Influence of monolayer amounts of HNO3 on the evaporation rate of H2O over ice in the range 179 to 208 K: a quartz crystal microbalance study

The evaporation flux J(ev) of H2O from thin H2O ice films containing between 0.5 and 7 monolayers of HNO3 has been measured in the range 179 to 208 K under both molecular and stirred flow conditions in isothermal experiments. FTIR absorption of the HNO3/H2O condensate revealed the formation of metastable alpha-NAT (HNO(3).3H2O) converting to stable beta-NAT at 205 K. After deposition of HNO3 for 16-80 s on a 1 mum thick pure ice film at a deposition rate in the range (6-60) x 10(12) molecules s(-1) the initial evaporative flux J(ev)(H2O) was always that of pure ice. J(ev)(H2O) gradually decreased with the evaporation of H2O and the concomitant increase of the average mole fraction of HNO3, chi(HNO3), indicating the presence of an amorphous mixture of H2O/HNO3 that is called complexed or (c)-ice whose vapor pressure is that of pure ice. The final value of J(ev) was smaller by factors varying from 2.7 to 65 relative to pure ice. Depending on the doping conditions and temperature of the ice film the pure ice thickness d(D) of the ice film for which J(ev) < 0.85J(ev)(pure ice) varied between 130 and 700 nm compared to the 1000 nm thick original ice film at 208 and 191 K, respectively, in what seems to be an inverse temperature dependence. There exist three different types of H2O molecules under the present experimental conditions, namely (a) free H2O corresponding to pure ice, (b) complexed H2O or c-ice, and (c) H2O molecules originating from the breakup of NAT or amorphous H2O/HNO3 mixtures. The significant decrease of J(ev)(H2O) with increasing chi(HNO3) leads to an increase of the evaporative lifetime of atmospheric ice particles in the presence of HNO3 and may help explain the occurrence of persistent and/or large contaminated ice particles at certain atmospheric conditions.     

Surface abundance change in vacuum ultraviolet photodissociation of CO2 and H2O mixture ices

Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O((3)P(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO(2). Two different adsorption states of CO(2), i.e., physisorbed CO(2) on the surface of amorphous solid water and trapped CO(2) in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H(2)O. Since the absorption cross section of CO(2) is smaller than that of H(2)O at 157 nm, the CO(2) surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.     

Vacuum ultraviolet photodissociation and surface morphology change of water ice films dosed with hydrogen chloride

Time-of-flight (TOF) spectra of photofragment H atoms from the photodissociation of water ice films at 193 nm were measured for amorphous and polycrystalline water ice films with and without dosing of hydrogen chloride at 100-145 K. The TOF spectrum is sensitive to the surface morphology of the water ice film because the origin of the H atom is the photodissociation of dimerlike water molecules attached to the ice film surfaces. Adsorption of HCl on a polycrystalline ice film was found to induce formation of disorder regions on the ice film surface at 100-140 K, while the microstructure of the ice surface stayed of polycrystalline at 145 K with adsorption of HCl. The TOF spectra of photofragment Cl atoms from the 157 nm photodissociation of neutral HCl adsorbed on water ice films at 100-140 K were measured. These results suggest partial dissolution of HCl on the ice film surface at 100-140 K.     

The photodissociation dynamics of NO2 at 308 nm and of NO2 and N2O4 at 226 nm

Velocity-map ion imaging has been applied to the photodissociation of NO(2) via the first absorption band at 308 nm using (2 + 1) resonantly enhanced multiphoton ionization detection of the atomic O((3)P(J)) products. The resulting ion images have been analyzed to provide information about the speed distribution of the O((3)P(J)) products, the translational anisotropy, and the electronic angular momentum alignment. The atomic speed distributions were used to provide information about the internal quantum-state distribution in the NO coproducts. The data were found to be consistent with an inverted NO vibrational quantum-state distribution, and thereby point to a dynamical, as opposed to a statistical dissociation mechanism subsequent to photodissociation at 308 nm. Surprisingly, at this wavelength the O-atom electronic angular momentum alignment was found to be small. Probe-only ion images obtained under a variety of molecular-beam backing-pressure conditions, and corresponding to O atoms generated in the photodissociation of either the monomer, NO(2), or the dimer, N(2)O(4), at 226 nm, are also reported. For the monomer, where 226 nm corresponds to excitation into the second absorption band, the kinetic-energy release distributions are also found to indicate a strong population inversion in the NO cofragment, and are shown to be remarkably similar to those previously observed in the wavelength range of 193-248 nm. Mechanistic implications of this result are discussed. At 226 nm it has also been possible to observe directly O atoms from the photodissociation of the dimer. The O-atom velocity distribution has been analyzed to provide information about its production mechanism.     

