Activity Of Such Catalysts Research Articles

Elevating hydrogen evolution performance with plasma-induced sulfur vacancies and heteroatom doping in hollow-structured MnS–CoS catalysts

Transition-metal sulfides doped with various elements show great promise as highly effective catalysts for the hydrogen evolution reaction (HER), potentially serving as alternatives to Pt-based electrocatalysts. Defect engineering has been identified as a key strategy to enhance the HER activity of such catalysts. This study proposes an effective strategy to generate numerous topological/geometric defects on hollow spherical Mn-doped cobalt sulfide (MnS–CoS) via plasma etching. MnS–CoS catalysts with well-defined cavity structures were successfully prepared using a combination of template-assisted and hydrothermal synthesis methods. Under plasma induction, these catalysts exhibited uniformly formed nanocavity structures with a built-in electric field, which uniformly enhanced their topological/geometric defect levels. The plasma-etched MnS–CoS catalyst demonstrated significantly enhanced HER activity, exhibiting an excellent overpotential of 172 mV at a current density of 10 mA·cm⁻2 in alkaline media and a low Tafel slope of 120 mV·dec⁻¹. These improvements can be attributed to the remarkable conductivity of Co and Mn, along with the unique hollow spherical nanostructure of the material, which contained numerous gaps that promoted electron movement and electrolyte penetration, thereby enhancing the overall catalytic performance. The synergistic effects of heteroatom doping and plasma modulation played a crucial role in achieving outstanding catalytic performance toward the HER. Density-functional theory calculations based on a structural model of MnS–CoS-p confirmed that the introduction of defects and heteroatoms reduced hydrogen adsorption: the Gibbs free energy decreased from 0.72 eV for CoS to 0.23 eV for MnS–CoS-p. The excellent HER activity of the MnS–CoS materials stemmed from the abundance of reactive heterostructured interfaces, which optimized the electronic configuration and, thus, accelerated H2O adsorption and dissociation. This study highlights the potential of combining plasma-assisted defect manipulation and transition-metal-element doping to design high-performance catalysts. The findings also offer valuable insights into the development of environment-friendly energy-conversion technologies.

Structural andelectronic characterization offluorine-doped La0.5Sr0.5CoO3-δ using electron energy-loss spectroscopy.

Perovskite oxides, ABO3, are potential catalysts for the oxygen evolution reaction, which is important in the production of hydrogen as a sustainable energy resource. Optimizing the chemical composition of such oxides by substitution or doping with additional elements is an effective approach to improving the activity of such catalysts. Here, we characterized the crystal and electronic structures of fluorine-doped La0.5Sr0.5CoO3-δ particles using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM imaging demonstrated the formation of a disordered surface phase caused by fluorine doping. In addition, spatially resolved EELS data showed that fluorine anions were introduced into the interiors of the particles and that Co ions near the surfaces were slightly reduced by fluorine doping in conjunction with the loss of oxygen ions. Peak fitting of energy-loss near-edge structure data demonstrated an unexpected nanostructure in the vicinity of the surface. An EELS characterization comprising elemental mapping together with an energy-loss near-edge structure analysis indicated that this nanostructure could not be assigned to Co-based materials but rather to the solid electrolyte BaF2. Complementary structural and electronic characterizations using STEM and EELS as demonstrated herein evidently have the potential to play an increasingly important role in elucidating the nanostructures of functional materials.

Deactivation of Cu SCR catalysts based on small-pore SSZ-13 zeolites: A review

Increasingly stringent regulation towards nitrogen oxides (NOx) emission is foreseen across the world to mitigate their detrimental effects on environment and human health. Selective catalytic reduction (SCR) of NOx with NH3 is regarded as the most efficient technology for NOx abatement in both mobile and stationary sources. As the state-of-art SCR catalyst, transition metals supported on small-pore zeolites, especially Cu on chabazite SSZ-13 (Cu-SSZ-13), have been successfully applied to NOx abatement in road transport. Their application can be expanded to stationary sources due to the gradual phase-out of petrol/diesel vehicles worldwide between 2030 and 2035. Accordingly, the stability and activity of such catalysts need to be revisited and tuned to accommodate the complex operating conditions in various industries such as wider temperature windows and the more complex compositions of the exhausts. This review focuses on the poisoning and deactivation of zeolite-based SCR catalysts, especially Cu-SSZ-13. State-of-the-art of different mechanisms of deactivation (including hydrothermal ageing, sulphur poisoning, hydrocarbon poisoning and chemical poisoning) are discussed critically, attempting to establish the link between current zeolite-based SCR catalysts and the possible application demands in industries.

