Direct STM, XPS and TPD observation of spillover phenomena over mm distances on metal catalyst films interfaced with solid electrolytes

Spillover and Migration of Surface Species on Catalysts Can Li and Qin Xin, editors 9 1997 Elsevier Science B.V. All rights reserved.

39

D i r e c t S T M , X P S and T P D o b s e r v a t i o n of s p i l l o v e r p h e n o m e n a o v e r m m distances on m e t a l catalyst films interfaced with solid electrolytes C. G. Vayenas a, R.M. Lambert b, S. Ladas a, S. Bebelis a, S. Neophytidesa,M.S. Tikhov b, N.C. Filkin b, M. Makri a, D. Tsiplakides a, C. CavalcaC and K. Besocked aDepartment of Chemical Engineering, University of Patras, GR-26500 Patras, Greece bDepartment of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK CDepartment of Chemical Engineering, Yale University, New Haven, CT 06520 - USA dBesocke Delta Phi GmbH, Tuchbleiche 8 - Postfach 2243, D 52402 Jiilich - Germany

Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) have been recently used to study electrochemically controlled spillover and backspillover of oxygen and sodium on polycrystalline Pt films and Pt single crystals interfaced with 0 2. and Na+-conducting solid electrolytes. In this paper we summarize and compare the findings of these investigations which have elucidated the physicochemical origin of electrochemical promotion (NEMCA effect) and have provided concrete evidence that spillover-backspillover of atomic oxygen and sodium can take place over mm distances on metals interfaced with solid electrolytes.

1. INTRODUCTION Spillover phenomena have been reported to play an important role in numerous catalytic systems I1,2]. Hydrogen spillover, in particular has attracted wide attention [1,2] but the spillover of oxygen and several other species [1-3] has also been invoked to rationalize several experimental observations. The evidence for spillover and backspillover phenomena is usually indirect, i.e., is based on adsorption-desorption measurements or on macroscopic catalytic kinetics [1-3]. One class of catalytic systems where spillover and backspillover has been invoked to rationalize the observed kinetic behaviour are metal catalysts interfaced with solid electrolytes [4-14]. In these systems it is known that electrical potential application between the catalyst film and a second (catalytically inert [7-101 or not-accesible to the reactants [4,9-12]) metal film also deposited on the solid electrolyte causes dramatic and reversible alterations in the catalytic activity and selectivity of the catalyst films [4-14] giving rise to the effect of nonFaradaic Electrochemical Modification of Catalytic Activity (NEMCA effect [4-7]) for which the synonyms Electrochemical Promotion [10] or In situ controlled promotion [8,10] are also used. The induced change in catalytic rate is up to a factor of 100 [ 101 or higher [8] than the open-circuit, i.e., unpromoted, catalytic rate and up to a factor of 3x 105 higher than the rate of ion (e.g. O 2-, H § Na § supply or removal to or from the catalyst film [4]. Work in this area has been reviewed [9,10].

40 Since the first studies of in situ controlled promotion, first with 02- conducting solid electrolytes and Pt catalysts [4], it was noticed that the time constant, x, of the catalytic rate increase upon application of constant currents, I, between the catalyst and counter electrodes was on the order of 2FN/I, where F is Faraday's constant and N is the catalyst film surface area, expressed in mol metal and measured via the surface titration technique [4,9,10]: x-- 2FN/I

(1)

Since the parameter 2FN/I corresponds to the time required to form a monolayer of oxygen (or of any oxidic species accompanied by its compensating charge in the metal) on a surface with N adsorption sites when oxygen is supplied as 02. through the solid electrolyte at a rate I/2F, Equation (1) provided evidence that the effect of electrochemical promotion (or NEMCA) is due to the electrochemically controlled migration (backspillover) of a promoting anionic oxygen species from the solid electrolyte onto the gas-exposed catalyst surface. It was also proposed that this anionic oxygen species reacts much slower than normally chemisorbed atomic oxygen with the oxidizable species (e.g. C2H 4 when studying the oxidation of ethylene [4,9,10]) and that the ratio of the reaction rates of atomic oxygen and of the promoter anionic oxygen equals [ 11] the enhancement factor or Faradaic efficiency, A, defined from [4-10]: A = Ar/(I/2F)

