Catalysis by Transition Metal Oxides

Abstract

Catalytic oxidation reactions are divided into two groups:electrophilic oxidation proceeding through activation of oxygen and nucleophilic oxidation in which activation of the hydrocarbon molecule is the first step, followed by consecutive hydrogen abstraction and nucleophilic oxygen insertion. Properties of individual cations and their coordination polyhedra determine their behaviour as active centers responsible for activation of hydrocarbon molecules. A facile route for nucleophilic insertion of oxygen into such molecules by group V, VI and VII transition metal oxides is provided by the crystallographic shear mechanism, catalytic properties are thus dependent upon the geometry of the surface. The catalyst surface is in dynamic interaction with the gas phase, and changes of the latter may thus result in surface transformations and appearance of surface phases, which influence the selectivity of catalytic reactions.The vast majority of catalysts used in modern chemical industry are oxides. Because of their ability to take part in the exchange of electrons, as well as in the exchange of protons or oxide ions, oxides are used as catalysts in both redox and acid-base reactions. They constitute the active phase not only in oxide catalysts but also in the case of many metal catalysts, which in the conditions of catalytic reaction are covered by a surface layer of a reactive oxide.Properties of oxides are also important in the case of preparation of many metal and sulphide catalysts, which are obtained from an oxide precursor.Very often, highly dispersed metals are prepared by reduction of an appropriate oxide phase, and sulphide catalysts are formed from the oxide precursor in the course of the hydrodesulphurization by interaction with the reaction medium.Finally, oxides play an important role in carriers for active metal or oxide phases, very often modifying strongly their catalytic properties. The present paper concerns

Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts

“…Electrophilic oxygen species are described as adsorbed peroxo groups and can bond to a lone metal atom as a peroxo (M>O 2 ), a superoxo (M-O 2 – ), or as a peroxo bridging two metal centers (M–O–O–M). Alternatively, nucleophilic oxygen species can bond to a lone metal atom as an oxo group (MO) or a bridging oxygen atom between metal centers (M–O–M). , Representative drawings of these oxygen species are shown in Figure . The preferred oxygen species varies by the application.…”

“…Alternatively, nucleophilic oxygen species can bond to a lone metal atom as an oxo group (MO) or a bridging oxygen atom between metal centers (M−O−M). 74,75 Representative drawings of these oxygen species are shown Scheme 2. Hydrogen Atom Transfer from an Alkane to the Active Site of a Metal Oxide Catalyst a a The H-atom transfer forms a hydroxyl site and alkyl radical while an electron adds to a vacant d orbital to create a reduced metal center.…”

Constructing A Rational Kinetic Model of the Selective Propane Oxidation Over A Mixed Metal Oxide Catalyst

“…It is clear that structures of active sites and/or active centres mean surface structures. The surface of a catalyst material (e.g., a metal or metal oxide) may significantly differ from its bulk structure and composition, and may additionally be affected strongly by the gas phase surrounding it [12,[23][24][25][26][27].…”

Influence of Hubbard U Parameter in Simulating Adsorption and Reactivity on CuO: Combined Theoretical and Experimental Study

“…23 These studies provide mechanistic insights into the C−H bond activation, which is central in paving the way for novel processes and catalysts to be used in value-added C 1 chemical processing. Most of the TMO-catalyzed hydrocarbon oxidation reactions are believed to occur primarily via the Mars-Van Krevelen type mechanism 24,25 with nucleophilic oxidation 26 (consumption of lattice oxygen 27 ). Catalytic properties of TMOs are largely dependent on its surface characteristics such as, but not limited to, the degree of unsaturation of the surface, the surface acid− base characteristics, the adsorbate−surface interactions, the binding energy of lattice oxygen, and the ease of vacancy formation and the presence of cationic and anionic vacancies.…”