Ž .Applied Surface Science 137 1999 142–149

Surface segregation behaviors of amorphous Ni Nb alloy under65 35

oxidation in O at various temperatures2

Zhen Song a, Dali Tan a, Fei He b, Xinhe Bao a,)

a State Key Lab of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Chinab State Key Lab of C1 Chemical Engineering, Tianjin UniÕersity, Tianjin 300072, China

Received 10 June 1998; accepted 26 August 1998

Abstract

Variations of surface morphology and chemical composition of amorphous Ni Nb alloy after oxidation in 1 atm O at65 35 2Ž .temperatures from room temperature to 773 K have been investigated using scanning electron microscopy SEM , X-ray

Ž . Ž .photoelectron spectroscopy XPS and Auger electron spectroscopy AES depth profile methods. Inverse segregationŽ .behaviors have been observed at two oxidation temperature ranges. At the lower temperature range F373 K , Nb

Ž .segregates at the surface as Nb O ; while at the higher temperature range G523 K , Ni segregates at the surface as metallic2 5

Ni and NiO. Different oxidation mechanisms have been suggested to operate in the two oxidation temperature ranges, whichmay be concerned with different oxidation rates of Nb and diffusion speeds of O and Ni ions in the Nb O layer at different2 5

temperatures. The affinities of Ni and Nb to oxygen may act as the driving force of the segregation. q 1999 Elsevier ScienceB.V. All rights reserved.

Keywords: Catalysis; Amorphous; Ni–Nb alloy; Oxidation; Segregation

1. Introduction

It is well known that surface structures and com-positions of amorphous alloys are easily changed byvarious chemical and physical treatments. Accord-ingly, amorphous alloys can be employed as samplesfor the study of some fundamental problems ofcatalysis, such as the relationship between catalyticactivities and catalyst structures, the interaction ofactive components with the supports, the function ofpromoters in catalysts, etc. For this purpose, it is

) Corresponding author. Tel.: q86-411-4686637; Fax: q86-411-4694447; E-mail: xhbao@gingko.dlut.edu.cn

important to make clear the surface structure ofamorphous alloys after various treatments.

Niobium and its compounds have been investi-gated in recent years for their special use in catalysis

w x w xas promoters 1 , catalyst supports 2 as well asw xunique solid acid catalysts 3,4 . Nickel is a conven-

tional catalyst, which shows good catalytic propertiesfor many hydrogenation and dehydrogenation reac-tions, and is widely used in industry. However, veryfew reports have been published on the surfacestructure and catalytic property of amorphous Ni–Nb

w xalloys so far. Li et al. 5 had compared the catalyticactivities of amorphous Ni Nb alloy powder with60 40

metallic Ni powder in the reaction of styrene hydro-genation. They found that the activity of the amor-phous Ni–Nb alloy was 10 times higher than that of

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00466-8

( )Z. Song et al.rApplied Surface Science 137 1999 142–149 143

w xthe metallic Ni powder. Teruuchi et al. 6 andw xSakany et al. 7 had studied the catalytic property of

an amorphous Ni–Nb alloy added with small amountŽ .of platinum group metals -at-2% in the reactions

of hydrogenation, COqO and COqNO etc. All2

those alloys showed good activities.In the present work, variations of the surface

structure and chemical composition of an amorphousNi Nb alloy after oxidation treatments by O at65 35 2

temperatures from room temperature to 773 K havebeen studied by XPS, SEM and AES depth profilemethods. The relevant chemical reactions and atommigration processes in the near surface region arediscussed.

2. Experimental

2.1. The sample

The sample used in the present experiment was aribbon of amorphous Ni Nb alloy with 5 mm65 35

width and 20 mm thickness. It was prepared by thesingle-roller melt-quenching method, with a coolingspeed of 106 Krs. The amorphous state of the

as-quenched ribbon was checked by X-ray diffrac-Ž . Ž .tion XRD and differential thermal analysis DTA .

