Synergistic photocatalytic treatment of aromatic hydrocarbons/NOx mixtures over TiO2: The activation and replenishment of lattice oxygen

Abstract

The photocatalytic oxidation (PCO) mechanisms of single aromatic hydrocarbons/NOx gases and the synergistic photocatalytic treatment of their mixtures over TiO2 (anatase) are proposed by analyzing the evolution of adsorbed and gas-phase intermediates through in-situ FTIR and mass-spectroscopy. Photo-activated surface lattice oxygen of TiO2 (TiO2(·OL)) is found to be a strong oxidant for the PCO of both o-xylene and NOx and their consumption led to the deactivation of photocatalysts. As an important coexist air pollutant, NO is firstly discovered as a helper for the persistent ring-opening and degradation of aromatics including o-xylene, toluene, styrene and benzene. NO2, a PCO intermediate of NO, acts as a powerful oxidant for replenishing lattice oxygen of TiO2, which guarantees the continuous generation of active TiO2(·OL) and prevents the deactivation of photocatalyst. With the introduction of NO, the PCO efficiency of o-xylene increases from ∼65% to 100% and no performance decay of TiO2 happens in 6 h. The existence of aromatics prevents the escape of toxic NO2 into air and promotes the harmless transformation of NO into nitrite/nitrate ions and N2. This work outlines the essential role of activation and replenishment of lattice oxygen in the PCO process by analyzing the synergistic and stable photocatalytic treatment of aromatic hydrocarbon/NOx mixtures, which explores the practical potentiality of photocatalysis in air purification.