Strong Metal-Support Interactions - American Chemical Society

21 Spectroscopic and Electrochemical Study of the State of Pt inPt-TiO2Catalysts

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 15, 2016 | http://pubs.acs.org Publication Date: February 10, 1986 | doi: 10.1021/bk-1986-0298.ch021

M. Spichiger-Ulmann, A. Monnier, M. Koudelka, and J. Augustynski Département de Chimie Minérale, Analytique et Appliquée, Université de Genève, 1211 Genève 4, Switzerland Previous studies performed in this laboratory have led to the observation of a strong metal-support interaction (SMSI) for a Pt/TiO2 catalyst heated in argon (i.e., in the absence of hydrogen) at temperatures higher than ~500°C. This SMSI effect, evidenced by a negative shift of binding energy for the Pt4f electrons (reaching 0.6 eV), presents the peculiarity of being persistent even after exposure of the samples to air for several days. Such a behavior, observed for the titanium supported Pt/TiO2 films, is thought to arise from the fact that Ti atoms from the underlying metal may act as a reducing agent during the thermal treatment. Because of a relatively small specific surface area of this Pt/TiO2 catalyst, an electrochemical technique - the cyclic voltammetry - was chosen to characterize the properties of platinum on the surface of various SMSI and non-SMSI samples. Results regarding H and CO chemisorption, obtained for different Pt/TiO2 films reduced or oxidized at various temperatures, are discussed. A tentative explanation of the SMSI effect exibited by the Pt/TiO2 films is proposed. A large number of studies, s t i r r e d up with the report i n 1978 by Tauster et a l . (J_) of strong metal-support interaction (SMSI) occurring for a series of Ti0 -supported noble metal c a t a l y s t s , has allowed successive refinements of the model of such a system. Both, geometrical and electronic factors are presently considered between possible causes of the altered adsorption behavior of SMSI c a t a l y s t s , appearing i n a q u a s i - t o t a l suppression of hydrogen and carbon monoxide chemisorption. Following an e a r l i e r suggestion of Mériaudeau et a l . (2), the high-temperature reduction of the c a t a l y s t , inducing an SMSI behavior, has been conclusively shown (3,4) to lead to at least p a r t i a l recovering of the metal p a r t i c l e s by the reduced oxide of the support. However, i t i s s t i l l not quite clear whether the encapsulation of metal p a r t i c l e s i s the cause or the consequence of an electronic interaction with the support. 2

0097-6156/86/0298-0212S06.00/0 © 1986 American Chemical Society

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

21.

SPICHIGER-ULMANN ET AL.

213

Electrochemical Study of Pt State

The existence of the electronic interaction i n a model P t / T i 0 system has been inferred (5,6) from our observation of a s i g n i f i c a n t and reproducible negative binding energy (BE) s h i f t for core l e v e l Pt electrons, following the reduction of the catalyst i n argon at 550°C. The kind of samples used i n these studies (5,6) , consisting of P t / T i 0 films supported on T i metal, allowed to minimize charging effects which usually render d i f f i c u l t precise interpretation of the photoelectron spectra for powder c a t a l y s t s . Apart from the extent of the observed Pt4f BE s h i f t , which reached -0.6 eV i n comparison with the BE value measured for a Pt f o i l , i t i s i t s persistence, i n spite of exposure of the samples to a i r , which constitutes the most pecul i a r feature of these P t / T i 0 f i l m s . In this paper we discuss i n more d e t a i l s the morphological c h a r a c t e r i s t i c s and adsorption propert i e s of this SMSI P t / T i 0 system in comparison with those of other non-SMSI P t / T i 0 f i l m s . In p a r t i c u l a r , the second part of this paper focuses on the informations, regarding chemisorption of hydrogen and of carbon monoxide, available from cyclic-voltammetric measurements at P t / T i 0 surfaces. 2


