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Ammonia synthesis and related reactions over iron-cobalt and iron-nickel alloy catalysts. Part I. Catalysts reduced at 853 K
Cutalysis, 3 (1982) 161-176 Eleevier Scientific Publishing Company, Amsterdam
161
Applied
AMMONIA ALLOY
SYNTHESIS
AND
CATALYSTS.
D.W. TAYLORa, Department aPresent
University
AND
IRON-NICKEL
853 K
C. KEMBALL
of Edinburgh,
Ltd., Paints Division,
I.C.I.
IRON-COBALT AT
and
D.A. WHAN
West Mains Road, Edinburgh,
Wexham
EH9 355
Road, Slough,
SL2 5DS.
address;
Cheshire, 'Present
OVER
REDUCED
D.A. DOWDENC,
P.J. SMITHb,
address;
REACTIONS
CATALYSTS
I.
of Chemistry,
Berkshire, bPresent
RELATED
PART
- Printed in The Netherlende
Ltd., Mond Division,
I.C.I.
Winnington,
Northwich,
CW8 4DJ.
address;
Cleveland,
(Received
12 Dunottar
Avenue,
Eaglescliffe,
Stockton-on-Tees,
TS16 OAB.
8 January
1982, accepted
22 March
1982)
ABSTRACT Studies nitrogen catalysts reduced
have been undertaken
isotope exchange containing
of ammonia
synthesis,
3% of alumina
as a structural
for 18 h at 853 K before use and metal
adsorption
with coverage
stabiliser.
chemisorption
so markedly
equilibrium
becoming
could be observed
that temperatures
within a reasonable
significant.
853 K were required the temperatures chemisorption reversible
for ammonia
adsorption
Catalysts
were by
below 673 K the rate decreased
of 773 K or more were required period.
of activity
synthesis
Armnonia synthesis
activity
0166.9834/82/0~00/$02.76
correspond
Thus
to the start of nitrogen needed for equilibrium.
for the cobalt alloys
showed a small
(5%) for all three processes
A similar
but less well-defined
from 5% to 15% of nickel.
of these alloys
by the back
isotope exchange.
of the adsorption
of iron (95%) cobalt
to attain
was observed
in the range from 773 K to
to the higher temperatures
with composition
containing
was affected
rates of nitrogen
synthesis
and exchange).
was found with catalysts for the enhanced
temperatures
and the establishment
at a composition
(adsorption,
Higher
to give measurable
and those for exchange
The pattern maximum
and
alloy
surface areas were measured
from 550 K but at 700 K or over the rate of synthesis reaction
chemisorption
and iron-nickel
of carbon monoxide.
While nitrogen
adsorption
nitrogen
over a series of iron-cobalt
Possible
are discussed.
0 1982 Elsevier Scientific Publishing Compeny
maximum reasons
162 INTRODUCTION The catalytic
synthesis
much fundamental chemisorption chemisorb
study.
of nitrogen
nitrogen
account
as synthesis
iron-nickel
alloys provide
are readily
prepared
on these systems an increased containing
(2).
up to 4% of cobalt.
(7) examined
in activity
for ammonia
then passed
through a maximum
of details
on the metal
The objective characterised
of this exchange
Subsequently, nitrogen
and iron-nickel
reaction
studies
et al. (4) report
containing
for additions
the iron-nickel
Interpretation
of
system and
for small additions
of nickel the
of these results
in all the catalysts
and by lack
synthesis
it was decided
adsorption
over well-
catalysts
and to examine activities of 14 The relevance N2 and 15N2. reaction of
is that its rate is almost certainly
dissociative
of nitrogen
determined
on the catalyst
to make some measurements
on samples of the catalysts
used for the catalytic
Catalytic
Artyukh
and
since they
at about 17% nickel although
promoters
for the isotope exchange
adsorption
(3).
in activity
of the present work was to study ammonia
the same catalysts
of the
surface areas.
iron-cobalt
of the reversible
synthesis
was less than for pure iron. of alkali
Iron-cobalt
et al. (5) for catalysts
(6) found no change
found a decrease
by the presence
have the optimum catalysis
over that of pure iron for catalysts
'but the activity
is complicated
of the Group VIII,
this possibility
literature.
to the extent.
with an adjacent
catalyst.
are available
synthesis
Tovbin and Zabuga
rate at the maximum
activity
and subsequent
synthesis
as do Kharchenko others
metals
to any appreciable
maximum
good systems for examining
for ammonia
although
readily
exists that alloying
in the Russian
attracted is the
to the left of Group VIII,
adsorption
and the phase diagrams
lo-20% cobalt,
and has reaction
Such metals may not necessarily
in an improved
have appeared
activity
1.5-17% cobalt,
metals
nitrogen
and the possibility
metal may result
importance
and tend to form stable nitrides;
ratio for nitrogen
of ammonia
transition
Transition
for the well-known
catalysts
electron/atom
formation
is of industrial
slow step in the synthesis
do not chemisorb
These generalisations
average
(1).
very strongly
right of Group VIII,
metals
of ammonia
The probable
treated
by the rate
(8).
of the rate and extent of in a similar manner
to those
reactions.
EXPERIMENTAL Catalyst
preparation
Catalysts nitrates
and characterisation
were prepared
by the addition
by co-precipitation
of ammonium
nitrate was added before precipitation to act as a structural sintering overnight,
during
stabiliser
reduction.
calcined
from aqueous
bicarbonate
solution.
solutions Sufficient
to give ~3 wt % of alumina
and to prevent
The precipitates
excessive
of mixed metal aluminium
in the catalysts
loss of metal area by
were washed with water, dried at 393 K
in air for 4 h at 723 K and then sieved with a 16 B.S. mesh sieve.
163 The catalysts
contained
metal compositions Before passage
use catalyst
through
nitrogen
less than 0.02 wt % of sulphur,
of the catalysts samples
were reduced
a Deoxo catalytic
halogen or alkali.
The
by X-ray fluorescence.
in situ with hydrogen,
purified
by
purifier (Englehard Industries) and a liquid 3 -1 s . The reduction procedure involved a
trap, at a flow rate of 1 cm
progressive
increase
after which
the catalyst
of the reduction
in temperature
catalysts
which
that no further temperature.
increase
temperatures
by raising
in activity
for a further
The choice
These experiments
the reduction
indicated
temperature
with a modest
were carried
practice,
exchange
on
that
to 853 K and rise in reduction
out on samples reduced
at
up to 1273 K and results will be given in a later paper.
Catalyst
samples for X-ray diffraction
transferred
to a dry box and sealed
re-oxidation.
measurements
were reduced
as above,
in the X-ray cell under dry nitrogen
were made using a Phillips -1 and a scan speed of 0.5' min .
radiation
18 h.
than that used in industrial
was associated
experiments
to 853 K over a 4 h period
on the rate of nitrogen
temperatures.
a plateau
Subsequently
hydrogen
is greater
experiments
reduced at different had reached
from room temperature
was left in flowing
temperature,
was made after some preliminary
activity
were determined
Measurements
diffractometer
to prevent
with Co-Ka,
Apparatus Adsorption
measurements
system evacuable and a mercury Catalyst
McLeod gauge
a silica reaction
vessel
mixture
(50 cm3) connected
without
appreciable reactant
grade gases) and reaction
The rates of ammonia
synthesis
to that described
mixture
circulation
was achieved
rates measured
vessel
bed of catalyst
between
(8).
Pyrex glass circulating
and mixing
of the gases. prepared
oxygen
Process
in
(1 cm x 1 cm2) was (10 cm x
The reaction from cylinder
and water vapour.
pump (Metal Bellows
(GEC Elliott
from the decrease
two beds of silica chips
to remove residual
by a bellows
by a rotameter
by standard methods
in a closed
of 3:1 of H2:N2 at 700 torr total pressure had been purified
Experiments
pressure.
at 50 torr of 4:l of '4N 2: 15N2 (BOC
rates were calculated
The packed
preheating
(BOC) which
to be made of the
by Ozaki et al. (9) in which arrmonia was condensed
pressure.
consisted
traps.
system with
leak to an A.E.I. MS 20
of the nitrogen
were measured
volume at constant
1 cm2) to ensure adequate
in a static
analysis
rates were calculated
traps and syntheses
in a silica reaction
by a capillary
continuous
respectively.
by liquid nitrogen
were followed
reduction
in liquid nitrogen
positioned
contamination
experiments
This system permitted
were made with a standard
system similar
doses of gas and pressures
from mercury
isotope equilibration
mass spectrometer.
research
to measure
samples were protected
Nitrogen
reaction
were made in a conventional Pyrex glass high vacuum -5 torr (1 torr = 133 Pa) including a gas burette
to better than 10
Corporation)
Instruments).
mixture gases Gas
and flow
164 RESULTS Catalyst
characterisation
Table 1 reports the catalysts
the composition,
X-ray analyses
and metal
surface areas of
reduced at 853 K.
