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Characterization of the Surface of MoS2 Based Catalysts Using Thermal Methods
K.J. Smith, E.C. Sanford (Editors), Progress in Catalysis 0 1992 Elsevier Science Publishers B.V. All rights reserved.
3
Characterization of the Surface of MoS2 Based Catalysts Using Thermal Methods G. B. McGarvey and S. Kasztelan Institut Fransais du PCtrole, B. P. 311,92506 Rueil-Malmaison Cedex, France
Abstract Temperature programmed reduction and temperature pro rammed desorption have been used to investigate the surface of a model Mo!$/$O, hydrotreatment catalyst. X-ray photoelectron spectra and conventional transmission electron microscopy were also used to characterize the sulfided catalysts. Changing the sulfidation temperature was found to result in changes in the TPR profiles. The effects have been associated with the modification of the MoS, slabs on the surface of the support. The effect of reduction and inert gas pretreatment on the surface of the sulfided catalysts was found to be significant in terms of the quantity and nature of the hydrogen detected during TPD experiments. The experimental evidence demonstrates the presence of two distinctly different types of hydrogenic species which are located on the edges of the MoS, slabs. The high temperature species is postulated to interact with exposed Mo. INTRODUCTION The sheer number of reactions and processes that are effectively catalyzed by supported molybdenum disulfide, MoS,, (or the similar catalyst tungsten disulfide, WS,) and related promoted catalysts is a testament to the utility of these materials as heterogeneous catalysts. Alumina supported MoS, catalysts promoted with nickel or cobalt are the workhorses of virtually all of the currently utilized hydroprocessing technologies including hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and aromatic hydrogenation (HYD) (1). When modified with alkali promoters, MoS, based catalysts have also proven to be effective for the synthesis of linear alcohols from carbon monoxide and hydrogen (2). Identification of the active catalytic sites on hydroprocessing catalysts remains a topic of considerable interest. While the promoted catalysts exhibit superior catalytic activity for the hydrotreatment reactions mentioned above, unpromoted Mo!$/&O, is also an active catalyst for these reactions. The study of unpromoted model catalysts offers the opportunity to investigate the active phase without the complications that arise from the inclusion of a promoter species.
4
Molybdenum disulfide has a hexagonal, lamellar structure with a theoretically infinite basal plane that is reminiscent of graphite. Each layer consists of molybdenum atoms bonded to two sheets of sulfur atoms. As a result of the high Mo-S bond strengths in the basal plane, only weak van der Waal's interactions exist between adjacent layers which gives MoS, its lubricating properties (3) and chemical inertness except under severe conditions (4). In marked contrast to the chemical inertness of the basal plane, the edges of the platelets are believed to possess the active catalytic sites. Several studies have provided evidence for the reactivity of the edges of MoS, and the relative inertness of the basal plane. Oxygen chemisorption measurements were found to correlate well with the HDS of dibenzothiophene while the change in surface area followed no similar correlation (5). This was interpreted to result from changes in the concentration of active sites along the edges of the platelets since the lack of correlation with the total surface area suggests that the larger basal plane does not participate. Other spectroscopic investigations including optical absorption (6) and scanning Auger measurements (7) have provided additional evidence for the activity of the edge planes of MoS,. Diene hydrogenation has been used as a model reaction to investigate the hydrogenation activity of alumina supported MoS, that has undergone reductive pretreatment at different temperatures (8-10). These studies demonstrated that the hydrogenation activity was a function of the S/Mo ratio which varied with the reduction temperature. The maximum in the activity was found for the catalyst reduced at 500 "C corresponding to a S/Mo ratio of 1.53. Despite the evidence demonstrating the presence of reactive hydrogen on the surface, the form or nature of the hydrogenic species has not been determined. The existence of sulfhydryl groups (S-H) on the surface of MoS, based catalysts has been demonstrated by infrared spectroscopy (1412). No experimental evidence has been obtained to confirm other hydrogenic species on the surface although theoretical calculations suggest that the formation of Mo-H bonds is another possibility (13). Attempts to control the quantity of hydrogen adsorbed on the catalyst surface by changing the reduction conditions are met with the complication of concomitant removal of sulfur from the edge planes (8). In order to gain a better undestanding of the nature of the hydrogenic species associated with the surface of Mo&/Al,O, catalysts, temperature programmed reduction and temperature programmed desorption techniques have been employed to investigate the modification of the surface as a function of the pretreatment conditions. TPR has been used in the study of the nature and amount of supported sulfide based catalysts, particularly those with promoter
