Metal Oxide Sulfide Catalysts and Their Mechanism of Catalytic Action
Basic Concepts
Metal oxide catalysts are often complex oxides (Complex oxides), i.e. multi-component oxides. For example, VO5-MoO3, Bi2O3-MoO3, TiO2-V2O5-P2O5, V2O5-MoO3-Al2O3, MoO3-Bi2O3-Fe2O3-CoO-K2O-P2O5-SiO2 (i.e., 7-component catalysts codenamed as C14 for the production of acrylonitrile in the third generation). At least one of the components is a transition metal oxide. The components may interact with each other, and the interaction often varies depending on the conditions. Complex oxide systems often coexist in multiple phases, such as Bi2O3-MoO3, which has α, β and γ phases. There is a so-called active phase concept. Their structures are very complex, with solid solutions, heteropolyacids, mixed crystals and so on.
As far as the catalyst role and function are concerned, some components are the main catalyst and some are co-catalysts or carriers. The main catalyst is active when it exists alone, such as MoO3 in MoO3-Bi2O3; the co-catalyst is inactive or rarely active when it exists alone, but it can enhance the activity of the main catalyst, such as Bi2O3. The co-catalysts can modulate the generation of new phases, or regulate the rate of electron migration, or promote the formation of active phases. Depending on the improvement of catalyst performance, there are structural additives, anti-sintering additives, mechanical strength enhancement and promotion of dispersion and other different co-catalyst functions. The purpose of modulation is always on the promotion of activity, selectivity or stability.
Metal oxides mainly catalyze the selective oxidation of hydrocarbons. It is characterized by: the reaction is highly exothermic, effective heat transfer, mass transfer is very important to consider the catalyst’s fly temperature; there is a reaction explosion zone exists, so there is a so-called “fuel-excess type” or “air-excess type” two kinds of conditions; the products of this type of reaction, relative to the The products of this type of reaction are more stable than the raw materials or intermediates, so there are the so-called “rapid cooling measures” to prevent further reaction or decomposition; in order to maintain high selectivity, it is often operated at a low conversion rate, with a second reactor or raw material recycling and so on.
This kind of oxide catalyst for oxidation can be divided into three categories: ① transition metal oxides, easy to pass out oxygen from its lattice to the reactant molecules, composed of more than two kinds of cations with variable valence, are non-metric compounds, and the cations in the lattice are often cross-mutual solubility, forming a rather complex structure. ② Metal oxides, the active component for oxidation of chemisorbed oxygen species, adsorption state can be molecular, atomic state or even interstitial oxygen (Interstitial Oxygen). (iii) The original state is not an oxide, but a metal, but its surface adsorption of oxygen to form an oxide layer, such as the oxidation of ethylene by Ag, the oxidation of methanol, the oxidation of ammonia by Pt, and so on.
Metal sulfide catalysts are also available in single-component and composite systems. They are mainly used in heavy oil hydrofinishing, hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodemetallization (HDM) and other processes. Metal oxides and metal sulfides are semiconductor catalysts. Therefore it is necessary to understand some basic concepts and terminology about semiconductors.
Energy band structure of semiconductors and their catalytic activity
Important semiconductors in catalysis are transition metal oxides or sulfides. There are three types of semiconductors: intrinsic semiconductors, n-type semiconductors and p-type semiconductors. Semiconductors with two types of carrier conduction, electron and hole, are called intrinsic semiconductors. This type of semiconductor is not important in catalysis because the temperature of the chemical change process, typically 300 to 700°C, is not sufficient to produce this electron jump. Semiconductors that conduct by electrons bound to metal atoms are called n-type (Negative Type) semiconductors. Positive ions and holes in the lattice by the transfer of conductive, called p-type (Positive Type) semiconductor.
Belong to the n-type semiconductor ZnO, Fe2O3, TiO2, CdO, V2O5, CrO3, CuO, etc., in the air when heated by the loss of oxygen, the cation oxidation number decreases until it becomes an atomic state. Those belonging to p-type semiconductors are NiO, CoO, Cu2O, PbO, Cr2O3, etc., which gain oxygen when heated in air, and the cation oxidation number is elevated, while causing an absence of positive ions in the lattice.