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Experimental Different kinds of platinum catalysts investigated i n t h i s study were supported on 10 to 15 ym thick T i 0 f i l m s . These films were prepared according to a standard procedure (6,7) involving a layer by layer hydrolytic decomposition of an alcoholic TiClt, s o l u t i o n , applied onto metallic titanium substrates. After each application the s p e c i mens were heated at 450°C i n a i r for 15 m i n . ; following the deposition of the l a s t layer the heating was prolonged up to 1 h . Thermal platinum deposits, referred as P t ( t h ) / T i 0 , were obtained by impregnating the T i 0 films with an aqueous solution of H PtCl6 and decomposing i t i n a i r , i n most cases at 450°C. The p l a t i n i z e d T i 0 films were then subjected for 40 min. to annealing i n ultra-pure argon, i n general at 300° or 550°C. The l a t t e r high-temperature reduction treatment led to an SMSI type behavior. The T i 0 films which served as supports for photochemical (6,8) and electrochemical (9) platinum deposits were, p r i o r to recovering them with Pt, reduced i n argon under conditions i d e n t i c a l with those used for SMSI P t ( t h ) / T i 0 samples, i . e . , at 550°C and during 40 min. X-ray photoelectron spectroscopic (XPS) measurements were taken on a Varian IEE-15 spectrometer, using MgKoti, radiation at 1253.6 eV. Basic pressure i n the sample chamber of this spectrometer i s not better than 10~ t o r r . A l l XPS analyses were performed ex s i t u , the samples being transferred to the spectrometer through a i r . Binding energies were referenced to a main C1s l i n e , due to residual pump o i l on the sample surface, taken at 285 eV. The Au4f(7/2) l i n e of metallic gold served as a second internal reference. The BE d i f f e rence between these two levels was remarkably constant, 201.2 ± 0 . 1 eV, in agreement with the results of an extensive comparative study published i n the l i t e r a t u r e (5). Relative surface concentrations of d i f f e r e n t species were determined as described previously (10). Several portions of the powder c a t a l y s t , detached from the surface region of P t ( t h ) T i 0 specimens, were examined by transmission 2

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214

STRONG METAL-SUPPORT INTERACTIONS

electron microscopy (TEM). Both, samples u l t r a s o n i c a l l y dispersed i n alcohol and extractive r e p l i c a with isolated platinum p a r t i c l e s were used. Titanium-supported P t / T i 0 specimens characterized by c y c l i c voltammetry had an apparent (geometrical) area of 0.28 cm . The measurements were carried out i n an aq. 0.5 M NaHSOit/0.5 M Na S0it solution kept at 20°C. The solution was saturated with Ar or CO under 1 atm. The potentials were monitored with respect to and are referred to the reversible hydrogen electrode (RHE) i n the same s o l u t i o n . The cyclic-voltammetry procedure followed that conventionally employed with the potential swept, i n general, at a rate of 0.1 V s . 2

