Composition and activity of high surface area PtRu catalysts towards adsorbed CO and methanol electrooxidation—

Electrochimica Acta 47 (2002) 3693 /3706 www.elsevier.com/locate/electacta

Composition and activity of high surface area PtRu catalysts towards adsorbed CO and methanol electrooxidation* A DEMS study /

Z. Jusys 1, J. Kaiser, R.J. Behm * Department of Surface Chemistry and Catalysis, University of Ulm, D-89069 Ulm, Germany Received 7 January 2002; received in revised form 1 May 2002

Abstract The activity of unsupported high surface area PtRu catalysts of different Pt:Ru ratio towards preadsorbed CO and the methanol oxidation reaction (MOR) in 0.5 M sulfuric acid solution has been studied at room temperature using differential electrochemical mass spectrometry (DEMS). Adsorbed CO monolayer stripping DEMS experiments show that (i) the contribution of double-layer charging increases with the Ru content, reaching up to 50% of the total stripping charge at approximately 40 at% Ru; (ii) the onset of COad oxidation for Ru containing catalysts starts at approximately 0.3 VRHE, about 0.15 V more negative compared with Pt; and (iii) both the onset potential and the peak potential for COad-stripping depends on the Ru content, reaching the most negative values at medium Ru contents, for 20 /60 at% Ru. COad-stripping was furthermore used to determine the active surface area of the PtRu catalysts. Based on the electron yield of 1.9 electrons per CO2 product molecule COad can be identified as the stable adsorbed product of methanol dehydrogenation on all PtRu catalysts. Potentiodynamic methanol oxidation experiments show a clear effect of the chemical composition of the PtRu catalysts. The onset of CO2 formation occurs most negative, at slightly below 0.3 VRHE, for PtRu catalysts containing about 40 /60 at% Ru. In the technologically interesting potential regime of 0.4 /0.5 VRHE PtRu catalysts containing small or medium amounts of Ru (15, 42, 46 at%) are most active, while at more positive potentials more Pt rich catalysts containing approximately 15 at% Ru are most active. These activities refer to the inherent chemical activity obtained by normalizing the oxidation current to the active surface area determined by COad-stripping. Without normalization, the Pt rich catalyst (15 at% Ru) would appear as the most active one, underlining the necessity to properly account for variations in the active surface area. For all compositions methylformate formation starts at potentials around 0.5 VRHE, about 0.2 V positive of the onset of CO2 formation, indicating that at low anodic potentials complete oxidation of methanol to CO2 is preferred. The high current efficiency of both the PtRu and the Pt catalysts for the MOR, with electron yields of slightly above six electrons per CO2 product molecule, is attributed to readsorption and complete oxidation of partially oxidized reaction intermediates, which is more facile for electrodes with high catalyst loadings, as used here, than for electrodes with lower loadings or smooth electrodes. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: PtRu catalysts; COad-stripping; Methanol oxidation; DEMS

1. Introduction Despite of an intense and ongoing search for novel catalytic materials of a comparable or even improved electrocatalytic performance PtRu alloys still remain the

* Corresponding author. Tel.: /49-731-502-5450; fax: /49-731502-5452 E-mail address: [email protected] (R.J. Behm). 1 Permanent address: Institute of Chemistry, A. Gosˇtauto 9, 2600 Vilnius, Lithuania.

superior material for anode catalysts in low-temperature polymer electrolyte fuel cells (PEFC), both for PEFCs operated by H2-rich reformate, i.e. by CO contaminated fuel gases generated by reforming of organic fuels, or for PEFCs driven by direct oxidation of methanol (direct methanol fuel cell (DMFC)). A key problem in these reactions is the gradual poisoning of the catalyst by CO, either by adsorption of CO trace impurities present in the feed or by CO generation during the stepwise dehydrogenation of methanol. The superior catalytic activity of PtRu alloys in these reactions was explained

0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 3 3 9 - 0

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by a bifunctional mechanism, according to which the electrocatalytic oxidation of adsorbed CO is facilitated at lower anodic overpotentials than on Pt due to reaction with OHad species on oxophyllic surface atoms such as Ru, which can be generated by electrochemical dissociation of water on these sites at more negative potentials compared with pure Pt [1,2]. As a result the catalyst composition and in particular its surface composition should be of outmost importance for its performance. This has indeed been demonstrated for CO electrooxidation and methanol electrooxidation in model studies on massive PtRu alloy electrodes (CO: [3 / 5], CH3OH: [4,6 /8]), PtRu surface alloys (CH3OH: [9,10]), Ru modified Pt electrodes (CO: [2,11,12], CH3OH: [1,13]) and high surface area carbon supported PtRu catalysts (CH3OH: [14 /17]). Adjacent Pt
/Ru sites and boundaries between domains of Pt and Ru surface atoms were proposed as active sites for CO electrooxidation [2], and this effect was recently reproduced in a kinetic Monte Carlo study [18]. The optimum surface composition for methanol oxidation varied between Pt/ Ru /9:1 and 1:1, depending on the nature of the substrate and the reaction conditions such as temperature, methanol concentration, potentiodynamic or potentiostatic measurements [1,6,7,9,10,13/15]. Systematic studies on the influence of the catalyst composition on the reaction kinetics and on the product distribution, however, are still scarce, in particular for well-defined high surface area catalysts, and the apparent discrepancy between the results for high surface area PtRu catalysts and massive PtRu bulk alloys or Ru modified Pt surfaces is still unresolved. This is a topic of an ongoing study in our laboratory, where the influence of the composition of unsupported high surface area PtRu catalysts on the activity for CO electrooxidation and on the activity and product distribution in methanol electrooxidation was investigated by differential electrochemical mass spectrometry (DEMS). This method allows to follow the rate and total amount specified products directly, without interference from other contributions such as HUPD, double-layer charging, OH adsorption/oxide formation etc., which are present in measurements of the electrochemical current. We here communicate results of potentiodynamic measurements on the electrooxidation of CO (‘COad-stripping’) and methanol (‘methanol adsorbate stripping’) and bulk methanol electrooxidation performed at room temperature. Potentiostatic measurements of the MOR at elevated temperatures will be presented in a forthcoming publication [19]. Following a brief description of the experimental setup and procedures we first determined the electrochemically active surface area of the PtRu catalysts with varying composition by electrooxidation of a saturated CO monolayer. These measurements also allow to determine the contribution of double-layer charging to

the total CO oxidation charge for varying Ru contents and to characterize the CO oxidation behavior, in particular the onset of COad oxidation and the stripping peak potential. Next we investigate the dehydrogenation of adsorbed methanol on the PtRu catalysts, by oxidation of the adsorbed stable residues resulting from the dehydrogenation process. In the third section we focus on bulk methanol oxidation, with special emphasis on the yields of oxidation products (CO2) and secondary products (methylformate) as a function of the electrode potential and Pt/Ru ratio. We compare the activities of high surface area catalysts with different Pt/Ru ratios on the basis of active surface area specific currents, normalizing the measured current signals versus the active surface area of the catalyst particles determined by preadsorbed CO monolayer stripping.

