30 Microanalysis of a Copper-Zinc Oxide Methanol Synthesis Catalyst Precursor Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 6, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch030
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P. B. Himelfarb , G. W. Simmons , Κ. Klier , and M. José-Yacamán 1
Center for Surface and Coatings Research and Department of Chemistry, Lehigh University, Bethlehem, PA 18015 Instituto de Fisica Universidad Nacional Autónoma de México, Apdo. Postal 20-264, México 20 DF, México
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The naturally occurring mineral aurichalcite has been used as a model for the methanol synthesis catalyst precursor that is formed by coprecipitation from aqueous copper and zinc nitrate solution by the addition of alkali carbonate. The chemical and morphological transformations that occur in both materials upon calcination and subsequent reduction have been monitored by transmission electron microscopy, selected area electron diffraction, and X-ray powder diffraction. The treatments did not change the platelet morphology of these samples, but produced platelets that were porous and polycrystalline, in contrast to the original single crystal materials. Calcination of the mineral and synthetic samples yielded ZnO crystallites in crystallographic registry, oriented with major zone axes of [1010] and [3031]. The preferred orientations of ZnO were in epitaxial registry to the original aurichalcite orientation having a [101] zone axis, such that, the aurichalcite [040] and [202] axes were aligned with the ZnO [1210] and [0002] axes, respectively. In the reduced materials, the Cu(211) planes were parallel to the ZnO(1010) planes such that the Cu[111] axis was aligned with the ZnO[0002] axis. The coprecipitated precursor of the most active binary Cu/ZnO methanol synthesis c a t a l y s t (1) has recently been shown to be a s i n g l e phase hydroxy carbonate, ( C u q . Z n -7)5(003)2(0H) , a u r i c h a l c i t e (2) . In the present i n v e s t i g a t i o n , the natural a u r i c h a l c i t e mineral of composition (Cu ^Ζη ) (0Ο ) (ΟΗ) , which consisted of large, t h i n p l a t e l e t s having dimensions on the order of micrometers, was used as a model compound f o r following the chemical, s t r u c t u r a l , and morpho l o g i c changes during c a t a l y s t preparation. The phase d i s t r i b u t i o n s and morphology of the synthetic and mineral samples were compared 3
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0097-6156/ 85/0288-0351 $06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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CATALYST CHARACTERIZATION SCIENCE
Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 6, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch030
throughout the preparation procedures. Although the p r e c i p i t a t e d precursors and the n a t u r a l mineral gave r i s e to e s s e n t i a l l y i n d e n t i c a l c a t a l y s t s , the l a r g e r p l a t e l e t dimensions of the natural mineral provided an i d e a l morphology f o r studying the genesis of the f i n a l Cu/ZnO c a t a l y s t by transmission electron microscopy (TEM). Techni ques of x-ray d i f f r a c t i o n (XRD), selected area electron d i f f r a c t i o n (SAD), and dark f i e l d and b r i g h t f i e l d imaging i n the TEM were used to characterize the mineral during and a f t e r c a l c i n a t i o n and a f t e r reduction. Experimental Copper ore containing a deposit of a u r i c h a l c i t e was obtained from Wards Natural Science Establishment. The mineral a u r i c h a l c i t e crys t a l l i t e s were gently scraped from the ore and rinsed i n ethanol p r i o r to use. The synthetic precursor was prepared by c o p r e c i p i t a t i o n from a mixture of 1M Cu and 1M Zn n i t r a t e s o l u t i o n s , such that a Cu/Zn mole r a t i o of 30/70 was prepared, by dropwise a d d i t i o n of 1M Na2C03 at 90°C u n t i l the pH increased from approximately 3 to 7. C a l c i n a t i o n and reduction of the mineral were performed as i n standard cata l y s t preparation procedures, which have been described i n d e t a i l e a r l i e r (1). A P h i l i p s EM 400T transmission electron microscope which included a scanning transmission mode was used i n the electron microscopic characterization studies. Samples were prepared by dispersing the a u r i c h a l c i t e mineral i n ethanol and p l a c i n g a drop of the dispersion on a carbon-coated titanium or copper g r i d . For reduced specimens exposure to a i r was minimized by preparing and transporting samples i n a nitrogen f i l l e d glove bag. Energy dispersive X-ray analysis (EDS) f o r the i d e n t i f i c a t i o n of elements and q u a n t i t a t i v e analysis was c a r r i e d out i n the manner described i n reference (3). Powder d i f f r a c t i o n patterns were obtained with a P h i l i p s XRG 3100 X-ray generator coupled with an APO 3600 c o n t r o l u n i t using CuK^ r a d i a t i o n . Scans were conducted with a step s i z e of 0.01° i n 2Θ and a counting time of 1.2 sec. Results Representative TEM micrographs and SAD patterns of the mineral and synthetic a u r i c h a l c i t e are given i n Figures 1 and 2, r e s p e c t i v e l y . The SAD patterns were indexed to a [101] zone a x i s , as described i n reference (2). The u n i t c e l l parameters of the mineral and synthetic a u r i c h a l c i t e are given i n Table I together with the Cu/Zn r a t i o s . The XRD data and Zn/Cu r a t i o are also given f o r a reference a u r i c h a l c i t e specimen reported i n the l i t e r a t u r e (4). A l l d-spacings i n the mineral and synthetic a u r i c h a l c i t e matched the l i t e r a t u r e values w i t h i n the l a t t i c e volume changes (
