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Secondary Interactions upon Thermal Desorption of Surface Oxides from Coal Chars Peter J. Hall and Joseph M. Calo* Chemical Engineering Program, Division of Engineering, Brown University, Providence, Rhode Island 02912 Received January 3, 1989. Revised Manuscript Received February 27, 1989
A comparison of temperature-programmed desorption (TPD) spectra from oxidized coal char samples, both with and without “CO loading”, is used to show that secondary reaction between CO and oxygen surface complexes can be a significant source of COz in TPD-like experiments. TPD of “thermally cleaned” and both “CO-loaded” and TOz-loaded”char samples c o n f i i s this conclusion and shows that CO can also be strongly chemisorbed on coal chars and/or formed by secondary reaction of C02with free active sites. It is suggested that these secondary mechanisms may be a t least partially responsible for the apparent high-temperature CO peak observed a t about 1250 K in typical TPD experiments. The degree to which secondary interactions affects resultant TPD spectra seems to be a complex function of the carbon pore structure, extent of surface coverage, and the integrated time-temperature history of the various species involved and, possibly, even the levels and types of impurities. It is concluded that secondary interactions can significantly affect TPD spectra of oxidized carbonaceous solids and that such spectra may not necessarily be reflective of the true original state of surface oxygen complexes.
Introduction Temperature-programmed desorption (TPD) has become a standard technique for the analysis of the state of adsorbed surface species’” and has also found application in the investigation of surface oxygen complexes on carbonaceous surfaces?*s When such complexes are subject to a program of increasing temperature, they desorb primarily as CO and COP. Recently, Hall et al.,B in performing linear TPD on various oxidized coal char samples at variable heating rates from 20 to 300 K/min, noted that the total integrated amount of COz evolved is a function of the heating rate and exhibits a broad maximum in the vicinity of about 100 K/min. Some representative data exhibiting this effect are presented in Figure 1. (It is noted that the curves in this figure are simply interpolations of the data points and have no other significance.) Oxygen balances for these chars show that this phenomenon amounts to a variation of the ratio of the evolved oxides of carbon, while the total amount of evolved oxygen remains constant. For example, the two Wyodak coal char runsa t 20 and 100 K/min were conducted on samples obtained from the same batch of char oxidized in a TGA for 12 h under 0.1 MPa of oxygen. The total evolved oxygen balance (i.e., CO + 2COZ)closes to within 8% for these two runs, whereas the ratio 2COZ/(C0 + 2CO2) increases from 0.22 to 0.36 for the 20 and 100 K/min runs, respectively. The total amount of chemisorbed oxygen recovered in the two runs was calculated to be 93% and 101%, respectively, with an estimated error on the order of *5%. In other words, practically all the chemisorbed oxygen was recovered as oxides of carbon. This behavior was attributed to secondary reaction via co + C(0) Cf + coz (1) where CO represents “free” carbon monoxide resulting from the desorption of a surface oxygen complex, C(0) is a surface oxygen complex, and Cf is an unoccupied surface active site. In view of the high carrier gas flow rates that
* To whom correspondence should be addressed.
were used in this work (see Experimental Section), it is quite certain that these reactions do not involve external bulk gas-phase species, but rather they must occur within the char pore structure (most probably in micropores, due to the relatively higher residence times and larger surface area-to-pore volume ratio) between desorbed oxides of carbon during transport out to the bulk phase and on both occupied and unoccupied active surface sites. The extent to which reaction 1 occurs seems to be a complex function of the carbon pore structure, extent of surface coverage, and the integrated time-temperature history of the various species involved and, possibly, even the levels and types of impurities. Obviously, this phenomenon complicates the interpretation of TPD spectra and indicates that considerable care must be taken to insure that resultant energetics are truly reflective of the original state of oxygen complexes on carbon surfaces, if indeed such an interpretation is the objective of the experiment. There has been speculation over a number of years concerning the possibility of secondary reactions occurring during the transport of gasification products within the pore structure of gasifying carbons. The forward and reverse reactions of reaction 1are well-known steps in the oxygen-exchange mechanism for COz gasification’-” at the relatively high temperatures required. Menster and Erg” (1)King, D.A. Surf. Sci. 1975,47, 384. (2) Ehrlich, G.Ado. Cat. 1963,14,255. (3) M e a d , P. A.; Hobson, J. P.; Komelsen, E. V. The Physical Basis of Ultrahigh Vacuum, 1968,Chapman and H a k London. . (4) ..Kyotani, T.; Zhang, Z.; Hyashi, S.; Tomita, A. Energy Fuels 1988,
2, 136.
