Reaction Kinetics and Mechanism: Model Studies on Metal Single Crystals [P.H. Emmett Award Address, 1983]

In the past decade remarkable progress has been made in the surface science of catalysis. Studies on both small single crystal metal particles on supporting materials and on large (bulk) single crystals point toward a more refined understanding of the fundamental processes underlying catalytic chemistry and, hopefully, toward a rational design of catalytic materials. It seems clear that concepts gleaned from studies on single crystal surfaces will strongly influence the research with supported metal catalysts and ultimately affect the nature of the materials used as practical catalysts.     

Surface Science and Catalysis—Studies on the Mechanism of Ammonia Synthesis: The P. H. Emmett Award Address

About 5 years ago a Battelle Colloquium on “The Physical Basis of Heterogeneous Catalysis” was held in honor of Professor Emmett where he presented an introductory talk on 'Fifty Years of Progress in the Study of the Catalytic Synthesis of Ammonia' [1], a field to which he had made significant contributions during this whole period. So why another report on this topic? Justification can perhaps partly be obtained by some of Professor Emmett's concluding remarks which were not completely decisive with respect to the mechanism of this reaction: 'The experimental work of the past 50 years leads to the conclusion that the rate-determining step in ammonia synthesis over iron catalysts is the chemisorption of nitrogen. The question as to whether the nitrogen species involved on the surface is molecular or atomic is still not conclusively resolved. In fact, our own work in this field was mainly induced by this paper, and continuing discussions and correspondence with Professor Emmett were extremely helpful during its progress. I am therefore very much honored to be this year's recipient of the Emmett Award.     

The Formation, Characterization, and Catalytic Activity of Transition Metal Complexes in Zeolites: The P. H. Emmett Award Address

The chemistry of transition metal complexes in zeolites has received considerable attention in our laboratory because we believe that such complexes have potential both in fundamental and applied catalysis. To scientists outside of the field of heterogeneous catalysis it often appears virtually hopeless to obtain meaningful chemistry from surface reactions because the active centers are so poorly characterized. Although most catalytic chemists would not totally agree with this viewpoint, one would have to admit that it contains an element of truth. The traditional approach to solving this problem has been to attempt better characterization of active sites on known catalysts, and while some significant advances have been made in this direction, the progress in general has been painfully slow. Another approach, and the one which will be described here, has been to synthesize surface complexes which are both well defined and active as catalysts. These then may serve as model systems for conventional catalysts, but more than this, they may be unique catalysts which are important in their own right.     

The Formation,Characterization, and Catalytic Activity of Transition Metal Complexes in Zeolites: The P. H. Emmett Award Address

The chemistry of transition metal complexes in zeolites has received considerable attention in our laboratory because we believe that such complexes have potential both in fundamental and applied catalysis. To scientists outside of the field of heterogeneous catalysis it often appears virtually hopeless to obtain meaningful chemistry from surface reactions because the active centers are so poorly characterized. Although most catalytic chemists would not totally agree with this viewpoint, one would have to admit that it contains an element of truth. The traditional approach to solving this problem has been to attempt better characterization of active sites on known catalysts, and while some significant advances have been made in this direction, the progress in general has been painfully slow. Another approach, and the one which will be described here, has been to synthesize surface complexes which are both well defined and active as catalysts. These then may serve as model systems for conventional catalysts, but more than this, they may be unique catalysts which are important in their own right.     

Acid Zeolites: An Attempt to Develop Unifying Concepts (P. H. Emmett Award Address, 1981)

The recent developments of zeolite chemistry are characterized by the appearance of attempts to explain certain properties of these porous crystalline solids in terms of general chemical properties developed for other solids or even for other states of matter. Transition metal complexes in zeolites sometimes exhibit the same properties as in more conventional solvents [1]. Thus the zeolite behaves as a mono to polydentate macroligand of the transition metal ion [2]. Although numerous observations [2] underline the role of the zeolite as a solid solvent or as a common solid matrix, its unique properties in terms of structure or cage geometry should not be overlooked. Indeed, the appropriate activation of Ru(III) hexammine in faujasite-type zeolites gives a very active water gas-shift catalyst [3]. A cationic Ru-complex seemed to be formed under these conditions, having no homogeneous equivalent, and its stabilization requires the specific geometry of a faujasite supercage.     