CeO2 Supported Gold Nanocluster Catalysts for CO Oxidation: Surface Evolution Influenced by the Ligand Shell.

Monolayer protected Au nanocluster catalysts are known to undergo structural changes during catalytic reactions, including dissociation and migration of ligands onto the support, which strongly affects their activity and stability. To better understand how the nature of ligands influences the catalytic activity of such catalysts, three types of ceria supported Au nanoclusters with different kinds of ligands (thiolates, phosphines and a mixture thereof) have been studied, employing CO oxidation as model reaction. The thiolate‐protected Au25/CeO2 showed significantly higher CO conversion after activation at 250 °C than the cluster catalysts possessing phosphine ligands. Temperature programmed oxidation and in situ infrared spectroscopy revealed that while the phosphine ligands seemed to decompose and free Au surface was exposed, temperatures higher than 250 °C are required to efficiently remove them from the whole catalyst system. Moreover, the presence of residues on the support seemed to have much greater influence on the reactivity than the gold particle size.

REGULARITIES OF HYDROGEN GENERATION BY HYDROLYSIS OF SODIUM BOROHYDRIDE ON IMMOBILIZED PLATINUM CATALYSTS

Sodium borohydride hydrolysisis one of the most productive hydrogengeneration methods, which can be used, inparticular, to power fuel cells with a hydrogenanode. In alkaline solutions, the interaction of NaBH4 with water practically does not occur, which makes it possible to store such solutions for a longtime. However, in the presence of catalysts, the borohydride hydrolysis reaction actively proceeds at room temperature with the formation of hydrogenand sodium metaborate. More than 300 homogeneous and heterogeneous catalysts for the hydrolysis of sodium borohydride are known, among which the most active metal catalysts are nanodispersed rhodium, ruthenium, and platinum immobilized on various substrates. The activity of such catalysts depends on the conditions of the reduction and immobilization of metals, the nature of used precursors, the amount of active phase of the catalyst, and the type of substrate. In this work, platinum catalysts immobilized on several substrates are developed, and the regularities of hydrolysis of sodium borohydride in alkaline solution when using such catalysts in flowing flat and cylindrical reactors are investigated. Using various methods, platinum was deposited on carbon black and carbon cloth, onactivated granular carbon, on titanium crumb, and on synthetic cordierite of honey combstructure with surface, previously modified by alumina. It is shown that nanodisperse platinum catalysts immobilized on carbon black and carbon cloth and especially on synthetic cordierite of cellular structure with surface previously modified by Al2O3 layer are active and reliable. The average rate of hydrogen evolution during hydrolysis of NaBH4 on such catalysts increases with increasing flow rate of its solution through the reactor, but the degree of conversion of sodium borohydride decreases due to the reduced duration of contact of the solution with the catalyst. Nanodisperse platinum catalysts on surface-modified cordierite provide a highandstable rate of hydrogengeneration with moderate heating of NaBH4 solution (60±5°C). The use of hydrogen generators with such catalysts in combination with fuel cell batteries is promising for the creation of autonomous powers ources.

