Another intriguing result from this work is the unexpected Brønsted acidity of TiO2-A-SG associated with the carboxylate layers auto-adsorbed from the air (Table ). It has long been known in surface science studies that hydrocarbon layers are formed under air on TiO2, altering its surface properties. The chemical identity of the layers was recently elucidated as mixed formate/acetate, formed by the strong dissociative adsorption of formic/acetic acids ubiquitously present in the air at ppm levels. They are removed by UV radiation or H2/air at ≥ 350 ℃, but stable during laboratory storage, and their presence is detected by XPS (Figure ) and IR as shown in our previous work. TiO2 is widely used in thermo-catalysis as the catalyst or the support, and it is not always treated in-situ at high temperature prior to the reaction.