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MnO2 catalyst
Applied
270
XPS CHARACTERIZATION V. DI CASTRO, Dipartimento
Cenrro
di Studio
OF THE CuO/MnO,
Science 28 (1987) 270-278 North-Holland, Amsterdam
CATALYST
C. FURLANI
di Chimica,
M. GARGANO
Surface
Umuersit~
“La
Sapienza”,
00185 Roma, Italy
and M. ROSS1 sulk
Metodologle
Innovative
di Sintesi
Organiche,
Via Amendola
173,
70126 Barr, Itah
Received
14 July 1986; accepted
for publication
25 November
1986
We followed by XPS the changes occurring at the surface of a CuO/MnOz catalyst during two different reductive pretreatments with H,. The difference in catalytic activity for the hydrogenation reaction between the two pretreated catalysts has been correlated with the surface concentration and oxidation state of copper.
1. Introduction Copper oxides dispersed on different supports are active catalysts for several reactions such as NO reduction, metanol synthesis, hydrogenation and dehydrogenation of olefines. The catalytic properties of these systems depend on the nature of the support, preparation procedure and possible pretreatments [l]. The wide-spread importance of copper-based catalysts has prompted an extensive physico-chemical characterization of the state of copper dispersed on different supports. Various techniques have been applied including chemisorption [2], ESR [3,4], X-ray diffraction [5,6], EXAFS [7], XPS [6-lo], etc.; nevertheless in many cases the nature of the copper phase active in the catalytic process is still controversial. We have studied for some years copper-based systems as catalysts for hydrogenation reactions [lo-131 and in this framework we found CuO/MnO, to be in some way an anomalous system. In fact, its catalytic activity is more sensitive to changes in reductive pretreatments than analogous systems containing the metal on other oxidic supports (e.g. Al,O,, SiO,, Cr,O,). The present paper reports an XPS investigation undertaken in order to correlate surface composition with catalytic activity in a test reaction, that is the hydrogenation of cyclododecatriene. The aim of this work is to obtain a better understanding of the copperrmanganese oxide interaction and of the structural changes occurring at the catalyst surface during the reductive pretreatments. 0169-4332/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
V. Di Castro et al. / XPS characterization
of CuO/MnO?
catalyst
271
2. Experimental 1,5,9-cyclododecatriene (CDT), supplied by Fluka, was distilled, purified by filtration on activated alumina and stored under nitrogen atmosphere at 0 o C. Reagent grade Nujol, deareated and stored under nitrogen, was used as a solvent. Copper oxide on manganese oxide (CuO/MnO,), with a Cu content of 8.6% was prepared by adsorbing on MnO, the tetramino complex of Cu(IT) according to the procedure described by Koritala for CuO/Al,O, [14]. The adsorption was followed by precipitation of the unadsorbed copper by dilution of the ammoniacal solution with water. The catalyst was filtered, dried at 110°C overnight, then calcinated in air for 5 h at 35O’C. Two different reductive pretreatments were performed: (i) A catalyst sample (0.2 g) was introduced into a glass reactor provided with a water-cooled arm and heated at 270°C for 15 min under vacuum in a thermostatted device. Then hydrogen at 1 atm was quickly introduced. A very fast reduction took place and water condensation in the cold arm of the reactor was observed. After 30 min water was evaporated under vacuum and the hydrogen-vacuum cycle was repeated twice. (ii) A catalyst sample (0.2 g) was heated at 110°C for 15 min under vacuum. Then the reactor was filled with hydrogen at 1 atm and the temperature was increased to 270°C in discontinuous rapid increments of 40°C and then held constant for 30 min. After each temperature step the condensed water was evaporated under vacuum. Finally, the reduced catalyst was allowed to cool at room temperature in hydrogen atmosphere. The catalytic activity of the two differently reduced samples was tested in the hydrogenation of a mixture of commercial 1,5,9-cyclododecatriene isomers and computed in terms of the turnover number from the initial rate of H, uptake. The turnover numbers were expressed as moles of H, consumed per mole of copper per hour. The reaction was carried out at 180 o C and 1 atm of H,, introducing a Nujol solution of substrate in the same reactor in which the catalyst was reduced. For XPS determination, samples of CuO/MnO, were either reduced with hydrogen or annealed in ultra-high vacuum (UHV) in the preparation chamber of the spectrometer by means of the above described procedure. XP spectra were recorded using a VG ESCA 3 spectrometer employing an Al Ka source (hv = 1486.6 ev). The catalyst samples were dusted as thin films onto a gold probe. C 1s (BE 285.0 eV) from pump-oil contamination was used as an internal standard for the correction of charging effects. Reported values are averages of at least three runs on three different samples; tabulated values are accurate to kO.2 eV. The investigated samples proved sufficiently stable to X-ray exposure under the experimental conditions.
