Energy & Fuels 1999, 13, 761-762
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Communications The Role of Surface Area in the NO-Carbon Reaction J. M. Calo,* E. M. Suuberg, and I. Aarna Division of Engineering, Brown University, Providence, Rhode Island 02912
A. Linares-Solano, C. Salinas-Martı´nez de Lecea, and M. J. Illa´n-Gome´z Deparamento de Quı´mica Inorga´ nica, Universidad de Alicante, Alicante, Spain Received November 4, 1998 In a recent article, Ruiz Machado and Hall1 (hereinafter referred to as RMH) reported on the reactivity behavior of a set of cellulose chars in NO and O2. It was observed that although the O2 reactivities seemed to correlate quite well with N2 BET surface area, the NO reactivities did not. From these results, it was concluded that, “‚‚‚there is no correlation between reactivity in NO and the BET surface area.... Clearly, the porosity in chars plays a different role in O2 and NO gasification.” These authors go on to state that, “This disagrees with the work of Illa´nGome´z et al.”,2 and further that, “The question arises as to why Illa´n-Gome´z et al.2 should have concluded that (NO) reactivity is proportional to surface area.” The issue of whether the NO-carbon reaction utilizes all the available or accessible porosity in a carbon, the details of the reaction mechanism, and how these vary with various parameters, are important issues from a number of fundamental and practical viewpoints, and, consequently, these are topics of current debate in the literature. In our view, however, the conclusions of RMH may be misleading to the general community of workers interested in this and related topics. Therefore, in the current communication we reiterate the significant evidence which exists that surface area is indeed a primary indicator of NO reactivity, and point out some potential pitfalls with the analysis and conclusions of RMH. Essentially, RMH call into question the observations based on data which originally appeared as Figure 8(b) in ref 2, presented here in modified form as Figure 1. These data were obtained for a very wide variety of carbons, including activated carbons produced from coals, olive stones, almond shells, phenolic resin, and activated carbon fibers, covering over an order of magnitude variation in surface area. Although there is significant scatter, it is apparent that the data are well characterized by a linear fit (R ) 0.92). Consequently, we believe that this answers the rhetorical question posed by RMH; that is, Illa´n-Gome´z et al.2 concluded that NO reduction activity is proportional to surface area because it is manifestly obvious in these data. In addition, in a more recent study on a wide range of carbons,4 it was also shown that BET surface area does indeed improve the correlation of * To whom correspondence should be addressed. (1) Ruiz Machado, W. A.; Hall, P. J. Energy Fuels 1998, 12, 958-962. (2) Illa´n-Gome´z, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; Calo, J. M. Energy Fuels 1993, 7, 146-154. (3) Teng, H. The NO-Char Reaction: Kinetics and Transport Aspects. Ph.D. Dissertation, Division of Engineering, Brown University, Providence, RI, 1992. (4) Aarna, I. A Study on Reaction Order and Micropore Utilization in the NO-Carbon Reaction. Ph.D. Dissertation, Division of Engineering, Brown University, Providence, RI, May, 1998.
Figure 1. NO reduction activity of a wide variety of activated carbons and chars at 873 K, 0.4 kPa NO, from Figure 8b of ref 2. To that data have been added three data points from Teng3 obtained for a char produced from phenolic resin, pyrolyzed at 1323 K for 1 h in helium, and reacted in 4.04, 6.06, and 10.1 kPa NO at 873 K. These latter points were “corrected” to 0.4 kPa, assuming first-order behavior in NO partial pressure, and are essentially superimposed on one another in the figure.
otherwise widely varying NO reactivities (cf. Figure S1 and discussion in Supporting Information). For the sake of comparison, the reactivities measured by RMH are also presented in Figure 1. To present them on this plot, the original reactivity values were corrected for NO partial pressure (assuming first-order behavior) and for temperature to 873 K (from 773 K), assuming an activation energy of 77 kJ/mol,3 for a net decrease of a factor of 3.2. In any case, on the scale presented, the reactivities of RMH are still significantly greater than the rest of the data. Two conclusions are readily apparent from Figure 1: (1) practically no surface area variation is exhibited by the RMH samples on this scale; and (2) the NO reactivities are considerably greater than those of the other carbons. We begin with the first point. In our opinion, at least part of the reason for the apparent disagreement with respect to the role of surface area in the NO-char reaction (if indeed there is any, as discussed further below) lies in the fact that if any experimental measurements are examined over a sufficiently narrow range of an independent variable, any correlation will be lost. The surface area range reported by RMH was 382-486 m2/g; a variation of only (12% from a mean value of 432 m2/g. On the scale of Figure 1, this range represents essentially a single value on the abscissa. If RMH wish to establish
10.1021/ef980244t CCC: $18.00 © 1999 American Chemical Society Published on Web 05/17/1999
762 Energy & Fuels, Vol. 13, No. 3, 1999
that there is no correlation with surface area in a general sense, then they must demonstrate this over a much larger range of surface area than reported. In point of fact, the data of RMH may admit to alternative interpretations. For example, the relative microporosity of the cellulose char samples, as indicated by the SAXS data presented in Figures 1 and 2 of RMH, appear to correlate reasonably well with the measured NO reactivities. (See discussion in Supporting Information.) Apparently, a significant portion of the pores responsible for NO reactivity at 773 K may not have been accessible to N2 at 77 K, and thus use of the latter might be too imprecise due to activated diffusion problems. Other adsorptives at higher adsorption temperature (for example, CO2 at 273 K5) should have been used to test for any such problems. The oxygen reactivity data of RMH exhibited a much better correlation with N2 BET surface area most probably because essentially the same pores that were accessible to N2 at 77 K were the ones primarily responsible for the much higher reactivity of