The reduction of diazonium salts has recently been proposed as a robust covalent modification scheme for graphene surfaces. While preliminary studies have provided indirect evidence that this strategy decorates graphene with aryl moieties, the molecular ordering and conformation of the resulting adlayer have not been directly measured. In this Article, we report molecular-resolution characterization of the adlayer formed via the spontaneous reduction of 4-nitrophenyl diazonium (4-NPD) tetrafluoroborate on epitaxial graphene on SiC(0001) using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS). An atomically flat inhomogeneous layer of covalently bonded organic molecules is observed after annealing the chemically treated surface at ∼500 °C in UHV. STM and STS results indicate that the adlayer consists predominantly of aryl oligomers that sterically prevent uniform and complete covalent modification of the graphene surface. The adsorbed species can be selectively desorbed by the STM tip above a threshold sample bias of -5 V and tunneling current of 1 nA, thus enabling the fabrication of a diverse range of graphene nanopatterns at the sub-5 nm length scale.
In
this study, we report a single-step continuous production of
straight-chain liquid hydrocarbons from oleic acid and other fatty
acid derivatives of interest including castor oil, frying oil, and
palm oil using Mo, MgO, and Ni on Al
2
O
3
as catalysts
in subcritical water. Straight-chain hydrocarbons were obtained via
decarboxylation and hydrogenation reactions with no added hydrogen.
Mo/Al
2
O
3
catalyst was found to exhibit a higher
degree of decarboxylation (92%) and liquid yield (71%) compared to
the other two examined catalysts (MgO/Al
2
O
3
,
Ni/Al
2
O
3
) at the maximized conditions of 375
°C, 4 h of space time, and a volume ratio of 5:1 of water to
oleic acid. The obtained liquid product has a similar density (0.85
kg/m
3
at 15.6 °C) and high heating value (44.7 MJ/kg)
as commercial fuels including kerosene (0.78–0.82 kg/m
3
and 46.2 MJ/kg), jet fuel (0.78–0.84 kg/m
3
and 43.5 MJ/kg), and diesel fuel (0.80–0.96 kg/m
3
and 44.8 MJ/kg). The reaction conditions including temperature,
volume ratio of water-to-feed, and space time were maximized for the
Mo/Al
2
O
3
catalyst. Characterization of the spent
catalysts showed that a significant amount of amorphous carbon deposited
on the catalyst could be removed by simple carbon burning in air with
the catalyst recycled and reused.
Current
interest in renewable fuel production is focused on high-performance
fuels such as jet fuel because of their premium value in the marketplace.
Currently, lower-value fuels such as biodiesel can be obtained using
a variety of feedstocks, but contain significant amounts of oxygen,
hence lowering their fuel value. In this work, we examined a one-pot
catalytic hydrothermal process for the decarboxylation with an activated
carbon catalyst of oleic acid as a model compound for free fatty acids.
Temperature (350–400 °C), water-to-oleic acid ratio (2:1–4:1,
v/v), catalyst, catalyst-to-total feed ratio (0.15–0.75), and
residence time (1–2 h) were found to be key factors for removing
oxygen from oleic acid. The complete removal of the carboxylic group
from the upgraded liquid phase was achieved at 400 °C with a
water-to-oleic acid ratio of 4:1 (v/v) and a residence time of 2 h
as confirmed by FTIR and 13C NMR results. The pseudo-first-order
reaction rate constant was found to follow Arrhenius behavior with
the activation energy determined to be 90.6 ± 3 kJ/mol. GC-FID
results showed a high selectivity to heptadecane conversion, whereas
the GC-TCD results indicated that decarboxylation was the dominating
chemical reaction. High heating values and fuel densities in the range
of commercial jet fuels were obtained using this approach without
the addition of high-pressure hydrogen or a hydrogen-donor solvent.
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