- Email: [email protected]
C catalysts
Nuclear Instruments and Methods in Physics Research A 448 (2000) 318}322
EXAFS study of Cu/C catalysts V.V. Kriventsov *, O.V. Klimov , O.V. Kikhtyanin, K.G. Ione , D.I. Kochubey Boreskov Institute of Catalysis, SB RAS, Pr. Laurentieva, 5, Novosibirsk 630090, Russia Scientixc Engineering Center **ZEOSIT++, SB RAS, Novosibirsk 630090, Russia
Abstract A local arrangement of copper in Cu/C catalysts for dimethylcarbonate synthesis was studied by EXAFS. The samples with various Cu content were prepared by impregnating of carbon carrier `Sibunita with the alcohol solution of CuCl . It was determined, that the oxygen atoms from surface groups of carrier always enter into copper surrounding and the relative content of oxygen drops with the increase of Cu content in the samples. The structure of surface copper compounds for initial catalysts was proposed. Thus, samples with low Cu content (9;10\ mol/g-cat) possess surface compounds [carrier-COO}CuCl] or [carrier}CO}CuCl], further, by increasing copper content a second surface layer consisting of hydrated CuCl non-bounded with carrier is formed. 2000 Elsevier Science B.V. All rights reserved. PACS: 41.60.Ap; 61.10.Ht; 82.65.Jv Keywords: Synchrotron radiation; Heterogeneous catalysis; Surface groups; EXAFS
1. Introduction It was shown [1] that catalysts prepared by impregnation of carbon carriers with copper compounds, especially chlorides, possess a good activity in dimethylcarbonate (DMC) synthesis. Earlier [2,3] an in#uence of copper content on both the activity and the selectivity of the catalysts was shown. In our opinion, surface oxygen-containing groups of a carrier also have a great e!ect. Obviously, a quantity of functional groups of the carrier in#uences `chemically bounded-to-non-boundeda
* Corresponding author. Tel.: #7-3832-39-40-13; fax: #73832-34-30-56. E-mail address: [email protected] (V.V. Kriventsov).
ratio of copper chloride of the samples. This ratio seems to be a reason for various catalytic behavior of such systems. The present work deals with the study of local arrangement as well as the state of copper in the catalysts by EXAFS.
2. Experiment The catalysts (Table 1) were prepared by impregnation of `Sibunita carbon carrier (S"220 m/g), fraction 0.25}0.5 mm, with CuCl ) 2H O solution in 96% ethanol followed by drying in a nitrogen #ow at 1003C for 2 h. The copper content before and after catalytic tests were determined by atomic spectroscopy technique using ASSIN unit. Chlorine concentration in the samples was determined as described in Ref. [1].
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 7 1 6 - 0
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319
Table 1 Chemical composition of the studied samples and interatomic distances and coordination numbers in the local arrangement of copper obtained from EXAFS spectra Sample
Type of bond
Distance (As )
Coordination numbers
C1 P } 0.95 ! P } 0.55 ! Cl/Cu }1.05
Cu}O Cu}Cl Cu}O Cu}Cu
1.95 2.27 2.74 3.35
2.80 0.40 0.30 0.05
C3 P } 3.10 ! P } 1.95 ! Cl/Cu } 1.15
Cu}O Cu}Cl Cu}O Cu}Cu
1.96 2.28 2.80 3.34
2.10 0.80 0.20 0.08
C5 P } 5.15 ! P } 3.05 ! Cl/Cu }1.05
Cu}O Cu}Cl Cu}O Cu}Cu
1.98 2.31 2.98 3.40
2.20 0.80 0.10 0.21
C10 P } 9.90 ! P } 7.50 ! Cl/Cu } 1.40
Cu}O Cu}Cl Cu}O Cu}Cu
1.98 2.32 2.89 3.38
1.50 0.90 0.20 0.23
C15 P } 14.95 ! P } 13.50 ! Cl/Cu } 1.63
Cu}O Cu}Cl Cu}O Cu}Cu
1.97 1.97 2.87 3.33
0.60 0.90 0.20 0.31
P } Cu content, wt%; P } Cl content, Wt%; Cu/Cl } molar ratio ! !
