J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1233-1240 Profiles of Adsorption during the Oxidation of Small Organic Molecules: Oxidation of Formic Acid at Polycrystalline Pt in Acid Solutions C. Paul Wilde* and Meijie Zhang Otta wa-Carleton Chemistry Institute, University of Ottawa Campus, Department of Chemistry, University of Ottawa, 10 Marie Curie Priv., Ottawa, Ontario, Canada KIN 6N5 The electrochemical quartz crystal microbalance has been used to monitor changes in adsorption at Pt elec- trodes during the oxidation of formic acid in 0.1 mol dm-3 HCIO, solutions. This has been achieved through cyclic voltammetric, injection and open-circuit potential decay experiments where mass profiles are recorded alongside the electrochemical response. Adsorption in the H underpotential deposition (UPD) region causes the mass to increase relative to the background electrolyte whereas in the double-layer region of potential, increased coverage of strongly adsorbed intermediates has the reverse effect.Removal of these latter species can be followed from the mass response because it leads to a characteristic mass step. Subsequent to this process there is a region of potential where oxidation of formic acid occurs through consumption of adsorbed OH or PtOH and mass responses reveal that, as concentration increases, there is increased adsorption of organic residues here. The consumption of surface oxy species (OH,,, , PtOH or PtO) by formic acid also results in irreversible oxidation of the electrode surface being shifted to higher potentials with increasing formic acid concentration, since it is only at the higher potentials that the rate of the place exchange process can compete effectively with the reaction with formic acid.The mass decrease associated with removal of the surface oxide is also accelerated at higher formic acid concentrations and occurs at higher potentials. The oxidation of formic acid at Pt is a much studied reaction for two reasons. The first is interest in the area of fuel cells where formic acid might constitute the organic species to be oxidised. The second is for the elucidation of the mechanism of oxidation of small organic molecules. In one respect, formic acid is a special case since it does not necessarily require a source of oxygen (as, for example, methanol does) to produce CO,.There are several reviews of the various investigations of formic acid oxidation,'-4 but as noted else- where,4 trying to assess comparative claims in the literature is difficult. Factors such as variations in the electrode surface (e.g. roughness and orientation), the anion of the background electrolyte and its concentration and the concentration of formic acid, not to mention the different potential regimes applied to electrodes can all influence the results obtained. Thus, it is most appropriate simply to state several salient features here. First, the concept of parallel pathways for the oxidation process is generally a~cepted.~-~ Thus there is one pathway that involves a reactive surface intermediate, perhaps CO,H, adsorbed at the electrode surface, and a second that involves a chemisorbed intermediate or 'poison ', most likely CO.Both give rise to CO,, but the latter is oxi- dised only at relatively high potentials in the double-layer region on Pt and with the assistance of a source of oxygen, such as water or adsorbed OH or, at more positive poten- tials, PtOH. There is abundant spectroscopic evidence7-' ' for the presence of CO (linear, bridged or multiple site) at the electrode surface, and CHO species may also be present.12-14 The principal spectroscopic methods used to date have been various IR techniques such as electrochemically modulated IR spectroscopy (EMIRS),7-9*' ' polarisation modulation IR reflection absorption spectroscopy (PM-IRRAS)' and single potential alteration IR spectroscopy (SPAIRS),' and mass spectrometry (differential electrochemical mass spectrometry, DE M S);' ,-' surface-enhanced Raman spectroscopy (SERS)' has also recently been applied to platinum-coated gold electrodes.However, it has been suggested that different methods may be biased towards detection of different species. EMIRS for example, may detect strongly adsorbed species (CO) while DEMS can only detect adsorbates that may be desorbed from the electrode surface, and so may be more likely to identify the weakly adsorbed or reactive interme- diate.4 These factors, as well as the likelihood that the exact distribution of surface species may well be strongly dependent on the concentration of substrate, potential and the other conditions of a given experiment, should be borne in mind when considering the literature and the results to be present- ed here.Our interests are in applying the electrochemical quartz crystal microbalance (EQCM) technique to the study of elec- trocatalytic phenomena. Here, mass profiles recorded along- side electrochemical experiments add a fresh perspective on adsorption phenomena in the systems under study and have been found to be particularly valuable in locating the point where strongly adsorbed