IR Spectroscopy as an In Situ Probe for Molecular Structure in

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IR Spectroscopy as an In Situ Probe for Molecular Structure in Electrocatalytic and Related Reactions Alan Bewick and Maher Kalaji Department of Chemistry, University of Southampton, Southampton, England

Techniques are described which obtain the IR absorption spectra of species, either adsorbed or free in the electrode/electrolyte solution interphase. Applications slanted towards topics relevant to electrocatalytic processes are discussed to illustrate the capabilities of the methods in probing molecular structure, orientation and interactions. Data obtained using spectroscopic methods have had a profound influence on the development of many areas of chemistry. In most cases the value of these methods stems from their high sensitivity and molecular specificity which can lead directly to a quantitative assessment and a structural characterisation of the species present in a chemical system and possibly also to information on their interactions and reactions. High vacuum surface science provides a particularly spectacular example of the value of spectroscopic methods and the rapid progress that can be engendered by their intelligent deployment; thus the use of electron diffraction techniques to determine the long range atomic or molecular order on solid surfaces and adlayers in conjunction with UPS, XPS, EELS and IR reflection spectroscopy to characterise molecular structure and orientation have produced rapid advances in the understanding of surface chemistry and surface physics at the fundamental level. There has also been a direct technological benefit in the use of this information to improve catalytic processes and to develop new catalysts. It is a relatively simple step to remove the gas in a solid surface/gas phase system without seriously violating the structural integrity of the interface which can then be characterised using the high vacuum techniques. It would be expected, however, that only in very few cases will i t be possible to remove the electrolyte from the electrode/electrolyte solution interface without producing major changes; thus the high vacuum methods have only limited applicability as ex-situ probes for the structure of the interface in electrochemical systems. Fortunately, the ease with which the electrode potential can be used to control and change, (i) the electrochemical potential of species in the electrical double layer, (11) the free energy of adsorption/desorption processes and ( i l l ) the rate constants 0097-6156/ 85/0288-0550S06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Molecular Structure in Electrocatalytic Reactions

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for electron transfer processes, allows the small number of species i n the electrode/electrolyte solution interphase to be distinguished from the overwhelming number i n the bulk e l e c t r o l y t e i n a manner which has enabled i n - s i t u spectroscopic methods of investigation to be developed. These methods now include ESR spectroscopy (1), u v / v i s i b l e r e f l e c tion (2), transmission (3) and Raman scattering spectroscopies (4) and IR r e f l e c t i o n spectroscopy ( 5 , 6 , 7 , 8 , 9 ) . This paper w i l l be r e s t r i c t e d to a discussion of the l a t t e r . Experimental techniques There are two major problems to be overcome i n developing i n - s i t u , external reflectance methods with submonolayer s e n s i t i v i t y towards adsorbates on smooth electrode surfaces. The f i r s t i s to design a c e l l allowing s u f f i c i e n t radiation to emerge from i t after passing twice through the e l e c t r o l y t e s o l u t i o n , the l a t t e r after being an aqueous s o l u t i o n . The second problem i s to detect the very small absorbance change, usually i n the range 10" to 10" , caused by the adsorbate i n the presence of the overwhelming quantity of absorbing species i n the t o t a l o p t i c a l path. Use of a t h i n layer c e l l overcomes the f i r s t problem and adequate s e n s i t i v i t y i s achieved by the marriage of electrochemical and spectroscopic techniques. The basic method has developed i n two d i s t i n c t d i r e c t i o n s : the f i r s t employs a grating spectrometer and i s c a l l e d electrochemically modulated i n f r a red spectroscopy (EMIRS) (5); the second i s subtractively normalized i n t e r f a c i a l Fourier transform infrared spectroscopy (SNIFTIRS) (6). Both methods obtain the necessary s e n s i t i v i t y by modulating the e l e c trode potential between two values which define two d i s t i n c t states of the electrode surface; thus the chemistry to be observed i s d i r e c t l y modulated and may be detected with great s e n s i t i v i t y by an appropriate form of synchronous detection. In the case of EMIRS, the modulation frequency i s made s u f f i c i e n t l y high compared to the wavelength scanning rate to enable a phase sensitive detection system to be used whereas, for SNIFTIRS, the electrode potential i s held for a s u f f i c i e n t period at each potential to accumulate data from several i n t e r f e r o metric scans and, after an adequate number, the two sets of data are ratioed. 6

