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Various forms of the carbonaceous deposit on the model cobalt catalyst studied in hydrogenation of ethylene
Studies in Surface Science and Catalysis, Vol. 139 J.J. Spivey, G.W. Roberts and B.H. Davis (Editors) 9 2001 Elsevier Science B.V. All rights reserved.
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Various forms of the carbonaceous deposit on the model cobalt catalyst studied in hydrogenation of ethylene Joanna Lojewska Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland Carbonaceous deposits, formed during the hydrogenation of ethylene (150-450~ over the cobalt foil model catalyst, were investigated by TPO, TPR and SEM methods. The catalytic tests performed under the transient conditions provided the information on the mechanism of ethylene hydrogenation. The quantitative results of the deposit oxidation were related to the number of metallic active centers measured by TPO and TPR. The reaction temperature was found to exert the most profound impact on both the deposit forms and the deposit functions in the catalytic reaction. Below 300~ only a fraction of the deposit was found to block the metallic active centers and the other part appeared non-reactive in the hydrogenation. Above 400~ the regeneration of the catalyst activity was associated with the diffusion of cobalt atoms to a deposit surface. The results were confronted with the model of deactivation postulated before for CO2 hydrogenation [ 1]. 1. O B J E C T I V E S The aim of this study was to develop a new experimental basis for testing the model of deactivation caused by the inactive carbonaceous deposit. Deactivation of the model cobalt foil catalyst was studied in the hydrogenation of ethylene. The focus was on the conditions of ethylene hydrogenation under which the deposits might occur as well as on the determination of their forms and functions in the reaction network. To do this temperature-programmed reactions (TPR and TPO) and microscopic methods (SEM) were applied in conjunction with catalytic tests and stop-flow transient kinetic experiments. In a previous paper, the model of deactivation via carbonaceous deposits has been proposed and tested for the hydrogenation of CO2 occurring on the same catalyst [ 1]. To expand the model either the catalyst or the reaction might be changed. Cobalt foil has been a promising material for further investigation since we have worked out and successfully verified the method of its activation via reduction/oxidation cycles, which allows to obtain a reproducible catalyst of a structure described in [2,3]. Thus, the hydrogenation of ethylene has been chosen as a new test reaction for further modeling of deactivation especially because af~er almost the century there is no agreement on the role the carbonaceous deposits play in it. The advantage of using this reaction is the absence of the surface oxygen amongst the reaction intermediates, which much simplifies the reaction network to be taken into account in considering the deactivation mechanisms. The model of deactivation describes the transformations of two boundary forms of the carbonaceous deposits during the catalytic hydrogenation of CO2. These are: the hydrocarbon-formed active deposit (CH)n and the graphitic inactive one (C)n. Thus deactivation is based on dehydrogenation of the active deposit into the inactive one that blocks active centers for hydrogenation. The active deposit, a product of polymerization of surface methane precursors (CH,), is simultaneously their consumer and producer. The mass balance of the active intermediates derived from the model assumptions gave the kinetic equation which quantitatively describes the deactivation. 2. E X P E R I M E N T A L
2.1. Materials The cobalt foil (Aldrich) of 99.99+% purity, 0.1 mm thick, catalyst preparation. Typical samples were rectangle pieces (ca. mg. Their activation procedure consisted of three steps: regeneration. The preactivation involved about 10 subsequent
was used as an initial material for the 6x5 mm) of a mass between 15 and 44 preactivation, activation proper and oxidation and reduction cycles which
14
were carried out under the following conditions: reduction in 5% (v/v) 1-12in Ar (Linde) at a flow rate of 8.5 cm3/min for 30 min, at 550~ oxidation in 5% (v/v) O2 in He (Linde) at a flow rate of 8.5 cm3/min for 30 min at the same temperature. The activation, used before each catalytic test, involved reducing of a preactivated oxidized sample in pure H2 (HG Packard) at a temperature of the catalytic test. The sample was regenerated by oxidation in 5% 02 at 550~ each time after a catalytic test. The regeneration procedure restored the initial activity of the catalyst. Before SEM analyses, the reduced samples were passivated in the gas mixture containing 5% 02 in He at a flow rate 8.5 cm 3/min for 15 min at 60~ An active sample of the catalyst, having undergone the whole activation procedure, exhibits a sandwich-like structure built of four layers: the original foil, the dispersed CoO layer, the dispersed Co layer, and finally, on top, the residual oxygen layer including O2, OH" and 1-120 molecules. The dimension of the Co layer, in which Co active centers are localized, increases with the temperature of activation proper [2, 3].
2.2.
Procedures and apparatus
TPO, TPR studies and catalytic tests were carried out in a microreactor unit where two methods of products' detection were used: by Shimadzu 14A GC equipped with FID and TCD detectors and a packed HayeSep DB column, and by VG SX 200 QMS. This system operates in a continuous flow of reactants at atmospheric pressure. The detailed description of the apparatus the reader can find in [3]. TPR/O were performed at a heating rate of 10~ in the temperature range usually from 20 to 550~ The catalytic tests were performed using a reaction mixture ofHz (HG Packard) and C2I-h (2:140:1) (Aldrich 99.9)at temperatures from the range 150-450~ Usually, ethylene hydrogenation is studied at higher temperatures than those used here, however, the upper temperature limit is a sort of a compromise so as to avoid sintering of the catalyst which would interfere with the deposit formation. Sintering is reported as a dominating deactivation mode at above 450~ [4]. Unless otherwise stated, a typical reaction mixture contained H2:C2I-h = 6:1 at a volumetric flow rate 25 ml/min. The catalytic reaction was initiated by switching on the flow of C2I-h to the reactor immediately after the activation of the samples in pure H2 was completed. The adsorption experiments were carried out using a reaction mixture 1% C2I-I4 in He (Aldrich) flowing over the samples of the activated catalyst. SEM images of the surface of the samples were taken ex situ in a scanning electron microscope (Philips XL20) equipped with X-ray probe for chemical analyses. 3. RESULTS
6
--a-Z25~ "-~-250~ -'~ 275~ --,-- 350~
=~_ = , % ~
E
2
50
100 t [rain]
150
200
Fig. 1. Temperature influence on ethylene hydrogenation H2:C2H4=6:1.