High‐entropy alloys and compounds for electrocatalytic energy conversion applications

AbstractHigh‐entropy materials, composed of five or more elements in near‐equiatomic percentage, have been attracting tremendous interests due to their advantageous properties in a variety of applications. Recently, electrocatalysis on high‐entropy alloys (HEAs) and high‐entropy compounds (HECs) has emerged as a new and promising material owing to the tailored composition and the disordered configuration of HEAs and HECs. Though extensive efforts have been devoted to investigating the catalytic nature of HEAs and HECs, the details related to the active sites and intrinsic activity of such catalysts still remain uncertain due to the complexity of the multicomponent systems. In this review, the recent progress of HEAs and HECs is systematically reviewed in terms of their synthetic strategies and electrocatalytic applications. Importantly, the computationally assisted methods (e.g., density functional theory [DFT]) are also presented to discover and design the optimum HEA‐ and HEC‐based catalysts. Subsequently, the applications of HEAs and HECs in electrocatalytic energy conversion reactions will be discussed, including hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, methanol oxidation reaction, and ethanol oxidation reaction (EOR). Moreover, the prospects and future opportunities for this research field are cautiously discussed. A series of upcoming challenges and questions are thoroughly proposed from the experimental and theoretical aspects as well as other future applications in electrocatalysis.

Highly Active Catalysts for the Dehydration of Isopropanol

Due to the high costs and low selectivity associated with the production of propylene, new routes for its synthesis are being sought. Dehydration has been widely investigated in this field, but, thus far, no study has produced efficient results for isopropanol. Vanadium-zirconia catalysts have been shown to be effective for the dehydration of ethanol. Therefore, we investigated the activity of such catalysts in the dehydration of isopropanol. The catalysts were synthetized on a SBA-15 base, supplemented with zirconia or combined zirconia and vanadium. Tests were conducted in a continuous flow reactor at 150–300 °C. Samples were analyzed using a gas chromatograph. The most active catalyst showed 96% conversion with 100% selectivity to propylene. XRD, SEM and Raman spectroscopy analyses revealed that as the vanadium content increases, the pore size of the catalyst decreases and both isopropanol conversion and propylene selectivity are reduced. Thus, without the addition of vanadium, the Zr-SBA-15 catalyst appears to be suitable for the dehydration of isopropanol to propylene.

In operando studies of CO oxidation on epitaxial SrCoO2.5+δ thin films

The high abundance and fast kinetics of select transition metal oxide catalysts are attractive features for many chemical and electrochemical device applications. However, the activity of such catalysts can be accompanied by phase instabilities that prevent their widespread usage. Furthermore, complexities associated with variations in phase behavior and oxygen stoichiometry have hindered studies on the true origins of catalytic activity. Here, we explore the interactions between activity, phase stability, and microstructure using in operando synchrotron X-ray techniques and gas chromatography/mass spectroscopy (GCMS) to probe the behavior of model SrCoO2.5+δ (SCO) catalysts. Pulsed laser deposition was used to prepare SCO thin films on (001) SrTiO3, (111) SrTiO3, and pseudocubic (001) DyScO3 substrates. The GCMS catalytic measurements were performed with a custom-built microreactor compatible with a synchrotron X-ray diffractometer at the Advanced Photon Source. The activity for carbon monoxide oxidation was determined as a function of temperature from 500 °C to 800 °C. We show that the SrCoO2.5+δ films are active for CO oxidation, most likely by direct reaction with lattice oxygen; consequently, the activity was observed to increase as the films become less stable, with the most active film being the one exhibiting the lowest surface and crystal quality. All films decompose at high temperatures, with in operando diffraction indicating the gradual formation of Sr-rich hexagonal and CoO phases. We find that real-time studies of model oxide systems with synchrotron X-rays is a powerful means of gaining insight into the varied processes taking place at catalytic surfaces.

Hydrogen Activation by Silica-Supported Metal Ion Catalysts: Catalytic Properties of Metals and Performance of DFT Functionals.