intermediates are oxidised and in monitoring the extent of oxidation of the electrode surface when this is obscured by high currents from oxidation of the organic substrate.We have applied the method previously to the oxidations of gluco~e,'~~'*and methan01'~ and to the competitive adsorption between glucose and UPD lead" and we now present results of a study of formic acid oxidation. Experimental 10 MHz AT cut quartz crystals with gold electrodes were purchased from International Crystal Manufacturing Co., Oklahoma City, OK and then platinised as described earlier.' Real surface areas of the platinum electrodes, mea- sured using integration of the H UPD charge, and the accepted conversion factor of 210 pC cm-, were typically found to be 15-20 times greater than the geometric area, which was 0.25 cm2. A saturated calomel electrode (SCE), separated from the main cell by a Luggin capillary was used as the reference electrode and a Pt wire was used as the counter (auxiliary) electrode.All stated potentials are referred to the SCE. The potentiostat used was from Oxford Elec- trodes (Abingdon, England) and current, voltage and fre- quency (mass) results were recorded on an XYY' chart recorder (Philips PM 8272 or Kipp and Zonen BD91). Control and measurement of frequency (through conver- sion of the frequency difference between working and refer- ence crystals to a voltage) was accomplished as described by Bruckenstein and Shay.21 The frequency to voltage converter has a voltage offset which allows small changes (mV) in output to be displayed when there is a high background voltage. Unless otherwise noted, this offset was not changed during a given experiment.Frequency changes were con-verted to mass as described previously,2i and for all figures mass increases up the page. Further experimental and theo- retical information on the EQCM is available in the liter- at~re.~l-~' Purified water for solutions was obtained from a Millipore Milli-Q system. Chemicals were obtained from BDH [HClO, , AnalaR; H2PtCl, , Analytical Reagent and formic acid, AnalaR (98-100%)] and Merck (H2S0,, Suprapure 96%) and were used without further purification. Stock solu- tions of 5.0 mol dm-3 (for final bulk concentrations >1 mmol dmP3) or 0.5 mol dm-3 (for final bulk concentrations below 1 mmol dmP3) formic acid were used for injection experiments. All experiments were carried out at room tem- perature, 22 & 1"C.Results and Discussion Some Generalities in the Effwt of Small Organic Fuels on Mass Responses There are several common traits in the oxidation processes of small organic molecules such as formic acid and methanol at platinum electrodes. Thus it is not surprising to find that when these processes are studied with the microbalance tech- nique some similar mass features are to be found too. These latter are presented here to provide a framework for con- sideration of the results to be discussed. (1) In general, the presence of organic adsorbates at the electrode surface causes only small differences in mass pro- files (relative to those of the background electrolyte). This is because of factors such as the small molecular weight of most organic fragments (for example CO and CHO), sub-monolayer coverages, and the fact that adsorption will involve displacement of adsorbed water and specifically adsorbed anions (typically perchlorate and sulfatelhydrogen sulfate).Furthermore, one CO, for example, may occupy one, two or three surface sites. In fact, for Pt electrodes in HClO, , it has been found that coverage of the electrode surface with poisoning adsorbates in the double-layer region of potential (whether they be derived from gluco~e,'~ methanolig or formic acid) leads to a mass decrease. It should also be noted here that because of the factors mentioned above, together with the difficulty in obtaining an exact indication of the nature of distribution of species adsorbed at the surface as a function of potential, the extraction of coverage values for adsorbates from mass data is exceptionally difficult.(2). The removal (by oxidation) of strongly adsorbed species (which is often associated with a sharp peak in the voltammogram) leads to a transition between a surface that is largely covered with organic adsorbates and one that is largely free from such adsorption (this is less true at concen- trations > lop2mol dmP3). As a consequence, a mass step that corresponds to a mass increase (see comment above) is often observed to accompany the removal process. However, the likelihood of adsorption subsequent to the removal of the strongly adsorbed intermediates increases with the concentra- tion of the organic reactant.Thus the mass step diminishes in size as concentration increase^.'