Article pubs.acs.org/JPCC
CO2 Adsorption As a Flat-Lying, Tridentate Carbonate on CeO2(100) Peter M. Albrecht,† De-en Jiang, and David R. Mullins* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201, United States ABSTRACT: Results of an experimental and computational study of CO2 adsorption onto a CeOX(100) thin-film surface are reported. For both oxidized CeO2(100) and reduced CeO1.7(100), a 5 L dose of CO2 at 180 K resulted in mainly carbonate ([CO3]2−) on the surface with a minute amount of physisorbed CO2 that desorbed by 250 K based on C 1s and O 1s photoemission and C k-edge NEXAFS. No evidence for the formation of a carboxylate intermediate was indicated. Angle-dependent C k-edge NEXAFS revealed that the carbonate species was oriented parallel to the surface suggesting a tridentate configuration. Various adsorption geometries were tested using DFT PBE+U calculations. The most stable configuration was a carbonate with its molecular plane parallel to the surface and each O atom bonded to two Ce cations. Through temperature-programmed desorption (TPD), it was determined that CO2 was the sole reaction product. CO was not detected in the TPD for the reduced surface, indicating that reoxidation of a reduced CeO2‑X(100) surface by CO2 did not occur. TPD and photoemission indicated that the coverage and the thermal stability of the [CO3]2− intermediate were greater on partially reduced CeO1.7(100) compared to CeO2(100).
1. INTRODUCTION For several industrially relevant and heterogeneously catalyzed processes, a detailed understanding of the surface chemistry of carbon dioxide is crucial for the design of optimal catalysts.1,2 Three reactions of particular interest are CO2 reduction to useful chemical feedstock (CO2 → CO + 1/2O2), water−gas shift (CO + H2O → CO2 + H2), and dry reforming of methane (CH4 + CO2 → 2CO + 2H2).3 In addition to its central role in these catalytic reactions, CO2 has also been used as a Lewis acid probe to characterize the base strength of metal oxide catalysts.4 Zhang et al. have shown that on alkaline earth oxides the strength of the basic sites is related to the CO2 TPD desorption temperature.5 The number of basic sites can be determined by integrating the area under the CO2 TPD desorption curve. It has also been suggested that the strength of the basic sites is related to the degree of coordination, or number of coordination vacancies, at the Oanion on a metal oxide surface.6 Cerium oxide (ceria) has proven to be a viable catalyst for both water−gas shift and methane reforming. Ceria is a reducible oxide, with the remarkable capability to store and release large amounts of oxygen. The release and uptake of oxygen coincide with a reversible transformation between two oxidation states (Ce3+ ↔ Ce4+).7 Ceria can accommodate a high concentration of oxygen vacancies and hence participates readily in many redox reactions. Hilaire et al. measured the water−gas-shift reaction rates for Pd/ceria, Ni/ceria, Fe/ceria, Co/ceria, and ceria.8 Otsuka and co-workers demonstrated the direct partial oxidation of methane to synthesis gas by cerium oxide.9 Sharma et al. asserted that CO2 is able to oxidize reduced ceria.10 Similarly, Demoulin et al. observed direct spectroscopic evidence of the oxidizing power of CO2 at mild (ca. 573 K) temperature on a Pd/Ce0.21Zr0.79O2 catalyst.11 © 2014 American Chemical Society
Bueno-Lopez and co-workers employed isotopically labeled C18O2 to study the oxygen exchange mechanism between carbon dioxide and ceria.12 One question that has arisen in the study of carbon dioxide on cerium oxide is the nature of the surface intermediate(s). Infrared vibrational spectroscopy is the most common method utilized to characterize the adsorbed species.13−17 The adsorbed species was generally identified as carbonate, (CO3)2−, adsorbed in mono-, bi-, and poly dentate configurations. Owing to the heterogeneous surface structure of the powder samples multiple adsorption states were often evident on any given sample. Complementary