B. Delmon and G.F. Froment (Editors), Catalyst Deactiuation 0 1980 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

141

THE LOSS IN SELECTIVITY OF A COBALT OXIDE AMMONIA OXIDATION CATALYST

S.P.S. ANDREW and G,C. CHINCHEN Agricultural Division, Imperial Chemical Industries Ltd., P.O. Box No. 1, aillingham, Cleveland, TS23 lLB, United Kingdam

ABSTRACT

Although it has long been known that gaseous ammonia can be oxidised with air

over non-precious metal catalysts to give nitric oxide, the platinum-rhodium gauze

has retained its popularity for this duty. The problem with non-precious metal

catalysts is that in the past they have not retained a high selectivity in use and

the ratio NO formed/NHs destroyed rapidly falls below the desired .94 to .97.

The work described in this paper using cobalt oxide catalysts indicates that the

loss in selectivity is entirely a result of loss in activity of the catalyst caused

by a fall in exposed surface of Co301,. This fall is partly caused by sintering,

and part by poisoning, Because the l o s s in selectivity is brought about by the

change in gas composition in contact with the catalyst as a result of its loss in

activity, this effect cannot be compensated for by an increased charge of catalyst

and it is essential to preserve activity by inhibiting sintering and reducing

poisoning.

INTRODUCTION

The most important requirement of a catalyst for the production of NO by the

air oxidation of NH3 is selectivity-the ability to produce NO rather than N2.

Platinum-rhodium gauze stacks give selectivities, measured as the ratio of NO

formed (in the product gas) to NH3 decomposed (at complete NH3 decomposition), of

from about 0.97 (at atmospheric pressure) to about 0.92 (at about 10 bar) when

operating with a 11% NH3 in air stream at about 900°C.

undertaken to discover whether a cobalt oxide catalyst was capable of performing

as well - and if not, why not.

This investigation was

Before investigation was commenced it was evident that the best non-precious

metal was Co301, and that for this species to be thermodynamically stable in the

product gas stream it would be necessary to ensure that the catalyst should

operate at below about 85OoC, otherwise the Co3Ot, would decompose to COO.

also known that high selectivities >0.95 could be obtained with Porous tablets of

CoeOk packed in a bed, but that within a short time the catalyst lost its good

selectivity.

It was

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Loss in selectivity of an apparently single phase catalyst such as Cos01, could,

it was hypothesised, be the result of one or more of the following phenomena:

a) Loss in activity of the catalyst causing ammonia slip which resulted in loss

of selectivity due to subsequent reaction of N H 3 and NO to form N 2 .

b) Change in the relative velocities of the reactions:

3 ) 2NWN2 i 02

Whilst (a) could be result of sintering, or poisoning, a suggestion was made

that (b) could be caused by a relative change in the density of the sorts of

active sites which it was presumed promoted the three reactions. It was further

hypothesised that this change in site density was a result of partial or incipient

reduction of the COSOL,, as it were, in anticipation of the relatively close, from

a thermodynamic standpoint, phase change to COO. A further hypothetical cause

of the loss in selectivity ascribed the change to the influence of changing gas

composition in contact with the catalyst as the catalyst lost activity, thus:

c) Loss in catalyst activity causes increased partial pressure of ammonia in

contact with the catalyst as the reaction velocity becomes more catalytically

limited and less limited by diffusional mass transfer of ammonia from the gas

stream to the outer surface of the catalyst tablets. If, in addition, the

dependence of the velocity of reaction (2) on ammonia partial pressure is

higher than that of reaction (1) then lowered catalyst activity would produce

lower apparent selectivity. This hypothesis has the virtue that it is much

more readily tested than that of (b), requiring no difficult investigation of

the surface oxidation state change hypothesis.

RATIONALE OF THE INVESTIGATION

The relevance of the various hypothetical causes of loss in selectivity can

be tested, relatively easily, by performing three types of test on a given

catalyst formulation at various stages during its life. These tests are:

1) To change the depth of catalyst bed at a constant gas linear velocity, constant

gas inlet composition and exit temperature and pressure and to measure the

selectivity. If increase in bed depth improves selectivity then hypothesis

(a) must be relevant and hypothesis (b) (3) irrelevant. Hypotheses (b) (1)

and (2) and (c) are also of less importance. If the efficiency falls with

increasing bed depth then reaction (b) ( 3 ) is of importance.

