1146
Znd. Eng. Chem. Res. 1991,30, 1146-1151
Mechanisms of Methanation of CO and COPover Ni Shin-ichiro Fujita,* Hiroyuki Terunuma, Masato Nakamura, and Nobutsune Takezawa Department of Chemical Process Engineering, Hokkaido University, Sapporo 060, Japan
The mechanisms of the title reactions over Ni were studied by the transient response and the temperature-programmed-reaction method. These reactions were markedly different in their kinetics under the transient states. At the steady state of the CO methanation, adsorbed carbon, C(a), and strongly adsorbed carbon monoxide, CO(a), were appreciably present together with a small amount of reversibly adsorbed CO. In the C 0 2 methanation, the amount of C(a) was much smaller than that of CO(a). The amount of reversibly adsorbed CO was negligible. On analysis of the response curves obtained, it was shown that the dissociation of CO(a) was urmaffeded by H2 The hydrogenation of C(a) proceeded in proportion to 0.39th order in the partial pressure of H2. This process was suggested to be greatly retarded in the presence of reversibly adsorbed CO.
Introduction CO and C02 methanation occurs over various transition-metal catalysts. A number of the reviews related to CO methanation have been reported by many authors (Mills and Steffgen, 1973; Vannice, 1976; Ponec, 1978; Bell, 1981; Biloen and Sachtler, 1981). It has been widely accepted that this reaction proceeds through adsorbed carbon species C(a) on Ni catalysts (Araki and Ponec, 1976; Ponec, 1978; McCarty and Wise, 1979; Biloen et al., 1979; Goodman et al., 1980; Bell, 1981; Biloen and Sachtler, 1981). C 0 2 methanation has also been considered to occur through this adsorbed species (Araki and Ponec, 1976; Falconer and Zagli, 1980, Solymosi et al., 1981; Weatherbee and Bartholomew, 1981; Peebles et al., 1983; Fujita et al., 1987; Zagli and Falconer, 1981). However, the activity and selectivity for the latter reaction are markedly different from those for the former reaction (Mills and Steffgen, 1973; Inui et al., 1978; Bardet and Trambouze, 1979; Solymosi and Erdohelyi, 1980; Iizuka et al., 1982). In the present paper, the transient response (Kobayashi and Kobayashi, 1974; Bennett, 1976) and the temperature-programmed-reaction method were applied to the study of the mechanism of CO and C02methanation over Ni. On analysis of the response curves obtained, the difference of the mechanisms of these reactions is elucidated. Experimental Section Catalyst. Basic nickel carbonate (Wako Pure Chemical Industry Ltd., extra pure grade) was pressed into pellets, crushed and screened to 24-60-mesh granules, and then decomposed to NiO at 773 K for 3 h in air. One gram of NiO thus prepared was packed in a reactor and reduced in H2 at a flow rate of 100 cm3(NTP)/min at 873 K for 13 h prior to the reactions. Methanation. Experiments were carried out in a differential flow reactor with a CO-H2 or a COz-Hz mixture at atmospheric pressure. For the reactions under the transient state, the gaseous composition in the inlet stream was changed stepwise at a constant total flow rate (100 cm3(NTP)/min)and that in the outlet stream was followed in time. Gases from the outlet of the catalyst bed were collected with autosampling devices (Hori et al., 1985) at computer-controlled intervals and analyzed by gas chromatography. Helium was used as diluent. Temperature-ProgrammedReaction. After exposure to a stream of CO, C02, a CO-H2 mixture, or a C02-Hz mixture at a total flow rate of 200 cm3(NTP)/min and at a given temperature for 2 h, the catalyst was rapidly cooled to room temperature. Gases in the reactor were subse-
quently flushed with a helium stream until no desorption occurred. The temperature-programmed reaction was then started at a heating rate of 5 K/min in a stream containing 30 vol % H2. Gaseous components at the outlet of the catalyst bed were determined by gas chromatography. Number of Surface Ni Atoms. The number of surface Ni atoms was determined by H2 chemisorption at room temperature (Bartholomew and Pannel, 1980). It amounted to 284 pmol/g of Ni for the present catalyst.
