Coadsorption Systems Research Articles

Adsorption performance and gas sensing of the Ru atomic catalyst on GaN sheets to transformer oil dissolved gases

The Ru atomic catalyst-doped GaN (Ru-GaN) model was established by density functional theory (DFT), and the adsorption models of transformer oil dissolved gas (CO, H2, CH4, C2H2, C2H4 and C2H6) by a Ru-GaN monolayer were optimised. From the adsorption energy, differential charge density, electron state density, molecular orbital and desorption properties, the modification mechanism of Ru-GaN and the gas-sensitive response mechanism of Ru-GaN to CO, H2, CH4, C2H2, C2H4 and C2H6 were systematically studied. The introduction of metal Ru increases the defect state of GaN to provide the active site for the adsorption of gas molecules and reduces the adsorption barrier of GaN to gas molecules. The results show that Ru metal-doped GaN can adsorb CO, C2H2 and C2H4 by chemisorption, with Eads of −2.068 eV, −2.422 eV and −1.877 eV and Qt of −0.1696 e, −0.0671 e, −0.1222 e. In addition, charge redistribution leads to changes in the density of states, molecular orbitals and the conductivity of CO, C2H2 and C2H4 adsorption systems, indicating excellent gas sensitivity to the three gases. The desorption time further verified that Ru-GaN is an ideal CO and C2H2 gas scavenger and a recyclable material for C2H4 gas sensors.

Dissolved Gas Analysis in Transformer Oil Using Ni Catalyst Decorated PtSe2 Monolayer: A DFT Study

In this paper, the first-principles theory is used to explore the adsorption behavior of Ni catalyst decorated PtSe2 (Ni-PtSe2) monolayer toward the dissolved gas in transformer oil, namely CO and C2H2. Some Ni atoms from the catalyst are trapped in the Se vacancy on the pure PtSe2 surface. The geometry configurations of Ni-PtSe2 monolayer before and after gas adsorption, the electronic property of Ni-PtSe2 monolayer upon gas adsorption, and the sensibility and recovery property of Ni-PtSe2 monolayer are explored in this theoretical work. Through the simulation, the Ead of CO and C2H2 gas adsorption systems are calculated as −1.583 eV and −1.319 eV, respectively, both identified as chemisorption and implying the stronger performance of the Ni-PtSe2 monolayer on CO molecule, which is further supported by the DOS and BS analysis. According to the formula, the sensitivity of Ni-PtSe2 monolayer towards CO and C2H2 detection can reach up to 96.74% and 99.91% at room temperature (298 K), respectively, which manifests the favorable sensing property of these gases as a chemical resistance-type sensor. Recovery behavior indicates that the Ni-PtSe2 monolayer is a satisfied gas scavenger upon the noxious gas dissolved in transformer oil, but its recovery time at room temperature is not satisfactory. To sum up, we monitor the status of the transformer to guarantee the stable operation of the power system through the Ni-PtSe2 monolayer upon the detection of CO and C2H2, which may realize related applications, and provide the basis and reference to cutting-edge research in the field of electricity in the future.

Two-dimensional graphitic carbon nitride (g-C4N3) for superior selectivity of multiple toxic gases (CO, NO2, and NH3)

Using first principles calculations, we investigated the possibility of selecting multiple toxic gases using one substrate material. Here, we explored the transport property of H2, N2, O2, CO, CO2, NO2, and NH3 gas molecules on two-dimensional graphitic carbon nitride (2D g-C4N3). The homonuclear molecule such as H2, N2, and O2 has very weak adsorption energy (equal to or less than 0.1 eV) and also CO2 has an adsorption energy of 0.23 eV. In the typical toxic gas molecule adsorption systems, we found an appreciable charge transfer. In CO and NH3 adsorption systems, the charge transfer of 0.397 and 0.418 electrons from the molecule to the substrate was found, while the NO2 molecule gained 0.124 electrons from the substrate. Due to this large amount of charge transfer, we obtained large adsorption energies of 4.57, 1.29, and 1.93 eV in CO, NO2, and NH3 systems. Moreover, through the I–V curve calculations, we found large difference in the current. The calculated current was 21, 13.11, and 16.16 μA for CO, NO2, and NH3 systems at the bias voltage of 0.5 V. Our results imply that the 2D g-C4N3 can be a superior substrate material for sensing of multiple toxic gases.