Acid Zeolites: An Attempt to Develop Unifying Concepts (P. H. Emmett Award Address, 1981)

Abstract

The recent developments of zeolite chemistry are characterized by the appearance of attempts to explain certain properties of these porous crystalline solids in terms of general chemical properties developed for other solids or even for other states of matter. Transition metal complexes in zeolites sometimes exhibit the same properties as in more conventional solvents [1]. Thus the zeolite behaves as a mono to polydentate macroligand of the transition metal ion [2]. Although numerous observations [2] underline the role of the zeolite as a solid solvent or as a common solid matrix, its unique properties in terms of structure or cage geometry should not be overlooked. Indeed, the appropriate activation of Ru(III) hexammine in faujasite-type zeolites gives a very active water gas-shift catalyst [3]. A cationic Ru-complex seemed to be formed under these conditions, having no homogeneous equivalent, and its stabilization requires the specific geometry of a faujasite supercage.     

Monodisperse Metal Nanoparticle Catalysts: Synthesis, Characterizations, and Molecular Studies Under Reaction Conditions

We aim to develop novel catalysts that exhibit high activity, selectivity and stability under real catalytic conditions. In the recent decades, the fast development of nanoscience and nanotechnology has allowed synthesis of nanoparticles with well-defined size, shape and composition using colloidal methods. Utilization of mesoporous oxide supports effectively prevents the nanoparticles from aggregating at high temperatures and high pressures. Nanoparticles of less than 2?nm sizes were found to show unique activity and selectivity during reactions, which was due to the special surface electronic structure and atomic arrangements that are present at small particle surfaces. While oxide support materials are employed to stabilize metal nanoparticles under working conditions, the supports are also known to strongly interact with the metals through encapsulation, adsorbate spillover, and charge transfer. These factors change the catalytic performance of the metal catalysts as well as the conductivity of oxides. The employment of new in situ techniques, mainly high-pressure scanning tunneling microscopy (HPSTM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) allows the determination of the surface structure and chemical states under reaction conditions. HPSTM has identified the importance of both adsorbate mobility to catalytic turnovers and the metal substrate reconstruction driven by gaseous reactants such as CO and O₂. APXPS is able to monitor both reacting species at catalyst surfaces and the oxidation state of the catalyst while it is being exposed to gases. The surface composition of bimetallic nanoparticles depends on whether the catalysts are under oxidizing or reducing conditions, which is further correlated with the catalysis by the bimetallic catalytic systems. The product selectivity in multipath reactions correlates with the size and shape of monodisperse metal nanoparticle catalysts in structure sensitive reactions.     

A Metabolic Activation Mechanism of 7 H -Dibenzo[ c,g ]carbazole Via o -Quinone. Part 1: Synthesis of 7 H -Dibenzo[ c,g ]carbazole-3,4-dione and Reactions with Nucleophiles

7 H -Dibenzo[ c,g ]carbazole (DBC) is a potent carcinogen present in the environment. The metabolic activation pathway of DBC has not been completely understood. The formation of the DBC- o -quinone was proposed and the novel DBC-3,4-dione was chemically synthesized from 4-OH- or 3-OH-DBC, using the Fremy's salt at room temperature. The DBC-3,4-dione was found to react readily with model nucleophile, 2-mercaptoethanol, to afford 1-(2'-hydroxyethylthio)-DBC-3,4-dione through 1,4-Michael addition. An adduct of DBC-3,4-dione with adenine was obtained; adducts formed with other DNA bases and nucleosides were partially characterized and are under further analysis. Under the same conditions, DBC-3,4-dione with guanine did not yield an adduct. These results are encouraging for further studies on DNA binding of DBC through 3,4-dione in vitro and in vivo.     

