J. Phys. Chem. 1986, 90, 4888-4893
4888
On the Structure of Hydrodesulfurization Catalystst S. H. Bauer,* N.-S. Chiu, Department of Chemistry. Cornel1 University, Ithaca, New York 14853
and Marvin F. L. Johnson* ARC0 Petroleum Products Company, Harvey Technical Center, Harvey, Illinois 60426 (Received: December 20, 1985; In Final Form: May 27, 1986)
A series of Mo/Co/A1203catalysts on a high-area alumina was prepared, with up to 30%MOO3and 12%COO. Characterization of the catalysts in the oxide form by XRD, by pore distribution, and by EXAFS showed that the molybdenums were extensively dispersed on the alumina surface, encased in distorted MOO, octahedra. For most catalysts UV-visible spectra showed the Co2+to be in octahedral coordination. Hydrodesulfurization (HDS) activities were measured with a petroleum fraction under conditions typical of refinery use. The EXAFS spectra were recorded at the CHESS facility and reduced according to an improved procedure that incorporates an objective test for the assumed background and corrections for termination errors. The resulting radial distribution functions have low noise, so that small peaks can be discerned, and with proper calibration their areas have quantitative significance. Comparisons of the RD patterns derived from Mo K-edge spectra for MOO,, MoS,, and freshly sulfided and extensively used catalysts lead to the following conclusions: (i) Even after extended use some Mo-0 bonding remains. (ii) The ratio of peak areas for ( M e M o ) vs. (Mo-S) scattering is a measure of the mean size of the active platelets. (iii) There are indications that while the Co was initially randomly distributed in the metal atom layer, on extended use they congregated at the platelet peripheries. (iv) As predicted, there is a good correlation between the catalytic activity and the squat root of the number of (Mo-S) [or (Mo-Mo)] pairs, as measured by the RD curves. Analysis of the EXAFS spectra at t .Co K-edge showed that observation (i) applied also to Coo-sulfiding was incomplete even after extended treatment in the HDS reactor. Of greater interest is the observation that Co-Co scattering associated with a Cogs, phase is particularly strong in catalysts derived from 12%COOpreparations. This indicator, that at high loadings the promoter tends to aggregate (partially) into a separate phase that partially blocks catalysis, is consistent with HDS activity data and with optical spectra
Introduction It is now recognized that high-precision, well-resolved records of the energy-dependent absorption coefficients of materials, in the vicinity of the K or L edges of an element of interest (EXAFS), provide three types of information: (i) the location of the steeply rising edge (which can be measured to within a fraction of an electron volt) is determined by the oxidation state of the element; (ii) the shape of the near-edge spectrum (NEXAFS) is a reflection of the symmetry of the local electric field around the central atom; (iii) the oscillations in the extended edge absorption are determined by the configurations of atoms around the central element that do the back-scattering. In this paper we present a summary of our structural studies of hydrodesulfurization (HDS) catalysts in which we utilize these three types of fingerprints. In general respects our conclusions do not differ from models proposed during the past decade for this important class of catalysts. Indeed, our data support the proposals that have been derived indirectly from a variety of techniques. Additionally, our improved EXAFS data reduction procedure provides quantitative descriptions of the structures of HDS precatalysts, the virgin sulfided materials, and the reactor-seasoned catalysts, with more detail than has been presented so far. Before summarizing the results of our EXAFS studies it will prove interesting to list briefly features of the improved data reduction program that we developed.' (i) Because of the availability of high counting rates and a large number of data points, smoothing of high-frequency "noise" is advisable. In most instances we used the Savitzky/Golay running-average program,2 but this procedure is limited by the number of available coefficients. Repeated smoothing with S / G imposes a loss of too many data points. For the K-edge fluorescence records of nickel and cobalt we found that an autocorrelation routine provides more efficient moo thing.^ (ii) The spectra were "decon~oluted"~ to correct for the finite energy interval transmitted by the monochromator. Also, we found a simple expression for correcting the background absorption due 'Presented at the Emmett Memorial Symposium, Sept. National Meeting of the American Chemical Society, Chicago, September, 1985. Present address: 1 I24 Elder Rd, Homewood, IL 60430.
