Article pubs.acs.org/JACS
Direct Neutron Spectroscopy Observation of Cerium Hydride Species on a Cerium Oxide Catalyst Zili Wu,*,† Yongqiang Cheng,‡ Franklin Tao,§ Luke Daemen,‡ Guo Shiou Foo,† Luan Nguyen,§ Xiaoyan Zhang,§ Ariana Beste,∥ and Anibal J. Ramirez-Cuesta‡ †
Chemical Science Division and Center for Nanophase Materials Sciences and ‡Chemical & Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemical and Petroleum Engineering and Department of Chemistry, University of Kansas, Lawrence, Kansas 66047, United States ∥ Joint Institute for Computational Sciences, The University of Tennessee, Oak Ridge, Tennessee 37831, United States S Supporting Information *
ABSTRACT: Ceria has recently shown intriguing hydrogenation reactivity in catalyzing alkyne selectively to alkenes. However, the mechanism of the hydrogenation reaction, especially the activation of H2, remains experimentally elusive. In this work, we report the first direct spectroscopy evidence for the presence of both surface and bulk Ce−H species upon H2 dissociation over ceria via in situ inelastic neutron scattering spectroscopy. Combined with in situ ambientpressure X-ray photoelectron spectroscopy, IR, and Raman spectroscopic studies, the results together point to a heterolytic dissociation mechanism of H2 over ceria, leading to either homolytic products (surface OHs) on a close-to-stoichiometric ceria surface or heterolytic products (Ce−H and OH) with the presence of induced oxygen vacancies in ceria. The finding of this work has significant implications for understanding catalysis by ceria in both hydrogenation and redox reactions where hydrogen is involved.
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one.1a,c,6 For CeO2, current IR studies showed the presence of O−H groups only upon H2 interaction and reduction, and thus a homolytic pathway was widely suggested.2a,3a,4a−c Recently, Lopez and co-workers3c,d proposed for the first time a different mechanism where the overall homolytic dissociation of H2 proceeds through the formation of Ce hydride intermediates and the heterolytic dissociation of H2 on the Ce−O bond followed by the transfer of a H atom on Ce to O to yield the homolytic product. This mechanism was later elaborated by other theory groups.3b,e However, there is still no direct experimental evidence for the presence of cerium hydride (Ce−H), expected in the range of 1500−2000 cm−1 in the vibrational spectrum,5,7 upon H2 dissociation over CeO2. Therefore, the pathway for H2 dissociation over CeO2 and the hydrogenation mechanism still remain elusive. Here, we report for the f irst time direct experimental observation of the formation of Ce−H upon H2 interaction over ceria nanocrystals by employing in situ inelastic neutron scattering (INS) spectroscopy. Supported by in situ IR, Raman, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and DFT calculation, we are able to show the presence of both surface and bulk cerium hydride upon H2− CeO2 interaction, which enables an in-depth understanding of
INTRODUCTION Ceria, an extensively studied redox catalyst,1 has recently attracted intense interest for its remarkable catalytic performance in hydrogenation reactions such as partial hydrogenation of light alkynes in the gas phase and complex alkynes in liquid phase.2 These studies showed that a large hydrogen excess in the feed mixture (H2/alkyne >25) and temperatures above 500 K were required, indicating that hydrogen dissociation over the ceria surface is involved in the rate-determining step in these reactions.2a,c Intrigued by the reactivity of ceria for partial hydrogenation, several density functional theory (DFT) works3 have since been devoted to understanding the reaction mechanism with particular interest in how dihydrogen is dissociated on the ceria surface. The H2−CeO2 interaction has actually long been of great interest both experimentally and theoretically for understanding the redox property of ceria-based catalysts, as the oxygen vacancies in ceria are normally created via hydrogen reduction.1a,4 Two mechanisms have been proposed from DFT calculations for the dissociation of H2 over CeO2: homolytic cleavage to form two O−H and heterolytic cleavage to produce Ce−H and O−H.2c,5 The experimental approach to a hydrogen dissociation mechanism over metal oxides (MOx) is largely based on spectroscopic observation such as infrared spectra of either both the O−H or M−H bands for the heterolytic pathway or only O−H bands for the homolytic © 2017 American Chemical Society