TABLE 1 Physical
properties
by X-ray diffraction
and CO adsorption
Actual
X-ray diffraction
Metal areab
compositiona
results
A/m2 g-'
Nominal composition
of the catalysts
Iron/% b.c.c. iron (a-Fe)
100
Fe
95.2
Fe(95)Co(5)
3.7
b.c.c. Fe-Co alloy
3.8
Fe(90)Co(lO)
91.5
b.c.c. Fe-Co alloy
4.3
Fe(80)Co(20)
84.7
b.c.c. Fe-Co alloy
4.3
Fe(60)Co(40)
63.8
b.c.c. Fe-Co alloy
3.5
Fe(40)Co(60)
43.3
b.c.c. Fe-Co alloy
5.7
Fe(20)Co(80)
21.6
b.c.c. Fe-Co alloy + CL & B - Co
4.8
cc&B-
8.5
0
co
co
Fe(95)Ni(5)
97.6
b.c.c. Fe-Ni alloy
2.8
Fe(gO)Ni(lO)
90.2
b.c.c. Fe-Ni alloy
2.0
Fe(85)Ni(l5)
85.5
b.c.c. Fe-Ni alloy
1.7
Fe(80)Ni(20)
85.3
b.c.c. Fe-Ni t f.c.c. Ni-Fe alloys
2.7
Fe(60)Ni(40)
63
b.c.c. Fe-Ni t f.c.c. Ni-Fe alloys
1.5
Fe(30)Ni(70)
31.2
f.c.c. Ni-Fe alloy
1.5
f.c.c. nickel
8.1
0
Ni aError 20.1% bAssuming
1.3~10-'~
unreduced
m 2 metal area for each CO molecule
catalyst;
error 28% but subject to an absolute
The surface areas were determined grade) at room temperature. monolayer
coverage
were expressed 1.3~10-'~
and expressed
by adsorption
were derived
uncertainty
of carbon monoxide
Good Type I isotherms
g
were obtained
catalyst
of
of 20.1 m2 g
Complete
to an absolute
alloy formation
the Fe(20)Co(80)
sample.
Metal areas
and were based on an assumed
area of
Reproducibility of the area measurements 2 -1 . uncertainty of 50.1 m g
was observed
.
for the
m2 for each CO molecule.
-+8% but subject
-1
(BOC research
and values
from the flat portion of the isotherm.
per 1 gram of unreduced
-1
with both series of catalysts
except
for
was
165 Nitrogen
chemisorption
Measurements
of the adsorption
on a number of the catalysts The intention
pure) were made at 673 and 773 K
as described
for catalytic
experiments.
of this part of the work was to obtain data on the rate and the
extent of nitrogen adsorption used in the catalytic
The uptake of nitrogen in pressure
of different
catalysts,
adsorbed
on the catalyst
under vacuum conditions
was followed
a dose of nitrogen
to those
and measuring
In order
to allow for the different
the nitrogen
coverage
e was expressed
divided
extent of the reversible
by admitting
with time.
adsorption
In some cases measurements of nitrogen
the system was evacuated
i.e. following
molecules
adsorbed
were made of the nitrogen
at that temperature
better than 10S5 torr and then a second uptake of nitrogen
metal areas
as the number of nitroge
by the total number of carbon monoxide
at room temperature.
at a given temperature
which were similar
experiments.
the decrease
molecules
(BOC >99.99%
pretreated
adsorption
for 15 h to
followed.
0.08 0.06 e 0.04
I
2
6 EXPOSURE
FIGURE a
1
Chemisorption
of nitrogen
adsorption
temperature
for 15 h.
10
/ 10gL
on iron as a function
at 773 K; - - - - repeat experiments
8
after evacuation
of exposure;
0
of the catalyst
at 673 K, at
166 The main features Figure
of the nitrogen
1 for iron catalysts.
extensive
nitrogen
catalysts
was greater
was greater
of molecules
in
was very slow but faster, more
2.
Results
for two of the
On both these alloys
than on iron and on the Fe(gO)Ni(lO) Table 2 summarises
the uptake of
sample the adsorption the results
for nitrogen
on all the systems studied; rates of uptake were expressed in terms -1 (exposure) using the Langmuir (1 L = lo6 torr s) as the unit of
to about 5 h.
Over cobalt, exposure.
are shown in Figure
and extent of adsorption
corresponded
are shown by the results
at 773 K than at 673 K.
at 673 K than at 773 K.
chemisorption
exposure,
The adsorption
and more reversible
iron/nickel
adsorption
was measured
after exposure
Data on reversibility
the uptake of nitrogen
Over nickel, adsorption
are also shown in the table.
was very slow but increased
was not detected
reached a steady value within 3 min.
to 10 lo L which
Details
linearly
for these metals
are included
Table 2.
I
I
I
I
2
4
6
8
EXPOSURE
FIGURE 2 773 K;
Chemisorption circles,
filled symbols
of nitrogen
Fe(gO)Ni(lO),
773 K.
I 10
/lO’L
on two iron/nickel
squares Fe(80)Ni(20);
with
at 673 K but at 773 K it
alloys at 673 K and
open symbols,
673 K,
in
16'7 TABLE 2 Results on nitrogen
chemisorption
Nominal
Initial
composition
rate of
L
-1
673K
Fe(95)Co(5)
0.104
0.14
0.082
0.024
_d
1
21 (2) 6 (0.8)
Ni
co.05
as nitrogen adsorbed
molecules
(0.136)
0.142
(0.138
0.132
(0.025)
0.190
(0.185
adsorbed
co.002
divided
0.05
by the number of carbon monoxide
coverage.
to an exposure
after evacuation
of the sample
of only 5~10~ L because virtually
all of the
had been adsorbed.
data.
exchange
experiments
Rates of reaction
the Arrhenius
were followed
on each catalyst in the temperature range from -2 m of metal surface and fitted to
in terms of molecules
equation
A, exp -E,/RT
The results
(1)
are summarised
well as the Arrhenius samples
0.234
refer to second adsorptions
773 K to 853 K expressed
obtain
43 (36)
temperature.
'This value refers
=
(0.089 I)
440 (440) 86
at complete
in parentheses
Nitrogen
0.100
>o.lgc
Fe(80)Ni(20)
dose of nitrogen
(0.031)
42
0.2
dInsufficient
0.095
84
Fe(gO)Ni(lO)
rX
30 (38)
773K
14
co
at the adsorption
673K
210
Fe(90)Co(lO)
bValues
mW2 773 K
6 (0.64)b
Fe
molecules
at lOlo L
adsorption r/lo8 molecules
aExpressed
Coveragea
containing the recorded
the errors
in Table 3 which
parameters.
includes
Duplicate
the values of rx at 823 K as
experiments
80% or more of iron and on cobalt, data.
mean values
The error in each rate measurement
being used to
was about +10X and
in E, are recorded.
The pressure
dependence
of the exchange
reaction
was measured
metals,
iron, cobalt and nickel using total pressures
between
10 and 200 torr.
and nickel
were made on all the
respectively.
The derived
on the single
of the nitrogen
mixture
orders were 0.7, 0.6 and 0.8 for iron, cobalt
168 TABLE 3 Nitrogen
exchange
Nominal
reactions
in the temperature
rx/1015 molecules
composition
at 823 K
co*
12.8
s
-1
range 773-853
m-'
K
E,/kJ mol-'
(9)a
137210
loglo
24.8
(160)a
Fe(ZO)Co(80)
8.8
151+12 -
25.5
Fe(60)Co(40)
8.1
184250
27.6
Fe(80)Co(ZO)*
17.4 (18)
175220
(160)
Fe(90)Co(lO)*
17.2 (17)
191+12
(182)
28.4
Fe(95)Co(5)*
25.1 (21)
157216
(173)
26.4
Fe*
16.3 (20)
171213
(177)
27.1
Fe(95)Ni(5)*
36.8 (40)
188217
(182)
28.5
Fe(gO)Ni(lO)*
16.9 (20)
153224
(160)
25.9
Fe(85)Ni(l5)*
26.4 (25)
153225
(140)
26.1
Fe(80)Ni(20)*
19.1
198531
28.9
182tlO -
27.3
Fe(60)Ni(40)
5.1
Fe(30)Ni(70)
4.6
Ni
2.4
107;10
refer to experiments
mixture of H2:N2 with rates corrected * Results of more than one experiment.
The effect of including
nitrogen
and adding
hydrogen
effect on rate was observed partial
pressure.
to a partial
other than that expected
Results for a 3:1 mixture
in Table 3 with the rates of reaction of 50 torr using the measured
with a 50 torr 3:1 of N2 of 50 torr.
mixture
were made by reducing
to keep the total pressure
22.2
(115)
pressure
with the nitrogen
These experiments hydrogen
26.5
17ot59 (3)
aValues of rx and E, in parentheses
was investigated.
