Low temperature decomposition of nitrous oxide over Fe/ZSM-5: Modelling of the dynamic behaviour

Kinetic modeling of nitrous oxide decomposition on Fe-ZSM-5 in the presence of nitric oxide based on parameters obtained from first-principles calculations

A variety of experiments for the N₂O decomposition over Fe-ZSM-5 catalysts have been simulated in the presence and absence of small amounts of nitric oxide and water vapor.

N2O Decomposition over Fe-ZSM-5: A Systematic Study in the Generation of Active Sites

We have carried out a systematic investigation of the critical activation parameters (i.e., final temperature (673–1273 K), atmosphere (He vs. O₂/He), and final isothermal hold (1 min–15 h) on the generation of “α-sites”, responsible for the direct N₂O decomposition over Fe-ZSM-5 (Fe content = 1200–2300 ppm). The concentration of α-sites was determined by (ia) transient response of N₂O and (ib) CO at 523 K, and (ii) temperature programmed desorption (TPD) following nitrous oxide decomposition. Transient response analysis was consistent with decomposition of N₂O to generate (i) “active” α-oxygen that participates in the low-temperature CO→CO₂ oxidation and (ii) “non-active” oxygen strongly adsorbed that is not released during TPD. For the first time, we were able to quantify the formation of α-sites, which requires a high temperature (>973) treatment of Fe-ZSM-5 in He over a short period of time (<1 h). In contrast, prolonged high temperature treatment (1273 K) and the presence of O₂ in the feed irreversibly reduced the amount of active sites.

Plasmonic Photocatalysis of Nitrous Oxide into N2 and O2 Using Aluminum-Iridium Antenna-Reactor Nanoparticles

Photocatalysis with optically active "plasmonic" nanoparticles is a growing field in heterogeneous catalysis, with the potential for substantially increasing efficiencies and selectivities of chemical reactions. Here, the decomposition of nitrous oxide (N₂O), a potent anthropogenic greenhouse gas, on illuminated aluminum-iridium (Al-Ir) antenna-reactor plasmonic photocatalysts is reported. Under resonant illumination conditions, N₂ and O₂ are the only observable decomposition products, avoiding the problematic generation of NO_x species observed using other approaches. Because no appreciable change to the apparent activation energy was observed under illumination, the primary reaction enhancement mechanism for Al-Ir is likely due to photothermal heating rather than plasmon-induced hot-carrier contributions. This light-based approach can induce autocatalysis for rapid N₂O conversion, a process with highly promising potential for applications in N₂O abatement technologies, satellite propulsion, or emergency life-support systems in space stations and submarines.

Mechanism and Kinetics of Direct N2O Decomposition over Fe−MFI Zeolites with Different Iron Speciation from Temporal Analysis of Products

The mechanism of direct N(2)O decomposition over Fe-ZSM-5 and Fe-silicate was studied in the temporal analysis of products (TAP) reactor in the temperature range of 773-848 K at a peak N(2)O pressure of ca. 10 Pa. Several kinetic models based on elementary reaction steps were evaluated to describe the transient responses of the reactant and products. Classical models considering oxygen formation via recombination of two adsorbed monoatomic oxygen species (*-O + *-O --> O(2) + 2*) or via reaction of N(2)O with adsorbed monoatomic oxygen species (N(2)O + *-O --> O(2) + N(2) + *) failed to describe the experimental data. The best description was obtained considering the reaction scheme proposed by Heyden et al. (J. Phys. Chem. B 2005, 109, 1857) on the basis of DFT calculations. N(2)O decomposes over free iron sites (*) as well as over iron sites with adsorbed monoatomic oxygen species (*-O). The latter reaction originates adsorbed biatomic oxygen species followed by its transformation to another biatomic oxygen species, which ultimately desorbs as gas-phase O(2). In line with previous works, our results confirm that the direct N(2)O decomposition is controlled by pathways leading to O(2). Our kinetic model excellently described transient data over Fe-silicalite and Fe-ZSM-5 zeolites possessing markedly different iron species. This finding strongly suggests that the reaction mechanism is not influenced by the iron constitution. The TAP-derived model was extrapolated to a wide range of N(2)O partial pressures (0.01-15 kPa) and temperatures (473-873 K) to evaluate its predictive potential of steady-state performance. Our model correctly predicts the relative activities of two Fe-FMI catalysts, but it overestimates the absolute catalytic activity for N(2)O decomposition.

Nitrous Oxide Decomposition over Fe-ZSM-5 in the Presence of Nitric Oxide: A Comprehensive DFT Study

A number of experimental studies have shown recently that ppm-level additions of nitric oxide (NO) enhance the rate of nitrous oxide (N(2)O) decomposition catalyzed by Fe-ZSM-5 at low temperatures. In the present work, the NO-assisted N(2)O decomposition over mononuclear iron sites in Fe-ZSM-5 was studied on a molecular level using density functional theory (DFT) and transition-state theory. A reaction network consisting of over 100 elementary reactions was considered. The structure and energies of potential-energy minima were determined for all stable species, as were the structures and energies of all transition states. Reactions involving changes in spin potential-energy surfaces were also taken into account. In the absence of NO and at temperatures below 690 K, most active single iron sites (Z(-)[FeO](+)) are poisoned by small concentrations of water in the gas phase; however, in the presence of NO, these poisoned sites are converted into a novel active iron center (Z(-)[FeOH](+)). These latter sites are capable of promoting the dissociation of N(2)O into a surface oxygen atom and gas-phase N(2). The surface oxygen atom is removed by reaction with NO or nitrogen dioxide (NO(2)). N(2)O dissociation is the rate-limiting step in the reaction mechanism. At higher temperatures, water desorbs from inactive iron sites and the reaction mechanism for N(2)O decomposition becomes independent of NO, reverting to the reaction mechanism previously reported by Heyden et al. [J. Phys. Chem. B 2005, 109, 1857].

Role of Adsorbed NO in N2O Decomposition over Iron-Containing ZSM-5 Catalysts at Low Temperatures

Transient response and temperature-programmed desorption/reaction (TPD/TPR) methods were used to study the formation of adsorbed NO(x) from N2O and its effect during N2O decomposition to O2 and N2 over FeZSM-5 catalysts at temperatures below 653 K. The reaction proceeds via the atomic oxygen (O)(Fe) loading from N2O on extraframework active Fe(II) sites followed by its recombination/desorption as the rate-limiting step. The slow formation of surface NO(x,ads) species was observed from N2O catalyzing the N2O decomposition. This autocatalytic effect was assigned to the formation of NO(2,ads) species from NO(ads) and (O)(Fe) leading to facilitation of (O)(Fe) recombination/desorption. Mononitrosyl Fe2+(NO) and nitro (NO(2,ads)) species were found by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) in situ at 603 K when N2O was introduced into NO-containing flow passing through the catalyst. The presence of NO(x,ads) does not inhibit the surface oxygen loading from N2O at 523 K as observed by transient response. However, the reactivity of (O)(Fe) toward CO oxidation at low temperatures (<523 K) is drastically diminished. Surface NO(x) species probably block the sites necessary for CO activation, which are in the vicinity of the loaded atomic oxygen.

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.