Electron Donation Enhanced CO Oxidation over Ru-Loaded 12CaO

Apr 20, 2015 - Department of Materials Structure Science, School of High Energy Accelerator Science, SOKENDAI (the Graduate University for Advanced St...
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Electron Donation Enhanced CO Oxidation over Ru-Loaded 12CaO·7Al2O3 Electride Catalyst Md Jafar Sharif,†,‡,¶ Masaaki Kitano,†,¶ Yasunori Inoue,§ Yasuhiro Niwa,∥ Hitoshi Abe,‡,∥,⊥ Toshiharu Yokoyama,†,‡ Michikazu Hara,‡,§,# and Hideo Hosono*,†,‡,§,# †

Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ‡ ACCEL, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ∥ Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ⊥ Department of Materials Structure Science, School of High Energy Accelerator Science, SOKENDAI (the Graduate University for Advanced Studies), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan # Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan S Supporting Information *

ABSTRACT: It is widely recognized that CO oxidation effectively proceeds over Au nanoparticles supported on a reducible oxide, such as TiO2 and Fe2O3, in which the support supplies oxygen during the reaction (Mars−van Krevelen (MvK) type mechanism). Here we examine the stable electride, [Ca24Al28O64]4+(e−)4 (C12A7:e−), with a low work function (2.4 eV), as a Ru catalyst support for CO oxidation. Ru-loaded C12A7:e− exhibited more than a 3-fold higher turnover frequency (TOF) and a lower activation energy than Ruloaded TiO2, Al2O3, and [Ca24Al28O64]4+(O2−)2 (C12A7:O2−) catalysts. The high catalytic performance of Ru/C12A7:e− is not due to the MvK type mechanism but to efficient electron transfer from C12A7:e− to the species adsorbed on the Ru catalyst, in which the association reaction between adsorbed CO molecules and O adatoms is promoted on the Ru surface.

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position using Ru-loaded C12A7:e−, where C12A7:e− functions as an efficient electron donor.10−13 This initial success in a catalytic application has inspired us to apply C12A7:e− as a support material for the activation of molecular oxygen. CO oxidation is one of the most studied reactions in heterogeneous catalysis to achieve a fundamental understanding of catalysis,14−17 in addition to its importance in green chemistry.18−20 After the pioneering work by Haruta et al.,21−23 a quick burst of research on CO oxidation began and is still in progress using noble metal nanoparticles (NPs) based catalysts supported on various types of oxide materials.24,25 One of the implicit steps in CO oxidation is the activation of molecular O2, which is strongly dependent on the nature of the support material.26 Despite much research on the role of the support in O2 activation, this remains a topic of active research as the reaction mechanism has yet to be clarified. It is widely accepted that oxygen adsorption occurs on the support or at

INTRODUCTION Electrides are ionic solids, in which electrons serve as anions.1,2 In the past three decades, electrides have been synthesized in the form of alkali-metal adducts of organic molecules or inorganic molecular sieves; however, these compounds are not generally stable under ambient conditions.1,3,4 As a result, application of these materials has been restricted. To date, among all the electrides, only [Ca24Al28O64]4+(e−)4 (C12A7:e−) is stable in an air atmosphere up to ca. 300 °C because of its unique crystal structure, which consists of positively charged subnanometer-sized cages and anionic electrons that are trapped inside cages with small openings.5 The anionic electrons are protected against the outer environment due to the stable lattice framework structure of densely packed cages with Ca−O and Al−O bonds. The trapped electrons occupy a cage conduction band formed by the three-dimensionally connected empty cages,6 which results in a low work function (2.4 eV) comparable to that of potassium metal.7−9 Therefore, the unique stability and low work function of C12A7:e− make it a promising candidate as an electron donor in chemical reactions. Recently, we reported ammonia synthesis/decom© XXXX American Chemical Society

