J. CHEM. SOC. FARADAY TRANS., 1991, 87(8), 1221-1227 1221

Fourier-transform Infrared Studies of Carbon Monoxide Adsorption on Platinum and Palladium Hydrosols : Influence of pH

Michael R. Mucalo and Ralph P. Cooney* Chemistry Department, University of Auckland, Private Bag, Auckland, New Zealand

Carbon monoxide adsorption on aqueous platinum and palladium sols has been studied by IR spectroscopy over the pH range 2.0-12.6. IR spectra of the CO-saturated platinum sols at pH x 5.0 show a single asymmetric band at ca. 2070 cm-' due to CO adsorbed linearly on platinum metal surface sites. The decrease in v(CO),,, as pH is increased has been attributed to a decrease in coverage of adsorbed CO induced by hydroxyl adsorption. At pH < 5.0 the platinum sols are unstable and exhibit broadening of the v(CO),,, band associated with aggre- gation. In contrast, IR spectra of CO-saturated palladium sols at pH x 6.0 exhibit bands due to adsorbed CO on two-fold bridge sites (1950 cm-') and to linearly adsorbed CO (2067 cm-'). The v(CO),,, for palladium sols is relatively insensitive to pH over the range 2.0-11.5 but decreases sharply at pH > 11.5. Correlations of v(CO),,, with 8 suggest that CO island formation is an important aspect of CO adsorption on both the platinum and palladium sols. It was found that CO adsorbed on colloidal platinum at pH < 4.0 does not exhibit similar behav- iour to that of CO adsorbed on platinum electrodes in acidic media.

IR spectra' of CO adsorbed on colloidal platinum in CH,Cl, revealed bands due to linearly adsorbed CO (2050 cm- ') and two-fold bridge adsorbed CO (1880 cm-'). A study of palla- dium sols in methylcyclohexane by infrared and I3C-NMR spectroscopy' established the assignment of infrared bands at 2062 and 1941 cm-' to linear and two-fold bridge adsorbed CO on palladium metal, respectively. A previous in situ IR study by the present authors3 reveals a band in the region 2065-2070 cm-' due to CO adsorbed linearly on platinum metal sols in aqueous media. The aim of the present study is to extend the characterisation of CO on metallic sols to include IR spectra of CO adsorbed on aqueous palladium sols and also to investigate the influence of pH on CO adsorption on metal sols. Early work on platinum solsL6 indicated that the negative zeta potential exhibited by the particles arose from the presence of a thin oxide layer which creates a dipole layer, the negative end of which is directed towards the liquid. This indicates that the sols may behave like microelectrodes. Metal sol particles could also be con- sidered as large clusters. For example, spectroelectrochemical experiments show that v(CO),,, in a [Pt24(C0)30]n (n = 0 to - 6) cluster' (dissolved in CH2C1,-[NBu4]+ [ClO,] -) decreases directly as a result of the decrease in surface-charge density on the cluster as it is electrochemically reduced at a platinum electrode. Therefore, it is of particular interest to compare CO adsorption on metal sols with CO adsorption at polycrystalline and single-crystal platinum electrode surfaces.

Experimental Reagents

Chloroplatinic acid (H'PtCl, * 6H20) was supplied by May and Baker. Anhydrous palladium(r1) chloride, sodium boro- hydride, and disodium hydrogen citrate were all BDH chemi- cals. Analytical reagent grade potassium hydroxide was supplied in pelletised form by Scientific Supplies Ltd., Auck- land. Millipore water generated by passing singly distilled water through a Milli-Q water-purifier system was generally used in the preparation of solutions and the metallic hydro- sols. Carbon monoxide gas (minimum purity, 99.5%) was supplied by New Zealand Industrial Gases Ltd.

Preparation of Platinum and Palladium Hydrosols

Platinum hydrosols generated by borohydride reduction were the principal colloidal substrates used in this study. Citrate-

generated platinum hydrosols were studied to a lesser extent. Black platinum hydrosols form immediately in the NaBH, reduction process' when 9.75 cm3 of ca. 0.03 mol dm-3 cold, freshly prepared, aqueous sodium borohydride is added dropwise to a cool (7' = 2-3 "C), stirred solution of 6.0 cm3 of 0.034 mol dm-3 chloroplatinic acid and 20-30 cm3 of water. The citrate-generated platinum sols were prepared as per the procedure given by Furlong et aL9 except that concentrations of reactants were scaled up to in order to acquire a higher surface area sol.

