Structural Characterization and Selective Catalytic Reduction of

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Structural Characterization and Selective Catalytic Reduction of Nitrogen Oxides with Ammonia: a Comparison between Co/ZSM-5 and Co/SBA-15 Feng Bin, Chonglin Song, Gang Lv, Jinou Song, Xiaofeng Cao, Huating Pang, and Kunpeng Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp303830x • Publication Date (Web): 29 Nov 2012 Downloaded from http://pubs.acs.org on December 1, 2012

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The Journal of Physical Chemistry

Structural Characterization and Selective Catalytic Reduction of Nitrogen Oxides with Ammonia: a Comparison between Co/ZSM-5 and Co/SBA-15

Feng Bin, Chonglin Song,* Gang Lv, Jinou Song, Xiaofeng Cao, Huating Pang, Kunpeng Wang State Key Laboratory of Engines, Tianjin University, Tianjin 300072, P. R. China *Corresponding author. Tel.: +86-22-27406840-8020; fax: +86-22-27403750 E-mail address: [email protected] (C.-L. Song)

Feng Bin: Postdoctor, State Key Laboratory of Engines, Tianjin University, P. R. of China; Email: [email protected] Chonglin Song: Professor, State Key Laboratory of Engines, Tianjin University, P. R. of China; Email: [email protected] Gang Lv: Associate Professor, State Key Laboratory of Engines, Tianjin University, P. R. of China; Email: [email protected] Jinou Song: Associate Professor, State Key Laboratory of Engines, Tianjin University, P. R. of China; Email: [email protected] Xiaofeng Cao: PhD., State Key Laboratory of Engines, Tianjin University, P. R. of China; Email: [email protected] Huating Pang: PhD., State Key Laboratory of Engines, Tianjin University, P. R. of China; Email: [email protected] Kunpeng Wang: MSc., Laboratory of Engines, Tianjin University, P. R. of China; Email: [email protected] 1

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Abstract: Cobalt-containing catalysts supported on ZSM-5 zeolite and mesoporous siliceous SBA-15 were prepared and characterized by nitrogen sorption, X-ray diffraction,

scanning

electron

and

transmission

electron

microscopies,

energy-dispersive X-ray, Fourier transform infrared, ultraviolet-visible diffuse reflectance, X-ray photoelectron spectroscopies and temperature-programmed desorption of ammonia measurement. The effect of cobalt loading ratio on the selective catalytic reduction (SCR) of Nitrogen oxides (NOx) with ammonia was investigated. The existing Brønsted acid sites contributed to the cobalt species finely dispersed within the ZSM-5 zeolite, either as isolated cobalt ions anchored at α, β and γ sites, or as amorphous cobalt oxides enriched on the ZSM-5 surface. NOx conversion profiles of Co/ZSM-5 exhibited two peaks. The low-temperature peak (300 °C) was assigned to the amorphous and crystalline cobalt oxides. With increasing cobalt content, the intensity of low-temperature peak was enhanced monotonously, and the peak position remained constant. Increasing cobalt content promoted the high-temperature peak to shift toward lower temperatures. NOx conversion profiles of Co/SBA-15 only exhibited a high-temperature peak. For Co/SBA-15, the poor dispersion of cobalt species was derived from the absence of Brønsted acid sites. The activity of Co/SBA-15 catalysts was lower than that of the Co/ZSM-5 catalysts due to inactive cobalt ions anchored on isolated Si-OH groups, and agglomerated cobalt oxides within the SBA-15 channels blocking the reactant pathway to active sites. Keywords: Co/ZSM-5; Co/SBA-15; Structural characterization; Nitrogen oxide; Selective catalytic reduction 2


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1. Introduction Selective catalytic reduction (SCR) of nitrogen oxides (NOx) by ammonia (NH3) or hydrocarbons is an attractive technology for removing NOx from stationary and mobile sources. Current commercial SCR catalysts are generally composed of V2O5 and WO3 loaded on anatase TiO2 supports. Although these catalysts can effectively eliminate NOx pollutants, problems still need to be overcome. Examples include the undesired oxidation of SO2 to SO3,1,2 the formation of N2O as a byproduct at high temperatures,3 and the inherent toxicity of vanadium. The use of TiO2 as a support is also discouraging because its poor specific surface area (SSA) significantly hinders the NOx removal efficiency.4 To overcome these problems, one approach is to replace vanadium with alternative metal ions, and to then disperse them within a porous host of higher SSA. ZSM-5 zeolite is a crystalline inorganic polymer, consisting of a three dimensional (3-D) network of SiO4 and AlO4 tetrahedra linked by interconnecting oxygen ions. Whenever an Al3+ cation replaces a Si4+ cation, an additional proton (H+) is required to maintain electrical neutrality. These additional protons provide the zeolite a high degree of acidity and many substitution sites. Through ion exchange procedures within the zeolite, ZSM-5 based materials can exhibit remarkably high hydrothermal stability and excellent catalytic activity for NOx reduction, even under a highly oxidizing atmosphere.5 Among ZSM-5 based materials, Co/ZSM-5 is one of the most active and studied catalytic systems for the SCR of NOx by NH3 or hydrocarbons.6-13 Mesoporous siliceous SBA-15 is another promising support that has also attracted

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considerable recent attention. Its interest arises from features including its ordered pore structure, high surface area and uniform pore size distribution. Thanks to its thick walls (3–6 nm), the SBA-15 exhibits a higher thermal stability, which renders it suitable as a support in catalytic processes where thermal treatments are frequently encountered. Moreover, due to the confinement of ordered mesoporous channels, the growth of CoOx nanoclusters to larger Co3O4 particles can be limited in the mesoporous channels. Pure siliceous SBA-15 materials have low catalytic activities, and hence active sites are usually provided by introducing metals or their oxides into the mesoporous SBA-15 silica.14 Due to significant amount of Si-OH groups inside the mesopores, active species can be anchored either onto the framework through the substitution of hydroxyl groups to form metal-O-Si bonds,15 or by filling the void space in the pore via a tethering group. 16 The tunable pore size of SBA-15 makes it possible to tailor the metal oxides size over a nanometer range, and consequently enhance the dispersion of the cobalt active species. Wang et al.

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reported that the

mesoporous structure of SBA-15 was retained after cobalt grafting with up to 10% cobalt loading, and the cobalt species could be highly dispersed on the surface without large cobalt oxide particles being formed. Several investigations have been carried out related to cobalt-, copper-, iron- and manganese-containing SBA-15 catalysts, and confirmed that the corresponding metal nanoparticles anchored to the silica matrix resulted in excellent catalytic performances for the SCR of NOx.18-22 However, studies involving the SCR process of SBA-15 based catalysts are limited in number. The aim of this work is to compare the effect of microporous ZSM-5 and

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mesoporous SBA-15 supports on the dispersion of cobalt species, the particle size and morphology of cobalt and cobalt oxide, state of cobalt oxidation and acidity of catalyst, etc. The catalytic performance of the modified samples is studied in the selective catalytic reduction of NOx by NH3. Experimental results validate that three type cobalt species co-existing over Co/ZSM-5 are responsible for the high activity: ion exchanged Co2+ ions, exhibiting the low-temperature activity (300 °C). The Co3O4 crystal, together with few dispersed cobalt species, is found to be stabilized on the mesoporous SBA-15, and leads to a low activity. This study will be helpful in designing commercial molecule-sieve catalysts for diesel exhaust purification.

