Coadsorption Of NO Research Articles

Mechanistic studies on defective hBN-supported metal-nonmetal synergistic catalysis for urea electrosynthesis

This study explores electrocatalytic urea synthesis promoted by a newly-designed bimetallic-tricenter catalyst consisting of defective hexagonal boron nitride (hBN) and embedded dimerized 3d metal, by means of density functional theory calculations. Among all candidates, Mn was proved to be the most feasible catalytic center originating from its unique electronic structure and metal-support interactions. The metal-nonmetal synergistic effects of the Mn2N center effectively drive site-selective co-adsorption of CO and NO, spontaneous C-N coupling, low-energy-demand NO hydrogenation, and adequate protection of the carbonyl group, achieving highly active and selective urea production. The calculated limiting potential of urea formation is as low as −0.19 V, making it more favorable energetically compared to other competing reactions such as hydrogen evolution and NO reduction reactions.

Revisiting Competitive Adsorption of Small Molecules in the Metal–Organic Framework Ni-MOF-74

To precisely evaluate the potential of metal-organic frameworks (MOFs) for gas separation and purification applications, it is crucial to understand how various molecules competitively adsorb inside MOFs. In this paper, we combine in situ infrared spectroscopy with ab initio calculations to investigate the mechanisms associated with coadsorption of several small molecules, including CO, NO, and CO2 inside the prototypical structure Ni-MOF-74. Surprisingly, we find that the displacement of CO bound inside Ni-MOF-74 (binding energy of 53 kJ/mol) is readily driven by CO2 exposure, even though CO2 has a noticeably weaker binding energy of only 41 kJ/mol; meanwhile, the significantly more strongly binding NO molecule (90 kJ/mol) is not able to easily displace bound CO inside Ni-MOF74. These results show that single-phase binding energies of a molecule inside the MOF cannot completely describe their interaction with the MOF in the presence of other guest molecules. We unveil many crucial factors, such as the kinetic barrier, partial pressure, secondary binding sites, and guest-host/lateral interactions that control the coadsorption process and, combined with the binding energy, are better descriptors of the behavior and adsorption of gas mixtures inside MOFs.

Adsorption of NO and O2 on MnO2 and (MnO2)3/Al2O3

Based on first-principles density functional theory (DFT), the adsorption performance of MnO2(110) surface for NO and O2 was calculated, and the deposition of (MnO2)3cluster on the γ-Al2O3(110) surface was studied. In addition, the adsorption and co-adsorption of NO and O2 on the surface of (MnO2)3/Al2O3 were further calculated. The results showed that NO was more inclined to adsorb on the four-coordinated Mn(Ⅳ) sites on the MnO2(110) surface (the maximum Eads was −75.35KJ·mol−1). There were very few O2 adsorption sites on the MnO2(110) surface, and only the vacancies between adjacent Mn(IV) could adsorb O2 stably. PDOS analysis showed that the hybridization between Mn 3d orbital and N 2p (or O 2p) orbital was the main reason for Mn-N bonds (or Mn-O bonds). The (MnO2)3 cluster tended to load on Al2O3(110) surface in a flat six-membered ring structure. There are abundant active oxygen sites on the (MnO2)3/Al2O3 surface, which had an excellent ability to adsorb NO (the maximum Eads was −278.24 KJ·mol−1). NO molecules interacted with surface-active oxygen to generate nitroso, and NO2 was easily desorbed from the (MnO2)3/Al2O3 surface. The co-adsorption of NO and O2 on the (MnO2)3/Al2O3 surface formed an ONOO* structure, which could decompose to form adsorbed NO2* and O*. Compared with single-phase manganese oxide crystals, manganese oxide clustered on γ-Al2O3 had superior NO adsorption performance and may have excellent catalytic oxidation properties.

