Catalytic gasification of carbon with steam, carbon dioxide and hydrogen

Carbon, 1977, Vol. IS, pp. 103-106. Pergamon Press.

Printed in Great Britain

CATALYTIC GASIFICATION OF CARBON WITH STEAM, CARBON DIOXIDE AND HYDROGEN YASIJKATSU TAM& HARUO WATANABEand AKIRA TOMITA Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Sendai, Japan

(Received 21 October 1976) A~~~-Nine t~nsition metals in Group VIII were examined as catalysts for the carbon gasification with steam, carbon dioxide and hydrogen. Metal-doped carbons were heated up to 950°Cat a constant rate of 2OO”C/hrin a flowingreactant gas. The relative activities of metal catalysts are nearly the same for all gases regardless of their quite different chemical nature. Metals like Ru, Rh, Ir and Pt are invariably active, whereas Fe, Co and Pd have small activities. The reaction pattern differs from catalyst to catalyst. Only four metals (e.g. Ni, Ru, Rh and OS) exhibit the maximum reactivity in the low-temperature region, and this behavior is common to three gases. It is soectdated from these observations that the metal-carbon interaction is more important than the metal-gas interaction in determining the reaction profile. 1. fNTRODIJCTION

A comparative study on activities of a series of catalysts for the gasification of carbon was initiated by Dayll], who examined the activities of a number of metal oxides for the oxidation of carbons prepared from sucrose, benzene, acetylene and others. Since then many workers investigated the relative activities of their particular catalysts for the gasification of their particular carbons[2-6]. They utilized oxidizing gases as gasifying agents, mainly O2 or COZ and less frequently HzO. Extensive studies on the catalytic reaction between carbon and H2 were not initiated until the early 1970’s when the increasing demand for the production of gaseous fuels from coal stimulated the study of hydrogasification of carbonaceous materials[7-101. Our previous work]71 has shown that a large portion of an active carbon can be gasified to methane at relatively low temperature (~7~OC) in the presence of some metal catalysts. Each catalyst shows its characteristic activity pattern on heating from ambient to 1050°C. The aim of this study is to extend the gasifying agent to Hz0 and co*. The relative activity has no absolute meaning but depends on a number of experimenta condi~ons, such as physical and chemical properties of both carbon and catalyst, intimacy of contact between them, and others. When these variables are controlled as well as possible, this kind of information may have a significant value. This paper attempts to compare the activities of nine metal catalysts in Group VIII for the gasification of an active carbon in HzO, CO* and H,. which are of quite different nature from each other. It might be interesting to know whether the nature of gas has a significant effect on the activity pattern or not, because it would tell us about the crucial step in the catalytic gasification reaction. 2. ANTS A steam-activated granular carbon, Shirasagi C (4 x 6 US mesh), from Takeda Chemical Industry was employed as a carbonaceous material. It was found to be composed of C, 93.5%; H, 0.4%; 0 (by difference) 1.4%;

ash, 4.7%. The following properties were listed in the catalogue: Total surface area, 1200m’lg; pore volume, 0.9 ml/g; particle density, 0.7 g/ml. The carbon was mixed with an aqueous solution of a metal chloride, the mixture being dried in a rotary evaporator. The impregnated carbon was then reduced in a stream of HZ.The reduction temperature was 300°C for Ru, Rh, OS, Pt; 350°C for Ni, Pd, Ir; 400°C for Co and 500°C for Fe. Samples contained 4.8% metal by weight upon loosing chloride anions and water of crystallization. In a typical run, one granule (around 1OOmg)was put into a quartz basket attached to a Cahn RG Electrobalance. After the hydrogen for the reduction of metal salt was evacuated away, the reactant gas was introduced and flowed at a rate of 60 ml/min. In the case of H20 gasification, helium was used as a carrier. Helium at atmospheric pressure was passed through a steam saturator which was maintained at 15.9”~. The partial pressure of steam in a resultant gas was checked and found to be approx. 13mmHg. No 0, was detected as impurity by gas chromatograph. In other cases, CO, and Hz were obtained from commercial cylinders and used at atmospheric pressure. The rate of gasification was dete~ined by recording continuously weight change and also by analyzing the effluent gas chromato~aphically at 10 min intervals. 3.

RESULTSAND DISCUSSION

3.1 Relative activities of catalysts Figures l-3 show temperature-pro~ammed gasification profiles in H20, CO, and H,. The reaction temperature shown on the abscissa means a scanning temperature at a constant heating rate of 200YJhr. The rate of weight loss shown as ordinate is calculated from the effluent gas analysis. In the case of H20 reaction, for example, the rate of production of CO2 plus CO corresponds to the rate of carbon weight loss. This rate is normalized on an initial carbon weight basis. The integral forms of these curves are essentially the same as TGA curves. A small discrepancy between these data tells us about some processes which do not involve

103

104

YASUKATSU TAMAI etal.

i y 0.3 E" - 0.2 0 Ti

K

400

500 600 700 Tomprraturr,

800

0. I

400

900

500

*C

600

700

800

Fig. 1. The rate of gasificationin H20.

