Copper Catalysts for Soot Oxidation - American Chemical Society

Sep 9, 2008 - A. ADAMSKI, ‡. B. URA, §. AND J. TRAWCZYNSKI §. Inorganic Chemistry Department, University of Alicante. Ap. 99, E-03080 Alicante, Sp...
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Environ. Sci. Technol. 2008, 42, 7670–7675

Copper Catalysts for Soot Oxidation: Alumina versus Perovskite Supports ´ PEZ-SUA ´ REZ,† F. E. LO ´ P E Z , * ,† A. BUENO-LO ´ N-GO ´ MEZ,† A. ADAMSKI,‡ M. J. ILLA B. URA,§ AND J. TRAWCZYNSKI§ Inorganic Chemistry Department, University of Alicante. Ap. 99, E-03080 Alicante, Spain, Faculty of Chemistry, ´ Jagiellonian University, 30-060 Krakow, ul. Ingardena 3, Poland, and Division of Chemistry and Technology Fuels, Wrocław University of Technology, ´ 50 - 344 Wrocław, ul. Gdanska 7/9, Poland

Received April 8, 2008. Revised manuscript received July 24, 2008. Accepted August 5, 2008.

Copper catalysts prepared using four supports (Mg- and Srmodified Al2O3 and MgTiO3 and SrTiO3 perovskites) have been tested for soot oxidation by O2 and NOx/O2. Among the catalysts studied, Cu/SrTiO3 is the most active for soot oxidation by NOx/O2 and the support affects positively copper activity. With this catalyst, and under the experimental conditions used, the soot combustion by NOx/O2 presents a considerable rate from 500 °C (100 °C below the uncatalysed reaction). The Cu/ SrTiO3 catalyst is also the most effective for NOx chemisorption around 425 °C. The best activity of Cu/SrTiO3 can be attributed to the improved redox properties of copper originated by Cusupport interactions. This seems to be related to the presence of weakly bound oxygen on this sample. The copper species present in the catalyst Cu/SrTiO3 can be reduced more easily than those in other supports, and for this reason, this catalyst seems to be the most effective to convert NO into NO2, which explains its highest activity for soot oxidation.

1. Introduction Soot and NOx emitted by diesel engines are responsible of severe environmental and health problems and their emission must be controlled. Diesel particulate filters (1, 2) remove soot from diesel exhaust, and the filters are continuously or periodically regenerated to their original state by burning off the trapped soot. The frequency of regeneration is determined by the amount of soot build-up, and to facilitate its removal, a catalyst is used either in the form of a coating on the filter or a catalyst added to the fuel (3). NOx removal is more complicated because of the high level of O2 in the exhaust and selective catalytic reduction (SCR) has been proposed as a potential solution (3-5). The preferred reductant is urea (which decomposes to NH3), but hydrocarbons, NH3, CO, H2, and even diesel fuel have been also proposed to reduce NOx to N2 (6). It has been recently demonstrated that several metal oxides with oxygen storage capacity, such as pure ceria (7) or mixed oxides of cerium and rare-earth metals (La and Pr, for instance) (8-11), are able to accelerate soot oxidation due to the effective participation of highly reactive oxygen * Corresponding author e-mail: [email protected]. † University of Alicante. ‡ Jagiellonian University. § Wrocław University of Technology. 7670

