CO oxidation under lean and stoichiometric conditions over ceria-zirconia with very low metal contents (Cu, Co, Ag and Pt)

Abstract

In this work, several ceria-zirconia based catalysts with very low (and equimolar) metal contents were prepared, characterised and tested for the CO oxidation reaction (under lean and stoichiometric conditions), trying to emulate those conditions found in a diesel oxidation catalyst (DOC) system from a diesel engine and those encountered under gasoline exhaust (λ = 1). The metals chosen are Cu, Co, Ag and Pt (as a reliable benchmark). The results reveal enormous differences among reducibility and catalytic activity despite quite similar structural and textural properties of the catalysts, showing differences among dispersion (Ag-catalyst seems to present a low level of dispersion). This catalyst seems to be characterised, as well, by a strong electronic interaction between Ce and Ag centres which is suggested to yield an improved reducibility under H2-TPR conditions. Nevertheless, the order in catalytic activity (Cu > Ag > Co ≈ Pt >> support) seems not to follow the order found in reducibility and the Cu-catalyst seems to be the most active independently on the reaction conditions, yielding nearly overlapped CO oxidation catalytic curves. Interestingly, a strong correlation between the catalytic activity under the two conditions tested and the OSC values of the Ce0.8Zr0.2O2-supported metal catalysts is found. Therefore, OSC parameter measured at 150 °C can be used as a relevant descriptor to evaluate the CO oxidation activity at low and medium conversions for the investigated catalysts, much better than the H2-TPR measurements.