J. Phys. Chem. B 2006, 110, 18725-18728
18725
Reply to “Comment on ‘Electrooxidation of COad Intermediated from Methanol Oxidation on Polycrystalline Pt Electrode’” Weilin Xu, Tianhong Lu, Changpeng Liu, and Wei Xing* State Key Laboratory of Electro-analytical Chemistry, Changchun Institute of Applied Chemistry, Graduate, School of the Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, Jilin, P. R. China ReceiVed: June 1, 2006 To make our experimental results more clear, it is necessary to illustrate the mechanism of normal pulse voltammetry (NPV) and the speciality of our system of COad oxidation, although it had been shown in our paper compendiously.1 Any maze about them obviously will lead to the misunderstanding of the results obtained in our experiments. The NPV theory truly is only applied for the systems under the conditions of semiinfinite linear diffusion. In previous works, we had applied this theory to study the oxidation of methanol into formate with appropriate potential control.2 As shown in Figure R1, typically the electrode is held at a base potential, Eb, at which negligible electrolysis occurs. After a fixed waiting period (or waiting time), the potential is changed abruptly to value E for a period typically about tens of milliseconds in duration (pulse width). The potential is ended by a return to the base value, Eb. The details can be seen in the book edited by Bard et al.,3 specifically pp 279-286. In a typical semiinfinite diffusion system, a current plateau should appear at high potential in NPV experiments.2,4-9 But, in ref 1, the oxidation of CO intermediated from methanol oxidation under the control of NPV is a special semiinfinite diffusion process. The speciality is that the reactant of CO came from the methanol oxidation at based potential (Eb). The detailed illustration can be seen in the following as shown in Figure R2. COad (Pt - CO) was produced from methanol oxidation at Eb ) -0.2 V vs Ag/AgCl during the waiting time. The intermediate of CO could not be oxidized into CO2 when the pulse potential was lower than 0.1 V vs Ag/AgCl.10 Then the CO molecules were accumulated gradually. With the production of CO from methanol oxidation, it was hypothesized that there is a dynamic equilibrium between Pt - CO and the free CO16
CH3OH + Pt f Pt - CO S Pt + CO
(R1)
With the oxidation of methanol at a low potential of Eb ) -0.2 V vs Ag/AgCl, the products of CO molecules were stored as both Pt - CO and free CO. As there is a special electrostatic field near the electrode during the potential scanning, the free CO was hypothesized to locomote just near the adsorbed COad layer (Pt - CO). The free CO could be regarded in the same monolayer with Pt - CO approximately shown in Figure R2. As the covalent radius of Pt is about 130 pm17 and the atomic radius of carbon is about 65 pm,18 on average, the space around every Pt atom could contain four CO molecules. With the transfer of CO from Pt - CO to the free CO, more and more * To whom correspondence should be addressed. E-mail: xingwei@ ciac.jl.cn.
Figure R1. Sampling scheme for normal pulse voltammetry.
Figure R2. (A) Scheme for the monolayer of CO on the electrode surface and concentration profile of reactant of CO near the electrode surface at limiting current. (B) The scheme for bulk liquid of CO.
CO molecules were stored as free CO near the electrode surface. So, it was approximated that most of the CO produced from methanol oxidation is stored in free
CCO-total ) CCOad + CCO-free ≈ CCO-free When the pulse potential is high enough, COad (Pt - CO) will be oxidized into CO2 during the pulse time. The consumed Pt - CO then was supplemented by the diffusion of free CO to the Pt surface simultaneously. The wastage of free CO could be partially supplemented in the next waiting time at Eb due to the slight methanol oxidation. Then the value of CCO-total (≈ CCO-free) is approximately kept constant. At this time, most of the electrode surface had been covered by CO and then only very few methanol molecules could reach the electrode surface to be oxidized into CO. This electrolysis could be neglected approximately. With the further increase of pulse potential, at some value of pulse potential COad (Pt - CO) could be oxidized into CO2 completely and the content of Pt - CO decreased to zero as
10.1021/jp0633852 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/31/2006
18726 J. Phys. Chem. B, Vol. 110, No. 37, 2006
Comments
Figure R3. Panel for the parameter setting on Model 273.