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species, which is the so-called “active oxygen”. High catalytic activity for soot oxidation has been also reported for other oxides with perovskite- (12-15) and hydrotalcite-like (16) structure, and in some cases (14), it has been suggested that their high activity could be also related with the participation of suprafacial, weakly chemisorbed oxygen, which contributes actively to soot combustion by spillover. On the contrary, the metal oxides that are not able to efficiently involve their oxygen in the soot oxidation reaction, such as TiO2 or ZrO2 (17), show in general, poor catalytic activity. On the other hand, several catalysts have demonstrated to be effective for the simultaneous removal of soot and NOx, and among them there are some copper-containing catalysts such as La0.8K0.2CuxMn1-xO3 (18), Cu/K/Al2O3 (19), and Cu/ K/beta zeolite (20). These catalysts are able to catalyze the reduction of NOx to N2 by soot (19-22), therefore increasing the regeneration rate of the soot filters and contributing simultaneously to NOx abatement. Additionally, copper is significantly cheaper than the noble metals, therefore lowering the price of the final catalysts and making copper catalysts more attractive for a general application to the wide diesel market. In this study, copper catalysts have been prepared, characterized and tested for soot oxidation by O2 and NOx/ O2, paying special attention to the role of the support in the activity of copper. Two different types of supports have been selected consisting of two titanates, with perovskite-like structure, and two aluminas. Perovskites are potentially active support, and they could involve their oxygen in the soot oxidation reactions (14), whereas aluminas are considered inert supports. Therefore, the selected supports allow comparing the activity of copper supported on potentially active and inert oxides.

2. Experimental Section Two stabilized aluminas, containing Mg or Sr, and two titanium-based mixed oxides, also containing Mg or Sr were prepared and used as copper supports. These supports are denoted by MgAl2O3, SrAl2O3, MgTiO3, and SrTiO3, respectively. These supports were further impregnated with a Cu(NO3)2 · 3(H2O) water solution of the required concentration to obtain a 5 wt. % copper loading dried overnight at 110 °C and calcined at 700 °C in air for 5 h. The amount of copper was selected based on previous results obtained with a series of Cu/Al2O3 catalysts used for soot oxidation by NOx/ O2 (23). The characterization techniques used were XRD, N2 adsorption, ICP, Temperature Programmed Reduction with H2 (TPR-H2), XPS and decomposition of cyclohexanol (CHOL) (24). Soot oxidation experiments were performed in a fixedbed reactor at atmospheric pressure under a gas flow (500 mL/min) containing 5% O2 or 500 ppm NOx + 5% O2. The experiments consisted of heating the soot-catalyst or sootsupport mixtures (1:4 wt. ratio, loose contact) from 25 to 800 °C at 10 °C/min. The model soot used is a carbon black from Cabot (Vulcan XC72). Blank experiments were performed with the catalysts and supports (without soot). The gas composition was monitored by specific NDIR-UV gas analyzers for NO, NO2, CO, CO2, and O2, and the gas composition was recorded every 10 s. In the figures corresponding to catalytic results (Figures 1-4), all the experimental data collected are represented as continuous lines and, when required, symbols are depicted above these lines to facilitate the figure interpretation. The soot conversion was determined 10.1021/es8009293 CCC: $40.75

 2008 American Chemical Society

Published on Web 09/09/2008

TABLE 1. Characterization of the Supports and Catalysts sample SrAl2O3 MgAl2O3 SrTiO3 MgTiO3 Cu/SrAl2O3 Cu/MgAl2O3 Cu/SrSrTiO3 Cu/MgTiO3

Cu (wt. %)

BET (m2/g)

4.0 3.2 3.8 3.8

127 131 11 9 122 109 11 9

TABLE 2. Temperature for 50% Soot Conversion (T50%) and CO2 Selectivity in Soot Oxidation Experiments FIGURE 1. Soot oxidation profiles under O2 obtained with catalysts.

sample no catalyst MgAl2O3 SrAl2O3 SrTiO3 MgTiO3 Cu/MgAl2O3 Cu/SrAl2O3 Cu/SrTiO3 Cu/MgTiO3

FIGURE 2. Soot oxidation profiles under NOx/O2 obtained with catalysts. from CO and CO2 evolved. The formation N2O as NOx reduction product was ruled out with additional experiments followed by gas chromatography. This setup is described in detail elsewhere (25). Additional details about the preparation of the supports and about the experimental set-ups used and procedures followed for characterization and catalytic tests are included as Supporting Information.

