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Porosity of carbon blacks
POROSITY
OF CARBON
BLACKS
ANDRIES VOET and PETER ABOYTES Research Laboratory, J. M. Huber Corporation, Borger, -Texas 79007, C!.S..4. 19 May 1970)
(Received
Abstract-The determination of pore sizes and pore size distribution of carbon blacks by the t curve procedure requires a nonporous reference material with a heterogeneous surface. It could be shown that treatment of a fine thermal black with methane at 105OY: leads to such a standard. It appears that the 1 numbers derived are identical to those proposed by de Boer and coworkers. In addition, it could be proven that the small positive intercept of the t curves for conventional furnace blacks is due to a residual homogeneity, in accordance with an earlier hypothesis. The same phenomena ha\,e been observed in other furnace blacks.
1. INTRODUCTION Nitrogen
adsorption
have been sizes
used
and
1l-51.
extensively
size distribution
The
analyses
porosity
isotherms
at -
196°C
to calculate
pore
in carbon
are based
blacks
on a compari-
of carbon
it is important a
truly
nonporous,
It is known phase
at residence
uncertainty
deposit
carbon
pores
in
in the
addition, surface
reference
that
and heterogeneous statistical
layer
a function data
of the
are known
ing a particularly adsorption isotherm
reference of
dif-
for homostandard
of absorbed units,
nitrogen
as
pressure.
as t numbers,
follow-
elegant procedure. analysis developed
are
a
prerequisite
however, differ Since correct for
the
a carbon
for by
significantly t numbers
calculation
pore
to active
filling.
of
sites[9].
At the same
should
be randomly
phase
at atmospheric
methane surface,
surtace
homogeneity
of after
Consequently, black
could
t number change
Since
with
time.
deposited
pores
since
carbon the gas
pressure
carbon such
a
f-t-om pure
be
an
in character
carbon residual
blacks [ 51 would
a methane
treatment.
methane-treated acceptable
determination.
are
from
on a nearly hererogeneous it was expected that the
disappear
de Boer and coworkers[S]. Recently an increasing series of t numbers have been published [fi-81, based on specific materials, but frequently held more universally valid. These numbers, in magnitude.
gas
1 set, but will
necessarily active sites, we concluded that exposure to methane at 1050°C must lead to
Angstrom
partial
on
below
shows the
In
surfaces.
thickness in
times
in
used.
for the nonporous
indicated
decomposition
preference
view
isotherms
negligible
of
the in
is the
for
carbon
absence
standards
heterogeneous,
Characteristic nitrogen,
however.
complete
in adsorption
geneous
‘l‘hese
results, for
it is essential be
ferences
is the
the
evidence
values
heterogeneous
that at 105O’Y methane
only
of
by the i method, precise
surface.
son with isotherms for a selected, presumably nonporous reference material. A marked lack
blacks
to establish
A
carbon
standard
for
study
the
of the nitrogen
of
adsorp-
tion isotherms upon exposure of a carbon black to methane at 1050°C~ for increasing periods of time should yield important information which could lead to a method For preparing a nonporous Ileterogeneous carbon 135
surface.
136
A. VOE?‘ and P. ABOYTES 2. EXPERIMENTAL
3. RESULTS AND DISCUSSION
The carbon black used was a fine thermal (FT) black, of an arithmetic mean diameter of about 1800 A, selected because of the low degree of particle chain formation and its relatively low surface area, minimizing the possibility of interparticle capillary condensation at higher partial nitrogen pressures. The pelletized sample was rigorously extracted with benzene, followed by heating in an oxygen-free nitrogen stream for one hour at 950°C. This calcination procedure eliminated all remnants of adsorbed materials as well as surface oxide complexes irrelevant for the present objectives. Thereafter, the sample was micropulverized at low temperatures by means of a high speed hammer mill in a nitrogen atmosphere, to obtain material with a low bulk density. This was done to eliminate condensation of nitrogen in capillary spaces between particles, which could possibly occur in the present measuring range with very dense materials. A pure grade of methane was passed over the sample at 1050°C in an electric furnace, the temperature of which was thermostatically controlled. The nitrogen adsorption isotherms were determined by a standard volumetric method which had been found to be in excellent accord with a more elaborate gravimetric procedure [5]. All samples were examined by electron micrography.
