Catalytic Materials - American Chemical Society

19 NMR Techniques for Studying Platinum Catalysts Downloaded by UNIV OF MASSACHUSETTS AMHERST on February 21, 2016 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch019

HAROLD T. STOKES Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602

We describe in some detail the techniques of nuclear magnetic resonance which are used for studying alumina-supported platinum catalysts. In particular, we describe the spin-echo technique from which the Pt lineshape can be obtained. We also discuss spin echo double resonance between surface Pt and chemisorbed molecules and show how the NMR resonance of the surface Pt can be separately studied. We present examples of experimental data and discuss their interpretation.

In recent years, the increased a v a i l a b i l i t y of superconducting magnets f o r n u c l e a r magnetic resonance (NMR) has made p o s s i b l e many new types of s t u d i e s r e q u i r i n g h i g h s e n s i t i v i t y . Among these i s the study of heterogeneous c a t a l y s i s . An unusual example i s the p l a t i n u m - c a t a l y s t s t u d i e s being c a r r i e d out by the r e s e a r c h group of P r o f e s s o r C. P. S l i c h t e r a t the U n i v e r s i t y of I l l i n o i s (1-6). I n these s t u d i e s , care has been taken t o achieve the l i m i t s of s e n s i t i v i t y . I n t h i s paper, we w i l l examine i n some d e t a i l the NMR techniques r e q u i r e d t o c a r r y out such s t u d i e s . We w i l l a l s o show some examples of experimental data and d i s c u s s their interpretation. High S e n s i t i v i t y

i n S o l i d - S t a t e NMR

During the l a s t decade, s e v e r a l t e c h n o l o g i c a l advances have enabled the e x p e r i m e n t a l i s t t o g r e a t l y improve the s e n s i t i v i t y of NMR. Foremost among these a r e superconducting magnets which produce very h i g h magnetic f i e l d s . These h i g h f i e l d s a f f e c t the NMR s e n s i t i v i t y i n two ways. F i r s t , the NMR s i g n a l i s p r o p o r t i o n a l t o the n u c l e a r magnetization M, which, f o l l o w i n g Curie's law, i s p r o p o r t i o n a l t o the magnetic f i e l d SQ. This means that i f we can double the magnetic f i e l d , then, from C u r i e ' s Law alone, we w i l l double the NMR s i g n a l . Since the superconducting 0097-6156/84/0248-0385$06.00/0 © 1984 American Chemical Society

In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, Thaddeus E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on February 21, 2016 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch019

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magnets commonly a v a i l a b l e today produce f i e l d s 3 or 4 times as l a r g e as that of conventional i r o n - c o r e magnets, we can see that a superconducting magnet would allow us to make s u b s t a n t i a l gains i n signal sensitivity. But high f i e l d s a f f e c t s i g n a l s e n s i t i v i t y even more s u b s t a n t i a l l y i n another way. Since the NMR frequency CDQ i s p r o p o r t i o n a l to the f i e l d HQ, a higher f i e l d allows us to operate at a higher frequency. At higher frequencies, the e l e c t r o n i c c h a r a c t e r i s t i c s of a pulsed NMR spectrometer are v a s t l y improved. For one t h i n g , the spectrometer recovers from the high-power r f pulse i n a much shorter time. This i s an important f a c t o r i n s o l i d - s t a t e s t u d i e s where the NMR s i g n a l o f t e n decays r a p i d l y f o l l o w i n g the p u l s e . A l s o , the problems of " a c o u s t i c r i n g i n g " of the sample c o i l v i r t u a l l y disappear at high f r e q u e n c i e s . [Acoustic r i n g i n g i s a poorly understood phenomenon a s s o c i a t e d with mechanical o s c i l l a t i o n s of the sample c o i l f o l l o w i n g an r f pulse (7).] In our spectrometer, the problem disappeared at frequencies greater than approximately 65 MHz. T h i s was perhaps the g r e a t e s t s i n g l e advantage high f i e l d s a f f o r d e d us i n our s t u d i e s of platinum c a t a l y s t s . Our 85-kG superconducting magnet allowed us to operate at 74 MHz. Most of the data acquired at 74 MHz would be impossible to o b t a i n at lower frequencies [even as high as 55 MHz, for example. We t r i e d i t (2) i n some of our field-dependence studies!]. Besides the e f f e c t of high magnetic f i e l d s , the c h a r a c t e r i s t i c s of the sample i t s e l f i s an important c o n s i d e r a t i o n i n improving s e n s i t i v i t y . A l a r g e number of n u c l e i are r e q u i r e d to produce an observable NMR s i g n a l . Heterogeneous c a t a l y s i s i s a surface phenomenon. Thus, i n NMR s t u d i e s , we need to be a b l e to observe the NMR s i g n a l from the n u c l e i at the s u r f a c e . In macroscopic s i n g l e c r y s t a l s , the number of surface n u c l e i i s very small, and NMR s t u d i e s on t h i s kind of sample are p r e s e n t l y impossible. Thus, we are forced to use s m a l l - p a r t i c l e samples which have a very l a r g e surface area. Our c a t a l y s t samples c o n s i s t of small platinum p a r t i c l e s supported on eta alumina. The p a r t i c u l a r NMR p r o p e r t i e s of Pt caused an a d d i t i o n a l problem. Due to the presence of surfaces near most of the n u c l e i , the NMR l i n e i s very broad (approximately 4 kG wide). T h i s means that only a small f r a c t i o n of the nuclear spins can be e x c i t e d by an r f pulse and thus c o n t r i b u t e to any given NMR s i g n a l . Given these v a r i o u s c o n s t r a i n t s , our NMR s t u d i e s of platinum c a t a l y s t s r e q u i r e d 1-gram samples c o n t a i n i n g 5-10% Pt by weight. NMR

