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USY Zeolites: Effect of Alloying Pt with Cu
Hydroconversion of n-Octane on Pt/USY Zeolites: Effect of Alloying Pt with Cu M. Dufaux, M. Lokolo, P. Meriaudeau, c. Naccache and Y. Ben Taarit Institut de Recherches sur la Catalyse - C.N.R.S., Laboratoire conventionne l'Universite Claude Bernard, LYON I - 2, avenue Albert Einstein 69626 Villeurbanne Cedex, France.
a
Hydroconversion of n-octane is performed over a series of catalysts basically composed of Pt (or Pt-Cu alloys) as the hydrogenating functions and HY zeolite as acid material. The addition of Cu to Pt HY enhances the overall activity and the selectivity towards isomerisation. For a given Pt loading, it is proposed that the addition of Cu to Pt has two effects: - an increase of the acid function giving rise to an enhancement of the activity, - an increase of the number of metallic particles and thus inducing an improved selectivity towards isomerisation. INTRODUCTION It is well established that the hydroconversion of alkanes on bifunctional catalysts strongly depends on the balance between the metallic and the acid function (1) (2) • For a given acid function, the overall activity and the selectivity towards isomerisation increase with the metal content until a plateau is reached. The purpose of this work was to investigate the effect of alloying Pt with Cu on the performances of Pt HY catalyst in the n-octane hydroconversion. EXPERIMENTAL 1. Catalyst preparation The ultrastable zeolite USY used in this study was obtained from Linde Division, Union Carbide: this material (LZY82) has a very low Na content (0.16 %). The samples were exchanged successively with Pt(NH3)~+ and with CU+2(H20)6 cations. The catalysts were washed with water, dried in air at 343 K and slowly heated (0.5°/mn) under a flow of 02 from room temperature to 523 K and then reduced under a flow of H2 at 773 K. 2. Hydrogen chemisorption H2 chemisorption was studied with a classical apparatus equipped with a Texas pressure gauge. Hydrogen uptake was estimated by extrapoling the isotherm to zero pressure , the dispersions were calculated with the relation D = H/Pt. In some cases, T.E.M. (Transmission Electron Microscopy) was used in order to estimate the metal particle diameters. 3. Infrared of adsorbed CO For I.R. measurements, samples were pressed in disks, H2 reduced at 773 K, evacuated at 773 K and then contacted with CO. Spectra were recorded on a Perkin Elmer IR 580 Spectrometer, after evacuation of the gas phase at R.T. 4. Hydrogenation of benzene, dehydrogenation of cyclohexane These reactions were studied in a microdifferential reactor in a flow of reactants (H2/HC = 40) in the temperature range 298 K - 320 K for the hydrogenation of C6H6 and 523 - 573 K for C6H12 dehydrogenation. 929
930 (CA-1l-2) 5. n-octane conversion
The reaction was studied in a dynamic flow reactor at pressures varying between 1 to 60 bar ; at high pressure, the H2/HC ratio was fixed to 60. Generally 20 to 50 mg of sample were used with a flow rate of 8 to 20 1 h- 1. Gas chromatography (I.G.C. Interstmat) allows the detection of Cl-C8 hydrocarbons. RESULTS AND DISCUSSION chemisorption On table 1 are reported H/Pt values together with the composition of the different samples, as measured by chemical analysis.
~
Table 1. Sample composition and hydrogen chemisorption results. Sample (a)
Pt (weight %)
Cu/Pt
(b)
T.E.M.
