˚C. Ceria nanorods were prepared by a hydrothermal method. The cerium (III) nitrate precursor was dissolved in a NaOH aqueous solution with stirring. The mixture was introduced into a stainless steel autoclave and kept at 100 ˚C for 24 hours. Both ceria supports were calcined in air at 500 ˚C for 2 hours after they were dried. Cu/CeO2 catalysts were prepared by incipient wetness impregnation using the as-prepared ceria supports and copper nitrate as the precursor, loaded with 5 wt% Cu. 2.2 Performance evaluation. Flow reactor studies of the reverse water gas shift reaction over the Cu/CeO2 catalysts were performed in a quartz tube reactor with an inner diameter of 4 mm under atmospheric pressure. 10 mgs of the catalyst (60~80 mesh) were mixed with ~10 mgs of inert material (acid-purified quartz which was pre-calcined at 900 oC for 2 hours, 60-80 mesh) and used in the reaction. The ratio of CO2 and H2 was set at 1:5 (2.5 ml/min CO2 with 12.5 ml/min H2) in the catalytic performance evaluation and diluted by Ar (10 ml/min). The catalysts were reduced under a H2/Ar mixture (10 ml/min H2 with10 ml Ar) at 400 oC for 2 hours prior to reaction. Their RWGS activity was measured at 150, 250, 350 and 450 oC respectively, and each step took 2 hours. The concentrations of gas products were analyzed with a gas chromatography instrument (Agilent 7890B) equipped with both flame ionization and thermal conductivity detectors. In this study the conversion of CO2 is defined as：

%

1

∗ 100%

The activity is defined as:

μmol/

∗s

∗ 1.344 ∗

And the selectivity of CO is defined as:

∗ 100%

%

2.3 Characterization 2.3.1 TEM. The high-resolution TEM images were collected at 200 kV using a Cs-corrected (1 mm) JEOL JEM 2100F instrument at the Center for Functional Nanomaterials at BNL. Powder samples were dispersed as a suspension in deionized water, sonicated for 60 s and introduced onto a Holey-C grid. The samples were air dried before imaging. 3



2.3.2 In-situ X-ray Adsorption Fine Structure (XAFS). The in-situ XAFS measurement for the Cu/CeO2 catalysts after reduction were performed at the 9BM beamline of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). In a typical study, 80 mgs of powder sample catalyst were loaded into a home-made micro-channel reactor with graphite paper windows. The cell was heated with cartridge heaters. The XAFS spectra were collected with a four-channel Vortex detector in fluorescence-yield mode. For each tested catalyst, the powder material was reduced in a flow of 50 % H2 in helium (20 ml/min) at 400 °C for 2 hours and cooled down to room temperature under the protection of diluted H2. The XAFS measurements were done at room temperature. Three parallel measurements were done and averaged together to improve the quality of each spectrum. The data processing and fitting were carried out using Ifeffit packages. 2.3.3 In-situ Time Resolved X-ray Diffraction (TR-XRD). The in-situ time-resolved X-ray diffraction measurement under the RWGS reaction were performed at the 17BM beamline ( = 0.45226 Å) of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). In a typical experiment, 3 mgs of powder sample of each as-prepared catalysts were loaded into a 1.0 mm diameter amorphous silica tube, which was installed into an in-situ gas flow cell with a resistance heater placed under the silica tube. The XRD measurements under RWGS reaction conditions were performed between 250 and 450 °C using stepwise heating and a time span of 30 min at each temperature stage. Two-dimensional XRD patterns were collected with a Perkin-Elmer amorphous silicon detector and the diffraction rings were integrated using the GSAS-II software. Lattice parameters, phase composition and particle size were determined with the Rietveld method using the GSAS-II package. 2.3.4 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 Department of BNL was used for XPS analysis (Resolution: ~0.4 eV). The Ce 3d photoemission line with the strongest Ce4+ feature (916.9 eV) was used for the energy calibration. The Cu/CeO2 catalyst powder was pressed on an aluminum plate and then loaded into the AP-XPS chamber. The sample was heated with 0.001 mTorr O2 at 500 ˚C for 30 min. 45 mTorrs of H2 were used to pre-treat the sample at 400 °C for 30 min, before 40 mTorrs of H2 and 8 mTorr of CO2 were introduced into the reaction chamber at 25 °C through a high precision leak valve. Ce 3d, Cu 2p, Cu LMM, O 1s, and C 1s XPS regions were collected at 500 °C after oxidation, at 400 °C after reduction, and at different temperatures (25 °C, 350 °C, 450 °C) under a reaction gas environment. 2.3.5 Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). In-situ DRIFTS spectra were collected using an FTIR spectrometer (Thermo Fisher Scientific, Nicolet 6700) equipped with a Harrick cell and an MCT detector at the Chemistry Department of BNL. The spectra were expressed in units of Kubelka-Monk (K-M) with liquid nitrogen. The conditions for the activation and reaction tests were similar to those used for performance evaluation, the catalyst was reduced in a mixture of 50%H2/50%He (20 mL/min) at 400 ˚C for 40 min, and then 4


