Inhibition by Sulfur Poisoning of the Heterogeneous Decomposition

11 Inhibition by Sulfur Poisoning of the Heterogeneous Decomposition of Acetone

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 8, 2016 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch011

M. J. BENNETT, G. H. CHAFFEY, B. L. MYATT, and D. R. V. SILVESTER Materials Development Division, A E R E Harwell, Didcot, Oxon, ΟΧ11 ORA, United Kingdom

Inhibition by sulphur poisoning of carbon deposition from the heterogeneous decomposition of acetone on an oxidised 20% Cr/25% Ni/Nb stainless steel and representative scale constituents (Fe O and Cr O ) has been studied. Two procedures examined were prior sulphiding and the continuous addition of sulphur compounds (thiophene, H S, SO and COS) to the gas phase. Deposition on Fe O was reduced, and in certain circumstances prevented, as catalytic intermediate formation was inhibited either by the preferential adsorption of the additives poisoning surface sites or by oxide conversion to Fe S. Sulphur poisoning did not inhibit acetone decomposition by Cr Ο . The oxide behaviour reflected that of the oxidised 20/25/Nb steel. Sulphiding reduced deposition for a limited period until new catalytic intermediates were generated by the simultaneous oxidation of the steel. A l l the gaseous additives were effective by extents which increased with their partial pressure. 3

2

4

3

2

2

3

4

1-N

2

3

The coolant f o r the UK advanced gas-cooled power reactors i s carbon dioxide containing minor c o n t r o l l e d l e v e l s of carbon monoxide, methane and water vapour. Radiation-induced degradation of the carbon monoxide and methane can r e s u l t i n gas-phase precursors, which i n turn can be decomposed c a t a l y t i c a l l y to produce carbonaceous deposits on the o x i d i s e d 20% Cr-25% N i niobium s t a b i l i s e d (20/25/Nb) a u s t e n i t i c s t a i n l e s s s t e e l f u e l cladding surfaces (J^, 2). This deposit forms as f l o c c u l e n t aggregates of interwoven f i l a m e n t s , and i r o n and n i c k e l are the p r i n c i p a l m e t a l l i c constituents of the c a t a l y s t s involved. This type of d e p o s i t i o n has been simulated out of reactor from the thermal degradation of acetone both on the 20/25/Nb s t e e l and a range of oxides (such as Cr203, Fe304 and other s p i n e l s ) (3)· These are the p r i n c i p a l c o n s t i t u e n t s of the scales formed on t h i s s t e e l

This chapter not subject to U.S. copyright. Published 1982 American Chemical Society.

Albright and Baker; Coke Formation on Metal Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

COKE FORMATION

224

during o x i d a t i o n in carbon dioxide over the temperature range of interest. Filamentary deposit growth on metals and a l l o y s from hydrocarbon degradation has been studied e x t e n s i v e l y (4) with a common feature being the high c a t a l y t i c a c t i v i t y of the t r a n s i t i o n elements. This i s a l s o one of the main types of carbonaceous de p o s i t formed on ESC p y r o l y s i s tubes during cracking operations


