La2O3 Catalyst for the Simultaneous Removal of

Article pubs.acs.org/JPCC

An Efficient Cu/BaO/La2O3 Catalyst for the Simultaneous Removal of Carbon Soot and Nitrogen Oxides from Simulated Diesel Exhaust Kohsuke Mori,*,†,‡ Yoshiro Iwata,† Makoto Yamamoto,† Naoto Kimura,† Akihiko Miyauchi,† Go Okamoto,† Takahiro Toyoshima,† and Hiromi Yamashita*,†,‡ †

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan ‡ Elements Strategy Initiative for Catalysts Batteries ESICB, Kyoto University, Katsura, Kyoto, 615-8520, Japan S Supporting Information *

ABSTRACT: A three-component Cu/BaO/La2O3 catalyst has been developed as a highly efficient catalyst for the simultaneous removal of carbon soot and NO gas from diesel emissions. The influence of transition metals (Cu, Pd, Ni, Co, and Pt) deposited on BaO/La2O3 supports and cosupports (BaO, SrO, CaO, and CeO2) combined with La2O3 were investigated. The Cu/BaO/La2O3 catalyst completely combusted 50 wt % carbon soot under an O2 atmosphere, with a T50% temperature of 375 °C. This catalyst also exhibited the highest activity among the investigated catalyst systems for the simultaneous removal of carbon soot and NO gas, even at low temperatures. The absence of carbon soot substantially decreased the NO conversion rate that suggested carbon soot acts as a solid reductant for the NO reduction. The high activity of the Cu/BaO/La2O3 system can be attributed to the high oxygen storage capacity (OSC) of the BaO phase on La2O3 and to the redox properties of the Cu species that are attributable to strong Cu− BaO interactions. The Cu/BaO/La2O3 also exhibits high durability and good recyclability without any appreciable loss of catalytic activity. We propose that the present catalytic reaction consists of two reaction pathways: (i) direct soot oxidation by surface oxygen species and (ii) NOx-assisted soot oxidation. The use of significantly cheaper Cu rather than noble metals makes the Cu/BaO/La2O3 catalyst a strong candidate to satisfy today’s industrial requirements because of its lack of a precious metal.

1. INTRODUCTION Diesel engines that operate under lean conditions have been widely employed in numerous kinds of vehicles because of their high fuel efficiency, high export power, and excellent durability.1 However, the exhausts of diesel engines contain carbon soot (particulate matter), which is composed of aggregated carbonaceous soot and adsorbed hydrocarbons with particle sizes less than 2.5 μm, and NOx gas; these particulates and gas have been causing severe damage to the environment and human health.2,3 Therefore, a technique that can efficiently remove carbon soot and NOx emissions is particularly desirable to satisfy upcoming diesel emission standards.4−6 Because the simultaneous removal of carbon soot and NOx cannot be accomplished by either engine modification or fuel pretreatment, a catalytic methodology should be developed. The most effective technique for controlling carbon soot is to use wall-flow diesel particulate filter (DPF) systems, in which soot from the exhaust is first trapped by the DPF and the filters are continuously or periodically regenerated by catalytic combustion.7,8 Catalytic NOx removal is complicated because of the high level of exhaust gases, and selective catalytic reduction (SCR) using urea, hydrocarbons, NH3, CO, or H2 as a preferred reductant has been proposed as a promising solution.9−11 However, noble-metal-based catalysts have been © 2014 American Chemical Society