(5) Causton, P.; McEnaney, B. Fuel 1985, 65, 1447. (6) Hall, P. J.; Calo, J. M.; Lilly, W. D. Proc. Zntl. Conf. Procedings of the International Conference on Carbon, Carbon ‘88, McEnaney, B.,
Mays, T. J., Eds.; IOP Publishing Co.: Bristol, UK, 1988;p 77. (7)Mentaer, M.; Ergun, S. J. Bull.-US. Bur. Mines 1973, 664. (8)Strange, J. E.; Walker, P. L., Jr. Carbon 1976, 14, 345. (9) Ergun, S. J. J. Phys. Chem. 1956, 60, 480. (10)Ergun, S.J. Bull.-US. Bur. Mines 1961, 598. (11)Reif, A. E. J.Phys. Chem. 1952, 56, 785.
08S~-0624/S9/2503-0370$01.50/0 0 1989 American Chemical Society
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concluded that secondary reactions were not important for the experimental conditions of the study. Prima facie then, there appears to be a fundamental contradiction among the preceding cited results. More information is needed concerning secondary reactions, such as reaction 1, in terms of their relative importance in both isothermal and nonisothermal TPD-type experiments and the conditions under which they become important. The possibility of readsorption of “desorbed” species, diffusing within the char porosity, with surface sites introduces a further complication into the interpretation of TPD spectra. Recently, Marchon et al.l9 have shown that CO can chemisorb on carbon surfaces to form complexes that are stable to high temperatures (i.e., >1200 K). The detailed mechanistic features of this interaction are still unclear, but it has been shown to be favored at high temperatures. CO adsorption on carbon has been measured and speculated upon for many years.ls It is the basis of the primary alternate mechanism for the well-known inhibition of COz gasification by CO. However, the complication introduced into TPD spectra is that CO desorbed at lower temperatures may be readsorbed to form a complex that is stable to higher temperatures, thereby effectively “shifting” the apparent energetics of desorption of the original CO-liberating surface complexes. To summarize then, in the current paper we focus on the convoluting effects on TPD spectra of carbon oxides of two “reinteraction” mechanisms: i.e., reaction 1and CO chemisorption. The primary objective is to show that caution should be exercised in the interpretation of TPD spectra from coal chars and, possibly, other carbonaceous materials as well.
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Figure 1. Total integrated C02evolved during TPD as a function of heating rate for various coal chars prepared in the manner described in the text from coals obtained from the Argonne Premium Coal Sample Bank. The CWM are simply interpolations of the data points and have no other significance. The legend indicates the parent coal and the oxidation temperature in O C in 0.1 MPa of oxygen for 12 h. Key: SL = Stockton-Lewiston; Wd = Wyodak; Pt = Pittsburgh No. 8; the designation “D” indicates a sample demineralized according to the precedure reported by Morgan et
determined AGO = -71 kJ/mol for reaction 1as written; i.e., it is exoergic and, therefore, favored a t lower temperatures. Similar results have been reported by Strange and Walker,8 among others. However, there also seems to be some evidence which suggests that this reaction is unimportant a t lower temperatures. For example, Vastola et a1.12 concluded that CO gas did not react with surface oxygen complexes under their experimental conditions. This conclusion was based on the results of an isothermal batch experiment in which partially gasified (11.4% weight loss) Graphon was reacted in a mixture of le1602and Cl80 a t 778 K. As the oxygen reacted with the carbon, a mixture of Cleo and Cle-l6Oz was evolved, together with the formation of a stable oxygen surface complex. However, the amount of Cl80 originally charged to the reactor did not change a t all over an 8-h period. Since no measurable C1&’SO2was formed, Vastola et al.12concluded that CO did not react with oxygen surface complexes under these conditions. More recently, the importance of reaction 1to the results and interpretation of TPD experiments was considered by Otake13for a highly microporous phenolic resin char. In this work, two techniques were used to study the potential effects of the forward and reverse Boudouard reactions. One consisted of performing TPD a t two heating rates of 2 and 5 K/min. These results revealed essentially no difference in the total amount of COBevolved. In another set of experiments, Otake13 carried out TPD on chars containing different amounts of surface oxide. This was achieved by reoxidizing a partially cleaned surface, (i.e., heated to 1000 K and outgassed, such that a significant amount of high-temperature-stable complexes remained) and comparing the total amount of COzliberated from this sample during T P D with that of a “normally” oxidized sample. Since no significant difference was observed in the total amount of COzevolved from these samples, it was (12)Vastola, F.J.; Hart, P. J.; Walker, P. L., Jr. Carbon 1964,2,65. (13)Otake, Y. Ph.D. Dissertation, 1986; The Pennsylvania State University. (14)Marchon, B.;Camarm, J.; Heinemann, H.: Somoriai,G. A. Carbon 1988, 26,507. (15)Gabby, J.; Long, F. J.; Sleightholm,P.; Sykes, K. W. Roc. R. SOC. London, A 1948,103,357. (16)Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. 1978,Carbon 16, 35. (17)Calo, 1. M.; Perkins, M. T. Carbon 1987,25,395. (18)Mehotra, S.P. Coal Sci. Technol. 1987,II,501.
