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Modification of surface activity of Cu–Zr amorphous alloys and Cu metal by electrochemical methods
Materials Science and Engineering A267 (1999) 227 – 234
Modification of surface activity of Cu–Zr amorphous alloys and Cu metal by electrochemical methods M. Janik-Czachor a,*, A. Kudelski b, M. Dolata a, M. Varga d, A. Szummer c, ´ . Molna´r d J. Bukowska b, A a
Institute of Physical Chemistry, Polish Academy of Science, Warsaw, Poland b Department of Chemistry, Uni6ersity of Warsaw, Warsaw, Poland c Department of Materials Science, Technical Uni6ersity, Warsaw, Poland d Department of Organic Chemistry, Attila Jozsef Uni6ersity, Szeged, Hungary
Abstract This paper summarizes our attempts to use some strictly controlled electrochemical processes of dissolution/redeposition of Cu (including disproportionation of Cu + to Cu metal and Cu2 + ) to modify Cu surfaces, as well as surfaces of Cu base amorphous alloys (AA), to produce active substrates for various phenomena of adsorption and catalytic reactions. We developed some new methods of activation of the Cu substrate for in situ investigations of adsorbates with SERS (Surface Enhanced Raman Spectroscopy). The first method developed produced an oxidized Cu surface. A distinct spectral shift of the bands characteristic of the adsorbate was observed, due to its interaction with Cu2O instead of interacting with metallic Cu. The second method produced a substrate with a clean surface and large specific surface area which resulted in a high quality SERS spectrum exhibiting a 10-fold increase in the signal-to-noise ratio, compared to the results for the surface pretreated by commonly used methods of surface roughening (oxidation–reduction cycling). The third method included an irreversible, diffusion-controlled Cu deposition onto a substrate and resulted in a rather complex, partially oxidized substrate with Cu clusters exhibiting a variety of SERS activities. The second method appeared also useful for the modification of the surface activity of Cu – Zr amorphous alloys. This method was combined with an ageing process of the AA to produce a partial devitrification of the substrate. The electrochemical pretreatment was then applied after this partial devitrification. The catalytic efficiency for dehydrogenation of 2-propanol on such a pretreated Cu–Zr substrate increased by a factor of two. A correlation has been found between the SERS activity of an electrochemically pretreated substrate and its catalytic efficiency. A tentative mechanism of surface activation is discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Surface activation; Adsorption; Cu–Zr amorphous alloys; Cu metal; Dissolution/redeposition; SERS activity; Catalytic activity
1. Introduction The catalytic properties of amorphous alloys (AA), a new class of catalysts, are of considerable interest [1–9]. Many of these alloys are excellent precursors of efficient and selective catalysts [1 – 4,6 – 8]. Much work has been done to understand the role and the effect of various activation processes which result in a transition of the given AA from a catalyst precursor into a stable, highly efficient catalyst [3 – 8]. Notably, Cu – Zr catalysts developed from the corresponding AA ribbons ap* Corresponding author. Tel.: +48-22-632-3221; fax: + 48-22-6325276. E-mail address: [email protected] (M. Janik-Czachor)
peared efficient catalysts of many chemical reactions including dehydrogenation and/or dehydration, isomerisation [4,5], and hydrogenation [6–8]. Detailed studies by Molna´r and coworkers [4,5,9] and by Baiker and coworkers [6–8] brought an insight into the effects of various pretreatments on activity, selectivity and stability of the Cu–Zr base catalysts. However, there is still an attempt to find new methods of activation of the ribbons to produce efficient and stable catalysts. In this paper, we summarize our efforts concerning the following subjects: 1. We report results on the development of a new efficient method for roughening Cu metal electrodes, which increases surface activities including
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treatment to give rise to a partial redeposition of metallic Cu from the disproportionation reaction of dissolving Cu1 + to Cu2 + and metallic Cu. Both pretreatments proved to enhance the activity of Cu metal and Cu–Zr ribbons as the SERS substrates [12–16]. In our attempts to achieve these goals, the following techniques were used for the investigations: electrochemistry, microscopy (optical, SEM, EDS/WDS X-ray electron microprobe), spectroelectrochemistry-SERS, catalytic tests.
2. Results and discussion
2.1. Cu metal
Fig. 1. Current density vs. electrode potential during the oxidation – reduction cycling (ORC) of Cu in LiCl + CuCl2 [19] at 20 mV/s between − 50 and + 5 mV (SCE). Only the first and 50th cycles are shown. This is the standard roughening procedure for SERS experiments.
