Materials Science for Energy Technologies 2 (2019) 607–623
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Materials Science for Energy Technologies
CHINESE ROOTS
GLOBAL IMPACT
journal homepage: www.keaipublishing.com/en/journals/materials-science-for-energy-technologies
Materials progress in the control of CO and CO2 emission at ambient
conditions: An overview
Subhashish Dey ⇑, Ganesh Chandra Dhal
Department of Civil Engineering, IIT (BHU), Varanasi, India
a r t i c l e
i n f o
Article history:
Received 31 May 2019
Revised 21 June 2019
Accepted 22 June 2019
Available online 24 June 2019
Keywords:
Carbon monoxide
Poisonous
Source
Catalyst
Parameters and applications
a b s t r a c t
Catalytic conversion of carbon monoxide (CO) is one of the most important process for human health protection. CO is also called the unnoticed poisons and silent killer of 21st century. The effect of inhaling CO
can cause of hypoxic injury, neurological break and even death. Due to CO poisoning decay vegetation life
and increases in global warming and ozone layer depletion. CO is produced into the environment by partial oxidation of carbon containing compounds. The main source of CO emission is the transportation sector. Thus, the oxidation of poisonous CO to nonpoisonous CO2 at ambient conditions is crucial for life
conservation in many applications. Further, low-temperature CO oxidation is vital in minimizing emissions at the cold start of an internal combustion engine. A variety of CO oxidation catalysts are investigated in this era, but a maximum of them have problems associated with the deactivation in presence
of moisture, cost-effective, lower stability, poison resistance, availability in the world. These accelerate
the investigation of CO oxidation over various catalysts. Although there are numerous research articles
present on this topic until now no one review are present for this demanding issue. So there is a space
in this area, it has been made an attempt to seal this hole by this review.
Ó 2019 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
Contents
1.
2.
3.
4.
5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sources of CO emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.
Natural sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.
Anthropogenic sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
An adverse effect of CO emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Impact of CO on health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Impact of CO on vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Impact of CO exposure on global warming and ozone-layer depletion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Control of CO emissions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Regulations of CO emissions and recommended limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1.
Motor vehicle emission standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2.
Recommended exposure limits (REL) for CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Formation of CO in internal combustion (IC) engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Control of CO emission by application of catalytic converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author.
E-mail address: subhasdey633@gmail.com (S. Dey).
Peer review under responsibility of KeAi Communications Co., Ltd.
Production and hosting by Elsevier
https://doi.org/10.1016/j.mset.2019.06.004
2589-2991/Ó 2019 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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7.
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S. Dey, G.C. Dhal / Materials Science for Energy Technologies 2 (2019) 607–623
Catalysts for CO oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.
Noble metal catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.
Base metal catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.
Groups of catalysts for CO oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1.
Hopcalite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.2.
Perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.3.
Spinel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.4.
Monel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.
Preparation of catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.1.
Co-precipitation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.2.
Sol-gel method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.3.
Impregnation method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.4.
Reactive grinding method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.5.
Hydrothermal method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.6.
Pyrolysis method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.5.
Metals promoted on catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.6.
Mechanism and kinetics study of catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.7.
Use of catalysts at cold start emission conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Carbon monoxide (CO) is also called carbonous oxide, is a colorless, odorless and tasteless gas, which creates very difficult for
humans to perceive [1,2]. CO has been called ‘‘the unnoticed poison
of the 21st century” and ‘‘the silent killer”, because it gives no clear
warning to its victims that they were at risk [3,4]. The small
amount of CO poisoning causes hypoxic injury and neurological
damage of humans. Due to CO exposure the plant respiration and
nitrogen fixation are failures [6,7]. The presence of CO on earth’s
atmosphere effects the atmospheric chemistry as well as the environment [5,8]. When CO enter into the ground level ozone, it can
creates serious respiratory problems and also increases the global
warming level. Therefore, CO levels in the atmosphere play a significant role in influential the air quality of region. Carbon monoxide is produced into the environment by partial oxidation of
carbon-containing compounds and also produced by the catalytic
cycle of heme degradation and approved by the enzyme heme
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oxygenase (HO-1) within the human body [11,12]. Endogenously
produced CO take part in a role of cytoprotection against damage
of body tissues. However, with outside exposure of CO formation
of carboxy-hemoglobin (COHb), which reduces the oxygen
percentages (Fig. 1) in the blood [13,14]. Thus, depending on the
concentration, the role of CO exposures depending on the range
from cyto-protective to cyto-toxic [15]. The main sources of CO
produces into the environments are transport, power plants and
industrial activities [16]. The transportation sector contributes
64% of the CO contamination in the urban area. Forest fires and
building fires also release a large quantity of CO in the atmosphere
[17,18]. Carbon monoxide is a molecule with three resonance
structures or Lewis structures. CO is one of the strongest diatomic
molecules and a weak electron donor.
Due to the poisoning of CO and its negative impacts on public
health and natural environment, the standards in legislation focus
on regulating pollutants from different sources [19,20]. For diesel
passenger cars, Legislation is progressively improved year by year
Fig. 1. Current and future therapeutic targets of CO poisoning.
S. Dey, G.C. Dhal / Materials Science for Energy Technologies 2 (2019) 607–623
to maintain the air quality and reducing CO such as: for Euro1/2,
medium HC & CO conversion, for Euro 3/4, medium HC conversion
and high CO conversion and for Euro 4/5, very high HC & CO conversion by improving engines and three way converters (TWC)
[21–23]. Many organizations such as WHOM, OSHA, NIOSHA,
ACGIH, ASHRAE etc., have its own terms to depict the type of limit
or level [24]. To control the automobile pollution was done by the
1970 Clean Air Act, for reduction in CO, HC and NOx productions
from automobiles [25]. Catalytic oxidation at lower or ambient
temperatures has stimulated a lot of research interest for its
abatement. The catalyst converts toxic CO into nontoxic product
CO2 form in the environment which was used for the photosynthesis by vegetation [26]. Low temperature CO oxidation has been
broadly studied over various types of catalysts like spinel structure,
perovskite structures, hopcalite and monel etc [27]. The noble
metal catalysts were challenging for their slower reproducibility,
high cost and deactivate easily [28]. The catalysts were deactivated
easily by moisture or humidity but removing moisture at various
temperatures simply improved the catalytic activity [29,30].