demonstrated to be active for the simultaneous removal of carbon soot and NOx.7,12,13 These catalysts efficiently catalyze the reduction of NO to N2 using carbon soot as a solid reductant, thus increasing the regeneration rate of the DPF system and contributing simultaneously to NOx abatement. However, from a raw-materials viewpoint, the extensive use of expensive and limited noble-metal catalysts should be restricted. To reduce the cost of catalysts, cerium oxides,14−16 perovskite-type complex oxides,17−19 and hydrotalcite-like structures20 are being developed. Among the noble-metal-free catalysts investigated, copper appears to be a promising candidate because of its superior redox properties, and some copper-based catalysts such as La0.8K0.2CuxMn1−xO3,21 Cu/K/ Al2O3,22 Cu/K/beta zeolite,23 Cu/ZnAl2O4,24 and Cu/ SrTiO325 have been reported. Moreover, copper is substantially less expensive than noble metals, thereby lowering the price of the final catalysts and making copper catalysts more attractive for general application in the broad diesel market. Alkaline earth metals, as exemplified by barium, exhibit high capacities for storing NOx; therefore, they have been widely investigated as NOx storage reduction (NSR) catalysts.26−28 Received: February 24, 2014 Revised: March 31, 2014 Published: April 7, 2014 9078

dx.doi.org/10.1021/jp501940f | J. Phys. Chem. C 2014, 118, 9078−9085

The Journal of Physical Chemistry C

Article

were performed at 77 K using a BEL-SORP Max (Bel Japan, Inc.). Each sample was degassed under vacuum at 343 K for 24 h prior to analysis. Combustion temperatures were monitored by thermogravimetric (TG) analysis (MAC Science Co. Ltd. TG-DTA2000S). X-ray absorption fine structure (XAFS) spectra were recorded at room temperature in fluorescence mode at the BL-9A facilities of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba (2012G126). A Si(111) double crystal was used to monochromatize the X-rays from the 2.5 GeV electron storage ring. In a typical experiment, a sample was loaded into an in situ cell with plastic windows. The extended X-ray absorption fine structure (EXAFS) data were examined using an EXAFS analysis program, Rigaku EXAFS. The pre-edge peaks in the Xray absorption near edge structure (XANES) regions were normalized for atomic absorption on the basis of the average absorption coefficient of the spectral region. Fourier transformation (FT) of k3-weighted normalized EXAFS data was performed over the 3.5 < k/Å−1 < 11 range to obtain the radial structure function. Temperature programmed reduction with H2 (H2-TPR) was carried out using BEL-CAT by heating a 0.1 g sample at 10 °C/min from 80 to 700 °C under a 4.75% H2/ Ar flow. Catalytic Activity Tests. The catalytic activity of the prepared catalysts toward soot combustion was evaluated using a thermal gravity (TG) technique. The mixture of catalyst/ carbon soot (10 mg) in a weight ratio of 1:1 was loaded into the sample chamber and heated from room temperature to 800 °C at a heating rate of 5 °C/min under a pure O2 atmosphere. The catalytic activity for NO gas conversion was continuously monitored with a NOx meter (Yanaco, ECL-77A). The catalyst/carbon soot mixture (0.3 g) in a weight ratio of 1:1 was placed in a quartz-tube reactor and heated at 400 °C in a gaseous mixture of NO (400 ppm), O2 (5%), and He (balance) (total flow rate: 100 mL/min). Measurement of Oxygen Storage Capacity (OSC). OSC was determined on the basis of the weight change during TG analysis. As a pretreatment, the catalyst (10 mg) was loaded into the sample chamber and heated from room temperature to 600 °C at a heating rate of 10 °C/min under an N2 atmosphere. The sample was cooled to 373 K, and the atmosphere was changed to pure O2. The weight change was then monitored while the sample was heated from 100 to 800 °C at a heating rate of 5 °C/min. Activation-Energy Measurements. The activation energy (Ea) for soot combustion was measured according to the Ozawa method using the TG technique.35,36 10 mg of a soot/ catalyst mixture with weight ratio of 10:1 was analyzed in air from 30 to 700 °C (heating rates: B = 2, 5, 10 °C/min). According to the Ozawa method, the activation energy can be calculated by the following equation:

Because the presence of NOx can accelerate the oxidation of carbon soot, the NOx storage characteristics of alkaline earth metals makes them promising components in NOx-assisted soot combustion catalyst systems. However, the development of mostly such catalysts has been focused on potassiumcontaining catalysts, such as Ba−K−Ce29 and Co−Ba−K.30,31 Despite their high catalytic activities, such potassium-containing catalysts suffer from low stability because of their low melting points and high solubility in H2O.32 Novel metal catalysts such as Pt−Ba−Al2O3 have also been reported;33,34 however, the high cost of noble metals restricts the global application of such catalysts. In this study, a novel Cu/BaO/La2O3 catalyst was developed for the simultaneous removal of carbon particulates and NO gas from diesel exhaust. We screened transition metals (Cu, Pd, Ni, Co, and Pt) over BaO/La2O3 supports and co-supports (BaO, SrO, CaO, and CeO2) with La2O3 over Cu-based catalysts. We observed that the Cu/BaO/La2O3 catalyst was able to completely combust a large amount of carbon soot (50 wt %) under an O2 atmosphere at a T50% temperature of 375 °C; the catalyst also exhibited high activity for the simultaneous removal of carbon soot and NO gas.

2. EXPERIMENTAL SECTION Materials. Cu(NO3)2 3H2O and Ba(NO3)2 were obtained from Wako Pure Chemical Ind. La2O3 was purchased from Shin-Etsu Chemical Co. Al2O3 was obtained from Sumitomo Chemical Co. A commercial carbon black, carbon ECP (SBET = 800 m2/g) obtained from the Lion Corporation, was used as a model of diesel soot. Synthesis of the Catalyst. La2O3 (5.0 g) was mixed with Ba(NO3)2 (1.41 g, 14.2 wt %) and deionized water (150 mL) and stirred at 60 °C for 1 h. The suspension was evaporated under vacuum; the obtained powder was subsequently dried at 110 °C overnight and calcined at 900 °C for 5 h under an O2 atmosphere resulting in BaO/La2O3 as a white powder. SrO/ La2O3, CaO/La2O3, and CeO2/La2O3 were prepared in a similar manner using Ba(NO3)2, CaCl2, and Ce(NO3)2, respectively. BaO/Al2O3 was also synthesized according to this procedure. Cu was deposited using impregnation methods. BaO/La2O3 (0.5 g) was mixed with Cu(NO3)2·3H2O (0.019 g, 1.0 wt %) and deionized water (150 mL) and stirred at 60 °C for 1 h. The suspension was evaporated under vacuum; the obtained powder was subsequently dried at 110 °C overnight and calcined at 550 °C for 5 h under an O2 atmosphere to give Cu/BaO/La2O3 as a gray powder. Pd/BaO/La2O3, Ni/BaO/La2O3, Co/BaO/ La2O3, and Pt/BaO/La2O3 were synthesized in a similar manner using PdCl2, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, and H2PtCl6·6H2O, respectively. Cu/SrO/La2O3, Cu/CaO/La2O3, Cu/CeO2/La2O3, Cu/BaO/Al2O3, and Cu/La2O3 with 1.0 wt % of Cu content were also prepared according to this procedure. The catalysts obtained were mixed well with a model carbon substance (ECP) in an aqueous solution and evaporated under vacuum. The catalyst (50 wt %) and carbon soot (50 wt %) was mixed with deionized water (100 mL) and stirred at 60 °C for 1 h. The suspension was evaporated under vacuum, and the obtained powder was dried at 80 °C overnight. Characterization. Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku Miniflex equipped with a Cu Kα radiation source with a wavelength of 1.5418 Å. Brunauer−Emmett−Teller (BET) surface area measurements

ln B = −0.4567

Ea 1 +C R Ta

where B represents the heating rate, Ta is the temperature corresponding to a carbon conversion of a%, Ea is the apparent activation energy in kJ/mol, and C is a constant. If several measurements are performed at various heating rates, the relationship between 1/T and ln B for each pair of these data can be plotted, and the activation energy of a specimen can be obtained from the slope of the plotted line. 9079



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3. RESULTS AND DISCUSSION 3.1. Effect of Transition Metals over BaO/La 2O3 Support. The BET surface area of BaO/La2O3 measured by N2 adsorption/desorption was 1.4 m2/g. Figure 1 shows the

