Effect of temperature and reduction on the activity of high temperature water gas shift catalysts

Applied Catalysis, 20 (1986) 3 13

3

Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands

EFFECT OF TEMPERATURE AND REDUCTION ON THE ACTIVITY OF HIGH TEMPERATURE WATER GAS SHIFT CATALYSTS J.C. GONZALEZI, M.G. GONZALEZ3, M.A. LABORDE2 and N. MORENOI Iyacimientos P e t r o l f f e r o s Fiscales - Gerencia de I n v e s t i g a c i 6 n y D e s a r r o l l o 1888 Florencio Varela - Argentina. 2pINMATE, Departamento de I n d u s t r i a s , F.C.E.N., Ciudad U n i v e r s i t a r i a 1428 Buenos Aires - Argentina. 3CINDECA, 47 No. 257 - 1900 - La Plata - Argentina. (Received 6 December 1984, accepted 4 September 1985) ABSTRACT The i n f l u e n c e of thermal treatments and reduction process on the a c t i v i t y of Fe/Cr c a t a l y s t s was studied. The temperature-programmed reduction method was used in the study of the c a t a l y s t a c t i v a t i o n by various reducing mixtures. Process and hydrogen gas seem to be the most adequate reducing agents. Temperature and water vapour a f f e c t the t e x t u r a l p r o p e r t i e s of the Fe/Cr c a t a l y s t s with a decrease in the s p e c i f i c surface area and the c a t a l y t i c a c t i v i t y . INTRODUCTION I t is known t h a t the a c t i v i t y

and the useful l i f e

of c a t a l y s t s depend mainly

on the a c t i v a t i o n process and the thermal h i s t o r y t h a t they s u f f e r during the operation. In the high temperatures water gas s h i f t process, c a t a l y s t s of Fe304 Cr203 are used. The a c t i v e agent is Fe304 which is obtained from a p a r t i a l reduction of Fe203. M e t a l l i c Fe formation must be avoided in the reduction step because i t can catalyse the h i g h l y exothermic methanation r e a c t i o n and t h i s can damage the catalyst.

One of the ways to avoid t h i s r i s k is adding steam to the reduction mixture.

In a previous paper [ I ]

i t was observed t h a t the c a t a l y t i c a c t i v i t y

decreased

when water vapour was added to the reduction mixture and also when the r e a c t i o n temperature was higher than 460°C. I t was shown t h a t both f a c t o r s a f f e c t the s i n t e r i n g , so a knowledge of these e f f e c t is useful in order to decide the procedure f o r the p r e p a r a t i o n , a c t i v a t i o n and regeneration of these c a t a l y s t s . The i n f l u e n c e of water vapour on the reduction of Fe203 to Fe304, though very important in the i n d u s t r i a l

process, was scarcely studied. The e f f e c t o f d i f f e r e n t

reduction mixtures, water vapour and temperature on the a c t i v i t y

and the t e x t u r a l

c h a r a c t e r i s t i c s of the Fe/Cr c a t a l y s t s are analyzed in t h i s paper.

0166-9834/86/$03.50

© 1986 Elsevier Science Publishers B.V.

? I

J

>

<

f J

®

<

j

®

I FIGURE I I

-

Experimental equipment.

Pressure gauge, 2 - On-off v a l v e s , 3 - Flow c o n t r o l l e r ,

4 - S i l i c a g e l bed,

5 - Bubble f l o w m e t e r , 6 - Reactor, 7 - Programmed temperature oven, 8 - Thermal conductivity detector. EXPERIMENTAL PART The r e d u c t i o n s t u d i e s were c a r r i e d out by means o f the temperature programmed r e d u c t i o n technique (TPR) in a f l o w equipment shown in Figure I . The heating r a t e was 6°C min - I and the r e a c t o r was fed w i t h a 10% reducing agent in a n i t r o g e n stream. Used gases were 99.998% p u r i t y . by the X-ray d i f f r a c t i o n

O r i g i n a l and reduced samples were analyzed

technique. The thermal treatments were c a r r i e d out in

a furnace w i t h c o n t r o l l e d atmosphere and both the s p e c i f i c surface area and the pore s i z e d i s t r i b u t i o n

were measured by the n i t r o g e n a d s o r p t i o n techniques

(BET) and mercury p o r o s i m e t r y , r e s p e c t i v e l y . The c a t a l y t i c

activity

o f the d i f f e r e n t

samples was determined in a f l o w r e a c t o r

described elsewhere [ I ] , a t temperature o f 633 K and a gaseous f l o w o f 1200 cm3 -I min . The s t u d i e s were c a r r i e d out on commercial c a t a l y s t s , which are c h a r a c t e r i z e d in Table I . l

