Many studies of ethanol steam reforming have been carried out over nickel-based catalysts, because these are used effectively on an industrial scale for the steam reforming of natural gas and higher hydrocarbons. Different carriers have been studied [7,31-34], a major concern being the catalyst deactivation by carbon deposition. In this context, basic supports give better results, concerning coke formation, than do acidic carriers.

Two nickel-based systems, Ni/La2O3 and Ni/MgO can be highlighted [7,34]. Ni/La2O3 showed a good performance in terms of activity, selectivity, and stability [7]. No dehydration products were detected in the course of catalytic tests. Below 573 K, only dehydrogenation of ethanol occurred. The ethanol steam reforming takes place above 673 K. At this temperature, the presence of CO2 and CH4 is caused by the WGSR and methanation reaction, respectively:

CO + H2O о CO2 + H2 AH0 = -41.1 kJ mol-1 CO + 3H2 о CH4 + H2O AH° = -205.6 kJ mol-1

Then, at higher temperatures, the yield of H2 increases and the selectivity to CO2 and CH4 decreases. On the one hand, high temperature does not favor the WGSR. On the other hand, the reforming of CH4 with H2O and/or CO2 (dry reforming of methane) could take place, because these reactions become thermo­dynamically feasible above 823 K. Moreover, the Ni/La2O3 catalyst was seen to be very active for these reforming reactions:

CH4 + H2O о CO + 3H2 AH° = 205.6 kJ mol-1

CH4 + CO2 о 2CO + 2H2 AH° = 246.9 kJ mol-1

It is worth mentioning that there is good long-term stability of Ni/La2O3 when it is compared with other nickel-supported catalysts. This feature has been attrib­uted to the lack of formation of carbon deposits on its surface, and a model has been proposed for this [7]. A lanthanum oxycarbonate species is formed when CO2 reacts with a La2Ox species:

La2O3 + CO2 ^ La2O2CO3

La2O2CO3, which decorates the nickel particles, removes the surface carbon located at its periphery; the following reaction has been proposed:

La2O2CO3 + C ^ La2O3 + 2CO

The Ni/MgO system has been proposed as appropriate to carry out the steam reforming of bioethanol to supply H2 to MCFC [34]. The addition of alkaline ions produces metal particles of larger size and higher specific activity. The authors claimed that potassium addition stabilizes nickel catalysts by depressing the metal sintering under steam reforming conditions [34]. It has been suggested that the addition of potassium could change the electronic properties of Ni/MgO catalysts by electronic transfer from alkali-oxide moieties to nickel particles, which may depress the Boudouard reaction (2CO ^ CO2 + C) and hydrocarbon decomposition, which can lead to coke formation during steam reforming [31].

On the other hand, several studies have been carried out over Ni-Cu-based catalysts [35-39]. When alumina was used as support [35-37]. Potassium was added to avoid dehydration reactions. The study of catalyst generation as a function of the calcination step and the reducibility of different phases has been analyzed. Copper has been proposed to be responsible for the fast ethanol dehy­drogenation to acetaldehyde and nickel for the C-C rupture of acetaldehyde to produce methane and CO. A new aspect was recently introduced when Cu-Ni catalysts supported on SiO2 were considered: the formation of Cu-Ni, alloys which may prevent the deactivation of catalysts by carbon deposits [40].

As stated above, cobalt is also considered an appropriate active phase for the steam reforming of ethanol. An early study on supported cobalt-based catalysts was reported by Haga et al. [41]. This study provided evidence that the support strongly influences the catalyst performance. However, most of the cobalt-based catalysts that were tested (ZrO2-, MgO-, SiO2- and C-supported) showed high yields of methane, probably coming from ethanol or acetaldehyde decomposition or CO hydrogenation, and these reactions were highly suppressed over Co/Al2O3. Several later studies have been devoted to clarifying the role of support and cobalt phases in the behavior of catalysts [21,42-45]. Unsupported or ZnO-supported Co3O4 transforms under steam reforming conditions to small Co particles, which show a high catalytic performance in ethanol steam reforming [43,45]. Using bioethanol-like mixtures (H2O/CH3CH2OH = 13, molar ratio), these catalysts can yield up to 5.5 moles of hydrogen per mole of ethanol reacted at 623 K. The use of relatively low reaction temperature and an excess of H2O makes it possible to obtain almost exclusively CO2 and H2 as reaction products with a low presence of by-products [43,45]. The addition of alkaline metals on Co/ZnO catalysts has been found to have a promoter effect because they stabilize the catalyst by inhibiting coke formation [25].