2. Experimental 2.1. Catalyst synthesis and characterization The catalysts were synthesized via a modified Adams route [20,21], which together with their characterization will be described in more detail in a forthcoming publication [19]. In short, a mixture of the noble metal nitrates was fused with an excess of sodium nitrate at 500 8C, followed by extensive washing with water and reduction of the mixed oxides by hydrogen gas (nominal composition, based on the ratio of precursors: 0, 20, 40, 50, 60 and 80 at%). Based on TEM and nitrogen adsorption BET measurements the mean size of the catalyst particles was between 3 and 5 nm. The overall Ru content of the catalysts was 0, 15, 42, 46, 61 and 82 at%, respectively, according to energy dispersive X-ray emission (EDX) data. Only part of the Ru was present as PtRu alloy. X-ray diffraction (XRD) measurements indicated that the alloy phase contained 0, 14, 17, 18, 40 and 23 at% of Ru, while the other part remained in oxidic phases. The active surface area of the catalysts was determined by electrooxidation of a preadsorbed CO monolayer (see Section 3.1). 2.2. Electrode preparation The thin-film electrodes for the DEMS measurements were prepared on mirror-finish polished glassy carbon disks (Sigradur G from Hochtemperatur Werkstoffe GmbH, 9 mm in diameter), using a recently developed thin-film electrode method [22]. The catalyst film, which was centered on the glassy carbon surface (diameter ca. 6 mm), was deposited by pipetting 20 ml of an aqueous (Millipore) catalyst suspension (2 mg ml 1) onto the substrate, yielding a catalyst loading of 140 mg cm 2. After evaporating the solvent under a stream of argon, an aqueous Nafion† solution was pipetted onto the

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catalyst film and dried in the Ar stream to fix the particles to the substrate. The resulting Nafion film is sufficiently thin to exclude significant transport limitations (ca. 0.1 mm thickness). The electrode was then mounted into the thin-layer flow-through DEMS cell. Prior to the stripping/oxidation experiments the electrode was cycled several times between 0.05 and 0.85 V (scan rate: 50 mV s 1) to warrant stable electrode conditions.

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acid solution, which was injected by a syringe through a separate inlet (2 ml solution). Methanol was preadsorbed at 0.26 V for 10 min from 0.5 M sulfuric acid solution containing 0.1 M methanol. After adsorption the thin-layer cell was carefully flushed with pure 0.5 M sulfuric acid and the adsorbed species were oxidized during a positive going potential scan. Continuous methanol oxidation was carried out in 0.5 M sulfuric acid solution containing 0.1 M CH3OH.

2.3. Thin-layer DEMS cell 3. Results and discussion The dual thin-layer flow-trough cell used in the present study was described in detail elsewhere [23,24]. It consists of two thin-layer compartments connected via four capillaries. Gaseous species produced at the electrode in the first compartment are transported by the electrolyte flow to the second compartment, where they can evaporate through a porous Teflon membrane (Scimat† , 60 mm thick, 50% porosity, 0.2 mm pore diameter) into the mass spectrometer. The electrolyte flow rate of approximately 5 ml s 1 is ensured by the hydrostatic pressure of the electrolyte in the supply bottle, which in turn is constantly purged with Ar. Two Pt wires serve as counter electrodes, a saturated calomel electrode is used as reference (all potentials given in this article, however, are referred to the reversible hydrogen electrode (RHE)). The experiments were performed at room temperature, with a potential scan rate of 10 mV s 1. In order to avoid dissolution of Ru the anodic limit in the potential scans was set to 0.85 V for all Ru containing catalysts. 2.4. DEMS set-up The DEMS set-up is based on a differentially pumped two-chamber system with a Balzers QMS 112 quadrupole mass spectrometer, a Pine Instruments potentiostat and a computerized data acquisition system. A high sensitivity of the set-up was achieved by positioning the ion source of the mass spectrometer between the differentially pumped pre-chamber and the main chamber containing the mass spectrometer. Further experimental details about the DEMS set-up are given elsewhere [25]. 2.5. Chemicals and solutions The electrolytes were prepared from high purity methanol (Merck, p.a.), superpure sulfuric acid (Merck, suprapur), and ultrapure water (Millipore Q). High purity gases (Ar: MTI Gase, N 6.0, CO: Messer / Griesheim N 4.7) were used for purging the electrolyte or preparing CO saturated solutions, respectively. CO was preadsorbed at a constant electrode potential of 0.11 VRHE for 10 min from CO-saturated 0.5 M sulfuric

3.1. Preadsorbed CO monolayer stripping on high surface area PtRu catalysts In the first section we present combined electrochemical and mass spectrometric DEMS measurements on the oxidative stripping of preadsorbed CO monolayers from the PtRu catalysts with varying Ru content in order to (i) determine their real surface area, (ii) evaluate contributions from double-layer charging to the COadstripping charge, and (iii) characterize the electrocatalytic behavior with respect to CO oxidation by the onset of COad oxidation and the stripping peak potential. This aims at elucidating trends in the electrocatalytic behavior of these bimetallic catalysts with increasing Ru content. A series of such DEMS measurements, following COad-stripping on high surface area PtRu catalysts with different Pt:Ru ratio, is shown in Fig. 1. The signals in this figure are normalized versus the electrochemically active surface area of the catalysts found from the COad-stripping data as described below. They will be discussed in the following. 3.1.1. Determination of the active surface area The commonly used method to determine the active surface area of metal electrodes by H adsorption in the underpotential deposition potential (UPD) regime, which works well for Pt electrodes, is not well suited for PtRu electrodes due to the poorly separated underpotential and overpotential hydrogen evolution regions and undefined double-layer contributions. Therefore the electrochemically active surface area of the different catalysts is determined from the total amount of adsorbed CO, assuming that the coverage of the saturated CO adlayer (number of CO molecules per metal surface atom) is identical on Pt and PtRu surfaces [26]. The adsorbed CO on the different surfaces is determined from the total amount of CO2 product generated during COad-stripping, by integration of the m /z/44 mass spectrometric signals in Fig. 1b. Problems caused by ill-defined contributions from double-layer charging etc. to the total Faradaic COad-stripping charge on Ru