Release of hydrogen molecules from the photodissociation of amorphous solid water and polycrystalline ice at 157 and 193 nm

The production of H(2) in highly excited vibrational and rotational states (v=0-5, J=0-17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H(2)(v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H(2)(v=0 and 1) populations in this case were below the detection limit. The H(2) products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H(2)O by a photolytically produced H atom yields vibrationally cold H(2) products, whereas exothermic recombination of two H-atom photoproducts yields H(2) molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations.     

The heterogeneous chemical kinetics of NO3 on atmospheric mineral dust surrogates

Uptake experiments of NO3 on mineral dust powder were carried out under continuous molecular flow conditions at 298 +/- 2 K using the thermal decomposition of N2O5 as NO3 source. In situ laser detection using resonance enhanced multiphoton ionization (REMPI) to specifically detect NO2 and NO in the presence of N2O5, NO3 and HNO3 was employed in addition to beam-sampling mass spectrometry. At [NO3] = (7.0 +/- 1.0) x 10(11) cm(-3) we found a steady state uptake coefficient gamma(ss) ranging from (3.4 +/- 1.6) x 10(-2) for natural limestone to (0.12 +/- 0.08) for Saharan Dust with gamma(ss) decreasing as [NO3] increased. NO3 adsorbed on mineral dust leads to uptake of NO2 in an Eley-Rideal mechanism that usually is not taken up in the absence of NO3. The disappearance of NO3 was in part accompanied by the formation of N2O5 and HNO3 in the presence of NO2. NO3 uptake performed on small amounts of Kaolinite and CaCO3 leads to formation of some N2O5 according to NO((3ads)) + NO(2(g)) --> N2O(5(ads)) --> N2O(5(g)). Slow formation of gas phase HNO3 on Kaolinite, CaCO3, Arizona Test Dust and natural limestone has also been observed and is clearly related to the presence of adsorbed water involved in the heterogeneous hydrolysis of N2O(5(ads)).     

Hydrogen peroxide formation following the vacuum ultraviolet photodissociation of water ice films at 90 K

The production of gaseous OH radicals from the 300-350 nm photodissociation of H(2)O(2) that was photolytically produced on a water ice surface following the 157 nm photolysis of water ice at 90 K was directly monitored using resonance-enhanced multiphoton ionization. The translational energy distribution estimated by the time-of-flight spectrum of the OH products is represented by a Maxwell-Boltzmann energy distribution with a translational temperature of 3750+/-250 K. The rotational temperature was estimated by a spectral simulation to be 225+/-25 K. Surface defects produced by HCl deposition on the water ice contributed to the higher production rate of H(2)O(2) in the 157 nm photoirradiation of water ice while surface coverage caused by CD(3)OH deposition decreased the H(2)O(2) production rate.     

Reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of polycyclic aromatic hydrocarbon surfaces

We investigated the reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of solid polycyclic aromatic hydrocarbons (PAHs) using a coated wall flow tube reactor coupled to a chemical ionization mass spectrometer. The PAH surfaces studied were the 4-ring systems pyrene, benz[a]anthracene, and fluoranthene. Reaction of NO3 radicals with all three PAHs was observed to be very fast with the reactive uptake coefficient, gamma, ranging from 0.059 (+0.11/-0.049) for benz[a]anthracene at 273 K to 0.79 (+0.21/-0.67) for pyrene at room temperature. In contrast to the NO3 reactions, reactions of the different PAHs with the other gas-phase species (N2O5, NO2, HNO3, and O3) were at or below the detection limit (gamma 相似文献   

Kinetic study of the daytime atmospheric fate of (z)-3-hexenal

Rate coefficients for three daytime atmospheric reactions of (Z)-3-hexenal (3HA)-photolysis (J(1)), reaction with OH radicals (k(2)), and reaction with ozone (k(3))-were measured at 760 Torr and 298 K using a 6 m(3) photochemical reaction chamber. The UV absorption cross sections (σ(3HA)(λ)) were obtained in the wavelength range 240-350 nm. The photodissociation rate of 3HA relative to that of NO(2) was measured by a solar simulator at 760 Torr and was determined to be J(1)/J(NO2) = (4.7 ± 0.4) × 10(-3). Using the obtained σ(3HA)(λ) and J(1)/J(NO2), the effective photodissociation quantum yield was calculated to be Φ(3HA) = 0.25 ± 0.06. The rate coefficient for the reaction with OH radicals was measured by the relative rate method with three reference compounds and was determined to be k(2) = (6.9 ± 0.9) × 10(-11) cm(3) molecule(-1) s(-1). The rate coefficient for the reaction with ozone was measured by an absolute method and was determined to be k(3) = (3.5 ± 0.2) × 10(-17) cm(3) molecule(-1) s(-1). Using the obtained rate coefficients, the daytime atmospheric lifetime of 3HA was estimated.     