H/Pt
(particle size nm) 1 1.02 1.14 1.06 1.68
PtlCuO PtlCuO.9 PtlCU1.7 PtlCu2.49 Ptl.68CuO
- 1.5
0.93 0.94 0.80 0.69 0.87
0 0.9 1. 72 2.49 0
1 1
-
1.5 1.5
(a) PtlCuO.9 means; 1 Pt atom for 0.9 Cu atom (b) atomic ratio. For all samples Na content is equal to 0.16 %. It appears that when Cu is added to Pt into the zeolite, the dispersion do not vary greatly from one sample to the other. In all cases, almost all platinum atoms are surface atoms able to chemisorb hydrogen. Transmission electron microscopy results confirm that the metallic particle diameters (1.0 - 1.5 nm) do not increase significantly when Cu is added to Pt. 2. Infrared of CO Infrared spectra of CO adsorbed on the different catalysts showed a band attributed to CO adsorbed linearly on Pt, the VCO decreasing when Cu contents increase (table 2). Table 2. Infrared of CO adsorbed onto Pt-Cu samples. sample
PtlCuO
Pt l CuO.9
Ptl CU1. 7
PtlCu2.49
(a) V CO
2095
2085
2070
2055
ClCO (b)
0.52
0.49
0.48
0.47
-1 (a) In em After brief evacuation of the gas phase at R.T. (b) As measured with the adsorbance of the band of the linear CO in arbatrary units. Upon increasing the Cu content, the VCO (linear CO on Pt) is shifted to lower frequency which is indicative, as already reported (3) (4) (5) of the formation of Cu Pt alloys. The intensity of this I.R. band remained roughly constant with increased Cu contents. These I.R. results indicate that upon alloying with Cu, Pt was diluted with Cu atoms (causing a frequency shift), the total number of surface Pt atoms remaining almost constant. In addition with the I.R. band of the linearly
M. Oufaux et al.
931
adsorbed CO on Pt, an I.R. band (\CO = 2165 - 2155 cm- 1) due to Cu+CO complexe was observed (6). Thus, hydrogen chemisorption, T.E.M. and infrared study of CO adsorption are in good agreement: upon alloying Pt with Cu in zeolites, the metal particle size remained constant and the number of surface Pt atoms accessible to H2 or CO decreased slightly : this suggest that the number of metallic particles would consequently increase. 3. Hydrogenation of c6H6. Dehydrogenation of C6H12. The results are reported in table 3. Table 3. Activities of Pt-Cu catalysts in the hydrogenation of C and dehydrogenation of C 6H6 6H12. Pt
Sample C T.O.N. (a) 6H6 C T.O.N. 6H12
~)
1
44 800
Pt 1cu O. 9 1.2 710
Pt 1cu1. 7
Pt 1Cu2. 49
< 0.2
a
270
250
-1 (a) in h at 298 K. (b) in h- 1 at 533 K. T.O.N. calculated by using data of table 1. The activity for C6H6 hydrogenation decreases drastically when Cu content increases ; similar results were reported for Pt Cu NaY (7) or Pt Au (8) and Pt Sn (9) and have been interpreted in terms of active ensemble, the benzene needing an ensemble of more than two atoms to be hydrogenated. In contrast, the dehydrogenation of cyclohexane appears to be less sensitive to the size of the ensemble since the results in table 3 show that the rate of the reaction decreased to a small extend. These properties will be discussed in a coming paper (10) and the change of the hydrogenating/dehydrogenating properties upon addition of Cu will only be considered as an indication of the incorporation of Cu into the Pt particles and as a reference for the dehydrogenating properties of the metallic sites. 4. Catalytic properties in the n-octane hydrocracking At atmospheric pressure the deactivation as a function of time on stream is important and the results reported here concern the data obtained at 40 atmosphere~ pressure at which'deactivation is not observed, even after several days on stream. On figure la are pictured the conversions of n-octane as a function of temperature for 3 different samples PtlCuO, PtlCu2.48' Pt 1.68' For clearity, the results concerning the other Pt-Cu catalysts are note reported in detail on figure 1 but it was checked that these catalysts exhibited catalytic properties (activities, selectivities) between PtlCuO and PtlCu2.49'
932 CCA-1l-2) Conversion % tot. 100
Conversion (Iso.) 50
so
25
473 593
Fig. la. Conversion of n-octane as a function of T (K) for (a) PtlCuo, (b) PtICu2.49' (c) Ptl.6SCUO. P = 40 atm. ; PH2/PHC = 60.
converslonccraCk~ 75
%
50
25
/:
4\
II I."
