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The adsorption of methylamine on Pt single crystal surfaces
Journal of Electroanalytical Chemistry 467 (1999) 105 – 111
The adsorption of methylamine on Pt single crystal surfaces F. Huerta, E. Morallo´n, C. Quijada, J.L. Va´zquez *, J.M. Pe´rez, A. Aldaz Departamento de Quı´mica Fı´sica, Uni6ersidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received 13 August 1998; received in revised form 13 November 1998; accepted 8 January 1999
Abstract The irreversible adsorption of methylamine at platinum single-crystal electrodes has been studied by a combination of electrochemical and in situ spectroscopic techniques. From purely electrochemical data, it has been stated that the adsorption of CH3NH2 molecules results in significant alteration of the Pt(h,k,l) adsorption capabilities. Voltammetric experiments show that the methylamine adlayer undergoes different surface processes depending on the orientation of the metallic substrate. This result contrasts intensely with the uniformity of the main product obtained from the open-circuit adsorption procedure, which has been identified as adsorbed cyanide for all surfaces. As concluded from the spectroscopic results, the interaction of methylamine with the platinum surfaces leads to the dehydrogenation (probably full dehydrogenation) of the molecule and to the formation of an adsorbed cyanide adlayer, which exhibits different surface reactivity on each basal surface. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Methylamine; Adsorbed cyanide; Platinum; Single crystal electrodes; FTIR spectroscopy; Cyclic voltammetry
1. Introduction Over the last 20 years, the study of the metal electrolyte interface has attracted the interest of an increasing number of electrochemists. An improved understanding of the influence of the electrode microstructure on the activity of many electrocatalytic metals has been possible because of the development of new methods to obtain clean and reproducible surfaces [1,2]. This and the successful combination of purely electrochemical with in situ spectroscopic techniques such as Fourier transform infrared spectroscopy (FTIRS), sum frequency generation (SFG) or surface enhanced Raman spectroscopy (SERS) has allowed the investigation of the interface at a molecular level. Among the more used spectroscopic techniques, FTIRS has contributed particularly to this purpose.
Dedicated to Jean Clavilier on the occasion of his retirement from LEI CNRS and in recognition of his contribution to Interfacial Electrochemistry. * Corresponding author. Fax: +34-6-590-3537. E-mail address: [email protected] (J.L. Va´zquez)
The adsorption and oxidation of amino compounds has been the subject of much interesting work in the field of electrocatalysis [3–7]. However, to our knowledge, the behaviour of methylamine on platinum singlecrystal electrodes has not yet been described. To date, studies on CH3NH2 have been limited either to polycrystalline Pt [3] and Au [6] surfaces in an electrochemical environment or to well-defined surfaces such as Ni(111) [8], Rh(111) [9], Pd(111) [10], Pt(111) [11] and Pt(100) [12] under ultra-high vacuum (UHV) conditions. In situ radiotracer results [3] have shown that in an alkaline medium methylamine behaves similarly to glycine (H2N–CH2 –COOH) and cyanide ions on Pt electrodes, all of them generating adsorbed residues which cannot be removed from the surface by reductive treatment. On the contrary, these adsorbates can be fully eliminated by reduction in acidic medium. The nature of the radiotracer techniques makes it difficult to assure the actual structure of the adsorbed species, but it seems that at least the C–N bond remains intact upon adsorption.
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UHV results on the adsorption of gaseous methylamine at Pt(111) surfaces demonstrated that this orientation is able to decompose the molecule to produce HCN, C2N2 and H2 as the main gaseous components. In addition, adsorbed species such as Hx CN and (CN)x seem to be formed, but no C – N dissociation occurs [11]. On the other hand, methylamine decomposes on the Pt(100) surface to yield HCN, C2N2 and substantial quantities of N2 (about 30% molecules of methylamine produce N2) [12]. Thus, Pt(100) exhibits higher activity to break the C–N bond of methylamine than Pt(111). On Pd(111) [10] methylamine dehydrogenates progressively at the metal surface to generate adsorbed cyanide, but the C–N bond remains intact. The adsorption structure of cyanide, as suggested by UHV results, shows the C–N bond parallel to the surface. In contrast, in an electrochemical environment a perpendicular or tilted geometry has been always reported [13,14]. The aim of the present work is 2-fold: to analyse the electrochemical response of the residues formed from the open-circuit adsorption of methylamine and to identify the adsorbed (or solution) species involved during the oxidation/reduction of these residues. Taking into account the possibility that the processes under inspection were structure-sensitive, the study will contemplate results for the three basal planes of platinum.
2. Experimental Pt single crystal electrodes were prepared according to the method developed by Clavilier et al. [1,2]. The surfaces were thermally cleaned in an air+propane flame and cooled in a H2 +Ar stream [15]. Then, the electrode surface was protected from the laboratory atmosphere by a droplet of ultrapure water saturated with the cooling gases. The open-circuit adsorption procedure was the following: firstly, the oriented surface was immersed in a 0.1 M CH3NH2 solution (Merck p.s) for 2 – 3 min. After that, it was rinsed with ultrapure water and, subsequently, transferred to the working solution (0.1 M HClO4, Merck suprapur) which was free of methylamine. All experiments were performed at room temperature. The water employed in the experiments was obtained from a Millipore – MilliQ plus system, which supplies it with a resistivity close to 18.2 MV cm. Cyclic voltammograms were recorded in a classical non-divided electrochemical cell, at a constant sweep rate of 50 mV s − 1. Potentials were measured against a reversible hydrogen electrode (RHE). A Nicolet Magna 850 spectrometer equipped with a liquid nitrogencooled mercury cadmium telluride (MCT) detector was employed for the in situ FTIR measurements. The spectroelectrochemical cell was made of glass and was
provided with a prismatic CaF2 window bevelled at 60°. Spectra were collected with a resolution of 8 cm − 1 and are presented in the usual form DR/R.