Single-site heterogeneous catalysts (SSHC) have received increasing attention due to their well-defined active sites and potentially high specific activity. Detailed computational studies were carried out on a set of potential SSHC's, i.e., silica-supported metal ions, to investigate the reactivity of these catalysts with H2 as well as to evaluate the performance of density functional theory (DFT) methods in conjunction with triple-ζ quality basis sets (i.e., cc-pVTZ) on reaction energetics. The ions considered include 4d and 5d metals as well as several post-transition metal ions. A representative cluster model of silica is used to calculate reaction free energies of the metal hydride formation that results from the heterolytic cleavage of H2 on the M-O bond. The hydride formation free energy is previously shown to be strongly correlated with the catalytic activity of such catalysts for alkene hydrogenation. ONIOM calculations (CCSD(T)//MP2) are used to assess the accuracy and reliability of the MP2 results and it is found that MP2 is a suitable level of theory for gauging the performance of DFT functionals. The performance of various DFT functionals is assessed relative to MP2 results and it is found that the wB97xd and PBE0 functionals have the lowest standard deviation (STD) value while the MN12SX and PBE functionals have the lowest mean absolute deviation (MAD) values. The B3LYP functional is shown to have similar MAD and STD values as the top performing functionals. Potential active SSHC's for exergonic hydrogen activation predicted in this study include mostly late and post transition metal ions, i.e., Au3+, Pd2+, Pt4+, Pd4+, Ir4+, Hg2+, Rh3+, Pb4+, Tl3+, In3+, Ir3+, Os4+, Cd2+, Ru2+, and Ga3+. This study provides important guidance to future computational studies of such catalyst systems.

Highly Active Low Cobalt Content-Based Bulk MoS2 Hydrodesulfurization Catalysts with a Unique Impact of H2S

A series of unsupported MoS2, Co9S8, and Co-promoted MoS2 catalysts have been synthesized by tuned impregnation and successive thermal annealing methods using a continuous flow of a mixture of H2 and H2S gases. The resulting catalysts were evaluated in terms of their activity and selectivity for the hydrodesulfurization of dibenzothiophene (DBT) both in the absence and the presence of H2S. The inclusion of Co onto MoS2 affected both the hydrogenation and direct desulfurization reactions, with the latter (production of biphenyl) being magnified to a much greater degree than the former. Interestingly, low cobalt/molybdenum ratio of ca. 0.05 of the catalyst exhibited outstanding promotion efficiency in the hydrodesulfurization reaction. However, as cobalt is added, the synergy effect drastically decreased. H2S in the reaction mixture led to a remarkable step up in the product from the direct desulfurization reaction route with the most notable increases occurring for the product from the hydrogenation reaction pathway. The HDS activity of such catalysts was much higher than that of the commercial CoMo/Al2O3. The promotion by H2S was discussed.

Pd core-shell alloy catalysts for high-temperature polymer electrolyte membrane fuel cells: Effect of the core composition on the activity towards oxygen reduction reactions

Pd-based core-shell alloy-supported catalysts were prepared sequentially via a microwave-assisted polyol method and galvanic replacement. To investigate the effect of the core composition on the catalytic activity of such catalysts, three different Pd alloy cores (PdNi, PdCu, and PdNiCu) were prepared on carbon supports using a polyol method. Then, Pd and Ir were introduced simultaneously to form shells on the Pd alloy cores by galvanic replacement in aqueous solution, thereby producing catalysts designated as PdNi@PdIr/C, PdCu@PdIr/C, and PdNiCu@PdIr/C. X-ray diffraction revealed that all three catalysts exhibited the face-centered cubic structure of Pd without the presence of individual phases for Ni, Cu, and Ir. The core-shell structure of the Pd-based alloy nanoparticles on the carbon support was verified by the electron energy loss spectroscopy line profile of a 25 nm nanoparticle of PdNiCu@PdIr/C. Among the three Pd-based core-shell catalysts, the highest electrochemical surface area and oxygen reduction reaction (ORR) activity was observed for PdNiCu@PdIr/C. In addition, the membrane electrode assembly employing the PdNiCu@PdIr/C catalyst displayed a significantly improved voltage compared to the other two catalysts under high-temperature polymer electrolyte membrane fuel cell conditions at 150 °C. Single-cell durability tests conducted to measure the voltage change at a constant current density of 0.2 A cm−2 showed a decay ratio of 12.3 μV h−1. These results suggest that the composition of the core in core-shell nanoparticles has an important influence on both the electronic properties in the Pd alloy core and compressive lattice strain on the PdIr shell. Control of these synergistic effects provides a new approach for developing catalysts with high ORR activity.

