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Borderless Science Publishing 381
Canadian Chemical Transactions Year 2015 | Volume 3 | Issue 4 | Page 381-409
Review DOI:10.13179/canchemtrans.2015.03.04.0227
Catalytic Abatement of Methane Emission from CNG
Vehicles: An Overview
Maninder Kumar1, Gaurav Rattan1,*, and Ram Prasad2
1Dr. S. S. Bhatnagar University Institute of Chemical Engineering & Technology, Panjab University,
Chandigarh, India. 2Department of Chemical Engineering & Technology, Indian Institute of Technology (BHU), Varanasi, UP,
India.
Corresponding author: E-mail: grattan@pu.ac.in +91-8288071498
Received: August 27, 2015 Revised: October 28, 2015 Accepted: October 29, 2015 Published: October 30, 2015
Abstract: Compressed natural gas (CNG) is a substitution of alternative fuel for automotive application
with significant environmental advantages as it is the only fuel cheaper than gasoline or diesel,
comparatively lower air pollution emissions, lesser CO2 emissions. However, emission of methane is 40%
more in CNG fuelled engines compared to gasoline and diesel. With the exponential growth in CNG fuelled
vehicles, CH4 concentrations have reached an alarming level in the environment. It affects human health
and climate change as the Global Warming Potential (GWP) of methane is 21 times more than CO2 which
means that CH4 will cause 21 times as much warming as an equivalent mass of CO2.
Stringent regulations have been adopted to curb the menace of vehicular pollution. In order to meet
the stringent regulations Catalytic convertor using noble metals proved to a boon in vehicular industry.
Noble metals are highly active for removal of methane as a pollutant. However, their expensiveness,
deterioration with time can generate even more toxic volatile pollutants. So, researchers have tried to
substitute the noble metals with transition base metals. Further, low cost, easy availability and advance
synthesis methods of catalyst preparation advocates for the use of base metals in an auto exhaust purification
catalysts.
Ample literature has been accumulated which constitutes of review articles, research papers, PhD
thesis, proceedings etc. on methane oxidation, but still there is a gap in literature for a review article which
directs the methane oxidation with CNG exhaust emissions. Therefore in order to fill this gap present review
updates and evaluates the progress of various catalysts for purification of CNG exhaust emissions. This
paper reveals a brief discussion on CNG as an alternative fuel, various catalysts, operating parameters
reported in the literature.
Keywords: CNG exhaust emissions, Methane oxidation, Review, Catalysts, Pollution abatement.
Abbreviations: CNG: Compressed Natural Gas; NMHCs: Non-Methane Hydrocarbons; IC: Internal
Combustion; THCs: Total Hydrocarbons; GHP: Green House Potential; IPCC: Intergovernmental Panel on
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Climate Change; EU: European Union; LPG: Liquefied Petroleum Gas; Calc. : Calcination; Temp. :
Temperature; STP: Standard Temperature & Pressure; GHSV: Gas Hourly Space Velocity; WHSV: Weight
Hourly Space Velocity; MHSV: Mass Hourly Space Velocity; ppm : Parts Per Million; mg: milligram; SV:
Space Velocity; Pepn: Preparation ; mL/min: millilitre per minute; HDP: Homogeneous Deposition
Precipitation Method; HCs: Hydrocarbons; NOx: Nitrogen Oxides; T100%= X°C: Means 100% oxidation
takes place at temperature of X°C; XRD: X-Ray Diffraction; TPR: Temperature Programmed Reduction;
XPS: X-Ray Photoelectron Spectroscopy; TGA-DSC: Thermal Gravity Analysis Differential Scanning
Calorimetry; BET: Brunauer-Emmett-Teller technique; TWC: Three Way Catalytic Convertor
1. INTRODUCTION
The combustion of conventional petroleum fuels leads to toxic emissions, global warming and
hence climate change which threatens the survival of life. Moreover with rising number of vehicles, the oil
reserves are exhausting at alarming rates. Therefore the challenge before the researchers is to investigate
alternatives for clean and efficient fuel. Natural gas provides an attractive fuel since it is available in
abundant supply. It is produced from gas wells or tied in with crude oil production. Natural gas has many
constituent gases such as methane, ethane, propane, nitrogen, helium, carbon dioxide, hydrogen sulphide,
and water vapour. Composition of these gases depends on the source of its origin. Composition of natural
gas is shown in table 1.
Table 1. Natural gas composition [1]
Composition Volume Fraction (%)
Methane(CH4) 94.00 92.07 94.39 91.82
Ethane(C2H6) 3.30 4.66 3.29 2.91
Propane(C3H8) 1.00 1.13 0.57 -
Iso-Butane(i-C4H10) 0.15 0.21 0.11 -
n-Butane(n-C4H10) 0.20 0.29 0.15 -
Iso-Pentane(i-C5H12) 0.02 0.10 0.05 -
n-Pentane(n-C5H12) 0.02 0.08 0.06 -
Nitrogen(N2) 1.00 1.02 0.96 4.46
Carbon Dioxide(CO2) 0.30 0.26 0.28 0.81
Hexane(C6+ (C6H14)) 0.01 0.17 0.13 -
Oxygen(O2) - 0.01 <0.01 -
Carbon
Monoxide(CO)
- <0.01 <0.01 -
Total 100 100 100
Natural gas can be easily compressed, so it can be stored and used as compressed natural gas
(CNG). CNG is colourless, odourless, non-toxic, lighter than air and inflammable. There are over 1,500,000
vehicles in the world produced by Honda, Ford, Toyota, Volvo, Mercedes Benz, Optare and Scania running
on CNG [2]. CNG engines guarantee considerable advantages over conventional gasoline and diesel
engines [3]. CNG is attractive for many reasons such as it is the only fuel cheaper than gasoline or diesel,
has comparatively lower air pollution emissions, lesser greenhouse gas emissions, large quantities of the
fuel is available. Being a gaseous-fuel it does not require cold-start enrichment; emissions from "cold"
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engine operation are higher than with liquid fuels [4]. As it is non-toxic gas and it will not contaminate
groundwater if spilled in comparison to gasoline and diesel. The CNG fuel properties are shown in Table
2.
Table 2. CNG fuel properties at 250C and 1 atm [24]
CNG Properties Value
Density (kg/m3) 0.72 0.68
Flammability limits (volume % in air) 4.3-15 4-14
Flammability limits (Ø) 0.4-1.6 -
Autoignition temperature in air (0C) 723 700
Minimum ignition energy (mJ) 0.28 -
Flame velocity (ms-1) 0.38 0.63
Adiabatic flame temperature (K) 2214 -
Quenching distance (mm) 2.1 -
Stoichiometric fuel/air mass ratio 0.069 0.058
Stoichiometric volume fraction % 9.48 -
Lower heating value (MJ/kg) 45.8 -
Heat of combustion (MJ/kgair) 2.9 -
Octane rating - 130
Advantages of CNG according to Jahirul et al. [5] are as follows:
Unique combustion and suitable mixture formation;
Due to high octane number, engine operates smoothly with high compression ratios without
knocking;
During lean burning conditions it will lead to low exhaust emissions and fuel operating cost;
It has a lower flame speed;
Engine durability is very high.
The formation of non-methane hydrocarbons and other air pollutants in CNG fuelled engines are one-
tenth than those of gasoline engines [6]. The formation of CO and NOx are 80% lower than gasoline engines
because of simple chemical structures of natural gas (primarily methane (CH4)) contain single Carbon
compare to diesel (C15H32) and gasoline (C8H18). In terms of economy CNG fuelled engines are 20% more
economical than gasoline and diesel engines.
However, some difficulties with CNG fuelled vehicles are: a) Fuel storage, b) Infrastructure costs, and
c) Ensuring sufficient supply. As CNG requires a much larger volume to store the same mass of natural gas
and a very high pressure of about 200 bars or 2,900 psi [4] which requires the additional chambers, storage
cylinders which in turn increases weight of the vehicle. With exponential rise in the CNG fuelled vehicles
would require new gas pipelines, CNG specific fuel stations and other infrastructure. Due to on-board
storage, the engine knocks at high loads [7]. CNG advantages are partially balanced by the emission of
unburned methane. The hydrocarbon composition in the exhaust gases of lean-burn CNG engines reflects
the composition of natural gas in methane and non-methanic hydrocarbons (NMHCs), typically 90–95%
methane [8]. The typical composition of CNG is given in Table 3.
