MCM-41 for the catalytic oxidation of diesel soot

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Microporous and Mesoporous Materials 113 (2008) 562–574 www.elsevier.com/locate/micromeso

Synthesis and characterization of CuO/Nb2O5/MCM-41 for the catalytic oxidation of diesel soot Fillipe A.C. Garcia a, Junia C.M. Silva a, Julio L. de Macedo a, Jose´ A. Dias a,*, Sı´lvia C.L. Dias a,*, Geraldo N.R. Filho b a

Universidade de Brası´lia,1 Instituto de Quı´mica, Laborato´rio de Cata´lise, Caixa Postal 4478, Brası´lia-DF 70904-970, Brazil Universidade Federal do Para´, Centro de Cieˆncias Exatas e Naturais, Rua Augusto Correa 1, Bele´m-PA 66075-010, Brazil

b

Received 18 May 2007; received in revised form 10 December 2007; accepted 14 December 2007 Available online 7 February 2008

Abstract This work deals with the preparation and application of new catalysts based on CuO, Nb2O5, and Si-MCM-41 for the abatement of diesel soot particulates. The properties of Si-MCM-41 mesoporous material were enhanced by studying the effect of the template removal method on specific surface area, terminal silanol groups, and long-range ordering. Template removal experiments indicated that thermal decomposition by double-step calcination at 300 °C for 3 h and 550 °C for 3 h produced a material with the best morphological characteristics. This material was used to prepare CuO/Nb2O5/MCM-41 catalysts with a 1:1 mass ratio (CuO:Nb2O5) and 2, 5, 10, 15, and 25 wt.% loadings. The incorporation of CuO and Nb2O5 leads to the formation of well-dispersed small crystallites and an amorphous phase, respectively, for the samples with lower oxide loadings. For the samples with higher oxide loadings, the crystallites increase in size, leading to the formation of copper–niobium mixed oxide phases and agglomerates. All the prepared materials presented considerable activity towards the oxidation of diesel soot particulates, shifting the maximum oxidation temperature (Tox) to lower values. The most promising catalyst was CuO/Nb2O5/MCM-41 with 25 wt.% of each oxide, which presented an onset temperature of 388 °C and appreciable activity at temperatures as low as 450 °C without the addition of any chemical promoter. Ó 2007 Elsevier Inc. All rights reserved. Keywords: MCM-41; Diesel soot; Supported catalyst; Copper oxide; Niobium pentoxide

1. Introduction Microporous materials are among the most used catalysts for industrial processes such as catalytic cracking and isomerization of crude oils [1–4]. Although possessing outstanding catalytic properties (e.g., high specific surface area, shape, and isomer selectivity), these materials have serious limitations regarding reactants with kinetic diameters higher than 0.75 nm [5]. Another limitation is related

*

Corresponding authors. Tel.: +55 61 3307 2162; fax: +55 61 3368 6901. E-mail addresses: [email protected] (J.A. Dias), [email protected] (S.C.L. Dias), [email protected] (G.N.R. Filho). 1 http://www.unb.br/iq/labpesq/qi/labcatalise.htm. 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.12.017

to mass diffusion of reactants and products inside their pores. Mobil Oil Company’s Research and Development Laboratory created a new family of silicon and silicon–aluminum mesoporous molecular sieves in order to solve the problems presented by microporous materials. MCM-41 (Mobil Composition of Matter) belongs to this family and exhibits a hexagonal system of adjustable pores from 1.5 to 10 nm, specific surface areas higher than 700 m2 g1 [6–8], and channels with variable depths depending on synthesis and template removal methods. In order to improve the catalytic activity of MCM-41, silicon atoms have been isomorphically substituted by several transition metals, such as titanium [9–14], zirconium [15], and iron [16–18]. Another method to incorporate such

