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Carbon gasification in the presence of metal catalysts
Carbon gasification of metal catalysts* Robert T. Rewick, Paul R. Wentrcek Solid-State Catalysis Laboratory, 94025, USA (Received 11 March 1974) (Revised 10 June 1974)
Stanford
in the presence
and Henry Wise Research Institute,
Menlo Park, California
The development of new processes for the production of gaseous fuels from carboncontaining solids is essential in meeting the energy needs of the nation. In this paper, catalysed carbon gasification is examined. The change in the reactivity of the interface between gaseous reactant (hydrogen or steam) and solid carbon has been measured in the presence of various metal catalysts. With platinum it is found that over a range of temperatures the specific rate of methane production is of the same magnitude as the rate of hydrogen atomization. The catalytic effect is interpretable in terms of an enhanced rate of hydrogen dissociation on the metal surface, followed by surface diffusion across the metal/carbon interface and reaction with carbon. The gas formation rate during the interaction of water vapour with catalyst-activated carbons has been increased by more than an order of magnitude by depositing small weight fractions of active metal catalyst on the carbon surface. At the temperaturesemployed in this study (975-l 175 K), carbon monoxide and hydrogen are the products of the catalysed reaction for each of the catalysts examined.
The increasing demand for natural gas and the predicted decline in natural gas reserves emphasizes the need for the development of new processes for coal gasification and other means of producing pipeline gas. Although a number of promising new processes are currently in various stages of development, the economic production of pipeline gas from coal remains a subject of further study. Specifically it is of considerable importance to explore the feasibility of augmenting the rate of coal gasification by the use of catalytic processes both for the hydrogasification and the steam-carbon reactions. During the last few years the catalytic effects of a number of metals and their compounds have been examined for coal However the mechanism of the catalytic gasification’. action has not been elucidated in sufficient detail to allow selection of an optimum catalytic system for coal gasification by reaction with hydrogen or water. In our work we have examined in considerable detail the relation between the kinetics of carbon gasification by reaction with hydrogen or steam as a function of (1) the type of metal catalyst, (2) the degree of catalyst dispersion, (3) the gaseous reactant concentration, and (4) the temperature of the reaction system. Selection of different catalysts was based on the hypothesis that in the production of gaseous fuels by heterogeneous reaction of carbon surfaces the slow step is to be found in the dissociative chemisorption of gaseous reactant molecules, namely hydrogen or water. A process that could lower this energy barrier for the sorption step, such as a catalysed reaction, Would result in the rapid formation of atomic species (H) and radicals (OH) which could readily surfacediffuse and react with the carbon sllhstrate. * This research was carried out under the sponsorship of the American Gas Association, whose support is gratefully acknowledged
274
FUEL, 1974, Vol 53, October
EXPERIMENTAL
DETAILS
Materials Three kinds of carbon were employed in this study: Graphon (a graphitized carbon black prepared by Cabot Corporation), Sterling FT (a fine thermal carbon black provided by Cabot Corporation), and Norit-A (a vegetable carbon obtained from Pfanstiehl Chemical Co.).
Gzialyst preparation Catalysts containing O-8 wt % platinum on Graphon and Sterling FT, and 5 wt % platinum on Sterling FT were The procedure involves burning off a fraction prepared’. of the carbon sample in air at 875 K until 50% of its original weight has been consumed. The carbon is mixed with a solution containing benzene, ethanol, and the metal salt (chloroplatinic acid or the respective metal chlorides). The slurry is evaporated to dryness with a stream of nitrogen and vigorous agitation. Subsequently the impregnated sample is dried under vacuum at 375 K and then reduced at 775 Kin a stream of hydrogen. Catalysts containing 0.8 wt % nickel and cobalt on Sterling FT and 5 wt % nickel and cobalt on Norit were prepared in a similar manner starting with NiC12.6H20 and CoCl2.6H2O respectively. Samples containing 5 wt % platinum, palladium, ruthenium and rhodium on Norit were purchased from Research Inorganic/ Organic Chemical Co.
Gasification measurements To measure quantitatively the rate of carbon gasification, a Cahn RG Electrobalance in a glass vacuum assembly was suitably modified to record continuously mass change of the
R. T. Rewick, P. R. Wentrcek and H. Wise: Carbon gasification in presence of metal catalysts
catalyst at various temperatures in a flow of hydrogen or water vapour at one atmosphere total pressure. Aliquots of the gas stream containing the reaction products were analysed by gas chromatography on a 12-ft X l/&in molecular sieve 5A column. In a typical run, a 20-mg sample of the catalyst was weighed into a quartz basket attached to the microbalance and evacuated to remove oxygen. The sample was then degassed at 1075 K for one hour in a stream of helium; after cooling to room temperature the helium atmosphere was replaced with hydrogen/helium mixtures or helium saturated with water vapour. Heat was then applied to the sample under flow conditions (26 cm3/min HzO/He, The temperature was then increased 60 cm3/min Hz). stepwise over a range of several hundred degrees to obtain the rate of mass loss and the concentration of products in the effluent gas stream at various temperature levels.
Surface-area measurements The surface areas of carbon and of the dispersed metal have been measured for samples of Sterling FT/5 wt % platinum and Sterling FT/O8 wt % platinum. The BET adsorption method using krypton as the physisorbate was employed for determination of the total surface area. The platinum surface areas were analysed by a new procedure involving a surface titration of metal-chemisorbed oxygen with carbon monoxide3.
84
10.0
9.0
Figure l
1
Net rate of mass change for different
Noritl5 wt 96 Pt. n Sterling FT/0.8 wt % Pt.
