C Catalyst at High Temperatures and

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Methanol Oxidation Over a Pt/C Catalyst at High Temperatures and Pressure: An Online Electrochemical Mass Spectrometry Study M. Chojak-Halseid,† Z. Jusys, and R. J. Behm* Institute of Surface Chemistry and Catalysis, Ulm UniVersity, 89069 Ulm, Germany ReceiVed: August 6, 2010; ReVised Manuscript ReceiVed: NoVember 7, 2010

The influence of elevated reaction temperatures (up to 100 °C) on the characteristics of the methanol electrooxidation reaction over carbon-supported Pt catalysts was investigated by high-temperature/high-pressure differential electrochemical mass spectrometry (HT/HP DEMS), at up to 100 °C and 3 bar overpressure. The total reaction rate and that for CO2 formation were followed by potentiodynamic and potentiostatic measurements, performed under fuel cell relevant, but nevertheless well-defined, reaction and transport conditions. Activation energies for the overall methanol oxidation process and for the specific pathway to CO2 formation as well as the current efficiency for CO2 production were determined over a wide range of potentials and temperatures; the influence of the electrolyte flow rates and of the catalyst loading was tested. Most important, the apparent activation energies for CO2 formation were generally larger than those for the overall reaction, leading to an increasing selectivity for CO2 formation with temperature and complete conversion to CO2 at ∼100 °C reaction temperature under present reaction conditions. The resulting trends are discussed in a mechanistic picture, including transport effects. Consequences for applications in direct methanol fuel cells (DMFCs) are discussed. 1. Introduction Driven by applications in direct methanol fuel cells (DMFCs), the electrochemical oxidation of methanol has become one of the most intensely studied reactions in electrocatalysis research. The most important kinetic and mechanistic aspects of this complex electrocatalytic process as well as the mechanisms limiting the reaction kinetics are summarized in excellent reviews.1-6 Nevertheless, despite tremendous efforts in both experimental and theoretical studies, the state-of-the-art understanding of the methanol oxidation reaction (MOR) is still not sufficient to even semiquantitatively understand the reaction kinetics in a DMFC on a molecular scale. The MOR is kinetically limited by the slow dissociative adsorption (dehydrogenation) of methanol and by the formation of COad species,7 which block the surface. Their oxidative removal is only possible at potentials where the OHad species required for the reaction with COad can be formed. Compared to reaction at a Pt electrode, the onset of OHad formation and COad oxidation can be shifted to lower potentials by alloying it with oxophilic metals.8-12 On the other hand, also thermal activation enhances the reaction kinetics. Therefore, the typical DMFC operation temperature is around 60-130 °C.3,13 For steady-state operation, both dissociative adsorption of methanol and also oxidation of COad must be enhanced to avoid poisoning of the catalyst surface by dissociative methanol adsorption products.14-16 In addition to thermally activated COad oxidation, thermal desorption of COad can also lower the steady-state COad coverage at elevated temperatures.15,17-19 These different processes will affect not only the MOR activity, given by the overall Faradaic reaction current but also most likely the selectivity of the reaction toward the different reaction products (complete * Author to whom correspondence should be addressed. E-mail: [email protected]. † Present address: Institute for Energy Technology (IFE), 2007 Kjeller, Norway.

methanol oxidation to CO2 vs incomplete oxidation to formaldehyde and formic acid).6,20-25 The need for a better understanding of the temperatureinduced changes in the reaction behavior for technical applications, but also for a better fundamental understanding of the methanol oxidation reaction, has stimulated studies at elevated temperatures. Electrochemical measurements in aqueous solution are limited, however, by the low boiling point of methanol (65 °C), which is well below that of water. This results in a rapid loss of methanol from a heated open electrochemical cell due to evaporation. Therefore, electrochemical measurements at elevated temperatures (>60 °C) require a closed reaction cell and operation at a suitable overpressure. Electrochemical measurements in a tightly closed, pressurized electrochemical cell and in pressurized flow cells were performed by a number of groups.14,15,26-30 These measurements showed a pronounced thermal activation of the MOR, and apparent activation energies were determined from the methanol oxidation current (for an excellent review on this topic, see ref 31).14,15,27 The overall reaction current, however, does not provide information on the contributions from the specific reaction pathways (reaction selectivity). This requires additional probing methods, in parallel to the electrochemical measurements. Elevated temperature (up to 60-80 °C) IR spectroscopy measurements were employed for studying methanol electrooxidation on polycrystalline Pt and PtRu electrodes,32-35 providing information on the nature and coverage of adsorbed species and their role in the reaction process. Direct quantitative information on volatile reaction products is obtained by differential electrochemical mass spectrometry (DEMS), as was demonstrated for ethanol36 and methanol37 oxidation at elevated temperatures recently. These measurements, however, were performed in an open reaction cell, where the onset of evaporation limits the measurements to temperatures between 60 and 80 °C. DEMS measurements at higher temperatures require, in addition to using a closed (and pressurized) reaction cell, modifications in the interface