\f
,\
15
1,1 /:
l: 1/ II II Ii
\ ~Pt1.88 \ \
10
.Ij
5113
823 K
Fig. lc. Conversion in cracking as a function of T. a, b, c as in fig. la.
0
CUo
\ \ - P t 1 CU2.s
'.'iA\
5
Ternperature 473
Fig. lb. Conversion in isomerisation as a function of T (K). a, b, c as in fig. la.
Conversion (ISO) 20 Converslonterac k J
..-
Pt1.88CuO
553
823
20
40
~
80
.
% Conversion tot.
Fig. Id. Ilc as a function of the conversion. a, b, c as in fig. la.
Figure la indicates that the activities of the catalysts are in the following sequence Ptl < PtlCu2.4S < Pt1.6S' It is known that the activity of an hydrocracking catalyst for a given acid function increases with the platinum loading until to reach a maximum and then a plateau is obtained (11). Thus it is quite reasonable to observe that the activity of Ptl.6SCuO is greater than that of PtICuO' Figure ld shows that the ratio Ilc increases with the addition of Cu. In order to have a more precise comparison between the different catalysts, some experiments were conducted at constant temperature, by varying the contact time
M. Dufaux et al.
933
to have the same conversion for samples having different activities. Table 4 summarizes the main results. Table 4. riC for different samples at the same temperature, for the same conversion Sample
ric
20
32
30
Conversion
26
26
26
ric Conversion
4.7
10
46.5
47
T
= 533
T
=
K
553 K
As one can observe in figure 1d and table 4, the increase of Pt content (with a constant dispersion) increase the IIC value as reported elsewhere (12). For Pt-Cu catalysts, let us first assumed that the general simplified mechanism for hydroconversion is valid : (2)
(1)
(3)
no ~--:~nO
nA ~
(MS)
~
(AS)
(5)
iO;"- _-_""'. iO
\6)
(AS)\ t,
--"...--
iA
(MS)
(CP) with (MS) (AS) (CP) iO iA ---+
metallic site acid site cracking products iso-olefin iso-alkane gas phase transport.
As indicated previously, with the catalyst (Pt1CuO) it is observe~ that the metallic function is the limiting step (step 1) since the increase in Pt content give rise to an increase of the total conversion. As the addition of Cu does not give an enhancement of the dehydrogenation function (table 3) the increase of the conversion with Cu cannot be attributed to a correlative increase of the rate of step 1. So the benefit increase of the activity due to the addition of Cu cannot be attributed to the change of the metallic function. The other factor governing the activity is the acid properties of the zeolite : an increase of the conversion would be related to an increase of the acid sites strength. This was in fact observed (13) in studying by microcalorimetry technique, the adsorption of NH3 at 423 K : the addition of Cu increases the acid site strength. This change in acid properties cannot be related to a decrease of Na content since chemical analysis (table 1) indicates that the sodium content does not change significantly with the addition of Cu. The acid properties of CUY zeolites have been studied (14) (15) and it has been observed (after H2 reduction) that the main part of Cu++ is reduced at Cuo and that acid OH groups are formed. These OH groups are more stable than the OH groups of the corresponding HY. This greater stability is attributed to a stabilisation by Cu++ ions (15). Thus, it is possible to attribute the increase of the n-octane conversion to a change in the acid properties of the material. In such a case, one can expect to observe a decrease of ric since the balance between the metallic and the acid function will be in favor of the last one but it is just the opposite situation which is observed. rf the metallic particles number (per gram of
934 (CA-1l-2)
catalyst) is increased when Cu is added, the distances between an acid centre and a metallic site would decrease : thus, step 4 (see above reaction diagram) corresponding to the gas phase transport of the iso-olefin from an acid site to a metallic site will be favoured. Step 6 could also be favoured. Since the acid strength is also increased, but as I/C increase is related to the Cu addition, it is suggested that step 4 is more increased than step 6. In addition, one could also suggest that the formation of Pt-Cu alloy which results in the decrease of hydrogenolysis reaction on the metal surface, would limit the possible coke deposit on the metal thus limiting the metal deactivation. Less deactivated Pt-Cu samples would have an apparent higher activity than Pt samples, this phenomenon being more apparent at higher temperatures. Similar results (increase of conversion and I/c with Cu addition) were obtained on ReY (rare earth) loaded with Pt and Cu but results will be reported in a coming paper. Acknowledgments Mr. Urbain (chemical analysis), G. Wicker (T.E.M.) are acknowledged for their technical assistance. REFERENCES 1. H. Pichler, H. Schulz, H.O. Reitemeyer and J. Weitkamp, Erdol, Kohle-Erdgar Petrochem., 25, 494 (1972). 