species such as cobalt (14). X-ray photoelectron spectra and transmission electron micrographs have been recorded to further characterize the catalyst system. EXPERIMENTAL The oxidic precursor catalyst was prepared by impregnating the Y-alumina (RhGne-Poulenc) with a solution of ammonium heptamolybdate ((NH,&Mo,O,J followed by drying at 100 "C and calcination at 500 "C for 4 hours. The loading, measured by x-ray fluorescence, was 15.2 wt% MoQ (10.2 wt% Mo). All temperature programmed reduction (TPR) and temperature programmed desorption (TPD)experiments were carried out using a X-SORB semi-automatic solid catalyst characterization unit. All catalyst sulfidation and pretreatments were performed in sifu using a by-pass gas handling circuit which circumvented the in-line traps and detector. For each experiment, the catalyst was sulfided using an HJ&S (85/15) mixture (Air Liquide) at a pressure of 1 bar for 2 hours. Sulfidation commenced at room temperature with a subsequent ramp of the temperature to the desired temperature (350-600 "C) at 5 "C/min. The samples were cooled in the sulfiding mixture and purged in argon for 1 hour before switching the flow to the HJAr (5/95) reduction mixture. TPR experiments were carried out at a heating rate of 5 "C/min to a maximum temperature of 800 or 1000 "C. An in-line molecular sieve trap was used to remove &S from the gas stream and ensure the protection of the catharometric detector. Blank TPR experiments on the virgin and sulfided (400 "C) support indicated that reducible impurities were not present in sufficient quantities to contribute to the observed reduction characteristics of the supported catalysts. For the TPD experiments, the sulfidation step was always carried out at 400 "C followed by cooling to room temperature in the sulfiding mixture. Reductive pretreatments were carried out in the Q / A r mixture using a temperature ramp of 15 "C/min to the desired temperature for 1 hour. The sample was cooled to below the desired purge temperature, the gas flow switched to pure argon, and the temperature maintained for 1 hour. TPD experiments were carried out between 50 and 1000 "C using a heating rate of 30 "C/min. Blank experiments performed with samples of the reduced and sulfided support did not indicate that significant quantities of hydrogen were desorbed from the surface of the support. X-ray photoelectron spectra (XPS) were recorded on a Kratos XSAM 800 spectrometer using an Al KEsource (1486.6 eV). The samples were sulfided using the methods described previously and transferred to the spectrometer under an inert atmosphere. Binding energies were measured relative to the Al 2p peak of the %03
6
support (74.5 eV). The Mo 3$/, and 3d,/, peaks of the supported oxide at 236.1 and 233.2 eV shifted following sulfidation to 232.3 and 229.2 eV respectively, indicative of the change in oxidation state of molybdenum from + 6 to + 4. Deconvolution of the Mo 3d envelope using Gaussian peaks for the Mo and S 2s peak, indicated that the catalyst was sulfided to greater than 90%. Conventional transmission electron microscopy (CTEM) was used to investigate the morphology of a freshly sulfided catalyst. Following sulfidation, the catalyst was transferred to the transfer chamber under n-heptane to avoid contact of the surface with air. The CTEM micrographs showed distinct layered regions which correspond to the MoS, platelets on the surface of the support. A survey of several regions of the sample indicated that the slabs had an average edge length of 30-40 A. Considerable stacking of the slabs was also observed with stacking heights ranging from a single slab up to 5 or 6 slabs with an average height of approximately 3 layers. RESULTS AND DISCUSSION Temperature Programmed Reduction Since the effect of reductive treatment on the surface of MoS,/&O, catalysts has been shown to effect large changes in the isoprene hydrogenation activity, the reduction behaviour of the catalysts was further investigated using temperature programmed reduction. A series of TPR measurements were made for catalysts sulfided at 350, 400, 500 and 600 "C and for each of the sulfidation temperatures two reduction peaks were observed. Moulijn and co-workers also observed two reduction peaks for MoS.JAI,O, with a broad peak observed at approximately 850 - 900 "C (14). The low temperature peak may corresponds to the removal of weakly bonded edge sulfur resulting in the formation of a stoichiometric MoS, slab (8). The high temperature peak may be associated with the removal of more strongly bonded sulfur atoms from the surface of the catalyst, possibly including a certain quantity of those in the basal plane. As is shown in Figure 1, the low temperature peak is centred at 190 "C for each of the sulfiding temperatures but the width of the peak was found to increase with increased sulfidation temperature between 400 and 600 "C. The high temperature peak was also found to undergo a change as the sulfidation temperature was increased, but in this case the temperature of the peak maximum shifted to higher temperatures as the sulfiding temperature was changed between 400 and 600 "C. Interestingly, the differences in the peak width of the low temperature peak and the peak maximum of the high temperature peak between the samples sulfided at 350 and 400 "C are minimal which suggests that this represents a minimum temperature for effective sulfiding.