Table 1 indicates surface area, porosity and pore size distribution for the original FT black as well as for samples subjected to treatment with methane at 1050°C for 30, 60 and 90 min respectively. Also included are the homogeneity factors H [5], characteristic of residual homogeneity, found from the intercept on the V, axis. Calculations were made according to the previously reported procedure[5]. Figure 1 indicates the t plots using the t values of de Boer[3]. The lines are either single straight lines or straight lines joined together at one or more points at specific t values. From these data it may be concluded that the original FT black is porous, with pores of a width of 9*0,12*5 and 16.0 A [5]. However, methane treatment at 1050°C causes the disappearance of the positive t curve intercept and the gradual filling up of the pores, starting at the smaller pores, until all pores are filled. After 90 minutes’ treatment, all pores have disappeared and the t plot on the basis of the t numbers of de Boer is a straight line through the ori,gin. The surface area was reduced to about one-third of its original value. Attempts to use published t numbers[6-81 differing from the de Boer values failed to obtain physically significant plots, in view of strongly negative intercepts, irregular lines, etc.
Table
1. Properties
of methane
treated
FT blacks
FT black, methane treatment in minutes 30
60
90
14.92 28.7
11.65 10.9
6.20 3.0
4.65 0.0
3.1 6.2 19.4 0.06
0.0 2.2 8.7 0.00
0.0 0.0 3.0 0.00
0.0 0.0 0.0 0.00
0
Surface area (t) Pore area, % Pore size distribution, 9.0 A width 12.5 A width 16-O 8, width Homog. factor H
%
POROSITY
OF CARBON
BLACKS
11./ 11 .,* ,/ p”/ ./ /” A’
.’
0-
/‘Y
I// /
6-
/-
.
.I*
I 6 t
Fig.
Electron
micrography,
visualize
pores
directly,
deposition
particle
deposits,
while
later
filling
In
addition,
completely. many particle pletely
with
giant
clumps a carbon
particle.
particle
while
unable
indicated
that
‘I-his
agglomerates
on
the
particle
a contributing
10
(de Boer)
particles
number
of
particles
was observed.
105OY: simple
new,
very
is an upper temperature process of pore filling
active
growth of carbon black. Tests were repeated
out the surface
more
blacks.
we
obser\-ed
had been covered deposit
to make
process
of
appeared growth
does
factor
that comone
irregularly It thus
on apparently
cause for the large reduction The
to sur-
growth, exactl) as deinitiated by chains of
first
I
8
1. V,-t Plot for F-I‘ blacks treated with methane at 1O:iOY:.
of carbon
had caused particle scribed by ‘I‘esner[9],
to be
.’ .’
.”
/’
4
2
area.
.”
I-
I
_,”
major
./’
I.
.
sites,
30’
.’
,:’ ,
4
face
STD.
/”
Results
elimination
were
quite
of pores
homogeneity surface area.
with
shaped
appears
HAF
similar,
furnace indicating
and of residual
as well as a strong
that
limit for a and surface
surface
reduction
in
covering to
be
only appeal
to the
4. CONCLUSIONS
the
in surface surface
(1) Treatment with methane nonporous
of commercial at 105OY: results
carbon
blacks
carbon
blacks
in completel),
of a heterogeneous
area reduction. No particles were seen which differed basically from the FT particles, indicating that new black formation by
surface. (2) FI‘he 1 numbers proposed coworkers are valid for carbon
methane decomposition alone was insignificant at 1050°C. However, after 2 hr treatment with methane at 1100°C: a significant
the use of an independent standard fog carbon blacks unnecessary. (3) The method of calculation of’ porosit!
b) de Hoer and blacks, making
138
A. VOET and P. ABOYTES
and surface homogeneity for carbon blacks proposed by Voet, Lamond and Sweigart leads to satisfactory results. Acknowledgment-The authors express their thanks to the J. M. Huber Corporation for their permission to publish this work. REFERENCES 1. Pierce C. and Smith R. N., J. Phys. Chem. 57, 64, 149 (1953). 2. Voet A., Rubber World 139,63,232 (1958).
3. de Boer J. H., Linsen B. G. and Osinga Th. J.,J. Catalysis4,643 (1965). 4. Atkins J. H., Carbon 3,299 (1965). 5- Voet A., Lamond T. G. and Sweigart D., Carbon 6,707 (1968). 6. Pierce C., J. Phys. Chem. 72, 3673 (1968) Summary. 7. Mitchell S. A. and Sing K. S. W., Chem. Ind. (London) 1772 (1968). 8. Smith W. R. and Kasten G. A., Rubber Chem. Technol. 43,960 (1970). 9. Tesner P. A., Seventh Symp. on Combustion, 546 (1959).