2. M. Guisnet and G. Perot; Zeolithes : Science and Technology, NATO ASI Series (F.R. Ribeiro et al., Eds) , Martinus Nijhoff Publishers, La Hague 1984, p. 397. 3. G. Blyholder, J. Phys. Chem., 68, 2772 (1964). 4. J.A. Dalmon, M. Primet, G.A. Martin et B. Imelik, Surf. Science, 50, 95 (1975). 5. F.J. Toolenaar, F. Stoop and V. Ponec, J. Catal., 82, 1 (1983). 6. G. Coudurier, T. Decamp and H. Praliaud, J. Chern. Soc. Farad. Trans., I, ~, 2661 (1982). 7. L. Tebassi, A. Sayari, A. Gorbel, M. Dufaux and C. Naccache, J. of Mol. Catal., 25, 397 (1984). 8. ~ Puddu and V. Ponec, Reac. Trav. Chim., Pays-Bas, ~, 255 (1976). 9. R. Bacaud, P. Bussiere, F. Figueras and J.P. Mathieu, in "Preparation of Catalysts", (B. Delmon, P.A. Jacobs and G. Poncelet Eds) , Elsevier Amsterdam, 1976, p. 509. 10. J. Bandiera and P. Meriaudeau, to be published. 11. G.P. Giannetto; thesis nO 420, Poitiers, France, 1985, p. 59. 12. G.P. Giannetto, G. Perot and M. Guisnet, in "Catalysis by acids and bases" (B. Imelik ed.). Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 20, 1985, p. 265. 13. M. Lokolo, A. Auroux, to be published. 14. J.C. Richardson, J. Catal., 9, 72 (1967). 15. C. Naccache and Y. Ben Taarit, J. Catal., ~, 171 (1971).
a
Hydroconversion of n-octane is performed over a series of catalysts basically composed of Pt (or Pt-Cu alloys) as the hydrogenating functions and HY zeolite as acid material. The addition of Cu to Pt HY enhances the overall activity and the selectivity towards isomerisation. For a given Pt loading, it is proposed that the addition of Cu to Pt has two effects: - an increase of the acid function giving rise to an enhancement of the activity, - an increase of the number of metallic particles and thus inducing an improved selectivity towards isomerisation. INTRODUCTION It is well established that the hydroconversion of alkanes on bifunctional catalysts strongly depends on the balance between the metallic and the acid function (1) (2) • For a given acid function, the overall activity and the selectivity towards isomerisation increase with the metal content until a plateau is reached. The purpose of this work was to investigate the effect of alloying Pt with Cu on the performances of Pt HY catalyst in the n-octane hydroconversion. EXPERIMENTAL 1. Catalyst preparation The ultrastable zeolite USY used in this study was obtained from Linde Division, Union Carbide: this material (LZY82) has a very low Na content (0.16 %). The samples were exchanged successively with Pt(NH3)~+ and with CU+2(H20)6 cations. The catalysts were washed with water, dried in air at 343 K and slowly heated (0.5°/mn) under a flow of 02 from room temperature to 523 K and then reduced under a flow of H2 at 773 K. 2. Hydrogen chemisorption H2 chemisorption was studied with a classical apparatus equipped with a Texas pressure gauge. Hydrogen uptake was estimated by extrapoling the isotherm to zero pressure , the dispersions were calculated with the relation D = H/Pt. In some cases, T.E.M. (Transmission Electron Microscopy) was used in order to estimate the metal particle diameters. 3. Infrared of adsorbed CO For I.R. measurements, samples were pressed in disks, H2 reduced at 773 K, evacuated at 773 K and then contacted with CO. Spectra were recorded on a Perkin Elmer IR 580 Spectrometer, after evacuation of the gas phase at R.T. 4. Hydrogenation of benzene, dehydrogenation of cyclohexane These reactions were studied in a microdifferential reactor in a flow of reactants (H2/HC = 40) in the temperature range 298 K - 320 K for the hydrogenation of C6H6 and 523 - 573 K for C6H12 dehydrogenation. 929
930 (CA-1l-2) 5. n-octane conversion
The reaction was studied in a dynamic flow reactor at pressures varying between 1 to 60 bar ; at high pressure, the H2/HC ratio was fixed to 60. Generally 20 to 50 mg of sample were used with a flow rate of 8 to 20 1 h- 1. Gas chromatography (I.G.C. Interstmat) allows the detection of Cl-C8 hydrocarbons. RESULTS AND DISCUSSION chemisorption On table 1 are reported H/Pt values together with the composition of the different samples, as measured by chemical analysis.
