Energy & Fuels 1992,6, 184-188
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which decompose at high temperatures, increased the yield of hydrocarbon gas at 850 OC. The key point of this method is the matching between the radical donation rate of the solvent vapor and the formation rate of the desired product. This method is one possible way to control the product distribution under mild conditions.
Acknowledgment. This work was financially supported by the Ministry of Education, Culture and Science of Japan through the Grant-in-Aid on Priority-Area Research (Grant No. 63603014, 01603012, and 02203112). Registry No. EB, 100-41-4; 2MIP, 78-83-1; Tet, 119-64-2; MeOH, 67-56-1; xylene, 1330-20-7; styrene, 100-42-5; benzene, 71-43-2; toluene, 108-88-3.
Calorimetric Study of Oxygen Adsorption on a High Surface Area Polymer-Derived Carbon A. S. Gowt and J. Phillips*J Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received November 5, 1991. Revised Manuscript Received December 17, 1991
Differential calorimetry is shown to be a powerful technique for probing the chemistry of reaction between oxygen and active sites on the carbon surface. Using calorimetry it was shown that for the polymer-derived carbon studied the temperature used to desorb surface groups greatly impacts the active site distribution. Following acid treatment and a low-temperature desorption (923 K)the oxygen adsorbs primarily onto one site type and the heats of adsorption indicate that a C02-likesurface group forms. In contrast, following a high-temperature desorption (1173 K)there are two principal types of surface sites. One is exactly the same type that is formed during the low-temperature treatment. Also following desorption at 1173 K, CO-like groups form on the second site type. The calorimetric results also suggest that most models of chemisorption are inadequate. Indeed, the majority of models cannot explain the observation that the heats of adsorption increased with increasing coverage.
Introduction High surface area carbons are used extensively as adsorbents, catalyst supports, and fuels. The adsorbing properties of the surface play a role in all of these uses. Thus,the surface of carbon has come under close scrutiny. For example, the adsorption of oxygen on high surface area carbons has been studied extensively.'"' In most cases the experiments are designed simply to measure the oxygen chemisorption capacity of the material. This measure has been found to correlate strongly with the reactivity of the carbon, whereas the total surface area measured using the BET and other techniques has been found to have a very poor correlation with rea~tivity.~ It is also clear that the strongly adsorbing sites help anchor metal particles on carbons used as catalyst supports,6 One of the major shortcomings of simply using oxygen as a probe molecule for counting the number of active sites is that it yields no data regarding the character of the adsorption sites. There is a need to use spectroscopic and other techniques to probe the surface to complement the oxygen adsorption studies. One of the most promising techniques for gaining information regarding the character of chemisorbing sites is differential microcalorimetry. In fact, several studies of the differential heat of oxygen adsorption on high surface area carbons have been carried out.'+ There have also been calorimetric studies of oxygen adsorption on coal chars and our new paper on potassi-
* To whom correspondence should be addressed.
'Current address: Department of Chemistry and Chemical Engineering, University of New Haven, 300 Orange Avenue, West Haven, CT 06516-1999. 0887-0624/92/2506-0184$03.00/0
um-promoted coal chars.* From the heat and kinetic data derived from these studies it has been possible to unravel partially the question of the identity of the adsorbing sites on carbons and to distinguish various site types. This paper is a report on the further use of differential calorimetry to probe the surface of high surface area carbons. Specifically, a novel calorimeter was used to study oxygen adsorption onto a high surface area carbon prepared from a proprietary phenol-formaldehyde copolymer (PFC). On the basis of this work it was possible to separately probe the sites from which CO and C02are removed during high-temperature outgassing treatments.
Experimental Section Calorimeter. The Calvert-type calorimeter used in this work has been described in detail elsewhere?# The most unusual aspect of the design is the shape of the sample cell. As can be seen in Figure l a it consists of a flat cylinder surrounded by two thermopiles which are in contact with two large metal heat sinks. This geometry has some distinct advantages over the more conventional long sample cylinder surrounded by cylindrical thermopiles. For (1) Radovic, L. R.; Walker, Jr., P. L. Fuel Process. Technol. 1984.8, 149.