2

C e l l design Figure 1 i l l u s t r a t e s a t y p i c a l c e l l design containing the three e l e c trodes required for control of the electrode potential and i n which the t h i n layer of e l e c t r o l y t e , t y p i c a l l y 1 urn to 50 urn, i s produced by pushing the working electrode surface up to the surface of the IR window into the c e l l . The window can be a disc with p a r a l l e l working surfaces or i t can be prismatic to allow normal incidence of the r a d i a t i o n at the surfaces; the l a t t e r design minimises r e f l e c t i o n losses at the air/window interfaces but i t requires a longer pathlength i n the window. The t h i n layer c e l l thus formed necessarily has a large value of uncompensated resistance due to the e l e c t r o l y t e l a y e r . T y p i c a l l y , t h i s leads to a time constant of the order of 10" s for the charging of the double layer i n response to a potential step. This factor together with the potential drop across the uncompensated resistance due to faradaic currents needs to be taken into account when designing experiments. 2

Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Reflection/absorption at a metal surface For a vibrating molecule to absorb radiation from an incident IR beam at the frequency of a p a r t i c u l a r normal mode i t must be situated at a position of f i n i t e intensity and with an orientation such that there i s a f i n i t e component of the dipole derivative du^/dQ^ i n the d i r e c t i o n of the e l e c t r i c vector of the r a d i a t i o n f i e l d , where d u i s the change of dipole for the change of normal mode coordinate dQ^. At a metal surface there i s always a zero intensity for radiation having the e l e c t r i c vector p a r a l l e l to the surface. As a result s-polarised radiation cannot interact with a molecule on a metal surface; ppolarised radiation has a f i n i t e intensity and can interact but the intensity of the resultant absorption band w i l l depend upon the o r i e n t a t i o n of the molecule with respect to the surface, i . e . t h e r e i s an apparent selection r u l e , the surface selection rule (10), which operates i n addition to the normal selection rules for IR absorption. In EMIRS and SNIFTIRS measurements the "inactive" s-polarised r a d i a t i o n i s prevented from reaching the detector and the r e l a t i v e intensi t i e s of the v i b r a t i o n a l bands observed i n the spectra from the remaining p-polarised radiation are used to deduce the orientation of adsorbed molecules. It should be pointed out, however, that v i b r a t i o n a l coupling to adsorbate/adsorbent charge transfer (11) and also an electrochemically activated Stark effect (7,12,13) can lead to apparent v i o l a t i o n s of the surface selection rule which can invalidate simple deductions of o r i e n t a t i o n . The surface active/surface inactive difference between p - p o l a r i s e d / s-polarised r a d i a t i o n has enabled an alternative modulation technique, p o l a r i s a t i o n modulation, to be developed (15,16). In electrochemical applications, i t allows surface s p e c i f i c i t y to be achieved whilst working at fixed potential and without electrochemical modulation of the interface. It can be implemented either on EMIRS or on SNIFTIRS spectrometers and can be very valuable i n dealing with electrochemically i r r e v e r s i b l e systems; however, the achievable s e n s i t i v i t y f a l l s well short of that obtained with electrochemical modulation. It should also be noted that i t s "surface s p e c i f i c i t y " i s not t r u l y surface but extends out into the e l e c t r o l y t e with decreasing s p e c i f i c i t y to about half a wavelength. i

