(707g) Variational Transition State Theory for Surface Reactions: A Case Study Involving Physisorbed Species

Elementary surface reactions are central to heterogeneous catalysis, where understanding the reaction mechanism is crucial for improving catalytic efficiency. Transition State Theory (TST) is a foundational framework that estimates reaction rates based on the system’s potential energy surface (PES). In its conventional form, known as fixed TST, the transition state is identified as the first-order saddle point on the PES. While this approximation works for many surface reactions, it may not hold in systems with substantial entropy changes.

A notable example involves a physisorbed reactant transforming into two chemisorbed products. In such cases, large configurational entropy changes can cause the position of the transition state to shift with temperature. Variational Transition State Theory (VTST) is more appropriate for these types of systems, as it identifies the transition state that minimizes the free energy barrier along the reaction path.

In this study, we examined surface reactions with significant entropy variations to understand their influence on the transition state location. We computed the intrinsic reaction coordinate (IRC) and validated it using the climbing image Nudged Elastic Band (CI-NEB) method. Multiple points near the saddle point were selected for frequency calculations to confirm the presence of a single imaginary vibrational mode.

Thermodynamic properties were then calculated over a temperature range using vibrational frequencies. The results showed that the transition state can shift notably with temperature, no longer aligning with the saddle point. Using the force-based Hessian, we verified that forces followed the reaction coordinate, and we rediagonalized the Hessian after projecting out the coordinate to obtain corrected vibrational modes.

Our findings highlight that fixed TST may fail to accurately describe reaction rates in high-entropy systems, even if a saddle point exists. VTST provides a better framework in such cases, especially at elevated temperatures.