Perovskites as base metal catalysts for TWC: is there any stretch in their activity?

Andrea Eva  Pascui, Johnson Matthey

Perovskites as base metal catalysts for TWC: is there any stretch in their activity?

The use of purely base metal oxide catalysts for mobile gasoline emission control has periodically been studied since the 1970s when perovskite-type oxides (ABO3) were reported to show reasonable oxidation activity for CO and HCs.1 Their interesting properties, particularly as PGM supports, were further expanded upon in the discovery of the ‘intelligent catalyst’ concept in the 1990s by Daihatsu.2 With the need to meet increasingly more stringent emission limits and interest in more cost-effective and sustainable TWC technology, identifying catalyst compositions which utilise either lower or no PGMs remains an active research topic. Although there has been a continuous effort to understand and improve the catalytic properties of perovskites for automotive applications, the bulk of what has been reported has focussed on evaluation under simple gas feeds. In addition, conventional synthesis methods such as co-precipitation and thermal crystallization give materials with low surface areas, which may hamper optimisation of catalytic properties.

In this talk, we present our findings on the use of high surface area, nanocrystalline perovskites as three-way catalysts; including their evaluation under simulated gasoline exhaust feeds, oxygen storage capacity properties and their stability to high temperature redox conditions.3 Using flame spray pyrolysis (FSP), a combustion method in which soluble precursors in organic solvents are combusted in a CH4/O2 flame to directly give nano-crystalline perovskite particles, the systematic investigation of changing both A site dopant and B site cation was explored. The ability to tune the redox properties of the perovskite material to allow for high thermal stability and flexibility in oxygen content is key to improving the catalytic properties.

An A-site deficient ferrite perovskite, namely La0.8FeO3, is identified as showing the most promise under the simulated gasoline exhaust feeds, therefore kinetic parameter estimation and atomistic modelling has been applied to gain a greater understanding of this non-PGM material as a gasoline three-way catalyst. The kinetic parameters were then used in a catalyst sizing study to compare to a ‘standard’ PGM based ceria catalyst, and tentative mechanistic information based on the CO-NO reaction has also been elucidated from the study. One of the limitations to this base metal technology, is the competitive adsorption between O2 and NO for the active vacancy surface site. Supporting PGMs (i.e. Rh or Pd) on the perovskite not only facilitates oxygen lability and mobility but also opens additional mechanistic pathways. The addition of relatively low amounts of PGM to the perovskite catalyst leads to significantly improved fresh TWC performance, however this stretch in activity is irrelevant when the catalysts are subjected to harsh high temperature hydrothermal redox conditions required by OEMs. Even with significant improvements in their resistance to sintering, its unlikely that these types of base metal catalysts can fully replace PGM based technology in automotive applications.

  1. A. Lauder, US Patent 4049583 September 20, 1977.
  2. Z. Nazarpoor, S. J. Golden, US Patent 2015/0105245 April 16, 2015.
  3. K. Simmance, D. Thompsett, W. Wang, B. Thiebaut, Catalysis Today, 320, 2019, 40-50.