Low temperature/ambient condition CO oxidation is gaining a
significant amount of interest at current situations, because of its
urgency for life preservation in many practical applications in various industrial and environmental fields [31,32]. These include
specialized heating, ventilation and air conditioning (HVAC) filters
for air purification in a cabin [33], rescue equipments [34], CO leak
detection sensors [35], automotive emission control [36], ice arenas, indoor and underground parking garages, confined spaces
[37], smoking [38], etc. It also has applications in regenerative
CO2 lasers used in orbiting applications for recombining dissociated CO2 for weather conditions monitoring and various other
applications [39]. Further, low temperature CO oxidation is vital
in minimizing emissions during the cold start of an internal combustion engine [40]. The oxidation of CO in H2 stream at low temperature is also significant for the effective operation of fuel cells in
production of electricity [41,42]. Special consideration has been
given to the personal protective equipment (PPE) application. It
has tremendous application assessment to the PPE in respiratory
protection systems for military, mining and fire fighting. It is also
very useful in confined space protection such as submarines, deep
sea welding and refuge chambers. Overall CO oxidation is vital in
protecting vegetation and environment [43,44]. To fulfill this
space, an endeavor has been made in this particular research interest to wrap all types of catalysts active, effects, sources, legislation,
controlling methodology and their application parts.
2. Sources of CO emissions
CO is an impartial oxide which combusted in atmosphere to
produce CO2, which is an excellent reducing agent. Plants can produce CO is an ubiquitous molecule as a trace component in the
ordinary environment [45]. The global concentrations of CO range
between 0.06 and 0.14 mg/m3. CO is a major product of volcanic
activity but also produced by the partial oxidation of carbon containing compounds like fuels/biomass, photochemical and biogenic
oxidation of organic matters [46]. Obviously, there are many
sources of CO as shown in (Fig. 2) which can be broadly classified
into natural and anthropogenic processes.
Fig. 2. Sources of CO emission.
garbage landfills. The sunlight-initiated oxidation of organic matter in waste-waters discharged in rivers/ocean is primarily responsible for producing CO [49].
Creation of CO in clouds is apprehensively recognized to the
photochemical oxidation of organic matter or the trivial dissociation of CO2 provoked by electrical discharge or both [50]. Submarine divers, space traveler and fire-fighters face a bulk amount
of CO. Other natural sources of CO are incomplete burning of
biomass: Forest/bushfires, building fires and fire-cleaning, smoke
contain many toxic gases including CO in substantial quantity
[51]. Depending upon the cases, fire can also contain as high as
3600 ppm of CO [52]. Each year an estimated 80,000 wild-land fire
fighters battle wildfires, often spend long periods at the fire front
where they are exposed to high levels of toxic smoke [53].
The sources of atmospheric CO have changed from seasons to
seasons. It is extremely difficult to measure accurately the variable
sources of CO emissions from year to year. Humans can also produce very small amounts of CO endogenously [54–56]. In human
metabolic processes the heme-catabolism produce (Fig. 3) only
very lower amount of CO. In both males and females, time to time
variations in CO formation. The pregnant women also showed
higher amount of endogenous CO production related to increased
breakdown of red blood cells [57,58]. Endogenously produced CO
is known to function in several important physiological processes,
including vasodilatation, apoptosis, inflammation and neurotransmission [59,60].
2.2. Anthropogenic sources
The major amount of CO contamination in the atmosphere is
contributed by the human-caused or anthropogenic sources.
Approximately 60% of the total CO emissions are thought to be
contributed by human endeavors and remainder by the natural
sources. Anthropogenic sources of CO emissions are categorized
as mobile, industrial and residential activities [61]. Mobile sources
account for the main CO emissions. It includes both on-road vehi-
2.1. Natural sources
In the environment the major amount of CO is produced in natural origin, due to the photochemical oxidation of methane emitted from wetland [47]. The huge amounts of CO are also
produced from the earth’s mantle, where it was dissolved in the
molten rock, by volcanic eruption [48]. In addition, CO is produced
from the oxidation of biogenic VOCs emitted by vegetation and
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Fig. 3. Global carbon monoxide cycle.
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cles (e.g., cars, buses, trucks) and off-road vehicles (e.g., bulldozers,
mobile excavators, farm machinery, trains, snowmobiles, domestic
lawn mowers etc.) [62]. Industrial sources are stationary sources;
include thermal power plants, steel plants, coke ovens, hydrogen
production, coal gasification and chemical production, petroleum
refining and other industries using various carbon-based fuels as
a source of energy [63]. The exhaust component of Diesel engines
and Gasoline engines are shown in Table 1.
Residential sources include space heaters (salamanders), stoves,
furnaces, heaters, generators, and others. Kitchen applications that
burn oil, LPG or other fuels [64]. Production of CO rises (Table 2)
when vehicle is moving slowly. A large concentration of CO is produced during the cold start of vehicle (Fig. 4), as catalytic converters take certain time to get the working temperature. The diesel
engines are widely applied in heavy-duty vehicles for its improved
fuel efficiency and power yield than a petrol engine vehicle [65].
Table 1
Exhaust component of Diesel engines and Gasoline engines.
Exhaust Component
Diesel Engines
Gasoline Engines
CO
HCs
NOX
Particulate Matter
300–1200 ppm
50–330 ppm
350–1000 ppm
20–200 mg/m3
1500–4000 ppm
50–5000 ppm
100–4000 ppm
12–40 mg/m3
Table 2
Composition of CO in dry atmosphere, by volume.
Concentration
Source
0.1 ppmv
0.5–5 ppmv
5–15 ppmv
Normal atmosphere level (MOPITT)
Average level in homes
Near-properly used to gas stoves in homes, modern
automobile emissions
Exhaust from a residence wood fire
Undiluted warm car exhaust without a catalytic converter
5000 ppmv
7000 ppmv
ppmv: parts per million by volume.
MOPITT: measurement of pollution in the troposphere.
In the assessment of diesel engine, the petrol engine emitted
more CO into the atmosphere. The CO emission from CNG vehicles
is two times less than the gasoline engine vehicles. The vehicle
emissions are also depending upon the vehicle design, maintenance, operation conditions and fuel composition, etc [66].
The CO emissions are also contributed to environmental from
the incineration of solid wastes in urban and other incinerators
[67,68]. Carbon monoxide is also encountered in mining operations
in which explosives are used in confined spaces. The exposure of
CO in warehouses propane-powered floor polishers are operated
[69,70]. The major hazardous levels of CO contamination generally
take place in air of buildings or enclosed spaces [71].
The higher amounts of CO can produces in indoor air (Fig. 5) as a
result of inappropriately installed or unvented and poorly maintained cooking/heating. Dangerous levels of CO can build up by
using older moveable propane heaters in enclosed indoor settings
[72]. Gasoline-powered tools like a power generator (Fig. 6), pressure washers inside homes and idle running of the car for long
periods of time in the garage lead to dangerous and deadly levels
of CO. Undetected releases of CO in indoor settings can present
serious health risks to building occupants [73].