3. Results and Discussion 3.1. Supports and Catalysts Characterization. The XRD spectra of the supports (Figure 1S, Supporting Information) show mainly the presence of the perovskite phase for MgTiO3 and SrTiO3 and the alumina or aluminate peaks for SrAl2O3 and MgAl2O3,, respectively. In the case of SrAl2O3 the XRD peaks for SrCO3 are also identified. For catalysts (Figure 1S, Supporting Information), the XRD spectra reveal the presence of CuO for all the catalysts except for Cu/MgAl2O3. The BET surface area of the supports and catalysts is included in Table 1 along with the copper content on the catalysts, which is between 3.2 and 4 wt. %. The BET surface areas of MgTiO3 and SrTiO3 are low (9 and 11 m2/g, respectively), but they are higher than the obtained previously by impregnation of TiO2 (1-3.6 m2/g) (15) instead of by the sol-gel method used in this study. The impregnation of Al2O3 with Mg or Sr precursor does not significantly affect the BET surface area of this material. In general, the addition of copper does not significantly modify the BET surface area of the supports. 3.2. Catalytic Tests with O2. The soot conversion profiles obtained during the catalytic tests performed with O2 are plotted as a function of temperature in Figure 1, including the curves obtained during the experiments performed with

T50% (°C) CO2 selectivity T50% (°C) CO2 selectivity reaction (%) reactions reaction (%) reaction in O2 in O2 in NOx/O2 in NOx/O2 715 708 705 705 710 681 695 670 629

36 41 57 60 31 94 100 97 100

665 628 643 653 664 628 630 600 605

36 91 88 62 34 93 99 99 87

catalyst-soot mixtures and only with soot. Additionally, the parameter T50%, which is the temperature required to convert 50% of the soot used in each experiment, is included in Table 2, as well as the selectivity toward CO2 formation. Under the reaction conditions used, the direct oxidation of soot (without catalyst or support participation) takes place between 575 and 775 °C, with a T50% temperature of 715 °C. The supports slightly decrease the soot conversion profiles a few degrees to lower temperatures. However, the catalysts lower the soot conversion temperature and, consequently, the T50% parameter. Among the different catalysts tested, those prepared with perovskite supports showed enhanced activity in comparison to alumina-based catalysts. Taking into account that the supports show little activity, these differences can be related to copper-support interactions, perovskites being more efficient supports for copper than aluminas. Regarding the selectivity toward CO2 formation, the uncatalyzed reaction yielded 36% CO2 and almost all the catalysts increase this value. For supports, the change in CO2 selectivity is low. The formation of CO and/or CO2 as carbon oxidation product depends on the nature of the oxygen complexes formed on soot surface upon oxidation, and the main parameters affecting the nature of these complexes are the carbon material properties, the oxidizing gas used, and the reaction temperature (26). Additionally, the selectivity toward CO2 formation could increase if the catalyst used is able to accelerate the oxidation of CO to CO2 once CO evolves as primary oxidation product. The results obtained in these experiments suggest that the supports, but mainly the catalyst, affect the nature of the oxygen groups formed on soot surface during the reaction and/or they could be also effective for CO oxidation to CO2. In fact CuO was found to enhance the formation of oxygen complexes on soot surface (27), also, the high activity of copper for CO oxidation is well-known (28). 3.3. Catalytic Tests in NOx/O2. 3.3.1. Soot Oxidation. Figure 2 compiles the soot conversion profiles obtained with the catalysts under the NOx/O2 gas flow, showing that all the catalysts significantly decreased the soot oxidation temperature (the T50% values are included in Table 2). Below VOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. NO conversion to NO2 profiles in blank experiments under NOx/O2 (without soot).