DFT study on the interactions between HCN and NO over char

A noteworthy part of HCN conversion is in reference to its potential behavior in NO reduction. In this work, a comprehensive density functional theory (DFT) study was carried out to obtain new insight into the mechanisms of HCNNO reaction on the char surface. A model modified with the hydroxyl group contributes to a better understanding of NO reduction with HCN oxidation. The adsorption modes are found to influence the adsorption energies and reaction paths significantly. For single adsorption, the HCN adsorption in dual-site mode is the most thermally favorable. The co-adsorption of HCN and NO is dramatically more significant than the sum of the corresponding single adsorption, mainly due to the reaction of adsorbates on the surface. In addition, the microscopic reaction paths for CO and N2 formation are summarized based on the most energetically stable adsorption and co-adsorption structures separately. The canonical variational transition-state theory (CVT) is adopted here to provide fruitful kinetic implications in the temperature range of 500–1800 K. The analysis indicates that the occurrence of HCN oxidation decreases the reaction energy barriers evidently, which is also responsible for the rates increasing. The co-adsorption mechanism is favorable for NO conversion, especially for the release of CO. The findings from this work are beneficial for better understanding and illustrating the ultra-low NOx emission processes. The co-adsorption mechanism that was always neglected before is strongly suggested in future computational studies.

DFT study for Structural and Electronic Properties of N2O3 Adsorption onto C20 Fullerene

Structural and electronic properties of N2O3 adsorbed on C20 fullerene were investigated by Density Functional Theory. Before N2O3 was adsorbed on C20 fullerene surface, the structural properties of the isomers of the N2O3 were calculated. According to the total energy calculations, asym-N2O3 is the most stable isomer since it has lower energy than the other two isomers. Data obtained for the structural properties of N2O3 molecule are in agreement with the previous studies. After full structural optimization without any restrictions, the most stable structures were obtained. The adsorption energies with no dissociation of N2O3 were in the range of −2.88 to −3.55 eV for LDA and −2.02 to −2.45 eV for GGA. On the other hand, the dissociative adsorption yields a lower total energy with Eads values of −3.72 and −2.93 eV in LDA and GGA, respectively. According to these values, adsorption can be evaluated as chemisorption for all stable structures. The adsorption occurs with no reaction barriers except for one configuration. Although the co-adsorption of NO and NO2 molecules on the same fullerene is energetically less favorable compared to their adsorptions on separate fullerenes, the dissociative co-adsorption occurs with no energy barrier. The electronic structures are dominated by charge transfer from the fullerene to the adsorbate. The obtained HOMO-LUMO gap (GapHL) values are in the range of 1.02 and 1.35 eV for both LDA and GGA, respectively. The results presented here can be expected to guide future experimental and theoretical studies as new hybrid material.

Deactivation of Pd/Zeolites passive NOx adsorber induced by NO and H2O: Comparative study of Pd/ZSM-5 and Pd/SSZ-13

Atomically dispersed Pd in Zeolites can adsorb NO to reduce NOx emission from vehicle exhausts at low temperature. In this contribution, low temperature NO adsorption abilities of Pd/ZSM-5 and Pd/SSZ-13 are comparatively studied. Co-adsorption of NO and H2O on Pd/ZSM-5 induced PdO sintering and deactivated NO adsorption ability. On the other hand, NO adsorption ability of Pd/SSZ-13 was maintained from the 500 °C treatment under NO and H2O where PdO sintering did not occur. Therefore, Pd/SSZ-13 would be the better candidate as NO storage material to reduce NOx emission from vehicle exhaust.

Investigation of an irreversible NOx storage degradation Mode on a Pd/BEA passive NOx adsorber

Passive NOx adsorbers (PNAs) are a proposed solution for cold start NOx emissions, via low temperature NOx trapping and subsequent release at a temperature where downstream NOx reduction catalysts are active. Pd/zeolites have been reported to have significant NOx storage capacities. However, their real exhaust compatibility has not been completely evaluated. In this work, Pd/BEA was used to study the mechanism of an observed PNA degradation. NO adsorption, NO and CO co-adsorption and temperature programmed desorption/oxidation were performed on a series of differently aged Pd/BEA catalysts to evaluate the NOx storage capacity and desorption temperature. An irreversible NOx storage degradation mode caused by low temperature CO exposure was observed. H2 temperature programmed reduction results indicate this irreversible degradation can be attributed to the loss of Pd2+. Additionally, STEM images revealed Pd particle migration/agglomeration after hydrothermal aging and CO/H2 exposure. Larger particles, formed during CO exposure, lead to the irreversible degradation.

