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Reforming of methane with carbon dioxide to synthesis gas over supported Rh catalysts
catalysis today
ELSEVIER
Catalysis Today 21 ( 1994) 579-587
Reforming of methane with carbon dioxide to synthesis gas over supported Rh catalysts V.A. Tsipouriari, Institute of Chemrcal Engmeenng
A.M. Efstathiou, Z.L. Zhang, X.E. Verykios * and High Temperature
Chetnrcal Processes.
Unrvermy
of Patras. GR-26500,
Patras. Greece
Abstract Reforming of methane with carbon dioxide to synthesis gas (CO/H*) has been investigated over rhodium supported on SiO,, TiO,, y-Al,Oa, MgO, Ce02, and Y SZ ( ZrOz (8 mol% Y,03) ) catalysts in the temperature range of 650-750°C at 1 bar total pressure. A strong carrier effect on the initial specific activity, deactivation rate, and carbon accumulation was found to exist. A strong dependence of the specific activity of the methane reforming reaction on rhodium particle size was observed over certain catalysts. Tracing experiments (using “CH,) coupled with temperature-programmed oxidation (TPO) revealed that the carbon species accumulated on the surface of the Rh/AI,O, catalyst during reforming reaction at 750°C are primarily derived from the CO2 molecular route. The amount of carbon present on the working catalyst surface which is derived from the CH4 molecular route is found to be very small.
1. Introduction The reforming reaction of methane with carbon dioxide is a very attractive route for the conversion of two of the cheapest carbon-containing materials into useful chemical products, such as synthesis gas (CO/HZ) [ 1,2]. As already discussed in the literature, this reaction offers important advantages over steam reforming of methane, namely: (a) the formation of a suitable H*/CO ratio for use in FischerTropsch synthesis to liquid hydrocarbons, (b) utilization of COT, which is considered to be a greenhouse gas, and (c) better use in chemical energy transmission systems [ 3-5 1. The reforming reaction of methane with carbon dioxide was first studied by Fischer and Tropsch using nickel- and cobalt-based catalysts [ 61, This reaction was later commercialized as the Calcor process [ 71 for which supported rhodium * Corresponding
author.
0920-5861/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved .SSD10920-5861(94)00125-l
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is one of the most effective catalysts [ 41. Based on the present knowledge, carbon formation during CO,-reforming appears to be far less on noble metals than on nickel [ 81, whereas rhodium and ruthenium are far more active than nickel [ 93. On the other hand, platinum and palladium may be considered having activity comparable to that of nickel [ 9, lo] . Solymosi and his coworkers [ 10,l l] have recently investigated the reforming of methane with carbon dioxide in the temperature range of 4OO-500°C over Rh and Pd supported on TiO,, A1,03, SiOz and MgO. For Pd, TiOz appeared to be the best support, in terms of specific activity, among those investigated. The authors have also reported that only very little amount of carbon was formed during reaction under the experimental conditions examined ( PCH4= 0.25 bar, CH,/C02 = 1.O, T= 400-500°C). The present work reports results of the investigation of the effects of support ( TiOz, y-A1203, MgO, SiOz, CeO, and yttria-stabilized zirconia (YSZ) ) and metal crystallite size of supported Rh catalysts on their methane reforming activity in the temperature range of 650-750°C. Various in-situ transient techniques (using also 13CH4) have been applied in order to quantify the support effects on activity and deactivation rate of supported Rh catalysts. Parameters such as the alteration of metal dispersion with time of reaction, and the amount of carbon species formed under integral reaction conditions (high COz conversions) were particularly studied.
2. Experimental Kinetic studies under differential conditions, and studies under integral reactor conditions were conducted in a conventional flow apparatus consisting of a flow measuring and control system, a mixing chamber, a quartz fixed-bed reactor, and an on-line gas chromatograph (Shimadzu 14A). Flow rates of gas streams were monitored and controlled by thermal mass flow meters and control valves (MKS, 247C). Kinetic experiments were conducted under conditions where interphase and intraparticle diffusional resistances did not influence the observed rates ( W,,, = 7-10 mg in powder form, Q = 300 cm3/min). Metal (0.5 wt.-% Rh) dispersion of fresh catalysts was determined by selective chemisorption of H2 at room temperature, following standard procedures. In-situ hydrogen chemisorption followed by temperature-programmed desorption (TPD), before and after reaction, as well as temperature-programmed oxidation (TPO) and various transient experiments have been conducted in a specially designed flow system which has been described elsewhere [ 121. Tracing experiments for measuring the amount of carbon derived from the CH4 and COz molecular routes under reforming reaction conditions have been conducted using an isotopic mixture consisting of 13CH4 (20%) / CO2 (20%) /He. Analytical techniques for this type of experiments have been reported elsewhere [ 131. Reduction of the fresh catalyst samples was performed
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at 400°C under Hz flow ( 1 bar) for 2 h, before the catalyst was brought to reaction temperature under He flow. Chemical analysis of gaseous streams during transients was done by an on-line mass spectrometer (VG Quadrupoles, SXP Elite) equipped with a fast response inlet capillary and data acquisition systems. Calibration of the mass spectrometer detector has been previously described [ 121. All gases during TPD, TPO, and tracing experiments were used at the flow rate of 30 ml/min (ambient). The quartz reactors used in kinetic and transient studies have been described elsewhere [ 12,141.