Background

In order to b e t t e r understand the NMR techniques described i n t h i s paper, l e t us f i r s t b r i e f l y review some fundamental concepts i n NMR. (For more d e t a i l s , see Reference 8.) Throughout the d i s c u s s i o n , we w i l l use a c l a s s i c a l treatment.


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A n u c l e a r s p i n has a magnetic moment μ which, when placed i n a magnetic f i e l d H, obeys the equation of motion,


du

->

->

, (1)

= μ x γ H

v

where γ i s the gyromagnetic r a t i o of that nucleus. I f Η i s simply an e x t e r n a l time-independent magnetic f i e l d HQ, the ^ o l u t i o n t o Equation 1 i s simple. The moment μ precesses about HQ w i t h frequency ωο = γΗ (see F i g u r e l a ) . For more complicated cases, a very u s e f u l t o o l f o r s o l v i n g Equation 1 i s the r o t a t i n g r e f e r e n c e frame (RRF^. Consider f i r s t of a l l the simple case a l r e a d y Created above: Η = HQ. I n a ^ r e f e r e n c e frame r o t a t i n g about HQ w i t h frequency ω ο , the moment μ appears t o be motionless and behaves as though i t were i n zero f i e l d (see F i g u r e l b ) . I f , however, we view the s i t u a t i o n from a r e f e r e n c e frame r o t a t i n g more s l o w l y a t a frequency ω = ω - Δω, the moment μ appears t o be precessing about HQ w i t h a frequency Δω and thus behaves as though i t were i n a f i e l d h = Δω/γ i n the d i r e c t i o n of HQ (see F i g u r e l c ) . S i m i l a r l y , i n a r e f e r e n c e frame r o t a t i n g a t a frequency ω = ωο + Δω, the moment μ appears t o be p r e c e s s i n g backwards w i t h a frequency Δω and behaves as though i t were i n a f i e l d h = Δω/γ which now p o i n t s i n the opposite d i r e c t i o n of Ho (see F i g u r e I d ) . Let us now apply t h i s concept of the RRF to the case where an r f f i e l d H i i s present. We choose a C a r t e s i a n c o o r d i n a t e system w i t h tlje ζ a x i s along the dc f i e l d HQ and the y a x i s along the r f f i e l d Ηι. The t o t a l f i e l d i s given i n the l a b o r a t o r y r e f e r e n c e frame by 0

0

H = HQZ + 2H y cos u>t x

(2)

