- Email: [email protected]
The rate of anion and hydrogen adsorption on Pt(111) and Rh(111)
PERGAMON
Electrochimica Acta 44 (1998) 909±918
The rate of anion and hydrogen adsorption on Pt(111) and Rh(111) T. Langkau, H. Baltruschat * Institut fuÈr Physikalische und Theoretische Chemie, UniversitaÈt Bonn, RoÈmerstrasse 164, D-53117 Bonn, Germany Received 20 November 1997; received in revised form 9 March 1998
Abstract Impedance spectra were measured for Pt(111) and Rh(111) electrodes in various electrolytes. They can all be interpreted by assuming one or two parallel adsorption resistances in addition to the double layer capacity. ÿ Adsorption of Clÿ and HSOÿ 4 at Pt(111) is very fast and at the limit of detectability. Adsorption of H2PO4 has to be described by two parallel adsorption reactions, one of which is possibly an adsorption connected with deprotonation. Adsorption of hydrogen at Rh(111) is much slower than at Pt(111); surface roughness has opposing eects on both metals and leads to an increased adsorption rate at Rh. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Anion adsorption on low index single crystal surfaces has been widely studied by structure and coverage sensitive methods such as STM [1±4], ex situ, LEED and other UHV-techniques [5±8], and IR spectroscopy [9, 10]. Much less attention, however, has been paid to the rate of ionic adsorption. Parsons and co-workers reported on the impedance spectra of an Ag(111) electrode in various electrolytes [11]. Aramata and co-workers [12] examined the kinetics of phosphate adsorption on Pt(111) by potential step experiments. Similarly, Wandlowski and co-workers [13] determined the adsorption rate of sulfate on Au(111) and observed a phase formation behaviour. The socalled ``frequency dispersion'', reported, for instance by Hamelin and co-workers on gold single crystal surfaces [14], has recently been explained by low adsorption rates of speci®cally adsorbing ions [15, 16]. Anion adsorption on Pt(111) leads to distinctive, well-shaped adsorption peaks, suggesting Frumkinlike, adsorption behaviour. Therefore, these systems seem to be ideally suited for the determination of the
* Corresponding author.
adsorption rate by impedance spectroscopy. For that reason we tried to study the rate of iodide adsorption on Pt(111) and found that it was too high to be determined [17]. In the following, we will describe the extension of this study to sulfate, chloride, and phosphate adsorption on Pt(111) and Rh(111). Due to its role in electrocatalysis, the adsorption of hydrogen is even more important. In our previous study, we found that its rate of adsorption in alkaline solution is higher by nearly one order of magnitude higher on Pt(111) than on a roughened Pt electrode. Conway and co-workers [18] studied the rate on other Pt single crystal surfaces in acidic solution and found that at the (311) stepped surface the adsorption rate of the (111) sites was higher than at the (100) sites. The latter showed no deviation from the homogeneous (100) surface. Here, we also extended our previous studies to hydrogen adsorption on Rh(111) to elucidate the role of the metal.
2. Experimental The Pt(111) single crystal (Metal Crystals and Oxides LTD, b = 10 mm) was prepared by ¯ame annealing and cooled down in ultra pure argon
0013-4686/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 1 9 4 - 7
910
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
(99.999%). The Rh(111) single crystal (Kristallhandel Kelpin, b = 10 mm) was annealed the same way, but cooled down in an argon hydrogen mixture (20% hydrogen[99.9%] and 80% argon[99.999%]). In some cases the Rh(111) was also cooled in pure argon; in these cases special care was necessary to avoid any oxygen contamination in argon. Cleanliness and orientation of the single crystals were checked by cyclic voltammetry (CV) before and after each measurement of one impedance spectrum. The potential was stepped to 0 V (RHE) after measurements of three frequencies to minimize contamination of the surface. All solutions were made of Millipore-Q-water purged by argon 5.0. The following chemicals were used: KOH (microselect) and KH2PO4 (microselect) obtained from Fluka and NaCl (p.a.), H2SO4 (supra pure) and HClO4 (p.a.) obtained from Merck. All potentials were measured against the reversible hydrogen electrode (RHE) except for experiments carried out in chloride-containing solution, for which a Hg/Hg2Cl2 reference electrode was used (E0=235 mV vs. NHE). Impedance spectra were obtained using an impedance system from EG & G (potentiostat 273A, lockin-ampli®er 5210). Experimental impedance spectra were ®tted using the calculation software ``Equivalent Circuit'' designed by B. A. Boukamp (bought from EG & G). The corresponding equivalent circuit for the simulation of the electrode impedance is given below.
The potential dependence of the adsorption resistance and the adsorption capacity obtained from the impedance spectrum were interpreted on the assumption that the adsorption processes follow Langmuir or Frumkin statistics. Therefore, they were simulated using the following equations: * Rad Rmin ad cosh 0:5f Cad C max ad
Rmin ad 2 C max ad
4ÿf
* ÿ f 4 cosh 0:5f
RT ad des ÿ0:5 k c1 k c2 n2 F 2
1 nF Qmax 4 ÿ f RT
* f ÿ f Y ÿ 0:5 f with kad and kdes, the rate constants for the adsorption and desorption reactions, c1 and c2, the concentrations of the involved species, Qmax, the charge corresponding to a full monolayer, f, the Frumkin parameter (it is zero under Langmuir conditions, negative values corre the (dimension f spond to repulsive interactions), E less) potential dierence to that potential Eh(fh) at 0:5 where Y is the fractional surface coverwhich Y age, E nF and E E ÿ E h: f RT More experimental and described in Ref. [17].
theoretical
details
were
3. Results 3.1. Pt(111): chloride adsorption The cyclic voltammogram of the Pt(111) electrode in NaCl solution at pH 4 is shown in Fig. 1a. The current density j of the CV, measured with a scan-rate of v = 50 mV sÿ1 versus Hg/Hg2Cl2, was converted into the corresponding capacity according to C j=v. A pH 4 solution was chosen to separate the chloride adsorption peaks from the peaks of hydrogen desorption. A better separation could be achieved by using even higher pH values, which would, however, necessitate the addition of a buer of anions which would compete with chloride for adsorption sites. The shape of the CV at Pt(111) in NaCl solution at pH = 4 is very similar to that at Pt(111) in the 0.5 M H2SO4 solution described in the next section. In this case, the hydrogen desorption overlaps with the chloride adsorption, resulting in a peak at ÿ122 mV (vs. Hg/Hg2Cl2). The spike at 172 mV versus Hg/Hg2Cl2 resembles that in sulfuric acid at 435 mV versus RHE and therefore can be assumed to be due to the completion of a chloride monolayer [19]. We veri®ed that with increasing concentration of chloride the spike shifts by 60 mV per decade into cathodic direction. A typical impedance spectrum in this region is shown in Fig. 1b. Only one capacity and one electrolyte resistance were found by curve ®tting with the help of the Boukamp ®t program. The capacities determined from their impedance spectra are shown in Fig. 1a as crosses. The inset in Fig. 1c shows a slight, but distinct decrease of the impedance around 10 kHz. From this decrease we tentatively calculated the adsorption resistances. The error of this calculation may reach up to 50% of the values obtained. The adsorption resistances
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
911
are reproduced in Fig. 1c. They decrease in the potential range ÿ150± ÿ 50 mV and increase in the range 50±172 mV. The corresponding double layer capacities were estimated from the frequency, where the impedance decrease occurs, using the common equation: CDL
1 : 2pnRe
The average value for the double layer capacity amounts to 4.52 2 mF cmÿ2 (n = 10 Hz, Re=3.5 O cm2). We simulated the curve for the adsorption resistances under Langmuir conditions (dashes) and under Frumkin conditions with the Frumkin parameter f = ÿ 5 (solid line). We obtained a minimal adsorption 2 h resistance of Rmin ad =0.07 O cm at a potential of E =0 V, corresponding to an exchange current density at half coverage of j0=382.6 mA cmÿ2 ( j0=RT/nFRmin ad ) and an adsorption rate of vad=3965 nmol cmÿ2 sÿ1 (vad=j0/nF). 3.2. Pt(111): sulfate adsorption
Fig. 1. Pt(111) 1 M NaCl, pH 4. (a) Solid line: current density converted into capacity, v = 50 mV sÿ1; crosses: capacities measured by impedance spectroscopy; doted line: simulated adsorption capacity for a Frumkin isotherm with Qmax=80 mC cmÿ2 and f = ÿ 5. (b) Impedance spectrum at a potential E = 150 mV; open symbols: ®t with an electrolyte resistance Re=4.85 O and a capacity C = 206.5 mF. (c) Crosses: potential dependence of the adsorption resistance; dashes: simulation of the adsorption resistance under Langmuir; solid line: for a Frumkin isotherm with f = ÿ 5 (solid line); inset: absolute impedance values in the range 1 to 50 kHz at a potential of 100 mV (Hg/Hg2Cl2) with an electrolyte resistance Re=3.4 O and an adsorption resistance Rad=0.16 O.
