Article pubs.acs.org/JPCC
Influence of Anion Adsorption on the Parallel Reaction Pathways in the Oscillatory Electro-oxidation of Methanol Raphael Nagao, Daniel A. Cantane, Fabio H. B. Lima, and Hamilton Varela* Institute of Chemistry of São Carlos, University of São Paulo, CP 780, CEP 13560-970, São Carlos, SP, Brazil S Supporting Information *
ABSTRACT: We report the experimental study of the impact of anion adsorption on the two parallel pathways of CO2 formation during the electro-oxidation of methanol on platinum. The effect of nature of the supporting electrolyte (HClO4 and H2SO4) was investigated at two methanol concentrations. Voltammetric profiles and oscillatory time series of the electrode potential were registered in conjunction with the evolution of the production of carbon dioxide and methylformate, as measured by means of on line differential electrochemical mass spectrometry (DEMS). In all conditions studied, the production of CO2 was higher in the presence of HClO4 rather than that in H2SO4. Importantly, the inhibition caused by anion adsorption was generally more pronounced in the direct pathway, i.e., the non-COad pathway. Furthermore, we have noted an additional peak commonly observed at high potentials during the oscillatory electro-oxidation of small organic molecules can be generally attributed to the oxidative removal of COad. Altogether, the effect of anion adsorption is discussed in connection with both the oscillatory kinetics and the dynamics of adsorbed species, contributing thus to a comprehensive physical-chemistry description of the surface processes.
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formation of both CO2 and COad.11−13 Methylformate, detected on line by means of differential electrochemical mass spectrometry (DEMS),14 has been extensively used to infer on the kinetics of the direct pathway.15 Nowadays, the formation of methylformate is believed to occur via the nucleophilic attack of a methanol molecule to an adsorbed intermediate16 instead of the homogeneous reaction between formic acid and methanol in solution.17,18 In the case of a nucleophilic attack of a H2O molecule instead of methanol to this adsorbed species, formic acid is formed. Besides the carbonaceous molecules illustrated in Figure 1, the presence of additional adsorbed species such as oxygenated species and adsorbed anions must be considered when studying the electro-oxidation reaction itself. The adsorption of such species is known to pose dramatic consequences on reaction rates, selectivity, product distribution, etc.19−24 It has been postulated that the first adsorption step of the methanol molecule dictates which pathway the reaction will proceed.18 If the adsorption takes place by the carbon atom, a rapid sequence of dehydrogenation steps, which requires at least three contiguous sites,25,26 will generate adsorbed carbon monoxide, leading to the indirect pathway. On the other hand, if the adsorption starts by the oxygen atom, methoxy species are formed and the production of formaldehyde and formic acid is
INTRODUCTION The electrocatalytic oxidation of methanol has been extensively used as anodic reaction in low-temperature fuel cells.1−6 On bare platinum and platinum-based surfaces, this reaction occurs via a parallel, dual pathway mechanism.7,8 A simplified schematic of the parallel pathways during methanol electrooxidation is given in Figure 1 (see ref 9 and Supporting Information for a more detailed description).
Figure 1. Simplified reaction scheme for the electro-oxidation of methanol.
The direct pathway proceeds via reactive intermediates such as formaldehyde and formic acid. The indirect pathway occurs in parallel and proceeds through the formation of adsorbed carbon monoxide (COad), which reacts with oxygenated species at high potentials via a Langmuir−Hinshelwood mechanism.10 Along the direct, non-COad pathway, adsorbed formic acid might result in bridge-bonded adsorbed formate (HCOOad), a reactive intermediate which is a common precursor for the © 2013 American Chemical Society
Received: March 21, 2013 Revised: May 20, 2013 Published: July 11, 2013 15098
dx.doi.org/10.1021/jp4028047 | J. Phys. Chem. C 2013, 117, 15098−15105
The Journal of Physical Chemistry C
Article
Figure 2. Simultaneously recorded cyclic voltammograms (index 1) and mass fragments for m/z = 44 (index 2) and 60 (index 3), during the electrooxidation of methanol on a sputtered platinum electrode at [H3COH] = 0.5 mol L−1 (item a) and 2.0 mol L−1 (item b) in acidic perchloric (black line) and sulfuric (red line) media. [HClO4] = [H2SO4] = 0.5 mol L−1. Sweep rate = 0.01 V s−1 and T = 20 °C.
