Dealuminated mordenites as catalyst in the oxidation and decomposition of nitric oxide and in the decomposition of nitrogen dioxide: characterization and activities

Catalysis Today, 4 (1989) 155-172 Elsevier Science Publishers B.V., Amsterdam

DEALUMINATED

MORDENITES

AS

155 -Printed

in The Netherlands

IN THE

CATALYSTS

NITRIC OXIDE AND IN THE DECOMPOSITION

OXIDATiON

OF NITROGEN

AND

DIOXIDE:

DECOMPOSITION

OF

CHARACTERIZATION

AND

ACTIVITIES

C.U.INGEMAR

ODENBRAND,

Department Institute

LARS A. H. ANDERSSON,

och Chemical of Science

Technology,

Chemical

and Technology,

*EKA Nobel AB, S-445 01 Surte,

JAN G.M. BRANDIN Center,

AND SVEN JXRAS*

Lund University,

P.O. Box 124, S-221 00 Lund, Sweden

Sweden

ABSTRACT Dealuminated mordenites were investigated in order to illustrate the effect of the aluminium content on catalytic and physicochemical characteristics. Chemical and physical characterizations of the catalysts were performed by means of X-ray diffraction, chemical analysis, adsorptionand desorption studies and IR-measurements. The catalysts were tested in the oxidation of NO and in the decomposition of NO2 and NO. Activities for the mordenites in both the oxidation of NO and the decompotition of NO2 were strongly dependent on the aluminium content of the catalyst. The highest activities were obtained for the original unleached catalyst. No direct decomposition of NO to N2 and 0 was observed in the temperature range from 420 to 690 K. Adsorbed amounts NO and NH3 showed a regular decrease with the amount of aluminium in the OP catalyst. The activities in the oxidatiof of NO and the decomposition of NO could be correlated to the amount of NO adsorbed on the catalyst and whit was detected by IR.

2

INTRODUCTION We have for some time been engaged tive catalytic

reduction

on the reduction

idea

arised

dioxide based

from

might

be

on the fact

and NO2 was

larger

information

was

of nitrogen

oxides

vanadium 650 K

at

oxidation

oxide a

of nitrogen

of NO with

was later extended

to cover this an

work

that

the

step

the rates

oxidation

yielding of

of

for the reduction

only

62.000

(1).

0 1989 Elsevier Science Publishers

oxide

to The

work.

(2). The nitrogen idea

amounts

of NO or NO2 alone. catalytic

was

of ND This

reduction

oxide was not very rapid over

a maximum h-l

as well

mechanism.

for improved

of nitric

earlier

(1):This

of equimolar

of So

are warranted.

og20-5861/89/$06.30

nitric

on the selec-

We reported

catalyst dioxide

reaction

application

research

ammonia. oxide

of nitrogen

in the

(3). The oxidation

velocity

with

for the reduction

in a patent

catalysts

space

the reactions

important

than

oxides

NH3 over a vanadium

that the rate

used

in the fundamental

B.V.

8% other

nitrogen

dioxide

catalysts

for

at the

156 The catalytic

oxidation

of NO has been the object

lier. Rao and Hougen determined 330 K (4). The influence sideration, being

a

behavior

is necessary

shows

the removal

the

behaviour

rate

of NO by wet induced

some

Co and Fe in supported Japanese These

patents

ion-exchanged especially The phase

decomposition at 470 K and

on

decomposition

also

The

of NO2 takes

NO and 02 being

A

used

rate

and

produced

from

at

was

methods

for

of NO

containing

seventies

some

which

oxidation

Co304.

high

takes

of

over

the

catalysts

or

unusual

330 K but

temperature

to

Cu, Mn,

a number

of NO to NO2 above

Cr203

80%

of

(8,9).

