Chemisorption and Catalysis over TiO2-Modified Pt Surfaces - ACS

3 Chemisorption and Catalysis over TiO -Modified Pt Surfaces 2

Downloaded by CORNELL UNIV on November 29, 2012 | http://pubs.acs.org Publication Date: February 10, 1986 | doi: 10.1021/bk-1986-0298.ch003

D. J. Dwyer, J. L. Robbins, S. D. Cameron, N. Dudash, and J. Hardenbergh Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801

The migration of titania from the support onto a Pt surface during high temperature reduction is clearly demonstrated and correlated with chemisorption suppression. The origin of chemisorption suppression is a simple loss in surface adsorption sites; no electronic or chemical modification of the Pt occurs. The rate of methanation (3/1 H2/CO, latm) over Pt blacks increases by well over 2 orders of magnitude when surface TiO i s added. This rate enhancement does not require high temperature reduction and is not directly related to chemisorption suppression. Ti+3 centers at the periphery of the titania-Pt interface are implicated in the mechanism of enhanced methanation. 2

The nature o f s t r o n g metal support i n t e r a c t i o n s (SMSI) has been t h e s u b j e c t o f debate and c o n t r o v e r s y over t h e past s e v e r a l y e a r s . SMSI i s c h a r a c t e r i z e d by t h e l o s s o f a supported m e t a l ' s a b i l i t y t o chemisorb CO o r H f o l l o w i n g high temperature r e d u c t i o n . ( l ) In a d d i t i o n , t h e r a t e o f methane s y n t h e s i s from C0/H can be enhanced over c e r t a i n t r a n s i t i o n metals by u s i n g SMSI s u p p o r t s . ( 2 - 5 ) Pt/Ti0 c a t a l y s t s e x h i b i t one of t h e l a r g e s t SMSI e f f e c t s i n terms o f e n hanced methanation r a t e s and have been e x t e n s i v e l y s t u d i e d i n t h e past.(6-12) E a r l y e x p l a n a t i o n s of the SMSI e f f e c t c e n t e r e d on an e l e c t r o n i c model wherein t h e chemical p r o p e r t i e s o f Pt were a l t e r e d v i a a charge t r a n s f e r between t h e support and t h e m e t a l . ( 1 3 , 1 4 ) More r e c e n t l y , t h e r e i s a growing body o f evidence t h a t a reduced form o f t i t a n i a m i g r a t e s onto t h e metal s u r f a c e d u r i n g high tempera ture reduction.(15-20) In t h i s work, we have s t u d i e d SMSI e f f e c t s i n t h e P t / T i 0 system u s i n g a v a r i e t y o f s u r f a c e s c i e n c e probes on both model ana c o n v e n t i o n a l c a t a l y s t s . T h i s work shows t h a t T i 0 i n c o n t a c t w i t h Pt i s r e a d i l y reduced t o a s u b s t o i c h i o m e t r i c TiOo . A s u b s t a n t i a l rearrangement o f t h e e l e c t r o n i c s t r u c t u r e a t t h e n i L / P t 2

2

2

2

2

χ

0097-6156/ 86/0298-0021 $06.00/ 0 © 1986 American Chemical Society

In Strong Metal-Support Interactions; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

22

S T R O N G M E T A L - S U P P O R T INTERACTIONS

i n t e r f a c e takes p l a c e d u r i n g t h i s r e d u c t i o n . However, t h i s e l e c t r o n i c r e o r g a n i z a t i o n does not induce c h e m i s o r p t i o n s u p p r e s s i o n v i a a charge t r a n s f e r mechanism as p r e v i o u s l y s u g g e s t e d . R a t h e r , chemi s o r p t i o n s u p p r e s s i o n i s due t o the s i m p l e l o s s of s i t e s a s s o c i a t e d w i t h the m i g r a t i o n of T i 0 _ onto the Pt s u r f a c e . When Pt b l a c k s are promoted by the a d d i t i o n o f TiOo o v e r l a y e r s , a two order o f magnitude i n c r e a s e i n the methanation r a t e i s o b s e r v e d . The r a t e enhancement does not r e q u i r e high temperature r e d u c t i o n and may be associated with special i n t e r f a c i a l reaction s i t e s . 2

