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 andCu metal by electrochemical methods

M. Janik-Czachor a,*, A. Kudelski b, M. Dolata a, M. Varga d, A. Szummer c,J. Bukowska b, A. Molnar d

a Institute of Physical Chemistry, Polish Academy of Science, Warsaw, Polandb Department of Chemistry, Uni6ersity of Warsaw, Warsaw, Poland

c Department of Materials Science, Technical Uni6ersity, Warsaw, Polandd 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 amorphousalloys (AA), to produce active substrates for various phenomena of adsorption and catalytic reactions. We developed some newmethods of activation of the Cu substrate for in situ investigations of adsorbates with SERS (Surface Enhanced RamanSpectroscopy). The first method developed produced an oxidized Cu surface. A distinct spectral shift of the bands characteristicof the adsorbate was observed, due to its interaction with Cu2O instead of interacting with metallic Cu. The second methodproduced a substrate with a clean surface and large specific surface area which resulted in a high quality SERS spectrum exhibitinga 10-fold increase in the signal-to-noise ratio, compared to the results for the surface pretreated by commonly used methods ofsurface roughening (oxidation–reduction cycling). The third method included an irreversible, diffusion-controlled Cu depositiononto a substrate and resulted in a rather complex, partially oxidized substrate with Cu clusters exhibiting a variety of SERSactivities. The second method appeared also useful for the modification of the surface activity of Cu–Zr amorphous alloys. Thismethod was combined with an ageing process of the AA to produce a partial devitrification of the substrate. The electrochemicalpretreatment was then applied after this partial devitrification. The catalytic efficiency for dehydrogenation of 2-propanol on sucha pretreated Cu–Zr substrate increased by a factor of two. A correlation has been found between the SERS activity of anelectrochemically pretreated substrate and its catalytic efficiency. A tentative mechanism of surface activation is discussed. © 1999Elsevier 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), anew class of catalysts, are of considerable interest [1–9].Many of these alloys are excellent precursors of effi-cient and selective catalysts [1–4,6–8]. Much work hasbeen done to understand the role and the effect ofvarious activation processes which result in a transitionof the given AA from a catalyst precursor into a stable,highly efficient catalyst [3–8]. Notably, Cu–Zr catalystsdeveloped from the corresponding AA ribbons ap-

peared efficient catalysts of many chemical reactionsincluding dehydrogenation and/or dehydration, iso-merisation [4,5], and hydrogenation [6–8].

Detailed studies by Molnar and coworkers [4,5,9]and by Baiker and coworkers [6–8] brought an insightinto 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 methodsof activation of the ribbons to produce efficient andstable catalysts.

In this paper, we summarize our efforts concerningthe following subjects:1. We report results on the development of a new

efficient method for roughening Cu metal elec-trodes, which increases surface activities including

* Corresponding author. Tel.: +48-22-632-3221; fax: +48-22-632-5276.

E-mail address: [email protected] (M. Janik-Czachor)

0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII: S0921 -5093 (99 )00096 -9

M. Janik-Czachor et al. / Materials Science and Engineering A267 (1999) 227–234228

Fig. 1. Current density vs. electrode potential during the oxidation–reduction cycling (ORC) of Cu in LiCl+CuCl2 [19] at 20 mV/sbetween −50 and +5 mV (SCE). Only the first and 50th cycles areshown. This is the standard roughening procedure for SERS experi-ments.

treatment to give rise to a partial redeposition ofmetallic Cu from the disproportionation reaction ofdissolving Cu1+ to Cu2+ and metallic Cu. Bothpretreatments proved to enhance the activity of Cumetal and Cu–Zr ribbons as the SERS substrates[12–16].

In our attempts to achieve these goals, the followingtechniques were used for the investigations: electro-chemistry, microscopy (optical, SEM, EDS/WDS X-rayelectron microprobe), spectroelectrochemistry-SERS,catalytic tests.

