Studies of Ni—Cu catalysts with parallel chemisorption of O2 and decomposition of N2O

Journal of Molecular Catalysrs, 49 (1988) 71 - 83

71

STUDIES OF Ni-Cu CATALYSTS WITH PARALLEL CHEMISORPTION OF O2 AND DECOMPOSITION OF N20 J ZIELtiSKI Instztute of Physrcal Chemistry, Pohsh Academy 01-224 Warsaw (Poland) (Recewed December 23,1987,

of Sciences, ul Kasprzaka 44/52,

accepted February 2,198s)

Chemisorption of molecular oxygen and decomposition of mtrous oxide on NrCu powders and (NrCu)/y-Al,Os catalysts were studied. O2 1s chemlsorbed on NrCu alloys snmlarly to on Ni powder. Decomposition of NzO depends on the composition of the Ni-Cu alloy surface, it proceeds similarly on nickel or on copper for Cu surface concentrations, expressed as Cu surface atomic fractions, below and above -0.5, respectively. Chemlsorption of O2 and decomposition of N,O may be used for the determination of metal surface area and for momtormg the surface cornpositron of Nl-Cu alloys, respectively. Introduction Selective chemisorption is the method most often used for characterization of supported metal catalysts. The technique mvolves fmdmg the conditions of temperature and pressure under which a gas is chemisorbed to a fixed coverage on the metal, but not to any appreciable extent on the support Selective chemisorption allows determination of the dispersion of monometalhc catalysts and sometimes of the surface composition of alloy systems [ 1 - 51 In contrast with surface-sensitive techniques requnmg ultra-high vacuum, such as Auger and photoelectron spectroscoples (LEIS or PAX), selectlve chemisorption yields surface composition data under conditions more closely resembling those of many catalytic reactions Chemlsorption studies have a number of mterpretation problems However, when applied takmg these complications fully mto consideration, they can provide very relevant results with simple and mexpenswe equipment Nickel-copper is one of the most studied blmetalhc systems, and the volume of work concernmg the alloy is large [2 - 171. Considerable mterest has been shown m the surface composition of various NrCu alloys as a model m studies of surface segregation phenomena The chemisorption of hydrogen, which is selectwely adsorbed on mckel but not on copper, has revealed strong segregation of copper to the surface over a wide range of bulk compositions [2 - 51. Later this segregation was confirmed by Auger and photoelectron spectroscoples, and also by LEIS, APFIM and PAX studies 0304-5102/88/$3

50

0 Elsevler Sequola/Prmted m The Netherlands

72

[12 - 171 The experimental results agree with theoretical predictions [18 - 201. In order to characterize an alloy catalyst, at least two parameters should be determined. -total surface area (dispersion) of the alloy, and - composition of the alloy surface. The task is relatively simple m the case of unsupported NrCu alloys. The total surface area may be found by the BET method, and Hz chemisorption enables determmation of the number of surface nickel atoms. The characterization of NrCu supported catalysts is a real problem, as no method is available for determmation of total surface area of supported NrCu alloys and, consequently, the surface concentration of nickel cannot be found from selective chemisorption of H,. Chemisorption of molecular oxygen is often used to determine the metal surface area m supported nickel catalysts. At room temperature, chemisorption is rapid and an O/Ni, ratio close to 2 is usually attamed [21]. Molecular oxygen is not used for determmation of copper surface area smce, even at low (subambient) temperature, chemisorption of oxygen on the surface is followed by considerable oxygen uptake m the bulk [ 221. The surface area of copper m supported catalysts is usually determined by means of oxygen chemisorption performed usmg mtrous oxide as an adsorbate [23 - 251, which mteracts with copper according to the equation NzO + Cu, -

C&---O + N,

(1)

