Catal Lett (2010) 137:16–27 DOI 10.1007/s10562-010-0327-y
Partial Oxidation and CO2-ATR of Methane over Rh/LaMnO3 Honeycomb Catalysts P. S. Barbato • G. Landi
Published online: 2 April 2010 Ó Springer Science+Business Media, LLC 2010
Abstract Catalytic partial oxidation of methane has been studied over structured bi-functional catalysts containing LaMnO3 perovskite and Rh. The effects of the noble metal content and of substrate morphology and thermal conductivity have been investigated under high temperature pseudo-adiabatic conditions. Several reaction conditions have been explored by means of changing CH4/O2 ratio and GHSV and of adding CO2. Synergistic effect between active phases has been detected even if best performance has been expressed by the catalyst containing the largest amount of Rh. Catalytic performance can be improved by a proper choice of the substrate; high cell density honeycombs showed increased syngas yields, while a better heat management can be obtained depositing the active phase onto high thermal conductivity substrate. H2/CO ratio can be modified by CO2 addition. Keywords Catalytic partial oxidation Hydrogen Gas-to-liquid processes Natural gas Rh-perovskite Structured catalyst
1 Introduction Syngas is the row material for the productions of H2 and synthetic fuels (through gas-to-liquid (GTL) processes, like methanol and Fischer-Tro¨psch syntheses). The hydrogen P. S. Barbato G. Landi (&) Institute of Researches on Combustion—CNR, P.le Tecchio 80, 80125 Naples, Italy e-mail:
[email protected] P. S. Barbato Department of Chemical Engineering, University of Naples Federico II, P.le Tecchio 80, 80125 Naples, Italy
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production is expected to grow in the next future due to the development of power generation systems using hydrogen as energy carrier [1]. On the other hand, the interest towards GTL processes has grown due to the possibility of converting gaseous into liquid fuels whose transport is less expensive. New GTL plants have been constructed in several countries and others are under construction [2]. Up-to-date, syngas is mainly produced by catalytic steam reforming (SR) of natural gas, through a highly endothermic reaction (DH° = 206 kJ/mol), suffering from thermodynamic and heat transfer limitations and requiring great amounts of catalyst and large energy input at high temperatures [3–5]. In order to produce syngas more efficiently, catalytic partial oxidation (CPO) of methane CH4 þ 1=2O2 ! CO þ 2H2 ðDH ¼ 36 kJ=molÞ
ð1Þ
has received an increasing interest during last decades [6], leading to the realisation of pilot plants in USA and Europe [7]. Through CPO, syngas can be produced with no energy consumption, no coke formation and at relatively low temperature, thus avoiding NOx emissions. Moreover, the slight esothermicity of the reaction allows coupling CPO together with steam and/or dry reforming of methane CH4 þ H2 O ¼ CO þ 3H2 ðDH ¼ 206 kJ=molÞ
CH4 þ CO2 ¼ 2CO þ 2H2 ðDH ¼ 247 kJ=molÞ
ð2Þ ð3Þ
leading to the so-called auto-thermal reforming (ATR) or oxidative steam/dry reforming (OSR/ODR). Through these processes, it should be possible to produce syngas with different H2/CO ratio, according to the downstream processes requirements. In a single adiabatic reactor, the heat produced by CPO can be used to drive the endothermic reactions, if a bi-functional catalyst is utilized. In this case, the heat transfer is more effectively managed.
Partial Oxidation and CO2-ATR of Methane
Starting from the first works [8–11] up to date, two main classes of catalysts have been studied for CPO and ATR, containing Ni [12–20] or noble metals [15, 21–33] as active phase. Although more expensive than Ni-based catalysts, noble metals (generally Pt and Rh) appear to be very attractive because of their high activity, allowing the development of processes at short contact time (SCT), and no coke formation [9]. However, this class of catalysts suffers from deactivation due to the formation of hot spots [9], caused by the rapid CH4 oxidation leading to a mixture of partial and total oxidation products, as proposed by Lyubovsky et al. [28], Horn et al. [32] and Liu et al. [34]. In order to obtain effective catalysts with an optimised use of the noble metal, precious metals deposition onto supports (eventually modified with oxygen carriers) able to increase metal dispersion and reduce deactivation by volatilisation and sintering has been investigated. In particular, it has been demonstrated that the addition of oxygen carriers like MgO [24, 30] and CeO2 [22, 31, 33] leads to higher performance for both Pt and Rh. On the other hand, in order to minimise pressure drops at the high flow rates typical of SCT, the realisation of high void fraction reactors is necessary. Several configurations have been proposed like honeycombs [25, 27, 34, 35], foams [21, 32], spheres [21] and wires [26, 28]. Recently we proposed a bi-functional Rh-perovskite honeycomb catalyst for CPO of methane [35, 36]; in particular, this catalyst showed good Rh dispersion, high activity and selectivity towards syngas. Moreover, the Rh/ LaMnO3 catalyst showed a synergistic effect between the noble metal and the mixed oxide, thus providing better performances than Rh alone catalysts in fuel rich methane combustion. Even if it has been demonstrated that Rh shows a poor activity towards dry reforming [32, 37], in our previous work on Rh/LaCoO3 catalysts we showed that it is possible to manage H2/CO ratio in the products by CO2 addition [35]. This characteristic is particularly important when syngas is used as the raw material for FischerTro¨psch synthesis; as a matter of fact, H2/CO ratio for this process ranges between 1 and 2, thus a CO enrichment with respect to partial oxidation leads to the best operating conditions for the Fischer-Tro¨psch catalysts. Recently, we investigated the effects of Rh loading and of substrate morphology and thermal conductivity on methane fuel rich combustion using Rh/LaMnO3 as catalyst [38]. Even if a positive trend of the catalytic performance as a function of Rh loading has been evidenced, the reduced differences among the samples suggested that the bi-functional catalysts show a significant synergistic effect. Moreover, the use of high thermal conductive substrate allows shifting the heat from the oxidation to the reforming zone, thus increasing the rate of endothermic reactions and, as a consequence, the catalytic performance. These results
17
encouraged us to extend this study to methane CPO in the N2 absence and CO2-ATR. In these cases, i.e. in the absence of nitrogen dilution or in the presence of CO2, the effects of the noble metal loading and of the substrate features could be more significant, due to the major importance of reforming reactions on the overall catalytic performance.
