J. Phys. Chem. 1996, 100, 14081-14086
14081
Oxidation of Carbon Monoxide and Methanol on a Platinum Electrode in Acid As Studied by Optical Second Harmonic Generation In Tae Bae Ernest B. Yeager Center for Electrochemical Sciences and the Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106 ReceiVed: January 29, 1996; In Final Form: June 3, 1996X
Oxidation of CO and methanol on a polycrystalline platinum electrode in 0.1 M HClO4 was examined using real time optical second harmonic generation (SHG). A large SHG signal was observed in the potential range where CO directly adsorbed or formed from dissociative chemisorption of methanol on the electrode surface. The potential dependence of the SHG intensity for the surface covered with CO at saturation was interpreted to be due to the redistribution of the π-electrons in CO rather than the changes in the free electron density on the metal side. A simplified kinetic model for the SHG intensity measured as a function of time at a given electrode potential step allowed to evaluate the rate of CO buildup on the surface from methanol decomposition. It was also found that weakly adsorbed hydrogen blocks effectively methanol from adsorbing on to the surface by eliminating the atop sites.
Introduction Electrocatalytic oxidation of methanol on a platinum electrode is one of the most frequently chosen subjects in many studies since it may provide some key information regarding the oxidation mechanism of such a simple organic molecule on catalytic electrodes.1 Although the advent of in situ infrared reflection spectroscopy has contributed greatly to the understanding the details of the reaction pathways2 in the oxidation of methanol to the final product CO2, questions as to the elementary processes in the hydrogen adsorption and the double layer (DL) regions of an platinum electrode have not been clearly answered. The recent chronoamperometric study by Wieckowski and co-workers has also brought the controversial issue of existence of direct 6-electron oxidation of methanol to CO2 without involving CO adsorbed on Pt.3,4 While an electrochemical technique itself has great surface sensitivity, it often lacks specificity toward the identity of the adsorbed species. In situ spectroscopic methods such as in infrared spectroscopy heavily rely on signal averaging over a certain period of time5 and, thus have limitation in gaining information about the reaction kinetics. In order to monitor the electrode surface during a faradaic process, it is necessary to implement a specific in situ probe which is highly sensitive to the surface yet indifferent to the bulk media. A proper timeresolving power compatible to the process on the electrode surface is also a crucial factor in implementing such a probe. Optical second harmonic generation (SHG)6 is suitable for such a purpose as appeared in the literature for studying adsorptiondesorption of anions,7 underpotential deposition layer,8 and various molecules9 on the electrode surface as a function of the electrode potential. SHG has been also used for examining the electrode surface during a vigorous faradaic process.10 In this work we employed the SHG technique in monitoring the Pt electrode surface during the methanol oxidation in an acidic solution and CO formation therein was compared with direct CO adsorption from the solution. The measurement scheme was intended particularly to address the following issues: (1) difference between direct adsorption of CO from the solution phase and formation of CO from methanol, (2) the X
Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00273-0 CCC: $12.00
rate of CO formation on the surface from methanol at different potentials, and (3) the catalytic activity of the Pt surface covered with hydrogen. The kinetic data for CO formation were analyzed based on a Langmuirian regime for simplicity. Experimental Section The incident optical beam (580 nm) was delivered at 2 MHz from a cavity-dumped rhodamine 6G dye laser (702, Coherent) that was pumped synchronously by a frequency-doubled CW mode-locked Nd:YLF laser (Antares, Coherent). The pulse width was adjusted to 3 ps by inserting a one-plate birefringent filter in the dye laser cavity, and the temporal shape was monitored on an oscilloscope using an autocorrelation tracer (FR-103, Coherent). The incident power was maintained at 60 mW and the beam sized was adjusted to ca. 250 µm at the electrode position using a focusing lens. The SHG signal from the electrode was refined by rejecting the fundamental using a filter (7-54, Corning) and a monochromator (H-10, Jobin-Yvon Optical) before being fed to a photoncounter (925 SCINT, EG&G). The incident beam at 65° with respect to the normal to the plane of the electrode was