J . Phys. Chem. 1987, 91, 6257-6269
t & I
3
4
5
6
-log
c
Figure 4. Effect of microdomains formation on the concentration of adsorbed ions. Number of adsorbed ions/number of charged lipids 6 vs logarithm of calcium concentration (M). Full lines refer to 0 values calculated with the present model; discontinuous lines have been calculated assuming a random distribution of charged and neutral lipids over the membrane surface. Upper and lower curves have been calculated at 0.10 and 0.50 M ionic strength, respectively.
curves (here not reported) were obtained for monovalent ions. The results suggest an appreciable increasing of the number of bound ions as a consequence of lipid aggregate formation, this effect taking place in a wide range of calcium concentrations and ionic strengths. On the basis of these results, the modulation of cellular functions induced by lateral phase separation could be mediated in some cases by the variation of bound ions on the membrane surface. ~ ~ its .~~ This hypothesis has been suggested in the l i t e r a t ~ r eand
6257
reliability is supported by the present figures. However, to our best knowledge, no direct measurements of binding constants have been performed on lipid membrane showing lateral phase separation. In conclusion, the present model is an useful tool in predicting, at a semiquantitative level, some general trends of the ionotropic lateral phase separation in lipid mixtures. The model shows a particular flexibility because the molecular aspects of the lipid bilayer, adsorbed ions, and surrounding medium can be taken into account through a set of semimacroscopic parameters ( W, K , 6 , etc.). The formalism could be improved in several ways, for example, taking into account a different ion concentration in the neighborhood of the membrane with respect to the bulk of solution, or using a more flexible variational function for the charged lipid concentration (see eq 1). The first point has been investigated by us calculating the effective ion concentration near the membrane surface as a function of the surface charge density in the Debye-Hiickel approximation. The effect of such an improvement on B and D numerical values is negligible (less than 10%) in the range of physiological ionic strength (lo-] M), becoming appreM). However, ciable only at low electrolyte concentrations (the mathematical complexity of the resulting formulas sharply increases, leading also to convergence problems in their numerical solution. The latter point concerning the flexibility of the variational function, as well as the linkage between the used parameters and the molecular properties of the system, is the object of further development in our laboratory.
Acknowledgment. This work has been partially supported by the Italian Minister0 della Pubblica Istruzione. ( 5 3 ) Ohki, S . Bioelecrrochem. Bioenerg. 1978, 5 , 204. (54) McLaughlin, S . ; Brown, J. J . Gen. Physiol. 1981, 77, 475.
Effect of Support Pretreatments on Carbon-Supported Fe Particles A. A. Chen, M. A. Vannice, and J. Phillips* Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: May 11, 1987; In Final Form: July 6. 1987)
Recent work on carbon-supported catalysts has shown that the nature of the carbon can influence the structure and stability of the supported metals. In this study, the effects of different high-temperature H2 treatments on a porous carbon black support were examined. Carbonyl-derived iron particles placed on differently treated carbon samples exhibited differing structure and chemical behavior as detected by Mossbauer spectroscopy, adsorption, and catalytic activity measurements. For carbon that has not been exposed to air following H2 treatment, a substantial fraction of the supported iron particles are highly dispersed and remain sinter resistant in a phase of iron formed during the initial carbonyl cluster decomposition. In contrast, iron supported on similarly treated but air-exposed carbon sinters more rapidly, while carbon not treated in H2 yields large particles reminiscent of those found on graphitic supports. A model is discussed whereby the observed behavior is attributed to the nature of the active sites that exist on the carbon black as a result of the pretreatments in H,.
Introduction Recent work has shown that the structure, stability, and chemical nature of metal particles on carbon supports can be influenced by the morphology and/or the surface chemical nature of the Large differences have been found, for example, between iron particles dispersed on graphitic supports and those dispersed on porous high surface area carbons. Previously it was shown that Fe particles supported on Grafoi13 or Vulcan 3 graphite1° exist in phases that are essentially identical with those *Author to whom correspondence should be addressed.
0022-3654/87/2091-6257$01.50/0
expected for bulk Fe. Moreover, these particles showed very little apparent interaction with the support, and they also sintered quite (1) Jung, H. J.; Walker, P. L. Jr.; Vannice, M. A. J . Card. 1982, 75,416. (2) Lin, S. C.; Phillips, J. J . Appl. Phys. 1985, 58, 1943. (3) Phillips, J.; Clausen, B.; Dumesic, J. A. J . Phys. Chem. 1980, 84, 1814.
(4) Linares-Solano, A,; Rodriguez-Reinoso, F.; Salinas-Martinez de Lecea, C.; Mahajan, 0. P.; Walker, P. L. Carbon 1982, 20, 177. (5) Jones, V. K.;Neubauer, L. R.; Bartholomew, C. H.J . Phys. Chem. 1986, 90, 4832.
(6) Ehrburger, P.; Walker, P. L. Jr. J . C a r d . 1978, 55, 63. (7) Ehrburger, P.; Mahajan, 0. P.; Walker, P. L. Jr. J . Cafal. 1976, 43, 61.