SERS (Surface Enhanced Raman Spectroscopy) phenomena. 2. We use our experience with Cu to enhance the surface activity of Cu – Zr AA, and to find a correlation between the catalytic and SERS activity of the substrate. 3. We describe a new and efficient method of activation of Cu–Zr AA ribbons which transforms them into active catalysts. The method combines an ageing/devitrification (exploiting the poor stability of these ribbons [10,11]), and an electrochemical pre-
2.1.1. Electrochemical measurements The following methods of surface pretreatment/ roughening were used: 1. Procedure 1. Oxidation–reduction cycling (ORC); a standard electrochemical method consisting of rapid cycling of the potential from a cathodic to an anodic side resulting in a dissolution/redeposition of the metal surface in 0.2 M LiCl+ 0.01 M CuCl2 solution (Fig. 1) [17–19]. 2. Procedure 2. Special electrochemical pretreatment (see [13,14] for details). 3. Procedure 3. A new procedure—anodic rapid dissolution (NP-AD); Cu electrode is anodized in 0.4 M H2SO4 + 0.2 M CuSO4, at E= 0.47 V (SCE) for a short time [12] (Fig. 2). 4. Procedure 4. Non-equilibrium cathodic deposition; Cu electrode was covered by Cu electrodeposited from 0.4 M H2SO4 + 0.2 M CuSO4 solution at E= − 0.5 V (SCE) (Fig. 3) [20].
Fig. 2. Typical current intensity vs. time plots for metallic Cu and for amorphous Cu – Zr alloys anodized in 0.4 M H2SO4 +0.2 M CuSO4, at E = 150 mV (SCE). The dramatic difference in the anodic behavior of the two kinds of materials is highlighted.
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Fig. 3. Current density vs. time curve for a non-equilibrium cathodic deposition of Cu onto a carefully polished Cu substrate from 0.4 M H2SO4 +0.2 M CuSO4, at E= − 0.5 mV (SCE). The slope confirms the diffusion-controlled deposition process.
2.1.2. Microscopic examinations SEM and EDS/WDS X-ray electron microprobe examinations revealed large morphological and compositional differences between the Cu surfaces pretreated with four different procedures listed above (Figs. 4 – 7). Highly developed surface morphologies were produced with the aid of procedures 1 (Fig. 4) and 4 (Fig. 7). Both surfaces were oxidized and contaminated with C. Moreover, some Cl − contamination from the electrolyte was found on the surface pretreated with procedure 1 (Fig. 4). Procedure 2 produced rough surfaces covered with Cu2O. Procedure 3 produced highly developed and clean surfaces with no oxygen signal detectable by WDS/EDS. The surface exhibited a peculiar morphology characteristic of a rapid dissolution of the substrate with some small metal particles randomly distributed on top (Fig. 6). 2.1.3. SERS measurements SERS results obtained from rough Cu surfaces produced by our developed electrochemical methods (including our new procedure of rapid anodic dissolution (NP-AD)) were compared with those obtained for a rough Cu surface made by the standard ORC method (Procedure 1) [12]. Pyridine was used as a probe molecule to study the surface activity of various rough Cu electrodes because the SERS enhancement factor is high for this molecule.
SERS confirmed that procedures 2 and 4 produced oxidized Cu surfaces with a spectral shift characteristic of an interaction of pyridine with Cu2O. The results are summarized in Table 1. There is no doubt that our new procedure 3 gives superior results. Therefore, this method has been used for the experiments with Cu–Zr AA.
2.2. Cu–Zr amorphous alloys 2.2.1. Electrochemical measurements There was a large difference in the anodic behaviour of Cu and Cu–Zr AA (Fig. 2). For the AA, the electrochemical pretreatment was terminated after 10 s, i.e. well before the anodic current density (c.d.) dropped down from about 20 mA/cm2 to a value of about 40 mA/cm2. This behaviour was dramatically different from that of pure Cu specimens where the same pretreatment resulted in a stationary c.d. of about 20 mA/cm2 (Fig. 2). These results showed that our attempt to modify the Cu microdomains on the aged AA ribbons by a prolonged anodization, as was done for pure Cu [12], was not successful. Due to the distinct c.d. decay in time for Cu–Zr AA (Fig. 2), the charge passed during a prolonged Cu metal dissolution is much larger than that which can be passed by anodization of Cu–Zr AA. So a much smaller surface alteration was achieved in the case of the AA.
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One should note here that the higher the Zr concentration in the alloy (Fig. 2b), and the longer the ageing time (Fig. 2a), the steeper is the c.d. decay. This suggests that a selective dissolution of Cu from the ribbon may result in a local increase in Zr concentration which then facilitates passivation of the devitrified alloy surface during anodization.
Fig. 6. SEM micrograph of the Cu surface pretreated with method 3 (NP-AD). A developed morphology is visible with some small particles on the longitudinal strips.
