A first-principles insight into Pd-doped MoSe2 monolayer: A toxic gas scavenger

Using first-principles theory, we investigate the Pd-doping effect on the geometric and electronic behaviors of MoSe₂ monolayer, and the adsorption behavior of Pd-MoSe₂ monolayer upon four toxic gases, namely NO, NO₂ SO₂ and H₂S. Desorption property of Pd-MoSe₂ monolayer upon four gases at diverse temperatures is analyzed as well to help explore its potential application. For Pd dopant adsorption onto MoSe₂ monolayer, somewhat n-type doping is determined, which accounts for the increased conductivity for intrinsic MoSe₂ monolayer. The strong adsorption ability and poor desorption performance of Pd-MoSe₂ monolayer upon four gases indicate its large potential as gas scavenger to remove these toxic gases from their surroundings. Moreover, it could be explored as a gas sensor for detection of NO, NO₂ and H₂S as well, given the obvious change in conductivity after gas adsorption based on band structure analysis. Our calculations would be beneficial to understand the TM doping effect on intrinsic MoSe₂ monolayer and to provide a first insight into the potential application as gas sensor or sweeper for Pd-MoSe₂ monolayer.     

An investigation of paddle wheel Cu2(μ2-O2CCH3)4 for gas molecule adsorptions

Metal organic frameworks (MOFs) have been well-known and extensively researched due to the high storage /good selectivity for gas molecules. Herein, the structures and electron paramagnetic resonance (EPR) spectra for dicopper paddle wheel MOF compound (Cu₂(µ₂-O₂CCH₃)₄ with various gas molecule are theoretically investigated by density functional theory (DFT) calculations. The adsorption energies and isotherms (including pure gas molecules and the mixed ones) are calculated for the gas molecules interacting with the unsaturated Cu₂(µ₂-O₂CCH₃)₄. Both quantities exhibit the roughly consistent orders (e.g. H₂S?>?NH₃?>?CO₂?>?CO?>?H₂O?>?N₂?>?NO?>?H₂ for isotherms and H₂S?>?NH₃?>?N₂?>?CO₂?>?NO?>?H₂O?>?H₂?>?CO for adsorption energies), possibly suggesting that this material may act as a potential adsorbent of these gas molecules. The catalytic property of Cu₂(µ₂-O₂CCH₃)₄ for oxidation of CO and NO into non-toxic molecules and splitting of H₂O into H₂ and O₂ in the solvent condition are uniformly discussed. Simulation of Grand Canonical Monte Carlo (GCMC) in MS 8.0 and calculations in Langmuir model reveal that Cu₂(µ₂-O₂CCH₃)₄ has good selectivity for CH₄ in natural gas (CH₄/CO₂/N₂) and SO₂ in fog (SO₂/NO/NO₂/H₂O/O₂), which would exhibit potential environmentally friendly applications.     

A first-principles study of gas molecule adsorption on hydrogen-substituted graphdiyne

Hydrogen-substituted graphdiyne (HsGDY) is a novel alkynyl carbon material with a structure similar to that of graphene. In this paper, the adsorption of four gas molecules (NO, NO₂, NH₃, and N₂) on HsGDY and B-doped HsGDY (B-HsGDY) was studied using density functional theory. The results show that the adsorption of NO and NO₂ on HsGDY and B-HsGDY is characterized by a larger charge transfer, stronger interaction, and higher adsorption energy compared with that of NH₃ and N₂. Based on the doping with B atoms, the adsorption energies of the gas molecules on HsGDY significantly improve, especially that of NO and NO₂. The gas molecule adsorption on both HsGDY and B-HsGDY is physical adsorption and the adsorption selectivity is good and thus may be applied for gas-sensitive NO and NO₂ materials.     

