Monolayer dispersion of CoO on Al2O3 probed by positronium atom

CoO/Al2O3 catalysts were prepared by wet impregnation method with CoO contents ranging from 0 wt% to 24 wt%. X-ray diffraction and X-ray photoelectron spectroscopy measurements suggest formation of CoO after calcined in N2. Quantitative X-ray diffraction analysis indicates monolayer dispersion capacity of CoO in CoO/Al2O3 catalysts to be about 3 wt%. Positron annihilation lifetime and coincidence Doppler broadening measurements were performed to study the dispersion state of CoO on Al2O3. The positron lifetime measurements reveal two long lifetime components τ3 and τ4, which correspond to ortho-positronium annihilation lifetime in microvoids and large pores, respectively. It was found that the positronium atom is very sensitive to the dispersion state of CoO on Al2O3. The presence of CoO significantly decreases both the lifetime and the intensity of τ4. Detailed analysis of the coincidence Doppler broadening measurements suggests that with the CoO content lower than the monolayer dispersion, spin conversion reaction of positronium is induced by CoO. When the cobalt content is higher than the monolayer dispersion capacity, inhibition of positronium formation becomes the dominate effect.