FIGURE 3. NOx removal profiles under NOx/O2 obtained with catalysts; (a) with soot and (b) without soot. 600 °C, Cu/SrTiO3 is the most active catalyst, and at this temperature, a huge raise in soot conversion is observed for the catalyst Cu/MgTiO3 as a consequence of the high activity of this catalyst for soot combustion by O2 (see Figure 1). In agreement with the results obtained with O2 (previous section), the results under NOx/O2 indicate that the coppersupport interaction determines the catalyst activity. For instance, Cu/SrTiO3 (Figure 2) is the most active catalyst below 600 °C but the activity of the support SrTiO3 for soot oxidation is quite poor (Figure 2S, Supporting Information). In fact, it has been probed (Figure 2S, Supporting Information) that any support presents activity at low temperature (T < 550 °C). Regarding the CO2 selectivity, under the reaction conditions of these experiments, the uncatalysed soot oxidation reaction yielded 36% CO2 (Table 2), which is the same value observed with O2. In experiments performed under NOx/O2, almost all the supports and catalysts enhance the selectivity for CO2 formation, but in a different extent depending on the sample considered. The most active catalyst (Cu/SrTiO3) is also one of the more selective ones, the formation of CO being almost negligible (99% CO2 selectivity). 3.3.2. NOx Removal. Figure 3a includes the NOx removal profiles obtained during the soot oxidation experiments performed with the catalysts under NOx/O2. For comparison, blank experiments under similar conditions were carried out with the catalysts but without soot, and the NOx removal profiles obtained are included in Figure 3b. Additionally, the NO2 percentage measured in the blank experiments (without soot) performed with supports and catalysts have been plotted in the Figure 4. As it is observed in the Figure 3a, NOx removal takes place above 300 °C during the soot oxidation experiments, and Cu/SrTiO3 is the most active catalysts for NOx removal with 7672

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a maximum removal peak centered at around 425 °C and a shoulder at higher temperature. The activity of supports is very low (Figure 3S, Supporting Information) revealing the catalytic effect of copper. The comparison of Figure 3a with the blank experiments, carried out in the absence of soot (Figure 3b), allows concluding that NOx chemisorption on the catalysts occurs around 425 °C and, thus, NOx reduction by soot takes place above 500 °C, approximately (Figure 3a). The uncatalyzed NOx reduction by soot occurs around 650 °C and, in agreement with the previously described soot conversion profiles, all the catalysts lowered this temperature. NOx chemisorption on the supports (Figure 4S, Supporting Information) is much lower and takes place at higher temperature than on the catalysts, pointing out the key role of copper, also, in the chemisorption process. Figure 4 reveals that all the catalysts are able to convert NO into NO2 above 400 °C approximately, whereas the supports do not catalyze the NO2 formation. This means that the catalytic conversion of NO to NO2 must be attributed to copper (27). Thus, the catalytic activity for soot oxidation seems to be related to the production of NO2, which is much more oxidant than NO and O2. The most active catalyst for soot oxidation (Cu/SrTiO3) is also the most effective for NO conversion to NO2. The chemisorption of NOx on the catalysts around 425 °C seems to occur also via NO2 because Cu/ SrTiO3 is also the sample with highest NOx chemisorption capacity. NOx chemisorption on the catalysts must be analyzed in detail. Comparison of Figure 3a (with soot) and 3b (without soot) allows us to conclude that the presence of soot diminishes the amount of NOx chemisorbed on the catalysts, since the NOx removal levels in the blank experiments are higher than those in the presence of soot. Considering all these experimental observations, the process occurring around 425 °C seems to be, first, the catalytic conversion of NO to gas phase NO2 or to NO2chemisorbed species. If soot is available, part of the NO2 produced reacts with soot and becomes NO again, and the remaining gas phase NO2/NO2-chemisorbed species is stored on the catalysts. On the contrary, if there is no soot available, NOx chemisorption is the only NOx removal pathway. At higher temperature (>500 °C) actual NOx reduction to N2 by soot takes place. The potential formation of N2O as NOx reduction product was ruled out in experiment followed by gas chromatography. 3.4. Temperature Programmed Reduction by H2. In order to understand the different activity of the catalysts prepared and tested, some characterization techniques were used, and TPR-H2 provided information about the reducibility of the different catalysts (Figure 5). H2 was consumed by all the catalysts around 200-350 °C due to copper oxide

FIGURE 5. TPR-H2 characterization of catalysts.

