Activated dissociative chemisorption is key reactive step in many catalytic processes and can serve as a useful model reaction that can be probed in exceptional detail by varying the properties of the impinging gas and the surface to yield multi-dimensional dissociative sticking coefficient data. Precursor mediated microcanonical trapping (PMMT) models of gas-surface reactivity will be discussed as a means to interpret non-equilibrium surface science experiments, extract transition state parameters, and to predict thermal dissociative sticking coefficients relevant to catalysis at high pressures. A complication is that the reactivity of small molecules may be dynamically biased such that energy in one degree of freedom may be more or less efficacious in promoting reactivity than another (e.g., normal translational energy vs vibrational energy). Generally, it has been possible to formulate dynamically-biased PMMT models that are in good quantitative accord with most experiments. The Polanyi rules anticipate that an early transition state barrier will favor normal translational energy over vibrational energy in promoting reactivity, whereas a late barrier will favor vibrational energy. Modeling indicates that such dynamical biases can introduce significant reactivity changes equivalent to a 10-30 kJ/mol change of activation energy at 700 K. Given electronic structure theory methods can calculate both reaction threshold energies and transition state geometries, optimizing dynamics in tuning catalyst performance is worth considering. Effusive molecular beam, supersonic molecular beam, and laser induced thermal associative desorption experiments are analyzed with a view towards revealing dynamical biases and making connection with thermal catalysis.

Towards closing the non-equilibrium gap between surface science and catalysis: Assessing the importance of dynamics, tunneling, and active sites in small molecule activation at surfaces

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ACS Spring 2023

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