0022-3654/86/2090-4888$01.50/0
to underlying wings from L-edges and general background due to absorption from other species. (iii) At that stage x l ( k ) ( p - pBK)/pBK is known (standard symbols, defined in ref l ) , and ) pI(R+). evaluation of the Fourier transform FT ( k 3 x l ( k )gives This function is a distorted radial distribution because the assumed background was arbitrary, there was no correction for the finite range of integration, and a phase shift is incorporated in the argument of the sine function [sin ( 2 k R + + ( k ) J ]that appears in the expression for the absorption coefficient; improvement is achieved by successive approximations. (iv) We developed an objective criterion for approximating the background and derived from the corrected X2(k)an improved radial distribution, designated p2(R+). The peaks in this distribution function can then be interpreted in terms of the structure anticipated for the material. This information can be used to partially correct for truncation errors. (v) We synthesize a composite intensity function [designated x 3 ( k ) ] . The missing low and high k sections [0-4 and 16-34 A-'], calculated from the structural data derived from p2(R+),are patched (with appropriate scaling) onto the recorded spectrum (4 Ik I16), so that the restructured x 3 ( k )covers the range 0 < k < 34. The Fourier transform p3(R+)= FT(k3x3(k)) has considerably less noise and better resolution than p l ( R + )or p2(R+).(vi) The distance scale can now be adjusted for the phase shift # ( k ) = A - 2Bk by calibrating with known (similar) structures. The back-transform of p3(R+)for both the unknown (un) and the reference structure (kn) provides a record of both the amplitude (H3j)and phase (&,) functions for each coordination shell 0'). A plot of the phase function 4,jk"(k) vs. k should be linear, with a slope 2 ( R p - B,) and an intercept A,. The phase shift from a specified atom in a coordination sheII, relative to the central atom, is assumed to be transferrable from the known to ( 1 ) Chiu, N.-S.; Bauer, S. H.; Johnson, M. F. L. J . Mol. Struct. 1984, 125,
33.
(2) Savitzky, A.; Golay, M. J. E. Anal. Chem. 1964, 36, 1627. (3) For noisy excursions that occur with frequencies much higher than the I(n) = sought for modulation in the function of interest, use: [ ' / s ~ , " f ~ ~ J ( m ) * J3 )(] 'm 1 2 , where I ( n ) is the smoothed function at point n. and the J(m)s are the recorded values that range from (n - 5) to ( n + 2). (4) Ergun, S . J . Appl. Crystallog. 1968, 1 , 19.
+
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4889
Structure of HDS Catalysts
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Figure 1. Sequence of distance spectra for a 12% Moo3 + 1.5% COOpreparation, oxide state, which demonstrates the improvement in resolution and noise reduction achieved by the program outlined in the text (steps iii-vi). The D(R) curves in parts e and f were included for comparison of two crystal structures with that of a typical Mo03/CoO/alumina catalyst: (e) (NH4)6M01024*4H20, (f) Moo3.
the unknown compound. The fully corrected absorption function is therefore the sum of the product of Gaussian fitted amplitude functions [ H ( k ) ]and the sine of the phase-corrected arguments (4uk- 4k") over all coordination shells
k 3 x 4 ( k )= C H 3 j ( k )sin [43juk - +,?+ 2 k R , 9
(1)
J
Then, p 4 ( R )= FT(k3x4(k)).The correctly weighted radial distribution function is D(R)= R2p4(R)exp(+ZR/A), where A is an estimated (constant) absorption length for the back-scattered waves. Figure 1 illustrates the improvement achieved by following through the indicated sequence for the absorption spectrum at the K-edge for 12% MOO, plus 1.5% COOon a high-area alumina support.
