The 3-way catalytic convertor for purification of gasoline exhaust and NH₃ selective catalytic reduction (SCR) for removal of NOx from lean diesel exhaust are two successes of heterogeneous catalysis research. The introduction of ceria-based supports as oxygen storage capacity (OSC) materials to expand the usable range of engine air-to-fuel ratios was critical to meeting ever more stringent regulations for NOx emissions and decreasing usage of costly platinum group metals. Similarly, utilization of Cu ion-exchanged zeolites, namely Cu-SSZ13, has become ubiquitous with NH₃-SCR; and more recently, zeolite-based materials are at the forefront of research into new emission control strategies such as passive NOx adsorbers (PNA). Despite trends in vehicle electrification, the need for exhaust catalysis remains critical to the automotive industry due to the continued proliferation of hybrid vehicles. Thus, to move beyond any limits of current materials, it is desirable to expand the exploration space to ceria-free, non-zeolitic, earth abundant materials. Specifically, we seek to improve performance and utilization of first-row transition metal oxides for automotive exhaust catalysis applications.
To that end, the emission catalyst research team at Toyota Research Institute of North America has spent the last several years conducting research on direct NOx decomposition over first-row transition metal spinels (AB₂O₄, A,B = Mn, Fe, Co, Ni, Cu, Zn). While this is an ideal reaction and far from commercialization, our work on Cu_xCo_3-xO₄ bulk oxides, CuOx/Co₃O₄ supported oxides, and other complex spinels has improved our fundamental understanding effects compositional changes impart on structure, stability, activity, and selectivity of the catalyst material. This new understanding subsequently led to improved OSC performance after accelerated aging protocols of iron-based spinels via compositional variations to modify phase stability. Additionally, this method of controlling functionality with well-defined compositional variation was then extended beyond the spinel structure to layered Ruddlesden-Popper oxides, where it became possible to tune the NOx desorption temperature window for PNA application. These results provide new insight into structure-function-reactivity relationships for the important catalyst material classes of bulk and supported metal oxides and represent progress toward utilization of earth-abundant materials in automotive exhaust catalysis.

Charles Roberts

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