IR study of the adsorption and decomposition of methanol on carbon surfaces and carbon-supported catalysts
Introduction
Adsorption of alcohols can be used both to understand the nature of the adsorbed state and to characterize the adsorbent by determining surface area, pore size distribution and surface heterogeneity. In previous IR studies [1], [2] on phenomena of aliphatic alcohols adsorption on carbons, the sorbent surface is usually considered as inert as to be unaffected by the presence of adsorbed molecules. In general, relatively little attention has been paid to strong specific interactions between adsorbed molecules and the surface of carbons.
The interaction of alcohols with adsorbent and catalyst surfaces is relevant with respect to a number of heterogeneously catalyzed reactions such as dehydrogenation and dehydration. The decomposition of methanol has been investigated on the most of the group VIII metals [3], [4], [5] and on the group IB metals [5], [6], [7]. These studies have demonstrated that methanol decomposition proceeds through methoxide species produced by the removal of the hydroxyl hydrogen. The conversion of methanol to formaldehyde as intermediate in the decomposition and oxidation of methanol vapor has been observed [7], [8], [9]. Although numerous investigations [10], [11], [12], [13], [14] employing spectroscopic methods have been carried out to establish the nature of adsorbed organic residues of methanol on catalysts, the subject is still a mater of controversy.
IR spectroscopy of carbon-supported metal catalysts has not received much attention due to the difficulty of obtaining IR spectra on these highly opaque samples. Due to the application of the carbon film technique, the problems of experimental spectroscopic investigations of surface phenomena on carbons have been solved to a great extent. FTIR vibrational spectroscopy provides a unique means to obtain information on concentration of reactants, intermediates and products at the surface of the catalysts. The method of recording IR spectra by using the vacuum cell has made it possible to analyze the reaction products quickly and with an absolute minimum of disturbance of the samples so as to avoid intervening secondary reactions. The in situ investigations of heterogeneously catalyzed reactions are among the most important experiments for testing the proposed reaction mechanisms.
Section snippets
Experimental
Carbon films were prepared from cellulose [1]. The raw material used for carbonization was cellophane which was previously purified. The following procedure was used to prepare an oxidized carbon film. Samples outgassed at 873 K (a standard pre-treatment) were exposed to 1000 hPa of pure O2 at 573 K for 1 h. In the present studies, carbon films differing in the chemical structure of surface functional groups were used. Methyl alcohol (99.9%) from Aldrich was used.
Metals-loaded carbon samples
Results and discussion
Fig. 1 shows the IR spectra of an oxidized carbon sample after being exposed to CH3OH at 7 hPa and after subsequent heating to various temperatures.
The presence of absorption bands at 1830, 1753 and 900 cm−1 [Fig. 1(a)] indicate that some of the acidic surface groups are cyclic anhydrides and probably lactone structures. The band at 1600 cm−1 and mutually overlapping absorption bands within the 1150–1450 cm−1 range have been discussed previously [1]. As a result of methanol adsorption, a band
Conclusions
Adsorption and thermal decomposition of methanol on carbon and carbon-supported metal catalysts have been investigated by FTIR and gas chromatography. The data reported in this work show that pure nonoxidized carbon itself is unreactive in the decomposition of methanol.
In the present study the vibrational spectra for methanol adsorbed on oxidized carbon surfaces provided evidence for participation of methoxides and formaldehyde intermediates in the thermal decomposition of methanol. On the
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