Encyclopedia of Environmental Science and Engineering, Volume I and II

706 MOBILE SOURCE POLLUTION

and hydrocarbons of a given carbon number increased in reac-

tivity according to the series: aromatic  branched paraffi n

or alicyclic  normal paraffi n  olefi nic  acetylenic. The

olefi nic hydrocarbons, generally considered the most unde-

sirable, are relatively easy to remove. Other results could be

summarized in the following manner:

1) With most of the catalysts tested, some crack-

ing occurs during the oxidation of hydrocarbons.

Oxidation without CH 4 formation was possible

with the oxides of Co, Cr, Cu, Mn and Ag only.

Zirconium oxides are unique in that they pro-

duce only cracking products, mainly methane and

smaller amounts of intermediate hydrocarbons.

2) Complete removal of all hydrocarbons was not

attained with some of the catalyst, even at 600
C.

The oxidation is not a simple function of temperature.

The potential of copper oxide-alumina catalyst for air pol-

lution control has been studied by Sourirajan and Accomazzo.^22

They stated that, “the simultaneous removal of hydrocarbons

and carbon monoxide present in the auto exhaust gases has

been tested making use of a six-cylinder Chevrolet engine

run on leaded gasoline fuel. The hydrocarbon and carbon

monoxide concentrations encountered in these studies varied

in the range 170–17,000 ppm and 1–7%, respectively. It was

found that the minimum initial temperature of the catalyst

bed required for the complete removal of both hydrocarbons

and carbon monoxide, simultaneously, was 226
C under no

load condition, 342
C, under an engine load of 2.5hp, 400
C,

under an engine load of 5.1hp or higher, and 236
C under

deceleration conditions. The catalyst showed no deterioration

in performance even after 100 hours of continuous service in

conjunction with the above auto exhaust gases. The extent of

removal of hydrocarbons from the exhaust gases was found

to depend on the initial temperature of the catalyst bed and

the engine load condition.

It is realized that a successful 100 hour run does not

constitute a life test on the catalyst, but it does indicate the

potential applicability of the catalyst in air pollution control

devices. The engineering design of the suitable converter for

any particular practical application of the catalyst should

naturally take into account the heat liberated during oxi-

dation. Instantaneous catalyst temperatures of the order of

900 
C have been encountered in this work with no deleteri-

ous effect on the subsequent effectiveness of the catalyst...

the heat liberated during the reaction can be advantageously

used to maintain the full effectiveness of the catalyst under

all conditions of engine operation encountered in normal

practice.”

When contaminants are passed through a Hopcalite

(unsupported coprecipitate of copper and manganese oxides)

catalyst burner, the results vary from almost complete oxida-

tion of some organics to very slight oxidation of the lower

molecular weight aliphatic hydrocarbons^23 at some 300
C.

Nitrogen compounds form N 2 O when oxidized and haloge-

nated compounds indicate a strong acid reaction when the

reactor effl uent is tested with detector paper.

An interesting example of the use of exhaust gas recy-

cle and catalysts has been presented by the Esso Research

Group. In Figure 3, typical hot cycle traces of CO, O 2 and

NO are presented for cases before the catalyst, after Monel

catalyst and after a 2nd stage Platinum-alumina catalyst,

for instance, with and without recycle. The major benefi cial

effect of recycle is on the NO concentration.

The combustion of gasoline is more or less incomplete

regardless of the quantity of excess air used. About 1% of the

exhaust gas is composed of harmful products chiefl y carbon

monoxide (CO), oxides of nitrogen (NO x ) and hydrocarbons

(HC). A signifi cant variable affecting each of these pollutant

concentrations is the air to fuel ratio (ATFR). The stoichio-

metric value, (ATFR) STO is about 14.7:1.0 on a weight basis.

Using a catalytic three way converter, more than 90%

of the pollutants can be converted to harmless substances.

To avoid catalyst contamination lead free gasoline must

be used. In the closed loop electro-mechanical control of

(ATFR) STO described by Robert Bosch,^34 1985, an oxygen

sensor in the exhaust gas transmits a signal which is used

to correct ATFR deviations. This control method is particu-

larly effective on fuel injection engines because they do not

have the additional delay times of carburetor engines. For

catalytic converter operation, the optimum ATFR range is

X X X X X X

100

50

k

SEC–1

INTRINSIC

120 SCFM

ACCELERATION

60

30

needed for

80%

conversion

10 IDLE

500 400 300 200 °C

AGED

FRESH

10

5

1

0.5

.25mm

1.5mm

I / T °K

FIGURE 2 Carbon monoxide activity of an eggshell catalyst.

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