Low-Temperature (553 K) Catalytic Growth of Highly Ordered Carbon Filaments during Hydrodechloriunation Reactions

Article abstract of DOI:10.1021/jp000048z

In the gas-phase hydrodechlorination of chlorobenzene to benzene over Ni/SiO2, a pretreatment of the catalyst with HCl or HBr gas induced the growth of highly ordered carbon filaments from the catalyst surface. This filamentous growth was observed to occu

Full text of DOI:10.1021/jp000048z

©
Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 18, MAY 11, 2000
LETTERS
Low-Temperature (553 K) Catalytic Growth of Highly Ordered Carbon Filaments during
Hydrodechlorination Reactions
Claudia Menini,†,‡ Colin Park, Rik Brydson, and Mark A. Keane*
†
‡
,†
Department of Chemical Engineering and Department of Materials, The UniVersity of Leeds,
Leeds LS2 9JT, United Kingdom
ReceiVed: January 5, 2000; In Final Form: March 9, 2000
2
In the gas-phase hydrodechlorination of chlorobenzene to benzene over Ni/SiO , a pretreatment of the catalyst
with HCl or HBr gas induced the growth of highly ordered carbon filaments from the catalyst surface. This
filamentous growth was observed to occur at 553 K, i.e., at least 150 K less than that reported previously in
the literature for the catalytic formation of graphitic carbon. The hydrogen halide pretreatment also resulted
in an appreciable increase in the average nickel particle size and a suppression of hydrodechlorination activity.
In the absence of this pretreatment the catalyst exhibited stable hydrodechlorination activity and there was no
evidence of any filamentous carbon growth. Analysis of the halide-treated samples revealed the presence of
potassium that was not evident in the freshly activated catalyst but was introduced as a result of a corrosive
attack of the hydrogen halide gas on the microreactor unit. The source of our low-temperature ordered carbon
growth must be electronic in nature and is attributed to an intimate combination of halogen/alkali metal on
the catalyst surface that facilitates a restructuring of the active sites where a destructive chemisorption of
chlorobenzene precedes the dissolution of carbon and precipitation in an ordered fashion.
4
Introduction
NiCl2 species. In our studies of hydrodehalogenation we probed
such effects by contacting a freshly activated Ni/SiO2 catalyst
As part of an ongoing program of “environmental catalysis”,
we have been examining the catalytic hydrodehalogenation of
directly with the pertinent hydrogen halide, monitoring the
subsequent catalytic activity and comparing it with the repro-
ducible conversions achieved using the untreated catalyst. We
observed a dramatic loss of activity that was unexpectedly
accompanied by the growth of filamentous carbon from the
surface of the catalyst. The growth of such carbonaceous
materials is certainly well established either by an arc discharge
method or by catalytic decomposition of hydrocarbons over both
supported and unsupported metal surfaces, but each route
typically requires temperatures in excess of 723 K.5 In our
system, the catalytic step was conducted at 553 K; i.e.,
appreciable carbon filament growth was achieved at a temper-
ature at least 150 K lower than that quoted for related catalyst
a range of halogenated aromatics in the gas phase over supported
nickel catalysts.1-3 Catalytic hydrodehalogenation represents a
nondestructive, low-energy treatment whereby known hazardous
compounds are transformed into recyclable products in a closed
system with negligible associated toxic emissions. Chloroben-
zene has emerged as a favored model reactant where hydro-
dechlorination yields HCl that can serve to induce catalyst
deactivation possibly via the formation of an unreactive surface
,6
*
Tel: +44 113 2332428. Fax: +44 113 233 2405. E-mail: chemaak@
leeds.ac.uk.
†
Department of Chemical Engineering.
Department of Materials.
‡
1
0.1021/jp000048z CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/14/2000