(2)

where Ar is the NEMCA induced catalytic reaction rate increase, expressed in mol O, and I/2F is the rate of supply of 02. to the catalyst through the solid electrolyte. This point has been confirmed by the magnitude of the measured catalytic rate transient time constants upon current interruption [ 12]. The above kinetic observations provided the first strong evidence for electrochemically controlled backspillover as the cause of electrochemical promotion. During the last few years this picture has been confirmed by several surface spectroscopic investigations, reviewed in the present paper, which have shown conclusively that electrochemically controlled spillover/backspillover of atomic oxygen and sodium can take place over atomically enormous (mm) distances. 2. EXPERIMENTAL The experimental setup for kinetic electrochemical promotion studies is shown schematically in Fig. 1. The gaseous reactants (e.g. C2H 4 plus O 2) are cofed over the working electrode of the solid electrolyte cell: Catalyst working gaseous reactants electrode (e.g.CzH4+O2) (e.g. Pt,Rh,Ag,lrO2)

solid electrolyte e.g. ZrOz-Y203 [~'-A1203

counter auxiliary electrode gas (e.g. Au) (e.g. 02)

In the single-pellet design (Fig. 1, right) the auxiliary gas is the gaseous reactant mixture. The working electrode serves simultaneously as an electrode and as the catalyst for the catalytic reaction under investigation, e.g., Call 4 oxidation. It is usually in the form of a porous film, 2-20 ~m in thickness, with a roughness factor 3 to 500 [10] deposited on the

41 surface of the ceramic solid electrolyte (e.g. Y203-stabilized-ZrO 2 (YSZ),

a n 0 2-

conductor,

Na-~"-A120 3, a Na + conductor, CaZr0.9In0.1Oa-ot, a H + conductor [13], or TiO 2, a mixed electronic-ionic conductor [ 14]). Catalyst, counter and reference electrode preparation and characterization details have been described previously [9,10] together with the gas-analysis system for on-line monitoring the rates of catalytic reactions via gas chromatography, mass spectrometry and infrared spectroscopy.

| \

R ~

lua

Electro

.)

G~

fN

EJecu~e t~

Reference Electrode ~:n

Figure 1. Experimental setup for electrochemical promotion studies using the fuel-cell type design (left) and the single-pellet type design (fight); G/P galvanostat-potentiostat. A galvanostat or potentiostat is used to apply constant currents between the catalyst and counter electrode or constant potentials, VWR, between the catalyst-working electrode (W) and the reference (R) electrode. In this way ions (02. in the case of YSZ, Na + in the case of Na-[3"-A120 3, H + in the case of CaZr0.9In0.103_~) are supplied from (or to) the solid electrolyte to (or from) the catalyst electrode surface. The single-pellet design has also been used, in various configurations, for the STM [ 15], XPS [16-19], TPD [20] and cyclic voltammetric [20,21] investigations reviewed here. In the case of STM the catalyst-electrode was a Pt monocrystal with an exposed Pt(111) surface interfaced with a Na-[3"-AI20 3 component using a Pt paste electrode to establish good mechanical and electrical contact along the perimeter [15]. Figure 2 shows the experimental setups used for the STM [15], XPS [16-19] and TPD [20] investigations. Further experimental details are given in the corresponding recent publications [ 15-20]. It is worth noting that the porous catalyst films used in most NEMCA studies are typically 20 mm in diameter and, as shown by SEM [9,10], typically 2-20 ~m thick and consist of grains typically l~tm in diameter. The Pt(ll 1) surface used for the STM investigation [15] was 10 mmx 10ram. 3. RESULTS AND DISCUSSION

3.1. Spillover and backspiilover of oxygen The use of X-ray photoelectron spectroscopy (XPS) has provided conclusive evidence for spillover/backspillover of oxygen and has shown that electrochemically controlled backspillover of oxide ions, O ~, is the origin of electrochemical promotion. The first XPS

42

X-Ray Source

Pt(lll)

Phofo el ec fro n

I 9

Energy Analyzer

/@ (a)

PI film .... (_..,Au ~ r

I

(b)

I v,qt

,,,

1,------

Pt-VCE 11-

Q

,

,

(c) u-RE

Figure 2. Experimental setup for X-ray photoelectron spectroscopic (XPS) [ 171 (a), scanning tunelling microscopic (STM) [151 (b), and temperature programmed desorption (TPD) 120] (c) investigation of electrochemical promotion. investigation of Ag electrodes on YSZ under electrochemical O 2 pumping conditions was published in 1983 and provided strong evidence for the creation of backspillover oxide ions on Ag (O1 s at 529.2 eV) upon positive current application [ 161. These results were confirmed by GtJpel and coworkers who used XPS, UPS and EELS to study Ag/YSZ catalyst surfaces under electrochemical bias conditions !181. Ek , eV

716

778.

720

I

i

'"

722

72~

725

728

i

I

i

I

~ 5J8

I

1

536

s3~

I

C

!