In a r e f e r e n c e frame r o t a t i n g about ζ w i t h frequency ω, the r f f i e l d Hj has two components: (1) a dc component H^y and (2) a component o s c i l l a t i n g w i t h frequency 2ω. The 2u)-component has v e r y l i t t l e e f f e c t on ]ϊ and can be d i s c a r d e d . I f the r f f i e l d i s "on resonance" (ω = ωο), then, i n t h e RRF, HQ d i s a p p e a r ^ completely, and we a r e l e f t w i t h o n l y the dc component of H i , H = Hiy

(3)

The moment μ appears to be i n a s t a t i c f i e l d H i which p o i n t s along the y a x i s . The s o l u t i o n to Equation 1 i n the RRF i s simple. The moment Î precesses about y w i t h frequency γΗι (see F i g u r e 2a). Free I n d u c t i o n Decay Consider a l a r g e number of nuclear spins i n some sample under ^tudy. Since Equation 1 i s l i n e a r i n μ, the n u c l e a r magnetization M = 2/M a l s o obeys t h i s equation of motion. I f the sample i s


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placed i n a magnetic f i e l d HQ, M f o l l o w s Curie's Law and^acquires a non-zero component along the ζ a x i s (the d i r e c t i o n of H Q ) . I f we then apply an r f f i e l d and view M i n the RRF, i t w i l l appear to precess about y with frequency yHj (see Figure 2a). The angle through which i t precesses i s given by Δθ = γ Η ^ Δ ί . I f we ajjply the r f only during a time i n t e r v a l At = π/2γΗ , the moment M w i l l have precessed through an angle Δθ = π/2 and w i l l be p o i n t i n g along the χ a x i s ( i n the RRF). Such a pulse of r f i s commonly c a l l e d a " 9 0 ° p u l s e " (see F i g u r e 2b). S i m i l a r l y , i f |he length of the pulse i s at = π/γΗ^, we have a " 1 8 0 ° p u l s e , " and M w i l l be p o i n t i n g along the negative ζ a x i s (see Figure 2 c ) . Now consider what happens f o l l o w i n g a 9 0 ° p u l s e . In the RRF, M i s p o i n t i n g along the χ a x i ^ . In the l a b o r a t o r y r e f e r e n c e frame, M i s precessing about HQ. T h i s induces an r f v o l t a g e i n a r e c e i v e r c o i l around the sample and thus produces an "NMR signal" i n the spectrometer. Such a s i g n a l i s commonly c a l l e d a " f r e e - i n d u c t i o n decay" (FID). I f a l l nuclear spins i n the sample were i n e x a c t l y the same f i e l d , t h i s s i g n a l would not decay, as the name i m p l i e s , but would p e r s i s t " f o r e v e r . " In any r e a l sample, though, l o c a l f i e l d s cause d i f f e r e n t n u c l e i to be i n s l i g h t l y d i f f e r e n t t o t a l f i e l d s , and t h e i r moments ί precess at s l i g h t l y d i f f e r e n t f r e q u e n c i e s . Thus, f o l l o w i n g a 9 0 ° pulse, the moments μ i n i t i a l l y precess coherently but e v e n t u a l l y get out of phase with each other, causing M to "decay" to zero. In s o l i d s , l o c a l f i e l d s are o f t e n r e l a t i v e l y l a r g e and thus cause very short decays. In order to observe the FID, the spectrometer must recover from the preceding r f pulse before M decays to zero. In most cases, t h i s c o n d i t i o n can be s a t i s f i e d , and FID s i g n a l s are normally observed. However, i n the case of our Pt-ca^talyst samples, the d i s t r i b u t i o n of l o c a l f i e l d s i s very l a r g e . M decays to zero long before the spectrometer can recover from the r f pulse, and no FID s i g n a l can be observed.