Both n-type semiconductors and p-type semiconductors are nonmetric compounds. In n-type semiconductors, such as nonmetric ZnO, there is an excess of Zn++ ions, which are in the interstices of the lattice. Since the lattice has to remain electrically neutral, the excess Zn++ ions at the gap pull an electron nearby and open into eZn++, forming an additional energy level near the conduction band. When the temperature rises, this eZn++ pulling electron is released and becomes a free electron, which is ZnOIf the catalyst is reduced, its activity decreases; when the oxygen supply is restored, the reaction again returns to its original steady state. These experimental facts indicate that it is the lattice oxygen (O=) that acts as a catalyst and the catalyst is reduced.
It is generally accepted that the catalyst is reduced to some extent in the steady state; different catalysts have their own optimal reduction states. Based on the numerous catalytic oxidations of complex oxides it is generalized that (A) selective oxidation involves effective lattice oxygen; (B) non-selective complete oxidation reactions, where both adsorbed and lattice oxygen participate in the reaction; and (C) for complex oxide catalysts involving two different cations, one cation, Mn+, assumes the activation and oxidation function of the hydrocarbon molecules, which then oxidize the O= ions that rely on the delivery of the O= ions along the lattice; so that the other One cation, Mn+, is responsible for the activation and oxidation of the hydrocarbon molecules, which are then oxidized by O= ions passing along the lattice; the other metal cation is in the reduced state to accept gas-phase oxygen. This is the mechanism of double-redox.
(2) Types of metal-oxygen bonding and M=O bonding
Take the oxidative bonding of Co2+ as an example.
Co2+ + O2 + Co2+ Co3+- O- Co3+
There can be 3 different types of bonding into M=O σ-π double bond bonding. (a) the eg orbital (,) of metal Co forms a σ-bond with the lone pair of electrons of O2; (b) the eg orbital of metal Co forms a σ-bond with the π molecular orbital of O2; and (c) the t2g orbitals (dxy, dxz, and dyz) of metal Co opens up into a π-bond with the π* molecular orbital of O2.
(3) M=O bond energy size and catalyst surface deoxygenation capacity
The ability of a complex oxide catalyst to give oxygen is a measure of its ability to perform selective oxidation. If the thermal effect of M=O bond to dissociate oxygen (grade to reactant molecules in the gas phase) △HD is small, it is easy to give, and the catalyst has high activity and low selectivity; if △HD is large, it is difficult to give, and the catalyst has low activity; only if the △HD is moderate, the catalyst has medium activity, but good selectivity.
Structural chemistry of composite metal oxide catalysts
The generation of a new chemistry with a particular lattice structure needs to satisfy three requirements: (i) valence equilibrium controlling the stoichiometric relationship; (ii) controlling the possibility of mutual substitution of sizes between ions; and (iii) modification of the ligand situation changes of the ideal structure, which is based on the assumption that the ions are rigid, impenetrable, and non-distorted spheres. The structures of actual composite metal oxide catalysts are often lattice defective, non-stoichiometric, and the ions are deformable.
Any stable compound must satisfy a balance of chemical valencies. When the substitution of a high valence ion for a low valence ion occurs in the lattice, it is necessary to combine the high valence ion and the lattice cation vacancies required due to the substitution to satisfy this requirement. For example, the Fe++ ion of Fe3O4, if balanced by the valence in γ-Fe2O3, can be written as Fe □1/3○4.
Cations are generally smaller than anions. The lattice structure is always determined by the number of anions configured around the cation. For binary compounds, the coordination number depends on the radius ratio of the anions, i.e. ρ = r cation/r anion.
Finally there is the polarization of the ions to consider. This is because polarization can shift the charge surrounding an electron away from the idealized three-dimensional lattice structure to such an extent that it forms a layered structure and eventually becomes a molecular crystal, changing ionic bonding to covalent bonding.
(1) Catalytic properties of spinel structure
Many metal oxides with spinel structure are commonly used as catalysts for oxidation and dehydrogenation processes. The general formula of the structure can be written as AB2O4. Its unit cell contains 32 O=negative ions, composed of cubic close-packed, corresponding to the formula A8B16O32. In the normal lattice, each of the 8 A atoms is coordinated by 4 oxygen atoms in a positive tetrahedral coordination; each of the 16 B atoms is coordinated by 6 oxygen atoms in a positive octahedral coordination. There are also spinel-type compounds with a completely chaotic distribution of A and B.