EXPERIMENTAL
RESULTS with
of hydrogen Interaction carbons Since this study of carbon gasification
metal-activated
involves several parameters, we examined first the effect of different carbon substrates on the gasification rate catalysed by finely dispersed metal at different weight loadings. These results are summarized in Figure 1 for the case of platinum. At constant hydrogen pressure the reaction was found to be first order with respect to carbon both in the presence and the absence of catalyst. The reaction kinetics follows the rate law -dm/d t = km, where m represents the mass of carbon and k the rate constant, which in turn is an exponential function of temperature as given by the Arrhenius expression [k = A exp (-L?‘/RT)] with A defined as the preexponential factor. Thus a semilog plot of the relative rate of mass loss -dm p/dt (
m
>
versus the reciprocal temperature (1 /T) yields a straight line whose slope is proportional to the apparent activation energy E. Although marked differences in reactivity of the different carbon samples containing dispersed platinum are apparent, possibly owing to different physical surface properties of the carbons, the slopes of the straight portion of the curves shown are identical (within the experimental error). They correspond to an overall activation energy of 5.5 + 3 kcal/mol *. Also, methane is the only detectable gaseous carbon-containing product. Another characteristic feature exhibited by these results is the gradual transition from an exponential increase in * 1 kcal/mol
= 4,187
kJ/mol
11.0
11.4
lO‘/T(K-‘I carbons/Pt
(1 atm Hz)
OSterling FT/5 wt % Pt, 0 Graphon/OB wt % Pt
reaction rate with temperature to a region where the rate passes through a maximum (Figures 1 and 2). A similar phenomenon has been observed in the interaction of pyrolytic graphite with oxygen4-* and hydrogen atoms’. Also, the results for Norit/platinum indicate an irreversible change in the reactivity of the carbon sample after exposure to high temperatures as demonstrated by the reduced rate of gasification during stepwise decrease in temperature. Similar behaviour is exhibited by Sterling FT/platinum, although to a much lesser degree than for Norit/platinum. While some recent measurements on the oxidation of basal planes of graphite” have shown hysteresis effects during temperature cycling, our data for Norit/platinum do not exhibit closure of the hysteresis loop. For carbon gasification, a measure of the reaction order with respect to hydrogen is shown by the data presented in Figure 2. In the region in which the reaction rate varies exponentially with temperature, the rate of carbon mass loss is nearly proportional to the square root of the partial pressure of hydrogen for the Norit/platinum system. In each of these measurements a fresh sample of Norit/ platinum was employed, and the time-temperature history of each sample was identical. Finally, ruthenium, rhodium, palladium, cobalt and nickel were investigated for their ability to promote the catalytic hydrogasification of carbon (Table 1). For the same weight loading, ruthenium and rhodium exhibit much higher activity than platinum under certain experimental conditions (Table 1). By way of comparison at 875 K (Figure 3), the observed rate of methane production is 100 times as great for ruthenium and 25 times as great for rhodium as for platinum. However, with increasing temperatures, the gasification rates observed with ruthenium and rhodium exhibit a maximum value, and drop below those observed for platinum. For catalysts containing
FUEL, 1974, Vol 53, October
275
Carbon gasification in presence of metal catalysts: R. T. Rewick, P. R. Wentrcek and H. Wise
5 wt % palladium, cobalt or nickel supported on Norit, no additional methane formation was detected up to 1175 K, in excess of that for a Norit carbon sample without these catalysts.
Interaction carbons
of
water
vapour
with metal-activated
in nearly equimolar amounts. For the Sterling FT/platinum system no detectable hysteresis was recorded during interaction with water vapour. Although nickel is known to be an active methanation catalyst, the high temperatures employed in our studies precluded the formation of methane. On a catalyst-weight basis the reaction appears to be more strongly promoted by platinum than by any of the other metals investigated. Also the platinum-catalysed
For the catalysed carbon-steam reaction the present experimental results (TabZe 2) are limited to Sterling-FT carbon supporting platinum and ruthenium and several transition metals (iron, cobalt, nickel). For each of the metals investigated the products of the carbon-steam reaction were found to be carbon monoxide and hydrogen
i
\
\ \
.
\ 10% Hz/He
.-*--’
-*
\
\
.*L . \-\
1% H2/He__..--• -0-e
i
d4 8
9
Figure Norit/
2
Effect of Hz partial pressure on the rare of gasification wt % Pt (total pressure = 1 atm)
Tab/e 7
Methane formation
of
Figure3 Net (I atm Hz)
11
10
IO‘/ T
10&/T (K-‘1 rate
13
12
14
( K -‘I
of mass change for
Norit/
wt %
Pt,
Rh,
Ru
kinetics by reaction of hydrogen with different carbons and metals Net rate* of CH4 formation (cm3/min g carbon)
Catalyst
Norit
Type
Wt %
None Pt Pt Ru Rh Pd co Ni
0.8 5 5 5 5 5 5
*
Difference
276
925 K 0 2.7 73 62 0 0 0
between
Sterling FT
Graphon
1025 K
1125K
925 K
1025 K
1125K
925 K
1025 K
1125 K
0.07 33 IO 68 0 0 0
5.8 57 7 9 0 0 0
0 0.07 0.21 -
0 2.0 6.6
0.06 8.4 21 -
0 0 -
0.02 0.12 -
0.04 1.6 -
observed rates of CH4 formation
FUEL, 1974, Vol 53, October
with and without
added catalyst
-
R. T. Rewick, P. R. Wentrcek and H. Wise: Carbon gasification in presence of metal catalysts
carbon-steam system does not exhibit the maximum in reaction rate observed for the carbon-hydrogen system as evidenced by the curves shown in Figure 4 in which we compare the carbon gasification rate due to reaction with 2.4 vol % water and 2.4 vol % hydrogen. The latter data were computed from those presented in Figure 2 by taking into account the observed 0.5power dependency on the partial pressure of hydrogen. The greater reactivity of hydrogen relative to steam observed in the exponential portion of the rate curves may be the result of the lower bond dissociation energy of hydrogen [D(H-H) = 102 kcal/ mol] as compared to that of water [D(HO-H) = 119 kcal/ mol] .