At room temperature the uptake of oxygen expressed as the O/C& ratio is restricted to about 0.25, and no bulk oxidation takes place. Oxidation occurs above 373 K. Hitherto there have been few studies on the decomposition of N,O on Ni surfaces [26 - 301; one of them concerns Ni powder and Ni supported on 7-A120s and SiOZ [30]. The reaction proceeds accordmg to an equation analogous to that for copper (eqn 1); oxygen is chemisorbed, but N,O itself and Nz are chemisorbed neghgibly. The kinetics and mechanism of N,O decomposition on the Ni surface are discussed [ 301. Recapitulatmg the above review, molecular oxygen and nitrous oxide can be used for the determination of Ni and Cu surface areas, respectively. This paper concerns the study of O2 chemisorption and N,O decomposition on NrCu unsupported and supported alloys This work has been undertaken m the belief that the parallel use of both reactants may be a useful method for characterization of Ni-Cu catalysts.

Experimental Apparatus

The interaction of molecular oxygen and nitrous oxide with NrCu alloys was studied m a flow system [ 311 equipped with a gradientless micro-

73

reactor [ 321. The gases were admitted by means of a sampling valve usmg hehum as a carrier gas flowmg through the reactor contammg a catalyst sample. The amount of O2 non-chemuorbed (or, alternatively, the amount of Nz formed) durmg N,O decomposition was determmed by means of a thermal conductivity detector The apparatus mentioned above was also used for (1) measurements of argon adsorption at 78 K to determine total surface area of Ni-Cu powders, and (ii) measurements of O2 uptake at 673 K to determine the amount of Ni and Cu metals m supported catalysts after their reduction. Materzals Hydrogen, helium and argon were of 99.99% purity. Hydrogen was additionally purified by diffusion through a palladium filter. Hehum and argon were purlfled by passing through successive columns packed with Cu/SiO, adsorbent, silica gel and molecular sieve 5A. Nitrous oxide and oxygen of 99.9% purity were further purified m cold traps mamtamed at 195 K. The NrCu powders and Ni-Cu supported catalysts were prepared via co-precipitation of the metal base nitrates by adding aqueous ammonia to an aqueous solution of mixed nitrates. In the case of supported catalysts, the precipitation was accomplished m the presence of y-A120s The alumina, prepared by hydrolysis of alummmm isopropoxide (surface area 111 m* g-’ and particle size 0 15 - 0.33 mm), was calcmed for 12 h at 973 K prior to use. The stirred suspension was evaporated to dryness, then calcmed m hehum under slowly mcreasmg temperature (0.7 K mm-‘) from room temperature to 673 K, and then kept at that temperature for another hour The studied materials are characterized m Tables 1 and 2. Method of measurements Typically about 200 mg of NrCu oxide or about 50 mg of (NrCu)/ y-Al,Os precursor were used. The sample was uniformly spread on the TABLE 1 Charactermatron of Nr-Cu powders Sample

1 2 3 4 5 6

Cu mean atomic rn

0 0 0 0 0 1

001 005 010 050

Prereductron temperature (K)

723 723 723 723 723 523

Reductron, desorption leb;lperature

673 673 673 673 673 473

Drspersrona of alloy (D)

0.0100 0 0101 0 0107 0.0124 0 0150 0 0047

aDetermmed from argon adsorptron at 78 K. bEstlmated by the equatron proposed by TomPnek et al [20].

Cu bulkb atomrc fraction

Cu surfaceb atomic fraction

fxb)

K3)

0 0 0 0 0 1

0 0 08 0.38 0.61 0 96 1

0002 001 003 036

10 10 10 10 10 0

1 2 3 4 5 6

0 0 54 108 2 16 3 25 5

Copper content (wt W)

0 0 05 0 10 0.20 0 30 -

CU/NI nommal atomic ratlo

773 773 773 773 773 673

F’rereductlon temperature (K) 673 673 673 673 673 573

Reduction, desorptlon temperature (K)

