Part 2 - Oxides, porosity and catalyst support materials

Question 1

What are the most common materials used for support in catalysis?

Answer 1

Alumina, silica and carbon.

Question 2

What are the desired properties of a surface?

Answer 2

High surface area, high stability (thermal, mechanical, chemical), promoting of catalysis (stabilizing metal particles, strong metal support interactions, assist the catalysis with reactant activation, oxygen storage and acidity) and processable to desired shapes.

Question 3

Name some other common materials used for supports in catalysis?

Answer 3

MgO, ZrO2 (zircona), TiO2 (titania), ZnO, SiC, zeolites

Question 4

What type of support are used for high temperature applications such as solid oxide fuel cells?

Answer 4

Mixed oxides.

Question 5

What two forms of alumina are there?

Answer 5

alpha - crystalline, low surface area, high thermal stability
gamma - porous, amorphous, spinel-like structure

Question 6

How is aluumina produced?

Answer 6

1) From Bauxite via different routes
2) Treatment with NaOH
3) From co-percipitation of salt solutions (in lab)

Question 7

What is the structure of silica?

Answer 7

Amorphous, but local order is possible. Though many crystalline forms exist.

Question 8

How does the stability of silica compare with alumina?

Answer 8

Less stable, particularily in the presence of steam.

Question 9

Name two methods silica can be produces, and the resulting products.

Answer 9

Through sol-gel percipitation -> xerogel

Flame hydrolysis -> fumed silica

Question 10

Name one desireable property of silica as a support?

Answer 10

It has a tuneable pore distribution and surface area.

Question 11

Name some good properties of carbon supports.

Answer 11

High surface area, high stability in certain environments (though not O2/H2O and high T). Electrical conductivity - makes them good as supports in fuel cells.

Question 12

What is the conventional fabrication method for carbon supports? What is one advanced fabrication method?

Answer 12

Conventional: pyrolysis of biomass or coal.
Advanced: hydrocarbon decompositions to CNF/CNT.

Question 13

What are the size ranges for micro, meso and macropores?

Answer 13

Micro 50 nm

Question 14

How can pores be distinguished from roughness?

Answer 14

If the features are deeper than the lateral dimensions

Question 15

Who invented BET?

Answer 15

Brunauer, Emmet, Teller.

Question 16

What is BET based on?

Answer 16

Physisorption of an inert gas (N2). It measures an adsorption isotherm, that is how the amount of adsorbed gas depends on the equilibrium pressure of the gas at constant temperature.

Question 17

What are the assumptions for BET?

Answer 17

• The rate of adsorption and desporption in any layer are equal (equilibrium)
• In the first layer molecules adsorb on equivalent sites.
• ∆Hads for the second and consecutive layers are the same. Adsoprtion heat is approx. same as condensation heat for the gas.
• ∆Hads for the first layer is independent and different from layer 2.
• The surface is constant during adsorption.

Question 18

Write the BET-equation.

Answer 18

P/[Va(P-P_0)] = 1/(xV_0) + (x-1)/(xV_0) * P/P_0

P - adsorption pressure
P_0 - equilibrium pressure of condensed gas (saturation pressure)
V_a - total volume of adsorbed gas
V_0 - volume of gas adsorbed in the first monolayer

Question 19

How do we linearize the BET-equation, and how do we get anything useful from it?

Answer 19

P/[Va(P-P_0)] = 1/(xV_0) + (x-1)/(xV_0) * P/P_0 = eta + alpha * P/P_0

We then plot P/[Va(P-P_0)] vs. P/P_0.

We know dependence of Va on P from experiment.
eta is intercept, alpha is slope.

Use these relations for the expression of specific surface area:

V_0 = 1/(alpha + eta)
V_molecule = M / (rho * N_A)
N_molecules = V_0 / V_molecule

S_0 = 16.2 (N2 area) * N_molecules (in Å)

Question 20

What are some limitations to the BET-method?

Answer 20

• Microporous materials - mono- multilayer adsorption cannot occur
• assumption about constant N2-packing not correct?

Question 21

What are some common oxides and sulfides used as support?

Answer 21

Oxides: MgO, TiO2, MoOx (+4,+5,+6)
Sulfides: MoS2, WS2

Question 22

What is an advantage of readily reduced oxides?

Answer 22

Supply of O-atoms to oxidation reactions
O-buffer for varying conditions.

CeOx in paritcular.

Question 23

What are zeolites?

Answer 23

Porous, crystalline materials built from oxygen tetrahedra connected by metal atoms (Si or Al). The tetrahedra are linked by by common O-atoms to form cages. These cages are building blocks for a range of structures.

Question 24

What are the three different mechanisms of selectivity in zeolites?

Answer 24

1) Reactant selectivity: only allows smaller molecules to transport through the structure.
2) Product selectivity: only certain products can diffuse out of the structure.
3) Restricted transition state selectivity: Some transition states can’t form inside the structure.