Showing papers in "Studies in Surface Science and Catalysis in 1995"
TL;DR: In this article, the authors highlight recent applications using both forcefield and electronic structure techniques to the study of problems in structure, sorption, synthesis, and reactivity of both zeolites and microporous aluminophosphates.
Abstract: Publisher Summary This chapter highlights recent applications using both forcefield and electronic structure techniques to the study of problems in structure, sorption, synthesis, and reactivity of both zeolites and microporous aluminophosphates. In particular, the chapter describes how simulation techniques may yield detailed and accurate information of the structure, and energies associated with framework modification by hetero-atoms; on the diffusion coefficients and migration mechanisms of sorbed molecules; on the nature of the interactions between host structures and templates used in zeolite synthesis; and on the use of electronic structure techniques in throwing light on the crucial questions of acid site–molecule interactions in solid acid catalysts. The chapter also summarizes the range and status of the current computational methodologies before describing the recent illustrative applications. Forcefield methods have been used extensively in modeling the structures of microporous solids, and in investigating docking and diffusion of sorbed molecules. The full range of simulation techniques—Energy Minimisation (EM), Monte Carlo (MC), and Molecular Dynamics (MD) have been employed, as have combinations of these methods, as in the widely used “docking” procedure for identifying low-energy sites for sorbed molecules. The chapter concludes with consideration of the likely future direction for the field.
195 citations
TL;DR: In this article, a new method of dispersed perovskites synthesis based upon mechanochemical activation of the solid starting compounds is elaborated, and the influence of defect structure of these compounds as well as surface segregation on their catalytic properties is discussed.
Abstract: New method of dispersed perovskites synthesis based upon mechanochemical activation of the solid starting compounds is elaborated. The influence of defect structure of these compounds as well as surface segregation on their catalytic properties is discussed. Basic stages of the monolith perovskite catalysts preparation are optimized. The experimental samples of monolith catalysts of various shapes are obtained, possessing high activity, thermal stability and resistance to catalytic poisons.
189 citations
TL;DR: In this paper, a review of recent developments in the study of intracrystalline diffusion in zeolites by novel macroscopic methods and the results obtained by some of these methods are reviewed.
Abstract: Recent developments in the study of intracrystalline diffusion in zeolites by novel macroscopic methods and the results obtained by some of these methods are reviewed. For many systems there is a significant discrepancy between the macroscopic and microscopic (QENS, PFG NMR) diffusivity values. A possible explanation is suggested.
184 citations
TL;DR: The physical characterization of MCM-36 is described in this article, which unequivocally establishes its existence as a novel large pore pillared material with zeolite properties, and the pore size distribution plot obtained by Ar physisorption shows two distinct peak in the 6-7 A region.
Abstract: Publisher Summary This chapter describes the physical characterization of MCM-36, which unequivocally establishes its existence as a novel large pore pillared material with zeolite properties. MCM-36 is a pillared material obtained from MCM-22 layers. The preparation of MCM-36 involves a lamellar intermediate, designated MCM-22 precursor, produced in a hydrothermal process. The structural information concerning pore system of MCM-36 is revealed by adsorption methods. The layers in MCM-22 posses two kinds of pore systems. One consists of 10-ring interconnected channels within the layers. The second are isolated 12-ring cages on the surface, which result in pockets on the outside of MCM-22 crystal and internal supercages, accessible through 10-ring apertures, inside the crystal. Accordingly, the pore size distribution plot obtained for MCM-22 by Ar physisorption shows two distinct peak in the 6–7 A region. The complex pore structure of MCM-22 is also reflected in the unique three step uptake profile of bulky 2,2-dimethylbutane (DMB) observed in the dynamic sorption experiment.
181 citations
TL;DR: In this paper, the catalytic activity of Au/TiO2 is sensitive to the structure of the perimeter interface between Au and TiO2, and an increase in calcination temperature not only caused particle coagulation but also brought about a stronger interaction with the TiO 2 support.