A huge amount of CO is released from firewood, charcoal, coal,
biomass and other fuels burning in cook-stoves indoors in various
rural areas. The domestic sources release openly into the spaces at
the phase of human residence [74,75]. The CO emissions resulting
from burning various fuels in stove, thus represent cleanliness of
the fuels in the subsequent order: charcoal < wood < dung < crop
residue < coal < kerosene < LPG < biogas. The CO concentrations
are very low for LPG and biogas, pointing them as cleaner fuels.
This indicates that the CO production from biomass cooking is
high; depends upon the biomass fuel activity pattern [76]. The
options for CO exposure reduction (Fig. 7), are the use of cook
stoves having flues or chimneys, or building hoods or exhaust fans,
and use of cleaner fuels such as LPG or biogas [77,78]. Cigarette
smoking is one of the major source of CO in indoor air contaminant
[79]. CO is produced in home from the tobacco burning during the
certain mainstream smoke by the flaming closing stages of the
cigarette [80]. Accordingly Moschandreas et al. [81] the one cigar-
Fig. 4. Schematic diagram of catalytic convertor for solution of cold start problems.
Fig. 5. Emission of pollutants from vehicles and control technologies.
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611
Fig. 6. Emission of CO from various sources.
ette can be produced 88.3 mg CO. The summary of major contributions of natural and anthropogenic sources of CO emissions is given
in Table 3.
CO readily react with hydroxyl radical creating a much stronger,
a greenhouse gas—CO2, it increases the percentages of CH4, another
strong greenhouse gas. So, that the production of CO2 leaves smaller amount OH for CH4 to react with, thus rising CH4 concentration
[81–84].
CO results from certain microbial fermentations, but even
under exceptionally favorable conditions, it generally does not
exceed 3% of the gases formed by bacterial fermentation of
organic wastes. CO can also be generated in surface waters by
ultraviolet photolysis of humic substances. The chemical reactivity of CO and relative ease with which various bacteria reduce it
to methane or oxidize in to CO2 probably account for its absence
or very low concentrations in most environments. Aerobic COoxidizing bacteria are distributed widely, especially in organicrich sediments. Species of Carboxydomonas, Hydrogenomonas,
Bacillus, and certain methane-oxidizing bacteria oxidize CO
[85–90].
CO þ 1=2O2 ! CO2 DG0 0 ¼ 66kcal=mol
ð1Þ
Additionally, the several aerobic bacteria in mixed populations
quantitatively convert CO into CH4 in the presence of hydrogen:
CO þ 3H2 ! CH4 þ H2 O
DG0 0 ¼ 46kcal=mol
ð2Þ
In the absence of H2, some anaerobic bacteria (Methanosarcina)
produce CO2 and CH4:
4CO þ 2H2 O ! 3CO2 þ CH4
Fig. 7. Diagrammatic representation showing common sources of CO in the indoor environment.
ð3Þ
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Table 3
Sources of carbon monoxide in the environment.
CO production (million tonnes per year)
Anthropogenic
Natural
Range
Directly from combustion
Fossil fuels: Transportation, coal, oil, natural gas
Biomass burning: Agricultural clearing, wood, refuse
Forest fires:
500
650
–
400
200
30
1000
800
10–50
Oxidation of hydrocarbons
Methane: Wetlands, rice cultivation, animal husbandry, landfills, coal mines
Non-methane hydrocarbons: alkanes, alkenes, aromatics, isoprene, terpenes
300
90
300
600
400–1000
300–1400
Other sources
Plants: Metabolic by-product
Oceans: Oxidation of organic substances
Total
–
–
1500
100
40
1100
50–200
20–80
2000–3000
3. An adverse effect of CO emissions
Higher concentration of CO emissions adversely affects on
human health, vegetation and the environment. However,
endogenously produced CO is taking part in a key role in cytoprotection against damage of body tissues [91,92].
3.1. Impact of CO on health
Carbon Monoxide (CO) is lethal to every aerobic form of life. It
was simply absorbed in the blood through the lungs [96]. CO affinity to hemoglobin is 240 times higher than that of oxygen so, this
prevents oxygen binding to hemoglobin. Carboxy-hemoglobin
reduces the oxygen-carrying ability of the blood and interferes
with oxygen discharge (Fig. 8) at the tissue level. Continued supplying of oxygen can interfere with cellular respiration and result
in tissue hypoxia [97,98]. Increased levels of Carboxy-hemoglobin
(COHb) in the blood decrease the oxygen carried by hemoglobin
around the body in red blood cells.
The acute CO poisoning includes headedness, uncertainty and
flu-like effects; higher exposures of CO can lead to higher toxicity
of the main nervous system and heart, impaired vision and coordination and even death. Following acute poisoning, long-term
squeal often occurs [103]. The chronic exposure to smaller levels
of CO regularly (such as at home, or in the workplace) can also
cause long-term health problems characterized by the effect on
pulse rate, respiration, blood pressure and neurological reflexes
which can lead to fatigue, headaches, dizziness, depression,
confusion and memory loss. Significant exposure of CO may lead
to a heart damage [104,105].
The signs and symptoms at various concentrations of CO in the
atmosphere and duration of exposure are illustrated in Fig. 9. At
concentrations more than 0.1% by volume in atmosphere can seriously affect human aerobic metabolism resultant in respiratory
system failure and death. CO poisoning is the most common type
of fatal air poisoning in many countries [106,109,110].
The effect of CO at various concentrations in normal air on
human health is representing in Table 4. According to the national
institute for occupational safety and health (NIOSH) has recommended that lowering the level of CO exposure is 35 ppm. Children
of age under 14 years are more probable to sustain poisoning than
adults at lower levels. On average, the exposures at 100 ppm or
more is dangerous to human health [111,112].
3.2. Impact of CO on vegetation
CO is a lower molecular weight diatomic gas; it’s capable to
influence the plant seed germination, bring on the adventitious
Fig. 8. Effects of CO emission on human health. (Source: https://en.wikipedia.org/wiki/Carbon_monoxide_poisoning).
S. Dey, G.C. Dhal / Materials Science for Energy Technologies 2 (2019) 607–623
613
Fig. 9. Effect of exposure to various concentrations of CO.
Table 4
The effects of carbon monoxide at various concentrations in air.
CO Concentration (ppm)
Physiological Effects on Humans
50
100
200
400
900
1500
Safe for continuous exposure
No Perceptible effect
Slight effect after six hours
Headache after three hours
Headache and nausea after an hour
Death after one hour
rooting development. Due to the CO poisoning, the plant leaf is curling, reduces the leaf size, premature ageing of the chlorophyll. It
reduces the cellular respiration system of plants and also reduces
the agriculture productivity [113]. The longtime CO exposure
causes reduce the length of primary roots; plant leaf is discolorization (Fig. 10), droops its shape and reduces the distance from seed
to tip of the leaf. The current indication suggests that the concentration of atmospheric CO is a major implication for plant physiology and growth [92–95]. The increasing of CO concentrations more
than 120 ppm in most plant species explain higher rates of photo-
synthesis decreased water use, better growth and lowered tissue
concentration of N2 and protein [114,115].