Black palladium hydrosols form immediately when 10.0 cm3 of ca. 0.03 mol dm-3 freshly prepared cool sodium boro- hydride is slowly added to a cool (T = 2-3 "C), stirred solu- tion of 20.0 cm3 of water and 6.0 cm3 of ca. 0.03 mol dm-3 palladium(I1) chloride solution [from anhydrous palladium(@ chloride dissolved in ca. 0.03 mol dm-' NaCl]. Both the platinum and palladium hydrosols were ion-exchanged for 10-15 min immediately after their preparation with (BDH) Amberlite MB 1 cation-anion ion-exchange resin until the conductivity of the sols had decreased to <4.0 pS cm-'. The ion-exchanging time was minimised to avoid significant loss of sol particles by adsorption on the ion-exchange resin." No protecting agents were added to either sol preparation. Platinum sols prepared in this manner were colloidally stable at ambient temperatures for many months while palladium sols at ca. 2 "C aggregated several hours after preparation.

Characterisation of the Hydrosols

An indirect measure of sol particle density was given by the sol turbidityg*" which was obtained from visible absorbance values at 450 nm (A45o) for five-fold dilutions of the platinum and palladium hydrosols in 1.0 cm cells. Values of A450 were used in the determination of normalised absorbance of adsorbed CO on platinum and palladium sols (see later). Platinum metal concentrations (in g dm-3) at 25°C were determined for the platinum sols in situ by using an Anton- Paar vibrating tube digital densimeter' combined with a Julabo thermostat/circulator/refrigerator unit to maintain the temperature in the densimeter at 25 & 0.02"C. The time period for an arbitrary number of vibrations (20 O00) was determined. The densimeter was calibrated by determining the time periods of pure water and aqueous sodium chloride. Assuming that 1.0 g of platinum occupies a negligible volume in 1 dm3 of water, the difference between the measured

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1222 J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87

density of the platinum sol in water and that of pure water should correspond to the platinum-metal concentration. Platinum-metal concentrations determined in this manner fell in the range 0.80 to 1.10 g dm3 and were approximately lin- early correlated with measured sol turbidities. Using similar densimetry techniques, palladium metal concentrations in palladium sols were determined to be ca. 0.50 g dm- '. Metal concentrations in the sols were also determined from the weight of residue left after evaporating the aqueous medium from pre-weighed sol samples. It was assumed that dissolved salts present in the sols after ion-exchanging would contrib- ute negligibly to the residue weight. These metal sol particle concentrations were subsequently used in the calculation of CO surface coverage parameters (0 and 0, see later).

of the platinum and palladium hydrosols evaporated on 40-mesh carbon-coated copper grids revealed necklacing of approximately spherical particles with the platinum hydrosol having an average particle size of 5.2 nm and the palladium hydrosol having an average particle size of 9.1 nm. X-Ray photoelectron spectral3 of the isolated (dry) hydrosol particles revealed predominant contributions from the metal in both sol systems but also the presence of a Pt" oxide on the platinum hydrosol particles, and Pd" and Pd" oxides on the palladium particles.

Electron micrographs'

Spectroscopy

All IR spectra were acquired at 4.0 cm-' resolution using a Digilab FTS-60 FTIR spectrometer purged with dry air from a Bio-Rad recirculating air-dryer. A conventional, uncooled infrared DTGS detector was employed to detect adsorbed carbon monoxide. Small volumes of the platinum and palla- dium hydrosols (0.10-0.15 cm3) were treated with carbon monoxide for 10 min in a small bubbling vessel of ca. 0.50 cm3 capacity. 1R transmission spectra were subsequently recorded of the aqueous sol samples held as a thin layer (5.s 15.0 pm) between calcium fluoride windows using the spec- trum of the windows as a background. The absorption maxima of v(co),d, in the IR spectra of the CO-saturated metallic sols emerged following weighted spectral subtraction of bands due to liquid water and water vapour. In general, subtraction was usually performed until complete nulling of the band of water at 2137 cm-' was achieved. The region (1900-2100 cm- ') in which v(co),d, appeared coincided with an energy 'window' ( A = 0.0660 at 2050 cm-' for a path length of ca. 8 pm) in the IR spectrum of water. In the adjust- ed pH IR studies, the pH of the colloid samples were altered to the desired values by potassium hydroxide or perchloric acid addition followed by CO treatment for 10 min.

Spectra were also obtained using a Harrick Scientific Cor- poration single-pass silicon ATR accessory which employs an angle of incidence of 45". The depth of penetration (d,) is e ~ p e c t e d ' ~ to be 369 nm at a wavelength of 4.83 pm (2070 cm- '). Spectra of CO-treated platinum sols were ratioed us a single-beam silicon ATR background of untreated platinum sol. The measured peak absorbance for v(CO),,, was found to be ca. 10% of that measured for v(co),d, bands from transmission-mode spectra.

