Understanding the mechanochemical synthesis of manganite perovskites and their catalytic behaviour

Abstract

The tightening of emission regulations and increasing prices of precious metals for existing commercial catalysts drives the need to develop of new sustainable catalysts at lower performance costs. Mechanochemistry offers a solventless, ‘one-step’ route to preparing metal oxide catalysts, however, there is limited information on the chemical steps involved. The use of multiple advanced characterisation techniques, such as HERFD, XES and NAP-XPS, has been applied in this work to understand the structure of the highly disordered and heterogeneous materials produced by ball milling. This thesis has provided further insights to begin to form generalised rules for the mechanochemical synthesis of mixed metal oxides, such as perovskites. In doing so, it can start to aid the rational design of new catalyst technologies. A predominant part of the work within this thesis has focused on the mechanochemical synthesis of LaMnO3. Preforming XAS on ex situ ‘time slices’ during the milling procedure was successful in providing insights into the underlying chemistry; not previously possible by lab-based XRD. The XAS data showed the La precursor to disperse readily over Mn2O3 at the early stages of milling. On increasing the milling time it allowed for mechanical activation of both precursors and the formation of powdered LaMnO3, with no calcination step required. Applying the same milling conditions as for LaMnO3 to the synthesis of other manganite perovskites, ErMnO3 and YMnO3, did not result in the desired phase, with a highly amorphous material produced. This highlights the intrinsic difficulties with regards to the mechanochemical syntheses; often each individual system requires specific optimisation to reach the desired material properties. However, exploring the effect of La dispersion at low mechanical energies was shown to be effective in another system, Au/Al2O3. Challenges regarding the in situ monitoring of mechanochemistry have been addressed within this work. Difficulties in the dynamic set-up of commercial milling equipment has meant in situ monitoring of planetary ball milling is not possible. However, the work here attempted a modified in situ mill and in situ high pressure experiments to mimic those experienced during mechanochemistry. It was able to show the challenges in (1) monitoring mixed metal oxide systems in real-time and (2) mimicking impact forces occurring as a result of mechanical action. Surface sensitive studies, such as XPS, have been crucial in determining improved catalytic activity towards deN2O for the ball milled LaMnO3. Following the reaction under working conditions via NAP-XPS, the increased activity at lower reaction temperatures was accredited, not to changes in the catalytic active site Mn, but to increased surface oxygen vacancies, and the presence -OH within the La 3d XPS region. This research demonstrates by collating a large variety of complementary characterisation techniques it has produced new insights into the mechanochemical synthesis of manganite perovskites. However, it also highlights the continued innovation and development required for the analysis of highly complex materials and equipment setups.

More information

Identifiers

ORCID for Peter Wells: orcid.org/0000-0002-0859-9172

ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

Catalogue record

Export record