Simulation of NO oxidation Catalysis on Perovskite Catalysts

Base metal oxides are of interest as low-cost replacements for Pt-based NO oxidation catalysts. While the mechanism of Pt-catalyzed NO oxidation is fairly well understood, such is not the case for the oxides. In this work, we report density functional theory (DFT) and kinetic modeling results for one class of potential oxide catalysts, the perovskites. The perovskite LaCoO₃ itself has been shown to have good NO oxidation activity, and Sr substitution improves NO oxidation rates and reduces NO₂ inhibition. We used periodic DFT calculations in the Vienna ab initio Simulation Package to compute the energies of vacancies, O, O₂, NO, NO₂, and NO₃ on the (100)-LaO and (100)-CoO₂ terminations of the parent LaCoO₃ and 8%-doped Sr-substituted material. Further, we used first-principles thermodynamic models to determine the most common surface species under NO oxidation conditions. Experimental TPD suggest O, NO, and NO₂ species are present on the perovskites. Thermodynamic models show nitrates and adsorbed NO₂ as the most stable species on the LaO and CoO₂ terminations, respectively. Based on the results above, we performed a DFT screening of vacancies, O, NO, NO₂, and NO₃ intermediates on the (100)-LaO and (100)-MO₂ terminations of the entire first row perovskite series, LaMO₃ (M = Sc-Cu). Vacancy and O energies vary nearly linearly across the series, while NOx adsorption energies show less variability. We constructed relative energy diagrams combining these results with both Langmuir Hinshelwood (LH, competitive surface adsorption) and Mars van Krevelen (MVK, vacancy-mediated) reaction mechanisms. These models show that the reaction mechanism is sensitive to the material, termination, and temperature. Surface pathway microkinetic models predict rates of the LH MO2>LaO pathway with Mn having the highest TOF. Finally, sulfur deactivation is a practical challenge for the application of perovskites as NO oxidation catalysts. However, recent reports have determined that depositing Pd onto LaCoO₃ can increase the sulfur tolerance. We explore the binding of SO_x on LaCoO₃ and various models of Pd deposition. The increasing Pd content in general decreases the binding of sulfur to the LaO termination. We also use thermodynamic modeling to explore the phase degradation in of LaCoO₃ in the presence of sulfur. The La₂O₂SO₄ is the lowest energy phase of La₂O₃, CoO₂, Co₂O₃, and LaCoO₃.