The asymmetric profile of each v(CO),,, band was sub- jected to smoothing (Savitsky-Golay) and sub-band analysis based on Gaussian sub-bands using Spectra-Calc (Galactic Industries) curve-fitting software. A minimum of three sub- bands was necessary to achieve acceptable curve fits. Opti- misation of fit was effected by an iterative auto-band synthesis procedure which operated until the root-mean- square fitting parameter k2) was < 1 x

Calculations utilising the Kramers-Kronig dispersion formulae15 to determine the real and imaginary parts n(1)

and k(A) over the asymmetric v(CO),,, bands were performed using the 'nk' program supplied by Digilab. These formulae apply to specular reflectance spectra from smooth dielec- trics.'

Quantification of CO Coverage on Platinum and Palladium Sols [A'(CO), 8 and 01

Calculation of Normalised Absorbance [ A'(C0)I (a)

Determination of A'(C0) values was carried out when values for the metal concentration [MI in the sols were not avail- able. In general, A'(C0) values are calculated from the follow- ing expression,

3

1 12.4 1 (Aco)i A'(C0) = i = l (1)

A450 137

where l ( A c o ) i refers to the sum of the sub-band absorbances of the chemisorbed CO (obtained from curve-fitting) and A 2 1 3 7 represents the absorbance at 2137 cm-' of the associ- ation band of water.17 The sample pathlength L is given in m by A2,3,/11240 which was interpolated from the calibration plot of A 2 1 3 7 us. known pathlengths of water. The slope of this plot was 11 240 t 5%. The turbidity that the colloid sample would have at the sample pathlength L is calculated from A450/(LUV/L) where L,, is the pathlength (0.010 m) of the sol in the UV-VIS spectrum. The incorporation of the scaling factor L,,/L into eqn. (1) explains the constant of 1 12.4.

(b) Calculation of 8

In the calculation of 0, it is assumed that an IR beam of radius R (0.0065 m) irradiates a cylinder (the irradiated cell volume, ICV) of height L m (L, sample pathlength) and dia- meter 2R containing spherical CO-treated metal particles which are predominantly metallic in surface character. The volume V of the ICV is thus given by the volume of a cylin- der, uiz. V = zR2L. This is used to determine the total surface area (Sd of the colloidal metal particles in the ICV assuming that the surface area of a sphere applies [see eqn. (2)],

~ V C M I s, = - rP

where [MI is the concentration of colloidal metal particles in the metal sol (expressed in g m-3) under examination, r is the average particle radius of the sol particles (obtained from TEM) and p is the density of the metal comprising the sol particles in g m-3.

The surface area occupied by the CO molecules (&) on the metal sol surfaces is determined on the assumption that the IR beam detects all adsorbed carbon monoxide on the surface of the particle. This condition arises for particles pos- sessing diameters which are small (e .g . 5-9 nm) compared with the wavelength of the infrared radiation'* (e.g. 5000 nm at 2000 cm-'). Each particle thus experiences a uniform infrared electromagnetic field over its entire surface. A conse- quence of this condition is that the IR beam would be negli- gibly attenuated by the colloidal particles. This is confirmed by the observation that absorbance of the association band of water at 2137 cm-' (A,,,,) obtained from the IR spectra of platinum sols is independent of the turbidity (measured by A450) of the sol samples (see Fig. 1). Assuming that the CO molecules occupy circular areas of radius rco equal to 5.65 x lo-" m (the radius of the CO mole~ule '~) on the

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0.700

0.600

0.500

t h

b ,- 0.400 ; - p. 0.300

q 0.200

0.100 t 0.000

0.000 0.500 1 .ooo 1.500 2.000

A 4 5 o ( W

Fig. 1 IR absorbance of the association band of water (0.010 x m pathlength) at 2137 cm-’ (A,,,,) us. the UV-VIS absorbance (10 x lo-’ m pathlength) at 450 nm (A450) for diluted samples of a platinum sol

metal surface and that Beer’s law applies, S,, is given by, 3

~ $ 0 1 (ACoIi VNA i = 1

sco = ECO = (3)

where N , is Avogadro’s number and E ~ , refers to the absorp- tion coefficient for CO adsorbed on platinum or palladium. For CO adsorbed linearly on platinum,20 cCO is 1200 dm3 mol- ’ cm - ’. For CO adsorbed in two-fold bridging mode on palladium,2’ cCO is 4100 dm3 mol-’ cm- ’.

The coverage (8) of CO on the colloidal metal surface is given by the ratio Sco/SM which when expressed in terms of C(A,o)i 9 A , 13 7 and [MI gives

3

K C (Aco)i

A21 37CMI 8 = i = l (4)

where K is a constant given by 11 2407cN,r&pr/3~~. For linear carbon monoxide adsorbed on platinum sols at natural pH, 0 is 0.2M.30 whereas for CO occupying B2 sites on pal- ladium sols at natural pH, 8 is ca. O.l(LO.20.

