Hydrothermal synthesis of bi-functional nanostructured manganese tungstate catalysts for selective oxidation.

Xuan Li,Frank Rosowski,Frank Girgsdies,Thomas Lunkenbein,Annette Trunschke,Verena Pfeifer,Robert Schlögl,Jutta Kröhnert

doi:10.1039/c5fd00191a

Abstract

The mechanism of C-H activation in selective oxidation reactions of short-chain alkane molecules over transition metal oxides is critically affected by the balance of acid-base and redox sites at the surface of the catalyst. Using the example of manganese tungstate we discuss how the relative abundance of these sites can be controlled via synthetic techniques. Phase-pure catalysts composed of the thermodynamic stable monoclinic MnWO4 phase have been prepared using hydrothermal synthesis. Variation of the initial pH value resulted in rod-shaped nano-crystalline MnWO4 catalysts composed of particles with varying aspect ratio. The synthesis products have been analysed using transmission electron microscopy, X-ray diffraction, infrared, and photoelectron spectroscopy. In situ Raman spectroscopy was used to investigate the dissolution-re-crystallization processes occurring under hydrothermal conditions. Ethanol oxidation was applied to probe the surface functionalities in terms of acid-base and redox properties. Changes in the aspect ratio of the primary catalyst particles are reflected in the product distribution induced by altering the fraction of acid-base and redox sites exposed at the surface of the catalysts in agreement with the proposed mechanism of particle growth by re-crystallization during ageing under hydrothermal conditions.

Highlights

Metal oxides are widely used as heterogeneous catalysts in the synthesis of chemicals, and in energy conversion and storage applications.[1,2,3,4] High aDepartment of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellscha, Faradayweg 4-6, 14195, Berlin, Germany
The preparation of monoclinic manganese tungstate composed of the wolframitetype structure (ICSD 67906) was performed by hydrothermal synthesis for 12 hours at 180 C in aqueous medium
Mn(OH)x species at the surface of the catalysts as indicated by an increasing intensity of the two bands at 619 and 665 cmÀ1 in the Raman spectra recorded during hydrothermal synthesis that are tentatively attributed to surface manganese oxide–hydroxide species.[23]

Summary

Introduction

Metal oxides are widely used as heterogeneous catalysts in the synthesis of chemicals, and in energy conversion and storage applications.[1,2,3,4] High aDepartment of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellscha , Faradayweg 4-6, 14195, Berlin, Germany. Mn(OH)x species at the surface of the catalysts as indicated by an increasing intensity of the two bands at 619 and 665 cmÀ1 in the Raman spectra recorded during hydrothermal synthesis that are tentatively attributed to surface manganese oxide–hydroxide species.[23] The appearance of the corresponding bands is more distinct in reaction mixtures that contain a higher concentration of OHÀ ions (compare Fig. 1a and c).

Results

Conclusion