Activation energy formic acid dehydrogenation

Flo. 2. Activation energy of formic acid dehydrogenation over silver on doped silicon carhide (19). (Copyright by the Universite de Liege. Reprinted with permission.)... 

Unimolecular pyrolysis of the tautomers of monothioformic acid (two conformers of thiol- and two conformers of thiono-) have been studied by ab initio methods with STO-3G and 6-31 G basis sets. The barrier heights for dehydrogenation (via a four-centre transition state) and dehydrogensulfldation (via a three-centre transition state) of thiol formic acid are 67.47 and 67.09 kcalmol" respectively. Dehydration of 5-cw-HCSOH occurs via a three-centre transition state with an activation energy of 81.18 kcalmoG this is much greater than for dehydration of the s-trans form, which occurs via a four-centre transition state with a barrier of only 68.83 kcalmol" ... 

The formate, formed by oxidative dehydrogenation of the acid, is quite stable and doesn t decompose until 480 K. This decomposition is a classical first-order case with a decomposition activation energy of 130 kJ mol-1 and a normal value pre-exponential of 1013 s-1. The great ability of the TPD technique is the separation of the individual steps in the reaction in temperature. It is clear that the step proceeding over the highest barrier in this case is the formate decomposition, and that in a catalytic oxidation of formic acid the most abundant surface intermediate is likely to be the formate with its decomposition being rate determining. 

The activities for the dehydrogenation of formic acid given in literature are compiled in Fig. 23 here the reaction rate per sq cm of the accessible catalyst surface area is given as a function of temperature. If not given by the authors, the value of the accessible surface area is estimated from experience. This figure shows that the specific activity as well as the activation energy of the reaction depend largely on the nature of the oxide (see also Table V). 

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