1

Spin Echo I f the l o c a l f i e l d s are s t a t i c and depend only on p o s i t i o n i n the sample ( t h i s kind of s i t u a t i o n i s c a l l e d "inhomogeneous broadening"), an observable NMR s i g n a l can be produced by a technique c a l l e d s p i n echoes. Consider a moment μ which i s i n a l o c a l f i e l d which causes i t to precess f a s t e r than the RRF. In the RRF, the apparent f i e l d h i n t h i s case p o i n t s along the p o s i t i v e ζ a x i s . F i g u r e 3a shows the spin-echo sequence. First, at t = 0 , we ajjply a 9 0 ° pulse which causes μ to p o i n t along the χ a x i s . Then μ precesses about h through an angle Δθ = γΐιτ during a time i n t e r v a l τ. At t h i s p o i n t (t = τ ) , we apply a 180° pulse which i n v e r t s the x-component of μ. The moment μ i s now at an angle π - Δθ with respect to i t s i n i t i a l d i r e c t i o n along the χ a x i s . Following the 1 8 0 ° pulse, μ precesses again through^an angle ΔΘ during a second time i n t e r v a l τ so that at t = 2τ, μ i s



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ω=ω

ω=ω -Δω

0

0

no precession h=0

precession at Δω h = Δω/γ

ω=ω +Δω 0

precession at -Δω h = -Δω/γ

F i g u r e 1. Precessing magnetic moment i n (a) the l a b o r a t o r y reference frame and i n (b-d) the r o t a t i n g reference frame.

t=0

yH,t=ir/2

yH,t=7r

90° pulse

180° pulse

RF Field H, in the Rotating Reference Frame μ precesses about H, with frequency γΗ,

F i g u r e 2. Magnetic moment precessing about the r f f i e l d i n the r o t a t i n g reference frame.



Figure 3. Spin echo formation w i t h (a-b) non-inverted (c) i n v e r t e d 90° pulse.

and



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at an angle π, i . e . , pointed along the negative χ a x i s . Note that t h i s r e s u l t does not depend on the value of ΔΘ. A l l moments μ, f a s t and slow, end up p o i n t i n g along the negative χ a x i s at the same time t = 2τ. Thus the coherence among the moments which was l o s t i n the FID f o l l o w i n g the i n i t i a l 90° pulse i s regained at t = 2τ. I t does not l a s t long, however, because the d i f f e r e n t moments μ precessing at d i f f e r e n t frequencies soon get out of phase w i t h each other again, and the s i g n a l decays to zero, j u s t as i n the FID. The s i g n a l has the appearance of an "echo" (see F i g u r e 3b). In F i g u r e 4a, we show the envelope of such a s p i n echo obtained from one of our samples. The t r a c e shown was obtained by averaging 20,000 s i g n a l s . (The s i g n a l s are d i g i t i z e d and then d i g i t a l l y added together.) The 90° and 180° pulses were separated by τ = 50 ys. The i n i t i a l t r a n s i e n t s i n the t r a c e a r i s e from recovery of the spectrometer from the 180° p u l s e . As can be seen, a major advantage of the spin-echo technique i s t h a t the s i g n a l can be moved away from those t r a n s i e n t s so that i t i s observable. By separating the two pulses by even a greater amount ( i n c r e a s e τ), the s p i n echo can be moved even f u r t h e r away from the 180° p u l s e . In our case, however, other e f f e c t s cause the amplitude of the echo to s t r o n g l y decrease w i t h i n c r e a s i n g τ so that we do not gain any advantage by i n c r e a s i n g τ f u r t h e r than shown. (This e f f e c t i s caused by the time-dependent part of the l o c a l f i e l d , i . e . , "homogeneous broadening.") The t r a c e shown i n F i g u r e 4a i s one of our strongest s i g n a l s observed. In order to observe much weaker s i g n a l s , we must average a l a r g e r number of s i g n a l s . However, when we do t h a t , the i n i t i a l t r a n s i e n t s a l s o grow l a r g e r and mask the smaller echo, no matter how many s i g n a l s we average. We thus r e f i n e the spin-echo technique to remove the t r a n s i e n t s . Consider what happens i f we i n v e r t the i n i t i a l 90° p u l s e . (We do t h i s by s h i f t i n g the phase of the r f by 180°. In the RRF, Ηχ i s then p o i n t i n g along the negative y a x i s . ) As can be seen i n F i g u r e 3c, the r e s u l t i n g μ at t = 2τ p o i n t s along the p o s i t i v e χ a x i s . This echo i s thus 180° out of phase w i t h the one produced by a non-inverted 90° p u l s e . With a spectrometer which uses phase d e t e c t i o n to produce the envelope of the s i g n a l , the s p i n echo i s " i n v e r t e d , " as seen i n Figure 4b. The t r a n s i e n t s , though, are not i n v e r t e d s i n c e they a r i s e from the 180° pulse which i s i d e n t i c a l f o r the two cases. Thus, i f we s u b t r a c t the two t r a c e s from each other, the s p i n echoes add and the t r a n s i e n t s c a n c e l , l e a v i n g us w i t h a r a t h e r " c l e a n - l o o k i n g " s p i n echo shown i n F i g u r e 4c. Up to t h i s p o i n t , we have ignored the e f f e c t of h during the r f p u l s e s . Of course, i f h > H} f o r many of the nuclear moments i n the sample, the e f f e c t becomes very s i g n i f i c a n t . The a n a l y t i c a l s o l u t i o n to Equation 1 under these c o n d i t i o n s i s very complicated. In e f f e c t , the pulse sequence described above s t i l l produces a s p i n echo (as can be seen i n l5


