Pilar Ramirez de la Piscina and Narcfs Homs

Inorganic Chemistry Department,

Universitat de Barcelona, Spain

CONTENTS

Background…………………………………………………………………………………………………… 233

Energetically Integrated Ethanol Reforming Processes………………………………… 236

Catalytic Systems…………………………………………………………………………………………. 238

Mechanistic Aspects…………………………………………………………………………. 240

Nickel — and Cobalt-Based Catalysts………………………………………………… 241

Noble Metal-Based Catalysts…………………………………………………………… 243

Catalysts for Autothermal Steam Reforming………………………………….. 244

Perspectives…………………………………………………………………………………………………… 244

References…………………………………………………………………………………………………….. 245

BACKGROUND

Energy is one of the main factors that must be taken into account when sustainable development of our society is envisioned because there is an intimate connection between energy, the environment and development. In response to the need for cleaner and more efficient energy technology, a number of alternatives to the current energy network have emerged. In this context, the general use of fuel cells for automotive purposes or stationary power generation is envisioned in the medium term. This is a promising advance in the production of electrical energy from chemical energy, since the efficiency of a fuel cell is much higher than that of a combustion engine.

The fuel most widely studied for use in a fuel cell is hydrogen. Although the ideal situation would be the production of hydrogen from water, using renewable energy sources (e. g., solar energy), this is unlikely to become extensively oper­ative in the short to medium term. At present, hydrogen is mainly produced by steam reforming of fossil fuel-derived feedstock, mostly natural gas and naphtha.

The main objective of the steam reforming process is to extract the hydrogen from the substrate. From hydrocarbons, hydrogen is obtained via the general equation:

CnH2n+2 + nH2O о nCO + (2n + 1)H2 AH0 > 0

Then, the production of hydrogen is completed by the successive water gas shift reaction (WGSR):

CO + H2O о CO2 + H2 AH° = -41.1 kJ mol1

Both reactions can only be carried out in a practical way by catalytic means. The steam reforming reaction is endothermic and the real amount of energy required depends on both the stability of the substrate to be reformed and the ability of the catalyst to activate and transform the substrate into the products. The WGSR is slightly exothermic, and the forward reaction is not favored at the temperature used for steam reforming, which is higher than 1000 K for CH4. Therefore, the overall process requires the use of different catalysts, which operate under different reaction conditions in separate reactors. In the case of natural gas and naphtha, many years of industrial practice have led the total process to become technologically mature. However, if a strong increase in the demand for hydrogen is contemplated, some advanced research and development in catalysis and tech­nology would still be needed in the next few years [1,2].

On the other hand, society has become environmentally conscious and sen­sitive to its oil dependency because petroleum is likely to become scarce and expensive and the reserves are concentrated in a few countries. If a long-term global solution is envisioned, other, nonfossil-derived fuels, which are renewable and environmentally friendly must be contemplated for the supply of hydrogen. In this context, ethanol is a very promising alternative. As has been stated in previous chapters, ethanol, which can be considered a renewable and ecofriendly hydrogen carrier, can be produced from a large variety of biomass-based sources.

The catalytic steam reforming of ethanol may provide up to 6 moles of hydrogen per mol of ethanol reacted:

CH3CH2OH + 3H2O о 2CO2 + 6H2 AH° = 173.4 kJ mol-1

If the primary production of CO is considered in the steam reforming of ethanol, the WGS reaction must be taken into account. The overall process, then, will be the combination of both reactions:

CH3CH2OH + H2O о 2CO + 4H2
CO + H2O о CO2 + H2

Although globally the reaction releases 2 moles of carbon dioxide, the total process is almost neutral from the point of view of CO2 generation, since it may be assumed that the CO2 produced is consumed in biomass growth. Consequently, the use of the steam reforming of ethanol as a source of hydrogen can contribute to the global reduction of CO2 emissions. Moreover, other emissions of green­house or polluting gases such as hydrocarbons and NOx could also be mitigated.

As we have just said, the reaction of ethanol steam reforming is highly endothermic. However, theoretical and experimental studies have shown that ethanol steam reforming can take place at temperatures above 500 K [3]. Table 13.1 shows that relatively high values of equilibrium constant (Kp) can be achieved for temperatures of over 600 K. On the other hand, it is worth mentioning that, in this case, the energy required per mol of hydrogen generated (Table 13.1) is lower than half of that required to obtain hydrogen from the steam reforming of hydrocarbons. As an example, values of H (kJ per mol of hydrogen generated) at 600 K can be considered; 32.33 kJ must be supplied when H2 is obtained from ethanol, and 72.82 kJ if methane is used [4].

An issue of major importance in ethanol steam reforming is the development of catalysts that operate with high levels of activity, selectivity, and stability. Several products that can be formed under reaction conditions could need other experimental conditions to be reformed. Consequently, the total process leading to an effluent that mainly contains H2 and CO2 and is free of undesirable products may be complex. Depending on the reaction conditions and catalyst used, the following reactions could contribute to a low selectivity of the process, among others:

CH3CH2OH ^ CH3CHO + H2
CH3CH2OH ^ CH2CH2 + H2O
CH3CH2OH ^ CH4 + CO + H2
CH3CHO ^ CH4 + CO

COx + (2 + x)H2 ^ CH4 + xH2O

Thus, after the steam reforming, an additional purification of the effluent could be necessary, but this will depend on the fuel cell to be fed. For hydrogen operating in a polymer membrane fuel cell (PEMFC) or phosphoric acid fuel cell (PAFC) the limit of CO concentration in the fuel is 50 ppm and 0.05%, respec­tively [5]. These low CO concentrations may be achieved by subsequent catalytic selective oxidation or methanation processes or by the use of H2 selective mem­branes. An additional purification of the reformed effluent might be unnecessary when a molten carbonate fuel cell (MCFC) or a solid-oxide fuel cell (SOFC) is used. Both fuel cells, which operate at high temperatures, may convert impurities of CH4 and CO in the anode chamber [5,6].

Moreover, to make the steam reforming of ethanol operative in practice it must be energetically integrated with other exothermic processes, e. g., combus­tion or partial oxidation, which may supply the energy required for the steam reforming.

In the following sections, some propositions for globally energetically inte­grated processes and the main catalytic systems used to date for the different reactions will be analyzed. Finally, relevant perspectives of the development of the ethanol reformation to hydrogen in the near future will be presented.