Catalyst catalytic role
1、Definition of catalyst
Catalyst is a kind of substance that can change the speed of chemical reaction but not the thermodynamic equilibrium position of the reaction, and it is not obviously consumed by itself.
2、Catalyst activity, expression
(1)Definition of activity: Generally, the reaction rate on a certain amount of catalyst under specified conditions (pressure, temperature) (to measure).
(2) Expression: For the reaction.
3. Catalyst selectivity, method of expression
(1)Definition:When a reaction can proceed in several thermodynamically possible directions, the catalyst can selectively accelerate one of the reactions.
4、What are the functions and roles of the carrier?
① Dispersing effect, increasing surface area and dispersing active components;
② Stabilizing effect, preventing the active component from melting or recrystallizing;
③Supporting role, so that the catalyst has a certain mechanical strength, not easy to break;
(iv) Heat transfer and dilution, can remove the heat in time and improve the thermal stability;
⑤Auxiliary catalytic effect, some carriers can induce the active component and assist the active component to catalyze.
5、Considerations for catalyst selection: Selectivity＞Life＞Activity＞Price
Industrial catalyst:
6、General composition of catalyst
1) Active component or main catalyst
2) Carrier or substrate
3) Co-catalyst
7. Catalyst classification
According to the homogeneity of physical phase: homogeneous catalysis, multiphase catalysis, enzyme catalysis According to the mechanism of action: redox catalysis, acid-base catalysis (ionic mechanism, generating positive carbon ions or negative carbon ions)
Coordination catalysis: catalyst and reactant molecules have coordination effect and activate the reactants.
Classified by reaction type: hydrogenation, dehydrogenation, partial oxidation, complete oxidation, water gas, syngas, acid catalysis, chlorine oxidation, carbonylation, polymerization
8、What are the process steps of multiphase catalytic reaction? How many steps can be divided into?
(1) External diffusion – internal diffusion – chemical adsorption – surface reaction – desorption – internal diffusion – external diffusion
(2) Physical process – chemical process – physical process
9、How is adsorption defined? What is the essential difference between physical adsorption and chemical adsorption?
Adsorption: When a gas comes into contact with a solid surface, the concentration of the gas on the solid surface is higher than the concentration of the main body of the gas phase.
Solid surface adsorption: physical adsorption:
Force: van der Waals force
Electrostatic force: electrostatic attraction between molecules with permanent dipole moments
induced force: easily polarized molecules are polar molecules induced by the generation of induced dipoles and permanent dipoles between the force of action
Dispersion force: transient electron density within an atom induces a dipole in a neighboring atom and results in an interaction force between two transient dipoles Chemisorption: force: valence bonding force, formation of chemical bonding
Essence: the difference between the two lies in their different forces, the former is van der Waals force, the latter is chemical bonding force, so the adsorption of adsorption formed by the adsorption of different species, and the adsorption process is different, and so many different.
10. The pore structure parameters of catalyst include
1) Density ρ=m/v V=V gap+V pore+V true
Heap density : ρheap=m/v
Particle density : ρP=m/(V-V gap)=m/(V pore+V trueor skeleton)
True Density (Skeleton Density): ρSkeleton = m/(V Heap – (V Pore + V Gap))
apparent density: ρ apparent = m/(V pile – (V pore + V gap))
2) Specific pore volume In a container of volume V filled with catalyst particles or powder of weight W, after evacuating, filled with helium, the volume of filled helium, VHe, was determined, i.e.: the volume of all spaces in the container excluding the volume of the catalyst skeleton. Then, the helium was withdrawn and mercury was charged at atmospheric pressure, and the volume of mercury charged VHg was determined, i.e., the volume remaining in the container after the volume of the catalyst skeleton and the volume of the pores in the particles were removed.
In other words, the pore volume of the catalyst: V pore = VHe – VHg
3) Porosity
4）Average pore size
5) Pore size distribution
6) Pore shape and connectivity
11、Why is Langmuir adsorption said to be ideal adsorption? What are the basic assumptions?
