Tomographic Energy Dispersive Diffraction Imaging To Study the Genesis of Ni Nanoparticles in 3D within γ-Al_2O_3 Catalyst Bodies

摘要

Tomographic energy dispersive diffraction imaging (TEDDI) is a recently developed synchrotron-based characterization technique used to obtain spatially resolved X-ray diffraction and fluorescence information in a noninvasive manner. With the use of a synchrotron beam, three-dimensional (3D) information can be conveniently obtained on the elemental composition and related crystalline phases of the interior of a material. In this work, we show for the first time its application to characterize the structure of a heterogeneous catalyst body in situ during thermal treatment. Ni/γ-Al_2O_3 hydrogenation catalyst bodies have been chosen as the system of study. As a first example, the heat treatment in N_2 of a [Ni(en)_3](NO_3)_2/ γ-Al_2O_3 catalyst body has been studied. In this case, the crystalline [Ni(en)_3](NO_3)_2 precursor was detected in an egg-shell distribution, and its decomposition to form metallic Ni crystallites of around 5 nm was imaged. In the second example, the heat treatment in N_2 of a [Ni(en)(H_2O)_4]Cl_2/γ-Al_2O_3 catalyst body was followed. The initial [Ni(en)(H_2O)_4]Cl_2 precursor was uniformly distributed within the catalyst body as an amorphous material and was decomposed to form metallic Ni crystallites of around 30 nm with a uniform distribution. TEDDI also revealed that the decomposition of [Ni(en)(H_2O)_4]Cl_2 takes place via two intermediate crystalline structures. The first one, which appears at around 180 ℃, is related to the restructuring of the Ni precursor on the alumina surface; the second one, assigned to the formation of a limited amount of Ni_3C, is observed at 290 ℃.