10.1021/jp107398g  2010 American Chemical Society Published on Web 12/03/2010

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to the vacuum chamber for online mass spectrometric detection of gaseous products. The commonly used porous membrane interface would cause excessive evaporation of water and volatile organic molecules into the vacuum chamber at elevated temperature and pressure.38 Recently, we have developed a high-temperature/high-pressure (HT/HP) DEMS setup allowing operation at up to 3 bar overpressure and up to 100 °C,39 which is based on a thin-layer channel flow cell with well-defined hydrodynamic characteristics.30 The flow cell was connected to a second thin-layer compartment, which in turn was interfaced to a mass spectrometer via a nonporous Teflon film.40,41 This limits the evaporation of water and organics into the vacuum system but is still permeable for gases which can be monitored online. This HT/HP DEMS setup was applied recently for studying the electrooxidation of different organic molecules over a Pt/C fuel cell catalyst at temperatures up to 100 °C and under continuous controlled electrolyte flow. First results on the electrooxidation of ethanol and ethylene glycol were published recently.42,43 In the present paper, we will report results of an extensive study on the temperature dependence of the methanol oxidation reaction over a Pt/C catalyst. After a brief description of the experimental procedures and setup, we will present results from potentiodynamic and potentiostatic measurements, where the overall methanol oxidation, the complete oxidation to CO2, and the resulting CO2 current efficiency were determined over a wide range of temperatures, from room temperature up to 100 °C, and reaction potentials. To gain additional information on transport effects, we also used different catalyst loadings and electrolyte flow rates. Apparent activation energies for the overall reaction and for methanol oxidation to CO2 are determined for the different reaction parameters. Consequences of the present findings on the mechanistic understanding of the MOR will be discussed. Finally, we briefly discuss the implications of the present findings for the development and operation of DMFCs. 2. Experimental Section A detailed description of the HT/HP DEMS setup was given in ref 39. In short, it consists of a HT/HP flow cell, a second flow cell containing the interface to the vacuum system and the mass spectrometer, the pressurized and heated electrolyte supply bottles, and a pressure-stable syringe pump (Harward PHD2000) for controlling the electrolyte flow rate. The design of the thin-layer channel flow cell for elevated temperatures (up to 100 °C) and overpressures (3 bar) was described in detail and characterized in ref 30. The pressurized and thermostatted electrolyte supply bottles were connected to the flow cell, which was located in a home-built air thermostat, via PEEK capillaries and an inert multiport valve. The exhaust of the flow cell was connected via a PEEK capillary to the second thin-layer compartment and via a 10 µm thick Teflon film to the vacuum chamber. The outlet capillary of the second compartment was connected to the syringe pump. The thin-film electrodes were prepared according to the experimental protocol described in ref 44 by subsequent pipetting/drying of an ultrasonically redispersed aqueous catalyst suspension and aqueous Nafion solution onto the surface of a rectangular, mirror-polished glassy carbon (GC) plate. The GC plate with the resulting circular catalyst thin film (ca. 5 mm in diameter, loading 10 or 40 µgPt cm-2) was pressed against the cell body via a gasket (thickness 50 µm), with the rectangular channel flow cut in the middle. The Pt counter electrode was located in front of the working electrode, in a separate

Chojak-Halseid et al. compartment behind a glass frit. An external saturated calomel electrode (SCE), operated at ambient temperature, was connected to the inlet of the thin-layer flow cell via a Teflon capillary. The SCE was calibrated vs a reversible hydrogen electrode (RHE) in separate hydrogen oxidation/evolution measurements at each temperature at 3 bar H2 overpressure.30 The experimental setup for DEMS measurements consists of a differentially pumped, dual chamber setup with a quadrupole mass spectrometer (Balzers 112) and a Pine Instruments potentiostat equipped with a computerized data acquisition system (for details, see refs 45 and 46). The mass spectrometric m/z ) 44 ion current was calibrated via the calibration constant K*, using the constant potential bulk oxidation of formic acid (0.1 M formic acid solution (99.5% Merck) in 0.5 M H2SO4) as a test reaction. The calibration constant K* for the m/z ) 44 ion current is defined as

K* ) nIMS/IF

(1)

where n is the number of electrons per CO2 molecule formation (n ) 2 for formic acid oxidation), and IMS and IF are the mass spectrometric and Faradaic currents, respectively. Changes in K* with temperature or with the electrolyte flow rate were corrected by normalizing the measured m/z ) 44 signal to the same K* value for all temperatures and flow rates. These normalized MS signals were used for all plots and calculations. The time delay between the formation of gaseous species and their mass spectrometric detection, which depends on the flow rate, was corrected accordingly. The mass spectrometric m/z ) 44 ion current was converted into a partial Faradaic current for CO2 formation, using eq 1 and assuming the release of six electrons (n ) 6) per CO2 molecule formed from methanol. The current efficiency for CO2 formation was calculated as the ratio of the partial current for CO2 formation and the measured (total) Faradaic current. The apparent activation energies were evaluated from the slopes in the Arrhenius plots of the Faradaic current and of the partial current for CO2 formation, respectively. Methanol bulk oxidation DEMS measurements (potentiodynamic measurements at a scan rate of 10 mV s-1 and constant potential measurements with a holding time of 5 min) were carried out at 23, 40, 60, 80, and 100 °C under 3 bar Ar (MTI, N 6.0) overpressure. The supporting electrolyte (0.5 M H2SO4) was prepared from suprapure sulfuric acid (Merck) and Millipore Milli-Q-water. A 0.1 M solution of methanol (Lichrosolv, Merck) in 0.5 M H2SO4 was used for bulk oxidation experiments. The electrolytes were first deaerated at ambient pressure by Ar (MTI, N 6.0) and subsequently pressurized with Ar. 3. Results and Discussion 3.1. Potentiodynamic Methanol Oxidation. Figure 1 depicts the Faradaic current (a), mass spectrometric m/z ) 44 ion current (b), and the current efficiency for CO2 formation (c) during the positive-going potential scan for methanol oxidation over a Pt/C catalyst electrode (10 µgPt cm-2 loading) under the conditions described above at reaction temperatures between room temperature and 100 °C. The Faradaic current (corrected for doublelayer contributions) is suppressed at potentials below ca. 0.4 V. Independent of the temperature, it then starts to increase, passes through a maximum at ∼0.75-0.8 V, and finally decays at more positive potentials due to the build-up of reaction inhibiting OHad species and platinum oxide species. Increasing the reaction temperature results in the following characteristic changes: (i) the onset of the reaction shifts to lower potentials

