Re-examination of the Diffuse Reflectance Spectra of Cu/A1203 Catalysts

BY JOHN J. FREEMAN AND ROBERT MARK FRIEDMAN"

Corporate Research Department, Monsanto Company, St. Louis, Missouri 63166, U.S.A.

Received 15th April, 1977

Diffuse reflectance optical spectra of Cu/AI2O3 catalysts measured over the range 300-2000 nm are presented which clearly show both octahedrally and tetrahedrally coordinated cupric ions. Earlier work which had overlooked the 4-coordinate Cu2+ ions is discussed.

In a paper presented at the Fifth International Congress of Catalysis, Summers and Klimisch (hereafter denoted S-K) interpreted optical spectra of Cu/A1,0, catalysts as showing the exclusive presence of octahedrally coordinated cupric ions dispersed on the high surface area supports. Earlier studies by Wolberg et aZ.,2 showed that ESCA and K-absorption edge shifts could distinguish between CuO and an aluminate phase on alumina supports. They found that loadings up to 10.5 wt % Cu on a high surface area alumina (332 m2 g-l) were typified by an aluminate, whereas at this level of metal on a lower surface area support (72 m2 g-l), CuO was pre- dominant. Since bulk CuAl,O, has both octahedral and tetrahedral sites occupied by cupric ionsY3 an apparent discrepancy arose when S-K reported their results. Roth proposed that the dilemma could be resolved if it was assumed that ESCA and K-edge techniques can distinguish between the copper in CuO and CuA1,O4, but not between the different geometric coordinations within the spinel. The quaginire deepened when Lytle and coworkers reported an EXAFS-1 study of CuCr/Al,O, catalysts. They concluded that both 4- and 6-coordinate cupric ions were present, consistent with the ESCA and K-edge, in direct conflict with the optical results, and negating the explanation of Roth. Although the presence of Cr obscures comparison with the prior experiments, the necessary supposition to explain consistently the diverse results had reached a disturbing level. We have reinvestigated the optical spectra of Cu/A1203 catalysts in an attempt to clarify the situation.

EXPERIMENTAL

Diffuse reflectance spectra were obtained with a Cary 14 spectrophotometer fitted with a Cary 1411 diffuse reflectance accessory. The accessory was equipped with a MgO-coated integrating sphere. To minimize the effect of particle size and specular reflection, samples were mixed in a steel ball mill with an equal weight of LiF ; the cupric aluminate sample was diluted with twice its weight of LiF.6 The finely ground mix was lightly compacted into a 3 mm deep well of an aluminium plate. The powdered sample surface was smoothened using a glass slide and the flat surface was mounted directly against the open port of the integrating sphere. Spectra were recorded in terms of the logarithm of the ratio of LiF reflectivity to sample reflectivity, [loglo (Ro/R)] . A baseline of LiF against LiF was also recorded for reference purposes.

t EXAFS = Extended X-ray Absorption Fine Structure. 758

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J . J . F R E E M A N A N D R . M . F R I E D M A N 759

A series of catalysts was prepared by minimum solution impregnation of Kaiser KA201 7-alumina (N2 B.E.T. surface area = 306 m2 g-l) by aqueous copper nitrate solutions. The preparations were oven-dried at 110°C overnight and then calcined at 500°C for 12 h in a static air muffle furnace. Our optical spectra consistently showed two peaks with maxima at - 750 and - 1450nm. One difference remained between our preparations and those of S-K ; these workers had only air-dried their catalysts before calcination. In an investiga- tion of supported Ni catalysts, LoJacono, et aZ.7 had claimed a higher degree of tetrahedrally coordinated transition metal ions in dry samples compared with samples calcined in a wet atmosphere. An additional catalyst, 7.5 wt % Cu/A1203, was prepared under conditions chosen to test the effect of this parameter. This material was dried overnight in air and subsequently calcined at 500°C for 5 h under a stream of oxygen which had been bubbled through water to raise its moisture content.

RESULTS

Spectrum l (a) shows the lower wavelength band typical of Cu2+ in an octahedral environment and spectrum l(6) shows the higher wavelength band which is charac- teristic of tetrahedrally coordinated cupric ions. The presence of both types of Cu2+

\

500 1000 1500 Alnm

FIG. l.-Diffuse reflectance optical spectra of copper catalysts and model compounds : (- - -), CuAI204 ; (- -), model compounds with octahedrally coordinated Cu2+ ; (-), catalysts. (a) 6.3 wt% Cu-doped in MgO. (b) CuA1204 (this work). (c) 7.5 wt% Cu/A1203 catalyst (this work). (d) and (e) are spectra (6) and (c) respectively shown over the limited range of wavelength between 300 and 1400 nm. cf), @), and (h) are spectra of CuS04 - 5H20, 6.5 wt% Cu/A1203

catalyst and CuA1204 respectively, reported in ref. (1).