Coadsorption of NO and other small gases (H2 and CO) on polycrystalline rhodium surface

The coadsorption of NO and other small gases (H₂ and CO) on a polycrystalline Rh filament has been studied by thermal desorption mass spectroscopy, using ¹⁵NO. The sample was exposed to a mixture of nitric oxide and other gases with various concentrations of ¹⁵NO at room temperature. It is indicated that NO, CO and H₂ coadsorbs on the rhodium surface, and NO desorbs as N₂ and O₂. NO is adsorbed mainly in the dissociation at lower coverage and molecular adsorption becomes dominant at higher coverage. But the amount of desorbed O₂ was very small. The chemisorption of CO is affected by the chemisorbed NO. Thermal desorption of hydrogen is detected when the value of P_15NO/P_CO is very small. The hydrogen adsorbed on the rhodium surface is replaced by NO with a longer exposure time.     

Surface science studies of selective catalytic reduction of NO: Progress in the last ten years

Selective catalytic reduction (SCR) of NO_x is one of the important strategies in regulating NO_x emissions. In the past several decades, the reactions of NO_x (mainly NO) with H₂, CO, NH₃ and hydrocarbons have been extensively investigated under ambient conditions and have been summarized in numerous reviews. Nonetheless, many questions appear to be difficult to answer under ambient conditions, e.g., the pathways through which the reactions proceed. The introduction and development of modern surface science technology has played an indispensable role and is widely employed in the studies of the SCR of NO_x, greatly helping to elucidate the mechanisms of the reactions with CO, H₂, and NH₃. However, so far, there are few review papers systematically summarizing the progress of such studies.Recently, systematic surface science studies have been conducted on the mechanisms of SCR of NO with organic molecules including ethylene, benzene, and ethanol, which are much more complicated than those with H₂, CO and NH₃ and have drawn much less attention before. It is confirmed that these reactions can be reliably and most importantly, reproducibly probed by surface science technology, but a great deal of work remains to be done.Since Delmon et al. have provided a very thorough review of the researches on catalytic removal of NO (reactions with H₂, CO and NH₃) up to 1998, in which the reactions conducted both under ambient and UHV conditions were included, this review mainly concentrates on the progress made since 1998.     

Adsorption and diffusion behaviors of H2, H2S,NH3, CO and H2O gases molecules on MoO3 monolayer: A DFT study

Molybdenum trioxide (MoO₃) with α-phase is a promising material for gas sensing because of its high sensitivity, fast response and thermodynamic stability. To probe the mechanism of superior gas detection ability of MoO₃ monolayer, the adsorption and diffusion of H₂, H₂S, NH₃, CO and H₂O molecules on two-dimensional (2D) MoO₃ layer are studied via density functional theory (DFT) calculations. Based on calculated adsorption energies, density of states, charge transfer, diffusion barriers and diffusion coefficient, MoO₃ shows a superior sensitive and fast response to H₂ and H₂S than CO, NH₃, H₂O, which is consistent with experimental conclusions. Moreover, the response of MoO₃ to H₂S and H₂ will be obviously enhanced at high gas concentration, and the incorporation of H₂ and H₂S results in an obvious increasing in DOS near Fermi level. Our analysis provides a conceptual foundation for future design of MoO₃-based gas sensing materials.     

Microwave absorption coefficients of atmospheric pollutants and constituents

Calculations of the transition frequencies and absorption coefficients of microwave rotational transitions are given for a number of atmospheric pollutants and constituents. New measurements of the absorption coefficients are made in the vicinity of 70 GHz. The apparatus used in these measurements is briefly described. The calculated absorption coefficients are compared with these measurements and with existing measurements at other frequencies where available. Transitions with frequencies up to about 200 GHz are considered for the molecules and radicals SO₂, O₃, H₂O, NO₂, H₂S, H₂CO, NH₃, CO, OCS, N₂O, NO, OH, O₂, SO. Also discussed are criteria for the selection of appropriate transitions for the development of high sensitivity monitors to be used in air pollution and combustion research.     

