The potential dependence of the rate of dehydration of formic acid to adsorbed CO (COad) on Pt at pH 1 has been studied on a polycrystalline Pt surface by time-resolved surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) with simultaneous recording of current transients after a potential step. A range of formic acid concentrations has been used to obtain a deeper insight into the mechanism of the reaction. The experiments have allowed us to confirm that the potential dependence of the rate of dehydration has a bell shape, going through a maximum around the potential of zero total charge (pztc) of the most active site. The analysis of the integrated intensity and frequency of the bands corresponding to COL and COB/M shows a progressive population of the active sites on the surface. The observed potential dependence of the rate of formation of COad is consistent with a mechanism in which the reversible electroadsorption of HCOOad is followed by its rate-determining reduction to COad.

The study of the formic acid oxidation reaction (FAOR) on platinum is a popular research topic in electrocatalysis that has received much interest and attention for a long time.1,2 This is because the formic acid oxidation is the simplest electrocatalytic reaction involving an organic molecule (only two electrons and two protons need to be transferred for its complete oxidation to CO2), and it can, therefore, be seen as a model reaction to understand the mechanism of more complex reactions such as methanol oxidation.3–6 Furthermore, formic acid can be used as fuel in direct formic acid fuel cells (DFAFCs).7,8 It is well known that the FAOR proceeds on Pt through a dual-path mechanism, first proposed by Capon and Parsons back in 1973,9 composed of a direct path that leads directly to the formation of CO2, and an indirect path through a poisoning intermediate, namely, adsorbed CO (COad).10,11

Some years ago, our group demonstrated that adsorbed formate (HCOOad) is the bifurcation point in the dual-path mechanism, the last intermediate common to both paths (see Scheme 1).11,12 This very important conclusion has been discussed, analyzed, and confirmed by our and other research groups during the last ten years.13–16 

SCHEME 1.

The dual path mechanism for the electrocatalytic oxidation of formic acid, where adsorbed formate is the last intermediate common to both paths.

SCHEME 1.

The dual path mechanism for the electrocatalytic oxidation of formic acid, where adsorbed formate is the last intermediate common to both paths.

Close modal

Formate can be adsorbed through either one or both of its oxygen atoms (mono- and bidentate adsorbed formate, respectively). Bi- and monodentate adsorbed formates should be expected to be in equilibrium with each other on the electrode surface, although the equilibrium is strongly displaced to the bidentate form, which is the most stable of these configurations and the only one that has been detected on electrode surfaces.11,17–20 The bidentate adsorbed formate was initially proposed to be the reactive intermediate in the direct path,21 but this has been disproved because the H atom is far from the surface and, thus, the kinetic barrier for the cleavage of the C–H bond is very high.20,22 The strong evidence that some form of HCOOad must still be the reactive intermediate in the direct path of the FAOR14,15,23–27 has led to the current consensus that the monodentate adsorbed formate is the actual reaction intermediate. Although the bidentate form can block sites on the electrocatalyst surface needed for the reaction to proceed, it is not a dead end, as it can act as a reservoir of monodentate adsorbed formate through the chemical equilibrium connecting the two forms and can stabilize the H-down form of neighboring monodentate adsorbed formates.14,28–30

In the so-called indirect path, COad was very early identified as the catalytic poison.10,31–33 COad forms around a quite narrow potential range of about 100 mV around the local point of zero total charge (pztc).11,34 Although the rates of COad formation are significantly slower than those of the direct path,14 from a practical perspective, the buildup of COad results in a considerable decrease in the DFAFC’s power and efficiency because it can be removed only at high overpotentials. The study of the mechanism of its formation can provide the information required to engineer electrocatalysts on which COad formation is inhibited. Our group has shown that a minimum of three contiguous Pt atoms are required for the dehydration of HCOOH on Pt,19 and later, we provided spectrokinetic evidence using time-resolved surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) that HCOOad is also the intermediate in the indirect path and, therefore, the last intermediate common to both paths in the reaction mechanism (see Scheme 1).11,12