Experimental Section For all the experiments reported here, chars were produced from two co& obtained from the Argonne Premium Coal Sample Bank; Le., Wyodak subbituminous and Pittsburgh No. 8, a me-
dium-volatile bituminous coal. These coals were carbonized by heating in an atmosphere of ultrahigh-purity helium to 1273 K and holding at this temperature for 2 h. The resultant 77 K N2 BET surface areas were 160 and 2 m2/g for the freshly pyrolyzed Wyodak and Pittsburgh No. 8 coal chars, respectively. For all the TPD experimenta reported here, the Pittsburgh No. 8 coal char was burned-off at 723 K to 11.5% weight loss in 0.1 MPa oxygen. The N2 BET surface area of this latter, partially gasified char was 90 m2/g. The TPD reador apparatus was constructed from a high-purity silica tube, 1-cm inside diameter, within which a close-fitting, circular silica sinter was used to support the sample. The carrier gas was ultrahigh-purity helium, which was passed over the sample in downflow. Heating was accomplished electricallyvia nichrome wire wrapped around the outer silica tube, powered by a highcurrent variable transformer. The heating regimen was controlled by a microcomputer. The resultant TPD reactor had a low thermal capacitance,which allowed linear heating rates of up to 500 K/min. A small portion of the main carrier flow was leaked into a quadrupole mass spectrometer. The output of the mass spectrometer was fed to a microcomputer,which provided for multiple species detection via mass programming. In order to minimize the delay between species desorption and detection and prevent potential reinteraction of external bulk gas-phase species with the char sample, a high helium carrier gas flow rate of 1.5 L/min (correspondingto a linear superficial velocity in the empty TPD reactor tube of 32 cm/s) was used, as well as a minimal length of small-bore sampling tubing (1/16-in.0.d.). The resultant high velocity in the sampling tubing minimized dispersion of the sampled gas. In addition, special attention was paid to the elimination of stagnant volume in the vicinity of the sampling (f9)Marchon, B.; Tysoe, W. T.; Carrazza, J.; Heinemann, H.; Somorjai, G. A. J. Phys. Chem. 1988,92,5447.
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point such that there was no noticeable integration of the desorption spectrum. The basic experimental procedure for TPD was as follows. The char samples were first “thermally cleaned” in a TGA at 1273K in ultrahigh-purity helium, and the samples were then cooled in helium. Oxidation or gasification was carried out in the TGA apparatus in 0.1 MPa of oxygen at the required temperature. Part of the oxidized sample (typically about 10 mg) was transferred to the TPD apparatus, the reaction chamber flushed with ultrahigh-purity helium, and the TPD performed. The resultant mass spectrometer peak heights were normalized for sample size and mass spectrometer sensitivity as determined by calibration with a known partial pressure of an equimolar mixture of CO and COP For most of the experiments reported here, a heating rate of 300 K/min was used since our previous dataBindicated that this rate allows reasonable peak resolution,while minimizing the time available for diffusion of “soak” gas out of the char pores during the gas-loading experiments described below. The results are all presented by using arbitrary gas rate desorption scales, but due to the calibration procedure described above, the relative rates of gas evolution are directly comparable among all the data presented here. Repeated experiments with char samples obtained from the same batch indicate that the reproducibility of gas desorption rates is approximately &lo%. This error is attributable to a combination of effects arising primarily from sample inhomogeneity, sample size, and mass spectrometer calibration. Since for the purposes of the current paper we are primarily concerned with qualitative spectral features, rather than quantitative analysis,only representative spectra are presented here instead of spectra averaged over multiple experiments. The procedure for filling the char pores with *soak” gas was as follows. About 10 mg of char was placed in the TPD reactor, and the sample was heated in an atmosphere of ultrahigh-purity helium for 3 h at 473 K. The “soak” gas (either CO or C02)was then introduced into the reactor, and the temperature was maintained at 473 K for an additional 2 h. The sample was then allowed to cool in the particular gas used, and the reaction chamber was flushed with helium before commencing with the TPD. In order to be sure that intrapore CO introduced in the preceding manner did not produce COzvia the CO-CO reaction (Le., the reverse Boudouard reaction), a control or “blank” experiment was conducted with a sample of the same Pittsburgh No. 8 coal char that had been “thermally cleaned” of carbonoxygen surface complexes (asdescribed previously) prior to f i g the pores with CO as indicated above. Three distinctly different types of coal char samples were prepared for the TPD experiments in the current work. Samples of Pittsburgh No. 8 coal char that were burned off in 0.1 MPa of oxygen at 723 K and then subjected to TPD with no intervening treatment are referred to as “normal” oxidized char samples. Samplesthat were burned off under the same conditions and then subjected to the ”s& gas procedure described above are referred to as “loaded” oxidized char samples. Samples that were bumed off and then “thermally cleaned” and subjected to the “soak” gas procedure are referred to as “thermally-cleanedand loaded” char samples. These latter samples were prepared primarily for “blank” experiments. In order to investigate the importance of the forward step of reaction 1under nonisothermal, TPD-like conditions, TPD spectra of a “normal” oxidized Pittsburgh No. 8 char sample were compared to those of a TO-loaded”oxidized char sample. “Normal” and “C02-loaded”oxidized char spectra were also compared in an attempt to explore the reverse step of reaction 1. TPD runs with “thermally cleaned”,nonoxidized “blank” coal char samples produced no discernible levels of carbon oxides. Therefore, the observed carbon oxides produced during TPD arose exclusively from oxygen surface complexesformed upon oxidation. In order to investigate the behavior of reaction 1under isothermal conditions, a variation of the experiment performed by Vastola et a1.12 was conducted in which CO was admitted isothermally into a TGA containing a sample of preoxidized Wyodak coal char. The 80-mg char sample was first “cleaned” of any adsorbed surface oxides by heating it in a flowing atmosphere of ultrahigh-purity helium at 1273K for 2 h. It was then cooled to 523 K and oxidized for 2 h in 0.1 MPa of pure oxygen. At this
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Figure 3. CO and COzTPD spectra obtained at 300 K/min from a gasified and TO-loaded” Pittsburth No. 8 coal char sample. The coal char was burned off to 11.5% weight loss in 0.1 MPa of oxygen at 723 K and then “soaked” in 0.1 MPa of CO at 473 K, as described in the text.