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e

- 1

Results and discussion Structural

studies

Both, X-ray d i f f r a c t i o n and electron m i c r o - d i f f r a c t i o n analyses showed the outer part of the reduced T i 0 films to consist mainly of p o l y c r y s t a l l i n e anatase domains with only small adjunction of r u t i l e . No structural changes were v i s i b l e for high-temperaturereduced P t ( t h ) / T i 0 films for which, besides anatase, metallic p l a tinum was also observed. Due to a p a r t i c u l a r mode of reduction of T i 0 and P t ( t h ) / T i 0 samples used i n this study, the i n t e r i o r of the f i l m s , close to the interface with titanium metal, i s expected to consist of lower titanium oxides including we11-conducting TiO and T i 0 . This was q u a l i t a t i v e l y confirmed by the presence i n this region of the f i l m of perceptible oxygen and titanium concentration gradients detected by means of electron microprobe (6). However, limited resolution of the l a t t e r method does not allow detailed i n terpretation of the observed concentration p r o f i l e s . The reduction at 550°C of the titanium oxide layer i s supposed to proceed mainly from the metal-oxide interface and to involve both the T i migration into T i 0 - and the 0 ~migration into the metal (Figure 1). As a r e s u l t , two kinds of defects, the oxygen vacancies, V Q , and the titanium atoms i n i n t e r s t i t i a l p o s i t i o n s , ( T i ) ^ , may be expected to coexist (11) i n thus reduced T i 0 f i l m s . An extensive XPS study, involving large number of T i 0 samples p l a t i n i z e d using thermal, photochemical and electrochemical methods, showed that, c h a r a c t e r i s t i c a l l y , Pt4f binding energies corresponding to the P t ( t h ) / T i 0 samples reduced at 550°C were substantially lower than those for bulk platinum metal and those for other P t / T i 0 samples. The main results of these measurements are summarized i n Table I. In order to avoid the uncertainties associated with a possible contribution to measured BE's from the extra-atomic relaxation energy (6,12) we chose for this comparison a r e l a t i v e l y thick photochemical Pt deposit, Pt(ph)/Ti0 (in f a c t , the T i signal from the T i 0 support could not be distinguished i n that case). The corresponding Pt4f(7/2) BE was of 71.4 eV, i . e . perceptibly higher than that of unsupported bulk Pt. As a r u l e , Pt4f(7/2) BE s s l i g h t l y higher than 71.1 eV were also found for other Pt(ph)/Ti0 samples containing less or much less platinum, l i k e the one shown i n Figure 2. Platinum appears on the low-magnification scanning electron micrograph as irregular islands dispersed on the T i 0 surface and i n the cracks. 2

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SPICHIGER-ULM ANN ET AL.


215

4o2-


Ti-MTi)

H-Pt Τ1 -(Τ1 "·"),-Τ1 4+ 3+

Ti

3

Ti0

metal

2

anatase

Pt crystallite

Figure 1. D i f f u s i o n processes expected to occur i n the Ti-supported, p l a t i n i z e d T i 0 f i l m during annealing i n argon at 550°C. 2

Figure 2. Scanning electron micrograph (magnification χ 5000) of photochemically p l a t i n i z e d , prereduced T i 0 f i l m . 2



216 Table I .

Surface composition and XPS binding energies f o r T i 0 and P t / T i 0 f i l m s , and for bulk P t .

2

2

surface atomic ratio

BE, eV Sample Pt4f(7/2) T i 0 ( A r , 550°C) Pt(ph)/Ti0


2

2

P t ( t h ) / T i 0 ( A r , 550°C) 2

smooth Pt f o i l

71 .4 (6-1.4) 70.5 (6=1.4) 71.1 (6=1.5)

Ti2p(3/2)

01s*

458.8

530.2 532.1

458.7

530

-

Pt/Ti

0/Ti 2.05

0.1

2.03

532.3

*The 01s BE s and 0 / T i ratios are given f o r the most intense oxygen 1s s i g n a l . 1

Table I I .

Surface composition and XPS binding energies for various P t ( t h ) / T i 0 (Ar, 550°C) samples. 2

surface atomic

Pt4f(7/2) 1 2 3 4 5 6 7 8 9

70.5 (6=1.6) 70.5 (6=1.5) 70.5 (6=1.4) 70.6 (6=1.6) 70.6 (6=1.5) 70.6 (6=1.5) 70.7 (6=1.5) 70.9 (6=1.6) 70.9 (6=1.5)

Ti2p(3/2)

01s

Pt/Ti

0/Ti

458.8

530

0.015

1 .96

458.8

530

0.04

2.04

458.7

530

0.1

2.03

458.8

530

0.04

2

458.8

530

0.07

1.92

458.8

530

0.15

1 .95

458.9

530

0.13

1.93

458.9

530.1

0.13

1.75

458.9

530.1

0.23

2.04


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SPICHIGER-ULMANN ET AL.

