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Propane dehydrogenation (PDH) to propene is an important alternative to oil-based cracking processes, to produce this industrially important platform chemical.sup.1,2. The commercial PDH technologies utilizing Cr-containing (refs. .sup.3,4) or Pt-containing (refs. .sup.5-8) catalysts suffer from the toxicity of Cr(vi) compounds or the need to use ecologically harmful chlorine for catalyst regeneration.sup.9. Here, we introduce a method for preparation of environmentally compatible supported catalysts based on commercial ZnO. This metal oxide and a support (zeolite or common metal oxide) are used as a physical mixture or in the form of two layers with ZnO as the upstream layer. Supported ZnO.sub.x species are in situ formed through a reaction of support OH groups with Zn atoms generated from ZnO upon reductive treatment above 550 °C. Using different complementary characterization methods, we identify the decisive role of defective OH groups for the formation of active ZnO.sub.x species. For benchmarking purposes, the developed ZnO-silicalite-1 and an analogue of commercial K-CrO.sub.x/Al.sub.2O.sub.3 were tested in the same setup under industrially relevant conditions at close propane conversion over about 400 h on propane stream. The developed catalyst reveals about three times higher propene productivity at similar propene selectivity. Propene is obtained through propane dehydrogenation using catalysts that are toxic, expensive or demanding to regenerate with ecologically harmful compounds, but the ZnO-based alternative reported here is cheap, clean and scalable., Author(s): Dan Zhao [sup.1] [sup.2] , Xinxin Tian [sup.2] [sup.3] , Dmitry E. Doronkin [sup.4] , Shanlei Han [sup.1] [sup.2] , Vita A. Kondratenko [sup.2] , Jan-Dierk Grunwaldt [sup.4] , [...]

Propane dehydrogenation (PDH) to propene is an important alternative to oil-based cracking processes, to produce this industrially important platform chemical1,2. The commercial PDH technologies utilizing Cr-containing (refs. 3,4) or Pt-containing (refs. 5–8) catalysts suffer from the toxicity of Cr(vi) compounds or the need to use ecologically harmful chlorine for catalyst regeneration9. Here, we introduce a method for preparation of environmentally compatible supported catalysts based on commercial ZnO. This metal oxide and a support (zeolite or common metal oxide) are used as a physical mixture or in the form of two layers with ZnO as the upstream layer. Supported ZnOx species are in situ formed through a reaction of support OH groups with Zn atoms generated from ZnO upon reductive treatment above 550 °C. Using different complementary characterization methods, we identify the decisive role of defective OH groups for the formation of active ZnOx species. For benchmarking purposes, the developed ZnO–silicalite-1 and an analogue of commercial K–CrOx/Al2O3 were tested in the same setup under industrially relevant conditions at close propane conversion over about 400 h on propane stream. The developed catalyst reveals about three times higher propene productivity at similar propene selectivity.Propene is obtained through propane dehydrogenation using catalysts that are toxic, expensive or demanding to regenerate with ecologically harmful compounds, but the ZnO-based alternative reported here is cheap, clean and scalable. [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]

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.