Nitric oxide reduction by CO ON Rh(111): Temperature programmed reaction

The reaction of NO with CO on Rh(111) has been studied with temperature programmed reaction (TPR). Comparisons are made with the reaction of O₂ with CO and the reaction of NO with H₂. The rate-determining step for both CO oxidation reactions is CO(a) + O(a) → CO₂(g). Repulsive interactions between adsorbed CO and adsorbed nitrogen atoms lead to desorption of CO in a peak at 415 K which is in the temperature range where the reaction between CO(a) and O(a) produces CO₂(g). Thus the extent of reaction of CO(a) with NO(a) is less than that between CO(a) and O(a) due to the lower coverage of CO caused by adsorbed N atoms and NO. A similar repulsive interaction between NO(a) and H(a) suppresses the NO + H₂ reaction. CO + NO reaction behavior on Rh(111) is compared to that observed on Pt(111).     

The effect of oxygen–oxygen repulsion in the production of water from hydrogen–oxygen reaction on the surface: A computer simulation study

The formation of water from hydrogen–oxygen reaction on a metal surface is of immense importance due to the technological reasons. This reaction has been studied via a thermal mechanism on a Pt single crystal surface where the two molecules, H₂ and O₂, have been adsorbed dissociatively in atomic form. The reaction takes place between the adsorbed atoms through an intermediate OH radical. We have studied this reaction via a thermal (Langmuir–Hinshelwood mechanism) as well as a non-thermal mechanism (precursor mechanism) by the Monte Carlo computer simulations. In this study, we have applied a novel approach based upon the experimental observations that the dissociated oxygen atoms do not sit next to one another on a catalytic surface. Some interesting results like the shifting of the phase transition points, the broadening of the reaction window width and the elimination of the second-order phase transition in the non-thermal reaction mechanism are obtained by considering various possibilities of the reaction scheme. The phase diagrams as well as the snapshots of the surface covered with the reacting species are presented.     

H2 分子在Li3N(110)表面吸附的第一性原理研究

采用第一性原理方法研究了H₂分子在Li₃N(110)晶面的表面吸附. 通过研究H₂/Li₃N(110)体系的吸附位置、吸附能和电子结构发现: H₂分子吸附在N桥位要比吸附在其他位置稳定,此时在Li₃N(110)面形成两个-NH基,其吸附能为1.909 eV,属于强化学吸附;H₂与Li₃N(110)面的相互作用主要是H 1s轨道与N 关键词： 第一性原理 3N(110)')" href="#">Li₃N(110) 2')" href="#">H₂ 吸附和解离     

TPD study of NO decomposition on Rh(111) by dynamic Monte Carlo simulation

The kinetics of NO desorption and its decomposition on Rh(111) surfaces have been simulated by using a dynamic Monte Carlo method. During the simulations, we used a triangular lattice that mimics the Rh(111) phase. NO decomposition was studied at low pressure and temperatures ranging from 120 to 1000 K. The present analysis incorporates recent experimental evidence showing that N₂ production occurs either from the classical N + N recombination step or by the formation and successive decay of an (N–NO)* intermediate species. Moreover, N₂ and NO desorption rates are enhanced and the NO dissociation rate is inhibited by coadsorbed NO, N, and O species as nearest neighbors. These effects are taken into account in this study, along with the experimental adsorption, desorption, dissociation, and diffusion rates of the reactants. Our simulations are consistent with the experimental results of TPD spectra and can explain the formation of two peaks, δ-N₂ and β-N₂, as a natural consequence of the reaction mechanism herein proposed. Comparisons with different mechanisms used by other authors are also made.     

Adsorption de l'éthyléne et de l'acétylène sur des films de rhénium evaporés sous ultra-vide; Hydrogénation de l'éthylène: II. Discussion des résultats expérimentaux et interprétation

A qualitative model is proposed in order to explain our experimental results on ethylene chemisorption on evaporated rhenium films and hydrogenation of ethylene (part I). The surface must present at least two kinds of surface sites (A and B). The second type (B), either preexists on the surface, or is induced by the adsorption phenomenon itself. On the most energetic ones (A), dissociation of ethylene and hydrogen is complete. Adsorption of ethylene is characterized by a sticking coefficient value of 0.1 if they are free and 1 if they are hydrogen covered. On sites B, ethylene is adsorbed without full dissociation (sticking coefficients equal to 0.015). independent on adsorption temperature. Hydrogen desorption is due to full dissociation of ethylene on the surface and a displacement reaction while ethane is produced by reaction between non-dissociated adsorbed ethylene and hydrogen in the gas phase. The same Rideal-Eley mechanism applies for hydrogenation of ethylene in quasi-stationary conditions, along with a self-poisoning mechanism involving dehydrogenation leading to C₂H₂ non-hydrogenable adsorbed species.     

