G Model
APCATA-15586; No. of Pages11
ARTICLE IN PRESS
2
P. Riani et al. / Applied Catalysis A: General xxx (2015) xxx–xxx
SAMPLE A
*
Co, cF4-Cu
BCo , tI12-CuAl2
2
*
*
*
*
*
b
c
a
d
2
0
30
40
50
60
70
80
90
100
2
Theta (º)
−
1
Fig. 1. XRD of catalyst A: fresh (a); after annealing at 773 K for 3 h in Ar (b); aged sample after annealing at 773 K for 3 h in Ar (c); exhaust sample after catalysis at 51,700 h
d).
(
Unsupported cobalt has been used in the past for Fischer Tropsch
tained for 15 min under the inert flux and stirring. The separation
from the reaction medium and as well the washing procedure were
carried out in different ways for the different samples prepared.
Four samples, hereinafter denoted A–D, will be considered here.
The different separation procedures will be described in the cor-
responding section. Depending on whether the sample has been
tested and characterized few days after the preparation, several
weeks or months after the preparation, or after catalytic runs, they
will be denoted as “fresh”, “aged” or “exhaust”, respectively.
synthesis [18], while Raney-type “sponge” cobalt is commercially
available for use in the hydrogenation of nitriles and nitro com-
pounds to amines [19].
Unsupported cobalt catalysts produced by reducing Co oxide
where found to be very active for ESR [20–22]. Recently, we found
that unsupported cobalt nanoparticles (NPs), prepared by reduc-
ing Co chloride with NaBH , may act as very good catalysts for ESR
4
at least upon short time on stream experiments [23]. In particular,
they allowed high hydrogen yield (over 85%) at moderate temper-
atures with low CO and CH4 coproduction. XRD analysis did not
show any crystalline phase in case of the fresh catalyst and the
characteristic pattern of cubic metallic Co after its use. However, the
fresh catalyst after annealing at the reaction temperature (773 K)
also shows, besides the pattern of cubic Co metal, some reflec-
2.2. Characterization techniques
X-ray diffraction patterns on dried nanoparticles, annealed at
7
73 K for 3 h, and on the exhaust samples (after the ESR experi-
ment) were carried out by using a vertical powder diffractometer
tions attributed to poorly crystallized cobalt boride Co B, showing
2
X’Pert with Cu K␣ radiation (ꢀ = 0.15406 nm). The patterns were
the presence of boron impurities and their strong interaction with
cobalt. Indeed, the presence of impurities arising from the prepa-
ration procedure and from the precursor salts and their roles in
modifying the catalyst behavior is a relevant point in the field of
heterogeneous catalysis. With the preparation adopted, boron and
sodium impurities arising from the reductant as well as chlorine
arising from the cobalt source can be expected.
As a development of our previous work, we wanted to investi-
gate whether the contamination of the nanoparticles could affect
the catalytic activity of the resulting Co nanoparticles. In particular
we will focus our attention on contamination by boron, the main
contaminant residue of the reducing agent.
◦
◦
usually collected in the 25–100 2ꢁ range with a step of 0.02 and
a counting time for each step of at least 12 s. Powder patterns were
indexed by comparing experimental results to the data reported in
the Pearson’s Crystal Data database [25].
IR spectra were recorded with ThermoFisher Instrument using
KBr pressed disks (1% wt/wt of sample, total weight 0.8080 g).
A scanning electron microscope ZEISS SUPRA 40 VP, with a field
emission gun, was used to study the morphology of all the prepared
catalysts (FE-SEM measurements). This instrument is equipped
with a high sensitivity “InLens” secondary electrons detector and
with an EDX microanalysis OXFORD “INCA Energie 450 × 3”. Sam-
ples for SEM analysis were suspended in ethanol and exposed to
ultrasonic vibrations to decrease the aggregation. A drop of the
resultant mixture was finally laid on a Lacey Carbon copper grid.
XPS measurements were performed using a Physical Electronics
PHI 5700 spectrometer with non-monochromatic Mg K␣ radiation
2
. Experimental
2
.1. Preparation of cobalt nanoparticles
(300 W, 15 kV, and 1253.6 eV) and Al K␣ radiation (300 W, 15 kV,
Co-based nanoparticles were prepared using
a
reduction
and 1456.6 eV) with a multi-channel detector. Spectra of fresh, aged
or exhausted samples were recorded in the constant pass energy
mode at 29.35 eV, using a 720 m diameter analysis area. Charge
referencing was measured against adventitious carbon (C 1s at
for acquisition and data analysis. A Shirley-type background was
subtracted from the signals. Recorded spectra were always fitted
method in aqueous solution. In a typical synthesis procedure, a con-
trolled excess of sodium borohydride is added as reducing agent to
−
2
a 10 M solution of CoCl ·6H O maintained at room temperature
2
2
under mechanical stirring and argon flux as described elsewhere
23,24]. The addition of the reductive agent is quickly followed by
the appearance of a black precipitate. The reaction is then main-
[