MgO solid solution catalysts — effect of calcination temperature and Co loading

Applied Catalysis A: General 209 (2001) 207–215

CO2 reforming of CH4 over Co/MgO solid solution catalysts — effect of calcination temperature and Co loading H.Y. Wang, E. Ruckenstein∗ Department of Chemical Engineering, State University of New York at Buffalo, Amherst, NY 14260, USA Received 9 June 2000; received in revised form 4 August 2000; accepted 5 August 2000

Abstract The effect of calcination temperature on the CO2 reforming of CH4 over Co/MgO of various Co loadings was investigated and the results can be summarized as follows: (i) the catalysts with Co loadings between 8 and 36 wt.% precalcined at 500 or 800◦ C provided high and stable activities; (ii) for Co loadings between 4 and 48 wt.%, the high calcination temperature of 900◦ C yielded low activities or very long activation periods; (iii) a high Co loading (48 wt.%) combined with a calcination temperature of 500 or 800◦ C resulted in unstable activities. Depending on the calcination temperature and Co loading, one, two or three of the following Co-containing species, Co3 O4 , MgCo2 O4 , and (Co, Mg)O (solid solution of CoO and MgO) were identified by combining temperature programmed reduction and X-ray diffraction (XRD) results. Their reducibility decreased in the sequence: Co3 O4 > MgCo2 O4 > (Co, Mg)O. It is suggested that the high and stable activities observed over most catalysts (except the 48 wt.% one) precalcined at 500 or 800◦ C as well as the low activities or the long activation periods observed over the 900◦ C calcined catalysts were induced by the formation of a solid solution between CoO and MgO. The activity decay observed over the 48 wt.% Co catalyst precalcined at 500 or 800◦ C was most likely caused by the large metallic particles, formed particularly through the reduction of Co3 O4 and MgCo2 O4 . These particles sintered and stimulated coke formation. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon dioxide reforming of methane; Synthesis gas; Co catalyst; Effect of calcination temperature; Effect of Co loading

1. Introduction Currently, the catalytic steam reforming is used for converting methane to synthesis gas (mixture of H2 and CO). This process produces, however, a synthesis gas with a too high ratio of H2 :CO (>3) which is unsuitable for the Fischer–Tropsch synthesis of liquid hydrocarbons. The CO2 reforming of CH4 which produces a synthesis gas with a H2 :CO ratio close to 1, appears to be a more suitable one. Numerous supported metal catalysts have been tested for this reaction including Ni and Co-based cata∗ Corresponding author. E-mail address: [email protected] (E. Ruckenstein).

lysts as well as supported noble metals (Rh, Ru, Pd, Pt, and Ir) [1–15]. The main problem encountered was the rapid deactivation caused by carbon deposition and/or sintering of the metal. In general, the noble-metal based catalysts can provide operations with lower carbon deposition. However, considering their high cost, it is desirable to employ non precious metals. In a previous paper [16], the effect of support on the performance of Co-based catalyst was investigated and it was found that the 12 wt.% Co/MgO catalysts provided a CO yield of 93% and a H2 yield of 90% at the high space velocity of 60,000 ml g−1 h−1 , with very high stability. It was suggested that the suppression of carbon deposition and the resistance to sintering of this catalyst were induced by the formation of

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a solid solution between CoO and MgO, because both have a NaCl type structure and close lattice parameters and bond distances [17]. The formation of a solid solution is expected to depend on the Co loading, calcination atmosphere, calcination temperature, and calcination time. To obtain information about their effect, experiments have been carried out in which Co loading was varied between 4 and 48 wt.%, and the calcination temperature between 500 and 900◦ C. Temperature-programmed reduction (TPR) and X-ray diffraction (XRD) were performed to characterize the structural properties of the calcined catalysts and CO chemisorption to determine the exposed metal surface areas of the reduced catalysts. The results showed that a solid solution between CoO and MgO was indeed formed in the calcined catalysts. However, depending upon the calcination temperature and Co loading, Co3 O4 and Co2 MgO4 could also be generated. The reduction behavior and the catalytic performance are affected by the species formed in the catalysts and their amounts. Possible explanations for the obtained results are provided.