7
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0,
C
630
0.60
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-Ef al
.-0 5
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430
0.20
230
2
0.00 0
160T----7800
t 1030
30
60
90
120
Time (minutes)
.
P
$
5 e&
1401
5
p 120 Y
mB a
100
8&l "V
150 180
6
' 400
'
500 S k o o '
Sulfidation Temperature ("C)
%,
Figure 1. (left) TPR profiles for Mo / G O ,following sulfidation at different temperatures: a) 350 "C, b) 400 "C, c) 500 d) 600 "C. Figure 2. (right) Relationship between the width of the low temperature reduction peak, and the temperature maximum of the high temperature reduction peak as a function of the sulfidation temperature. For each of the sulfidation temperatures, it was observed that the TPR profile passed through a minimum following the low temperature peak. The temperature at which this behaviour was observed increased with increasing sulfidation temperature and as the width of the low temperature peak increased. Since the detection system is designed to measure the consumption of hydrogen, this phenomenon must be the result of hydrogen desorption from the MoS, surface. Several supplementary experiments confirmed that the behaviour was a feature of the surface rather than an artifact of the experimental conditions. The same basic features were observed for catalysts sulfided at 400 "C and reduced at programming rates of 2 and 10 "C/min. Similarly, when the TPR experiment was performed in a 10% HJAr mixture the same behaviour was observed. The effectiveness of the reductive treatment was investigated by recording a second TPR profile on a sample that had been previously reduced to 800 "C. The profile was featureless except for a small low temperature peak which is believed to be
8
the result of a small amount of surface reoxidation that occured during the time between TPR experiments (approx. 14 hr). This experiment demonstrated that the surface had been effectively reduced during the initial reduction to 800 "C such that all available edge sulfur (and presumeably some basal plane sulfur) was removed. As was mentioned above, increasing the sulfidation temperature resulted in the broadening of the low temperature reduction peak and the shifting of the high temperature peak. Figure 2 shows these two phenomena and demonstrates the parallel nature of the changes. At this preliminary stage, the source of the behaviour is unclear although several speculative interpretations can be proposed. The effect of the sulfidation temperature on the catalyst morphology could affect the distribution of M o 3 slab sizes and the stacking of the platelets. It is conceivable that changes of this type can influence the reduction characteristics of the edges since intuitively it is to be expected that the energy required for sulfur removal will be a function of the number of neighbouring sulfur atoms. In the case of the low temperature peak this appears to manifest itself as a broader distribution of energies to remove the more weakly bound sulfur. Using the slab size arguement, it can be proposed that changes to the particle size will affect the strength of the bonds between the molybdenum and the sulfur. Experiments are currently in progress to address this question. Temperature Proerammed Desorption Temperature programmed desorption experiments demonstrate the effect of reductive pretreatment on the characteristics of the catalyst surface. In order to investigate the effect of reductive pretreatment on the nature of the hydrogenic species on the surface, a series of TPD profiles were recorded for MoS,/Al,O, samples which had been reduced at temperatures in the range 100 to 800 "C. Figure 3 shows representative TPD profiles for MoS,/Al,O, reduced at 200, 400 and 600 "C. The differences between the profiles are quite dramatic and represent what appear to be distinctly different states of the catalyst surface. Over the range of temperatures studied, there is an evolution in the nature of the envelope of desorption peaks. For a reduction temperature of 100 "C, the lowest temperature peak at approximately 300 "C is the most intense but, as the reduction temperature is increased the peak envelope evolves such that the high temperature peak at approximately 520 "C becomes the more intense. These features are illustrated in Figure 3 with the differences between the three reduction treatments quite evident. In addition to the changes in the relative intensities of the two peaks, the high temperature peak undergoes a shift as the reduction temperature is increased. For samples reduced
9
at low temperatures (below 400 "C), the high temperature peak remains relatively stable, but as the reduction temperature is increased above 400 "C, the shift is quite significant (from 520 to 950 "C for reduction temperatures between 400 and 800 "C). Concurrent with the increase of intensity of the high temperature peak was the shift of the peak maximum to higher temperatures. From the evolution of the TPD profiles it appears as if there are two hydrogenic species which, depending on the precise structure of the surface, are bonded more or less strongly.
