NO Dimer Research Articles

Unusual C-N coupling mechanism via NO dimerization for the electrochemical synthesis of urea on transition metal dimers supported by two-dimensional expanded phthalocyanine

Electro-synthesis of urea is a powerful alternative to traditional demanding industrial engineering by selective reduction of N2 and CO2. Due to the existence of inert NN bond, usual C-N coupling has always hindered the design and performance of catalysts. Herein, we present an unusual C-N coupling mechanism for highly electrochemical catalytic synthesis of urea via NO dimerization on transition metal dimers supported by two-dimensional expanded phthalocyanine (TMTM-Pc) as a prototype through first principles calculations. The key is that the adsorbed *NO dimer can directly jump over challenging step of N-N bond breaking and associate with CO. Particularly, FeFe-Pc is singled out in terms of activity, selectivity, and stability via proposed reaction pathways. What's more, we discovered an easy descriptor, namely, the difference of the adsorption energies of *NO-CO-NO* and *NO-NO*, ∆E(*NO-CO-NO* - *NO-NO*), to consummately depict the thermal-kinetic relationships for the C-N coupling. This work may broaden the understanding of urea formation mechanism and provide more options for future research.

A new insight on the NO-CO reaction at the electronic level: homogeneous, E-R, and L-H mechanisms.

Carbonaceous surface, as one of the major carriers in coal combustion, was found to exert great influence on the nitric oxide with carbon monoxide (NO-CO) reaction. Although there have been some studies addressing the NO-CO reaction, the inherent mechanism remains obscure. In this work, some updated mechanisms with details were proposed at the electronic level. Using density functional theory calculations, the preferred pathways were identified with three channels consisting of homogeneous, Eley-Rideal (E-R), and Langmuir-Hinshelwood (L-H) heterogeneous reactions. The reasons for the difference in energy barrier among the three mechanisms were revealed by analyzing the chemical bond and electronic transfer. Results show that among these channels, the NO-CO reaction is more likely to occur along the E-R mechanism, due to its lower energy barrier of the rate-determining step. Compared to the L-H mechanism, there is a higher degree of electronic localization between NO molecules at the initial stage of the E-R mechanism. As a result, the NO dimer formation of the E-R mechanism has a lower energy barrier than that of the L-H mechanism. Meanwhile, a large number of electrons floods into the N-N, N-O, and O-O bonds of NO dimer in the homogeneous reaction, which certainly gets more difficult for the dissociation of O atoms in the gas phase. Accordingly, the following stage of N2 formation in the homogeneous reaction has a higher energy barrier than that in both the E-R and L-H reactions. Compared to the L-H mechanism, the E-R mechanism exhibits a lower degree of electronic localization between N2O and carbonaceous surface, suggesting that the interfacial interaction in the E-R mechanism is weaker. As a result, N2 is easier to remove from the carbonaceous surface in the E-R mechanism than in the L-H mechanism. To sum up, the results deepen the knowledge about the NO-CO reaction, which will help to further develop the oxy-fuel combustion technology.

Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)-Platinum(II) Olefin Complexes as Catalysts.

The nitrogen oxides NO2, NO, and N2O are among the most potent air pollutants of the 21st century. A bimetallic RhI–PtII complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3–4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N2O) to water and N2 was recorded, making these RhI–PtII complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON=38) or nitrogen dioxide (NO2, TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NOx were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO2 proceeds stepwise, to first give NO and H2O, followed by the generation of N2O and H2O, which is then further converted to N2 and H2O. The nitrogen−nitrogen bond‐forming step takes place in the conversion from NO to N2O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N2O2 2−) complex, which was detected as an intermediate.

Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)–Platinum(II) Olefin Complexes as Catalysts

AbstractThe nitrogen oxides NO2, NO, and N2O are among the most potent air pollutants of the 21st century. A bimetallic RhI–PtII complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3–4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N2O) to water and N2 was recorded, making these RhI–PtII complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON=38) or nitrogen dioxide (NO2, TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NOx were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO2 proceeds stepwise, to first give NO and H2O, followed by the generation of N2O and H2O, which is then further converted to N2 and H2O. The nitrogen−nitrogen bond‐forming step takes place in the conversion from NO to N2O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N2O22−) complex, which was detected as an intermediate.