^ (3) For the oxidations of 0.1 mol dm-3 glucose in alkaline media,'* of 0.1 mol dm-3 methanol in perchloric acid solu- tions,lg and of 0.1 mol dm-3 formic acid (to be presented here) the mass responses reveal a significant positive shift in J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the point at which irreversible oxidation of the electrode surface begins. (The large current due to organic oxidation prevents detection of this phenomenon in the voltam-mogram.) In both background electrolyte and when organic fuels are present, there is a region of constant mass before the onset of irreversible surface oxidation.The mass increases again only when the place exchange process [insertion of OH (and 0)into the Pt latti~e~~.~'] starts, because the initial stages of oxidation of the surface, where PtOH is produced from adsorbed water, do not lead to a mass change. The point where mass begins to increase again after the double- layer region of potential, and its variation with increasing amounts of organic fuel in the electrolyte are thus easily iden- tified. A related item of interest is that the rate of increase of mass (and hence oxide growth) is more rapid when shifted towards more positive potentials, presumably because of the increased field at the higher onset potential. We will now illustrate and elaborate upon some of the above comments by examining the mass responses resulting from the oxidation of formic acid at various concentrations in 0.1 mol dm-3 HClO, .Oxidation of Formic Acid at Very Low Concentrations, 0.5 mmol dm-3 Fig. 1 shows a voltammogram and accompanying mass response, recorded after the potential profile in the inset, for 0.5 mmol dmV3 formic acid. Essentially there is a broad oxi- dation of HC02H across almost the entire potential range. This oxidation current first increases above the background at around -0.1 V. Current densities are low, and the roughly constant oxidation is disturbed only by the sharp peak in the double-layer region of potential on the anodic scan. The first 40 20 0 -20 5.: -40 I 1 -0.2 0.2 0.6 1.o E/V vs.SCE Fig. 1 Cyclic voltammogram and mass response for the oxidation of 0.5 mmol dmP3 formic acid in 0.1 mol dm-3 HC10,. Results for the background electrolyte alone are also shown (dotted line). El = 1.15 V,t, = 120 s, E, = -0.25 V and tpds= 60s. The scan rate was 5 mV s-and the electrode area 3.82 an2.Mass responses are shown exactly as recorded. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 stages (to -0.1 V) of the mass response accompanying the anodic scan are seemingly unaffected by the presence of formic acid. (A discussion of the mass response in 0.1 mol dm-3 HClO, alone has been presented elsewhere.I7) This is not unexpected, given that the value of E, was -0.25 V where H coverage is nearly unity. Capon and Parsons6 have shown that the reaction between Hadsand HC0,H (bulk) is slow in such a circumstance.From -0.1 V on, however, the mass response is flat and lower than in the background elec- trolyte over a range of nearly 400 mV (indicated as region A) until the step (feature B) that accompanies the sharp peak. Here then are two of the standard features described earlier. First, the steady region of mass reflects what is probably a constant (small) coverage of strongly adsorbed species, pre- sumably developed in the earliest stages of the anodic scan and then the step corresponds to their removal. The electrode surface thereafter seems to be quite similar to that seen in the background electrolyte since there is little difference in mass. (Note that the two mass traces are shown exactly as recorded and it is most likely that in fact the masses should be equal at the anodic scan limit.Some evidence for this is presented later, in the discussion of mass transients. The difference in the figure is probably a result of a small drift in the frequency between the recording of the scans.) When present at this low level, formic acid has no influence on the oxidition-reduction of the electrode surface and so the next point of interest follows oxide removal on the negative-going scan. Here, there is another flat region, yet the mass is larger than in the corre- sponding section of the positive-going scan. This difference arises because the double-layer region is approached from opposite directions.On the anodic scan the initial sections involve some residue formation (removed in the sharp peak) before the double-layer region is encountered, whereas on the cathodic scan the surface in the double layer region is ‘clean’ since it is reached after oxide removal. These data indicate that, under the conditions of this experiment, the formation of strongly adsorbed species occurs principally in the region of weakly adsorbed H. Fig. 2 supports this assumption. Here, 1 I 1 I I I 1 I 0.0 0.4 0.8 EfV vs.SCE Fig. 2 Details as for Fig. 1 except that E, was -0.10 and 0.10 V for curves 1 and 2, respectively. Electrode area 4.88 cm’. The vertical arrow labelled 2 indicates the starting point for the mass for curve 2. the potential E, was increased to -0.1 V and 0.1 V as com- pared to -0.25 V in Fig.1. For E, =-0.1 V, the mass profile mirrors that of Fig. 1 and the sharp current peak is seen (though there is less charge involved). For E, =0.1 V, however, the peak is negligible (the current is constant almost throughout the double-layer region) and the mass on the anodic scan actually follows the path of the signal from the cathodic scan