The EXAFS spectra of Cu-K edge for all samples were obtained at the EXAFS Station of Siberian Synchrotron Radiation Center through the previously described methods [4]. The radial distribution of the atoms (RDA) function was calculated from the spectra in k (k) by using Fourier analysis Q for the intervals of wave numbers 4.0}12.0 As \. Curve "tting using EXCURV92 program [5] was realized for k (k) in similar wave number intervals Q taking into account the known XRD data of bulk compounds; CuCl [6], CuCl ) 2H O [7], Cu Cl(OH) [8]. It is known that Cu}Cl and Cu}O distances may vary up to 0.1 As for the same substance, for instance, for terminal or bridged bonds. However, when modeling the bonds Cu}Cl, Cu}O, Cu}Cu it was assumed that there is the single distance, i.e. the single coordination sphere. The reason is connected with methodical limitations of EXAFS spectroscopy. In detail we discussed this in Ref. [9] when analysing various phases of manganese oxides. This
limitation results in some uncertainty of de"nition of coordination numbers, since the determined coordination number may be lowered due to splitting of several close interatomic distances, for example, at changing of ratio of terminal and bridged bonds. Since the carbon surface contains a number of various surface groups, it is likely that the spectra should consist of a number, close to each other, within the Cu}O distances. So, further we do not consider absolute values of coordination numbers, but only their change due to both alteration of coordination number and alteration of copper distribution between various surface compounds or structure distortion.
3. Results When drying the catalysts, HCl elimination due to interaction of impregnated CuCl with surface carrier groups takes place. According to chemical
SECTION VIII.
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analysis data (Table 1) copper concentrations up to 5 wt%. give a constant atomic ratio Cl/Cu+1. Higher Cu contents result in an increase of this ratio. According to N"P !((P !P *M /M )*M /M ) (1) ! ! ! ! ! ! ! where P is the Cu content in catalyst, wt%; ! P the Cl content in catalyst, wt%; M the Cu ! ! atomic weight (63.54); M the Cl atomic weight ! (35.46); N the content of copper bounded with one chlorine atom, wt%; and the samples C10 and C15 have N the values 5.5}6 wt% what corresponds to mole concentration 9;10\ mol/g-cat. RDA curves (Fig. 1) of C1}C15 samples contain four peaks which, according to [6}8], are attributed to bonds Cu}O, Cu}Cl, Cu}O and Cu}Cu with the distances 1.97$0.02, 2.29$0.03,
2.80$0.05 and 3.37$0.03 As , correspondingly. When Cu concentration in the samples increases, a decrease of relative intensity of a peak for the shortest Cu}O distance is observed due to decrease of coordination number of O surrounding copper atoms (Table 1). Intensity of peaks for Cu}Cl distances, on the other hand, slightly increases and reaches maximum for C10 and C15 samples. Intensities of peaks attributed to Cu}Cu distances also increase, but their coordination numbers do not exceed 0.3. At adsorption of copper on the surface of a carbon carrier with various O-containing groups a number of surface copper complexes with Cu}O distances close to 1.97 As should be observed. This gives a single peak on RDA curve corresponding to an average value. The same is also valid for average distance Cu}Cl and Cu}Cu. Due to this reason, as described earlier, lowered coordination numbers are obtained.
4. Discussion
Fig. 1. Curves of radial distribution of atoms (RDA) for C1}C15 samples.