· Previous work (6) has i n d i c a t e d that carbon d i s u l p h i d e and other sulphur compounds (hydrogen sulphide, e t h y l mercaptan, t h i o g l y c o l l i c a c i d , a l l y l i s o t h i o c y a n a t e and t r i t h i o a c e t a l d e h y d e ) , at a 0.1% concentration, were e f f e c t i v e gas phase poisons of the thermal decomposition of ketones on heated nickel-chromium heat r e s i s t a n t a l l o y r e a c t i o n v e s s e l s . In a d d i t i o n , sulphur compounds prevented the d e p o s i t i o n of carbon on i r o n and various s t e e l s by the Boudouard r e a c t i o n from carbon monoxide (7)· On t h i s b a s i s two approaches to reduce acetone decomposition by the 20/25/Nb s t e e l , Fe304 and 0 ^ 0 3 have been examined. The f i r s t was p r e t r e a t ment to form a sulphide surface l a y e r , which could be l e s s c a t a l y t i c a l l y a c t i v e . The second was to add a sulphur bearing compound continuously to the gas phase, which would be adsorbed on the a c t i v e surface s i t e s p r e f e r e n t i a l l y to acetone. The r e l a t i v e e f f e c t i v e n e s s has been compared of hydrogen sulphide, sulphur d i o x i d e , carbonyl sulphide and the h e t e r o c y c l i c sulphur c o n t a i n i n g organic compound, thiophene. The l a t t e r was s e l e c t e d because f o r some r e a c t i o n s the r e l a t i v e t o x i c i t y of a number of poisons, each c o n t a i n i n g the same t o x i c atom, increased with the covering power of the r e s p e c t i v e molecules, i . e . with the complexity of the chain or r i n g attached to the t o x i c atom ( 8 ) . Experimental M a t e r i a l s . The 20/25/Nb s t a i n l e s s s t e e l specimens, with dimensions 2 cm χ 1 cm, were cut from 250 μπι t h i c k annealed s t r i p with the a n a l y s i s and g r a i n s i z e given in Table I . The specimen TABLE I - A n a l y s i s and G r a i n Size of 20/25/Nb S t a i n l e s s S t e e l

ppm

Wt.%

Grain s i z e um Cr

Ni

Μη

Nb

Si

C

S

19.8

22.6

0.9

0.70

0.56

400

100

18 - 25

surfaces were abraded with 600 g r i t s i l i c o n carbide paper and then were o x i d i s e d , in carbon d i o x i d e , f o r 1000 h, at e i t h e r 600° or 700°C. The oxygen uptakes by the i n d i v i d u a l specimens are given in


11.

BENNETT

ET AL.

Inhibition

by Sulfur

225

Poisoning

Tables I I I and V. The Fe3Û^ powder was a composite of three batches, prepared by the o x i d a t i o n of Johnson-Matthey s p e c t r o s c o p i c grade i r o n sponge in dry carbon d i o x i d e , at 350°-500°C, f o r 2000-3000 h. A n a l y s i s and X-ray d i f f r a c t i o n (Table I I ) i n d i c a t e d the presence of some Fe203, in a d d i t i o n to Fe304» The 2 ° 3 powder was a high p u r i t y grade ( > 99.999%) s u p p l i e d by Koch-Light L t d . The surface areas of both oxides, a l s o given in Table I I , were determined by the BET method from the n i t r o g e n a d s o r p t i o n measurements, at 78°K, using a Perkin-Elmer sorptometer.


C r

TABLE I I - Analyses and Surface Areas of Fe^04 and Crg03

Analyses Oxide

F e

3°4

C r

2°3

Surface Area m /g

Fe

C

0.61

71.5%

1.0%

43.8

50

ppm

100

S

ppm

20

ppm

20

ppm

The thiophene was supplied by Koch L i g h t L t d . , while the other sulphur bearing gases (hydrogen sulphide,sulphur dioxide and carbonyl sulphide) were obtained from BOC ( S p e c i a l Gases) L t d . Experimental Procedure. As gold does not c a t a l y s e acetone decomposition, i t was used to f a b r i c a t e c r u c i b l e s (1 cm diam., 1 cm high) f o r the containment of oxide specimens and as support wires f o r the 20/25/Nb specimens. The oxide sample weight used was 50 mg. The specimens were exposed in s i l i c a r e a c t i o n v e s s e l s . Acetone at p a r t i a l pressures o f ^ 0.15 and 0.31 atm, was introduced i n t o the carbon dioxide c a r r i e r gas by passage through a bubbler at 0°C and ambient temperature r e s p e c t i v e l y . The carbon d i o x i d e pressure was 1 atm and i t s flow rate was 200 cc/min. For the experiments in which thiophene was added to the gas, the bubbler was charged with a mixture of acetone and thiophene. Although the acetone p a r t i a l pressure in the gas phase was maintained e f f e c t i v e l y constant, that of the thiophene was a l t e r e d by changing the thiophene c o n c e n t r a t i o n in the bubbler. Hydrogen s u l p h i d e , sulphur dioxide and carbonyl sulphide were added to the carbon d i o x i d e c a r r i e r gas p r i o r to s a t u r a t i o n with acetone. The p a r t i a l pressures of the r e s p e c t i v e c o n s t i t u e n t s , other than sulphur d i o x i d e , both before and a f t e r the r e a c t i o n v e s s e l , were measured by gas chromatography. The r e s p e c t i v e sulphur dioxide concentrations were evaluated by gas a n a l y s i s , i n v o l v i n g a b s o r p t i o n by hydrogen peroxide and subsequent sulphur determination by a Technicon autoanalyser. There was no marked d e p l e t i o n in any ?