Formation of nitric oxide dimers on MgO-supported gold particles

We present density functional theory (DFT) calculations on the formation of nitric oxide dimers (N₂O₂) on Au atoms, dimers and trimers adsorbed on regular O^2 ? sites and neutral oxygen vacancies (F_s sites) of the MgO(100) surface. The study of the N₂O₂ species is of great interest since it has been detected in the NO reduction reaction as an intermediate towards the formation of N₂O. We found that the coupling of a NO molecule with a previously adsorbed one on Au/MgO is energetically favorable on Au₁ and Au₃, but unfavorable on Au₂. The stability of N₂O₂ is in direct relation with the amount of charge taken from the support. Furthermore, one of the N―O bonds can be activated as a result of the attraction between the negatively charged NO dimer and the ionic oxide surface. In fact, for Au₁ anchored on the F_s site a barrierless reaction occurs between N₂O₂ and a third NO molecule, forming adsorbed N₂O and NO₂.     

Exploring the mechanism of NO–C2H4 reactions on the surface of stepped Pt(3 3 2)

Fourier transform infra red reflection–absorption spectroscopy (FTIR-RAS), thermal desorption spectroscopy (TDS), and auger electron spectroscopy (AES), were employed to explore the mechanism of NO reduction in the presence of C₂H₄ on the surface of stepped Pt(3 3 2). Both NO–Pt and C₂H₄–Pt interactions are enhanced when NO and C₂H₄ are co-adsorbed on Pt(3 3 2). As a result, C₂H₄ is dissociated at surface temperatures as low as 150 K, and the N–O stretch band is weakened. The presence of post-exposed C₂H₄ leads NO desorption from steps to decrease significantly, but the same effect on NO desorption from terraces becomes appreciable only at higher post-exposures of C₂H₄, e.g., 0.6 L and 1.2 L, and proceeds to a much slighter extent. Auger spectra indicate that as a result of the reaction with O from NO dissociation, the amount of surface C species is greatly reduced when NO is post-exposed to a C₂H₄ adlayer. It is concluded that reduction of NO in the presence of C₂H₄ proceeds very effectively on the surface of the Pt(3 3 2), through a mechanism of NO dissociation and subsequent O removal. Following this mechanism, the significant dissociation of adsorbed NO molecules on steps at surface temperatures below 400 K, and subsequent rapid reaction between the resultant O and C-related species, accounts for the considerable amount of N₂ desorption at temperatures below 400 K.     

Promotion of surface SOx on the selective catalytic reduction of NO by hydrocarbons over Ag/Al2O3

The promotion of sulfur oxides on the selective catalytic reduction (SCR) of NO by hydrocarbons in the presence of a low concentration of sulfur oxides over Ag/Al₂O₃ has been investigated by a flow reaction test and in situ infrared spectroscopy. When the C₃H₆ (or C₁₀H₂₂) + NO + O₂ feed-flow reaction was tested, maximum NO reduction was below 30% over fresh Ag/Al₂O₃. After the addition of SO₂ to the feed flow, conversion increased slightly. Conversion increased further after SO₂ was cut-off from the feed flow. This demonstrated that the increase in NO reduction activity of the catalyst was related to SO_x adsorbed on the catalyst. SO_x adsorbed on the catalytic surface (1375 cm⁻¹) was detected by IR spectroscopy and was stable within the temperature range. NCO species, as an intermediate in NO reduction, on SO_x-adsorbed Ag/Al₂O₃ in a C₃H₆ + NO + O₂ feed flow was observed in in situ IR spectra during the elevation of the reaction temperature from 473 to 673 K, while it was only observed at 673 K on fresh Ag/Al₂O₃ under the same experimental conditions. We suggest that SO_x in low concentrations depressed the combustion of reductants by contaminating hydrocarbon combustion active sites on the catalyst, resulting in an increase in NO reduction efficiency of the reductants.     

