| dc.description.abstract | The main aspects of the study on the adsorption of
argon, nitrogen, carbon monoxide, carbon dioxide and hydrogen
have been carried out on iron?kieselguhr powders in the
temperature range of ?191 to 450°C. They are summarised
below:
The iron?kieselguhr samples were prepared by
the method of impregnation and also precipitation. In all
about ten samples were prepared in the present study. In
the impregnation method, the kieselguhr was impregnated
with a solution of ferric nitrate and the nitrate on kieselguhr
was decomposed to the oxide at 400–450°C. The metal
on kieselguhr was then obtained by reduction of the oxide
on kieselguhr in a stream of hydrogen at 450°C. In the
precipitation method, the hydroxide of the metal was precipitated
in the presence of kieselguhr by the addition of
ammonia. After filtration and washing, the wet mass was
dried and the metal oxide reduced to the metal by hydrogen
at 450°C.
The surface areas of the various unreduced and
the reduced catalysts and also the supports were determined
by measurement of the adsorption of nitrogen, argon, and
carbon monoxide at ?191°C and of carbon dioxide at ?70°C
and application of the BET equation to the data obtained.
The surface areas of the various samples prepared by
different methods were compared on the basis of the nitrogen
areas. These studies have shown that with impregnated
catalysts there will be a decrease in the surface area of
kieselguhr, whereas for the precipitated catalysts, there
will be an increase in the area. Further, the surface
area of an impregnated catalyst increases on reduction due
to channelling and formation of cracks whereas the area of
a precipitated catalyst decreases due to sintering.
The surface areas of the iron?kieselguhr samples
determined by employing nitrogen and argon have been compared.
It has been shown that the higher values for the
area obtained by nitrogen adsorption is not due to the
chemisorption of nitrogen on iron at ?191°C, but is due to
the differences in the adsorbent–adsorbate interactions.
It is suggested that nitrogen undergoes greater polarisation
than argon near the surface of a metal resulting in a closer
packing of the adsorbate molecules or larger adsorption.
The surface areas obtained by the application
of the Harkins and Jura equation to the adsorption of
nitrogen compare very well with the areas obtained by the
BET method for several samples. However, differences of
20–30% between the two methods have been noticed with some
samples, which cannot be explained easily.
The differences in the adsorption isotherms of
CO and H? on iron?kieselguhr samples represents the chemisorption
of CO and gives an estimate of the fraction of
surface occupied by metal atoms. It has been shown that
these values do not differ to a large extent from the
values obtained by taking into consideration the suppression
of the physical adsorption of CO by a chemisorbed layer of
the same gas (CO).
Techniques like differential thermal analysis
and electron microscopy were employed to study the properties
of the unreduced iron?kieselguhr samples. These
studies have shown that the fine pore structure of natural
kieselguhr is completely lost on its impregnation with
ferric nitrate solution. Further the precipitation method
employed in the preparation of some ferric oxide?kieselguhr
samples does not give rise to any iron hydro?silicates as
suggested by earlier workers.
The rate of adsorption of hydrogen has been
measured at different temperatures under conditions of
constant pressure and constant volume. The kinetic data
have been examined in the light of equations proposed from
time to time. The Elovich equation represents the kinetic
data over a considerable range of surface coverage. Elovich
plots reveal multiple kinetic stages and explanations have
been offered to account for these.
The effect of temperature and pressure on the
Elovich constants revealed certain interesting results.
The plot of the Elovich constant (?) against the initial
pressure showed a break around 15 cm Hg. In the pressure
range beyond 15 cm Hg, ? varied relatively slowly with
the initial pressure. The adsorption–initial pressure
plots also indicated a break at 15 cm Hg, indicating a
change from one type of adsorption sites to another. The
temperature coefficients of ? and log ? are found to be
either positive or negative depending on the iron?kieselguhr
sample employed and other experimental conditions—constant
pressure and constant volume.
The kinetic data can also be represented by
the application of the equation proposed by Polanyi and co?workers.
It has been shown that the Elovich equation and
the equation of Polanyi et al. reduce to the same form after
expansion, which means that both equations are equally
applicable for any gas–solid system but in view of the
various mechanisms proposed for the Elovich equation, the latter
should be preferred.
The power rate law of Kwan could not be applied
to the present experimental data. This is partly due to
the uncertainty involved in deriving momentary rates, which
are necessary to apply the power rate law.
The various mechanisms put forward to explain
the slow adsorption of hydrogen (the rate of which follows
the Elovich equation) have been classified into those based
on the concept of site generation followed by site decay
and those based on surface heterogeneity. A suitable
mechanism has been proposed to explain the kinetics of
hydrogen adsorption on iron?kieselguhr.
Adsorption isotherms of hydrogen have been
determined on iron?kieselguhr in the temperature range of
?191 to 350°C. At ?191°C, there was considerable adsorption
on both kieselguhr and on iron?kieselguhr. It
was noticed that the hydrogen adsorbed at ?191°C could be
completely desorbed at the same temperature with two
samples of IK–Kg (II and IV), whereas with another sample,
IK–Kg I, about 1.5 c.c. of hydrogen was retained at ?191°C.