2

fi^S treatment

2

1st exposure to CO^ + acetone (0.31 atm. pressure)

0.801

0.0%

600

700

lh, 600°C

lh, 600°C

24h, 20°C

7

7.5

6

0.03

0.18

0.10

N.D. = not determined

700

600

0.113

0.010

700

N.M.

600

600

600

6-160

4-270

20-560

0.13

0.13

115

213

Nil

2.08

28

10-375

Nil

50.0

115

Weight Deposition Temp. Time gair^ Reduction h mg/cm Factor °C (b) (a)

2

2nd exposure to C0 + acetone (0.15 atm. pressure)

0.018

0.120

N.D.

2

Sulphur remaining after exposure to 00 + agetone mg/cm

(b) Accumulative values for successive exposures.

(a) The ratio of the extents of deposition on oxidised and sulphided 20/25/Nb stainless steel for similar conditions. The former data are frcm reference (3).

N.M. = none measurable

0.095

Deposition Carbon Weight Weigjht Reduction Temp. Time Exposure uptake, gain gain Factor h °C Conditions mg/cm mg/cm mg/cm (a)

700

Temp, °C

2

Oxidation by 00 , 1000 h

Table III - The Effect of Sulphiding Upon Carbon Deposition from Acetone on Oxidised 20/25/Nb Stainless Steel


H δ

Ο m ^ g

11.

BENNETT ET AL.

Inhibition

by Sulfur

Poisoning

227

experiment of e i t h e r acetone or a sulphur-bearing gas c o n s t i t u e n t across the r e a c t i o n v e s s e l . As a consequence only the i n l e t values are quoted in Tables I I I - V I I I . The d e p o s i t i o n k i n e t i c s were determined, e i t h e r d i s c o n t i n u ously by weighing the specimens a f t e r i n t e r m i t t e n t exposure, or continuously by measuring the extension of a s i l i c a s p r i n g balance.


Results Sulphiding. Oxidised 20/25/Nb s t a i n l e s s s t e e l and FeÔ^ were sulphided by exposure to hydrogen sulphide, e i t h e r f o r 24 h at room temperature or f o r 1 h at 600°-625°C. The ( ^ 0 3 , however, was only sulphided at the higher temperature. The r e s p e c t i v e sulphur uptakes are given in Tables I I I , IV and V. X-ray d i f f r a c t i o n examination revealed that the more severe s u l p h i d i n g at the higher temperature r e s u l t e d in the p a r t i a l formation of Fej_flS and Cr2S3 from F e 3 0 4 and 0 ^ 0 3 r e s p e c t i v e l y . The extents of d e p o s i t i o n on the sulphided 20/25/Nb s t a i n l e s s s t e e l during 6-7.5 h exposure to carbon dioxide c o n t a i n i n g acetone (0.31 atm pressure) at 600° and 700°C are given in Table I I I . For s i m i l a r exposure c o n d i t i o n s these were lower by f a c t o r s between 4 and 560 than those on o x i d i s e d 20/25/Nb s t a i n l e s s s t e e l ( 3 ) . However, during longer exposures of the sulphided s t e e l to carbon d i o x i d e c o n t a i n i n g 0.15 atm acetone, p r o t e c t i o n ceased on two out of the three specimens as comparable d e p o s i t i o n occurred to that expected f o r the o x i d i s e d s t e e l . Nevertheless, i n h i b i t i o n was s t i l l maintained on the t h i r d specimen. Loss of p r o t e c t i o n was not due to the removal of sulphur, as the amounts analysed a f t e r exposure to acetone were s i m i l a r to the weight gains of sulphiding. The sulphide l a y e r on Fe304 d r a m a t i c a l l y reduced the extent of d e p o s i t i o n at 600 and 700°C (Table I V ) . For comparable exposures at the lower temperature the f a c t o r by which d e p o s i t i o n was reduced increased with the sulphur content of the Fe304« This c o r r e l a t i o n could not be a s c r i b e d to any change in surface area, as that of the most completely sulphided FeÔ^ (0.97 ^/g) was s i m i l a r to that of the o r i g i n a l m a t e r i a l (0.61 m^/g). As with the 20/25/Nb s t a i n l e s s s t e e l , the sulphur contents of the sulphided Fe304 before and a f t e r exposure to the d e p o s i t i n g gas were s i m i l a r to w i t h i n a f a c t o r of two. The exposure of C r 0 3 to hydrogen sulphide r e s u l t e d in i t s almost complete conversion to Cr2S3 with a corresponding r e d u c t i o n in surface area from 43.8 to 8 m^/g. The k i n e t i c s of d e p o s i t i o n on the sulphided m a t e r i a l at 600°C were s i m i l a r to those on Cr203 ( F i g . 1) and w h i l s t the extent of d e p o s i t i o n was at l e a s t comparable, i t could have been s l i g h t l y increased (Table V ) . m

