Langmuir-Hinshelwood Reaction Research Articles

Development and validation of a finite-rate model for carbon oxidation by atomic oxygen

Molecular beam experiments of hyperthermal oxygen atoms scattering from a vitreous carbon surface were simulated and analyzed using direct simulation Monte Carlo (DSMC). The purpose of this study was to construct and validate a surface chemistry model in DSMC by matching observed features in experimental time-of-flight (TOF) and angular distribution data. A variety of reaction mechanisms including adsorption, desorption, and several types of Langmuir-Hinshelwood (LH) reactions are used to characterize the surface processes in the DSMC surface chemistry framework. This framework is used to numerically simulate the beam-surface scattering experiments of oxygen on vitreous carbon with surface temperatures ranging from 600 to 2000 K. Detailed analysis of the experimental data including TOF and angular distributions, and product fluxes derived from these data, is used to propose modifications to the carbon surface oxidation model developed by Poovathingal et al. This revised model successfully captures the important features in the experimental TOF distributions, including: (i) the impulsively scattered and thermally desorbed components in the oxygen (O) TOF distributions, and (ii) the relatively slow long-tail components in the carbon monoxide (CO) TOF distributions. Comparisons between the simulated (DSMC) and experimental TOF and angular distributions, as well as product fluxes, show excellent agreement.

Kinetic investigation for the catalytic reduction of nitrophenol using ionic liquid stabilized gold nanoparticles.

We demonstrate the synthesis of gold nanoparticles (AuNP) stabilized by 1-butyl-3-hexadecyl imidazolium bromide (Au@[C4C16Im]Br) and their use as a catalyst for the reduction of nitrophenol. The AuNPs show excellent stability in presence of [C4C16Im]Br ionic liquids for the reduction of 4-nitrophenol and 2-nitrophenol using NaBH4 as a reducing agent. The detailed kinetics for the reduction of 4-nitrophenol and 2-nitrophenol were investigated and the catalytic activity of Au@[C4C16Im]Br was evaluated. The pseudo first-order rate constant (kapp) values for 4-nitrophenol was observed to be greater than that of 2-nitrophenol and explained on the basis of hydrogen bonding present in 2-nitrophenol. Au@[C4C16Im]Br showed good separability and reusability and hence, it can be used for the complete reduction of nitrophenols in multiple cycles. The Langmuir–Hinshelwood reaction mechanism is elucidated for reduction of 4-nitrophenol by Au@[C4C16Im]Br nanocatalyst on the basis of the kapp values. The thermodynamic activation parameters such as activation energy, enthalpy of activation and entropy of activation were determined and explained using the temperature dependent kinetics for the reduction of nitrophenol using Au@[C4C16Im]Br. The above results reveal that the Au@[C4C16Im]Br nanocatalyst demonstrates excellent catalytic performance for the reduction of nitrophenol by NaBH4 at room temperature.

A kinetic study on methylation of naphthalene over Fe/ZSM-5 zeolite catalysts

2,6-Dimethylnaphthalene is an important dimethylnaphthalene isomer which can be used in the production of polyethylene naphthalate. The novelty of this study is to reveal Langmuir–Hinshelwood and Eley–Rideal reaction rate equations for the methylation of naphthalene over Fe/ZSM-5 zeolite catalysts besides the proposed reaction rate equation. To investigate the kinetics and mechanisms of naphthalene methylation, the methylation experiments were carried out in a gas–solid catalytic fixed-bed reactor in the presence of Fe/ZSM-5 zeolite catalysts at two different temperatures (450 and 500 °C) and five different weight hourly space velocities (0.5, 1.0, 1.5, 2.0, 2.5, 3.0 h−1). A naphthalene:methanol:1,2,4-trimethyl benzene mixture having a 1:3:10 molar ratio was used as a feed stream. The methylation products were identified by using GC–MS. For the methylation kinetics of naphthalene, the reaction rates depending on the naphthalene and methanol concentration were determined. Furthermore, the effects of temperature and weight hourly space velocity on the conversion of naphthalene, the selectivity of 2,6-dimethylnaphthalene, and the ratio of 2-methylnaphthalene/1-methylnaphthalene were determined. The results of this study demonstrate that the Langmuir–Hinshelwood reaction mechanism for naphthalene methylation is more compatible at 450 °C and the Eley–Rideal reaction mechanism at 500 °C. Moreover, in addition to 2,6-dimethylnaphthalene, other dimethylnaphthalene and tri-methylnaphthalene isomers were formed in the methylation of naphthalene. The conversion of naphthalene reached approximately 70%. Moreover, the highest selectivity of 2,6-dimethylnaphthalene was almost 40%. The ratios of 2-methylnaphthalene/1-methylnaphalene demonstrate that the methylation of naphthalene to 2-methynaphthalene is much higher than to 1-methynaphthalene.

