Synthesis and support composition effects on CH4 partial oxidation over Ni–CeLa oxides
Graphical abstract
Introduction
The production of synthesis gas from natural gas is an important step in the gas to liquid (GTL) technology, accounting for 50–75% of the capital cost. The syngas may be actually produced by different processes such as steam reforming (SR), dry reforming (DR), partial oxidation of methane (POM) and autothermal reforming (ATR) which is a combination of the endothermic reforming reaction (SR) and the exothermic oxidation reaction [1], [2], [3]. Dry reforming is an interesting and promising process since it contributes to the removal of the greenhouse CO2 present in the natural gas in a 2% amount [4]. However the difficulty to activate the stable CO2 molecule and to avoid catalyst poisoning by the large amount of carbon deposition still prevents its application at industrial level. In fact, each of the mentioned processes presents peculiarity in terms of the produced H2/CO ratio, carbon poisoning and energy efficiency. However, steam reforming (SR) and catalytic partial oxidation of methane (POM) are currently the preferred reactions to produce synthesis gas. With respect to the endothermic steam reforming, which is highly energetic and capital intensive [5], [6] the partial oxidation reaction (1)offers the following advantages: (a) H2/CO ratio of 2, suitable for methanol or Fischer–Tropsch synthesis; (b) being mildly exothermic the CPO process does not require large heat exchange reactors and needs a more compact plant technology [7]. Nevertheless, as for all the reactions involving conversion of methane the choice of a catalyst, able to activate the molecule, is crucial due to the high dissociation energy of the first CH3H(g) bond (440 kJ/mol) [8]. Noble metals are appropriate for this kind of reaction, exhibiting high activity/selectivity and long term stability. However, since noble metals are rather expensive their use is prohibitive for industrial application. A more convenient alternative is represented by nickel which is also active but less stable, suffering from carbon deposition and metal sintering [5], [9]. Still, in terms of cost and availability, the nickel based catalysts are the most interesting systems and much research is being addressed to their stability improvement [9]. To this respect the choice of an appropriate support is very important as the supports affect the metal dispersion, inhibit particle sintering and in the case of basic character they may minimize carbon formation [10]. Different types of oxides like Al2O3, MgO, La2O3 and CeO2 have been investigated as nickel supports for CPO applications [11], [12], [13], [14], [15], [16], [17]. These oxides differ for oxygen mobility and for their different degree of interaction with nickel, both properties affecting the catalyst reducibility and their final catalytic behavior. According to the literature no definite conclusion on the most suitable support can be drawn. Indeed, the use of a specific support has yielded different catalytic results related to various factors such as the precursor of the catalyst components, the preparation methods, the nickel loading and the CPO reaction conditions.
The objective of the present study was to analyze the effect of the synthesis procedure and the effect of the nature of the support on the catalytic behavior of Ni over CeO2, La2O3 and mixed oxide CeO2–La2O3. The two oxides were selected because of their peculiar properties. CeO2 is characterized by large oxygen mobility, playing an important role in the oxidation type of reaction [18], [19]. La2O3 is characterized by a strong chemical interaction with nickel, forming several La–Ni oxide phases and therefore stabilizing the metal and possibly avoiding carbon formation [20], [21]. The catalytic results in terms of methane conversion and CO selectivity were related to the structural and chemical properties of the samples investigated by XRD, XPS and TPR techniques.
Section snippets
Support preparation
La2O3, CeO2 and La2O3/CeO2(Ce/La = 50/50 wt%) oxides were prepared by (co)-precipitation from the corresponding nitrate precursors using K2CO3 to precipitate the corresponding hydroxides. After washing several times to remove the potassium and drying at 100 °C, the obtained solids were calcined at 800 °C for 4 h.
Catalyst preparation
Two series of catalysts were prepared by following two procedures. One series was prepared by wet impregnation (WI) consisting of impregnating the supports with Ni(NO3)2 aqueous solution, of
Catalyst activity
Preliminary tests were performed on reduced and on just oxidized samples. The reduced one were pretreated in H2/Ar at 750 °C as described above. The oxidized ones were simply treated in 5% O2 in He at 400 °C for 30 min. As shown in Fig. 1 for the selected sample, Ni–CeLa(WI), the methane conversion and the CO selectivity during a long run at 800 °C were definitively better on the reduced sample as compared to the unreduced one. Based on these results, all the catalytic tests reported here were
Conclusion
The structural investigation on the Ni catalysts supported over CeO2, La2O3 and mixed La2O3–CeO2 indicated different interaction between the nickel and the support oxides modulated by the preparation procedure. In the case of the La2O3 oxide, the strong chemical interaction between nickel and lanthanum yielded a variety of mixed oxides going from LaNiO3 to La3Ni12O7. The interaction occurred with both, wet impregnation and co-precipitation, procedures. On the contrary, the CeO2 support did not
Acknowledgments
The Bilateral Collaboration Program supported by Italian CNR and Indian CSIR is kindly acknowledged.
The authors are thankful to Dr. Francesco Giordano from ISMN for performing the XRD measurements.
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