Results and Discussion CO Methanation. Figure 1A shows the outlet partial pressures of CHI and H 2 0 formed in a CO-H2 mixture (Pco = 0.1 atm, PHp= 0.9 atm) against time. On feeding of the mixture, the outlet partial pressure of H20 overshoots rapidly and decreases until a steady state is achieved. The formation of CHI proceeds slowly as compared with that of H20. The outlet partial pressure of CHI increases to a steady value in a monotonic manner. The selectivity for the CHI formation at the steady state was estimated to be 50% on a carbon basis. In separate runs, the steady-state rate was found to decrease appreciably with increasing the partial pressure of CO, as observed by many authors (Mills and Steffgen, 1973; Vannice, 1976; Bell, 1981; Biloen and Sachtler, 1981). Figure 1B shows how the outlet partial pressures of C&, H20, and CO change when the stream of the mixture was switched to that of H2 at the steady state of the reaction. On the step change of the feed to H2, CO in the outlet stream diminishes within a few minutes, suggesting that reversibly adsorbed CO desorbs rapidly. The amount of this species was estimated to be 12 pmol/g of catalyst by integration of the response curve of CO. In contrast, the CH, and H,O formation overshoot appreciably, decreasing with time. The CHI formation at a maximum reaches rapidly to a factor of about 20 times that at the steady state. The HzO formation rises slowly to about 3 times as much as that in the steady state and then falls off with time. In a similar manner, the methanation was carried out in a stream of a CO-H, mixture with lower partial pressure of CO (Pco = 0.02 atm). When the mixture was switched to H2 at the steady state of the reaction, CO in the outflow diminished rapidly. The amount of the reversively adsorbed CO was much smaller than that in the preceding run. On the other hand, the CHI and H20 formation was enhanced to 4 and 1.5 times those in the steady state, respectively, and then decreased. The extent of the enhancement was therefore much smaller than that observed for the previous mixture (Figure 1B). On the basis of these findings, we concluded that the reversibly adsorbed CO inhibited the CHI and H20 formation. When
0888-588519112630-1146$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 6,1991 1147 Table I. Coverages of Strongly Adsorbed CO and Adsorbed Carbon on Ni during CO Methanation Reaction in the Steady States at 473 K
(I) (11)
0.9 0.1 0.9 -
0.1
I
I
I
I
1
1
(6) (1
t h i n
Figure 1. Change of outlet partial pressure of CHI, HzO, and CO with time. (A) HZstream was switched to stream I [CO + H21. (B) Stream I was switched to stream I1 [H,].
l5
I
8-
--t
. E
2
6.
m
N i Cat.
1
Ni cat. T = 413 K
0 x
-
+ 0
CH4
-
H20 ‘H2
&o-
‘C02 ‘He
(1) 0.9 0.1 (11) 0.9 -
4-
0.1
-
t Imin
Figure 3. Change of outlet partial pressure of CHI and H20 with time. (A) H2stream was switched to stream I [COz + H21. (B) Stream I was switched to stream I1 [H2].
which underwent the hydrogenation to CHI through the steps CO(a) C(a) + O(a) and C(a) + H2 CHI In these respects, H20 formed in the TPR run was ascribed to the hydrogenation of adsorbed oxygen O(a) which was arisen from the dissociation of CO(a). As found by Underwood and Bennett on Ni/A1203 (Underwood and Bennett, 1984),C(a) and CO(a) were appreciably present in the CO methanation on Ni. On the basis of the results described above, the amounts of C(a) and CO(a) present in the steady state of the reaction can be evaluated from thotie of CHI and H 2 0 produced after a switch to H2 or He at the steady state of the reaction. Denoting VI and V2 as the total amounts of CH4 and H20 respectively formed in a H2stream and V, as that of H 2 0 formed in a helium stream under the transient states, we have the equations = v, + (1) and (2) v, = + v, where V, and V, designate the amounts of CO(a) and C(a) present in the steady state of the reaction, respectively. VI, V,, and V3can be evaluated by integration of response curves obtained in the transient state, so that V , and V, are readily determined by the use of eqs 1 and 2. Table I summarizes the values of V, and V, thus determined and the surface coverages of these species 8, and 8, The values of 8, and 8, were estimated from V, V,, and the number of surface Ni atoms. The table shows that the amount of C(a) present under the steady state of the reaction increases appreciably m the partial pressure of H2 decreases. This suggests that the hydrogenation of C(a) was enhanced in preference to the dissociation of CO(a) in the presence of H2.
