Sample homogeneity on the microscopic scale is critical for the reliable interpretation of x-ray absorption spectra collected in transmission mode. Unfortunately, it is not always easy to ensure it in practice. Especially in operando studies of catalysts and functional materials, the microstructure of the sample can evolve during its operation and even become the key descriptor for understanding structure-property relationships of the material, as exemplified by the transformations taking place in technical iron-based catalysts for ammonia synthesis under operating conditions. Here we present a simple approach for the identification and quantification of the material's microgranular structure effect on its x-ray absorption spectrum. We demonstrate that the quantitative information on the sizes of microscopic sample particles can be extracted from the observed distortions in the x-ray absorption near-edge structure spectra. The obtained insight can also be used to correct for the artifacts in extended x-ray absorption fine structure fitting, associated with the presence of microscopic inhomogeneities in the sample.

Ammonia is industrially synthesized over multi-promoted Fe-based catalysts for more than a century. Although ammonia synthesis reflects a prototypical catalytic reaction, rational catalyst design is still impossible as the full structural complexity of this catalyst system often referred to as ammonia iron and its structural entanglement is barely understood. Here, the mesoscopic structure of a technical, multi-promoted ammonia synthesis catalyst is uncovered using a scale-bridging electron microscopy approach complemented by X-ray diffraction and spectroscopy to explore the structural integrity of ammonia iron. Amorphous contributions and structures of the melilite type and tricalcium aluminate as additional phases are identified. Furthermore, the understanding of the ammonia iron family by unveiling the role of the platelet-Fe perimeter, framework Fe, thin film Fe, and refractory Fe is extended. Their interconnectedness is highlighted, suggesting that each component has to be present to fulfill a specific task. The study demonstrates that catalysis science can only proceed if it openly explores the full complexity of catalytic systems.

Ammonia is industrially produced by the Haber-Bosch process over a fused, multi-promoted iron-based catalyst. Current knowledge about the reaction has been derived from model systems of reduced structural complexity, impeding any clear-cut structure-activity correlation relevant for the industrial counterpart. Here, we unveil the structural evolution of complex, technical, multi-promoted ammonia synthesis catalysts by operando scanning electron microscopy and near-ambient pressure X-ray photoelectron spectroscopy. We highlight that the activation is the critical step in which the catalyst is formed and decode the pivotal role of the promoters. We discover that the active structure consists of a nanodispersion of Fe covered by mobile K-containing adsorbates, so called "ammonia K". The porous catalyst is stabilized by mineral cementitious phases containing oxides of Al, Si, Ca, and Fe. The synergism between the different promoters contributes simultaneously to the structural stability, hierarchical architecture, catalytic activity, and poisoning resistance. The confluence of these aspects is the key for the superior performance of technical catalyst formulations.

Despite the Haber-Bosch process being more than 100 years old, only incremental improvements have been achieved until recently. Now, by combining the catalyst expertise of CLARIANT and the engineering knowledge of CASALE, a breakthrough has been realized. AmoMax®-Casale is a new ammonia synthesis catalyst jointly developed by Casale and Clariant particularly for use in Casale ammonia converters. AmoMax®-Casale is a customized evolution of the well-known, wustite-based catalyst, AmoMax® 10. While retaining the same superior resistance to ageing, poisoning and mechanical strength, AmoMax®-Casale is significantly more active. This feature allows to reduce the loop recycle rate and the loop pressure and/or to increase the ammonia production. The higher activity of AmoMax®-Casale contributes to improve the overall operating efficiency either by saving energy, or by increasing significantly the plant capacity. This article will describe in detail the successful development of AmoMax®-Casale, explain advantages and commercial benefits based on concrete plant simulations and share the start-up experience of the first commercial reference.

Ammonia synthesis remains one of the most important catalytic processes since it enables efficient hydrogen storage and provides the basis for the production of fertilizers. Herein, complementary bulk and local analytical techniques were combined to investigate the effect of selected promoters (Al, K, Ca) on the reduction of wuestite into alpha-iron and their catalytic performance for ammonia synthesis. The use of promoters appears to have a positive effect on the wuestite-derived catalyst in ammonia synthesis. The promoters seemingly act as a binder for wuestite grains and impede the reduction and disproportionation events of wuestite precursors resulting in an increased catalytic performance. This effect is associated with an increase of surface area and mesoporosity. The study delivers new insights into the interplay of structure and promoters in wuestite-based catalysts.