The role of adsorbed sulfur in the H2D2 equilibration reaction on Pt single crystals

The H₂D₂ equilibration on Pt single crystals was investigated under intermediate pressure (100–400 Torr) and temperature (50–250°C), as a function of sulfur coverage. On Pt(110) and Pt(111), adsorbed sulfur modifies the kinetic parameters, activation energy and pre-exponential factor; the latter depends on the temperature on Pt(110) only. The clean Pt(110) face was found to be 5 times more active than the clean Pt(111). On both faces, adsorption of sulfur induces electronic effects on the neighbouring reactional sites. The difference in the behaviour of the two faces and a clear influence of the arrangement of the adsorbed sulfur atoms, deduced from LEED diagrams, tend to prove the structure dependency of the H₂D₂ reaction. A consistent reaction mechanism could be proposed, involving the dissociative adsorption and surface recombination of hydrogen and deuterium, and the reaction between adsorbed molecules for high sulfur coverages. The value of the sulfur coverage which makes the platinum inactive towards H₂D₂ is lower for the (111) than for the (110) orientation; this is in correlation with the roughness of the surface; the denser at atomic scale a surface is, the further is the extent of the lateral interactions due to adsorbed sulfur.     

Interactions of hydrogen and nitric oxide in outwardly propagating spherical flame: Insight into non-hydrocarbon NOX reduction mechanism

Co-firing ammonia (NH₃) and hydrogen (H₂) or H₂-rich fuel and partially cracking NH₃ are promising non-carbon combustion techniques for gas turbines and marine engines, raising a growing need to understand the interactions of H₂ and nitric oxide (NO) as well as the non-hydrocarbon nitrogen oxides (NO_X) reduction mechanism under flame conditions. In this work, the outwardly propagating spherical flame method was used to investigate the laminar flame propagation of H₂/NO and H₂/NO/nitrogen (N₂) mixtures at initial pressure (P_u) of 2 atm, initial temperature (T_u) of 298 K and equivalence ratios of 0.2-1.4. The laminar burning velocities (LBVs) of H₂/NO mixtures are generally 5-10 times lower than those of H₂/air mixtures, while the dilution of N₂ can dramatically inhibit the laminar flame propagation. A kinetic model of H₂/NO combustion was constructed and validated against the new data in this work and other types of experimental data in literature. The modeling analyses reveal that NO+H=N+OH becomes the most important chain-branching reaction in H₂/NO reaction system, while the LBV data of H₂/NO mixtures in this work can provide highly sensitive validation targets for the kinetics in H₂ and NO interactions. Furthermore, the NO reduction to N₂ mainly proceeds through NO+N=N₂+O under various H₂/NO ratios, and NO+O=N+O₂ is found to have a significant contribution to NO reduction under NO-rich conditions.     

A thermal desorption study of thiophene adsorbed on the clean and sulfided Mo(100) crystal face

The adsorption of thiophene (C₄H₄S) on the clean and sulfided Mo(100) crystal surface has been studied. A fraction of the adsorbed thiophene desorbs molecularly while the remainder decomposes upon heating, evolving H₂ and leaving carbon and sulfur deposits on the surface. The reversibly adsorbed thiophene exhibits three distinct desorption peaks at 360, 230–290 and 163–174 K, corresponding to binding energies of 22, 13–16 and 7–9 kcal/mol respectively. Sulfur on the Mo(100) surface preferentially blocks the highest energy binding state and causes an increase in the amount of thiophene bound in the low binding energy, multilayer state. The thiophene decomposition reactions yield H₂ desorption peaks in the temperature range 300–700 K. We estimate that 50–66% of the thiophene adsorbed to the clean Mo(100) decomposes. The decomposition reaction is blocked by the presence of c(2 × 2) islands of sulfur and is blocked completely at θ_s = 0.5, at which point thiophene adsorption is entirely reversible.     

