- Email: [email protected]
Nature and distribution of coke formed on mono-metallic platinum and bimetallic platinum-rhenium catalysts
APPLIED CATALYSIS
::~.~.~~i~
A: GENERAL
ELSEVIER
Applied Catalysis A: General 159 (1997) 1-7
Nature and distribution of coke formed on mono-metallic platinum and bimetallic platinum-rhenium catalysts M.R. Jovanovi6 a, P.S. Putanov b'* ~Development Department, Oil Refine~, Pan(evo, Yugoslavia bSerbian Academy of Sciences and Arts, Belgrade, Knez Mihailova St. 35, Yugoslavia Received 18 March 1996: received in revised form 28 February 1997; accepted 3 March 1997
Abstract In this work the amount and dispersion of carbon deposits, as well as their distribution between active metal and support were investigated on Pt/A1203 and Pt,Re/A1203 commercial catalysts, using real industrial feed under industrial conditions, simulated in fixed bed laboratory reactor. The conclusions about textural characteristics and quality of coke have been based on textural changes of used catalyst studied by sorption methods and mercury porosimeter. The coke location and its distribution between metal and support centres have been estimated by DTA of used catalysts. The significant difference between mono- and bimetallic catalysts has been noticed in the quality of coke deposits, as well as in distribution factor among metal and support centres. The coke formed on bimetallic catalysts has been fine-dispersed "soft" coke, located mainly on the support. The distribution factor metal/support has been higher for monometallic catalysts than for bimetallic ones.
Keywords: Mono- and bimetallic catalysts; Naphtha reforming; Coke deposition: Porous structure: Exothermal effects; Metal/support distribution factor
1. Introduction The studies on carbon deposition as a factor of deactivation of naphtha reforming catalysts cover a broad spectrum of problems, starting from the role of coking in catalyst degradation to the influence of numerous process parameters on the rate of carbon deposition and its mechanism, i.e. the nature and morphology of carbon
* Corresponding author. 0926-860X/97/$17.00 :;"J 1997 Published by Elsevier Science B.V. All rights reserved. Pll S0926-860X(97)00095- 1
2
M.R. Jovanovi(, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7
deposits, their quantity and distribution between metal clusters and the support. In these studies special attention has been paid to bimetallic catalysts, especially to platinum-rhenium catalysts, because of their improved reforming performance and enhanced coke tolerance. Investigation made on Pt,Re/A1203 models, with monocomponent hydrocarbon feed, resulted in various assumptions on promotional effect of rhenium: (i) the formation of high melting point at Pt-Re alloy prevents sintering of active platinum sites, (ii) rhenium modifies alumina support, enhancing the resistance of platinum surface area to harmful coke laydown, (iii) rhenium removes coke precursors which would block the active surface [1]. The assumption that coke precursors are converted on rhenium to harmless products can be related to the activity of rhenium as a promoter in aromatic ring opening [2]. The role of rhenium in preventing polymerization reactions has been understood in terms of weak adsorption of methyl cyclohexane from the feed [3]. Synergistic effect of platinum, rhenium and sulphur was also explained by prevention of polymerization reactions on Pt-Re catalysts; the growth of polycrystalline coke deposits should be shown by sulphur joint to rhenium atoms [4]. The fact that the state of Pt,Re/A1203 catalyst is highly sensitive to the conditions of its preparation and activation makes the speculations about the nature of this catalyst under actual reforming conditions rather ambiguous. In contrast to experimentally proved propensity of Pt and Ir to form