0 1987 American Chemical Society
6258 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
rapidly during heating.3,'0 In contrast, on high surface area porous carbons, there is evidence which indicates that the support can influence the morphology and chemistry of the particles. For example, unusual phases of Fe have been observed via Mossbauer spectroscopy for particles supported on such carbons,l0%I1 and some authors have postulated the existence of metal-carbon electronic interacti~ns.~.''*'~ Additionally, several reports indicate that very high metal dispersions can be obtained when porous carbon supports are used.l*10 Although enough information is available to state that porous carbons much more strongly influence the nature of supported metal particles than graphite, it is difficult to determine how or why. Pore structure, pore size distribution, impurities, and ordered domain structures are highly variable between different porous carbons, and an additional complicating factor is that activated carbons and carbon blacks are known to stabilize surface functional groups, which have already been shown to influence the polarity of the surface.13 This study focuses on the properties of carbon-supported metal particles as a function of support pretreatment prior to metal impregnation. Although the effects of burnoff treatments and activation procedures in various atmospheres have been studied for blank c a r b o n ~ , ~ *few ' ~ Jstudies ~ to date have concentrated on the effects that such treatments may have on supported meta l ~ . ~ , In ~ *this ~ - study, ~ it is shown on the basis of Mossbauer spectroscopy, C O chemisorption, CO hydrogenation kinetics, and transmission electron microscopy (TEM) measurements that the properties of Fe particles supported on a particular high surface area carbon black can vary markedly with the pretreatment given the carbon prior to metal impregnation. To help explain the major differences noted, analyses of the pore structure (N, and C 0 2 adsorption) and domain size and structure (X-ray diffraction) of the blank supports following each pretreatment were conducted. This had not been done in previous studies of carbon-supported iron particle^.',^,^^'"^^ On the basis of the data collected, it was found that, on carbons that were heat-treated in H 2 and never air exposed, most of the Fe particles have an unusual structure and resist sintering, possibly as a result of an interaction with unsaturated active sites created on the carbon. In contrast, particles that form on either untreated or H2-heat-treated, airexposed carbon have a structure, stability, and chemistry more similar to that of Fe particles on graphite.
Experimental Procedures Catalyst Preparation. Three types of Fe/carbon samples were prepared. The principal difference among the samples was the pretreatment given the carbon (CSX-203 from Cabot Corp.) prior to the metal loading. Each sample was impregnated in the same fashion with Fe3(CO)12(Pressure Chemical Co.) dissolved in dried, degassed THF. This solution was added dropwise to the continuously stirred carbon support until the point of incipient wetness was reached (3.4 cm3/g carbon). In all cases, the final Fe loading was approximately 10 wt %. The samples were then evacuated overnight at 0.13 Pa to remove the THF. To prevent exposure (81 Peters. U.:Greb. H.: Jockers. R.: Klein. J. In Prenarafiono f Cafalvsfs IV:' ScientiJc Bases for Preparation of Heterogeneous 'Cafalysfs~Delm~n et al., Eds.; Elsevier: Amsterdam, 1987; p 493. (9) Aika, K.; Hori, H.; Ozaki, A. J . Cafal. 1972, 27, 424. (IO) Jung, H. J.; Vannice, M. A,; Mulay, L. N.; Stanfield, R. M.; Delgass, W. N. J . Catal. 1982, 76, 208. (11) Hegenberger, E.; Wu, N. L.; Phillips, J. J . Phys. Chem. 1987, 91, 51367
(1 2) Rodriguez-Ramos, I.; Rodriguez-Reinoso, F.; Guerrero-Ruiz, A,; Lopez-Gonzalez, J. J . Chem. Technol. Biotechnol. 1986, 36, 61. (13) Puri, B. R. In Chem. Phys. Carbon 1970, 6, 191. (14) Martin-Martinez, J. M.; Linares-Solano, A,; Rodriguez-Reinoso, F.; Lopez-Gonzalez, J. D. Adsorpf. Sci. Technol. 1984, 1, 195. (15) Moreho-Castilla, C.; Domingo-Garcia, M.; Lopez-Garzon, F. J.; J . Colloid Inferface Sei. 1986, 112, 193. (16) Chen, A. A.; Kaminsky, M.; Geoffroy, G. L.; Vannice, M. A. J . Phys. Chem. 1986, 90, 4810. (17) Venter, J. J.; Kaminsky, M.; Geoffroy, G. L.; Vannice, M. A. J . Cafal. 1987, 103, 450. (18) Kaminsky, M.; Yoon, K. J.; Geoffroy, G. L.; Vannice, M. A. J . Cufal. 1985, 91, 338.
Chen et al. TABLE I: Analvsis of Ashed CSX-203 As Received' ~
~~~
~
elem
amt, ppm
elem
amt, ppm
elem
amt, ppm
Si
110 42 2 34
Mg
7 2154 3
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.
*.
..P . C
.;,..
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098 l . 1
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.
..
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,
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,
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0.98
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0.96 -
0.94
0.92
-
-10
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I
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VELOCITY
I 5
0
I
10
m m /see 1
Figure 6. Mossbauer spectra at 77 K following 643 K reduction in HZ: (a) LTE sample after 5 h of reduction; (b) HTA-1 sample after 17 h of reduction; (c) HTA-2 sample after 17 h of reduction; (d) HTE sample after 17 h of reduction.