First-principles investigation of CO adsorption on pristine, C-doped and N-vacancy defected hexagonal AlN nanosheets

The optimized atomic structures, energetics and electronic structures of toxic gas CO adsorption systems on pristine, C-doped and N-vacancy defected h-AlN nanosheets respectively have been investigated using Density functional theory (DFT-D2 method) to explore their potential gas detection or sensing capabilities. It is found that both the C-doping and the N-vacancy defect improve the CO adsorption energies of AlN nanosheet (from pure −3.847 eV to −5.192 eV and −4.959 eV). The absolute value of the system band gap change induced by adsorption of CO can be scaled up to 2.558 eV or 1.296 eV after C-doping or N-vacancy design respectively, which is evidently larger than the value of 0.350 eV for pristine material and will benefit the robustness of electronic signals in potential gas detection. Charge transfer mechanisms between CO and the AlN nanosheet have been presented by the Bader charge and differential charge density analysis to explore the deep origin of the underlying electronic structure changes. This theoretical study is proposed to predict and understand the CO adsorption properties of the pristine and defected h-AlN nanosheets and would help to guide experimentalists to develop better AlN-based two-dimensional materials for efficient gas detection or sensing applications in the future.

Synergistic adsorption of Cd(II) with sulfate/phosphate on ferrihydrite: An in situ ATR-FTIR/2D-COS study

Elucidation of the co-adsorption characteristics of heavy metal cations and oxyanions on (oxyhydr)oxides can help to better understand their distribution and transformation in many geological settings. In this work, batch adsorption experiments in combination with in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were applied to explore the interaction mechanisms of Cd(II) with sulfate or phosphate at the ferrihydrite (Fh)–water interface, and the two-dimensional correlation spectroscopic analysis (2D–COS) was used to enhance the resolution of ATR-FTIR bands and the accuracy of analysis. The batch adsorption experiments showed enhanced adsorption of both sulfate (S) and phosphate (P) on Fh when co-adsorbed with Cd(II); additionally, the desorbed percentages of Cd(II) were much lower in the P+Cd adsorption systems than those in the S+Cd adsorption systems. The spectroscopic results suggested that in the single adsorption systems, sulfate primarily adsorbed as outer-sphere complexes with a small amount of bidentate inner-sphere complexes, while the dominant adsorbed species of phosphate were largely the bidentate nonprotonated inner-sphere complexes, although there was significant pH-dependence. In the co-adsorption systems, the synergistic adsorption of Cd(II) and sulfate was dominantly attributed to the electrostatic interaction, as well as the formation of Fe–Cd–S (i.e., Cd-bridged) ternary complexes. In contrast, Fe–P–Cd (i.e., phosphate-bridged) ternary complexes were found in all of the co-adsorption systems of phosphate and Cd(II); furthermore, electrostatic interaction should also contribute to the co-adsorption process. Our results show that in situ ATR-FTIR in combination with 2D–COS can be an efficient tool in analyzing the co-adsorption mechanisms of anions and heavy metal cations on iron (oxyhydr)oxides in ternary adsorption systems. The co-existence of Cd(II) with sulfate or phosphate can be beneficial for their accumulations on Fh, and phosphate is more efficient than sulfate for the long-term immobilization of Cd(II).

Structure and Dynamics of Reactant Coadsorption on Single Crystal Model Catalysts by HP-STM and AP-XPS: A Mini Review

Understanding the reaction mechanism of various heterogeneous catalytic reactions is of fundamental importance in catalysis science. In the past, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) have proved to be powerful surface-sensitive techniques to characterize surface reactions on model catalysts under UHV conditions. The recent development of high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has largely extended the application of these two excellent surface-sensitive imaging and electron spectroscopy techniques to a variety of catalytic systems under realistic conditions. In this mini review, we will review a series of catalytic systems studied by HP-STM and AP-XPS, including reactant coadsorption systems, coadsorption + reaction systems, and poisoned reaction systems. We will also illustrate one of the main difficulties in the practical execution of experiments where the initial surface cleanliness is easily compromised by the adsorption of adventitious contaminants. All of these examples will demonstrate that the combined use of HP-STM and AP-XPS can provide a deeper understanding of the structure and dynamics of reactant coadsorption on model catalysts, although great care has to been taken to maintain the cleanness of the in situ instrumentation.

