MOBILE SOURCE POLLUTION 705
In this system, when the engine operation is too rich (too
little oxygen) nitrogen oxide reduction is found to be excel-
lent, but carbon monoxide conversion poor. The nitrogen
oxide will readily combine with Monel, forming Monel oxide
and nitrogen since there is little oxygen to compete with the
nitrogen oxide for active sites on the Monel. However, since
the CO:NO ratio is so large, there will be insuffi cient Monel
oxide formed to give up its oxygen to the carbon monoxide.
On the other hand, if the engine operation is too lean, NO
reduction will be poor and CO conversion excellent since the
reaction becomes signifi cant and oxygen will compete with
the nitrogen oxide for sites on the Monel.
Some typical conversion data^1 at space velocities of
50,000 v/vhr indicated that conversion of NO in synthetic
gas mixture (2% CO, 1000 ppm NO) was over 95% if the
temperature was above 700C, but fell off sharply at temper-
ature below 650C. Problems associated with this technique
are the formation of NH 3 if any residual H 2 is present and
back pressure buildups with the current catalyst structure.
In addition, dusts form which are emitted as particulates. It
is essential for effi cient performance of this catalyst that it
warms up to operating temperatures very rapidly. Lead in the
fuel reduces the chemical activity and ultimately increased
the rate of deterioration of the catalyst.
Another catalyst which showed promise for the same
reaction but at lower temperature (200 to 350C) has also
been mentioned.^18 Despite the fact that conversions of better
than 90% were reported equal amounts of CO and NO were
used. Automotive exhaust have about 16 times more CO thanNO. Activated carbon has been used^19 successfully with H 2
gas at 600C. However, activated carbon lost a considerable
portion of its activity after only 7 hours service.
CO and Hydrocarbon Removal Major automotive and
petroleum companies have combined efforts in the develop-
ment of an inexpensive multi-thousand mile catalytic pack-
age for reducing CO and HC exit concentrations. Since the
input of gasoline and hence the effl uent gases are rarely at
steady state, any study of a catalytic reactor must consider
the dynamic situation. To give some idea of magnitude, the
exhaust gas fl ow in standard cubic feet per minute (SCFM)
is roughly twice the miles per hour equivalent of an auto-
mobile and the temperatures of the exhaust gas change from
that of the ambient to well over 600C. Figure 1 shows typi-
cal CO and temperature levels in the exhaust stream after
engine startup in a Federal cycle.
For balancing pollution problems with no catalytic or
afterburner control an air–fuel ratio of about 16 is recom-
mended. Above this level the NO x level increases markedly
and below it the amount of unburned HC and CO substan-
tially increases. With catalytic devices this ratio may no
longer be optimum, since the catalyst selectivity may be
greater towards removal of one of the pollutants than any of
the others.
We i^20 has noted that the kinetics of CO oxidation over
an egg-shell catalyst turn out to be fi rst order for CO and
0.2 order for O 2 in the range of 1 to 9% oxygen. The curva-
ture in the Arrhenius plot (Figure 2) is believed to be caused
by a pore diffusional phenomena. As the catalyst ages and
activity falls the reaction rate becomes controlling and the
Arrhenius plot becomes a straight line. The catalyst of the
fi gure is best above 350C, but a lower operating tempera-
ture may be preferred for longer catalyst life. Wei^20 found
that, “As the catalyst lost 90% of its activity, the emission
rose by only 30%; but the last 10% of activity loss would
result in a precipitous rise of carbon monoxide emission.
A catalyst with 50% reduction in heat capacity performs much
better; a reactor with 50% reduction in volume performs better
when the catalyst is fresh and worse then it is aged.” His phi-
losophy is, “It is our engineering goal to produce a low-cost
and convenient solution. However, any solution requires some
inconvenience and cost from everyone. Quick warm-up is no
problem if we are willing to sit and wait in the car for 2 min
for an auxiliary heater to warm up the catalyst bed before the
car moves. We have ninety million cars on the road, and a
$100 device will cost us nine billion dollars. How much are
we willing to pay for 90% cleaner air? These decisions belong
to the public, not the engineers. For the sake of everyone we
hope to be able to present to the public an economical and
convenient solution in the near future.”
Stein et al.^21 have evaluated the effectiveness of possible
catalysts by a microcatalytic technique based on gas chroma-
tography. The technique which is described in detail allows
a large variety of hydrocarbons and catalysts to be rapidly
tested over a wide range of temperatures (100–600C). In
general oxides of cobalt, chromium, iron, manganese, and
nickel are the most effective catalysts. The higher molecular
weight hydrocarbons are more easily oxidized than the lowerTABLE 3
The equilibrium constants for some reduction reactions of nitric oxide^17log Kp
Reactions 600 700 800 900 1000K
NO 5H 2 → 2NH 3 2H 2 O 49.0 48.1 32.13 26.44 21.8
NO 2H 2 → N 2 2H 2 O 51.7 43.4 37.1 32.2 28.3
NO CO → ½N 2 CO 2 27.3 22.6 19.7 16.46 14.29
NO CH 4 → ½N 2 2H 2 19.59 18.1 17.6 16.2 15.5
NO Hat → HNO 12.4 9.77 7.79 6.24 5.0
NO ½ H 2 → HNO 3.7 3.6 3.52 3.46 3.427 Temperature 600CO %(^00)
Time, min.^7
0
°C
FIGURE 1
C013_005_r03.indd 705C013_005_r03.indd 705 11/18/2005 10:42:27 AM11/18/2005 10:42:27 AM