indicating that the average particle size was less than about 15 nm. Indeed, on the basis of a simple formula3 and a reasonable estimate of the anisotropy energy constant (1 X lo6 erg/cm3), the average particle sizes in the magnetically split portions of the HTA-1 and HTA-2 samples were found to be 7 and 8.5 nm, respectively, based on the approximate hyperfine splittings noted in Table IV. The spectra of the HTA-1 and HTA-2 catalysts are fairly similar despite the fact that the HTA-1 sample was deliberately exposed to the air and rereduced, whereas the HTA-2 sample was reduced at 643 K directly after the low-temperature
reduction with no air exposure. This implies that the differences ~ among the particles supported on differently treated supports are principally due to the carbon surface prior to Fe impregnation, rather than air exposure subsequent to the impregnation step. In contrast to the LTE and HTA samples, the reduction process had very little influence on the iron in the HTE sample (Figure 6d). After 17 h of reduction treatment at 643 K, a significant fraction of the spectral area remained in the D structure, while the fraction of Fe associated with the large particles remained approximately constant. An additional 13 h of reduction at 643 K caused no further changes in the Mossbauer spectrum. To confirm the high dispersion (fraction exposed) obtained on the HTA and HTE samples, C O was again adsorbed on all samples by using the same procedure as mentioned earlier. For the LTE sample, only the growth of a small inner peak of the a-Fe sextuplet was observed, while a substantial amount of Fe(CO),, approximately 21% of the spectral area, was formed on the HTE sample (Figure 7). The formation of Fe(C0)5 in this sample is again strong evidence for the presence of highly dispersed Fe in this catalyst. For the HTA-1 and HTA-2 samples, the small peak at -1.5 mm/s in combination with an increased intensity of the 1.0 mm/s peak indicates that a small amount of Fe(C0)5 may have formed in these samples as well. Given that the Mossbauer data for the a-Fe phase for both the HTA samples and HTE sample indicate the presence of fairly large particles, it is reasonable to suggest that it is the D structure which is highly dispersed. One aspect of the Mossbauer spectra that requires additional comment is the broad dip seen in the background of many of the spectra. This phenomenon is associated with superparamagnetism and is consistently seen in both experimental and spectral modeling s t ~ d i e s . ~ , ~ , ~ ' ( 4 ) Exposure of Catalysts to Reaction Conditions. Treatment of the samples in a flow of 3: 1 H 2 : C 0 at 500 K for 2 h gave the following results (Figure 8). (1) The LTE sample gave a combination of €'-carbide lines superimposed on bulk a-Fe lines, according to the hyperfine splittings of 340 and 187 kOe, re~ p e c t i v e l y . ~(2) ~ . ~The ~ HTA- 1 sample was completely converted into a species which appears to be t'-carbide, while some residual a-Fe still existed in the HTA-2 catalyst. (3) The HTE sample still showed a predominant D structure peak. Although the previously observed outer lines of the a-Fe sextuplet are visible, the remainder of the spectrum background had become broad and featureless, making species identifkation impossible. Computer fitting was not attempted, and it should be noted that a similar spectrum has been observed previously on 2-nm Fe particles supported on carbon following exposure to a CO/H2 mixture at 510 K.39 For all of the samples except HTA-1, the presence of metallic Fe can again be attributed to the presence of some large, magnetically split particles. Previous studies have shown that the incomplete carburization of these particles could be due to carbon diffusional limitations at the s ~ r f a c e . ~ ~ . ~ ' CO Chemisorption Measurements. CO chemisorption was performed following both low- and high-temperature reductions in H2 To approximate the conditions used in the Miissbauer study, fresh catalyst samples were decomposed for 2 h at 473 K in a 40 cm3/min H2flow. Chemisorption isotherms were then obtained at 195 and 298 K, with a 2-h rereduction period at 473 K between measurements to remove adsorbed CO. Following these uptakes, the samples were reduced at 673 K for 16 h and the CO adsorption measurements were repeated. As indicated in Table V, these adsorption results are fairly consistent with the observations of the Mossbauer runs. After the 473 K reduction all four samples showed apparent dispersions near one-half. After extended reduction at 673 K the dispersion of the LTE catalyst dropped markedly, the HTA-1 and HTA-2 exhibited some mild sintering, (37) Gatte, R. R.; Phillips, J. J . Cural. 1987, 104, 365. (38) Amelse, J. A.; Butt, J. B.; Schwartz, L. H. J . Phys. Chcm. 1978, 82, 558.
(39) Niemantsverdriet, J. W.; Van der Kraan, A. M.; Delgass, W. N.; Vannice, M. A. J . Phys. Chem. 1985, 89, 67. (40) Raupp, G. B.; Delgass, W. N. J . Cutal. 1979, 58, 348. (41) Tau, L. M.; Bennett, C. 0. J . Phys. Chem. 1986, 90, 4825.
The Journal of Physical Chemistry, Vol. 91, No. 24. 1987
6264 r-201
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Chen et al.
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Figure 7. Mossbauer spectra at 77 K following 643 K reduction and CO adsorption at 298 K: (a) LTE sample; (b) HTA-1 sample; (c) HTA-2 sample; (d) HTE sample.
and the dispersion of the H T E sample showed little change. The fact that the CO uptake at 298 K routinely exceeded that found at 195 K implies that some subcarbonyl formation may be occurring on these samples,10 which is very consistent with the Fe(CO)5 Mossbauer signals. It should be noted that the C O uptake is high on the HTE sample despite the presence of some large particles, and it is probable that extensive subcarbonyl formation on the small crystallites more than compensates for the low CO uptakes expected on the larger particles.
I mm 1 sec 1
Figure 8. Mossbauer spectra at 77 K after 2 h at 498 K in a 3/1 H 2 / C 0 flow: (a) LTE sample; (b) HTA-1 sample; (c) HTA-2 sample; (d) HTE
sample.