Quantum chemical study of NO reduction mechanism on Ag /Al2O3 catalysts

It was shown that N2O content among NO reduction products increases with an increase of the silver concentration in the catalyst because the nature of the catalytic centers changes and leads to a subsequent change in the mechanism of the reaction. Two reaction mechanisms were proposed and studied by means of quantum chemistry: a two-stage mechanism that proceeds via NO dimer formation on catalysts with high (above 2 wt. %) silver concentration and a parallel mechanism with isocyanates involved on catalysts with low (below 2 wt. %) silver concentration. It was demonstrated that on catalysts with high silver concentration mechanism that involves stepwise NO reduction via N2O to N2 is realised. Moreover, the final stage is complicated by the fact that formed intermediates and N2O are likely to desorb from the catalyst surface. In the case of catalysts with low silver concentration, the formation of both products (N2O and N2) proceeds in parallel and the lower activation barriers of the reaction leading to N2, as well as the thermodynamic profitability of its formation, lead to the predominance of the target product. The competition between the proposed mechanisms was studied in the case of catalytic centers represented by silver dimers. It was shown that activation barriers of reaction proceeding via NO dimer formation are lower than the corresponding barriers of the reaction with isocyanates involved, which confirms the prevalent realisation of the first process and the predominance of N2O among the final products. The obtained results explain the experimental data and are significant for further modelling of the mechanism of nitrogen oxides catalytic reduction considering the Al2O3 support.

Visible photodissociation study of NO dimer cation using ion imaging technique combined with theoretical calculations

Visible photodissociation of (NO)2+ at 532–692 nm was investigated by ion imaging experiments and theoretical calculations. From images of photofragment NO+, we obtained the translational energy, which increased with excitation energy. Anisotropy parameter increased from β = 0.7 at 692 nm to 1.4 at 532 nm. These results qualitatively agreed with theoretical calculations, in which (NO)2+ was predicted to be directly dissociated after the C∼←X∼ transition. However, a simple description with an impulsive dissociation model cannot reproduce photon energy dependence of the photofragment energy and angular distributions, implying the importance of multidimensional analyses of potential energy surfaces.

Reaction Behavior of the NO Molecule on the Surface of an Mn Particle (M = Ru, Rh, Pd, and Ag; n = 13 and 55): Theoretical Study of Its Dependence on Transition-Metal Element.

Reaction of NO molecule on M13 and M55 clusters (M = Ru, Rh, Pd, and Ag) was theoretically investigated to elucidate why its reaction behavior depends on the position of metal element in the periodic table. DFT computations show that NO dissociative adsorption occurs on M = Ru and Rh, NO molecular adsorption occurs on M = Pd, and NO dimerization occurs on M = Ag, which agree with experimental findings. The d-band center and d-band top become lower in energy following the order Ru > Rh > Pd > Ag; this is one of the characteristic features of the periodic table. In the Ag cluster, the valence band-top consists of Ag 5s orbital and its energy is higher than the d-band top of Pd. For NO dissociative adsorption, the M-N and M-O bond strengths are crucially important at the transition state and the product, to which the metal d orbital contributes very much. Ru and Rh clusters have a high energy d-band center and d-valence band top, leading to the formation of strong M-N and M-O bonds. Pd and Ag clusters have a low energy d-band center and d-band top, leading to the formation of weak M-N and M-O bonds. Because the Ag cluster has a high energy 5s valence band that can overlap well with the π* + π* MO of ONNO (NO dimer) moiety due to the same symmetry, charge transfer (CT) occurs from the Ag cluster to the π* + π* MO, which is indispensable for NO dimerization. The 4d-valence band top of Ru and Rh clusters does not fit to the π* + π* MO because of the different symmetry. Though the d-valence band top of the Pd cluster can overlap with the π* + π* MO, its energy is low, which is not good for the CT. Thus, the reactivity of metal cluster for NO is determined by the energy and type (4d or 5s) of the valence band top, which both depend on the position of element in the periodic table; accordingly, Ru and Rh clusters are reactive for NO dissociative adsorption, the Ag cluster is reactive for NO dimerization, but the Pd cluster is not reactive for both and only NO molecular adsorption is possible.

Catalysis of Cu Cluster for NO Reduction by CO: Theoretical Insight into the Reaction Mechanism.

