Part 2 - Basics of catalysis

Question 1

What is the definition of a catalyst, according to Ostwald?

Answer 1

A catalyst is a substance that enhances the rate of a reaction without itself being consumed.

Question 2

Name four industrial areas in which catalysts are used.

Answer 2

Peteroleum and energy production
Chemicals and polymer production
Pollution control (removal of gases)
Pharmaceutical and Food industry

Question 3

In brief terms, how does a catalyst help a reaction go faster?

Answer 3

It introduces another pathway with a lower activation energy barrier.

Question 4

What are the four types of catalysis?

Answer 4

Heterogenous catalysis, homogenous catalysis, electrocatalysis and biocatalysis.

Question 5

What is heterogenous catalysis?

Answer 5

Catalysis where reactant and catalyst is in different phases, for example gas reactants and solid catalyst.

Question 6

What is homogenous catalysis?

Answer 6

Catalysis where reactant and catalyst is the same phase.

Question 7

What is electrocatalysis?

Answer 7

It is associated with electrode reactions and electron transfer (redox)

Question 8

What is biocatalysis?

Answer 8

Enzymes that work as catalysis. Huge protein molecules. High selectivity, low temperature activity.

Question 9

What is activitiy of a catalyst?

Answer 9

The catalyst’s ability to convert the reactants (amount of reactant converted / amount of catalyst and time)

Question 10

What is selectivity of a catalyst?

Answer 10

That catalyst’s ability to convert the reactants into the desired product. (amount of product / amount of reactant converted)

Question 11

What is the lifetime of a catalyst?

Answer 11

The time during which a certain activity and/or selectivity is maintained.

Question 12

Explain the normal pathway of a heterogenous catalysis reaction.

Answer 12

• Reaction initiated by adsorption of reactants on the surface of the active material (the catalyst)
• Molecules may or may not dissociate (CO can be split into C* and O, or stay as CO)
• The adsorbed species react to form new, adsorbed molecules (products)
• The products desorb.

Question 13

Since several of the most used catalysts are very expensive, how do one enhance the activity of a given amount of catalyst material?

Answer 13

Make small nanoparticles dispersed on a highly porous support.

Question 14

Why do we often say “small is better” in catalysis?

Answer 14

Because the reaction occurs on the surface. Smaller particles yields a higher surface/bulk ratio, so the material is better utilized. It also exposes more edge and corner-sites, which are often more catalytic active.

Question 15

What are some other differences of small particles compared to larger ones?

Answer 15

• Small particles lose their metallic properties.
• Charge transfer between particle and support will influence properties.
• Interface effects and spillover are important.
• Strong metal support interactions

Question 16

What are some developments that have given us insights into catalysis?

Answer 16

• Better instruments, that can investigate ever closer to the working temperatures.
• Spectroscopies that give detailed information on bonds, and electronic states
• Computations that elucidate some of the findings, and enable predictions of good candidate materials.
• Parallelization and miniaturization of material synthesis, allowing high-throughput-screening.

Question 17

Name some classical examples of heterogenous catalysis.

Answer 17

• Ammonia synthesis
• Upgrading of crude oil
• Natural gas conversion
• Polymer production
• Exhaust cleanup

Question 18

What is the three-way catalyst?

Answer 18

It is the catalyst used to clean up the exhaust from an ICE. Lowers the emissions of NOx, CO and uncombusted hydrocarbons. There are three reactions here (most efficient catalysts in paranthesis):

CO + O2 -> CO2 (Pt, Pd)
CxHy + O2 -> CO2 (Pt, Pd)
NO + CO -> N2 + CO2 (Rh, Pd)

Question 19

If the three-way catalysts mainly produces CO2, why would we want to use it?

Answer 19

Because the other exhausts are much worse. CO blocks the transfer of oxygen in the blood, and suffocates you. NOx is environmentally acidic, induces the formation of ground level ozone and smog and is acutely poisonous in moderate concentrations. And uncombusted hydrocarbons are so callde volatile organic compounds, that also can induce ground-level ozone and some are also carcinogenic.

Question 20

What role does ceria (CeO2) play in a TWC?

Answer 20

Added to the support to act as an oxygen buffer. The TWC needs to have almost stoichiometric ratio to function properly. Works in conjuction with a lambda-probe.

Question 21

What are the typical properties of the support material in a TWC?

Answer 21

• High surface area, often achieved by adding a washcoat in a honeycomb structure. The washcoat is porous (alumina, titania, silica, or a alumina/silica mix).
• Ability to stabilize small metal particles
• high mechanical strength
• no effect on reaction

Question 22

What is the typical measurement of catalyst activity in the lab?

Answer 22

Turnover-frequency. TOF = # reactant molecules reacted / site * time

Not always a practical measure for industry.

Question 23

What are the requirements of a good catalyst in the industry?

Answer 23

• High activity per volume
• High selectivity under the conditions applied.
• Long lifetime under the conditions applied
• Reproduceable, low-cost preperation even at large scale
• Sufficient mechanical strength and attrition resistance
• Low cost materials, minimum by-products
• Shape, structure and composition that facilitates heat and mass transfer.

Question 24

Give four ways you can measure the catalyst activity.

Answer 24

• Turnover-frequency. TOF = # reactant molecules reacted / site * time
• Rate of reactants converted/products formed (mol / g cat * s)
• Conversion, X [%]: reactants converted / reactants fed
• Yield, Y [%]: products desired formed / reactants fed

Question 25

Can the catalyst change the thermodynamic equilibrium?

Answer 25

No! Only the rate at which the equilibrium is reached.