Received: May 27, 2017 Published: June 27, 2017 9721
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through the catalyst surface design by Tao and Nguyen,12a which allows XPS characterization under reaction conditions and during catalysis up to 25 Torr of gas. The in situ cell is equipped with a diode laser heater, which can heat samples up to 1073 K in gas. In this study, powder CeO2 catalyst was immobilized onto a clean, roughened Au foil. Due to natural charging of CeO2 at 298 K, the XPS study of the CeO2 sample needed to be done at a sample temperature above 393 K if not stated otherwise. Sample temperature was monitored using a K-type thermocouple placed behind the Au foil substrate. At each reaction condition, the sample was incubated for 1 h to reach equilibrium before collecting data. All XPS data in this work were collected while catalysts were in a gas environment at the mentioned temperature. Quantification of the XPS data was done using the CasaXPS software package. All spectra were background subtracted using the Shirley equation. Deconvolution of Ce 3d spectra was based on reference information from Mullins et al.13 and Paparazzo.14 For each Ce 3d spectrum, four components (u′, u0, v′, and vo) were used for Ce3+ cations and six components (u‴, u″, u, v‴, v″, and v) were used for Ce4+ cations. In Situ INS. INS experiments were performed at the VISION beamline of Spallation Neutron Source, Oak Ridge National Laboratory. The CeO2 rod sample (about 1.3 g) was wrapped in aluminum foil and loaded in a stainless steel sample container. The sample was first activated at 673 K under vacuum for 1 h, and the blank sample (in the sample holder) was then measured at 5 K to collect the background INS spectrum. The sample was further exposed to 1 bar of H2 at various temperatures (533, 623, and 673 K in order). After exposure at each temperature for 2 h, the sample was then cooled to 393 K and pumped briefly to remove unreacted H2 and possibly water. It was subsequently quenched to 5 K to collect INS spectra, and the difference spectra were calculated by subtracting the background (blank sample). After all these procedures, the sample was then exposed to 1 bar of O2 at room temperature for 1 h and then pumped at 393 K. The O2-exposed sample was measured to collect the final spectrum. For the bulk CeH3 spectrum, about 3 g of cerium metal (99.9%) was placed in a sealed aluminum can and heated under vacuum at 363 K for about 1 h. Then hydrogen gas was loaded and the sample was heated at 363 K overnight under 2 bar of hydrogen until the reaction saturated. The sample was then quenched in liquid nitrogen before measuring at VISION. Simulation of INS Spectra. DFT simulations of bulk CeH2, CeH3, and the CeO2 surface of orientations (100), (110), and (111) were conducted with VASP.15 The generalized gradient approximation (GGA), as implemented by Perdew−Burke−Ernzerhof (PBE),16 was used to describe the exchange−correlation interactions. The projector augmented wave (PAW) method17 was employed to account for the effects of core electrons. The energy cutoff for the plane waves was 400 eV. For the surface models of CeO2, a Hubbard U term of 3.0 eV was used for the GGA+U calculation for the localized 4f electrons. Atoms in the bottom layer of the surface were fixed, while the rest of the atoms were allowed to fully relax until the maximum force was below 0.01 eV/Å. The phonon calculations were performed on a gammacentered 8 × 8 × 8 mesh for the bulk hydrides and gamma point only for the surface models. The aClimax software18 was used to convert the DFT-calculated phonon results to the simulated INS spectra.
not only the hydrogenation reaction but also the redox property of CeO2-based catalysts.
