Ind. Eng. Chem. Res. 1987, 26, 379-383
379
Transient Temperature Technique Applied To Study Activity Pattern Changes during the Hydrogenation of CO over Nickel/Silica Catalysts The hydrogenation of carbon monoxide over Ni/SiOz catalysts a t a high Hz/CO ratio has revealed a reversible change in the rate of methane formation a t 473 K. Such effects have been observed by many other investigators by employing bulk and supported nickel catalysts. Two recent models have been proposed to account for this behavior. The first considers a direct surface suppression of active catalytic centers by the deposition of carbon. T h e second proposes a change in a rate constant for one of the steps in the model for the mechanism. Neither proposal has been confirmed experimentally. A variation of the multiple-cycle transient analysis (MCTA) technique has been employed to determine which model is valid. In this study the temperature of the catalyst bed is modulated in the form of a sinusoidal wave. The time-dependent methane signal becomes asymmetric a t the transition temperature, 473 K. Comparison between experimental and predicted results demonstrates that the surface suppression model due to carbon deposition does not provide adequate agreement. On the other hand, the model that considers a change in mechanism does provide a reasonable prediction of the time-dependent methanation rate. Since nickel was found to be an effective catalyst for the methanation of carbon monoxide, numerous studies have been directed toward revealing the surface reaction mechanism. Reaction intermediates, such as CH,O, C, and CH,, have been proposed in those mechanisms. Recently, Lee and Schwarz (1986) summarized the salient findings for nickel catalysts by previous investigators. One intriguing feature of those studies directed toward determining possible models for the mechanism of the methanation reaction is the observation of a change in the activity pattern for the catalyst as a function of temperature. Depending on the reaction conditions, two kinetic regimes characterized by different activation energies are found. Provided the reaction temperature is maintained low enough, the transition between the two kinetic regimes is entirely reversible. Table I summarizes the results from previous investigators, defines the reaction conditions, and provides their proposed explanation for this behavior. Two conclusions emerge to explain this phenomenon. The first proposes that there is an increase in surface carbon that either limits the number of active sites or is, itself, converted to a less active carbon (Ho and Harriott, 1980; Goodman et al., 1980). The second proposes a change in the rate constant of one of the pathways leading to methanation (Dalmon and Martin, 1983; Polizzotti et al., 1978). The objective of this study is to provide data which will allow testing the applicability of these two proposed models for the change in the kinetics of the methanation reaction. In order to accomplish this objective, a variation of the multiple-cycle transient analysis technique is employed in conjunction with steady-state and other transient procedures. Multiple-cycle transient analysis temperature (MCTAT) periodically forces a reaction system to change from one temperature to another while perfused by a constant reactant supply. A sinusoidal temperature wave that is symmetrical about a *common” temperature is the perturbation to the reaction system. As will be demonstrated, the time response of the product (CH,) signal is periodic, but its symmetry is a function of experimental conditions. In light of the observed effect of temperature on the activity change of the catalyst, the MCTA-T technique appears to be suitable to employ for study of these activity changes that occur at the transition temperature. A high H2/C0 ratio has been chosen to ensure that carbon formation would occur at the elevated temperatures, but its accumulation would not lead to irreversible changes in the steady-state activity of the catalyst over the range of temperatures studied (Lee and Schwarz, 1986). Thus, 0888-588518712626-0319$01.50/0
experimental conditions suitable to distinguish between the two proposed models cited above are met.