RESULTS AND DISCUSSION E f f e c t o f w a t e r steam, temperature and r e d u c t i o n agent on the t e x t u r a l c h a r a c t e r istics

o f the c a t a l y s t s

In Figure 2 is shown the CO conversion (X A) versus time f o r d i f f e r e n t

tempera-

t u r e s , f o r the r e a c t i o n

CO + H20 = CO2 + H2

(I)

TABLE I Properties of catalysts Catalyst

A

B

C

D

Fe203 content/wt%

74.16

83.99

89.01

79.40

Cr203 content/wt% Surface area/m 2 g-1

5.92 91

8.42 72

7.63 78

6.94 99

Pore volume/cm 3 g-1

0.22

0.27

0.29

0.23

Apparent density/g cm-3

0.21

1.60

1.52

1.96

4O

I

I

20

360 °C

0

FIGURE 2

210

I

40

6~0

810

I

100

1~0

160 t i m e (hs)

Effect of temperature on c a t a l y t i c a c t i v i t y .

Reaction conditions: flow

rates; H20:198 cc min-1; H2:235 cc min - I " N2:732 cc min - I - ~ CO: 50 cc min -I

°

Catalyst A weight: I g. I t can be seen that when the c a t a l y s t is submitted to high temperature by some time the conversion increases but, when the reaction temperature is turned to the i n i t i a l

value (360°C), the conversion is lower than the o r i g i n a l value.

Hoogschagen and Zwietering [2] a t t r i b u t e the loss of a c t i v i t y

in these catalysts

to a decrease of the s p e c i f i c area. For t h i s reason, in t h i s study the c a t a l y s t was submitted to d i f f e r e n t thermal and hydrothermal treatments in order to analyze t h e i r influence on the t e x t u r a l c h a r a c t e r i s t i c s . One series of samples was treated for 4 hours at temperatures between 360 and 500°C in a nitrogen stream and the

6

TABLE 2 Textural changes in Catalyst D Gas

Pore volume

/°C

composition

Vp/cm3 g-1

Surface area Sg /m2 g-1

360

N2

0.31

74.5

400

N2

0.20

61.4

450

N2

0.17

44.5

500

N2

0.21

33.6

360

N2/H20

0.26

60.5

400

N2/H20

0.30

45.0

450

N2/H20

0.37

32.1

500

N2/H20

0.34

34.5

Temperature

IOO vp °/o

8o

6o

4G

2~

1o FIGURE 3

~o

c0(~)

Pore size d i s t r i b u t i o n of Catalyst D. ( B )

treated to 400°C in N2. ( X )

Original sample. ( - - )

Sample treated to 500°C in N2. ( 0 )

Sample

Sample treated

to 400°C in N2/H20. other was heated in a stream of nitrogen saturated with water vapour at 20°C. The values of the s p e c i f i c surface area and the pore volume of the samples are shown in Table 2. I t may be observed that in a l l cases the thermal treatment modifies the values of these parameters. This e f f e c t is s t i l l

evident in samples

submitted to the normal operation temperature in industry (360%), showing that the structure of these catalysts is very sensitive to thermal e f f e c t s . The most important decrease of the s p e c i f i c surface area was observed in the catalysts (A and D) which consist of goethite (FeOOH) in i t s o r i g i n a l composition. The s i n t e r i n g is favoured by the presence of steam, as i t may be seen in Table 2.

TABLE 3 Effect of reduction mixture on t e x t u r a l p r o p e r t i e s of Catalyst D Reduction mixture