Experiments and RD Analysis We found no reports of HDS-catalyst investigations that covered a matrix of compositions, wherein both the Mo and Co contents were systematically varied. Hence a series of Co/Mo/alumina catalysts was prepared to constitute an internally consistent set, with both metal atom compositions varied in a regular manner. The alumina as received (boehmite alumina monohydrate) was converted to y-alumina by calcination; its surface area was 289 m2/g and its pore volume 0.552 cm3/g. Portions were impregnated with ammonium molybdate solutions by the incipient wetness technique and again calcined in flowing air at 550 O C to give preparations of 2%, 4%, 6%, and up to 30% of MOO, in 6% increments. Portions of each of the Mo/aluminas were then
additionally impregnated with C O ( N O , ) ~solutions, dried, and calcined to yield catalysts with 1.5%, 3.0%, 6.0%,and 12.0% COO. These preparations were fully characterized in the oxide state5 by measurements of the surface area, pore size distribution, diffuse reflectance spectra, X-ray diffraction, and an occasional Raman spectrum. The surface area for up to 30% MOO, changed as expected for simple dilution of the alumina with the metal oxides. The pore sizes were only slightly reduced. No characteristic X-ray diffraction patterns could be recorded except for those given by the alumina base. Sulfiding decreased the pore volume slightly. These data indicate that the MOO, was deposited essentially as a monolayer and that the COO behaved as an equal quantity of MOO, with respect to surface coverage. The reflectance spectra showed an intense peak at 17200 cm-', which corresponds to octahedral Co2+. The amplitude of this peak depended not only on the Co loading but also on the percent Mo, which leads to the conclusion that there is an interaction between the metals in the oxide states. The near-edge absorption functions at the Mo K-edge were resolved into three overlapping Gaussians, one of which corresponds (nominally) to the 1s 5p transition.6 Its area, which is a measure of the departure of the coordination shell from a regular octahedron, increased with percentage Mo, a trend that
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( 5 ) Johnson, M . F. L.; Voss, A. P.; Bauer, S. H.; Chiu, N . 4 . J . Cafal. 1986, 98, 5 1. (6) (a) Chiu, N.-S.;Bauer, S . H.; Johnson, M. F. L. J . Cafal.1984, 89, 226. (b) 1986, 98, 32.
4890
Bauer et al.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 m
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Figure 2. Time-temperature conversion sequence for 18% MoO,/O% COO. FTs c-g are at the p3(RS)stage to provide the reader with estimates of the direct transforms for (18/0). The upper curves are D(R) functions: (a) 18% MoO3/6% COO sulfided for 10 h; (b) MoS,, a mechanical mixture with AI,O, (50%). Note the change in vertical scale.
parallels the decrease in the apparent coordination number as derived from the EXAFS spectra. Addition of Co to these preparations leads to a decrease in the distortion as measured by each of these independent indices. Typical radial distributions centered at the Mo atoms for the oxide state of the hydrodesulfurization catalysts (Figure 1) differ significantly from that for crystalline MOO, and from the other reference compounds; at short range they resemble the radial distribution of ammonium heptamolybdate tetrahydrate. The principal peak at 1.75-1.69 8, is due a short Mo-0 bond. The small peak designated a (2.53-2.57 A) may be assigned to the longer Mo-0 separations in the distorted Moo6 octahedra. The peak b at e2.8 A (Figure 2) appears to arise from back-scattering by the aluminum atoms in the support. Finally, c is most likely due to Mo-Mo scattering, since its amplitude increases with Mo
2.393A(Mo-Mo)/A(Mo-S)
vs. moles of metal, per 100
