Relevance of the Drying Step in the Preparation by Impregnation of Zn/SiO₂ Supported Catalysts

Abstract

Zinc supported over silica materials, which are widely used in heterogeneous catalysis, are prepared in most cases by incipient wetness impregnation with solutions of zinc nitrate. After calcination, the formation of ZnO is usually assumed, although not always fully demonstrated. The goal of this paper is to clarify the nature of the zinc phase present in Zn/SiO 2 samples not only after calcination, but also after drying. To achieve this goal, the supported zinc phase was characterized in such systems after drying (between 25 and 200 °C) and after calcination (450 °C). The use of four complementary techniques, UV-visible, diffuse reflectance infrared, and X-ray absorption spectroscopies and X-ray diffraction, allowed us to show that the ZnO phase supposed to be formed during calcination is a minor phase and that the main phase is a poorly crystallized zinc silicate Zn 4 Si 2 O 7 (OH) 2 ‚H 2 O, also called hemimorphite. ZnO formation strongly depends on the drying temperature after impregnation: after drying at 25, 50, and 200 °C, ZnO does not form after calcination. After drying at 25 or 50 °C, the supported zinc phase is an amorphous nitrate, whereas after drying at 200 °C, it is almost already fully transformed into zinc silicate. At intermediate drying temperatures between 90 and 150 °C, part of the zinc forms crystallized zinc hydroxynitrates, which lead to the ZnO phase after calcination; almost all of the remaining zinc is a zinc silicate.

Influence of the synthesis parameters on the cationic distribution of ZnFe2O4 nanoparticles obtained by forced hydrolysis in polyol medium

“…Chemical homogeneity of the particles at a nanoscale level was checked by EDX microanalysis using a FEG (Field Emission Gun) -TEM equipment (probe diameter: 4 nm) operating also at 200 kV and EELS measurements, using a Scanning Transmission Electron Microscope (STEM) operating at 100 kV (probe diameter: 0.5-1 nm) and equipped with a parallel EELS spectrometer optically coupled to a CCD camera. Cationic distribution was inferred from EXAFS at 10 K at the Zn and Fe K-edges and from XCMD at 4 K at the Fe L 2,3 -edges measurements according to the general procedures described in the literature [7,8].…”

Towards ‘greener’ catalyst manufacture: Reduction of wastewater from the preparation of Cu/ZnO/Al2O3 methanol synthesis catalysts

“…Several strategies have been proposed to prevent metal agglomeration during the thermal treatment of ex-nitrate catalyst precursors. These include careful tuning of heating rate and temperature profiles [31,33,34], and the use of glow-discharge plasma-assisted nitrate decomposition [35]. The gas-solid hydrodynamics has also a significant impact on the resulting metal dispersion.…”

Structural and optical characterization of hemimorphite with flower-like morphology synthesized by a novel low-temperature method

“…It is clear that our findings are in satisfactory agreement with the so far observed PL bands. On the other hand, only Chouillet et al [14] observed and proved the minor ZnO phase in the synthesized zinc silicate. They also found that the optical absorption could be assigned completely to the minor ZnO phase.…”