Thermal-Mechanical History and the Strength of Magnesium Oxide Single Crystals: II, Etch Pit and Electron Transmission Studies

Mechanical tests show that the room-temperature flow strength of magnesia single crystals varies with heat treatment. Flow strength depends on the size, density, and distribution of precipitate particles which inhibit the motion of dislocations. The present paper describes optical and electron transmission microscope evidence for a precipitation reaction in magnesium oxide. The observations correlate fairly well with mechanical test data. Mechanical tests also show that mobile dislocations are locked by heating above 600°C. The room-temperature yield strength increases with time and temperature particularly when the aging temperature exceeds 1000°C. Studies to determine the nature of this locking mechanism are described. Etch pit studies show that dislocations aged above 600°C etch at a slower rate than fresh dislocations. This corresponds precisely with the change in mechanical behavior and both effects are attributed to impurity diffusion to dislocation lines. Electron transmission studies indicate a redistribution of point defects, left by moving dislocations, above 600°C. Eventually these result in a change in dislocation configuration which is considered responsible for the strong locking observed above 1000°C.     

Effect of Heat-Treatment on the Mechanism and Kinetics of the Hydrogen Evolution Reaction on Ni—P + TiO2 + Ti Electrodes

Composite Ni—P + TiO₂ + Ti layers were prepared by codeposition of Ni—P alloy with TiO₂ and Ti powders from a solution containing suspension of TiO₂ and Ti particles. The electrodeposition was carried out under galvanostatic conditions at room temperature. The layers exhibited an amorphous Ni—P matrix in which crystalline TiO₂ and Ti were embedded. On the deposit surface, the nonstoichiometric Ti oxide, Ti₁₀O₁₉, and intermetallic compounds, NiTi, formed during the electrodeposition, were also present. The heat treatment of these layers in argon leads to the crystallization of Ni—P matrix and formation of nonstoichiometric Ti oxides, detected by XRD. Electrolytic activity towards the hydrogen evolution reaction (HER) was studied on these electrode materials before and after heat treatment. The mechanism of the HER was also studied, and the kinetic parameters were determined using steady-state polarization and electrochemical impedance spectroscopy (EIS). An increase in activity occurring after heating of Ni—P + TiO₂ + Ti layers is related to TiO₂ reduction and formation of nonstoichiometric Ti oxides: Ti₁₀O₁₉(400 °C), Ti₇O₁₃(500 °C) and Ti₄O₇(800 °C). It is postulated that the increase in electrochemical activity is related to the properties of these oxides and a facility for H reduction/adsorption on their surface, as well as to the presence of NiTi intermetallics as compared with the Ni—P + TiO₂ + Ti electrode.     

Oxidation of Magnesiowustite Single Crystals: II, Dependence on Temperature, Oxygen Pressure, and Crystal Composition

Single crystals of magnesiowustite (Mg,Fe)O containing up to 79 at.% Fe were prepared, and the proportions of ferrous and ferric ions were determined by chemical analysis. The rate of oxidation of individual crystals was studied at temperatures from 1000° to 1100°C and oxygen partial pressures of 2 × 10⁻¹ to 3 × 10⁻⁶ atm. The reaction follows an equation based on diffusion through a product layer. The rate constant varies with temperature according to the Arrhenius relation, and activation energies range from 52 kcal/mole with 13 at.% Fe to about 17 to 20 kcal/mole when Fe atoms occupy one third or more of the cation positions. The rate constant is independent of oxygen partial pressure within the pressure range studied. Spectrographic measurements show that Fe in magnesiowustite occupies mainly octahedral sites, but some tetra-hedral positions appear to be occupied. The mechanism of the oxidation reaction is considered.     

