Decomposition HCOOH

Formic acid can decompose either by dehydration, HCOOH — H2O + CO (AG° = —30.1 kJ/mol AH° = 10.5 kJ/mol) or by dehydrogenation, HCOOH H2 + CO2 (AG° = —58.6 kJ/mol AH° = —31.0 kJ/mol). The kinetics of these reactions have been extensively studied (19). In the gas phase metallic catalysts favor dehydrogenation, whereas oxide catalysts favor dehydration. Dehydration is the predominant mode of decomposition ia the Hquid phase, and is cataly2ed by strong acids. The mechanism is beheved to be as follows (19) ... 

The hydride phase may be present in a catalyst as a result of the method of its preparation (e.g. hydrogen pretreatment), or it may be formed during the course of a given reaction, when a metal catalyst is absorbing hydrogen (substrate—e.g. in H atom recombination product—e.g. in HCOOH decomposition). The spontaneous in situ transformation of a metal catalyst (at least in its superficial layer) into a hydride phase is to be expected particularly when the thermodynamic conditions are favorable. 

V vs. Ag/AgCl were collected as functions of decomposition time. The electrode potential was initially held at 0.75 V to produce a clean Pt(lll) surface, and was next switched to monitor the CO uptake. Starting at Os, where CO adsorption (from HCOOH decomposition) had not yet begun, the potentiostatic experiment lasted until about 500 s of the progress of reaction. The spectral position is typical of the... 

Figure 12.14 SFG spectra of the carbonyls formed during formic acid decomposition on a Pt(lll) electrode in 0.1 M H2SO4 electrolyte containing 0.1 M formic acid. The spectral position is typical of atop CO on the Pt(l 11) surface. Times at which the spectra have been recorded are from 2 to 496 s, yielding HCOOH decomposition kinetics at three electrode potentials, -0.200, -0.025, and 0.225 V vs. Ag/AgCl.

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 ... 

HCOOH decomposition, 29 25-26 schematic apparatus, 29 18-19 small metal particles, 36 108, 109 spectra, 29 7-9, 25-26 titania-supported catalysts, 36 203-205, 208-209... 

FIGURE9.t. Volcano Plot of formic acid decomposition. Abscissa Calculated A HadsofHCOOH Ordinate Temperature at which rate of HCOOH decomposition reaches the same value for all metals. 

W. M. H. Sachtler and J. F. Fahrenfort, Catalytic decomposition of HCOOH vapour on metals, Actes du 2ieme Congres Intern, de Catalyse 1960, (Editions Technip., Paris) pp. 831-863 (1961). 

The decomposition of formic acid was studied on clean Cu(l 10) by TPRS (75). Formic acid adsorbed at either 190 K or 300 K with an initial sticking probability of unity. The adsorption roughly followed Langmuirian behavior. Multilayers were not adsorbed down to 190 K condensed HCOOH was reported to desorb from copper films below 190 K (90). Chemisorbed... 

Fig. 18. AES for the HCOOH decomposition on W(IOO) (96). (a) Clean surface (b) clean surface exposed to HCOOH at 300 K and heated to 800 K (c) heated further to 1500 K to desorb carbon and oxygen atoms. The carbon and oxygen peaks in (b) indicate dissociation of the HCOOH.

The decomposition behavior of formic acid on the close-packed Ru(lOTO) surface parallels the reaction on nickel, except that the autocatalytic process was not observed (lOJ). Water was desorbed at 183 K by apparent second-order kinetics following adsorption of HCOOH at 100 K. Subsequent desorption of Hj, COj, and CO suggested the formation of the surface anhydride. The rate constant for decomposition was 2.6 x 10 sec exp —26.9 kcal/mol// r. ... 

Formic acid did not adsorb on clean Ag(llO) above 180 K. In order to obtain the adsorbed species, the surface was predosed with oxygen to produce adsorbed oxygen atoms. Surface species could then be stabilized by reacting formic acid with surface oxygen to produce water and the formate (102). Subsequent heating of the surface produced decomposition near 425 K. Only CO2 and H2 were observed as products from the HCOO intermediate. Additionally, some back reaction to reform HCOOH occurred between HCOO(a) and the H(a) liberated by the decomposition. The rate constant for the decomposition was... 