In plants, the effects of CO has been considered to illuminate in
Fig. 10. Accumulating the confirmation in plants has revealed that
CO is used for a numeral intracellular biological functions. The
rising of CO concentration is highly affected both agricultural
production and food quality [100–102]. CO exerted a beneficial
effect on improving seed germination in a dose-dependent way
in various plants. [117]. Stomata function significantly control
the plant water status and produce by many environment or hormonal factor [119,120].
3.3. Impact of CO exposure on global warming and ozone-layer
depletion
Carbon monoxide is individual one of major reactive trace-gases
in the earth’s atmosphere. The amount of CO is increasing in the
atmosphere day by day by the increasing of fossil fuel consumptions. CO is a weak direct greenhouse gas and an important indirect
effects on global warming (Fig. 11) [116–118,121]. The initiating
reaction for CO destruction is given by Eq. (4).
Fig. 10. Adverse effects of CO emission on plant. (Source: https://www.skepticalscience.com/co2-plant-food.htm).
614
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Fig. 11. Adverse effects of CO emission on Green house effects. (Source: https://myassignmenthelp.com/free-samples/ozone-layer-depletion-and-global-warming).
CO þ OH ! CO2 þ H
ð4Þ
The troposphere photochemical oxidation of CO (Eq. (5)), in the
presence of enough NOx (i.e., above 3–8ppbv at ground level and
20ppbv in the upper troposphere) also lead to the creation of
significant amount of ground-level ozone (a secondary pollutant).
Excess quantities of ozone can trigger serious respiratory problems, damage crops and natural ecosystem [107,108,124].
CO þ 2O2 þ hm ! CO2 þ O3
To address this problem, significant efforts have been made since
1970. The emissions from gasoline-fueled vehicles can be condensed through the changes in engine design, burning conditions
and catalytic after treatment. The emissions of CO can be
controlled by A/F ratio, turbulence in the combustion chamber
and exhaust gas re-circulation [121–124].
4.1. Regulations of CO emissions and recommended limits
ð5Þ
Since CO has not direct effect on global warming and is complicated to evaluate the global warming potential of the emission. CO
is reported to have a Global-warming potential (GWP) about 3.2 on
a 20 years’ time span [140]. It means that CO is 3.2 times more
potent than CO2.
CO þ O3 ! CO2 þ O2
ð6Þ
Similarly, the incidence of CO, at concentrations ranging from
15 to 30ppmv, leads to major decreases in the rate of oxidation
of atmospheric SO2 to form sulfate aerosols as a result of reducing
the amount of OH present. In the troposphere, it act as cloudcondensation nuclei (Fig. 11) which reflect sunlight back to space,
therefore neutralize the global warming effect of the greenhouse
gases [125,126].
4. Control of CO emissions
The control of CO emissions from automobile vehicles has
become a global challenge in achieving improved urban air quality.
The emission standards are set by the certain limits to the concentration of CO and other pollutants that can be free in the air that
people breathe. Many emission standards focus on the variable
pollutants released by automobile vehicles and from industry,
small equipment such as forklifts, lawn moters generators, etc.
4.1.1. Motor vehicle emission standards
The increasing of number of automobile vehicles on roads, the
CO concentrations have reached an alarming level in metropolitan
areas. To regulatory actions have been adopted to restrain the danger of automobile pollution [166,167]. There had been a much perceptible concern in the early 1980s on the adverse environmental
impact of increased automobile traffic in developing countries like
India. India has started adopting European emission norms and
fuel regulations for four-wheeled light-duty and heavy-duty vehicles [127].
All vehicles produced after the exploit of norms have to be compliant with the regulations. At present, Bharat Stage IV (BS IV) parallel to Euro IV regulations since April 1st, 2010 is applicable for
various types of vehicles; this is given in Table 5 for CO emissions
Table 5
Emission norms in India parallel to EU emission standards.
Norms
European
Year
CO (g/Km)
HC + NOx (g/Km)
1991Norms
1996 Norms
1998Norms
India Stage 2000 norms
Bharat Stage-II
Bharat Stage-III
Bharat Stage-IV
Bharat Stage-V
Bharat Stage-VI
–
–
–
Euro
Euro
Euro
Euro
Euro
Euro
–
–
–
2000
2001
2005
2010
2017
2020
14.3–27.1
8.68–12.40
4.34–6.20
2.72
2.2
2.3
1.0
0.63
0.50
2.0(Only HC)
3.00–4.36
1.50–2.18
0.97
0.5
0.35
0.18
0.10
0.07
1
2
3
4
5
6
615
S. Dey, G.C. Dhal / Materials Science for Energy Technologies 2 (2019) 607–623
(REL) for CO (mostly for the workplace). EPA and WHO has recommended exposure of ambient air quality guideline values for CO at
9 ppm and 25 ppm as an 8 h and 1 h time-weighted avg. concentration respectively. The exhaust pollutants regulations of CO
emissions from various countries is represented in Table 7. In the
United States (US), the OSHA limits long-term exposure levels of
CO has been less than 50 ppm averaged over an 8-hour period.
[128]. India has recognized limits on CO exposures (at idle) for
motorcycles, cars and innovative emission standards for gasolinefueled cars took effective in 1991. The automobile emissions are
affected by driving pattern; overcrowding, temperature, traffic
speed, vehicle’s engine conditions and emissions control equipment and its maintenance [129,130].
4.1.2. Recommended exposure limits (REL) for CO
The natural concentration of CO in air is around 0.9 ppm, and
that amount is not harmful to humans. The higher amount of CO
can occur in various settings, include those at home or at work.
Average levels of CO in homes without gas stoves vary from 1 to
5 ppm [133]. CO concentrations indoors automobile vehicles are
usually around 9–25 ppm and irregularly over 35 ppm. The peak
exposures of CO over 200 ppm in the vehicle exhaust. People in
certain occupations (e.g., car, bus and taxi drivers etc.) can have
long-term CO exposure (Table 6). Acute as well as chronic exposure
of CO has concern for potential effects [134,135]. Studies indicates
that the auto mobile vehicle interior has the maximum level of CO
concentration (averaging 10–29 mg/m3 (9–25 ppm)) of all environments [136].
There are several government organizations and professional
organizations that have posted recommended exposure limits
5. Formation of CO in internal combustion (IC) engine
The internal combustion (I.C.) engine is operating by the combustion of fossil fuels like petrol or diesel, both contain a mixture
of hydro carbon (HC). When an I.C. engine gets a stoichiometric
mixture of air and fuels (A:F = 14.7:1), it produces the smallest
amount of pollutants. The CO is produced as an intermediate reaction during the partial combustion of HC. When the air–fuel (A/F)
ratio was too low and there was not enough oxygen to convert
all CO present in a fuel into the CO2. The CO concentration was
raises as the A/F ratio decreases and its maximum when the automobile was an idled conditions [137]. The ratio of A/F acts as a significant role in the combustion process. There are different
situation in the combustion chamber which prevents perfect combustion and also causes of unnecessary chemical reactions. There-
Table 6
Exhaust Pollutants Regulations in India (g/Km).
