494
Energy & Fuels 1992,6,494-497
Transient Kinetic Techniques for Detailed Insight in Gas-Solid Reactions Freek Kapteijn,*?+Ronald Meijer, and Jacob A. Moulijnt Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Received November 5, 1991. Revised Manuscript Received March 30, 1992
Transient kinetic studies with application of labeled molecules form a powerful tool to investigate in situ gas-solid interactions,to obtain fundamental and mechanistic information, like concentrations of active sites and intrinsic rate constants. Especially in the field of carbon gasification, this approach is extremely valuable; due to the high temperatures involved, other techniques are difficult to apply in situ. A short review is given on the kinetic studies of the carbon gasification with C02 and the potential value of transient techniques in the study of gasification reactions.
Introduction Gas-solid interactions play an enormous important role in nature. In many chemical-industrial activities this is easily recognized by the large-scale application of heterogeneous catalytic processes, the combustion and gasification of coal and carbon, adsorption processes, reduction of ores, and processes based on chemical vapor deposition, as in the semiconductor industry. All examples involve the interaction between gaseous species and solid surfaces, either of a physical or of a chemical nature. A clear picture of these interactions and, as a consequence, of the chemical transformations can add to the improvement of the aforementioned processes by the development of better catalysts, better control over the surface layer formation, or optimal choices of the experimental or operating conditions. This demands fundamental mechanistic investigations. In this article the statement is advanced that transient kinetic techniques, in combination with the use of labeled molecules, will potentially contribute to the mechanistic insight of gas-solid reactions. This is illustrated with an example from the uncatalyzed gasification of carbon. Mechanistic Studies Apart from characterization of the solid material and its surface, the real information about the gas-solid reaction and its mechanism lies in the investigation of the system under reaction conditions, in situ studies. This limits the number of techniques that can be applied considerably. The in situ techniques can be divided into chemical reactivity/ kinetic studies and spectroscopic techniques. Kinetic studies can be performed under steady-stateand under transient conditions. The latter including pulse, stepresponse, and temperature-programming experiments. Generally, transient or unsteady-state conditions are realized by changes in concentration or temperature levels. Spectroscopic techniques that have been proven useful and applicable to investigate gas-solid systems under controlled conditions are FTIR, ESR, NMR, magnetic susceptibility, XRD, Raman spectroscopy, and electron microscopy. In a majority of the cases these have been developed and applied in catalyst studies,' but applications in the field of coal and carbon conversion processes exist." Present address: Department of Chemical Engineering and Materiala Science, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
In both types of techniques the isotopic labeling of reacting molecules will contribute to the information obtained. Either the pathway of atomic species can be followed6or shifts in the spectra or patterns may be discernible. A major problem with all spectroscopictechniques is the uncertainty of what is observed, and the associated question is this information relevant to the mechanism of the reaction. Steady-state kinetic studies consider the reaction system as a black box. Reaction conditions are varied over a predetermined range and the response of the system in the form of gas-phase conversions or reaction rates is measured. This mainly serves to develop rate expressions that can be used in the operation and development of reactors, but, in principle, it will not give mechanistic insight. Transient kinetic studies with application of labeled molecules form an ideal combination that gives direct information about chemical activity, pathways of atomic species, number of intermediates present on and/or in the solid surface, and rate data of elementary processes, and, hence, constitutes an ideal in situ technique to study gas-solid interactions. This has been suggested for catalytic studies6as well as for carbon gasification studies.' It should be stressed, however, that no single technique in itself is the Holy Grail for mechanistic studies, but generally the combined evidence from different techniques will lead to a clear picture of the reaction. Transient Kinetic Techniques In these techniques a change in concentration or temperature is impoaed on the reaction system and its response is studied. This can be a gradual, a sudden, or a periodic (1) Catal. Today 1991,9 (1-2). (2) Petrakis, L.; Grandy, D. W. Free Radicals in Coak and Synthetic Fuels; Coal Sci. Technol. Ser. 5; Elsevier: Amsterdam,1989, Chapter 111. (3) Cerfontain, M. B.; Moulijn, J. A. In Advances in Coal Chemistry; Vasilakos, N. P., Ed.; Theophrastus Publishers: Athens, 1988; pp 233-264. (4) Baker, R. T. K. In Carbon and Coal Gasification: Science and Technology; Figueiredo, J. L., Moulijn, J. A., Eds.; NATO AS1 Series E Appl. Sci. Vol. 177; Martinus Nijhoff: Dordrecht, 1986; pp 231-268. (5) Happel, J. Isotopic Assessment of Heterogeneow Catalysis; Academic Press: New York, 1986. (6) Mirodatos, C. Catal. Today 1991, 9, 83-95. (7) Kapteijn, F.; Moulijn, J. A. In Carbon and Coal Gasification: Science and Technology; Figueiredo, J. L., Moulijn, J. A., Eds.; NATO AS1 Series E: Appl. Sci. Vol. 177; Martinus Nijhoff: Dordrecht, 1986; pp 291-360.