27.4
on the rate of exchange
the partial
constant
from the change
of hydrogen:total
corrected
orders of reaction
to the standard or an assumed
pressure
at 50 torr.
of
No
in the nitrogen
nitrogen pressure
are included of nitrogen
order of 0.7 for the
alloy catalysts.
Ammonia
synthesis
There are two problems the rate of the catalytic inhibiting
associated synthesis
with the determination of ammonia.
effect of small quantities
second is the influence the conversion
of the reverse
increased.
of meaningful
The first is associated
of atmnonia on the rate of reaction reaction
as the temperature
results with the and the
is raised or
on
169 For ammonia
synthesis
the rate equation
r
S
=
under conditions
where the back reaction
due to Temkin and Pyzhev
(10) is generally
k
expressed
PN2
(2)
b;$,3)a
=
(2) can be
(Za+l)c(l/v)
(3) of ammonia
gas flow rate and c is a constant. a straight equation
system equation
as
where x is the mole fraction
for our iron catalyst
temperatures
The validity
was shown by measuring
the results
(671 K) are in agreement equation
be extrapolated
the catalyst
Thus, a plot of log x against
according
with literature
values
values
(9).
bed, v is the
log (l/v) should be
of the Temkin-Pyzhev
conversions
to equation
are given in Figure 3 and the derived
on the Temkin-Pyzhev and cannot
in the gas leaving
line from which CL can be derived.
flow rates and plotting
0.69
used - this is
dPNH3 at=
Ozaki et al. (9) have shown that for a circulating
x2u+l
is negligible,
(3).
at a series of The plots for two
of a of 0.65
(613 K) and
An important
to obtain
initial rates of synthesis
67iK
Plots according
Temkin-Pyzhev
equation
to equation
for the synthesis
(3) to test the validity of ammonia
of ammonia
in the absence
ammonia.
FIGURE 3
limitation
is that it only holds with finite pressures
of the
at two temperatures.
of
170 The dramatic our results
effect of the back reaction
at higher temperatures
is illustrated
by
with temperature in the percentage of ammonia formed -1 under a fixed flow rate of 6 cm3 s over an iron catalyst shown in Figure 4. Similar
for the variation
behaviour
the percentage controlled
by the constraints
superimposable region
was observed
with all the catalysts
of ammonia formed
in the gas leaving
of equilibrium
the efficiency
conforms
in activity
to normal Arrhenius
was found with catalyst
and in region A where bed is largely
the data for all catalysts
on a single line and no differences
B where the reaction
studied
the catalyst
composition
were
were apparent.
behaviour,
and this region
variations
In in
was used to compare
catalysts.
0.20-
0.10:
0.05-
ql%
-
0.02-
1
1
I
I
12
14
16
IO'K/T
Arrhenius
FIGURE 4 percentage
of ammonia
As the purpose
plot for the synthesis
in the exit gas and - - - - represents
of this work was to compare
relative
efficiency
detailed
kinetic analyses,
temperatures
of different
in the simplest
alloy catalysts
each of the catalysts
but only at a fixed flow rate.
rates of synthesis
of ammonia over iron - q is the
were determined
possible
for the synthesis was studied
Measurements
in the temperature
a flow rate of 6 cm3 s-' for each catalyst.
equilibrium
at a series of
of a were not made and
range from 550 K upwards for
The rates of synthesis
i.e. up to 700 K, were fitted to the Arrhenius
way the
and not to make
equation
in region B,
171
r
=
S
A, exp (-E,/RT)
and the derived together
with
synthesis
parameters,
its temperature
were accurate
It must be stressed of summarising significance
TABLE
(4) the rate of reaction of occurrence
are given
to +lO% and the uncertainty
that these Arrhenius
parameters
a large number of experimental
otherwise
at 673 K and the maximum
will be discussed
in Table 4.
in the values
rate
The rates of of Es are given.
were used as a convenient
results
and the limitations
method
on their
below.
4
Synthesis
of ammonia
above 550 K
Nominal
molecules
E,/kJ mol-'
log,0(As)
rs'l:l: mm2
composition
at 673 K
co
0.5
Fe(20)Co(80) Fe(40)Co(60)
max. rs/1015 molecules s-l .-2 (at T/K)
115220
23.6
4.8 (795)
2.8
97219
23.0
14.0 (750)
3.6
77+9
21.5
11.7 (745)
Fe(60)Co(40)
22.4
68+7 -
21.5
44.7
Fe(80)Co(20)*
22.9
79+5 -
22.5
31.2 (695)
Fe(90)Co(lO)
28.7
68+9 -
21.7
31.2 (700)
Fe(95)Co(5)
36.1
65+5 -
21.6
76.6 (735)
Fe*
18.9
5628
20.6
28.7 (715)
Fe(95)Ni(5)
27.8
63210
21.3
47.6
Fe(gO)Ni(lO)*
23.3
6526
21.4
38.7 (715)
Fe(85)Ni(15)
32.9
62+9 -
21.3
52.7 (735)
Fe(80)Ni(20)*
17.7
61+6 -
21.0
27.9 (735)
Fe(60)Ni(40)
11.2
8455
22.6
40.4
Fe(30)Ni(70)*
16.7
60+4 -
20.9
32.2 (720)
0.5
50+5 _
18.6
Ni
aNo maximum
observed
in the temperature
the catalyst. *
Duplicate
results
obtained.
(730)
(725)
(745)
(a)
range studied due to the low activity
of
172 DISCUSSION Nitrogen
adsorption
The results extensive,
for the adsorption
conform
and confirmed
of nitrogen
to the ideas established
more recently
by Ertl (12).
on our catalysts,
originally
The dissociative
on iron is very slow, the rate of adsorption increases,
and temperatures
adsorption
equilibrium
evidence
within
for this failure
apparently
greater
of nitrogen
becomes
become equal.
the activation
energies
coverage
for adsorption
of nitrogen
the
The clearest is
now to the desorption
covered
surface
(12).
because
of the
As the surface
increase while the rate of
isapproached
factors
the rate of the
are the relative
and desorption
These quantities
(11)
changes
in
(EA and E,,) and the heat of
are related
by the equation
qtEA
(5) EA increases
increase
and the reverse
exchange
in excess
reaction
desorption
below temperatures Our results
with coverage
involves
at which the adsorption
of 773 K for an appreciable of ammonia
and nitrogen desorption
a means of converting leading to the eventual
exchange;
of chemisorption
synthesis
temperatures hand for
rate of nitrogen
because
the presence
nitrogen
of ammonia.
atoms
but the exchange (and the reverse
into
than
where the initial requires
of
These arguments
lower temperatures
can occur at temperatures
is adequate
such that the final rate of chemisorption sufficiently
desorption
occurs at substantially
the synthesis
of nitrogen
the chemisorbed
of nitrogen it to occur
is established.
On the other
per se is not required
species
why ammonia
for nitrogen
to have a reasonable
provides
adsorption
on iron requires
rate of reaction.
it is sufficient
hydrogenated
nitrogen
ED will fall
we should not expect
equilibrium exchange
the hydrogen
explain
than q decreases,
both the dissociative
process and so in general
bear this out in that nitrogen
the synthesis adsorption
less rapidly
of coverage.
The nitrogen
rate
higher temperatures
process of desorption)
is
fast.
We have some evidence
that alloying
the rate of nitrogen
chemisorption
lower temperatures.
The results
were
Turning
will gradually
The controlling
(q) with coverage.
and provided with
the rate of desorption
to establish
experiment.
is in excess of 200 kJ mol-'
will fall as the equilibrium
two processes
ED =
which
chemisorption
at 673 K is that adsorption
this should be very slow from a sparsely
covered
adsorption
of a typical
equilibrium
not
very sharply as the coverage
of 673 K are required
the duration
to attain
decreasing
on iron at 773 K than at 673 K.
high heat of adsorption
adsorption
in excess
although
by Elrmett and Brunauer
the only catalysts
for a fixed exposure
of iron with either cobalt or nickel
and enables
the equilibrium
in Table 2 show that Fe(95)Co(5)
for which the adsorption
although
with the other alloy catalysts.
some enhancement
increases
to be established
at
and Fe(gO)Ni(lO)
at 673 K was greater
of rates of adsorption
than at 773 K were obtained
173 The small amount of nitrogen nickel
adsorption
on cobalt and the even smaller
amount on
are not unexpected.
