Metal Synergy in Pd-Ni Bimetallics: Assessing and Controlling Metal Segregation

Abstract

This thesis aims to enhance our understanding of metal segregation in supported bimetallic catalysts, to maximize the synergy between metals. It covers the assessment, effects, and origin of metal segregation as exemplified by supported alloyed Pd-Ni nanoparticles. To assess the degree and type of metal segregation in detail, we introduce a novel analytical technique to study industrial mesoporous catalysts – atom probe tomography (APT). Due to its nature, measuring mesoporous catalysts was not yet possible with APT, thus we have developed a resin-based sample preparation approach. This enables to unravel sub-nm compositional heterogeneities within/between nanoparticles, which is crucial for the optimization of industrial catalysts. With this upgraded arsenal of analytical techniques, we investigated the origins and effects of metal segregation for bimetallic catalysts prepared via industrially relevant incipient wetness impregnation. It spans most of the catalyst’s lifetime from precursor decomposition during calcination, alloy formation during reduction, and restructuring under steady and modulating operating conditions. Hereby, we elucidate the pathways for alloy formation for less uniform catalyst materials, describe the origin of reduction promotion, and showcase the catalyst’s state under operando conditions. However, next to revealing pathways which lead to alloy formation, we shed light on the origins leading to metal segregation. Therefore, we followed the metal precursor decomposition using different gas atmospheres. This allowed to assess the degree of metal segregation and pinpointed which role the decomposition of precursors plays. Finally, we elucidated how metal segregation/alloying between Pd and Ni affects catalysis mechanistically for both CO2 hydrogenation and chitosan depolymerization.