2) To increase the ammonia content of the gas at a given gas velocity and bed

exit temperature to determine whether the maximum selectivity (at optimum gas

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linear velocity) falls with increase in % ammonia. If it does and also there

is no effect of change in depth of bed as obtained by test 1, then reaction

(2) has a higher dependence on ammonia partial pressure than reaction (1).

3) To measure the activity of the catalyst, by measuring the reaction extinction

velocity of gas flow over a single pellet maintained at reaction temperature

by the exothermic heat of reaction. If this catalyst activity for tablets of

different age and also of different formulations (ie having different specific

surfaces of cobalt oxide) correlates with the selectivity in the manner expected

using a mathematical model to calculate the surface ammonia concentration then

hypothesis (c) is true,

APPARATUS AND CATALYSTS

Two types of test apparatus were employed: a small adiabatic packer bed reactor

operating at atmospheric pressure and a single tablet reactor, The arrangements

of these are shown in Figures 1 and 2 respectively. Catalysts were also tested

in a small commercial size burner operating at atmospheric pressure using a

50 cm x 50 cm square bed.

SILICA PREHEATER

NH,tAIR ,/ ,THERMOCOUPLE I

THERMO- COUPLE

CATALYST BED

/ SUPPORT GRID

HEATER WINDINGS

4 TO CONDENSER 6 SCRUBBER

Fig. 1. Adiabatic packed bed atmospheric pressure reactor for NH3 oxidation

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When using the single tablet reactor, the tablet temperature was measured as

a function of the gas rate, the tablet and surrounding gas and apparatus having

first been heated to reaction ignition temperature, the gas flo\s. rate was then

gradually increased until, quite suddenly the temperature fell to virtually the

gas temperature as ignition was lost. The gas rate at which extinction of the

oxidation process occurred was taken as a measure of the catalyst activity.

The cobalt oxide catalysts were prepared by precipitation of hydrated basic

cobalt nitrate and ammonium carbonate solutions. Following drying at a temperature

of about 14OoC the basic carbonate, now containing some C o 3 0 4 , was mixed with 2%

graphite and tabletted. The tablets were then calcined at temperatures of

between 7OO0C to 900°C to give tablets with surface areas of between 3 m2/g and

0 . 2 m2/g respectively.

final tablets were about 5.4 mm x 5.4 mm,

Typical porosities were in the range 0.20 to 0.40 and the

'L METERED NH,/AIR UXILME

GAS PREHEAT THERMOCOUPLE-

-GAS MIXTURE PREHEATER

-HEATER WlNMNG

L A T A L I S T WLLET

-CATALYST PFLLET THERMOCOUPLE

GAS OUT kz

Fig. 2. Reactor f o r determining activity of a single pellet of NH3 oxidation

catalyst by measurement of feed gas flow rate at autothermic limit.

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EXPERIMENTAL RESULTS

The results of the tests made in the small atmospheric pressure test unit

(Fig. 1) with variable depths of catalyst bed are shown in Fig. 3 . as a plot of

selectivity against gas flow rate per unit cross section of reactor for two

different bed depths of 4 ern and 7 cm,

10% N H 3 were employed. For both bed depths the selectivity shows a maximum but

this is much lower for the deeper bed.

volume expressed at 2OoC) the 4 cm bed has a selectivity of 0.99 whereas the 7cm

bed has a selectivity of only 0.95, thus indicating that the lower 3cm of the bed

must have decomposed to nitrogen and oxygen some of the NO made in the top 4 cm.

The rising portions of the curves thus may be explained as due to a reducing

fraction of the NO being lost by decomposition whilst the falling parts can be

explained as due to the slip of unreacted ammonia. There is thus some evidence

for the importance of both hypothesis (a) and of reaction (3) of hypothesis (b).

However, it should be noted that the catalyst used in these tests was freshly

prepared and all the selectivities measured were relatively high.

An exit gas temperature of 75OoC and

At a gas flow rate of 350 l/hrzcm (gas

042L I x)O 2 5 0 300 3 5 0 400 4 5 0

GAS RATE LITRES &/HR.CM*

Fig. 3. Influence of gas rate and bed depth on selectivity of c0304 catalyst

bed for N H 3 oxidation to NO.

