(113c) Combined Experimental and Theoretical Analysis for the Low-Temperature Formation of Nitrous Oxide on Pt/Al2O3

Precise measurement of active catalyst site density and consequently, turnover frequency can provide fundamental understanding and rationale for better catalyst design. However, existing techniques for active site measurements such as chemisorption provide a lower bound for rate measurements as they assume a homogenous site distribution. In this work, we combine density functional theory (DFT) and kinetic experiments to demonstrate that the structure-sensitive nature of N₂O formation via NO dissociation can serve as a method to probe stepped sites on Pt/Al₂O₃.

Two mechanisms for N₂O formation have been proposed: the first mechanism involves direct dissociation of NO* to form N₂O, and the other mechanism, the NO-assisted NO dissociation mechanism, proceeds via (NO)₂* decomposition to form N₂O. We used DFT to calculate the reaction barriers for relevant reactions on model terrace (Pt(111), Pt(100)) and step (Pt(211)) surfaces at varying NO* coverages. Our findings suggest that at high NO* coverages, the NO-assisted NO mechanism is favored on Pt(211) because of the lower barriers for the overall reaction, whereas N₂O formation is inhibited on terrace sites.

To test this predicted structure-sensitive behavior, we performed kinetic experiments by flowing NO over pre-reduced Pt/Al₂O₃ catalysts of varying particle sizes. We quantified the fraction of metal surface sites responsible for N₂O for­mation by calculating the ratio of moles of N₂O formed to surface metal sites. We observed that, for a given metal loading, as the particle size increased, the fraction of catalyst sites responsible for this reaction first increased, reached a maximum, and then decreased to a constant value. Finally, we employed microscopy to show that sites responsible for N₂O formation follow the same trend as the step-edge ensemble of the nanoparticles. In summary, these findings suggest that low-temperature N₂O formation by NO dissociation occurs on step sites over Pt, suggesting the potential for quantifying these sites.