3. Results and discussion A first step in a fundamental study on the behaviour of methylamine at Pt(h,k,l) electrodes should prove whether or not this compound adsorbs on the surface. If the compound adsorbs, it would be necessary to test the kind of adsorption that takes place. Methylamine molecules could adsorb intact or suffer chemical bond cleavage during their interaction with the surface. It is known that the dissociative adsorption of many organic compounds results in the formation of different adsorbed species. The so-called poisons modify strongly the adsorption properties of the electrode, blocking a significant number of surface active sites.
3.1. Pt(111) The voltammetric profile recorded for a Pt(111) electrode after the open-circuit adsorption of methylamine is shown in Fig. 1a. This profile remains stable during a few cycles, but a continuous sweeping in the whole range of potential employed gives rise to the almost complete removal of the adlayer. A comparison between this voltammogram and that recorded for the bare Pt(111) in a clean 0.1M HClO4 solution (Fig. 1b) reveals interesting differences. On the one hand, the adsorption states between 0.06 and 0.45 V appear modified for the electrode covered with methylamine. A loss of part of the voltammetric charge associated with the adsorption of hydrogen is observed (about 40% hydrogen sites seem blocked). In addition, a positive potential shift of this process seems to occur. On the other hand, two very broad anodic peaks appear in the potential region between 0.7 and 1.3 V, the first centred at 0.85 V and the second at 1.22 V. The latter peak
Fig. 1. Cyclic voltammograms recorded in a 0.1 M HClO4 test solution. (a) Pt(111) electrode after methylamine adsorption at opencircuit. (b) Clean Pt(111).
F. Huerta et al. / Journal of Electroanalytical Chemistry 467 (1999) 105–111
Fig. 2. FTIR spectra collected for the Pt(111) surface, after the adsorption of methylamine, at the sample potentials indicated. The reference potential was 0.2 V. p-Polarised light. 500 Scans.
seems to be related to the surface oxidation process that the Pt(111) electrode undergoes under the sharp peak at about 1V in a perchloric acid solution (see Fig. 1b). Regarding the first peak, one can notice its high reversibility with the cathodic peak appearing between the same potential limits. The voltammetric profile described above is very similar to that recorded for a cyanide-covered Pt(111) surface in perchloric medium, in which the reversible voltammetric wave centred at 0.85 V was ascribed to the adsorption/desorption of hydroxyl species [16]. Relying on purely voltammetric experiments it is difficult to determine the nature of the adsorbed species coming from the adsorption of methylamine, but the formation of a cyanide adlayer can be postulated. This assumption will be checked spectroscopically below although it is in close agreement with the previous studies on the adsorption of CH3NH2 in the UHV environment. Under our experimental conditions, the coverage achieved by the adsorbed cyanide derived from methylamine is lower than that obtained from the adsorption of cyanide ions and this fact could be the reason for the lower stability of the adlayer. Fig. 2 shows a set of FTIR spectra for the methylamine-covered Pt(111) electrode. After the open-circuit adsorption the electrode was rinsed with ultrapure water and then immersed in the test solution at 0.2 V. Subsequently, the electrode surface was pressed against the CaF2 window and the reference spectrum was acquired at this potential. The applied potential was then systematically stepped in the positive direction up to the maximum value of 0.9 V and sample spectra were collected for every step. Fig. 2 shows the computed transmittance spectra for some of the sample potentials. At 0.5 V, only a negative-going band centred at 2105
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cm − 1 appears in the spectrum. The frequency of this band coincides with that observed for the CN stretching vibration of cyanide adsorbed molecularly on Pt(111) surfaces in acidic medium [16]. When the applied potential reaches 0.7 V, the band becomes bipolar and, simultaneously, a small absorption band appears at 2345 cm − 1. The intensity of this band increases at 0.9 V. This feature is related to the presence of small amounts of dissolved CO2, probably arising from slight oxidation of cyanide. The observation of a negative character in the band at around 2100 cm − 1 for the spectrum taken at 0.5 V could indicate a certain overlapping of the absorption bands for the sample and reference spectra at this potential. It is worth noting that the adsorbed cyanide remains at the surface at the reference potential during the whole experiment (note the bipolar character of the bands at 0.7 and 0.9 V). Similar behaviour has been reported for carbon monoxide adsorbed on platinum electrodes, see Ref. [17] and references cited therein. The results presented support the voltammetric assignment suggested above and, consequently, it might be accepted that methylamine interacts at open-circuit with the Pt(111) surface yielding a cyanide adlayer. As no absorption bands attributable to N–H vibrations can be distinguished in the spectra, we assume that methylamine undergoes a full dehydrogenation reaction during its adsorption on Pt(111) to generate CNads.
3.2. Pt(100) We will now analyse the behaviour of the adsorbed residues formed during the open-circuit dosage of methylamine on Pt(100) electrodes. Voltammograms recorded for the CH3NH2-covered surface are presented in Fig. 3a and the blank voltammogram for the Pt(100) in a 0.1 M HClO4 solution, in Fig. 3b. After the adsorption procedure at open-circuit, the electrode was immersed in the test solution at 0.4 V and then polarised to less positive potential values. The lower potential limit was 0.06 V while the upper limit was set firstly at 0.4 V. In this way, the initial oxidation of the adlayer can be avoided and it is possible to check the blockage of the adsorption sites from the voltammetric profile. This experiment is shown in the inset to Fig. 3. It can be observed that a broad cathodic peak (labelled as peak a) appears during the initial sweep down to 0.06 V and its peak current decreases abruptly in the subsequent cycle. At the same time, the voltammetric charge between 0.06 and 0.4 V grows slowly by cycling in this potential interval. These results point to the reductive elimination of adsorbed species from the electrode surface and to the simultaneous liberation of adsorption sites. However, one should note the differences in the voltammetric charge below 0.3 V for this voltammetric profile and that for the blank (Fig. 3b). Part of the
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adsorption processes that take place above 0.3 V for the bare surface could be shifted towards less positive potentials. From the inspection of this profile, it can be concluded that significant amounts of adsorbed species, derived from methylamine, remain at the surface. After a steady voltammogram was reached in the potential region below 0.4 V, the upper potential limit was shifted up to 0.9 V to check the possibility of removing the adlayer by oxidation. This experiment is shown in Fig. 3a (main figure). During the first sweep up to 0.9 V (solid line), two partially overlapped anodic peaks, centred at 0.69 V (peak b) and 0.78 V (peak c), can be clearly distinguished. The first one lies in the potential region for the oxidation of adsorbed CO obtained at low coverage [18], while the latter appears always associated with the sharp cathodic peak at about 0.6 V (peak d) in the subsequent negative-going sweep. During the sweep towards less positive potentials, the cathodic peak a arises again at around 0.2 V (see the inset to Fig. 3). This result strongly suggests that the species which gives rise to the peak a is produced in the oxidation of some adsorbate above 0.5 V and can be reduced below 0.4 V. The voltammetric profile described here is very similar to that recorded during the oxidation of a cyanide adlayer on Pt(100) electrodes [16]. Cyanide adlayers formed on these substrates are very reactive. They oxidise to adsorbed nitric oxide and carbon dioxide above 0.6 V or produce adsorbed carbon monoxide below 0.4 V. It is worth mentioning that when nitric oxide is present on Pt(100) substrates at moderate coverage, the voltammogram
Fig. 3. Cyclic voltammograms recorded for a Pt(100) electrode immersed in a 0.1 M HClO4 test solution. (a) After methylamine adsorption at open-circuit. Solid line: first potential sweep up to 0.9 V. Dashed line: second sweep. Inset: Evolution of the voltammetric profile between 0.06 and 0.4 V. (b) Clean Pt(100).
