Showing papers on "Catalysis published in 1994"

Introduction to surface chemistry and catalysis

Abstract: Preface. Introduction. 1 Surfaces: An Introduction. 2 The Structure of Surfaces. 3 Thermodynamics of Surfaces. 4 Dynamics at Surfaces. 5 Electrical Properties of Surfaces. 6 Surface Chemical Bond. 7 Mechanical Properties of Surfaces. 8 Polymer Surfaces and Biointerfaces. 9 Catalysis by Surfaces. Index.

Titanium-containing mesoporous molecular sieves for catalytic oxidation of aromatic compounds

Abstract: Titanium silicalite is an effective molecular-sieve catalyst for the selective oxidation of alkanes, the hydroxylation of phenol and the epoxidation of alkenes in the presence of H2O2 (refs 1-3). The range of organic compounds that can be oxidized is greatly limited, however, by the relatively small pore size (about 0.6 nm) of the host framework. Large-pore (mesoporous) silica-based molecular sieves have been prepared recently by Kresge et al. and Kuroda et al.; the former used a templating approach in which the formation of an inorganic mesoporous structure is assisted by self-organization of surfactants, and the latter involved topochemical rearrangement of a layered silica precursor. Here we describe the use of the templating approach to synthesize mesoporous silica-based molecular sieves partly substituted with titanium--large-pore analogues of titanium silicalite. We find that these materials show selective catalytic activity towards the oxidation of 2,6-di-tert-butyl phenol to the corresponding quinone and the conversion of benzene to phenol.

Low-barrier hydrogen bonds and enzymic catalysis

Abstract: Formation of a short (less than 2.5 angstroms), very strong, low-barrier hydrogen bond in the transition state, or in an enzyme-intermediate complex, can be an important contribution to enzymic catalysis. Formation of such a bond can supply 10 to 20 kilocalories per mole and thus facilitate difficult reactions such as enolization of carboxylate groups. Because low-barrier hydrogen bonds form only when the pKa's (negative logarithm of the acid constant) of the oxygens or nitrogens sharing the hydrogen are similar, a weak hydrogen bond in the enzyme-substrate complex in which the pKa's do not match can become a strong, low-barrier one if the pKa's become matched in the transition state or enzyme-intermediate complex. Several examples of enzymatic reactions that appear to use this principle are presented.

Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia

Abstract: Manganese oxides of different crystallinity, oxidation state and specific surface area have been used in the selective catalytic reduction (SCR) of nitric oxide with ammonia between 385 and 575 K. MnO2 appears to exhibit the highest activity per unit surface area, followed by Mn5O8, Mn2O3, Mn3O4 and MnO, in that order. This SCR activity correlates with the onset of reduction in temperature-programmed reduction (TPR) experiments, indicating a relation between the SCR process and active surface oxygen. Mn2O3 is preferred in SCR since its selectivity towards nitrogen formation during this process is the highest. In all cases the selectivity decreases with increasing temperature. The oxidation state of the manganese, the crystallinity and the specific surface area are decisive for the performance of the oxides. The specific surface area correlates well with the nitric oxide reduction activity. The nitrous oxide originates from a reaction between nitric oxide and ammonia below 475 K and from oxidation of ammonia at higher temperatures, proven by using 15NH3. Participation of the bulk oxygen of the manganese oxides can be excluded, since TPR reveals that the bulk oxidation state remains unchanged during SCR, except for MnO, which is transformed into Mn3O4 under the applied conditions. In the oxidation of ammonia the degree of oxidation of the nitrogen containing products (N2, N2O, NO) increases with increasing temperature and with increasing oxidation state of the manganese. A reaction model is proposed to account for the observed phenomena.