‘Determined from 02 chemlsorptlon or N20 decomposltlon at 273 K bDetermmed from O2 uptake at temperature 673 K CEstlmated by the equation proposed by TomPnek et al [20]

Nickel content (wt W)

Sample

Charactenzatlon of (NI-CU)/~-AI~O~ catalysts

TABLE 2

0 0 0 0 0 0

20 24 26 27 28 13

Dlsperslor? of alloy (D) 0 0 0 0 0 1 056 105 19 26

Cu meanb atomic fraction (W 0005 001 004 015

0 0 0 0 0 1

23 41 71 90

Vs)

(xb)

0 0 0 0 0 1

Cu surfaceC atomic fraction

Cu bulk’ atomic fraction

2

75

porous disk of the reactor, dried m a helium stream (40 cm3 mm-‘, 673 K, 0 5 h) and prereduced. The reduction was carried out m a hydrogen stream (40 cm3 mm-‘) under lmearly mcreasmg temperature (2 K mm-‘) from ambient to the temperature mdicated m Table 1 or 2. At that temperature the sample was reduced for one hour longer. The samples of NrCu powders agglomerated during their prereduction. In order to regain uniformity of the NrCu powder layer, the samples were passivated with oxygen, ground up outside the reactor, agam placed mto the reactor, and finally rereduced After the reduction, hydrogen was desorbed from the samples by flushmg the reactor with a hehum stream (40 cm3 mm-‘, 0 5 h at the temperature shown m Table 1 or 2). On coohng the reactor to 273 K, portions of O2 or N,O were introduced every 2 mm. The amount of O2 non-chemisorbed (or, alternatively, the amount of N, formed) was used to calculate rates of the reactions. The rate was expressed as conversion probability (CP), defined as the ratio of the number of molecules converted to the number of molecule-metal surface collisions. The number of molecules reacted was found from the degree of conversion of O2 or N,O portions. The number of collisions was calculated assummg that the concentration of reacting gas close to the metal surface was equal to the concentration m the stream leaving the reactor. A complete analysis of the methods of measurement has been given previously [ 301. The surface area of the NrCu powders was found from measurements of argon adsorption at 78 K. In the calculation it was assumed that one Ar atom occupies 0.138 nm* [33] Dispersion of supported Ni and supported NrCu alloys was determined from O2 chemisorption at 273 K, assuming that the O/Nb and O/(Ni + Cu), ratio attamed 1.7 (the grounds for this value for (NrCu)/r-A1,03 catalysts are discussed below). Dispersion of copper supported on r-Al,O, was determined from the measurement of N,O decomposition at 273 K, assuming that the O/C& ratio reached 0.25. In the calculation it was assumed that the number of metal atoms per square metre of NrCu alloys and of pure Cu was equal to the value reported for pure nickel, z e 1.55 X 10lg [5]. Results and discussion Reaction of O2 and N,O with Nl-Cu powders Figure 1 shows the probabrhty of conversion for the reaction of O2 and N,O with NrCu powders of various copper contents The probability for oxygen chemisorption on pure NI powder (Fig. la) is high until the O/Ni, ratio attams ca 1 5, then it drops to a low value as the O/Nb approaches 1.7. The chemisorption of O2 on NrCu powders (Figs. lb - e) proceeds m a manner similar to that on pure Ni powder (Fig. la). Henceforth the term ‘high’ conversion or chemisorption probability is used to mean that the probability is higher than lo-‘. The probability m this range cannot be measured exactly m the flow system used m this work.

