XPS study of copper aluminate catalysts

Applications of Surface Science 5 (1980) 49—64 © North-Holland Publishing Company

XPS STUDY OF COPPER ALUMINATE CATALYSTS G. ERTL, R. HIERL, H. KNOZINGER, N. THIELE and H.P. URBACH Institut für Physjkalische Chemie, Universitàt Mdnchen, 8000 München 2, W. Germany Received 30 July 1979

The oxidation states and compositions of the surfaces of CuAl

2O4 and NiO promoted CuA12O4 were investigated by X-ray photoelectron spectroscopy (XPS). It is shown that an appreciable enrichment of the copper content within the probing depth of CuA12O4 occurs as compared to the bulk concentration. This trend is still enhanced by the addition ofsmall amounts of NiO. The aluminates are being reduced in vacuo at temperatures ~ 200°C to form Cu20 and Al2 03. Reduction in CO and H2 at 500°C leads to the formation of metallic Cu within the probing depth, while the nickel appears to be only partially reduced. Prereduced catalysts can be reoxidized by NO and 02 at temperatures ~‘ 200°C. The original spinel structure can be reformed under fairly mild conditions (approximately 450°C),which is explained by the assumption of very small and highly reactive particles being formed during reduction—reoxidation cycles. Treatment of the 2t The catalysts low oxidation in NO + COstates reaction seemmixtures to be essential produces for Cu thespecies catalytic in low activity oxidation of the system. states and Cu

1. Introduction Cupric ions dispersed on alumina supports are being used as catalysts for oxidation reactions [1] and alkane dehydrogenation [2]. Moreover, cupric ions play an important role as principal components -of catalysts for emission control, namely for CO oxidation and NO reduction [3—5].Cupric ions have also proven to promote the reduction of Ni2~[6,7]. This wide-spread importance of cupric ions for catalytic systems has prompted an extensive physico-chemical characterization of the state of the copper dispersed on the support surface. The techniques applied include chemisorption [8], extended X-ray absorption fine structure [9], X-ray edge shifts [9,10], X-ray diffraction [9,11], ESR [9—13], XPS [9,14], optical spectroscopy [9,13], SIMS [15], inelastic ion scattering spectroscopy (ISS) [16] and electron probe microanalysis [171. These studies, in particular the extensive work by Friedman et al. [9], lead to the following description of the system: (a) low metal loading on moderate surface area supports or moderate loadings on high surface area supports calcined at temperatures ~ 500°C for less than 12 h did not show any copper-containing phase. Under these conditions the so-called “surface spinels” as described by LoJacono and Schiavello [18] are being formed.

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G. Erti et al.

/ XPS study of copper aluminate catalysts

(b) low calcination temperatures (~600°C)of samples with higher loadings (~‘4% per 100 m2/g) may lead to the formation of supported CuO. Bulk CuA12O4, however, was not detected. (c) Calcination at 900°C leads to the formation of bulk aluminate. Copper aluminate has been found to develop high adsorption capacity for NO [8,16] and it is an active catalyst for NO reduction [4]. Recently, it has been shown that activity and catalysts stability of dispersed CuO on alumina is increased by addition of small amounts of NiO (~1% wt) [191. We have therefore initiated research with the aim to understand the action of NiO as a promoter. In the present paper, we wish to report on XPS-studies of CuA12O4 and NiO promoted CuAl2O4. These studies include investigations of the reduction and reoxidation behavior and of the surface state after contacting the catalysts with stoichiometric and non-stoichiometric NO+CO mixtures. Some studies of NiA12O4 are also reported for comparison.

2. Experimental 2.1. Catalysts The ~~~203 support was obtained from CATAPAL SB (Südchemie AG, Moosburg, W. Germany) by calcination at 800°C for were 15 h.prepared The N2 BET surface area 2/g. The catalysts by impregnation of this support material was 70 m the support with aqueous solutions of Cu(NO 3)2 or Ni(NO3)2, or appropriate

mixtures of the two nitrates. The concentrations of the solutions were adjusted so that the final catalysts contained a nominal concentration of 10% wt of CuO or NiO or of 10% wt CuO + 3% wt2)NiO. aqueous suspensions driedair at for 130—160°C and The subsequently calcined at were 800°Cin 15 h. The under vacuum (< 10 Nm high calcination temperature has been chosen to ensure the aluminate formation. CuA1 2O4 has been shown to be thermodynamically unstable relative to A1203 and 2 [201. CuOcatalysts at temperatures <600°Cat an oxygen partial pressure 2 X io~Nm— The obtained show a spinel structure as shown byofXRD and optical spectroscopy (see section 3.2.1 .). Possible CuO and/or NiO formation remains below the limits of detectability. The oxide catalysts are designated 10 CuAl, 10 NiA1 and Table 1 Surface areas and compositions ofthe aluminate catalysts Catalyst

