Catalysts Paper

7/30/2019 Catalysts Paper

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Challenges encountered in the catalytic converter industry:

Today, the most fuel efficient engines run with excess air and hence cannot be used with the

proven three-way catalysts which reduce pollutant emissions by 96%. This poses the problem of

developing catalyst concepts that can work with fuel efficient engines while retaining the low

pollutant levels reached by three-way catalysts (1). It is known that three-way catalytic

converters are a good and efficient method to control the emissions of pollutant. However, for

lean burn engines the large amount of oxygen in exhaust gas, and the low temperatures of the

exhaust gas negates the use of the three-way catalysts and lead to a negative impact on engineenergy efficiency. Also, it was found that all after-treatment systems are not reliable or efficient

without a proper operating temperature and an accurate flow control (6). Furthermore the need

for a development of a technology for an efficient catalytic combustion of lean gaseous fuels is

needed in industrial practice (17). In summary catalytic converters play an important role in

reducing pollutant emissions from cars however the converters present a various number of

limitation including that they do not perform very well in oxygen rich conditions, they are

susceptible to poisoning by impurities in the fuel mix, they are not active at cold start conditions,

the formation of undesirable products such as N 2O or ammonia, instead of the N 2 desired is

another concern in operating catalytic converters. It has been found that a detailed knowledge of

the underlying catalytic chemistry can also help in finding and building better catalytic

converters (24). Because there is challenging combination of physical and chemical processes in

a catalytic converter, improvement in the design of the monolith require a good understanding of

the influence of the many designs and operating variables on the performance of the reactor;


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even more particularly, the ability to determine the light-off location and the thickness of the

light-off front. Let's explain what light-off location and light-off front means:

When an automobile equipped with a catalytic converter is turned on, the initial response of the

converter from cold has two stages. During the first stage, the converter heats up because of the

presence of the hot inlet gas, but no chemical reaction occurs. After short amount of time the

temperature in the converter rises and becomes sufficiently high for chemical reaction to occur.

This generates heat faster than can be swept away by the gas stream. The converter then divides

into a region upstream where the gas flow makes the temperature high and the reaction rates are

large. The narrow transition between these regions is called the light-off front. The second stageof the response is a relatively small slow movement of light-off front towards the converter inlet.

Because this process can take some time up to several minutes for the catalytic converter to get

to a working temperature, an understanding of this initial phase is crucial in the control of

pollutant emissions. And from an environmental point of view it is clear that how to cope with

motor vehicle exhaust emissions is an increasing concern in automobile engineering (23).

Determining the light-off front can help in minimizing cold-start emissions from three way

catalysts, and hence improving their design, but developing a method or model to do this is a

challenge (3). Overall, catalyst efficiency depends on the cold start behavior and the conversions

at normal temperature above the ignition temperature, which represent the operating temperature.

It has been found that cold start emissions, which means the emissions of pollutants when the

catalyst has not reached normal operating temperature represented 80% of the total emissions;

for this reason, to improve the design of catalytic converters researchers have been focused on

improving the low temperature activity. However the design and improvement of a catalytic

converter is not an easy task because of the complex interaction between transport phenomena


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and chemical reactions (17).This complexity led to the development of mathematical models to

solve the problem through the use of numerical solutions. However, the fact that numerical

solutions of such detailed models with complex catalytic chemistry is possible with present day

computers; it may be difficult in terms of time and memory requirements, to solve them. This is

especially true for real time simulations that may be needed in the control and optimization

schemes; in addition, detailed solution , even when available, have to be coarse grained to obtain

quantities of practical interest, such as the average exit conversions of a reactant; added to this it

was shown that for strongly linear reactor models showing multiple solutions and boundary

layers, determination of the different types of solutions that may exist in the multidimensionalparameter space may be computationally prohibitive (28)./

Even though catalytic converters are used to reduce emission of pollutants such as CO,

hydrocarbons, and NOx by converting those to less toxic gases, it has been found that catalytic

converters are a source of pollution due to the platinum group elements contained in them.

