J. Phys. Chem. 1995, 99, 13247-13256
13247
Electrochemical Reactions of Ethyne Adsorbed on Polycrystalline Au Electrodes Volkmar M. Schmidt*$+and Elena Pastore Institut jiir Energieve$ahrenstechnik (IEV), Forschungszentrum Jiilich GmbH (KFA), 52425 Jiilich, Germany, and Institut jiir Physikalische und Theoretische Chemie, Uniuersitat Bonn, Wegelerstr. 12, 53115 Bonn, Germany Received: April 24, 1995; In Final Form: June 19, 1995@
Ethyne dissolved in H2SO4 and HC104 adsorbs on smooth and porous polycrystalline gold electrodes in a wide potential range between 0 and 1.00 V vs RHE. The adsorption process is accompanied by an anodic current transient when the adsorption potential is adjusted above the potential of zero charge. Electrooxidation and reduction reactions of the ethyne adsorbate were studied by means of differential electrochemical mass spectrometry (DEMS) and in-situ Fourier transform infrared spectroscopy (FTIRS). It was found that the only oxidation product of the adsorbate is CO2, which is confirmed by the potential-dependent mass signal for m/z = 44 in DEMS as well as the C02 band at 2343 cm-’ in FTIR spectra. The adsorption and oxidation charge densities provide evidence for adsorbed species that are more highly oxidized than the initial ethyne molecule. The mass signal for m/z = 26 recorded during the reduction of ethyne demonstrates that adsorbates are electrodesorbed as intact ethyne molecules. A simultaneously observed small contribution of m/z = 28 reveals the formation of ethyne in a parallel reaction pathway. C2HD and C2D2 were identified after the reduction of adsorbed C2H2 in 0.1 M DClOdD20. These findings suggest that species with stoichiometries such as ( C Z H ) and ~ ~ (C2)ad were previously formed upon the adsorption of ethyne on polycrystalline Au in acid solution. The also- studied electrooxidation of CO showed a weaker interaction with Au than ethyne since no adsorbate remained on the surface after electrolyte exchange. It was found that an adlayer of adsorbed ethyne formed on the electrode surface completely inhibits the electro-oxidation of dissolved CO.
Introduction The interaction of organics with electrified interfaces is of considerable interest in electrochemical surface science. The most important motivation is to understand electrocatalytic processes, including such factors as the molecular adsorption of the species, the structure of adsorbates and intermediates, the influence of the surface crystallographic orientation on electrochemical kinetics, and the identification of reaction products. In the past decade numerous in-situ spectroscopic methods have been developed in order to characterize the structure of electrode surfaces and their adsorbates.’.2 Another approach to the study of the electrochemical process may be the identification of intermediates and products during the reaction. In this regard, differential electrochemical mass spectrometry (DEMS) tums out to be a powerful m e t h ~ d .By ~ analogy to thermal desorption spectroscopy (TDS) in ultrahigh vacuum (UHV) studies, adsorbates formed at electrodes can be desorbed by electrooxidation or -reduction and identified by their typical mass signals provided that the products are volatile or gaseous. Thus, for the study of surface reactions of adsorbates, this DEMS technique can be called “potential controlled electrodesorption spectrometry” (PEDS). In recent communications, we reported data obtained by using DEMS to study the reactivity and bonding of unsaturated hydrocarbons (ethyne, ethene, p r ~ p e n e )and ~ . ~alcohols (propen1-01, propyn-l-ol)6 at polycrystalline Au electrodes in acid solutions. As a complementary method, in-situ infrared spectroscopy (FTIRS) was also e m p l ~ y e d . ~Compounds .~ with a
’ Forschungszentrum Jiilich GmbH (KFA). Universitat Bonn. On leave from Departamento de Quimica Fisica, Universidad de La Laguna, 38204, Spain. Abstract published in Advance ACS Absrracts, August 1, 1995. @
C=C bond (ethene, propene, propen- l-ol) show reversible adsorption characterized by the experimental fact that no adsorbate remains on the surface after an exchange by pure supporting electrolyte. On the other hand, ethyne and propenl-ol form stable adsorbates on the surface, surviving a complete electrolyte exchange. More recently, the influence of the crystallographic orientation on the adsorption and electrochemical reactions of ethyne on Au(hkl) has been demon~trated.~For example, on the Au( 111) surface the lowest peak potential for adsorbate oxidation was observed about 0.30 V more negative than on the Au( 110) face. The adsorption of organics, except for sulfur-containing species, on gold electrodes is mainly characterized as a weak interaction in comparison to adsorption on Pt.* The influence of the electronic structure of the metal (Ag, Au, Hg, and Pt) on the adsorption of pyridine has been recently discu~sed.~ In this context, ethyne on Au provides an interesting model system in which chemisorbed species are formed in acid electrolytes. In the present paper new experimental data on the electrooxidation and -reduction of adsorbed ethyne are reported with PEDS and some supplementary measurements with FTIRS. The PEDS technique involves the formation of an adsorbate at a defined potential followed by one reduction or oxidation cycle with the simultaneous detection of reaction products by mass spectrometric cyclic voltammograms (MSCVs) for typical m/z values. Additionally, using isotopically labeled compounds, valuable information on the nature of adsorbed ethyne can be obtained. Furthermore, the electrooxidation of CO on Au is considered in comparison to electrooxidation of ethyne. Experimental Section Measurements were carried out in 0.05 M H2S04 and 0.1 M HC104 (Merck, pa.) electrolyte solutions prepared with Milli-
0022-3654t95/2099-13247$09.00/0 0 1995 American Chemical Society
13248 J. Phys. Chem., Vol. 99, No. 35, 1995