Abstract: Gold can be deposited on TiO2 as hemispherical fine particles with diameters smallerthan 4nm by deposition precipitation. Gold hydroxide precipitates with high and homogeneous dispersion on TiO2, most probably on specific surface sites, in the pH range 6 to 10. The calcination of TiO2 with Au(OH)3 in reducing gas atmospheres, such as H2 and CO led to smaller gold particles than in air. In the case of Au/TiO2 samples prepared by deposition precipitation, the catalytic activity for low-temperature CO oxidation was very high and could be observed even at temperatures below 0°C. Physically mixed Au/TiO2 samples though of much lower catalytic activity, showed gradual improvement with increasing calcination temperature. An increase in calcination temperature not only caused particle coagulation but also brought about a stronger interaction with the TiO2 support. The above results therefore indicate that the catalytic activity of Au/TiO2 is sensitive to the structure of the perimeter interface between Au and TiO2.
165 citations
TL;DR: In this article, MCM-41 type silicas were covalently grafted with various functional alkoxysilanes (RO)3Si(CH2)3 X with X = Cl, NH(CH 2)2NH2 and NHC(O)N-Salpr.
Abstract: MCM-41 type silicas were covalently grafted with various functional alkoxysilanes (RO)3Si(CH2)3 X with X = Cl, NH(CH2)2NH2 and NHC(O)N-Salpr The functions of the first two organic moieties already attached were further transformed into respectively 2-(NHCH2)Pyr, 4-(NH(CH2)2NHCO)Pyr also known as organic ligands of transition metals The grafted mesoporous silicas were characterized by IR, MAS-NMR spectroscopies, thermogravimetry, nitrogen adsorption and elemental analysis The coupling reactions of organic ligands do not modify the covalent grafting bonds Pore opening shrinks at increasing organic coverage The diameter of the channels accessible to nitrogen varies from 33 to 13 A total surface lining by concentrically oriented organic chains is suggested by the decrease in the nitrogen adsorption enthalpy as a function of the organic coverage
159 citations
TL;DR: In this article, NO and CO molecules were used as IR probes of the copper oxidation state in H-Cu-ZSM-5 catalysts and their characteristic band at 2158 cm −1 provides quantitative results on integrating its molar extinction-coefficient.
Abstract: NO (reactant) and CO (product) molecules were used as IR probes of the copper oxidation state in H-Cu-ZSM-5 catalysts. CO adsorption is specific to Cu I sites. Its characteristic band at 2158 cm −1 provides quantitative results on integrating its molar extinction-coefficient (e) NO decomposes oxidizing Cu I to Cu II . Propane and oxygen in a special IR reactor cell always yielded chemisorbed CO. Use of e indicates the NO influence on Cu state.
53 citations
TL;DR: In this article, the authors present the thermodynamics, kinetics and a proposed synthesis mechanism for the crystallization of a prototypical high-silica zeolite, namely ZSM-5.
Abstract: Publisher Summary This chapter presents the thermodynamics, kinetics and a proposed synthesis mechanism for the crystallization of a prototypical high-silica zeolite, namely ZSM-5 This information is used to develop strategies for the design of zeolite syntheses and a few of these strategies are outlined Two aspects of the design process are described below: (i) The design of pore architectures and (ii) the design of framework atom positioning In order to develop zeolite pore architecture by design, the proper choice of organic structure-directing agent and inorganic composition must be made Next, the size and rigidity of the molecule must be controlled It is clear that as the size of the organic molecule is increased, the specificity for producing zeolite pore architectures also increases Very recently, Zones has shown that there is a good correlation between structure direction and the rigidity of the organic structure-directing agent as measured by the number of tertiary and quaternary connectivities in the molecule for a series of organics with C/N Thus, size, rigidity, and hydrophobicity, simultaneously, must be correctly specified in order to prepare a good structure-directing agent by design For the inorganic compositions, it is clear that for pure-silica preparations a small amount of alkali-metal cations is necessary for practical kinetics; however, large amounts will lead to layered compounds
48 citations
TL;DR: Titanium modified beta zeolite (Ox-Ti-β) was prepared by treating aluminum containing β zeolites (Na-K-, NH 4 - or H-form) with ammonium titanyl oxalate solutions of different concentrations.
Abstract: Titanium modified beta zeolite (Ox-Ti-β) was prepared by treating aluminum containingbeta zeolite (Na-K-, NH 4 - or H-form) with ammonium titanyl oxalate solutions of different concentrations. The obtained catalysts were characterized by atomic absorption, XRD, IR, UV-Vis and XPS. The incorporation of Ti into the zeolite framework was evidenced by (i) the increase in the [302], interplanar d-spacing, (ii) the increase in the intensity of the 960 cm −1 IR band, (iii) the presence of an absorption band at 220 nm and the absence of absorption band due to TiO 2 in UV-Vis, (iv) the high binding energy of Ti 2p 3/2 (459.9 eV) and (v) the catalytic activity in epoxidation reactions. The amount of Ti in the samples was found to depend on the concentration of ammonium titanyl oxalate solutions and also on the nature of the counter-cations of the parent beta zeolite.