Probing the functionality of nanostructured MnCeOx catalysts in the carbon monoxide oxidation: Part II. Reaction mechanism and kinetic modelling

The kinetics of the CO oxidation on a typical nanocomposite MnCeOx catalyst (M5C1) were probed by temperature programmed catalytic reaction (TPCR) tests in the range of 293–533K, varying reagent partial pressure (i.e., pCO and pO2) between 0.00625 and 0.025atm (P, 1atm) (Arena et al., 2017). Experimental data indicate kinetic orders of 0.6 and 0.4 on pCO and pO2 respectively, with apparent activation energy of 40±3kJ/mol (Arena et al., 2017). A systematic study of the interaction pattern of catalyst with reagent and product molecules shows easy reactivity of surface oxygen to CO, low mobility of lattice oxygen and weak surface affinity to CO2. Systematic evidences on reaction mechanism and surface intermediates signal an extrafacial redox path, triggered by abstraction of oxygen atoms in the neighbouring of active MnIV sites (Arena et al., 2017), and sustained by O2 species adsorbed on those surface oxygen vacancies. A Langmuir-Hinshelwood (L-H) reaction mechanism leads to a formal kinetic model explaining the CO oxidation functionality of bare and promoted MnOx catalysts.

Reactions of 2-octanol on Amberlyst® 15: concurrent Eley–Rideal and Langmuir–Hinshelwood reactions on two different types of active sites

On Amberlyst® 15 (dry) at 75 °C, 2-octanol undergoes concurrent esterification and dehydration in the presence of heptanoic acid in the liquid phase. Kinetic data and 18O labelling studies show that the esterification reaction most likely occurs by an Eley–Rideal (ER) mechanism between adsorbed carboxylic acid and bulk phase alcohol, while the dehydration reaction occurs by a Langmuir–Hinshelwood (LH) mechanism requiring interaction of adsorbed alcohol with a vacant site. The two reactions occur on two different types of catalytic sites: while all the sites are sulfonic acid groups, kinetic data suggest that not all of the acidic sites are positioned appropriately to a neighboring site to assist with dehydration. A kinetic model fits the observed data for the simultaneous ER and LH reactions over at least 23 h of observation.

Azo dyes degradation using TiO2-Pt/graphene oxide and TiO2-Pt/reduced graphene oxide photocatalysts under UV and natural sunlight irradiation

The photocatalytic degradation of azo dyes with different structures (amaranth, sunset yellow and tartrazine) using TiO2-Pt nanoparticles (TPt), TiO2-Pt/graphene oxide (TPt-GO) and TiO2-Pt/reduced graphene oxide (TPt-rGO) composites were investigated in the presence of UV and natural sunlight irradiation. The composites were prepared by a combined chemical-thermal method and characterized by Transmission Electron Microscopy (TEM), X-ray powder diffraction (XRD), Infrared (FTIR) and UV–Vis spectroscopy. The modification of TiO2-Pt with graphene oxide shifted its optical absorption edge towards the visible region and increased its photocatalytic activity under UV and natural sunlight irradiation. The efficiency of catalysts on azo dyes degradation (in similar conditions) reached high values (above 99%) under sunlight conditions, proving the remarkable photocatalytic activities of obtained composites. TPt-GO nanocomposite exhibited higher photoactivity than TPt or TPt-rGO, demonstrating degradation efficiencies of 99.56% for amaranth, 99.15% for sunset yellow and 96.23% for tartrazine. The dye photodegradation process follows a pseudo-first-order kinetic with respect to the Langmuir-Hinshelwood reaction mechanism. A direct dependence between azo dyes degradation rate and chemical structure of dyes has been observed.

Time resolved study of hydroxyl radical oxidation of oleic acid at the air-water interface

The ubiquity of oleic acid (OA) renders it a poster child for laboratory investigations of environmental oxidation chemistry. In the current study, mechanistic details of the oxidation of OA by hydroxyl radicals at the air-water interface are investigated using field-induced droplet ionization mass spectrometry (FIDI-MS). Products from OH oxidation of both unsaturated and saturated carbon atoms are identified, and mechanisms for both types of oxidation processes are proposed. Uptake of oxygen in the interfacial layer increases linearly with time, consistent with Langmuir-Hinshelwood reaction kinetics. These results provide fundamental knowledge relating to OH initiated degradation of fatty acids in atmospheric aerosols.