bimetallic clusters, in Pt-Re system were observed both highly dispersed alloys and bimetallic clusters, depending on work conditions [5]. On the other hand, it has been shown that in Re-A1203 system at low rhenium concentration the interaction between rhenium and alumina might be very strong [6]. In view of the location of coke deposits it has been shown that coke formed during naphtha reforming can occupy both metal centres and the support [7]. However, the centres of coke deposition on mono- and bimetallic catalysts have not been investigated sufficiently so far. The morphology, the textural properties and the crystallinity of carbon deposits have not been thoroughly examined either. Several different types of carbon deposits have been described, like polycrystal graphite, cylindric graphite fibres, dendrites, sponge form, etc. [8]. It is generally accepted that in the case of oxide and alumosilicate catalysts the polycrystal graphite is predominant [9]. The subject of this work is the study of coke formation on platinum-rhenium catalysts under conditions close to their industrial exploitation. Commercial Pt/ A1203 and Pt,Re/A1203 catalysts were investigated using industrial feed, under conditions which correspond to industrial naphtha reforming. Criteria used for distinguishing between effects of coke deposition on monometallic and bimetallic catalysts were: the amount and the dispersion of carbon deposits as well as their distribution between the active metal and support. These criteria were established on the basis of data obtained for the quantity of coke deposited under identical conditions, the change of textural properties of the catalyst during naphtha
M.R. Jovanovid, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7 Table 1 Active metal content in catalyst samples Sample sign
Pt content (wt%)
Re content (wt%)
Support
BM 1 BM 2 MM 1 MM 2
0.38 0.20 0.40 0.75
0.38 0.20 ---
~-A1203 ")-A1203 "v-Al20~ ")-A1203
reforming, the density of carbon deposits, dispersion factors calculated from the above data and thermal effects of coke burning. 2. Experimental Two monometallic platinum catalysts (sign MM 1 and MM 2) were compared with two bimetallic platinum-rhenium catalysts (sign BM 1 and BM 2). All these catalysts were of commercial origin, with ")'-A1203 as support. The content of active metals in the catalyst is presented in Table 1. As shown, the platinum weight ratio within the same type was 1 : 2. The catalysts BM 1 and MM 1 had the same platinum content: the total metal content was equal in the case of BM 1 and MM 2, as well as in the case of BM 2 and MM 1. Catalysts were activated by sulphurization, using the standard industrial procedure [10]. Characteristic of the feed were the following: density d2o=0.744 g/cm3; paraffins 59.3 vol%; naphtenes 31.6 vol%; aromatics 9.1 vol%. The RON was 45.9. Naphtha reforming process was simulated in fixed bed laboratory reactor with 100 ml catalyst, at the temperature 500°C, under pressure 2.9 MPa, with LHSV 1.5 h -~ and H2/HC ratio 5" 1. The time of testing was 100 h. After catalytic reaction and before characterization catalyst samples were treated by the procedure usual for each method. The quantity of coke on catalyst samples was estimated by standard volumetric method [ 11 ]. The analysis of coke burning effects was done by temperature programmed oxidation, using DTA apparatus Du Pont model 1090. The catalyst samples were heated in oxygen, from ambient temperature to 750°C. The heating speed was 20°/min. Textural properties were analysed before and after reforming, by Sorptomatic Carlo Erba model 2000. 3. Results and discussion The quantity of carbon deposited on the catalysts after 100 h testing is presented in Table 2. It is given in wt% and in g/100 ml of used catalysts. The initial and residual activities (RON) are listed, too.