To allow relative comparisons, particle sizes were calculated from the C O chemisorption data assuming a spherical geometry and a CO/M, ratio of 1 at 300 K. The particle sizes calculated for the HTA-1 and HTA-2 samples are much smaller than the values indicated by Mossbauer spectroscopy. This discrepancy could be due to subcarbonyl formation and/or the existence of a particle size distribution favoring small, nonmagnetically split particles. CO Hydrogenation Measurements. A kinetic study of these supported Fe catalysts was done under differential reactor con-
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Carbon-Supported Fe Particles
6265
TABLE V Chemisorption Results'
wt %
sample LTE HTA-1 HTA-2 HTE
Fe* 9.2
low-temp reduction (473 K) uptake, pmol of co/g c0298/ ~ 0 2 195 K 298 K COi95 M, 376 531 554 586
9.3 9.0 10.1
791 871 1172 967
2.10 1.64 2.12 1.65
9 8
0.47 0.52 0.72 0.52
high-temp reduction (673 K) uptake, pmol ~ of co/g C0298/ C0298/ 195 K 298 K COi95 M, 138 462 467 662
179 707 452 986
1.30 1.53 0.97 1.49
particle sizes, A after after low-temp high-temp redn redn (473 K) (673 K)
0.1 1 0.42 0.28 0.53
16 15 10 15
70 20 27 14
'All values were corrected for weight loss due to decarbonylation and solvent evaporation. bMetal loadings determined by ashing at 750 OC in air. 20
E -P
a
18
c
c Q
12
0
.-2
.-c>
8
-0 Q 0
HTE
HTA- 1
LTE
v ) 4
Figure 9. CO hydrogenation turnover frequencies (TOF) at 498 K under 3/1 H2/C0 flow, based on CO chemisorption values at 298 K, following reduction at 473 K (a) (LTR) and 673 K (Z(HTR) ) and 2 h of carburization (E)(CARB).
0
c2-
c2
c3-
c3
c4-
c4
c5
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e5
ce
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20
=
16
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C 0
n 9)
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%
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HTA- 1
LTE
Figure 10. CO hydrogenation activation energies for hydrocarbon production following LTR (a), HTR E)and , CARB (r).
ditions to investigate the catalytic behavior of the phases detected in the Mossbauer experiments. In particular, it was of interest to determine if the C O hydrogenation behavior of the HTE catalyst, in which a large fraction of the Fe remained in the D structure, was markedly different that of the HTA and LTE catalysts, which formed €'-carbide. The activities and selectivities of the HTE, HTA-1, and LTE catalysts were measured following each of three treatments. The fresh catalysts were first given a 2-h low-temperature reduction (LTR) at 473 K. Second, the catalysts were given a high-temperature reduction (HTR) at 673 K for 16 h. During the final treatment the catalysts were carburized (CARB) under the H,/CO mixture for 2 h at 498 K. None of the catalysts was exposed to air between the LTR and HTR pretreatments. Several trends are evident from the results shown in Figures 9-12. As seen in Figure 9, the turnover frequencies (TOF), based on the C O chemisorption at 300 K, were very similar to each other following the LTR; however, the T O F rose quite noticeably for the LTE catalyst following the HTR treatment. This is consistent with the Mossbauer and chemisorption results which indicated that some crystallite growth probably occurred in that catalyst, as previous investigations have shown that the TOF for CO hy-
.-c 0 -0
$
8
4
a
c2-
c2
c3-
c3
c4-
e4
Hydrocarbon Product
Figure 11. CO hydrogenation selectivities for C2-C6 hydrocarbons at 2-3% conversion to following LTR (a), HTR (E),and CARB (E) hydrocarbons. (a) HTE sample; (b) LTE sample.
drogenation increases with increasing Fe particle ~ i z e . ' . ' ~ The .~~ activation energies for hydrocarbon production (Figure 10) were again similar after LTR, all increased after the HTR treatment, and they increased further following the CARB treatment. In general, the activation energies obtained are in agreement with values which have been observed in other studies of carbon-supported Fe;'~5~10 however, the values after LTR are lower than the 75-100 kJ/mol values typically reported. Previous investigations have shown that activation energies for CO hydrogenation decrease with decreasing particle size;5consequently, the low E, values after (42) Boudart, M.; McDonald, M. A. J . Phys. Chem. 1984, 88, 2185.
6266 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
LTE
HTA- 1
HTE
Figure 12. CO hydrogenation C2-C3 olefin/paraffin ratios after LTR (m), HTR (@), and CARB (3at 2-3% conversion to hydrocarbons.