Methanol Oxidation Over a Pt/C Catalyst

Figure 1. Simultaneously recorded Faradaic (a) and mass spectrometric m/z ) 44 ion currents (b) recorded during potentiodynamic (positivegoing scan) methanol oxidation on a Pt/Vulcan catalyst electrode at different reaction temperatures, as well as the current efficiency for CO2 formation calculated from these data (c) (for assignments see figure). Electrolyte, 0.1 M methanol in 0.5 M H2SO4, pressurized at 3 bar Ar overpressure; electrolyte flow rate, 15 µL s-1; potential scan rate, 10 mV s-1; catalyst loading, 10 µgPt cm-2.

(see inset in Figure 1a), from ca. 0.6 V at room temperature to about 0.4 V at 100 °C, (ii) the peak maxima shift negatively by about 50 mV, from ca. 0.8 V at room temperature to ca. 0.75 V at 100 °C; and (iii) the maximum current increases by a factor of about 40 from room temperature to 100 °C. At the highest temperature, the significant gas evolution and its insufficient removal from the thin-layer gap and/or its accumulation at the counter electrode resulted regularly in the current oscillations and loss of potential control. Therefore, for 100 °C controlled signals were only obtained up to potentials shortly above the peak maximum. The general shape of the CV recorded at room temperature agrees well with results of previous studies on the same catalyst under similar reaction conditions,6,23,46 and we refer to them for a more detailed discussion of the characteristic features. The considerable thermal activation of the methanol oxidation reaction agrees well with reports from previous studies,14-16,27,31,47-50 which covered the temperature range between room temperature and 60 °C,31,49 up to 100-140 °C,14-16,27 or in the range 100-250 °C in an autoclave cell in aqueous electrolyte,47 or in a fuel cell (single cell) using a solid cesium dihydrogenphosphate electrolyte.50 It should be noted that the increase in reaction rate at elevated temperatures may partly be related to a decrease of the steady-state coverage of reaction inhibiting COad species with increasing temperature, in addition to the activation of the rate-determining step (rds) in the dominant reaction pathway.15,32-34,51 Figure 1b shows simultaneously recorded mass spectrometric signals of CO2 formation during the MOR experiments described above. Similar to the Faradaic current response, the onset of the mass spectrometric m/z ) 44 ion current and the peak maximum shift to lower potentials with increasing temperature,

J. Phys. Chem. C, Vol. 114, No. 51, 2010 22575 and also the peak height increases significantly. Comparison of 10-fold magnified Faradaic and mass spectrometric current traces (see corresponding insets in Figures 1a and 1b) shows that the shifts in the onset potential with increasing temperature are of comparable order of magnitude for the CO2 formation current and the Faradaic current. A similar shift of the onset of CO adlayer oxidation with increasing temperature was reported also for the oxidation of adsorbed CO.16,17,52-55 The about 4-fold (15fold) increase in the CO2 formation rate from room temperature to 40 °C (60 °C) reaction temperature observed in the present measurements reveals a significantly more pronounced temperature dependence compared to the ∼3.5-fold increase in the maximum CO2 formation rate (positive-going scan) reported in a recent DEMS study for going from room temperature to 50 °C.37 This difference can be explained by a combination of three effects: (i) a slightly higher catalyst loading in the present study (the complete conversion of methanol to CO2 depends on the catalyst loading, with increasing CO2 formation at higher catalyst loading23), (ii) the full temperature control of both the electrolyte (electrolyte supply bottle) and the electrochemical reactor in the present experiments, and (iii) the loss of methanol in an open system as used in refs 20 and 37, which is excluded under the pressurized reaction conditions in the present study. Going to even higher temperatures, the maximum CO2 formation rate at 100 °C is ca. 2 orders of magnitude higher compared to room temperature reaction, which is significantly more than the ∼40fold increase in Faradaic current in the same temperature range. This indicates that the MOR selectivity changes toward complete conversion of methanol to CO2 with increasing temperature. (This also explains the extensive gas evolution at 100 °C mentioned above.) To quantify the relative increase in CO2 formation, the current efficiency for CO2 formation was calculated from the ratio of the partial Faradaic current for CO2 formation (see Experimental Section) and the overall Faradaic current. The resulting values are plotted in Figure 1c. It should be noted that at very low mass spectrometric currents these values become increasingly noisy and are therefore smoothed by averaging. The higher current efficiency for CO2 formation at the onset of methanol oxidation may be correlated with contributions from the oxidation of adsorbed species, which had been formed before at lower potentials. In the calculation of the CO2 formation current efficiency, we assumed a transfer of six electrons per CO2 molecule formation, while only about two electrons are delivered during oxidation of preformed COad (“methanol adsorbate”). Therefore, the calculated CO2 current efficiencies appear higher than they are in reality in the potential range where CO2 formation from preformed methanol adsorbate contributes significantly to CO2 formation.6 With increasing potential, the CO2 formation current efficiencies approach approximately constant values, which gradually increase with reaction temperature, from ca. 40% at room temperature to about 60% at 60 °C and ca. 100% at 100 °C. Hence, at 100 °C, complete conversion of methanol to CO2 is reached under the present reaction conditions. The current efficiency for CO2 formation at 60 °C is significantly larger than that reported in ref 37, which was around 30% averaged over the whole potential cycle. Interestingly and in contrast to our findings, the latter study did not detect any significant change in the CO2 current efficiency for going from room temperature to 50 °C, which we relate to the differences in the experimental protocol in both studies as discussed above. The complex reaction behavior during the potentiodynamic scans, with the formation and oxidation of methanol adsorbate

