An
attractive solution to mitigate tars and also to decompose lighter
hydrocarbons in biomass gasification is secondary catalytic reforming,
converting hydrocarbons to useful permanent gases. Albeit that it
has been in use for a long time in fossil feedstock catalytic steam
reforming, understanding of the catalytic processes is still limited.
Naphthalene is typically present in the biomass gasification gas and
to further understand the elementary steps of naphthalene transformation,
we investigated the temperature dependent naphthalene adsorption,
dehydrogenation and passivation on Ni(111). TPD (temperature-programmed
desorption) and STM (scanning tunneling microscopy) in ultrahigh vacuum
environment from 110 K up to 780 K, combined with DFT (density functional
theory) were used in the study. Room temperature adsorption results
in a flat naphthalene monolayer. DFT favors the dibridge[7] geometry
but the potential energy surface is rather smooth and other adsorption
geometries may coexist. DFT also reveals a pronounced dearomatization
and charge transfer from the adsorbed molecule into the nickel surface.
Dehydrogenation occurs in two steps, with two desorption peaks at
approximately 450 and 600 K. The first step is due to partial dehydrogenation
generating active hydrocarbon species that at higher temperatures
migrates over the surface forming graphene. The graphene formation
is accompanied by desorption of hydrogen in the high temperature TPD
peak. The formation of graphene effectively passivates the surface
both for hydrogen adsorption and naphthalene dissociation. In conclusion,
the obtained results on the model naphthalene and Ni(111) system,
provides insight into elementary steps of naphthalene adsorption,
dehydrogenation, and carbon passivation, which may serve as a good
starting point for rational design, development and optimization of
the Ni catalyst surface, as well as process conditions, for the aromatic
hydrocarbon reforming process.
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