Partial oxidation of methane to synthesis gas over MgO-supported Rh catalysts: the effect of precursor of MgO

Applied Catalysis A: General 198 (2000) 33–41

Partial oxidation of methane to synthesis gas over MgO-supported Rh catalysts: the effect of precursor of MgO E. Ruckenstein∗ , H.Y. Wang Department of Chemical Engineering, State University of New York at Buffalo, Amherst, NY 14260, USA Received 1 September 1999; received in revised form 28 October 1999; accepted 28 October 1999

Abstract Five magnesium-containing precursors were used to prepare magnesium oxides of different surface areas. With these oxides as supports, catalysts (1 wt.% Rh loading) with different Rh dispersions after reduction were prepared. At a Tfurnace of 750◦ C and 1 atm, all these catalysts provided a conversion >80% and selectivities of 95–96% to CO and 96–98% to H2 , at the high space velocity of 7.2 × 105 ml g−1 h−1 , with very high stability. No significant deactivation of the catalyst was observed for up to 96 h of reaction. However, no notable effect of the precursor of MgO was noted and possible explanations are provided. Temperature-programmed reduction (TPR) experiments indicated the presence of up to three kinds of rhodium-containing species, Rh2 O3 , Rh2 O3 -interacting with the support, and MgRh2 O4 , in the oxidized MgO-supported Rh catalysts. The strong interactions between rhodium and magnesium oxides are suggested to be responsible for the high stability of the catalyst. It was found that the methane conversion increased with the amount of catalyst when it was below 5.0 mg, but remained almost unchanged when it was greater than 5.0 mg. The catalytic assays also provided some information about the reaction mechanism. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Methane partial oxidation; Syngas; Rh catalyst; MgO

1. Introduction In a previous paper [1], the effect of the support on the performance of Rh-based catalysts in the partial oxidation of methane to synthesis gas was investigated. It was found that, in general, the reducible oxide supports provided much lower methane conversions and selectivities to CO and H2 than the irreducible ones. Among the irreducible metal oxides, MgO provided the highest catalytic activity, with high ∗ Corresponding author. Tel.: +1-716-645-2911/2214; fax: +1-716-645-3822. E-mail address: [email protected] (E. Ruckenstein)

selectivities and stability. It was suggested that the strong interactions between rhodium and magnesium oxides (especially the formation of MgRh2 O4 ) were responsible for the high stability of MgO-supported Rh catalyst. In the present paper, the effect of the precursor of magnesium oxide on the partial oxidation of methane over the MgO-supported Rh catalysts was investigated. It was found that even though the precursor affected to notable extents the dispersion of rhodium, the reduction profile of the oxidized catalyst and the resistance to deactivation caused by carbon deposition during CH4 decomposition, it had almost no effect on the partial oxidation reaction.

0926-860X/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 4 9 5 - 0

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E. Ruckenstein, H.Y. Wang / Applied Catalysis A: General 198 (2000) 33–41