4282 J. Phys. Chem. B, Vol. 104, No. 18, 2000
Letters
Figure 1. Low- and high-magnification SEM micrographs of the carbon filaments formed during the hydrodechlorination of chlorobenzene over
HBr-pretreated Ni/SiO
2
.
TABLE 1: Effect of Contacting the Ni/SiO
HCl and HBr on the Resulting Hydrodechlorination Rate
r), the Weighted Average Supported Nickel Particle Size (d),
and the Observed Range of Nickel Particle Sizes
2
Catalyst with
systems. We are unaware of any published account of ordered
carbonaceous filament growth at such a low temperature. In
this Letter, we report our experimental observations, provide
some preliminary characterization results for the filamentous
carbon growth that we have generated in this serendipitous
manner and identify a possible combination of promoters for
this growth.
(
freshly activated HCl-pretreated HBr-pretreated
catalyst catalyst catalyst
r, mol h-
d, nm
1
g
-1
Ni
1.5 × 10
-4
8.0 × 10
-6
7.9 × 10
-6
1.6
23
75
size range, nm
0.4-3.3
11-50
10-180
Experimental Procedure
field emission SEM where the sample was deposited on a
standard SEM holder and double coated with gold. The nature
of the carbon deposition was examined by temperature-
programmed oxidation (TPO). The demineralized sample was
ramped (10 K min ) to 1248 K in a 5% v/v O2/He mixture,
and the effluent gas was analyzed by on-line TCD; the sample
temperature was independently monitored using an on-line data
logging system (Pico Technology, model TC-08).
The catalyst was prepared by the homogeneous precipitation/
deposition of nickel (15.2% w/w) onto a Cab-O-Sil 5M silica
2
-1
(
2
1
BET surface area ) 194 m g ) and dried in air at 383 K for
7
0 h. The catalyst precursor, sieved in the mesh range 150-
-
1
-
1
3
25 µm, was activated by heating at 5 K min in a 100 cm
-
1
min stream of dry hydrogen (99.9%) to 673 K, maintaining
this temperature for at least 14 h. All the catalytic reactions
were carried out in situ under atmospheric pressure in a fixed
8
bed glass reactor (i.d. ) 15 mm) at 553 K. A Model 100
Results and Discussion
(kd Scientific) microprocessor-controlled infusion pump was
used to deliver the chloroaromatic feed at a fixed molar rate
The catalytic hydrogen treatment of chlorobenzene generated
benzene as the sole product, and there was no detectable ring
reduction; i.e., the process is 100% selective in terms of
hydrodechlorination. Conversion was stable with respect to time
for up to 6 h on stream, and there was no evidence of any short-
term catalyst deactivation with the untreated catalyst. Steady
state hydrodechlorination rates with and without HCl/HBr
pretreatment are recorded in Table 1, where it is clear that
contacting the catalyst with the hydrogen halide resulted in a
marked loss of activity. The latter proved to be irreversible in
that a catalyst regeneration in flowing hydrogen at elevated
temperatures was ineffective in restoring the original activity.
-
3
-1
(
6 × 10 h , partial pressure ) 0.04 atm), which was carried
through the catalyst bed in a stream of purified hydrogen. In
the case of the HCl/HBr pretreatments, the catalyst was
contacted with flowing HCl and HBr gas (in the range 20-150
cm min for 0.25-5 min) at 548 K; HCl/HBr contact was
exothermic and gave rise to an increase in the catalyst bed
temperature of up to 34 K. The pretreated catalyst was then
3
-1
3
-1
flushed with hydrogen (100 cm min ) for 2 h prior to catalysis.
The catalyst system was operated with negligible internal or
external diffusion retardation of the reaction rate, and heat
transport effects can be ignored. The reactor effluent was frozen
in a liquid nitrogen trap for subsequent analysis, which was
made using an AI Cambridge GC94 chromatograph equipped
with a flame ionization detector and employing a DB-1 50 m
The freshly activated Ni/SiO was characterized by a narrow
2
distribution of the supported nickel metal particle size, as noted
7
elsewhere; selected area electron diffraction (SAED) and dark
×
0.20 mm i.d., 0.33 µm capillary column (J&W Scientific).
All the reactants were AnalaR grade and were used without
further purification.
field imaging have revealed that the nickel particles are crystal-
line. The hydrogen halide treatment induced an appreciable
growth of the nickel crystallites, where the shift in the average
size was significantly greater in the case of the HBr treatment
(see Table 1). Moreover, the treated samples were characterized
by a wide size distribution of geometrically shaped (square/
hexagonal) nickel particles. An agglomeration of nickel particles
(supported on carbon) after a HCl treatment has been noted
The freshly activated and hydrogen halide pretreated catalyst
samples were analyzed by transmission electron microscopy
(TEM) using a Philips CM20 TEM/STEM equipped with a
UTW energy-dispersive X-ray (EDX) detector (Oxford Instru-
ments) operated at an accelerating voltage of 200 keV. The
specimens were prepared by suspending the catalyst in isopropyl
alcohol with ultrasound and evaporating a drop of the resultant
suspension onto a holey carbon support grid. Scanning electron
microscopic (SEM) analysis was performed on a Hitachi S700
9
elsewhere and was attributed to a surface mobility of Ni-Cl
species. Indeed, larger nickel metal particles are typically
generated with a nickel chloride precursor, regardless of the
nature of the support or reduction temperature due to crystallite