x


Experimental The experimental apparatus used i n t h i s study i s shown s c h e m a t i c a l l y in F i g . 1. It c o n s i s t s o f a m u l t i - t e c h n i q u e UHV s u r f a c e s c i e n c e chamber coupled d i r e c t l y t o a r e a c t o r system. This configuration p e r m i t s s u r f a c e c h a r a c t e r i z a t i o n and r e a c t o r s t u d i e s t o be c a r r i e d out on both model and c o n v e n t i o n a l c a t a l y s t systems. The r e a c t o r and the down stream a n a l y t i c a l t r a i n have been d e s c r i b e d e l s e where. (21^22) The UHV s u r f a c e probes used i n t h i s study were x - r a y p h o t o e l e c t r o n s p e c t r o s c o p y ( X P S ) , i o n s c a t t e r i n g spectroscopy (ISS) and tem p e r a t u r e programmed d e s o r p t i o n (TPD). XPS measurements were c a r r i e d out w i t h Mg Κ r a d i a t i o n ( h v = 1253.6 eV) and a h e m i s p h e r i c a l e l e c t r o n energy a n a l y z e r set at 50 eV pass energy. ISS measurements were made w i t h the same a n a l y z e r but w i t h r e v e r s e d p o l a r i t y on the hemispheres and i n p u t l e n s e s . The i n c i d e n t H e i o n beam c u r r e n t was 5 χ 1 0 " amps at 500 e V . T h i s beam energy and i o n c u r r e n t caused no change i n s u r f a c e c o m p o s i t i o n d u r i n g the ISS measurements. TPD experiments were c a r r i e d out on a l i q u i d n i t r o g e n c o o l e d sample m a n i p u l a t o r whose temperature c o u l d be l i n e a r l y ramped at 10 K s " over the range 100 Κ t o 1200 K . The desorbed molecules were mass a n a l y z e d by a m u l t i p l e x e d quadrupole mass s p e c t r o m e t e r . Three types of samples were used i n the s t u d y : Pt p a r t i c l e s supported on T i 0 t h i n f i l m s , TiOo c r y s t a l l i t e s on Pt s u b s t r a t e s and 0.8 wt % T i 0 on Pt b l a c k . The r i 0 t h i n f i l m s (200 nm f i l m t h i c k n e s s ) were prepared on a metal s u b s t r a t e heater assembly by chemical vapor d e p o s i t i o n (CVD). The f i l m s were c h a r a c t e r i z e d by x - r a y d i f f r a c t i o n and o p t i c a l t e c h n i q u e s . Pt was d e p o s i t e d on these t h i n f i l m s i n UHV v i a e v a p o r a t i o n from a r e s i s t i v e l y heated Pt w i r e . The TiOo o v e r l a y e r s on Pt s u b s t r a t e s were prepared i n the UHV system; the d e t a i l s of t h i s p r e p a r a t i o n are r e p o r t e d e l s e w h e r e . ( 1 2 ) The T i 0 m o d i f i e d Pt b l a c k was prepared by suspending two grams o f Pt powder (Johnson-Matthey P u r a t r o n i c : 2 ppm S i , 1 ppm Fe) i n a solution of T i ( 0 - C g H ) (0.106g) i n s p e c t r o g r a d e heptane (4.0 ml). The heptane was evaporated i n a n i t r o g e n s t r e a m , the dry s o l i d heated i n f l o w i n g a i r at 675 Κ f o r 2.5 hr and then i n f l o w i n g 10% H , 90% He at 450 Κ f o r 4 h r . The sample was p a s s i v a t e d i n a i r at 300 Κ and i t s s u r f a c e area measured by Kr BET. X-ray d i f f r a c t i o n measurements showed no evidence f o r c r y s t a l l i n e TiOo domains g r e a t e r than 50 A. 4

+

9

1

2

2

2

2

1 9

4

2

D i r e c t Evidence f o r the M i g r a t i o n of T i t a n i a A model c a t a l y s t system i d e a l i z e d t o permit meaningful TPD and ISS s t u d i e s o f Pt/Ti02 prepared as shown s c h e m a t i c a l l y i n F i g . 2 . w a s



3.

DWYER E T A L .

Figure 1. system.