~
Table 1. Sample composition and hydrogen chemisorption results. Sample (a)
Pt (weight %)
Cu/Pt
(b)
T.E.M.
H/Pt
(particle size nm) 1 1.02 1.14 1.06 1.68
PtlCuO PtlCuO.9 PtlCU1.7 PtlCu2.49 Ptl.68CuO
- 1.5
0.93 0.94 0.80 0.69 0.87
0 0.9 1. 72 2.49 0
1 1
-
1.5 1.5
(a) PtlCuO.9 means; 1 Pt atom for 0.9 Cu atom (b) atomic ratio. For all samples Na content is equal to 0.16 %. It appears that when Cu is added to Pt into the zeolite, the dispersion do not vary greatly from one sample to the other. In all cases, almost all platinum atoms are surface atoms able to chemisorb hydrogen. Transmission electron microscopy results confirm that the metallic particle diameters (1.0 - 1.5 nm) do not increase significantly when Cu is added to Pt. 2. Infrared of CO Infrared spectra of CO adsorbed on the different catalysts showed a band attributed to CO adsorbed linearly on Pt, the VCO decreasing when Cu contents increase (table 2). Table 2. Infrared of CO adsorbed onto Pt-Cu samples. sample
PtlCuO
Pt l CuO.9
Ptl CU1. 7
PtlCu2.49
(a) V CO
2095
2085
2070
2055
ClCO (b)
0.52
0.49
0.48
0.47
-1 (a) In em After brief evacuation of the gas phase at R.T. (b) As measured with the adsorbance of the band of the linear CO in arbatrary units. Upon increasing the Cu content, the VCO (linear CO on Pt) is shifted to lower frequency which is indicative, as already reported (3) (4) (5) of the formation of Cu Pt alloys. The intensity of this I.R. band remained roughly constant with increased Cu contents. These I.R. results indicate that upon alloying with Cu, Pt was diluted with Cu atoms (causing a frequency shift), the total number of surface Pt atoms remaining almost constant. In addition with the I.R. band of the linearly
