Adsorption and desorption of methyl alcohol on magnesium oxide

Abstract

Methyl alcohol absorbs on MgO to give a coverage of one molecule for each " ion pair " on the surface. Infra-red studies of successive additions of the alcohol indicate the build-up of various alcohol species on the surface which can be characterized mainly by their CO stretching bands and also by the presence of OH and CH bands. Infra-red and gas chromatographic desorption studies in vucuo at room temperature and at elevated temperatures are used to investigate the stability of the adsorbed species. The four major adsorbed species are characterized by CO stretching frequencies at 1030,1060,1114 and 1080 cm-'. This latter species is stable to above 600 K and its disappearance is accompanied by the evolution of alcohol decomposition products. Comparative studies with deuterated alcohols confirm the assignments and indicate that some exchange reactions involving hydrogens of the methyl group occur at elevated temperatures.

Thermal decomposition of methanol adsorbed on magnesia*1

“…On the basis of infrared absorption studies , Tench et al [2) have proposed a type ( Although we cannot categorically rule out the possibility of type ( 1) heterogeneity , our high resolution thermal deeorption studies are consistent with the proposal of ref. ( 2) and serve to clarif y and amplify certain of its features .…”

“…The T~ data described above makes it clear that partial desorption produces a state of the surface that differs quali- (2) has been proposed by Pen [ 7] for hydroxyl groups on alumina , and i ts correc tness for methoxyl on alumina has been verified by Matsushima and White [ 4], who…”

“…Thus the temperature dependence of desorption of species III reported in ref. [2] is probably determined primarily by the rate of surface migration , whereas our data reflects primarily the relative ease of desorption of various subspecies of III having different local concentrations. The origin of the formaldehyde produced during desorption cannot be unambiguously assigned to one of the two surface methoxy l species discussed above , but the evidence suggests species IV as its most likely source.…”

“…showed that the C13 3 0 remaining af ter the fir st cycle of a One impor tant piece of evidence for model (2) for alumina is the fac t that the onse t tempera ture for methanol desorption and the position of the peak maximum are independent of exposure and evacuation time [4]. In the case of MgO , however , the evidence is some what less clear .…”

“…( 2) and serve to clarif y and amplify certain of its features . The low-temperature therma l desorption of methanol , which increases more slowly with exposure than the higher-temperature d.sorption, is to be attributed to species I and II.…”