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CO monolayer oxidation at Pt nanoparticles supported on glassy carbon electrodes
Journal of Electroanalytical Chemistry 554 /555 (2003) 241 /251 www.elsevier.com/locate/jelechem
CO monolayer oxidation at Pt nanoparticles supported on glassy carbon electrodes O.V. Cherstiouk a,b, P.A. Simonov a, V.I. Zaikovskii a, E.R. Savinova a,b,* a
b
Boreskov Institute of Catalysis, Pr. Akademika Lavrentieva 5, 630090 Novosibirsk, Russia Technische Universita ¨ t Mu ¨ nchen, Physik-Department E19, James-Franck-Str. 1, D-85748 Garching bei Munchen, Germany Received 8 November 2002; received in revised form 18 February 2003; accepted 21 February 2003
Abstract CO monolayer oxidation on glassy carbon supported 1 /2 nm Pt nanoparticles is studied using potential sweep and potential step methods. The CO stripping peak on the nanoparticles is significantly shifted to positive potentials vs. the corresponding feature at bulk polycrystalline Pt. Current transients at nanoparticulate electrodes are highly asymmetric with a steep rise, maximum at uCO :/ 0.8 /0.9, and a slow decay following t1/2. The experimental results are compared to the theoretical models of adsorbed CO oxidation described in the literature. A tentative model is suggested to account for the experimental observations, which comprises spatially confined formation of oxygen containing species at active sites, and slow diffusion of CO molecules to the active sites, where they are oxidized. The upper limit of the CO surface diffusion coefficient at Pt nanoparticles is estimated as approximately 4 /10 15 cm2 s 1. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Oxidation; Carbon monoxide; Chronoamperometry; Nanoparticles; Platinum; Glassy carbon
1. Introduction Catalytic and electrocatalytic properties of nanoparticles have been the subject of numerous investigations (see e.g. Ref. [1] and references therein). The incentives have been twofold: first, to gain fundamental knowledge concerning the properties of small confined systems and, second, to develop better catalysts for practical applications, in particular, fuel cells. One of the most intriguing questions in nanoparticle (electro)catalysis is how the reaction kinetics and mechanisms are influenced by the particle size and morphology. In the present paper we continue our efforts to examine the electrocatalytic properties of model Pt nanoparticles. Armed with the experimental procedure of supporting nanometer sized Pt nanoparticles on the surface of glassy carbon (GC) described in Refs. [2,3], we study CO monolayer oxidation at Pt/GC electrodes. CO oxidation is a model surface reaction both in
* Corresponding author. Tel.: /49-89-289-12538; fax: /49-89-28912536. E-mail address: [email protected] (E.R. Savinova).
catalysis and electrochemistry, which is also relevant to the development of CO tolerant anodes for direct methanol and PEM fuel cells. We study CO monolayer oxidation under potentiodynamic (CO stripping) as well as potentiostatic conditions (chronoamperometry). Since the rate of CO monolayer oxidation is affected both by potential and time, investigation of the reaction under potentiostatic conditions is of particular interest and offers information on the reaction dynamics and mobility of the adsorbates on the metal surface. This paper is in memory of Weaver, whose work has to a large extent determined the progress of modern interfacial electrochemistry. This concerns also his significant contribution to the understanding of electrochemical properties of nanoparticles and nanoclusters (see e.g. Refs. [4 /7]).
2. Experimental Solutions were prepared from Milli-Q water (18 MV cm), H2SO4 (puriss. Suprapur, Merck). GC rods (7-mm diameter) were obtained from Alfa (Johnson Mattey)
0022-0728/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0728(03)00198-0
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and Sigradur. GC was pre-activated via anodic oxidation in 0.1 M H2SO4 at 2.23 V (reversible hydrogen electrode, RHE) for 3 min. To prepare Pt/GC electrodes a defined volume of H2PtCl4 aqueous solution (2.6 / 105 M in 6.5 /105 M HCl) was deposited on a support and dried in air. Then the samples were reduced in a quartz reactor in a hydrogen flow at 250 8C for 2 h. Thus prepared electrodes were cooled down in pure nitrogen or argon and immediately immersed in Milli-Q water to avoid their contamination. Electrochemical measurements were carried out in three-electrode Pyrex cells at 219/1 8C in Ar. The potential was controlled using an Autolab PGSTAT30 potentiostat. Pt foil was used as a counter electrode and a mercury sulfate electrode (MSE) Hg/Hg2SO4/0.1 M H2SO4(aq) connected to the cell via a Luggin capillary */as a reference electrode. All the potentials below are given vs. the RHE scale (EMSE /0.73 V vs. RHE). Prior to the measurements, Cl ions, which could remain on the surface after the preparation procedure, were desorbed from the electrode at approximately 0 V and the potential was cycled between 0.03 and 1.23 V at a 0.1 V s 1 scan rate to obtain a stable cyclic voltammogram (CV). To avoid contamination of the electrodes all the measurements were performed only with freshly prepared samples. If not otherwise stated, for CO oxidation experiments CO was bubbled through the electrolyte for 15 min at 0.1 V, then dissolved CO was removed by purging the solution with Ar for 35 min. The real surface area of the bulk polycrystalline Pt(pc) electrode was determined using hydrogen upd assuming 210 mC cm 2 per Hupd monolayer. By comparing the CO stripping with the Hupd charge CO coverage uCO was estimated as approximately 0.7. The latter is in agreement with the CO coverage reported for Pt surfaces [8]. However, application of the same procedure to the Pt/GC electrodes resulted in uCO /1.2. Higher than unity CO coverage at Pt nanoparticles has been reported also by Friedrich et al. [9]. uCO /1 might result from binding more than one CO molecule per Pt site, which is characteristic of Pt carbonyl clusters [4,10,11]. However, it could also be a consequence of the uncertainty in the determination of the Hupd charge incurred by a significant contribution of the support charging current. Hence, in the following the real surface area of Pt nanoparticles was calculated on the basis of CO stripping, assuming 420 mC cm 2 per CO monolayer. Current densities are referred to a square centimeter of Pt. The electrodes were examined with transmission electron microscopy (TEM) using a JEM-100C and high resolution transmission electron microscopy (HRTEM) using a JEM-2010 instrument. The samples for electron microscopy were prepared by scraping the active catalyst layer off the surface. Size distributions of
Pt particles were reconstructed from TEM data and used for calculation of the square weighted mean particle size d¯S (ai ni di2 =ai ni )1=2 :/
3. Results 3.1. Characterization of the Pt/GC electrodes Fig. 1 shows a typical HRTEM image of the Pt/GC sample, which shows that Pt nanoparticles are not agglomerated and are uniformly distributed on the surface. Analysis of multiple probes under different magnifications proved that thorough preparation of the Pt/GC electrodes using the chemical deposition method described above indeed offered Pt nanoparticles with a very high degree of dispersion and zero agglomeration extent. Depending on the Pt loading, the average particle size d¯S increased from approximately 1.3 at 0.9 mg cm 2 to 2.8 nm at 5.1 mg cm 2 (Pt loading is given in microgram Pt per geometric surface area of GC). A typical particle size distribution for a Pt/GC sample is given in Fig. 2. As confirmed by microdiffraction, Pt nanoparticles exhibit a face centered cubic (fcc) structure. The inset in Fig. 1 shows a high resolution electron micrograph of a representative Pt nanoparticle. The metal lattice is clearly discernable, although it is somewhat blurred by the structure of the GC support described e.g. in Ref. [12]. As is evident from HRTEM, Pt particles have a
Fig. 1. HRTEM image of Pt/GC, Pt loading 5.1 mg cm2. The insert shows a magnified 4.6 /4.6 nm2 image of a representative nanoparticle.
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Fig. 3 represents a CO stripping voltammogram for the Pt nanoparticulate electrode at a sweep rate of
0.1 V s1 in comparison to the corresponding voltammogram of Pt foil. Two striking differences between Pt/ GC and Pt voltammograms can be observed. First, the onset of the CO stripping peak at 1.79/0.5 nm Pt nanoparticles on GC is shifted by approximately 0.1 V positive vs. that at a Pt(pc) electrode. Second, the CO stripping peak at the nanoparticulate electrode is very wide so that stripping of the full monolayer of CO from Pt nanoparticles is achieved only at approximately 1.2 V at a sweep rate of 0.1 V s1 vs. approximately 0.9 V at polycrystalline Pt. Decrease of the sweep rate results in a negative shift of CO stripping peak both for Pt/GC and for Pt foil, but does not cancel out the difference between them. A positive shift of the CO monolayer stripping has been previously observed by Takasu and coworkers for Pt nanoparticles deposited on GC [13] and Friedrich et al. for Pt particles on Au [9]. Comparison of CO stripping from a commercial ETek 20% Pt/Vulcan catalyst (see e.g. Ref. [14]) with the corresponding voltammogram of polycrystalline Pt foil reveals the same phenomenon: CO oxidation on Pt/ Vulcan requires a higher overpotential than on Pt(pc). It is interesting to compare the CO stripping voltammograms with the CVs in supporting 0.1 M H2SO4 electrolyte. The analysis of CVs of the Pt/GC electrode is complicated by the relatively high contribution of the support charging and redox transformations of the oxygen-containing groups (mainly quinone [12]) of the GC surface. Hence, in Fig. 4 we compare the CV of the Pt foil electrode with that obtained at Pt/GC after the subtraction of the current originating from the GC support. Although a wide peak in the Pt double layer
Fig. 3. CO stripping voltammograms in 0.1 M H2SO4 at a sweep rate of 0.1 V s 1 at: (a) Pt/GC; d¯S/ /1.79/0.5 nm; Pt loading 1.8 mg cm 2; (b) Pt foil.
Fig. 4. CVs in 0.1 M H2SO4 at a sweep rate of 0.1 V s 1 at: (a) Pt/GC after the support current subtraction; d¯S/ /1.79/0.5 nm; Pt loading 1.8 mg cm2; (b) Pt foil.
