4832
J . Phys. Chem. 1986, 90, 4832-4839
In previous studies the value of x in an assumed intermediate CH, has been found by an oxygen titration, measuring the water formed after removal of surface hydrogen and correction for support effect^.^^.^^ This experiment is the source of the statement made above that the active intermediate on Fe/Al,O, is CH.35 For Fe/C, this experiment is not possible, for the carbon support is burned by the oxygen and nonlabile hydrogen from the carbon makes too much water to permit a satisfactory experiment. Thus we must be content with the general formula CH, for the 50
pmol/g of active carbon found for Fe/C. Since 50 pmol/g of C H was found for Fe/A1203, it is tempting to suppose that x = 1 for Fe/C also, but there is no experimental evidence.
Acknowledgment. The support of the National Science Foundation (Grant No. C E P 81-20499) is gratefully acknowledged. We also express our appreciation to Professor W. N. Delgass for helpful discussions of the Mssbauer spectroscopy. Registry No. CH4, 74-82-8; CO, 630-08-0; Fe, 7439-89-6.
Effects of CrystaiHte Size and Support on the CQ Hydrogenation Activity/Selectivlty Properties of Fe/Carbon Valiant K. Jones,* Loren R. Neubauer, and Calvin H. Bartholomew B W Catalysis Laboratory, Department of Chemical Engineering, Brigham Young University, Provo. Utah 84602 (Received: February 26, 1986; In Final Form: May 16, 1986)
The objective of this work was to determine how adsorption, catalytic, and electronic properties of iron are affected by crystallite size and metalsupport interactions. Adsorption, physical/chemical,and catalytic properties of 1, 3, and 10 wt 5% Fe-activated carbon catalysts were investigated. From measurements of activity and selectivity for CO hydrogenation at 1 atm, H2/C0 = 2 and 450-530 K, it is evident that initial and steady-state specific activities and the olefin/paraffin ratio decrease with decreasing metal crystallite size. The activity of well-dispersed Fe/C decreases very significantly with time at a H2/C0 ratio of 2. Miissbauer spectroscopy data provide evidence that small superparamagnetic clusters of iron metal are the predominant iron phases of 1% and 3% Fe/carbon. The electron density of these tiny metal clusters is different from that of large iron crystallites, suggesting an electronic interaction between the support and metal.
Introduction Carbon-supported iron catalysts have been the subject of recent scientific interest because of (i) their high selectivities for olefins in Fischer-Tropsch ~ynthesisl-~ and (ii) their utility as highly dispersed iron systems in studying magnetism and lattice dynamic~.*,~ In addition to their having high olefin selectivities Jung et reported that poorly dispersed Fe/carbon catalysts are more active for CO hydrogenation than Fe/alumina and that well-dispersed iron on porous carbon black has a high activity stability at 1 atm and a H,/CO ratio of 3. Jung et aL3 also reported a decrease in specific activity with decreasing crystallite size for the Fe/carbon system. While Sommen et aL4 also observed high olefin selectivities for Fe/carbon catalysts, they found a strong tendency for these catalysts to deactivate due to formation of carbon deposits at a H,/CO ratio of 1 . Thus, it is not clear from this previous work whether iron/carbon catalysts are stable under typical synthesis conditions, Le., at a H2/C0 ratio of 2. There are also a number of other interesting, unresolved questions raised by these previous studies.'-' Since Jung et aL2s3 measured initial specific activities for iron metal based on C O adsorption, is it possible that a different order of activity for Fe/carbon and Fe/alumina or a different trend in activity vs. dispersion might be observed if the specific activities were based on hydrogen adsorption and/or measured at steady-state reaction conditions after several hours of reaction? Since the stoichiometry of C O adsorption on base metals can vary from 0.5 to 2-3 molecules per metal atom depending upon dispersion and prepa r a t i ~ n ,the ~ , ~use of this technique for determining specific ac(l),Vannice, M. A.; Walker, P. L.; Jung, H.-J.; Moreno-Castilla, C.; Mahajan, 0.P. Proc. Int. Cong. Catal. 7th 1980, paper A31. (2) Jung, H.-J.; Walker, P. L., Jr.; Vannice, M. A. J . Catal. 1982, 75, 416. (3) Jung, H.-J.; Mulay. L. N.; Vannice, M. A,; Stanfield, R. M.; Delgass, W. N. J. Catal. 1982, 76, 208. (4) Sommen, A. P. B.; Stoop, F.; Van der Wiele, K. Appl. Cutul. 1985, 14, 277. ( 5 ) Niemantsverdriet, J . W.; Van-der-Kraan, A. M.; Delgass, W. N.; Vannice, M. A J. Phys. Chem. 1985, 89, 61.