Q887-0624/92/ 25Q6-Q494$Q3.QQ/Q 0 1992 American Chemical Society
Gas-Solid Reactions
change and in general the change in gas-phase composition is monitored, preferably by mass spectrometry, in view of ita speed. In most catalytic and gasification studies the (porous) solid is contained as a packed bed in a tubular reactor through which continuously a gas mixture flows. In step-response experiments two gas flows are interchanged nearly instantaneously, e.g., from a reactive to a nonreactive mixture, and the elution of reactants and products is followed as a function of time? This changes the reaction conditions completely and one might wonder if the results can be extrapolated to reaction conditions. To overcome this problem also steady-state-transient experiments can be perform& a step change from a reacting mixture to a mixture of the same composition but in which one of the components is replaced by its labeled component. Steady-state conditions are maintained, but there is a transient of the isotopic species. Also, the addition of small amounts of radioactively labeled molecules to a system falls in this category. The negligible addition does not disturb the steady state, whereas a high sensitivity is achieved. Recently, the use of positron emitters has been reported. In combination with the tomography technique, well-known in the medical environment,the time-resolved concentration profile of the positron emitting element along the packed bed is obtained? Generally, these types of experiments are performed around atmospheric pressure. In pulse experiments a well-defined amount of reacting species is injected into vacuum or in a carrier gas and passes over a packed bed of the solid material. The time-resolved production of gaseous products is analyzed. It will be clear that in these experiments steady-state conditions are never approached. Examples are the TAP systemlo and the Multitrack." Temperature-programmed experiments are generally applied to study the thermal desorption of reactants or products in an inert atmosphere or in vacuum. All techniques will yield quantitative information on the amount of species and their production or conversion rates. However, the intrinsic kinetic response of the reaction system can be disguised by an improper experimental setup (e.g., detector response times, dead zones, pressure waves during switching) or parasitic phenomena (e.g., nonideal reactor behavior, intraparticle diffusion, readsorption, temperature gradients). In the mathematical treatment of the observed response signals this has to be taken into consideration by deconvolution. In general it is advisable to avoid these problems. Several criteria can be applied to verify the presence of these disg~ises.'~J~
Carbon Gasification In studying the mechanism of the gasification of carbon in situ the high temperature levels constitutes a major problem for spectroscopic studies, and in this respect most information is obtained by post-mortem analysis of the carbon samples. Direct information from the reaction system is in general obtained by kinetic studies with the aim of developing a kinetic model that describes best the observed rate behavior of the carbon as a function of the gasification (8)Biloen, P. J. Mol. Catal. 1983,21, 17-24. (9) (a) Vonkeman, K. A. Ph.D. Thesis, Technical University of Eindhoven, 1990. (b) Jonkere, G.; Vonkeman, K. A.; Wal, S. W. A. van der; Santen, R. A. van Nature 1992,355. (10) Gleavea, J. T.; Ebner, J. R.; Kuechler, T. K. Catal. Rev.-Sci. Eng. 1988,30,49-116. (11) Moulijn, J. A.; Langeveld, A. D. van. Patent applied for. (12) Gorte, R. J. J. Catal. 1982, 75, 164-174; 1984, 90,32-39. (13) Dautzenberg, F. M. ACS Symp. Ser. 1989,411,99-119.
