6074
J. Phys. Chem. 1995,99, 6074-6083
Methanol Oxidation at p-Si/Pt Electrodes. Evidence for Hot Hole Reactivity K. W. Frese, Jr.,* and C. Chen Interfacial Sciences, Inc., Santa Clara, California 95051 Received: August 24, 1994; In Final Form: January 24, 1 9 9 9
Electrochemical methanol oxidation was performed using p-Si( lOO)/Pt, p-Si( 11l)/Pt, and graphite foil/Pt electrodes in 0.1 M H2SO4 electrolyte at 80 "C. For p-Si( 100) substrates, the real-area 10 min current density peaked at 1.1 mA cmr-2 at a Pt thickness of 110 at 0.1 M and 1.7 mA cmr-2 at 0.5 M methanol. The peak current was observed near 100 for all voltages from 0.05 to 0.45 V vs Ag/AgCl. Structural and area effects were ruled out as the cause for the peak. No currentkhickness peak was observed with the graphite foil substrate. A surface state model for the main reaction intermediate and poison combined with hot holes accounted for the peak in the steady-state current density and the dependence of the measured activation energy on Pt thickness. A kinetic model was used to determine rate constants as well as transient and time evolution of the poison coverage. Steady-state poison coverage ranged from 0.8 for a Pt foil to a 0.35 minimum for 110 ii~t on p-si(100).
Introduction We have reported studies of one electron reduction and oxidation reactions using n- or p-Si substrates coated with thin films of Au,' Ag,2 and Pt.394 The main goal of this research is to demonstrate, if possible, enhanced reactivity5 for electrochemical oxidation and reduction with thin metal films on electron and hole-injecting semiconductor substrates. The electrochemical reaction site is expected to be at the metalelectrolyte junction. A hole injected by a p-type substrate at the metal-semiconductor interface should enter the film at the energy corresponding to the top of the semiconductor valence band. By virtue of the Schottky barrier in the semiconductor, the carrier energy level at the metal-electrolyte interface should be different from the metal fermi energy if the injected carriers arrive at the interface with a partial loss of energy (cooling). Such carriers are termed hot with respect to the metal fermi energy. For ballistic transport, the hole at the reaction site has the energy level of the substrate valence band. Theoretically, the energy level difference between a hole and an electrolyte or adsorbed donor state determines the rate of hole capture by the state (oxidation rate). Therefore one can envision cases where a more favorable energy level matching may be obtained with a hot hole rather than a vacant state at the metal fermi energy. In such a case, hot holes might accelerate the slow step in a reaction sequence. An illustration is shown in Figure 1. The hole is injected into the metal at (vbi 4-p ) volts below the fermi level. A ballistic hole reaches the Pt surface with no loss in energy, 6 = 0. The partially cooled holes reach the surface at a voltage (vbi 4-p ) - 6 below the fermi level. Consider the energy levels, VI and VP, representing surface states for an adsorbed intermediate and poison respectively. An example intermediate is Pt-CH = 0: and a known poison is the incompletely oxidized adsorbate, toad. Holes cooling to VI should react with optimum rate, that is, faster than a ballistic hole or a fully cooled hole at V f e ~ . According to the energy matching model, if (vbi p ) were close to Vp, the poison might be oxidized rapidly. Since holes are cooled more fully for thicker metal films, we expect an optimum or limiting current density at a certain metal thickness. References 2 and 5 may be consulted for more details.
+
@
Abstract published in Advance ACS Abstracts, March 15, 1995.
-
bbi ..______.....__.____......-.....----
Adsorbed Intermediate Surfacestates
' Pt Film
Ws '
p-type Silicon
Figure 1. Level diagram of Wp-Si electrolyte interface. The energetic relation between hot holes and donor surface state levels, corresponding to a reaction intermediate, I, are illustrated. A maximum rate of hole capture and oxidation rate is expected when VI, reaches VI. Vp refers to the energy level of a poison that is difficult to oxidize.
A possible catalysis by hot holes at a Pt/p-Si electrode may occur as follows. Consider a multistep charge-transfer oxidation that is limited by oxidation of an adsorbed intermediate, I, with coverage, 8. The reaction velocity on an ideal thin film Schottky barrier metal-semiconductor electrode at overpotential, q, is vsc8exp[q{Vsc - q (VI - vh)2/4a}/kr]. The energy level of the hot hole is v h = (vbi p)exp[-t/L~], where (vbi p ) is the Schottky barrier height, t is the metal thickness and LB is the ballisti5 mean free path. Typical values are t = 30 A and LB = 100 A. Both VI and v h are measured relative to the fermi energy VO. The rate of the same step at the bulk metal electrode is vm8exp[-q(V~ - VO- q)2/4;lkr]. On the basis of densities of states, we expect the preexponential, vm, is 10-100 times larger than vsc. The ratio of rates on the two types of electrodes at the same q is R = {vscexp[-q{Vsc - q [VI - (vbi p ) exp(-t/LB)I2/4a}/kr)/{v~ exp[-q(V1 - Vo - q)2/41kr]}. R may be larger or smaller than unity, depending on the kinetic
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0022-3654/95/2099-6074$09.00/0 0 1995 American Chemical Society
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Methanol Oxidation at p-Si/Pt Electrodes
J. Phys. Chem., Vol. 99, No. 16, I995 6075
TABLE 1: Raw Integrated Intensities and Breadth Parameters for Pt Films on p-Si r(A) (111) (100) (110) B(rad) meansize@) 50 80 100 120 150 500
97.89 130.3 113.6 123.6 203.3 286.6
48.5 57.4 34.2 38.4 69.7 91.9
19.9 27.1 31.4 32.4 92.4 139.4
0.0325 0.0292 0.0221 0.0220 0.0242 0.0142
50 59 72 72 65 112
properties. Typically, R > 1 occurs when is small, 0.1-0.3 eV, and VI - VOis about 0.5 V or greater. This means hot carriers are most effective when the reaction rate on the bulk metal is slow and the donor surface state energy level spread is sufficiently small. In this report, we aim to show that methanol oxidation at p-Si/Pt electrodes can be described by the energy level and hot hole model described in Figure 1. The oxidation of methanol with thin Pt films on p-Si, graphite, and other carbon substrates was investigated. Thin films were necessary to minimize hole cooling within the metal film.