Density functional theory calculations here elucidated that Cu38-catalyzed NO reduction by CO occurred not through NO dissociative adsorption but through NO dimerization. NO is adsorbed to two Cu atoms in a bridging manner. NO adsorption energy is much larger than that of CO. N–O bond cleavage of the adsorbed NO molecule needs a very large activation energy (ΔG°‡). On the other hand, dimerization of two NO molecules occurs on the Cu38 surface with small ΔG°‡ and very negative Gibbs reaction energy (ΔG°) to form ONNO species adsorbed to Cu38. Then, a CO molecule is adsorbed at the neighboring position to the ONNO species and reacts with the ONNO to induce N–O bond cleavage with small ΔG°‡ and very negative ΔG°, leading to the formation of N2O adsorbed on Cu38 and CO2 molecule in the gas phase. N2O dissociates from Cu38, and then it is readsorbed to Cu38 in the most stable adsorption structure. N–O bond cleavage of N2O easily occurs with small ΔG°‡ and significantly negative ΔG° to form the N2 molecule and the O atom adsorbed on Cu38. The O atom reacts with the CO molecule to afford CO2 and regenerate Cu38, which is rate-determining. N2O species was experimentally observed in Cu/γ-Al2O3-catalyzed NO reduction by CO, which is consistent with this reaction mechanism. This mechanism differs from that proposed for the Rh catalyst, which occurs via N–O bond cleavage of the NO molecule. Electronic processes in the NO dimerization and the CO oxidation with the O atom adsorbed to Cu38 are discussed in terms of the charge-transfer interaction with Cu38 and Frontier orbital energy of Cu38.

Electronic processes in NO dimerization on Ag and Cu clusters: DFT and MRMP2 studies.

Experimentally observed NO dimerization on Cu and Ag surfaces is surprising because binding energy of NO dimer is very small in gas phase. MRMP2, MP2 to MP4, CCSD(T), and DFT studies of NO dimerization on Ag2 and Cu2 clusters disclosed that the CCSD(T) method could be applied to this reaction on Ag2 and Cu2 unlike NO dimerization in gas phase which exhibits significantly large nondynamical electron correlation effect. Charge-transfer (CT) from Ag2 and Cu2 to NO moieties plays important role in NN bond formation between two NO molecules. This CT considerably decreases nondynamical correlation effect. Also, the DFT method could be applied to this NO dimerization, if appropriate DFT functional is used; all pure functionals examined here and most of the hybrid functionals underestimated the activation barrier (Ea ), while only ωB97X provided Ea similar to CCSD(T)-calculated value. NO dimerization on similar Cu2 and Cu5 needs moderately larger Ea than those on Ag2 and Ag5 , because frontier orbital participating in the CT exists at lower energy in Cu2 and Cu5 than in Ag2 and Ag5 . The Ea decreases in the order Ag2 >> Ag38 > Ag7 ∼ Ag5 and the reaction energy (ΔE) is positive (endothermic) in Ag2 but significantly negative in Ag38 , Ag7 , and Ag5 , indicating that various Ag clusters could be effective for NO dimerization except for Ag2 . The decreasing order of Ea and increasing order of exothermicity are attributed to increasing order of the frontier orbital energy of Ag2 < Ag38 < Ag7 ∼ Ag5 . © 2018 Wiley Periodicals, Inc.

Vibrational predissociation dynamics of the nitric oxide dimer

We have studied the photofragmentation of NO dimer employing quasi-classical trajectories propagation using quantum probabilities distributions and Wigner distribution functions to determine the initial conditions of the system. The distributions of initial conditions in phase space, between Wigner distribution functions and quantum probabilities distributions, are not so different. The photofragmentation of NO dimer depends strongly on the excitation of the initial state, especially for the lowest of them. These results are in concordance with experimental values where we can see a similar tendency for the lifetimes different vibrational modes that have been studied. In this work we explore more intramolecular and intermolecular vibrational modes. We have increased the number of trajectories and the dissociation distance in the simulation as well.

