Energy & Fuels 1988,2, 719-720
relatively little has been reported concerning the decomposition of NH, over quartz. This reaction was first observed during the early part of this century, but no activation energy was given." A subsequent study,lg however, examining the NH3 decomposition over Cu and Fe surfaces, included the contribution to this reaction from the quartz reactor, and their data is in reasonable agreement with this present investigation. In order for the determined rate constants to be applicable in a wider context, a knowledge of the available surface area of the Si02 involved in the reaction is necessary. The rate constants observed can thus be divided by the calculated active quartz surface area (1812 cm2)and expressed in terms of reciprocal seconds per unit area of catalyst (k 9. Using a smaller bed of quartz sand (4.00 g) a t 900,920 and 940 "C allowed a check of the K'and E, values from the 8.00-g quartz sand bed at these temperatures to be undertaken. Although k 'showed some deviation (ca. 9% error) possibly originating from uncertainties in the internal surface area of the reactor, the E, determined (150 kJ mol-l) was in close agreement. The appearance of Figure 3 indicates the absence of any change from a kinetic-controlled to a diffusion-controlled regime often observed in heterogeneous reactions.22 This is partly to be expected since the nonporous nature of the quartz sand particles would prevent any diffusion processes within the particles. Furthermore, since the rate estimated
for film mass transfer was considerably greater than the observed reaction rate,22>23 film resistance effects were neglected.
Conclusion The heterogeneous catalyzed decomposition reaction of NH, over quartz sand has been shown to occur with a first-order rate constant k'between 2.89 X lo-* and 15.56 X lo4 s-l cm-2 over the temperature interval 840-960 OC. For the most part, previous studies involving solid-catalyzed reactions pertaining to fluidized-bed combustion have not incorporated details of the active surface area of the solid c a t a l y ~ t ; ~thus, - ~ . ~this ~ precluded an estimation of the relative importance of NH, decomposition over quartz sand under such conditions. An awareness of the rate of this reaction is however of significance in the determination of kinetic data of other gas-solid reactions involving NH3 where quartz materials are used in the apparatus. Acknowledgment. We express our gratitude to the Swedish National Energy Administration for Financial support and to Prof. H. Wallman and S. Ghardashkani for helpful discussions. Registry No. NH,, 7664-41-7; quartz, 14808-60-7. (23)Petrovic, L. J.; Thodos, G. Znd. Eng. Chem. Fundam. 1968, 7, 274-280. ~ . . (24)Furusawa, T.; Kunii, D.; Tsujimura, M. Proc. Znt. Conf. Fluid. Bed Combust. 1982, 7th, 525-533. (25)Iler, R. K.; The Chemistry of Silica; Wiley: New York, 1979;pp 637-644. ~
(21)Sjbberg, J.; Rundgren, K.; Oaten-Sacken, J.; Pompe, R.; Larsson, B. Sci. Ceram. 1988,14, 205-210. (22)Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; M.1.T Press: London, 1970; pp 11-92.
719
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L'ommunxat6ons Pore Structure of Illinois No. 6 Coal
Sir: The surface area and pore structure of coals are among their most important physical properties. They are well studied, and good reviews are available.14 We here present data on the adsorption (BET) of C02and a set of aliphatic hydrocarbons on Illinois No. 6 coal from the Argonne Sample Bank. These data are inconsistent with the generally accepted structure of coal pore systems, that is, an interconnected network of bottle-necked pores. They suggest that most pores are closed and to reach them an adsorbate must diffuse through solid coal, rather than through a pore network. C02 is soluble in ~ o a l s(however ~*~ see ref 7 for a conflicting view), diffuses rapidly through them, and we believe gives approximately accurate values for the total pore surface areas. Hydrocarbon gases that (1)Marsh, H.Carbon 1987,25,49-58. (2)Mahajan, 0.P.In Coal Structure; Meyers, R. A., Ed.: Academic: New York, 1982. (3)Mahajan, 0.P.; Walker, P. L. Jr. In Analytical Methods for Coal and Coal Products, Karr, C., Jr., Ed.; Academic: New York 1979;Vol. 1. (4)Sharkey, A. G. Jr.; McCartney, J. T. In Chemistry of Coal Utilizatton, Second Supplementary Volume; Elliott, M. A., Ed.; Wiley: New York, 1981. (5)Reucroft, P. J.; Patel, H. Fuel 1986, 65, 816-820. (6) Reucroft, P. J.; Sethuraman, A. R. Energy Fuels 1987,1, 72-75. (7)Stacy, W.0.;Jones, J. C. Fuel 1986, 65, 1171-1173.
0887-0624/88/2502-0719$01.50/0
are not soluble in coals can reach a much smaller portion of the coal's pores and thus report a much smaller surface area. Coals are molecular sieves, and it has been universally accepted that they contain an interconnected pore network of high surface area, the pores having constricted openings that result in the observed size discrimination.14 This pore structure was first proposed by Bond8 and has been generally accepted. The surface is the gateway to the coal, and all reagents either pass through it or react at it. It is a most important aspect of coal's physical structure. We measured the surface area of Illinois No. 6 coal (77.8% C, 5.7% H, 9.1% O(diff), 1.4% N, 16.2% ash) from the Argonne Sample Bankg using standard multipoint BET te~hniques,~JO a set of alkanes, and C02. Six hours equilibration time between individual points was allowed. The data and measurement conditions are presented in Figure 1. All data are duplicates and the worst precision observed was *3 m2/g. The excellent relationship (cor~______
(8) Bond, R. L. Nature (London) 1956, 178, 104-105. Bond, R. L.; Spencer, D. H. T. Brennst.-Chem. 1956,37, 233-234.