Direct observation of OH radicals ejected from water ice surface in the photoirradiation of nitrate adsorbed on ice at 100 K

Production of gaseous OH radicals in the 248-350 nm photoirradiation of NO3(-) doped on amorphous ice at 100 K was monitored directly by using resonance-enhanced multiphoton ionization. The translational energy distribution of the OH product was represented by a Maxwell-Boltzmann energy distribution with the translational temperature of 3250 +/- 250 K. The rotational temperature was estimated to be 175 +/- 25 K. We have confirmed that the OH production should be attributed to the secondary photolysis of H2O2 produced on ice surface on the basis of the results of controlled photolysis experiments for H2O2 doped on ice surface.     

Formation of nitric acid in the gas-phase HO2 + NO reaction: effects of temperature and water vapor

A high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer was used to investigate the minor channel (1b) producing nitric acid, HNO3, in the HO2 + NO reaction for which only one channel (1a) is known so far: HO2 + NO --> OH + NO2 (1a), HO2 + NO --> HNO3 (1b). The reaction has been investigated in the temperature range 223-298 K at a pressure of 200 Torr of N2 carrier gas. The influence of water vapor has been studied at 298 K. The branching ratio, k1b/k1a, was found to increase from (0.18(+0.04/-0.06))% at 298 K to (0.87(+0.05/-0.08))% at 223 K, corresponding to k1b = (1.6 +/- 0.5) x 10(-14) and (10.4 +/- 1.7) x 10(-14) cm3 molecule(-1) s(-1), respectively at 298 and 223 K. The data could be fitted by the Arrhenius expression k1b = 6.4 x 10(-17) exp((1644 +/- 76)/T) cm3 molecule(-1) s(-1) at T = 223-298 K. The yield of HNO3 was found to increase in the presence of water vapor (by 90% at about 3 Torr of H2O). Implications of the obtained results for atmospheric radicals chemistry and chemical amplifiers used to measure peroxy radicals are discussed. The results show in particular that reaction 1b can be a significant loss process for the HO(x) (OH, HO2) radicals in the upper troposphere.     

n(H_2O)(n=1,2)在HO_2+NO→HNO_3反应中的催化机制研究

采用CCSD(T)/aug-cc-p VTZ//B3LYP/6-311+G(2df,2p)方法对n(H_2O)(n=0,1,2)参与HO_2+NO→HNO_3反应的微观机理和速率常数进行了研究.结果表明,由于水分子与HO_2形成的复合物(H_2O…HO_2,HO_2…H_2O)结合NO与水分子形成的复合物(NO…H_2O,ON…H_2O)的反应方式具有较高能垒和较低有效速率,其对HO_2+NO→HNO_3反应的影响远小于双体水(H_2O)2与HO_2(或NO)形成复合物然后再与另一分子反应物NO(或HO_2)的反应方式,因此n(H_2O)(n=1,2)催化HO_2+NO→HNO_3反应主要经历了HO_2…(H_2O)_n(n=1,2)+NO和NO…(H_2O)_n(n=1,2)+HO_22种反应类型.由于HO_2…(H_2O)_n(n=1,2)+NO反应的低能垒和高速率,HO_2…(H_2O)_n(n=1,2)+NO反应优于NO…(H_2O)_n(n=1,2)+HO_2反应.与此同时,由于计算温度范围内HO_2…H_2O+NO反应的有效速率常数比HO_2…(H_2O)2+NO反应对应的有效速率常数大了10~12数量级,可推测(H_2O)_n(n=1,2)催化HO_2+NO→HNO_3反应主要来自于单个水分子.此外,在216.7~298.6 K范围内水分子对HO_2+NO→HNO_3反应起显著的正催化作用,且随温度的升高有明显增大的趋势,在298.2 K时增强因子k'RW1/ktotal达到67.93%,表明在实际大气环境中水蒸气对HO_2+NO→HNO_3反应具有显著影响.     