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metals in MCM-41 is to support metal promoters in the silicon matrix and to calcine the solids in order to produce the metal oxides [19–22]. Promising catalytic properties were observed for the selective oxidation of large organic molecules using TiMCM-41 catalyst and H2O2 or tert-butylhydroperoxide (TBHP) as the oxidative agent. It was observed that the catalyst was more active in those reactions than Ti-Beta zeolite due to better diffusion characteristics [9]. Other applications of modified MCM-41 materials include Friedel–Crafts alkylations [23], polyethylene, n-heptane and naphta cracking [24,25], acetylations [26], and Diels–Alder reactions [27]. Catalysts containing supported metal oxides have been studied because of the variety of interactions among the different phases [28,29]. These materials exhibit new properties and applications depending on the oxide loading and specific surface area of the support [30–32]. Niobium pentoxide and copper oxide are among the most studied oxides. Nb2O5 is a strong solid acid even though it has a high hydrophilic character, which allows various interesting applications [33–35]. CuO supported on different matrices is applied in many areas including environmental catalysis [36,37]. Copper oxide is highly dispersed in acidic supports, which gives rise to high activity and selectivity for these materials [38,39]. The diesel engine is responsible for emission of particulate matter, nitrogen and sulfur oxides, and unburned hydrocarbons [39]. Particulate matter (PM) is fine solid or liquid particles suspended in a gaseous phase, which form aerosols [40]. Stricter legislation concerning particulate emissions were developed because of the environmental and health issues often associated with them. The improvement in emission control technologies frequently involves catalytic oxidation reactions carried out on particulate filters [39–42]. The most active catalysts reported in the literature included oxides such as: V2O5, CuO, MoO3, MnO2, Fe2O3, WO3, La2O3, and CeO2 [43,44]. Although, both silicon and non-silicon mesoporous materials are widely applied to important oxidation processes, their applications to the oxidation of diesel soot particulates are very limited [45]. The goal of this work is to enhance the properties of SiMCM-41 mesoporous material through the study of template removal methods and their effects on specific surface area, percentage of terminal silanol groups, and long-range ordering. The material with the best morphological properties was used as a support to produce CuO/Nb2O5/MCM41 catalysts, which were characterized and applied in the abatement of diesel soot particulates. The structural characterization was performed using X-ray powder diffraction (XRD), elemental analysis by X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), Fourier transform Raman spectroscopy (FT-Raman), magic angle spinning nuclear magnetic resonance (MAS-NMR) of 29 Si, thermal analysis (TG and DTA), and scanning electron microscopy (SEM).

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2. Experimental 2.1. Synthesis and characterization of Si-MCM-41 Si-MCM-41 was synthesized at room temperature using a modified precipitation method [46] based on the work of Cai et al. [47]. The modified method consisted of adding 510 mL of concentrated ammonium hydroxide (NH4OH, Vetec), 675 mL of quartex water and 18.25 mL of cetyltrimethylammonium chloride (CTMA – CH3(CH2)15N(CH3)3Cl, Aldrich) to a 3 L round-bottom flask. The solution was equilibrated under stirring conditions at 30–35 °C for 30 min and 25 mL of tetraethylorthosilicate (TEOS – Si(OC2H5)4, Fluka) were added. The precipitated solid was stirred continuously for 2 h, filtered, and washed several times with quartex water to remove the remaining chloride ions. The presence of these ions was tested in the wash water using a 0.2 mol L1 solution of silver nitrate (AgNO3, Fluka). The molar ratio used in the synthesis was 525:69:0.125:1 (H2O:NH4OH:CTMA:TEOS). The solid was treated at 100 °C for 24 h in a vacuum oven (National Appliance Company), with a heating rate of about 5 °C min1, before the removal of the organic template. The template was removed using either a thermal or an extraction method. The thermal method consisted of the calcination of the sample in a muffle furnace (EDG model EDG3PS) at 14 °C min1, using either a single step at 550 °C for 6 h or a double step (the first at 300 °C for 3 h followed by heating up to 550 °C and equilibrating for another 3 h). The template extraction was performed by mixing the as-prepared sample with 50 mL of ethanol (Vetec) and placed in a reflux system for 4 h. Then it was filtered and washed with 60 mL of hydrogen peroxide to oxidize any remaining template. This sample was extracted once again for another 4 h using the same method. The samples treated by the thermal method will be referred to as either single step (SS) or double step (DS), while the samples extracted with ethanol will be referred to simply as extracted once (X) or twice (X2), depending on the conditions described above. The Si-MCM-41 was characterized before and after template removal using XRD, FTIR, FT-Raman, 29Si MAS-NMR, TG/DTG/DTA, and surface area measurements using the thermal desorption method [48].

2.2. Preparation of CuO/Nb2O5/MCM-41 materials The supported catalysts were prepared by the impregnation method with a 1:1 mass ratio (CuO:Nb2O5). The metal precursors, Cu(NO3)  3H2O (Vetec) and NH4[NbO (C2O4)2(H2O)2]  (H2O)n (CBMM), were added to the prepared Si-MCM-41 (DS) and ethanol in order to obtain a 1:10 ratio (mass to volume) of solid to solvent. The amounts of copper and niobium oxides were 2, 5, 10, 15, and 25 wt.%. Each slurry sample was kept at 80 °C under ultra-sonic stirring (Branson, model 1210) until all ethanol