temperature, the predominant reaction was chemisorption of oxygen as surface complexes, rather than gasification, since only a net weight increase was observed. The oxygen was then flushed from the apparatus and the sample was heated to 773 K in ultrahigh-purity helium. Then, 10 kPa of CO was then introduced into the system, and the sample weight and the amount of COz in the outlet carrier gas were recorded as functions of time.
Results The results of a 300 K/min TPD on 11.5% gasified (in 0.1 MPa of oxygen at 723 K) “normal” Pittsburgh No. 8 coal char are presented in Figure 2. Although the primary focus of the current paper is not on the general aspects of carbon-oxygen complex TPDs, these results are noted to be fairly typical for oxygen-gasified chars, and are remarkably similar to those of Otake13 and Marchon et al.,14 among others, obtained with very different carbons. The low-temperature C02peak has been attributed to primary C02-producing oxygen complexes, e.g., lactones (Marchon et all4) and/or carboxylic acid anhydrides (Otake13). Hall et al.? however, have claimed that the higher temperature COz peak from porous coal chars is at least partly due to secondary reactions of CO according to reaction 1,because most of the variation in total evolved C02with heating rate was observed to occur in this region of the TPD spectrum. The CO production consists of a peak centered at about 1000 K, and a higher temperature shoulder. Tremblay et al.16 and other workers have explained this type of behavior by postulating two distinct kinds of CO-producing groups. Marchon et al.14 observed evidence for three peaks or shoulders and speculated that these features are due to semiquinone groups on three differents types of graphitic edge sites, i.e., zigzag, armchair, and other more complex structures.
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Figure 4. Comparison of CO TPD spectra obtained at 300 K/mh from both “normal” (11.5% burn-off) and “CO-loaded”oxidized Pittsburgh No. 8 coal char samples taken from Figures 2 and 3,
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Figure 5. Comparison of C02 TPD spectra obtained at 300 K/min from both “normal” (11.5% burn-off) and “CO-loaded” oxidized Pittsburth No. 8 coal char samples from Figures 2 and 3, respectively.
The TPD spectra for the partially gasified, “CO-loaded” Pittsburgh No. 8 coal char is presented in Figure 3, and for ease of comparison, the differences in the evolution rates of CO and COz between the “normal” and “COloaded” oxidized char samples are presented in Figures 4 and 5, respectively. The low-temperature CO peak in Figure 3 is attributable to diffusion of free gaseous CO “soak“ gas out of the char. Apart from this feature, the major difference between the TPD spectra in Figure 4 for mass number 28 is that for the “CO-loaded” oxidized coal char the principal peak is broadened and the high-temperature shoulder is significantly less distinct. This result may be interpreted in terms of the presence of additional CO-producing surface oxygen complexes at the same temperature as the shoulder observed in the TPD spectrum for the “normal” oxidized coal char. This hypothesis is considered further below. Figure 5 shows that the major difference in the evolution rates of C 0 2 between the “normal” and “CO-loaded” oxidized chars is the relatively large increase in the lower temperature COz peak; the higher temperature C02 peak does not appear to be affected as much by the CO loading. Since the only difference between the treatment of the two samples is the low-temperature CO exposure, the increase in C 0 2 can only be due to secondary reactions between pore-trapped CO and either oxygen surface complexes or other pore-trapped CO molecules. (It is noted that the pore-trapped CO may be present in either a “free” or an associated/adsorbed form.) Figure 6, which shows the TPD spectra from “thermally-cleaned and CO-loaded” Pittsburgh No. 8 char, resolves this question. As can be seen, a negligible amount of low-temperature C02 is produced in this case, certainly not enough to explain the large
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Figure 6. CO and C02TPD spectra obtained at 300 K / m h from burned off, “thermally-cleaned and CO-loaded”Pittsburgh No. 8 coal char samples. The coal char was burned off to 11.5% weight loss in 0.1 MPa of oxygen at 723 K, thermally degassed at 1273 K for 2 h, to remove all carbon-oxygen surface complexes, and then “soaked”in 0.1 MPa of CO at 473 K, as described in the text.