M. Oufaux et al.
931
adsorbed CO on Pt, an I.R. band (\CO = 2165 - 2155 cm- 1) due to Cu+CO complexe was observed (6). Thus, hydrogen chemisorption, T.E.M. and infrared study of CO adsorption are in good agreement: upon alloying Pt with Cu in zeolites, the metal particle size remained constant and the number of surface Pt atoms accessible to H2 or CO decreased slightly : this suggest that the number of metallic particles would consequently increase. 3. Hydrogenation of c6H6. Dehydrogenation of C6H12. The results are reported in table 3. Table 3. Activities of Pt-Cu catalysts in the hydrogenation of C and dehydrogenation of C 6H6 6H12. Pt
Sample C T.O.N. (a) 6H6 C T.O.N. 6H12
~)
1
44 800
Pt 1cu O. 9 1.2 710
Pt 1cu1. 7
Pt 1Cu2. 49
< 0.2
a
270
250
-1 (a) in h at 298 K. (b) in h- 1 at 533 K. T.O.N. calculated by using data of table 1. The activity for C6H6 hydrogenation decreases drastically when Cu content increases ; similar results were reported for Pt Cu NaY (7) or Pt Au (8) and Pt Sn (9) and have been interpreted in terms of active ensemble, the benzene needing an ensemble of more than two atoms to be hydrogenated. In contrast, the dehydrogenation of cyclohexane appears to be less sensitive to the size of the ensemble since the results in table 3 show that the rate of the reaction decreased to a small extend. These properties will be discussed in a coming paper (10) and the change of the hydrogenating/dehydrogenating properties upon addition of Cu will only be considered as an indication of the incorporation of Cu into the Pt particles and as a reference for the dehydrogenating properties of the metallic sites. 4. Catalytic properties in the n-octane hydrocracking At atmospheric pressure the deactivation as a function of time on stream is important and the results reported here concern the data obtained at 40 atmosphere~ pressure at which'deactivation is not observed, even after several days on stream. On figure la are pictured the conversions of n-octane as a function of temperature for 3 different samples PtlCuO, PtlCu2.48' Pt 1.68' For clearity, the results concerning the other Pt-Cu catalysts are note reported in detail on figure 1 but it was checked that these catalysts exhibited catalytic properties (activities, selectivities) between PtlCuO and PtlCu2.49'
932 CCA-1l-2) Conversion % tot. 100
Conversion (Iso.) 50
so
25
473 593
Fig. la. Conversion of n-octane as a function of T (K) for (a) PtlCuo, (b) PtICu2.49' (c) Ptl.6SCUO. P = 40 atm. ; PH2/PHC = 60.
converslonccraCk~ 75
%
50
25
/:
4\
II I."
\f
,\
15
1,1 /:
l: 1/ II II Ii
\ ~Pt1.88 \ \
10
.Ij
5113
823 K
Fig. lc. Conversion in cracking as a function of T. a, b, c as in fig. la.
0
CUo
\ \ - P t 1 CU2.s
'.'iA\
5
Ternperature 473
Fig. lb. Conversion in isomerisation as a function of T (K). a, b, c as in fig. la.
Conversion (ISO) 20 Converslonterac k J
..-
Pt1.88CuO
553
823
20
40
~
80
.
% Conversion tot.
Fig. Id. Ilc as a function of the conversion. a, b, c as in fig. la.
Figure la indicates that the activities of the catalysts are in the following sequence Ptl < PtlCu2.4S < Pt1.6S' It is known that the activity of an hydrocracking catalyst for a given acid function increases with the platinum loading until to reach a maximum and then a plateau is obtained (11). Thus it is quite reasonable to observe that the activity of Ptl.6SCuO is greater than that of PtICuO' Figure ld shows that the ratio Ilc increases with the addition of Cu. In order to have a more precise comparison between the different catalysts, some experiments were conducted at constant temperature, by varying the contact time
M. Dufaux et al.
933
to have the same conversion for samples having different activities. Table 4 summarizes the main results. Table 4. riC for different samples at the same temperature, for the same conversion Sample
ric
20
32
30
Conversion
26
26
26
ric Conversion
4.7
10
46.5
47
T
= 533
T
=
K
553 K
As one can observe in figure 1d and table 4, the increase of Pt content (with a constant dispersion) increase the IIC value as reported elsewhere (12). For Pt-Cu catalysts, let us first assumed that the general simplified mechanism for hydroconversion is valid : (2)
(1)
(3)
no ~--:~nO
nA ~
(MS)
~
(AS)
(5)
iO;"- _-_""'. iO
\6)
(AS)\ t,
--"...--
iA
(MS)
(CP) with (MS) (AS) (CP) iO iA ---+
metallic site acid site cracking products iso-olefin iso-alkane gas phase transport.
