1004
Energy & Fuels 1994,8, 1004-1005
Effect of Agglomerate Size on the Catalytic Activity of an Iron Oxyhydroxide Nanocrystalline Powder toward Carbon-Carbon Bond Scission in Naphthylbibenzylmethane John G. Darab, John C. Linehan,' and Dean W. Matson Pacific Northwest Laboratory,+Richland, Washington 99352 Received October 29, 1993. Revised Manuscript Received April 18, 1994 In recent years much effort has been aimed at developing methods to prepare and characterize highly dispersible, iron-based, nanocrystalline catalyst precursor powders (e.g., ferrihydrite, magnetite, etc.) for use in slurry-based, first-staqe direct coal liquefaction processes.'-' The term nanocrystalline used to describe these powders can be defined in this context to mean particulate material containing individual primary crystallites of diameter C 100 nm (0.1pm). For such catalyst precursor powders, however, the nanocrystallites are usually aggregated (strongly bondedheacted together9 or agglomerated (crystallites and/or aggregates held together by surface forces*) into larger porous secondary particles. Thus, as is the case for nanocrystalline powders in general, the particle size is larger than the crystallite size. We show here that, in addition to crystallite size, agglomerate size is an important factor in determining the overall catalytic activity of a six-line ferrihydrite (5Fe203-9H20)powder. Ideally, the use of individual, nonaggregated, nonagglomerated precursor crystallites (i.e., the particle size is the same as the crystallite size) in the nanometer size range offers several advantages over catalyst powders consisting of particles/crystallites in the micrometer range: (1) the diffusivity (via Brownian motion, convection, etc.) of the particles within the reaction mixture is enhanced due to smaller particle sizes; (2) the greater number of particles per unit weight of powder provides reactants and products with shorter diffusion distances to and from catalytic sites; and (3) the increased surface areas associated with small particles provides more reactive sites per unit weight of powder. In small-scalemodel studies the increased surface area provided by the small particles may be the more important of these factors. However, real catalyst precursor powders are highly aggregated/agglomerated (see for example the SEM micrographs in refs 1, 2, 6, 7, and 9). Although these powders are often readily redispersed into their constituent t Pacific Northwest Laboratory ie operated for the US.Department of Energy by the Battelle Memorial Institute under contract DEACW 76RLO 1830. (1)Pradhan, V.R.;Tiemey, J. W.; Wender, I.; Huffman,G. P. Energy Fuels 1991,5,497-507. (2)Pradhan, V. R;Herrick, D. E.;Tiemey, J. W.; Wender, I. Energy Fuels 1991,5,712-720. (3)Ibrnhim, M. M.; Zhao, J.; Seehra, M. S. J. Mater. Rea. 1992,7, 1856.1880. (4)Mateon, D.W.; Linehan, J. C.; Bean,R. M. Mater. Lett. 1992,14, 222-226. (6) Matson, D. W.; Linehan, J. C.; Geusic, M. E.Part. Sci. Technol. 1992,10,143-164. (6) Huffman,G.P.;Ganguly,B.;Zhno, J.;Rao,K.R.P.M.;Shah,N.; Feng, Z.; Huggins, F. E.;Taghiei, M. M.; Lu, F.; Wender, I.; Pradhan, V. R.; Tiemey, J. W.; Seehra, M. S.;Ibrahii, M. M.; Shabtai, J.; Eying, E. M. Energy Fuels 1993,7,285-296. (7) Pradhan, V. R.; Hu, J.; Tiemey, J. W.; Wender, 1. Energy Fuels 1993,7,446-464. (8) Yan,M.,F. In Advances in Powder Technology; Chin,G. Y., Ed.; American Society for Me& Metala Park,OH, 1982;pp 99-133.
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aggregates and/or individual crystallitesfor TEM analysis, e.g., by ultrasonic agitation, it is not yet clear whether they are equally redispersed under conditions at which they are used as catalysts. Perpetuation of aggregation and agglomeration of nanocryetallitesunder liquefactionConditionswill decrease the number of particles per unit weight of powder compared to the ideal case where crystallites are highly dispersed. If under liquefaction conditions the internal porosity of the catalyst agglomerates is isolated as a result of the sintering and phase transformations which occur, then agglomerate size, not crystallite size, will determine the ultimate number of surface sites per unit weight of powder. Thus,in addition to crystallite size, which may affect the ultimate catalytic activity of a powder, the particle size (i.e., aggregate and/or agglomerate size) of the powder may dictate the degree of attenuation of the catalytic activity. If there is a barrier to agglomerate dispersion under liquefaction conditions, then the smaller catalyst precursor agglomerates might be expected to show greater catalytic activity than the larger ones. Common techniques used to characterize nanocrystalline iron-based powders such as X-ray diffraction (XRD) and Mwbauer spectroscopy give an indication of, among other things, crystallite size. These techniques, however, do not directly probe aggregateor agglomeratesize. Thus, any claims made regarding catalyst dispersion or diapersability in a reaction medium based purely on XRD linebroadening results are speculative. Another technique used to characterize"crystalline powders is BET surface area analysis, usually with N2 gas. Although this gas adsorption technique is very sensitive to the internal surfaces of typical porous agglomerates that are accessibleby N2 gas, it does not necessarily follow that the surfaces that result from heating these agglomerates under liquefaction conditions are similarly accessible by thelarger organic molecules present in typical reaction schemes of interest here. Additionally, the internal surface area of porous agglomeratesof nanocrystalsis much larger than the external surface area of thesesame agglomerates. Thus BET surfacearea analysis on such powders is largely insensitive to the degree of agglomeration. A highly agglomerated nanocrystalline powder composed predominantly of six-line ferrihydrite with a minor amount of hematite was produced usingthe rapid thermal decomposition of precursors in solution (RTDS) process developed at PNL.10 The dried powder was briefly milled to a coarse powder to demonstrate the importance of agglomerate size on the catalytic activity of the powder. (9)Darab, J. G.; Fulton, J. L.; Linehan, J. C. Prep. Pap.-Am. Chem.
Soc., Diu. Fuel Chem. lSS3,98,27-33.
(10)Mntson,D.W.;Lmehan,J.C.;Darab,J.G.;Buehler,M.F.Energy Fuels 1994,8,10-18.
0 1994 American Chemical Society
Energy & Fuels, Vol. 8, No. 4, 1994 1005
Communications Table 1. Iron Oxyhydroxide Powder Characterization A1
fraction B
C
screen analysis -325 +325/-230 +230 mesh size agglomerate diameter (pm), < 4 5 f 3 4 5 f 3 t o 6 3 f 4 > 6 3 * 4 as defined by screen opening size 13 53 135 mean agglomerate diameter (Irm) 2 50 48 percentage by weight of starting powder XRD analysis six-line ferrihydrite (major) phases detected hematite (minor)
