Direct observation of carbon dioxide adsorption and binding at the air/aqueous interface

Carbon dioxide removal (CDR) involves reducing carbon dioxide (CO₂) concentrations. Developing new technologies and enhancing existing ones for extracting and converting CO₂ are ongoing areas of research. In all these technologies, the movement of CO₂ molecules through an interface is a common process. At liquid surfaces, the nanometer-thick interfacial region is expected to play a fundamental role in enhancing or hindering the process. The interface can have significantly different conditions, such as pH, ion concentration, and ion speciation, compared with the bulk. Despite this, our knowledge of the molecular-scale details of CO₂ capture and conversion at liquid interfaces is limited. Here, we report direct observation of CO₂ surface adsorption and conversion to bicarbonate at the air/aqueous interface of potassium orthovanadate solutions using vibrational sum frequency generation spectroscopy. We show that orthovanadate ions enhance the hydrated CO₂ population at the interface, indicated by a strong peak at 2,336 cm^-1. DFT calculations suggest that CO₂ molecules are bent with respect to their original linear structure, demonstrating the initiation of CO₂ to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>HCO</mml:mi></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:math> conversion. With increasing orthovanadate concentration and/or time of exposure, the CO₂ peak disappears, and (bi)carbonate peaks appear. The characterization of the bulk solutions as well as the precipitated products suggests that the observed interfacial species are transient, different from the final products. This study provides a better understanding of CO₂ transport into aqueous media, not only for CDR technologies but also for environmental and atmospheric chemistry in general.