M. Sansotera et al.
JournalofFluorineChemistry205(2018)22–29
undergo abrupt corrosion-related breakdown in the presence of reactive
fluorinated species [26]. A passivation layer made of metal fluorides,
rather than metal oxides or hydroxides, is required on the inner surface
of reactors used for fluorination reactions in order to prevent corrosion.
In stainless steel reactors, a passivation film is obtained by direct
fluorination of the vessel, followed by an appropriate thermal treatment
[28–30]. In Al- or Cu-based reactors, a protective layer of Ni is usually
deposited on the inner surface and then passivated via mild direct
fluorination [29–31]. The typical Ni-based coatings are prepared by
chemical and electrochemical deposition methods coupled with colloid-
based strategies, including the use of additives, surfactants, and tem-
plates [32,33]. The design of a microreactor chamber can be improved
by structuring the catalyst support surface, providing important ad-
vantages such as increased surface area thanks to micro- and nano-
structured morphologies, sensible wettability control, and availability
of grafting sites suitable for the immobilization of catalytic species
This work reports on the fabrication of a working catalytic micro-
reactor for the synthesis of trifluoromethyl hypofluorite, CF3OF, via
direct fluorination of carbonyl difluoride. Trifluoromethyl hypofluorite
is an interesting trifluoromethoxylating agent as CF3O represents a
highly valuable functional group in pharmaceutical and agrochemical
applications owing to its peculiar physicochemical properties and high
metabolic stability [18–25,38]. Moreover, CF3OF can be directly syn-
thesized from CO and F2 immediately before use, circumventing
handling/manipulation hazards along with the special conditions re-
quired for storage [39]. Indeed, as shown in the present work, a cata-
lytic microreactor meets the safety and throughput requirements of
industry processes, potentially leading to the introduction of CF3OF as a
readily available trifluoromethoxy transfer agent. αtrifluorinated ani-
soles are, for example, useful as intermediates for dyes and agro-
chemicals and they are industrially prepared on the large scale from
other aromatic substrates, such as phenol or α,α,α-trichloroanisole, by
treatment with HF [40–45]. Substituted α,α,α-trifluoroanisols can be
directly synthesized also by reaction between CF3OF and appropriate
aromatics in good to medium yields [38]. Insights obtained from this
research were applied to the fabrication of a hierarchically nanos-
tructured nickel coatings acting as protective layers as well as catalyst
support in the catalytic microreactors. In particular, the effect of elec-
trolyte pH on morphology and wetting properties was investigated. The
resulting coatings were exposed to elemental fluorine in order to obtain
a fluorine-resistant passivation layer suitable for application in direct
catalytic fluorination reactions. Bare and passivated nanostructured
nickel coatings were characterized by scanning electron microscopy
(SEM), atomic force microscopy (AFM), X-ray photoelectron spectro-
scopy (XPS), and X-ray diffraction spectroscopy (XRD). The wettability
of the coatings was also evaluated by measurements of static water
contact angle.
2.2. Electrodeposition of hierarchically nanostructured nickel coatings
Before the electrodeposition, Cu plates were pre-treated by etching
in a 20 wt.% H2SO4 solution for 10 min at room temperature. Ni thin
films were electrodeposited on Cu substrates from 1.0 M NiCl2 solutions
at different pH: 1.0, 2.0, 3.0, and 4.0. In each deposition, the electro-
lytic solution was augmented by addition of H3BO3 0.5 M as pH buffer
agent, while the desired pH was achieved by addition of an appropriate
amount of HCl. pH values of 1, 2, 3, and 4, were considered. DEA
(0.5 M), as crystal modifier, and sodium lauryl sulphate (1 g/L), as anti-
pitting additive, were also dissolved into the solution. The electro-
deposition was performed under galvanostatic conditions in two sepa-
rate steps. A first treatment at 20 mA cm−2 for 10 min produced the
primer layer, while a successive treatment at 50 mA cm−2 for 1 min.
The total charge was 15C cm−2 for a final average thickness of about
5 μm. A constant deposition temperature of 60 °C was maintained
throughout. A Ti mesh was used as counter electrode. After electro-
deposition, the samples were rinsed with distilled water and dried
under N2 flux. Stirring of the electrolytic solution was deemed un-
necessary, and thus switched off after reaching uniform thermal con-
ditions prior to deposition, as it was found to be detrimental to the
homogeneity of the final coatings.
2.3. Passivation towards fluorine resistance
Each nanostructured Ni coated sample was initially loaded into a
previously-passivated stainless steel reactor connected to a vacuum
line, and was exposed to a set amount of gaseous F2 for a specific
amount of time. The passivation procedure included four steps at in-
creasingly aggressive conditions: 60 mbar of F2 for 30 min (step 1);
150 mbar for 15 min (step 2); 200 mbar for 10 min (step 3); 300 mbar
for 5 min (step 4). Vacuum conditions (under 8 mbar) were re-estab-
lished in the reactor between each step.
2.4. Characterization
The morphology of the nickel coatings on copper plates was ob-
served with the aid of a ZEISS EVO50 EP Scanning Electron Microscope.
The samples were analysed without performing surface etching or
conductive layer coating. The SEM parameters were as follows: working
distance of 20.0 mm, beam current of 100 pA, acceleration voltage of
20.00 kV, magnifications of 1, 5 and 10 kx.
Atomic force microscopy (AFM) was performed using a Bruker
Caliber scanning probe microscope. Scans of 5 × 5 μm and 10 × 10 μm
were acquired for each sample in tapping mode, using Sb n-doped si-
licon tips. Surface roughness was evaluated by flattening the images
(first order) using WSxM software.
XPS spectra were obtained by using an M-probe apparatus (Surface
Science Instruments). The source was monochromatic Al Kα radiation
(1486.6 eV). A spot size of 200 × 750 μm and a pass energy of 25 eV
were used. 1 s level hydrocarbon-contaminant carbon was taken as the
internal reference at 284.6 eV. Fits were performed using pure Gaussian
peaks, Shirley’s baseline, and without any constraints. For each sample,
a survey analysis in the whole range of the X-ray spectrum and high-
resolution analyses in the typical zone of C-1s, Ni-2p, O-1s, and F-1s,
were performed.
2. Experimental
2.1. Materials
Ni thin films for the preliminary optimization of electrodeposition
and passivation conditions were deposited on a 30 × 20 mm area on
commercial pure Cu plates (size: 60 × 30 × 0.6 mm). Nickel(II)
chloride hexahydrate, NiCl2∙6H2O, boric acid, H3BO3, hydrochloric
acid, HCl, diethanolamine (DEA), NH(CH2CH2OH)2, sodium lauryl
sulphate, CH3(CH2)11SO4Na, and sulfuric acid, H2SO4, were used to
prepare the solutions for electrodeposition. All the chemicals were of
reagent-grade purity (supplied by Sigma Aldrich) and were used
without further purification; doubly distilled water passed through a
Milli-Q apparatus was used to prepare the solutions.
X-ray diffraction (XRD) patterns were obtained with thin film con-
figuration on a Philips PW 1830 X-ray Diffractometer equipped with a
Philips PW 3020 Goniometer with Cu Kα radiation (λ = 1.54058 Å) at
a scan rate of 0.02° s−1
.
Static contact angle (SCA) values with water were measured by
sessile drop method in order to evaluate the hydrophilic/hydrophobic
properties of the coatings immediately after rinsing and drying. SCA
measurements were performed with a Dataphysics OCA contact angle
instrument and elaborated with SCA20 software. The averaged contact
angle for each coating was calculated as the average over five values
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