The hydroisomerization of two long-chain n-alkane mixtures was investigated over bifunctional Pt/H-ZSM-5 catalysts before and after dealumination of preshaped zeolite/binder pellets. The hydroisomerization over the dealuminated catalysts leads to more isomers and less cracking products. Consequently, higher ratios of multi- to mono-branched isomerization products are formed as expected for large- rather than medium-pore zeolites. This indicates a higher availability of space in the vicinity of the active sites and provides an attractive route to make medium-pore zeolites suitable for upgrading higher boiling hydrocarbon feeds.

The hydroisomerization of a C-10-C-13 n-alkane mixture was investigated over six bifunctional platinum-containing zeolite catalysts with 10-and 12-membered ring pore systems. The catalysts with 12-membered-ring zeolites are more active than those with 10-membered-ring zeolites. In Contrast, higher yields of the desired mono-branched isomers are obtained on the 10-membered-ring zeolites due to shape-selectivity effects. For the 10-membered-ring zeolites, higher reaction temperatures are required for achieving the maximum isomer yields, which leads to more cracking products compared to the 12-membered-ring zeolites.

The hydroisomerization of a C-10-C(13)n-alkane mixture was investigated over six bifunctional platinum-containing zeolite catalysts with 10- and 12-membered-ring pore systems. The catalysts with 12-membered-ring zeolites are more active than those with 10-membered-ring zeolites. In contrast, higher yields of the desired monobranched isomers are obtained over the 10-membered-ring zeolites due to shape-selectivity effects. For the 10-membered-ring zeolites, higher reaction temperatures are required for achieving the maximum isomer yields, which leads to more cracking products compared to the 12-membered-ring zeolites.

A model for the phosphorus dynamics in vanadium-phosphorus oxide (VPO) catalysts for the oxidation of n-butane to maleic anhydride was developed. According to the model, reversible sorption processes determine the phosphorus content of the catalyst. Simulations reveal that several phenomena can be successfully described. If no phosphorus is added to the reactant feed, the catalytic activity increases until runaway occurs. With addition of a proper amount of phosphorus, the loss can be compensated while excessive phosphorus addition results in complete catalyst deactivation. Adjusting the model parameters to experimental data may result in a model that can be used to optimize the performance of maleic anhydride reactors.

Vanadia/titania catalyst particles were made by flame-spray pyrolysis and deposited onto ceramic sponge monoliths either by direct deposition of the flame-made particles or by a dip-coating technique. In the partial oxidation of o-xylene, the influence of the coating thickness and porosity on the catalytic performance was investigated. It was found that the highly porous coatings obtained by direct deposition exhibit insufficient heat transfer properties, while dip-coated layers are prone to internal mass transfer limitations if a certain thickness of the coating is exceeded. In the absence of transport limitations, kinetic experiments were carried out to derive a reaction network and to develop a quantitative kinetic model. The resulting model describes well the influences of reactant concentrations and temperature on the product distributions in the oxidation of o-xylene to phthalic anhydride over the novel flame-made catalyst and can be used for reactor simulations.

Layered double hydroxides (LDHs) and derived mixed oxides with different Mg/Al/Fe contents were investigated. Two super-saturation precipitation methods were used for the synthesis of LDHs with general formula [Mg1−x M(III)x (OH)2](CO3)x/2⋅mH2O where M(III) presents Al and/or Fe. The content of trivalent ions x = M(III)/[M(II) + M(III)], was varied between 0.15 < x < 0.7. Such a wide range of trivalent ions was chosen with the aim to induce the formation of different multiphase mixed oxides. Iron was introduced as constituent metal in order to obtain redox properties. LDHs and their derived mixed oxides were characterized with respect to their crystalline structure (XRD), thermal stability (TG/DTA), textural (N2 adsorption), redox (H2 TPR) and acid properties (NH3 TPD) as well as the nature of the iron species (Mössbauer spectroscopy). Catalytic behavior was studied in two test reactions: N2O decomposition and reduction with NH3. It has been demonstrated that extended M(III) substitution influences the structure and surface properties of Mg–Al–Fe LDHs and derived mixed oxides, weakens Mg–Al–Fe–O interactions and improves catalytic behavior correlated with the presence of Fe–O–Fe–O–Fe entities providing possibility for facilitated extraction of oxygen with simultaneous redox Fe3+ Fe2+ conversion. The catalytic behavior is mainly determined by redox properties, nature of iron species in mixed oxides and by structural properties of initial LDHs. The best catalytic results were obtained when the amount of M(III) was near the limit for the incorporation into LDH matrix.