increase observed for the “CO-loaded” over the “normal” oxidized char. Therefore, the additional low-temperature C 0 2 formed from the “CO-loaded” oxidized char sample could only have been formed by the interaction of intrapore CO with oxygen surface complexes on carbon and/or impurity sites. It should be carefully noted at this point that although the lower temperature C 0 2peak is the one primarily amplified by “CO-loading”, due to secondary reaction via reaction 1, for the “normal” oxidized char TPDs it is the second, or higher temperature C02 peak that is affected. This is because free CO “soak” gas is present in the char porosity from the inception of the TPD to about 650 K (see Figure 6). Above this latter temperature, the CO “soak” gas has already reacted with surface complex, chemisorbed, or diffused out of the porosity. In TPD of “normal” oxidized samples, however, C02 cannot be produced by reaction 1 until there is a sufficient density of CO in the char porosity, and this does not occur until the primary CO-producing complexes are undergoing desorption, i.e., at temperatures greater than 650 K. It is also noted from Figures 4 and 5 that both evolved CO and C02fall off more rapidly in the TO-loaded” TPDs then in the “normal” TPDs. This is consistent with scavenging of surface oxygen complexes early in the TPD by free CO, which then reduces the surface complex population available for desorption as CO a t the higher temperatures late in the TPD. C02 will exhibit a similar effect if the higher temperature C02 is primarily due to reaction 1.
The only consistent conclusion from the results presented in Figures 4-6 is that CO does indeed interact significantly with surface oxygen complexes formed on carbon and/or impurity sites under the experimental conditions examined. Although the relative importance of the role of inorganic impurity sites is not known a t this time, it is pointed out that in Figure 1 the demineralized Wyodak coal char exhibited the most severe variation of secondary C02 production with heating rate, with absolute amounts exceeding that of all the coal chars examined. Therefore, if catalysis by and/or oxygen originating from impurity sites were a predominant mechanism for secondary C02 production in this coal char, it is expected that this effect should have been reduced, not accentuated as was observed. We explain the observed behavior in terms of the effects of the acid demineralization treatment in significantly increasing the porosity of this coal char. The hypothesis that secondary C02 production is associated with char porosity is also supported by the fact that the amount
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374 Energy & Fuels, Vol. 3, No. 3, 1989
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Figure 7. CO and COz TPD spectra obtained at 300 K/min from burned off, “thermally-cleaned and C02-loaded”Pittaburth No. 8 coal char samples. The coal char was burned off to 11.5% weight loss in 0.1 MPa of oxygen at 723 K, thermally degassed at 1273 K for 2 h, to remove all carbon-oxygen surface complexes, and then “soaked”in 0.1 MPa of COSat 473 K, as described in the text.
of COz produced from the least porous coal char, freshly pyrolyzed Pittsburgh No. 8 (Nza t 77 K and COz a t 194 K BETS of 2 m2/g) does not vary with heating rate. A comparison between the TPD spectra of “normal” and TOz-loaded”,(not shown here) oxidized coal char samples indicated no significant additional production of CO at low temperatures via the reverse step of reaction 1. This result can be understood in terms of the reverse reaction being strongly endothermic and, therefore, not favored a t lower temperatures. In addition, in the regime where the intrapore COz concentration is highest, prior to loss via diffusion (i.e., a t low temperatures), the concentration of unoccupied active sites is quite low as a result of the preoxidation. Consequently, this reaction is quite difficult to observe by using the present experimental procedure. Figure 6 also exhibits another feature that is a potential complicating factor in the interpretation of TPD spectra: i.e., the existence of strongly chemisorbed CO, as indicated by the very high desorption temperatures evident for CO. As mentioned in the Introduction, this behavior has been recently confirmed by Marchon et al.,Ig who suggested that the high-temperature CO produced upon exposure of polycrystalline graphite to CO is a species “formed by incorporation of the CO in the graphite lattice”.lg Figure 6 shows that the maximum rate of desorption of these species corresponds to the high-temperature shoulder for CO in Figure 2. Therefore, the possibility exists that the entire high-temperature CO shoulder need not necessarily arise from the desorption of stable, carbon-oxygen complexes produced by primary oxidation prior to TPD, but may also include a significant amount of complexes formed by rechemisorption of intrapore CO following thermal desorption. If this occurs to a significant extent, it would represent an experimental artifact, insofar as the interpretation of TPD spectra of coal chars is concerned. This spurious mechanism is further supported by the noticeable broadening of the high-temperature CO peak upon “COloading” evident in Figure 4. The TPD of the “thermally-cleaned and C02-loaded” coal char in Figure 7 exhibits CO evolution over two temperature ranges: one relatively small peak centered a t about 600 K and another larger one centered a t ca. 1200 K. The high-temperature CO may be due to thermal decomposition of C(0) surface complexes formed by the reverse step of reaction 1 and/or to desorption of rechemisorbed intrapore CO formed during the same reaction. The slight amount of CO evolution evident ca. 600 K most probably arises from a fraction of the gaseous CO
produced by the reverse step of reaction 1 that does not rechemisorb and escapes the char porosity. In any case, it is noted that the “thermally-cleaned and CO-loaded” char sample yielded significantly more desorbed CO a t high temperature than the “thermally-cleaned and COzloaded” char sample. The mechanism of CO readsorption during TPD appears to be a complex function of surface coverage, char pore structure, density of active sites, the presence of mineral matter, carbon aromaticity, and a number of other factors. An important corollary of this result is that it may be difficult to determine, from TPD experiments alone, the original energetic distribution of CO complexes formed by CO adsorption or during gasification, since it may be altered during the course of TPD.