OF CARBON
BLACKS
ANDRIES VOET and PETER ABOYTES Research Laboratory, J. M. Huber Corporation, Borger, -Texas 79007, C!.S..4. 19 May 1970)
(Received
Abstract-The determination of pore sizes and pore size distribution of carbon blacks by the t curve procedure requires a nonporous reference material with a heterogeneous surface. It could be shown that treatment of a fine thermal black with methane at 105OY: leads to such a standard. It appears that the 1 numbers derived are identical to those proposed by de Boer and coworkers. In addition, it could be proven that the small positive intercept of the t curves for conventional furnace blacks is due to a residual homogeneity, in accordance with an earlier hypothesis. The same phenomena ha\,e been observed in other furnace blacks.
1. INTRODUCTION Nitrogen
adsorption
have been sizes
used
and
1l-51.
extensively
size distribution
The
analyses
porosity
isotherms
at -
196°C
to calculate
pore
in carbon
are based
blacks
on a compari-
of carbon
it is important a
truly
nonporous,
It is known phase
at residence
uncertainty
deposit
carbon
pores
in
in the
addition, surface
reference
that
and heterogeneous statistical
layer
a function data
of the
are known
ing a particularly adsorption isotherm
reference of
dif-
for homostandard
of absorbed units,
nitrogen
as
pressure.
as t numbers,
follow-
elegant procedure. analysis developed
are
a
prerequisite
however, differ Since correct for
the
a carbon
for by
significantly t numbers
calculation
pore
to active
filling.
of
sites[9].
At the same
should
be randomly
phase
at atmospheric
methane surface,
surtace
homogeneity
of after
Consequently, black
could
t number change
Since
with
time.
deposited
pores
since
carbon the gas
pressure
carbon such
a
f-t-om pure
be
an
in character
carbon residual
blacks [ 51 would
a methane
treatment.
methane-treated acceptable
determination.
are
from
on a nearly hererogeneous it was expected that the
disappear
de Boer and coworkers[S]. Recently an increasing series of t numbers have been published [fi-81, based on specific materials, but frequently held more universally valid. These numbers, in magnitude.
gas
1 set, but will
necessarily active sites, we concluded that exposure to methane at 1050°C must lead to
Angstrom
partial
on
below
shows the
In
surfaces.
thickness in
times
in
used.
for the nonporous
indicated
decomposition
preference
view
isotherms
negligible
of
the in
is the
for
carbon
absence
standards
heterogeneous,
Characteristic nitrogen,
however.
complete
in adsorption
geneous
‘l‘hese
results, for
it is essential be
ferences
is the
the
evidence
values
heterogeneous
that at 105O’Y methane
only
of
by the i method, precise
surface.
son with isotherms for a selected, presumably nonporous reference material. A marked lack
blacks
to establish
A
carbon
standard
for
study
the
of the nitrogen
of
adsorp-
tion isotherms upon exposure of a carbon black to methane at 1050°C~ for increasing periods of time should yield important information which could lead to a method For preparing a nonporous Ileterogeneous carbon 135
surface.
136
A. VOE?‘ and P. ABOYTES 2. EXPERIMENTAL
3. RESULTS AND DISCUSSION
The carbon black used was a fine thermal (FT) black, of an arithmetic mean diameter of about 1800 A, selected because of the low degree of particle chain formation and its relatively low surface area, minimizing the possibility of interparticle capillary condensation at higher partial nitrogen pressures. The pelletized sample was rigorously extracted with benzene, followed by heating in an oxygen-free nitrogen stream for one hour at 950°C. This calcination procedure eliminated all remnants of adsorbed materials as well as surface oxide complexes irrelevant for the present objectives. Thereafter, the sample was micropulverized at low temperatures by means of a high speed hammer mill in a nitrogen atmosphere, to obtain material with a low bulk density. This was done to eliminate condensation of nitrogen in capillary spaces between particles, which could possibly occur in the present measuring range with very dense materials. A pure grade of methane was passed over the sample at 1050°C in an electric furnace, the temperature of which was thermostatically controlled. The nitrogen adsorption isotherms were determined by a standard volumetric method which had been found to be in excellent accord with a more elaborate gravimetric procedure [5]. All samples were examined by electron micrography.