Experimental Section
Pt films were magnetron sputtered at 3-5 mTorr 99.9995%
Ar on device quality p-Si (100) and (1 11) substrates and 99.99% graphite foil. The doping levels of two (1 11) crystals and one (100) were 1.0 x 1015, 3.0 x lo"-?, and 1.0 x l O I 5 cmV3, respectively. The terms hd and Id in the text refer to the higher and lower doping levels. Si crystals were etched with 5% Fisher Optima HF before Pt deposition. The Si crystals were masked with Teflon tape so that 0.5 cm2 of Pt was deposited. Sputtering was stopped automatically when the film thickness reached a preset value determined by a Kurt Lesker thickness monitor. Back contacts were 400 8, Pt films. The crystals were attached to glass slide backings using silver paint. Wires were soldered to an exposed portion Ag-painted slide. General Electronic Silicone 11 sealant was used to cover all surfaces except the back side of the glass slide and the Pt film. Current-voltage curves were acquired with a Pine RDE4 potentiostat and computer with 12 bit A/D conversion. XRD measurements were performed by Signetics Materials Analysis Laboratory, Sunnyvale, CA, using a Siemens D500 XRD system with Cu radiation in 1" low-angle mode, 0.05' increment, and 1 s count time. Methanol, oxidation was performed at 80 "C. All measurements were made on 0.05 M high-purity H2SO4 made from Nanopure I1 bioresearch grade water and Merck Suprapure acid.
Results Low-Angle XRD. It will be shown that the methanol oxidation current on p-Si substrates is sharply peaked in Pt thickness for all three p-Si crystal types. Given the known dependence of oxidation current on crystal orientation, it is important to consider structural characteristics of our films. Deviations in the relative intensities of X-ray reflections from the random power pattem is a sign of preferred orientation. Lowangle (1") X-ray diffraction measurements were made on Pt films that were sputtered on an etched hd p-Si (1 11) substrate. The grazing angle effectively masked the Si substrate X-ray reflections. The data consisted of integrated intensity, Z, and breadth parameter? B . Table 1 shows the intensities, B values, and the calculated mean particle sizes. Mean size = 1.386 h B cos(@, where B and 8 are in radians. The relative intensities of a random sample are Z111:ZlW:Zllo = 1:0.53:0.31. Table 1 shows the intensities do not follow the random pattem, indicating that there is some texture (preferred
: 0.551
0.5-
8 111
100 A 'lo
0.15 i 40
A
100 Pt Thlckners
(A)
600
Figure 2. Estimated XRD distribution of low index planes for Pt films sputtered on etched p-Si (111) crystals. Mean particle diameter, (4,
versus thickness is shown. orientation) to the Pt grains. The relative intensities also depend on film thickness. An approximate analysis of the distribution of low index planes parallel to the Si surface was made. We normalized the observed intensities by dividing by the intrinsic response: unity for ( l l l ) , 0.53 for (loo), and 0.31 for (110). The fractional amount of each plane was then calculated as the ratio of normalized intensity to the sum of normalized intensities for all three low-index planes. Figure 2 shows the results for six thicknesses. Below, we will use these data to analyze Pt area and predict the average methanol oxidation current versus Pt film thickness. The importance of the growth in the population of (1 10) planes with Pt thickness will then become clear. The inset to Figure 2 shows the dependence of mean particle diameter on thickness. The data are represented by the relation (d) = 16t03,d and t in angstroms. The average X-ray particle size is the order of the film thickness, especially below 100 A. This means the surface orientation of planes would, qualitatively at least, follow that found by the volumetric analysis of grain orientation. Electron diffraction measurements8 of 1000-2000 8, thick evaporated metal films showed that the particle size was 100-200 A, in agreement with our empirical (d)from XRD analysis of thinner films. Metal particle sizes tend to fall in the range 50-200 A for films of 50-2000 A thickness. Pt Area. All methanol oxidation current densities in this report are based on the real area of Pt obtained by standard electrochemical methods at 30 "C. We determined the charge (hI corresponding to oxidative desorption of adsorbed hydrogen. In standard fashion, the real area was assumed to be given by where q" = 210 p C ~ m , - ' . ~Assuming one Had per Pt site, 210 pC cm,-2 is a good approximation for a random distribution of all three low index planes. The error in current density caused by small variations in qo will be addressed below. Figure 3 shows the results for the ratio of the real area to the geometrical area calculated on the basis of 210 pC cm,-2 for R on p-Si (100) Id. These results were not effected by illumination with a 500 watt tungsten halogen lamp, ruling out kinetic limitations in the formation of Had under space charge depletion conditions. Geometrical areas were 0.5 cm2. On p-Si, the ratio of real area to geometrical area reached a maximum of 1.7 at 40 A, and saturated at about 0.84 for thicker films. In the limit of zero thickness, the data suggest the area ratio is about unity. Although the films were mirror smooth, the 0.84 ratio requires that the distribution of low index crystal planes was not random for thicker films.
e&",
Frese and Chen
6076 J. Phys. Chem., Vol. 99, No. 16, 1995 A
0
100
200
300
400
500
600
P! thickness (A)
Figure 3. Ratio of real area to geometrical area for sputtered Pt films
on graphite and p-Si crystals. Area calculated from Hd oxidation charge. Potentiodynamic sweep rate, 0.05 V s-l, 0.05 M HzS04, 30 "C, under Nz. Geometrical area 0.50 cm2. If we assume at 40 A, the Pt film has no roughness, then we would require a qo of 1.7 x 210 p C cmr-2 = 360 pC cmr-2 to account for the area ratio. We are not aware of any Pt arrangement that would have such a high Had concentration per real unit area. We can qualitatively explain the area ratio peak at 40 8, on p-Si in Figure 3 as follows. We envision the film formation process to proceed in stages. It is known that Pt and Pd form silicides10 at room temperature. This means the structure of first several layers of Pt will be significantly influenced by bonding with the Si substrate. Such valence forces and energy release will tend to cause the Pt to cover the Si and form a compact layer. This explains area ratios near unity for the thinnest films. As more Pt is added, the results show the film becomes less compact leading to an increase in area (film porosity). Porosity can occur by imperfect stacking of Pt clusters causing kink sites and other faults. We have presented capacitance and electrical conductivity data3 that shows that complete coverage of Pt on &lass or Si occurs when the film thickness reaches about 150 A. Figure 3 shows that area ratio becomes constant near 125 8,. This suggests that the Pt site density becomes constant for t > 120 8,. However, this constant site density apparently represents only 84% of geometrical area. We believe this is physically impossible. Therefore, we have to conclude that 210 pC cmr-2 is too large for films thicker than 120 8,. A value of 176 pC cmr-2 would give a real area equal to the geometrical area, reasonable for a mirror film on a mirror smooth substrate. The ideal Pt atom densities arell 1.30 x 1015 cm-2 (loo), 0.930 x 1015 cm-* (110), and 1.51 x 1015 cmW2(111). Assuming one H atom per Pt atom, the corresponding q" values are 208, 149, and 242 pC cmr-2, respectively. The 176 pC cm,-2 value, applicable for t > 120 A, suggests that (1 10) planes, having the lowest Pt site density, make the dominant contribution to the surface orientation. The XRD data in Figure 2 confirm that the fraction of (1 10) planes increases and then becomes dominant as films become thicker. We can estimate q" using our XRD data in Figure 2 and the known Pt site densities on the three low index planes. For 500 A, we calculated q" = 189 pC cmr-2. We can make another estimate of surface crystal plane population by integration of the individual (100) and (1 10) Had desorption eaks. Typical current voltage curves for a Pt foil and a 110 film on p-Si(100) Id are shown in Figure 4. The
1
V
I/
1 l O A Pi/p-Si(lOO)ld
I
I
I
I
I
I
0.0
0.2
0.4
0.6
0.0
1.0
V (SHE)
Figure 4. Potentiodynamiccurrentholtage curves for surface oxidation of Pt foil and Pt film on p-Si(100) Id. Sweep rate, 0.05 V s-l, 0.05 M HzS04, 30 "C, under N2.
