The Journal of Physical Chemistry, Vol. 82, No. 21, 1978 2347
Communications to the Editor
COMMUNICATIONS TO THE EDITOR Comment on “Effect of Gallium Ions and of Preparation Methods on the Structural Properties of Cobalt-Molybdenum-Alumina Catalysts”
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Publication costs assisted by Phil@ Morris U.S.A.
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Sir: In a recent article, Lo Jacono et al.’ have presented a detailed examination of the structure of cobalt-molybdenum-alumina catalysts with additions of gallium. Their conclusion that gallium enhances the formation of tetrahedral Co2+ is based in part on optical reflectance data, which are not suited for these materials, and proper analysis suggests that gallium has little effect on the site preference. Figure 3 of their paper contains plots of the optical density a t 578 nm as a function of cobalt concentration. The optical density as recorded on the Beckman DK1 is the negative logarithm of the reflectance. Since the reflectance spectra of particles contain contributions from scattering, it is improper to use the optical density as a relative measure of absorption center concentration. Commonly, the individual particle scattering theory of Melamed2y3is used to convert reflectance values (using the Lambert cosine law, the Fresnel relations, and the Lambert-Beer law) into products of the absorption coefficient with the particle diameter. If the powder can be reasonably approximated as a collection of uniformly sized.spherica1 particles, then the Melamed conversion gives absolute absorption values which are in excellent agreement with transmission experiments. The absorption coefficients so calculated also provide an appropriate comparison with color center concentration via the Lambert-Beer law. In Figure 1 are shown the recalculated curves of Figure 3 of Lo Jacono et al. after applying the Melamed technique, using a nominal refractive index of 1.65 for alumina. The plots of absorption vs. cobalt content are similar throughout the reported range of refractive indices for alumina, 1.55-1.75. The curves are generally concave upward, with the exception of that for B-AyGaMoCo (0.6:5:x), which is almost linear. While it would be preferable to use integrated peak areas (if possible from a curve-resolved absorption spectrum) the absorption peak heights are acceptable if the contributions at a particular wavelength are from a common source. This is certainly not true a t 720 nm where measurements of the shoulder height from base line, as in Figure 4 of Lo Jacono et al., cannot be associated solely with Co304. Indeed, even at 578 nm the assumption of contributions from only tetrahedral Co2+might be ruled out by the true nonlinearity of the absorption curves. If the absorption at 1560 nm, where little is expected to interfere with the tetrahedral bands, is investigated instead, then the data of Lo Jacono et al.’y4 (converted through the Melamed method) appear as in Figure 2. The plots are essentially linear, obey the Lambert-Beer law, and show no effect of gallium addition except for experimental scatter from point to point in concentration. This is inconsistent with the nonlinear plots of Figure 1,which suggest the growth in intensity from a source other than tetrahedral Co2+. If the data were plotted through the full concentration range, with the exception of B-AyGaMoCo (0.6:5:x), it would be apparent 0022-365417812082-2347$0 1.OO/O
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Flgure 1. A plot of absorption (obtained by use of the Melamed conversion procedure and the optical density plots reported in ref 1) vs. concentration of cobalt (atom percent) at 578 nm: (A)AyCox; (0)AyGaCo ( 0 . 6 : ~ )(; 0 )B-AyGaMoCo (0.6:5:~);(0) AyGaCo ( 4 : ~ ) ; (0)A-AyGaMoCo (0.6:5:~). 1.1
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Co CONTENT (ATOMIC PERCENT) Figure 2. A plot of absorption (obtained by use of the Melamed conversion procedure and the optical density plots reported in ref 1 and 4) vs. concentration of cobalt (atom percent) at 1560 nm: (A) AyCox; (0)AyGaCo ( 0 . 6 : ~ ) ;( 0 )A-AyGaMoCo (0.6:5:~).
that they are all nonlinear with similar intensity and suggest no effect of gallium on the site preference. The nonlinearity of the absorption curves in Figure 1 may in fact point up the effects of the preparation conditions on the catalysts. The grinding of the samples prior to firing may allow fracture and subsequent segregation 0 1978 American Chemical Society
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The Journal of Physical Chemistry, Vol. 82, No. 21, 1978
of surface impregnated species from the substrate. Thus, the formation of Co304and/or CoMo04 may indeed be taking place, but as a distinct bulk phase rather than as a surface phase. This is supported by the fact that the absorption of the coimpregnated B-AyGaMoCo (0.6:5:x) is linear, in comparison to the nonlinear absorption of the sequentially prepared A-AyGaMoCo (0.6:5:x) which already contains molybdenum intimately bound to the substrate prior to cobalt addition. Some support for this hypothesis is obtained from the X-ray powder diffraction’ results which suggest the formation of a discrete bulk CoMoOl phase in the B-AyGaMoCo (0.6:5:x) series. In addition, experiments we have performed5 with A-yGaCo (x:0.5), using Reynolds’ RA-1 y-Al,03 and no grinding, have indicated no significant change in the relative absorption coefficients with x = 0.7-8.0%.