pore-MilliQ* water. Saturated solutions of ethyne (cSat= 0.05 mol L-’)’O were obtained by bubbling ethyne (Linde) through the solution previously deaerated with argon (5.0, MesserGriessheim). In order to avoid traces of acetone, ethyne was passed through three water traps before it was introduced into the electrochemical cell. DC104 (Aldrich, 68% solution in D20, D 99%) and D20 (Cambridge Isotope Laboratories, D2 98%) were used for isotopic labeling studies. CO for comparative measurements was obtained from Messer-Griessheim and was bubbled through the solution. DEMS studies were performed in an electrochemical flow cell directly attached to the vacuum chamber of the mass spectrometer. The working electrode was a porous gold layer sputtered onto a microporous ethylene-tetrafluoroethylene copolymer membrane (Scimat 200/40/60, mean thickness 60 nm, 50% porosity, mean pore diameter 0.17 pm). The geometric area of the electrode was 0.5 cm2, and the roughness factor varied between 5 and 6, calculated from the charge under the AuO reduction peak assuming that a monolayer oxide requires 400 p C cm-2.11 A gold wire served as the counter electrode, and the reference electrode was a reversible hydrogen electrode (RHE) in the same electrolyte solution. All potentials are given relative to this electrode. The mass spectrometer was a QMG 412 (Balzers) provided with a Faraday cup detector. The hydrophobic membrane interfaces the electrochemical reaction layer in the solution and the ionization chamber of the quadrupole mass spectrometer. In this way, it is possible to detect volatile and gaseous products generated during electrochemical reactions within a time delay of less than 0.05 s. Faradaic cyclic voltammograms (CVs) are recorded simultaneously with the dynamic mass spectrometric cyclic voltammograms (MSCVs). More details of the DEMS technique have been given elsewhere. l 2 FTIR experiments were performed in a small glass flow cell containing a gold ring as the counter electrode and an RHE as the reference. The working electrode was a polycrystalline gold disk (geometric area: 0.79 cm2) fixed in a Teflon holder and mechanically polished to a mirror finish. The FTIR spectrometer was a Digilab FTS-40 with a mercury-cadmium telluride detector. A BaF2-supported A1 grid polarizer was employed in order to select p- and s-polarized light, and a CaF2 window was used. FTIR spectra were obtained at selected potentials by applying single potential steps from a reference potential (reflectance Ro) to more positive potentials where a sample potential (reflectance R) was collected. Finally, spectra were calculated at each potential as the reflectance ratio WRo. Thus, positive-going bands correspond to the consumption of species in the thin-layer cavity, whereas negative-going bands are associated with the production of species at the sample potential. More experimental details are described in refs 13 and 14. The working electrodes for DEMS and FTIRS measurements were activated before each experiment by a triangular potential scan program between the onset of hydrogen and oxygen evolution. Adsorption experiments were performed using an electrochemical flow cell combined with DEMS. Adsorption potentials were choosen in the 0.10-1.20 V range. Each experiment consists of the following steps4 (i) after activation, the potential was set to the adsorption potential (Uad); (ii) the electrolyte solution was replaced by ethyne-saturated electrolyte (or COsaturated solution, see below) at constant potential; simultaneously the current transient was recorded; (iii) after 5 min, the ethyne-containing solution was replaced by the base solution at Uad; (iv) positive- or negative-going potential scans were recorded at 0.01 V s-], and the electrooxidation and -reduction
Schmidt and Pastor TABLE 1: Ethyne Adsorption and Adsorbate Oxidation Charge Densities in 0.05 M HzS04 on Porous Au at Different Adsorption Potentials u a d (V) 0.10 0.20 0.30 0.40 0.60 0.70 0.75 0.80 0.90 1.oo 1.20
Uad
Qad
OLC cm-? -82 -46 30 49 86 177 23 1 268 666 1250 14380
QoxOLC cm-*Y 116 91 133 203 422 413 46 1 512 457 42 1 145
QodQad
4.4 4.1 4:9 2.3 2.0 1.9 0.7 0.3 -
a Qad, adsorption charge density determined by integration of the current transient during 5 min adsorption. Qox, oxidation charge density required to oxidize the adsorbate to CO? in the first positive-going potential scan.
TABLE 2: Ethyne Adsorption and Adsorbate Oxidation Charge Densities in 0.1 M HC104 on Porous Au at Different Adsorption Potentials Uad Uad
(v)
0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.oo 1.10
Qad
@cCm-?)a 32 52 74 105 124 203 839 2816 11510
QoxCUC cm-’Y’
QoxIQad
229 334 345 430 469 466 460 434 265
7.1 6.4 4.7 4.1 3.8 2.3 0.6 0.2 -
“ Qad, adsorption charge density determined by integration of the current transient during 5 min adsorption. Qox, oxidation charge density required to oxidize the adsorbate to CO’ in the first positive-going potential scan. products from the adsorbed residues were identified by corresponding MSCVs. All experiments were performed at room temperature. Results Electrooxidation of the Ethyne Adsorbate in 0.1 M HC104/ H20 and 0.05 M H2SO&I20. As indicated before, during the adsorption of ethyne the current transient was recorded. The values for the charge densities obtained by integrating the current transients @ad) are summarized in Table 1 for 0.05 M H2S04 and in Table 2 for 0.1 M HC104. As shown in Table 1, cathodic current transients were recorded when the adsorption potential was adjusted below the potential of zero charge (pzc), which is known to be 0.17 V vs RHE for polycrystalline g01d.l~ However, when v a d > V,,,, only anodic transients were observed. For Vad > 0.80 V the high adsorption charge densities (see Tables 1 and 2) were accompanied by large ion currents for m/z = 44, which is associated with the radical cation [CO$+. This is explained by the oxidation of dissolved ethyne to C02, in parallel to the adsorption process for Uad > 0.80 v.4 Figures l a and 2a show typical CVs for the first positivegoing potential scan after ethyne adsorption at different Uad in H2S04 and HC104, respectively. Figures l b and 2b display the simultaneously recorded MSCVs for d z = 44, attributed to the formation of C02 as the sole oxidation product since no other potential-dependent mass signals could be found. It should be noted that the second cycle in the CVs shows the voltammetric profile obtained in pure supporting electrolyte, and no potential-dependent ion currents for m/z = 44 in the MSCVs could be measured, thus providing evidence for a complete adsorbate oxidation in the first positive-going potential scan.