76

0.5

1.0

1.5

20

100

h Li!izl e

50 0

100,

,

I

m;5jGOO’,\

100

I

1 50

&=o

050

N20

02

1.0

0.5

f

1.5

2.0

1.5

2.0

C”

02

N20

0.5 1.0 O/(NI+CU&

1.5

ratlo

2.0

Fig 1 Probablhty of convemon

0

a5 1.0 O/(NI+CUI~

ratlo

for reactlon of O2 and N20 with NrCu

powders

The chemlsorptlon of O2 on pure copper powder (Fig. If) proceeds differently than on pure Nl and NrCu powders (Figs. la - e). The probablllty of O2 chemlsorptlon on Cu powder is mltlally high and decreases to a measurable value as the O/C& ratlo approaches ca 0.25. However, the uptake of O2 still continues at a slowly decreasmg rate. Under the experlmental condltlons of this work (0, exposure of an order of lo9 L, 1 L = 133.3 X 10V6Pa s-l), the O/C& ratlo attains a value close to 1, z e. almost two tnnes lower than that attamed for Nl powder. The relatlonshlps of conversion probablhty for the reaction of NzO with NrCu powders are more complex (Fig. 1). In the case of pure Cu powder, the decomposltlon of N,O (Fig If) 1s m&ally fast and then slows to a very low value as the O/C!& ratio attams 0.25. This ratio 1s close to those reported by Dell et al [23] and Scholten and Konvahnka [24]. The decomposltlon of N,O on Nl powder (Fig la) has been extensively studied [30]. Sunultaneous measurements of the kmetlc and thermal effects of the reaction have shown that durmg N,O decomposition on the NI surface, the uptake of oxygen proceeds m stages. -fast chemlsorptlon of oxygen on the surface until the O/Nl, ratio approaches 0.3 - 0.4, -nucleation and growth of the surface nickel oxide islands until a closed layer 1sobtamed; the O/NI, ratlo attams 1.15; - slow thlckenmg of the surface oxide layer, the process 1s slow, so its rate cannot be measured exactly by means of the flow system used m this work.

77

Fmres lb - e show also the probablhty of N20 conversion on NrCu powders having various copper concentrations. Imtlally the decomposltlon of N20 1s fast regardless of Cu concentration. In the second stage the reaction rate depends on copper content. (1) low concentration of copper (Fig. lb) does not mfluence the rate of N,O decomposltlon, (11) moderate Cu concentration (Fig. lc) hampers N20 decomposltlon, but the O/(Nl + Cu), ratio attams the same fmal value as for pure Nl powder, and (in) relatively high Cu concentration (Figs. Id, e) obviates the second stage of N,O decomposition. The measurements of O2 chemlsorptlon and N20 decomposltlon carried out m this work on NrCu powders have revealed two unportant facts - chemlsorptlon of O2 on NrCu powders 1s mdependent of copper concentration up to ca 5 at.%. This suggests that such chemlsorptlon may be used for determmatlon of surface area of NrCu alloys, for low copper content alloys, the O/(Nl + Cu), ratio attams ca 1.7, - decomposltlon of N,O on NrCu alloys 1s sensltlve to copper concentration, which suggests that the reaction may be used for monitoring surface composltlon of the alloys; the apphcatlon of this method needs asslgnatlon of a correlation between the reactivity of N20 and the surface composition of the alloy The surface composltlon of NrCu alloys mvestlgated m this work was estunated on the basis of literature studies In this work the problem concerned polycrystalhne samples of very low copper content (below 5 at.%) and annealed at 673 K. Table 3 presents available expenmental data on Cu surface concentration (the topmost layer) for low copper content NrCu alloys. Henceforth, surface and bulk concentrations of copper are expressed as atomic fractions. An extensive dlscusslon of these results 1s beyond the scope of this paper. The analysis done by Weber et al [15] emphasized that to some extent all measured surface concentrations of copper can be regarded as mmmmm values; unless great care 1s taken, experunental results can show less than equlhbnum segregation of copper TABLE 3 Summary of surface composltlons for various NrCu Cu bulk concentration (at %) 93

10 5 5 1 3

7 (111) (a) (100) (110)

alloys

Ref

Temperature F)

