Industrial & Engineering Chemistry Research - ACS Publications

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7498

Ind. Eng. Chem. Res. 2009, 48, 7498–7504

KINETICS, CATALYSIS, AND REACTION ENGINEERING Syngas Production via CO2 Reforming of Methane over Sm2O3-La2O3-Supported Ni Catalyst W. D. Zhang, B. S. Liu,* Y. P. Zhan, and Y. L. Tian Department of Chemistry, School of Science, Tianjin UniVersity, Tianjin 300072, P.R. China

CO2 reforming of methane into syngas over Sm2O3-La2O3 (SL)-supported Ni catalyst was investigated in a fixed-bed quartz reactor and the best catalytic activity was observed over Ni/SL (sol-gel) due to the reduction of the perovskite LaNiO3 and spinel La2NiO4 to form small nickel particles. The conversion of CH4 and CO2 over Ni/SL(sol-gel) were 55% and 57%, respectively, the selectivity of H2 and CO were 96% and 98%, significantly higher than those over Ni/SL (imp) catalyst at 700 °C and a GHSV of 4.8 × 104 mL · h-1 · g-1cat. In the meantime, the Sm2O3-La2O3-supported Ni catalysts were characterized by means of techniques, such as BET, XRD, H2-TPR, TG/DTA, HRTEM, XPS, and XAES. 1. Introduction

2. Experimental Section

In recent years, there has been an increasing interest in the catalytic transformation of carbon dioxide, a greenhouse gas, into more useful or valuable chemicals. Especially important is the CO2 reforming of methane, the cheapest carbon-containing feed, into synthesis gas (i.e., CO + H2), which can be used in chemical energy transmission systems or utilized in the Fischer-Tropsch synthesis to produce liquid hydrocarbons.1,2 The CO2/CH4 reforming has been studied over numerous supported metal catalysts, such as Ni-based3-7 and noble metal catalysts.8,9 However, when one considers the high cost and limited availability of noble metal, it is more practical, from the industrial standpoint, to choose nickel as the active component in catalytic reforming catalysts. But coke formation is a major problem of Ni catalyst in CO2/CH4 reforming reactions.10 In many literatures, rare earth oxides are usually added to the nickel-based catalysts for CO2/CH4 reforming, as a promoter to optimize the activity of the catalysts.11-14 Gallego et al.11 studied the CO2/CH4 reforming over La2NiO4 catalyst, indicated that perovskite La2NiO4 caused the formation of small nickel particles with an average diameter of 7 nm which would be responsible for the high activity. Recently, we reported that the samarium oxide could stabilize the formation of Ni nanometer particles to prevent carbon deposition while the Ni/ Sm2O3/CaO catalyst exhibited excellent stability within time on stream of 100 h.15 The strong interaction of CO2 with basic La2O3 resulted in CO2 decomposition and the generation of CO and O; the latter reacted with the surface carbon species (CHx) to form CO.3,6,16,17The CH4/CO2 reforming reaction over Nibased catalyst18 has been investigated in pilot plant scale.

2.1. Catalyst Preparation. The Ni/SL (sol-gel) catalyst was prepared from Ni(NO3)2 · 6H2O, La (NO3)3 · 6H2O and Sm2O3 by means of a sol-gel technique. Nickel nitrate (0.50 g), lanthanum nitrate (0.52 g) and Sm2O3 (0.70 g) were dissolved in small amount of dilute nitric acid, and then citric acid and ethylene glycol were added to the solution. The molar amounts of citric acid and ethylene glycol were 1.5 times that of the total metal ions. The solution was then heated to 60 °C with constant stirring. A translucent green gel was formed after the removal of water by evaporation. Next, the Cogel obtained was aged and dried in a beaker at room temperature (RT) for 3 days. Finally, the gel was calcined in air at 500 and 800 °C for 5 h, respectively according to report in the literature.6 This sol-gel generated catalyst, 10% Ni/(Sm2O3)0.77(La2O3)0.23 will be referred to as “Ni/SL (sol-gel)” hereafter. For comparison, 10% Ni/ (Sm2O3)0.77(La2O3)0.23 catalyst was prepared by impregnating Sm2O3-La2O3 (0.90 g) with a solution of Ni(NO3)2 · 6H2O (0.50 g). After the excess water was evaporated at 80 °C, the catalyst was calcinated using the same procedure mentioned above. This impregnated catalyst will be referred to as “Ni/SL (imp)” hereafter. All catalysts were finally pressed, crushed and sieved through 40-60 meshes. 2.2. Catalyst Characterization. The specific surface area, average pore diameter and pore volume of catalysts were determined using N2 as the adsorbate on a NOVA-1200 instrument at -196 °C. Prior to the measurement, the sample was outgassed in a flowing N2 at 350 °C for 2 h. The morphology of fresh and used Ni/SL (sol-gel) catalysts was observed by a high resolution transmission electron microscopy (HRTEM, TECNAI G2 F20, Philip) operated at 200 kV. Samples were prepared by ethanol dispersion and transferred to a carbon-coated copper grid. The powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a BDX 3300 diffractometer with a Cu KR radiation source (λ ) 0.154056 nm) at 30 kV and 20 mA. The average size of Ni particles was calculated using Scherrer equation. The temperature-programmed reduction (TPR) was performed in a quartz microreactor and gas chromatograph equipped with a thermal conductivity detector (TCD). The sample was heated linearly to 800 °C at a rate of 10 °C min-1 in 8% H2/N2 atmosphere (60 mL/

On the basis of those ideas, we prepared a new Sm2O3-La2O3 supported Ni catalyst which could proper for the dispersion of nickel and increase the selectivity of CO and H2 as well as suppress the formation of coke. The aim of this work is to investigate the activity and stability of Sm2O3-La2O3 supported Ni catalyst for CO2 reforming of methane. * To whom correspondence should be addressed. E-mail: bingsiliu@ tju.edu.cn. Tel.: +86-22-27892471. Fax: +86-22-87892946.

10.1021/ie9001298 CCC: $40.75  2009 American Chemical Society Published on Web 07/20/2009

Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009

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Figure 1. Pore size distributions of fresh Ni/SL (imp), fresh and used Ni/ SL (sol-gel).

min). Thermogravimetric (TG) and differential thermal analysis (DTA) for used Ni/SL (sol-gel) catalyst was carried out in air (30 mL/min) with a STA 409 PC analyzer (NETZSCH Instruments Co. Ltd.). About 8 mg of sample was heated from RT to 800 °C at 15 °C /min. X-ray photoelectron spectroscopy (XPS) was adopted for surface analysis. Each powder sample was pressed onto a piece of double-sided sticking tape and mounted on the sample probe; the sample was then outgassed (∼10-5 Torr) before being transferred into the analyzer chamber (