the conventional BET sorption technique, evaluation of the surface area of the metal itself is a difficult task. We have evaluated the surface areas of the carbonsupported platinum-systems3. The results of the surface-area measurements for a carbon sample of Sterling-FT containing two weight loadings of platinum are expressed in terms of specific surface area per unit mass of platinum and of carbon (Table 3). Also shown are the total surface areas of the samples (carbon and catalyst) as determined by BET
DISCUSSION Since the enhancement of carbon gasification by the presence of the various catalysts is a surface-initiated process, a comparison of the effectiveness of different metals is inadequate on the basis of catalyst weight loading. Quantitative interpretation of the catalytic effect requires detailed knowledge of the surface area of each catalyst so that the gasification process may be expressed in terms of specific rates of mass loss. While the measurement of total surface area (carbon t catalyst) can be accomplished by means of
Tab/e 2 Gas-formation kinetics by reaction of water vapour (2.4 vol %) with Sterling FT carbon and different metal catalysts Net gas formation rate* (cm3/min g carbon) Cata- Loading lyst (wt X) 975 K None Pt5 Pt Ru Ni Co Fe *
0 0.37 0.41 0.10 -
0.8 O-8 O-8 O-8 O-8
1025K
1075K
1125K
1175K
0.20 I.1 0.80 0.43 -
0.22 4.6 2.8 2.1 I.1 0.22 0
0.65 18 7-o 5.3 2.2 O-97 0.09
I.9 38 15 11 3.1 l-6 926
lO’/T
The rate data are expressed in terms of net CO or H2 formation
Table 3
(K-l)
Net rate of mass change for Sterling Figure4 with 2.4 vol % H20/He and 2.4 vol % Hz/He
FT/Pt
(0.8
wt o/o)
Surface-area measurements” Surface area Sterling FT/5 wt % Pt Total
Temperature exposure*
Sterling FT/0.8 wt % Pt
Pt
Total
Pt
m2/g C
m2/g Pt
m2/g C
m2/g C
m2/g Pt
m2/g C
K K K K
34.8 -
12.5 IO.5 7.57 6.7
0.66 0.54 0.40 0.36
45.2 -
-
-
3.94 -
0.030 -
1 hat 1065 K 2hat 1065K
34.6
1.48 1.48
0.079 0.079
-
1.09 -
0.0086 -
1 hat 2 hat 4 hat 1 hat
*
625 625 745 855
Heating
in vacuum
(P < 10e6
torr)
(1 torr = 133 Pa)
FUEL, 1974, Vol 53, October
277
Carbon gasification in presence of metal catalysts: R. T. Rewick, P. R. Wentrcek and H. Wise
(krypton sorption). Since, preceding each of the carbon gasification experiments, the carbon sample was preheated in helium at 1060 K for various periods of time, the entries in Table 4 corresponding to these temperature exposures are of primary interest.
Tab/e 4 Specific methane formation rates by reaction of hydrogen with platinum supported on Sterling FT CH, formation rate (cm3/min m2 Pt) Pt (wt %)
925 K
1025 K
1125K
0.8 5
8.14 2.66
233 83.5
977 266
Tab/e5 Specific gasification rates for the catalysed carbon *-steam reaction Rate of total gas production+ Icm3/min m2 Pt) Catalyst
Wt %
1075 K
1125 K
1175 K
Pt Pt
5 0.8
116 651
456 1630
962 3490
* +
Sterling FT 50 vol % co,
50 vol % l-l*
Tab/e 6 Comparison between observed rate of methane and atomic hydrogen formation
H formation* It is to be noted
that considerable sintering of the platinum crystallites on the carbon support takes place as a result of exposure to elevated temperatures for prolonged periods of time, particularly for the platinum (5 wt %) sample. For the original material of platinum Sterling-FT (5 wt %), we estimate a surface area of about 15 m2/g platinum which decreases by a factor of ten on exposure to conditions comparable to those encountered during our catalytic studies, while the carbon surface area is hardly affected. At the same time the dispersion of platinum is nearly the same for the two catalyst weight loadings employed. (Dispersion is defined as the ratio of the number of surface metal atoms to the total number of metal atoms in the crystallite.) The gasification rates of carbon may be examined on the basis of either the mass of carbon or the surface area of catalyst. Since we are interested in examining catalyst efficiency we have calculated the specific carbon gasification rates in terms of the volumetric production of methane per unit area of catalyst (Table 4). These results demonstrate the marked degree of enhancement resulting from the increased platinum dispersion. Similar considerations apply to the catalysed carbon-steam reaction for which the total specific rates of gas production (carbon monoxide + hydrogen) are shown in Table 5. For the carbonhydrogen reaction catalysed by platinum we are able to compare the experimental rate of methane production with the net rate of formation of hydrogen atoms by dissociation on the platinum surface. In a study of hydrogen dissociation on platinum, Brennan and Fletcher” demonstrated that the rate of hydrogen-atom production is proportional to the square root of the hydrogen pressure and exhibits an activation energy of 5 1 .l * 1.2 kcal/mol. For the rate of methane production on Sterling FT/platinum (5 wt %), we obtain the same kinetic dependence on the hydrogen partial pressure - [H21112 - and an activation energy of 5.5 f 3 kcal/mol. Even more striking is the agreement between the rate of methane formation and the rate of hydrogen-atom production (Table 6). These results point to the formation of atomic hydrogen as the ratecontrolling step in the carbon-hydrogen gasification process. The reaction between carbon and atomic hydrogen (reaction (3)) generated at the catalyst site is in competition with the hydrogen recombination reaction (2) which syphons off hydrogen atoms in accordance with:
278
FUEL, 1974, Vol 53, October
Temp. (K) 925
1025 l
(atom.cm-2
s-l)
3.0 f 0.3 x 10’4 4.5 f 0.7 x 10’5
CH4 formation+ (molecules.cm-2
1.2 x 10’4 3.8 X 1015
Rate of atomization of hydrogen: RH = (1.3 lpexp (-51000 f 12OO)lRT (Ref.1 1)
Rate of methane 4.6 x 1O25 (Pt,,,)~
H2+
production (Sterling exp (-55OOOlRn.