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Figure 3. Dependence of the apparent activation energy for the overall methanol oxidation (triangles) and for methanol oxidation to CO2 on the electrode potential during potentiodynamic (positive-going scan) methanol oxidation on a Pt/Vulcan catalyst at different reaction temperatures (derived from the Arrhenius plots in Figure 2; data from Figures 1a,b).

Figure 2. Arrhenius plots of the Faradaic (a) and mass spectrometric m/z ) 44 ion currents (b) at selected potentials during potentiodynamic (positive-going scan) methanol oxidation on a Pt/Vulcan catalyst electrode (data from Figures 1a,b; for assignments, see figure).

occurring at different potentials, and with the reaction rate and selectivity being strongly affected or even dominated by the composition and coverage of the adlayer, explains straightforwardly why Tafel plots of the Faradaic MOR current derived in potentiodynamic measurements (see Figure S1, Supporting Information) yield little information on the actual molecularscale reaction mechanism.56-59 For the partial currents for CO2 formation, the situation is somewhat less complex. The finding of a Tafel slope of around 60 mV dec-1 in the potential range right after the onset of CO2 formation, over ca. 0.1 V, reflects a two-electron transfer in the rate-determining step (rds) for CO2 formation, as would be expected for the oxidation of methanol adsorbate species, mainly COad, which had been formed at lower potentials.15,60 The apparent activation energies of the MOR during potentiodynamic oxidation were determined in Arrhenius plots of the Faradaic currents (Figure 2a) and of the mass spectrometric m/z ) 44 ion currents in the positive-going scan (Figure 2b) at various electrode potentials. (Note that at lower potentials Faradaic current measurements are only possible at higher temperatures, resulting in an increasingly smaller number of data points in the Arrhenius plots). The variation of the apparent activation energies with potential is illustrated in Figure 3. The apparent activation energies for the overall reaction (Faradaic current) in the positive-going scan (Figure 2a) decrease from ca. 90 kJ mol-1 at 0.45 V to about half (∼45 ( 2 kJ mol-1) at 0.7 V. A comparably high value of around 70 kJ mol-1 was determined for methanol oxidation over a Pt/C catalyst at a constant potential of 0.35 V (50-100 °C, pressurized conditions) previously.14 The value of 55 ( 5 kJ mol-1 at potentials of 0.6-0.65 V (Figure 3) fits reasonably well the value of 54 ( 5 kJ mol-1 reported in refs 31 and 61. The general trend of the potential dependence of the apparent activation energy derived from the data in this study is, however, opposite to that reported recently for methanol oxidation in sulfuric acid solution on polycrystalline Pt electrodes, where lower Ea′ values were

determined at lower electrode potentials.31 In the latter study, however, the data were derived from the negative-going scan, and the methanol concentration was 5-fold higher. The potential dependence of the apparent activation energy for CO2 formation (Figure 3) is qualitatively similar to that for the overall reaction, decaying with increasing potential from about 145 kJ mol-1 at 0.45 V to ca. 65 ( 3 kJ mol-1 at 0.7 V. Overall, this effective barrier is significantly higher than that for the overall reaction. The difference between the apparent activation energy values for CO2 formation and for the overall methanol oxidation at the same potential, however, decreases from ∼55 kJ mol-1 at 0.45 V to ∼10 kJ mol-1 at 0.6-0.7 V. The high value of the apparent activation energy (ca. 145 kJ mol-1) at the onset of the MOR corresponds quantitatively to that determined for COad monolayer oxidation on a Pt(111) electrode.52 The Ea′ value of ca. 70 kJ mol-1 at 0.6 V is in reasonable agreement with the apparent activation energy of ca. 55 kJ mol-1 determined for CO2 formation during methanol oxidation on a platinized Pt electrode at 0.6 V in the temperature range up to 35 °C reported by Ota et al.20 The significantly lower value of the apparent activation energy for CO2 formation via methanol oxidation of around 40 kJ mol-1, which was reported in a recent DEMS study of the MOR on a polycrystalline Pt electrode (room temperature and 50 °C),37 is mainly attributed to the different data evaluation (in that study, the currents were averaged over a full potential cycle). In total, the differences between the present HT/HP DEMS data and previous data can largely be explained by the different experimental conditions such as the pressurized vs ambient pressure electrochemical cell, the temperature range applied, and different materials (carbon-supported catalyst vs smooth Pt electrode). Important is also the procedure used for calculating the apparent activation barrier, using currents at a specific potential or averaging the current over a full potential cycle. Because of the pronounced potential dependence of the apparent activation barrier, values determined by averaging over the complete potential cycle can at best be used as a rough estimate. We want to note already at this place, however, that because of the dynamic variation of the surface composition during potentiodynamic measurements the use of potentiodynamic data for the determination of apparent activation energies is generally limited. Well-defined values of the kinetic data, including the apparent activation energy, can be obtained only under steady-