[Ni-Mo-Si]:Nb Bulk Metallic Glasses: Microstructure,Mechanical and Corrosion Studies

Silicon - We examine the effect of substituting Niobium (Nb), on the glass-forming abilities (GFA), mechanical and corrosion properties of bulk metallic glasses (BMGs). BMGs are synthesized using...     

Dynamics and Thermodynamics Studies on the Lead and Cadmium Removal from Aqueous Solutions by Padina sp. Algae: Studies in Single and Binary Metal Systems

Raw and modified biomasses prepared from Padina sp. algae have been used as sorbent for the removal of lead and cadmium from single and binary aqueous solutions. The effects of chemical pretreatment, exposure time, initial solution pH, initial metal concentration, and temperature on the metal uptake by the algae were investigated. It was observed that initial solution pH considerably influenced Pb and Cd uptake. The maximum removal occurred at initial pH of 5.0 for lead and 6.0 for cadmium. Also, alkali modified biomass has been shown to have a high uptake capacity for both lead and cadmium. The kinetic and equilibrium experimental data fitting tested with various models. The pseudo-first-order kinetic model and Langmuir isotherm provided the best correlation of the kinetic and equilibrium experimental data, respectively. The maximum uptake estimated from the Langmuir isotherm was 264 mg g^?1 for lead and 164 mg g^?1 for cadmium ions. Experimental biosorption data in binary system were well described by the extended Langmuir model. Various thermodynamic parameters, such as ΔG°, ΔH°, and ΔS° were calculated.     

The Effect of 7 H -Dibenzo[ c,g ]carbazole and Benzo[ a ]pyrene Mixture on DNA Adduct Formation in Strain A/J Mouse Model: Preliminary Report

Complex mixtures consist of homocyclic and heterocyclic polycyclic aromatic compounds (PACs) represented by benzo[ a ]pyrene (B a P) and 7 H -dibenzo[ c,g ]carbazole (DBC), respectively. To exert their biological effects, PACs are metabolized into reactive intermediates, which can form DNA adducts. In this preliminary report, male A/J mice were given a single intraperitoneal injection. Groups of three animals were treated with DBC (2 or 10 mg/kg) or B a P (10 or 100 mg/kg). Mixtures of DBC:B a P were given at doses of 2:10, 2:100, 10:10, or 10:100 mg/kg. DNA adduct levels in lungs collected three days posttreatment were determined by the 32 P-postlabeling method. The results indicate that, in the lungs, exposure to mixtures containing more B a P than DBC resulted in the absence of adduct 3 (DBC) and significantly higher total adduct levels. This suggests that B a P is being preferentially metabolized, resulting in less DBC adduction.     

Radiochemical studies of chemisorption and catalysis: XII. Studies of the adsorption of [14C]acetylene, [14C]ethylene and [14C]carbon monoxide on silica- and alumina-supported rhodium catalysts

The interaction of ¹⁴C-labeled acetylene, ethylene and carbon monoxide with rhodium supported on silica or alumina catalysts has been studied at room temperature at pressures of a few Torr. Adsorptions of acetylene, carbon monoxide and ethylene were performed separately: adsorptions of each of these gases followed by another were also studied. The behavior of the system was observed by counting of surface radioactivity. Acetylene gave adsorption isotherms with two well-defined regions, one of which, the primary region, was steeply sloping. [¹⁴C]CO as a probe for metal sites showed that few were left after adsorption of acetylene. CO-precoverage of the metal surface showed that the extent of secondary adsorption of acetylene was unchanged and that this adsorption was probably associated with the support. C₂H₂ had the ability to displace about 40% of the adsorbed CO. This study is to be taken in conjunction with Part XIII of the series, in which mass spectrometry was used to elucidate the nature of the reactions occurring.     