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The second of the set of tests is shown in Fig. 4 . The apparatus of Fig. 1

was again used but with a constant bed depth of 4 cm, a constant exit temperature

of 75OoC and two gas rates of 380 and 250 l/hr cm2. The object of the test was

to determine how selectivity changed with % N H 3 in the range 9% to 12%. It can

be seen that at both gas rates there is a sharp fall off in selectivity with % NH,

and that, furthermore, this fall off is greater at the high loading than it is at

the low. This fall off cannot therefore be due primarily to the N O decomposition

reaction having a higher order than the ammonia oxidation reaction (ie reaction

( 3 ) having a higher order in NO than reaction (1) has in N H 3 ) otherwise the fall

off would be greatest at the lower gas rate. These results suggest that reaction

(2) has a higher order in ammonia than reaction (1). If this is so then hypo-

thesis (c), relating loss in selectivity with catalyst use to loss in activity

should be correct, for during operation of this catalyst the velocity of the total

ammonia oxidation reaction is almost, though not quite, gas film mass transfer

limited and therefore the ammonia concentration at the outside of the tablet

should be inversely proportional to the catalyst activity. Increasing the

activity would thus reduce the ammonia concentration in contact with the catalyst

and increase its selectivity.

1.00 1

0 8 4 I I 9 10 I1 12

NH, CONCENTRATION %

Fig. 4. Influence of ammonia concentration on selectivity of Co301, catalyst

bed for N H 3 oxidation to NO.

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The third of the series tests therefore compared the selectivity of catalysts

measured in the small plant reactor and aged by plant running over many months with

their activities as measured by the gas rate at extinction as measured f o r single

pellet samples in the apparatus of Fig. 2 . The results of this work are shown

in Fig. 5 .

A s can be seen the selectivity falls rapidly when the activity becomes low.

The curve drawn has been derived from a mathematical model of these phenomena

which is not detailed in this paper. We may thus conclude that the predominant

effect causing loss in selectivity of this cobalt oxide whilst in operation is

l o s s in activity operating through the combined effects of gas film mass transfer

and the higher order dependence of reaction (2) on ammonia concentration than

of reaction (1).

092

150OC PREHEAT 6MM TUBE 0.84

0.96 -

150OC PREHEAT 6MM TUBE

0.80 100 200 4 0 0 800 IMX,

GAS RATE FOR EXTINCTION OFo SINGLE C0,O. TALLET LITRES 2 0 CIHR.

Fig. 5. Relation between selectivity of poisoned and sintered C o 3 0 1 , catalyst

and activity as measured by gas rate at extinction.

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CAUSE OF LOSS IN ACTIVITY

We may now enquire why activity has been lost remembering however that the

inherent selectivity of the surface catalysed reactions (l), ( 2 ) and ( 3 ) remains

unchanged - only the apparent selectivity of the catalyst bed changes. This

suggests that there is no inherent change in the characteristic of unit surface

of cobalt oxide as the catalyst loses activity, only a loss in the quantity of

accessible surface per tablet of catalyst. In brief, two causes may be envisaged.

sintering and poisoning. Inspection of the catalyst from the plant test showed

that both had undoubtedly occurred. Sintering was particularly noticeable at

the top (inlet) of the bed, with some catalyst agglomeration. Measurements of

the specific surface of the tablets also showed a marked fall and ESCA showed

that there was a significant accumulation of oxides of calcium, iron, lead,

magnesium on their surface. There was also a partial sulphation due to SO2

in the feed air stream.

Subsequent work to that descrioed above was also undertaken under higher

pressure conditions ( 5 bar and 9 bar) which showed that the general pattern of

phenomena and their quantitative effects as predicted from the atmospheric

pressure work, detailed above, were in all ways confirmed.

CONCLUSIONS

The relevance of the above findings to the design of a commercial catalyst is

to emphasize the necessity to exclude poisons as rigorously as possible. A study

of the mechanism discovered above shows that because the majority of the ammonia

is lost by its conversion to nitrogen in place of NO over catalyst at the inlet

of the bed, particularly if this catalyst has a reduced activity as a result of

sintering and poisoning, then an increase in bed depth cannot be employed to

offset lost activity in a hope that by this means selectivity can be improved.

Indeed, excess bed depth is detrimental to selectivity as some of the NO produced

in the inlet regions of the bed will be lost by decomposition.