We believe that certain requirements exist for the carboxylate layers on TiO2 to exhibit Brønsted acidity. All three types of TiO2 we tested have the carboxylate layers (Figure ), but only TiO2-A-SG shows hydrocracking activity. Rutile, or even anatase from the commercial source do not. The observations resonate with our previous report that the layers on TiO2-Acomm deactivate Pt/Al2O3 in CO2 hydrogenation while those on TiO2-R do not. We propose two possible structures of the BAS on TiO2-A-SG: 1) the surface -OH formed along with the carboxylates in the dissociative adsorption of acids; 2) the re-protonation of carboxylates under reaction conditions (≥ 5 bar H2, 260 ℃). Because physisorbed acetic acids are much less active for PP hydrocracking than the chemisorbed carboxylate layers (Table ), we suggest that the first possibility is more likely. Regardless, the properties of the carboxylate layers should be sensitive to the strength and configuration of their chemisorption, which vary with the properties of TiO2.