Fig. 4. (a) Typical secondary electron image of Cu surface roughened by the standard method (1), ORC. The image was obtained by the X-ray electron microprobe analyser. The ellipsoidal particles formed on the surface during the redeposition of Cu are easily visible. (b) Electron microprobe/EDS results of a line scan across the surface between points ‘1’ and ‘2’, marked in Fig. 5a, showing an enrichment of oxygen and some Cl contamination of the particles.
Fig. 5. SEM micrograph of the Cu surface pretreated with method 2.
Fig. 7. SEM micrograph of the Cu surface pretreated with method 4 (non-equilibrium deposition). ‘Flower-like’ deposits are visible with some small particles in between. Table 1 Estimated SERS intensities for Cu roughened with various electrochemical procedures Procedure
ISERS
1. ORC, oxidation–reduction cycling in LiCl+CuCl2 1a. ORC, oxidation–reduction cycling in KCl 2. Oscillating reaction roughening 3. NP–AD, new procedure; anodic dissolution in CuSO4+H2SO4 4. Cu deposition, non-equilibrium conditions
1 0.2 0.06 6–10 0.3
2.2.2. Microscopic examinations One side of the ribbon looked very different from the other after ageing (or after thermal devitrification [15]), and also differed in the degree of segregation and, hence, composition, as discussed in detail elsewhere [4].
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Fig. 8. A typical X-ray electron microprobe image of a 60Cu – 40Zr amorphous alloy after ageing for 10 years at room temperature. Circular, ‘flower-like’ deposits rich in Cu form during partial devitrification; (a) secondary electron image; (b) backscattered electron image (c) distribution of Cu Ka; (d) distribution of Zr La.
Characteristic ‘flower-like’ microdomains rich in Cu were formed on the wheel (dull) side (Fig. 8). No distinct morphological changes were produced by the anodization, as revealed by microscopic examinations up to a magnification of 10 000× . However, the wheel side of the ribbon changed in colour from red to brownish, whereas the free side did not exhibit any optical change. Electron microprobe/EDS results have shown that the wheel side of the electrochemically pretreated samples was distinctly oxidized, particularly near ‘flowerlike’ Cu-rich areas, therefore affecting surface activity. Furthermore, extrapolating from the previous work [12,13], we assume that: 1. the new pretreatment may increase the specific surface area of the Cu-rich domains on a submicroscopic scale, thus making them more efficient in affecting the interaction of the adsorbed molecules during the catalytic reaction; 2. it may redeposit some of the metallic Cu (originating from the disproportionation reaction of the dissolving Cu + ) on the Zr-rich areas, thus increasing the total Cu-enriched surface area available for the catalytic reaction; 3. during the catalytic test, the hydrogen may reduce the remaining oxides completely and activate the
electrochemically developed surface for the catalytic reaction. The increase in the catalytic efficiency ratio at the beginning of the catalytic test (Fig. 10) agrees with this hypothesis which requires further experimental verification using high lateral resolution microscopic techniques.
2.2.3. Catalytic studies The results of catalytic tests for a 5-year-old Cu–Zr sample are given in Fig. 9. A twofold increase in the
Fig. 9. Conversion of 2-propanol as a function of reaction time over an aged amorphous 60Cu – 40Zr alloy (a), over the same material additionally modified by an electrochemical pretreatment (b).
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8-month-old specimens. However, the efficiency ratio for pretreated and untreated samples depends distinctly on the ageing time (Fig. 10). It decreases with the catalytic test duration for the 8-month-old sample, whereas it reaches a stable value of about 1.8 for the 5-year-old sample, suggesting that there is an interrelation between the ageing and the electrochemical pretreatment which affects the final surface activity of the substrate.
2.2.4. SERS in6estigations
Fig. 10. Efficiency ratio between the conversion of 2-propanol over an aged amorphous 60Cu–40Zr alloy, and its value over the same material additionally modified by an electrochemical pretreatment as a function of reaction time. Ageing time: 8 months or 5 years. Full lines are eye guidelines only.