p-polarized and p-polarized SHG signal was selected in all measurements. The in situ cell was constructed by pressing a Pt foil (1 mm thick, 4N, Aldrich) onto a circularly machined Kel-F cell body equipped with a flat fused silica window. A pair of Teflon tubes were connected to the cell body for the Luggin capillary and the gas purging inlet, respectively. A gold wire (1 mm in diameter, 15 cm long, 4N, Aldrich) served as a counter electrode. The Pt foil was polished with alumina successively down to 0.05 µm and cleaned in hot nitric acid followed by rinsing with pure water prior to mounting. The exposed area of the Pt electrode was 1.37 cm2. A more detailed description of the experimental setup can be found elsewhere.9d Concentrated perchloric acid (Ultrex, Baker) was diluted with pure water to 0.1 M and methanol was the HPLC grade (Aldrich). Both carbon monoxide and nitrogen were 4N grade (HP, Metheson). The water used was freshly collected from the Nanopure system (Barnsted). Electrochemical control and measurements were performed with the PARC 173-175 potentiostat-programmer pair. The electrode potential was measured against a saturated calomel electrode and quoted so. © 1996 American Chemical Society
14082 J. Phys. Chem., Vol. 100, No. 33, 1996
Figure 1. Cyclic voltammogram and SHG response simultaneously obtained for the Pt electrode in 0.1 M HClO4. The sweep rate was 50 mV/s and the apparent electrode area was 1.37 cm2. The filled circles and open circles in the SHG signal denote the positive and the negative sweep, respectively.
Figure 2. Changes in the SHG intensity under open circuit conditions: (A) 0.1 M HClO4 bubbled with CO, and (B) 0.1 M HClO4 after injecting CH3OH (50 mM). The 0’s indicate the moments of CO bubble initiation and of CH3OH injection, respectively. See the text for details.
Results Clean Pt Surface in 0.1 M HClO4. The electrode potential was cycled over the full Pt electrochemical window while the SHG intensity was measured simultaneously. The result is given in Figure 1. The cyclic voltammetric curve shows a typical feature for the clean polycrystalline Pt surface in 0.1 M HClO4. The SHG intensity in the DL region was small and increased slightly as the Pt surface is oxidized, which indicates that the interaction of the perchlorate anion7c with the surface is relatively weak compared to other types of anions such as Clor HSO4-. The signal increases markedly as the hydrogen coverage increases. This SHG feature can be favorably compared with the earlier data in the literature.7c Although the relationship between the SHG intensity and the hydrogen coverage is known to be quadratic, such a treatment is not attempted. Adsorption of CO and Methanol on Pt under OpenCircuit Conditions. Figure 2A shows SHG intensity changes with time for the Pt electrode under open-circuit conditions while purging CO through the electrolyte solution. The signal increased to a plateau in 400 s showing a sigmoidal shape after the initiation of bubbling, which indicates that the CO coverage
Bae reaches a saturation value. Meanwhile, the open-circuit potential (OCP) initially decreased from ca 0.75 V and reached ca. 0.10 V vs SCE as CO saturation was achieved. The SHG signal level for the CO-saturated surface was much higher than for the bare surface in the DL region but smaller than for the surface in the weakly adsorbed hydrogen region (see Figure 1). When methanol was injected into the neat electrolyte purged with N2, the SHG intensity, unlike in the case of CO bubbling, initially decreased quickly and then increased slowly, reaching a level close to that for the bare surface in the DL region as shown in Figure 2B. Even after 2000 s, the signal persisted at such a low level. On the other hand, the OCP decreased fast, after passing a minimum (0.10 V) increased slowly, and then stayed at 0.18 V. As noticed in Figure 2, the initial intensities for the two cases are not identical since the initial surface condition is highly uncertain as the electrode is prepared. It is believed that the Pt surface may be covered initially by oxygen giving a high OCP value (0.7-0.9 V) supplied from the electrolyte solution which contains a trace amount of oxygen and this adsorbed oxygen is replaced with CO or methanol. This may explain the initial fast decrease in the SHG signal for the solution containing methanol as well as the changes in the OCP. With applied electrode potentials, methanol undergoes dissociative chemisorption on Pt in the DL and hydrogen regions forming CO on the surface and this has been well documented in many in situ infrared reports.2b,c In this regard, it is interesting to note that methanol does not undergo such dissociative chemisorption on the Pt surface under open circuit