2

Gas Phase Poisons. The e f f e c t i v e n e s s was examined of four sulphur compounds, thiophene, hydrogen sulphide, sulphur d i o x i d e and carbonyl sulphide, as gas phase poisons.


COKE FORMATION

228

TABLE IV - The Effect of Sulphiding by H?S on Carbon Deposition from Acetone on F e ^

H S treatment


2

Exposure to carbon dioxide containing acetone

Carbon Sulphur Acetone Temp Time Temp Time pressure uptake content h h °C °C atm. % (a) % (a)

6

700 20

24

185 0.003

600 (b)

115

0.15

11.5

0.31

8

0.31

1

9.2

700

600

1

10.3

600

1

1.3

0.06

600

600

0.31

Sulphur Deposition content reduction after factor exposure (c) % (a)

38.4

378

5.1

62

40

N.D.

90

250 6.7

600

221 (b)

0.15

266

0.15

468

3.5

90

3

9.4 χ 10

35.5

N.D. = Not determined

(a)

Expressed as a percentage of the original F e ^ sample weight. Value for complete conversion to FeS = 41.5.

(b) Successive exposures. (c)

The ratio of the extents of deposition on untreated control and sulphided Fe3Û4 for comparable exposure conditions. The former data are from reference (3).


Albright and Baker; Coke Formation on Metal Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 1.1

13.4

0.49

0.44

(

C

)

0.92

0.39

0.39

0.92

0.68 0.43

0.92

0.39

2

2

Cr 03

C a l c u l a t e d from C r 0 3 surface area, except ( c ) c a l c u l a t e d from sulphided surface area.

2

69

72

0.23

)

(b)

3

0.31

0.11

73

c

Expressed as a percentage of the C r 0 3 weight; value f o r complete conversion to C r S = 63.2%.

3

4

0.11

(

Without sulphur treatment

1.02

0.99

With sulphur treatment

Carbon uptake a f t e r 75h Ug/cm (b)

(a)

2

4 χ 10~

so

2

2

6 χ 10"

HS

2

8 χ 10"

6

0.95

50

0.20

0.10

0.013

50

0.38

Decreased from 56.1 to 40.4 during exposure

8 χ 10"

266

Sulphur uptake (a)

0.15

Acetone

Exposure duration h

-

4

Sulphur additive

P a r t i a l pressure (atm)

HS

Thiophene

Thiophene

?

Sulphided by by H S ( l h , 600°C)

Sulphur treatment/ gas phase additive

TABLE V - Influence of S u l p h i d i n g and the Continuous A d d i t i o n of a Sulphur Compound to the Gas Phase upon the I n t e r a c t i o n between CrpO/i and Acetone at 600°C


230

COKE FORMATION


TABLE VI - Influence of the Continuous Gas Phase Addition of a Sulphur Compound on the Decomposition of Acetone by 20/25/Nb Stainless Steel at 6Q0°C

Deposition exposure conditions Weight gain during 1000h Acetone oxidation Sulphur Partial Duration partial in (XL at bearing pressure pressure h mere additive atm. atm.

