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Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 523
CORRESPONDENCE Comments on “Nickel-Support Interactions: Their Effects on Particle Morphology, Adsorption, and Activity Selectivity Properties” Sir: I have read with interest the paper of Bartholomew et al. (1981). I would like to comment on the validity of application of TEM and XRD in determining the size and concentration of reduced metal, reported in this paper. It was observed that metals (NiO and Cuo) formed on reduction of Ni2+and Cu2+present in the interlayer space of vermiculite (Patel, 1978) and montmorillonite (Patel, 1982) are highly active as a result of which they are reoxidized on exposure to the atmosphere. Reoxidation kinetic study of such metals (Patel, 1976) indicated that even in the first minute, a significant proportion of it is reoxidized. It is assumed that the same problem should exist in the present work. In order to avoid such reoxidation phenomena, a special in situ method for XRD was adapted where the treated samples can be transferred to Lindmann tubes and sealed under vacuum or hydrogen as desired (Patel, 1978). Examination with TEM was suspected to be unsuitable as in the usual method of sample preparation, exposure to air for long periods is unavoidable and the finely divided
metal particles risk even being lost in the solvent. As the dimension of reduced metals formed in montmorillonite was -100 A, direct examination with SEM was rather found to be more reliable (Patel 1982). The authors have claimed an accuracy of k0.3 nm, which is questionable unless strict in situ methods of measurement have been adapted. Unfortunately, nothing is mentioned about in situ examination procedures in the above article.
Literature Cited Bartholomew. C. H.; Pannell, R. B.; Butler, J. L.; Mustard, D. Ind. f n g . Chem. Prod. Res. Dev. 1981, 2 0 , 296. Patel, M. Clays Clay Miner In press, 1982. Patel, M. Doctoral Thesis, University of Pierre and Marie Curie, Paris, 1978. Patel. M. Unpublished data, 1976. Patel, M. X-Ray Spectrom. In press, 1982.
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& D Laboratory Tats Chemicals Ltd. Mithapur 361345 (Gujrat) India
M. Patel
Response to Comments on “Nickel-Support Interactions: Their Effects on Particle Morphology, Adsorption, and Activity Selectivity Properties” Sir: Patel (1982) has questioned the accuracy of our crystallite size data (Bartholomew, 1981) determined by X-ray diffraction and TEM for nickel supported on alumina, silica, and titania. He states that because of possible bulk metal oxidation, the reported “accuracy of h0.3 nm is questionable unless strict in situ methods of measurement have been adapted.” While we agree that it is desirable to perform in situ measurements of crystallite size for reduced catalysts (in practice difficult by X-ray diffraction and impossible by conventional TEM), we do not feel that our X-ray and TEM methods of crystallite size measurement were affected within experimental accuracy by the exposure to air. First, our reduced catalysts were carefully passivated in air prior to x-ray and TEM measurements, a treatment designed to create a passive surface oxide layer thereby preventing further bulk oxidation of the metal particles. Second, we were able to easily rereduce and successfully reproduce the Hz uptake measurements of passivated nickel catalysts, thereby demonstrating that the careful controlled exposure to air has not changed the average particle size. Thirdly, we observed only metal peaks (no metal oxides) in our X-ray diffraction scans of passivated supported nickel. Finally it is not reasonable to expect that 1-3 layers of metal oxide would significantly alter the average crystallite size for metal particle sizes of 2 3 nm. In another related paper from this laboratory (Mustard and Bartholomew, 1981) we reported the accuracies and limitations for determination of average nickel crystallite size by Hzadsorption, TEM, and X-ray diffraction. In the
0198-4321/82/1221-0523$01.25/0
case of Ni/SiOz, we found that crystallite size estimates from H2 adsorption agreed with those from TEM within h10% or h0.3 nm. This excellent agreement demonstrates that particle size estimates from TEM are indeed accurate within h0.3 nm for this particular catalyst. However, the agreement between H2 adsorption and TEM for Ni/Al,O3 and Ni/Ti02 was not as good. Furthermore, the agreement between estimates from X-ray diffraction and Hz adsorption was generally h50% for Ni/AlZO3, Ni/SiOz, and Ni/Ti02. Accordingly, we did not claim an accuracy for our X-ray data of zt0.3 nm, as Patel (1982) obviously erroneously inferred. In view of Patel’s observation of Ni and Cu catalyst systems in which reoxidation is apparently important, we could encourage him (and other workers) to perform a careful study to determine if complete bulk reoxidation does affect average crystallite size in these systems, since to our knowledge a quantitative study of this sort using in situ X-ray diffraction has yet to be reported.
Literature Cited Patel, M. Ind. f n g . Chem. Prod. Res. D e v . 1982, preceding paper in this issue. Bartholomew, C. H.; Pannell, R. 6.;Butler, J. L.; Mustard, D. G. Ind. Eng. Chem. Prod. Res. D e v . 1981, 20, 296. Mustard, D. 0.;Bartholomew, C. H. J. Catal. 1981, 6 7 , 186.
B Y U Catalysis Laboratory Department of Chemical Engineering Brigham Young Universtiy Provo, U t a h 84602
0 1982 American Chemical Society
C. H. Bartholomew