212
V. DI Castro rt al. / XPS characteruatlon
of CuO/Mn02
catalyst
The quantitative composition was determined by considering a solid solution and using sensitivity factors measured in our spectrometer. X-ray diffraction was performed on powdered catalyst samples with a Philips PW 1729 instrument using Ni-filtered Cu Ka radiation.
3. Results and discussion Table 1 reports the Cu 2p,,, and Mn 2p,,, binding energies and the Cu LMM kinetic energy for the CuO/MnO, precursor and some manganese and copper oxides for reference purposes. The measured spectra of the oxides are in good agreement with previous literature data [15,16]. In the CuO/MnO, precursor, prepared as reported, the Mn 2p,,, p eak (BE 642.2 eV) is characteristic for a Mn(IV) species in a MnOz structure, while the Cu 2p,,, peak is typical of a Cu(I1) species, in terms of both BE value and presence of intense satellite structures. Some minor differences with respect to the unsupported CuO, namely the 0.2 eV shift of the main peak to lower BE, the increase of the satellite intensity to 36% and energy separation (AE) from the main peak to 9.2 eV, seem to indicate some interaction between CuO and the MnO, support. Further information on the catalyst structure comes from the quantitative analysis of the XPS peaks. In this sample the atomic ratio between Cu and Mn at the sample surface measured by XPS is 0.6, that is five times higher than the value (0.13) expected on the basis of the chemical analysis. This means that Cu does not diffuse homogeneously in the sample, but forms a Cu-rich layer at the catalyst surface. A similar behaviour has been previously reported for other catalytic systems prepared in the same way by using different supports, namely SiOz [17] and Al,O, [18]. Table 1 XPS binding compounds Compound
CuO/Mn02 cue cu,o cu Mn,O, Mn,O, Mn02 a’ Full widths
energies
and
Cu Auger
kinetic
energies
of CuO/MnO,
Cu 2P,,, (eV)
Cu satellite
Cu LMM
A E (eV)
(W)
(eV)
934.2 934.4 932.7 932.8
9.2 8.6 _ _
36 31
(3.8) ‘) (4.0) (1.8) (1.9)
917.4 917.3 916.0 918.3
and
of some
reference
Mn 2~3,~ (eV) 642.2 (3.8)
641.6 (4.2) 642.0 (4.6) 642.4 (3.8) at half maximum
(FWHM)
of peaks given in parentheses.
V. Di Castro et al. / XPS characterization of CuO/MnO, Table 2 Experimental T
XPS binding
(“C)
Cu 2P3,2 (ev)
25 150 190 230 270 350
934.2 934.2 933.6 933.4 932.7 932.7
270 + H, latm 30 min ‘) Full widths
energies
and Auger kinetic energies Cu LMM
for CuO/MnO,
273
heated
in vacuum
AE (ev)
(%)
(ev
Mn 2pj,, (ev)
Cu/Mn atomic ratio
9.2 9.2 10.2 10.6 11.2 11.5
36 34 30 25 18 16
917.4 917.2 917.2 917.2 916.5 916.4
642.2 642.2 642.0 641.8 641.5 641.5
(3.8) (4.0) (4.6) (4.6) (4.4) (4.4)
0.59 0.54 0.50 0.31 0.17 0.15
932.8 (2.4)
_
_
918.3
641.4 (4.6)
0.09
at half maximum
(FWHM)
(3.8) a) (3.6) (4.0) (3.7) (3.0) (2.8)
Satellite
catalyst
of peaks given in parentheses.
The catalytic activity exhibited by the tested samples in the hydrogenation of 1,5,9-cyclooctadiene at 180°C and 1 atm of H,, is strongly dependent on the reductive pretreatment. In fact, by warming this system up to 270 o C in vacuum and then treating it with H, at atmospheric pressure, an almost inactive material is obtained as indicated by its low turnover number (< 5 hh’). Conversely, when a carefully controlled reduction is carried out, as described in detail in section 2, a very active catalyst results showing a turnover number of 137 h-‘. In order to understand this vastly different behaviour in terms of changes occurring at the catalyst surface during the pretreatments, we followed by XPS the changes of the sample surface during the warming up both in vacuum and in H,. Table 2 reports the main XPS results for the sample heated in UHV, together with the kinetic energy of the Cu LMM Auger peak and the Cu/Mn atomic ratio calculated from the XPS spectra. Upon warming to a temperature of 150°C in the absence of H,, we found no significant changes in the CuO/MnO, system as shown by the copper and manganese spectra. At higher temperature the Cu 2p,,, and Mn 2p3,* p eaks progressively shift towards lower BE values, due to a progressive reduction of these elements to lower oxidation states. The Mn 2p peak, characteristic for Mn(IV) in the untreated catalyst, is shifted at first to 641.8 eV in the temperature range between 190 and 230” C, and then to 641.4 eV at temperatures higher than 230” C. A comparison of the shape and position of the Mn 2p,,, peak measured in the annealed catalyst with the Mn 2p 3,2 peaks measured in the pure oxides allows one to verify that Mn(IV) is reduced at first to Mn(II1) and then an oxide spine1 structure similar to Mn,O, is formed which remains stable up to a temperature of 350 o C. The observed reduction trend of manganese is in good agreement with previously reported results on the reduction of pure MnO, in vacuum [15].