Transition-metal

activities

place

reaction

reactions

were

obtained

and

homogeneous low

gas

(11)

and

in this paper. Catalytic

and Ce02/A1203

reaction NO2

in the

is relatively

presented

CuO/A1203

sequential

reaction,

the

X and Y (10).

dioxide

place

(12).

and

Zeolite

the catalytic

675 K respectively

below

with

the early

conversions

Mn02

nitrogen

above.

has

(5). The heterogeneous

requires

base metal

(7). During

showing

were

of

temperature

methods

with

over copper-exchanged

would not influence

work;

scrubbing

based

zeolites

reaction,

increase

from 300 to

The homogeneous

order

355 and 410 K (6). Flue gas treatment

on A1203

were

to

carbon

ear-

was also taken into con-

at temperatures

starts

studies

appeared

catalysts

third

rate with increasing

shown for silica gel between

N02, which

reaction

at low temperatures.

on a formal

a similar

temperature

over activated

of the homogeneous

example

of decreasing

reaction higher

which

typical

the kinetics

of but a few studies

scheme

finally

was

above

assumed

N2 and

02

575 and in

from

this

NO. At

790 K and a SV of 11.000 h-l 99 % of the inlet 1260 ppm NO2 was decomposed.

Of

these were

55% dissociated

1%

NO2

to

in He

temperature Although suitable for

N2 and

decomposition

This

is due

reaction

the temperature

limited

extent

NiO also

of

(14).

inhibition

Copper-exchanged

573 - 823 K. In

range

of NO directly

from flue gas little to the

and with a reciprocal

of elimination

only

dissociates

7% at

an optimal

to N2 and O2 is the most success

by oxygen

Y-type

the presence

has been reported formed

zeolites

during

are

of the CuNaY catalyst

SV of 7.8 g(catalyst)

s cm -3(gas)

the

active

in (69%

the extent

of NO (4 vol.% in He) was 90% at 773 K. Below 673 K the activireaction

ty declined

with

consecutive

reaction

(15). Using

copper(I1)

even higher steady

path

time was

reaching

because

presented

ion-exchanged

state activities

NO was 97% at a reciprocal rapidly

to a

decomposition

for NO removal

method.

exchanged)

but

to N2 and 02.

of 620 K and at a SV of 1250 h-l (13). the catalytic

method

this

O2

completely

a conversion

of

inhibition

with

ZSM-5

N20 as an

zeolites

by

02.

A

parallel

intermediary

Iwamoto

et al.

-

product observed

(16). At 823 K the degree of conversion of -3 . Below 723 K the rate declined

SV of 10 g s cm

of 57% at 673 K.

157 The aim of this

paper

oxidation

of NO

selective

catalytic

zation

was

in order

used

activities

is to study

to get a better

reduction

as

a

the decomposition

of NO and NO2 and

understanding

of their

of NOx with annnonia. The influence

suitable

means

of

studying

of the number and the character

the

of the active

role

the

in the

of dealumini-

effect

on

catalytic

sites for these reac-

tions.

EXPERIMENTAL

METHODS

Preparation A

of catalysts

series

900 H

of

(Norton

treated

with

described

dealuminated A crushed

HCl

(AnalaR)

below.

12 M. About heated

was

decanted

at its boiling and

washed

added and heating

and

was

the

text

samples

before

where

(SLEl).

once more

(SLE2).

The samples

for

contacted

molar

was

The final

leaching as SLEy

was

checked

was

50 cm3

as 1 to

mordenite water

was

proce-

by pH measurements

The samples

for 6 hours and

stored

at

in a

are designated

In method

HCl.

To 27 g mordenite

in

2 the

120 cm' 2M HCl was

washed,

mordenite

from

The washing

air for 3 days

of

was

and the mixture

performed

for 4 hours,

Zeolon

durations

varied

Then

hour as above.

tests.

From the dealuminated

are designated

water.

concentration

boiled

of

1 hour. The dealuminated

with

and catalytic

and

of HCl was

Drying

AgN03.

leaching

of the mordenite

to 10 g mordenite

distilled

with HCl was varied.

The mixture

and stored as above.

added

the efficiency

x means

by

concentrations

for another

with

were

analysis

as SLx

number of treatments added

was

with

and

ion detection

395 K. The dried

various

temperature twice

4 times

prepared

l.OZ1.27mn,

1 the concentration

was continued

repeated

chloride

desiccator

using

In method

were

fraction,

30 cn? HCl solution

was

dure

mordenites

Co).

dried,

rehydrated

18 g was leached with HCl

was performed

with 8 g mordenite

in which y means

the number

(SLE3).

of acid treat-

ments.

X-ray diffraction The

leached

Philips

goniometer. 3:l.

were

examined

diffraction

grounded

Cu Karadiation

unit

mordenite the samples

by

X-ray

equipped

was mixed were

at 20 = 45.827

degrees,

in this investigation

were analyzed sample holder.