2 Experimental 2.1 Catalyst Preparation Rh/LaMnO3 catalysts have been prepared according to the procedure described elsewhere [35, 38]. Commercial monoliths in cordierite, provided by Saint-Gobain Ceramics (200 cpsi), Corning (400 cpsi) and NGK (600, 900 and 1200 cpsi), and in SiC, provided by CTI (190 and 290 cpsi), in the shape of disks (L = 11 mm, D = 17 mm) were coated using a modified dip-coating procedure with a thick La2O3-stabilised c-Al2O3 layer and calcined in air at 800 °C [35]. Rh and perovskite precursors were deposited on the stabilised alumina wash coat through impregnation with an aqueous solution (0.23 M) of La(NO3)36H2O (Aldrich,[99.99%), (CH3CO2)2Mn4H2O (Aldrich,[99%) and Rh(NO3)22H2O (Fluka, purum) (0.0027–0.027 M). The samples were dried in a MW oven and in a stove at 120 °C and calcined at 800 °C for 3 h under flowing air. The process was repeated 10 times in order to achieve the target loading (*30 wt% perovskite and 0.1–1 wt% Rh with respect to the active wash coat layer, monolithic substrate excluded). Samples were labelled as LMR-x–y, where x is the nominal Rh content as wt% and y the cell density; ‘‘SiC’’ suffix denotes the catalysts with a SiC substrate. As reference, a structured catalyst containing 1 wt% Rh but without perovskite has been also prepared. In Table 1 the prepared structured catalysts are reported. ICP analysis (Table 1) performed on both fresh and used catalysts by Agilent 7500 ICP-MS instrument, after microwave-assisted digestion of samples in nitric/hydrochloric acid solution, revealed that samples compositions are in good accordance with theoretical ones. A full catalyst characterisation is reported elsewhere [38]. 2.2 Catalytic Tests Even if the main purpose of this work is the study of the effects of Rh loading and of substrate features, each catalyst has been tested using different CH4/O2, space velocities and adding CO2 (CO2-ATR). Catalytic tests were carried out under pseudo-adiabatic conditions using the experimental rig described in [35]. The catalytic monoliths were stacked between two inert radiation shields (mullite
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P. S. Barbato, G. Landi
Table 1 Sample features: cell density, substrate material, ICP Rh content
a
Referred to total weight of active phase, monolith substrate excluded
b
After tests of self-sustained methane catalytic partial oxidation
Sample
Cell density (cpsi)
sH 2 ¼
yH2
600
Cordierite
0.98
600
Cordierite
0.092b
LMR–0.5–600
600
Cordierite
0.48b
LMR–1–200
200
Cordierite
1.89
–
LMR–1–400
400
Cordierite
3.30
0.90
LMR–1–600
600
Cordierite
4.14
–
LMR–1–900
900
Cordierite
5.00
–
LMR–1–1200
1200
Cordierite
4.85
–
LMR–1–190–SiC
190
Silicon carbide
1.76
–
LMR–1–290–SiC
290
Silicon carbide
2.69
1.01
100 H2 OUT COOUT s ¼ 100 CO 2 CH4 IN CH4 OUT CH4 IN CH4 OUT yCO
COOUT ¼ 100 CH4 IN
Thus, xCO2 can be negative, if the produced carbon dioxide is more than the consumed one. Moreover, methane has been chosen as reference species also in CO2-ATR tests; in this case CO selectivity and yield can overcome 100% if CO2 conversion is positive. Partial oxidation catalytic tests have been carried out in the absence of nitrogen dilution at CH4/O2 ratio between 1.6 and 2.2 and gas hourly space velocities (GHSV) in the range 22,000–65,000 h-1, while the pre-heating temperature was fixed at 450 °C. On the contrary, CO2-ATR tests have been conducted both at fixed CH4/O2 ratio (=1.9), substituting nitrogen with increasing amounts of CO2, and at fixed C/O ratio (=1), in the absence of nitrogen dilution; in the last case, at increasing CO2 content methane and oxygen concentrations in the feed mixture were,
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Rh by ICPa (wt%)
1%-Rh/Al2O3
CH4 IN CH4 OUT CO2 IN COOUT 2 x ¼ 100 CO 2 CH4 IN CO2 IN
100 H2 OUT ¼ 2 CH4 IN
Catalyst loading (g/cm3)
LMR–0.1–600
foams, 45 ppi) and tightly sealed in a quartz tube that was inserted in an electric furnace. Reactor temperatures were measured by means of thermocouples in the centre of each monolith, in close contact with the solid, and in the exit gas after the last radiation shield. Exit gases passed through a condenser and a CaCl2 trap to remove water, prior to splitting to a Hartmann and Braun Advance Optima continuous analyser for H2, CO, CO2, CH4, and to an on line GC (HP5890 Series II). Carbon balance was always closed within ± 4%. No other hydrocarbons except from methane were detected in the products, whereas O2 was always completely converted. Methane and carbon dioxide conversions, hydrogen and carbon monoxide selectivities and yields were calculated according to the following equations: xCH4 ¼ 100
Substrate
respectively increased and decreased. The pre-heating temperature of CO2-ATR tests has been varied from 450 to 650 °C, while GHSV has been kept constant and equal to 42,000 h-1. 2.3 Thermodynamic Equilibrium Calculation Thermodynamic equilibrium compositions and temperatures have been calculated using CHEMKIN 4.0.3 software [39]. Data have been obtained at constant pressure and enthalpy, thus simulating adiabatic conditions. Calculations have been carried out considering gaseous species only, thus excluding solid carbon formation on the catalyst, ruled out as previously reported [36].