Figure 2. (A) Cu K-edge XANES spectra and (B) FT−EXAFS spectra of (a) Cu foil, (b) Cu2O, (c) CuO, (d) Cu/BaO/La2O3, (e) Cu/BaO/ Al2O3, (f) Cu/La2O3, and (g) recovered Cu/BaO/La2O3. Figure 1. XRD patterns of (a) Cu/BaO/La2O3, (b) Pt/BaO/La2O3, (c) Pd/BaO/La2O3, (d) Ni/BaO/La2O3, (e) Co/BaO/La2O3, and (f) BaO/La2O3.

strong interaction between the copper species and the BaO phase on the La2O3 support. Either Cu ions are incorporated within the BaO lattice, thereby leading to a different electronic environment from that of the CuO standard, or isolated copper ions that strongly interact with BaO and form a mixed copper barium oxide with electronic and structural properties that differ from those of the CuO standard. On the basis of the edge energy, the copper species in Cu/BaO/Al2O3 and Cu/La2O3 also exist in a 2+ oxidation state (Figure 2A(e) and (f)); however, the white line intensity of the Cu/La2O3 is slightly weaker than those of the Cu/BaO/La2O3 and Cu/BaO/Al2O3. The FTs of Cu K-edge EXAFS data are depicted in Figure 2B. The peak positions of the Cu/BaO/La2O3 are similar to those of CuO but different from those of the Cu foil and Cu2O, indicating that most copper species exist as CuO. However, compared to the spectrum of the reference CuO standard, the second peaks in the spectrum of the Cu/BaO/La2O3 are substantially more intense (Figure 2B(d)). This peak is attributable to the contiguous Cu−O−Cu bonds due to CuO6 octahedra. The increased intensity of the Cu/BaO/ La2O3 in this region can be explained by the formation of Cu− O−Ba bonds due to the strong interaction between small CuOx clusters and the BaO phase. The formation of these bonds is indicated by the absence of the edge transition peak, the greater white line intensity, and the different shape in the XANES spectrum of the Cu/BaO/La2O3. Such unique phenomena cannot be observed in cases of Cu/BaO/Al2O3 and Cu/La2O3 (Figure 2B(e) and (f)), where the peak positions and relative intensity coincide with those of the CuO standard. We concluded that the CuO phase formed on the BaO/La2O3 support is completely different from those on the BaO/Al2O3 and on the original La2O3.

XRD patterns of BaO/La2O3 and various metal-loaded BaO/ La2O3 catalysts. The pattern of the original BaO/La2O3 showed distinct peaks associated with La2O3 and small BaO phases, accompanied by a small peak due to BaCO3. Peaks attributable to the transition metal and/or oxide phases could not be confirmed, even after transition metals (Cu, Pt, Pd, Ni, and Co) were deposited, presumably because of the highly dispersed nature of the metal species on the BaO/La2O3 surface. In the TEM image of the Cu/BaO/La2O3, no appreciable nanoparticles are observed (Figure S1). Because XRD cannot be used to characterize highly dispersed Cu species, XAFS analysis was performed to confirm the local structure of the deposited Cu species. Figure 2A shows the Cu K-edge XANES spectra of the Cu/BaO/La2O3, Cu/BaO/ Al2O3, and Cu/La2O3 as well as those of Cu foil, Cu2O, and CuO as reference copper compounds. The different edge energies and edge shapes of the three standards provide important information for identifying the oxidation state and local structure of copper. A small pre-edge peak of the CuO standard at 8.974 keV is ascribed to the dipole-forbidden electronic transition of 1s → 3d for Cu2+; the more intense peaks between 8.982 and 8.986 keV are attributed to the dipole-allowed 1s → 4p electron transition of Cu+ and Cu2+, respectively, which are indicative of the copper oxidation state.37,38 The spectrum of Cu/BaO/La2O3 (Figure 2A(d)) indicated that this catalyst exhibits the same edge energy as the CuO standard, which suggests that the copper species exist in a 2+ oxidation state. In addition, the absence of the edge transition peak, the greater white line intensity, and the different shape of the catalyst spectrum can be attributed to a 9080



