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16.9: Hydrogenation/dehydrogenation reactions and renewable energy technology

Hydrogen gas and hydrogenation chemistry both play a role in the vision of a future society that is no longer dependent on fossil fuels. Hydrogen gas, it must be stressed, is not an energy source - there are no large reserves of H2 that can be tapped in the way that oil or natural gas is. Rather, the promise of hydrogen gas is as an energy carrier. The earth's supply of hydrogen is virtually unlimited - but in the energy-poor form of water. If enough energy is supplied, water can be broken into its two elemental components - hydrogen and oxygen - by a process called electrolysis:

\[2H_2O + \text{energy} \rightarrow 2H_2 + O_2 \tag{16.9.1}\]

When hydrogen gas is burned in oxygen to form water, the energy is released again - thus H2 has been used to store energy.

Traditionally, H2 has been produced with electrical energy derived from burning coal, oil,  or natural gas. Although, extensive efforts are being made to make it easier to use energy from solar and wind power to produce hydrogen.

There is also considerable interest in developing new biotechnological methods to produce hydrogen using energy derived from organic waste material. One idea, for example, is to use the enzymes of the oxidative pentose phosphate pathway (section 16.7B) to generate NADPH through the oxidation of glucose that is in turn derived from cellulose in biomass (waste cornstalks, for example) (Nature 2000, 405, 1014; Curr. Op. Biotechnol. 2004, 15, 343). Then, certain hydrogenase enzymes can be used to to combine the transferable hydride ion from NADPH with a proton from water to form hydrogen gas.

\[ \ce{NADP-H + H^+ \rightarrow H-H + NADP^+} \tag{16.9.2}\]

Whether the energy to produce it came from fossil fuels, solar, wind, or biomass, once we have hydrogen, the biggest question is what to do with it. Because it is a highly explosive gas, hydrogen can be difficult and dangerous to transport. One major area of hydrogen research is the development of more efficient fuel cells, which could use hydrogen to power automobiles. Efforts are also underway to improve the technology by which hydrogen produced from a renewable source (eg. solar, wind, or biomass) could be used at the site of production to:

  1. reduce nitrogen gas to anhydrous ammonia for use as agricultural fertilizer \[N_2 + 6H_2 \rightarrow 2NH_3 \tag{16.9.3}\]
  2. reduce carbon dioxide to methanol for use as automobile fuel. \[CO_2 + 3H_2 \rightarrow CH_3OH + H_2O \tag{16.9.4}\]

Compared to hydrogen, liquid ammonia and methanol are much less of a challenge to transport safely and efficiently.