Lloyd L. Handbook of Industrial Catalysts

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458 Chapter 11

drogen that improves the performance of the catalyst under rich condi-

tions.

• Barium oxide has often been included in the washcoat because it can also

absorb oxygen under lean conditions. The barium peroxide that formed

also provides oxygen as it decomposes during rich operation.

• Zirconia is added to stabilize the ceria and to ensure than oxygen remains

available from the surface oxides even if the ceria surface has been sin-

tered at the high operating temperature of about 850°–900°C. Zirconia al-

so plays an essential role in the three-way catalyst formulation as the sup-

port for rhodium.

Ceria and zirconia have been shown by electron microprobe spectroscopy to

combine the preparation of the catalyst, forming a very thermally stable phase,

and platinum group metals deposit preferentially on the alumina.

The washcoat

has often been applied and fired with more alumina before the platinum group

metals are impregnated. Alternatively, the washcoat and metals can be applied at

the same time. In either case, the conditions for the deposition can be adjusted to

provide a variable surface layer of alumina and the oxides, which absorb trace

poisons and protect the active metals.

11.2.2.4 Platinum Group Metal Catalysts

When automobile emission regulations were first introduced, only platinum and

palladium catalysts were used for the oxidation of carbon monoxide and residual

hydrocarbons. Both metals were active oxidation catalysts although palladium

was more temperature resistant than platinum but was more readily poisoned by

sulfur. Rhodium became an important part of the three-way autocatalysts be-

cause it had high activity for the reduction of nitric oxide. Three-way catalysts

were used in all cars from 1981 because of US Federal regulations although they

had been introduced by California for 1978 model cars.

Although platinum and palladium have some activity for the reduction of

NOX under stoichiometric conditions, the presence of oxygen in exhaust gas

inhibits the conversion. Nitric oxide is, however, strongly adsorbed on the rho-

dium (III) surface to form nitrosyl groups.

These are reduced by adsorbed car-

bon monoxide and the nitrogen produced is desorbed at temperatures between

200°–300°C. Rhodium is less catalytically active than either platinum or palla-

dium for the oxidation of carbon monoxide, because the adsorption of carbon

Rhodium also has a relatively high water gas shift activity, does not sinter at high

temperature and resists sulfur poisoning.

Some modifications to the operation of the car engine were made to resolve

early difficulties in the optimisation of the removal of all three pollutants from

the exhaust emission. Different combinations of the three catalysts were used in

monoxide by rhodium is inhibited by high concentrations of nitrogen atoms.

Environmental Catalysts 459

The exact composition of the operating

literature, as this type of information is kept confi-

typical ranges are as follows:

• Oxidation catalyst: platinum/palladium ratio of ~5:2 with 1.5 g of metals

per liter of monolith volume.

• Three-way catalyst: platinum/rhodium ratio of 5:1 to 20:1 with 0.9–2.2 g

of metal per liter of monolith volume.

• Three-way catalyst (containing palladium): metal contents of about 0.9–

3.1 g Pt, 0–3.1 g Pd, and 0.15–0.5 g Rh per monolith.

• Palladium light-off catalyst: with 1.8–10.6 g of metal per liter of monolith

volume.

• Three-way catalyst (all or most of the platinum replaced by palladium):

platinum/palladium/rhodium ratio of 0–1:8–16:1 with 2–5.5 g of metal

per liter of honeycomb volume.

The switch from platinum to palladium has been driven by the better

availability and lower price of palladium, together with a higher level of com-

bustion of the hydrocarbons. The improved performance of palladium has arisen

because the amount of sulfur and lead impurities in the gasoline has decreased.

11.2.2. Catalyst Poisons

The performance of the catalyst is adversely affected by the presence of residual

sulfur in the gasoline, and by tetraethyl lead, which used to be added to improve

the octane rating of the gasoline. By the late 1990s, the use of lead additives had

almost completely been discontinued in many parts of the world. Furthermore,

the amount of sulfur in gasoline was also limited by environmental legislation.