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evaporated. The solid was then placed in a vacuum oven at 120 °C for 24 h and stored for further treatment. The CuO/Nb2O5/MCM-41 catalysts were characterized before and after the decomposition of the metal precursors using XRD, XRF, FTIR, TG/DTG/DTA, SEM, and surface area measurements using the thermal desorption method [48]. 2.3. Characterization techniques 2.3.1. X-ray diffraction (XRD) X-ray powder diffraction patterns were recorded on a Rigaku D/Max-2A/C spectrometer with Cu Ka radiation ˚ (40 kV and 20 mA). The Bragg angle range of 1.5418 A was scanned from 2° to 90° at 2° min1. Crystalline phases were identified by comparison with PDF database from ICDD included in the software JADE 3.0 and diffraction data reported in literature. 2.3.2. X-ray fluorescence (XRF) X-ray fluorescence spectra were obtained using an EDX 700 instrument (Shimadzu) with a rhodium X-ray source tube (40 kV). The analytical curves for copper and niobium elemental analysis were built using CuSO4 pellets diluted with H3BO3 and a niobium atomic absorption standard solution (Aldrich). 2.3.3. Thermal analyses (TG/DTG/DTA) Thermal analysis data were obtained using a 2960 simultaneous DSC-TGA (TA Instruments), with a heating rate of 10 °C min1 from room temperature (ffi25 °C) up to 700 °C under synthetic air flow (99.999%) of 100 mL min1. Platinum pans loaded with about 15 mg of sample were used in all runs with a-Al2O3 as reference. 2.3.4. Fourier transform infrared spectroscopy (FTIR) Infrared spectra were recorded on a BOMEM MB-100 (Hartman & Braun) with 128 scans and spectral resolution of 4 cm1 in dried 1 wt.% KBr (Merck) pellets. 2.3.5. Fourier transform raman spectroscopy (FT-Raman) FT-Raman spectra of samples were obtained at room temperature (ffi25 °C) with 256 scans and resolution of 4 cm1 on a Bruker FRA 106/S module attached to a Bruker Equinox 55 spectrometer. The wavelength and laser (Nd-YAG) power were 1064 nm and 100 mW, respectively. A liquid N2-cooled Ge detector collected the Raman signal.

NMR data were deconvoluted using a Gaussian mathematical fit. 2.3.7. Scanning electron microscopy (SEM) The scanning electron micrographs were obtained in a ZEISS equipment model LEO 1430, with 10 kV and 90 mA of beam current. The samples were metallized and supported on a carbon tape and the micrographs were obtained under vacuum conditions. 2.3.8. Surface area measurements Surface area measurements were made through the thermal desorption of probe molecules. In this method, the sample is heated in a helium atmosphere in a TG/DTG apparatus from room temperature up to 300 °C with a heating rate of 5 °C min1. After the oven is cooled to 30 °C, two drops of distilled n-butanol are added to the sample pan, and the oven is closed for 10 min to equilibrate the sample with the alcohol. Then, the oven is opened again to check if the sample presents a humid aspect. If the sample is dry, another drop of n-butanol is added, and the oven is closed. Finally, a second TG/DTG run is made at the same conditions as the first. This relatively new method has been validated in modified mesoporous materials [48,49] and has many advantages over the classical BET method, such as faster analysis and lower operational cost. The results obtained from the second TG/DTG run provide information to calculate the pore volumes and surface areas, which are similar to the ones obtained by the BET method. 2.4. Catalytic tests for diesel soot oxidation The catalyst activities were evaluated in the oxidation reaction of standard soot produced in a fuel burner (PrintexÒ U – Degussa, control number 5124998). The PrintexÒ U was mixed with the catalyst at a 1:20 mass ratio, respectively. The mixing process was carried out in an agate mortar in order to promote a tight contact between the components. The mixture (ffi15 mg) was submitted to a temperature ramp from room temperature (ffi25 °C) up to 800 °C under air flow (110 mL min1) at a heating rate of 10 °C min1. The curves (TG/DTG/DTA) were recorded using a 2960 simultaneous DSC-TGA. 3. Results and discussion 3.1. Characterization of Si-MCM-41

2.3.6. Nuclear magnetic resonance (MAS-NMR) The NMR experiments were performed at 7.05 T with a Varian Mercury Plus spectrometer equipped with a 7 mm Varian probe (zirconia rotors with torlon cap). Spectra of 29 Si (59.609 MHz) were recorded at the magic angle spinning (MAS) at speed of 3 kHz, single pulse duration of 5.5 ls (p/2), a recycle delay of 20 s and 500 scans. The spectra were indirectly referenced to kaolin (91.5 ppm). MAS-

The XRD patterns of the prepared materials presented a peak at 2h ffi 2.6° associated with the (1 0 0) plane of the hexagonal MCM-41 crystalline lattice. There was an increase in the intensity of the peak at 2h ffi 2.6° after all template removal experiments, and the samples treated by thermal methods (single-step – SS or double-step – DS) presented all the main peaks of MCM-41 [50], indicating the