Fig. 3. The rate of gasificationin H,.

Temperature. F

Fig. 2. The rate of gasificationin CO,.

carbon as a reactant; the oxidation of Fe upon admission of CO2 can only be detected by a TGA curve. Although these profiles give only qualitative information, they are enough to discuss the characteristics of catalytic activity of metals or the difference of gasification behavior in various atmospheres. One of the most striking features is that catalysts behave very similarly in three different gasifying agents. Nine metals

can be classified into the following three groups: (A) Ni, Ru, Rh and OS; (B) Ir and Pt; (C) Fe, Co and Pd. The first group shows a reactivity peak at relatively low temperature and the reactivity is fairly high. The group (B) has a high activity at higher temperatures but exhibits no low temperature peak. The metals in the last class show the smallest activities. Palladium exhibits almost no activity in Hz0 and in CO*. It should be emphasized that the above classification is valid for all three gasification systems. Table 1 summarizes the fraction of carbon gasified during heating up to 750°C (or 650°C in the case of H,) and to 950°C. The former temperature was chosen in order to distinguish the low-temperature reactivity from the overall reactivity. It can be seen that the order of catalytic activity does not depend much on the nature of gasifying agent. In general, Rh, Ru, Ir and Pt are active for all gases, and they are followed by a group of OS and Ni. This order is in a good agreement with our previous result [7]. The gasification rate, however, is considerably lower in the present study. The conversion of carbon up to 950°C at the same heating rate is almost 80% for the Rh-catalyzed hydrogastication in the previous work, whereas it is only 18% here. This discrepancy is due mainly to the difference in carbon source. The reaction pattern also is influenced by many factors, such as the

Table 1. The fraction of carbon gasified at different temperature regions in H,O, CO2 and H, Weight Loss of Carbon, % Catalyst

900

Temprraturr.~~

R20 -75ooc -95ooc

-75ooc

CO2 -95ooc

HP -65O'C

-95ooc

NOllS

1

6

0

8

0

2

Fe

1

12

1

25

0

3

co

2

13

1

36

0

5

Ni

7

17

6

39

2

7

RU

10

23

25

68

8

25

Rb

14

26

27

66

8

18

Pd

3

9

0

11

0

8

OS

6

13

11

61

4

13

1r

8

22

16

86

3

27

Pt

6

19

4

46

2

46

Catalytic gasification of carbon with steam, carbon dioxideand hydrogen

105

catalyst. At SOO“C,the highest ratio, 1.3, was obtained for the Fe-catalyzed reaction, while Ir catalyst gave the lowest value of 0.08. The equilibrium COKO ratio was calculated for the reaction C + H,O&O + H, and CO t H,OZXO, : Hz, taking carbon as &graphite, at a total pressure of 1 atm. The partial pressure of inert gas, He, was taken at 0.983atm, which is the same as of the present study. The calculated ratio was roughly 0.94, 0.12, 0.014, 0.0041 and 0.0025 at 500, 600, 700, 800 and 9WC, respectively. The ratio decreases with increasing temperature. Experimental values are always larger than the calculated ratios by one or two orders of magnitude. They generally show the same temperature dependence, but in cases of group (A) metals some maxima and 3.2 Effect of gasifying agent At 800°C and a pressure of 0.1 atm, the relative rates minima are observed in the CO,/CO ratio vs temperature of the uncatalyzed gasification of carbon with HZO, COZ plots. The higher the reaction rate, the lower the ratio. and H, have been estimated as 3, 1 and 3 x lo-‘, When the catalyst is so active that the conversion of respectively [ 1I]. The sequence indicated in Table 1 is steam is more than 50%, the ratio approaches the COZ> HzO> Hz. In view of the fact that the partial theoretical one. For example, the CO&O ratio for the pressure of HZ0 (13 mmHg) is considerably lower than Ni-catalyzed reaction was found to be 1.0 at 55O”C,while of the others, this sequence is not inconsistent with the the theoretical value is around 0.36. In other words, the above estimate. At high temperature the rate in Hz0 does products ratio depends on the degree of completion of not increase as rapidly as in COZ,perhaps because of an the reaction, which, in turn, depends on the activity of insufficient feed of H,O. In the case of P&catalyzed catalyst. reaction, the conversion of H,O is around 65% at 95O”C, whereas the conversion of CO2 to CO is only 4%. The restriction in equipment gas makes some part of feed 3.4 Mechanism of catalysis Proposed mechanisms to account for the catalytic pass through the clearance between the basket and the tube wall without involving gasification. Thus, in the gasification of carbon are broadly divided into two above case, Hz0 might be almost consumed in the categories according to the partner of principal interaction vicinity of the sample. The decrease of the rate in Hz at with catalyst. If the partner is a gas molecule, a catalyst may act as a dissociation center of gas molecule. The higher temperature is attributed to the equilibrium well-known oxygen transfer theory is an example restriction. The first stage gasification in the low temperature belonging to this category[5]. The catalysts form with gas region was characteristic for the catalyzed reaction by intermediates, such as metal oxides. The intermediates group (A) metals. The maximum reaction rate was serve to oxidize the carbon and, in doing so, return to their original form. The spill-over theory is another example. observed at the temperature listed in Table 2. Generally, the lowest peak temperature was observed for the HZ Hydrogen molecules dissociate on the catalysts surface, gasification, and the highest one for the gasification in and then migrate to the active site on carbon surface COZ. This may have some correlation with the disso- where they react to evolve CH [4,9]. Under some ciation energy of gas molecules as shown in Table 2. circumstances, dissociated species may react at the However, since the peak temperature is a function of interface between catalyst and carbon without diffusing to carbon surface. The spill-over theory has also been many parameters, more data should be accumulated before drawing a conclusion on this point. suggested for the CO, reaction1131 and Ot reaction[l4]. Another category involves the interaction of catalyst 3.3 Selectivity in the H,O gasification firstly not with gas, but with carbon. Long and Sykes’s Main reaction products in the Hz0 gasification are CO, electronic theory[l5] is based on the electron transfer CO, and Hz. From a practical point of view, the products from the carbon matrix to metal catalysts. As a result of ratio, COJCO, is important, because it directly affects this transfer, the carbon-carbon bonds are weakened and the thermal efficiency in a coal conversion process. The are easily broken to release product gases. On the other ratio varies with temperature and depends on the type of hand, if the enhanced reactivity of carbon is ascribed to