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Figure 3 shows the effect of transition metals on carbon soot conversion under an O2 atmosphere. Carbon soot conversion

Figure 4. NO removal efficiency and oxygen storage capacity of various catalysts. NO removal was performed in a quartz-tube reactor using catalyst/carbon soot (0.3 g, 50 wt %/50 wt %) in a gaseous mixture of NO (400 ppm), O2 (5%), and He (balance) (total flow rate: 100 mL/min). The NO removal efficiency was determined after 30 min.

Figure 3. Carbon soot conversion in the presence of various catalysts under an O2 atmosphere. Catalyst (50 wt %)/carbon soot (50 wt %).

was determined at 400 and 450 °C. Under the reaction conditions used, the direct oxidation of carbon soot in the absence of a catalyst begins at 580 °C, with a T50% temperature (the temperature required to convert 50% of the carbon soot) of 675 °C. The carbon conversion using BaO/ La2O3 without a transition metal was 7% at 400 °C, which slightly changed after the deposition of transition metals. The deposition of copper substantially improved the catalytic activity of BaO/La2O3: the soot combustion started at approximately 300 °C, with a T50% temperature of 375 °C. The T50% temperatures of the other metal catalysts, except Pd/BaO/La2O3, were greater than 450 °C. The apparent activation energy (Ea) of soot oxidation was determined using the Ozawa method (Figure S2). The measured Ea values of 110−140 kJ mol−1 were lower than those obtained without a catalyst (i.e., the noncatalytic oxidation of carbon black with gaseous oxygen), 150−160 kJmol−1. Notably, the T50% temperature achieved by the Cu/BaO/ La2O3 catalyst is lower than those reported for other copperbased catalyst systems, including Cu/MgTiO3 (629 °C),25 Cu/ ZnAl2O4 (600 °C),24 Cu/γ-Al2O3 (600 °C),24 Cu/SrTiO3 (600 °C),25 SrTi0.89Cu0.11O3 (594 °C),39 and (La1.7Rb0.3CuO4)20/ nmCeO2 (401 °C).40 More significantly, the present Cu/BaO/ La2O3 system also exhibits high soot combustion ability, even in the presence of a large amount of carbon soot (i.e., a catalyst/ carbon soot mixture in a weight ratio of 1:1). This catalyst/ carbon soot ratio is substantially higher than those combusted by other reported systems, where the catalyst/carbon soot mixture was typically in a weight ratio of approximately 4:1 to 20:1.24,25,39,40 This high soot combustion capacity, the low combustion temperature, and the use of inexpensive copper as a catalyst satisfy the criteria of the upcoming diesel emission standards. Figure 4 shows the NO removal efficiency and oxygen storage capacity of the various catalysts during the soot combustion experiments in the presence of NO (400 ppm), O2 (5%), and He (balance) at 400 and 500 °C. No significant reaction was observed in the blank experiment with no catalyst under the present reaction conditions. We also found that Cu/