No crystalline diffraction peaks were detected in theŽ .X-ray diffraction Fig. 1a . The DTA result shows

that the crystallization temperature of the amorphousŽ .sample begins near 930 K Fig. 2 .

2.2. The treatments

SEM, XRD and XPS investigations of an oxi-dized sample were carried out after the oxidationtreatments in a flow micro-reactor, with O flow rate2

of 30 mlrmin. Oxidation temperatures were mea-sured by a thermal-couple near the sample inside thereactor.

AES investigations were carried out after oxida-tion treatments under the same conditions as abovein a reactor connected with the detection chamber.The thermal-couple in the reactor was contacted tothe surface of the sample. The sample could betransferred under high vacuum from the reactor tothe detection position without being exposed to air.Except the analysis for the as-quenched alloy, thesamples were all cleaned by Arq bombardment be-fore the oxidation treatments.

Ž . Ž . Ž .Fig. 1. XRD patterns of amorphous Ni Nb alloy; a oxidized in air at room temperature as-quenched alloy ; b oxidized in O at 77365 35 2

K for 30 min.

( )Z. Song et al.rApplied Surface Science 137 1999 142–149144

Fig. 2. Crystallization behavior of amorphous Ni Nb alloy65 35

studied by DTA.

In order to compare clearly the changes of thesurface morphologies by SEM, the samples werepolished before the oxidation treatments. The sur-faces of the polished samples were flat, and nospatial structure could be detected under SEM obser-vation by a magnification of 5000 times.

2.3. Characterization

2.3.1. XRDX-ray diffraction analysis was performed on a

Japan Rigaku Drmax-rb X-ray diffractometer underthe following condition: CuKa , 50 mA, 40 kV andgraphite filter.

2.3.2. SEMThe surface morphologies of the alloy before and

after oxidation treatments were observed on a Hi-tachi-600 scanning electron microscope.

2.3.3. AES depth profileAES investigations were performed on a VG

HB50 A electron scanning spectrometer. The depthprofiles were obtained by recording the AES spectraafter pre-selected times of Arq bombardment. Thecurrent density of the ion beam used for bombard-ment was 20 mArcm2 at 4 keV. Under these condi-tions, the erosion speed of the ion gun was ;2nmrmin. The relative surface contents were esti-

Žmated by the peak–peak heights of Ni LMM 848. Ž . Ž .eV , Nb MNN 167 eV and O KLL 510 eV

spectra. The influence of carbon and the difference

in the sputtering speed for the elements mentionedabove are neglected here.

2.3.4. XPSXPS spectra were recorded with a VG ESCALAB

X-ray photoelectron spectrometer using AlKa radia-tion. The analyzer pass energy was fixed at 20 eV,corresponding to a resolution of 1.15 eV FWHM ofAg3d line. Binding energies were calibrated us-5r2

ing C1ss284.5 eV. The atomic sensitivity factorsfor Ni2p and Nb3d are 3.653 and 2.517, respectivelyw x8 , which were used to estimate the relative surfacecontents of Ni and Nb containing species from theXPS data.

3. Results

Fig. 1 presents the XRD patterns for the amor-Ž .phous Ni Nb alloy oxidized: a in air at room65 35

Ž . Ž .temperature the as-quenched sample , and b in 1atm O at 773 K for 30 min. Neither of the XRD2

Fig. 3. SEM image of the sample oxidized in O at 673 K for 302

min.

( )Z. Song et al.rApplied Surface Science 137 1999 142–149 145

patterns exhibited crystalline diffraction peaks. Itindicates that the structure of the alloy is very stable,and the sample maintains an amorphous state up to773 K under an O atmosphere.2

However, the SEM image in Fig. 3 shows that thesurface morphology changed obviously, with a lot ofsmall particles appearing on the surface, when theoxidation temperature was elevated to 673 K. SEMimages of the samples oxidized at 523 K and lowerdo not show any changes when compared with theas-quenched polished sample, on which no spatialstructure could be observed at the same magnifica-tion as that in Fig. 3. Crystallization to a certainextent must have happened at the surface under hightemperature oxidation, although the bulk still re-mained in the amorphous state.