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EXPERIMENTAL SECTION
Synthesis of CeO2 Rods. Ceria rods have been prepared by a hydrothermal method described in detail in our recent paper.8 After the hydrothermal treatment, fresh white precipitates were separated by centrifugation, and washed with deionized water and ethanol several times, followed by drying at 333 K in air overnight. Na impurities were removed by dispersing the products in 2 mL of 0.1 M NH4OH, which was suspended in an ultrasonic bath for 2 min. The product was isolated via centrifugation, followed by three warm water washes. The products were then dispersed in 2 mL of 0.1 M HNO3, which was suspended in an ultrasonic bath for 2 min. The product was isolated via centrifugation followed by three warm water washes. The products were dried under vacuum overnight and then calcined at 673 K for 4 h in air. The rods have a surface area of 72 m2/g. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) of ceria rods can be found in our previous work.8,9 These rods are ∼10 nm in diameter and several hundred nanometers in length. H2 Temperature-Programmed Reduction (TPR). H2-TPR was carried out in a plug-flow, temperature-controlled microreactor (Altamira AMI 200). Prior to TPR, the sample (ca. 40 mg) was pretreated in a 5% O2/He gas mixture for 1 h at 673 K. After being cooled to room temperature, the sample was purged with high-purity Ar (30 mL/min) for 15 min. Subsequently, a flow (30 mL/min) of 4% H2/Ar was switched into the system, and the sample was heated under this reducing gas flow from room temperature to 1100 K at a rate of 10 K/min. The consumption of H2 was recorded using a TCD detector located downstream from a cold trap that removed H2O. The TCD signal was calibrated by integrating pulses of 4% H2/Ar from a calibrated loop sent through the TCD. In Situ IR. IR spectra were collected using a Thermo Nicolet Nexus 670 spectrometer in diffuse reflectance mode (DRIFTS), while the exiting stream was analyzed by an online quadruple mass spectrometer (QMS) (OmniStar GSD-301 O2, Pfeiffer Vacuum).10 A Pike Technologies HC-900 DRIFTS cell with a nominal cell volume of 6 cm3 was used. The ceria sample was pretreated in the DRIFTS cell in flowing 5% O2/He (25 mL/min) at 673 K for 1 h and then cooled to 298 K before switching to He. The sample was then exposed to 4% H2/He flow (25 mL/min), heated to different temperatures (533, 623, and 673 K), and held there for 1 h each. IR spectra of the H2-treated ceria sample were collected after the sample was cooled to at room temperature. For O2 adsorption at room temperature, the ceria sample was pretreated with 4% H2/He or 4%D2/He at 673 K for 1 h. After cooling to room temperature, the sample was subjected to He purging before exposed to 5% O2/He flow. IR spectra were collected continuously during the O2 adsorption process, while the effluent was monitored by the online QMS. In Situ Raman. The experimental procedure is very similar for the Raman and IR studies of H2 treatment of the ceria sample. A Raman catalytic reactor (Linkam CCR1000) was used for the in situ study. Raman scattering was collected via a customized ellipsoidal mirror and directed by a fiber-optic bundle (Princeton Instruments) to the spectrograph stage of a triple Raman spectrometer (Princeton Instruments Acton Trivista 555).11 An edge filter (Semrock) was used in front of the UV−vis fiber-optic bundle to block the laser irradiation. The 532 nm excitation (10 mW at sample position) was emitted from a solid-state laser (Princeton Scientific, MSL 532-50). A UV-enhanced liquid-N2-cooled CCD detector (Princeton Instruments) was employed for signal detection. Cyclohexane was used as a standard for the calibration of the Raman shifts. Ambient-Pressure X-ray Photoelectron Spectroscopy. APXPS experiments were performed on the lab-based system built in the Tao group.12 A detailed description of the AP-XPS system can be found in the literature. In brief, the AP-XPS system is equipped with an monochromatized Al Kα X-ray source and differentially pumped energy analyzer with a reaction cell allowing gaseous reactant flow
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RESULTS AND DISCUSSION In Situ IR and Raman of H2 Interaction with CeO2. The CeO2 rods used in our previous work9,19 were employed in this work to study the H2−CeO2 interaction because of their defined surface structure and high surface area (72 m2/g). The high surface area is essential for providing enough hydrogen species to ensure the successful INS measurement with a reasonable amount (a few grams) of ceria sample. The H2-TPR profile of CeO2 (Figure 1) shows two low-temperature reduction peaks at 707 and 762 K and a high-temperature one at 1048 K, due to the reduction of surface and bulk oxygen, 9722
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Figure 1. H2-TPR of CeO2 rods.