r 1050
I
.650 .450
a
5 E a f
c
a
- 250 0
10
20
30 40 50 Time (minutes)
60
70
Figure 3. TPD profiles for MoS,/AI 0, catalysts sulfided at 400 "C and subsequently pretreated in Ar/H, at: a) 200 "C,b) 400 "C,c) 600 "C. Studies of a series of catalysts which were reduced at 800 "C and subsequently purged in argon at different temperatures showed that the inert gas treatment had a strong influence on the hydrogen adsorbed on the surface. Following low temperature purges a shoulder on the low temperature side of the dominant desorption peak was observed which decreased in intensity as the purge temperature was raised above 400 "C. Unlike the series of catalysts that had undergone reductive treatments at different temperatures, the position of the dominant peak remained constant at approximately 950 "C regardless of the change of purge temperature. This indicates that a strongly bonded hydrogen species is formed on MoS, particles where all of the edge sulfur species have been previously removed.
10
The TPR and TPD studies have demonstrated the sensitivity of the MoS, surface to the pretreatment conditions and indicate that distinctly different surfaces are present. TPR profiles show that low temperature reduction (in fact it is probably more accurate to refer to these experiments as temperature programmed reaction with hydrogen since unlike reduction processes with metal catalysts, there is evidently significant structural change induced by the reaction with hydrogen) results in the removal of weakly bonded S-H groups from the edges of the slabs. Further reduction at higher temperatures results in the progressive removal of more strongly bonded edge sulfur in conjuction with the adsorption of hydrogen (as demonstrated by the higher quantity of desorbed hydrogen following higher temperature reductions). Reduction at 800 "C was found to yield a clean surface which was free of any remaining reducible species. While it is not yet possible to assign a structure or bonding location for the surface hydrogen species, the most probable arrangement is on the edge planes of the slabs at defect or surface sites formed during the reductive removal of sulfur rather than on the basal planes. The high temperature required to remove the second, more strongly bonded species suggests the possiblity of the formation of Mo-H bonds exposed following the removal of edge sulfur species. ACKNOWLEDGEMENTS Experimental assistance of H. Ajot and C. Russmann is gratefully acknowledged. REFERENCES
:I
3
7) 8) 9)
13 14
M. J. Girgis and B. C. Gates, Ind. Eng. Chem. Res., 30 (1991) 2021. P. Forzatti, E. Tronconi,and I. Pasquon, Cata1.-Rev. Sci. Eng., 33 (1991) 109. D. E. Pierce, R. P. Burns, H. M. Dauplaise and L.J. Mizerka, Tribol. Trans., 34 (1991) 205. J. R. Linee, D. J. Carre and P. D. Fleischauer, Langmuir, 2 (1986) 805. S. J. Tauster, T. A. Pecoraro and R. R. Chianelli, J. Catal., 63 (1980) 515. C. B. Roxlo, M. Daage, A. F. Ruppert and R. R. Chianelli, J. Catal., 100 (1986) 176. R. R. Chianelli, A. F. Ruppert, S.K. Behal, B. H. Kear, A. Wold and R. KershiIW, J. Catal., 92 (1985) 56. A. Wambeke, L. Jalowiecki, S. Kasztelan, J. Grimblot and J. P. Bonnelle, J. Catal., 109 (1988) 320. S. Kasztelan, A. Wambeke, L. Jalowiecki, J. Grimblot and J. P. Bonnelle, J. Catal., 124 (1990) 12. L. Jalowiecki, J. Grimblot and J. P. Bonnelle, J. Catal., 126 (1990) 101. P. Ratnasamy and J. J. Fripiat, J. Chem. SOC.,Farad. Trans. I, (1970) 2897. N. Topsoe, J. Catal., 64 (1980) 235. A. B. Anderson, Z. Al-Saigh and W. K. Hall, J. Phys. Chem., 92 (1988) 803. B. Scheffer, N.J.J. Dekker, P.J. Magnus and J.A. Moulijn, J. Catal., 121 (1990) 31.
