Adsorbate formation

The mechanism is thought to involve dissociation of hydrogen, which reacts with molecularly adsorbed CO2 to form formate adsorbed on the surface. The adsorbed formate is then further hydrogenated into adsorbed di-oxo-methylene, methoxy, and finally methanol, which then desorbs. The reaction is carried out under conditions where the surface is predominately empty and the oxygen generated by the process is quickly removed as water. Only the forward rate is considered and the process is assumed to go through the following elementary steps ... 

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. 

Recently, it has been demonstrated that coordination vacancies on the surface metal cations are relevant to the unique redox reactivity of oxide surfaces]2]. Oxidation of fonnaldehyde and methyl formate to adsorbed formate intermediates on ZnO(OOOl) and reductive C-C coupling of aliphatic and aromatic aldehydes and cyclic ketones on 1102(001) surfaces reduced by Ar bombardment are observed in temperature-prognunmed desorption(TPD). The thermally reduced 1102(110) surface which is a less heavily damaged surface than that obtained by bombardment and contains Ti cations in the -t-3 and +4 states, still shows activity for the reductive coupling of formaldehyde to form ethene]13]. Interestingly, the catalytic cyclotrimerization of alkynes on TiO2(100) is also traced in UHV conditions, where cation coordination and oxidation states appear to be closely linked to activity and selectivity. The nonpolar Cu20( 111) surface shows a... 

Figure 6.16 Attenuated total reflection surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS) spectra for the oxidation of 0.1 M HCOOH in 0.5 M H2SO4 on a polycrystaUine electrode. The bands at 2055 -2075 and 1800-1850 cm are assigned to linear- and bridge-bonded CO, whereas the band at 1323 cm corresponds to adsorbed formate. (Reproduced from Samjeske et al. [2006].)...

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]. 

The adsorbed acetaldehyde (CHsCHOad), adsorbed acetate (CHsCOO ad) and adsorbed formate (HCOO ad) relative IR intensity during the first 20 min of photocatalytic oxidation are presented in Figure 52.3 for the Pt/Ti02 catalyst and Ti02 catalyst (Aeroxide P25S Degussa). Results of Ti02 are included for... 

Diagne, Kiennemann, and coworkers—TPD study of adsorbed formate... 

It is essential to understand the mechanisms of methanol oxidation including the adsorbate formation and removal. To this purpose, the electrochemical oxidation of adsorbed carbon monoxide (COad) and methanol was studied using electrochemical and two spectroscopic... 

Perchloric add, which is less adsorbing to platinum, showed a smaller sustained current than sulfuric and phosphoric adds, which more likely adsorb on platinum- Anions are considered to play important roles in the methanol oxidation, the adsorbate formation and its removal. 

Ni(lOO) stabilized the adlayer and only isolated formate ions decomposed easily at this temperature. Thus, inhibition of the reaction by adsorbed formate ions is possible, if they form islands. 

High expenses for adsorbents formation of secondary hazardous waste. 

The compensation relationships mentioned here for the decomposition of formic acid on metals (Table III, K-R and Figs. 6 and 7) cannot be regarded as established, meaningful kinetic descriptions of the reactions concerned, since the magnitudes of the calculated values of B and e depend on the selection of data to be included in the calculation. While there is evidence of several sympathetic interrelationships between log A and E, the data currently available do not accurately locate a specific line and do not define values of B and e characteristic of each system, or for all such systems taken as a group. The pattern of observations is, however, qualitatively attributable to the existence of a common temperature range within which the adsorbed formate ion becomes unstable. The formation of this active intermediate, metal salt, or surface formate, provides a mechanistic explanation of the observed kinetic behavior, since the temperature dependence of concentration of such a participant may vary with the prevailing reaction conditions. 

Had the first oxidative coupling not taken place, the final product would have been adsorbed formate. 

The term chemisorption was coined in order to classify the interaction between a particle in the gas phase and a solid surface, i.e. the result of the adsorption process [1]. If the interaction leads to the formation of a chemical bond the adsorbate formed is called a chem-isorbate. Where chemical bond formation is not important the process is classified as physisorption. There are several conceptual problems with such a differentiation which we briefly address in the following, and which indicate that a more detailed look at the entire process of adsorbate formation is needed before a reliable classification may be carried out. In fact, as it turns out, for a conclusive classification one would need the full theoretical and experimental understanding of the system under investigation. Such an approach must include the static aspects, i.e. the energies involved, as well as the dynamic aspects, i.e. the processes involved in the formation of the adsorptive interactions. 

Fig. 33. Schematic representation of the geometry of adsorbed formate on Ti02(l 10) [165].

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

Methoxide may also combine with surface hydrogen to form methanol. This step is similar to the production of formic acid at higher temperatures from adsorbed formate. 

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