Shock tube study of the interaction between ammonia and nitric oxide at high temperatures using laser absorption spectroscopy

The interaction between ammonia (NH₃) and nitric oxide (NO) at high temperatures is studied in this work using a shock tube combined with laser absorption diagnostics. The system simultaneously measured the NH₃ and NO time-histories during the reaction processes of the shock-heated NH₃/NO/CO/Ar mixtures (NH₃:NO ≈ 0.9:1.0 and 1.4:1.0). The absorption cross-sections of NH₃ near 1122.10 cm^–1 and NO at 1900.52 cm^–1 (characterized in this study) were used for measuring NH₃ and NO time-histories with the temperature measured by two CO absorption lines. The measured NH₃ and NO time-histories at 1614–1968 K and 2.4–2.8 atm were compared with predictions of seven recent kinetics models. The predictions that based on different mechanisms are very different and the measured profiles are within the range of the predictions. The Glarborg, NUI Galway Syngas-NOx, and Mathieu mechanisms give the closest predictions to the measurements. Kinetics analyses indicate that the NH₃ and NO consumption rates are extremely sensitive to the rate constants and branching ratio of NH₂ + NO = N₂ + H₂O and NH₂ + NO = NNH + OH, which are more reliably represented in the Glarborg and NUI Galway Syngas-NOx mechanisms. The performances of Glarborg mechanisms at lower initial temperatures can be apparently improved by revising the rate constants and branching ratio of NH₂ + NO = N₂ + H₂O and NH₂ + NO = NNH + OH. These two reactions are also the primary pathways for NO reduction and NH₃ is mainly consumed via NH₃ + OH = NH₂ + H₂O and NH₃ + H = NH₂ + H₂. Trace amounts of NO₂ and N₂O impurities decompose to form O radical followed by the generation of OH radical via H-abstraction reactions, which significantly affects the predictions of NH₃ and NO according to kinetics analyses.     

Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review

In the recent past a great deal of research efforts were directed toward the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. In this article a critical parameter analysis of different metal oxides that are known to be sensitive to various gaseous species are thoroughly examined. This includes phase of the oxide, sensing gaseous species, operating temperature range, and physical form of the material for the development of integrated gas sensors. The oxides that are covered in this study include oxides of aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, indium, iron, manganese, molybdenum, nickel, niobium, ruthenium, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or multi-component metal oxides. They cover gases such as CO, CO₂, CH₄, C₂H₅OH, C₃H₈, H₂, H₂S, NH₃, NO, NO₂, O₂, O₃, SO₂, acetone, dimethylamine (DMA), humidity, liquid petroleum gas (LPG), petrol, trimethylamine (TMA), smoke, and many others. Both doped and undoped oxides are analyzed for the compatibility with silicon processing conditions and hybrid microcircuit fabrication techniques. In silicon processing conditions, they are further analyzed for the suitability for simple silicon surfaces, silicon-on-insulator (SOI) surfaces, and micromachined silicon geometries for different operating temperatures. Discussion on gas-sensing properties of each material and its applications are described in the text in alphabetical order of the elemental oxides. Further, the gas-sensing properties like sensitivity, detection limits, operating temperature, and so on are summarized in tables al ong with relevant references. The figures incorporated in the present review are primarily based on discussions and data in tables. However, these figures provide a qualitative comparison and present a pictorial view to examine suitability of a material for a particular application. From the known parameters, the present study clearly indicates the suitability of certain materials and the gases that they cover for the development of integrated micro gas sensors. A clear picture has been brought out for the development of silicon-based processing technology. Various parameters are discussed for the selection of these materials, to examine their suitability and practical problems that are being associated. Etching of these metal oxides and the reliability of devices are also discussed.     

A photoelectron spectroscopic observation of iron surfaces exposed to N2, N2O,NO, NO2 and air at 200 torr or 1 atm

Photoelectron spectra from core levels are presented for adsorption of nitrogen-containing gases at 200 torr or 1 atm on iron surfaces. Assignments of each band and the adsorption process are discussed. On the substrate at normal temperature, each gas forms nitride and nitrogen oxide groups (NO, NO₂ and NO₃) which appear at 398.6, 400.0, 404.5 and 407.1 eV, respectively. The relative intensifies of each species depend on the gases adsorbed. NO and NO₂ are considered to dissociate on the surface and the oxygen atom adsorb preferentially. The oxygen on the surface can be considered to contribute to the formation of each surface ligand.     