LTR are another indication of the high initial dispersion for the catalysts in this study. With regard to the selectivities, all the catalysts produced 40-50 mol % CH,, and the c& selectivities for the LTE and H T E samples are shown in Figure 11. The HTA-1 and H T E samples behave more similarly to each other than to the LTE catalyst; in particular, the olefin selectivity is higher on the catalysts supported on heat-treated carbons (Figure 12), especially after the H T R and CARB treatments, and the olefin/paraffin ratios near 2 agree well with previous resu1ts.l On the basis of these results, it appears that the D-structure particles in the HTE sample produce essentially the same catalytic behavior as the €'-carbide particles in the HTA-1 sample, even though their bulk structure appears to differ, as shown in the Mossbauer results. Transmission Electron Microscopy. To determine the approximate particle sizes of the used catalysts, air-exposed samples were examined by TEM. The Fe particles in the LTE catalyst appear in Figure 13a, and Figure 13b shows the Fe particles observed in the HTE catalyst. For the LTE catalyst, the particles were evenly sized, about 30-50 nm in diameter, whereas the HTE sample particles are fairly large in size, ranging from 20 to 100 nm. For both these samples, the observation of large particles was in agreement with the appearance of the sextuplet in the Mijssbauer spectra. The alternating light and dark rings concentric with the particle circumference are consistent with a three-dimensional structure for these large particles.43 The highly dispersed (D-structure) portion of the H T E particles could not be discerned because of poor contrast. In the HTA-1 catalyst, the areas containing Fe showed up as the darker regions in the low-magnification photograph of Figure 13c. Although the individual particles could not be clearly imaged even at higher magnifications, the presence of Fe in these regions was confirmed by using energy dispersive X-ray analysis.
Discussion In the following discussion, a qualitative interpretation of the effects of the different pretreatments on the structure and surface chemistry of this carbon black is presented. Specifically, the changes in the micropore structure, the effect of unsaturated active sites, and the nature of oxygen functional groups will be examined. Subsequently, an analysis of the D structure observed in the Mossbauer spectra is presented, and it is suggested that the stability and size of the D-structure particles are more easily determined than their actual structure or composition. Finally, on the basis of all the data, a tentative model for the behavior of iron on all three supports is advanced. It is postulated that D-structure particles are stabilized by unsaturated active sites on the carbon such that they resist sintering, whereas iron particles which are (43) Treacy, M. M. J.; Howie, A. J . Coral. 1980, 63, 265.
Chen et al. not formed near active sites sinter more readily and show phase behavior similar to that seen for large iron crystallites. Carbon Structure. According to the XRD results, there was a decrease in the two-dimensional layer width La as a result of heat treatments. At the temperatures used, however, no graphitization was found, and there was very little difference between the La values found for the HTA-1 and HTE sample (Table IIc). A more significant change was found in the supermicropore volumes-the adsorption measurements indicate that the HTE sample had a 30% higher supermicropore volume than either the HTA or LTE samples. Previous investigators have indicated at least three mechanisms by which heat treatment can increase pore volume. First, it has been shown that a Hz treatment at 1273 K can increase the pore diameters in charcoal, presumably by the etching of carbon atoms from the carbon s ~ r f a c e , 'to ~ .produce ~~ methane. Second, treatments in Ar at 1310 K were shown to create a "surface porosity", apparently as the result of the desorption of surface functional groups.31 Finally, it has been shown that water continues to be removed from some types of carbon at temperatures as high as 1223 K.4s,46 This implies that carbons outgassed at lower temperatures may have a fraction of their micropores filled with water, thus decreasing the N, adsorption. For the small micropores, this last possibility seems less likely for the carbon used in this study, since the small-micropore volumes seem unchanged by the pretreatments. For the supermicropores, however, the HTA-1 sample does not show the same increase in porosity as the HTE sample. Furthermore, N2 adsorption on an air-exposed HTE carbon showed a reduced porosity relative to the original HTE carbon, thus implying that air exposure is one cause for the reduced supermicroporosity. The reason for the reduction of the porosity on air exposure is not known; however, it could easily be related to the adsorption of oxygen and/or water, especially the latter, from the ambient atmosphere. Freshly heat-treated carbon has been shown in previous studies to adsorb significant amounts of oxygen and Carbon Surface Chemistry. Previous experiments have shown that carbon blacks give off first C 0 2 and then CO as they are outgassed at progressively higher temperature^.^^,^^ In addition, almost all the oxygen is removed via these two gases at 1123-1273 K.46 In the case of heat treatments in H2, the oxygen groups have been found to leave at even lower temperature^.^^^^^ Thus, the HTE and both of the HTA samples were essentially oxygen-free immediately after the high-temperature H2 pretreatments. The creation of active sites on carbon occurs simultaneously with the elimination of oxygen-containing groups. The nature of these sites is not entirely clear; they may exist in the form of olefinic bonds between carbon atom^'^^^' or as basal plane edge site^.^^,^^ In any case, these active sites have been shown to adsorb oxygen over a wide temperature range. For example, the work of Walker et al. showed that up to 3% of the BET surface area of Graphon could strongly adsorb 0, at 573 K,s2 and these workers have frequently used oxygen chemisorption to measure the active surface areas (ASA) of their sample^.^^^^"^^ Other workers have studied oxygen adsorption on carbon and come to similar con(44) Emmett, P. H. Chem. Rev. 1948, 43, 69. (45) Puri, B.; Bansal, R. C. Curbon 1964, I, 451. (46) Anderson, R. B.; Emmett, P. H. J . Phys. Chem. 1952, 56, 753. (47) McDermot, H. L.; Arnell, J. C. J . Phys. Chem. 1954, 58, 492. (48) Kamishita, M.; Mahajan, 0.P.; Walker, P. L. Jr. Fuel 1977, 56,444. (49) Puri, B. R.; Kumar, B.; Singh, D. D. J . Sci. Ind. Res. (India) 1961, 200, 366. (50) Smith, R. N.; Duffield, J.; Pierotti, R. A,; Mooi, J. J . Phys. Chem. 1956, 60, 495. (51) Puri, B. R. Carbon 1966, 4, 391. (52) Laine, N. R.; Vastola, F. J.; Walker, P. L. Jr. J . Phys. Chem. 1963, 67, 2030. (53) Polley, M. H.; Schaeffer, W. D.; Smith, W. R. J . Phys. Chem. 1953, 57, 469.