Ethyne Adsorbed on Polycrystalline Au Electrodes
J. Phys. Chem., Vol. 99, No. 35,1995 13249
< E 2 e2
0.04
-0.04
U
t
-0.12
-0.20
b
L
0.0
0.4
L
I
0.8
I
I
I
1
I
I .2
I .6 PotentiallV vs RHE
Figure 1. Electrooxidation of adsorbed ethyne in 0.05 M H2SO4at 0.01 v s-' on porous Au; tad = 5 min: (a) cv, first and second cycle after adsorption; (b) corresponding MSCV for d z = 44, first cycle after adsorption. (- * -), Uad = 0.20 V; (- - -), Uad = 0.40 V; (-), Uad = 0.60 v; p'*), L/ad = 0.70 v.
The onset potential for adsorbate oxidation is observed to be the same for both electrolytes at about 0.85-0.90 V independent of Uad. The first positive-going potential scan exhibits a broad peak at 1.24-1.26 V in the CVs in both electrolytes when starting from the same adsorption potential, suggesting that the oxidation of the adsorbate is not affected by anion adsorption. However, the potentiodynamic profiles reveal differences when H2S04and HC104 are compared. In sulfuric acid solution the adsorbate is mostly oxidized at potentials below the formation of gold oxide, whereas in perchloric acid a higher current contribution is observed in the gold oxide region. In this respect it should be noted that the onset potential for oxide formation in sulfuric acid is shifted 0.10 V positive as compared to that of perchloric acid (compare Figure 1, second cycle, and Figure 2, dotted line). This well-known finding is associated with the stronger adsorption of HS04-/S0d2-, thus partially displacing preadsorbed H;,0/OH-.I6 Another interesting aspect is the dependence of the CV and MSCV profiles on the initial adsorption potential. When Uad is adjusted in the 0.20-0.40 V range, only a small anodic current contribution is observed in the CV, which is more easily visible in the MSCV through a broad peak at about 1.25 V. For Uad = 0.60-0.80 V, peaks at about 1.25 V are observed in the MSCV in both electrolytes (perchloric acid produces the sharper peak), with a small shoulder at 1S O V. At higher Uad the peak in the MSCV shifts positively, parallel to the faradaic current in the CV. Even at u a d = 1.20 V, at which potential the oxidation of dissolved ethyne to CO;, is observed, the surface is partially covered by adsorbate species being oxidized at > 1.40 V (Figure 2b). It should be mentioned that in the reverse scans small signals for COz formation are apparent at 1.15 V as soon as the Au surface becomes free from oxide. The charge density required to completely oxidize the adsorbate to C02, Qox,was calculated by integrating the anodic
0.0
0.4
1.2
0.8
1.6
PotentiaVV vs RHE
Figure 2. Electrooxidation of adsorbed ethyne in 0.1 M HC104 at 0.01 V s-' on porous Au; fad = 5 min: (a) CV, first positive-going potential scan after adsorption, p * *) pure supporting electrolyte; (b) corresponding MSCV for d z = 44, first cycle after adsorption, (- * -), Uad = 0.80 V; (-), Uad = 1.00 v; ( - - - ) , Uad = 1.20 v . currents in the first cycle according to the relationship
Q,, = QT
- QA~O
where QT is the total oxidation charge density in the presence of adsorbed ethyne and Q A ~ O is the charge density for oxide formation in pure supporting electrolyte. The formation of the AuO was not affected by the ethyne adsorbate, as judged by the oxide reduction charge density calculated from the peak in the negative-going potential scan. This charge density is found to be the same as that in pure supporting electrolyte. The real gold surface area was calculated with the assumption that a monolayer formation requires 400 p Q cm-;, for polycrystalline Au. Data are summarized in Tables 1 and 2. The error for determining all charge densities, including those presented in Table 3 (see below), was estimated to be +lo-15%. Figure 3a shows the corresponding plots of Qox vs Uad, and Figure 3b shows the integrated ion current for m/z = 4.4 vs Uad. For both electrolytes the oxidation charge densities increase for Uad > 0.20 V, attaining a maximum at about 0.75 V followed by a decrease for Uad > 0.90 V. The integrated ion currents show qualitatively the same dependence on Uad for HC104 and H2SO4. It should be mentioned that the integrated ion currents are given in arbitrary units, owing to the experimental conditions, so that the curve for sulfuric acid in Figure 3b is only apparently shifted downward compared to the curve for perchloric acid.
Schmidt and Pastor
13250 J. Phys. Chem., Vol. 99, No. 35, 1995 TABLE 3: Oxidation and Reduction Charge Densities for Ethyne Adsorbates Formed in 0.05 M HzS04 on Porous Au at Different Adsorption Potentials Uad
0.40 0.60 0.75 0.90
203 422 46 1 457
28 28 103 151
22 61 71 85
-
0.55
- = /
v
7.9 5.9
5.0 3.6
Pox.oxidation charge density required to oxidize the adsorbate to cO2 in the first positive-going potential scan.
Qred, reduction charge density determined in the first negative-going potential scan. (Q0,),, oxidation charge density required to oxidize to CO2 the residual adsorbate which remains on the surface after two reduction cycles between 0 V and U a d .
a N
I
400
I2343
0
3 .
\
300
0
t
200
.
I
I
I
1
3000
2500
2000
1500
r
1000
Wavenumber / cm'l
$
Figure 4. FTIR spectra of ethyne in 0.1 M HClOl (saturated solution) on a smooth polycrystalline Au electrode. Reference spectrum was collected at 0.10 V. Resolution = 8 cm-I; 256 scans; p-polarized light.
