Tailoring Active O₂^– and O₂^2– Anions on a ZnO Surface with the Addition of Different Alkali Metals Probed by CO Oxidation

To explore the promotional effects of alkali metals on a ZnO surface, a series of ZnO-based samples doped by Li, Na, K, and Cs were fabricated and analyzed with various methods. Moreover, their reactivity was probed by CO oxidation. Wurtzite ZnO is formed as the predominant phase in all the samples, and the alkali metal species distributed finely on the catalyst surface. With the addition of alkali metals, the amount of ZnO surface oxygen vacancies increases significantly, which induces a more effective formation of active surface O₂^– and/or O₂^2– anions. Raman, O₂ temperature-programmed desorption, electron paramagnetic resonance, X-ray photoelectron spectroscopy, and in situ diffuse reflectance Fourier transform spectroscopy studies have demonstrated that a sample with richer surface oxygen vacancies usually possesses more abundantly these active surface oxygen species. The promotional effects of the alkali metals on ZnO follow nearly the sequence of K > Cs > Na > Li. It was found that the abundance and diversity of surface active oxygen anions determine the reactivity of ZnO-based catalysts. K-ZnO and Cs-ZnO, the K and Cs-modified catalysts, possess much larger quantities of both O₂^– and O₂^2– sites, thus displaying far better CO oxidation activity than that of other catalysts.