~32 s3o E~ , eV

s28

526

ff2c

Figure 3. XPS demonstration of electrochemically controlled back-spillover of oxygen on a Pt catalyst electrode deposited on YSZ upon electrochemical 0 2- pumping to the catalyst : Ols spectra at 673 K (A) I=0; AVwR=0, (B) I=40 I.tA, AVwR=I.2 V (C) Ols difference spectrum (adapted from Fig. 2 of ref. [17].)

43 A similar detailed XPS study of Pt films interfaced with YSZ [17] has shown conclusively that: I. Backspillover oxide ions (Ols at 528.8 eV) are generated on the gas-exposed Pt electrode surface upon positive current application (peak ~i in Fig. 3 c). II. Normally chemisorbed atomic oxygen (Ols at 530. 2 eV) also forms upon positive current application (peak y in Fig. 3 c). The maximum coverages of the y and ~5 states are comparable and of the order of 0.5 each. III. Oxidic backspillover oxygen (~i-state) is significantly less reactive with the reducing (H 2 and CO) ultra high vacuum background than normally chemisorbed atomic oxygen. These observations provide a direct explanation of electrochemical promotion when using O2--conducting solid electrolytes [10,17], as discussed below in conjunction with the TPD results. The two types of electrochemically formed chemisorbed oxygen on Pt films interfaced with YSZ is also clearly manifest via solid state linear potential sweep voltammetry (Fig. 4, Ref. [21]): The first oxygen\reduction peak corresponds to normally chemisorbed oxygen (ystate) and the second reduction peak which appears only after prolonged positive current application [21] corresponds to the ~i-state of oxygen, i.e. backspillover oxidic oxygen, which is significantly less reactive than the y-state.

~=3[~ mV ~ 5

Pt

l 000

0

...5

Figure 4. Linear potential sweep voltammogram of a Pt electrode deposited on YSZ at T=653 K and Po2=0.1 kPa showing the effect of holding time t H at VWR=300 mV on the

-1 -800

-to -t5-

8OO

6 I

-700

-400

~,

0 mV

'

|

J

400

reduction of the y- and ~5-states of adsorbed oxygen; sweep rate=30 mV/s (adapted from ref. [21 l).

The creation of two types of chemisorbed oxygen on Pt surfaces interfaced with YSZ and subject to NEMCA conditions is also manifest clearly by temperature-programmed-desorption (TPD) [20] as shown in Fig. 5. The strongly bonded backspillover oxygen species (peak desorption temperature Tp=750-780 K) displaces the normal chemisorption state of atomic oxygen obtained via gas phase adsorption (Tp=740 K) to a significantly more weakly bonded state (Tp=680 K). The pronounced rate enhancement in NEMCA studies of catalytic oxidations with positive potentials (electrophobic behaviour) is due to the very fast oxidative action of this weakly bonded oxygen. The strongly bonded backspillover anionic oxygen is significantly (A times) less reactive and acts as a sacrificial promoter [ 10].

44

20

16

Figure 5. TPD demonstration of electrochemically controlled backspillover of oxygen on a Pt catalystelectrode of surface area 2x10 -7 mol Pt deposited on YSZ. Oxygen TPD spectra after gaseous oxygen adsorption at 673 K and Po2 =5. 3x 10 -6 mbar for 1800 s (7.2 kilolangmuirs) followed by e l e c t r o c h e m i c a l 0 2. supply I/2F (I=15ktA) for various time periods (reprinted with permission from ref.

d8 4

0 9500

600

700

800

900

~K

[201.

Table 1 summarizes the XPS, TPD and cyclic voltammetric features of the two types of oxygen on P t ~ S Z . Similar observations have been made recently for Ag films interfaced with YSZ.(Fig. 6, ref. [221). Table 1 XPS, TPD and cyclic voltammetric features of oxygen formed on Pt/YSZ with electrochemically controlled oxygen backspillover XPS O 1s binding energy/eV Atomic oxygen "Oxidic" oxygen

TPD peak desorption temperature at 2K/s, Tp/K

Cyclic voltammetry peak reduction potential at 30 mV/s

740 to 680 750-780

0 to -0.2 V -0.3 V

530 528.8

Po2-$xlO'~abm",t,,L.4=15rain

T~t,= 300~

t.,,.~==3mia

r.n O

6

z"

,

Figure 6. TPD demonstration of electrochemically controlled backspillover of oxygen on a Ag catalystelectrode deposited on YSZ. Oxygen TPD spectra after gaseous 0 2

i

/

~

+IOILA +5

2

+2 0 250

300

350

400

T, ~

450

500

550

adsorption at 573 K and Po2=8x10 -6 mbar for 900 s followed by electrochemical 0 2. supply I/2F for 180 s and various applied currents.