46 citations
TL;DR: In this paper, the authors studied the sorption of light n- and iso-alkanes on H-ZSM5 and H-MOR by means of calorimetry, gravimetry and in situ infrared spectroscopy.
Abstract: 1. Abstract The sorption of light n- and iso-alkanes on H-ZSM5 and H-MOR was studied by means of calorimetry, gravimetry and in situ infrared spectroscopy. Alkanes sorb preferentially localized on the strong Bronsted acid sites of H-ZSM5 and H-MOR. Two n-alkane, but only one iso-alkane molecule can be simultaneously accommodated on the Bronsted acid sites of H-ZSM5. In contrast, only one alkane molecule interacts with a strong Bronsted acid site of H-MOR, and, moreover, only a third of these sites are strongly interacting at all with alkanes because of steric reasons. For both zeolites, the heat of adsorption increases with increasing size of the hydrocarbon. The linear relationship between the enthalpies and the entropies of sorption suggests that the molecular motions of the sorbed molecules are restricted in proportion to the strength of interaction with the zeolite.
43 citations
TL;DR: The use of zeolites for the immobilization of catalytically active guests (i.e., ship-in-the-bottle complexes, chiral guests, enzymes), applications in environmental protection and the development of catalytic zeolite membranes is discussed in this article.
Abstract: Catalysis by zeolites is a rapidly expanding field. Beside their use in acid catalyzed conversions, several additional areas can be identified today which give rise to new catalytic applications of zeolites. Pertinent examples are oxidation and base catalysis on zeolites and related molecular sieves, the use of zeolites for the immobilization of catalytically active guests (i.e., ship-in-the-bottle complexes, chiral guests, enzymes), applications in environmental protection and the development of catalytic zeolite membranes. Selected examples to illustrate these interesting developments are presented and discussed in the paper.
TL;DR: In this article, a noncontact atomic force microscope (AFM) and N2 absorption measurements on beta zeolites reveal the extreme irregularity of the external crystal surface which can make up a considerable proportion of the total surface area.
Abstract: Non-contact atomic force microscope (AFM) and N2 absorptionmeasurements on beta zeolites reveal the extreme irregularity of the external crystal surface which can make up a considerable proportion of the total surface area A catalytic test, the acylation of 2-methoxynaphthalene, shows that active sites on the outer surface play an important role in the catalytic activity of the zeolites Attempts to influence the external surface area and its catalytic activity through synthesis or post-synthesis modification such as dealumination show that the principle influence on the external surface comes from the synthesis procedure
TL;DR: Ceramic foams are sponge-like porous structures and are made by filling flexible open-cell organic polymer foams with slurries of ceramic particles such as alpha alumina, zirconia, silicon nitride, etc as discussed by the authors.
Abstract: Ceramic foams are preformed materials used extensively as filters, advanced burners, diffusers and mixers, but catalytic applications are now beginning to appear. These include catalytic solar receivers, partial oxidation, catalytic combustion, and diesel exhaust catalytic filters. The foams are sponge-like porous structures and are made by filling flexible open-cell organic polymer foams with slurries of ceramic particles such as alpha alumina, zirconia, silicon nitride, etc.. The plastic is burned off and the ceramic sintered to yield either a positive or negative replica of the original foam depending on exact loading procedures. Mega-pore openings range from 0.02 to 1.5 mm, apparent densities from 0.5 to 1.5 g cm −3 , porosities from 40 to 85%, and the number of pores per cm from 4 to 30. With appropriate moulding or machining of the plastic foam, the ceramic may be fabricated in any shape or size. Megaporosity enhances turbulence in gases flowing through the foam and produces better mixing with lower pressure drop. The foam may be impregnated with catalytic agents, with or without an appropriate washcoat. Although pressure drop correlations follow the Ergun equation, the high porosity gives much lower pressure drop than equivalent beds of particles. Mass transfer follows standard correlations but turbulent flow is seen at much lower flow rates. Heat transfer is enhanced by the superior conductivity of the web structure. These feature combine to make ceramic foams attractive possibilities for many applications. The only disadvantage is the relatively low strength, a feature which may be controlled in some cases.