3. Results The variation of the intrinsic reaction rate of the methane reforming reaction with time on stream over Rh dispersed on different carriers is illustrated in Fig. 1. In the estimation of TOF the dispersion of the fresh catalyst, as obtained by selective chemisorption of hydrogen at room temperature, following H2 reduction at 400°C for 2 h, was used (see Table 1) . A strong influence of the support on the specific activity of Rh as well as on the deactivation characteristics is observed. Methane in the following order: Rh/ reforming activity of Rh decreases YSZ > A120, > Ti02 > SiOz > MgO. It must be pointed out, however, that the results shown in Fig. 1 do not account for possible sintering of the catalysts under reaction conditions, or for possible influence of metal crystallite size on specific activity, issues which are discussed below. The deactivation characteristics of the various catalysts are quantified by the ratio of TOF after 500 min of reaction over TOF after 10 min of reaction, which is shown in Table 1. The values of this ratio are relatively high when Rh is dispersed on Al,O,, YSZ, and SiOz, indicating that these catalysts do not deactivate very rapidly, and rather low when Rh is dispersed on TiO*, CeOz and MgO. The deac-
60 50 a 0.6 Cl 0 3 0.4 e c
0 Fig. 1. Variation of TOF,, bar; ‘3&/C02 = 1.
200 400 Time (min)
0.0 600
with time on stream over the 0.5 wt.-% supported Rh catalysts. T= 650°C; PcHI= 0.2
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V.A. Tsipounari et al. /Catalysrs Today 21 (1994) 579-587
Table 1 Metal dispersion, deactivation characteristics, at 650°C over Rh-supported catalysts Catalyst
Rh/y-A&O, Rh/YSZ Rh/TiOZ Rh/SiOz Rhf CeO, Rh/MgO
Metal dispersion
and amount of carbon accumulated
a (%)
TOFWJ,,,/TOF,,
during CO,/C&/He
reaction
0, ( monolayers)h
m,n
r= 10 min
t=2h
100 45 75 100 48
0.7 0.7 0.3 0.7 0.4
0.64 0.003 0.008 _c _c
0.60 0.03 0.007 _c _c
50
0.3
0.03
0.03
a Following H, reduction at 400°C for 2 h. b Equivalent amount of carbon in monolayers ’ Not measured.
of surface Rh atoms (C/Rh,=
1)
tivation characteristics of selected catalysts were also investigated under integral reactor conditions (high CH4 conversion) in the temperature range 650-750°C for an extended period of time (50 h) . The rate of deactivation was found to be lower at higher conversions and to decrease with increasing reaction temperature. The variation of the average Rh crystallite size of Rh/A1203, TiOz, YSZ and MgO catalysts with time on stream under integral reactor operation (Xc,, - 40%) at 650°C is shown in Fig. 2. The average metal crystallite size was determined with the following methodology: After the catalyst was exposed to the reaction mixture for the specified period of time, the feed was switched to He for 3 min and then to O2 for 15 min in order to remove the carbon accumulated during reaction. The 10
0 WA!203 ). RhNsz I Rhmgo 0 IWTiO2
8
0
50
100
150
Time (mm) Fig. 2. Variation of average Rh crystallite catalysts.
size with time on stream, at 65O”C, of Rh/AI,O,,
TiOZ, YSZ and MgO
V.A. Tsipouriari et al. /Catalysis Today 21 (1994) 579-587
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0 Rh/Al2O3 A Rh/Si02
A
n
WiO2
n
Oo 1
1
2
T=650°C
3
4
5
,
d(nm) Fig. 3. Dependence of initial specific activity (TOF) of the C&/CO, reaction on the average Rh particle size of Rh/ y-Al,O,, SiOz and TiO, catalysts. T= 65O’C; PcHI= 0.2 bar; CHJCO, = 1.O.