Received: March 10, 2015 Revised: April 8, 2015

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DOI: 10.1021/acs.jpcc.5b02342 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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KEK (Proposal No. 2013S2-002). A Si(311) double-crystal monochromator was used to obtain the monochromated X-ray beam, and the spectra were obtained in the transmission mode. The Athena and Artemis softwares37 and the FEFF6 code38 were used to analyze XAFS spectra. The Fourier transformation of k3-weighted EXAFS oscillation was carried out over the range of 3−15 Å−1. The Ru−Ru interactions were fitted in the range of 1.9−2.7 Å. Ru dispersion was determined by the CO-pulse chemisorption method using an automatic gas-adsorption apparatus (BELCAT-A, BEL, Japan). Prior to CO-pulse chemisorption, the samples were pretreated with a He flow (50 mL min−1) at 400 °C for 15 min, followed by a H2 flow (50 mL min−1) at 400 °C for 15 min. Hydrogen atoms adsorbed on the reduced catalysts were removed by purging with He (50 mL min−1) at 400 °C for 15 min. To calculate the metal dispersion, a stoichiometry of Ru/CO = 1 was assumed.39 3. Catalytic Reaction. CO oxidation was conducted in a fixed-bed continuous-flow quartz-tube reactor set inside a temperature-controlled furnace. Typically, 100 mg of catalyst was loaded into the tubular reactor with quartz wool and pretreated with H2 and N2 at 250 °C. The activities of the catalysts were measured in a gas mixture of 4.72% O2/He (34 mL min−1) and 9.52% CO/He (6.9 mL/min), which corresponds to a space velocity of 25200 mL h−1 gcat−1. The temperature was ramped at a rate of 6 °C min−1 to the operation temperature by thermostat control with a thermocouple and held at the desired temperature for 20 min to measure the steady-state conversion level. The outlet gas was analyzed using online gas chromatography (490-MicroGC, Varian) with a thermal conductivity detector.

the metal−support interface in the case of reducible oxide supports such as TiO2 (metal−support interface mechanism).17,27,28 The catalytic activity is also influenced by charge transfer from the support to the metal.29,30 For example, Au nanoclusters on oxygen-deficient support materials catalyze low-temperature CO oxidation due to electron transfer from the vacancies to the metal clusters.31 This effect has recently been referred to as an electronic metal−support interaction (EMSI) and has attracted much attention in research on various heterogeneous catalysis reactions.32−34 Here, we focused on the latter factor to obtain a clear insight into the EMSI effect in CO oxidation using C12A7:e− with strong electron-donation properties. Other metal oxides, such as C12A7:O2− (without electrons in the cage, i.e., an insulator without a low work function), TiO2, and Al2O3 are also used as supports to elucidate the role of the oxide or charge transfer from the oxide to the metal catalyst. Ru was used as a catalyst because efficient electron donation from C12A7:e− to Ru was observed in our previous study.10 It was recently reported that Ru NPs exhibit unique catalytic properties for CO oxidation; e.g., the catalytic activity increases with an increase in the particle size in the range of 2−6 nm, which is opposite to the size dependence trend observed for Au catalysts.23,35 Ru/ C12A7:e− was demonstrated to function as an efficient and stable catalyst for CO oxidation. The present findings might pave the way to applying the inorganic electride for lowtemperature oxidation reactions.