Schematic

o f t h e UHV s u r f a c e

analysis/mircoreactor

Sintered Pt Particles

PT Film 10A Ï2000A

23

Catalysis over TiU2-Modified Pt Surfaces

Ti0 Thin Film 2

600°C

Ti0

2

R, =1 x10 Mbar e

Figure 2. the P t / T i 0

2

Schematic catalyst.

representation

of the t h i n

f i l m model f o r


24


The TiOo t h i n f i l m s were f i r s t c l e a n e d i n UHV by A r i o n s p u t t e r i n g f o l l o w e d by oxygen a n n e a l i n g (1100 Κ, 1 0 " mbar Oo) t o r e s t o r e s u r face stoichiometry. A Pt f i l m o f a p p r o x i m a t e l y u) A (based on the a t t e n u a t i o n of the Ti XPS s i g n a l s ) was vapor d e p o s i t e d on t h i s d e fect free T i 0 surface. The Pt f i l m was then s i n t e r e d by h e a t i n g t o 875 Κ (2 h r . , 1 0 " mbar Oo). The s i z e of the Pt p a r t i c l e s formed i n t h i s manner are e s t i m a t e d t o be 2 t o 5 nm i n diameter based on com p a r i s o n w i t h microscopy s t u d i e s i n the literature.(18) CO TPD r e s u l t s from the p r e s i n t e r e d Pt/TiÛ2 samples before and a f t e r high temperature r e d u c t i o n are shown i n the l e f t hand panel o f F i g . 3. D e s o r p t i o n from the f r e s h l y d e p o s i t e d Pt p a r t i c l e s ( F i g . 3a) e x h i b i t s the c h a r a c t e r i s t i c two peak spectrum o f p o l y c r y s t a l l i n e Pt.(12) The lower temperature peak around 400 Κ i s due t o d e s o r p t i o n r r o m Pt (111) s u r f a c e s ; the high temperature peak i s due t o Heating t h i s s u r f a c e i n H (10"" mbar, 825 K) had a s u b s t a n t i a T i m p a c t on the CO TPD ( F i g . 3b + 3c). There i s a decrease i n the t o t a l amount of CO c h e m i s o r p t i o n on the Pt p a r t i c l e s and a change i n t h e l i n e s h a p e of the d e s o r p t i o n spectrum. The decrease i n c h e m i s o r p t i o n c a p a c i t y i s a d i r e c t m a n i f e s t a t i o n of the SMSI e f f e c t . The changes i n l i n e s h a p e and peak maxima are p r o b a b l y a s s o c i a t e d w i t h a change i n p a r t i c l e morphology as suggested by White and c o w o r k e r s ( l l ) and observed by Baker et al.(24,25) The o r i g i n of c h e m i s o r p t i o n s u p p r e s s i o n i n t h i s model system can be d i r e c t l y a s c e r t a i n e d by the h i g h l y s u r f a c e s p e c i f i c t e c h n i q u e of ISS. The ISS s p e c t r a a s s o c i a t e d w i t h t h e s e TPD s p e c t r a are d i s p l a y e d i n the r i g h t hand panel o f F i g . 3 . The spectrum l a b e l e d 3a i s t h a t o b t a i n e d from the p r e s i n t e r e d Pt p a r t i c l e s which e x h i b i t normal c h e m i s o r p t i o n . A s t r o n g s i g n a l at a r e c o i l energy o f 0.88 i s r e a d i l y a s s i g n e d by the b i n a r y c o l l i s i o n model(26) t o s c a t t e r i n g from Pt s u r f a c e atoms. The a n t i c i p a t e d s i g n a l s from Ti and 0 s u r f a c e atoms are a l s o c l e a r l y present i n the spectrum. Upon ex posure t o high temperature hydrogen ( F i g . 3b, 3c) t h e r e i s a c l e a r decrease i n the Pt s c a t t e r i n g s i g n a l concomitant w i t h the decrease i n CO c h e m i s o r p t i o n . The decrease i n the ISS s i g n a l i n t e n s i t y i n d i c a t e s a l o s s of Pt s c a t t e r i n g c e n t e r s i n the outermost atomic l a y e r of the sample. I t should be noted t h a t the i n t e n s i t y of the XPS s p e c t r a ( T i , 0 , P t ) from the sample remained v i r t u a l l y constant d u r i n g t h i s sequence. The o n l y p o s s i b l e e x p l a n a t i o n f o r these r e s u l t s i s the c r e e p i n g of t i t a n i u m o x i d e onto the Pt s u r f a c e d u r i n g high temperature r e d u c t i o n . The SMSI induced c h e m i s o r p t i o n s u p p r e s s i o n c o u l d be reversed i n t h e s e t h i n f i l m samples by h e a t i n g i n o x y g e n . T h i s f a c t i s demon s t r a t e d i n F i g . 4 where the l e f t hand panel c o n t a i n s CO TPD r e s u l t s from the p r e v i o u s l y e n c a p s u l a t e d samples a f t e r o x i d a t i o n (875 K, PQ = 10" mbar). I t i s c l e a r l y e v i d e n t from the TPD r e s u l t s t h a t the c h e m i s o r p t i o n c a p a c i t y o f the Pt p a r t i c l e s c o u l d be r e s t o r e d by oxidation. In a d d i t i o n , the ISS r e s u l t s ( r i g h t hand panel of F i g . 4) show the reemergence of Pt i n the outermost atomic l a y e r of t h e sample. In t h i s manner i t was p o s s i b l e t o r e p e a t e d l y c y c l e the sample i n t o and out o f the SMSI s t a t e . During the f i r s t r e d u c t i o n o x i d a t i o n c y c l e a permanent l o s s of some of the ISS and TPD s i g n a l s occured. However, i n a l l subsequent c y c l e s both the ISS and TPD s i g n a l i n t e n s i t y c o u l d be f u l l y r e c o v e r e d by o x i d a t i o n . The i n i t i a l permanent l o s s of some 10 - 15% of the c h e m i s o r p t i o n c a p a c i t y i s +