Fig. 11. SERS spectra of pyridine adsorbed from 0.05 M pyridine+ 0.1 M KCl solution on a 50Cu–50Zr alloy partially devitrified by ageing during 10 years in air; (a) measurement outside the Cu ‘flowers’, i.e. on the Zr-rich area; (b) measurement on Cu-rich, ‘flower-like’ domains; the SERS intensity ratio on the two sides is 1 and 15; (c) electrochemically roughened Cu surface, with the aid of procedure (1) (ORC); (d) normal Raman spectrum of 1 M pyridine aqueous solution.
catalytic activity due to the new electrochemical pretreatment is observed. Similar results were found for
2.2.4.1. Effect of ageing/de6itrification. Investigations of the local enhancement of the Raman spectra on the catalyst surface were carried out with pyridine as a probe molecule, since the SERS enhancement factor for this molecule is very high [12–21]. Both the frequencies and the relative intensities in the SERS spectra (Fig. 11a–c) differ from those of bulk pyridine (Fig. 11d) indicating that indeed we are measuring the Raman spectra of the adsorbed molecules. Fig. 11b shows a part of the SERS spectrum of pyridine adsorbed on a Cu-rich domain of the 10-yearold 50Cu–50Zr catalyst. Comparison of this spectrum with another measured on the rough surface of Cu metal (Fig. 11c) shows that they are almost identical. This confirms that the SERS signals measured for the Cu–Zr catalyst originated from the pyridine molecules adsorbed on the Cu domains. The microscopic facility associated with our Raman spectrometer enables measurement of SERS activity of various areas of the surface of the Cu–Zr catalysts with a lateral resolution of 1 mm. It has been found that the surface is highly inhomogeneous: large SERS signals are measured from the Cu-rich, ‘flower-like’ domains, whereas the intensity drops by a factor of 9–15 when one attempts to measure the SERS signals from Zr-rich areas, outside the Cu ‘flowers’ (see Fig. 11a,b where this ratio is 1 and 15). Moreover, the effect of the electrode potential on the SERS signal for various surface domains (of about 1 mm2) differs significantly suggesting that the activity of Cu clusters at different surface sites differs accordingly [21]. SERS investigations provided an insight into the degree of surface segregation of Cu–Zr amorphous alloys in the course of the ageing/devitrification process. It has been found that the SERS intensity attains a maximum on Cu-rich ‘flowers’, but its value depends on the devitrification time [21]. 2.2.4.2. Effect of electrochemical pretreatment on SERS. The most pronounced effect of electrochemical pretreatment was the disappearance of domains of poor SERS activity for electrochemically activated samples. SERS intensity increased considerably outside the Cu-
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Fig. 12. Schematic representation of the surface of the aged Cu – Zr AA before and after the electrochemical pretreatment with procedure (3) (NP-AD); IR is the Raman intensity.
rich ‘flowers’ leading to some kind of homogenization of the surface. Hence, the situation reported in Fig. 11 was not observed any more. This strongly suggests that the electrochemical activation creates new surface clusters of Cu atoms in the Zr-rich areas, which then become SERS active. Probably they are formed during the disproportionation reaction of the dissolving Cu + to Cu metal and Cu2 + [12,16] and are the result of a redeposition of Cu on the Zr-rich domains (i.e. outside the Cu ‘flowers’ as shown schematically in Fig. 12).
there is a correlation between the SERS and the catalytic activities of Cu-containing substrates.
Acknowledgements This work was financially supported by grant KBN 7 T08C 035 14 and by the Institute of Physical Chemistry PAS.
References 3. Conclusions We have developed a new pretreatment involving an ageing process followed by electrochemical dissolution of Cu microdomains of a partly devitrified metal which distinctly enhances the catalytic efficiency of Cu–Zr amorphous alloys for the dehydrogenation of 2-propanol. The mechanism of this enhancement, as the SERS investigations suggest, is the formation of some new Cu-rich sites in the Zr-rich areas during the course of the electrochemical activation (deposition of Cu metal due to the disproportionation reaction of the dissolving Cu1 + : Cu1 + Cu2 + +Cu0). SERS appears to be an excellent method to study the local activity of Cu-containing catalysts since
[1] [2] [3] [4] [5] [6] [7]
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´ . Molna´r, G.V. Smith, M. Barto´k, Adv. Catal. 36 (1989) 229. A K. Hashimoto, Mater. Sci. Eng. A226-A228 (1997) 891. ´ . Molna´r, M. Barto´k, T. Katona, Z. Hegedus, C. Kopasz, A Catal. Lett. 5 (1990) 351. ´ . Molna´r, T. Katona, M. Barto´k, K. Varga, J. Mol. Catal. 64 A (1991) 41. ´ . Molna´r, J. Catal. 153 (1995) 333. T. Katona, A F. Vanini, St. Buechler, X.N. Yu, M. Erbudak, L. Schlapbach, A. Baiker, Surf. Sci. 189/190 (1987) 1117. A. Baiker, H. Baris, M. Erbudak, F.Vanini, New frontiers in catalysis, in: L. Guczi, F. Solymosi, P. Tetenyi (Eds.), Stud. Surf. Sci. Catal. 75 (1993) 1928. D. Gasser, A. Baiker, Appl. Catal. 48 (1989) 279. ´ . Molna´r, M. Barto´k, Mater. Sci. Eng. A181-182 T. Katona, A (1994) 1095. J. Latuszkiewicz, M. Strzeszewska, A. Calka, P.G. Zielinski, H. Matyja, Metal. Technol. (Lond.) 10 (1978) 329.