peak positions at 0.12 and 0.25 V (SHE) indicate12 (110) and (100) planes, respectively. Assuming no (111) planes are present, integration of the 110 8, film ilV curves gives 68% (110) and 32% (loo), which translates to q" = 168 pC cmr-2. This value implies a residual roughness factor of 1.065 for t > 120 A. The average of the two estimates, 189 pC cmr-2 from XRD and 168 pC cmrP2from direct electrochemical data, is 179 f 10p C cm,-2. This value gives the required geometrical area of 0.50 cm2 for thicker films. For comparison, the Pt foil data in Figure 4 gave 63% (110) and 37% (100). The area ratio data in Figure 3 not give defietive information about the texture of films on p-Si below 120 A. However, the XRD and area data clearly show that there is a tendency for the (1 10) orientation as the films thicken. This finding is very significant because studies of methanol oxidation on single crystals show that the (110) plane gives the highest current density. We assume the single-crystal plane order of reactivity is reflected in our steady-state results on polycrystalline films. Since we found the current drops sharply for t > 110 A, the peak current appears not to be controlled by structural effects. Similar area ratios (0.5 cmz) were determined for Pt films on graphite foil. In this case, the apparent roughness factors for Pt on 99.9% graphite foil were 1.8 to 2.8. The higher area ratios are consistent with the higher roughness of the graphite foil substrate. In summary, the dependence of the electrochemically determined Pt real area and the XRD results on Pt thickness allow us to conclude that for t > 120 A, the fraction of (110) planes increases. The XRD data alone indicate that (1 10) orientation is always increasing in the thickness range 50-120 A in the as-deposited films. The (111) oriented graints are not evident in the cyclic voltammetry as evidenced by the lack of the unique anomalous current spikes13between 0.4 and 0.5 V (SHE) caused
J. Phys. Chem., Vol. 99, No. 16, 1995 6077
Methanol Oxidation at p-Si/Pt Electrodes 2
1.2
1.8
1 1.6
aoA
600
1.4
0.8
v-
N- 1.2
8
a E
L
'-
g0.6
a
-
1
E
0.8
0.4
0.6
0.2
0.4
0.2 0 0 0
io
1
200
300
460
560
100
200
time (9)
Figure 5. Real area steady-state methanol oxidation current density versus time for Pt films on p-Si(100) Id 0.35 V vs Ag/AgCl. 0.1 M CH3OH, 0.05 M HzS04, 80 "C. by sulfate adsorption. The (111) orientation, shown to be present by XRD,must be easily transformed by limited voltage cycling (*IO cycles) which was always performed before area measurement and methanol oxidation. Indeed the (1 11) poles of a Pt single crystal sphere disappear under our typical cycling conditions, giving rise to (100) oriented grains.14 Effect of t'F Thickness, Potential, Temperature, and Methanol Concentration with Wp-Si. Example plots of methanol oxidation currents versus time for various thicknesses of Pt on p-Si(100) are shown in Figure 5 . The potential was held constant at 0.35 V vs Ag/AgCl at 80 "C. Note that the films had been cycled *IO times between -0.45 and 0.9 V Ag/AgCI. The magnitude of the current per real area of Pt depends on the thickness for all times measured. The peaks in the jltime data at very early times are caused by a lag in adjustment of the voltage drop in the p-Si/Pt electrodes after a jump from open circuit to 0.35 V. The drop in current with time is caused by the gradual buildup of an incompletely oxidized poison, likely to be toad. The 10 min methanol oxidation currents on p-Si substrates at 0.35 V are plotted versus Pt thickness in Figure 6. The main features are the prominent maxima for 90-110 A films and the nearly constant current densities for thicker films. The maximum current density on p-Si substrates followed the order (100) Id =- (111) Id > (111) hd. The effect of electrode potential on the current density/ thickness curve for p-Si (100) Id substrate is shown for potentials ranging from 0.05 to 0.45 V in Figure 7. The peak current increased for potentials of 0.05-0.35 V and dropped at 0.45 V. The thickness corresponding to the maximum current was always about 110 A. A drop in catalytic current at more anodic potentials is caused by loss of free metal sites due to the formation of an oxidized overlayer, likely Pt-OH. The current-thickness peaks shifted a small amount with potential. The activation energy for methanol oxidation current was determined in the temperature range 25-80 "C using 0.1 M CH30H. We raised or lowered the temperature 1-2 W m i n and recorded the current at 0.3 V. Table 2 shows the activation energy declined for thinner films ranging from 19.2 to 43.3 kJ mol-'. The decline of the activation energy was more rapid for films below 150 A. Very similar activation energies were found with increasing or decreasing temperature. The measurements discussed so far refer to a methanol concentration of 0.1 M. We selected the Pt thickness of 110 8, for a study of the effect of methanol concentration. The results
300
400
500
600
Pl lhlcknass (A)
600
Figure 6. Real area steady-state methanol oxidation current density versus thickness for Pt films on p-Si(100) Id, psi(111) Id, p-Si( 111) hd, 0.35 V vs Ag/AgCl. 0.1 M CH3OH, 0.05 M H2so4,80 "C, current after 10 min 1
0 0.45V 0 0.35 V
b 0.25V 7 0.15
n
v
0.05~
0.25
0
lb
0
200
360
460
500
600
PI thickness (A)
Figure 7. Real area steady-state methanol oxidation current density for Pt films on p-Si(100) Id versus thickness and electrode potential, 0.05-0.45 V vs AgJAgCl. 0.1 M CHsOH, 0.05 M HzS04, 80 "C, current after 10 min. TABLE 2: Activation Energy for Methanol Oxidation Current at 0.35 Vu t (A) E, (M mol-') t (A) Ea(Mmol-') 25 19.2 250 38.0 50 23.1 400 42.4 75 21.1 600 43.3 150 36.1 Versus an isothermal AgJAgC1 reference electrode in the anode compartment. ~
~~
(1
are summarized as follows: The peak current increased from zero to 1.2 mA cm,-2 at 0.1 M, then peaked at 1.75 mA cm,-* at 0.5 M. A slow decline to 1.3 mA cmr-2 at 2 M followed. Graphite Foil and Other Substrates. We measured real area methanol oxidation currents using two forms of carbon as substrate. We wanted a comparison between the p-Si substrate, graphite foil, Vulcan XC-72R carbon, and literature results for Pt supported on carbons. The hole energy levels of p-Si are very different from carbon and this property could cause higher reactivity. Figure 8 gives the 10 min Pt/C currents as a function of thickness at 0.25 and 0.35 V (Ag/AgCl). Two features of the graphite data are the absence of a strong peak near 100 8, thick on p-Si and the much lower current compared to p-Si substrates (see Figures 6 and 7). At 100 A, the current on Pt/
6078 J. Phys. Chem., Vol. 99, No. 16, 1995 1.2
fl
1-
0.8-
v-
6 2
-.-
. 0.6-
I
p-si(100) 0.35Y
graphite
100
200
I
300
I
1 0.25V
r . L
0
Frese and Chen
400
500
600
700
Pt thickness (A)
Figure 8. Real area steady-state current density for methanol oxidation with Pt films on graphite sheet versus film thickness at 0.20 and 0.35 V vs Ag/AgCl. 0.1 M CH30H, 0.05 M H2S04, 80 "C. Results on Wp-Si(100) Id are shown for comparison.