Fig. 2. Typical size distribution for a Pt/GC sample. Pt loading 3.6 mg cm 2, d¯S/ /1.59/0.5 nm.
facetted rather than a round shape. The specific surface area of 1 /2 nm nanoparticles is very high and exceeds 100 m2 g1 (Pt). 3.2. CO stripping voltammetry
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region of Fig. 4a shows some contribution of the GC support, the Pt features can readily be recognized. The onset of the Pt surface oxidation does not seem to be noticeably affected by the decrease of the particle size, while the oxygen desorption peak at the Pt nanoparticles is shifted approximately 0.2 V negative. The latter suggests that oxygen is adsorbed more strongly at Pt nanoparticles than at bulk Pt. A negative shift of the oxygen desorption peak for Pt nanoparticles supported on GC has also been observed in Ref. [15]. Small Pt particles demonstrate a lower amount of strongly adsorbed hydrogen (Fig. 4a) in agreement with Ref. [16]. 3.3. CO monolayer oxidation at constant potential Fig. 5 shows typical current transients for the CO monolayer oxidation obtained at nanoparticulate Pt/GC electrodes after stepping the potential from the CO adsorption to the increasing oxidation potentials Eox. As one may see, the current transients exhibit a initial steep decay followed by a current maximum (or a shoulder), which shifts towards shorter times with the increase in the oxidation potential. Friedrich et al. [9] have previously observed a dramatic difference between the kinetics of CO monolayer oxidation at Pt nanoparticles immobilized at a gold substrate and bulk Pt. Oxidation transients at 3 nm Pt nanoparticles on the Au substrate demonstrated exponential decay rather than the bell-shaped curve typical for bulk single crystalline [17 /19] and Pt(pc) electrodes [20]. A maximum showed up in the current transients only upon aggregation of Pt nanoparticles (average particle size estimated as 16 nm) [9]. This was interpreted by invoking an Eley /Rideal mechanism of CO adlayer oxidation with some solution species (e.g. interfacial water) at small Pt nanoparticles vs. a
Fig. 5. Current transients for CO monolayer oxidation at Pt/GC in 0.1 M H2SO4 after a potential step from 0.1 V (RHE) to (a) 0.73; (b) 0.76; (c) 0.78 and (d) 0.80 V RHE. d¯S/ /1.79/0.5 nm; Pt loading 1.8 mg cm 2.
Langmuir /Hinshelwood (L /H) mechanism at bulk Pt electrodes and large Pt particles. Keeping in mind the above data, special efforts have been undertaken to exclude the possible influence of nanoparticle agglomeration. Potential step measurements were performed at different Pt/GC samples. The latter were thoroughly examined with TEM after the experiments in order to detect possible particle agglomeration. The results of these measurements prove undoubtedly that the maximum in the current transients is characteristic of Pt nanoparticles finely dispersed on GC. It is important to note however that a pronounced current maximum could be observed only in a certain window of the oxidation potentials. At high Eox it merged into the initial current decay, while at low Eox it became indistinguishable from the current tailing (see Fig. 5). It has recently been noticed by Lebedeva et al. [18] that contamination of Pt(1 1 1) electrodes may result in asymmetric current transients. We must clearly point out that the striking differences observed in the present paper between CO monolayer oxidation on Pt nanoparticles and flat surfaces were induced by the particle size rather than by the presence of impurities (e.g. chloride ions resulting from the preparation procedure) or other artefacts. Indeed, we have found that other conditions (including the preparation procedure) being equal, the properties of Pt particles approached those of flat surfaces as their size increased. Systematic investigation of the effect of the particle size on electrocatalytic properties of Pt is in progress and will be subject of forthcoming publications [21]. To elucidate the nature of the initial current decay, the current transients were recorded both on Pt/GC and on bare GC electrodes (pretreated in the same way as Pt/ GC). As one may see from Fig. 6, the currents at Pt/GC and at bare GC support decay in a similar manner, testifying that the initial current decrease is mainly concerned with the GC support rather than with Pt nanoparticles. In order to understand the origin of the current decay let us estimate the double layer (DL) charging time constant tDL for Pt/GC. We estimate the upper limit of the capacitance of oxidized GC from the CV CGC /200 mF. Assuming that the resistance between the working electrode and Luggin capillary in 0.1 M H2SO4 should not exceed 1 /2 V, we estimate tDL / RCGC :/200 /400 ms. Hence, the charging current should drop by a factor of 20 at t/3t 5/1.2 ms. It is thus obvious that in the time range of interest (from 0.1 to 10 s) the DL charging current must be negligible and we suppose that the main contribution to the initial current decay is the oxidation of the surface groups at the GC surface. Hence, the current transients may be represented as a superposition of a decaying current, originating from the GC support, and a curve with a maximum (Imax) at
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Fig. 6. Current transients following a double potential step from 0.1 V to 0.68 V (2 s) and then 0.73 V (RHE) (a) at Pt/GC after the adsorption of a CO monolayer at 0.1 V for 15 min and purging with Ar for 35 min at the same potential; (b) after the same experimental protocol but at the bare GC support; (c) at Pt/GC in the absence of CO and (d) difference current obtained by subtraction of (c) from (a). d¯S/ /1.79/ 0.5 nm; Pt loading 1.8 mg cm 2. A pre-pulse to 0.68 V was made in order to decrease the background current.
t/tmax corresponding to CO oxidation at Pt nanoparticles (Fig. 6d). The maxima in the current transients strongly suggest that CO oxidation at nanoparticles, similarly to bulk Pt electrodes, proceeds via a L /H reaction. It should be pointed out, however, that in contrast to essentially symmetric transients exhibited by bulk single and polycrystalline Pt electrodes [17 /20], those observed at Pt nanoparticles are rather asymmetric, exhibiting a steep rise at t B/tmax and very slow decay at t /tmax. Fig. 7 shows current transients for Pt/ GC in comparison to those observed at Pt foil at selected potentials. It is interesting that at lower oxida-
Fig. 7. Current transients for CO monolayer oxidation at Pt/GC (solid line) and Pt foil (dashed line) at 0.73 (a) and 0.83 (b) V RHE. d¯S/ / 1.79/0.5 nm; Pt loading 1.8 mg cm2.
245
tion potential (0.73 V) tmax is shifted to larger values and current relaxation at t /tmax is much slower at Pt nanoparticles than at bulk Pt(pc). Meanwhile, at a high oxidation potential of 0.83 V CO monolayer oxidation is completed essentially in the same time interval at nanoparticles and bulk Pt electrode. As is evident from Fig. 8, jmax and tmax for Pt nanoparticles depend on Eox exponentially, similarly to bulk Pt(pc) [20] and Pt single crystal electrodes [17 /19]. The slope of log tmax vs. Eox varied from (1039/8 mV) 1 to (899/15 mV) 1, and the slope of log jmax vs. Eox */ from (1269/5 mV)1 to (989/11 mV) 1, while the Pt loading increased from 1.8 to 2.5 mg cm 2. Note that as revealed by TEM, the particle size distribution at the Pt/ GC electrode with the higher loading was broader and shifted somewhat towards a larger particle size. The latter may account for a considerably shorter tmax (Fig. 8a) and larger jmax (Fig. 8b). Comparison of the experimental slopes with the literature data will be given in Section 4. Fig. 9 shows the evolution of the charge densities q during CO monolayer oxidation at Pt nanoparticles calculated from the current transients at different Eox. It is remarkable that q at nanoparticles increases instantaneously, while at bulk Pt an induction period is observed at lower Eox. It is worth mentioning that at long time q approached 410 mC cm 2, corresponding to a full monolayer of adsorbed CO (COads). This confirms the validity of the background current subtraction.
Fig. 8. log tmax (top) and log jmax (bottom) vs. Eox for Pt/GC electrodes with different Pt loadings: (1) 1.8 mg cm 2 Pt, d¯S/ /1.79/ 0.5 nm; (2) 2.5 mg cm2 Pt, d¯S/ :/1.9 nm.
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ing cases can be distinguished depending on the mobility of CO on the surface. (I) The first limiting case is based on the assumption that the surface mobility of COads is very high and thus COads and OHads species are intermixed. This may be addressed as a simple L /H mechanism (the so-called ‘mean-field approximation’ [23]) with the reaction rate being proportional to the product of OHads and COads coverages (W 8/uOHuCO). The model predicts symmetric current transients with the maximum corresponding to the conversion of half of the CO monolayer. This model has been considered in a number of publications [23,24] and has been shown to describe CO monolayer oxidation at (stepped) single crystal Pt surfaces adequately [18,19]. Assuming simple L /H kinetics, in a mean-field approximation the reaction rate W can be expressed as: W k2 uOH uCO k2 uCO (1uCO ) Fig. 9. Evolution of the charge after potential steps from 0.5 V RHE to different oxidation potentials Eox for (a) Pt foil: Eox set at 0.73 V (1) and 0.83 V (2) and (b) Pt/GC: Eox set at 0.78 V (1), 0.80 V (2), 0.83 V (3) and 0.88 V (4). Current transients were corrected to the background current recorded under the same conditions in the absence of a CO monolayer. d¯S/ :/1.9 nm. Pt loading 2.5 mg cm 2. CO adsorption potential 0.5 V (RHE).
4. Discussion 4.1. Models of CO monolayer oxidation We start the discussion section by considering the mechanism of CO monolayer oxidation. According to the present understanding, oxidation of adsorbed CO on Pt electrodes proceeds in a L/H reaction between COads and surface oxygen-containing species, which is usually formulated as OHads1, to form CO2: H2 O(sol)SUOHads H (sol)e COads OHads 0 CO2 (gas)H (sol)e 2S
(1) (2)
Here S is a free site on the surface. It has recently been proposed [22] that reaction (2) proceeds through a chemical rate-determining (rds) step (2a) followed by fast electron transfer (2b): COads OHads 0 COOHads COOHads 0 CO2 (gas)H (sol)e 2S
(2a) (2b)
A number of approaches have been suggested in order to describe CO monolayer oxidation at Pt and bimetallic electrode surfaces quantitatively. Essentially two limit1 Although there is no direct indication that OHads rather than Oads is involved in the oxidation process.
