Differentiated crystal structures generally affect the surface physicochemical properties of catalysts, causing variety in catalytic activity between polymorphs. However, the underlying mechanism has not been completely revealed, especially the influence of surface physicochemical properties on photocatalytic redox activity and the reaction mechanism. In this work, we reveal the mechanism of surface redox properties on different crystal forms of gallium oxide from a molecular level. α-Ga2O3 and β-Ga2O3 exhibit a slight difference in catalytic oxidation of organic pollutants due to comprehensive influencing factors, including their valence band position, reactive oxygen species, and pore structure properties related to the adsorption-reaction-desorption process. But the catalytic reduction ability of CO2 is obviously different due to the large differences of interaction between the surface of crystal structures and CO2 molecules, which are critical to determine the catalytic performance and reaction pathways. The enhanced adsorption and activation of CO2 on the α-Ga2O3 surface could promote the reduction reaction efficiency. Moreover, the large energy barrier of CH2* formation on β-Ga2O3 makes the formation of methane (CH4) relatively difficult compared to that on α-Ga2O3. The yield rate of CH4 (1.8 μmol·g-1·h-1) on α-Ga2O3 is three times better than that on β-Ga2O3 (CH4: 0.6 μmol·g-1·h-1). The current findings can offer novel insights into the understanding of crystal-structure-dependent photocatalytic performances and the design of new catalysts applied in energy conversion and environmental purification by crystal structure-tuning.
Keywords: Ga2O3; crystal structure; photocatalytic redox activity; reaction pathways; surface properties.