(54) Laine, N. R.; Vastola, F. J.; Walker, P. L. Jr. Proceedings ofrhe 5th Carbon Conference, Pennsylvania State University; Pergamon: New York,
1963; Vol. 2, p 211. (55) Hart, P. J.; Vastola, F. J.; Walker, P. L. Jr. Carbon 1967, 5, 363. (56) Lussow, R. 0.; Vastola, F. J.; Walker, P. L. Jr. Carbon 1967, 5, 591.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Carbon-Supported Fe Particles
1
6267
\-pi,
4
Figure 13. Transmission electron micrographs of air-exposed catalysts following reduction at 673 K: (a, top left) LTE sample; (b, top right) HTE sample; (c, bottom) HTA-1 sample.
wt % of O2 c l u ~ i o n s . ~ ~Studies * ~ * at, 300 K show that up to can adsorb on charcoals, and up to 50% of this remains strongly adsorbed until the charcoal is outgassed at 1173 K.59*60 In view of these studies, it reasonable to expect that some of the active (57) ONeill, M.; Lovrien, R.; Phillips, J. Reo. Sci. Instrum. 1985, 56, 2312. (58) O”ei1, M.; Phillips, J. J . Phys. Chem. 1987, 91, 2867. (59) Lowry, H. H.; Hulett, G. A. J . Am. Chem. SOC.1920, 42, 1408. (60) Allardice, D. J. Carbon 1966, 4, 255.
sites present in the HTA-1 and HTA-2 samples were probably filled during air exposure, while they were available for interaction during incipient wetness impregnation for the HTE sample. In summary, it is suggested that the high-temperature H2 pretreatment moderately increases the supermicropore volume and active-site concentration of the HTE and HTA samples, but air exposure results in significant coverage of the ASA, probably by oxygen or H 2 0 (HTA samples), and filling of much of the extra supermicropore volume created by the high-temperature heat treatment.
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The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Iron Phases. The Mossbauer data allow the identification of most of the iron phases present on the different samples. However, the composition of the D structure present in all samples following initial decomposition and stabilized in the H T E sample is considerably more difficult to identify. Earlier workers have postulated that the asymmetric D-structure doublet is a combination of an Feo singlet and an Fe2+ doublet.5,9*39*61 In the present study, Fe is most likely to form compounds with carbon and oxygen, as the elemental analysis indicates that only small amounts of other materials are present in the H2-treated carbons. The formation of bulk FeO is not thermodynamically possible under the conditions used in this study,62 and indeed the Mossbauer parameters of the D structure do not correspond to those of FeO reported previously at either 85,6377,61 or 300 K.64”7 Also, the D structure is different from the Fe2+ ions reported previously for Fe supported on S O 2 , Ti02, or Mg06’968,69as the latter samples showed a large, weakly temperature-dependent quadrupole splitting at either 77 or 300 K, whereas a stronger collapse of the quadrupole splitting for the D structure was seen at 300 K (spectrum not shown). For these reasons, it is not possible to unambiguously identify the D structure at the present time. It is important to note that the D structure apparently adsorbs considerable amounts of CO (Table V) and is active for CO hydrogenation. The relatively large amounts of C O adsorbed by the H T E sample as well as the activation energy and TOF data found for this catalyst cannot be ascribed solely to the large iron particles that are present. Considering the Fe(CO), formation, the high CO chemisorption capacity, and the Miissbauer spectrum obtained under CO/H2 conditions for this sample, it seems likely that the D structure is highly dispersed and is the principal active component for C O hydrogenation on the H T E support. Proposed Model. Any model of the behavior of supported iron as a function of pretreatment accorded the carbon used in this study must explain the difference in behavior of the iron on HTA and LTE supports and iron on the H T E support. That is, iron on the HTA and LTE supports behaves in a similar fashion, the only major difference being the slower sintering on the HTA support. In contrast, on the HTE carbon only the initial carbonyl cluster decomposition process is very similar to that on the HTA and LTE supports. After that, the behavior of the iron on the H T E support is fundamentally different than that on the other supports during high-temperature hydrogen reduction; moreover, the D structure present after the low-temperature reduction persists under reaction conditions, whereas €’-carbide forms on the other samples. The explanation should be traced to the difference in supermicropore structure and/or surface chemistry, as these two aspects of the supports differed most significantly. The difference in pore structure of the three samples is restricted to an increase in the supermicropore volume of the HTE sample. The adsorption results show that all of the carbons do possess some supermicroporosity, yet widely differing sintering rates and particle sizes were found. By itself, however, the 0.05 cm3/g increase in supermicorpore volume of the H T E sample does not seem to explain why the majority of the Fe (over 5 wt %) was stabilized against sintering in the H T E sample, whereas the LTE sample sintered fairly rapidly at 673 K. Furthermore, different dispersions and CO hydrogenation rates existed between the HTA-1 and LTE samples despite the fact that their supermicropore volumes were equal. (61) Hugues, F.; Bussiere, P.; Basset, J. M.; Commereuc, D.; Chauvin, Y.; Bonnevoit, L.; Olivier, D. Proceedings of the 7th International Congress on Catalysis; Elsevier: New York, 1977; p 418. (62) Darken, L. S.; Gurry, R. W. J . A m . Chem. SOC.1945, 67, 1398. (63) Shechter, H.; Hillman, P.; Ron, M. J . Appl. Phys. 1966, 37, 3043. (64) Greenwood, N . N.; Howe, A. T. J . Chem. SOC.,Dalton Trans. 1 , 1972, 110. (65) Shirane, G.; Cox, D. E.; Ruby, S. L. Phys. Reo. 1962, 125, 1158. (66) Johnson, D. P. Solid State Commun. 1969, 7 , 1785. (67) Elias, D. J.; Linnett, J. W. Trans. Faraday SOC.1969, 65, 2673. (68) Lazar, K.; Matusek, K.; Mink, J.; Dobos, S.; Guczi, L.; Vizi-Orosz, A,; Marko, L.; Reiff, W. M. J . Catal. 1984, 87, 163. (69) Van der Kraan, A. M.; Nonnekens, R. C. H.; Stoop, F.; Niemantsverdriet, J. W. Appl. Catal. 1986, 27, 285.