Cation pair formation in copper and palladium exchanged MFI zeolite frameworks - a theoretical study.

The coordination of copper and palladium cations in the channels of MFI frameworks is examined by the QM/MM approach within the ONIOM method. Density functional theory is applied to the high layer of the ONIOM models, and different rings and ring associations are considered with the aim to find the optimal positions for cation pair formation. The Al atoms were located at tetrahedral sites which are known to have high occupancy factors from crystal structure studies. Divalent cations have a strong preference towards six-member ring coordination, and while the energy differences between five-member ring coordination and six-member ring coordination are not that large for the monovalent cations and hydroxyl-cations (M+ and M(OH)+, M = Cu, Pd) they still prefer six-member ring coordination in the straight channel. The monovalent cations form dimers [Pd-Pd]2+ and [Cu-Cu]2+ with bond lengths of 2.50-2.57 Å, but while palladium cations form dimers independent of the Si, Al ordering, the copper cation dimers are stable only with closely positioned negative framework charges. Separation of the two Cu+ cations at distances >4 Å is preferred, unless an oxygen bridge is formed. The cation dimers are exposed in the intersection volume of the straight and sinusoidal channels of MFI, which greatly favors the accessibility of reactants. The oxygen bridged Pd cations accept protons from Brønsted acid sites to form Pd-OH+-Pd and they are able to transfer this proton to reactants: upon co-adsorption of NO and NO2 on Pd-OH+-Pd, protonation of NO2 occurs. In contrast, Cu-OH+-Cu groups reach stabilization by forming hydrogen bonds to framework oxygen atoms. [Pd-Pd] cation dimers alone, or in oxygen-bridged form, provide an energetically favorable route for NO dissociation. Two different configurations with wide and narrow angle ∠MOM were examined for partial oxidation of methane to methanol. The narrow angle [Cu-O-Cu] was found to ease methanol desorption. The active site [Pd-O-Pd] is superior to [Cu-O-Cu], providing lower energy barriers for methane activation and methanol desorption.

Catalytic properties of CuO/CeO2-Al2O3 catalysts for low concentration NO reduction with CO

Cu/CeAl catalysts with different Ce/Al molar ratios were prepared for low concentration NO removal by CO in the presence of O2 and SO2. The flash and used catalysts were characterized by XRD, BET, XPS, H2-TPR, CO-TPD, NO-TPD, in situ DRIFTS and catalytic properties for NO + CO model reaction. Cu/CeAl catalysts with appropriate ceria doping contents showed excellent catalytic performance and superior resistance to O2 and SO2. Ceria was conducive to reduce the active temperature and improve N2 selectivity. Cu/Ce0.1Al catalyst had stable NO conversion over 90% within the temperature range from 330 °C to 650 °C. After incorporation of Ce4+, CeO2 replaced Al2O3 and gradually became the major phase structure in XRD, specific surface area and pore volume decreased with Ce content increasing. Based on XPS analysis, the synergistic interaction of Ce3+ + Cu2+ ↔ Ce4+ + Cu+ was beneficial to generate oxygen vacancies, improving the reducing property and adsorption capacity. Therefore, the reduction temperature in H2-TPR and desorption temperature in TPD both shifted to low temperature. Moreover, the characterizations of used Cu/Ce0.1Al-A catalyst were almost unchanged with flash catalyst. The reaction over Cu/Ce0.1Al catalyst was an Eley-Rideal (E-R) mechanism according to the CO and NO co-adsorption in situ DRIFTS results.

Effect of Ti doping on heterogeneous oxidation of NO over Fe3O4 (1 1 1) surface by H2O2: A density functional study

Magnetite is considered a promising catalyst for catalytic oxidation of nitrogen oxide (NOx) by vaporized H2O2. The NO oxidation mechanisms on original and Ti-doped Fe3O4 (111) catalyst surfaces were analyzed by density functional theory, and the individual and combined adsorption characteristics of H2O2 and NO on the original and Ti-doped surfaces were investigated. Results revealed that H2O2 disassociates on the original and Ti-doped surfaces with large adsorption energies. Ti doping significantly augmented the adsorption intensity of H2O2 and promoted the decomposition of H2O2 on the catalyst surface. NO was adsorbed on the original and Ti-doped surfaces in molecular form. The adsorption intensity of H2O2 was greater than that of NO. HNO2 was generated on the original and Ti-doped surfaces through NO and H2O2 co-adsorption, thereby confirming that NO was oxidized. Analysis of electronic structures demonstrated that H2O2 was decomposed according to the Haber-Weiss mechanism. The oxidation of NO by surface oxygen atoms over the original and Ti-doped surfaces was also calculated, and the low energy barrier obtained suggests that NO is easily oxidized by active O over catalyst surfaces.