The role of surface Pt on the coadsorption of hydrogen and CO on Pt monolayer film modified Ru(0001) surfaces

We have investigated the impact and role of the Pt surface modification on the coadsorption of hydrogen and CO on structurally well defined bimetallic Pt monolayer island/film modified Ru(0001) surfaces with Pt contents up to a complete Pt layer, employing temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS). Kinetic limitations in the surface diffusion are shown to play an important role for adsorption at 90K, and lead to profound effects of the dosing sequence on the adsorption and desorption characteristics. Furthermore, they are responsible for spill-over effects during the TPD measurements, where COad becomes mobile and can spill-over from weakly bonding Pt monolayer areas to strongly bonding Pt-free Ru(0001) areas, which displaces Dad from these surface areas. The present findings are discussed in comparison with previous results on related metallic and bimetallic adsorption and coadsorption systems.

Metal oxides as dual-functional adsorbents/catalysts for Cu2+/Cr(VI) adsorption and methyl orange oxidation catalysis

The LDO, calcined product of the Mg/Al-CO3 hydrotalcite, was used as a dual-functional adsorbent/catalyst material for the removal of binary heavy metal ions and the catalytic degradation of methyl orange (MO). The adsorption properties for Cu2+ cations and Cr(VI) anions were investigated in depth in single and co-adsorption systems. The results showed that the LDO demonstrated 0.80 and 2.90 mmol/g adsorption capacities in single Cu2+ (1.6 mmol/L) and Cr(VI) (4.0 mmol/L) systems, respectively. In co-adsorption system, Cu2(OH)3Cl precipitation, isomorphic substitution of Mg2+ sites by Cu2+ and intercalation of CrO42– into the interlayers played important roles. On the LDO, Cu2+ and Cr(VI) presented synergetic effect, resulting in an increase in both Cu2+ and Cr(VI) adsorption capacities. In addition, the spent adsorbents were employed as a catalyst for methyl orange (MO) degradation. The Cr-Cu-RLDO, a calcined product of the spent adsorbent in the co-adsorption system, indicated the high activity with 86.5% degradation rate of MO in the presence of H2O2. The existence of Cu and Cr elements in the catalyst was one of the causes of the enhanced degradation of MO.

Desorption Temperature Control of Palladium-Dissolved Hydrogen through Surface Structural Manipulation

To assess the possibility of controlling the desorption temperature of palladium-absorbed hydrogen (Habs) through surface structural manipulation, we investigated coadsorption systems of H and CO on Habs-charged Pd(110) surfaces through temperature-programmed desorption, low-energy electron diffraction, and H-depth profiling by nuclear reaction analysis (NRA). A CO coverage of 0.5 ML lifts the H-induced (1 × 2) pairing-row (PR) reconstruction on Habs precharged Pd(110), and, as on clean Pd(110), heating Pd(110) (1 × 1) holding 0.3–1.0 ML of CO gives rise to a missing-row (MR) structure. Whereas Habs desorbs through surface defects of clean, PR-reconstructed Pd(110) at 160 K, CO coadsorption onto Habs-loaded Pd(110) gives rise to three new high-temperature shifted Habs desorption modes at 200, 270, and 375 K that are assigned to different exit sites for resurfacing Habs atoms at regular terraces of the individual Pd(110) structures, i.e., the (1 × 2) PR, bulk-terminated (1 × 1), and (1 × 2) MR reconstructi...