2. Results and discussion

2.1. Cu metal

2.1.1. Electrochemical measurementsThe following methods of surface pretreatment/

roughening were used:1. Procedure 1. Oxidation–reduction cycling (ORC); a

standard electrochemical method consisting of rapidcycling of the potential from a cathodic to an anodicside resulting in a dissolution/redeposition of themetal surface in 0.2 M LiCl+0.01 M CuCl2 solu-tion (Fig. 1) [17–19].

2. Procedure 2. Special electrochemical pretreatment(see [13,14] for details).

3. Procedure 3. A new procedure—anodic rapid disso-lution (NP-AD); Cu electrode is anodized in 0.4 MH2SO4+0.2 M CuSO4, at E=0.47 V (SCE) for ashort time [12] (Fig. 2).

4. Procedure 4. Non-equilibrium cathodic deposition;Cu electrode was covered by Cu electrodepositedfrom 0.4 M H2SO4+0.2 M CuSO4 solution atE= −0.5 V (SCE) (Fig. 3) [20].

SERS (Surface Enhanced Raman Spectroscopy)phenomena.

2. We use our experience with Cu to enhance thesurface activity of Cu–Zr AA, and to find a correla-tion between the catalytic and SERS activity of thesubstrate.

3. We describe a new and efficient method of activa-tion of Cu–Zr AA ribbons which transforms theminto active catalysts. The method combines an age-ing/devitrification (exploiting the poor stability ofthese ribbons [10,11]), and an electrochemical pre-

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, atE=150 mV (SCE). The dramatic difference in the anodic behavior of the two kinds of materials is highlighted.

M. Janik-Czachor et al. / Materials Science and Engineering A267 (1999) 227–234 229

Fig. 3. Current density vs. time curve for a non-equilibrium cathodic deposition of Cu onto a carefully polished Cu substrate from 0.4 MH2SO4+0.2 M CuSO4, at E= −0.5 mV (SCE). The slope confirms the diffusion-controlled deposition process.

2.1.2. Microscopic examinationsSEM and EDS/WDS X-ray electron microprobe ex-

aminations revealed large morphological and composi-tional differences between the Cu surfaces pretreated withfour different procedures listed above (Figs. 4–7). Highlydeveloped surface morphologies were produced with theaid of procedures 1 (Fig. 4) and 4 (Fig. 7). Both surfaceswere oxidized and contaminated with C. Moreover, someCl− contamination from the electrolyte was found on thesurface pretreated with procedure 1 (Fig. 4). Procedure2 produced rough surfaces covered with Cu2O. Procedure3 produced highly developed and clean surfaces with nooxygen signal detectable by WDS/EDS. The surfaceexhibited a peculiar morphology characteristic of a rapiddissolution of the substrate with some small metalparticles randomly distributed on top (Fig. 6).

2.1.3. SERS measurementsSERS results obtained from rough Cu surfaces pro-

duced by our developed electrochemical methods (includ-ing our new procedure of rapid anodic dissolution(NP-AD)) were compared with those obtained for arough Cu surface made by the standard ORC method(Procedure 1) [12].

Pyridine was used as a probe molecule to study thesurface activity of various rough Cu electrodes becausethe SERS enhancement factor is high for this molecule.

SERS confirmed that procedures 2 and 4 producedoxidized Cu surfaces with a spectral shift characteristicof an interaction of pyridine with Cu2O. The results aresummarized in Table 1. There is no doubt that our newprocedure 3 gives superior results. Therefore, this methodhas been used for the experiments with Cu–Zr AA.

2.2. Cu–Zr amorphous alloys

2.2.1. Electrochemical measurementsThere was a large difference in the anodic behaviour

of Cu and Cu–Zr AA (Fig. 2). For the AA, theelectrochemical pretreatment was terminated after 10 s,i.e. well before the anodic current density (c.d.) droppeddown from about 20 mA/cm2 to a value of about 40mA/cm2. This behaviour was dramatically different fromthat of pure Cu specimens where the same pretreatmentresulted in a stationary c.d. of about 20 mA/cm2 (Fig. 2).These results showed that our attempt to modify the Cumicrodomains on the aged AA ribbons by a prolongedanodization, as was done for pure Cu [12], was notsuccessful. Due to the distinct c.d. decay in time forCu–Zr AA (Fig. 2), the charge passed during a prolongedCu metal dissolution is much larger than that which canbe passed by anodization of Cu–Zr AA. So a muchsmaller surface alteration was achieved in the case of theAA.