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Table 3. Typical Composition (Vol %) of Compressed Natural gas [2]
Component Volumetric %
Methane (CH4) 94.42
Ethane (C2H6) 2.29
Propane (C3H8) 0.03
Butane (C4H10) 0.25
Nitrogen (N2) 0.44
Carbon dioxide (CO2) 0.57
Others 2
Methane is a potent greenhouse gas and its global warming potential is about 21 times than CO2
[9]. It contributes more to global atmosphere warming than carbon dioxide at equivalent emission rates. It
leads to climatic change which is due to the presence of such greenhouse gases. Methane has the most stable
structure compared to other hydrocarbons and is more difficult to be oxidized than most HCs. The catalytic
oxidation of methane is an easy way of automobile emission control as shown by the equation (1).
CH4 [g] + 2O2 [g] → CO2 [g] + 2H2O [g] + heat (890 kJ/mol)……. (1)
The catalytic conversion of CH4 to CO2 and H2O helps in reducing global warming. Further CO2
and H2O found in the atmosphere are useful for vegetation. CH4 oxidation has been studied broadly over
various types of catalysts such as gold based catalyst [9-10] noble metals (Pt, Pd, Rh, Au, etc.) [11-12], base
metals [13-14], mixture of noble and base metals [15], perovskites [16-18] etc. Various review articles has
been published [8, 19], number of PhD [20-21] has been awarded by various universities on the CNG
exhaust emissions and control. Also various controlling techniques and methods, catalysts have been
depicted in many patents [22-23] but still there is a gap in literature for a review article which relates the
methane emissions from CNG vehicles with the various catalysts for its abatement and control.
Therefore this paper aims to summarize the latest research results regarding the catalysts used for
methane oxidation. Complete analyses have been made of CNG properties, its advantages, disadvantages,
etc. Various parameters used for catalytic oxidation of methane such as calcinations temperature, flow rate,
other experimental operating parameters are presented in a tabular form. The sources and effect of CH4 on
humans, environment and ways to minimize emissions have also been discussed.
2. SOURCES OF METHANE EMISSIONS
Methane is colourless, odourless; tetrahedron structure (figure 2) gas having boiling point of 111.66
K. It is lighter than air which makes it very easily escapable gas from wherever it is stored or processed.
Hydrogen sulphide (which smells like rotten egg) is usually mixed with methane by the commercial energy
companies for the detection of its leakage. CH4 is a “greenhouse gas,” meaning that it traps infrared
radiation (heat) from the earth’s surface and increases the temperature of the earth. During the past century,
humans have substantially added to the amount of greenhouse gases in the atmosphere through activities
such as burning fossil fuels and deforestation. According to a study CNG fuelled transit buses have higher
methane emissions than diesels [25]. CNG engine emission results are shown in figure 1 [25].
CH4 is emitted from both natural and anthropogenic sources. Large portion of the gas is emitted
from the industrial sector such as petrochemicals [27-28]. It is generally emitted to the atmosphere during
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the production, processing, storage and distribution which accounts for the huge amount to the atmosphere.
Apart from industries methane is also emitted from the flora and fauna which adds great amount of methane
as pollutant to the atmosphere. Methane is also emitted from wetland rice fields [29-30], terrestrial plants
under aerobic conditions [31], cattle [32], landfills and biomass burning.
Figure 2. Structure of Methane
2.1. CH4 from Internal Combustion Engine
CH4 is emitted from gasoline, diesel, methanol, ethanol, LPG, and natural gas internal-combustion-
engine vehicles. These emissions occur due to incomplete or partial fuel combustion, which produces CH4,
CO, PM along with other unburned hydrocarbons. This usually occurs when the ratio of air to fuel in
combustion chamber is too low for complete combustion i.e. there is inadequate oxygen to convert all CH4
present in the fuel to CO2 and H2O and heat. When internal combustion engine gets a stoichiometric mixture
of air and fuel, 17.2 parts by weight of air and one part by weight of CNG (almost methane), it emits
minimum amount of pollutants. But the combustion process is never found to be 100% efficient in IC engine
(Equation-1). Hence, under ideal conditions only the engine operates efficiently and would generate CO2,
H2O and heat. However, emissions depends upon so many factors [33], such as (i) Type of fuel used, (ii)
The design of engine, (iii) Tuning of the engine, (iv) Type of emission control system, (v) Age of the vehicle.
Figure 1 shows the comparison of exhaust gases of CNG and gasoline fuelled engines.
Figure 1. Comparison of exhaust gases of CNG and gasoline fuelled engines [25]
[g/km]
CNG
Gasoline
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Moreover, CH4 has the most stable structure compared to other organic gases and is difficult to
oxidise catalytically, the systems used to control emissions of NMHCs and total hydrocarbons (THCs) do
not have the same effectiveness in controlling CH4 emissions as they do in controlling NMHCs. According
to a report, CH4 emissions from vehicles with hydrocarbon controls might be about 3 times less than CH4
emissions from vehicles with no controls. Methane from Internal-combustion-engine vehicles using fossil
fuels is presented in table 4. Vehicles without a catalytic converter emit 0.3 g/mile CH4 in comparison with
0.1 g/mile CH4 for vehicles provided with a catalytic converter. As expected that vehicles using natural gas
would emit considerably more CH4 than gasoline engines. CH4 emissions from natural gas vehicles (NGVs)
range from 0.6 to 4 g/mile for dual-fuel vehicles (which carry and use two fuels, gasoline and natural gas),
and between 0.13 and 3 g/mile for dedicated vehicles (which carry and use only natural gas) [33]. During
cold start period, gasoline engines emit much higher amount of hydrocarbons compared with the natural
gas [27]. This is mainly because CH4 is a gas at all ambient temperature and hence does not have to be
vaporized as in the case of gasoline, temperature-dependent process. Content of methane emission by
various tests is shown in table 5.
Table 4. The percentage contribution of individual GHGs to lifecycle CO2-equivalent emissions for
alternative transportation fuels for light-duty vehicles [33]
Fuel→
Conventional
gasoline
Reformulated
gasoline
Low
sulphur
diesel
85%
methanol
Compressed
natural gas
Compressed
hydrogen
LPG
CH4 3% 3% 3% 5% 17% 7% 4%
Table 5. Content of methane emission [33]
CH4 content of natural gas 86% 90% 94% 97%
CH4 emissions from vehicles in grams per mile
(REP05 cycle/FTP cycle)
0.47/0.91 0.50/0.93 0.48/0.96 0.49/0.92
FTP: Federal Test Procedure, REP05: the EPA’s high-speed, high-load driving cycle used
2.2. Adverse effect of methane
Methane emissions adversely affect humans and environment. It has drastic effects on nature as GHP and
hence climate change.
2.2.1. Effects on humans
In normal circumstances, methane is not toxic to humans. It causes asphyxiation by displacing oxygen.
However long-term exposure of methane to humans can cause loss of consciousness, depression,
suffocation, emotional upset, fatigue, nausea or even death can occur.
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2.2.2. Effects on nature
The presence of methane in the atmosphere has been known since the 1940’s when Migeotte [34]
observed strong absorption bands in the infra-red region of the solar spectrum which were attributed to the
presence of atmospheric methane. Methane has a much greater impact on global warming than carbon
dioxide and it is the most damaging greenhouse gas produced by human activity after carbon dioxide [26].
According to IPCC 2013 [74], the surface mixing ratio of CH4 has increased by 150% since pre-industrial
times with some projections indicating a further doubling by 2100. A recent report by IPCC 2014 [92]
shows the trends of temperature increase from the past time. Climatic changes due to greenhouse effect are
likely to have an effect on water sources and supplies and the increase in temperature will induce a new
distribution of deserts and wet areas in the world.
Methane is also the most abundant reactive trace gas in the troposphere and its reactivity is
important to both tropospheric and stratospheric chemistry. The oxidation of CH4 by hydroxyl (OH) in the
troposphere leads to the formation of formaldehyde (CH2O), carbon monoxide (CO), and ozone (O3), in the
presence of sufficiently high levels of nitrogen oxides (NOx). Along with CO, methane helps control the
amount of OH in the troposphere. Methane affects the concentrations of water vapour and ozone in the
stratosphere, and plays a key role in the conversion of reactive chlorine to less reactive HCl in the
stratosphere [35]
2.2.3. Formaldehyde emission
Compared to low-emitting diesel, the CNG exhaust had higher levels of six toxic air contaminants
(TAC) listed by the California Air Resources Board (ARB)—acetaldehyde, acrolein, benzene,
formaldehyde, methyl ethyl ketone, and propionaldehyde—and did not have lower emissions of any TAC.