The sulfur components of gasoline, which were oxidised to sulfur dioxide in the

engine and subsequently reduced to hydrogen sulfide in the exhaust, adversely

affected the performance especially of palladium catalysts. Oil additives such as

zinc dialkyl-dithiophosphates also have a poisoning effect on the catalyst due to

the presence of zinc and the oxides of phosphorus in the exhaust gas. The total

exposure of the catalyst to poisons

during a typical lifetime of 100,000 miles is

shown in Table 11.11.

The addition of nickel oxide to a catalyst washcoat can minimize the for-

mation of hydrogen sulfide during lean operation by absorbing some of the sul-

fur dioxide from exhaust gas. Nickel oxide can, therefore, store the sulfur diox-

ide as sulfate under reducing conditions and then release the sulfur dioxide un-

der oxidizing conditions.

various locations behind the engine.

catalysts is not available in the

dential within the business, but

460 Chapter 11

TABLE 11.11: Poisons in Catalysts Used Over 100,000 Miles.

Poison Approximate poison content of fuel (g/kg catalyst)

Sulfur oxides from gasoline 6–20

Phosphates 0.08–0.2

Zinc (as dialkyl dithiophosphate) 0.32–0.48

Lead (as tetraethyl) Depends on gasoline; ideally zero

11.2.3 Platinum Metal Group Availability

The platinum group metals are usually found as sulfides, arsenides or as the

native metal, usually in conjunction with base metals. The concentration is

almost always too low to justify mining for the precious metals alone, and the

worldwide availability of the precious metal component tends to be determined

by the demand for the other metal. For example, platinum is most commonly

associated with nickel and copper sulfide deposits, and it is the extraction of the

base metals from their ores that provides an economic route to the precious met-

al. In a typical operation in the US, 10 lbs copper can be extracted from a ton of

ore, but the content of palladium is 0.000029 tr.oz and the platinum content is

only 0.0000029 tr.oz. Significant quantities are only available in South Africa,

Canada, Russia and the United States.

Spent honeycomb exhaust catalysts contain a relatively high concentra-

tion of the precious metal, and these can be recycled. It is certain that they will

become an important and economic source of the metals as the use of automo-

biles continues to increase. About 15% platinum and 5% rhodium were recycled

in 1990. The predicted demand for the metals to be used in autocatalysts, com-

pared with the potentially available supplies, is shown in Table 11.12.

11.2.4 Catalyst Operation

Three way auto-catalysts containing a high proportion of palladium were even-

tually preferred, because platinum was more expensive, potentially in short

supply, and palladium is more active than platinum for hydrocarbon oxidation.

Early palladium catalysts were easily deactivated by the sulfur and phos-

phorous impurities in the gasoline but new engines operating at higher tempera-

ture limited the adverse effect of these poisons. It is now common practice to

replace the platinum with palladium completely in some catalysts although the

loading of palladium is usually higher.

Different catalysts or combinations of catalysts can be used as outlined in

the basic principles:

Environmental Catalysts 461

TABLE 11.12. Demand for Platinum Group Metals.

Year

Platinum Palladium Rhodium

World

supply

(tonne)

Catalyst use

% total

World

supply

(tonne)

Catalyst

use

% total

World

supply

(tonne)

Catalyst

use

% total

1983 77 31 78 11 6 11

1988 107 34 104 7 10 85

1990 130 39 135 9 10 75

1993 144 39 131 10 12 85

1994 143 43 136 ~10 12 ~90

2000 137 24 224 70 – –

• Palladium catalysts are very effective for the conversion of carbon mon-

oxide and hydrocarbon residues, but the conversion of NO

is low.

• Catalysts based on platinum and rhodium give good NO

conversion, but

an increased metal loading is required to give a satisfactory oxidation of

the carbon monoxide and hydrocarbon components.