The results of the catalytic tests allowed us to distinguish the temperature ranges in which a radically different catalyst behavior was observed. To illustrate this, the time-on-stream profiles of deactivation recorded at various temperatures ranging from 225450~ are presented in Figure 1. The experiments were performed in the excess of hydrogen (6:1) to prevent retarding of the reaction by ethylene (see Fig. 2 A and B). The main gaseous product of the reaction was ethane with some amount of methane whose content increased with temperature from 0.5 to 10% and with traces of C3 and C4 hydrocarbons. Below 300~ deactivation was relatively fast and the reaction rate showed the reverse temperature dependence.
15
0,8 B
A 0,6
,....,
"E" 6 E
12:1 34:1
~0,4
2:1
He
2:1
50
100
150
t [min]
200
C2H6
34:1
'
~ 0,2 0,0
0
C3
x
6:1 6:1
overall
=
---o--- CH4
._
~-4 o E E '- 2
=
250
300
,
0
50
100
150
200
250
t [min]
Fig. 2. Reactants concentration influence on the overall rate of ethylene hydrogenation A. at 250~ 0
16
regeneration towards C3 hydrocarbons was also observed. What is worth pointing out is that no ethane was formed over the reaction time A similar effect of regeneration was noted during ethylene hydrogenation in the reaction mixture diluted 100 times. The surface of the deactivated catalyst was examined in situ by temperature-programmed reduction and oxidation. TPO method proved more sensitive for the deposit characterization than TPR and for the sake of brevity of this paper only the results of TPO are presented. The deposits of different reactivity and chemical composition, found on the catalyst at different reaction temperatures, are represented by the 44 mass line of CO2 (Fig. 4 A). The amount of other oxidation products was negligibly small. The general trend was a decrease of the deposit reactivity with the catalytic reaction temperature below 400~ This is exhibited by the shift of the TPO profiles towards higher temperatures. However, above 400~ the regeneration of the deposits activity was observed similar to that reported from the catalytic tests; the maximum of the deposit from 450~ emerges at temperature by 60~ lower than the maximum of the one from 400~ (curves d and c Fig. 4 A). It was also noted that the deposit reactivity increased with the increase of hydrogen content in the reaction mixture (curves d and e Fig. 4 A). There is a striking difference in shapes between TPO profiles of the deposits from 250~ (Fig. 4 B) and of those from higher temperatures (Fig. 4 A). The broad TPO maximum with a slow linear decay observed for the former is characteristic of a diffusion-inhibited process. The quantitative results of TPO experiments are presented in Table 1. The amount of oxygen consumed by the reduced layer of the active catalyst is a direct measure of the number of the metallic active centers created at a given temperature. Thus the deposit amount normalized to the corresponding number of active centers can be understood in terms of the number of C-monolayers on the catalyst surface. Several fractions of carbon deposits differing in content and amount were collected after the reaction at 250~ (Fig. 4 B). The time of flushing in He after the reaction was a criterion to distinguish between these fractions: the longer the eluting time, the more strongly the deposit is bonded to the surface. The fraction will be called stable if the catalyst has been flushed for 12 h at room temperature. The stable deposit occupied only 20% of available metallic active centers and was mainly composed of carbon (curve a in Fig. 4B and Tab. 1) as evidenced by the very small amount of water measured by TPO (<0.01 mmol/g cat). On the contrary, the hydrogen-rich hydrocarbon deposit, whose amount several times exceeded the number of active centers, was found on the working catalyst. A huge amount of water evolved during TPO (12 mmol/g cat.) can be regarded at least partly as originating from this deposit. An analogous set of TPO experiments were performed on the activated samples which were preadsorbed with ethylene for 2 h in the mixture 1 % C2H4 in He (Fig. 5). In the initial short period of adsorption monitored by QMS, ethylene decomposes into acetylene, ethane and methane. 50
1,6 ,..., 1,2 "E" E 0,8 "6 E
,~ =_ E
t
0
0,4
1
0,0
\
1
-0,4 0
200
E ~"
1 400
600
I
40
B
\C
\
30 20 10 0
~
'
-10
800
T [~
Fig. 4. A. TPO of the deposits obtained at: a - 250~ b- 350~ c - 400~ d-450~ e- 450~ 6% Hz:C2I-I4=6:1 in He. The catalyst flushed in He, 12 h, 25~
0
200
400
600
800
T ["C]
B. TPO of the deposits obtained at 250~ followed by flushing in He for: a - 12 h at 25~ b - 2 h at 25~ c - without flushing.
