Calcination of pd nanocatalysts on alumina: ex‐situ analysis versus in‐situ environmental TEM

Abstract

Sustainable chemistry, reduction of pollution, greenhouse gases control, oil refinery, liquid and solid waste management, are essential societal topics related to what is called Environmental catalysis [1], a sector which is booming rapidly for the past 10 years. So, developing innovative catalysts is a very important aspect ahead of us. To develop more advanced catalysts, we have to understand the catalysts genesis in all the stages of preparation (impregnation, drying, calcination and reduction of the active phase). Nowadays Environmental Transmission Electron Microscopy (ETEM) enables dynamic studies of the catalysts down to the nanometer and even atomic scale., at high temperatures and under gaseous environment. This contribution deals with the dynamic evolution of palladium nanoparticles (NP) supported on alumina during the catalyst preparation process. Here, the calcination step is studied in a FEI TITAN‐ETEM microscope operated at 300 kV. A Pd/delta alumina catalyst was investigated at different temperatures and under different gas pressures in order to follow the particles size evolution. Owing to the small size of the Pd NPs in the range of 1‐3 nm, we mainly used the STEM‐ADF imaging mode. Preliminary observations during heating under high vacuum up to 600°C show that particles as well as the supporting media remain stable and apparently not damaged by the electron beam under nominal and atomic resolution imaging STEM conditions (see Fig. 1). Some EELS (Electron Energy‐Loss Spectroscopy, Gatan Imaging Filter Tridiem Ers) analysis was performed in situ in order to ascertain the chemical nature of the observed particles. During this work it appeared essential to perform measurements systematically on the same areas at different temperatures. Fig. 2 is a typically illustration of the in situ evolution of palladium particles under oxygen partial pressure at different temperatures. From such micrographs, the NP size evolution was quantified and compared with post‐mortem TEM observations after ex‐situ experiments performed at same temperatures but atmospheric pressure in the course of the catalyst synthesis (Fig. 3). This comparison shows that all measurements appear to be consistent except those performed in the bright field TEM imaging mode, where larger particles sizes are obtained, most probably due to irradiation effects which were further evidenced by a high‐induced mobility of NPs on their supporting media. This ETEM study brings direct in situ information on the transient stages that cannot be followed by post mortem experiments after ex‐situ treatments; in particular, we will also discuss about the crystallographic evolution of the Pd NPs during the calcination process [2,3].