Comparisons of 6 values with those of A’(C0) show that the two methods for calculating CO coverage gave either almost identical numerical values (Pt sol/CO) or values that differed by a constant factor for a given sol preparation (Pd sol/CO). When eqn. (4) is compared with eqn. (l), it is found that 8 and A‘(C0) differ only by a constant which depends on the relationship between A,,, and [MI.

(c) Calculation of 0 (Metal-face-averaged Coverage) An alternative method explored for determining CO coverage on platinum and palladium sols is based on the average number of CO adsorption sites on the various metal crystal faces [i.e. (1 1 l), (1 10) and (loo)]. Metallic platinum and palla- dium crystallise in face-centred cubic (f.c.c.) lattices for which the lattice vector lengths (a) are 0.3923 and 0.3859 nm, respec- tively.22 The surface atom densities of the f.c.c. ( l l l ) , (110) and (100) surfaces are given by 2.31/a2, 1.4/a2 and 2.0/aZ, respectively.22 The surface atom density averaged over the (111)’ (110) and (100) faces is 1.90/a2. Surface area per atom (Sat,,) is given by the inverse of the density, viz. a2/1.90. Thus, for Pt, the average surface area per atom (SPtatom) is 0.081 nm2 while for Pd, the average surface area per atom (SPdatom) is 0.078 nm2. The total number of surface metal atoms avail- able for CO adsorption in the ICV (Nsurf) is given by SM/Satom. The value of the metal-face-averaged coverage 0 is calculated from the ratio NCO/Nsurf, where N,, refers to the

total number of adsorbed CO molecules in the ICV. An expression identical to eqn. (4) results:

3

K’ C (Aco)i

where K’ is a constant given by 11 240NAa2pr/5.7~,, . The calculation of 0 in eqn. (5) assumes that one CO molecule per metal surface atom is adsorbed as for the Pt sol/CO case23 with one CO per two metal surface atoms in the Pd sol/CO case (B, mode23) so that the value of 0 determined in eqn. (5) is divided by two.

In general, this second method of calculation of 0 appeared to overestimate the CO coverage. For example, for CO adsorbed on Pt at natural pH, 0 = 2.0 whereas for CO on Pd at natural pH, 0 = 2.2. The overestimation of 0 stems from the difference in the constants K and K’ from eqn. (4) and (5 ) , the ratio of which gives K/K’ = nr~o/Sato,. When numerically evaluated, K’ > K since K‘ = 8.3K (Pt sol/CO) and K’ = 3.9K (Pd sol/CO). On physical grounds, the over- estimation of @ may be due to CO not exhibiting equal pref- erence to the ( l l l ) , (110) and (100) crystal planes when chemisorption on a metal sol takes place. For example, non- dissociative adsorption of CO has been claimed to occur on (1 11) and (100) planes with dissociative adsorption occurring on high-index plane^.^ The assumption of sphericity may also affect the calculation of 0. For example, a recent EXAFS/HREM study on platinum describes platinum sol particles as f.c.c. crystallites which exhibit roughly hex- agonal outlines in projection when viewed down the (110) direction. The former calculation of 8 values is thus regarded as leading to apparently more realistic values for CO cover- age on the metal sols.

Results and Discussion (a) Natural pH studies

(i) Pt sol/CO (PH z 5.0) The IR transmission spectra of borohydride and citrate- generated platinum hydrosols (dispersion pH z 5.0 after ion exchange) saturated with carbon monoxide generally exhibit a single asymmetrical band in the region 2065-2070 cm-’ (see Fig. 2). Spectra of CO-treated platinum sols obtained with optically reproducible pathlengths (using Si ATR

r

I 2067

2100 2000 1900 1800 vjcm - ’

Fig. 2 IR absorbance spectra of CO adsorbed on metal sols at natural pH after spectral subtraction of water: (a) IR transmission spectrum of CO adsorbed on a platinum sol, (b) spectrum of CO adsorbed on a platinum sol obtained using Si ATR sampling and (c) IR transmission spectrum of CO adsorbed on a palladium sol

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7 2060-

2 2050--

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J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87

--

--

sampling) also give v(co),d, in an almost identical position with a very similar band shape as that obtained in transmis- sion mode but with much weaker intensity (see Fig. 2). The observed band has been assigned to carbon monoxide adsorbed linearly on platinum-metal surface sites.23 Leung et aL2' have observed that v(co),d, is lower for CO adsorbed on Pt( 11 1) electrodes than for CO adsorbed under ultra-high vacuum on Pt(ll1) and have attributed the frequency decrease to the screening effect26 of dipoles of co-adsorbed water which diminishes the dipolar coupling between adjac- ent CO molecules. It is likely that a similar H,O screening mechanism operates when CO is adsorbed on aqueous plati- num sols.