TL;DR: Y-type zeolites with different Si/Al ratios, surface areas and pore volumes have been modified by grafting with organosiloxane molecules as mentioned in this paper, which can be readily functionalized by further reaction with suitable reagents to generate imino and benzamido groups.
Abstract: Y-type zeolites with different Si/Al ratios, surface areas and pore volumes havebeen modified by grafting with organosiloxane molecules. Volumetry, 13C NMR and infrared spectroscopy, thermal and elemental analyses confirm the covalent attachment of the organic species inside the micro- and mesopore voids. The grafted species are strongly bound to the surface and can be readily functionalized by further reaction with suitable reagents to generate imino and benzamido groups.
TL;DR: In this paper, carbon fibrils are used as supports for noble metal particles in liquid phase catalytic processes, and a homogeneous deposition-precipitation procedure is used to obtain carbon fibil-supported palladium catalysts.
Abstract: Carbon fibrils can be produced smoothly by exposing finely dispersed iron or nickel particles to reducing carbon containing gas flows. Alumina-supported iron particles were used to grow carbon fibrils with cylindrical graphite layers parallel to the fibril axis. The carbon fibrils interweave during growth, resulting in the formation of tangled skeins. Owing to the size of the skeins (about 3 μm), the pore volume, the pore size distribution, the surface area, the filterability and the mechanical strength, the skeins are very appropriate as supports for, e.g., noble metal particles, in liquid phase catalytic processes. Therefore, we examined the properties of the bare carbon fibrils, as, well as the properties of catalysts supported on carbon fibrils. To obtain carbon fibril-supported palladium catalysts, a homogeneous deposition-precipitation procedure was utilized. Varying preparation conditions, such as, the atmosphere during drying and the pretreatment of the carbon fibrils, affect the activity in the liquid-phase hydrogenation of nitrobenzene significantly.
TL;DR: Saturated gas-solid reactions known from Atomic Layer Epitaxy (ALE) were used to process various catalysts as discussed by the authors, demonstrating the achievement of surface saturation, reproducibility of processes, selection of process parameters, growth of oxides to modify the support and binding of two metal compounds.
Abstract: Saturated gas-solid reactions known from Atomic Layer Epitaxy (ALE) were used to processvarious catalysts. Good homogeneity of metal species was verified both along the entire catalyst bed and inside the particles. A variety of volatile metal compounds including metal chlorides, alkoxides and β-diketonates were successfully used as reactants. The ALE processing is described with reference to examples demonstrating the achievement of surface saturation, reproducibility of processes, selection of process parameters, growth of oxides to modify the support and the binding of two metal compounds.
TL;DR: In this article, the X type zeolites were synthesized around the RuF 16 Pc complexes and were characterized by XRD, FT-IR and UV-Vis spectroscopy as well as elemental analysis.
Abstract: Summary Hexadecafluorophthalocyanine (F 16 Pc) complexes of Ru(II) which were prepared bythe reaction of tetrafluorophthalonitrile and Ru 3 (CO) 12 , have been encapsulated in the supercages of zeolites NaX. The X type zeolites were synthesized around the RuF 16 Pc complexes. The zeolites modified with the metal complexes were characterized by XRD, FT-IR and UV-Vis spectroscopy as well as elemental analysis. The oxidation of cyclohexane using t-butylhydroperoxide was catalyzed by the intrazeolite RuF 16 Pc complexes. Complete conversion to cyclohexanone and cyclohexanol was achieved with nearly 3000 turnovers per day. These ship-in-a-bottle RuF 16 Pc complexes show no signs of deactivation in contrast to the iron analogs, regardless of how the peroxide is administered during the reaction.
TL;DR: In this paper, synthetic saponites were synthesized at 90°C and 1 atmosphere from a Si/Al-gel and a solution containing ure and M2+-nitrate (M2+=Zn, Mg, Ni, Co and Co) in only a few hours.