CATALYTIC MATERIALS


rf pulses

IslCM

0

sequence

(b)

"Subtract" sequence

^

NMR signal

|5

I

Π

L

t a> a. > ι

>

c

-El-

(c)

Difference

Signal — *

/

\

F i g u r e 4. Spin-echo technique f o r removing i n i t i a l t r a n s i e n t s from s i g n a l . Reproduced w i t h permission from Réf. 1. Copyright 1982, The American P h y s i c a l S o c i e t y .



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Figure 4 ) , but only those n u c l e a r moments μ w i t h small values of h p a r t i c i p a t e i n the s p i n echo. (These a r e the moments μ whose frequency of precession i n the l a b o r a t o r y reference frame i s near the r f frequency ω.) In a very broad l i n e , then, the s i z e of the s p i n echo i s p r o p o r t i o n a l to the number of nuclear moments near ω. I f we change HQ, we change h by the same amount and thus sample nuclear moments a t a d i f f e r e n t l o c a l f i e l d . By measuring the s i z e of the s p i n echo as a f u n c t i o n of HQ, we o b t a i n the NMR l i n e s h a p e , i . e . , i n t h i s case, the d i s t r i b u t i o n of the l o c a l f i e l d among the nuclear spins i n the sample. Experimental Data Using Spin Echoes In Figure 5, we show some lineshapes obtained from some of our samples a t 77 K. Each p o i n t represents the s i z e of a s p i n echo obtained a t t h a t f i e l d . The d i s p e r s i o n of these samples ( f r a c t i o n of P t atoms which a r e on the surface of the p a r t i c l e s ) were 4, 11, 15, 26, 46, and 58% f o r F i g u r e s 5a through 5 f , r e s p e c t i v e l y . These samples were u n t r e a t e d , i . e . , exposed t o a i r . The width of most of these l i n e s i s about 4 kG. Compared t o El = 100 G, these l i n e s a r e indeed broad! The source of such l a r g e l o c a l f i e l d s i n our samples i s the i n t e r a c t i o n between the P t n u c l e i and the p o l a r i z e d conductione l e c t r o n s p i n s . This i n t e r a c t i o n i n Pt metal s h i f t s the P t NMR resonance to higher f i e l d s . This s h i f t i s c a l l e d the Knight s h i f t and i s r a t h e r l a r g e compared t o t h e s h i f t s observed i n n o n - m e t a l l i c diamagnetic Pt compounds. I n b u l k Pt metal, the resonance i s a t H / v = 1.1380 kG/MHz (VQ ω /2π, and t h e r e f o r e H / v =2π/γ, which i s independent of f i e l d f o r a given substance), whereas the resonance i n n o n - m e t a l l i c P t compounds ranges between approximately 1.085 and 1.100 kG/MHz (9-10), depending on the compound (see F i g u r e 6). The s h i f t s among the n o n - m e t a l l i c compounds a r e c a l l e d chemical s h i f t s and a r i s e from the interaction with polarized electron o r b i t a l s . In a sample of bulk P t metal, a l l of the n u c l e i have the same i n t e r a c t i o n w i t h the conduction e l e c t r o n s and thus see the same l o c a l f i e l d . The r e s u l t i n g NMR l i n e i s q u i t e narrow. However, i n our samples of s m a l l P t p a r t i c l e s , many of the n u c l e i a r e near a surface where the s t a t e of the conduction e l e c t r o n i s d i s t u r b e d . This tends to reduce the Knight s h i f t f o r these n u c l e i . Since the Pt p a r t i c l e s i n our samples a r e of many d i f f e r e n t s i z e s and shapes, t h i s r e d u c t i o n i n the Knight s h i f t i s not the same f o r every nuclear s p i n near a s u r f a c e . Thus, we o b t a i n a broad "smear" of Knight s h i f t s r e s u l t i n g i n the lineshapes of F i g u r e 5. These lineshapes e x h i b i t some i n t e r e s t i n g f e a t u r e s . Some of them have a peak a t 1.138 kG/MHz, which i s the p o s i t i o n of the resonance i n b u l k P t metal. This peak a r i s e s from P t n u c l e i which are deep w i t h i n the i n t e r i o r of the l a r g e s t P t p a r t i c l e s . T h e i r e l e c t r o n i c environment looks very much l i k e t h a t of b u l k P t metal. ξ