Discussion The isothermal experiment in which gaseous CO was exposed to an oxidized Wyodak coal char sample in the TGA, as described in the Experimental Section, produced no measurable weight change or production of COz, thereby appearing to confirm the conclusion of Vastola et al.12 This result, taken together with the preceding analysis of the current TPD results, still begs the question as to what is the correct interpretation of the results of Vastola et al.,12 Otake,13and others, including our own isothermal experiment, which all seem to reject the importance of reaction 1 in the low-temperature regime. The key to reconciling the results of the isothermal and nonisothermal experiments lies in the implicit assumption made by these workers (including ourselves, initially) that a t a particular constant temperature, CO should be capable of interacting with all carbon-oxygen surface complexes that are present if reaction 1 were significant. However, the most prominent conclusion of almost all the TPD studies of carbonoxygen complexes to date is that the binding energy of these species is a distributed function,l6ls such that at any arbitrary temperature (below some upper limit) there is always an inventory of complexes that are thermally stable a t that temperature. This observation provides the framework for a possible explanation of the seeming contradiction between isothermal and nonisothermal experiments with regard to the relative importance of reaction 1. With the preceding in mind, consider the case of a carbon surface undergoing isothermal, steady-state gasification. At this temperature, there exist inventories of thermally stable and unstable complexes and unoccupied active sites. The latter two types of sites taken together represent the portion of the active site distribution that is “turning-over”, that is, the low-temperature portion of the active site distribution that is being continually oxidized by the oxidant species a t the same rate that the complexes formed are thermally decomposing. If the oxidant were removed, after a transient period, the resultant carbon surface would be covered by only high-temperature, stable complexes, and the fraction of the active carbon sites that was participating in the gasification reaction would be left unoccupied as a result of thermal decomposition. If the remaining, thermally stable complexes are also inactive with respect to reduction by CO under isothermal conditions at the same temperature, then this is consistent with the “null” results of our own isothermal experiment cited above. Increasing the temperature, however, (as in a TPD experiment, for example) would tend to make a fraction of the originally stable complexes thermally labile to desorption and, presumably, also reactive to CO. In this manner, access to the more stable complexes to reduction by CO can be provided in a nonisothermal process, whereas
Thermal Desorption of Surface Oxides isothermal conditions would exhibit little or no reactivity. The isothermal experiment of Vastola et a1.12 cited above, however, is actually not the same as the one conducted by us and reported on here. In the former experiment, the Graphon sample was exposed to a gas mixture initially composed of 6 Pa of oxygen and 0.6 Pa of Cl80. During the course of the 8-h experiment, the oxygen partial pressure decreased to 1.1P a due to reaction and surface complex formation, while the C1*O partial pressure was reported to remain constant. In keeping with the preceding scenario, the labeled CO would not react with any stable complex that was formed during the experiment (which in most cases ends up being the majority of the surface complex). However, since oxidation was occurring simultaneously at a low rate, there most certainly had to be an inventory (albeit a small one) of thermally labile surface oxygen complexes at the reaction temperature with which CO could possibly react. The observation, however, is that it did not. On the other hand, this result is completely understandable in terms of relative rates. In a competition of reaction pathways for the same oxidized active sites, the relative steady-state rates of thermal desorption (i.e., gasification) versus reduction by CO can be expressed as
r = kdes[C(0)]/krd[C(0)l [ c o l = kdes/kred[CO] = kox[021"/kred[C(0) 1[Col where n is the observed overall steady-state gasification rate order, which is positive and usually less than unity.21 From these expressions, it is obvious that thermally labile oxygen complexes will react preferentially via the gasification pathway rather than via reduction by CO (i.e., r >> 1)for conditions of high oxygen-to-CO ratios, low oxygen complex surface coverages, and/or low absolute concentrations of CO. Since the experiment of Vastola et a1.12 fulfills all these conditions, it is not surprising that the labeled CO was observed not to react. In view of the work presented here, another potential sink for the labeled CO in the Vastola et al.12 experiment might have been direct chemisorption of CO, as discussed previously. However, a similar consideration of the relative rates of CO and oxygen chemisorption on free active sites during the experiment also explains why labeled CO did not seem to chemisorb at all, as well as not react with C(0) surface complexes. The argument once again is concerned with parallel, competitive rates for free active sites, via oxygen gasification and CO chemisorption in this case:
