Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N2 and N2O formation in the direct decomposition and reduction of NO by NH3 were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying 15NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH3–NO interaction, N2O formation was most marked between 573 and 673 K, whereas N2 formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the 15NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N2O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide

Effect of different oxygen species originating from the dissociation of O2, N2O and NO on the selectivity of the Pt-catalysed NH3 oxidation

An effect of oxygen species formed from O2, N2O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O2, N2O and NO) and NH3. In contrast to adsorbed oxygen species formed from NO, those from O2 and N2O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O2, N2O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O2 and N2O rather than from NO. These species may be of bi-atomic nature

Mechanistic aspects of N2O and N2 formation in NO reduction by NH3 over Ag/Al2O3: the effect of O2 and H2

A mechanistic scheme of N2O and N2 formation in the selective catalytic reduction of NO with NH3 over a Ag/Al2O3 catalyst in the presence and absence of H2 and O2 was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N2 and N2O are formed via direct or oxygeninduced decomposition of surface NH2NO species. These intermediates originate from NO and surface NH2 fragments. The latter NH2 species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N2O and O2. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgOx particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N2O production in the presence of H2 is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N2O decomposition

Partial Oxidation of Methane to Syngas Over γ‑Al₂O₃‑Supported Rh Nanoparticles: Kinetic and Mechanistic Origins of Size Effect on Selectivity and Activity

A series of supported Rh/γ-Al₂O₃ catalysts with an overall metal loading of 0.005 wt % was synthesized by impregnation of γ-Al₂O₃ with a toluene solution containing colloidally prepared well-defined (1.1, 2.5, 2.9, 3.7, and 5.5 nm) Rh nanoparticles (NP). The size of NP was not found to change after their deposition on γ-Al₂O₃ and even after performing partial oxidation of methane (POM) to synthesis gas at 1073 K for 160 h on stream. Apparent CO formation turnover rates and CO selectivity strongly decrease with an increase in this size. Contrarily, the overall scheme of POM is size-independent, i.e. CO and H₂ are mainly formed through reforming reactions of CH₄ with CO₂ and H₂O at least under conditions of complete oxygen conversion. The size effect on the activity and selectivity was related to the kinetics of interaction of CH₄, O₂, and CO₂ with Rh/γ-Al₂O₃ as concluded from our microkinetic analysis of corresponding transient experiments in the temporal analysis of products reactor. The rate constants of CH₄, O₂, and CO₂ activation decrease with an increase in the size of supported Rh NP thus influencing both primary (methane combustion) and secondary (reforming of methane) pathways within the course of POM

Mechanistic origin of the diverging selectivity patterns in catalyzed ethane and ethene oxychlorination

In the context of vinyl chloride monomer (VCM) production, an oxychlorination catalyst that allows direct VCM formation from gas-derived ethane instead of expensive oil-derived ethene is intensively sought after. A wide range of stable ethane oxychlorination catalysts for this purpose have recently been reported, yet they mainly yield ethene, while VCM remains a minor by-product. Strikingly, the same catalysts are active in ethene oxychlorination, resulting in selective VCM formation under equivalent reaction conditions. This work reveals the origin of these diverging selectivity patterns by combining quantitative catalytic tests, temporal analysis of products (TAP), and density functional theory (DFT) on iron phosphate. Ethane oxychlorination is found to proceed sequentially through ethyl chloride (EtCl) dehydrochlorination to ethene, while ethene oxychlorination directly yields VCM without formation of the intermediate dichloroethane (EDC) on iron phosphate. Furthermore, by co-feeding ethane in ethene oxychlorination, we demonstrate that the alkane suppresses the formation of VCM in ethene oxychlorination. The reason for this VCM inhibition is found in the hydrocarbon competition for a combination of the active, free and chlorinated iron centers. As ethane activation exhibits half of the barrier of ethene activation, the presence of ethane leads to active site depletion, hindering VCM formation. These observations are extended by ethane co-feeding tests in ethene oxychlorination over a wide range of known oxychlorination catalysts (EuOCl, LaOCl, CeO2, and CuCl2-KCl-LaCl3/c-Al2O3), and corresponding DFT calculations, indicating that the described phenomenon is material independent. The gathered molecular-level understanding explains the major hurdle of using ethane as feedstock for vinyl chloride production

Effect of Support and Promoter on Activity and Selectivity of Gold Nanoparticles in Propanol Synthesis from CO₂, C₂H₄, and H₂

Direct propanol synthesis from CO₂, H₂, and C₂H₄ was investigated over TiO₂- and SiO₂-based catalysts doped with K and possessing Au nanoparticles (NPs). The catalysts were characterized by scanning transmission electron microscopy and temperature-programmed reduction of adsorbed CO₂. Mechanistic aspects of CO₂ and C₂H₄ interaction with the catalysts were elucidated by means of temporal analysis of products with microsecond time resolution. CO₂, which is activated on the support, is reduced to CO by hydrogen surface species formed from gas-phase H₂ on Au NPs. C₂H₄ adsorption also occurs on these sites. In comparison with TiO₂-based catalysts, the promoter in the K–Au/SiO₂ catalysts was found to increase CO₂ conversion and propanol production, whereas Au-related turnover frequency of C₂H₄ hydrogenation to C₂H₆ decreased with rising K loading. The latter reason was linked to the effect of the support on the ability of Au NPs for activation of C₂H₄ and H₂. The positive effect of K on CO₂ conversion was explained by partial dissolution of potassium in silica with formation of surface potassium silicate layer thus inhibiting formation of potassium carbonate, which binds CO₂ stronger and therefore hinders its reduction to CO

Mechanism of Ethylene Oxychlorination on Ceria

ISSN:2155-543