(ii) Pd Sol/CO (PH = 6.0) IR spectra of CO saturated palladium sols (pH 5.0-7.0 after ion exchange) generally exhibited two asymmetrical bands in the region 1900-2100 cm-' (Fig. 2). The weaker band at 2067 cm- ' may be assigned to carbon monoxide adsorbed linearly on palladium metal surface sites with the more intense band at 1950 cm-' being assigned to carbon monoxide bridging two metal atoms (i.e. on B, sites).23 The frequencies observed for the Pd sol/CO system are slightly higher than those observed for Pd sol/CO in methylcyclohexane by Bradley et a/.' The band due to linear adsorption of CO on palladium ([Pd'] x 0.50 g drnp3) is too weak to be monitored for fre- quency changes. The unstable nature of palladium sols meant that colloid samples were aggregated prior to CO treatment. However, CO treatment was found to cause colloidal redispersal of the palladium sols, except where dispersion pH was too low (< 3.0) or too high (> 12.0) (see Pd sol/CO, Adjusted pH Studies). This observation suggests that the de- stabilisation of the unprotected palladium sol leads to the formation of weakly bound aggregates (flocs). On a free- energy curve for palladium sols, the formation of weak flocs would be represented" by the presence of a secondary minimum of the order of a few k,T. Carbon monoxide adsorption on the palladium particles may promote their separation by the displacement of destabilising cations from the surfaces of the particles. The heat of chemisorption of carbon monoxide on palladium is of the order 69 k , T at 293 K, thereby making redispersion energetically favourable.

(iii) Band Asymmetry ~ f v ( C o ) , ~ , The bandshape in the IR spectrum for carbon monoxide adsorbed on platinum and palladium sols (see Fig. 2) is asym- metric. Band asymmetry tends to become less significant at high pH (see Pt sol/CO and Pd sol/CO, adjusted pH Studies). Thus, it is very probable that band asymmetry is molecular in origin and due to adsorption of carbon monoxide on mul- tiple, more highly energetic metal surface sites.3 Tailing of v(co),d, bands t o low wavenumbers may also suggest a dis- tribution of CO island sizes29 (see later). The influence of optical effects on the bandshape of the v(Co),d, band was considered tentatively since the asymmetric v(co),d, bands are similar to the typical reflection spectrum recorded from a metal diele~tric, '~ suggesting that there may be dispersion in the refractive index in the vicinity of the absorption band. The k(A) determination from the Kramers-Kronig calcu- lations (see Experimental) on the v(co),d, bands generally increased v(co) ,d , by 5-6 cm-'. However, the results of the Kramers-Kronig calculations are not supported since the metal sol does not conform to the model of a smooth dielec- tric. Also, previous workers have not employed the Kramers- Kronig dispersion formulae in the interpretation of asymmetric v(CO),,, bands from electrode3' or c a t a l y ~ t ~ ' * ~ ~

surfaces. The former interpretation of band asymmetry in terms of site distribution is, therefore, believed to be correct.

(6) Adjusted pH Studies

(i) Pt sol/CO (adjusted p H ) Fig. 3 illustrates a series of v(Co),d, spectra at various adjust- ed pH values. Fig. 4 is a plot of v(Co),d, us. pH for CO adsorbed on a platinum sol. The v(Co),d, band for CO on platinum sols appears to be slightly sensitive to pH changes for pH 2.0 to 10.0 (see Fig. 3). However, for pH > 10.0, v(CO),,, decreases more significantly until at very high pH (ca. 12.O), v(co),d, approaches 2030 cm- ' (see Fig. 4).

In general, when pH was adjusted to <5.0 by perchloric acid addition, the platinum sol was stable over a rather restricted pH range. This was true especially at pH < 4.0 where rapid aggregation occurred. This observation supports the generally held viewL6 that the metal sol particles are negatively charged (see Introduction). Recent work by Furlong et a1.' has demonstrated that platinum sols at pH 5.0 behave as a typical negative sol with coagulation being induced mainly by the electrolyte cation with a marked dependence of critical coagulation concentration on the valence of the cation. Carbon monoxide treatment of the per- chloric acid-aggregated platinum sol particles produced

2070

n L - - - - 0 7 0 w n

2037

2100 2050 2000 v/cm - '

Fig. 3 IR absorbance spectra of CO adsorbed on a platinum sol at pH: (a) 2.41, (b) 3.81, (c) natural pH (4.86), (6) 7.31, (e) 9.15, cf) 10.75, (9) 11.66 and (h) 12.04. Spectra (a), (b) and (h) represent IR spectra of CO adsorbed on aggregated sol samples

2020 I \ - 0 2 4 6 8 10 12 14

PH

Fig. 4 v(CO),,, us. pH for the Pt sol/CO system (collated data). A polynomial curve of order x5 (truncated at pH 2.70) has been fitted to the data

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broader v(co),& bands at slightly lower frequency (see Fig. 3). The broadening may be due to the increased heterogeneity of the particle surfaces as a result of aggregation and removal of the surface-adsorbed hydroxyls by neutralisation in acidic media. It has been shown voltammetrically that CO interacts very strongly with platinum electrode surfaces in acidic (0.1 mol dmP3 HC10,) media.31 The removal of competing surface hydroxyls on the platinum sol particles may lead to stronger CO adsorption. The presence of a broader asym- metric v(co),d, band for CO adsorbed on acid-aggregated sols (see Fig. 3) would tend to indicate the presence of more strongly bonded CO species.