SBET

CuO

NiO

(m2 g~4)

%wt

%wt

1OCuA1 lONiAl

75 80

10.28

—

—

8.96

3NilOCuAl

76

10.23

2.84

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/ XPS study of copper aluminate catalysts

51

3 NilOCuAl in the following, where the numbers indicate the nominal concentrations in %wt of the transition metal oxides in the aluminate. Exact compositions as determined electrolytically (Cu) and gravimetrically (Ni) and N2 BET surface areas in table 1. Copper metal was used for reference measurements in form of a thinthe highreaction purity 4 Nrn2 02 at 400°Cin metal plate. This was exposed to 5.6 X i0 chamber of the spectrometer to obtain CuO, from which Cu 2O was prepared by thermal treatment at 400°C in UHV. A thin polycrystalline nickel plate of 99,999% purity was used to measure the Ni°binding energies after Ar+ sputtering of the prosur2 02 face. A thermal ducedNiO [21].treatment of the metallic Ni at 500°Cin 1.3 X 10—2 Nm— SBET are summarized

2.2. XPS measurements The experiments were performed with a modified commercial XPS apparatus (Vacuum Generators, ESCA 3). The experimental setup has been described in detail elsewhere [22]. The spectrometer chamber was equipped with an Al Kct X-ray source (liv = 1486.6 eV) and a hemispherical energy analyzer. It can be separated from the preparation and reaction chamber by a Viton-sealed valve. Chemical treatments of the samples are carried out within the preparation chamber at pressure up to i05 Nm—2, while the spectrometer chamber is held at Ul-IV conditions. The samples were mounted onto a manipulator which allowed the transfer from the reactor or preparation chamber into the spectrometer section and heating up to about 550°C. The temperature was measured by means of a chromel—alumel thermocouple. The X-ray source was operated with a stabilized electron current of 20 mA at an acceleration voltage of 12 kV. A binding energy of 83.6 eV for the Au 4f 712 levels was used as standard which always led to a constantC isbinding energy of 284.1 eV. As an internal standard of the catalysts a binding energy of 73.5 eV was thus obtained for the Al 2p level which was found to be independent of the catalysts composition and treatments. The accuracy of the binding energies as determined with respect to these standard values was within 0.2 eV. Quantitative analysis was achieved by the use of tabulated ionisation cross sections for the corresponding levels of the various elements [23,24]. The data obtained concern the weighted average of the composition in the surface region across the probing depth of the technique and have to be considered at best as semi-quantitative values reflecting the trends of surface segregation if compared with the bulk composition. Variations of the relative escape depths of photoelectrons of different kinetic energy have been taken into account. High purity gases (H2, 02, CO, NO) were introduced through leak valves. Pressures were5Nm2), measureda with membrane manometer P2OXPartial + UM8O, 65 to Pirania gauge and an ionization(Kontron manometer. pressures 1.3 X be l0 recorded with a quadrupole mass spectrometer (Vacuum Generators Q7) could

52

G. Erti eta!.

/ XPS study of copper alu,ninate catalysts

which was attached through a variable

leak valve and was pumped through a by-pass

line.

