ABSTRACT
Many heterogeneous catalysts of industrial use consist of metals and promoters supported by porous alumina of very high specific surface. In order to optimize the catalyst, it is of great practical importance to know the space distribution of active elements inside the matrix. Electron Probe Micro Analysis (EPMA) is a particularly useful technique for studying local variations of composition. Although the concentrations of metals in our catalysts are usually less than 2 % in weight, experimental EPMA measurements clearly reveal that the intensity of X-rays (A1 Koc and O Ka) emitted from porous alumina is less than the intensity from compact mono-crystalline alumina (sapphire). Moreover, the loss of X-rays depends on the particular line being measured, the accelerating voltage, the measurement time, the embedding medium, the preparation of the sample, etc. Consequently, when traditional EPMA quantitative procedures are employed, they yield unrealistically low concentrations of metals. Some authors [1,2] have suggested that the Xray signal loss might be a consequence of charge trapping in the insulating material. On the other hand, measured x-ray intensities also depend on the porosity of the sample, which indicates that the signal loss may be partially due to the porous structure of the material.