is evident in the spectrum of the 7.5 wt% Cu/Al,O, catalyst [fig. l(c)]. This spec- trum, if anything, shows a higher ratio of tetrahedral to octahedral band intensities than the spectra of catalysts which were oven-dried before calcinationYg the reverse observation of that previously noted for nickel cataly~ts.~

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760 SPECTRA OF Cu/A120, C A T A L Y S T S

In order to compare our results more easily with those of S-K, we display our CuA1204 [fig. l(d)] and copper catalyst [fig. l(e)] spectra over their more limited range of wavelength, as well as reproducing the spectra [fig. lCf)-(h)] from ref. (1). The catalyst spectra over the 300-1400 nm range [fig. l(e) and l(g)] are very similar. The minimum between the octahedral and tetrahedral peaks is not very prominent and the low energy side of the tetrahedral peak is in the omitted portion of the spec- trum. On this basis the copper appears uniquely in an octahedral environment by comparison to the CuS0,-5H20 [fig. l(f)] and Cu2+/Mg0 [fig. l(a)] standards.

DISCUSSION The tetrahedral peak in CuAl,O, clearly dominates the spectrum although X-ray

diffraction structural studies show 60% of the copper to be 4-coordinate and 40% to be 6-coordinate. This reflects the order-of-magnitude greater extinction co- efficient for the tetrahedral band compared to the octahedral band.g Furthermore, since the higher energy absorption is located on the tail of a charge transfer band, there is no discrete octahedral peak and the presence of octahedrally coordinated cupric ions might be overlooked. The reverse situation occurred in the case of the copper catalysts. The approximately equal band intensities indicate that - 90 % of the cupric ions are located at the octahedral sites and a smaller, yet significant, fraction of the metal ions occupy tetrahedral sites. The large overlap of the two bands and the limited range of the spectrum resulted in their oversight.

Careful examination of fig. l(e) shows some indication of the tetrahedral peak which is totally absent from fig. l(g). A possible explanation might be found by comparing the two aluminate spectra. The spectrum from S-K shows the absorption maximum for the tetrahedrally coordinated Cu2+ between 1300-1400 nm whereas the peak from our work appears between 1400-1600 nm, consistent with the litera- ture.lo Apparently S-K's instrument response is falling off in the region of the tetrahedral band resulting in the observed maximum when the absorption was actually still increasing. Since experimental details presented by S-K are sparse, it is difficult to pinpoint the origin of this problem. In this communication, we have presented spectra of similar catalysts to those of S-M but whose spectra were measured over the wavelength range from 300 to 2000nm. Our spectra clearly show the band characteristic of tetrahedrally coordinated Cu2+ ions. From comparison of our results with those of S-K, we conclude that they were misled by the limited range of their data (300 to 1400 nm) which was further compounded by the difference in relative tetrahedral and octahedral site occupancies between bulk CuAl,O, and Cu/Al,O, catalysts and the relative molar absorption coefficients for the two different coordinate cupric ions.g The essential conclusions of S-K that typical bulk spinel is not present in the virgin catalysts is borne out by our more extensive studies to be reported

We thank Mr. D. J. Bauer for preparing the 7.5 wt% catalyst and Dr. J. F. Roth for bringing the dilemma that initiated this work to our attention.

J. C . Summers and R. L. Klimisch, Proc. 5th Int. Congr. Catalysis, ed. J. W. Hightower (North- Holland/American Elsevier, N.Y., 1973), vol. 1, p. 293.

* (a) A. Wolberg and J. F. Roth, J . Catalysis, 1969, 15, 250; (b) A. Wolberg, J. L. Ogilvie and J. F. Roth, J . Catalysis, 1970,19,86. (a) F. Bertaut and C . Delorme, Compt. rend., 1954,239, 504 ; (b) R. M. Friedman and D. J. Dahm, to be published. J. F. Roth, Proc. 5th Int. Congr. CataZysis, ed. J. W. Hightower (North Holland/American Elsevier, N.Y., 1973), vol. 1, p. 302.

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J . J . F R E E M A N AND R . M . F R I E D M A N 76 1

F. W. Lytle, D. E. Sayers and E. B. Moore, Jr., Appl. Phys. Letters, 1974, 24, 45. The use of LiF as a diffuse reflectance matrix and reference is discussed by S. P. Tandon and J. P. Gupta, Spectr. Letters, 1970, 3, 297 and references therein.

R. M. Friedman, F. W. Lytle and J. J. Freeman, to be published. R. Pappalardo, J. Mol. Spectr., 1961, 6, 554.

l o F. H. Chapple and F. S. Stone, Proc. Brit. Cerarn. Suc., 1964, 1, 45.

' M. LoJacono, M. Schiavello and A. Cimino, J. Phys. Chem., 1971, 75, 1044.

(PAPER 7/649)

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