6 times each after

a-Si02,

with

scanned

a

PW

silica

in weight

at a speed

a

ratios

of l/8 degree reference

(17). The

- 46.8 degrees.

recompressing

using

1050 wide-angle

The silica

d = 1.980

was at 20 = 46.6 intermediate

diffraction

with

over the range 20 = 47.0 to 45.5 degrees.

obtained

peak" used

powder

Finely

Using

per minute was

mordenites

PW 1300

peak

"mordenite All samples

of samples

in the

158 Chemical

analysis

AAS of Al, Si, Fe and Na in the mordenites AA-1275

Atomic

Absorption

of acetylene-N20 preparation scribed

and

Fe and

of the solutions

below.

the contents

Na by means for analysis

of the crucible

For Si analysis

50 cm3 with water. in 6 g molten

the dissolved

Adsorption The

mordenite

surface

used

de-

was dis-

After

cooling,

water.

This

to 1.5. - 2.0 with

to 25 cm3 solution

The amount

HCl or

and diluted

0.95 g mordenite

to

was dissolved

of cont. HCl used was then 9 cm3.

as to yield

similar

compositions

as in

samples.

studies area

and

using N2-adsorption also

the procedure mordenite

in distilled-deionized

was adjusted

were prepared

The

of 17 cm3 cont. HCl in 750 cm3 water

When Na and Al was analysed

solutions

grounded

du,ring 30 minutes.

0.1 g NaOH was added

KOH for 6 minutes.

'The calibration

Series

by means

gas mixtures.

was done following

was dissolved

with a solution

at 283 K. The pH of the final solution NaOH.

of acetylene-air

NaOH in a Ni crucible

was the combined

on a Varian

Al and Si were analysed

For Fe and Si 0.45 g of the finely

so'ived in 7.5 g molten

solution

was performed

Spectrophotometer.

to determine

The. dehydration

the

total

technique

of

pore

the micropore

the of

volume

at p/p0 = 0.99 were

on a Cahn 2000 microbalance structure

as described

mordenite

samples

50 K from

273 K to 633 K under

temperature

in steps

(2.7 mPa)).

The evacuation

was

was

followed

before

continued

determined

(2). These data were by Spitzer

by

(21).

increasing

vacuum

measurements

the

(2*10m5

torr

of the surface

area for 16 hours. The same

adsorption gravimetric

above. the then the

of NH3

NH3 was

increase

samples

while were

introduction a measure

equipment admitted

of weight

followed

(Alfax and

was

degassed

>99.96%)

by means

in small

amount

performed

The

to a final

by means

pre-treatment

up to a pressure increased

the temperature

again

on NH3 was repeated

was

of the same

increments

registered.

decreasing

of the adsorbed

N36,

adsorption

of

6-8*10m5

the

as stated

of 85 torr

to 375 K. At this pressure

of

of

and

NH3 was

temperature torr.

The

as above to 85 torr. The increase was used as of NH3.

Acid strength The acid the powder briefly pKa

strength

of the

(18). The following

ranges

Bromophenol

sites

into a test tube, a dding in brackets. Blue

isooctane

indicators

Methyl

(3.0-4.6).

on the mordenite

Violet

were

was determined

containing

indicator

used and are presented

(O.O-1.6),

Thymol

Blue

by placing and shaking with

(1.2-2.8)

their and

159 Adsorption

and desorption

of NOx

In the NO oxidation experiments -1 flow of 830 cm:TPmin containing was raised

in increments

the reactor constant

outlet

inlet

the catalyst

was treated with a steady-state

600 ppm NO and 2% 02 in N2. The temperature

of 30 K to study the desorption.

was monitored

concentrations

peak, was used as a measure

while

of

increasing

NO

and

of the desorbed

the temperature

The

02.

The NO, signal

area

amount

under

from

maintaining

the

desorption

of NOx.

IR measurements IR spectra of the leached mordenites coupled

to a data station

pressed

to 2 cm discs

situ

IR cell

in the

catalyst

a pressure

of 30 MPa.

at 675 K for 30 minutes.

at room temperature.

spectra

recorded

were

on a Perkin

3600. 20 mg of the finely grounded

under

torr in order to remove The

were

After

interference

recorded

580 B

samples were com-

Degassing

Then

30 minutes

Elmer

was performed

NO was

introduced

the cell was evacuated

in

to the to 10-l

from gas phase species.

before

and

after

admission

of

NO.