Chen et al. The ASA on these carbons, on the other hand, could be dramatically different. This ASA could explain all the observed differences if one postulate is made: the stability of the D structure is the result of interaction with the unsaturated active sites of the carbon. This hypothesis is attractive for a few reasons: First, it explains why the amount of D structure remaining after the 643 K reduction correlates, at least qualitatively, with the amount of active sites expected for each carbon a t the time of impregnation. Second, it is consistent with the observation that the carbonyl precursor was found to decompose more rapidly on the HTE support (no Fe(CO), was found for this sample after 423 K, whereas detectable amounts were present in the other samples). This is in agreement with earlier work which showed that the rate of Fe carbonyl decomposition on graphite is proportional to the active-site concentrati~n.~ Third, it provides a possible explanation for the bimodal particle size distribution in the HTE sample. If it is assumed that the active sites are limited in number, then some iron carbonyl clusters therefore must decompose elsewhere. The clusters that decompose away from an active site should have more mobility and could give rise to the large particles in the HTE sample after the reduction at 473 K. Previous studies’O have shown that iron-phthalocyanine particles supported on carbon surfaces devoid of oxygen functional groups sintered quite rapidly as compared to carbons where the groups existed. The presence of some functional groups on the HTA and LTE carbons apparently inhibited sintering following the 473 K reduction, possibly by interaction with the initial carbonyl clusters, as no magnetically split lines occur in the Mossbauer spectra. Following the impregnation and initial cluster decomposition process, it was seen that the D structure formed on all the samples, even the HTE sample where surface oxygen had been removed. It is therefore possible that the D structure is a result of the interaction of the Fe with the C O liberated during the cluster decomposition. Indeed, CO is known to dissociate on the more open crystallite planes of Fe at 300 K,” and the resulting presence of C and 0 atoms may have led to the oxidation and/or surface carburization of some of the small metal particles formed. These particles would be available for interaction with the active sites, and it is quite possible that the mode of interaction is through C-0-Fe bonds on the carbon surface. During the high-temperature reductions metal particles not bound to the active sites sinter and become reduced, thereby giving a-Fe lines in the spectra (Figure 6a,b). For the HTE sample, the strong interaction is presumed to stabilize the D structure, so that the support-particle interface remains unreduced. For extremely well dispersed particles, a large fraction of the Fe atoms are thus stabilized and give rise to the Mossbauer spectrum of the D structure. It is not possible to be definitive about the D structure; however, one possible model is that it consists of two phases coexisting within highly dispersed particles. Following the initial decomposition these are iron metal and/or €’-carbide (singlet) and iron “oxide” (doublet). As mentioned earlier, it is unlikely that this oxide is FeO, and its true identity is unknown. The oxygen (and carbon) is supplied by the disproportionation of CO from the ligands. Both phases must be very small in extent ( d C 1 nm) such that they have superparamagnetic Mossbauer spectra even at 77 K. Considering the small quadrupole splitting of superparamagnetic carbide^,*.^'.^* it would not be possible to know if the singlet phase is due to iron metal or a combination of metal and carbide. Indeed, the similar kinetics of the D structure on the HTE support and the €’-carbide on the HTA and LTE catalysts indicates that following exposure to syngas the zerovalent component of the D structure probably belongs to iron carbide, whereas after reduction the formation of Fe(C0)5 would indicate the presence of metallic Fe. Either phase presumably coexists with the “oxide” state, which gives the doublet. In summary, a model is presented where small particles of oxide/carbide are generated from the decomposition of Fe3(CO),, (70) Ehrburger, P.; Lahaye, J., ACS Symp. Ser., 1986, 303, 310 (71) Brucker, C.; Rhodin, T.J . Catal. 1977, 47, 214.
J. Phys. Chem. 1987, 91, 6269-6271
on carbon. These particles can interact with the active sites of the carbon to form a species at the metal-carbon interface which is stable against high-temperature reduction. This interaction may occur via oxygen atoms bonding with both the carbon surface and iron atoms contacting the carbon. This oxygen could be provided by dissociation of the C O ligands during decomposition. The remainder of the surface Fe can exist in either a reduced or carburized state, but the small size of these particles results in either case in a superparamagnetically collapsed Mossbauer spectrum. For carbons with fewer active sites, more of the particles
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are unbound, and these particles sinter and carburize in a manner more similar to that of Fe particles supported on graphite.