Nitrogen
exchange
The requirement
of a high temperature
and the fact that most of the catalysts 150 kJ mol-'
is evidence
under the equilibrium Perhaps
the most
catalysts,
coverages
surprising
the fastest
reaction
efficient
even although
subject of dispute. accelerated
the reaction
due to further significant exchange
reduction
process
Fe(95)Co(5)
and would
catalyst
equilibration
(14) suggested
poorly reduced
of agreement
that any effect was
catalyst.
reaction
for catalysts
Our results
show no
energy of the
between
exchange
is faster over the
and there
containing
is evidence
of a
up to 15% Ni.
the results for nitrogen
iron with small amounts
rates of nitrogen
has been a
of hydrogen
the latter view. that the exchange
in activity
data in that alloying
leads to faster
the
is small, see Table 2.
isotope
than over the pure iron catalyst
maximum
there is some measure adsorption
support
Clearly
is comparatively
on either the rate or the activation
in Table 3 suggest
less well-defined
on nickel
(13) found that the presence
but Kummer and Emmett of initially
than the slowest.
of nitrogen
on the rate of nitrogen
effect of hydrogen
The results
adsorption
Jones and Taylor
reaction.
, show rather similar activity at 823 K -
the extent of the adsorption
of hydrogen
the exchange
of nitrogen
in Table 3 is that all the
being only about 10 times greater
of the dissociative
The effect
of the results
cobalt and nickel
been discussed
energy of more than
energy for the adsorption
which will exist during
feature
has already
an activation
for the high activation
even including
reversibility
for this reaction exhibit
Thus
exchange
and the
of either cobalt or nickel
as well as more rapid or increased
nitrogen
chemisorption.
Ammonia
synthesis
The inhibiting mentioned derived
and this effect
from changes
Consider equation
effect of ammonia
on the rate of its synthesis
has a consequential
of rs with temperature
an increase of temperature
(2) to derive
the ratio
k'/k.
influence
has already
on the Arrhenius
but measured
at a constant
from T to T' such that rs'/rs Since conversions
been
parameters
flow rate.
= x and use
are small changes
in
p,, and pN can be ignored but as the rate of synthesis rises from rs to rs' there 2 2 will be a corresponding rise of PNH to P~H~ such that P~H~/PNH~ = x. 3 If a remains constant, we have
k’/k or
=
(‘s’/rs)(P~H3hNH3)
2a
=
(x)'+2a
(6)
174 In k'/k
=
(1+2a) In x
The activation
(7)
energies
true activation
reported
energies
in Table 4 are related
related
to In k'/k would be greater
i.e. 2.4 if we assume a typical value of a = 0.7. parameters
in Table 4 are useful
for the synthesis activation
at constant
energies
superimposable correspond
(15).
in the measurement
region A where the results
position
It is possibly
rate as the percentage
cobalt, as expected,
are much less active
containing
containing
synthesis
at a fixed temperature required
method may be less influenced
cobalt
basis of comparison
alloys compared
and nitrogen composition despite
exchange.
than iron for ammonia
in Figure 6. parallel
effect of ammonia
For the two reactions, activity
A similar comparison
Results
to
The latter
and give a more
for the iron-rich
5 for both ammonia
the catalyst
are required
synthesis
of nominal
and the curves are broadly
for the iron-rich
for the two reactions
for
rates of
that it may be preferable
catalysts.
The
than iron
defined,
Instead of comparing
by the inhibiting
For this system the results
behaviour
more active
to attain a given rate of synthesis.
shows maximum
similar
for the exchange nickel alloys
show more scatter
than
is shown
but again there is
with a broad maximum
centred
at
Fe(gO)Ni(lO).
Influence
of alloying
It seems to be established
that some cobalt or some nickel
on iron for nitrogen
synthesis
and that while the effects are not marked
(5%) to achieve The effect important twice
Apparently
the maximum
propertywerethe
has a beneficial
more nickel
they are broadly
similar
(10%) is needed than cobalt
influence.
does not appear
as beneficial
chemisorption
, nitrogen exchange and ammonia
influence
for the three processes.
to be purely electronic
electron/atom
as cobalt whereas
e.g.
on the
synthesis.
but less clearly
it is arguable
the fact that much higher temperatures
for the synthesis.
errors,
data used, but alternatively
affect of ammonia
in this way are shown in Figure
Fe(95)Co(5)
of systematic
display
is increased.
of activity,
of different
are
in Table 4 show that both nickel and
from 5% to 15% of nickel.
the temperatures
reliable
inhibiting
in the literature
5% or 10% of cobalt are somewhat
catalysts
catalysts. catalysts
that the line does not seem to
but other results
of ammonia
at 673 K given
and there is a similar enhancement
compare
for the various
due to a combination
by the enhanced
The rates of synthesis
alloy catalysts
region B and the true
of the flow rate or the equilibrium
it could be partly caused
that the Arrhenius
the results we observed
140 to 160 kJ mol-' for the active
on a single line, it is surprising
the same effect
synthesis
flow in the temperature
with the equilibrium
by a factor of (1+2a),
It follows
solely for summarising
would be around
In the high temperature
to the size of In x but the
in origin
ratio small amounts the reverse
is found.
because
of nickel
if the should be
176
,780 0
-600 T& -820
640
I 10
I 20
I 30
LO
Co/% FIGURE 5 Temperatures required to attain rates of reaction of 1016 molecules s-l .-2 on iron-cobalt alloys for amnonia synthesis, Ts (@). or for nitrogen exchange, TX (0).
700
1
TX/K -820
-8L0 660-
10
20 Nil%
30
LO
FIGURE 6 Temperatures required to attaiorates of reaction of 1016 molecules s-l ,-2 on iron-nickel alloys for amaonia synthesis, Ts (a), or for nitrogen exchange, Tx (0).
176 A possible
explanation
the crystallisation
may be that small amounts
of iron such that a greater
occurs as (111) faces than would otherwise
of cobalt or nickel affect
proportion
of the surface area
There
be the case.
that sites present on the (111) face of iron are those mainly chemisorption
(16) and ammonia
We cannot exclude
synthesis
the possibility
ensembles
consisting
important we can be reasonably atoms are beneficial
because
that mixed ensembles
purely of iron atoms.
sure that only a limited
the enhanced
involved
in nitrogen
(17). containing
atoms but one or more cobalt or nickel atoms are marginally corresponding
is strong evidence
activity
mainly
more effective If
iron than the
this possibility
is
number of cobalt or nickel
is found only with iron-rich
alloys. The final cause of the increased of the other metals achieved
by our treatment
more detail catalysts
activity
on the ease of reduction for 18 h at 853 K.
in a later paper concerned
might
be attributable
to the influence
of iron and the extent of the reduction This possibility
with attempts
will be considered
to compare
the behaviour
in
of
reduced at higher temperatures.
ACKNOWLEDGEMENTS The authors
wish to thank L. Watson and E.G. Clingly
and J.D. Rankin, Chemical I.C.I.
I.R.
Industries
for experimental
Shannon and S.A. Topham of Agricultural Ltd. for helpful discussions.
Joint Research
Division,
assistance Imperial
The work was financed
Project and one of the authors
by an
(PJS) held an S.R.C.
studentship.
REFERENCES
P.H. Emmett, The Physical Basis for Heterogeneous Catalysis, Plenum Press, New York, p.3, (1975). G.C. Bond, Heterogeneous Catalysis, Clarendon Press, Oxford, p.27, (1974). M. Hansen, Constitution of Binary Alloys, McGraw-Hill, London, (1958). Yu.N. Artyukh, M.T. Rusov and N.A. Boldyreva, Kinet. Katal., 8 (1967) 1319. E.V. Kharchenko, M.V. Tovbin, K.I. Prisyazhnyuk, N.F. Mazmitsa and A.I. Grin'ko, Khim. Prom. (Ukr.) 1 (1969) 7. P.D. Rabina, L.D. Kuznetsov, E.Kh. Enikeev and M.M. Ivanov, Kinet. Katal., 8 (1967) 167. M.V. Tovbin and V.Ya. Zabuga, Kinet. Katal., 3 (1962) 370. A. Ozaki, Isotopic Studies of Heterogeneous Catalysis, Academic Press, New York, p.139, (1977). A. Ozaki, Sir H. Taylor and M. Boudart, Proc. R. Sot. London, Ser. A, 258 (1960) 47. M. Temkin and V. Pyzhev, Acta Phys. Chim. URSS, 12 (1940) 327. P.H. Enznett and S. Brunauer, J. Am. Chem. Sot., 56 (1934) 35. G. Ertl, Catal. Rev., 21 (1980) 201. G.G. Jones and H.S. Taylor, J. Chem. Phys., 7 (1939) 893. J.T. Kurmier and P.H. Ennnett, J. Chem. Phys., 19 (1951) 289. A.M. Mearns, Chemical Enqineerinq Process Analysis, Oliver & Boyd, Edinburgh, p.77 (1973). R. Brill, E.L. Richter and E. Ruth, Angew Chem. Int. Ed. Engl., 6 (1967) 882. J.A. Dumisec and M. Boudart, Catalytic Chemistry of Nitrogen Oxides, Plenum Press, New York, (1974).
