Understanding Catalysts at the Atomic Scale Using In Situ Scanning Transmission Electron Microscopy

Lead Research Organisation: University of Manchester
Department Name: Materials

Abstract

Heterogeneous catalysis is a cornerstone of the chemical industry and plays a vital role in improving energy efficiency and reducing unwanted pollutants for a wide range of industrially important chemical reactions. Consequently, the extensive beneficial reach of this process in industry also implicates it in influencing world-wide economic stability.

Bimetallic catalysts often show enhanced performance over their monometallic equivalents in many aspects, such as activity, selectivity, and stability. The mechanism behind improved functionality with the addition of a secondary element is generally poorly understood and is further complicated by variations in particle morphologies demonstrating different activities for a given reaction.

Working closely with BP Ltd in the development of supported metallic/bimetallic nanoparticle characterization under realistic environmental conditions (elevated temperatures and pressures; gaseous environments), initially, the project will focus on the cobalt oxide (Co-Mn/Ru/Re) for the 'fresh' catalyst and the catalyst after the reduction process (cobalt metal). Ideally, a series of analyses from the fresh catalyst from synthesis to operation (via reduction and reactor start-up steps) would provide a logical progression in understanding the transformation processes.

With the use of cutting-edge analytical research methods in correlating the use of STEM with EDS, in situ FTIR spectroscopy, X-ray computation tomography and X-ray absorption spectroscopy, it is hoped that the dramatic improvement in the reaction conversion efficiency by alloying elementary nanoparticles with a secondary metal can be established.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R512035/1 30/09/2017 29/09/2022
1964164 Studentship EP/R512035/1 16/09/2017 29/09/2021 Matthew Lindley
 
Description The main aim of this project is to apply recently developed in situ gas cell scanning transmission electron microscopy (STEM) imaging and analytical capabilities to observe the evolution of the structure and elemental distribution of cobalt-based nanoparticle catalysts during exposure to reactive gas environments and range of elevated temperatures. These catalysts are industrially important to the Fischer-Tropsch process, the synthesis of long-chain hydrocarbons, which can be processed to yield a range of clean fuels, such as petrol and diesel. It is already well understood that the size and morphology of cobalt nanoparticles is critical for their effective catalytic operation. Catalytic testing has also shown that the addition of elemental promotors, such as manganese, can change the nanoparticle size distribution as well as the selectivity shift from methane to C5+ products.

STEM observations of 10 wt% cobalt catalysts supported on titania and promoted with increasing amounts of manganese have been made. High angle annular dark field (HAADF) imaging and energy dispersive X-ray spectroscopy (EDS) elemental mapping have revealed stark morphological variations, both before and during reduction conditions. Large porous cobalt structures identified in the unpromoted catalyst during ex-situ characterization (i.e. in vacuum) have been shown to consist of an agglomeration of small crystallites 2-3 nm in size, as previously calculated by XRD analysis. The structures begin to fragment below 150°C and remain relatively localised to their original position on their titania support. Increased temperatures in a hydrogen environment provide the means for these crystallites to then sinter into relatively large particles, first via Ostwald ripening (>150°C) and then particle coalescence (>250°C). In the promoted catalyst, the effect of Mn is to increase the Co dispersion during catalysts synthesis, resulting in a retardation of the particle size growth during the in-situ reduction treatment.
Exploitation Route Current work has focussed on the hydrogen induced reduction process of the synthesized catalysts during in-situ gas cell STEM observation. The cause of the stark differences observed between the fresh, calcined catalyts as a function of manganese loading could be studied during the synthesis process in much the same way as established during this project. However, as the catalysts are manufactured from aqueous solutions, the use of in-situ liquid cell system would be required. This recently developed technique could provide novel insights into the how the cobalt and manganese interact during synthesis and result in increased dispersion across the titania support.
Sectors Chemicals