3 Results 3.1 Effect of Rh Loading In Fig. 1 the catalytic performance of bi-functional catalysts (substrate: cordierite, 600 cpsi) containing 0.1, 0.5 and 1 wt% Rh and of 1%Rh/Al2O3, as reference, are reported as a function of CH4/O2 ratio at a fixed preheating temperature (450 °C). Methane conversion and products yields show the same trend of equilibrium ones at CH4/O2 C 1.9; the thermodynamically previewed maxima in H2 and CO productions are not observed because CH4 conversion is relatively far from equilibrium. In particular, methane conversion steadily increases decreasing CH4/O2 ratio, due to the increased O2 partial pressure, which is the limiting reactant. Also yields to desired products increase almost linearly increasing oxidant in the feed, according to the conversion trend; because the selectivities (not reported) appear slightly affected by feed ratio (a little increase with CH4/O2 ratio is observed, as expected), the main effect on H2 and CO production is related to the conversion.
Partial Oxidation and CO2-ATR of Methane 100
95
95
CO yield, %
100
CH4 conversion, %
Fig. 1 Partial oxidation performance of LMR [0.1–600 ), 0.5–600 ( ), 1–600 ( )] (ds) and 1%Rh/Al2O3 ( catalysts as a function of CH4/O2. Solid lines correspond to thermodynamic equilibrium. Tpre: 450 °C; GHSV: 36,000 h-1
19
90 85 80 75 70 1.7
90 85 80 75 70
1.8
1.9
2.0
2.1
2.2
65 1.7
2.3
1.8
1.9
Temperature, °C
95
H2 yield, %
2.1
2.2
2.3
1200
100
90 85 80 75 70 65 1.7
2.0
CH4/O2
CH4/O2
1.8
1.9
2.0
2.1
CH4/O2
Figure 1 shows that performance of bi-functional catalysts are close to each other, suggesting a significant contribution of perovskite to the catalytic activity, but not superposable; as a matter of fact, among bi-functional samples catalytic performance is directly related to Rh content, conversion and yields increasing with noble metal amount. Thus, in the overall reaction mechanism, at least one step appears kinetically controlled and strictly related to the number of Rh sites. Comparing these results with those obtained on the catalyst without perovskite (Fig. 1), the synergistic effect of noble metal and mixed metal oxide is evident. Rh alone sample shows performance comparable with the bi-functional one containing one half noble metal. Similar effect has been detected on Rh–Ni catalysts [40]. The temperatures measured inside the catalyst, reported in Fig. 1, are in the range 800–1000 °C and appear affected by the feed ratio. It must be underlined that the co-presence of partial (Rh) and total (LaMnO3) oxidation catalysts with a reduced noble metal amount does not cause hot-spots or over-heating even at relatively high oxygen partial pressures [41]. The comparison with equilibrium temperature, shows that the behaviour is similar down to CH4/O2 & 1.9, where a sudden increase of the adiabatic temperature is observed corresponding to complete CH4 conversion and subsequent decrease of partial oxidation products selectivity. This behaviour is not observed on the catalysts, due to the incomplete methane conversion and the relatively unchanged selectivities to CO and H2. The adiabatic temperatures corresponding to the gas mixtures downstream to
2.2
2.3
1100 1000
Teq Tad_mix
900 Tcat
800 700 600 1.7
Tout 1.8
1.9
2.0
2.1
2.2
2.3
CH4/O2
the LMR–0.5–600 have been also evaluated (Fig. 1) and result higher than the temperature measured in the centre of the catalyst (Tcat). It has to be pointed out that the temperature at the catalyst exit is between Tcat and Tout because of the existence of an oxidation (the first part of the catalyst) and a reforming zone (the second part), as recently demonstrated by Schmidt et al. [32] by high resolution spatial concentration profiles. As a consequence, the real difference between gas exit temperature and the calculated adiabatic one is still higher, indicating significant heat losses. According to [38], the best activity of higher Rh loading samples among the bi-functional catalysts in the methane partial oxidation is due to their ability to convert more efficiently methane in the second part of the reactor by steam reforming, which is kinetically controlled and exclusively activated on Rh sites. On the other hand, considering that perovskite is active towards hydrocarbon combustion [41], the synergistic effect of bi-functional catalysts should be attributed to a different behaviour in the oxidation zone. Even if on Rh catalysts with high noble metal content (5 wt%) the oxygen mass transfer is rate limiting in the oxidation zone [32], on our samples, loaded with 1 wt% Rh maximum, the product distribution strongly depends on the catalyst composition, suggesting that, even if in a limited extent, kinetics affects the overall oxidation rate. Under this hypothesis, the presence of a second active phase, i.e. the perovskite, could contribute to enhance the oxidation kinetics and to shorten the oxidation zone, thus resulting in an increase of space time for reforming
