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Catalysts

The catalysts of more general use for the methane and methanol steam reforming are formed of Ni or noble metals supported on MgO [4]. Although the same catalysts can also work for the partial oxidation reactions, the use of mixed oxides of La and other metals (Fe, Co, Ni, etc.) minimizes the coke formation improving the metal dispersion [7], The methanol and ethanol reforming have been also studied on mixed catalysts based on Cu [8], which operate with high yields in C0X at even T is lower than 600 K. Some catalysts, specifically used in the past for the synthesis of the alcohols, are, also, suitable for the decomposition of the ethanol into carbon dioxide and hydrogen. This group comprises catalysts based on Zn, Cr, chromium/manganese/zinc oxides, opportunely doped by alkaline metals. Further examined catalysts, based on Cu0/Zn0/Al203 (BASF S3-85), seem applicable to the ethanol steam reforming, but they show lower stability to the molten carbonate fuel cell temperature. At last, catalysts based on silver are still used for the dehydrogenation of the ethanol into acetaldehyde.

The use of an high operative pressure, although not recommended from a thermodynamic point of view, allows the interposition of metallic or ceramic membranes between the catalyst and the anode, reducing in such a way the poisoning of the catalyst due to the electrolyte alkali vapours. The investigation on the employment of membranes, began since the sexties, demonstrated that they are able to catalyze the dehydrogenation of hydrocarbons or the steam reforming reactions, when they supported some appropriate catalysts.

Results and discussion In Fig.2 are reported the gas reformed compositions as a function of the steam/ethanol molar ratio, ranging from 1.0 to 3.5, that represent the typical behaviour of the selectivity yielded by the ethanol steam reforming reaction. The hydrogen production enhances with this ratio, but it presents a maximum at S/EtOH of about 2.0. Otherwise, the increase of S/EtOH depresses the production of CO and CHi, because it moves the reaction equilibrium towards a further production of hydrogen. Furthermore, it is opportune to consider that the risk of carbon deposition, on the active catalyst sites, increases heavily when values of S/EtOH lower than 2.0 will be applied. In Fig.3 are reported the polarization curves calculated for a molten carbonate fuel cell with direct internal reforming of ethanol. The curves have been determined considering a flow of ethanol corresponding to a fuel utilization coefficient equal to 75% for a current density of 160 mA/cm2 and for operational temperature ranging from 873 K to 973 K.

As well as other type of fuel cell, the influence of the temperature on the cell performance is significant also for the determination of the open circuit cell voltage. In fact, this value decreases of about 32 mV/100 K. Anyway, for every considered temperature, the limit current results equal to about 270 mA/cm2. TTie operational pressure does not influence the cell voltage. In fact, its increase, unadvisable from a thermodynamic point of view (see above), enhances the electrochemical cell reaction, through a rise in the partial pressure of the reactants. These two effects are compensated and the overall results is a difference of few millivolts in the cell potential. Thus, a rise in the operational pressure of 1 to 5 atm, produces a detrimental effect for current densities lower than 50 mA/cm2, while for higher current densities values there is an increase of the cell voltage in the order of 10 mV.

Conclusions

At the present time, various catalysts are available for the ethanol steam reforming, which can be performed at the typical operative conditions of a molten carbonate fuel cell. In addition to the traditional catalysts based on noble metals or Ni/MgO, catalysts based on mixed oxides (ZnCrOx and Cu0/Zn0/Al203) may also be utilized to produce hydrogen from ethanol. However, the endurance of these catalysts at long-term operative conditions and high temperature, and their tolerance in the presence of eventual electrolyte vapours, needs to be verified. Considering the favourable molar ratio between the produced hydrogen and the raw fuel to be reformed, ethanol represents an efficacious alternative to the use of methane to feed fuel cells. The use of membranes separating the anodic compartment from the reforming section is also possible at intermediate or high pressures, since this parameter doesn't heavily affect the overall cell performance.

References

[1] B. Baker, D. Burns, C. Lee, H. Maru, P. Patel, Internal Reforming for Natural Gas Fuelled Molten Carbonate Fuel Cells, prep, for GRI Contr. No 5080-344-0302, (1981)

[2] M. A. Rosen, Int. J. of Hydrogen Energy, 16, 3,207, (1991)

[3] S. Cavallaro, S.Freni, R.Cannistraci, M.Aquino and N.Giordano, Int. J. of Hydrogen Energy, 181, (1992)

[4] D. Qin, J. Lapszewicz, X. Jiang, J. of Catalysis, 159, 140, 1996.

[5] C.J. Jiang, D.L. Trimm, M. S. Wainwright, N. W. Cant, Applied Catalysis A: General, 93,245, 1993.

[6] S. Freni, G. Maggio,S. Cavallaro, La TermotecnicaA, 53-59, 1996.

[7] A. Slagtern, U. Olsby, Applied Catalysis A: General, jJO, 99, 1994.

[8] H. Agaras, G. Cerrella, M.A. Laborde, Applied Catalysis, 45,53, 1988.

[9] S. Cavallaro, S. Freni, Int. J. Hydrogen Energy, 21, in press, 1996.

Steam/EtOH molar ratio Fig.2: Selectivity of ethanol steam reforming reaction vs. steam/EtOH molar ratio.

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