The cyclic voltammogram of the Pt(111) electrode in 0.5 M H2SO4 solution is shown in Fig. 2a. Again, the current density was converted into capacity values. It is generally accepted that in the range 50±350 mV the hydrogen adsorption takes place, and that from 350 mV up to the spike at 435 mV (bi)-sulfate adsorption occurs [20]. The spike is assigned to the development 1 of a highly symmetrical adlayer 21p of (bi-)sulfate 2 p with the lattice parameters 3a and 7a and angles of 308 and 198, respectively, with respect to the substrate lattice [1]. It is still a matter of debate whether the adsorbate consists of a sulfate or a bisulfate species [2, 21, 22]. The impedance spectra were measured in the potential range of the (bi)-sulfate adsorption. They are very similar to the spectra of Pt(111) in NaCl solution at pH = 4, and therefore are not shown here. The capacity values thus obtained are included (crosses) in Fig. 2a. They are identical to those given by cyclic voltammetry. As in the case of chloride adsorption, a slight decrease of the impedance at high frequencies points to a low but measurable adsorption resistance (inset in Fig. 2b). Their potential dependence is shown in Fig. 2b. At about 380 mV there is a minimum of the adsorption resistances as expected from the maximum of the capacity. We simulated adsorption curves under Langmuir conditions (dashes) and under Frumkin conditions with a Frumkin parameter of f = ÿ 2 (solid line). The average value for the double layer capacity, calculated as described above, amounts to 6 2 2 mF cmÿ2
912
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
Fig. 2. Pt(111) in 0.5 M H2SO4. (a) Solid line: current density converted into capacity; v = 50 mV sÿ1; crosses: capacities measured by impedance spectroscopy; dotted line: simulated adsorption capacity for a Frumkin isotherm with Qmax=80 mC cmÿ2 and f = ÿ 2. (b) Crosses: potential dependence of the adsorption resistance; dashes: simulation of the adsorption resistance for a Langmuir isotherm. Solid line: for a Frumkin isotherm f = ÿ 2; inset: absolute impedance values in the range 1±50 kHz at a potential of 410 mV (RHE) with an electrolyte resistance Re=2.15 O and an adsorption resistance Rad=0.04 O.
(n = 11 kHz, Re=2.3 O cm2). A minimal adsorption 2 at a potential of resistance of Rmin ad =0.04 O cm Eh=390 mV versus RHE corresponding to an exchange current density of j0=657.3 mA cmÿ2 and an adsorption rate of nad=6605 nmol cmÿ2 sÿ1 were calculated. 3.3. Pt(111): phosphate adsorption Since the rate of phosphate adsorption was studied by the potential pulse techniques in 0.3 M KH2PO4 solution pH 4.6 [9], we studied the adsorption rate in the same solution by impedance spectroscopy for reason of comparison. The cyclic voltammogram is shown in Fig. 3a. Again, the range 50±390 mV is the hydrogen adsorption region. In accordance with Weber et al. [10] and Ye et al. [9] we assign the peak
Fig. 3. Pt(111) in 0.3 M KH2PO4, pH 4.6. (a) Solid line: current density converted into capacity; v = 50 mV sÿ1; crosses: capacities measured by impedance spectroscopy. (b) Impedance spectrum at a potential E = 450 mV; open symbols: ®t with an electrolyte resistance Re=27 O, a capacity C = CDL+Cad1=113.7 mF and an adsorption resistance Rad2=67.95 kO at a ®xed value for Cad2=232.5 mF. (c) Crosses: potential dependence of the adsorption resistance; dashes: simulation of the adsorption resistance for a Langmuir isotherm; solid line: for a Frumkin isotherm with f = 2.
at the potential more positive than 390 mV to the phosphate adsorption. An impedance spectrum typical for the phosphate adsorption region is shown in Fig. 3b. Its shape is
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
mainly determined by one capacity in the range 0.5±50 Hz (Fig. 3a) and one resistance (electrolyte resistance) above 100 Hz. The capacity values obtained by a ®t using a capacity and a resistance in series are also shown in Fig. 3a. They are far too high to be solely due to the double layer capacity. Therefore, we have to assume a fast adsorption process with a very low adsorption resistance which cannot be determined under our conditions. On the other hand, they are much lower than the capacitance obtained from the cyclic voltammetry, which points to a very slow adsorption process. The decrease of the phase angle at lower frequencies (which was observed at all potentials in the phosphate adsorption region) points to such a second very slow adsorption process, indeed. To ®nd the adsorption resistance for the slow adsorption process, an equivalent circuit consisting of a parallel capacitance and resistance in series with the electrolyte resistance was used, i.e. the adsorption capacity of the slow adsorption process was omitted and the adsorption capacity of the fast adsorption process was combined with the double layer capacity. The obtained adsorption resistances for the slow adsorption process are shown in Fig. 3c. We simulated an adsorption curve under Langmuir (dashed line) and under Frumkin conditions for f = 2 (solid line). The corresponding adsorption capacity was calculated as follows. The values for the total capacity for each potential were taken from the CV. To get the adsorption capacity of the slow process, the dierence between the total capacity values (from the CV) and the values obtained from the ®rst ®t (corresponding to the double layer capacity plus that of the fast adsorption) were formed. These values were used to calculate another ®t with the equivalent circuit program where these values were ®xed using the equivalent circuit described in Section 2. We obtained a minimal adsorption resistance of 2 h Rmin ad =2377 O cm at a potential of E =505 mV, corresponding to an exchange current density of j0=5.4 mA cmÿ2 and an adsorption rate of Vad=0.028 nmol cmÿ2 sÿ1. We also examined the phosphate adsorption at other pH values (0.1 M H3PO4/0.44 M KH2PO4 solution, 0.2 M H3PO4/0.1 M HClO4 solution, 0.1 M H3PO4/ 0.05 M HClO4 solution and 0.3 M NaH2PO4/10ÿ3 M HClO4 solution) but these systems were not stable enough during one impedance measurement. Cyclic voltammetry showed that the state of the surface changed, possibly due to contamination. 3.4. Rh(111) In acidic solutions, hydrogen desorption and anion adsorption strongly overlap in the case of Rh(111)
913
except for HClO4 electrolyte. HClO4, on the other hand, has the disadvantage of being reduced at Rh electrodes. Therefore, we studied the rate of hydrogen adsorption on Rh(111) in alkaline solutions. This also allows a better comparison with our previous study of hydrogen adsorption on Pt(111) where we used an alkaline electrolyte, because the rate in acidic solutions was too high to be measured. Preliminary experiments in HClO4 electrolyte will also be described. The cyclic voltammogram of a roughened Rh(111) electrode in 0.5 M KOH solution, where the current density was converted to capacity values, is shown in Fig. 4a. No double layer region exists because the adsorption peak of the hydrogen adsorption nearly overlaps with the peak of the oxygen adsorption. In the cathodic scan, oxygen reduction overlaps with hydrogen adsorption due to the strong irreversibility of oxygen adsorption. Moreover, the hydrogen adsorption peaks are shifted with respect to each other by 60 mV. Therefore, to simulate the apparent irreversibility in cyclic voltammetry in the impedance spectroscopy data, the potential was stepped before and after a period of measurements at three dierent frequencies to a ``conditional potential''. The potential values of the upper and lower potential limit in the CV were chosen for the conditional potentials, in this case 0 and 1500 mV. A typical impedance spectrum of a roughened Rh(111) electrode in 0.5 M KOH solution is shown in Fig. 4b. In the range 1±30 kHz the electrolyte resistance determines the overall impedance. The slope in the range 2±1000 Hz can be assigned to the double layer capacity which interferes at some potentials with a second adsorption capacity. In the range 0.22±2 Hz another plateau can be seen corresponding to the sum of the electrolyte and the adsorption resistance. A slight increase in the absolute value of the impedance below 0.2 Hz is due to the adsorption capacity. The capacity values obtained from impedance spectroscopy are included in Fig. 4a. The capacity values measured at a conditional potential of 0 mV are shown as positive values to compare them with the anodic sweep of the CV, the capacity values measured with a conditional potential of 1500 mV are shown as negative values in order to compare them with the cathodic sweep although they are not really negative. The sum of all observed capacities obtained from the impedance spectroscopy are shown as crosses (``low frequency capacity''). The high frequency limit of the capacity which usually corresponds to the double layer capacity is shown as squares. At potentials below 100 mV this capacity amounts to 50 mF cmÿ2 and therefore can be identi®ed as a double layer capacity, but at more positive potentials this capacity becomes too high (up to 180 mF cmÿ2). Another very fast adsorption process must be assumed, whose
914
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
Fig. 4. Roughened Rh(111) in 0.5 M KOH solution. (a) Solid line: current density converted into capacity; v = 100 mV sÿ1; crosses: total capacity values (Cad+CDL) measured by impedance spectroscopy; squares: double layer capacity values (positive values for a conditional potential (CP) of 0 mV, negative values for CP = 1500 mV). (b) Impedance spectrum at a potential E = 200 mV and CP = 0 mV; open symbols: ®t with an electrolyte resistance Re=3.9 O, a capacity CDL=118 mF, an adsorption resistance Rad=838.6 O and an adsorption capacity Cad=1277 mF. (c) Crosses: potential dependence of the adsorption resistance (positive values for CP = 0 mV, negative values for CP = 1500 mV); solid line: simulation of the adsorption resistance for a Langmuir isotherm.