until it merges with the mass profile from the first anodic scan at 0.4 V at the beginning of the plateau region. Thus under the conditions of these experiments, we may conclude that a small amount of adsorption occurs prin- cipally at the extreme negative limits of the scan, and there is little further adsorption in the double-layer region of poten- tial.This is not unexpected, given the small bulk concentra- tion used. The general picture of a constant small oxidation current across virtually the whole potential range, together with the sharp peak, seems to fit well with the accepted view that the oxidation of formic acid occurs by parallel routes with a reactive intermediate and a strongly adsorbed interme- diate. The differences in mass response in the double-layer region of potential also illustrate the fact that increased coverage here causes the mass to decrease and a comparison of the mass responses can thus give a qualitative picture of the variation of coverage between the two scan directions. We do not believe that changes in surface roughness (with possible increased solvent entrapment in pores) play any role in causing these differences in the mass response in the double-layer region of potential. This is for two reasons.First, the surfaces are already rough (see Experimental section) and significant changes in roughness upon cycling are thus not to be expected. Secondly, the response in back- ground electrolyte (Fig. 1, dotted line) shows no difference (within experimental error) between the mass responses accompanying the anodic-going and cathodic-going halves of the cycle. Changes in roughness upon cycling through oxidation-reduction of the electrode would not result in the mass response forming a closed loop as it does here. As a final point, identical experiments to those of Fig.1 and 2 but with methanol” showed that the formation of strongly bound intermediates began at a more positive potential (than for formic acid) and continued through most of the double- layer region leading to a continued decrease in mass, rather than the flat response seen here. Thus subtle differences between the behaviour of different reactants can be discerned from the mass responses. Cyclic Voltammograms for the Oxidation of 9 mmol dm-3 Formic Acid When the bulk formic acid concentration is increased to 9 mmol dm-3, changes in both the voltammetry and mass response become apparent. Currents are larger and an anodic scan is dominated by the sharp peak at 0.4 V with a steady increase in current prior to this and a shoulder thereafter. [Note: The current before the sharp peak is often apparent as another peak, frequently referred to as the first anodic peak.6 Under the conditions used here, namely slow scan rates and 0.1 mol dmP3 HClO, as electrolyte, this first anodic peak was not observed (cf:ref.15, Fig. 7). However, at faster scan rates in 0.5 mol dm-3 H,SO, it was present.] On the cathodic scan, there is a reactivation peak which is followed by a dip due to surface oxide reduction and then a current plateau before the final decline in current begins at 0.1 V. Fig. 3 also shows the second cycle recorded directly after the first. The only significant difference between the voltammograms is the suppression of the current in the region from -0.25 V to -0.1 V. This is a result of the higher coverage of strongly adsorbed species formed in the latter stages of the first cycle.1236 0.2 0.1 0.0 5-0.1 --.\ 0.0 0.4 0.8 E/V vs. SCE Fig. 3 Cyclic voltammogram and mass response (upper mass trace) for the oxidation of 9 mmol dm-3 formic acid in 0.1 mol dm-3 HClO,. Results were recorded after a potential profile like that of Fig. 1. The second cycle was recorded directly after the first with no pause. The electrode area was 4.88 cm2 and the scan rate was 5 mV s-'. The background mass response is shown only for comparison purposes and is displaced downwards for purposes of clarity. The vertical dotted line is drawn to illustrate the slight shift in the point where irreversible surface oxidation begins when formic acid is present.Point C indicates where the two anodic-going mass responses coincide and point D illustrates the mass plateau that follows the step. Mass Responses for 9 mmol dm-3 Formic Acid, Variation of Electrode Mass with Coverage in the H UPD Region and the Double-layer Region The mass responses for the two cycles with formic acid, in the middle of Fig. 3, are in excellent agreement, except during the initial stages of each of the anodic halves of the two cycles. The reasons for this difference will now be addressed. The state of the surface at a potential of -0.25 V is affected by the potential perturbation applied to the electrode before it reaches -0.25 V. In the first cycle there is a step from 1.15 V and a pause of 60 s at -0.25 V, where some adsorption will take place.This is likely to be small, as discussed above. Further adsorption can take place once the potential scan begins and adsorbed H is removed from the surface. In con- trast, on the second cycle the anodic scan is preceded by a cathodic scan where, once