2M+2M-H
s-l)
FT/Pt
_ + 0.6)
x lO25
(5 wt o/o)): RCH,’
(1)
2M-H-+2M+H2
(2)
M-H+C+M+C-H
(3)
C-H+H-+...-+CH4
(4)
where M represents a metal site and C a carbon site. Therefore the role of the catalyst is to provide the reactive atomic hydrogen which surface-diffuses to a neighbouring carbon site. There it can react to form methane by a mechanism previously examined for the case of atomic hydrogen-graphite’. Activated surface migration of chemisorbed hydrogen on platinized carbon has been shown to occur readily at much lower temperatures than encountered in our studies12. Differences in the reactivity of the carbons employed are probably due to variations in surface area of the carbons and distribution of edge carbon atoms. The maxima in gasification rate exhibited by the carbonhydrogen reaction, as shown in Figure 3, have been noted in a number of surface reactions involving different forms of carbon and various gaseous reactants such as oxygen and halogens13. Thus the cause of this reaction pattern is not necessarily associated with changes in the physical or chemical properties of the catalyst, such as sintering of the metal crystallites, since pretreatment of the carbon-catalyst samples has stabilized the surface area preceding the kinetic studies with hydrogen or water. Most likely the reversible maxima noted for Sterling FT are due to the competition between two surface processes, one the continuous formation of a highly reactive carbon surface owing to the gasification reaction (formation of edge carbon atoms as a
R. T. Rewick, P. R. Wentrcek and H. Wise: Carbon gasification in presence of metal catalysts Table 7 Comparison of gas production rates at 1125 K for different catalytic additives
Additive, 5wt% Pt Ni co Liz03 Pb02, Pb304
Relative increase in gasification rate (R,r
- Rcarbon)lRcarbon
Present work*
28 3.4 1.5 0.48
8aO
0.35 0.33
Bi203 coo
0.31 0.29
* Carbon = Sterling FT; l-l*0 (1 atm = 101.3 kPa) t Carbon = bituminous coal total pressure = 20 atm
Reference
It may be of interest to compare the catalysed carbonsteam gasification rates for highly dispersed transition metal catalysts as measured in our studies with those recently reported for a number of compounds in admixture with a bituminous coal” (Table 7). It is apparent that dispersion of the catalyst as well as the type of catalyst employed must play an important role in the catalytic process of gas production. The results point to the degree of enhancement with coal that one might expect under the most favourable experimental conditions.
15+
REFERENCES 1
Forney,
A. J., Haynes, W. P. and Corney, R. C. Sot. of Petrol.
Engrs, AIME, Preprint SPE 3586,197l; = 2.4 vol %, total (Bruceton,
Pa.);
Walker, P. L. Jr, Shelef, M. and Anderson, R. A. ‘Chemistry and Physics of Carbons’, Vol.4 (Ed. P. L. Walker, Jr), Marcel Dekker, New York, 1968 Bartholomew, C. H. and Boudart, M. J. Catal. 1972,25,173 Wentrcek,P., Kimoto, K. and Wise, H. J. Catal. 1974,33,279 Duval, X. Ann. Chim. (Ser.12) 1955, 10,905 Blyholder, G., Binford, J. S. Jr and Eyring, H. J. phys. Chem.
pressure = 1 atm Hz0
=6,9
vol %;
1958,62,263 6
result of C-C bond breakage’, and the other, thermal annealing or removal of edge atoms owing to the mobility of the carbon atoms in the surface layers at the higher temperatures, where the levelling off in reaction rate is observed. Such a mechanism has been examined in some detail by Olander and co-workers*. Qualitatively our results exhibit similar patterns in reaction rate as described recently for ‘active’ carbon14. However, direct comparison is difficult since the degree of dispersion of the metal catalyst was not cited in the latter study.
I ; 10
11
Walls, J. R. and StricklandConstable, R. F. Carbon 1964, 1, 333 Rosner, D. E. and Allendorf, H.D. Carbon 1969, 7,515 Olander, D. R. et al J. them. Phys. 1972,57,408 Wood, B. J. and Wise, H. .J. phys. Chem. 1969,73,1348 Coad, J. P. and Riviere, J. C. Surface Sci. 197 1, 25, 609 Brennan, D. and Fletcher, P. C. Trans. Faraday Sot. 1960, 56,1662
12
Robell, A. J., Ballou, E. V. and Boudart, M. J. J. phys. Chem.