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Figure 4. Simultaneously recorded Faradaic (a, d, g) and mass spectrometric m/z ) 44 ion currents (b, e, h) and CO2 formation current efficiency (c, f, i) during constant-potential methanol oxidation on a Pt/Vulcan catalyst electrode at different reaction temperatures, as well as the current efficiency for CO2 formation calculated from these data (c) (for assignments, see figure). Electrolyte: 0.1 M methanol in 0.5 M H2SO4, pressurized at 3 bar Ar overpressure. Electrolyte flow rates: 10 µL s-1 (a-c), 15 µL s-1 (d-f), and 20 µL s-1 (g-i). Catalyst loading: 10 µgPt cm-2.

state reaction conditions, i.e., in potentiostatic measurements, which will be presented and discussed in the following section. 3.2. Potentiostatic Methanol Oxidation. Prior to the potentiostatic MOR measurements, the negative-going potential scan was held at 0.06 V for 5 min, and then the potential was stepped to increasingly higher values (0.1 V per step, 5 min per potential). This experimental protocol results in a catalyst surface which is initially (partly) covered by a CO adlayer, whose coverage decreases during the reaction when approaching the steady-state situation. To gain further information on mass transport effects during the MOR,23-25 similar series of constant potential experiments were performed (i) at different electrolyte flow rates (10, 15, and 20 µL s-1) and (ii) with a 4-fold higher catalyst loading (40 µg cm-2) at the same electrolyte flow rate and oxidation potentials. Representative Faradaic (Figures 4a,d,g) and mass spectrometric current (Figures 4b,e,h) transients as well as the resulting current efficiencies for CO2 formation (Figures 4c,f,i) recorded at different constant potentials and reaction temperatures for 10 µgPt cm-2 Pt/C catalyst loading and at electrolyte flow rates of 10, 15, and 30 µL s-1, respectively, are shown in Figure 4. In most cases, the currents increase steeply upon stepping up the potential and then quickly approach their steady-state value. Measurable Faradaic currents (see insets Figures 4a,d,g) and mass spectrometric CO2 signals (see insets Figures 4b,e,h) appear at 0.4 V and an electrolyte flow rate of 10 µL s-1 at higher temperatures (g80 °C). At higher electrolyte flow rates (Figures 4d,g and 5e,h), however, methanol oxidation is barely detectable at this potential. Both the overall reaction rate and that for CO2 formation decrease gradually with increasing electrolyte flow rate, equivalent to a lower conversion of methanol at higher flow rates. The current efficiency for CO2 formation also decreases with increasing electrolyte flow rate.

These results will be discussed in detail below. Going to more positive potentials, both the overall oxidation currents and the CO2 formation rates increase. Higher reaction rates are achieved also at increased temperatures. For the highest rates, during reaction at 100 °C/0.7 V and at an electrolyte flow rate of 10 µL s-1, extensive gas evolution on both the working electrode and counter electrode (the latter is located in a separate chamber with a volume of ca. 0.5 cm3) in combination with the slow electrolyte flow led to a decrease and eventually disruption of the current and a loss of potential control. Therefore, under these conditions the measurement time was limited to ca. 2 min. At higher flow rates, these problems were less pronounced, mainly because of the lower CO2 formation rates at high potentials obtained under these conditions (Figures 4d,g and 4e,h; see discussion below). Finally, the resulting CO2 formation current efficiencies are plotted in Figures 4c,h,i for the different potentials, temperatures, and electrolyte flow rates, respectively. It is important to note that the current efficiencies are calculated from steady-state data, where the rates for methanol decomposition (adsorbate formation) and methanol adsorbate oxidation are identical, and the assumption of a six-electron transfer per CO2 molecule is fully justified, in contrast to the potentiodynamic data (see Figure 1c and section 3.1). The current efficiencies for CO2 formation increase with increasing electrode potential and reaction temperature. In contrast, they decrease with increasing electrolyte flow rate. For instance, with increasing reaction temperature the current efficiency for CO2 formation increases from ca. 40% at room temperature to ∼100% at 100 °C at 0.7 V and 10 µL s-1 electrolyte flow rate. At the higher flow rate of 20 µL s-1, but otherwise similar reaction parameters, in contrast, the CO2 current efficiency increases only from ca. 30% to about 80%. At the slowest electrolyte flow (10 µL s-1), the current efficiency

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Figure 5. Arrhenius plots of the Faradaic current (open symbols), of the current for CO2 formation (filled symbols) (a-c), and of the CO2 current efficiency (d-f) under a steady state during constant-potential oxidation of methanol over a Pt/Vulcan catalyst at different reaction potentials (data from Figure 4 and similar measurements; for assignments, see figure).