3. Results and discussion 3.1. Support and reference compounds The binding energies of the y-A12O3 support and of a number of reference compounds are summarized in table 2, which contains also some literature data for comparison. These data have been corrected taking into account different standards and standard binding energies used by different research All4fvalues in table 2 are T or groups. to the Au thus related to a C ls binding energy of 284.1 e\ 712 binding energy of 83.6 eV, as used in this work. 2P3/2 peaks of Cu2~in A and chemical shift of 1.7 eV is observed between the CuO metallic Cu°.The width at half maximum of Cu the Cu 2P 0 is 1.7 3/2is due in Cuto a muleV, while that in CuO amounts to 3.5 eV. This apparent broadening tiplet splitting in Cu2~[25]. Peak position and width at half maximum for all peaks of Cu~in Cu 2O are identical with those in Cu° within the limits of experimental 2~ accuracy. Cu~and Cu°oxidation canpeaks, thus not be distinguished. Cu peaks are The accompanied by the typical states satellite which are separated All from the main peak by approximately 8.5 eV. These satellite peaks can be assigned as 3d—~4s shake-up transitions [25]. The chemical shift between the Ni 2P3/2 peaks of Ni2~in NiO and metallic Ni° was found to be 2.0 eV. The width of the 2P3/2 peak at half maximum is increased from 2.0 eV of Ni°to 4.2 eV due to a multiplet splitting in Ni2~[25]. All Ni peaks are accompanied by satellite peaks at a separation of approximately 7 eV. These are assigned as either 3d -÷4s shake-up transitions [25] or interatomic charge-transfer transitions [21,26—281. As demonstrated by Wolberg et al. [14] and by Friedman et al. [9], Cu2p 312 binding energies can be used to 2P3/ distinguish between CuO and CuA12O4 phases in alumina supported CuO. The Cu 2 binding energy in CuA12O4 is higher than in 2~3/2 CuO by energies 1 eV (see 2). Although the absolute values of the reported Ni binding fortable NiO and NiAl 2O4 differ appreciably from each other, probably due to the uncertainty of the C is standard binding energy, an analogous increased binding energy for the NiA12O4 phase as compared to NiO can be observed if the corresponding values of individual research groups are considered (table 2). Differences of 1.0—1.7 eV are reported [31,33], while Ng and Hercules [291 3~in reporttheir the very high 2.3 eV, and whichCook might[32] suggest the presence of energy Ni sample (seedifference table 2).ofMcIntyre observed a binding of the Co 2P 3/2 level in CoAl2 04 which was also shifted by 0.6 eV to higher energies from CoO. A clear discrimination between oxide and aluminate phases in the catalysts is 2P therefore possible using the Me 3/2 binding energies.

G. Err! et a!. / XPS study of copper alurninate catalysts

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G. Err! eta!.

/ XPS study of copper aluminate

catalysts

~3,o

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~NIO

EB(eV) Fig. 1. Valence band spectra of ‘y-A1

203, Cu, CuO and NiO. (Ordinates displaced for clarity.)

Valence band spectra of y-A12O3, Cu, CuO, and NiO are shown in fig. 1. The valence band of ‘y-A12O3 clearly develops two peaks at 5.2 eV (A) and 8.9 eV (B), which are assigned as 0 2p bands. Sharpe and Vickerman [35] observed a similar band structure in spinel solid solutions of the type Cu~Mg1~Al2O4. They suggested the possibility that the two peaks could be attributed to surface and bulk0 2p bands, but this assignment appears to be rather unlikely because of the similar intensities of peaks A and B and other interpretations may be equally possible. The valence band of CuO develops a peak at 3.6 eV with a width at halfmaximum of 4.4 eV. This peak is shifted by 0.6 eV to higher energies as compared to metallic Cu,2~which also in has also occur thea narrowerband valence bandspectra width of andonly are 2.7 centered eV. The at typical 11.8 eV.satellites The valence for Cuband of NiO is narrower than that of CuD and shows a peak at 3.0 eV which is accompanied by a satellite at 9.8 eV. The NiO valence band spectrum in fig. I very closely resembles that reported by Wertheim et al. [361 who interpreted the asymmetry of the principal peak as being due to a crystal field splitting.