0
:
I
-0.0
0.2
0.4
Potential 0.0
0.2
0.4
0.0
0.6
/
1.0
1.2
1.4
0.f
0.6
/
1.0
1.2
1.4
V v s RHE
Figure 5. Band intensity integrated from spectra in Figure 4 for the
Figure 3. Oxidation of adsorbed ethyne previously formed at different
bands at 1260 cm-' (e)and 2343 cm-I (W), related to the consumption of C2H2 and the production of CO2, respectively.
In Figure 4 a series of FTIRS spectra is given which were obtained at different sample potentials in 0.1 M HC104 saturated with ethyne. In FTIRS experiments perchloric acid was preferred since sulfate ions adsorb more strongly than perchloric anions, leading to additional bands for adsorbed HS04- and S042-, which cause more difficulties in interpreting the IR spectra. Only three characteristic vibration modes were observed using both s- and p-polarized IR light, indicating that these bands are related to solution species. Two negative-going bands appear at 1109 cm-I, owing to the C1-0 bending mode of C104- ions, and at 2343 cm-I, associated with the asymmetric 0-C-0 stretching vibration of C02. Finally, a positive-going band is apparent at 1260 cm-'. The latter can be assigned to an IR-active combination of two C-H vibrations of the ethyne
molecule." Therefore, this band can be attributed to the consumption of ethyne from the solution in the thin-layer cell during the adsorption process. No IR-active vibration modes of adsorbed ethyne were observed in our measurements. This fact is in agreement with the interpretation of SERS data, which indicate that adsorbed ethyne is oriented with the C-C chain parallel to the Au For this orientation ethyne adsorbate bands are not expected according to the IR surface selection rule.20 It is interesting to note that within the resolution limit of the spectrometer and the experimental conditions no band for adsorbed CO at about 2100 cm-' (see below) could be observed as a possible intermediate during ethyne adsorbate oxidation. The intensities of the bands at 1260 and 2343 cm-I determined by integration are plotted vs Uad in Figure 5. The appearance of the C02 band at U > 0.80 V and the increase of the intensity at higher potentials are in agreement with the CVs
Potential
V vs RHE
adsorption potentials in 0.1 M HC104 (V)and 0.05 M H2S04 (e):(a) oxidation charge required to oxidize the adsorbate in the first positivegoing potential scan, Qox, vs U a d ; (b) integrated ion current for d z = 44 vs Uad.
Ethyne Adsorbed on Polycrystalline Au Electrodes
. = O.O( 4
a
E
2
6 -0.0;
T
E
J. Phys. Chem., Vol. 99, No. 35, 1995 13251
a < 0.01 E
e
2
U
-0.01 d
;
C
: 2
6
-
U
-
E
F:
mlz =26 0.0
0.8
0.4
1.2
PotentiallV vs RHE
b
-0.01
u 0.0
0.2 0.4 0.6 PotentialiV vs RHE
Figure 6. Electroreduction of adsorbed ethyne in 0.1 M HClOl on porous Au at 0.01 V s-l; U a d = 0.60 V; fad = 5 min: (a) CV, first and second cycle after adsorption (negative-going potential scan starting and d z = 28 [C2HJ+), at U a d ) ; (b) MSCV for d z = 26 ([C2H2]'+) first cycle after adsorption.
and corresponding MSCVs for d z = 44, as presented in Figure 2. Furthermore, the behavior of the band at 1260 cm-' fits well with the curve for the oxidation charge densities displayed in Figure 3a since the intensity of the band increases as well between 0.20 and 0.80 V. This fact provides evidence for a strong correlation of the band intensity at 1260 cm-' and Qox, and therefore, it is reasonable to assume that the charge density required to oxidize the adsorbate completely to C02 is proportional to the coverage of ethyne on the Au surface for Uad = 0.20-0.80 V. This can be confirmed by the fact that the integrated ion current for COz formation exhibits the same potential dependence as shown in Figure 3b. Thus the highest ethyne coverage is attained at adsorption potentials between 0.60 and 0.80 V, in agreement with the dependent of Qox on the adsorption potential in Figure 3a. In the 0.80-1.00 V range the intensity of the 1260 cm-I band does not increase further and becomes independent of Uad in the same range in which the band at 2343 cm-I appears. This is interpreted by a mechanism for ethyne electrooxidation involving an adsorbed intermediate as a precursor for the formation of COz as the final product. Electroreduction of the Ethyne Adsorbate in 0.1 M HC104/HzO and 0.05 M HzS04/H20. Figure 6a shows the first negative-going potential scan starting from Uad = 0.60 V in perchloric acid solution. The reduction peak centered at about 0.31 V is accompanied by a sharp peak of the ion current for d z = 26, which is assigned to the molecular peak [CzH2]'+ of ethyne.*' In parallel to this signal, a small contribution for d z = 28 is measured and assigned to [C,HJ'+, indicating the formation of ethene as a second reduction product. Even at high adsorption potentials, for example Uad = 1.OO and 1.20 V as seen in Figure 7, C2H2 can be produced in the first negativegoing potential scan. However, in contrast to the adsorbate oxidation as described above, the reduction is not complete. After the fist reduction cycle about 10% of the adsorbate formed
2
c
1
I
I
mlz = 26
0.0
0.4
0.8 I .2 PotentiaVV vs RHE
Figure 7. Electroreduction of adsorbed ethyne in 0.1 M HCIO4 on
porous Au at 0.01 v S K I ; tad = 5 min: (top) CV, first and second cycle after adsorption (negative-going potential scan starting at U a d ) ; (bottom) z = 26 ([CZH~]'+), first cycle after adsorption. (a) Liad = MSCV for d 1.00 v and (b) U a d = 1.20 v .