Catalytic Air Pollution Control: Commercial Technology

Abstract: Preface. ACKNOWLEDGEMENTS. ACKNOWLEDGEMENTS, FIRST EDITION. ACKNOWLEDGEMENTS, SECOND EDITION. I. FUNDAMENTALS. 1. Catalyst Fundamentals. 1.1 Introduction. 1.2 Catalyzed Verses Non-Catalyzed Reactions. 1.3 Catalytic Components. 1.4 Selectivity. 1.5 Promoters and their Effect on Activity and Selectivity. 1.6 Dispersed Model for Catalytic Component on Carrier: Pt on Al 2 O 3 . 1.7 Chemical and Physical Steps in Heterogeneous Catalysis. 1.8 Practical Significance of knowing the Rate-Limiting Step. 2. The Preparation of Catalytic Materials: Carriers, Active Components, and Monolithic Substrates. 2.1 Introduction. 2.2 Carriers. 2.3 Making the Finished Catalyst. 2.4 Nomenclature for Dispersed Catalysts. 2.5 Monolithic Materials as Catalyst Substrates. 2.6 Preparing Monolithic Catalysts. 2.7 Catalytic Monoliths. 2.8 Catalyzed Monoliths Nomenclature. 2.9 Precious Metal Recovery from Monolithic Catalysts. 3. Catalyst Characterization. 3.1 Introduction. 3.2 Physical Properties of Catalysts. 3.3 Chemical and Physical Morphology Structures of Catalytic Materials . 3.4 Techniques for Fundamental Studies. 4. Monolithic Reactors for Environmental Catalysis. 4.1 Introduction. 4.2 Chemical Kinetic Control. 4.3 The Arrhenius Equation and Reaction Parameters. 4.4 Bulk Mass Transfer. 4.5 Reactor Bed Pressure Drop. 4.6 Summary. 5. Catalyst Deactivation. 5.1 Introduction. 5.2 Thermally Induced Deactivation. 5.3 Poisoning. 5.4 Washcoat Loss. 5.5 General Comments on Deactivation Diagnostics in Monolithic Catalysts for Environmental Applications. II. MOBILE SOURCE. 6. Automotive Catalyst. 6.1 Emissions and Regulations. 6.2 The Catalytic Reactions for Pollution Abatement. 6.3 The Physical Structure of the Catalytic Converter. 6.4 First-Generation Converters: Oxidation Catalyst (1976-1979). 6.5 NOx, CO and HC Reduction: The Second Generation: The Three Way Catalyst (1979 - 1986). 6.6 Vehicle Test Procedures (U.S., European and Japanese). 6.7 NOx, CO and HC Reduction: The Third Generation (1986 - 1992). 6.8 Palladium TWC Catalyst: The Fourth Generation (Mid-1990s). 6.9 Low Emission Catalyst Technologies. 6.10 Modern TWC Technologies for the 2000s. 6.11 Towards a Zero-Emission Stoichiometric Spark-Ignit Vehicle. 6.12 Engineered Catalyst Design. 6.13 Lean-Burn Spark-Ignited Gasoline Engines. 7. Automotive Substrates. 7.1 Introduction to Ceramic Substrates. 7.2 Requirements for Substrates. 7.3 Design Sizing of Substrates. 7.4 Physical Properties of Substrates. 7.5 Physical Durability. 7.6 Advances in Substrates. 7.7 Commercial Applications. 7.8 Summary. 8. Diesel Engine Emissions. 8.1 Introduction. 8.2 Worldwide Diesel Emission Standards. 8.3 NO x -Particulate Tradeoff. 8.4 Analytical Procedures for Particulates. 8.5 Particulate Removal. 8.6 NOX Reduction Technologies. 8.7 2007 Commercial System Designs (PM Removal Only). 8.8 2010 Commercial System Approaches under Development (PM and NO x Removal). 8.9 Retrofit and Off-Highway. 8.10 Natural Gas Engines. 9. Diesel Catalyst Supports and Particulate Filters. 9.1 Introduction. 9.2 Health Effects of Diesel Particulate Emissions. 9.3 Diesel Oxidation Catalyst Supports. 9.4 Design/Sizing of Diesel Particulate Filter. 9.5 Regeneration Techniques. 9.6 Physical Properties and Durability. 9.7 Advances in Diesel Filters. 9.8 Applications. 9.9 Summary. 10. Ozone Abatement within Jet Aircraft. 10.1 Introduction. 10.2 Ozone Abatement. 10.3 Deactivation. 10.4 Analysis of In-Flight Samples. 10.5 New Technology. III. STATIONARY SOURCES. 11. Volatile Organic Compounds . 11.1 Introduction. 11.2 Catalytic Incineration. 11.3 Halogenated Hydrocarbons. 11.4 Food Processing. 11.5 Wood Stoves. 11.6 Process Design. 11.7 Deactivation. 11.8 Regeneration of Deactivated Catalysts. 12. Reduction of NO x . 12.1 Introduction. 12.2 Nonselective Catalytic Reduction of NOx. 12.3 Selective Catalytic Reduction of NOx. 12.4 Commercial Experience. 12.5 Nitrous Oxide (N 2 O). 12.6 Catalytically Supported Thermal Combustion. 13. Carbon Monoxide and Hydrocarbon Abatement from Gas Turbines. 13.1 Introduction. 13.2 Catalyst for CO Abatement. 13.3 Non-Methane Hydrocarbon (NMHC) Removal. 13.4 Oxidation of Reactive Hydrocarbons. 13.5 Oxidation of Unreactive Light Paraffins. 13.6 Catalyst Deactivation. 14. Small Engines. 14.1 Introduction. 14.2 Emissions. 14.3 EPA Regulations. 14.4 Catalyst for Handheld and Nonhandheld Engines. 14.5 Catalyst Durability. IV. NEW AND EMERGING TECHNOLOGIES. 15. Ambient Air Cleanup. 15.1 Introduction. 15.2 Premair (R) Catalyst Systems. 15.3 Other Approaches. 16. Fuel Cells and Hydrogen Generation. 16.1 Introduction. 16.2 Low-Temperature PEM Fuel Cell Technology. 16.3 The Ideal Hydrogen Economy. 16.4 Conventional Hydrogen Generation. 16.5 Hydrogen Generation from Natural Gas for PEM Fuel Cells. 16.6 Other Fuel Cell Systems. INDEX.