Cesium, oxygen coadsorption on AlGaN(0001) surface: experimental research and ab initio calculations

Cs, O activation experiment of Al0.24Ga0.76N photocathodes was carried out and the photocurrent variation was recorded. Then first principle calculations were performed to study Cs, O coadsorption on Al0.25Ga0.75N(0001) surface. Three surface models were built, one model represents pure surface, and the other two are defective surface models with Ga or N vacancy on the topmost layer. Cs only adsorption systems were built based on clean surface, and Cs, O coadsorption systems were built based on the three surfaces. Work function, average dipole charge, average dipole length, and dipole moments were calculated. Results show there exist double dipoles in the Cs, O coadsorbed Al0.25Ga0.75N(0001) surfaces. One is Cs-induced dipole [Cs–Al0.25Ga0.75N], which was caused by electrons transferring from Cs adatoms to Al0.24Ga0.75N substrate. This dipole is perpendicular to surface, and it is helpful for electrons escaping to vacuum. The other dipole [Cs–O] is composed of Cs and O adatoms, which is induced by electrons transferring from the Cs adatoms to O adatoms. Cs, O coadsorption results in lower work function on defective Al0.24Ga0.75N(0001) surfaces than the pure one. The reason is that the direction of [Cs–O] dipole on the defective surface is oriented to the substrate. Nevertheless, the direction difference between [Cs–O] dipole and surface without defect is too small to lower work function significantly. This work gives theoretical explanation for photocurrent variation and dipole moments formation of Cs, O coadsorbed AlGaN photocathodes.

Coadsorption of CO and O on Ir(1 0 0): First principles calculations

Density functional theory (DFT) calculations with generalized gradient approximation (GGA) are performed to investigate the structural, electronic and energetic properties of Ir(1 0 0)–( mCO+ nO), m=1, 2; n=1, 2 and 3, coadsorption systems. The stability of coadsorbed oxygen is not affected by the increase of CO coverage from 0.25 ML to 0.50 ML. However, the stability of CO increases by increasing O coverage from 0.25 ML to 0.50 ML and 0.75 ML as a result of the rumpling of the topmost Ir layer.

Vibrational Investigation of Catalyst Surfaces: Change of the Adsorption Site of CO Molecules upon Coadsorption

The understanding of the elementary steps occurring in catalytic reactions in the heterogeneous phase is one of the foremost goals of surface science. Adsorption on solid surfaces is the first step in catalytic reactions. Therefore, the individuation of adsorption sites for reactive chemical species is essential information to tailor catalytic properties of surfaces and interfaces. As a matter of fact, the change in adsorption site often implies a different reactivity for chemisorbed adsorbates and a selective catalytic activity. In this Feature Article, we evidence how vibrational spectroscopy can be used for individuating adsorption sites in coadsorption systems on catalyst surfaces. In particular, we studied CO coadsorption with oxygen, nitrogen, and hydrogen on the Ni(111) and the Ni(100) surfaces. Our attention was focused on the determination of CO adsorption sites in the various investigated systems. For CO adsorbed alone on the substrate, the preferential adsorption sites are the 3-fold hollow on the (111) face and the atop and bridge for the (100) surface. Striking changes in the CO adsorption site occur whenever CO is coadsorbed with other chemical species. In the CO + O coadsorption system, atop sites are populated by CO on the Ni(111) surface, while bridge and 4-fold hollow sites are occupied on Ni(100). In the CO + N phase on Ni(111), CO molecules occupy the bridge site of the pseudo-(100) reconstructed surface. For a H-precovered Ni(100) surface at 150 K, the C–O stretching frequency is near its gas-phase value, thus suggesting the occurrence of a weakly bonded CO phase without changes of adsorption sites.

Coadsorption of cesium and iodine on Pt(111): Structure and ionicity

Coadsorption of cesium and iodine on Pt(111) has been studied with Low Energy Electron Diffraction, Auger Electron Spectroscopy and Thermal Desorption Spectroscopy. By adsorbing I 2 onto a Cs layer with saturation coverage (with an incommensurate hexagonal close packed structure), two distinct structures were prepared. First, Pt(111)(4 × 4)–Cs,I was observed after dosing until the work function had increased by 1 eV. Further I 2 dosing gives mixed structures and then saturates at Pt ( 111 ) ( 7 × 7 ) R 19.1 ∘ –Cs,I. The Pt(111)(4× 4)–Cs,I was assigned to a Cs 2I 2D ionic crystal. Adsorption of Cs onto an I layer and the reactivity of selected coadsorption systems to oxygen were also investigated. Electrostatic calculations were performed to compare the stability of various structures, and it is shown that bilayer structures and mixed monolayer structures can both be consistent with ionic principles.

