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Keywords: Ammonia Oxidation; Au/CeO2; Metal-Support Interface; Nitrous Oxide; Reaction Mechanisms EN Ammonia Oxidation Au/CeO2 Metal-Support Interface Nitrous Oxide Reaction Mechanisms 1 1 1 04/27/22 20220502 NES 220502 B Ein Spiel zweier Hälften b Die direkte Oxidation von Ammoniak eröffnet einen Weg zur kostengünstigen und effizienten Produktion von Stickstoffoxid, einem vielversprechenden Reagens für selektive Oxidationsreaktionen. Rücktitelbild: Ceria-Supported Gold Nanoparticles as a Superior Catalyst for Nitrous Oxide Production via Ammonia Oxidation (Angew. Ammonia Oxidation, Au/CeO2, Metal-Support Interface, Nitrous Oxide, Reaction Mechanisms. [Extracted from the article]

Ammonia Oxidation, Au/CeO2, Metal-Support Interface, Nitrous Oxide, Reaction Mechanisms Keywords: Ammonia Oxidation; Au/CeO2; Metal-Support Interface; Nitrous Oxide; Reaction Mechanisms EN Ammonia Oxidation Au/CeO2 Metal-Support Interface Nitrous Oxide Reaction Mechanisms 1 1 1 04/27/22 20220502 NES 220502 B A game of two halves b Direct ammonia oxidation unlocks a pathway towards affordable and efficient production of nitrous oxide, a highly promising agent for selective oxidation reactions, but necessitates the development of suitable catalysts. Back Cover: Ceria-Supported Gold Nanoparticles as a Superior Catalyst for Nitrous Oxide Production via Ammonia Oxidation (Angew. [Extracted from the article]

The simple deposition of tailored Rh or Pt nanoparticles (NP) on γ-Al2O3 results in active, selective, and stable catalysts for partial oxidation of methane (POM) to syngas. The NP were prepared by the ethylene glycol method in strong alkaline solution. This approach was found to be a promising way of providing active metallic NP for catalyzing the POM reaction. NP sizes determined by small angle X-ray scattering (SAXS) in solution and by transmission electron microscopy (TEM) on alumina were very close. This result highlights the possibility of easy pre-characterization of NP by the former method. Supported Rh NP are intrinsically more active in the POM reaction than Pt NP and also showed a superior performance compared with a conventionally prepared Rh catalyst, even if the latter had been pre-reduced. Mechanistic investigations in the temporal analysis of products (TAP) reactor indicate that the higher selectivity of well-defined Rh-NP is determined by their lower activity for consecutive CO transformations. [ABSTRACT FROM AUTHOR]

Abstract: The mechanisms of direct N2O decomposition and N2O reduction by H2 or NH3 on RuO2 has been studied by means of steady-state catalytic tests at ambient pressure, transient experiments in a temporal analysis of products (TAP) reactor, and density functional theory (DFT) simulations. Bulk RuO2 is very active for N2O decomposition into N2 and O2, achieving full conversion at 723K. According to the DFT calculations, coordinatively unsaturated ruthenium (Rucus) sites are active for the decomposition and oxygen elimination from the surface is rate-limiting step. When pre-reducing RuO2 with H2, the de-N2O activity in the low-temperature region increases. This is attributed to the higher reactivity of surface oxygen vacancies created by H2 at bridge sites compared to Rucus sites. Co-feeding of hydrogen or ammonia with nitrous oxide strongly accelerates the rate of N2O elimination. TAP studies show that oxygen species formed upon N2O decomposition over RuO2 react with NH3 and H2 yielding N2, NO, N2O, and H2O. Both TAP experiments and DFT simulations show that the N-containing intermediates coming from ammonia adsorb much stronger on the catalyst surface than H-containing intermediates coming from hydrogen. This causes blockage of active sites for N2O decomposition by the former species, accounting for the higher efficiency of H2 as reductant for N2O. The presence of O2 in the feed cancels the reducing effect of H2 or NH3 due to the more favorable re-oxidation of reduced RuO2 by O2 compared to N2O. [Copyright &y& Elsevier]

