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CATTOD-8448; No. of Pages9
ARTICLE IN PRESS
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N.W. Cant et al. / Catalysis Today xxx (2013) xxx–xxx
measurements were carried out using a single sample comprising
65 mg of Co/SiO2 made up to 200 mg in total with the silica sup-
port. The mixture was packed in a 6 mm internal diameter Pyrex
tube with a thermocouple placed close to the centre of the catalyst
bed. The Pyrex reactor passed through an aluminium sleeve located
at the centre of a 25 mm bore tube furnace, with the temperature
maintained constant using a Shinko PID temperature controller
operating from a second thermocouple positioned in the sleeve.
The standard test mixture, 2.5% C2H4, 3% H2, 0–500 ppm (0.05%)
CO in a He carrier with a combined flow rate of 50 cm3(STP)/min
(GHSV ∼15,000 h−1), was supplied by mixing streams delivered
by separate mass flow controllers (Brooks-type 5850E). The lat-
ter were connected to cylinders of high purity gases (CoreGas,
Australia) or, in the case of carbon monoxide, a 2500 ppm CO
in helium mixture. Products were analysed by two gas chro-
matographs. One was an MTI model M200 fitted with a Poraplot
U column, which was used to analyse C1 to C3 hydrocarbons, with
a detection limit of ∼10 ppm. It could also be used for the anal-
ysis of CO in the feed stream on bypass (detection limit 30 ppm)
but not under reaction conditions due to interference by hydrocar-
bons. The second chromatograph was a Shimadzu GC17 fitted with
a Restek RtQ PLOT column for analysis of C1 to C6 hydrocarbons
with analysis for C7 and C8 also possible in some situations. The MTI
instrument could be set to operate unattended with a cycle time as
short as 3 min. The Shimadzu instrument was operated manually
every 30 min.
The 20 wt% Co/SiO2 catalyst was made by conventional incipient
wetness impregnation of a commercial silica gel (Davison grade
62, 285 m2/g). The resultant material was dried and calcined in air
for 10 h at 300 ◦C. Samples, 65 mg in most cases, 20 mg in a few,
each diluted to 200 mg with the silica support, were loaded into
the reactor and reduced in 90% H2/He on a temperature ramp of
3 ◦C/min that ended with 10 h at 300 ◦C.
Each reaction was run for a period of one to seven days. The
usual procedure between successive reactions was to flush the sam-
ple with helium, cool to below 50 ◦C and then carry out an in situ
temperature programmed oxidation (TPO). This comprised ramp-
ing in a stream of 0.9% O2/He on a temperature ramp of 3 ◦C/min
to a final temperature of 300 ◦C where it was held for 1 h. The con-
centration of CO2 produced during the TPO, and with somewhat
less accuracy that of the O2 consumed, was monitored by periodic
analyses using the MTI chromatograph. At the TPO conclusion the
sample was cooled to below 50 ◦C in helium alone and re-reduced
as before. A total of 30 reactions were carried out on one 65 mg
sample of Co/SiO2. The sample was then subjected to a single TPO
to a final temperature of 450 ◦C to confirm that the 300 ◦C used in
the previous TPOs had been sufficient to combust all carbonaceous
matter.
same way as for catalytic testing and then passivated by overnight
exposure to 0.9% O2 in He prior to measurement.
3. Results and discussion
3.1. Catalyst characterisation
The XRD patterns of calcined samples as prepared showed sharp
peaks due to cubic Co3O4 alone (JCPDS-IUDD pattern number 00-
from the peak half-width using the Scherrer equation. Peaks in the
patterns of reduced and then passivated samples were considerably
weaker and broader and similar to those of Srinivasan et al. [19] and
of Ducreux et al. [20]. The pattern obtained after standard reduction
showed a mixture of the hcp (pattern 00-005-0727) and fcc (pattern
00-015-0806) phases of cobalt, the fcc phase in slight excess, and
the presence of substantial faulting as indicated by considerable
variations in peak-widths. The corresponding apparent crystallite
dimensions ranged from 4 to 13 nm. There were also weak broad
peaks attributable to an oxide phase, probably non-stoichiometric
as the line positions were not an exact match to those of CoO or
Co3O4. The apparent domain dimension was 2–4 nm, which may
correspond to the creation of oxidised outer layer around a metallic
core during the passivation process.