For pH > 5.0, platinum sols studied were stable to addi- tions of KOH up to pH 12.0, after which aggregation of the colloid occurred. The decrease in v(co),& with increasing pH is probably associated with a decrease in carbon monoxide coverage (see Fig. 4). The data for 8 and A’(C0) us. pH (Fig. 5) are scattered but generally exhibit a decrease towards high and low pH. Benziger et aL3’ reported a downward freqency shift in v(co),d, for carbon monoxide adsorbed on a plati- num electrode (in NaOH or HClO,) adjusted to a potential corresponding to the onset of OH adsorption. This was inter- preted as arising from blocking of CO by hydroxyl electro- sorption. For the platinum sols, it is likely that the decrease in v(Co),& is also due to the adsorption of hydroxyl ions on the high-surface-area platinum sol particles. An early study3 reports that hydroxide in 0,-purged aqueous media is appar- ently adsorbed by platinised platinum. Passage of 0, was postulated to lead to the formation of a strong hexa- aquoplatinic acid which subsequently neutralised the hydrox- ide. However, this neutralisation behaviour is consistent with the ionisation phenomena typically shown by oxide sur- f a c e ~ . ~ ~ Furlong et a/.’ have suggested that at natural pH the platinum sol particles already possess an oxide layer contain- ing hydroxyl functionalities with dissociation behaviour related to pH. As pH increases (see Fig. 4) and in the pre- sence of atmospheric oxygen, the oxide layer is likely to grow on the platinum sol particles (as predicted from the Pourbaix diagram of the Pt-H,O system3’). Therefore, carbon mon- oxide adsorption would be effectively blocked leading to lower v(co),d, as a result of loss of dipolar coupling (see Natural pH Studies, Pt sol/CO). Correlations of v(Co),d, with 8 and A’(C0) indicate the clustering of points which suggests island formation of adsorbed CO. The distribution of v(co),& values exhibited by the CO islands appears to indicate that CO island size is influenced by hydroxide com- petition for surface sites.

(ii) Pd sol/CO (adjusted pH) The trend in v(co),d, us. pH for bridge-bonded CO adsorbed on a palladium sol is shown in Fig. 6. Similar data for CO

0.500 I 0’500 t 0.400 . 0.400 . t e

0.200 0.200 k

o.loo/ 0.100 I 0.000 \ 0.000

0 2 4 6 8 1 0 1 2 1 4

PH Fig. 5 0 and A‘(C0) us. pH for the Pt sol/CO system (collated data). The fitted parabola illustrates tentatively the trend of CO coverage on Pt sols with pH

1960 , I

0- 19201

19101- ,. 1900 0 2 4 6 8 1 0 1 2 1 4

PH

Fig. 6 v(CO),,, us. pH for the Pd sol/CO system (collated data). Two straight lines determined from independent least-squares analyses have been drawn through data points in the pH ranges 2 . e 11.5 and 11.5-13.0

adsorbed linearly on palladium are not reported as the band was not intense enough to be monitored with precision. Unlike platinum sols, v(co),& for the Pd so l /co system is insensitive to pH over the range 2.0 to 11.5 and is located at 1950 f 3 cm-’. However, at very high pH (> l lS) , v(CO),,, decreases very sharply to values as low as 1900 cm-’ (see Fig. 6). The palladium sol is usually unstable at pH > 12.0 and at pH < 4.0. For the palladium sol, these pH regions of colloid instability are evident when the colloid fails to become redispersed during CO treatment (see Experimental). As pH is adjusted below 6.0, the palladium sol is not redispersed as easily by CO treatment as it is at pH > 6.0 (see later). This indicates that the palladium sol particles achieve colloidal stability by the existence of negative charges at their surfaces (see Introduction). Carbon monoxide treatment of the aggregated particles yields IR spectra in which v(co),d, bands are broad as in the Pt sol/CO system at pH < 5.0 (see Fig. 3). Surface heterogeneity and stronger metal-CO inter- action induced by sol aggregation and neutralisation of surface hydroxyls may again be offered as the origin of the band broadening.