• Catalysts based on all three metals give an overall satisfactory perfor-

mance, but conversion of NO

is even higher when a binary plati-

num/rhodium catalyst is followed by a palladium catalyst.

Detailed information is never released by the catalyst supplier, as it is very

difficult to guarantee adequate protection of proprietary information. Metal load-

ings vary depending on the engine size and the required performance. The cata-

lyst monolith in a 1.8 liter car would have a volume of about 1.25 liters with 400

channels per square inch, or 60 channels per square centimeter. Up to 300 grams

of an alumina washcoat containing 30% ceria and about 1% precious metals

would be used, corresponding to about 0.1wt% of metal in the monolith. Some

monoliths contain about 600 channels per square inch or 90 channels per square

centimeter.

In common with all catalysts, there is a temperature, often referred to as

the “light-off” temperature, below which the reaction does not proceed. As the

temperature is increased above the light-off temperature, the reaction then pro-

ceeds with increasing rate. In the case of automobile exhaust catalysts, the tem-

perature of the exhaust and the catalyst is too low for a period of about 30 se-

conds for the oxidation of exhaust hydrocarbon residues to take place. However,

once the temperature reaches about 300ºC, the catalyst becomes very active and

the required reactions then take place. The composition of a typical exhaust gas

under different conditions is shown in Table 11.13.

Various modifications have been considered to decrease the time between

starting the engine and reducing emissions, as shown below:

462 Chapter 11

TABLE 11.13. Automobile Emissions at Increasing Engine Temperature.

Cold Warm Normal

Temperature (°C)

0–250 250–300 500–900

Carbon monoxide (g/mile) 20–10 10–0.2 <1.0

Hydrocarbons (g/mile) 10 – <1 <1 – <0.1 <0.1

Nitrogen oxides (g/mile) 4 – <1 <1 – 0.1 0.1–0.2

Oxygen (%vol) >1.0 <1.0 <0.5

• Move the catalyst closer to the engine. There is not much space to install

a small catalyst bed in modern automobiles, and there are additional prob-

lems associated with the selection of a catalyst with sufficient thermal

stability to operate at temperatures close to 1000ºC. Catalysts derived

from palladium may have the required thermal properties, but this ap-

proach is not favoured by the industry.

• Indirect electrical heating. This would certainly decrease the time before

the catalyst became active, but the electrical load would be excessive, and

the lifetime of the heater is likely to be too short to offer a viable solution.

• Use of a hydrocarbon adsorption trap. This approach has the potential to

store unburnt hydrocarbons until the catalyst is sufficiently hot to enable

oxidation of the hydrocarbon.

Compounds such as zeolites, perhaps containing a suitable metal promoter,

can adsorb exhaust hydrocarbons that consist largely of olefin and aromatic

compounds. The pore structure of many zeolites is broken down by the removal

of alumina from the crystal lattice, particularly in the presence of steam at high

temperatures. A practical solution to this problem would be to add a low alumi-

na zeolite to the washcoat, prior to impregnation onto the monolith, and then to

add an oxidation catalyst derived from palladium. Much of the hydrocarbon

would be contained by the zeolite at low temperature and then oxidized over the

palladium catalyst when the temperature became sufficiently high, both to de-

sorb from the zeolite and to light off the catalyst.

A more direct way to avoid hydrocarbon escape during the heat up period

has been the development of a very short monolith with about 400 channels per

square centimeter (2500 channels per square inch) followed by a conventional

monolith. The high gas velocity in the small channels allows faster heat and

mass transfer so that the catalyst heats more rapidly to reaction temperature.

Light-off is reported to take place within fifteen seconds of starting the engine.

The use of a distillation device heated by the engine has also been sug-

gested as a way of collecting some of the very lowest boiling fractions of gaso-

line. The low boiling fuel can be stored until the next cold start. The low-boiling

components of gasoline are oxidised over the catalyst at a much lower tempera-

ture than the other higher-boiling components, and this approach

can lead to a

reduction of up to 50% of the normal emissions during a cold-start.