2. Experimental 2.1. Catalyst preparation 2.1.1. Preparation of MgO supports The following five magnesium-containing precursors were used: magnesium nitrate hexahydrate (Mg(NO3 )2 ·6H2 O), magnesium hydroxide (Mg(OH)2 ), and magnesium peroxide (MgO2 · xMgO) (Aldrich); magnesium carbonate (Major) (4MgCO3 ·Mg(OH)2 ·4H2 O), and magnesium citrate (MgC6 H6 O7 ·xH2 O) (Alfa). MgO supports were prepared by calcination in the open air of a furnace at 800◦ C for 4 h. 2.1.2. Preparation of MgO-supported Rh catalysts The supported rhodium catalysts were prepared by impregnating the support with a 0.10 M Rh(NO3 )3 ·2H2 O (Alfa) ethanol solution for 1 h, followed by overnight drying at 110◦ C and calcination in the open air of a furnace at 800◦ C for 4 h. The calcined catalysts will be denoted Rh(O)/MgO. After reduction with H2 the catalysts will be denoted Rh/MgO. The reduced catalysts had a 1 wt.% Rh loading. 2.2. Catalytic reaction All the catalysts were tested under atmospheric pressure in a fixed-bed vertical quartz reactor (ID 3 mm), which was operated in a down flow mode. In the reactor, the catalyst was held on a quartz wool bed and the temperatures were monitored using two thermocouples: one placed in the catalyst bed (the end of the thermocouple was just below the top surface of the catalyst bed) and the other one after the quartz wool bed to measure the temperature of the gas phase. For the reactions carried out at a Tfurnace of 500◦ C, the catalysts were reduced in an H2 flow (20 ml/min) by increasing the temperature from room temperature to 500◦ C and holding at 500◦ C for 0.25 h; for those at a Tfurnace of 750◦ C, the reduction was carried out by increasing the temperature from room temperature to 750◦ C at 20◦ C/min in an H2 flow (20 ml/min) without holding at 750◦ C. After reduction, the feed gases (CH4 /O2 = 2) were introduced into the catalyst bed at a flow rate of 60 ml/min, which for 5.0 mg of supported catalyst corresponds to a space velocity

of 7.2 × 105 ml g−1 h−1 . The reactants and products were analyzed with an on-line gas chromatograph equipped with Porapak Q and 5A molecular sieve columns. An ice-cold trap was set between the reactor exit and the GC sampling valve to remove the water formed during reaction. Methane (99.97% purity, from Matheson) and oxygen (99.9% purity, from Cryogenic) were used without further purification. The gases were premixed before being introduced into the reactor. At room temperature, the upper limit of flammability of the methane/oxygen mixture is at about 60% methane, but rises significantly as the temperature increases [3]. In the absence of catalyst, the blank runs with quartz wool showed almost no methane conversion below 700◦ C, and about 0.5% and 4.0% of methane conversions at 750◦ C and 850◦ C, respectively. Consequently, the contribution of the gaseous phase reaction can be neglected under the conditions employed in the present paper. 2.3. Catalyst characterization 2.3.1. Surface area The surface areas of the calcined catalysts were determined via nitrogen adsorption on a Micromeritics ASAP2000 instrument. The samples were degassed at 200◦ C for at least 5 h in high vacuum before measurements. 2.3.2. Rhodium dispersion The Rh dispersion of the reduced catalyst was determined via CO chemisorption at room temperature by assuming a 1/1 stoichiometry. 50 mg of calcined catalyst powder, held on a quartz wool bed in a vertical quartz tube reactor (ID 4 mm), was reduced in an H2 flow (20 ml/min) at 650◦ C for 1 h. Further, at the same temperature, the reduced catalyst was purged with an ultra high purity helium flow (35 ml/min) for 1 h. After the temperature was cooled down to room temperature, CO (10 ␮l per pulse) was pulsed over the catalyst until no further adsorption of CO was detected. The CO left in CO chemisorption was determined quantitatively with a thermal conductivity detector (TCD). Before use, the hydrogen and helium were additionally purified, as in all other experiments, by employing Hydro-Purge II and Oxy-Trap columns.

E. Ruckenstein, H.Y. Wang / Applied Catalysis A: General 198 (2000) 33–41

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Table 1 BET surface areas of magnesium oxides prepared from various precursors MgO

Precursor and preparation condition

BET surface area (m2 /g)

MgO-1 MgO-2 MgO-3 MgO-4 MgO-5

Mg(NO3 )2 ·6H2 O, calcined in air at 800◦ C for 4 h 4MgCO3 ·Mg(OH)2 ·4H2 O, calcined in air at 800◦ C for 4 h MgC6 H6 O7 ·xH2 O, calcined in air at 800◦ C for 4 h Mg(OH)2 , calcined in air at 800◦ C for 4 h MgO2 ·xMgO, calcined in air at 800◦ C for 4 h

