Chapter 30
X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) Spectra of Pt—Sn—Alumina Catalysts Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 23, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch030
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Yong-Xi Li , N.-S. Chiu , W.-H. Lee , S. H . Bauer , and B. H . Davis
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Baker Laboratory of Chemistry, Cornell University, Ithaca, N Y 14853-1301 Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, K Y 40511 2
This report concerns one of four groups of Pt/Sn loaded catalysts, prepared according to selected protocols, which incorporated a range of Sn loadings (0.4 to 3.4%), with Pt maintained at 1%. The X-ray fluorescence and absorption spectra of these preparations were recorded, for both the calcined and reduced states, at the CHESS f a c i l i t y . The near-edge and extended-edge spectra, at the Pt LIII and Sn K-edges, were analyzed to ascertain the states of the metallic constituents. Spectra of the pure metals, of alloys (Pt/Sn 1:1 and 3:1), of their oxides and chlorides were used for calibration. Both the near and extended edge spectra show that reduction with H at 773K (one atm) for 5 hours leaves major fractions of oxides and chlorides unreduced, but enhances dispersion. Differences due to t i n loading are discernible. A radial distribution peak may be assigned to (Sn-Pt) scattering, but by i t s e l f i s not sufficient to demonstrate the development of alloy clusters upon reduction. 2
Supported Pt/Sn b i m e t a l l i c catalysts a r e assuming i n c r e a s i n g l y s i g n i f i c a n t r o l e s i n the p r o c e s s i n g o f petroleum, o p e r a t i n g as reforming c a t a l y s t s . They pr o v i d e high s e l e c t i v i t y f o r aromatics, they are s t a b l e , and permit o p e r a t i o n a t high r e a c t i o n temperatures (-780K) a t r e l a t i v e l y low pressures o f hydrogen (10-15 p s i g ) . The many c u r r e n t s t u d i e s o f these m a t e r i a l s (112) a r e a measure o f t h e i r p r a c t i c a l importance. Nonetheless, the atomic c o n f i g u r a t i o n s about the m e t a l l i c c o n s t i t u e n t s and t h e i r s t r u c t u r a l c o o r d i n a t i o n have not yet been c l a r i f i e d . Indeed, d i f f e r e n t d i a g n o s t i c techniques l e a d t o incompatible c o n c l u s i o n s r e g a r d i n g the formal o x i d a t i o n s t a t e s , extents o f d i s p e r s i o n , Pt/Sn 0097-6156/89/0411-0328$06.00/0 © 1989 American Chemical Society
In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 23, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch030
30. LI ET AL.
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Pt—Sn—Alumina Catalysts
a s s o c i a t i o n , e t c . I t appears t h a t the c r i t i c a l molecular parameters may be h i g h l y s e n s i t i v e t o d e t a i l s o f t h e p r e p a r a t i v e procedures, and t o the nature o f t h e support. Therefore we have undertaken a comparative study o f f o u r groups o f Pt/Sn c a t a l y s t s prepared a c c o r d i n g t o s e l e c t e d p r o t o c o l s . These i n c o r p o r a t e a range o f Sn l o a d i n g s (0.4 t o 3.4%) w i t h Pt maintained a t about 1 % . As a d i a g n o s t i c of b u l k s t r u c t u r e we measured both the near and extended X-ray f l u o r e s c e n c e and absorptions a t the Pt L//j and Sn K-edges, and c o r r e l a t e d these with a v a i l a b l e Mossbauer, XPS and XRD data. Here we r e p o r t on one group o f c a t a l y s t s , prepared a c c o r d i n g t o the acetone-complexation method, on low and h i g h area alumina. Experimental 2
Twenty grams o f support [low area alumina: 1 1 0 m /g; h i g h area alumina: 250 m /g] were wetted w i t h 2 0 ml o f acetone, f o l l o w e d by slow a d d i t i o n (with s t i r r i n g ) o f 2 0 ml o f an acetone s o l u t i o n c o n t a i n i n g the d e s i r e d amounts of H2PtCl