Abstract: Saponites were synthesised at 90°C and 1 atmosphere from a Si/Al-gel and a solutioncontaining ure and M2+-nitrate (M2+=Zn, Mg, Ni and Co) in only a few hours. The products were characterised by XRD, TEM, BET, 27Al- and 29Si-MASNMR. Furthermore, the catalytic properties of the synthetic saponites in the Friedel-Crafts alkylation of benzene with propylene to cumene were tested. Incorporation of Zn, Mg, Ni, Co, or a combination of Zn and Mg, in the octahedral layer, as well as controlling the Si/Al,-ratio in the tetrahedral layer between 7.9 and 39 could easily be established. The specific surface areas and the pore volumes of the saponites are extremely high, viz., 100–750 m2/g and 0.03–0.32 ml/g, respectively. Zn-saponite with Al3+in the interlayer exhibited a higher catalytic activity as compared to a commercial SPA-catalyst (Solid Phosphoric Acid).
TL;DR: In this article, the hydrogen chemisorption on two Rh/CeO 2 catalysts prepared respectively from rhodium nitrate and RCl 3 was studied by means of a magnetic balance.
Abstract: The hydrogen chemisorption on two Rh/CeO 2 catalysts prepared respectively from rhodium nitrate and rhodium chloride has been studied. The evolution of the ceria oxidation state with the series of treatments applied was monitored by means of a magnetic balance. Temperature Programmed Reduction and Oxidation (TPR/TPO) as well as Volumetric Adsorption techniques have also been used. Upon treating with H 2 , at 623 K, the Precursor/Support systems, the reduction level reached by ceria was 11.4% for the sample prepared from Rh(NO 3 ) 3 and much higher, 22.1%, for the chlorine containing catalyst. In the later case, the reduction degree varied very slightly with succesive evacuation/hydrogen adsorption cycles. This contrasts with the behaviour observed for the sample prepared from rhodium nitrate, for which the reduction process is to a much larger extent reversible. The results reported here for RhCl 3 /CeO 2 are interpreted as due to the substitution of the ceria lattice oxygen ions by Cl − , thus blocking the operation of both the direct and incorporation of Cl − into the support. This induces a much higher irreversible reduction of ceria as well as the blocking of both the direct and back spillover processes responsible for the reversiblity of the ceria reduction by hydrogen.
TL;DR: In this paper, the results of a development program for Pd-only and Pd/Rh automotive emission control catalysts are discussed and a review of former experiences with Pdcontaining systems, especially their poor tolerance against lead and sulfur poisoning is given.
Abstract: The results of a development program for Pd-only and Pd/Rh automotive emission control catalysts are discussed. A review of former experiences with Pd-containing systems, especially their poor tolerance against lead and sulfur poisoning is given. Model gas experiments were conducted with improved Pd-based catalysts. Not only the activity to convert simultaneously CO, NO x and hydrocarbons (HC) was investigated, but also the formation of so-called secondary emissions such as N 2 O or NH 3 . The catalyst development was supported by XPS-measurements paying special attention to the oxidation state, spread and the dispersion of Pd. Promising Pd and Pd/Rh-catalyst candidates were tested fresh and after engine aging on engine- and vehicle dynamometers. The results are compared to Pt/Rh-catalysts. For Pd-only catalysts the ratio of catalyst volume to vehicle mass seems to be one important factor to guarantee high conversion levels after aging. The Pd/Rh-technologies reported in this study showed similar activities on different vehicles and in different test cycles as Pt/Rh catalysts. With the new developed Pd/Rh-catalyst the European legislation for model year 1996 could be achieved thus representing an economical and technical alternative to Pt/Rh-catalysts. Major potential for further improving the Pd/Rh-technologies performance must be seen in an optimization work regarding fuel quality and A/F-management.
TL;DR: In this paper, a novel, high silica zeolite (UTD-1) could be prepared using bis(pentamethylcylcopentadienyl) cobalt(III) hydroxide, Cp* 2 CoOH.
Abstract: Summary As part of our effort to advance the application of metal complexes as structure directing agents for molecular sieve synthesis, we discovered that a novel, high silica zeolite (UTD-1) could be prepared using bis(pentamethylcylcopentadienyl) cobalt(III) hydroxide, Cp* 2 CoOH. UTD-1 has been characterized as a large pore molecular sieve having one dimensional channels running in paralle. The incorporation of titanium into the synthesis gel results in the formation of the titanosilicate, Ti-UTD-1 as well as the novel phase Ti-UTD-8. The preliminary characterization of these materials is described.