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cooled down to 25 ˚C. The gas flow was changed to 50%H2/10%CO2/He (20 mL/min) at 25 °C. The reaction was performed following stepwise heating to 250, 350, and 450 ˚C with a heating rate of 10 ˚C/min and stayed at each temperature stage for 30 min. To further investigate the active intermediates during the RWGS, a “CO2 on-CO2 off” test was carried out with in-situ DRIFTS. The activation process was described above. First, the activated catalysts were exposed to a reactant gas stream (vH2: vCO2: vHe = 5:1:4, total volume=20 ml/min) at 250 °C for half an hour to reach steady state. Then, we removed the CO2 gas from the reactant stream, keeping the sample in a H2/He (v/v=1/1, total volume=20 ml/min) gas stream for another half an hour. This cycle of “CO2 on-CO2 off” was repeated at least twice while measuring DRIFTS spectra. 3. Results and Discussion 3.1 Structure and Performance. In previous studies, three kinds of nano-structured CeO2 (nanospheres, nanorods and nanocubes) have been frequently selected as the supports of model catalysts to understand morphological effects 12, 20-21. In Figure 1a-c, we display images for Cu/CeO2-NR and Cu/CeO2-NS. For comparison, we also include data for Cu/CeO2-nanocubes 21. The TEM images of the fresh catalysts with 5% Cu loading show that the three nanostructured ceria supports keep their representative morphology after the deposition of the copper oxide. The CuO/CeO2-NR is composed of nanorods with a uniform diameter of ca. 10 nm (Figure 1a). The CuO/CeO2-NS has a sphere like-shape with average size 5-10 nm (Figure 1b), while the particle size of CuO/CeO2-NC with cubic shape is around 20 nm (Figure 1c)21. In previous studies of our group, the BET surface of CuO/CeO2-NR and CuO/CeO2-NS was similar (ca. 70 m2/g). The dispersion of Cu on the CeO2 substrates was estimated using XAFS as shown in Figure 2a and 2b. The results from fitting the Cu K edge (8979 eV) EXAFS spectra show that the Cu-Cu coordination number in the CuO/CeO2-NR and CuO/CeO2-NS samples was almost the same (C.N.Cu-Cu=6.7) (Table 1). Based on a semi-sphere model, it could be estimated that the average diameter of the Cu particles in these catalysts was around 1.5 nm 32. In comparison, the BET surface area of CuO/CeO2-NC was much smaller (ca. 29 m2/g) than its counterparts 21, and the particles size of the copper particles deposited on the cubes were determined to be over 4 nm (C.N. =10.3). Such a difference in the BET surface area and in the average particles size of copper makes difficult to establish a reasonable comparison on the performance of the three nano-structured ceria supports. We were not able to synthesize CuO/CeO2-NC samples with the desired properties. Therefore, in the rest of the paper, only Cu/CeO2-NS and Cu/CeO2-NR catalysts with similar sizes and surface areas were used to investigate morphological effects and determine the role of the ceria surface termination on CO2 hydrogenation reactions.

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Figure 1. Representative TEM and HR-TEM images of nanostructured Cu/CeO2 catalysts. (a) Cu/CeO2-nanorods, (b) Cu/CeO2-nanospheres, and (c) Cu/CeO2-nanocubes.

Figure 2. (a) XANES and (b) EXAFS R space of Cu/CeO2-NR, Cu/CeO2-NS, Cu/CeO2-NC and Cu foil, Cu2O and CuO references.