According to a number of reasons a surface of carbon carriers contains a lot of functional Ocontaining groups [10,11]. At some conditions protons of these groups may be substituted with cations whose concentrations in the samples may reach 1.4;10\ mol/g. Impregnation of carbon with CuCl results in HCl elimination due to chemical interaction with surface groups of the carrier. It is likely that this interaction proceeds via substitution of chlorine ion with surface O-containing group of the carrier and is accompanied by the formation of chemical bond between copper and oxygen which is seen in EXAFS spectra for all catalysts (Fig. 1) as an intensive peak corresponding to a distance Cu}O" 1.97$0.02 As . This bond is typical of catalysts prepared by the interaction of metal halogenides with carbons [12]. All the catalysts do not contain basic copper chloride Cu Cl(OH) as there are no peaks near 3.05 As for Cu}Cu distance on the RDA curves. Another distance Cu}O (2.8 As ) is attributed to sterically coordinated oxygen for COO groups which stabilize copper. In accordance with EXAFS data and elemental analysis, an interaction of copper dichloride with
V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 318}322
carrier surface may be described as follows: (carrier)!COOH#CuCl N(carrier)!COOCu Cl#HCl (carrier)!OH#CuCl N(carrier)!OCuCl #HCl. In the catalyst heating stage in a nitrogen #ow [1] initial portions of copper dichloride are more likely to react with surface functional groups with the most easily substituting proton. When copper concentration increases more stable surface groups are involved. In the case of low copper contents ((5%) all supported metal is bounded on the surface as monomolecular species, a substitution of only one Cl ion from each CuCl molecule occurs. According to elemental analysis all the samples with copper concentrations less than 1% have the atomic ratio Cl/Cu close to 1. On the other hand, according to EXAFS, data oxygen coordination numbers for the samples with low copper content essentially exceed 2. However, the bond Cu}O cannot be explained only by coordination of water molecules adsorbed from the surrounding air by hydroscopic surface species, because increase in the copper concentration results in the decrease of coordination number for this distance. So, a peak corresponding to average distance Cu}O "1.97$0.02 As contains at least 2 distances: 1. a distance between Cu and oxygen atoms in coordinated water. This is equal to 1.97 As for CuCl ) 2H O [7]; 2. a distance between copper atoms and oxygen atoms of surface groups of the carrier. This may vary from 1.95 to 2.10 As and is proved by the fact that assymmetrical factor is not equal to 0. The existence of several distances results in the lowering of e!ective coordination number which is determined by a "tting procedure but does not
321
change a behaviour of alteration of coordination number when Cu concentration increases. In the samples with high Cu content an increase of Cu/Cl atomic ratio is observed (Table 1). Also it is seen (Fig. 1) that there is an increase of relative intensities of peaks for distances Cu}Cl and Cu}Cu and a decrease of relative intensities for distances Cu}O. This may be caused for several reasons: 1. An increase of coordination number for distance Cu}Cu is connected with an interaction of CuCl , with two nearly situated surface groups with the formation of binuclear compounds as in molybdenum complex compounds on SiO [13]. 2. An increase of coordination number for Cu}Cl distance as well as an increase of atomic ratio Cu/Cl is caused by the appearance of substances with two chlorine atoms. Using Eq. (1), a concentration of copper bounded with one chlorine atom is close to 6 wt% or 9;10\ mol Cu/g-cat. Obviously, this is determined by the concentration of surface groups of the carrier ready for chlorine substitution. This value is in the range 6;10\ } 15;10\ mol/g-carrier for the active hydrogen found in Ref. [10] for various carriers. The area of the single molecule CuCl ) 2H O is equal to ca. 3;10\ m, hence, the total area of supported CuCl is close to speci "c surface area of our carrier (220 m/g). Taking into account EXAFS data (Table 1) the area of separate molecule of surface compound [carrierCOO}Cu}Cl ) nH O] is close to the area of CuCl ) 2H O, hence, a monolayer "lling of the carrier surface takes place. Further increase in copper concentration results in the formation of the second layer without interaction with the surface groups:
SECTION VIII.