BaO/La2O3 was the most active catalyst for NO removal, with a maximum conversion efficiency of 70% at 400 °C. At 500 °C, the conversion efficiency of NO approached 90%. The absence of carbon soot substantially decreased the NO conversion, which suggests that carbon soot acted as a solid reductant for the NO reduction that accompanied the oxidation to CO2. The deposition of other transition metals, such as Pd, Ni, Co, and Pt on BaO/La2O3 did not improve the catalytic activity by a significant extent at 400 °C, which suggests that the CuO species is important as a catalytically active site. In the reaction at 500 °C, the addition of transition metals slightly increased the catalytic activity. With respect to the Cu loading, best catalytic activity was obtained at 1.0 wt %, whereas greater amounts of Cu retarded both reactions (Figure S3). The Cu Kedge FT−EXAFS spectra revealed that only the Cu/BaO/ La2O3 that contained 1.0 wt % of Cu exhibited enhanced Cu− O−Ba bonding at approximately 3.3 Å because of the strong interaction between small CuOx clusters and the BaO phase, which may explain the high catalytic activity of the 1.0 wt % Cu/BaO/La2O3 (Figure S4). The catalytic activities for carbon soot combustion and NO removal correspond well to the oxygen storage capacity (OSC) of the catalysts. The OSC is known to be an important characteristic of soot combustion catalysts.25,41,42 First, the transfer of active oxygen across the soot surface is proposed to be a key issue for soot oxidation activity. Second, the OSC properties of catalysts continuously supply active oxygen to the structural defect sites. Cu/BaO/La2O3 exhibited the highest OSC, whereas the deposition of Pd, Ni, Co, or Pt significantly decreased the OSC compared to that of the inherent BaO/ La2O3. In our previous works, we demonstrated that BaO/ La2O3 exhibits a high OSC and that O22− species, whose presence was confirmed by XPS analysis, were active toward the combustion of carbon soot and the oxidative coupling of methane.43 These results inevitably suggest that the deposition of Cu species creates interesting synergetic effects for catalysis and that both the original BaO/La2O3 and Cu species on the BaO/La2O3 act as oxygen storage sites that stem from its redox properties (i.e., 2CuO ⇄ Cu2O + 1/2O2).44 9081



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La2O3. These results suggest that the Cu species are mainly highly dispersed on the co-support, not on the La2O3 surface. The support and co-support strongly influence the activity of the Cu catalysts. Cu/SrO/La2O3 exhibits moderate activity, whereas Cu/CaO/La2O3 and Cu/CeO2/La2O3 gave poor results for both carbon soot combustion and NO removal (Figures 6 and 7). Notably, not only the Cu site, but also the

We explored the potential catalytic durability of the Cu/ BaO/La2O3 in the combustion of carbon soot and in NO removal (Figure S5). The recovered catalyst could be recycled without substantial loss of its inherent activity; it exhibited almost complete combustion of carbon soot and a NO removal efficiency in >85% yields at least three times at 500 °C for 30 min. This characteristic renders the Cu/BaO/La2O3 a pivotal contribution to the development of a catalyst for the purification of actual diesel exhaust. In our preliminary experiments, the Cu K-edge XANES spectrum of the recovered Cu/BaO/La2O3 after the reaction is almost identical to that of the fresh catalyst (Figure 2A(d) and (e)). However, the white line intensity decreased, accompanied by the appearance of a small peak attributable to the dipole-allowed 1s → 4p electron transition for Cu+ at approximately 8.982 keV. In the FT− EXAFS spectrum, the relative intensity of a second peak associated with the contiguous Cu−O−Cu bonds decreased compared to that in the spectrum of the fresh sample (Figure 2B(d) and (e)). These results indicate Cu2+ is partially reduced to Cu+ and that the CuO/Cu2O redox couple is critical for the catalytic reaction. 3.2. Effect of Supports over Cu-Based Catalysts. Next, the effect of supports on the performance of Cu-based catalysts was investigated. The XRD patterns of Cu/BaO/La2O3 and Cu/CeO2/La2O3 showed peaks attributable to BaO and CeO2, respectively (Figure 5a and d). However, the deposition of SrO

Figure 6. Carbon soot conversion in the presence of various catalysts under an O2 atmosphere: catalyst (50 wt %)/carbon soot (50 wt %).

Figure 7. NO removal efficiency and oxygen storage capacity of the various catalysts. NO removal was performed in a quartz-tube reactor using catalyst/carbon soot (0.3 g, 50 wt %/50 wt %) in a gaseous mixture of NO (400 ppm), O2 (5%), and He (balance) (total flow rate: 100 mL/min). The NO removal efficiency was determined after 30 min.