Acknowledgment. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and a Texaco Philanthropic Foundation fellowship. We thank Dr. Jod-Miguel Martin-Martinez for interpreting the physisorption results. Registry No. Fe, 7439-89-6; N2, 7727-37-9; C02, 124-38-9; CO, 630-08-0.
The Missing Term in Effective Pair Potentialst H. J. C. Berendsen,* J. R. Grigera,t and T. P. Straatsma Laboratory of Physical Chemistry, The University of Groningen, 9747 AG Groningen, The Netherlands, and Instituto de Fisica de Liquidos y Sistemas Biologicos, 1900 La Plata, Argentina (Received: January 5, 1987; In Final Form: August 25, 1987)
Effective pair potentials used for simulations of polar liquids include the average effects of polarization. Such potentials are generally adjusted to produce the experimental heat of vaporization. It has not been recognized before that the self-energy term inherent in any polarizable model should be included in effective pair potentials as well. Inclusion of the self-energy correction with a consequent reparametrization of the SPC (simple point charge) model of water yields an improvement of the effective pair potential for water, as exemplified by density, radial distribution functions, and diffusion constant.
Introduction For the purpose of Monte Carlo and molecular dynamics (MD) simulations of the condensed state, effective pair potentials are widely used. Such potentials incorporate the average many-body interactions into the interaction between pairs. In particular for polar molecules, with water as the most extensively studied example, effective pair potentials deviate significantly from pure pair potential^.'-^ The main reason that pure pair potentials cannot reproduce condensed-state properties for polar molecules is that such potentials neglect the effect of polarizability beyond the level of pair interactions. In water-and in other polar liquids-there is a considerable average polarization, leading to a cooperative strengthening of intermolecular bonding. Thus effective pair potentials invariably exhibit larger dipole moments than the isolated molecules have and produce second virial coefficients larger (in absolute value) than the experimental ones.2vs These dipole moments include the average induced moments in the condensed phase. It is the purpose of this article to show that in the design of effective pair potentials the self-energy due to polarization has been consistently overlooked. The total Coulombic interaction, normally taken as a representation of the electrical interaction, includes the full interaction of induced dipole moments, instead of only half that term, as should be appropriate if the self-energy were included. This leads to too large heats of vaporization and thus to wrong parametrization if the heat of vaporization is used for parametrization. We will show that the simple point charge (SPC) effective pair model for water4 is considerably improved by including a self-energy correction. It is through the pioneering work of A. Rahman and F. H. Stillinger on the simulation of liquid water in the early 1970~~3' that the importance of effective pair potentials became clear. The first simulation of liquid water used the Ben Naim-Stillinger (BNS) model that had been derived on the basis of both gas-phase
* Author to whom correspondence should be addressed. 'This paper was invited to be part of the Rahman Festschrift but was received too late to be included with the others which were published in the September 10 issue of this Journal. f Instituto de Fisica de Liquidos y Systemas Biologicos. 0022-365418712091-6269$01.50/0
data (second virial coefficient related to the pure pair potentials) and condensed-phase data (ice). This pair potential appeared too weak for the liquid phase and could be improved by a simple scaling of energy. When a modified version, the ST2 potential,' was devised, the notion of an effective pair potential was already developed. In the mean time a CECAM Workshop8 was held in Orsay in 1972, where polarizability was extensively discussed and the effect of introducing polarizability into a modified BNS model was e v a l ~ a t e d . ~This workshop, with Aneesur Rahman as the leading expert on simulation, has been seminal to many subsequent activities in simulations,including biological systems.I0
Theory The basic theory of interactions within a system of polarizable polar molecules is well-known." The contribution of the induced dipoles to the energy corresponds to a self-energy term and hence equals half of the corresponding electrostatic interaction term. This can also be formulated as a positive self-energy necessary to create the induced dipole moments, compensating for half the total electrostatic energy of the induced moments. This formulation has, among others, been used by Wertheim12J3in a re(1) Stillinger, F. H. Adu. Chem. Phys. 1975, 31, 1. (2) Finney, J. L.; Quinn, J. E.; Baum, J. 0. In Water Science Reuiews; Franks, F., Ed.; Plenum: New York, 1985; Vol. 1, p 93. (3) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J . Chem. Phys. 1983, 79, 926. (4) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; Hermans, J. In Intermolecular Forces; Pullmann, B., Ed.; Reidel: Dordrecht, 1981; p 331. (5) Postma, J. P. M. Ph.D. Thesis, University of Groningen, 1985. (6) Rahman, A.; Stillinger, F. H. J. Chem. Phys. 1971, 55, 3336. Stillinger, F. H.; Rahman, A. J . Chem. Phys. 1972, 57, 1281. (7) Stillinger, F. H.; Rahman, A. J. Chem. Phys. 1974, 60, 1545. (8) Berendsen, H J. C., Ed. M D and MC on Water, CECAM Workshop 1972; Centre Europeen de Calcul Atomique et Moleculaire: Orsay, France, 1972. (9) Berendsen, H. J. C.; Van der Velde, G. A,, ref 8, p 63. (10) Berendsen, H. J. C., Ed. Protein Dynamics, CECAM Workshop 1976; Centre Europeen de Calcul Atomique et Moleculaire: Orsay, France, 1976. (11) Bottcher, C. J. F. Theory ofElectric Polarization; Elsevier: Amsterdam, 1973; Vol. 1, p 110. (12) Wertheim, M. S. Mol. Phys. 1979, 37, 83.
0 1987 American Chemical Society