17
The number of adsorbed forms of ethylene 60 030 ascribed to the peaks in a TPO spectrum, 50 500 increases with the adsorption temperature from 2 (or 3) observed at 250~ to 5 observed at r .t: 350~ This implies the growing tendency IE ~ 300 E towards ethylene dissociative adsorption with Eo 2 0 200 ~E the reaction temperature. However, ethylene 10 1oo 8 adsorption is affected not only by temperature tO ~but also by a redox state of the catalyst "- 0 0 surface; ethylene molecules are much more -I0 -I00 strongly bonded to the surface of the oxidized 0 200 400 600 800 catalyst than of the active one, which is TI~ manifested by the temperature of the TPO maxima (compare curve a and b in Fig. 5). Fig. 5. TPO of the active catalyst preadsorbed in From the oxygen 32 line included in the Figure ethylene at: a - at 250~ b - at 250~ on the oxidized catalyst, c - at 350~ The catalyst flushed in He, 2 we might infer that during oxidation first are 25~ consumed oxygen atoms diffusing to the catalyst surface from the deeper oxide layer of the catalyst, and later oxygen from the gas feed. The catalyst surface exposed to the reaction mixture at various temperatures was studied by SEM (Fig. 6). At 250~ only the traces of carbon were found in the catalyst pores by the X-ray probe (picture A). The deposits grown at higher temperatures were similar in texture and all contained metallic cobalt crystallites on the surface. As an example, the deposit grown at 450~ is envisaged in picture B and C, respectively; cobalt crystallites can be spotted as white dots in picture B. The results of X-ray analyses taken at 100 points of the sample revealed that the higher amount of cobalt on the deposit produced at 450~
Table 1. Results of TPO of carbon deposits Reaction temperature Tmax Deposit amount Nc Active centers numberNdNmax [mmol C/g cat.] Nmax [mmol Co/g cat] loci loci Ethylene hydrogenation 250 200, 310 0.05 0.25 0.20 250 ~) 320 0.48 2 2502) 310 0.95 7 350 330 2.4 0.33 15 400 510 11 0.36 31 450 450 19 0.42 45 4503) 550 6.8 16 550 0.51 Ethylene adsorption 2504) 230, 330 0.03 0.25 0.12 3505) 200, 230, 0.41 0.33 1.2 280, 330 1) TPO started after 2-h flushing in He 2) TPO started immediately atier the reaction without flushing the catalyst 3) Reaction performed in the reaction mixture H2:C2H4=6:1 diluted in He to 6% 4.5)TPO started after 2-h flushing in He
18
Fig. 6. SEM of the surface of the cobalt catalyst after ethylene hydrogenation, H2:C2H4=6:1 for 2 h, at various temperatures: A. at 250~ (10000x), B. at 450~ deposit in the light of secondary electrons (5000x), C. the same in the light of back scattered electrons (5000x).
4. D I S C U S S I O N 4.1. Deposits at T<300~ As hinted above, two kinds of the carbonaceous deposits were found on the catalyst surface after ethylene hydrogenation in this temperature range: the stable and the unstable one. The questions arise whether they can cause deactivation of the catalyst and what is their role in ethylene hydrogenation. One of the suspects to deactivate the catalyst is the stable deposit. The quantitative results of TPO indicate that this deposit is composed mainly of C species and it resembles the adsorbed forms of ethylene (Fig. 5). This becomes evident when we take into account that, during adsorption at 250~ ethylene decomposes into acetylene, ethane and methane as measured by QMS. Therefore more firmly bonded hydrocarbon species can be expected on the surface of the deactivated catalyst rather than n or 6-bonded ethylene [6]. For the related Fe catalyst these have been recognized as ethyne (=HC-CH=), ethynyl (-C-CH =) [6]. These may also constitute the stable deposit on the cobalt catalyst. Alternatively, cobalt carbides can be considered as possible products of ethylene decomposition on the catalyst surface, although they are said to be active intermediates in hydrogenation of CO [7]. Cobalt carbides in low amounts were observed by AES on the deactivated surface of the same catalyst after CO2 hydrogenation [8,9]. Despite its chemical nature, the stable deposit can only partly explain deactivation of the catalyst. Let us note that at the moment the 20 %fraction of the metallic active centers is occupied by the stable carbon deposit, the overall reaction rate dropped by 70+8% of its initial value (Fig. 1 and Tab. 1). Since hydrogen adsorption proved a rate determining step, rate of ethylene hydrogenation can be expressed by a simple kinetic equation: r = k PH2[,]2, where [,] is an instantaneous surface concentration of the Co active centers. Suppose the deposit blocks 20 % of the active centers, the rate would drop only by 36 % and conversely, the amount of the deposit should have been about 45 % if it were the only reason for the observed deactivation. This simple recalculation, even including the experimental error, shows that there must be another deactivation pathway in this temperature range. According to the SEM, TPR, XPS and TG results published in [3] the deactivation can be assigned to the surface reconstruction of the catalyst, which, during the catalytic reaction in hydrogen-rich atmosphere, loses its dispersion and the number of active centers. To conclude briefly, it is either the deposit located in the catalyst pores or sintering that can cause the catalyst deactivation and account for the diffusion inhibited oxidation observed at elevated temperatures during TPO (Fig. 4 B). The role of the hydrocarbon deposit found on the working catalyst appears more a reaction bystander than a reactive intermediate. In the light of the results this deposit is a weakly bonded hydrocarbon whose amount several times exceeds the number of active centers on the catalyst surface (Tab. 1). Creation of such a 3-dimentional structure requires ethylene molecules gradually adsorbing
19