conditions, although the Nernst potential corresponds to the double region. However, the variations in the SHG signal and the OCP indicate undoubtedly methanol adsorption on the Pt electrode surface. SHG Measurements during Potential Cycling. The SHG intensity for the Pt electrode was monitored as the electrode potential was continually cycled at the scan rate of 5 mV/s for each of the solutions, CO-saturated 0.1 M HClO4 and 50 mM methanol in the same electrolyte. The results are displayed in Figure 3A and 3B, respectively. The abrupt drop in the SHG signal at 0.59 V for the CO-saturated solution was well timed with the sharp current peak for oxidative removal of CO from the surface. In the negative sweep, reduction of the anodic film, Pt(OH)2, occurs first, and then CO oxidation is mixed at more negative potentials as the fresh Pt surface is gradually exposed to the solution. This is seen as switching from the negative current to the positive in the voltammetric curve. The CO buildup onto the surface initiates at 0.40 V and the oxidation current becomes essentially nil as the SHG intensity reaches the plateau region. The CO buildup rate was relatively slow compared to its oxidation rate. It is noteworthy that the SHG signal for the CO-covered Pt surface decreases monotonically as the potential moves positively while the CO coverage is maintained at saturation.11 This feature in conjunction with the origin of SHG for the CO-covered Pt surface will be discussed later. For the solution containing methanol, the pattern of increase and decrease in the SHG signal as well as in the oxidation current was much more chronic than that for the COsaturated solution. The SHG intensity at -0.2 V was about 70% of that for the electrode covered with CO at the same potential, indicating that some of the adsorption sites were not occupied by CO, most likely due to relatively slow decomposition of methanol. This is also evidenced by the increased SHG intensity in the positive sweep (filled circles) from that in the negative sweep (open circles). It is obvious from the results that the SHG intensity for CO depends not only on the coverage but also on the electrode potential. The fast decrease in the SHG intensity commenced as early as at 0.25 V where the
Oxidation of Carbon Monoxide and Methanol on Pt
J. Phys. Chem., Vol. 100, No. 33, 1996 14083
Figure 4. Coverage-dependent parts of the SHG intensities upon negative potential steps from +0.900 V to +0.100 (A), -0.100 (B), and -0.275 V (C) after subtracting the background intensity. The solid curves represent the theoretical fits based on the Langmuir adsorption kinetics. See the test for details.
Figure 3. Cyclic voltammograms and SHG responses of the Pt electrode in 0.1 M HClO4: (A) CO-saturated and (B) 50 mM CH3OH. The sweep rates were 5 mV/s for both.
oxidation current started to increase and the signal reached the background level at the current maximum 0.53 V. The position of the current maximum was highly dependent on the sweep direction whereas the SHG intensity change was relatively reversible. A detailed look shows that a broad current shoulder near 0.40 V in the anodic sweep coincides with the fast drop in the SHG signal (inflection point). This indicates that the current shoulder is due to the oxidative removal of CO which poisons the Pt electrode in the DL and the hydrogen regions. The current beyond this shoulder is believed to be due to the direct oxidation of methanol to CO2. The current eventually decreased at more positive potentials because of the slow kinetics of methanol oxidation on the Pt anodic film and the SHG intensity in this region was not much different from that for neat electrolyte solution. Overall, the rate of CO formation from methanol was much slower compared to the rate of direct adsorption from the solution containing CO. Oxidation of CO in the former case occurs at a much less positive potential because of the low coverage. CO Buildup Rates Measured by SHG. Potential steps were applied to the electrode in order to measure the rate of CO buildup from methanol on the Pt surface. As shown by the SHG intensity in the cyclic potential sweep in Figure 3B, the methanol oxidation in the Pt anodic film region is kinetically controlled; i.e., the current is much smaller than a value expected in diffusion control. The concentration gradient of methanol at the electrode would be minimal for the solution containing methanol at a high concentration (50 mM) stirred vigorously by nitrogen bubbling. An arbitrary potential in this region, 0.90 V in the present case, can be chosen as an initial rest potential without developing a significant difference in the methanol concentration at the vicinity of the electrode from the bulk value.