ФАЗОВЫЙ СОСТАВ И МОРФОЛОГИЯ ПОВЕРХНОСТИ Ni,Zn СПЛАВА, ЭЛЕКТРООСАЖДЕННОГО ИЗ СУЛЬФАТНО АММОНИЙНОГО РАСТВОРА

Хронопотенциометрическим и рентгенофазовым анализом показано, что никель-цинковое покрытие толщиной 1.1 мкм, осажденное на гладкую никелевую пластину в потенциостатических условиях Ek = -1.050 В из сульфатно-аммонийного электролита, содержит интерметаллические фазы NiZn и NiZn3. При помощи сканирующей электронной микроскопии установлено, что Ni,Zn-сплав осаждается равномерно по всей поверхности электрода, и молярное соотношение Ni и Zn составляет 1:4. По данным атомно-силовой микроскопии размер частиц дисперсного никеля составляет 40‑60 нм.
 
 Результаты исследований получены на оборудовании Центра коллективного пользования научным оборудованием ВГУ

Gold Doping in PtCu3/HSAC Nanoparticles and Their Morphological, Structural, and Compositional Changes during Oxygen Reduction Reaction Electrochemical Cycling

AbstractPt‐based nanoparticles supported on high‐surface‐area carbon (HSAC) show very good properties as catalysts in polymer electrolyte membrane fuel cells (PEMFC). In many cases, however, the initial high activity of such catalysts rapidly drops as caused by various detrimental phenomena occurring in a typical electrochemical environment. In this work, a detailed study of a highly active system composed of PtCu3/HSAC nanoparticles with partially ordered structure and Pt skin with the addition of Au is performed. By using aberration‐corrected scanning transmission electron microscopy, the effects of adding small amounts of gold compared to the nonmodified sample are followed through the different stages of an electrochemical cycling degradation protocol. Various morphological changes such as faceting, reshaping, and thickening of the Pt skin are investigated for both sets of samples on the atomic level. Interesting features such as well‐defined shapes and surface defects are observed after degradation. However, the most important differences in terms of durability seem related to particle porosity. Finally, the experimental morphological observations are successfully reproduced by dealloying simulation with kinetic Monte Carlo modeling.

Visible light driven mineralization of spiramycin over photostructured N-doped TiO2 on up conversion phosphors

A novel visible light-active photocatalyst formulation (NdT/OP) was obtained by supporting N-doped TiO2 (NdT) particles on up-conversion luminescent organic phosphors (OP). The photocatalytic activity of such catalysts was evaluated for the mineralization process of spiramycin in aqueous solution. The effect of NdT loading in the range 15–60wt.% on bulk and surface characteristics of NdT/OP catalysts was investigated by several chemico-physical characterization techniques. The photocatalytic performance of NdT/OP catalysts in the removal of spyramicin from aqueous solution was assessed through photocatalytic tests under visible light irradiation. Total organic carbon (TOC) of aqueous solution, and CO and CO2 gas concentrations evolved during the photodegradation were analyzed. A dramatic enhancement of photocatalytic activity of the photostructured visible active NdT/OP catalysts, compared to NdT catalyst, was observed. Only CO2 was detected in gas-phase during visible light irradiation, proving that the photocatalytic process is effective in the mineralization of spiramycin, reaching very high values of TOC removal. The photocatalyst NdT/OP at 30wt.% of NdT loading showed the highest photocatalytic activity (58% of TOC removed after 180min irradiation against only 31% removal after 300min of irradiation of NdT). We attribute this enhanced activity to the high effectiveness in the utilization of visible light through improved light harvesting and exploiting. OP particles act as “photoactive support”, able to be excited by the external visible light irradiation, and reissue luminescence of wavelength suitable to promote NdT photomineralization activity.

