Formation Formic acid decomposition

Anhydrous, monomeric formaldehyde is not available commercially. The pure, dry gas is relatively stable at 80—100°C but slowly polymerizes at lower temperatures. Traces of polar impurities such as acids, alkahes, and water greatly accelerate the polymerization. When Hquid formaldehyde is warmed to room temperature in a sealed ampul, it polymerizes rapidly with evolution of heat (63 kj /mol or 15.05 kcal/mol). Uncatalyzed decomposition is very slow below 300°C extrapolation of kinetic data (32) to 400°C indicates that the rate of decomposition is ca 0.44%/min at 101 kPa (1 atm). The main products ate CO and H2. Metals such as platinum (33), copper (34), and chromia and alumina (35) also catalyze the formation of methanol, methyl formate, formic acid, carbon dioxide, and methane. Trace levels of formaldehyde found in urban atmospheres are readily photo-oxidized to carbon dioxide the half-life ranges from 35—50 minutes (36). 

As early as 1923 Hinshelwood and Topley (27) noted the exceptionally erratic behavior of palladium foil catalyst in the formic acid decomposition reaction within 140-200°C. The initially very high catalytic activity decreased 102 times during the exposure of palladium to hydrogen, which is a product of the reaction. Though the interpretation does not concern the /3-hydride formation, the authors observation deserves mentioning. 

Overall, we demonstrated electrode potential- and time-dependent properties of the atop CO adsorbate generated from the formic acid decomposition process at three potentials, and addressed the issues of formic acid reactivity and poisoning [Samjeske and Osawa, 2005 Chen et al., 2003,2006]. There is also a consistency with the previous kinetic data obtained by electrochemical methods the maximum in formic acid decomposition rates was obtained at —0.025 V vs. Ag/AgCl or 0.25 V vs. RHE (cf. Fig. 12.7 in [Lu et al., 1999]). However, the exact path towards the CO formation is not clear, as the main reaction is the oxidation of the HCOOH molecule ... 

In reviews on formic acid decomposition, Mars and coworkers194,198 wrote that the formation and decomposition of formate anions were monitored by infrared spectroscopy. These studies were carried out by Fahrenfort, Sachtler, and coworkers188,193 for the case of formates on metals produced by formic acid adsorption—Cu, Ni, Pd, Rh, Pt, and Zn and in the case of metal oxides, Hirota et al. investigated ZnO,187,189,190,197 while Scholten et al. studied MgO.199,200 The infrared... 

The autocatalytic process observed on Ni(l 10) was very similar to results reported previously on nickel powder (82). In that work the rate of decomposition was measured as a function of the amount of formic acid adsorbed and the formic acid pressure. At 60°C and pressures from 0.4 to 1.4 Torr the rate of decomposition decreased as the formate coverage increased. Such behavior can be accounted for if the attractive interactions between the adsorbed species observed for the formic acid decomposition on Ni(llO) or... 

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). 

Fahrenfort et al. 34) not only observed the formation of formate during the acid decomposition, but also studied the decomposition of the formate-covered surface by means of infrared spectroscopy and also by measuring the increase in gas pressure. They found that the rate and the activation energy of the decomposition of the surface formate both coincide with those of the overall reaction of formic acid decomposition. This was not necessarily the case in the data obtained by Tamaru (21) at lower temperatures. They accordingly concluded that the decomposition of formic acid on a nickel surface proceeds via nickel formate as a key intermediate. 

The formic acid decomposition on metal catalysts accordingly seems to proceed via surface formate, though the difference in basicity of the metal surface would result in different coverage during the reaction, which in turn leads to a different reaction order. It is widely known that Fahrenfort et al. 34) thus correlated the catal3d ic activities of the... 

The importance of formate ions is also stressed by Komarov et al. (92). These workers analyzed the composition of the catalyst after this had been used for a certain period in the decomposition of formic acid. The analyses showed Fe, Mg, and Mn formate to be present on the separate catalysts. From the fact that the selectivity for the formate decomposition resembles that for formic acid decomposition they concluded that in the last reaction formate ions occur on the surface as an intermediate. 

In Table XIII a survey is given of data on the decomposition of bulk formates and of the direction of the formic acid decomposition on the corresponding metals or oxides. 

Energy of activation values calculated from rate data between 75 and 150°C ranged from 22.7 to 24.4 kcal/mole with the minimum at 90°C. This agrees well with the value of 22.8 kcal/mole for formic acid decomposition on nickel powder between 125 and 150°C. The interpretation of the infrared data was further supported by calorimetric measurements. Thus, the heat of adsorption of formic acid at monolayer coverage was 18 kcal/mole which compares favorably with the heat of formation for mole of Ni (OOCH)2, 13 kcal. 

This process can be considered the first plasma-chemical stage of a two-step process for ly drogen production from water. The second stage in this case is formic acid decomposition with formation of hydrogen and carbon dioxide ... 

If one is virtually certain that a certain surface species is a reaction intermediate, measurement of the concentration of that species can yield the site density. For example, in 1969-1971 Gonzalez and coworkers ( 365 37, 28) related formic acid decomposition over transition metal formates to surface formate concentration. 

The influence on the activation enCTgy by formation of a water containing activated complex is different in the two steps of reaction pathway [40]. As a consequence the decomposition of formic acid proceeds differently in the gas phase and in supercritical water. In the gas phase nearly complete reaction to CO and v/ater is observed. In supercritical water formic acid decomposes nearly completely to carbon dioxide and hydrogen [39,40]. This is shown experimentally [39] and by calculations [40]. The increase of the water gas shift reaction by water is measured in pure wato, when the density of water is raised above 0.35 g/cm [41]. This is a rafher high water density, thCTefore it is not clear wheth very small amounts of water are able to lead to the measured difference in the CO content (Figure 2a) by influencing the formic acid decomposition or not. 

Similar to methanamide, ammonium formate decomposition proceeded in a wide temperature window. As ammonium formate does in fact not need to be hydrolyzed, formic acid decomposition is the limiting reaction for the quantitative conversion of the precursor solution to NH3 and CO2. However, if mixtures with urea, such as Denoxium are being used, the temperature needs to be sufficient to enable urea hydrolysis. Therefore, an in-pipe or side stream system equipped with an Au/Ti02 hydrolysis catalyst could in fact be used to provide NH3 from either AdBlue , Denoxium or Admide(R). This would enable the flexibility to ensure efficient NH3 production, and, consequently, NO reduction, even in cold climates or during long refill intervals. Guanidinium formate solution would also be an optional NH3 precursor solution for a side stream reactor if the reactor could be electrically heated at temperatures below 250 °C, but would not qualify for direct injection into the main exhaust pipe. 

Kinetic studies of the decomposition of metal formates have occasionally been undertaken in conjunction with investigations of the mechanisms of the heterogeneous decomposition of formic acid on the metal concerned. These comparative measurements have been expected to give information concerning the role of surface formate [522] (dissociatively adsorbed formic acid) in reactions of both types. Great care is required,... 

Isothermal a—time curves were sigmoid [1024] for the anhydrous Ca and Ba salts and also for Sr formate, providing that nucleation during dehydration was prevented by refluxing in 100% formic acid. From the observed obedience to the Avrami—Erofe ev equation [eqn. (6), n = 4], the values of E calculated were 199, 228 and 270 kJ mole"1 for the Ca, Sr and Ba salts, respectively. The value for calcium formate is in good agreement with that obtained [292] for the decomposition of this solid dispersed in a pressed KBr disc. Under the latter conditions, concentrations of both reactant (HCOJ) and product (CO3") were determined by infrared measurements and their variation followed first-order kinetics. 

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