Recent pulsed-voltammetry studies of the kinetics of the direct and indirect pathways of formic acid oxidation on Pt(111) and Pt(100)15,16 confirm, in agreement with Grozovski et al.34 and previous predictions,11 that the rate of dehydration of formic acid depends on the applied potential and goes through a maximum and that the rate of poisoning by COad on Pt(111) is much slower than on Pt(100), as expected from the difference between their pztc’s.11 The more positive pztc on Pt(111) implies that more positive potentials, at which the rate constant for the reduction of HCOOad to COad is smaller, are needed to have a sufficiently high coverage of HCOOad. On the contrary, on Pt(100), sufficiently high coverages of HCOOad can be reached at more negative potentials, at which the rate constant for the reduction of HCOOad to COad is larger and the buildup of COad happens faster.15 

We have also recently studied the oxidation of methanol to COad on Pt using the high sensitivity and the absence of transport limitations offered by ATR-SEIRAS,35 which allowed us to monitor in real-time the evolution of the COad coverage (θCO). Our results allowed us to infer which sites on the Pt surface are the active ones in which potential region for the methanol dehydrogenation reaction and provided Tafel plots that revealed the effect of adsorbed spectator species on the reaction rate. Following the same methodology, we present here a detailed study that combines chronoamperometry with time-resolved ATR-SEIRAS. These experiments have allowed us to carefully monitor the progressive population of COad on the Pt surface at pH 1 as a function of potential and formic acid concentration. These results are compared with those recently obtained for methanol dehydrogenation35 and discussed within the context of our recent results using pulsed voltammetry.15,16

Electrolytes were prepared by dissolving HClO4 (70%, Merck p.a. EMSURE) in ultrapure water (Milli-Q) up to a concentration of 0.1M (approximate pH 1). The solutions containing formic acid (Sigma-Aldrich ≥ 98%) were prepared by adding it to the desired concentration (8 × 10−3, 4 × 10−3, 2 × 10−3, 10−3, and 5 × 10−4M). All the experiments were performed using N2 purging at room temperature. A flame-annealed Pt wire (Alfa Aesar, 99.997% metals basis) was used as a counter electrode and a homemade Ag/AgCl (KClsat) electrode as a reference. However, all the potentials in the text are referred to the reversible hydrogen electrode (RHE) unless otherwise stated. The working electrode was a Pt film deposited following a previously reported procedure21 on the totally reflecting plane of a Si prism beveled at 60°.

The ATR-SEIRA spectra were recorded using a Nicolet iS50R FTIR spectrometer equipped with a liquid nitrogen-cooled MCT detector and a homemade ATR accessory, using unpolarized light. A new reference spectrum was recorded before each spectral series at 0.96 V vs RHE. This potential was chosen for the background spectrum because it is positive enough to have any present COad oxidized before starting the experiment17,36 while minimizing changes in the baseline of the spectra. Differential spectra are reported in absorbance units (a.u.), calculated as logRsampleRreference, where Rreference and Rsample are the reference and sample spectra, respectively. The positive bands correspond to species present in the sample spectrum that were absent or present at a lower concentration in the reference spectrum, while the negative bands correspond to species present in the reference spectrum that are absent or present at a lower concentration in the sample spectrum. A series of differential spectra after a potential step were obtained in the kinetics mode by accumulating 1 interferogram per spectrum with a spectral resolution in the range between 4 and 32 cm−1. An adequate combination of these two parameters allowed for achieving a time resolution high enough to monitor the speed of the COad band growth with a good signal-to-noise ratio for each specific experiment. The specific parameters used in each of the experiments presented in this work are provided in the corresponding figure captions. The Si prism where the working electrode is deposited was attached to the spectroelectrochemical cell using an O-ring seal, and electrical contact with the film was made by pressing onto it a circular platinum wire. Before any IR measurements, the film was cycled in the corresponding electrolyte to check its stability and then rinsed with Milli-Q water before filling the cell with the solution containing the target concentration of formic acid.