G. Ert! eta!.

/ XPS study of copper aluminate

catalysts

55

3.2. Aluminate catalysts 3.2.1. Oxidized samples The untreated I OCuAI sample gave rise to a binding energy of the Cu 2p3/2 level of 934.5 eV and the 0 is peak developed a shoulder towards high binding energies. This can be interpreted as being, due to the presence of surface hydroxyl groups [32] and, in fact, on heating the sample lost H2O as well as CO and CO2. All sam4Nm2O ples were therefore heat treated at 400°Cin 2.66 X i0 2 before XPS spectra were recorded. The binding energies measured after this treatment are summarized in table 3. The 0 lslevel has a slightly lower binding energy than in y-A1203 for all2psamples, peak is its band width atand halfcatalysts maximum equal in support (2.4being eV). 2.6 eV. The band width of the Al The 2P3/2 levels of Cu and Ni are significantly shifted to higher binding energies as compared to the corresponding oxides. The shift for Cu is 0.8 eV in 10 CuAI and 0.6 eV in 3 NilOCuAl, while one obtains smaller shift of 0.2 eV in 10 NiAl and of 0.4 eV in 3 Ni1OCuA1 for Ni. These shifts confirm the aluminate formation in the catalysts as proved by XRD and optical spectroscopy. Two characteristic bands at 750 nm and 1450 nm are observed in the optical spectra which are typical for CuA1 2~in octahedral and tetraand which are assigned as d—d transitions of Cu hedral2O4 oxygen environment, respectively [9,37]. The band width at half maximum of the Cu 2P3/2 remains practically unchanged as compared to CuO, while that of Ni 2P3/2 in the catalysts is reduced by I eV as compared to NiO. The valence bands of the three catalysts are shown in fig. 2. Two peaks are developed similar to those in the y-M 2O3 support, although the separation of the peaks is increased and the onset of the valence band emission is shifted to lower energy by 0.6 to 1 eV. These observations can be explained by assuming as a rough approximation an additive behaviour of the contributions of O2p levels and metal 3d levels to the overall band structure. It has tobe emphasized that the relative intensities —

—

Table

3 Binding energies (eV) of

Sample

Al 2p

oxidized catalysts

0

is

10 CuA1

73.5

530.1

10 NiAI

73.5

530.0

3Ni1OCuAI

73.5

530.i

Cu 2p312 + satellites

Cu 2p112

934.0, 942.0 (3.2 eV) a)

953.7, 96i.6

+

satellites

—

—

933.8, 94i.7 (3.6 eV)

953.6, 961.6

a) Values in parentheses indicate peak width at half maximum.

Ni 2p312 +

Ni 2p112

satellites —

—

855.0, 861.5 (3.2 eV)

872.9

855.2, 861.7 (2.6 eV)

872.9

56

G. Err! eta!.

/ XPS study of copper aluminate

catalysts

3~5 9,8

N (E)

0

Fig. 2.

5

Valence band spectra of samples

10 EB(eV)

15

(1): 10 CuA1, (2): 10 NiA1, and (3): 3NiiOCuAl.

of the metal 3d bands are higher by approximately one order of magnitude than that of the 0 12p band, so that the 3d band contributions may significantly alter the valence band emission even though the metal concentration might be low. Since the positions of the 3d band of CuD and NiO occur at lower values than peak A for ‘yAl203 (see fig. 1), the superposition of both contributions leads to a shift of peak A in the aluminates to lower energies. Results of the quantitative XPS surface analysis are summarized in table 4. Despite the relatively high uncertainty of the CuD concentrations in particular (approximately 30%), there is a clear enhancement of the Cu concentration in the surface layer of 10 CuAi as compared to the overall bulk composition (see table 1). If at all present, the surface enrichment is definitely much less pronounced in 10 NiA1. This result is in complete agreement ion in scattering experiments 2~iswith present the surface of CuAI by Shelef et 2~ al.ions [16] who have shown that Cu 2O4 cannot be detected in the surface of NiAl while Ni 2 04. Moreover, the adsorption capacity for NO of CuA12 04 is much higher than of NiAl2 04 [161 and CO adsorption can easily be monitored by infrared spectroscopy on 10 CuA1, while only Table 4 Surface composition of catalysts in the oxidized state Sample

Composition (%wt) Al2 03

CuO

NiO

10 CuA1

82.5

17.5

—

iONiAl

87.7

—

12.3

3Ni1OCuA1

76.5

20.3

___________

3.2

G. Ert! eta!. / XPS study of copper aluminate catalysts

2200 —

2100

57

2000

1)

wavenumber (cm

Fig. 3. Infrared spectra (CO stretching) of CO (1.3 X i0~Nm2) adsorbed on (1) 10 NiAl, (2) 10 CuAl and (3) 3Ni1OCuA1 (self-supporting wafers of identical weight have been used, spectral slit width was 5 cm1).

