The obvious question here is why TiO2-A-SG can catalyze C-C bond cleavage, as TiO2 is not expected to have BAS required for it. Therefore, we impregnated Ni onto various oxides (0.07 Ni/nm 2 if not specified), and tested them in PP upcycling under the same conditions to verify that the second pathway is hydrocracking. Table shows that Ni only converts PP and forms yellow solid residues on supports known for having BAS, i.e., Nb2O5 and SSZ-13 zeolite (entries 4-6), while other supports showing relatively good PP upcycling performance with Ru in Figure , i.e., CeO2, rutile TiO2, "TiO2-R" and fumed SiO2, do not convert PP with Ni (entries 1-3). These results further support that the superior performance of Ru/TiO2-A-SG in Figure is due to its unique bifunctionality that enables hydrocracking. Hydrocracking allows efficient PP conversion using non-noble metals with good (de)hydrogenation ability, such as Ni, another advantage over hydrogenolysis besides the desired selectivity. We note that TiO2-A-SG is a particularly effective acidic support for PP hydrocracking, showing higher activity than SSZ-13 with even 10 times Ni loading (entry 7 compared to 6). This could be due to the absence of micropores on TiO2-A-SG (Figure ) causing less diffusion limits of the long-chain polymers. C This sample uses a second batch of TiO2-A-SG synthesized following the same procedure. d Calcination was performed at 500 ℃ for 4 h before the reaction (~5 min air exposure in between). e Acetic acid exposure was performed using the procedure described previously after 3 weeks RT air exposure. Since TiO2 surfaces were known to auto-adsorb ppm-level formic/acetic acids in the air to form mixed carboxylate layers, we suspected that the unexpected BAS on TiO2-A-SG are associated with these chemisorbed layers. M/TiO2-A-SG were calcined at 400 ℃ for 4 h in the final step of the synthesis. This step would remove the carboxylic layers, which re-accumulate during the storage (Figure mid showing their presence after long-term storage by C 1s XPS).