Sulphur Carbon Deposition uptakg uptake^ reduction ug/em mg/cm f a c t o r ^

14.0

1-6

0.19

17

4

2

0.15

48

6

6

0.09

4

N.D.

2xl0~

5

0.11

72

15

0.13

6-1100

¥

3xl0"

5

0.14

50

than would have been expected f o r the acetoneFeÔ/^ r e a c t i o n under the same c o n d i t i o n s . C o n t r o l experiments were c a r r i e d out during each s e r i e s of experiments with the v a r i o u s i n h i b i t o r s and e s t a b l i s h e d that the e f f e c t i v e n e s s of the lower a d d i t i v e p a r t i a l pressures was not a t t r i b u t a b l e to any r i g memory e f f e c t , e.g. d e s o r p t i o n of sulphur compounds or degradation products from the r e a c t i o n v e s s e l s u r f a c e s . Neither hydrogen sulphide (3.7-570 uatm pressure) nor sulphur d i o x i d e (4000 uatm pressure) a d d i t i o n s a f f e c t e d the k i n e t i c s or i n h i b i t e d the extent of decomposition of acetone by C r 0 3 , at 600°C ( F i g . 1 and Table V ) . The sulphur uptakes were 1.1% of the o r i g i n a l C r 0 3 weight, except in the experiment with the highest hydrogen sulphide p a r t i a l pressure, when i t was 13.4%. Since the C r 0 3 surface area was decreased by s u l p h i d i n g t h i s was used to c a l c u l a t e the carbon uptake on t h i s specimen. The value so obtained, normalised to 75 h, 0.44 ug/cwr was in c l o s e agreement with that (0.39 ug/cm ) f o r C r 0 (Table V ) . 2

2

2

2

2

2

3

Discussion Before c o n s i d e r i n g the p o s s i b l e r o l e s of sulphur, some of the e s s e n t i a l f e a t u r e s of acetone degradation on the two oxides and the 20/25/Nb s t a i n l e s s s t e e l (3) need to r e i t e r a t e d . Fe304 was