Local dynamics of copper active sites in zeolite catalysts for selective catalytic reduction of NOx with NH3

In Cu-zeolite based selective catalytic reduction of NOx with NH3 (NH3-SCR), Cu species (in particular CuI) solvated by NH3 molecules are predicted theoretically to be highly mobile with their mobility being decisive for the NH3-SCR reactivity at low temperatures (<250 °C). Direct experimental observation of the Cu mobility after NH3 solvation, however, has not been achieved yet. Here we show that complex impedance-based modulus spectroscopy, performed by following the corresponding dielectric relaxation processes at high frequencies (104 to 106 Hz), can be applied to monitor directly the dynamic local movement of Cu ions in zeolite catalysts under NH3-SCR related reaction conditions. Simultaneous in situ impedance and infrared spectroscopy studies, assisted by periodic DFT calculations with reliable van der Waals dispersion corrections, allowed us to identify the key factors determining the local dynamics of Cu ions in two representative Cu-zeolites, i.e. Cu-ZSM-5 and Cu-SAPO-34. The co-adsorption and interaction of NO and NH3 on CuII sites led to the formation of highly mobile CuI species and NH4+ intermediates, and, consequently, significantly enhanced local dynamics of Cu ions in both zeolite catalysts. The re-oxidation of CuI, which is the rate-determining step of NH3-SCR reaction, was more favorable in Cu-SAPO-34 than in Cu-ZSM-5, which can be attributed to the close coupling of NH4+ intermediate and Cu site promoting the formation of CuII-NO2/NH4+. As a result, the overall local dynamics of Cu, largely determined by CuI species, is less dependent on the NH4+ intermediate in Cu-SAPO-34 than in Cu-ZSM-5.

Species formed during NO adsorption and NO + O2 co-adsorption on ceria: A combined FTIR and DFT study

Adsorption of NO and co-adsorption of NO and O2 on ceria have been re-investigated by FTIR spectroscopy. To provide unambiguous assignments of the IR bands, adsorption of 15NO and co-adsorption of 14NO + 15NO isotopic mixtures have also been studied and DFT calculations performed. At the initial adsorption stages mainly symmetric nitrite species (2472, 1304, ca. 1160 and 822 cm‐1) and trans-hyponitrites ([N2O2]2−, ca. 1100 cm‐1) are simultaneously produced as a result of NO disproportionation. At further stages non-symmetric NO2− species (1393 and 1014 cm‐1), N2O (2240, 1252 cm‐1) and small amount of nitrates are formed. Nitrates are the only surface products of the thermal decomposition of the nitrites and hyponitrites. In the presence of small amounts of oxygen, the hyponitrites are very easily oxidized. In contrast, all nitrites increase in concentration after addition of small amount of O2 to the NO/CeO2 system but are gradually oxidized in the presence of higher amount of oxygen giving rise to different surface nitrates.

Mechanistic investigations on NO reduction with CO over Mn/TiO2 catalyst at low temperatures