TABLE 3. Quantitative Characterization of the Surface of Catalysts Determined by XPS element

Cu/SrTiO3 (atomic %/wt. %)

Cu/MgTiO3 (atomic %/wt. %)

Cu O Ti Sr or Mg C

4.0/7.1 54.5/24.2 5.9/7.9 23.3/56.7 12.2/4.1

1.6/4.9 67.4/52.1 13.5/31.3 2.6/3.1 14.9/8.6

reduction, and the support significantly affects the reducibility of copper, being that copper catalysts supported on perovskite are the most easily reduced. For instance, copper oxide supported on SrTiO3 is reduced at 50 °C lower than copper oxide supported on SrAl2O3. Among the catalysts, Cu/SrTiO3 (the most active catalyst for soot oxidation by NOx/O2) is reduced by H2 at the lowest temperature. 3.5. XPS and CHOL Decomposition Characterization. XPS analysis provided additional information about the surface of the Cu/perovskite catalysts, and the surface composition determined by this technique is included in the Table 3. The surface concentration of Sr is significantly higher than that of Mg, which could be related to the cations size (Mg2+ 1.03 Å versus Sr2+ 1.40 Å) since the larger size of Sr2+ could be difficult to introduce into the bulk. The CHOL decomposition experiments performed with the SrTiO3 and MgTiO3 supports are consistent with this fact. Both titanates present similar activity in CHOL conversion (20% conversion), suggesting that they contain the same amount of active sites for this reaction, including both acid and basic sites. Nevertheless, SrTiO3 has to exhibit higher contribution of basic sites at the surface than MgTiO3, since the selectivity toward CHON formation is higher for SrTiO3 than for MgTiO3 (CHON/CHEN ) 1.58 and 1.05, respectively). Taking into account the basic character of alkaline-earth elements, the accumulation of Sr at the surface of SrTiO3, detected by XPS, explains the suggested higher concentration of basic sites at the surface of this catalyst. XPS results (Table 3) also point out the accumulation of copper at the surface of the catalysts studied, since copper wt. % values obtained by XPS (7.1 and 4.9 for Cu/SrTiO3 and Cu/MgTiO3, respectively) are higher than those determined by ICP (close to 4 wt. %). Moreover, the most active catalyst (Cu/SrTiO3) presents a higher copper surface concentration than Cu/MgTiO3. These results are in agreement with the XRD results commented above (Section 3.1) because XRD peaks of CuO are more defined for Cu/MgTiO3, which indicates a larger crystalline size than those corresponding to Cu/SrTiO3.