2. Experimental section 2.1. Catalyst preparation H/ZSM-5 with an atomic Si/Al ratio of 25 and crystallinity of 100% was supplied by Nankai University, Tianjin, P. R. China. SBA-15 was synthesized by hydrothermal treatment according to a report by Zhao et al.

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The chemical composition of the

reaction mixture was 4 g Pluronic P123 [(EO)20(PO)70(EO)20, Aldrich]: 0.041 mol tetraethyl orthosilicate: 0.24 mol HCl: 6.67 mol H2O. The mixture was maintained at 35 °C for 24 h and then transferred in a Teflon-lined autoclave and held at 100 °C for 48 h. The white solids were recovered by filtration, washed with deionized water, dried, and calcined at 550 °C for 6 h, yielding the final SBA-15 material. Co/ZSM-5

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and Co/SBA-15 were both prepared by a conventional ion-exchange technique. 9 An appropriate amount of cobalt nitrate was dissolved in deionized water and mixed with 0.5 g of H/ZSM-5 or SBA-15. The resulting solution was stirred at 80 °C for 24 h, at a pH of about 7.0. After being dried by evaporation, the sample was calcined in air at 550 °C for 4 h. The cobalt concentration of each catalyst calcined was determined by Atomic Absorption in a PerkinElmer AAnalyst 300 spectrometer (AAS). 2.2. Characterization Nitrogen sorption was measured with a NOVA 2000 gas sorption analyzer at liquid nitrogen temperature (-196 °C). Prior to measurement, each sample was degassed under vacuum for 8 h at 300 °C. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area using adsorption data acquired at a relative pressure (P/P0) range of 0.05-0.25. The total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure of about 0.99. Pore size distribution curves were calculated from the analysis of the adsorption region of the isotherm, based on the Barrett-Joyner-Halenda (BJH) algorithm. The crystalline phase was determined by powder XRD using a Rigaku D/MAC/max 2500v/pc instrument with Cu Kα radiation (40 kV, 200 mA, λ=1.5418 Å). Diffractometer data were acquired with a step size of 0.02° for 2θ values over two angular domains from 0.5-4° (small-angle) and from 5-80° (wide-angle). SEM images and semi-quantitative analyses of micron-sized spots were determined by a Hitachi S-4800 field emission scanning electron microscope, in combination with an EDAX Genesis 4000 energy-dispersive X-ray spectrometer (EDX). Specimens for SEM investigation were

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fixed on aluminum holders with adhesive conductive graphite tape. TEM images of catalysts were observed with a Philips Tecnai G2 F20 microscope operating at 200 kV coupled with an Oxford-1NCA EDX detector. Prior to TEM analysis, samples were dispersed in ethanol by sonication and deposited on a copper grid coated with a carbon film. FT-IR spectra were obtained over the 4000-600 cm-1 region using a Bruker Tensor 27 spectrophotometer. Samples were prepared as KBr pellets with a 1/10 weight ratio of sample to KBr. Ultraviolet-visible diffuse reflectance spectra (UV-vis DR) were measured on a Hitachi U-4100 UV-vis spectrophotometer with an integration sphere diffuse reflectance attachment. Powder samples were loaded into a transparent quartz cell and were measured in the region of 200-800 nm at room temperature. The reflectance of BaSO4 was used as the baseline for the corresponding catalysts. XPS spectra were recorded on a Perkin-Elmer PHI-1600 ESCA spectrometer using a Mg Kα X-ray source. The binding energies were calibrated using C1s peak of contaminant carbon (BE = 284.6 eV) as an internal standard. Temperature-programmed desorption of NH3 (NH3-TPD) test was performed in a Micromeritics Autochem 2920 II analyzer with the thermal conductivity detector (TCD). After being pretreated at 300 °C under flowing helium (50 ml/min) for 1 h, the sample (100 mg) was cooled to 50 °C, and then adsorbed to saturation by pulses of ammonia for 0.5 h. Physically adsorbed ammonia on catalyst was removed by flushing the sample with helium (50 ml/min) for 1h at the adsorption temperature. Thermal desorption of ammonia was carried out in the temperature range of 50-600 °C at an increasing temperature rate of 10 °C/min.

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2.3. Catalytic activity testing Catalytic experiments were performed at atmospheric pressure in a flow-type apparatus designed for continuous operation. Before each test run, the catalyst powder was first pressed into a wafer and sieved into 20-40 meshes, and then 0.5 g of the catalyst was packed into a fixed-bed reactor made of a quartz tube with an internal diameter of 10 mm. A K-type thermocouple was located inside the catalyst bed to monitor reaction temperature. The reaction was carried out across the temperature range 50-600 °C, and the feed gas (1000 ppm NO, 1000 ppm NH3, 10% O2 and N2 to balance; space velocity (SV) of 15,000 h-1) was metered using calibrated electronic mass flow controllers. An online multi-component measurement system (AVL SESAME FTIR) was used to monitor the effluent NO, NO2, N2O and NH3. From the concentration of the gases at steady state, the NOx conversion was defined as: NO x conversion (%) =

[NO x ]in − [NO x ]out

× 100 ,

[NO x ]in

[NOx]=[NO]+[NO2]

(1)

Because the large excess of nitrogen present in our background gas make it impossible to directly measure nitrogen as a NOx reduction product, N2 selectivity was estimated from the following formula: N 2 selectivity (%) =

[NO x ]in -[NO x ]out -[N 2 O]out [NO x ]in -[NO x ]out

× 100

(2)

3. Results and discussion 3.1. Structure and morphology Nitrogen adsorption-desorption isotherms for pure and cobalt-containing ZSM-5

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catalysts are shown in Fig. 1. The isotherm for the pure ZSM-5 sample is similar to a type-I isotherm characteristic of microporous materials, and has a narrow pore size distribution with an average pore diameter of 2.041 nm (see Table 1). The adsorption-desorption isotherm shape remains unchanged after the modification of ZSM-5 with cobalt. However, cobalt incorporation leads to a dramatic decrease in BET surface area and micro-pore volume, from 376.4 m2g-1 and 0.1275 cm3g-1 respectively for ZSM-5, to 294.6 m2g-1 and 0.0107 cm3g-1 respectively for 12.1% Co/ZSM-5, as shown in Table 1. The reason is that cobalt species cover the external surface of ZSM-5 and impede N2 entry into the pores. The SBA-15 isotherm (Fig. 2) exhibits well-defined type-IV isotherm behavior with a H1 broad hysteresis loop.24 This is characteristic for mesoporous materials containing 1-D cylindrical channels that facilitate the condensation of N2. A sharp inflection in the range of 0.6-0.8 relative pressure corresponds to capillary condensation of nitrogen within uniform mesopores, where the inflection point correlates to the pore diameter in the mesopore range. Table 2 lists the textural properties of pure and cobalt-containing SBA-15 catalysts. Both BET surface area and micro-pore volume undergo a gradual loss with increasing cobalt loading, while the average pore diameter is only enhanced slightly. This indicates the incorporation of metal atoms into the SBA-15 framework, or the formation of extra-framework cobalt oxide species located at the support pore structure. These suppositions will be discussed by means of XRD, TEM and FT-IR measurements. Fig. 3 shows the XRD patterns of Co/ZSM-5 catalysts with different cobalt loading