Improvement of Nb Doping on SO2 Resistance of VOx/CeO2 Catalyst for the Selective Catalytic Reduction of NOx with NH3

The influence of sulfation treatment on Nb–VOx/CeO2 and VOx/CeO2 catalysts for the selective catalytic reduction (SCR) of NOx with NH3 was fully investigated. The Nb–VOx/CeO2 catalyst showed higher catalytic activity and stronger resistance to SO2 than VOx/CeO2. The formation of sulfates, small specific surface area, and reduction in the number of active sites were all responsible for the low catalytic activity over VOx/CeO2 after sulfation under SCR conditions. On the contrary, Nb–VOx/CeO2 adsorbed much more nitrate than sulfate when sulfated under SCR conditions and showed much higher NH3-SCR activity than VOx/CeO2 after the same treatment. After sulfation by SO2 + O2 only, instead of sulfation under SCR conditions, both of the samples exhibited decreased NH3-SCR activity, mainly due to the formation of sulfates and the blockage of the Langmuir–Hinshelwood reaction pathway.

Mechanistic Investigation into the Effect of Sulfuration on the FeW Catalysts for the Selective Catalytic Reduction of NOx with NH3.

Iron tungsten (FeW) catalyst is a potential candidate for the selective catalytic reduction (SCR) of NOx with ammonia because of its excellent performance in a wide operating window. Sulfur poisoning effects in SCR catalysts have long been recognized as a challenge in development of efficient catalysts for applications. In this paper, the impact of sulfuration on catalyst structure, NH3-SCR reaction performance and mechanism was systematically investigated through spectroscopic and temperature-programmed approaches. The sulfuration inhibited the SCR activity at low temperatures (<300 °C), while no evident effect was observed at high temperatures (≥300 °C). After sulfuration for FeW oxides catalyst, the organic-like with covalent S═O bonds sulfate species were mainly formed over the FeW catalysts. Combining TPD with in situ DRIFTS results, it was found that the Lewis and the Brønsted acidity were enhanced by the interaction between metal species and sulfate species due to the strong electron withdrawing effect of the S═O double bonds. The in situ DRIFTS study showed that the formation of NO2 was hindered, leading to the "fast-SCR" pathway was partly cut off by the sulfuration process and thereby the loss of SCR activity at low temperatures. However, the Langmuir-Hinshelwood reaction pathway between adsorbed NH3/NH4+ species and nitrate species was facilitated and dominated at high temperatures, making the as-synthesized FeW catalysts resistant to SO2 poisoning.

Simultaneous Adsorption–Desorption Processes in the Conductance Transient of Anatase Titania for Sensing Ethanol: A Distinctive Feature with Kinetic Perception

A distinctive feature in the operating temperature-dependent conductance transients of titania sensor is identified during chemi-resistive type detection of ethanol vapor. Precisely, simultaneous adsorption of ethanol and desorption of the corresponding oxidized product over anatase titania sensor are reflected systematically in the response transients measured with the rise in operating temperature. An attempt is made here to understand the origin of the said feature based on the reaction sequences of ethanol over sensor surface. For a fixed ethanol concentration (200 ppm), the conductance transients obtained for response process are modeled using Langmuir–Hinshelwood reaction mechanism and the characteristic time constants are estimated for adsorption and desorption of ethanol. From the temperature variations of these characteristic time constants, the activation energies for the adsorption and desorption of ethanol over sensor surface is estimated. In addition, the general ethanol sensing characteristi...

The Reduction of Alkynes Over Pd-Based Catalyst Materials - A Pathway to Chemical Synthesis

Many reactions, including selective hydrogenation of alkynes, take place on solid surfaces. These reactions are vital in many areas of industry including the manufacture of polymers and fine chemicals such as vitamins, fragrances, and drugs. The choice of a catalyst is a trade-off between activity, selectivity and costs. Palladium-based heterogeneous catalysts are traditionally used for these processes as they provide the activation of hydrogen at room temperatures and offers reasonable selectivity, but these catalysts have a number of practical drawbacks. This review discusses recent research work in the selective hydrogenation of alkynes on palladium-based catalysts, emphasises the mechanism and catalytic materials and important applications including alkyne removal from gasphase alkene precursors for polymer synthesis and liquid phase selective hydrogenation for the synthesis of fine chemicals. Langmuir-Hinshelwood reaction kinetic models, reaction intermediates, formation of carbonaceous layer, the nature of active sites and the effects of reversible and irreversible adsorbates over Pd surface are discussed as well as the factors affecting catalyst activity and selectivity and how these can be optimised in synthetic protocols for these reactions.

