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J. Phys. Chem. C 2010, 114, 18494–18500
Open Circuit Interaction of Formic Acid with Oxidized Pt Surfaces: Experiments, Modeling, and Simulations Bruno C. Batista† and Hamilton Varela*,†,‡ Instituto de Quı´mica de Sa˜o Carlos, UniVersidade de Sa˜o Paulo, C.P. 780, CEP 13560-970, Sa˜o Carlos, SP, Brazil, and Ertl Center for Electrochemistry and Catalysis, GIST, Cheomdan-gwagiro 261, Buk-gu, Gwangju 500-712, South Korea ReceiVed: May 20, 2010; ReVised Manuscript ReceiVed: July 9, 2010
We report in this work the study of the interaction between formic acid and an oxidized platinum surface under open circuit conditions. The investigation was carried out with the aid of in situ infrared spectroscopy, and results analyzed in terms of a mathematical model and numerical simulations. It has been found that during the first seconds of the interaction a small amount of CO2 is produced and absolutely no adsorbed CO was observed. A sudden drop in potential then follows, which is accompanied by a steep increase first of CO2 production and then by adsorbed CO. The steep transient was rationalized in terms of an autocatalytic production of free platinum sites which enhances the overall rate of reaction. Modeling and simulation showed nearly quantitative agreement with the experimental observations and provided further insight into some experimentally inaccessible variables such as surface free sites. Finally, based on the understanding provided from the combined experimental and theoretical approach, we discuss the general aspects influencing the open circuit transient. 1. Introduction An important problem in low-temperature polymer electrolyte fuel cells is the fuel crossover from the anodic to the cathodic chamber. Fuels such as methanol or ethanol can cross the ionic membrane and react in the cathodic compartment, causing a mixed potential and loss of efficiency. A simple yet powerful method to investigate this problem consists of studying the open circuit transients observed during the interaction between platinum oxides and species in solution, using half-cell setup. The current interpretation of this chemical interaction, via conjugated pair reactions, was provided long ago by Breiter.1 Since then many subtitles have been reported according to the organic species and experimental conditions. Varying experimental parameters such as mass transport conditions, surface roughness, composition, and concentration of electrolyte, as well as the use of in situ and online techniques, have revealed some mechanistic aspects.2 The rigorous work undertaken by Podlovchenko and coworkers4,7-10 provided a greater level of understanding about some factors influencing open circuit transients. They worked with solutions containing different species such as CO,7 formic acid,8 formaldehyde,9 and methanol,10 and explored the scheme of conjugated reactions to obtain kinetic parameters associated with the interaction between the organic molecule and oxidized platinum surfaces. Recently, some other factors concerning mechanistic aspects of those interactions have been brought to light. Sitta and Varela11 studying the interaction between methanol and oxides grown at different polarization times suggested that the phenomenon of place-exchange between surface and subsurface oxygenated species plays a central role in the observed transients. This conjecture was based on the * To whom correspondence should be addressed. E-mail: varela@ iqsc.usp.br. † Universidade de Sa˜o Paulo. ‡ Ertl Center for Electrochemistry and Catalysis.