previously at Uad = 0.60 v remains on the surface. l%is residue is then oxidized completely to C02 in the first positive-going scan as demonstrated in Figure 8. The same behavior for adsorbate reduction was observed when the experiments were performed in sulfuric acid solution. Table 3 comprises a series of reduction as well as oxidation experiments at different adsorption potentials. The oxidation and reduction charge densities (eox and Qred) were determined independently as well as the charge density required to oxidize the remaining residue after the reduction cycle ((eo&,.). Figure 9 shows the corresponding plots of the electrochemical charges and the integrated ion currents for C02 ( d z = 44) and C2H2 ( d z = 26). The increase of (Qo& with Uad in Figure 9a is parallel to the increase of the ion current for d z = 44 after reduction (CO& in Figure 9b) and is accompanied by an increase of electrodesorbed C2H2 between Uad = 0.40 and 0.60 V. This signal has a maximum at Uad = 0.60 V and decreases at higher adsorption potentials. A possible explanation for this observation could be the formation of species having higher oxidation states with increasing adsorption potential. Accordingly, the electroreduction of these adsorbates to initial ethyne is inhibited for Uad > 0.60 V. Further evidence for the possible formation of oxygen-containing species at higher adsorption potentials will be given in the Discussion section. Electroreductionof the Ethyne Adsorbate in 0.1 M DCIOJ D20. The observation of an anodic transient during adsorption and the fact that the adsorbate is reduced by a faradaic reaction around 0.30 V yielding C2H2 clearly prove a more highly oxidized adsorption species compared to the initial ethyne molecule. In order to obtain more information on the nature
13252 J. Phys. Chem., Vol. 99, No. 35, 1995
Schmidt and Pastor
a N
-0.04
L!
0
a
300
\
-0.12
-0.20
I
U
I
/
\
200
100
0
b miz = 44
601 \
0.0
0.4
0.8
I .2 I .6 PotentiallV vs RHE
Figure 8. Electrooxidationof adsorbed ethyne remaining on the surface after reduction as in Figure 6 in 0.1 M HC104 on porous Au; scan rate = 0.01 V s-I; Uad = 0.60 V: (a) CV, first and second positive-going z = 44, potential scan after reduction; (b) corresponding MSCV for d first positive-going scan. of the adsorbate, adsorption experiments using isotopic labeling were performed. Ethyne was adsorbed first at 0.75 V in 0.1 M HC10&20, and then the supporting electrolyte was replaced by a pure solution containing 0.1 M DClOl in D20. In the following first negative-going potential scan (see Figure lo), ion currents for d z = 27 and 28 were observed simultaneously with the faradaic current in the CV. Both signals reveal nearly the same intensity. The peak height for d z = 28 is higher than the signal in Figure 6 by a factor of 2. A new signal for m/z = 27 was observed which was not apparent in HC104/H20. On the other hand, no signal for d z = 26 could be obtained owing to C2H2. Therefore, the mass signals obtained in DC104/ D20 are assigned to [C2HD['+ (dz = 27) and [C2D2]'+ (dz = 28). A detailed interpretation of these findings will be given in the Discussion section. Reactivity of CO on Au and A U / ( C ~ H Z )In ~ ~order ~ . to compare the reactivity of ethyne on polycrystalline Au, we selected the electrooxidation of carbon monoxide as one of the most important systems for the electrocatalysis of organics on transition metals. The CV and MSCV show that the oxidation of CO dissolved in 0.1 M HC104 commences at about 0.50 V, yielding CO;, (see Figure 11). Thus the onset potential for CO oxidation is about 0.30 V more negative as compared to the onset of ethyne adsorbate oxidation (see Figure 2). In the potential range where AuO is formed, the oxidation current decreases while increasing again in the reverse potential scan when the reduction of AuO starts. Figure 12 shows FTIR spectra of dissolved CO in 0.1 M HC104 in the frequency region 2600-1800 cm-'. The positivegoing band at 2107 cm-' for U > 0.40 V is consistent with CO coordinated to a single Au atom.22 The observation of a bipolar band between 0.20 and 0.30 V indicates that, even at the reference potential at 0 V, CO should be partially adsorbed. The electrooxidation already starts at about 0.20 V, yielding '
401
20
0.0
0.2
0.4
0.6
Potential
0.8
/
1.0
1.2
1.4
V v s RHE
Figure 9. Oxidation and reduction of adsorbed ethyne previously formed at different adsorption potentials in 0.05 M H2S04: (a) Qox, oxidation charge density required to oxidize the adsorbate in the first positive-going potential scan directly after adsorption vs Uad (0);(Qo&, oxidation charge density of remaining species after reduction vs Liad (V);Qred, reduction charge density involved in the first negative-going scan vs Uad (m);(b) integrated ion current for d z = 44 (coz)during electrooxidation of the adsorbates vs Uad, directly after adsorption ( 0 ) or after reduction (V);integrated ion current for d z = 26 (C?H?)during electroreduction of the adsorbates vs L/ad(.). the corresponding negative-going band at 2343 cm-' associated with C02 formation. When adsorption experiments are performed in a similar way to that of C2H2, no adsorbate remains on the surface after electrolyte exchange. This is demonstrated in Figure 13 by means of an FTIRS experiment. The band at 2107 cm-' observed in CO-containing perchloric acid disappeared when the solution was replaced by pure acid. Thus the interaction of CO with Au is characterized by a reversible adsorption in contrast to the formation of a chemisorbate in the case of CrH2. The ethyne adsorbate is stable in an electrochemical sense in a potential range between 0.40 and 0.80 V without being oxidized or reduced. This potential window is available for the study of other electrochemical reactions on an ethynemodified Au electrode. In particular, the electrooxidation of dissolved CO offers an interesting system. Therefore, the following series of experiments was performed: CO is oxidized in HC104 at a constant potential of U = 0.75 V, yielding C02 (see Figure 14a). Then the electrolyte was replaced by pure perchloric acid followed by ethyne adsorption at the same potential. It should be emphasized that the highest ethyne coverage is attained in this potential range (see Figure 3). After renewed electrolyte exchange, CO was introduced and the current was recorded again. The result as presented in Figure 14b exhibits neither faradaic current nor CO? formation in the
Ethyne Adsorbed on Polycrystalline Au Electrodes
m/z = 21
J. Phys. Chem., Vol. 99, No. 35, 1995 13253 also Figure 3a). As mentioned above, the oxidation charge density determined for u a d < 0.60 V is correlated with the adsorbate coverage. It should be mentioned in this respect that the number of electrons required to oxidize one adsorbate molecule to c02 is assumed to be independent of U a d in this potential range. Nevertheless, the lower values for Qox in sulfuric acid can be associated with specific adsorption of HS04-/S0d2- species commencing at U > 0.20 V.23.24 On the other hand, the same onset potential for adsorbate oxidation is observed in sulfate and perchloric solution as well as the same peak potential for identical Uad, indicating no influence of the anion. It can be concluded that the adsorption strength of ethyne at U a d > 0.60 V is distinctly higher than anion adsorption (including OH-), suggesting the order C2H2 >> HS04-/S042- > OH- > C104-.'6 The adsorption of ethyne on Au for both electrolytes studied modifies the electrode/ electrolyte interface, thus providing the preferential coadsorption of OH- species necessary for the oxidation of the adsorbate to
coz.
