What is a catalyst ?

A catalyst is a substance that speeds up a chemical reaction, but is not consumed by the reaction.

Hence, a catalyst can be recovered physically or chemically unchanged at the end of the reaction it has been used to speed up, or catalyze. For chemicals to react, their bonds must be rearranged, because the bonds in the products are different from those in the reactants.

The slowest step in the bond rearrangement produces what is termed a transition state - a chemical species that is neither a reactant nor a product, but is an intermediate between the two.

Reactant ⇄ Transition State ⇄ Product

Energy is required to form the transition state.

This energy is called the activation energy, or Ea. Reading the diagram below from left to right shows the progress of a reaction as reactants pass through the transition state to become products. The activation energy can be thought of as a barrier to a chemical reaction. If the barrier is high, only few molecules have sufficient kinetic energy to collide, form a transition state, and cross the barrier.

Reactants with energy lower than Ea cannot pass through the transition state to react and become products.

A catalyst works by providing a different route, with lower Ea, for the reaction. Catalysts lower the energy barrier. The different route allows the bond rearrangements needed to convert reactants to products to take place more easily, with a lower energy input. In any given time interval, the presence of a catalyst allows a greater proportion of the reactant species to pick up sufficient energy to pass through the transition state and become products.

Catalysts cannot shift the position of a chemical equilibrium. The forward and backward reactions are both accelerated so that the equilibrium constant Keq remains unchanged. However, by removing products from the reaction mixture as they form, the overall rate of product formation can in practice be increased.

Example: The Haber-Bosch process for ammonia synthesis

The Haber–Bosch process is the main industrial procedure for the production of ammonia today, with a significant contribution to humanity via the development of fertilizers. It is named after its inventors, the German chemists Fritz Haber and Carl Bosch, who developed it in 1908-1913. Ammonia is synthesized by hydrogen and nitrogen at high pressure and temperature over an iron-based catalyst, which provides atomic sites on which the reactant bonds can rearrange more easily to form the transition state.

Heterogeneous Catalysis

Heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. The great majority of practical heterogeneous catalysts are solids and the reactants are mainly gases or liquids. Heterogeneous catalysis is of paramount importance in many areas of the chemical and energy industries. Worth noting is that heterogeneous catalysis has attracted 4 Nobel prizes so far; Fritz Haber in 1918, Carl Bosch in 1931, Irving Langmuir in 1932, and Gerhard Ertl in 2007.

A heterogeneous catalytic process goes through several steps of diffusion and chemical reactions on the catalytic surface as shown in Figure 2; the reactant species diffuse across the gas film towards the catalyst surface (external diffusion), continue to diffuse in the catalyst pores (internal diffusion) and finally reach the catalytically active sites where they adsorb or react with other adsorbed species and form the products. The latter species desorb to the gas phase and diffuse across the pores and the gas film.

Application in automobile exhaust purification:

The three-way catalytic converter

A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine to less toxic pollutants by catalyzing the possible chemical reactions. Catalytic converters are mainly used since 1980s with internal combustion engines fueled by petrol (gasoline). However, over the last decade similar catalysts have been introduced to diesel engine exhaust gas purification. The first widespread introduction of catalytic converters was in the United States automobile market. To comply with the U.S. Environmental Protection Agency's stricter regulation of exhaust emissions, most gasoline-powered vehicles starting with the 1975 model year had to be equipped with catalytic converters. These "two-way" converters combine oxygen with carbon monoxide (CO) and unburned hydrocarbons (HC) to produce carbon dioxide (CO2) and water (H2O). In 1981, two-way catalytic converters were rendered obsolete by "three-way" catalytic converters that also reduce oxides of nitrogen (NOx) to N2.

The “two-way” converters are still used in lean-burn engines, due to high oxygen excess present in the exhaust gas that severely poisons the metal catalyst (Rh) which conducts the NOx reduction.

These three reactions occur most efficiently when the catalytic converter operates under exhaust conditions slightly above the stoichiometric point. For gasoline combustion, this ratio is between 14.6 and 14.8 parts of air to one part of fuel, by weight. The ratio for LPG, natural gas and ethanol fuels is slightly different for each, requiring modified fuel system settings when using these fuels. Conversion efficiency drops rapidly when the engine is operated outside of this band. Under lean engine operation, the exhaust contains excess oxygen, and the reduction of NO is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, leaving only the oxygen contained in the catalyst support available for the oxidation function. The electronic control system prevents the NO reduction catalyst from becoming fully oxidized, yet replenish the oxygen storage material so that its function as an oxidation catalyst is maintained. Figure 3 shows a schematic of a three-way catalytic converter used in automobiles; it consists of a perforated ceramic carrier (cordierite honeycomb) enclosed in a metal casing. The ceramic-supported metal catalyst (washcoat) is deposited on the cordierite walls inside the channels of the cordierite.

The use of oxygen storage materials, like ceria, in automotive catalysis has been applied over the last two decades, due to the scientifically proven active role of oxygen vacancies and over-stoichiometric oxygen on the catalytic properties of supported catalysts. At high temperatures (>300oC) these oxygen species can migrate (back-spillover) onto the metal particles (e.g. Pt or Pd or Rh in automotive catalysis) and either participate in the oxidation reactions, or serve as promoters, or both. Critical is also the role of the oxygen vacancies on the oxide support near the three phase boundaries (metal oxide / metal / gas), where NO dissociation seems to take place.

Although catalytic converters are most commonly applied to exhaust systems in automobiles, their application platform includes electrical power generators, forklifts, mining equipment, trucks, buses, locomotives, motorcycles, etc. In automobiles a typical lifetime of a catalytic converter is 150,000 - 200,000 km. Spent catalysts can be recycled to recover the precious metals.

[1] https://en.wikipedia.org/wiki/Catalysis
[2] https://en.wikipedia.org/wiki/Heterogeneous_catalysis
[3] http://ect.jmcatalysts.com/catalyst-technologies-johnson-matthey/three-way-catalysts
[4] Handbook of Heterogeneous Catalysis, 2nd Edition, by G. Ertl (Ed), H. Knözinger (Ed), F. Schüth (Ed) and J. Weitkamp (Ed), ISBN: 978-3-527-31241-2
[5] A.M. Beale, et al., Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials, Chem. Soc. Rev., 2015,44, 7371-7405.
[6] Ph. Vernoux, et al., Ionically Conducting Ceramics as Active Catalyst Supports, Chem. Rev., 2013, 113 (10), pp 8192–8260.

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This project has received funding from the European Union’s Horizon 2020 Research and Innovation program under Grant Agreement n° 778893