observation that the concentration of surface oxide would be approximately invariant for small decreases in total oxide content. In an early article we pointed out a new mechanistic understanding of the process of fast potential variation during the transients.5 By employing in situ infrared spectroscopy, it was shown that the rates of production of CO2, in the case of methanol, and also acetaldehyde, in the case of ethanol, increase exponentially when the fast transient of potential takes place. Through detailing the probable pathways for the interaction of methanol or ethanol and platinum oxides, under the guide of spectroscopic data, we were able to show that during the fast transient there can take place an autocatalytic production of free sites of platinum for both methanol and ethanol. Since the rate of reaction of these molecules is proportional to the number of available surface sites, there is an explosive increase in reaction rates accompanying the steep potential decay. It is well-known that methanol and ethanol react electrochemically in a very complex way and that its oxidation process presents several possible pathways and also many adsorbed intermediates. This work tries to take advantage of the comparatively simple scenario offered by formic acid oxidation in order to gain mechanistic insight and quantitative knowledge about the players involved during the chemical interaction. In situ spectroscopy was employed to follow the turnover frequencies for CO2 production and the formation of adsorbed CO, concomitantly to the record of the open circuit potential. In addition, a comprehensive model accounting for all known essential steps was set and validated by numerical simulations. The paper is organized as follows. After the description of all experimental aspects, a model based on previously published experimental data is proposed bearing the essential elementary steps that the interaction between formic acid and oxidized platinum could possess. It is emphasized that all the reasonably conceivable net reactions proposed led to a scenario showing an autocatalytic production of free catalyst sites. The chemical steps introduced initially are then translated into a set of ordinary
10.1021/jp104650x 2010 American Chemical Society Published on Web 08/19/2010
Interaction of Formic Acid with Oxidized Pt Surfaces differential equations along with the assumptions made for its construction. Experimental results are then presented alongside with the simulated behavior first for the important variable integral curves and second for their time derivatives. Turnover frequency for formic acid dehydrogenation under open circuit condition is calculated and discussed in light of other articles concerning this quantity. Finally, it is showed how simulation can enhance our knowledge about this system by following the numerical behavior of free metal sites (a hidden experimental variable) and analyzing how the elementary steps of interaction influence the overall transient characteristics. 2. Experimental Section The potentiostat employed was a Solartron 1286 and for the in situ spectroscopic experiments a Nicolet Nexus 670 spectrometer with MCT detector was used. In situ measurements were done with an adapted cell described elsewhere.12 The cell was equipped with a ring-shaped platinum counter electrode possessing large surface area and a reversible hydrogen reference electrode in the same solution (RHE, to which all potentials are referred to). The working electrode was a platinum disk having 0.8 cm2 real surface area, which was polished to a mirror finish by the employment of alumina with average diameters ranging from 9 to 0.05 µm. A solution of 0.5 mol L-1 H2SO4 purged with argon was used as electrolyte for all experiments. Formic acid was obtained from J. T. Baker (99.9%), sulfuric acid from Mallinckrodt (99.8%), argon and carbon monoxide from White Martins (5.0 and 4.0 N, respectively), and purified water collected from a Milli-Q system (Millipore). Before objective measurement, a cyclic voltammogram was obtained in the absence of formic acid in order to check the cleanliness of the system. The open circuit potential experiment was carried out as follows: First, the electrode was immersed in solution and polarized at 1.40 V for 10 min while the formic acid concentration at solution was 0.10 mol L-1. Close to the end of this procedure the electrode was pressed against a CaF2 prismatic window producing a thin layer of solution between both. Immediately, the circuit was opened and thus the electrode potential was allowed to evolve while a series of spectra were collected with use of a rapid scan mode of acquisition. A total of 10 scans (equivalent to a sampling rate of 1 s) were coadded for spectra construction and analysis. Polarization of the infrared beam was perpendicular to the surface, allowing bands for both adsorbed and dissolved species to be observed. The spectrum used as reference was obtained for the first seconds of open circuit condition, before adsorbed carbon monoxide was produced. This led to absolute bands for COad. Data are presented in reflectance mode for which negative bands represent production of a given species and positive bands mean its consumption. 3. Model In spite of mechanistic details, the open circuit interaction between platinum oxides and different organic compounds retains several similarities. In a previous work we addressed some universal features based on the cases of methanol and ethanol.5 Specifically, we described an autocatalytic process responsible for the explosion in the oxide consumption and the consequent steep decrease in the open circuit potential. In the next we translate the current understanding of those processes into a simple mathematical model accounting for the interaction between formic acid and an oxidized platinum surface. Under conditions adopted here (namely below the limit of two monolayers13), part of the oxygen is located within the platinum surface, and referred to as subsurface oxygen, Osub. It