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ratios. All Co/ZSM-5 samples exhibit the typical peaks of ZSM-5 zeolite, indicating that the original zeolite structure remains intact. The intensity of the ZSM-5 principal diffraction peaks decreases with the increasing cobalt content, due to the higher absorption coefficient of cobalt compounds for the X-ray radiation.25 No secondary phases are detected at 0.6%, 1.1%, 1.7%, 3.5% and 7.3% cobalt loadings. At 12.1% cobalt loading, low intensities of diffraction peaks at 2θ = 36.8°, 59.3° and 65.2° (PDF 42-1467) can be observed, which is attributed to crystallite Co3O4 existing on the zeolite support. These results show that the cobalt species are finely dispersed on the ZSM-5 support as amorphous cobalt oxides, or that the cobalt species consist of cobalt oxide particles in the form of minicrystals. Small-angle diffraction patterns of Co/SBA-15 samples (Fig. 4A) exhibit three well-resolved peaks, which can be indexed to the (100), (110) and (200) planes of the high crystallinity hexagonal space group p6mm. The small-angle diffraction patterns of Co/SBA-15 samples are similar to that of pure SBA-15, though the weak (110) and (200) diffraction peaks are less intense. Here, we calculated the unit-cell parameters of SBA-15 using the formula: a0 = 2d100 / 3 , where the d100 denotes the interplanar spacing of the (100) plane. With increasing cobalt content, the diffractions shift slightly towards smaller angles. The resulting increase in the unit cell parameter (a0), from 11.79 nm for SBA-15 to 12.63 nm for 14.7% Co/SBA-15 (Table 2), represents the dilation of the SBA-15 structure. Such expansion of the unit cell has been reported by Jha et al. and Kong et al.,26,27 who ascribed it to the incorporation of metal ions into the silica framework.

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Wide-angle XRD patterns for Co/SBA-15 catalysts are shown in Fig. 4B. The crystallized Co3O4 nanoparticles are not detected until the cobalt loading ratio reaches 5.6%. Upon further increasing the cobalt content, the characteristic peaks of Co3O4 narrow and sharpen due to the gradual increase of Co3O4 crystallite size. By contrast, the diffraction peaks of cobalt oxides for Co/ZSM-5 samples in Fig. 3 are not observed until the cobalt content reaches 12.1%. The Co3O4 crystallite sizes (see Tables 1 and 2) are calculated from the width of XRD peaks (2θ = 65.2°) using the Sherrer equation.28 The average Co3O4 crystallite size (DCO) in the Co/SBA-15 series shows a clear dependence on the cobalt content, with larger particles formed at higher loading. The DCO value is 7.186 nm at 5.6 % cobalt loading, and increases to 9.965 nm at 14.7% cobalt loading. For the 12.1% Co/ZSM-5 sample, the DCO value is merely 4.763 nm. Cobalt species are clearly less dispersed on SBA-15 with larger crystallite size than those on ZSM-5, partly because the SBA-15 support has a larger pore size (Table 2) than that of ZSM-5 (Table 1). The precursors may be more likely to congregate into cobalt oxides clusters on the SBA-15 support, rather than evenly disperse on the mesoporous walls with silanols. It must be noted that the Co3O4 crystallite sizes at 9.4% and 14.7% cobalt content (9.32-9.96 nm, see Table 2) exceed the average pore diameter of the SBA-15 support (about 8.8 nm, see Table 2), which indicates that larger Co3O4 particles are located either on the external surface or in the channels of the SBA-15 support. These larger Co3O4 particles located in the channels may partially block the channels and widen the pore size. The SEM image in Fig. 5 shows that the pure ZSM-5 sample is composed of

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irregularly localized, distinct edged and bright polycrystalline aggregates within the regular geometry. This is virtually unchanged in the morphology of zeolite samples after cobalt incorporation. The simultaneous EDX results collected from different regions (Point 1 and Point 2 in Fig. 5B) give similar compositions, and average cobalt loadings of 7.33-7.55% are detected for the 7.3% Co/ZSM-5 sample, in good agreement with loadings determined by AAS analysis. Figure 6A reveals that pure SBA-15 consists of well-defined worm-like macro-structures aggregated together with rope-like domains.23 The incorporation of cobalt species leads to a significant degradation in macrostructure (Fig. 6B), but the rope-like morphology is maintained. EDX analysis of the Co/SBA-15 sample (Point 1 and Point 2 in Fig 6B) confirms the macroscopically homogeneous distribution of cobalt within the SBA-15 support, but the cobalt content measured by EDX is slightly lower than that detected by AAS analysis due to parts of cobalt species deposited probably in the deep layer of SBA-15 support. The TEM image in Fig. 7A shows interference fringes of the ZSM-5 crystal structure. In Fig. 7B, the cobalt oxides on the 3.5% Co/ZSM-5 sample are not discernible. The EDX analyses (Point 1 and Point 2 in Fig. 7B) indicate the cobalt species are well-dispersed and cobalt contents are consistent with those measured by AAS. At cobalt loading ratios of up to 7.3%, some darker spots (marked as circles in Fig. 7C and D) are apparent. EDX spectra of these particles indicate a composition of Co and O rich phases (not shown). At 7.3% cobalt content, these particles have a diameter of less than 2 nm, which can not be observed from the XRD pattern due to

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the detection limit. At 12.1% cobalt content, the diameter of particles reaches 5 nm, similar to the Co3O4 diameter detected by the XRD technique (4.763 nm in Table 1). Moreover, an inhomogeneous distribution of cobalt oxides (marked as ellipses in Fig. 7C and D) suggests that a large fraction of the cobalt oxide particles are probably located on the outer surface of ZSM-5 grains. Representative TEM images of pure SBA-15 and Co-containing SBA-15 samples are shown in Fig. 8. For the pure SBA-15 sample, a well-ordered hexagonal array with uniform mesopores and side channels is clearly visible. The 1-D channels in the rods run parallel to the long axes of the rope, apparent as alternating clear and dark stripes due to electronic density contrast between empty mesopores and silica walls. The corresponding mesopores diameter is estimated to be approximately 8.8 nm, which is slightly larger than the pore diameter of 8.356 nm in Table 2. In Fig. 8B and C, the regular silica structure is retained after cobalt introduction, but the increase in cobalt content leads to a decline in the long-range order of hexagonally arranged porosity. Cobalt species are evidenced by regions of darker contrast and identified by EDX where the cobalt peak is clearly observed. Cobalt oxide species mainly form inside the pores, where they are often attached to the silica walls and aggregated to large particles, with particle sizes of 10–30 nm for the 5.6% Co/SBA-15 (Fig. 8B). Only a minor number of cobalt species (marked with a white arrow in Fig. 8C) are obvious on the external surface of the silica grains. When the cobalt content is up to 9.4%, cobalt oxide particles agglomerate together at the vertical direction of light path, and it is difficult to distinguish them from TEM images. Figure 8D displays a