Fig. 4. FTIR spectrum for the Pt(100) covered with the residues coming from methylamine. The spectrum was acquired after potential cycling in the 0.45 – 0.1 V range. Sample potential 0.45 V. Reference 0.1 V. 500 Scans. p-Polarised light
recorded between 0.5 and 0.9 V displays a characteristic pair of peaks. These peaks are very similar to those labelled c and d in Fig. 3a. On the other hand, the elimination of considerable amounts of adsorbed species can be clearly observed from the voltammetric profile of the second cycle, although the adsorption states above 0.4 V still seem blocked. During this sweep, the anodic peaks b and c lose a great part of their peak intensity and the same occurs for the cathodic peaks a and d. The recovery of most of the adsorption capabilities of the Pt(100) substrate is effective in a few cycles (not shown). These results reveal that the voltammetric response of the adsorbed species coming from methylamine at Pt(100) electrodes is rather similar to that of adsorbed cyanide. From this information, it can be pointed that the open-circuit adsorption of the amine generates adsorbed CN, which would subsequently suffer different surface reactions as a function of the applied potential. To clarify this point, new open-circuit adsorption experiments were performed and the surface probed by means of in situ FTIR spectroscopy. Fig. 4 shows the spectrum acquired after the Pt(100), covered with the residues formed at open-circuit, was cycled in the potential region between 0.1 and 0.45 V. Reference interferograms were collected at 0.1 V while those corresponding to the sample, at 0.45 V. This experiment allows the voltammetric profile of the inset to Fig. 3 to be related with the current spectrum. Thus, the negativegoing absorption band, which appears at 2083 cm − 1, is clearly related to the C–N stretching of adsorbed cyanide. The presence of CNads at the surface implies the full dehydrogenation of methylamine molecules, as was found for the Pt(111) substrate. On Pt(111) cyanide
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was the unique adsorbed species detected by FTIR in the whole range of potentials. However, this spectrum shows two more features, both ascribable to adsorbates different from CNads. On the one hand, the intense bipolar band centred at 1830 cm − 1 is characteristic of the C–O stretching of carbon monoxide in either a bridge or multibonding adsorption geometry. On the other hand, the positive-going band at 2005 cm − 1 seems related with the presence of linearly bonded CO at the reference potential. Taking into account that at these potentials CO adlayers are rather stable, the absence of linearly bonded CO at 0.45 V is not easy to explain. An eventual potential-induced conversion of bridge to linearly bonded CO at 0.1 V could justify the positive character of the band at 2005 cm − 1, although this kind of transformation only takes place under certain conditions. The site occupancy of adsorbed CO at the Pt(100) surface is dependent on both coverage and electrode potential [19,20]. Shifting the potential to more positive values results in a conversion from bridge to linearly bonded CO, although such a conversion does not take place by decreasing the applied potential. Moreover, as the spectra were collected alternately between 0.1 and 0.45 V, the opposite conversion (COL to COB) should be fast enough to avoid the observation of bipolar character in the absorption band at ca. 2000 cm − 1. Even in this case, the feature for bridge bonded CO (around 1830 cm − 1) should present significant negative character. A better explanation may be that of a surface reaction of CN to yield adsorbed CO at low potentials. This reaction has been already observed for cyanide adlayers formed from the open-circuit adsorption of cyanide molecules on Pt(100) [16]. Let us assume that cyanide adsorbs in clusters at the Pt(100) surface (as has been reported for Pt(111) electrodes by means of scanning tunnelling microscopy [21,22]); if the geometry of adsorbed cyanide is preserved for adsorbed carbon monoxide during the surface reaction CN CO, the expected CO resulting species will be linearly bonded due to its high local coverage. Then, CO molecules could diffuse on the surface to accommodate in a less energetic adsorption geometry at low coverages, i.e. the bridge or multibonded one. In a new experiment, after methylamine was adsorbed at the Pt(100) surface, the electrode was immersed into the spectroelectrochemical cell at 0.5 V to avoid the initial reduction of the adsorbed species. Subsequently, the potential was cycled between 0.5 and 0.9 V until a steady voltammetric profile was obtained. Then, the electrode surface was pressed against the window and reference (0.5 V) and sample (0.9 V) spectra were alternately collected in groups of 100 interferograms. Fig. 5 shows the resulting FTIR spectrum, in which the presence of three absorption bands is clearly noted. The negative-going band at 2345 cm − 1 is assigned to dissolved CO2 while the bipolar one
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centred at around 2100 cm − 1 corresponds to the C–N stretching of adsorbed cyanide. On the other hand, the frequency of the absorption band at 1603 cm − 1 lies in the frequency region of the N–O vibrations in nitrosylmetal complexes [23]. This band was already assigned to adsorbed nitric oxide, which was generated during the oxidation of a cyanide adlayer at a Pt(100) surface [16]. This fact and the observation of the characteristic voltammetric profile in Fig. 3 make it possible to ascribe the vibrational band at around 1600 cm − 1 to the presence of adsorbed NO. As concluded from these results, the cyanide adlayer produced during the opencircuit adsorption of methylamine is also oxidised at the Pt(100) surface. This oxidation yields carbon dioxide and adsorbed nitric oxide, which remains at the surface unless the potential reaches values below 0.3–0.4 V. The adsorbed NO may be reduced (peak a in Fig. 3a) to some N-containing species, probably in the form of NH4+ [24].