Fig. 4 presents the depth concentration profiles of:Ž .a the as-quenched sample which had been oxidizedin air at room temperature for several months, and

Ž . qb–d the samples which had been cleaned by Arbombardments and then oxidized in 1 atm O at 3732

K, 523 K and 673 K, respectively, for 30 min. It isobvious that within the same oxidation time theoxide layer is thicker at higher temperatures than thatat lower temperatures, which indicates that oxygendiffuses more rapidly from the surface to the bulk athigher temperatures. The surface atomic ratios ofNirNb vs. sputtering time at different oxidationtemperatures are given in Fig. 5. The depth profilesreveal that the as-quenched sample had a very thickoxide layer on it, and the surface is extensively Nb

Ž .rich Fig. 4a and curve for RT in Fig. 5 . Oxidationof a cleaned amorphous Ni–Nb alloy at 373 K also

Žresulted in a slightly Nb rich surface curve for 373.K in Fig. 5 . As the oxidation temperature was

elevated to 523 K, a Ni rich surface began to formŽ .curve for 523 K in Fig. 5 . Oxidizing at 673 K, the

Žsurface became extensively Ni rich curve for 673 K

Ž . Ž . Ž .Fig. 4. AES concentration depth profile of the samples: a oxidized in air at room temperature; b oxidized in O at 373 K for 30 min; c2Ž .oxidized in O at 523 K for 30 min; d oxidized in O at 673 K for 30 min.2 2

( )Z. Song et al.rApplied Surface Science 137 1999 142–149146

Fig. 5. Atomic ratio of NirNb vs. Arq sputtering time forŽ . Ž .samples: a oxidized in air at room temperature; b oxidized in

Ž .O at 373 K for 30 min; c oxidized in O at 523 K for 30 min;2 2Ž .d oxidized in O at 673 K for 30 min.2

.in Fig. 5 . The oxidation-induced segregation at vari-ous temperatures indicates that there may be at leasttwo competition processes tending to result in differ-ent segregation during the oxidation of Ni–Nb al-loys.

Fig. 6 presents the Nb3d and Ni2p XPS spec-3r2

tra of the samples oxidized in air at room tempera-Ž . Ž .ture b and oxidized in O at 673 K c . For2

comparison, the Nb3d and Ni2p spectra of a3r2

‘clean’ surface of the alloy prepared by polishingand Arq bombardment for 12 min was also pre-

Ž . w xsented in Fig. 6 by curve a . According to Ref. 8 ,the peaks in the spectra can be assigned as follows:207.1 and 209.8 eV for Nb5q 3d and 3d ,5r2 3r2

202.8 and 205.5 eV for Nb2q 3d and 3d ,5r2 3r2

respectively; 852.7, 854.5 and 856.4 eV for Ni0,Ni2q and Ni3q, respectively. Correspondingly, onthe surface of the sample oxidized at room tempera-

Ž .ture curve b , the major component within the XPS

Ž . q Ž .Fig. 6. Ni2p and Nb3d XPS of the Ni Nb alloys; a polished and Ar bombarded in vacuum for 12 min; b oxidized in air at room3r2 65 35Ž .temperature; c oxidized in O at 673 K for 30 min.2

( )Z. Song et al.rApplied Surface Science 137 1999 142–149 147

Table 1Relative atomic concentrations of species on Ni Nb alloy surfaces after being oxidized at different temperatures65 35

Species Relative surface atomic concentration

Cleaned Sample oxidized Sample oxidizedsurface at RT in air at 673 K in O2

0Ni 29.2% 0.2% 25.6%2qNi 11.4% 0.1% 60.4%3qNi 0 1.9% 3.7%5qNb 10.2% 97.8% 10.3%2qNb 49.2% 0 0

investigation depth was Nb O , with traces of Ni O .2 5 2 3

On the surface of the sample oxidized at 673 KŽ .curve c , the major components were metallic Niand NiO, with traces of Nb O and Ni O . On the2 5 2 3

Arq bombarded surface, metallic Ni and NbO arethe major components with small amount of NiO andNb O . This indicates that Nb was first oxidized to2 5

NbO, and then to Nb O . Table 1 lists the relative2 5

atomic concentrations of the components on the threesample surfaces calculated from the XPS spectra.Inverse segregation behaviors on the Ni–Nb alloysurface oxidized at lower and higher temperatureswere also revealed by XPS results.