Even when the electrode potential is stepped to the region where methanol oxidation occurs, the CO formation rate is not affected significantly by the small concentration gradient if the reaction is slow. Thus, the effect of the concentration gradient on the rate measured can be safely ruled out. The SHG profiles measured with time for the potential steps from 0.90 V to 0.10, -0.10, and -0.275 V are displayed in Figure 4A, B, C, respectively. These plots were made for only adsorbate coverage-dependent parts by subtracting the average background intensity ca. 0.03 in the DL region. Since this value is very small compared to the signal corresponding to the surface covered with CO, its potential dependence is neglected. The rates of CO formation on the Pt surface from methanol in all three cases were surprisingly slow, so that the coverage in each case reached a certain steady value after more than 200 s. The SHG intensities at the end of the time sweep were ca. 0.27, 0.41, and 0.52 for the potential steps to 0.10, -0.10, and -0.275 V, respectively, which is consistent with the potential dependence of the SHG intensity as seen in Figure 3B. The SHG intensity change at -0.275 V unlike the other two cases, shows a distinctive decaying feature from a high initial value 0.82 which corresponds to that for the surface fully covered with hydrogen at this potential (see Figure 1). This implies that the Pt surface is instantly covered with the both types of strongly (s-H) and weakly adsorbed hydrogen (w-H) upon the potential step and then slowly replaced with CO produced from methanol decomposition. Although SHG signal does not distinguish between hydrogen and CO, this view based on comparison of the SHG intensity changes is agreement with in situ IR data,2 in which a slow buildup of CO on the surface is found while holding the potential in this region. To extract kinetic information as to CO formation on the surface, the three sets of data were analyzed based on a simple model. Langmuirian Approach. For the Pt surface in contact with 0.1 M HClO4, the substrate contribution to the second-order nonlinear susceptibility12 is very small, particularly in the potential region negative to the anodic film and, subsequently, the cross term13 is also negligible, so that the measured SHG intensity can be mostly due to the adsorbate. Considering only the coverage-dependent term after subtracting the background
14084 J. Phys. Chem., Vol. 100, No. 33, 1996
Bae
TABLE 1: Adsorption Rate, Desorption Rate, and Other Parameters Obtained from Curve Fittings for SHG Responses in Potential Stepsa E, V
a
0.100 -0.100 -0.275
0.105 0.107 0.828
k(atop CO), s-1
k(bridge CO), s-1
1.15 0.0287 0.0360 2.07 0.0541b
k(H), s-1
r
γ, deg
0.0589
0.621 1.00 0.791
13.4 28.5 17.2
Equation 4 was used for 0.10 and -0.10 V, and eq 5 was used for -0.275 V. The parameter r represents the ratio of the two contributions. b The two types of CO’s were not separately treated. a
value,14 the SHG intensity would be
I(2ω) ∝ |χ(2)a|2
(1)
χ(2)a ) Aϑ1 + Bϑ2 + ...