Formaldehyde emission is very high with CNG vehicles. Formaldehyde is classified as a toxic air
contaminant and is known carcinogenic. Inhalation of high doses of formaldehyde has produced nasal
tumors in laboratory rats, and lower concentrations have irritated eyes and air passages in humans.
Thresholds for sensory irritation determined by controlled exposure studies, are reported as 0.6-1.2 mg/m3
[0.5-1.0 ppm] (formaldehyde); 0.1-0.2 mg/m3 [0.04-0.09 ppm] (acrolein); and 90 mg/m3 [50 ppm]
(acetaldehyde) [70].
3. EMISSION REGULATIONS
With exponential growth of vehicles on roads, CH4 concentration has reached an alarming level in
the environment. According to the IPCC 2014 [92] “With high levels of warming that result from continued
growth in GHG emissions, risks will be challenging to manage, and even serious, sustained investments in
adaptation will face limits”. In order to manage harmful exhaust emissions stringent regulations have been
adopted from time to time. Environmental protection agency (EPA) setup in 1970 in order to reduce
automobile pollution and is still in development process. In October, 2003, the National Auto Fuel Policy
was announced, in which Indian automobile emissions are revised according to European Union,
introducing Euro 2–4 emission as Bharat Stage (BS II-IV) and fuel regulations by 2010 [36]. The
implementation schedule of EU emission standards in India is summarized in Table 6. For detail description
of various emissions norms read [37].
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Table 6. Emission norms in India parallel to EU emission standards
NORMS Year CO (g/Km) HC + NOx (g/Km)
1991Norms - - 14.3-27.1 2.0(Only HC)
1996 Norms - - 8.68-12.40 3.00-4.36
1998Norms - - 4.34-6.20 1.50-2.18
India stage 2000 norms Euro 1 2000 2.72 0.97
Bharat stage-II Euro 2 2001 2.2 0.5
Bharat Stage-III Euro 3 2005 2.3 0.35(combined)
Bharat Stage-IV Euro 4 2010 1.0 0.18(combined)
Bharat Stage-V Euro 5 2011
However the emission standards for CNG fuelled vehicles runs parallel to gasoline and diesel fuelled
engines with certain modifications. Mass emission standards for CNG fuelled vehicles are same as
applicable for gasoline vehicles with an exception that HC shall be replaced by NMHC, where NMHC=
0.3 x HC [37]. Table 7 represents a brief detail of CNG emission standards.
Table 7. CNG EMISSION STANDARDS [37]
Category Applicable emission norms
OE CNG/ LPG Category M and Category N Vehicles
with GVW 3500kg, 3 wheelers and 2 wheelers
Prevailing gasoline norms *
CNG/LPG Category M and Category N Vehicles with
GVW 3500kg, 3 wheelers and 2 wheelers retro fitment
from Gasoline
Prevailing gasoline norms
CNG/LPG Category M and Category N Vehicles with
GVW 3500kg, 3 wheelers and 2 wheelers retro fitment
from Diesel
Prevailing diesel norms**
CNG/LPG Category M and Category N Vehicles with
GVW > 3500kg, manufactured upto 1st April 2010
Prevailing diesel engine norms based on 13-
mode steady-state engine dynamometer test
or 13 -mode Engine steady state cycle as
applicable **
CNG/LPG Category M and Category N Vehicles with
GVW > 3500kg, manufactured on and from 1st April
2010
Prevailing diesel engine norms **
*-Vehicle having option for bi-fuel operation and fitted with limp-home gasoline tank of capacity not
exceeding 2 liters, 3 liters and 5 liters respectively on 2W, 3W and 4W are exempted from emission test,
crankcase emission test and SHED test in gasoline mode.
**-PM limit is not applicable
It is important to note that although methane emissions not regulated in the above given table.
Regulations are adopted for hydrocarbon and Non-Methane hydrocarbons where as methane is not
regulated. Methane is also not regulated in many parts of the developing countries, USA etc.
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4. ACTIONS TO REDUCE CH4 EMISSIONS
It is likely that the emission legislation will be more stringent in the near future, since the current
limits lead to unacceptable emissions from a health point of view [38]. As emissions are a result of many
parameters and these parameters can be optimised in order to reduce or meet the stringent legislations.
There are two principal approaches for decreasing CH4 emission from internal combustion engines.
1. Optimisation of the combustion process, fuel modification or fuel additives, modification to motor
engines.
2. Cleaning of exhaust gases: catalytic oxidation using catalytic convertor.
The present review is primarily concerned with catalytic convertor used in exhaust of vehicles, hence
emphasis is on various catalysts used in the catalytic convertor.
4.1. Catalytic Converter
Frenchman Eugene Houdry a mechanical engineer and expert in catalytic oil refining invented the
catalytic converter in 1950s. Catalytic convertor was introduced in USA in 1974 [39] and in European
vehicles in 1985. Catalytic converter is fitted before the vehicular exhaust in order to reduce harmful
pollutants such as unburned Hydrocarbon (HCs), Carbon monoxide (CO) and Oxides of nitrogen (NOx)
into less harmful components. A three-way catalytic converter has three simultaneous tasks: reduction of
NOx [equation-(2)], Oxidation of CO [equation- (3)] and unburned HCs [equation- (4)].
2NOx → xO2 + N2 (2)
2CO + O2 → 2CO2 (3)
CXH2X+2 + [(3x + 1)/2]O2 → xCO2 + (x+1)H2O (4)
The three noble metals that are used in Catalytic converter are platinum, palladium and rhodium
[40]. Catalytic converter consists of two ceramic blocks with micro ducts which are used in order to increase
the contact zone between gases and catalyst. The increase in surface area is greater than the area of soccer
field. The ducts consist of platinum and rhodium in one block while platinum and palladium in the other
block, acting as catalysts. As the gas enters inside the catalyst the above reactions (equations 2-4) take
place.
When the automobile starts, both the engine and catalyst are cold. After some time, the heat of combustion
is transferred from the engine and the exhaust piping begins to heat up. Finally, a temperature is reached
within the catalyst that initiates the catalytic reactions. This light-off temperature and the concurrent
reaction rate is kinetically controlled; i.e., it depends on the chemistry of the catalyst since the transport
reactions are fast. Typically, the CO reaction begins first followed by the HC and NOx reaction [41]. There
is decrease in activity of the catalyst with time. One would expect that pollutants including CH4 to increase
somewhat as the engine and the emission-control system age and deteriorate or deterioration of catalytic
convertor. The data do suggest that for most petroleum fuels as well as nonpetroleum fuels, CH4 emissions
increase with the age of the catalyst [33].
Various factors are responsible for the deactivation of the catalyst such as 1) Deposited poisons that
cover the active sites of the catalyst and partially plug the pore entrance. 2) Chemisorbed poison which is
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a result of the compounds that are present in the reactant gasses such as sulphur which chemisorbed on the
catalyst surface. The three way catalytic convertor has same problem of poisoning. 3) Diffusion poisons
that results due to the blocking of the pore mouths of the catalyst and prevents the reactant from diffusing
into the inner surface of the catalyst.
5. CATALYSTS FOR CH4 COMBUSTION/OXIDATION
5.1. Gold-Based Catalysts
Gold is the least reactive metal and it has been regarded as poorly active as a heterogeneous catalyst
[42]. However, it was only since the discovery of gold nano particles that gold has been getting considerable
interest in the field of catalysis. Since then number of patents [43-44], PhD degree [20] and various articles
has been published [15, 45-46]. Early work of Haruta et al. on catalysis showed that Au catalysts are very
active for CO oxidation even at sub-ambient temperatures [47]. Also the application of supported gold
catalysts for the total oxidation of methane has been addressed in earlier studies [9, 42, 46, 48]. Waters and
his colleagues made a considerable effort on methane combustion over transition metal oxide supported
gold catalysts prepared by co-precipitation, and concluded that the best catalytic performance was obtained
with Co3O4 as the support [45].
Various supports were tested for methane oxidation that was active for methane combustion in the
absence of gold at higher temperatures. Au/CoOx prepared by co-precipitation method can oxidise 100%
CH4 at temperature of 350°C [49] and it was concluded that the catalytic activity of Au/Co3O4 (Au loading
was 2–5 wt. %) towards methane combustion could be enhanced with addition of small amount of Pt, e.g.
0.2 wt. %, and the temperature for 100% conversion of methane could be decreased by 50 °C. Liotta studied
the effect of cerium addition on AuCo catalyst. The CeO2 in AuCoCe plays the role of a structural promoter,
limiting the Au sintering and the Co3O4 decomposition at temperature above > 600°C [15]. Moreover, the
activity of Au/Al2O3 and Au/MOx/Al2O3 (M = Cr, Mn, Fe, Co, Ni, Cu and Zn) catalysts for oxidation of
CH4 has also been studied [10].