the surface oxide is removed, there is a considerable time for coverage of adsorbates to develop. Comparison of current and mass in this section of the cathodic scan is interesting since in a general sense the two signals are similar. Thus the region of constant current from 0.4 V downwards is accompanied by a region of almost con- stant mass and as the current decreases more sharply so does the mass, at least for a short time.Finally from about 0.0 V the mass increases, until it reaches -0.25 V. It is clear from a comparison of the voltammetry and mass that a higher coverage of adsorbates leads (in the H UPD region) to an J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 increase in mass, whereas the reverse is true in the double- layer region of potential. However, the fact that responses for the two cycles merge at ca. 0.15 V implies that at this point (C) there is a comparable coverage (the voltammetry is quite similar too) which is reached by different routes on the two scans. It is more difficult to explain why the mass responses take the shape they do and why an increased coverage of adsorbates leads to a mass increase in the H UPD region and a mass decrease in the double-layer region.Undoubtedly some of the factors described in the introduction play a part. For example, the coverage of water and specifically adsorbed anions is potential dependent and thus one might not neces- sarily expect that the occupation of an equal number of sites by a given adsorbate results in the same net mass change at two different potentials. We must also recognize the roles of time and formic acid concentration as additional influences because the rate of adsorption will vary with both of these factors. Thus the scan rate and the bulk formic acid concen- tration will influence the mass response. Further information on this point can be obtained from mass-transient experi- ments, but before presenting those results the remainder of the cycles of Fig.3 will be discussed. Mass Responses for Oxidation of 9 mmol dm-3 Formic Acid, Removal of Strongly Adsorbed Species and Electrode Surface Oxidation Once the sharp peak is reached in the voltammogram, the mass step is seen. There is then a plateau region (D) before the mass increases again. An interesting question is whether or not there is any adsorption in this plateau region. Certain- ly we know that prior to the mass step there is a significant coverage of strongly adsorbed intermediates, but what is the situation thereafter? If one chooses a particular potential in the mass plateau region, say 0.5 V, and then measures the mass difference between this point and the mass at the poten- tial of scan reversal, the difference is larger than when a similar measurement is made for the background electrolyte alone.If one assumes that the mass at scan reversal is the same in both cases (i.e. that there is a similar coverage of oxide and that there is little adsorption of organic residues; both of these points are substantiated by later results) then this implies a small amount of adsorption in the plateau region around 0.5 V. This makes the electrode mass lower than in 0.1 mol dmP3 HClO,. It is interesting to note that the mass plateau region corresponds to a broad shoulder in the voltammogram. It may be that (some of) the oxidation here takes place with the assistance of adsorbed OH or PtOH at the electrode surface. The likelihood of adsorption in this region is also indicated by the small shift in the onset of the final mass increase of the anodic scan (dotted line).This implies some blockage or shifting of the irreversible stages of surface oxidation. There is little apparent effect of the formic acid on the reduction of the surface oxide, but it is interesting to note that the reactivation peak on the cathodic half of the voltammogram occurs before any noticeable decrease in mass implying that no significant reduction of the oxidised surface is necessary to allow the oxidation of formic acid to begin again. Mass Transient Experiments One simple means of examining adsorption processes at the EQCM is to add a small amount of the adsorbate to the electrolyte and follow the frequency (mass) signal with time at constant potential. In general, changes in solution viscosity and density during such experiments are too small to influ- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 4 Mass transients resulting from the addition of formic acid (sufficient to give a bulk concentration of 9 mmol dm-3) to the back- ground electrolyte at different constant potentials. Each injection was performed at a clean electrode surface. Electrode area 4.30 an2. Responses are displaced for purposes of clarity and are not intended to represent relative mass values. Constant potentials used were (a) -0.15, (b)0.0, (c) 0.15, (6)0.30, (e) 0.55 and (f) 1.0 V. The arrows indicate when the formic acid was added to the electrolyte, which was stirred with nitrogen so that the time of mixing is just a few seconds.ence the mass response, although this was found not to be the case when concentrated glucose