(9)Samples were dried by evacuation to lo-' Torr at room temperature in the BET apparatus. They were otherwise untreated. Further characterization data for this coal are available from Dr. Karl Vorres at the Argonne National Lab. (10) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. SOC.1938, 60, 309-319.
0 1988 American Chemical Society
720 Energy & Fuels, Vol. 2, No. 5, 1988
Communications
0.0
-1.0 h
,M
Y
0
E
.\
E
-
M
-2.0
- t a t . 5 m2/p
-3s
1.30
1.40
1.50
log Cross Sectional Area
(1
1
I
Figure 1. Dependence of surface area (monomolecular surface coverage) on adsorbate cross-sectional area.
relation coefficient 0.9996) between adsorbate size and coal surface area is striking, and the steepness of the slope is surprising. Some of the measurements were carried out at different temperatures, which seems not to have contributed to the scatter, possibly because the activation energy for diffusion is very small. The coal surface areas obtained by using C02,ethane, and cyclopropane, all of similar cross-sectionalarea, cannot be rationalized with a constricted pore model. Ethane (cylindrical) and cyclopropane (planar) have different shapes and either slitlike or cylindrical pore openings should discriminate between them. Both report the same coal surface area. Ethane and COz have closely similar shapes (cylindrical) and similar diameters (ethane's is bigger by 16%) but give very different coal surface areas. Diffusion through constricted openings cannot be responsible for this size discrimination. To see whether adsorption on the coal surface might be responsible for the observed dependence of measured surface area on adsorbate size, a fractal analysis of the data was performed by using the approach developed by Avnir and Pfeifer.l1-l5 A plot of log N (monolayer surface coverage) vs log Q (cross-sectional area of the adsorbate) has a slope of - 0 1 2 where D is the fractal dimensionality of the solid and must be between 2 and 3.*13 The plot in Figure 1 yields a fractal dimensionality of 23.5, an impossible value. The observed surface areas are not controlled by a coal surface property. We believe the dependence of the observed coal surface areas on adsorbate size is due to sharply differing diffusion rates of the adsorbates through solid coal to reach the pores. The extraordinarily steep dependence of measured surface area on adsorbate size is similar to the dependence of diffusion rates through glassy polymers on molecular size.ls The hydrocarbons reach only a small portion of
(11)Pfeifer, P.; Avnir, D. J. Chem. Phys. 1983,79,3558-3565. (12)Avnir, D.;Farin, D.; Pfeifer, P. J. Chem. Phys. 1983, 79, 3566-3571. (13)Avnir, D.;Farin, D.; Pfeifer, D. Nature (London) 1984, 308, 261-263. (14)Farin, D.;Volpert, A.; Avnir, D. J. Am. Chem. SOC.1985,107, 3368-3370. (15)Farin, D.;Peleg, S.; Yavin, D.; Avnir, D. Langmuir 1985, I , 399-407. (16)Van Krevelen, D.W. Properties of Polymers; Elsevier: New York, 1976;pp 403-425.
the pores, that portion depending on their diffusion rates. Those diffusion rates fall very rapidly as molecular size increases, as expected for diffusion through a glassy polymer. Illinois No. 6 coal is a g h y polymer under these c~nditions.'~J*COPis soluble in coals, will diffuse rapidly reaching closed pores, and so will report the total pore surface area. These surface areas will be slightly overestimated due to the solubility of C 0 2 in the coal, but this effect is thought to be small (although this may be becoming contr~versial).'.~~ Very small molecules like He can diffuse reasonably rapidly through coals, resulting in accurate He densities and valid pore volumes determined from them. Apparently, COPis sufficiently polar to have an efficient interaction-based diffusion mechanism. To verify slow diffusion, ethane uptake was followed for 130 h at 25 "C. Adsorption was continuous during this time, and equilibrium had not been reached when the experiment was terminated. The only other explanation of these data is that coal surface areas are not being measured, just the solubility of the probe molecules in the coal. This argument is controverted by recent neutron diffraction studies of coal pores and by the agreement between COzcoal surface areas and those obtained from low-angle X-ray scattering experiment~.~~!~~ We propose that most pores in this coal are closed. They cannot be reached by diffusion through the pore network but can only be reached by diffusion through solid coal. COPgives accurate total surface areas because it dissolves in and rapidly diffuses through coals, reaching all the pores. This surface area is irrelevant to materials that are slightly soluble or insoluble in coals for these can reach only a small portion of the pores. Thus, diffusion rate controls the pore size determined. For the large molecules involved in direct liquefaction and most organic reactions, the effective surface area of this coal is very small, only a few square meters per gram. The consequences of this structure model for coal processing and reaction are significant. This coal cannot be considered a high (hundreds of square meters per gram) surface area material that is easy to access. It will usually be a low surface area, impermeable solid. The advantages of fine grinding are obvious. Using solvents that will cause coal to become rubbery rather than glassy will enhance diffusion and thus reactivity. Acknowledgment. Helpful conversations with Om Mahajan and Eric Suuberg are gratefully acknowledged. This paper was prepared with the support of the U. S. Department of Energy, Grant No. De-FG22-85PC80527. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE. Registry No. C02, 124-38-9;C2H6, 74-84-0;cyclopropane, 75-19-4.
(17)Brenner, D.Fuel 1985,64,167-173. (18)Lucht, L. M.;Larson, J. M.; Peppas, N. A. Energy Fuek 1987,I, 56-68. (19)Spitzer, Z.;Ulicky, L. Fuel 1977,55, 212-224. (20)Winans, R.E.; Thiyagarajan, P. Energy Fuels 1988,2,355-358.
John W. Lamen,* Patrick Wernett Department of Chemistry, Lehigh University Bethlehem, Pennsylvania 18018 Received February 15, 1988 Revised Manuscript Received June 21, 1988
