A review of ethanol reactions over the surface of supported noble metal catalysts has recently been published [46]. The noble metal-based catalysts most widely studied in ethanol steam reforming are those based on Pd, Pt, Ru, and Rh, and their behavior also depends, in this case, on the support. Comparative studies of g-Al2O3-supported catalysts showed Rh to be the most active metallic phase [47,48]. Taking into account that ethylene and methane were the main by-prod­ucts, the performance of different metals in the reformation of ethylene and methane is a key aspect to take into account. Rh turned out to be the most active in ethylene steam reforming, whereas Pd was almost inactive. In this context, the selectivity to ethylene of an Rh/Al2O3 catalyst showed a maximum at 973-1023 K, and then dropped at higher temperatures because ethylene reforming took place [47]. To optimize the hydrogen production by steam reforming of bioethanol on Rh/Al2O3 catalyst, high temperature and long contact times (high reactor volume/volumetric flow rate ratios) are required [8]. Although Rh is highly active in hydrogenation and its presence may reduce coke formation, under steam reforming conditions the catalyst deactivates by sintering and coke formation. The introduction of a small amount of O2 (0.4 vol%) has been proposed to reduce coke formation by combustion of carbonaceous species forming during the reac­tion. However, this combustion could be responsible for the formation of larger metal particles and consequently of the decrease in activity [8].

Several papers have been published in which the reforming of ethanol is carried out over Rh on supports other than alumina, namely, CeO2, ZrO2, and derived systems [29,49]. A high yield in H2 is found for catalysts containing ZrO2. This is related to the available oxygen on the surface, which participates in the WGS reaction [49].

As for palladium catalysts, it has recently been shown that the steam reform­ing of ethanol can be effectively carried out over a commercial Pd/g-Al2O3 catalyst, which does not produce ethylene as a by-product [23]. At 473-623 K, ethanol is dehydrogenated to acetaldehyde, which is decomposed to CH4 and CO. Then, at temperatures higher than 733 K, CH4 can be reformed as a function of the H2O/ethanol ratio.

Other promising systems for the steam reforming of ethanol could contem­plate the concurrence of different catalysts, with an appropriate active phase to catalyze each step of the total process. A two-layer fixed-bed reactor containing Pd/C and a Ni-based catalyst has been proposed to produce COx:H2 mixtures from ethanol steam reforming [50]. Ethanol can be converted to carbon oxides, CH4 and H2 over the Pd/C catalyst (608 K). Then, over the second layer containing the Ni-based catalyst, methane can be reformed with steam (923-1073 K) [50].