3.3. Pt(110) Fig. 6a shows cyclic voltammograms obtained for a Pt(110) electrode after the open-circuit dosage of methylamine. The adsorption properties of this surface are influenced by the presence of adsorbed species derived from methylamine. The first potential cycle (solid line) reveals that the most noticeable difference from the bare electrode (which voltammetric response is presented in Fig. 6b) arises from the blockage of the adsorption states between 0.2 and 0.3 V. Also, little modification of the hydrogen adsorption/desorption process below 0.2 V can be noticed. Part of this process is recovered during the following sweeps, but the voltammetric profile remains altered. This could indi-
Fig. 5. FTIR spectrum the Pt(100) covered with the residues coming from methylamine. The spectrum was acquired after potential cycling in the 0.9 – 0.5 V range. Sample potential 0.9 V. Reference 0.5 V. 500 Scans. p-Polarised light.
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Fig. 6. Cyclic voltammograms obtained for a Pt(110) electrode in a 0.1 M HClO4 test solution. (a) After methylamine adsorption at open-circuit. Solid line: first potential sweep up to 0.85 V. Dashed line: fifth sweep. (b) Clean Pt(110).
Fig. 7. FTIR spectra collected for the Pt(110) electrode after the open-circuit adsorption of methylamine. Sample potentials are indicated in each spectrum. The reference was acquired at 0.05 V. p-Polarised light. 200 Scans.
cate either the presence of adsorbed residues on the surface or the induction of surface defects in the metal substrate. The alteration of well-defined electrode surfaces by adsorbed residues has been already detected, among others, for CN adsorbed on Pt(110) [14], CO adsorbed on Pt(110) [25] or SO2 on Pt(111) [26]. The Pt(110) orientation seems much less active than Pt(100) for the oxidation or reduction of the adsorbed species coming from the adsorption of methylamine. The voltammetric profile shown in Fig. 6a is rather similar to that recorded after the open-circuit adsorption of cyanide on the same surface [16]. If CNads was also the species formed on Pt(110) after the open-circuit contact of methylamine, this species may desorb when the electrode is polarised below 0.1 V, as was found for cyanide ions [16]. In situ FTIR experiments confirm that adsorbed cyanide is the main species present at the Pt(110) surface. Fig. 7 shows spectra in the 1200 – 2500 cm − 1 range obtained after the adsorption of methylamine on this surface. The covered electrode was immersed in the test solution at 0.2 V to avoid an eventual desorption of adsorbed species. It was subsequently pressed against the window and sample interferograms were acquired from 0.3 to 0.8 V. The reference was taken at 0.05 V after all sample spectra were collected. The negative-going band at around 2100 cm − 1 is clearly related with the C–N stretching of adsorbed cyanide. Its negative character indicates the absence of cyanide at the reference potential. No other bands ascribable to adsorbed species can be discerned in the spectral range studied.
produces adsorbed cyanide as the main adsorbed species. This accounts for the considerable modification in the adsorption capabilities of the Pt(h,k,l) and suggests that a full dehydrogenation of the amine is the fundamental process occurring at the electrode surface under open-circuit conditions, independently of its orientation. Spectroscopic evidence for the presence of non-dissociated methylamine molecules on the surfaces were not found. The reactivity of the resulting cyanide adlayer strongly depends on the crystallographic orientation of the substrate. Pt(111) and Pt(110) preserve the C–N bond, as was reported in UHV experiments. Also in agreement with UHV results, the Pt(100) orientation shows higher catalytic activity to break the C–N bonds. This has made it possible to observe a CO adlayer at potentials below 0.5 V and a NO adlayer above 0.5 V, both arising from the surface reaction of adsorbed cyanide. The tendency of the amino group to dehydrogenate on platinum surfaces has been described previously [7,27]. Nevertheless, this observation is opposite to the behaviour shown by ammonia, which is rather stable on the same substrates [28,29]. The substitution of one hydrogen atom by a methyl group in the molecule of ammonia produce higher reactivity of the N–H bonds in the resulting methylamine species. This chain effect cannot be extrapolated to the following amine (ethylamine), for which poor reactivity on Pt(h,k,l) electrodes has been observed [30].
4. Conclusions
Acknowledgements
The interaction of methylamine molecules with Pt(h,k,l) surfaces in an electrochemical environment
Francisco J. Huerta thanks the Generalitat Valenciana for the award of his FPI grant.