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In the second layer, copper is present as isolated molecules or associates of small dimensions. This does not result in a big increase of coordination number for Cu}Cu"3.37$0.03 As , because copper ions do not possess constant number of water molecules in the "rst coordination sphere which is the reason for both considerable disorder of surface composition of the catalyst and more variations of Cu}Cu distances in the associates. In this case, the samples do not contain large particles of any compounds with distances of 3}4 As . 5. Conclusion All the catalysts prepared by impregnation of carbon carrier with copper dichloride may be considered as the three groups varying by their surface composition: (A) Catalysts with low copper content (1}3%) contain surface compounds [carrier}COOCuCl ) H O] or [carrier}CO}CuCl ) H O], where copper is connected with the surface O-containing groups via the easiest substituting protons. (B) Catalysts with average copper content (5%) contain compounds of group (A) and surface compounds of the same composition and structure but coordinated with more stable groups of the carrier. A part of copper ions
possesses a distance of 3.4 As which in reaction conditions may result in the formation of binuclear compounds with Cu}Cu bond. (C) Catalysts with high copper content contain compounds of groups (A) and (B) as well as the non-chemically bounded hydrated copper dichloride. References [1] O.V. Klimov, O.V. Kikhtyanin, A.V. Kalinkin, K.G.Ione, J. Catal. (in press). [2] G. Lee, A. Curnutt, A. Dale Harley, In Oxygen Complexes and Oxygen Activation by Transition Metals, Plenum Press, New York, 1988, p. 215. [3] Eur. Pat. 0584785 A2. [4] D.I. Kochubey, EXAFS spectroscopy of catalysts, Nauka, Novosibirsk, 1992 (in Russian). [5] N. Binsted, J.V. Campbell, S.J. Gurman, P.C. Stephenson, SERC Daresbury Laboratory EXCURV92 program (1991). [6] P.C. Burns, F.C. Hawthorne, Am. Mineral. 78 (1993) 187. [7] A. Engberg, Acta Chemica Scand. 24 (1970) 3510. [8] M.E. Fleet, Acta Crystallogr. B 31 (1975) 183. [9] D.I. Kochubey, V.V. Kriventsov, G.N. Kustova, G.V. Odegova, P.G. Tsyrulnikov, E.N. Kudrya, Kinet. Catal. 39 (1998) 294. [10] H.P. Boehm, Carbon 32 (1994) 759. [11] E. Czaran, J. Finster, H. Schnabel, Z. Anorg. Allg. Chem. 443 (1978) 175. [12] H.P. Boehm, Angew. Chem. 78 (1966) 617. [13] Y. Iwasawa, H. Ichinose, S. Ogasawara, J. Chem. Soc. Faraday Trans. 1 (77) (1981) 1763.
EXAFS study of Cu/C catalysts V.V. Kriventsov *, O.V. Klimov , O.V. Kikhtyanin, K.G. Ione , D.I. Kochubey Boreskov Institute of Catalysis, SB RAS, Pr. Laurentieva, 5, Novosibirsk 630090, Russia Scientixc Engineering Center **ZEOSIT++, SB RAS, Novosibirsk 630090, Russia
Abstract A local arrangement of copper in Cu/C catalysts for dimethylcarbonate synthesis was studied by EXAFS. The samples with various Cu content were prepared by impregnating of carbon carrier `Sibunita with the alcohol solution of CuCl . It was determined, that the oxygen atoms from surface groups of carrier always enter into copper surrounding and the relative content of oxygen drops with the increase of Cu content in the samples. The structure of surface copper compounds for initial catalysts was proposed. Thus, samples with low Cu content (9;10\ mol/g-cat) possess surface compounds [carrier-COO}CuCl] or [carrier}CO}CuCl], further, by increasing copper content a second surface layer consisting of hydrated CuCl non-bounded with carrier is formed. 2000 Elsevier Science B.V. All rights reserved. PACS: 41.60.Ap; 61.10.Ht; 82.65.Jv Keywords: Synchrotron radiation; Heterogeneous catalysis; Surface groups; EXAFS
1. Introduction It was shown [1] that catalysts prepared by impregnation of carbon carriers with copper compounds, especially chlorides, possess a good activity in dimethylcarbonate (DMC) synthesis. Earlier [2,3] an in#uence of copper content on both the activity and the selectivity of the catalysts was shown. In our opinion, surface oxygen-containing groups of a carrier also have a great e!ect. Obviously, a quantity of functional groups of the carrier in#uences `chemically bounded-to-non-boundeda
* Corresponding author. Tel.: #7-3832-39-40-13; fax: #73832-34-30-56. E-mail address: [email protected] (V.V. Kriventsov).
ratio of copper chloride of the samples. This ratio seems to be a reason for various catalytic behavior of such systems. The present work deals with the study of local arrangement as well as the state of copper in the catalysts by EXAFS.