to studies of CO2 adsorption onto highsurface-area CeO2 catalysts under reaction conditions are studies which take a surface science approach. In these, clean, well-ordered CeO2 single crystals or oriented thin films are prepared in ultrahigh vacuum and subsequently exposed to CO2. To date only experiments on CeO2(111) surfaces have been reported. In their study of the redox pathways for formic acid decomposition over CeO2(111), Senanayake and Mullins also examined the adsorption of CO2 on CeO2(111) in order to distinguish between carbonate and formate species.18 Carbonate and formate have similar C 1s binding energies at ca. 290 eV. However, C k-edge NEXAFS was able to unambiguously distinguish between carbonate and formate which have their π* resonances at 290.5 and 288 eV, respectively. More recently, Senanayake et al. probed the reaction intermediates for the water−gas shift reaction over inverse CeOx/Au(111) catalysts, where x = 1.95−1.98 (i.e., almost fully oxidized).19 C 1s coreReceived: February 3, 2014 Revised: April 3, 2014 Published: April 3, 2014 9042
dx.doi.org/10.1021/jp501201b | J. Phys. Chem. C 2014, 118, 9042−9050
The Journal of Physical Chemistry C
Article
adsorption configurations using DFT PBE+U calculations revealed a stable, flat-lying, tridentate adsorption state. Through temperature-programmed desorption (TPD), it was determined that CO2 was the only reaction product. CO was not detected in the TPD for the reduced surface, indicating that reoxidation by CO2 did not occur.
level spectra again contained a peak at 290 eV that was ascribed to carbonate, and NEXAFS at the C k-edge revealed a π* C−O resonance at 290.7 eV. For both CeO2(111)18 and CeOx/ Au(111),19 heating the sample above the exposure temperature of 100 K resulted in the decomposition of [CO3]2− and desorption as CO2 by 300 K. Libuda et al. have investigated CO2 adsorption and activation on ceria and magnesia/ceria thin films20,21 and have also demonstrated the reoxidation of reduced ceria by CO2.22 For pure CeO2−x(111), exposure to CO2 at room temperature resulted in two peaks in the C 1s region: 289.5 eV (assigned to [CO 3 ] 2− , i.e. carbonate) and 285.8 eV (assigned to carboxylate).20 However, the carboxylate peak appeared only after exposure to large (>4000 Langmuirs) doses of CO2. For such large CO2 exposures, partial reoxidation of CeO2−x occurs with high reaction probability even at temperatures as low as 300 K, on pure ceria in the absence of supported noble metal particles, and in the absence of surface hydroxyl groups.22 Libuda and co-workers also determined that the carbonate species decomposes upon annealing the partially reduced ceria sample to >400 K. On the other hand, CO2 did not adsorb onto stoichiometric CeO2(111) at room temperature.21 No C 1s signal was observed even after exposing CeO2(111) to 14,200 L of CO2. Providing theoretical insight into the surface-science studies of CO2 on ceria surfaces are the recent density-functional calculations of CO2 adsorption mechanisms on CeO2(110) and CeO2(111) by Cheng et al.23 and Hahn et al.,24 respectively. Cheng and co-workers examined both oxidized and reduced (110) surfaces and determined that stoichiometric CeO2(110) does not activate CO2.23 All of the adsorption configurations on the fully oxidized surface were classified as “weak physisorption” based on an adsorption energy of only 0.26 eV, bond lengths within the CO2 molecule that remained largely unchanged and the separation between the CO2 molecule and the ceria surface being >0.252 nm, which is too great for a Ce−O bond to form. On the other hand, carbonate anions formed on reduced CeO2−x(110) with an adsorption energy of 1.043−1.223 eV. The strength of the adsorption, and whether the carbonate species was monodentate or bidentate, depended upon the geometry of the oxygen vacancy site. Cheng et al. emphasized that the mechanism and energetics of CO2 adsorption depended sensitively on the structure of the ceria surface.23 Hahn and co-workers