0

0

0

0

0



CATALYTIC MATERIALS

H /v (kG/MHz) 0

H /i/

0

0

0

(kG/MHz)

Figure 5. P t NMR lineshapes f o r s i x untreated samples. Reproduced w i t h permission from Réf. 1. Copyright 1982, The American P h y s i c a l S o c i e t y .

Chemical Shifts

Knight Shift

CVJ

H /i/ 0

0

(kG/MHz)

F i g u r e 6. P o s i t i o n of P t NMR l i n e s f o r v a r i o u s compounds. Reproduced w i t h permission from Réf. 1. Copyright 1982, The American P h y s i c a l S o c i e t y .



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395

As the average p a r t i c l e s i z e decreases (moving from F i g u r e 5a through 5 f ) , the i n t e n s i t y of t h i s peak decreases s i n c e there are fewer l a r g e p a r t i c l e s . In F i g u r e 5, we a l s o see a peak at 1.089 kG/MHz which increases w i t h decreasing p a r t i c l e s i z e . From Figure 6, we f i n d that t h i s peak i s a t the p o s i t i o n of the resonance i n H2Pt(0H)g. Chemical s h i f t s are very s e n s i t i v e to the species of l i g a n d forming the bonds as w e l l as to the c o o r d i n a t i o n of the l i g a n d . This i s e s p e c i a l l y t r u e f o r the OH l i g a n d which produces one of the l a r g e s t paramagnetic s h i f t s known f o r P t . Thus, we f e e l i t i s reasonable to conclude that t h i s peak i n our lineshapes a r i s e s from s u r f a c e Pt which are each bonded to s i x OH groups. Since there are more surface Pt i n the samples of smaller p a r t i c l e s , the i n t e n s i t y of t h i s peak i n c r e a s e s , as observed. The r e a c t i o n s w i t h the atmosphere which cause t h i s r e s u l t are s t i l l under i n v e s t i g a t i o n . Note t h a t our c o n c l u s i o n r e q u i r e s that the s u r f a c e r e c o n s t r u c t and t h a t a l l P t - P t bonds at the surface be broken. This r e s u l t i s c o n s i s t e n t w i t h recent EXAFS s t u d i e s of S i n f e l t , V i a , and L y t l e (11). In v e r y h i g h - d i s p e r s i o n Pt samples which had been exposed to a i r , they found that there are no P t - P t bonds. Spin Echo Double Resonance We show i n F i g u r e 7 the Pt lineshapes (the s o l i d l i n e ) obtained by Makowka (5-6) f o r two d i f f e r e n t samples (26% and 76% d i s p e r s i o n f o r F i g u r e s 7a and 7b, r e s p e c t i v e l y ) at 77 Κ which were cleaned by oxygen and hydrogen c y c l e s at 300°C and then exposed to CO (90% enriched C ) at room temperature. Approximately h a l f a monolayer of CO was chemisorbed. As can be seen, t h i s chemical treatment g r e a t l y a f f e c t e d the lineshape at the l o w - f i e l d end. (Compare the l i n e s h a p e of the untreated 2 6 % - d i s p e r s i o n sample i n Figure 5d w i t h t h a t of the CO-treated sample i n F i g u r e 7a.) The e l e c t r o n i c environment of the Pt n u c l e i near or at the surface of the p a r t i c l e s i s s i g n i f i c a n t l y a l t e r e d by the presence of the CO molecules. With C n u c l e i on the s u r f a c e of the p a r t i c l e s , we can use a technique c a l l e d s p i n echo double resonance (SEDOR) to separate the resonance of surface Pt from the r e s t of the l i n e s h a p e . The s p i n - s p i n i n t e r a c t i o n between Pt and C n u c l e a r spins produces a l o c a l f i e l d Ah at the Pt s p i n . During the f i r s t τ-interval of the spin-echo pulse sequence (see F i g u r e 3a), t h i s a d d i t i o n a l f i e l d causes the Pt moment μ to precess through an angle ΔΘ which i s l a r g e r than before by γΔητ. Now, consider the e f f e c t of a p p l y i n g a 180° pulse to the C n u c l e i . This i n v e r t s the C nuclear s p i n s , causing Ah to reverse d i r e c t i o n , i . e . , Ah -> -Ah. I f we apply t h i s 180° p u l s e to the C spins simultaneously w i t h the 180° pulse a p p l i e d to the P t , then during the second τ i n t e r v a l , the Pt moment μ w i l l precess through an angle ΔΘ which i s smaller than before by γΔητ. At t = 2τ, the moment μ w i l l not be at angle π as before, but at angle π - 2γΔητ. Since the C-Pt c o u p l i n g randomly 13



