Energy & Fuels 1988,2,750-756
750
i
t
5 :
csc
(b) CSC (1850 K )
5 30
.
R16 (1880 K)
R16 ( 1 4 0 0 K )
CBC ( 1 4 1 0 K )
I5
18
17
18
LOG ([02], cm-3)
Figure 12. Dependence of carbon black reaction probability on
[O,]:RA,ref 19 for pyrolytic (pyro) graphite.
values predicted by NSC and observed by RA may be present for R16, but are not suggested by the CSC data. Figure 12 displays the probability of reaction, 5, per O,-surface collision calculated by using kinetic theory to determine the number of collisions per second per unit surface area and the measured Re values. The RA data on pyrolytic graphite over a range of low [O,] at =1500 K are shown for comparison in Figure 12a. Figure 12b shows 5 for R16 and CSC for most of the wide range of [O,] covered in this work a t the extremes of the temperature ranges investigated. The low reactivity of these carbon
blacks translates into collision efficiencies as low as -1 X lo* at [O,] = 1 X 10l8 cm-3 and as high as -4 X loe3 at [O,] = 1X 1015~ m - ~These . values are comparable to those obtained for graphitic carbons. Thus,the present resulta indicate low reactivity for R16 and CSC for [O,] values of practical interest. The complex [O,] dependence of Re required by NSC kinetics is not observed over the temperature range studied, despite extremely wide variations in [02].The data show no significant reactivity differences that can be attributed to metallic content. We speculate that the high sulfur content in these carbon blacks may be the cause of their low reactivity and the failure of the two-site model to describe their oxidation kinetics. This sulfur content (mole fraction =40 ppm) may be sufficient to poison potential metallic catalytic sites (on a molar basis, sulfur is -20 and 10 times more abundant than metah in R16 and CSC, respectively) as well as to interfere with active sites in both carbon blacks. The furnace black studied by PA" had a sulfur cohtent similar to that of R16, but the higher temperature regime they studied may have allowed this impurity to be volatilized (at least from the surface) and thus not to interfere with the oxidation. If correct, this speculation suggests that it is important to avoid sulfur contamination where carbonaceous burnout is desired. Significant further study is required to verify this speculation. Acknowledgment. This work was sponsored by the U.S. Army Research Office, under Contract No. DAAG 29-83-C-0023; the content of the information does not necessarily reflect the position or policy of the U.S. Government, and no official endorsement should be inferred. We acknowledge helpful discussions with Drs. N. M. Laurendeau (Purdue), S. J. Harris (GM), H. F. Calcote (AeroChem), and D. M. Mann (ARO).
Interaction between Potassium Carbonate and Carbon Substrate at Subgasification Temperatures. Migration of Potassium into the Carbon Matrix M. Matsukata,* T. Fujikawa, E. Kikuchi, and Y. Morita Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 160, Japan Received April 27, 1988. Revised Manuscript Received August 2, 1988 The interaction between potassium carbonate and carbon substrate in an inert atmosphere was investigated by using carbon black with an amorphous structure and graphite. Qualitative and quantitative changes of potassium species on carbon in the course of heat treatment were monitored by means of a temperature-programmed reaction, extraction of potassium with a HC1 solution, Auger electron spectroscopy, and electron probe microanalysis. Potassium carbonate impregnated on carbon black decomposed to give potassium oxide and COz in the temperature range 470-900 K. We found that potassium species migrated into the carbon matrix in the temperature range 670-900 K. At higher temperatures potassium oxide remaining on the surface was reduced by reaction with carbon. Although metallic potassium on graphite was last due to evaporation, no pronounced loss of potassium was observed from carbon black containing leas than 5 w t % of potassium. The migration of potassium into bulk carbon was not observed on graphite. Introduction The catalytic gasification of carbonaceous materials is one of the potential routes to produce industrially useful gases such as hydrogen, methane, and syngas. Many in0887-0624/88/2502-0750$01.50/0
vestigatorsl-' have reported that alkali-metal carbonate is catalytically effective, whereas the catalytic structure and (1) Wen, W. H.Catal. Rev.-,%
Eng. 1980, 22, 1-28.
@ 1988 American Chemical Society
K2COrCarbon Substrate Interaction role under gasification conditions have not sufficiently been elucidated. The factors governing the catalysis are the structure of carbon, oxygen content, and mineral matter content. In addition, the loss of catalyst due to evaporation of alkali metal has frequently been observed. It has generally been acceptable that the chemical interaction between potassium salts and carbon substrate at subgasification temperatures is important for the formation of catalytically active specie^.^*^*^'^ Although the oxygencontaining species is considered to be responsible for gasification activity, several types of the species, such as carbonate, the oxidic and metallic species involved in redox cycle, a surface C-0-K complex, and intercalation compounds, have been proposed. On the other hand, we consider that knowledge of the amount of potassium actually existing on the carbon surface under gasification conditions is also essential to understand the catalysis of potassium compounds. Most of the proposed potassium species described above are considered to be soluble in acidic solution. We expect that the amount of potassium exsisting on carbon surface can be determined by extraction with acidic solution, if neither the formation of intercalation compounds nor the reaction of potassium with mineral matter is important. We chose a carbon black containing very little mineral matter and a graphite as carbon substrates in the present study. In this study, we investigated the interaction between potassium species and carbon substrate in an inert atmosphere at subgasification temperatures. I t will be established that potassium migrates into amorphous carbon in the course of heat treatment.