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importance of the temperature on the condensation of the hexagonal channels. The DTG curve of the material as prepared showed three mass losses at about 238, 298, and 348 °C, associated with Hoffman’s elimination of trimethylamine, desorption and pyrolysis of the organic chains, and the complete oxidation of the template, respectively [51]. Mass loss of water by desorption was also observed at about 100 °C (not shown), and dehydroxylation of silanol groups was observed (above 400 °C). The samples treated by the thermal method did not present any of the mass losses cited above, except the one at 100 °C indicating the oxidation of the organic template and the presence of silanol groups that are more thermally stable than the groups from extracted samples. The low stability of the Si–OH groups on the extracted samples is expected, since these materials are heated above 400 °C soon after their generation in the TG/DTG equipment. The DTG curves of the extracted samples showed the incomplete removal of the template. The sample extracted twice (X2) had less mass losses compared to the one extracted only once, which is expected since the amount of template extracted increases with the number of extractions. The quantitative analysis of the TG/DTG curves showed that the mass loss associated with the dehydroxylation of the silanol groups of the thermally treated materials is smaller than that for the extracted materials. This result is in agreement with XRD data, which showed that the extracted materials presented only a bi-dimensional ordering and, consequently, more Si–OH groups [52]. In addition, the DS material presents silanol groups that are more stable than the SS material regarding thermal dehydroxylation. Another interesting result regarding the silanol mass loss of the materials extracted once (X) and twice (X2) was observed. Since a higher amount of the organic template was removed by double extraction, a higher mass loss in the temperature range of 400–800 °C would be expected. However, a small decrease in mass loss was observed for the X2 sample, probably because of the thermal treatment at 100 °C for 24 h that was used to dry the sample before it undergoes the second extraction. The Raman spectra of the solid as prepared presented strong bands associated with the stretching of C–H from CH3 and CH2 of the non-polar chain and C–N bonds and bending of CH3 from the trimethylammonium group, respectively. There were also weak bands in the region from 1000 to 750 cm1, which are characteristic of the fingerprint of CTMA [53]. These bands decreased in intensity for the extracted samples (X and X2) and disappeared completely for the samples treated by the thermal method (SS and DS). MCM-41 produces only a poor signal due to the presence of hot bands [54]. The FTIR spectra of the solid as prepared presented bands at 3456 and 1630 cm1 related with stretching and bending OH vibrations from physisorbed water; 2920, 2854, 1470 cm1 related to CH2 and CH3 vibrational

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modes from CTMA [55]; and 1230, 1080, 966, 805, and 460 cm1 related to the silicon matrix of MCM-41 [56– 59]. The CTMA bands decrease in intensity for the extracted samples and disappear completely for those treated by the thermal method, which was also observed on Raman spectra. The 29Si MAS-NMR spectra of the samples treated with different methods showed peaks at about 104.3, 94.5, and 86 ppm (when present). These peaks are related to the chemical environments in which the silicon atom is bonded to four oxygen atoms that are connected to four other (Q4), three other (Q3), and two other (Q2) silicon atoms, respectively. The rest of the coordination sphere is completed with OH terminal groups [60]. The deconvolution of the MAS-NMR spectra provided the condensation degree, through the ratio (Q2 + Q3)/Q4 [61], the relative proportion of different silicon environments and the molar percentage of silanol groups. The MAS-NMR results showed that thermal elimination of CTMA causes a decrease in the condensation degree and of the percentage of silanol groups due to their dehydroxylation forming siloxane surface groups and collapsing some of the mesopores. The solid sintering, condensation of hexagonal channels, and the dehydration are also important factors [61]. The Q2 chemical environment is probably formed after the elimination of CTMA. The ethanol extraction causes an increase in condensation degree as well as the formation of Q2 chemical environments in the samples treated by this method. This occurs because the temperature of ethanol reflux does not cause dehydroxylation of silanol groups already present on MCM-41, increasing Q3, Q2, and the molar percentage of Si–OH, as well as the condensation degree due to template removal. The surface area and pore volumes were calculated using a thermodesorption method [48,49]. The calculated pore volumes of all samples were within the expected range for mesoporous materials (between 0.7 and 0.9 cm3 g1). The SS and DS samples presented higher surface areas than the X and X2 samples, different from the behavior reported by Gomes et al. in the template extraction of Al-MCM-41 material [62]. However, Hitz et al. reported that Si/Al-MCM-41 samples, which were ion exchanged and calcined, presented higher surface areas than the ones extracted with salt solutions [63]. This behavior probably occurs due to the lower efficiency of the Si-MCM-41 extraction in comparison with the Al-MCM-41 extraction, which leads to a higher amount of CTMA adsorbed on the surface and a lower calculated surface area. The modified precipitation method associated with the thermal elimination of the organic template produced a material with surface areas well above the usual range reported in the literature (1000–1100 m2 g1) [49]. It should be noted that the calculated surface area results of Si-MCM-41 as prepared, extracted once (X) and twice (X2), possess a positive bias due to the decomposition of CTMA, which means that these values should not