-
Figure 2. Methane formation in temperature-programmedreaction of carbon-containing species adsorbed on Ni. Experimenb were carried out after (A) CO adsorption at room temperature and (B)CO methanation at 473 K, respectively.
the supply of CO was cut off at the steady state of the CO methanation, the reversibly adsorbed CO species desorbed rapidly and hence the formation of CHI and H20 would be markedly enhanced. When the CO-H2 mixture was switched to helium at the steady state of the reaction, the outlet partial pressures of CHI and H 2 0 declined rapidly. As compared with the results in Figure lB, the total amount of CHI formed in the He stream was estimated to be less than 0.1% of that formed in the H2stream. The total amount of H20 formed was somewhat higher than that of CHI formed. However, this amount of H 2 0 still remained below 1% of that formed in the H2stream. Hence, we concluded that the transient production of CHI and H20 shown in Figure 1B was resulted from the hydrogenation of the surface species present in the steady state of the reaction. TPR runs were conducted after CO was adsorbed at room temperature or CO methanation was carried out at 473 K. As Figure 2 shows, one peak of CHI formation occurs at 473 K over the catalyst pretreated with CO. Over the catalyst previously subjected to the methanation, another peak of C H I formation is discernible at 430 K on a shoulder of the former one. The peak at 473 K wm always accompanied by H20 formation. When C(a) species were previously deposited on the catalyst by disproportionation of CO at 473 K, a strong CHI peak was found to occur at 430 K in a TPR run. The CHI peak at 430 K, shown in Figure 2, was therefore ascribed to the hydrogenation of C(a). On the other hand, the CHI peak at 473 K was originated from strongly adsorbed carbon monoxide CO(a),
-
v,
v,
v,,
1148 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 10
-
I
1
1
-
-
N i cat.
9 8 \
0
PH,latm 0.9 0.5 0.2 0.1
6 -
o 7
x
-A
Table 11. Coverages of Strongly Adsorbed CO and Adsorbed Carbon on Ni Catalyst during C 0 2 Methanation Reaction in the Steady States with Various Partial Pressures of H,at 473 K
-
V I
2 . n 173
373
573
T/K
Figure 4. Methane formation in temperature-programmed reaction of carbon-containing species adsorbed on Ni. Experiments were carried out after (A) C02 methanation at 473 K and (B)COz adsorption at 473 K, respectively.