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favored, following the direct pathway. Considering the different site requirements for each pathway, it becomes clear that the competition for surface sites by dissolved species strongly impacts the efficiency and selectivity of the whole process. Iwasita et al.23,24 investigated the impact of anion adsorption on the product distribution along the electro-oxidation of methanol on platinum by means of high performance liquid chromatography (HPLC). They carried out a 1000 s electrolysis at 0.6 V vs RHE and observed a significant inhibitory behavior of sulfate compared to perchlorate ions with respect to the CO2 yield on Pt(111) surfaces. In contrast, the production of HCHO and HCOOH remains virtually unaffected by anion adsorption. They further concluded that although adsorbed sulfate inhibits the dissociative adsorption of methanol, it barely affects the further oxidation of adsorbed CO. The high specific adsorption energy of sulfate anions prevents the carbon monoxide formation on the surface and decreases the overall faradaic current. Baltruschat et al.,27 using a dual thin-layer flow-through cell in an on line DEMS apparatus, also observed higher oxidation reaction rates for methanol in perchloric acid. However, a slight change in the current efficiency of CO2 was verified when perchloric acid was replaced by sulfuric acid. We have recently reported the experimental investigation of the oscillatory electro-oxidation of methanol on platinum using on line DEMS.9 By means of an integrated approach consisting of experiments, modeling, and simulations, we were able to decouple the CO2 production from direct and indirect pathways. Unlike previous investigations under close to equilibrium conditions (using mainly voltammetric and chronoamperometric experiments), investigating the system under oscillatory regime allowed us to gain important mechanistic information on the parallel pathways. In the present contribution, we profounder those investigations9 and study the influence that the nature of supporting electrolyte exerts on the parallel pathways during the oscillatory electrooxidation of methanol on platinum.
EXPERIMENTAL SECTION
The experiments were carried out in a conventional electrochemical cell coupled with DEMS apparatus for volatile intermediates mass detection.28 The working electrode was prepared by a platinum sputtering procedure in a Teflon membrane (Gore-Tex, PTFE) with thickness and pore size at around 50 nm and 0.02 μm, respectively. The real area and surface roughness were evaluated by means of CO stripping and amounts to 3.5 ± 0.3 cm2 and 8.8 ± 0.8, respectively. A high area platinized-platinum sheet was used as a counter electrode, and all potentials were measured by and are quoted with respect to a reversible hydrogen electrode (RHE). The solutions were prepared with high purity water (Milli-Q, 18.2 MΩ cm), HClO4 (Sigma-Aldrich, 71%), H2SO4 (Merck, 98%), and H3COH (J.T. Baker, 99.9%). The cell temperature was maintained constant at 20.0 ± 0.1 °C with the aid of a ColeParmer Polystat temperature controller. On line temporal resolution of mass response smaller than 0.1 s, as calculated by the procedure proposed by Wolter and Heitbaum,28 was achieved by adapting the electrochemical cell through two chambers system (Pfeiffer, Vacuum). The mass/ charge (m/z) ratios 44 for carbon dioxide and 60 for methylformate were analyzed by a quadrupole/Faraday cup/ electron multiplier (Pfeiffer, QMA 200). Acquisition frequencies of 5.7 and 10 Hz were used for mass and electrochemical measurements, respectively. The mass spectrometer was operated in a negative mode. Both mass signals were recorded simultaneously along with the electrochemical data, and an excellent synchronization was reached; for more details see ref 9. The system was controlled by the potentio/galvanostat PGSTAT 30 by Autolab. Prior to each galvanostatic measurement, the system was cycled ten times between 0.05 and 1.5 at 0.10 V s−1 (ending at 0.05 V), and subsequently, a current step to the desired value was then applied. Oscillations during the electro-oxidation of methanol on platinum were usually very robust and reproducible. When the measurement finished, we left the 15099
dx.doi.org/10.1021/jp4028047 | J. Phys. Chem. C 2013, 117, 15098−15105
The Journal of Physical Chemistry C
Article
Figure 3. Period-one potential time series (black line) during electro-oxidation of methanol at [H3COH] = 0.5 mol L−1 (item a) and 2.0 mol L−1 (item b) in perchloric (index 1 to 3) and sulfuric (index 4 to 6) acid media at j = 0.30 mA cm−2 accompanied by the mass fragments of m/z = 44 (red line) and 60 (blue line). Peak i or i′ for indirect; peak ii and iii or ii′ and iii′ for direct pathway. Δt accounts for the oscillatory period. Remaining details as in Figure 2.