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be interpreted as exact values but as parameters obtained at special conditions. The use of Si-MCM-41 as a catalyst and, more importantly, as a support is highly dependent on the morphological properties explored in this study, mainly the specific surface area and molar percentage of terminal Si–OH groups. Based on these characteristics the double step calcination process was the most efficient way to remove the organic template producing materials with high surface area, long-range ordering, and more stable silanol groups. The detailed experimental data are provided as supplementary material. 3.2. Characterization of CuO/Nb2O5/MCM-41 The CuO/Nb2O5/MCM-41 materials were synthesized by the wet impregnation method using ethanol as the solvent. The solids obtained after drying the slurry were characterized using elemental analysis (XRF), XRD, FTIR, surface area measurements, and thermal analysis (TG/ DTG/DTA). The elemental analysis of the supported materials was performed after calcination in order to determine the actual loadings of each prepared material (Table 1). The XRF results demonstrated that the copper and niobium contents were all slightly below the expected nominal value with experimental errors within 5%. These results are mainly related to water content on copper nitrate and niobium oxalate precursors. In addition, the CuO:Nb2O5 ratio did not differ significantly among all the samples. Thus, the samples will be referenced in the following sections using the nominal values. The XRD patterns of the samples as prepared presented a decrease in the intensity of the peaks associated with MCM-41 structure proportional to the increase in CuO and Nb2O5 loadings (Fig. 1). In addition to that, the sample with 25 wt.% of CuO did not present any reflections in the low angle region (2° < 2h < 10°), which suggests that the support was completely covered by the metal precursors. Peaks were observed at 2h ffi 13°, 14.9°, 17.3°, and 22.9° in the sample with 25 wt.% oxide loadings. The first peak at 2h ffi 13° is related with the 0 0 1 plane of copper nitrate (JCPDS No.74-1749), while the other four peaks are associated with ammonium niobium oxalate complex (JCPDS = 83–1993).

Table 1 Elemental analysis of CuO/Nb2O5/MCM-41 calcined at 300 °C for 6 h Catalysta

Cu

Nb

Cu/Nb ratio

2 5 10 15 25

1.89 4.71 9.63 14.11 24.43

1.92 4.85 9.82 14.90 24.95

0.98 0.97 0.98 0.95 0.98

a

Nominal catalyst oxide loadings (wt.%).

The niobium complex main peak was observed only in the samples with oxide loadings higher than 5 wt.%. This indicates that the complex remains mainly on the external surface since the niobium complex has a high kinetic diameter, which facilitates the coverage of the hexagonal channels of MCM-41 pores. This interpretation is in agreement with XRD results in the low angle region that showed the decrease in all peaks associated with MCM41 structure. In addition, the copper nitrate reflection was barely observed even in the sample with 25 wt.%. This might have happened due to the low kinetic diameter of copper nitrate allowing the occurrence of diffusion processes through those channels and the formation of small and well-dispersed nanocrystallites on the samples with lower oxide loadings. The differential diffusion between copper and niobium precursors is related to the kinetic diameter and to the steric hindrance of the metal complexes, which are higher for the niobium complex. This interpretation is confirmed by SEM micrographs that did not present any large copper agglomerates. The XRD pattern in the low angle region of the samples after calcination at 300 °C for 6 h showed all the main peaks of MCM-41 structure (2h ffi 2.6°, 4.4°, and 5.1°) for the sample with 2 wt.% (Fig. 2). The peaks at 2h ffi 4.4° and 5.1° were not present for the samples with higher oxide loadings, while the main peak decreased in intensity but remained present even for the sample with 25 wt.%. In addition, the XRD pattern showed two distinct peaks, which are associated with the formation of CuO crystallites (PDF #05-0661), at 2h ffi 35.5° and 38.7° only on the sample with 25 wt.%. No Nb2O5 peaks were observed, which is in agreement with a previous work that reported the formation of an amorphous niobium phase at this temperature [19]. The absence of CuO peaks in the samples with oxide loadings equal to or lower than 15 wt.% does not exclude the formation of CuO crystallites, since it was also not observed for CuO/SiO2–Al2O3 any peaks associated with those species below the monolayer coverage [64]. The appearance of CuO peaks in the sample with 25 wt.% is not evidence of monolayer coverage because of the partial blocking of the hexagonal channels by the niobium precursor, which precludes the deposition of niobium pentoxide and copper oxide in the inner pore walls. The XRD results of all samples are consistent with the formation of small and well-dispersed CuO and Nb2O5 species in the samples with oxide loadings up to 15 wt.%, which retains the hexagonal structure of MCM-41. In addition, the sample with 25 wt.% presented well-defined CuO crystallites and possible CuO–Nb2O5 mixed phases covering almost all the channels. The DTG curves of the samples without calcination showed a thermal behavior dependent on the oxide loadings (Fig. 3). The sample with 2 wt.% exhibited a broad mass loss associated with the decomposition of copper nitrate and some niobium complex. A peak at about 67 °C is associated with adsorbed water, and the other peak

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Fig. 1. XRD patterns of CuO/Nb2O5/Si-MCM-41, before calcination, with: 2 wt.% (a), 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and 25 wt.% of oxides (e).