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reaction. A similar behaviour has been evidenced by Schmidt et al. [32] on Rh and Pt foams, the latter showing a partially kinetic regime, leading to a longer oxidation zone. Besides its intrinsic oxidation activity, the perovskite could offer a good environment in order to highly disperse the noble metal. Because steam reforming activity strongly depends on Rh structure [42, 43], enhanced dispersion could result in higher reforming reaction rates. The effect of perovskite on Rh dispersion is under investigation. Due to the absence of nitrogen, temperatures and performance of LMR catalysts are higher than those reported in our previous work [38]. Moreover, we detected a more significant synergistic effect between Rh and perovskite, probably due to a more important contribution of the noble metal to methane conversion at relatively lower temperatures, i.e. under conditions where the amount of catalyst can play a major role. Contrary to that observed using air as oxidant [38], we did not observe a maximum in the hydrogen and carbon monoxide productions, due to the relative short range of investigated composition. As a matter of fact, our investigations in [38] and in this work have been carried out using compositions outside the flammability limits, in particular above the UFL; because UFL significantly increases reducing dilution, i.e. substituting air with pure oxygen, the CH4/O2 range of interest is reduced. Figure 2 shows the catalytic performance of bi-functional catalysts as a function of GHSV at fixed pre-heating temperature (450 °C) and feed ratio (2.0); methane conversion and products yields increase increasing GHSV for all the samples. In this case, the enhanced methane 100
100 95
95
CO yield, %
CH4 conversion, %
Fig. 2 Partial oxidation performance of LMR [0.1–600 ( ), 0.5–600 ( ), 1–600 (d)] catalysts as a function of GHSV. Solid lines correspond to thermodynamic equilibrium. CH4/O2 = 2; Tpre: 450 °C
conversion involves a more than linear increase on syngas production. Apart from the linear yield dependence on conversion, at complete O2 conversion the limiting oxygen availability shifts product distribution towards partial oxidation products, thus enhancing selectivity, when hydrocarbon conversion increases at fixed feed ratio. At the same time, catalyst and outlet temperatures slightly raise for each sample, suggesting that the real gas temperature at the exit of the reactor increases as well. The comparison with adiabatic temperature, shows that their difference is reduced, implying a decrease of heat losses. Generally, by increasing GHSV, higher velocities are obtained in the channels and, as a consequence, longer oxidation zones are expected. Notwithstanding, an increase of reforming activity, as revealed by enhanced CH4 conversions and syngas selectivity, is detected in our experiments, since the temperature increase due to the higher thermal power generated counterbalances the shorter reforming length. Therefore, the improved performance is related to a more adiabatic operation achieved at high space velocity. The comparison among the catalysts containing different Rh amounts shows that the catalytic activity increasing with noble metal content similarly to the previously reported behaviour at constant GHSV. Moreover, it appears that at the highest GHSV performance reaches a plateau, whose value depends on the Rh amount, indicating that the maximum allowable conversion is related to steam reforming kinetic limitations, oncoming at the highest space velocity (i.e. at the shortest contact time). Consequently, these results indicate that at GHSV & 60,000 h-1 (corresponding to a contact time equal to about 16 ms) the
90 85 80
90 85 80 75 70
75
65 20
30
40
50
60
70
20
30
50
60
70
1100
Temperature, °C
100
H2 yield,%
40
GHSV, (10-3 h-1)
GHSV, (10-3 h-1)
90
80
70
Tad_mix
1000 Tcat
900
Teq
800 700
Tout
600 20
30
40
50
60
GHSV, (10-3 h-1)
123
70
20
30
40
50
GHSV, (10-3 h-1)
60
70
Partial Oxidation and CO2-ATR of Methane
N2, % 40
30
N2, %
20
10
0
40
30
20
10
0 2.5
100 80
2.0
CH4
60
1.5
40 20
CO2
H2 /CO
Conversion, %
Fig. 3 CO2-ATR performance ), of LMR [0.1–600 ( ), 1–600 (ds) and 0.5–600 ( )] catalysts as 1%Rh/Al2O3 ( a function of CO2/O2. Solid lines correspond to thermodynamic equilibrium. CH4/O2 = 1.9; N2 ? CO2 = 43 vol.%; Tpre: 450 °C; GHSV: 42,000 h-1
21
1.0
Selectivity, %
140
Tcat
120
CO
800
100
Tad
700
80 60
900
H2
600
Tout
Temperature, °C
0
500
40 0.0
0.5
1.0
1.5
CO2 /O2
contact time begins to be too short to carry on steam reforming, compensating the beneficial effect of the higher temperature level. Figure 3 shows the performance of LMR–x–600 and 1%Rh/Al2O3 in CO2-ATR as a function of CO2/O2 ratio at fixed CH4/O2 (1.9) and pre-heating temperature (450 °C). Catalytic tests have been carried out starting with roughly 43% N2 dilution and substituting progressively nitrogen by CO2 at constant flow rate. In this case, the catalytic behaviour is independent of the partial pressures of CH4 and O2 and can be attributed only to reactions involving CO2. Moreover, the heat generated by oxidation does not change, thus the thermal effects not linked to endothermic reactions are minimised. For all the samples, methane conversion appears quite unaffected by CO2 content in the feed, slightly decreasing as carbon dioxide increases and showing the same trend of equilibrium conversion. CO2 conversion is negative at low CO2/O2 ratio, thus indicating that its production is greater than its consume. Increasing CO2, conversion becomes positive and reaches a plateau (about 20%). Comparing to the corresponding equilibrium line, it appears that the amount of converted carbon dioxide is much lower than predicted, particularly at low CO2/O2 ratio, suggesting a low rate of reactions involving CO2. Selectivity to CO steadily increases substituting N2 by CO2 and overcomes 100%, indicating that part of carbon monoxide is derived from CO2 reactions. On the contrary, H2 selectivity shows a specular behaviour, steadily decreasing down to about 70%; it must be underlined that, considering quite constant methane conversion, the above results indicate an inter-conversion ratio between products equal to 1 (i.e. one mole of hydrogen consumed per mole of
2.0
0.0
0.5
1.0
1.5
2.0
CO2 /O2