274
K Di Castro et al. / XPS characterizatton
of CuO/MnO,
catalyst
b
940
930
Fig. 1. Cu 2p spectra
of CuO/MnO,:
(a) untreated,
950
(b) heated in vacuum,
eV (c) heated in H,
and a The Cu 2~,,, sP ectrum shows a decrease of the satellite intensity shift to lower BE of the main peak (fig. 1). These modifications of the XPS spectrum, together with the shift of the Auger peak, indicate that the major part of copper is reduced progressively to Cu(1) although a minor amount of Cu(I1) remains stable up to 350°C. The Cu/Mn atomic ratio measured by XPS decreases with increasing temperature by a drastic change in the range 190-270°C as evidenced in fig. 2. At 270” C the copper concentration measured by XPS is very close to the value expected on the basis of the chemical analysis. This means that the copper-rich phase at the catalyst surface does not exist any more and that copper has diffused homogeneously in the substrate during the Mn and Cu reduction. The reported experimental results seem to indicate the formation of a copper-manganite mixed oxide such as Cu,Mn3_,04 where Cu(1) and Cu(I1) oxidation states have been reported [19-211 to be in equilibrium with each other in a spine1 structure similar to that of Mn,O,.
V. Di Castro et al. / XPS characterization of CuO/MnO,
catalyst
0=1n x=in
275
“a‘““m
H2tlalm)
R *
P Q
0.2
Q
1
oL-*-
I
I 200
0
II300
LOO
Temperature
Fig.
2. Variation
of Cu/Mn
atomic
ratio
at surface: 1 atm (X).
heating
("Cl
in vacuum
(0)
and
in H,
of
The XPS and Auger data measured during the heating in H, (1 atm) are listed in table 3. It is worth noting that at room temperature and up to 150 o C the CuO/MnO, system is stable in the presence of H,, while at higher temperature both Mn(IV) and Cu(I1) undergo a progressive reduction. Comparing the changes due to heating in H, with the ones due to heating in vacuum, it can be seen that the manganese reduction follows the same trend and is not influenced by the presence of H,. On the other hand, copper reduction exhibits a strong dependence on the presence of H,. In fact the XPS signal due to Cu(I1) progressively disappears between 1.50 and 230 o C and at a temperature of 23O’C only the XPS peak due to reduced copper is present, while two components due to Cu(1) and Cu(0) can be identified in the Auger spectrum. By increasing the temperature, the component at 918.2 eV increases, showing that copper is completely reduced to Cu(0) at 270 o C (fig. 3). Copper reduction to Cu(0) coincides with a progressive decrease of the Cu/Mn ratio from 0.58 to 0.16, in the temperature range 190-270°C. The decrease of the Cu signal during the reduction process can be due either to
Table 3 XPS binding T
energies
(“C)
Cu 2P,,, (eV)
25 150 190 230 270
934.2 934.0 933.5 932.7 932.8
(3.8) (3.7) (4.0) (2.0) (2.3)
and Cu Auger kinetic
energies
for CuO/MnO,
Cu LMM
Satellite
annealed
AE (ev)
(W)
(eV?