Fig. 2. XRD patterns of CuO/Nb2O5/Si-MCM-41, calcined at 300 °C for 6 h, with: 2 wt.% (a), 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and 25 wt.% of oxides (e).

at ffi230 °C is related to the decomposition of the niobium precursor. The samples with 5–15 wt.% presented a small peak close to 200 °C related to the decomposition of the copper nitrate and two distinct peaks at ffi272 and 293 °C, which are characteristic of the two-step decomposition of ammonium niobium oxalate. The sample with

25 wt.% showed four distinct peaks at ffi68, 232, 272, and 293 °C associated with the desorption of water, decomposition of copper nitrate, and the two-step decomposition of the niobium complex. The DTG experiments were repeated after calcination at 300 °C in order to prove that all metal precursors were

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Fig. 3. DTG curves of CuO/Nb2O5/Si-MCM-41, before calcination, with: 2 wt.% (a), 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and 25 wt.% of oxides (e).

Fig. 4. DTG curves of CuO/Nb2O5/Si-MCM-41, calcined at 300 °C for 6 h, with: 2 wt.% (a), 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and 25 wt.% of oxides (e).

completely decomposed to the oxides after 6 h (Fig. 4). The DTG curves of the materials after calcination presented only one peak at ffi55 °C, associated with desorption of water, and a broad mass loss at ffi490 °C related with the

dehydroxylation of the terminal silanol groups. This mass loss is higher for the materials with lower oxide loadings, which is in agreement with the interpretation that the metal precursors bond to silanol groups [20], decreasing their

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number after calcination and thus, the extension of the dehydroxylation process. The thermal analysis (TG/DTG/DTA) of the prepared materials provides evidence that all metal precursors are indeed decomposed after calcination at 300 °C for 6 h and that Nb2O5 is in the amorphous phase, since no phase transition was observed on the DTA curves (not shown). The FTIR spectra of the samples before calcination presented strong bands at 3440, 3180, 1716, 1630, 1400, 1232, 1080, 955, 800, and 460 cm1 (Fig. 5). The bands at 3440, 3180, and 1630 cm1 are related to O–H stretching vibrations of water interacting through hydrogen bonding with silanol groups, Debye–Hu¨ckel interactions with the oxalate groups of the niobium complex, and O–H bending, respectively. The bands at 1232, 1080, 955, 800, and 460 cm1 are characteristic of the silicon matrix of MCM-41 and are associated with the asymmetric stretching vibrations of the tetrahedron bonds [56], Si–O–Si groups, Si–OH stretching [57,58], symmetric stretching, and bending of Si–O–Si [58,59]. The bands at 1716 and 1400 cm1 are related to two vibrational modes: C@O and N–H stretching from the counter ion (ammonium) [65–67]. The other band expected at 1680 cm1 and associated with a combination of C–O and C–C stretching vibrations could not be observed in Fig. 5 due to the overlap with the bands at 1716 and 1630 cm1. The characteristic bands of copper nitrate are usually observed at 1450, 1298, and 950 cm1, and are associated with different stretching vibrations of the N–O bonds [67]. These vibrations could not be clearly observed

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due to overlap with the vibrations at 1400, 1232, and 955 cm1. The FTIR spectra of the samples after calcination at 300 °C for 6 h presented important differences regarding the metal precursors bands (Fig. 6). It can be observed that the peaks at 3180, 1716, and 1400 cm1 completely disappeared, indicating that all niobium oxalate was decomposed during thermal treatment, while a band at 808 cm1 appeared. This band is associated with Nb–O– Nb and Nb–O stretching vibrations [68] and with the shift of the band at 950–966 cm1, which indicates the presence of Nb2O5 over the MCM-41 surface. The copper oxide bands, usually present at 588, 534, and 480 cm1 [69], could not be clearly identified because of the background produced by the MCM-41 support. However, the small shoulder at 580 cm1 might be attributed to copper oxide species. The specific surface area was calculated after impregnation of the metal precursors and after the calcination procedure in order to evaluate this parameter of the materials in their final form. The results from the desorption curves are shown in Table 2, so that it can be observed that the surface area decreases as the oxide loadings increase. This result is expected since the incorporation of metal oxide species forms crystallites with lower surface area, which may even block the hexagonal channels for the samples with 25 wt.% loadings. In addition, the sinterization of the oxide crystallites during calcination contributes to the condensation of catalyst particles, which produces materials with lower surface areas. Despite all the factors mentioned above, all the synthesized materials

Fig. 5. FTIR spectra of CuO/Nb2O5/Si-MCM-41, before calcination, with: 2 wt.% (a), 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and 25 wt.% of oxides (e).

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Fig. 6. FTIR spectra of CuO/Nb2O5/Si-MCM-41, calcined at 300 °C for 6 h, with: 2 wt.% (a), 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and 25 wt.% of oxides (e).