6

2


6

2

b

b



3.

DWYER E T A L .

Catalysis over TiOi-Modified

25

Pt Surfaces

Figure 3. TPD and ISS r e s u l t s o b t a i n e d from the t h i n f i model c a t a l y s t as a f u n c t i o n o f r e d u c t i o n time at 875 Κ i n 10"' mbar Ho. a) 0 m i n . b) 15 m i n . c) 30 m i n .

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Figure 4. TPD and ISS r e s u l t s o b t a i n e d from a prereduced t h i n f i l m model as a f u n c t i o n of o x i d i a t i o n time at 875 Κ i n 1 0 " ° mbar 0 d) 0 m i n . e) 20 m i n . f ) 40 m i n . 2


26


probably due t o the i r r e v e r s i b l e d e c o r a t i o n of the Pt p a r t i c l e s by migrating t i t a n i a . The subsequent r e v e r s i b l e c h e m i s o r p t i o n s u p p r e s s i o n i s due t o t i t a n i a t h a t spreads a c r o s s the Pt s u r f a c e d u r i n g high temperature r e d u c t i o n and then s p a l l s up on the s u r f a c e d u r i n g oxidation.


The Nature of C h e m i s o r p t i o n Suppression The m i g r a t i o n of t i t a n i a onto the metal s u r f a c e d u r i n g high tempera t u r e r e d u c t i o n does not r u l e out chemical m o d i f i c a t i o n of the metal as a f a c t o r i n SMSI b e h a v i o r . A second type of sample was s t u d i e d t o probe the d e t a i l s o f c h e m i s o r p t i o n s u p p r e s s i o n and t o t e s t whether TiOg induces long range e l e c t r o n i c e f f e c t s on P t . This sample c o n s i s t e d of an u l t r a h i g h p u r i t y Pt f o i l decorated w i t h a T i 0 overlayer. The r e s u l t s of t h i s study have been p u b l i s h e d i n d e t a i l e1sewhere(12) but are i n c l u d e d here f o r the sake of c o m p l e t e ness. T h i s e a r l i e r study found t h a t T i 0 c r y s t a l l i t e s i n c o n t a c t w i t h Pt are r e a d i l y reduced t o a s u b s t o i c h i o m e t r i c form by hydrogen or carbon monoxide treatment i n the vacuum chamber. The r e d u c t i o n process r e s u l t s i n a s u b s t a n t i a l rearrangement of the electronic s t r u c t u r e at the P t / T i 0 i n t e r f a c e . These changes can be seen i n XPS s p e c t r a from t h e s e s u r f a c e s as shown i n F i g . 5 . The s p e c t r a l a b e l e d 5a are those o b t a i n e d from a f r e s h l y o x i d i z e d TiOo o v e r l a y e r on the Pt f o i l . The c e n t e r panel c o n t a i n s the 2 P ' , Z p ' ' s p i n o r b i t d o u b l e t from t i t a n i u m i n the +4 o x i d a t i o n s t a t e . Upon r e d u c t i o n w i t h H or CO a set of s h o u l d e r s due t o T i appear i n t h i s spectral region ( F i g . 5b). In a d d i t i o n t o t h i s t r u e chemical s h i f t , t h e r e i s a r i g i d s h i f t [ £ = 0.7 ± 1 eV) i n the e n t i r e T i 0 band s t r u c t u r e r e l a t i v e t o the Pt fermi l e v e l . This r i g i d s h i f t i s caused by a change i n the fermi l e v e l p o s i t i o n i n the s e m i c o n d u c t i n g o x i d e and the e s t a b l i s h m e n t of an ohmic c o n t a c t at the metal s e m i conductor j u n c t i o n . ( 1 2 ) Since the T i c e n t e r s produced d u r i n g the r e d u c t i o n are donor s t a t e s ( 2 7 ) , t h i s r e o r g a n i z a t i o n undoubtedly i n v o l v e s charge t r a n s f e r f r o m t h e semiconductor t o the m e t a l . How e v e r , the charge t r a n s f e r does not p e r t u r b the c h e m i s o r p t i v e p r o p e r t i e s of the metal as was suggested i n e a r l i e r s t u d i e s . ( 1 3 , 1 4 ) This f a c t can be seen i n CO TPD r e s u l t s from t h e s e samples as a f u n c t i o n of T i 0 coverage. The l e f t hand panel o f F i g . 6 c o n t a i n s CO TPD s p e c t r a at t h r e e coverages of Τ ι 0 . The presence of TiOo on the s u r f a c e o n l y suppresses the t o t a l amount of chemisorbed CO. No new c h e m i s o r p t i o n s t a t e s are produced when T i 0 _ i s added t o the s u r face. The r i g h t hand panel of F i g . 6 demonstrates the l i n e a r r e l a t i o n s h i p between the amount of c h e m i s o r p t i o n s u p p r e s s i o n and the number o f b l o c k e d s u r f a c e s i t e s as measured by ISS. It i s c l e a r from these data t h a t the presence of t i t a n i u m oxide on Pt suppresses c h e m i s o r p t i o n v i a a s i m p l e s i t e b l o c k i n g mechanism. 2