respectively.1a,19 To investigate surface H species on ceria, we chose three reduction temperatures (533, 623, and 673 K) below the first major reduction peak at 707 K to follow the surface hydroxylation and initial reduction of CeO2. We employed conventional in situ IR and Raman spectroscopy to follow any possible surface H species, O−H, and Ce−H, upon H2 treatment at these temperatures. Special attention was paid to the spectral region 1500−2000 cm−1 for the stretching mode of Ce−H species. Neither IR nor Raman spectra (Figure S1) show any evident sign for the generation of the Ce−H band at these reduction temperatures, consistent with previous studies.1c,3d,5 Instead, changes to the O−H stretching region were observed in both IR and Raman spectra (Figure 2) collected at room temperature (RT) after CeO2 was treated with H2 at the three temperatures. IR bands (Figure 2A) of bridging OH (3650 cm−1) and terminal OH (3721 cm−1) are observed for the freshly oxidized (673 K) CeO2.1c,20 The IR band at 3520 cm−1, not observable in the corresponding Raman spectrum, is assigned to hydrogen-bonded OH on CeO2.1a Upon exposure to H2 at 533−673 K, the terminal OH bands gradually disappear with the increasing intensity of the bridging OH at 3690 cm−1, which was associated with an OH group adjacent to O vacancies.20b The removal of terminal OH was considered as a sign of CeO2 reduction,1c which occurs slightly at 533 K. Both bands at 3650 and 3521 cm−1 slightly blue shift to 3655 and 3525 cm−1, respectively, as a result of the surface reduction of ceria at 623 and 673 K.1c,20b RT exposure of O2 to the 673 K H2-treated sample does not result in an obvious change to the OH features, indicating these OH modes are not sensitive to O2 adsorption. A similar observation was made in the OH region of the Raman spectra (Figure 2B) upon H2 treatment of CeO2: increasing intensity of the bridging OH at 3691 cm−1 and blue shift of the bridging OH mode from 3656 to 3669 cm−1 with increased intensity, again indicative of the reduction of the ceria surface. Notably, the two bridging OH bands at 3669 and 3691 cm−1 increase in intensity considerably after 533 and 623 K treatment, implying increased hydroxylation via hydrogen dissociation on the ceria surface. The slight decrease of the intensity of the OH groups for the 623 K treated sample indicates the onset of thermal removal of surface OHs. Further
Figure 2. IR (A) and Raman (B) spectra of CeO2 collected at room temperature after calcination at 673 K, H2 treatment at 533, 623, and 673 K, and exposure to O2 at room temperature. The Raman peak marked with an asterisk (*) represents a laser plasma line and was used to normalize the Raman spectra intensity.