catalyst was rapidly cooled to 3OO”C, it was exposed to H2 flow for 1 h and was then cooled to 30°C and maintained for 30 min under H2 flow. The feed was subsequently switched to He and H2 TPD was initiated (p= 35Wmin). The desorbing H, was detected by on-line mass spectrometry. The quantity of H2 desorbed was used to estimate the average size of the Rh particles, which is shown in Fig. 2. It is apparent that in the case of the Rh/A1203 catalyst, a substantial reduction of the dispersion of Rh (sintering) occurs under the stated reaction conditions. The degree of sintering of the Rh/TiOz, Rh/YSZ, and Rh/MgO catalysts is significantly smaller. Sintering of the Rh particles under reaction conditions accounts, at least partially, for the reduction of methane conversion with time on stream which was observed under the same conditions. A similar experiment as the one described above but without the 15min oxygen treatment of the catalyst was performed over the Rh/MgO which accumulates only very small amounts of carbon. It was found that the 15-min oxygen treatment at 650°C had only a very small effect (less than 5%) on H2 chemisorption. The sensitivity of the intrinsic activity of Rh on particle size, or degree of dispersion, under conditions of methane reforming with CO*, was also investigated employing the Rh/ y-A1203, Rh/SiO, and Rh/TiO* catalysts. Different dispersions were achieved by variation of the metal loading between 0.2 and 10 wt.-%. Fig. 3 presents results, in the form of TOF versus average Rh particle size, obtained at 650°C. TOF values correspond to initial activity which was obtained by extrapolation of TOF versus time curves to zero time. It is apparent from Fig. 3 that structure sensitivity of this catalytic system is a strong function of the carrier employed for the dispersion of Rh. The Rh/Ti02 catalyst exhibits strong structure sensitivity, with TOF decreasing rapidly with increasing Rh particle size in the range of 1 to 4 nm, the Rh/A1203 catalyst a moderate one, while the Rh/Si02 catalyst exhibits a facile behaviour. These results illustrate the fact that the carrier
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0.15 T=750°C t=lO min s 2
0.10
E CJ
uo
0.05
0.00
I
3 _^^
IOU
400 5uo 200 300 Temperature (WY)
HeI--)0,/h
mo)
Fig. 4. Temperature-programmed oxidation (TPO) of carbon species formed after 10 min of reaction with CH, (20%) /CO, (20%) /He at 750°C over the 0.5 wt.-% Rh/AI,O1 catalyst. Q = 30 ml/min; p = 20Wmin; W,,, = 0.5 g.
plays an important role in defining intrinsic parameters of the specific catalytic system, probably participating in some mode in the chemistry of reaction steps and surface transformations. The amount of carbon species formed during the CH4/C02 reforming reaction in the temperature range of 65O”C-750°C and its reactivity towards oxygen were studied with the following methodology: Following reaction for a certain period of time, the reactor was purged with He for 10 min, followed by cooling in He flow to lOO”C, followed by temperature-programmed oxidation (TPO) in O,/He flow at a heating rate of 20Wmin. Fig. 4 shows the CO2 response obtained during TF’O over the Rh/y-Al,O, catalyst after reaction at 750°C for 10 min. At the conditions of the experiment, it is possible to distinguish two peaks with maxima at T= 110 and 320°C while a shoulder in the range of 340-500°C appears on the second COz peak (Fig. 4). The total amount of carbon deduced from the TRO experiment of Fig. 4 is 17.8 mmol/g,,,, which corresponds to an equivalent amount of monolayers, based on the Rh surface, 0, =0.35 (assuming C/Rh, = 1). Table 1 reports the amount of carbon accumulated over various supported Rh catalysts, after 10 min and 2 h of reaction time at T= 650°C. Except for the Rh/A1203 catalyst, the amount of carbon formed on the other catalysts is very small ( 0, < 0.03). It is interesting to observe that the amount of carbon accumulated on the catalysts is nearly independent of reaction time, while, in the case of Rh/Al,O, catalyst, it was observed that the amount of carbon accumulated after 10 min on stream decreases with increasing reaction temperature ( 0, = 0.6 at 650°C vs. 6, = 0.35 at 750°C). Fig. 5 presents an isotopic experiment for deconvoluting the COz response observed in Fig. 4 during the TPO experiment. The purpose of this experiment was to determine the source of carbon accumulation (CH, or COz molecules) during reforming reaction over the Rh/ y-Al,O, catalyst. The experimental sequence of catalyst gas treatments applied was similar to that described in Fig. 4. The ‘*CO2 response observed in Fig. 5 is due to the oxidation of carbon species formed during
V.A. Tsipouriari et al. /Catalysis Today 21 (1994) 579-587
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t=lO min 0.10 8
Temperature (Oc) He I+ 02/He (TPO) Fig. 5. Temperature-programmed oxidation (TPO) of carbon species formed after 10 min of reaction with ‘3CH,(20%)/C0,(20%)/He at 750°C over the 0.5 wt.-% Rh/Al,O, catalyst. Q=30 mUmin; p=20”C/min; w,,, = 0.5 g.
reforming reaction ( ‘3CH,/C02/He treatment) via the CO2 reaction route, while the 13C02 response via the CH4 reaction route. It is clearly demonstrated that most of the carbon species accumulated on the catalyst during reaction are derived from the CO2 molecule. The sum of the “CO2 and 13C02 responses shown in Fig. 5 is in good agreement with the CO2 response shown in Fig. 4.