Difference

All samples were scanned from obtained using the microcomputer. -1 -1 -1 4000 cm-l to 300 cm at a scanning speed of 10 cm min . The resolution of spectra

the

were

spectrometer

was

used as a measure

Activity

The

area

under

the following

were

- 2170

cm-'

peak was

measured,

in a tubular

2.

The decomposition

of.N02

to NO and O2

The decomposition

of NO

to=N2 and O2

experimental

100.000 h-l. Other was performed

reactor

as before

(l),

in

of NO to NO2

3.

a capillary

flow

reactions:

The oxidation

The

the 2150

of NO adsorbed.

measurements

The activities

1.

21 cm-l.

of the amount

conditions details

were:

will

0.5

be given

g catalyst below.

with a Balzer QMG 311 Quadrupole

leak inlet and a chopper

device

and

a space

In addition,

velocity

analysis

mass spectrometer

of

of N20

equipped

with

(19).

RESULTS AND DISCUSSION Chemical

analysis

Leaching

with

bonded metallic linity.

This

and X-ray diffraction HCl

results

in a selective

ions, from the zeolite is

intact but where

reflected

in

the interplanar

the

removal

structure

X-ray

spacings

without

diffraction changed

of aluminium,

and

a decreased

pattern

somewhat.

which

Figure

to it

crystalremains

1 shows the

,

160 dependence There

of the interplanar

is a nearly

perfect

In further

increases.

spacings

linear

studies

of mordenite

decrease

this

on the amount of aluminium.

in the spacing

deoendence

could

as the Si/Al

be used

ratio

to predict

the

Si/Al ratio.

0.1949

E

0.1948

$2

0.1947

-

2

0.1946

-

% 9

0.1945

-

=4

0.1944

-

P

0.1943

-

F z

0.1942

-

5

6

7

6

SVAI ATOMIC

Figure

1

Interplanar

dealuminated

$ L

spacings

9

10

RATIO

as a function

of the Si/Al atomic

ratio in

mordenites.

0.04

-

0.03

-

0.02

-

0.01

-

.E g 5 P 5 0 2 if

0

!

5

6

7 S/Al

Figure 2

Metal

in dealuminated

The

amounts

shown in Figure

ion content

Fe and

Na

9

RATIO

as a function

mordenites,w

of

8 ATOMIC

Fe/Al,.

af the Si/Al atomic

in the mordenites

2. As expected

ratio

Na/Al.

they decrease

were

also

along with

determined

the decreasing

and

are

amount

161 of aluminium. mordenite

Table

These

ions are directly

structure

1: Chemical

and should

composition

bonded

decrease

to negative

aluminium

ions in the

in a similar way as did aluminium.

of catalysts

by AAS, XRF and by thermogravimetry

(wt % hydrous) Sample:

SLl

M

Al

5.58

Si

SL5

5.06

31.0

SLconc

4.64

32.3

4.64

31.3

SLEE

SLEl 4.56

32.0

SLE3

3.97

32.2

3.31

32.4

Fe

0.50

0.28

0.25

0.23

0.23

Na

0.180

0.133

0.130

0.114

K2°

0.18

0.18

-

0.12

34.0

0.16

0.09

0.087

0.061

0.041

-

0.13

0.10

CaO

0.13

0.08

-

0.05

-

0.03

<0.03

Ti02

0.43

0.41

-

0.41

-

0.43

0.43

'2'5

0.05

0.03

-

0.03

-

0.03

0.05

____________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~H20(NH3)

10.95

12.55

H20(BET)

11.97

12.71

H20(Ign.)15.4

12.14

12.99

-

-

-

-

16.0

15.1

16.3

12.72

11.14

12.39

11.71

15.8

16.4

____________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Si/Al(AAS)

5.338

6.133

6.479

6.624

6.874

7.842

9.866

____________________~~_~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ d (A)

The

results

given was

1.9489

Ti02 Nor

from

in Table also

calcium

used

the chemical

to

are around

Si/Al

0.4 wt.