Figure 7. Pt NMR lineshapes f o r two samples w i t h chemisorbed CO. Reproduced w i t h permission from Ref. 5. Copyright 1982, The American P h y s i c a l S o c i e t y .



19.

STOKES

Platinum Catalysts: NMR

397

Techniques

v a r i e s throughout the sample, so w i l l the v a l u e of Ah. The d i r e c t i o n of the Pt moments thus a f f e c t e d w i l l be randomly s c a t t e r e d about the angle π and w i l l not r e f o c u s p e r f e c t l y a t t = 2τ. I f Ah i s l a r g e enough, we w i l l observe a r e d u c t i o n i n the s i z e of the s p i n echo. Now, the s i z e of Ah i s l a r g e enough o n l y f o r nearest-neighbor Pt-C p a i r s . Thus, any r e d u c t i o n i n the s p i n echo must come from surface Pt bonded to CO molecules. The r e s u l t s are shown i n F i g u r e 7. The l i n e s h a p e i n d i c a t e d by the dashed l i n e i s the d i f f e r e n c e between s p i n echoes obtained w i t h and without the 180° pulse a p p l i e d to the C spins and represents the lineshape due to the surface Pt alone. Since t h i s d i f f e r e n c e was o n l y 10-20% of the s i g n a l , a l a r g e number of s i g n a l s (more than a m i l l i o n ! ) needed to be averaged to o b t a i n the data shown. Even so, a l a r g e amount of s c a t t e r i n the data i s evident. The "SEDOR" data was scaled to match the " s p i n echo" data at the v i c i n i t y of the peak. I f we compare Figures 7a and 7b, we see that although the lineshape due to a l l Pt n u c l e i i n the sample are very d i f f e r e n t (the s o l i d l i n e s ) , the l i n e s h a p e due to surface Pt alone are s t r i k i n g l y s i m i l a r (the dashed l i n e s ) . The NMR c h a r a c t e r i s t i c s of the surface Pt are l a r g e l y determined by the nature of the chemical bonding to the nearby CO molecules and not by the s i z e of the p a r t i c l e i t s e l f . We compared our r e s u l t s i n Pt c a t a l y s t s w i t h t h a t of P t - c a r b o n y l molecules (12), which are o f t e n used as models of the Pt c a t a l y t i c s u r f a c e . The data f o r [ P t 3 8 ( C O ) 4 4 ] ~ was obtained from a sample prepared by Dahl and Murphy at the U n i v e r s i t y of Wisconsin. The data f o r the remaining three molecules was obtained from Brown et a l . (12). As can be seen i n Table I , we found t h a t the p o s i t i o n of the surface Pt resonance i n our c a t a l y s t s i s very c l o s e to those i n v a r i o u s Pt c a r b o n y l s . (The v a r i a t i o n s among the p o s i t i o n s shown i n Table I are small compared to t y p i c a l v a r i a t i o n s among Pt compounds. See F i g u r e 6.) Thus, 2