Figure 1(a) shows the cyclic voltammogram (CV) of polycrystalline Pt in 0.1M HClO4 within the potential range between 0.05 and 0.46 V, which encompasses the Hupd region plus the negative end of the double-layer region. Characteristic chronoamperometric transients recorded simultaneously with the time-resolved ATR-SEIRA spectra in the presence of 2 × 10−3M HCOOH after a potential step from 0.96 V to selected potentials within this potential region are presented in Fig. 1(b).

FIG. 1.

(a) Zoom into the region between 0.05 and 0.5 V of the cyclic voltammogram of a polycrystalline Pt electrode in 0.1M HClO4 at 0.05 V s−1, which corresponds to the potential region where the formation of COad was monitored using the time-resolved ATR-SEIRAS. The whole CV is shown in the inset. (b) Characteristic current transients recorded simultaneously with an ATR-SEIRA spectral series in 2 × 10−3M HCOOH solutions in 0.1M HClO4 after a potential step from 0.96 V to selected potentials within this region.

FIG. 1.

(a) Zoom into the region between 0.05 and 0.5 V of the cyclic voltammogram of a polycrystalline Pt electrode in 0.1M HClO4 at 0.05 V s−1, which corresponds to the potential region where the formation of COad was monitored using the time-resolved ATR-SEIRAS. The whole CV is shown in the inset. (b) Characteristic current transients recorded simultaneously with an ATR-SEIRA spectral series in 2 × 10−3M HCOOH solutions in 0.1M HClO4 after a potential step from 0.96 V to selected potentials within this region.

Close modal

Despite not involving any net electron transfer, the rate of the dehydration of formic acid to COad on Pt,

HCOOHCOad+H2O,
(1)

has been known for decades to be potentially dependent. As we have shown,11,12 this potential dependence of a purely chemical reaction can be explained if a two-step electrochemical mechanism is followed, in which an oxidative reversible electroadsorption,

HCOOHHCOOad+H++e,
(2)

is followed by the rate-determining reduction of formate (which may, and most likely does, occur in more than one elementary step),

HCOOad+H++eCOad+H2O,
(3)

Because this reaction sequence leads to a zero net electron transfer, the rate of this reaction cannot be determined directly from electrochemical experiments, although it can be indirectly determined from the decay of the current corresponding to either the formic acid oxidation reaction (FAOR) or the hydrogen evolution reaction (HER) at the selected potential using an appropriate kinetic model.13,15,20,34 The extrapolation of the so-obtained rate of formation of COad to t = 0 leads to the intrinsic rate of poisoning of the catalyst, i.e., corresponding to zero COad coverage (θCO = 0). This extrapolation is, however, not free from interference, as the current measured will contain contributions from double-layer charging, the reduction of the PtOx layer created at the initial potential, and the adsorption and then COad-induced desorption of Hupd.35 

The transients in Fig. 1(b) all show a continuous current decay, clear evidence of self-poisoning due to the buildup of COad. At 0.51 V, on the contrary, after an initial decay, a constant current is reached, which suggests that a stationary state has been reached at which θCO remains constant because it is being oxidized at the same rate at which it is being formed. The shape of the transient at 0.46 V suggests that it also tends toward a steady state, but our experiment was not long enough for that steady state to be reached. The noisy transients at both 0.46 and 0.51 V (and, to a lesser extent, also at 0.41 V) suggest a mixed kinetic-transport controlled oxidation of formic acid (experiments were performed while bubbling N2 through the solution). The current is higher at 0.46 than at 0.51 V because (i) the steady state at the former potential has not been reached and (ii) 0.51 V is located after the peak potential in the oxidation of formic acid (see CVs in Refs. 15 and 16 and the discussions therein).

In contrast to the limitations in the study of these current transients, the high time resolution achievable with ATR-SEIRAS allows us to follow the growth of the infrared COad bands after a potential step and to extrapolate the rate of COad formation to t = 0 without interference from any other process. We have recently demonstrated how powerful this approach can be and the wealth of information that can be gathered from this kind of experiment in the case of the oxidation of methanol to COad on Pt electrodes.35 As shown there, with the time resolution achieved in our ATR-SEIRAS experiments, the recording of the time evolution of the COad bands in the spectral series starts once the interfacial potential profile has been properly established and is, therefore, an accurate reflection of the kinetics of the dehydration reaction at the corresponding applied potential.