adsorption capacity cannot be separated from the double layer capacity because the adsorption resistance of this adsorption process is too small. Above 100 mV there is a signi®cant dierence to be noticed between the dierent conditional potentials. The values of the total capacity (adsorption capacity + double layer capacity) are higher for the conditional potential of 0 mV. In Fig. 4c the adsorption resistances with the two dierent conditional potentials (CP) are shown (negative values CP = 1500 mV, positive values CP = 0 mV). Obviously, the dierent conditional potentials have dierent values for the adsorption resistance as an eect. With a conditional potential of 0 mV the resistance above 150 mV increased as expected. The adsorption resistance at the potential of the capacity maximum (Rmin ad ) is about 2.5 times higher in the case 2 of a conditional potential of 0 mV. Rmin ad is 321 O cm for a conditional potential of 0 mV and for a conditional potential of 1500 mV it is 114 O cm2. Below 100 mV, the adsorption resistance further decreases instead of increasing as one would expect for Frumkin and Langmuir type adsorption processes. There seems to be another adsorption process with a minimal adsorption resistance in the hydrogen evolution region. A similar behaviour was observed for hydrogen adsorption on roughened Pt(111) [17]. A theoretical curve for Langmuir conditions is also shown in Fig. 4b. For the conditional potential of 0 mV, a current exchange density of j0=80 mA cmÿ2 and an adsorption rate of Vad=0.829 nmol cmÿ2 sÿ1 were calculated, and for the conditional potential of 1500 mV a current exchange density of j0=225.1 mA cmÿ2 and an adsorption rate of Vad=2.33 nmol cmÿ2 sÿ1 were calculated. The situation at the smooth, freshly annealed Rh(111) electrode in 0.5 M KOH solution is very similar to the roughened Rh(111) electrode as can be seen in the cyclic voltammogram shown in Fig. 5a. As before, the current density is converted into the capacity values. This system is even more irreversible although potentials of strong oxygen adsorption were avoided. The hydrogen adsorption process overlaps with the oxygen adsorption process and no double layer region is observed. In this case the conditional potentials 0 and 800 mV were used. The impedance spectra are very similar to the spectra at the roughened single crystal shown in Fig. 4b. The capacity values obtained from the impedance spectroscopy are shown in Fig. 5a where the total capacity is presented as crosses and the ``double layer capacity'' as open squares. Above 250 mV the values for the capacity shown as squares are too high to be identi®ed as a double layer capacity. Again, a very fast adsorption process has to be assumed which interferes with the charging of the double layer
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
Fig. 5. Smooth Rh(111) in 0.5 M KOH solution. (a) Solid line: current density converted into capacity; v = 50 mV sÿ1; crosses: total capacities values (Cad+CDL) measured by impedance spectroscopy; squares: double layer capacity values (positive values for CP = 0 mV, negative values for CP = 800 mV); inset: CV at a scan rate of 1 mV sÿ1. (b) Crosses: potential dependence of the adsorption resistance (positive values for CP = 0 mV, negative values for CP = 800 mV). (c) Crosses: potential dependence of the adsorption capacity (positive values for CP = 0 mV, negative values for CP = 800 mV); solid line: simulation of the adsorption capacity for a Frumkin isotherm with Qmax=256 mC cmÿ2 and f = 1.
915
capacity and whose adsorption resistance is too small to be detected. The total capacity shows its maximum around 250 mV for both conditional potentials. Both conditional potentials lead to similar values in the whole potential range. For comparison a cyclic voltammogram was recorded with a scan rate of 1 mV sÿ1 (inset in Fig. 5a). This cyclic voltammogram shows an almost reversible peak at about 250 mV. Fig. 5c shows the adsorption capacity values with a simulation of an adsorption curve under Frumkin conditions with a Frumkin parameter of f = 1, the maximum of the adsorption capacity at 235 mV and Qmax=256 mC cmÿ2 (the charge for the adsorption of one monolayer of hydrogen at a Rh(111) electrode was calculated by Clavilier et al. [23]). The corresponding adsorption resistances for the slow process are described in Fig. 5b. The adsorption resistances are identical for both conditional potentials, too. A mini2 mal adsorption resistance Rmin ad =900 O cm at a potenh tial of E =235 mV was calculated. As in the case of the roughened electrode the adsorption resistance decreases below 200 mV. Again, this is due to another adsorption process with its minimum in the hydrogen evolution region. For both conditional potentials a current exchange density of j0=28.6 mA cmÿ2 and an adsorption rate of Vad=0.297 nmol cmÿ2 sÿ1 was calculated. Due to the reduction of HClO4 on rhodium electrodes, we only performed preliminary experiments in this electrolyte. Clavilier et al. [23] found that a reduction, probably to chloride, takes place in the potential range 0.25±0.3 mV in the anodic and in the cathodic sweep. Because of the reduction product a signi®cant change in the CV can be observed (cf. Fig. 6). Impedance spectra were recorded in very fresh electrolytes and in electrolytes which already contained some reduction product. In all cases, they correspond to the simple equivalent circuit for an adsorption process. Capacitance values are included in Fig. 6. The lowest adsorption resistance was found at Eh=150 mV 2 with Rmin ad =70.3 O cm in an electrolyte nearly free of 2 reduction product and Rmin ad =83.1 O cm in the presence of some reduction product. In the electrolyte almost free of the reduction product a current exchange density of j0=365.1 mA cmÿ2 and an adsorption rate of Vad=3.8 nmol cmÿ2 sÿ1 were calculated, in the presence of the reduction product a current exchange density of j0=308.9 mA cmÿ2 and an adsorption rate of Vad=3.2 nmol cmÿ2 sÿ1 were calculated. The adsorption values are summarized in Table 1.
916
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
(H2PO4)ad and (HPO4)ad. Actually, these processes also might be subsequent ones, i.e. the (H2PO4)ad could slowly be deprotonated after ®rst being adsorbed fast. In this case, the corresponding equivalent circuit is the following:
Fig. 6. Smooth Rh(111) in 0.1 M HClO4 solution. Solid line: current density converted into capacity; v = 100 mV sÿ1 nearly free of reduction product; dashes: current density with reduction product; crosses: total capacities values (Cad+CDL) measured by impedance spectroscopy; squares: double layer capacity values (positive values for CP = 0 mV, negative values for CP = 800 mV).
