Formate species

The carbonate can also be compared with adsorbed formate species prepared by reacting methanol with the alumina surface at 350 C ( ). The spectrum for adsorbed formate. Figure 4d, shows the asymmetric carboxylate stretches at 1565 and 1440 cm l respectively, the CH stretch at 2832 cm , and the CH bending mode at 1505 cm . The Al-OC stretching mode is seen at 1060 cm", and the out-of-plane deformation at 750 cm . The signal to noise ratio in the low frequency end of the spectrum is insufficient to see the planar deformation, which should occur around 630 cm . It should be noted that the carbonate and formate species are very similar, the main distinction being the vibrations associated with the CH bond. 

Starting from n-butane, 2-butoxides that rapidly convert to 2-butanone are found over MgCr204 [24]. However, the further oxidation of adsorbed 2-butanone only gives rise to the acetate species, while starting from n-butane, formate species are also observed. This can be explained assuming that sec-butoxides can partly isomerize to rert-butoxides before further oxidation. This implies that the C-O bond formed is partly ionic and the alkyl moiety has the... 

The mjz = 60 ion current (Fig. 13.7c), which results from methyl formate and hence is indicative of formic acid formation, equally increases in a pronounced step after raising the potential, and then decays slowly with time. (The higher noise in these data is due to the very low concentration of methyl formate species.)... 

Whereas in the indirect pathway, COad is clearly identified as a reaction intermediate, the specific nature of the intermediate(s) in the direct pathway is under debate. For methanol oxidation, species such as COH [Xia et al., 1997 Iwasita et al., 1987, 1992 Iwasita and Nart, 1997], CHO [Zhu et al., 2001 Willsau and Heitbaum, 1986 Wilhehn et al., 1987], COOH [Zhu et al., 2001], and adsorbed formate species [Chen et al., 2003] have been proposed. Adsorbed formate species were identified during formaldehyde oxidation [Samjeske et al., 2007], methanol oxidation [Nakamura et al., 2007 Chen et al., 2003, unpublished], and fornfic acid oxidation [Miki et al., 2002, 2004 Samjeske and Osawa, 2005 Chen et al., 2006a, b, c Samjeske et al., 2005, 2006]. 

Figures 19.6 through 19.11 detail the isotopic exchange rates during water-gas shift for the formate, C02, and Pt-CO bands, in switching from the 12C to 13C label. In all cases, the reactive exchange rates of formate and C02 were virtually identical, implicating the formate species as the likely intermediate to the water-gas shift catalytic mechanism over Pt/Zr02 and PtNa/Zr02 catalysts. The DRIFTS spectra at the top of each figure show the switching of these species from...

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. 

Puschmann et al. also carried out a SEXAFS analysis giving an adsorption site shown as A in the inset to Figure 8. The adsorption site of the formate species on copper surfaces in general has, however, proved to be controversial [22,23]. Most recent... 

Figure 7 0 1s NEXAFS from the surface formate species on Cu 110. The E vector is aligned in (a) the <110> azimuth and in (b) the <100> azimuth. After [20].

Having established the orientation of the formate species on the surface we can proceed to examine the photoemission data. 

Figure 9 shows the effect of deprotonation of adsorbed formic acid which occurs on warming the surface to above 270 K. Spectrum (a) can be assigned by comparison with the photoelectron spectrum of the free molecule. The formate species also gives rise to four spectral bands and, since the number of expected orbitals is the same, it is tempting to assume a one-to-one correspondence, allowing of course for the change in symmetry from Cs to C2v- The application of selection rules proves, however, that such an... 

Figure 8 Photoelectron diffraction data (normal emission) for the surface formate species on (a) Cu 100] and (b) Cu 110). Insets A) The aligned atop site and B) the aligned bridge site. After [51.

They focused their research on answering the question as to whether catalysis proceeds via formate anion as an intermediate, such that dissociation of a CO ligand is the first step in the mechanism Cr(CO)6 -> Cr(CO)5 + CO followed by nucleophilic attack by formate anion i.e., from CO + OH- -> HCOO-) to produce the formate species Cr(CO)5 + HCOO- > Cr(CO)5(OOCH) , according to King et al.5 in Scheme 18 or whether a metallocarboxylic acid forms upon... 

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). 

Campbell and coworkers287,297 also published work on methanol synthesis. In that case, they also found that the active site is metallic copper, but they suggest that the mechanism in that case does involve formate intermediates, and that the ratedetermining step is likely the hydrogenation of the formate species to methanol. 

The authors use some arguments to rule out the formate intermediate before proceeding to the discussion regarding the redox mechanism. The basis was not the activation energy barrier for OCOH formation, as it is quite low relative to other possible pathways. They rule out the OCOH as an intermediate on the basis of its proposed decomposition selectivity, which they indicate will always preferentially decompose back to CO and OH. And the isomerization to a true formate species (i.e., with a C-H bond) they indicate cannot occur by reacting OH on Cu(110) with CO. Yet intense formate bands have been observed by infrared spectroscopy upon... 

In 1999, Binet et al.395 published a review on the response of adsorbed molecules to the oxidized/reduced states of ceria. In light of recent infrared studies on ceria, the assignments for OH groups, methoxy species, carbonate species, and formates are highly instructive. The OH and methoxy species have been briefly discussed. Characteristic band assignments of carbonate and formate species are provided below, the latter formed form the dissociative adsorption of formic acid, the reaction of CO with H2-reduced ceria surface, or via selective oxidation of methanol. Formate band intensities were a strong function of the extent of surface reduction of ceria. 

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