Experimental Section
A TPD apparatus with a microprocessor-controlled infrared oven (Micricon 823) was employed in the MCTA-T studies. The temperature of the bed was measured at the center. A uniform temperature distribution has been demonstrated to exist through the catalyst bed (Huang, 1984; Huang et al., 1986a). A flow rate of 100 cm3/min of premixed H2/C0, having a ratio of 201, was used. The catalyst contained 47 wt % Ni/Si02; its physical properties have been described elsewhere (Lee and Schwarz, 1982) and are summarized in Table 11. A fresh catalyst sample of 100 mg was dehydrated at 373 K in He at a flow rate of 100 cm3/min for 2 h. After dehydration the catalyst was cooled to room temperature in flowing He. The gas stream was switched to pure hydrogen at a flow rate of 100 cm3/min. The catalyst was brought to 723 K by a 5 K/min temperature ramp and reduced at 723 K for 3 h. The catalyst was purged in pure He and cooled to room temperature. The H 2 / C 0 gas mixture was introduced into the system at room temperature, and the temperature was gradually increased. Previous studies showed that at such low CO pressures carbonyl formation was negligible (Lee and Schwarz, 1985). Throughout the experiment the m / e 15 peak was continuously monitored by an on-line UTI-100C mass spectrometer. After the system reached steady state for 10 min, the rate of methanation was recorded. The temperature was then modulated in a sinusoidal wave around this common temperature with an amplitude of f 5 K. The procedures used to accomplish the desired temperature variation required a balance between the power input to the oven and the water flow rate in a coil surrounding the reactor. The cycle time for the temperature wave was 240 s. For this modulation time and these reaction conditions, it was previously shown (Huang et al., 1986a) that the effective thermal conductivity of the catalyst bed was 2.6 W/(m K) which ensured a uniform temperature through the bed and that the conversion would be less than 10% over the wide range of reaction temperatures studied. Kinetic data similar to those reported in detail here were found for cycle times of 300 and 360 s, which was also consistent with the earlier measurements (Huang et al., 1986a). Within 10 cycles the system reached a “cyclic” steady state. Modulation was then stopped and the catalyst kept at the common temperature for 10 min to allow the system 0 1987 American Chemical Society
380 Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987 Table I. Summary of Literature Survey author reaction condition Dalmon and Martin, 1983 Pcn = 1-180 torr, Pw,= 150-630 torr, T = 413-673 K, 4.5%-i'3% Ni/SiO, Goodman et al., 1980 H,/CO = 4, P = 1, 10 torr, T = 450-800 K, Ni(100) Goodman and Yates, 1983 PCO= 2 torr, PH>= 8 torr, T = 300-800 K, Ni( 100) Ho and Harriott, 1980 PC0 = 0.07 atm, PH2= 1 atm, T = 400-800 K, 2% and 10% Ni/Si02 Huang and Schwarz, 1986b, 1986c H,/CO = 3, T = 433-800 K, P = 1 atm, 0.84%-8.29% Ni/A1,OB H,/CO = 3, P = 1 atm, T = 550-800 K, Ni Polizzotti et al., 1978 powder H,/CO = 3. 15. P = 10 torr. T = 350-800 K, Polizzotti and Schwarz, 1982 Ni foil _ i
Table 11. ProDerties of t h e Catalyst Ni content, wt '70 apparent particle density, g/cm3 porosity, cm3/g of catalyst Ni surface aream, m2/g of catalyst total surface area (BET), m2/g of catalyst av of catalyst crystallite size, A
45-49 1.57-1.61 0.30-0.38 70-80 270-290 20-22
A. Steady-State Methanation Rate. The circled points in Figure 1 show an Arrhenius plot of the experimental steady-state methanation rate. The difference between the first steady-state methanation rate (before temperature modulation) and the second steady-state methanation rate (after temperature modulation) was negligible for all temperatures within the range 435-534 K. The apparent activation energy for the methanation rate was approximately 25 kcal/mol for the temperature region lower than 473 K. Above 473 K, the apparent activation energy was approximately 3 kcal/mol. B. Time-Dependent Methanation Rate. Figure 2 shows the time-dependent methanation rate during the tenth cycle of the MCTA-T for different common temperatures. When the temperature was lower than the transition