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18495 has been suggested11 that the availability of oxygen atoms at the electrode surface is controlled by the emergence of subsurface oxygen, k1
Osub-Pt 98 Pt-O
(r.1)
The term k1 is the rate constant for the emergence of subsurface oxygen, and k-1 would represent its counterpart for the oxygen insertion due to the place exchange. In this scenario, only surface oxygen, Pt-O species, are available to interact with formic acid. Furthermore, the release of subsurface oxygen is limited to about one-third of one monolayer, as previously suggested by Conway.13 As in the case of other organic molecules,10 at high oxide content, formic acid molecules are expected to interact with the electrode via an Eley-Rideal step involving adsorbed oxygenated species and dissolved formic acid: k2
HCOOH + Pt-O 98 CO2 + H2O + Pt
(r.2)
The associated rate of reaction should be slow, however, due to the fact that the activated complex is not being stabilized via the platinum bond. This fact is indeed reflected in the initial slow decrease of the open circuit potential along the region of high oxide content.8 After the initial consumption of oxygenated species, adsorption of formic acid on platinum comes into play. It often has been assumed that adsorbed formic acid occupies two platinum sites:14,15 k3
HCOOH + 2Pt 98 (HCOOH)Pt2
(r.3)
Adsorbed formic acid can be oxidized basically through two different routes. First, the complete electro-oxidation to carbon dioxide can be coupled to the electro-reduction of oxygen: k4
(HCOOH)Pt2 98 CO2 + 2H+ + 2e- + 2Pt
k5
2H+ + 2e- + Pt-O 98 H2O + Pt
(r.4)
(r.5)
Given that no net current flows through the open circuit, steps r.4 and r.5 are virtually coupled and can thus be represented by the overall net reaction: k6
(HCOOH)Pt2 + Pt-O 98 CO2 + H2O + 3Pt
(r.6) The second possible pathway consists of that occurring via adsorbed carbon monoxide:
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k7
(HCOOH)Pt2 + Pt 98 Pt-CO + H2O + 2Pt
(r.7) Reaction r.7 is responsible for the buildup of the COad coverage after the fast potential transient has been finished, since in this condition there is no oxide at the surface of the electrode. Adsorbed CO can be oxidized by surface oxygen in a Langmuir-Hinshelwood step: k8
Pt-CO + Pt-O 98 CO2 + 2Pt
(r.8)
Since the active pathway determines the current density observed under voltammetric conditions16 it is reasonable to assume that it will have a similar contribution in the open circuit behavior. Combining step r.3 with steps r.4 and r.5 or with steps r.7 and r.8 results in the following overall step:
HCOOH + Pt-O + 2Pt f CO2 + H2O + 3Pt
(r.9) in which the autocatalytic production of free sites of platinum is evident: two free sites produces three during the process. The autocatalytic step becomes more prominent when oxide content reaches a certain value for which formic acid adsorption can take place allowing reactions r.4 and r.5 to happen. On the basis of the above-described reactive scenario and having established the most probable reactions, a model scheme can now be proposed for the reaction investigated. Figure 1 depicts an illustrative representation of the main aspects of the interaction between formic acid and an oxidized platinum surface and labels the kinetic constants involved. To translate the ideas just presented into a set of ordinary differential equations (ODEs), the time evolution of the oxide coverage above and below the surface and the coverage with adsorbed formic acid and that of adsorbed monoxide were considered. Each case was associated to a rate law consisting of a sum of factors involved in the variation of the concentration of those species, according to the mass action rate law. The following conditions were involved in the establishment of the set of equations: (a) The number of platinum free sites available for formic acid adsorption corresponds to those platinum atoms which are not connected to subsurface or surface oxygen. This is based on the low catalytic activity observed in the cyclic voltammograms for elevated potentials, the same range of potentials for which the open circuit interaction happens. (b) Formic acid concentration was held constant at solution, and diffusional limitations were not considered. As the exact nature of the intermediates in the process of CO and CO2 production is not entirely clear, a generic adsorbed formic acid was chosen to represent it. (c) The coupled reaction of oxide reduction (eq r.5) and the reactions associated with the place-exchange mechanism have high kinetic constants. Taking all these assumptions into consideration, two sets of differential equations were constructed, according to the degree of surface oxidation. The first set accounts for the initial consumption of oxide. In our model it corresponds to the range in which the total oxide content varies from 1.2 to 0.99. Under
Figure 1. Schematics of the model for the interaction between formic acid and platinum oxides. This scheme was used for the elaboration of a set of differential equations representing the system.