Mn 2~~~~ (eV)
9.2 9.5 10.0 -
35 33 26 _ _
917.4 917.3 917.3 918.2-916.2 918.2
642.2 642.0 641.8 641.7 641.4
(3.5) (4.0) (4.4) (4.4) (4.5)
in H,
(1 atm)
Cu/Mn atomic ratio 0.58 0.53 0.48 0.30 0.16
216
V. Di Castro et al. / XPS characteriraiion
of CuO/MnO,
catalyst
I
920
910
eV
Fig. 3. Cu LMM Auger spectra of CuO/MnO, heated (a) at 270 o C in vacuum, (b) at 270 o C in H,, and (c) at 270 o C in vacuum and then exposed to H, (1 atm).
diffusion in the substrate or to agglomeration of metallic Cu in crystallites bigger than the escape depth of the emitted electrons. Further information on the state of copper can be obtained from X-ray diffraction measurements. In the reduced sample, no signal due to metallic copper was observed, indicating that copper remains well dispersed in the reduced sample. The absence of the signal in the XRD spectra and the manner in which the Cu/Mn ratio decreases - very similar to that observed in vacuum (fig. 2) seem to indicate that copper diffuses in the substrate during the reduction process. It is worth noting that copper diffuses in the substrate as Cu(0) in the presence of H,, while an oxidized form is incorporated in vacuum. When the
V. Di Castro et al. / XPS characterization of CuO/MnO,
catalyst
211
sample annealed in vacuum is exposed to H, at 270 o C a further reduction of copper is observed together with a strong decrease of the Cu/Mn atomic ratio (table 2) to a value lower than the one expected from the chemical analysis. The X-ray diffraction spectrum shows in this case an intense peak due to metallic copper, suggesting that the decrease of Cu/Mn ratio is due to copper agglomeration during the fast reduction to Cu(0). Moreover, copper is not completely reduced to Cu(0) as in the sample slowly annealed in Hz, and an intense component due to Cu(1) is still observed in the Auger spectrum (fig. 3). It is worth noting that a similar treatment, that is a warming up in vacuum to 270°C and exposure to H, (1 atm), was able to reduce completely to Cu(0) the copper in CuO/Al,O, and in “copper chromite” catalysts [lo-131. The higher catalytic activity observed in the hydrogenation of cyclododecatriene using the CuO/MnO, sample annealed in H, can be correlated either to the better copper dispersion or to the more complete reduction to Cu(O), since it has been previously proved that Cu(0) is the active species in hydrogenation reactions [lo-131.
4. Conclusions We followed by XPS the changes occurring at the CuO/MnO, catalyst surface by heating it in vacuum and in H,. In both cases the progressive reduction of Mn(IV) and Cu(I1) to lower oxidation states is observed together with the concomitant decrease of the Cu/Mn atomic ratio at the sample surface which we attributed to diffusion of copper in the substrate. The reduction of manganese and the diffusion of copper are not influenced by the presence of H *; on the contrary copper is reduced almost completely to Cu(0) on heating in H,, while just a partial reduction to Cu(1) is observed in vacuum. Thus, copper diffuses in the substrate as Cu(0) in the presence of H,, while in vacuum an oxidized form is incorporated, probably forming a structure similar to copper manganite. Exposing the sample heated in vacuum to H,, a reduction and sintering of copper is observed. The catalytic activity of the pretreated catalysts can be correlated with the dispersion and oxidation state of copper. In fact the most active one, that is the sample annealed in H,, shows a better dispersion of copper at the surface and a more complete reduction to Cu(0).
Acknowledgements The authors are grateful to Mr. G. Minelli for his assistance in carrying out the XRD measurements. This work was supported by “Progetto finalizzato de1 CNR, Chimica fine e secondaria”.
278
V. Di Castro et al. / XPS characterization
of CuO/MnO,
catalyst