Energy & Fuels, Vol. 6, No. 4, 1992 495
Table I, Kinetic Models Proposed in the Literature for Gasification of Carbon in CO, Ergunll~l* 1. Cf =!e C(0) 2. C(0) co Cf
-
cos +
co + +
Keyzo 1. coz + -c-c == c-co
2.
-c-co * co + -c-
+ co
--
Gadsby e t al.13J4 1. co2 + Cf C ( 0 ) + co 2. C(0) co + Cf 3. co + Cf = C(C0)
--cop++
Blackwood and Ingemepl added two steps to the Gadsby model
4. coz + C(C0) 5. co + C(C0)
2co
C(0)
2 Cf
McCarthy3' 1. coz + Cf = co + C(0) 2. C(0) == c-0 3. c-0 co + Cf
-
Koenig e t aLZ5 1. co2 + 2Cf + c * 2. c * + C(0) + C(C0) 3. C(C0) + co + Cf 4. C(0) co + Cf
cop + - co + -- c-0co + c-0 - co +
Adschiri e t al.% 1. Cf C(0) 2. C(0) 3. C(0) Cf 4. Cf Radovic e t al.36
coz + Cf + co + C ( 0 ) 2. C ( 0 ) + c-0 3. C(0) co + Cf 1.
+ co + = co + - co +
Meijer e t al.40,41 a 1. COZ Cf + C(O)/C(CO) 2. C(C0) Cf 3. C(0) Cf (I
See text.
condition^.^ This kinetic model consists of a series of proposed elementary processes pertaining to the overall reaction. Under the assumption of steady-state conditions and a homogeneous surface, rate expressions can be derived on which the experimental data are fitted. Based on statistical and physicochemical considerations the best expression(s)is (are) selected. Also in these experiments one assumes that intrinsic kinetic data have been gathered. Several models for the gasification of carbon with COz (eq 1) have been proposed and are listed in Table I.
c + coz 2co
(1)
+
Much research into the mechanism of the gasification reaction was performed in the fourties and fifties by Reif,ls E r g ~ n , ~Key,20 ' - ~ ~ Blackwood and Gadsby et Ingeme,2l Strickland-Constable,22and Walker et al.= and al.,14916
~
~
~~
~
~~
(14) Long,F. J.; Sykes, K. W. h o c . R. SOC.(London) Ser. A 1952,215, 100-110. (15) (a) Gadsby, J.; Long, F. L.; Sleightholm, P.; Sykes, K. W. Proc. R. SOC.(London) Ser. A 1948,193,357-376. (b) Long, F. J.; Sykes, K. W. Proc. R. SOC.(London) Ser. A 1948,193, 377-399. (16) Reif, A. E. J. Phys. Chem. 1952,56,77&784,785-788. (17) Ergun, S. J . Phys. Chem. 1956,60,480-485. (18) Ergun, S.; Mentaer, M. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1965; Vol. 1, pp 203-263. (19) Ergun, S. US.Bur. Mines, Bull. 1962,598, 1-38. (20) Key, A. Gas Res. Board Commun. 1948,40, 36. (21) Blackwood,J. D.; Ingeme, A. J. A u t . J . Chem. 1960,13,194-209. (22) Strickland-Constable, R. F. J. Chim. Phys. 1950, 47, 356-378.