(2) Laine, N. R.; Vastola, F. J.; Walker, Jr., P. L. J.Phys. Chem. 1963,
67, 2030.
(3) Radovic, L. R.; Walker, Jr., P. L.; Jenkins, R. G. Fuel 1983,62,849. (4) Gow,A. S.; Phillips, J. J. Catal. 1991, 132. (5) Radovic, L. R.; Steczko,K.; Walker, Jr., P. L.; Jenkins, R. G. Fuel Process. Technol. 1985, 10, 311. (6)Chen, A. A.; Vannice, M. A.; Phillips, J. J. Phys. Chem. 1987,91,
6257. (7) O'Neil, M.; Lovrien, R.; Phillips, J. Reo. Sci. Instrum. 1985, 56, 2312. (8) ONeil, M.; Phillips,J. J. Phys. Chem. 1987, 91, 2867. (9) Keyes, F. G.; Marshall, M. J. J. Am. Chem. SOC.1927, 49, 156.
0 1992 American Chemical Society
Energy & Fuels, Vol. 6, No. 2, 1992 185
Oxygen Adsorption on Polymer-Derived Carbon
Table I. Samples Studied in Microcalorimetric Characterization of Surface Functional Groups starting sample material pyrolysis of raw material prepn of surface complex off-gassing treatment 1 PF copolymer flowing N,; 10 K/min to 1273 K; oxidized in conc HNO,; 340 K, 5 h; 3 h 650 “C 1 X Torr 1 h at 1273 K; cooled to RT dried in vacuum; 383 K, 12 h 2 PF copolymer flowing N,; 10 K/min to 1273 K; oxidized in conc HNO,; 340 K, 5 h; 3 h 900 “C 1 X lo4 Torr dried in vacuum; 383 K, 12 h 1h at 1273 K; cooled to RT 3 and 4 PF copolymer flowing N,; 10 K/min to 1273 K; no additional treatment 3 h 500 “C 1 X lo4 Torr 1 h at 1273 K; cooled to RT
example, it allows the use of shallow sample beds which shorten diffusion length. This permits the impact of diffusion on results to be minimized. Another special feature of the calorimeter is that the gas is admitted from a dosing volume which is kept at the same temperature as the sample cell. In most instruments the gas is dosed directly from the laboratory. The precision, accuracy, and reliability of the instrument are all discussed in previous publication^.^^ In sum, the calorimeter has been shown to have both high accuracy and precision. The procedure for using the calorimeter is straightforward. Once the sample pretreatment was complete, the sample was loaded into the sample cell from the quartz pretreatment reactor by tilting the entire instrument. Gas was next loaded into the dosing volume. After it has had a chance to equilibratethermally, the valve connecting the sample and dosing volumes was opened (Figure lb). In all cases the gas and sample were kept at 303 K. When adsorption was complete, as determined from the heat output, the residual pressure in the dosing volume was measured with a Texas Instruments precision pressure gauge. This procedure was repeated until the sample will adsorb no more gas. The size of the sample and the pressure of the gas doses (generally of the order of 1 Torr) were selected such that it takes about 20 doses for all chemisorbing sites to be filled. This yields nearly differential heats. Note that the number of moles adsorbed with each dose was computed from initial and fiial pressures, measured values of the initial and final volumes and the ideal gas law. Sample Preparation. Carbon samples were made from the phenol-formaldehyde (PF) copolymer using a several step procedure. Large quantities (e.g., more than 10 g) of the granular copolymer were first pyrolyzed at a temperature of 1273 K under flowing N, for 1 h in a tube furnace in order to remove volatile material and produce a high surface area carbonaceous char. The second step consisted of crushing the cooled devolatilized char with a mortar and pestle to yield enough char for a number of fine powder samples (approximately2 g/sample). About half of the fine powder was nitric acid treated for 5 h at 340 K. The slurry which remains was cooled, fiitered, and washed with a large excess of C02-freedistilled water in order to remove residual acid. The char was then dried in vacuum at 383 K for 12 h, cooled, and stored in a desiccator. The final pretreatment step for both the acid-treated and non-acid-treatedsamples is a heat treatment designed to remove some surface groups. This treatment was performed in the quartz cell attached to the calorimeter. Samples were treated at either 923 or 1173 K for 3 h, under a diffusion pump vacuum (ultimate pressure 1 X lo4 Torr) and then gradually (1.5 h) cooled to room temperature. Samples were held under vacuum an additional 10 h, and then transferred to the calorimeter cell. The sample preparation sequences described are very similar to those used by earlier workers studying the desorption of CO and C02 from the same material.lOJ1
Results Four calorimetric experiments were carried out for this study as listed in Table I. The sample treatments were selected deliberately to duplicate those used in earlier studies of the same material.lOJ1 Specifically, oxygen adsorption was carried out on two acid-treated samples, one of which was subsequently outgassed at a high tempera(10) Otake, Y.; Jenkins, R. G. Prepr. Pup.-Am. Chem. SOC.,Diu.Fuel Chem. 1987,32,310. (11)Otake, Y. Ph.D. Thesis, Pennsylvania State University, University Park, PA 1987.