Non-PGM ORR Catalysts Prepared from Polyaniline-Type Polymers with Strong Affinity to Iron

Platinum-based catalysts play an important role in electrocatalysis of the generally sluggish oxygen reduction reaction (ORR) at the polymer electrolyte fuel cell (PEFC) cathode. However, the expensive Pt-based catalysts contribute to more than 40% of the cost of the PEFC stack for automotive applications, constraining the commercialization of fuel cell power systems.1 To lower the fuel cell catalyst cost, non-platinum group (non-PGM) catalysts have drawn attention as a promising low-cost replacement for Pt-based ORR catalysts.2The development of non-PGM catalysts have been the focus of significant research effort for years, aimed at obtaining active and durable materials, typically via the heat treatment of N-containing organic compounds and transition metal salts in inert atmosphere. One of the keys to the ultimate success of those efforts is the selection of suitable nitrogen precursors for the synthesis. In this presentation, we will demonstrate a rational design of polyaniline-type (PANI-type) polymers, as the precursors for active and durable non-PGM catalysts for oxygen reduction. Recently, we have developed well-performing non-PGM catalysts by heat-treating PANI, Fe salts, and carbon in N2 atmosphere.3 The studies of this and similar non-PGM catalysts suggest that Fe-N defects in a carbon matrix may be vital for the ORR activity of such catalysts.4 If so, it is essential to promote the formation of Fe-N bonds during the catalyst synthesis. However, current approaches are still quite rudimentary, largely limited to screening of N-rich precursors and optimizing the Fe salt loading. The rationale behind selecting N-rich precursors is still lacking. In many cases, the N-containing groups in polymers, such as PANI, have poor affinity to Fe salts limiting Fe-N bond formation in the polymer precursor before the heat treatment. An approach to increasing the population of such bonds will be given in this presentation. The subject of this study are PANI-type polymers with N-containing side groups having high affinity to Fe. By this approach, we allow Fe-N bonds to form evenly in the precursor polymers before the subsequent heat treatment. A higher Fe-N bond content is observed compared to previously used PANI-Fe precursors. Following the precursor synthesis, the polymers are converted into a carbon-based non-PGM catalyst via a heat treatment under N2. The catalyst undergoes further characterization by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy, followed by electrochemical characterization and fuel cell testing. The efficiency of the Fe uptake is also determined. This new approach paves the way to rational synthesis of non-PGM ORR catalysts via a rational design of polymer precursors with strong Fe-N interaction, ultimately resulting in a higher ORR active sites. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. References Spendelow, J.; Marcinkoski, J., DOE Fuel Cell Technologies Office, Fuel Cell System Cost-2014, (2014).Wu, G.; Zelenay, P., Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 46, 1878-1889 (2013).Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 332, 443-447 (2011).Holby, E. F.; Wu, G.; Zelenay, P.; Taylor, C. D., Structure of Fe–Nx–C Defects in Oxygen Reduction Reaction Catalysts from First-Principles Modeling. J. Phys. Chem. C 118, 14388-14393 (2014).

Evolution of self-sustained kinetic oscillations in the catalytic oxidation of propane over a nickel foil

The evolution of self-sustained reaction-rate oscillations in the catalytic oxidation of propane over a nickel foil has been studied in situ using X-ray photoelectron spectroscopy coupled with online mass spectrometry and gas chromatography. Changes in the effective surface area and in the catalyst morphology under reaction conditions have been examined by scanning electron microscopy and a krypton adsorption technique. It is shown that the regular kinetic oscillations arise under oxygen-lean conditions. CO, CO2, H2, H2O, and propylene are detected as products. The conversion of propane oscillates in a range from 1% to 23%. During the half-periods with high activity, the main reaction pathway is the partial oxidation of propane: selectivity toward CO achieves 98%. In contrast, during the half-periods with low activity, the reaction proceeds through three competitive pathways: the partial oxidation of propane, the total oxidation of propane, and the dehydrogenation of propane to propylene. The driving force for the self-sustained kinetic oscillations is the periodic reoxidation of nickel. According to the Ni2p and O1s core-level spectra measured in situ, the high-active catalyst surface is represented by metallic nickel, whereas during the inactive half-periods the catalyst surface is covered with a thick layer of NiO. The intensity of O1s spectra follows the oscillations of O2 in the gas phase during the oxidation of propane. It is found that during the induction period before the regular oscillations appear, a rough and porous structure develops because of strong reconstruction of the catalyst surface. The thickness of the reconstructed layer is approximately 10–20μm. This process is accompanied with at least an 80-fold increase in the effective surface area compared with a clean, non-treated nickel foil, which undoubtedly leads to a drastic increase in the number of active sites. We believe that it is the main reason for the induction period always being observed before the appearance of self-sustained oscillations in the catalytic oxidation of light hydrocarbons over catalysts with a low specific surface area (single crystals, foils, or wires). Moreover, without such reconstruction, the oscillations cannot arise due to low activity of such catalysts.