Vehicle Type
CO
CO2
NOX
SOX
HC
PM
Two wheelers
Four wheelers (Petrol engine)
Four wheelers (Diesel engine)
Six wheelers (Petrol engine)
Six wheelers (Diesel engine)
1.5
2.25
1.0
3.20
3.20
1.5
5
5
12
12
0.25
0.40
0.85
0.60
1.20
0.02
0.04
0.04
0.10
0.10
0.12
0.28
0.28
0.40
0.40
–
–
0.10
–
0.18
Table 7
Exhaust pollutants regulations of CO emissions from various countries.
Vehicle Type
India (gm/Km)
U.S. (gm/Km)
Canada(gm/Km)
China (gm/Km)
Europe (gm/Km)
Japan (gm/Km)
Two wheelers
Four wheelers (Petrol engine)
Four wheelers (Diesel engine)
Six wheelers (Petrol engine)
Six wheelers (Diesel engine)
0.5
1.25
1.00
2.20
2.20
0.40
1.0
0.5
2.60
2.40
1.42
2.11
1.50
2.20
2.00
0.75
2.1
1.50
2.0
2.0
0.80
1.22
0.70
2.20
1.0
1.36
2.0
1.60
2.20
2.00
Fig. 12. Formation of CO in internal combustion engine.
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The reactions done on the automobile exhaust catalysts are extremely complex as listed below in Eqs. (8)–(10):
2NOx þ CO þ HC ! CO2 þ H2 O þ N2
ð8Þ
2CO þ O2 ! 2CO2
ð9Þ
CX H2Xþ2 þ ½ð3x þ 1Þ=2O2 ! XCO2 þ ðX þ 1ÞH2 O
Fig. 13. Catalytic Converter.
fore the harmful exhaust gasses are emitted into the environment
[169,170]. The excess HC can also be influenced by the amount of
A/F mixture as it enters into the incineration chamber as shown in
Fig. 12. If the A/F combination does not have sufficient oxygen
present during the ignition, therefore it will be not burned completely [138].
When burning takes place in an oxygen starved environment
(Fig. 13), there was inadequate oxygen present to convert completely CO into CO2. When carbon atom bonded only with one oxygen atom, therefore, CO was formed [171,172]. A composition of
main primary pollutants from the engine exhaust, complex reactions in I.C. Engines:
FuelðHCÞ þ AirðN2 þ O2 Þ ! CO2 þ H2 O þ N2 þ O2 þ CO
þ HCðUnburnedÞ þ NOx þ PM
ð7Þ
The engine design modification, clean fuels, proper maintenance and post-combustion control device reduces the CO emissions from vehicles. The diesel combustion is heterogeneous in
nature, but petrol combustion is homogeneous in nature [139].
The hydrocarbon (HC) pollutant also enters into the atmosphere
through the fuel evaporation. The fuel evaporation is done from
the fuel tank, carburetor and tailpipe of the vehicle. When the
engine is at warm idle, therefore very little CO was produced.
The reason behind this the presence of sufficient oxygen during
the combustion process to fully oxidize all the carbon atoms
[140,141].
6. Control of CO emission by application of catalytic converter
A catalytic converter is an automobile emissions control device
that converts more contaminated pollutants present in the exhaust
gasses to the lower poisonous pollutants by a catalyzing redox
reaction. The basic reactions of HC and CO in the exhaust are oxidation (Table 8) with the certain products being CO2 and H2O,
while the NOx reaction is a reduction with preferred products of
N2 [142]. The major three pollutants (CO, HC and NOx) are concurrently impassive from the exhaust by a sole converter. These converters often function at 90% efficiency, almost removing the diesel
odor and decreases the particulates (soot).
During this period, since the temperature of tailpipe-gases is
relatively low the conventional three-way catalysts illustrate especially low catalytic efficiency to convert all the harmful pollutants.
The catalyst employs in a ceramic honeycomb structure or monolith apprehended in a converter apply for an exhaust stream [143].
Table 8
Typical exhaust gas Composition.
Component
Concentration
Component
Concentration
CO
CO2
HC
NOX
PM (Soot)
SOX
100–1000 ppm
2–12 vol%
50–500 ppm
30–1000 ppm
20–200 mg/m3
2–7 ppm
Ammonia
Aldehydes
Benzene
Cyanides
PAH
Toluene
2.0 mg/mile
0.0 mg/mile
6.0 mg/mile
1.0 mg/mile
0.3 mg/mile
2.0 mg/mile
ð10Þ
At the time when the automobile vehicle initially starts up both
the engine and catalyst are in cold conditions. After startup, the
high temperature combustion reaction is move from the engine
to the exhaust piping initiate to warm up. At last, a temperature
is reached inside the catalyst that start the catalytic reactions
[144]. The reaction temperature and kinetic reaction rate depends
much upon the chemistry of catalyst. Although the catalytic converters are most frequently apply to the mining equipment, electrical generators, trucks, buses, locomotives, motorcycles, forklifts
and airplanes [145].
7. Catalysts for CO oxidation
The catalytic conversion of CO at lower temperature has significant attention due to its values in human safety in mining, deep
sea diving, space investigation, masks for fire fighters, CO sensors
and in reducing the CO build-up in CO2 lasers [130–132]. The CO oxidation also applications in industrial processes such as water–gas
shift reaction and production of methanol. The capacity to convert
CO at ambient temperatures is an important in many applications
[50–52]. At lower temperature the CO conversion was difficult to
sustain due to the strong adsorption and self-poisoning (which preferentially accumulates CO on the surface) versus surface oxidation
kinetics which removes CO from the surface [146–148].
Commercially available CO oxidation catalysts fall into three
categories.
1. Noble metal catalyst
2. Transition metal catalyst
3. Mixed metal oxide catalyst
The activity, selectivity and stability of catalyst are a fundamental step for improving the combustion reaction process between the
catalyst and reaction gasses. The base metals (Cu, Mn, Co, Cr, Ni, Fe
etc.), noble metals (Pt, Pd, Rh, Au etc.) and metal oxide (Cu2O,
CeO2, ZnO, ZrO2, TiO2 etc.) are broadly used as a catalyst in the catalytic converter [140–143]. The platinum group metal catalysts have
a huge activity and thermal stability. The effectiveness of catalytic
converter is also depending upon the temperature [145–147].