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content may partly be attributed to the lower atomic mass of Ru as compared with Pt, which leads to a higher number of metal (surface) atoms for similar loadings. The active surface areas were used to normalize the Faradaic and mass spectrometric currents shown subsequently (Figs. 1 and 4d /f; Figs. 6 and 7d /f), both for CO oxidation as well as for the MOR (see paragraph Section 3.2.1). 3.1.2. Contribution of double-layer charging to the COadstripping charge Contributions from double-layer charging and reversible OH adsorption/oxide formation to the COadstripping charge are determined by comparing the mass spectrometric signal for CO2 product formation, QMS, with the electrochemical signal QF for the different catalysts (Fig. 2). They correspond to the difference between QF and the true COad-stripping charge QF;CO. The latter can be determined via QF;CO /2QMS/K *, where K * is the calibration constant of the DEMS setup for CO adlayer oxidation and 2 is the number of electrons per CO molecule oxidation to CO2. The

Fig. 1. Preadsorbed CO monolayer oxidation on high surface area PtRu catalysts in 0.5 M H2SO4 solution at room temperature: CV (a) and MSCV (b) (catalyst loading 140 mg cm 2, potential scan rate 10 mV s 1, electrolyte flow rate 5 ml s 1). Currents are normalized to the active surface area of the respective catalysts.

containing electrodes are avoided this way. Absolute values of the respective surface areas are calculated by comparison with the mass spectrometric signals of Pt electrodes, whose surface area was determined via independent measurements of the HUPD adsorption charge (for more details see ref. [25]). The Faradaic and mass spectrometric charges measured for COadstripping and the resulting active surface areas obtained for the different catalysts are listed in Table 1. The increase in the active surface area with increasing Ru

Fig. 2. Relative contribution of COad-stripping charge (DQCO: m) and double layer charge (DQdl: k), and relative Ru surface concentration uRu (j) as a function of Ru content in high surface area PtRu catalyst derived from the DEMS data in Fig. 1. uRu is calculated according to Q (0:35  0:6 V) ref. [14] ðuRu  dl Þ; for Pt uRu is set to zero. QH

Table 1 Faradaic and mass spectrometric (MS) charges for oxidation of preadsorbed CO on PtRu catalysts, the calibration constant K *, and the active surface area found from the COad-stripping data (Fig. 1) Nominal composition (at%)

Real composition (at%)

Faraday charge (mC)

MS charge (nC)

105  K *

Active surface area (cm2)

100% Pt; 0% 80% Pt; 20% 60% Pt; 40% 50% Pt; 50% 40% Pt; 60% 20% Pt; 80%

100% Pt; 0% 85% Pt; 15% 58% Pt; 42% 54% Pt; 46% 39% Pt; 61% 18% Pt; 82%

3.10 4.73 11.16 12.10 11.15 14.77

21.6 27.0 41.1 45.6 42.8 51.0

1.40 1.14 0.74 0.75 0.77 0.69

7.5 9.4 14.9 17.2 15.9 18.5

Ru Ru Ru Ru Ru Ru

Ru Ru Ru Ru Ru Ru

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calibration constant K * is derived from the Faradaic and mass spectrometric charges of the CO2 signal, respectively, determined for the pure Pt catalyst (K * / 2QMS/QF) (note that double-layer charging contributions to the total stripping charge on Pt, which had previously been determined to be about 20% [27,28], are not taken into account in this calculation). The resulting contributions of double-layer charging (filled circles) and COad oxidation (open circles) to the Q  QF;CO  electrochemical charge, DQdl;rel  F QF 100 (%) and DQCO,rel /100/DQdl (%), respectively, are plotted in Fig. 2 versus the Ru content in the catalysts, taking the pure Pt catalyst as reference. The double-layer contribution to the total COad-stripping charge grows with increasing Ru content, reaching up to 50 at% approximately 40 at% Ru content and remains approximately constant with further increase in the Ru content. The contribution of the COad oxidation decays correspondingly with increasing Ru content. The general increase in double-layer charge to higher Ru contents agrees perfectly with recent findings for preadsorbed CO oxidation on an electrochemically deposited PtRu alloy [23] and on unsupported PtRu ETEK catalyst (nominal composition: Pt54Ru46) [29], where the contribution from double-layer charging to the total charge amounted to about 50%. A qualitatively similar behavior can also be concluded from the variation of the apparent calibration constant K * versus Ru content, which was reported for the oxidation of preadsorbed CO on porous, electrodeposited PtRu alloys [8] (see also our data in Table 1). By combined Auger electron spectroscopy (AES) and electrochemical measurements on Ru modified Pt(111) Lin et al. had shown that for Ru modified Pt surfaces the double-layer charge, measured in the potential range between 0.35 and 0.6 V, increases linearly with the amount of Ru deposited [11]. The results are in good agreement with earlier suggestions by Watanabe and Motoo, according to which the charge in the doublelayer region of the CV, Qdl, normalized by the charge in the HUPD region, QH, can be used as a direct measure for the Ru coverage on Ru modified Pt electrodes [1]. The apparent discrepancy to the non-linear relation in our data can result from different effects. First and most important the surface composition of the particles may deviate substantially from their bulk composition [30]. In fact, the oxidative treatment during catalyst formation will lead to a pronounced Ru surface enrichment, due to the much stronger Ru
/O bonds as compared with Pt
/O, and it is likely that this is not completely reversed during subsequent reduction. In that case the Ru surface coverage will increase steadily at lower bulk concentrations, and saturate at higher bulk concentrations, where the surface is mostly covered by Ru and