2

2

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2

3

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Methanation over T i 0

2

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M o d i f i e d Pt

C h e m i s o r p t i o n s u p p r e s s i o n v i a a s i m p l e s i t e b l o c k i n g mechanism i s not c o n s i s t e n t w i t h enhanced methanation r a t e s observed f o r P t / T i 0 c a t a l y s t s i n the SMSI s t a t e . ( 2 - 5 ) A simple l o s s of CO a d s o r p t i o n s i t e s w i t h o u t a s u b s t a n t i a l cïïârTge i n the a d s o r p t i o n energy s h o u l d r e s u l t i n a decrease i n c a t a l y t i c a c t i v i t y . I t i s l i k e l y then t h a t 2


3.

27

Catalysis over T1O2-Modified Pt Surfaces

DWYER ET AL.

ΔΕ = .6

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ΔΕ = 0.7

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Ti(2p)

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Binding Energy (eV)

F i g u r e 5 . XPS data o b t a i n e d from a T i 0 on Pt p o l y c r y s t a l sample, a) F r e s h l y o x i d i z e d sample b ) A f t e r r e d u c t i o n by CO i n the UHV chamber. Reproduced w i t h p e r m i s s i o n from R e f . 12 ( F i g . 3). 2

Pt(foil) + Ti0

200

300

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400

500

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600

700

800

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Temperature (K)

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coverage.

TPD o f CO from Pt p o l y c r y s t a l The f r a c t i o n a l

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Exposed Platinum (Pt/Pto ISS)

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o b t a i n e d by r a t i o i n g t h e Pt ISS s i g n a l surface P t . Reproduced w i t h p e r m i s s i o n and 1 0 ) .