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EXPERIMENTAL SECTION 1. Preparation of Catalysts. Powdered C12A7:e− was prepared by solid-phase reaction according to the procedure described in the literature.36 Ru deposition was conducted by the following procedure. Ru/C12A7:e− was obtained from the physical mixture of Ru3(CO)12 and C12A7:e−; Ru3(CO)12 was mixed thoroughly with C12A7:e− in an Ar glovebox, and the mixture was sealed in an evacuated silica glass tube. The glass tube was then heated stepwise from 40 to 250 °C in a furnace and then cooled to ambient temperature to form the Ru/ C12A7:e − catalyst. Ru NPs were also deposited on [Ca24Al28O64]4+(O2−)2 (C12A7:O2−), TiO2 (P25, Degussa), and α-Al2O3 using a similar experimental protocol to form the Ru/C12A7:O2−, Ru/TiO2, and Ru/Al2O3 catalysts. All the support materials were dehydrated under vacuum at 150 °C prior to Ru deposition. 2. Characterization of Catalysts. The surface morphologies of the catalyst samples were investigated using field emission scanning electron microscopy (FE-SEM; S-5200, Hitachi) and high-resolution transmission electron microscopy (TEM; JEM-2010F, JEOL). Nitrogen adsorption−desorption isotherms were measured at −196 °C using a specific surface area analyzer (Nova 4200e, Quantachrome) after evacuation of the sample at 300 °C. The amount of Ru loading was estimated using inductively coupled plasma atomic emission spectrometry (ICP−AES; ICPS-8100, Shimadzu). X-ray photoelectron spectroscopy (XPS; Electron Spectrometer ESCA-3200, Shimadzu) measurements of Ru 3p were performed using Mg Kα radiation at 700 °C.5 Accordingly, the MvK mechanism is not possible over Ru/C12A7:e− and Ru/ C12A7:O2− as with the case of Ru/Al2O3. On these catalysts, both CO and O2 molecules are adsorbed on the Ru particles. It is important to note that the TOF value of Ru/C12A7:e− (0.19) is an order of magnitude higher than that of Ru/ C12A7:O2− (0.016) at a reaction temperature of 120 °C (Figure 3). The pronounced difference in the catalytic activity

those of Ru references such as Ru foil and RuO2. The FT for Ru/C12A7:e− shows an intense peak in the range of 2−3 Å, which corresponds to Ru−Ru interactions. The peak was analyzed by the Athena and Artemis softwares to obtain the structural parameters, and the values are listed in Table 1. The Table 1. EXAFS Analysis for Ru/C12A7:e− Catalyst and Ru Foil sample Ru/ C12A7:e− Ru foil a

Ru−Ru distance (Å)

coordination number

Debye−Waller factor (Å2)

R-factor (%)

2.68

8.4

0.0043

0.59

2.68

12a

0.0036

0.19

The coordination number was fixed at 12.

bulk metallic Ru (Ru foil) data were used as the standard for the analysis. The Ru−Ru bond for Ru/C12A7:e− is identical to that for the bulk Ru, but the reduced coordination number (8.4) was obtained, indicating that the Ru particle on C12A7:e− was in the state of metallic NPs. The XPS and XAFS results suggest that both the surface and bulk of Ru on C12A7:e− are in the metallic form, and Ru3(CO)12 is completely decomposed into metallic Ru during the deposition process. 2. Catalytic Performance for CO Oxidation. The catalytic activities of 2 wt % Ru-loaded samples for CO oxidation at various temperatures are shown in Figure 2. Among these catalysts, Ru/C12A7:e− and Ru/TiO2 exhibited similar trends in terms of conversion and reached 100% CO conversion at around 140 °C, although the surface area of Ru/ C1A7:e− is much lower than that of Ru/TiO2 (Table S1, Supporting Information). In contrast, the Ru/Al2O3 and C12A7:O2− catalysts exhibited respective CO conversion of 58.6 and 8.5% at the same reaction temperature. Due to the difference in the surface areas of the support materials, the dispersion of Ru in all four catalysts was also different, as shown C

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Figure 3. Apparent activation energies and TOFs for various Ru catalysts at 120 °C.