Hydroxide addition to the palladium sols leads to a loss of intensity in the v(co),d, bands which may be due to the dis- placement of CO by adsorbed hydroxyls as described for the Pt sol/CO system. The correlation of v(co),& with 8 and A’(C0) (see Fig. 7) indicates a more dramatic degree of clus- tering of v(co),& values than observed for the Pt sol/CO system. The reduction in 8 without change in v(co),& at ca. 1950 cm- ’ (Fig. 7) suggests that relatively large island films are shrinking while average dipolar coupling36 is essentially constant. This indicates that CO island formation is more important for the Pd sol/CO system. The insensitivity of

1.500 7 1.500 I

1 2501

i 1.000

0.750

0.500 t 0.250

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1.250 I 4 1.000 h

0.500

1900 1920 1940 1960 1980

v ( CO ) a ,$cm - ’ Fig. 7 data)

0 and A’(C0) us. v(CO),,, for the Pd sol/CO system (collated

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1226 J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87

v(CO),,, over the wide pH range (Fig. 6) is also indicative of CO island formation. The sharp decrease in v(Co),d, at very high pH (see Fig. 6) may imply a sudden change in the palla- dium surface (e.g. oxidation) that prevents the coalescence of CO into islands leading to a more randomised CO adsorp- tion on the surface. The probable involvement of CO island formation on the palladium sols prompted an attempt to determine a critical CO island size. Internal island CO mol- ecules are expected to be contributing to the higher frequency components of the asymmetric v(co),d, band represented by the two highest frequency sub-bands, v1 and v2, (see Fig. 8) since the stretching frequency of adsorbed CO molecules is influenced by dipolar coupling with neighbouring molecules in the island. In terms of such coupling, chemisorbed CO molecules at the perimeter of the CO islands are distinct from those at the centre. The v(co),d, spectra ~ a l c u l a t e d ~ ~ for large island sizes (7, 19 and 37 CO per island) on Cu surfaces exhibit shoulders or tails at the low-frequency end of the band which have been assigned to perimeter CO molecules. The third fitted sub-band at lowest frequency (see Fig. 8) may, therefore, be assigned to CO molecules situated at the island perimeters. Thus, the integrated absorbance data [~A(v,) dv,] of the three sub-bands fitted to each v(Co),d, band may be used to compute the ratio of populations ( P ) of internal CO molecules to external CO molecules (PinJPext) using eqn. (6).

J A W dv, + JA(v2) dv,

J 4 v 3 ) dv3 (6) - Pint

p e x t

--

The ratio PinJPext us. 6' should tend towards a limiting value with coverage from which island size can be estimated by geometric considerations of CO adsorption on the metal surface. Most of the data from the present study, however, exhibit much scatter of PinJPext values implying a wide range of island sizes so that the determination of a limiting island size is not possible.

The coexistence of many different island sizes is supported by the breadth of bands observed in IR spectra of the Pd sol/CO samples. Difficulties in determining island dimensions may arise because island formation is influenced36 by a com- bination of thermodynamic and kinetic factors. Thus, island size distribution and hence intensity and bandwidth of the v(Co),& band may all exhibit some temperature dependence. An additional factor complicating the determination of island size involves the interactions between CO molecules

1950 1900

vlcm - ' Fig. 8 Sub-band analysis of the spectrum of CO adsorbed on a Pd sol at natural pH. The spectral profile was subjected to a two-point baseline correction and Savitsky-Golay smoothing of degree 7 prior to curve-fitting

adsorbed on non-equivalent (defect) sites. It is known36 that CO adsorbed on defect sites may become occupied at very low coverage which gives rise to a significantly intense high- frequency component in the v(co),& band due to dipolar coupling intensity transfer effects.

(iii) Comparison with the ElectrodelElectrolyte Interface In theory, comparisons with CO adsorption at the electrode/ electrolyte interface may be made by using the frequency us. pH data obtained from the Pt sol/CO and Pd sol/CO adjust- ed pH studies to determine dv/d$, from dv/dpH. This has been achieved by assuming that the metal colloid particles possess a thin oxide surface layer in aqueous solution in which the hydrogen ion is considered as a primary surface- potential-determining ion for these particles.' The following expression [eqn. (7)] for the surface potential ($,) may be written given that the surface activity of H + is constant and i n d e ~ e n d e n t ~ ~ of t,bo :

-2.303RTCpH - pH($, = O)] F (7) t,bo =

Where pH($, = 0) = -log ao(H+) is the pH at which t,ho = 0.

Differentiation of eqn. (7) with respect to pH yields the expression for d$,/dpH given by

d$o -2.303RT -- dpH - F

Multiplying the inverse of eqn. (8) by dv and rearranging leads to an expression for dv/d$, given by

dv (- dv/dpH)F (9) -- -

dlCl0 2.303R T

Thus a value for dv/d$, may be obtained by determining the slope of the v(co),d, us. pH graphs from the Pt sol/CO (Fig. 4) and Pd sol/CO (Fig. 6) adjusted pH experiments.

