Ketene as a gas phase intermediate in acetic acid ketonization on TiO₂

Reactive gas-phase intermediates are often underappreciated in heterogeneous catalysis, despite their potential to control key catalytic steps. Here, we provide direct experimental evidence that ketene can act as an intermediate in acetic acid ketonization, a mechanism that has long been debated. Using faceted TiO₂(101) nanoparticles assembled into layers of controlled thickness, we show that ketene, formed from surface acetate, migrates through interparticle voids via desorption-readsorption events before reacting with surface acetate to yield acetone. Surface science and reactor studies reveal a direct correlation between ketene and acetone production and the depth to which the particle layers are saturated. Ab initio calculations support ketene's reactivity via low-barrier coupling pathways that converge on a common β-ketoacid intermediate. Here, we show that ketene can act as a mobile gas-phase intermediate and underscore the broader importance of spatially decoupled elementary steps in determining reactivity and selectivity in catalytic systems.