2. Experimental 2.1. Catalyst preparation The MgO supported Co catalysts were prepared by impregnating MgO (32 m2 /g) with aqueous solutions of Co(NO3 )2 ·6H2 O, followed by overnight drying at 110◦ C and calcination in the open air of a furnace for 8 h at 500, 800 or 900◦ C. The calcined catalysts are denoted as Co(O)/MgO (500, 800 or 900◦ C), and the catalysts reduced in H2 are denoted as Co/MgO (500, 800 or 900◦ C). The temperature inside the parenthesis indicates the calcination temperature. The Co loading means wt.% Co in the completely reduced catalyst. 2.2. Catalytic reaction All the catalysts were tested under atmospheric pressure in a fixed-bed vertical quartz reactor (i.d. 3 mm) with the catalyst held on a quartz wool bed. The reactor was operated in a down flow mode, and the temperatures were monitored using two thermocouples: one located in the middle of the catalyst bed and the other one immediately below the quartz wool

bed to measure the temperature of the gas phase. The catalyst was reduced in a H2 flow (20 ml/min) by increasing the temperature from room temperature to 600◦ C at a rate of 20◦ C/min and then to 900◦ C at a rate of 10◦ C/min without holding at 900◦ C. After reduction, the feed gases (CH4 :CO2 = 1:1) were allowed to flow through the catalyst bed at a rate of 20 ml/min. For a 20.0 mg catalyst, this corresponds to a space velocity (SV) of 60,000 ml h−1 g−1 . The reactants and products were analyzed with an on-line gas chromatograph (GC) equipped with a Porapak Q column. An ice-cold trap was located between the reactor exit and the GC sampling valve to remove the water formed during reaction. In the absence of the catalyst, the blank runs with quartz wool and two thermocouples indicated that at 900◦ C, the CH4 conversion was below 3%. Consequently, the contribution of the homogeneous reaction was insignificant under the conditions employed. 2.3. Catalyst characterization 2.3.1. CO adsorption The exposed Co metal surface area of the reduced catalyst was determined by CO pulse adsorption in a quartz tube (i.d. 4 mm) at room temperature. Even though CO might also have been adsorbed in bridged and twinned forms besides the linear one, a 1:1 stoichiometry of CO adsorption on Co was assumed. The sample (50 mg) of calcined catalyst powder was held on a quartz wool bed in a vertical quartz tube reactor and its reduction was carried out as already described. The reduced catalyst was purged with an ultra high purity helium flow (35 ml/min) at 900◦ C for 0.25 h. After the sample was cooled to room temperature, CO was injected as pulses (10 ␮l per pulse) into the helium flow until no further adsorption of CO was detected. The CO left after CO adsorption was determined quantitatively with a thermal conductivity detector (TCD). Before use, H2 and He were purified using Hydro-Purge II and Oxy-Trap columns. 2.3.2. Temperature-programmed reduction During each TPR run, a high-purity 2.5% H2 /Ar mixture (35 ml/min) which was additionally purified using Hydro-Purge II and Oxy-Trap columns was passed over the calcined samples in a vertical quartz tube reactor (i.d. 4 mm). The sample (10.0 mg, unless

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otherwise indicated) was held on a quartz wool bed. The temperature of the sample was increased from 40 to 1000◦ C at a rate of 20◦ C/min. The water, formed during reduction was trapped in a Hydro-Purge II column. The hydrogen consumed in TPR was monitored continuously with a TCD which was calibrated using known amounts of CoO and Co3 O4 . 2.3.3. X-ray powder diffraction The XRD patterns, obtained on a Siemens D500 X-ray diffractometer instrument with a Cu K␣ radiation at 40 kV and 30 mA were used to identify the major phases present in the calcined catalysts.