Communications to the Editor
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References and Notes (1) Lo Jacono, M.; Schiavello, M.; DeBeer, V. H. J.; Minelli, G. J . Phys. Chem. 1977, 87, 1583. (2) Melamed, N. T. J. Appl. Phys. 1963, 34, 560. (3) Monahan, E. M.; Nolle, A. W. J. Appl. Phys. 1077, 48, 3519. (4) Lo Jacono, M.; Cimlno, A.; Schuit, G. C. A. G a u . Chim. Ital. 1973,
703, 1281.
(5) Losee, D. B.; Crawford, M.; Kassman, A. J. to be published. Philip Morris U.S.A. Research Center Richmond, Virginia 2326 1
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Flgure 1. A plot of absorption (obtained by use of the Melamed conversion procedure and the optical density plots reported in ref 1 of KL) vs. concentration of cobalt (atom percent)at 578 nm: (A) AyCox; (0)AyGaCo (0.6:~); (0)B-AyGaMoCo (0.6:5:~); (0)AyGaCo ( 4 : ~ ) ; (0)A-AyGaMoCo ( 0 . 6 ~ 5 : ~ ) .
A. J. Kassman’ D. B. Losee‘
Received November 28, 7977; Revised Manuscript Received June 79, 1978
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Reply to the Comment by A. J. Kassman and D. B. Losee on the “Effect of Gallium Ions and of Preparation Methods on the Structural Properties of Cobalt-Molybdenum-Alumina Catalyst” Publication cost assisted by Consiglio Nazionale delle Ricerche, Italy
Sir: The more proper treatment of the reflectance spectra, done by Kassman and Losee (KL),3allows confirmation of the effect of gallium ion addition. First of all we wish to remark that both absorption bands at 578 and 1560 nm are strongly affected, but not to the same extent, by the presence of C0304(Figures 1 and 6 of ref l),a normal spinel, with Co2+ions in tetrahedral positions. Accordingly, in order to evaluate the effect of Ga3+ions we must consider separately the low Co content specimens (0 < Co < 3) and the high Co content specimens (Co > 3), where the influence of Co304 is more strongly felt. Thus, from Figure 1 (redrawn from ref 3) it can be observed that, in the low Co content region, the specimens containing Ga3+ ions have a higher absorption than the Ga3+-freespecimens despite the presence in the latter of a small amount of Co30,. Moreover, quite acceptable linearity was visible, even though peak heights were used instead of peak areas. As for the high Co content region (Figure 1, ref 3), the presence of bulk phase Co304does not allow one to make a meaningful comparison between A,Co and A,GaCo samples, since Co30, has a broad, very intense absorption over the whole range from 730 to 210 nm (see Figure 6, ref 1). The same reasoning can be applied to the absorption at 1560 nm (see Figure 2, redrawn from ref 3) by taking into account the minor influence of Co304. Indeed, the plots of Figure 2 are essentially linear at low Co content, for all 0022-365417812062-2348$0 1.0010
Co CGNTENT(ATGM1C PERCENT)
Flgure 2. A plot of absorption (obtained by use of the Melamed conversion procedure and the optical density plots reported in ref 1 and 4 of KL) vs. concentration of cobalt (atom percent) at 1560 nm: (A)ATCOX; (0)AyGaCo ( 0 . 6 : ~ ) ; (0)A-AyGaMoCo (0.6:5:~).
specimens. Moreover, it is still observed that the A,GaCo specimens have higher absorptions. A t high Co content, while the specimens A,Co4 and A,Co5 show a jump in absorption due to the massive presence of Co304,as visible by X-rays, magnetic susceptibilities, and reflectance spectra,’ absorption for the Ga3+-containingspecimens are still linear. However, this linearity, as compared with the less linear plots of Figure 1,can be understood by considering the different influence of c0304 absorption in the two regions (578 and 1560 nm). Furthermore, it is not surprising that KL have not found any significant differences in the relative absorption coefficients for their specimens, A,Co and A GaCo, with a Co content of 0.5 at. 70,since, as is seen in hgure 2, for this Co content the difference in absorption between A,GaCo and A,Co is very small (-0.02). Finally, it must be pointed out that the conclusion about the effect of Ga3+ions on the phase formation and cation distribution was based on the results obtained by several techniques, such as X-rays, magnetic susceptibilities, and reflectance spectra.
0 1978 American Chemical Society