minute bands are developed on 10 NiAl [38] (see fig. 3). Thus, all these results confirm the surface enrichment ofCu in CuAl 2~for chemiand thealuminate accessibility ofCu sorption. The incorporation of Ni2~into 2O4 the copper (3NilOCuAl) further enhances the surface enrichment with Cu2+, the surface concentration of CuD in sample 3 Ni1OCuAJ amounts to approximately 20% wt (table 4) as compared to the overall bulk concentration of 10.23% wt. Infrared spectroscopy of adsorbed CO has confirmed these trends [38] as shown in fig. 3. The intensity of the ~COstretching vibration of CO coordinated onto Cu21’ is strongly enhanced in 3NilOCuAl as compared to sample 10 CuAl. (Wafers of equal weight have been used in these experiments). It should be mentioned that CO adsorbs only very weakly on the y-A1 2O3 support. As will be discussed in detail in a forthcoming publication [38], the addi2~by the effect of Ni2~is accompanied by an intional surface enrichment with Cusites by Cu2~.The ratio of the extinctions of the creasing occupation of tetrahedral bands in the optical spectra at 1450 nm (tetrahedral Cu2~)and 750 nm (octahedral Cu2~)is increased by a factor of 2.2 in sample 3NilOCuAi as compared to sample 10 CuAl. Ni2~enters preferentially octahedral sites due to its high octahedral site preference. 3.2.2. Vacuum reduction Heat treatment of the copper aluminate catalysts even at moderate temperatures (200—250°C)in vacuo leads to reduction of Cu2+ to lower valence states. This can easily be followed by the shift of the Cu 2P3/2 peak to lower binding energies and the gradual disappearance of the satellite bands which are typical for the +2 oxidation state (fig. 4). The final Cu2p 312 binding energy is 931.8 to 931.6 eV for cata-

58

G. Ert! eta!.

/ XPS study

of copper a!uminate catalysts

N CE)

930

935

9~0

9~5

EB (eV)

Fig. 4. Cu 2p3/

2 levels of sample 10 CuAI during vacuum reduction: (1) oxidized state, (2) room temperature, 3 h, (3) 120°C,15 h, (4) 250°C,45 mm, (5) 550°C,45 miss.

lysts lOCuAl and 3Ni1OCuA1, respectively, with a width at half maximum of 2.8 eV. This binding energy closely corresponds to that of bulk metal Cu°or Cu20 (see table 2). It seems reasonable to assume the formation of Cu2O (although a distinction between Cu+ and Cu°cannot be made on the basis of the binding energies), since a decomposition of Cu2O into the elements under such mild conditions does not usually occur. As a consequence, Cu20 and A12O3 must have been formed by vacuum reduction in a surface layer of a thickness which is at least equal to or larger than the electron escape depth (30—40 A), since Cu’~ions cannot be stabilized in a spinel lattice for electroneutrality reasons. The relatively large width of the Cu 2p312 peaks of 2.8 eV as compared to only 1.8 eV for bulk Cu2O suggests relatively small and poorly crystallized particles. Corresponding changes in the valence band structure are also produced, peak A being shifted towards the position of the valence2~is band Cu20 close to 3these eV. mild conditions neither in 3NiiOCuAl nor in notof reduced under Ni lONiAl. However,Ni2~in 3Ni1OCuA1 clearly reduced the rate of reduction of Cu2~, although the reduction rates could only be followed qualitatively by XPS. 3.2.3. Reduction in CO and H 2 Reduction experiments have of been performed in eitherdetectable CO or H2 at atmospheres at 4Nm2. The reduction Cu2~ by CO becomes temperatures 1.6 X 10

G. Err! eta!.

/ XPS study of copper a!uminate catalysts

59

I 200

100

300

400

500

Temperature 1C I Fig. 5. C0

2-evolution 2 CO) at during 15°/mmtemperature-programmed heating rate and total flow reduction rate of of 46.610 cm3/min CuAl in (CO CO/He flow (4 X l0~Nm 2 partial pressure was measured with a quadrupole mass spectrometer which was connected to a thermal conductivity detector via a jet separator).

of approximately 100°Cand final reproducible reduction state is obtained after treatment at 500°Cfor 2 h, indicating that the reduction is completed within the probing depth under these conditions. Temperature programmed reduction experiments confirmed these XPS observations [39]. An is given 2~satellites vanish, theexample band half width in offig. the5.CuDuring 2P3/2 the reduction the typical Cuto 1 eV as compared to the oxidized state and its posipeaks is diminished by 0.6 tion at 930.9 to 931.3 eV is at lower binding energies than for the vacuum reduced samples and also slightly lower than for metallic Cu0 (see table 2). Peak A of the valence band is also shifted to still lower binding energies than in the vacuum reduced state. The reduction of Ni2~by CO is much more difficult and becomes detectable

8554

N(El

861.6

850

855

860

865

EB (eV) Fig. 6. Ni 2p 312 levels of sample 3Ni1OCuA1 after reduction in 112 at

550°C.