Letters
J. Phys. Chem. B, Vol. 104, No. 18, 2000 4283
Figure 2. Representative TEM micrographs of the carbon filaments
formed during the hydrodechlorination of chlorobenzene over HBr-
pretreated Ni/SiO .
2
growth involving volatile nickel chloride in the presence of
hydrogen and hydrogen chloride.10
While the growth of nickel crystallites in the presence of
hydrogen halide was not altogether unexpected, the presence
of an appreciable carbon deposition on these samples was
certainly curious, particularly as this carbon was in the form of
a “hedgehog-like” filamentous growth from the catalyst surface,
as shown by the SEM micrographs presented in Figure 1. The
filaments, which range in diameter from between 10 and 180
nm, appear, from TEM analysis, to be highly ordered and exhibit
little or no curvature; this ordered structure is evident in the
representative TEM micrographs shown in Figure 2. In addition
to this highly ordered form of carbon, a small amount (ca. 5%
w/w) of amorphous carbon, deposited along the filament edges,
was observed along with trace quantities of carbon nanotubes.
Nickel metal particles, with a high degree of dispersion, were
observed on the surface of these carbon filaments. It is now
generally accepted that the growth of filamentous carbon from
a hydrocarbon feedstock on supported metal catalysts involves
adsorption of the hydrocarbon with decomposition on specific
faces of the metal followed by a dissolution of the carbon atoms
into the metal particle and diffusion to precipitate at other faces
as graphitic layers.6,11 The growth and characteristics of the
carbon are very much dependent on catalyst composition and
the reaction conditions. Carbon deposition can be bidirectional
in nature on opposite faces of the metal particle, which then
remains within the carbon fiber throughout the growth process.
Alternatively, deposition may be monodirectional where the
metal particle is either carried at the tip of the growing fiber or
remains attached to the substrate with the fiber growing from
an exposed face.11 In our case, while there is some evidence of
particle fragmentation, the nickel metal crystallites largely
remain anchored to the support during carbon growth. The latter
effect can be attributed to the strong metal support interaction-
Figure 3. STEM/EDX elemental maps of the “hedgehog” sample
showing (a) a STEM annular dark field image of the catalyst exhibiting
growth of a number of individual carbon filaments together with EDX
maps illustrating the distribution of (b) Ni, (c) Si, (d) Cl, (e) Br, and
(f) K on the sample.
and the carbon filaments while the halogen component (from
both the pretreatment and reaction steps) is also present on the
catalyst and the carbonaceous growth. It must be pointed out
that the untreated catalyst after extended use (in excess of 200
h on stream) did exhibit an appreciable carbon content and
preliminary XPS analysis has revealed the presence of a nickel
carbide species. Temperature-programmed oxidation of the used
and demineralized catalyst samples revealed an amorphous
carbon content of up to 8% w/w. In the absence of a
pretreatment with HCl or HBr there was no evidence of any
graphitic carbon formation on the used catalysts regardless of
the reaction conditions, i.e. contact time, temperature, aromatic/
hydrogen partial pressure(s), etc. The direct interaction of
hydrogen halide with the catalyst must then induce a reconstruc-
tion of the metal crystallites to expose orientations that promote
12
the growth of ordered carbon. Yang and Chen have shown
that the specific orientation of surface metal particles influences
the crystallinity of the carbon deposits. It is certainly the case
that the HCl/HBr treatment brings about an appreciable change
in metal dispersion and this surface reconstruction may well
generate an arrangement that favors dissociative chemisorption
(
s) associated with Ni/SiO2 catalysts prepared by precipitation/
7
deposition. The “hedgehog-like” growth, however, serves to
occlude the active metal face(s), and this may be responsible,
at least in part, for the dramatic loss of hydrodechlorination
activity. Given the involvement of electron-withdrawing halo-
gens, the promotion of filamentous carbon growth at such a
low temperature must be linked to electronic effects. Indeed,
prolonged and continuous use of Ni/SiO2 catalysts in hydro-
dechlorination processes has been shown to result in a disruption
of the hydrogen chemisorption characteristics as a result of Cl/
1
3
of chlorobenzene. Indeed, Chambers and Baker have noted
that the presence of trace quantities of chlorine in an ethylene
feed promoted the carbon deposition activity (at 673 K) of cobalt
and iron powders.
The EDX analysis also revealed the unexpected presence of
potassium that was largely associated with the catalyst rather
than the carbon filaments (see Figure 3). Potassium (even in
trace amounts) was not detected in the freshly activated sample,
and its presence must arise as a serendipitous impurity intro-
duced through a corrosive leaching from the walls of the
microreactor unit in the flow of HBr gas. It is well established
3
catalyst electronic interactions. The STEM/EDX elemental
maps of the “hedgehog” (HBr-treated) sample, shown in Figure
3, highlight the dispersion of nickel particles on both the catalyst

4284 J. Phys. Chem. B, Vol. 104, No. 18, 2000
Letters
that alkali metal doping markedly affects the reactivity of
supported metal catalysts via an electronic effect. The possible
involvement of the potassium “impurities” in promoting the
formation of graphitic carbon does find a precedence in the
recent work of Stevens et al.14,15 involving the interaction of
cesium with nanoporous carbon. They observed that their Cs/C
system had a high affinity for hydrogen and promoted the
splitting of the very energetic C-H bond in benzene where
electropositive cesium provides electrons necessary to initiate
the process and the carbon stabilizes the intermediate radical
partial financial support from the Engineering and Physical
Sciences Research Council (Quota Studentship No. 97302706).
References and Notes
(
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(
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(
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1
5
temperature. It seems that we have hit upon a combination of
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(
(
(
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built-in” alkali metal/halogen combinations as a first step to
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Acknowledgment. This work was supported by a grant
RSRG 20833) from the Royal Society. C.M. acknowledges
(
(