Surface Structure-Dependent Molecular Oxygen Activation Of BiOCl

As a byproduct of the developments of the society, environmental pollutions are increasingly pressing. To relieve this pressure, photocatalytic degradation of contaminants has become a promising strategy for environmental pollution control. Solar energy, semiconductors, and dioxygen are the main elements utilized in this method. Among these elements, molecular oxygen, the most natural and green oxidant, is expected to be the promising source of environmental remediation, but limited by its triplet state which forbid its reactions with most pollutants. Photocatalysis can promote the molecular oxygen activation on semiconductor surfaces, exciting the triplet state to doublet or singlet state. However, the mechanisms of the activation processes are still not so explicit, especially the correlations between the surface properties of semiconductors and the molecular oxygen activation processes. Therefore, we focus on the first step of the interactions between the surfaces and dioxygen, i.e., the processes of adsorption and chemical bonds alteration.1. Molecular oxygen activation processes are different with the distinct structures of semiconductor surfaces. According to recent density functional theory (DFT) calculations and scanning tunneling microscope (STM) experiments, there are two representatively active surface structures:surface oxygen vacancies and surface terraces. These two structures are the prerequisites of adsorption reactions, and share the common characters of exposing with unsaturated-coordinated metal ions on the surfaces. Based on this kind of coordinated adsorption, the following activation processes of molecular oxygen are conducted by the electron transfer (ET), spin selection, and electron tunneling mechanisms, resulting in producing ·O2-, O2-, O4-, and O4- species eventually. To selectively obtain the activated products, we choose the typically layer-structural photocatalyst, BiOCl, as the tunable substrate, because BiOCI has (001) surface with high energy and high oxygen density, and has (010) surface with low energy and open channel. These characters of BiOCl allow us to control the molecular oxygen activation paths.2. DFT calculations was been employed to systematically investigate the geometric and electronic structures of BiOCl bulk, crystal surfaces, surface oxygen vacancies, and the interactions between the surfaces and dioxygen. It was found that BiOCl (001) surface was exposed with high-density O-termination, which was stabilized by the ambient H+. This feature of high density of O atoms promoted the exothermic formation of surface oxygen vacancies. The oxygen vacancies induced a redistribution of electrons. As a result, two adjacent Bi atoms were accumulated some negative electrons to become the active sites. After the adsorption of the molecular oxygen, the electrons transferred from surface to dioxygen to produce ·O2-. (010) surface was a low energy surface because its surface atom numbers were stoichiometric and the charges of hanging bonds were self-compensated. In contrast to the cases on (001) surface, oxygen vacancies created on (010) was endothermic. Three adjoining Bi atoms were reduced by the surface oxygen vacancies so that O22- species produced through two-electron transfer.3. Upon the DFT calculation results, experimental methods, including in-situ FT-IR, Raman, EPR, PL, et.al, were applied to detect BiOCl (001) and (010) surface structures and the products of activated molecular oxygen at the real reaction conditions. It was revealed that BiOCl (001) surface was covered by hydroxyl groups. Under UV light irradiation, oxygen vacancies produced among the samples, together with the photogenerated electrons reducing the adsorbed O2 to ·O2-. (010) surface also exhibited oxygen vacancies after UV light excitation. At the same time, the photogenerated electrons preferred to activate the adsorbed O2 to ·O22-. Thus, the experiments and the calculations corroborated mutually. These findings helped us to understand BiOCl surface structure-dependent molecular oxygen activation from atomic level.4. Cosidering the oxidation from alchol to aldehyde via two electrons and protons transfer, BOC-001 and BOC-010 were expected to exhibit selection for the adehyde synthesis. Thereby we endowed BOC-001 and BOC-010 to catalyze the selective oxidation of benzyl alchol (BA) to benzaldehyde (BAD). Through simiulations, we found that the adsorption reactions of BA on BOC-001 and BOC-010 were similar, which could not determine the different reactivities between them. Further investigations by experiments indicated that the first step of BA reaction began with photogenerated hole oxidation, but not with the reaction with the surface activated O2. Besides, the reactivity of BOC-010 was better under full light irradiation, while BOC-001 performed better under under UV illumination.