Kinetic Parameters for the Decomposition of HCOOD or HCOOH on Single Crystal Surfaces... 

Fig. 20. The volcano plot for HCOOH decomposition on high surface area catalysts ( ) and single crystals ( ) (102a). Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1979.

Thus, the main impurities are water, HCOOH and (CH3)2NH. Small amounts of HCN, formed by photolysis, and CO, formed by thermal decomposition, are also present. In the purification of a commercial product, it is kept with molecular sieves (4A or 5A) for 1-4 days to dehydrate. Then it is shaken with BaO for 1-2 days and the supernatant is distilled twice at 20 mmHg under nitrogen. All these operations must be carried out in the absence of light. The distillate should be stored under nitrogen gas and used as soon as possible. DMF has toxic effects, particularly on the liver and kidney and care should be taken in its handling. 

The simplest or lowest member of the fatly acid series is formic acid, HCOOH. followed by acetic acid, CHiCOOH. propionic acid with three carbons, butyric acid with four carbons, valeric acid with five carbons, and upward to palmitic acid with sixteen carbons, stearic acid with eighteen carbons and melissic acid with thirty carbons. Fatty acids are considered to be the oxidation product of saturated primary alcohols. These acids are stable, being very difficult [with the exception of formic acid) to convert to simpler compounds they easily undergo double decomposition because of the carboxyl group they combine with alcohols to form esters and water they yield halogen-subslitulion products they convert to acid chlorides when reacted with phosphorus pcntachloridc and Iheir acidic qualities decrease as their formula weight increases. 

Na2C03, the H2 evolution rate increased dramatically. Furthermore, at the same time, 02 evolution occurred in an exactly stoichiometric ratio of H20 decomposition, i.e. H2 02=2 l. Production of CO, CH4, HCOOH, CH3OH and carbon was not observed. It is clear that stoichimetric water splitting was established in the case of Pt/Ti02 photocatalyst system by the addition of Na2C03. 

Several reports of catalysis of the decomposition of formic acid involving homogeneous transition metal complexes and proceeding by means of metalloformate intermediates have recently appeared in the literature. For example, Rh(C6H4PPh2)(PPh3)2 (8) catalyzes the decomposition of formic acid to C02 and H2 via the intermediacy of the product of oxidative-addition of HCOOH, Rh(HC02)(PPh3)3 (56). -Elimination of the hydride from the... 

HCOOH (liq.). The heat of combustion of gaseous formic acid was measured by Favre and Silbermann1 and Thomsen,15 and of liquid formic acid by Berthelot and Matignon510 and Jahn.2 The data yield the following values for the heat of combustion of liquid formic acid Thomsen,15 63.0 Berthelot and Matignon,5-10 62.5 Jahn 2 63.0. Berthelot,25 150 measured the heat of decomposition of formic acid with aqueous sulfuric acid. 

A theoretical study of the thermal isomerization and decomposition of oxalic acid has attempted to account for the predominant formation of C02 and HCOOH from the vapour at 400-430 K.41 Transition-state theory calculations indicate that a bimolecular hydrogen migration from oxygen to carbon of intermediate dihydroxycarbene (formed along with C02) achieved through a hydrogen exchange with a second oxalic acid... 

Table 2. Energies of activation for the decomposition of HCOOH over metal-phthalocyanines (0-Modification)...

For the formate radical HCOO, we use Eq. (10b) with DAB =110 kcal/ mol for the 171 coordination and Eq. (20b) with DAX = 166 kcal/mol for the rj1 coordination. We have calculated the relevant values of (2hcoo f°r Ag(lll), Ni(lll), Fe(110), and W(110). Because no direct experimental data are available, we shall discuss our projections later in the general context of HCOOH decomposition on metal surfaces. Here we shall only mention that for HCOO the 172 coordination was found to be always preferred over the 17 coordination, in full agreement with experiment (see Section IV,C). 

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