k1 k1 k1 k2 uCO
(3)
where k1, k 1 and k2 stand for the rate constants of OH adsorption, OHads desorption and OHads/COads interaction, respectively. k1 and k 1 obey the Butler /Volmer law and comprise the water and proton concentrations near the surface, respectively: a1 FE 0 (4) k1 k1 exp RT (1 a1 )FE 0 k1 k1 exp (5) RT k2 may or may not be a function of the electrode potential, depending on whether COads oxidation proceeds in the electrochemical (Eq. (2)) or chemical step (Eq. (2a)). The effect of the electrode potential on the overall reaction rate W is determined by the ratelimiting step. Reaction rate equations for a number of limiting cases along with the corresponding Tafel slopes are given in Table 1. Although Eq. (3) is oversimplified, it allows some preliminary conclusions on the Tafel slope and its variation depending on the potential interval and type of rds. Note that Koper et al. predicted a slope variation from approximately 40 mV dec 1 for low to 119 mV dec1 for high Eox [23]. (II) The second model is based on an assumption that the reaction occurs via a nucleation-and-growth mechanism with OHads islands nucleating at the free sites of the CO-covered metal surface and growing via consumption of COads at the rims of the islands. Then, if the OHads formation is fast, the surface will be covered with OHads islands separated by the areas of still unreacted COads. The OHads islands will ultimately overlap, the reaction rate being controlled by nucleation, growth and
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Table 1 Rate equations and Tafel slopes for COads oxidation in the mean-field approximation for limiting cases a Case OH adsorption
COads oxidation
rds Rate equation
A
Chemical (Eq. (2a))
2a
k2a(k01/k01)uCO(1/uCO)exp(FE /RT )
(2.3RT )/F (2.3RT )/((1/a2)F ) 2.3RT /a1F
120 120
Reversible
Tafel slope
B
Reversible
Electrochemical (Eq. (2))
2
k02(k01/k01)uCO(1/uCO)exp((1/a2)FE /RT )
C
Irreversible
Chemical (Eq. (2a))
1
k01(1/uCO)exp(a1FE /RT )
D
Irreversible
Electrochemical (Eq. (2))
1
k01(1/uCO)exp(a1FE /RT )
2.3RT /a1F
E
Irreversible
Chemical (Eq. (2a))
2a
k2auCO(1/uCO)
0
2
k02uCO(1/uCO)exp(a2FE /RT )
2.3RT /a2F
F
Irreversible a b
Electrochemical (Eq. (2))
Tafel slope for ai /0.5/mV dec 1 b 60 40
0 120
The rates of electrode reactions are assumed to obey the Butler /Volmer law; the parameters have their usual meaning. At 298 K.
overlap of the OHads islands. Two cases have been described in the two-dimensional nucleation-andgrowth model. These are so-called (a) instantaneous nucleation, with the reaction rate W proportional to: 2 2 2 W 8NN kG t exp(pNN kG t)
(6)
and progressive nucleation, with the reaction rate proportional to: 2 2 2 3 W 8NN kN kG t exp(pkN kG t =3)
(7)
Here NN is the number of nucleation centers, kN is the nucleation rate constant and kG is the growth rate constant. The nucleation-and-growth model has been shown to give an adequate description of the current transients at Pt (pc) [20] and single crystal Pt(1 0 0), Pt(3 1 1) and Pt(1 1 1) electrodes [17]. Usually the current (and as a consequence the charge) rises after an induction period, which is interpreted in terms of progressive nucleationand-growth (cf. Fig. 9 top and bottom). The ‘classical’ nucleation-and-growth model is based on an assumption of immobile COads, the reaction front propagating along the surface via growth of OHads islands provided by reaction of OHads and COads at the rims of the islands. Formation of COads islands on Pt surfaces (which presumes restricted CO mobility) has been proven e.g. by FTIR spectroscopy by Chang and Weaver [25]. Koper et al. have shown that deviations from the simple nucleation-and-growth model may occur [23] if COads diffuses on the surface resulting in intermixing COads-covered and OHads-covered areas. The relations between the nucleation-and-growth model and mean-field approximation have been analyzed also by Korzeniewski and Kardash [26]. (III) It has recently been pointed out, in particular by Petukhov et al. [24], Koper et al. [27] and Baltruschat and co workers [28] that surface defects (e.g. steps) play a key role in the CO monolayer oxidation. Thus, yet another model has been proposed [27], which like the first one, assumes fast surface diffusion of COads, but
implies the reaction to take place only at specific surface sites, i.e. between OHads adsorbed at steps and COads diffusing to the steps from the terraces. This model provides very good quantitative agreement with the experimental chronoamperometric results of COads monolayer oxidation at stepped Pt [n(1 1 1) /(1 1 1)] surfaces. Note that in the case of fast diffusion one would expect symmetric current transients, which have indeed been observed at Pt surfaces [27]. 4.2. CO stripping voltammograms Let us now analyze how our experimental data can be related to the above models and start the discussion by considering CO stripping voltammograms at Pt foil and Pt nanoparticles. It is instructive to compare the evolution of COads (uCO) and oxygen species (uOH)
Fig. 10. Evolution of the adsorbate coverages on (a) Pt/GC and (b) Pt foil. Solid lines */uCO as calculated from the stripping voltammograms of Fig. 3a and b; dash-dotted lines */(1/uCO); dashed lines */uOH as calculated from the CVs of Fig. 4a and b. See text for details.
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coverage on Pt surface. Fig. 10 shows uCO calculated from the stripping voltammograms of Fig. 3 along with uOH calculated from the CVs of Fig. 4 (in the absence of CO). uOH is calculated neglecting the actual complex mechanism of Pt surface oxidation assuming OHads as a sole oxidation product. This gives an upper limit of uOH in the presence of CO. Indeed, uOH in the presence of COads will be either equal to (if (i) there are enough ‘free’ sites at the electrode surface and (ii) the reaction of COads and OHads is slow) or less than uOH in the supporting electrolyte as plotted in Fig. 10. Comparison of the uCO and uOH shows that the CO monolayer oxidation at Pt foil starts below the visible onset of surface oxidation and is completed at uOH /0.15. This testifies that very low oxygen coverage is necessary to sustain the reaction.2 This actually means that the ‘classical’ nucleation-and-growth model, which presumes immobility of COads, is not applicable for Pt(pc). Indeed, if COads were immobile, the only way for the reaction to proceed would be by consumption of COads molecules adjacent to the rims of OHads islands. In this case the amount of free sites on the surface would be negligible and uOH :/1/uCO. Since uOH in the presence of COads is expected to be less than that on a bare Pt electrode, one would expect uOH plotted in Fig. 10 to be ]/1/uCO. However, this is not the case, which strongly suggests that COads is mobile on Pt(pc) in agreement with the conclusions of Refs. [23,24,26,28,29] and the reaction of COads and OHads is faster than OHads propagation over the surface (otherwise one would expect an absence of ‘free’ sites on the electrode). At Pt nanoparticles the decay of a uCO is shifted very much positive on the potential scale. However, 1/uCO still considerably exceeds uOH, which implies that oxidation of a CO monolayer on Pt nanoparticles requires the mobility of COads. The latter, as will be shown below, may be considerably lower on Pt nanoparticles than on flat surfaces. The fact, which has to be rationalized, is why the onset of CO monolayer oxidation on Pt/GC is shifted positive vs. that on Pt (Figs. 3 and 10). Special experiments performed at submonolayer COads coverages proved that it did not stem from higher uCO at Pt nanoparticles vs. Pt(pc). We suggest that the positive shift of the CO oxidation onset is due to slow interaction of OHads and COads at Pt nanoparticles. The latter might be a consequence of strong bonding of oxygen to the nanoparticle surfaces [30], which has indeed been confirmed by a number of techniques, in particular in situ EXAFS and XANES [31,32].
2 This may also be due to participation of some adsorbed ‘activated’ water molecules in the CO oxidation.
4.3. CO monolayer oxidation at a constant potential We start the discussion with the Tafel slopes calculated from the plots of log tmax vs. Eox, which for Pt nanoparticles were equal to 90/100 mV dec 1. Since tmax can be related to the rate constant of COads oxidation [19,23,33], the slope of log tmax vs. Eox has been extensively discussed in the literature for Pt electrodes of different crystallographic orientations: from perfect single crystals [17 /19,33,34] to stepped Pt[n (1 1 1) /(1 1 1)] [19] and polycrystalline Pt [20]. It has recently been pointed out, however, that it may not be totally correct to determine the Tafel slope from a plot of log tmax vs. Eox because not only CO oxidation, but also reaction initiation, contributes to the dependence of tmax on the oxidation potential [18]. Most of the authors agree that at low potential values (between :/0.6 and 0.7 V RHE) the Tafel slopes lie between 60 and 80 mV dec 1. Thus, Love and Lipkowski [17] found 80 mV dec 1 for Pt(1 0 0), Pt(3 1 1) and Pt(1 1 1); McCallum and Pletcher [20] found 60 mV for Pt(pc) (see Fig. 8); Lebedeva et al. [19] found approximately 80 mV dec 1 for stepped Pt single crystals. Tafel slopes between 60 and 80 mV dec 1 observed for Pt single crystals have been interpreted in terms of the limiting case (A) (Table 1) [19]. However, at higher potential values (above :/0.7 V RHE) there is considerable disagreement between the literature data: from 240 mV dec1 for single crystals [17] to 180 mV dec 1 for polycrystalline [20] and 70 /80 mV dec 1 for stepped Pt single crystals [19]. Despite considerable experimental error, the Tafel slopes of 90 /100 mV dec 1 observed in this work hardly conform to the limiting case (A), and are, rather, consistent with the limiting cases (C), (D) or (F) (Table 1) and imply irreversible formation of OHads. The latter is in line with the shape of the CV in supporting electrolyte (Fig. 4). However, neither irreversible formation of OHads, nor slow interaction of COads with OHads is sufficient to account for the asymmetry of the current transients. Indeed, slow reaction of OHads and COads alone would result in a shift of the current transients towards longer time rather than in a change in their symmetry. Apparently, the shape of the current transients is the result of a combination of a number of effects induced by the particle size. uCO at the tmax calculated from the charge consumed in the electrode reaction varied for nanoparticles between :/0.93 and 0.8 (assuming maximum uCO /1). This is at variance with the results at single crystals, which usually exhibit current maxima around uCO /0.5. Note that the nucleation-and-growth models predict maxima at uCO /0.51 for progressive and at 0.61 for instantaneous nucleation [28]. The asymmetry of the current transients at Pt/GC is not explained by any of
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Fig. 11. Normalized current (I /Imax) vs. (t /tmax) 1/2 for Pt/GC electrode. d¯S/ :/1.9 nm, Pt loading 2.5 mg cm2. CO adsorption potential 0.5 V (RHE).
the mechanistic models described above. The current decay cannot be fitted either by exp(/t2/t), characteristic of the instantaneous growth model or by exp(/t3/t ), typical for progressive nucleation. As one can see from Fig. 11, the current decay follows t 1/2 dependence in a wide interval of normalized time from t/ tmax /3 to 20, which is indicative of a diffusion limitation. It should be mentioned however that at longer times and/or lower Eox the current decay deviates from t 1/2 dependence. One may argue that the tailing of the current transients towards longer times is the consequence of a superposition of current transients from particles of different sizes rather than a characteristic of a nano-
Fig. 12. Current transients at Pt/GC following a multiple potential step program (a) 0.1 V0/0.68 (2 s) 0/0.73 V (200 s) and (b) 0.1 V 0/ 0.68 (2 s) 0/0.73 V (20 s) 0/0.63 V (10 s) 0/0.73 V (RHE) (200 s). d¯S/ / 1.79/0.5 nm; Pt loading 1.8 mg cm2. The insert shows the potential programs.