Abstract: Mechanistic and kinetic aspects of direct N2O decomposition over BaFeAl11O19 hexaaluminate were investigated in the Temporal Analysis of Products (TAP) reactor and compared with those previously determined for Fe-MFI zeolites. The catalysts were chosen due to their de-N2O operation in significantly different temperature regimes. Several micro-kinetic models were evaluated for describing the transient responses of N2O, N2, and O2 obtained in N2O pulse experiments at 823–973K. Thorough discrimination between these models enabled us to conclude that the preferred models of N2O decomposition over BaFeAl11O19 and Fe-MFI zeolites differ in the reaction pathways leading to O2 and N2. Gas-phase N2 and O2 are simultaneously formed over BaFeAl11O19 upon interaction of gas-phase N2O with a bi-atomic surface oxygen (*–O2) species. Contrarily, the formation of O2 over Fe-MFI occurs via a sequence of three elementary heterogeneous steps and limits the overall rate of N2O decomposition. Despite the easy O2 formation, BaFeAl11O19 is less active for N2O decomposition below 973K than the Fe-MFI zeolites due to the low coverage by *–O2. According to our quantitative micro-kinetic analysis, this species is formed when gas-phase N2O reacts with a mono-atomic oxygen (*–O) species. This reaction pathway is strongly influenced by the degree of isolation of iron species. The higher the degree of iron isolation in the catalyst, the lower the de-N2O activity. [ABSTRACT FROM AUTHOR]

A temporal analysis of products (TAP) reactor was used to study relationships between the mechanism of direct N2O decomposition over metal-loaded zeolites and their resulting activity. Rh-ZSM-5 (prepared by incipient wetness) and Fe-ZSM-5 (prepared by liquid-ion exchange) were chosen as prototypic catalysts displaying low (<550 K) and high (>650 K) temperature activity, respectively. Transient studies at the same contact time revealed the higher activity of Rh-ZSM-5 below 623 K and significantly stronger N2O adsorption over Rh species than over Fe species in the zeolites. Several microkinetic models were applied for simultaneous fitting the transient responses of N2O, N2, and O2. Classical reaction schemes failed to describe the experimental data. The preferred models of N2O decomposition over Rh-ZSM-5 and Fe-ZSM-5 differ in the reaction pathways of O2 formation. For both catalysts, free active metal sites (*) and those occupied by monoatomic oxygen species (*-O) from N2O participate in the decomposition of gas-phase N2O. Gas-phase O2 is formed directly on N2O interaction with *-O over Rh-ZSM-5, whereas the latter reaction over Fe-ZSM-5 leads to a surface bi-atomic oxygen species (O-*-O), followed by its transformation to *-O2. The latter species desorbs as molecular oxygen. Comparison of ion-exchanged and steam-activated Fe-ZSM-5 [J. Phys. Chem. B 110 (2006) 22586] revealed that the reaction mechanism is independent of the iron constitution induced by the preparation and activation routes, despite important differences in catalytic activity. Our quantitative microkinetic analysis demonstrated that both the stronger reversible N2O adsorption and, most importantly, the faster desorption of O2 are distinctive mechanistic features of Rh-ZSM-5, likely indicating its high de-N2O activity. [Copyright &y& Elsevier]

Abstract: 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 973K 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 673K, whereas N2 formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the 15NO pulse at 473K 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. [Copyright &y& Elsevier]

Abstract: Mechanistic aspects of low-temperature (423–723 K) selective catalytic reduction of NO with NH3 (NH3-SCR) over an Ag(1.7 wt%)/Al2O3 (2Ag/Al2O3) catalyst in the presence and absence of O2 and H2 were studied using a transient low-pressure (peak pressure < 10 Pa) technique, the temporal analysis of products (TAP) reactor, in combination with isotopic traces. Preoxidized 2Ag/Al2O3 showed very low activity in the NH3-SCR reaction. The activity increased tremendously after ex situ reduction of 2Ag/Al2O3 in a hydrogen flow (5 vol% H2 in Ar) at 373 K for 30 min. This observation was related to the creation of reduced Ag species, which catalyze O2 and NO dissociation, yielding adsorbed oxygen species. O2 is a better supplier of oxygen species. Oxygen species played a key role in NH3 dehydrogenation, yielding reactive NH x fragments that are important intermediates for nitrogen formation via a coupling reaction between NO and NH3. This reaction pathway predominated over direct NO decomposition to N2 in the presence of O2. In addition to generation of active oxygen species, gas-phase oxygen accelerated transformation of surface N-containing intermediates into gas-phase reaction products. The role of hydrogen in the NH3-SCR reaction is to transform oxidized Ag species into reduced species that are active sites for O2 and NO adsorption. Our findings suggest that the reduction of oxidized Ag is responsible for the boosting effect of H2 in the NH3-SCR reaction, and also that H2 helps decrease total N2O production. [Copyright &y& Elsevier]