4
M.R. Jovanovid, P..S. Putanov/Applied Catalysis A: General 159 (1997) 1-7
Table 2 Quantity of carbon deposited during catalyst testing Catalyst
Carbon content
BM 1 BM 2 MM 1 MM 2
RON
wt%
g/100 ml
Initial
Residual
3.2 3.0 3.0 3.8
1.92 1.65 1.59 2.01
98 94 92 95
96 92 88 91
Table 3 Textural properties of catalyst samples before and after testing Catalyst
SA (m2/g)
Total pore volume (mUg)
Bulk density (g/ml)
Total porosity (%)
Mean pore diameter (nm)
Pores>3.7 nm (%)
BM 1
Fresh 175 Used 123 Fresh 230 Used 110 Fresh 150 Used 109 Fresh 174 Used 139
0.92 0.81 0.80 0.77 0.87 0.40 0.80 0.63
0.65 0.60 0.72 0.55 0.62 0.53 0.63 0.53
66.22 60.40 57.60 42.35 53.75 21.20 50.40 33.39
21 27 13 28 24 15 19 10
94 99 93 98 92 88 88 80
BM 2 MM 1 MM 2
The analysis of the quantity of carbon deposits in correlation with active metal contents in the catalyst shows the following: on the samples with equal, lower, total metal content (BM 2 and MM 1) equal percent ratio of carbon deposits was measured. However, on the samples with higher total metal content (BM 1 and MM 2) noticeable higher percent ratio of deposited carbon was measured on the catalyst MM 2, which points out predominant role of platinum in carbon deposition. The significant difference between textural properties of fresh and used monometallic and bimetallic catalysts, listed in Table 3, illustrates different nature of carbon deposits accumulated in the pores of the catalyst during reforming. In the case of bimetallic catalysts, the carbon completely filled the micro pores; consequently, the mean pore diameter moved towards higher values. At monometallic catalysts the carbon is deposited in wider pores; it occupies the pore walls, but does not fill entirely the pores. Therefore the mean pore diameter in monometallic catalysts moved towards lower values. This picture is in agreement with data obtained in this work for the density and dispersion of coke deposits. The real density of carbon settled on the catalyst was estimated using the adsorption procedure based on the additivity of the volumes of the support skeleton and coke deposit, respectively [ 12]. According to the accepted criterion the values of density under 1 g/cm 3 mean well dispersed carbon, data presented in Table 4 confirm significantly better carbon dispersion on bimetallic catalysts, especially on the sample BM 2.
M.R. Jovanovi(, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7 Table 4 Characteristics of deposited coke and factors of SA changes Catalyst
Real carbon density (g/ml)
Carbon volume (ml/100 ml cat.)
Part of coke in pores r>3.7 nm (%1
Change of SA to carbon weight appr.
MM 1 BM 2 MM 1 MM 2
0.769 0.302 1.071 0.976
2.50 5.46 1.48 2.09
4.5 0.6 17.0 15.0
1600 4000 1100 900
As the density of deposited carbon is in inverse proportion with its dispersion, the ratio of the change of surface area to the quantity of carbon deposit may serve as a measure of carbon dispersion. This dispersion factor was taken into account considering catalyst degradation. It has been admitted that the values of this factor over 1000 indicate well dispersed carbon [13]. In Table 4 parallel to the values of carbon real density the approximate ratios of surface area change to the quantity of carbon deposit are given. Noticeably higher values of above mentioned dispersion factor obtained for bimetallic catalysts, espeeially for sample BM 2, proved that carbon deposited on bimetallic catalysts is better dispersed than the carbon on monometallic catalysts. The data about carbon volume show more voluminous carbon on bimetallic catalysts, as an additional evidence of its "soft" nature. Percentages of coke accumulated in catalyst pores of radius higher than 3.7 nm calculated on the basis of volume and density of carbon and the changes of catalyst porous structure [13], provide another proof of fine small grain structure of coke deposits on bimetallic catalysts. The results of differential thermal analysis made after 100 h testing are presented in Table 5 and Fig. 1. It has been generally accepted that the temperature regions of the first observed effect corresponds to burning coke settled on platinum [ 14]. In our experiments this first effect is more pronounced on monometallic catalysts, especially on MM 2 sample, characterized by the highest platinum content. The appearance of exothermal effects at 330°C and 375°C could not be ascribed to burning carbon deposits on alumina. It is known that coke on alumina burns off
Table 5 DTA of used catalysts (oxygen atmosphere) Catalyst
Platinum content
Temperatures of exothermal effects in 'C
BM 1 BM 2 MM 1 MM 2
0.38 0.20 0.40 0.75
215 280 230 320
Number of * indicates relative peak intensity.