on the already formed deposit, which is generally accepted in the literature [ 10]. It has been shown that ethylene competes with hydrogen for the metallic centers in this way reversibly retarding the reaction to the extent depending on ethylene partial pressure in the reaction mixture (Fig. 2 A). Thus, at a certain pressure, when a quasi-steady state of the reaction is achieved, the hydrocarbon deposit appears to occupy only a part of the metallic active centers leaving the other part empty for ethylene hydrogenation. A support for this interpretation can be found in the work of Jacobs et al. [11] who using SF IR spectroscopy proved that some forms of adsorbed ethylene (ethylidine and di-o ethylene), are non-reactive in ethylene hydrogenation over Pt catalysts at room temperature, although they occupy a majority of the metallic surface. Additionally, ethylene hydrogenation was shown to equally proceed without a deposit overlayer on metallic surfaces fully covered with H atoms [ 12]. A fraction of this bystander deposit presumably undergoes dehydrogenation into the stable carbon deposit that blocks active centers. 4.2. Deposits at T>300~ On the contrary, a huge amount of the stable deposit was found on the catalyst surface at temperatures above 300~ Apparently temperature facilitates both the polycondensation and dehydrogenation of hydrocarbon species [6]. The temperature of 300~ appears boundary for carbon graphitization as shown by the XPS results of Lahtinen et al. [13] and Nakamura et al. [14]. This most obviously elucidates the break-down of the reaction rate observed at 350~ (Fig. 1). However, as shown above, the deposits obtained in the narrow temperature range between 300 and 400~ differ in physical and chemical properties from those obtained above 400~ An interesting issue is why the higher-temperature deposit is more reactive both in the catalytic reaction and in the oxidation (Fig. 1 and Fig. 4 A). There are several facts which have to be taken into account discussing this. Firstly, all collected deposits included metallic cobalt crystallites on their surface as observed by SEM (Fig. 6), but when perceived with an unarmed eye the lower-temperature deposit appears much thicker than the higher-temperature one. The deposits most probably have a filamentous structure with metallic cobalt clusters built in on top of hydrocarbon fibers. Such filamentous deposits have been reported mainly for nickel reforming catalyst and also for Fe-Ni and Co-Cu catalyst [15,16]. It must also be pointed out that 450~ is the temperature of phase transition between cubic fee and hexagonal hop cobalt [ 17]. As this transition is based on the distortion of every third atomic layer, this must result in the increased mobility of cobalt atoms at this temperature region. Metal particles migrating inside the deposit fibers were observed by TEM for Fe-Ni catalyst [ 15]. As a consequence, the increased activity of the catalyst at above 400~ can be accounted for by the increased ability of metallic atoms to migrate throughout the deposit to its surface in order to form metallic centers for hydrogenation. Formation of the new metallic centers does not, however, explain the change in the mechanism of ethylene hydrogenation observed at higher temperatures (Fig. 2 B). Moreover, as was mentioned above, they appear not to be the only active centers for this reaction in the higher temperature range [6,10]. The deficit in the metallic centers can force mobile hydrogen atoms to leave them immediately after adsorbing and dissociating, and to diffuse over the deposit until they meet an ethylene molecule to hydrogenate it into ethane. In this way, the deposit becomes some sort of hydrogen reservoir for the ethane formation. This conclusion is consistent with the change of the rate- determining step observed at 400~ (Fig. 2 B). The new hydrogenation route towards ethane occuring on the deposit sites can elucidate the initial increase in the reaction rate during a typical catalytic test above 400~ (Fig. 1). The opposite impact of ethylene pressure on the methane formation (Fig. 2 B) indicates that the methanation route proceeds through dissociative adsorption of ethylene on the metallic centers or, which is also possible, through hydrogenolysis of the hydrocarbon agglomerates. The observations made during ethylene hydrogenation on the deactivated catalyst (Fig. 3) led me to postulate methane precursors (CH2,) being the intermediates both for methanation and the deposit growth. A slow increase of the overall rate of ethylene hydrogenation with time-on-stream can be accounted for by the migration of cobalt atoms to the deposit surface to form active centers for hydrogen adsorption without which the reaction could not proceed.
20
4.3. Model expanding Figure 7 summarizes the conclusions made from the results of ethylene hydrogenation over the cobalt foil catalyst. The experimental basis prepared here for the new reaction system supports the model of deactivation by the inactive carbonaceous deposit [ 1] adding some new facts that allow its expansion. Comparing the model assumptions with these findings it has to be pointed out that the inactive carbon deposit is formed on the catalyst surface at temperatures as low as 250~ but due to its relatively low amount it could not be the only reason for the catalyst deactivation. However, with the temperature raise enhancing polycondensation of methane precursors, the inactive deposit amount grows to the extent that causes a complete hindrance of ethylene hydrogenation observed in a narrow temperature range between 300 and 400~ The hydrogenolysis of the active deposit might be responsible for the observed impact of hydrogen partial pressure on the methanation rate. In order to broaden the model, for the temperatures above 400~ a new type of the hydrocarbon active centers for ethylene adsorption has to be postulated along with the regeneration of the metallic centers by diffusion of the cobalt atoms along deposit fibers.
Fig. 7. The model of ethylene decompositionand hydrogenationover the cobalt foil catalyst
ACKNOWLEDGEMENTS This study was supported by the grant from KBN (PB0217/T09/97). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17.