A series of titania-supported transition metal oxide catalysts were evaluated for NO reduction with CO as reductant at low temperature (200°C) in the presence of excess oxygen. Among the investigated systems, MnOx/TiO2 has been found to be the preeminent catalyst. In situ FT-IR and transient studies were carried out in order to identify the chemisorbed species which could act as catalytic intermediates, and in consequence to propose the low temperature reduction of NO pathway over the Mn/TiO2. The NO adsorption and NO+O2 co-adsorption of in situ FT-IR experiments revealed the formation of surface reaction intermediate N2O species along with the formation of monodentate and bidentate nitrates, whereas CO adsorption on the catalyst leads to the formation of carbonate species. Interestingly, in the case of NO and CO co-adsorption over MnOx/TiO2, the formation of CO2 (CO oxidation with gas phase oxygen) is inhibited due to the surface reaction competition between NO and CO. The formation of N2O as an intermediate was evident from the occurrence of peaks at 1286 and 1335cm−1 region. There is no formation of NO2 as a surface intermediate or gas phase stable product in the present in situ Fourier transform infrared spectroscopy (FT-IR) and transient mass spectroscopic studies, respectively. Remarkably, a neat absorption peak at 2178cm−1 ascribed to the isocyanate (–NCO) species was not observed during in situ infrared spectroscopic studies of NO+CO and CO+NO co-adsorptions. Based upon this evidence it is proposed that the formation of NCO− species from the reaction between CO and Nads is completely forbidden over the Mn/TiO2 catalysts. These results indicate that the reaction mechanism follows a different pathway for our catalyst, from that of the other metal based catalysts. The role of lattice oxygen in the reaction mechanism is substantiated by isotopic labeling and transient analysis studies. Lewis acid sites act as the active sites for the reaction.

DFT study of NO and H2O co-adsorption on CumCon(m+n=2∼7) clusters

A theoretical study was carried out of NO and H2O co-adsorption on CumCon(2≤m+n≤7) clusters using density functional method. It can be found that doping H2O can generally promote activity of NO molecules except Co2NOH2O and Cu2CoH2O clusters. Co4NOH2O, CuCo4NOH2O and Cu2Co3NO clusters are much more stable than their neighbors, while CuCoNOH2O,Cu2CoNOH2O,Cu2Co2NOH2O,Cu4CoNOH2O,Cu3Co3NOH2O,Cu6CoNOH2O clusters display stronger chemical stability. Magnetic and electronic properties are also discussed. It turns out that the change of magnetic moment after H2O adsorption is affected not only by negative magnetic moments provided by NO and H2O, but also by the degree of spin polarization of Co atoms.

FTIR Study of CO and NO Coadsorption on Cu-ZSM-5: Does a Cu+(CO)(NO) Complex Exist?

Metal cations that are exchanged in zeolites are generally characterized by a low coordination number and can adsorb more than one small molecule. For instance, Cu+ ions exchanged in ZSM-5 (and in other zeolites) can bind, at low temperature, up to three CO and up to two NO molecules. Although mixed aqua-carbonyl complexes are easily formed, here we show that no mixed carbonyl-nitrosyl species are produced. Instead, NO adsorption on Cu-ZSM-5 with pre-formed Cu+-CO species leads to formation of dicarbonyls and dinitrosyls according to the reaction: 2 Cu+-CO + 2 NO  Cu+(CO)2 + Cu+(NO)2. The role of the ligand and the nature of the bond formed on the production of mixed ligand complexes are discussed. Keywords: Adsorption, carbon monoxide, coadsorption, Cu-ZSM-5, FTIR spectroscopy, nitrogen monoxide, zeolites.

Precise sensing and selection of molecules by the interface between the metal nanocluster and the oxide support

Molecules being adsorbed on the surface dramatically change the physics and the chemistry of the substrate, thus offering an opportunity for precise sensing of the molecules via monitoring the transformation of the state of the adsorbent. Combination of different types of substrates, forming the surface interface border, generally exhibits synergistic effect dramatically enhancing sensitivity factor. In this regard, the aim of the present work is to demonstrate how the metal/oxide interface, formed in-vacuo (10-10 Torr) by deposition of metal nanoclusters on oxide support, affects behavior of NO and CO molecules being adsorbed on the surface. Coadsorption of NO and CO molecules on the Ni clusters deposited on MgO(111) film formed on Mo(110) crystal has been studied by reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Observed high sensitivity of metal/oxide interface to the molecular behaviour suggests an opportunity for design of new molecular sensors based on the metal/oxide nanostructures.