3
Characterization of the Surface of MoS2 Based Catalysts Using Thermal Methods G. B. McGarvey and S. Kasztelan Institut Fransais du PCtrole, B. P. 311,92506 Rueil-Malmaison Cedex, France
Abstract Temperature programmed reduction and temperature pro rammed desorption have been used to investigate the surface of a model Mo!$/$O, hydrotreatment catalyst. X-ray photoelectron spectra and conventional transmission electron microscopy were also used to characterize the sulfided catalysts. Changing the sulfidation temperature was found to result in changes in the TPR profiles. The effects have been associated with the modification of the MoS, slabs on the surface of the support. The effect of reduction and inert gas pretreatment on the surface of the sulfided catalysts was found to be significant in terms of the quantity and nature of the hydrogen detected during TPD experiments. The experimental evidence demonstrates the presence of two distinctly different types of hydrogenic species which are located on the edges of the MoS, slabs. The high temperature species is postulated to interact with exposed Mo. INTRODUCTION The sheer number of reactions and processes that are effectively catalyzed by supported molybdenum disulfide, MoS,, (or the similar catalyst tungsten disulfide, WS,) and related promoted catalysts is a testament to the utility of these materials as heterogeneous catalysts. Alumina supported MoS, catalysts promoted with nickel or cobalt are the workhorses of virtually all of the currently utilized hydroprocessing technologies including hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and aromatic hydrogenation (HYD) (1). When modified with alkali promoters, MoS, based catalysts have also proven to be effective for the synthesis of linear alcohols from carbon monoxide and hydrogen (2). Identification of the active catalytic sites on hydroprocessing catalysts remains a topic of considerable interest. While the promoted catalysts exhibit superior catalytic activity for the hydrotreatment reactions mentioned above, unpromoted Mo!$/&O, is also an active catalyst for these reactions. The study of unpromoted model catalysts offers the opportunity to investigate the active phase without the complications that arise from the inclusion of a promoter species.