2. Experiment The catalysts (Table 1) were prepared by impregnation of `Sibunita carbon carrier (S"220 m/g), fraction 0.25}0.5 mm, with CuCl ) 2H O solution in 96% ethanol followed by drying in a nitrogen #ow at 1003C for 2 h. The copper content before and after catalytic tests were determined by atomic spectroscopy technique using ASSIN unit. Chlorine concentration in the samples was determined as described in Ref. [1].
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 7 1 6 - 0
V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 318}322
319
Table 1 Chemical composition of the studied samples and interatomic distances and coordination numbers in the local arrangement of copper obtained from EXAFS spectra Sample
Type of bond
Distance (As )
Coordination numbers
C1 P } 0.95 ! P } 0.55 ! Cl/Cu }1.05
Cu}O Cu}Cl Cu}O Cu}Cu
1.95 2.27 2.74 3.35
2.80 0.40 0.30 0.05
C3 P } 3.10 ! P } 1.95 ! Cl/Cu } 1.15
Cu}O Cu}Cl Cu}O Cu}Cu
1.96 2.28 2.80 3.34
2.10 0.80 0.20 0.08
C5 P } 5.15 ! P } 3.05 ! Cl/Cu }1.05
Cu}O Cu}Cl Cu}O Cu}Cu
1.98 2.31 2.98 3.40
2.20 0.80 0.10 0.21
C10 P } 9.90 ! P } 7.50 ! Cl/Cu } 1.40
Cu}O Cu}Cl Cu}O Cu}Cu
1.98 2.32 2.89 3.38
1.50 0.90 0.20 0.23
C15 P } 14.95 ! P } 13.50 ! Cl/Cu } 1.63
Cu}O Cu}Cl Cu}O Cu}Cu
1.97 1.97 2.87 3.33
0.60 0.90 0.20 0.31
P } Cu content, wt%; P } Cl content, Wt%; Cu/Cl } molar ratio ! !
The EXAFS spectra of Cu-K edge for all samples were obtained at the EXAFS Station of Siberian Synchrotron Radiation Center through the previously described methods [4]. The radial distribution of the atoms (RDA) function was calculated from the spectra in k (k) by using Fourier analysis Q for the intervals of wave numbers 4.0}12.0 As \. Curve "tting using EXCURV92 program [5] was realized for k (k) in similar wave number intervals Q taking into account the known XRD data of bulk compounds; CuCl [6], CuCl ) 2H O [7], Cu Cl(OH) [8]. It is known that Cu}Cl and Cu}O distances may vary up to 0.1 As for the same substance, for instance, for terminal or bridged bonds. However, when modeling the bonds Cu}Cl, Cu}O, Cu}Cu it was assumed that there is the single distance, i.e. the single coordination sphere. The reason is connected with methodical limitations of EXAFS spectroscopy. In detail we discussed this in Ref. [9] when analysing various phases of manganese oxides. This
limitation results in some uncertainty of de"nition of coordination numbers, since the determined coordination number may be lowered due to splitting of several close interatomic distances, for example, at changing of ratio of terminal and bridged bonds. Since the carbon surface contains a number of various surface groups, it is likely that the spectra should consist of a number, close to each other, within the Cu}O distances. So, further we do not consider absolute values of coordination numbers, but only their change due to both alteration of coordination number and alteration of copper distribution between various surface compounds or structure distortion.
3. Results When drying the catalysts, HCl elimination due to interaction of impregnated CuCl with surface carrier groups takes place. According to chemical
SECTION VIII.