between these two catalysts is mainly due to the electrondonating power. In our previous study,13 it was demonstrated that N2 dissociation is significantly enhanced by charge transfer from C12A7:e− to Ru. Similarly, efficient electron transfer from C12A7:e− to Ru perturbs the electronic state of Ru, which leads to higher catalytic activity for CO oxidation than in the case of Ru/C12A7:O2−. The apparent activation energies, summarized in Figure 3, were determined from the Arrhenius plots for CO oxidation rate in the temperature range 100−140 °C. Ru/ C12A7:e− has a lower activation energy than that for Ru/TiO2, Ru/Al2O3, or Ru/C12A7:O2− by 27−72 kJ mol−1. A similar trend has been observed for ammonia synthesis and decomposition reactions.10,11 Therefore, it can be concluded that the electron donation from C12A7:e− to Ru lowers the activation energy and thereby enhances the catalytic activity at lower reaction temperatures. 3. Reaction Mechanism of CO Oxidation. To clarify the reaction mechanism, the orders of the reaction with respect to CO and O2 were examined for these catalysts. Figure 4 shows the dependence of the CO oxidation rate on the partial pressures of CO and O2. The reaction orders with respect to CO and O2 pressure are summarized in Table 2. The reaction rates for the Ru/C12A7:e−, Ru/C12A7:O2−, and Ru/Al2O3 catalysts are proportional to the CO pressure but decrease with an increase in O2 pressure, which indicates that CO oxidation proceeds over these catalysts via a similar reaction mechanism. This tendency is completely different from that for the Au/ TiO2 catalyst (CO order: 0.05, O2 order: 0.24) reported by Haruta et al.42 It is reported that the oxygen adsorption energy of the Ru step surface (−4.98 eV) is a larger negative value than that of the Au step surface (0.54 eV).43 Accordingly, negative reaction orders with respect to O2 are attributed to strong oxygen adsorption on the Ru surface. In contrast, inverse dependence of the reaction rate on the CO and O2 pressures was observed for only the Ru/TiO2 catalyst, which suggests that oxygen adsorption is the rate-determining step because the dominant reaction pathway is the MvK type mechanism. Figure 5 shows a possible reaction mechanism for CO oxidation over Ru/TiO2 and Ru/C12A7:e−. On Ru/TiO2, CO molecules are preferentially adsorbed on the Ru surface, as demonstrated by the kinetic analysis. The adsorbed CO molecules react with lattice oxygen to form CO2 and oxygen vacancies, and lattice oxygen is then restored by O2 in the gas phase (MvK type mechanism). In the case of the other Ru

Figure 4. Dependence of CO oxidation rate on the partial pressure of (a) CO and (b) O2. The reaction was conducted at 120 °C for Ru/ C12A7:e− (red) and Ru/TiO2 (black), at 140 °C for Ru/Al2O3 (green), and at 150 °C for Ru/C12A7:O2− (blue).

Table 2. Orders of Reaction for CO Oxidation over Various Ru Catalysts catalyst

CO order

O2 order

Ru/TiO2 Ru/Al2O3 Ru/C12A7:e− Ru/C12A7:O2−

−0.59 1.45 1.32 1.94

1.10 −0.57 −1.16 −1.38

catalysts (Ru/C12A7:e−, Ru/C12A7:O2−, and Ru/Al2O3), both CO and O2 molecules are adsorbed on the Ru surface, and molecular O2 is readily dissociated into atomic oxygens. The Ru surface is more densely populated with O adatoms than CO species as evidenced by the kinetic analysis. It was recently reported that the bond formation between CO and O is the transition state in CO oxidation over the Ru catalyst; i.e., the energy barrier of this step is the highest among all elementary steps in the reaction of CO with O.44 Therefore, the CO−O bond formation step is the rate-determining step of CO oxidation over these catalysts. The rate-determining barrier is lowered by strong electron-donation properties of C12A7:e− according to the following mechanism. Polarized CO species and negatively charged O adatoms are formed on the Ru surface because the work function of Ru particles is lowered by electron transfer from C12A7:e− to Ru. The C−O bond is weakened, and the bond length is increased by electron transfer from the Ru d-orbitals to the 2π* antibonding orbitals of CO, which leads to the formation of Cδ+−Oδ− species.45 When N2 molecules are adsorbed on Ru/C12A7:e−, N−N bond weakening is confirmed by a strong red-shift of the N−N D

DOI: 10.1021/acs.jpcc.5b02342 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Finally, the stability of the Ru/C12A7:e− catalyst was investigated in a continuous flow of the reactant gas for 20 h. Figure 6 shows the time course for the reaction at 140 °C. The