(2)
where A and B are complex numbers related to the Fresnel coefficients and the tensor elements from the second-order susceptibility, and ϑ’s are coverages of adsorbates. We used the Langmuir kinetics for a simplicity. Since the concentration of methanol at the vicinity of the electrode is assumed to be the same as the bulk value by the reason mentioned earlier, the situation is very similar to that for a gas/ solid reaction interface. Hence
ϑi(t) ) Γi(t)/Γi(s) ) 1 - e-k(i)t
(3)
where Γi(s) is the saturated surface concentration of species i and ki is its adsorption rate constant. Based on this formalism, the kinetics of the surface buildup of adsorption intermediates during methanol decomposition on the Pt electrode can be analyzed. In the region examined by potential steps, two types of CO’s form on Pt, i.e., atop CO and bridged CO (on the twofold site) are found from infrared studies.2 Therefore, the coveragedependent SHG intensity can be written using eqs 1, 2, and 3
I(2ω,CO) ) a[(1 - e-k(1)t)2 + r2(1 - e-k(2)t)2 + 2r(1 - e-k(1)t)(1 - e-k(2)t) cos γ] (4) where B/A ) reiγ and γ is the phasor between the two complex numbers A and B. Fittings of eq 4 to the profiles at 0.10 and -0.10 V are shown as solid lines in Figure 4, A and B, respectively. For the SHG profile at -0.275 V, using ϑH ) e-k(H)t and ϑCO ) 1 - e-k(CO)t:
I(2ω,CO,H) ) a[e-2k(H)t + r2(1 - e-k(CO)t)2 + 2re-k(H)t(1 - e-k(CO)t)cos γ] (5) In this case, the rising part was treated as a single component, namely, only one type of CO since it was not possible to distinguish one from the other in the current data alone. The best fit is shown as a solid line in Figure 4C. The values for the parameters are summarized in Table 1. The rate constants k(1) and k(2) are believed to be due to bridged CO and atop CO, respectively for the SHG profile at -0.10 V and the individual contributions of the two species are nearly identical since the ratio r is 1.00. SHG is a nonspecific toward the identity for the type of CO; however, the opposite assignment would be true for the profile at 0.10 V; i.e., the majority species would be atop CO since r ) 0.62. This is so because atop CO is found as the majority in the DL region while both atop and
Figure 5. Cyclic voltammogram and SHG response for the Pt electrode held at -0.25 V in the absence of CH3OH, injected CH3OH waiting for 4 min, and then swept positively at 20 mV/s.
bridged CO species are found in the hydrogen region by IR studies.2c,15 In the w-H region, the hydrogen desorption rate k(H) is very close to the CO formation rate k(CO). Observation of the interference due to the w-H led to the following measurement. Influence of Weakly Adsorbed Hydrogen. In the absence of methanol in the electrolyte solution, the electrode potential was held at -0.25 V where the Pt surface was fully covered with s-H and w-H, and then methanol (50 mM) was injected into the solution stirred vigorously. After waiting for 4 min (this is longer than the time required to saturate the surface with CO in the potential step measurement), the electrode potential was swept at a rather fast rate 20 mV/s. The SHG intensity change is shown together with the voltammetric curve in Figure 5. Surprisingly, the initial SHG intensity due to the w-H did not change while holding the potential after injecting methanol. Upon the potential sweep the pattern of the decrease in the SHG intensity up to 0.0 V was nearly identical to that for the neat electrolyte solution in Figure 1. The CO buildup started at 0.0 V finally due to the activity of the surface toward methanol decomposition and the rest of the feature is similar to that for the methanol solution in Figure 3B. This observation indicates that w-H is a strong poison against methanol activation while s-H is not. This is also supported by the small current peak at 0.0 V in the voltammogram in Figure 5. Discussion The origin of SHG for the CO/Pt interface in contact with the electrolyte solution is not immediately clear. First, whether the signal is due to CO itself, i.e., high polarizability of π-electrons,12 or due to modification in the distribution of free electrons on the metal side caused by CO adsorption can be discussed. In ultrahigh vacuum (UHV), the surface nonlinear susceptibility for a metal is dominated by the surface free electron density16 and in consequence, a large SHG signal is expected for the bare metal surface. Such a large background SHG signal was not observed for the present electrochemical interface employing a polycrystalline electrode and CO injection into the solution led to the SHG profile in a sigmoidal shape unlike in the case of the experiments in UHV. A diminished polarizability of surface free electron caused by localization of free electrons upon contacting with the solution can account for the small background SHG signal. Therefore, the net SHG signal observed upon CO injection is believed to be directly due to the high polarizability of π-electrons in CO. This would also explain the monotonic decrease in the SHG intensity with increasing potential in Figure 2A. According to the atom