Table 8. Recent literature review at glance of methane combustion on gold based catalysts.
Catalyst, Pepn. Method
Exp Operating Parameters Remark Reference
Au/TiO2, Au/SiO2, Au/CeO2,
Au/ZrO2, Au/Al2O3,
Deposition Precipitation, Calc.
temp. 400 °C
Quartz tubular fixed bed, 50
mL/min. (20% CH4 and 5% O2
rest is He.
Au/TiO2 shows T80%=
400°C, where as
Au/Al2O3 shows
T20%=400°C.
[46]
Co3O4/CeO2 and Co3O4/CeO2–
ZrO2
Co-precipitation, Calc. temp.
650 °C
quartz U-shaped,
0.3 vol.% of CH4 + 4.8 or 0.6
vol.% O2 , in He. 50 mL/min
(STP),
WHSV of 12000 or 60000
mL/g h.
Co3O4/CeO2 shows
T100%= 800°C
[14]
Au/CoOx, Au/MnOx co-
precipitation,
Calc. temp. 400 °C
50 mg, fixed-bed
S. S. tubular flow, 50 mL/min.,
GHSV = 15,000 h-1, alkane/air
Au/CoOx
Shows T100%= 350 °C.
[48]
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( molar ratio) of 0.5/99.5
Au/Co3O4–CeO2
co-precipitation,
Calc. temp. 600 °C.
50 mg, U-shaped quartz,
0.3 vol.%CH4, 2.4 vol.%O2 in
He.
WHSV=60,000 mL/ gh.
AuCeO2 shows
T100%=750°C.
[15]
Au/MgO,
Impregnation,
Calc. temp. 850 °C.
200 mg, microreactor of quartz
glass, CH4 (46%, O2 8%, He
46%. GHSV=750 h-1.
Enhanced activity when
gold was employed
[51]
Au–Pt/Co3O4
coprecipitation technique
Calc. temp. 400 °C.
0.10 g, fixed bed microreactor,
1 vol.% CH4,
5 vol.% O2, and rest N2., hourly
space velocity of 10,000 per
hour
Activity of Au/Co3O4
enhanced when Pt, e.g.
0.2 wt.% was added,
and conv. Temp.
decreased by 50°C.
[49]
Au/MOx/Al2O3
where M is Cr, Mn, Fe, Co, Ni,
Cu, and Zn, homogeneous
deposition precipitation, Calc.
temp. 400 °C
lab-scale fixed-bed, 0.8 vol.%
CH4 and 3.2 vol.% O2 in He, 30
mL/min.
Au/MnOx/Al2O3 shows
T100%= 25°C for CO.
whereas MnOx/Al2O3
shows T80%= 375°C
[10]
CoxMny
co-precipitation,
Calc. temp. 400 °C
0.5 g, fixed-bed quartz,
1 vol.% CH4, 10 vol.% O2 and
N2 balance gas, flow rate =150
mL/min, WHSV= 36,000
mL/hg.
Co5Mn exhibits good
result with T50%=290°C
T90%=320°C
T100%=400°C
[52]
Au/Fe2O3
Deposition precipitation,
0.1 g, fixed bed quartz,
1 vol.% CH4 in air, flow
rate=100 mL/min, GHSV =
51,000 h-1.
Au/Fe2O3 prep. By HDP
shows T50%=375°C,
T100%=500°C.
[9]
AuOx/Ce0.6Zr0.3Y0.1O2
Co-precipitation,
Calc. temp. 700 °C
0.12 mL, quartz fixed-bed
microreactor,
5% CH4/95% He (v/v, 40
mL/min) + O2 (8 mL/min) + He
(52 mL/min),
SV= 50,000 h-1.
Catalyst with 6% AuOx
shows T50%=590°C
T100%=680°C
[53]
AgMnLa,
Coprecipitation,
Calc. temp. of 800 °C
0.15 g,
2 vol% CH4 in air,
300 cm3/min,
SV=120,000 cm3/(h gcat),
Catalyst with 0.3 %
mole fraction of Ag
shows T50%=600°C and
T100%=700°C
[54]
A higher methane conversion was observed for catalysts with small gold particles. However work
reported by Chaudhary and his colleagues [9] on gold based catalysts showed that Au/Fe2O3 prepared by
HDP method is the best catalyst for CH4 oxidation among various other metal oxide (viz. MnO2, CoOx,
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CeO2, Ga2O3, Al2O3, TiO2 and MgO) supported gold catalysts. Apart from the oxidation studies, various
routes have also been suggested for preparation of gold based catalysts. Ivanova reported the preparation
Table 9. Parameters of some publication related to noble metal based catalysts
Catalyst, Pepn. Method
exp operating parameters REMARK Ref
monolith catalysts of
5 wt% Pd//Al2O3; 1:1 molar ratio of
Pd:Co, Rh, Ir, Ni, Pt, Cu, Ag,
Au,//Al2O3;
Ni//Al2O3; 2.5 wt% Pd//Al2O3;
Pt//Al2O3
Co-impregnation, Calc. temp. of 1000
°C
1.5 vol% CH4 in air
SV=250000 h-1.
5 wt% Pd//Al2O3 shows
good results with
T50%=600°C and
T90%=850°C
[62]
Pd/SnO2, Impregnation method,
Calc. temp. of 600-1000 °C
CH4 (1 vol.%), O2 (20
vol.%), H2O (0–20
vol.%), and N2, SV=
48000 h-1, quartz tube
1 wt.% Pd/SnO2 shows
T50%=350°C,
T100%=450°C
[63]
Pd/ZrO2,wet impregnation,
Calc. temp. of 600 °C
50 mg,
alumina tubular reactor,
CH4=1%, O2=4%,
He=95%.
Effect of water vapour
studied, Shows
T50%=650°C ,T90%=800°C
[64]
Pd/Al2O3, Pd/SiO2, Pd/Al2O3-SiO2
Impregnation, Calc. temp. 500 °C
0.01g, Pyrex glass,
100 mL/min of gas
mixture of 10%CH4,
20% O2, 70%N2.
The stabilized, activity of
the palladium follows
SiO2> A1203> SiO2-
A12O3.
[65]
Ce0:67Zr0:33O2, Pt/Ce0:67Zr0:33O2,
Pd/Ce0:67Zr0:33O2, Precipitation,
Calc. temp. 700 °C.
0.50 g catalyst, 1
vol% CH4, 4 vol%
O2, and N2, flow
rate=6.4 L/h.
Activity varies
Pd/Ce0:67Zr0:33O2 >
Pt/Ce0:67Zr0:33O2 >
Ce0:67Zr0:33O2, where
Pd/Ce0:67Zr0:33O2 shows
T50%=300 ,T100%=550°C
[11]
Pd/SiO2, sol-gel, Calc. temp. 500 °C.
200mg, 1 vol% CH4, 2
vol% O2, balance He.
Pd/SiO2 shows T50%=650,
T100%=800°C
[66]
Pd/Co3O4, Impregnation, Calc. temp.
280 °C
horizontal quartz tubular,
1.2% CH4, 12% O2,
balance N2, flow rate= 33
ml/min.
10 wt.% Pd/Co3O4
exhibits T50%=250°C,
T100%=300°C
[67]
Pd/[SO42-ZrO2, MgO, SiO2-ZrO2, SiO2,
SiO2-Al2O3, ZrO2], impregnation,
Calc. temp. 500 °C
100 mg, conventional
flow, (CH4 + O2 + He),
flow rate= 200 cm3/ min.
Pd/SiO2 shows
T50%=625°C, T90%=850°C
[68]
of gold catalysts supported on alumina by direct anionic exchange which showed a better activity than the
catalysts prepared by deposition-precipitation in the CO oxidation reaction. However gold catalysts are not
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favaurable where high temperature of around 1000°C are employed because of low melting point of gold
at 1064°C. Whereas the Au/TiO2 shows a conversion of 100% at temperature of 600°C [50]. Table 8 shows
some of the developments on gold based catalysts.