(*) ('1 (**~ (***)
500 490 375 330
(***~ (***~ (**) (*)
6
M.R. JovanoviF, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7
10
h
'10 ....
|
MMI MM2
|
20O
,(00 T'C
Fig. 1. DTA of used catalysts.
at higher temperatures. It could be rather an indication of carbon deposited near platinum centres. Two types of coke on platinum have been described: (i) coke deposited directly on platinum and (ii) coke in the vicinity of platinum centres [ 15]. Coke can be also deposited on the support, but weakly bonded or less polymerized [16]. In any case it is obvious that in monometallic catalysts a part of coke deposited on platinum is predominant, compared to coke bonded elsewhere. DTA results obtained for bimetallic catalysts show noticeably lower deposition of carbon on platinum centres. Intensive exothermal effects at 500°C, observed on bimetallic catalysts, are ascribed to burning coke joint with alumina support. Data given in Table 5 indicate wider coke dispersion on alumina at the surface of bimetallic catalysts, already indicated by the change of textural properties.
4. Conclusion Results obtained in this work lead to the conclusion that the essential difference between coke formation on monometallic platinum and bimetallic platinumrhenium catalyst is not in the quantity of deposited carbon, but in its nature and distribution. The coke formed on bimetallic catalysts is small grain, finedispersed, so called "soft" coke, located mainly on the support. As such coke does not disturb reforming reactions, bimetallic Pt-Re catalysts are more coke tolerant and do not show sharp decline of activity in the presence of relatively great quantity of coke [17,18].
M.R. Jovanovi(, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7
7
Different dimensions of carbon aggregates formed on monometallic and bimetallic catalysts result also from different sintering conditions. The aggregation of coke demands larger metal centres and better mobility of coke itself [ 19]. The coke formed on monometallic catalysts forms easily larger aggregates on bimetallic catalysts metal dispersion and carbon mobility are modified by catalyst component interactions: platinum-rhenium interaction stabilizes platinum dispersion while platinum-rhenium-sulphur and rhenium-alumina interactions effect coke mobility.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
R.J. Bertolacini and R.J. Pellet, Stud. Surf. Sci. Catal., 6 (1980) 73. R.J. Bertolacini and R.J. Pellet, Stud. Surf. Sci. Catal., 6 (1980) 75. D.A. Selman and A. Voorbies, Ind. Eng. Chem. Prod. Res. Dev., 14(2) (1975) 118. W.K. Schumm, J.B. Butt and W.M.H. Sachtler, J. Catal., 99 (1986) 126. J.H. Sinfelt, in: J. Anderson, M. Boudart (Eds.I, Catal. Sci. Technol., vol. 1, Acad. Verlag, Berlin, 1982, Ch. 5. A.N. Webb, J. Catal., 39 (1975) 485. R.A. Cabrol and A. Oberlin, J. Catal., 89 (1984) 256. J.E Brooks, G.H. Taylor, Chemistry and Physics of Carbon, vol. 4, Arnold, New York, 1968. O. Bragin, A.L. Liberman, Prevra~6enie uglevodorodov na metallsoder~a~6ih katalizatorah, Himia, Moskva, 1981. UOP Procedure, Inter, Publ. E Delannay, Characterization of Heterogeneous Catalysts, Marcel Dekker, New York, 1984. V.J. Gavrilov, V.B. Fenelonov, L.N. Rackovskaja, I.I. Frolova and I.A. Ovsjanikova, Kinet. Katal.. XXVII(3) (1986) 685. V.J. Gavrilov, V.B. Fenelonov and L.N. Rackovskaja, Kinet. Katal., XXIV(5) (1983) 1149. R.L. Mieville, J. Catal., 100 (1986) 482. J.M. Parera, N.S. Figoli and E.M. Traffano, J. Catal., 79 (19831 481. J. Barbier, E Marecot, N. Martin, L. Elassal and R. Maurel, Stud. Surf. Sci. Catal., 6 (1980) 53. R.A. Cabrol and A. Oberlin, J. Catal., 89 (1984) 256. M.R. Jovanovi6, Ph.D. Thesis, Univ. of Novi Sad, 1996. J. Barbier and P. Marecot, J. Catal., 102 (1986) 21.
