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| Introduction to the Catalytic Combustion of Diesel Soot |
| Jian Liu, Zhen Zhao* and Chunming Xu* |
| State Key Laboratory of Heavy Oil, College of Science, China University of Petroleum, Beijing, 102249, P.R. China |
| *Corresponding author: |
Dr. Zhen Zhao
State Key Laboratory of Heavy Oil
College
of Science, China
University of Petroleum, Beijing
102249, P.R. China
E-mail: zhenzhao@cup.edu.cn |
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Chunming Xu
State Key Laboratory of Heavy Oil
College of Science, China
University
of Petroleum, Beijing
102249, P.R. China
E-mail: xcm@cup.edu.cn |
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| Received August 18, 2011; Accepted November 14, 2011; Published November
16, 2011 |
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| Citation: Liu J, Zhao Z, Xu C (2011) Introduction to the Catalytic Combustion of
Diesel Soot. J Pet Environ Biotechnol 2:e102. doi:10.4172/2157-7463.1000e102 |
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| Copyright: © 2011 Liu J, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited. |
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| The heterogeneous catalytic reaction containing solid particle as
reactant is a class of important and complex chemical reaction. The
solid particles such as soot emitted from diesel engines, are becoming
increasingly important in the context of cleaning air and lowering
automotive emissions. Diesel soot particulates are defined by the US
Environmental Protection Agency (EPA) as "all compound collected
on a pre-condition filter in diluted diesel exhaust gases at a maximum
temperature of 50°C". The catalytic combustion is considered as one of
effective method for purifying these diesel soot pollutants. Supported
oxide catalysts were originally developed for the catalytic combustion,
but they are not especially active at low temperatures (<400°C) under
the loose contact condition between catalysts and soot [1,2]. Noble
metal catalysts such as platinum and gold, on the other hand, are
efficient and exhibits high activity [3,4]. However, Pt is very expensive
and causes the great emissions of sulfates. Thus, the development
of active and stable catalysts without noble metals for the catalytic
combustion of soot particles at low-temperature remains a significant
challenge. |
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| Many other attempts have been made to develop catalyst for
diesel soot combustion on the basis of simple metal oxides and mixed
oxides. As we consider the results of this study, two challenges remain
for future research, i.e., to design appropriate catalytic materials that
possess strong redox ability and to design new systems that improve
the contact between solid particles and catalyst. On the one hand, the
catalysis nature of solid particle combustion is a deep oxidation process.
Therefore, the redox property of the catalyst determines its' intrinsic
activity. Some ideas should be considered to get high redox property.
One is to select the transition metal elements, whose ions have several
oxidation states and possess good redox properties such as, Mn, Co, Fe
etc. The other one is to tune the redox property of complex oxides with
the alkali metal or alkali earth metal ions. |
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| On the other hand, the reactions of diesel soot oxidation is three phase
catalyzing reactions. The reaction supposedly takes place at
the three-phase boundary among a solid catalyst, a solid reactant
(soot) and gaseous reactants (O2, NO). The efficiency of a catalyst is
strongly influenced by the contact between solid particles and catalyst.
Though by using many other mechanical methods, such as ball milling,
mechanic mixing, part of the catalyst particles may be contacted with
soot particle materials, this method gave only tight contact condition
between soot and catalyst. The condition cannot be easily obtained
under the real exhaust gases emitted from diesel engine. Therefore, the
catalytic effect is weakened in the real application. Some strategies have
been proposed for getting the good contact between the catalyst and
soot [5,6]: |
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| a) To seek the oxides or mixed oxides with low melting points
or low eutectic melting points. Molten salt catalysts under
realistic conditions give promising results due to their active
components with high mobility. |
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| b) To synthesize nano-particle oxide catalyst. Since the nano particle
catalysts have higher fraction of surface atoms than the
conventional catalysts with large particle size, the catalyst is in close proximity with solid particles and more catalyst surface
is exposed to reaction atmosphere. A decrease in particle size
from 1 µm to 10 nm will increase the probability of contact
each other by 100 times. For a given catalyst, the more the
surface of catalyst is exposed to solid particles, the stronger the
catalytic effect. Therefore, the catalytic effect of nano-particle
catalyst will be stronger than that of the big particles when the
same amount of catalysts is used. |
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| Similarly to noble metal Pt catalysts, to search the efficient
oxide catalysts for catalyzing the NO to NO2. A strategy successfully
developed by Johnson Matthey for their DPF is to use the oxidizing
properties of NO2 to combust the soot particulates (PM), i.e. the
famous CRT technology (continuous regeneration traps) 7. In the CRT
system the removal of PM by oxidation with NO2 is achieved by placing
an Pt-based oxidation catalyst upstream of the trap in order to convert
NO in the exhaust into NO2. The oxidizing ability of NO2 is much
stronger than that of O2 and it can directly oxidize the soot particle
to CO2. If the cheap metal oxide catalysts might oxidize NO to NO2
like Pt catalyst they could subtly change solid (soot) -solid (catalyst)
contact into solid (soot) - gas (NO2) - solid (catalyst) contact. Then, the
catalyst should have good activity for oxidizing soot to CO2 and CO
under loose contact conditions. |
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| It is noted that the pore size of catalyst would affect the catalytic
combustion of solid particles. The pore sizes of normal catalysts are
less than 10 nm, therefore, big soot particles are difficult to enter
inner pores of the conventional catalysts. Thus, the contact area and
the exposed active site amounts are limited and only the outer surface
area is the valid reactive surface area for the title reactions. And then
the catalytic performances of catalysts would be largely affected.
Three-dimensionally ordered macroporous (3DOM) materials with
big pore size (>50 nm) can permit solid reactant to enter the inner
pores of materials and to easily transport and diffuse. Therefore, the
tangible active points between reactant and catalysts will be increased
dramatically. |
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| In the end, the catalysts to be the used in the combustion of
soot particles must be stable both at high temperatures. In any case,
the catalyst stability is a key factor in determining its applicability in
a commercial scale. Another key factors to be taken into account is the presence of other gaseous products that may affect the catalytic
behavior. In the case of diesel exhaust, the presence of CO2, NO2, and
water is unavoidable, and in the majority of cases, SO2 is also important. |
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| References |
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- Krishna K, Bueno-López A, Makkee M, Moulijn JA (2007) Appl Catal B 75:
189-195.
- Liu J, Zhao Z, Xu C, Duan A, Jiang G (2010) Ind Eng Chem Res 49: 3112-3119.
- Oi-Uchisawa J, Obuchi A, Ohi A, Nanba T, Nakayama N(2008) Powder Technol
180: 39-44.
- Wei Y, Liu J, Zhao Z, Chen Y, Xu C, et al. (2011) Angew Chem Int Ed 50: 2326-
2329.Russo N, Furfori S, Fino D, Saracco G, Specchia V (2008) Appl Catal B
83: 85-95.
- Sophie A. Cottilard (editor) 2011 Catalytic Combustion. Chapter 2: The
Combustion of Soot. Nova Science Publishers, Inc: 1 - 29.
- Burch R (2004) Catal Rev 46: 271-334.
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