Oxidation of Carbon Monoxide and Methanol on Pt superposition electron delocalization MO theory,17 the degree of decrease in the back-donation of Pt d-electrons to the π* level of CO with anodic polarization overcomes the increase in the 5σ donation of CO to Pt. Hence, decreasing net charge transfer from Pt to the π* level of CO with increasing potential causes the decrease in the SHG intensity with positive polarization of the electrode. If the surface free electron density was responsible for the SHG, a rather increasing signal with positive polarization would be observed due to the decreasing backdonation. Second, adsorption of CO on the Pt surface makes the empty density of states composed significantly of 5σ* and 2π* levels of CO, ranging 1-5 eV above the Fermi level.18 These empty states have been probed by inverse photoemission19a and laser desorption studies.19b It may be possible to facilitate resonance enhancement in SHG at the photon energies of the fundamental (2.14 eV) and the second harmonic (4.28 eV) used in the present study. This resonance enhancement may be responsible for the large signal observed for the Pt surface covered with CO. Measurements with carefully tuned laser energies would be required to manifest such a possibility. The charge under the voltammetric curve for CO oxidation obtained by integrating up 0.60 V where the surface CO was persistent (see Figure 3A) was 1.9 mC/cm2. This value is nearly 2 times of the charge corresponding to the monolayer2b 0.4 mC/ cm2 even after considering a typical roughness factor of 2 or 3 for a polished electrode, which indicates that at some Pt surface sites, the turnover number is larger than unity and CO oxidation on such sites occurs at relatively lower overpotentials. For the methanol-containing solution, the charge was 63 mC/cm-2. This corresponds to ca. 3 × 10-8 mol/cm2 as methanol. The turnover number becomes 15 assuming that the same number of sites are involved, the oxidation releases six electrons for each methanol molecule and the roughness factor is 3. Therefore, there exists at least one reaction route3b giving the final product CO2 without generating long-lived CO species as intermediates on the surface. As shown in the SHG measurements under open-circuit conditions, methanol does not undergo decomposition without an externally applied potential but simply adsorbs on the Pt surface even though the OCP reaches 0.15 V. This implies that the dissociative chemisorption of methanol on Pt is an electrochemical process. In the hydrogen adsorption region, strong reductive adsorption of proton or methanol onto the surface reaction sites possibly provides a driving force for the C-H bond activation resulting in CO formation whereas in the DL region the overpotential is large enough to overcome the activation energy for the C-H bond breakage. When the electrode potential was stepped to the w-H region in the presence of methanol, the surface would be covered mostly with hydrogen due to its fast kinetics. However, it is expected that there be some adsorption defects since some surface sites are initially occupied by directly contacting methanol molecules. Such a defect site as a “hot spot” would accommodate further extension of methanol activation eventually replacing all adsorbed hydrogen. The very similar values for k(H) and k(CO) at -0.275 V also indicate that there is no time delay between the two processes and there is a close interaction between the two species on the surface. The fact that methanol decomposition is absent for a prolonged period of time when the surface is fully covered with w-H in addition to s-H eliminating any adsorption defects (see the last section of Results) supports the model in which the methanol activation is induced by methanolic hydrogen adsorption at the hydrogen-absent sites. Furthermore, such a hot spot created by initial defective hydrogen adsorption (methanol adsorption) is believed to be the same site as for w-H;