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Ru-oxide. Also, the presence of Ru oxides on the surface may play a role [19]. It should be noted here that Pt and Ru oxides can be fully or partially reduced in the electrochemical environment [30]. Incomplete reduction of RuO2 can lead to enhanced capacitive contributions in the double-layer region [31]. Second, in the study by Lin et al. Ru is present in small islands on the Pt surface rather than forming a dispersed surface alloy [32]. Finally, the procedure for determining the (normalized) double-layer charging, Qdl/QH, was different. Lin et al. calculated this from the charge in the double-layer region (0.35 /0.6 V), whereas in our measurements contributions from the entire potential range for COadstripping are included. For comparison we also evaluated the normalized double-layer charge from the base voltammograms of the different PtRu catalysts in the same way as described above, in the potential range between 0.35 and 0.6 V, and calculated the ratio Qdl:QH (full squares in Fig. 2). Overall, these values exhibit a similar dependence on the Ru content as those derived from the difference between mass spectrometric and Faradaic charge (full squares). Therefore, contributions from the different evaluation schemes are insignificant and the difference to the results by Lin et al. and to Watanabe and Motoo must be due to segregation effects or oxide formation. In general, the large contributions from double-layer charging/OH adsorption will lead to significant overestimates (up to 2-fold) of the active surface area of PtRu catalysts if they are based on the Faradaic charge for COad oxidation. 3.1.3. Activity of PtRu catalysts in COad oxidation The electrocatalytic activity of the PtRu catalysts determines the position and shape of the CO oxidation peaks in Fig. 1. It can be approximately described by the onset potential and the potential of the maximum rate (peak potential). Due to the absence of background contributions in the mass spectrometric signals the onset potential can be evaluated with high sensitivity from the mass spectrometric DEMS measurements. COad-stripping on a pure Pt catalyst (Fig. 1a) leads to a sharp and symmetric COad oxidation peak centered at approximately 0.73 V. Such peaks are characteristic also for COad electrooxidation on low-index Pt single crystal surfaces, with slightly different peak potentials for the different orientations [33 /35], but were observed also for carbon supported Pt catalysts [36,37]. In contrast, previous studies on polycrystalline Pt reported a distinct shoulder at higher potentials or at least a broadening of the peak [3,38,39], which was attributed to contributions from different crystallite orientations. The corresponding mass spectrometric CO2 formation peak (Fig. 1b) is shifted positive by approximately 20 mV compared with the faradaic current signal due to the time constant (1 /2 s) of the thin-layer flow-through DEMS cell. These

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measurements also show the well known ‘pre-wave’ in COad-stripping current [3], which for the present catalysts starts at about 0.45 V. Interestingly, the onset potential is much higher than the very low value of 0.15 V we determined recently for carbon supported Pt catalyst [37]. It resembles more that of polycrystalline Pt [3,40]. A more detailed discussion of the COadstripping behavior of pure Pt can be found in our previous study [37]. The presence of 15 at% of Ru in the catalyst shifts the onset potential of COad oxidation approximately 0.2 V more negative compared with pure Pt and results in a structured oxidation peak with two maxima centered at approximately 0.5 and 0.58 V, respectively, and a shoulder at approximately 0.7 V (Fig. 1a). Since according to the XRD and EDX data [19] the catalysts employed in the present study contain Pt, PtRu and Ru(RuO2) phases, the shoulder at 0.7 V is attributed to CO oxidation on Pt domains. The shape and position of the remaining peak agrees well with that observed for COad electrooxidation on well-characterized, massive PtRu alloy electrodes: For PtRu alloys with low (7 /8 at%) Ru content a previous study reported a shift of the COad-stripping peak potential to more negative values and multiple overlapping features, causing a broad stripping peak [3]. The shift was interpreted by those authors as evidence of COad surface diffusion to a limited number of active Ru sites. Multiple features were found also for COad electrooxidation on Ru-decorated stepped Pt single crystal surface and were equally explained as a result of lateral migration of CO towards reactive Ru-decorated steps [12,41,42] (in ref. [12] the authors assumed adsorbed OH to be the migrating species). More insight came from Kinetic Monte Carlo simulations, which showed that multiple features occur for either limited mobility of COad species or on surfaces containing larger Pt and Ru islands [18]. In light of the XRD results mentioned above the latter is the more likely explanation for the multi-peak structure in our measurement. With increasing Ru content the total width of the peak remains about constant, but the first peak becomes more pronounced, while the second one decays in intensity. This results in peaks with a pronouncedly asymmetric shape for PtRu catalysts with approximately equal Pt and Ru content, with a steep increase in the COad oxidation current in the positive-going scan, while the subsequent current decay at more positive potentials is slower (Fig. 1). Although not resolved, the long decay of the signal clearly indicates contributions from the peak at approximately 0.58 V and possibly also from the shoulder at 0.7 V mentioned above. Finally, at very high Ru contents (Pt18Ru82), the peak becomes very broad and shifts back to more positive values, peaking at 0.65 V.

The results resemble those reported for PtRu bulk alloys with comparable composition, with the only difference that the peaks are about double as wide in our case [3]. The larger peak width can again be explained by a more pronounced phase separation in the present catalysts [18]. Based on its width and position the COad-stripping peak from the Ru rich Pt18Ru82 comes close to that for pure Ru in ref. [3], the shift to more positive potentials is even more pronounced. This similarity indicates that the Pt18Ru82 catalyst is practically Ru covered. For comparison with similar plots in a previous study [4] the peak potentials and their shifts with Ru content, as determined from the Faradaic current signal (Fig. 1a), are plotted in Fig. 3(circles). The plot illustrates the reduction in peak potential for the PtRu catalysts for small and medium Ru contents as compared with pure Pt or the most Ru rich catalyst. The mass spectrometric data in Fig. 1b largely follow these trends, but allow a more sensitive evaluation of the onset potential for COad-stripping, due to the absence of double-layer charging contributions. This is better

Fig. 3. Peak potential (a) and onset potential (b) for CO2 formation as a function of Ru content in high surface area PtRu catalysts for COadstripping (k, data from Fig. 1), oxidation of adsorbed methanol dehydrogenation products (‘methanol adsorbate stripping’) (I, data from Fig. 6), and methanol bulk oxidation (^, data from Fig. 7). (a) m: Potential of the second peak for COad-stripping from Pt85Ru15 catalyst; (b) 2: onset potential for methylformate formation in methanol bulk oxidation.