J. Lojewska and R. Dziembaj, Studies in Surface Science and Catalysis, 126 (1999) 121-128. J. Lojewska and R. Dziembaj, J. Mol. Catal. A: Chemical 122 (1997) 1-11. J. t_~jewska, W. Makowski, T.Tyszewski and IL Dziembaj, accepted to Catal. Today. C.H. Bartholomew, Appl. Catal. 107 (1993) 1-32. E. Myazaki, J. Catal. 65 (1980) 84-94. G.C. Bond, Appl. Catal. A: General 149 (1997) 3-25. G. Weatherbee and H. Bartholomew, J. Catal. 77 (1982) 460. G. Fr6hlich, U. Kestel, J. Lojewska, T. Lojewski, G. Meyer, M. VoB, D. Borgmann, R. Dziembaj and G. Wedler, Appl. Catal. A: General 134 (1996) 1-19. B. Klingenberg, G. Fr6hlich, U. Grellner, U. Kestel, J. Lojewska, T. Lojewski, G. Meyer, M. VoB, D. Borgmann, R. Dziembaj and G. Wedler, Studies in Surface Science and Catalysis 101 (1996) 961-969 G.A. Somorjai, Introduction to surface chemistry and catalysis, Wiley-Interscience 1994. P.W. Jacobs, G.A. Somorjai, J. Mol. Catal. A: Chemical 131 (1998) 5-18. R.L. Augustine, D.R. Baum, K.G. High L.S. Shivos and S.T. O'Leary, J. Catal. 127 (1991) 675683. J. Lahtinen, T. Arnaku, G. Somorjai; J. Catal. 142 (1993) 206. J. Nakamura and I. Toyoshima, Surface Sci., 201 (1988) 185. C. Park, R.T. Baker, J. Catal. 179 (1998) 361-374. A. Chambers, R.T. Baker, J. Catal. 158 (1996) 356-360. B.W. Lee, R. Alsenz, A. Ignatiev and M. A. Van Hove, Physical Review B 17 (1978) 1510-1520.
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Various forms of the carbonaceous deposit on the model cobalt catalyst studied in hydrogenation of ethylene Joanna Lojewska Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland Carbonaceous deposits, formed during the hydrogenation of ethylene (150-450~ over the cobalt foil model catalyst, were investigated by TPO, TPR and SEM methods. The catalytic tests performed under the transient conditions provided the information on the mechanism of ethylene hydrogenation. The quantitative results of the deposit oxidation were related to the number of metallic active centers measured by TPO and TPR. The reaction temperature was found to exert the most profound impact on both the deposit forms and the deposit functions in the catalytic reaction. Below 300~ only a fraction of the deposit was found to block the metallic active centers and the other part appeared non-reactive in the hydrogenation. Above 400~ the regeneration of the catalyst activity was associated with the diffusion of cobalt atoms to a deposit surface. The results were confronted with the model of deactivation postulated before for CO2 hydrogenation [ 1]. 1. O B J E C T I V E S The aim of this study was to develop a new experimental basis for testing the model of deactivation caused by the inactive carbonaceous deposit. Deactivation of the model cobalt foil catalyst was studied in the hydrogenation of ethylene. The focus was on the conditions of ethylene hydrogenation under which the deposits might occur as well as on the determination of their forms and functions in the reaction network. To do this temperature-programmed reactions (TPR and TPO) and microscopic methods (SEM) were applied in conjunction with catalytic tests and stop-flow transient kinetic experiments. In a previous paper, the model of deactivation via carbonaceous deposits has been proposed and tested for the hydrogenation of CO2 occurring on the same catalyst [ 1]. To expand the model either the catalyst or the reaction might be changed. Cobalt foil has been a promising material for further investigation since we have worked out and successfully verified the method of its activation via reduction/oxidation cycles, which allows to obtain a reproducible catalyst of a structure described in [2,3]. Thus, the hydrogenation of ethylene has been chosen as a new test reaction for further modeling of deactivation especially because af~er almost the century there is no agreement on the role the carbonaceous deposits play in it. The advantage of using this reaction is the absence of the surface oxygen amongst the reaction intermediates, which much simplifies the reaction network to be taken into account in considering the deactivation mechanisms. The model of deactivation describes the transformations of two boundary forms of the carbonaceous deposits during the catalytic hydrogenation of CO2. These are: the hydrocarbon-formed active deposit (CH)n and the graphitic inactive one (C)n. Thus deactivation is based on dehydrogenation of the active deposit into the inactive one that blocks active centers for hydrogenation. The active deposit, a product of polymerization of surface methane precursors (CH,), is simultaneously their consumer and producer. The mass balance of the active intermediates derived from the model assumptions gave the kinetic equation which quantitatively describes the deactivation. 2. E X P E R I M E N T A L
2.1. Materials The cobalt foil (Aldrich) of 99.99+% purity, 0.1 mm thick, catalyst preparation. Typical samples were rectangle pieces (ca. mg. Their activation procedure consisted of three steps: regeneration. The preactivation involved about 10 subsequent
was used as an initial material for the 6x5 mm) of a mass between 15 and 44 preactivation, activation proper and oxidation and reduction cycles which
14
were carried out under the following conditions: reduction in 5% (v/v) 1-12in Ar (Linde) at a flow rate of 8.5 cm3/min for 30 min, at 550~ oxidation in 5% (v/v) O2 in He (Linde) at a flow rate of 8.5 cm3/min for 30 min at the same temperature. The activation, used before each catalytic test, involved reducing of a preactivated oxidized sample in pure H2 (HG Packard) at a temperature of the catalytic test. The sample was regenerated by oxidation in 5% 02 at 550~ each time after a catalytic test. The regeneration procedure restored the initial activity of the catalyst. Before SEM analyses, the reduced samples were passivated in the gas mixture containing 5% 02 in He at a flow rate 8.5 cm 3/min for 15 min at 60~ An active sample of the catalyst, having undergone the whole activation procedure, exhibits a sandwich-like structure built of four layers: the original foil, the dispersed CoO layer, the dispersed Co layer, and finally, on top, the residual oxygen layer including O2, OH" and 1-120 molecules. The dimension of the Co layer, in which Co active centers are localized, increases with the temperature of activation proper [2, 3].
2.2.