those conditions, the adsorption of any carbonaceous species is negligible (see ref 14 and also experimental results below). The two ODE are
dλOsub ) -k1λOsub(0.33 - κOsur) + k-1κOsur dt
(1.1)
dκOsur ) k1λOsub(0.33 - κOsur) - k-1κOsur - k2cFAκOsur dt (1.2) Equation 1.1 describes the evolution of subsurface oxide, λOsub, and possesses a term representing the emergence of subsurface oxygen to the surface and another accounting for the reverse process. The value 0.33 corresponds to the maximum coverage for surface oxide. The variation of surface oxide, κOsur, is represented by eq 1.2, which contains a place exchange term and one relative to the direct interaction between surface oxides and formic acid in solution following an Eley-Rideal mechanism. The initial oxide content was set to 1.2 monolayer (which will be referred to as ML from now on) in order to match experimental data, see below. Numerical integration was halted when total oxide coverage reached a value of 0.99 ML. From this point on a new set of differential equations was employed to describe the dynamics of oxide consumption as well as adsorbed carbon monoxide and formic acid production and consumption. Equation 2.1 describes the evolution of subsurface oxide, λOsub, and is identical to eq 1.1.
dλOsub ) -k1λOsub(0.33 - κOsur) + k-1κOsur dt
(2.1)
The second differential equation, 2.2, shows how a generic adsorbed formic acid intermediate, f(t), behaves during the transient. It is composed of a term describing formic acid adsorption from an inexhaustible source, blocking two Pt sites (*), the second term refers to the formation of COad (step r.7), and a last term for the direct oxidation of adsorbed FA through the so-called direct pathway of CO2 formation, which is also responsible for the production of 2 electrons and the consequent coupled oxide reduction reaction (r.6):
Interaction of Formic Acid with Oxidized Pt Surfaces
df ) k3cFA(*)2 - k6 fκOsur - k7 f(*) dt
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(2.2)
TABLE 1: Kinetic Constants Employed for the Numerical Experiment Performed constant
The number of free platinum sites (*) represents the fraction of platinum atoms which are not blocked by neither CO, formic acid, or oxygen, thus following the conservation law:
k1 k-1 k2
(*) ) 1 - θCO - f - κOsur - λOsub
k3
As already mentioned, it is explicitly stated that both surface and subsurface oxygen blocks platinum sites. The third differential equation, 2.3, describes how adsorbed carbon monoxide, θCO(t), is produced via formic acid dehydration and consumed by reaction with platinum oxides:
dθCO ) k7 f(*) - k8θCOκOsur dt
(2.3)
Finally, eq 2.4 shows the pathways by which surface oxide is consumed. It consists of expressions for the interaction between atomic oxygen and COad and the coupled reaction of oxide reduction along with adsorbed formic acid direct oxidation as well as a term accounting for the direct interaction with the organic species described by an Eley-Rideal mechanism:
dκOsur ) k1λOsub(0.33 - κOsur) - κOsur(k-1 + k2cFA + dt k6 f + k8θCO) (2.4) Equations 2.1-2.4 were numerically integrated from an initial oxygen content of 0.99 on, using the following set of initial conditions:
λOsub ) 0.66;
κOsur ) 0.33;
θCO ) 0;
f)0
Table 1 summarizes all terms of our model, including physical meaning and numerical values. The CO2 production was calculated via numerical integration of the different steps involved:
NCO2 NPt
)
∫0t κOsur(k2cFA + k6 f + k8θCO) dt
This equation comprises the pathways of formation of CO2 by adsorbed CO, conjugated reactions, and direct oxidation via Eley-Rideal mechanism. Since the parameters involved on the calculation are normalized by the total number of platinum sites (as they represent coverages) the above sum also results in a normalized quantity. Some words should be spent on the values attributed to the individual rate constants. First, there are parameters which are clearly more determinant for the simulation to have a quantitative resemblance with experimental data. Those are the rate constants associated with CO2 production, namely, that related to the direct interaction with oxides (k2), another to the CO oxidation (k8), and that one concerning the direct pathway of oxidation (k6). The rate of CO production (k7) is also important as its relation with the associated oxidation constant will give the characteristics of the observed CO curve.