shown in Figure R2 (blue line). Instantaneously, there is a maximal grads of CO content on the electrode surface. The maximum current appears here. At higher pulse potential, as the wastage of COad is larger than the production of CO at previous waiting time, a great deal of free CO molecules diffuse to the electrode surface to form COad (Pt - CO). This will lead to an obvious decrease of CCO-free or CCO - total. This decrease of CCO-free further leads to the decrease of current. Obviously, the decrease of current reflects the speciality of this system. Under this condition, the diffusion of free CO to the electrode surface to form COad also is a semiinfinite diffusion process in approximation. With this premise, we then applied the extended NPV theory2 to study the electrode kinetics of this process. As the CO was intermediated from methanol oxidation at base potential (Eb), “This method is similar to stripping voltammetry, so we call this experiment as ‘stripping’ NPV”.1 Noticeably, it is the first try of applying NPV theory to this kind of special semiinfinite diffusion process, in which the reactant was produced during the waiting time at Eb. From the results obtained in our work,1 it can be found that this try under appropriate potential control is reasonable to a certain extent. The new try also had been ratified by the reviewers of ref 1. It should be noted that this special semiinfinite diffusion process is different from that in which the solution was saturated by pure CO simply. According to the linear relationship between the limiting currents and the inverse square roots of the sampling times (Figure 8 in ref 1), it could be concluded that the limiting current is diffusion controlled by free CO,4-9 while, in our system, the production of CO from methanol oxidation is the premise for the diffusion of free CO to the electrode surface to form COad in the next step. So, we claimed, “...the limiting current was controlled by the content of COad produced on the electrode surface and this behavior was similar to semiinfinite”. The linearity also partially validates the rationality of applying NPV theory to study this special semiinfinite diffusion process. The three references given here only show that there exists some common ground among them. The following are the replies to the comments from Dr. Gutie´rrez addressed indivdually. It should be noted that as in ref 1 we did not analyze the above special semiinfinite diffusion
Figure R4. CV results of Pt in 0.5 M sulfuric acid solution with a scan rate of 100 mV/s.
model in detail and “COad” in ref 1 and the following denotes both Pt - CO and free CO obtained according to eq R1. 1. Reply to the First Point. As shown in Figure R3, all our experimental data were recorded with the reference of Ag/AgCl (222 mV vs RHE) and the CV scans were performed from -0.2 V to 1.2 V vs Ag/AgCl. This region should be 0.022-1.422 V vs RHE. According to the literature,11 the current plateau should appear near 1.14 V vs RHE or 0.92 V vs Ag/AgCl. Figure R4 shows the characteristic wave of the Pt electrode. Curve a shows that the current plateau for Pt oxidation appears near 0.92 V, which is consistent with other reports.11 When the scan is extended to a higher potential than 0.92 V vs Ag/AgCl, as shown in Figure R4b, a higher slope of CV appears in the region of Pt oxidation. Obviously, the reason for this higher slope should be attributed to the further oxidation of Pt and then the oxidation of water into oxygen catalyzed by Pt oxide. Therefore, this is the main reason for the higher slope of CV in the region of Pt oxidation observed in our results. Certainly, as shown in CVs in our reports,1 the peak for COad oxidation overlapped with the current plateau of Pt oxidation; the residual COad (very little) after the first CV can also cover the current plateau of Pt oxidation. But, in this situation, the residual COad should not be regarded as impurity. Due to the slight residual COad, we
Comments
J. Phys. Chem. B, Vol. 110, No. 37, 2006 18727 3. Reply to the Third Point. To characterize the property of COad on Pt, according to the results shown in Figures 4 and 5 in ref 1, the COad on the Pt surface was hypothesized to be a monolayer. Naturally, one can think of the widely used Langmuir equation, which had been extensively used in different systems in previous works.13,14 Herein, according to the properties of our system,1 we rebuilt this equation slightly and named it the “Langmuir monolayer model.” In this name, we used the word “Langmuir” to indicate that this equation is based on the Langmuir equation rather than our origination. The content of COad on the Pt electrode was calculated according to the monolayer model as shown in Figure R2A.15 The molar amounts (N) of COad on the Pt electrode surface at different waiting times were calculated approximately based on the peak area of COad oxidation. Then the molarity of COad on the electrode was calculated from the following equation
C (mol/cm3) ) Figure R5. Results cited from ref 10.
“hypothesized approximately that it was a monolayer of COad on the electrode surface and COad had been oxidized to CO2 completely in the first positive scan.” 1 In previous work, a host of researchers had reported the electrochemical studies about methanol or adsorbed CO. But herein it is very necessary to study the electrochemistry of COad intermediated from methanol oxidation.1 Obviously, it is the premise for the next step to study the oxidation of COad with the NPV method. 2. Reply to the Second Point. In this part, Dr. Gutie´rrez concluded that the adsorbed CO was formed only “in the preceding negative scan”. As shown in Figure R5, it was cited from the results reported by Ye et al.10 To make the points more clearly observable, some points corresponding to the positive scan had been dyed red by our group. Obviously, it can be found, just as the red points show, during the positiVe scan from 0.05 to 0.9 V vs RHE, in the lower potential region less than 0.4 V, the intensity of COad increased gradually from 0.82 to 0.9 with the increase of potential. At 0.05 V, the intensity of COad is up to 0.82. So, if they extended the onset potential from 0.05 to 0.022 V vs RHE, then the intensity of COad at 0.022 V vs RHE could be evaluated to be near 0.8. In fact, Jusys et al.11 had also observed the COad intermediated from methanol oxidation at fixed electrode potential near this potential region. All in all, these recent results reported by others10,11 firmly validate the great existence of COad intermediated from methanol oxidation in acid at a low potential region. On the other hand, we observed a similar phenomena about the methanol oxidation on Pt in acidic condition as shown in the CV results in our paper.1 Combining the results reported by Ye and Jusys, one could conclude safely that the intermediate from methanol oxidation in acid on Pt at a fixed potential of -0.2 V vs Ag/AgCl (222 mV vs RHE) or 0.022 V vs RHE mainly is the COad. As shown in Figure 2b in ref 1, in the low potential region of the first cycle on Pt in 0.5 M sulfuric acid and 1.0 M methanol solution, the hydrogen sorption/desorption had been weakened. This indicated that the adsorbed CO had formed in the first scan and was not only “formed in the second and following CVs or the preceding negative scan.” Probably, the SEIRA results reported by Ye et al. as shown in Figure R5 corresponded to the second or the following CVs,12 but never can one conclude there is no COad on the first positive scan of CV or the adsorbed CO was formed in the preceding negative scan.