CO, Methanation. In Figure 3A are illustrated the response curves for CHI and HzO formed in CO, methanation. When a stream of Hz is switched to that of a C02-H2 mixture, the CH, formation increases with time to a steady state in a monotonic manner. As compared with this, the HzO formation occurs at a rapid rate. The outlet partial pressure of HzO overshoots rapidly, decreasing to a steady value. The rate of CH4formation at the steady state is about 6 times that in the CO methanation. The by-products were mainly composed of CO. The selectivity for the CHI formation reached 92% on a carbon basis. As reported by a number of investigators (Mills and Steffgen, 1973; Inui et al., 1978; Bardet and Trambouze, 1979; Weatherbee and Bartholomew, 1981; Fujita et al., 1987),the CO, methanation was much more selective than the CO methanation. After a steady state was attained for the CO, methanation, the C02-H2 mixture was switched to H,. In response to this, CO in the outlet stream was found to vanish instantly, suggesting that the amount of the reversibly adsorbed CO was practically negligible. As evidently seen from Figure 3B, the responses of CH, and HzO are strikingly different from those observed in the CO methanation. The outlet partial pressure of CH, decreased with time in a monotonic manner whereas that of H 2 0 decreased instantly to half of the steady value and then fell off slowly with time. When the C02-H2 mixture was switched to helium at the steady state of the reaction, the formation of CHI ceased instantly whereas that of HzO decreased somewhat slowly and ceased within 2 min. The total amount of HzO desorbed was estimated to be about onetenth that desorbed in the H2 stream. These findings strongly suggest that CH, was produced by hydrogenation of adsorbed species. On the other hand, HzO was suggested to form through two distinct steps; one occurred only in the presence of C02and Hz in the gaseous phase whereas the other occurred by hydrogenation of adsorbed species. Figure 4 illustrates TPR profiles obtained after the C02 methanation and COOadsorption were carried out at 473 K, respectively. A strong peak of CHI formation, which is arisen from CO(a), occurs at 473 K over the catalyst subjected to the former pretreatment. However, the peak shape is asymmetric and its slope on the lower temperature side extends to 400 K. These suggest that a very small amount of C(a) is present together with CO(a). Over the catalyst subjected to the latter pretreatment, the CHI peak is also observed at 473 K and is somewhat symmetric. The intensity of the peak is much weaker than that obtained after the pretreatment with the C02 methanation. This showed that CO(a) was predominantly present in the C02 methanation and was enhanced in the presence of H2. In
ha
4
0.22 0.22 0.25 0.27
0.049 0.054 0.061 0.070
these connections, it is to be noted that some researchers reported that adsorbed CO was formed by dissociation of CO, and the amount of CO formed was appreciably enhanced in the presence of Hz (Falconer and Zagli, 1980; Solymosi et al., 1980; Iizuka and Tanaka, 1981; Zagli and Falconer, 1981; Henderson and Worley, 1985). Falconer and Zagli showed that H2rapidly reacted with O(a) formed by CO, dissociation (Falconer and Zagli, 1980),and Henderson and Worley claimed that H2 rapidly regenerates vacant sites by the hydrogenation of O(a) (Henderson and Worley, 1955). This would be why the formation of CO(a) is enhanced in the presence of H2 In these respects, it was highly probable that H20 is produced through two different pathways in the CO, methanation; one occurred through the hydrogenation of O(a) formed by CO, dissociation COz CO(a) + O(a) and O(a) + Hz H 2 0 and the other occurred through that formed by CO(a) dissociation CO(a) C(a) + O(a) and O(a) + Hz HzO As shown in Figure 3B, the H20 formation decreased rapidly in response to a switch of the C02-H2 mixture to H,. Since COPin the gaseous phase rapidly diminished, the COPdissociation, i.e., COz CO(a) + O(a), would be ceased rapidly. This reflected upon the rapid decrease in the HzO formation at the initial period of the transient state. The responses of CH4and HzO formation were followed in time when a stream of the C02-H2 mixture was switched to that of Hz or helium at the steady state of the reaction. As has been done for the CO methanation, the surface coverages of CO(a) and C(a) present in the steady state of the reaction were estimated by using eqs 1and 2. Table I1 lists the results. As compared with the results for the CO methanation, the amount of C(a) was extremely lower than that of CO(a). However, the former species still increases appreciably with the decrease in the partial pressure of H2as observed in the CO methanation (Table I). This again suggests that the hydrogenation of C(a) is enhanced more than the dissociation of CO(a) in the presence of H,. Simulation. When a CO-H2 or a C02-H2 mixture is switched to H,, CO(a) and C(a) present are hydrogenated via a sequence of the steps as H, + 2* 2H(a)