temperature, the methanation rate wave form was symmetric and similar to the input sinusoidal temperature wave. Asymmetric wave forms of the methanation rate occurred above the transition temperature as shown in parts d and e of Figure 2. Methanation Models A. Basic Model. Steady-state experiments showed a reversible transition in the methanation rate a t approximately 473 K. MCTA-T wave forms became asymmetric above this temperature. The TPR results provide confirmation that the surface distribution of carbon-containing species undergoes a unique change at this temperature. Figure 3 shows a typical TPR spectrum obtained after the MCTA-T experiments. There was one asymmetric peak
surface carbide and graphite formation inhibitation of H2adsorption by carbon monoxide due to change in temperature change in activity pattern change in rate constant competative adsorption between H2 and CO on surface
\
.- :.I-
to reach a second steady state. The rate of methanation was recorded. The catalyst was cooled to room temperature and flushed with He to desorb physisorbed H2 and CO. A temperature-programmed reaction (TPR) with a 5 K/min temperature ramp and a H2flow rate of 100 cm3/min was employed to obtain the m / e 15 TPR spectra and to clean the catalyst surface. A single peak was observed in the TPR spectra. A second TPR was performed to ensure that the catalyst surface was free of adsorbed carbon. A series of experiments were performed at different common temperatures ranging from 435 to 534 K.
Experimental Results
interpretation change in apparent activation energy due to change in surface coverage of CO surface carbide and graphite formation
,
-0- Exwrimcntol
I6 '
regardless of the common temperatures at which MCTA-T was conducted. Due to the experimental procedures, both surface carbon and undissociated CO were likely to remain on the surface. Previous studies (Lee, 1983) have demonstrated that TPR spectra of CO and surface carbon are similar, although the latter has a larger half-width at half-height than the former. This could account for the asymmetry shown in Figure 3. The shape of the TPR spectra was analyzed by a shape ratio, defined as the ratio of the width at halfheight to the peak height. Figure 4 shows the plot of the shape ratio vs. temperature. The shape ratio indicates a discontinuity at 473 K. For temperatures greater than 473 K the shape ratio increases which suggests that the dominant carbon speciation is surface carbon. A suitable basis is required to distinguish whether the methanation reaction mechanism is influenced by the presence of surface carbon or a reflection of a change in the rate constant for one of the steps in the overall reaction model at the transition temperature. The model of Lee et al. (1983, 1986) is used as a basis. The results of Lee et al. (1983, 1986) using the same catalyst showed reversibility as the temperature was increased or decreased but no loss in catalytic activity, because the temperature was confined to be below 475 K. Their proposed model was found to represent well the experimental data obtained under the following conditions: H2/C0 ratios from 1.34/1 to 10/1 and flow rates from 88 to 176 cm3/min. Their model is taken as a basis for predicting activity data for T > 475 K. It is described as
Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987 381 1. adsorption and desorption
CO,,)
k
+ Ni & CONi k-I
k
Hz + 2Ni .&
2HNi
k-2
(1) (2)
2. initiation of surface reaction (hydrogen-assisted CO dissociation)
CONi
+ 2HNi
CNi
3. propagation
CNi CH,Ni
+ nHNi
k4
+ (4 - n)HNi
4. termination CH4Ni HzONi
k6
k
+ HzONi + Ni + nNi CH4Ni + Ni
CH,Ni
k5
(3)
(4) (5)
CH4 + Ni
(6)
HzO + Ni
(7)
Steps 5-7 were assumed to be faster than steps 3 and 4, and the majority of the surface was assumed to be covered by CO, C, and H. The time-dependent mass balance equations for each species can be written as