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representative TEM image of the 14.7% Co/SBA-15 sample with an inhomogeneous cobalt distribution. EDX analysis performed in the bright area of the mesopores (Point 1 in Fig. 8D) detects no trace of cobalt, in contrast to the dark area (Point 2 in Fig. 8D) where a cobalt content of 38.42% is detected (much higher than the 14.7% as measured by AAS analysis). Hence, it is clear that part of the channels remains empty after cobalt introduction, and only a fraction of the channels are actually employed to disperse the loaded cobalt. 3.2. FT-IR analysis FT-IR spectra of Co/ZSM-5 and Co/SBA-15 samples are shown in Fig. 9 and Fig. 10 respectively. The spectra of Co/ZSM-5 and Co/SBA-15 samples exhibit similar structure characteristics. The band at around 1057 cm-1 corresponds to the asymmetric stretching vibration of Si-O-Si, and the bands close to 813 cm-1 are assigned to the symmetric stretching and deformation modes of the Si-O-Si framework. The broad peak in the 3000-3700 cm-1 range and a band at around 1628 cm-1 for all spectra are due to the stretching vibrations of hydrogen-bonded silanols and water, and to the deformation vibration of water, respectively.29,30 The 3751 cm-1 absorption band is attributed to the stretching vibrations of terminal silanol groups (Si-OH). The appearance of a new band characteristic for the Si-O-Co bond (970 cm-1) is accompanied by an intensity decrease of the Si-OH (3751 cm-1) and Si-O-Si (813 cm-1) bands. This result supports the conclusion of cobalt incorporation into the framework. The intense band at 1200 cm-1 for all Co/ZSM-5 samples in Fig. 9 is associated with the asymmetric internal tetrahedron vibrations of SiO4.31,32 The band at 3628 cm-1 for

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the Co/ZSM-5 attributes to bridging hydroxyl groups (Si-(OH)-Al), which is associated with Brønsted acid sites. As the cobalt content increases, the intensity reduces slightly, indicating that partial cobalt ions are anchored on the bridging oxygen of Si-(OH)-Al groups. By contrast, due to absence of bridging hydroxyl groups, the cobalt ions can enter the SBA-15 structure through the substituting for H+ at the isolated Si-OH (3751 cm-1) groups. The adsorption peak at about 680 cm-1 is a conventional fingerprint of Co3O4 on the ZSM-5 and SBA-15 supports,33 and its enhancement correlates with the cobalt content. Additionally, for the Co/ZSM-5 series the Co3O4 crystal can be detected by XRD only at the cobalt content of 12.1%, while detected by FT-IR when the cobalt content is up to 1.7%. This discrepancy is caused by the sensitivity of XRD and FT-IR techniques. The detection limit of the XRD analysis is about 4 nm, depending on the periodic structure of crystals. FT-IR is renowned for its sensitivity to structures of short-range structural order and low crystallinity. To avoid erroneous conclusions, UV-vis spectra are further employed to throw more light on the nature of cobalt species formed in Co/ZSM-5 and Co/SBA-15 catalysts. 3.3. XPS analysis The chemical states and surface compositions of elements for the catalysts were characterized by XPS. The XPS survey scan in Fig. 11 shows that carbon, oxygen, silicon, cobalt and aluminum (only in the Co/ZSM-5 series) are present on the surfaces of the Co/ZSM-5 and Co/SBA-15 catalysts, based on their binding energy signatures. Typical narrow scan spectra for the Co 2p peaks are shown in Fig. 12. The

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Co 2p3/2 and Co 2p1/2 peaks are present at 780.2 and 795.3 eV, respectively. The doublet separation between the 2p3/2 and 2p1/2 signals may be 15.1 eV which is consistent with the standard spectra of elemental cobalt, indicating the presence of Co2+ or Co3+. Moveover, a satellite peak located at 787.5 eV confirms the existence of Co2+ species.34 Peak deconvolution and fitting to experimental data show that the Co 2p peak could be fitted well by two peaks, corresponding to the chemical states Co3+ at 781.7 eV of 2p3/2 35 and Co2+ at 780.2 eV of 2p3/2.36-38 Elemental compositions have been calculated from the area of O 1s, Si 2p, Co 2p and Al 2p peaks, and are shown in Table 3. The results confirm that the superficial cobalt content increases with cobalt loading ratio for both the Co/ZSM-5 and Co/SBA-15 catalysts. XPS is a surface technique that allows to detect species preferentially located on the surface sample (sampling depth is about 3-10 nm). The cobalt appears to become enriched on the surface of ZSM-5 grains, since the Co/Si atomic ratios for all Co/ZSM-5 samples are considerably larger than those obtained from AAS. For the Co/SBA-15 series, the surface Co/Si ratios are generally lower than those from AAS, indicating that most of cobalt species are located in the SBA-15 channels. The area ratio of Co3+/Co2+ calculated according to the XPS spectra are also shown in Table 3. The Co3+/Co2+ ratio is 0.37 for the 0.6% Co/ZSM-5 sample and increases with the cobalt loading. For the 12.1% Co/ZSM-5 catalyst, the highest Co3+/Co2+ ratio is 0.59. As mentioned above, the cobalt species are prone to migrate on the ZSM-5 surface when the cobalt content increases. These Co2+ species enriched on the ZSM-5

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surface are partly oxidized to Co3+ ones, and thus form the Co3O4 spinel during the calcination process. In contrast, when the cobalt are introduced into SBA-15, the Co3+/Co2+ ratios for all of the Co/SBA-15 samples keep stable within the range of 0.63–0.68 regardless of the cobalt content, which are apparently higher than those for the Co/ZSM-5. 3.4. Chemical nature of cobalt species UV–vis DR spectroscopy was applied to understand the nature and coordination of cobalt oxide species in the samples. Fig. 13 shows the UV–vis absorption spectra of Co/ZSM-5 samples. The band at about 225, 280 and 361 nm is assigned to the zeolite ZSM-5 structure and the adsorption band at 471 nm is attributed to β-site Co2+ ions located at the intersection of the straight and sinusoidal channels, close to the plane of the 6-member ring.39 The band at around 520 nm is characteristic of γ-site Co2+ ions located in the sinusoidal channels of ZSM-5.40 The intensity of these two peaks increases with increasing cobalt loading. As for α-site Co2+ ions (situated in the straight, 10-membered ring channels), the representative band is near 650 nm,41 which is absent in the 0.1% and 1.1% Co/ZSM-5 samples, but present in the 1.7%, 3.5%, 7.3% and 12.1% Co/ZSM-5 samples. This suggests that H+ ions at the α site are more difficult to exchange with Co2+ ions than at the β and γ sites. A new band at 320 nm, clearly apparent for 3.5% and 7.3% Co/ZSM-5, is assigned to distorted tetrahedral Co3+, presumably corresponding to well-dispersed extra-framework CoOx clusters.42 In comparison to the samples with lower cobalt content, the 12.1% Co/ZSM-5 sample displays two intense absorptions in visible range. The broad peak centered at 420 nm

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is attributed to the octahedral coordination of Co3+, and the corresponding ligand-to-metal charge transfer transitions from the p orbital of the ligand oxygen atoms to the d orbital of the cobalt ion.43 Another broad peak in the region of 650-800 nm shows the d-d transition bands originating from surface Co2+/Co3+ species. Both bands verify the presence of Co3O4 in the 12.1% Co/ZSM-5 sample. According to the XPS and UV-vis results, a significant amount of Co2+ ions presented in low-loaded Co/ZSM-5 can be related to the isolated cobalt ions located on α-, β- and γ-sites of the ZSM-5 framework. Hence cobalt ions are preferentially located at ion-exchange sites in ZSM-5. With the cobalt content increasing to 12.1%, the rest Co2+ ions are partly oxidized to Co3+ ones. TEM and EDX analyses show that a part of the cobalt oxide particles are located on the outer surface of ZSM-5 grains. Also according to XPS analyses, the cobalt species enrich on the ZSM-5 surface. Therefore, the rest cobalt species tends to migrate to the ZSM-5 surface and evolve from cobalt ions to the amorphous CoOx, and then aggregate to form the Co3O4 crystals. The UV-vis spectra for the Co/SBA-15 catalysts are shown in Fig. 14. The SBA-15 shows no absorption at all in the wavelength region of 400–800 nm as can be seen from the straight-line pattern of the sample in this figure. In the case of cobalt-containing SBA-15 samples, cobalt incorporation increases the absorption in both visible and UV range compared with undoped SBA-15. Two broad adsorption peaks at about 420 and 740 nm indicate that Co3O4 is the predominant phase in the SBA-15 catalysts.43 3.5. Temperature-programmed desorption of NH3