graphite substrate was 6 times lower than on the best Pt/p-Si(100) Id substrate. For films thicker than 120 A, the currents were the same for both p-Si and graphite substrates. For further comparison with supported Pt, we measured methanol oxidation currents using a commercial Teflon-bonded gas diffusion fuel cell electrode. A Wporous carbon paper fuel cell electrode with 5 mg of F't cm-2 was obtained from JohnsonMatthey. The area of Pt was found by the usual electrochemical method of H adsorption and stripping. The Pt real area was 490 cm2 on 0.45 cm2 of exposed geometrical area. The area was determined immediately after the methanol oxidation measurements. The main result was the real area steady-state methanol oxidation current density was 0.125 mA cmIP2at 0.35 V Ag/AgCl and 80 "C. The electrolyte contained 0.1 M CH3OH and 0.05 M H2SO4. The Pt supported-on-carbonelectrode had 8 times lower oxidation current than 110 A Wp-Si(100) Id. Literature results also support the lower currents with Pt supported on high surface area carbon. Wantanabe et al.15 reported 0.39 mA cm,-2 at 60 "C using a methanol vapor feed. We can assume the concentration is high, in the saturation current region. According to our data the current increases 1.45 times from 0.1 M to saturation current conditions. Therefore the comparable current is 0.39A.45 = 0.27 mA cmI-2. Attwood et a1.16 used Pt supported on carbon-fiber paper and 60 "C and 1.0 M CH30H. Their real area galvanostatic current corrected to our slightly different conditions using17 Ea = 42 kJ mol-', was either 0.056 or 0.27 mA cm,-2, depending on whether the Pt was prepared in H2 or air. The latter result is in good agreement with the corrected results of Wantanabe et al. Finally, Wang and Fedkiw18 reported a current of 0.15 mA cmI-2 using 1 M CH30H and 50 "C and 0.6 V EtHE. Correction to our conditions gives 0.38 mA cmI-2. In summary our results on two different types of Pt/C electrodes and three studies in the literature on similar electrodes show that the real area methanol oxidation current under our conditions is in the range 0.120.38 mA cmI-2, whereas on Wp-Si we found peak currents up to 1.7 mA cmI-2. p-Si/Pt Schottky Barrier Height. The Schottky barrier height ( v b i p ) is required in the hole-transfer model to explain the current-thickness peak. As shown below, the theory of activation energy of the current also requires the barrier height. Attempts to measure VScor v b i , by capacitance in 0.1 M H2SO4 and 0.5 M K2HP04, did not give satisfactory results. A strong
+
frequency dispersion at -0.5 to 0.5 V vs Ag/AgCl was observed in the range loz-lo6 Hz, precluding any extraction of barrier height in the available frequency range. The problem persisted for thickness of 50-600 A Pt on p-Si(100) and -(111) with 1015 cm-3 acceptors. We found no such problem with n-Si/Pt electrodes that were treated similarly. A requirement for reliable barrier height determination is that the true space charge capacitance is measured; if so, by definition the measured capacitance would be independent of frequency. We concluded it was impossible to measure the barrier height by capacitance. Although barrier heights have been tabulated'O for 10 metals on p-Si, a value for p-Si/Pt is absent, suggesting some difficulty. A value may be obtained by subtracting the known results for n-Si from the bandgap. Using our average result for n-Si/Pt of 0.81 f 0.01 eV,394we find a Schottky barrier of 1.12-0.81 = 0.31 eV. Another approach relies on the correlation between barrier heights and metal work functions. Plotting the 10 p-Si barrier height values in ref 10, versus metal work function^'^ gave 0.35 V for Pt work function, 5.3 eV. Therefore a Schottky barrier height of 0.30-0.35 eV is supported by indirect experimental data. A third calculation of the p-Si/Pt barrier height was made using the n-type barrier height-work function model presented by Sze.l0 The final result of this theory is shown in eq 1. For vbi
+ p = C24m + c3
(1)
+
Si substrates, the constant cz = 1/(1 NJ1.1 x 1013)where N,, is the surface state density in cm-2 eV-'. Similarly c3 = (1 - cz)[E,/q - $01 where Eg and $0 are the bandgap and the fermi level position above the valence band before metal contact. Analysis of barrier heights with many metals gives $0 = 0.3 eV l/3Eg.10 With eq 1 we arrived at a barrier height for p-Si/ Pt of 0.28 eV in approximate agreement with two estimates above. This calculated result has an uncertainty of f 0 . 3 eV, mainly due to imprecise knowledgelo of $0. Our inability to measure C,, and the strong frequency dispersion supports the supposition that a high density of surface states is operative at the fermi level of the p-Si/Pt electrode. A preliminary analysis of admittance-frequencydata confirms the surface state density is 2 x 1014 cm-2 eV-' positive of 0.0 V vs Ag/AgCl. Since the space charge capacitance could not be measured, we cannot assign the position of the fermi level above the valence band. However the very large state density can help explain the frequency dispersion and, in view of the barrier height estimations above, suggests fermi level pinning at 0.300.35 v above Vvb.