treatment at 673 K leads to a large decrease in the intensity of all OH bands, due to the removal of OH groups via water formation, i.e., formation of oxygen vacancies (O-vacancies), and thus further reduction of ceria at 673 K. RT O2 exposure results in a slight intensity increase of the two bridging OH bands but without any frequency shift. The increased intensity can be due to two possible factors: (1) the decreased selfabsorption21 of ceria due to RT partial oxidation; (2) additional OH groups formed upon O2 adsorption. The second factor is possible if there are surface Ce−H species around so that the hydride may transfer to adjacent O that fills the vacancy, a process similar to what Lopez et al.3c,d proposed for the conversion of a heterolytic to a homolytic pathway for H2 dissociation on the ceria surface. We show in the following section by INS spectra that this is likely the case. In Situ AP-XPS of H2 Interaction with CeO2. To follow the oxidation state of Ce upon H2 interaction, AP-XPS was conducted on ceria rods during the H2 treatment at different temperatures. The AP-XPS spectra in the Ce 3d and O 1s regions are shown in Figure S2, and the quantified Ce3+ % is shown in Figure 3A. The pristine CeO2 rod possesses a few percent of Ce3+, due to the presence of intrinsic O-vacancies.9 9723
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groups and generation of O-vacancies, in general agreement with the observed dehydroxylation in the Raman spectra (Figure 2B) for the two samples. In Situ INS of H2 Interaction with CeO2. INS spectroscopy, very sensitive to H-containing species on catalysts’ surfaces,23 was demonstrated capable of detecting surface hydride species on solid surfaces such as Pd upon interaction with hydrogen.23b,24 Since both IR and Raman spectroscopic studies failed to directly probe Ce−H species, we employed INS spectroscopy to study the CeO2−H2 interaction. Figure 4
Figure 4. INS spectra of CeO2 collected at 5 K after H2 treatment at (a) 533 K, (b) 623 K, (c) 673 K, and (d) 393 K under vacuum after (c) and (e) exposure to O2 at RT and then 393 K vacuum after (d). All spectra are difference spectra using the spectrum of CeO2 after 673 K O2 treatment as background.
Figure 3. Quantified percentage of Ce3+ (A) and O/Ce ratio (B) from the AP-XPS study of CeO2 after O2 treatment at 673 K and H2 treatment at 533, 623, and 673 K.
presents the spectra collected at 5 K after the ceria rods were treated with H2 at different temperatures. These are difference spectra between the measured spectra and the background spectrum of CeO2 after oxidation treatment at 673 K. The H2 interaction with CeO2 at 533 K results in a broad feature centered around 575 cm−1, assignable to additional surface OH groups24a on CeO2 generated upon H2 dissociation, in line with the above Raman and AP-XPS results (Figure 2B and Figure 3). Increasing the H2 treatment temperature to 623 K leads to a stronger band at ca. 540 cm−1 with some weak features spanning from 700 to 1500 cm−1. The strong band increases in intensity and shifts to ∼495 cm−1 at a H2 treatment temperature of 673 K. Meanwhile, a more prominent broad feature is further developed in the range 650−1300 cm−1, with another one centered at around 1500 cm−1. A further vacuum treatment of the 673 K H2-treated sample at 393 K produces essentially the same spectral features, indicating that the surface species with these INS bands are relatively stable and are not due to adsorbed water. Interestingly, exposure to O2 at RT followed by a vacuum treatment at 393 K leads to significant spectral changes: the strong band at ∼495 cm−1 and the broad features at 650−1300 and ∼1500 cm−1 are nearly diminished. The remaining spectrum (e) with a weaker band at ∼560 cm−1 resembles very well that from the 533 K H2-treated CeO2 (spectrum a in Figure 4). The drastic spectral changes upon O2 exposure indicate that the surface species responsible for the strong bands at ∼495 cm−1 and the broad features at higher wavenumbers are not