Environmental Catalysts 463

11.2.5 Nitrogen Oxide Removal in Lean-Burn Engines

Lean-burn engines are more fuel-efficient when there is an excess of oxygen in

the combustion gas and emit less carbon dioxide per mile travelled. Lean condi-

tions lead to lower levels of NO

in the exhaust gas, but the introduction of fu-

ture legislation on the NO

content of the exhaust will require that further reduc-

tion will have to be considered. NO

removal in the presence of excess oxygen is

more difficult than removal under stoichiometric conditions because normally

the NO

is reduced by some of the unburnt carbon monoxide and hydrocarbons

in the exhaust.

A partial solution to the NO

removal problem with lean-burn engines may

be the use of a platinum/rhodium catalyst combined with a barium oxide trap

that can absorb nitrogen dioxide as nitrate. The mechanism of the reaction is:

• nitric oxide is oxidized to nitrogen dioxide on a platinum site;

• the nitrogen dioxide forms barium nitrate on the trap before it can leave

the catalytic converter;

• alternating rich operation for a few seconds every minute or so, releases

the nitrogen dioxide which can then be reduced by carbon monoxide or

hydrocarbon at a rhodium site.

The cycle then repeats during subsequent lean/rich operation.

The main problem with the barium oxide approach is that sulfur dioxide

competes with NO

for the basic sites, and is converted irreversibly to barium

sulphate, which is quite inert. Thus, the active barium sites are quickly saturated,

and the removal of NO

from the emission is severely restricted. The sulfur con-

tent of the gasoline would need to be much lower for this approach to provide a

long-term solution to the NO

problem. There may also be problems associated

with the thermal stability of the barium oxide traps during the many redox cy-

cles, which the catalyst would be expected to experience over the lifetime of an

autocatalyst.

Other procedures have been suggested for the treatment of the exhaust

from lean-burn engines. One possibility is the absorption of NO

on a suitable

zeolite, followed by desorption and recycle of the NO

back to the engine, where

it would be reduced by some of the fuel in the combustion chambers. The prob-

lem with this approach is that most zeolites suffer from dealumination in the

presence of steam at high temperature. An alternative approach could be the

direct reduction of NO

with hydrocarbons using a copper/ZSM-5 zeolite cata-

lyst, but this has not yet been feasible, because the catalyst is deactivated at tem-

peratures above 450ºC. Tin oxide, supported on alumina, is also active for the

reduction of NO

by olefins in lean-burn exhaust gases. The olefin intermediate

formed on the tin oxide surface can react with nitrogen oxide on an adjacent

alumina site. Although such catalysts have not yet been used in autocatalysts, it

is a further example of how autocatalysts, which already contain several compo-

nents, must now be designed for specific duties.

464 Chapter 11

The use of lean-burn engines will therefore be limited until a viable solution

to the NOX removal problem has been developed.

11.2.6 Diesel Engines

The efficiency of gasoline engines operating under lean-burn conditions has led

to a resurgence of interest in diesel engines. The operation of the engine has

been improved considerably by the use of better fuel injection systems, the use

of turbo-chargers, and the recirculation of exhaust gases. These changes have

led to a much lower concentration of pollutants in the exhaust, but there is still a

need to reduce the concentrations of carbon monoxide, hydrocarbons and nitro-

gen oxides even further. The main problems have been caused by the presence

of oxygen and sulfur dioxide in the exhaust gas from the diesel engine. It has not

been possible to remove NO

in the presence of oxygen to a satisfactory extent,

and the sulfur dioxide is converted to sulphate on the washcoat, also contaminat-

ing any soot formed by partial combustion of the diesel fuel.

