CO oxidation by Ti- and Al-doped ZnO: Oxygen activation by adsorption on the dopant

Using a combination of theory and experiments, we show that ZnO substitutionally doped with Ti or Al oxidizes CO by several parallel reaction pathways that differ from the traditional Mars–van Krevelen (MVK) mechanism. In one, the dopant atoms at the surface of the doped oxide adsorb and activate O2 so that it reacts with CO. In the other, a surface oxygen atom from the lattice next to the dopant, D, moves onto the dopant and creates an O-D group and an oxygen vacancy. The O-D group is capable of oxidizing a reductant. To test these predictions made by theory, we have prepared Ti- and Al-doped ZnO and have shown that these compounds oxidize CO at temperatures at which pure-phase ZnO, Al2O3, or TiO2 do not. The proposed mechanisms were made plausible by studying CO oxidation with 18O2.