Figure 2(a) shows a typical time-resolved ATR-SEIRA spectral series recorded after a potential step from 0.96 to 0.11 V in 0.1M HClO4 containing 2 × 10−3M HCOOH with a time interval of 0.15 s. Please note that t = 0 corresponds to the last spectrum in the series just before the first observation of a COad band. The spectra separated by 0.3 s are shown as a representative sample of those recorded during the first 5 s, while the spectra separated by intervals of 70 s have been selected for the period between 5 and 1000 s.

FIG. 2.

(a) A typical time-resolved ATR-SEIRA spectral series showing the buildup of COad on Pt in 0.1M HClO4 containing 2 × 10−3M HCOOH, recorded after a potential step from 0.96 to 0.11 V vs RHE. Each spectrum consists of a single interferogram recorded with a spectral resolution of 16 cm−1, resulting in a time interval between spectra of 0.15 s. The reference spectrum was taken at 0.96 V. Only selected spectra are shown. Between 0 and 5 s (red spectrum), a spectrum every 0.3 s is shown, whereas from that point onwards, a spectrum every 70 s is shown. (b) Time dependence of ICOL (red) and ICOB/M (black) during the whole experiment. (c) Time evolution of the stretching frequency of COL (black line) and of ICOL (red line) during the first 80 s.

FIG. 2.

(a) A typical time-resolved ATR-SEIRA spectral series showing the buildup of COad on Pt in 0.1M HClO4 containing 2 × 10−3M HCOOH, recorded after a potential step from 0.96 to 0.11 V vs RHE. Each spectrum consists of a single interferogram recorded with a spectral resolution of 16 cm−1, resulting in a time interval between spectra of 0.15 s. The reference spectrum was taken at 0.96 V. Only selected spectra are shown. Between 0 and 5 s (red spectrum), a spectrum every 0.3 s is shown, whereas from that point onwards, a spectrum every 70 s is shown. (b) Time dependence of ICOL (red) and ICOB/M (black) during the whole experiment. (c) Time evolution of the stretching frequency of COL (black line) and of ICOL (red line) during the first 80 s.

Close modal

The characteristic bands corresponding to the C–O stretching of linearly adsorbed CO (COL, appearing initially at 2010 cm−1) and to bridge-bonded (COB) plus multiply-bonded (COM) adsorbed CO (the very asymmetric band initially appearing at 1790 cm−1) are clearly visible in the spectra. For the sake of simplicity and following Yan et al.,37 COM and COB will be considered jointly (COB/M) when integrating the band intensity for analysis of the rate of formation of COad.

As expected, the integrated intensity of both COL (ICOL, red) and COB/M (ICOB/M, black) increases with time until they reach a constant value when the maximum θCO is reached (ICOB/M reaches its maximum value at shorter times). Due to dipole–dipole coupling between neighboring adsorbed oscillators,38,39 the continuously decreasing distance between oscillating COad dipoles as θCO increases should lead to a continuous increase in the CO-stretching frequency parallel to the increase in the integrated absorbance [shown in Fig. 2(c) for COL]. However, the frequency of the COL band remains constant within our experimental error during the first 5 s [the vertical lines in Fig. 2(a) show that this is true both for COL and COB], although the integrated absorbance increases during this time. This suggests that, during this initial 5 s, θCO is low enough for the surface to accommodate the newly formed COad without a significant decrease in the distance between oscillating dipoles. This result is in good agreement with our recent work on the dehydrogenation of methanol on Pt, where the frequency was found to grow in a staircase manner with time too during the initial stages of COad buildup. However, in that case, the rate of COad growth was slower, and more than one period of constant or nearly constant frequency, despite continuous growth in θCO, could be observed.35 In addition, contrary to what we observed during the oxidation of methanol to COad on Pt electrodes,35 the COL band shows a clear inhomogeneous broadening already in the first spectrum recorded, suggesting that more than one kind of adsorption sites have already been occupied after 0.1 s.38,39

