The use of TPD and TPR to study subsurface mobility: Diffusion of oxygen in Mo 2C

Abstract

The application of temperature-programmed desorption (TPD) and reduction (TPR) to the study of subsurface diffusion in high surface area powders is considered. Although subsurface diffusion may be important in catalysis, relatively few techniques have been developed to study this process in high surface area materials. Since oxygen is very mobile in Mo 2C, we chose to study Mo 2C to illustrate how TPD and TPR can be used to study subsurface diffusion. In particular, the effect of the Mo: C ratio on the oxygen mobility is investigated. The TPR spectrum of oxygen adsorbed on Mo 2C contains two water peaks, a narrow one at 479 K, and a second much broader peak near 573 K. While the first peak is produced by surface oxygen, the second peak is caused by oxygen that diffuses into the subsurface region of the catalyst during the temperature ramp. As the surface becomes depleted at higher temperatures, this oxygen diffuses back to the surface, reacts with the gas-phase hydrogen, and desorbs as water. For carbon-deficient catalysts the two peaks merge into one asymmetric peak at 510 K with a tail extending to higher temperatures. A model is presented which gives a semiquantitative description of the effect of subsurface diffusion on TPD and TPR spectra. The model is able to explain the changes in the spectrum caused by varying the Mo: C ratio; the data cannot be explained by a model which assumes that the two peaks are due to two distinct adsorption sites on the surface. An important result of this work is that in cases in which a peak produced by subsurface diffusion is resolved, the activation energy for diffusion in the subsurface can be measured.