The molecules of forgetfulness.

2. Productivity at this level means that the production costs of H 2 (expressed as the cost in US dollars per joule of calorific value contained in the molecule) are two to three times higher than for natural gas 4,5. However, taking into account the transportation and distribution costs of natural gas, and how pure you want the H 2 to be, the comparative cost of H 2 production can vary. Biomass, the product of photosynthesis, could become a feedstock of choice for H 2 production, even in the absence of environmental premiums. At present, two strategies are being intensively pursued. The first is fractionation, which leads to the isolation and depolymerization of constitutive biomass fractions. An advantage of this approach is that the production of marketable goods (such as chemicals and fibres) can be integrated with the conversion of particular residual fractions to H 2 , as well as to other biofuels or to bioener-gy 6. In fact, in the absence of subsidies or carbon credits (through which countries can trade off carbon-producing industry against carbon-absorbing activities, such as the planting of crops), co-products from biomass are necessary to make bioenergy or biofuels economically viable 7. The second approach is through biosyn-theses that produce H 2 directly. This requires photosynthesis to be conducted, as in algae, either with the removal of oxygen (to avoid poisoning the natural hydrogenases), or with oxygen-tolerant hydrogenases, which would need genetic engineering 8. Alternatively , certain wet biomass fractions can be fermented by hydrogen-producing microorganisms in anaerobic environments where methanogenic pathways are suppressed 9. Efforts in all of these directions are being pursued actively in academic and industrial laboratories. Realistically, the introduction of a clean fuel such as H 2 is most likely to be achieved if the production technology can be integrated within the existing infrastructure used for natural-gas reforming. To use biomass as the source of H 2 , several objectives must be achieved. To begin with, reaction and reactor productivities must match those achieved in hydrocarbon reforming. According to the results of Cortright et al. 1 , this is almost achieved with glycerol as substrate, but in the case of sugars as substrate much lower water–sugar ratios for aqueous-phase reforming will need to be used. Heat inputs to balance the endothermic reforming reaction must also be equivalent to those used in hydrocarbon reforming. In steam reforming, a steam– carbon molar ratio …