CATALYTIC CONVERSION OF ETHANOL TO HYDROGEN USING COMBINATORIAL METHODS (Senkan)

Methane, methanol and gasoline, all of which are derived from fossil fuels, have long been studied as possible liquid feed stocks to produce hydrogen for automotive fuel cell applications.2-4 In contrast, ethanol steam reforming has been studied to a more limited extent. Ethanol has several advantages over fossil-fuel derived hydrocarbons as a source for hydrogen production in fuel cell applications. First, it represents a renewable and CO2-neutral source that can readily be obtained from biomass fermentation.5

The economic future of ethanol production looks even more favorable when one considers the likely increases in the price of petroleum and other fossil fuels as world reserves are depleted. Although ethanol has only 65-70% of the energy density of hydrocarbon fuels, it represents a reliable and sustainable energy source which is decoupled from geopolitical developments and will result in significant net reduction in CO2 emissions when it replaces fossil fuels. All these considerations render ethanol economically, environmentally and strategically attractive energy source.

 In addition, ethanol can be a particularly attractive energy and hydrogen source for countries that lack fossil fuel resources, but have significant agricultural economy. This is feasible because virtually any biomass can now be converted into ethanol as a result of recent advances in biotechnology. ethanol reforming proceeds at temperatures in the range 300-600oC, which is significantly lower than those required for CH4 or gasoline reforming. This is an important consideration for the improved heat integration of fuel cell vehicles. Third, ethanol is significantly less toxic than methanol, and as such provides less risk to the population. The fact that methanol is derived from fossil fuel resources also renders it an unreliable energy source in the long run.

The thermodynamics of steam reforming of ethanol has been extensively studied.6-9 The preferred ethanol steam reforming process is represented by the following endothermic reaction with the formation of CO2 as the desired product:

C2H5OH + 3H2O = 2CO2 + 6H2 HR = 173.4 kJ/mole, at 300K (1)

The formation of CO, which is undesirable as it poisons the Pt catalyst of the electrochemical cell, must also be considered under steam reforming conditions:

C2H5OH + H2O = 2CO + 4H2 HR = 255.7 kJ/mole, at 300K (2)

Furthermore, some steam reforming catalysts can also catalyze the following water gas shift reaction:

CO + H2O = CO2 + H2 HR = -41.1 kJ/mole, at 300K (3)

which, at high steam concentrations enhances CO2 production over CO. The formation of other byproducts such CH4, C2H4 and CH3CHO have also been observed in ethanol reforming processes.10 An important by-product that must be considered in the design and operation of all reforming catalysts is solid carbon formation. Because of its accumulative nature, carbon formation can lead to catalyst deactivation and in the limit can even result in the plugging of the reforming reactors with potentially catastrophic consequences. Carbon formation is a problem at high temperatures and at low H2O/C2H5OH ratios. Consequently, the discovery and development of new catalytic materials that can efficiently convert ethanol to hydrogen at low temperatures and at low H2O/C2H5OH ratios is crucial for the practical utilization of fuel cells in the transportation industry.

Previously a variety of oxide support materials and metals have been considered for the steam reforming of ethanol.11 Most of the earlier catalysts investigated were Ni based materials with the addition of Cu, Cr, Zn or K.12-17 This was due to the generally accepted belief that nickel promotes C-C bond rupture,13-15 thus should be a good ethanol reforming catalyst. More recently, cobalt supported on ZnO has also been determined to be a promising catalytic material for the conversion of ethanol to hydrogen.20 However, in spite of excellent initial results, carbon deposition remained a persistent problem in Co/ZnO at the 450-500oC temperatures associated with these catalysts, necessitating the use of dopants, such as alkali metals, to suppress coke formation.21 In summary, there exists a need for the development of low temperature catalytic materials for the efficient synthesis of hydrogen by the steam reforming of ethanol.

Combinatorial catalysis, an effective methodology for the accelerated discovery and optimization of functional materials, has been applied for the discovery of low temperature catalysts for the production of hydrogen from ethanol. The implementation of combinatorial methods generally entails a two-phase approach.22,23 In the first, i.e. primary, screening phase, libraries of catalytic materials are rapidly evaluated to identify “leads” or “hits” exhibiting superior activities, and equally desirable, superior product selectivities. In the secondary screening phase, new leads are thoroughly evaluated, characterized and optimized using traditional catalysis research methods and tools. It is also in the secondary screening phase that issues related to durability and resistance to poisoning are addressed in order to develop industrially significant catalysts.

We have reported the results of the primary screening phase as applied to the title reaction. Libraries of catalytic materials were prepared by impregnating porous pellets of ã-Al2O3, SiO2, TiO2, CeO2 and Y-ZrO2, with individual aqueous salt solutions of 42 elements from the periodic table at 4 different loadings in the range 0.5-5 wt%. Ethanol steam reforming activities and H2 selectivities of these 840 distinct materials were then evaluated using a computerized array channel microreactor system and mass spectrometry. Catalysts were screened under identical operating conditions of 300 °C, 1 atm, and a GHSV of 60,000 h-1, using a feed gas composition of 2% C2H5OH and 12% H2O in helium carrier gas. This systematic investigation, completed over a period of several months, provided both confirmatory results and produced new leads of superior catalytic materials. Pt/TiO2 and Pt/CeO2 were the most significant new leads, both of which gave the highest ethanol conversions (+90%) and hydrogen selectivities (about 30%) at 300 °C among all the single component catalytic materials explored. Although these catalytic materials were highly active, their H2 production levels require further improvement.

Therefore, future studies must focus on increasing the selectivities for H2 production, while decreasing the same for CH4 and CO, and other by-products. This could be accomplished, for example, by considering binary, ternary and higher order combinations of Pt on these supports, using the genetic algorithm approach.22

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