COKE FORMATION

236

extremely a c t i v e in promoting t h i s r e a c t i o n . G r a p h i t i c filaments were formed through which i r o n became f i n e l y d i s t r i b u t e d . X-ray d i f f r a c t i o n i n d i c a t e d that t h i s was present as Fe3C and FeO, although n e i t h e r n e c e s s a r i l y was the a c t i v e c a t a l y t i c s t a t e . Both the t h r e s h o l d temperature (,,-550°C) and the i n d u c t i o n period proceeding r e a c t i o n were a s s o c i a t e d with c a t a l y s t formation. The extent of r e a c t i o n on chromia, in c o n t r a s t , was extremely small and the amount of carbon formed was i n s u f f i c i e n t f o r d e t a i l e d characterisation. Chromia was not a f f e c t e d chemically by the r e a c t i o n with acetone, whose degradation was b e l i e v e d to have been c a t a l y s e d by the oxide, with the primary adsorption s i t e p o s s i b l y being the chromic i o n . Oxidation of the 20/25/Nb s t a i n l e s s s t e e l in carbon dioxide at 600-700°C r e s u l t e d in the formation of a scale c o n s i s t i n g of C r 0 3 where the g r a i n boundaries i n t e r a c t e d with the surface and an (FeCrNi) s p i n e l over the g r a i n s . Following an i n d u c t i o n p e r i o d , again a t t r i b u t e d to c a t a l y s t formation and probably i n v o l v i n g the r e d u c t i o n of the s p i n e l , acetone was decomposed f o l l o w i n g pseudo-parabolic k i n e t i c s . The deposit produced was again f i l a m e n t a r y in s t r u c t u r e , with the main m e t a l l i c c o n s t i t u e n t being i r o n , present as Fe3C. The r o l e of the oxide scale formed on the 20/25/Nb s t e e l mirrored, t h e r e f o r e , the r e s p e c t i v e behaviour of Fe304 and C r 0 3 . Two p o s s i b l e complementary mechanisms were r e s p o n s i b l e f o r the r e d u c t i o n , or in some circumstances complete i n h i b i t i o n , by sulphur of carbon d e p o s i t i o n from acetone on Fe30^. The f i r s t was operative on m a t e r i a l sulphided before exposure to acetone, and p o s s i b l y a l s o when high p a r t i a l pressures of hydrogen sulphide and sulphur dioxide were added continuously to the gas phase, as t h i s a l s o r e s u l t e d in appreciable F e j ^ S formation. In these instances reduced d e p o s i t i o n could be a t t r i b u t e d , at l e a s t p a r t l y , to a slower rate of formation of the r e a c t i o n intermediary, e.g. i r o n c a r b i d e s , on Fej_jjS as compared with Fe304» Thiophene carbonyl sulphide, together with hydrogen sulphide and sulphur dioxide at lower p a r t i a l pressures, i n h i b i t e d d e p o s i t i o n by a second mechanism. These compounds were adsorbed p r e f e r e n t i a l l y on the a c t i v e surface s i t e s to the e x c l u s i o n of acetone and thereby, prevented the i n i t i a l c a t a l y s t formation. That t h i s was p r i m a r i l y a surface phenomenon i s demonstrated by the sulphur uptakes which were extremely s m a l l . A value of 100 ppm represented ~ 2 0 % coverage of the i r o n s i t e s on the surface of F e 3 0 ^ and t h e r e f o r e , the measured uptakes corresponded to surface coverages of t h i s order extending up to the formation of a few monolayers of sulphide. Thiophene was s l i g h t l y more e f f e c t i v e in preventing d e p o s i t i o n than the simpler sulphur compounds, p o s s i b l y due to i t s l a r g e r molecular area b l o c k i n g adjacent s i t e s . The c o n t i n u i n g i n h i b i t i o n of acetone decomposition was considerably greater than would have been obtained by the a s s o c i a t e d s u l p h i d a t i o n alone. The extents of adsorption of the poison would be pressure dependent, so that i t was not s u r p r i s n g that at low p a r t i a l pressures


#

2

2


11.

BENNETT

ET AL.

Inhibition

by

Sulfur

Poisoning

237

(e.g. 0.19 uatm thiophene) the surface coverage was i n s u f f i c i e n t to block c r i t i c a l adsorption s i t e s . With C r 0 3 , n e i t h e r s u l p h i d i n g nor sulphur-containing gas phase a d d i t i v e s , which a l s o sulphided the surface, i n h i b i t e d acetone d e p o s i t i o n . This r e f l e c t e d the d i f f e r e n t mechanism involved in carbon d e p o s i t i o n on t h i s oxide such that whether the surface anion was e i t h e r oxygen or sulphur, d i d not a f f e c t e i t h e r acetone adsorption or the subsequent decomposition of the adsorbed molecules. The r e a c t i o n between acetone and p r e o x i d i s e d 20/25/Nb s t a i n l e s s s t e e l was c a t a l y s e d p r i m a r i l y by i r o n . I t would have been a n t i c i p a t e d , t h e r e f o r e , as observed that i t would have been i n h i b i t e d a l s o by sulphur poisoning. However, the i n f l u e n c e of sulphur was not so marked as f o r FeÔ^, The smaller amount of d e p o s i t i o n on the s t e e l c a t a l y s e d by the chromium-containing oxides would not have been a f f e c t e d s i g n i f i c a n t l y by sulphur poisoning, as demonstrated f o r C r 0 3 « These r e a c t i o n s would account probably f o r the small carbon uptakes always observed, even when that p r i n c i p a l l y responsible f o r d e p o s i t i o n was i n h i b i t e d . Sulphiding was e f f e c t i v e i n i t i a l l y as again t h i s i n h i b i t e d the formation of the c a t a l y t i c i n t e r m e d i a r i e s . The decreased p r o t e c t i o n on extended exposure to the carbon dioxide containing environment was not caused by sulphur removal. This r e s u l t e d probably from outward i r o n d i f f u s i o n from the underlying s t e e l to form a surface oxide l a y e r and the r e s u l t i n g generation of c a t a l y s t s f o r acetone decomposition. High p a r t i a l pressures of sulphur a d d i t i v e s to the gas phase, as on Fe304, again r e s u l t e d in sulphide formation. However, lower p a r t i a l pressures, which prevented d e p o s i t i o n on Fe304 were not e f f e c t i v e in poisoning the o x i d i s e d s t e e l surface p o s s i b l y because absorptive bonding was l e s s strong on the mixed (FeCrNi) s p i n e l than on Fe304.


