(99c) Microkinetic Analysis of Low Temperature Nitrous Oxide Formation on Pt: Understanding the Role of Catalyst Surface Structure and Coverage

Advances in combustion engines and decreased exhaust temperatures have made understanding the mechanism for N₂O formation on the diesel oxidation catalyst (DOC) at low temperatures imperative. Here, we perform kinetic experiments by flowing NO over pre-reduced Pt/Al₂O₃ catalyst to study the formation of N₂O at low temperatures (400 K-500 K). We observe transient formation of N₂O that coincides with the flow of NO through the reactor, as shown in Figure 1. The decline in N₂O formation may be attributed to O* poisoning active sites.

To find the surface species and reaction steps responsible for N₂O formation over Pt, we performed microkinetic modeling. The microkinetic model (MKM) is parameterized using the reaction energies and activation barriers of the N₂O formation mechanism proposed by Burch et.al.[1]. We calculate reaction energies and kinetic barriers of relevant surface intermediates on different Pt facets at varying surface coverages using Density Functional Theory (DFT) calculations. The reaction energies are used as free parameters that are adjusted by comparing the reaction rates predicted by DFT trained MKM to that of kinetic experiments. The model predictions suggest that N₂O formation depends strongly on NO dissociation barriers and N₂O formation most likely occurs on high-index surfaces. The combination of kinetic experiments and DFT calculated reaction energies allows the MKM to be coverage aware at reaction conditions of interest. Further, the comparison allows the recognition of missing reaction steps in the hypothesized reaction mechanism.

References:

[1] R. Burch, G. A. Attard, S. T. Daniells, D. J. Jenkins, J. P. Breen, and P. Hu., “Low-temperature catalytic decomposition of N2O on platinum and bismuth-modified platinum: identification of active sites,” Chem. Commun., no. 22, pp. 2738–2739, Nov. 2002.