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Fig. 15 shows the NH3-TPD profiles from Co/ZSM-5 samples. Two desorption peaks at 183 °C and 418 °C are observed for parent ZSM-5, corresponding to the weak and the strong acid sites, respectively. The desorption peak at the lower temperature is attributable to physisorbed NH3, while the peak at the higher temperature is assigned to NH3 strongly adsorbed on the Brønsted acid sites.44 The intensity of the peak at 418 °C is lower than that of ZSM-5 material alone and gradually diminished with the cobalt content increasing to 3.5%. The Brønsted acid sites created by the aluminum centers in ZSM-5 not only bind and disperse the cobalt ions,45-47 but also absorb and activate the ammonia.48,49 In addition, a new and distinct NH3 desorption peak appeared at a high temperature of 437 °C for 7.3% and 12.1% Co/ZSM-5, indicating the creation of the stronger acid sites. Considering a poor capacity of NH3 adsorption for pure Co3O4 (Fig. 16), we deduce that the stronger acid sites for 7.3% and 12.1% Co/ZSM-5 mainly originated from the dispersion of cobalt species on ZSM-5 support.50 In Fig. 16, the pure SBA-15 shows negligibly small adsorption of NH3. Upon the introduction of cobalt into SBA-15, the NH3-TPD profiles of Co/SBA-15 samples display a two-stage desorption behavior. Herein, the low-temperature desorption peak between 80−470 °C is due to the surface hydroxyl group attached with silicon; the high temperature peak around 557 °C is attributed to the Co3O4 supported on SBA-15 silica to form the surface phase oxide,51 which has been testified from the XRD and TEM analyses. The agglomeration of Co3O4 may result from the absence of Brønsted protons in the SBA-15 framework.

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3.6. Catalytic activity testing The catalytic activities of the catalysts are evaluated in terms of the temperature (Tp) at which the catalysts reach their peak NOx conversion (PNO), and the corresponding formation of N2O (FN2O), as well as the selectivity to N2 (SN2). NOx conversion profiles of Co/ZSM-5, shown in Fig. 17, exhibit a low temperature peak (LT-peak) at approximately 250 °C and the high temperature peak (HT-peak) between 350 -550 °C. The identification of the LT-peak has been the subject of some debate. Sullivan et al.52 and Kieger et al.53 assigned it to a transient peak due to the temperature-programmed reaction condition, whereas Li et al.54 attributed it to a permanent peak. With respect to our results, we consider this peak as a permanent peak because the reactions are running at a steady state. The main catalytic data for these samples are compiled in Table 4. It can be seen that the pure ZSM-5 sample shows poor catalytic activity. Upon the addition of cobalt, the PNO of the LT-peak is enhanced monotonously from 66.6% for 0.6% Co/ZSM-5, to 97.6% for 12.1% Co/ZSM-5, while the Tp of the LT-peak remains almost unchanged. The increase in cobalt content promotes the Tp of HT-peak shifting toward lower temperatures, from 557 °C (pure ZSM-5) to 351 °C (12.1% Co/ZSM-5). However, the PNO of HT-peak shows a declining trend after an initial ascent, achieving a maximum of 99.1%, at 3.5% Co/ZSM-5. The conversion of NOx on Co/SBA-15 as a function of temperature is depicted in Fig. 18, and complied data of initial activities are given in Table 5. Pure SBA-15 is inactive over the entire temperature range studied. Upon the introduction of cobalt,

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the SCR profile shifts toward low temperature. The Tp decreases from 560 °C for pure SBA-15 to 425 °C for 9.4% Co/SBA-15, and the corresponding PNO rises from 30.8% to 98.0%. Activity saturation is observed in the cobalt loading ratio of 9.4%-14.7%. Compared with the Co/ZSM-5 samples, the most distinguishing feature of the Co/SBA-15 samples is the absence of LT-peak. Moreover, under the same cobalt loading ratio Co/SBA-15 catalysts present a lower activity than Co/ZSM-5 catalysts. The stability of Co/ZSM-5 and Co/SBA-15 catalysts is evaluated during 50 h under reaction at 600 °C. The NOx conversion with time is presented in Fig. 19. It is clear that at 600 °C Co/ZSM-5 and Co/SBA-15 catalysts give about 62 and 66% initial NOx conversion respectively, and the activities remain almost unchanged after reaction for 50 h. This result indicates that both Co/ZSM-5 and Co/SBA-15 catalysts are durable under high-temperature conditions. As a by-product, N2O is detected during the SCR process. N2O can be formed by the reaction between NOx and NH3 or by the direct oxidation of NH3. The SCR process induced by ZSM-5 or SBA-15 catalyst produces a considerable amount of N2O, despite the PNO being low. For the Co/ZSM-5 series, the FN2O values are in the range of 72-98 ppm at the LT-peak, and do not exceed 80 ppm at the HT-peak. With increasing cobalt loading ratio, the FN2O at both the LT- and HT-peak decreases, which contributes to the SN2 of the LT-peak being enhanced from 78.1% to 92.6%, and that of the SN2 of the HT-peak from 89.7% to 96.2 %, respectively. Similarly, the FN2O for the Co/SBA-15 catalysts declines as the cobalt content rises from null to

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14.7%, which brings about a continuous increase of SN2 values from 78.2% to 96.4%. In general, the SCR process for Co/ZSM-5 and Co/SBA-15 catalysts follows the Mars and van Krevelen mechanism38,55-57: (і) Co2+ species are preferentially oxidized by O2 to form Co3+ species; (ii) the oxygen in the atmosphere then reacts with NOx to produce a surface nitrogen oxide intermediate bound to Co3+, namely “Co3+-NxOy”; (iii) the resulting Co3+-NxOy active intermediate reacts with adsorbed ammonia to yield N2 and H2O, accompanied by the regeneration of Co2+. For Co/ZSM-5 catalysts, it is widely accepted that the isolated Co2+ cations anchored on α, β and γ sites and cobalt oxide clusters and Co3O4 particles are the active phases for the SCR of NOx.41,58 At temperatures below 300 °C, the SCR activity is primarily caused by Co2+ ions on α, β and γ sites,59 corresponding to the LT-peak in Fig. 17. The PNO value at the LT-peak increases with increasing cobalt content, but becomes saturated when the cobalt content is higher than 7.3%. Similar results were also obtained by Smeets et al.60 and Lucas et al.,61 who ascribed it to the exchange utmost of cobalt cations. With further increasing the cobalt loading ratio, excess cobalt may coexisting in small CoxOy clusters (CoO, Co2O3, Co3O4...) constitute the majority of the species in over-exchanged catalysts,33,62-64 and are potentially a good candidate for performing SCR activity at the HT-peak (Fig. 17), resulting in the HT-peak shifting toward lower temperatures. With increasing cobalt content, the PNO at the HT-peak shows a declining trend after an initial ascent. The reason for this activity loss is attributed to the growth of grains of Co-oxides with increasing cobalt content, since the activity for NO reduction decreases in the sequence: isolated cobalt ions > small CoxOy clusters >

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large CoxOy clusters.65-66 The presence of Co3O4 particles inevitably promotes the oxidation of NH3 (i.e., non-selective reduction of O2 with NH3) to NO at the low temperature region although they exhibit the less activity to the SCR process.18,66 Co/SBA-15 catalysts exhibit lower SCR activity than Co/ZSM-5 catalysts. Two factors may be responsible for this. The first is that inactive Co2+ ions are anchored on the isolated Si-OH groups on the SBA-15 surface. The second is that the cobalt oxide particles in the channels of SBA-15 partially or completely obstruct access to the mesopores and limit transport of the reactants to active sites.

