*2.2. NO Oxidation Activity*

The oxidation of NO occurs as a homogeneous gas-phase reaction with a second order dependency in NO concentration. For this reason, contributions from gas phase conversion have been detected for studies performed at lean NO concentrations [28–30]. This necessitates the quantification of the gas phase contribution to NO oxidation in the current study, which involves such a high concentration of NO (10%). The blank run performed without catalyst is given in our previous publication [5]. The gas phase conversion gradually decreases with increasing temperature in the studied temperature range (150–450 ◦C); with a conversion of 6.1% at 350 ◦C.

The conversion of NO over LaCo1−xMnxO3 as a function of temperature is shown in Figure 6. The NO conversion and rate of reaction are summarized in Table 2. At low temperature, only gas phase conversion is observed for all perovskites. The catalytic activity for LaCoO3 starts at about 270 ◦C and increases gradually until it becomes limited by the thermodynamic equilibrium. This is in contrast to studies performed at lean NOx conditions, where the catalytic activity starts at significantly lower temperatures (150–200 ◦C); increases with a steeper slope, and becomes thermodynamically limited at ca 300 ◦C [14,15]. The catalytic conversion starts at a lower temperature (240 ◦C) for *x* = *0.25* in comparison to other perovskites and a significant increase in conversion was observed. A further increase in *x* to *0.5* leads to a substantial decrease in NO conversion to conversion levels even lower than what is observed for LaMnO3. Manganese is stable in valence states +III and +IV while cobalt is stable in valence states +II and +III. Ghiasi et al. [21] used X-ray absorption spectroscopy (XAS) to study the valence state of Mn and Co in a series of LaCo1−xMnxO3 perovskites and found that the average trivalent metal site is conserved by shifting the balance between Mn4+/Mn3<sup>+</sup> in combination with Co3+/Co2+. The ratio between Mn<sup>4</sup>+/Mn3<sup>+</sup> and Co3+/Co2<sup>+</sup> decreases with a sequential increase in manganese content, the highest value being observed for LaCo0,75Mn0,25O3. The best activity of LaCo0.75Mn0.25O3 in the series of LaCo1−xMnxO3 perovskites may be attributed to the presence of highest content of Mn4<sup>+</sup> as amongst different valence states of manganese, Mn4<sup>+</sup> (MnO2) exhibits the highest activity for NO oxidation followed by Mn3<sup>+</sup> (Mn2O3) and Mn2<sup>+</sup> (Mn3O4) [31].

**Figure 6.** Conversion of NO over LaCo1−xMnxO3 as a function of temperature.


**Table 2.** NO oxidation activity of perovskites at 350 ◦C.

<sup>1</sup> Catalytic activity obtained by subtracting the gas phase conversion of NO.

Figure 7 shows the conversion of NO over LaCo1−yNiyO3 as a function of temperature. Minor difference in gas phase conversion is observed at lower temperatures due to the differences in the packing of the catalyst bed. The substitution with 25 mol% Ni in LaCoO3 had a significant positive impact on the activity exhibiting a conversion of 30% at 350 ◦C. With a further increase in *y*, the activity gradually decreased until 75% nickel content. Similar conversion curves are exhibited by *y* = *0.5* and *y* = *1* perovskites. Although our results differ from Zhong et al. [13], who reported 70 mol% nickel substitution in LaCoO3 to yield the best results for NO oxidation, it can be argued that they used a co-precipitation method for preparation of perovskites and the activity was tested at substantially different feed concentration (400 ppm NO and 6% O2) compared to the present study. The increase in activity for LaCo0.75Ni0.25O3 can in part be attributed to the 25% increase in surface area. However, for LaCo0.25Ni0.75O3 the surface area increased with 50% compared to LaCoO3 and 33% compared to LaNiO3, while the catalytic activity decreased. Thus, it seems likely that the change in catalytic activity is more related to the chemical oxygen dynamics and to the redox properties than to changes in the surface area in this range. Ivanova et al. [32] reported a maximum in defect structures for *x* = *0.25* in a series of LaCo1−xNixO3, detected by EPR due to the presence of magnetic Ni clusters. However, this depends on the method of preparation and pre-treatment procedure. The highest activity of LaCo0.75Ni0.25O3 may thus be related to lattice defects not detectable by bulk XRD. The NO oxidation activity follows the order LaCoO3 > LaNiO3 > LaMnO3 for the undoped perovskites.

**Figure 7.** Conversion of NO over LaCo1−yNiyO3 as a function of temperate.

Figure 8 shows the rate of reaction as a function of crystallite size for LaCo1−xMnxO3 and LaCo1−yNixO3 perovskites. The rate of reaction increases linearly with crystallite size with *x* = *0.25* and *y* = *0.25* samples being the exception and not included in the linear fit. It should be kept in mind that the crystallite sizes were calculated using the Scherrer equation assuming spherical geometry. Though, the crystallites are not spherical as revealed through SEM images. The crystallite size estimates from the Scherrer equation might not be 100% accurate but they provide a fair comparison.

Partial substitution of LaCoO3 with either manganese or nickel leads to a modification in the redox properties, morphology, structure, crystallite size and valence state of the metal site. Therefore, it is difficult to pinpoint one governing factor determining the catalytic activity. In fact, it is a combination of several factors which contribute to dictating catalytic activity.

**Figure 8.** Rate of reaction as a function of crystallite size: (**a**) LaCo1−xMnxO3; (**b**) LaCo1−yNiyO3 perovskites.