Surface area /m 2 g-1

Pore volume /cm 3 g-1

CO - N2

51.7

0.33

H2 - N2 - H20

51.9

0.31

CO - N2 - H20 Reaction mixture

52.6

0.34

59.0

0.34

Used c a t a l y s t

37.2

0.28

a c t i v a t e d with H2 - N2 - H20 In general, f o r both thermal and hydrothermal treatment a decrease of the s p e c i f i c surface area and an increase of the pore volume w i t h respect to fresh c a t a l y s t s is observed. Besides, because of the temperature e f f e c t , the d i s t r i b u t i o n of the pore sizes is s h i f t e d to higher values. The steam enhances t h i s tendency e l i m i n a t i n g o

those pores smaller than 100 A (Figure 3). The e f f e c t of the reduction agent on the s p e c i f i c surface area and the pore volume f o r sample D is shown in Table 3. I t may be appreciated that the reduction produces a decrease o f the s p e c i f i c surface area and also an increase of the pore volume, although these v a r i a t i o n s are independent of the reducing agent used. I t is possible to observe t h a t during the process of CO conversion these e f f e c t s become more important, although they do not reach the extreme values obtained with the hydrothermal treatment at 500°C (see Table 2). These r e s u l t s i n d i c a t e the presence of s i n t e r i n g processes in the c a t a l y s t , which would be responsible f o r the decrease in the c a t a l y t i c a c t i v i t y . Effect of steam, temperature and reduction agent on the c a t a l y s t reduction The study was c a r r i e d out on fresh samples at programmed temperature with the f o l l o w i n g mixtures: nitrogen/hydrogen; CO/N2 and N2/H2/water vapour. The mixtures containing CO and steam could not be analysed by t h i s technique since the CO conversion r e a c t i o n occurs simultaneously with the c a t a l y s t reduction and, as a consequence, i t is impossible to maintain a base l i n e due to the f a c t t h a t the composition o f the r e a c t o r e f f l u e n t changes during the run. The possible reduction reactions are: 3 Fe203 + H2

=

2 Fe304 + H20

3 Fe203 + CO = 2 Fe304 + CO2

(2) (3)

203

3~0 FIGURE 4

l

5~0

l

Temperature-programmed r e d u c t i o n c u r v e s .

H2 - N2 m i x t u r e :

2 CrO3 + 3 H2

700l

I

I[°C]

Sample w e i g h t :

50 mg. Flow r a t e

100 cm3 min - 1 .

= Cr203 + 3 H20

(4)

2 CrO3 + 3 CO = Cr203 + 3 CO2

(5)

Fe304 + 4 H2

(6)

= 3 Fe +

4 H20

Fe304 + 4 CO = 3 Fe + 4 CO2

(7)

Reactions (6) and (7) ought to be avoided since m e t a l l i c Fe i s , not only i n a c t i v e f o r CO conversion reaction but also i t may catalyse the methanation and the carbon formation reactions [3]. The methanation is a highly exothermic reactior and the c a t a l y s t can be damaged due to the increase of temperature and, on the other hand, the carbon formation contributes to i t s d e a c t iv a t io n . The reduction p r o f i l e s of a Fe/Cr c a t a l y s t , and the pure oxides, Fe203 and CrO3, in a mixture N2/H2 (10:1) are shown in Figure 4. Fe203 shows a c h a r a c t e r i s t i ( curve with two peaks, the f i r s t

at 420°C and the second at a temperature of about

700°C. Simultaneously, the thermal effects which are produced in the reduction were detected by the DTA technique; at 420°C an exothermic peak which would correspond to reaction (2) and an exothermic peak about 700°C which was assigned to the formation of m e t a l l i c Fe are observed. The TPR diagram of CrO3 shows two peaks which correspond to a highly exothermic e f f e c t by DTA according to reaction (4) with a reaction heat of = -163.6 kcal mol -! The X-ray analysis of the reduced oxide shows the presence of Cr203 and CrOOH, but n e i t h e r Cr nor CrO3 was found. This would indicate that both peaks correspond to the reduction of Cr (Vl) to Cr ( I I I ) .

I

I

200

FIGURE 5

300

I

I

400

500 T[°c]

DTA curve f o r c a t a l y s t D.

O/N2

200

FIGURE 6

I

I

300

400

T[°C]

TPR curves f o r c a t a l y s t D with various reduction mixtures.

