Al2O3 catalysts for partial oxidation of methane to syngas*

NATURAL GAS CONVERSIONV Studies in Surface Science and Catalysis,Vol. 119 A. Parmalianaet al. (Editors) 9 1998 Elsevier Science B.V. All rights reserved.

855

Reactivity of Pt/A1203 and Pt/CeO2/A1203 catalysts for partial oxidation of methane to syngas* Qian-Gu Yan a

Wei Chu a

Li-Zhen Gao a

Zuo-Long yuat

Song-Yue Yuan b

aChengdu Institute of Organic Chemistry, The Chinese Academy of Sciences Chengdu, Sichuan 610041 P.R.China bChangchun Institute of Applied Chemistry, The Chinese Academy of Sciences Changchun, Jilin 130022 P.R.China

Abstract: The ceria modified Pt/CeO2/A1203 and Pt/A1203 catalysts were studied in the partial oxidation of methane to syngas. The SEM, XRD, TPR and TPD techniques were used for the catalyst characterization. The addition of ceria could enhance the Pt dispersion and decrease the Pt crystallise size; the activity and selectivity of catalyst for partial oxidation were improved significantly, and the methane total oxidation was suppressed sharply.

The ceria

effect was also discussed in a detailed way.

Keywords:

Partial oxidation of methane (POM), Syngas, Pt catalyst, Ceria promoter,

Characterization INTRODUCTION Partial oxidation of methane (POX) to produce syngas has more potential than the steam reforming (SR) process. In POX process, supported metal catalysts, including noble metals and transition metal catalysts are often used [1'2]. Under the operation conditions, supported Pt catalyst showed great stability and carbon deposition resistance, however, it showed lower conversion and selectivities than Rh and Ni, especially for H2 selectivity. Cerium oxide and CeO2-containing materials have been used as catalysts and as structural and electronic promoters for heterogeneous catalytic reactions since the past decades. Ceria is added as a promoter to noble metals in many reactions. Several possible mechanisms of these promoting effects have been proposed, such as ceria affecting the dispersion of supported metals, promoting the WGS reaction and SR reaction, increasing the thermal stability of the support, and preventing the sinter of the metal particles [3"~5]. The potential of CeO2 to oxide hydrocarbons has been known for several years [6]. Total oxidation of hydrocarbons on ceria * This project is supported by the National Natural Science Foundation of China, No. 29573128. t To whom correspondence should be addressed.

856 have been examined. Several studies on methane oxidation have been conducted using CeO2 supported on A1203 or SiO2 with noble metals.

This study reports the effect of ceria on

Pt/A1203 catalyst for partial oxidation of methane to syngas.

The TPR and H2-TPD

techniques were used to elucidate the effect of ceria on Pt/A1203 catalyst and the possible mechanism. EXPERIMENTAL

Preparation of catalysts The support used in this study was alpha alumina (5 m2/g) with the particle size of 40-50 mesh. Six catalysts were prepared with different amounts of ceria of 0, 0.5, 1.0, 2.0, 4.0 and 8.0%(wt). The incipient wetness impregnation method was used. The support was first impregnated with a solution of cerium nitrate, dried at 383 K for 5 h, calcined at 873 K for 10 h and cooled, then it was impregnated with a solution of Pt(NH3)4(NO3)2 and dried at 423 K for 5 h. The platinum amounts are all 1.0 %(wt).

Test of catalyst The catalytic performance tests were carried out in a fixed quartz microreactor (8 mm i.d.). The apparatus has been described previously [7]. The experimental conditions for catalyst performance testing were as follows: 250 mg catalyst, total space velocity of 2• 105 h "l, mole ratio of CH4/O2=2, reaction system pressure of 0.05 MPa, temperature range 1023-~1073 K.

