I3 Flashcards
What are the 5 steps in the Catalysis cycle?
Transport to catalyst
Adsorption to catalyst
Run to form product
Desorption of product
Transport away from catalyst
What are the rate limiting steps in the catalytic cycle?
Step 2-4 chemical processes Ea>40KJmol(-1) chemical kinetics
1+5 transport process Ea>4-10KJmol(-1) chemical diffusion
What are the parts of the catalyst?
Active site: responsible for the catalyst
Support: disperses the active phase, give final shape, mechanical strength, bulk of catalyst, not active on its own.
What support types are there?
Alumina (alpha, gamma)- most common- cheap, inert, thermally stable, SA-1<->500m^2g(-1)
Silica - amorphous, cheap, inert, SA <-> 800 m^2g(-1)
Carbon- cheap, used in fine chem/pharma, bad reproducibility, SA-<4,000 m^2g(-1)
What is Sabtier’s Principle + graph derived from it?
Principle- The best catalyst binds a key intermediate strongly enough so that it reacts but weakly enough so that the product will easily desorb.
Graph- Volcano curve can be found with Y-axis rxn temp(K), X-axis delta fH(standard). Shows activity of catalyst materials.
What is the Raney/Skeletal method? How is it made?
Finely divided catalyst, metal-alumna alloys, NiAl, CuAl. Once Al is dissolved out it becomes highly porous, high SA, High activity.
‘Finger diagram’
What are the uses of Co-precipitation
Cu/ZnO/Al2O3 —> Methanol Synthesis
Cu-Cr oxides —> Hydrogenation
Fe2O3–> ethyl benzene to styrene
What is the process of Co-precipitation
Precipitation - Physical - Cooling/heating, Chemical- carbonate to precipitate out metal hydroxide, foreign ions replace some of the ions in the crystal lattice, the precipitate is filtered, washed, dried and calcinated to form the catalyst or catalyst precursor.
What are the 4 forms of catalytic imoregnation?
Uniform, Eggshell (Fast rxn with surface), Egg white, Egg yolk
Slow rxn by changing surface sites what would you add instead of PtCl6(2-)
Add HCl first
How is egg yolk distribution achieved?
Add oxalic acid/citric acid - strong absorbers - preferring to go to the pore mouth then, Pt solution, CH2PtCl6, skips blocked sites.
Steps for imoregnation?
Inject into support, drying 20-100, heat treatment 200-700 converts salt to oxide (calcination), activation treatments 200-500.
Industry example of impregnation
Fischer Tropsch- Fe or Co on Al2O3
Ethene to ethylene oxide - Ag on Al2O3
How do you know you’ve made the right catalyst?
Microscopy- size info, XRF + SEM-EDX, Raman spectroscopy - metal oxides
What’s the difference between the Elay Rideal mechanism and the Langmuir Hinshelwood?
ER- One molecule adsorbs + reacts with gas phase molecule- adsorbs
LH- 2 molecules adsorb on neighbouring sites-react-desorb.
What is the Mars-Van Krevelen mechanism?
Molecule adsorbs- surface O atom reacts - product desorbs- O vacancy replenished by gas phase oxidant.
What is the Langmuir adsorption isotherm?
Theta=(KPa)/(1+KPa) where K=Ka/Kd
How do you calculate dispersion?
1G of 5 wt% PdAl2O3 => 5/100 =0.05 g of Pd use that to figure out moles then multiply by Na for the total atoms.
Dispersion= (no. Pd atoms with CO2 adsorbed/ total no. Pd atoms)
What is the methanol production process?
Desulphurisation
Steam Reforming —>Steam Raising
I
Compression. <–I
Methanol Synthesis
Distillation
What methods can be used to better understand the mechanism?
XRD- see changes in phase of the starting vs working catalyst
XAS- Oxidation state change.
What is Poisoning?
Selectively blocking the edge + corner which inhibit some reactions
How do you investigate a catalytic rxn?
1)Understand rxn profile- Sun addition of H to double bond
2)What’s controlling selectivity- formation of PdCx phase inhibiting reactive H
3)Form a hypothesis + test
What is the rxn for exhaust emissions involves NO2?
3NO2 + H2O —-> HNO3 + NO
What is in a three way catalytic converter?
Monolith: (Mg,Fe)2 Al4Si5O18
Catalyst: Pt or Pd + Rh
Support: Al2O3
Promotor: CeZrO2- helps prevent sintering