loading. These spectra confirm the model that the molybdenum is extensively dispersed over the large area support and that the MOO, octahedra are highly distorted but not as much as are the octahedra in crystalline molybdenum oxide. In the precatalysts, the inner-most coordination shell consists of two types of oxygens; for the closely bound group the mean distance decreases with Mo loading. There is negligible effect of cobalt on these structures but the regenerated catalyst shows somewhat shorter Mo-0 separations. The oxide preparations were sulfided following four distinct protocols (to obtain some insight into the kinetics of transformation) and their Mo K-edge absorption spectra recorded. In the oxide state, we found that the K-edges for Mo in the 0, octahedra appear approximately 8 eV above that of the metal, characteristic of Mo6+. With partial reduction, the edge moves toward lower voltages and after extended treatment reaches about 1 eV above that of the metal, characteristic of Mo4+. The shapes of the near-edge absorption spectra also change. From the areas of the resolved Gaussians one can estimate the first-order rate Co reduces constant for the conversion of oxide to the the initial conversion rate. There is a higher level of microcrystalline order in the sulfided state than in the initially calcined preparations. An initial rapid rise in temperature facilitates growth of a molybdenum sulfide phase, so does higher Mo loading, but this effect is countered by the presence of Co. Our EXAFS spectra provide information in response to the following questions: (a) Is conversion to MoS2 complete in the active catalysts? (b) How extensive is aggregation of the molybdenum atoms over the high-area support on sulfidation? (c) Are there significant structural parameters that correlate with the observed activity rates? That conversion of oxidic molybdenum to the sulfide upon H,S/H, reduction of Co-Mo/A1203 HDS catalysts was incomplete has been reported by several authors.’-I0 Our studies via EXAFS directly confirm this conclusion, since residual Mo-0 peaks are present in the RD curves of samples subjected to extended sulfiding. A time-temperature conversion sequence is illustrated in Figure 2. The oxide catalysts were treated with H,S/H, (l/lO) for the indicated minutes at the listed temperatures (in-situ scans). As sulfiding progresses the principal Mo-0 peak ($) decreases in intensity and is transformed to smaller peaks, (7) Schrader, G. S.; Cheng, C. P. J . Catal. 1983, 80, 369. (8) Massoth, F. E. J . Catal. 1975, 36, 164. (9) (a) Schuit, G. C. A.; Gates, B. C, AIChE J . 1973, 19, 417. (b) Mitchell, P. C. H.; Trifiro, F. J . Catal. 1974, 33, 350. (c) deBeer, V. H. J.; Bevelander, C.; VanSint Fiet, T. H. M.; Werter, P. G. A. J.; Amberg, C. H. J . Catal. 1976, 43, 68-77. ( I O ) (a) Okamoto, Y.; Nakano, H.; Shimokawa, T.; Imanaka, T.; Teranishi, S. J . Catal. 1977, 50, 447. (b) Okamoto, Y . ;Imanaka, T.; Teramshi, S. J . Catal. 1980, 65, 448. (c) Okamoto, Y . ;Tomika, T.; Katoh, Y . ;Imanaka, T.: Teranishi. S. J . Phys. Chem. 1980, 84, 1833.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4891
Structure of HDS Catalysts
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Figure 4. p3(R,) curves at the Co K-edge for Co304,COO,and several catalyst compositions.
which remain even after continued sulfiding. For a MoO,/CoO preparation 18/0, the Mo-S peak appears after about 40 min, but approximately 70 and 120 min are required for the 18/3 and 18/6 samples, respectively. To establish the quantitative basis for estimating the extent of nonreducible oxygens, EXAFS scans were obtained for three mechanical mixtures of (MoS2/oxide/ A1203). By using strictly uniform data reduction programs, we established a basis for determining from the areas under the respective peaks the fraction of Mo06's converted from the oxide to the sulfide state. On our scale, the area under the Mo-S peak in MoS, is 2648. None of the catalysts attained that value. From the calibrating spectra, we estimate a net conversion of 60%70%, depending on the extent of treatment in the HDS reactor. The second significant feature that appears in Figure 2 is the slow growth of the Mo-Mo peak; it never obtains an intensity 2.4 times that of Mo-S, as is present in crystalline MoS,. The Mo-S peaks appear first, but no Mo-Mo peaks are discernible under mild sulfiding conditions. This suggests that sulfiding initially involves replacement of the more distant oxygen atoms by sulfur with minor adjustment of the spacing. A one-to-one replacement of oxygens by sulfurs at some of the more reactive bridging oxygen sites is p o s ~ i b l e . ~Oxygen .~ vacancies may also form under these reducing conditions. Increasing the temperature and/or lengthening the H2S/H2treatment leads to the appearance of Mo-Mo scattering indicative of further structural rearrangement. A layer of MoS, ( x < 2) on the surface of the support is formed at that time. The observation that peak b remains in all the sulfided samples indicates that some molybdenum-substrate bonds are not significantly disturbed by sulfiding. In support of this Okamoto et a1.I0 and Grimbolt et al." compared the Mo/AI and Co/AI