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ture, and a second which was subsequently outgassed at a low temperature. Experiments were also carried out on two samples which were not acid treated. Both of these samples were outgassed at a low temperature. In Figure 2 the differential heat of oxygen adsorption and the “normalized cooling width at half-maximum” (NCWHM) are plotted versus the amount adsorbed on sample 3. (This sample is one of two which was not acid treated.) The NCWHM is the length of time it takes each ballistic heat curve to go from its maximum value to its half-maximum value, divided by this value for the first ballistic curve in the series. It has been shown previously that this is a good qualitative measure of relative rates of adsorption? Note that the heat of adsorption, initially low, first decreases and then increases and reaches a plateau before declining and giving off heats consistent with physical adsorption. Also note that the NCWHM is vir-
Gow and Phillips
186 Energy & Fuels, Vol. 6, No. 2, 1992
75
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Figure
Heat of adsorption versus uptake for acid treated samples following low temperature outgassing (sample 1) (0), and following high temperature outgassing (e) (sample 2). 4.
copolymer char.
Table 11. Total Surface Area Measurements
sample 1
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BET," m2/g 505 6271582 4131401
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0 10
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40
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Figure 3. Differential heat of oxygen adsorption versus the quantity of gas adsorbed for two identical samples of the 500 "C raw phenol-formaldehyde copolymer char. tually constant until physical adsorption begins to play a role at which point the rate of adsorption appears to increase slighlty. There is some scatter in the data, and this raises the question of accuracy. There are two possible explanations. Either the scatter is real and reflects the complexity of the adsorption process, or it is an indication of the inherent uncertainty of the measurement. The former view is favored for a couple of reasons. First, the heat curve of another sample (sample 4) treated in the same fashion had (Figure 3) the same shape (fall-rise-plateau-fall) and virtually the same minimum, maximum, and plateau values of the heat of adsorption. Almost precisely the same total amount of gas chemisorbed in both cases. Sample 4 had a slightly different shape to the scatter than sample 3, but this is expected for two samples of a complex material, each possibly containing a slightly different distribution of site types. Second, many other samples examined as part of this work or as part of previous investigations using the same c a l ~ r i m e t e did r ~ ~not ~ ~show significant scatter in the heats of adsorption. The differential heats of adsorption versus amount adsorbed are plotted in Figure 4 for the two acid-treated samples. There are several important features to each curve. Sample 1, which was outgassed at a low temperature (Table I), has only a single plateau to the heat of
adsorption and the value of that heat (approx 75 kcal/g mol of O2 adsorbed) is clearly higher than that on the samples which were not acid treated. In contrast sample 2, which was outgassed at a high temperature, has two heat of adsorption plateaus. The lower plateau is at only 50 kcal/g mol, but the higher plateau is at virtually the same value as that found for sample 1. Surprisingly, the lower heat plateau f h first. Sample 2 also clearly adsorbed more gas than any other sample. In order that kinetic data be available for the two acid-treated samples, both heats of adsorption of NCWHM values are plotted versus amount adsorbed in Figure 5. The BET and Polyani-Dubinin (CO,) surface areas were measured for all samples as well, using techniques which are described in detail elsewhere.* There data are presented in Table 11. Discussion On the basis of the earlier temperature-programmed desorption (TPD) studies OtakelOJ1 suggested that there are four types of surface groups on acid-treated PFC char samples. These were identified as (i) low-temperature CO, groups, (ii) high-temperature C 0 2 groups, (iii) low-temperature CO groups, and (iv) high-temperature CO groups. The vast majority of the desorption was from only two of the site types; either low-temperature CO,, or high-temperature CO sites. On the basis of the experimental results Otake estimated average values of 50 and 74 kcal/mol for the decomposition activation energies (decomposition followed by desorption) for these two groups, respectively. He also found that the majority of the COz type groups desorbed by about 925 K, whereas the majority of the CO groups did not desorb until the temperature reached above 1150 K. The desorption treatments used in the present study were designed on the basis of Otake's results, That is, the temperature chosen for low-temperature outgassing was