Table 2 Surface area measurements of CuO/Nb2O5/MCM-41 calcined at 300 °C for 6 h Catalysta

Surface area (m2 g1)

Pure MCM-41 2 5 10 15 25

1308.6 917.5 906.1 836.1 708.9 494.3

a

Nominal oxide loadings (wt.%) on the catalyst.

presented specific surface areas close to or greater than 500 m2 g1, which is considerably higher than the ones obtained for common supported catalysts [64,70–72]. The scanning electronic microscopy (SEM) results of the supported catalysts are presented in Fig. 7. The formation of spherical particles was observed for Si-MCM-41, which exhibited also a homogeneous particle size distribution (d ffi 500 nm). The influence of the copper oxide and niobium pentoxide on the catalyst morphology can be observed by the presence of small white dots on the micrographs of the samples with low oxide loadings (Fig. 7b and c). In addition, the impregnation of 10 wt.% on Si-MCM41 generated an increase in the number and size of these white dots but also the formation of small placket aggregates with widths of approximately 2 lm (Fig. 7d). These agglomerates cover completely the catalyst surface with 15 wt.% loadings, which indicates that the monolayer coverage is reached between 10 and 15 wt.% and corroborates the interpretation of XRD, FTIR, and surface area mea-

surements. The SEM micrograph of the catalyst with 25 wt.% showed the formation of small copper and niobium species over a mixed oxide monolayer and some large aggregates with micrometric size (Fig. 7f). 3.3. Catalytic oxidation of diesel soot particulates The evaluation of catalytic activity was conducted out in a model oxidation reaction of particulates from diesel engines. It should be mentioned that the comparison of experimental results is not always straightforward since different particulate sources, as well as other catalytic test parameters, lead to different catalytic activities. The catalysts were mixed with a model particulate material (PrintexÒ U – Degussa) with a 1:20 (soot:catalyst) mass ratio. The mixture was submitted to a TG/DTG run, from room temperature up to 700 °C, in order to record its thermal profile. The activity of the catalysts can be compared by several parameters such as the onset temperature, oxidation range, and/or the temperature of the oxidation peak. These parameters will be used in the present work since they are the most frequently used in the literature [73,74]. The oxidation profiles of isolated PrintexÒ U, as well as the activity of Si-MCM-41, CuO, and Nb2O5, were also studied so that the thermal profiles of each catalyst could be compared with these materials in order to confirm the improvement in activity with the preparation of the supported materials. The DTG curve of PrintexÒ U showed mass losses at about 532, 566, 600, and 622 °C, which are associated with an initial oxidation step of the hydrocarbons adsorbed on the particulates (532 °C), their complete

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Fig. 7. SEM images of Si-MCM-41 (DS – double step calcined) (a) and the supported catalysts containing: 2 wt.% (b), 5 wt.% (c), 10 wt.% (d), 15 wt.% (e) and 25 wt.% of oxides (f), calcined at 300 °C for 6 h.

oxidation (566 and 600 °C), and the oxidation of the particulates themselves (622 °C) (Fig. 8a). The temperature at which all catalytic activities are compared is 622 °C, since it was the highest oxidation temperature of PrintexÒ U. The DTG curves of Si-MCM-41, Nb2O5 and CuO showed a single oxidation step in which both the adsorbed hydrocarbons and the particulates oxidize to CO and CO2 (Fig. 8b–d). It should be noted that an increase in activity is associated with a decrease in the maximum oxidation

temperature (Tox). Thus, the materials can be arranged in the following activity order: Si-MCM-41 < Nb2O5 < CuO. The low temperature shift observed for Si-MCM-41 was expected since it was reported in the literature that similar materials (SiO2, SiO2–Al2O3) present low oxidation activities [20,75]. The activities of CuO and Nb2O5 were previously reported in the literature [20]. However, they were never obtained, as far as we know, in the same conditions used in the present work, which is of great importance in

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Fig. 8. DTG curves for PrintexÒ U (a) and mixtures 1:20 of PrintexÒ U and: Si-MCM-41 (DS) (b), Nb2O5 (c) and CuO (d). Maximum temperatures (Tox) are listed for each material.

Fig. 9. DTG curves for the mixtures of 1:20 PrintexÒ U:CuO/Nb2O5/Si-MCM-41 (air at 110 mL min1 and rate of 10 °C min1) with: 2 wt.% (a), 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and 25 wt.% (e) of oxides. Maximum temperatures (Tox) are listed for each catalyst.

the comparison of experimental results, as mentioned above. The DTG curves of CuO/Nb2O5/MCM-41 presented a considerable temperature shift roughly proportional to the oxide loadings and interesting differences in thermal

behavior at lower temperatures (Fig. 9). It was observed that the materials with low oxide loadings (2–10 wt.%) presented almost the same Tox and the formation of a small peak at about 570 °C, which could be attributed to the oxidation of the particulates on isolated Nb2O5 species. The

F.A.C. Garcia et al. / Microporous and Mesoporous Materials 113 (2008) 562–574 Table 3 Thermal parameters of CuO/Nb2O5/MCM-41 calcined at 300 °C for 6 h when mixed with PrintexÒ U Catalysta

Onset (°C)

Tox (°C)

Offset (°C)

O.R. (°C)

