The intermediate steps of catalytic mechanisms are challenging to identify experimentally, but are critical to understanding the speed, stability, and selectivity of product evolution. In the laboratory, we employ photo-triggered vibrational and electronic spectroscopy to time-resolve the catalytic cycle at a surface, identifying meta-stable intermediates and critical transition states which connect one to another. The focus is on the highly selective oxygen evolution reaction (OER) at the semiconductor (SrTiO3)-aqueous interface, triggered by an ultrafast light pulse in an electrochemical cell. A short summary of past work will be given, which resolved the structure and picosecond formation kinetics of the first meta-stable electron-transfer intermediates (Ti-OH*) of OER. The main topic will concern the recent results that connect the time-resolved kinetics to a thermodynamic free energy difference often used to differentiate the activity of materials. In particular, the picosecond optical spectra of the Ti-OH* population on lightly doped SrTiO3 are ordered by their surface hydroxylation. A Langmuir isotherm as a function of electrolyte pH extracts an effective equilibrium constant relatable to the free energy difference of the first OER reaction step. The Langmuir isotherm is highly dependent on the surface hydroxylation at neutral pH, tuned by the Nb-doping of SrTiO3 and measured by Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS). Overall, the work shows that in applying ultrafast spectroscopy to a catalytic reaction, conditions that shift the individual equilibria of separable reaction steps become important, independent axes to the traditional time & energy axes of the spectroscopy.

Resolving catalytic mechanism at an electrode surface by ultrafast spectroscopy while tuning surface hydroxylation

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.