Formic acid, decomposition heterogeneous reactions

Hydrocarbons and carboxylic acids. J Catal 65 49-58 Benziger JB, Schools GR (1984) Influence of absorbate interactions on heterogeneous reaction kinetics. Formic acid decomposition on nickel. J Phys Chem 88 4439-/ / / / ... 

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,... 

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... 

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. 

A heterogeneously catalysed reaction involves several steps (Mady et al, 1976) (i) mass transport of fluid reactants to the surface, (ii) chemisorption of reactants on the surface, (iii) diffusion and chemical reaction at the surface and (iv) desorption and diffusion of products from the surface. Step (iii), involving the formation of surface intermediates, is the key step. Formation of surface intermediates, which ultimately give rise to products, was first proposed by Sabatier (see Burwell, 1973) and strikingly demonstrated by Sachtler Fahrenfort (1960) in the decomposition of formic acid... 

Zeolite catalysts may also be regarded as mixed oxides, but the crystallographic structures differ from the solids discussed above in that active sites for catalysis occur within the open lattice framework. In consequence, rate data are not directly comparable with similar observations for other heterogeneous reactions since the preexponential factors are calculated and reported on a different basis. For completeness, however, it is appropriate to mention here that instances of compensation behavior on zeolite catalysts are known. Taylor and Walker (282) described such an effect for the decomposition reactions of formic acid and of methyl forma te on cation-exchanged 13X molecular sieves, and comparable trends may be found in data reported for reactions of propene on similar catalysts (283). 

During the heterogeneous decomposition of formic acid on copper, the active metaUic phase undergoes sintering, sublimation and modifications of surface texture [25], By comparison with the behaviour of copper(II) formate [13], it is concluded that copper(l) formate is formed. Thus variations in the values of A and E, reported for the catalytic process may be attributed to a dependence of kinetic behaviour on the lifetime of the volatile participant, and thus upon metal mobility and reaction conditions [5], The decompositions of the copper formates and the catalytic decomposition of formic acid on copper metal thus include the participation of common intermediates, but these different reactions each consist of a sequence of several interdependent processes. 

Reports of investigations of metal-catalyzed decompositions of formic acid have discussed the participation of the metal formate as a reaction intermediate, for example in the decomposition of Ni(OOCH)2 [73]. This indicates the possible value of complementary investigations of both classes of heterogeneous rate processes crystolysis and metal surface chemistry. There is an extensive literature concerned with the decompositions of adsorbed species, including formic acid, at low coverages on almost perfect metal crystal faces. Results of such work may not, however, be directly applicable to the more crowded states existing at solid state reaction interfaces. 

IR spectroscopy was also helpful in elucidating the mechanism of the decomposition of formic acid, a well-known model reaction in heterogeneous catalysis. On me-... 

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