::~.~.~~i~
A: GENERAL
ELSEVIER
Applied Catalysis A: General 159 (1997) 1-7
Nature and distribution of coke formed on mono-metallic platinum and bimetallic platinum-rhenium catalysts M.R. Jovanovi6 a, P.S. Putanov b'* ~Development Department, Oil Refine~, Pan(evo, Yugoslavia bSerbian Academy of Sciences and Arts, Belgrade, Knez Mihailova St. 35, Yugoslavia Received 18 March 1996: received in revised form 28 February 1997; accepted 3 March 1997
Abstract In this work the amount and dispersion of carbon deposits, as well as their distribution between active metal and support were investigated on Pt/A1203 and Pt,Re/A1203 commercial catalysts, using real industrial feed under industrial conditions, simulated in fixed bed laboratory reactor. The conclusions about textural characteristics and quality of coke have been based on textural changes of used catalyst studied by sorption methods and mercury porosimeter. The coke location and its distribution between metal and support centres have been estimated by DTA of used catalysts. The significant difference between mono- and bimetallic catalysts has been noticed in the quality of coke deposits, as well as in distribution factor among metal and support centres. The coke formed on bimetallic catalysts has been fine-dispersed "soft" coke, located mainly on the support. The distribution factor metal/support has been higher for monometallic catalysts than for bimetallic ones.
Keywords: Mono- and bimetallic catalysts; Naphtha reforming; Coke deposition: Porous structure: Exothermal effects; Metal/support distribution factor
1. Introduction The studies on carbon deposition as a factor of deactivation of naphtha reforming catalysts cover a broad spectrum of problems, starting from the role of coking in catalyst degradation to the influence of numerous process parameters on the rate of carbon deposition and its mechanism, i.e. the nature and morphology of carbon
* Corresponding author. 0926-860X/97/$17.00 :;"J 1997 Published by Elsevier Science B.V. All rights reserved. Pll S0926-860X(97)00095- 1
2
M.R. Jovanovi(, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7
deposits, their quantity and distribution between metal clusters and the support. In these studies special attention has been paid to bimetallic catalysts, especially to platinum-rhenium catalysts, because of their improved reforming performance and enhanced coke tolerance. Investigation made on Pt,Re/A1203 models, with monocomponent hydrocarbon feed, resulted in various assumptions on promotional effect of rhenium: (i) the formation of high melting point at Pt-Re alloy prevents sintering of active platinum sites, (ii) rhenium modifies alumina support, enhancing the resistance of platinum surface area to harmful coke laydown, (iii) rhenium removes coke precursors which would block the active surface [1]. The assumption that coke precursors are converted on rhenium to harmless products can be related to the activity of rhenium as a promoter in aromatic ring opening [2]. The role of rhenium in preventing polymerization reactions has been understood in terms of weak adsorption of methyl cyclohexane from the feed [3]. Synergistic effect of platinum, rhenium and sulphur was also explained by prevention of polymerization reactions on Pt-Re catalysts; the growth of polycrystalline coke deposits should be shown by sulphur joint to rhenium atoms [4]. The fact that the state of Pt,Re/A1203 catalyst is highly sensitive to the conditions of its preparation and activation makes the speculations about the nature of this catalyst under actual reforming conditions rather ambiguous. In contrast to experimentally proved propensity of Pt and Ir to form bimetallic clusters, in Pt-Re system were observed both highly dispersed alloys