1.9447

1.9469

by AAS,

other

of P205 which

ratio

AAS

elements.

to the amount

% and are not

0.1% which

atomic by

1.9471

analysis

determine

in parallel

is the amount

determined

1.9466

XRF

and

1. Al, Si, Fe and Na were determined

decrease

MnO are below The

1.9477

is around

is the detection determined

but

give

by XRF

slightly

The

0.04

amounts

by the

are

potassium

the

leaching

for these

same

values

at

and

in the form of

% The amounts

in XRF

follows

higher

of

Impurities

wt.

limit

thermogravimetry

by both AAS and XRF. XRF

of Al.

influenced

1.9415

trend

procedure. of MgO and compounds. as the

intermediate

one acid

leachings. The total by ignition

amount

of gravimetry surface

including

at 630 K in the

area measurements.

increased, somewhat

of water,

at 1470 K. The amount

reached

at higher

a maximum ratios.

NH3

the surface

OH groups,

of "zeolitic

water"

adsorption

studies

The amount

increased

as well

somewhat

the mordenite

was heated

as

by means

in the

BET

when the Si/Al ratio

of just below 13% at Si/Al around When

was determined

was determined

7 and decreased

to 1470 K the amount

162 of water

lost was about

terials the

in the order

difference

findings

Surface

The

amount

of

900 H and water

11.0%

water

areas were obtaind pore

dealuminized total

found,

analysis

the BET surface

pore

by means

developed

this compares

removed

was

in

at

area

increases

volume,

isotherm

zeolitic

is of type

materials

I in the

agreement

for ordinary

be determined

at low relative

classifies

micro

the size

4000 A.

We

supermicro

Table 2:

pores

630 K

with

earlier

a

pores

nonlinear

volumes

experimental

Surface

Samole

as

with

pores

from

Vmicro

to

12 A,

There

was

area has to

pores

pores

above

microa very

and good

as shown in Figure 3.

for dealuminated

mordenites

SBET

Smicro*

-1 Cm29 1

(m2 g-l)

0.1799

0.0056

402.5

472.1

SLl

0.211

0.1887

0.0061

414.7

493.0

SLEZ

0.223

0.1951

0.0071

414.1

505.7

SLE3

0.239

0.2030

0.0095

430.0

528.7

pore model

The analysis portion

shows that the mordenite of the surface

10%) is contained

area

in pores with

the micro

pores is about 0.18 3 -1 calculated 0.178 cm g

calculated

for

the

0.207

the main

is common

supermicro

Mordenite

*cylindrical

in-

used here). Dubinin

determine

isotherm.

values

VSUD

(cm3 g

(cm3 g-l)

which

also

30 to 4000 A and macro

-1)

is

pores. This type of isotherm

below

method

the adsorption

and calculated

= 0.99,

and the BET surface

openings

area and pore structure

vtot

p/p0

(p/p0 = 0.001-0.03

regression

from

at

classification

with micro

pressures

the mordenite

from 472 to 529 m2 g-I (Table 2).

pore size analysis

12 to 30 A, meso

used pore

fit between

value

with

and

and also by means

(22). When

adsorption

BDDT

and other materials

is not suited

small

nicely

1470 K

of the BET method

by Dubinin

measured by N2 3 -1 from 0.207 to 0.239 cm g .

creases

with

and

from dry ma-

area and pore structure

of the micro

The

15.4

losses on ignition

(20,21).

The surface

The

reports

of 4.1% for Zeolon

between

respectively.

15-16%. Norton

channels

(8-15%)

cylindrical

higher values for the surface

as well

and that only a

as the pore volume

(around

diameters greater than 16 A. The volume of -1 and this is close to the theoretical cm3 g from

the structure

and the side pockets

assuming

is highly microporous

pores.

areas

of the mordenite

(23). The micro

(22).

A

spherical

including

pore surface pore

model

areas

would

are give

163

cl

0.04

0.08

0.12

0.16

RELATIVE

Figure

3

Determination

dealuminated

The effect surface

around

nite. We assume

ferentially found

in

measured

the

This would

experiments.

by means

Acid strength

of ammonia

dealuminization.