Catalyst characterization In the TPR and TPD of this study, 100 mg catalyst sample was placed into a quartz tube (3 mm o.d.). The sample was preheated in situ with He at 423 K for 1 h and then cooled to room temperature. A gas stream of 8%(V) H2 in argon at a total flow rate of 40 mL/min passed through the catalyst. The temperature was increased at a rate of 10 K/min from 303 K to 1023 K. The water produced by reduction was trapped into a column of silica gel. The amount of H2 consumption was detected with a thermal conductivity detector (TCD). The reduced sample was cleaned at 1023 K in He for 2 h, then a pure hydrogen flow of 30 mL/min (purified before using) passed through the catalyst for 0.5 h. The catalyst sample was cooled to room temperature in pure hydrogen stream, then the catalyst was blown with He to remove the excess gas. Desorption experiments were carried out with He flowing at 30 mL/min by increasing catalyst bed temperature from 303 K to 1023 K at the rate of 20 K/rain. SEM was carried out on a Jeol TXA-800 scanning electron microscopy. The Pt crystal sizes of catalysts were measured on a powder X-ray diffractometer (D/max-rB).

857 RESULTS AND DISCUSSION TPR and TPD experimentals The TPR profiles of catalysts are shown in Fig. 1. A blank test of CeO2/AI203 sample was performed for the TPR analysis, there were two reduction peaks: one peak was around 773 K, the higher was at 923 K. The peak area was bigger when the ceria content was higher. For the Pt/AI203 catalyst, there was only one peak at 458 K; for the Pt/CeO2/A1203 catalyst with different content of ceria, the reduction profiles were illustrated in Fig. 1. It was shown that the presence of Pt decreased the low temperature peak of CeO2 on alumina to a range between 483 K and 623 K; and this peak became broader and more multiple when the ceria content augmented; that of Pt was almost unchanged. As it was knows, the platinum could activate hydrogen and this activation could be enough to decrease the reduction temperature of ceria through a spillover mechanism. In other aspect, some of the reduced ceria could cover a part of platinum.

d C

9 _4..___

373

473

573 673 773 Temperatm'e, K

_

_

873

~

973

Fig.1 TPRspectra of Pt/Al203 and Pt/CeO2/A1203 catalysts (a) 0%CeO2, (b) 0.5%CEO2, (c) 1.0% CeO2 (d) 2.0%CEO2, (e) 4.0% CeO2, (f) 8.0%CEO2 The TPD profiles of catalysts with H2 were discussed. In the TPD experiment, a blank test of CeO2/AI203 sample was operated and there was only one peak at 823 K for H2 desorption, and it was explained as that the ceria could behave as a container of hydrogen which desorbed at 823 K. The profiles of Pt/A1203 and Pt/CeO2/AI203 catalysts were detected. Three broad peaks were shown for the Pt/A1203 catalysts, at 393 k, 623 K and 803K respectively. When the ceria was added, there were some obvious changes: (a) a much .broader region between 373 K and 723 K was obtained rather

858 than two peaks at 393 K and 623 K, this new desorption could be from Pt surface with a ceria environment. Another explanation could be that during the reduction the partially reduced CeO2-Ce203 could move and cover the surface of platinum and this could diminish the desorption amount of hydrogen at low temperature. (b) however, the H2 desorption amount increased with the content of ceria at high temperature of 823 K. SEM and XRD analyses

Table 1 Pt crystallite sizes of catalysts before and after reaction Pt particle size/A CeO2 / % (wt) 0 0.5 1.0 2.0 4.0 8.0

Before reaction 210 115 90 85 83 80

After reaction 282 121 92 86 87 83

The dispersion of Pt over catalyst was studied using SEM-EDX and it was found that the Pt on Pt/CeO2/AI203 is well-distributed but it is bad-distributed on Pt/A1203. The Pt crystallite size was calculated from the half-width of the Pt (111) peak in XRD curve of the catalysts (Table 1). From Table 1, we find that the Pt crystallite size on Pt/CeO2/AI203 is smaller than that on Pt/AI203, which indicates that ceria can decrease the Pt crystallite size and inhibit Pt crystallite growth during reaction. This may be derived from the high temperature reduction, partially reduced ceria species are able to migrate through the Pt particles to separate them to smaller ones or to cover part of the Pt particle surface. Methane partial oxidation reaction