3. Results 3.1. Carbon dioxide reforming of methane The initial (after 0.5 h on-stream) catalytic activities (conversions of CH4 and CO2 and yields to CO and H2 ) as well as the temperatures of the catalyst and the gas phase are listed in Table 1. With the exception of the 4 wt.% Co/MgO (800◦ C), the catalysts

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precalcined at 500 and 800◦ C provided initially high activities (CH4 conversion >75%) and H2 :CO ratios close to 1.0. For all the catalysts precalcined at 900◦ C, the initial activities were low (CH4 conversion <20%) and the H2 :CO ratio was much lower than 1.0 but both characteristics increased with increasing loading. The time-dependent activities are plotted in Figs. 1–3 (only the conversion of CH4 was plotted because that of CO2 and the yields to CO and H2 followed the same pattern with time on stream). The results can be summarized as follows: (i) at a calcination temperature of 500◦ C, the catalysts with Co loadings between 4 and 36 wt.% provided high and stable activities; deactivation with time on stream was, however, observed over the 48 wt.% catalyst; (ii) at a calcination temperature of 800◦ C, the catalysts with a Co loading between 8 and 36 wt.% yielded high and stable activities; deactivation with time on stream was, however, observed over the 48 wt.% catalyst; (iii) at a calcination temperature of 900◦ C, the catalysts with a Co loading ≤12 wt.% provided low activities (CH4 conversion <15%), the 24 wt.% catalyst needed a very long activation period to reach a high steady-state activity (85%), and the 36 and

Table 1 Effect of calcination temperature and Co loading on initial (after 0.5 h) activity of Co/MgO for CO2 reforming of methane at a Tfurnace of 900◦ Ca Co loading (wt.%)

Calcination temperature (◦ C)

Temperature (◦ C) bed/gas

CH4 conversion (%)

CO2 conversion (%)

CO yield (%)

H2 yield (%)

H2 :CO ratio

4

500 800

887/895 895/898

85.7 7.6

87.6 17.4

86.7 12.6

84.3 2.5

0.97 0.20

8

500 800 900

878/894 881/891 894/891

94.7 76.0 5.3

96.7 77.8 12.9

95.7 77.0 9.1

91.8 74.6 1.5

0.96 0.97 0.16

12

500 800 900

870/890 870/894 896/892

95.7 91.9 5.5

97.4 93.9 11.9

96.6 92.9 8.7

92.4 89.8 2.2

0.96 0.97 0.25

24

500 800 900

865/892 860/894 894/893

95.5 93.0 11.1

97.1 94.9 22.8

96.3 94.0 17.0

93.6 90.9 4.9

0.97 0.97 0.29

36

500 800 900

868/890 857/890 897/894

95.2 94.0 14.2

97.3 95.9 26.7

96.3 95.0 20.5

91.9 91.7 7.9

0.95 0.96 0.39

48

500 800 900

866/890 861/891 895/894

96.0 96.1 19.2

97.3 97.2 37.6

96.7 96.6 28.4

94.2 94.4 9.8

0.97 0.98 0.34

a

Catalyst: 20.0 mg; flow rate: 20 ml/min (CH4 :CO2 =1:1); space velocity: 60,000 ml h−1 g−1 .

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Fig. 1. Effect of Co loading on CH4 conversion as a function of time on stream over the Co/MgO catalysts precalcined at 500◦ C.

Fig. 3. Effect of Co loading on CH4 conversion as a function of time on stream over the Co/MgO catalysts precalcined at 900◦ C.

48 wt.% catalysts yielded high and stable activities (93 and 87%, respectively) after a 10 h activation period. Due to the endothermicity of the reforming reaction, the temperature of the catalyst bed is expected to be lower than that of the gas phase. Indeed, this occurred over all the catalysts (Table 1) for high conversions. At low conversions (as for the 900◦ C calcined catalysts), the temperature of the bed was a little higher. Because at low conversions, the conversion of CO2 was much higher than that of CH4 , it is likely that a side reaction related to CO2 (such as the reoxidation of the metallic sites) took place.

3.2. Physico-chemical characterization 3.2.1. BET surface area of the calcined catalyst and the exposed metallic Co surface area of the reduced catalyst The surface area of some calcined catalysts was determined by the BET method. The results obtained are listed in Table 2 which shows that at a given Co loading the surface area decreased with increasing calcination temperature. After the calcined catalysts were reduced with H2 , the exposed metal surface areas were determined by CO chemisorption and the results are listed in Table 3. For the Co/MgO (500◦ C) catalysts, the Co surface area increased with increasing Co loading from 4 to 36 wt.%; for the 48 wt.% catalyst it was comparable to that for the 36 wt.% catalyst. For the Co/MgO (800◦ C) catalysts, the exposed Co surface area increased with increasing Co loading, more moderately between 8 and 36 wt.%. For the Co/MgO (900◦ C) catalysts, those with Co loadings between 8 and 24 wt.% Table 2 The BET surface area of some Co(O)/MgO catalysts precalcined at different temperatures (m2 /g-catalyst)

Fig. 2. Effect of Co loading on CH4 conversion as a function of time on stream over the Co/MgO catalysts precalcined at 800◦ C.