0.0 0.2 0.4 0.6 0.8 PotentialiV vs RHE
Figure 10. Electroreduction of adsorbed ethyne in 0.1 M DClOdD20 on porous Au at 0.01 V s-'; Uad = 0.75 V. CzH2 was previously adsorbed for 5 min in 0.1 M HClOdHzO: (a) CV, first and second cycle after adsorption (negative-going potential scan starting at Uad); (b) MSCV for d z = 27 ([C2HD]'+) and d z = 28 [CzD$+), first cycle after adsorption.
For U a d < 0.30 V negative adsorption currents are observed (Table 1). These adsorption potentials are more negative than the potential of adsorbate reduction which was observed at approximately 0.30 V (see Figures 6 , 7, and 10). We assume that the negative charge densities are associated with an adsorption process rather than with a bulk reduction reaction of dissolved ethyne. This suggestion can be confirmed by the fact that an adsorbate remains on the surface after electrolyte exchange. These species are oxidized in the first positive-going scan, revealing small currents in the CVs and traces of C02 formation in the MSCVs (see Figure 1 for U a d = 0.20 V). An interpretation of the nature of the adsorbate formed at these potentials seems to be rather speculative at this moment, and this aspect is not further considered for the purpose of the present paper. Two distinct steps in ethyne electrooxidation are perceptible, that is, the adsorption followed by the oxidation of the adsorbed intermediate. Using on-line mass spectrometry, C02 was found to be the sole reaction product for the direct oxidation of both dissolved and adsorbed C Z H ~Thus . ~ the adsorption process can be written as
-
C2H2 I .2 I .6 PotentiaUV vs RHE Figure 11. Electrooxidation of CO in 0.1 M HClOl (saturated solution) on a porous Au electrode; scan rate = 0.02 V s-I: (a) CV; (b) MSCV 0.0
0.4
0.8
for d z = 44.
mass spectrometer. This observation clearly indicates that the Au surface is completely blocked by the ethyne adsorbate, leading to the inhibition of CO electrooxidation.
Discussion The aim of the present work is to study the adsorption of ethyne and the surface reactions of the adsorbate on polycrystalline Au in acid solution. In order to obtain some mechanistic information, the charge densities involved during adsorption and oxidationlreduction will be discussed in combination with mass spectrometric data. For a defined adsorption potential in the range 0.20-0.60 V, nearly the same adsorption charge densities Qad were obtained in sulfuric and perchloric acid (see Tables 1 and 2 ) . However, oxidation charge densities required to oxidize the adsorbate completely to COz (eox) are somewhat higher in HC104 (see
+
[C2H~2-,,]ad xH+
+ xe- (0 < x < 2 )
(1)
assuming that the anodic current during adsorption is accompanied by proton abstraction where x represents the number of protons. The oxidation of the adsorbate should occur by a surface reaction with preadsorbed water molecules according to [C2H[Z-x)lad
+ 4(H20)ad
-
+
2 ~ 0 , (IO - X)H+
+ (IO - x)e-
(2)
Regarding the adsorption charge density Qad and the oxidation charge density Qox involved in eqs 1 and 2, x can be determined as follows:
(3) According to this relationship the abstraction of one proton should lead to Qox/Qad = 9 and for two protons to = 4. Tables 1 and 2 display values for Qox/Qad of about 4 for 0.30 V < Uad < 0.60 V in sulfuric acid and for 0.50 V < Uad < 0.70 V in perchloric acid. This suggests the abstraction of two protons during adsorption at these potentials. This result is in agreement with values of Qox/Qad = 4 obtained at U a d =
Schmidt and Pastor
13254 J. Phys. Chem., Vol. 99, No. 35, 1995 0.10
I
I
I
I
I
v
I
2500 2400 2300 2200 2100 2000
Wavenumber / cm" Figure 13. ITIR spectra of CO in 0.1 M HC104 on a smooth polycrystalline Au electrode at Uad = 0.04 V. Reference spectrum was collected at 0.00 V. Resolutrion = 8 cm-I; 256 scans; p-polarized light. (a) Spectrum was obtained in a saturated solution, (b) spectrum was obtained after adsorption at Uad followed by complete electrolyte exchange.