Procedures and apparatus
TPO, TPR studies and catalytic tests were carried out in a microreactor unit where two methods of products' detection were used: by Shimadzu 14A GC equipped with FID and TCD detectors and a packed HayeSep DB column, and by VG SX 200 QMS. This system operates in a continuous flow of reactants at atmospheric pressure. The detailed description of the apparatus the reader can find in [3]. TPR/O were performed at a heating rate of 10~ in the temperature range usually from 20 to 550~ The catalytic tests were performed using a reaction mixture ofHz (HG Packard) and C2I-h (2:140:1) (Aldrich 99.9)at temperatures from the range 150-450~ Usually, ethylene hydrogenation is studied at higher temperatures than those used here, however, the upper temperature limit is a sort of a compromise so as to avoid sintering of the catalyst which would interfere with the deposit formation. Sintering is reported as a dominating deactivation mode at above 450~ [4]. Unless otherwise stated, a typical reaction mixture contained H2:C2I-h = 6:1 at a volumetric flow rate 25 ml/min. The catalytic reaction was initiated by switching on the flow of C2I-h to the reactor immediately after the activation of the samples in pure H2 was completed. The adsorption experiments were carried out using a reaction mixture 1% C2I-I4 in He (Aldrich) flowing over the samples of the activated catalyst. SEM images of the surface of the samples were taken ex situ in a scanning electron microscope (Philips XL20) equipped with X-ray probe for chemical analyses. 3. RESULTS
6
--a-Z25~ "-~-250~ -'~ 275~ --,-- 350~
=~_ = , % ~
E
2
50
100 t [rain]
150
200
Fig. 1. Temperature influence on ethylene hydrogenation H2:C2H4=6:1.
The results of the catalytic tests allowed us to distinguish the temperature ranges in which a radically different catalyst behavior was observed. To illustrate this, the time-on-stream profiles of deactivation recorded at various temperatures ranging from 225450~ are presented in Figure 1. The experiments were performed in the excess of hydrogen (6:1) to prevent retarding of the reaction by ethylene (see Fig. 2 A and B). The main gaseous product of the reaction was ethane with some amount of methane whose content increased with temperature from 0.5 to 10% and with traces of C3 and C4 hydrocarbons. Below 300~ deactivation was relatively fast and the reaction rate showed the reverse temperature dependence.
15
0,8 B
A 0,6
,....,
"E" 6 E
12:1 34:1
~0,4
2:1
He
2:1
50
100
150
t [min]
200
C2H6
34:1
'
~ 0,2 0,0
0
C3
x
6:1 6:1
overall
=
---o--- CH4
._
~-4 o E E '- 2
=
250
300
,
0
50
100
150
200
250
t [min]
Fig. 2. Reactants concentration influence on the overall rate of ethylene hydrogenation A. at 250~ 0
16
regeneration towards C3 hydrocarbons was also observed. What is worth pointing out is that no ethane was formed over the reaction time A similar effect of regeneration was noted during ethylene hydrogenation in the reaction mixture diluted 100 times. The surface of the deactivated catalyst was examined in situ by temperature-programmed reduction and oxidation. TPO method proved more sensitive for the deposit characterization than TPR and for the sake of brevity of this paper only the results of TPO are presented. The deposits of different reactivity and chemical composition, found on the catalyst at different reaction temperatures, are represented by the 44 mass line of CO2 (Fig. 4 A). The amount of other oxidation products was negligibly small. The general trend was a decrease of the deposit reactivity with the catalytic reaction temperature below 400~ This is exhibited by the shift of the TPO profiles towards higher temperatures. However, above 400~ the regeneration of the deposits activity was observed similar to that reported from the catalytic tests; the maximum of the deposit from 450~ emerges at temperature by 60~ lower than the maximum of the one from 400~ (curves d and c Fig. 4 A). It was also noted that the deposit reactivity increased with the increase of hydrogen content in the reaction mixture (curves d and e Fig. 4 A). There is a striking difference in shapes between TPO profiles of the deposits from 250~ (Fig. 4 B) and of those from higher temperatures (Fig. 4 A). The broad TPO maximum with a slow linear decay observed for the former is characteristic of a diffusion-inhibited process. The quantitative results of TPO experiments are presented in Table 1. The amount of oxygen consumed by the reduced layer of the active catalyst is a direct measure of the number of the metallic active centers created at a given temperature. Thus the deposit amount normalized to the corresponding number of active centers can be understood in terms of the number of C-monolayers on the catalyst surface. Several fractions of carbon deposits differing in content and amount were collected after the reaction at 250~ (Fig. 4 B). The time of flushing in He after the reaction was a criterion to distinguish between these fractions: the longer the eluting time, the more strongly the deposit is bonded to the surface. The fraction will be called stable if the catalyst has been flushed for 12 h at room temperature. The stable deposit occupied only 20% of available metallic active centers and was mainly composed of carbon (curve a in Fig. 4B and Tab. 1) as evidenced by the very small amount of water measured by TPO (<0.01 mmol/g cat). On the contrary, the hydrogen-rich hydrocarbon deposit, whose amount several times exceeded the number of active centers, was found on the working catalyst. A huge amount of water evolved during TPO (12 mmol/g cat.) can be regarded at least partly as originating from this deposit. An analogous set of TPO experiments were performed on the activated samples which were preadsorbed with ethylene for 2 h in the mixture 1 % C2H4 in He (Fig. 5). In the initial short period of adsorption monitored by QMS, ethylene decomposes into acetylene, ethane and methane. 50
1,6 ,..., 1,2 "E" E 0,8 "6 E
,~ =_ E
t
0
0,4
1
0,0
\
1
-0,4 0
200
E ~"
1 400
600
I
40
B
\C
\
30 20 10 0
~
'
-10
800
T [~
Fig. 4. A. TPO of the deposits obtained at: a - 250~ b- 350~ c - 400~ d-450~ e- 450~ 6% Hz:C2I-I4=6:1 in He. The catalyst flushed in He, 12 h, 25~
0
200
400
600
800
T ["C]
B. TPO of the deposits obtained at 250~ followed by flushing in He for: a - 12 h at 25~ b - 2 h at 25~ c - without flushing.