Co loading (wt.%)

Calcination temperature (◦ C) 500

800

900

24 48

58 48

20 12

11 3

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211

Table 3 Exposed Co surface area of the reduced Co/MgO catalysts precalcined at different temperatures ((m2 /g-catalyst) × 100) Co loading (wt.%)

Calcination temperature (◦ C) 500

800

900

4 8 12 24 36 48

38.3 73.4 95.3 155.6 200.6 198.3

5.8 10.9 11.2 12.7 13.2 30.3

–a 8.9 8.7 8.2 3.0 2.4

a

Not available.

possessed larger exposed metal surface areas than those with loadings of 36 and 48 wt.%. For the same Co loading, the metal surface area decreased in the sequence: Co/MgO(500◦ C) >> Co/MgO(800◦ C) > Co/MgO(900◦ C). 3.2.2. Reduction characteristics of the calcined supported Co catalysts The TPR traces of the calcined supported Co catalysts, presented in Figs. 4–6 were strongly dependent on both the calcination temperature and the Co load-

Fig. 4. The TPR profiles of the Co(O)/MgO (500◦ C) catalysts with Co loadings of (a) 4 wt.% (20.0 mg of catalyst), (b) 8 wt.%, (c) 12 wt.%, (d) 24 wt.%, (e) 36 wt.%, and (f) 48 wt.%.

Fig. 5. The TPR profiles of the Co(O)/MgO (800◦ C) catalysts with Co loadings of (a) 4 wt.% (20.0 mg of catalyst), (b) 8 wt.%, (c) 12 wt.%, (d) 24 wt.%, (e) 36 wt.%, and (f) 48 wt.%.

ing. A low temperature reduction peak between 260 and 300◦ C and a high temperature reduction peak between 660 and 690◦ C could be observed for most of the Co(O)/MgO (500◦ C) catalysts (Fig. 4). For

Fig. 6. The TPR profiles of the Co(O)/MgO (900◦ C) catalysts with Co loadings of (a) 8 wt.%, (b) 12 wt.%, (c) 24 wt.%, (d) 36 wt.%, and (e) 48 wt.%.

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Table 4 Reduction degree (%) of the calcined catalysts after TPR Co loading (wt.%)

12 24 36 48 a

Calcination temperature (◦ C) 500

800

900

22 24 32 45

3 3 14 39

–a 1 2 6

Too small to be detected.

all the Co(O)/MgO (800◦ C) catalysts, a peak started to form at a temperature higher than 800◦ C; an additional low temperature reduction peak between 380 and 430◦ C was observed only over the catalysts with Co loadings of 36 and 48 wt.% (Fig. 5). For all the Co(O)/MgO (900◦ C) catalysts, a peak started to form at a temperature higher than 800◦ C (Fig. 6). Table 4 lists the reduction degrees of the calcined catalysts in percentage. In all cases, the reduction degree was much less than 100%. At each calcination temperature, the reduction degree increased with increasing Co loading; for the same loading, it decreased with increasing calcination temperature. 3.2.3. Compounds present in the calcined supported Co catalysts The Co-containing species present in the calcined catalysts were identified by XRD. The data and the assignments of the XRD patterns for some Co(O)/MgO catalysts are listed in Table 5. For the 48 wt.% Co(O)/MgO (500◦ C), the XRD patterns can be assigned to Co3 O4 and/or MgCo2 O4 as well as to (Co, Mg)O (a solid solution between CoO and