4.7 38.2 29.8 33.1 35.2

2.3.3. Temperature-programmed reduction (TPR) TPR of the calcined catalyst was conducted by heating the sample (50.0 mg), held on a quartz wool bed in a vertical quartz tube reactor (ID 4 mm), from 50 to 850◦ C, at a rate of 20◦ C/min in a flow of 2.5% H2 /Ar mixture (35 ml/min). The hydrogen consumed in TPR was determined with a TCD. Before each TPR experiment, the sample was pretreated by heating it up to 800◦ C in an air flow (30 ml/min), and maintaining that temperature for 1 h, unless otherwise indicated. 2.3.4. X-ray powder diffraction (XRD) X-ray powder diffraction (XRD) was carried out on a Siemens D500 X-ray diffractometer, using the Cu K␣ radiation, at 40 kV and 30 mA. 2.3.5. Methane decomposition over the reduced catalyst The decomposition of pure methane over the reduced catalyst was carried out in a pulse microreactor. A quartz tube (ID 4 mm), in which the calcined catalyst was held on a quartz wool bed, was employed as reactor. In each experiment, 50.0 mg of catalyst was used and the pulse volume was 250 ␮l of CH4 . The catalyst was reduced at 650◦ C for 1 h in an H2 flow (20 ml/min) before reaction. During experiment, a carrier gas, helium, (35 ml/min) was allowed to flow through the reactor, and CH4 was injected in the carrier gas. The products were analyzed with an on-line gas chromatograph equipped with a TCD and a Porapak Q column. 3. Results 3.1. Physical characterizations of MgOs and 1 wt.% Rh(O)/MgO catalysts As shown in Table 1, five precursors were used to prepare magnesium oxide and its formation was

identified by XRD (Fig. 1). The surface area decreased in the sequence: MgO-2 > MgO-5 > MgO-4 > MgO-3  MgO-1. The MgO prepared from magnesium carbonate had the largest surface area, while that obtained from magnesium nitrate the smallest one. With the exception of MgO-3, which had a light gray color, all other ones were white. The gray color of MgO-3 was probably caused by the carbon generated from [C6 H6 O7 ]2− during the high-temperature calcination. The surface areas of the calcined 1 wt.%Rh(O)/MgO catalysts are listed in Table 2, which shows that most of them have a somewhat lower surface area than the corresponding magnesium oxides, with the exception of 1 wt.% Rh(O)/MgO-1, which has a somewhat larger surface area. The Rh dispersions on the reduced 1 wt.% Rh/MgO catalysts are also listed in Table 2. The Rh dispersion decreased in the sequence: MgO-2 > MgO-5 > MgO-4 > MgO-1 > MgO-3. For most of them, the larger the surface area of the support, the higher the Rh dispersion of the catalyst; MgO-3 constitutes an exception. 3.2. Reduction characteristics of the oxidized MgO-supported Rh catalysts The results of the TPR experiments over the oxidized MgO-supported Rh catalysts are presented in Fig. 2, which shows that: (1) the 1 wt.% Rh(O)/MgO-1 provides two peaks, one at about 280◦ C and the other one at about 680◦ C; (2) the 1 wt.% Rh(O)/MgO-2 provides a very flat peak; (3) the 1 wt.% Rh(O)/MgO-3 and the 1 wt.% Rh(O)/MgO-4 exhibit a single peak between 530 and 550◦ C; (4) the 1 wt.% Rh(O)/MgO-5 exhibits two peaks, one at about 200◦ C and the other one at about 510◦ C. The effect of calcination temperature on the TPR profile was investigated and the results for the 1 wt.% Rh(O)/MgO-1 catalyst are presented in Fig. 3, which

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E. Ruckenstein, H.Y. Wang / Applied Catalysis A: General 198 (2000) 33–41

Fig. 1. X-ray diffraction patterns of MgO. (a) MgO-1, (b) MgO-2, (c) MgO-3, (d) MgO-4, (e) MgO-5.