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[11] K. Asami, E. Akiyama, Y. Murakami, H.M. Kimura, M. Kikuchi, K. Hashimoto, Mater. Sci. Forum 185-188 (1995) 789. [12] A. Kudelski, J. Bukowska, M. Janik-Czachor, W. Grochala, A. Szummer, M. Dolata, Vibr. Spectrosc. 16 (1998) 21. [13] A. Kudelski, W. Grochala, M. Janik-Czachor, J. Bukowska, A. Szummer, M. Dolata, J. Raman Spectrosc. 29 (1998) 431. [14] M. Dolata, A.L. Kawczynski, Pol. J. Chem. 71 (1997) 1699. [15] J. Bukowska, A. Kudelski, M. Janik-Czachor, Chem. Phys. Lett. 268 (1997) 481. [16] A. Kudelski, M. Janik-Czachor, M. Varga, M. Dolata, J.
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Modification of surface activity of Cu–Zr amorphous alloys and Cu metal by electrochemical methods M. Janik-Czachor a,*, A. Kudelski b, M. Dolata a, M. Varga d, A. Szummer c, ´ . Molna´r d J. Bukowska b, A a
Institute of Physical Chemistry, Polish Academy of Science, Warsaw, Poland b Department of Chemistry, Uni6ersity of Warsaw, Warsaw, Poland c Department of Materials Science, Technical Uni6ersity, Warsaw, Poland d Department of Organic Chemistry, Attila Jozsef Uni6ersity, Szeged, Hungary
Abstract This paper summarizes our attempts to use some strictly controlled electrochemical processes of dissolution/redeposition of Cu (including disproportionation of Cu + to Cu metal and Cu2 + ) to modify Cu surfaces, as well as surfaces of Cu base amorphous alloys (AA), to produce active substrates for various phenomena of adsorption and catalytic reactions. We developed some new methods of activation of the Cu substrate for in situ investigations of adsorbates with SERS (Surface Enhanced Raman Spectroscopy). The first method developed produced an oxidized Cu surface. A distinct spectral shift of the bands characteristic of the adsorbate was observed, due to its interaction with Cu2O instead of interacting with metallic Cu. The second method produced a substrate with a clean surface and large specific surface area which resulted in a high quality SERS spectrum exhibiting a 10-fold increase in the signal-to-noise ratio, compared to the results for the surface pretreated by commonly used methods of surface roughening (oxidation–reduction cycling). The third method included an irreversible, diffusion-controlled Cu deposition onto a substrate and resulted in a rather complex, partially oxidized substrate with Cu clusters exhibiting a variety of SERS activities. The second method appeared also useful for the modification of the surface activity of Cu – Zr amorphous alloys. This method was combined with an ageing process of the AA to produce a partial devitrification of the substrate. The electrochemical pretreatment was then applied after this partial devitrification. The catalytic efficiency for dehydrogenation of 2-propanol on such a pretreated Cu–Zr substrate increased by a factor of two. A correlation has been found between the SERS activity of an electrochemically pretreated substrate and its catalytic efficiency. A tentative mechanism of surface activation is discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Surface activation; Adsorption; Cu–Zr amorphous alloys; Cu metal; Dissolution/redeposition; SERS activity; Catalytic activity
1. Introduction The catalytic properties of amorphous alloys (AA), a new class of catalysts, are of considerable interest [1–9]. Many of these alloys are excellent precursors of efficient and selective catalysts [1 – 4,6 – 8]. Much work has been done to understand the role and the effect of various activation processes which result in a transition of the given AA from a catalyst precursor into a stable, highly efficient catalyst [3 – 8]. Notably, Cu – Zr catalysts developed from the corresponding AA ribbons ap* Corresponding author. Tel.: +48-22-632-3221; fax: + 48-22-6325276. E-mail address: [email protected] (M. Janik-Czachor)
peared efficient catalysts of many chemical reactions including dehydrogenation and/or dehydration, isomerisation [4,5], and hydrogenation [6–8]. Detailed studies by Molna´r and coworkers [4,5,9] and by Baiker and coworkers [6–8] brought an insight into the effects of various pretreatments on activity, selectivity and stability of the Cu–Zr base catalysts. However, there is still an attempt to find new methods of activation of the ribbons to produce efficient and stable catalysts. In this paper, we summarize our efforts concerning the following subjects: 1. We report results on the development of a new efficient method for roughening Cu metal electrodes, which increases surface activities including
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treatment to give rise to a partial redeposition of metallic Cu from the disproportionation reaction of dissolving Cu1 + to Cu2 + and metallic Cu. Both pretreatments proved to enhance the activity of Cu metal and Cu–Zr ribbons as the SERS substrates [12–16]. In our attempts to achieve these goals, the following techniques were used for the investigations: electrochemistry, microscopy (optical, SEM, EDS/WDS X-ray electron microprobe), spectroelectrochemistry-SERS, catalytic tests.