for CO2 formation is ∼100% over the entire potential range from 0.5 to 0.7 V at 100 °C reaction temperature, equivalent to complete conversion of methanol to CO2. Hence, the selectivity of the MOR toward CO2 formation depends sensitively on all three parameters tested so far, temperature, potential, and mass transport (electrolyte flow). The variation in selectivity is visible also in the overall methanol oxidation current since the change in the MOR selectivity, from 100% conversion to CO2 toward increasing contributions from incomplete oxidation, results also in a decrease in the electron yield per methanol molecule turnover from six (full conversion) to less than six electrons (contribution from four electrons for methanol oxidation to formic acid and two electrons for methanol oxidation to formaldehyde) and hence in a decay of the total MOR reaction current. The effect of the mass transport (electrolyte flow) on the MOR selectivity can be understood in terms of the “desorptionreadsorption-reaction model” described recently.62 In that model, weakly adsorbed reaction intermediates can undergo further reaction after desorption, by readsorption of the dissolved species from the diffusion layer and their further reaction to another reaction intermediate or to the stable final product.62 Due to the shorter residence time of the reaction intermediate in the diffusion layer at the electrode surface at faster electrolyte flow rates (thinner diffusion layer63), the probability for readsorption decreases with increasing mass transport and electrolyte flow rate. Therefore, also the probability for incomplete oxidation products to be transported out of the flow cell is expected to increase with increasing flow rate. Accordingly, CO2 formation should decrease, as is observed experimentally (see also ref 25). Comparable effects of the mass transport on the MOR activity and selectivity were reported also in refs 58 and 64. Similar effects are expected also for varying the catalyst loading on the electrode.22,23 The higher the catalyst loading, the lower the probability of incomplete oxidation products to

escape from the reaction cell. This agrees with predictions based on the more general term of the space velocity.65 According to rules developed in heterogeneous catalysis, the product distribution shifts toward the thermal equilibrium distribution with decreasing space velocity (increasing catalyst mass, decreasing reactant flow), which is equivalent to an increasing contact time between reactants/reaction intermediates and the catalyst.65 Similar concepts should be applicable also in electrocatalysis62 and have been verified in previous model studies on the MOR and EOR on supported catalyst electrodes23,36 and on formaldehyde and methanol oxidation on nanostructured electrodes at room temperature.24,25 To test this hypothesis also at elevated reaction temperatures, the effect of the catalyst loading on the MOR activity and selectivity was also studied as a function of temperature. As expected from the desorption-readsorptionreaction model, increasing the catalyst loading from 10 to 40 µgPt cm-2 indeed results in a higher conversion of methanol to CO2. Under these conditions, 100% current efficiency for CO2 formation is reached already at 80 °C at an electrolyte flow rate of 15 µL s-1 (see Figures S2 and S3, Supporting Information). On the basis of these findings, we expect essentially full conversion of methanol to CO2 at high catalyst loadings, high operation temperatures, and low electrolyte flow rates, i.e., under reaction conditions which are characteristic for realistic DMFCs, at least considering the catalyst loading and reaction temperature, which agrees fully with results of product analyses at the DMFC exhaust.66-68 Apparently, the effects induced by the higher reaction temperature and catalyst loading apparently overcompensate counteracting effects caused by the higher electrolyte flow in a realistic DMFC. The resulting kinetic data are summarized and compared in Arrhenius plots of the respective Faradaic currents (overall methanol oxidation current) and of the partial current for methanol oxidation to CO2 (derived from the mass spectrometric data; see Experimental Section) in Figures 5a-5c. The related current


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TABLE 1: Apparent Activation Energies for the Overall Methanol Oxidation Reaction (Faradaic Current) and Oxidation to CO2 (Partial Current for CO2 Formation) at Different Electrolyte Flow Rates, Electrode Potentials, and Catalyst Loadings, Determined from the Arrhenius Plots of the Steady-State Reaction Ratesa Pt loading, 10 µg cm-2

Pt loading, 40 µg cm-2

flow rate/µL s-1

Ea′ (IF)/kJ mol-1 (0.6 V)

Ea′ (ICO2)/kJ mol-1 (0.6 V)





10 15 20

56 ( 3 49 ( 2 47 ( 3

66 ( 3 58 ( 2 56 ( 3

42 ( 3 38 ( 2 38 ( 1

53 ( 3 49 ( 2 46 ( 2

58 ( 3 -

68 ( 3 -

a

See Figure 5 and Figure S3, Supporting Information.