XPS intensity ratios of the oxidic precatalysts with the sulfided catalysts and found that the surface structures of the Mo/Co catalysts do not alter significantly upon sulfiding. Models proposed by Massoths-l2and Schrader et aL7*13show a Mo-S surface layer that is bound extensively to the alumina by Mo-0-AI bonds. However, the extent of structure rearrangement is limited by the temperature so that for any specified set of conditions the formation of Mo-S and Mo-Mo structural units eventually slows down. Our data clearly show that the reaction temperature has a greater effect than reaction time; this is consistent with Massoth's results8 that a limiting catalyst sulfur content is attained for each selected temperature. Pollack et aI.l4 reported that upon extended use, or sulfiding at higher temperatures, recrystallization takes place. The transition of the two-dimensional MoS, to three-dimensional MoS2 was observed by X-ray diffractionI4J5 and by high resolution electron microscopy.16
Structural Deductions (Mo K-Edge, Sulfided State) We propose that the low areas under the MG-S peaks of the sulfided catalysts compared to those in crystalline MoS2 measure the limited reduction of the MOO, units, while the low ratios of (1 1) Grimbolt, J.; Dufresne, P.; Grengembre, L.; Bonnelle, J -P. Bull. SOC. Chim. Belg. 1981, 90, 1261-1269. (12) Massoth, F. E. J . Less-Common Met. 1977, 54, 343. (13) Schrader, G. L.; Cheng, C. P. J . Phys. Chem. 1983,87, 3675-3681. (14) Pollack, S. S.; Makovsky, L. E.; Brown, F. R. J . Cural. 1979, 59, 452. (15) Topsae, N.-Y. Bull. SOC.Chim. Befg. 1981, 90, 1311. (16) (a) Thomas, J. M.; Millward, G . R.; Bursill, L. A. Philos. Trans. R . SOC.London, A 1981, 300, 43. (b) Topsae, H. Advances in Catalytic Chemistry II; American Chemical Society: Washington, D.C., 1982; ACS Symp. Ser.
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Figure 5. p3(RG)curves a t the Co K-edge for Co& and several catalyst compositions, after sulfiding.
A(Mo-Mo)/A(Mo-S) measure the limited aggregation into a molybdenum sulfide phase. Unlike bulk MoS2, which consists of continuously stacked Mo and S layers, in the catalysts rafts of S,-Mo-0, with some S,-Mo-S, are indicated. In bulk MoS, each Mo atom is surrounded by six S and six Mo, so that the ratio of their peak areas should be approximately proportional to the ratio of their atomic numbers (2.6). Indeed, our EXAFS RDs show 2.4 for crystalline MoS2. However, for all the catalysts this ratio is much smaller. Thus, while Mo atoms located within the platelets have six neighboring Mo's, those that reside at the edges have smaller numbers of neighbors, so that the average is well below six. Our data also indicate that in the catalysts on the average there are considerably fewer S atoms around each Mo, since not all the oxygens have been replaced by sulfur. The magnitudes recorded for the respective areas in MoS2 (given in our calibration spectra) are 2648 and 6337 [arbitrary but consistent units] for A(Mo-S) and A(Mo-Mo), respectively. A quantitative measure of conversion is 0 = A(M0-S),~,~/2648, whereas a quantitative measure of mean raft size is
R=
A(Mo-Mo) 6331
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- 0.418A(Mo-M0) A (Mo-S)
(2)
A plot of R for various metal loadings is presented in Figure 3. An interesting dichotomy appears. The open symbols were derived from spectra of freshly sulfided catalysts while the filled symbols give values for R for the same catalysts after extented use in the test HDS units. The virgin samples show an upward trend with Mo loading, indicating that larger rafts develop when more of the metal is present, but strikingly, f o r the same Mo levels, added Co decreases the Mo-Mo scattering. This is expected for a
random distribution of Mo/CO atoms during the initial aggregation stage. On exposure of the catalysts to sulfur-bearing fuels, R increases, indicative of some raft growth, with mean sizes nearly independent of metal loading. Significantly, there is little effect of Co at this stage, suggesting that these atoms had migrated to the peripheries of the platelets and no longer interfer with Mo-Mo scattering. This is consistent with observations by Behal et aLL7 that in single crystals of MoS2, doped with Co, Auger spectra indicate that the Co atoms tend to segregate to the growth steps (edge planes) of the faceted crystals.