1979; Martin et al., 1978) and interact with a larger number of sites (Wentrcek et al., 1976). The parameter x is introduced to test the direct surface suppression model based on the possibility that each surface carbon can be “catalytically” transformed on more than one Ni surface site. The mass balance equations are very similar to those of the basic model except for the mass balance on the total number of surface sites. The same procedures used by Lee and Schwarz (1986) are used here to generate the numerical simulation results. C. Effect of a Change in Rate Constant. Yang et al. (1984) recently studied the abundancy and reactivity of surface intermediates in methanation by using an isotopic tracing transient technique. They proposed that methane is formed from the reaction mechanism
They concluded that surface blocking by less reactive carbon is not controlling the coverage of reactive intermediates and proposed that the reaction rate constant, kl*, is hydrogen coverage dependent. This proposal implies that the methanation reaction mechanism changes with temperature and thus might result in a lower activity above some threshold temperature. In light of the conclusions drawn from Table I and consistent with step 3 in the basic model by Lee and Schwarz (1986), a modified form for the surface carbon formation step is written as CONi
+ 2HNi --%CNi + HzONi + Ni
It is important to note that in the basic model, the rate
and the rate of formation of methane is
The kinetic parameters used in this model were determined experimentally by T P D and T P R (Lee and Schwarz, 1982, 1986). They are listed in Table 111. B. Effect of Surface Carbon. Wolf and Alfani (1982) recently reviewed the relationship between catalyst activity and coking by two limiting cases: (i) direct surface suppression and (ii) indirect surface suppression. The former case assumes coke precursors are irreversibly adsorbed on the active sites and that activity changes occur because of direct subtraction of active sites from the catalytic cycle. The latter case assumes that the formation of coke deposits near the pores’ mouth decreases the effective diffusivity and, in the limiting case, blocks access to the active sites. Because the reaction conditions used in this study were free from mass- and energy-transfer limitations (Huang et al., 1986a), only the direct surface suppression model was considered. Here the surface is considered to be altered by a less active carbon, which is formed from the reactive carbon pool by a unidirectional reaction. An additional step in the basic model described earlier is designated as the less active surface carbon formation step:
When x = 0, one reactive active surface carbon is transformed to one less active surface site. Several reports demonstrate that the surface carbon may occupy three active sites (Wentrcek et al., 1976; McCarty and Wise,
constant k, is assumed to be only dependent on temperature (see Table 111). In each of the models proposed to explain the reversible change in the steady-state activity above 473 K, an additional rate constant has been added when compared to the basic model by Lee and Schwarz. There does not exist any direct measurement techniques to determine the activation energy and preexponent for this rate constant. Therefore, a fitting procedure had to be developed. In the Lee model all rate parameters were determined experimentally. The kinetic parameters for the additional rate constants k,‘ and k8 were determined in the following manner. At each temperature studied, the steady-state turnover number (TON) was measured. Each model was solved numerically at that temperature, and the “best” (to within 1%) value of the unknown ki was determined iteratively. Here ‘‘2’ refers to the unknown constant at temperature “i”. This operation was carried out at each temperature for which experimental steady-state data were available. Thus, for each model, k, vs. 1/T was obtained. The slope of this plot gave E,; A, was determined by A i= kieE;/RTt. The rate constants determined were used in the numerical solution of the time-dependent mass balances for each of the models when subjected to MCTA-T conditions, and the results were compared with those found experimentally. The temperature modulation changed the magnitude of the rate constant, which was not insignificant over a cycle. For example, k,’ and k, changed by a factor of 1.9 and 2.8, respectively, over one cycle modulated around a common temperature of 435 K with a f 5 K amplitude.
