Question 1

How do catalysts increase reaction rates?

Accepted Answer

Catalysts increase reaction rates by providing an alternative mechanistic pathway with lower activation energy, the minimum energy required for a reaction to proceed. They are not consumed in the reaction, making them valuable for industrial-scale processes. Heterogeneous catalysts, typically solid nanoparticles on support materials, work through adsorption of reactants, reaction, and desorption of products on active sites.

Question 2

What is the difference between homogeneous and heterogeneous catalysts?

Accepted Answer

Homogeneous catalysts exist in the same phase as reactants, while heterogeneous catalysts exist in a different phase. Heterogeneous catalysts are typically solid, nano-scale entities dispersed on support materials. For gas-phase reactions, transition metal nanocrystals are the most common heterogeneous catalysts, operating through the Langmuir-Hinshelwood Mechanism involving surface adsorption and desorption steps.

Question 3

What happens during the Langmuir-Hinshelwood mechanism?

Accepted Answer

The Langmuir-Hinshelwood mechanism begins with reactants adsorbing to the catalyst surface, where their bonding electrons delocalize into empty orbitals of transition metal atoms, often causing dissociation. Several intermediate steps follow, concluding with a bimolecular elementary reaction forming products that desorb from the surface. The overall reaction rate depends on the slowest elementary step and the availability of catalytic reactive sites.

Question 4

Why is a plug flow reactor appropriate for this catalytic hydrogenation?

Accepted Answer

A plug flow reactor is appropriate because high conversions and spatially variable reaction rates are expected in the ethylene hydrogenation process. The tubular reactor is packed with catalyst and operated continuously with reactants added and products withdrawn. This model allows researchers to apply kinetic analysis to reactor effluent data, yielding power law rate expressions that provide evidence for the reaction mechanism.

Question 5

What does the power law rate expression reveal about the catalyst?

Accepted Answer

The power law rate expression reveals information about how reactants interact with the catalyst surface. In ethylene hydrogenation, the best-fit expression includes ethylene concentration raised to the first power and hydrogen concentration to the one-quarter power. This indicates hydrogen adsorbs strongly to the catalyst while ethylene adsorbs weakly, consistent with a kinetically controlled Langmuir-Hinshelwood mechanism.

Question 6

What safety systems are essential in a catalytic reactor setup?

Accepted Answer

Essential safety systems include high-pressure relief valves, high-temperature shutdown controls, bypass and venting systems, and combustible gas leak detectors. Diluted reactants are used to reduce hazards. The reactor operates in an electrically heated, temperature-controlled sand bath with continuous monitoring of flow rates and temperature. Emergency shutdown procedures include closing reactant valves, shutting off heaters, and maintaining air flow until the system cools.

Question 7

How are catalytic reactors applied in industrial processes?

Accepted Answer

Pilot-scale catalytic reactors study effects of catalysts and reaction conditions on chemical synthesis. The Fischer-Tropsch Synthesis produces alkane fuels from carbon monoxide and hydrogen using iron, cobalt, or ruthenium catalysts. The Haber-Bosch Process produces ammonia from hydrogen and nitrogen using iron or cobalt-molybdenum catalysts. Research continues into long-life, high-selectivity catalysts to improve efficiency and reduce operational pressure requirements.

Catalytic Reactor: Hydrogenation of Ethylene - Assessment Quiz