7.1. Noble metal catalysts
The noble metals are most commonly considered to be (Pt, Rh,
Ru, Ag, Pd, Ir and Au) and utilize of this element in vehicle as a catalytic converter. In the noble metal catalyst, the Rh was used as a
reduction catalyst, Pd was used as an oxidation catalyst and Pt is
use for both the reduction and oxidation catalyst. Gold (Au) is
shows good performances for low-temperature CO oxidation, if
isolated on suitable metal oxides and composite oxides [145–
148]. Au supported on reducible oxides is known to catalyze the
CO oxidations efficiently at very low temperatures (even below
0 °C). The Pt/SnO2 and Pd/SnO2 catalysts are widely used for ambient temperature oxidation of CO [149]. The catalytic performance
of PtOx, PdOx, RhOx and RuOx is strongly influenced by the oxygen
coordination around their surfaces. The oxidized noble-metal catalyst (Fig. 14) has been indeed more active than the completely
reduced particles. The disadvantage of noble metal catalyst is a
S. Dey, G.C. Dhal / Materials Science for Energy Technologies 2 (2019) 607–623
Fig. 14. Noble metal catalysts.
high-cost and lower availability. Silver (Ag) is an outstanding catalyst for different catalytic reactions for a long time. The activity
of silver based catalysts strongly depends on the surface structure
and active sites. The performances of AgO catalyst is often regarded
as a result of the incidence of various Ag-O interactions [150–154].
Out of noble metal catalysts, the Pd is mostly used and changing
the Rh and Pt metals. The noble metal catalysts are moisture tolerant but generally require a temperature above 100 °C for better
operation. The Pt/Pd/SnO2 catalysts need a high priced metal loadings therefore it was extremely costly materials to be used in this
application [151–152].
7.2. Base metal catalysts
The base metal catalysts are very active for catalytic reactions.
The base metal oxides like (Co3O4, CuO and MnO2) have a large catalytic activity per unit surface area for CO oxidation. The low price
and high performance of supported base metal oxide catalysts
may have high perspective to find its appliance to the catalytic reaction [153]. The different supported base metal oxide catalysts used
for CO oxidation is as follows Co3O4, Cu2O, Fe2O3, MnO, NiO, Cr2O3
and V2O5, etc and represented in Fig. 15. The cobalt-based catalysts
have been found to be mostly active for CO conversion and highly
susceptible to the presence of moisture. It has outstanding thermal
resistance [155–158]. In comparison to Co3O4, the CuO/Cu2O have
been found rarely use in CO conversion. This is necessary the distinctive structure of cobalt oxide, which contains both Co2+ and Co3+,
whereas CuO or Cu2O have only either Cu2+ or Cu+ ions. The CuO is
not very steady; the oxidation state of Cu may differ in path of the
reaction, CO/O2 ratio and temperature [153]. The outstanding
behavior of Cu2O in CO oxidation at ambient temperature has studied the kinetics of O2 chemisorptions over the same cuprous oxide.
The MnOx (MnO2 or Mn2O3) nano-particles are willingly to synthesized by self-assembly in a one-pot reaction under ambient conditions for conversion of CO to CO2 at low temperatures [150–152].
The iron (Fe) based catalyst main advantage is that it has a long
life-time, therefore, it could resist more poisoning from CO2. Presence of Fe2+ or Fe3+ ion has been identified as one of the key factors
governing the structure and texture property of Fe based products.
Iron was frequently used as the active metal in the catalyst for
industrial scale due to their lower cost [154]. The high activities
of nickel (Ni) were correlated to the existence of both intra and
inter-particle porosity of the catalysts and this lead to high Ni dispersion on the catalyst surfaces. Nickel is a cheap, active and suitable catalyst to be produced economically. Nickel oxide has a
bimodal pore structure, which entails high activity for CO oxidation [155]. Chromium (Cr) also belongs to the group of refractory
metals, which includes all metals with a higher melting point than
platinum (1772 °C). The reducibility of chromium (III) oxide, Cr2O3
is highly application in CO oxidation [156].
Fig. 15. Base metal oxide catalysts.
617
Fig. 16. Catalysts for CO oxidation.
7.3. Groups of catalysts for CO oxidation
In the exhaust gases, CO proves the most perilous so that more
attention has been paying on catalytic control of CO emissions
(Fig. 16) from vehicles exhaust. A wide variety of catalysts used
in a catalytic converter, which initiates the oxidation of CO such
as pyrochlores, spinel, hopcalite and monel based catalyst, etc.
The success of catalyst is highly depends upon the role played by
each element and nature of active sites.
7.3.1. Hopcalite
In 1920 Bray, Lamb and Frazer discovered that various oxides
mixture of Cu and Mn, recognized as a group of catalysts known
as a hopcalite (CuMnOx). Jones and Taylor confirm the catalytic
activity of such a arrangement called hopcalite in the year 1923.
It can well catalyze the oxidation of dry CO even at room temperature [15–18]. In recent times, the ambient temperature CO oxidation has seen a revival in its importance to the catalytic
community. The addition of Co, Au, Ce and Ag improves the catalytic activity of hopcalite catalyst. A lot of interest has been given
to the adaptation of CuMnOx catalyst to remove its faults of moisture deactivation and lower activity [156,157]. The preparation of
catalysts by other methods including anti-solvent precipitation
method, sol–gel method is reported to give better CO conversion
than commercial hopcalite [158]. The morphological, structural
and catalytic activity of hopcalite catalyst is also depending upon
the synthesis methods. The recent work shows that the addition
of gold into the hopcalite catalyst not only improved its activity
but also prevents the deactivation of catalyst [159,160].
7.3.2. Perovskites
Many perovskite catalysts are shows good catalytic active for
CO oxidation at ambient temperature. The general formula of perovskites is ABO3; typically the A elements are rare earth alkaline
(Ce, La, Pr etc.), alkaline earth metals (Ca, Cs, Sr, Ba etc.) and the
B sites are usually occupied by transition metals (Fe, Co, Cu, Mn,
Ni and Cr). The main benefit of perovskites lies in the fact that they
can possess the higher activity and stability compared to pure oxides [161]. There are many problems related to perovskite-based
catalyst such as thermal stability, catalytic activity and deactivation of catalyst by the potential poisons of sulfur and lead in the
fuel. Very few numbers of perovskite was until reported, which
was active for CO oxidations at the ambient conditions [162].
7.3.3. Spinel
The spinels are natural resources minerals and formulation
2
A2+B3+
2 O4 which crystallize in the cubic (isometric) crystal system.
The A and B can be divalent, trivalent cations, including Mn, Zn or
Fe, Mg, Al, Cr, Ti and Si. Spinels are transition metal oxide catalysts.
The aluminum and ferric based spinels are homogeneous due to
their great size difference. The NiFe2O4 spinel catalyst obtained by
combustion reaction method and preferential for CO oxidation. The
spinel catalysts are highly active for CO oxidation at high temperature [163,164].