THE THEORY OF DEVELOPMENT OF SUPPORTED METAL-COMPLEX CATALYSTS

Some results of the investigations for the purpose of development of supported metal-complex catalysts for phosphine and carbon monoxide oxidation as well as for ozone decomposition are summarized. The activity of such catalysts has been found to depend not only on a nature of a central atom and ligands but also on a nature of supports. The theoretical model explaining mechanisms of surface complex formation taking into account the influence of physicochemical and structural-adsorption properties of the supports (SiO2, Al2O3, carbon materials, zeolites, dispersed silicas, lamellar aluminosilicates, etc.) has been proposed. For quantitative description of the support effect, such a thermodynamic parameter as the adsorbed water activity assignable with the help of water vapor adsorption isotherms has been introduced. Successive stability constants of the surface metal complexes have been calculated by the kinetic method and, hence, compositions and partial catalytic activity of the latter have been determined. Taking into account the competitive adsorption of metal ions on the supports, some schemes of formation of surface bimetallic complexes have been suggested. The compositions of the supported metal-complex catalysts have been optimized to meet requirements of their use in respirators and plants for air purification from foregoing gaseous toxicants.

SHS-produced catalysts for in-situ generation of syngas in combustion chambers of heat engines: A review

Harmful emissions from internal combustion engines and gas turbines can be markedly lowered upon addition of synthesis gas to hydrocarbon fuels such as natural gas. It has been suggested to perform the reactions of partial oxidation of methane as well as of dry and steam carbon dioxide reforming of methane in order to generate syngas directly in the combustion chamber of aircraft or car engines. In this communication we attempted to classify and characterize the SHS-produced catalysts that were suggested for the purpose. High activity of such catalysts was associated with a highly deficient structure of combustion products formed in specific conditions of SHS reactions.

Synthesis and photocatalytic activity of mesoporous nanocrystalline Fe-doped titanium dioxide

Synthesis of mesoporous nanocrystalline iron-doped titania following the sol–gel method is presented in this work. Samples with various molar ratios (0.1, 0.25, 0.5, 1.0, 2.5, 5.0, 10 and 20%) of Fe to Ti were prepared. The particle size was found to be in the range of ∼12nm while mesopores were approximately near to ∼5.5nm. The effect of Fe as doping element on titania properties, such as crystallite size, surface area, pore size, pore volume and d-spacing was investigated. Moreover, distribution of Fe in TiO2 matrix was determined by elemental mapping whereas change in absorption properties was evaluated by diffuse reflectance spectroscopy. It was observed that as the Fe content was increased, a partial phase transformation from anatase to rutile and pseudorutile took place. Effect of ultraviolet, ultraviolet-visible and visible radiations on the photocatalytic activity of these catalysts was studied by removal of Methyl Orange as model pollutant. As results, it was found that the photocatalytic activity of such catalysts depends strongly on Fe amount and type of radiation.

Ammonia synthesis from dinitrogen and dihydrogen over the catalysts based on supported mononuclear potassium carbonyl ruthenate. Promoting effect of alkyllithium compounds

New catalysts for the ammonia synthesis from dinitrogen and dihydrogen based on supported mononuclear potassium carbonyl ruthenate K2Ru[(CO)4] as a precursor of catalytically active particles have been developed. Magnesium oxide and graphite-like active carbon Sibunit were used as supports, while aliphatic organolithium compounds (BunLi and ButLi) were employed as electron promoters in these catalysts. The systems with MgO as a support are the most efficient. The introduction of RLi into these systems allows one to considerably increase the ammonia synthesis rate. When using carbon Sibunit, the promoting effect of organolithium compounds is much weaker but the activity of such catalysts can be essentially increased by the introduction of an additional electron promoter, viz., metallic potassium, into the system. All the catalysts tested are active in the ammonia synthesis at atmospheric pressure and temperatures ≥250 °C.