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particle of the average size. To rule out such a possibility multiple step experiments have been performed as follows. The electrode potential was stepped from the adsorption (0.1 V (RHE)) to the oxidation potential Eox for a limited time (we will call it a pre-pulse), then to a lower potential value where no oxidation occurs and then back to the Eox. The results of one of these experiments with Eox /0.73 V (RHE) are presented in Fig. 12. One may see that when approximately 20% of the CO monolayer is oxidized in the pre-pulse, tmax of the pulse shifts to smaller values. Meanwhile, our recent experimental results prove that oxidation of COads at large nanoparticles proceeds faster with a current maximum shifted to smaller values. Thus, if there were a fraction of large Pt particles on Pt/GC, CO adsorbed on their surface would have been oxidized in the prepulse, and tmax of the curve b of Fig. 12 would have shifted positive vs. tmax of the curve a. Since this is not the case, we infer that the current transients presented in this paper are indeed characteristic of CO monolayer oxidation at Pt nanoparticles with an average size of 1/2 nm. Another important conclusion from Fig. 12 is that the difference of the shapes of the current transients between Pt foil and Pt/GC electrodes is not the result of higher uCO at nanoparticles. 4.4. Model of CO monolayer oxidation at Pt nanoparticles Taking into account the above observations, we suggest the following qualitative model of COads oxidation at 1/2 nm Pt nanoparticles. We assume that in the potential interval of CO monolayer oxidation OHads formation occurs only at specific active sites3, which might be, e.g. particle vertices and/or edges. This is in agreement with the mechanism of COads oxidation at stepped Pt [n(1 1 1) /(1 1 1)] single crystals, where the reaction has been suggested to occur at steps [18,19]. It is interesting to mention that for an ideal 2 nm cuboctahedral Pt nanoparticle the ratio of vertices amounts to approximately 7% and edges to approximately 40% [32]. At high oxidation potentials OHads formation at these sites is fast, accounting for a steep rising of the current. As COads adjacent to these ‘active’ sites is oxidized, further evolution of the reaction would be possible either (i) via growth of the OHads islands, or (ii) diffusion of more distant COads to the ‘active sites’ occupied by OHads. If the former were true, the current transients would have exhibited a shape typical for the nucleation-and-growth mechanism, which is not the case. If the latter were true and the diffusion of the CO molecules were fast, the transients would have been symmetric as shown in Ref. [27]. We were able to 3
At least in the potential interval explored: 0.73 VB/Eox B/0.88 V.
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account for the highly asymmetric shape of the transients only under an assumption that the OHads formation is spatially confined and the reaction proceeds via slow diffusion of COads to the active sites occupied by OHads. The COads diffusion coefficient can be estimated roughly from the current transients as follows. At 0.83 V oxidation of the CO monolayer proceeds on the time scale of 5 /10 s (Fig. 7), the current decay following a t 1/2 dependence (Fig. 11). The diffusion mean free path D at 1 /2 nm Pt nanoparticles should not exceed 1 /2 nm. Thus from D /2Dt we can estimate the upper limit of DCO as 4 /10 15 cm2 s 1. On Pt(1 1 1) in UHV DCO was estimated as approximately 10 9 cm2 s1 at 300 K and high uCO [35]. In electrolyte solutions DCO is supposed to be lower due to the presence of water molecules and adsorbed anions. Koper et al. have recently obtained 10 11 cm2 s 1 as a lower limit for the COads diffusion at stepped Pt [n (1 1 1) /(1 1 1)] [27]. The results of this work suggest that decrease of the Pt particle size to 1 /2 nm may lead to a considerable decrease of the COads diffusion coefficient (by three to four orders of magnitude). We would like to refer to the work of Sinfelt and co workers, who used 13C-NMR to investigate the mobility of COads on alumina supported Pt clusters [36]. These authors have found that the activation energy for the COads diffusion depends on the Pt particle size, increasing from 279/2 kJ mol 1 for the large particles of 10 nm in diameter (in agreement with the values reported for CO on Pt(1 1 1)) to 449/4 kJ mol 1 for small clusters 1.2 nm in size. At 300 K such a difference in the activation energy would correspond to approximately three orders of magnitude difference in the apparent diffusion coefficients. Sinfelt and coworkers attributed the influence of the Pt cluster size on the mobility of CO to the large fraction of the edge and corner sites in small Pt clusters. The binding energy of COads at low coverage has also been found to increase when the cluster size decreases below 5 nm [37 /39]. This result has been attributed to a stronger adsorption on low co-ordinated atoms (edges and corners), the proportion of which increases with the decrease of the cluster size. Hence, a diminution of the DCO with the decrease of the Pt particle size observed in Ref. [36] and in this work may result from higher ratio of low co-ordination sites at small metal particles. However, the possibility of an electronic effect cannot be ruled out for small nanoparticles and needs further investigation. A possible origin of such an effect could be contraction of the lattice parameter usually observed for 1 /2 nm metal nanoparticles and attributed to large surface strain induced by their high curvature. According to the literature data the difference of the lattice parameter between 1 and 2 nm nanoparticles and bulk metal may reach 5% [38]. Meanwhile, Nørskov and coworkers [40] have shown
that strain results in a considerable change of chemisorption and catalytic properties. A likely reason for that is a shift in the d-band center. It is interesting to mention that DCO on Ru-modified Pt(1 1 1) has been estimated by Friedrich et al. as ]/4/ 1014 cm2 s 1 [29], which is considerably smaller than for the bare Pt(1 1 1) surface. Baltruschat and coworkers [28] report a still lower value of 3/1017 cm2 s 1 as an upper limit for DCO on Ru-decorated Pt(6 6 5). Slow diffusion of COads on the Ru-modified surface has been confirmed also by model simulations [27]. It has been attributed to the COads binding energy gradient at the Ru-decorated surface, in particular the high activation barrier of CO diffusion from ‘distant’ Pt sites to the electronically modified Pt sites adjacent to the rims of Ru islands. In a way this model may apply also to bare Pt nanoparticles, which due to high surface heterogeneity may exhibit a high COads binding energy gradient. More work is currently being done to (i) understand the bonding and reactivity of COads at nanoparticles using FTIR spectroscopy and (ii) establish a mathematical model of COads oxidation at nanoparticle surfaces (M. Eikerling, TU Muenchen).
5. Conclusions One to two nanometers sized Pt nanoparticles have been prepared using a chemical precursor method via adsorption of H2PtCl4 on a pre-oxidized GC surface and subsequent reduction in the hydrogen flow at 250 8C. Pt nanoparticles exhibit an fcc structure and narrow particle size distribution as revealed by HTREM. CO monolayer oxidation at GC supported Pt nanoparticles has been studied using potential sweep and potential step techniques. Both methods shown significant differences between the reaction at bulk electrodes and Pt nanoparticles. Oxidation of a full CO monolayer during CO stripping is achieved at nanoparticles at much more positive potentials (at :/1.2 V (RHE) vs. :/0.9 V (RHE) at polycrystalline Pt at a sweep rate of 100 mV s 1). Current transients at nanoparticulate electrodes demonstrate a steep rising front, a maximum at uCO / 0.8 to 0.9, and a long tail following t 1/2. A tentative model is suggested, which assumes spatially confined formation of OHads at active sites and slow diffusion of the COads molecules to the active sites, where they are oxidized. These active sites can be attributed tentatively to the particle corners and/or edges. The upper limit of the COads surface diffusion coefficient is estimated on the basis of the current transients as :/4 /1015 cm2 s 1.
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Acknowledgements The authors express their gratitude to M. Eikerling (TU, Muenchen) for stimulating discussions, thorough reading of the manuscript and many valuable comments, to U. Stimming (TU, Muenchen) for his interest in the work and to S. Schreier, S. Weinkauf and M. Heinzlik (TU, Muenchen) for their kind assistance with TEM measurements. We would also like to thank F. Maillard (TU, Muenchen) for performing experiments on the influence of COads coverage and sweep rate on the CO stripping transients at Pt/GC and Y.-Y. Tong (Georgetown University) for providing useful references. Financial support from DFG under contract Sti 74-8-3 is gratefully appreciated.
References [1] A. Wieckowski, E. Savinova, C.G. Vayenas (Eds.), Catalysis and Electrocatalysis at Nanoparticle Surfaces, Marcel Dekker, NY, 2003. [2] E.R. Savinova, N.P. Lebedeva, P.A. Simonov, G.N. Kryukova, Russ. J. Electrochem. 36 (2000) 952. [3] O.V. Cherstiouk, P.A. Simonov, E.R. Savinova, Electrochim. Acta, in press. [4] J.D. Roth, G.J. Lewis, L.K. Safford, X. Jiang, L.F. Dahl, M.J. Weaver, J. Am. Chem. Soc. 114 (1992) 6159. [5] S. Park, S.A. Wasileski, M.J. Weaver, J. Phys. Chem. B 105 (2001) 9719. [6] S. Park, Y.Y. Tong, A. Wieckowski, M.J. Weaver, Langmuir 18 (2002) 3233. [7] S. Park, A. Wieckowski, M.J. Weaver, J. Am. Chem. Soc. 125 (2003) 2282. [8] R. Gomes, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf. Sci. 410 (1998) 48. [9] K.A. Friedrich, F. Henglein, U. Stimming, W. Unkauf, Electrochim. Acta 45 (2000) 3283. [10] G. Longoni, P. Chini, J.Am. Chem. Soc. 98 (1976) 7225. [11] Q. Xu, Coord. Chem. Rev. 231 (2002) 83. [12] K. Kinoshita, Carbon. Electrochemical and Physicochemical Properties, Wiley, New York, 1988. [13] K. Yahikosawa, N. Tateishi, K. Nishimura, Y. Takasu, Chem. Express 7 (1992) 437. [14] T.J. Schmidt, M. Noeske, H.A. Gasteiger, R.J. Behm, P. Britz, H. Bonnemann, J. Electrochem. Soc. 145 (1998) 925.