solutions were generated by injection.20 Thus it is possible to see whether adsorption leads to a simple mass increase or decrease relative to the background electrolyte. The progression of the mass to a steady state or pseudo-steady state can also be followed. For CO adsorption, for example, initial adsorption at Pt leads to a mass decrease but as coverage develops the mass change reverses direction and finally a net increase is seen.28 These types of experiment thus provide further insight into adsorp- tion processes at constant potential and to the relative posi- tions of mass responses accompanying vol tammograms for background electrolyte containing different amounts of formic acid from zero upwards. Results of some mass transient experiments are shown in Fig.4. Additions of formic acid were chosen to provide a final bulk concentration of 9 mmol dm-3. Each experiment was carried out at a clean electrode. The results suggest the following general conclusions. First, at the oxidised electrode surface (1.0 V) there does not appear to be any significant adsorption. This was found to be true for concentrations up to 1.0 mol dm-3. Secondly, in the double-layer region of potential (0.15,0.30 and 0.55 V) there is adsorption such that the mass of the electrode is less in the presence of formic acid.Both these conclusions agree with those derived from mass responses that accompanied the voltammetry. (The potential dependence of the mass change also reinforces the conclusion that there is no effect of the small change in solution viscosity and density on the mass response.) Finally, at 0.0 V and -0.15 V a small increase is seen in the mass of the electrode after addition of formic acid. This too is what would be expected from the data in Fig. 3, although it should be appre- ciated that the mass response accompanying the CV is depen- dent upon processes occurring at lower potentials. The result at 0.0 V also shows the effect of time, since the initial mass change is a decrease followed by a steady increase. Again this is most likely to be a result of the competition between anions and water and the adsorbate and perhaps also of the changing nature of the adsorbate with coverage (e.g. CO may [HCO,H]/mmol dm-3 II' 1 I I B I1 I 1 I I 0.3 0.5 0.7 0.9 1.I E/V vs.SCE Fig. 5 A, Variation of the potential (E,) of the anodic (m) and cathodic (reactivation) peaks (0)with formic acid concentration. Scan rate was 5 mV s-' and all voltammograms were recorded after a potential profile like that shown in Fig. 1. B, Influence of formic acid concentration on mass response. (a) 20 mmol dm-3, (b)40 mmol dm-3, (c) 60mmol dm-3 and (d) 0.1 mol dm-'. The concentration of formic acid was increased in steps and then each response recorded after the potential profile shown in Fig.1 at 5 mV s-'. Traces are drawn aligned so that the masses coincide at the upper potential limit. Electrode area 4.55 cm2. shift from being multiply bound to being singly bound as coverage increases). The combined data provide a useful illus- tration of the fact that mass changes upon adsorption can be either positive or negative. Influence of Increasing Amounts of Formic Acid on the Mass Profile An increase in the amount of formic acid present in the elec- trolyte does not cause significant changes in the cyclic volt- ammetric response, but the following general trends are observed (when all voltammograms are recorded after a potential profile like that seen in Fig. 1). First the anodic peak (identified with the second anodic peak of Capon and Parsons6) that is associated with the mass step (and with removal of strongly adsorbed species) is shifted steadily in the positive direction from 0.42 V at 20 mmol dm-3 to 0.70 V at 0.1 mol dm-3.The shoulder following the peak also grad- ually disappears until there is, at 0.1 mol dm-3 formic acid, an abrupt drop in current after the peak. At the same time the reactivation peak (where formic acid is oxidised on fresh sites generated by some removal of the surface oxide) becomes much sharper. It also shows a positive shift, but to a lesser extent. These data are summarised in Fig. 5A. Needless to say, current densities also increase. The evolution of the mass response is more interesting. Fig. 5B shows part of the mass responses (restricted to potentials in the double-layer region and above) for a series of experiments on the same electrode.There are three general effects. First, the mass step shifts in a positive direction, just as the anodic current peak does. It also diminishes in size and the subsequent mass plateau shortens. Second, the final mass increase of the scan is also shifted in the positive direction. Finally, the rate of removal of surface oxide on the cathodic scan is increased and thus the mass decreases more sharply than in the back- ground electrolyte and at a more positive potential. This behaviour is very similar to that reported earlier for methanol’ and leads to several conclusions. (1) The removal of strongly adsorbed species is shifted towards more positive potentials as the bulk concentration increases.This