4. Discussion Both the adsorption of HSOÿ 4 (possibly connected with a deprotonation predominating species HSOÿ 4 at pH = 0) and Clÿ on Pt(111) behave as expected like simple adsorption processes. Their adsorption kinetics are satisfactorily described assuming Frumkin conditions with a slight repulsion. The reactions, however, are very fast and at the detection limit of our instrumentation. The adsorption rates (at the potential of half coverage) are comparable, although Clÿ is more strongly bound to the surface as indicated by the lower potential at which half coverage is achieved. This hints to our previous observation that the adsorption of Iÿ ÿ onto Pt(111) is very fast (Rmin ad <0.5 O), although I is one of the most strongly bound anionic adsorbates. However, it has to be kept in mind that the diering adsorption strength is balanced by corresponding half coverage potentials. Wandlowski et al. found a minimal adsorption resistance of 5 O cm2 for the adsorption of Brÿ on Au(111) from an 0.5 mM Brÿ-solution. Assuming a transfer coecient of a = 0.5, the minimal adsorption resistance in a 1 M solution would be 0.1 O cm2, i.e. in the same order of magnitude as our values for Clÿ and HSOÿ 4 on Pt(111). The adsorption of the phosphate species is much more complicated. At the pH used (pH = 4.6), the anion predominating in solution is H2POÿ 4 . As in the case of sulfate adsorption from sulfuric acid, the adsorption might also be connected with a deprotonation. A natural explanation for the two adsorption processes might therefore be a parallel formation of
Since Rad1 is negligible as compared to Rad2, this equivalent circuit is indistinguishable from one with two parallel adsorption processes. It is tempting to assume that such a deprotonation would also be slow in the case of H2SOÿ 4 adsorption; since we did not ®nd such a slow adsorption process for H2SOÿ 4 on Pt(111), one would have to conclude that such a deprotonation on Pt(111) does not occur. In this context, it would be interesting to know the adsorption rate of H2SOÿ 4 on Au(111) which is now generally believed to proceed without such deprotonation. The splitting of the phosphate adsorption peak, which occurs at higher pH values [9, 24], might be interpreted in a similar way as being due to a subsequent adsorption of H2PO4 and HPO2ÿ 4 : above a pH of 7, HPO2ÿ 4 becomes the predominating species in solution. On a logarithmic scale, its concentration does not change much with pH. Since also the position of the second splitted peak becomes pH-independent above pH 7 on an NHE scale (as opposed to the ®rst phosphate adsorption peak which is pH-independent over a large pH-range), one might interpret this peak as being due to the adsorption of HPO2ÿ 4 adsorption. Although it is tempting to interpret the ®rst of the two splitted phosphate peaks as being due to H2POÿ 4 adsorption, the situation is more complicated, since its potential is not independent of pH below pH 7 as it should be with the same reasoning. Moreover, it has to be kept in mind that cations have a large in¯uence on the pH at which splitting occurs [24]. Fukuda and Aramata measured the rate of phosphate adsorption using potential step experiments [12]. Transients during potential steps to potentials positive of the adsorption peak suggested repulsive interaction, whereas desorption at potentials negative of the desorption peak followed a nucleation growth mechanism, which is indicative of an ordered phosphate structure (with attractive interactions) forming slowly at potentials below the peak potential. This slow process might therefore be identi®ed with the slow reaction we observed in the case of the (attractive) interaction parameter of f = 2. As already mentioned, anion adsorption on Rh is much stronger than on Pt(111); therefore, the rate of
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
917
Table 1 Substrate
Adsorbate
Hydrogen adsorption Rh(111) hydrogen Rh(111) rough hydrogen Rh(111) rough hydrogen Pt(111)a hydrogen Pt polyk.a hydrogen Anion adsorption Pt(111) sulfate Pt(111) chloride Pt(111) phosphate a b
2 Rmin ad (O cm )
Solution
Eh (mV)
KOH KOH KOH KOH KOH
235 157 157 250 100
896 321 114 20 4.6
H2SO4 NaCl KH2PO4
390 0b 505
0.04 0.07 2377
vad (nmol cmÿ2 sÿ1) j0 (mA cmÿ2) 0.297 0.829 2.33 13 57 6605 3965 0.028
28.6 80 225.1 1300 5500
CP (mV)
0 1500
6.6 105 3.8 105 5.4
Oelgeklaus et al. [17]. Versus Hg/Hg2Cl2.
anion adsorption, e.g. Clÿ, is not separable from that of hydrogen desorption. Preliminary experiments for Clÿ adsorption showed as a result an adsorption resistance of 108 O cm2 at 90 mV. This value is close to that found for hydrogen adsorption in perchloric acid mentioned above. Therefore, it is not clear whether this value corresponds to hydrogen adsorption in both cases, or to Clÿ adsorption, formed by HClO4 reduction. Nevertheless, taking into account that the hydrogen adsorption rate in alkaline solution on Rh(111) is smaller than on Pt(111) by two orders of magnitude, and assuming a similar relation to hold for acidic solutions, one would expect a value for the adsorption resistance of hydrogen on Rh(111) of several 10 O. We can only speculate about the reasons for this large dierence in adsorption rates. One reason might be that hydrogen desorption and oxygen adsorption nearly overlap, and that the coadsorbed anions lead to a decrease of the hydrogen exchange current as we found for Pt(111) in KI containing KOH electrolyte. The lower adsorption rate obtained with the more positive conditional potentials, which result in a surface still being partially covered by hydroxide species, support this interpretation. A further factor might be that water in the double layer is more strongly bound in the case of Rh. Interestingly, the in¯uence of surface roughness on the hydrogen adsorption rate is also dierent on Pt and Rh: on the roughened Rh(111) the rate is higher by a factor of 3 whereas the contrary is the case on Pt(111) as compared to polycrystalline Pt [17]. Therefore, hydrogen adsorption rates on the rough surfaces only dier by one order of magnitude. More information about other adsorbate systems is necessary to obtain an insight into the factors in¯uencing the adsorption rate such as repulsion of water
from the interface, and the partial desolvation of the adsorbing ions. Acknowledgements Thanks are due to the Fonds der chemischen Industrie for ®nancial support and also to K. H. Langen who has manufactured the glass cell for the impedance measurements. References [1] A.M. Funtikov, U. Linke, U. Stimming, R. Vogel, Surf. Sci. 324 (1995) L343±L348. [2] A.M. Funtikov, U. Stimming, R. Vogel, J. Electroanal. Chem. 428 (1997) 147±153. [3] L.-J. Wan, S.-L. Yau, K. Itaya, J. Phys. Chem. 99 (1995) 9507±9513. [4] L.-J. Wan, S.-L. Yau, G.M. Swain, K. Itaya, J. Electroanal. Chem. 381 (1995) 105±111. [5] I. Albers, H. Baltruschat, I. Kamphausen, J. Electroanal. Chem. 395 (1995) 99±105. [6] Shingaya, Y., Ito, M., 1996. Chem. Phys. Lett. (1996) 438±444. [7] D.A. Stern, H. Baltruschat, M. Martinez, J.L. Stickney, D. Song, S.K. Lewis, D.G. Frank, A.T. Hubbard, J. Electroanal. Chem. 217 (1987) 101±110. [8] M.S. Zei, D. Scherson, G. Lehmpfuhl, D.M. Kolb, J. Electroanal. Chem. 229 (1987) 99±105. [9] S. Ye, H. Kita, A. Aramata, J. Electroanal. Chem. 333 (1992) 299±312. [10] M. Weber, F.C. Narat, I.R. de Moraes, T. Iwasita, J. Phys. Chem. 100 (1996) 19933±19938. [11] V.D. Jovic, R. Parsons, B.M. Jovic, J. Electroanal. Chem. 339 (1992) 327±337. [12] Fukuda., T., Aramata, A., J. Electroanal. Chem., in press. [13] T. Dretschkow, T. Wandlowski, Ber. Bunsenges. Phys. Chem. 101 (1997) 749±757.
918
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
[14] Hamelin, A., 1987. In: Conway, B.E., White, R.E., Bockris, J.O.M. (Eds.), Modern Aspects of Electrochemistry, vol. 16. Plenum Press, New York 1987, p. 1. [15] T. Pajkossy, T. Wandlowski, D.M. Kolb, J. Electroanal. Chem. 414 (1996) 209±220. [16] D. Eberhardt, E. Santos, W. Schmickler, J. Electroanal. Chem. 419 (1996) 23±31. [17] R. Oelgeklaus, J. Rose, H. Baltruschat, J. Electroanal. Chem. 376 (1994) 127±133. [18] S. Morin, H. Dumont, B.E. Conway, J. Electroanal. Chem. 412 (1996) 39±52.
[19] Baltruschat, H., Stern, D.A., Hubbard, A.T., unpublished. [20] R.R. Acizic, F. Feddrix, B.Z. Nikolic, E. Yeager, J. Electroanal. Chem. 341 (1992) 287±306. [21] S. Thomas, Y.-E. Sung, A. Wieckowski, J. Phys. Chem. 99 (1995) 13513. [22] H. Ogasawara, Y. Sawatari, J. Inukai, M. Ito, J. Electroanal. Chem. 358 (1993) 337. [23] J. Clavilier, M. Wasberg, M. Petit, L.H. Klein, J. Electroanal. Chem. 374 (1994) 123±131. [24] J.M. Feliu, M.J. Valls, A. Aldaz, M.A. Climent, J. Clavilier, J. Electroanal. Chem. 345 (1993) 475±481.