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[15] Y. Takasu, N. Ohashi, X.-G. Zhang, I. Murakami, H. Minagawa, S. Sato, K. Yahikosawa, Electrochim. Acta 41 (1996) 2595. [16] Y. Takasu, Y. Fujii, K. Yasuda, Y. Iwanaga, Y. Matsuda, Electrochim. Acta 34 (1989) 453. [17] B. Love, J. Lipkowski, in: M. Soriaga (Ed.), Electrochemical Surface Science, ACS Symposium Series, vol. 378, Washington, 1988, p. 484. [18] N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van Santen, J. Electroanal. Chem. 524 /525 (2002) 242. [19] N.P. Lebedeva, M.T.M. Koper, J.M. Feliu, R.A. van Santen, J. Phys. Chem. B 106 (2002) 12938. [20] C. McCallum, D. Pletcher, J. Electroanal. Chem. 70 (1976) 277. [21] F. Maillard, M. Eikerling, O.V. Cherstiouk, S. Schreier, E.R. Savinova, U. Stimming, Faraday Discuss., submitted. [22] E. Herrero, J.M. Feliu, S. Blais, Z. Radovic-Hrapovic, G. Jerkiewicz, Langmuir 16 (2000) 4779. [23] M.T.M. Koper, A.P.J. Jansen, R.A. van Santen, J.J. Lukkien, P.A.J. Hilbers, J. Chem. Phys. 109 (1998) 6051. [24] A.V. Petukhov, W. Akemann, K.A. Friedrich, U. Stimming, Surf. Sci. 402 /404 (1998) 182. [25] S.-C. Chang, M.J. Weaver, J. Chem. Phys. 92 (1990) 4582. [26] C. Korzeniewski, D. Kardash, J. Phys. Chem. B 105 (2001) 8663. [27] M.T.M. Koper, N.P. Lebedeva, C.G.M. Hermse, Faraday Discuss. 121 (2002) 301. [28] G. Samjeske, X.-Y. Xiao, H. Baltruschat, Langmuir 18 (2002) 4659. [29] K.A. Friedrich, K.P. Geyzers, A.J. Dickinson, U. Stimming, J. Electroanal. Chem. 524 /525 (2002) 261. [30] A. Russel (Comment) and E. Savinova (Response), Faraday Discuss. 121 (2002) 353. [31] S. Mukerjee, J. McBreen, J. Electroanal. Chem. 448 (1998) 163. [32] J. McBreen, S. Mukerjee, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, 1999, p. 49. [33] M. Bergelin, E. Herrero, J.M. Feliu, M. Wasberg, J. Electroanal. Chem. 467 (1999) 74. [34] E. Santos, E.P.M. Leiva, W. Vielstich, Electrochim. Acta 36 (1991) 555. [35] E.G. Seebauer, C.E. Allen, Prog. Surf. Sci. 49 (1995) 265. [36] L.R. Becerra, C.A. Klug, C.P. Slichter, J.H. Sinfelt, J. Phys. Chem. 97 (1993) 12014. [37] C.R. Henry In: A. Wieckowski, E. Savinova, C.G. Vayenas (Eds.), Catalysis and Electrocatalysis at Nanoparticle Surfaces, Marcel Dekker, New York, 2003, p. 239. [38] C.R. Henry, Surf. Sci. Rep. 31 (1998) 235. [39] A.K. Santra, D.W. Goodman, Electrochim. Acta 47 (2002) 3595. [40] M. Mavrikakis, B. Hammer, J.K. Nørskov, Phys. Rev. Lett. 81 (1998) 2819.
CO monolayer oxidation at Pt nanoparticles supported on glassy carbon electrodes O.V. Cherstiouk a,b, P.A. Simonov a, V.I. Zaikovskii a, E.R. Savinova a,b,* a
b
Boreskov Institute of Catalysis, Pr. Akademika Lavrentieva 5, 630090 Novosibirsk, Russia Technische Universita ¨ t Mu ¨ nchen, Physik-Department E19, James-Franck-Str. 1, D-85748 Garching bei Munchen, Germany Received 8 November 2002; received in revised form 18 February 2003; accepted 21 February 2003
Abstract CO monolayer oxidation on glassy carbon supported 1 /2 nm Pt nanoparticles is studied using potential sweep and potential step methods. The CO stripping peak on the nanoparticles is significantly shifted to positive potentials vs. the corresponding feature at bulk polycrystalline Pt. Current transients at nanoparticulate electrodes are highly asymmetric with a steep rise, maximum at uCO :/ 0.8 /0.9, and a slow decay following t1/2. The experimental results are compared to the theoretical models of adsorbed CO oxidation described in the literature. A tentative model is suggested to account for the experimental observations, which comprises spatially confined formation of oxygen containing species at active sites, and slow diffusion of CO molecules to the active sites, where they are oxidized. The upper limit of the CO surface diffusion coefficient at Pt nanoparticles is estimated as approximately 4 /10 15 cm2 s 1. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Oxidation; Carbon monoxide; Chronoamperometry; Nanoparticles; Platinum; Glassy carbon
1. Introduction Catalytic and electrocatalytic properties of nanoparticles have been the subject of numerous investigations (see e.g. Ref. [1] and references therein). The incentives have been twofold: first, to gain fundamental knowledge concerning the properties of small confined systems and, second, to develop better catalysts for practical applications, in particular, fuel cells. One of the most intriguing questions in nanoparticle (electro)catalysis is how the reaction kinetics and mechanisms are influenced by the particle size and morphology. In the present paper we continue our efforts to examine the electrocatalytic properties of model Pt nanoparticles. Armed with the experimental procedure of supporting nanometer sized Pt nanoparticles on the surface of glassy carbon (GC) described in Refs. [2,3], we study CO monolayer oxidation at Pt/GC electrodes. CO oxidation is a model surface reaction both in
* Corresponding author. Tel.: /49-89-289-12538; fax: /49-89-28912536. E-mail address: [email protected] (E.R. Savinova).
catalysis and electrochemistry, which is also relevant to the development of CO tolerant anodes for direct methanol and PEM fuel cells. We study CO monolayer oxidation under potentiodynamic (CO stripping) as well as potentiostatic conditions (chronoamperometry). Since the rate of CO monolayer oxidation is affected both by potential and time, investigation of the reaction under potentiostatic conditions is of particular interest and offers information on the reaction dynamics and mobility of the adsorbates on the metal surface. This paper is in memory of Weaver, whose work has to a large extent determined the progress of modern interfacial electrochemistry. This concerns also his significant contribution to the understanding of electrochemical properties of nanoparticles and nanoclusters (see e.g. Refs. [4 /7]).
2. Experimental Solutions were prepared from Milli-Q water (18 MV cm), H2SO4 (puriss. Suprapur, Merck). GC rods (7-mm diameter) were obtained from Alfa (Johnson Mattey)
0022-0728/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0728(03)00198-0
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and Sigradur. GC was pre-activated via anodic oxidation in 0.1 M H2SO4 at 2.23 V (reversible hydrogen electrode, RHE) for 3 min. To prepare Pt/GC electrodes a defined volume of H2PtCl4 aqueous solution (2.6 / 105 M in 6.5 /105 M HCl) was deposited on a support and dried in air. Then the samples were reduced in a quartz reactor in a hydrogen flow at 250 8C for 2 h. Thus prepared electrodes were cooled down in pure nitrogen or argon and immediately immersed in Milli-Q water to avoid their contamination. Electrochemical measurements were carried out in three-electrode Pyrex cells at 219/1 8C in Ar. The potential was controlled using an Autolab PGSTAT30 potentiostat. Pt foil was used as a counter electrode and a mercury sulfate electrode (MSE) Hg/Hg2SO4/0.1 M H2SO4(aq) connected to the cell via a Luggin capillary */as a reference electrode. All the potentials below are given vs. the RHE scale (EMSE /0.73 V vs. RHE). Prior to the measurements, Cl ions, which could remain on the surface after the preparation procedure, were desorbed from the electrode at approximately 0 V and the potential was cycled between 0.03 and 1.23 V at a 0.1 V s 1 scan rate to obtain a stable cyclic voltammogram (CV). To avoid contamination of the electrodes all the measurements were performed only with freshly prepared samples. If not otherwise stated, for CO oxidation experiments CO was bubbled through the electrolyte for 15 min at 0.1 V, then dissolved CO was removed by purging the solution with Ar for 35 min. The real surface area of the bulk polycrystalline Pt(pc) electrode was determined using hydrogen upd assuming 210 mC cm 2 per Hupd monolayer. By comparing the CO stripping with the Hupd charge CO coverage uCO was estimated as approximately 0.7. The latter is in agreement with the CO coverage reported for Pt surfaces [8]. However, application of the same procedure to the Pt/GC electrodes resulted in uCO /1.2. Higher than unity CO coverage at Pt nanoparticles has been reported also by Friedrich et al. [9]. uCO /1 might result from binding more than one CO molecule per Pt site, which is characteristic of Pt carbonyl clusters [4,10,11]. However, it could also be a consequence of the uncertainty in the determination of the Hupd charge incurred by a significant contribution of the support charging current. Hence, in the following the real surface area of Pt nanoparticles was calculated on the basis of CO stripping, assuming 420 mC cm 2 per CO monolayer. Current densities are referred to a square centimeter of Pt. The electrodes were examined with transmission electron microscopy (TEM) using a JEM-100C and high resolution transmission electron microscopy (HRTEM) using a JEM-2010 instrument. The samples for electron microscopy were prepared by scraping the active catalyst layer off the surface. Size distributions of
Pt particles were reconstructed from TEM data and used for calculation of the square weighted mean particle size d¯S (ai ni di2 =ai ni )1=2 :/
3. Results 3.1. Characterization of the Pt/GC electrodes Fig. 1 shows a typical HRTEM image of the Pt/GC sample, which shows that Pt nanoparticles are not agglomerated and are uniformly distributed on the surface. Analysis of multiple probes under different magnifications proved that thorough preparation of the Pt/GC electrodes using the chemical deposition method described above indeed offered Pt nanoparticles with a very high degree of dispersion and zero agglomeration extent. Depending on the Pt loading, the average particle size d¯S increased from approximately 1.3 at 0.9 mg cm 2 to 2.8 nm at 5.1 mg cm 2 (Pt loading is given in microgram Pt per geometric surface area of GC). A typical particle size distribution for a Pt/GC sample is given in Fig. 2. As confirmed by microdiffraction, Pt nanoparticles exhibit a face centered cubic (fcc) structure. The inset in Fig. 1 shows a high resolution electron micrograph of a representative Pt nanoparticle. The metal lattice is clearly discernable, although it is somewhat blurred by the structure of the GC support described e.g. in Ref. [12]. As is evident from HRTEM, Pt particles have a
Fig. 1. HRTEM image of Pt/GC, Pt loading 5.1 mg cm2. The insert shows a magnified 4.6 /4.6 nm2 image of a representative nanoparticle.