results in a similar shift of both mass step and current peak. This may occur because there are less oxy species (OH,,, or PtOH) at the surface to assist in oxidation of CO as a result of more extensive occupation of the surface by the reactive intermediate and possibly by CO itself. (2) There is probably more adsorption subsequent to the mass step as concentration increases, but there comes a point where the mass plateau region has almost disappeared (and so has the shoulder in the voltammogram after the peak, as noted above). The gradual shift in the potential at which oxi- dation of strongly adsorbing species occurs leads to a situ- ation where the removal process is immediately followed by irreversible surface oxidation.This effectively ‘switches off’ the formic acid oxidation process, although it is well known that the oxidation begins again at more positive potentials on the fully oxidised surface. (3) The more rapid reduction of the oxidised surface, illus- trated by the rapid decline of the mass earlier in the cathodic scan (and the positive shift in the reactivation peak) is most likely to be associated with an increased reaction between adsorbates derived from formic acid and PtOH as concentra- tion increases. This point is dealt with further in the next section. A complete cycle for 0.1 mol dm-3 formic acid with both voltammetric and mass responses can be seen in Fig.6, com-pared to the background responses. This illustrates more clearly the changes that occur as the formic acid concentra- tion is increased, particularly the almost complete absence of the mass step on the anodic scan and the significant shifts in the potentials of oxide formation and removal. Thus the final increase in mass which corresponds to the beginning of irre- versible surface oxidation is shifted to 0.79 V compared to 0.57 V in the background electrolyte. It is more rapid at first and then slows to a rate similar to the background response. The faster removal of the surface oxide is also seen to coin- cide clearly with the sharp reactivation on the negative-going scan. Two further small points should be made about the response at this concentration.In the double-layer region of potential the mass is flat, and there is no difference between the anodic and cathodic sections. It is likely that at this con- centration the steady-state coverage of adsorbates is reached rapidly (clearly this is the case after oxide removal) and is the same on both halves of the cycle. As before, the mass increases as the potential proceeds into the H UPD region (on a cathodic scan) since a higher coverage of adsorbates in this region leads to a mass increase. The slight discrepancy between the beginning and end of the mass loop may be accounted for by differing coverages. Finally the relative posi- tions of the two mass responses are again substantiated by injection (mass transient) experiments which may be sum-J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 4.0 3.0 2.0 41.o .-.\ 0.0 Fig. 6 Cyclic voltammogram and mass response for 0.1 rnol dm-3 formic acid in 0.1 mol dm-3 HCIO,. Scan rate 5 mV s-l, electrode area 4.55 cm2. The background mass response (dotted line) is pre-sented for purposes of comparison and is drawn so that masses coin- cide at the point of scan reversal. The vertical arrow between points F and G represents the mass change when 0.1 mol dm-3 formic acid is added to the electrolyte with the potential held at 0.7 V. See text for details. marised thus. In the H UPD region the mass of the electrode is larger than in the background. (The observed increase is in fact larger than that seen for the same potential with 9 mmol dm-3 formic acid.) In the double-layer region the mass of the electrode is less than in the background electrolyte and finally, on the oxidised electrode surface any mass difference is at the limit of experimental significance.Role of Surface Oxy-species in the Oxidation Reaction, Experiments with 0.1 mol dm-3 Formic Acid Injection and open-circuit potential decay experiments, where mass is recorded in parallel, provide some interesting further information regarding the interaction between formic acid and the electrode surface when it is at least partially oxidised. The injection experiment was carried out with the electrode held at 0.7 V (after a step up from 0.15 V) in background electrolyte. Formic acid is then added to give a final concen- tration of 0.1 mol dm-3 while the potential is maintained at 0.7 V.This causes a drop in mass which is larger than could be accounted for by simple reduction of the (partially) oxi- dised surface alone. The mass decreases because the partial coverage of oxide is removed and then there is adsorption of formic acid or intermediates. This process can be represented in Fig. 6 by a shift from a point at 0.7 V on the background response (F) to a corresponding point on the response with 0.10 mol dm-3 formic acid (G).At this potential, electro- chemical oxidation (turnover of PtOH to OHPt) does not appear to be able to compete effectively with removal of OH or PtOH by reaction