Electrochimica Acta 44 (1998) 909±918
The rate of anion and hydrogen adsorption on Pt(111) and Rh(111) T. Langkau, H. Baltruschat * Institut fuÈr Physikalische und Theoretische Chemie, UniversitaÈt Bonn, RoÈmerstrasse 164, D-53117 Bonn, Germany Received 20 November 1997; received in revised form 9 March 1998
Abstract Impedance spectra were measured for Pt(111) and Rh(111) electrodes in various electrolytes. They can all be interpreted by assuming one or two parallel adsorption resistances in addition to the double layer capacity. ÿ Adsorption of Clÿ and HSOÿ 4 at Pt(111) is very fast and at the limit of detectability. Adsorption of H2PO4 has to be described by two parallel adsorption reactions, one of which is possibly an adsorption connected with deprotonation. Adsorption of hydrogen at Rh(111) is much slower than at Pt(111); surface roughness has opposing eects on both metals and leads to an increased adsorption rate at Rh. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Anion adsorption on low index single crystal surfaces has been widely studied by structure and coverage sensitive methods such as STM [1±4], ex situ, LEED and other UHV-techniques [5±8], and IR spectroscopy [9, 10]. Much less attention, however, has been paid to the rate of ionic adsorption. Parsons and co-workers reported on the impedance spectra of an Ag(111) electrode in various electrolytes [11]. Aramata and co-workers [12] examined the kinetics of phosphate adsorption on Pt(111) by potential step experiments. Similarly, Wandlowski and co-workers [13] determined the adsorption rate of sulfate on Au(111) and observed a phase formation behaviour. The socalled ``frequency dispersion'', reported, for instance by Hamelin and co-workers on gold single crystal surfaces [14], has recently been explained by low adsorption rates of speci®cally adsorbing ions [15, 16]. Anion adsorption on Pt(111) leads to distinctive, well-shaped adsorption peaks, suggesting Frumkinlike, adsorption behaviour. Therefore, these systems seem to be ideally suited for the determination of the
* Corresponding author.
adsorption rate by impedance spectroscopy. For that reason we tried to study the rate of iodide adsorption on Pt(111) and found that it was too high to be determined [17]. In the following, we will describe the extension of this study to sulfate, chloride, and phosphate adsorption on Pt(111) and Rh(111). Due to its role in electrocatalysis, the adsorption of hydrogen is even more important. In our previous study, we found that its rate of adsorption in alkaline solution is higher by nearly one order of magnitude higher on Pt(111) than on a roughened Pt electrode. Conway and co-workers [18] studied the rate on other Pt single crystal surfaces in acidic solution and found that at the (311) stepped surface the adsorption rate of the (111) sites was higher than at the (100) sites. The latter showed no deviation from the homogeneous (100) surface. Here, we also extended our previous studies to hydrogen adsorption on Rh(111) to elucidate the role of the metal.
2. Experimental The Pt(111) single crystal (Metal Crystals and Oxides LTD, b = 10 mm) was prepared by ¯ame annealing and cooled down in ultra pure argon
0013-4686/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 1 9 4 - 7
910
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
(99.999%). The Rh(111) single crystal (Kristallhandel Kelpin, b = 10 mm) was annealed the same way, but cooled down in an argon hydrogen mixture (20% hydrogen[99.9%] and 80% argon[99.999%]). In some cases the Rh(111) was also cooled in pure argon; in these cases special care was necessary to avoid any oxygen contamination in argon. Cleanliness and orientation of the single crystals were checked by cyclic voltammetry (CV) before and after each measurement of one impedance spectrum. The potential was stepped to 0 V (RHE) after measurements of three frequencies to minimize contamination of the surface. All solutions were made of Millipore-Q-water purged by argon 5.0. The following chemicals were used: KOH (microselect) and KH2PO4 (microselect) obtained from Fluka and NaCl (p.a.), H2SO4 (supra pure) and HClO4 (p.a.) obtained from Merck. All potentials were measured against the reversible hydrogen electrode (RHE) except for experiments carried out in chloride-containing solution, for which a Hg/Hg2Cl2 reference electrode was used (E0=235 mV vs. NHE). Impedance spectra were obtained using an impedance system from EG & G (potentiostat 273A, lockin-ampli®er 5210). Experimental impedance spectra were ®tted using the calculation software ``Equivalent Circuit'' designed by B. A. Boukamp (bought from EG & G). The corresponding equivalent circuit for the simulation of the electrode impedance is given below.
The potential dependence of the adsorption resistance and the adsorption capacity obtained from the impedance spectrum were interpreted on the assumption that the adsorption processes follow Langmuir or Frumkin statistics. Therefore, they were simulated using the following equations: * Rad Rmin ad cosh 0:5f Cad C max ad
Rmin ad 2 C max ad
4ÿf
* ÿ f 4 cosh 0:5f
RT ad des ÿ0:5 k c1 k c2 n2 F 2
1 nF Qmax 4 ÿ f RT
* f ÿ f Y ÿ 0:5 f with kad and kdes, the rate constants for the adsorption and desorption reactions, c1 and c2, the concentrations of the involved species, Qmax, the charge corresponding to a full monolayer, f, the Frumkin parameter (it is zero under Langmuir conditions, negative values corre the (dimension f spond to repulsive interactions), E less) potential dierence to that potential Eh(fh) at 0:5 where Y is the fractional surface coverwhich Y age, E nF and E E ÿ E h: f RT More experimental and described in Ref. [17].
theoretical
details
were
3. Results 3.1. Pt(111): chloride adsorption The cyclic voltammogram of the Pt(111) electrode in NaCl solution at pH 4 is shown in Fig. 1a. The current density j of the CV, measured with a scan-rate of v = 50 mV sÿ1 versus Hg/Hg2Cl2, was converted into the corresponding capacity according to C j=v. A pH 4 solution was chosen to separate the chloride adsorption peaks from the peaks of hydrogen desorption. A better separation could be achieved by using even higher pH values, which would, however, necessitate the addition of a buer of anions which would compete with chloride for adsorption sites. The shape of the CV at Pt(111) in NaCl solution at pH = 4 is very similar to that at Pt(111) in the 0.5 M H2SO4 solution described in the next section. In this case, the hydrogen desorption overlaps with the chloride adsorption, resulting in a peak at ÿ122 mV (vs. Hg/Hg2Cl2). The spike at 172 mV versus Hg/Hg2Cl2 resembles that in sulfuric acid at 435 mV versus RHE and therefore can be assumed to be due to the completion of a chloride monolayer [19]. We veri®ed that with increasing concentration of chloride the spike shifts by 60 mV per decade into cathodic direction. A typical impedance spectrum in this region is shown in Fig. 1b. Only one capacity and one electrolyte resistance were found by curve ®tting with the help of the Boukamp ®t program. The capacities determined from their impedance spectra are shown in Fig. 1a as crosses. The inset in Fig. 1c shows a slight, but distinct decrease of the impedance around 10 kHz. From this decrease we tentatively calculated the adsorption resistances. The error of this calculation may reach up to 50% of the values obtained. The adsorption resistances
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
911
are reproduced in Fig. 1c. They decrease in the potential range ÿ150± ÿ 50 mV and increase in the range 50±172 mV. The corresponding double layer capacities were estimated from the frequency, where the impedance decrease occurs, using the common equation: CDL
1 : 2pnRe
The average value for the double layer capacity amounts to 4.52 2 mF cmÿ2 (n = 10 Hz, Re=3.5 O cm2). We simulated the curve for the adsorption resistances under Langmuir conditions (dashes) and under Frumkin conditions with the Frumkin parameter f = ÿ 5 (solid line). We obtained a minimal adsorption 2 h resistance of Rmin ad =0.07 O cm at a potential of E =0 V, corresponding to an exchange current density at half coverage of j0=382.6 mA cmÿ2 ( j0=RT/nFRmin ad ) and an adsorption rate of vad=3965 nmol cmÿ2 sÿ1 (vad=j0/nF). 3.2. Pt(111): sulfate adsorption
Fig. 1. Pt(111) 1 M NaCl, pH 4. (a) Solid line: current density converted into capacity, v = 50 mV sÿ1; crosses: capacities measured by impedance spectroscopy; doted line: simulated adsorption capacity for a Frumkin isotherm with Qmax=80 mC cmÿ2 and f = ÿ 5. (b) Impedance spectrum at a potential E = 150 mV; open symbols: ®t with an electrolyte resistance Re=4.85 O and a capacity C = 206.5 mF. (c) Crosses: potential dependence of the adsorption resistance; dashes: simulation of the adsorption resistance under Langmuir; solid line: for a Frumkin isotherm with f = ÿ 5 (solid line); inset: absolute impedance values in the range 1 to 50 kHz at a potential of 100 mV (Hg/Hg2Cl2) with an electrolyte resistance Re=3.4 O and an adsorption resistance Rad=0.16 O.