5.2. Noble Metal Catalysts
Noble metal catalysts are highly active for methane combustion when supported on suitable
support. Thermal stability at high temperature makes it a favourable candidate when high temperature
conditions are employed or when there is deterioration of active sites of catalysts at high temperature [12,
55-56]. The deposition of noble metals on the support such as Al2O3, TiO2, MgO, ZrO2, La2O3 etc. which
are generally cheap compounds having very high surface area per unit mass of the catalyst reduces the
amount of the precious metal with enhanced activity. Noble metals like Pt, Pd and Rh have been extensively
studied with supports like ceria, zirconia, alumina, titania, etc. for methane oxidation [11]. Supported
palladium catalysts have been extensively studied for the catalytic combustion of methane as palladium is
more active for methane combustion than other noble metals [57]. Alumina has been widely used as a
support for the palladium catalyst due to its high specific surface area and low cost. The Pd/Al2O3 catalyst
is active at medium temperatures above 400°C, though its activity is insufficient for the low temperature
ignition, e.g. at 300°C.
The catalytic performance of PdO catalysts supported on various metal oxides (MOx; M = Al, Ga,
In, Nb, Si, Sn, Ti, Y, Zr, Ni) was studied, out of which highest activity was achieved on the most dispersed
catalysts, i.e. Pd/Al2O3–NiO and Pd/SnO2 [57]. Catalytic convertor mainly uses palladium for treatment of
the exhausts, therefore considerable work has been done on palladium in order to increase its activity. Ceria
is a well-known promoter in automotive catalysts due to its enhanced oxygen storage capacity (OSC) [58].
In particular, ceria-supported 2 wt% Palladium catalyst prepared by a deposition–precipitation method was
reported [59] to be highly active for total methane oxidation at low temperature of 300°C. Moreover effect
of ceria on palladium was studied by many authors [11, 60-61] in order to increase the activity i.e.
attainment of 100% conversion of CH4 at low temperature. But none has revealed temperature below 300°C.
However the light off temperature was revealed a bit low at 150°C [11]. Table 9 reveals some of the
development in catalyst on noble metals.
5.3. Base metal catalysts
Because of the expensiveness and non-availability of noble metals, transition metal (Ni, Cu, Mn,
Co, Fe, Cr, etc.) oxides have been considered as practical alternative materials to prepare the catalysts for
the combustion of lean methane [69, 52]. The series of base metals have been supported on different
supports in order to attain the activity as high as that of noble metals in order to decrease the high cost of
noble/gold based catalysts. The combustion/oxidation of CH4 has been explored since 1960 [71]. Since
then many advances have been made primarily for methane combustion. Recently low temperature
oxidation of methane has been reported [72-73] by the oxides of base metals. A series of MnOx(m)–NiO
composite oxide catalysts were prepared by co-precipitation method. Compared with the corresponding
single NiO and MnOx oxides, the MnOx(m)–NiO composite oxide exhibits much higher catalytic activity
in the combustion of lean methane at low temperature. The activity of MnOx(m)–NiO catalysts is related
to the content of manganese in MnOx(m)–NiO. MnOx(0.13)–NiO with an atomic ratio of n(Mn)/(n(Mn) +
n(Ni)) being 0.13 performs the best in the lean methane combustion. Methane conversion over
MnOx(0.13)–NiO reaches 96% at 396°C and 100% at 450°C, which is outstanding for a non-noble catalyst
compared with those reported in the literature.
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Transition metal has been reported recently with the mixture of noble metals which is as good as
that of noble metal catalyst. Cobalt has been reported many times and still in the development process [22,
Table 10. Base metal catalysts used for methane oxidation
Catalyst, Pepn. Method
Exp Operating
Parameters
Remark Ref
Cu, Mn-BaAl12O19, Sol-
gel,
Calc. temp. 1200 °C
0.50 g, Quartz
microreactor,
1 vol.% CH4, 4 vol.%
O2 and 95 vol.% N2 ,
SV= 20000–25000 h-
1.
Cu sites are intrinsically more active than
Mn sites, BaMn3 shows T50%=600,
T100%=800, Where BaCu shows T50%=700,
T100%=800
[78]
Co-Mg/Al, prepared by
calc. of CoxMg3−x/Al
hydrotalcites,
precipitation at 800 °C.
0.5 g, a mixture of 1%
vol of CH4 in air, total
flow rate of 400
mL/min,
GHSV= 50,000 h-1.
catalysts with molar ratio of 1.5 of Co/Al,
shows T50%=540 °C, T100%=800 °C.
[79]
MnOx–CeO2,
Coprecipitation, 500 °C,
Plasma, Modified
coprecipitation.
150 mg, fixed-bed
reactor, 20% CH4 and
40% O2 in Ar, and
SV=40,000 mL/g h.
Ppep. T50%(°C) T80%
(°C)
T100%
(°C)
MP 350 360 480
CP 410 480 NA
PP 380 410 NA
[80]
oxides of
cobalt/manganese,
sol–gel, Calc. temp. 850
°C
0.1 g, quartz tubular
reactor, flow rate of
100 mL/min, CH4 (0.5
vol.%), O2 (0–8
vol.%), water vapor
(5 vol.%) and argon.
CoOx shows good results with T50%=425
°C and
T100%=550 °C.
[77]
CuO/AI2O3,
Wet impregnation,
Calc. temp. 500 °C
500 mg, quartz micro
reactor, 1 % CH4, 4%
02, and 95% N2, flow
rate 6.41 h-1
4.8% CuO
shows T50%=450 °C, T100%=570 °C
[82]
CeO2,
ZrO2,HfO2,Ce0.8Zr0.2O2,
and Ce0.8Hf0.2O2,
Precipitation,
coprecipitation, calc.
temp. 920 °C
quartz mic roreactor,
CH4 (l%), O2(4%) and
He, SV=34000 h-1.
Ce0.8Zr0.2O2 shows
T50%=520 °C, T100%=670 °C
[83]
Co/MgO, Wet
impregnation, Calc.
temp. 800 °C.
0.05 g, quartz reactor,
2% CH4, 8% O2 and
N2, SV=480,000 h-1.
Cobalt exhibit good activity when MgO
employed
[84]
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MnOx–NiO, co-
precipitation, Calc. temp.
500 °C.
200 mg, quartz tubular
flow micro reactor,
1.0 vol.% CH4, 19
vol.% O2, and
balanced argon,
WHSV= 32 000 mL/g
h.
MnOx(0.13)–NiO exhibits
T50%=360 °C and
T100%=525 °C.
[72]
CoCr2O4, co-
precipitation , Calc.
temp. 700 °C.
250 mg, 2000 ppm
CH4, 10 vol.% O2, and
N2, GHSV= 36000
mL/h g.
Catalyst containg 5% Ce, shows T50%=375
°C and
T100%=525 °C.
[76]
CoxCry, Coprecipitation,
Calc. temp. 700 °C.
500 mg, 2000 ppm
CH4, 10 vol.% O2, 10
vol.% and N2, flow
rate= 300 mL/min,
GHSV= 36000 mL/h
g.
Co/Cr=0.5 shows, T50%=400 °C and
T100%=550 °C.
[73]
Co3O4/CeO2 and
Co3O4/CeO2–ZrO2, co-
precipitation,
impregnation, Calc.
temp. 750 °C.
0.3% of CH4 + 4.8%
of O2 in He,
WHSV= 12,000 mL/
g h,
SV= 60000 mL/ g h
Catalyst with 30% Co3O4 shows
T50%=400 °C, T100%=700 °C
[13]
Cerium-chromium/γ-
Al2O3
incipient
wetness impregnation,
Calc. temp. 500, 800 °C.
300 mg, stainless-
steel fixed bed, 2.0
vol.% CH4,
8 vol.% O2, 90 vol. %
N2, MHSV= 20,000
mL/g h.
3 wt.% Ce displayed T50%=375°C,
T100%=475 °C
[85]
CeO2-ZrO2, urea
hydrolysis,
Calc. temp. 500-900 °C.
100 mg, differential
quartz tube
microreactor, 2.0
vol.% CH4,
21 vol.% O2 and He,
flow rate= 100
mL/min.
Ce0.75Zr0.25O2 calcined at 500 °C shows
good activity with T50%=550 °C,
T100%=650 °C
[86]
CeO2-ZrO2,
Precipitation, Calc. temp.
700 °C.
500 mg, Microreactor,
1 vol.% CH4, 4 vol.%
O2 diluted in N2, Flow
rate=6.4 l h-1,
GHSV=20 000 h-1.
CeO2 shows T50%=575 °C, T100%=750 °C [87]
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CuO/ZrO2, Wet
impregnation,
Calc. temp. 600 °C
200 mg, continuous
flow fixed-bed, 20
vol.% CH4, 40 vol.%
O2, 40 Ar.