with formic acid or intermediates. However, if the potential is increased to 1.0 V and the same injection made, there is no change in mass (note that this is J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 effectively the experiment shown in Fig. 4 except that the bulk concentration then was 9 mmol dm-3). Here, the rate of electrochemical regeneration of any oxide removed in a chemical reaction is sufficient to prevent a noticeable decrease in mass. It is also noted that at this potential there is no difference between the mass responses as drawn in Fig. 6. That the chemical reaction between the oxidised surface and formic acid is possible is well known2 and the results of Fig. 7 illustrate this. The potential was taken to 1.0 V (in back- ground electrolyte) and the circuit opened.Once a stable potential was attained, formic acid was added to give a chosen final concentration and the potential and mass were followed with time. Results are shown for two concentrations 0.5 and 100 mmol dmP3. Addition of formic acid in both cases leads to a shift in the potential to the region of H UPD, as expected, and to a decrease in mass. Two further points emerge. The rate of decrease is slower for the lowest concen- tration as might be expected, but there is also a larger mass decrease for the lower concentration. Part of this decrease is accounted for by the removal of the oxide, but this should be equal in both cases, The remainder represents the difference between the stable relative masses of the electrode surface in the presence of differing amounts of formic acid.It is there- fore suggested that at the potentials of interest (in the H UPD region) there is a larger coverage of adsorbates devel- oped from 0.1 mol dm-3 formic acid and hence the mass is larger and so the change in mass seen after the potential- decay experiment is smaller for 0.1 mol dmP3 formic acid. Again one can carry out an injection experiment to investi- gate this and indeed at a constant potential of -0.15 V titra-I \ I0.20 v \ \ Fig. 7 Mass (upper) and potential (lower) transients observed upon addition of sufficient formic acid to yield a final bulk concentration of 0.5 mmol dm-3 (dashed line) and 0.1 mol dm-3 (solid line) in 0.1 rnol dm-HCIO,. The electrode was held first at 1.0 V and then the circuit was opened.The starting potential was 0.8 V in both cases, and the final potentials were -0.11 V (0.5 mmol dm-3) and -0.18 V (0.1 rnol dmP3). The arrow indicates when formic acid was added. Electrode area 3.50 cm2. tion of the formic acid concentration to higher amounts leads to successive increases in mass. Conclusions Despite the most obvious drawback of the technique, namely the inability to identify adsorbed species present at the elec- trode surface, significant information can be obtained through the careful application of the EQCM to electro- catalytic reactions. Interactions at the electrode surface are clearly complex and the fact that the net mass change upon adsorption depends upon the species displaced from the surface as well as those adsorbed is well illustrated.Thus in the H UPD region of potential, increased coverage of the electrode surface by adsorbates derived from the reaction of formic acid causes an increase in the mass of the electrode. In contrast, in the double-layer region of potential the reverse is true and differences in the mass in this region on the two halves of a scan can reveal the differing extents of adsorption. The mass step is found to be a feature common to several processes and is useful in locating the point where oxidative removal of strongly bound adsorbates occurs. The sub- sequent mass plateau region is also informative since it can be correlated with a shoulder on the voltammogram and it is most likely that oxidation here involves consumption of surface OH, either adsorbed or as PtOH.The evolution of these two features with concentration shows a shift of the step to higher potentials, and increased adsorption following the step (so that it becomes smaller in size). Perhaps the most useful data are those furnished from considering the oxidation-reduction of the electrode surface, and how it is influenced by varying amounts of formic acid. Mass results accompanying cyclic voltammetric and injection experiments data show that higher concentrations of formic acid appear to consume surface oxy species (perhaps PtOH) at a rapid rate in the mass plateau region so that irreversible surface oxidation is shifted towards more positive potentials until the rate of the place-exchange process will have increased sum- ciently to compete with consumption by formic acid (it is also possible that there is some blockage of surface sites by stongly adsorbed species).The rate of growth of the oxide (as represented by the rate of increase of mass) is then faster than usual when it does occur because of the higher field. In addi- tion, removal of the oxide occurs at a more positive potential. This is also a result of the swift consumption of PtOH and PtO by formic acid. 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