efficiencies for CO2 formation are plotted in Figures 5d-5f. The resulting apparent activation energies for both processes, and the current efficiencies for CO2 formation, as a function of the electrode potential and reaction temperature, are listed in Table 1. In all cases, the logarithmic rates decay about linearly with the inverse temperature, indicating that the mechanism and the rds of the reaction do not change over this temperature range. Similar to the trends observed in the apparent activation energies derived from the potentiodynamic data, the apparent activation energies for the total reaction (Faradaic current) are lower than those for the pathway leading to CO2 formation. This must be due to lower barriers in the pathways leading to formaldehyde and formic acid formation, which contribute significantly to the total methanol conversion measured by the Faradaic current. Also, the decay of the apparent activation energies with increasing potential (0.6 f 0.7 V) resembles the trend in the potentiodynamic data. The actual values of the apparent activation energies, however, are different in both cases, with the barriers derived from the steady-state data being generally lower than those calculated from the potentiodynamic measurements. Most likely, this results from convolution of the kinetic effects with effects induced by the variation of the potential and the changes in the adlayer composition and coverage with time in the potentiodynamic measurements, in contrast to the steadystate reaction conditions achieved in constant potential measurements. Furthermore, the apparent activation energy also increases when going to the higher Pt/C catalyst loading, by about 10 kJ mol-1 (from 49 ( 2/58 ( 2 to 58 ( 3/68 ( 3 kJ mol-1) for otherwise identical parameters. The activation barriers for the overall reaction and for methanol oxidation to CO2 at 40 µgPt cm-2 loading and 15 µL s-1 electrolyte flow rate are comparable to those for 10 µgPt cm-2 loading and 10 µL s-1 flow rate. Hence, both electrolyte flow and the amount of catalyst are equally important for the reaction kinetics and hence the product distribution (see also below). The current efficiencies for CO2 formation (for calculation, see the Experimental Section) are plotted in a logarithmic scale vs 1/T in Figures 5d-f for potentials of 0.6 and 0.7 V at different electrolyte flow rates. This representation is chosen because of the linear relation between ln IF or ln ICO2 and 1/T (Figures 5a-c). The plots show an about linear decrease in the logarithmic CO2 formation current efficiency with the inverse temperature, from 80-100% (depending on the flow rate) at 100 °C to ca. 40% at room temperature. The increase in the current efficiency for CO2 formation with increasing temperature reflects the more pronounced temperature dependence (higher apparent activation energy) of the pathway for CO2 formation compared to the total rate for methanol conversion given by the Faradaic current. More specifically, the barriers in the rds for methanol oxidation to formaldehyde and formic acid must be lower than that of the rds for oxidation to CO2. The somewhat lower current efficiencies at higher electrolyte flow rates can be explained again by the decreasing probability for readsorption and further reaction of desorbed reaction intermediates under

these conditions (desorption-readsorption-reaction model) or, analogously, by expectations based on the decreasing space velocity.65 Accordingly, higher conversion can be equally reached by either decreasing the electrolyte flow rate or increasing the catalyst loading. It is important to note that the apparent activation energy values reflect the integral response of a complex reaction network not only for overall methanol oxidation (to several reaction products, i.e., formaldehyde, formic acid, and CO2) but also for the case of methanol oxidation to CO2. The latter can proceed either via an indirect pathway (COad formation/ oxidation) or via a direct pathway (methanol f formaldehyde f formic acid f CO2), with both pathways including several elementary reaction steps. Furthermore, COad oxidation could be favored from both partial thermal desorption of COad (decrease in COad coverage) and thermal activation of water splitting, both of which would shift the onset of the reaction to lower potentials.17,69 Finally, COad formation due to dissociative adsorption of methanol32-35 and its oxidative intermediates (formaldehyde and formic acid70) can also be thermally activated, with consequences for the COad coverage. The interplay of these contributions is summarized in the apparent activation energy values. The present data do not allow us to discriminate between the individual contributions even for complete methanol oxidation to CO2, although from the variation of the apparent activation energy values with reaction conditions the predominant factors can be speculated upon. The energetics of methanol dissociative adsorption on Pt(111) was also extensively studied by DFT calculations (see, e.g., refs 71-73 and references therein). The dissociation of the C-H bond was found to be favorable compared to O-H bond dissociation. The dissociative adsorption of methanol at the electrochemical interface is influenced also by the presence of water and the electrode potential applied.74-77 Ensemble requirements are more stringent for C-H bond dissociation compared to O-H dissociation, resulting in methanol oxidation to proceed either via COad formation or to formaldehyde depending on the electrode potential.75,77 The oxidative removal of COad proceeds via water activation to form OHad and COOH intermediate formation in the transition state.78-83 The experimentally studied electrooxidation of the preadsorbed CO monolayer was reported to be thermally activated,17,52-54 with an apparent activation of ca. 130 kJ mol-1.52 On this basis, one can argue that the higher apparent activation energies for methanol oxidation to CO2 refer to the barrier for COad oxidation as the reaction rate-determining step, while the lower Ea′ values apparently correspond to the kinetic limitions for the COad formation: larger Ea′ values appear at lower electrooxidation potential (0.6 V), reflecting the insufficient OHad formation, which hinders the oxidation of COad; however, at higher potential (0.7 V), hydroxide formation/adsorption is fast, and the reaction rate becomes limited by COad formation.84 The effect of the electrolyte flow rate on the apparent activation energy (decrease in the Ea′ values with increasing electrolyte flow) can be rationalized by the

