Density functional theory study of the NO2-sensing mechanism on a WO3 (0 0 1) surface: the role of surface oxygen vacancies in the formation of NO and NO3

Investigations of the ODS process utilizing CNT- and CNF-based WO3 catalysts for environmental depollution: experimental and theoretical aspects

In this contribution, CoW/X materials (X = CNT or CNF) were utilized as oxidative desulfurization (ODS) catalysts for the removal of dibenzothiophene (DBT) from a model fuel (n-decane), incorporating the H₂O₂ as an efficient oxidant. Different operating conditions were investigated. Both compounds revealed high desulfurization efficiency using milder operating conditions leading to low levels of the DBT compound since only 1 h while using a low ratio of H₂O₂/S = 6. Among synthesized compounds, the CoW (15)/CNT showed superior DBT conversion through the ODS process. In other words, the highest sulfur removal efficiency of 100% for a feed sulfur content of 500 ppm was determined in a 40-min duration under optimum conditions. This was satisfyingly more effective than a recently reported CoW (20)/rGO catalyst. The characterization of synthesized catalysts was performed in order to evaluate their physicochemical properties. Moreover, product identification of the oxidation desulfurization process was performed using the GC-Mass, FTIR, and NMR techniques where it was found that this process was that of a single product. These experimental studies were complemented with density functional theory (DFT) investigations, which indeed shed important light on understanding the adsorption mechanisms as well as electronic properties of the system undertaken.

Insights into the Interfacial Contact and Charge Transport of Gas-Sensing Liquid Metal Marbles

Understanding the interfacial contacts between liquid metals and substrate materials is becoming increasingly important for the fast-rising liquid metal-enabled technologies. However, for such technologies, probing the contact behavior and interfacial charge transport has remained challenging due to the deformable nature of liquid metals and the presence of the surface oxide layer. Here, we encapsulate eutectic gallium indium (EGaIn) micro-/nanodroplets with tungsten trioxide (WO₃) nanoparticles to form a WO₃/EGaIn liquid metal marble network, in which the interfacial contact of the intrinsically semiconducting WO₃ governs the charge transport. We investigate the interfacial structures and charge transport characteristics under different contact conditions and various gaseous environments. The results suggest that establishing a WO₃/EGaIn heterostructure leads to near-ohmic contact behaviors and also the emergence of localized surface plasmon resonance. Density functional theory calculations of the WO₃/EGaIn interface support the experiments by revealing atomistic attractions between EGaIn alloy and the O atoms from WO₃, resulting in a Fermi level shift. We also show that the efficient interfacial charge transport of the liquid metal marble network results in an enhanced gas-sensing response. This work paves the way for the possibility of studying other liquid metal/semiconductor contacts for applications in soft electronics and optics.

Nanoplasmonic NO2 Sensor with a Sub-10 Parts per Billion Limit of Detection in Urban Air

Urban air pollution is a critical health problem in cities all around the world. Therefore, spatially highly resolved real-time monitoring of airborne pollutants, in general, and of nitrogen dioxide, NO₂, in particular, is of utmost importance. However, highly accurate but fixed and bulky measurement stations or satellites are used for this purpose to date. This defines a need for miniaturized NO₂ sensor solutions with detection limits in the low parts per billion range to finally enable indicative air quality monitoring at low cost that facilitates detection of highly local emission peaks and enables the implementation of direct local actions like traffic control, to immediately reduce local emissions. To address this challenge, we present a nanoplasmonic NO₂ sensor based on arrays of Au nanoparticles coated with a thin layer of polycrystalline WO₃, which displays a spectral redshift in the localized surface plasmon resonance in response to NO₂. Sensor performance is characterized under (i) idealized laboratory conditions, (ii) conditions simulating humid urban air, and (iii) an outdoor field test in a miniaturized device benchmarked against a commercial NO₂ sensor approved according to European and American standards. The limit of detection of the plasmonic solution is below 10 ppb in all conditions. The observed plasmonic response is attributed to a combination of charge transfer between the WO₃ layer and the plasmonic Au nanoparticles, WO₃ layer volume expansion, and changes in WO₃ permittivity. The obtained results highlight the viability of nanoplasmonic gas sensors, in general, and their potential for practical application in indicative urban air monitoring, in particular.

Tungsten-Based Catalysts for Environmental Applications

This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO2) and precious metals. WOx particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO3-based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO3 catalysts for DeNOx applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis).

WO3-Based Gas Sensors: Identifying Inherent Qualities and Understanding the Sensing Mechanism

Semiconducting metal oxide-based gas sensors are an attractive option for a wide array of applications. In particular, sensors based on WO₃ are promising for applications varying from indoor air quality to breath analysis. There is a great breadth of literature which examines how the sensing characteristics of WO₃ can be tuned via changes in, for example, morphology or surface additives. Because of variations in measurement conditions, however, it is difficult to identify inherent qualities of WO₃ from these reports. Here, the sensing behavior of five different WO₃ samples is examined. The samples show good complementarity to SnO₂ (the most commonly used material)-based sensors. A surprising homogeneity, despite variation in morphology and preparation method, is found. Using operando diffuse reflectance infrared Fourier transform spectroscopy, it is found that the oxygen vacancies are the dominant reaction partner of WO₃ with the analyte gas. This surface chemistry is offered as an explanation for the homogeneity of WO₃-based sensors.