4. Discussion The results presented in the previous section strongly indicate that the performance of Rh catalysts, under conditions of methane reforming with COZ, in terms of specific activity and deactivation characteristics, is a strong function of the carrier employed to disperse the metal. In certain cases, the reaction also appears to be structure sensitive, i.e., the specific activity of Rh is affected by the average size of the metal particles. Structure sensitivity is also affected by the carrier. Th&se findings strongly suggest that the carrier, directly or indirectly, participates in reaction steps and surface transformations. The observed structure sensitivity could also be explained as a carrier effect since the higher the dispersion the higher the metalcarrier gas interfacial area, where the interaction between the carrier and the metal would be strongest (the case of Rh/TiO* system). This idea is supported by the observation that the reaction is facile over Rh/Si02, since SiO, is one of the most inert carriers. With the exception of Rh/Al,O, catalyst, very small quantities of carbon accumulate on the catalyst surface when the reaction is carried out at 650°C (Table 1) . Thus, the deactivation which is observed with time of exposure to the reaction mixture should be attributed mostly to sintering of the metal particles and to a smaller extent to carbon deposition. Other processes could also contribute towards the deactivation of these catalysts, such as slow induction of the SMSI phenomenon
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et al. /Catalysis
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in the Rh/TiOz catalyst, accumulation of inactive adsorbed oxygen originating from the carrier in the Rh/YSZ catalyst [ 15 1, or poisoning of active sites by sulphur originating from the support in the case of Rh/MgO, as has been observed by Wang et al. [ 161 on a similar catalyst. In the case of Rh/A1203, sintering can account for the deactivation pattern, while the significant quantities of carbon deposited during reaction do not seem to influence significantly the reaction rate (accumulation of total carbon species on the catalyst surface is very rapid and does not change with time of exposure to reaction conditions, Table 1) . It is speculated that some of this carbon might be located on the Al,O, carrier. However, other hydrogenation experiments of carbon over the Rh/A1,03 catalyst indicated that some transformation of active carbon to a less active form occurs, a result which explains the role of carbon on catalyst deactivation [ 151. In spite of the fact that a significant amount of inactive carbon is formed during reaction on Rh/Al,O,, this catalyst exhibits very high TOF. This result could be understood by comparing the surface coverage of active carbon, which is in the sequence of steps to form CO, with that of the other catalysts shown in Table 1. Based on results reported elsewhere [ 151, it is found that the active carbon over Rh/YSZ, Rh/TiO,? and Rh/MgO is less than 0.02 of a monolayer, while that over Rh/Al,O, is one order of magnitude higher. Therefore, if this active carbon participates in one of the rate-limiting steps of the reaction sequence, then the high TOF observed over Rh/A1203 as compared to the Rh/Ti02 or Rh/MgO seems reasonable. On the other hand, the rate constant, k, of the rate-limiting step(s) plays an important role in determining the activity of the catalyst. This is illustrated by comparison of the TOF of Rh/A1203 and Rh/YSZ catalysts. Assuming that the rate-limiting step(s) of both systems are the same, and carbon species participate in these steps, then the comparable TOF values over the two systems can be explained to be due to larger values of k over Rh/YSZ than Rh/Al,O,. The same argument may also apply in comparing the large differences in TOF among Rh/ YSZ and Rh/MgO. It could also be stated that sulphur poisoning effects might be responsible for the very low TOF over the Rh/MgO catalyst. This could also explain the difference in reactivity order with respect to the carrier, between the present study and the work of Erdbhelyi et al. [ 111. The isotopic deconvolution TPO experiment shown in Fig. 5 suggests that reaction steps found in the sequence from CH4 to CO formation may be faster than reaction steps found in the sequence from CO2 to CO formation. This argument arises from the observations that much lower quantities of carbon (precursor species of CO formation) stay on the surface in the case of CH, than CO* reaction route, and appreciable amounts of active carbon, derived from the CO* molecule, participate in the formation of CO [ 15 1. The results shown in Figs. l-3 clearly demonstrate that structural and kinetic parameters of the present catalytic system are strongly dependent on the carrier employed for the dispersion of Rh. Specific catalytic activity, rate of sintering, and the apparent structural sensitivity of the reaction are found to depend strongly on
V.A. Tsipouriari et al. /Catalysis Today 21 (1994) 579-587
587
the carrier. The fact that the Rh/SiO,! catalyst exhibits facile behaviour might indicate that the structural sensitivity observed over Rh/A1203 and, primarily, over Rh/Ti02 (Fig. 3) might be a manifestation of a carrier effect which is more pronounced at higher dispersions.
