Abatement of NOx emissions remains a challenge especially in fuel-efficient lean-burn engine conditions. Current diesel oxidation catalysts are noble metal based promoting the oxidation of NO, CO and residual hydrocarbons.LaCoO3 is a promising low price alternative to noble metals due to its similar catalytic activity. Partial substitution has significant effects on both physicochemical properties and catalytic activity of such materials. Hence, an array of differently doped phase pure perovskite oxides LaCo0.8X0.2O3 (LCX) has been synthesized with X representing dopant ions of different stable valence (Zn2+, Mg2+, Al3+, Ge4+, Zr4+) or other transition metal ions (Cr, Mn, Fe, Ni) in order to elucidate the origin of partial substitution effects on the B-site. The dopant strongly affects the NO oxidation rate. (Fig. 1A) Transition metal doped materials expose higher NO oxidation performance than non-d-block elements or Zn (3d10) with the exceptions LCAl and LCCr. The most active catalyst LCNi exhibits a reaction rate increased almost by a factor of two compared to non-doped LC. The adsorption of reactant and product gases as analyzed by TPD is not significantly affected by the dopant presence and, hence, is no factor in the varying catalytic activity. Instead, the effects are attributed to a change in the Co oxidation state induced by the dopant, possibly accompanied by formation of oxygen vacancies, as reported similarly for A-site substitution. The dopant effect manifests further in changes in the reducibility as evident in TPR experiments (Fig. 1B). The first reduction step is shifted to lower termperatures for more active catalysts. Also, stability against the irreversible reduction at elevated temperature can be strongly increased through partial substitution e.g. with Al or Fe while upholding similar catalytic activity. The partial substitution with lower-valent ions such as Zn2+ and Mg2+ does not induce a catalytic effect as reported for Sr2+ on the A-site.

B-site substitution effects in La-Co based perovskites

251st National Meeting (2016)

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