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[15] A. Rodes, M.A. Zamarkhchari, K. El Achi, J. Clavilier, J. Electroanal. Chem. 305 (1991) 115. [16] F. Huerta, E. Morallo´n, J.L. Va´zquez, A. Aldaz, Surf. Sci. 396 (1998) 400. [17] E. Morallo´n, A. Rodes, J.L. Va´zquez, J.M. Pe´rez, J. Electroanal. Chem. 391 (1995) 149. [18] L. Palaikis, D. Zurawski, M. Hourani, A. Wieckowski, Surf. Sci. 199 (1988) 193. [19] S.C. Chang, M. J. Weaver, Surf. Sci. 238 (1990) 142. [20] F. Kitamura, M. Takahashi, M. Ito, J. Phys. Chem. 92 (1988) 3320. [21] Y. Kim, S. Yau, K. Itaya, J. Am. Chem. Soc. 118 (1996) 729. [22] C. Stuhlmann, I. Villegas, M.J. Weaver, Chem. Phys. Lett. 219 (1994) 319. [23] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, NY, 1986. [24] A. Rodes, R. Go´mez, J.M. Pe´rez, J.M. Feliu, A. Aldaz, Electrochim. Acta 41 (1996) 719. [25] E. Morallo´n, J.L. Va´zquez, R. Du´o, A. Aldaz, Surf. Sci. 265 (1992) 95. [26] C. Quijada, J.L. Va´zquez, J.M. Pe´rez, A. Aldaz, J. Electroanal. Chem. 372 (1994) 243. [27] G. Hora´nyi, E.M. Rizmayer, J. Electroanal. Chem. 198 (1986) 393. [28] G.B. Fisher, Chem. Phys. Lett. 79 (1981) 452. [29] I. Villegas, M.J. Weaver, Surf. Sci. 367 (1996) 162. [30] F. Huerta, PhD Thesis, University of Alicante, 1999.
The adsorption of methylamine on Pt single crystal surfaces F. Huerta, E. Morallo´n, C. Quijada, J.L. Va´zquez *, J.M. Pe´rez, A. Aldaz Departamento de Quı´mica Fı´sica, Uni6ersidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received 13 August 1998; received in revised form 13 November 1998; accepted 8 January 1999
Abstract The irreversible adsorption of methylamine at platinum single-crystal electrodes has been studied by a combination of electrochemical and in situ spectroscopic techniques. From purely electrochemical data, it has been stated that the adsorption of CH3NH2 molecules results in significant alteration of the Pt(h,k,l) adsorption capabilities. Voltammetric experiments show that the methylamine adlayer undergoes different surface processes depending on the orientation of the metallic substrate. This result contrasts intensely with the uniformity of the main product obtained from the open-circuit adsorption procedure, which has been identified as adsorbed cyanide for all surfaces. As concluded from the spectroscopic results, the interaction of methylamine with the platinum surfaces leads to the dehydrogenation (probably full dehydrogenation) of the molecule and to the formation of an adsorbed cyanide adlayer, which exhibits different surface reactivity on each basal surface. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Methylamine; Adsorbed cyanide; Platinum; Single crystal electrodes; FTIR spectroscopy; Cyclic voltammetry
1. Introduction Over the last 20 years, the study of the metal electrolyte interface has attracted the interest of an increasing number of electrochemists. An improved understanding of the influence of the electrode microstructure on the activity of many electrocatalytic metals has been possible because of the development of new methods to obtain clean and reproducible surfaces [1,2]. This and the successful combination of purely electrochemical with in situ spectroscopic techniques such as Fourier transform infrared spectroscopy (FTIRS), sum frequency generation (SFG) or surface enhanced Raman spectroscopy (SERS) has allowed the investigation of the interface at a molecular level. Among the more used spectroscopic techniques, FTIRS has contributed particularly to this purpose.
Dedicated to Jean Clavilier on the occasion of his retirement from LEI CNRS and in recognition of his contribution to Interfacial Electrochemistry. * Corresponding author. Fax: +34-6-590-3537. E-mail address: [email protected] (J.L. Va´zquez)
The adsorption and oxidation of amino compounds has been the subject of much interesting work in the field of electrocatalysis [3–7]. However, to our knowledge, the behaviour of methylamine on platinum singlecrystal electrodes has not yet been described. To date, studies on CH3NH2 have been limited either to polycrystalline Pt [3] and Au [6] surfaces in an electrochemical environment or to well-defined surfaces such as Ni(111) [8], Rh(111) [9], Pd(111) [10], Pt(111) [11] and Pt(100) [12] under ultra-high vacuum (UHV) conditions. In situ radiotracer results [3] have shown that in an alkaline medium methylamine behaves similarly to glycine (H2N–CH2 –COOH) and cyanide ions on Pt electrodes, all of them generating adsorbed residues which cannot be removed from the surface by reductive treatment. On the contrary, these adsorbates can be fully eliminated by reduction in acidic medium. The nature of the radiotracer techniques makes it difficult to assure the actual structure of the adsorbed species, but it seems that at least the C–N bond remains intact upon adsorption.
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UHV results on the adsorption of gaseous methylamine at Pt(111) surfaces demonstrated that this orientation is able to decompose the molecule to produce HCN, C2N2 and H2 as the main gaseous components. In addition, adsorbed species such as Hx CN and (CN)x seem to be formed, but no C – N dissociation occurs [11]. On the other hand, methylamine decomposes on the Pt(100) surface to yield HCN, C2N2 and substantial quantities of N2 (about 30% molecules of methylamine produce N2) [12]. Thus, Pt(100) exhibits higher activity to break the C–N bond of methylamine than Pt(111). On Pd(111) [10] methylamine dehydrogenates progressively at the metal surface to generate adsorbed cyanide, but the C–N bond remains intact. The adsorption structure of cyanide, as suggested by UHV results, shows the C–N bond parallel to the surface. In contrast, in an electrochemical environment a perpendicular or tilted geometry has been always reported [13,14]. The aim of the present work is 2-fold: to analyse the electrochemical response of the residues formed from the open-circuit adsorption of methylamine and to identify the adsorbed (or solution) species involved during the oxidation/reduction of these residues. Taking into account the possibility that the processes under inspection were structure-sensitive, the study will contemplate results for the three basal planes of platinum.