Table I . P o s i t i o n of NMR peaks i n Pt c a t a l y s t s and v a r i o u s P t - c a r b o n y l molecules

H /v 0

0

(kG/MHz)

Reference

1.096

Our work

1.096

Our work (77 K)

1.0975

12

[Pt (C0) ] "

1.0975

12

[pt (co) ] -

1.0975,1.0980

12

Pt c a t a l y s t s

[pt (coK ] [pt (co) ] 3 8

2

4

3

6

2

2

6

9

9

1 8

2


398

CATALYTIC MATERIALS

from the NMR data alone, we conclude t h a t the Pt atoms i n the P t - c a r b o n y l molecules behave very much l i k e those on the s u r f a c e of P t - c a t a l y s t p a r t i c l e s .


Acknowledgments I wish to acknowledge my co-workers a t the Department o f P h y s i c s , U n i v e r s i t y of I l l i n o i s : C. P. S l i c h t e r , H. E. Rhodes, P.-K. Wang, C. D. Makowka, S. L. Rudaz, and J . P. Ansermet. I e s p e c i a l l y want to thank J . H. S i n f e l t of Exxon Research L a b o r a t o r i e s f o r p r o v i d i n g the Pt c a t a l y s t samples used i n these s t u d i e s and a l s o f o r the h e l p f u l a d v i c e he has given us. I a l s o thank L. Dahl and M. Murphy of the Chemistry Department, U n i v e r s i t y of Wisconsin, f o r p r o v i d i n g us w i t h samples of P t c a r b o n y l s . This r e s e a r c h was supported by the U.S. Department of Energy under Contract No. DE-AC02-76ER01198. Literature 1. 2.

3. 4. 5. 6. 7. 8. 9.

10.

11. 12.

Cited

Rhodes, H. E.; Wang, P.-K.; Stokes, H. T.; Slichter, C. P.; Sinfelt, J. H. Phys. Rev. Β 1982, 26, 3559. Rhodes, Η. E.; Wang, P.-K.; Makowka, C. D.; Rudaz, S. L.; Stokes, H. T.; Slichter, C. P.; Sinfelt, J. H. Phys. Rev. Β 1982, 26, 3569. Stokes, H. T.; Rhodes, H. E.; Wang, P.-K.; Slichter, C. P.; Sinfelt, J. H. Phys. Rev. Β 1982, 26, 3575. Rhodes, Η. E. Ph.D. Thesis, University of Illinois, Urbana, 1981. Makowka, C. D.; Slichter, C. P.; Sinfelt, J. H. Phys. Rev. Lett. 1982, 49, 379. Makowka, C. D. Ph.D. Thesis, University of Illinois, Urbana, 1982. Fukushima, E.; Roeder, S. B. W. J. Mag. Reson. 1979, 33, 199. Slichter, C. P. "Principles of Magnetic Resonance"; Springer, New York, 1980. Carter, G. C.; Bennett, L. G.; Kahan, D. J. in "Progress in Material Science"; Chalmers, B.; Christian, J. W.; Massalski, T. B., Eds.; Pargamon, New York, 1977; Vol. 20, Part I, pp. 295-302. Kidd, R. G.; Goodfellow, J. in "NMR and the Periodic Table"; Harris, R. K.; Mann, Β. E., Eds.; Academic, New York, 1978; p. 251. Sinfelt, J. H., personal communication. Brown, C.; Heaton, B. T.; Chini, P.; Fumagalli, Α.; Longini, G. J. Chem. Soc. Chem. Commun. 1977, 309.

R E C E I V E D September 26, 1983


Catalytic Materials - American Chemical Society

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