and bimetallic clusters, depending on work conditions [5]. On the other hand, it has been shown that in Re-A1203 system at low rhenium concentration the interaction between rhenium and alumina might be very strong [6]. In view of the location of coke deposits it has been shown that coke formed during naphtha reforming can occupy both metal centres and the support [7]. However, the centres of coke deposition on mono- and bimetallic catalysts have not been investigated sufficiently so far. The morphology, the textural properties and the crystallinity of carbon deposits have not been thoroughly examined either. Several different types of carbon deposits have been described, like polycrystal graphite, cylindric graphite fibres, dendrites, sponge form, etc. [8]. It is generally accepted that in the case of oxide and alumosilicate catalysts the polycrystal graphite is predominant [9]. The subject of this work is the study of coke formation on platinum-rhenium catalysts under conditions close to their industrial exploitation. Commercial Pt/ A1203 and Pt,Re/A1203 catalysts were investigated using industrial feed, under conditions which correspond to industrial naphtha reforming. Criteria used for distinguishing between effects of coke deposition on monometallic and bimetallic catalysts were: the amount and the dispersion of carbon deposits as well as their distribution between the active metal and support. These criteria were established on the basis of data obtained for the quantity of coke deposited under identical conditions, the change of textural properties of the catalyst during naphtha
M.R. Jovanovid, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7 Table 1 Active metal content in catalyst samples Sample sign
Pt content (wt%)
Re content (wt%)
Support
BM 1 BM 2 MM 1 MM 2
0.38 0.20 0.40 0.75
0.38 0.20 ---
~-A1203 ")-A1203 "v-Al20~ ")-A1203
reforming, the density of carbon deposits, dispersion factors calculated from the above data and thermal effects of coke burning. 2. Experimental Two monometallic platinum catalysts (sign MM 1 and MM 2) were compared with two bimetallic platinum-rhenium catalysts (sign BM 1 and BM 2). All these catalysts were of commercial origin, with ")'-A1203 as support. The content of active metals in the catalyst is presented in Table 1. As shown, the platinum weight ratio within the same type was 1 : 2. The catalysts BM 1 and MM 1 had the same platinum content: the total metal content was equal in the case of BM 1 and MM 2, as well as in the case of BM 2 and MM 1. Catalysts were activated by sulphurization, using the standard industrial procedure [10]. Characteristic of the feed were the following: density d2o=0.744 g/cm3; paraffins 59.3 vol%; naphtenes 31.6 vol%; aromatics 9.1 vol%. The RON was 45.9. Naphtha reforming process was simulated in fixed bed laboratory reactor with 100 ml catalyst, at the temperature 500°C, under pressure 2.9 MPa, with LHSV 1.5 h -~ and H2/HC ratio 5" 1. The time of testing was 100 h. After catalytic reaction and before characterization catalyst samples were treated by the procedure usual for each method. The quantity of coke on catalyst samples was estimated by standard volumetric method [ 11 ]. The analysis of coke burning effects was done by temperature programmed oxidation, using DTA apparatus Du Pont model 1090. The catalyst samples were heated in oxygen, from ambient temperature to 750°C. The heating speed was 20°/min. Textural properties were analysed before and after reforming, by Sorptomatic Carlo Erba model 2000. 3. Results and discussion The quantity of carbon deposited on the catalysts after 100 h testing is presented in Table 2. It is given in wt% and in g/100 ml of used catalysts. The initial and residual activities (RON) are listed, too.