Thus

by 31%. In an earlier

agreement. structure

the micro

porous

values.

pore

structure

in the main channels

reports

the pore volumes pore

for pellets

volume

volumes

of

and

of the morde-

rings of the side pockets

and that

(24) are preby the amounts 0.274

cm3

g-l

of Zeolon 900 H with 35% above

0.18 cm3 in the smaller

ones (down to 65 A).

at 375 and 635 K are displayed

at 375 K decreases

to the Si/Al

in

computed

and atmnonia adsorption

The adsorption adsorption

both

then increase

Norton

leaving

pore volumes

atoms located

of Hg-porosimetry

1400 A pore diameter,

0.28

and -

the micro

in the four-membered

removed.

points

is to increase

12% maintaining

that the aluminium

located

0.24

and supermicro

experimental

of dealumination

area

the ones

of micro-

mordenites..

0.2

PRESSURE

ratio

The

linearly

50 to 35 mg

the number

of acid

publication

(25) the amount

and comparing

molar

from

ratio

sites

that value

of adsorbed

is 1.16 and in good agreement

in Figure -1

with

of this strength

4. The

inc'reasing

is decreased

of acid sites was correlated

(30.5%)

ammonia

g

to ours yields

to aluminium

with earlier

findings

excellent

present (26,20).

in the

164

S/AI ATOMIC

Figure 4

n

The adsorption

375 K and.635

The number

of ammonia

of acid sites equivalent slowly

Si/Al

ratio of 6.8 thereafter

nated

mordenite.

The

1.2-1.6).

The change of adsorption

reflecting

ratio has been investigated

a

results

diffusion

based

The amount

and desorption of adsorbed

in the presence

are displayed temperature determined

that

there

(pKa = 1.2-2.8)

capability

process.

was studied

The

by means

between from

an

controlled

the

of NH,, measured

was

a small

in-

and SLE 2 (pKa.=

as a function

of the

variation

440

correlation

acid

strength

with

increased of

from 5 to 10. There

Broenstedt

results

to

acidity

to

the

(28).

of NOx

NOx was determined

as the area under

in the NO oxidation

5. The amount and

of

of H MAS NMR (27) and was shown

which give similar

of oxygen

in Figure

the reciprocal

diffusion

on

electronegativity

The adsorption

versus

showed

Mordenite

by a small amount when the Si/Al ratio

Sanderson's

peak

amount

635 and 375 K. The heat of adsorption calculated from the -1 dependence was around 8 kJ mol which is a very low value pro-

Si/Al

other

at

between

temperature

increase

to the adsorbed

indicator

between

bably

are

mordenites

from 25.5 mg NH3 g-l in mordenite toJ26.9 at a -1 decreasing to 18.3 mg g in the most dealumi-

use of an

in the acid strength

temperature

on dealuminated

K.

at 635 K is increasing

crease

RATIO

decreases

620 K. The

Arrhenius-plot temperature, as the ammonia

of

monotoneously

activation the

logaritm

the desorption

experiments.

energy of

for

the

The

with

results

increasing

this

adsorbed

process, amount

was 19 kJ mol -l, This low value is probably adsorption

above.

0-l 420

I

I 440

I

I 460

I

I 460

I

I 500

I

DESORPTION

The desorption

I 520

I

I 640

I

I 560

TEMPERATURE

I

I 580

I1

600

I

620

(K)

of NO, from mordenite

as a function

of

the temperature.

IR-spectra before

and

played.

of adsorbed after

NO were

recorded.

the adsorption

of

NO

In Figure 6 the difference

on dealuminated

Several

peaks are present.

The most

can be assigned

to the NO+ species

(29).

important

mordenites

spectra are

dis-

one is at 2165 cm-l and

l-,

1000

1500

2000

3000

FREQUENCY (cm-l) Figure 6

The

Infrared

integrated

spectra

absorbance

of adsorbed

of the

0.9 to 0.15 with an increased

Si/Al

NO on a) Mordenite

2165 cm-l

peak

and b) SLE3.

is decreasing

ratio from 5.3 to 7 (Figure

rapidly

from

7). At higher

166 degrees amount

of dealuminization of chemisorbed

tion. The low decrease aluminium

atoms

necessary

in

to understand

There

were

the

decomposition

product

gases

in

were

is known (30).

to occur

Our

9

the

studies

of are

mordenites.

