Research Article pubs.acs.org/acscatalysis
A Thermally Stable Cr−Cu Nanostructure Embedded in the CeO2 Surface as a Substitute for Platinum-Group Metal Catalysts Hiroshi Yoshida,†,‡ Noriko Yamashita,† Shota Ijichi,† Yuri Okabe,† Satoshi Misumi,† Satoshi Hinokuma,†,‡,§ and Masato Machida*,†,‡ †
Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan ‡ Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, 1-30 Goryo-Ohara, Nishikyo, Kyoto 615-8245, Japan § Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: High catalytic activities for CO−O2 and CO−NO reactions that were superior to or comparable with those of platinum-group metal catalysts were achieved by synchronous dual-mode arc plasma deposition of a very small amount of Cr and Cu (0.07 wt % each) onto CeO2, followed by subsequent thermal aging at 900 °C for 25 h. The turnover frequency for CO oxidation over Cr−Cu/CeO2 was 3-fold higher than that over Cu/CeO2 and exceeded values for the Rh, Pd, and Pt catalysts loaded on CeO2, despite a significant decrease in the surface area from 169 to 5 m2 g−1 caused by thermal aging. Experimental structure characterization and density functional theory calculations based on CeO2 (111) surface slab models revealed that Cu+ substitution for surface Ce atoms leads to the formation of asymmetric 3-fold oxygen coordination sites capable of efficient CO chemisorption and catalytic activity. In addition, Cr3+ was incorporated into the surface structure of CeO2; it plays an important role in enhancing the surface concentration of Cu+. A CO oxidation rate with nearly zero partial orders with respect to O2 and isotopic C16O−18O2 reactions yielding C16O2 as the primary product demonstrated that the reaction proceeds via the Mars−van Krevelen mechanism. KEYWORDS: chromium, copper, CeO2, surface structure, CO oxidation
1. INTRODUCTION Substituting platinum-group metals for abundant transition metals in catalysis is significant from both scientific and industrial viewpoints. Copper is one such promising element, and in particular, Cu catalysts loaded on CeO2 have attracted much attention because their CO oxidation activity is competitive with or superior to that of Pt catalysts. In the past two decades, a number of studies have been devoted to CO oxidation in stoichiometric or excess O2, preferential CO oxidation in excess H2 (PROX), and CO−NO reactions over Cu/CeO2 catalysts.1−27 The chemical states and local structures of active Cu sites have been investigated in relation to their interactions with CeO2 using both experimental and theoretical approaches. Nevertheless, a clear understanding of this catalysis has not been attained. One of the reasons for the complexity of these catalysts is the variety of Cu species in different structures and oxidation states obtained depending on the chemical composition, preparation route, and calcination temperature. For the first time, Liu et al.2 proposed the presence of three types of Cu species: CuO particles, Cu2+ incorporated in the bulk structure of CeO2, and Cu+ incorporated in the surface structure of CeO2; they suggested that Cu+ species are responsible for CO chemisorption and catalytic oxidation. © XXXX American Chemical Society
For the CO−PROX reaction, it is generally agreed that optimal catalytic properties for CO oxidation over Cu/CeO2 are achieved in the presence of well-dispersed copper oxide patches over CeO2 nanoparticles.13,15,28 In addition, using in situ infrared spectroscopy, CO oxidation has been linked to CO adsorption on Cu+ sites.15,19 Several density functional theory (DFT) calculations have been performed to elucidate Cu−CeO2 interactions.29−31 Shapovalov and Metiu reported that when Cu occupies a Ce site on the CeO2 (111) surface, it induces a distorted asymmetric coordination with three nearest neighbor oxygens.30 Yang et al. performed calculations for the structure of a Cu-doped CeO2 (111) surface and found that Cu is stable both as an adsorbed atom (Cu+) on the surface and as a dopant (Cu2+) on the CeO2 surface.31 Calculations by Wang et al. for a Ce1−xCuxO2−y solid solution system indicated that Cu approaches the planar geometry characteristics of Cu2+ with a strongly perturbed local order.29 All these results considered together suggest that Cu species may be accommodated in the Received: August 20, 2015 Revised: October 4, 2015
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DOI: 10.1021/acscatal.5b01847 ACS Catal. 2015, 5, 6738−6747
Research Article
ACS Catalysis
(Ulvac Inc., ARL-300). An electric capacity of 360 μF and a discharge voltage of 125 V were applied. The Cr−Cu bimetal nanoparticles were prepared using a dual arc plasma technique with synchronized generation of plasma from each source.35,39 The plasma from the cathodes entered into a container containing CeO2 powder (99.99%, Anan Kasei Co., Ltd.) that was mechanically stirred at ambient temperature. Arc pulses were generated repeatedly with a period of 0.2 ms, a current amplitude of 2 kA, and a frequency of 1 Hz until the total metal loading reached approximately 0.14 wt %. The as-prepared samples were thermally aged at 900 °C in a flow of 10% H2O in air for 25 h prior to evaluation of their thermal stability and catalytic activity. Other bimetal catalysts loaded on CeO2 were also prepared in the same manner. 