4
M.R. Jovanovid, P..S. Putanov/Applied Catalysis A: General 159 (1997) 1-7
Table 2 Quantity of carbon deposited during catalyst testing Catalyst
Carbon content
BM 1 BM 2 MM 1 MM 2
RON
wt%
g/100 ml
Initial
Residual
3.2 3.0 3.0 3.8
1.92 1.65 1.59 2.01
98 94 92 95
96 92 88 91
Table 3 Textural properties of catalyst samples before and after testing Catalyst
SA (m2/g)
Total pore volume (mUg)
Bulk density (g/ml)
Total porosity (%)
Mean pore diameter (nm)
Pores>3.7 nm (%)
BM 1
Fresh 175 Used 123 Fresh 230 Used 110 Fresh 150 Used 109 Fresh 174 Used 139
0.92 0.81 0.80 0.77 0.87 0.40 0.80 0.63
0.65 0.60 0.72 0.55 0.62 0.53 0.63 0.53
66.22 60.40 57.60 42.35 53.75 21.20 50.40 33.39
21 27 13 28 24 15 19 10
94 99 93 98 92 88 88 80
BM 2 MM 1 MM 2
The analysis of the quantity of carbon deposits in correlation with active metal contents in the catalyst shows the following: on the samples with equal, lower, total metal content (BM 2 and MM 1) equal percent ratio of carbon deposits was measured. However, on the samples with higher total metal content (BM 1 and MM 2) noticeable higher percent ratio of deposited carbon was measured on the catalyst MM 2, which points out predominant role of platinum in carbon deposition. The significant difference between textural properties of fresh and used monometallic and bimetallic catalysts, listed in Table 3, illustrates different nature of carbon deposits accumulated in the pores of the catalyst during reforming. In the case of bimetallic catalysts, the carbon completely filled the micro pores; consequently, the mean pore diameter moved towards higher values. At monometallic catalysts the carbon is deposited in wider pores; it occupies the pore walls, but does not fill entirely the pores. Therefore the mean pore diameter in monometallic catalysts moved towards lower values. This picture is in agreement with data obtained in this work for the density and dispersion of coke deposits. The real density of carbon settled on the catalyst was estimated using the adsorption procedure based on the additivity of the volumes of the support skeleton and coke deposit, respectively [ 12]. According to the accepted criterion the values of density under 1 g/cm 3 mean well dispersed carbon, data presented in Table 4 confirm significantly better carbon dispersion on bimetallic catalysts, especially on the sample BM 2.
M.R. Jovanovi(, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7 Table 4 Characteristics of deposited coke and factors of SA changes Catalyst
Real carbon density (g/ml)
Carbon volume (ml/100 ml cat.)
Part of coke in pores r>3.7 nm (%1
Change of SA to carbon weight appr.
MM 1 BM 2 MM 1 MM 2
0.769 0.302 1.071 0.976
2.50 5.46 1.48 2.09
4.5 0.6 17.0 15.0
1600 4000 1100 900
As the density of deposited carbon is in inverse proportion with its dispersion, the ratio of the change of surface area to the quantity of carbon deposit may serve as a measure of carbon dispersion. This dispersion factor was taken into account considering catalyst degradation. It has been admitted that the values of this factor over 1000 indicate well dispersed carbon [13]. In Table 4 parallel to the values of carbon real density the approximate ratios of surface area change to the quantity of carbon deposit are given. Noticeably higher values of above mentioned dispersion factor obtained for bimetallic catalysts, espeeially for sample BM 2, proved that carbon deposited on bimetallic catalysts is better dispersed than the carbon on monometallic catalysts. The data about carbon volume show more voluminous carbon on bimetallic catalysts, as an additional evidence of its "soft" nature. Percentages of coke accumulated in catalyst pores of radius higher than 3.7 nm calculated on the basis of volume and density of carbon and the changes of catalyst porous structure [13], provide another proof of fine small grain structure of coke deposits on bimetallic catalysts. The results of differential thermal analysis made after 100 h testing are presented in Table 5 and Fig. 1. It has been generally accepted that the temperature regions of the first observed effect corresponds to burning coke settled on platinum [ 14]. In our experiments this first effect is more pronounced on monometallic catalysts, especially on MM 2 sample, characterized by the highest platinum content. The appearance of exothermal effects at 330°C and 375°C could not be ascribed to burning carbon deposits on alumina. It is known that coke on alumina burns off
Table 5 DTA of used catalysts (oxygen atmosphere) Catalyst
Platinum content
Temperatures of exothermal effects in 'C
BM 1 BM 2 MM 1 MM 2
0.38 0.20 0.40 0.75
215 280 230 320
Number of * indicates relative peak intensity.