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potential) evolution of the rate. This would lead to smaller currents for a lower number of reactive sites, as observed experimentally. Also, the shifts in the onset potential with Ru content are included in Fig. 3(open circles). 3.2. Methanol oxidation reaction (MOR) at room temperature

Fig. 4. Magnified MSCV traces for the onset of CO2 formation from preadsorbed CO (a), preadsorbed methanol (b) and bulk methanol oxidation (c) on PtRu catalysts of different composition (magnified data from Fig. 1b, Fig. 6b, Fig. 7e).

visible from higher magnification mass spectrometric cyclic voltammograms (MSCV) for the different PtRu catalysts, which are presented in Fig. 4a. They clearly show that the onset potential for CO2 formation on PtRu catalysts of different composition shifts approximately 0.15 V more negative in PtRu catalysts (0.3 V), compared with pure Pt (0.45 V), but does not depend significantly on the Ru content. As already mentioned, the onset potential for the pure Pt catalyst is significantly more positive than that of a carbon supported Pt catalyst, which we found to be around 0.15 V [37]. The Ru content is important only for the size and slope of the increasing signal, with the highest slopes for about equal Pt and Ru contents. Qualitatively, this is consistent with a mechanistic picture where COad oxidation starts at Pt/Ru boundaries and where transport effects, via surface diffusion of COad, are negligible for the onset of COad oxidation, but play a role for the time (and

In this section we present results of potentiodynamic DEMS measurements*/Faradaic current and mass spectrometric currents of the reaction product CO2 (m /z/44) and of the secondary reaction product methylformate (m /z /60) */for the MOR on high surface area PtRu catalysts of different composition. Methylformate is formed by reaction between methanol and formic acid, which is a reaction intermediate in the MOR [43 /45] (direct MS detection of the reaction intermediates formic acid and formaldehyde is complicated due to overlap with the mass spectra of methanol (educt) and CO2 (main product)). The activity of the PtRu catalysts and the product distribution in the MOR are determined for the different catalyst compositions. By investigating both the remaining stable adsorption products after methanol adsorption at 0.26 V and subsequent electrolyte exchange to methanol-free sulfuric acid solution (0.5 M), and the continuous methanol oxidation process we try to separate between activity for dehydrogenation and that for continuous methanol oxidation. Similar to the procedure for CO oxidation we first evaluated an unsupported Pt high surface area catalyst, which was synthesized in a similar way, as reference for subsequent measurements on the PtRu catalysts. 3.2.1. Methanol oxidation on unsupported Pt catalyst The methanol oxidation characteristics on an unsupported high surface area Pt catalyst (Pt loading 140 mg cm 2) are shown in Fig. 5. The signals are normalized versus the active surface area determined by COadstripping on the same catalyst (see Section 3.1.1). The MOR rate dependence on the electrode potential resembles closely that reported in previous measurements on massive and high surface area Pt electrodes (see ref. in [25,46] as well as [44]). The onset of the reaction (in the positive-going scan) at 0.55 V, the peak position at
/0.81 V, and the pronounced hysteresis between positive-going and negative-going scan agree perfectly with the reaction characteristics observed on a carbon supported Pt catalyst [25]. Only the active surface area normalized total charge in the positivegoing oxidation peak is significantly higher on the unsupported Adams-type Pt catalyst (9.7 mC cm 2 active surface area at a Pt loading of 140 mg cm 2 geometric electrode area) than on the carbon supported Pt catalyst (5.2 mC cm 2 active surface area at a Pt

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positive-going and negative-going scan is much more pronounced. This difference and the intensity ratio between the signals for CO2 and methylformate of about 2000:1 are similar to findings on a carbon supported Pt catalyst [25,44] (note that this ratio is not a direct measure for the efficiency of this side reaction, since it includes also contributions of the different vapor pressures and ionization probabilities of the two species detected). The much more pronounced difference between the intensities in the positive and the negativegoing scan indicates that on Pt methylformate formation and hence formic acid formation is more efficient at lower potentials than at the higher potentials for methanol oxidation encountered in the positive-going scan [25].

3.2.2. Oxidation of stable methanol adsorption products on PtRu catalysts The decomposition of methanol on the PtRu catalysts upon adsorption at low potentials and the nature of the reaction product were investigated in similar experiments as reported previously for carbon supported Pt catalysts [25], by exposing the catalyst to methanol

Fig. 5. Methanol oxidation on high surface area Pt catalyst in 0.5 M H2SO4/0.1 M CH3OH solution at room temperature: CV (a) and MSCVs (b, c) (catalyst loading 140 mg Pt cm2, potential scan rate 10 mV s 1, electrolyte flow rate 5 ml s 1). Currents are normalized to the active surface area of the catalysts.

loading of 28 mg cm 2 geometric electrode area). This is tentatively explained by the higher amount of complete oxidation on the unsupported catalyst, due to readsorption of desorbed reaction intermediates, which will be discussed in more detail later (see Section 3.2.3). The decay of the reaction at higher potential is generally attributed to the formation of Pt oxides, which are inert for methanol dehydrogenation. Correspondingly, the hysteresis in the negative-going scan is explained by the well-known hysteresis in the formation/reduction of oxides on Pt [47]. This is also responsible for the narrower peak width and slightly higher peak height in the negative-going scan: The higher rate of oxide reduction at the higher overpotential for this process causes a more rapid increase in the MOR during the negative-going scan, and the decline due to CO poisoning is also much faster due to the low rate of CO oxidation at these potentials. The mass spectrometric (m /z/44) signal closely follows the voltammetric signal. The mass spectrometric signal for methylformate formation (m /z /60) also follows the CV, but in this case the difference in the integrated charge between

Fig. 6. Oxidation of methanol dehydrogenation products (‘methanol adsorbate stripping’) on high surface area PtRu catalysts of different composition in 0.5 M H2SO4 solution at room temperature: (a) CV and (b) MSCVs traces of m /z /44 (CO2) (catalyst loading 140 mg cm 2, potential scan rate 10 mV s 1, electrolyte flow rate 5 ml s 1). Currents are normalized to the active surface area of the respective catalysts.

