Decomposition of formic acid

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

If sulfuric acid, H2SO4, is added to an aqueous solution of formic acid, carbon monoxide bubbles out rapidly. This also occurs if phosphoric add, HjPO, is added instead. The common factor is that both of these acids release hydrogen ions, H+. Yet, careful analysis shows that the concentration of hydrogen ion is constant during the rapid decomposition of formic acid. Evidently, hydrogen ion acts as a catalyst in the decomposition of formic acid. 

This ease with which we can control and vary the concentrations of H+(aq) and OH (aq) would be only a curiosity but for one fact. The ions H+(aq) and OH (aq) take part in many important reactions that occur in aqueous solution. Thus, if H+(aq) is a reactant or a product in a reaction, the variation of the concentration of hydrogen ion by a factor of 1012 can have an enormous effect. At equilibrium such a change causes reaction to occur, altering the concentrations of all of the other reactants and products until the equilibrium law relation again equals the equilibrium constant. Furthermore, there are many reactions for which either the hydrogen ion or the hydroxide ion is a catalyst. An example was discussed in Chapter 8, the catalysis of the decomposition of formic acid by sulfuric acid. Formic acid is reasonably stable until the hydrogen ion concentration is raised, then the rate of the decomposition reaction becomes very rapid. 

The similar phenomenon of poisoning in situ of a palladium catalyst by hydrogen which was in this case the product of a reaction was observed by Brill and Watson (37). The reaction studied was the decomposition of formic acid... 

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,... 

There is an extensive literature relating to the role of surface intermediates in the heterogeneous catalytic decomposition of formic acid on metals and oxides (see Refs. 36, 522,1030,1042—1045). 

Figure 5. Isokinetic relationship in the coordinates log k versus T" decomposition of formic acid on various catalysts (189).

While very limited data ate presented here, the kinetics of adsorption/decomposition of formic acid molecules [Rice et al., 2002] have been measured by BB-SFG, as shown in Fig. 12.14. A Pt(lll) electrode and a 0.1 M H2SO4 electrolyte containing 0.1 M formic acid were used. The families of spectra at 0.200, 0.025, and 0.225... 

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. 

The decomposition of formic acid over evaporated Pd-Au alloy films has been studied by Clarke and Rafter (69) the same reaction on Pd-Au alloy wires was studied by Eley and Luetic (128). The alloy films were prepared in a conventional high vacuum system by simultaneous evaporation of the component metals from tungsten hairpins. The alloy films were characterized by X-ray diffraction and electron microscopy. The X-ray diffractometer peaks were analyzed by a method first used by Moss and Thomas (SO). It was found that alloys deposited at a substrate temperature of 450°C followed by annealing for one hour at the same temperature were substantially homogeneous. Electron microscopy revealed that all compositions were subject to preferred orientation (Section III). 

The possible intermediacy of the formate ion (eqs. 6 and 18) in the WGSR has been considered (2,6,10), but its involvement has not been clearly demonstrated. The Group VI metal carbonyl complexes are effective in the decomposition of formic acid (as sodium formate), as shown in Table VII. Some heterogeneity is observed in those reactions carried out under nitrogen pressure, but in no case was CO detected. The similarity in rates for WGSR... 

The Activity of the Group VI Metal Carbonyls in Decomposition of Formic Acid as Formate... 

P3.01.02. TRUE CONTACT TIME OF FORMIC ACID REACTION The gas phase decomposition of formic acid,... 

Cuf1101-HC00 The decomposition of formic acid on metal and oxide surfaces is a model heterogeneous reaction. Many studies have since shown that it proceeds via a surface formate species. Thus on Cu 110) adsorbed formic acid is found at low temperature. On heating to 270 K deprotonation occurs, giving rise to the surface formate, which in turn decomposes at 450 K with evolution of H2 and C02- In previous studies, particularly with vibrational spectroscopy, it had been demonstrated that the two C-0 bonds are equivalent and that the symmetry is probably C2v [19]. A NEXAFS study by Puschmann et al. [20] has subsequently shown that the molecular plane is oriented perpendicular to the surface and aligned in the <110> azimuth. 

Catalytic decomposition, of formic acid, 14 35 Catalytic effects, in isocyanate reactions, 13 393... 

Sabatier and Balandin had predicted a relationship between catal)dic activity and heat of adsorption. If a solid adsorbs the reactants only weakly, it will be a poor catalyst, but if it holds reactants, intermediates or products too strongly, it wiU again perform poorly. The ideal catalyst for a given reaction was predicted to be a compromise between too weak and too strong chemisorption. Balandin transformed this concept to a semiquantitative theory by predicting that a plot of the reaction rate of a catal)Tic reaction as a function of the heat of adsorption of the reactant should have a sharp maximum. He called these plots volcano-shaped curvesl This prediction was confirmed by Fahrenfort et al." An example of their volcano-shaped curve is reproduced in Fig. 9.1. They chose the catalytic decomposition of formic acid... 

The decomposition of formic acid is one of the most extensively studied catalytic reactions. Several excellent review articles have been written on the subject (76- 78). From a large number of studies on supported metal catalysts the following, and sometimes contradictory, observations were made ... 

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... 

The decomposition of formic acid on nickel single crystals showed unusual features not observed on Cu(llO), Fe(lOO), Ag(110), or W(100) surfaces. Adsorption of isotopically labeled formic acid, HCCOD, or Ni(llO)... 

The Catalytic Decomposition of Formic Acid P. Mars, J. J. F. Schollen, and P. Zwietering... 

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