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7.3.4. Monel
Monel is a nickel-copper mixture catalyst and used for controlling the emissions of CO, HC and NOx from vehicle exhaust. The
composition of monel catalyst which usually contains 31.5% by
weight of copper, 66.5% by weight of nickel and 2% by weight of
impurities. It has a low cost, short life and less durable catalyst
[165]. These compounds prepared from mixed oxides after calcination treatment and unique properties like high surface area, porosity and good thermal stability.
7.4. Preparation of catalysts
The structure of catalyst is highly affected by the synthesis
methods. The various methods have been applied for the synthesis
of catalyst. The property of heterogeneous catalyst was also
depending upon the synthesis conditions. There are three fundamental stages of catalyst preparation may be illustrious:
I. Synthesis of primary solid (or first precursor solid) associate
all the valuable components (e.g., impregnation or coprecipitation or amorphous, crystallization).
II. Processing of that primary solid to get the catalyst precursor,
for Ex. by heat treatment.
III. Activation of the precursor to provide the active catalyst and
activation may take place instinctively at the beginning of
the catalytic reaction (selective oxidation catalyst).
The effect of preparation conditions including metal ions concentration, ageing time, pH, drying temperature and calcination
temperature is highly effective on the catalytic activity of the catalyst. The various parameters which highly effected on the activity
of resulting catalyst, it includes calcination temperature, reduction
temperature and reduction time, etc [167,168]. The calcination
temperature has a strong influence on the chemical composition
and physicochemical properties of the resulting catalyst. The using
of different supports in the catalyst for improved their performance in CO oxidation reactions [172–174]. The promoters are
the substances that increase the activity of catalyst; they are creating the ideal conditions of the catalyst and even enhance the life of
catalyst by saving them from poison [172].
There are several methods used for the preparation of catalysts
and each methods effect on the activity of resulting catalyst for CO
oxidation was as follows:
7.4.1. Co-precipitation method
The co-precipitation is one of the simplest and most commonly
used methods for the synthesis of catalyst. The synthesis of catalyst
by co-precipitation of soluble salts of the constitute metals in a right
solvent then co-precipitated by the addition of an acid/base or
another reagent to cause the precipitation [10,11]. The precipitation
of catalyst has three main stages: Super saturation, nucleation and
growth. After ageing, filtration, washing thoroughly with deionizer
water rather than the precipitate solution was obtained in an amorphous phase or crystalline phase. Then after further steps followed
it: drying, calcination and activation [2,28]. These precursors can be
voluntarily improved into the catalyst by thermal treatment. The
different characterization techniques confirm that the coprecipitation method created minor crystalline and more surface
area than the other methods. In the co-precipitation method the
phase association contains two main elements, if one of them contained an anion and following one contained cation, the precipitate
would have a fixed or smallest amount very nonflexible composition [29,30]. There are many procedures use for the coprecipitation processes; one of the easy processes is that adding
drop-wise to the solution containing the active component to the
precipitate solutions. This is the easiest method to synthesized the
catalysts based on more than one component. The bulk catalysts
as well as support catalysts are prepared by this method [32,38].
7.4.2. Sol-gel method
The sol–gel method is a method for creating the solid materials
from small molecules. The catalyst also prepare by a sol–gel
method is as follows: the reagents (acetate and nitrate salts of preferred oxides) initially dissolve in deionizer water in stochiometric
amount [35]. A known amount of completing agents dissolve in
deionizer water was added over the solution as a ligand. The solution was maintained pH to (7.5–8.0) with acid, stirring and heating
to a temperature of 60–80 °C for 4 h, then after the syrup was
obtained heated to 100 °C for 24 h in air, followed by calcination
at 400 °C for 2 h or 500 °C for 2 h [179].
7.4.3. Impregnation method
The impregnation was associated to an ion exchange or adsorption processes, and applied in contact of solid with fluid components to be deposited on the catalyst surfaces. Throughout
impregnation, several different processes have done at different
rates. The individual adsorption species by coulomb force; ion
exchange between the electrolyte and charged surface; incomplete
suspension on the surface of solid [180]. The type of product is
depending on the nature of reactants (solid and liquid surfaces)
and reaction conditions. The major parameters affecting the liquids
are pH and concentrations of dissolved substances. It is one of the
simplest methods of preparing supported catalysts [181].
7.4.4. Reactive grinding method
The reactive grinding is an important preliminary procedure
and sometime used for the production of particular catalysts. In
fine particles, this is a assortment of particles of very small size
(typically 0.5 mm–l mm), the particles may have diverse shapes
influences the catalytic operation strongly. The grinding operations
producing particles of smaller size than after forming the operation
pores of desired sizes. It is use with the materials of natural origin
or with the products of action category [97]. This is a unique and
valuable process to make a catalyst. The fused mass is crush into
unequal lumps and sieved to the appropriate size. The powder
obtained after reactive grinding, it has a broad particle size distribution [182].
7.4.5. Hydrothermal method
The hydrothermal method is considering the modification of
precipitates and gels produce by the temperature under ageing in
the presence of water. The hydrothermal transformations usually
occur within the liquid phases. The variable of operations for a
specified solid are pH, temperature, time, pressure and concentration [183]. In other instance, the hydrothermal transformations
done by the unit operations such as precipitation, washing, drying
and extrusion [8–12].
7.4.6. Pyrolysis method
Pyrolysis is a thermo chemical disintegration of organic materials at high temperature in the lack of oxygen. It concerned with the
instantaneous modify of chemical composition and physical phase
are irreversible. Pyrolysis is a type of thermolysis and usually
observed in organic materials expose to higher temperatures
[99]. The flame spray pyrolysis use to create a broad range of pure
nano powders ranging from unique metal oxides to extra complex
mixed oxides. It is a one-step process in which a liquid feed – a
metal precursor dissolved in a solvent – is spray with an oxidizing
gas into a flare zone [183,184].
S. Dey, G.C. Dhal / Materials Science for Energy Technologies 2 (2019) 607–623
619
Fig. 17. Metal promoted catalyst.
7.5. Metals promoted on catalysts
The addition of noble metals or base metals by a Coprecipitation method into the catalyst has been considered for
low temperature CO oxidation. The addition of promoters into
the catalyst, it will raise the amount of active sites presence on
the catalyst surface; therefore, the performance of catalyst for CO
oxidation has been increases [47]. The metal oxides and supported
noble metal oxides are active for several deep oxidation reactions.
The mechanism for CO oxidation over noble metal promoted catalysts (Fig. 17) may be different from the other metal oxide catalysts
[48].