-

60

G. Ert! eta!. / XPS study of ccpper aluminate

catalysts

Table 5 Binding energies (eV) of H2 reduced catalysts Sample

Reduction conditions

Cu2p312

350°C, 2h

930.8

950.6

440°C,15 h 520°C,18 h 550°C,16 h 10 NiAl 550°C,16 h 3Ni1OCuAI 550°C,16 h

931.1 931.0 930.8

950.8 950.8 950.6

—

—

930.9

950.9

10

CuAI

Cu2p112

Ni2p312

Ni2p112

~E112 (eV)

Cu —

—

—

—

2.3 2.4

—

—

2.5

—

—

2.7

855.4, 851.3 855.4, 851.3

873 872.5—873

—

2.5

only at temperatures as high as 500°C.After treatment at 550°Cfor 16 h only 10% of the surface nickel in sample 3Ni1OCuA1 was reduced. H2 reduction turns 2~ outinto much layer easiercan than reduction. temperature thebesurface be CO reduced, whileAtthea reduction of of 350°Cthe total Cu Ni2~only becomes detectable above 500°C.About 20—25% of the surface nickel in 3NilOCuAl are reduced after treatment at 550°Cfor 16 h, as shown in fig. 6. Hoste et al. [33], Ross and Steel [40] and Ng and Hercules [9] also reported on the difficulty to reduce Ni2~in spinel matrices. Characteristic XPS data of l~I2reduced catalysts are summarized in table 5. The 2P3/2 binding energies of Cu and Ni in the reduced catalysts are clearly below the values of the metal (table 2), while the width at half height of the peaks is larger. This can be seen for the Cu 2P3/2 peaks in fig. 7, where the spectra of the oxidized and H 2-reduced catalyst 3Ni1OCuA1 are compared with that of metallic Cu°.The valence band emission occurs already near the Fermi level in samples containing Ni°,

9309~ 9315

NIEI~/I~9~7~

930

935

940

945

I eV I 0 metal: (1) oxidized state, (2) after reFig. 7. Cu duction in 2p312 H levels of sample 3Ni1OCuAI 0 metal. and of Cu 2 at 550°C, (3) bulk Cu

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/ XPS study of copper a!uminate catalysts

5,3

61

9,4

N (El

E5 leVI Fig. 8. Valence band spectra of sample 3Ni1OCuAI

after reduction

in

112

at 550°C.

peak A occurs near 2 eV and only a weak feature is observed at 5.3 eV, while peak B at 9.4 eV remains unchanged (fig. 8). The intensity loss of peak A and its shift towards the Fermi level can be attributed to the disappearance on reduction of the O 2p bands of NiO and CuO and to the appearance of the 3d bands of Ni and Cu. Thus it seems reasonable to suggest the formation of zero valent Cu°and Ni°on reduction of the catalysts in either CO or H2. This assignment is confirmed by the CO stretching frequency which occurs at 2097 cm~ on reduced samples0 [38]. [41]. Bands nearthe 2100 have been assigned to CO adsorbed metallic However, lowcm—’ intensities of peak A in the valence band on spectra and Cu the 2P3/2 binding energies, which are lower by > 0.5 eV than for the bulk metals, is surprising. Analogous phenomena have been found for Fe 2P3/2 levels of reduced NH 3 synthesis catalysts [22,42] and have been explained by the small particle size, the paracrystallinity and the enclosure of alumina and aluminate particles. It seems likely to suggest the same interpretation for the present systems. 3.2.4. Reoxidation by 02 and NO Reoxidation experiments performed with the reduced by exposure 4 Nm—2 02 or NOwere at different temperatures. The Cucatalysts containing catalysts to 1.6readily X i0 reoxidized by both gases within a few minutes at temperatures above were 200°C.NO was decomposed to form N 2P 2 + 02 to some extent. The Cu 3/2 peaks appeared at 933.2 eV, which corresponds to the respective binding energy in CuD after 02 treatment at 450°Cfor 0.5 h of catalyst 10 CuAl. After a total oxidation time of 3 h a value of 934.0 eV was obtained which is very close to the typical vàlue of CuA12O4 and which coincides exactly with the value of the original oxidized catalyst 10 CuAI. Also the width at half height of the Cu 2p3/2 peak of the original and reoxidized samples was identical and equal to 3.2 eV. This suggests an initial reoxidation of the reduced catalysts to form supported CuO, which provided temperatures > 400°Care applied for ~ 2 h is transformed into the original spinel —