Discussion A. Steady-State Methanation Rate. Figure 1 shows the experimental results (circles) and those for the numerical simulations for the different models. Curve A
382 Ind. Eng. Chem. Res,, Vol. 26, No. 2, 1987 Table 111. Summary of the Rate Constants Used in t h e Basic Model method TPD TPD TPD TPD TPR TPR
perturbation and variable linear linear linear linear linear linear 151
ramp ramp ramp ramp ramp ramp
on on on on on on
temp temp temp temp temp temp
carrier gas He He He He
reactive species Dreadsorbed CO preasdorbed CO preadsorbed H2 preadsorbed H2 preadsorbed C,,, preadsorbed C(s)and H,,,
H2
He
a
r
5 4 28
rate constant A , s-l 5.0 X 101*Pm __ 3.5 x 1013 1.5 x 1 0 - 1 4 ~ ~ ~ 1.5 x 1014 4.5 x 10" 7.0 X lo8
ki
k-l
k2
k-2
k, k4
C
i
I
216
220
224
228 232 236 TIME IsecJ
240x IO2
20
216
220
224
228
232
236
#Oxd
I
-270
220 224 228 232 236 240x12 TI ME isecJ
216
TIMEisecJ
e
d
756
E, kcal/mol 0 26-108,, 0 22 ( 1 4 . 5 8 ~ ' ) 25.4 17.0
'I 15
IS
Figure 2. Time-dependent methanation rate during the tenth cycle of MCTA-T operation. Curves labeled A-D correspond to (A) basic model, (B) direct surface suppression model ( x = O), (C) direct surface suppression model ( x = 2), and (D) change in rate constant model, respectively. The common temperatures are (a) 435, (b) 455, (c) 473, (d) 498, (e) 506 K, respectively. See text for detail.
-
CI
0 14-
-p
12-
k
08-
5E
a6-
W
04-
B
0.2-
6
10-
00
' 350
400
450
500
301
440
460
I
480
500
52U
I
549
C
T(KJ
Figure 4. Shape ratio vs. temperature. Note discontinuity at 473 K.
for each of the two alternative models considered. It was obtained by the parameter-fitting procedures described. The agreement between experimental results and numerical "fitting" is apparent for each model. The time-
Ind. Eng. Chem. Res. 1987,26, 383-386
dependent data are required to distinguish between the two models. The kinetic parameters (k3/and k,) were used in the numerical simulation, incorporating 10 cycles of temperature modulation. B. Time-Dependent Methanation Rate. Figure 2 shows the time-dependent methanation rates during the tenth cycle of MCTA-T for different common temperatures. When the temperature is lower than the transition temperature, the methanation rate wave form is symmetric and similar to the input sinusoidal temperature wave. For this region there is good agreement with the experimental data. This is indicated by Figure 2a-c. Asymmetric wave forms of the methanation rate occurred above the transition temperature. For this region, the model based on direct surface suppression by carbon predicts a lower methanation rate than that of the experimental results. The basic model predicts a rate higher than that found experimentally because no steps in the overall model account for a possible transition in the kinetics. The kinetic parameters determined from the steady-state methanation data support the proposal by Yang to explain the transition in kinetics. When this model is employed, the simulation results show good agreement with the experimental results.
Conclusions The results reported here demonstrate that unidirectional site-blocking models are not applicable in the experimental range studied. Only small differences between the initial steady-state rate and that measured after temperature modulation indicate that the rate of formation of site-blocking material, less active carbon, is small, and virtually all the surface sites are still active after MCTA-T experiments. The deposited carbon can be removed by TPR. This procedure results in a single asymmetric peak. In addition to steady-state data, MCTA-T provides the time-dependent rate of the methanation reaction when the reactor is subjected to temperature modulation. Figure 2 shows the time-dependent differences between model predictions and the experimental results. The effect of a change in the rate constant in the model predicts the experimental results well.
383
Acknowledgment This work was supported by the Division of Chemical Sciences, Office of Basic Energy Research, under the Department of Energy Contract DE-AC02-84ER 13158.