References [I] D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Eds.), Methane Conversion, (Studies in Surface Science and Catalysis, Vol. 36). Elsevier, Amsterdam, 1988. [ 21 W.M. Ayers (Ed. ), Catalytic Activation of CO>, ACS Symp. Ser., American Chemical Society, Washington, DC, 1988. [3] S. Tenner, Hydrocarbon Process., 66 (1987) 52. [4] J.T. Richardson and S.A. Paripatyadar, Appl. Catal., 61 (1990) 293 [5] J. Stal, D. Hanson, J.-H. Bak Hansen and N Undengaard, Oil Gas J., 90 (1992) 62. [6] F. Fischer and H. Tropsch, Brennstoff Chem., 3 (9) ( 1928) 39. [7] S. Tenner, Hydrocarbon Process., 64 ( 1985) 106. [8] J.R. Rostrup-Nielsen, J. Catal., 33 ( 1974) 184. [9] J.R. Rostrup-Nielsen, J. Catal., 31 ( 1973) 173 [ 101 A. Erdohelyi, J. Cresenyi, E. Papp and F. Solymosi, Appl Catal. A, 108 ( 1994) 205. [ 1 l] A. Erddhelyi, J. Cresenyi and F. Solymosi, J. Catal., 141 ( 1993) 287. [ 121 A.M. Efstathiou, D. Papageorgiou and X.E. Verykios, J. Catal., 141 (1993) 612. [ 131 A.M. Efstathiou and C.O. Bennett, J. Catal., 120 (1989) 137. [ 141 A.M. Efstathiou, D. Boudouvas, N. Vamvouka and X. Vetykios, J. Catal., 140 ( 1993) 1. [ 151 A.M. Efstathiou, V.A. Tsipouriari, A. Kladi, and X.E. Verykios, J. Catal., submitted for publication. [ 161 Wang, J.A. Lercher, and CL. Haller, J. Catal., 88 (1984) 18.
ELSEVIER
Catalysis Today 21 ( 1994) 579-587
Reforming of methane with carbon dioxide to synthesis gas over supported Rh catalysts V.A. Tsipouriari, Institute of Chemrcal Engmeenng
A.M. Efstathiou, Z.L. Zhang, X.E. Verykios * and High Temperature
Chetnrcal Processes.
Unrvermy
of Patras. GR-26500,
Patras. Greece
Abstract Reforming of methane with carbon dioxide to synthesis gas (CO/H*) has been investigated over rhodium supported on SiO,, TiO,, y-Al,Oa, MgO, Ce02, and Y SZ ( ZrOz (8 mol% Y,03) ) catalysts in the temperature range of 650-750°C at 1 bar total pressure. A strong carrier effect on the initial specific activity, deactivation rate, and carbon accumulation was found to exist. A strong dependence of the specific activity of the methane reforming reaction on rhodium particle size was observed over certain catalysts. Tracing experiments (using “CH,) coupled with temperature-programmed oxidation (TPO) revealed that the carbon species accumulated on the surface of the Rh/AI,O, catalyst during reforming reaction at 750°C are primarily derived from the CO2 molecular route. The amount of carbon present on the working catalyst surface which is derived from the CH4 molecular route is found to be very small.
1. Introduction The reforming reaction of methane with carbon dioxide is a very attractive route for the conversion of two of the cheapest carbon-containing materials into useful chemical products, such as synthesis gas (CO/HZ) [ 1,2]. As already discussed in the literature, this reaction offers important advantages over steam reforming of methane, namely: (a) the formation of a suitable H*/CO ratio for use in FischerTropsch synthesis to liquid hydrocarbons, (b) utilization of COT, which is considered to be a greenhouse gas, and (c) better use in chemical energy transmission systems [ 3-5 1. The reforming reaction of methane with carbon dioxide was first studied by Fischer and Tropsch using nickel- and cobalt-based catalysts [ 61, This reaction was later commercialized as the Calcor process [ 71 for which supported rhodium * Corresponding
author.
0920-5861/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved .SSD10920-5861(94)00125-l
580
V.A. Tsipouriari et al. /Catalysis
Today 21(1994) 579-587
is one of the most effective catalysts [ 41. Based on the present knowledge, carbon formation during CO,-reforming appears to be far less on noble metals than on nickel [ 81, whereas rhodium and ruthenium are far more active than nickel [ 93. On the other hand, platinum and palladium may be considered having activity comparable to that of nickel [ 9, lo] . Solymosi and his coworkers [ 10,l l] have recently investigated the reforming of methane with carbon dioxide in the temperature range of 4OO-500°C over Rh and Pd supported on TiO,, A1,03, SiOz and MgO. For Pd, TiOz appeared to be the best support, in terms of specific activity, among those investigated. The authors have also reported that only very little amount of carbon was formed during reaction under the experimental conditions examined ( PCH4= 0.25 bar, CH,/C02 = 1.O, T= 400-500°C). The present work reports results of the investigation of the effects of support ( TiOz, y-A1203, MgO, SiOz, CeO, and yttria-stabilized zirconia (YSZ) ) and metal crystallite size of supported Rh catalysts on their methane reforming activity in the temperature range of 650-750°C. Various in-situ transient techniques (using also 13CH4) have been applied in order to quantify the support effects on activity and deactivation rate of supported Rh catalysts. Parameters such as the alteration of metal dispersion with time of reaction, and the amount of carbon species formed under integral reaction conditions (high COz conversions) were particularly studied.