-
-
-
-
-
Cqa)
+
*
kl
-
C(a)
+ qa)
ka H(a)
____)
cH4
fast
H(a)
HzO
+
*
+
*
Lnd. Eng. Chem. Res., Vol. 30, No. 6,1991 1149
1
where kl and k2 denote the rate constants. CO(a) is dissociated to C(a) and oxygen species O(a) is adsorbed on vacant sites *. The C(a) and the O(a) are hydrogenated to CHI and H20,respectively. Under these conditions, the rate constanb of the reaction steps for these adsorbed species can be determined by analysis of the response curves of the products in the H2 stream under the transient state. Material balance for a product i in a differential reactor at temperature T yields the equation dPi/dt = r(i)'(l - c)RT/c - Pi(F/V)(l/t) (3)
2 c
0l
-
2
where Pi is the average partial pressure of product i in the reactor, Pi is the outlet partial pressure of product i, r(i)' is the reaction rate per unit volume of the catalyst, t is the reactor void fraction, R is the gas constant, F is the total feed rate, and V is the volume of the catalyst bed. Under the present experimental conditions, the value of the term in the left-hand side of eq 3 is negligible as compared with those in the right-hand side, so that the equation may be rewritten as di)' = Pi@'/V)(1/(1 - 4)(l/RT) (4) Hence the reaction rate for the product i per unit weight of the catalyst, r(i), can be obtained by the equation as r(i) = PiF/(wRT)
(5)
where w is the weight of the catalyst used. Assuming that dominant species on the sites for the CO(a) dissociation are CO(a) and C(a) after Ozdogan et al. (Ozdogan et al., 1983),we have equations for CO(a) and C(a) species at a given partial pressure of H2 as d8,,/dt = -k18,(l - ,8 - 8,) (6)
_
.
1
P
H-,
0
-
0.1 atm
10
5
15
tlnin
Figure 5. Change of outlet partial pressure of CHI in transient states. C02-H2mixture (Pm = 0.1 atm, PHz= 0.9 atm) was switched to hydrogen stream of various PH2 values, and outlet partial preasure of CHI was followed in time. Solid circles show the experimental values. Solid lines are determined by simulation with kl and kz values indicated in the figure.
and de,/dt = kle,,(i - e, - e,) - k2ec
(7)
where, ,e etc., denote the average surface coverage of CO(a), etc. The rate of methane formation r(CH4)can be written as r(CH4) = k26$ (8) where S denotes the number of surface Ni atoms per unit weight of the catalyst. The rate of CHI formation at t = 0, r(CH4)0,can be expressed by the use of eq 8 as r(CH4)0 = k20,0S (9) where 82 denotes the surface coverage of C(a) at t = 0. The value of k 2 was immediately estimated from 62 and PCH at t = 0 by the use of eqs 5 and 9. Hence, the values o! Be and 8, in the transient state can be computed numerically by the use of eqs 6 and 7 for various kl values with known initial surface coverages of C(a) and CO(a). The outlet partial pressure of CH4 was then estimated by using eqs 5 and 8 for 8, computed. The optimum value of k, can be finally determined by fitting the partial pressure of CHI simulated to the response curve of the CHI formation in the transient state. Figure 5 compares the calculated values with the observed ones for the outlet partial pressure of CHI in the transient states of the C02methanation. In these experiments, a mixture with 0.1 atm of C02 and 0.9 atm of H2 was switched to H2 with various partial pressures. Solid circles represent the experimental data. Solid lines were determined by the simulations at kl and k2 values which are indicated in the figure. It is seen that the lines are well fitted with the experimental results. In a similar manner, the response curves were analyzed for the experiments in
f" - -
Y c Y
0.1
1
I
I
I
I
I
-,
kl kZ
I I I l
-
Table 111. Initial Coverages of CO(a) and C(a) after CO, Methanation Reaction with Various Partial Pressures of HI and k,and k 2Values Determined by Simulation PH."/atm PH,6/atm emo :8 k,/min-l k9/min-l 0.9 0.9 0.22 0.049 0.33 1.06 0.5 0.9 0.22 0.049 0.33 0.91 0.9 0.1 0.22 0.049 0.33 0.53 0.5 0.5 0.22 0.054 0.33 0.84 0.2 0.2 0.25 0.061 0.30 0.52 0.1 0.1 0.27 0.07 0.33 0.45 a Partial pressure of H2in the COz methanation. *Partial pressure of Hz in the H2 stream fed over the catalyst following the methanation reaction.
which the supply of C02was cut off for the mixtures with various partial pressures of H2. Table I11 lists the surface coverages of CO(a) and C(a) at t = 0 and the rate constants estimated in these experiments. In Figure 6, the k l and k2 values estimated are plotted against the partial pressure of H2at which the responses of the products were obtained.