determined that fully oxidized CeO2(111) is able to support up to 1/3 monolayer of CO2 as a monodentate carbonate species.24 However, the adsorption energy was relatively weak (0.31 eV). A bidentate carbonate was also observed, although the energetics were even less favorable. CO2, adsorbed as either a monodentate or bidentate carbonate, acted as a Lewis acid accepting electronic charge from the stoichiometric ceria surface at all coverages. Further CO2 adsorption beyond 1/3 ML resulted in the formation of a linear, physisorbed CO2 species. In this paper, we report the first experimental and computational results of CO2 adsorption onto a CeOX(100) thin-film surface. For both oxidized CeO2(100) and partially reduced CeO1.7(100), C 1s and O 1s sXPS and C k-edge NEXAFS indicated that at 180 K CO2 adsorbed predominantly as a carbonate on the surface with a small amount of physisorbed CO2 that desorbed by 250 K. Angle-dependent NEXAFS indicated that the carbonate orientation was predominantly parallel to the surface. Testing various
2. EXPERIMENTAL SECTION The experiments were conducted in two different ultrahigh vacuum (UHV) chambers. The temperature-programmed desorption (TPD) experiments were performed in a UHV chamber at Oak Ridge National Laboratory (ORNL) using a Hiden HAL/3F 301 mass spectrometer. The temperature was increased at a rate of 2 K/s. The sample was biased −70 V to prevent electrons generated by the mass spectrometer ionizer from stimulating reactions at the ceria surface. The TPD measurements were made in a line-of-sight geometry in which the mass spectrometer aperture was positioned ca. 2 cm from the sample face. Soft X-ray photoelectron spectroscopy (sXPS) and near-edge X-ray absorption fine structure (NEXAFS) were performed in a UHV end-station located at Beamline U12A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. C 1s and O 1s sXPS spectra were recorded using 400 and 600 eV synchrotron radiation, respectively. The instrumental resolution was ca. 0.5 eV. The binding energy was referenced against the Ce 4d X‴ peak at 122.3 eV.25 The O 1s, Ce 4d, and background intensities at kinetic energies slightly greater than the C 1s or O 1s photoemission peaks were used to normalize C 1s and O 1s spectra on the CeOX(100) surfaces.26 This enabled the comparison of spectra of CO2 not only at different temperatures and substrate oxidation states but also to spectra for other C- or O-containing molecules on CeOX(100). NEXAFS was carried out at the C k-edge. The energy resolution was less than 0.5 eV. The photon energy was calibrated using the dip in the photon flux at 284.7 eV.27 The X-ray absorption was measured using a partial yield electron detector. The high-pass retarding grid was biased −230 V. Insight into the orientation of the surface intermediate was obtained by recording NEXAFS for two different angles of incidence: normal incidence (which aligns the polarization of the light parallel to the surface) and grazing incidence (70° angle of incidence with respect to the surface normal). Thirdorder X-ray excitation at the Ce MIV and MV edges resulted in apparent absorption peaks at photon energies of 295 and 301.5 eV, respectively. The absorption due to only higher-order radiation was determined by collecting spectra with a retarding grid voltage between −310 and −320 V (i.e., greater than the first-order photon energy). The background resulting from the higher-order excitation was then subtracted from the NEXAFS spectra. CeO2(100) thin films were grown ex situ at the Center for Nanophase Materials Sciences at ORNL by pulsed laser deposition on 0.05% Nb-doped SrTiO3(100) (Nb-STO).28 The as-grown films were epitaxial and 19 ± 1 nm in thickness. Auger electron spectroscopy and sXPS indicated that the asgrown CeO2(100) surface contained S, C, K, and Cl impurities. S and C were removed by annealing in an oxygen background (1 × 10−6 Torr O2, 800 K, 5−10 min). K and Cl were removed by gentle sputtering (5 × 10−5 Torr Ar or Ne, 300 K, 1 keV, 1 μA,
