Dispersed Complex Phosphates as Tailored Acid Catalysts: Synthesis and Properties
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calcined at 500 'C for 3 h. For comparison, a sample with composition Ca 0 .5Sr 0.5Zr 4 P6 O24-2.062.08 sintered at 1300 'C, ball milled and annealed at 1000 'C was used. The water content in samples was estimated using thermal analysis (a Q-1500 device, heating rate 10 degree/min). Bulk structure of all samples was characterized by EXAFS (spectra were acquired at the EXAFS Station of the Siberian Center of Synchrotron Radiation, Novosibirsk using the same procedure as in [5] and fitted with the help of EXCRUV-92 program) and IR spectroscopy of the lattice modes (a BOMEM MB-102 IR-Fourier spectrometer, samples were pressed in wafers with CsI). Crystalline samples were also studied by TEM (JEM 2010 C, 200 keV) and XRD (URD-63 diffractometer, Cu Ka radiation). 31p NMR MAS (a Bruker CXP-300 spectrometer operating at 121,469 Mhz, the MAS rate 2.5 kHz, pulse length lIts, recycle time 10 or 20s) was used to characterize composition and local environment of the phosphate groups in amorphous and crystalline samples. Surface properties were probed by the IR spectroscopy of surface hydroxyls (Broensted acid centers) and CO test molecule absorbed at coordinately unsaturated cations-Lewis acid centers. In these experiments, samples were pressed in wafers with densities 4.4-22.7 mg/cm and vacuum pretreated in the IR cell at 400 C for I h. CO was absorbed at 150-160 K at 5 Torr, and spectra were recorded using a IFS-1 13 v Bruker spectrometer. Catalytic properties in the reaction of Nox reduction by propane and methane were studied in the plug-flow microreactor in the temperature range 200-500 C at space rates ca 50,000 h and feed composition 300-700 ppm NO, + 0.17%CH 4 (C3H8 ) + 180 ppm SO2 + 2% H2 0 + 5% 02 + 5% CO 2 in He. RESULTS Bulk structure of dispersed low-temperature complex phosphates, According to thermal analysis data, all samples contained a great amount of water approaching 5-6 molecules for the MeZr 4P6 0 24 formula unit. Approximately from one third to a half of water is desorbed in a peak at around 120-150 'C, while remaining water is removed continuously at temperatures from 600 to 1000 'C depending on its location within the lattice. Removal of this structural water was followed by exoeffects at 700-800 °C and 900-1000 °C indicating structures relaxation due to annealing of rather highly energetic defects (point defects). In general, structural water can be assigned to different lattice sites/structural groups. Combination of the IR and NMR data allowed one to select the most probable models. Thus, NMR data are in favor of the absence of the acidic phosphate groups in the lattice: only single narrow peak at 824.1 (relative to H3PO 4 ) was observed for samples sintered at 1300 'C corresponding to PO 43" groups in a symmetrical environment, while for dispersed samples, superposition of several peaks situated in the range of 8--30-21 and assignable only to phosphate groups in a different environment [6] was revealed. It agrees with the data of the IR spectroscopy of lattice modes which exclude presence of any a
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