On the role of alkali metals in ammonia synthesis

Applied Elsevier

Catalysis, 18 (1985) 405-408 Science Publishers B.V., Amsterdam

ON THE ROLE OF ALKALI

H. BOSCH,

of Chemical

7500 AE Enschede,

(Received

Bowker

et al. [I] recently

potential

diagram

adsorption

ward the necessity This

rates of ammonia

of nitrogen,

hydrogen

P.O. Box 217,

be raised.

kinetics

based on the

[21. Agreement

that, under

with

known

these conditions,

and, therefore,

the potential

In their concluding

their calculations

draws attention

at industrially

of detailed

and ammonia,

of nitrogen

by assuming

of quantifying

synthesis

They used a compilation

for low coverage

remarks

the

well for

they put for-

for iron catalysts

containing

to the role of K in the mechanism

of the

reaction. communication

measurements

K-contents

from this laboratory,

for the N2+H2 reaction

ranging

Fe catalysts

Altenburg

over doubly

from 0.55 to 3.8 wt%. We showed

from 620 to 720 K and at pressures promoted

of Technology,

1985).

predicted

should

(again)

In a previous

with

University

of N2 is activated

N and H atoms

potassium.

17 June

could only be achieved

dissociative adsorbed

Twente

and pressures.

and desorption

energy

activities

SYNTHESIS

The Netherlands.

temperatures

of adsorption

in The Netherlands

GELLINGS

P.J.

Technology,

8 May 1985, accepted

relevant

synthesis

IN AMMONIA

J.G. VAN OMMEN and

Department

kinetic

METALS

405 -Printed

et al. [3] reported

promoted

commercial

that in a temperature

from 0.5 to 20 MPa the reaction

can be expressed

catalysts

as

(1)

r = k.f

where

range

rate of doubly

k = reaction

rate constant

fi = fugacity w = order

of species

i

in hydrogen

a = constant K = equilibrium

constant

of synthesis

a and W were found to be a function function

of temperature

for all the catalyst For catalysts

only. This rate equation

formulations

0166-9834/85/$03.30

increased

formulation was found

only, whereas

k was a

to give the best fit

examined.

with a very low K-content

0.5 and this order

reaction

of catalyst

with

0 1985 Elsevier

the order

increasing

Science

Publishers

in hydrogen

K-content. B.V.

was found to be

On a catalyst

with

’

406 optimum

formulation

it was found

from the hydrogenation surface,

assuming

determining

the chemisorption

the resulting

equilibrium

order

(NH2)a + 0.5

derivation

formation species. alumina

orders

results,

state,

o = vacancy):

amount

we concluded

the steps

Altenburg

whereas

higher

to form H-bridges,

et al. gave a full

them

N species

in catalysts with

higher

was thought

cristallites.

the presence

catalysts

hand,

Recently,

for the formation

of (N), and (NH), species

ESCA pictures

was hydrogenated

by hydroxyl

species.

from that used of (NH), and

in ammonia

(NH2)a groups

for the mechanism mentioned

experimental to the quite

their different

same molar A singly an aqueous

results

our commercial always

revealed could

Although

by hydroxylic

species

this system

of the ammonia

evidence

in our opinion

formation

and speci-

above. discussed

above

and to discuss

it is now relevant

some recent which

influences

the nature

of OH-groups,

charge/radius

that N chemisorbed

this spectroscopic

hypothesis

influence

in which

synthesis,

speculative

of potassium

of the IA group would

was prepared

to reduce

with

2+ amounts of Fe . Also, the presence of OH-groups

In the light of recent developments

the presence

(N), in

on pure iron

of our samples

spectroscopy

implications

may contribute

(apparent

hydrogenated

of mainly

by electron

for the hypothesis

some additional

potassium

by the alkaline

Ertl [4] was able to detect

et al. [51 showed

surface

different

has important

through

either.

Roberts

on a Zn(0001)

At higher

of predominantly

it was not possible

completely;

of considerable

not be confirmed

of (N), species

of (N), to (NH),

nor the presence

Among others,

the presence

On the other

promoted

(NH), and (N),

that

to be changed

for conversion

the rapid

(order = 0.5).

low K-content,

K-content.

UHV conditions

favour

but only at low K-contents.

less effective

with

would

favpur

by the speculation

we were not able to prove the presence

catalysts

would

role in the hydrogenation

of these OH-groups

of K, making

K-contents

these findings

in H'= 1) or even to (NH2)a

ESCA under

that a low K-content

play an important

the nature

is quite

to 1.5. Analogously

+ 0

to explain

OH-groups

However,

If

on the

of N2 to be the rate

(a = adsorbed

of 1 and 0.5 respectively.

of (NH2)a species, We tried

their ability

fically

species

of the equations.