J. Phys. Chem., Vol. 100, No. 33, 1996 14085 i.e., the atop site since CO formation occurs on the surface near the potential where the oxidative desorption of w-H terminates (see Figure 5) exposing the atop sites to methanol. The CO formation rates from methanol at different potentials were evaluated based on a simple Langmuirian despite the site inhomogeneity was introduced. This is not an ideal approach since the assumptions made for the analysis are in question. The adsorbate was assumed to be solely CO species although fragmental species such as -CHxOH and -CHO have been found as transient species under certain conditions.20 Interconversion15b between atop CO and bridged CO also occurs in the potential range examined. The subtraction of the small background SHG signal may not be justified when a possible phase difference is involved even if the contribution is minimal. In addition, the contribution from s-H was not taken into account at all. Consideration of all such factors would make the analysis very complicated and a strict treatment may not be possible with current data alone. Nevertheless, we believe that the rate constants obtained from the present analysis represent reasonably the gross decomposition process for methanol on Pt under the given condition. The scheme for adsorption kinetics proposed by Wieckowski and co-workers3a was also tested; however, the result was not acceptable, particularly in the initial rising part of the SHG intensity. From the above observations and arguments, we propose the following oxidation pathways for methanol on Pt in the sequence of increasing electrode potentials:
Pt-CH3OH + Pt(s-H)m f Pt2CO + (m + 4)H+ + (m + 4)e- w-H region Pt + Pt(s-H)m + CH3OH f Pt2CO + (m + 4)H+ + (m + 4)e- s-H region CH3OH + nPt f PtnCO + 4H+ + 4e- DL region Pt2CO f PtCO + Pt CH3OH + H2O ff CO2 + 6H+ + 6eThe site for s-H is believed to be multiple-fold;21 thus, m is 1/2 or 1/3. If s-H goes underneath the plane of the first atomic layer, m would be smaller than 1/3. In the w-H region, the reaction may be facilitated by methanol molecules initially adsorbed on the surface (hot spot). The number n is 1 or 2 for atop CO or bridged CO, respectively. At higher potentials and at higher coverages, atop CO is more favored. Since the turnover number for methanol is much larger than for CO, there must be a reaction path giving the final product CO2 in the DL region. Additionally, the electrode surface in the anodic film region did not show any significant SHG intensity in the presence of methanol indicating that adsorption of methanol or CO was not facilitated on this surface. Conclusions We employed the SHG technique to examine the Pt electrode surface during methanol oxidation and the result was compared with that for the CO-saturated solution. In contrast to the UHV studies, the SHG signal for the surface covered with CO in the electrolyte solution was interpreted to be due to the high polarizability of the π electrons rather than the free electron redistribution on the metal side. Contribution of resonance enhancement to the SHG signal may be also possible due to electronic transitions to the empty density of states a few
14086 J. Phys. Chem., Vol. 100, No. 33, 1996 electronvolts above the Fermi level. The SHG intensity was appeared to be a function of both the CO coverage and the electrode potential. A cross examination of the voltammetric feature with the corresponding in situ SHG profile allowed us to resolve the detailed processes, particularly in the potential region where methanol oxidation occurs vigorously. The data also supported the mechanistic model involving direct sixelectron oxidation of methanol in the DL region. Such a process is believed to exist in the potential region where CO oxidation and Pt oxidation occur concurrently. The rates of CO buildup from methanol were measured by monitoring the SHG intensity in potential steps and analyzed based on a simple Langmuirian approach. The rate of methanol decomposition as an electrochemical process on the Pt surface covered with w-H was rather slow compared to those in the s-H and the DL regions. The surface fully covered with w-H inhibited methanol oxidation and the slow process was explained by the model of hot spot, i.e., the atop site initially occupied by a methanol molecule. Acknowledgment. The author thanks Prof. Daniel A. Scherson for his valuable comments. This work was partially supported by the U.S. Office of Naval Research through ARPA contract No. N00014-92-J-1848. British Petroleum Research, America, is also greatly acknowledged for generous donation of laser-optical equipment. References and Notes (1) Parsons, R.; VandeNoot, T. J. Electroanal. Chem. 1988, 257, 9. (2) (a) Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K. J. Electroanal. Chem. 1981, 121, 343. (b) Kunimatsu, K.; Kita, H. J. Electroanal. Chem. 1987, 218, 155. (c) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 241, 143. (3) (a) Franaszczuk, K.; Herrero, E.; Zelanay, P.; Wieckowski, A.; Wang, J.; Masel, R. I. J. Phys. Chem. 1992, 96, 8509. (b) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074. (4) (a) Vielstich, W.; Xia, X. H. J. Phys. Chem. 1995, 99, 10421. (b) Herrero, E.