A new unifying structural model of heteropolymolybdate salts: Microstructure and thermal stability of a series of molecular oxides

Abstract

Powder X-ray diffraction and cry0 electron diffraction techniques supported by computer simulations were used to determine the secondary structures of the series of heteropoly acid W A ) compounds Cs,%, P V M o 1~0 a . n H~0 (05x14). The structures of the caesium salts (25x14) are simple cubic with space group Pn3m. The free acid exhibits a tricliic lattice (P-1) for the majority of all crystals. A small fraction of thin crystals was detected with simple cubic structure (Pn3m). Computer simulations of the X-ray diffraction patterns suggested that the structure of Cs2H2PVh4011040~5H20 contains defects in the cation lattice. The caesium ion occupancy is U3. [H5O2lf-groups occupy 1/3 of the cation positions. The diffraction pattern of Cs4PVM01~0~5H20 could be simulated when only 3/4 of the anion positions are occupied. C~~H P V M O ,~O~.~H~O crystallises in a simple cubic lattice without any defects.The secondary structures of the investigated heteropoly salts can, therefore, be described by a single structure with variable site occupations, with the exception of Cs$€PVMolIOa~7H2O (caesium ion occupancy of 1 and polyanion occupancy of I). All salts belong to one phase with large stoichiometric variations. A mixture of acid and caesium salt, with the acid epitactly grown on the salts, an intergrowth-structure with varying caesium ion content, mixed crystals as well as the coexistence of a nanocrystalline X-ray-invisible form of free acid besides caesiurn salt could be excluded for the partly neuaalised materials. Selected physico-chemical properties of the HPA compounds, which depend on the caesium ion content, will be rationalised with the knowledge of the secondary structures.

Nanocatalysis: Mature Science Revisited or Something Really New?

“…These processes are strongly structure-determining and occur under kinetic control. [59][60][61] A characteristic example is the "glow phenomenon" in the preparation of the sulphated zirconia catalyst for butane isomerization, [62] which operates at 400 K but requires calcination to 773 K and accurate control of extrinsic preparation parameters (batch size) in the furnace.…”

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

“…Mixed solids form with a variable amount of Cs + per Keggin unit. This arrangement is thermally unstable and tends to segregate into a core shell-structure [24,25]. The formal composition of the Cs(2)HPA transforms during initial thermal treatment (drying above 473 K) into a nucleus, formed by the stable Cs(3)-salt and a shell of less stable but catalytically active phases of Cs(3-x) salts [24].…”

“…It has been proposed that the active phase of the HPA under reaction conditions corresponds to the intact and undistorted Keggin structure and, hence, that the catalytic reactivity of the material could be understood based on the initial structure of the HPA [22,23]. Conversely, the stability of the Keggin anions during thermal treatment and under catalytic conditions, the homogeneity of the partially salified HPA, and the correlation of the structure of the HPA and its catalytic activity are under debate [24,25,26,27]. Mixed phases of partially salified HPA have been proposed forming a core-shell system of the Cs 3 A-salt and the free acid under catalytic conditions [25].…”

UV/Vis/near-IR spectroscopic characteristics of H4–xCsxPVMo11O40 (x = 0, 2) catalyst under different temperatures and gas atmospheres

“…Many efforts have been made to investigate the thermally unstable state, for example, by XRD [2,4], TG-DTA coupled with IMR-MS and DSC [2,5], IR [4,[6][7][8], EPR [9,10] and Raman [11][12][13]. To improve the stability and catalytic activity of such compounds, cations were introduced in the secondary structure [14,15]. Recently, the caesium containing compound with x = 2 proved to be the best catalyst among the different title compounds, for instance for the partial oxidation of ethene and propene by molecular oxygen [14][15][16].…”