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Table 1. Cu K edge EXAFS fitting results for Cu-based catalysts Sample

Shell

Bond length (Å)

C. N.

σ2 (Å2)

E0 shift

R factor

Cu/CeO2-NS

Cu-Cu

2.53±0.01

6.7±1.8

0.010

2.9

0.009

Cu/CeO2-NR

Cu-Cu

2.53±0.01

6.7±1.3

0.008

2.7

0.008

Cu/CeO2-NC

Cu-Cu

2.54±0.01

10.3±1.0

0.008

3.6

0.001

Cu foil

Cu-Cu

2.540±0.001

12

0.009

3.7

0.002

Cu-O

1.85±0.01

4

0.003

Cu-Cu

3.02±0.01

12

0.015

9.5

0.03

Cu-O

3.54±0.01

12

0.015

7.8

0.04

Cu2O

Cu-O CuO

1.95±0.03

4

0.003

Cu-O

2.80±0.03

2

0.003

Cu-Cu

2.91±0.03

4

0.010

Cu-Cu

3.10±0.03

4

0.010

Cu-Cu

3.19±0.03

2

0.010

The catalytic performance for the RWGS reaction of the as-prepared Cu/CeO2 catalysts with different morphologies was evaluated in the temperature range from 150 to 450 °C, under a relatively high space velocity (WHSV) of 150,000 mL/gcat/h. As shown in Figure 3a, the Cu/CeO2-NR catalyst exhibited higher CO2 conversion than the Cu/CeO2-NS catalysts at temperatures of 250, 350, 450 °C (the activity for the bare ceria supports was negligible based on performance tests under the same reaction condition). At 250 °C, the conversion of both Cu/CeO2 catalysts are below 5%, which are in the kinetic region. At this temperature, the catalytic activity of Cu/CeO2-NR catalyst (1.8 μmol/g/s) is 4.3 times higher than the activity of Cu/CeO2-NS catalyst (0.42 μmol/g/s) (Figure 3b). As the temperature increases to 350, and 450 °C, the Cu/CeO2-NR catalyst exhibits higher CO2 conversion than the Cu/CeO2-NS catalyst, even though the catalytic activity is limited by the equilibrium. In a previous study19, it has been reported that a Cu/CeO2-NR catalyst exhibits an excellent performance for methanol synthesis when using elevated pressures of CO2 and H2. To understand the influence of the morphology of the ceria support on the reactivity and to further establish relationships between the catalytic behavior with respect to a particular exposed surface, a series of structural characterizations of the bulk phase, surface properties as well as adsorption species have been done.

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Figure 3. (a) CO2 conversion and (b) CO generated rates of two Cu/CeO2 catalysts (Cu/CeO2-NR, Cu/CeO2-NS).

3.2 In-situ Characterization. The crystal structure changes of the Cu/CeO2-NR and Cu/CeO2-NS catalysts under a hydrogen temperature program reduction (H2-TPR) and under steady-state working conditions were monitored by time-resolved in-situ X-ray powder diffraction (XRD). Initially, the as-prepared catalysts contained CuOx species dispersed on ceria. The diffraction patterns of the hydrogen reduction process suggest that copper oxide crystallites in both catalysts were reduced into metallic copper near 150 °C (Figure 4a, b). After switching to the reactant gas (10%CO2/50%H2/He,10 ml/min), copper remained in a metallic state over both catalysts from room temperature to 450 °C (Figure 4c, d), indicating that metallic copper was not oxidized by the CO2 in the reactant stream. The lattice parameter of cerium oxide obtained from Rietveld refinement under different condition are shown in Figure 5. The non-thermal lattice expansion of cerium oxide (reflecting the formation of Ce3+ in the bulk phase) was used to evaluate the reduction degree of the support. The lattice constant of the fresh Cu/CeO2-NR and Cu/CeO2-NS at 25 °C were determined as 5.4043 and 5.4094 Å, respectively. After reduction, the ceria lattice expanded to 5.4395 and 5.4229 Å at room temperature. Exposing the reduced catalysts to reactant gas at 25 °C would cause the lattice parameter of ceria to decrease rapidly, which suggested the oxygen vacancies were recovered by CO2 decomposition on the catalyst surface. When the temperature increased, the CeO2 lattice of Cu/CeO2-NR shrinked dramatically at the temperature of 45-110 oC under reaction condition (red rectangle in Figure 5). In contrast, the lattice parameter of Cu/CeO2-NS increased slightly. This phenomenon implied that CO2 activation over Cu/CeO2-NR was easier than over the Cu/CeO2-NS catalyst at low temperature (45-110 oC). The comparison of ceria lattice parameters at elevated reaction temperature (above 250 °C) in H2 and a H2-CO2 mixture demonstrated that the lattice constants of the ceria support in both catalysts in the presence of a H2-CO2 mixture were closed to the ones in H2 flow, implicating that the bulk phase of CeO2 tended to 8


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keep a highly reduced state under reaction condition other than being oxidized by CO2, while copper keeps a metallic state during the RWGS reaction.