References [l] C.L. Thomas, Catalytic Processes and Proved Catalysts (Academic Press, New York, 1970). [2] N. Takezawa, H. Kobayashi, Y. Kamegai and M. Shimokawabe, Appl. Catalysis 3 (1982) 381. [3] H. Tominaga, Y. Ono and T. Keii, J. Catalysis 40 (1975) 197. [4] CF. Aissi, G. Wrobel, A. D’Huysser, M. Guelton and J.P. Bonelle, J. Chem. Sot. Faraday Trans. II, 81 (1985) 1367. [5] R.G. Herman, K. Kher, G.W. Simmons, B.P. Finn and J.B. Bulko, J. Catalysis 56 (1979) 407. [6] J.R. Monnier, M.J. Hanrahan and G. Apai, J. Catalysis 92 (1985) 119. [7] R.M. Friedman and J.J. Freeman, J. Catalysis 55 (1978) 10. [8] G. Ertl, R. Hierl, R. Knozinger, N. Thiele and H.P. Urbach, Appl. Surface Sci. 5 (1980) 49. [9] Y. Okamoto, K. Fukino, T. Imanaka and S. Teranishi, J. Phys. Chem. 87 (1983) 3740. [lo] F.M. Capece, V. Di Castro, C. Furlani, G. Mattogno, C. Fragale, M. Gargano and M. Rossi, J. Electron Spectrosc. 27 (1982) 119. [ll] C. Fragale, M. Gargano and M. Rossi, J. Am. Oil Chem. Sot. 59 (1982) 465. [12] C. Fragale, M. Gargano, N. Ravasio, M. Rossi and I. Santo, Inorg. Chim. Acta 82 (1984) 157. [13] V. Di Castro, C. Furlani, C. Fragale, M. Gargano, M. Rossi and N. Ravasio, Gazz. Chim. Ital., submitted. [14] S. Koritala, J. Am. Oil. Chem. Sot. 49 (1972) 83. [15] M. Oku, K. Hirokawa and S. Ikeda, J. Electron Spectrosc. 7 (1975) 465. [16] K. Wandelt, Surface Sci. Rept. 2 (1982) 1, and references therein. [17] V. Di Castro and G. Piredda, Chem. Phys. Letters 114 (1985) 109. [18] V. Di Castro, G. Piredda, M. Gargano and C. Fragale, in: Proc. Congr. Nazionale Chimica Inorganica, Ferrara, 1983, p. 230. [19] A.P.B. Sinha, N.R. Sanjana and A.B. Biswas, J. Phys. Chem. 62 (1958) 191. [20] N.K. Radha Krishnan and A.B. Biswas, J. Ind. Chem. Sot. Ll (1974) 274. [21] A.D.D. Broemme and V.A.M. Bramers, Solid State Ionics 16 (1985) 171.
270
XPS CHARACTERIZATION V. DI CASTRO, Dipartimento
Cenrro
di Studio
OF THE CuO/MnO,
Science 28 (1987) 270-278 North-Holland, Amsterdam
CATALYST
C. FURLANI
di Chimica,
M. GARGANO
Surface
Umuersit~
“La
Sapienza”,
00185 Roma, Italy
and M. ROSS1 sulk
Metodologle
Innovative
di Sintesi
Organiche,
Via Amendola
173,
70126 Barr, Itah
Received
14 July 1986; accepted
for publication
25 November
1986
We followed by XPS the changes occurring at the surface of a CuO/MnOz catalyst during two different reductive pretreatments with H,. The difference in catalytic activity for the hydrogenation reaction between the two pretreated catalysts has been correlated with the surface concentration and oxidation state of copper.
1. Introduction Copper oxides dispersed on different supports are active catalysts for several reactions such as NO reduction, metanol synthesis, hydrogenation and dehydrogenation of olefines. The catalytic properties of these systems depend on the nature of the support, preparation procedure and possible pretreatments [l]. The wide-spread importance of copper-based catalysts has prompted an extensive physico-chemical characterization of the state of copper dispersed on different supports. Various techniques have been applied including chemisorption [2], ESR [3,4], X-ray diffraction [5,6], EXAFS [7], XPS [6-lo], etc.; nevertheless in many cases the nature of the copper phase active in the catalytic process is still controversial. We have studied for some years copper-based systems as catalysts for hydrogenation reactions [lo-131 and in this framework we found CuO/MnO, to be in some way an anomalous system. In fact, its catalytic activity is more sensitive to changes in reductive pretreatments than analogous systems containing the metal on other oxidic supports (e.g. Al,O,, SiO,, Cr,O,). The present paper reports an XPS investigation undertaken in order to correlate surface composition with catalytic activity in a test reaction, that is the hydrogenation of cyclododecatriene. The aim of this work is to obtain a better understanding of the copperrmanganese oxide interaction and of the structural changes occurring at the catalyst surface during the reductive pretreatments. 0169-4332/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
V. Di Castro et al. / XPS characterization
of CuO/MnO?
