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[Display omitted] • Alkali metal in MnO x -M 2 WO 4 /SiO 2 affects activity under anaerobic conditions. • Lattice oxygen of MnO x -M 2 WO 4 /SiO 2 can convert CH 4 to hydrocarbons. • OCM over MnO x -M 2 WO 4 /SiO 2 and M 2 WO 4 /SiO 2 proceeds via adsorbed oxygen species. • Presence of MnO x does not create more active sites but changes their reactivity. This work unveils the fundamentals relevant for activity and product selectivity in the oxidative coupling of methane (OCM) over the (MnO x)-M 2 WO 4 /SiO 2 (M = Na, K, Rb, or Cs) catalysts. The presence of the molten phase of Na 2 WO 4 and the ability of the catalysts to release lattice oxygen were not found to be the performance-governing factors. As proven by oxygen isotopic exchange experiments combined with thorough catalytic tests, the performance of both tri- and bimetallic catalysts is defined by the involvement of adsorbed oxygen species formed from gas-phase O 2. Lattice oxygen of M 2 WO 4 /SiO 2 is not capable of converting CH 4 to C 2 H 6 , while that of MnO x -M 2 WO 4 /SiO 2 is active for this reaction. The OCM activity of M 2 WO 4 /SiO 2 decreases with the atomic weight of alkali metal, whereas there is no such effect in the presence of MnO x , which, however, increases the activity. The enhancing effect was explained by improving the active site turnover. [ABSTRACT FROM AUTHOR]

Catalyst characteristics affecting product selectivity and mechanistic aspects of formation of Mo-carbene species in the metathesis of ethylene and trans -2-butene to propylene over MoO x /AlSiO x were elucidated by complementary techniques. Steady-state catalytic tests revealed that reaction pathways leading to propene formation depend on the kind (highly dispersed or polymerized) of MoO x species, from which Mo-carbene species are formed in situ. C 3 H 6 is produced directly through metathesis of C 2 H 4 and trans -2-C 4 H 8 on Mo-carbenes originated from polymerized MoO x species, whereas the metathesis of 1- and 2-butenes plays an important role for propene formation on Mo-carbenes formed from highly dispersed MoO x . In situ FTIR spectroscopy and kinetic analysis suggest that the pathways of propene formation and accordingly the selectivity are determined by the ability of C 2 H 4 to react with oxidized MoO x species and Mo-carbenes. This feed component interacts with highly dispersed MoO x species more weakly than with polymerized ones. Moreover, C 2 H 4 adsorbs more strongly on Mo-carbenes formed from highly dispersed MoO x than on those originating from polymerized MoO x species. [ABSTRACT FROM AUTHOR]

Non-oxidative propane dehydrogenation (PDH) was investigated over unconventional catalysts, i.e. binary La 2 O 3 -ZrO 2 and Y 2 O 3 -ZrO 2 materials with or without tiny amounts (0.005 or 0.05 wt.%) of Ru in form of nanoparticles with the size of about 1.1 nm. The results of catalytic tests and catalyst characterization with complementary techniques enabled us to conclude that coordinatively unsaturated zirconium cations (Zr cus ) and neighboring lattice oxygen are catalytically active sites responsible for C 3 H 6 formation. Supported Ru species contribute to a minor extent to PDH but promote generation of Zr cus sites upon catalyst reductive treatment or in situ under PDH conditions. The concentration of Zr cus sites can also be increased through doping of ZrO 2 with La 2 O 3 or Y 2 O 3 with the latter being a more efficient dopant. High activity, selectivity, and durability of our catalysts were verified by comparing their performance under industrially relevant conditions with that of a conventional K-CrO x /Al 2 O 3 reference material in a series of 60 PDH/oxidative regeneration cycles at 550, 600, and 625 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • Well-defined Fe x O y C z compositions promoted with Li, Na, K, Rb or Cs were prepared. • They were tested in CO 2 hydrogenation to higher hydrocarbons. • Stationary phases and their spatial distribution depend on alkali metal type. • Steady-state Fe 5 C 2 /Fe 3 C ratio determines the selectivity to C 2+ -hydrocarbons. • The kind of alkali metal affects reaction pathways of CH 4 formation. A series of catalysts for CO 2 hydrogenation to higher (C 2+) hydrocarbons were prepared through controlled decomposition of iron (II) oxalate dihydrate impregnated with Li, Na, K, Rb or Cs carbonate. They consist of Fe 3 O 4 , Fe 5 C 2 , Fe 3 C and Fe. Their steady-state spatial phase distribution is, however, affected by the presence and the kind of the promoter. These structural characteristics determine catalyst activity and product selectivity. Fe 3 C seems to favor CH 4 formation, while C 2+ -hydrocarbons are formed over Fe 5 C 2. A positive relationship between the rate of C 2+ -hydrocarbons formation and the rate constant of dissociation of adsorbed CO 2 was established, while an opposite dependence is valid for the rate of CO 2 methanation. The rate constant of this reaction increases in the presence of alkali metal and with an increase in its atomic number. Therefore, CH 4 is mainly formed over Rb- or Cs-doped catalysts through CO hydrogenation, while CO 2 methanation prevails over the remaining catalysts. [ABSTRACT FROM AUTHOR]