The derivative of the spectroscopic transient (i.e., of the time dependence of the integrated absorbance of the COL and COB/M bands) provides a direct measure of the rate of formation of COad. Because the product of the reaction (COad) blocks sites on the surface, any kinetic study must be based on the rate of formation of COad at t = 0, when all the surface is active, which can be obtained by extrapolating the derivative of the spectroscopic transients to t = 0. Figure 3 shows plots of the time evolution of ICOL (red), ICOB/M (black), and (ICOL+ICOB/M) (green), as well as the time dependence of the corresponding derivatives with respect to time (dICOLdt, dICOB/Mdt, and d(ICOL+ICOB/M)dt, respectively), obtained at 0.11, 0.21, and 0.26 V from a series of time-resolved ATR-SEIRA spectra in 0.1M HClO4 containing 2 × 10−3M HCOOH.

FIG. 3.

Time dependence of ICOL (red), ICOB/M (black), and ICOL + ICOB/M (green) (top panels) and of the corresponding derivatives with respect to time (lower panels) obtained from a series of time-resolved ATR-SEIRA spectra in 0.1M HClO4 containing 2 × 10−3M HCOOH, recorded after a potential step from 0.96 to 0.11 (top left), 0.21 (top right), and 0.26 V (bottom). The integrated intensity vs time data were smoothed by fitting to a fifth-order polynomial, and the result of the fit (dotted yellow lines) was used to obtain the derivatives in the bottom panels.

FIG. 3.

Time dependence of ICOL (red), ICOB/M (black), and ICOL + ICOB/M (green) (top panels) and of the corresponding derivatives with respect to time (lower panels) obtained from a series of time-resolved ATR-SEIRA spectra in 0.1M HClO4 containing 2 × 10−3M HCOOH, recorded after a potential step from 0.96 to 0.11 (top left), 0.21 (top right), and 0.26 V (bottom). The integrated intensity vs time data were smoothed by fitting to a fifth-order polynomial, and the result of the fit (dotted yellow lines) was used to obtain the derivatives in the bottom panels.

Close modal

As we have discussed recently,35 when analyzing the rate of COad formation at short times, θCO needs to be estimated from the integrated intensities of both COL and COB/M, because at this initial reaction stages, ICOB/M and ICOL are similar, and at some potentials, COB/M is the only band present in the spectra. Ignoring this band when analyzing the rate of COad formation could, therefore, lead to a considerable error. This can be clearly seen in the results at 0.26 V reported in Fig. 3, where dICOLdt (red line) is seen to initially increase until a maximum is reached, after which it slowly decreases to zero. If COL is used as a proxy for the total θCO, the wrong and unreasonable conclusion would be reached that, in the initial stages of the reaction, increasing θCO results in an increase in the surface activity for the dehydration of formic acid. This can be avoided if the sum of the two observable bands and the corresponding derivative, d(ICOL+ICOB/M)dt, are used instead (Fig. 3, green lines). The fact that, under certain conditions, the rate of the reaction is seen to initially remain constant for a relatively long time suggests that, during this time, θCO remains low enough so as to have a negligible blocking effect. In other words, it is a confirmation that we are analyzing the kinetics of the reaction when θCO is extremely low.

The same analysis as shown in Fig. 3 has been performed at each applied potential with all the different concentrations of formic acid (data provided in the supplementary material, Figs. S1–S5), and the results have been used to plot the dependence of the rate of formic acid electrooxidation on the potential at several concentrations of HCOOH (Fig. 4).

FIG. 4.

Potential dependence of the rate of dehydration of HCOOH on Pt calculated by extrapolating to t = 0 the derivate of ICOL+ICOB/M at different concentrations of formic acid (ranging between 5 × 10−4 and 8 × 10−3M, as indicated in the figure) in 0.1M HClO4. Note that the error bars do not correspond to the standard deviation within a set of similar experiments but to the uncertainty in the rate of dehydration obtained by extrapolation at t = 0, due to the noise present in the integrated intensity vs time curves.