496 Energy h Fuels, Vol. 6, No. 4, 1992
more recently by Lbwe?' Koenig et aLz6and Walker et The two-step kinetic model (Table I) generally associated with Ergunl' but earlier proposed by Semechkova and F'rank-Kamenetakii,Bresulting in rate expression 2 which, by assuming the first step to be at quasiequilibrium, reduces to expression 3, predicts that the gasification rate is dependent on the p ( C O ) / p ( C 0 2 )ratio. al.26727
r=
klNtP(CO2) 1+ (k-i/k2)p(CO)+ ( k i / k ~ ) p ( C O ) z
(2)
(3)
0
10
20
30
40
50
Time ((I)
Ergun found substantial proof for this assumption by performing oxygen exchange experimentsbetween COPand 14C0,18as Temkin reported for the reaction between C 0 2 and Cl8Oin the same decennium.29 Both research groups found a much faster oxygen exchange than the gasification reaction. This model accounts for CO inhibition by lowering of the amount of C ( 0 ) complexes through the reversed first reaction step. In the same period, Keympublished a mechanism that accounted for CO inhibition by a reversible CO decomplexation (Table I). The rate expression 4 only differs from the Ergun expression by an extra term in the numerator, indicating the reversibility of the overall reaction. At low
p(CO), i.e., low COP conversion, both rate expressions converge to the same equation. Gadsby, Long, and Sykes14J5proposed a model (Table I), in which oxidation of carbon is irreversible and which contains a reversible CO adsorption step, leading to a rate expression that only differs from expression 2 in the composition of the coefficients in the denominator (eq 5). r=
kiNg(C02) 1 + (k3/k-&(CO) + (ki/k2)p(COz)
(5)
The models proposed by Ergun and Gadsby et al. both imply that CO lowers the gasification rate by decreasing the number of active C ( 0 ) complexes at steady-state gasification conditions either by removing them through the reversible first step or by blocking free sites, respectively, whereas in the model proposed by Key CO also inhibits by insertion of gas-phase CO, i.e., the incorporation of CO in the carbon matrix. Additional evidence for this assumption was provided by studies using radioactive 14C0 and 14C02ws2and 13C0.33 These studies indicated that (23) Walker, Jr., P. L.; Rueinko, F.; Auetin, L. G. Advances Catalysis; Academic Prees: New York, 1959; Vol. 11, pp 133-221. (24) LBwe, A. Carbon 1974,12, 335-348. (25) (a) Koenig, P. C.; Squires, R. G.; Laurendeau, N. M. Carbon 1986, 23,531-536. (b) Koenig, P. C.; Squires, R. G.; Laurendeau, N. M. Fuel 1986,65,412-416. (26) Strange, J. F.; Walker, P. L., Jr. Carbon 1976,14, 345-350. (27) Biederman, D. L.; Miles, A. J.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1976,14, 351-356. (28) Semechkova, A. F.; Frank-Kamenetakii,D. A. Acta Phys.-Chim. URSS 1940,12,87+898. (29) Temkin, M. I. Advances in Catalysis; Academic Press: New York, 1976; Vol. 28, pp 173-291. (30) Bonner, F.; Turkevich, J. J. Am. Chem. SOC.1951, 73,561-564. (31) Brown, F. Tram. Faraday Soc. 1962,48, 1005-1014. (32) Oming, A. A.; Sterling, E. J. Phys. Chem. 1954, 58, 1044-1047. (33) Phillips, R.; Vastola, F. J.; Walker, P. L., Jr. R o c . Third Ind. Carbon Graphite Conf., London 1971, 257-263.
Figure 1. Elution rates Gmolls) as a function of time (a) at 1250 K after a step change of 10% I3CO2 in Ar to Ar. Sample: 100 mg of Norit RX extra activated carbon.
carbon from the gas-phase molecules became incorporated in the carbon sample. From the results of a kinetic study in CO2,CO mixtures Koenig et aLP5proposed a model containing a reversible adsorption and dissociation of COP,leading to the formation and subsequent decomposition of two types of carbon-oxygen complexes (Table I), a carbonylic and a semiquinone structure. Blackwood and IngemeZ1studied the Boudouard reaction at higher pressures (up to 40 bar) and observed higher reactivities than predicted by the Ergun or Gadsby model. They proposed a five-step model (Table I), leading to rate expression 6, containing a second order dependency on P(C02.)