The TPR diagram of the Fe/Cr c a t a l y s t shows three peaks, a l l at temperatures lower than 400°C and a fourth at a temperature about 700°C. I t would correspond to the superposition of the reduction p r o f i l e s of the pure components. In the c a t a l y s t spectra the peaks are s h i f t e d to lower temperatures with respect to the pure oxides. This indicates that chromium, as dispersing agent, makes the hematite reduction easier. Partial reductions were carried out in order to analyse each of the species already formed and the f i n a l samples were analysed by the X-ray d i f f r a c t i o n technique. The presence of Cr203, Fe203 and Fe304 was observed in the X-ray

10

z

/./

z/

l/

Il iii I

200

3 0

/~",

\

\

\

"1~ II

40O

T [°C]

500

FIGURE 7 TPR curves f i r c a t a l y s t D with H2/N2 mixture ( - - ) - u n t r e a t e d sample; ( - . - ) treated in N2 stream to 460°C; ( - - - ) treated in steam to 460°C. diffractogram of the samples reduced up to 350°C, which includes the f i r s t

two

peaks. No peaks corresponding to m e t a l l i c Fe or to CrO3 were observed, so a p a r t i a l reduction of the hematite was indicated. The sample reduced up to 450°C, which displays the t h i r d peak, shows a more defined X-ray diffractogram where the peaks corresponding to Fe203 disappeared and, as a consequence, magnetite is the only species of Fe in the catalyst. I f the reduction continues up to 900°C, over reduction of the catalyst is seen which is v e r i f i e d by the peak of m e t a l l i c Fe in the X-ray analysis. The hydrogen stoichiometric amount for the reduction of Fe203 to Fe304 is 2.2 mole g-1 Fe203. The hydrogen consumed in the TPR experiments, corresponding to the second and t h i r d peak was, for the d i f f e r e n t experiments, from 1.8 to 2.2 mol g-1 Fe203. These results would indicate that the f i r s t

peak is due to the

reduction of CrO3 to Cr203 and the two others to the reduction of hematite to magnetite, while the high temperature peak is produced by the reduction to m e t a l l i c iron. In Figure 5 is shown the DTA spectra for catalyst D. Endothermic peaks at 275 and 300°C are observed in absence of reducing gas mixture. A low AH associated with these peaks would indicate that they are produced by structural changes. I t would suggest that the same species (Fe203) were present in the c a t a l y s t with d i f f e r e n t degrees of r e d u c i b i l i t y . This fact could explain the two peaks in the TPR diagram corresponding to the reduction of hematite to magnetite. The DTA diagram in the presence of H2 produces only exothermic peaks at temperatures lower than 550°C; the absence of endothermic peaks in the range of temperature indicates

11 TABLE 4 Effect of reduction mixture on c a t a l y s t performance Reaction rate

Reduction mixture/cm 3 min -I Hydrogen

Nitrogen

230

700

230

700

C monoxide

r = k Pco/Cm3 min -I g -I 10.8

190

7OO

230a

Water vapour

5O

8.4 10.0

700

50

190

6.5

700

220

190

7.6

700

50

190

10.8

aprocess gas composition. that i t is impossible to produce m e t a l l i c iron at these reduction conditions. These results agree with those obtained by means of hydrogen consumed in the TPR experiments. Our results d i f f e r from those obtained by Pernicone and Traina [4] who assign the t h i r d TPR peak to m e t a l l i c iron. Besides, Brown et al. [5],,by means of calculations of hydrogen consumed, a t t r i b u t e the low temperature peaks to the reduction of hematite to magnetite. The influence of reducing mixtures on the c a t a l y s t is shown in Figure 6. I t is evident that the reduction mixture saturated with water vapour (N2/H2/H20) s h i f t s the reduction peaks towards higher temperatures without the formation of m e t a l l i c Fe. However, according to our experiences, even i f the reduction is carried out in hydrogen-nitrogen stream, m e t a l l i c iron s o l e l y appears at temperatures higher than 700°C. Moreover, when the reduction mixture is CO/N2 the second and t h i r d peak appear superimposed and at lower temperature than when the reducing gas is hydrogen. As i t was previously mentioned, the s t o i c h i o m e t r i c consumption of the reducing gas by reactions (2) and (3) is 2.2 mol g-1 Fe203. On the other hand, according to the f o l l o w i n g reactions: Fe203 + 3 H2

= 2 Fe = H20

(8)

Fe203 + 3 CO = 2 Fe + CO2

(9)

the reducing gas necessary f o r complete reduction would be nine t i m e s g r e a t e r ; that is 19.8 mol g-1 Fe203. The CO consumed, calculated from TPR spectra, was

12

13 mol g-1 Fe203, notably higher than the H2 consumed during the experiments with the H2/N2 mixture. I t indicates that CO, which is a stronger reducing agent than H2, enhances the formation of m e t a l l i c iron at temperatures lower than 400°C. The X-ray analysis of the samples reduced with CO/N2 reveals the presence of metallic

iron according to the semiquantitative results of TPR experiments.