13 14 15
Rosner, D. E. and Strakey, J. P.SurfaceSci. 1971,24, 77 Tomita, A. and Tamai, Y. J. Catal. 1912,27, 293 Haynes, W. P., Gasior, S. J. and Forney, A. J. Div. of Fuel Chem. Am. Chem. Sot. Preprints of Papers presented at Dallas, Texas 1973, 18, (No.21, 1
1964,68,9
FUEL, 1974, Vol 53, October
279
Stanford
in the presence
and Henry Wise Research Institute,
Menlo Park, California
The development of new processes for the production of gaseous fuels from carboncontaining solids is essential in meeting the energy needs of the nation. In this paper, catalysed carbon gasification is examined. The change in the reactivity of the interface between gaseous reactant (hydrogen or steam) and solid carbon has been measured in the presence of various metal catalysts. With platinum it is found that over a range of temperatures the specific rate of methane production is of the same magnitude as the rate of hydrogen atomization. The catalytic effect is interpretable in terms of an enhanced rate of hydrogen dissociation on the metal surface, followed by surface diffusion across the metal/carbon interface and reaction with carbon. The gas formation rate during the interaction of water vapour with catalyst-activated carbons has been increased by more than an order of magnitude by depositing small weight fractions of active metal catalyst on the carbon surface. At the temperaturesemployed in this study (975-l 175 K), carbon monoxide and hydrogen are the products of the catalysed reaction for each of the catalysts examined.
The increasing demand for natural gas and the predicted decline in natural gas reserves emphasizes the need for the development of new processes for coal gasification and other means of producing pipeline gas. Although a number of promising new processes are currently in various stages of development, the economic production of pipeline gas from coal remains a subject of further study. Specifically it is of considerable importance to explore the feasibility of augmenting the rate of coal gasification by the use of catalytic processes both for the hydrogasification and the steam-carbon reactions. During the last few years the catalytic effects of a number of metals and their compounds have been examined for coal However the mechanism of the catalytic gasification’. action has not been elucidated in sufficient detail to allow selection of an optimum catalytic system for coal gasification by reaction with hydrogen or water. In our work we have examined in considerable detail the relation between the kinetics of carbon gasification by reaction with hydrogen or steam as a function of (1) the type of metal catalyst, (2) the degree of catalyst dispersion, (3) the gaseous reactant concentration, and (4) the temperature of the reaction system. Selection of different catalysts was based on the hypothesis that in the production of gaseous fuels by heterogeneous reaction of carbon surfaces the slow step is to be found in the dissociative chemisorption of gaseous reactant molecules, namely hydrogen or water. A process that could lower this energy barrier for the sorption step, such as a catalysed reaction, Would result in the rapid formation of atomic species (H) and radicals (OH) which could readily surfacediffuse and react with the carbon sllhstrate. * This research was carried out under the sponsorship of the American Gas Association, whose support is gratefully acknowledged
274
FUEL, 1974, Vol 53, October
EXPERIMENTAL
DETAILS
Materials Three kinds of carbon were employed in this study: Graphon (a graphitized carbon black prepared by Cabot Corporation), Sterling FT (a fine thermal carbon black provided by Cabot Corporation), and Norit-A (a vegetable carbon obtained from Pfanstiehl Chemical Co.).
Gzialyst preparation Catalysts containing O-8 wt % platinum on Graphon and Sterling FT, and 5 wt % platinum on Sterling FT were The procedure involves burning off a fraction prepared’. of the carbon sample in air at 875 K until 50% of its original weight has been consumed. The carbon is mixed with a solution containing benzene, ethanol, and the metal salt (chloroplatinic acid or the respective metal chlorides). The slurry is evaporated to dryness with a stream of nitrogen and vigorous agitation. Subsequently the impregnated sample is dried under vacuum at 375 K and then reduced at 775 Kin a stream of hydrogen. Catalysts containing 0.8 wt % nickel and cobalt on Sterling FT and 5 wt % nickel and cobalt on Norit were prepared in a similar manner starting with NiC12.6H20 and CoCl2.6H2O respectively. Samples containing 5 wt % platinum, palladium, ruthenium and rhodium on Norit were purchased from Research Inorganic/ Organic Chemical Co.
Gasification measurements To measure quantitatively the rate of carbon gasification, a Cahn RG Electrobalance in a glass vacuum assembly was suitably modified to record continuously mass change of the
R. T. Rewick, P. R. Wentrcek and H. Wise: Carbon gasification in presence of metal catalysts
catalyst at various temperatures in a flow of hydrogen or water vapour at one atmosphere total pressure. Aliquots of the gas stream containing the reaction products were analysed by gas chromatography on a 12-ft X l/&in molecular sieve 5A column. In a typical run, a 20-mg sample of the catalyst was weighed into a quartz basket attached to the microbalance and evacuated to remove oxygen. The sample was then degassed at 1075 K for one hour in a stream of helium; after cooling to room temperature the helium atmosphere was replaced with hydrogen/helium mixtures or helium saturated with water vapour. Heat was then applied to the sample under flow conditions (26 cm3/min HzO/He, The temperature was then increased 60 cm3/min Hz). stepwise over a range of several hundred degrees to obtain the rate of mass loss and the concentration of products in the effluent gas stream at various temperature levels.
Surface-area measurements The surface areas of carbon and of the dispersed metal have been measured for samples of Sterling FT/5 wt % platinum and Sterling FT/O8 wt % platinum. The BET adsorption method using krypton as the physisorbate was employed for determination of the total surface area. The platinum surface areas were analysed by a new procedure involving a surface titration of metal-chemisorbed oxygen with carbon monoxide3.
84
10.0
9.0
Figure l
1
Net rate of mass change for different
Noritl5 wt 96 Pt. n Sterling FT/0.8 wt % Pt.
