Benzene Oxidation in Air by an Amine-Functionalized Metal-Organic Framework-Derived Carbon- and Nitrogen-Loaded...

To learn more about the behavior of amine (NH₂)-functionalized metal-organic framework (MOF)-derived noble metal catalysts in the removal of aromatic volatile organic compounds in air, benzene oxidation at low temperatures has been investigated using 0.2-, 0.8-, and 1.5%-platinum (Pt)/Universitetet i Oslo (UiO)-66-NH₂. The benzene conversion (X_B) of x%-Pt/UiO-66-NH₂-R under dry conditions (175 °C) was 23% (x = 0.2%) < 52% (x = 0.8%) < 100% (x = 1.5%): 'R' suffix denotes reduction pretreatment using a hydrogen (10 vol %) and nitrogen mixture at 300 °C for the generation of metallic Pt (Pt⁰) sites and simultaneous partial MOF decomposition into carbon- and nitrogen-loaded zirconium dioxide. The prominent role of reduction pretreatment was apparent in benzene oxidation as 1.5%-Pt/UiO-66-NH₂ did not exhibit catalytic activity below 175 °C (dry condition). The promotional role of moisture in benzene oxidation by 1.5%-Pt/UiO-66-NH₂-R was evident with a rise in the steady-state reaction rate (r) at 110 °C (21 kPa molecular oxygen (O₂)) from 1.3 × 10^-3 to 5.0 × 10^-3 μmol g^-1 s^-1 as the water (H₂O) partial pressure increased from 0 to 1.88 kPa. In contrast, the activity was lowered with increasing RH due to catalyst poisoning by excess moisture (r (110 °C) of 6.6 × 10^-04 μmol g^-1 s^-1 at 2.83 kPa H₂O (21 kPa O₂)). Kinetic modeling suggests that X_B proceeds through the Langmuir-Hinshelwood mechanism on the Pt/UiO-66-NH₂-R surface (dissociative O₂ chemisorption and the involvement of two oxygen species in benzene oxidation). According to the density functional theory simulation, the carbon and nitrogen impurities are to make the first X_B step (i.e., hydrogen migration from the benzene molecule to the substrate) energetically favorable. The second hydrogen atom from the benzene molecule is also extracted effectively, while the oxygen derived from O₂ facilitates further X_B. The Pt⁰ sites dissociate the O₂ and H₂O molecules, while the product of the latter, i.e., free hydrogen and hydroxyl, makes the subsequent X_B steps energetically favorable.