Abstract: A simple cellulose-templating (CT) method was used to prepare Na–CaO catalysts for high-temperature N2O decomposition to N2 and O2 in excess of NO, O2, and H2O. According to this method, cellulose materials were soaked in aqueous solution of calcium and/or sodium nitrates followed by burning out the template material. Depending on the nature of cellulose template and calcination procedure, the obtained catalysts consisted of regularly shaped particles of 100–400nm. Conventionally prepared Na–CaO reference materials had randomly shaped particles on a micron scale. Compared to the latter catalysts, the CT ones showed higher de-N2O activity and resistance against inhibition by NO and O2, which make them interesting from industrial point of view. Transient analysis of NO interaction with the studied catalysts in the presence and the absence of O2 enabled us to conclude that the superior de-N2O performance of the CT materials is due to improved desorption of surface NO x species acting as inhibitors for N2O decomposition. [ABSTRACT FROM AUTHOR]

Abstract: In order to identify factors governing selectivity of an oxygen-conducting perovskite BaCo x Fe y Zr z O3−δ (BCFZ) membrane in the partial oxidation of methane and ethane, mechanistic aspects of product formation in these reactions were investigated with a millisecond time resolution using the temporal analysis of products (TAP) reactor. It was found that the selectivity depends on: (i) reduction degree of the perovskite surface; the higher the reduction degree, the higher the ethane and ethylene selectivity in methane and ethane oxidation, respectively, and (ii) contact time; short contact times favor partial selective oxidation. The influence of contact time on the ethylene selectivity in ethane oxidation at degrees of ethane conversion above 85% was experimentally proven in hollow fiber and disk membranes, which differ in the contact times. The low activity and selectivity in methane oxidation in the BCFZ perovskite membrane reactor were significantly increased, when the membrane on the hydrocarbon side was coated by a Ni-based steam reforming catalyst. This catalyst fulfils a double role: (i) it increases the oxygen transport through the perovskite membrane due to the high oxygen consumption, and (ii) it accelerates syngas production via deep methane oxidation followed by dry and steam reforming of methane. The syngas selectivity increases with an increase in the catalyst reduction degree, which is determined by the ratio of the rate of methane oxidation to the rate of oxygen permeation through the membrane. [Copyright &y& Elsevier]