From these

triply

dissociation

established

in H2 would

H2 = NH3

lead to apparent

nature

can be derived

t 1 H2 = NH3 + o

(NH),

order

and/or

orders

N-containing

+ 1.5 He = NH3 + o,

(N),

loads,

of the predominant

step.

Were the following

and

to be 1.5. These apparent

equilibria

those OH-groups

ratio z/r. To test this

the K was replaced

we previously

in a different

by other

the other

1A elements

which

put forward.

way because

idea, a series

group

to add

observations

elements of

of catalysts but with

the

percentages. promoted solution

catalyst of 2 molar

(3 wt% A1203) Fe(N03)3

was prepared

and 0.1 molar

by coprecipitation

Al(N03)3

with ammonia.

from This

407

FIGURE

1

The activity

function culated

from equation

constant reference

singly

solution,

and the fraction

The activity commercial

Thus, the catalyst

activity

method,

k_ = reaction

rate constant

3 wt% A1203.0impregnated

of alkali

0.3-0.6

carbonate,

were calrate

of the

from a

was pressed,

mm was used for activity are described

precipitation

one reported

is governed

mainly

in which

the potassium

by its chemical

with the findings

with the

stage of the

was added

composition,

of Dmitrenko

cata-

[3].

was comparable

no matter

this work)

crushed

measurements,

previously

catalysts

previously,

(KOH, K2C03,

in agreement

and in the case of potassium

at 110°C the product

measurements

form

at IO MPa and 673 K as a rate constants

from a KOH solution.

of the K-containing

or in which

the preparation

with

in a solution

promoted

Reaction

a = 0.5 and w = 1.5. kAM = reaction

of KOH. After drying

with diameter

doubly

preparation,

catalyst

and activity

iron catalysts

metal.

catalysts,

@impregnated

was suspended

lyst reduction

promoted

promoted

also in a solution

promoted

of the alkali

1 with the values

of the doubly

carbonate

product

of alkali

of charge/radius

[31.

not by

[63 for molten

iron catalysts. The activity

of the promoted

one, the results the influence

of the alkali

is in agreement

catalysts

of these measurements additives

with the mechanism

motor may influence

the surface

formation

of NH and/or

H-bridges

between

of the alkali

hydroxide.

on the oxygen

atom

decreases

proposed

already

that of the unpromoted 1. It can be seen that

z/r of the alkali

OH group

metal.

This

, which is based on how the pro-

prevented, present

At high z/r ratios,

in the surface

with

earlier

with

in Figure

If z/r of the alkali metal

OH-groups.

NH2 is apparently

the OH-groups

was compared

being given

possibly

is small the

by the formation

on the surface

as for Li and Na, the electron

is lower,

of

and hydroxyl-groups

so that H-bridge

density

formation

will

408 be less favourable. thus decreasing constants doubly

This means

the promotor

in Figure

promoted

1 are calculated

catalysts.

is best described

that N-H (and/or

effect.

For reasons with

formation

of comparison

behaviour

can take place,

the reaction

the value of w = 1.5 which

In fact, the kinetic

by equation

NH2) group

of Li-containing

1 with the value w = 0.5, typical

rate

applies

of singly

to

catalysts promoted

catalysts. We agree with behaviour surface function

the idea of Bowker

of commercial

techniques

catalysts,

in order

of catalyst

et al. [I] of paying but we also suggest

to study the degree

more attention

the application

of hydrogenation

to the of modern

of N-species

as a

formulation.

REFERENCES M. Bowker, I.B. Parker and K.C. Waugh, Applied Catalysis, 14 (1985) 101. Vol. 4, J.R. Anderson and M. G. Ertl, "Catalysis, Science and Technology", Boudart (Editors), Springer Verlag, Berlin, 1983, p.273. K. Altenburg, H. Bosch, J.G. van Ommen and P.J. Gellings, J. Catal., 66 (1980) 326. G. Ertl and N. Thiele, Appl. Surface Sci., 3 (1979) 99. C.T. Au, M.W. Roberts and A.R. Zhu, Chem. Comm., (1984) 737. L.M. Dmitrenko, L.D. Kuznetsov, P.D. Rabina, T.Ja.Malisheva, M.M. Ivanov, Sh. Sh. Mischenko and V.S. Sobolevski, Proc. 4th Int. Cong. on Catal., Moscow, 1968, vol.1 (1968) 404.