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Fig. 3 represents a CO stripping voltammogram for the Pt nanoparticulate electrode at a sweep rate of
0.1 V s1 in comparison to the corresponding voltammogram of Pt foil. Two striking differences between Pt/ GC and Pt voltammograms can be observed. First, the onset of the CO stripping peak at 1.79/0.5 nm Pt nanoparticles on GC is shifted by approximately 0.1 V positive vs. that at a Pt(pc) electrode. Second, the CO stripping peak at the nanoparticulate electrode is very wide so that stripping of the full monolayer of CO from Pt nanoparticles is achieved only at approximately 1.2 V at a sweep rate of 0.1 V s1 vs. approximately 0.9 V at polycrystalline Pt. Decrease of the sweep rate results in a negative shift of CO stripping peak both for Pt/GC and for Pt foil, but does not cancel out the difference between them. A positive shift of the CO monolayer stripping has been previously observed by Takasu and coworkers for Pt nanoparticles deposited on GC [13] and Friedrich et al. for Pt particles on Au [9]. Comparison of CO stripping from a commercial ETek 20% Pt/Vulcan catalyst (see e.g. Ref. [14]) with the corresponding voltammogram of polycrystalline Pt foil reveals the same phenomenon: CO oxidation on Pt/ Vulcan requires a higher overpotential than on Pt(pc). It is interesting to compare the CO stripping voltammograms with the CVs in supporting 0.1 M H2SO4 electrolyte. The analysis of CVs of the Pt/GC electrode is complicated by the relatively high contribution of the support charging and redox transformations of the oxygen-containing groups (mainly quinone [12]) of the GC surface. Hence, in Fig. 4 we compare the CV of the Pt foil electrode with that obtained at Pt/GC after the subtraction of the current originating from the GC support. Although a wide peak in the Pt double layer
Fig. 3. CO stripping voltammograms in 0.1 M H2SO4 at a sweep rate of 0.1 V s 1 at: (a) Pt/GC; d¯S/ /1.79/0.5 nm; Pt loading 1.8 mg cm 2; (b) Pt foil.
Fig. 4. CVs in 0.1 M H2SO4 at a sweep rate of 0.1 V s 1 at: (a) Pt/GC after the support current subtraction; d¯S/ /1.79/0.5 nm; Pt loading 1.8 mg cm2; (b) Pt foil.
Fig. 2. Typical size distribution for a Pt/GC sample. Pt loading 3.6 mg cm 2, d¯S/ /1.59/0.5 nm.
facetted rather than a round shape. The specific surface area of 1 /2 nm nanoparticles is very high and exceeds 100 m2 g1 (Pt). 3.2. CO stripping voltammetry
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region of Fig. 4a shows some contribution of the GC support, the Pt features can readily be recognized. The onset of the Pt surface oxidation does not seem to be noticeably affected by the decrease of the particle size, while the oxygen desorption peak at the Pt nanoparticles is shifted approximately 0.2 V negative. The latter suggests that oxygen is adsorbed more strongly at Pt nanoparticles than at bulk Pt. A negative shift of the oxygen desorption peak for Pt nanoparticles supported on GC has also been observed in Ref. [15]. Small Pt particles demonstrate a lower amount of strongly adsorbed hydrogen (Fig. 4a) in agreement with Ref. [16]. 3.3. CO monolayer oxidation at constant potential Fig. 5 shows typical current transients for the CO monolayer oxidation obtained at nanoparticulate Pt/GC electrodes after stepping the potential from the CO adsorption to the increasing oxidation potentials Eox. As one may see, the current transients exhibit a initial steep decay followed by a current maximum (or a shoulder), which shifts towards shorter times with the increase in the oxidation potential. Friedrich et al. [9] have previously observed a dramatic difference between the kinetics of CO monolayer oxidation at Pt nanoparticles immobilized at a gold substrate and bulk Pt. Oxidation transients at 3 nm Pt nanoparticles on the Au substrate demonstrated exponential decay rather than the bell-shaped curve typical for bulk single crystalline [17 /19] and Pt(pc) electrodes [20]. A maximum showed up in the current transients only upon aggregation of Pt nanoparticles (average particle size estimated as 16 nm) [9]. This was interpreted by invoking an Eley /Rideal mechanism of CO adlayer oxidation with some solution species (e.g. interfacial water) at small Pt nanoparticles vs. a
Fig. 5. Current transients for CO monolayer oxidation at Pt/GC in 0.1 M H2SO4 after a potential step from 0.1 V (RHE) to (a) 0.73; (b) 0.76; (c) 0.78 and (d) 0.80 V RHE. d¯S/ /1.79/0.5 nm; Pt loading 1.8 mg cm 2.
Langmuir /Hinshelwood (L /H) mechanism at bulk Pt electrodes and large Pt particles. Keeping in mind the above data, special efforts have been undertaken to exclude the possible influence of nanoparticle agglomeration. Potential step measurements were performed at different Pt/GC samples. The latter were thoroughly examined with TEM after the experiments in order to detect possible particle agglomeration. The results of these measurements prove undoubtedly that the maximum in the current transients is characteristic of Pt nanoparticles finely dispersed on GC. It is important to note however that a pronounced current maximum could be observed only in a certain window of the oxidation potentials. At high Eox it merged into the initial current decay, while at low Eox it became indistinguishable from the current tailing (see Fig. 5). It has recently been noticed by Lebedeva et al. [18] that contamination of Pt(1 1 1) electrodes may result in asymmetric current transients. We must clearly point out that the striking differences observed in the present paper between CO monolayer oxidation on Pt nanoparticles and flat surfaces were induced by the particle size rather than by the presence of impurities (e.g. chloride ions resulting from the preparation procedure) or other artefacts. Indeed, we have found that other conditions (including the preparation procedure) being equal, the properties of Pt particles approached those of flat surfaces as their size increased. Systematic investigation of the effect of the particle size on electrocatalytic properties of Pt is in progress and will be subject of forthcoming publications [21]. To elucidate the nature of the initial current decay, the current transients were recorded both on Pt/GC and on bare GC electrodes (pretreated in the same way as Pt/ GC). As one may see from Fig. 6, the currents at Pt/GC and at bare GC support decay in a similar manner, testifying that the initial current decrease is mainly concerned with the GC support rather than with Pt nanoparticles. In order to understand the origin of the current decay let us estimate the double layer (DL) charging time constant tDL for Pt/GC. We estimate the upper limit of the capacitance of oxidized GC from the CV CGC /200 mF. Assuming that the resistance between the working electrode and Luggin capillary in 0.1 M H2SO4 should not exceed 1 /2 V, we estimate tDL / RCGC :/200 /400 ms. Hence, the charging current should drop by a factor of 20 at t/3t 5/1.2 ms. It is thus obvious that in the time range of interest (from 0.1 to 10 s) the DL charging current must be negligible and we suppose that the main contribution to the initial current decay is the oxidation of the surface groups at the GC surface. Hence, the current transients may be represented as a superposition of a decaying current, originating from the GC support, and a curve with a maximum (Imax) at
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Fig. 6. Current transients following a double potential step from 0.1 V to 0.68 V (2 s) and then 0.73 V (RHE) (a) at Pt/GC after the adsorption of a CO monolayer at 0.1 V for 15 min and purging with Ar for 35 min at the same potential; (b) after the same experimental protocol but at the bare GC support; (c) at Pt/GC in the absence of CO and (d) difference current obtained by subtraction of (c) from (a). d¯S/ /1.79/ 0.5 nm; Pt loading 1.8 mg cm 2. A pre-pulse to 0.68 V was made in order to decrease the background current.
t/tmax corresponding to CO oxidation at Pt nanoparticles (Fig. 6d). The maxima in the current transients strongly suggest that CO oxidation at nanoparticles, similarly to bulk Pt electrodes, proceeds via a L /H reaction. It should be pointed out, however, that in contrast to essentially symmetric transients exhibited by bulk single and polycrystalline Pt electrodes [17 /20], those observed at Pt nanoparticles are rather asymmetric, exhibiting a steep rise at t B/tmax and very slow decay at t /tmax. Fig. 7 shows current transients for Pt/ GC in comparison to those observed at Pt foil at selected potentials. It is interesting that at lower oxida-
Fig. 7. Current transients for CO monolayer oxidation at Pt/GC (solid line) and Pt foil (dashed line) at 0.73 (a) and 0.83 (b) V RHE. d¯S/ / 1.79/0.5 nm; Pt loading 1.8 mg cm2.
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tion potential (0.73 V) tmax is shifted to larger values and current relaxation at t /tmax is much slower at Pt nanoparticles than at bulk Pt(pc). Meanwhile, at a high oxidation potential of 0.83 V CO monolayer oxidation is completed essentially in the same time interval at nanoparticles and bulk Pt electrode. As is evident from Fig. 8, jmax and tmax for Pt nanoparticles depend on Eox exponentially, similarly to bulk Pt(pc) [20] and Pt single crystal electrodes [17 /19]. The slope of log tmax vs. Eox varied from (1039/8 mV) 1 to (899/15 mV) 1, and the slope of log jmax vs. Eox */ from (1269/5 mV)1 to (989/11 mV) 1, while the Pt loading increased from 1.8 to 2.5 mg cm 2. Note that as revealed by TEM, the particle size distribution at the Pt/ GC electrode with the higher loading was broader and shifted somewhat towards a larger particle size. The latter may account for a considerably shorter tmax (Fig. 8a) and larger jmax (Fig. 8b). Comparison of the experimental slopes with the literature data will be given in Section 4. Fig. 9 shows the evolution of the charge densities q during CO monolayer oxidation at Pt nanoparticles calculated from the current transients at different Eox. It is remarkable that q at nanoparticles increases instantaneously, while at bulk Pt an induction period is observed at lower Eox. It is worth mentioning that at long time q approached 410 mC cm 2, corresponding to a full monolayer of adsorbed CO (COads). This confirms the validity of the background current subtraction.
Fig. 8. log tmax (top) and log jmax (bottom) vs. Eox for Pt/GC electrodes with different Pt loadings: (1) 1.8 mg cm 2 Pt, d¯S/ /1.79/ 0.5 nm; (2) 2.5 mg cm2 Pt, d¯S/ :/1.9 nm.
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ing cases can be distinguished depending on the mobility of CO on the surface. (I) The first limiting case is based on the assumption that the surface mobility of COads is very high and thus COads and OHads species are intermixed. This may be addressed as a simple L /H mechanism (the so-called ‘mean-field approximation’ [23]) with the reaction rate being proportional to the product of OHads and COads coverages (W 8/uOHuCO). The model predicts symmetric current transients with the maximum corresponding to the conversion of half of the CO monolayer. This model has been considered in a number of publications [23,24] and has been shown to describe CO monolayer oxidation at (stepped) single crystal Pt surfaces adequately [18,19]. Assuming simple L /H kinetics, in a mean-field approximation the reaction rate W can be expressed as: W k2 uOH uCO k2 uCO (1uCO ) Fig. 9. Evolution of the charge after potential steps from 0.5 V RHE to different oxidation potentials Eox for (a) Pt foil: Eox set at 0.73 V (1) and 0.83 V (2) and (b) Pt/GC: Eox set at 0.78 V (1), 0.80 V (2), 0.83 V (3) and 0.88 V (4). Current transients were corrected to the background current recorded under the same conditions in the absence of a CO monolayer. d¯S/ :/1.9 nm. Pt loading 2.5 mg cm 2. CO adsorption potential 0.5 V (RHE).
