Novel Phases in the V-P-O Catalytic System from Oxovanadium Hydrogenphosphate Precursors
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involving different hydrogenphosphate structures, showed that all these hydrates evolve finally to y-(VO) 2 P2 0 7 [9]. The present study was undertaken looking to gain insight into the course of the structural rearrangements involving VOPO4 . We have now thermally induced the solid state reactions under air atmosphere, which, as stated above, would finally result in the formation of [3-VOPO 4 . The starting precursors were different oxovanadium(IV) hydrogenphosphate hydrates and also the ammonium salt of the vanadium(V) derivative 13-NH 4 (V0 2)(HPO 4 ) [10]. EXPERIMENTAL Details of the preparation of powder samples of the different precursors have been reported elsewhere [8,10,10]. To follow the course of the reactions we have used time resolved X-ray thermodiffractometric techniques (XRTD) [11]. X-ray Powder diffraction patterns were obtained by means of a conventional angle-scanning Siemens D501 diffractometer using Cu Ka radiation and having the Pt peaks as standard. The apparatus is equipped with a variable temperature device (Anton Paar HTK-10) working from room temperature to ca. 1200 'C. The time interval between successive powder patterns was 3000 sec., including a measuring time of 2000 sec per diffractogram (recorded at constant temperature). The temperature was increased by steps of 20 'C starting from 20'C up to ca. 780 0C. The powder patterns were recorded in steps of 0.05'(20) over the short angular range 10-30'(20) for 5 sec. per step. Patterns used for indexation were collected in steps of 0.020(20) over the angular range 10-70 0(20) for 10 sec. per step. The diffraction patterns were analyzed numerically and the relative percentage of each phase was determined from the thermal evolution of the intensity of well-isolated Bragg peaks. The (hkl) indices of the reference peaks used for this semiquantitative analysis are: VO(HPO 4 )-0.5H 2 0 [(001), d=5.719A], a-VO(HPO 4 )'2H 2 0 [(100), d=7.578A], 13-VO(HPO 4).2H 2 0 [(011), d= 5.525K], VO(HPO 4 ).4H-0 [(022), d=6.503,],o13-NH 4(V0 2 )HPO 4 [(002), d=8.866A], calVOPO 4 [(101), d=3.569A], 6-VOPO 4 [d= 4.03A], 7-VOPO 4 [d=3.915A], 13-VOPO 4 [(210), d=3.340A] and o-VOPO4 [(111), d=4.212A]. Thermal reduction was carried out using a Perkin Elmer thermogravimetric analyzer TGA7. Samples were heated up to 700'C at 3 0/min under flowing N2 atmosphere (50 cc/min). The final solid was characterized by X-ray powder diffraction techniques. RESULTS AND DISCUSSION Concerning VO(HPO 4 )-0.5H 20 (type I), the results of our XRTD experiment confirm the main conclusions of previous work based on a variety of techniques (f e., static XRD, SEM, TEM, thermal analysis). In fact, this is the only oxovanadium hydrogenphosphate whose thermal behaviour under an oxidizing atmosphere has been described in the literature. X-ray powder diffraction data and DTA observations were interpreted in terms of successive hemihydrate -ý 5 -> Y-ý 13-VOPO4 transitions [5,7]. In our case, peaks associated with the 8-phase appear at 460'C; after further heating, peaks due to 'Y-VOPO4 are dete
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