Platinum group elements are a new kind of pollutant concentrated in earths core. Platinum

group metals for example are critical metals whose extraction is associated with a high

environmental burden (14). Environmental concentrations of these metals now exceed geogenic

background concentrations by factors of 10 5 to 10 6 and they are continuing to grow. However,

some metals are more harmful than others. For example a low concentration of palladium causes

more serious effects to cells and organisms than platinum, yet it has been found that palladium is

now being used as a substitute of platinum and rhodnium in catalytic converters (15). This is

because it is very difficult to analyze and trace palladium in complex systems, and other

analytical techniques such as graphite furnace atomic absorption spectrometry are not sensitive

enough to detect palladium in biological samples (15). Platinum group elements have been found


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in roadside dust, airborne particles, soils, plants, sediments, and snow which confirms the fact

that they are emitted from converters. Hence, these platinum group elements were considered to

be harmless, but recent studies have shown that some species have high level of cytotoxicity and

mutagenicity and can lead to significant negative effects on human health after chronic exposure

at very low level (4). Moreover due to the increasing price of noble metals (Pt, Pd, Rh) different

methods for catalytic oxidation have to be found (11). Also, due to the scarcity and the high

value of these metals, there is an increased interest towards their recovery from wastes such as in

three way catalyst. Concentration of platinum in catalysts ranges from 300 to 1000 g g -1; in the

case of palladium. Concentration can vary from 200 to 800 g g-1

; and for rhodium concentrationin catalyst have been found to be between 50 to 100 g g -1; even though these are very small

quantities, they are richer than those found in mined ores (20). Because used catalytic converters

are usually disposed of in landfills, which represent a great waste in terms of the minerals

contained in it; a method to exploit and recover the precious metals left in it should be

developed. The amount of platinum present in earth's crust is estimated at a concentration of

5g/kg in the upper 16km of the crust. Platinum is the 76th most abundant element and can

occur in oxidation states, and in solutions. The reason why platinum are used in catalytic

converters is because when it is finely dispersed it exhibits excellent catalytic properties. Even

though platinum are used in various application such as in the manufacture of jewellery and in

the chemical and petrochemical industry, studies have shown that most of the platinum produced

in the world is utilized to manufacture catalytic converters (21). A study showing the level of

exposure of people working in the fabrication of catalytic converter would be useful to set

standards regarding the wellbeing of these individuals. For other metals used in catalytic

converters such as gold, hardly nothing is know about the consequences of exposure to it and


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there is every small information on the background levels of exposure to gold in the general

population. The only negative effects that have been found include allergies and skin

sensitiveness. However, this does not rule out the fact that gold element present in catalytic

converters have to be retrieved after the use of the converter at least for economic purposes.

Investigations to determine platinum group content in general populations could be of interest to

study their harmful effects in more details. Accurate methods to determine the content of these

platinum group elements in urine, blood, and plasma in the concentration range which occurs as

a result of ecological exposure or exposure at the workplace must also be developed (22). /

Even though much research has been done in developed countries such as in Europe and northAmerica, there are some countries that do not have much data concerning the emission factors of

pollutants, oil rich countries for example that are experiencing rapid industrialization and

expanding urbanization do not have this information available besides their significant growth

(30). Research in the emission of pollutant is very important to be able to reduce pollution and

protect our environment, but it also has some other uses: indeed, nowadays there are global

budget accorded to the emission of pollutants. Let's take the example of the emission of N 2O on

the environment. There is a global N 2O budget that is not yet completely understood, and the

reason is that nitrous oxide emissions from vehicles have only been estimated based on fuel

consumption, and emission factors, and that amount is nearly equal to those obtained from a

simple global extrapolation of the nitrous oxide emission factor deduced from observations in

roadway tunnels (31). So, it is clear that the emission factors presented are taken from limited

studies and there for are not much reliable. Hence, the behavior of nitrous oxide isotopomers in

automobile combustion and subsequent catalytic treatment must be studied with respect to other

factors such as driving conditions, temperature and aging of catalysts so that estimated values


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can be used to distinguish automobile source of nitrous oxide from other sources and to be able

to determine an accurate global nitrous oxide budget (31)./

As it is indicated in the literature, hydrogen is a potential energy source for the future, so its

production could help reduce the emission of pollutant caused by catalytic converters. From

various investigation that were made, one of the byproduct of catalyst converters is hydrogen ,

and hydrogen production from synthesis gas for fuel cell applications has become a big focus of

attention. All this attention lead to the need of developing new catalysts that can obey strict

safety requirements while improving water-gas shift reactions for on-board hydrogen processing

(9).Another challenge that faces the development of improved design is regulations. A historical

case study has shown that outcomes such as the adoption of a new design are determined by

factors largely beyond the control of innovators (16).

