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X-ray photoelectron spectroscopy (XPS) using a KRATOS Axis ultra electron spectrometer. After analysis of the film surface, about 15 nm of the film was removed by argon sputtering for 30 minutes and the sample was analyzed again. This procedure was repeated in order to measure the depth profile in the film. Nitrogen adsorption and desorption at liquid nitrogen tempera- ture was measured using a Micromeritics ASAP 2010 instrument after evacuation of the sample at 300 °C for 12 h. The specific sur- face area, m2 per g sample, was calculated from the adsorption iso- therm using the Brunauer–Emmet–Teller (BET) equation. By using the specific surface area (415 m2 g1) for ZSM-5 powder [9,16], the mass of zeolite on the sample (g), and zeolite loading (mass zeolite/ mass sample, g/g) were estimated from the measured surface area of the samples [9]. The surface area of the support was very small compared to the surface area of the zeolite film and the former was therefore safely neglected. The pore size distribution was calcu- lated using the Barret–Joyner–Halenda (BJH) method. The micro- pore volume per gram sample of the MFI film samples was determined from a t-plot. X-ray diffraction (XRD) data using CuKa radiation was collected using a Siemens D5000 diffractometer running in Bragg-Brentano geometry. NO2 sorption was measured by first mounting the zeolite film coated cordierite monoliths in a quartz tube with a length of 880 mm. The tube was equipped with an electrical heater on the outside and a temperature regulator connected to a thermocouple in the inlet gas. Prior to tests, the sample was activated at 500 °C for15mininafeedof8%O2 inArwithaflowrateof 3000 cm3 min1 (STP). The sample was then cooled to 30 or 200 °C for NO2 adsorption. At the beginning of the adsorption experiment (time = 0), the feed was changed to 600 ppm NO2 in Ar with a flow rate of 2600 cm3 min1 (STP). The concentration of NO and NO2 in the reactor effluent was measured continuously by a chemiluminescence detector (Ecophysic CLD 700 EL ht). After adsorption, the reactor was flushed with pure argon for 4 min. Temperature programmed desorption was then carried out using a heating rate of 20 °C/min up to 550 °C. Thermal analysis was per- formed on discrete MFI crystals with different Si/Al ratios (samples DCC96 and DCG) on a Netzsch STA 409 C balance. The samples were heated in Ar from room temperature to 500°C/min at 10 °C/min, kept at 500 °C/min for 15 min (mimicking the activation of the samples prior to NOx adsorption measurements) and there- after heated to 1400 °C/min at a heating rate of 20 °C/min. Mass spectrometry was carried out simultaneously with the thermal analysis. 3. Results and discussion 3.1. Characterization of MFI crystals by SEM, ICP – AES Fig. 1 shows SEM images of the MFI crystals grown by seed addition to the clear synthesis mixture and the gel. The diameter of the seed crystals grown in one step in the clear solution is about 240 nm after 48 h (a) and about 450 nm after 96 h (b). This shows that the diameter of the seed crystals (60 nm) increases linearly and that the growth rate is about 5 nm/h, at least up to a synthesis time of 96 h. The crystals are 90° rotational intergrowths, which is typical when TPA+ is used as template molecule [22]. Aggregates of crystals formed after growth of seed crystals in the gel (Fig. 1c). The Si/Al and Na/Al ratios of the crystals grown by seed addition to the synthesis mixture were measured by ICP-AES and the results are given in Table 2. Note that since the seed crystals were as small as about 60 nm, they hardly affect the average chemical composi- tion of the grown seed crystals. The measured Si/Al ratios thus re- flect the average composition of the skin grown around the seed crystals. As the size of the crystals grown in the clear synthesis solution increases, the Si/Al ratio decreases. These crystals are thus zoned, with increased Al concentration in the outer parts of the crystals, as expected for MFI crystals grown using TPA+ [19,28,40,41] as template molecule. The Al content (Si/Al = 178) and the size of the MFI crystals grown in the clear solution for 96 h, see Table 2, are about two times higher than for the crystals A. Mosca et al. / Microporous and Mesoporous Materials 120 (2009) 195–205 197 Fig. 1. SEM images of MFI crystals grown (a) for 48 h (DCC48) and (b) 96 h (DCC96) by adding 10 ppm silicalite seeds to the clear synthesis mixture. (c) MFI crystals grown for 14 h by adding 10 ppm silicalite seeds to the gel (DCG).PDF Image | Structured Zeolite Adsorbents for PSA Applications
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