as a f u n c t i o n o f T i 0

coverage

of

Ti0

2 - x

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was

to that of the clean from R e f . 12 ( F i g s . 8


28


enhanced methanation r a t e s over P t / T i 0 are not d i r e c t l y l i n k e d t o the SMSI e f f e c t as d e f i n e d by c h e m i s o r p t i o n s u p p r e s s i o n . To study the e f f e c t s of T i 0 o v e r l a y e r s on the c a t a l y t i c response of Pt s u r f a c e s , Pt black samples w i t h and w i t h o u t 0.8 w t . % TiOo were i n v e s tigated. The 0.8 w t . % l o a d i n g o f T i 0 was e q u i v a l e n t t o 10 mono l a y e r s o f T i 0 i f t o t a l l y d i s p e r s e d on the Pt b l a c k s u r f a c e . The methanation a c t i v i t i e s of these samples as a f u n c t i o n of temperature are shown i n an A r r h e n i u s format i n F i g . 7. The r e a c t i o n c o n d i t i o n s were 3:1 H : C 0 , latm and a maximum c o n v e r s i o n of 3%. In agreement w i t h e a r l i e r s t u d i e s , ( 8 , 1 0 ) c l e a n Pt b l a c k was an extremely poor methanation c a t a l y s t . The t u r n o v e r frequency of our m a t e r i a l at 548 Κ was a p p r o x i m a t e l y 1 0 " molecules s i t e " S" (assuming 8 χ 1 0 site cm' ). T h i s v a l u e i s somewhat lower than e a r l i e r s t u d i e s ( 8 , 1 0 ) but was not due t o s u r f a c e i m p u r i t i e s as v e r i f i e d by XPS and ISS s t u d i e s of the m a t e r i a l . The a d d i t i o n of TiOo t o the Pt b l a c k causes a dramatic i n c r e a s e i n the methanation a c t i v i t y (> 2 o r d e r s of magnitude). I t i s important t o note t h a t the r a t e s r e p o r t e d i n F i g . 7 are s c a l e d by the p h y s i c a l s u r f a c e area of the c a t a l y s t and not by the Pt s u r f a c e s i t e d e n s i t y . T h e r e f o r e , the r a t e i n c r e a s e r e p r e s e n t s a t r u e i n c r e a s e i n the c a t a l y t i c y i e l d of the m a t e r i a l . T h i s r e s u l t i s remarkable i n l i g h t of the massive l o a d i n g of s u r f a c e TiOo but i s i n e x c e l l e n t agreement w i t h e a r l i e r s t u d i e s of t h i s type.(10,28) S u r f a c e a n a l y s i s of the T i 0 promoted Pt b l a c k r e v e a l e d i m p o r t a n t i n f o r m a t i o n c o n c e r n i n g the nature of the c a t a l y t i c surface. F i g . 8 c o n t a i n s the Ti 2p r e g i o n of the XPS spectrum of the T i 0 promoter a f t e r H r e d u c t i o n (450 K, l a t m H ) . Two o x i d a t i o n s t a t e s o f t i t a n i u m (+3 and +4) are r e a d i l y v i s i b l e i n the spectrum. The a b s o l u t e b i n d i n g e n e r g i e s are h i g h e r than those i n F i g . 5 due t o d i f f e r e n c e s i n the l o c a t i o n o f the fermi l e v e l of the o x i d e . ( 1 2 ) The presence of s u b s t a n t i a l T i c h a r a c t e r i n the XPS spectrum iriôTcates that T i 0 i n c o n t a c t w i t h Pt i s r e a d i l y reduced t o a subs t o i c h i o m e t r i c form even under m i l d r e d u c i n g c o n d i t i o n s (450 K ) . These c o n d i t i o n s are w e l l below those r e q u i r e d t o induce t i t a n i a m i g r a t i o n and c h e m i s o r p t i o n s u p p r e s s i o n i n t h i n f i l m s . Another i m p o r t a n t o b s e r v a t i o n was o b t a i n e d from ISS (see t a b l e i n s e r t i n F i g . 5 ) , which shows t h a t o n l y 5% of the outermost atomic l a y e r s o f the sample i s P t . In s p i t e of the e x t e n s i v e T i 0 _ ^ o v e r l a y e r which b l o c k s 95% of the Pt a d s o r p t i o n s i t e s , the methanation r e a c t i o n r a t e i n c r e a s e d by over two o r d e r s of magnitude. The o r i g i n of t h i s e n hancement i s not immediately e v i d e n t from these measurements but t h e r e i s i n d i r e c t evidence t h a t T i c e n t e r s l o c a t e d at the p e r i phery of the t i t a n i a - P t i n t e r f a c e may be i m p o r t a n t . T h i s evidence was o b t a i n e d when the c a t a l y t i c response of the T i 0 _ promoted sample was i n v e s t i g a t e d at a h i g h e r temperature/higher c o n v e r s i o n (~ 5%) c o n d i t i o n s . Prolonged (16 h r ) o p e r a t i o n of the T i 0 / P t at 625 Κ r e s u l t e d i n a 50% l o s s of c a t a l y t i c a c t i v i t y . The d e a c t i v a t e d ma t e r i a l s t i l l e x h i b i t e d l i n e a r arrhenius behavior with a slope i n d i s t i n g u i s h a b l e from t h a t of the f r e s h l y charged c a t a l y s t (Fig. 9). S u r f a c e a n a l y s i s showed the a c t i v i t y l o s s was not due t o carbon d e p o s i t i o n or the a d s o r p t i o n of any o t h e r i m p u r i t y . In f a c t , ISS and XPS r e v e a l e d the d e a c t i v a t e d c a t a l y s t c o n t a i n e d l e s s a d v e n t i t i o u s s u r f a c e carbon than the f r e s h one. Significantly, catalyst d e a c t i v a t i o n accompanied both a 30% d e c l i n e i n the Ti" "' s i g n a l and a 2

