Thus, we controlled the post-synthesis history of Ni/TiO2-A-SG to test the hypothesis. Table shows that the PP conversion on Ni/TiO2-A-SG increases with longer post-synthesis air exposure at room temperature (entries 8-9), and drops to ~0 after calcining the catalyst again before the reaction (entry 10). The results indicate that the BAS on TiO2-A-SG are formed by contacting air at room temperature, and removed by high-temperature calcination, aligning well with the behaviors of the carboxylate layers on TiO2 surfaces. We also exposed Ni/TiO2-A-SG to acetic acid vapor to saturate TiO2 surfaces with the layers, which increases the PP conversion (entry 11 compared to entry 9), further supporting the hypothesis. We note that after the acetic acid vapor exposure, the catalyst also carries a significant amount of physisorbed acetic acid (reflected by the significant mass increase and strong smell). The fact that the PP conversion increase caused by it is not significant (46% to 57%) indicates that the chemisorbed carboxylate layers are much more active in PP hydrocracking than physisorbed acetic acid. Calcining acetic-acid-saturated Ni/TiO2-A-SG still eliminates its hydrocracking activity, i.e., BAS on it, as we expected (entry 12). Overall, the control experiments strongly imply that the BAS on TiO2-A-SG originate from the carboxylates it adsorbs from the air. We will discuss the carboxylate layers and BAS in more details in the Discussions section.