Since the 1980s, the main effort has been on the use of filters to remove

up to 90% of the soot particles, which themselves contain a soluble organic frac-

tion and some sulphuric acid. There is no regular regeneration of the filters, but

as the filter takes up the soot particles, there is a gradual increase in pressure

drop through the filter. This leads to an increase in the temperature of the filter,

which in turn causes the soot to burn. High temperatures can be generated, and

these can lead to damage to the filter.

Catalysts to remove carbon monoxide and hydrocarbons from exhaust gas

have been used in Europe since 1991 with limited application to trucks in the

US. Platinum or palladium have been used on ceramic monoliths with a special

washcoat which minimizes sulfate formation. The main characteristics required

by the catalysts are as follows:

• High carbon monoxide and hydrocarbon oxidation activity at a low en-

gine temperature.

• Low sulfur dioxide oxidation activity and low reactivity with sulfur triox-

ide to form sulfate.

• Thermal stability at maximum engine temperature.

The concentration of the active metal usually lies in the range 0.35–1.76 g

liter

-1

. Smaller volumes of catalyst are required for diesel engines than for gaso-

line engines, and this results in a higher space velocity. Palladium catalysts have

a lower activity for the oxidation of sulfur dioxide than platinum catalysts. Nev-

ertheless, a lower metal content is used at the present time to avoid excessive

oxidation of sulfur dioxide to the trioxide.

The activity of the catalyst is adversely affected by the deposition of soot

particles. It has been shown that a filter that contains a platinum catalyst is ac-

tive for the oxidation of nitric oxide to nitrogen dioxide at 195ºC, when using

Environmental Catalysts 465

low-sulfur fuel. The nitrogen dioxide then reacts with the soot at the same tem-

perature with the formation of carbon dioxide and nitrogen, so that the filter is

regenerated continuously, and the levels of NO

are lowered.

11.3 VOLATILE ORGANIC COMPOUNDS

The oxidation of trace amounts of volatile organic compounds (VOCs) by cata-

lytic processes was used as early as the 1940s.

At that time, the main use of the

procedure was to recover energy or remove unpleasant odors from waste gas

streams. The US Clean Air Act (CAA) of 1970 led to a greater interest in the

recovery and removal of VOC as oil prices increased and environmental stand-

ards became more stringent. As a result of the Clean Air Act, from 1990 an

amendment required that a list of areas which did not comply with the National

Ambient Air Quality Ozone Standard of 0.12 ppm over a one hour period was

introduced.

A federal register was then issued for improvements to be made within a

period of 3–20 years.

This meant that VOC removal from a wide range of pro-

cess effluents became necessary. Of the 189 toxic compounds identified by the

1990 amendments, up to 154 can be removed by oxidation processes and more

than 20,000 facilities in the US were reporting data to the EPA by 1994.

The main classification of VOCs include:

• Aliphatic and aromatic hydrocarbons;

• Organic oxygen and nitrogen compounds;

• Organic chlorine compounds.

A list of some of the main activities which could use catalytic oxidation pro-

cesses, together with some of the VOCs emitted, is given in Table 11.14.

Both thermal and catalytic oxidation processes have now been used for

nearly thirty years and almost 100% removal efficiency can be achieved. Cata-

lytic processes are usually felt to be more efficient and economical for the fol-

lowing reasons:

• lower operating temperature required;

• significantly lower fuel consumption;

• lower carbon dioxide emissions;

• much lower residence time and therefore a smaller reactor;

• lower capital and operating costs.

Oxidation catalysts can last for up to twenty years and are easily regenerated, if

necessary, to remove coke formed by incomplete combustion or particles enter-

ing the bed with the gas stream.

466 Chapter 11

TABLE 11.14. Sources of Volatile Organic Compounds.