NO Electrochemical Reduction on Pt Electrocatalysts: A DFT Approach

Electrochemical denitrification is a promising technology for the removal of toxic nitrate and nitrite species from groundwater due to the process’s environmental compatibility, energy efficiency, safety, and product selectivity[1]-[3]. The adsorbed NO is generally considered to be a selectivity-determining species during this electrochemical potential-dependent reaction[4], [5]. Atomic-scale studies using density functional theory (DFT), in turn, can provide powerful molecular-level information about the elementary mechanisms of reductive NO electrocatalysis, and these basic building blocks can further serve as a starting point for future trends-based analysis on different transition metal surfaces. In this work, we employ periodic, self-consistent DFT calculations to clarify the adsorption structures and thermochemistry of NO and its related reaction intermediate species on different Pt single crystal surfaces ((111) and (100)). Kinetics are further determined by calculating the activation barriers for N-O dissociation, protonation, and N-N bond formation. We begin by describing results for the reaction at low NO coverage (ca. 0.11 ML). After describing the thermodynamics (the adsorption) and kinetics (N-O bond breaking and protonation barrier) for the dentirification reaction scheme, a plausible mechanistic reaction pathway is proposed. Water-assisted protonation barriers are generally shown to be lower than barriers for non-electrochemical reactions, such as surface hydrogenation or N-O bond breaking[6]. Consistent with available experimental evidence, from the most probable reaction pathway, we conclude that ammonium ion would be the most favorable product that would evolve. There is no evidence for the formation of hydroxylamine (H2NOH) at low coverage. At higher NO coverages (ca. 0.45 ML), the relative energy states of the intermediates show a similar trend for the energetics when it is compared with the study with lower NO coverage. Next, N2O formation is examined. We find that trans-(NO)2 species could be a precursor state for the N­2O formation due to its lower kinetic barrier than barriers that are found for cis-type NO dimers. The simulation demonstrates that N2O could be formed at higher potentials through two protonations of trans-(NO)2 species, and the reaction is thermodynamically more favorable at higher NO coverage. This barrier could be surmountable at room temperature, and these mechanistic conclusions hold on both the (111) and (100) surfaces. Finally, using the DFT results to determined rate constants, we construct a detailed kinetic Monte Carlo model of the overall NO electroreduction network. The model accurately tracks experimental NO stripping curves and provides estimates of the degrees of rate control of elementary reaction steps.

Temperature-programmed desorption study of NO reactions on rutile TiO2(110)-1 × 1

Systematic temperature-programmed desorption (TPD) studies of NO adsorption and reactions on rutile TiO2(110)-1×1 surface reveal several distinct reaction channels in a temperature range of 50–500K. NO readily reacts on TiO2(110) to form N2O, which desorbs between 50 and 200K (LT N2O channels), which leaves the TiO2 surface populated with adsorbed oxygen atoms (Oa) as a by-product of N2O formation. In addition, we observe simultaneous desorption peaks of NO and N2O at 270K (HT1 N2O) and 400K (HT2 N2O), respectively, both of which are attributed to reaction-limited processes. No N-derived reaction product desorbs from TiO2(110) surface above 500K or higher, while the surface may be populated with Oa's and oxidized products such as NO2 and NO3. The adsorbate-free TiO2 surface with oxygen vacancies can be regenerated by prolonged annealing at 850K or higher. Detailed analysis of the three N2O desorption yields reveals that the surface species for the HT channels are likely to be various forms of NO dimers.

Kinetics of oxygen transfer reactions on the graphene surface: Part I. NO vs. O2

Oxygen transfer reactions on carbon surfaces are of great interest for a variety of industries, as well as for the development of new technologies (e.g., NO gas detectors). They are also at the heart of the chemical surface properties and behavior of graphene-based materials. In this study we use quantum chemistry to discover the most probable ground and transition states of reactants, surface intermediates and products in the transfer of oxygen from nitric oxide and O2 to form N2 and CO/CO2. The graphene-NO reaction is more complex and dependent on site availability. Whereas O2 chemisorption is barrierless, adsorption of NO is a complex function of geometric and electronic factors. Thus, the desorption of N2 is highly favored both kinetically (from a C(NN) complex and O-down adsorption of N2O) and thermodynamically (especially via the N2O intermediate). These results are analogous to the experimental findings with a potassium catalyst. Adsorption of the NO dimer on H-saturated sites may account for the reversible chemisorption at low temperatures. These findings allowed us to propose a concise mechanism of oxygen transfer, which highlights the similarities and differences with respect to the graphene-O2 reaction. It is also consistent with the experimentally observed concentration dependence of these reactions.

Covalent intermolecular interaction of the nitric oxide dimer (NO)2**Project supported by the National Natural Science Foundation of China (Grant Nos. 90403007 and 10975044), the Key Subject Construction Project of Hebei Provincial Universities, China, the Research Project of Hebei Education Department, China (Grant Nos. Z2012067 and Z2011133), the National Natural Science Foundation of China (Grant No. 11147103), and the Open Project