2. Experimental Pt single crystal electrodes were prepared according to the method developed by Clavilier et al. [1,2]. The surfaces were thermally cleaned in an air+propane flame and cooled in a H2 +Ar stream [15]. Then, the electrode surface was protected from the laboratory atmosphere by a droplet of ultrapure water saturated with the cooling gases. The open-circuit adsorption procedure was the following: firstly, the oriented surface was immersed in a 0.1 M CH3NH2 solution (Merck p.s) for 2 – 3 min. After that, it was rinsed with ultrapure water and, subsequently, transferred to the working solution (0.1 M HClO4, Merck suprapur) which was free of methylamine. All experiments were performed at room temperature. The water employed in the experiments was obtained from a Millipore – MilliQ plus system, which supplies it with a resistivity close to 18.2 MV cm. Cyclic voltammograms were recorded in a classical non-divided electrochemical cell, at a constant sweep rate of 50 mV s − 1. Potentials were measured against a reversible hydrogen electrode (RHE). A Nicolet Magna 850 spectrometer equipped with a liquid nitrogencooled mercury cadmium telluride (MCT) detector was employed for the in situ FTIR measurements. The spectroelectrochemical cell was made of glass and was
provided with a prismatic CaF2 window bevelled at 60°. Spectra were collected with a resolution of 8 cm − 1 and are presented in the usual form DR/R.
3. Results and discussion A first step in a fundamental study on the behaviour of methylamine at Pt(h,k,l) electrodes should prove whether or not this compound adsorbs on the surface. If the compound adsorbs, it would be necessary to test the kind of adsorption that takes place. Methylamine molecules could adsorb intact or suffer chemical bond cleavage during their interaction with the surface. It is known that the dissociative adsorption of many organic compounds results in the formation of different adsorbed species. The so-called poisons modify strongly the adsorption properties of the electrode, blocking a significant number of surface active sites.
3.1. Pt(111) The voltammetric profile recorded for a Pt(111) electrode after the open-circuit adsorption of methylamine is shown in Fig. 1a. This profile remains stable during a few cycles, but a continuous sweeping in the whole range of potential employed gives rise to the almost complete removal of the adlayer. A comparison between this voltammogram and that recorded for the bare Pt(111) in a clean 0.1M HClO4 solution (Fig. 1b) reveals interesting differences. On the one hand, the adsorption states between 0.06 and 0.45 V appear modified for the electrode covered with methylamine. A loss of part of the voltammetric charge associated with the adsorption of hydrogen is observed (about 40% hydrogen sites seem blocked). In addition, a positive potential shift of this process seems to occur. On the other hand, two very broad anodic peaks appear in the potential region between 0.7 and 1.3 V, the first centred at 0.85 V and the second at 1.22 V. The latter peak
Fig. 1. Cyclic voltammograms recorded in a 0.1 M HClO4 test solution. (a) Pt(111) electrode after methylamine adsorption at opencircuit. (b) Clean Pt(111).
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Fig. 2. FTIR spectra collected for the Pt(111) surface, after the adsorption of methylamine, at the sample potentials indicated. The reference potential was 0.2 V. p-Polarised light. 500 Scans.
seems to be related to the surface oxidation process that the Pt(111) electrode undergoes under the sharp peak at about 1V in a perchloric acid solution (see Fig. 1b). Regarding the first peak, one can notice its high reversibility with the cathodic peak appearing between the same potential limits. The voltammetric profile described above is very similar to that recorded for a cyanide-covered Pt(111) surface in perchloric medium, in which the reversible voltammetric wave centred at 0.85 V was ascribed to the adsorption/desorption of hydroxyl species [16]. Relying on purely voltammetric experiments it is difficult to determine the nature of the adsorbed species coming from the adsorption of methylamine, but the formation of a cyanide adlayer can be postulated. This assumption will be checked spectroscopically below although it is in close agreement with the previous studies on the adsorption of CH3NH2 in the UHV environment. Under our experimental conditions, the coverage achieved by the adsorbed cyanide derived from methylamine is lower than that obtained from the adsorption of cyanide ions and this fact could be the reason for the lower stability of the adlayer. Fig. 2 shows a set of FTIR spectra for the methylamine-covered Pt(111) electrode. After the open-circuit adsorption the electrode was rinsed with ultrapure water and then immersed in the test solution at 0.2 V. Subsequently, the electrode surface was pressed against the CaF2 window and the reference spectrum was acquired at this potential. The applied potential was then systematically stepped in the positive direction up to the maximum value of 0.9 V and sample spectra were collected for every step. Fig. 2 shows the computed transmittance spectra for some of the sample potentials. At 0.5 V, only a negative-going band centred at 2105
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cm − 1 appears in the spectrum. The frequency of this band coincides with that observed for the CN stretching vibration of cyanide adsorbed molecularly on Pt(111) surfaces in acidic medium [16]. When the applied potential reaches 0.7 V, the band becomes bipolar and, simultaneously, a small absorption band appears at 2345 cm − 1. The intensity of this band increases at 0.9 V. This feature is related to the presence of small amounts of dissolved CO2, probably arising from slight oxidation of cyanide. The observation of a negative character in the band at around 2100 cm − 1 for the spectrum taken at 0.5 V could indicate a certain overlapping of the absorption bands for the sample and reference spectra at this potential. It is worth noting that the adsorbed cyanide remains at the surface at the reference potential during the whole experiment (note the bipolar character of the bands at 0.7 and 0.9 V). Similar behaviour has been reported for carbon monoxide adsorbed on platinum electrodes, see Ref. [17] and references cited therein. The results presented support the voltammetric assignment suggested above and, consequently, it might be accepted that methylamine interacts at open-circuit with the Pt(111) surface yielding a cyanide adlayer. As no absorption bands attributable to N–H vibrations can be distinguished in the spectra, we assume that methylamine undergoes a full dehydrogenation reaction during its adsorption on Pt(111) to generate CNads.