Mechanism of the Selective Catalytic Reduction of Nitric Oxide by Ammonia Elucidated by in Situ On-Line Fourier Transform Infrared Spectroscopy

Abstract: The selective catalytic reduction reaction of nitric oxide bv ammonia over vanadia-titania catalysts is one of the methods of removing NOx pollution. In the present study, it has been possible to identify the reaction mechanism and the nature of the active sites in these catalysts by combining transient or steady-state in situ (Fourier transform infrared spectroscopy) experiments directly with on-line activity studies. The results suggest a catalytic cycle that consists of both acid and redox reactions and involves both surface V-OH (Bronsted acid sites) and V=O species. A fundamental microkinetic model is proposed, which accounts for the observed industrial kinetics performance.

Homogeneous catalytic hydrogenation of supercritical carbon dioxide

Abstract: THE use of carbon dioxide as a starting material for the synthesis of organic compounds has long been a goal for synthetic chemists. The hydogenation of carbon dioxide to formic acid, methanol and other organic substances is particularly attractive, but has remained difficult. This route to formic acid has been described recently, based on the use of organometallic rhodium catalysts in dimethyl sulphoxideII and aqueous2 solvents. We report here the efficient production of formic acid in a supercritical mixture of carbon dioxide and hydrogen containing a catalytic ruthenium() phosphine complex. The use of a supercritical phase, in which hydrogen is highly miscible, leads to a very high initial rate of reaction up to 1,400 moles of formic acid per mole of catalyst per hour. The same reaction under identical conditions but in liquid organic solvents is much slower. Our results suggest that supercritical fluids represent a promising medium for homogeneous catalysis.

Nitrogen donors in organometallic chemistry and homogeneous catalysis

Abstract: Homogeneous catalysis has been responsible for many major recent developments in synthetic organic chemistry. The combined use of organometallic and coordination chemistry has produced a number of new and powerful synthetic methods for important classes of compounds in general and for optically active substances in particular. For this purpose, complexes with optically active ligands have been used, most of them coordinating through phosphorus. More recent developments have highlighted the use of “nitrogen-donors”, particularly as they are easily obtained from the “chiral pool”. However, the remarkable achievements in this area have been based on an empirical approach. This article attempts to bridge the gap between the synthetic and the coordination chemist. The first section discusses the rates of formation and dissociation of complexes with nitrogen donors, their relationship to the rates of product formation, and presents the factors which induce homolytic cleavage of MC bonds. It also provides a summary of the main types of organometallic complexes formed by metal centers coordinated to nitrogen donors and their reactivity patterns. The second section highlights the most significant, homogeneously catalyzed reactions involving complexes with nitrogen ligands. Foremost among them are the asymmetric aspects of hydrogenation (particularly those involving boranes as reducing agents), hydrosilylation, cyclopropanations, Diels-Alder reactions, aldol condensations, alkylation of aldehydes, conjugate addition reactions, Grignard cross-coupling reactions, allylic alkylations, oxidation reactions, olefin epoxidations, and di-hydroxylation of olefins.