4
Molybdenum disulfide has a hexagonal, lamellar structure with a theoretically infinite basal plane that is reminiscent of graphite. Each layer consists of molybdenum atoms bonded to two sheets of sulfur atoms. As a result of the high Mo-S bond strengths in the basal plane, only weak van der Waal's interactions exist between adjacent layers which gives MoS, its lubricating properties (3) and chemical inertness except under severe conditions (4). In marked contrast to the chemical inertness of the basal plane, the edges of the platelets are believed to possess the active catalytic sites. Several studies have provided evidence for the reactivity of the edges of MoS, and the relative inertness of the basal plane. Oxygen chemisorption measurements were found to correlate well with the HDS of dibenzothiophene while the change in surface area followed no similar correlation (5). This was interpreted to result from changes in the concentration of active sites along the edges of the platelets since the lack of correlation with the total surface area suggests that the larger basal plane does not participate. Other spectroscopic investigations including optical absorption (6) and scanning Auger measurements (7) have provided additional evidence for the activity of the edge planes of MoS,. Diene hydrogenation has been used as a model reaction to investigate the hydrogenation activity of alumina supported MoS, that has undergone reductive pretreatment at different temperatures (8-10). These studies demonstrated that the hydrogenation activity was a function of the S/Mo ratio which varied with the reduction temperature. The maximum in the activity was found for the catalyst reduced at 500 "C corresponding to a S/Mo ratio of 1.53. Despite the evidence demonstrating the presence of reactive hydrogen on the surface, the form or nature of the hydrogenic species has not been determined. The existence of sulfhydryl groups (S-H) on the surface of MoS, based catalysts has been demonstrated by infrared spectroscopy (1412). No experimental evidence has been obtained to confirm other hydrogenic species on the surface although theoretical calculations suggest that the formation of Mo-H bonds is another possibility (13). Attempts to control the quantity of hydrogen adsorbed on the catalyst surface by changing the reduction conditions are met with the complication of concomitant removal of sulfur from the edge planes (8). In order to gain a better undestanding of the nature of the hydrogenic species associated with the surface of Mo&/Al,O, catalysts, temperature programmed reduction and temperature programmed desorption techniques have been employed to investigate the modification of the surface as a function of the pretreatment conditions. TPR has been used in the study of the nature and amount of supported sulfide based catalysts, particularly those with promoter
species such as cobalt (14). X-ray photoelectron spectra and transmission electron micrographs have been recorded to further characterize the catalyst system. EXPERIMENTAL The oxidic precursor catalyst was prepared by impregnating the Y-alumina (RhGne-Poulenc) with a solution of ammonium heptamolybdate ((NH,&Mo,O,J followed by drying at 100 "C and calcination at 500 "C for 4 hours. The loading, measured by x-ray fluorescence, was 15.2 wt% MoQ (10.2 wt% Mo). All temperature programmed reduction (TPR) and temperature programmed desorption (TPD)experiments were carried out using a X-SORB semi-automatic solid catalyst characterization unit. All catalyst sulfidation and pretreatments were performed in sifu using a by-pass gas handling circuit which circumvented the in-line traps and detector. For each experiment, the catalyst was sulfided using an HJ&S (85/15) mixture (Air Liquide) at a pressure of 1 bar for 2 hours. Sulfidation commenced at room temperature with a subsequent ramp of the temperature to the desired temperature (350-600 "C) at 5 "C/min. The samples were cooled in the sulfiding mixture and purged in argon for 1 hour before switching the flow to the HJAr (5/95) reduction mixture. TPR experiments were carried out at a heating rate of 5 "C/min to a maximum temperature of 800 or 1000 "C. An in-line molecular sieve trap was used to remove &S from the gas stream and ensure the protection of the catharometric detector. Blank TPR experiments on the virgin and sulfided (400 "C) support indicated that reducible impurities were not present in sufficient quantities to contribute to the observed reduction characteristics of the supported catalysts. For the TPD experiments, the sulfidation step was always carried out at 400 "C followed by cooling to room temperature in the sulfiding mixture. Reductive pretreatments were carried out in the Q / A r mixture using a temperature ramp of 15 "C/min to the desired temperature for 1 hour. The sample was cooled to below the desired purge temperature, the gas flow switched to pure argon, and the temperature maintained for 1 hour. TPD experiments were carried out between 50 and 1000 "C using a heating rate of 30 "C/min. Blank experiments performed with samples of the reduced and sulfided support did not indicate that significant quantities of hydrogen were desorbed from the surface of the support. X-ray photoelectron spectra (XPS) were recorded on a Kratos XSAM 800 spectrometer using an Al KEsource (1486.6 eV). The samples were sulfided using the methods described previously and transferred to the spectrometer under an inert atmosphere. Binding energies were measured relative to the Al 2p peak of the %03
