Surface Acidity of Oxygen Evolution Intermediates by Excited State Optical Spectroscopy

Area of Science:

• Surface Chemistry
• Catalysis
• Photochemistry

Background:

• Protonation of homogeneous metal-oxide catalysts is understood, but surface acidity's role in heterogeneous metal oxides remains unclear.
• Current understanding of surface acidity is limited to rationalizing pH-dependent product evolution due to a lack of metrics for transient intermediates.

Purpose of the Study:

• To investigate the pH dependence of metastable intermediates in water oxidation catalysis.
• To establish a link between surface acidity and the chemical steps of water oxidation at a heterogeneous metal oxide interface.

Main Methods:

• Utilized time-resolved visible broadband probe spectroscopy to detect emissive intermediates.
• Studied photoexcitation of the oxygen evolution reaction (OER) at an electron-doped SrTiO3/aqueous interface.
• Analyzed the pH dependence of intermediate populations over their entire lifetime (<2 ps to 10 μs).

Main Results:

• The pH dependence of a transient intermediate population (<2 ps) was preserved as it decayed over 10 μs, with a pH onset at 11.4.
• A distinct surface acidity was ascribed to the first water oxidation intermediate, with a pKa indicating a transition between Ti-OH• and Ti-O•- species.
• Excited state optical spectroscopy proved sensitive to protonation of metastable intermediates.

Conclusions:

• Surface acidity plays a critical role in the mechanism of water oxidation catalysis at heterogeneous metal oxide interfaces.
• The hydration layer's properties significantly influence the stability of intermediates with varying acidities, impacting subsequent catalytic steps.
• This work highlights the utility of excited state spectroscopy in probing proton transfer dynamics in catalytic systems.