Another area of chemistry that is deeply interwoven throughout all areas of sustainable energy is catalysis. As noted with respect to green chemistry in Section 4.6, catalysis enables more environmentally benign production methods and cuts the cost of manufacture by dramatically reducing the amount of energy to carry out chemical transformations. In addition, progress in catalysis has allowed the synthesis of evermore complex materials with greater selectivity. In the energy arena, catalysts are fundamental to the rening of petroleum (as we have already seen), the operation of fuel cells, hydrogen production, and the synthesis of most polymers. This chapter will provide a general overview of how catalysts work and then specically focus on the production of hydrogen gas, where catalysts play a crucial role. The efcient generation of hydrogen gas is necessary for its use in fuel cells, as we will see in Chapter 6.