Table 1 indicates surface area, porosity and pore size distribution for the original FT black as well as for samples subjected to treatment with methane at 1050°C for 30, 60 and 90 min respectively. Also included are the homogeneity factors H [5], characteristic of residual homogeneity, found from the intercept on the V, axis. Calculations were made according to the previously reported procedure[5]. Figure 1 indicates the t plots using the t values of de Boer[3]. The lines are either single straight lines or straight lines joined together at one or more points at specific t values. From these data it may be concluded that the original FT black is porous, with pores of a width of 9*0,12*5 and 16.0 A [5]. However, methane treatment at 1050°C causes the disappearance of the positive t curve intercept and the gradual filling up of the pores, starting at the smaller pores, until all pores are filled. After 90 minutes’ treatment, all pores have disappeared and the t plot on the basis of the t numbers of de Boer is a straight line through the ori,gin. The surface area was reduced to about one-third of its original value. Attempts to use published t numbers[6-81 differing from the de Boer values failed to obtain physically significant plots, in view of strongly negative intercepts, irregular lines, etc.
Table
1. Properties
of methane
treated
FT blacks
FT black, methane treatment in minutes 30
60
90
14.92 28.7
11.65 10.9
6.20 3.0
4.65 0.0
3.1 6.2 19.4 0.06
0.0 2.2 8.7 0.00
0.0 0.0 3.0 0.00
0.0 0.0 0.0 0.00
0
Surface area (t) Pore area, % Pore size distribution, 9.0 A width 12.5 A width 16-O 8, width Homog. factor H
%
POROSITY
OF CARBON
BLACKS
11./ 11 .,* ,/ p”/ ./ /” A’
.’
0-
/‘Y
I// /
6-
/-
.
.I*
I 6 t
Fig.
Electron
micrography,
visualize
pores
directly,
deposition
particle
deposits,
while
later
filling
In
addition,
completely. many particle pletely
with
giant
clumps a carbon
particle.
particle
while
unable
indicated
that
‘I-his
agglomerates
on
the
particle
a contributing
10
(de Boer)
particles
number
of
particles
was observed.
105OY: simple
new,
very
is an upper temperature process of pore filling
active
growth of carbon black. Tests were repeated
out the surface
more
blacks.
we
obser\-ed
had been covered deposit
to make
process
of
appeared growth
does
factor
that comone
irregularly It thus
on apparently
cause for the large reduction The
to sur-
growth, exactl) as deinitiated by chains of
first
I
8
1. V,-t Plot for F-I‘ blacks treated with methane at 1O:iOY:.
of carbon
had caused particle scribed by ‘I‘esner[9],
to be
.’ .’
.”
/’
4
2
area.
.”
I-
I
_,”
major
./’
I.
.
sites,
30’
.’
,:’ ,
4
face
STD.
/”
Results
elimination
were
quite
of pores
homogeneity surface area.
with
shaped
appears
HAF
similar,
furnace indicating
and of residual
as well as a strong
that
limit for a and surface
surface
reduction
in
covering to
be
only appeal
to the
4. CONCLUSIONS
the
in surface surface
(1) Treatment with methane nonporous
of commercial at 105OY: results
carbon
blacks
carbon
blacks
in completel),
of a heterogeneous
area reduction. No particles were seen which differed basically from the FT particles, indicating that new black formation by
surface. (2) FI‘he 1 numbers proposed coworkers are valid for carbon
methane decomposition alone was insignificant at 1050°C. However, after 2 hr treatment with methane at 1100°C: a significant
the use of an independent standard fog carbon blacks unnecessary. (3) The method of calculation of’ porosit!
b) de Hoer and blacks, making
138
A. VOET and P. ABOYTES
and surface homogeneity for carbon blacks proposed by Voet, Lamond and Sweigart leads to satisfactory results. Acknowledgment-The authors express their thanks to the J. M. Huber Corporation for their permission to publish this work. REFERENCES 1. Pierce C. and Smith R. N., J. Phys. Chem. 57, 64, 149 (1953). 2. Voet A., Rubber World 139,63,232 (1958).
3. de Boer J. H., Linsen B. G. and Osinga Th. J.,J. Catalysis4,643 (1965). 4. Atkins J. H., Carbon 3,299 (1965). 5- Voet A., Lamond T. G. and Sweigart D., Carbon 6,707 (1968). 6. Pierce C., J. Phys. Chem. 72, 3673 (1968) Summary. 7. Mitchell S. A. and Sing K. S. W., Chem. Ind. (London) 1772 (1968). 8. Smith W. R. and Kasten G. A., Rubber Chem. Technol. 43,960 (1970). 9. Tesner P. A., Seventh Symp. on Combustion, 546 (1959).