3.2. Pt(100) We will now analyse the behaviour of the adsorbed residues formed during the open-circuit dosage of methylamine on Pt(100) electrodes. Voltammograms recorded for the CH3NH2-covered surface are presented in Fig. 3a and the blank voltammogram for the Pt(100) in a 0.1 M HClO4 solution, in Fig. 3b. After the adsorption procedure at open-circuit, the electrode was immersed in the test solution at 0.4 V and then polarised to less positive potential values. The lower potential limit was 0.06 V while the upper limit was set firstly at 0.4 V. In this way, the initial oxidation of the adlayer can be avoided and it is possible to check the blockage of the adsorption sites from the voltammetric profile. This experiment is shown in the inset to Fig. 3. It can be observed that a broad cathodic peak (labelled as peak a) appears during the initial sweep down to 0.06 V and its peak current decreases abruptly in the subsequent cycle. At the same time, the voltammetric charge between 0.06 and 0.4 V grows slowly by cycling in this potential interval. These results point to the reductive elimination of adsorbed species from the electrode surface and to the simultaneous liberation of adsorption sites. However, one should note the differences in the voltammetric charge below 0.3 V for this voltammetric profile and that for the blank (Fig. 3b). Part of the
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adsorption processes that take place above 0.3 V for the bare surface could be shifted towards less positive potentials. From the inspection of this profile, it can be concluded that significant amounts of adsorbed species, derived from methylamine, remain at the surface. After a steady voltammogram was reached in the potential region below 0.4 V, the upper potential limit was shifted up to 0.9 V to check the possibility of removing the adlayer by oxidation. This experiment is shown in Fig. 3a (main figure). During the first sweep up to 0.9 V (solid line), two partially overlapped anodic peaks, centred at 0.69 V (peak b) and 0.78 V (peak c), can be clearly distinguished. The first one lies in the potential region for the oxidation of adsorbed CO obtained at low coverage [18], while the latter appears always associated with the sharp cathodic peak at about 0.6 V (peak d) in the subsequent negative-going sweep. During the sweep towards less positive potentials, the cathodic peak a arises again at around 0.2 V (see the inset to Fig. 3). This result strongly suggests that the species which gives rise to the peak a is produced in the oxidation of some adsorbate above 0.5 V and can be reduced below 0.4 V. The voltammetric profile described here is very similar to that recorded during the oxidation of a cyanide adlayer on Pt(100) electrodes [16]. Cyanide adlayers formed on these substrates are very reactive. They oxidise to adsorbed nitric oxide and carbon dioxide above 0.6 V or produce adsorbed carbon monoxide below 0.4 V. It is worth mentioning that when nitric oxide is present on Pt(100) substrates at moderate coverage, the voltammogram
Fig. 3. Cyclic voltammograms recorded for a Pt(100) electrode immersed in a 0.1 M HClO4 test solution. (a) After methylamine adsorption at open-circuit. Solid line: first potential sweep up to 0.9 V. Dashed line: second sweep. Inset: Evolution of the voltammetric profile between 0.06 and 0.4 V. (b) Clean Pt(100).
Fig. 4. FTIR spectrum for the Pt(100) covered with the residues coming from methylamine. The spectrum was acquired after potential cycling in the 0.45 – 0.1 V range. Sample potential 0.45 V. Reference 0.1 V. 500 Scans. p-Polarised light
recorded between 0.5 and 0.9 V displays a characteristic pair of peaks. These peaks are very similar to those labelled c and d in Fig. 3a. On the other hand, the elimination of considerable amounts of adsorbed species can be clearly observed from the voltammetric profile of the second cycle, although the adsorption states above 0.4 V still seem blocked. During this sweep, the anodic peaks b and c lose a great part of their peak intensity and the same occurs for the cathodic peaks a and d. The recovery of most of the adsorption capabilities of the Pt(100) substrate is effective in a few cycles (not shown). These results reveal that the voltammetric response of the adsorbed species coming from methylamine at Pt(100) electrodes is rather similar to that of adsorbed cyanide. From this information, it can be pointed that the open-circuit adsorption of the amine generates adsorbed CN, which would subsequently suffer different surface reactions as a function of the applied potential. To clarify this point, new open-circuit adsorption experiments were performed and the surface probed by means of in situ FTIR spectroscopy. Fig. 4 shows the spectrum acquired after the Pt(100), covered with the residues formed at open-circuit, was cycled in the potential region between 0.1 and 0.45 V. Reference interferograms were collected at 0.1 V while those corresponding to the sample, at 0.45 V. This experiment allows the voltammetric profile of the inset to Fig. 3 to be related with the current spectrum. Thus, the negativegoing absorption band, which appears at 2083 cm − 1, is clearly related to the C–N stretching of adsorbed cyanide. The presence of CNads at the surface implies the full dehydrogenation of methylamine molecules, as was found for the Pt(111) substrate. On Pt(111) cyanide
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was the unique adsorbed species detected by FTIR in the whole range of potentials. However, this spectrum shows two more features, both ascribable to adsorbates different from CNads. On the one hand, the intense bipolar band centred at 1830 cm − 1 is characteristic of the C–O stretching of carbon monoxide in either a bridge or multibonding adsorption geometry. On the other hand, the positive-going band at 2005 cm − 1 seems related with the presence of linearly bonded CO at the reference potential. Taking into account that at these potentials CO adlayers are rather stable, the absence of linearly bonded CO at 0.45 V is not easy to explain. An eventual potential-induced conversion of bridge to linearly bonded CO at 0.1 V could justify the positive character of the band at 2005 cm − 1, although this kind of transformation only takes place under certain conditions. The site occupancy of adsorbed CO at the Pt(100) surface is dependent on both coverage and electrode potential [19,20]. Shifting the potential to more positive values results in a conversion from bridge to linearly bonded CO, although such a conversion does not take place by decreasing the applied potential. Moreover, as the spectra were collected alternately between 0.1 and 0.45 V, the opposite conversion (COL to COB) should be fast enough to avoid the observation of bipolar character in the absorption band at ca. 2000 cm − 1. Even in this case, the feature for bridge bonded CO (around 1830 cm − 1) should present significant negative character. A better explanation may be that of a surface reaction of CN to yield adsorbed CO at low potentials. This reaction has been already observed for cyanide adlayers formed from the open-circuit adsorption of cyanide molecules on Pt(100) [16]. Let us assume that cyanide adsorbs in clusters at the Pt(100) surface (as has been reported for Pt(111) electrodes by means of scanning tunnelling microscopy [21,22]); if the geometry of adsorbed cyanide is preserved for adsorbed carbon monoxide during the surface reaction CN CO, the expected CO resulting species will be linearly bonded due to its high local coverage. Then, CO molecules could diffuse on the surface to accommodate in a less energetic adsorption geometry at low coverages, i.e. the bridge or multibonded one. In a new experiment, after methylamine was adsorbed at the Pt(100) surface, the electrode was immersed into the spectroelectrochemical cell at 0.5 V to avoid the initial reduction of the adsorbed species. Subsequently, the potential was cycled between 0.5 and 0.9 V until a steady voltammetric profile was obtained. Then, the electrode surface was pressed against the window and reference (0.5 V) and sample (0.9 V) spectra were alternately collected in groups of 100 interferograms. Fig. 5 shows the resulting FTIR spectrum, in which the presence of three absorption bands is clearly noted. The negative-going band at 2345 cm − 1 is assigned to dissolved CO2 while the bipolar one
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centred at around 2100 cm − 1 corresponds to the C–N stretching of adsorbed cyanide. On the other hand, the frequency of the absorption band at 1603 cm − 1 lies in the frequency region of the N–O vibrations in nitrosylmetal complexes [23]. This band was already assigned to adsorbed nitric oxide, which was generated during the oxidation of a cyanide adlayer at a Pt(100) surface [16]. This fact and the observation of the characteristic voltammetric profile in Fig. 3 make it possible to ascribe the vibrational band at around 1600 cm − 1 to the presence of adsorbed NO. As concluded from these results, the cyanide adlayer produced during the opencircuit adsorption of methylamine is also oxidised at the Pt(100) surface. This oxidation yields carbon dioxide and adsorbed nitric oxide, which remains at the surface unless the potential reaches values below 0.3–0.4 V. The adsorbed NO may be reduced (peak a in Fig. 3a) to some N-containing species, probably in the form of NH4+ [24].