2

2

2

2

5


e

1

#

X

1 4

z

2

2

2

2

+ 3

2

2

+ 5

2

x

2

1

3


3.

DWYER E T A L .

Catalysis over Ti0 -Modified

29

Pt Surfaces

2

Temperature Κ


700

t 1.4

600

I 1.5

I 1.6

500

I 1.7

I 1.8

Ë 1.9

I 2.0

1 2.1

1/Kx103

Figure 7. Methanation rate as a function of temperature o b t a i n e d from Pt b l a c k compared t o T i 0 m o d i f i e d Pt b l a c k . E 25 ± 1 k c a l m o l e ' f o r P t , E = 20. ±1 kcal m o l e " f o r T i 0 _ 2

1

X

x

a

1

a

2

/Pt. Ti 2p Ti0 _ /Pt H 175°C 1 atm 2

x

Atom

%

0 Ti Pt

39 56 5.0

2

459.1 eV

l

470.0

Figure 8. reduction.

Ti

Τ

.

XPS s i g n a l

I

I

I

I

460.0 Binding Energy eV

from

TiOo / P t X

I

black

I

450.0

sample


after

30


d o u b l i n g of the Pt ISS i n t e n s i t y ( F i g . 1 0 ) . These data c l e a r l y show t h a t methanation a c t i v i t y of the T i 0 / P t does not c o r r e l a t e w i t h Pt metal s u r f a c e a r e a . A p p a r e n t l y , water produced under high c o n v e r s i o n c o n d i t i o n s i s o x i d i z i n g the t i t a n i a o v e r l a y e r c a u s i n g i t t o s p a l l up on the s u r f a c e . The concomitant l o s s i n c a t a l y t i c a c t i v i t y w i t h i n c r e a s i n g Pt s i t e d e n s i t y suggests t h a t the key c h e m i s t r y i s o c c u r r i n g at s i t e s a s s o c i a t e d w i t h the T i c e n t e r s and not on the average Pt s i t e . These s i t e s presumably e x i s t at the p e r i p h e r y o f t h e Pt t i t a n i a i n t e r f a c e and decrease i n c o n c e n t r a t i o n as the oxide o v e r l a y e r c o n t r a c t s back i n t o t h r e e d i m e n s i o n a l c r y s t a l l i t e s . It i s i m p o r t a n t t o note t h a t even a f t e r 16 h r . under C0/H at 5% c o n v e r sion, T i i s c l e a r l y e v i d e n t by XPS. These r e s u l t s show t h a t T i c e n t e r s and SMSI e f f e c t s are q u i t e d u r a b l e under hydrocarbon s y n t h e sis conditions. 2