0022-3654/86/2090-4832$01.50/0
tivities is certainly open to question. Although hydrogen adsorption on supported iron can be highly activated,*s9techniques have been developed recently for measurement of iron surface areas of supported iron catalysts in which a stoichiometry of one hydrogen atom per metal surface atom has been demonstrated.IoJl Nevertheless, there are potential problems with hydrogen spillover in carbon-supported metal systems.I2 Thus, can hydrogen adsorption be used to measure iron surface areas of iron/carbon catalysts? Another question concerns whether the high olefin selectivity of iron/carbon changes with time during reaction or with metal crystallite size. The data of Jung et al.233suggest that olefin selectivity increases with decreasing particle size; however, these data were obtained after short periods of reaction for different catalysts prepared with different techniques on different carbon supports. Is it possible that some of the differences in selectivity might have been influenced by the differences in support or preparation? While Sommen et aL4 obtained selectivity data at steady state for poorly dispersed catalysts, they did not determine the time-dependent activity or selectivity behavior for well-dispersed catalysts. The purpose of this investigation was to determine the effects of dispersion on the initial activity, the activity maintenance, and the steady-state activity and selectivity properties of a given iron/carbon system in which loading and dispersion were varied systematically. Experimental Section Catalyst Preparation. Activated carbon (Barneby-Cheney Type UU-1636 1000 m2/g, 0.7% sulfur, 4% ash containing silica, (6) Bartholomew, C. H.; Pannell, R. B. J. Cutal. 1980, 65, 390. (7) Reuel, R. C.; Bartholomew, C. H . J. Catal. 1984, 85, 63. (8) Topsoe, H.; Topsoe, N.; Bohlbro, H.; Dumesic, J. A,, Proc. Int. Cong. Catal., 7th 1980, paper A15. (9) Weatherbee, G. D.; Rankin, J. L.; Bartholomew, C. H. Appl. Catal. 1984, 11, 73. (10) Amelse, J. A,; Schwartz, L. H.; Butt, J. B. J . Cutul. 1981, 72, 95. (1 1) Rankin, J. L.; Bartholomew, C. H. J. Catal., in press. (12) Robell, A . J.; Ballou, E. V.; Boudart, M. J. Phys. Chem. 1964, 68, 2748.