ZrO2- 5% CuO shows
T50%=400 °C
T90%=425 °C
[88]
75]. From the past literature it could be concluded that complete combustion of CH4 can be attained at
temperature of around 650°C [11, 73, 76-77]. However the conversion temperature can be decreased by
varying the composition, preparation method, doping of other metals etc [76]. The minimum temperature
reported for 100% conversion of methane is 450°C by [72] using MnOx(0.13)–NiO, where 0.13 represents
the atomic ratio of [Mn]/[Mn + Ni]. i.e. the content of Mn in composite oxide. Various other parameters
such as addition of water vapour, SO2 [73] have also been studied. Manganese oxide has been reported for
the lean oxidation of methane at low temperature [72]. The author has given a brief explanation about the
structure and various properties using the advanced techniques of characterisation such as XRD, XPS,
TEM, HRTEM, XAS, and FTIR. It was demonstrated that activity varies as MnOx(0.13)–NiO >
MnOx(0.10)–NiO > MnOx(0.17)–NiO > MnOx(0.25)–NiO > NiO > MnOx. Table 10 shows some of the
developments on base metal catalysts.
5.4. Perovskites
The catalytic properties of perovskites were systematically studied since 1977 and these materials
have been used for the total oxidation of methane [89-90]. Perovskites are mixed oxides of general formulae
ABO3, whereas A = Lanthanide ion, and B= Transition metal ion. Perovskites type transition metal oxide
mixtures showed comparable activity, stability when properly prepared. They are attractive with respect to
noble metals because of their lower cost and the absence of problems such as sublimation and volatilization.
The perovskite structure allows for a number of substitutions i.e. partial substitution of either A or B cation
and compositional modifications, so that different oxidation states for the transition metal are possible, as
well as anionic or cationic defectivity [91]. Considerable amount of work has been done on perovskites in
order to enhance the catalytic activity of the structures [23, 93, 94]. According to the data accumulated in
literature complete oxidation of CH4 takes place at temperature of 600 °C or above [16]. This is a bit high
when compared with other catalysts i.e noble or base metal catalysts.
Surface area plays a major role for the methane oxidation and therefore umpteen efforts have been
made to increase the surface area. Considerable efforts have been done by Kaliaguine and colleagues [23]
for developing new sized high surface area of 100 m2/g using high energy milling. The process is simple,
efficient, inexpensive and does not require any heating step for producing a perovskite that may easily show
a very high specific surface area. Another advantage is that the obtained perovskite has a high density of
lattice defects thereby showing a higher catalytic activity. Series of perovskites have been reported [16] for
methane conversion which increases the surface area and hence the catalytic activity using gold in the
substitution of La, instead of Sr, in LaFeO3 and LaFe0.5Co0.5O3 perovskite oxides, at low temperature (below
700 °C). Series of many efforts have been done using many metals [95-96]. However, when compared with
other catalysts; catalytic activity, poisoning, low surface area are some of the other facts where perovskites
cannot withstand with the other catalysts [93]. Gold was also depicted in many perovskites for increasing
the activity of the catalyst. Chaudhary [16] reported series of perovskites for methane conversion in which
Ag was doped in substitution of La in LaFeO3 and LaFe0.5Co0.5O3 perovskite. This causes large increase in
the catalytic activity of the perovskite in the complete combustion of methane. Ag-doped LaFe0.5Co0.5O3
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shows the highest methane combustion activity i.e. T100%=670°C. Table 11 depicts some of the advances
on perovskites catalysts.
Table 11. Accumulated literature on perovskites for methane oxidation
Catalyst, Pepn. Method
Exp Operating
Parameters
Remark Ref
Pd/ZrO2,
Pd/LaMnO32ZrO2
Solution combustion synthesis
(SCS),
Calc. temp. 900 °C
0.1 g, U-shaped quartz
tube, 50 cm3/min, CH4
(2%), O2 (14%) and He
(balance)
Pd/LaMnO32ZrO2
Shows T50%=375°C
T90%=425°C, T100%=475°C
[97]
La0.9FeO2.85,
La0.8FeO2.70,
La0.7FeO2.55,
LaFeO, low-temperature thermal
decomposition,
Calc. temp. 473 K
400 mg, tubular
continuous flow,
30mL/min,
CH4 and O2 (ratio 1:6)
At temp. of 748 La0.9FeO2.85
shows T34%,
La0.8FeO2.70 shows T30%,
La0.7FeO2.55 shows T25%,
LaFeO shows T20%
[96]
LaMnO3 ,
LaMnO3·17MgO
combustion synthesis
0.05 g, quartz reactor,
2% CH4, 8% O2 and
N2.
LaMnO3·17MgO shows
T50%= 526 °C,
LaMnO3 shows 461°C
[95]
Ce1-xLaxO2-x/2/
Al2O3/FeCrAl, monolith catalysts,
where x varies from 0 to 1. Calc.
temp. 500 °C
quartz flow reactor,
2 vol.% CH4 in air,
SV= 7802, 15,604 and
31,208 mL/g
Ce1-xLaxO2-x/2/ Al2O3/FeCrAl
shows T50%=550 °C, T100%=700
°C.
[98]
LaAl1-xFexO3(0<X<1), citerate,
calc. temp. 800 °C
0.4 g, 0.4%
CH4, 10% O2, and N2
as balance, SV=
40,000 cm3 STP h-1 g-1
Catalyst with x=0.6 shows
T50%=800 °C, T100%=950 °C.
[18]
LaFeO3,
LaFe0.5Co0.5O3,
La0.7 Sr (or Ag)0.3FeO3 and La0.7Sr
(or Ag)0.3Fe0.5Co0.5O3
Co-precipitation,
Calc. b/w 750-900 °C.
0.1 g, quartz micro-
reactor, 4 mol%
methane in air, SV= 51
000 cm3/g h.
La0.7Sr (or Ag)0.3Fe0.5Co0.5O3
and La0.7 Sr (or Ag)0.3FeO3
shows T100%=700 °C
[16]
LaMnO3
La2O3/Al2O3MgO
30% LaMnO3/(5% La2O3/Al2O3)
quartz down flow
reactor,
0.4 vol% CH4 and 10
30% LaMnO3/(5%
La2O3/Al2O3) shows T100%=650
°C
[17]
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20% LaMnO3/MgO
30% LaMnO3/(5% La2O3/Al2O3)
20% LaMnO3/MgO,
Calc. temp. of 800 and 1100 °C
vol.% O2 remaining
gas being nitrogen,
Flow rate 0.09 g s/N
cm3.
whereas 20% LaMnO3/MgO
exhibits T100%=625 °C
6. METHANE OXIDATION OVER COBALT BASED CATALYST
6.1 Early progress in catalyst development
Exploration of cobalt based catalysts started in nineteenth century [101] and is still in the
development process [102]. Apart from oxidation/ combustion, various advance studies have been made on
cobalt supported catalyst. Such as synergistic effect [103], effect of modification (supported and
unsupported catalyst) [13, 14, 15] and new synthesis methods [104], nano-particles exhibit promising
catalytic activities [105]. Cobalt has been extensively studied particularly for the purification of automotive
exhaust which has been depicted in several reports [106, 55].
6.2 Catalyst Activity and Stability of cobalt based catalyst
Bray et al. in 1920 [107] investigated many cobalt based catalysts for the first time for CO
oxidation. Since then the catalytic behaviour of a number of Cobalt based catalyst for methane oxidation
have attracted much interest. Among the catalyst for methane oxidation, cobalt based catalyst has received
a lot of attention due to its remarkable properties, such as moderate saturation magnetization, relatively
large magnetic anisotropy, high Curie temperature, incredible mechanical hardness, excellent chemical
stability and last but not least low price. Many patents were filed [71] in early stage of catalyst development
for use as exhaust control catalyst for internal combustion engine. Many researchers [108] around the globe
tested and explored cobalt based catalyst and found that it is a best suited catalyst for methane oxidation.
Cobalt is very active for CO oxidation even at temperature of -77°C [109]. However according to the
literature data minimum temperature required for methane oxidation is around 550°C [110, 103]. McCarty
et al. 1997 [111] studied specific rates for catalytic combustion of dilute methane on various oxides and it
was demonstrated that activity for methane combustion with the supported oxides follows the order: PdO
> RuO2 > Co3O4 > CuO > NiO > Fe2O3 > Mn2O3 > Cr2O3.