+ 3

2


+ 3

+ 3

Conclusions The data present i n t h i s paper support the e n c a p s u l a t i o n model (15) f o r SMSI b e h a v i o r . It i s c l e a r t h a t reduced t i t a n i a can m i g r a t e onto the supported metal d u r i n g high temperature r e d u c t i o n (see F i g . 3). T h i s m i g r a t i o n r e s u l t s i n the s u p p r e s s i o n of c h e m i s o r p t i o n by a s i m p l e p h y s i c a l blockage of s u r f a c e s i t e s . The chemical p r o p e r t i e s o f Pt a d s o r p t i o n s i t e s which are not b l o c k e d remain u n a l t e r e d by the presence of reduced t i t a n i a and e x h i b i t normal c h e m i s o r p t i v e be havior ( F i g . 6). No evidence e x i s t s f o r e l e c t r o n i c or chemical m o d i f i c a t i o n o f the Pt s u r f a c e as i s the case f o r a l k a l i adl a y e r s . (29^31) T h i s simple l o s s of metal a d s o r p t i o n s i t e s causes c h e m i s o r p t i o n s u p p r e s s i o n and may account f o r r e d u c t i o n s i n r a t e s of ethane h y d r o g e n o l y s i s over SMSI c a t a l y s t s . ( ^ 2 ) On the o t h e r hand, temperaenhanced methanation r a t e s over t u r e r e d u c t i o n and are not d i r e c t l y l i n k e d t o SMSI b e h a v i o r (as d e f i n e d by c h e m i s o r p t i o n s u p p r e s s i o n ) . The h i g h e r methanation r a t e s appear t o r e l a t e t o some i n t r i n s i c p r o p e r t y of the P t / T i 0 system and not coupled t o the m i g r a t o r y b e h a v i o r of reduced T i 0 _ . The o r i g i n of methanation r a t e enhancement s t i l l remains an enigma but i n d i r e c t evidence suggests t h a t T i c e n t e r s at the p e r i p h e r y of metal p a r t i c l e s are i m p o r t a n t . XPS data ( F i g . 8) shows t h a t TiOo i n c o n t a c t w i t h Pt i s r e a d i l y reduced t o T i 0 _ even at m i l d r e d u c t i o n temperatures (450 K ) . T h i s r e d u c t i o n t a k e s p l a c e w e l l below the temperature (> 650 K) where T i 0 migration is f a c i l e . The p r e s ence o f T i c e n t e r s at the p e r i p h e r y o f the metal p a r t i c l e s may a c t i v a t e CO m o l e c u l e s v i a an unusual a d s o r p t i o n geometry as s u g gested by Vannice and o t h e r s . ( 8 , 3 3 , 3 4 ) The proposed a d s o r p t i o n geometry i n v o l v e s c o o r d i n a t i o n or the oxygen end of Pt-CO t o a T i center. Presumably, t h i s bonding weakens the CO bond and f a c i l i t a t e s CO s c i s s i o n ( m e t h a n a t i o n ) . However, no d i r e c t evidence f o r t h i s novel CO b i n d i n g geometry was o b t a i n e d i n t h i s s t u d y . No new f e a t u r e s were observed i n the CO TPD spectrum of T i 0 m o d i f i e d Pt ( F i g . 6) and IR s t u d i e s r e v e a l e d no unusual CO s t r e t c h i n g f r e q u e n cies.^) I t c o u l d be t h a t the c o n c e n t r a t i o n of such a d s o r p t i o n s i t e s " i s below our d e t e c t i o n l i m i t s but the s i t e s are e x t r e m e l y a c t i v e and dominate the c a t a l y s i s . However t h i s e x p l a n a t i o n i s not very s a t i s f y i n g and u n t i l more d i r e c t evidence i s o b t a i n e d , the e x a c t r o l e of T i c e n t e r s i n CO hydrogénation remains s p e c u l a t i v e . 2

2

x

+ 3

2

x

2 - x

+ 3

+ 3

2 - x

+ 3


3.

DWYER ET AL.

Catalysis over Ή0 -λϊοώβ"Ί

31

Pt Surfaces

2

Temperature Κ 400

500

600 10i« ρ — τ

—ι—

—ι—

3:1 H :CO 1 atms Ti0 _ /Pt 2

2

x


625K Over Night

ο) ί ο "

1010 1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

1/KX103

Figure 9. a c t i v i t y at

Arrhenius plot for high t e m p e r a t u r e . 1

1

Ti0

2 - x

showing

1

"~ι

Γ"

/Pt

From ISS Atom %

Ti 2p Ti0 _x/Pt 3:1 H :CO 350 °C 1 atm 2

2

loss

of

Γ"

53 37 10

0 Ti Pt

459.2 eV

Ti*

3

456.3 eV

470.0

Figure 10. Ti XPS data temperature d e a c t i v a t i o n .

450.0

460.0 Binding Energy eV

from

Ti0

2 - x

/Pt

sample

after


high

32

S T R O N G METAL-SUPPORT

INTERACTIONS

Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

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DWYER ET AL.

33. 34. 35.

Catalysis over Ti0 -Modified 2

Pt Surfaces

33

Sachtler, W. M. H . ; Shriver, D. F.; Hollenberg, W. B.; Lang, A. F. J . Catal. 1985, 92, 429. Burch, R.; Flambard, A. R. J. Catal. 1982, 78, 389. Robbins, J . L . , unpublished data.


RECEIVED September 17, 1985


Chemisorption and Catalysis over TiO2-Modified Pt Surfaces - ACS

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