EXAFS at K-Edge of Co Of the seven Co/S and Co/O calibrating compounds for which EXAFS spectra were recorded, those of Co304and Cogs8 are most similar (respectively) to the spectra of the oxide and sulfide states of the catalysts. The oxide has a spinel structure with a mean value for the (Co-0) distance of 1.92 8, and (Co-Co) separations of 2.86 8,. Of the 24 metal atoms in the unit cell, 8 are in tetrahedral sites while 16 have octahedral coordinations [Figure 4a]. The structure of C O ~ isS more ~ complex. In each unit cell there are eight tetrahedrally coordinated Co atoms, with Co-S distances that range from 2.13 to 2.21 8;there is also one octahedral Co-S at 2.39 8. There are nearby Co-Co at 2.50 A and 3.54 8,. Clearly, additional Co-Co spacings that span 3.48 conversion of the oxide to sulfide necessitates extensive restructuring [Figure sa]. The initial catalyst preparations consist of mixed Coz+ with Co3+;this is based on the observed small differences in the edge position, as determined for Co,O,. If the preparations that contain
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(17) Behal, S. H.; Chianelli, R. R.; Kear, B. H. Muter. Lett. 1985, 3, 381.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4893
Structure of HDS Catalysts 12% COOare excluded, the edges of the others were found to lie within a relatively narrow range, less than 1 eV, which indicates that the environments of these cobalt atoms are similar. The magnitudes of the Co/Mo loadings appear to control the variations in the [Co2+/Co3+]ratio. In the ideal COO structure all the octahedral sites are filled with Co2+. When a small portion of the Co2+is replaced by Co3+ a defect structure is generated. The K-edge locations of preparations with high molybdenum loadings (>18%) appear at the high side of the I-eV range, possibly due to some of the Co being forced into tetrahedral sites. When the Co loading exceeds the 6% level the mean oxidation state increases, as indicated by the edge positions, which appear about 0.5 eV above the highest value for the low (16%) COOpreparations. The shapes of the near-edge spectra of the 12% preparations differ from those with lower loadings; they are similar to the near-edge features of CO304. The following discussion is a preliminary analysis of p3(R+) curves, in which some of the small peaks may be either satellites of large peaks or were developed by superposition of a nois background. The peak positions are unshifted (add 0.3 to 0.4 to all peaks, based on calibration curves for CO(ACAC)~ and Co304; also, one should mentally multiply the peak areas by R2 to obtain estimates of the relative scattering pair contributions). The curve for MoO,/CoO = 12% is essentially identical with that for Co304 except that all the peaks are reduced in area ( ~ 7 5 % and ) the background fluctuations are larger, indicative of slightly distorted structures. In all the other preparations, the major peak is located at approximately 1.6 A. Its position, as well as those of the adjacent small peaks at larger R+'s,is dependent on the ratio of octahedral. to tetrahedral cobalt. The strongest indication that Co,O, is present in high Co loadings is the appearance of a peak at 4.7 A; no such peak appears in the preparations with low COO content. In general, the shapes of the RD curves, even for the lower loadings of COO,are more like those of Co304than of COO, and it appears that the level of Mo does influence their locations. The higher loadings of cobalt lead to sulfided structures more like CO& than COS. The shape of the near-edge absorption function for low Co preparations is very similar to the shapes in the oxide state showing an almost negligible (1s 3d) transition 4p transition. The shapes of the with no indication of a 1s near-edge absorption functions for sulfided 12% COOpreparations have features similar to those of Co9Ss. The p3(R+)transforms of the sulfided preparations have many similarities. (The phase shift correction is +0.36 A, derived from Co(S,CN(Et),),.) The main peak is located at 1.7-1.8 A, which is 0.3 A shorter than Co-S in COSand 0.1 0.2 8, shorter than in Cogs8 but it is 0.2 0.3 A longer than C o - 0 in c0304. We interpret these displacements as measures of incomplete sulfiding of the cobalt oxide, previously observed with higher resolution, for