3.3. Pt(110) Fig. 6a shows cyclic voltammograms obtained for a Pt(110) electrode after the open-circuit dosage of methylamine. The adsorption properties of this surface are influenced by the presence of adsorbed species derived from methylamine. The first potential cycle (solid line) reveals that the most noticeable difference from the bare electrode (which voltammetric response is presented in Fig. 6b) arises from the blockage of the adsorption states between 0.2 and 0.3 V. Also, little modification of the hydrogen adsorption/desorption process below 0.2 V can be noticed. Part of this process is recovered during the following sweeps, but the voltammetric profile remains altered. This could indi-
Fig. 5. FTIR spectrum the Pt(100) covered with the residues coming from methylamine. The spectrum was acquired after potential cycling in the 0.9 – 0.5 V range. Sample potential 0.9 V. Reference 0.5 V. 500 Scans. p-Polarised light.
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Fig. 6. Cyclic voltammograms obtained for a Pt(110) electrode in a 0.1 M HClO4 test solution. (a) After methylamine adsorption at open-circuit. Solid line: first potential sweep up to 0.85 V. Dashed line: fifth sweep. (b) Clean Pt(110).
Fig. 7. FTIR spectra collected for the Pt(110) electrode after the open-circuit adsorption of methylamine. Sample potentials are indicated in each spectrum. The reference was acquired at 0.05 V. p-Polarised light. 200 Scans.
cate either the presence of adsorbed residues on the surface or the induction of surface defects in the metal substrate. The alteration of well-defined electrode surfaces by adsorbed residues has been already detected, among others, for CN adsorbed on Pt(110) [14], CO adsorbed on Pt(110) [25] or SO2 on Pt(111) [26]. The Pt(110) orientation seems much less active than Pt(100) for the oxidation or reduction of the adsorbed species coming from the adsorption of methylamine. The voltammetric profile shown in Fig. 6a is rather similar to that recorded after the open-circuit adsorption of cyanide on the same surface [16]. If CNads was also the species formed on Pt(110) after the open-circuit contact of methylamine, this species may desorb when the electrode is polarised below 0.1 V, as was found for cyanide ions [16]. In situ FTIR experiments confirm that adsorbed cyanide is the main species present at the Pt(110) surface. Fig. 7 shows spectra in the 1200 – 2500 cm − 1 range obtained after the adsorption of methylamine on this surface. The covered electrode was immersed in the test solution at 0.2 V to avoid an eventual desorption of adsorbed species. It was subsequently pressed against the window and sample interferograms were acquired from 0.3 to 0.8 V. The reference was taken at 0.05 V after all sample spectra were collected. The negative-going band at around 2100 cm − 1 is clearly related with the C–N stretching of adsorbed cyanide. Its negative character indicates the absence of cyanide at the reference potential. No other bands ascribable to adsorbed species can be discerned in the spectral range studied.
produces adsorbed cyanide as the main adsorbed species. This accounts for the considerable modification in the adsorption capabilities of the Pt(h,k,l) and suggests that a full dehydrogenation of the amine is the fundamental process occurring at the electrode surface under open-circuit conditions, independently of its orientation. Spectroscopic evidence for the presence of non-dissociated methylamine molecules on the surfaces were not found. The reactivity of the resulting cyanide adlayer strongly depends on the crystallographic orientation of the substrate. Pt(111) and Pt(110) preserve the C–N bond, as was reported in UHV experiments. Also in agreement with UHV results, the Pt(100) orientation shows higher catalytic activity to break the C–N bonds. This has made it possible to observe a CO adlayer at potentials below 0.5 V and a NO adlayer above 0.5 V, both arising from the surface reaction of adsorbed cyanide. The tendency of the amino group to dehydrogenate on platinum surfaces has been described previously [7,27]. Nevertheless, this observation is opposite to the behaviour shown by ammonia, which is rather stable on the same substrates [28,29]. The substitution of one hydrogen atom by a methyl group in the molecule of ammonia produce higher reactivity of the N–H bonds in the resulting methylamine species. This chain effect cannot be extrapolated to the following amine (ethylamine), for which poor reactivity on Pt(h,k,l) electrodes has been observed [30].
4. Conclusions
Acknowledgements
The interaction of methylamine molecules with Pt(h,k,l) surfaces in an electrochemical environment
Francisco J. Huerta thanks the Generalitat Valenciana for the award of his FPI grant.
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