Literature Cited Dalmon, J. A.; Martin, G. A. J. Catal. 1983, 84, 229. Goodman, D. W.: Kelly, R. D.; Madey, T. E.; Yates, J. T., Jr. J. Catal. 1980, 63, 226.Goodman. D. W.: Yates. J. T.. Jr. J. Catal. 1983.82. 255. Ho, S. V.{Harriott, P. C a t h 1980, 64, 272. Huang, Y. J. Master Thesis, Syracuse University, Syracuse, NY, 1984. Huang, Y. J.; Schwarz, J. A.; Heydweiller, J. C. Ind. Eng. Chem. Fund. 1986a, 25(3), 402. Huang, Y. J.; Schwarz, J. A. Appl. Catal. 1986b, in press. Huang, Y. J.; Schwarz, J. A., submitted for publication in Appl. Catal. 1986~. Lee, P. I. Ph.D. Dissertation, Syracuse University, Syracuse, NY, 1983. Lee, P. I.; Schwarz, J. A. J. Catal. 1982, 73, 272. Lee, P. I.; Schwarz, J. A.; Heydweiller, 3. C. Chem. Eng. Sci. 1985, 40(3), 509. Lee, P. I.; Schwarz, J. A. Ind. Eng. Chem. Process Des. Dev. 1986, 25(1),76. Martin, G. A.; Primet, M.; Dalmon, J. A. J . Catal. 1978, 53, 321. McCarty, J. G.; Wise, H. J. Catal. 1979, 57, 406. Polizzotti, R. S.; Schwarz, J. A.; Kugler, E. L. Proceeding of the Symposium on Advances in Fischer-Tropsch Chemistry; American Chemical Society: Washington, DC, 1978. Polizzotti, R. S.; Schwarz, J. A. J. Catal. 1982, 77, 1. Wentrcek, P. R.; Wood, B. J.; Wise, H. J . Catal. 1976,43, 363. Wolf, E. E.; Alfani, F. Catal. Rev.-Sci. Eng. 1982, 24(3), 329. Yang,C. H.; Soong, Y.; Biloen, P. Presented at the 6th International Congress on Catalysis, Burlin, Germany, 1984.
i.
,
I
*Author to whom correspondence should be addressed.
Yao-Jyh R. Huang, James A. Schwarz* Department of Chemical Engineering and Materials Science Syracuse University Syracuse, New York 13244 Received for review January 2, 1986 Reuised manuscript received September 19, 1986 Accepted October 30, 1986
Catalytic Gasification of Rice Hull. 3. Measurement of Reaction Efficiency in the Steam Reforming of Carbonaceous Material Steam reforming of hydrocarbons provides an effective method of releasing hydrogen from a water molecule. The efficiency of the reaction depends on the structure of the reactant and catalysts and the reaction conditions. T h e traditional index, AT, for measuring reaction efficiency using the temperature difference between the actual reaction temperature and the thermodynamic equilibrium temperature of the product gases is inadequate. A new index, REST, is proposed t o indicate the reaction efficiency of steam participation in the steam reforming reaction; this index is calculated from the ratio of actual steam consumption vs. the theoretical requirement of steam. With both AT and REST indexes, the efficiencies of steam reforming reactions of biomass (rice hull), methanol, and hydrocarbons (methane and naphtha) are analyzed with respect t o the effects of temperature, H,O/C ratio, and Ni surface area. T h e results indicate t h a t REST approach provides greater sensitivity and more universal applicability in the general scope of this reaction. Steam reforming of methane to produce hydrogen is usually carried out in the presence of catalyst at a reaction temperature higher than 600 OC (Allen et al., 1975; Karim and Metwally, 1979; Singh and Saraf, 1979). At these conditions the reaction is virtually complete and the reaction efficiency is often measured by the temperature differential approach (AT) to compare with thermodynamic equilibrium (Akers et al., 1970; Anderson et al., 1984). The temperature differential approach is defined 0888-5885181/2626-0383$0l.50/0
as the difference between the actual reaction temperature (TRx)and the corresponding temperature (TEq)of the equilibrium constant calculated from the composition of the actual reaction products (Akers et al., 1970). The smaller the value of AT, the closer the reaction is to the thermodynamic equilibrium. When heavier hydrocarbons are used as feedstock, the reaction becomes more complicated. Unless prior formation of methane is involved (Yarze and Lockerbie, 1962) 0 1987 American Chemical Society