Abstract: Tailored Pt nanoparticles were prepared in strong alkaline ethylene glycol solution and deposited on α-Al2O3, γ-Al2O3, and CeO2–TiO2. The resulting supported catalytic materials were applied for ammonia oxidation between 473 and 1073K in the temporal analysis of products reactor. Although the catalysts possessed only 0.005wt.% Pt, they were active and selective for nitric oxide formation. The NO selectivity obtained over Pt/CeO2–TiO2 between 873 and 1073K was around 98% and exceeded that of industrially relevant Pt wire. Both alumina-supported catalysts were less selective. Pt/CeO2–TiO2 also showed the highest NO selectivity in steady-state ambient pressure tests. The superior performance of Pt/CeO2–TiO2 is related to a synergy effect between the noble metal and the support since they provide adsorbed and lattice oxygen species for selective NH3 oxidation to NO in a complementary fashion. [Copyright &y& Elsevier]

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: 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]

• The mechanism of ethane and ethene oxychlorination is assessed on iron phosphate. • Ethane yields ethene via ethyl chloride, while ethene directly forms vinyl chloride. • Ethane acts as a vinyl chloride suppression agent in ethene oxychlorination. • Active site competition favors ethane, depleting sites for ethene activation. • The diverging selectivity patterns are translatable to other catalytic systems. 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, CeO 2 , and CuCl 2 -KCl-LaCl 3 / γ -Al 2 O 3), 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Approach for anchoring of Pt atoms on isolated Sn4+ is introduced. • Using this approach PtSn/SiO 2 (hexagonal mesoporous silica) catalysts were prepared. • Pt-Sn/atoms/sub nanoclusters were identified on the surface of catalysts. • The catalysts are active, selective, and stable in C 3 H 8 dehydrogenation to C 3 H 6. • Sn is essential to enhance H 2 desorption from Pt that is the rate-limiting step. Owing to propane availability in shale gas and the growing demand for propene, non-oxidative dehydrogenation of this alkane has attracted much attention. Although, catalysts with supported PtSn species are applied on the large scale, a further increase in their activity without loss in propene selectivity at low Pt content would certainly improve process economy. Herein, we introduce a molecularly defined strategy for controlling catalyst activity through anchoring of Pt atoms on isolated Sn4+ firmly confined in the framework of hexagonal mesoporous silica. In addition to the dispersion of Pt species, the presence of Sn4+ in close contact with Pt is essential for increasing the rate of propane dehydrogenation through enhancing H 2 desorption, which is the rate-limiting step. As the anchored Pt atoms/sub nanoclusters do not sinter under reducing conditions, the developed catalysts show high on-stream stability and propene selectivity of above 99 % under industrially relevant conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Highly dispersed VO x species showed high apparent TOF value but low C 3 H 6 selectivity. • VO x species in low-polyvanadate form are deemed to be more active and stable in PDH. • VO x -C 3 H 5 species was speculated to be the important intermedia during PDH. • H 2 formation is deemed as the rate-limiting step over as-prepared V-HMS. Vanadium was introduced to hexagonal mesoporous SiO 2 (HMS) to prepare supported vanadium-based catalysts (V-HMS). Their catalytic performance in the non-oxidative propane dehydrogenation to propylene (PDH) was modulated with V surface density varying from 0.12 to 15.08V atom·nm−2. VO x species with different structure were analyzed by UV–vis spectroscopy, NH 3 -TPD, H 2 -TPR and XPS. High propylene space time yield of 0.73 kg·h−1·kg cat −1 and excellent regeneration behavior were achieved. Correlating VO x polymerization degree with catalytic performance, VO x aggregates, mainly in low-polyvanadate form, were established to reveal high intrinsic activity and stability in comparison with their counterparts, including isolated or/and highly polymerized VO x as well as V 2 O 5. Temporal analysis of products (TAP) identified hydrogen formation as the rate-limiting step in the PDH reaction. In-situ DRIFTS was employed to investigate the reaction mechanism. V species containing C=C bond, i.e., VO x -C 3 H 5 , was proposed to be an intermediate of gas-phase propylene. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co/ZrO 2 catalysts with single Co2+ are efficient for C 3 H 8 dehydrogenation to C 3 H 6. • 1Co/ZrO 2 performed durable over multiple dehydrogenation/regeneration cycles. • 1Co/ZrO 2 showed higher activity than a commercial analogue of K-CrO x /Al 2 O 3. • The high activity results from a synergy effect between Co2+ and Zr cus. • Recombination of surface H species is the rate-limiting step in C 3 H 6 formation. Due to the increased production of shale gas containing propane, the share of non-oxidative propane dehydrogenation (PDH) in the large-scale production of propene is expected to continue to grow. There are, however, some ecofriendly and cost shortcomings associated with the currently applied Cr- or Pt-containing catalysts. Against this background, we present here Co/ZrO 2 alternatives and reveal the fundamentals affecting their activity, which can be used for purposeful catalyst design. Co2+ species homogeneously distributed within the lattice of ZrO 2 significantly increase the activity of coordinatively unsaturated Zr4+ for the PDH reaction. The increase is caused by accelerating both the cleavage of CH bonds in propane and the recombination of surface H species, with the latter reaction being the rate-limiting step. The best-performing catalyst outperformed an analogue of commercial K-CrO x /Al 2 O 3 in terms of the rate of propene formation and demonstrated durable performance over a series of 10 PDH/regeneration cycles under industrially relevant conditions. It also outperformed most previously developed Co-containing catalysts in terms of propene productivity. The space–time yield of propene formation achieved at 57 % equilibrium propane conversion at 550 °C was 0.71 kg·h−1·kg cat -1. [ABSTRACT FROM AUTHOR]