Proposed solutions to the challenges faced in the catalytic converter industry:

Special types of catalysts have been developed and these catalysts can remove NOx from lean

exhaust. This could solve the problem or starts to solve the problem of catalytic converters not

being able to work with fuel efficient engine. Three methods were proposed to reduce the

emission of NOx in lean exhaust; and they include: NOx absorber catalysts, selective catalytic

reduction (SCR) of NOx by ammonia, and Nox reduction by hydrocarbons (HC-DeNOx). Due to

technical problems associated with the DeNOx method, it has not been used in serial

applications. Regarding the NOx absorber catalyst, the major technical problem with it is the

accumulation of sulfur on the catalyst and consequently the lowering of the NOx adsorption

capacity. The selective catalytic reduction seems to be the best methods since it can be used with

ammonia, with vanadium, or with zeolite-Based SCR catalyst; the reason why this method is


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better is because a technical issue encountered with ammonia for example is solved by using

vanadium, and a technical issue encountered with vanadium is solved when Zeolite is used.

However the specific method and the most common after treatment system used for fuel efficient

engine is a three-way catalyst combined with a NOx absorber catalyst as shown in Figure 1.

Other systems based on SCR catalysts are being researched. Even though the system shown in

Figure 1, only describes the aftertreatment system for a gasoline direct-injection engine, it is also

assumed that it can be used with fuel efficient engine due to the fact that these so-called direct-

injection gasoline engines face the same NOx reduction problems as Diesel engines and do

require a special catalyst for lean NOx removal (1).

Figure 1

Furthermore, to solve this problem of lean engine catalytic converter an innovative active flow

control has been researched and proposed to maintain a proper thermal window for energy-


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efficient aftertreament and to reduce the need for additional fuel. this new method is based on

reversed flow systems and also on the control of the exhaust gas flow path through the

aftertreament mechanism Figure 2. What happens is that the exhaust gases are periodically

switched between the two system ends by using valves, with a cycle consisting of forward and

backward operations, the cycle is defined as symmetric reverse flow when the two consecutive

processes last the same time and asymmetric reverse flow when it is the opposite.

Figure 2

To better understand underlying catalytic chemistry that can help in developing better catalytic

converters, detailed kinetic models consisting of elementary reaction steps have been used,

however these detailed kinetic models are only useable in a limited set of operating conditions,

so to solve this problem validated reaction mechanism consisting of several steps, such as in

microkinetics models, consisting of a sequence of detailed elementary reaction have been

developed with a focus on reduced reaction rate models (24).

To cope with motor vehicle exhaust emissions various mathematical models for the study of

interphase heat-transfer problems in catalytic converters have been developed and one of them is

the one suggested by Leightion and Chang where the vehicle temperature and the converter


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temperature are governed by a coupled system of a first-order partial differential equation as

indicated below (23).

On a simplified form, the equations in above become:

When we let:

As we can see developing a method or model to minimize cold-start emissions from three way

catalyst is not an easy task, however mathematical modeling and simulations are useful in

identifying the optimal designs and reducing the amount of experimentation needed to achieve

this goal (3).


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Because it is difficult in terms of time and memory requirements of present day computers to

solve complex mathematical models of catalytic chemistry, it is desirable to have simplified low-

dimensional models having same qualitative behavior as the full partial differential equation

models and sufficient accuracy for practical applications (28). Taking consideration of this fact,

simpler models were developed, including 1-D two phase models, models including detailed

kinetics along with washcoat diffusional limitations, and models that solve the full 3-D

governing equations for a single representative channel. Figure 3 gives the example of a detailed

mathematical model of a catalytic monolith, it depicts one of the assumption used to derive the

models; here it is that the cross section of the channel is invariant with the axial position, with avariable washcoat thickness.

Figure 3

Investigations were also done to reduce the environmental pollution of some platinum group

elements. For palladium for example, in the electronics industry the development of multilayered

ceramic capacitors that can work with nickel or silver were developed. From this observation it is

projected that similar investigation will be developed for the automotive industry (14).


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To solve the problem of analyzing and tracing platinum group element such as palladium,

techniques such as the combination of graphite furnace atomic absorption spectrometry with

modified selective separation and preconcentration procedure were developed (15).

To further understand the harmful effects of platinum group elements studies are constantly

determining and updating information on platinum group elements concentration and enrichment

on urban areas. These studies often investigate the spatial distribution of platinum group

elements in different functional districts (4). Figure 4 shows the sampling sites of an

investigation that was performed in Beijing, China.

Figure 4

Regarding the issues related to the increasing price of noble metals, and the need for different

methods for catalytic oxidation, a promising reaction that has been developed in the literature is

CO oxidation over metals and studies have examined it over simple and mixed oxide catalysts

(11). Table 1 shows the heat of chemisorption values of CO on metals including platinum,


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rhodium and palladium. We observe an increase in chemisorption for platinum, rhodmium and

ruthenium, and other behavior for the other metals.

