17 Effects of Reduction Temperature onH2Adsorption by Pt on Various Supports Jacek A.Szymura1and Sieghard E. Wanke
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Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 The influence of reduction temperature on subsequent hydrogen chemisorption for Pt supported on magnesias, aluminas and silicalite is examined in this paper. Significant decreases in hydrogen chemisorption capacities with increasing reduction temperatures were observed for most catalysts. X-ray diffraction results showed that sintering of Pt was not the major cause for the suppression in adsorption uptakes. Poisoning of the Pt surface by substances which originate in the support and which migrate onto the Pt surface during reduction at elevated temperatures is the probable cause for the suppression of adsorption observed for Pt/MgO and Pt/silicalite. Adsorption suppression for Pt supported on sulfur-containing MgO was observed after reduction at temperatures as low as 350°C. For Pt/Al2O3, reduction at 700°C was required before significant decreases in hydrogen adsorption were observed. Hydrogen chemisorption has been used extensively f o r over two decades for the determination of metal surface areas, i . e . metal dispersions of supported platinum group metal c a t a l y s t s . In order to convert measured hydrogen adsorption uptakes to metal dispersions, the amount of hydrogen adsorbed per surface metal atom has to be known, i . e . the adsorption stoichiometry has to be known. Considerable amount of research was carried out i n the 1960s and 1970s to determine adsorption stoichiometrics for various adsorbates on d i f f e r e n t metals f o r a range of adsorption pressures and temperatures (1-7). Based on these and other studies, i t was concluded that hydrogen adsorption on p l a t inum at temperatures close to room temperature and low pressures corresponds to approximately one hydrogen atom per surface platinum atom. 1Current address: Institute of Technology, Technical and Agricultural Academy, 85-326 Bydgoszcz, Poland 0097-6156/86/0298-0169S06.00/0 © 1986 American Chemical Society
Baker et al.; Strong Metal-Support Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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STRONG METAL-SUPPORT INTERACTIONS
However, i n the past few years many investigators have reported hydrogen adsorption uptakes on supported platinum catalysts which correspond to adsorption stoichiometries of much less than one hydro gen atom per surface platinum atom. This suppression of hydrogen ad sorption i s usually brought about by reducing the supported catalyst i n hydrogen at elevated temperatures. Tauster and co-workers (8-9) were the f i r s t to report dramatic decreases i n hydrogen adsorption capacity as a result of high temperature reduction for various p l a t i num metals on reducible supports such as T i 0 , V 0 and N b 0 . They coined the phrase 'Strong Metal-Support Interactions (SMSI) to denote this behavior which was attributed to the r e d u c i b i l i t y of the supports. Decreased hydrogen adsorption uptakes as a r e s u l t of high temperature reductions have also been reported for platinum supported on alumina, a much more d i f f i c u l t support to reduce (10-11). Dautzenberg and co-workers also attributed the suppression i n hydrogen adsorption to metal-support i n t e r a c t i o n s , i . e . the formation of P t - A l 0 complexes. I t i s now w e l l established that reduction tem perature can have a s i g n i f i c a n t effect on subsequent hydrogen adsorp t i o n uptakes (12), but no single explanation for this phenomenon has been universally accepted. In the present paper, results on the influence of reduction tem perature on subsequent hydrogen adsorption are reported for platinum supported on three d i f f i c u l t to reduce supports ( γ - a l u m i n a , magnesia and s i l i c a l i t e ) . The results indicate that the observed suppression i n hydrogen uptakes, a f t e r high temperature reduction, are due to poisoning of the platinum surface by species o r i g i n a t i n g from the support. This interpretation i s s i m i l a r to that of Wang et a l . (13). 2
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Downloaded by AUBURN UNIV on December 26, 2017 | http://pubs.acs.org Publication Date: February 10, 1986 | doi: 10.1021/bk-1986-0298.ch017
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Experimental Methods Supports and Catalysts. The c a t a l y s t supports used i n this work are described i n Table I . The surface areas, except for the s i l i c a l i t e , were measured by the multi-point BET method. The surface area for the s i l i c a l i t e was obtained from the manufacturer. S i l i c a l i t e i s an e s s e n t i a l l y aluminum-free p e n t a s i l z e o l i t e (14) manufactured by Union Carbide. The chlorine contents of the supports were determined by neutron activation a n a l y s i s , and s u l f u r contents were obtained with a Leco s u l f u r analyser. Sulfur and chlorine contents were measured since these elements may influence subsequent hydrogen adsorption on the supported platinum catalysts (15). Support MgO-3 was obtained by steam s t r i p p i n g MgO-2 at atmospheric pressure for 40 h at 500°C, 105 h at 550°C and 110 h at 600°C, followed by dehydration i n flowing, dry nitrogen at 600°C for 4 h . This treatment reduced the chlorine content from 0.41 to 0.03 wt% (see Table I ) . Support A l 0 - 2 was obtained by steam s t r i p p i n g A l 0 - 1 at atmospheric pressure for 24 h at 350°C and 24 h at 4 0 0 ° C . This treatment reduced the chlorine content from 1.5 to 0.04 wt%. Supported platinum c a t a l y s t s , described i n Table I I , were prepared by impregnation of MgO and A 1 0 supports and by exchange for the s i l i c a l i t e support. Impregnations were done with aqueous solutions of hexachloroplatinic acid and with acetone solutions of platinum acetylacetonate. The impregnation procedures with aqueous c h l o r o p l a t i n i c a c i d have been described previously (16). The impreg nation procedure with P t ( C H 0 ) - acetone consisted of wetting the support with acetone, addition of Pt(C5H 0 )2*" acetone s o l u t i o n , 2
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Baker et al.; Strong Metal-Support Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
17.
SZYMURA AND WAN Κ Ε
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Table I . Support
Description of Supports
Surface Area (m /g)
Chlorine Content (wt%)
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Downloaded by AUBURN UNIV on December 26, 2017 | http://pubs.acs.org Publication Date: February 10, 1986 | doi: 10.1021/bk-1986-0298.ch017
MgO-1 MgO-2 MgO-3 MgO-4 Al 0 -1 Al 0 -2 Silicalite 2
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H Adsorption for Pt
30 90
Sulfur Content (wt%) 0.32 0.02 0.02 n.d.* n.d. n.d. n.m.//