Investigation of competitive COS and HCN hydrolysis reactions upon an industrial catalyst: Langmuir-Hinshelwood kinetics modeling

Distinct and simultaneous COS and HCN hydrolysis reactions over an industrial TiO2 based catalyst were extensively studied in this work in the scope of synthesis gas purification applications. 144 experiments were carried out, including 92 experiments that allowed to achieve partial conversion rates and showed reaction kinetics sensitivity to operating parameters. Significant crossed influences were evidenced between both COS and HCN hydrolysis reactions. The concomitant occurrence of both reactions showed to detrimentally affect each other upon COS and HCN conversion rates, and therefore upon kinetic rates. This was explained through a competitive adsorption of HCN and COS reactants upon catalyst surface active sites. Inhibition of catalytic activity by the presence of NH3 and H2O was also evidenced and explained through competitive adsorption phenomena. For the operating conditions ranges explored, H2S and CO2 had no sensitive impact on the kinetics of the COS and HCN hydrolysis reactions. However, the moderate impact of CO2 upon COS and HCN conversion rates might be explained by the large CO2 excess compared to COS and HCN levels. A reaction model has been fully developed considering hydrodynamic, external mass transfer and intra particle diffusion limitations, and Langmuir-Hinshelwood reaction mechanisms for both COS and HCN hydrolysis reactions. Langmuir-Hinshelwood kinetic rate laws were indeed considered to account for the detrimental effect of gaseous species upon COS and HCN conversion kinetic rates, through competitive adsorption upon catalyst active sites of COS, HCN, H2O, and NH3. Collected kinetic data as a function of reactor size, gas residence time, temperature and reactants partial pressures were used to validate and fit kinetic and adsorption constants. Very good agreement was achieved between experimental and calculated COS and HCN conversion rates from the model developed, that allowed complete validation of the Langmuir-Hinshelwood based modeling. The coupled hydrodynamic-reaction model also constitutes a complete industrial reactor model taking into account all the potential limitations, and can be used as a powerful predicting tool for industrial process design, i.e. fully usable for industrial process scale-up and optimization purposes.

A Comparison of CO Oxidation by Hydroxyl and Atomic Oxygen from Water on Low-Coordinated Au Atoms

The catalytic oxidation of CO is studied at low-coordinated Au atoms using a single-crystal approach. Electron irradiation activates an otherwise unreactive overlayer of undissociated D2O on Au(310). A low-coverage D2O/O mixture is subsequently allowed to react at surface temperatures from 105 K upward, with CO supplied from the gas phase. X-ray photoelectron spectroscopy shows the absence of Au oxides and quantifies various O-containing species during the reaction. The dependency of the reaction rate on the surface temperature yields an activation energy for the Langmuir–Hinshelwood reaction of O(ads) and CO(ads) between 26 ± 4 and 42 ± 5 kJ/mol. The presented results provide evidence that O(ads) and not OH(ads) is the active reactant on small Au nanoparticles. In addition, the observations suggest that water has a negative effect on the reactivity of O(ads).

Cover Image, Volume 116, Issue 21

International Journal of Quantum ChemistryVolume 116, Issue 21 p. i-i Cover ImageFree Access Cover Image, Volume 116, Issue 21 First published: 16 September 2016 https://doi.org/10.1002/qua.25288AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract On page 1575, Matteo Bonfanti and Rocco Martinazzo examine reactions at surfaces under a magnifying glass. Fruitful combination of theory, modelling, and simulations provides a powerful tool to understand the dynamics of atoms and molecules at the gas-solid interface. This is pictured on the cover with a simple illustration of two prototypical recombination processes, the Eley-Rideal (left) and the Langmuir-Hinshelwood (right) reactions. Image credit goes to Matteo Bonfanti. (DOI: 10.1002/qua.25192) Volume116, Issue21Special Issue: Theoretical and Computational Chemistry in ItalyNovember 5, 2016Pages i-i RelatedInformation

Photocatalytic reduction of CO 2 with H 2 O over graphene oxide-supported oxygen-rich TiO 2 hybrid photocatalyst under visible light irradiation: Process and kinetic studies