the oxygenchar reaction. That is, under the experimental conditions examined, oxygen could not utilize the very finest pores which NO apparently could. This is the inverse of the conclusion of RMH that, “Both SAXS and TPD suggest that there are some mass transfer limitations that exclude some of the surface from being active (for NO).” Since the oxygen reactivity was far greater than for NO, it appears that this could have been the case with respect to oxygen, but perhaps not for NO. We now address the issue of the magnitude of the NO reactivities reported by RMH, which are significantly greater than for the rest of the carbons in Figure 1. There are other reports of NO reactivities of cellulosic chars of similar magnitude to those reported by RMH. For example, DeGroot and Richards6 reported 4.4 × 10-5 g/g s for a cellulose char (@ 773 K, corrected to 5.05 kPa), which is comparable to the higher values reported by RMH. Aarna and Suuberg7 report values of 4.7, 6.1, and 7.7 × 10-6 g/g s for CF-11 R-cellulose char at burnoffs of 10, 30, and 50% (@ 773 K, corrected to 5.05 kPa), which are quite similar to the three lower reactivities reported by RMH, although the corresponding surface areas are considerably greater by a factor of 2 to three. We can only speculate on the reasons for what appears to be the very high reactivities of these cellulosic chars in comparison to all the other carbons in Figure 1. The char samples investigated by Illa´n-Gome´z et al.2 and Teng3 were all treated at high temperature for an appreciable time (i.e., “stabilized”) prior to reaction, while the cellulosic chars of RMH were not. It is well-known that following carbonization, char samples contain a certain amount of highly reactive amorphous material, as a result of tar deposition and decomposition.8 The initial reactivity can be controlled by selectively reacting away this material first. There are a number of reports of such behavior in the literature, including even a recent, small angle neutron scattering study by Hall and co-workers for a phenolic resin char.9 The relative importance of such effects can vary considerably from char to char, depending on their nature and method of preparation. In the case of the initially oxidized, zero burnoff, cellulosic char used by RMH, such effects may be especially pronounced due to the good accessibility to any amorphous carbon material which is formed (unlike for a glassy carbon at zero burnoff). The fractal nature of the cellulosic char surfaces, as reported by RMH, is also consistent with a high degree of amorphous surface carbon. It is also well-known that for most char-gas reactions, very high transient reactivity is almost always observed.
Communications
This is due to a variety of factors, including the variation in oxygen surface complex population during the establishment of steady-state conditions, initial enhanced reactivity due to high concentrations of dangling bonds, etc., as well as the influence of disordered carbon, as discussed above. Very high reactivities were observed in all cases by Illa´n-Gome´z et al.2 of up to two to six times the eventual steady-state values. For this reason, all the reactivities from that paper presented in Figure 1 were determined at 80 min following initiation of reaction, at which time all the samples clearly exhibited steady-state behavior (although the various samples required a wide variety of times to attain the eventual steady-state reaction rate). Although RMH reported that they also waited for steady-state conditions, in similar TGA measurements Teng3 observed transient periods of high reactivity for the NO-char reaction on phenolic resin char samples, manifested by practically linear mass loss curves characteristic of pseudo-steady-state conditions. This transient period of high reactivity was always followed by a significantly slower steady-state gasification rate. Consequently, if one is not sufficiently patient, the apparent “steady-state” value recorded could easily be transient in nature, as well as anomalously high. RMH used very small samples (2-5 mg) at a low temperature (773 K). Under these conditions, the reproducibility and precision of the measurements may be problematic. For example, for a 2 mg sample and a reactivity of 0.018 g/g h (5 × 10-6 g/g s), in 1 h in the TGA the mass loss would have been only 36 µg. This is much too small to allow for adequate accuracy in reactivity measurements; a larger sample size and higher temperature would have been more advisable. Another potential problem with the use of mass loss data in this case is that it is known that NO can cause the desorption of other, preexisting oxygen surface complexes.2 The pretreatment and preparation conditions used by RMH are consistent with a relatively large population of such surface oxygen complexes. It is conceivable that a significant portion of the mass loss could have been attributable to this source and not NO reduction. In this case, the direct measurement of NO reduction would have been a better choice. In summary, we feel that RMH have not proven their theses that NO reactivity does not correlate with surface area, and that char porosity plays a different role in NO and O2 gasification. Their range of BET surface areas is insufficient to provide a statistically significant variation, and their reactivity measurements may not be sufficiently accurate to support these conclusions. Moreover, N2 BET surface areas may not have been suitable for correlating NO reactivity data for the cellulosic char samples used. Their data may actually support the inverse conclusion; i.e., that NO reactivity correlates well with (the “relevant”) surface area for their samples. Supporting Information Available: Additional NO reactivity data correlated by surface area, and a discussion of the SAXS data of RMH. This material is free of charge via the Internet at http://pubs.acs.org.
EF980244T (5) D. Cazorla-Amoro´s, D.; Alcan˜iz-Monge, J.; De la Casa-Lillo, M. A.; Linares-Solano, A. Langmuir 1998, 14, 4589-4596. (6) DeGroot, W. F.; Richards, G. N. Carbon 1991, 29, 179. (7) Aarna, I.; Suuberg, E. M. Private communication, unpublished results, 1998. (8) Rodriguez-Reinoso, F. In Introduction to Carbon Technologies; Marsh, H., Heintz, E. A., Rodriguez-Reinoso, F., Eds.; University of Alicante Publications: Alicante, Spain, 1997; Chapter 2. (9) Antxustegi, M.; Hall, P. J.; Calo, J. M. Colloid Interface Sci. 1998, 202, 490-498.