catalyst
271
2. Experimental 1,5,9-cyclododecatriene (CDT), supplied by Fluka, was distilled, purified by filtration on activated alumina and stored under nitrogen atmosphere at 0 o C. Reagent grade Nujol, deareated and stored under nitrogen, was used as a solvent. Copper oxide on manganese oxide (CuO/MnO,), with a Cu content of 8.6% was prepared by adsorbing on MnO, the tetramino complex of Cu(IT) according to the procedure described by Koritala for CuO/Al,O, [14]. The adsorption was followed by precipitation of the unadsorbed copper by dilution of the ammoniacal solution with water. The catalyst was filtered, dried at 110°C overnight, then calcinated in air for 5 h at 35O’C. Two different reductive pretreatments were performed: (i) A catalyst sample (0.2 g) was introduced into a glass reactor provided with a water-cooled arm and heated at 270°C for 15 min under vacuum in a thermostatted device. Then hydrogen at 1 atm was quickly introduced. A very fast reduction took place and water condensation in the cold arm of the reactor was observed. After 30 min water was evaporated under vacuum and the hydrogen-vacuum cycle was repeated twice. (ii) A catalyst sample (0.2 g) was heated at 110°C for 15 min under vacuum. Then the reactor was filled with hydrogen at 1 atm and the temperature was increased to 270°C in discontinuous rapid increments of 40°C and then held constant for 30 min. After each temperature step the condensed water was evaporated under vacuum. Finally, the reduced catalyst was allowed to cool at room temperature in hydrogen atmosphere. The catalytic activity of the two differently reduced samples was tested in the hydrogenation of a mixture of commercial 1,5,9-cyclododecatriene isomers and computed in terms of the turnover number from the initial rate of H, uptake. The turnover numbers were expressed as moles of H, consumed per mole of copper per hour. The reaction was carried out at 180 o C and 1 atm of H,, introducing a Nujol solution of substrate in the same reactor in which the catalyst was reduced. For XPS determination, samples of CuO/MnO, were either reduced with hydrogen or annealed in ultra-high vacuum (UHV) in the preparation chamber of the spectrometer by means of the above described procedure. XP spectra were recorded using a VG ESCA 3 spectrometer employing an Al Ka source (hv = 1486.6 ev). The catalyst samples were dusted as thin films onto a gold probe. C 1s (BE 285.0 eV) from pump-oil contamination was used as an internal standard for the correction of charging effects. Reported values are averages of at least three runs on three different samples; tabulated values are accurate to kO.2 eV. The investigated samples proved sufficiently stable to X-ray exposure under the experimental conditions.
212
V. DI Castro rt al. / XPS characteruatlon
of CuO/Mn02
catalyst
The quantitative composition was determined by considering a solid solution and using sensitivity factors measured in our spectrometer. X-ray diffraction was performed on powdered catalyst samples with a Philips PW 1729 instrument using Ni-filtered Cu Ka radiation.
3. Results and discussion Table 1 reports the Cu 2p,,, and Mn 2p,,, binding energies and the Cu LMM kinetic energy for the CuO/MnO, precursor and some manganese and copper oxides for reference purposes. The measured spectra of the oxides are in good agreement with previous literature data [15,16]. In the CuO/MnO, precursor, prepared as reported, the Mn 2p,,, p eak (BE 642.2 eV) is characteristic for a Mn(IV) species in a MnOz structure, while the Cu 2p,,, peak is typical of a Cu(I1) species, in terms of both BE value and presence of intense satellite structures. Some minor differences with respect to the unsupported CuO, namely the 0.2 eV shift of the main peak to lower BE, the increase of the satellite intensity to 36% and energy separation (AE) from the main peak to 9.2 eV, seem to indicate some interaction between CuO and the MnO, support. Further information on the catalyst structure comes from the quantitative analysis of the XPS peaks. In this sample the atomic ratio between Cu and Mn at the sample surface measured by XPS is 0.6, that is five times higher than the value (0.13) expected on the basis of the chemical analysis. This means that Cu does not diffuse homogeneously in the sample, but forms a Cu-rich layer at the catalyst surface. A similar behaviour has been previously reported for other catalytic systems prepared in the same way by using different supports, namely SiOz [17] and Al,O, [18]. Table 1 XPS binding compounds Compound
CuO/Mn02 cue cu,o cu Mn,O, Mn,O, Mn02 a’ Full widths
energies
and
Cu Auger
kinetic
energies
of CuO/MnO,
Cu 2P,,, (eV)
Cu satellite
Cu LMM
A E (eV)
(W)
(eV)
934.2 934.4 932.7 932.8
9.2 8.6 _ _
36 31
(3.8) ‘) (4.0) (1.8) (1.9)
917.4 917.3 916.0 918.3
and
of some
reference
Mn 2~3,~ (eV) 642.2 (3.8)
641.6 (4.2) 642.0 (4.6) 642.4 (3.8) at half maximum
(FWHM)
of peaks given in parentheses.
V. Di Castro et al. / XPS characterization of CuO/MnO, Table 2 Experimental T
XPS binding
(“C)
Cu 2P3,2 (ev)
25 150 190 230 270 350
934.2 934.2 933.6 933.4 932.7 932.7
270 + H, latm 30 min ‘) Full widths
energies
and Auger kinetic energies Cu LMM
for CuO/MnO,
273
heated
in vacuum
AE (ev)
(%)
(ev
Mn 2pj,, (ev)
Cu/Mn atomic ratio
9.2 9.2 10.2 10.6 11.2 11.5
36 34 30 25 18 16
917.4 917.2 917.2 917.2 916.5 916.4
642.2 642.2 642.0 641.8 641.5 641.5
(3.8) (4.0) (4.6) (4.6) (4.4) (4.4)
0.59 0.54 0.50 0.31 0.17 0.15
932.8 (2.4)
_
_
918.3
641.4 (4.6)
0.09
at half maximum
(FWHM)
(3.8) a) (3.6) (4.0) (3.7) (3.0) (2.8)
Satellite
catalyst
of peaks given in parentheses.