FIG. 4.

Potential dependence of the rate of dehydration of HCOOH on Pt calculated by extrapolating to t = 0 the derivate of ICOL+ICOB/M at different concentrations of formic acid (ranging between 5 × 10−4 and 8 × 10−3M, as indicated in the figure) in 0.1M HClO4. Note that the error bars do not correspond to the standard deviation within a set of similar experiments but to the uncertainty in the rate of dehydration obtained by extrapolation at t = 0, due to the noise present in the integrated intensity vs time curves.

Close modal

The dependence of the rate of dehydration of formic acid on the potential has a bell shape, with a maximum of around 0.2 V, in good agreement with previous work.15,16,34

As discussed in previous work,11 the potential at which the rate of dehydration is maximum corresponds to the point where the product of the formate coverage (θHCOO) times the rate constant of reaction (3), k3, is maximum and must, therefore, be associated with the potential of zero total charge (pztc) of the active site. As also discussed previously, because the value of k3 at any given potential can be expected to depend weakly on the atomic structure of the active site, but θHCOO at any potential and any concentration will be higher on the site with the most negative pztc, the rate of COad formation at the maximum is also the higher the more negative the pztc is. The pztc of Pt(111) is more positive than that of Pt(100), which is more positive than that of Pt(110).40–43 It is, therefore, not surprising that the maximum rate of formation of COad in Fig. 4 is around 0.2 V for all concentrations, which is closer to the pztc of (110)-oriented defect sites than to that of (100)-oriented sites and far away from the pztc of (111) sites. The contributions from one-dimensional and small two-dimensional (100)-oriented sites are expected to be around 0.3 V, while dehydration on larger two-dimensional (100)-oriented sites is expected to emerge around 0.4 V. These two contributions lead to the broadening of the peak in Fig. 4 with increasing HCOOH concentration. However, the negative shift of the potential at which the rate of formic acid dehydration is maximum with increasing HCOOH concentration, expected for the reaction mechanism composed of reactions (2) and (3)11 and recently observed when the reaction is catalyzed by Pt(100) electrodes,15 cannot be observed in Fig. 4, which shows a broad maximum between 0.15 and 0.2 V for all but the two lowest concentrations of HCOOH. The potential at which the rate of dehydration of formic acid on polycrystalline Pt reaches its maximum coincides very well with the completion of the first peak in the hydrogen adsorption region of the CV (see Fig. 1), which suggests that the observed lack of dependence of the position of the maximum on the HCOOH concentration is due to the overlap on (110)-oriented defect sites of the adsorption of formate with that of hydrogen. It should be noted that the peak potential for this peak should be almost independent of the formate concentration since the peak potential is almost identical in perchloric and sulfuric acid solutions (sulfate and formate adsorb similarly strongly on Pt).

At concentrations higher than 2 × 10−3M, the rate of dehydration in the potential region between 0.2 and 0.3 V [the latter corresponding to the second peak in the hydrogen region of the CV and attributed to the desorption of hydrogen from (100)-oriented sites] grows faster than at E < 0.2 V. We attribute this behavior to the contribution of (100)-oriented sites to the reaction. Because the potential at which the rate of dehydration is maximum on these sites will shift negatively with increasing HCOOH concentration,15 the contribution of these sites is responsible for the broadening of the peak in Fig. 4 as the concentration of formic acid increases.

Assuming that the reduction of HCOOad to COad [Reaction (3)] is the rate-determining step and taking into account the site-demanding nature of the formic acid dehydration reaction,19 the rate equation for the dehydration of formic acid is

dθCOdt=k3θHCOO(1θHCOOθHθCO),
(4)

However, because the reaction rate in Fig. 4 corresponds to that at t = 0, when θCO = 0, Eq. (4) reduces to

dθCOdt=k3θHCOO(1θHCOOθH).
(5)

Assuming that the adsorption of formate, reaction (2), obeys the Langmuir isotherm, and taking into account that formic acid dehydration occurs mostly within the hydrogen adsorption region, the coverage of formate will be given by

θHCOO=k2cHCOOH1θHk2+k2cHCOOH,
(6)

where k2 and k−2 are the rate constants of formate adsorption and desorption, respectively. The rate of formation of COad from formic acid dehydration on Pt should hence be

dθCOdt=k3k2cHCOOH1θHk2+k2cHCOOH(1θHCOOθH)
(7)

(note that because the pH is constant throughout the experiment, both k−2 and k3 contain the term corresponding to the H+ concentration, cH+).

