Elsevier

Catalysis Today

Volume 157, Issues 1–4, 17 November 2010, Pages 257-262
Catalysis Today

Synthesis of γ-Bi2MoO6 catalyst studied by combined high-resolution powder diffraction, XANES and Raman spectroscopy

https://doi.org/10.1016/j.cattod.2010.02.071Get rights and content

Abstract

The formation of crystalline γ-Bi2MoO6 during hydrothermal synthesis was followed by a combination of HRPD, XANES and Raman scattering in situ techniques. The environment of Mo species changes from isolated tetrahedral MoO42− in the initial gel to distorted octahedra in γ-Bi2MoO6 (Raman, XANES). In the initial stages, spherical (Bi,Mo)2O3 particles form with a fluorite structure, and the growth of anisotropic platelets of γ-Bi2MoO6 follows in a second step (HRPD). The presence of molybdenum specie in the intermediate (Bi,Mo)2O3 mixed oxide was ascertained by EDS during complementary ex situ experiments. These particles are assumed to be involved in the crystal growth process, their structure acting as a skeleton for the formation of γ-Bi2MoO6 koechlinite.

Introduction

It is well-known that the textural and structural properties of a catalytic material influence strongly its catalytic properties. In the case of structure-sensitive reactions like selective oxidation of light alkanes and olefins to chemical intermediates, the morphology of particles must be controlled. The sensitivity of, e.g., methanol which yields formaldehyde or dimethylether when reacting with (1 0 0) or (1 0 1) crystal faces of MoO3 respectively was first demonstrated [1]. Other examples as the (0 0 1) faces of MoVNbTeO (M1) phase (propane to acrylic acid or acrylonitrile) or the (1 0 0) faces of vanadyl pyrophosphate (n-butane to maleic anhydride) claimed to be selective are well-documented [2], [3]. In the latter case it was shown that VOHPO4·0.5H2O precursor obtained by the slurry method and (VO)2P2O7 final catalyst are pseudomorphs owing to the topotactic dehydration, so that the catalyst morphology is directly driven by the precursor morphology [4], [5]. During the heat treatment also the catalyst morphology may change, which results in the formation of specific V(V) phases related to increase (or decrease) of selectivity during catalysis [6]. Several methods of preparation can be used in order to improve the purity, surface area, surface acidity, crystallinity of the catalyst particles.

The power of in situ methods applied to synthesis has been demonstrated several times, including in the case of VPO catalysts [7]. This was soon understood for another case which is bismuth molybdate, active and selective in the oxidation of propene to acrolein [8], [9], [10]. Conventional methods of preparation of bismuth molybdate catalysts like solid-state reactions, sol–gel, co-precipitation have been investigated over the years [11], [12], [13], [14], [15], [16]. All these techniques necessitate a calcination step at ca. 400–700 °C to obtain the final crystalline material, the purity, morphology, surface texture, and grain shape of which are not easily controlled. In the special case of koechlinite γ-Bi2MoO6, a drawback can be the formation of the catalytically inactive form γ′-Bi2MoO6 during the heating process. One way to overcome this difficulty is to prepare γ-Bi2MoO6 by hydrothermal synthesis. By this method, crystalline products with high purity and narrow particle size distribution are generated under mild conditions, at temperatures below 200 °C [8], [15].

Thanks to the fast development of in situ methods of characterization, the catalyst formation during hydrothermal synthesis can be followed with the aim to optimize the conditions of production of γ-Bi2MoO6 catalyst. The target is to be able to control its crystal growth and morphology, as well as to minimize the amount of other phases that could form during the synthesis. In the first studies, the crystallisation of γ-Bi2MoO6 under hydrothermal conditions was followed in situ at 110–150 °C by EDXRD (Energy dispersive X-ray diffraction), and the changes of the coordination of Mo(VI) species were revealed using combined XRD/XAS techniques [8], [9], [10]. The formation of an intermediate compound was observed, but its nature and structure, or the structural modifications that may take place during the growth process, could not be precisely determined. It is noteworthy that this intermediate compound was not mentioned in ex situ studies of the crystallization mechanism performed by XRD, TEM and Raman spectroscopy [17].

It was therefore challenging to determine the nature of the intermediate phase and to elucidate how the layered structure of γ-Bi2MoO6 forms along time of synthesis. For this purpose, we carried out an in situ study using combined High-Resolution Powder Diffraction (HRPD), X-ray Absorption Spectroscopy (XAS) and Raman experiments. Preliminary results were presented in [18]. By combination of these techniques, it was possible to obtain information on both short-range and extended-range structure modifications. The former study is here completed by ex situ characterizations aimed at elucidating the nature of the intermediate that would lead to a better understanding of the crystal growth of γ-Bi2MoO6 catalyst.

Section snippets

Preparation of γ-Bi2MoO6 by hydrothermal route

According to the procedure of in situ hydrothermal synthesis described by Beale and Sankar [9], two solutions, A (2.32 g of Bi2O3 dissolved in 5.8 ml concentrated HNO3) and B (0.88 g of (NH4)6Mo7O24·4H2O dissolved in 5.6 ml ammonia) were prepared. The solution B was slowly added into the solution A. The pH of this mixture was adjusted with ammonia to be higher than 6 before loading about 1 cm3 in a specially designed hydrothermal synthesis cell. This cell was introduced into the pre-heated block

In situ experiments

Fig. 1, Fig. 2 show stacked HRPD patterns and Raman spectra collected in situ during the hydrothermal reaction at 180 °C. Both techniques reveal a two-step reaction whatever the temperature. A crystalline phase with wide peaks was observed since the very first minutes. After 21 min of reaction, its pattern was characterized by lines at 2θ ca. 8.8°, 10.2° and 14.4° (λ = 0.5 Å) (Fig. 1, and inset) and could be assigned to a structure close to that of δ-Bi2O3. Then, this phase gradually disappeared

Conclusion

The combination of HRPD, XANES and Raman in situ techniques during hydrothermal synthesis is a powerful tool to understand the formation and transformations of phases, which will further enable to control the crystal growth of a crystalline catalytic material. By combining such an in situ study with the ex situ characterization of the precipitates during reaction, we can conclude that Bi2MoO6 forms via an intermediate phase of fluorite-type structure which contains molybdenum. EDS showed that

Acknowledgments

The Alliance program for financial support, and ESRF, in particular SNBL for beam time, are gratefully acknowledged. Nora Djelal is acknowledged for SEM experiments. C. Kongmark is grateful to the Royal Thai Government for her PhD grant.

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