4. Conclusions A series of Co/ZSM-5 and Co/SBA-15 catalysts with different cobalt content were prepared, characterized and evaluated for the selective catalytic reduction of NOx with ammonia. The cobalt species are dispersed finely within the ZSM-5 zeolite. The Co3O4 phase is not detected in the Co/ZSM-5 catalysts until the cobalt loading ratio reaches 12.1%. Cobalt species within SBA-15 agglomerate easily to form cobalt oxide crystals due to the weak cobalt-support interaction. The introduction of cobalt results in a decline in the long-range order of hexagonally arranged porosity. For the Co/ZSM-5 series, cobalt species are either anchored at α, β and γ sites to form isolated cobalt ions, or enriched on the ZSM-5 surface to yield amorphous cobalt oxides. By comparison, for Co/SBA-15 catalysts most of the cobalt species reside in the channels, while some enter the SBA-15 framework leading to an increase in pore diameter. NOx conversion profiles of Co/ZSM-5 exhibit two peaks. The LT-peak is caused by

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cobalt ions on α, β and γ sites, while the HT-peak is assigned to amorphous and crystal cobalt oxides. The PNO of the LT-peak is enhanced monotonously with increasing cobalt loading ratio, and becomes saturated at 7.3% cobalt content due to the exchange utmost of ion sites at ZSM-5 zeolite. The Tp of the LT-peak remains largely unchanged. The increase in cobalt content promotes the HT-peak shifting toward lower temperatures, and the PNO shows a maximum of 99.1% at 3.5% cobalt loading into ZSM-5. NOx conversion profiles of Co/SBA-15 only exhibit a high-temperature peak. Because inactive cobalt ions are anchored on isolated Si-OH groups, and cobalt species in the SBA-15 channels limit the access of the reactants to active sites, Co/SBA-15 catalysts show lower activity than Co/ZSM-5 catalysts.

Acknowledgments This study was supported by the National Natural Science Foundation of China (No.51176139) and China Postdoctoral Science Foundation funded project (2011M500045).

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Table 1. Physicochemical properties of pure ZSM-5 and Co/ZSM-5 samples DCO BET surface area (m2g-1)/ Average pore Micro-pore volume Sample 3 -1 (nm) Standard deviation diameter (nm) (cm g )/Standard deviation Pure ZSM-5 Nd 376.4/6.85 2.041 0.1513/0.0020 0.6% Co/ZSM-5 Nd 363.6/4.42 2.013 0.1482/0.0075 1.1% Co/ZSM-5 Nd 355.7/5.84 1.997 0.1346/0.0055 1.7% Co/ZSM-5 Nd 345.8/5.24 1.893 0.1312/0.0014 3.5% Co/ZSM-5 Nd 321.4/6.31 2.044 0.1269/0.0066 7.3% Co/ZSM-5 Nd 312.7/2.80 1.975 0.1217/0.0042 12.1% Co/ZSM-5 4.763 294.6/6.38 1.966 0.1017/0.0031

Table 2. Physicochemical properties of pure SBA-15 and Co/SBA-15 samples BET surface area Average pore d100 a0 DCO Sample (m2g-1)/Standard diameter (nm) (nm) (nm) deviation (nm) Pure SBA-15 10.21 11.79 Nd 584.9/6.94 8.356 3.2% Co/SBA-15 10.24 11.82 Nd 569.4/8.57 8.338 5.6% Co/SBA-15 10.38 11.99 7.186 541.0/8.12 8.480 7.8% Co/SBA-15 10.67 12.32 8.232 536.3/6.23 8.561 9.4% Co/SBA-15 10.81 12.48 9.319 504.2/7.59 8.807 14.7% Co/SBA-15 10.94 12.63 9.965 407.3/5.46 8.847

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Micro-pore volume (cm3g-1)/Standard deviation 0.0415/0.0008 0.0367/0.0005 0.0185/0.0008 0.0166/0.0006 Nd Nd

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Table 3. Surface composition of the Co/ZSM-5 and Co/SBA-15 catalysts derived from XPS analyses Surface atomic composition Co/Si (atomic %) Sample Co3+/Co2+ O Si Co Al AAS XPS Pure ZSM-5 0.6% Co/ZSM-5 1.1% Co/ZSM-5 1.7% Co/ZSM-5 3.5% Co/ZSM-5 7.3% Co/ZSM-5 12.1% Co/ ZSM-5 Pure SBA-15 3.2% Co/SBA-15 5.6% Co/SBA-15 7.8% Co/SBA-15 9.4% Co/SBA-15 14.7% Co/SBA-15

32.9 30.6 27.9 27.8 27.9 27.2 26.8 55.7 55.2 54.3 50.8 44.7 32.6

15.5 13.4 13.4 12.7 11.8 10.4 9.9 20.6 20.4 20.3 19.9 18.5 13.2

0 0.4 0.5 0.8 1.3 1.2 1.3 0 0.1 0.2 0.2 0.4 0.5

0.8 1.3 0.7 1.1 1.8 0.9 0.5 0 0 0 0 0 0

0 0.34 0.62 0.96 1.96 4.08 6.64 0 1.68 2.94 4.16 4.99 7.71

0 2.99 3.73 6.30 11.02 11.54 13.13 0 0.49 0.99 1.01 2.16 3.79

Table 4. Tp, PNOx, FN2O and SN2 catalytic data for Co/ZSM-5 catalysts LT-peak HT-peak Sample Tp PNO FN2O SN2 Tp PNO FN2O (°C) (%) (ppm) (%) (°C) (%) (ppm) Pure ZSM-5 247 34.8 76 78.1 557 94.8 98 0.6% Co/ZSM-5 246 66.6 98 85.3 497 97.5 76 1.1% Co/ZSM-5 241 84.6 96 88.7 474 98.8 59 1.7% Co/ZSM-5 241 91.2 84 90.8 454 98.8 47 3.5% Co/ZSM-5 242 95.5 82 91.4 402 99.1 36 7.3% Co/ZSM-5 237 97.4 76 92.2 386 88.4 34 12.1% Co/ZSM-5 238 97.6 72 92.6 351 85.9 33 Table 5. Tp, PNOx, FN2O and SN2 catalytic data for Co/SBA-15 catalysts Tp PNO FN2O Sample (°C) (%) (ppm) Pure SBA-15 560 30.8 67 3.2% Co/SBA-15 497 53.6 65 5.6% Co/SBA-15 469 69.3 53 7.8% Co/SBA-15 464 92.6 48 9.4% Co/SBA-15 425 98.0 42 14.7% Co/SBA-15 396 95.9 35

