(627c) A Mechanistic Understanding of Photocatalytic NOx Oxidation over TIO2 Hybrids

A promising approach to address the limited activity of TiO₂ in NOx oxidation is through coupling with nanomaterials. However, the limited understanding of the photocatalytic mechanisms of the resulting hybrids under relevant conditions is still an impediment to the rational design process. In this study, we have conducted NOx oxidation experiments in a laminar flow reactor using TiO₂ hybrid photocatalysts and pristine TiO₂ as a reference under different conditions, including NOx composition, relative humidity, gas-phase environment, and presence of a hole scavenger. The hybrid photocatalysts were TiO₂ coupled with carbon nanotubes (CNTs), Mxenes, or TiC. Scenarios were designed that hindered specific reaction pathways to develop a deeper understanding of the differences between the mechanisms of each catalyst by comparing their NO conversion and rate constants for NO and NO₂ oxidation reactions. Two scenarios of significance are the reaction in the absence of water and in the absence of oxygen. P25 showed favorability towards the superoxide mechanism, with an NO conversion of 46% in the absence of water and an NO conversion of 25% in the absence of oxygen. Meanwhile, both the TiO₂-CNTs and the TiO₂-Mxenes showed similar conversions of approximately 50% in both scenarios. The most dramatic difference found in the mechanism pathways is the mechanism of the TiO­₂-TiC, which showed no conversion in either scenario. While P25 showed favorability of superoxide, TiO_2­-CNTs and TiO₂-Mxenes show equal weight towards both; the results of TiO₂-TiC suggest that the superoxide pathway is the primary mechanistic pathway with very little involvement from the hydroxyl pathway. It is believed that the enhancement due to the addition of water is due to the stabilization effect that physisorbed water has on superoxide radicals.