k6 k7 k8 cFA
meaning rate constant for emergence of subsurface oxide rate constant for insertion of oxide into the surface rate constant for the Eley-Rydeal step rate constant for HCOOH adsorption rate constant for reaction involving active intermediary rate constant for COad production rate constant for CO oxidation bulk formic acid concentration
value 20 s
-1
1 s-1 0.42 mol-1 L s-1 100 mol-1 L s-1 100 s-1 4 s-1 10 s-1 0.1 mol L-1
Although the model is very robust and several values of rate constants could produce a comparable qualitative behavior, quantitative agreement with experimental data was found for the set given at Table 1. In the process of fitting, the parameters mentioned above were the more relevant ones and their values could be changed by about only 5%. As for the other parameters it was found that the rate constant for formic acid adsorption (k3) should display a relatively large value in order to account for a steep transient to be observed. The relationship between the oxide emergence (k1) and insertion (k-1) should be comparatively large so that the surface would be always saturated with oxygen. Finally formic acid concentration (cFA) was arbitrarily chosen equal to the experimental value. Those last four constants were not individually determinant to a good quantitative fitting and their associated values could be changed individually by about 10%. 4. Results and Discussion The open circuit experiments were carried out as follows. First, oxide content was built up at the polycrystalline platinum electrode by setting its potential at 1.40 V for 10 min in the presence of 0.10 mol L-1 formic acid. Second, the circuit was opened and electrode potential followed along with a continuous collection of in situ infrared spectra. Results are depicted in Figure 2. Figure 2A shows the time evolution of the open circuit potential and the integrated band intensities for CO and CO2. Panel B shows the collection of spectra obtained during the transient. In Figure 2A it is seen that the open circuit potential initially decreases at a slow rate for a range of potentials at which platinum oxide is present. Podlovchenko and co-workers had observed a decrease in the oxygen content of about 0.2 ML for this region of the transient.8 This small kinetic activity has been attributed to the slow emergence of subsurface oxygen atoms.11 A region of sudden change in potential follows at about 17s. During this period, which lasts for only 2 s, the remaining oxide is reduced at a fast rate. This behavior was modeled by the mechanism of conjugated pair reactions of formic acid oxidation coupled to oxide reduction by Manzhos et al.8 Finally a region of monotonic increase in potential is observed for longer times. In situ IR spectroscopy provided a great deal of mechanistic information on this system. It can be observed at panels A and B of Figure 2 that for the region of high potentials there cannot be observed any band associated to adsorbed CO (2050 cm-1) indicating that this species is either not produced or consumed at a higher rate than the temporal resolution of our experimental setup. The spectroscopic feature associated to dissolved CO2
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(
θO ) 1.2 1 -
Figure 2. (A) Transient of the open circuit potential during interaction between platinum oxides and 0.1 mol L-1 formic acid and the associated temporal evolution of the integrated bands for CO2 and COad. (B) Collection of spectra during the transient. Electrolyte was 0.5 mol L-1 H2SO4.
(2340 cm-1) can be seen from the beginning of the transient, but with a rather small intensity up to the fast transition region. A massive production of CO2 is observed at 17s during the sudden drop of the open circuit potential, cf. Figure 2B. The band at 1730 cm-1 is due to the fast consumption of formic acid from the thin layer.17 Then follows a region of slow increase in potential that is associated with the appearance of a band at 2050 cm-1, related to on-top adsorbed carbon monoxide. The COad band gradually increases to a stationary value. The CO2 band continuously increases after the transient, a fact that could be explained by the mechanism of conjugated pair reaction between formic acid oxidation and, in this case, proton reduction. This latter observation is a signal that formic acid is not exhausted from the thin layer during the fast transient. An independent CO stripping experiment carried out under nearly identical optic conditions revealed a total amount of CO2 of 42.0 au, as measured by band integration, while the sum for integrated signals from linear and bridge bonded CO was 40.0 au. Charge measurement for the stripping experiment revealed that the proportion between CO molecules and Pt sites was 0.85. Therefore, if all platinum sites were involved in the reaction, the measured quantity for CO2 would be 49.4 au. As can be seen in Figure 2A the amount of CO2 produced during the fast transient was 60 au, which corresponds to about 1.2 ML of oxide content. As for adsorbed CO, its amount reaches a maximum value at 28 au, which represents approximately 0.7 ML. In spite of some controversies concerning the conversion of integrated bands for CO to coverages (see ref 18 and references therein), this procedure has been adopted in other comparable contexts.18,19 Since there is a simple 1:1 relation between consumed O and produced CO2, either via step r.2 or step r.4, the temporal evolution of the oxide content can be reconstructed from the data for carbon dioxide. It is easy to find the relation:
ICO2 ICO2,max
)
for which θO represents the total oxygen content in monolayers and ICO2 stands for the integrated signal for carbon dioxide. Results of the temporal evolution of the total oxygen content, carbon monoxide coverage, and carbon dioxide produced per platinum site are given in Figure 3, for (a) experiments and (b) simulations. Visual comparison between experimental and simulated data reveals an almost quantitative agreement. It can be seen in Figure 3a that during the slow transient at high potentials, oxide coverage drops by about 0.2 ML, 17% of the total content, which is essentially the same result obtained by Manzhos et al.8 using an electrochemical methodology. Figure 3b shows the temporal evolution for the oxygen content (θo ) λOsub + κOsup), the coverage with adsorbed CO, and the CO2. It can be observed that surface oxide content does not change appreciably for the first instants of reaction. This is a result of the high constant associated with withdraw of oxide from the crystallite of platinum (k1) that guarantees a high surface coverage. As long as formic acid is reacting with surface oxide via the indirect pathway the oxide content is being kept constant by the place-exchange mechanism. This characteristic is necessary for the observation of an acute transient. The total oxygen coverage, however, experiences a monotonic decrease in value since the beginning of the transient, with its speed accelerating with time. The curve for the adsorbed carbon monoxide shows an essentially zero coverage in the high potential region and gradually increases, achieving a saturation value for long times. The set of rate constants employed generated a simulated behavior qualitatively corresponding to that observed experimentally. Particularly for Figure 3b, low values for the CO coverage when surface oxides are present are observed, and its convergence to a stable value after the fast transient. The temporal evolution of total oxide content is also equivalent to that found experimentally by Podlovchenko and co-workers.8 The main discrepancy between experiments and simulations consists of the continuous increase of the CO2 production observed in the experiments and not represented in the simulations. In their seminal paper Capon and Parsons20 acknowledged the fact that CO2 was evolved when formic acid was inserted into the cell while the electrode has been at open circuit conditions. That result and ours show that the molecule could undergo a disproportionation reaction producing CO2 alongside H2. Such reaction was not accounted in the model for simplification purposes and therefore is not reproduced in the simulation results. Finally, the time derivative of the coverage evolution profiles for adsorbed CO and CO2 was calculated for evaluating the turnover frequencies (referred to as TOF) for the dehydration and oxidation processes. The results are shown at Figure 4a along with the corresponding simulated values in Figure 4b. Turnover frequencies were obtained by Chen et al.21 for the buildup of a monolayer of CO originating from formic acid dehydration at potentiostatic control. TOF values were found to be potential dependent ranging from 3.2 × 10-4 at 0.6 V to 1.8 × 10-3 at 0.5 V and 2.0 × 10-2 at 0.4 V. These values were obtained calculating the time derivative of the coverage at t ) 0 s and show that for the conditions influencing the study, CO production from formic acid is a slow reaction. The results obtained for OCP transients were quantitatively different from the potentiostatic experiments. First, no bridged bonded CO was observed under open circuit conditions. Second, TOF values
Interaction of Formic Acid with Oxidized Pt Surfaces
Figure 3. (a) Time evolution of the oxygen and carbon monoxide coverages and the amount of carbon dioxide produced (per platinum sites) during the open circuit transient depicted in Figure 1 for 0.1 mol L-1 formic acid and 0.5 mol L-1 H2SO4 electrolyte. (b) Simulated temporal evolution of COad, CO2, surface oxides, total oxides, and free sites of platinum according to the proposed model and kinetic constants given in Table 1.