One should note here that the higher the Zr concen-tration in the alloy (Fig. 2b), and the longer the ageingtime (Fig. 2a), the steeper is the c.d. decay. This sug-gests that a selective dissolution of Cu from the ribbonmay result in a local increase in Zr concentration whichthen facilitates passivation of the devitrified alloy sur-face during anodization.

Fig. 6. SEM micrograph of the Cu surface pretreated with method 3(NP-AD). A developed morphology is visible with some small parti-cles on the longitudinal strips.

Fig. 4. (a) Typical secondary electron image of Cu surface roughenedby the standard method (1), ORC. The image was obtained by theX-ray electron microprobe analyser. The ellipsoidal particles formedon the surface during the redeposition of Cu are easily visible. (b)Electron microprobe/EDS results of a line scan across the surfacebetween points ‘1’ and ‘2’, marked in Fig. 5a, showing an enrichmentof oxygen and some Cl contamination of the particles.

Fig. 7. SEM micrograph of the Cu surface pretreated with method 4(non-equilibrium deposition). ‘Flower-like’ deposits are visible withsome small particles in between.

Table 1Estimated SERS intensities for Cu roughened with various electro-chemical procedures

ISERSProcedure

1. ORC, oxidation–reduction cycling in LiCl+CuCl2 10.21a. ORC, oxidation–reduction cycling in KCl

2. Oscillating reaction roughening 0.063. NP–AD, new procedure; anodic dissolution in 6–10

CuSO4+H2SO4

4. Cu deposition, non-equilibrium conditions 0.3

Fig. 5. SEM micrograph of the Cu surface pretreated with method 2.

2.2.2. Microscopic examinationsOne 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].


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) distributionof Cu Ka; (d) distribution of Zr La.

Characteristic ‘flower-like’ microdomains rich in Cuwere formed on the wheel (dull) side (Fig. 8).

No distinct morphological changes were produced bythe anodization, as revealed by microscopic examina-tions up to a magnification of 10 000× . However, thewheel side of the ribbon changed in colour from red tobrownish, whereas the free side did not exhibit anyoptical change.

Electron microprobe/EDS results have shown thatthe wheel side of the electrochemically pretreated sam-ples was distinctly oxidized, particularly near ‘flower-like’ 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 sur-

face area of the Cu-rich domains on a submicro-scopic scale, thus making them more efficient inaffecting the interaction of the adsorbed moleculesduring the catalytic reaction;

2. it may redeposit some of the metallic Cu (originat-ing from the disproportionation reaction of thedissolving Cu+) on the Zr-rich areas, thus increas-ing the total Cu-enriched surface area available forthe catalytic reaction;

3. during the catalytic test, the hydrogen may reducethe remaining oxides completely and activate the

electrochemically developed surface for the catalyticreaction. The increase in the catalytic efficiency ratioat the beginning of the catalytic test (Fig. 10) agreeswith this hypothesis which requires further experi-mental verification using high lateral resolution mi-croscopic techniques.

2.2.3. Catalytic studiesThe 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 overan aged amorphous 60Cu–40Zr alloy (a), over the same materialadditionally modified by an electrochemical pretreatment (b).


Fig. 10. Efficiency ratio between the conversion of 2-propanol overan aged amorphous 60Cu–40Zr alloy, and its value over the samematerial additionally modified by an electrochemical pretreatment asa function of reaction time. Ageing time: 8 months or 5 years. Fulllines are eye guidelines only.

8-month-old specimens. However, the efficiency ratiofor pretreated and untreated samples depends distinctlyon the ageing time (Fig. 10). It decreases with thecatalytic test duration for the 8-month-old sample,whereas it reaches a stable value of about 1.8 for the5-year-old sample, suggesting that there is an interrela-tion between the ageing and the electrochemical pre-treatment which affects the final surface activity of thesubstrate.