MgO) and/or MgO. Since Co3 O4 can be reduced in H2 below 500◦ C [18,19] and as shown later a complete reduction of (Co, Mg)O requires a temperature higher than 1000◦ C, the peak at about 290◦ C can be attributed to Co3 O4 and the one at about 690◦ C to MgCo2 O4 . Though only two reduction peaks were observed for this catalyst (Fig. 4f), its reduction degree of only 45% (Table 4) suggested that (Co, Mg)O which is much less reducible was also present besides Co3 O4 and MgCo2 O4 . The absence of a TPR peak for the solid solution was most likely due to the insufficiently high upper-limit temperature during the TPR experiment. Because for the calcination temperature of 500◦ C and Co loadings between 12 and 48 wt.% all TPR spectra are similar (Fig. 4) and the reduction degree is much less than 100% (Table 4), one can conclude that the Co(O)/MgO (500◦ C) catalysts with Co loadings ≥12 wt.% contain (as did the 48 wt.% one) Co3 O4 , MgCo2 O4 and (Co, Mg)O. For the 24 wt.% Co(O)/MgO (800◦ C), the XRD patterns (Table 5) could be attributed to (Co, Mg)O) and/or MgO. The TPR spectrum (Fig. 5d) which exhibited a reduction peak that started to form at a temperature higher than 800◦ C, allowed one to conclude that a solid solution was formed. Similarly, the XRD patterns (Table 5) coupled with the TPR spectrum (Fig. 5f) indicated that Co3 O4 and (Co, Mg)O were present in the 48 wt.% Co(O)/MgO (800◦ C) catalyst. Among the 800◦ C calcined catalysts, the TPR spectra for loadings between 4 and 12 wt.% are similar to that for 24 wt.%, and the TPR spectrum for the 36 wt.% is similar to that for the 48 wt.% (Fig. 5). Consequently, Co3 O4 and (Co, Mg)O were present in the 36 and 48 wt.% Co(O)/MgO (800◦ C) catalysts but only (Co, Mg)O in the Co(O)/MgO (800◦ C) catalysts with Co loadings ≤24 wt.%. For the 48 wt.% Co(O)/MgO

Table 5 Data and assignments of XRD patterns of calcined Co(O)/MgO catalysts Co loading (wt.%)

Pre-treatment, (calcined for 8 h)

d (Å)

24 48

at 800◦ C at 500◦ C

48

at 800◦ C

48

at 900◦ C

2.110, 2.456, 2.038; 2.111, 1.429, 2.110,

Assignments 1.493, 1.272, 1.218, 2.438 1.440, 2.879, 1.568, 2.098, 1.488, 2.456 1.493, 1.274, 1.219, 2.438; 2.438, 2.860, 1.556, 2.022, 1.652 1.490, 1.272, 1.218, 2.435

(Co, Mg)O, MgO Co3 O4 , MgCo2 O4 ; (Co, Mg)O, MgO (Co, Mg)O, MgO; Co3 O4 (Co, Mg)O, MgO

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(900◦ C), only (Co, Mg)O was identified by coupling XRD (Table 5) with TPR (Fig. 6e) results. From the similarity of the TPR spectra, one can infer that only (Co, Mg)O was present in all Co(O)/MgO (900◦ C) catalysts.

4. Discussion 4.1. Effects of calcination temperature and Co loading on the species present in the calcined catalysts Depending upon the calcination temperature and Co loading, XRD and TPR provided evidence for the existence of one, two or three of the following compounds: Co3 O4 , MgCo2 O4 and (Co, Mg)O. At the calcination temperature of 900◦ C, only the solid solution (Co, Mg)O was present in all the catalysts. At the low calcination temperature of 500◦ C, CoO (formed through the decomposition of Co(NO3 )2 ) either diffused into the MgO matrix to form a solid solution or was oxidized to Co2 O3 ; the latter reacted either with CoO to form Co3 O4 or with MgO to generate MgCo2 O4 . At the high calcination temperature of 800◦ C, Co3 O4 and MgCo2 O4 partially decomposed because they are unstable at high temperatures; a complete solid solution was formed in those with loadings ≤24 wt.%, but the Co3 O4 phase was still present for loadings ≥36 wt.% and its amount increased steeply with increasing Co loading from 36 to 48 wt.%. At the high calcination temperature of 900◦ C, the Co3 O4 phase was no longer present and (Co, Mg)O was the only Co-containing phase in all the catalysts. Regarding the reduction characteristics of Co3 O4 , MgCo2 O4 and (Co, Mg)O, Co3 O4 can be reduced below 500◦ C, and MgCo2 O4 below 700◦ C (Fig. 4). However, a temperature higher than 1000◦ C was required to reduce the solid solution (Co, Mg)O (Figs. 5 and 6). This happened because of the irreducibility of MgO and because both Co and Mg share the oxygen in the solid solution which thus becomes more strongly bound. Because the diffusion of CoO which generates the solid solution was deeper at higher temperatures, the reduction degree of the catalysts with the same loading decreased with increasing calcination temperature (Table 4).