carbon monoxide produced). As for Rh/LaCoO3 catalysts [35], these results confirm a poor activity of bi-functional catalysts towards dry reforming of methane (methane conversion is unaffected by CO2 partial pressure), while carbon dioxide appears to be converted only by reverse water gas shift (RWGS), accordingly to that reported for Rh catalysts by Schmidt et al. [44]. As a matter of fact, these Authors reported that in the reforming zone of the catalyst methane preferentially reacts with H2O even if reforming reactions appear 0-th order in the co-reactant and the CO2 co-feeding reduces the thermodynamic driving force of methane reaction rates. The effectiveness of the Rh in the RWGS leads to catalytic performance less far from equilibrium at high CO2/O2, especially in term of products selectivity. For this reason, the H2/CO ratio (Fig. 3), higher than the equilibrium one at low CO2 content in the feed, approaches the predicted one, suggesting that the factors favouring CO2 with respect to CO are counterbalanced by the high RWGS reaction rate. The occurrence of endothermic reactions is confirmed by the temperature profiles (Fig. 3); catalyst and outlet temperatures, in fact, decrease increasing CO2/O2 with a trend similar to the equilibrium one. It must be underlined that H2/CO ratio varied from about 2 in the absence of carbon dioxide to about 1 at the highest CO2/O2 ratio; hence, even at low CO2 conversion, the H2/CO ratio can be managed by adding CO2 according to the requirements of syngas users in a single auto-thermal operation. About the effect of Rh amount, also in this case, the catalytic activity is directly related to the noble metal content, LMR–1–600 providing the best conversions and selectivities, indicating that steam reforming and reverse water gas shift are activated on the same Rh sites. The
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beneficial effect of bi-functional catalyst is also detected in CO2-ATR, 1%Rh/Al2O3 providing the same catalytic activity of LMR–0.5–600. Figure 4 shows the performance of LMR–x–600 and 1%Rh/Al2O3 in CO2-ATR as a function of CO2/O2 ratio at C/O = 1 and fixed pre-heating temperature (450 °C). In this case, the atom balance at complete reactants conversion would impose the production of only CO and H2, thus maximising syngas yield. Moreover the H2/CO ratio could be regulated simply changing the relative amounts of CH4, CO2 and O2 in the feed. On the other hand, increasing CO2/O2 ratio means a reduction of the heat that can be generated through oxidation reactions. First of all, also in this set of experiments the beneficial effect of bi-functional catalyst with respect to Rh alone is detected. In addition, even if increasing Rh amount leads to higher performance, very low noble metal content catalysts show activity approaching that of the most Rh rich one. For all the samples, methane conversion strongly decreases from about 85% in the absence of CO2 down to around 35% at the highest CO2/O2 ratio, according to the trend of equilibrium conversion. On the contrary, CO2 conversion is negative at very low CO2/O2 ratio and approaches a quite constant value (&12%) for a large range of feed compositions. H2 selectivity continuously decreases, while CO selectivity steadily increases; with respect to previous results, it appears that H2 selectivity departs from equilibrium line more significantly than CO one. This is confirmed looking at the H2/CO ratio, which appears lower than that predicted by thermodynamics at CO2/O2 C 1. As expected, catalyst and outlet temperatures, compared to
140
80
120
60 40
CH4
20
CO2
0
Selectivity, %
100
Conversion, %
Fig. 4 CO2-ATR performance ), of LMR [0.1–600 ( ), 1–600 (ds) and 0.5–600 ( )] catalysts as 1%Rh/Al2O3 ( a function of CO2/O2. Solid lines correspond to thermodynamic equilibrium. C/O = 1; N2 = 0 vol.%; Tpre: 450 °C; GHSV: 42,000 h-1
adiabatic equilibrium temperature, show a significant decrease related to the low heat generable at increased CO2/O2. As for previous results, the catalyst temperature depends on the catalyst, increasing by reducing the noble metal content. In order to better understand the above results, in Table 2 the performances of LMR–1–600 in the two experimental sets have been reported in terms of conversions, H2 selectivity, reacted CH4 and H2 in the outlet steam. Moreover, the H2 amount consumed by RWGS is calculated from the CO2 consumption and added to the outlet H2 in order to calculate the total H2 produced from CH4. In the last column the ratio between the as calculated H2 and reacted CH4 is reported. As shown in the table, this ratio is directly related to methane conversion, suggesting that H2 selectivity is affected not only by the RWGS extent, but also by SR reaction involving unconverted CH4. So, in the experimental set at CH4/O2 = 1.9, due to the high and quite constant CH4 conversion, H2 and CO selectivities appear practically related only to product inter-change through RWGS, as suggested by the quite constant H2/CH4 ratio reported in Table 2. On the contrary, the wide range of conversion obtained in the experimental set at C/O = 1 does not allow to exclude the reduced SR reaction rate in order to explain the reduction of the H2 selectivity, which appears only qualitatively but not quantitatively related to the CO selectivity increase due to RWGS. Moreover, the above results suggest that SR is more sensitive to temperature modifications than RWGS, as expected considering the great difference between their heats of reaction (DH° = 41.2 kJ/mol for RWGS compared to 206 kJ/mol for SR). The effect of Rh amount is
CO
100 80 H2
60
-20 40 0.0
-40 0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.5
1.0
2.5
Temperature, °C
H 2 /CO
1.5 1.0
0.5
1.0
1.5
2.0
CO2 /O2
123
2.0
2.5
3.0
1000
2.0
0.5 0.0
1.5
CO2 /O2
CO2 /O2
2.5
3.0
900 800 700
Tcat Teq
600 500 0.0