6
support (74.5 eV). The Mo 3$/, and 3d,/, peaks of the supported oxide at 236.1 and 233.2 eV shifted following sulfidation to 232.3 and 229.2 eV respectively, indicative of the change in oxidation state of molybdenum from + 6 to + 4. Deconvolution of the Mo 3d envelope using Gaussian peaks for the Mo and S 2s peak, indicated that the catalyst was sulfided to greater than 90%. Conventional transmission electron microscopy (CTEM) was used to investigate the morphology of a freshly sulfided catalyst. Following sulfidation, the catalyst was transferred to the transfer chamber under n-heptane to avoid contact of the surface with air. The CTEM micrographs showed distinct layered regions which correspond to the MoS, platelets on the surface of the support. A survey of several regions of the sample indicated that the slabs had an average edge length of 30-40 A. Considerable stacking of the slabs was also observed with stacking heights ranging from a single slab up to 5 or 6 slabs with an average height of approximately 3 layers. RESULTS AND DISCUSSION Temperature Programmed Reduction Since the effect of reductive treatment on the surface of MoS,/&O, catalysts has been shown to effect large changes in the isoprene hydrogenation activity, the reduction behaviour of the catalysts was further investigated using temperature programmed reduction. A series of TPR measurements were made for catalysts sulfided at 350, 400, 500 and 600 "C and for each of the sulfidation temperatures two reduction peaks were observed. Moulijn and co-workers also observed two reduction peaks for MoS.JAI,O, with a broad peak observed at approximately 850 - 900 "C (14). The low temperature peak may corresponds to the removal of weakly bonded edge sulfur resulting in the formation of a stoichiometric MoS, slab (8). The high temperature peak may be associated with the removal of more strongly bonded sulfur atoms from the surface of the catalyst, possibly including a certain quantity of those in the basal plane. As is shown in Figure 1, the low temperature peak is centred at 190 "C for each of the sulfiding temperatures but the width of the peak was found to increase with increased sulfidation temperature between 400 and 600 "C. The high temperature peak was also found to undergo a change as the sulfidation temperature was increased, but in this case the temperature of the peak maximum shifted to higher temperatures as the sulfiding temperature was changed between 400 and 600 "C. Interestingly, the differences in the peak width of the low temperature peak and the peak maximum of the high temperature peak between the samples sulfided at 350 and 400 "C are minimal which suggests that this represents a minimum temperature for effective sulfiding.
7
* 2
I
1.oo
830
a 0.80
v
0,
C
630
0.60
2
-Ef al
.-0 5
3 0.40
430
0.20
230
2
0.00 0
160T----7800
t 1030
30
60
90
120
Time (minutes)
.
P
$
5 e&
1401
5
p 120 Y
mB a
100
8&l "V
150 180
6
' 400
'
500 S k o o '
Sulfidation Temperature ("C)
%,
Figure 1. (left) TPR profiles for Mo / G O ,following sulfidation at different temperatures: a) 350 "C, b) 400 "C, c) 500 d) 600 "C. Figure 2. (right) Relationship between the width of the low temperature reduction peak, and the temperature maximum of the high temperature reduction peak as a function of the sulfidation temperature. For each of the sulfidation temperatures, it was observed that the TPR profile passed through a minimum following the low temperature peak. The temperature at which this behaviour was observed increased with increasing sulfidation temperature and as the width of the low temperature peak increased. Since the detection system is designed to measure the consumption of hydrogen, this phenomenon must be the result of hydrogen desorption from the MoS, surface. Several supplementary experiments confirmed that the behaviour was a feature of the surface rather than an artifact of the experimental conditions. The same basic features were observed for catalysts sulfided at 400 "C and reduced at programming rates of 2 and 10 "C/min. Similarly, when the TPR experiment was performed in a 10% HJAr mixture the same behaviour was observed. The effectiveness of the reductive treatment was investigated by recording a second TPR profile on a sample that had been previously reduced to 800 "C. The profile was featureless except for a small low temperature peak which is believed to be
8