shows that the increase in the calcination temperature from 650 to 950◦ C, decreases the intensity of the high-temperature reduction (HTR) peak, and shifts the low-temperature reduction (LTR) peak from about 220 to about 300◦ C.

carbon-containing product. The amount of surface carbon (C) generated during CH4 decomposition was calculated from the carbon balance. The yield of surface carbon is presented in Fig. 4, together with the CH4 conversion, as a function of the number of CH4 pulses. Excluding the first pulse, the CH4 conversion decreases with increasing number of pulses. Over the 1 wt.% Rh/MgO-1, the decay of CH4 conversion experiences two stages: the first is rapid and the second is slow (Fig. 4a). Over the other catalysts, it experiences three stages, a slow one, followed by a rapid one and finally again by a slow one (Fig. 4a). Fig. 4b

3.3. Methane decomposition over the reduced MgO-supported Rh catalysts The decomposition of CH4 pulses over the reduced MgO-supported Rh catalyst was investigated at 800◦ C. CO was detected as the only gaseous Table 2 Physical data for 1 wt.% Rh/MgO catalysts Support

Surface areaa (m2 /g-cat.)

Rh dispersionb (%)

Rh surface areab (m2 /g-cat. × 100)

MgO-1 MgO-2 MgO-3 MgO-4 MgO-5

6.0 36.2 25.6 29.9 32.6

14.0 25.0 12.0 16.7 17.5

64.7 115.5 55.4 77.2 80.8

a b

After calcination in air at 800◦ C for 4 h. After reduction in an H2 flow (20 ml/min) at 650◦ C for 1 h.

E. Ruckenstein, H.Y. Wang / Applied Catalysis A: General 198 (2000) 33–41

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Fig. 2. TPR profiles of the oxidized 1 wt.% Rh(O) supported on MgO-1 (a), MgO-2 (b), MgO-3 (c), MgO-4 (d) and MgO-5 (e).

Fig. 4. CH4 conversions (a) and surface carbon (C) yields (b) in the decomposition of pure CH4 over the reduced 1 wt.% Rh supported on MgO-1 (䊏), MgO-2 (䊉), MgO-3 (䉱), MgO-4 (䉲), and MgO-5 (䉬) at 800◦ C.

Fig. 3. Effect of calcination temperature on the TPR profile of the 1 wt.% Rh(O)/MgO-1. (a) 650◦ C, (b) 750◦ C, (c) 850◦ C and (d) 950◦ C.

shows that similar patterns occur for the C yield. It is clear that the coverage of the metallic sites by carbon species is responsible for the decay of methane conversion observed in Fig. 4a. From Fig. 4a and b, one can conclude that for the reduced MgO-supported Rh catalysts, the resistance to deactivation due to carbon deposition decreases in the sequence: MgO-2 > MgO-5  MgO-4 > MgO-3  MgO-1. One may notice that the first two have relatively large surface areas and the last the smallest surface area.

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Table 3 Activities of 1 wt.% Rh/MgO catalysts for methane partial oxidationa Support

Temperature (◦ C) F/B/Gb

CH4 conversion (%)

CO selectivity (%)

H2 selectivity (%)

H2 /CO ratio

MgO-1

500/667/495 750/801/740

59.6 84.7

76.9 95.6

90.1 97.9

2.34 2.05

MgO-2

500/667/498 750/819/747

59.0 82.6

75.0 95.1

87.2 97.5

2.32 2.05

MgO-3

500/662/509 750/816/750

57.7 85.7

74.7 95.7

85.96 97.8

2.30 2.04

MgO-4

500/646/490 750/804/743

63.4 86.3

80.4 95.7

89.8 98.2

2.23 2.05

MgO-5

500/633/500 750/791/738

60.7 83.4

77.6 95.6

87.5 95.9

2.25 2.00

a b

Data obtained after 24 h of reaction; 5.0 mg catalyst, 7.2 × 105 ml g−1 h−1 space velocity. F denotes furnace, B catalyst bed and G gas phase.