the molybdenum oxy sulfides. The position of this peak may be used to estimate the extent of sulfiding (roughly 50-70%). The peak in the low Mo loading preparations appears at a shorter distance than in the higher Mo's, which suggests that the molybdenum and cobalt are competing for the incoming sulfur source; the extent of cobalt sulfiding is lower at greater molybdenum contents. The small peak at 2.2-2.3 A, which has been assigned to Co-Co scattering in Cogss, appears clearly in all the sulfided catalysts, but its position varies, since there is also a contribution from Co-Co of the oxide residue, due to incomplete conversion, at 2.5 A. There is a persistent small peak (marked X in Figure 5 ) , also somewhat variable in location, at ~ 2 . %, 9 that might be assigned to Co-Mo scattering. From the p3(R+)curves it appears that when the initial preparation is high in cobalt (12% COO), the Co-Co peaks are much stronger than in the 6% COO, indicating partial segregation into a Cogs8 phase, associate with a decrease in catalytic activity. Platelet Structure of the Active Catalyst A detailed analysis of the structure of the O,-Mo-S,,/Co species spread over the support surface requires the introduction of a distribution of platelet sizes. An abbreviated discussion based on a dimensional analysis suggests the following. The total number of Mo-S atom pairs, per unit volume, as recorded by EXAFS is
K
-
-
-
-
0) 0
r
s 0
N 0
h
* ?
0
W
2 P)
2 8 0
0.000
0.020
0.040
0.060
0.080 G I Ys
0.100
0.120
0.140
0.160
Figure 6. Hydrcdesulfurization rate constant (activity/unit area) plotted vs. the function Ys,eq 4.
given by [Mo](A(Mo-S)/2648, where [Mol is the total number of molybdenums per unit volume. Similarly the number of Mo-Mo atom pairs is given by [Mo](A(Mo-M0)/6337. In practical catalysts Co is an essential promoter and our data indicate that during residence in the HDS reactor the Co atoms had migrated to the peripheries of the rafts. It follows that the measured catalytic activity should be primarily dependent on the number of Mo-S [or Mo-Mol atom pairs located at the peripheries of the rafts, modulated by some function of the Co content. This leads to the expectation that there is a primary linear dependence of the specific rate constant for hydrodesulfurization on Y,, defined by (mol Mo/100 g cat./unit area)
2336
Conversion rate constants were calculated for each catalyst at each temperature, using an integrated form of the generalized rate equation governing irreversible reactions. This relates conversion to space-time values at a given temperature
[S,'-" - Sl-"] / ( n - 1) = kt = k/WHSV
(4)
where Sfis the weight percent sulfur in the feed, S , is the weight percent sulfur in the product, n is the reaction order, k is the nth order rate constant in h-I, and WHSV is the grams of feed per hour, per gram of catalyst. While thiophene and other singlecomponent sulfur hydrocompounds can be described by a firstorder equation, a complex mixture follows higher order kinetics, depending on the relative amounts of lumped types and on conversion levels. It was found that reasonably good fits could be obtained by using a reaction order of 2.0 for data gathered at 343 "C and a reaction order of 2.75 for those obtained at 316 OC. Sulfur conversions, expressed as S f / S , , ranged between 1.1 and 8.3 at 316 "C and between 1.4 and 25 at 343 "C. Since the apparent reaction order at 343 "C is 2, eq 4 reduces to S i 1- Si' = k/WHSV = K . Figure 6 is a plot of K vs. Y, for the 12 samples prepared for this study; G is a constant scaling factor. The predicted dependence of K on a diffraction derived measure of the number of peripheral Mo-S (or Mo-Mo) atom pairs is evident, as is also the promoting effect of Co when present in levels of up to 6% (CoO/g of catalyst in the oxide state); catalysts with 12% are clearly inferior. The deleterious effect is presumed to be due to the development of an overlaying phase of Co9S8. This is supported by an analysis of the Co K-edge spectra.
Acknowledgment. This work was supported by a grant from the A R C 0 Petroleum Products Company, a Division of the Atlantic Richfield Company. The EXAFS spectra were recorded at the Cornel1 High Energy Synchrotron Source supported by NSF Grant No. DMR-780/267. Registry No. COO,1307-96-6; MOO,, 13 13-27-5.