The use of a 90% H2 in He stream and a slow temperature ramp
precluded an accurate estimate for hydrogen consumption during
the standard reduction procedure. However, further TPR (to 900 ◦C
in 10% H2/Ar) of a sample reduced by the standard method showed
an additional H2 consumption of ∼0.3 mmol/g which is ∼7% of
that expected for complete reduction if the calcined preparation
contained Co3O4 and SiO2 alone. On this basis reduction by the
standard method was ∼93% complete.
Carbon monoxide uptake on reduced samples was
∼0.13 mmol/g corresponding to a dispersion of ∼6.5% using a
COadsorbed/Cosurface ratio of ∼0.6 for 20 wt% Co/SiO2 obtained
showing a ratio of 0.4 for bulk Co and 0.7 for 10% Co/SiO2. The
stoichiometry of <1.0 implies the existence of bridge-bound CO
surface species for which there is diffuse reflectance infrared evi-
dence for Co/SiO2 catalysts [22] and both RAIR and LEED evidence
for adsorption on a Co(1 0 1 0) single crystal surface [23].
A dispersion of 6.5% corresponds to an apparent crystallite size
of ∼15 nm, assuming the cobalt was present as uniform spheres.
This is in reasonable agreement with the XRD measurements if the
latter determinations (i.e. 13 nm or less) correspond to the metal-
core of crystallites in which the outer layer was converted to an
oxide shell during passivation.
Carbon monoxide chemisorption measurements were carried
out on the 65 mg Co/SiO2 sample while in the reactor. After reduc-
tion in the standard way it was exposed to a stream of 100 ppm
CO/He with the MTI chromatograph used to monitor the emergence
of CO, which occurred near step-wise after ∼38 min.
3.2. Example reaction
Ethylene oligomerisation experiments were carried out using
the standard C2H4/H2 mixture and 65 mg sample of Co/SiO2 at tem-
peratures from 50 ◦C to 180 ◦C and CO concentrations from zero to
500 ppm. The results for a typical experiment, carried out at 120 ◦C
with 25 ppm CO in the feed, are shown in Fig. 1. As may be seen,
the conversion of ethylene, XC 4 , calculated as
Independent temperature programmed reduction (TPR) mea-
surements were carried out on 0.2 g samples of Co/SiO2 using
a Micromeritics Autochem 2920 instrument with a dry ice trap
for water removal ahead of the thermal conductivity detector.
The samples were ramped at 10 ◦C/min in 10% H2/Ar flowing at
50 cm3(STP)/min to a final temperature of 900 ◦C. Comparison mea-
surements were made on samples of the starting calcined material
and of material pre-reduced to 300 ◦C using the standard procedure
applied prior to the activity tests (i.e. 10 h in 90% H2/He).
H
2
C2H4(out)
C2H4(in)
XC 4 (%) = 100 ×
,
H
2
was complete for the first 100 min and then fell steadily to ∼50%
after 1500 min of reaction. While the selectivity to all higher prod-
ucts in total, C3+, calculated as
Powder XRD patterns of both calcined and reduced samples
were obtained using a Philips X’pert Pro MPD diffractometer oper-
ating with a Cu K␣ source, a step size of 0.013◦ (2ꢀ basis) and a scan
rate of 5◦/min. Reduced samples were prepared by reduction in the
ꢀ
Cn(n > 2)
C2H4(in) − C2H4(out)
SC + (%) =
3
Please cite this article in press as: N.W. Cant, et al., Ethylene oligomerisation over Co/SiO2 in the presence of trace carbon monoxide: The Eidus