The cyclic voltammogram of the Pt(111) electrode in 0.5 M H2SO4 solution is shown in Fig. 2a. Again, the current density was converted into capacity values. It is generally accepted that in the range 50±350 mV the hydrogen adsorption takes place, and that from 350 mV up to the spike at 435 mV (bi)-sulfate adsorption occurs [20]. The spike is assigned to the development 1 of a highly symmetrical adlayer 21p of (bi-)sulfate 2 p with the lattice parameters 3a and 7a and angles of 308 and 198, respectively, with respect to the substrate lattice [1]. It is still a matter of debate whether the adsorbate consists of a sulfate or a bisulfate species [2, 21, 22]. The impedance spectra were measured in the potential range of the (bi)-sulfate adsorption. They are very similar to the spectra of Pt(111) in NaCl solution at pH = 4, and therefore are not shown here. The capacity values thus obtained are included (crosses) in Fig. 2a. They are identical to those given by cyclic voltammetry. As in the case of chloride adsorption, a slight decrease of the impedance at high frequencies points to a low but measurable adsorption resistance (inset in Fig. 2b). Their potential dependence is shown in Fig. 2b. At about 380 mV there is a minimum of the adsorption resistances as expected from the maximum of the capacity. We simulated adsorption curves under Langmuir conditions (dashes) and under Frumkin conditions with a Frumkin parameter of f = ÿ 2 (solid line). The average value for the double layer capacity, calculated as described above, amounts to 6 2 2 mF cmÿ2
912
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
Fig. 2. Pt(111) in 0.5 M H2SO4. (a) Solid line: current density converted into capacity; v = 50 mV sÿ1; crosses: capacities measured by impedance spectroscopy; dotted line: simulated adsorption capacity for a Frumkin isotherm with Qmax=80 mC cmÿ2 and f = ÿ 2. (b) Crosses: potential dependence of the adsorption resistance; dashes: simulation of the adsorption resistance for a Langmuir isotherm. Solid line: for a Frumkin isotherm f = ÿ 2; inset: absolute impedance values in the range 1±50 kHz at a potential of 410 mV (RHE) with an electrolyte resistance Re=2.15 O and an adsorption resistance Rad=0.04 O.
(n = 11 kHz, Re=2.3 O cm2). A minimal adsorption 2 at a potential of resistance of Rmin ad =0.04 O cm Eh=390 mV versus RHE corresponding to an exchange current density of j0=657.3 mA cmÿ2 and an adsorption rate of nad=6605 nmol cmÿ2 sÿ1 were calculated. 3.3. Pt(111): phosphate adsorption Since the rate of phosphate adsorption was studied by the potential pulse techniques in 0.3 M KH2PO4 solution pH 4.6 [9], we studied the adsorption rate in the same solution by impedance spectroscopy for reason of comparison. The cyclic voltammogram is shown in Fig. 3a. Again, the range 50±390 mV is the hydrogen adsorption region. In accordance with Weber et al. [10] and Ye et al. [9] we assign the peak
Fig. 3. Pt(111) in 0.3 M KH2PO4, pH 4.6. (a) Solid line: current density converted into capacity; v = 50 mV sÿ1; crosses: capacities measured by impedance spectroscopy. (b) Impedance spectrum at a potential E = 450 mV; open symbols: ®t with an electrolyte resistance Re=27 O, a capacity C = CDL+Cad1=113.7 mF and an adsorption resistance Rad2=67.95 kO at a ®xed value for Cad2=232.5 mF. (c) Crosses: potential dependence of the adsorption resistance; dashes: simulation of the adsorption resistance for a Langmuir isotherm; solid line: for a Frumkin isotherm with f = 2.
at the potential more positive than 390 mV to the phosphate adsorption. An impedance spectrum typical for the phosphate adsorption region is shown in Fig. 3b. Its shape is
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
mainly determined by one capacity in the range 0.5±50 Hz (Fig. 3a) and one resistance (electrolyte resistance) above 100 Hz. The capacity values obtained by a ®t using a capacity and a resistance in series are also shown in Fig. 3a. They are far too high to be solely due to the double layer capacity. Therefore, we have to assume a fast adsorption process with a very low adsorption resistance which cannot be determined under our conditions. On the other hand, they are much lower than the capacitance obtained from the cyclic voltammetry, which points to a very slow adsorption process. The decrease of the phase angle at lower frequencies (which was observed at all potentials in the phosphate adsorption region) points to such a second very slow adsorption process, indeed. To ®nd the adsorption resistance for the slow adsorption process, an equivalent circuit consisting of a parallel capacitance and resistance in series with the electrolyte resistance was used, i.e. the adsorption capacity of the slow adsorption process was omitted and the adsorption capacity of the fast adsorption process was combined with the double layer capacity. The obtained adsorption resistances for the slow adsorption process are shown in Fig. 3c. We simulated an adsorption curve under Langmuir (dashed line) and under Frumkin conditions for f = 2 (solid line). The corresponding adsorption capacity was calculated as follows. The values for the total capacity for each potential were taken from the CV. To get the adsorption capacity of the slow process, the dierence between the total capacity values (from the CV) and the values obtained from the ®rst ®t (corresponding to the double layer capacity plus that of the fast adsorption) were formed. These values were used to calculate another ®t with the equivalent circuit program where these values were ®xed using the equivalent circuit described in Section 2. We obtained a minimal adsorption resistance of 2 h Rmin ad =2377 O cm at a potential of E =505 mV, corresponding to an exchange current density of j0=5.4 mA cmÿ2 and an adsorption rate of Vad=0.028 nmol cmÿ2 sÿ1. We also examined the phosphate adsorption at other pH values (0.1 M H3PO4/0.44 M KH2PO4 solution, 0.2 M H3PO4/0.1 M HClO4 solution, 0.1 M H3PO4/ 0.05 M HClO4 solution and 0.3 M NaH2PO4/10ÿ3 M HClO4 solution) but these systems were not stable enough during one impedance measurement. Cyclic voltammetry showed that the state of the surface changed, possibly due to contamination. 3.4. Rh(111) In acidic solutions, hydrogen desorption and anion adsorption strongly overlap in the case of Rh(111)
913
except for HClO4 electrolyte. HClO4, on the other hand, has the disadvantage of being reduced at Rh electrodes. Therefore, we studied the rate of hydrogen adsorption on Rh(111) in alkaline solutions. This also allows a better comparison with our previous study of hydrogen adsorption on Pt(111) where we used an alkaline electrolyte, because the rate in acidic solutions was too high to be measured. Preliminary experiments in HClO4 electrolyte will also be described. The cyclic voltammogram of a roughened Rh(111) electrode in 0.5 M KOH solution, where the current density was converted to capacity values, is shown in Fig. 4a. No double layer region exists because the adsorption peak of the hydrogen adsorption nearly overlaps with the peak of the oxygen adsorption. In the cathodic scan, oxygen reduction overlaps with hydrogen adsorption due to the strong irreversibility of oxygen adsorption. Moreover, the hydrogen adsorption peaks are shifted with respect to each other by 60 mV. Therefore, to simulate the apparent irreversibility in cyclic voltammetry in the impedance spectroscopy data, the potential was stepped before and after a period of measurements at three dierent frequencies to a ``conditional potential''. The potential values of the upper and lower potential limit in the CV were chosen for the conditional potentials, in this case 0 and 1500 mV. A typical impedance spectrum of a roughened Rh(111) electrode in 0.5 M KOH solution is shown in Fig. 4b. In the range 1±30 kHz the electrolyte resistance determines the overall impedance. The slope in the range 2±1000 Hz can be assigned to the double layer capacity which interferes at some potentials with a second adsorption capacity. In the range 0.22±2 Hz another plateau can be seen corresponding to the sum of the electrolyte and the adsorption resistance. A slight increase in the absolute value of the impedance below 0.2 Hz is due to the adsorption capacity. The capacity values obtained from impedance spectroscopy are included in Fig. 4a. The capacity values measured at a conditional potential of 0 mV are shown as positive values to compare them with the anodic sweep of the CV, the capacity values measured with a conditional potential of 1500 mV are shown as negative values in order to compare them with the cathodic sweep although they are not really negative. The sum of all observed capacities obtained from the impedance spectroscopy are shown as crosses (``low frequency capacity''). The high frequency limit of the capacity which usually corresponds to the double layer capacity is shown as squares. At potentials below 100 mV this capacity amounts to 50 mF cmÿ2 and therefore can be identi®ed as a double layer capacity, but at more positive potentials this capacity becomes too high (up to 180 mF cmÿ2). Another very fast adsorption process must be assumed, whose
914
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
Fig. 4. Roughened Rh(111) in 0.5 M KOH solution. (a) Solid line: current density converted into capacity; v = 100 mV sÿ1; crosses: total capacity values (Cad+CDL) measured by impedance spectroscopy; squares: double layer capacity values (positive values for a conditional potential (CP) of 0 mV, negative values for CP = 1500 mV). (b) Impedance spectrum at a potential E = 200 mV and CP = 0 mV; open symbols: ®t with an electrolyte resistance Re=3.9 O, a capacity CDL=118 mF, an adsorption resistance Rad=838.6 O and an adsorption capacity Cad=1277 mF. (c) Crosses: potential dependence of the adsorption resistance (positive values for CP = 0 mV, negative values for CP = 1500 mV); solid line: simulation of the adsorption resistance for a Langmuir isotherm.