Oxygen Adsorption on Polymer-Derived Carbon
Energy & Fuels, Vol. 6, No. 2, 1992 187 I20
a0 I
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9
postulated in many are reasonable. For example, structures with 0:C ratios of 2:l and 3:2 have been postulated. It is easy to imagine that the stable species which would desorb when these types of structures decompose would be COz. Finally, it must be noted that the activation energy for decomposition/desorption .of a surface group is not the same as the heat of adsorption/formation of the same group. On the high-temperature acid-treated sample (sample 2) there are two plateaus. The second, and higher heat, plateau can be explained in the same fashion as the single plateau observed on sample 1. Indeed, the heat of adsorption and the width of this plateau are virtually identical with that seen on the low-temperature acid-treated sample. This suggests C02-like species are forming on these sites. The initial, and low heat, plateau can be associated with the formation of CO-like surface groups. This identification is made for two reasons. First, these are the "new sites" created by the higher temperature of the sample 2 heat treatment. As such, they can be associated with the CO group decomposition/desorptionwitnessed by Otake during the same high-temperature treatment. It is pussible that the same type of species will form at the vacated sites. Second, the heats of adsorption are more consistent with the formation of CO-like surface structures than with C02-likespecies. The heats observed are just slightly lower than the heat of formation of gaseous CO at the same temperature. This is a little higher than expected if the surface groups have a perfect 1:l C O stoichiometry. That is, as the carbon atoms are bound to the lattice and are not "free", it is expected that the heat of formation of bound CO groups will be lower than that of free CO groups. Thus, it is suggested that groups form in which the carbon atoms are bound to more than a single oxygen. For example, earlier have suggested that carbonoxygen chains can form in which each carbon is bound to two oxygens, yet the overall stoichiometry of the chain is 1:l. The observation of heats of adsorption which increase with coverage, rather than decrease, always raises questions. This is because the vast majority of models assume that adsorption takes place in an equilibrium fashion,21-24 such that the observed heat of adsorption should decrease monotonically with increasing coverage. This is not the first time that heats of adsorption which increase with coverage have been detected for oxygen adsorption on carbon: and it was suggested that this takes place because on some materials adsorption at relatively low temperatures is controlled by kinetics rather than by equilibrium. More recently the qualitative model used to explain rising heats observed on carbon was generalized to many different surface types and made quantitative.12J3According to this new model, for surfaces consisting of well-distributed sites there is no mechanism of intersite (14) Shilov, N. A.; Shatunovska, H.; Chmutov, K. 2.Phys. Chem. 1930, A149,211. (15) Langmuir, I. J. Am. Chem. SOC.1918,40, 1361. (16) Blench, E. A.; Garner, W. E. J. Am. Chem. SOC.1924,46,1288. (17) Garner, W. E. Nature 1924,114,932. (18) Garner, W. E.; McKie, D. J. Am. Chem. SOC.1927,49. (19) Puri, B. R. Chemistry and Physics of Carbon; P. L. Walker, Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, p 191. (20) Tucker, B. G.; Mulcahy, M. F . R. Trans. Faraday SOC.1969,65, 274. (21) Porter, A. S.;Tompkins, F. C. R o c . R. SOC.London 1953, A217, 529. (22) Stone, F. S. In Chemistry of the Solid State; Garner, W . E., Ed.; Butterworths: London, 1955; p 367. (23) Roberta, J. K. h o c . R. SOC.1935, A152,445. (24) Klachko, A. L. Kinet. Katal. 1978,19, 1218.