Temperature‐Dependent Methanol Electro‐Oxidation on Well‐Characterized Pt‐Ru Alloys

Abstract: The kinetics of methanol electro-oxidation on well-characterized Pt-Ru alloy surfaces were measured in sulfuric acid solution as a function of temperature. The alloy surfaces were prepared in ultrahigh vacuum with the surface composition determined by low energy ion scattering. It was found that the activity of Ru towards the dissociative adsorption of methanol is a strong function of temperature. This change in the adsorptive nature of the Ru sites with temperature produced a variation in the optimum surface composition with temperature. The optimum surface had an Ru content which increased with increasing temperature, from close to [approximately]10 atomic percent (a/o) Ru at 25 C to a value in the vicinity of [approximately]30 a/o at 60 C. The shift in optimum composition with temperature was attributed to a shift in the rate-determining step from methanol adsorption/dehydrogenation at low temperature to the surface reaction between the dehydrogenated intermediate and surface oxygen at high temperature. The apparent activation energies were consistent with this change in the rate-determining step.

Development of the Copper-Catalyzed Olefin Aziridination Reaction

Abstract: Soluble Cu(1) and Cu(I1) triflate and perchlorate salts are efficient catalysts for the aziridination of olefins employing (N-@-tolylsulfonyl)imino)phenyliodinane, PhI=NTs, as the nitrene precursor. Electron-rich as well as electron-deficient olefins undergo aziridination with this reagent in 55-958 yields, at temperatures ranging from -20 OC to +25 OC. The catalyzed nitrogen atom-transfer reaction to enol silanes and silylketene acetals has also been developed to provide facile syntheses of a-amino ketones. Other metal complexes were found to be less effective at catalyzing the reaction, while PhI=NTs proved to be superior to other imido group donors as the nitrene precursor. Reaction rates and yields are enhanced in polar aprotic solvents such as MeCN and MeN02. Reaction stereospecificity in the aziridination of cis and trans disubstituted olefins was evaluated and found to be both catalyst and substrate dependent. Intermolecular competition experiments between pairs of monoand disubstituted olefins indicate that the olefin selectivity profile for the reaction is independent of the oxidation state of the copper catalyst employed. It is concluded that these reactions are proceeding through the 2+ catalyst oxidation state under the conditions employed

Autopoietic Self-Reproduction of Fatty Acid Vesicles

Abstract: Conditions are described under which vesicles formed by caprylic acid and oleic acid in water are able to undergo autopoietic self-reproduction-namely an increase of their population number due to a reaction which takes place within the spherical boundary of the vesicles themselves. This is achieved by letting a certain amount of the neat water-insoluble caprylic or oleic anhydride hydrolyze at alkaline pH: the initial increase of the concentration of the released acidcarboxylate is extremely slow (several days to reach the conditions for spontaneous vesicle formation), but afterwards, the presence of vesicles brings about a rapid second phase leading to more and more vesicles being formed in an overall autocatalytic process. The catalytic power of the caprylic acid and oleic acid vesicles toward the hydrolysis of the corresponding anhydride is documented in a set of independent experiments. In these experiments, the hydrolysis was carried out in the presence of vesicles at a pH corresponding approximately to the pK of the acid in the vesicles. The process of autopoietic self-reproduction of caprylic acid and oleic acid vesicles is studied as a function of temperature: by increasing temperature (up to 70 “C), the exponential time progress of vesicle formation tends to become steeper while the long initial slow phase is significantly shortened. The caprylic acid and oleic acid vesicles are characterized by electron microscopy and by determining their internal volume. The question whether and to what extent these vesicles form a classic chemical equilibrium system-in which namely the free surfactant is in equilibrium with the aggregates-is also investigated.