Information accessible The EMIRS and SNIFTIRS methods provide the IR v i b r a t i o n a l spectra ( r e a l l y the difference spectra - see later) of a l l species whose population changes either on the electrode surface or i n the e l e c t r i c a l double layer or i n the diffusion layer i n response to changing the electrode p o t e n t i a l . Spectra w i l l also be obtained for adsorbed species whose population does not change but which undergo a change i n orientation or for which the electrode potential a l t e r s the i n t e n s i t y , the p o s i t i o n or shape of IR absorption bands. Shifts i n band maxima with potential at constant coverage ( < ^ / 9 ) 0 very common for adsorbed species and they provide valuable information on the nature of adsorbate/absorbent bonding and hence also additional data on adsorbate o r i e n t a t i o n . In p r i n c i p l e , therefore, these valuable techniques can provide a l l of the information needed to specify the molecular structure of the electrode/electrolyte solution interphase, the dynamics of adsorption/ E

a

r

e

m a x


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desorption processes and the nature of reaction intermediates. In many cases, i t w i l l not be easy unambiguously to deduce quantitative molecular populations; t h i s aspect has been neglected to date but w i l l certainly command increasing attention i n future. It w i l l be clear that EMIRS and SNIFTIRS spectra are difference spectra and can be somewhat complex (5). T y p i c a l l y they w i l l contain positive absorption bands from species present i n excess at potential Ei compared to potential E and negative absorption bands from species whose polulation changes oppositely with p o t e n t i a l . In addition, bands which s h i f t with potential w i l l appear as a single bipolar band either with one lobe of each sign, figure 2, (or even more complex structures with three lobes). 2

Electrocatalytic

reactions

E l e c t r o c a t a l y s i s necessarily involves adsorption, often d i s s o c i a t i v e , and the characterisation of the molecular fragments produced i s essent i a l for an adequate understanding of the reaction pathways. Many of these reactions are important technologically; both the anodic and the cathodic reactions i n fuel c e l l s are good examples i n t h i s category. Hydrogen evolution and hydrogen oxidation are both of interest i n t h i s context. The exact characterisation of the adsorbed hydrogen atom involved i n these reactions i s not l i k e l y to stimulate technological progress but EMIRS studies have yielded information of considerable interest (16,17). Hydrogen adsorption On platinum group metals, hydrogen adsorption/desorption i s reversible and thus readily lends i t s e l f to investigation by EMIRS. The two kinds of adsorbed hydrogen on p o l y c r y s t a l l i n e platinum, strongly adsorbed hydrogen (H ) formed at less negative potentials and weakly adsorbed hydrogen (H^) produced nearer the reversible potential for hydrogen evolution, are c l e a r l y distinguishable spectroscopically either using u v - v i s i b l e (18) or infrared radiation (16). In the i n f r a r e d , the formation of H substantially increases the r e f l e c t i v i t y of the electrode and t h i s i s observed as a featureless negative absorption over a wide range of wavelength. When the electrode i s covered with both H and H^, absorption bands are observed superimposed on this baseline s h i f t but, over the range 4000 cm" to 1250 cm" , these are a l l at the v i b r a t i o n a l frequencies of hydrogen bonded water; no bands corresponding to Pt-H vibrations are observed. The bands a l l had the same sign: increased absorption when the metal was covered by H . Their assignment to v i b r a t i o n a l modes of water associated with the adsorbed were confirmed from isotopic s h i f t s using H 2 O / D 2 O mixtures; examples are i l l u s t r a t e d i n figure 3. The d i f f i c u l t task of seeing t h i s surface water through the large amount i n the ambient e l e c t r o l y t e was f a c i l i t a t e d by using the H 2 O / D 2 O mixtures to maximise energy throughput over the spectral region. Nine bands were c l e a r l y i d e n t i f i e d and measured from the range: V i , V 2 , V 3 , (V2 + V 3 ) , 2V3,(2V2 + V 3 ) for H 2 O , HDO and D2O. The r e l a t i v e i n t e n s i t i e s of these bands indicated a p a r t i c u l a r orientation for the water structure interacting with the adsorbed H atoms. Application of the surface selection rule leads to the model shown i n figure 4(a) i n which oriented water dimer units are hydrogen bonded to on one side g

s

g

1

w


1


Reference electrode

Figure 1.

