Decomposition Mechanism of Anisole on Pt(111): Combining Single-Crystal Experiments and First-Principles Calculations

To
valorize lignin as a renewable source of aromatics, it is necessary
to develop selective heterogeneous catalysts for the hydrodeoxygenation
reaction of aromatic oxygenates such as anisole. Most of the metal-supported
catalysts tested so far exhibit a high conversion but a low selectivity
toward valuable aromatic hydrocarbons, yielding mainly phenolic compounds.
To gain insights into that catalytic system, we performed surface
science experiments (X-ray photoelectron spectroscopy and temperature-programmed
desorption) under ultrahigh-vacuum conditions (UHV). Dosing anisole
on Pt(111) surprisingly gave benzene, carbon monoxide, and hydrogen
as the main desorbing products of decomposition. With the help of
density functional theory (DFT) we successfully explain the unexpected
selectivity. In the present work we show in particular that phenoxy
(PhO) stands as a key intermediate. Although the UHV conditions do
not allow the hydrogenation of phenoxy into phenol, i.e. the catalytic
product, they reveal the key role of both hydrogen and carbonaceous
species. Under UHV conditions, anisole becomes extensively dehydrogenated:
it results in the formation of carbonaceous fragments, which can actually
perform the deoxygenation of phenoxy into benzene, but also, more
importantly, coke. This detailed study opens the door to a rational
design of hydrodeoxygenation catalysts based on supported metals.