obtained from Figure 3 were of approximately 0.5 molecules s-1 site-1, which is 1 order of magnitude higher than that obtained for 0.4 V.21 Also, it can be seen that the saturation time was 90 s, while in those studies it could take as much as 300 s. This difference in behavior can be reasonably explained taking into account that under open circuit conditions, CO electro-oxidation is inhibited, which thus makes saturation time lower and saturation coverage higher. Thus, the TOF value obtained under open circuit conditions can be viewed as fundamental during the dehydration of formic acid since it is independent of the CO oxidation reaction, which could lead to subestimation of the turnover frequencies as well as a dependency of these values on potential, for a reaction that is not directly associated with charge transfer (dehydration). However, we point out that the obtained TOF value of 0.5 molecules s-1 site-1 possesses significance only while keeping the pretreatment executed in mind. As discussed by Osawa and collaborators16 adatoms and holes produced during reduction of an oxidized electrode may be very reactive for formic acid oxidation and thus lead to an enhanced rate of reaction. In Figure 4b the time derivative of the evolution of the simulated CO and CO2 coverages is presented. It is seen that numerically calculated behavior is quantitatively equivalent to that observed experimentally for the set of parameters employed. Besides the comparison with experimentally obtained data, we were also able to extract information from the simulations that was not accessible in experiments, namely the curve of evolution of free platinum sites. During the first moments of theoretical transient its values remain small in rather a stationary state. Then suddenly, as the fast transient approaches, its value starts increasing exponentially, to a maximum when the coverage with
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Figure 4. (a) Time derivative of carbon monoxide coverage and the amount of carbon dioxide produced (per platinum sites) during the open circuit transient depicted in Figure 2 for 0.1 mol L-1 formic acid and 0.5 mol L-1 H2SO4 electrolyte. (b) Same analysis as in part a for the simulated results, along with the calculated evolution for the free platinum site variable.
TABLE 2: Effect of Increasing Values of Several Parameters on Time of Transient, Stationary CO Coverage, and CO2 Curvea constant
time of transient
stationary θCO
k1 v k-1 v k2 v k3 v k6 v k7 v k8 v cFA v
V v V v V V V V
≈ ≈ v V v v ≈ V
CO2 curve fr T fr T fr fr fr T
a Key: v increase, V decrease, ≈ not sensible, T broadening, f r stretching.
CO and oxide are the same. Such behavior is a signature of an autocatalytic process of production of free platinum sites.22 Table 2 brings the qualitative effects produced on some relevant aspects of the transient curve for varying parameters. Analyzing the data presented in Table 2 it is possible to infer on the individual weight of the several steps constituting the mechanism on its general behavior. This understanding is fundamental for electrocatalysis and chemistry in a broader sense, since it represents the way in which the system can be controlled. In the case of the open circuit transients, for example, which is related to the problem of crossover of organic species from the anodic to the cathodic chamber of a fuel cell, it is interesting that the time the cathode expends at high potentials is as high as possible. As though, strategies that employ catalysts which inhibit CO production and had low interaction with organic species could be of value.
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The model proposed for the open circuit interaction of formic acid with oxidized platinum surfaces could be extended for the case of methanol. The easiest way to do this would be to increase the value of the sites necessary for adsorption mimicking the greater steric factor for this molecule, or increasing the number of sites necessary for dehydrogenation. The experimental evidence of an explosive reaction between the organic species studied here and in other work5 can be related to the results obtained under ultrahigh vacuum conditions which show essentially second order autocatalysis.23 In electrochemical systems, however, higher order autocatalysis can take place, since the condition of reactants proximity is not necessarily a condition for reaction. By the mechanism of conjugated reactions, organic oxidation and oxide reduction can be coupled and, however, happen at distant sites. The increase in the rate of earlier reaction steps by later products, namely autocatalysis, is an enticing phenomenon. It plays a central role in the development of chemical oscillations and traveling waves and also for the enhancement of fluctuations which eventually lead a system to a new state of organization.22 If explored rationally it could have profound and “high rating” consequences for heterogeneous catalysis.24 5. Conclusions We presented in