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4.2. Effect of calcination temperature on the exposed metal surface area of the reduced catalysts Table 3 shows that for any Co loading the exposed metal surface area decreased with increasing calcination temperature. This is caused by the decreases in the reducibility and in the surface area of the catalyst. Regarding the former, the increase in the calcination temperature from 500 to 800◦ C tremendously decreased the amounts of Co3 O4 and MgCo2 O4 which are much more reducible than the solid solution. The moderate decrease from 800 to 900◦ C for Co loadings ≤24 wt.% occurred because in this range only a solid solution was present and CoO diffused deeper in the support at the higher temperature. The notable decrease from 800 to 900◦ C for Co loadings ≥36 wt.% occurred because the catalysts contained only a solid solution at the calcination temperature of 900◦ C but also Co3 O4 at the 800◦ C one. 4.3. Effects of calcination temperature and Co loading on the activity and stability of the catalysts The present results show that most of MgO supported Co catalysts calcined at 500 and 800◦ C provided high and stable activities for the CO2 reforming of CH4 . However, the high calcination temperature of 900◦ C decreased tremendously the activities of the catalysts with loadings ≤12 wt.%, and generated long activation periods for those with loadings ≥24 wt.%. The high Co loading of 48 wt.% combined with a calcination temperature of 500 or 800◦ C resulted in an unstable activity; a stable activity was, however, reached at the high calcination temperature of 900◦ C. At the calcination temperature of 500◦ C, most of the catalysts provided high and stable activities; significant deactivation, however, occurred over the 48 wt.% catalyst (Fig. 1). The XRD and TPR indicated that Co3 O4 and MgCo2 O4 which are more reducible than the solid solution were present in most Co(O)/MgO (500◦ C) catalysts and that their amounts increased notably with increasing Co loading (Fig. 4). Thus, a large number of metallic sites were generated during reduction particularly over the 48 wt.% catalyst. During reaction, the metallic sites were oxidized by CO2 and H2 O and reduced by CH4 and H2 . At the beginning, the oxidation is expected to dominate. The oxidized sites formed diffused during reaction into the MgO

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matrix in which they are soluble particularly if the CoO content of the matrix is not too high. This process is expected to be rapid because of the high reaction temperature (900◦ C). At steady-state which is reached after a short time (<0.5 h) by most catalysts (except the 48 wt.% one), the rate of site oxidation becomes equal to that of site reduction. The formation of the solid solution is responsible for the stability of these catalysts for reasons explained later in the paper. The rapid deactivation observed over the 48 wt.% catalyst (Fig. 1) occurred most likely because large amounts of Co3 O4 and MgCo2 O4 were present in the calcined catalyst. Being more reducible than the solid solution, they generated large metal clusters which stimulated coke formation. In addition, due to the relatively weak interactions between these clusters and the substrate, enhanced sintering could occur easily. Indeed, one can see from Table 3 that the Co surface area of the reduced 48 wt.% Co(O)/MgO (500◦ C) was near that of the 36 wt.% catalyst. This is an indication that larger metallic clusters were generated over the former after reduction. The sintering and the coke formation are responsible for the deactivation observed over the 48 wt.% catalyst precalcined at 500◦ C. The 48 wt.% Co/MgO (800◦ C) catalyst was unstable (Fig. 2) for the same reason (a large amount of Co3 O4 was present in the calcined catalyst). The Co/MgO (800◦ C) catalysts with a Co loading between 8 and 24 wt.% provided high and stable activities (Fig. 2). As indicated by XRD and TPR, a solid solution (Co, Mg)O was present in the calcined catalyst (Table 5 and Fig. 5). The high stability can be explained as follows. Due to the inherent difficulty of reducing a solid solution, the size of the metal clusters generated via the reduction of the solid solution is expected to be small. Being small, these clusters did not stimulate coke formation. Being formed from a solid solution, the clusters were at least partially embedded in the substrate, and thus were more resistant to sintering than the crystallites on the conventional supports (such as Al2 O3 , SiO2 , etc.). As indicated by XRD and TPR, Co was present in all the Co(O)/MgO (900◦ C) catalysts solely as (Co, Mg)O (Table 5 and Fig. 6). Table 3 shows that their exposed Co surface area decreased with increasing Co loading while Table 1 reveals that the conversions of CH4 and CO2 after 0.5 h increased with increasing loading. This means that the activity attained after 0.5 h was