Adsorption behavior and reaction properties of NO and CO on Rh(1 1 1)

Reactions between NO and CO on Rh(1 1 1) surfaces were investigated using infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption. NO adsorbed on the fcc, atop, and hcp sites in that order, whereas CO adsorbed initially on the atop sites and then on the hollow (fcc + hcp) sites. The results of experiments with NO exposure on CO-preadsorbed Rh(1 1 1) surfaces indicated that the adsorption of NO on the hcp sites was inhibited by preadsorption of CO on the atop sites, and NO adsorption on the atop and fcc sites was inhibited by CO preadsorbed on each type of site, which indicates that NO and CO competitively adsorbed on Rh(1 1 1). From a Rh(1 1 1) surface with coadsorbed NO and CO, N₂ was produced from the dissociation of fcc-NO, and CO₂ was formed by the reaction of adsorbed CO with atomic oxygen from dissociated fcc-NO. The CO₂ production increased remarkably in the presence of hollow-CO. Coverage of fcc-NO and hollow-CO on Rh(1 1 1) depended on the composition ratio of the NO/CO gas mixture, and a gas mixture with NO/CO ? 1/2 was required for the co-existence of fcc-NO and hollow-CO at 273 K.     

Mechanism of catalytic reduction of NO by H2 or CO on a Pd foil; Role of chemisorbed nitrogen on Pd

The adsorption of NO and its reaction with H₂ over polycrystalline Pd were investigated using flash desorption technique and ultraviolet photoelectron spectroscopy under 10^?5 Pa pressure range of reactants and surface temperatures between 300 and 900 K. NO was adsorbed dissociatively onto the Pd surface above 500 K, and the heat of dissociative adsorption was ca. 126 kJ/mol. Atomic nitrogen was observed to accumulate on the Pd surface during the NO-H₂ reaction, whose desorption rate exhibited second order kinetics and is expressed as follows: V_d = 10^{?9.8 ± 0.3}exp(?67(kJ/mol)/RT) (cm²/atom·s). Hydrogenation of the adsorbed nitrogen proceeded rapidly at 485 K. It was confirmed from these results that formation of N₂ and NH₃ in the NO-H₂ reaction proceeds through this atomically adsorbed nitrogen. Pd-N bond energy and enthalpies of some intermediate states of the NO-H₂ reaction were estimated.     

The catalytic reduction of NO by H2 on Ru(0 0 0 1): Observation of NHads species

Adsorption of NO and the reaction between NO and H₂ were investigated on the Ru(0 0 0 1) surface by X-ray photoelectron spectroscopy (XPS). Surface composition was measured after NO adsorption and after the selective catalytic reduction of nitric oxide with hydrogen in steady-state conditions at 320 K and 390 K in a 30:1 mixture of H₂ and NO (total pressure = 10⁻⁴ mbar). After steady-state NO reduction, molecularly adsorbed NO in both the linear on-top and threefold coordinations, NH_ads and N_ads species were identified by XPS. The coverage of the NH_ads and N_ads species was higher after the reaction at 390 K than the corresponding values at 320 K. Strong destabilisation of N_ads by O_ads was detected. A possible reaction mechanism is discussed.     

Adsorption of NO and CO on a Ru(1010) surface

A combination of modern surface measurement techniques such as LEED, AES and Thermal Desorption Spectroscopy were used to study the chemisorptive behavior of NO and CO on a $\text{(10}\text{1}\text{0)Ru}$ surface. The experimental evidence strongly favors a model in which NO adsorbs and rapidly dissociates into separate nitrogen and oxygen adsorbed phases, each exhibiting ordered structures: the C(2 × 4) and (2 × 1) structures at one-half and full saturation coveilage, respectively. At temperatures as low as 200°C, the nitrogen phase begins to desorb, and continuous exposure to NO in this temperature range results in an increasing oxygen coverage until the surface is saturated with oxygen and no further NO dissociation can take place. The nitrogen desorption spectrum depends strongly on coverage and exhibits several peaks which are related to structure of the adsorbed phase. There is evidence that once the surface is saturated with the dissociated NO phase further NO adsorption occurs in a molecular state. Carbon monoxide adsorbs in a molecular state and does not exhibit an ordered structure. The implications of the results with respect to the catalytic reduction of NO by H₂ and CO and the N₂ selectivity of Ru catalysts are discussed.     