Hydrogen atom formation from the photodissociation of water ice at 193 nm

The TOF spectra of photofragment hydrogen atoms from the 193 nm photodissociation of amorphous ice at 90-140 K have been measured. The spectra consist of both a fast and a slow components that are characterized by average translational energies of 2k(B)T(trans)=0.39+/-0.04 eV (2300+/-200 K) and 0.02 eV (120+/-20 K), respectively. The incident laser power dependency of the hydrogen atom production suggests one-photon process. The electronic excitation energy of a branched cluster, (H(2)O)(6+1), has been theoretically calculated, where (H(2)O)(6+1) is a (H(2)O)(6) cyclic cluster attached by a water molecule with the hydrogen bond. The photoabsorption of this branched cluster is expected to appear at around 200 nm. The source of the hydrogen atoms is attributed to the photodissociation of the ice surface that is attached by water molecules with the hydrogen bond. Atmospheric implications are estimated for the photodissociation of the ice particles (Noctilucent clouds) at 190-230 nm in the region between 80 and 85 km altitude.     

HNO3 forming channel of the HO2 + NO reaction as a function of pressure and temperature in the ranges of 72-600 Torr and 223-323 K

A high-pressure turbulent flow reactor coupled with a chemical ionization mass-spectrometer was used to determine the branching ratio of the HO(2) + NO reaction: HO(2) + NO --> OH + NO(2) (1a), HO(2) + NO --> HNO(3) (1b). The branching ratio, beta = k(1b)/k(1a), was derived from the measurements of "chemically amplified" concentrations of the NO(2) and HNO(3) products in the presence of O(2) and CO. The pressure and temperature dependence of beta was determined in the pressure range of 72-600 Torr of N(2) carrier gas between 323 and 223 K. At each pressure, the branching ratio was found to increase with the decrease of temperature, the increase becoming less pronounced with the increase of pressure. In the 298-223 K range, the data could be fitted by the expression: beta(T,P) = (530 +/- 10)/T(K) + (6.4 +/- 1.3) x 10(-4)P(Torr) - (1.73 +/- 0.07), giving beta approximately 0.5% near the Earth's surface (298 K, 760 Torr) and 0.8% in the tropopause region (220 K, 200 Torr). The atmospheric implication of these results is briefly discussed.     

Photochemical processes induced by vibrational overtone excitations: dynamics simulations for cis-HONO, trans-HONO, HNO3, and HNO3-H2O

Photochemical processes in HNO3, HNO3-H2O, and cis- and trans-HONO following overtone excitation of the OH stretching mode are studied by classical trajectory simulations. Initial conditions for the trajectories are sampled according to the initially prepared vibrational wave function. Semiempirical potential energy surfaces are used in "on-the-fly" simulations. Several tests indicate at least semiquantitative validity of the potential surfaces employed. A number of interesting new processes and intermediate species are found. The main results include the following: (1) In excitation of HNO3 to the fifth and sixth OH-stretch overtone, hopping of the H atom between the oxygen atoms is found to take place in nearly all trajectories, and can persist for many picoseconds. H-atom hopping events have a higher yield and a faster time scale than the photodissociation of HNO3 into OH and NO2. (2) A fraction of the trajectories for HNO3 show isomerization into HOONO, which in a few cases dissociates into HOO and NO. (3) For high overtone excitation of HONO, isomerization into the weakly bound species HOON is seen in all trajectories, in part of the events as an intermediate step on the way to dissociation into OH + NO. This process has not been reported previously. Well-established processes for HONO, including cis-trans isomerization and H hopping are also observed. (4) Only low overtone levels of HNO3-H2O have sufficiently long liftimes to be spectrocopically relevant. Excitation of these OH stretching overtones is found to result in the dissociation of the cluster H hopping, or dissociation of HNO3 does not take place. The results demonstrate the richness of processes induced by overtone excitation of HNO(x) species, with evidence for new phenomena. Possible relevance of the results to atmospheric processes is discussed.     