As indicated previously, with the catalyst (Pt1CuO) it is observe~ that the metallic function is the limiting step (step 1) since the increase in Pt content give rise to an increase of the total conversion. As the addition of Cu does not give an enhancement of the dehydrogenation function (table 3) the increase of the conversion with Cu cannot be attributed to a correlative increase of the rate of step 1. So the benefit increase of the activity due to the addition of Cu cannot be attributed to the change of the metallic function. The other factor governing the activity is the acid properties of the zeolite : an increase of the conversion would be related to an increase of the acid sites strength. This was in fact observed (13) in studying by microcalorimetry technique, the adsorption of NH3 at 423 K : the addition of Cu increases the acid site strength. This change in acid properties cannot be related to a decrease of Na content since chemical analysis (table 1) indicates that the sodium content does not change significantly with the addition of Cu. The acid properties of CUY zeolites have been studied (14) (15) and it has been observed (after H2 reduction) that the main part of Cu++ is reduced at Cuo and that acid OH groups are formed. These OH groups are more stable than the OH groups of the corresponding HY. This greater stability is attributed to a stabilisation by Cu++ ions (15). Thus, it is possible to attribute the increase of the n-octane conversion to a change in the acid properties of the material. In such a case, one can expect to observe a decrease of ric since the balance between the metallic and the acid function will be in favor of the last one but it is just the opposite situation which is observed. rf the metallic particles number (per gram of
934 (CA-1l-2)
catalyst) is increased when Cu is added, the distances between an acid centre and a metallic site would decrease : thus, step 4 (see above reaction diagram) corresponding to the gas phase transport of the iso-olefin from an acid site to a metallic site will be favoured. Step 6 could also be favoured. Since the acid strength is also increased, but as I/C increase is related to the Cu addition, it is suggested that step 4 is more increased than step 6. In addition, one could also suggest that the formation of Pt-Cu alloy which results in the decrease of hydrogenolysis reaction on the metal surface, would limit the possible coke deposit on the metal thus limiting the metal deactivation. Less deactivated Pt-Cu samples would have an apparent higher activity than Pt samples, this phenomenon being more apparent at higher temperatures. Similar results (increase of conversion and I/c with Cu addition) were obtained on ReY (rare earth) loaded with Pt and Cu but results will be reported in a coming paper. Acknowledgments Mr. Urbain (chemical analysis), G. Wicker (T.E.M.) are acknowledged for their technical assistance. REFERENCES 1. H. Pichler, H. Schulz, H.O. Reitemeyer and J. Weitkamp, Erdol, Kohle-Erdgar Petrochem., 25, 494 (1972). 2. M. Guisnet and G. Perot; Zeolithes : Science and Technology, NATO ASI Series (F.R. Ribeiro et al., Eds) , Martinus Nijhoff Publishers, La Hague 1984, p. 397. 3. G. Blyholder, J. Phys. Chem., 68, 2772 (1964). 4. J.A. Dalmon, M. Primet, G.A. Martin et B. Imelik, Surf. Science, 50, 95 (1975). 5. F.J. Toolenaar, F. Stoop and V. Ponec, J. Catal., 82, 1 (1983). 6. G. Coudurier, T. Decamp and H. Praliaud, J. Chern. Soc. Farad. Trans., I, ~, 2661 (1982). 7. L. Tebassi, A. Sayari, A. Gorbel, M. Dufaux and C. Naccache, J. of Mol. Catal., 25, 397 (1984). 8. ~ Puddu and V. Ponec, Reac. Trav. Chim., Pays-Bas, ~, 255 (1976). 9. R. Bacaud, P. Bussiere, F. Figueras and J.P. Mathieu, in "Preparation of Catalysts", (B. Delmon, P.A. Jacobs and G. Poncelet Eds) , Elsevier Amsterdam, 1976, p. 509. 10. J. Bandiera and P. Meriaudeau, to be published. 11. G.P. Giannetto; thesis nO 420, Poitiers, France, 1985, p. 59. 12. G.P. Giannetto, G. Perot and M. Guisnet, in "Catalysis by acids and bases" (B. Imelik ed.). Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 20, 1985, p. 265. 13. M. Lokolo, A. Auroux, to be published. 14. J.C. Richardson, J. Catal., 9, 72 (1967). 15. C. Naccache and Y. Ben Taarit, J. Catal., ~, 171 (1971).