3.4. Partial oxidation of methane over the reduced MgO-supported catalysts 3.4.1. Effect of the precursor of MgO The partial oxidation of methane over the reduced MgO-supported Rh catalysts was investigated at 500 and 750◦ C (Tfurnace ) and the results are summarized in Table 3. The experiments were repeated and the reproducibility was within a few percents. At each of the temperature investigated, the five catalysts provided similar methane conversions and product selectivities. Thus, no notable effect of the precursor of MgO on the methane conversion could be found. A syngas with an H2 /CO ratio of about 2 was obtained at 750◦ C, and with a ratio of around 2.3 at 500◦ C. The effect of time-on-stream at a Tfurnace of 750◦ C is presented in Fig. 5, which clearly reveals that no significant catalyst deactivation occurred over any of the catalysts, for up to 96 h of reaction. In summary, at a Tfurnace of 750◦ C and atmospheric pressure, all the catalysts provided a conversion >80% and selectivities of 95–96% to CO and 96–98% to H2 at a space velocity of 7.2 × 105 ml g−1 h−1 (5.0 mg supported catalyst) with very high stability. This is in agreement with our previous results [1,2]. Hot layers were visually observed at the top of the catalyst bed in all reaction experiments. As shown in Table 3, the temperature of the catalyst bed measured by the in-bed thermocouple was always much higher than that determined by the thermocouple inserted af-

ter the quartz wool bed, and the latter temperature was close to that of the furnace. The temperature difference between the catalyst bed and gas phase was much higher for a Tfurnace of 500◦ C than for a Tfurnace of 750◦ C. At the temperatures measured in the catalyst bed, the observed methane conversions were less than the equilibrium values [4] in both cases. They were, however, much higher than the equilibrium value of 37.5% [4] at the gas phase temperature of 500◦ C (Tfurnace of 500◦ C) and close to the equilibrium value

Fig. 5. Effect of time-on-stream on the methane conversions of the 1 wt.% Rh/MgO catalysts. Tfurnace = 750◦ C, P = 1 atm, amount of catalyst = 5.0 mg, CH4 /O2 = 2.0, space velocity = 7.2 × 105 ml g−1 h−1 .

E. Ruckenstein, H.Y. Wang / Applied Catalysis A: General 198 (2000) 33–41

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4. Discussion

Fig. 6. Effect of the amount of catalyst on the methane conversion of 1 wt.% Rh/MgO-1 (a) and 1 wt.% Rh/MgO-5.(b) Data obtained after 24 h of reaction; reaction conditions: P = 1 atm, Tfurnace = 750◦ C, CH4 /O2 = 2.0 (60 ml/min).

of 86.0% [4] at the gas phase temperature of 750◦ C (Tfurnace of 750◦ C).

3.4.2. Effect of the amount of catalyst As shown in Fig. 6a, at a Tfurnace of 750◦ C, over the 1 wt.% Rh/MgO-1, the methane conversion increased with increasing amount of catalyst from 1.0 to 5.0 mg; it remained, however, almost unchanged when the amount of catalyst was increased from 5.0 to 20.0 mg. In each case, a syngas with an H2 /CO ratio of about 2 was obtained. As shown in Fig. 6b, the results observed over the 1 wt.% Rh/MgO-5 were quite similar to those over the 1 wt.% Rh/MgO-1.