4. Discussion

Compared to other popular amorphous alloys,such as Ni–Zr, Ni–P, Cu–Zr, etc., amorphousNi Nb alloy is very stable for its amorphism. The65 35

Ž .crystallization temperature is very high 930 K ,while for other alloys, the temperature is usuallybelow 750 K. From this point, the amorphousNi Nb alloy is visualized to be stable during65 35

catalytic reactions. Structural unstability in catalyticreactions has been found to be the fatal problem forother amorphous alloys.

According to the above investigation, the amor-phous Ni Nb alloy has different segregation be-65 35

haviors under oxidation in O at different tempera-2Ž .tures. At lower temperatures -373 K , Nb segre-

gates at the surface in the form of Nb O ; while at2 5Ž .higher temperatures )523 K , Ni segregates at the

surface as metallic Ni and NiO. Therefore, theremust be different oxidation mechanisms for the oxi-dation process at the two temperature ranges.

A segregation process is concerned with the driv-ing forces to the segregated atoms and the diffusionbehaviors of the pertinent atoms. In vacuum, if thealloy temperature is high enough so that the atoms inthe bulk can migrate, the atoms with smaller diame-ter will segregate onto the alloy surface by a drivingforce for reducing the surface free energy. In thecase of gas phase existence, chemical reaction be-tween the gas molecules and the atoms in the alloywill play a dominant role in the surface segregation.In the present study of the oxidation of Ni–Nb alloy,the attraction of oxygen to metallic Ni and Nb maybe the primary driving force in surface segregation.When gas phase oxygen exists, both Ni and Nbatoms in the Ni–Nb alloy subject the attractiontowards the surface. However, the affinities of Niand Nb to oxygen are different. Since Nb is moreelectropositive, Nb has stronger affinity to oxygenthan that for Ni, so that it will be preferentiallyoxidized. Additionally, the oxidation and migrationprocesses of Nb in the near surface region may becompetitive. The difference in Nb oxidation rates atdifferent temperatures may be crucial to the segrega-tion.

At lower oxidation temperatures, the strongeraffinity of Nb to oxygen resulted in the segregationof Nb on the surface and formed a Nb O layer. For2 5

amorphous alloys of Ni–Zr, Cu–Zr, Pd–Zr, etc.,such preferential oxidation-induced segregation has

w xalso been found 9–11 . This phenomenon indicatesŽ .that the migration of Nb or Zr was faster than the

Ž .oxidation reaction to form Nb O or ZrO .2 5 2

At higher oxidation temperatures, the segregationof Ni and the formation of Nb O implied that2 5

oxidation of Nb became so fast that Nb might beoxidized before it could migrate. The leaving Ni in

( )Z. Song et al.rApplied Surface Science 137 1999 142–149148

the Nb O lattice then began to migrate towards the2 5

surface due to the attractive force from the abundantw xoxygen ions on the surface. Walz et al. 9 had found

similar phenomenon on the surface of amorphousNi–Zr alloys. They observed that the surface of anamorphous Ni Zr alloy was relatively Zr rich when91 9

the alloy was oxidized in 10y6 Pa O at 523 K or2

lower. As the oxidation temperature increased to 573K, the surface became Ni rich with respect to thebulk. For Ni–Zr alloys with higher concentration ofZr, the segregation turn-over temperatures becomesrather high, which can hardly be attained by oxida-tion at elevated temperatures. People found that treat-ing the sample first in H , then in O at appropriate2 2

temperature resulted in the segregation of Ni on thew xNi–Zr alloy surface 12,13 . An explosion reaction

between O and hydride was assumed to happen2

when the O was introduced to the H exposed2 2

surface. During the explosion reaction, the surfacetemperature could become very high instantly, at

w xwhich the sample became illuminant 13 . It can beassumed that it was the instant high temperatureoxidation that might result in a fast oxidation of Zrbefore it could migrate to the surface, and Ni segre-gation happened. It can also be assumed that themigration of Ni towards the surface begins just after