Cu surface atomic fraction measured

Cu surfaceb atomic fraction calculated

12 13 14 15 16 17

600 673 550 850 - 920 873 700 - 800

0 0 0 0 0 0

0 99

34 86 54 85 - 1.00 45 68 - 0.70

*Various low and high mdex planes bEstlmated by the equation proposed by Tomanek et al [ 201

0 98 99

0 0 0 0

89 - 0 84 57 93 - 0 86

78

Table 3 shows that the experimental data on Cu surface concentration are scattered. Apphcation of these results for the determination of surface composition of the samples mvestigated m this work needs some recalculation, since the literature studies and measurements carried out m this work were made at different experimental conditions. This calculation requires a knowledge of additional factors: (1) the form of the relation between copper concentration m the bulk and on the surface, and/or (n) the effect of temperature on the segregation The available experimental studies do not supply this mformation, so the recalculation cannot be accomplished without the help of a theoretical relation. Segregation m Ni-Cu alloys has been the subIect of several theoretical studies [ 18 - 201, U-I which various models of the alloy were applied The crucial point m these works was verification of theory by comparison of calculated values for Cu surface concentration with experimental data. The surface composition of NrCu alloys studied m this work was determined using the semiempirical equation proposed by Tomanek et al

WI*

XS l-xx,

/

xb

Q seg

l-x,, -=exp

RT

(2)

m which X, and Xb = fractions of copper atoms on the surface and m the bulk, respectively, Qseg= heat of segregation, 35 5 kJ mol-’ [20]; T = temperature, K The magnitude of the heat of segregation Qseerwas estimated [20] takmg mto account two contributions (1) the difference m the heats of vaporization of nickel and copper, and (11) the stram energies arismg from different atomic sixes of the constituents The equation proposed by Tomanek was verified by comparison of calculated values for Cu surface concentration with the experimental data (Table 3). The comparison shows that the calculated values are m reasonable agreement with more recent experimental results [15 - 171 This indicates that Tominek’s equation may be used to calculate segregation m low copper content Ni-Cu alloys, however, it is impossible to Judge the precision of the values obtained. In estlmatmg segregation m dispersed Ni-Cu alloys, apart from eqn. (2), an equation describmg mass balance was also used. The equation had the form X,*D+Xb*(l---)=X

(3)

m which X = atomic fraction of copper m the alloy, and D = dispersion of the alloy as a fraction of surface atoms exposed Segregation m Ni-Cu powders was calculated for 673 K, at which temperature the samples were heated m hehum before testmg their reactivity to O2 and N,O. The calculated values of surface and bulk concentrations of copper are presented m Table 1 The calculation indicates strong segregation of copper to the surface. In the case of Ni-Cu powder of the lowest Cu content, about 80% of the copper IS present on the surface. For