system at open circuit conditions (i.e., ∼0.33 V) for 60 s in order to obtain a well-defined baseline (set to zero detection ionic current) for mass normalization criterion. The results presented here were obtained with approximately the same values of roughness, real area, and identical experimental conditions in order to ensure a semiquantitative discussion of the gaseous products detection. Nevertheless, the reported behavior was reproduced at different sputtered electrodes and also for slightly distinct experimental parameters.
studied. The main current shoulder along the forward (backward) sweep was shift toward more positive potentials by 80 mV (60 mV), as the concentration of methanol is increased to 2.0 mol L−1. In contrast to the relatively known aspects of anion adsorption and methanol concentration along a regular voltammetric experiment, the effect of such parameters becomes rather intricate under oscillatory regime. As already reported for this system,29−31 large amplitude, period-one potential oscillations emerge abruptly at intermediate applied currents. Examples of such oscillations are given in Figure 3 for [H3COH] = 0.50 mol L−1 (item a) and 2.0 mol L−1 (item b), in perchloric acid (index 1 to 3), and in sulfuric acid (index 4 to 6) aqueous solution. Oscillatory profiles of the electrode potential and of the production of methylformate (blue line) have very similar amplitude and morphology for both electrolytes, but the oscillation period is about 25% longer for sulfuric acid. This increase in the period has been generally associated with the poisoning process caused by anion adsorption.32 The oscillatory production of carbon dioxide is much more susceptible to the nature of the supporting electrolyte, viz. anion adsorption. The experimental time series for the electrode potential and for the evolution of CO2 and HCOOCH3 have been rationalized in terms of modeling and
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RESULTS The effect of supporting electrolyte was studied at two concentrations of methanol. Figure 2 shows results for [H3COH] = 0.50 mol L−1 (item a) and 2.0 mol L−1 (item b) in terms of the voltammetric current (index 1), production of carbon dioxide (CO2, m/z = 44 and index 2), and methylformate (HCOOCH3, m/z = 60 and index 3). As a whole, the use of sulfuric acid (red line) instead of perchloric acid (black line) results in a generalized decrease in the faradaic current as well as in the production of both CO2 and HCOOCH3. The voltammetric and mass fragments profiles seem to be independent of the concentration of methanol and of the nature of the electrolyte with a dominant production of carbon dioxide over methylformate in the range of potentials 15100
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numerical simulations.9 In particular, focusing on the multipeaked CO2 production, we have recently found that peak i results mainly from the indirect pathway, whereas the peaks ii and iii reflect the contribution of the direct pathway.9 The same reasoning can be used to explain the mechanism associated with peaks i′ to iii′ for sulfuric acid. One important observation is the absence of peak iii and a considerable suppression of the peaks ii′ and iii′ when methanol concentration is decreased to 0.5 mol L−1. Although the order of magnitude of CO2 production is basically the same under different methanol concentration and electrolyte compositions studied, the HCOOCH3 production amount is considerably affected by the concentration of methanol: the production of methylformate is around 10 times higher for [H3COH] = 2.0 mol L−1. Additionally, methanol concentration hardly affects the oscillatory period in both electrolytes. The applied current influences the relative intensities of peaks in the CO2 time series. Figure 4 shows the potential time
Figure 5. CO2 contributions from (a) indirect (peak i or i′) and (b) direct pathway (peaks ii and iii or ii′ and iii′) as a function of time at j = 0.30 mA cm−2 for experiments in perchloric (black line) and sulfuric acid (red line) media with concentrations of methanol of 0.5 mol L−1 (filled circles) and 2.0 mol L−1 (open circles). Data obtained from time series given in Figure 3.
described in this figure: (a) overall, the relative CO2 production is higher in the presence of HClO4 rather than H2SO4 in both concentrations of methanol; (b) the contributions of parallel pathways to the CO2 production have very similar weights in nearly all cases; (c) as the oscillations evolve in time, there is a slight increase in the CO2 production from the indirect and direct pathways in all cases, except for initial decrease observed for direct route for [H3COH] = 2.0 mol L−1; and (d) the inhibition caused by anion adsorption was generally more pronounced in the direct pathway, i.e., the non-COad pathway. At considerably high currents, the period-one pattern is spontaneously replaced by period-two oscillations.31 Figure 6 illustrates these period-two structures in the electrode potential, which are characterized by a small modulation in the high potential region for both electrolytes. The production of methylformate remains very similar to that found for periodone response, and no trace of the modulation at high potentials is detected in the quiescent region. As pointed out for periodone series, more pronounced changes are detected in the time series for CO2 production.