adsorption capacity cannot be separated from the double layer capacity because the adsorption resistance of this adsorption process is too small. Above 100 mV there is a signi®cant dierence to be noticed between the dierent conditional potentials. The values of the total capacity (adsorption capacity + double layer capacity) are higher for the conditional potential of 0 mV. In Fig. 4c the adsorption resistances with the two dierent conditional potentials (CP) are shown (negative values CP = 1500 mV, positive values CP = 0 mV). Obviously, the dierent conditional potentials have dierent values for the adsorption resistance as an eect. With a conditional potential of 0 mV the resistance above 150 mV increased as expected. The adsorption resistance at the potential of the capacity maximum (Rmin ad ) is about 2.5 times higher in the case 2 of a conditional potential of 0 mV. Rmin ad is 321 O cm for a conditional potential of 0 mV and for a conditional potential of 1500 mV it is 114 O cm2. Below 100 mV, the adsorption resistance further decreases instead of increasing as one would expect for Frumkin and Langmuir type adsorption processes. There seems to be another adsorption process with a minimal adsorption resistance in the hydrogen evolution region. A similar behaviour was observed for hydrogen adsorption on roughened Pt(111) [17]. A theoretical curve for Langmuir conditions is also shown in Fig. 4b. For the conditional potential of 0 mV, a current exchange density of j0=80 mA cmÿ2 and an adsorption rate of Vad=0.829 nmol cmÿ2 sÿ1 were calculated, and for the conditional potential of 1500 mV a current exchange density of j0=225.1 mA cmÿ2 and an adsorption rate of Vad=2.33 nmol cmÿ2 sÿ1 were calculated. The situation at the smooth, freshly annealed Rh(111) electrode in 0.5 M KOH solution is very similar to the roughened Rh(111) electrode as can be seen in the cyclic voltammogram shown in Fig. 5a. As before, the current density is converted into the capacity values. This system is even more irreversible although potentials of strong oxygen adsorption were avoided. The hydrogen adsorption process overlaps with the oxygen adsorption process and no double layer region is observed. In this case the conditional potentials 0 and 800 mV were used. The impedance spectra are very similar to the spectra at the roughened single crystal shown in Fig. 4b. The capacity values obtained from the impedance spectroscopy are shown in Fig. 5a where the total capacity is presented as crosses and the ``double layer capacity'' as open squares. Above 250 mV the values for the capacity shown as squares are too high to be identi®ed as a double layer capacity. Again, a very fast adsorption process has to be assumed which interferes with the charging of the double layer
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
Fig. 5. Smooth Rh(111) in 0.5 M KOH solution. (a) Solid line: current density converted into capacity; v = 50 mV sÿ1; crosses: total capacities values (Cad+CDL) measured by impedance spectroscopy; squares: double layer capacity values (positive values for CP = 0 mV, negative values for CP = 800 mV); inset: CV at a scan rate of 1 mV sÿ1. (b) Crosses: potential dependence of the adsorption resistance (positive values for CP = 0 mV, negative values for CP = 800 mV). (c) Crosses: potential dependence of the adsorption capacity (positive values for CP = 0 mV, negative values for CP = 800 mV); solid line: simulation of the adsorption capacity for a Frumkin isotherm with Qmax=256 mC cmÿ2 and f = 1.
915
capacity and whose adsorption resistance is too small to be detected. The total capacity shows its maximum around 250 mV for both conditional potentials. Both conditional potentials lead to similar values in the whole potential range. For comparison a cyclic voltammogram was recorded with a scan rate of 1 mV sÿ1 (inset in Fig. 5a). This cyclic voltammogram shows an almost reversible peak at about 250 mV. Fig. 5c shows the adsorption capacity values with a simulation of an adsorption curve under Frumkin conditions with a Frumkin parameter of f = 1, the maximum of the adsorption capacity at 235 mV and Qmax=256 mC cmÿ2 (the charge for the adsorption of one monolayer of hydrogen at a Rh(111) electrode was calculated by Clavilier et al. [23]). The corresponding adsorption resistances for the slow process are described in Fig. 5b. The adsorption resistances are identical for both conditional potentials, too. A mini2 mal adsorption resistance Rmin ad =900 O cm at a potenh tial of E =235 mV was calculated. As in the case of the roughened electrode the adsorption resistance decreases below 200 mV. Again, this is due to another adsorption process with its minimum in the hydrogen evolution region. For both conditional potentials a current exchange density of j0=28.6 mA cmÿ2 and an adsorption rate of Vad=0.297 nmol cmÿ2 sÿ1 was calculated. Due to the reduction of HClO4 on rhodium electrodes, we only performed preliminary experiments in this electrolyte. Clavilier et al. [23] found that a reduction, probably to chloride, takes place in the potential range 0.25±0.3 mV in the anodic and in the cathodic sweep. Because of the reduction product a signi®cant change in the CV can be observed (cf. Fig. 6). Impedance spectra were recorded in very fresh electrolytes and in electrolytes which already contained some reduction product. In all cases, they correspond to the simple equivalent circuit for an adsorption process. Capacitance values are included in Fig. 6. The lowest adsorption resistance was found at Eh=150 mV 2 with Rmin ad =70.3 O cm in an electrolyte nearly free of 2 reduction product and Rmin ad =83.1 O cm in the presence of some reduction product. In the electrolyte almost free of the reduction product a current exchange density of j0=365.1 mA cmÿ2 and an adsorption rate of Vad=3.8 nmol cmÿ2 sÿ1 were calculated, in the presence of the reduction product a current exchange density of j0=308.9 mA cmÿ2 and an adsorption rate of Vad=3.2 nmol cmÿ2 sÿ1 were calculated. The adsorption values are summarized in Table 1.
916
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
(H2PO4)ad and (HPO4)ad. Actually, these processes also might be subsequent ones, i.e. the (H2PO4)ad could slowly be deprotonated after ®rst being adsorbed fast. In this case, the corresponding equivalent circuit is the following:
Fig. 6. Smooth Rh(111) in 0.1 M HClO4 solution. Solid line: current density converted into capacity; v = 100 mV sÿ1 nearly free of reduction product; dashes: current density with reduction product; crosses: total capacities values (Cad+CDL) measured by impedance spectroscopy; squares: double layer capacity values (positive values for CP = 0 mV, negative values for CP = 800 mV).