The catalytic activity exhibited by the tested samples in the hydrogenation of 1,5,9-cyclooctadiene at 180°C and 1 atm of H,, is strongly dependent on the reductive pretreatment. In fact, by warming this system up to 270 o C in vacuum and then treating it with H, at atmospheric pressure, an almost inactive material is obtained as indicated by its low turnover number (< 5 hh’). Conversely, when a carefully controlled reduction is carried out, as described in detail in section 2, a very active catalyst results showing a turnover number of 137 h-‘. In order to understand this vastly different behaviour in terms of changes occurring at the catalyst surface during the pretreatments, we followed by XPS the changes of the sample surface during the warming up both in vacuum and in H,. Table 2 reports the main XPS results for the sample heated in UHV, together with the kinetic energy of the Cu LMM Auger peak and the Cu/Mn atomic ratio calculated from the XPS spectra. Upon warming to a temperature of 150°C in the absence of H,, we found no significant changes in the CuO/MnO, system as shown by the copper and manganese spectra. At higher temperature the Cu 2p,,, and Mn 2p3,* p eaks progressively shift towards lower BE values, due to a progressive reduction of these elements to lower oxidation states. The Mn 2p peak, characteristic for Mn(IV) in the untreated catalyst, is shifted at first to 641.8 eV in the temperature range between 190 and 230” C, and then to 641.4 eV at temperatures higher than 230” C. A comparison of the shape and position of the Mn 2p,,, peak measured in the annealed catalyst with the Mn 2p 3,2 peaks measured in the pure oxides allows one to verify that Mn(IV) is reduced at first to Mn(II1) and then an oxide spine1 structure similar to Mn,O, is formed which remains stable up to a temperature of 350 o C. The observed reduction trend of manganese is in good agreement with previously reported results on the reduction of pure MnO, in vacuum [15].
274
K Di Castro et al. / XPS characterizatton
of CuO/MnO,
catalyst
b
940
930
Fig. 1. Cu 2p spectra
of CuO/MnO,:
(a) untreated,
950
(b) heated in vacuum,
eV (c) heated in H,
and a The Cu 2~,,, sP ectrum shows a decrease of the satellite intensity shift to lower BE of the main peak (fig. 1). These modifications of the XPS spectrum, together with the shift of the Auger peak, indicate that the major part of copper is reduced progressively to Cu(1) although a minor amount of Cu(I1) remains stable up to 350°C. The Cu/Mn atomic ratio measured by XPS decreases with increasing temperature by a drastic change in the range 190-270°C as evidenced in fig. 2. At 270” C the copper concentration measured by XPS is very close to the value expected on the basis of the chemical analysis. This means that the copper-rich phase at the catalyst surface does not exist any more and that copper has diffused homogeneously in the substrate during the Mn and Cu reduction. The reported experimental results seem to indicate the formation of a copper-manganite mixed oxide such as Cu,Mn3_,04 where Cu(1) and Cu(I1) oxidation states have been reported [19-211 to be in equilibrium with each other in a spine1 structure similar to that of Mn,O,.
V. Di Castro et al. / XPS characterization of CuO/MnO,
catalyst
0=1n x=in
275
“a‘““m
H2tlalm)
R *
P Q
0.2
Q
1
oL-*-
I
I 200
0
II300
LOO
Temperature
Fig.
2. Variation
of Cu/Mn
atomic
ratio
at surface: 1 atm (X).