10

RATIO

-1

band for dealuminated

entire

mordenites.

of 600 ppm NO into N2 and O2 over any

temperature

by both

range

from

chemiluminiscence

on zeolites have

but were

shown

only detected

that

this

420

to

and mass

were formed. The disproportionation

experiments

that

by depletion

Further

of NOx on dealuminated

conversions

the

analysed

and no other products

structure.

shown

of dealuminiza-

of NO

no measurable

catalysts

It is thus

by the degree

can be explained

the

8

7

IR intensity of the 2165 cm

7

The catalytic

of

af

the adsorption

SilAl ATOMIC

Figure

ratios

four-rings

6

is lower.

influenced

at high Si/Al

the

5

the decrease

NO is strongly

The

spectrometry

of NO to N20 and NO2

at ambient

reaction

680 K.

does

not

temperatures occur

at

our

conditions.

The catalytic

oxidation

The oxidation Figure the

8 shows

selectivity

thermodynamic centrations

results

were

performed

for mordenite.

to NO2 and data

the equilibrium conversion

of NO, increases

from about

conversion

with

The figure

(2). The

at 620 K. The agreement

of NO to NO2

experiments

drops

with the equilibrium

600 ppm NO and shows

conversion

computed

at higher

temperatures

of NO,

of NO calculated

from

inlet

an outlet

5% at 420 K to a maximum

at the highest

2% O2 in N2.

the conversion

of about 52%

but attains

temperature.

from con-

values

in

167

420

460

500

540

560

TEMPERATURE

Figure 8 @the

Oxidation

seletivity

The selectivity

explanation

separate

measurements

could

this idea. The explanation

reaction

decrease

attributed

to diffusion

catalyst

type,

was

of

was

temperatures

could

24

amount

by

kJ mol-l

spectrometry

decreases

oxidation

8 for all

535

no change that

The

and

the

lower

NO,

catalysts.

above which

the formation

temperature

is not influenced

but

1:st

activation

of

475 and might

energy

sites,

energy

influence

at

not

be as the

higher

but an effect

should aldo be present.

has

of NO2 decreases

fashion

the

There

of the temperature

As the aluminium

ratios.

in a regular of

between

in activation

only by diffusion

atomic

assuming

595 K. This

number

of NO2 formed as a function

Si/Al

NO

investigations.

of 46 t4 kJ mol-'

was

indicating

not be explained

of

has not confirmed

region of 475 to 590 K. There was a

between

There

below 525 K. A

reaction

of NO2 was calculated

values

dealumination.

various

in the

in Figure

of mass

the gas phase equilibrium

of NO2 formed

conversion, shown

with

with

effects.

Figure 9 shows the amount mordenites

disproportionation

the temperature

dealuminated

changed

of approaching

21

of NO,

of NO.

needs to be found by further

energy

values

conversion

of NO2 is low at temperatures the

for the formation

in NO over

in activation

535 K and

the

be

of N20 by means

energy

conversion

the equilibrium

for the formation

possible

order

660

of NO over Mordenite.athe

to N02, -

The activation

620

(K)

same

at all temperatures. temperature

is always with

by dealumination.

is removed

dependence

an optimal

the temperature.

for the The as

temperature This optimal

168

240 220 200 P

160

s

160

9

140

9z

120 100

g

80 60 40 20 0 5

6

6

7 Si/AI ATOMIC

Figure 9

n

Effects

420 K,O

of acid leaching K and A570

470 K,h520

The

decrease

trend

as does

in oxidation the amount

could be coordinated

not

correlation

of the oxidation

In

mordenite.

from

Si/Al

not effect

the oxidation

activity.

atoms

in the 4-rings

and that this pair

abrupt change

in activity

The catalytic

decomposition

The decomposition

various

is

a

in N2.

ion in the crystal

in

temperatures

at Si/Al atomic

calcula-

ions present

in

is that one of by the leaching

activity.

Thus

the

ratio above 8 could be explained.

of NO2 to NO was

studied

10 shows

between

495

using

and

of NO increases

of NO over

695 K. Below

but the amount

an empty

reactor.