Upon H2 treatment at 533 K, a significant increase of Ce3+ %, to ∼28%, is observed, indicating that H2 can already dissociate at this temperature and thus lead to the reduction of CeO2. This is consistent with the increased hydroxylation observed in the Raman spectra (Figure 2B) for the 533 K treated sample. Further increase in H2 treatment temperatures to 623 and 673 K results in similar Ce3+ %, ∼34% on the ceria sample. This is inconsistent with the different intensity of the OH groups observed in the Raman spectra (Figure 2B) for the two samples, suggesting that the reduction of ceria at higher temperatures such as 673 K is a result of both O-vacancy formation and the surface hydroxylation. This is supported by the decrease of the O/Ce ratio to 1.84 at 673 K in H2 obtained from the AP-XPS measurements, as shown in Figure 3B. The ratio was obtained by calibrating the photoelectron intensities of Ce 3d and O 1s with the mean free paths of Ce 3d and O 1s in H2 and O2. The mean free paths of Ce 3d and O 1s in 0.5 Torr of H2 are 11.95 and 17.75 mm, respectively; they are 1.98 and 2.82 mm in 1 Torr of O2, respectively.22 The O/Ce ratio is normalized to 2 for the 673 K O2-treated sample for simplicity, although it has a few percent of Ce3+. The O/Ce ratio is similar for the calcined and 533 K H2-treated ceria, implying the reduction of ceria at 533 K is purely due to surface hydroxylation upon H2 dissociation instead of formation of O-vacancies. A decrease of the O/Ce ratio is observed for the 623 and 673 K treated sample, due to the removal of OH 9724
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cm−1 band observed for the 673 K H2-treated sample, suggesting possible contributions of CeH3 to this intense peak at 495 cm−1. But the very different relative intensity of the bands at ∼495 to ∼900 cm−1 for the bulk CeH3 and 673 K H2treated CeO2 points to additional sources for the intense band at ∼495 cm−1, such as surface Ce−H. We then simulated the spectra for surface hydride on ceria. The surface termination of CeO2 rods is still in debate, proposed as either (110) + (100) or (111) + (100).8 To be conclusive, we simulated hydride with DFT on all three surfaces of ceria. It is notable that the surface Ce−H is only stable when an O-vacancy is introduced into the slab model in our DFT calculation; otherwise the hydride in Ce−H would transfer to a neighboring O to form OH. This is consistent with the recent DFT work3c,d showing that homolytic product (OH) is more stable than the heterolytic counterpart for initial H2 dissociation on CeO2. The simulated spectra for surface Ce−H on CeO2 (100), (111), and (110) with a surface O-vacancy give major vibrational features at energies generally lower than those of the bulk hydrides: 300−750 cm−1 for Ce−H on (100) and 600− 900 cm−1 for Ce−H on (111) and (110). Some weaker features are also observed in the range 1300−1800 cm−1 for Ce−H on the (111) surface. Considering the strong band at 540 cm−1 is formed at 623 K prior to the broad features at higher wavenumbers (evident after 673 K H2 treatment), we assign the bands at ∼495−540 cm−1 (at least a large fraction of the intensities) to surface Ce−H formed upon CeO 2 −H 2 interaction at 623 and 673 K. The formation of bulk Ce−H preceded by surface Ce−H after the high-temperature reduction of CeO2 (673 K) is supported by recent DFT calculation4d that showed that diffusion of H at the reduced surface or into the bulk can occur more readily than that at stoichiometric CeO2. The large spectral change upon O2 exposure to H2-treated CeO2 at 673 K provides further evidence for the assignment of the strong bands at ∼495−540 cm−1 and the broad features at higher wavenumbers to Ce−H species. These surface and bulk Ce−H species can be observed only after O-vacancies are present on reduced CeO2 at 623 and 673 K, as supported by the IR, Raman, and AP-XPS results (Figures 2 and 3). This is consistent with our DFT calculation, which showed that Ce−H, not stable on the oxidized CeO2 surface, is stable only when an adjacent O-vacancy is introduced. Upon exposure to O2 at RT, the O-vacancies are mostly healed via O2 adsorption, as evidenced by the decrease of the Ce 3+-associated IR absorbance1c at ∼2130 cm−1 (see Figure S4A) and the decreased Ce3+ % from the AP-XPS measurement (Figure S2). Due to the instability of Ce−H on the stochiometric surface, i.e., oxygen-vacancy-free, of CeO2, the hydride H atoms transfer to neighboring O to form OH or water. Since the resulting intensity of the OH band (spectrum e in Figure 4) is similar to that of the 533 K H2-treated sample (spectrum a in
stable under an O2 atmosphere and thus are not likely surface OH species. These species may undergo reaction with O2, thus bringing the CeO2 surface back to low-temperature H2-treated status (spectrum a). Since these stronger features (around 500 cm−1 and the broad features at 650−1300 and ∼1500 cm−1) are not likely due to surface OH groups on ceria, we propose that they are due to Ce−H species. This is supported by our DFT calculations of the INS spectra for both surface and bulk Ce−H as shown in Figure 5. The computed spectra for bulk
Figure 5. Simulated INS spectra of bulk hydride of CeH2 and CeH3 and surface hydride on reduced (111), (110), and (100) surfaces. The experimental spectra from CeO2 after H2 treatment at 673 K and bulk CeH3 are also shown for comparison.