BaO phase plays a critical role in attaining high catalytic activity. Cu/BaO/Al2O3 exhibited very low activity compared to that of Cu/BaO/La2O3; this result suggests that the BaO phase does not form on γ-Al2O3, which is consistent with the XRD results. The importance of the BaO phase is also evident because of the low catalytic activity of Cu/La2O3. Similar tendencies in catalytic activity were observed for carbon soot combustion at 450 °C and for NO removal efficiency at 500 °C. Additionally, the mismatch of ion sizes of Cu2+ with the co-support elements (Ba2+, Sr2+, Ca2+, and Ce3+) may play an important role in catalytic activity; the ionic radius (Å) increased in the order of Ca2+ (1.00) < Ce3+ (1.01) < Sr2+ (1.18) < Ba2+ (1.35), which is almost consistent with the increase of catalytic activity.

Figure 5. XRD patterns of (a) Cu/BaO/La2O3, (b) Cu/SrO/La2O3, (c) Cu/CaO/La2O3, (d) Cu/CeO2/La2O3, (e) Cu/BaO/Al2O3, and (f) Cu/La2O3.

or CaO as a cosupport instead of BaO did not result in any significant new peaks, suggesting that amorphous phases formed (Figure 5b and c). No peaks attributable to the BaO phase were observed in the pattern of Cu/BaO/Al2O3, whereas distinct peaks due to BaAl2O3 and γ-Al2O3 were observed. Peaks attributable to Cu metal and/or oxide phases were not confirmed in the patterns of any of the samples except Cu/ 9082



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play a predominant role in the catalytic reaction. The active oxygen species released from CuO (2CuO ⇄ Cu2O + O) can be continuously supplemented by atmospheric O2 through the oxygen vacancies in the vicinal BaO phase; these vacancies can be induced by the strong Cu−BaO interactions, as confirmed by XAFS analysis. This assumption is supported by the high OSC properties of Cu/BaO/La2O3 and the original BaO/ La2O3 compared to those of other transition-metal-supported BaO/La2O3. Because the original BaO/La2O3 is also active toward the direct combustion of carbon soot (Figure 3), the direct contribution of the O22− species generated on the BaO phase cannot be ruled out. In our previous works, we demonstrated that BaO/La2O3 exhibits high OSC and confirmed the existence of O22− species by XPS analysis.43 The O22− species could be converted to O2− (eq 1) or O− (eq 2), which is active toward the combustion of carbon soot and the oxidative coupling of methane52

Again, the catalytic activity was found to be well correlated with the OSCs of the samples. Cu/SrO/La2O3 exhibited a moderate OSC, whereas other catalysts with poor OSC properties resulted in low catalytic activity. CeO2 has been identified as a promising catalyst for model soot oxidation in the presence of NO/O2,14,45,46 and the performance of Ag/ CeO2 and Pt/CeO2 catalysts has thus far been reported.15,47 One of the key features of CeO2 is its ability to switch between the Ce4+ and Ce3+ oxidation states and to incorporate oxygen into its crystal structure depending on the atmosphere, temperature, and pressure. However, the CeO2 phase on La2O3 did not exhibit substantial OSC properties. The deposition of Ba on Al2O3 was also insufficient to achieve high catalytic activity as well as a high OSC because of the formation of BaAl2O3. In addition, alkaline earth metal compounds such as BaO also exhibit high capacity for storing NOx at low temperatures because of their strong basicity. The loss of the NOx storage capacity of the Cu/BaO/Al2O3 caused by the formation of BaAl2O3 may also seriously diminish its catalytic activities. H2-TPR experiments were also carried out in order to evaluate the reducibility of Cu species deposited on different supports, since such reducibility plays a key role in the catalytic activity for soot combustion of this metal.48 A bimodal shape of peaks was observed between 220 and 420 °C for Cu/BaO/ La2O3 and Cu/La2O3 (Figure 8), which are ascribed to the

O2 2 − + O2 → 2O2−

(1)

O2 2 − → 2O−

(2)