161
Applied
AMMONIA ALLOY
SYNTHESIS
AND
CATALYSTS.
D.W. TAYLORa, Department aPresent
University
AND
IRON-NICKEL
853 K
C. KEMBALL
of Edinburgh,
Ltd., Paints Division,
I.C.I.
IRON-COBALT AT
and
D.A. WHAN
West Mains Road, Edinburgh,
Wexham
EH9 355
Road, Slough,
SL2 5DS.
address;
Cheshire, 'Present
OVER
REDUCED
D.A. DOWDENC,
P.J. SMITHb,
address;
REACTIONS
CATALYSTS
I.
of Chemistry,
Berkshire, bPresent
RELATED
PART
- Printed in The Netherlende
Ltd., Mond Division,
I.C.I.
Winnington,
Northwich,
CW8 4DJ.
address;
Cleveland,
(Received
12 Dunottar
Avenue,
Eaglescliffe,
Stockton-on-Tees,
TS16 OAB.
8 January
1982, accepted
22 March
1982)
ABSTRACT Studies nitrogen catalysts reduced
have been undertaken
isotope exchange containing
of ammonia
synthesis,
3% of alumina
as a structural
for 18 h at 853 K before use and metal
adsorption
with coverage
stabiliser.
chemisorption
so markedly
equilibrium
becoming
could be observed
that temperatures
within a reasonable
significant.
853 K were required the temperatures chemisorption reversible
for ammonia
adsorption
Catalysts
were by
below 673 K the rate decreased
of 773 K or more were required period.
of activity
synthesis
Armnonia synthesis
activity
0166.9834/82/0~00/$02.76
correspond
Thus
to the start of nitrogen needed for equilibrium.
for the cobalt alloys
showed a small
(5%) for all three processes
A similar
but less well-defined
from 5% to 15% of nickel.
of these alloys
by the back
isotope exchange.
of the adsorption
of iron (95%) cobalt
to attain
was observed
in the range from 773 K to
to the higher temperatures
with composition
containing
was affected
rates of nitrogen
synthesis
and exchange).
was found with catalysts for the enhanced
temperatures
and the establishment
at a composition
(adsorption,
Higher
to give measurable
and those for exchange
The pattern maximum
and
alloy
surface areas were measured
from 550 K but at 700 K or over the rate of synthesis reaction
chemisorption
and iron-nickel
of carbon monoxide.
While nitrogen
adsorption
nitrogen
over a series of iron-cobalt
Possible
are discussed.
0 1982 Elsevier Scientific Publishing Compeny
maximum reasons
162 INTRODUCTION The catalytic
synthesis
much fundamental chemisorption chemisorb
study.
of nitrogen
nitrogen
account
as synthesis
iron-nickel
alloys provide
are readily
prepared
on these systems an increased containing
(2).
up to 4% of cobalt.
(7) examined
in activity
for ammonia
then passed
through a maximum
of details
on the metal
The objective characterised
of this exchange
Subsequently, nitrogen
and iron-nickel
reaction
studies
et al. (4) report
containing
for additions
the iron-nickel
Interpretation
of
system and
for small additions
of nickel the
of these results
in all the catalysts
and by lack
synthesis
it was decided
adsorption
over well-
catalysts
and to examine activities of 14 The relevance N2 and 15N2. reaction of
is that its rate is almost certainly
dissociative
of nitrogen
determined
on the catalyst
to make some measurements
on samples of the catalysts
used for the catalytic
Catalytic
Artyukh
and
since they
at about 17% nickel although
promoters
for the isotope exchange
adsorption
(3).
in activity
of the present work was to study ammonia
the same catalysts
of the
surface areas.
iron-cobalt
of the reversible
synthesis
was less than for pure iron. of alkali
Iron-cobalt
et al. (5) for catalysts
(6) found no change
found a decrease
by the presence
have the optimum catalysis
over that of pure iron for catalysts
'but the activity
is complicated
of the Group VIII,
this possibility
literature.
to the extent.
with an adjacent
catalyst.
are available
synthesis
Tovbin and Zabuga
rate at the maximum
activity
and subsequent
synthesis
as do Kharchenko others
metals
to any appreciable
maximum
good systems for examining
for ammonia
although
readily
exists that alloying
in the Russian
attracted is the
to the left of Group VIII,
adsorption
and the phase diagrams
lo-20% cobalt,
and has reaction
Such metals may not necessarily
in an improved
have appeared
activity
1.5-17% cobalt,
metals
nitrogen
and the possibility
metal may result
importance
and tend to form stable nitrides;
ratio for nitrogen
of ammonia
transition
Transition
for the well-known
catalysts
electron/atom
formation
is of industrial
slow step in the synthesis
do not chemisorb
These generalisations
average
(1).
very strongly
right of Group VIII,
metals
of ammonia
The probable
treated
by the rate
(8).
of the rate and extent of in a similar manner
to those
reactions.
EXPERIMENTAL Catalyst
preparation
Catalysts nitrates
and characterisation
were prepared
by the addition
by co-precipitation
of ammonium
nitrate was added before precipitation to act as a structural sintering overnight,
during
stabiliser
reduction.
calcined
from aqueous
bicarbonate
solution.
solutions Sufficient
to give ~3 wt % of alumina
and to prevent
The precipitates
excessive
of mixed metal aluminium
in the catalysts
loss of metal area by
were washed with water, dried at 393 K
in air for 4 h at 723 K and then sieved with a 16 B.S. mesh sieve.
163 The catalysts
contained
metal compositions Before passage
use catalyst
through
nitrogen
less than 0.02 wt % of sulphur,
of the catalysts samples
were reduced
a Deoxo catalytic
halogen or alkali.
The
by X-ray fluorescence.
in situ with hydrogen,
purified
by
purifier (Englehard Industries) and a liquid 3 -1 s . The reduction procedure involved a
trap, at a flow rate of 1 cm
progressive
increase
after which
the catalyst
of the reduction
in temperature
catalysts
which
that no further temperature.
increase
temperatures
by raising
in activity
for a further
The choice
These experiments
the reduction
indicated
temperature
with a modest
were carried
practice,
exchange
on
that
to 853 K and rise in reduction
out on samples reduced
at
up to 1273 K and results will be given in a later paper.
Catalyst
samples for X-ray diffraction
transferred
to a dry box and sealed
re-oxidation.
measurements
were reduced
as above,
in the X-ray cell under dry nitrogen
were made using a Phillips -1 and a scan speed of 0.5' min .
radiation
18 h.
than that used in industrial
was associated
experiments
to 853 K over a 4 h period
on the rate of nitrogen
temperatures.
a plateau
Subsequently
hydrogen
is greater
experiments
reduced at different had reached
from room temperature
was left in flowing
temperature,
was made after some preliminary
activity
were determined
Measurements
diffractometer
to prevent
with Co-Ka,
Apparatus Adsorption
measurements
system evacuable and a mercury Catalyst
McLeod gauge
a silica reaction
vessel
mixture
(50 cm3) connected
without
appreciable reactant
grade gases) and reaction
The rates of ammonia
synthesis
to that described
mixture
circulation
was achieved
rates measured
vessel
bed of catalyst
between
(8).
Pyrex glass circulating
and mixing
of the gases. prepared
oxygen
Process
in
(1 cm x 1 cm2) was (10 cm x
The reaction from cylinder
and water vapour.
pump (Metal Bellows
(GEC Elliott
from the decrease
two beds of silica chips
to remove residual
by a bellows
by a rotameter
by standard methods
in a closed
of 3:1 of H2:N2 at 700 torr total pressure had been purified
Experiments
pressure.
at 50 torr of 4:l of '4N 2: 15N2 (BOC
rates were calculated
The packed
preheating
(BOC) which
to be made of the
by Ozaki et al. (9) in which arrmonia was condensed
pressure.
consisted
traps.
system with
leak to an A.E.I. MS 20
of the nitrogen
were measured
volume at constant
1 cm2) to ensure adequate
in a static
analysis
rates were calculated
traps and syntheses
in a silica reaction
by a capillary
continuous
respectively.
by liquid nitrogen
were followed
reduction
in liquid nitrogen
positioned
contamination
experiments
This system permitted
were made with a standard
system similar
doses of gas and pressures
from mercury
isotope equilibration
mass spectrometer.
research
to measure
samples were protected
Nitrogen
reaction
were made in a conventional Pyrex glass high vacuum -5 torr (1 torr = 133 Pa) including a gas burette
to better than 10
Corporation)
Instruments).
mixture gases Gas
and flow
164 RESULTS Catalyst
characterisation
Table 1 reports the catalysts
the composition,
X-ray analyses
and metal
surface areas of
reduced at 853 K.