Improving the selectivity in the oxidative coupling of methane to ethane/ethylene poses a significant challenge for commercialization. The required improvements are hampered by the uncertainties associated with the reaction mechanism due to its complexity. Herein, we report about 90 % selectivity to the target products at 11 % methane conversion over Gd 2 O 3 -based catalysts at 700 °C using N 2 O as the oxidant. Sophisticated kinetic studies have suggested the nature of adsorbed oxygen species and their binding strength as key parameters for undesired methane oxidation to carbon oxides. These descriptors can be controlled by a metal oxide promoter for Gd 2 O 3 ., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The development of selective catalysts for direct conversion of ammonia into nitrous oxide, N 2 O, will circumvent the conventional five-step manufacturing process and enable its wider utilization in oxidation catalysis. Deviating from commonly accepted catalyst design principles for this reaction, reliant on manganese oxide, we herein report an efficient system comprised of isolated chromium atoms (1 wt %) stabilized in the ceria lattice by coprecipitation. The latter, in contrast to a simple impregnation approach, ensures firm metal anchoring and results in stable and selective N 2 O production over 100 h on stream up to 79% N 2 O selectivity at full NH 3 conversion. Raman, electron paramagnetic resonance, and in situ UV-vis spectroscopies reveal that chromium incorporation enhances the density of oxygen vacancies and the rate of their generation and healing. Accordingly, temporal analysis of products, kinetic studies, and atomistic simulations show lattice oxygen of ceria to directly participate in the reaction, establishing the cocatalytic role of the carrier. Coupled with the dynamic restructuring of chromium sites to stabilize intermediates of N 2 O formation, these factors enable catalytic performance on par with or exceeding benchmark systems. These findings demonstrate how nanoscale engineering can elevate a previously overlooked metal into a highly competitive catalyst for selective ammonia oxidation to N 2 O, paving the way toward industrial implementation., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Nitrous oxide, N 2 O, exhibits unique reactivity in oxidation catalysis, but the high manufacturing costs limit its prospective uses. Direct oxidation of ammonia, NH 3 , to N 2 O can ameliorate this issue but its implementation is thwarted by suboptimal catalyst selectivity and stability, and the lack of established structure-performance relationships. Systematic and controlled material nanostructuring offers an innovative approach for advancement in catalyst design. Herein low-valent manganese atoms stabilized on ceria, CeO 2 , are discovered as the first stable catalyst for NH 3 oxidation to N 2 O, exhibiting two-fold higher productivity than the state-of-the-art. Detailed mechanistic, computational and kinetic studies reveal CeO 2 as the mediator of oxygen supply, while undercoordinated manganese species activate O 2 and facilitate N 2 O evolution via NN bond formation between nitroxyl, HNO, intermediates. Synthesis via simple impregnation of a small metal quantity (1 wt%) predominantly generates isolated manganese sites, while full atomic dispersion is achieved upon redispersion of sporadic oxide nanoparticles during reaction, as confirmed by advanced microscopic analysis and electron paramagnetic resonance spectroscopy. Subsequently, manganese speciation is maintained, and no deactivation is observed over 70 h on stream. CeO 2 -supported isolated transition metals emerge as a novel class of materials for N 2 O production, encouraging future studies to evaluate their potential in selective catalytic oxidations at large., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Alkali metal promoters have been widely employed for preparation of heterogeneous catalysts used in many industrially important reactions. However, the fundamentals of their effects are usually difficult to access. Herein, we unravel mechanistic and kinetic aspects of the role of alkali metals in CO 2 hydrogenation over Fe-based catalysts through state-of-the-art characterization techniques, spatially resolved steady-state and transient kinetic analyses. The promoters affect electronic properties of iron in iron carbides. These carbide characteristics determine catalyst ability to activate H 2 , CO and CO 2 . The Allen scale electronegativity of alkali metal promoter was successfully correlated with the rates of CO 2 hydrogenation to higher hydrocarbons and CH 4 as well as with the rate constants of individual steps of CO or CO 2 activation. The derived knowledge can be valuable for designing and preparing catalysts applied in other reactions where such promoters are also used., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The production of nitrous oxide, N 2 O, via NH 3 oxidation is not on a practical scale due to the lack of a suitable catalyst. Instead, it is produced via thermal decomposition of NH 4 NO 3 , rendering N 2 O too costly and limiting its prospective uses. Herein, we report CeO 2 -supported Au nanoparticles (2-3 nm) as a highly selective catalyst for low-temperature NH 3 oxidation to N 2 O, exhibiting two orders of magnitude higher space-time yield than the state-of-the-art Mn-Bi/α-Al 2 O 3 and remarkable stability over 70 h on stream. The reaction proceeds via a Mars-van Krevelen mechanism, with the density of interfacial Au δ+ species and the oxygen storage capacity of CeO 2 identified as the key performance descriptors. The latter could be enhanced by cobalt doping, improving the catalytic activity and setting a new benchmark for N 2 O productivity. These findings establish NH 3 oxidation as an efficient process for N 2 O manufacture and facilitate its broader utilization in selective oxidations., (© 2022 Wiley-VCH GmbH.)

Conversion of propane or butanes from natural/shale gas into propene or butenes, which are indispensable for the synthesis of commodity chemicals, is an important environmentally friendly alternative to oil-based cracking processes. Herein, we critically analyse recent developments in the non-oxidative, oxidative, and CO 2 -mediated dehydrogenation of propane and isobutane to the corresponding olefins over metal oxide catalysts. Particular attention is paid to (i) comparing the developed catalysts in terms of their application potential, (ii) structure-activity-selectivity relationships for tailored catalyst design, and (iii) reaction-engineering aspects for improving product selectivity and overall process efficiency. On this basis, possible directions for further research aimed at the development of inexpensive and environmentally friendly catalysts with industrially relevant performance were identified. In addition, we provide general information regarding catalyst preparation and characterization as well as some recommendations for carrying out non-oxidative and CO 2 -mediated dehydrogenation reactions to ensure unambiguous comparison of catalysts developed in different studies.

This paper demonstrates the potential of Eu2O3 and Gd2O3 as catalysts for non-oxidative propane dehydrogenation to propene. They reveal a higher activity than the state-of-the-art bare ZrO2-based catalysts due to the higher intrinsic activity of Gdcus or Eucus in comparison with that of Zrcus (cus = coordinatively unsaturated).

The impact of formaldehyde (HCHO, formed in vehicle exhaust gases by incomplete combustion of fuel) on the performance of a commercial V 2 O 5 -WO 3 /TiO 2 catalyst in NH 3 -SCR of NO x under dry conditions has been analyzed in detail by catalytic tests, in situ FTIR and transient studies using temporal analysis of products (TAP). HCHO reacts preferentially with NH 3 to a formamide (HCONH 2 ) surface intermediate. This deprives NH 3 partly from its desired role as a reducing agent in the SCR and diminishes NO conversion and N 2 selectivity. Between 250 and 400 °C, HCONH 2 decomposes by dehydration (major pathway) and decarbonylation (minor pathway) to liberate toxic HCN and CO, respectively. HCN was proven to be oxidized by lattice oxygen of the catalyst to CO 2 and NO, which enters the NH 3 -SCR reaction.

