Temperature intermediates, adsorption kinetic

In Sect. 3.2.2, the effects of precursor states on adsorption kinetics have been discussed. Since even the earliest adsorption experiments show evidence for the influence of weakly bound intermediate states, following the principle of microscopic reversibility it might be expected that desorption kinetics would show the influence of such species. However, desorption experiments are usually carried out at considerably higher temperatures and average lifetimes in such states will be much lower. 

At present we have evidence for the complexity of higher temperature adsorption/desorption phenomena while, in general, the kinetic characteristics observed for many catalytic reactions are perhaps deceptively simple. The estimations of the concentrations of the participating surface intermediates are, in contrast, experimentally very difficult. Mechanistic investigations of many heterogeneous catalytic processes yield insufficient information to allow clear distinctions to be drawn between alternative reaction modelsf 125). 

Kinetic evidence for synergic adsorption of carbon monoxide and water on the low-temperature shift catalyst Cu/ZnO/Fe203 was obtained by van Herwijnen and deJong (113), and IR spectra of surface formate were detected on several oxide catalysts, including CuO/MgO, at temperatures as low as 20 JC and pressures of 20 Torr, as reported by Davydov et al. (104). Decomposition of the surface formate to C02 and H2 occurred at 100-150°C over the Cu/MgO catalyst and at 250 300°C over the MgO catalyst, and the promotion effect of copper was attributed to the formation and decomposition of a labile surface formate (HCOO)2Cu. Ueno et al. (117) have shown earlier that surface formates are formed on zinc oxide, from CO and H20 as well as from C02 and H2, and hence an associative mechanism of the shift and reverse-shift reaction, involving formate intermediate, is believed to operate on many oxide catalysts. 

The effect of adsorption temperature on metals or supported metals on the mobility of adsorbed probe molecules has not received as much attention as on metal oxides. Gelin and co-workers (96) used adsorption microcalorimetry at 296 and 423 K and IR spectroscopy to study the adsorption of CO on Ir supported on NaY zeolite reduced from 383 to 923 K and on Ir supported on silica. At 296 K it was observed that for intermediate coverages (6 > 0.3) the kinetics of adsorption changed, with the thermograms displaying long tails... 

As mentioned above no adsorption of CO is observed at around room temperature. When both reactants, O2 and CO, are introduced into the ion trap, the reaction kinetics of Au2 changes drastically, as seen by the offset in the Au2 signal. This offset increases when the partial pressure of CO is augmented. In addition, at temperatures below 200 K the intermediate with the stoichiometry Au2(C0)02 could be isolated (Figure 17.2A-b). The ion stoichiometry clearly shows that CO and O2 are able to coadsorb onto an Au2 dimer. From the kinetics of all observable ions, Au2, Au202, and Au2(C0)02, measured under a multitude of different reaction conditions the catalytic conversion of CO to CO2 could unambiguously be detected. The reaction mechanism that fulfills all the prerequisites and fits all kinetic data measured under all the different reaction conditions could be described by the reaction equations below ... 

To reveal the complete reaction mechanism, the reaction was investigated at lower temperatures. The product ion mass spectrum recorded at 100 K with O2 and CO in the ion trap (Fig. 1.63b) shows the appearance of the coadsorption complex Au2(C0)02 discussed above. This complex represents a key intermediate in the reaction mechanism of the catalytic oxidation of CO to CO2 as has been predicted in the earlier theoretical study [382]. The experimental evidence obtained so far demonstrates that O2 adsorption is likely to be the first step in the observed reaction mechanism. Subsequent CO coadsorption yields the observed intermediate (Fig. 1.63b) and finally the bare gold dimer ion must be reformed. The further strategy to reveal the full reaction mechanism consists in varying the available experimental parameters, i.e., reaction temperature and reactant partial pressures. This procedure leads to a series of kinetic traces similar to the one shown in Fig. 1.64b and c [33]. The goal then is to find one reaction mechanism that is able to fit all experimental kinetic data obtained under the various reaction conditions. This kinetic... 

Recently there have appeared papers by other authors in which volcano-shaped curves have been obtained. Fahrenfort, van Reijen, and Sachtler (467) have carried out complex kinetic, IR spectroscopic, calorimetric, and mass spectrometric investigations on the decomposition of formic acid on various metals. The authors come to the conclusion that the reaction proceeds via the intermediate formation of an adsorption complex of the surface nickel formate type. By comparing the heat of formation of the formate of the corresponding metal with the temperature Tr at which a fixed depth of conversion r (log r = —0.8) is reached, the authors have obtained a broken line similar to the Balandin volcano-shaped curves (Fig. 63). The catalyst half-covered with the adsorption complex is the most active one. The reaction investigated by the authors differs from those investigated by us. It is characteristic, however, that in the case of oxides the selectivity is the same with respect to the decomposition of alcohols and of formic acid [Fig. 1 in Mars (468)). In their report at the Paris Congress on Catalysis Sachtler and Fahrenfort (469) give additional data on volcano-shaped curves for a number of reactions and point out that this relationship between the catalytic activity and the stability of the intermediate complex has been qualitatively predicted by Balandin. ... 

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