Coadsorbate effects on adsorbate vibrational properties

A quantitative analysis of infrared absorption spectra to determine coadsorbate induced relative changes of the vibrational polarizability α v of an adsorbate mode and of the dielectric screening ϵ due to this extra species is presented. Four (ternary) coadsorption systems consisting of the Ru(0 0 0 1)-(2 × 2)-(X + CO + O) layer with additional coadsorbates X = H, NO, CO, or O (all of them occupying the remaining empty fcc site) have been studied with FT-IRAS, TDS, LEED and work function change measurements. On-top CO is thereby used as a probe molecule to monitor coadsorbate effects on the dielectric properties of the layer. The vibrational polarizability α v associated with the internal C–O stretch mode ( ν C–O) of on-top CO is lowered by all coadsorbates. The dielectric screening ϵ within the adsorbate layer is reduced in the presence of the atomic coadsorbates O and H whereas an increase of ϵ is found for the molecular coadsorbates, threefold coordinated CO and NO. The derived changes of α v and dielectric screening ϵ, as well as the involved line shifts of ν C–O and ν Ru–CO can be understood in terms of the standard Blyholder backbonding model, i.e. CO 5 σ charge donation to the metal combined with a backdonation to electronic states with 2π∗ character.

Challenges and opportunities for glycerol as a renewable feedstock

We have investigated the impact and role of the Pt surface modification on the coadsorption of hydrogen and CO on structurally well defined bimetallic Pt monolayer island/film modified Ru(0001) surfaces with Pt contents up to a complete Pt layer, employing temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS). Kinetic limitations in the surface diffusion are shown to play an important role for adsorption at 90 K, and lead to profound effects of the dosing sequence on the adsorption and desorption characteristics. Furthermore, they are responsible for spill-over effects during the TPD measurements, where COad becomes mobile and can spill-over from weakly bonding Pt monolayer areas to strongly bonding Pt-free Ru(0001) areas, which displaces Dad from these surface areas. The present findings are discussed in comparison with previous results on related metallic and bimetallic adsorption and coadsorption systems.

Low Temperature H2O and NO2 Coadsorption on θ-Al2O3/NiAl(100) Ultrathin Films

The coadsorption of H(2)O and NO(2) molecules on a well-ordered, ultrathin theta-Al(2)O(3)/NiAl(100) film surface was studied using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS). For H(2)O and NO(2) monolayers adsorbed separately on the theta-Al(2)O(3)/NiAl(100) surface, adsorption energies were estimated to be 44.8 and 36.6 kJ/mol, respectively. Coadsorption systems prepared by sequential deposition of NO(2) and H(2)O revealed the existence of coverage and temperature-dependent adsorption regimes where H(2)O molecules and the surface NO(x) species (NO(2)/N(2)O(4)/NO(2)(-),NO(3)(-)) form segregated and/or mixed domains. Influence of the changes in the crystallinity of solid water (amorphous vs crystalline) on the coadsorption properties of the NO(2)/H(2)O/theta-Al(2)O(3)/NiAl(100) system is also discussed.

Simultaneous characterization of acidic and basic properties of solid catalysts by a new TPD method and their correlation to reaction rates

We propose a new TPD method for simultaneously characterizing the acidic and basic properties of solid catalysts by utilizing the co-adsorption of NH 3 and CO 2 on catalysts. First CO 2 was adsorbed on the catalyst sample; then NH 3 was adsorbed on it. Another adsorption sequence of NH 3 and CO 2, and CO 2 and NH 3 single adsorptions were also conducted. The TPD measurements were carried out by heating the catalyst sample from 373 to 773 K at a heating rate of 2.5 K min −1 in a helium stream under a total pressure of 1.3 kPa. In solid acid catalysts, there is little difference in the NH 3-TPD spectra between single and co-adsorption systems. This results from the absence of any induction effect between the acid and base sites, because the number of base sites in the solid acid catalyst is very small. In contrast, in a solid acid–base catalyst of alumina, a remarkable difference in the NH 3-TPD spectra was observed between single adsorption and co-adsorption systems. The difference in the TPD spectra between single and co-adsorption systems was ascribed to a strong induction effect appearing on the acid and base sites, which was proved by an in situ IR measurement. The validity of the TPD method was examined by correlating the number of the strong acid sites to catalytic activities of dehydrolysis of ethanol over solid acid and solid acid–base catalysts. In solid acid–base catalysts, the number of strong acid sites was calculated from the activation energy distribution for the desorption of NH 3 in a co-adsorption system because of the strong induction effect. A proportional relationship between the intrinsic reaction rate constant, which is based on the concentration of ethanol within the catalyst, and the number of strong acid sites could be obtained, regardless of the catalysts or their types or pore structure.