Experimental Section The carbon samples used in the present study were graphite (Union CarbideCo., spectrmopic powder SP-1) and carbon black (Tokai Carbon Co., Seast S, SRF). Carbon black was crushed, and a portion of particles in the range 24-42 mesh was washed with decationated water, dried at 383 K, and treated at 1173 K for 2 h in a stream of nitrogen. Ultimate analysis showed that the resulted carbon black was predominantlycomposed of carbon with a small amount of ash (0.1 wt %) and without any detectable hydrogen, oxygen, nitrogen, and sulfur. We observed by means of X-ray diffraction (XRD) that the carbon black has an amorphous structure. Potassium carbonatewas impregnated or physically mixed with the carbon substrate. K2COs-impregnatedcarbon was prepared by dissolving a desired amount of potassium carbonate in decationated water, permitting the solution to soak into the carbon, and then dryidg at 383 K in a stream of nitrogen. The amount of potassiumin the impregnatedsample was determined by means of flame spectrochemical analysis, after extracting potassium (2) McKee, D. W.; Chatterji, D.Carbon 1978, 16, 53-57. (3) Veraa, M. J.; Bell, A. T. Fuel 1978,57, 194-200. (4) b o , Y. K.; Adjorlolo, A.; Haberman, I. H. Carbon 1982, 20, 207-212. (5) McKee, D. W. Carbon 1982,20,59-66. (6) Spiro, C. L.; McKee, D.W.; Kosky,P. G.;Lamby, E. J. Fuel. 1983, 62,180-184. (7) Kikuchi, E.; Adachi, H.; Hirose, M.; Morita, Y. Fuel 1983, 62, 226-230.
(81 Freriks, I. L. C.; van Wechem, H. M. H.; Stuvier, J. C. M.; Bouwnmann, R. Fuel 1981,60,463-470. (9) Wigmans, T.; van Doom, J.; Moulijn, J. A. Fuel 1983, 62, 190. (10) Wood, B. J.; Fleming, R. H.; Wise, H. Fuel 1984,63,16OC-1603. (11) Saber, J. M.; Falconer, J. H.; Brown, L. F. Fuel 1986, 65, 1356-1359. (12) Wigmans, T.; Goebel, J. C.; Moulijn, J. A. Carbon 1983, 21, 295-301. (13) Cerfontain, M. B.; Moulijn, J. A. Fuel 1983,62, 256-258. (14) Mime, C.A.; Chludzinski, J. J., Jr.; Pabst, J. K.; Baker, R.T. K. J. Catal. 1984,88,97-106. (15) Keleman, S. R.; Freund, H. J. Catal. 1986, 102, 80-91. (16) Shadman, F.; Sams, D. A.; Punjak, W. A. Fuel 1987, 66, 1658-1663.
Energy & Fuels, Vol. 2, No. 6,1988 751
A
270
470
670
a70
Desorption temerature
1070 (K)
Figure 1. TPR spectra for the carbon black sample prepared (a) by impregnationand (b) by physical mixing (heating rate, 10
K min-I): open circle, CO,; solid circle, CO.
deposited on the carbon sample in the reflux of a 1N HC1solution for 3 h. A temperature-programmed reaction (TPR) in an inert atmosphere was performed in order to monitor the structural change of potassiumspecies caused by the interaction with carbon at high temperatures. The TPR of K2COs-impregnatedcarbon was performed in a stream of argon at atmosphericpressure by using a continuous-flowreactor. The reactor is a 9.0-mm4.d. alumina tube, in the middle of which 0.6 g of K2COs-impregnatedcarbon was placed. The carbon sample was heated from 300 to 1123 K at a heating rate of 10 K m i d , unless otherwise stated. The composition of effluent gases was determined by means of gas chromatography. The carbon sample was analyzedfor the amount of potassium remainingon the surface by extractionwith 1N HC1 solution, after the sample was heated up to a desired temperature and was subsequently quenched by removing the reactor from the furnace. The sample was soaked in HCl solution in an argon atmosphere. The extracted amount of potassium will be referred to as the term “Ks”. Extraction using concentrated HC1solution (11.6 N)or aqua regia gave the same value of Ks with that determined with l N HC1solution. The TRP of the physical mixture of carbon and potassium carbonate was carried out by using a Tanmann tube as a reactor, where the mixture was placed at the bottom. The Tanmann tube is made of alumina and is shaped like a test tube. The quantitative change of potassium on the carbon sample was measured by means of Auger electron spectroscopy (AES). AES experiments were performed in an ion-pumped UHV chamber (PHI Model 50) at a base pressure of