The comparison of reduction p r o f i l e s , obtained with H2/N2 mixture, between a fresh c a t a l y s t sample and two others submitted previously to thermal treatment can be seen in Figure 7. I t can be observed that the reduction peaks in the samples treated thermally in a nitrogen atmosphere, are s h i f t e d towards higher temperatures and become less defined than in the fresh c a t a l y s t . This tendency is more important when the sample is heated in presence of steam, as only one peak is observed. The X-ray analysis of these samples, a f t e r reduction, shows the presence of m e t a l l i c iron only in the sample previously submitted to thermal treatment in an i n e r t atmosphere. According to Pernicone and Traina [4] for h i g h l y disordered samples as f o r a c a t a l y s t of 100 m2 g-1 of surface area, Fe203 is reduced f i r s t

to Fe304 and then

to Fe. On the other hand, in ordered samples, not only because they are n a t u r a l l y ordered but because they are treated at higher temperature, both stages occur simultaneously (Fe203 ÷ Fe). This would be v e r i f i e d by our r e s u l t s . Catalytic activity In order to r e l a t e the reduction stage of the c a t a l y s t with i t s performance, a c t i v i t y measurements on activated samples with d i f f e r e n t reducing mixture were performed. The r e s u l t s expressed as reaction rate, assuming a f i r s t

order k i n e t i c s , are

shown in Table 4. The maximum reaction rate is obtained when the a c t i v a t i o n is carried out with hydrogen or process gas. When steam is added to the reduction mixture, the c a t a l y t i c a c t i v i t y decreases noticeably. On the other hand, the s p e c i f i c surface area also decreases in presence of steam (Table 2); t h i s e f f e c t could be one of the reasons f o r the a c t i v i t y decrease in samples in presence of steam, and i t is enhanced when CO is used instead of H2. Dowden [6] suggests that water concentrations of approximately 100 ppm may produce deactivation by s i n t e r i n g and by a l t e r a t i o n of the promoters d i s t r i b u t i o n due to s o l i d state reactions catalysed by water. I t could explain our r e s u l t s . CONCLUSIONS I t was shown that the reduction of Fe/Cr catalysts with hydrogen does not lead necessarily to Fe formation, unless they are submitted to a previous thermal treatment or an a c t i v a t i o n at temperatures higher than 500°C is carried out. The a d d i t i o n of vapour in standard conditions of hydrogen reduction is not justified

due to the fact that Feformation is not possible. Besides, i t produces

a less active c a t a l y s t since the decrease of the s p e c i f i c surface area is higher in the presence of water.

13 Carbon monoxide is a more powerful reducing agent than hydrogen, so i t may lead to Fe formation at temperatures lower than 500°C. In t h i s case i t is convenient to add steam to the reducing m i x t u r e . Process and hydrogen gas are the best reducing agents in order to obtain higher a c t i v i t i e s without having an over-reduction at temperatures lower than 500°C. Temperature and steam a f f e c t the t e x t u r a l properties of the c a t a l y s t , decreasing the s p e c i f i c surface area and s h i f t i n g the pore size d i s t r i b u t i o n upwards. During the reduction process, both effects are observed but they are not so marked and are not affected by the composition of reducing mixture. ACKNOWLEDGEMENTS The authors wish to acknowledge Mr. F. Morris f o r the experimental determinations. They thank the f i n a n c i a l assistance f o r the Consejo Nacional de Investigaciones C i e n t i f i c a s y Tecnicas (CONICET) of Argentina. REFERENCES I 2 3 4 5 6

O. F e r r e t t i , J.C. Gonz~lez, M.A. Laborde and N. Moreno, to be published. J. Hoogschagen and P. Zwietering, J. Chem. Phys., 21 (1953) 2224. D. Newsome, Cat. Rev., 21 (1980) 275. N. Pernicone and F. Traina, Preparation of Catalysts I I , ed. B. Delmon, P. Grange, P. Jacobs and G. Poncelet, Elsevier S c i e n t i f i c Publ. Co., 3 (1979) 321. R. Brown, M.E. Cooper and D.A. Whan, Appl. Catal., 3 (1982) 177. D.A. Dowden, Progress in Catalyst Deactivation, ed. J.L. Figuereido, Nato Adv. Study Inst. Series E. Applied Science 54.