4. Discussion 4.1. Models of CO monolayer oxidation We start the discussion section by considering the mechanism of CO monolayer oxidation. According to the present understanding, oxidation of adsorbed CO on Pt electrodes proceeds in a L/H reaction between COads and surface oxygen-containing species, which is usually formulated as OHads1, to form CO2: H2 O(sol)SUOHads H (sol)e COads OHads 0 CO2 (gas)H (sol)e 2S
(1) (2)
Here S is a free site on the surface. It has recently been proposed [22] that reaction (2) proceeds through a chemical rate-determining (rds) step (2a) followed by fast electron transfer (2b): COads OHads 0 COOHads COOHads 0 CO2 (gas)H (sol)e 2S
(2a) (2b)
A number of approaches have been suggested in order to describe CO monolayer oxidation at Pt and bimetallic electrode surfaces quantitatively. Essentially two limit1 Although there is no direct indication that OHads rather than Oads is involved in the oxidation process.
k1 k1 k1 k2 uCO
(3)
where k1, k 1 and k2 stand for the rate constants of OH adsorption, OHads desorption and OHads/COads interaction, respectively. k1 and k 1 obey the Butler /Volmer law and comprise the water and proton concentrations near the surface, respectively: a1 FE 0 (4) k1 k1 exp RT (1 a1 )FE 0 k1 k1 exp (5) RT k2 may or may not be a function of the electrode potential, depending on whether COads oxidation proceeds in the electrochemical (Eq. (2)) or chemical step (Eq. (2a)). The effect of the electrode potential on the overall reaction rate W is determined by the ratelimiting step. Reaction rate equations for a number of limiting cases along with the corresponding Tafel slopes are given in Table 1. Although Eq. (3) is oversimplified, it allows some preliminary conclusions on the Tafel slope and its variation depending on the potential interval and type of rds. Note that Koper et al. predicted a slope variation from approximately 40 mV dec 1 for low to 119 mV dec1 for high Eox [23]. (II) The second model is based on an assumption that the reaction occurs via a nucleation-and-growth mechanism with OHads islands nucleating at the free sites of the CO-covered metal surface and growing via consumption of COads at the rims of the islands. Then, if the OHads formation is fast, the surface will be covered with OHads islands separated by the areas of still unreacted COads. The OHads islands will ultimately overlap, the reaction rate being controlled by nucleation, growth and
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247
Table 1 Rate equations and Tafel slopes for COads oxidation in the mean-field approximation for limiting cases a Case OH adsorption
COads oxidation
rds Rate equation
A
Chemical (Eq. (2a))
2a
k2a(k01/k01)uCO(1/uCO)exp(FE /RT )
(2.3RT )/F (2.3RT )/((1/a2)F ) 2.3RT /a1F
120 120
Reversible
Tafel slope
B
Reversible
Electrochemical (Eq. (2))
2
k02(k01/k01)uCO(1/uCO)exp((1/a2)FE /RT )
C
Irreversible
Chemical (Eq. (2a))
1
k01(1/uCO)exp(a1FE /RT )
D
Irreversible
Electrochemical (Eq. (2))
1
k01(1/uCO)exp(a1FE /RT )
2.3RT /a1F
E
Irreversible
Chemical (Eq. (2a))
2a
k2auCO(1/uCO)
0
2
k02uCO(1/uCO)exp(a2FE /RT )
2.3RT /a2F
F
Irreversible a b
Electrochemical (Eq. (2))
Tafel slope for ai /0.5/mV dec 1 b 60 40
0 120
The rates of electrode reactions are assumed to obey the Butler /Volmer law; the parameters have their usual meaning. At 298 K.
overlap of the OHads islands. Two cases have been described in the two-dimensional nucleation-andgrowth model. These are so-called (a) instantaneous nucleation, with the reaction rate W proportional to: 2 2 2 W 8NN kG t exp(pNN kG t)
(6)
and progressive nucleation, with the reaction rate proportional to: 2 2 2 3 W 8NN kN kG t exp(pkN kG t =3)
(7)
Here NN is the number of nucleation centers, kN is the nucleation rate constant and kG is the growth rate constant. The nucleation-and-growth model has been shown to give an adequate description of the current transients at Pt (pc) [20] and single crystal Pt(1 0 0), Pt(3 1 1) and Pt(1 1 1) electrodes [17]. Usually the current (and as a consequence the charge) rises after an induction period, which is interpreted in terms of progressive nucleationand-growth (cf. Fig. 9 top and bottom). The ‘classical’ nucleation-and-growth model is based on an assumption of immobile COads, the reaction front propagating along the surface via growth of OHads islands provided by reaction of OHads and COads at the rims of the islands. Formation of COads islands on Pt surfaces (which presumes restricted CO mobility) has been proven e.g. by FTIR spectroscopy by Chang and Weaver [25]. Koper et al. have shown that deviations from the simple nucleation-and-growth model may occur [23] if COads diffuses on the surface resulting in intermixing COads-covered and OHads-covered areas. The relations between the nucleation-and-growth model and mean-field approximation have been analyzed also by Korzeniewski and Kardash [26]. (III) It has recently been pointed out, in particular by Petukhov et al. [24], Koper et al. [27] and Baltruschat and co workers [28] that surface defects (e.g. steps) play a key role in the CO monolayer oxidation. Thus, yet another model has been proposed [27], which like the first one, assumes fast surface diffusion of COads, but
implies the reaction to take place only at specific surface sites, i.e. between OHads adsorbed at steps and COads diffusing to the steps from the terraces. This model provides very good quantitative agreement with the experimental chronoamperometric results of COads monolayer oxidation at stepped Pt [n(1 1 1) /(1 1 1)] surfaces. Note that in the case of fast diffusion one would expect symmetric current transients, which have indeed been observed at Pt surfaces [27]. 4.2. CO stripping voltammograms Let us now analyze how our experimental data can be related to the above models and start the discussion by considering CO stripping voltammograms at Pt foil and Pt nanoparticles. It is instructive to compare the evolution of COads (uCO) and oxygen species (uOH)
Fig. 10. Evolution of the adsorbate coverages on (a) Pt/GC and (b) Pt foil. Solid lines */uCO as calculated from the stripping voltammograms of Fig. 3a and b; dash-dotted lines */(1/uCO); dashed lines */uOH as calculated from the CVs of Fig. 4a and b. See text for details.
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coverage on Pt surface. Fig. 10 shows uCO calculated from the stripping voltammograms of Fig. 3 along with uOH calculated from the CVs of Fig. 4 (in the absence of CO). uOH is calculated neglecting the actual complex mechanism of Pt surface oxidation assuming OHads as a sole oxidation product. This gives an upper limit of uOH in the presence of CO. Indeed, uOH in the presence of COads will be either equal to (if (i) there are enough ‘free’ sites at the electrode surface and (ii) the reaction of COads and OHads is slow) or less than uOH in the supporting electrolyte as plotted in Fig. 10. Comparison of the uCO and uOH shows that the CO monolayer oxidation at Pt foil starts below the visible onset of surface oxidation and is completed at uOH /0.15. This testifies that very low oxygen coverage is necessary to sustain the reaction.2 This actually means that the ‘classical’ nucleation-and-growth model, which presumes immobility of COads, is not applicable for Pt(pc). Indeed, if COads were immobile, the only way for the reaction to proceed would be by consumption of COads molecules adjacent to the rims of OHads islands. In this case the amount of free sites on the surface would be negligible and uOH :/1/uCO. Since uOH in the presence of COads is expected to be less than that on a bare Pt electrode, one would expect uOH plotted in Fig. 10 to be ]/1/uCO. However, this is not the case, which strongly suggests that COads is mobile on Pt(pc) in agreement with the conclusions of Refs. [23,24,26,28,29] and the reaction of COads and OHads is faster than OHads propagation over the surface (otherwise one would expect an absence of ‘free’ sites on the electrode). At Pt nanoparticles the decay of a uCO is shifted very much positive on the potential scale. However, 1/uCO still considerably exceeds uOH, which implies that oxidation of a CO monolayer on Pt nanoparticles requires the mobility of COads. The latter, as will be shown below, may be considerably lower on Pt nanoparticles than on flat surfaces. The fact, which has to be rationalized, is why the onset of CO monolayer oxidation on Pt/GC is shifted positive vs. that on Pt (Figs. 3 and 10). Special experiments performed at submonolayer COads coverages proved that it did not stem from higher uCO at Pt nanoparticles vs. Pt(pc). We suggest that the positive shift of the CO oxidation onset is due to slow interaction of OHads and COads at Pt nanoparticles. The latter might be a consequence of strong bonding of oxygen to the nanoparticle surfaces [30], which has indeed been confirmed by a number of techniques, in particular in situ EXAFS and XANES [31,32].