Covalent bonds arise from the overlap of the electronic clouds in the internucleus region, which is a pure quantum effect and cannot be obtained in any classical way. If the intermolecular interaction is of covalent character, the result from direct applications of classical simulation methods to the molecular system would be questionable. Here, we analyze the special intermolecular interaction between two NO molecules based on quantum chemical calculation. This weak intermolecular interaction, which is of covalent character, is responsible for the formation of the NO dimer, (NO)2, in its most stable conformation, a cis conformation. The natural bond orbital (NBO) analysis gives an intuitive illustration of the formation of the dimer bonding and antibonding orbitals concomitant with the breaking of the π bonds with bond order 0.5 of the monomers. The dimer bonding is counteracted by partially filling the antibonding dimer orbital and the repulsion between those fully or nearly fully occupied nonbonding dimer orbitals that make the dimer binding rather weak. The direct molecular mechanics (MM) calculation with the UFF force fields predicts a trans conformation as the most stable state, which contradicts the result of quantum mechanics (QM). The lesson from the investigation of this special system is that for the case where intermolecular interaction is of covalent character, a specific modification of the force fields of the molecular simulation method is necessary.

NO Electrochemical Reduction on Pt Electrocatalysts: A DFT Approach

Electrochemical denitrification is a promising technology for the removal of toxic nitrate and nitrite species from groundwater due to the process’s environmental compatibility, energy efficiency, safety, and product selectivity. The adsorbed NO is generally considered to be a selectivity-determining species during this electrochemical potential dependent reaction. Atomic-scale studies using density functional theory (DFT), in turn, can provide powerful molecular-level information about the elementary mechanisms of reductive NO electrocatalysis. These basic building blocks can further serve as a starting point for future trends-based analysis on different transition metal surfaces. In this work, we employ periodic, self-consistent DFT calculations to clarify the adsorption structures and thermochemistry of NO and its related reaction intermediate species on different Pt single crystal surfaces ((111) and (100)). Kinetics are further determined by calculating the activation barriers for N-O dissociation, protonation, and N-N bond formation. We begin by describing results for the reaction at low NO coverage (ca. 0.11 ML). After describing the thermodynamics (the adsorption) and kinetics (N-O bond breaking and protonation barrier) for the dentirification reaction scheme, a plausible mechanistic reaction pathway is proposed. Water-assisted protonation barriers are generally shown to be lower than barriers for non-electrochemical reactions, such as surface hydrogenation or N-O bond breaking. Consistent with available experimental evidence, from the most probable reaction pathway, we conclude that ammonium ion would be the most favorable product that would evolve. There is no evidence for the formation of hydroxylamine (H2NOH) at low coverage. At higher NO coverages (ca. 0.45 ML), the relative energy states of the intermediates show a similar trend for the energetics when it is compared with the study with lower NO coverage. Finally, N2O formation is examined. We find that trans NO dimer species could be a precursor state for the N­2O formation due to its lower kinetic barrier than barriers that are found for cis-type NO dimers. The simulation demonstrates that N2O could be formed at higher potentials through two protonations of trans NO species, and the reaction is thermodynamically more favorable at higher NO coverage. This barrier could be surmountable at room temperature, and these mechanistic conclusions hold on both the (111) and (100) surfaces. Based on our study, we find that reduction of NO proceeds through the water-assisted protonations which will finally lead to ammonia. The primary pathway for N2O formation occurs through trans-type NO dimer species. The proposed pathways occur with different applied electrochemical potentials and NO coverages.

An update on the mechanism of the graphene–NO reaction

Cognizant of the key experimental facts from studies of carbonaceous solids ranging from soot to graphite, we performed a quantum chemistry study of the interaction of NO monomer or dimer with one or more zigzag sites. Thermodynamic and kinetic results were used to examine two alternative mechanisms proposed in the literature, and to compare them with the graphene–O2 reaction mechanism. The chemisorption stoichiometry similarities are striking; but the differences, especially regarding the intermediate role of N2O, have important practical implications. Monomer chemisorption on an isolated site is a dead-end and temporarily inhibiting process, similar to that of formation of a stable C–O surface complex in the graphene–O2 reaction. When two sites are available, successive monomer adsorption eventually leads to N2O formation subsequent to parallel reorientation of the first NO molecule. If three contiguous sites are available, N2 and CO are the principal products. Chemisorption of the dimer provides a straightforward path to N2 and CO2 when one site is available and to N2 and CO when two sites are available. The formation of N2O is also feasible in this case, both during adsorption and desorption; in the adsorption phase it is very sensitive to the details of the electron pairing processes.