r' = k1[021Cf/k2[COlCf = ~l[o,l/k,[COI where k1 is the oxygen chemisorption rate constant (observed to be first order21)and k2 is the CO chemisorption rate constant. We have observed that the rate of chemisorption of CO from the bulk gas phase a t 473 K is orders of magnitude less than the rate of oxygen uptake under similar conditions. Marchon et al.I9 also report that CO adsorbs onto graphite with a very low sticking coefficient. It was observed that the amount of CO obtained upon TPD after adsorption of CO at 13.3 kPa for 30 s is an order of magnitude less than the one obtained after O2adsorption under similar conditions.14 Thus, under the conditions of the Vastola et a l . I 2 experiment, for example, a comparative rate analysis leads to the conclusion that CO (20) Phillips, R.;Vastola, F. J.; Walker, P. L., Jr. Carbon 1970,88 197. (21) Suuberg, E.M.; Wojtowicz, M.; Calo, J. M. Submitted for publication in Twenty-Second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA. (22) Morgan, M. E.;Jenkins, R. G.; Walker, P. L., Jr. Fuel 1981,60, 189.
Energy & Fuels, Vol. 3, No. 3, 1989 375 cannot effectively compete with O2 for free active sites. Enhanced C02 gasification reactivity was noted by Phillips et aL20during nonisothermal heating of Graphon. This effect was attributed to an increased rate of production of highly reactive "nascentn carbon sites upon thermal decomposition of oxygen surface complexes. Although the purpose of the current paper is not to explicitly examine mechanisms for reaction 1, it is certainly possible that such sites could be participating in the conversion of CO to C02 by facilitating the chemisorption of intrapore CO, which could then be more easily oxidized by an oxygen surface complex, and/or by providing a mechanism for "destabilizing" otherwise stable oxygen complexes in the vicinity of the nascent site. In fact, these sites might also explain the observed enhanced rechemisorption of intrapore CO upon T P D i.e., a fraction of these highly energetic sites could be "capped off" by CO. In any case, it is noted that the existence of reactive nascent carbon sites does not qualitatively alter the preceding competitive rate analysis. The results of Otake13are also consistent with the simple qualitative model set forth above. The particular experiment under consideration consisted of maintaining a partial surface coverage of the most stable oxygen complexes (i.e., those that survive loo0 K) and reoxidizing the sample. Subsequent slow linear TPD experiments indicated no additional C02 was evolved from the partially cleaned sample than from the initially oxidized sample prepared following complete thermal cleaning. This result is understandable in terms of the microscopic reversibility of reaction 1 and is understandable also, because thermally stable C(0) complexes do not participate in secondary reactions a t temperatures significantly below their desorption temperature. That is, a t the temperatures required to activate (i.e., thermally decompose) the hightemperature complexes (>lo00 K),the forward step of reaction 1 (Le., C 0 2 formation) is not favored, and the carbon-oxygen complexes become effectively unreactive toward reduction by CO; they simply desorb as CO. This "cutoff" in C02 production via reaction 1is also favored by the low intrapore CO concentrations maintained by the low heating rates employed (i.e., 2 and 5 K/min). Moreover, a t the lower temperatures where the forward step of reaction 1 is favored, the high-temperature (>lo00 K) complexes are effectively unreactive toward reduction by CO. These two effects taken together account for the results of Otake,13 who observed no difference in C02 formation between his two experiments. Thus, during the course of a TPD, there appears to be a "thermal windown produced by the kinetic and thermodynamic characteristics of reaction 1, where the net production of C02 is favored.
Conclusions It has been shown that secondary reaction between "free" CO and surface oxygen complexes can be a significant source of C02 in TPD experiments. TPD of "thermally cleaned and CO-labeled" coal char samples demonstrated that the C O C O reaction is not a significant source of C02under the experimental conditions examined. The latter experiments, as well as the "thermally-cleaned and C02-loaded" coal char sample experiments also revealed the existence of strongly chemisorbed CO and/or additional, secondary C(0) due to the reverse of reaction 1.