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analysis data (Table 1) copper concentrations up to 5 wt%. give a constant atomic ratio Cl/Cu+1. Higher Cu contents result in an increase of this ratio. According to N"P !((P !P *M /M )*M /M ) (1) ! ! ! ! ! ! ! where P is the Cu content in catalyst, wt%; ! P the Cl content in catalyst, wt%; M the Cu ! ! atomic weight (63.54); M the Cl atomic weight ! (35.46); N the content of copper bounded with one chlorine atom, wt%; and the samples C10 and C15 have N the values 5.5}6 wt% what corresponds to mole concentration 9;10\ mol/g-cat. RDA curves (Fig. 1) of C1}C15 samples contain four peaks which, according to [6}8], are attributed to bonds Cu}O, Cu}Cl, Cu}O and Cu}Cu with the distances 1.97$0.02, 2.29$0.03,
2.80$0.05 and 3.37$0.03 As , correspondingly. When Cu concentration in the samples increases, a decrease of relative intensity of a peak for the shortest Cu}O distance is observed due to decrease of coordination number of O surrounding copper atoms (Table 1). Intensity of peaks for Cu}Cl distances, on the other hand, slightly increases and reaches maximum for C10 and C15 samples. Intensities of peaks attributed to Cu}Cu distances also increase, but their coordination numbers do not exceed 0.3. At adsorption of copper on the surface of a carbon carrier with various O-containing groups a number of surface copper complexes with Cu}O distances close to 1.97 As should be observed. This gives a single peak on RDA curve corresponding to an average value. The same is also valid for average distance Cu}Cl and Cu}Cu. Due to this reason, as described earlier, lowered coordination numbers are obtained.
4. Discussion
Fig. 1. Curves of radial distribution of atoms (RDA) for C1}C15 samples.
According to a number of reasons a surface of carbon carriers contains a lot of functional Ocontaining groups [10,11]. At some conditions protons of these groups may be substituted with cations whose concentrations in the samples may reach 1.4;10\ mol/g. Impregnation of carbon with CuCl results in HCl elimination due to chemical interaction with surface groups of the carrier. It is likely that this interaction proceeds via substitution of chlorine ion with surface O-containing group of the carrier and is accompanied by the formation of chemical bond between copper and oxygen which is seen in EXAFS spectra for all catalysts (Fig. 1) as an intensive peak corresponding to a distance Cu}O" 1.97$0.02 As . This bond is typical of catalysts prepared by the interaction of metal halogenides with carbons [12]. All the catalysts do not contain basic copper chloride Cu Cl(OH) as there are no peaks near 3.05 As for Cu}Cu distance on the RDA curves. Another distance Cu}O (2.8 As ) is attributed to sterically coordinated oxygen for COO groups which stabilize copper. In accordance with EXAFS data and elemental analysis, an interaction of copper dichloride with
V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 318}322
carrier surface may be described as follows: (carrier)!COOH#CuCl N(carrier)!COOCu Cl#HCl (carrier)!OH#CuCl N(carrier)!OCuCl #HCl. In the catalyst heating stage in a nitrogen #ow [1] initial portions of copper dichloride are more likely to react with surface functional groups with the most easily substituting proton. When copper concentration increases more stable surface groups are involved. In the case of low copper contents ((5%) all supported metal is bounded on the surface as monomolecular species, a substitution of only one Cl ion from each CuCl molecule occurs. According to elemental analysis all the samples with copper concentrations less than 1% have the atomic ratio Cl/Cu close to 1. On the other hand, according to EXAFS, data oxygen coordination numbers for the samples with low copper content essentially exceed 2. However, the bond Cu}O cannot be explained only by coordination of water molecules adsorbed from the surrounding air by hydroscopic surface species, because increase in the copper concentration results in the decrease of coordination number for this distance. So, a peak corresponding to average distance Cu}O "1.97$0.02 As contains at least 2 distances: 1. a distance between Cu and oxygen atoms in coordinated water. This is equal to 1.97 As for CuCl ) 2H O [7]; 2. a distance between copper atoms and oxygen atoms of surface groups of the carrier. This may vary from 1.95 to 2.10 As and is proved by the fact that assymmetrical factor is not equal to 0. The existence of several distances results in the lowering of e!ective coordination number which is determined by a "tting procedure but does not
