Application of Wolkenstein’s Electronic Theory to Size Effects in CO Oxidation over ZnO Nanocatalysts

The volcano-shaped dependence of the catalytic activity of the magnesia-supported ZnO nanoparticles on their diameter in CO oxidation was considered in the framework of Wolkenstein’s electron theory of catalysis on semiconductors. By analyzing the diffuse reflectance UV-Vis spectra of the ZnO nanoparticles in catalysts, we demonstrate that a narrow range of particle diameters (4.0–4.6 nm) leads to changes in the Fermi level due to quantum confinement of free electrons. As the diameter of the ZnO nanoparticles decreases, the Fermi level rises, resulting in an accelerated acceptor stage and a decelerated donor stage involving free electrons interacting with atomic oxygen and carbon dioxide on the catalyst surface, respectively. This opposing change in the rates of the donor and acceptor stages during the CO oxidation reaction, influenced by the diameter of the ZnO nanoparticles, gives rise to a volcano-shaped size dependence of the reaction rate. Furthermore, an optimal catalyst particle diameter is identified, at which the reaction rate reaches its maximum.