The amounts

a gas mixture

the formation

both at the inlet and at the exit while

rapidly

of NO, formed

less than 10 ppm. These small amounts rate data.

removed

of

with

and iron does

Our suggestion

for the catalytic

the

content

11, a theoretical

is preferentially

reaction

tic reaction

between

activity

of aluminium

about

structure.

is necessary

reaction Figure

a similar NO+ species

distinguish

decrease

removal

detected

usually

to

rapid

At Si/Al

the aluminium

2% O2

data,

follows

7). This

(24) that, there will be no more pair of aluminium rings of the mordenite

and

9,

to the iron or the aluminium

there

the 4-mebered

process

our

activity

cases

in Figure

by IR (Figure

ratio to about 7. Further

increasing

tion showed

shown

an iron ion or an aluminium

possible,

both

of NO to NO2

K.

activity,

to either

It

10

on the oxidation

of NO+ measured

structure.

the

is

9

RATIO

of 600 ppm NO2 the catalyst

595 K there

at higher flowing

is almost

temperatures. the reactants

on the reactor

walls,

do not to influence

at no

NO was through

were

small

the cataly-

169

TEMPERATURE

Figure

10.

Influence

decomposition @SLEW

ASLEI, The

figure

perature the

NO

(K)

of temperature

of NO2 to NO and 02.

and acid leaching

on the catalytic

n equilibrium, qMordenite&L5,

andOSLE3.

shows

that

approaching formed

the formation

equilibrium

amounts

to

89%

of NO

values to

is increasing

at the highest

the

equilibrium

rapidly

temperatures.

value

over

the

with

tem-

At 675 K unleached

mordenite. As

the

decreased tained.

degree

of

(Figure

There

dealumination

11).

Similar

was a rapid

is

increased

curves

decrease

as

for

the the

in activity

decomposition oxidation

of

at low Si/Al

of

NO2

NO were

ratios

is ob-

levelling

out at Si/Al above around 8. The activation

energies

were

of 615 to 655 K the activation on

the

Si/Al

obtained only.

ratio

activities

of

the

which

determined energy

catalyst.

described

conclude

that

In both these

are directly

the

on other catalysts

further

In the temperature

was 88 $9 kJ mol-'

related

In no case was there a decrease

therefore

as above.

in

again

total

decomposition

(13) does not occur.

not dependent

reactions

to the number NO, of

over NO

we

have

of active the

to

range

O2

thus sites

reactor. and

N2

We as

170

. 280 -

P s

240 -

8 2

200 -

5 E

160 \

5

6

7

6

SVAI ATOMIC

Figure

11.

Influence

decomposition

9

of the Si/Al molar

n

of NO2 to NO.

10

RATIO

570 K, 0

ratio on the catalytic 620 K andA

K.

CONCLUSIONS Mordenites

and dealuminated

ties in the oxidation direct decomposition The

dealuminated

characterized

both micropore

is also

oxides,

content

Si/Al

is the determining

areas.

also influences

fashion

NO+ has been suggested

from

5.3

to

9.9

The greatest

the ability

have

effects

were

to adsorb

as an important

to the amount

Futher

discussions

on the mechanisms

will be dealt with

species

in

observed

both ammonia

content.

content.

the number

By means

and

increases

ions such as Fe and Na.

with the aluminium

for the activities.

been

is maintained

leads to small

along with the aluminium

in parallel

oxides

ratios

activi-

of NO2 to NO. No

420 and 690 K.

The crystallinity

on dealumination

decrease

nitrogen

to exhibit

(02) of NO to NO2 and the decomposition

do not change factor

between

and to it bonded metallic

both of which decrease

in a regular

energies

been shown

The dealumination

and surface

in the oxidation

both decrease activation

with

intact.

of aluminium

The lower aluminium

Activities

mordenites

volumes

for the content

have

of NO to N2 and O2 occurs

by means of various methods.

the pore system

and nitrogen

mordenites

of NO to NO2 and in the decomposition

Since the

of active

of infrared

for both reactions.

of NO2

sites

technique Activities

of NO+ determined. as well as impacts

in separate

papers

on the reduction

(30,31).

of

171 ACKNOWLEDGEMENTS We are grateful of Sweden. Europe

The

for financial

catalyst

in England.

by Mrs B. Svensson. fully performed

support

material

The skillful

was

from the National kindly

supplied

work with the nitrogen

The preparations

Energy Administration

by Dr Mills adsorption

of Norton

was performed

and the X-ray characterizations

was skill-

by Mr. M. Persson.

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11 12 13

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26 27

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