NO dissociation and reduction by H2 on Pd(1 1 1): A first-principles study

Periodic density functional theory (DFT) calculations were carried out to elucidate the reaction mechanisms of NO reduction by H2 and possible products on Pd(111). The results show that direct NO dissociation is unlikely due to high-energy barrier; alternatively, NO dimer exists on Pd(111) surface followed by the N–O bond scission to form N2O. The presence of H2 promotes the NO dissociation. The N2 formation pathway is NO+N→N2+O rather than N+N→N2. Besides, N2 is formed preferentially than N2O from the coadsorbed state of NO+N. The NH3 formation comes from the successive hydrogenation reactions of nitrogen and the NH formation is the rate-determining step. The microkinetic analysis further confirms that N2O is major at low temperature while N2 becomes dominant as temperature increases. The selectivities toward N2 and NH3 shift to slightly lower temperature as H2/NO ratio increases.

Role of Acidic Proton in the Decomposition of NO over Dimeric Cu(I) Active Sites in Cu-ZSM-5 Catalyst: A QM/MM Study

The influence of protons on the mechanism of the direct decomposition of NO over adjacent dimeric Cu(I) active sites in zeolite is theoretically investigated by using ONIOM (QM/MM) calculations with two dicopper model systems 1T and 2T, where the Cu(I) atoms are separated by one and two SiO4 tetrahedra, respectively. The reaction proceeds through the formation of N2O as a reaction intermediate and further its decomposition into oxygen and nitrogen. The present study shows that the presence of proton plays an important role in the production of N2O from two NO molecules. In the proton-free mechanism, this process requires a large activation barrier of 56.3 and 55.3 kcal/mol on 1T and 2T, respectively, while the inclusion of protons reduces it to 31.4 and 17.3 kcal/mol. The significant decrease in the activation barrier is due to the strengthening of the N–N bond of the formed NO dimer upon protonation, which facilitates the formation of N2O. On the other hand, the presence of protons disfavors the decomposition of N2O and needs an activation barrier of 6–9 kcal/mol higher than that of the corresponding reaction in the absence of protons. The stable intermediate Cu–OH+–Cu formed in the proton-assisted mechanism is responsible for the larger activation energy for N2O decomposition. The proton-assisted NO decomposition mechanism is in agreement with the experimental observation that the decomposition of N2O as well as O2 desorption are the governing reaction steps in the decomposition of NO. The present study explains the role of the Cu–O–Cu species in the NO decomposition reaction. The results disclosed herein will also pave a way to understanding the mechanism of the reductive N–N coupling of NO molecules catalyzed by metalloenzymes and transition-metal catalysts.

Revealing the Adsorption Mechanisms of Nitroxides on Ultrapure, Metallicity-Sorted Carbon Nanotubes

Carbon nanotubes are a natural choice as gas sensor components given their high surface to volume ratio, electronic properties, and capability to mediate chemical reactions. However, a realistic assessment of the interaction of the tube wall and the adsorption processes during gas phase reactions has always been elusive. Making use of ultraclean single-walled carbon nanotubes, we have followed the adsorption kinetics of NO2 and found a physisorption mechanism. Additionally, the adsorption reaction directly depends on the metallic character of the samples. Franck–Condon satellites, hitherto undetected in nanotube–NOx systems, were resolved in the N 1s X-ray absorption signal, revealing a weak chemisorption, which is intrinsically related to NO dimer molecules. This has allowed us to identify that an additional signal observed in the higher binding energy region of the core level C 1s photoemission signal is due to the C=O species of ketene groups formed as reaction byproducts . This has been supported by density functional theory calculations. These results pave the way toward the optimization of nanotube-based sensors with tailored sensitivity and selectivity to different species at room temperature.

Molecular Beam Depletion: A New Approach

During the last years some interesting experimental results have been reported for experiments in N2O, NO, NO dimer, H2, Toluene and BaFCH3 cluster. The main result consists in the observation of molecular beam depletion when the molecules of a pulsed beam interact with a static electric or magnetic field and an oscillating field (RF). In these cases, and as a main difference, instead of using four fields as in the original technique developed by I.I. Rabi and others, only two fields, those which configure the resonant unit, are used. That is, without using the nonhomogeneous magnetic fields. The depletion explanation for I.I. Rabi and others is based in the interaction between the molecular electric or magnetic dipole moment and the non-homogeneous fields. But, obviously, the change in the molecules trajectories observed on these new experiments has to be explained without considering the force provided by the field gradient because it happens without using non-homogeneous fields. In this paper a theoretical way for the explanation of these new experimental results is presented. One important point emerges as a result of this development, namely, the existence of an, until now unknown, spin-dependent force which would be responsible of the aforementioned deviation of the molecules.