N (mol) A (cm2)‚l (cm)
A is the area of electrode, l is the thickness of the monolayer of COad on electrode surface, which approximately equals the diameter of CO. Reasonably, the value of l should be 2(rO + rC), where rO and rC are the covalent radius of oxygen and carbon atoms, respectively. But, unfortunately in our treatment process, the l was valued as rO + rC (≈ 150 × 10-10cm) due to our carelessness. So, the evaluated contents of COad on the electrode surface should be the half (1/2) of the value presented in that paper,1 namely, the maximum of COad (C0) should be 18 mol/L. This value is smaller than the molarity of liquid CO, 28.3 mol/L, indicating the electrode surface had not been covered up thoroughly by COad even at the maximum equilibrium content. The reason probably is that there are some other species on surface, such as the adsorbed hydrogen or water.15 We thank Dr. Gutie´rrez very much for his advertence on this minutia. As we did not find out the value of the molarity of liquid CO, this standard value could not be used in time to check on the details in the data treatment. As shown in eqs 9, 9b, and 10 in ref 1
k0 ∝ xDapp ∝ 1/C0,
Dapp ∝ 1/C02
then all the values of k0 should be twice that shown in Figure 11 and the value of Dapp should be 4-fold of that shown in Figure 9.1 As for the value of R, as shown in eq 9*,1 its value is independent of the reactant concentration, so the error in C0 has no effect on the value of R. Figure R2B is the scheme for the bulk of liquid CO. In this situation, as for the ruleless thermic movement of CO molecules, some space is always empty on average, while if these molecules were arranged as monolayers on a surface in an orderly and compact manner, as there is less free space than that in bulk liquid, then in this situation, it is possible that the surface volume concentration for the ordered monolayer molecules is higher than that in bulk liquid. 4. Reply to the Fourth Point. As shown in the Cottrell equation,2 the maximal current is correlated with many parameters, such as the number of electron transfers (n), reactant concentration (C), sampling time (τ), and apparent diffusion coefficient (Dapp). Obviously, the values for these parameters are different between methanol and COad; this difference certainly will lead to different currents. On the other hand, as shown in Cottrell eq 9,2 the (id)Cott value is the unstable diffusion-limited current and is correlated
18728 J. Phys. Chem. B, Vol. 110, No. 37, 2006 with the reaction time or sampling time (τ) in NPV experiments.2,4-9 With the decrease of τ, the (id)Cott value increases fast as shown in Figure 3,2 while the stationary diffusion-limited current is a theoretical maximum, a constant for a certain system. The (id)Cott value and stationary diffusion-limited current are two completely different concepts. What is more, the methanol oxidation into formate in our NPV process is a four-electron process while the stationary diffusion-limited current mentioned by Dr. Gutie´rrez is a six-electron process. Then, there is no comparability between the two kinds of limiting currents. 5. Reply to the Fifth Point. This point has been addressed in the beginning of this paper. 6. Reply to the Sixth Point. As shown in Figure 8 in ref 1, there are three straight lines at different waiting times. Some points departed the straight lines slightly; obviously, this could be attributed to experimental error. In fact, this slight departure also appeared in other NPV treatments4-9 or other linear fitting.14 In summary, we have replied to the comments from Dr. Gutie´rrez on our papers in detail. Some important illustrations have been made to eliminate the maze of information to readers. We believe that after this comment-reply process our work could be understood easily by readers. In the end, we are grateful to Dr. Gutie´rrez forhis pertinent comments on our papers. References and Notes (1) Xu, W.; Lu, T.; Liu, C.; Xing, W. J. Phys. Chem. B 2006, 110, 4802. (2) Xu, W.; Lu, T.; Liu, C.; Xing, W. J. Phys. Chem. B 2005, 109, 7872.
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