2. Results and discussion
2.1. Cu metal
Fig. 1. Current density vs. electrode potential during the oxidation – reduction cycling (ORC) of Cu in LiCl + CuCl2 [19] at 20 mV/s between − 50 and + 5 mV (SCE). Only the first and 50th cycles are shown. This is the standard roughening procedure for SERS experiments.
SERS (Surface Enhanced Raman Spectroscopy) phenomena. 2. We use our experience with Cu to enhance the surface activity of Cu – Zr AA, and to find a correlation between the catalytic and SERS activity of the substrate. 3. We describe a new and efficient method of activation of Cu–Zr AA ribbons which transforms them into active catalysts. The method combines an ageing/devitrification (exploiting the poor stability of these ribbons [10,11]), and an electrochemical pre-
2.1.1. Electrochemical measurements The following methods of surface pretreatment/ roughening were used: 1. Procedure 1. Oxidation–reduction cycling (ORC); a standard electrochemical method consisting of rapid cycling of the potential from a cathodic to an anodic side resulting in a dissolution/redeposition of the metal surface in 0.2 M LiCl+ 0.01 M CuCl2 solution (Fig. 1) [17–19]. 2. Procedure 2. Special electrochemical pretreatment (see [13,14] for details). 3. Procedure 3. A new procedure—anodic rapid dissolution (NP-AD); Cu electrode is anodized in 0.4 M H2SO4 + 0.2 M CuSO4, at E= 0.47 V (SCE) for a short time [12] (Fig. 2). 4. Procedure 4. Non-equilibrium cathodic deposition; Cu electrode was covered by Cu electrodeposited from 0.4 M H2SO4 + 0.2 M CuSO4 solution at E= − 0.5 V (SCE) (Fig. 3) [20].
Fig. 2. Typical current intensity vs. time plots for metallic Cu and for amorphous Cu – Zr alloys anodized in 0.4 M H2SO4 +0.2 M CuSO4, at E = 150 mV (SCE). The dramatic difference in the anodic behavior of the two kinds of materials is highlighted.
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Fig. 3. Current density vs. time curve for a non-equilibrium cathodic deposition of Cu onto a carefully polished Cu substrate from 0.4 M H2SO4 +0.2 M CuSO4, at E= − 0.5 mV (SCE). The slope confirms the diffusion-controlled deposition process.
2.1.2. Microscopic examinations SEM and EDS/WDS X-ray electron microprobe examinations revealed large morphological and compositional differences between the Cu surfaces pretreated with four different procedures listed above (Figs. 4 – 7). Highly developed surface morphologies were produced with the aid of procedures 1 (Fig. 4) and 4 (Fig. 7). Both surfaces were oxidized and contaminated with C. Moreover, some Cl − contamination from the electrolyte was found on the surface pretreated with procedure 1 (Fig. 4). Procedure 2 produced rough surfaces covered with Cu2O. Procedure 3 produced highly developed and clean surfaces with no oxygen signal detectable by WDS/EDS. The surface exhibited a peculiar morphology characteristic of a rapid dissolution of the substrate with some small metal particles randomly distributed on top (Fig. 6). 2.1.3. SERS measurements SERS results obtained from rough Cu surfaces produced by our developed electrochemical methods (including our new procedure of rapid anodic dissolution (NP-AD)) were compared with those obtained for a rough Cu surface made by the standard ORC method (Procedure 1) [12]. Pyridine was used as a probe molecule to study the surface activity of various rough Cu electrodes because the SERS enhancement factor is high for this molecule.
SERS confirmed that procedures 2 and 4 produced oxidized Cu surfaces with a spectral shift characteristic of an interaction of pyridine with Cu2O. The results are summarized in Table 1. There is no doubt that our new procedure 3 gives superior results. Therefore, this method has been used for the experiments with Cu–Zr AA.
