Sites for the Dehydrogenation of Formic Acid on Gold

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Abstract
The dehydrogenation of formic acid on gold in the pressure range 5–15 mm Hg and the temperature range 130° to 200°C follows the rate law: −dP M /dt=aP M /(1+bP M ) ; a and b are constants and PM is the monomer pressure. On heating, the catalyst acquires enhanced activity for the dehydrogenation. The enhanced activity depends on the temperature to which it is heated, approaches a limiting value after about 40 h of heating and decays at an annealing temperature (130°C) by zero, first, second, or higher order in a manner similar to the extra electrical resistivity induced by quenching. The decay is a function of time alone at constant annealing temperature, the dehydrogenation of formic acid having no apparent effect. Repeated quenchings, however, leave the catalyst with a small, apparently irreversible, enhanced activity. A mechanism for the dehydrogenation is proposed and is analyzed in terms of the energy of activation of the constituent steps. The results indicate that surface terminations of dislocations, vicinal faces, and vacancies or other point defects are perhaps the catalytically active sites.