0 1986 American Chemical Society
C O Hydrogenation Properties of Fe/Carbon alumina, 0.7% alkali, 0.17% alkaline earth oxides, and 0.04% Fe) was pretreated in flowing H2 at 950 'C for 12 h to remove sulfurI3 and alkali and then reactivated by heating in air at 550 "C to 30% burnoff. Flame photometric analysis confirmed the absence of gas-phase sulfur following this treatment. Vannice and cow o r k e r s ' ~ ~have * ~ ~shown that the high-temperature hydrogen pretreatment successfully removes all residual sulfur available to the gas phase and thereby prevents sulfur poisoning of the iron surface. Moreover, Mossbauer data of the reduced catalysts indicated only iron metal and iron oxides to be present (there were no iron sulfides present). Iron was deposited at 1, 3, and 10 wt % levels by evaporative deposition14 since good metal dispersions on carbon have been achieved with this t e c h n i q ~ e . ~For * ' ~ the 3% and 10% catalysts our modification of this technique included vacuum drying at about 80 OC of a continuously stirred slurry of the support and a solution of iron nitrate dissolved in a 4 to 1 mixture of benzene and methanol and then further heating to about 120 "C to remove residual solvent. This was followed by continued drying overnight in air at 95 OC. All samples were later further dried under vacuum at about 110 OC before weighing to remove any water absorbed from the air during storage. For the 1% Fe/C catalyst, half of the iron nitrate was prepared by dissolving 95.45% enriched 57Fe (New England Nuclear) in 3.9 M H N 0 3 , heated to boiling. This acidic solution was added to a mixture of the support and natural iron nitrate dissolved in methanol, and preparation proceeded as for the other catalysts. The enriched 57Fewas used in the case of the 1% Fe/C to facilitate collection of Mossbauer data. Precise metal loadings were determined by atomic absorption analysis (Kimball Laboratories) and were found to be 0.99, 3.18, and 10.14 wt % Fe, respectively. Catalyst Reduction. On the basis previous experience,15 the catalysts in the study were reduced in a flow-through Pyrex ce11I6 in flowing hydrogen at a high space velocity (2000 h-l) and at a low heating rate (4 K/min) in order to achieve maximum dispersion. The heating schedule included a 2-h constant temperature hold at 473 K and a 24-h hold at 623 K. Higher temperatures were not used in order to avoid gasification of the support. The hydrogen (99.99%) was purified by passing through an Engelhard Deoxo catalytic purifier followed by a molecular sieve trap at 190 K or a Linde Molecular Sieve 5A trap. Adsorption Measurements. H2 adsorption measurements were carried out with a flow system with a thermal conductivity detector described elsewherei7according to a procedure recently developedI7 for measuring total chemisorbed hydrogen while minimizing spillover This procedure involves cooling the catalyst sample in hydrogen after reduction from the reduction temperature to 180 K, purging in inert gas for 15 min at 180 K, and heating rapidly to 675 K to remove chemisorbed hydrogen. This method provides metal surface area measurements for supported cobalt, iron, and metal catalysts in very good agreement with those determined by transmission electron microscopy and by static, volumetric adsorption, except in the case of 1% Fe/C samples where spillover was a complicating pr0b1em.I~ Reproducibility of this technique was better than *lo%. Since hydrogen uptake on the UU carbon was negligible," no support corrections were necessary. Metal dispersion and crystallite size calculations were based on the assumptions that iron metal is present as spherical particles of uniform size6*"and that unreduced iron is present in a separate dispersed oxide layer in intimate contact with the support."%'* Thus, percentage dispersion (%D) was calculated according to the equation (13) Moreno-Castilla, C.; Mahajan, 0. P.; Jung, H.-J.; Vannice, M. A,; Walker, P. L., Jr.; Abstracts; 14th American Carbon Conference, Pennsylvania State University, College Park, PA, June, 1979. (14) Bartholomew, C. H.; Boudart, M. J . Cutal. 1972, 25, 173. (15) Bartholomew, C. H.; Farrauto, R. J. J . Coral. 1976, 45, 41. (16) Erekson, E. J.; Bartholomew, C. H. Appl. Cutul. 1983, 5, 323. (17) Jones, R. D.; Bartholomew, C. H., Appl. Catal., submitted. (18) Boudart, M.; Delboulle, A.; Dumesic, J. A.; Khammouma, S.;Topsoe, H. J . Curd. 1975, 37, 486.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4833