Efficient manganese catalysts for low-temperature bleaching

Abstract: THE detergents of the next century will be routinely required to contain bleaching agents that are not only more active than those currently available but also environmentally safe and cost-effective. Hydrogen peroxide, the traditional bleaching agent1, loses its activity as the washing temperature decreases. Peroxyacetic acid maintains acceptable bleaching activity down to 40–60 °C (ref. 2), but still lower temperatures are desirable. It is generally recognized that manganese and iron complexes are less environmentally damaging reagents than other transition-metal compounds, and such complexes have received considerable attention as bleaching catalysts3–11. Here we show that manganese complexes derived from l,4,7-trimethyl-l,4,7-triazacyclononane and related ligand systems act as highly effective catalysts for the bleaching of stains by hydrogen peroxide at low temperatures. These complexes also catalyse the epoxidation of alkenes and the oxidation of poh phenolic substrates by hydrogen peroxide. Our results demonstrate the considerable potential of these systems for clean and efficient low-temperature bleaching.

Activity and Stability of Ordered and Disordered Co‐Pt Alloys for Phosphoric Acid Fuel Cells

Abstract: Co-Pt alloys were studied in detail by a corrosion test under phosphoric acid fuel cell (PAFC) conditions on the well-defined crystallographic structures for a typical combination of the alloy catalysts used in PAFCs, examining the long-life stabilities of the structures and the catalytic activities for O[sub 2] electroreduction. The ordered (O) and disordered (D) alloys at the same particle sizes can be obtained by heat-treating the mother alloy in different temperature sequences. The O-alloy exhibits a specific activity 1.35 times higher than the D-alloy before the corrosion test, but shows less activity after the corrosion test, due to a higher degradation in the O-alloy activity as compared with that of the D-alloy. It was found that the Co atoms on particle surfaces of both alloys dissolved easily in the acid. This is followed by a second slow dissolution from inside the alloy particles probably due to the protective action by a monolayer thickness of Pt remaining on the alloy surfaces, but the loss of Co in the second stage dissolution for the O-alloy is higher by several percentage points compared to that of the D-alloy. It was also found that the Pt content does not change on the catalyst supportmore » even after 50 h of corrosion test, but the pure Pt phase is formed in the corrosion product, where the phase for the O-alloy grows faster than that for the D-alloy with corrosion time. Based on these results, obtained by chemical, x-ray diffraction, and transmission electron microscopy with energy dispersive spectroscopy analyses, the corrosion for Pt alloy catalysts is clearly explained: after the dissolution of Co atoms in the first surface layer of alloys, both Co and Pt dissolve out simultaneously from small-size alloy particles and the Pt redeposits on the surfaces of large-size alloy particles. It is concluded that the D-alloy is preferable to the O-alloy from the viewpoint of the stabilities in the structure and the electrocatalytic activity.« less

Electrochemistry of Methanol at Low Index Crystal Planes of Platinum: An Integrated Voltammetric and Chronoamperometric Study

Abstract: We have studied a catalytic decomposition of methanol on low Miller index platinum surfaces, Pt(111), Pt(110), and Pt(100) in perchloric, sulfuric, and phosphoric acids at room temperature. The instantaneous methanol oxidation current is unaffected by the methanolic CO formation (surface poisoning) and depends on platinum surface structure and composition of supporting electrolyte with respect to the anions. The highest oxidation current, 156 mA-cm[sup [minus]2], is observed with the Pt(110) electrode in perchloric acid solution at 0.200 V vs Ag/AgCl reference. In terms of turnover, this current translates to 163 molecules-(Pt site)[sup [minus]1][center dot]s[sup [minus]1], a high rate exceeding previous expectations in methanol electrode kinetics. Overall, the oxidation current changes by 3 orders of magnitude between the extreme cases examined in this study. Breaking up the total effect into individual components shows that the surface geometry and anionic effects are roughly comparable. Therefore, we have an evidence that anion-platinum interactions are as important in determining the methanol oxidation rate as is the surface geometry of the Pt catalyst. 52 refs., 13 figs., 4 tabs.