Various ZSM-5 zeolites with iron contents ranging from of 0.86 to 4.98 and Si/Al ratios of 11.5–140 were prepared by solid-state ion exchange with FeCl2. The catalysts were characterised by XRD, H2-TPR and NH3-TPD. The formation and stability of surface oxygen over these zeolites were investigated with a transient multipulse technique combined with subsequent temperature-programmed desorption (TPD). Higher iron contents enhance the formation of surface oxygen. However, when a critical iron content is reached, larger iron oxide clusters and particles are formed and these larger clusters do not contribute significantly to surface oxygen formation. Moreover, the amount of desorbed oxygen is more than that formed from N2O due to the additional desorption of oxygen present in the lattice of such oxide clusters. TPD studies indicate the presence of different surface oxygen species depends on both the zeolite iron content and Si/Al ratio.

Layered double hydroxides (LDHs) and their thermally derived mixed oxides have reached growing attention in past decades due to their wide application as catalysts or catalyst supports in organic/pharmaceutical synthesis, clean energy and environmental pollution control (decomposition of volatile organic compounds, photodecomposition, DeNox and DeSOx). Desired properties of LDHs can easily be tailored using different synthesis methods and introducing different bivalent and trivalent constituting metals. In this study, Mg–Al and Mg–Al–Fe LDHs were synthesized by low supersaturation (LS) and high supersaturation (HS) coprecipitation methods. The content of trivalent ions was varied in a wide range between 0.15 < x < 0.7 exceeding the optimal range for the single LDH phase synthesis (0.20 < x < 0.33). The intention was to induce the formation of different LDHs and consequently obtain, after thermal treatment, different multiphase mixed oxides. The properties of the precipitates were investigated by structural (XRD), chemical (AAS and EDS) and thermal analysis (TG–DTA). The study revealed that the LS method allows the formation of LDHs with an extended M(III) substitution (x = 0.5). Although, a more disordered structure in the stacking of layers was detected for HS samples, LS samples with the same initial composition showed lower thermal stability estimated by lower temperature of both LDH thermal decomposition transition stages. The thermal stability of LDHs was not influenced considerably with the introduction of a small amount of iron as ternary metal even though lower crystallinity of Mg–Al–Fe LDHs was observed.

The natural zeolite obtained from the Sivas-Yavu region in Turkey and iron modified forms were studied for the decomposition of N2O and selective catalytic reduction of N2O with NH3. The natural and iron modified zeolites were characterised by XRD, SEM, H2-TPR, NH3-TPD and low temperature nitrogen sorption. The effect iron loading, precursor and valency on the catalytic performance of catalysts were studied. The catalytic activity of the zeolites increased up to about 7.0wt.% Fe. Above this value, the activity decreased as a result of a reduction in the surface area and pore volume of the zeolite. The highest catalytic activity was observed using catalysts prepared with FeCl2 due to the formation of more reducible iron species in the zeolites. When FeSO4 was used as the iron precursor, sulphate remained on the surface even after extensive washing resulting in a decrease in the N2O decomposition activity and a shift the N2O reduction temperature to higher values. Since the natural and iron exchanged natural zeolites prepared using FeCl2 have comparable activity with synthetic zeolites, the offer a promising alternative catalyst for the abatement of N2O, particularly for the selective reduction of N2O with NH3.

The vapor phase epoxidation of propene with nitrous oxide (N2O) was experimentally investigated in a fixed bed reactor using different Cs-x/Fe-y/SiO2 catalysts. This was done with a systematic approach which comprises the derivation of kinetic parameters for directly comparing catalyst performance. Therefore, kinetic measurements were made for each catalyst by variation of the residence time. It was found that the addition of an alkali promoter to the Fe-y/SiO2 catalyst is essential for the formation of propylene oxide and a proper alkali/Fe molar ratio is crucial for both activity and selectivity. Maxima in both activity and selectivity were observed for alkali/Fe ratios in the region 1.2-1.7. A further increase in activity without any loss in selectivity was obtained by adjusting the calcination temperature to 783 K. The conversion of PO was used as a tool to measure the product stability and a minimum reaction rate was also found for alkali/Fe ratios in the region 1.2-1.7. The promoter is responsible for the formation of active centers and it reduces surface acidity which leads to an increased stability of PO through the inhibition of the consecutive conversion. Maximum selectivities to PO of about 40% at 5-10% conversion were achieved at moderate reaction temperatures of 648 K. Because of parallel and consecutive formation of carbonaceous deposits on the catalyst, the catalyst deactivated within 2 h of operation to a remaining activity of around 40%. Neglecting the carbonaceous deposits as a reaction product and considering only the vapor phase products, PO selectivity is more than 75% at 5-10% propene conversion. The attempt to slow down the deactivation through the addition Of Supplementary gases (H-2, O-2, NH3, H2O) was partially Successful, but unfortunately this is always accompanied by lower PO selectivity.

