In-Situ Dynamic Transformation of Vanadyl Hydrogen Phosphate Hydrate, VOHPO 4 -1/2H 2 O, to Vanadyl Pyrophosphate Cataly
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P. L.GAI and C. C. TORARDI Central Research & Development, DuPont Science and Engineering Laboratories, Experimental Station, Wilmington, Delaware 19880-0356. ABSTRACT We report, for the first time, direct studies of the dynamic VOHPO 4 .1/2H 2 0 precursor to (VO) 2 P207 catalyst transformation using recently developed in-situ environmental-cell, high-resolution, electron microscopy (in-situ ECELL-HREM) under controlled environments. Our observations provide fundamental evidence concerning the nature of the topotactic transformation and associated temperature regimes critical to the formation of active catalysts. The direct ECELL-HREM studies show that the structural transformation begins at - 0400 0 C, and0 a mixture of the precursor and pyrophosphate phases exists at - 425 C. At 450 C, most of the conversion to VPO has taken place. These atomic-scale studies reveal no amorphous phases during the transformation, and that the atomic periodicity is maintained throughout. No other phases have so far been identified in the transformation. The direct studies are important in the development of selective catalysts. INTRODUCTION The selective catalyzation of n-butane to maleic anhydride over vanadium phosphorus oxides is an important commercial process [1-4]. The active phase in this selective oxidation is identified as the vanadyl pyrophosphate (VO) 2 P20 7, hereafter
referred to as VPO [2]. For good performance, it is critical that VPO be prepared from a
vanadyl hydrogen phosphate hydrate precursor, VOHPO 4.1/2H 2 0. Transformation of the precursor (hereafter referred to as VHPO) to VPO is of considerable importance in the synthesis of the catalyst and has been the subject of a number of studies [5-6]. A structural association clearly exists between the two phases [7-8]. Based on their structures, and X-ray and electron diffraction studies, a simple conversion mechanism has been proposed [7,8] that allows the framework V-O and P0 bonds to remain intact, whilst only weak V-OH 2 and P-OH2 bonds are broken. In this model, the hydrogen phosphate groups condense into pyrophosphate moieties through an inversion of the P atoms without bulk atomic diffusion, or sliding of the precursor layers. However, this has been challenged by studies based on neutron diffraction [9] which are interpreted as indicative of atomic diffusion and high activation energy. Although the precursors and the resulting catalysts have been studied extensively, these studies are post-reaction examinations of the static samples. This is not necessarily the same as observing dynamically reacting samples under gaseous conditions at elevated temperatures. Therefore, uncertainty exists as to the active nature of the transformation. Understanding of the dynamic structural transformation is further hampered because of the lack of microstructural studies of the precursor samples which contain water. The samples are extremely beam sensitive and highresolution microstructural analyses are difficult to achieve experimentally. A better understanding of the precurs
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