321
change a behaviour of alteration of coordination number when Cu concentration increases. In the samples with high Cu content an increase of Cu/Cl atomic ratio is observed (Table 1). Also it is seen (Fig. 1) that there is an increase of relative intensities of peaks for distances Cu}Cl and Cu}Cu and a decrease of relative intensities for distances Cu}O. This may be caused for several reasons: 1. An increase of coordination number for distance Cu}Cu is connected with an interaction of CuCl , with two nearly situated surface groups with the formation of binuclear compounds as in molybdenum complex compounds on SiO [13]. 2. An increase of coordination number for Cu}Cl distance as well as an increase of atomic ratio Cu/Cl is caused by the appearance of substances with two chlorine atoms. Using Eq. (1), a concentration of copper bounded with one chlorine atom is close to 6 wt% or 9;10\ mol Cu/g-cat. Obviously, this is determined by the concentration of surface groups of the carrier ready for chlorine substitution. This value is in the range 6;10\ } 15;10\ mol/g-carrier for the active hydrogen found in Ref. [10] for various carriers. The area of the single molecule CuCl ) 2H O is equal to ca. 3;10\ m, hence, the total area of supported CuCl is close to speci "c surface area of our carrier (220 m/g). Taking into account EXAFS data (Table 1) the area of separate molecule of surface compound [carrierCOO}Cu}Cl ) nH O] is close to the area of CuCl ) 2H O, hence, a monolayer "lling of the carrier surface takes place. Further increase in copper concentration results in the formation of the second layer without interaction with the surface groups:
SECTION VIII.
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In the second layer, copper is present as isolated molecules or associates of small dimensions. This does not result in a big increase of coordination number for Cu}Cu"3.37$0.03 As , because copper ions do not possess constant number of water molecules in the "rst coordination sphere which is the reason for both considerable disorder of surface composition of the catalyst and more variations of Cu}Cu distances in the associates. In this case, the samples do not contain large particles of any compounds with distances of 3}4 As . 5. Conclusion All the catalysts prepared by impregnation of carbon carrier with copper dichloride may be considered as the three groups varying by their surface composition: (A) Catalysts with low copper content (1}3%) contain surface compounds [carrier}COOCuCl ) H O] or [carrier}CO}CuCl ) H O], where copper is connected with the surface O-containing groups via the easiest substituting protons. (B) Catalysts with average copper content (5%) contain compounds of group (A) and surface compounds of the same composition and structure but coordinated with more stable groups of the carrier. A part of copper ions
possesses a distance of 3.4 As which in reaction conditions may result in the formation of binuclear compounds with Cu}Cu bond. (C) Catalysts with high copper content contain compounds of groups (A) and (B) as well as the non-chemically bounded hydrated copper dichloride. References [1] O.V. Klimov, O.V. Kikhtyanin, A.V. Kalinkin, K.G.Ione, J. Catal. (in press). [2] G. Lee, A. Curnutt, A. Dale Harley, In Oxygen Complexes and Oxygen Activation by Transition Metals, Plenum Press, New York, 1988, p. 215. [3] Eur. Pat. 0584785 A2. [4] D.I. Kochubey, EXAFS spectroscopy of catalysts, Nauka, Novosibirsk, 1992 (in Russian). [5] N. Binsted, J.V. Campbell, S.J. Gurman, P.C. Stephenson, SERC Daresbury Laboratory EXCURV92 program (1991). [6] P.C. Burns, F.C. Hawthorne, Am. Mineral. 78 (1993) 187. [7] A. Engberg, Acta Chemica Scand. 24 (1970) 3510. [8] M.E. Fleet, Acta Crystallogr. B 31 (1975) 183. [9] D.I. Kochubey, V.V. Kriventsov, G.N. Kustova, G.V. Odegova, P.G. Tsyrulnikov, E.N. Kudrya, Kinet. Catal. 39 (1998) 294. [10] H.P. Boehm, Carbon 32 (1994) 759. [11] E. Czaran, J. Finster, H. Schnabel, Z. Anorg. Allg. Chem. 443 (1978) 175. [12] H.P. Boehm, Angew. Chem. 78 (1966) 617. [13] Y. Iwasawa, H. Ichinose, S. Ogasawara, J. Chem. Soc. Faraday Trans. 1 (77) (1981) 1763.