Study of CO2 gasification of activated carbon catalysed by molybdenum oxide and potassium carbonate

Study of CO, gasification catalysed by molybdenum potassium carbonate* Isabel

of activated oxide and

carbon

F. Silva and Luis S. Lobo

Department0 de Caparica,

de Quimica, Portugal

Universidade

Nova

de Lisboa

Quinta

da Torre, 2825

Monte

A kinetic study is made of the reaction of activated carbon gasification by CO, catalysed by MOO, and K&O,, using a microbalance to record the loss of weight as a function of time. Orders and activation energies were measured. MOO, is found to be a good catalyst at low temperatures and moderate pressures. The effect of loading on reactivity shows saturation above z 0.3 wt %. The catalytic effect of the mixture Mo0,/K,C03 two catalysts.

is found to be slightly above the effect expected by the addition of the separate A brief reference is made to the possible mechanism for these reactions.

(Keywords: gasification

of carbon; Boudouard catalyst; reaction mechanism)

Carbon gasification and carbon formation are opposite reactions. A good catalyst for one reaction should also be a catalyst for the opposite reaction, providing it is still stable and active at the different thermodynamic conditions that might be required to favour either reaction. The present paper describes a study of MOO, and K,C03, singly and admixed, as catalysts for gasification of an active carbon by CO,.

EXPERIMENTAL The kinetic experiments were conducted in a microbalance, with continuous recording of changes in weight. Ground charcoal with a measured specific surface of 605m2g-’ and a porous volume of 0.48 cm3 g- ’ was used. The elemental analysis gave: C-78, 8 wt %; H-1, 1 wt %; N-O, 4 wt %. MOO, was obtained by calcination of ammonium molybdate at 600°C for 6 h. The catalyst was mixed with the activated carbon in a mortar, and the mixtures so prepared used in each series of experiments. Most experiments were conducted with the following percentages of catalyst (by weight): 2.5 K,CO,; 2.5 MOO,; 2.5 K,CO,-t-0.17 MOO,. Other formulations were used solely in experiments designed to reveal the effect of the amount of catalyst on reactivity (‘loading’). The gases used were purified CO2 and N,. By diluting withN,, CO, partial pressures from 0.01 to 0.1 MPa were obtained. After testing for absence of external mass transfer limitations, a gas flow of 2.5 cm3 s- ’ was used throughout the work. The experiments were carried out in the range 700 to 950°C. The rate of gasification refers to the 20 wt % burn-off point, as change of weight per unit time and per mass of carbon present (s- ‘). * This paper was presented at the International Symposium ‘Fundamentals of Catalvtic Coal and Carbon Gasilication’, Rolduc. The Netherlands, 5-7 May i986 0016-2361/86/101400JI4$3.00 0 1986 Butterworth & Co. (Publishers) Ltd.

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FUEL,

effects of the

1986, Vol 65, October

RESULTS Induction depending

periods lasting up to 1 h could be observed on temperature and other conditions. Figures 14 show the continuous recording of weight versus time elapsed from the moment where the onset of reaction was observed. The induction time is not represented. Experiments were carried out in the range of temperatures 700 to 850°C at 0.1 MPa pressure. The linearity of the curves in Figure 1 corresponds to zero order kinetics with relation to the amount of carbon. Figure 2 shows similar curves for samples loaded with 2.5 wt “, K,CO, and 0.17 wt y0 MOO,. The curves are linear for temperatures up to 764°C. Above that, S shaped curves are observed. Similar curves were obtained for the uncatalysed reaction and for 2.5 wt Y0 K2C03 doped samples. Similar behaviour has been reported by previous researchers. Figures3 and 4 show a comparison of the previously mentioned data for the four types of samples used at 802 and 764°C. With decreasing temperature, the activity of MOO, approaches that of K,CO,. The catalytic activity of the mixture of the two catalysts appears to be additive. Figure 5 shows the Arrhenius plots at 20 wt % burn-off for the four samples. It can be seen that the activation energy for the MOO, catalysed reaction is much lower than that with K,CO, and MOO, is the better catalyst of the two at 0.1 MPa pressure for temperatures below 730°C. Above 850°C the activity of MOO, (m.p. 795°C) vanishes. The reactivities for the mixture of the two catalysts is further increased, as shown in the figure. Table 1 summarizes the kinetic parameters for the four cases studied. The activation energies for the uncatalysed and the K,CO, catalysed reactions are similar, as reported by previous researchers. The activation energy with 2.5 wt % MOO, is much lower (130 kJ mol-I). The reaction catalysed by 2.5 wt % K,CO, and 0.17 wt Y0 MOO, shows an intermediate value (205 kJmol_‘).