heating
("Cl
in vacuum
(0)
and
in H,
of
The XPS and Auger data measured during the heating in H, (1 atm) are listed in table 3. It is worth noting that at room temperature and up to 150 o C the CuO/MnO, system is stable in the presence of H,, while at higher temperature both Mn(IV) and Cu(I1) undergo a progressive reduction. Comparing the changes due to heating in H, with the ones due to heating in vacuum, it can be seen that the manganese reduction follows the same trend and is not influenced by the presence of H,. On the other hand, copper reduction exhibits a strong dependence on the presence of H,. In fact the XPS signal due to Cu(I1) progressively disappears between 1.50 and 230 o C and at a temperature of 23O’C only the XPS peak due to reduced copper is present, while two components due to Cu(1) and Cu(0) can be identified in the Auger spectrum. By increasing the temperature, the component at 918.2 eV increases, showing that copper is completely reduced to Cu(0) at 270 o C (fig. 3). Copper reduction to Cu(0) coincides with a progressive decrease of the Cu/Mn ratio from 0.58 to 0.16, in the temperature range 190-270°C. The decrease of the Cu signal during the reduction process can be due either to
Table 3 XPS binding T
energies
(“C)
Cu 2P,,, (eV)
25 150 190 230 270
934.2 934.0 933.5 932.7 932.8
(3.8) (3.7) (4.0) (2.0) (2.3)
and Cu Auger kinetic
energies
for CuO/MnO,
Cu LMM
Satellite
annealed
AE (ev)
(W)
(eV?
Mn 2~~~~ (eV)
9.2 9.5 10.0 -
35 33 26 _ _
917.4 917.3 917.3 918.2-916.2 918.2
642.2 642.0 641.8 641.7 641.4
(3.5) (4.0) (4.4) (4.4) (4.5)
in H,
(1 atm)
Cu/Mn atomic ratio 0.58 0.53 0.48 0.30 0.16
216
V. Di Castro et al. / XPS characteriraiion
of CuO/MnO,
catalyst
I
920
910
eV
Fig. 3. Cu LMM Auger spectra of CuO/MnO, heated (a) at 270 o C in vacuum, (b) at 270 o C in H,, and (c) at 270 o C in vacuum and then exposed to H, (1 atm).
diffusion in the substrate or to agglomeration of metallic Cu in crystallites bigger than the escape depth of the emitted electrons. Further information on the state of copper can be obtained from X-ray diffraction measurements. In the reduced sample, no signal due to metallic copper was observed, indicating that copper remains well dispersed in the reduced sample. The absence of the signal in the XRD spectra and the manner in which the Cu/Mn ratio decreases - very similar to that observed in vacuum (fig. 2) seem to indicate that copper diffuses in the substrate during the reduction process. It is worth noting that copper diffuses in the substrate as Cu(0) in the presence of H,, while an oxidized form is incorporated in vacuum. When the
V. Di Castro et al. / XPS characterization of CuO/MnO,
catalyst
211
sample annealed in vacuum is exposed to H, at 270 o C a further reduction of copper is observed together with a strong decrease of the Cu/Mn atomic ratio (table 2) to a value lower than the one expected from the chemical analysis. The X-ray diffraction spectrum shows in this case an intense peak due to metallic copper, suggesting that the decrease of Cu/Mn ratio is due to copper agglomeration during the fast reduction to Cu(0). Moreover, copper is not completely reduced to Cu(0) as in the sample slowly annealed in Hz, and an intense component due to Cu(1) is still observed in the Auger spectrum (fig. 3). It is worth noting that a similar treatment, that is a warming up in vacuum to 270°C and exposure to H, (1 atm), was able to reduce completely to Cu(0) the copper in CuO/Al,O, and in “copper chromite” catalysts [lo-131. The higher catalytic activity observed in the hydrogenation of cyclododecatriene using the CuO/MnO, sample annealed in H, can be correlated either to the better copper dispersion or to the more complete reduction to Cu(O), since it has been previously proved that Cu(0) is the active species in hydrogenation reactions [lo-131.
4. Conclusions We followed by XPS the changes occurring at the CuO/MnO, catalyst surface by heating it in vacuum and in H,. In both cases the progressive reduction of Mn(IV) and Cu(I1) to lower oxidation states is observed together with the concomitant decrease of the Cu/Mn atomic ratio at the sample surface which we attributed to diffusion of copper in the substrate. The reduction of manganese and the diffusion of copper are not influenced by the presence of H *; on the contrary copper is reduced almost completely to Cu(0) on heating in H,, while just a partial reduction to Cu(1) is observed in vacuum. Thus, copper diffuses in the substrate as Cu(0) in the presence of H,, while in vacuum an oxidized form is incorporated, probably forming a structure similar to copper manganite. Exposing the sample heated in vacuum to H,, a reduction and sintering of copper is observed. The catalytic activity of the pretreated catalysts can be correlated with the dispersion and oxidation state of copper. In fact the most active one, that is the sample annealed in H,, shows a better dispersion of copper at the surface and a more complete reduction to Cu(0).
Acknowledgements The authors are grateful to Mr. G. Minelli for his assistance in carrying out the XRD measurements. This work was supported by “Progetto finalizzato de1 CNR, Chimica fine e secondaria”.
278
V. Di Castro et al. / XPS characterization
of CuO/MnO,
catalyst
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