Tout 0.5
1.0
1.5
2.0
CO2 /O2
2.5
3.0
Partial Oxidation and CO2-ATR of Methane
23
Table 2 Effect of CO2 content on LMR–1–600 performance in CO2-ATR experiments CO2/O2
CH4/O2
xCH4 (%)
xCO2 (%)
sH2 (%)
CH4 react.a
H2 prod.a
H2 cons.b
Total H2
H2/CH4
CO2-ATR at C/O = 1 (N2 = 0%) 0.00
2.00
87.6
94.8
58.4
110.7
110.7
1.90
0.17
2.17
80.7
-17.0
93.2
52.4
97.7
-0.9
96.8
1.85
0.50
2.50
69.2
16.5
90.0
43.3
77.9
2.1
79.9
1.85
1.00
3.00
53.9
15.8
82.5
32.4
53.4
3.2
56.6
1.75
1.83
3.83
42.0
13.7
70.0
24.2
33.8
3.8
37.6
1.56
2.50
4.50
35.3
11.9
62.6
19.9
24.9
3.7
28.6
1.44
97.3
32.5
63.2
63.2
1.95
CO2-ATR at CH4/O2 = 1.9 (N2 ? CO2 = 43%) 0.00
1.90
86.8
0.32
1.90
86.7
-7.3
92.6
32.4
60.0
-0.5
59.6
1.84
0.96 1.61
1.90 1.90
86.6 85.9
19.7 22.5
88.0 81.2
32.4 32.1
57.0 52.1
3.7 7.1
60.7 59.2
1.88 1.84
2.22
1.90
84.6
25.8
71.0
31.6
44.9
11.3
56.2
1.78
On 100 mol feed base
b
Calculated according to RWGS stoichiometry
temperatures. In order to overcome these limitations, a higher pre-heating could be applied; in Fig. 5 the results obtained on LMR–1–600 at three different pre-heating temperatures and the corresponding equilibrium conditions are reported. The positive effect of higher pre-heating is detected, but it appears limited, showing a low conversion increase for a pre-heating temperature 100 °C higher (CH4 conversion increases from 54 to 56% at CO2/O2 = 1); the temperature increase is limited too, corresponding to maximum 40 °C. The results are in accordance with the thermodynamic calculations, showing adiabatic temperature
100
160
80
140
60 40
CH4
20
CO2
0 -20 -40 0.0
0.5
1.0
1.5
2.0
2.5
100 80
H2
60 40 0.0
3.0
CO
120
0.5
1.0
2.2
2.0
2.5
3.0
900
Temperature, °C
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.0
1.5
CO2 /O2
CO2 /O2
H2 /CO
Fig. 5 CO2-ATR performance of LMR–1–600 as a function of CO2/O2 at different pre-heating temperatures [Tpre: 450 °C (d, s), 500° ( , ), 550 °C ( , )]. Solid lines correspond to thermodynamic equilibrium (Tpre: 450 °C [ ], 500° [ ], 550 °C [ ]). C/O = 1; N2 = 0 vol.%; GHSV: 42,000 h-1
Conversion, %
related to the increase of reaction rates of SR and RWGS, both occurring on the same noble metal sites. Due to the relatively low temperatures, at C/O = 1 higher Rh loadings appear connected to higher conversion levels for SR reaction, as suggested by the more pronounced difference in H2 than in CO selectivity. In conclusion, the significant decrease of catalytic performance is related to the relatively low temperatures obtained in the catalysts, adversely affecting SR reaction, due to the lower heat released by oxidation reactions in the first part of the catalyst and the increased importance of heat losses at decreased average
Selectivity, %
a
0.5
1.0
1.5
2.0
CO2 /O2
2.5
3.0
800
Teq
700 600 Tcat
500
Tin
400 300 0.0
0.5
1.0
1.5
2.0
2.5
3.0
CO2 /O2
123
24
(b) 100 CO selectivity, %
(a) 100 CH4 conversion, %
Fig. 6 Partial oxidation (a–c) and CO2-ATR (d) performance of LMR [1–200 (d), 1–400 ( ), 1–600 ( ), 1–900 ( ), 1–1200 ( )] catalysts. Solid lines correspond to thermodynamic equilibrium. Tpre: 450 °C; GHSV: 36,000 h-1 (a–c), 42,000 h-1 (d)
P. S. Barbato, G. Landi
95 90 85 80 75 1.6
1.7
1.8
1.9
2.0
2.1
2.2
95 90 85 80 75 1.6
2.3
1.7
1.8
95
2.1
2.2
2.3
90 85 80
1.7
1.8
1.9
2.0
2.1
2.2
80
40 20
CO2
0 -20 0.0
2.3
CH4
60
0.5
CH4/O2
3.2 Effect of Honeycomb Substrate The influence of substrate features on catalytic performance has been studied on the LMR–1, being the most active catalysts as reported above. In the Fig. 6a–c the effect of the cell density of cordierite honeycombs on the catalytic performance towards CH4 partial oxidation has been reported. Conversion steadily increases with cell density from 200 to 1200 cpsi. The curves corresponding to the different catalysts are parallel to each other, differences slightly decreasing as cell density increases. CO production is favoured at high cell density, reducing the gap from equilibrium selectivity; on the contrary, H2 selectivity shows an unexpected maximum for 600 cpsi substrate (very close to equilibrium), leading to a corresponding maximum for H2/CO ratio. These results strongly suggest a significant role of channel dimension on products inter-conversion through RWGS reaction, thus explaining the increment of CO selectivity and the decrement of H2 selectivity. These considerations are confirmed by calculating the Kp/Keq ratio for the RWGS alone at the catalyst exit temperature and for each experimentalQconditions, limited to higher cell density catalysts (Kp ¼ pmi i , where pi are the outlet partial pressures of H2, CO, CO2 and H2O and mi the corresponding stoichiometric coefficient; Keq is the thermodynamic constant). These values are reported in Fig. 7. Increasing cell density results in increased Kp/Keq, approaching 1 for 1200 cpsi catalyst, clearly suggesting
1.0
1.5
2.0
2.5
CO2/O2
that RWGS is favoured by reducing channel dimension. In the literature it has been reported that WGS and, as a consequence, RWGS reach equilibrium conditions on Rh catalysts [45]. Moreover, it has been reported that the occurrence of WGS or RWGS strongly depends on the local balance among CO, CO2, H2 and H2O [44]. In our experiments it clearly appears that equilibrium conditions for RWGS can be reached only on the monolith with the smallest channels. With respect to our previous results under fuel-rich combustion conditions [38], the trend obtained changing cell density appears more defined, differences among catalysts performance being more evident. This is due to the higher temperature level corresponding to undiluted feeds; as a matter of fact, also on bi-functional 1.2 1.0 0.8
Kp/Keq
increments lower than those of pre-heating and slight increases of performance.