Biosorption of Zinc on Palm Tree Leaves: Equilibrium,Kinetics, and Thermodynamics Studies

Abstract

The efficiency of using palm tree leaves to remove zinc ions from aqueous solution was studied. Adsorption isotherms, kinetics, and thermodynamics studies were conducted. The influence of different experimental parameters, such as equilibrium pH, shaking rate, temperature, and the presence of other pollutants such as chelating agents on the biosorption of zinc on palm tree leaves was investigated.

Batch biosorption experiments showed that palm tree leaves used in this study proved to be suitable for the removal of zinc from dilute solutions where a maximum uptake capacity of 14.7 mg/g was obtained at 25°C. Zinc biosorption on palm tree leaves was found to be highly pH dependent. The biosorption process was found to be rapid with 90% of the adsorption completed in about 10 min. Dynamics studies of the biosorption of zinc on palm tree leaves showed that the biosorption process followed the pseudo second‐order kinetics with little intraparticle diffusion mechanism contribution. The equilibrium results indicated that zinc biosorption on palm tree leaves could be described by the Langmuir, Freundlich, Gin et al., and Sips models. Using the Langmuir equilibrium constants obtained at different temperatures, the thermodynamics properties of the biosorption (ΔG⁰, ΔH⁰, and ΔS⁰) were also determined. The values of these parameters indicated the spontaneous and endothermic nature of zinc biosorption on palm tree leaves.     

Heats of Chemisorption of O2, H2, CO, CO2, and N2 on Polycrystalline and Single Crystal Transition Metal Surfaces

One of the important physical-chemical properties that characterizes the interaction of solid surfaces with gases is the bond energy of the adsorbed species. The determination of the bond energy is usually performed indirectly by measuring the heat of adsorption (or heat of desorption) of the gas [1, 2], In order to define the heat of adsorption, let us consider the chemisorption of a diatomic molecule, X₂, onto a site on a uniform solid surface, M. The molecule may adsorb without dissociation to form MX₂. M represents the adsorption site where bonding occurs to a cluster of atoms or to a single atom. In this circumstance, the heat of adsorption, ΔH_ads, is defined as the energy needed to break the MX₂ bond:     

Heats of Chemisorption of O2, H2, CO,CO2, and N2 on Polycrystalline and Single Crystal Transition Metal Surfaces

Abstract

One of the important physical-chemical properties that characterizes the interaction of solid surfaces with gases is the bond energy of the adsorbed species. The determination of the bond energy is usually performed indirectly by measuring the heat of adsorption (or heat of desorption) of the gas [1, 2], In order to define the heat of adsorption, let us consider the chemisorption of a diatomic molecule, X₂, onto a site on a uniform solid surface, M. The molecule may adsorb without dissociation to form MX₂. M represents the adsorption site where bonding occurs to a cluster of atoms or to a single atom. In this circumstance, the heat of adsorption, ΔH_ads, is defined as the energy needed to break the MX₂ bond:     

The Synthesis of Cresol from Toluene and N2O on H[Al]ZSM-5: Minimizing the Product Diffusion Limitation by the Use of Small Crystals

The direct hydroxylation of toluene with nitrous oxide to cresol has been studied on two different H[Al]ZSM-5 zeolites with an Si/Al ratio of around 25 and different crystal sizes (30-70 nm and 1-3 m). The samples were activated by calcination and characterized by X-ray diffraction, temperature programmed desorption of ammonia, adsorption of nitrogen and transmission electron microscopy. For the two different crystal sizes, different macroscopic cresol yields and time on stream behaviours are observed. The sample having larger crystals shows a decrease in toluene conversion with increasing reaction temperature. For the smaller crystals an increase in toluene conversion, selectivity to cresol and amount of para-cresol in the cresol fraction with increasing reaction temperature is observed. The para-cresol selectivity is lower on the sample with the longer diffusion path. The findings are explained by product diffusion limitation caused by high reactivity and strong adsorption of the polar product cresol on H[Al]ZSM-5, resulting in a rapid deactivation of the larger crystals.