0.60 V on a smooth polycrystalline Au electrode as well as on Au(l1I), Au( 1lo), and A u ( ~ I O ) .However, ~ higher values were determined for Uad = 0.30-0.40 v (7.1 and 6.4, respectively) in HC104 (Table 2). According to eq 3 this is interpreted in terms of a mixture of adsorbate species formed by the abstraction of one and two protons. On the other hand, in the case of Qox/Qad 4 as observed for Uad = 0.70-0.80 V (see Tables 1 and 2), the number of abstracted protons x should become >2, which cannot correspond to the reaction sequence 1 and 2. This can be explained by the formation of adsorbates already containing oxygen, such as for example [C2HOIad, which are more stabilized than the C2H(2-,) adsorbed species. This suggestion is supported by examining the CV and MSCV curves in Figures 1 and 2 for Uad =- 0.60 V, revealing a positive shift of the peak potential for adsorbate oxidation with increasing wad. Furthermore, a shoulder appears at about 1.50 V which can be perceived in the MSCV for Uad = 0.70 V in sulfuric acid (see Figure lb) and for Uad = 0.80 V in perchloric acid (Figure 2b). This peak multiplicity gives evidence for the existence of various adsorbate
L
0
b 1
40
1
I
SO
1
I20
1
I
160
I
1
200
1
' 240 rime I s
Figure 14. Faradaic current and ion current for d z = 44 (COz) during CO oxidation at 0.75 V on porous Au in 0.1 M HCIOI: (a) clean Au surface; (b) Au surface modified by ethyne adsorbates. species being present at the surface which probably have different oxidation potentials. However, at u a d > 0.80 V the high adsorption charge densities measured are partially associated with the complete oxidation of dissolved ethyne to C02, thus leading to Qox/Qad 1 (Tables 1 and 2). The reaction sequence 1 and 2 proposed above for adsorption potentials below 0.60 V is corroborated by the reduction of the adsorbate to C2H2 identified by mass spectrometry. The reduction charge density Qred (see Table 3) is consumed according to the following reaction: [C2Ho-,,]a,
+ (2 - x)H+ + (2 - x)e- - C2H2( d z = 26) (4)
The protons necessary to form C2H2 orginate from the acid electrolyte as confirmed unequivocally by the reduction of [ C ~ H ( Z - in ~ )deuterated ]~~ perchloric acid:
C2HD (
d z = 27), C2D2( d z = 28) ( 5 )
Ethyne Adsorbed on Polycrystalline Au Electrodes yielding the isotopically labeled ethyne molecules H C W D and D C W D with approximately the same intensities (see Figure 10). According to the interpretation presented above, the amount of C2D2 should be higher. However, the apparently high intensity of the ion current for C2DH is explained by the incomplete electrolyte exchange during the adsorption experiment, leading to small amounts of H20/HC104 in the deuterated solution. The partial reduction of the adsorbate to C2H2 (compare Figures 6 and 8) and the values of Qox/Qad 4 at Uad = 0.700.80 V determined are interpreted in terms of the formation of oxygen-containing adsorbate species which cannot be reduced. A further approach in order to distinguish between these species and adsorbates such as ( C Z H )and ~ ~ (C2)ad is made with the ratio - (Qox)ar)/Qred in the last column of Table 3. This is a relationship similar to the ratio Qox/Qaddiscussed above, but only if one considers the adsorbate species which can be reduced to C2H2 in the first negative-going potential scan. In this respect, the value of (Qo,)arcorresponds to a charge density required to oxidize oxygen-containing species. Therefore, the relation (Qox - (Qox)ar)/Qred in Table 3 can be compared with Qox/Qadin Tables 1 and 2. Values for - (Qox)ar)/Qred decrease from 7.9 at Uad = 0.40 V to 3.6 at Uad = 0.90 V, indicating the presence of (C2H)adand (C2)ad according to eqs 1 and 2 even at high adsorption potentials. The observation of a small signal for d z = 28 during reduction of the adsorbate in HC10m20 (see Figure 6) indicates the formation of ethene in a parallel reaction pathway. This fact can be explained by a probable hydrogenation reaction of the adsorbate with adsorbed hydrogen atoms. Hydrogenation of propyn-1-01 on polycrystalline Au in acids was also observed in a previous paper.6 However, the coverage of (H)ad on Au is low, and the reactivity for hydrogenation reactions should be low as compared to when Pt is used. The reduction of ethyne on Pt single-crystal electrodes was recently studied using DEMS.25 After continuous cycling of the Au electrode in saturated ethyne solution, SERS spectra show vibration modes typical of polyethyne.I8 However, under the present experimental conditions no indication of polymer formation on the electrode could be found. Additionally, it seems to be unlikely that a complex polymer structure can be easily dismantled by reduction, forming initial ethyne monomers. Two interesting aspects arise when the electrooxidation of CO is considered with respect to ethyne. One of these is the interaction strength of an organic species with a metal electrode. An assessment can in principle be made by classifying them into two groups. First, a reversible adsorption is characterized by the experimental observation that the adsorbate is rinsed away after electrolyte exchange. For the second group of organics, described as having undergone irreversible adsorption, chemisorbed intermediates remain on the surface.26 According to this classification, ethyne clearly belongs to the latter. The interaction of CO with the Au surface indicates that this molecule is attached to the first group since no stable adsorbate could be found after electrolyte exchange, as confirmed by IR spectroscopy (Figure 13), clearly in contrast to CO on platin~m.~’ The second aspect is the coadsorption of CO and C2H2. The experimental results show a stronger interaction of ethyne than CO with gold. Nevertheless, it proves to be surprising that an adlayer of adsorbed ethyne completely inhibits the electrooxidation of dissolved CO (Figure 14). This can be interpreted in terms of geometric considerations. As reported previously, the ethyne adsorbate at Uad = 0.60 v (maximum coverage) requires
