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Flame pyrolysis prepared catalysts for the steam reforming of ethanol
No full textIntroduction
Among the methods to produce H2, Steam Reforming (SR) is one of the most common and feasible to use [1-7]. One challenge for SR at high temperature is catalyst deactivation by sintering, so that high thermal resistance is a pressing need. By contrast, it is envisaged to operate at lower temperature, to lower the heat input to the reactor. Nevertheless, catalyst deactivation may be impressive by coking, due to the formation of carbon filaments and occurs mainly over big nickel particles [8]. Additional coking may occur over acidic sites of the support. Therefore, an appropriate catalyst formulation should be found, which allows to reach the highest catalytic performance at low operating temperature (i.e. ca. 500°C) together with proper resistance.
The aim of this work is the synthesis of catalysts for the SR of ethanol. A set of catalysts was synthesized by Flame Spray Pyrolysis (FP) and another set was prepared by impregnation of the active phase on the FP-prepared support. This high temperature synthesis was here adopted to impart suitable thermal resistance to the samples and to provide a high metal support interaction, which showed a pivotal importance to improve resistance towards coking.
2 Experimental/methodology
A first set of samples was directly prepared by FP, inserting in one-step the Ni-based active phase into the selected support, TiO2 and La2O3 with different metal loading of 5, 10 and 15 wt%. For comparison, the same formulations have been prepared by impregnation of Ni on the FP prepared supports. The catalysts were reduced for 1h at 800°C in a 20 vol% H2 / N2 gas mixture.
The samples characterization was carried out by conventional methods of XRD, BET, SEM, EDX, TEM and TPR.
Finally the activity test of the samples were performed by means of a micro pilot plant constituted by an Incoloy 800 continuous down flow reactor heated by an electric oven. Catalyst activation was accomplished by feeding 50 cm3/min of a 20 vol% H2/N2 gas mixture, while heating by 10 °C/min up to 800 °C, then kept for 1 hour. For activity testing 0.017 cm3/min of a 3:1 (mol/mol) H2O:CH3CH2OH liquid mixture were feed to the reactor by means of a HPLC pump (Waters, mod. 501). The activity tests were carried out at atmospheric pressure, GHSV=2.500 h-1 (referred to the ethanol + water gaseous mixture) at 500, 625 and 750 °C. The analysis of the out-flowing gas was carried out by a gas chromatograph (Agilent, mod. 7980) ca. 8h.
3 Results and discussion
Nickel-based catalysts at three different loadings (5, 10 and 15wt%), supported over lanthana and titania were synthesized and tested for ethanol steam reforming at 500 and 750 °C. All of them were more active and stable at the latter temperature while at the former the impregnated catalysts with low Ni loading exhibited low H2 productivity, mainly due to unreformed CH4. By contrast, the FP ones demonstrated superior catalytic activity and satisfactory stability, especially with lanthana support, which effectively reduced deactivation by coking at the lowest operating temperature.
The catalytic activity has been correlated to metal dispersion and to the metal-support interaction strength. Both parameters affected also catalyst resistance to coking at 500°C. Overall, lanthana demonstrated and interesting support due to its basic character, which prevented significant coke formation related to the acidic properties of the support. Furthermore, high metal dispersion and proper stabilization on the support allowed to limit the formation of carbon nanofilaments as deactivation mode.
References
[1] Ni, M., Leung, D.Y.C., Leung, M.K.H., Int. J. Hydrogen Energy, 2007, 32, 3238.
[2] Muroyama, H., Nakase, R., Matsui, T., Eguchi, Eguchi, K., Int. J. Hydrogen Energy 2010, 35, 1575.
[3] Llorca, J., Homs, N., Sales, J., De la Piscina, P.R., J. Catal. 2002, 209, 306.
[4] Fatsikostas, A.N., Verykios, X.E., J. Catal. 2004, 225, 439.
[5] Cheekatamarla, P.K., Finnerty, C.M., J. Power Sources 2006, 160, 490.
[6] Frusteri, F., Freni, S., Spadaro, L., Chiodo, V., Bonura, Donato, S., Catal. Commun. 2004, 5, 611.
[7] Wang, C.B., Lee, C.C., Bi, J.L., Siang, Liu, J.Y., Yeh, C.T., Catal. Today 2009, 146, 76.
[8] Yoshida, H. Yamaoka, R., Arci, M., J. Mol. Science, 2015, 16, 350-362
Flame pyrolysis prepared catalysts for the steam reforming of ethanol: effect of support basicity
No full textA set of Ni catalysts supported over ZrO2 for the steam reforming of ethanol were prepared by two different procedures to evaluate the influence of the preparation method on the catalytic performance. Flame pyrolysis (FP) is a one step high temperature synthesis and usually imparts a strong metal-support interaction, besides high thermal resistance. It was compared with traditional precipitation/impregnation method (multistep synthesis). The samples were thoroughly characterized by AAS, XRD, N2 adsorption-desorption, TPR, SEM, TEM and FT-IR. Activity testing for the steam reforming of ethanol was performed after activation at 500°C in H2 flow by means of a continuous micropilot plant, by feeding a 3:1 (mol:mol) mixture of water/ethanol. Testing has been carried out at 500°C, 400°C and 300°C for 8 h-on-stream. Spent catalysts were characterized by TPO, TEM, FE-SEM and Raman analysis. Very high activity has been observed at high reaction temperature (>600°C), but it would be interesting to operate under milder conditions, in order to lower the energy input to the process and to improve H2 productivity by favoring the water gas shift reaction. The main issue at low operating temperature is coke accumulation, which leads to fast catalyst deactivation. The catalytic performances, in terms of stability towards coking, were related to the metal–support interaction. Acidity was tuned by doping ZrO2 with alkali and alkali-earth oxides, helping to prevent ethanol dehydration and coking by ethylene polymerization. The FP method allowed to prepare samples very resistant to coking, and alkali doping was a successful strategy to improve catalyst life
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