31


/ 0.37 0.44 0.48 0.52 0.56 0.59 / 0.62 0.63 0.61 0.65 0.68

SN2 (%) 89.7 92.2 94.0 95.2 96.3 96.2 96.2

SN2 (%) 78.2 87.9 92.4 94.8 95.7 96.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Captions：： Fig. 1. Nitrogen adsorption-desorption isotherms of Co/ZSM-5 samples with different cobalt loading ratios: (a) 0% (pure ZSM-5); (b) 0.6%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%. Fig. 2. Nitrogen adsorption-desorption isotherms of Co/SBA-15 samples with different cobalt loading ratios: (a) 0% (pure SBA-15); (b) 3.2%; (c) 5.6%; (d) 7.8%; (e) 9.4%; (f) 14.7%. Fig. 3. XRD patterns of Co/ZSM-5 catalysts with different cobalt loading ratios: (a) 0% (pure ZSM-5); (b) 0.6%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%. Fig. 4. Small- (A) and wide-angle (B) XRD patterns of Co/SBA-15 catalysts with different cobalt loading ratios: (a) 0% (pure SBA-15); (b) 3.2%; (c) 5.6%; (d) 7.8%; (e) 9.4%; (f) 14.7%. Fig. 5. SEM images and EDX analyses for pure ZSM-5 (A) and 7.3% Co/ZSM-5 (B). Fig. 6. SEM images and EDX analyses for pure SBA-15 (A) and 5.6% Co/SBA-15 (B). Fig. 7. TEM images and EDX analyses for Co/ZSM-5 catalysts with different cobalt loading ratios: (A) 0% (pure ZSM-5); (B) 3.5%; (C) 7.3%; (D) 12.1%. Fig. 8. TEM images and EDX analyses for Co/SBA-15 catalysts with different cobalt loading ratios: (A) 0% (pure SBA-15); (B) 5.6%; (C) 9.4%; (D) 14.7%. Fig. 9. FT-IR absorbance spectra of Co/ZSM-5 catalysts with different cobalt loading ratios: (a) 0.6%; (b) 1.1%; (c) 1.7%; (d) 3.5%; (e) 7.3%; (f) 12.1%. Fig. 10. FT-IR absorbance spectra of Co/SBA-15 catalysts with different cobalt loading ratios: (a) 3.2%; (b) 5.6%; (c) 7.8%; (d) 9.4%; (e) 14.7%. Fig. 11. XPS survey scan spectra of (a) 12.1% Co/ZSM-5 and (b) 14.7% Co/SBA-15 catalysts.

32


Page 32 of 47

Page 33 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 12. XPS narrow scan spectra of the Co 2p region for the 12.1% Co/ZSM-5 (A) and 14.7% Co/SBA-15 (B) catalysts. Fig.13. UV-vis DR spectra of Co/ZSM-5 catalysts with different cobalt loading ratios: (a) 0% (pure ZSM-5); (b) 0.1%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%. Fig.14. UV-vis DR spectra of Co/SBA-15 catalysts with different cobalt loading ratios: (a) 0% (pure SBA-15); (b) 5.6%; (c) 7.8%; (d) 9.4%; (e) 14.7%. Fig. 15. NH3-TPD curves of Co/ZSM-5 samples with different cobalt content: (a) 0% (pure ZSM-5); (b) 0.6%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%. Fig. 16. NH3-TPD curves of Co3O4 (a) and Co/SBA-15 samples with different cobalt content: (b) 0% (pure SBA-15); (c) 3.2%; (d) 5.6%; (e) 7.8%; (f) 9.4%; (g) 14.7%. Fig. 17. Catalytic activities for NOx reduction by NH3 for Co/ZSM-5 catalysts with different cobalt loading ratios: (a) 0% (pure ZSM-5); (b) 0.6%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%. The standard deviations for the NOx conversion are within the range of 0.6-4.9. Fig. 18. Catalytic activities for NOx reduction by NH3 for Co/SBA-15 catalysts with different cobalt loading ratios: (a) 0% (pure SBA-15); (b) 3.2%; (c) 5.6%; (d) 7.8%; (e) 9.4%; (f) 14.7%. The standard deviations for the NOx conversion are within the range of 0.8-4.9. Fig. 19. Time on stream behavior of 3.5% Co/ZSM-5 (a) and 9.4% Co/SBA-15 (b) catalysts for NOx conversion at 600 °C.

33


Page 34 of 47

g f e

3

15

d c b a

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Relative pressure ( P/P0)

Fig. 1. Nitrogen adsorption-desorption isotherms of Co/ZSM-5 samples with different cobalt loading ratios: (a) 0% (pure ZSM-5); (b) 0.6%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%.

f e

300

d

3

Volume adsorbed ( cm /g, STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volume adsorbed ( cm /g, STP)


c b a

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Relative pressure ( P/P0)

Fig. 2. Nitrogen adsorption-desorption isotherms of Co/SBA-15 samples with different cobalt loading ratios: (a) 0% (pure SBA-15); (b) 3.2%; (c) 5.6%; (d) 7.8%; (e) 9.4%; (f) 14.7%.

34


Page 35 of 47

Co3O4

1000

g f

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


e d c b a 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 2 Theta (degree)

Fig. 3. XRD patterns of Co/ZSM-5 catalysts with different cobalt loading ratios: (a) 0% (pure ZSM-5); (b) 0.6%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%.

35



1×10 (200)

(110)

(100)

4

Intensity (a.u.)

A

f e d c b a

0.5

B

1.0

1.5 2.0 2.5 3.0 2 Theta (degree)

3.5

4.0

Co3O4

100

f

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 47

e d c b a

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2 Theta (degree)

Fig. 4. Small- (A) and wide-angle (B) XRD patterns of Co/SBA-15 catalysts with different cobalt loading ratios: (a) 0% (pure SBA-15); (b) 3.2%; (c) 5.6%; (d) 7.8%; (e) 9.4%; (f) 14.7%.

36


Page 37 of 47

A

B Point 1

Point 2

1.00 µm

1.00 µm

3.2

3.7

Si 2.5

Point 1 2.9

Kcnt

O

1.3

Wt%

At%

OK

51.82

67.23

AlK

03.14

02.42

SiK

37.48

27.70

CoK

07.55

02.66

Element

Kcnt

Element

O

2.2 1.5

0.6

Wt%

At%

OK

48.89

64.61

AlK

03.96

03.09

SiK

39.81

29.88

CoK

07.33

02.62

0.7

Co Al 0.0

Point Point 2 1

Si

O

1.9

Co

Co Co

0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.0

Al Co Co

0.0

0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.0 Energy (keV)

Energy (keV)

Fig. 5. SEM images and EDX analyses for pure ZSM-5 (A) and 7.3% Co/ZSM-5 (B).