this work a comprehensive study of the open circuit interaction between formic acid and an oxidized platinum surface. The study was carried out in terms of the transients of the open circuit and in situ IR spectroscopy. In addition we have also set a model that describes the main aspects found in experiments, as evidenced by numerical simulations. The main aspects are summarized in the following. For initial seconds of interaction there is a small decrease in potential associated to a small increase in the quantity of CO2 produced. This production augments explosively at the middle of transient while the open circuit potential decreases steeply. After the fast transient, adsorbed CO starts being produced and its coverage as well as the potential value develops to a saturation value. Detailed analysis of the possible reaction pathways demonstrated an autocatalytic production of free sites of platinum, whose role would be important particularly for the middle region of transients. Autocatalytic production of free sites for adsorption can explain the sudden rise in concentration of CO2 since the production of this species is directly proportional to the former quantity. We have also proposed a model for the open circuit reaction and derived a set of representative differential equations. Numerical integration of these equations revealed a nearly quantitative agreement with the experimental observation for COad and CO2 evolution. Numeric experiments also shed some light on the free platinum site “hidden” variable showing that its production is exponentially increased in the region of fast transient. The effect of altering values for the different kinetic parameters on the overall behavior of the simulated curves was also explored revealing great dependence for the transient
Batista and Varela features on the parameters related to place exchange and direct oxidation of formic acid. In a broader sense the results presented in this article revealed the importance autocatalytic production of catalyst’s free sites can have on the enhancement of rate values for heterogeneous reactions. Through its knowledge, modeling and numerical simulation can help uncover the weight the individual steps have on overall behavior allowing us to look for optimization, selectivity, and control of those processes with a more secure eyes. Acknowledgment. The authors acknowledge Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP, HV no. 09/07629-6) and Conselho Nacional de Desenvolvimento Cientı´fico (CNPq, BCB no. 141723/2009-2, HV no. 302698/20078) for financial support. We are also very thankful to the reviewers for bringing us interesting points on the modeling. References and Notes (1) Breiter, M. W. J. Electrochem. Soc. 1962, 109, 425–427. (2) Oxley, J. E.; Johnson, G. K.; Buzalski, B. T. Electrochim. Acta 1964, 9, 897–910. (3) Jusys, Z.; Behm, R. J. J. Phys. Chem. B 2001, 105, 10874–10883. (4) Manzhos, R. A.; Podlovchenko, B. I.; Maksimov, Yu. M. Russ. J. Electrochem. 2007, 43, 1268–1272. (5) Batista, B. C.; Sitta, E.; Eiswirth, M.; Varela, H. Phys. Chem. Chem. Phys. 2008, 10, 6686–6692. (6) Varela, H.; Sitta, E.; Batista, B. C. Methanol oxidation on oxidized Pt surface. In Handbook of Fuel Cells Fundamentals, Technology and Applications; Vielstich, W., Yokokawa, H., Gasteiger, H. H., Eds.; John Wiley & Sons, Ltd.: Chichester, UK, 2009; Vol. 5, Chapter 13, pp 1-15. (7) Podlovchenko, B. I.; Manzhos, R. A.; Maksimov, Yu. M. Russ. J. Electrochem. 2002, 38, 1292–1298. (8) Manzhos, R. A.; Podlovchenko, B. I.; Maksimov, Yu. M. Russ. J. Electrochem. 2006, 42, 658–664. (9) Smolin, A. V.; Podlovchenko, B. I.; Maksimov, Yu. M. Russ. J. Electrochem. 2009, 45, 246–251. (10) Podlovchenko, B. I.; Manzhos, R. A.; Maksimov, Yu. M. Russ. J. Electrochem. 2006, 42, 1061–1066. (11) Sitta, E.; Varela, H. J. Solid State Electrochem. 2008, 12, 559– 567. (12) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271–340. (13) Conway, B. E. Prog. Surf. Sci. 1995, 49, 331–452. (14) Mukouyama, Y.; Kikuchi, M.; Samjeske´, G.; Osawa, M.; Okamoto, H. J. Phys. Chem. B 2006, 110, 11912–11917. (15) Angelucci, C. A.; Varela, H.; Herrero, E.; Feliu, J. M. J. Phys. Chem. C 2009, 113, 18835–18841. (16) Samjeske´, G.; Miki, A.; Ye, S.; Osawa, M. J. Phys. Chem. B 2006, 110, 16559–16566. (17) Iwasita, T.; Xia, X.; Herrero, E.; Liess, H.-D. Langmuir 1996, 12, 4260–4265. (18) Boscheto, E.; Batista, B. C.; Lima, R. B.; Varela, H. J. Electroanal. Chem. 2010, 642, 17–21. (19) Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem. 1988, 92, 4019– 4022. (20) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973, 45, 205–231. (21) Chen, Y. X.; Heinen, M.; Juzys, Z.; Behm, R. J. Angew. Chem. 2006, 45, 981–985. (22) Gray, P.; Scott, S. K. Chemical Oscillations and Instabilities; Oxford University Press: New York, 1990; Chapter 1, pp 1-31. (23) Sharpe, R. G.; Bowker, M. J. Phys.: Condens. Matter 1995, 7, 6379–6392. (24) Tributsch, H. Electrochim. Acta 1994, 39, 1495–1502.
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