Synchrotron XPS and desorption study of the NO chemistry on a stepped Pt surface

The interaction of NO with Pt(4 1 0) was studied using high-energy resolution fast XPS and temperature programmed desorption/reaction mass spectroscopy. LEED studies show that the surface in the clean state restructures, which results in the formation of some larger {1 0 0} terraces. STM measurements show, that most terraces are small, ∼1 nm. Two different binding energy (BE) components were observed in the N 1s region of the core level spectra, both assigned to molecular forms of NO. NO dissociation starts between 350 and 400 K. This is a significantly higher temperature than previous literature reports suggested. This difference is thought to be caused by the restructuring of the surface used in our experiments. The reaction of NO with H₂, NH₃ and CO was also studied. The onset of these NO reduction reactions is determined by the NO_ad dissociation temperature (between 350 and 400 K) and NO_ad dissociation is the rate limiting step for all the reactions that were studied. Reaction with H₂ yields NH₃ below 600 K, but the selectivity shifts towards N₂ at higher temperatures. We did not find any indication that reaction between NO_ad and NH_3 ad proceeds via a special NO-NH₃ intermediate. A new surface species was detected during the reaction between NO and CO, both in the N 1s and the C 1s spectrum. It is tentatively assigned to either CN or CNO. The reactivity of NO on Pt(4 1 0) is compared with the reactivity that was observed for Pt(1 0 0) and other noble metal surfaces, such as Pd and Rh.     

SERS and XPS observation of the adsorption behavior of NO and NO2 on a silver powder surface

The influence of H₂O on the adsorption behavior of NO or NO₂ on a silver powder surface was studied by SERS and XPS at room temperature. Water vapor was found to be responsible for the adsorption of NO on the silver powder surface. When surface species such as Ag₂O are present on the surface, some of the NO₂ molecules are adsorbed on the surface species to produce NO^-₃, whereas NO molecules are adsorbed on a different site to produce NO^-₂.     

X-ray photoelectron spectroscopic observation of nitrogen-containing gases adsorbed at high pressures on some transition metals

The chemisorbed species formed by reaction of nitrogen-containing gases (NO, N₂O, N₂ and dry air) on some transition metals (Ni, Cu, Ti, Co and Pd) at high pressures (1–200 torr) are examined by XPS. At least four adsorption species are observed on the surface at ambient temperature. They can be assigned to ?NO₃ (407.3 eV), ?NO₂ (404.5 eV), ?NO (400.0 eV) and the nitrogen bound directly to metal, which shows a characteristic energy value for each metal. This feature differs from the reported results of low pressure adsorption experiments. Relative abundances among the chemisorbed species vary with individual metals and gases.     

Atomic and molecular adsorption on Pd(111)

The adsorption properties of a variety of atomic species (H, O, N, S, and C), molecular species (N₂, HCN, CO, NO, and NH₃) and molecular fragments (CN, NH₂, NH, CH₃, CH₂, CH, HNO, NOH, and OH) are calculated on the (111) facet of palladium using periodic self-consistent density functional theory (DFT–GGA) calculations at ¼ ML coverage. For each species, we determine the optimal binding geometry and corresponding binding energy. The vibrational frequencies of these adsorbed species are calculated and are found to be in good agreement with experimental values that have been reported in literature. From the binding energies, we calculate potential energy surfaces for the decomposition of NO, CO, N₂, NH₃, and CH₄ on Pd(111), showing that only the decomposition of NO is thermochemically preferred to its molecular desorption.     