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solution at the adsorption potential (10 min, 0.26 V), subsequent potential change to more cathodic potential (0.05 V) and electrolyte exchange to methanol-free sulfuric acid solution (0.5 M), and finally sweeping the potential to 0.85 V. During the sweep the Faradaic current and mass spectrometric currents were monitored. The adsorption potential is sufficiently high that site blocking of Pt sites due to HUPD is avoided [25], but still below the onset potential for CO oxidation. At the even lower potential during electrolyte exchange (0.05 V) CO oxidation is quantitatively excluded. The resulting signals (electrochemical current and mass spectrometric current for m /z /44), normalized to the active surface area, are plotted in Fig. 6. The number of electrons consumed per CO2 product molecule, calculated from the ratio of Faradaic and mass spectrometric currents, gives information on the nature of the stable adsorbed decomposition products. For all catalysts we obtain values of about 1.9 electrons per CO2 molecule, similar to our findings on carbon supported Pt catalysts [25]. Hence, also for the PtRu catalysts the stable, adsorbed decomposition product resulting from methanol adsorption at cathodic potentials (0.26 V) is identified as COad. The onset of CO2 formation is, except for the most Ru rich catalyst, practically independent of the composition of the PtRu catalysts slightly below 0.30 V (see magnified traces in Fig. 4b). For Pt18Ru82 it appears to be slightly higher. The onset potentials determined here (see Fig. 3b, empty squares) are in the range of those measured for COad-stripping after saturation from CO adsorption. This contrasts the trends observed for carbon supported Pt catalysts, where the onset potential for methanol adsorbate stripping exhibited a pronounced shift to negative potentials compared with that found upon CO adsorption, which was attributed to the lower CO saturation coverage reached by methanol dehydrogenation as compared with CO adsorption [25]. Apparently, such effects play no major role for the PtRu catalysts. The peak potential shows subtle shifts with varying Ru content, with the most negative potentials reached at medium Ru contents (40 /60 at% Ru) (Fig. 3a, empty

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squares). More interestingly, for low to medium Ru contents the peak position is shifted positively compared with COad-stripping, supporting the above conclusion that CO coverage effects play no major role here. A last result of these experiments to be discussed is the amount of adsorbed CO formed during methanol dehydrogenation at the adsorption potential, before the surface is poisoned for further reaction by the CO adlayer. The COad coverages resulting from methanol decomposition on the different catalyst compositions, relative to the saturation coverage of the respective catalyst determined by COad-stripping, are listed in Table 2. For all catalyst compositions the saturation coverages achieved by methanol dehydrogenation are significantly below those obtained by CO adsorption (uCO,rel B/1), similar to findings for supported Pt catalysts [25]. The CO saturation coverage shows a pronounced composition dependence, with the highest coverage of (uCO,rel :/0.67) obtained for the Pt-rich Pt85Ru15 catalyst. With increasing Ru content the CO saturation declines steadily, reaching a value of below 0.2 at the most Ru rich catalyst (Pt18Ru82: uCO,rel :/0.17) (see Table 2). On a first view this behavior agrees well with the decreasing activity for methanol oxidation at higher Ru contents, which was reported by Gasteiger et al. for PtRu bulk electrode, based on cyclic voltammetry measurements [3]. In a microscopic picture, this result can be explained by assuming that methanol dehydrogenation requires a critical ensemble of several Pt surface atoms-values of 4 Pt atoms were suggested previously [3,46], which become increasingly rare for higher Ru concentrations. This does not explain, however, why the CO formed in the dehydrogenation process does not diffuse to Ru sites, thereby allowing dehydrogenation of another methanol molecule on the Pt ensemble. The CO adsorption experiments clearly show that CO adsorption on these sites is possible, as evidenced by the much higher CO saturation coverage. In recent calculations it was even found that on a PtRu surface CO adsorption on Ru sites is energetically favorable compared with adsorption on Pt sites [48]. Surface diffusion of CO should also be fast enough to

Table 2 Faradaic and mass spectrometric (MS) charges, electron yield per product CO2 molecule and adsorbate coverage for oxidation of the stable, adsorbed dehydrogenation product on PtRu catalysts after methanol adsorption and dehydrogenation (Fig. 6) Nominal composition (at%) Real composition (at%) Faraday charge (mC) MS charge (nC) Electron yield per CO2 CO Coverage uCO,rel (ML) 80% 60% 50% 40% 20%

Pt; Pt; Pt; Pt; Pt;

20% 40% 50% 60% 80%

Ru Ru Ru Ru Ru

85% 58% 54% 39% 18%

Pt; Pt; Pt; Pt; Pt;

15% 42% 46% 61% 82%

Ru Ru Ru Ru Ru

0.0264 0.0095 0.0076 0.0067 0.0036

0.209 0.076 0.056 0.047 0.027

1.8 1.8 1.9 2.0 1.9

The adsorbate coverage is obtained by normalizing to the active surface area determined by COad-stripping.

0.67 0.39 0.34 0.36 0.17

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allow migration to adjacent Ru sites, as discussed in the CO stripping experiments. A possible explanations may be blocking of Ru sites by adsorbed OH species at the higher potential used for methanol dehydrogenation, but this needs to be verified. Overall, a convincing explanation for this composition effect, in a microscopic picture, is still lacking.

3.2.3. Methanol oxidation activity of PtRu catalysts of varying composition Continuous methanol oxidation measurements on the PtRu catalysts (loading 140 mg cm 2, positive-going scan only), similar to that on Pt in Section 3.2.1, are plotted in Fig. 7. In this case Fig. 7a /c show uncorrected, absolute currents. According to these measurements the highest currents for the potentiodynamic oxidation of methanol at technically relevant low anodic potentials (0.4 /0.5 V) are achieved on catalysts of approximately equal Pt:Ru ratio (Pt58Ru42, Pt54Ru46), while at lower or higher Ru contents the MOR rate decreases (Fig. 7a). Since the metal loading was identical in all measurements, the data in Fig. 7a are analogous to the mass specific currents (activity per 1 mg metal of the catalyst) used for characterization of catalysts in fuel cell measurements. In order to separate contributions from possible variations in the surface area from chemical effects, we also show surface area specific current values in Fig. 7d,