FIGURE 6. XPS characterization of selected catalysts: (a) Cu 2p3/2 and (b) O 1s. The XPS surface characterization suggests that the heterogeneity of the Cu/SrTiO3 surface has a positive effect on the catalytic activity of copper because of improved surface oxygen mobility that allows better reduction and further oxidation of copper during the soot oxidation reactions. This is in line with the Cu 2p3/2 and O 1s XPS analysis of Cu/SrTiO3 and Cu/MgTiO3 catalysts included in Figure 6. The shapes of the Cu 2p3/2 bands suggest differences in the amount of copper species present on each catalyst. Reduced copper species, such as metallic copper or Cu2O usually appear below 933 eV (27), and only a very small band centered at 931 eV is observed in the catalyst Cu/SrTiO3. This observation rules out the presence of significant amounts of reduced copper species in the fresh catalysts, as expected. CuO typically appears with binding energies higher than 933 eV (29), and this is the case of the catalysts studied. The presence of CuO in both samples is confirmed by the values of the copper Auger parameter (R), which is calculated as the sum of XPS binding energy and Auger peak kinetic energy, being 1851.0 and 1851.1 eV for Cu/SrTiO3 and Cu/MgTiO3, respectively. These are values typically reported for bulk CuO, and lower values have been reported for more reduced copper species such as Cu2O (R ) 1849.6 eV), Cu+ in tetrahedral and octahedral positions in spinels (R ) 1848.6 and 1849.6 eV, respectively) or Cu+ ions exchanged in zeolites (R ) 1847.2 eV) (30). The deconvolution of the normalized Cu 2p3/2 bands (Figure 6) shows two main bands with maxima at 933.2 ( 0.2 eV and 934.9 ( 0.1 eV, respectively. This shape suggests the presence of CuO species with different Cu/support interaction. CuO with binding energy around 933.2 eV (more abundant in Cu/SrTiO3 than in Cu/MgTiO3) is expected to be reduced more easily than CuO with binding energy around 934.9 eV. This is in agreement with the easiest reduction of VOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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copper in the Cu/SrTiO3 catalyst during TPR-H2 experiments and also with the highest activity of this catalyst for soot oxidation. Figure 6b shows the O 1s spectra for the as-prepared samples, and they are similar to those obtained by Tang et al. (31) for CuO/CeO2 catalysts. In the CuO/CeO2 catalysts, the band at lower energy (around 529.5 eV) was attributed to oxygen in ceria, and the second band at higher energy to the lattice oxygen vacancies, which were related to highly polarized oxide ions at the surface (and interfaces) of the nanocrystallites with an unusual low coordination. Figure 6b shows that the relative intensity of the band centered on 532.5 eV is higher for Cu/SrTiO3 than for Cu/MgTiO3, which could be related to the presence of a higher concentration of weakly bound oxygen in Cu/SrTiO3. This is also in agreement with TPR-H2 characterization of support and catalysts and also with the highest catalytic activity of Cu/ SrTiO3. To summarize, it can be concluded that the activity of copper catalysts for soot oxidation by O2 and NOx/O2 is notably affected by the support, and as a general conclusion, perovskite-supported copper is more active than copper supported on alumina. The supports tested do not contribute directly to the activity of copper catalysts, since their activity is poor in comparison to that of the Cu-containing catalysts, but they affect the activity of copper. Among the samples studied, Cu/SrTiO3 is the most effective for soot oxidation by NOx/O2. Under the experimental conditions used, the uncatalyzed soot oxidation by NOx/O2 presents a considerable rate from 600 °C, and Cu/SrTiO3 lowers this threshold by 100 °C, approximately. This catalyst is also the most effective for NOx chemisorption around 425 °C. The best activity of the catalyst Cu/SrTiO3 for soot oxidation in NOx/O2 gas mixtures can be attributed to the improved redox properties of copper originated by Cusupport interactions. This could be related to the presence of weakly bound oxygen on this sample, and the heterogeneity of the support surface due to the incorporation of large Sr2+ cations, hardly introduced into the bulk, could promote the presence of this weakly bound oxygen. It is proposed that the role of copper is to convert NO into NO2 following a redox cycle where the copper active sites are oxidized and reduced consecutively. The copper species present in the catalyst Cu/SrTiO3 can be reduced more easily than those in other supports, and for this reason, this catalyst seems to be the most effective way to convert NO into NO2, which explains its high activity for soot oxidation.

Acknowledgments We thank the financial support of the MEC (project CTQ200501358). F.E.L.S. thanks the University of Alicante (International Cooperation Office) for his thesis grant and A.B.L. thanks the Ramon y Cajal Program and the GV for the contract funding.

Supporting Information Available XRD characterization, catalytic activity of the supports, NOx removal in blank experiments, details about the preparation of the supports and about the experimental set-ups used and procedures followed for characterization and catalytic tests. This material is available free of charge via the Internet at http://pubs.acs.org.

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