Figure 4. Time evolution of the m/z = 44 mass fragment during electro-oxidation of methanol at different applied current densities, from j = 0.20 to 0.40 mA cm−2, and [H3COH] = 2.0 mol L−1 in (a) perchloric and (b) sulfuric acid media. The dashed line in both cases locate peak i (or i′ for panel b), which accounts for the CO2 peak from carbon monoxide oxidation. Additional details as in Figure 2.
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DISCUSSION The initial voltammetric characterization presented in Figure 2 attests to the synchrony between current and mass fragment signals and also the quality of our results. The general aspects of the voltammetric results have been already reported; see, for instance, refs 14, 15, and 27. The following discussion is structured as follows. We initially examine the general aspects of the period-one oscillatory response and the main effects of anion adsorption. The long-term evolution of the system’s dynamics and its relation with the anion adsorption and CO2 production are briefly discussed. In the next section we focus on the period-two potential structures observed at higher applied currents. We finalize our discussion with some mechanistic considerations on the role of adsorbing anions on the electro-oxidation reaction under close and far-fromequilibrium regimes. Period-One Potential Oscillations. The electro-oxidation of small organic molecules on platinum is very susceptible to oscillatory instabilities.33−35 When operated under galvanostatic regime, most systems undergo a bifurcation at a critical value and enter a self-organized state in which the coverage of different adsorbates oscillates. Despite the earlier investigations on the oscillatory electro-oxidation of formic acid and
series for five different applied currents obtained under similar conditions to that given in Figure 3 in (a) perchloric and (b) sulfuric acid solution. The vertical dashed line locates the main contribution for the indirect pathway, i.e. peak i (for perchloric acid) and i′ (for sulfuric acid). As the applied current increases, the production of CO2 via this pathway becomes pronounced and a higher definition of the peaks is observed in the presence of sulfate anions. It is noteworthy in this figure the robustness of the whole system, as reflected by the stability of the limit cycle. Peaks discriminated in Figure 3 were deconvoluted (see Supporting Information) and the contribution to the CO2 formation evaluated for direct and indirect pathways, assuming that the major contribution for indirect pathway comes from peak i (and i′), and that for direct pathway from peaks ii and iii (ii′ and iii′).9 Results are depicted in Figure 5 as a function of time, at j = 0.30 mA cm−2, and for experiments in perchloric and sulfuric acid aqueous solutions. Four main lessons can be learnt from the long-term evolution of individual contributions 15101
dx.doi.org/10.1021/jp4028047 | J. Phys. Chem. C 2013, 117, 15098−15105
The Journal of Physical Chemistry C
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Figure 6. Period-two potential time series (black-line) during methanol electro-oxidation at [H3COH] = 0.5 mol L−1 (item a at j = 0.35 mA cm−2) and 2.0 mol L−1 (item b at j = 0.40 mA cm−2) in perchloric (index 1 to 3) and sulfuric (index 4 to 6) acidic medias accompanied by the mass fragments of m/z = 44 (red line) and 60 (blue line). Further details as in Figure 2.
formaldehyde using on line DEMS,36,37 the results we reported for methanol9 could be classified as rather unusual. Luckily enough, we found for this system a multipeaked time series for the CO2 evolution, in contrast to the single-peaked time series for the electrode potential and for the production of methylformate. We were able to model and simulate most of such experimental observation and found that the multipeaked production of CO2 actually allows at decoupling the parallel direct and indirect pathways.9 Mass spectrometry has been used to monitor the CO2 production during the oscillatory oxidation of carbon monoxide at the solid/gas interface.38,39 But, as far as the more complex molecules are concerned, the electrooxidation of methanol is apparently the only one to date to allow the separation of parallel oxidation pathways. In short, we observed that the CO2 production in the indirect pathway occurs in the portion of the potential time series corresponding to U > ∼0.76 V. The rate of CO2 production via this pathway, i.e. through the reaction between COad and (H)xOad (x = 0, 1, or 2, depending on the nature of the oxygenated species), increases explosively during the fast potential increase and slowly decreases as the electrode potential decreases and, finally, drops nearly to zero when U becomes smaller than about 0.76 V. On the other hand, the CO2 production along the direct pathway prevails in the range of the oscillating potential