EXPERIMENTAL
RESULTS with
of hydrogen Interaction carbons Since this study of carbon gasification
metal-activated
involves several parameters, we examined first the effect of different carbon substrates on the gasification rate catalysed by finely dispersed metal at different weight loadings. These results are summarized in Figure 1 for the case of platinum. At constant hydrogen pressure the reaction was found to be first order with respect to carbon both in the presence and the absence of catalyst. The reaction kinetics follows the rate law -dm/d t = km, where m represents the mass of carbon and k the rate constant, which in turn is an exponential function of temperature as given by the Arrhenius expression [k = A exp (-L?‘/RT)] with A defined as the preexponential factor. Thus a semilog plot of the relative rate of mass loss -dm p/dt (
m
>
versus the reciprocal temperature (1 /T) yields a straight line whose slope is proportional to the apparent activation energy E. Although marked differences in reactivity of the different carbon samples containing dispersed platinum are apparent, possibly owing to different physical surface properties of the carbons, the slopes of the straight portion of the curves shown are identical (within the experimental error). They correspond to an overall activation energy of 5.5 + 3 kcal/mol *. Also, methane is the only detectable gaseous carbon-containing product. Another characteristic feature exhibited by these results is the gradual transition from an exponential increase in * 1 kcal/mol
= 4,187
kJ/mol
11.0
11.4
lO‘/T(K-‘I carbons/Pt
(1 atm Hz)
OSterling FT/5 wt % Pt, 0 Graphon/OB wt % Pt
reaction rate with temperature to a region where the rate passes through a maximum (Figures 1 and 2). A similar phenomenon has been observed in the interaction of pyrolytic graphite with oxygen4-* and hydrogen atoms’. Also, the results for Norit/platinum indicate an irreversible change in the reactivity of the carbon sample after exposure to high temperatures as demonstrated by the reduced rate of gasification during stepwise decrease in temperature. Similar behaviour is exhibited by Sterling FT/platinum, although to a much lesser degree than for Norit/platinum. While some recent measurements on the oxidation of basal planes of graphite” have shown hysteresis effects during temperature cycling, our data for Norit/platinum do not exhibit closure of the hysteresis loop. For carbon gasification, a measure of the reaction order with respect to hydrogen is shown by the data presented in Figure 2. In the region in which the reaction rate varies exponentially with temperature, the rate of carbon mass loss is nearly proportional to the square root of the partial pressure of hydrogen for the Norit/platinum system. In each of these measurements a fresh sample of Norit/ platinum was employed, and the time-temperature history of each sample was identical. Finally, ruthenium, rhodium, palladium, cobalt and nickel were investigated for their ability to promote the catalytic hydrogasification of carbon (Table 1). For the same weight loading, ruthenium and rhodium exhibit much higher activity than platinum under certain experimental conditions (Table 1). By way of comparison at 875 K (Figure 3), the observed rate of methane production is 100 times as great for ruthenium and 25 times as great for rhodium as for platinum. However, with increasing temperatures, the gasification rates observed with ruthenium and rhodium exhibit a maximum value, and drop below those observed for platinum. For catalysts containing
FUEL, 1974, Vol 53, October
275
Carbon gasification in presence of metal catalysts: R. T. Rewick, P. R. Wentrcek and H. Wise
5 wt % palladium, cobalt or nickel supported on Norit, no additional methane formation was detected up to 1175 K, in excess of that for a Norit carbon sample without these catalysts.
Interaction carbons
of
water
vapour
with metal-activated
in nearly equimolar amounts. For the Sterling FT/platinum system no detectable hysteresis was recorded during interaction with water vapour. Although nickel is known to be an active methanation catalyst, the high temperatures employed in our studies precluded the formation of methane. On a catalyst-weight basis the reaction appears to be more strongly promoted by platinum than by any of the other metals investigated. Also the platinum-catalysed
For the catalysed carbon-steam reaction the present experimental results (TabZe 2) are limited to Sterling-FT carbon supporting platinum and ruthenium and several transition metals (iron, cobalt, nickel). For each of the metals investigated the products of the carbon-steam reaction were found to be carbon monoxide and hydrogen
i
\
\ \
.
\ 10% Hz/He
.-*--’
-*
\
\
.*L . \-\
1% H2/He__..--• -0-e
i
d4 8
9
Figure Norit/
2
Effect of Hz partial pressure on the rare of gasification wt % Pt (total pressure = 1 atm)
Tab/e 7
Methane formation
of
Figure3 Net (I atm Hz)
11
10
IO‘/ T
10&/T (K-‘1 rate
13
12
14
( K -‘I
of mass change for
Norit/
wt %
Pt,
Rh,
Ru
kinetics by reaction of hydrogen with different carbons and metals Net rate* of CH4 formation (cm3/min g carbon)
Catalyst
Norit
Type
Wt %
None Pt Pt Ru Rh Pd co Ni
0.8 5 5 5 5 5 5
*
Difference
276
925 K 0 2.7 73 62 0 0 0
between
Sterling FT
Graphon
1025 K
1125K
925 K
1025 K
1125K
925 K
1025 K
1125 K
0.07 33 IO 68 0 0 0
5.8 57 7 9 0 0 0
0 0.07 0.21 -
0 2.0 6.6
0.06 8.4 21 -
0 0 -
0.02 0.12 -
0.04 1.6 -
observed rates of CH4 formation
FUEL, 1974, Vol 53, October
with and without
added catalyst
-
R. T. Rewick, P. R. Wentrcek and H. Wise: Carbon gasification in presence of metal catalysts
carbon-steam system does not exhibit the maximum in reaction rate observed for the carbon-hydrogen system as evidenced by the curves shown in Figure 4 in which we compare the carbon gasification rate due to reaction with 2.4 vol % water and 2.4 vol % hydrogen. The latter data were computed from those presented in Figure 2 by taking into account the observed 0.5power dependency on the partial pressure of hydrogen. The greater reactivity of hydrogen relative to steam observed in the exponential portion of the rate curves may be the result of the lower bond dissociation energy of hydrogen [D(H-H) = 102 kcal/ mol] as compared to that of water [D(HO-H) = 119 kcal/ mol] .