17
The number of adsorbed forms of ethylene 60 030 ascribed to the peaks in a TPO spectrum, 50 500 increases with the adsorption temperature from 2 (or 3) observed at 250~ to 5 observed at r .t: 350~ This implies the growing tendency IE ~ 300 E towards ethylene dissociative adsorption with Eo 2 0 200 ~E the reaction temperature. However, ethylene 10 1oo 8 adsorption is affected not only by temperature tO ~but also by a redox state of the catalyst "- 0 0 surface; ethylene molecules are much more -I0 -I00 strongly bonded to the surface of the oxidized 0 200 400 600 800 catalyst than of the active one, which is TI~ manifested by the temperature of the TPO maxima (compare curve a and b in Fig. 5). Fig. 5. TPO of the active catalyst preadsorbed in From the oxygen 32 line included in the Figure ethylene at: a - at 250~ b - at 250~ on the oxidized catalyst, c - at 350~ The catalyst flushed in He, 2 we might infer that during oxidation first are 25~ consumed oxygen atoms diffusing to the catalyst surface from the deeper oxide layer of the catalyst, and later oxygen from the gas feed. The catalyst surface exposed to the reaction mixture at various temperatures was studied by SEM (Fig. 6). At 250~ only the traces of carbon were found in the catalyst pores by the X-ray probe (picture A). The deposits grown at higher temperatures were similar in texture and all contained metallic cobalt crystallites on the surface. As an example, the deposit grown at 450~ is envisaged in picture B and C, respectively; cobalt crystallites can be spotted as white dots in picture B. The results of X-ray analyses taken at 100 points of the sample revealed that the higher amount of cobalt on the deposit produced at 450~
Table 1. Results of TPO of carbon deposits Reaction temperature Tmax Deposit amount Nc Active centers numberNdNmax [mmol C/g cat.] Nmax [mmol Co/g cat] loci loci Ethylene hydrogenation 250 200, 310 0.05 0.25 0.20 250 ~) 320 0.48 2 2502) 310 0.95 7 350 330 2.4 0.33 15 400 510 11 0.36 31 450 450 19 0.42 45 4503) 550 6.8 16 550 0.51 Ethylene adsorption 2504) 230, 330 0.03 0.25 0.12 3505) 200, 230, 0.41 0.33 1.2 280, 330 1) TPO started after 2-h flushing in He 2) TPO started immediately atier the reaction without flushing the catalyst 3) Reaction performed in the reaction mixture H2:C2H4=6:1 diluted in He to 6% 4.5)TPO started after 2-h flushing in He
18
Fig. 6. SEM of the surface of the cobalt catalyst after ethylene hydrogenation, H2:C2H4=6:1 for 2 h, at various temperatures: A. at 250~ (10000x), B. at 450~ deposit in the light of secondary electrons (5000x), C. the same in the light of back scattered electrons (5000x).
4. D I S C U S S I O N 4.1. Deposits at T<300~ As hinted above, two kinds of the carbonaceous deposits were found on the catalyst surface after ethylene hydrogenation in this temperature range: the stable and the unstable one. The questions arise whether they can cause deactivation of the catalyst and what is their role in ethylene hydrogenation. One of the suspects to deactivate the catalyst is the stable deposit. The quantitative results of TPO indicate that this deposit is composed mainly of C species and it resembles the adsorbed forms of ethylene (Fig. 5). This becomes evident when we take into account that, during adsorption at 250~ ethylene decomposes into acetylene, ethane and methane as measured by QMS. Therefore more firmly bonded hydrocarbon species can be expected on the surface of the deactivated catalyst rather than n or 6-bonded ethylene [6]. For the related Fe catalyst these have been recognized as ethyne (=HC-CH=), ethynyl (-C-CH =) [6]. These may also constitute the stable deposit on the cobalt catalyst. Alternatively, cobalt carbides can be considered as possible products of ethylene decomposition on the catalyst surface, although they are said to be active intermediates in hydrogenation of CO [7]. Cobalt carbides in low amounts were observed by AES on the deactivated surface of the same catalyst after CO2 hydrogenation [8,9]. Despite its chemical nature, the stable deposit can only partly explain deactivation of the catalyst. Let us note that at the moment the 20 %fraction of the metallic active centers is occupied by the stable carbon deposit, the overall reaction rate dropped by 70+8% of its initial value (Fig. 1 and Tab. 1). Since hydrogen adsorption proved a rate determining step, rate of ethylene hydrogenation can be expressed by a simple kinetic equation: r = k PH2[,]2, where [,] is an instantaneous surface concentration of the Co active centers. Suppose the deposit blocks 20 % of the active centers, the rate would drop only by 36 % and conversely, the amount of the deposit should have been about 45 % if it were the only reason for the observed deactivation. This simple recalculation, even including the experimental error, shows that there must be another deactivation pathway in this temperature range. According to the SEM, TPR, XPS and TG results published in [3] the deactivation can be assigned to the surface reconstruction of the catalyst, which, during the catalytic reaction in hydrogen-rich atmosphere, loses its dispersion and the number of active centers. To conclude briefly, it is either the deposit located in the catalyst pores or sintering that can cause the catalyst deactivation and account for the diffusion inhibited oxidation observed at elevated temperatures during TPO (Fig. 4 B). The role of the hydrocarbon deposit found on the working catalyst appears more a reaction bystander than a reactive intermediate. In the light of the results this deposit is a weakly bonded hydrocarbon whose amount several times exceeds the number of active centers on the catalyst surface (Tab. 1). Creation of such a 3-dimentional structure requires ethylene molecules gradually adsorbing
19