TABLE 1 Physical
properties
by X-ray diffraction
and CO adsorption
Actual
X-ray diffraction
Metal areab
compositiona
results
A/m2 g-'
Nominal composition
of the catalysts
Iron/% b.c.c. iron (a-Fe)
100
Fe
95.2
Fe(95)Co(5)
3.7
b.c.c. Fe-Co alloy
3.8
Fe(90)Co(lO)
91.5
b.c.c. Fe-Co alloy
4.3
Fe(80)Co(20)
84.7
b.c.c. Fe-Co alloy
4.3
Fe(60)Co(40)
63.8
b.c.c. Fe-Co alloy
3.5
Fe(40)Co(60)
43.3
b.c.c. Fe-Co alloy
5.7
Fe(20)Co(80)
21.6
b.c.c. Fe-Co alloy + CL & B - Co
4.8
cc&B-
8.5
0
co
co
Fe(95)Ni(5)
97.6
b.c.c. Fe-Ni alloy
2.8
Fe(gO)Ni(lO)
90.2
b.c.c. Fe-Ni alloy
2.0
Fe(85)Ni(l5)
85.5
b.c.c. Fe-Ni alloy
1.7
Fe(80)Ni(20)
85.3
b.c.c. Fe-Ni t f.c.c. Ni-Fe alloys
2.7
Fe(60)Ni(40)
63
b.c.c. Fe-Ni t f.c.c. Ni-Fe alloys
1.5
Fe(30)Ni(70)
31.2
f.c.c. Ni-Fe alloy
1.5
f.c.c. nickel
8.1
0
Ni aError 20.1% bAssuming
1.3~10-'~
unreduced
m 2 metal area for each CO molecule
catalyst;
error 28% but subject to an absolute
The surface areas were determined grade) at room temperature. monolayer
coverage
were expressed 1.3~10-'~
and expressed
by adsorption
were derived
uncertainty
of carbon monoxide
Good Type I isotherms
g
were obtained
catalyst
of
of 20.1 m2 g
Complete
to an absolute
alloy formation
the Fe(20)Co(80)
sample.
Metal areas
and were based on an assumed
area of
Reproducibility of the area measurements 2 -1 . uncertainty of 50.1 m g
was observed
.
for the
m2 for each CO molecule.
-+8% but subject
-1
(BOC research
and values
from the flat portion of the isotherm.
per 1 gram of unreduced
-1
with both series of catalysts
except
for
was
165 Nitrogen
chemisorption
Measurements
of the adsorption
on a number of the catalysts The intention
pure) were made at 673 and 773 K
as described
for catalytic
experiments.
of this part of the work was to obtain data on the rate and the
extent of nitrogen adsorption used in the catalytic
The uptake of nitrogen in pressure
of different
catalysts,
adsorbed
on the catalyst
under vacuum conditions
was followed
a dose of nitrogen
to those
and measuring
In order
to allow for the different
the nitrogen
coverage
e was expressed
divided
extent of the reversible
by admitting
with time.
adsorption
In some cases measurements of nitrogen
the system was evacuated
i.e. following
molecules
adsorbed
were made of the nitrogen
at that temperature
better than 10S5 torr and then a second uptake of nitrogen
metal areas
as the number of nitroge
by the total number of carbon monoxide
at room temperature.
at a given temperature
which were similar
experiments.
the decrease
molecules
(BOC >99.99%
pretreated
adsorption
for 15 h to
followed.
0.08 0.06 e 0.04
I
2
6 EXPOSURE
FIGURE a
1
Chemisorption
of nitrogen
adsorption
temperature
for 15 h.
10
/ 10gL
on iron as a function
at 773 K; - - - - repeat experiments
8
after evacuation
of exposure;
0
of the catalyst
at 673 K, at
166 The main features Figure
of the nitrogen
1 for iron catalysts.
extensive
nitrogen
catalysts
was greater
was greater
of molecules
in
was very slow but faster, more
2.
Results
for two of the
On both these alloys
than on iron and on the Fe(gO)Ni(lO) Table 2 summarises
the uptake of
sample the adsorption the results
for nitrogen
on all the systems studied; rates of uptake were expressed in terms -1 (exposure) using the Langmuir (1 L = lo6 torr s) as the unit of
to about 5 h.
Over cobalt, exposure.
are shown in Figure
and extent of adsorption
corresponded
are shown by the results
at 773 K than at 673 K.
at 673 K than at 773 K.
chemisorption
exposure,
The adsorption
and more reversible
iron/nickel
adsorption
was measured
after exposure
Data on reversibility
the uptake of nitrogen
Over nickel, adsorption
are also shown in the table.
was very slow but increased
was not detected
reached a steady value within 3 min.
to 10 lo L which
Details
linearly
for these metals
are included
Table 2.
I
I
I
I
2
4
6
8
EXPOSURE
FIGURE 2 773 K;
Chemisorption circles,
filled symbols
of nitrogen
Fe(gO)Ni(lO),
773 K.
I 10
/lO’L
on two iron/nickel
squares Fe(80)Ni(20);
with
at 673 K but at 773 K it
alloys at 673 K and
open symbols,
673 K,
in
16'7 TABLE 2 Results on nitrogen
chemisorption
Nominal
Initial
composition
rate of
L
-1
673K
Fe(95)Co(5)
0.104
0.14
0.082
0.024
_d
1
21 (2) 6 (0.8)
Ni
co.05
as nitrogen adsorbed
molecules
(0.136)
0.142
(0.138
0.132
(0.025)
0.190
(0.185
adsorbed
co.002
divided
0.05
by the number of carbon monoxide
coverage.
to an exposure
after evacuation
of the sample
of only 5~10~ L because virtually
all of the
had been adsorbed.
data.
exchange
experiments
Rates of reaction
the Arrhenius
were followed
on each catalyst in the temperature range from -2 m of metal surface and fitted to
in terms of molecules
equation
A, exp -E,/RT
The results
(1)
are summarised
well as the Arrhenius samples
0.234
refer to second adsorptions
773 K to 853 K expressed
obtain
43 (36)
temperature.
'This value refers
=
(0.089 I)
440 (440) 86
at complete
in parentheses
Nitrogen
0.100
>o.lgc
Fe(80)Ni(20)
dose of nitrogen
(0.031)
42
0.2
dInsufficient
0.095
84
Fe(gO)Ni(lO)
rX
30 (38)
773K
14
co
at the adsorption
673K
210
Fe(90)Co(lO)
bValues
mW2 773 K
6 (0.64)b
Fe
molecules
at lOlo L
adsorption r/lo8 molecules
aExpressed
Coveragea
containing the recorded
the errors
in Table 3 which
parameters.
includes
Duplicate
the values of rx at 823 K as
experiments
80% or more of iron and on cobalt, data.
mean values
The error in each rate measurement
being used to
was about +10X and
in E, are recorded.
The pressure
dependence
of the exchange
reaction
was measured
metals,
iron, cobalt and nickel using total pressures
between
10 and 200 torr.
and nickel
were made on all the
respectively.
The derived
on the single
of the nitrogen
mixture
orders were 0.7, 0.6 and 0.8 for iron, cobalt
168 TABLE 3 Nitrogen
exchange
Nominal
reactions
in the temperature
rx/1015 molecules
composition
at 823 K
co*
12.8
s
-1
range 773-853
m-'
K
E,/kJ mol-'
(9)a
137210
loglo
24.8
(160)a
Fe(ZO)Co(80)
8.8
151+12 -
25.5
Fe(60)Co(40)
8.1
184250
27.6
Fe(80)Co(ZO)*
17.4 (18)
175220
(160)
Fe(90)Co(lO)*
17.2 (17)
191+12
(182)
28.4
Fe(95)Co(5)*
25.1 (21)
157216
(173)
26.4
Fe*
16.3 (20)
171213
(177)
27.1
Fe(95)Ni(5)*
36.8 (40)
188217
(182)
28.5
Fe(gO)Ni(lO)*
16.9 (20)
153224
(160)
25.9
Fe(85)Ni(l5)*
26.4 (25)
153225
(140)
26.1
Fe(80)Ni(20)*
19.1
198531
28.9
182tlO -
27.3
Fe(60)Ni(40)
5.1
Fe(30)Ni(70)
4.6
Ni
2.4
107;10
refer to experiments
mixture of H2:N2 with rates corrected * Results of more than one experiment.
The effect of including
nitrogen
and adding
hydrogen
effect on rate was observed partial
pressure.
to a partial
other than that expected
Results for a 3:1 mixture
in Table 3 with the rates of reaction of 50 torr using the measured
with a 50 torr 3:1 of N2 of 50 torr.
mixture
were made by reducing
to keep the total pressure
22.2
(115)
pressure
with the nitrogen
These experiments hydrogen
26.5
17ot59 (3)
aValues of rx and E, in parentheses
was investigated.