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If one assumes that platinum, present on the surface of P t ( t h ) / T i 0 samples during their annealing i n argon, did not affect in a s i g n i f i c a n t way the extent of reduction of the T i 0 support, the Pt4f BE difference occurring between Pt(ph)/Ti0 and P t ( t h ) / T i 0 samples (equal to 0.9 eV) can be assigned neither to relaxation energy nor, more generally, to a matrix effect (6,13). On the contra r y , since the size of Pt p a r t i c l e s observed with TEM for high-tempe rature-reduced (HTR) P t ( t h ) / T i 0 deposits (Figure 3) was much smaller than that of the photochemically formed Pt islands shown i n Figure 2, the actual chemical BE s h i f t could possibly be even larger than the experimentally observed Pt4f BE s h i f t (6,12). However, i t i s to be noted that despite a r e a l l y heterogeneous metal p a r t i c l e size d i s t r i bution characterizing the above mentioned P t ( t h ) / T i 0 deposits, the corresponding Pt4f photoelectron signals had the same width at h a l f maximum (indicated i n Tables as δ) as the signals recorded for Pt f o i l (Tables I and I I ) . This i s inconsistent with d i v e r s i f i e d c o n t r i butions to the Pt4f electron spectra and thus renders doubtful per ceptible differences i n relaxation energy between the P t / T i 0 samples examined during this study. XPS measurements performed with a large number of HTR P t ( t h ) / T i 0 samples, containing d i f f e r e n t amounts of platinum showed certain dispersion of the Pt4f(7/2) BE s examplified i n Table I I . The values ranged from 70.5 to 70.9 eV (respectively 0.6 to 0.2 eV lower than for Pt f o i l ) and tended to increase for the P t / T i ratios higher than 0.1-0.15. In contrast with d i s t i n c t differences between the Pt4f BE*s for the HTR thermal and the photochemical P t / T i 0 deposits, the Ti2p(3/2) BE s l i s t e d i n Tables I and II (including that for nonp l a t i n i z e d T i 0 ) a l l range from 458.7 to 458.9 eV. Also the 01s BE (corresponding to the main signal due to 0 " ions i n the T i 0 l a t t i c e ) was almost constant at 530-530.1 eV. Importantly, there was no s i g n i f i c a n t change i n the positions of Ti2p and 01s signals conse cutive to HTR treatment of the P t / T i 0 f i l m , following which the Pt4f(7/2) l i n e shifted 0.4 eV to lower BE s (Table I I I ) . Table III examplifies also an unexpected and interesting effect exhibited by a few samples, namely the persistence of the low BE Pt4f(7/2) value i n spite of prolonged ( i n one case, for 75 days) exposure to a i r . S t i l l , another Pt(th)/Ti0 specimen, maintained i n a i r for 4 months, showed f i n a l l y a positive s h i f t of the Pt4f(7/2) BE (Table I V ) . This s h i f t was accompanied by a s i g n i f i c a n t increase, up to 2 eV, of the width at half-maximum (WHM) of the corresponding photoelectron signal (Figure 4). Since, as just mentioned, the WHM of the Pt4f(7/2) l i n e s were very reproducible, close to 1.5 eV, a value of 2 eV can be taken as an evidence for the presence of non-equivalent platinum atoms on the surface of that partly "oxidized" sample. Other HTR P t ( t h ) T i 0 samples exposed to a i r for a similar period of time exhibited a shoulder to the main Pt4f s i g n a l , due to Pt(II) oxide ( i n comparison, the oxidation i n a i r of the non-SMSI samples was i n general much more r a p i d , needing few days). As shown i n Table IV, the i n i t i a l , lower Pt4f BE could be restored by high-temperature reduction i n argon and then increased 2

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218


Figure 3. Transmission electron micrograph (replica) of Pt p a r t i c l e s isolated from the P t ( t h ) / T i 0 f i l m annealed i n 2

76

74

72

70

Binding energy (eV)

Figure 4. X-ray photoelectron spectra of the Pt4f l e v e l for the reduced (550°C) P t ( t h ) / T i 0 deposit : (a) as prepared, (b) after 4 months of exposure to a i r . 2


21.

SPICHIGER-ULMANN ET A L .

TABLE I I I .

SURFACE COMPOSITION AND XPS BINDING

ENERGIES

SURFACE ATOMIC

BE, EV

RATIO

SAMPLE

Pt4f(7/2) a) P t ( t h ) / T i 0 ( a i r ,9 ο ρ χ . / 450 C) before annealing i n Ar

_ 71

2


/

ς

2

a

( t

Pt/Ti

^ 530.1

0.04

«8.8

530

0.04

458.7

529.9

0.03

^

^

Q

e

.

