Transducer-Aware Hydroxy-Rich-Surface Indium Oxide Gas Sensor for Low-Power and High-Sensitivity NO2 Gas Sensing.

Low-power metal oxide (MOX)-based gas sensors are widely applied in edge devices. To reduce power consumption, nanostructured MOX-based sensors that detect gas at low temperatures have been reported. However, the fabrication process of these sensors is difficult for mass production, and these sensors are lack uniformity and reliability. On the other hand, MOX film-based gas sensors have been commercialized but operate at high temperatures and exhibit low sensitivity. Herein, commercially advantageous highly sensitive, film-based indium oxide sensors operating at low temperatures are reported. Ar and O2 gases are simultaneously injected during the sputtering process to form a hydroxy-rich-surface In2O3 film. Conventional indium oxide (In2O3) films (A0) and hydroxy-rich indium oxide films (A1) are compared using several analytical techniques. A1 exhibits a work function of 4.92 eV, larger than that of A0 (4.42 eV). A1 exhibits a Debye length 3.7 times longer than that of A0. A1 is advantageous for gas sensing when using field effect transistors (FETs) and resistors as transducers. Because of the hydroxy groups present on the surface of A1, A1 can react with NO2 gas at a lower temperature (∼100 °C) than A0 (180 °C). Operando diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) shows that NO2 gas is adsorbed to A1 as nitrite (NO2-) at 100 °C and nitrite and nitrate (NO3-) at 200 °C. After NO2 is adsorbed as nitrate, the sensitivity of the A1 sensor decreases and its low-temperature operability is compromised. On the other hand, when NO2 is adsorbed only as nitrite, the performance of the sensor is maintained. The reliable hydroxy-rich FET-type gas sensor shows the best performance compared to that of the existing film-based NO2 gas sensors, with a 2460% response to 500 ppb NO2 gas at a power consumption of 1.03 mW.

In situ spectroscopic studies of the effect of water on the redox cycle of Cu ions in Cu-SSZ-13 during selective catalytic reduction of NOx.

The effect of water on the NH3-assisted selective catalytic reduction of NOx (NH3-SCR) has been largely neglected, despite the inevitable presence of water vapor in real emissions produced by fuel combustion. In this work, we investigated the role of water in the behavior of active Cu2+ ions in Cu-SSZ-13 in the NH3-SCR reaction. The addition of water to the reactant feed leads to significantly increased NOx reduction over the catalyst. By combining in situ DRIFTS and XANES analyses during the NH3-SCR reaction, we found that the redox cycle of Cu ions is promoted by the presence of water.

Mobility of Cu Ions in Cu-SSZ-13 Determines the Reactivity of Selective Catalytic Reduction of NOx with NH3.

Selective catalytic reduction of NOx with NH3 (NH3-SCR) in Cu-SSZ-13 has been proposed to have a unique homogeneous-like mechanism governed by the spatial proximity of mobile Cu ions. Among factors that determine the proximity, the effect of ion density on the SCR reaction is well established; however, it has not been verified how the different mobility of the Cu ion influences the SCR reaction. Herein, we try to reveal the mobility-dependent SCR reaction by controlling the Cu species with different ion mobilities in Cu-SSZ-13. Since the reaction kinetics is governed by the diffusion of Cu ions, the Cu ion mobility determines the reactivity of the Cu-SSZ-13. In terms of this correlation, enhanced ion mobility leads to improved NH3-SCR activity. These findings help understand the behavior of Cu ions in Cu-SSZ-13 under a catalytic reaction and provide insights to design rational catalysts by tuning the ion mobility.

Enhanced activity of vanadia supported on microporous titania for the selective catalytic reduction of NO with NH3: Effect of promoters.

Vanadium oxide-based catalysts are considered a promising catalyst for selective catalytic reduction (SCR) of NO with NH3, which is an effective NOx removal technology. As environmental issues have garnered more attention, however, improvements to vanadium-based SCR catalysts are strongly required. In a previous study, we found that vanadium oxide on microporous titania as a support (V/MPTiO2) has certain advantages, such as improved thermal stability and more suppressed N2O formation, over the use of conventional nanoparticle titania (DT-51) as a support. In this study, widely used promoters, such as W, Sb, and Mo, were added to V/MPTiO2 to investigate whether they have promoting effects on V/MPTiO2 as well. Among these promoters added catalysts, the W and Mo were found to have significant promoting effects on the enhancement of deNOx activities at low temperatures, while the addition of Sb to V/MPTiO2 tended to have a negative effect on the SCR activity. Based on the characterizations, including laser Raman, H2-temperature programmed reduction (H2-TPR), and in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) analysis, we found that the addition of W and Mo increased the degree of polymerization in V/MPTiO2, which generated more reactive vanadia species. Hence, such changes, resulting from the addition of W and Mo promoters to V/MPTiO2, yielded enhanced catalytic activity at low temperatures.

Simple physical mixing of zeolite prevents sulfur deactivation of vanadia catalysts for NOx removal.

NOx abatement has been an indispensable part of environmental catalysis for decades. Selective catalytic reduction with ammonia using V2O5/TiO2 is an important technology for removing NOx emitted from industrial facilities. However, it has been a huge challenge for the catalyst to operate at low temperatures, because ammonium bisulfate (ABS) forms and causes deactivation by blocking the pores of the catalyst. Here, we report that physically mixed H-Y zeolite effectively protects vanadium active sites by trapping ABS in micropores. The mixed catalysts operate stably at a low temperature of 220 °C, which is below the dew point of ABS. The sulfur resistance of this system is fully maintained during repeated aging/regeneration cycles because the trapped ABS easily decomposes at 350 °C. Further investigations reveal that the pore structure and the amount of framework Al determined the trapping ability of various zeolites.

Time-resolved observation of V2O5/TiO2 in NH3-SCR reveals the equivalence of Brønsted and Lewis acid sites.

The involvement of Lewis and Brønsted acid sites on V2O5/TiO2 catalyst in the selective catalytic reduction of NO with NH3 (NH3-SCR) is under debate. Here, a Li doping strategy is applied to selectively block Brønsted sites, which aims to prepare model catalysts with the same V loading but different ratios of the two acid sites. Time-resolved in situ DRIFTS observation demonstrates that the surface ammonia species pre-adsorbed on Lewis and Brønsted sites can participate equally in the reaction. Consideration of site redistribution in the early stages of the transient reaction is key to accurate measurement of the ammonia consumption rate.