%D = 1.117X/(WJ)
(1)
where X is the average H2 uptake in micromoles per gram of catalyst, W is the weight percentage of iron, andfis the fraction of iron reduced to the metal as determined by Mossbauer spectroscopy. Average crystallite diameters in nanometers were calculated from %D assuming spherical metal crystallites of uniform diameter d with a site density of 17.3 atoms/nm2. This site density was calculated from a weighted average of the site densities of the most dense planes of bcc iron, (loo), (1 lo), and (1 11). The weighting was based on the occurrence of these planes in the unit cell in a 6:6:8 ratio. Thus
d = 122.5/%D
(2)
Electron Microscopy. Reduced, passivated catalyst samples were crushed to a fine powder, ultrasonicated in 1-butanol, and impregnated on fine-mesh copper screens coated with holey carbon as outlined by Mustard and Bartholomew.19 Transmission electron microscopy (TEM) measurements were made with a Philips EM400HTG electron microscope, with better than 1-nm resolution. The micrograph negatives were photographed at a magnification of 117000 and enlarged 2.5X in printing. The micrographs were then analyzed under a low-power microscope with a calibrated eyepiece. Resolution as small as 1.1 nm was possible. In the case of 10% Fe/C approximately 750 crystallites were counted and sized. A surface mean diameter (for comparison with diameters calculated by H2 adsorption) and a volume mean diameter (for comparison with the calculated from XRD) were calculated from the crystallite size distribution according to conventional methods.I9 X-ray Diffraction. X-ray powder diffraction (XRD) measurements were performed at the University at Utah using a Philips diffractometer with Cu K a radiation and a graphite monochromator. Analysis of the line broadening based on the half-maximum breadth of the (110) peak ( l / S o , 20/min scan) using the Sherrer equation, according to Klug and Alexander,*O produced a volume 'mean diameter for comparison with T E M results. These calculations included corrections for Kcu doublet broadening and Warren's correction for instrumental broadening. Mossbauer Spectroscopy. Extents of reduction to iron metal were measured by Mossbauer spectroscopy with a constant acceleration spectrometer system with 57Fe in a Pd source and procedures described elsewhere.21 With the use of an absolute laser velocity calibrator it was possible to measure isomer shifts to within an absolute accuracy of f0.005 mm/s. Mossbauer spectra were corrected during the collection procedure to remove the C U N ~background of instrumental origin. Spectra of reduced catalysts contained in a controlled-atmosphere cell2' were measured at 298 and 77 K in 1 atm of hydrogen to ensure that no oxidation of the samples would occur. Mossbauer spectra were computer-fitted to Lorentzian lines with a least-squares optimization procedure.21 Resonant absorption areas were found from integration of the background curvature-corrected spectra. Activity and Selectivity Tests. Materials and Apparatus. Measurements of CO hydrogenation activity and selectivity were performed in a differential, fixed-bed Pyrex reactor.16 High-purity hydrogen and nitrogen (both 99.997%) were obtained from Whitmore, and high-purity carbon monoxide (99.99%) was obtained from Matheson. The hydrogen was further purified by passing it through an Engelhard Deoxo catalytic Purifier followed by a molecular sieve 5A (Linde) trap. The nitrogen and carbon monoxide were passed through similar molecular sieve traps to remove water and iron carbonyl. Gas flow rates were held constant with Brooks mass flow controllers, and reactor temperatures were fixed with a temperature controller constructed in-house. (19) Mustard, D. G.; Bartholomew, C. H. J . Cutal. 1981, 67, 186. (20) (a) Klug, H. P.; Alexander, L. E. X-ruy Diffruction Procedures, Wiley: New York 1954; Chapter 9, pp 491-538. (b) Klug, H. P.; Alexander, L. E. X-ruy Diffraction Procedures, 2nd ed.;Wiley: New York, 1974; Chapter 9; pp 687-704. (21) Bartholomew, C . H., final report to NSF, No. CPE-8010780, January 31, 1982.
4834
T h e Journal of Physical Chemistry, Vol. 90, No. 20, 1986
TABLE I: Extents of Reduction, H, Uptakes, Dispersions, and Average Crystallite Diameters for Fe/C Catalvsts Reduced at 623 I(
ave cr)st'illite diameter, nm
_ ._ l_l
H 2
uptake,b %' €12 reductn" pmol/g dispersion ads' ____
TEh4
XRDd
0.6 1.5 7,9