The results of catalytic performance testing are shown in Table 2. From Table 2, we can find the improvement both in catalytic activity and in selectivity of products, especially in H2 selectivity over Pt/CeO2/AI203 catalyst with increasing of the amount of Ce. The effect was obvious at the beginning and became slight after the addition of 1% CeO2. The measured axial temperatures along catalyst bed (center temperature was all at 1023 K) were compared. Over the CeO2/A1203 sample, the combustion activity of methane oxidation was very high and the center temperature could increase sharply to more than 1273 K. For the unpromoted Pt/AI203 catalyst, there was the selective oxidation of methane to CO and H2, together with a part of total oxidation to CO2 and H20. And the temperature at the inlet could

859 arrive to 1171 K. When the ceria was added, there were both dispersion effect and electronic effect; the activity of total oxidation was suppressed and the temperature at the inlet arrived only at 1109 K when 1% CeO2 was added. Table 2 The conversion and selectivity (at 1023 K and 1073 K) of catalyst in the partial oxidation of methane to syngas 1023 K

1073 K

%(wt)

C(CH4) %

S(CO) %

S(H2) %

C(CH4) %

S(CO) %

S(H2) %

0 0.5 1.0 2.0 4.0 8.0

751 80 5 82 0 82 8 83.0 82.5

86.5 89.0 90.5 91.0 91.2 91.5

80.3 84.6 86.0 87.2 87.1 87.0

83.8 88.2 89.5 90.2 90.6 90.4

90.2 93.3 93.4 93.0 93.3 93.0

84.5 90.6 91.2 92.8 91.8 92.2

CeO2

L.D.Schmidt [1] pointed out that the monolith-supported Pt catalyst exhibited less activity and H2 selectivity than Rh and Ni catalysts for partial oxidation reaction. It was shown the total oxidation ( combustion reaction ) activity of methane over Pt/A1203 catalyst decreased significantly as ceria was added, CH4 conversion and the selectivity of partial oxidation products were increased.

S.H.OH [6] found that the total oxidation of CH4 over

Pt/CeO2/A1203 was suppressed. Discussion of ceria effect

According to the results given above, the following discussions are proposed. The strong interaction between Pt and ceria may result in a larger extent of boundary area between cerium oxide and platinum. We believe that the active sites involve both Pt and ceria. It was found that during the SR process or under the reductive atmosphere, some of the cerium were in the state of Ce(III) [8] Ce 3+ sites formed after reduction of Pt based catalyst, and those sites are at the metal -ceria interface, the lower-valence-state cerium may dissociatively adsorb oxygen or water, and the resulting adspecies -O or -OH may be transferred to adjacent platinum and reacted with surface carbon species (CHx, x=0-~3) to give out CO, CO2, H2 and H20. So the activity of Pt/CeO2/A1203 is higher than Pt/A1203. Otsuka |9] reported that cerium oxide itself can oxidize methane to syngas. Though its activity is several orders lower than other transition metal catalysts, however, when noble metals coexist with cerium oxide, the reaction may be much accelerated. The mechanism of redox cycle of CeO2 is proposed as follows:

860 CeO2 + nCH4 CeO2-n + n/2 02

~ CeO2-n +nCO + 2nil2

(1)

~ CeO2

(2)

When the amount of ceria was increased on the surface of catalyst, and following hightemperature reduction, CeOx species are able to migrate through the Pt particles to separate them to smaller ones or to cover part of the particle surface, the activation energy of CH4 on Pt would increase, or the chemisorption sites of CH4 on Pt decrease, therefore, in some extent, the dissociative adsorption of CH4 onto Pt would be suppressed. So, the reaction over Pt/CeO2/A1203 is not as violent as that over Pt/Al203, this depressed the combustion reaction of methane over the catalyst, avoided high temperature for local catalyst bed and decreased the temperature gradient of catalyst bed (decreased more than 100~

The selectivity increase for

Pt/CeO2/A1203 catalyst should be resulted from the inhibition of the total oxidation activity of methane over catalyst. The suppressing of total oxidation to avoid high temperature benefits also to the stability of catalyst. Moreover, another role of ceria in this reaction is promoting the water-gas-shifted (WGS) reaction by a redox cycle mechanism, which benefits the increase in selectivity to hydrogen. REFERENCES

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