123
2.0
(d) 100 Conversion, %
H2 selectivity, %
(c) 100
75 1.6
1.9
CH4/O2
CH4/O2
0.6 0.4 0.2 0.0 1.7
1.8
1.9
2.0
2.1
2.2
2.3
CH4/O2 Fig. 7 Kp/Keq of RWGS reaction for LMR [1–600 ( ), 1–900 ( ), 1–1200 ( )] catalysts. Equilibrium constants have been evaluated at reactor exit temperature
Partial Oxidation and CO2-ATR of Methane
25
catalysts the limiting step is methane steam reforming occurring in the second part of the reactor and kinetically controlled. As a consequence, steam reforming kinetics is increased due to higher temperatures and this effect is enhanced increasing catalyst amount, as a consequence of the increase of the cell density. A similar behaviour has been detected in CO2-ATR experiments (Fig. 6d); for the sake of brevity only reactant conversion are reported, selectivities showing the same trends reported above. CH4 conversion increases with cell density, appearing related to higher steam reforming activity, accordingly with the results reported in the previous paragraph. Also CO2 conversion increases with cell density due to a higher extent of RWGS and not of DR, highlighting a distinct reaction pathway for each reactant for all the samples. A larger cell density results in increased geometrical surface area, which provides enhanced mass and heat transfers and larger loadings of active phase (more than doubled passing from 200 to 1200 cpsi). In the first part of the catalyst oxidation reactions occur under partially mass transfer control; the enhancement of transport coefficient leads to higher reaction rates and, consequently, to a shorter oxidation zone. Steam reforming, which is under kinetic control as demonstrated above, benefits of longer fraction of the reactor and increased catalyst amount, both factors leading to higher contact time for residual methane in the steam reforming zone. As stated above, a clear order in RWGS activity can be established, especially at similar CH4 conversion; the results strongly suggest a transport limitation for this reaction, indicating, as a consequence,
RWGS [ SR [ DR Inside ceramic substrates heat transport limitations along the catalyst axis result into significant hot-spots during high exothermic reactions [32]. Using high cell density honeycombs, catalyst surface temperature in the oxidation zone, which is higher than the gas one, benefits of increased heat transfer because cold gases remove heat more efficiently; as a consequence, the temperature peak is reduced and shifted downstream [37]. In the steam reforming zone, the occurrence of endothermic reactions on the surface makes the catalyst cooler than the gas phase; in this part of the reactor the enhanced heat transfer increases the heat available for endothermic reactions, thus resulting in a higher activity. For low thermal conductivity substrates the heat transfer between gas and solid is the main path to drive thermal power along the catalyst axis, especially with ultra thin wall monoliths [46]. The previous considerations suggest that a good heat management in the axial direction could be a proper way in order to enhance catalytic performance; in particular, the axial heat transfer can be increased by using high thermal conductivity substrates. In Fig. 8 the performance of cordierite and SiC-based catalysts are compared; LMR–1–200 and LMR–1–190–SiC show similar geometric and fluid dynamics properties and about the same catalyst loading (Table 1), differing only for their thermal conductivity (10 times higher for SiC than for cordierite). A significant
CH4 conversion, %
100
100 95
CO yield, %
95 90 85 80
90 85 80 75 70
75 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
65 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
CH4/O2
CH4/O2 1400
Temperature, °C
100 95
H2 yield, %
Fig. 8 Partial oxidation performance of LMR [1–200 ), 1–190–SiC (ds), ( )] catalysts as a 1–290–SiC ( function of CH4/O2. Solid lines correspond to thermodynamic equilibrium. Tpre: 450 °C; GHSV: 36,000 h-1
that Rh is more active towards RWGS than SR. Coupling with the above results, it can be concluded that Rh activity order in the absence of an oxidant is
90 85 80 75 70 65 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
CH4/O2
1200
Teq
1000 800
Tcat
600
Tout
Tin 400 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
CH4/O2
123
26 100
160
80
140 CH4
60 40 20 CO2
0 -20 0.0
0.5
1.0
1.5
2.0
CO2 /O2
performance increase can be noted in terms of both CH4 conversion and syngas yield in the whole range of compositions investigated. In Fig. 8 the temperatures measured upstream, in the middle and downstream the catalysts are also reported. The inlet temperature is always higher for cordierite sample, while catalyst and outlet temperatures are higher for SiC-based one, suggesting a reduction of the peak temperature and its shift downstream due to the enhanced axial heat transfer related to the high thermal conductivity of SiC honeycombs. Performance data of LMR–1–290 sample (Fig. 8) confirm that the increase in the cell density of honeycombs is effective to enhance syngas yield also in the case of SiCbased material. As reported in [38], in high conductivity substrates the heat is more efficiently driven from hot (oxidation) zone to ‘‘cool’’ (reforming) zone, thus favouring the occurrence of endothermic reactions and, furthermore, preserving catalyst from hot spot formation. With respect to fuel-rich combustion conditions reported in [38], in the absence of nitrogen the effect of the thermal conductivity appears stressed, due to the higher temperatures obtained using oxygen as oxidant. Moreover, under these experimental conditions, the peak temperature reduction appears a more stringent goal, due to the highest maximum temperature achievable without nitrogen dilution. The positive effect of high thermal conductivity has been detected also in CO2-ATR experiments. As reported in Fig. 9, both CH4 and CO2 conversions increase. Because it has been clarified that the surplus of reacted methane is due to enhanced steam reforming extent and the reacted CO2 surplus to enhanced RWGS, it is confirmed that high thermal conductivity allows to ‘‘feed’’ the heat produced in the oxidation zone to the reforming one, increasing the activity towards endothermic reactions.
4 Conclusions Experimental results showed that mixed Rh–LaMnO3 catalysts are suitable for syngas production by methane partial
123
Selectivity, %
Conversion, %
Fig. 9 CO2-ATR performance ), 1–190– of LMR [1–200 ( )] SiC (ds), 1–290–SiC ( catalysts as a function of CO2/O2. Solid lines correspond to thermodynamic equilibrium. C/O = 1; N2 = 0 vol.%; Tpre: 450 °C; GHSV: 42,000 h-1
P. S. Barbato, G. Landi
2.5
CO 120 100 80 60 40 0.0
H2 0.5
1.0
1.5
2.0
2.5
CO2 /O2
oxidation. This class of catalysts showed enhanced performance with respect to Rh catalysts due to a synergistic effect between phases even at very low noble metal content. Nevertheless, overall catalytic activity appeared directly related to Rh amount in LMR catalysts under pseudo adiabatic conditions; this behaviour seemed due to an increased extent of kinetically controlled steam reforming in the second part of the honeycombs. Carbon dioxide cofeeding did not result in increased methane conversion, due to the low activity towards dry reforming; however, H2/CO ratio has been deeply modified due to the good activity of the noble metal towards reverse water gas shift reaction, evidenced also by the positive effect of Rh amount. On the other hand, an increase of the activity towards endothermic reactions (i.e. increase of CH4 and CO2 conversions) was obtained modifying geometrical and physical features of the substrate. In particular, the use of high cell density honeycombs, characterised by high geometrical surface and reduced channel dimension, other than supporting more active phase, improves transport properties thus obtaining a more uniform temperature profile. Besides the H2/CO ratio could be modified with the appropriate choice of the substrate features. A similar effect can be obtained by improving axial heat transfer by deposing the active phase onto substrates characterised by high thermal conductivity. As a matter of fact, the higher performance detected on this class of catalyst appears related to a larger temperature profile and, in particular, to a higher temperature in the reforming zone, increasing the rate of endothermic reactions, principally steam reforming.