2. Experimental Kinetic studies under differential conditions, and studies under integral reactor conditions were conducted in a conventional flow apparatus consisting of a flow measuring and control system, a mixing chamber, a quartz fixed-bed reactor, and an on-line gas chromatograph (Shimadzu 14A). Flow rates of gas streams were monitored and controlled by thermal mass flow meters and control valves (MKS, 247C). Kinetic experiments were conducted under conditions where interphase and intraparticle diffusional resistances did not influence the observed rates ( W,,, = 7-10 mg in powder form, Q = 300 cm3/min). Metal (0.5 wt.-% Rh) dispersion of fresh catalysts was determined by selective chemisorption of H2 at room temperature, following standard procedures. In-situ hydrogen chemisorption followed by temperature-programmed desorption (TPD), before and after reaction, as well as temperature-programmed oxidation (TPO) and various transient experiments have been conducted in a specially designed flow system which has been described elsewhere [ 121. Tracing experiments for measuring the amount of carbon derived from the CH4 and COz molecular routes under reforming reaction conditions have been conducted using an isotopic mixture consisting of 13CH4 (20%) / CO2 (20%) /He. Analytical techniques for this type of experiments have been reported elsewhere [ 131. Reduction of the fresh catalyst samples was performed
V.A. Tsipouriari et al. /Catalysis
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at 400°C under Hz flow ( 1 bar) for 2 h, before the catalyst was brought to reaction temperature under He flow. Chemical analysis of gaseous streams during transients was done by an on-line mass spectrometer (VG Quadrupoles, SXP Elite) equipped with a fast response inlet capillary and data acquisition systems. Calibration of the mass spectrometer detector has been previously described [ 121. All gases during TPD, TPO, and tracing experiments were used at the flow rate of 30 ml/min (ambient). The quartz reactors used in kinetic and transient studies have been described elsewhere [ 12,141.
3. Results The variation of the intrinsic reaction rate of the methane reforming reaction with time on stream over Rh dispersed on different carriers is illustrated in Fig. 1. In the estimation of TOF the dispersion of the fresh catalyst, as obtained by selective chemisorption of hydrogen at room temperature, following H2 reduction at 400°C for 2 h, was used (see Table 1) . A strong influence of the support on the specific activity of Rh as well as on the deactivation characteristics is observed. Methane in the following order: Rh/ reforming activity of Rh decreases YSZ > A120, > Ti02 > SiOz > MgO. It must be pointed out, however, that the results shown in Fig. 1 do not account for possible sintering of the catalysts under reaction conditions, or for possible influence of metal crystallite size on specific activity, issues which are discussed below. The deactivation characteristics of the various catalysts are quantified by the ratio of TOF after 500 min of reaction over TOF after 10 min of reaction, which is shown in Table 1. The values of this ratio are relatively high when Rh is dispersed on Al,O,, YSZ, and SiOz, indicating that these catalysts do not deactivate very rapidly, and rather low when Rh is dispersed on TiO*, CeOz and MgO. The deac-
60 50 a 0.6 Cl 0 3 0.4 e c
0 Fig. 1. Variation of TOF,, bar; ‘3&/C02 = 1.
200 400 Time (min)
0.0 600
with time on stream over the 0.5 wt.-% supported Rh catalysts. T= 650°C; PcHI= 0.2
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Table 1 Metal dispersion, deactivation characteristics, at 650°C over Rh-supported catalysts Catalyst
Rh/y-A&O, Rh/YSZ Rh/TiOZ Rh/SiOz Rhf CeO, Rh/MgO
Metal dispersion
and amount of carbon accumulated
a (%)
TOFWJ,,,/TOF,,
during CO,/C&/He
reaction
0, ( monolayers)h
m,n
r= 10 min
t=2h
100 45 75 100 48
0.7 0.7 0.3 0.7 0.4
0.64 0.003 0.008 _c _c
0.60 0.03 0.007 _c _c
50
0.3
0.03
0.03
a Following H, reduction at 400°C for 2 h. b Equivalent amount of carbon in monolayers ’ Not measured.
of surface Rh atoms (C/Rh,=
1)
tivation characteristics of selected catalysts were also investigated under integral reactor conditions (high CH4 conversion) in the temperature range 650-750°C for an extended period of time (50 h) . The rate of deactivation was found to be lower at higher conversions and to decrease with increasing reaction temperature. The variation of the average Rh crystallite size of Rh/A1203, TiOz, YSZ and MgO catalysts with time on stream under integral reactor operation (Xc,, - 40%) at 650°C is shown in Fig. 2. The average metal crystallite size was determined with the following methodology: After the catalyst was exposed to the reaction mixture for the specified period of time, the feed was switched to He for 3 min and then to O2 for 15 min in order to remove the carbon accumulated during reaction. The 10
0 WA!203 ). RhNsz I Rhmgo 0 IWTiO2
8
0
50
100
150
Time (mm) Fig. 2. Variation of average Rh crystallite catalysts.
size with time on stream, at 65O”C, of Rh/AI,O,,
TiOZ, YSZ and MgO
V.A. Tsipouriari et al. /Catalysis Today 21 (1994) 579-587
583
0 Rh/Al2O3 A Rh/Si02
A
n
WiO2
n
Oo 1
1
2
T=650°C
3
4
5
,
d(nm) Fig. 3. Dependence of initial specific activity (TOF) of the C&/CO, reaction on the average Rh particle size of Rh/ y-Al,O,, SiOz and TiO, catalysts. T= 65O’C; PcHI= 0.2 bar; CHJCO, = 1.O.