The noble metal oxide catalysts may follow either a LangmuirHinshelwood type of mechanism or an Eley-Rideal mechanism. On
silver (Ag), the oxygen chemisorptions are relatively very strong,
with a transmit of electrons to the oxygen taking place and on
Pd, however, oxygen adsorption was comparatively weak [50–
53]. On Pt show that the oxidation done more rapidly in the air
than in pure oxygen due to the development of moderately stable
species in extremely oxidizing environments [68]. The precious
metals are also able of reducing under atmospheric conditions to
some extent. The addition (doping) of Co, Ce, Fe, Zn, Ni and other
substituted metals into the catalyst has enhanced their activity
for CO oxidation. The synergy effects are arises, the contact of promoters with catalyst being the binary catalyst, which was more
active than each component [20].
7.6. Mechanism and kinetics study of catalyst
The performance of catalyst for CO oxidation was measured by
the activation energy of the process. These data are desirable for
modeling and design of catalytic converter. It is significant to
improves the kinetic expressions for catalytic conversion of CO also
because they can be applied into computational fluid dynamics
(CFD) models useful for optimization and reactor design [143–
148]. The catalyst initially oxidized CO before it’s oxidized by air,
and this is an investigation of a Mars van Krevelen-type mechanism, which has consequently found support [175–178]. The conversion of CO by the Marsvan Krevelen mechanism would shows
the relationship between the catalyst activity and reducibility. In
the first step of reaction, the CO and O2 adsorb into the catalyst surface. Then after, the adsorbed O2 molecules separate into the individual O atoms. The CO molecules and O atoms are initiate to
disperse on the metal surface, when a CO molecule and O atom
combine each other, they recombine and form CO2. In the last step,
CO2 desorbs into the gas phase. Under reaction conditions, the rate
was proportional to the O2 pressure and independent of CO pressure [150–154]. The dissociation of oxygen molecules followed
by the reaction between adsorbed O2 atom and CO to form CO2
is individual one of the usual mechanisms for CO oxidation. In this
condition, the reaction rate was restricted by the dissociation of O2.
The molecular adsorption of CO occurs at the upper temperatures,
which shows that the presence of reactive oxygen forms. The O
ions are more reactive and reactivity of superoxide is also high,
though much lower as compared to O. In oxygen species, the
CO molecules from gas phase can be directly oxidized. The rate
of reaction (Eqs. (11)–(13)) will be proportional to the surface coverage of Oads and COads [172–175].
O2 þ 2 ! 2Oads
ð11Þ
CO þ ! COads
ð12Þ
COads þ Oads ! CO2 þ 2
ð13Þ
Fig. 18. Mechanism of CO chemisorptions over catalysts.
620
S. Dey, G.C. Dhal / Materials Science for Energy Technologies 2 (2019) 607–623
Fig. 19. Adsorption of CO and dissociation of O2 over Pd catalysts.
In the oxidation state, CO was oxidized by surface lattice oxygen
in the metal oxide. An oxygen vacancy was created, reducing the
metal ions to a poorer oxidation state. In the next step, the surface
metal atoms are oxidized by gas-phase oxygen. The process of CO
oxidation does not take place as long as the adsorbed molecules of
O2 transform to the reactive form of oxygen.
The rate of CO oxidation (Fig. 18) was measured by following
either the rate of production of CO2 or, when the CO was captured
and rate of removing of CO. The reaction rate was compacted when
CO was added into the gas phase during the oxidation. The amount
of CO2 molecules adsorbed corresponded exactly to the amount of
oxygen atoms (Fig. 19) pre-adsorbed on the catalyst surfaces. The
equivalent concentration of oxygen atoms in the gas phase over
the surface, therefore, the heterogeneous reaction was taking
places [160–165]. The inconsistent oxygen stoichiometry of transitional metal structure was responsible for the unusual performance of these palladium catalysts. For non-reducible supports,
such as SiO2, TiO2 and Al2O3, the catalytic performance is much
more dependent on the metal diffusion; in fact, the oxygen adsorption occurs only on the metal sites [150–154].
The Langmuir-Hinshelwood model has been applied to illustrate the CO oxidation mechanism taking place on the metal catalysts. In the Langmuir-Hinshelwood model, the CO oxidation done
on the surface in the following steps are involved:
COðgÞ ! COads
ð14Þ
O2 ðgÞ ! 2Oads
ð15Þ
COads þ Oads ! CO2 ðgÞ
ð16Þ
The CO oxidation over an unsupported catalysts proceeds
according to a Langmuir–Hinshelwood mechanism, in which the
certain steps take place in the adsorbed phase. Both reactants are
adsorbed. The interface of metal oxide and support is certain interest since metal can act as a good promoter to transition metal oxides for CO oxidation reaction [157–160]. The kinetic data are taken
by the steady-state reaction conditions at a low conversion of CO.
Monometallic catalysts fast surface oxidation in ambient air, so
that the CO chemisorptions experiments was done after in situ
reduction, with oxygen being excluded [162–165].
7.7. Use of catalysts at cold start emission conditions
Catalysts operating efficiently at the ambient conditions are a
different class of materials, in application as different as the cold
start of engines to inside air quality. The 60%–80% of CO and HCs
emissions from automobile occurs during this ‘‘cold-start” period.
In the cold start period, the catalytic converter is entirely inactive,
because the catalytic converter has not warmed up. The cold start
phase is also depending upon the characteristics of vehicles [31].
The entire amount of fuel required for a cold start of the engine
is a function of ambient air, engine design and coolant temperatures. The catalytic converter is uses in an automobile vehicle are
capable to reach the reductions of CO, HCs and NOx up to 95%
when they are completely warmed up [40]. The cold start
emissions can be subdivided into two parts: the first was much
more emissions due to the initiation of engine and second was over
emissions during the warm-up procedure of the engine and catalyst. The amount of catalyst was required to entrap the toxic pollutants throughout the cold-start period is usually much less than
that needed in catalytic converters so that added volume of catalyst in a adsorbed devices is comparatively low [173].
8. Conclusions
Carbon monoxide is one of the most poisoning gas present in
the atmosphere. The main contribution of CO in the air is transportation section therefore catalytic converter has been invented.
Catalytic oxidation has been widely studied for two main reasons
first CO is a toxic gas which should be removed from the exhaust
gases exist in mobile and stationary sources second CO oxidation
is a simple reaction mostly used for evaluating the catalytic activity
of new materials. The improvement of catalytic converters with
noble metals led to an extremely high number of publications on
metal catalysts. The activity of catalyst is strongly dependent on
the reaction conditions (CO/O2 ratio, presence of steam or other
impurities in the gas). The oxide catalysts show much more adaptable than metal catalysts for CO oxidation reaction. This review
over CO oxidation at ambient conditions will be useful to researchers who are interested in a more comprehensive overview. There is
an ever present need to develop new, effective CO oxidation catalyst composition working at ambient conditions with improved
processes.
Declaration of Competing Interest
None.
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