A recent work reported by Raluca et al. [104] argued that cobalt is a rival of platinium based catalyst
for methane oxidation. In their work they demonstrated that the two oxides CoFe2O4 (I) and CoFe2O4 (II)
show same activity as that of platinum based catalyst (1% wt. Pt/Al2O3) [104]; also in terms of stability
CoFe2O4 (II) is equally stable as that of 1% wt. Pt/Al2O3. Further the work of Fino and colleagues [112]
on CNG exhaust emissions on various Spinel-type-oxide catalysts states that cobalt (CoCr2O4) comes out
to be the best catalyst among the other catalysts i.e. activity varies as CoCr2O4 > MnCr2O4 > CoFe2O4 >
MgFe2O4. Cobalt is also reported on various supports for methane oxidation [103]. Xiao et al. [103]
prepared series of cobalt-based catalysts with different supports for methane oxidation, the supports used
are TiO2, Al2O3, MgO, and ZrO2 and it was demonstrated that ZrO2 supported cobalt catalyst (1% Co) was
found to have the highest activity amongst other supported and bulk Co3O4 catalysts. Whereas, tremendous
results in terms of activity of cobalt based catalyst, the author has described the Influence of cobalt precursor
and fuels on the performance of combustion synthesized Co3O4/γ-Al2O3 catalyst for methane oxidation
[113]. A series of nanosized Co3O4/γ-Al2O3 catalysts have been prepared using a combination of wetness
impregnation, the obtained catalyst has complete conversion of methane in between 400–425°C which is
very less in comparison with other catalyst prepared by traditional methods. Moreover the stability of the
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catalyst was also not disturbed as after two days of catalytic performance, no considerable deactivation of
combustion-synthesized catalysts was observed and their particle size remained the same. A recent work,
demonstrated that the addition (Co/Sm molar ratio=0.98/0.02 and 0.95/0.05) of samarium (Sm) increased
the activity of spinel Co3O4 catalyst for CH4 oxidation prepared by co-precipitation method. The complete
conversion of methane takes place at temperature of 450°C [102].
6.3 Characterization studies on cobalt based catalyst
In order to elucidate the relationship between the performance and its physico-chemical properties
of catalyst, characterisation of the catalyst is a fundamental step which provides various parameters.
Characterization includes textural properties like surface area (N2 adsorption), pore volume and pore size
distribution [49], active surface area, and dispersion of active components (chemisorptions) [114], phase
identification (X-ray Diffraction, XRD) [115], surface composition (XPS), redox properties (temperature
programmed reduction/oxidation, TPR/TPO) [114], morphology (scanning electron microscopy-electron
dispersive spectroscopy, SEM-EDS/transmission electron microscopy, TEM) [116] etc. All of these
properties help in better understanding of the catalyst.
Surface area, pore volume and pore size distribution is important tool in determining the activity
of particular catalyst moreover with variation of these three parameters activity can be increased or
decreased and hence can be modified accordingly. A study reports that the catalyst (Co3O4/samarium)
prepared by co-precipitation method and characterization was done by various techniques such as N2
adsorption-desorption with Brunauer-Emmett-Teller technique (N2-BET), X-ray powder diffraction (XRD),
thermal gravity analysis differential scanning calorimetry (TGA-DSC), H2 temperature programmed
reduction (H2-TPR) and X-ray photoelectron spectroscopy analysis (XPS). BET specific surface areas of
pure Co3O4 was observed about 20 m2/g, pore volume of 0.24 cm3/g and crystalline size of 65 nm. Moreover
the addition of samarium increased all these parameters i.e. BET becomes for sample having molar ration
of 0.90/0.10, BET specific surface areas becomes 75 m2/g, pore volume of 0.38 cm3/g and crystalline size
of 21 nm [102]. However the characterization results shown by Miao et al. [49] of Au–Pt/Co3O4 and Au–
Pd/Co3O4 catalyst prepared by co-precipitation technique, reveals some of the different from Xu et al. [102].
Pure Co3CO4 have BET surface area of 57.1 m2/g. The surface area increases tremendously to 139.8 m2/g
with the addition (1.58%) of palladium to Co3CO4. Calcinations temperature plays an important role in the
physio-chemical properties of the catalyst.
6.4 Cold Start Emission Control
Several studies confirmed that during the cold start phase of the engine, unburned HCs, CO is
emitted even if the vehicle is fitted with TWC [117-118]. Cold start is that phase, when the engine is just
turned on i.e. when the temperature of the tailpipe or engine is under transformation phase between ambient
temperature and fully warmed up condition. It has been also concluded that the extra emissions of cars
during cold start period depend on three major parameters: 1) The combustion principle (diesel/gasoline);
2) The ambient temperature which is considered to be equal to the vehicle’s temperature at test start; and
3) The test cycle driven [117]. Further Cold start extra emissions can be subdivided into two parts: (i) excess
emissions due to the starting of the engine and (ii) excess emissions during the warming-up process of the
engine and the catalyst [119]. The extent of this cold-start phase is also dependent on the characteristics of
the vehicle.
During cold start, TWC is totally inactive; because the catalytic converter has not yet warmed up
i.e TWC has not attained light-off temperature which is required (350°C) for catalytic conversion of CO
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and HCs. TWC will not be able to function effectively until it reaches the light-off temperature as the
conversion efficiency depends strongly on the working temperature and is practically zero during the
starting and warming up period. Vehicles equipped with TWCs in catalytic converters presently are able to
achieve the reductions of CO, NOx and unburned HCs. However these emissions are reduced up to 95%
when TWC is fully warmed up [120]. Together, these two important factors are responsible for the higher
tailpipe emissions during the cold-start phase. The extent of this cold-start phase is also dependent on the
outside temperature and characteristics of the vehicle. Cobalt based catalysts can oxidise CO even at the
temperature of -77°C [109] which is a paramount in vehicular industry. Cobalt is reported by numerous
authors for its oxidation ability even at low temperature [73]. Thus it can be used in TWC converter to
overcome the problem of cold start phase.
7. CATALYST DEACTIVATION
Deactivation refers to loss of activity of the catalyst with time. This time can vary from a few
minutes for a laboratory catalyst to 150,000 miles duration for an auto-exhaust catalyst. Deactivation is a
complex phenomenon as it depends upon number of parameters such as poisoning, fouling, thermal
degradation (sintering, evaporation) initiated by the often high temperature, mechanical damage and
corrosion/leaching etc [122]. Catalyst activity and stability plays an important role in three-way catalysis
because it controls the economics of the vehicular industry. Loss of catalyst activity can occur due to loss
of the catalytically active surface, the change in the metal surface, changes in the catalyst structure or may
be the combination of all three[123]. Numerous parameters are responsible for the deactivation of the
catalyst however they can be described in terms of Mechanical, Chemical and Thermal phenomenon as
described in Table 12.
Table 12. Cause of catalyst deactivation [124]
Type Cause Results
Mechanical Particle failure channelling, plugging
Fouling Loss of surface
Thermal Component volatization Loss of component
Phase changes Loss of surfaces
Compound formation Loss of component and surface
Sintering Loss of surface
Chemical Poison adsorption Loss of active sites
Coking Loss of surface, plugging
7.1. Thermal Deactivation and Stability
Loss of activity in metal catalysts via loss of surface area owing to sintering is a universal
phenomenon. Hughes [125] gave the following increasing order of stability for metals:
Ag < Cu < Au < Pd < Fe < Ni < Co < Pt < Rh < Ru < Ir < Os < Re.
Lower the metal is in this series the more troublesome deactivation by sintering will be and the more care
has to be taken to minimize the effect. For example, it is not surprising that copper based catalysts are more
susceptible to sintering than the cobalt and nickel catalysts used in various oxidation or hydrogenation
process. Generally a minimum of 1m2/g surface area is required for useful field application.
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7.2. Chemical Poisoning
Catalyst deactivation is inevitable; it occurs when the poison molecule becomes reversibly or
irreversibly chemisorbed to active sites. The poisoning molecule may be a reactant, by-product or product
in the main reaction or it may be an impurity in the feed stream. For example water is produced during the
catalytic oxidation of methane, which sticks to the number of active site and inhibits the reaction [77]. This
decreases the surface area of catalyst available for reaction so more un-reacted pollutants such as NOx, CO,
and hydrocarbons are released into the atmosphere. However the data presented in literature is contradictory
sometimes [77]. Other catalyst poisons include manganese, silicone, phosphorus, and zinc [126].
Li [77] studied promoting effect of water vapor on catalytic oxidation of methane over
cobalt/manganese mixed oxides water vapour. When 5% water vapour is introduced into the reaction feed,
methane conversion on MnOx decreased from 47 to 41% at 500°C, whereas only minor changes were
observed on CoOx catalyst under the similar conditions. However, it was found that a remarkable increase
in methane conversion (70 to 83%) was observed on the CoMn and CoMn2 catalysts when water vapour
(5%) was introduced into the feed gas.