These results indicate that deconvolution of T P D spectra to deduce the original energetic distribution of oxygen surface complexes is complex. In order to accomplish such a deconvolution it would appear to be necessary
376
Energy & Fuels 1989,3, 376-381
to know the detailed kinetics of the secondary interactions represented by reaction 1and CO chemisorption/desorption as a function of experimental conditions. The kinetics of these processes depend upon the nature of the char pore structure in terms of pore size distribution and the locations within this structure where the complexes are formed. This, in turn, depends upon the manner in which porosity is developed by gasification, the rate of gasification, the degree of order of the subject carbon, and the type and amount of inorganic impurities. A fundamental understanding of carbon gasification via oxidizing agents, however, requires the resolution of these issues.
In any case, the most important conclusion to be drawn and the caution that is given here are that secondary interactions can significantly affect TPD spectra of oxidized coal chars and, perhaps, other carbonaceous materials and that such spectra may not necessarily be reflective of the true original state of surface oxygen complexes.
Acknowledgment. We wish to gratefully acknowledge the support of this work by the Morgantown Energy Technology Center of the U.S. Department of Energy under Contract No. DE-AC21-87MC23284. Registry No. COz, 124-38-9;CO, 630-08-0.
Role of Induction Time and Other Properties in the Recovery of Coal from Aqueous Suspensions by Agglomeration with Heptane C.-W. Fan, Y . 4 . Hu, R. Markuszewski, and T. D. Wheelock* Ames Laboratory and Department of Chemical Engineering, Iowa State University, Ames, Iowa 50011 Received November 14, 1988. Revised Manuscript Received February 27, 1989
The percent recovery of fine coal or graphite particles suspended in water by agglomeration with heptane was highly dependent on the measured induction time, i.e., the gas bubble to particle attachment time of the material. The induction time was found to correlate closely with the heat of immersion of the solids in water, another indicator of the hydrophobic/hydrophilic character of the material. For a series of coals and graphite, the agglomeration recovery decreased exponentially with increasing induction time. For the more oleophilic coal or graphite particles, an increase in salt (NaC1) concentration of the suspending medium caused an increase in agglomeration recovery and a decrease in induction time. For the less oleophilic coal or pyrite particles, an increase in salt concentration caused a decrease in agglomeration recovery apd an increase in induction time. Due to the opposing effects of salt concentration on the recoveries of a highly hydrophobic coal and pyrite, it was possible to improve the separation of these materials by an increase in salt concentration. On the other hand, because the recoveries of pyrite and a weakly oleophilic coal were affected similarly by an increase in salt concentration, it was not possible to improve the separation of these materials.
Introduction The selective agglomeration of ultrafine coal particles suspended in water by bonding the particles with oil or hydrocarbon liquid such as pentane or heptane is the basis for several proposed coal cleaning processes.14 Following agglomeration, the suspension can be screened to recover the agglomerated coal from unagglomerated mineral particles. The method takes advantage of the greater hydrophobicity and oleophilicity of the coal compared to that of the mineral matter. Generally the competition between oil and water for the particle surfaces favors oil in the case of coal and water in the case of mineral matter. Among (1) Capes, C. E.; Gemain, J. R. In Physical Cleaning of Coal; Liu, Y . A., Ed.; Marcel Dekker: New York, 1982; pp 293-351. (2)Mehrotra, V. P.; Sastry, K. V. S.;Morey, B. W. Int. J. Miner. Process. 1983,11, 176-201. (3) Wheelock, T.D.;Markuszewski, R. In The Science and Technology of Coal and Coal Utilization; Cooper, B. R., Ellingson, W. A., Eds.; Plenum: New York, 1984; pp 47-123. (4) Steedman, W.G.; Krishnan, S. V. In Fine Coal Processing; Mishra, S . K.; Klimpel, R. R., Eds.; Noyes Publications: Park Ridge, N.J. 1987; pp 179-204.
0887-0624/89/2503-0376$01.50/0
the most commonly occurring minerals in coal, only iron pyrite appears to be sufficiently oleophilic to interfere with separation from coal by this method. However, the oleophilicity of pyrite is quite variable and depends on treatment conditions. Strongly oleophilic materials respond more favorably to oil agglomeration than weakly oleophilic materials.',"' Thus, when heptane was employed as an agglomerant, Drzymala et al.7 found that the amount of material recovered per gram of heptane (i.e., the specific recovery) increased with increasing oleophilicity of the material as indicated by the oil receding three-phase contact angle for the heptane-solid-water system. Other work8igshowed (5) Venkatadri, R.; Markuszewski, R.; Wheelock, T. D. Energy Fuels 1988,2, 145-150.
(6) Sadowski, 2.;Venkatadri, R.; Druding, J. M.; Markuszewski, R.; Wheelock, T. D. Coal. Prep. 1988,6, 17-34. (7) Dnymala, J.; Markuszewski, R.; Wheelock, T. D. Miner. Eng. 1988, 1, 351-358. (8) Yang,G. C. C.; Markuszewski,R; Wheelock, T. D. Coal Prep. 1988, 5, 133-146. (9) Fan,C.-W.;Markuszewski, R.; Wheelock, T. D. Fizykochem. Probl. Mineralurgii 1987,19, 17-26.
0 1989 American Chemical Society