hydrides, CeH2 and CeH3, give major features ranging from 750 to 1100 cm−1, due to the deformation mode of Ce−H in various directions and relative phases (see Figure S3 for illustrations of some representative modes and Table 1 for assignments), and additional weaker features at 1300−1800 cm−1 (combination modes). These features coincide generally well with the broad features observed at 650−1300 and ∼1500 cm−1 for 673 K H2-treated CeO2. As such, it is reasonable to assign these broad features to bulk Ce−H species, which is also in good agreement with previous INS measurement of bulk hydride compounds25 with peaks in the range 500−900 cm−1. But it should be noted that although DFT calculation of CeH2 yields very accurate results, the vibrational frequencies calculated for CeH3, especially the broad band in the range 500−900 cm−1, tend to be overestimated.26 We thus made bulk CeH3 to measure the INS, and the experimental spectrum is also shown in Figure 5. Indeed, the Ce−H bands are now extended to much lower frequencies, consistent with a previous experiment.25 The peak at lower energy aligns well with the 495 Table 1. Assignment of the Ce−H Associated INS Bandsa label
a
frequency range (cm−1)
B1
400−650
B2
750−1100
B3
1300−1800
experimental observations
assignment of the vibrational modes
sharp peak, more intense than peak B2; appears at 623 K and above broad band, less intense than peak B1; appears at 673 K very weak; appears at 623 K and above
Ce−H deformation of surface hydride, possible additional contribution from bulk CeH3-like local structure Ce−H deformation in bulk hydride with CeH2-like and CeH3-like local structures combination and overtone of deformation modes of B1 and B2 peaks
See Figures 4 and 5. Examples of the deformation modes are visualized in Figure S3. 9725
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resonance (ESR),27 and DFT4a,d indicated the incorporation of H into ceria bulk, there was no direct identification of the chemical nature of included H species, either as bulk OH or hydride. Our INS work presents the first direct evidence for the coexistence of both surface and bulk Ce−H, associated with partially reduced ceria, which can be expected at 673 K from the H2-TPR profile (Figure 1). The presence of bulk OH is not supported by our INS result: the facile reactivity to O2 of these species excludes the presence of bulk OH, as it is not expected that OH groups would react with O that fills the vacancy in the bulk. The presence of both surface and bulk Ce−H cautions how one may accurately determine the reduction degree of ceria based on H2 consumption in TPR and the oxygen storage capacity based on O2 adsorption on prereduced ceria, which is the current practice in the literature.1a Similar problems can arise when ceria is used as a support for metal nanoparticles.6a The spillover H can adsorb on the surface and in the bulk of ceria, resulting in inaccurate quantification of chemisorbed H on the metal nanoparticles. It remains an interesting question if and what kind of catalytic role the bulk Ce−H may play in ceria catalysis. It was shown that hydride on surfaces can play an important role in heterogeneous catalysis.5 Our brief experiment of RT O2 exposure, showing reactivity of both surface and bulk Ce−H, does not discriminate the reactivity of the two different hydride species. Further in situ INS experiments are planned in our lab for alkyne hydrogenation over CeO2 with the goal of differentiating the catalytic role of surface hydride from bulk hydride.