the conventional BET sorption technique, evaluation of the surface area of the metal itself is a difficult task. We have evaluated the surface areas of the carbonsupported platinum-systems3. The results of the surface-area measurements for a carbon sample of Sterling-FT containing two weight loadings of platinum are expressed in terms of specific surface area per unit mass of platinum and of carbon (Table 3). Also shown are the total surface areas of the samples (carbon and catalyst) as determined by BET
DISCUSSION Since the enhancement of carbon gasification by the presence of the various catalysts is a surface-initiated process, a comparison of the effectiveness of different metals is inadequate on the basis of catalyst weight loading. Quantitative interpretation of the catalytic effect requires detailed knowledge of the surface area of each catalyst so that the gasification process may be expressed in terms of specific rates of mass loss. While the measurement of total surface area (carbon t catalyst) can be accomplished by means of
Tab/e 2 Gas-formation kinetics by reaction of water vapour (2.4 vol %) with Sterling FT carbon and different metal catalysts Net gas formation rate* (cm3/min g carbon) Cata- Loading lyst (wt X) 975 K None Pt5 Pt Ru Ni Co Fe *
0 0.37 0.41 0.10 -
0.8 O-8 O-8 O-8 O-8
1025K
1075K
1125K
1175K
0.20 I.1 0.80 0.43 -
0.22 4.6 2.8 2.1 I.1 0.22 0
0.65 18 7-o 5.3 2.2 O-97 0.09
I.9 38 15 11 3.1 l-6 926
lO’/T
The rate data are expressed in terms of net CO or H2 formation
Table 3
(K-l)
Net rate of mass change for Sterling Figure4 with 2.4 vol % H20/He and 2.4 vol % Hz/He
FT/Pt
(0.8
wt o/o)
Surface-area measurements” Surface area Sterling FT/5 wt % Pt Total
Temperature exposure*
Sterling FT/0.8 wt % Pt
Pt
Total
Pt
m2/g C
m2/g Pt
m2/g C
m2/g C
m2/g Pt
m2/g C
K K K K
34.8 -
12.5 IO.5 7.57 6.7
0.66 0.54 0.40 0.36
45.2 -
-
-
3.94 -
0.030 -
1 hat 1065 K 2hat 1065K
34.6
1.48 1.48
0.079 0.079
-
1.09 -
0.0086 -
1 hat 2 hat 4 hat 1 hat
*
625 625 745 855
Heating
in vacuum
(P < 10e6
torr)
(1 torr = 133 Pa)
FUEL, 1974, Vol 53, October
277
Carbon gasification in presence of metal catalysts: R. T. Rewick, P. R. Wentrcek and H. Wise
(krypton sorption). Since, preceding each of the carbon gasification experiments, the carbon sample was preheated in helium at 1060 K for various periods of time, the entries in Table 4 corresponding to these temperature exposures are of primary interest.
Tab/e 4 Specific methane formation rates by reaction of hydrogen with platinum supported on Sterling FT CH, formation rate (cm3/min m2 Pt) Pt (wt %)
925 K
1025 K
1125K
0.8 5
8.14 2.66
233 83.5
977 266
Tab/e5 Specific gasification rates for the catalysed carbon *-steam reaction Rate of total gas production+ Icm3/min m2 Pt) Catalyst
Wt %
1075 K
1125 K
1175 K
Pt Pt
5 0.8
116 651
456 1630
962 3490
* +
Sterling FT 50 vol % co,
50 vol % l-l*
Tab/e 6 Comparison between observed rate of methane and atomic hydrogen formation
H formation* It is to be noted
that considerable sintering of the platinum crystallites on the carbon support takes place as a result of exposure to elevated temperatures for prolonged periods of time, particularly for the platinum (5 wt %) sample. For the original material of platinum Sterling-FT (5 wt %), we estimate a surface area of about 15 m2/g platinum which decreases by a factor of ten on exposure to conditions comparable to those encountered during our catalytic studies, while the carbon surface area is hardly affected. At the same time the dispersion of platinum is nearly the same for the two catalyst weight loadings employed. (Dispersion is defined as the ratio of the number of surface metal atoms to the total number of metal atoms in the crystallite.) The gasification rates of carbon may be examined on the basis of either the mass of carbon or the surface area of catalyst. Since we are interested in examining catalyst efficiency we have calculated the specific carbon gasification rates in terms of the volumetric production of methane per unit area of catalyst (Table 4). These results demonstrate the marked degree of enhancement resulting from the increased platinum dispersion. Similar considerations apply to the catalysed carbon-steam reaction for which the total specific rates of gas production (carbon monoxide + hydrogen) are shown in Table 5. For the carbonhydrogen reaction catalysed by platinum we are able to compare the experimental rate of methane production with the net rate of formation of hydrogen atoms by dissociation on the platinum surface. In a study of hydrogen dissociation on platinum, Brennan and Fletcher” demonstrated that the rate of hydrogen-atom production is proportional to the square root of the hydrogen pressure and exhibits an activation energy of 5 1 .l * 1.2 kcal/mol. For the rate of methane production on Sterling FT/platinum (5 wt %), we obtain the same kinetic dependence on the hydrogen partial pressure - [H21112 - and an activation energy of 5.5 f 3 kcal/mol. Even more striking is the agreement between the rate of methane formation and the rate of hydrogen-atom production (Table 6). These results point to the formation of atomic hydrogen as the ratecontrolling step in the carbon-hydrogen gasification process. The reaction between carbon and atomic hydrogen (reaction (3)) generated at the catalyst site is in competition with the hydrogen recombination reaction (2) which syphons off hydrogen atoms in accordance with:
278
FUEL, 1974, Vol 53, October
Temp. (K) 925
1025 l
(atom.cm-2
s-l)
3.0 f 0.3 x 10’4 4.5 f 0.7 x 10’5
CH4 formation+ (molecules.cm-2
1.2 x 10’4 3.8 X 1015
Rate of atomization of hydrogen: RH = (1.3 lpexp (-51000 f 12OO)lRT (Ref.1 1)
Rate of methane 4.6 x 1O25 (Pt,,,)~
H2+
production (Sterling exp (-55OOOlRn.