As indicated by the XRD experiments (Fig. 1), all five magnesium-containing precursors listed in Table 1 were decomposed into magnesium oxide after the high-temperature calcination. The surface area of MgO was affected by its precursor (Table 1). With the exception of the 1 wt.% Rh/MgO-3 catalyst, the Rh dispersion increased with increasing support surface area. Over the 1 wt.% Rh/MgO-3 catalyst, which has a large surface area, the migration of the carbon present in the support to and over the surface of the metallic particles is most likely the reason for the decreased number of metallic rhodium sites exposed when compared to the 1 wt.% Rh/MgO-1 catalyst, which has the smallest surface area (as already noted, MgO-3 had a gray color, caused by the carbon generated from [C6 H6 O7 ]2− during calcination). The reduction profiles of the oxidized MgOsupported Rh catalysts were affected to some extent by the MgO precursor (Fig. 2). This indicates that the rhodium-oxide species formed during the calcination process were somewhat different from catalyst to catalyst. As previously reported [1,2], a magnesium rhodium oxide (MgRh2 O4 ) is formed over MgO-supported Rh catalyst after a high-temperature calcination, which is reduced with higher difficulty than rhodium oxide (Rh2 O3 ). The latter can be usually reduced at (or below) about 200◦ C [5–8]. It is therefore, reasonable to conclude that the LTR and HTR peaks observed over the 1 wt.% Rh(O)/MgO-5 catalyst are caused by Rh2 O3 and MgRh2 O4 , respectively. Similarly, the single peaks between 530 and 550◦ C observed over the 1 wt.% Rh(O)/MgO-3 and 1 wt.% Rh(O)/MgO-4 catalysts are caused by MgRh2 O4 . The very flat TPR peak observed over the 1 wt.% Rh(O)/MgO-2 catalyst might be due to various stages of interactions between rhodium and magnesium oxides. Unexpectedly, the LTR and HTR peaks appeared at much higher temperatures over the 1 wt.% Rh(O)/MgO-1 catalyst than over the 1 wt.% Rh(O)/MgO-5. To gain some insight about the nature of the peaks for the 1 wt.% Rh(O)/MgO-1, the effect of calcination temperature was investigated (Fig. 3). The decrease of the intensity of the HTR peak with increasing calcination temperature indicates that this peak is not due to MgRh2 O4 . By carrying out a TPR experiment with MgO-1, a single peak at about 720◦ C

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E. Ruckenstein, H.Y. Wang / Applied Catalysis A: General 198 (2000) 33–41

was detected. Consequently, the peak at about 680◦ C over the 1 wt.% Rh(O)/MgO-1 is most likely due to an impurity undetectable by XRD. The temperature of about 280◦ C of the LTR peak over the 1 wt.% Rh(O)/MgO-1 is higher than that of rhodium oxide but much lower than that of magnesium rhodium oxide. For this reason, we attribute this peak to a rhodium-oxide interacting with the support without formation of a definite compound [5]. The positive shift by about 80◦ C of this peak, observed when the calcination temperature was increased from 650 to 950◦ C, indicates that the higher the calcination temperature, the stronger the interaction generated between rhodium oxide and support. In conclusion, though the rhodium-containing species formed after high-temperature calcinations were somewhat different when different magnesium oxides were used, strong interactions were, in general, present between rhodium and magnesium oxides over the oxidized MgO-supported Rh catalysts. The strong interactions might be responsible for the absence of any significant deactivation of the catalyst for up to 96 h of reaction (Fig. 5). The sintering of the metal, one of the main reasons for the deactivation of the catalysts, was delayed by the strong interactions [9]. In the partial oxidation of methane, the carbon deposition is another important factor for catalyst deactivation. As indicated by the decomposition of pure methane (Fig. 4a and b), the resistance to deactivation due to carbon deposition was strongly dependent on the magnesium oxide used. The catalyst prepared from a magnesium oxide of large surface area was more resistant to the decay of the CH4 decomposition than that prepared from a magnesium oxide of small surface area. This probably occurred because of the higher dispersion of the carbon over the former substrate. Even though the number of metal sites exposed after reduction over catalysts prepared from different magnesium oxides were different (Table 2), Table 3 and Fig. 5 reveal that the precursor of MgO had no notable effect on the methane conversion in the partial oxidation of methane. The nearly equal methane conversions might occur either because the reaction is diffusion limited or even because the number of metallic sites was in each of the catalysts sufficiently large for thermodynamic equilibrium to be achieved at its exit. Indeed, at a Tfurnace of 750◦ C, the methane