not directly related to the exposed Co surface area of the fresh reduced catalyst. Most likely this occurred because the number of metallic sites was too small and most of them were oxidized by CO2 , thus, the system became catalytically less active. Only after a sufficiently large number of sites were generated by CH4 and/or H2 through reduction, could the catalyst become more active. For a solid solution catalyst, the generation of metallic sites is strongly affected by the calcination temperature and the Co loading. At the same calcination temperature, the lower the Co loading the smaller is its concentration near the surface and the fewer the metallic sites formed through reduction during reaction. As a result, the catalysts with higher Co loadings can be more easily activated than those with lower ones. This is the reason why the 24 wt.% Co/MgO (900◦ C) catalyst had a much longer activation period than the 36 and 48 wt.% catalysts (Fig. 3).

5. Summary In summary, when the active component of the catalyst forms a solid solution with the substrate, the pretreatment of the catalyst plays a major role regarding its structural properties and the reduction behavior, and hence the catalytic performance. The variables investigated in this paper are the calcination temperature used during the preparation of the impregnated catalyst and the metal loading. The results demonstrate that the Co/MgO solid solution catalysts with loadings between 8 and 36 wt.%, precalcined at 500 and 800◦ C, provide high and stable activities for the CO2 reforming of CH4 . However, a too high calcination temperature (e.g. 900◦ C) may cause a low activity or a long activation period, and a too high Co loading (48 wt.%) combined with a calcination temperature ≤800◦ C may result in an unstable activity. References [1] A.M. Gadalla, B. Bower, Chem. Eng. Sci. 43 (11) (1988) 3049. [2] J.T. Richardson, S.A. Paripatyadar, Appl. Catal. 61 (1990) 293. [3] A.T. Ashcroft, A.K. Cheetham, M.L.H. Green, P.D.F. Vernon, Nature 352 (1991) 225. [4] J.R. Rostrup-Nielsen, J-H.B. Hansen, J. Catal. 144 (1993) 38.

H.Y. Wang, E. Ruckenstein / Applied Catalysis A: General 209 (2001) 207–215 [5] A. Erdöhelyi, J. Cserényi, F. Solymosi, J. Catal. 141 (1993) 287. [6] E. Ruckenstein, Y.H. Hu, Appl. Catal. A: Gen. 133 (1995) 149. [7] Z.L. Zhang, X.E. Verykios, J. Chem. Soc., Chem. Commun. 71 (1995). [8] V.C.H. Kroll, H.M. Swaan, C. Mirodatos, J. Catal. 161 (1996) 409. [9] Y.H. Hu, E. Ruckenstein, Catal. Lett. 36 (1996) 145. [10] R.N. Bhat, W.M.H. Sachtler, Appl. Catal. A: Gen. 150 (1997) 279. [11] H.Y. Wang, C.T. Au, Appl. Catal. A: Gen. 155 (1997) 239.

215

[12] S.M. Stagg, E. Romeo, C. Padro, D.E. Resasco, J. Catal. 178 (1998) 137. [13] J.H. Bitter, K. Seshan, J.A. Lercher, J. Catal. 183 (1998) 336. [14] K. Tomishige, Y. Chen, K. Fujimoto, J. Catal. 181 (1999) 91. [15] H.Y. Wang, E. Ruckenstein, Appl. Catal. A: Gen. 204 (2000) 143. [16] E. Ruckenstein, H.Y. Wang, Appl. Catal. A: Gen. 204 (2000) 257. [17] I. Náray-Szabó, Inorganic Crystal Chemistry, Akadémiai Kiadó Budapest, 1969, 237 pp. [18] K.S. Chung, F.E. Massoth, J. Catal. 64 (1980) 320. [19] P. Arnoldy, J.A. Moulijn, J. Catal. 93 (1985) 38.