(*) ('1 (**~ (***)
500 490 375 330
(***~ (***~ (**) (*)
6
M.R. JovanoviF, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7
10
h
'10 ....
|
MMI MM2
|
20O
,(00 T'C
Fig. 1. DTA of used catalysts.
at higher temperatures. It could be rather an indication of carbon deposited near platinum centres. Two types of coke on platinum have been described: (i) coke deposited directly on platinum and (ii) coke in the vicinity of platinum centres [ 15]. Coke can be also deposited on the support, but weakly bonded or less polymerized [16]. In any case it is obvious that in monometallic catalysts a part of coke deposited on platinum is predominant, compared to coke bonded elsewhere. DTA results obtained for bimetallic catalysts show noticeably lower deposition of carbon on platinum centres. Intensive exothermal effects at 500°C, observed on bimetallic catalysts, are ascribed to burning coke joint with alumina support. Data given in Table 5 indicate wider coke dispersion on alumina at the surface of bimetallic catalysts, already indicated by the change of textural properties.
4. Conclusion Results obtained in this work lead to the conclusion that the essential difference between coke formation on monometallic platinum and bimetallic platinumrhenium catalyst is not in the quantity of deposited carbon, but in its nature and distribution. The coke formed on bimetallic catalysts is small grain, finedispersed, so called "soft" coke, located mainly on the support. As such coke does not disturb reforming reactions, bimetallic Pt-Re catalysts are more coke tolerant and do not show sharp decline of activity in the presence of relatively great quantity of coke [17,18].
M.R. Jovanovi(, P.S. Putanov/Applied Catalysis A: General 159 (1997) 1-7
7
Different dimensions of carbon aggregates formed on monometallic and bimetallic catalysts result also from different sintering conditions. The aggregation of coke demands larger metal centres and better mobility of coke itself [ 19]. The coke formed on monometallic catalysts forms easily larger aggregates on bimetallic catalysts metal dispersion and carbon mobility are modified by catalyst component interactions: platinum-rhenium interaction stabilizes platinum dispersion while platinum-rhenium-sulphur and rhenium-alumina interactions effect coke mobility.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
R.J. Bertolacini and R.J. Pellet, Stud. Surf. Sci. Catal., 6 (1980) 73. R.J. Bertolacini and R.J. Pellet, Stud. Surf. Sci. Catal., 6 (1980) 75. D.A. Selman and A. Voorbies, Ind. Eng. Chem. Prod. Res. Dev., 14(2) (1975) 118. W.K. Schumm, J.B. Butt and W.M.H. Sachtler, J. Catal., 99 (1986) 126. J.H. Sinfelt, in: J. Anderson, M. Boudart (Eds.I, Catal. Sci. Technol., vol. 1, Acad. Verlag, Berlin, 1982, Ch. 5. A.N. Webb, J. Catal., 39 (1975) 485. R.A. Cabrol and A. Oberlin, J. Catal., 89 (1984) 256. J.E Brooks, G.H. Taylor, Chemistry and Physics of Carbon, vol. 4, Arnold, New York, 1968. O. Bragin, A.L. Liberman, Prevra~6enie uglevodorodov na metallsoder~a~6ih katalizatorah, Himia, Moskva, 1981. UOP Procedure, Inter, Publ. E Delannay, Characterization of Heterogeneous Catalysts, Marcel Dekker, New York, 1984. V.J. Gavrilov, V.B. Fenelonov, L.N. Rackovskaja, I.I. Frolova and I.A. Ovsjanikova, Kinet. Katal.. XXVII(3) (1986) 685. V.J. Gavrilov, V.B. Fenelonov and L.N. Rackovskaja, Kinet. Katal., XXIV(5) (1983) 1149. R.L. Mieville, J. Catal., 100 (1986) 482. J.M. Parera, N.S. Figoli and E.M. Traffano, J. Catal., 79 (19831 481. J. Barbier, E Marecot, N. Martin, L. Elassal and R. Maurel, Stud. Surf. Sci. Catal., 6 (1980) 53. R.A. Cabrol and A. Oberlin, J. Catal., 89 (1984) 256. M.R. Jovanovi6, Ph.D. Thesis, Univ. of Novi Sad, 1996. J. Barbier and P. Marecot, J. Catal., 102 (1986) 21.