At potentials at which either θH or θHCOO are very high, the rate of dehydration should be very small and approach zero at any HCOOH concentration, as is, indeed, the case in Fig. 4 at both the negative and positive potential ends of the plot. At any other potential, the dependence of the reaction rate on the formic acid concentration will be complex. However, at potentials at which θH and θHCOO remain constant [as expected between the two peaks in the hydrogen adsorption region of Pt, where replacement of Had by HCOOad on (110)-oriented defects must be complete but has barely started yet on (100)-oriented sites], the reaction is expected to be first order with respect to the formic acid concentration as long as k2cHCOOHk−2. As shown in Fig. 5, this seems to be, indeed, the case between 0.19 and 0.26 V, although at the lowest concentration used, the experimental data clearly deviate from the trend at higher concentrations.

FIG. 5.

Double logarithmic plot of the dependence on the formic acid concentration of the rate of dehydration of HCOOH to COad on Pt at t = 0 within the potential region between 0.19 and 0.26 V. The values in the legend correspond the slope at the indicated potential in the linear region of the plot.

FIG. 5.

Double logarithmic plot of the dependence on the formic acid concentration of the rate of dehydration of HCOOH to COad on Pt at t = 0 within the potential region between 0.19 and 0.26 V. The values in the legend correspond the slope at the indicated potential in the linear region of the plot.

Close modal

The high sensitivity and the absence of transport limitations characteristic of ATR-SEIRAS have permitted us to follow in detail the electrocatalytic formation of COad from formic acid on platinum. A real-time analysis of the intensity and shape of the COad characteristic bands, COL and COB/M, and of their corresponding derivatives has enabled us to study the rate of formation of COad as well as its dependence on potential at constant concentration and concentration at a constant potential. The comparative analysis of the time evolution of both the integrated intensity and the frequency of the bands corresponding to COL and COB/M suggests a progressive population by COad of the active sites on the surface.

The time-resolved spectra recorded simultaneously with current transients after a potential step in the Hupd region have confirmed the relationship between the potential of zero total charge (pztc) of the active site and the rate of dehydration of formic acid. In agreement with previous works,15,16 the rate of dehydration has a bell-shaped dependence on the potential and goes through a broad maximum extending between 0.15 and 0.2 V whose position is independent of the formic acid concentration. This is due to the polycrystalline nature of the platinum film, which contains a large multiplicity of sites, each with its own pztc, but is dominated by (110) and (100) defect sites. In addition, a kinetic mechanism depending on the potential and θH, θHCOO, and θCO has been proposed where the reaction would be expected to be first order with respect to the formic acid concentration when θCO ≈ 0, θH, and θHCOO are approximately constant, and k2cHCOOHk−2.

These results provide additional support to the proposal that HCOOad is the crucial intermediate in the electro-oxidation of HCOOH.

See the supplementary material for Figs. S1–S5.

L.P.-M. acknowledges a doctoral scholarship within the Leverhulme Centre for Doctoral Training in Sustainable Production of Chemicals and Materials (Grant No. DS-2017-073).

The authors have no conflicts to disclose.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Laura Pérez-Martínez: Formal analysis (lead); Investigation (lead); Methodology (equal); Writing – original draft (lead); Writing – review & editing (equal). Enrique Herrero: Formal analysis (equal); Writing – review & editing (equal). Angel Cuesta: Conceptualization (lead); Formal analysis (equal); Funding acquisition (lead); Project administration (lead); Resources (lead); Supervision (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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