What is apparent from this overview, and emphasized by Walker et al.,23is that different models might lead to similar pressure dependencies of the overall reaction rate and discrimination will be nearly impossible without additional information. Moreover, from steady-state kinetic meaauremente, the concentration of active sites (NJ is not obtained individually but always in combination with a rate ~ a r a m e t e rwhile , ~ determination of the number of active sites during gasification is an important issue at present, since ASA methods do not give conclusive results." A general rule states that steady-state kinetic modeling never yields the proof of a mechanism, although it can support it. By contrast, a valid mechanism should always provide the basis of a proper kinetic model. As is apparent from the literature, gasification research is stuck to the point of kinetic modeling, probably by the lack of input from other techniques. In the kinetic models surface oxygen complexes are involved, which are generally not or only tentatively specified. Only in recent years, due to the application of transient kinetic techniques, has new information become available,which has led to the proposal of new models, with at least two surface intermediates (Table I). Step-response experiments, switching from C 0 2 to an inert gas, have been applied to study the reaction in more detail.3637 As is apparent from the data presented in the (34) Radovic, L. R.; Lizzio, A. A.; Jiang, H. In Fundamental Issues in Control of Carbon Gasification Reactivity, Lahaye, J., Ehrburger, P., Eds.; NATO AS1 Series E Appl. Sei. Vol. 192; Kluwer: Dordrecht, 1991; pp 235-256. (35) (a) Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990,28,7-20. (b) Radovic, L. R.; Jiang, H.; Lizzio, A. A. Energy Fuels 1991,5, 68-74.
Gas-Solid Reactions literature, but not always mentioned, the elution of CO cannot be described by a single-exponential decay. The initial fast decay, followed by the slower, suggests the involvement of two different intermediates. Meijer et al.38-41applied 13C02to distinguish between the pathways of the CO from the gaseous reactant and from the carbon. Figure 1is an example of a step-response experiment over an activated carbon (Norit RX extra) at 1250 K. The 13C02has disappeared the fastest after the step. This signal is the same as that obtained with an Sic sample under similar conditions and can be regarded as the system reference. Both 13C0and CO are observed to decay initially fast, followed by a slower decay. The CO decays at a higher rate than the 13C0. Since 13C0 is a gas-phase product, the 13C0 signal indicates that it has adsorbed at the carbon surface, probably at vacancies at the edges of the graphene layers, and desorbs now in the SRE. Reversible desorption processes indeed may result in decay patterns as observed here, but the contribution of different surface complexes, a heterogeneous surface,28142 or surface diffusion phenomena43cannot be excluded in this stage of the research. Since CO is formed in similar amounts as 13C0, the unlabeled CO should adsorb to a similar extent and the difference between CO and 13C0 represents clearly another CO production. The difference signal can be described as a single-exponential decay. At this point it is interesting to mention that the experiments of Bonner and Turkevichmcan be regarded as stepresponse experiments. They used a batch system and admitted 14C02,which reacted nearly completely within a short period with the carbon, resulting in 14C0and an oxidized surface. So, in fact they followed the decomposition of surface complexes in a 14C0 atmosphere and observed an initial fast CO production, followed by a slower one that lasted for a long period. This is in excellent agreement with our resulta. Also from their data one can conclude that 14C must have been accumulated on the sample, most probably by CO adsorption, either molecular or dissociative. Based on this type of experimental evidence obtained in SRE, using 13Cand l80labeled CO and C02,41a fourstep model, eqs 7-10, has been proposed to describe the C02gasification. Two different surface species are pro-
c02 + Cf == co + C(0) c02 + Cf, + C(0) ?= co + 2C-(CO)