-
1150 Ind. Eng. Chem. Res., Vol. 30, No. 6 , 1991
11
l4
N i cat.
12
T
1
= 473 K
10
ke = 1.06 min-l 6
2tl \ 0
t 0
-
i
l 10
5
This work was supported in part by an Incentive Grant for Scientific Research of Ministry of Education, Science and Culture which S.F. gratefully acknowledges. Nomenclature F = total feed rate, cm3/min kl = rate constant of dissociation of strongly adsorbed carbon monoxide (CO(a) + * C(a) + O(a)),min-I k2 = rate constant of hydrogenation of adsorbed carbon (C(a) + H, CHI), min-' Pi = average partial pressure of product i in the reactor, atm Pi = outlet partial pressure of product i, atm R = gas constant, atmcm3/(moEK) r(i)' = reaction rate for product i per unit volume of the catalyst, pmol/ (min.cm3) r(i) = reaction rate for product i per unit weight of the catalyst, pmol/(min.g of catalyst) r(i)O = r(i) at t = 0 under the transient state, pmol/(min.g of catalyst) S = the number of the surface Ni atoms, pmol/g of catalyst V = volume of the catalyst bed, cm3 V, = amount of adsorbed carbon, pmol/g of catalyst V, = amount of strongly adsorbed carbon monoxide, pmol/g of catalyst VI = total amount of methane formed in a hydrogen stream under the transient state, pmol/g of catalyst V2 = total amount of water formed in a hydrogen stream under the transient state, pmol/g of catalyst V , = total amount of water in a helium stream under the transient state, pmol/g of catalyst w = weight of the catalyst used, g * = vacant site
-
kl = 0.32 min-
x
Acknowledgment
15
tlmm
Figure 7. Change of partial pressure of CHI in a transient state. = 0.1 atm, P H=~0.9 atm) was switched to H2 CO-H2 mixture (PCO (0.9 atm), and outlet partial pressure of CHI was followed in time. Solid circles show experimental values. Solid line is obtained by simulation with kl and k2 values determined in C02 methanation.
It is seen that k , is practically constant irrespective of the partial pressure of H2. This led to the conclusion that CO(a) is dissociated without participation of HZ. Although somewhat scattered, k 2 is strongly affected by the partial pressure of H2 and approximated as k 2 = k 2 0 P ~ > 3(k," 9 is a constant). By the use of the k l and k2 values obtained above, the response curves for CO methanation were also examined. Figure 7 illustrates the outlet partial pressure of CH, against time elapsed in the transient state. A solid line is obtained by the use of eqs 5-8 with the kl and kz0 values estimated in COPmethanation. Solid circles represent the experimental data. As seen from the figure, the line is well fitted to the experimental results. Hence, we concluded that C(a) and CO(a) formed in the CO methanation are transformed to CHI in the H2 stream in the same way as those in COz methanation. Difference between CO and C 0 2Methanation Reactions. As shown above, in the absence of CO in the gaseous phase both C(a) and CO(a) derived from CO methanation exhibited the same reactivity toward H2 as those from C02methanation. As already shown in Figure lB, on desorption of reversibly adsorbed CO the formation of CH, was initially enhanced to 20 times that in the steady state whereas that of H 2 0 was only increased to 3 times that in the steady state. This suggested that in the CO methanation the CHI formation was strongly inhibited by the reversibly adsorbed CO as compared with the H20 formation. On the basis of the reaction mechanism shown above, it was therefore concluded that the hydrogenation of C(a) was markedly retarded as compared with other steps such as the dissociation of CO(a) and the hydrogenation of O(a). In these respects, it is readily understood that an appreciable amount of C(a) was present together with CO(a) in the CO methanation. In great contrast to this, in the C02 methanation the amount of reversibly adsorbed CO was practically negligible. Under these circumstances, C(a) was rapidly hydrogenation to CH, in the steady state of the COPmethanation, so that the amount of C(a) was much lower than that of CO(a).