2 This may also be due to participation of some adsorbed ‘activated’ water molecules in the CO oxidation.
4.3. CO monolayer oxidation at a constant potential We start the discussion with the Tafel slopes calculated from the plots of log tmax vs. Eox, which for Pt nanoparticles were equal to 90/100 mV dec 1. Since tmax can be related to the rate constant of COads oxidation [19,23,33], the slope of log tmax vs. Eox has been extensively discussed in the literature for Pt electrodes of different crystallographic orientations: from perfect single crystals [17 /19,33,34] to stepped Pt[n (1 1 1) /(1 1 1)] [19] and polycrystalline Pt [20]. It has recently been pointed out, however, that it may not be totally correct to determine the Tafel slope from a plot of log tmax vs. Eox because not only CO oxidation, but also reaction initiation, contributes to the dependence of tmax on the oxidation potential [18]. Most of the authors agree that at low potential values (between :/0.6 and 0.7 V RHE) the Tafel slopes lie between 60 and 80 mV dec 1. Thus, Love and Lipkowski [17] found 80 mV dec 1 for Pt(1 0 0), Pt(3 1 1) and Pt(1 1 1); McCallum and Pletcher [20] found 60 mV for Pt(pc) (see Fig. 8); Lebedeva et al. [19] found approximately 80 mV dec 1 for stepped Pt single crystals. Tafel slopes between 60 and 80 mV dec 1 observed for Pt single crystals have been interpreted in terms of the limiting case (A) (Table 1) [19]. However, at higher potential values (above :/0.7 V RHE) there is considerable disagreement between the literature data: from 240 mV dec1 for single crystals [17] to 180 mV dec 1 for polycrystalline [20] and 70 /80 mV dec 1 for stepped Pt single crystals [19]. Despite considerable experimental error, the Tafel slopes of 90 /100 mV dec 1 observed in this work hardly conform to the limiting case (A), and are, rather, consistent with the limiting cases (C), (D) or (F) (Table 1) and imply irreversible formation of OHads. The latter is in line with the shape of the CV in supporting electrolyte (Fig. 4). However, neither irreversible formation of OHads, nor slow interaction of COads with OHads is sufficient to account for the asymmetry of the current transients. Indeed, slow reaction of OHads and COads alone would result in a shift of the current transients towards longer time rather than in a change in their symmetry. Apparently, the shape of the current transients is the result of a combination of a number of effects induced by the particle size. uCO at the tmax calculated from the charge consumed in the electrode reaction varied for nanoparticles between :/0.93 and 0.8 (assuming maximum uCO /1). This is at variance with the results at single crystals, which usually exhibit current maxima around uCO /0.5. Note that the nucleation-and-growth models predict maxima at uCO /0.51 for progressive and at 0.61 for instantaneous nucleation [28]. The asymmetry of the current transients at Pt/GC is not explained by any of
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Fig. 11. Normalized current (I /Imax) vs. (t /tmax) 1/2 for Pt/GC electrode. d¯S/ :/1.9 nm, Pt loading 2.5 mg cm2. CO adsorption potential 0.5 V (RHE).
the mechanistic models described above. The current decay cannot be fitted either by exp(/t2/t), characteristic of the instantaneous growth model or by exp(/t3/t ), typical for progressive nucleation. As one can see from Fig. 11, the current decay follows t 1/2 dependence in a wide interval of normalized time from t/ tmax /3 to 20, which is indicative of a diffusion limitation. It should be mentioned however that at longer times and/or lower Eox the current decay deviates from t 1/2 dependence. One may argue that the tailing of the current transients towards longer times is the consequence of a superposition of current transients from particles of different sizes rather than a characteristic of a nano-
Fig. 12. Current transients at Pt/GC following a multiple potential step program (a) 0.1 V0/0.68 (2 s) 0/0.73 V (200 s) and (b) 0.1 V 0/ 0.68 (2 s) 0/0.73 V (20 s) 0/0.63 V (10 s) 0/0.73 V (RHE) (200 s). d¯S/ / 1.79/0.5 nm; Pt loading 1.8 mg cm2. The insert shows the potential programs.
249
particle of the average size. To rule out such a possibility multiple step experiments have been performed as follows. The electrode potential was stepped from the adsorption (0.1 V (RHE)) to the oxidation potential Eox for a limited time (we will call it a pre-pulse), then to a lower potential value where no oxidation occurs and then back to the Eox. The results of one of these experiments with Eox /0.73 V (RHE) are presented in Fig. 12. One may see that when approximately 20% of the CO monolayer is oxidized in the pre-pulse, tmax of the pulse shifts to smaller values. Meanwhile, our recent experimental results prove that oxidation of COads at large nanoparticles proceeds faster with a current maximum shifted to smaller values. Thus, if there were a fraction of large Pt particles on Pt/GC, CO adsorbed on their surface would have been oxidized in the prepulse, and tmax of the curve b of Fig. 12 would have shifted positive vs. tmax of the curve a. Since this is not the case, we infer that the current transients presented in this paper are indeed characteristic of CO monolayer oxidation at Pt nanoparticles with an average size of 1/2 nm. Another important conclusion from Fig. 12 is that the difference of the shapes of the current transients between Pt foil and Pt/GC electrodes is not the result of higher uCO at nanoparticles. 4.4. Model of CO monolayer oxidation at Pt nanoparticles Taking into account the above observations, we suggest the following qualitative model of COads oxidation at 1/2 nm Pt nanoparticles. We assume that in the potential interval of CO monolayer oxidation OHads formation occurs only at specific active sites3, which might be, e.g. particle vertices and/or edges. This is in agreement with the mechanism of COads oxidation at stepped Pt [n(1 1 1) /(1 1 1)] single crystals, where the reaction has been suggested to occur at steps [18,19]. It is interesting to mention that for an ideal 2 nm cuboctahedral Pt nanoparticle the ratio of vertices amounts to approximately 7% and edges to approximately 40% [32]. At high oxidation potentials OHads formation at these sites is fast, accounting for a steep rising of the current. As COads adjacent to these ‘active’ sites is oxidized, further evolution of the reaction would be possible either (i) via growth of the OHads islands, or (ii) diffusion of more distant COads to the ‘active sites’ occupied by OHads. If the former were true, the current transients would have exhibited a shape typical for the nucleation-and-growth mechanism, which is not the case. If the latter were true and the diffusion of the CO molecules were fast, the transients would have been symmetric as shown in Ref. [27]. We were able to 3
At least in the potential interval explored: 0.73 VB/Eox B/0.88 V.
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account for the highly asymmetric shape of the transients only under an assumption that the OHads formation is spatially confined and the reaction proceeds via slow diffusion of COads to the active sites occupied by OHads. The COads diffusion coefficient can be estimated roughly from the current transients as follows. At 0.83 V oxidation of the CO monolayer proceeds on the time scale of 5 /10 s (Fig. 7), the current decay following a t 1/2 dependence (Fig. 11). The diffusion mean free path D at 1 /2 nm Pt nanoparticles should not exceed 1 /2 nm. Thus from D /2Dt we can estimate the upper limit of DCO as 4 /10 15 cm2 s 1. On Pt(1 1 1) in UHV DCO was estimated as approximately 10 9 cm2 s1 at 300 K and high uCO [35]. In electrolyte solutions DCO is supposed to be lower due to the presence of water molecules and adsorbed anions. Koper et al. have recently obtained 10 11 cm2 s 1 as a lower limit for the COads diffusion at stepped Pt [n (1 1 1) /(1 1 1)] [27]. The results of this work suggest that decrease of the Pt particle size to 1 /2 nm may lead to a considerable decrease of the COads diffusion coefficient (by three to four orders of magnitude). We would like to refer to the work of Sinfelt and co workers, who used 13C-NMR to investigate the mobility of COads on alumina supported Pt clusters [36]. These authors have found that the activation energy for the COads diffusion depends on the Pt particle size, increasing from 279/2 kJ mol 1 for the large particles of 10 nm in diameter (in agreement with the values reported for CO on Pt(1 1 1)) to 449/4 kJ mol 1 for small clusters 1.2 nm in size. At 300 K such a difference in the activation energy would correspond to approximately three orders of magnitude difference in the apparent diffusion coefficients. Sinfelt and coworkers attributed the influence of the Pt cluster size on the mobility of CO to the large fraction of the edge and corner sites in small Pt clusters. The binding energy of COads at low coverage has also been found to increase when the cluster size decreases below 5 nm [37 /39]. This result has been attributed to a stronger adsorption on low co-ordinated atoms (edges and corners), the proportion of which increases with the decrease of the cluster size. Hence, a diminution of the DCO with the decrease of the Pt particle size observed in Ref. [36] and in this work may result from higher ratio of low co-ordination sites at small metal particles. However, the possibility of an electronic effect cannot be ruled out for small nanoparticles and needs further investigation. A possible origin of such an effect could be contraction of the lattice parameter usually observed for 1 /2 nm metal nanoparticles and attributed to large surface strain induced by their high curvature. According to the literature data the difference of the lattice parameter between 1 and 2 nm nanoparticles and bulk metal may reach 5% [38]. Meanwhile, Nørskov and coworkers [40] have shown
that strain results in a considerable change of chemisorption and catalytic properties. A likely reason for that is a shift in the d-band center. It is interesting to mention that DCO on Ru-modified Pt(1 1 1) has been estimated by Friedrich et al. as ]/4/ 1014 cm2 s 1 [29], which is considerably smaller than for the bare Pt(1 1 1) surface. Baltruschat and coworkers [28] report a still lower value of 3/1017 cm2 s 1 as an upper limit for DCO on Ru-decorated Pt(6 6 5). Slow diffusion of COads on the Ru-modified surface has been confirmed also by model simulations [27]. It has been attributed to the COads binding energy gradient at the Ru-decorated surface, in particular the high activation barrier of CO diffusion from ‘distant’ Pt sites to the electronically modified Pt sites adjacent to the rims of Ru islands. In a way this model may apply also to bare Pt nanoparticles, which due to high surface heterogeneity may exhibit a high COads binding energy gradient. More work is currently being done to (i) understand the bonding and reactivity of COads at nanoparticles using FTIR spectroscopy and (ii) establish a mathematical model of COads oxidation at nanoparticle surfaces (M. Eikerling, TU Muenchen).
5. Conclusions One to two nanometers sized Pt nanoparticles have been prepared using a chemical precursor method via adsorption of H2PtCl4 on a pre-oxidized GC surface and subsequent reduction in the hydrogen flow at 250 8C. Pt nanoparticles exhibit an fcc structure and narrow particle size distribution as revealed by HTREM. CO monolayer oxidation at GC supported Pt nanoparticles has been studied using potential sweep and potential step techniques. Both methods shown significant differences between the reaction at bulk electrodes and Pt nanoparticles. Oxidation of a full CO monolayer during CO stripping is achieved at nanoparticles at much more positive potentials (at :/1.2 V (RHE) vs. :/0.9 V (RHE) at polycrystalline Pt at a sweep rate of 100 mV s 1). Current transients at nanoparticulate electrodes demonstrate a steep rising front, a maximum at uCO / 0.8 to 0.9, and a long tail following t 1/2. A tentative model is suggested, which assumes spatially confined formation of OHads at active sites and slow diffusion of the COads molecules to the active sites, where they are oxidized. These active sites can be attributed tentatively to the particle corners and/or edges. The upper limit of the COads surface diffusion coefficient is estimated on the basis of the current transients as :/4 /1015 cm2 s 1.
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Acknowledgements The authors express their gratitude to M. Eikerling (TU, Muenchen) for stimulating discussions, thorough reading of the manuscript and many valuable comments, to U. Stimming (TU, Muenchen) for his interest in the work and to S. Schreier, S. Weinkauf and M. Heinzlik (TU, Muenchen) for their kind assistance with TEM measurements. We would also like to thank F. Maillard (TU, Muenchen) for performing experiments on the influence of COads coverage and sweep rate on the CO stripping transients at Pt/GC and Y.-Y. Tong (Georgetown University) for providing useful references. Financial support from DFG under contract Sti 74-8-3 is gratefully appreciated.
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