Figure 4. Time-resolved XRD patterns for (a) CuO/CeO2-NR and (b) CuO/CeO2-NS catalysts with H2 pretreatment (50%H2/He), and pre-reduced (c) Cu/CeO2-NR and (d) Cu/CeO2-NS under RWGS reaction condition at different temperature (10%CO2/50%H2/He).

Figure 5. CeO2 lattice constant for Cu/CeO2-NR and Cu/CeO2-NS catalysts reduced in molecular hydrogen or under the reaction conditions for the hydrogenation of CO2. The variations in the ceria lattice were calculated through Rietveld refinement of the corresponding in-situ XRD data. 9



Understanding the surface structure and properties of the catalysts with surface sensitive techniques is crucial to unravel the origin of the catalytic effect, as the catalytic reaction mainly occurs at the gas-solid interface. However, traditional surface sensitive characterization methods usually encounter the systematic problem of “pressure gap”, which prevents the investigation of the surface properties in the presence of reactants with sufficiently high partial pressure. Herein, we applied ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) to determine the surface composition and the valence state of the elements of the Cu/CeO2 catalysts with different morphology under elevated temperature and a background pressure of CO2/H2. The Cu LMM Auger electron spectra of the two Cu/CeO2 catalysts under different reaction condition are shown in Figure 6a and 6b. The Cu LMM Auger peaks centered at 918.8, 917.7 and 916.8 eV were assigned to the Cu0, Cu1+ and Cu2+, 33 respectively. They demonstrated that Cu0 (ca. 918.8 eV) was the only surface specie under RWGS reaction conditions, which suggested that metallic Cu was the active state for the RWGS reaction in the two Cu/CeO2 catalysts. A deconvolution of the Ce 3d core-level XPS spectra 34 was used to monitor the valence of surface cerium under reaction conditions (Figure 7a, 7b). The reduction degree of the CeO2 support is quite similar among the two Cu/CeO2 catalysts, as the Ce3+ species were determined to be around 25% after H2 pretreatment. However, when exposed to both reactant gases (8 mTorr of CO2, and 40 mTorr of H2) at room temperature, the amount of Ce3+ present in the two catalysts decreased, which agrees well with the XRD results. When the reaction temperature was raised to 350 and 450 °C, the surface concentration of Ce3+ on Cu/CeO2-NS (25.4 and 25.0%) catalysts was significantly enhanced. On the contrary, the concentration of Ce3+ on Cu/CeO2-NR was determined as 18.2% and 17.9% at the elevated temperature (Figure 7c). This irregular phenomenon over Cu/CeO2-NR catalyst suggested that in the mixture of H2-CO2, the ceria rod support showed much stronger affinity for CO2 with a tendency to generate surface intermediate species or O adatoms from the dissociation of CO2. In comparison, the sphere support tends to be strongly reduced by H2 35. Even though CeO2-NS possessed more oxygen vacancies than CeO2-NR under the same conditions, the dissociation and activation of CO2 were actually inhibited, which further rendered loss of total RWGS activity. Therefore, the strong affinity of the surface oxygen vacancies in the CeO2-NR surface for CO2 is probably the reason of its excellent RWGS performance.

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Figure 6. Cu LMM Auger profiles of (a) Cu/CeO2-NR, (b) Cu/CeO2-NS catalysts at different reaction condition. (i) 500 oC oxidation condition: O2 (1E-6 Torr) pretreatment at 500 oC for 30 min to oxidize all Ce3+ to Ce4+. (ii) 500 oC reduction condition: H2 (45 m Torr) pretreatment at 400 oC for 30 min. (iii) 25 oC: after reduction process cool down to room temperature, reduced catalysts exposed with reactant gas at 25 oC for 30 min. (iv) 350 oC: exposed with reactant gas at 350 oC for 30 min. (v) 450 oC: exposed with reactant gas at 450 oC for 30 min. Reactant gas: CO2/H2=1:5 (8 m Torr CO2, and 40 m Torr H2).