-
Greek Symbols = reactor void fraction 0, = average surface coverage of adsorbed carbon Oc0 = average surface coverage of strongly adsorbed carbon 0: = average surface coverage of adsorbed carbon at t = 0 Be: = average surface coverage of strongly adsorbed carbon monoxide at t = 0 Registry No. CO, 630-08-0; Ni, 7440-02-0; COz, 124-38-9.
Literature Cited Araki, M.; Ponec, V. Methanation of Carbon Monoxide on Nickel and Nickel-Copper Alloys. J. Catal. 1976,44,439-448. Bardet, R.; Trambouze, Y. Methanation des Oxides de Carbone sur Catalyseure Ni-MgO. C. R. Acad. Sci. Paris, Ser. C 1979, 288, 101-102.
Bartholomew, C. H.; Pannel, P. B. The Stoichiometry of Hydrogen and Carbon Monoxide Chemisorption on Alumina- and Silicasupported Nickel. J. Catal. 1980,65,390-401. Bell, A. T. Catalytic Synthesis of Hydrocarbons over Group VI11 Metals. A Discussion of the Reaction Mechanism. Catal. Reo.Sci. Eng. 1981, 23, 203-232. Bennett, C. 0. The Transient Method and Elementary Step in Heterogeneous Catalyst. Catal. Reo.-Sci. Eng. 1976,13,121-148. Biloen, P.; Sachtler, W. M. H. Mechanism of Hydrocarbon Synthesis over Fischer-Tropsch Catalyst. In Advances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic: New York, 1981; Vol. 30, pp 165-216. Biloen, P.; Helle, J. N.; Sachtler, W. M. H. Incorporation of Surface Carbon into Hydrocarbons during Fischer-Tropsch Synthesis. J. Catal. 1979,58,95-107.
Falconer, J. L.; Zagli, A. E. Adsorption and Methanation of Carbon Dioxide on a Nickel/Silica Catalyst. J. Catal. 1980,62,280-285. Fujita, S.; Terunuma, H.; Kobayashi, H.; Takezawa, N. Methanation of Carbon Monoxide and Carbon Dioxide over Nickel Catalyst under the Transient State. React. Kinet. Catal. Lett. 1987,33, 179-184.
Goodman,D. W.; Kelly, R. D.; Madey, T. E.; Yates, J. T., Jr. Kinetica of the Hydrogenation of CO over a Single Crystal Nickel Catalyst. J. Catai. 1980,63, 226-234.
Znd. Eng. Chem. Res. 1991, 30,1151-1157 Henderson, M. A.; Worley, S. D. An Infrared Study of the Hydrogenation of Carbon Dioxide on Supported Rhodium Catalysk An Inverse Spillover. J. Phys. Chem. 1986,89, 1417-1423. Hori, B.; Takezawa, N.; Kobayashi, H. Sampling Devices for the Transient Response Study of the Catalyst Reaction. Ind. Eng. Chem. Fundam. 1986,24, 397-398. Iiiuka, T.; Tanaka, Y. Dissociative Adsorption of C02 on Supported Rhodium Catalyst. J. Catal. 1981, 70,449-450. Iizuka, T.; Tanaka, Y.; Tanabe, K. Hydrogenation of CO and COz over Rhodium Catalyst Supported on Various Metal Oxides. J. Catal. 1982, 76, 1-8. Inui, T.; Funabiki, M.; Suehiro, M.; Sezume, T.; Iwana, T. Methanation of Carbon Dioxide and Carbon Monoxide over Supported Ni-Laz03-Ru Catalyst. Nippon Kagakukaishi 1978, No. 4, 517-524.
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Phys. Chem. 1983,87,437&4387. Ponec, V. Some Aspecta of the Mechanism of Methanation and Fischer-Tropsch Synthesis. Catal. Rev.-Sci. Eng. 1978, 18, 151-171.
Solymosi, F.; Erdohelyi, A. Hydrogenation of C02 and CO over Alumina-Supported Nobel Metals. J. Mol. Catal. 1980, 6, 471-474.