DFT investigation of gas sensing characteristics of Au-doped vanadium dioxide

Vanadium dioxide compounds are affordable and effective materials with large potential in gas sensing applications. However, it is still very challenging for available experiments to provide an in-depth understanding of sensing mechanism of VO₂-based materials. In this work, density functional theory and molecular dynamics are applied to explore adsorption and diffusion of H₂, CO₂, CO and CH₄ gases molecules in Au-VO₂. Based on calculated adsorption energy, change transfer, charge density difference and density of state, a strong sensing characteristics of Au-VO₂ toward CH₄ gas is concluded, which is consistent with experimental conclusions. It is also inferred that H₂, CO and CO₂ relate physical adsorption, and CH₄ corresponds to a chemical adsorption. The diffusion of CH₄ in Au-VO₂ is more difficult than the other gases due to the chemical adsorption of CH₄.     

Atomic and molecular fluorescence as a stratospheric species monitor

The results of an extensive evaluation are presented assessing the potential of atomic and molecular fluorescence as a stratospheric monitor of the concentrations of any one of eighteen minor species. These include Cl, Cl₂, ClO, ClO₂, CO, H₂, HCHO, HCl, HNO₂, HNO₃, H₂S, NH₃, NO, NO₂, N₂O, O, OH and SO₂. All spectral regions from the vacuum u.v. through to the i.r. have been included. Where appropriate, detection limits (signal/noise ratio of unity) are presented for each species under various sample pressure conditions and are based on practical systems that could be constructed using current technology.The most promising systems, with typical detection limits indicated either as parts per million, billion or trillion by volume, are for CO(5ppb), NO₂(<1ppb), OH(0.2ppt) and O(50–200ppt). The fluorescence sensitivities for Cl(0.5–1 ppt), H₂(0.2 ppm at 10 torr sample pressure) and SO₂(1–10 ppb) are marginally insufficient at present for such a stratospheric application. Likewise HCHO(10 ppb) and NO(100 ppb) fluorescence detection may be of interest in other applications where sensitivity demands are not as severe. There are no promising analytical possibilities using direct fluorescence techniques for Cl₂, ClO₂, HCl, HNO₂, HNO₃, H₂S, NH₃ or N₂O. ClO fluorescence has not yet been characterized.It has been noted, for various reasons, that i.r. fluorescence techniques in general cannot be exploited in the development of sensitive analyzers. However, by far the most surprising outcome of this study has been the recognition of the analytical potential of vacuum u.v. fluorescence. For some species, under certain conditions, extremely high sensitivities are possible even with samples in air at atmospheric pressure.     

Millimeter and submillimeter wave absorption by atmospheric pollutants and constituents

Calculated absorption coefficients and rotational transition frequencies ara given for a number of polar molecules of interest to pollution and energy research. The results, which are presented in graphical form for microwave frequencies up to 1400 GHz, illustrate the increased absorption line intensities occurring in the submillimeter region. For most species, these absorption coefficients attain their maximum values in this region. Included in the calculations are SO₂, H₂CO, O₃, H₂O, H₂S, OCS, CO, NO, OH, SO, NH₃, and CS. A discussion of the techniques currently available for detection in the submillimeter region of these species is also given.     

A tunable diode laser system for the remote sensing of on-road vehicle emissions

2 O, and NO₂ emissions have been measured with this instrument. The system is also capable of measuring CO, NH₃, H₂CO, CH₃OH, and other small molecules in vehicle exhaust. Received: 18 May 1998/Revised version: 1 July 1998     

Mott insulator-superfluid phase transition in p-band triangle optical lattices with on-site rotation

In order to exploit the potential applications of graphene as gas sensors, the adsorptions of a series of small gas molecules (such as CO, O₂, NO₂ and H₂O) on pristine graphene (PG) and Si-doped graphene (SiG) have been investigated by ab initio calculations. Our results indicate that the electronic properties of PG are sensitive to O₂ and NO₂ molecules, but not changed much by the adsorption of CO and H₂O molecules. Compared with PG, SiG is much more reactive in the adsorption of CO, O₂, NO₂ and H₂O. The strong interactions between SiG and the adsorbed molecules induce dramatic changes to the electronic properties of SiG. Therefore, we suggest that SiG could be a good gas sensor for CO, O₂, NO₂ and H₂O.