The IR spectroelectrochemical c e l l .

Ε 1

Wavenumber

Figure 2.

Wavenumber

Origin of a bipolar difference band.


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Figure 4 . The structure and orientation of: (a) weakly bound hydrogen and i t s associated water on a Pt or Rh electrode; (b) water on the electrode surface at potentials i n the double layer region.


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and, on the other, to water molecules further out from the surface. Figure 4(b) shows the average water orientation assumed for a bare Pt surface at potentials i n the double layer region; one lone pair o r b i t a l i s assumed to be roughly normal to the surface. This o r i e n t a t i o n i s i n agreement with other electrochemical data and was used as the reference state for the EMIRS difference spectra. E l e c t r o c a t a l y t i c oxidations Interest i n fuel c e l l s has stimulated many investigations into the detailed mechanisms of the e l e c t r o c a t a l y t i c oxidation of small organic molecules such as methanol, formaldehyde, formic a c i d , e t c . The major problem using platinum group metals i s the rapid b u i l d up of a strongly adsorbed species which e f f i c i e n t l y poisons the electrodes. Thus a substantial effort has been made by electrochemists to identify this species; use of non-spectroscopic techniques to evaluate the number of surface s i t e s occupied by each molecule of poison and the number of electrons required i n i t s oxidation to CO2 led to a concensus view that i t was :COH. One of the f i r s t successful applications of EMIRS was an attempt to corroborate t h i s conclusion (19); however, the spectroscopic data c l e a r l y i d e n t i f i e d CO as the poison and no evidence for COH could be obtained. Further detailed spectroscopic measurements using a variety of organic fuels ( C H 3 O H , HCHO, HCOOH and CH2OH.CH2OH) and electrode materials (Pt, Rh, Pd, Au) substantiated t h i s finding as a general result i n such systems (20,21,22). Examples of EMIRS spectra obtained from f u l l y poisoned electrodes are shown i n figures 5, 6 and 7. In every case, absorption bands are seen from one or more adsorbed CO species, the types and t h e i r exact v i b r a t i o n frequencies depending upon the nature of the metal. Thus, for example, Pt supports both l i n e a r l y adsorbed CO, the band near 2070 cm" , and also CO s i t t i n g i n a higher coordination s i t e with a v i b r a t i o n frequency near 1850 cm" . On Pd bridge-bonded and more highly coordinated CO can be i d e n t i f i e d . These assignments have been made on the basis of the c l a s s i f i c a t i o n which has developed from gas phase adsorption of CO followed by the use of high vacuum spectroscopic techniques (23). There i s a very close agreement between these data and the EMIRS spectra. Direct adsorption of CO on the electrodes from gas dissolved i n the e l e c t r o l y t e gave almost i d e n t i c a l spectra, at high coverage, to those obtained after organic oxidation. The same adsorbed CO species were obtained on Pt by reduction of C0 (24). This was observed to take place only i n the range of potentials where adsorbed hydrogen i s formed, i n d i c a t i n g that the reduction takes place i n d i r e c t l y v i a the adsorbed atomic hydrogen. In t h i s case, i t was noticed that the more highly coordinated adsorbed CO, seen near 1850 cm"" , was formed very r e a d i l y , whereas i t i s formed d i r e c t l y from dissolved CO gas only after potential c y c l i n g of the electrode. There i s evidence that t h i s species i s usually formed by reduction of C 0 ; i t should be noted that, at non-poisoned electrodes, some CO2 i s produced from the organic substrates at quite low potentials and many of these also form adsorbed hydrogen during t h e i r dissociative e l e c t r o sorption. 1

1

2

1

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Adzic et a l (25) have shown that p a r t i a l coverage of the e l e c trode by adsorbed Pb can substantially reduce the effects of poisoning, presumably by blocking the surface s i t e s required by the adsorbed CO. This i s nicely confirmed by spectroscopic measurements. Figure 8


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IR Spectroscopy as an In Situ Probe for Molecular Structure in

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