(eox
(eox
J. Phys. Chem., Vol. 99, No. 35, I995 13255 four sites on a porous DEMS electrode4 as well as on a smooth polycrystalline Au surface and on single-crystal faces.’ The same value is determined in the present paper at this potential regarding the oxidation charge densities in Tables 1 and 2. Although the small CO molecule is only coordinated to a single Au atom as confirmed by IR data (Figure 12), the ethyne adlayer occupies all active Au sites. On the other hand, this finding demonstrates that the oxidation of CO to C02 on a Au surface should occur via a weak bonding to at least one Au atom. Concluding Remarks Ethyne turns out to be an interesting model system for chemisorption studies on Au in acids. Surface oxidation/ reduction reactions at the Adelectrolyte interface can be analyzed by potential-controlled electrodesorption spectrometry by analogy to thermal desorption spectrometry at the solid/ vacuum interface. The interpretation of these on-line mass spectrometric data in combination with adsorption and oxidation/ reduction charge densities provide valuable information on the nature of the ethyne adsorbate. The experimental results and their interpretation are summarized as follows. (i) During ethyne adsorption an anodic current transient is observed for Uad > Up,,. (ii) The highest adsorbate coverage is obtained for Uad = 0.60-0.80 V. (iii) Adsorbed ethyne is oxidized completely in the first positivegoing potential scan, yielding C02 as the sole product. (iv) The adsorbate can be partially reduced depending on Uad, yielding C2H2 and small amounts of C2&. (v) The stoichiometry of the adsorbate layer predominantly formed at Uad -= 0.60 v consists of species such as (C2H)ad and (CZ)ad, as confirmed by reduction of the adsorbate in DC10@20. (vi) CO exhibits a weaker interaction with Au compared to C2H2. (vii) A layer of ethyne adsorbate completely inhibits the electrooxidation of CO on Au. Acknowledgment. E.P. thanks the Ministerio de Educacion y Ciencia (Spain) for a research fellowship. The authors are grateful to Professor Dr. W. Vielstich and Dr. T. Iwasita for their continual encouragement during this work. References and Notes (1) Lipkowski, J., Ross, P. N., Eds. Adsorption ofMolecules at Metal Electrodes; VCH: Weinheim, 1992. (2) Lipkowski, J., Ross. P. N., Eds. Structure of Electr8ed Interfaces;
VCH: Weinheim, 1993. (3) Wolter, 0.;Heitbaum, J. Ber. Bunsen-Ges. Phys. Chem. 1984.88, 2. (4) Schmidt, V. M.; Pastor, E. J . Electroanal. Chem. 1994, 369, 271; 1994, 376, 65. ( 5 ) Schmidt, V. M.; Pastor, E. J . Electroanal. Chem., in press.
(6) Pastor, E.; Schmidt, V. M.; Iwasita, T.; Arevalo, M. C.; Gonzalez, S.; Arvia, A. J. Electrochim. Acta 1993, 38, 1337. (7) Schmidt, V. M.; Stumper, J.; Schmidberger, J.; Pastor, E.; Hamelin, A. SUI$ Sci. In press. (8) Soriaga, M. P. In Structure of Electrijied Interfaces; Lipkowski, J., Ross, P. N., Eds.: VCH: Weinheim. 1993; p 103. (9) Lipkoswki, J.; Stolberg, L.: Yang, D.-F.; Pettinger, B.; Minvald, S.; Henglein, F.: Kolb, D. M. Electrochim. Acta 1994, 39, 1045. ( I O ) Stephen, H.; Stephen, T. Solubilities of Inorganics and Organic Species, Vol. I ; Pergamon Press: Oxford, 1963. (1 1) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A,; Stoicoviciu, L. Electrochim. Acta 1986, 31, 1951. (12) Bittins-Cattaneo, B.; Cattaneo, E.; Konigshoven, P.; Vielstich, W. In Electroanalytical Chemistry: A Series of Advances, Vol. 17; Bard, A,, Ed.; Marcel Dekker: New York, 1991; p 181. (13) Nart, F.C.; Polligkeit, H.: Iwasita, T. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 638. (14) Iwasita, T.; Rasch, B.; Cattaneo, E.; Vielstich, W. Electrochim. Acta 1989, 34, 1073. (15) Clavilier, J.; Van Huong, C. N. J . Electroanal. Chem. 1977, 80, 101.
13256 J. Phys. Chem., Vol. 99, No. 35, 1995 (16) Borkowska, Z.; Stimming, U. J . Electroanal. Chem. 1991, 312, 209. (17) Herzberg, G. Molecular Spectra and Molecular Structure Vol. 11: Infrared and Raman Spectra of Polyatomic Molecules: Van Nostrand Reinhold: New York, 1975: p 288. (18) Patterson, M. L.: Waever, M. J. J . Phys. Chem. 1985, 89, 5046. (19) Feilchenfeld, H.; Weaver, M. J. J. Phys. Chem. 1989, 93, 4276. (20) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107. (21) Stenhagen, E., Abrahamsson, S., McLafferty, F. W., Eds. Atlas of Mass Spectral Data; Interscience Publishers: New York, 197 1. (22) Chang, S. C.; Hamelin, A.: Weaver, M. J. J . Phys. Chem. 1991, 95, 5560.
Schmidt and Pastor (23) Shi, Z.; Lipkowski, J.: Gamboa, M.; Zelenay, P.; Wieckowski, A. J . Electroanal. Chem. 1994, 366, 317. (24) Edens, G. J.: Gao, X.; Weaver, M. J. J . Elecrroanal. Chem. 1994, 375, 357. (25) Gao, Y.: Tsuji, H.; Hattori, H.: Kita, H. J. Electroanal. Chem. 1994, 372, 195. (26) Michelhaugh. S. L.; Bhardwaj, C.; Cali, G. J.; Bravo, B. G.: Bothwell, M. E.; Berry, G. M.: Soriaga, M. P. Corrosion 1991, 95, 7771. (27) Stanners. C. D.: Gardin, D.; Somorjai, G. A. J . Electrochem. SOC. 1994, 141, 3278. Jp95 1 150C