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Figure 7. Ce 3d profiles of (a) Cu/CeO2-NR, (b) Cu/CeO2-NS catalysts at different reaction condition. (c) Ce3+ concentration of Cu/CeO2-NR and Cu/CeO2-NS catalysts at different reaction conditions. The concentration of Ce3+ was obtained by deconvolution of the corresponding Ce 3d XPS spectra.34 (i) 500 oC oxidation condition: O2 (1E-6 Torr) pretreatment at 500 oC for 30 min to oxidize all Ce3+ to Ce4+. (ii) 500 oC reduction condition: H2 (45 m Torr) pretreatment at 400 oC for 30 min. (iii) 25 oC: after reduction process cool down to room temperature, reduced catalysts exposed with reactant gas at 25 oC for 30 min. (iv) 350 oC: exposed with reactant gas at 350 oC for 30 min. (v) 450 oC: exposed with reactant gas at 450 oC for 30 min. Reactant gas: CO2/H2=1:5 (8 m Torr CO2, and 40 m Torr H2).

In order to further understand the influence of cerium oxide morphology on the surface adsorbates and intermediates, in-situ diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS) was applied to investigate the surface species under a steady-state reaction. As shown in Figure 8a and 8b, after the injection of reactants (10%CO2/50%H2/He) over pre-reduced Cu/CeO2 at room temperature, abroad band composed by multiple adsorption peaks from 1700 to 800 cm-1 wavenumbers increased rapidly over the two studied catalysts, which reflected the most intense C-O vibration modes of surface carbonate and other oxo-species. The 12


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formation of surface oxygenates over oxygen vacancies caused the disappearance of Ce3+ sites and further induced the structural changes observed by TR-XRD and AP-XPS methods. The detailed assignment of IR peaks is summarized in Table 2. IR spectra under steady state were collected under reaction conditions at 250, 350 and 450 °C, respectively. At 250 °C, the formation of surface formate occurred on both catalysts as the typical C-H stretch vibration peaks of formate at 2848 and 2945 cm-1 were observed. Correspondingly, the C=O vibration signals were also generated near 1565 and 1374 cm-1, which are associated with a bidentate formate 24, 36-37,38,39. Cu/CeO2-NR showed the most intense features of these bidentate oxo groups. When the temperature increased to 350 and 450 °C, the bidentate formate species gradually got weaker and finally almost disappeared, indicating that those species are possible active surface intermediates. Formates are typically seen after the hydrogenation of CO2 on copper and ceria surfaces14,23,26,40. The major surface species, at a high temperature of 450 °C, were identified as polydentate surface carbonates, which were relatively stable and hard to be converted 38. Cu/CeO2-NS catalyst was discovered to be covered by plenty of polydentate carbonates (1474,1367, 1037cm-1), which is probably the reason for its low activity at low temperature. Table 2. Assignment of absorbance peaks observed on FT-IR spectra36-39 Frequencies (cm-1)

Vibration mode

2845

ν(CH)

1565

ν(CO2)

1372

ν(CO2)

771

δ(CO2)

1580

ν(CO3)

1292

ν(CO3)

851

δ(CO3)

1474

ν(CO3)

Polydentate

1367

ν(CO3)

carbonate

1037

ν(CO3)

856

δ(CO3)

Species

Bidentate formats

Bidentate carbonate

13


Structure


Figure 8. In-situ DRIFT spectra collected over (a) Cu/CeO2-NR, (b) Cu/CeO2-NS catalysts during RWGS reaction (10%CO2/50%H2/He) at different temperature.

To further investigate if the bidentate oxo species are active surface intermediates for the RWGS on Cu/CeO2 catalysts, more experiments were designed to observe the dynamic behavior of bidentate formate and carbonate species during the RWGS reaction over the Cu/CeO2 catalysts. On the basis of the steady state IR spectra, 250 °C was chosen as the experiment temperature, as bidentate oxo species showed relative high surface coverage and appropriate IR signals at this temperature. The consumption and decomposition behavior of surface species were evaluated by switching off CO2 in the reactants (CO2:H2:He=1:5:4) after the IR spectra of Cu/CeO2 catalyst reached steady state. As shown in Figure 9a, after the cutoff of CO2, the band at ca.1580 cm-1 decreased rapidly over Cu/CeO2-NR catalyst in the H2/He (1:1) flow, indicating that the bidentate species were able to react with H2 and transformed into CO and water. Judging from the subtracted spectra listed in the bottom of figure 9a, bands at ~1580 cm-1 (in the rectangle) were composed by two components. The higher component at 1580 cm-1 and lower component at 1565 cm-1, which are associated with bidentate carbonate and bidentate formate respectively. The assignment was further confirmed by the related IR peaks at 1290, 856 cm-1 (for carbonate) and 2845, 14