Vanadia/titania particles with a specific surface area (SSA) up to 195 m2 g−1 and a V2O5 content up to 40% (w/w) or V coverage up to 59 V nm−2 were prepared by flame spray pyrolysis (FSP) under various conditions. The catalysts were characterized by nitrogen adsorption, X-ray diffraction, temperature-programmed reduction, and in situ Raman spectroscopy and tested for partial oxidation of o-xylene. Depending on vanadia content, monomeric, polymeric, and crystalline vanadia species were formed on TiO2 support particles by FSP. Increasing the high-temperature particle residence time and concentration (production rate) during FSP reduced the SSA and increased the vanadia coverage of TiO2 beyond a theoretical “monolayer” (>8–10 V nm−2) while retaining amorphous (monomeric and polymeric) VOx surface species. Controlling liquid precursor and dispersion gas feed rates, precursor concentration, and V2O5 content allowed the tailoring of SSA and the population of the different VOx species in these vanadia/titania mixed oxides. For comparison, vanadia/titania catalysts containing 10% (w/w) V2O5 with comparable SSA and V coverage were prepared by impregnation, resulting in typical amorphous (<10 V nm−2) and crystalline (>10 V nm−2) VOx species. Catalysts containing 7, 10, and 20% (w/w) V2O5 were deposited directly from the flame on ceramic foams that were tested for the partial oxidation of o-xylene to phthalic anhydride. The global activity of flame-made and conventionally impregnated catalysts depended mainly on SSA and vanadia loading (number of V surface sites), whereas the amorphous or crystalline nature of the VOx species seemed to be less critical. In contrast, selectivity to phthalic anhydride was significantly affected by the nature of the VOx species; amorphous species exhibited higher selectivity for conversion <90% compared with catalysts containing crystalline V2O5.

Chemie Ingenieur TechnikVolume 80, Issue 9 p. 1263-1263 PosterFree Access Epoxidierung von Propen mit N2O – Reaktionsbedingungen, Katalysator- zusammensetzung und ihr Einfluss auf die Performance T. Thömmes Dipl.-Ing., T. Thömmes Dipl.-Ing. thomas.thoemmes@cvt.uni-karlsruhe.de Institut für Chemische Verfahrenstechnik, Universität Karlsruhe (TH), Kaiserstraße 12, D-76128 KarlsruheSearch for more papers by this authorA. Reitzmann Dr.-Ing., A. Reitzmann Dr.-Ing. Süd-Chemie AG, Waldheimer Straße 15, D-83052 BruckmühlSearch for more papers by this authorB. Kraushaar-Czarnetzki Prof. Dr., B. Kraushaar-Czarnetzki Prof. Dr. Institut für Chemische Verfahrenstechnik, Universität Karlsruhe (TH), Kaiserstraße 12, D-76128 KarlsruheSearch for more papers by this author T. Thömmes Dipl.-Ing., T. Thömmes Dipl.-Ing. thomas.thoemmes@cvt.uni-karlsruhe.de Institut für Chemische Verfahrenstechnik, Universität Karlsruhe (TH), Kaiserstraße 12, D-76128 KarlsruheSearch for more papers by this authorA. Reitzmann Dr.-Ing., A. Reitzmann Dr.-Ing. Süd-Chemie AG, Waldheimer Straße 15, D-83052 BruckmühlSearch for more papers by this authorB. Kraushaar-Czarnetzki Prof. Dr., B. Kraushaar-Czarnetzki Prof. Dr. Institut für Chemische Verfahrenstechnik, Universität Karlsruhe (TH), Kaiserstraße 12, D-76128 KarlsruheSearch for more papers by this author First published: 12 September 2008 https://doi.org/10.1002/cite.200750605AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume80, Issue9Special Issue: ProcessNet Jahrestagung 2008September, 2008Pages 1263-1263 ReferencesRelatedInformation