Application of infrared spectroscopy to the measurement of surface and bulk oxidation/reduction states of MoO3

Abstract

The oxidation/reduction state of finely divided MoO₃ catalyst has been studied by measuring the background transmission of infrared radiation. Experimental results have shown that the background transmission at lower wavenumber is mainly sensitive to the surface oxidation/reduction state of MoO₃, and the transmission at higher wavenumber is more sensitive to the subsurface layers. Assuming a different penetration depth of infrared radiation through the MoO₃ particle at a different frequency, based on electromagnetic theory, it is possible to explain the above results by a simple model. A normalized absorbance is defined and used to measure the degree of reduction on the surface and in the bulk of MoO₃. When MoO₃ is reduced by hydrogen, by methanol alone, and by methanol–O₂ mixtures below 140 °C, the surface is more reduced than the bulk. Above 140 °C exposure of MoO₃ to methanol–oxygen mixtures produces particles for which the subsurface layers are generally more reduced than the surface. The degree of reduction is dependent on oxygen concentration and is controlled by the way in which the reduction force by methanol is balanced by the oxidation force by oxygen. The existence of a space-charge barrier caused by ionosorbed oxygen is a characteristic of the catalyst during the reaction.