2 5 10 15 25

469.51 453.19 427.96 401.58 388.25

523.47 525.45 527.08 514.43 536.25

583.28 577.47 594.23 589.77 591.72

113.77 124.28 166.27 188.19 203.47

The values are related to the onset, offset, maximum (Tox) and range of oxidation (O.R.) for PrintexÒ U. a Nominal oxide loadings (wt.%) of catalysts.

catalyst with 15 wt.% oxide loading presents a small shift in comparison to the materials with lower loadings and a more asymmetrical profile towards lower temperatures. The catalyst with 25 wt.% presented a wide oxidation range with three broad bands at ffi463, 536, and 576 °C. These bands could be related to mixed phases of CuO–Nb2O5 with different Cu:Nb molar fractions, which were formed due to the blocking of the hexagonal channels. The presence of these mixed phases of CuO–Nb2O5 could not be identified in the present work, but they are expected since the gradual blockage of the hexagonal channels favors the interaction between the active species. In addition, Braga et al. reported the presence of mixed phases on CuO/Nb2O5/SiO2–Al2O3 catalysts with 25 wt.% loadings [20]. The DTG curves of the catalysts were further analyzed in order to obtain other oxidation parameters (onset, offset temperatures and the oxidation range), which allow a better understanding of the catalytic process (Table 3). The detailed analysis of the DTG curves showed that the onset temperature presents a better correlation of activity and oxide loadings than the maximum temperature (Tox), since the catalyst with 25 wt.% presented a Tox higher than the one with 15 wt.%, and that the catalysts containing CuO/ Nb2O5/MCM-41 present a broader oxidation range as the oxide loadings increase. The results show that CuO/Nb2O5/MCM-41 with 25 wt.% oxide loadings is the most promising catalyst because it presented the lowest onset temperature, the broadest oxidation range, and considerable activity at temperatures as low as 450 °C. In addition, these results were obtained in the absence of chemical promoters usually used with this kind of catalysts [76–78], which is remarkable. In addition to that, this material showed considerable advantages, such as increased activity, due the high surface area of Si-MCM-41 and lower operational cost associated with the support in comparison with copper–niobium catalysts supported on SiO2–Al2O3 [20] and La2O3 [79], respectively. 4. Conclusions New CuO/Nb2O5/MCM-41 catalysts were prepared in a wide oxide concentration range (2–25 wt.%) and

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applied in the oxidation of diesel soot particulates. The effect of different template removal techniques was studied in order to obtain a support with higher specific surface area, number of silanol terminal groups, and long range ordering. This study indicated that among the methods, the thermal decomposition by double-step calcination at 300 °C for 3 h and 550 °C for 3 h produced a material with the best morphological characteristics. The characterization of the supported materials showed that the incorporation of CuO and Nb2O5 compromise the long range ordering and the access to the hexagonal pores by covering them, which is dependent on the concentration of the oxides. In addition, the copper and niobium species form well-dispersed small crystallites and an amorphous phase, for the samples with lower oxide loadings, respectively. On the other hand, they form bigger CuO crystallites and niobium pentoxide particles, as well as copper–niobium mixed oxide phases for the samples with higher oxide loadings. All the prepared materials presented considerable activity towards the oxidation of diesel soot particulates, shifting the maximum oxidation temperature (Tox) to lower values. The most promising catalyst was CuO/Nb2O5/MCM-41 with 25 wt.% presenting an onset temperature of 388 °C and appreciable activity at temperatures as low as 450 °C without the addition of any chemical promoter. Acknowledgments We acknowledge the financial support given by UnB-IQ (FUNPE), FINATEC, FINEP/CTPetro, FINEP/CTInfra, CAPES, and MCT/CNPq. The authors would like to thank Professor Edi Mendes Guimara˜es (IG/UnB) for XRD and Professor Ivoneide C.L. Barros (DQ/UFAM) for XRF measurements. Also, we want to thank CBMM and DEGUSSA for the niobium reagents and the PrintexÒ U, respectively. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso. 2007.12.017. References [1] F.J. Llopes, G. Sastre, A. Corma, J. Catal. 242 (2006) 195. [2] A.G. Bhavani, A. Pandurangan, J. Mol. Catal. A: Chem. 267 (2006) 209. [3] N. Kumar, O.V. Masloboichikova, L.M. Kustov, T. Heikkila¨, T. Salmi, D.Y. Murzin, Ultrason. Sonochem. 14 (2007) 122. [4] G.F. Ghesti, J.L. Macedo, V.C.I. Parente, J.A. Dias, S.C.L. Dias, Micropor. Mesopor. Mater. 100 (2007) 27. [5] L. Martins, C. Dı´lson, Quim Nova 29 (2006) 358. [6] A. Corma, Chem. Rev. 97 (1997) 2373. [7] J.S. Beck, J.C. Vartuli, W.J. Roth, J. Am. Chem. Soc. 114 (1992) 10834. [8] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710.

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