Energy of formic acid

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

Apelblat, A. Tamir, A. Wagner, M. Excess Gibbs energy of (formic acid + benzyl alcohol) and (acetic acid + benzyl alcohol) J. Chem. Thermodyn. 1984,16, 891-895... 

Smit, P. H., Derissen, J. L., 8c van Duijneveldt, R B. (1978). Role of distortion energy in ab initio calculated dimerization energy of formic acid. Journal of Chemical Physics, 69, 4241-4244. 

Fig. 8-10. Potential energy diagram for the uncatalyzed decomposition of formic acid.

Table 6 Energy values (in a.u.) of M(n,7r ) excited state of formic acid for the two minima and two planar conformations.

From a prachcal standpoint, formic acid or its salts are the least valuable reaction products. The energy content of formic acid upon its reverse oxidation to CO2 is insignificant, and its separation from the solutions is a labor-consuming process. At present, maximum effort goes into the search for conditions that would ensure purposeful (with high faradaic yields) synthesis of methanol, hydrocarbons, oxalic acid, and other valuable products. 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

Each of the fuels other than methanol have serious problems hydrazine price and safety formaldehyde safety and energy density formic acid energy density and safety. 

Temperature programmed desorption studies of formic acid decomposition by metals was reviewed recently by Madix (7d) the significance of formate formation is paramount to the discussion. This is also apparent in the recent electron energy loss spectra of formic acid adsorption on Cu(lOO) reported by Sexton (77). 

On the assumption that the catalytically active area covered by the reactant remains unchanged over the experimental temperature range Hinshelwood and Topley (J.G.S. cxxiil. 1014, 1923) have determined the following energies of activation from the temperature coefficient and the relative reaction velocities for the decomposition of formic acid according to the equation... 

Schwab and co-workers (5-7) found a parallel between the electron concentration of different phases of certain alloys and the activation energies observed for the decomposition of formic acid into H2 and CO2, with these alloys as catalysts. Suhrmann and Sachtler (8,9,58) found a relation between the work function of gold and platinum and the energy of activation necessary for the decomposition of nitrous oxide on these metals. C. Wagner (10) found a relation between the electrical conductivity of semiconducting oxide catalysts and their activity in the decomposition of N2O. 

If these considerations are correct and the observed scattering spectrum of formic acid crystal in the 255-170 cm-1 is really due to the hydrogen bond, one can try to estimate the value of the dissociation energy D of the hydrogen bond in the crystal. We assume that one can make use of the formula for a diatomic molecule ... 

It must be repeated that this argument depends upon the assumption that there is only one way in which the molecules of formic acid can be attached to the surface of the catalyst. There is, however, some evidence against this assumption. Constable finds that the two simultaneous reactions undergone by allyl alcohol when passed over heated copper are differently influenced by the physical state of the catalyst. This points to the conclusion that there are two independent centres of activity on the catalyst surface with two different modes of adsorption, or, at any rate, centres where the energy of adsorption is so different that different reactions are facilitated. Hoover and Rideal f find that the two alternative decompositions of ethyl alcohol by thoria show a different behaviour towards poisons, which points to the same conclusion. 

It should be noted that the production of formic acid from C02 and H2 proceeds with a net increase in free energy under ambient conditions, but that an increased pressure of H2 and C02 will shift the equilibrium favorably. When alkyl formates or formamides are produced from C02, H2, and the amine or alcohol, the stability of the water formed in the reaction provides a powerful driving force, and the thermodynamics of these reaction are favorable under ambient conditions. 

Another interesting problem related to the thiol/thione tautomerism in thioformic acid is the relative stability of the s-cis (4a) and s-lrans (4b) conformers of the thiol form. As we have mentioned in the preceding section, the former is 0.7 kcal mol 1 more stable than the latter. A much larger energy difference is predicted to exist, at the ab initio level, between the s-cis (23a) and s-trans (23b) forms of formic acid (oc6.2 kcal mol 1) or between the same conformers of the thione form of thioformic acid (4c, 4d) (oc6.5 kJmol l)llx. 

Sosnovsky (37) studied the decomposition of formic acid on single-crystal sheets of silver at several temperatures. The specimens were used as they came from the mold, and there was no special preparation or examination of the surface. Thus, the over-all orientation of the crystal was the only controlled structural factor. The rates of reaction between faces were found to vary by less than a factor of 10, and for most temperatures by less than a factor of 2. Activation energies, varying from 16.0 to 30.4 kcal. for the different specimens, were obtained. 

Rossetti GFI, Albizzati ED, Alfano OM. Modeling of a flat-plate solar reactor. Degradation of formic acid by the photo-Fenton reaction. Solar Energy 2004 77 461-70. 

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