188 Energy & Fuels, Vol. 6, No. 2, 1992
“communication”, nor reapportionment of adsorbate molecules. Thus, the model indicates that on these surfaces gas remains where it initially adsorbed. Moreover, the model continues, it will adsorb most rapdily on sites with the lowest activation energy barriers to adsorption, which are not necessarily the sites with highest heats of adsorption. This raises a question. Is there evidence in the present work of a decrease in the rate of adsorption, and thus kinetic rather than equilibrium control of the adsorption process? The answer to the question is a qualified no. As explained elsewhere, if adsorption is controlled by kinetics, rather than by equilibrium, a decrease in the rate of adsorption should be detected? The NCWHM values do not change significantly in measured value for any of the samples. However, the NCWHM are not a good measure of adsorption rate for extremely fast processes. This is because the minimum half-width of the instrument (given a “spike” heat input) has been measured to be about 90 s. Thus, if the initial pulses adsorb on a time frame of about 10 s and the final pulses adsorb on a time frame of about 100 s, there will be only a small measured difference in the NCWHM values. Yet, in fact there will be a significant decrease in the rate of adsorption. Indeed, the absolute values of the half-widths were all relatively close to 90 8. A second explanation for the finding that the heats of adsorption increase with increasing coverage is that there is an inhomogeneous distribution of site types. For example, the following model could explain the observations made in this study. To wit, all of the sites responsible for the formation of CO-like structures are near the char particle surface and all of the sites responsible for the formation of C02-like structures are formed in the char particle interior. This is an intriguing alternative explanation. Unfortunately, it is not possible to distinguish the two possible models with the current data. Recently, calorimeters have been developed which allow the simultaneous measurement of the differential heat and true rate of adsorption of gases on high surface area mat e r i a l ~ . In ~ ~these instruments the kinetics of adsorption are measured using baratron pressure gauges linked to a computer. These gauges are used as the ballast volumes from which gas is admitted to the cells containing the high surface area samples. In this configuration changes in system pressure are measured almost instantaneously. (25) G t t e , R. R.; Phillips, J. Langmuir 1989, 5, 758.
Gow and Phillips These calorimeters yield excellent differential kinetics, virtually undistorted by instrument transfer functions. In order to test the notion that kinetics allows low heat sites to be filled first on the high-temperature outgassed acidtreated PF chars, this type of calorimeter will need to be employed in future studies. Another question which must be addressed is the difference between the acid-treated and the non-acid-treated samples. It is clear that the latter adsorbs less and with a different pattern to the heat of adsorption than either acid-treated sample. However, there is some evidence to support the proposition that some of the sites which are available on that sample are similar in nature to the COtype sites found on the acid surface treated at a high temperature. That is, both have sites which adsorb at around 55 (i5) kcal/mol of 02,and in both cases the kinetics of adsorption on these sites is rapid. Finally, it is not reasonable to make a correlation between the active surface area (ASA) as generally measured and the active sites measured in the present case. This is because the value of ASA is generally determined at much higher temperatures (ca. 575 K) than that employed in the present s t ~ d y . l -On ~ ~carbons, ~ far more sites are tritrated at high temperatures than at ambient. Past experience leads to the suggestion that there may be sites on the char studied in the present case with activation barriers to adsorption high enough to prevent significant adsorption at 300 K. To make valid comparisons between the calorimetric data and standard ASA measurements future calorimetric studies will need to be carried out at far higher temperatures.
Summary In some cases microcalorimetry can be used to distinguish between and “count” different site types on high surface area materials. This type of information can be used to supplement or even replace detailed spectroscopic studies. In the case of carbons this type of information many be sufficient for understanding surface chemical behavior.26 Acknowledgment. We are grateful to the National Science Foundation (Grant No. CTS-9011275) for partial support of this work. Registry No. 02,7782-44-7; C, 7440-44-0. (26) Leon y Leon, C. A.; Radovic, L. R. J’repr. Pap-Am. Chem. SOC., Diu. Fuel Chem. 1991, 36 (3), 1007.
