Formic acid, decomposition on metal

D. Reactions for Formic Acid Decomposition ON Metal-Adlayer Surfaces... 

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... 

Shustorovich E, Bell AT (1989) An analysis of formic acid decomposition on metal surfaces by the bond-order-conservation-order-potential approach. Surface Sci 222 371-382... 

B. Results for Formic Acid Decomposition on Clean Metals. 21... 

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). 

Sachtler, W. M. H., Fahrenfort, J. The Catalytic Decomposition of Formic Acid Vapor on Metals. Proc. Second. Intern. Congr. Catalysis. Technip. Paris 1961, p. 831. 

Catalytic reactions on a metal oxide single crystal switchover of the reaction paths in formic acid decomposition on titanium dioxide TiO2(110). J. Am. Chem. Soc., 115, 10460-10461. 

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. 

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 points for Ag and Pd-Ag alloys lie on the same straight line, a compensation effect, but the pure Pd point lies above the Pd-Ag line. In fact, the point for pure Pd lies on the line for Pd-Rh alloys, whereas the other pure metal in this series, i.e., rhodium is anomalous, falling well below the Pd-Rh line. Examination of the many compensation effect plots given in Bond s Catalysis by Metals (155) shows that often one or other of the pure metals in a series of catalysts consisting of two metals and their alloys falls off the plot. Examples include CO oxidation and formic acid decomposition over Pd-Au catalysts, parahydrogen conversion (Pt-Cu) and the hydrogenation of acetylene (Cu-Ni, Co-Ni), ethylene (Pt-Cu), and benzene (Cu-Ni). In some cases, where alloy catalysts containing only a small addition of the second component have been studied, then such catalysts are also found to be anomalous, like the pure metal which they approximate in composition. 

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... 

Formic acid decomposition has been examined on a number of metal oxides using a wide assortment of surface analytical techniques to probe the decomposition pathways. One might expect the most facile adsorption to occur... 

Figure 5-13 shows the relative activity of various metal catalysts for the decomposition of formic acid [T20], The y-axis gives the temperature required to achieve a particular catalytic activity the lower the temperature, the higher the activity of the catalyst. On the x-axis the heats of formation of the corresponding metal formates are plotted. This so-called volcano plot shows a very good correlation between the strength of adsorption of the formic acid as a metal formate and the heat of formation of the individual compounds. 

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). 

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). 

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. 

Grenoble and coworkers229 reported an important influence of the support on the water-gas shift activity of various metal catalysts. For example, the rate increased an order of magnitude when Pt was supported on alumina versus silica. Turnover numbers for alumina-supported metal catalysts decreased in the order Cu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh, and Ir, whereby the activity varied by 3 orders of magnitude, suggesting a correlation between activity of the metal and the heat of adsorption. To describe these differences in activity, the authors used a bifunctional model, involving chemisorption of water on alumina and CO on the metal, followed by association of the CO with the water to form a formic acid-like formate species, with subsequent decomposition via dehydrogenation on the metal sites (Scheme 55). 

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