on the already formed deposit, which is generally accepted in the literature [ 10]. It has been shown that ethylene competes with hydrogen for the metallic centers in this way reversibly retarding the reaction to the extent depending on ethylene partial pressure in the reaction mixture (Fig. 2 A). Thus, at a certain pressure, when a quasi-steady state of the reaction is achieved, the hydrocarbon deposit appears to occupy only a part of the metallic active centers leaving the other part empty for ethylene hydrogenation. A support for this interpretation can be found in the work of Jacobs et al. [11] who using SF IR spectroscopy proved that some forms of adsorbed ethylene (ethylidine and di-o ethylene), are non-reactive in ethylene hydrogenation over Pt catalysts at room temperature, although they occupy a majority of the metallic surface. Additionally, ethylene hydrogenation was shown to equally proceed without a deposit overlayer on metallic surfaces fully covered with H atoms [ 12]. A fraction of this bystander deposit presumably undergoes dehydrogenation into the stable carbon deposit that blocks active centers. 4.2. Deposits at T>300~ On the contrary, a huge amount of the stable deposit was found on the catalyst surface at temperatures above 300~ Apparently temperature facilitates both the polycondensation and dehydrogenation of hydrocarbon species [6]. The temperature of 300~ appears boundary for carbon graphitization as shown by the XPS results of Lahtinen et al. [13] and Nakamura et al. [14]. This most obviously elucidates the break-down of the reaction rate observed at 350~ (Fig. 1). However, as shown above, the deposits obtained in the narrow temperature range between 300 and 400~ differ in physical and chemical properties from those obtained above 400~ An interesting issue is why the higher-temperature deposit is more reactive both in the catalytic reaction and in the oxidation (Fig. 1 and Fig. 4 A). There are several facts which have to be taken into account discussing this. Firstly, all collected deposits included metallic cobalt crystallites on their surface as observed by SEM (Fig. 6), but when perceived with an unarmed eye the lower-temperature deposit appears much thicker than the higher-temperature one. The deposits most probably have a filamentous structure with metallic cobalt clusters built in on top of hydrocarbon fibers. Such filamentous deposits have been reported mainly for nickel reforming catalyst and also for Fe-Ni and Co-Cu catalyst [15,16]. It must also be pointed out that 450~ is the temperature of phase transition between cubic fee and hexagonal hop cobalt [ 17]. As this transition is based on the distortion of every third atomic layer, this must result in the increased mobility of cobalt atoms at this temperature region. Metal particles migrating inside the deposit fibers were observed by TEM for Fe-Ni catalyst [ 15]. As a consequence, the increased activity of the catalyst at above 400~ can be accounted for by the increased ability of metallic atoms to migrate throughout the deposit to its surface in order to form metallic centers for hydrogenation. Formation of the new metallic centers does not, however, explain the change in the mechanism of ethylene hydrogenation observed at higher temperatures (Fig. 2 B). Moreover, as was mentioned above, they appear not to be the only active centers for this reaction in the higher temperature range [6,10]. The deficit in the metallic centers can force mobile hydrogen atoms to leave them immediately after adsorbing and dissociating, and to diffuse over the deposit until they meet an ethylene molecule to hydrogenate it into ethane. In this way, the deposit becomes some sort of hydrogen reservoir for the ethane formation. This conclusion is consistent with the change of the rate- determining step observed at 400~ (Fig. 2 B). The new hydrogenation route towards ethane occuring on the deposit sites can elucidate the initial increase in the reaction rate during a typical catalytic test above 400~ (Fig. 1). The opposite impact of ethylene pressure on the methane formation (Fig. 2 B) indicates that the methanation route proceeds through dissociative adsorption of ethylene on the metallic centers or, which is also possible, through hydrogenolysis of the hydrocarbon agglomerates. The observations made during ethylene hydrogenation on the deactivated catalyst (Fig. 3) led me to postulate methane precursors (CH2,) being the intermediates both for methanation and the deposit growth. A slow increase of the overall rate of ethylene hydrogenation with time-on-stream can be accounted for by the migration of cobalt atoms to the deposit surface to form active centers for hydrogen adsorption without which the reaction could not proceed.
20
4.3. Model expanding Figure 7 summarizes the conclusions made from the results of ethylene hydrogenation over the cobalt foil catalyst. The experimental basis prepared here for the new reaction system supports the model of deactivation by the inactive carbonaceous deposit [ 1] adding some new facts that allow its expansion. Comparing the model assumptions with these findings it has to be pointed out that the inactive carbon deposit is formed on the catalyst surface at temperatures as low as 250~ but due to its relatively low amount it could not be the only reason for the catalyst deactivation. However, with the temperature raise enhancing polycondensation of methane precursors, the inactive deposit amount grows to the extent that causes a complete hindrance of ethylene hydrogenation observed in a narrow temperature range between 300 and 400~ The hydrogenolysis of the active deposit might be responsible for the observed impact of hydrogen partial pressure on the methanation rate. In order to broaden the model, for the temperatures above 400~ a new type of the hydrocarbon active centers for ethylene adsorption has to be postulated along with the regeneration of the metallic centers by diffusion of the cobalt atoms along deposit fibers.
Fig. 7. The model of ethylene decompositionand hydrogenationover the cobalt foil catalyst
ACKNOWLEDGEMENTS This study was supported by the grant from KBN (PB0217/T09/97). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
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