Abstract The photoreduction of carbon dioxide (CO2) into hydrocarbon fuels was studied in a homemade photocatalytic system over 5 wt.% graphene oxide-doped oxygen-rich TiO2 (5GO-OTiO2) photocatalyst. The CO2 transformation process is a sequential combination of both water oxidation and CO2 reduction. As these processes can be affected by parameters such as radiant flux intensity and the partial pressures of both CO2 and water vapour, these factors were systematically varied and studied in order to determine the most suitable process conditions for achieving high photocatalytic activity. Based on results from the CO2 photoreduction experiments, a total methane (CH4) yield of 3.450 μmol gcat−1 was successfully attained over 5GO-OTiO2 after 8 h of reaction time under visible light irradiation. The experimental data obtained was then fitted into the Langmuir-Hinshelwood surface reaction mechanism, wherein both CO2 and H2O adsorbed simultaneously on the photocatalyst surface to form the CH4 product. Regression fitting was performed to determine the kinetic parameters such as reaction rate constant and adsorption equilibrium constants. The reaction rate as well as CO2 and H2O adsorption equilibrium constants were determined to be 84.42 μmol gcat−1 h−1, 0.019 bar−1 and 8.07 bar−1, respectively. The significantly smaller CO2 adsorption equilibrium constant implied that the adsorption of CO2 was very weak while water strongly adsorbed on the photocatalyst surface.

Pd-embedded graphene: An efficient and highly active catalyst for oxidation of CO

Despite numerous efforts performed on the elimination of toxic gases from the air, the oxidation of carbon monoxide (CO) with metal-embedded nanostructures still remains a challenge. The geometry, electronic structure and catalytic properties of Pd-doped graphene (Pd-graphene) are investigated by means of density functional theory (DFT) calculations. The large atomic radius of Pd dopant in graphene can induce the local surface curvature and modulate the electronic structure of the sheet through the charge redistribution effects. Also, Pd-graphene can facilitate the O2 adsorption. Therefore, the catalytic activity of the Pd-graphene for CO oxidation reaction is enhanced. Moreover, the complete CO oxidation reactions on the Pd-graphene include a two-step process of the Langmuir-Hinshelwood (LH) reaction, in which the first step is almost barrier-less (Eact = 0.002 eV) and the second step exhibits an energy barrier of 0.2 eV. The results indicate that the surface activity of graphene-based materials can be drastically improved by introducing the Pd dopants, so Pd-graphene can be a clue for fabricating graphene-based catalysts with high activity toward the oxidation of CO molecule.

Quantitative Aspects of the Interfacial Catalytic Oxidation of Dithiothreitol by Dissolved Oxygen in the Presence of Carbon Nanoparticles.

The catalytic nature of particulate matter is often advocated to explain its ability to generate reactive oxygen species, but quantitative data are lacking. We have performed molecular characterization of three different carbonaceous nanoparticles (NP) by 1. identifying and quantifying their surface functional groups based on probe gas-particle titration; 2. studying the kinetics of dissolved oxygen consumption in the presence of suspended NP's and dithiothreitol (DTT). We show that these NP's can reversibly change their oxidation state between oxidized and reduced functional groups present on the NP surface. By comparing the amount of O2 consumed and the number of strongly reducing sites on the NP, its average turnover ranged from 35 to 600 depending on the type of NP. The observed quadratic rate law for O2 disappearance points to a Langmuir-Hinshelwood surface-based reaction mechanism possibly involving semiquinone radical. In the proposed model, the strongly reducing surface site is assumed to be a polycyclic aromatic hydroquinone whose oxidation to the corresponding conjugated quinone is rate-limiting in the catalytic chain reaction. The presence and strength of the reducing surface functional groups are important for explaining the catalytic activity of NP in the presence of oxygen and a reducing agent like DTT.

Determination of Model Kinetics for Forced Unsteady State Operation of Catalytic CH4Oxidation