TL;DR: In this paper, a scheme based on competitive adsorption has been proposed which accounts for the effect of the nature of the hydrocarbons on their own oxidation and on CO oxidation.
Abstract: The exhaust gas of spark ignited engines contains a hydrocarbon mixture of complex composition ranging from C 1 to C 9 molecules including alkanes, olefins, diolefins, alkynes and aromatics. It has been evidenced that among the various unsaturated, hydrocarbons, alkynes show the largest inhibiting effect on all oxido-reduction reactions occuring on a PtRh catalyst, i.e. non-alkyne hydrocarbons, CO and NO are not converted until alkynes are eliminated. A scheme based on competitive adsorption has been proposed which accounts for the effect of the nature of the hydrocarbons on their own oxidation and on CO oxidation. It has also been shown that the inhibition effect exists even, for very low C 2 H 2 concentrations, representative of real exhausts gases. Furthermore, the addition of 15 ppm SO 2 slows down C 2 H 2 oxidation, which, increases the C 2 H 2 , inhibiting effect. From these results a laboratory test has been defined, enabling the simulation of engine bench tests.
TL;DR: In this article, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line of automotive converters, as an attempt to simulate real operating conditions of converters.
Abstract: As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeAlO3. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed.
TL;DR: In this paper, the basic and polyfunctional catalysts are tested in Knoevenagel condensation and one step synthesis of methylisobutylketone (MIBK) from acetone.
Abstract: Preparation of new aluminophosphate oxynitride (AlPON) and nickel modifiedAlPON are presented. Such basic and polyfunctional catalysts are tested in Knoevenagel condensation and one step synthesis of methylisobutylketone (MIBK) from acetone. It is evidenced that the nitrogen content of the AlPON, which controls the rate of benzaldehyde-ethylcyanoacetate condensation depends on the activation procedure. 90% selectivity in the MIBK synthesis is obtained on Ni formate impregnating AlPON activated at 400°C for 4 hours. TPD of CO 2 and NH 3 , DRIFT analysis and quantum mechanical calculation evidence that the O/N substitution creates strong basic sites.
TL;DR: In this paper, the acid-base pairs in cation exchanged zeolites are defined as Lewis acids (cations) and Lewis bases (framework oxygens) pairs.
Abstract: Publisher Summary This chapter describes Lewis basic and Lewis acidic sites in zeolites. Acidic and basic sites should be regarded as paired concepts always conjugated to each other. This general relationship should be also true in the case of zeolites. The extra frame work protons and cations in zeolites are the Br6nsted and Lewis acid sites, respectively, while the framework oxygens are their conjugated basic sites. The alkali-exchanged zeolites are designated as basic zeolites. Indeed, the acid–base pairs in cation exchanged zeolites are Lewis acids (cations) and Lewis bases (framework oxygens) pairs. Many factors are known to affect the Lewis acidity and Lewis basicity in cation exchanged zeolites. They are the bond angles and bond lengths, the location of A1, the crystallographic sitting of the oxygen, the framework Si/A1 ratio, the electronegativity of both the framework and extra framework atoms, and so on.
TL;DR: In this article, a transition between the pillared interlayer clays and the new surfactant-induced nano-phase materials has been discussed, and it is shown that these materials constitute a transition from the smectite clays to the new nano phase materials.
Abstract: Since their discovery in the 1970's, Pillared Layered Structures (PLS) have been evaluated for numerous catalytic applications. Though FCC and other refinery processes still constitute the greatest area of investigation, these materials are now being explored for applications as fine chemical and environmental catalysts. Much of this work has centered around the smectite clays. Layered non-alumino silicate inorganic oxides containing manganese, titanium and niobium, with the potential for redox chemistry form an exciting new area of PLS catalysis though a subtantial number of these materials have yet to be tested as actual catalysts. Recent investigations into the pillared pure silicates hint that these materials constitute a transition between the pillared interlayer clays and the new surfactant-induced nano-phase materials.
TL;DR: M/CeO2-ZrO2 (M=Rh, Pt) solid solutions are investigated as catalysts for the reduction of NO by CO in this article, and it is shown that incorporation of CeO2 into a ZrO 2 framework strongly promotes the reduced support of the metallized support.