Discussion Kinetic Analysis of CH30H Oxidation Currents. To relate the large effect of Pt film thickness on the real area current for methanol oxidation to mechanistic steps and degree of poisoning, we derived a kinetic model based on the mechanism given below. The goal of the analysis was to obtain insights into the kinetic factors that are changing with Pt thickness. The theory provides a link between the measured current and rate constants and the energy levels of hot carriers (holes): CH3OH
-ki
I
k2
toad diagram of key mechanisitic steps in methanol oxidation
The key steps in the proposed mechanismZoare illustrated in the diagram. The composite rate constant kl contains all rate
J. Phys. Chem., Vol. 99, No. 16, 1995 6079
Methanol Oxidation at p-Si/Pt Electrodes
TABLE 3: Kinetic Parameters Derived from Model for Pt/p-Si(lOO~and WGlb at 0.35 V kiR
kdkp
t (A) ( m cm-2) ~ (x10-2)
10 20 40 60 80 100 110 120 150 200 300 600
0.13 0.24 0.53 0.86 0.81 2.1 2.3 0.85 0.41 0.40 0.47 0.44
Columns 1-4.
0.44 0.90 1.9 3.7 5.4 8.0 9.1 7.5 3.2 3.2 3.4 3.7
k3
kiR
k&
k3
t (A) ( m cm-2) ~ (x10-2) (s-1)
(s-1)
10 5.4 4.0 20 8.9 40 6.4 60 15 80 16 100 16 110 11 120 11 150 9.6 200 5.4 300 5.1 600
0.060 0.10 0.34 0.34 0.38 0.38 0.38 0.39 0.47 0.43 0.48 0.49
0.14 0.31 0.92 2.0 2.0 1.7 1.6 1.6 1.7 1.8 1.9 2.3
23 10 12 7.6 7.4 8.7 9.8 11 17 19 15 12
Columns 5-8.
constants leading up to the formation of the intermediate, I. A branch reaction is postulated in which I transforms to a poison, in this case CO,d, with rate constant kp. In the other branch I is oxidized to CO2 with rate constant kz. Although I is assumed to be the precursor of the poison, k2 does not necessarily refer to the rate-determining step. The poison is slowly removed by electrochemical oxidation with rate constant k3. The poison is considered to be a blocking agent toward chemisorption of reactant methanol. At 200-250 "C, above the normal operating temperatures of fuel cells, CO can desorb thermally. Equations 2-4 were used to derive expressions for the coverage 8 ~ and 0
+
j = k21 k30c,
(3) (4)
the total current as a funtion of time, z, including the steadystate values. For simplicity, the elementary rates are written without explicit voltage dependences or electron numbers. The model current density j and coverage of C o d with the time dependence are given in eqs 5 and 6: j=-+
--
(Bkfk3
kzB2 )[I - exp[-(B kp(B k3)
+
+ k3)z]] ( 5 )
The quantity B is defined in eq 7.
(7) We determined B, k3, and k2/kp by curve fitting the jltime data using the Levinberg-Marquardt algorithm implemented in the software package Delta Graph Professional.z1 The fitting function was of the f o r m j = a b[l - exp(-cz)], where constants a, b, and c follow from eq 5 . From these results we also determined klR and the poison coverage 8~0. The model gave excellent fits to the time evolution of the methanol oxidation current density for times beyond about 1 min. Table 3 summarizes the kinetic parameters derived from the experimental data for Wp-Si( 100) Id and Wgraphite using the above model. The k3 defined in eq 2 in current density units, was converted to s-l by dividing by (1.602 x C S ) , where the latter is the total number of Pt sites/cmz, the order of 1 x 1015 cm-z. Each set of best fit values of klR and kzlkp as a
+
TABLE 4: Initial Real Area Current DensitiM for CHJOH on Single Crystal Pt Surface* t(T) [MI (M) V'(SHE) J(mAcm-2) plane 25 25 25 25 25 25 25
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.50 0.60 0.70 0.60 0.70 0.60 0.70
0.1 0.23 0.47. 0.50 1.6 0.59 3.5
(111) (1 11) (111) (100) (100) (110) (1 10)
These are initial scans so magnitudes are not to be compared with our 10 min results. The order of reactivity with crystal planes is assumed to be preserved in our 10 min steady state. (I
function of film thickness for the three types of p-Si had a maximum near 100 8, where the respective current densities were optimum. In each case the klR and kdkP for graphite were 5-6 times smaller and did not exhibit the pronounced peak. The small dependence o f j with W C on thickness for very thin Pt might be due to Pt structure changes. At present, we assume the rate constant kp refers to a chemical surface process and is independent of Pt thickness or potential. If so, from the behavior of k2/kp,we may conclude that the rate constant kz also passes through a maximum at about 110 A. The values of kdkP W p-Si(100) ranged from 44 to 900 for thickness 10-110 8,. Similarly for graphite, the range was 14-200. For Wp-Si( 11l), the peak k2/kp, was 1040 at 100 8,. If kp varies, it probably depends mainly on Pt structure and to a lesser extent on the substrate. Since the composite kl and k2 have similar thickness dependencies, the rate-determining step hole transfer could be the oxidation of intermediate I. This assumption is supported by the observation that model rate constants deduced from the data show that the partial currents, j 1 = j z = j >> j 3 . Typically j 3 values were 0.5-2 pA cm,-z, showing the difficulty of oxidizing the poison. Although we have not specified the number of electron transfers before the intermediate I, it should resemble the poison. According to Conway,22 Pt-COH or Pt-HC-0 is the intermediate reactant in the rate-limiting step. If we assume the poison is C0,23 then it could be formed chemically by dehydration of Pt-COOH or H abstraction from Pt-COH. Formate has been observed spectroscopicallyzoain methanol electrooxidation. Of the two possible intermediates, either Pt-OCH3 or Pt-CHZOH, the latter seems more likely because in the methanol molecule, the H-CH20H bond in 42 kJ mol-' weaker than the 0-H bond. The methylene hydrogens in Pt-CH20H are also likely to be more reactive than hydrogen bonded to oxygen. Removal of the former would lead to Pt-COH or Pt-CH=O. Comparison with Literature Single-Crystal Results. The XRD data indicate that there are structural changes as Pt thickness increases. The crystal plane data in Figure 2 was used to calculate a weighted current density using known current densities on the (loo), (1 lo), and (1 11) single crystal surfaces.6 The single-crystal initial currents from refs 6 and 7 are given in Table 4. At 0.6 and 0.7 V vs SHE, no peak in the weighted current was evident. We concluded that the peak at 110 8, is not caused by an optimal structure. The XRD evidence for a continual increase in (110) with Pt thickness does not support the sharp decline in current for t > 120 8, because (1 10) has the highest current density. The evidence for a particle size effect is unclear. Reference 6 cites two contradictory papers on this question. 20 8, particles are supposed to be advantageous; our optimum currents correspond to about 70 8, particle size. Area Effects. An estimation of the maximum error in methanol oxidation current density due to uncertainties about
Frese and Chen