2.2. Cu–Zr amorphous alloys 2.2.1. Electrochemical measurements There was a large difference in the anodic behaviour of Cu and Cu–Zr AA (Fig. 2). For the AA, the electrochemical pretreatment was terminated after 10 s, i.e. well before the anodic current density (c.d.) dropped down from about 20 mA/cm2 to a value of about 40 mA/cm2. This behaviour was dramatically different from that of pure Cu specimens where the same pretreatment resulted in a stationary c.d. of about 20 mA/cm2 (Fig. 2). These results showed that our attempt to modify the Cu microdomains on the aged AA ribbons by a prolonged anodization, as was done for pure Cu [12], was not successful. Due to the distinct c.d. decay in time for Cu–Zr AA (Fig. 2), the charge passed during a prolonged Cu metal dissolution is much larger than that which can be passed by anodization of Cu–Zr AA. So a much smaller surface alteration was achieved in the case of the AA.
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One should note here that the higher the Zr concentration in the alloy (Fig. 2b), and the longer the ageing time (Fig. 2a), the steeper is the c.d. decay. This suggests that a selective dissolution of Cu from the ribbon may result in a local increase in Zr concentration which then facilitates passivation of the devitrified alloy surface during anodization.
Fig. 6. SEM micrograph of the Cu surface pretreated with method 3 (NP-AD). A developed morphology is visible with some small particles on the longitudinal strips.
Fig. 4. (a) Typical secondary electron image of Cu surface roughened by the standard method (1), ORC. The image was obtained by the X-ray electron microprobe analyser. The ellipsoidal particles formed on the surface during the redeposition of Cu are easily visible. (b) Electron microprobe/EDS results of a line scan across the surface between points ‘1’ and ‘2’, marked in Fig. 5a, showing an enrichment of oxygen and some Cl contamination of the particles.
Fig. 5. SEM micrograph of the Cu surface pretreated with method 2.
Fig. 7. SEM micrograph of the Cu surface pretreated with method 4 (non-equilibrium deposition). ‘Flower-like’ deposits are visible with some small particles in between. Table 1 Estimated SERS intensities for Cu roughened with various electrochemical procedures Procedure
ISERS
1. ORC, oxidation–reduction cycling in LiCl+CuCl2 1a. ORC, oxidation–reduction cycling in KCl 2. Oscillating reaction roughening 3. NP–AD, new procedure; anodic dissolution in CuSO4+H2SO4 4. Cu deposition, non-equilibrium conditions
1 0.2 0.06 6–10 0.3
2.2.2. Microscopic examinations One side of the ribbon looked very different from the other after ageing (or after thermal devitrification [15]), and also differed in the degree of segregation and, hence, composition, as discussed in detail elsewhere [4].
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Fig. 8. A typical X-ray electron microprobe image of a 60Cu – 40Zr amorphous alloy after ageing for 10 years at room temperature. Circular, ‘flower-like’ deposits rich in Cu form during partial devitrification; (a) secondary electron image; (b) backscattered electron image (c) distribution of Cu Ka; (d) distribution of Zr La.
Characteristic ‘flower-like’ microdomains rich in Cu were formed on the wheel (dull) side (Fig. 8). No distinct morphological changes were produced by the anodization, as revealed by microscopic examinations up to a magnification of 10 000× . However, the wheel side of the ribbon changed in colour from red to brownish, whereas the free side did not exhibit any optical change. Electron microprobe/EDS results have shown that the wheel side of the electrochemically pretreated samples was distinctly oxidized, particularly near ‘flowerlike’ Cu-rich areas, therefore affecting surface activity. Furthermore, extrapolating from the previous work [12,13], we assume that: 1. the new pretreatment may increase the specific surface area of the Cu-rich domains on a submicroscopic scale, thus making them more efficient in affecting the interaction of the adsorbed molecules during the catalytic reaction; 2. it may redeposit some of the metallic Cu (originating from the disproportionation reaction of the dissolving Cu + ) on the Zr-rich areas, thus increasing the total Cu-enriched surface area available for the catalytic reaction; 3. during the catalytic test, the hydrogen may reduce the remaining oxides completely and activate the
electrochemically developed surface for the catalytic reaction. The increase in the catalytic efficiency ratio at the beginning of the catalytic test (Fig. 10) agrees with this hypothesis which requires further experimental verification using high lateral resolution microscopic techniques.
2.2.3. Catalytic studies The results of catalytic tests for a 5-year-old Cu–Zr sample are given in Fig. 9. A twofold increase in the
Fig. 9. Conversion of 2-propanol as a function of reaction time over an aged amorphous 60Cu – 40Zr alloy (a), over the same material additionally modified by an electrochemical pretreatment (b).
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8-month-old specimens. However, the efficiency ratio for pretreated and untreated samples depends distinctly on the ageing time (Fig. 10). It decreases with the catalytic test duration for the 8-month-old sample, whereas it reaches a stable value of about 1.8 for the 5-year-old sample, suggesting that there is an interrelation between the ageing and the electrochemical pretreatment which affects the final surface activity of the substrate.