conversions and product selectivities were close to the equilibrium values at the gas phase temperature at the bed exit, which was about 750◦ C for a catalyst bed of 5.0 mg and also about 750◦ C for catalyst beds of 1.0–20.0 mg. For the 1 wt.% Rh/MgO-1 and 1 wt.% Rh/MgO-5 catalysts, the effect on the methane conversion of the amount of catalyst at a constant gas flow rate for a Tfurnace of 750◦ C is presented in Fig. 6. For 1.0 mg of catalyst, the entire oxygen of the feed gas was consumed and the methane conversion became as high as about 75%, with an H2 /CO ratio of 2.0. As the amount of catalyst was increased, the CH4 conversion increased but remained almost unchanged when the amount of catalyst surpassed 5.0 mg. Consequently, the reactions occurred essentially within a thin layer of about 5.0 mg (or even less) at the beginning of the catalyst bed and equilibrium was reached. However, for Tfurnace = 500◦ C, the conversions and selectivities to CO and H2 were, much higher than the equilibrium values at the gas phase temperature at the exit of the wool bed (about 500◦ C). Because the temperature of the catalyst bed was by about 150◦ C higher than that of the gas phase (Table 3), it is clear that the heat released by the reaction played an important role in this case. It is therefore, likely that the combustion-reforming mechanism (an initial strong exothermic oxidation of CH4 to CO2 and H2 O, followed by the endothermic reforming of the remaining methane by CO2 and H2 O) [3,10–14] played a role. In the first step, a large amount of heat was released, which generated a hot layer at the top of the catalyst bed and the reforming reactions that followed proceeded at a temperature higher than that of the gas phase. Consequently, methane conversions much higher than the equilibrium value at the temperature of the gas phase (of about 500◦ C) were reached. Because for Tfurnace = 750◦ C, the temperature near the entrance of the catalyst bed differed much less from the former (by only 50–70◦ C), it is likely that the combustion-reforming mechanism played a less important role than in the previous case and that the pyrolysis-oxidation mechanism (a catalytic pyrolysis of methane, followed by oxidation) [15–19] played an important role. These conclusions are compatible with those reported in our previous paper [20], based on the CH4 /CD4 isotope effect in the methane partial oxidation in a pulse reactor. In that paper, we suggested

E. Ruckenstein, H.Y. Wang / Applied Catalysis A: General 198 (2000) 33–41

that over a 1.0 wt.% Rh/␥-Al2 O3 catalyst, a combination between the combustion-reforming and pyrolysis-oxidation mechanisms was responsible for the reaction at relatively low temperatures and the pyrolysis-oxidation mechanism became more important at high temperatures.

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750◦ C enabled us to suggest that at low temperatures the combustion-reforming mechanism played a role and that at high temperatures the pyrolysis-oxidation mechanism was more significant than at lower temperatures. References

5. Conclusions The effect of the precursor of MgO on the partial oxidation of methane over the MgO-supported Rh catalysts was investigated. Even though the precursor affected to notable extents the dispersion of rhodium, the reduction profile of the oxidized catalyst and the resistance to deactivation due to carbon deposition during CH4 decomposition, it had almost no effect on the partial oxidation reaction. The nearly equal methane conversions over the catalysts might have occurred either because the reaction was diffusion limited or because the number of metallic sites was in each of the catalysts sufficiently large for thermodynamic equilibrium to be achieved at the exit of the catalyst bed. By investigating the effect of the amount of catalyst, it was shown that at Tfurnace = 750◦ C the reactions occurred essentially within a thin layer of about 5.0 mg (or even less) at the beginning of the catalyst bed after which equilibrium was reached. All these MgO-supported Rh catalysts exhibited high performances (high conversion, selectivity and stability) in the partial oxidation of methane under the conditions employed. The strong interactions, as identified by TPR experiments, between rhodium and magnesium oxides might be responsible for the high stability of the catalyst. A comparison between the behaviors of the catalysts at furnace temperatures of 500 and

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