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52 2. Adsorption, Ion Exchange, and Catalysis

The main advantages and disadvantages of catalysts are

age Advants

• high efficiency in the treatment of gas emissions
• large variety of catalysts
• simple installation

Disadvantages

• ater treatment) w deactivation phenomena (mainly in the w aste
• v spent catalysts hae to be appropriately disposed
• a specific temperature is required for their operation
• for low concentrations of VOCs, the heating of larolumes of emissions to the tem- ge v
e perature required for catalytic activity is e v xpensi

A look into thr ee-way catalysis
The need for controlling the exhaust emissions from automotie v v ehicles
has been recognized since 1975. The most effective and tested method
proved to be the installation of three-way catalysts at the e xhaust
emission system of cars. The deelopment and the impro v v ement
of such catalysts was and will be a complicated effort, since a cat-
alyst placed in a vehicle should simultaneously accelerate oxida-
tion and reduction reactions, under continuously changing
elocity
conditions of temperature and space v in contrast to
,
industrial applications where catalysts operate under fed and ix
,
v
controlled conditions. Generally the catalytic conerter of a
vehicle has to satisfy the follo wing requirements:
• facilitate the oxidation reactions of carbon monoxide and unburned hydrocarbons and
the reduction of nitrogen oxides (three reactions to perform; hence they are named
“three-way catalysts”)
• start its operation at the lowest possible temperature, since the emissions are high dur-
ing the first minutes of engine operation, where the temperature is still lo w
• show resistance for a short time at temperatures up to 1000 °C
• exhibit a satisfactory operation for at least 150,000 km
• be highly active in order to achieve the desired conversions for high volumetric feed of
emissions that take place at the engine e xhaust
• all the aboe to take place at continuously changing air-to-fuel ratios. v e ha v

v The presence of a three-way catalyst is mandatory for eery car produced in the United
e and one of the States and Europe since 1981 and 1993, respecti v . It is the most massi v ely
most successful stories in the history of catalysis.
The demand for occupying less space, operation at high volumetric feed, and lo w
loss of power led to the adoption of monoliths for the automobile catalyst. A monolith