Adsorption and Reaction of CO and NO on Ir(111) Under Near Ambient Pressure Conditions

The adsorption and reaction of CO and NO on Ir(111) have been studied by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) together with low-energy electron diffraction, scanning tunneling microscopy, and mass spectroscopy (MS). Under both ultrahigh vacuum (UHV) and NAP conditions CO molecules occupy on-top sites of the Ir(111) surface at room temperature (RT) by forming two-dimensional clusters. Exposure to NO under UHV conditions at RT induces partially dissociative adsorption, while NAP NO exposure leads to a Ir(111) surface that is covered by molecular NO. We conducted in-operando NAP-XPS/MS observation of the NO + 13CO reaction under a NAP condition as a function of temperature. Below 210 °C adsorption of NO is inhibited by CO, while above 210 °C the CO inhibition is released due to partial desorption of CO and dissociative adsorption of NO starts to occur leading to associative formation of N2. Under the most active condition studied here the Ir surface is covered by a dense co-adsorption layer consisting of on-top CO, atomic N and O, which suggests that this reaction is not a NO-dissociation-limited process but a N2/CO2 formation-limited process.

In situ IR studies of Co and Ce doped Mn/TiO2 catalyst for low-temperature selective catalytic reduction of NO with NH3

The Mn–Co–Ce/TiO2 catalyst was prepared by wet co-impregnation method for selective catalytic reduction of NO by NH3 in the presence of oxygen. The adsorption and co-adsorption of NH3, NO and O2 on catalysts were investigated by in situ FTIR spectroscopy. The results suggested that addition of cobalt and cerium oxides increased the numbers of acid and redox sites. Especially, the cobalt oxide produced lots of Brønsted acid sites, which favor to the adsorption of coordinated NH3 through NH3 migration. Ce addition improved amide ions formation to reach best NO reduction selectivity. A mechanistic pathway over Mn–Co–Ce/TiO2 was proposed. At low-temperature SCR reaction, coordinated NH3 reacted with NO2−, and amide reacted with NO (ad) or NO (g) to form N2. NO2 was related to the formation of nitrite on Co-contained catalysts and the generation of NH2− on Ce-contained catalysts. At high temperature, the other branch reaction also occurred between the coordinated NH3 and nitrate species, resulting in N2O yield increase.

Ammonia Formation from NO Reaction with Surface Hydroxyls on Rutile TiO2(110)-1 × 1

The reaction of NO with the hydroxylated rutile TiO2(110)-1 × 1 surface (h-TiO2) was investigated as a function of NO coverage using temperature-programmed desorption. Our results show that NO reaction with h-TiO2 leads to formation of NH3, which is observed to desorb at ∼400 K. Interestingly, the amount of NH3 produced depends nonlinearly on the dose of NO. The yield increases up to a saturation value of ∼1.3 × 1013 NH3/cm2 at a NO dose of 5 × 1013 NO/cm2, but subsequently decreases at higher NO doses. Preadsorbed H2O is found to have a negligible effect on the NH3 desorption yield. Additionally, no NH3 is formed in the absence of surface hydroxyls (HOb’s) upon coadsorption of NO and H2O on a stoichiometric TiO2(110) (s-TiO2(110)). On the basis of these observations, we conclude that nitrogen from NO has a strong preference to react with HOb’s on the bridge-bonded oxygen rows (but not with H2O) to form NH3. The absolute NH3 yield is limited by competing reactions of HOb species with titanium-bound oxygen...

Adsorption and co-adsorption of NO and water on LaCu-ZSM5

The effect of water on NO adsorption on LaCu-ZSM5 has been studied by in-situ FTIR at 50°C identifying for which copper sites NO adsorption competes with water. The relative thermal stability of NO and H2O ad-species have been also determined by increasing the sample temperature from 50 to 400°C. Water was removed from zeolite at temperature lower than that necessary to desorb NO, as both bridged and chelating nitrates, from copper sites.A quantitative analysis of the adsorption of NO on the zeolite previously contacted with water was made by carrying out adsorption tests at temperatures far from those activating any DeNOx reaction in a lab-scale PFR reactor. The variation of the amount of adsorbed NO with the temperature of H2O desorption has been evaluated increasing the desorption temperature from 125 to 500°C after zeolite saturation with water at 125°C. The total recovery of the amount of NO adsorbed on the dry zeolite demonstrated the reversibility of water deactivation.