Figure 4), we postulate that some of the hydride species are reacting with adsorbed O2 species to form water, which is removed upon 393 K vacuum treatment and thus not observed in the INS spectrum. This is supported by our IR-MS study following the same experiment. Upon exposure of the 673 K H2- or D2-treated CeO2 to O2 at RT, the difference IR spectra (Figure S4A) clearly show the production of surface-bound water, as evidenced by the broad feature at 3500−3000 cm−1 (2600−2100 cm−1 for the D2-treated case). The corresponding online MS profile (Figure S4B) also exhibits a water (H2O or D2O) evolution peak when O2 is exposed at RT to the H2- or D2-treated sample. The result implies that both the surface and bulk Ce−H species are quite unstable under oxidative condition and readily react with adsorbed O2 species (presumably as superoxide and peroxide species,9 the former being observed in the IR spectra at ∼1128 cm−1 in Figure S4A) to form water. Catalytic Implications of Ce−H Species. The direct identification of Ce−H has significant implications for ceriabased catalysis in at least two aspects. First, it strongly suggests the dissociation mechanism of H2 on CeO2 to be a heterolytic pathway (see Scheme 1). Yet the Ce−H species is only stable Scheme 1. Schematic Description of H2 Interaction with CeO2 with the Formation of Surface OH and Ce−H
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CONCLUSIONS In conclusion, we have shown for the first time direct inelastic neutron spectroscopic evidence for the presence of both surface and bulk cerium hydride species upon H2 interaction with CeO2 at elevated temperatures. Coupling with other in situ spectroscopy approaches including IR, Raman, and AP-XPS, we confirm that H2 dissociates to form homolytic products (OHs) on the O-vacancy-free ceria surface, while the dissociation would result in heterolytic products (Ce−H and OH) when Ovacancies are generated in ceria. This finding not only deepens our understanding of CeO2−H2 interaction but also has general implications for catalysis by ceria in hydrogenation and redox reactions. The valuable insights will facilitate the design of more effective ceria-based catalysts in H-related reactions. The capability of INS in detecting and differentiating surface and bulk hydrides in ceria represents a unique opportunity to study similar phenomena over other oxide catalysts and hydrogenation catalysts in general.
on a reduced CeO2 surface with O-vacancies and transfers readily to lattice O when the O-vacancies are filled, directly confirming the hypothesis raised in the recent DFT calculation3b−e that the homolytic product is more energetically stable than the heterolytic one. Under hydrogenation reaction condition, CeO2 may be partially reduced with O-vacancies, and thus the Ce−H species will be present. Although spectroscopically observed features are not necessarily related to reaction intermediates, the INS observation of the reactivity of Ce−H to O2 at room temperature indicates that these Ce−H species are quite reactive and thus may participate in the hydrogenation reaction. Even though a previous study2b indicated that the presence of oxygen vacancies in ceria does not promote the hydrogenation reaction via comparing ceria nanocubes and polyhedra, it has not been demonstrated that the ceria polyhedra (the better hydrogenation catalyst) are free from oxygen vacancies under hydrogenation reaction conditions with high H2 to hydrocarbon ratio. Thus, our result calls for a reconsideration of the current mechanistic view on the hydrogenation reaction over the CeO2 surface where only surface O was considered as the binding sites for surface H species.3a,c Second, the presence of both surface and bulk Ce−H is critical for understanding the redox property of ceria, as it is often accessed via H2-TPR. Although previous studies employing nuclear magnetic resonance (NMR), electron spin
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05492. Additional IR, Raman, and AP-XPS spectra with QMS profiles, and demonstration of the modes observed in INS (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Zili Wu: 0000-0002-4468-3240 9726
DOI: 10.1021/jacs.7b05492 J. Am. Chem. Soc. 2017, 139, 9721−9727
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Journal of the American Chemical Society
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Franklin Tao: 0000-0002-4916-6509 Ariana Beste: 0000-0001-9132-792X Funding
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paidup, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is sponsored by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Part of the work including in situ IR and Raman spectroscopy and H2-TPR measurement was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The INS study used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. F.T. acknowledges Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, under Grant No. DE-SC0014561. The computing resources were made available through the VirtuES and the ICE-MAN projects, funded by the Laboratory Directed Research and Development Program at ORNL.
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DOI: 10.1021/jacs.7b05492 J. Am. Chem. Soc. 2017, 139, 9721−9727