2

2

Conclusions (1)

The heterogeneous decomposition of acetone by FeÔ^, at 600 and 700°C r e s u l t i n g in carbon d e p o s i t i o n , was reduced s i g n i f i c a n t l y , and in some instances prevented, by sulphur poisoning. Two procedures examined were p r i o r s u l p h i d i n g and the continuous a d d i t i o n to the gas phase of e i t h e r thiophene, H S, S0 or COS. These i n h i b i t e d the formation of the c a t a l y t i c intermediates responsible f o r d e p o s i t i o n e i t h e r by p r e f e r e n t i a l adsorption poisoning a c t i v e surface s i t e s or by oxide conversion to F e i - ^ S . Thiophene was the most e f f e c t i v e a d d i t i v e as i t s l a r g e r molecular area than the simple sulphur compounds enhanced surface a c t i v e s i t e blockage. A c t i v e decomposition on C r 0 3 at 600°C was not diminished e i t h e r by p r i o r s u l p h i d i n g or sulphur bearing gaseous a d d i t i v e s , which r e f l e c t e d the d i f f e r e n t r e a c t i o n mechanism than with Fe304. 2

(2)

2

2


238


(3)

COKE FORMATION

The e f f e c t i v e n e s s of these procedures to i n h i b i t acetone decomposition by an o x i d i s e d 20% Cr/25% Ni/niobium s t a b i l i s e d s t a i n l e s s s t e e l , at the same temperatures, r e f l e c t e d the behaviour of the two oxides which were r e p r e s e n t a t i v e p r i n c i p a l c o n s t i t u e n t s of the scale on the s t e e l . Prior s u l p h i d i n g reduced d e p o s i t i o n f o r a l i m i t e d period u n t i l new c a t a l y t i c intermediates were generated by the simultaneous o x i d a t i o n of the s t e e l by the carbon dioxide c a r r i e r gas. D e p o s i t i o n was decreased throughout the exposure by sulphur bearing gaseous a d d i t i v e s , whose e f f e c t i v e n e s s increased w i t h p a r t i a l pressure.

Acknowledgement s We g r a t e f u l l y acknowledge the a n a l y t i c a l work c a r r i e d out by A. Parker, G.S. S p i c e r , A. Mead and D.J. Darke, and f o r d e t a i l e d d i s c u s s i o n s with Dr. J.E. A n t i l l .

Literature Cited 1. 2.

3. 4.

5. 6. 7. 8.

Campion, P.J., "Gas chemistry in nuclear reactors and large industrial plant"; Dyer A. Ed; Heyden, London, 1980, 53. Bennett, M.J.; Faircloth, R.L.; Firth, R.J.; Houlton, M.R.; Norwood, K.S.; Prior, H.A. "Gas chemistry in nuclear reactors and large industrial plant"; Dyer A. Ed; Heyden, London, 1980, 81. Bennett, M.J.; Chaffey, G.H.; Langford, A.J.; Silvester, D.R.V. UKAEA Report AERE-R7407; HMSO, London, 1973. Baker,R.T.K.;Harris, P.S. "Chemistry and Physics of Carbon", Walker J r . , P.L.: Thrower, P.A. Eds; M. Dekker, New York, 1978, 14, 83. Bennett, M.J.: Price, J.B. J. Mater. Sci. 1981, 16, 170. B r i t i s h Patents 629,211 (1949); 698,121 (1953). Karcher, W.; Glaude, P. Carbon, 1970, 9, 617. Maxted, E.B. Advances i n Catalysis, 1951, 3, 129.

RECEIVED June 28,

1982.


Inhibition by Sulfur Poisoning of the Heterogeneous Decomposition

Recommend Documents