The low OSC properties and catalytic activities of the Cu/SrO/ La2O3 and Cu/CaO/La2O3 can be explained by the easy combustion of such O22− species at temperatures less than 600 °C.43 The NO-assisted soot oxidation can be expressed according to the following equations (eqs 3 and 4):53,54 NO + 1/2O2 → NO2

(3)

NO2 + C → CO2 + NO

(4)

The NO2 produced by NO oxidation acts as a more powerful oxidant than NO or O2 for soot oxidation at relatively low temperatures. The above reactions do not produce N2. It can be proposed that the surface nitrite and nitrate species generated from the reaction between NO/NO2 and O2−/O− species according to the following equations participate in the reaction (eqs 5−7). Such surface nitrite and nitrate species are reduced by soot and are converted into N2 (eqs 8−9).32,40 Since the chemisorption of NO on the catalyst surface is also a crucial step in such reaction pathway,55 the alkaline earth metals with NOx storage characteristics is an important components in NOx-assisted soot combustion catalyst systems.

Figure 8. H2-TPR profiles of (a) Cu/BaO/La2O3, (b) Cu/La2O3, and (c) Cu/BaO/Al2O3.

stepwise reduction of surface dispersed CuOx clusters, i.e., Cu2+ → Cu+ and Cu+ → Cu0. The first peak of Cu/BaO/La2O3 shifted toward slightly higher temperature and the intensity of the second peak was smaller than those of the Cu/La2O3. These results suggest that the presence of BaO phase slightly retards the onset temperature of the Cu2+ → Cu+ and efficiently suppresses the subsequent Cu+ → Cu0 reduction, which may be unfavorable in the carbon soot combustion. On the other hand, the Cu/BaO/Al2O3 did not show any appreciable reduction peak under the experimental conditions. It can be said that the Cu species on BaAl2O3 completely lack reducibility.

NO + 2O2− → NO3− −

NO2 + O → −

NO + O → NO2 NO3−

(5)

NO3−

(6)

−

(7) −

+ C → N2 + COx + O

NO2− + C → N2 + COx + O−

(8) (9)

5. CONCLUSION The catalytic removal of carbon particulates and NO from simulated diesel exhaust was effectively promoted over the three-component Cu/BaO/La2O3 catalyst. The soot combustion starts at approximately 300 °C, with a T50% temperature of 375 °C, and a NO conversion greater than 90% was attained at 500 °C in the presence of carbon soot. Such high activity can be attributed to the high OSC of the BaO phase on La2O3 and to redox properties of Cu species that result from strong Cu−

4. MECHANISM On the basis of our catalyst characterization and evaluation of the catalytic performance of the Cu/BaO/La2O3 system, this system contains two reaction pathways: (i) direct soot oxidation by surface oxygen species and (ii) NO-assisted soot oxidation.49−51 In the direct oxidation route, soot is oxidized on the solid−solid interface between the catalyst and soot by the active oxygen species, where the CuO/Cu2O redox couple may 9083



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BaO interactions. The Cu/BaO/La2O3 system also exhibits high robustness and recyclability while maintaining its catalytic activity. Such a potentially useful Cu catalyst compared to noble metals contributes to lowering the price of the final catalysts and to making Cu catalysts more attractive for possible application in the broad diesel market.

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ASSOCIATED CONTENT

* Supporting Information S

TEM image, apparent activation energy, effect of the loading amount of Cu on catalytic activity, Cu K-edge FT−EXAFS spectra of Cu/BaO/La2O3 with different Cu loadings, and recycling experiments. This material is available free of charge via the Internetat http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax & Tel: +81-6-68797460, +81-6-6879-7457 *E-mail: [email protected]. Fax & Tel: +81-66879-7460, +81-6-6879-7457. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed under a management of ‘Elements Strategy Initiative for Catalysts & Batteries (ESICB) supported by MEXT. XAFS spectra were recorded at room temperature in fluorescence mode at the BL-9A facilities of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba (2012G126).

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REFERENCES

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La2O3 Catalyst for the Simultaneous Removal of

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