7 0

70.6

2

01s

, 458.8

4

Λ

SnoJ? , (Ar,550°C) sample (b) 75 days l a t e r d) P t ( t h ) / T i 0 (Ar,550°C)^sample C >

Ti2p(3/2)

r Î r ^ o - ÎC) ? sample , (Ar,550 ^ · 6^* (a) after reduction

b >

219


.

n

n

e

4

2

(C)

^

AFTER 2nd

Q

^

(6=1.5)

PROLONGED R E D U C TION I N AR

TABLE I V .

SURFACE

COMPOSITION AND XPS BINDING

ENERGIES SURFACE ATOMIC

B E

SAMPLE

Pt(th)/Ti0 (Ar,550°C) 2

A) B)

AS PREPARED AFTER A 4 MONTHS EXPOSURE

C)

TO A I R

AFTER ANNEALING

IN AR AT 550°C D)

AFTER ANNEALING

IN AIR AT 450°C

p

t

4

£

(

7

/

2

)

70.5 (6=1.4) 70.9 (6=2) 70.6 (6=1.5) 70.9 (6=1.7)

-

T i 2 p

e

ratio

V

(3/2)

01s

Pt/Ti

458.7

530

0.1

458.9

530.1

0.1

458.7

530

0.09

458.8

530

0.09


220


again by oxidation i n a i r at 450°C. The same p a r t i a l r e v e r s i b i l i t y of BE changes was also observed consecutive to electrochemical oxidation and reduction of the P t ( t h ) / T i 0 samples i n an aq.sodium sulfate solution at room temperature (6). 2


Electrochemical

investigations

The usefulness of c y c l i c voltammetry as a tool for characterization of noble metal catalysts has been demonstrated by several authors (14-17). The application of this method requires the metal/support system to be e l e c t r i c a l l y conducting, the condition which has been shown to be f u l f i l l e d by the reduced P t / T i 0 / T i samples (5). When the potential of a smooth platinum electrode immersed i n an a c i d i c aq. solution i s swept i n cathodic d i r e c t i o n , two reduction peaks, due to formation of atomic hydrogen, are usually observed at potentials preceding H evolution (Figure 5). Three oxidation peaks are v i s i b l e on the reverse(anodic) sweep, corresponding to removal of H atoms from different submonolayer adsorption states. It i s to be mentioned that four states of H adsorption at Pt could be resolved on c y c l i c voltammograms recorded i n d i l u t e acid solutions, i n the absence of s i g n i f i c a n t s p e c i f i c adsorption of anion (18). The occurrence of these multiple states of H chemisorption has been explained (18) by a combination of i n t r i n s i c and induced heterogeneity f a c t o r s . The former are mainly associated with the presence of various c r y s t a l planes on the p o l y c r y s t a l l i n e Pt surface whilst the l a t t e r arise from progressive increase of H coverage. 2

2

The region of the c y c l i c voltammogram, corresponding to anodic removal of H j atoms, looks quite similar to the thermal desorption spectra of platinum c a t a l y s t s . However, unlikely the thermal desorption spectra, the cyclic-voltammetric p r o f i l e s for H chemisorbed on Pt are usually free of k i n e t i c e f f e c t s . In addition, the e l e c t r o chemical techniques offer the p o s s i b i l i t y of cleaning eventual impur i t i e s from the platinum surface through a combined anodic oxidationcathodic reduction pretreatment. Comparative gas-phase and e l e c t r o chemical measurements, performed for dispersed platinum c a t a l y s t s , have previously demonstrated similar hydrogen and carbon monoxide chemisorption stoichiometries at both the l i q u i d and gas-phase interfaces (14). C y c l i c voltammograms recorded for a series of photochemically and electrochemically p l a t i n i z e d T i 0 samples (Figure 6) exhibited p r i n c i p a l features t y p i c a l of smooth platinum. There were some minor differences, especially for small Pt coverages, regarding the d i s t r i bution of H atoms amongst various adsorption states or the p o s i t i o n of the Pt oxide reduction peak. In contrast, the behavior of HTR P t ( t h ) / T i 0 electrodes was diametrically d i f f e r e n t , the multiple H adsorption peaks being replaced by smooth increase of the current both i n the cathodic and anodic directions (Figure 7). The voltammograms of this kind are known, for example, for electrochromic W0 films (19) and are associated with the proton i n j e c t i o n i n t o , r e s p e c t i v e l y , the proton removal from the oxide l a t t i c e . Prereduced T i 0 films also display similar behavior but the corresponding cathodic a(