2.2.4. SERS in6estigations
Fig. 10. Efficiency ratio between the conversion of 2-propanol over an aged amorphous 60Cu–40Zr alloy, and its value over the same material additionally modified by an electrochemical pretreatment as a function of reaction time. Ageing time: 8 months or 5 years. Full lines are eye guidelines only.
Fig. 11. SERS spectra of pyridine adsorbed from 0.05 M pyridine+ 0.1 M KCl solution on a 50Cu–50Zr alloy partially devitrified by ageing during 10 years in air; (a) measurement outside the Cu ‘flowers’, i.e. on the Zr-rich area; (b) measurement on Cu-rich, ‘flower-like’ domains; the SERS intensity ratio on the two sides is 1 and 15; (c) electrochemically roughened Cu surface, with the aid of procedure (1) (ORC); (d) normal Raman spectrum of 1 M pyridine aqueous solution.
catalytic activity due to the new electrochemical pretreatment is observed. Similar results were found for
2.2.4.1. Effect of ageing/de6itrification. Investigations of the local enhancement of the Raman spectra on the catalyst surface were carried out with pyridine as a probe molecule, since the SERS enhancement factor for this molecule is very high [12–21]. Both the frequencies and the relative intensities in the SERS spectra (Fig. 11a–c) differ from those of bulk pyridine (Fig. 11d) indicating that indeed we are measuring the Raman spectra of the adsorbed molecules. Fig. 11b shows a part of the SERS spectrum of pyridine adsorbed on a Cu-rich domain of the 10-yearold 50Cu–50Zr catalyst. Comparison of this spectrum with another measured on the rough surface of Cu metal (Fig. 11c) shows that they are almost identical. This confirms that the SERS signals measured for the Cu–Zr catalyst originated from the pyridine molecules adsorbed on the Cu domains. The microscopic facility associated with our Raman spectrometer enables measurement of SERS activity of various areas of the surface of the Cu–Zr catalysts with a lateral resolution of 1 mm. It has been found that the surface is highly inhomogeneous: large SERS signals are measured from the Cu-rich, ‘flower-like’ domains, whereas the intensity drops by a factor of 9–15 when one attempts to measure the SERS signals from Zr-rich areas, outside the Cu ‘flowers’ (see Fig. 11a,b where this ratio is 1 and 15). Moreover, the effect of the electrode potential on the SERS signal for various surface domains (of about 1 mm2) differs significantly suggesting that the activity of Cu clusters at different surface sites differs accordingly [21]. SERS investigations provided an insight into the degree of surface segregation of Cu–Zr amorphous alloys in the course of the ageing/devitrification process. It has been found that the SERS intensity attains a maximum on Cu-rich ‘flowers’, but its value depends on the devitrification time [21]. 2.2.4.2. Effect of electrochemical pretreatment on SERS. The most pronounced effect of electrochemical pretreatment was the disappearance of domains of poor SERS activity for electrochemically activated samples. SERS intensity increased considerably outside the Cu-
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Fig. 12. Schematic representation of the surface of the aged Cu – Zr AA before and after the electrochemical pretreatment with procedure (3) (NP-AD); IR is the Raman intensity.
rich ‘flowers’ leading to some kind of homogenization of the surface. Hence, the situation reported in Fig. 11 was not observed any more. This strongly suggests that the electrochemical activation creates new surface clusters of Cu atoms in the Zr-rich areas, which then become SERS active. Probably they are formed during the disproportionation reaction of the dissolving Cu + to Cu metal and Cu2 + [12,16] and are the result of a redeposition of Cu on the Zr-rich domains (i.e. outside the Cu ‘flowers’ as shown schematically in Fig. 12).
there is a correlation between the SERS and the catalytic activities of Cu-containing substrates.
Acknowledgements This work was financially supported by grant KBN 7 T08C 035 14 and by the Institute of Physical Chemistry PAS.
References 3. Conclusions We have developed a new pretreatment involving an ageing process followed by electrochemical dissolution of Cu microdomains of a partly devitrified metal which distinctly enhances the catalytic efficiency of Cu–Zr amorphous alloys for the dehydrogenation of 2-propanol. The mechanism of this enhancement, as the SERS investigations suggest, is the formation of some new Cu-rich sites in the Zr-rich areas during the course of the electrochemical activation (deposition of Cu metal due to the disproportionation reaction of the dissolving Cu1 + : Cu1 + Cu2 + +Cu0). SERS appears to be an excellent method to study the local activity of Cu-containing catalysts since
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