Activity Volatile Organic Compound

Chemical Phthalic/maleic anhydrides

Purified terephthalic acid

Formaldehyde/methanol

Ethylene oxide

Cumene/acetone

Polyethylene/polypropylene

Acrylonitrile/acrylic acid

Acetates/alcohols

Coating Alcohols, ketones, aromatics, ethers

Cyclohexanol, cellusolve

Bakeries Ethanol

Oils, fats, greases

Printing Ink compounds, alcohols, glycols

Electronics Ketones, cellusolve

Commercial Coffee fumes

Heavy oils, odours

Refining Gasoline, volatile hydrocarbons

11.3.1 VOC Removal Processes

Early procedures to control VOC emissions included physical adsorption in beds

of carbon which could be regenerated with steam or hot air to recover the organ-

ic impurity. Thermal combustion took place at temperatures in the range 800°-

900°C. Catalytic oxidation procedures, however, had the advantage of operating

at high conversion at much lower temperatures and higher space velocities. This

meant that temperature control was easier and that the smaller reactor led to a

Aliphatic compounds can be removed at temperatures between 200°–300°C

while aromatic compounds oxidize at slightly higher temperature in the range

250°–400°C. An important benefit of catalytic oxidation is that the reaction

tends to take place in the temperature range 200–350ºC, compared with tem-

peratures ranging between 600ºC and 980ºC for thermal oxidation. This has the

advantage that any carbon monoxide that might be formed in the thermal pro-

cess would easily be removed at very low temperature in the catalytic process.

The temperature of the catalytic process is also too low for the formation of ni-

tric oxide to occur by direct combination of the elements. Chlorinated com-

pounds can also be oxidised at temperatures up to 450ºC. A comparison of the

differences between the operating temperatures for the oxidation of some VOCs

in catalytic and thermal processes is shown in Table 11.15.

40,41

reduction in the capital cost.

Environmental Catalysts 467

TABLE 11.15. Catalytic and Thermal Oxidation Temperatures.

Organic Compound Catalytic Temperature (

C) Thermal Temperature (

Benzene 200–300 800

Toluene 250–300 900

Xylene 250–300 –

Styrene 200–250 –

Methanol 200–250 –

Ethanol 200–250 –

Butanol 300–350 960

Acetone 200–250 –

Methyl ethyl ketone 300–350 980

Formaldehyde 150–200 –

Ethyl acetate 250–300 750

Carbon tetrachloride 300–350 780

Chlorobenzene 300–350 –

Carbon monoxide <200 600

Acrylonitrile 250–300 –

Phthalic anhydride 250–300 –

When the concentration of hydrocarbons in the feed gas is greater than

about 0.2%, it may be necessary to carry out the oxidation in tube-cooled con-

verters, to provide adequate control over the temperature of the reaction.

Simple catalytic combustion units operate by preheating the contaminated

gas with oxygen to the appropriate operating temperature before it passes direct-

ly to the catalyst bed. The VOCs should be less than 1% volume and the heat of

combustion can be recovered by heat exchange with cold feed gas. It is usually

recommended that the concentration of VOCs is less than 25% of the lower ex-

plosive limit.

In a typical operation, 0.3–0.4% of mixed aromatic and oxygenated hydro-

carbons are almost completely removed at a space velocity of 40,000 hrs

-1

and

an inlet temperature of 280°C. The increase in temperature is about 100°C per

0.1% hydrocarbon. When the concentration of hydrocarbons in the feed gas is

TABLE 11.16. Selected Operating Experiences with VOC Catalysts.

VOC

Catalyst Life % Conversion Efficiency

Years Initial Pre-regen

Post-regen

Alcohols >10 >95 70–90 >95

Cumene/acetone > 5 >95 – –

Phthalic anhydride >17 95–98 70 92

Methanol/formaldehyde >10 95–100 – –

Toluene/xylene > 5 >95 70 93

Operation at up to at least 100,000 Nm

/hour.

greater than about 0.2%, it may be necessary to carry out the oxidation in

tube-cooled converters, to provide adequate control over the temperature of the

reaction. Some examples of operation with VOC oxidation catalysts are given in

Table