Sulphur is the most problematic among all the other poisons [127] and is usually found in all
commercially available fuels as an impurity. It is reported that concentration of around 150 ppm sulphur
present in fuel, gives an exhaust concentration of around 10 ppm of sulphur [127]. The decrease in
concentration is due to chemical poisoning in the engine converting Sulphur to SO2.
8. EFFECT OF CATALYST PREPARATION PARAMETERS ON CATALYTIC ACTIVITY
8. 1 Cobalt as a support
Liotta et al. did extensive study devoted to cobalt catalyst in a series of publications. In 2005, Liotta
et al. [13] prepared Gold-based catalysts supported on cobalt oxide (Co3O4), Ceria (CeO2) and mixed oxides
(Co3O4–CeO2) using co-precipitation method for methane oxidation. It was demonstrated that Au supported
on Co3O4 was the most active for total oxidation of methane i.e. T100%=600°C. The activity of the catalysts
was described i.e. the presence of Co2+ and Co3+ ions are active sites for methane activation whereas CeO2
in AuCoCe plays the role of a structural promoter. Further cobalt-ceria catalyst was tested for methane
oxidation by the same author [14] over Co3O4/CeO2 composite oxides with different cobalt loading (5, 15,
30, 50, 70 wt.% as Co3O4) prepared by co-precipitation method. Complete oxidation of methane takes place
at 750°C by composite catalyst containing 30% by weight of Co3O4 ( Co and Ce in atomic ratio close to
1:1) which is bit higher in comparison to the gold based catalyst. Furthermore, the optimised catalyst of
same composition prepared by co-precipitation method was again reported for methane oxidation but in
cordieritic Honeycomb support with Pd/Pt for methane oxidation [81]. The bimetallic, Pd–Pt catalyst
obtained by impregnation of the supported Co3O4 (30%)–CeO2 (70%) with Pd and Pt with total metal
loading of 50 g/ft3. However the addition of palladium and platinum does not cause much change in the
complete combustion of methane as the conversion temperature is not reduced significantly.
Junhua Li et al. 2009 [52] prepared manganese cobalt oxides by co-precipitation method with
different Co/Mn ratios for methane combustion. A significant improvement in the conversion temperature
for methane oxidation was achieved i.e. T100%=360°C in comparison to the previous reported cobalt catalyst.
This low oxidation temperature was observed for cobalt –manganese catalyst (Co/Mn molar ratio of 5:1).
The increase in catalytic activity of the catalyst is due to manganese (as the dopant), which caused disorder
in the spinel structure of cobalt oxides. This disorder consequently enhanced the activity of the reactive
ions in the octahedral sites and probably facilitated the de-hydroxylation steps, thus leading to increase in
the catalytic performance of the Co/Mn mixed oxides.
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8.2 KINETICS OF CH4 OXIDATION OVER COBALT BASED CATALYST
The typical graph of conversion of CH4 as a function of temperature of the catalyst bed is shown
in Figure 3, with three distinct regions, viz. kinetic controlled region-X, light-off region-Y (uncontrolled)
and steady state operation (diffusion controlled) region-Z.
In the first region-X (area X in Fig. 3), the rate of reaction increases relatively slowly with increasing
temperature and the rate is controlled by the kinetics of the chemical reaction. The reaction occurs only on
the catalyst surface in this region and the catalyst performance is dictated by its intrinsic activity. A further
increase in temperature leads to an exponential increase in rate (area Y in Fig. 3) to the point where heat
generated by combustion is much greater than heat supplied as a result of rapid exothermic CH4 oxidation.
Beyond region-Y oxidation reaction continues with increasing temperature and the system regains a self-
sustained steady state region-Z (area Z in Fig. 3) at very high conversion. In this region mass transfer of
gases to the catalysts is the rate determining step. One important factor in the catalytic combustion of
Methane is ‘light off’. This can be defined in various ways but refers to the temperature at which mass
transfer control becomes rate controlling. Because of the shape of the curve (Fig. 3), the definition of light-
off temperatures as the temperature at which conversion reaches 10%, 20% or 50% makes little difference.
The kinetic analysis involves exclusive data corresponding to conversions below 10-15%, therefore in order
to establish kinetic model various Light-off curves are required [128]. Moreover ideal plug flow conditions
are required in packed bed reactor to generate meaningful kinetic data otherwise the data is of no use [129,
130].
Figure 3. Effect of Temperature on Methane Oxidation [103]
A number of kinetic studies have been conducted on various cobalt based catalysts [114, 121, 106,
71]. The steady-state values of methane and oxygen partial pressures are used to describe the kinetics of
methane oxidation over the catalysts studied. Various correlations are available for the calculation of
average methane reaction rate. However the information available in the literature on the kinetics is often
conflicting. The reaction is reported to be of the first order or less in CH4 concentration (partial pressure).
Because of overheating as a result of high conversions of methane, chemical kinetics are often affected by
mass and heat transfer [55]. Below given are some re-arranged correlations for the evaluation of reaction
rate [121].
W𝑐𝑎𝑡
FCH4,0
= ∫ 𝑑𝑋𝐶𝐻4
−𝑟𝐶𝐻4
𝑋𝐶𝐻4,𝑜𝑢𝑡𝑋𝐶𝐻4,𝑖𝑛
= 1
(−𝑟𝐶𝐻4)𝑎𝑣𝑔 ∫ 𝑑𝑋𝐶𝐻4
𝑋𝐶𝐻4,𝑜𝑢𝑡𝑋𝐶𝐻4,𝑖𝑛
= 𝑋𝐶𝐻4,𝑜𝑢𝑡
− 𝑋𝐶𝐻4,𝑖𝑛
(−𝑟𝐶𝐻4)𝑎𝑣𝑔
(5)
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Where Wcat is the catalyst weight (g), FCH4,in is the initial methane flow-rate (mol/min), XCH4 is the
conversion of methane, and -rCH4 is the rate of methane reaction. Many researchers [121, 100] have reported
CH4 oxidation to be of first order as given below:
(−𝑟𝐶𝐻4)
𝑎𝑣𝑒= 𝑘𝑃𝐶𝐻4
𝛼 𝑃𝑂2
𝛽 (6)
where k is the rate constant, PCH4 is the average partial pressure of methane, PO2 is the average partial
pressure of oxygen, and 𝛼 and 𝛽 are the apparent reaction orders for methane and oxygen, respectively. A
kinetic study made by Klvana, D. et al. 1994 [99] for methane combustion over cobalt perovskites, has
revealed that a simple first-order model gave a good fit to the experimental data at low temperatures of 375
to 650 oC.
𝑟𝐶𝐻4= 𝑘𝑃𝐶𝐻4
(7)
However, two-term model was also related to the reaction mechanism, but is not fruitful for the cobalt based
catalysts.
𝑟𝐶𝐻4= 𝑘1 + 𝑘2𝑃𝑂2
0.5𝑃𝐶𝐻4 (8)
9. CONCLUSION
To curb the menace of vehicular pollution, the choice of the appropriate catalyst for TWC
converters is a fundamental step in terms of activity, selectivity, durability, availability and cost, for the ever
increasing number of vehicles on roads. The use of noble metals has detrimental effects on the commercial
cost of the catalyst, so focus has recently turned to transition metal base catalysts. Cobalt based catalysts
are reasonable and appropriate in the total oxidation of methane under lean-burn conditions. It exhibits
comparable activity for CH4 oxidation to that of precious metal auto exhaust purification catalyst. Further,
it is comparatively very cheap compared to noble metals. Moreover, cobalt based catalyst can
simultaneously remove all the three major pollutants (CO, HCs and NOx) from the exhaust in the
temperature region considerably lower than flame or explosion temperatures. However the poisoning
compounds with vehicular exhaust reduce or affect the activity of the catalyst. Modification of the catalyst
with the addition of suitable support, promotor, pretreatment and advance synthesis methods would lead to
the desirable performance of cobalt based catalyst. Performance of cobalt based catalyst considerably
improved when prepared as nano-structured materials.
Although cobalt based catalyst is a well-studied catalyst, still further research is required in order
to develop this catalyst following newer routes investigated recently for oxide catalysts, suitable for
vehicular exhaust TWC converter. It is, therefore, proposed to thoroughly investigate unsupported as well
as supported cobalt based nano-sized catalysts for future application of CH4 oxidation and also in TWC
converters. The present paper opens a new horizons or opportunity for the “cobalt based catalyst” as
competitive catalysts in methane combustion reaction.
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