Model assumptions:
① adsorption surface is uniform, each adsorption center has the same energy;
② No interaction between adsorbed molecules;
③ monomolecular layer adsorption, adsorption molecules and adsorption center collision for adsorption, a molecule occupies only one adsorption center;
④ under certain conditions, adsorption and desorption can establish a dynamic equilibrium.
12. Types of diffusion
(1) Definition of diffusion: The phenomenon of molecules propagating from a high concentration to a low concentration by random motion.
Conventional diffusion (volumetric diffusion) Pore size of porous solid media ≥ 100 nm; pore size > molecular mean free range; collision between molecules > collision between molecules and pore wall; diffusion resistance is mainly due to collision between molecules.
② Knudsen (Knudsen) diffusion porous solid media pore size ≤ 100nm; pore size < molecular mean free range; intermolecular collision rate < collision rate between molecules and pore wall; diffusion resistance is mainly molecular collision with the pore wall.
(iii) Conformational diffusion Zeolite micropore size and diffusion molecular size close to the molecular size, small changes in molecular size can make the diffusion coefficient change significantly. (pore size <1.5nm) The diffusion resistance is related to a variety of factors such as molecular shape, critical size, molecular interaction with the pore wall, molecular rotation and torsion.
④ Surface diffusion
13. Solid Surface Properties
Atoms on the surface of the solid: the existence of free valence, surface unsaturated sites, with a tendency to saturation; adsorbate molecules and adsorbate surface free valence interactions; solid surface free energy decreases
Solid surface free energy: condensed matter in equilibrium, its shape tends to have the smallest possible surface area, so that the surface free energy tends to be minimized
Adsorption on solid surfaces: spontaneous process, surface free energy decreases (driving force for adsorption)
14. Adsorption states of various compounds on different solid surfaces
(1) Adsorption state of hydrogen:
(2) Adsorption state of oxygen: negative ion state: O2-*, O2 2-*, O-*, O2-*, molecular oxygen: O2*, unstable O3-*
(3) Nitrogen adsorption state: ditopic and polynuclear adsorption, low temperature: ditopic adsorption
(4) Adsorption state of CO: linear structure, bonding, bridge structure, twin adsorption
(5) hydrocarbon adsorption state: saturated hydrocarbons (CH4): dissociative adsorption, unsaturated hydrocarbons: non-dissociative adsorption of the main
15、What are the experimental methods to determine the specific surface of catalyst?
(1) Specific surface area measurement by BET method
(1) Determination principle and calculation method
Based on the multilayer adsorption theory proposed by BET and BET adsorption isothermal curve for measurement and calculation. The BET equation is utilized for graphing, and the data is collected using tests and calculated using the graphical method.
2) Most commonly used method: N2 adsorption method
3) BET theoretical assumptions: the solid surface is uniform; adsorption and desorption are not affected by surrounding molecules; due to van der Waals forces, it is not necessary for the first layer to be full before multilayer adsorption occurs.
(2) Determination of Specific Surface Area by Chromatography The carrier gas for the determination of specific surface area by chromatography is generally He or H2, and N2 is used as the adsorbent, and the adsorption is carried out at the temperature of liquid nitrogen.
16. Properties of metal catalysts
Mostly d-region elements (IB, VIB, VIIB, VIII group elements).
Outer layer: 1~2 S electrons, 1~10 d electrons in the second outer layer Unpaired d orbitals that can be paired by S electrons or p electrons to form chemisorption Characteristics:
(1) bare surface, coordination unsaturated, substable state
(2) cohesion between metal atoms (is the thermal conductivity, electrical conductivity, ductility and mechanical strength of the reason, chemical bonding non-deterministic and thus obtain additional conjugate stabilization energy, the metal is difficult to be dispersed at the atomic level)
3) Participation in reactions as “phases
Why it acts as a catalyst: Transition group metals have empty d-orbitals that are capable of accepting pairs of electrons and conjugate bonds, and at the same time can revert to empty orbitals. This reduces the activation energy between the reactants and catalyzes the reaction.
17. Chemisorption and catalysis
Escape work Ф electron from the metal surface of the minimum energy required; or energy bands for the highest empty energy level and the highest filling energy level of the energy difference.