References 1. Aasberg-Petersen K, Bak Hansen JH, Christensen TS, Dybkjaer I, Seier Christensen P, Stub Nielsen C, Winter Madesen SEL, Rostrup-Nilsen JR (2001) Appl Catal A: Gen 221:379 2. ReddyKeshav T, Basu S (2007) Fuel Process Technol 88:493 3. Pena MA, Gomez JP, Fierro JLG (1996) Appl. Catal. A 144:7 4. Cavani F, Trifiro` F (1998) Stud Surf Sci Catal 119:561 5. Jones CA, Leonard JJ, Sofranko JA (1987) Energy Fuels 1:12
Partial Oxidation and CO2-ATR of Methane 6. Basini L (2006) Catal Today 117:384 7. Basini L (2006) Tøpsoe catalysis forum 2006. http://www.topsoe. com/site.nsf/vALLWEBDOCID/EOTT-6TF8BM/$file/TCF%20 Aug%202006.pdf 8. Ashcroft AT, Cheetham AK, Foord JS, Green MLH, Grey CP, Murrell AJ, Vernon PDF (1990) Nature 344:319 9. Vernon PDF, Green MLH, Cheetham AK, Ashcroft AT (1990) Catal Lett 6(2):181 10. Ashcroft AT, Cheetham AK, Green MLH, Vernon PDF (1991) Nature 352:225 11. Torniainen PM, Chu X, Schmidt LD (1994) J Catal 146:1 12. Slagtern A, Olsbye U (1994) Appl Catal A Gen 110:99 13. Zhang Y, Xiong G, Sheng S, Yang W (2000) Catal Today 63:517 14. Requies J, Cabrero MA, Barrio VL, Gu¨emez MB, Arias PL, Pen˜a MA, LaParola V, Fierro JLG (2006) Catal Today 116:304 15. Basile F, Fornasari G, Trifiro` F, Vaccari A (2001) Catal Today 64:21 16. Choudhary VR, Mamman AS (2000) Appl Energ 66:161 17. Matsuo Y, Yoshinaga Y, Sekine Y, Tomishige K, Fujimoto K (2000) Catal Today 63:439 18. Tomishige K, Matsuo Y, Sekine Y (2001) Catal Commun 2:11 19. Nicchio NN, Casella ML, Santori GF, Ponzi EN, Ferretti OA (2000) Catal Today 62:231 20. Liu S, Xion G, Dong H, Yang W (2000) Appl Catal A Gen 202:141 21. Hohn KL, Schmidt LD (2001) Appl Catal A Gen 211:53 22. Mattos LV, de Oliveira ER, Resende PD, Noronha FB, Passos FB (2002) Catal Today 77:245 23. Tavazzi I, Beretta A, Groppi G, Maestri M, Tronconi E, Forzatti P (2007) Catal Today 129(3–4):372 24. Ruckenstein E, Wang HY (2000) Appl Catal A Gen 198:33 25. Hickmann DA, Schmidt LD (1992) J Catal 138:267 26. Hofstad KH, Sperle T, Rokstad OA, Holmen A (1997) Catal Lett 454:97 27. Bodke AS, Bharadwaj SS, Schmidt LD (1998) J Catal 179:138 28. Lyubovsky M, Karim H, Menacherry P, Boorse S, LaPierre R, Roychoudhury S (2003) Catal Today 83:18
27 29. Erdo¨helyi A, Cserenyi J, Solymosi F (1993) J Catal 141:287 30. Mateos-Pedrero C, Cellier C, Eloy P, Ruiz P (2007) Catal Today 128:216 31. Silva FA, Martinez DS, Ruiz JAC, Mattos LV, Hori CE, Noronha FB (2008) Appl Catal A Gen 335:145 32. Horn R, Williams KA, Degenstein NJ, Bitsch-Larsen A, DalleNogare D, Tupy SA, Schmidt LD (2007) J Catal 249:380 33. Eriksson S, Rojas S, Boutonnet M, Fierro JLG (2007) Appl Catal A Gen 326:8 34. Liu T, Snyder C, Veser G (2007) Ind Eng Chem Res 46:9045 35. Cimino S, Landi G, Lisi L, Russo G (2005) Catal Today 117:718 36. Cimino S, Landi G, Lisi L, Russo G (2006) Catal Today 105:454 37. Donsı` F, Cimino S, DiBenedetto A, Pirone R, Russo G (2005) Catal Today 105:551 38. Landi G, Barbato PS, Cimino S, Lisi L, Russo G (in press) Catal Today. doi:10.1016/j.cattod.2009.01.020 39. Kee RJ, Rupley FM, Miller JA, Coltrin ME, Grcar JF, Meeks E, Moffat HK, Lutz AE, Dixon-Lewis G, Smooke MD, Warnatz J, Evans GH, Larson RS, Mitchell RE, Petzold LR, Reynolds WC, Caracotsios M, Stewart WE, Glarborg P, Wang C, Adigun O (2004) Chemkin collection, release 4.0. Reaction Design, Inc, San Diego, CA 40. Basile F, Fornasari G, Trifiro` F, Vaccari A (2002) Catal Today 77:215 41. Cimino S, Colonna S, De Rossi S, Faticanti M, Lisi L, Pettiti I, Porta P (2002) J Catal 205:309 42. Wei J, Iglesia E (2004) J Catal 225:116 43. Eriksson S, Rojas S, Boutonnet M, Fierro JLG (2007) Appl Catal A Gen 326:8 44. Michael BC, Donazzi A, Schmidt LD (2009) J Catal 265:117 45. Donazzi A, Beretta A, Groppi G, Forzatti P (2008) J Catal 255:241 46. Schneider A, Mantzaras J, Jansohn P (2006) Chem Eng Sci 61:4634
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