27.4
on the rate of exchange
the partial
constant
from the change
of hydrogen:total
corrected
orders of reaction
to the standard or an assumed
pressure
at 50 torr.
of
No
in the nitrogen
nitrogen pressure
are included of nitrogen
order of 0.7 for the
alloy catalysts.
Ammonia
synthesis
There are two problems the rate of the catalytic inhibiting
associated synthesis
with the determination of ammonia.
effect of small quantities
second is the influence the conversion
of the reverse
increased.
of meaningful
The first is associated
of atmnonia on the rate of reaction reaction
as the temperature
results with the and the
is raised or
on
169 For ammonia
synthesis
the rate equation
r
S
=
under conditions
where the back reaction
due to Temkin and Pyzhev
(10) is generally
k
expressed
PN2
(2)
b;$,3)a
=
(2) can be
(Za+l)c(l/v)
(3) of ammonia
gas flow rate and c is a constant. a straight equation
system equation
as
where x is the mole fraction
for our iron catalyst
temperatures
The validity
was shown by measuring
the results
(671 K) are in agreement equation
be extrapolated
the catalyst
Thus, a plot of log x against
according
with literature
values
values
(9).
bed, v is the
log (l/v) should be
of the Temkin-Pyzhev
conversions
to equation
are given in Figure 3 and the derived
on the Temkin-Pyzhev and cannot
in the gas leaving
line from which CL can be derived.
flow rates and plotting
0.69
used - this is
dPNH3 at=
Ozaki et al. (9) have shown that for a circulating
x2u+l
is negligible,
(3).
at a series of The plots for two
of a of 0.65
(613 K) and
An important
to obtain
initial rates of synthesis
67iK
Plots according
Temkin-Pyzhev
equation
to equation
for the synthesis
(3) to test the validity of ammonia
of ammonia
in the absence
ammonia.
FIGURE 3
limitation
is that it only holds with finite pressures
of the
at two temperatures.
of
170 The dramatic our results
effect of the back reaction
at higher temperatures
is illustrated
by
with temperature in the percentage of ammonia formed -1 under a fixed flow rate of 6 cm3 s over an iron catalyst shown in Figure 4. Similar
for the variation
behaviour
the percentage controlled
by the constraints
superimposable region
was observed
with all the catalysts
of ammonia formed
in the gas leaving
of equilibrium
the efficiency
conforms
in activity
to normal Arrhenius
was found with catalyst
and in region A where bed is largely
the data for all catalysts
on a single line and no differences
B where the reaction
studied
the catalyst
composition
were
were apparent.
behaviour,
and this region
variations
In in
was used to compare
catalysts.
0.20-
0.10:
0.05-
ql%
-
0.02-
1
1
I
I
12
14
16
IO'K/T
Arrhenius
FIGURE 4 percentage
of ammonia
As the purpose
plot for the synthesis
in the exit gas and - - - - represents
of this work was to compare
relative
efficiency
detailed
kinetic analyses,
temperatures
of different
in the simplest
alloy catalysts
each of the catalysts
but only at a fixed flow rate.
rates of synthesis
of ammonia over iron - q is the
were determined
possible
for the synthesis was studied
Measurements
in the temperature
a flow rate of 6 cm3 s-' for each catalyst.
equilibrium
at a series of
of a were not made and
range from 550 K upwards for
The rates of synthesis
i.e. up to 700 K, were fitted to the Arrhenius
way the
and not to make
equation
in region B,
171
r
=
S
A, exp (-E,/RT)
and the derived together
with
synthesis
parameters,
its temperature
were accurate
It must be stressed of summarising significance
TABLE
(4) the rate of reaction of occurrence
are given
to +lO% and the uncertainty
that these Arrhenius
parameters
a large number of experimental
otherwise
at 673 K and the maximum
will be discussed
in Table 4.
in the values
rate
The rates of of Es are given.
were used as a convenient
results
and the limitations
method
on their
below.
4
Synthesis
of ammonia
above 550 K
Nominal
molecules
E,/kJ mol-'
log,0(As)
rs'l:l: mm2
composition
at 673 K
co
0.5
Fe(20)Co(80) Fe(40)Co(60)
max. rs/1015 molecules s-l .-2 (at T/K)
115220
23.6
4.8 (795)
2.8
97219
23.0
14.0 (750)
3.6
77+9
21.5
11.7 (745)
Fe(60)Co(40)
22.4
68+7 -
21.5
44.7
Fe(80)Co(20)*
22.9
79+5 -
22.5
31.2 (695)
Fe(90)Co(lO)
28.7
68+9 -
21.7
31.2 (700)
Fe(95)Co(5)
36.1
65+5 -
21.6
76.6 (735)
Fe*
18.9
5628
20.6
28.7 (715)
Fe(95)Ni(5)
27.8
63210
21.3
47.6
Fe(gO)Ni(lO)*
23.3
6526
21.4
38.7 (715)
Fe(85)Ni(15)
32.9
62+9 -
21.3
52.7 (735)
Fe(80)Ni(20)*
17.7
61+6 -
21.0
27.9 (735)
Fe(60)Ni(40)
11.2
8455
22.6
40.4
Fe(30)Ni(70)*
16.7
60+4 -
20.9
32.2 (720)
0.5
50+5 _
18.6
Ni
aNo maximum
observed
in the temperature
the catalyst. *
Duplicate
results
obtained.
(730)
(725)
(745)
(a)
range studied due to the low activity
of
172 DISCUSSION Nitrogen
adsorption
The results extensive,
for the adsorption
conform
and confirmed
of nitrogen
to the ideas established
more recently
by Ertl (12).
on our catalysts,
originally
The dissociative
on iron is very slow, the rate of adsorption increases,
and temperatures
adsorption
equilibrium
evidence
within
for this failure
apparently
greater
of nitrogen
becomes
become equal.
the activation
energies
coverage
for adsorption
of nitrogen
the
The clearest is
now to the desorption
covered
surface
(12).
because
of the
As the surface
increase while the rate of
isapproached
factors
the rate of the
are the relative
and desorption
These quantities
(11)
changes
in
(EA and E,,) and the heat of
are related
by the equation
qtEA
(5) EA increases
increase
and the reverse
exchange
in excess
reaction
desorption
below temperatures Our results
with coverage
involves
at which the adsorption
of 773 K for an appreciable of ammonia
and nitrogen desorption
a means of converting leading to the eventual
exchange;
of chemisorption
synthesis
temperatures hand for
rate of nitrogen
because
the presence
nitrogen
of ammonia.
atoms
but the exchange (and the reverse
into
than
where the initial requires
of
These arguments
lower temperatures
can occur at temperatures
is adequate
such that the final rate of chemisorption sufficiently
desorption
occurs at substantially
the synthesis
of nitrogen
the chemisorbed
of nitrogen it to occur
is established.
On the other
per se is not required
species
why ammonia
for nitrogen
to have a reasonable
provides
adsorption
on iron requires
rate of reaction.
it is sufficient
hydrogenated
nitrogen
ED will fall
we should not expect
equilibrium exchange
the hydrogen
explain
than q decreases,
both the dissociative
process and so in general
bear this out in that nitrogen
the synthesis adsorption
less rapidly
of coverage.
The nitrogen
rate
higher temperatures
process of desorption)
is
fast.
We have some evidence
that alloying
the rate of nitrogen
chemisorption
lower temperatures.
The results
were
Turning
will gradually
The controlling
(q) with coverage.
and provided with
the rate of desorption
to establish
experiment.
is in excess of 200 kJ mol-'
will fall as the equilibrium
two processes
ED =
which
chemisorption
at 673 K is that adsorption
this should be very slow from a sparsely
covered
adsorption
of a typical
equilibrium
not
very sharply as the coverage
of 673 K are required
the duration
to attain
decreasing
on iron at 773 K than at 673 K.
high heat of adsorption
adsorption
in excess
although
by Elrmett and Brunauer
the only catalysts
for a fixed exposure
of iron with either cobalt or nickel
and enables
the equilibrium
in Table 2 show that Fe(95)Co(5)
for which the adsorption
although
with the other alloy catalysts.
some enhancement
increases
to be established
at
and Fe(gO)Ni(lO)
at 673 K was greater
of rates of adsorption
than at 773 K were obtained
173 The small amount of nitrogen nickel
adsorption
on cobalt and the even smaller
amount on
are not unexpected.