Study of CO2 gasification of activated carbon catalysis: I. F. Silva and L. S. Lobo Time (h) 3 I

2 I

-

4 I

Figures 6 and 7 show the effect of loading on reactivity for the two catalysts used, at 802°C and 0.1 MPa pressure. With K,CO, a s-shaped non-linear dependance was observed. With MOO,, fast reaction rates are obtained with loadings as low as 0.2 wt %. Above 0.5 wt “/d, the effect of increasing loading is almost nil. A clear saturation effect is observed.

5 I

--.___704~C zY--7~a~C

DISCUSSION

CO2 Activated

Generul comment,s

carbon

Moo3 (2.5 wt %)

36

Figure I Reaction temperatures

7.2

catalysed

IO 8 Time x 10e3 (S)

by

Moo,

14 4

Molybdenum oxide is a good catalyst for the gasification of carbon by CO,. The activation energy of 130 kJ mol-’ is probably the lowest reported for CO, gasification. At 0.1 MPa, the MOO, catalysed reaction is faster than that catalysed by K,CO, below 750°C. At higher pressures the MOO, reaction is favoured, due to its 1st order kinetics. It is interesting that the mixture of K,CO, and MOO, gives an even faster reaction, which an intermediate activation energy and shows a I st order with respect to CO, pressure, while the K,CO, catalysed reaction shows zero order. The latter is insensitive to pressure increases under the conditions used in this work. The results on the uncatalysed and K,C03 catalysed reaction are in accordance with the findings of previous researchers ’ -9.

18

(2.5 wt”;)

at

various

Time(h)

08

03 “L 0. I

c \\ \

Stability studies Many catalysts for carbon gasification seem to be active at temperatures near their melting points2~‘0-12.

817OC

85l’C

I

832’C

h\

3.6

. CO2 Activated carbon K2C03(25wt%) I

I

72

IO.8 Time x lO-3(s)

Figure 2 Reaction catalysed by a mixture K,CO, (2.5 wt “,) at various temperatures

I

I

14.4

I8

of MOO,

I

Mo03(017wt%)

Time (h) 3

2

4

5

I

(0.17 wt “,“) and

Time(h)

i

Activated

kY 0.43

carbon

T = 764OC CO2

0.2 -

Various

catalysts

I 3.6

Figure 4

Reaction

I

72

I IO 8 Time x 10m3(s)

at 764°C with various

I

I

I4 4

I8

catalysts

Temperature (“Cl 900

I I mp

_7

850

I

800

1I

750

I

700

’

10-3

K2FF 3.6

Figure 3

Reaction

72

10.8 Time x lO-3(~)

at 802,-C with various

14.4

18

catalysts

Orders to carbon are 0 with MOO, (Figure 1) and 1 for uncatalysed reaction. For and the K&O, K,CO, +MoO,, the order is zero up to ca. = 780°C (Figure2). Above that temperature the curves are nonlinear. Orders to CO, were evaluated in all cases in the range 0.01 to 0.1 MPa partial pressure. Order zero was observed with K,CO,, and orders 1 or close to 1 in the other cases.

8.5

Figure 5

8.7

Arrhenius

8.9 9. I Rectprocal temperature x 104( K-‘I

plots for various

catalysts

FUEL,

Vol 65, October

1986,

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Study of CO2 gasification of activated carbon catalysis: I. F. Silva and L. S. Lobo Table 1

Summary

of kinetic data Order

Catalyst

Uncatalysed

K&O, Catalytic

‘S shaped

curves above

In (Ko set-‘)

CO,

at

19

1

0.8

800°C

254+5 130+5

18.9 3.96

0” 0

0.0 1.0

164°C 784°C

205klO

13.98

0”

1.0

764°C

Loading

0

272i8 (126, above

2.5”/, 2.5% 2.5 :/, 0.17%

MOO, K,COx +MoO,

to

Order to carbon

Activation energy (kJ mol-‘)