Solymosi, F.; Erdohelyi, A.; Kocsis, M. Surface Interaction between Hz and H2on Rh/A1203, Studied by Adsorption and Infrared Spectroscopic Measurement. J. Catal. 1980,65,428-436. Solymosi, F.; Erdohelyi, A.; Bansagi, T. Methanation of C02 on Supported Rhodium Catalyst. J. Catal. 1981,68,371-382. Underwood, R.P.; Bennett, C. 0. The CO/H2 Reaction over Nickel-Alumina Studied by the Transient Method. J. Catal. 1984,86, 245-253.
Kobayashi, H.; Kobayashi, M. Transient Response Method in Heterogeneous Catalysis. Catal. Reu. 1974,10, 139-176. McCarty, J. G.; Wise, H. Hydrogenation of Surface Carbon on Alumina Supported Nickel. J. Catal. 1979,57,406-416. Mills, G. A.; Steffgen, F. W. Catalytic Methanation. Catal. Rev. 1973,83, 159-210.
Ozdogan, S. Z.; Gochis, P. D.; Falconer, J. L. Carbon and Carbon Monoxide Hydrogenation on Nickel: Support Effects. J. Catal.
Vannice, M. A. The Catalytic Synthesis of Hydrocarbons from Eng. 1976,14, Carbon Monoxide and Hydrogen. Catal. Rev.-% 153-191.
Weatherbee, G. D.; Bartholomew, C. H. Hydrogenation of C02 on Group VI11 Metals. J. Catal. 1981, 68, 67-76. Zagli, E.; Falconer, J. L. Carbon Dioxide Adsorption and Methanation on Ruthenium. J. Catal. 1981,69, 1-8. Received for review September 26,1989 Reoised manuscript receioed August 27, 1990 Accepted January 6, 1991
1983,83,257-266.
Peebles, D. E.; Goodman, D. W.; White, J. M. Methanation of Carbon Dioxide on Ni(100) and the Effects of Surface Modifiers. J.
MATERIALS AND INTERFACES NMR and Intrinsic Viscosity Study of Two Different Phenol-Formaldehyde Resol Resins Moon G.Kim* Forest Products Laboratory, Mississippi State University, Mississippi State, Mississippi 39762
Larry W.Amos Weyerhaeuser Company, Tacoma, Washington 98477
Two different phenol-formaldehyde (PF) resol resins used as binders in wood products manufacture were subjected to acetylation and fractionation, and the fractions were studied by quantitative NMR, intrinsic viscosity, and number average molecular weight measurements. The NMR results described the structures of these resin fractions reasonably well as polymeric methylene (hydroxymethy1)phenols. However, the solution 13C NMR results, obtained with long delay times and nuclear Overhauser effect (NOE) suppression, showed significant deviations from the structure, apparently due to the limitations of the method used. While higher molecular weight fractions deviated more than low molecular weight fractions, resin A fractions deviated less than resin B fractions, reflecting the structural differences between the two resins. Mark-Houwink correlation results for resin A fractions indicated nonuniform structures, and for resin B fractions the correlation resulted in an “a” value of 0.21 in chloroform and 0.12 in benzene, indicating a compact structure typical of branched polymers. Intrinsic viscosities of resin A fractions expanded less than those of resin B fractions upon a solvent change from benzene to chloroform. Resin B fractions showed higher, more variable Huggins’ constants than those of resin A fractions, indicating their higher tendency of molecular associations in solution, which was interpreted to give rise to their higher deviations in solution l9C NMR results. Introduction Phenol-formaldehyde (PF) resol resins used as binders in the wood products industry are known to be composed of polymeric methylene (hydmxymethy1)phenols(Megson, 1958). But defining the polymer molecular structure and 0888-5885/91/2630-1151$02.50/0
properties with respect to the performance or synthetic procedures has been difficult due to the lack of adequate analytical procedures and in part due to the instability of resol resins. The reported molecular weight determinations by gel permeation chromatography or ultracentrifuge 0 1991 American Chemical Society