79

the alloy of the highest Cu content, the surface atomic fraction of copper attams 0.96. Figure 2 shows the fmal O/(Ni + Cu), ratio attamed m the course of O2 chemisorption and NzO decomposition on Ni-Cu powders. In the case of O2 chemisorption, the O/(Ni + Cu), ratio is independent of copper concentration and equal to -1.7 This suggests that chemisorption of molecular oxygen may be used to determine the surface area of NrCu alloys The fmal O/(NI + Cu), ratio attamed during NzO decomposition (Fig 2) depends on the surface composition of the NrCu alloy. It equals 1.15 and 0.25 for Cu surface concentrations below 0.4 and above 0.6, respectively Somewhere m the range 0 4 - 0 6 the O/(Ni + Cu), ratio drops from the value characteristic of mckel to the value characteristic of copper Elucidation of the effect of copper on the decomposition of N20 on NrCu alloys needs further studies. The chemisorption of O2 on various Ni-Cu crystallographic planes has been extensively studied [16, 34, 351 It was found that the mteraction proceeds m two stages: (1) dissociative chemisorption of oxygen on the surface, and (ii) formation of surface NrCu oxide, the rate of both processes decreases with Cu surface concentration. The stepwise progress of O2 chemisorption on NrCu alloys was not observed m the measurements performed m this work, due to the high rate of the mteraction This question has been thoroughly discussed previously [ 301. A study of 0, chemisorption on CuNi(100) and CuNi(110) planes [16,35] showed that during both the chemisorption and the oxidation stages an enrichment of the surface with nickel occurred. This migration likely also happens durmg the course of O2 chemisorption and NzO decomposition performed m this work At the same time, it should be noted that the fmal O/(NI + Cu), ratio presented m Fig. 2 (and also Fig. 5) is related to the surface composition of NrCu alloys that was estabhshed during heatmg of the samples m hehum prior to any contact with the adsorbates. The final O/(Ni + Cu), ratio attamed m the course of N,O decomposition on NrCu powders (Fig. 2) decreases m a narrow range (0.4 - 0.6) of Cu surface concentration, mdicatmg that the surface composition of the samples is quite uniform. This fact is amazmg, since both (1) a difference m the mean concentration of copper m separate NrCu crystalhtes, and (11) a difference m the crystalhtes size m the samples should cause non-umformity of the surface concentration of copper. It seems that the surface composition of NrCu powders is uniform as a result of mtercrystalhte migration of copper, which occurs durmg the reduction and Hz desorption processes. Besides, it seems that, m the case of ideally uniform surface composition of Ni-Cu alloys, the fmal O/(Ni + Cu), ratio should decrease rapidly as Cu surface concentration exceeds ca 0.5 The study of N20 decomposition on a sample of NrCu powder can supply information concernmg its surface composition* -the final O/(Ni + Cu), ratio equal to 1 15 indicates that Cu surface concentration of any fraction of the alloy is lower than ca 0.5,

80

0 surface

copper atomtc

froctton

v

. CU/NI ratlo

Fig 2 The final O/(Nl + Cu), ratlo for reactlon of O1 and NzO with NI-Cu powders Fig 3 Composbon

of reduced (Nl-Cu)/y-Al203

catalysts

-the final O/(Ni + Cu), ratio equal to 0.25 indicates that Cu surface concentration of any fraction of the alloy is higher than cc 0 5, -the fmal O/(Ni + Cu), ratio between 0 25 and 1.15 indicates one of the two posslbihtles. either (1) Cu surface concentration is uniform and equal to ca 0.5, or (11) the surface composition of the sample is non-uniform: a fraction of the surface has a Cu surface concentration lower and a fraction higher than ca 0 5. Reactzon of O2 and N20 with (Nz-Cu)/r-A1203 catalysts The catalysts are characterized m Table 2 The amount of nickel and copper produced during prereduction of the catalysts were determined from Oz uptake at 673 K, assuming that NiO and CuO were formed. The measurements (Fig. 3) suggest that copper is completely reduced and the degree of nickel reduction is constant. The conclusion was supported by temperature-programmed reduction studies of a 5% Cu/r-Al,O, precursor, which showed that the sample was reduced completely below 623 K Dispersion of NrCu alloys supported on alumma increases with copper content (Table 2) This suggests that copper enhances the rate of nucleation of metal crystalhtes durmg the prereductlon process, and thus a larger number of smaller NrCu crystalhtes is produced Figure 4 shows the probability of O2 and N,O conversion on (NrCu)/ r-Al,Os catalysts. The relations obtamed for O2 chemlsorptlon are slmllar to those for NrCu powders (Fig 1). The results support the previous assumption that the O/(Ni + Cu), ratio attams 1.7 for O2 chemlsorptlon on NrCu alloys supported on alumma The chemlsorptlon of O2 on 5% Cu/y-AlzOs catalyst (Fig. 4f) proceeds sunllarly to that on Cu powder (Fig. If), though a difference between these relations 1s found m the early stage of the mteractlon. In the case of 5% Cu/y-AIZOs catalyst, the probability of O2 chemlsorptlon decreases to a