A polynomial curve (of order x5) was fitted to the v(co),d, us. pH curve in Fig. 4 for the Pt sol/CO adjusted pH study. Slope values dv/dpH were determined at key points of the fitted polynomial. Table 1 is a summary of the results obtained in the calculation of dv/d$, for the Pt sol/CO system. It was not necessary to fit a polynomial to the nearly rectangular v(co),& us. pH graph for the Pd sol/CO study in Fig. 6. Instead, the slopes of two straight lines resulting from

Table 1 Calculated values of dv/d$, from dv/dpH for the Pt sol/CO and Pd sol/CO adjusted pH experiments

(a) Pt sol/CO 3.37 2069.2 2.54 - 43.4 4.82 2070.8 - 0.082 4.0 8.22 2067.0 - 1.54 26.0

10.36 2060.9 - 5.55 94.9 12.09 2041.7 - 18.81 321.7

(b) Pd sol/COb 2-1 1.5 ca. 1950 0.188 - 3.22 > 11.5 > 1900 - 26.97 461.3

a This frequency was calculated from the equation of the fitted x5 polynomial to the v(CO),~, us. pH for the Pt sol/CO system (Fig. 4) at the pH stated. The values of dv/dpH and dv/d$, were calculated from the slopes of the two straight lines resulting from an indepen- dent least-squares fit through the points in the pH range 2.0-11.5 and 11.5-13.0 in v(CO),,, us. pH for the Pd sol/CO system (Fig. 6).

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J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87 1227

independent least-squares analyses drawn through the points in the pH range 2.0-11.5 and through the points in the pH range 11.5-13.0 were determined which led to the calculation of two values of dv/dt,bo, shown in Table 1.

For the Pt sol/CO adjusted pH system, dv/d$, is < zero at pH < 4.0. Earlier, it was reported that depression of pH by acid addition led to almost immediate flocculation of the sol. The i.e.p. (isoelectric point) of platinum sols' is known to occur at pH < 2.0. From eqn. (7), this implies that $, has increased from a previously negative value (see Introduction) to zero. An increase in v(co),d, is, therefore, expected to reduce the d,(Pt)-.n*(CO) ba~k-bonding~' due to the increase of positive charge on the platinum particle. For CO adsorbed at a platinum electrode3' in 1.0 mol dm-3 perchloric acid, dv/dt,b, = 30 cm-' V - ' in contrast to the value of -43 cm-' V - for the Pt sol/CO system at pH 3.37. Thus CO adsorbed on aggregated colloidal platinum particles at pH < 4.0 does not behave as it would on a platinum electrode surface in acidic media.

For the Pt sol/CO system at pH > natural pH, dv/d$o exhibits positive values. From eqn. (7), $o becomes more negative because pH is moving further from the pH for the i.e.p. Thus, a hypothetical graph of v(co),d, us. t,bo would have positive slope as t,bo becomes more positive (i.e. as pH decreases). Hence, the decline in v(CO),,, as pH of the plati- num sols is increased could be interpreted in terms of an increase in the d,(Pt)-n*(CO) b a ~ k - b o n d i n g ~ ~ due to the increase in negative charge at the particles from hydroxide adsorption. However, as pH increases, dv/d$, becomes larger in magnitude. Increases in dv/dE with decreasing 8 for CO adsorbed at ordered Pt( 1 1 l)/aqueous interfaces have been attributed3' to changes in the local surface environment, from primarily adsorbed CO to coadsorbed H/H,O. In the present study, the increase in dv/d$, with pH may reflect the competition of OH- with CO adsorption sites on the plati- num sol particles, which appears to indicate the existence of smaller, more coverage dependent islands.

For the Pd sol/CO adjusted pH system, dv/dpH is close to zero (dv/d$, = -3.22 cm-' V-') (see Table 1). This result highlights the different frequency us. pH behaviour for adsorbed CO on platinum and palladium sols. The insensi- tivity of v(co),d, in the pH range 2.0-11.5 may be associated with the more strongly bound nature of the bridge-bonded CO species on palladium. In the pH range 11.5-13.0, v(co),d, decreases sharply, with dv/dt,b, = 461 cm-' V-' (see Table 1). This implies a pronounced change in the palladium surface characteristics which appears to prevent the forma- tion of CO islands. Such a change in surface characteristics may be manifest at a negative value of t,b0 which is likely to arise from the competitive adsorption of hydroxide to form palladium oxides.

We thank the New Zealand University Grants Committee for funding the purchase of the FTIR spectrometer and for the award of a Postgraduate scholarship and William Georgetti scholarship to M.R.M. We thank Professors Graham Wright, Milton Kerker, John White, John Bertie, Norman Sheppard and Stuart Bradley for valuable discussions. We are grateful to Dr. Alan Easteal for his assistance and advice regarding densimetry .

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Paper 0/03733H; Received 14th August, 1990

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