Figure 6. Time course for CO conversion at 140 °C over 2 wt % Ru/ C12A7:e− catalyst.

initial conversion rate was 98%, which then decreased to almost 97% within 2 h, after which it was steady for approximately 20 h. After 20 h of continuous reaction, the catalyst was again characterized using XPS. Figure S2 (Supporting Information) shows the XPS Ru 3p spectrum for Ru/C12A7:e− after reaction for 20 h. The peak position around the 3p3/2 and 2p1/2 is similar to that for the original catalyst, which confirmed the catalyst was stable against possible oxidation.

Figure 5. Possible reaction mechanisms for CO oxidation over (a) Ru/TiO2 and (b) Ru/C12A7:e−. (a) CO adsorbed on Ru particles reacts with surface lattice oxygen near the Ru−TiO2 interface, which results in the formation of CO2 and oxygen vacancies (dotted open circles). The oxygen vacancies are recovered by the incorporation of oxygen from the gas phase. (b) Both CO and O2 molecules are adsorbed on Ru particles, and the adsorbed O2 is readily dissociated into O adatoms. The C−O bond of adsorbed CO is elongated by charge transfer from the Ru d-orbitals to the CO 2π*-antibonding orbitals. The charge transfer is enhanced by strong electron donation from C12A7:e−, which has a very small work function, to the Ru particles. As a consequence, polarized CO species and negatively charged O adatoms are formed on the Ru surface and react with one another to form CO2.

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CONCLUSIONS We have examined the CO oxidation reaction using Ru NPs loaded on various support materials. Among all the catalysts, Ru/C12A7:e− exhibits the highest catalytic activity in terms of TOF and the lowest activation energy, which is ascribed to efficient electron donation from C12A7:e−. Therefore, the primary role of electronic metal−support interaction (EMSI) was demonstrated using C12A7:e− as a support material for a Ru catalyst. The present findings also reveal that C12A7:e− is useful as a support material for low-temperature oxidation reaction.

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ASSOCIATED CONTENT

* Supporting Information

stretching vibration in the Fourier transform-infrared (FT-IR) spectrum.10 It is reported that the adsorption mechanisms of CO and N2 are qualitatively similar; i.e., both bonds involve charge donation from molecule to metal and back-donation from metal to molecule.45,46 Therefore, the C−O bond of CO adsorbed on Ru/C12A7:e− would be weakened. Furthermore, the electron transfer from C12A7:e− to Ru increases the charge of O adatoms due to the high electron affinity of oxygen. Thus, the association reaction effectively takes place between the absorbed Cδ+−Oδ− and Oδ− species, which results in high catalytic activity and a low activation energy (Figure 3). On the other hand, the association reaction is not promoted on the Ru/C12A7:O 2− and Ru/Al 2 O 3 catalysts because both C12A7:O2− and Al2O3 have little electron-donating properties.10 As a consequence, these two catalysts exhibit lower catalytic activity and higher activation energy than Ru/ C12A7:e−. A similar reaction mechanism was also proposed in a previous study,31 in which charge transfer from MgO with oxygen vacancies to adsorbed CO and oxygen species promoted the catalytic activity of Au nanoparticles for CO oxidation at a low reaction temperature.

S

Characterization of catalysts, XANES spectra, and XPS spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02342.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ¶

J.S. and M.K. contributed equally. H.H. conceived this research. Y.N., Y.I., and H.A. performed EXAFS measurements and analysis. J.S., M.K., and H.H. wrote the manuscript, and all the authors discussed the results. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr T. Yokoi from Tokyo Institute of Technology for FE-SEM analysis and Dr. D. Lu from Material Analysis Suzukake-dai Center, Technical Department, Tokyo E

DOI: 10.1021/acs.jpcc.5b02342 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Institute of Technology for TEM analysis. The XAFS study was carried out under the approval of PF-PAC No. 2013S2-002.

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