Time-slice velocity-map ion imaging studies of the photodissociation of NO in the vacuum ultraviolet region

The time-slice velocity-map ion imaging and the resonant four-wave mixing techniques are combined to study the photodissociation of NO in the vacuum ultraviolet (VUV) region around 13.5 eV above the ionization potential. The neutral atoms, i.e., N((2)D(o)), O((3)P(2)), O((3)P(1)), O((3)P(0)), and O((1)D(2)), are probed by exciting an autoionization line of O((1)D(2)) or N((2)D(o)), or an intermediate Rydberg state of O((3)P(0,1,2)). Old and new autoionization lines of O((1)D(2)) and N((2)D(o)) in this region have been measured and newer frequencies are given for them. The photodissociation channels producing N((2)D(o)) + O((3)P), N((2)D(o)) + O((1)D(2)), N((2)D(o)) + O((1)S(0)), and N((2)P(o)) + O((3)P) have all been identified. This is the first time that a single VUV photon has been used to study the photodissociation of NO in this energy region. Our measurements of the angular distributions show that the recoil anisotropy parameters (β) for all the dissociation channels except for the N((2)D(o)) + O((1)S(0)) channel are minus at each of the wavelengths used in the present study. Thus direct excitation of NO by a single VUV photon in this energy region leads to excitation of states with Σ or Δ symmetry (ΔΩ = ±1), explaining the observed perpendicular transition.     

Model of uptake of OH radicals on nonreactive solids

A model of adsorption and recombination of OH radicals was developed for nonreactive solid surfaces of atmospheric interest. A parametrization of this heterogeneous mechanism was carried out to determine the role of the catalytic properties of these solid surfaces, taking into account the adsorption energy, defects, surface diffusion, and chemical reactions in the gas-solid interface. The uptake process was simulated for diffusion-controlled chemical reactions on the surface on the basis of Langmuir-Hinshelwood and Eley-Rideal mechanisms. Using an analytical approach and the Monte Carlo technique, we show the dependencies of the uptake probability of the heterogeneous reactions on the OH concentration and adsorption energy. The model is employed in the analysis of the empirically derived uptake coefficient for water ice, Al(2)O(3), NaCl, NH(4)NO(3), NH(4)HSO(4), and (NH(4))(2)SO(4). We found the following values for the free energy of adsorption of OH radicals: E(ice) = 7.3-7.6 kcal/mol, E(Al)(2)(O)(3) = 11-11.7 kcal/mol, E(NH)(4)(NO)(3) = 10.2 kcal/mol, E(NaCl) = 10.2 kcal/mol, E(NH)(4)(HSO)(4) = 9.8 kcal/mol, and E((NH)(4))(2)(SO)(4) = 9.8 kcal/mol. The atmospheric implications of the catalytic reactions of OH with adsorbed reactive molecules are discussed. The results of the modeling of the uptake process showed that the heterogeneous decay rate can exceed the corresponding gas-phase reaction rate under atmospheric conditions.     

Photodissociation of polycrystalline and amorphous water ice films at 157 and 193 nm

The photodissociation dynamics of amorphous solid water (ASW) films and polycrystalline ice (PCI) films at a substrate temperature of 100 K have been investigated by analyzing the time-of-flight (TOF) mass spectra of photofragment hydrogen atoms at 157 and 193 nm. For PCI films, the TOF spectrum recorded at 157 nm could be characterized by a combination of three different (fast, medium, and slow) Maxwell-Boltzmann energy distributions, while that measured at 193 nm can be fitted in terms of solely a fast component. For ASW films, the TOF spectra measured at 157 and 193 nm were both dominated by the slow component, indicating that the photofragment H atoms are accommodated to the substrate temperature by collisions. H atom formation at 193 nm is attributed to the photodissociation of water species on the ice surface, while at 157 nm it is ascribable to a mixture of surface and bulk photodissociations. Atmospheric implications in the high latitude mesopause region of the Earth are discussed.     

Molecular dynamics simulations of D2O ice photodesorption

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D(2)O in an amorphous D(2)O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H(2)O, the main processes after UV photodissociation are trapping and desorption of either fragments or D(2)O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D(2)O molecule photodesorption. D(2)O desorption takes places either by direct desorption of a recombined D(2)O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D(2)O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D(2)O in the top four monolayers and are compared quantitatively with those for H(2)O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5-2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D(2)O photodesorption probability is larger than that of H(2)O (by about a factor of 1.4-2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D(2)O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D(2)O ice, because the D atom formed after D(2)O photodissociation has a larger momentum than photogenerated H atoms from H(2)O, and D transfers momentum more easily to D(2)O than H to H(2)O. The total (OD + D(2)O) yield has been compared with experiments and the total (OH + H(2)O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D(2)O ice than when we compare with calculated yields for H(2)O ice.     

Formation of the surface NO during N2O interaction at low temperature with iron-containing ZSM-5

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.