SSO’C)

780°C

T = 802°C

T = 802’C

:t

5

IO

15

Weight % Moo3

Figure 7

Table 2 s-1) Weight % K$03 Figure 6

Effect of loading

for K,C03

The results of the stability studies are presented in Table 2, together with gasification rates under similar conditions. The m.p. of MOO, is 795°C and if column 3 is compared with column 5 in Table 2, it can be seen that MOO, loses weight very rapidly at 800°C (zero order sublimation recorded), but is quite stable in the presence of carbon. N, was used as an inert gas. The volatility of K,CO, at its m.p. (891’C) was also found to be low in the presence of carbon (Table 2, column 7) as reported earlier”. The difference in MOO, volatility was found to be lower under CO, than under N, (columns 3 and 4). A similar finding was reported earlier for the alkaline metals*. This may be attributed to changes in surface tension and to the corresponding changes in vapour pressure. Active phase

MOO, is a non-stoichiometric oxide, being a mixture of various oxides, two of which are shown in Table 3. Above 700°C these oxides dissociate to MOO, and Mo0213. MOO, itself sublimes apprciably above 650°C. The bulk vapour pressure of melted MOO, at 800°C was calculated to be 800Pa. Calculations for the standard free energy changes for the reduction of the main oxides (Moo3 and MOO,) by carbon and for the oxidation by CO, at 800°C are shown in Table 3. The AG” values show that reduction of both bulk Moo3 and bulk MOO, by bulk carbon (graphite) are thermodynamically possible. However, direct reactions between two solids are usually very sluggish. The oxidation of MO by CO, to give MOO, is

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FUEL, 1986, Vol 65, October

Effect of loading

for MOO,

Results of the stability

Solid Exp. no. Catalyst wt% Gas Temp. 704°C 802°C 891°C

Carbon

(c)

experiments

MOO,

(loss of weight rate x lo’,

MoO,+C

K,CO,+C

1 0

2 0

3 loo

4 100

5 12.8

6

7 2.5

8

2.5

N,

CO,

N,

CO2

N,

CO,

N,

CO,

0 0

0.04 0.95

1.37 12.3 _

1.1

0.41 73.1

1.48 76

0 37 _

0 0.63 _

2.5

also possible, but oxidation to MOO, is forbidden. MOO, is the only solid phase that is near or above its m.p. in the range of temperatures used. However, preliminary X-ray diffraction studies suggest that MOO, is the main phase present on samples of partially burnt-off carbon. This is consistent with the calculations. Either MOO, or MO can be the active phase. Preparation

of mixture for reaction

It is considered that the mobility of MOO, near its melting point and its vapour pressure at the reaction temperature provides the explanation for the quick spreading of the catalyst over the activated carbon surface. The phenomenon can be pictured as a sequence of events in which vapour penetrates easily into the carbon structure, and droplets are formed throughout the pore surface. Droplets have been seen under reaction conditions with K,CO,*,‘* and SnO, and PbO”. Moving metal particles were also detected by electron microscopy in many carbon formation and gasification systems by Baker 2o. In the present case, it is possible that inital droplets of MOO, tend to be converted to MOO, if the contact with carbon is good enough, (and further reduced to MO).

Study Table 3

Calculations

for standard

of CO, gasification

of activated

carbon catalysis:

I. F. Silva and L. S. Lobo

free energy changes

r Free

energy

change

AGO (kcal

mole-l)

1300°C

mp Remarks

Reduction by C

Oxidation

Phase

Moo3

by CO2

OC

795

3

SublImes

above

65OT,

Non-stolchiometic, (MO,

023)

\<

\<

(700)