2.2. Characterization. Crystal structures were determined via powder X-ray diffraction (XRD) using monochromatic Cu Kα radiation (30 kV, 20 mA, Multiflex, Rigaku). The metal loading on CeO2 was determined using X-ray fluorescence (XRF) spectroscopy (EDXL300, Rigaku). The oxidation states of the surface metal species were examined via XPS using monochromated Al Kα radiation (12 keV, K-Alpha, Thermo Fisher Scientific). The binding energies were charge-referenced to C 1s at 285 eV. XPS combined with depth profiling was conducted with Ar+ ion beam bombardment (3000 eV), which was used for 10 s for each etch step, corresponding to a depth per etch step of 0.4 nm for a Rh reference. Extended X-ray absorption fine structure (EXAFS) analysis of the Cu K-edge spectra obtained using 0.2 g of each catalyst was conducted on the BL-9A station of the Photon Factory (PF), High Energy Accelerator Research Organization (KEK), at Tsukuba (Proposal 2014G567). A Si(111) double-crystal monochromator and a Rh-coated mirror were used. The Cu K-edge spectra were recorded at room temperature in fluorescence mode using a 19-element solid-state detector, and the XAFS data were processed using the IFEFFIT software package (Athena and Artemis). The k3-weighted EXAFS oscillations in the 2.0−12.0 Å region were Fourier transformed. A Cu foil, Cu2O, CuO, and CuCrO2 were used as references to extract the amplitudes and phase shift functions for the Cu−O bonds. The coordination numbers (CN), atomic distances (r), ΔE0 values, and Debye− Waller factors were fitted. The XPS and EXAFS measurements were performed without any pretreatment of the catalysts. Brunauer−Emmet−Teller (BET) surface areas (SBET) were determined from N2 adsorption isotherms measured at −196 °C (Belsorp-mini, Bel Japan, Inc.). CO chemisorption was performed using the pulsed CO technique (Belcat-B, Bel Japan, Inc.). Before the CO pulses were applied, each catalyst was preheated in flowing He at 200 °C for 30 min and subsequently cooled to a constant temperature of 50 °C. The molar ratio of CO chemisorbed per Cu loaded is expressed as CO/Cu. The H2-TPR experiment was performed in a flow system. The thermally aged catalyst was heated in a flow of 5% H2/Ar at a constant rate of 10 °C min−1. In situ FT-IR spectra of the CO chemisorbed on each catalyst were acquired on a Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific) using a temperature-controllable diffuse reflectance reaction cell and a mercury cadmium telluride (MCT) detector. The catalyst sample was ground well in a mortar and then placed into the cell making a flat surface. The cell was heated to 200 °C at a constant rate of 10 °C min−1 and maintained at this temperature for 30 min under a supply of He (50 cm3 min−1). After the cell had cooled to 50 °C, the background spectrum was collected. The catalyst sample was exposed to a
CeO2 surface and/or bulk structure to some extent. It should also be noted that a Cu loading corresponding to >1 wt % was mostly used in these previous experimental and theoretical studies. Given that no solid solutions or compounds have been reported in equilibrium Cu−Ce binary oxide phase diagrams, the deposition of various Cu species outside the CeO2 lattice should be inevitable at such high loadings. Lower calcination temperatures (≤600 °C) for these previous Ce/CeO2 catalysts may allow the presence of various metastable pseudosolid solutions. Another interesting approach to further enhance catalytic activity for CO oxidation is the design of doubly loaded CeO2 catalysts. Harrison et al. reported that CeO2 doubly loaded with Cr and Cu exhibited a high catalytic activity for lowtemperature CO oxidation under lean and stoichiometric conditions.32 Because the samples contained large loadings (20−40 mol % in total) of Cr and Cu, heat treatment above 600 °C deposited binary oxides (CuCr2O4 and/or CuCrO2), which are known as active catalysts for CO oxidation.33 However, the interaction between these binary oxides and CeO2 was not detected at up to 1000 °C. Li et al. suggested that the addition of Mn and Fe played a beneficial role in the catalytic activity for the selective oxidation of CO in H2-rich streams over CuO−CeO2 catalysts, whereas the addition of Cr and Co led to negative effects.34 More recently, Hinokuma et al. reported a higher CO oxidation activity for Fe−Cu/CeO2 than Cu/CeO2 when the catalyst with a very small metal loading (≤0.2 wt %) was prepared using pulsed arc plasma deposition.35 Interestingly, the catalytic activity was enhanced by thermal aging at 900 °C, which led to a significant decrease in the surface area. It is therefore expected that as-aged catalysts have a thermodynamically stable structure containing a low concentration of Cu that should be suitable for characterization. In addition, the thermally stable activity is essential for practical applications in automotive emission control systems, where the catalysts are exposed to high-temperature exhausts (∼1000 °C). In the study presented here, therefore, various Cu−transition metal binary systems supported on CeO2 were prepared via pulsed arc plasma deposition, and it was found that Cr−Cu/ CeO2 is the most efficient CO oxidation catalyst with a high thermal stability. It is noteworthy that a very small amount (0.14 wt % in total) of Cr−Cu codeposited onto CeO2 followed by thermal aging at 900 °C for 25 h achieved a high activity comparable to those of platinum-group metals. The structure of the active phase thus formed was studied using a combined experimental and theoretical approach to explore the origin of the active and thermally stable Cr−Cu species on the CeO2 surface. The experimental characterization was conducted using X-ray absorption fine structure (XAFS) analysis, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and Fourier-transformed infrared spectroscopy (FT-IR). DFT calculations were performed to generate a structural model of the Cr−Cu species on the CeO2 surface. These results led to novel thermostable catalytic materials that are useful alternatives to platinum-group metal catalysts.