6080 J. Phys. Chem., Vol. 99, No. 16, 1995 real area is needed to show that the peak currents near 110 8, are not caused by gross under estimation of the real area. All current densities reported below are based on qo = 210 pC cm-2. This is an upper limit to the true value that may in fact change slightly with thickness. It is found that qo 2 179 f 10 p C cm,-2 for any thickness because there is always evidence for a 35-40% (100) planes in the Had desorption characteristic for voltage cycled electrodes. Therefore the maximum error for current densities at t > 120 A would be an overestimate of the current density by 25%. For t < 120 A where the peak currents occur, the correction would be smaller because the fraction of (1 10) planes decreases, giving way to planes with q" values much closer to 210 pC cm-2. Surface State Model for Reaction Intermediates and Hot Hole Oxidation, Interpretation of k's. We showed above the peak current density cannot be adequately explained by area, particle size, or crystal plane effects. We propose a surface state model to explain the methanol oxidation rates. The model can explain the peak in the methanol oxidation current with Pt thickness and the lack of a peak on carbon substrates. The key ingredients are the energetic relationship between the intermediate surface state levels and hot holes and transport properties of hot holes as already indicated in Figure 1. The most important feature is the assumption of the applicability of the Marcus-Gerischer modelz4 for capture of carriers (holes) on surface states. The states of interest are intermediates or poisons in the methanol oxidation pathway. They are likely to be of the type Pt-CO,H, for example: Pt-CH20H, Pt-COH (carbinol), Pt-CHO (formyl), Pt-COOH (formate), and the poison, Pt-CO (linear or bridged). These species are bonded with molecular orbitals formed mainly from Pt and C atomic orbitals. This means that they have roughly the same bonding orbital ionization potential and as donor surface states should be located within about 1 eV or less of the fermi energy of Pt. When hot holes are captured by the donor surface states, they represent broken bonds leading to further oxidation. According to our previous calculation^,^^ we expect adsorbed formate to be the most reducing level and COad to be the least. This means that more energetic holes would be required to oxidize Pt-CO than Pt-COOH. We outline the main aspects of the model below. Following Morrison,24the rate (A cm-*) of capture of holes on an intermediate surface state is given in eq 8. N,, is the
reaction intermediate surface state area density, NA is the volume doping level of the p-Si, v is thermal velocity of holes, u is the capture cross section, A is the reorganization energy,26and V,, is the surface barrier (band bending). Inspection of eq 8 shows a peak in the reaction rate is expected when vh, the hole energy level, reaches V,,, the potential of the intermediate surface state. In Figure 1, the surface states, V,,, are VI and VP. Since they are physically located on top of the Pt film, we do not expect the energy levels V,, to depend on Pt thickness. We argued above that ( v b i p) is 0.30-0.35 eV. To observe the peak current, we must have v h changing with thickness. However, because rapid cooling processes always occur, we need thin films to observe the kinetic effect of hot holes. We related2 the hole energy level to the mean free path, the doping energy level, p, surface barrier and hole ballistic mean free path, LB and fermi level, VO. Equation 9 gives the result27
+
+ + AVi) exp(-t/&) + Vo
Vh = (Vbi p
(9)
for vh, showing the expected variation of the hot hole energy level with film thickness, t. Importantly, the thickness scale may be regarded to be an energy scale. In eq 9, AVi = V,(v) - V," (17 = 0), is the change in metal-semiconductor potential drop relative to the equilibrium value (see Figure l). Assuming fermi level pinning, it may be shown that the change in metal-semiconductor interfacial drop with electrode potential is, dVi/dV = a/[l a], with a = EHXi/E,XH, where E and x are the dielectric constant and thickness of the Helmholtz layer and the interfacial layer in Figure 1. Reasonable values are XH = 3 A, x, = 4 A, EH = 6, and E, = 10, the last value derives from the dielectric constant of silicide-type interfacial layer. With these values, dVi/dV = 0.44. The overpotential for our measurements was 0.70 V, giving AVi = 0.31 V. With oxidation of I being rate limiting, we may express eq 3 as follows, with v h given by eq 9 and VO = 0.60 V vs SHE:
+
S is 1 x 1015 cm-2, the total site density on Pt. According to eq 10, the peak current at 110 A occurs when v h = VI. The observation of the peak in methanol oxidation rate with thickness requires that VI must be in the range VOto [VO (Vsc p) AV,]. Using LB = 85 A, VO = 0.57 V (SHE), AV, = 0.31, we calculated the energy level of I to be 0.75 V vs SHE or 0.18 eV below the fermi energy. This value refers to pH = 1.2 and pcoz = 1 x atm. The steady-state coverage of I may be found from the current peak at 110 A. At the maximum, j = ~ ( ~ ~ ~ ) B I s ( ~ T / z A ) O ~ N A cm-2, v = lo7 cm exp(-qV,,/kT). Inserting 0 = 1.4 x s-l, V,, = 0.05 eV, A = 0.18 eV (see activation energy analysis below), we calculate 81 = 1 x The low coverage can explain the apparent absence28of Pt-COH in steady-state FTIR surface spectra. The model can also account for the thick film and Pt/graphite results where no hot carriers are possible. When Pt thickness reaches several mean free path lengths, nearly all the injected holes will cool to the fermi energy of Pt. If so, the Wp-Si electrodes will have the electronic characteristics of Pt on any substrate of similar structure. Under this condition, the current will be lower because of the energy mismatch between V,, and the fermi energy. When the substrate is carbon, there are no hot holes possible because there is no Schottky barrier. The term (V,, - v h ) in eq 7 a constant since v h is independent of the thickness and equal to the Pt fermi energy. This condition is also obtained for thick films on p-Si. The behavior of activation energy with Pt thickness will also be quantitatively explained with the above model. Poison Coverage. The increase of poison coverage with time derived with the model equations is shown in Figure 9. Model steady-state values for graphite and p-Si substrates are listed in Table 5. After 10 min Op becomes constant, close to the saturation coverage of CO on Pt, namely, 8, = 0.62. We found steady-state 8, was 0.8 for a Pt foil electrode, consistent with the lower current densities observed on this form of Pt. The inset of Figure 9 shows the minimum 8 p = 0.4 for p-SiPt electrodes occurs near 100 A Pt thickness. Chang et al.29 concluded CO,d, formed in methanol oxidation, occurs in the terminal configuration, Pt-CO. They reported CO poison coverages at 23 "C as follows: Pt(lll),0.2; Pt(lOO), 0.15; Pt(1 lo), 0.35. Our values are up to 2 times higher. Two factors
+
+ +
Methanol Oxidation at p-Si/Pt Electrodes
J. Phys. Chem., Vol. 99,No. 16, 1995 6081
0.6
1
MOA
20
A 25A 400A
7-
f
q
0.1
E
0.45
5 0.35 E-*
5 095 0.15
-
0
0
100
200
300
400
500
0.01 2 5
700
600
time ( 8 )
200
I
2.9
"
A 404 '
600
,
'
3.05 (10001T'K)
,
I
3.2
3.35
Figure 9. Derived values of poison coverage versus time for methanol oxidation at Wp-Si(100) Id electrodes. The steady-state poison coverage versus thickness shown in inset.