A

B Point 1

Point 2

25.0 µm

25.0 µm

5.00 µm

4.1

4.4

O

Si

Point 1

Si

3.3

Point 2

3.5

2.5 1.6

Wt%

O

At%

OK

67.90

79.88

AlK

01.52

01.06

SiK

26.54

17.77

CoK

04.04

01.29

Kcnt

Element

Kcnt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.6 1.8

0.8

Element

Wt%

At%

OK

63.50

76.41

AlK

01.47

01.05

SiK

31.02

21.23

CoK

04.01

01.31

0.9

Co Al 0

Co Al

Co Co

0

0

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.0

0

Co Co

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.0

Energy (keV)

Energy (keV)

Fig. 6. SEM images and EDX analyses for pure SBA-15 (A) and 5.6% Co/SBA-15 (B). 37



B

A

Point 1

Point 2

40 nm

40 nm

C

D

Cobalt oxide crystallites


50 nm

40 nm

Cobalt surface enrichment

1200

Si

1000

O

600 400 200

Point 2

1000 Element

Wt%

At%

OK

52.63

67.39

AlK

02.41

01.84

SiK

41.72

30.48

CoK

03.24

01.13

800

Counts

800

Cobalt surface enrichment

1200

Point 1

Si

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 47

Element

O

600 400

Al Co

Co Cu Cu Co 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Energy (KeV)

200

Wt%

At%

OK

53.01

AlK

02.79

67.99 02.11

SiK

40.31

29.47

CoK

03.89

01.35

Al Co

Co Cu Co Cu 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Energy (KeV)

Fig. 7. TEM images and EDX analyses for Co/ZSM-5 catalysts with different cobalt loading ratios: (A) 0% (pure ZSM-5); (B) 3.5%; (C) 7.3%; (D) 12.1%.

38


Page 39 of 47

A

B


8.81 nm

8.76 nm

50 nm

100 nm

20 nm

C

D

50 nm

Point 1 Point 2

Cobalt oxide outside the pores 50 nm

100 nm

200 nm

50 nm

1250

Point 1 1000

OK SiK

Wt% 52.43 47.57

1000

At% 65.91

Counts

750

34.09

500

Cu 250

Point 1

1250

Si Element

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O Cu

750 500 250

Cu

Si

Element

Wt%

At%

OK

28.36

49.15

SiK

33.22

32.79

CoK

38.42

18.06

O Co Co

Cu Co

0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Energy (KeV)

Cu Cu 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Energy (KeV)

Fig. 8. TEM images and EDX analyses for Co/SBA-15 catalysts with different cobalt loading ratios: (A) 0% (pure SBA-15); (B) 5.6%; (C) 9.4%; (D) 14.7%.

39



683

Absorbance

0.5

1200 813 1057 1628 970

3628 3751

f e d c b a

4000 3500 3000 2500 2000 1500 1000 -1

Wavenumber ( cm )

Fig. 9. FT-IR absorbance spectra of Co/ZSM-5 catalysts with different cobalt loading ratios: (a) 0.6%; (b) 1.1%; (c) 1.7%; (d) 3.5%; (e) 7.3%; (f) 12.1%.

1057 813 970 680

0.5 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 47

3751

1624

e d c b a

4000 3500 3000 2500

2000

1500 1000 -1

Wavenumber ( cm )

Fig. 10. FT-IR absorbance spectra of Co/SBA-15 catalysts with different cobalt loading ratios: (a) 3.2%; (b) 5.6%; (c) 7.8%; (d) 9.4%; (e) 14.7%.

40


Page 41 of 47

4

5×10

O 1s C 1s

O KLL

b

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co 2p3

O KLL C KLL Co 2p1

Si 2s Si 2p O 2s O 1s C 1s

a Co 2p3 Co LMM

Si 2s Si 2p O 2s Al 2s Al 2p

1000

800

600

400

200

0

Binding Energy (eV)

Fig. 11. XPS survey scan spectra of (a) 12.1% Co/ZSM-5 and (b) 14.7% Co/SBA-15 catalysts.

41



A 250

Intensity (a.u.)

Co 2p1/2

Co 2p3/2

2+

Co shake-up

800

796

792 788 784 Binding Energy (eV)

780

776

B Co 2p1/2

Co 2p3/2

250

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 47

2+

Co shake-up

800

796

792 788 784 Binding Energy (eV)

780

776

Fig. 12. XPS narrow scan spectra of the Co 2p region for the 12.1% Co/ZSM-5 (A) and 14.7% Co/SBA-15 (B) catalysts.

42


Page 43 of 47

g

Absorbance

f

e d c b a

200

300

400

500

600

700

800

Wavelength (nm) Fig.13. UV-vis DR spectra of Co/ZSM-5 catalysts with different cobalt loading ratios: (a) 0% (pure ZSM-5); (b) 0.1%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%.

e d

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c b

a 200

300

400

500

600

700

800

Wavelength (nm)

Fig.14. UV-vis DR spectra of Co/SBA-15 catalysts with different cobalt loading ratios: (a) 0% (pure SBA-15); (b) 5.6%; (c) 7.8%; (d) 9.4%; (e) 14.7%. 43



183 °C

437 °C

0.005 Intensity (a.u.)

418 °C

g f e d c b a

100

200

300 400 Temperature ( °C)

500

600

Fig. 15. NH3-TPD curves of Co/ZSM-5 samples with different cobalt content: (a) 0% (pure ZSM-5); (b) 0.6%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%.

557 °C 0.005 g f e d

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 47

c b a 100

200

300

400

500

600

700

Temperature ( °C)

Fig. 16. NH3-TPD curves of Co3O4 (a) and Co/SBA-15 samples with different cobalt content: (b) 0% (pure SBA-15); (c) 3.2%; (d) 5.6%; (e) 7.8%; (f) 9.4%; (g) 14.7%.

44


Page 45 of 47

NOx conversion (%)

100 90

d

80

e

70

f g

60 50 40 30

c

20

b a

10 0 100

200 300 400 Temperature (°C)

500

600

Fig. 17. Catalytic activities for NOx reduction by NH3 for Co/ZSM-5 catalysts with different cobalt loading ratios: (a) 0% (pure ZSM-5); (b) 0.6%; (c) 1.1%; (d) 1.7%; (e) 3.5%; (f) 7.3%; (g) 12.1%. The standard deviations for the NOx conversion are within the range of 0.6-4.9.

100 90

NOx conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70

d

60

e

50

f

c

40 30

b

20 10

a

0 100

200 300 400 Temperature ( °C)

500

600

Fig. 18. Catalytic activities for NOx reduction by NH3 for Co/SBA-15 catalysts with different cobalt loading ratios: (a) 0% (pure SBA-15); (b) 3.2%; (c) 5.6%; (d) 7.8%; (e) 9.4%; (f) 14.7%. The standard deviations for the NOx conversion are within the range of 0.8-4.9.

45



75

NOx conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 47

70

b

65

a 60 55 50 45 0

10

20 30 40 Time on stream (h)

50

Fig. 19. Time on stream behavior of 3.5% Co/ZSM-5 (a) and 9.4% Co/SBA-15 (b) catalysts for NOx conversion at 600 °C.

46


Table of Contents Image: Co/SBA-15

Co/ZSM-5

Partial empty channels Cobalt surface enrichment

Aggregation of cobalt oxide crystals Cobalt oxide crystallites

40 nm

100 90 80 70 60 50 40 30 20 10 0

1.7% Co 3.5% Co 7.3% Co 12.1% Co

1.1% Co 0.6% Co 0% Co

100

200

300

100 90 80 70 60 50 40 30 20 10 0

NOx conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NOx conversion (%)

Page 47 of 47

400

500

600

200 nm

7.8% Co 9.4% Co 14.7% Co

5.6% Co 3.2% Co 0% Co

100

200

300

400

Temperature (°C)

Temperature (°C)

47


500

600

Structural Characterization and Selective Catalytic Reduction of

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