(7) (8) c-(CO) co + Cf (9) C(0) co + 2Cf (10) posed to contribute to the gasification rate. One is a (36) (a) Adschiri, T.; Zhu, Z.-B.; Furueawa, T. Proc. Int. Conf. Coal Sci., Maastricht, The Netherlands 1987,515-518. (b) Zhu, 2.-B.; Furuaawa, T.; Adschiri, T.; Nozaki, T. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987,32, 132-139. (37) McCarthy, D. J. Carbon 1986,24,652-653. (38) Meijer, R.; Eck, S. C. van; Kapteijn, F.; Moulijn, J. A. Proc. Int. Carbon Conf Carbone '90,Paris 1990,538-539. (39) KapAjn, F.; Meijer, R.; Moulijn, J. A. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Ede.; NATO AS1 Seriea E Appl. Sci. Vol. 192; Kluwer: Dordrecht, 1991; pp 221-233. (40) Kapteijn, F.; Meijer, R.; Moulijn, J. A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991, 36 (3) 906-913. (41) Meijer, R. Ph.D. Thesis,University of Amsterdam, 1992; Chapter
Energy & Fuels, Vol. 6,No. 4, 1992 497
semiquinone that represents the slowly desorbing species (10); the other is the carbonyl complex, desorbing fast and reversibly (9). During steady-state gasification the semiquinones are formed by oxidation of a vacant carbon site. The carbonylic species are, apart from the reversible adsorption, produced by the oxidation of two adjacent sites on the armchair edge of the graphene layers, yielding a 1,a-diketonewhich readily breaks up under formation of two carbonyl complexes (8). In the latter case the formation of these adjacent groups is the rate determining process, whereas in the former it is the decomposition process (10). Hence, the gasification rate consists of a contribution proportional to the concentration of semiquinone groups and one proportional to this concentration times the C02pressure. Their relative contribution will depend on the C02pressure, as has been confiied by the SRE results. This model relates the SRE rate data with the steady-state gasification rates. It is interesting to note that this model implies that there might be a preferential gasification of one type of the graphene edges, depending on the experimental conditions. Further studies should confirm, of course, whether this holds for other carbons too,and if the model is applicable to other reactions, like H 2 0 gasification.44 It should be mentioned that the rate expression that can be derived from the proposed model is fairly similar to the one of Blackwood et aL,2l whose expression seem begt able to describe all (especially high pressure) C02gasification rate data.45 This mult illustrates the power of the transient response technique, namely that with a limited amount of experimental effort a good kinetic model has been derived, valid far outside the experimental range investigated.
Concluding Remarks Transient kinetic experiments with labeled molecules will provide detailed information on the mechanism of gas-solid reactions in general, as has been illustrated for the gasification of carbon as a specific application. The technique is relatively novel in its application and is still under development, as well with respect to the equipment, as to the mathematical modeling of the results. In the near future this technique will be increasingly applied in mechanistic research and probably constitutes an efficient route to kinetic modeling of heterogeneous reaction systems. Acknowledgment. Support by NATO grant CRG 910237 is gratefully acknowledged. Nomenclature rate constant of elementary step i of the corresponding model in Table I (units depend on the elementary process) Ki equilibrium constant of elementary step i (Table I) (units depend on the elementary process) Nt total concentration of active sites on the carbon, mol/g p(C0) partial pressure of CO, bar p ( C 0 2 ) partial pressure of COz, bar r gasification rate, mol/(g 8 ) Registry No. C, 7440-44-0; C02, 124-38-9. ki
9.
(42) Calo, J. M. In Fundamental Issues in Control of Carbon Gasification Reactiuity; Lahaye, J., Ehrburger, P., Eds.; NATO AS1 Series E Appl. Sci. Vol. 192; Kluwer: Dordrecht, 1991; pp 329-368,369-376. (43) Cazorla-Amorb, D.; Kapteijn, F. Proc. Int. Carbon Conf., Carbon '92, Essen, Germany 1992.
(44)Nozaki, T.; Adschiri, T.; Fujimoto, K. Energy Fuels 1991, 5 , 610-611. (45) Heek, K. H. van; Miihlen, H. J. In Fundamental Issues in Control
of Carbon Gasification Reactiuity; Lahaye, J.,&burger, P., Eds.;NATO AS1 Series E: Appl. Sci. Vol. 192; Kluwer: Dordrecht, 1991; pp 1-34.