sort of anion in metal salt, the reduction temperature, and the source of carbon. Such dependence makes the comparison difficult between different systems, but it may not be out of place to outline briefly the relative activities reported by other investigators. Rewick et ~1.191show a quite similar activity sequence to ours. Others have presented somewhat different results. In most cases, the activity of Ni is less than those of Fe and Co in OarHz0 or CO,[2-6, lo]. More surprisingly, Ru and Rh have relatively small activities among Group VIII metals[4,6], although this was found only in the O2 gas~cation.

Table 2. Peak temperaturesin the first stage gasification Gasifying Agent

Dissociation

Peak Temperature,

"C

Energy, kcal,'mol 1121

Ni

RU

Rh

OS

510

630

HZ

103

535

600

Hz0

116

540

680

690

700

550

710

670

BOO

CO2

127

106

YASUKATSU

the presence of the dissolved carbon in metals[K, 171, such a mechanism also falls into this category. There have been many attempts to decide between these possibilities, but so far no unambiguous conclusion has been reached. This is no wonder at all: a single mechanism must not necessarily account for all the reactions. Mechanisms can be different depending on the reactant gas and the kind of catalyst. Even for the same reaction system, it may change as the expe~men~ condition, such as temperature, varies with time. With this point in mind, it is still of interest to discuss the mechanism from the present data. The catalytic reaction pattern seems to be independent of the type of reactant gas, in view of the close resemblance among the patterns in Figs. l-3 and the similar activity sequence of catalysts in Table 1. Of course, there are several observations which differ from gas to gas: thp peak temperature (Table 2) depends on the nature of gas, and the activity sequence (Table 1) is not exactly the same for three reactant gases. However, these differences seem to be minor. It may be safe to conclude that the catalyst-gas interaction is less important than the catalyst-carbon interaction. If the former interaction is a principal prerequisite for the catalytic action, the mode of interaction would significantly affect the reaction profile. The behavior of chemisorption of CO,, HZ0 and Hz on metals are quite different from each other. The chemiso~tion of CA&on Rh, Pt and Pd has been reported to be almost negligible compared to that on Fe or Ni, whereas Hz is known to be readily chemisorbed by every metals in Group VIII[18]. Thus, in the second category some mechanism is responsible for the crucial step in the gasification reaction. It is now desirable to determine what type of ~te~ction between carbon and catalyst affects the observed reactivity profile. The carbide formation or carbon dissolution in metal may be an attractive explanation, because such carbons are known to have a high reactivity [19). The ability of a metal in carbide formation or in carbon ~ssolution, however, cannot be correlated with the observed catalytic activity. Only Fe, Co and Ni are known to form carbides at the temperature below 1ooo”C. Among them, only Ni exhibits the low-temperature activity. Rakszawski et al. [ 131 clearly showed that the iron carbide acts as inhibitor in the CO2 gas~cation. Thus, this hypothesis is not suitable to

TAMAl

@t al.

account for the mechanism in general. The electronic theory is more reasonable, because the catalytic activity increases with increasing the d character of the metal within a given triad in Group VIII. This discussion does not necessarily rule out the possibility of spill-over phenomena and others in the lirst category. For example, an explanation coupled with the spill-over theory may also be possible. Boudart ef al.[20] stated that conration of the surface by carbon plays a decisive role in providing bridges for dissociated atoms to be transported away from metal to the carbon. Such bridge formation also is an example of carbon-metal interactions. Many other possibilities may exist, but at the present moment one is incapable of interpreting all data by a unified mecha~sm, if any. Much work still remains to be done before the detailed mechanism is established. REFERENCES

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