*3.3. Catalyst Activity Testing*

Details of the reactor and experimental setup can be found in our previous publication [5]. Briefly, the activity of the catalysts was measured in a vertical stainless steel tubular reactor (internal diameter = 9.7 mm) operated at atmospheric pressure. The temperature was measured and controlled by a thermocouple, protected by a stainless steel jacket, inserted into the catalyst bed. A MKS MultiGas 2030-HS FTIR Gas Analyzer (path length 5.11 m, Cheshire, United Kingdom), calibrated at 1 bar and 191◦C was used to analyze the composition of NO and NO2 in the product stream.

For activity measurements, 0.5 g of catalyst diluted with 2.75 g SiC was loaded into the reactor and held in place by quartz wool plugs. Prior to the activity tests, the catalysts were pretreated at 500 ◦C for 1 h in 200 Ncm3/min flow of 10% O2/Ar and subsequently cooled down in inert argon atmosphere. The activity of the catalysts was investigated by heating from 150 to 450 ◦C at a rate of 5 ◦C/min under a flow of 200 Ncm3/min of feed gas (10% NO, 6% O2 in balance Ar).

Conversion of NO to NO2 was calculated by the following equation:

$$\text{NO} \cdot \text{conversion} = \alpha \times [\text{NO}\_2]\_{\text{outlet}} / [\text{NO}]\_{\text{irlet}} \tag{2}$$

where [NO]inlet and [NO2]outlet are concentrations of NO at inlet and NO2 at the outlet of the reactor. Volume changes arising from the reaction is taken into account by the constant "α" [36] where α = 0.99. The closure of nitrogen balance across the reactor (99.5–100%) confirmed that all nitrogen is present as NO and NO2. Comparison of catalyst activity is performed at 350 ◦C where the reaction is in the kinetic regime, away from equilibrium. The reaction rate (rNO) calculations were performed by subtracting the homogeneous gas phase conversion of NO; hence, reflecting only the activity provided by the catalyst.

#### **4. Conclusions**

A series of lanthanum-based perovskites have been investigated for oxidation of NO using a feed containing 10% NO and 6% O2, thus partially simulating nitric acid plant conditions. Among the undoped perovskites, the NO oxidation activity follows the order LaCoO3 > LaNiO3 > LaMnO3. A significant increase in NO oxidation activity was achieved by partial substitution of cobalt in LaCoO3 with 25 mol% of either nickel or manganese. Further increase in the degree of Co substitution had a negative impact on activity.

From this work, perovskites are shown to be promising catalysts for oxidizing NO to NO2 at conditions representative of nitric acid plant operation. Low cost, ease of production and significant catalytic activity make perovskites attractive candidates as alternatives to noble metal catalysts.

**Author Contributions:** Conceptualization, A.u.R.S., D.W., B.C.E., R.L. and M.R.; methodology, A.S., S.M.H.; formal analysis, A.u.R.S., S.M.H., S.K.R. and M.Z.; investigation, A.u.R.S., S.M.H., S.K.R. and M.Z.; writing—original draft preparation, A.u.R.S.; writing—review and editing, S.M.H., S.K.R, M.Z., B.C.E., R.L, D.W. and M.R.; visualization, A.u.R.S.; supervision, B.C.E., R.L., D.W. and M.R.; project administration, M.R.; funding acquisition, B.C.E., R.L., D.W. and M.R.

**Funding:** This research was funded by iCSI (industrial Catalysis Science and Innovation) Centre for Research-based Innovation, which receives financial support from the Research Council of Norway, grant number 237922. The Research Council of Norway is also acknowledged for financial support to the Swiss-Norwegian Beamlines at ESRF, grant number 273608.

**Conflicts of Interest:** The authors declare no conflict of interest.