which are normalized with respect to the active surface area determined by COad-stripping. The surface area normalized activity of the PtRu catalysts in the MOR (Fig. 7d) differs substantially from their nominal activity, uncorrected for the differences in surface area (Fig. 7a). At the potentials of MOR onset (0.4 /0.5 V), the most active catalysts PtRu catalysts are those with low to medium Ru contents (Pt85Ru15, Pt58Ru42, Pt54Ru46), with no significant difference between these three, while pure Pt and the Ru-rich (Pt38Ru62, Pt18Ru82) alloys are significantly less (Pt38Ru62) or much less (Pt, Pt18Ru82) active under these conditions (potentiodynamic measurement, room temperature). For more positive potentials (0.6 /0.7 V) the Pt-rich Pt85Ru15 catalyst shows the highest electrocatalytic activity (Fig. 7d). At even higher, but technically irrelevant potentials pure Pt will be most active. Our findings are in good agreement with room temperature MOR data on well-characterized massive PtRu alloy electrodes and PtRu films. For the former, Gasteiger et al. found a higher catalytic activity on a Rupoor alloy (ca. 7 at% surface Ru) at more positive potentials (0.6 V), while at a less positive potentials (0.4 /0.5 V), at the onset of the MOR, two PtRu alloys with a higher Ru content (33 and 46 at% surface Ru) show a slightly better performance (potentiostatic measurements, room temperature, 0.5 M CH3OH) [6,7]. Similar trends were observed by Iudice de Souza et al.

Fig. 7. Methanol oxidation on high surface area PtRu catalysts of different composition in 0.5 M H2SO4/0.1 M CH3OH solution at room temperature: CV (a, d) and MSCVs traces of m /z/44 (CO2) (b, d) and m /z/60 (methylformate) (c, f) (catalyst loading 140 mg cm 2, potential scan rate 10 mV s 1, electrolyte flow rate 5 ml s 1). Fig. 7a /c: measured currents, Fig. 7d /f: surface area normalized currents.

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for the surface area normalized MOR activity of porous, electrodeposited PtRu films (potentiodynamic measurements, room temperature) [8]. They found that in the potential range between 0.4 and 0.5 V PtRu films with low to medium Ru contents (7 /33 at% Ru) were about equally active, while at more positive potentials (0.6 /0.7 V) the Pt75Ru25 film had the highest activity. For Ru-modified low-index Pt single crystal surfaces the highest catalytic activity was found for the Pt(111) face with a Ru deposit coverage of 0.29/0.05 ML (potentiostatic current densities after 30 min at 0.49 V) [13,49] or at coverages between 0.2 and 0.4 ML [9,10] (potentiostatic current densities after 300 s at 0.5 V). A rather similar result was obtained for Pt samples modified by Ru evaporation [9,10]. Finally, PtRu surface alloys, prepared by Ru deposition on Pt(111) under vacuum conditions and subsequent annealing, also showed an enhanced activity in the MOR, with the optimum composition around 0.2 ML Ru (20 min chronoamperometric, 0.5 V) [10]. Watanabe et al. obtained the highest current densities at low potentials (around 0.45 V) for medium Ru contents (36 /60 at%), both for PtRu alloy electrodes and for Ru modified Pt electrodes (potentiodynamic measurement, 40 8C, 1 M CH3OH) [1]. Above 0.65 V again the more Pt rich alloys and finally pure Pt become most active. The overall tendency from these data is that the surface area normalized methanol oxidation activity of PtRu catalysts and electrodes is highest for low Ru contents around 15 at% at more positive potentials (0.6 /0.65 V), while at the technically interesting potentials at the onset of the MOR (0.4 /0.5 V) catalysts with higher Ru contents (40 /50 at%) become equally active. Finally it should be noted that the optimum composition of PtRu catalysts in the MOR strongly depends on the reaction conditions. It had been demonstrated for PtRu alloy electrodes that the optimum surface composition shifts to higher Ru contents with increasing temperature [3]. We will show in a forthcoming paper [19] that for continuous methanol oxidation under potentiostatic conditions and at elevated temperatures (60 8C) the highest MOR activity on these high surface area PtRu Adams catalysts is achieved for catalysts containing about equal amounts of Pt and Ru, in good agreement with literature reports for the MOR on carbon supported high surface area PtRu catalysts under comparable conditions [14,15,50]. 3.2.4. Reaction characteristics of the MOR over PtRu catalysts of varying composition The data in Fig. 7 allow also a more detailed evaluation of the MOR characteristics of the PtRu catalysts. Similar to the stripping experiments in Fig. 6, the onset potential for CO2 formation falls into the same regime as that for CO2 formation from preadsorbed CO oxidation (see magnified traces in Fig. 4c). Its depen-

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dence on the catalyst composition, however, is more pronounced than in the other reactions, reaching the most negative values for PtRu catalysts containing approximately equal amounts of Pt and Ru. Compared with CO and methanol adsorbate stripping the onset potential in the MOR is more positive at low and high Ru contents, while it is slightly more negative at medium Ru contents, between 40 and 60 at% Ru. The small magnitude and varying direction of these shifts in onset potential indicate that also for the MOR CO coverage effects play a minor role for determining the onset potential on PtRu catalysts, different from our findings for carbon supported Pt catalysts [25]. Further information on the MOR characteristics comes from the formation of side products and secondary products, in this case from methylformate production, which can be directly followed by DEMS (Fig. 7c and f). While formaldehyde and formic acid formation are well known for Pt and PtRu electrodes/catalysts [51], very little is known about the relation between the MOR product distribution and the composition of PtRu catalysts. The data in Fig. 7c demonstrate that methylformate formation starts at about the same potential (0.5 V) for all PtRu catalysts and also the Pt catalyst. The onset of methylformate formation is shifted positively compared with the onset potential of CO2 evolution (0.1 /0.25 V, depending on Ru content). Its dependence on the catalyst composition is less pronounced than that for CO2 formation. The positive shift in the onset of methylformate formation can be explained in different ways, either by a more efficient oxidative dehydrogenation of formic acid relative to formic acid desorption in the low potential regime of each catalyst, leaving no time for formic acid desorption, or by rapid desorption of formaldehyde, resulting in low formic acid formation rates under these conditions. Unfortunately, the present experiments do not allow to distinguish between these possibilities. A delay between formic acid formation/desorption and subsequent homogeneous reaction to methylformate is highly unlikely, since for the Pt catalyst this reaction sets in instantaneously, together with the onset of CO2 formation. As expected from the shift in the onset potential the relative amount of methylformate formation increases with more anodic potentials. It is, e.g. higher for 0.7 V than for 0.6 V. The higher activity for methylformate formation at higher potentials differs distinctly from the behavior on Pt, where methylformate formation becomes active already before the onset of significant CO2 formation (see Section 3.2.1). The formation and desorption of weakly adsorbed, not fully oxidized reaction intermediates such as formaldehyde and formic acid leads to a loss in efficiency of the MOR. This can be described by the electron yield per consumed CH3OH molecule or per CO2 molecule formed. Since the amount of desorbing reaction inter-