Abstract: M/CeO2-ZrO2 (M=Rh, Pt) solid solutions are investigated as catalysts for the reduction of NO by CO. It is shown that incorporation of CeO2 into a ZrO2 framework strongly promotes the reduction of the metallized support. The reduced support efficiently decomposes NO suggesting a direct participation of the support in the NO conversion.
TL;DR: In this paper, the variation of Si-O-Al bond angles after deammoniation and the interaction of SiOHAl groups with cationic species and organic guest-molecules in dehydrated zeolites has been studied by 1 H and 29 Si MAS NMR and static 27 Al NMR spectroscopy.
Abstract: The variation of Si-O-Al bond angles after deammoniation and the interaction of SiOHAl groups with cationic species and organic guest-molecules in dehydrated zeolites has been studied by 1 H and 29 Si MAS NMR and static 27 Al NMR spectroscopy. After deammoniation of zeolite NH 4 ZSM-5 a high-field shift of the 29 Si MAS NMR lines of Si(nAl) sites by about 2–3 ppm is observed which corresponds to an increase in the Si-O-Al bond angle by 3.5–5°. However, no corresponding shift appears for zeolites Y with Si/Al ratios of 2.6 and 8.0, i.e., the framework relaxation depends on the structure type and not on the framework composition. Rotation-synchronized 1 H NMR spin-echo sequences were applied to study the mobility of adsorbed species. The 1 H MAS NMR shift and the mobility of hydroxyl protons interacting with methanol indicate that proton transfer to methanol is the dominant interaction in zeolites HY and HZSM-5. The distribution of electric charges in the vicinity of framework aluminium atoms in Si-O-Al bridges was investigated by static 27 Al NMR quadrupole-echo experiments. The different quadrupole interactions determined for framework aluminium in dehydrated zeolites NH 4 Y and NH 4 ZSM-5, and in dehydrated zeolites HY and HZSM-5 before and after adsorption of guest-molecules support the results of 1 H MAS NMR spectroscopy.
TL;DR: In this paper, the host-guest interactions in the cavities and in the channels are dominated by hydrogen bonding in neutral Z-H…B adducts between the Bronsted groups Z-bH (Z = zeolite framework) and the base.
Abstract: When molecules of low-medium proton affinity (B = N2, CO, C2H4, C2H2, propene, methylacetylene, acetonitrile and H2O) are adsorbed on acidic zeolites, the resulting host-guest interactions in the cavities and in the channels are dominated by hydrogen bonding in neutral Z-H…B adducts between the Bronsted groups Z-bH (Z = zeolite framework) and the base. If the interaction is sufficiently strong and the protonated products stable (ethylene, acetylene, propene and methylacetylene) the hydrogen bonded precursors are slowly consumed with formation of olygomeric products entrapped into the zeolite cavities. Acetonitrile gives a stronger hydrogen bonding interaction not followed by protonation. Water initially gives neutral complexes; however when more than one water molecule per protonic site is present, H5O2+·nH2O species are formed. When molecules of high proton affinities are adsorbed (NH3 and Py), B-H+···Z− ionic pairs readily appear. The modifications induced on the spectrum of B-H+ by interaction with Z− can be used to probe the basicity of the negatively charged framework. At higher filling conditions B-H+···B dimers are also observed. The basic IR spectroscopy of all these hydrogen bonded systems is discussed in detail.
TL;DR: In this article, the authors present a synthesis employing a highly effective nucleating (reducing) agent, tetrakis(hydroxymethyl)phosphonium chloride (THPC), which produces fine gold particles of about 1.5 nm mean diameter.
Abstract: Publisher Summary The preparation of noble metal catalysts via colloidal solutions of metals has been investigated in recent years because the method promises enhanced control over particle size and morphology with a direct preparation of the metal component by the reduction of the metal ions in solution, which is independent of the nature of the support. Low-temperature oxidation catalysis by gold particles, however, is reported to rely on the particles being smaller than usually achieved by preparations of colloidal sols of gold. This chapter presents a synthesis employing a highly effective nucleating (reducing) agent, tetrakis(hydroxymethyl)phosphonium chloride (THPC), which produces fine gold particles of about 1.5 nm mean diameter. These nanoparticles can be used as nuclei for the preparation of coarser catalytic gold colloids. The technique of diffuse reflectance provides a quick and qualitative check of the particle size characteristics of catalysts containing gold on white support powders, and it can be used as a complement to transmission electron microscopy (TEM) observations.