45 Surface enhanced Raman spectroscopy (SERS) has also been used to investigate Ag/YSZ [23] and Pt/YSZ [241 surfaces under NEMCA conditions. The A g ~ S Z study focused on the oxygen peak at 780 cm -] which was the main peak of the SERS spectrum [23]. Electrochemical 0 2- supply to the catalyst film led to a significant, up to 60%, increase in the peak signal with a concomitant small peak displacement. The increase in the oxygen peak intensity is consistent with electrochemically controlled oxygen backspillover from the YSZ to the Ag surface. The Pt/YSZ study detected upon electrochemical oxygen supply a Raman band with an unusually low vibrational frequency (113 to 136 cm -]) ascribed to the transient formation of Pt-O bonds with a strongly negatively charged oxygen [24]. Although the exact locations of this Pt-O species is still a subject of discussion [25] this observation appears also to be consistent with the electrochemically controlled backspillover of oxygen.

3.2. Spillover and backspillover of sodium The use of XPS has also confirmed recently that electrochemically controlled Na backspillover is the origin of electrochemical promotion when using Na+-conducting solid electrolytes such as ~"-A1203 [19]. This is shown clearly in Fig. 7 which presents Na ls XPS spectra of Pt~a-~"-Al203 aquired at 500 K under electrochemical bias conditions. Spectrum A was obtained with VWR=0.6 V (VwR is the catalyst potential with respect to a reference Au electrode) corresponding to an electrochemically cleaned surface. Spectrum B was obtained with VWR=-0.4 V corresponding to the Na-promoted surface. The high binding energy feature at 1072.8 eV is assigned to Na on the Pt surface and the smaller feature at 1071.3 eV is ascribed to Na at the [3"-A1203 surface, visible through cracks in the porous polycrystalline Pt film. 350 300 ZSO " ~

"i"

9 /

~

9

l

Z00 150

9

~ ,,, .'~~ ,~.-" . ' ~ .'.~ -''

~

IlO'rl~

/

~

I

.~'-.

I

100

Figure 7. XPS demonstration of electrochemically controlled backspillover of Na on a Pt catalyst electrode deposited on ~3"-A1203:

50

9

""

Vwr:+6OOrnV[

"~

1065

1071

Binding Energy/eV

10"/7

Nals spectra of electro-chemical Na-promoted permission from

Pt/13"-Al203 acquired under bias. (A) cleaned surface, (B) surface (reprinted with ref. [ 19].)

A very clear demonstration of ion backspillover as the cause of NEMCA when using Na~"-A1203 as the solid electrolyte was recently obtained via atmospheric pressure scanning tunneling microscopy (STM) [15]. A Pt monocrystal with an exposed Pt(111) surface

46 (10mm xl0mm) was interfaced with a Na-lY'-AI203 component using a Pt paste electrode along the perimeter [ 15]. Negative current application was found to cause Na backspillover on the P t ( l l l ) surface forming at low surface coverage (<0.01) a Pt(lll)-(12xl2)-Na adlattice on the previously existing Pt(111)-(2x2)-O adlattice. Positive current application was found to totally remove the Na adlattice leaving the Pt(111)-(2x2)-O adlattice intact [15] (Fig. 8).

Figure 8. Atomic resolution STM images (unfiltered) showing reversible electrochemically controlled backspillover of Na on an oxygen covered Pt(111 ) surface of a Pt single crystal interfaced with 13"-A1203: Left: Electrochemically sodium-cleaned surface showing the Pt(l 11)-(2x2)-O adlattice formed by chemisorbed oxygen. Right: Electrochemically sodiumdosed surface (0Na=0.01) showing the creation of an ordered Pt(111) -(12x12) - Na adlattice (Total scan size 159 A, Vt=100 mV, It=l.8 nA) (reprinted with permission from ref. i151).

This study, in addition to explaining NEMCA with Na+-conducting solid electrolytes, provided the first STM confirmation of spillover/backspillover phenomena.

4. CONCLUSIONS The use of XPS [16-19] (Figs. 3 and 7), cyclic voltammetry [21] (Fig. 4), TPD [20,22] (Fig. 5 and 6) and STM [15] (Fig. 8) has clearly shown reversible electrochemically controlled spillover/backspillover of oxygen and sodium on metal surfaces interfaced with solid electrolytes. These observations underline the importance of spillover/backspillover phenomena and provide the means to interprete the key features of electrochemical promotion in catalysis. ACKNOWLEDGEMENT: We thank the PENED and EPET II Programmes of the Hellenic Secretariat of Research and Technology for partial financial support.

47

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