2. MATERIALS AND METHODS 2.1. Catalyst Preparation. The Cr, Cu, and Cr−Cu nanoparticles loaded on CeO2 were prepared using a cathodic pulsed arc plasma technique under vacuum as reported previously.35−42 Metal rods of Cr and/or Cu (99.99%, ϕ 10 mm, Furuya Metals Co., Ltd.) were used as source cathodes 6739
DOI: 10.1021/acscatal.5b01847 ACS Catal. 2015, 5, 6738−6747
Research Article
ACS Catalysis
1.25 kPa was maintained. The O2 partial pressure was then varied from 0.25 to 1.25 kPa while a constant CO partial pressure of 0.1 kPa was maintained. Pulsed isotopic C16O−18O2 reactions over thermally aged Cr−Cu/CeO2 were performed using gaseous C16O and 18O2 at 100−400 °C. The catalyst (W = 0.05 g) was fixed in a reactor and preheated at 400 °C for 30 min in a He stream (F = 50 cm3 min−1). The C16O (0.088 cm3) and 18O2 (0.64 cm3) were then simultaneously pulsed into the reactor three times, and the gas species in the effluent were monitored using a mass spectrometer (Omnistar, Pfeiffer) for m/z values of 28 (C16O), 30 (C18O), 32 (16O2), 34 (16O18O), 36 (18O2), 44 (C16O2), 46 (C16O18O), and 48 (C18O2).
gas stream containing 1% CO balanced with He for 10 min, and then the cell was purged with pure He for 10 min to remove the gaseous CO and collect the spectrum. Subsequently, the gas stream was changed to 2.5% O2 balanced with He for 10 min, and the spectrum was recorded again. 2.3. DFT Calculations. The structures of the Cu- and Cr− Cu-incorporated CeO2 surfaces were analyzed using periodic DFT calculations. Spin-polarized generalized gradient approximation GGA+U (U = Hubbard U) electronic structure calculations were performed with the Vienna Ab Initio Simulation Package (VASP)43 using projector augmentedwave (PAW) potentials for the Cr, Cu, Ce, and O atoms. A plane-wave basis set with a cutoff of 520 eV was used. The Perdew−Burke−Ernzerhof GGA was employed for the exchange and correlation functionals.44 Sums over occupied electronic states were performed using the Monkhorst−Pack scheme45 on a 2 × 2 × 1 k-point mesh for a (2 × 2) supercell of the (111) surface of CeO2 as described below. The simplified GGA+U method reported by Dudarev et al.46 was utilized. Literature U-J values of Cu d (6.52 eV), Cr d (2.13 eV), and Ce f (4.5 eV) were used.47 The unit cell parameters and atomic coordinates for cubic CeO2 (a = 5.48 Ǻ ) were optimized with a convergence condition of 0.02 eV Å−1, and the initial crystallographic parameters used in the optimization were taken from the literature.48 The optimized unit cell parameters and atomic coordinates for bulk CeO2 agreed well with the experimental crystallographic data, ensuring the validity of the GGA+U and PAW potentials used in this study. The (111) surface, which is well-known as the most stable among the low-index surfaces of CeO2,49−51 was used as the surface slab model. This is supported by powder XRD data of a thermally aged sample, which indicate crystallites with an isotropic shape (Supporting Information). The (111) surface slab was modeled by a (2 × 2) supercell with fixed cell axes based on the optimized lattice parameter for bulk CeO2 and consisted of 12 atomic layers (Ce16O32) with 15 Å of vacuum separation. The top six atomic layers were allowed to relax until the forces on the atoms were 600 °C). Although the irregular shapes of light-off curves were commonly observed at higher conversions, these behaviors are not indications of catalyst deactivation and are associated with weak chemisorption of CO as discussed later. The Cr−Cu/ CeO2 catalyst demonstrated three important features. (i) Its catalytic activity was remarkably enhanced by thermal aging at 900 °C despite a decrease in its SBET value from 169 to 5 m2 g−1.52 (ii) Its turnover frequency (TOF), 3.5 × 10−2 s−1 [at 100 °C (see Table 4)], was greater than those of Pd/CeO2 (2.8 × 10−2 s−1), Rh/CeO2 (0.9 × 10−2 s−1), and Pt/CeO2 (