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1370, 790 or 771 cm-1 (for formate) 39. In contrast, although the dynamic behavior on CeO2-NS is quite similar to the CeO2-NR, the surface concentration of active bidentate species was relatively low (figure 9b), indicating these species were relatively less favorable to form on the CeO2 nanosphere (CeO2(111) main termination). Furthermore, the switching-off experiment also revealed that some of the active species tend to convert into stable polydentate carbonates over the Cu/CeO2-NS catalyst, which is undesirable for low temperature activity. The experiment of “CO2 on- and off-cycles” were further performed (figure 9c, d). Respective peaks at 1580, 1370, 1290 cm-1 diminished over both Cu/CeO2-NR and Cu/CeO2-NS catalysts when CO2 was switched off during the RWGS reaction, while those peaks appeared when CO2 gas was introduced, which firmly confirmed that bidentate carbonate and bidentate formates were active species that could be effectively consumed and reproduced.

Figure 9. In-situ DRIFT spectra collect after the RWGS reaction during 30 min with subsequent switching off CO2 over (a) Cu/CeO2-NR, (b) Cu/CeO2-NS catalysts at 250 oC. DRIFT spectra of “CO2 on-CO2 off - CO2 on - CO2 off” collect over (c) Cu/CeO2-NR, (d) Cu/CeO2-NS catalysts at 250 oC, each step keep 30 min. The bottom spectra of Figure (a) and (b) were obtained from every spectrum in the upper by subtraction of the first spectrum to get a clear insight into the variation of adsorbed intermediate species.

DFT calculations indicate that H adatoms facilitate the adsorption of CO2 on copper and oxide surfaces by forming formates and carboxylates.23,40-42 The results of in-situ DRIFTS demonstrated that the preferential formation of active bidentate carbonate 15



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and bidentate formate species over CeO2-NR is probably the reason for the excellent low temperature performance of the Cu/CeO2-NR catalyst. The reason behind the preferential formation of bidentate oxo-species on the CeO2-NR surface could be in the surface geometry of a CeO2(110) termination24, which is expected for the nanorods but it could be distorted by the formation of some facets that expose a (111) termination 22, 30. The results of theoretical calculations have shown that bidentate formate binds stronger on CeO2(110) than on CeO2(111) 24. The nearest distance of surface oxygen atoms on a CeO2(110) surface is 2.71 Å, which is a more suitable spacing for the formation of bidentate oxo-intermediates 24. On the contrary, the distance between oxygen atoms on CeO2(111) is too large to accommodate such species (3.81 Å) 23, 25. A geometric effect probably influences the formation of polydentate carbonates but the bonding of those species is more complex and one must consider particular arrays of surface sites in addition to the Ce-Ce and O-O separations 36,39. 4. Conclusion The morphological effect of the cerium oxide support was evaluated over Cu/CeO2 RWGS catalysts. A system generated by depositing Cu on CeO2 nanorods exhibited better activity than catalysts involving ceria nanospheres. Time-resolved XRD and AP-XPS measurements studies showed that metallic copper and reduced CeO2 were the active phases for the RWGS in all the tested catalysts. AP-XPS measurements also revealed that Cu/CeO2-NR showed stronger affinity to CO2 at high temperature. in-situ DRIFTS spectra pointed to bidentate oxo-species as the active intermediates for CO2 hydrogenation. The preferential formation of a high coverage of bidentate carbonate and bidentate formate on CeO2-NR could be the main reason for the excellent performance of a Cu/CeO2-NR catalyst. The comparative study of the morphology effect of Cu-CeO2 catalysts hints that CeO2(110) is a suitable surface for the RWGS reaction due to its efficient O vacancies and ideal surface atomic configuration for CO2 activation. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The research carried out at the Chemistry Department of Brookhaven National Laboratory (BNL), was supported by the division of Chemical Science, Geoscience, and Bioscience, Office of Basic Energy Science of the US Department of Energy. Use of the Advanced Photon Source (beamline 17B-M and 9BM)

was

supported

by

the

U.S.

Department

of

Energy

under

Contract

Nos.

DE-AC02-06CH11357. This electric microscope (EM) work used resources of the Center for Functional Nanomaterials, which is a US DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. The scholarship under the International Postdoctoral 16


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Exchange Fellowship Program 2017 by the Office of China Postdoctoral Council (document number: No.32 Document of OCPC, 2017) is also gratefully acknowledged. Thanks are also due to MINECO (Spain, project CTQ2015-71823-R) for financial support.

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Active Sites for CO2


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