81 100,

I

I

IO1 ‘1.: %=020 io2 /

0

0.5 1.0 ls O/(NI+CU)~ ratlo

20

Fig 4 F’robablllty of convemon catalysts

0

0.5 1.0 1.5 O/(NI*CUI~ratio

20

for reaction of 01 and NzO with (NrCu)/y-A1203

measurable value at a higher O/Cu, ratio than for Cu powder It seems that the drfference 1s due to an effect of temperature, which rises during O2 chemlsorptlon more m the case of small crystalhtes of supported catalysts than m the case of large crystalhtes of Cu powder. The decomposltlon of N,O on nickel and copper supported on alumina (Figs 4a, f) proceeds as on N1 and Cu powders (Figs la, f) This indicates that neither the dlsperslon of these metals nor the y-A1,03 has a significant effect on the chemical properties of N1 and Cu crystalhtes as monitored by NzO decomposltlon. These results suggest that alumma also does not affect the chemical properties of NrCu crystalhtes, as 1s observed for N1 and Cu crystalhtes. This conclusion 1s unportant m view of the mtentlon to apply this reaction to the study of NrCu supported catalysts. The relations obtamed for NzO decomposltlon on (NrCu)/y-Al,O, catalysts (Figs. 4b - e) are unlike those for NrCu powders (Figs. lb - e) Figure 5 shows the values of the final O/(Nl + Cu), ratio attamed on (NrCu)/y-Alz03 catalysts uersus the surface concentration of copper The surface composltlon of these samples was calculated m the same way as for NrCu powders The relation for supported catalysts (Fig. 5) is different from that obtamed for NYCu powders (Fig 2). It 1s seen that m the case of supported catalysts even the lowest copper content dlmmlshes the final O/(Nl + Cu), ratio below the value character&x of pure mckel and, m contrast, even the highest content of copper does not lower the ratio to the value characterlstlc of pure copper.

82

0

I

0.2 surface

0.4

0.6

copper atomic

Q6

0

froctton

Fig 5 The final O/(Nl + Cu), ratio for reaction of 02 and N20 with (NI-Cu)/r-A1203 catalysts

The mterpretatlon of the relation presented m Fig 5 1s based on the assumption that the apparent relation between the final O/(Nl + Cu), ratio and the Cu surface concentration of NrCu alloy supported on 7-A1203 1s identical to that of NrCu powder. Thus the relation obtamed for supported catalysts (Fig. 5) suggests that surface composltlon of NrCu crystalhtes m these samples IS non-umform The catalyst with the lowest Cu content has a fraction of the alloy crystalhtes of Cu surface concentration already higher than ca 0 5 On the other hand, the catalyst with the highest Cu content has a fraction of alloy crystalhtes of Cu surface concentration still lower than ca 0 5. Assuming that as a result of N,O decomposltlon on (NrCu)/y-Al,O, catalysts the final O/(Nl + Cu), ratio attams locally one of the two characterlstlc values, z e 0.25 or 1 15, then from the experimental, I e average value of the fmal O/(Nl + Cu), ratlo for a supported catalyst, it 1s possible to fmd the fraction of the alloy surface with Cu surface concentrations lower and higher than cu 0.5 The exammatlon 1s complementary to H2 chemlsorptlon studies of Nl-Cu alloys which permit determination of the number of Nl surface atoms, but 1s msensltlve to their dlstrlbutlon on the alloy surface The study of NrCu alloys usmg parallel O2 chemlsorptlon and NzO decomposltlon seems to be a promlsmg way to characterize NrCu catalysts. It will be mterestmg to apply this approach for testmg other NrCu catalysts prepared m a different manner. The method developed m this work may be useful m research on the preparation of NrCu supported catalysts with a desired uniform surface composltlon of the alloy

Acknowledgement This work was carried out wlthm Research ProJect 03.20

83

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