2M+2M-H
s-l)
FT/Pt
_ + 0.6)
x lO25
(5 wt o/o)): RCH,’
(1)
2M-H-+2M+H2
(2)
M-H+C+M+C-H
(3)
C-H+H-+...-+CH4
(4)
where M represents a metal site and C a carbon site. Therefore the role of the catalyst is to provide the reactive atomic hydrogen which surface-diffuses to a neighbouring carbon site. There it can react to form methane by a mechanism previously examined for the case of atomic hydrogen-graphite’. Activated surface migration of chemisorbed hydrogen on platinized carbon has been shown to occur readily at much lower temperatures than encountered in our studies12. Differences in the reactivity of the carbons employed are probably due to variations in surface area of the carbons and distribution of edge carbon atoms. The maxima in gasification rate exhibited by the carbonhydrogen reaction, as shown in Figure 3, have been noted in a number of surface reactions involving different forms of carbon and various gaseous reactants such as oxygen and halogens13. Thus the cause of this reaction pattern is not necessarily associated with changes in the physical or chemical properties of the catalyst, such as sintering of the metal crystallites, since pretreatment of the carbon-catalyst samples has stabilized the surface area preceding the kinetic studies with hydrogen or water. Most likely the reversible maxima noted for Sterling FT are due to the competition between two surface processes, one the continuous formation of a highly reactive carbon surface owing to the gasification reaction (formation of edge carbon atoms as a
R. T. Rewick, P. R. Wentrcek and H. Wise: Carbon gasification in presence of metal catalysts Table 7 Comparison of gas production rates at 1125 K for different catalytic additives
Additive, 5wt% Pt Ni co Liz03 Pb02, Pb304
Relative increase in gasification rate (R,r
- Rcarbon)lRcarbon
Present work*
28 3.4 1.5 0.48
8aO
0.35 0.33
Bi203 coo
0.31 0.29
* Carbon = Sterling FT; l-l*0 (1 atm = 101.3 kPa) t Carbon = bituminous coal total pressure = 20 atm
Reference
It may be of interest to compare the catalysed carbonsteam gasification rates for highly dispersed transition metal catalysts as measured in our studies with those recently reported for a number of compounds in admixture with a bituminous coal” (Table 7). It is apparent that dispersion of the catalyst as well as the type of catalyst employed must play an important role in the catalytic process of gas production. The results point to the degree of enhancement with coal that one might expect under the most favourable experimental conditions.
15+
REFERENCES 1
Forney,
A. J., Haynes, W. P. and Corney, R. C. Sot. of Petrol.
Engrs, AIME, Preprint SPE 3586,197l; = 2.4 vol %, total (Bruceton,
Pa.);
Walker, P. L. Jr, Shelef, M. and Anderson, R. A. ‘Chemistry and Physics of Carbons’, Vol.4 (Ed. P. L. Walker, Jr), Marcel Dekker, New York, 1968 Bartholomew, C. H. and Boudart, M. J. Catal. 1972,25,173 Wentrcek,P., Kimoto, K. and Wise, H. J. Catal. 1974,33,279 Duval, X. Ann. Chim. (Ser.12) 1955, 10,905 Blyholder, G., Binford, J. S. Jr and Eyring, H. J. phys. Chem.
pressure = 1 atm Hz0
=6,9
vol %;
1958,62,263 6
result of C-C bond breakage’, and the other, thermal annealing or removal of edge atoms owing to the mobility of the carbon atoms in the surface layers at the higher temperatures, where the levelling off in reaction rate is observed. Such a mechanism has been examined in some detail by Olander and co-workers*. Qualitatively our results exhibit similar patterns in reaction rate as described recently for ‘active’ carbon14. However, direct comparison is difficult since the degree of dispersion of the metal catalyst was not cited in the latter study.
I ; 10
11
Walls, J. R. and StricklandConstable, R. F. Carbon 1964, 1, 333 Rosner, D. E. and Allendorf, H.D. Carbon 1969, 7,515 Olander, D. R. et al J. them. Phys. 1972,57,408 Wood, B. J. and Wise, H. .J. phys. Chem. 1969,73,1348 Coad, J. P. and Riviere, J. C. Surface Sci. 197 1, 25, 609 Brennan, D. and Fletcher, P. C. Trans. Faraday Sot. 1960, 56,1662
12
Robell, A. J., Ballou, E. V. and Boudart, M. J. J. phys. Chem.
13 14 15
Rosner, D. E. and Strakey, J. P.SurfaceSci. 1971,24, 77 Tomita, A. and Tamai, Y. J. Catal. 1912,27, 293 Haynes, W. P., Gasior, S. J. and Forney, A. J. Div. of Fuel Chem. Am. Chem. Sot. Preprints of Papers presented at Dallas, Texas 1973, 18, (No.21, 1
1964,68,9
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279