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mediates cannot be quantified from these DEMS measurements and the number of consumed CH3OH molecules is unknown, the loss in efficiency will be characterized by evaluating the electron yield per formation of one CO2 product molecule. This can be determined from the ratio of Faradaic current and mass spectrometric charges, using the calibration factor K * for conversion of mass spectrometric charge into number of CO2 molecules (see Section 3.1.2) (while this method appears to be straightforward, it does not allow to separate and remove charge contributions from double layer charging and Ru oxidation, which cannot be specified exactly. Hence, the actual charge per CO2 molecule will be less than or equal to the numbers derived here). For complete oxidation of methanol to CO2 one would obtain six electrons per CO2 molecule. If part of the methanol is only partly oxidized, this number will increase. For instance, for carbon supported Pt catalysts electron yields of nine electrons per CO2 product molecule were reported, indicating a significant loss of efficiency due to desorption of reaction intermediates [25]. In the present experiments we obtain values of slightly above six electrons per CO2 for the PtRu catalysts, independent of their composition. A comparable value (6.9 electrons) is obtained also for the Adams type Pt catalyst. Apparently, the deviation from the much higher values for carbon supported Pt catalyst is more typical for the type of catalyst than for their composition. Tentatively we attribute that to a higher probability for readsorption on the unsupported catalysts [45]. If desorbed reaction intermediates can readsorb again on a different catalyst particle, where they can either be completely oxidized or desorb again, this increases the fraction of total oxidation and hence the electron efficiency. For high loadings such effects should be more pronounced than for low loadings. A more detailed study on such effects is currently underway. The remaining (smaller) difference between the Adams type Pt and PtRu catalysts, with slightly lower electron numbers per CO2 and hence higher current efficiencies for the latter ones, agrees well with the higher tendency for complete oxidation in the potential regime of the main MOR peak for the PtRu catalysts. Despite of the very different current efficiencies for the MOR on carbon supported Pt catalysts and the high surface area unsupported catalysts the methylformate signal is of comparable relative intensity in both cases. This can be understood by looking at the product balance. For carbon supported Pt catalysts it was shown that the electron yield can be explained by assuming 25% loss due to desorption at each oxidation step [25]. If this loss is reduced to around 10% in the present case, the methylformate signal would change by a factor of 2.5, which is within the data range. Also the loss in efficiency is compatible with our findings.

4. Conclusions Extensive DEMS measurements on unsupported high surface area PtRu catalysts of different composition led to the following conclusions on the activity of these catalysts towards CO adsorption /oxidation and methanol dehydrogenation/oxidation and the underlying reaction mechanism: 1) COad-stripping DEMS experiments allow to determine the chemically active surface area of the different PtRu catalysts, where the active surface area is defined as the surface area able to adsorb CO, and enable to discriminate between COad electrooxidation and double-layer charging effects. 2) For adsorbed CO monolayer stripping the contribution of double-layer charging increases with Ru content, reaching up to 50% of the total stripping charge at approximately 40 at% Ru. The onset of COad oxidation for 15 /60 at% Ru containing catalysts starts approximately about 0.3, 0.15 V more negative compared with Pt. The COad-stripping peak potential depends on the Ru content, the most cathodic onset potential is reached for about 50 at% Ru. In agreement with results from Kinetic Monte Carlo simulations and our XRD results the broad, structured peak shapes are attributed to larger domains of different surface phases. 3) The mechanism for methanol dehydrogenation on the PtRu and Pt catalysts is similar in so far as in all cases COad is the identified as the stable, adsorbed dehydrogenation product. The COad coverage resulting upon methanol adsorption and dehydrogenation at low potentials decreases with increasing Ru content. 4) The relative inherent chemical activity of the different PtRu catalysts for bulk methanol oxidation, determined by normalizing the measured MOR currents versus the active surface area of the catalysts, depends on the potential: at low anodic potentials (0.4 /0.5 V) PtRu catalysts of low (15 at%) and medium Ru contents (42, 46 at%) show about equal chemical activities, higher than that of the Ru rich catalysts (61, 82 at%) and pure Pt, while at more positive potentials (0.6 /0.65 V) the Pt-rich catalyst containing approximately 15 at% Ru is most active in the MOR. The use of normalized currents allows to discriminate between physical (surface area) and chemical (catalyst composition) effects. Without normalization to the active surface area catalysts with medium Ru contents (42, 46 at%) are most active also in the low potential regime. 5) On PtRu catalysts methylformate formation starts for all compositions at about 0.5, 0.1 /0.25 V positive of the onset of CO2 formation. Hence, at

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low anodic potentials complete oxidation of methanol (to CO2) is preferred. In contrast, on Pt catalysts methylformate formation is more active at less anodic potentials. 6) The distinctly higher electron efficiency of the high surface unsupported catalysts as compared with our recent results on carbon supported Pt catalysts, with slightly above six electrons per CO2 product molecule for all catalysts, is attributed to increased readsorption effects, due to the much higher loading in the present case. Desorbed, partially oxidized reaction intermediates such as formaldehyde or formic acid can readsorb and then be completely oxidized, reducing the probability for effective desorption of these intermediates. This increases the amount of complete oxidation and thus the current efficiency.

Acknowledgements We gratefully acknowledge K. Lasch and Dr. L. Jo¨rissen (Center for Solar Energy and Hydrogen Research, Ulm) for providing the catalysts. Financial support came from the Deutsche Forschungsgemeinschaft (Be1201/8-2), by the State of Baden Wu¨rttemberg, through the ‘Zukunftsoffensive Junge Generation’ , and by the German Ministry of Economy (BMWi), within the ‘Verbundprojekt DMFC-Brennstoffzelle’.

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