Reaction Mechanism Dominated by the Hard–Soft Acid–Base Theory for the Oxidation of CH2Cl2 and CH3Br over a Titanium Oxide-Supported Ru Catalyst

Highly Selective Ionic Gel-Based Gas Sensor for Halogenated Volatile Organic Compound Detection: Effect of Dipole-Dipole Interaction

Halogenated volatile organic compounds (abbreviated as X-VOCs) are a class of hazardous gas pollutants that are difficult to detect due to their thermal stability, chemical inertness, and poisoning effect on gas sensors at high temperatures. In this work, room-temperature detection of X-VOCs is achieved using a surface acoustic wave (SAW) gas sensor coated with a 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide (EMIM-TFSI)-based ionic gel film. We experimentally verify that the high selectivity of the ionic gel-based SAW gas sensor for X-VOCs is due to the presence of halogen atoms in these gas molecules. Meanwhile, the sensor has very little response to common organic gases such as ethanol, isopropanol, and acetone, reflecting a low cross-sensitivity to nonhalogenated VOCs. This unique advantage shows potential applications in selective detection of X-VOCs and is validated by comparison with a commercial metal oxide semiconductor (MOS) sensor. Furthermore, the internal sensing mechanism is explored by the density functional theory (DFT) method. The simulation results demonstrate that the X-VOC molecules are highly polarized by the inductive effect of halogen atom substitution, which is beneficial for being adsorbed by the EMIM-TFSI ionic liquid via dipole-dipole interaction.

Mechanistic insights into the contribution of Lewis acidity to brominated VOCs combustion over titanium oxide supported Ru catalyst

CH₃Br catalytic oxidation as the probe reaction was investigated over Ru supported on TiO₂ with different crystalline phases. 1% Ru/anatase TiO₂ (a-TiO₂) exhibited superior stability at 240 °C after a 180 h time-on-stream run. And there was an induced activation for 1% Ru/a-TiO₂ during the initial 60 h reaction. Then the activity sustained stable. To elucidate the intrinsic mechanism, a series of characterizations were performed such as XRD, CO-Pulse, H₂-TPR, XPS and NH₃-TPD etc. Results showed that the Ru particle size increased and the Ru⁰ content decreased as the reaction proceeded, which were not conductive to the reaction. It was assumed that the catalytic activity was strongly dependent on other factors. In combination with NH₃-TPD and Py-FTIR measurements, it was confirmed that the enhanced activity and stability was strongly associated with the surface acidity, especially moderate strong Lewis acid (L acid). The increase of the acid amount and acidity strength was led by the generation and adsorption of HBr, Br₂ and RuO_xBr_y during the reaction, among which HBr and Br₂ was easier to desorb at 250 °C. While moderate strong L acid was sourced from the formation of RuO_xBr_y. The addition of transition metal (Ce, Co, Mn, Nb and Ni) further validated that the moderate strong L acid played a decisive role in the CH₃Br catalytic oxidation.