Selective catalytic oxidation by heterogeneous transition metal catalysts

A review with 21 refs.; The reaction mechanisms of two transition metal catalyzed reactions are discussed: epoxidn. of ethylene prodn. and vinylacetate. In addn. short ref. will be made to methanol and CO oxidn. The surface reactions are related to the corresponding reactions in organometallic complexes. Also a relation between surface science model studies and surface reactivity will be made. Elementary surface reaction steps on surfaces will be highlighted; subsequently the main mechanistic issues in oxygen CH and OH bond activation will be describe

Quantum chemistry of surface chemical reactivity

The quantum chemist's and surface physicist's view of the surface chem. bond are illustrated by means of chemisorption of CO as an example with 23 refs. Between adsorbate and metal surface, bonding as well as antibonding orbital fragments are formed. For mols. the adsorption geometry is a sensitive function of the balance of the repulsive atop directing interaction, resulting from the occupation of antibonding orbital fragments, and the high coordination directing bonding interaction. The bonding contribution to the surface chem. bond energy relates to the surface group orbital local d. of states around the Fermi level, as long as the orbital interactions are weak. The concepts of Pauli repulsion and group orbital LDOS can be used to provide a quantum-chem. basis to metal promotio

Chemical basis of metal catalyst promotion

A survey is presented of the different modes of action of metal catalyst promotors. Broadly two main categories can be distinguished: structural and bifunctional promotors. In structural promotion the promoting atoms do not participate themselves in the elementary steps of a catalytic reaction. Bifunctional promotors participate themselves chemically in the catalytic reaction. Five different kinds of structural promotion can be recognized: ensemble-size regulating; valence-state promotion; polarizing-structural promotion; surface-phase and cluster complex promotion; particle size stabilizing. Each subcategory is discussed shortly and when relevant illustrated with a practical example. Three different kinds of bifunctional promotors can be distinguished: Polarizing-bifunctional promotion; direct-bifunctional promotion; chemical-bifunctional promotion. Whereas conceptually different, subcategories are not always easily experimentally distinguishable. Of most of the different kinds of promotion theoretical treatments exists. Quantum-chemical studies are referred to in the tex

Transition states of NH, CH and OH bond activation

COLL 40 Based on computational studies of stepped and non-stepped transition metal surfaces the reaction paths and enrgetics of ammonia, methane and water activation have been studied, Clean surfaces as well as surfaces with coadsorbed oxygen have been studied. A coherent framework for the interpretation of activation-free energy relationships will be presented. Transition state energetics is mainly determined by variations in the interaction of reacting fragments in the transition state with the metal surface. For example ammonia dissociation on Pt is a specific case where transition state and final state conformation is very different. This has as a consequence that activation of ammonia on Pt is similar on different surfaces and independent of surface topology. For methane activation this appears to be quite different. Such differences in behaviour will be analysed on the basis of the underlying electronic factors that control the surface chemical bonds. Bronsted-Eyring-Polanyi proportionality constants in the corresponding activation-free energy relationship relate to the relative energy of (sometimes transient) reaction fragments. Knowledge of the transition state structures in combination with an understanding of the chemical bonding features that determine site preference of reaction frangments provide a rational basis to analyse trends in activation energy as a function of metal composition as well as surface topology

Theory of heterogeneous catalytic reactivity using the cluster approximation

A review with 41 refs.; d. functional theory enables quant. computational anal. of reaction intermediates. The cluster approach makes application to heterogeneous catalysis possible. Examples from transition metal catalysis and zeolite catalysis will be discussed. In the case of transition metals cluster choices will be analyzed to simulate properties of the extended surface. The issue of coverage dependent reactivity and surface reconstruction will be discussed. The concept of adatom basicity in metal catalysis will be analyzed. It will be shown that the information obtained on the elementary reaction steps from cluster can be used to predict the overall rate of a catalytic reaction. In transition metal catalysis for an oscillatory reaction as the CO oxidn. reaction catalyzed by (100) it will be shown how knowledge on the elementary reaction rate consts. of surface reactions can be used to predict the overall rate of the reaction with help of the time dependent Monte Carlo method. In zeolite catalysis the anal. of the reaction mechanism of methanol conversion will be used to show the strength of the cluster approxn. for the case of a refractory oxide. The hydroisomerization reaction will be used to relate microscopic mol. information to the overall rate of a zeolite catalyzed reactio

Theory of Brønsted Acidity in Zeolites

The nature of the chem. bond of protons in a zeolite is analyzed on the basis of theor. and spectroscopic results. Of interest is the dependence on zeolite structure as well as compn. The zeolitic OH bond is mainly covalent. Proton attachment to the zeolite lattice causes a weakening of neighboring Si-O and Al-O bonds. The effective increase in vol. of the bridging oxygen atom causes a local deformation, that changes the strength of the lattice-chem. bonds over a few bond distances. Proton concn. effects as well as lattice-compn. effects can be understood on the basis of the lattice-relaxation model. The energetics of proton transfer is controlled by the need to stabilize the resulting zwitter-ion. The pos. charge on the cation becomes stabilized by contact with basic lattice-oxygen atom

The active site of promoted ethylene-epoxidation catalysts

The cooperative effect of chlorine moderation and alkali promotion on the initial selectivity of silver-catalysed epoxidation of ethylene have been investigated. To this end a study was made of the conversion ethylene catalysed by silver doped with alkali and non-doped in the presence and absence of chlorine. The silver powders were characterized by temperature programmed reduction as well as by oxygen adsorption studies. Also, the exchange reaction of C2H40 and C2D4 was studied. The data were interpreted with the epoxidation model according to which the elementary step of the selective reaction is electrophylic attack of an adsorbed oxygen atom to the pi-bond of ethylene and the non-selective reaction occurs by electro-positive attack of a different atomic oxygen species to the CH bond of ethylene. The role of alkali appears to be stabilization of that silver oxychloride phase that contains predominantly atomic electrophilic oxygen

The Ostwald step rule

The empirical observation that crystn. from a soln. occurs in steps in such a way that often thermodynamically unstable phases occur first, followed by the thermodynamically stable phase (Ostwald step rule), still has no theor. foundation. Ostwald's step rule is demonstrated to be related to irreversible thermodn. Ostwald's step rule minimizes entropy prod