Figure 10. Arrhenius plots of methanol oxidation currents at p-Sj/Pt Id electrodes at 0.3 V vs Ag/AgCl. 0.1 M CHjOH, 0.05 M HzS04, 80
TABLE 5: Derived Poison Coverage for Graphite Foil and
Activation Energy. The dependence of activation energy on thickness also points to hot holes. We observed a progressively lower activation energy for methanol oxidation at constant potential with thinner films. The activation ener y declined about three times in the thickness range 600-20 . A lower activation energy is consistent with a more oxidizing energy level (more positive potential) expected for a hot hole that has a high probability of traversing a thin film. The measured activation energy at constant electrode potential, AHm'* = -R dlnu/d( 1/77], consists of several contributions as shown in eq 11. This result was obtained by differentiation of eq 8:
p-Si(100) Substrate@ R thickness (A) ep(graphite) 10 20 40 60 80 100 120 150 200 300 600
0.50 0.66 0.65 0.58 0.61 0.61 0.58 0.49 0.43 0.51 0.53
R thickness (A)
e, p-Si(100)
10 20 40 60 80 110 125 150 300 600
0.81 0.66 0.69 0.38 0.49 0.40 0.41 0.61 0.59
0.77
"C.
1
80 "C, 0.05 M HzS04, 0.1 M CH30H, 0.35 V vs Ag/AgCl.
that contribute to the differences are our higher temperature, 80 "C, and polycrystalline surfaces, known to give higher CO coverage.29 The current for poison removal was found to be 1% of the total current which means that the Pt surface always has a relatively large population of poison. Since the rate of electrosorption of methanol is expected to be partially controlled by the (1 - 0p) term, the minimum in the steady-state poison coverage contributes to the peak in current density. However, the ratio of (1 - 0,) at 110 8, and for thick films is only 1.5 times larger. The peak current was 6.1 times higher than the thick film value. Therefore the dominant kinetic factor must be an acceleration of the slow step in the main oxidation pathway rather than removal of the poison. Qualitatively, the poison energy level is expected to be well below the fermi energy because of the large overpotential, 0.7-0.8 V, required for oxidizing CO on Pt. If correct, the holes available in the p-Si/Pt system would not be sufficient energetic to react efficiently with Cod. The minimum poison coverage in Figure 9 may be due to a faster rate of formation of another oxidant derived from water such as OHd or it may be a structural effect. The derived values of Qp for graphitem support also varied with thickness. 0p declined from 0.66 at 20 8,Pt to a minimum of 0.42 at 200 A, finally saturating at 0.52. Although the poison coverage on graphite substrates was similar to p-Si/Pt, the ratio of the peak current on p-Si to the current on graphite at the same thickness was 1.18/0.150 = 7.8. Again variations of the poison coverage does not account for the large difference in rates between the two substrates. The small differences in steady-state coverage is probably caused by minor structural differences in the films.
The energy terms are the surface barrier at the overpotential of the measurement, the barrier to charge transfer to the surfacestate intermediate, and terms arising from the temperature dependencies of Vsc, v h , and VI.The hole energy level is given by eq 9, causing the activation energy to depend on thickness as observed. Equation 11 was employed to fit the measured activation enthalpy-thickness data in Table 2. The curve-fitting routine in Delta Graph Professionalz1 was used to obtain the best fit value of A. The Schottky barrier height was fixed at 0.33 eV, the ballistic mean free path at 85 A, and VI= 0.20 V below the fermi energy (see j / z analysis above). The theoretical mean free path for warm electrons in Pt was 110 A, but e~periencel-~ has shown measured mean free path values for Au and Ag are lower. Therefore a 30% lower value is very reasonable. With the above parameters, the best-fit reorganization energy was 0.18 f 0.05 eV. A low reorganization energy for a neutral organic surface state is not surprising if polar water reorientation is not involved in reaching the transition state. Tine small must also mean little internal reorganization of the adsorbate is required for oxidation. A mode130 to justify IO times smaller reorganization energies than expected, featuring weak coupling between the transferring electron and the heat bath (electrolyte) has been derived. eV K-' and The fitted constants a and b were 2.5 x -5.8 x lop4eV K-l, respectively. The correlation coefficient and x2 values of the curve fit were 0.99 and 6.0 x respectively. The input data from Table 2 and the fitted curve are shown in Figure 10 along with representative Arrhenius plots. The trend in AHm'* with Pt thickness is reproduced very
Frese and Chen
6082 J. Phys. Chem., Vol. 99, No. 16, 1995
well as shown by the comparison of the measured activation enthalpies and the theoretical fitted curve in Figure 10. Kinetics of Poison Oxidation. Surface vibrational spectroscopic studiesz9 indicate the poison in methanol oxidation is very likely toad. The derived value of the rate of C o d poison removal was 2.0 x low6A cmW2at 80 "C, 0.35 V AgtAgC1, pH = 1. The equivalent (SHE) potential was 0.64 V. This rate applies to thick films (no hot holes) and OCO = 0.7 coverage. The surface-state model of the poison, pictured in Figure 1, is the basis for the following analysis. The oxidation of toad, ultimately giving coz,, is expected to proceed by reactions i and ii. With the aid the Sanderson
Another possible mechanism that needs consideration is rate control by partial oxidation of water forming the oxidant, OH,,, followed by 0 transfer to toad and H ionization. We will consider the energetics of this mechanism in a future report. However as pointed out by Conway,22the coverage with oxidant derived from water in a potential range where methanol oxidation occurs is exceedingly small, as low as lo-*, near room temperature.