Ž .all the surrounding Nb or Zr had been completelyoxidized. The small particles on the surface, as ob-served by SEM shown in Fig. 3, might be the

Žsegregated Ni aggregating at high temperature 673.K on the surface.

w xStudies on oxidation of metallic Nb 14,15 re-vealed that the final oxidation product on the surfacewas Nb O , and the stable structure of it was NbO2 5 6

in octahedral form. This structure makes Nb O to2 5

be a good conductor for ions. Thus, from the aboveinvestigation, Nb O should be a good conductor for2 5

Ni ions.The formation of a small amount of Ni O on the2 3

Ž Ž . .as-quenched sample see curve a in Fig. 6 mayhave the same mechanism as that at high tempera-tures. It can be assumed that oxidation at roomtemperature in air in the near surface region of thealloy is so fast that the Nb in this region can beoxidized before it migrates. Then, the interstitial Niis attracted to the surface. With the increase inthickness of the oxide layer, the concentration ofoxygen at the interface of the oxide layer and the

alloy will decrease. Therefore, the oxidation rate ofNb will decrease and migration of Nb before oxida-tion becomes possible. In other words, segregation ofNb begins to occur. Since the Ni is in a disperedstate and its concentration is small on the as-quenched

Ž .surface, it is easy to be oxidized to the Ni III state.When large amount of Ni migrates to the surface athigh oxidation temperature, only part of them can be

Ž .oxidized to the Ni II state within 30 min of theŽ Ž . .treatment see curve b of Fig. 6 .

It can be inferred from the above analysis that theoxidation of Nb may occur at the interface of theoxide layer and the metallic alloy. Evidence for thisis that the small amount of Ni being formed at thebeginning of the oxidation on the as-quenched alloywas not buried by the large amount of segregatedNb. Oxidation of Ni may occur at the topmostsurface of the alloy because a large amount ofmetallic Ni was observed on the surface under highoxidation temperatures.

5. Summary

From the above results and discussion, the follow-ing conclusions can be obtained:

Ž .1 The amorphous Ni Nb alloy is stable in the65 35

bulk.Ž .2 Surface segregation induced by oxidation has

two temperature ranges. When oxidation occurs attemperatures lower than 373 K, Nb segregates at thesurface; while for oxidation at temperatures higherthan 523 K, Ni segregates at the surface. The affini-ties of Nb and Ni to oxygen may be the driving forceof the segregation.

Ž .3 Different oxidation mechanisms may operateat the two temperature ranges. At lower temperaturesŽ .-373 K , Nb migrates towards the surface becauseof its stronger affinity to oxygen and forms Nb O2 5

layer on the alloy surface. At higher temperaturesŽ .)523 K , Nb is oxidized rapidly to Nb O before2 5

it migrates. The leaving Ni in the Nb O lattice2 5

migrates to the surface by the attraction of oxygen,and then reacts with oxygen at the surface. Theoxidation of Nb may happen at the interface of theoxide layer and the metallic alloy. The oxidation ofNi may happen at the topmost surface.

Ž .4 Nb O may be a good conductor for Ni ions.2 5

( )Z. Song et al.rApplied Surface Science 137 1999 142–149 149

Acknowledgements

We want to thank Mr. Wensheng Sun for thepreparation of the samples. Financial support by theResearch Institute of Petroleum Processing, China

Ž .Petrochemical SINOPEC and the National NaturalŽ .Science Foundation of China NSFC are gratefully

acknowledged. One of the authors, Z. Song, thanksthe financial support for her post-doctorate from theScience and Technology Foundation of LiaoningProvince, China.

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