4. Discussion Both the adsorption of HSOÿ 4 (possibly connected with a deprotonation predominating species HSOÿ 4 at pH = 0) and Clÿ on Pt(111) behave as expected like simple adsorption processes. Their adsorption kinetics are satisfactorily described assuming Frumkin conditions with a slight repulsion. The reactions, however, are very fast and at the detection limit of our instrumentation. The adsorption rates (at the potential of half coverage) are comparable, although Clÿ is more strongly bound to the surface as indicated by the lower potential at which half coverage is achieved. This hints to our previous observation that the adsorption of Iÿ ÿ onto Pt(111) is very fast (Rmin ad <0.5 O), although I is one of the most strongly bound anionic adsorbates. However, it has to be kept in mind that the diering adsorption strength is balanced by corresponding half coverage potentials. Wandlowski et al. found a minimal adsorption resistance of 5 O cm2 for the adsorption of Brÿ on Au(111) from an 0.5 mM Brÿ-solution. Assuming a transfer coecient of a = 0.5, the minimal adsorption resistance in a 1 M solution would be 0.1 O cm2, i.e. in the same order of magnitude as our values for Clÿ and HSOÿ 4 on Pt(111). The adsorption of the phosphate species is much more complicated. At the pH used (pH = 4.6), the anion predominating in solution is H2POÿ 4 . As in the case of sulfate adsorption from sulfuric acid, the adsorption might also be connected with a deprotonation. A natural explanation for the two adsorption processes might therefore be a parallel formation of
Since Rad1 is negligible as compared to Rad2, this equivalent circuit is indistinguishable from one with two parallel adsorption processes. It is tempting to assume that such a deprotonation would also be slow in the case of H2SOÿ 4 adsorption; since we did not ®nd such a slow adsorption process for H2SOÿ 4 on Pt(111), one would have to conclude that such a deprotonation on Pt(111) does not occur. In this context, it would be interesting to know the adsorption rate of H2SOÿ 4 on Au(111) which is now generally believed to proceed without such deprotonation. The splitting of the phosphate adsorption peak, which occurs at higher pH values [9, 24], might be interpreted in a similar way as being due to a subsequent adsorption of H2PO4 and HPO2ÿ 4 : above a pH of 7, HPO2ÿ 4 becomes the predominating species in solution. On a logarithmic scale, its concentration does not change much with pH. Since also the position of the second splitted peak becomes pH-independent above pH 7 on an NHE scale (as opposed to the ®rst phosphate adsorption peak which is pH-independent over a large pH-range), one might interpret this peak as being due to the adsorption of HPO2ÿ 4 adsorption. Although it is tempting to interpret the ®rst of the two splitted phosphate peaks as being due to H2POÿ 4 adsorption, the situation is more complicated, since its potential is not independent of pH below pH 7 as it should be with the same reasoning. Moreover, it has to be kept in mind that cations have a large in¯uence on the pH at which splitting occurs [24]. Fukuda and Aramata measured the rate of phosphate adsorption using potential step experiments [12]. Transients during potential steps to potentials positive of the adsorption peak suggested repulsive interaction, whereas desorption at potentials negative of the desorption peak followed a nucleation growth mechanism, which is indicative of an ordered phosphate structure (with attractive interactions) forming slowly at potentials below the peak potential. This slow process might therefore be identi®ed with the slow reaction we observed in the case of the (attractive) interaction parameter of f = 2. As already mentioned, anion adsorption on Rh is much stronger than on Pt(111); therefore, the rate of
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
917
Table 1 Substrate
Adsorbate
Hydrogen adsorption Rh(111) hydrogen Rh(111) rough hydrogen Rh(111) rough hydrogen Pt(111)a hydrogen Pt polyk.a hydrogen Anion adsorption Pt(111) sulfate Pt(111) chloride Pt(111) phosphate a b
2 Rmin ad (O cm )
Solution
Eh (mV)
KOH KOH KOH KOH KOH
235 157 157 250 100
896 321 114 20 4.6
H2SO4 NaCl KH2PO4
390 0b 505
0.04 0.07 2377
vad (nmol cmÿ2 sÿ1) j0 (mA cmÿ2) 0.297 0.829 2.33 13 57 6605 3965 0.028
28.6 80 225.1 1300 5500
CP (mV)
0 1500
6.6 105 3.8 105 5.4
Oelgeklaus et al. [17]. Versus Hg/Hg2Cl2.
anion adsorption, e.g. Clÿ, is not separable from that of hydrogen desorption. Preliminary experiments for Clÿ adsorption showed as a result an adsorption resistance of 108 O cm2 at 90 mV. This value is close to that found for hydrogen adsorption in perchloric acid mentioned above. Therefore, it is not clear whether this value corresponds to hydrogen adsorption in both cases, or to Clÿ adsorption, formed by HClO4 reduction. Nevertheless, taking into account that the hydrogen adsorption rate in alkaline solution on Rh(111) is smaller than on Pt(111) by two orders of magnitude, and assuming a similar relation to hold for acidic solutions, one would expect a value for the adsorption resistance of hydrogen on Rh(111) of several 10 O. We can only speculate about the reasons for this large dierence in adsorption rates. One reason might be that hydrogen desorption and oxygen adsorption nearly overlap, and that the coadsorbed anions lead to a decrease of the hydrogen exchange current as we found for Pt(111) in KI containing KOH electrolyte. The lower adsorption rate obtained with the more positive conditional potentials, which result in a surface still being partially covered by hydroxide species, support this interpretation. A further factor might be that water in the double layer is more strongly bound in the case of Rh. Interestingly, the in¯uence of surface roughness on the hydrogen adsorption rate is also dierent on Pt and Rh: on the roughened Rh(111) the rate is higher by a factor of 3 whereas the contrary is the case on Pt(111) as compared to polycrystalline Pt [17]. Therefore, hydrogen adsorption rates on the rough surfaces only dier by one order of magnitude. More information about other adsorbate systems is necessary to obtain an insight into the factors in¯uencing the adsorption rate such as repulsion of water
from the interface, and the partial desolvation of the adsorbing ions. Acknowledgements Thanks are due to the Fonds der chemischen Industrie for ®nancial support and also to K. H. Langen who has manufactured the glass cell for the impedance measurements. References [1] A.M. Funtikov, U. Linke, U. Stimming, R. Vogel, Surf. Sci. 324 (1995) L343±L348. [2] A.M. Funtikov, U. Stimming, R. Vogel, J. Electroanal. Chem. 428 (1997) 147±153. [3] L.-J. Wan, S.-L. Yau, K. Itaya, J. Phys. Chem. 99 (1995) 9507±9513. [4] L.-J. Wan, S.-L. Yau, G.M. Swain, K. Itaya, J. Electroanal. Chem. 381 (1995) 105±111. [5] I. Albers, H. Baltruschat, I. Kamphausen, J. Electroanal. Chem. 395 (1995) 99±105. [6] Shingaya, Y., Ito, M., 1996. Chem. Phys. Lett. (1996) 438±444. [7] D.A. Stern, H. Baltruschat, M. Martinez, J.L. Stickney, D. Song, S.K. Lewis, D.G. Frank, A.T. Hubbard, J. Electroanal. Chem. 217 (1987) 101±110. [8] M.S. Zei, D. Scherson, G. Lehmpfuhl, D.M. Kolb, J. Electroanal. Chem. 229 (1987) 99±105. [9] S. Ye, H. Kita, A. Aramata, J. Electroanal. Chem. 333 (1992) 299±312. [10] M. Weber, F.C. Narat, I.R. de Moraes, T. Iwasita, J. Phys. Chem. 100 (1996) 19933±19938. [11] V.D. Jovic, R. Parsons, B.M. Jovic, J. Electroanal. Chem. 339 (1992) 327±337. [12] Fukuda., T., Aramata, A., J. Electroanal. Chem., in press. [13] T. Dretschkow, T. Wandlowski, Ber. Bunsenges. Phys. Chem. 101 (1997) 749±757.
918
T. Langkau, H. Baltruschat / Electrochimica Acta 44 (1998) 909±918
[14] Hamelin, A., 1987. In: Conway, B.E., White, R.E., Bockris, J.O.M. (Eds.), Modern Aspects of Electrochemistry, vol. 16. Plenum Press, New York 1987, p. 1. [15] T. Pajkossy, T. Wandlowski, D.M. Kolb, J. Electroanal. Chem. 414 (1996) 209±220. [16] D. Eberhardt, E. Santos, W. Schmickler, J. Electroanal. Chem. 419 (1996) 23±31. [17] R. Oelgeklaus, J. Rose, H. Baltruschat, J. Electroanal. Chem. 376 (1994) 127±133. [18] S. Morin, H. Dumont, B.E. Conway, J. Electroanal. Chem. 412 (1996) 39±52.
[19] Baltruschat, H., Stern, D.A., Hubbard, A.T., unpublished. [20] R.R. Acizic, F. Feddrix, B.Z. Nikolic, E. Yeager, J. Electroanal. Chem. 341 (1992) 287±306. [21] S. Thomas, Y.-E. Sung, A. Wieckowski, J. Phys. Chem. 99 (1995) 13513. [22] H. Ogasawara, Y. Sawatari, J. Inukai, M. Ito, J. Electroanal. Chem. 358 (1993) 337. [23] J. Clavilier, M. Wasberg, M. Petit, L.H. Klein, J. Electroanal. Chem. 374 (1994) 123±131. [24] J.M. Feliu, M.J. Valls, A. Aldaz, M.A. Climent, J. Clavilier, J. Electroanal. Chem. 345 (1993) 475±481.