Summary
Transient methanol oxidation currents and a kinetic model were used to derive rate constants and poison coverage versus Pt thickness. The rates at constant overpotential and time peaked with Pt thickness at about 110 A. The maximum real area current was explained by cooling of warm holes as the Pt (ii) film thickened. Under identical conditions, no current-thickness peak was observed with graphite substrates. Real area Polar Covalence mode131afor bond e n e r g i e we ~ ~calculated ~ ~ ~ ~ ~ ~ current ~ densities with graphite and other carbon supports were electrode potentials for 80 "C, 1 atm COz, pH = 0, unit activity at least 5 times lower. The activation energy-Pt thickness curve water and 0 = 0.5 with no interactions between adsorbed with p-Si substrates was successfully modeled with the key particles. They are +1.65 V (SHE) for reaction i and -1.19 V assumption that the hole level, v h , varies with thickness (SHE) for reaction ii. This latter value is known to be very according to eq 8. The reorganization energy for hole transfer negative from research on the electrochemical reduction of COz. in the rate-limiting step was 0.18 eV. The coverage of the The Nemst equations for the elementary steps, I and 11, are eqs intermediate in the rate limiting step was 1 x The poison 12a and 12b, respectively. Reaction i is expected to be slow coverage ranged from 0.8 to 0.4 and varied with Pt thickness, being a minimum near 100 A Pt. Other semiconductor systems that have larger Schottky baniers and hotter holes may allow more effective poison removal at appreciable overpotentials. According to the above model, a single energy level may not be sufficient to optimize both the oxidation of methanol and its vji= - 2303RT log intermediates and the poison. We plan to investigate other (12b) nF semiconductors to further enhance the methanol oxidation current with hot holes. because of its very positive AGO = 1.65 eV. When AG 1. 2, the activation energy of the step is simply the formal free energy Acknowledgment. The authors thank the National Science change.32 There are reasons for believing the condition AG 2 Foundation for support of this research under Grant CTSA is met in the surface oxidation of toad. We reasoned by 90 14394. breaking reaction i into several stages. We calculated33 the reorganization energy of the reaction, CO, h+ CO+,, to References and Notes be 0.03 eV, the order of kT. Here a nonbonding electron is (1) Chen, C.; Frese, Jr., K. W. J . Electrochem. Soc. 1992, 139, 3243. removed so that the force constants and bond lengths in the (2) Chen, C.; Frese, Jr., K. W. J . Electrochem. Soc. 1993, 140, 1355. reactant and product are only slightly perturbed. Similarly, we (3) Frese, Jr., K. W.; Chen, C. J. Electrochem. SOC.1994, 141, 2402. c a l c ~ l a t e Ad ~=~0.21 ~ ~ eV ~ for hole injection into a Pt-carbon (4) Frese, Jr., K. W.; Chen, C. J . Electrochem. Soc. 1994, 141, 3375. (5) Frese, Jr., K. W.; Chen, C. J. Electrochem. SOC.1992, 139, 3234. bond according to Pt-CO h+ [Pt- -CO]+. It may be shown (6) Leger, J. M.; Lamy, C. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, that AGO for this stage is approximately equal to AGO for step 1021. i. In view of the similar energetic requirements for hole (7) Cullity, B. D. Elements of X-Ray Diffraction, 2nd ed.; AddisonWesley: Reading, MA, 1978. Ohring, M. The Materials Science of Thin injection into Pt-CO and the overall reaction i, we expect the Films; Academic Press: San Diego, CA, 1992; p 273. transition state of i to resemble [Pt- -CO]+. Therefore, the (8) Eastman, D. E. Phys. Rev. B, Solid State 1970, 2, 1. overall process should have a small A, certainly less than 1.65 (9) Conway, B. E.; Angerstein-Kozlowska,H. Electrocaralysis on NoneV. We point out the agreement between the model reorganizaMetallic Surfaces; NE3S Publication 455, U.S.Department of Commerce, 1976. tion energy, 0.21 eV, and A = 0.18 f 0.05 eV, derived from (10) Sze, S. M. Physics ofSemiconductor Devices, 2nd ed.; John Wiley our experimental activation energy data. and Sons: New York, 1981; Chapter 5. Silverman, J.; Pellegrini, B.; Comer, The rate of oxidation of the poison is then given by the J.; Golbivic, A,; Weeks, M.; Mooney, J.; Fitzgerald, J. Mater. Res. Soc. Symp. 1986, 54, 515. Roth, J. A.; Crowell, C. R. J. Vac. Sci. Technol. simplified eq 10, eq 13, with V,, = 0.05 eV, u = 1.4 x 1978, 15, 1317. cm2, v = lo7 cm s-l, 8co = 0.7. The formal value of V, that
]1:::[
vij
+
+
-
-
satisfies eq 13 withj3 = 2.0 x lop6A cm,-* is 1.23 V (SHE). Taking Vio = 1.64 V in eq 12a, we find 1 9 ~ 0=0 1.6 ~ ~x lop5 at 80 "C. This value represents the steady-state coverage of formate radical at 0.64 V (SHE). It is the same order as the coverage value, 1.0 x deduced for the intermediate, I, which has been suggested to be Pt-COH but could be PtCOOH.
(11) Neidrach, L.W. J . Electrochem. Soc. 1964, 111, 1309. (12) Clavilier, J. Electrochemical Suface Science; Soriaga, M. P., Ed.; American Chemical Society: Washington, DC, 1988. (13) Al Jaaf-Golze, K.; Kolbe, D. M.; Scherson, D. J . Electroanal. Chem. 1986, 200, 353. (14) Canullo. J. C.: Triaca. W. E.: Arvia. A. J. J . Electroanal. Chem. 1986,200, 397. (15) Wantanabe. M.: Saemsa, S.: Stonehart, P. J . Electroanal. Chem. 1989, 271, 213. (16) Attwood, P. A,; McNicol, B. D.; Short, R. T.; Van Amstel, J. A. J . Chem. Soc., Faraday Trans. 1 1980, 76, 2310. (17) We found E, = 42 kJ mol-' for thick Pt films at 0.35 V vs Ag/ AgCl. (18) Wang, S.; Fedkiw, P. S. J . Electrochem. SOC.1992, 139, 3151. I
J. Phys. Chem., Val. 99, No. 16, 1995 6083
Methanol Oxidation at p-Si/F't Electrodes (19) Michaelson, H. J. Appl. Phys. 1950, 21, 536. Trasatti, S. J . Electroanal. Chem. 1971, 33, 351. (20) (a) Beden, B.; Leger, J. M.; Lamy, C. Modem Aspects of ElectrochemistryNo. 22, p 216. Bockris, J. O'M., Conway, B. E., white, R. E., Eds.; Plenum Press: New York, 1992. (b) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973, 44, 1; 1973, 45, 205. (21) DeltaPoint Software, Monterey, CA. (22) Conway, B. E. Electrodes of Conductive Metal Oxides; Trasatti, S . , Ed.; Elsevier Publishing Co.: Amsterdam, 1981; Part B, p 482. (23) (a) Beden, B.; Lamy, C.; Bewick, A,; Kinumatsu, K. J . Electroanal. Chem. 1981, 121, 343. (b) Wilhelm, S.;Iwasita, T.; Veilstich, W. J. Electroanal. Chem. 1987, 238, 383. 124) Morrison. S. R. Chemical Phvsics of Surfaces: Plenum Press: New York, '1977; Chapter 2. (25) Frese. Jr., K. W. Electrochemical and Electrocatalytic Reactions of Cahon Dioxide; Sullivan, B. P., Krist, K., Guard, H. E., Eds.; Elsevier: Amsterdam, 1993; Chapter 6. ~