2.2.4. SERS in6estigations

2.2.4.1. Effect of ageing/de6itrification. Investigations ofthe local enhancement of the Raman spectra on thecatalyst surface were carried out with pyridine as aprobe molecule, since the SERS enhancement factor forthis molecule is very high [12–21]. Both the frequenciesand 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 Ramanspectra of the adsorbed molecules.

Fig. 11b shows a part of the SERS spectrum ofpyridine adsorbed on a Cu-rich domain of the 10-year-old 50Cu–50Zr catalyst. Comparison of this spectrumwith another measured on the rough surface of Cumetal (Fig. 11c) shows that they are almost identical.This confirms that the SERS signals measured for theCu–Zr catalyst originated from the pyridine moleculesadsorbed on the Cu domains.

The microscopic facility associated with our Ramanspectrometer enables measurement of SERS activity ofvarious areas of the surface of the Cu–Zr catalysts witha lateral resolution of 1 mm. It has been found that thesurface is highly inhomogeneous: large SERS signalsare measured from the Cu-rich, ‘flower-like’ domains,whereas the intensity drops by a factor of 9–15 whenone attempts to measure the SERS signals from Zr-richareas, outside the Cu ‘flowers’ (see Fig. 11a,b where thisratio is 1 and 15). Moreover, the effect of the electrodepotential on the SERS signal for various surface do-mains (of about 1 mm2) differs significantly suggestingthat the activity of Cu clusters at different surface sitesdiffers accordingly [21].

SERS investigations provided an insight into thedegree of surface segregation of Cu–Zr amorphousalloys in the course of the ageing/devitrification process.It has been found that the SERS intensity attains amaximum on Cu-rich ‘flowers’, but its value depends onthe devitrification time [21].

2.2.4.2. Effect of electrochemical pretreatment on SERS.The most pronounced effect of electrochemical pre-treatment was the disappearance of domains of poorSERS activity for electrochemically activated samples.SERS intensity increased considerably outside the Cu-

Fig. 11. SERS spectra of pyridine adsorbed from 0.05 M pyridine+0.1 M KCl solution on a 50Cu–50Zr alloy partially devitrified byageing 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 1and 15; (c) electrochemically roughened Cu surface, with the aid ofprocedure (1) (ORC); (d) normal Raman spectrum of 1 M pyridineaqueous solution.

catalytic activity due to the new electrochemical pre-treatment is observed. Similar results were found for


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 homogenizationof the surface. Hence, the situation reported in Fig.11 was not observed any more. This strongly suggeststhat the electrochemical activation creates new surfaceclusters of Cu atoms in the Zr-rich areas, which thenbecome SERS active. Probably they are formed dur-ing the disproportionation reaction of the dissolvingCu+ to Cu metal and Cu2+ [12,16] and are the re-sult of a redeposition of Cu on the Zr-rich domains(i.e. outside the Cu ‘flowers’ as shown schematicallyin Fig. 12).

3. Conclusions

We have developed a new pretreatment involvingan ageing process followed by electrochemical dissolu-tion of Cu microdomains of a partly devitrified metalwhich distinctly enhances the catalytic efficiency ofCu–Zr amorphous alloys for the dehydrogenation of2-propanol. The mechanism of this enhancement, asthe SERS investigations suggest, is the formation ofsome new Cu-rich sites in the Zr-rich areas duringthe course of the electrochemical activation (deposi-tion of Cu metal due to the disproportionation reac-tion of the dissolving Cu1+: Cu1+�Cu2+ +Cu0).

SERS appears to be an excellent method to studythe local activity of Cu-containing catalysts since

there is a correlation between the SERS and the cata-lytic activities of Cu-containing substrates.

Acknowledgements

This work was financially supported by grant KBN7 T08C 035 14 and by the Institute of PhysicalChemistry PAS.

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Modification of surface activity of Cu–Zr amorphous alloys and Cu metal by electrochemical methods