Equivalent ordered-mixed-surface-structures of p(4 × 4)- p4 gm formed on Cu(0 0 1) by coadsorptions of Bi + Mg and Sb + Mg

Two coadsorption systems of Bi + Mg and Sb + Mg on Cu(0 0 1) have been studied by means of low-energy electron diffraction (LEED). A (4 × 4) LEED pattern with glide planes was observed in either case at ∼400 K. The two (4 × 4) structures were determined by a tensor LEED analysis. Both (4 × 4) structures are equivalent, and consist of a grid network of Mg 2Bi 2 (Mg 2Sb 2) plane clusters on a partially Bi (Sb) substituted Cu(0 0 1) layer. Structural parameters show that not only the formation of the clusters but also the arrangement of the clusters into the grid network contributes to the stabilization of this ordered-mixed-surface-structure.

A density functional theory study on the water formation at high coverages and the water effect in the Fischer–Tropsch synthesis†

The O removal through water formation is an important process in the Fischer–Tropsch synthesis. In this study, both steps in water formation (O+H→OH, OH+H→H2O) are studied on the stepped Co(0001) at high coverages using density functional theory. We find the following. (i) In both O–O and O–OH co-adsorption systems, two transition states (TSs) were located for the O hydrogenation: in one TS, both O and H are on the same terrace, and in the other they are at the interface between the step edge and the terrace below. (ii) In both the O–O and O–OH co-adsorption systems, the O hydrogenation at the interface is easier (E a = 0.32 eV in the O–O system, E a = 1.10 eV in the O–OH system) than that on the same terrace (E a = 1.49 eV in the O–O system, E a = 1.80 eV in the O–OH system). (iii) In both the O–O and O–OH co-adsorption systems, only one TS for the OH hydrogenation was located, in which both OH and H are on the same terrace. (iv) Compared to the OH hydrogenation in the O–OH system (E a = 1.46 eV), the reaction in the OH–OH system (E a = 0.64 eV) is much easier. The barrier differences and the water effect on the Fischer–Tropsch synthesis are discussed. A possible route with low barriers for water formation is proposed.

Ordered mixed surface structures formed by coadsorption of dissimilar metal atoms on Cu(0 0 1)

We have found that various ordered mixed surface structures are formed by coadsorption of two dissimilar metal atoms on Cu(0 0 1) at room tepmerature, using low-energy electron diffraction (LEED) I– V analysis. As coadsorbates, we employed Mg, Bi, Li and K, and surface structures formed by the coadsorption systems of (Mg, Li), (Mg, K) and (Mg, Bi) are presented. A tensor LEED analysis provided detailed geometries of the coadsorbates and the substrate surface. It was found that the surface structures in the above three coadsorption sytems exhibit the restructuring of the Cu(0 0 1) surface. The phase separation into individual adsorbates does not take place, implying that some additional stabilization arises. We demonstrate two origins for the stabilization of the ordered mixed surface structures on Cu(0 0 1). Structures and features formed by the individual adsorption of Mg, Bi, Li and K atoms on Cu(0 0 1) are described first, then those of (2√2×√2)R45°-Mg,Li, (√5×√5)R26.7°-Mg,K, c(2×2)-Mg,Bi, and c(6×4)-Mg,Bi structures formed by the coadsorption are presented. We consider on the basis of the determined structural parameters the question why ordered mixed surface structures are formed instead of the phase separation.