Surface enhanced raman scattering (SERS) from silver,copper, and gold films in UHV: excitation spectra.

SER excitation spectra of different vibrational lines of various molecules (C₅H₅N, C₂H₄, O₂, CO) adsorbed to Ag-, Cu-, and Au-films in UHV have been studied. The experimental results are explained by assuming an electromagnetic origin of the observed excitation profile resonances. Within this frame we estimate a size of ～ 1 – 2 nm of the ‘SERS relevant’ surface roughness (bumps) and consequently a short range ‘classical’ enhancement restricted to mainly the first layer of adsorbed molecules.     

Surface reaction mechanism of atomic layer deposition of HfO2 on Ge(1 0 0)-2 × 1: A density functional theory study

Density functional theory is employed to investigate atomic layer deposition mechanism of HfO₂ on Ge(1 0 0)-2 × 1 surface. Both the HfCl₄ and H₂O half-reactions proceed through an analogous trapping-mediated mechanism. The neighboring hydroxyl in the reaction of HfCl₄ with two Ge-OH* sites has a major effect on the formation of HfCl₄ adsorbed complex. In addition, both the Ge and Si reaction pathways are qualitatively similar, however, adsorption of HfCl₄ is favorable on Ge than on Si surface hydroxyl sites. By comparison of the reactions of H₂O on the different surfaces, the differences in energy are negligible to alter the reaction mechanism.     

The temperature dependence of the interaction of NO + CO on Pt{1 0 0}

Infrared reflection absorption spectroscopy together with mass spectrometry has been used to investigate the interaction of NO and CO on Pt{1 0 0}, initially prepared in the reconstructed ‘hex’ phase, under ambient pressures of these gases, in the temperature range 300-500 K. The results allow the local and total coverages of adsorbed CO and NO to be related to the rate of reaction to produce gas phase CO₂, and provide insight into the species present on the surface during the so-called low temperature oscillatory reaction regime of this process. At temperatures below that at which NO dissociation occurs (approximately 390-400 K) adsorption is controlled by the non-reactive displacement of NO by CO and results in a CO-poisoned surface. Above 400 K when significant CO₂ production occurs, the NO coverage increases to produce a surface with NO and CO fully intermixed; the increase in NO coverage is attributed to the higher rate of NO arrival from the gas phase (with a partial pressure ratio of P_NO:P_CO>1) at free surface sites created by NO dissociation and subsequent reaction with CO. The competition between these two processes of non-reactive NO displacement by CO and reactive displacement of CO by NO is proposed to determine the parameter space of the low temperature oscillatory regime. Rapid equilibration between bridged and atop CO species leads to them appearing to exhibit identical reaction behaviour. Particularly at the lowest reaction temperatures (around 400 K), islands of pure CO may coexist on the surface but not participate in the reaction. Under conditions corresponding to the high temperature oscillatory regime, small quantities of absorbed CO, but no NO, are seen on the surface.     

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.     

Microscopic insight into catalytic HCN removal over the CuO surface in chemical looping combustion

Hydrogen cyanide (HCN) is well-accepted as a main nitrogen-containing precursor from fuel nitrogen to nitrogen oxides. When using coal as fuel with a CuO-based oxygen carrier in chemical looping combustion (CLC), complex heterogeneous reactions exist among the system of HCN, O₂, NO, H₂O, and CuO particles. This work performs density functional theory (DFT) calculations to systematically probe the microscopic HCN heterogeneous reactions over the CuO particle surface. The results indicate that HCN is chemisorbed on the CuO surface, and the third dissociation step within the consecutive three-step HCN dissociations (HCN*→CN*→NCO*→N*) is the rate-determining step. Namely, the CN*/NCO* radicals can be deemed as an indicator of the performance of HCN removal due to their quite higher dissociation energies. With the existence of O₂, H₂O, and NO, the reaction mechanism of HCN conversion becomes extremely complex. Both DFT calculations and kinetic analyses determine that O₂, NO, and H₂O all significantly accelerate the consumption of CN*/NCO* radicals to produce various N-containing species (NOx or NH₃) to different extents. Finally, a skeletal reaction network in a system of O₂/NO/H₂O/HCN is concluded, which clearly elucidates that CuO exhibits excellent catalytic activity toward HCN removal.