The catalytic oxidation of methane for abating the emission vented from coal mine or natural gas transportation has been known as most reliable method. A reverse flow reactor operation has been widely used to oxidize this methane emission due to its capability for autothermal operation and heat production. The design of the reverse flow reactor requires a proper kinetic rate expression, which should be developed based on the operating condition. The kinetic rate obtained in the steady state condition cannot be applied for designing the reactor operated under unsteady state condition. Therefore, new approach to develop the dynamic kinetic rate expression becomes indispensable, particularly for periodic operation such as reverse flow reactor. This paper presents a novel method to develop the kinetic rate expression applied for unsteady state operation. The model reaction of the catalytic methane oxidation over Pt/-Al 2 O 3 catalyst was used with kinetic parameter determined from laboratory experiments. The reactor used was a fixed bed, once-through operation, with a composition modulation in the feed gas. The switching time was set at 3 min by varying the feed concentration, feed flow rate, and reaction temperature. The concentrations of methane in the feed and product were measured and analysed using gas chromatography. The steady state condition for obtaining the kinetic rate expression was taken as a base case and as a way to judge its appropriateness to be applied for dynamic system. A Langmuir-Hinshelwood reaction rate model was developed. The time period during one cycle was divided into some segments, depending on the ratio of CH 4 /O 2 . The experimental result shows that there were kinetic regimes occur during one cycle: kinetic regime controlled by intrinsic surface reaction and kinetic regime controlled by external diffusion. The kinetic rate obtained in the steady state operation was not appropriate when applied for unsteady state operation. On the other hand, the kinetic rate expression obtained in the unsteady state operation fitted quite well. It was proven that in one cycle period the kinetic rate would always shift according to the ratio of CH 4 /O 2 .

Pyridine hydrodenitrogenation kinetics over a sulphided NiMo/γ-Al2O3 catalyst

Gas phase pyridine hydrodenitrogenation (HDN) kinetics have been determined over a commercial, sulphided NiMo/γ-Al2O3 catalyst in a Berty type Continuous Stirred Tank Reactor (CSTR). Temperatures and total pressures varied from 573 to 633K and from 1.5 to 4.0MPa, in a space time range from 350 to 1800kgcatsmol−1P. Pyridine was first hydrogenated into piperidine. The subsequent denitrogenation went via piperidine ring opening to pentyl amine followed by the actual nitrogen removal, yielding C5 hydrocarbons and ammonia as end products. The C–N bond scission rate was enhanced by the presence of H2S, which could be rationalized based on a substitution mechanism. Langmuir Hinshelwood reaction mechanisms accounting for two types of sites, i.e., coordinatively unsaturated sites (∗) and sulphur anions (S2−), have been developed to assess the experimentally measured kinetics. These models explicitly accounted for the substitution mechanism in the C–N bond scission in terms of elementary steps. Model discrimination has allowed assessing the importance of hydrogen addition to pyridine through heterolytically or homolytically chemisorbed hydrogen and hydrogen sulphide, as well as of sulphydril or hydrogen assisted C–N bond scission in piperidine denitrogenation. Chemisorption entropies were calculated a priori such that, in addition to the kinetic parameters, only chemisorption enthalpies had to be estimated by regression. The third hydrogen addition and the naphthenic C–N bond breaking were found to be rate determining in pyridine hydrogenation and piperidine denitrogenation, with activation energies amounting to 41kJmol−1 and 185kJmol−1. A distinct linear relationship was established between the proton affinity of the various nitrogen components and their free energy of chemisorption.

Oxidation of Nitric Oxide on Gas-Phase Cerium Oxide Clusters via Reactant Adsorption and Product Desorption Processes.

The reactivity of cerium oxide cluster cations, CenO2n+x(+) (n = 2-9, x = -1 to +2), with NO was investigated using gas-phase temperature-programmed desorption (TPD) combined with mass spectrometry. Target clusters were prepared in the gas phase via the laser ablation of a cerium oxide rod in the presence of oxygen, which was diluted using helium as a carrier gas. NO adsorbed onto stoichiometric and oxygen-rich clusters of CenO2n+x(+) (x = 0-2), forming CenO2n+x(NO)(+) (x = 0-2) species. Gas-phase TPD was measured for the NO-adsorbed clusters, revealing that CenO2n(NO)(+) released NO2 at 600-900 K, forming CenO2n-1(+). Therefore, the overall reaction was the oxidation of NO by the CenO2n(+) clusters, which was explained in terms of a Langmuir-Hinshelwood type reaction. An activation barrier existed between the initial complex (CenO2n(NO)(+)) and the final oxidation products (CenO2n-1(+) + NO2). To determine the nature of the intermediates and the activation barrier, TPD was also performed on CenO2n-1(NO2)(+), which had been prepared through the adsorption of NO2 on CenO2n-1(+) for comparison. The activation barrier was associated with the release of NO2 from the intermediate complex (CenO2n-1(+)-NO2 → CenO2n-1(+) + NO2) rather than the structural rearrangement that formed NO2 in the other intermediate complex (CenO2n(+)-NO → CenO2n-1(+)-NO2).