catalyst was rapidly cooled to 3OO”C, it was exposed to H2 flow for 1 h and was then cooled to 30°C and maintained for 30 min under H2 flow. The feed was subsequently switched to He and H2 TPD was initiated (p= 35Wmin). The desorbing H, was detected by on-line mass spectrometry. The quantity of H2 desorbed was used to estimate the average size of the Rh particles, which is shown in Fig. 2. It is apparent that in the case of the Rh/A1203 catalyst, a substantial reduction of the dispersion of Rh (sintering) occurs under the stated reaction conditions. The degree of sintering of the Rh/TiOz, Rh/YSZ, and Rh/MgO catalysts is significantly smaller. Sintering of the Rh particles under reaction conditions accounts, at least partially, for the reduction of methane conversion with time on stream which was observed under the same conditions. A similar experiment as the one described above but without the 15min oxygen treatment of the catalyst was performed over the Rh/MgO which accumulates only very small amounts of carbon. It was found that the 15-min oxygen treatment at 650°C had only a very small effect (less than 5%) on H2 chemisorption. The sensitivity of the intrinsic activity of Rh on particle size, or degree of dispersion, under conditions of methane reforming with CO*, was also investigated employing the Rh/ y-A1203, Rh/SiO, and Rh/TiO* catalysts. Different dispersions were achieved by variation of the metal loading between 0.2 and 10 wt.-%. Fig. 3 presents results, in the form of TOF versus average Rh particle size, obtained at 650°C. TOF values correspond to initial activity which was obtained by extrapolation of TOF versus time curves to zero time. It is apparent from Fig. 3 that structure sensitivity of this catalytic system is a strong function of the carrier employed for the dispersion of Rh. The Rh/Ti02 catalyst exhibits strong structure sensitivity, with TOF decreasing rapidly with increasing Rh particle size in the range of 1 to 4 nm, the Rh/A1203 catalyst a moderate one, while the Rh/Si02 catalyst exhibits a facile behaviour. These results illustrate the fact that the carrier
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0.15 T=750°C t=lO min s 2
0.10
E CJ
uo
0.05
0.00
I
3 _^^
IOU
400 5uo 200 300 Temperature (WY)
HeI--)0,/h
mo)
Fig. 4. Temperature-programmed oxidation (TPO) of carbon species formed after 10 min of reaction with CH, (20%) /CO, (20%) /He at 750°C over the 0.5 wt.-% Rh/AI,O1 catalyst. Q = 30 ml/min; p = 20Wmin; W,,, = 0.5 g.
plays an important role in defining intrinsic parameters of the specific catalytic system, probably participating in some mode in the chemistry of reaction steps and surface transformations. The amount of carbon species formed during the CH4/C02 reforming reaction in the temperature range of 65O”C-750°C and its reactivity towards oxygen were studied with the following methodology: Following reaction for a certain period of time, the reactor was purged with He for 10 min, followed by cooling in He flow to lOO”C, followed by temperature-programmed oxidation (TPO) in O,/He flow at a heating rate of 20Wmin. Fig. 4 shows the CO2 response obtained during TF’O over the Rh/y-Al,O, catalyst after reaction at 750°C for 10 min. At the conditions of the experiment, it is possible to distinguish two peaks with maxima at T= 110 and 320°C while a shoulder in the range of 340-500°C appears on the second COz peak (Fig. 4). The total amount of carbon deduced from the TRO experiment of Fig. 4 is 17.8 mmol/g,,,, which corresponds to an equivalent amount of monolayers, based on the Rh surface, 0, =0.35 (assuming C/Rh, = 1). Table 1 reports the amount of carbon accumulated over various supported Rh catalysts, after 10 min and 2 h of reaction time at T= 650°C. Except for the Rh/A1203 catalyst, the amount of carbon formed on the other catalysts is very small ( 0, < 0.03). It is interesting to observe that the amount of carbon accumulated on the catalysts is nearly independent of reaction time, while, in the case of Rh/Al,O, catalyst, it was observed that the amount of carbon accumulated after 10 min on stream decreases with increasing reaction temperature ( 0, = 0.6 at 650°C vs. 6, = 0.35 at 750°C). Fig. 5 presents an isotopic experiment for deconvoluting the COz response observed in Fig. 4 during the TPO experiment. The purpose of this experiment was to determine the source of carbon accumulation (CH, or COz molecules) during reforming reaction over the Rh/ y-Al,O, catalyst. The experimental sequence of catalyst gas treatments applied was similar to that described in Fig. 4. The ‘*CO2 response observed in Fig. 5 is due to the oxidation of carbon species formed during
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t=lO min 0.10 8
Temperature (Oc) He I+ 02/He (TPO) Fig. 5. Temperature-programmed oxidation (TPO) of carbon species formed after 10 min of reaction with ‘3CH,(20%)/C0,(20%)/He at 750°C over the 0.5 wt.-% Rh/Al,O, catalyst. Q=30 mUmin; p=20”C/min; w,,, = 0.5 g.
reforming reaction ( ‘3CH,/C02/He treatment) via the CO2 reaction route, while the 13C02 response via the CH4 reaction route. It is clearly demonstrated that most of the carbon species accumulated on the catalyst during reaction are derived from the CO2 molecule. The sum of the “CO2 and 13C02 responses shown in Fig. 5 is in good agreement with the CO2 response shown in Fig. 4.