the result of a small amount of surface reoxidation that occured during the time between TPR experiments (approx. 14 hr). This experiment demonstrated that the surface had been effectively reduced during the initial reduction to 800 "C such that all available edge sulfur (and presumeably some basal plane sulfur) was removed. As was mentioned above, increasing the sulfidation temperature resulted in the broadening of the low temperature reduction peak and the shifting of the high temperature peak. Figure 2 shows these two phenomena and demonstrates the parallel nature of the changes. At this preliminary stage, the source of the behaviour is unclear although several speculative interpretations can be proposed. The effect of the sulfidation temperature on the catalyst morphology could affect the distribution of M o 3 slab sizes and the stacking of the platelets. It is conceivable that changes of this type can influence the reduction characteristics of the edges since intuitively it is to be expected that the energy required for sulfur removal will be a function of the number of neighbouring sulfur atoms. In the case of the low temperature peak this appears to manifest itself as a broader distribution of energies to remove the more weakly bound sulfur. Using the slab size arguement, it can be proposed that changes to the particle size will affect the strength of the bonds between the molybdenum and the sulfur. Experiments are currently in progress to address this question. Temperature Proerammed Desorption Temperature programmed desorption experiments demonstrate the effect of reductive pretreatment on the characteristics of the catalyst surface. In order to investigate the effect of reductive pretreatment on the nature of the hydrogenic species on the surface, a series of TPD profiles were recorded for MoS,/Al,O, samples which had been reduced at temperatures in the range 100 to 800 "C. Figure 3 shows representative TPD profiles for MoS,/Al,O, reduced at 200, 400 and 600 "C. The differences between the profiles are quite dramatic and represent what appear to be distinctly different states of the catalyst surface. Over the range of temperatures studied, there is an evolution in the nature of the envelope of desorption peaks. For a reduction temperature of 100 "C, the lowest temperature peak at approximately 300 "C is the most intense but, as the reduction temperature is increased the peak envelope evolves such that the high temperature peak at approximately 520 "C becomes the more intense. These features are illustrated in Figure 3 with the differences between the three reduction treatments quite evident. In addition to the changes in the relative intensities of the two peaks, the high temperature peak undergoes a shift as the reduction temperature is increased. For samples reduced
9
at low temperatures (below 400 "C), the high temperature peak remains relatively stable, but as the reduction temperature is increased above 400 "C, the shift is quite significant (from 520 to 950 "C for reduction temperatures between 400 and 800 "C). Concurrent with the increase of intensity of the high temperature peak was the shift of the peak maximum to higher temperatures. From the evolution of the TPD profiles it appears as if there are two hydrogenic species which, depending on the precise structure of the surface, are bonded more or less strongly.
r 1050
I
.650 .450
a
5 E a f
c
a
- 250 0
10
20
30 40 50 Time (minutes)
60
70
Figure 3. TPD profiles for MoS,/AI 0, catalysts sulfided at 400 "C and subsequently pretreated in Ar/H, at: a) 200 "C,b) 400 "C,c) 600 "C. Studies of a series of catalysts which were reduced at 800 "C and subsequently purged in argon at different temperatures showed that the inert gas treatment had a strong influence on the hydrogen adsorbed on the surface. Following low temperature purges a shoulder on the low temperature side of the dominant desorption peak was observed which decreased in intensity as the purge temperature was raised above 400 "C. Unlike the series of catalysts that had undergone reductive treatments at different temperatures, the position of the dominant peak remained constant at approximately 950 "C regardless of the change of purge temperature. This indicates that a strongly bonded hydrogen species is formed on MoS, particles where all of the edge sulfur species have been previously removed.
10
The TPR and TPD studies have demonstrated the sensitivity of the MoS, surface to the pretreatment conditions and indicate that distinctly different surfaces are present. TPR profiles show that low temperature reduction (in fact it is probably more accurate to refer to these experiments as temperature programmed reaction with hydrogen since unlike reduction processes with metal catalysts, there is evidently significant structural change induced by the reaction with hydrogen) results in the removal of weakly bonded S-H groups from the edges of the slabs. Further reduction at higher temperatures results in the progressive removal of more strongly bonded edge sulfur in conjuction with the adsorption of hydrogen (as demonstrated by the higher quantity of desorbed hydrogen following higher temperature reductions). Reduction at 800 "C was found to yield a clean surface which was free of any remaining reducible species. While it is not yet possible to assign a structure or bonding location for the surface hydrogen species, the most probable arrangement is on the edge planes of the slabs at defect or surface sites formed during the reductive removal of sulfur rather than on the basal planes. The high temperature required to remove the second, more strongly bonded species suggests the possiblity of the formation of Mo-H bonds exposed following the removal of edge sulfur species. ACKNOWLEDGEMENTS Experimental assistance of H. Ajot and C. Russmann is gratefully acknowledged. REFERENCES
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