—

62

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/XPS study of copper a!uminate catalysts

phase even at these low temperatures. The colour and infrared spectroscopic studies of the CO adsorption also confirmed the spinel formation [38]. It should be emphasized at this point that a calcination temperature of 800°Cfor 15 h was necessary for the preparation of the original catalysts. The reasons for the ease of this spinel formation are not clear yet, one i-night, however, assume the formation of very small, highly disordered and reactive CuO and Al203 particles during the reduction—reoxidation cycles which might increase the rate of the spinel formation. Reoxidation by NO at temperatures around 200°Capparently produced a mixture of CuO and CuA12O4. Quantitative analysis of XPS intensities gave surface compositions of the reoxidized catalysts which were identical with those of the original oxidized state.

3.2.5. Effect ofNO + CO mixtures Since the catalysts are active for NO reduction and CO oxidation, it was interesting to test the surface state after contacting the catalysts with NO + CO mixtures. Stoichiometric and non-stoichiometric mixtures into the reaction 3Nm2were andadmitted 200°C. Reaction product chamber at a total pressure of 3.3 X 10 as determined by mass spectrometry were N 2 and CD2, while the formation of N2O could not be detected. Small amounts of NH3 were formed during H2 reduction of samples which had previously been contacted with NO. Total conversion was usually obtained after 1 h for the Cu-containing catalysts, the XPS spectra were then recorded after pumping off the gas. 1ONiA1 had only very low activity (30% conversion within 16 h) in agreement with reported data [8, 161. The Cu species after the catalytic reaction were always found to be partially reduced irrespective of the initial state of the catalyst surface and irrespective of the composition ofprethe 2+ were also gas phase oxidizing). varyingstate. amounts Cu sent. The (reducing nickel wasoralways in theHowever, +2 oxidation Whileofprereduced catalysts were active from the very beginning, an oxidized catalyst 3NilOCuAl developed its full activity only after an induction period. These results suggest that lower oxidation states of Cu are essential for the catalytic reaction to occur, a clear distinction between Cu+ and Cu°,however, could not be made in the present study. On the basis of Auger spectroscopic studies, Sakurai et al. [421 concluded that CuD catalysts developed a high activity when CuD and Cu 20 coexisted on the surface, while the 0 metal was formed. activity decreased when Cu 4. Conclusions The following main conclusions can be drawn from the experimental results: (a) The Cu concentration in CuA1 2O4 within the probing depth of XPS experiments is increased as compared to the overall bulk concentration. (b) Addition of small amounts of NiO enhances the surface segregation of Cu, while Ni does not seem to have a preference for regions close to the surface.

G. Ert! eta!.

/ XPS study of copper a!uminate catalysts

63

(c) Cu2~within the probing depth is reduced to Cu~by heat treatment in vacuo, while Ni2~is not affected. (d) Reduction in CO and H 0. 2 atmospheres leads to the formation of metallic Cu (e) Cu 2O and Cu°particles in reduced catalysts are probably small and poorly crystallized, their dispersion should be fairly high. (I) Lower valent oxidation states of Cu, probably the +1 state, are detected after contacting the catalysts with NO + CO mixtures. The existence of Cu+ seems to be essential for the catalytic activity. (g) NO adsorption leads to a partial decomposition and formation of N2 + 02. A reaction sequence can thus tentatively be formulated as: NO

-~

O

Cu2O

+

CuO

N +0

+

CO

-~

CuO

-~

Cu20

+

CO2

(h) The aluminate does not seem to be a catalyst for the NO reduction or CO oxidation reaction, since Cu~cannot be stabilized in the spinel lattice. Catalytic activity may be brought about by coexisting Cu20 and CuD particles on the support surface. It may be presumed, that when starting off from a CuAI2O4 phase one might obtain a better dispersion of the supported oxides and a rather high thermal and mechanical stability of the catalysts.

Acknowledgement Financial support of this work by the Deutsche Forschungsgemeinschaft, by the Fonds der Chemischen Industrie and by the Max-Buchner-Forschungsstiftung is gratefully acknowledged. We also wish to thank Dr. Kochloefl and Mr. Bock, Slldchemie AG, for carrying out the elemental analysis of the catalysts.

References

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[31 L.

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