Control of Dispersion of Ni²⁺ Ions via Chelate Ligands in the Preparation of Ni/SiO₂ Materials. A XAFS Study

Abstract

The influence of the Ni deposition mode on the dispersion of Ni2+ precursors was investigated in the preparation of Ni/SiO2 catalysts. The coordination sphere of Ni complexes was mainly studied by XAFS spectroscopy in the initial step (dried samples) and after a vacuum activating treatment at 700 °C. Four modes of deposition were compared, two leading to supported silicate phases (exchange with ammine ligands and deposition−precipitation) and the other ones using the ethanediamine ligand (exchange and impregnation modes) which produced isolated Ni2+ precursors in electrostatic interaction with the silica support. In this work, EXAFS spectroscopy has been found to be a suitable technique to probe metal−support interactions in the first step of the preparation (dried samples). For samples activated at 700 °C, this spectroscopy showed the presence of several categories of atoms in the first (oxygen backscatterers) and second (Ni and Si backscatterers) shells. A distribution of long (d ≈ 2.04 Å) and short (d ≈ 1.75 Å) Ni−O distances was found, corresponding to hexacoordinated (Ni2+ 6c) and isolated tricoordinated (Ni2+ 3c) ions, respectively. Modeling of the structure of the Ni2+ 3c site indicated a distorted site with two short and one long Ni−O distance. The exchange mode with ethanediamine ligand led to isolated Ni2+ 3c ions which could then be photoreduced into Ni+ 3c ions with the highest yield. Impregnation with ethanediamine Ni nitrate was also found advantageous as it led after thermal activation at 700 °C to NiO particles smaller than those produced from impregnation with Ni nitrate and therefore to smaller Ni particles after the subsequent reduction step.

Conversion of Ethene to Propene on Nickel Ion-loaded Mesoporous Silica Prepared by the Template Ion Exchange Method

“…Table 2 and Fig. 6 reveal several important points from the comparison with the literatures [28][29][30][31][32][33][34][35][36][37]. The distance and the coordination number of the first shells (oxygen backscatterer) on Ni-M41 indicate the presence of hexacoordinated Ni 2+ 6c ions [29] in the TIE sample.…”

How nitric oxide affects the decomposition of supported nickel nitrate to arrive at highly dispersed catalysts

“…The dried impregnate is heat treated in air, called calcination, to convert the precursor into the desired metal oxide, optionally followed by a high temperature reduction treatment to obtain the metal in question. State-of-the-art studies at low Ni loadings and strong precursor-support interactions have revealed much insight into the interplay between metal, ligands, counterions and supports, and their impact on the activity, selectivity and stability of the final catalyst [13,16,17,19]. Moreover, these studies have yielded fundamental insight into the importance of liquid-solid interactions between precursor solution and support on the formation of poorly reducible mixed Ni phases like aluminates or phyllosilicates.…”

Monitoring Transport Phenomena of Paramagnetic Metal‐Ion Complexes Inside Catalyst Bodies with Magnetic Resonance Imaging

“…Besides metal precursor salts, organic complexing agents such as citric acid and nitrilotriacetic acid are often added to impregnation solutions. [9][10][11] Different metal chelate complexes can thus be formed in the impregnation solution depending on pH and concentration. Moreover, it has been shown that pH gradients can occur inside catalyst bodies after impregnation, which can be expected to influence the interaction between metal-ion complexes and the support surface.…”