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. 1995, 99, 10423. (5) In general, a signal averaging of several hundred interferometric scans which takes minutes is needed to achieve a monolayer sensitivity. Even for a system highly optimized for CO studies (see for example: Leung, L. H.; Weaver, M. J. J. Phys. Chem. 1988, 92, 4019), it takes 15 s to collect a spectrum. Moreover, in in situ IR reflection spectroscopy where an external reflection mode is employed, the electrode is pushed against the IR window, forming a thin electrolyte layer of a few microns to minimize the absorption by the bulk medium. A time constant of a few tenths of a second is commonly observed for such a configuration in the absence of electroactive species due to a large iR drop along with the electrode surface, which causes
Bae a highly nonuniform current distribution particularly in the case of faradaic processes involving solution phase species. Thus, a kinetic study is severely limited. (6) For reviews: (a) Shen, Y. R. Annu. ReV. Phys. Chem. 1989, 40, 327. (b) Richmond, G. L.; Robinson, J. M.; Shannon, V. L. Prog. Surf. Sci. 1988, 28, 1. (7) (a) Chen, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y. R. Phys. ReV. Lett. 1981, 46, 1010. (b) Richmond, G. L. Langmuir 1986, 2, 132. (c) Campbell, D. J.; Corn, R. M. J. Phys. Chem. 1988, 92, 5796. (d) Rojhantalab, H. M.; Richmond, G. L. J. Phys. Chem. 1989, 93, 3269. (e) Campbell, D. J.; Lynch, M. L.; Corn, R. M. Langmuir 1990, 6, 1656. (8) (a) Corn, R. M.; Romognoli, M.; Levenson, M. D.; Philpott, M. R. J. Chem. Phys. 1984, 81, 4127. (b) Koos, D. A.; Richmond, G. L. J. Chem. Phys. 1990, 93, 869. (c) Lakkaraju, S.; Bennahmias, M. J.; Borges, G. L.; Gordon II, J. G.; Lazaga, M.; Stone, B. M.; Ashley, K. Appl. Opt. 1990, 29, 4943. (d) Koos, D. A.; Richmond, G. L. J. Phys. Chem. 1992, 96, 3770. (9) (a) Voss, D. F.; Nagumo, M.; Goldberg, L. S.; Bundig, K. A. J. Phys. Chem. 1986, 90, 1834. (b) Campbell, D. J.; Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1990, 94, 3681. (d) Bae, I. T.; Choi, K. -J. J. Electroanal. Chem. 1992, 339, 187. (e) Kim, S.; Zhao, M.; Scherson, D. A.; Choi, K.J.; Bae, I. T. J. Phys. Chem. 1994, 98, 9383. (10) (a) Campbell, D. J.; Corn, R. M. J. Phys. Chem. 1987, 91, 5668. (b) Biwer, B. M.; Pellin, M. J.; Schauer, M. W.; Gruen, D. M. Langmuir 1988, 4, 121. (11) The sweep rate is slow enough to have the saturation coverage as early as 0.30 V in the negative sweep. If it is not saturated, the SHG signal should increase since CO adsorbs continually in this potential range. (12) Shen, Y. R. The Principles of Nonlinear Optics, Wiley: New York, 1984; p 479. (13) Lynch, M. L.; Corn, R. M. J. Phys. Chem. 1990, 94, 4382. (14) A phase difference between the Pt background contribution and the adsorbate contribution is expected since they are complex numbers. In the case of Pt contacting an aqueous solution, the background contribution is small enough to ignore the phase difference. The error caused by the simple subtraction of this background intensity for easy mathematical treatment of the data would not be serious. However, it is not possible to obtain the background intensity in the hydrogen region. Nevertheless, the same intensity as in the DL region was assumed. (15) (a) It was also predicted theoretically that bridged CO is relatively favored in this potential region. See for example: Anderson, A. B.; Awad, M. K. J. Am. Chem. Soc. 1985, 107, 7854. (b) Kitamura, F.; Takahashi, M.; Itoh, M. J. Phys. Chem. 1988, 92, 3320. (16) Grubb, S. G.; DeSantolo, A. M.; Hall, R. B. J. Phys. Chem. 1988, 92, 1419. (17) Ray, N. K.; Anderson, A. B. J. Phys. Chem. 1982, 86, 4851. (18) Wong, Y.-T.; Hoffmann, R. J. Phys. Chem. 1991, 95, 859. (19) (a) Bertel, E.; Memmel, N.; Rangelov, G.; Bischler, U. Chem. Phys. 1993, 177, 337. (b) Fukutani, K.; Song, M. -B.; Murata, Y. J. Chem. Phys. 1995, 103, 2221. (20) CHxOH or CHO are short-lived species and these species are not believed to affect the current analysis. For such findings refer to: (a) Iwasita, T.; Vielstich, W.; Santos, E. J. Electroanal. Chem. 1987, 229, 367. (b) Nichols, R. J.; Bewick, A. Electrochim. Acta 1988, 33, 1691. (21) Bewick, A.; Nichols, R. J. J. Electroanal. Chem. 1988, 243, 445.
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