Nitrogen
exchange
The requirement
of a high temperature
and the fact that most of the catalysts 150 kJ mol-'
is evidence
under the equilibrium Perhaps
the most
catalysts,
coverages
surprising
the fastest
reaction
efficient
even although
subject of dispute. accelerated
the reaction
due to further significant exchange
reduction
process
Fe(95)Co(5)
and would
catalyst
equilibration
(14) suggested
poorly reduced
of agreement
that any effect was
catalyst.
reaction
for catalysts
Our results
show no
energy of the
between
exchange
is faster over the
and there
containing
is evidence
of a
up to 15% Ni.
the results for nitrogen
iron with small amounts
rates of nitrogen
has been a
of hydrogen
the latter view. that the exchange
in activity
data in that alloying
leads to faster
the
is small, see Table 2.
isotope
than over the pure iron catalyst
maximum
there is some measure adsorption
support
Clearly
is comparatively
on either the rate or the activation
in Table 3 suggest
less well-defined
on nickel
(13) found that the presence
but Kummer and Emmett of initially
than the slowest.
of nitrogen
on the rate of nitrogen
effect of hydrogen
The results
adsorption
Jones and Taylor
reaction.
, show rather similar activity at 823 K -
the extent of the adsorption
of hydrogen
the exchange
of nitrogen
in Table 3 is that all the
being only about 10 times greater
of the dissociative
The effect
of the results
cobalt and nickel
been discussed
energy of more than
energy for the adsorption
which will exist during
feature
has already
an activation
for the high activation
even including
reversibility
for this reaction exhibit
Thus
exchange
and the
of either cobalt or nickel
as well as more rapid or increased
nitrogen
chemisorption.
Ammonia
synthesis
The inhibiting mentioned derived
and this effect
from changes
Consider equation
effect of ammonia
on the rate of its synthesis
has a consequential
of rs with temperature
an increase of temperature
(2) to derive
the ratio
k'/k.
influence
has already
on the Arrhenius
but measured
at a constant
from T to T' such that rs'/rs Since conversions
been
parameters
flow rate.
= x and use
are small changes
in
p,, and pN can be ignored but as the rate of synthesis rises from rs to rs' there 2 2 will be a corresponding rise of PNH to P~H~ such that P~H~/PNH~ = x. 3 If a remains constant, we have
k’/k or
=
(‘s’/rs)(P~H3hNH3)
2a
=
(x)'+2a
(6)
174 In k'/k
=
(1+2a) In x
The activation
(7)
energies
true activation
reported
energies
in Table 4 are related
related
to In k'/k would be greater
i.e. 2.4 if we assume a typical value of a = 0.7. parameters
in Table 4 are useful
for the synthesis activation
at constant
energies
superimposable correspond
(15).
in the measurement
region A where the results
position
It is possibly
rate as the percentage
cobalt, as expected,
are much less active
containing
containing
synthesis
at a fixed temperature required
method may be less influenced
cobalt
basis of comparison
alloys compared
and nitrogen composition despite
exchange.
than iron for ammonia
in Figure 6. parallel
effect of ammonia
For the two reactions, activity
A similar comparison
Results
to
The latter
and give a more
for the iron-rich
5 for both ammonia
the catalyst
are required
synthesis
of nominal
and the curves are broadly
for the iron-rich
for the two reactions
for
rates of
that it may be preferable
catalysts.
The
than iron
defined,
Instead of comparing
by the inhibiting
For this system the results
behaviour
more active
to attain a given rate of synthesis.
shows maximum
similar
for the exchange nickel alloys
show more scatter
than
is shown
but again there is
with a broad maximum
centred
at
Fe(gO)Ni(lO).
Influence
of alloying
It seems to be established
that some cobalt or some nickel
on iron for nitrogen
synthesis
and that while the effects are not marked
(5%) to achieve The effect important twice
Apparently
the maximum
propertywerethe
has a beneficial
more nickel
they are broadly
similar
(10%) is needed than cobalt
influence.
does not appear
as beneficial
chemisorption
, nitrogen exchange and ammonia
influence
for the three processes.
to be purely electronic
electron/atom
as cobalt whereas
e.g.
on the
synthesis.
but less clearly
it is arguable
the fact that much higher temperatures
for the synthesis.
errors,
data used, but alternatively
affect of ammonia
in this way are shown in Figure
Fe(95)Co(5)
of systematic
display
is increased.
of activity,
of different
are
in Table 4 show that both nickel and
from 5% to 15% of nickel.
the temperatures
reliable
inhibiting
in the literature
5% or 10% of cobalt are somewhat
catalysts
catalysts. catalysts
that the line does not seem to
but other results
of ammonia
at 673 K given
and there is a similar enhancement
compare
for the various
due to a combination
by the enhanced
The rates of synthesis
alloy catalysts
region B and the true
of the flow rate or the equilibrium
it could be partly caused
that the Arrhenius
the results we observed
140 to 160 kJ mol-' for the active
on a single line, it is surprising
the same effect
synthesis
flow in the temperature
with the equilibrium
by a factor of (1+2a),
It follows
solely for summarising
would be around
In the high temperature
to the size of In x but the
in origin
ratio small amounts the reverse
is found.
because
of nickel
if the should be
176
,780 0
-600 T& -820
640
I 10
I 20
I 30
LO
Co/% FIGURE 5 Temperatures required to attain rates of reaction of 1016 molecules s-l .-2 on iron-cobalt alloys for amnonia synthesis, Ts (@). or for nitrogen exchange, TX (0).
700
1
TX/K -820
-8L0 660-
10
20 Nil%
30
LO
FIGURE 6 Temperatures required to attaiorates of reaction of 1016 molecules s-l ,-2 on iron-nickel alloys for amaonia synthesis, Ts (a), or for nitrogen exchange, Tx (0).
176 A possible
explanation
the crystallisation
may be that small amounts
of iron such that a greater
occurs as (111) faces than would otherwise
of cobalt or nickel affect
proportion
of the surface area
There
be the case.
that sites present on the (111) face of iron are those mainly chemisorption
(16) and ammonia
We cannot exclude
synthesis
the possibility
ensembles
consisting
important we can be reasonably atoms are beneficial
because
that mixed ensembles
purely of iron atoms.
sure that only a limited
the enhanced
involved
in nitrogen
(17). containing
atoms but one or more cobalt or nickel atoms are marginally corresponding
is strong evidence
activity
mainly
more effective If
iron than the
this possibility
is
number of cobalt or nickel
is found only with iron-rich
alloys. The final cause of the increased of the other metals achieved
by our treatment
more detail catalysts
activity
on the ease of reduction for 18 h at 853 K.
in a later paper concerned
might
be attributable
to the influence
of iron and the extent of the reduction This possibility
with attempts
will be considered
to compare
the behaviour
in
of
reduced at higher temperatures.
ACKNOWLEDGEMENTS The authors
wish to thank L. Watson and E.G. Clingly
and J.D. Rankin, Chemical I.C.I.
I.R.
Industries
for experimental
Shannon and S.A. Topham of Agricultural Ltd. for helpful discussions.
Joint Research
Division,
assistance Imperial
The work was financed
Project and one of the authors
by an
(PJS) held an S.R.C.
studentship.
REFERENCES
P.H. Emmett, The Physical Basis for Heterogeneous Catalysis, Plenum Press, New York, p.3, (1975). G.C. Bond, Heterogeneous Catalysis, Clarendon Press, Oxford, p.27, (1974). M. Hansen, Constitution of Binary Alloys, McGraw-Hill, London, (1958). Yu.N. Artyukh, M.T. Rusov and N.A. Boldyreva, Kinet. Katal., 8 (1967) 1319. E.V. Kharchenko, M.V. Tovbin, K.I. Prisyazhnyuk, N.F. Mazmitsa and A.I. Grin'ko, Khim. Prom. (Ukr.) 1 (1969) 7. P.D. Rabina, L.D. Kuznetsov, E.Kh. Enikeev and M.M. Ivanov, Kinet. Katal., 8 (1967) 167. M.V. Tovbin and V.Ya. Zabuga, Kinet. Katal., 3 (1962) 370. A. Ozaki, Isotopic Studies of Heterogeneous Catalysis, Academic Press, New York, p.139, (1977). A. Ozaki, Sir H. Taylor and M. Boudart, Proc. R. Sot. London, Ser. A, 258 (1960) 47. M. Temkin and V. Pyzhev, Acta Phys. Chim. URSS, 12 (1940) 327. P.H. Enznett and S. Brunauer, J. Am. Chem. Sot., 56 (1934) 35. G. Ertl, Catal. Rev., 21 (1980) 201. G.G. Jones and H.S. Taylor, J. Chem. Phys., 7 (1939) 893. J.T. Kurmier and P.H. Ennnett, J. Chem. Phys., 19 (1951) 289. A.M. Mearns, Chemical Enqineerinq Process Analysis, Oliver & Boyd, Edinburgh, p.77 (1973). R. Brill, E.L. Richter and E. Ruth, Angew Chem. Int. Ed. Engl., 6 (1967) 882. J.A. Dumisec and M. Boudart, Catalytic Chemistry of Nitrogen Oxides, Plenum Press, New York, (1974).