Table 1

To recover precious metals from used catalytic converters, several methods were developed and

studies focused on recovering the metals from the synthetic effluents and the industrial effluents.

The methods to recover precious metals from used catalytic converters include ion exchange

methods, a system that combines two hollow-fiber liquid membrane, and one that uses supported

liquid membrane that contain a selective carrier. Some other methods include Sorption by

chitosan, selective dissolution in aqua regia, and many more. But the methods that are preferred

are the ones that recover the metal in an economical and environmentally friendly way. The


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apparatus depicted in figure 5 for example shows the apparatus of an extraction method that uses

sc CO2 modified with a complexing agent (20).

Figure 5

Even though most studies show that the pollution of platinum group elements is most ubiquitous

in vehicles, because it is these vehicles that carry catalytic converters, some other studies have

focused on the places where these converters are made, and the amount of exposure people

working in these place were subjected to was determined. Table 2 shows the results of platinum

determination in the urine of people who are occupationally exposed to platinum (21).


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Table 2

A more detailed study was also done to look at the exposure level of platinum in the workplace

the summary of the findings are shown in table 3 (22).

Table 3

Data concerning emission factors of pollutants is increasingly being generated around the world,

even in countries where similar research was non-existent. The country of Kuwait where there

was very limited information available regarding pollutant emissions from the transportation

system has pursued similar investigation as in Europe and the United states (30). In this case a

very elaborate investigation was done including the speed profiles of different types of drivers,

full cycle emissions profile for different types of vehicles. A summary of the statistic is collected

in table 4.


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Table 4

To better understand the global nitrous oxide budget the behavior of N 2O isotomers in car

combustion and the following catalytic treatments of exhaust have been studied with respect to

driving conditions, temperature, and aging of catalysts. The analysis showed that N 2O is

consumed in three-way catalysts at a rate that is similar to the rate at which it is produced (31).

The estimated values presented in table 5 can then be helpful in distinguishing automobile source

of nitrous oxide from other sources and hence help us deduce its contribution to the global

nitrous oxide budget.

Table 5

To solve the problem of emission of pollutant, some other route besides the improvement of the

design of catalytic converters is being pursued; one of these routes is the route that leads to the


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production of hydrogen as an energy carrier. And one of the most promising ways of achieving

that is through the water-gas shift reaction. The water-gas shift reaction is related to the

production of usable gas when steam is passed through a bed of incandescent coke (9). The

method was then developed through the years till it was used as part of the Haber-Bosch process

to make ammonia as shown in figure 6. More investigations showed that there were some pretty

good perspectives regarding water-gas shift reaction including the improvement of catalytic

activity on precious metals and development of new formulas.

Figure 6

Some other area of study that would help in developing catalytic converter is the area of

regulations. As it was indicated in the literature, outcome such as the adoption of new

technologies or design do not only depend on the innovator but it is also largely affected by

regulations. It can be government regulation or states regulations. A look at these regulations and

their impact on development could also help innovators understand the proper method to apply to

navigate through all of them in a minimum amount of time. Figure 7 shows the development of

catalytic converters through the years and some of the entities that were involved in it.


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Figure 7

Conclusion:

We conclude from our literature review and the research we conducted that there are three main

challenges facing the catalytic converter industry and its development:

A problem of efficiency:

o Catalytic converters are not very efficient during the first couple of minutes of

their run. This poses a big problem because most of the pollutants that are

supposed to be reduced by catalytic converter are emitted on the environment

during that small window of time. Hence, the development of new technologies to

reduce this time is required.

o Catalytic converters are not efficient when used with fuel efficient engines

because of the oxygen rich conditions. Consequently, the creation of a catalytic

converter that could be used with lean engines should be underway.


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Catalytic converters are used to reduce pollutant emissions, but research found that the

converters themselves pose a threat to the environment, due to the platinum group

element they contained.

From a historical perspective, regulations could slow down the adoption of new designs

and technologies

The solutions that were investigated to solve these problems are various and include:

The development of novel system to use catalytic converters in fuel efficient engine

The development of mathematical models to better understand the mechanism of

conversion and hence attempt to reduce cold start emissions of pollutant

Various investigations aimed at better understanding the effect of platinum group

elements on the human body and on the environment for the purpose of developing future

regulations.

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Catalysts Paper