Various

l68

- 144 (Moq O,,)

mtermedlate

decompose (700)

above

dlsproportlonation

1927

Above

930°C

of oxides

oxldes 700°C

wth

to

Moo3

Moo2

-92

Dissoclatlon mixture

1000”

partly

partly

subllmes

dlsproportionotes

2610

Mrchanisrn

The linearity of the loss of weight uersus time curves observed with MOO, (Figure 1) and with K,CO, at low temperatures (Figure 2) suggests that a constant ‘reaction front’ is operating throughout the reaction process. That reaction front should be the sum of the contact areas of all particles or droplets acting as catalyst. It is pertinent to pose the question of where the oxidizing gas meets the carbon surface. Carbon and/or CO, should be in a particularly active form, in line with the catalytic effect observed. Considering a catalyst particle on a carbon surface, several different situations can be envisaged but it is considered that the likely mechanism involves a normal gas/surface catalytic process at the exposed catalyst surface. Three phases would take part in the reaction. A gas phase, consisting mainly of CO, with some product CO. A solid phase which is the second reactant, carbon. Finally, a third phase (liquid or solid) is operating in between as a catalyst. If carbon atoms are even slightly soluble in the catalyst and diffuse through it easily, this mechanism explains the kinetics and behaviour observed. A gas-surface Langmuir-Hinshelwood (L-H) type of kinetics is operative. This is consistent with the observations of several researchers in other carbon gasification systems, both catalytic9,14~‘5 and noncatalytic1*3,5*16- 18. Inhibition by CO is explained by a LH type of surface reaction ‘,’ . There is no evidence that carbon can diffuse easily in this case. That proof must come from non-kinetic experiments, as was the case with nickel’ I. Loading effects, including saturation, can be explained by the way in which the active carbon pore surface is progressively covered with the catalyst. In the case of K,C03, the change from zero order to carbon at low temperatures, to S-shaped curves at higher temperatures (Figure2) could be attributed to a change from a nonwetting (with penetration) to a wetting (full pore recession) situation. Wetting is governed by the relative values of the three surface tensicns involved, and these change with temperature1’*20. The mechanism proposed here has points in common

with those previously proposed’,’ 1,12*22,25. It was also inspired by the reversible mechanism proposed for on transition carbon formation and gasification metals19,23.24

ACKNOWLEDGEMENTS This work was sponsored by JNICT, Junta National de Investigacao Cientitica e Tecnologica, whose support is gratefully acknowledged. REFERENCES 1 2 3

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Austin, G. and Walker, P. L., Jr. AICHE J. 1963, 9(3), 303 McKee, D. W. and Chatterji, D. Carbon 1975, 13, 381 Dutta, S., Wen, C. Y. and Belt, R. J. Ind. Eng. Chem. Process Des. Det.. 1977, 16(l), 20 McKee, D. W. AIP CONF PROC 1981, 70, 236 Wigmans, Ph.D. Thesis 1982, University of Amsterdam Adjorlolo, A. A. and Rao, Y. K. Carbon 1984, 22(2), 173 McKee, D. W. Carbon 1982, 20(l), 59 Mims, A., Chluzinski, J. J., Pabst, J. K. and Baker, R. T. J. Catulvsis 1984, 88(l), 97 Kapteijn, F. and Moulijn, J. A., ‘Carbon and Coal Gasification’, 1985, Algarve, Portugal Huhn, F., Klein, J. and Jiintgen, H., Funcat 1982, Amsterdam Yamada, T.. Homma, T., Tomita, A. and Tamai, Y. Carbon 1984, 22(2), 135 Rao, Y. R., Adjorlolo, A. A. and Haberman, J. H. Carbotl 1982, 20(3), 207 The Oxide Handbook, Samsonov, G. V., 1973 Miihlen, H. J., Schumacker, W. and van Heek, K. H., ‘Carbon and Coal Gasification’, 1985, Algarve, Portugal Holstein, W. L. and Boudart, M. J. Catalysis 1982, 75, 337 Meguro, T. and Torikai, N., Yokohama National University 1983, Japan Ergun, S. J. Phys. Chem. 1956,60,480 Walker, P. L., Jr., Rusinko, F., Jr. and Austin, L. G. Adr. Caral. 1959, 11, 133 Figueiredo, J. L., Progress in Catalyst Deactivation 1982, Algarve, Portugal Baker, R. T. K., ‘Carbon and Coal Gasification’, 1985, Algarve, Portugal Bernardo, C. A. and Lobo, L. S. Carbon 1976, 14, 287 Fox, D. A. and White, H. H. Ind. Eng. Chem. 1931, 23, 259 Lobo, L. S., Ph.D. Thesis, London University 1971 Bernardo, C. A. and Lobo, L. S. Catalyst Deuctiration 1980, Elsevier Yang, R. T. and Wong, C. J. Catalysis 1984, 85, 154

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