Hydrogen retained by IK–Kg I at ?191°C may be chemisorbed
on the small iron particles situated in the pores of the
kieselguhr. The adsorption on IK–Kg II and IV at ?191°C is
mostly due to physical adsorption. In order to get an
idea about the distribution of adsorbed hydrogen between
the metal and the support, it is convenient to employ the
ratio (VCO ? VH?) / VT which gives an estimate of the fraction
of surface occupied by the metal.
At temperatures of ?70°C and above, there is
no adsorption of hydrogen on the kieselguhr samples, so that
any adsorption noticed with iron?kieselguhr samples must
only take place on the iron surface. There is considerable
adsorption of hydrogen at ?70°C and 0°C on the samples
IK–Kg I and IV, whereas the adsorption on the low surface
area sample IK–Kg V is negligible.
At temperatures of 97°C and above, the retention
of hydrogen is determined with a Töpler pump on the
samples IK–Kg I and IV. The H??adsorption isotherm on the
powder having adsorbed hydrogen has been determined at all
the temperatures studied. Based on the studies on retention
and readsorption, the adsorption of hydrogen on IK–Kg I
and IV above 97°C can be classified into groups:
(1) Irreversibly chemisorbed hydrogen which is not desorbed
at the temperature of adsorption and
(2) Reversibly chemisorbed hydrogen which comes off when evacuated at the
temperature of adsorption.
The effect of temperature on the adsorption
of hydrogen on iron?kieselguhr is different from that noticed
with other iron catalysts. The adsorption isobar of hydrogen on
iron?kieselguhr shows a minimum at 0°C and an
ascending region in the temperature range of 100 to 350°C,
whereas on pure iron powder and other iron catalysts, the
adsorption maximum has been reported around 100 or 200°C.
The anomalous behaviour of the iron?kieselguhr system has
been attributed to the presence of the support, which can
influence the adsorption on account of the following
reasons:
(1) Because of the distribution of metal on kieselguhr,
the physical condition of the metal may correspond
more to a film than to a cube catalyst.
(2) The support
may give rise to an interface between the metal and the
support and hydrogen can slowly enter into these portions only
at higher temperatures.
(3) There may be a slow diffusion or
solution of hydrogen at the metal surface which always
increases with temperature.
With increase in temperature above 97°C, there
is an increase in the retention and readsorption of hydrogen
on iron?kieselguhr. The increase in retention with temperature
shows it to be an activated process and this has been
attributed to an increase in the number of active sites with
temperature.
The adsorption of carbon monoxide has been
investigated in the temperature range of ?191°C to 450°C
on iron?kieselguhr powders. At ?191°C, the carbon monoxide
is chemisorbed at the iron surface on all the iron?kieselguhr
samples.
At 0°C and 25°C, the iron?kieselguhr samples prepared
from the high surface area kieselguhr did not form any iron
carbonyl (as with pure iron powder investigated in this
laboratory), whereas the low surface area iron?kieselguhr
gave small amounts of iron carbonyl. The composition of
the iron carbonyl could not be determined on account of the
minute amounts formed. The difference in the behaviour
of iron powder and iron?kieselguhr has again been attributed
to the presence of the kieselguhr support.
In the temperature range of 0 to 97°C, there
was adsorption of carbon monoxide on the high surface
samples IK–Kg I and IV and therefore adsorption isotherms
were determined at this temperature. The calculation of
the differential heats of adsorption in this temperature
range showed that the adsorption of carbon monoxide was mostly
physical in nature.
The catalytic conversion of carbon monoxide to
carbon dioxide has been noticed at temperatures of 200°C
and above. It was found that the rate of catalytic conversion
increased with increasing temperature. The solid
products formed during this reaction between carbon monoxide
and iron?kieselguhr are iron carbide, iron oxide (Fe?O?)
and free carbon (in small amounts).
At 400°C and above, it was found that the iron?
kieselguhr powder took up large quantities of carbon monoxide
without producing any gaseous products of reaction. When
the rate of uptake of carbon monoxide became very slow,
carbon dioxide was detected as one of the products in the
gas phase. At these temperatures, iron oxide (Fe?O?) and
free carbon formed in large amounts, and iron carbide was
also one of the solid products.
A mechanism has been proposed for the reaction of
carbon monoxide with iron?kieselguhr in this temperature
range.
The adsorption of carbon dioxide has been
studied on iron?kieselguhr in the temperature range of ?78°C
to 450°C. A slow uptake of carbon dioxide decreasing with
increase in temperature has been noticed up to 200°C. At
temperatures of 300°C and above, there is a continuous
uptake of CO? by the iron?kieselguhr sample resulting in
the formation of iron carbide, iron oxide, free carbon and
different amounts of carbon monoxide. The formation of
these products has been explained on the basis of the
dissociation of carbon dioxide to carbon monoxide and oxygen
at higher temperatures.
Adsorption isotherms of nitrogen have been
determined on the iron?kieselguhr samples in the temperature
range of 200 to 450°C. It is noticed that the adsorption
decreases with temperature up to 350°C, above which it
increases. The ascending region in the isobar above 350°C
is due to an activated adsorption of nitrogen and involves
probably a partial dissociation of the nitrogen molecule
to atoms at the catalyst surface. | |