4. Discussion The results presented in the previous section strongly indicate that the performance of Rh catalysts, under conditions of methane reforming with COZ, in terms of specific activity and deactivation characteristics, is a strong function of the carrier employed to disperse the metal. In certain cases, the reaction also appears to be structure sensitive, i.e., the specific activity of Rh is affected by the average size of the metal particles. Structure sensitivity is also affected by the carrier. Th&se findings strongly suggest that the carrier, directly or indirectly, participates in reaction steps and surface transformations. The observed structure sensitivity could also be explained as a carrier effect since the higher the dispersion the higher the metalcarrier gas interfacial area, where the interaction between the carrier and the metal would be strongest (the case of Rh/TiO* system). This idea is supported by the observation that the reaction is facile over Rh/Si02, since SiO, is one of the most inert carriers. With the exception of Rh/Al,O, catalyst, very small quantities of carbon accumulate on the catalyst surface when the reaction is carried out at 650°C (Table 1) . Thus, the deactivation which is observed with time of exposure to the reaction mixture should be attributed mostly to sintering of the metal particles and to a smaller extent to carbon deposition. Other processes could also contribute towards the deactivation of these catalysts, such as slow induction of the SMSI phenomenon
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in the Rh/TiOz catalyst, accumulation of inactive adsorbed oxygen originating from the carrier in the Rh/YSZ catalyst [ 15 1, or poisoning of active sites by sulphur originating from the support in the case of Rh/MgO, as has been observed by Wang et al. [ 161 on a similar catalyst. In the case of Rh/A1203, sintering can account for the deactivation pattern, while the significant quantities of carbon deposited during reaction do not seem to influence significantly the reaction rate (accumulation of total carbon species on the catalyst surface is very rapid and does not change with time of exposure to reaction conditions, Table 1) . It is speculated that some of this carbon might be located on the Al,O, carrier. However, other hydrogenation experiments of carbon over the Rh/A1,03 catalyst indicated that some transformation of active carbon to a less active form occurs, a result which explains the role of carbon on catalyst deactivation [ 151. In spite of the fact that a significant amount of inactive carbon is formed during reaction on Rh/Al,O,, this catalyst exhibits very high TOF. This result could be understood by comparing the surface coverage of active carbon, which is in the sequence of steps to form CO, with that of the other catalysts shown in Table 1. Based on results reported elsewhere [ 151, it is found that the active carbon over Rh/YSZ, Rh/TiO,? and Rh/MgO is less than 0.02 of a monolayer, while that over Rh/Al,O, is one order of magnitude higher. Therefore, if this active carbon participates in one of the rate-limiting steps of the reaction sequence, then the high TOF observed over Rh/A1203 as compared to the Rh/Ti02 or Rh/MgO seems reasonable. On the other hand, the rate constant, k, of the rate-limiting step(s) plays an important role in determining the activity of the catalyst. This is illustrated by comparison of the TOF of Rh/A1203 and Rh/YSZ catalysts. Assuming that the rate-limiting step(s) of both systems are the same, and carbon species participate in these steps, then the comparable TOF values over the two systems can be explained to be due to larger values of k over Rh/YSZ than Rh/Al,O,. The same argument may also apply in comparing the large differences in TOF among Rh/ YSZ and Rh/MgO. It could also be stated that sulphur poisoning effects might be responsible for the very low TOF over the Rh/MgO catalyst. This could also explain the difference in reactivity order with respect to the carrier, between the present study and the work of Erdbhelyi et al. [ 111. The isotopic deconvolution TPO experiment shown in Fig. 5 suggests that reaction steps found in the sequence from CH4 to CO formation may be faster than reaction steps found in the sequence from CO2 to CO formation. This argument arises from the observations that much lower quantities of carbon (precursor species of CO formation) stay on the surface in the case of CH, than CO* reaction route, and appreciable amounts of active carbon, derived from the CO* molecule, participate in the formation of CO [ 15 1. The results shown in Figs. l-3 clearly demonstrate that structural and kinetic parameters of the present catalytic system are strongly dependent on the carrier employed for the dispersion of Rh. Specific catalytic activity, rate of sintering, and the apparent structural sensitivity of the reaction are found to depend strongly on
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the carrier. The fact that the Rh/SiO,! catalyst exhibits facile behaviour might indicate that the structural sensitivity observed over Rh/A1203 and, primarily, over Rh/Ti02 (Fig. 3) might be a manifestation of a carrier effect which is more pronounced at higher dispersions.
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