Ionization potential I will move electrons from the reactants to the outside world the minimum work required. Or the ease with which a reactant can lose an electron
Ф > I: reactant e → catalyst positive ion adsorption
Ф＜I: catalyst e→ reactant Negative ion adsorption
Ф≈I: covalent bond
18. Adsorption Strength and Catalytic Activity: Volcano Type
Example: HCOOH →H2+CO2
☆IR proves the generation of metal-like formate intermediates
☆Predicted catalytic activity is related to the stability of formate, and its stability is related to the heat of generation.
☆The greater the heat of generation, the higher the stability (→ adsorption strength)
☆Pt,Ir,Pd,Ru medium heat of adsorption, high catalytic activity. ☆Pt,Ir,Pd,Ru medium heat of adsorption, high catalytic activity. It can generate sufficient amount of surface intermediates and trade for subsequent decomposition reaction.
19. Electronic Factors and Catalysis: Energy Band Theory
Atomic state: discrete energy levels exist in the structure of the electron layer
Crystal form: atomic orbitals overlap, discrete electronic energy levels expand into energy bands
Electrons belong to the whole crystal and travel freely throughout the crystal – electron sharing
Electron sharing: outermost or second outermost electrons are more prominent. Can only occur at energy levels with similar energies.
20. Fermi energy levels: highest occupied molecular orbitals
Energy band theory: energy levels are continuous, electron sharing.
S orbitals synthesize S energy bands with strong interactions, wide energy bands, and low electron density.
d orbitals synthesize d energy bands with weak interactions, narrower energy bands, high electron density.
The highest energy level occupied by electrons is the Fermi energy level.
21. Energy Band Structure of Transition Metal Crystals
Valence electrons are involved in the s and d energy bands; the s and d energy bands overlap, and the s band electrons can populate the d band to lower the energy
22. d-band holes and magnetization
d-band holes are empty energy levels in the d-band that are not filled with electrons.
The denser spacing of the energy levels in the d-band allows electrons to remain unpaired, and the saturation moment is numerically equal to the number of unpaired electrons in the d-band.
The magnetization of a metal is determined by the number of unpaired electrons, and the magnetization of a metal can be expressed in terms of the size of the d-band holes.
23. d-band hole number and heat of adsorption
The d-band holes or empty energy levels can be used for bonding with adsorbates.
The lower the Fermi energy level, the larger the number of d-band holes and the stronger the adsorption.
Adsorption electron transfer number and d-band hole is about equal to the heat of adsorption is moderate.
24, the valence bond theory of metals that the transition metal atoms to hybridized orbitals combined, hybridized orbitals are usually s, p, d and other atomic orbitals of the linear combination, called spd or dsp hybridization. The percentage of d atomic orbitals in the hybridized orbitals is called the d characteristic percentage, which is a characteristic parameter used by the valence bond theory to correlate the catalytic activity of the metal and other physical properties. The larger the d% of a metal, the more electrons are filled in the corresponding d energy band and the smaller the d holes. For hydrogenation catalysts, a d% of 40-50% is generally desirable.
d characteristic % – the extent to which the d electrons of metal atoms participate in dsp bonding orbitals.
25. influence of crystal structure on catalysis: geometrical factors
Multisite theory- the study of multisite adsorption, geometrical adaptation and energetic adaptation is called multisite theory.
Chemisorption activity of metals: electronic factors or geometric or collective factors
26. Geometric adaptation of multisite theory
Soviet Union Баландин Barankin proposed in 1929 There are multiple active centers on the catalyst that affect the reactant molecules.
When the spatial structure (distribution and spacing) of the catalyst crystal lattice is in geometrical correspondence with the part of the structure of the reactant molecules that will be changed, the adsorbed molecules will be easily deformed and activated, i.e., old chemical bonds will be easily relaxed and new ones will be easily formed. Because the interaction force between the atoms of the reactant molecule and the atoms of the active center is close (zero-point few nanometers), geometric factors affect the distance and thus the close interaction, which becomes the principle of geometric correspondence.
Geometrical factors: number of ligands in the closest neighborhood of the adsorption site and two-dimensional symmetry