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Figure 6. C y c l i c voltammogram of a Pt(ph)/Ti0 f i l m electrode, recorded i n a 0.5 M NaHSO^/0.5 M Na S0i solution saturated with argon. 2

2

f


222


and anodic peaks are shifted to much more negative, non-equilibrium potentials (Figure 7, dotted l i n e curve). This suggests that, i n the case of HTR P t ( t h ) / T i 0 electrodes, the role of Pt c r y s t a l l i t e s i s to mediate entering of protons into the T i 0 f i l m and/or to enhance the f i l m conductivity. Voltammetric experiments, involving p o l a r i zation of the HTR P t ( t h ) / T i 0 electrode at constant potential corresponding to the end of the cathodic sweep, before starting the anodic sweep, showed that the proton i n j e c t i o n was r e s t r i c t e d only to the surface of the f i l m . The anodic oxidation peak recorded i n the l a t t e r experiment was, in f a c t , unchanged with respect to that observed during continuous potential c y c l i n g . These features of electrochemical behavior of the HTR P t ( t h ) / T i 0 samples are to be associated with the results of gas-phase experiments indicating that, whilst SMSI Pt catalysts become inactive with respect to H d i s s o c i a t i o n , they are s t i l l able to catalyse reduction of the T i 0 support by H atoms (C. Naccache, this Symposium). It should also be mentioned that a P t ( t h ) / T i 0 electrode, which had been subjected to the annealing i n argon at 300°C, i . e . , below the c r i t i c a l temperature necessary for inducing SMSI properties, showed voltammetric p r o f i l e s (Figure 8, dotted l i n e curve) of the kind of those e x h i b i ted by the Pt(ph)/Ti0 and smooth Pt electrodes. The differences i n behavior of the HTR P t ( t h ) / T i 0 SMSI samples and of other non-SMSI P t / T i 0 films were even more marked i n the case of carbon monoxide chemisorption experiments. In Figure 9 i s shown a t y p i c a l c y c l i c voltammogram recorded for a smooth Pt e l e c trode i n the solution saturated with CO. The strong adsorption of CO results both i n a q u a s i - t o t a l suppression of the current i n the hydrogen region and i n the appearance of a sharp anodic peak, at ^ 0.9 V vs.RHE, corresponding to the removal of CO j species from the Pt surface. The l a t t e r peak, associated with a two-electron oxidation of adsorbed carbon monoxide, usually serves as a basis for estimating the coverage of the electrode by C 0 ^ . Again, the behavior of Pt(ph)/Ti0 and of LTR (Ar,300°C) P t ( t h ) / T i 0 samples (Figures 10 and 11, respectively) was similar to that of smooth P t . On the other hand, i n the case of HTR P t ( t h ) / T i 0 electrodes, the anodic peak corresponding to the CO j oxidation was absent from the voltammogram (Figure 12). In a d d i t i o n , the l a t t e r voltammogram, i n contrast with those for the non-SMSI P t / T i 0 samples, shows pronounced cathodic and anodic currents i n the hydrogen region, which are also inconsistent with strong CO adsorption on the surface of HTR P t ( t h ) / T i 0 samples. These currents were, however, less intense than those observed i n the absence of CO, suggesting the occurrence of an apparently weak adsorption of carbon monoxide. 2

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Strong Metal-Support Interactions - American Chemical Society

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