Tertiary Alcohols as Mechanistic Probes for Photocatalysis: the Gas-Phase Reaction of 2-Methyl-2-Pentanol on Titania P25 in a Microphotoreactor

Despite intense research in heterogeneous photocatalysis, a lack of mechanistic understanding still hinders the rational design of efficient photocatalysts to make them competitive with thermal processes that currently dominate the industry. This study elucidates the underlying mechanism of photoreactions by employing tertiary alcohols as probe molecules on a titania P25 catalyst for the understanding of photocatalytic reactions on a molecular scale. We show that the reactions do not follow the commonly assumed reaction mechanism of separate but coupled redox reactions. Instead, the gas-phase reaction occurs selectively via a homolytic bond cleavage of the long alkyl chain, leading to the formation of the corresponding ketone and an alkane, as exemplified for 2-methyl-2-pentanol at ambient pressure. The alkane stems predominantly from the recombination of the alkyl-moiety with surface hydrogen. Additionally, we demonstrate that the alkyl moiety can also undergo a dimerization reaction forming a long chain alkane, which is facilitated on bare TiO₂. The high time-resolution enabled by the used microreactor allowed us to confirm that this side reaction is a higher-order process, which is governed by the alcohol surface coverage on TiO₂. The parallels of the observed reaction properties with studies performed on a TiO₂(110) single crystal in vacuum reveal that no significant pressure and material gap exists. On the one hand, this strongly suggests that also the reaction mechanism for the conversion of other alcohols must be reconsidered on titania-based photocatalysts and, on the other hand, demonstrates the potential of tertiary alcohols as mechanistic probes in photocatalysis. Moreover, the highly selective reactions of tertiary alcohols may open up alternative routes for chemical synthesis.