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change of the reaction selectivity toward incomplete oxidation due to the more efficient removal of incomplete oxidation products from the electrode region, thus decreasing their adsorption probability and hence the probability to further increase the COad coverage. Finally, we would like to comment on the implications of these results for the understanding of the MOR in an operating DMFC. Previous model studies of the MOR on smooth Pt electrodes always arrived at much lower conversions to CO237,64,85,86 than measured at the exhaust of the operating fuel cell, where in most cases complete conversion to CO2 was reached.51,66-68,87 Based on the results of the present study, which was preferred under realistic fuel cell relevant conditions (continuous mass transport, elevated temperatures), and on fuel cell relevant materials (carbon-supported catalysts), we can explain this discrepancy mainly by two effects, the generally higher reaction temperatures in a DMFC (operating temperatures 60-130 °C3,13) compared to most model studies and the larger amount of catalyst in technical membrane electrode assemblies (MEAs). Furthermore, our data imply that upon reducing the anode catalyst loading specific attention has to be paid to avoiding formaldehyde in the product stream, because of its toxicity, and the same is true for reducing the operation temperature. Specifically, these issues will become important in micro-DMFCs (small amounts of catalyst, lower operation temperatures) and DMFCs for portable applications. In total, our findings underline the importance of using fuel cell relevant reaction and transport conditions in model studies aiming at the quantitative understanding of electrocatalytic reactions in a fuel cell, which includes (i) the use of realistic carbon-supported catalysts, however, at well-defined catalyst loading and up to 100% utilization;44 (ii) measurements under continuous reaction and continuous electrolyte flow conditions with well-defined mass transport properties; and (iii) measurements at relevant elevated temperatures/pressures up to 100 °C. 4. Summary and Conclusions The influence of elevated reaction temperatures on the oxidation of methanol over carbon supported Pt/C catalysts was investigated systematically by high-temperature/high-pressure DEMS measurements under conditions which are relevant for fuel cell applications (elevated temperatures up to 100 °C, continuous enforced reactant flow, continuous reaction, supported catalysts) but, nevertheless, well-defined with respect to catalyst utilization, diffusion resistances, reactant transport, etc. Faradaic currents and the partial currents for CO2 product formation were determined for a wide range of reaction parameters, including potentiodynamic and potentiostatic measurements at different potentials, systematic variation of the reaction temperature between room temperature and 100 °C, variation of the electrolyte flow rate, and finally measurements at two different Pt/C catalyst loadings. Current efficiencies for CO2 formation were calculated from these data. These measurements led to the following main results and conclusions: 1. Increasing the temperature results in a continuous increase of the reaction rate (at otherwise fixed parameters); the increase is in the range of 2 orders of magnitude for methanol oxidation at 0.6 and 0.7 V when raising the temperature from room temperature to 100 °C. A similar behavior is observed also for the partial current for CO2 formation, which increases even more steeply. 2. The increase of the rates for the overall methanol oxidation (Faradaic current) and for the conversion of methanol to CO2 (mass spectrometric current for CO2) both follow a simple Arrhenius-type behavior, with a linear dependence of the logarithmic rate vs 1/T over the entire temperature range investigated. This indicates that the rate-determining steps

Chojak-Halseid et al. do not change under these conditions. The resulting apparent activation energies depend sensitively on the reaction parameters: they decrease (i) with increasing potential, (ii) with increasing electrolyte flow rate, and (iii) with increasing catalyst loading. The latter two effects directly demonstrate that the apparent activation energies can not be associated with the reaction barrier in an elementary reaction step but represent effective values describing the temperature dependence of the complex reaction network. 3. The apparent activation energy for the conversion of methanol to CO2 is significantly higher than that for the overall methanol oxidation reaction, typically by about 10 kJ mol-1 at otherwise identical reaction conditions. This results in a distinct increase of the selectivity for CO2 formation with increasing temperature, reaching about complete conversion to CO2 (100% selectivity for CO2 formation) at 100 °C, while at room temperature the current efficiency for CO2 formation is typically in the range of 30-40%, depending on the exact reaction conditions. The contribution of the incomplete oxidation products formaldehyde and formic acid decreases accordingly. 4. The current efficiency and hence the selectivity for CO2 formation increase with (i) decreasing electrolyte flow and (ii) increasing catalyst loading at otherwise constant reaction conditions. These changes are explained by mass transport effects, by increasing readsorption and further reaction of incompletely oxidized, desorbed reaction intermediates (desorption-readsorption-reaction model). 5. For practical applications in a DMFC, the reaction temperature is the most important parameter. It not only drastically increases the overall activity for the MOR measured by the Faradaic current but also changes the product distribution toward increasing CO2 contents. At 100 °C, complete conversion of methanol to CO is reached under the present reaction conditions. This agrees closely with findings in measurements of the exhaust of fuel cells, which showed mainly or exclusively CO2 as the reaction product. Because of the sensitivity of the product distribution to the catalyst loading, further reduction of the amount of catalyst employed in technical applications (DMFC anodes) has to consider also possible effects on the product distribution, in addition to the change in activity. Because of its toxicity, this is in particular true for the undesirable emission of the incomplete oxidation product formaldehyde. In total, the results clearly demonstrate the importance of using relevant, close-to-realistic reaction conditions in model studies for conclusions on the reaction characteristics in a technical application. Acknowledgment. This work was supported by the Federal Ministry of Education and Research (project 03SF0311C) and by the Helmholtz Association (project VH-VI-139). Supporting Information Available: Tafel plots of the Faradaic and mass spectrometric m/z ) 44 ion currents during potentiodynamic (positive-going scan) methanol oxidation on a Pt/Vulcan catalyst electrode at different reaction temperatures (data from Figures 1a,b) are available in Figure S1. Furthermore, simultaneously recorded Faradaic and mass spectrometric m/z ) 44 ion current as well as the current efficiency for CO2 formation during potentiostatic oxidation of methanol over a Pt/Vulcan catalyst at different reaction temperatures are plotted in Figure S2. Finally, Arrhenius plots of the Faradaic current, the partial current for CO2 formation, and of the CO2 current efficiency under steady state during potentiostatic oxidation of

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