Au/TiO 2 and Au/SiO 2 catalysts containing 2 wt % Au and different amounts of K or Cs were tested for alcohol synthesis from CO 2 , H 2 , and C 2 H 4 /C 3 H 6 . 1-Propanol or 1-butanol/isobutanol were obtained in the presence of C 2 H 4 or C 3 H 6 . Higher yields of the corresponding alcohols were obtained over TiO 2 -based catalysts in comparison with their SiO 2 -based counterparts. This is caused by an enhanced ability of the TiO 2 -based catalysts for CO 2 activation, as concluded from in situ fourier-transform infrared (FTIR) spectroscopy and temporal analysis of products (TAP) studies. The synthesized carbonate and formate species adsorbed on the support do not hamper CO 2 conversion into CO and the hydroformylation reaction. The transformation of Au δ+ to active Au 0 sites proceeds during an activation procedure. As reflected by CO adsorption and scanning transmission electron microscopy, the accessible Au 0 sites are influenced by the amount of alkali dopants and the support. FTIR data and TAP tests reveal a very weak interaction of C 2 H 4 with the catalyst, suggesting its quick reaction with CO and H 2 after activation on Au 0 sites to form propanol and propane., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Carbon-supported gold catalysts have the potential to replace the toxic mercuric chloride-based system applied industrially for acetylene hydrochlorination, a key technology for the manufacture of polyvinyl chloride. However, the design of an optimal catalyst is essentially hindered by the difficulties in assessing the nature of the active site. Herein, we present a platform of carbon supported gold nanostructures at a fixed metal loading, ranging from single atoms of tunable oxidation state and coordination to metallic nanoparticles, by varying the structure of functionalised carbons and use of thermal activation. While on activated carbon particle aggregation occurs progressively above 473 K, on nitrogen-doped carbon gold single atoms exhibit outstanding stability up to temperatures of 1073 K and under reaction conditions. By combining steady-state experiments, density functional theory, and transient mechanistic studies, we assess the relation between the metal speciation, electronic properties, and catalytic activity. The results indicate that the activity of gold-based catalysts correlates with the population of Au(i)Cl single atoms and the reaction follows a Langmuir-Hinshelwood mechanism. Strong interaction with HCl and thermodynamically favoured acetylene activation were identified as the key features of the Au(i)Cl sites that endow their superior catalytic performance in comparison to N-stabilised Au(iii) counterparts and gold nanoparticles. Finally, we show that the carrier (activated carbon versus nitrogen-doped carbon) does not affect the catalytic response, but determines the deactivation mechanism (gold particle aggregation and pore blockage, respectively), which opens up different options for the development of stable, high-performance hydrochlorination catalysts.

The promotional effect of H 2 on the oxidation of CO is of topical interest, and there is debate over whether this promotion is due to either thermal or chemical effects. As yet there is no definitive consensus in the literature. Combining spatially resolved mass spectrometry and X-ray absorption spectroscopy (XAS), we observe a specific environment of the active catalyst during CO oxidation, having the same specific local coordination of the Pd in both the absence and presence of H 2 . In combination with Temporal Analysis of Products (TAP), performed under isothermal conditions, a mechanistic insight into the promotional effect of H 2 was found, providing clear evidence of nonthermal effects in the hydrogen-promoted oxidation of carbon monoxide. We have identified that H 2 promotes the Langmuir-Hinshelwood mechanism, and we propose this is linked to the increased interaction of O with the Pd surface in the presence of H 2 . This combination of spatially resolved MS and XAS and TAP studies has provided previously unobserved insights into the nature of this promotional effect., Competing Interests: The authors declare no competing financial interest.

Non-oxidative dehydrogenation of propane to propene is an established large-scale process that, however, faces challenges, particularly in catalyst development; these are the toxicity of chromium compounds, high cost of platinum, and catalyst durability. Herein, we describe the design of unconventional catalysts based on bulk materials with a certain defect structure, for example, ZrO2 promoted with other metal oxides. Comprehensive characterization supports the hypothesis that coordinatively unsaturated Zr cations are the active sites for propane dehydrogenation. Their concentration can be adjusted by varying the kind of ZrO2 promoter and/or supporting tiny amounts of hydrogenation-active metal. Accordingly designed Cu(0.05 wt %)/ZrO2 -La2 O3 showed industrially relevant activity and durability over ca. 240 h on stream in a series of 60 dehydrogenation and oxidative regeneration cycles between 550 and 625 °C., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)