Methanol adsorption stoichiometry

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

Coadsorption of reactants and subsequent thermal decomposition of the surface complexes formed have been used to resolve the mechanisms in several studies (81-84). Mutual enhancement of the adsorbed amounts of the reactants is indicative of their interaction, and if the adsorption of separately admitted components is negligible, the stoichiometry of the adsorbed complex can be determined. Further evidence for the formation of an adsorbed complex, employed in a mechanistic study of methanol synthesis over ZnO (84), is obtained by thermal decomposition of the adsorbed complex if the reactants appear simultaneously at one temperature upon thermal desorption from a coadsorbed layer, but if each reactant adsorbed separately gives a thermal desorption peak at a different temperature, the existence, although not necessarily the structure or com-... 

Carbon dioxide and hydrogen also interact with the formation of surface formate. This was documented for ZnO by the IR investigation of Ueno et al. (117) and, less directly, by coadsorption-thermal decomposition study (84). Surface complex was formed from C02 with H2 at temperatures above 180°C, which decomposed at 300°C with the evolution of carbon monoxide and hydrogen at the ratio CO Hs 1 1. When carbon dioxide and hydrogen were adsorbed separately, the C02 and H2 desorption temperatures were different, indicating conclusively that a surface complex was formed from C02 and H2. A complex with the same decomposition temperature was obtained upon adsorption of formaldehyde and methanol. Based upon the observed stoichiometry of decomposition products and upon earlier reported IR spectra of C02 + H2 coadsorbates, this complex was identified as surface formate. Table XVI compares the thermal decomposition peak temperatures and activation energies, product composition, and surface... 

The catalysts used to oxidize alcohols take as a first consideration the mechanism of the oxidation reaction. The mechanism will be illustrated with reference to the methanol oxidation on platinum but, with different stoichiometries, the principles can be extended to other alcohols. The first step is believed to be the adsorption of the alcohol ... 

Section 11.4.1 to Section 11.4.4 discuss the stoichiometry of adsorption, kinetic parameters, reactivity, and surface morphology of several catalytic materials that have been obtained through the application of methanol chemisorption to determine Ns and TOP values. 

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